tanning chemistry: the science of leather tanning chemistry

www.rsc.org/books

Tanning ChemistryThe Science of Leather

Anthony D Covington

9 780854 041701

ISBN 978-0-85404-170-1

Tanning Chemistry: The Science of Leather is the only current text on tanning science, and addresses the scientific principles which underpin the processes involved in making leather. It is concerned with the chemical modification of collagen, prior to tanning and the tanning reactions in particular.

The aim of the book is to provide leather scientists and technologists with an understanding of how the reactions work, the nature of their outcomes and how the processes can be controlled and changed. The objective is to synthesise a scientific view of leather making and to arrive at an understanding of the nature of tanning - how the wide range of chemistries employed in the art can change the properties of collagen, making leather with different properties, especially conferring different degrees of stabilisation as measured by the hydrothermal stability. The impact of leather making on the environment is a thread running through the text, with the assumption that better understanding of the science of leather making will lead to improved processing. The book also reflects on the ways leather technology may develop in the future based on the foundation of understanding the scientific principles.

The book also provides the reader with insights into the role science plays in leather technology. It provides fundamental understanding, which should be the basis for scientific and technological research and development for the benefit of the global leather industry. Tanning Chemistry: The Science of Leather is ideal for students, leather scientists and technologists, in both academia and industry, in leather production and in chemical supply houses.

Tanning Chemistry

The Science of LeatherCovington


Tony CovingtonThe University of Northampton, Northampton, UK

ISBN: 978-0-85404-170-1

A catalogue record for this book is available from the British Library

r Tony Covington 2009

All rights reserved

Apart from fair dealing for the purposes of research for non-commercial purposes or forprivate study, criticism or review, as permitted under the Copyright, Designs and PatentsAct 1988 and the Copyright and Related Rights Regulations 2003, this publication may notbe reproduced, stored or transmitted, in any form or by any means, without the priorpermission in writing of The Royal Society of Chemistry or the copyright owner, or inthe case of reproduction in accordance with the terms of licences issued by the CopyrightLicensing Agency in the UK, or in accordance with the terms of the licences issued by theappropriate Reproduction Rights Organization outside the UK. Enquiries concerningreproduction outside the terms stated here should be sent to The Royal Society ofChemistry at the address printed on this page.

Published by The Royal Society of Chemistry,Thomas Graham House, Science Park, Milton Road,Cambridge CB4 0WF, UK

Registered Charity Number 207890

For further information see our web site at www.rsc.org

Preface

Even in the twenty-first century, the manufacture of leather retains an air of thedark arts, still somewhat shrouded in the mysteries of a millennia old, craftbased industry. Despite the best efforts of a few scientists over the last centuryor so, much of the understanding of the principles of tanning is still based onreceived wisdom and experience. It has been my mission to contribute tochanging the thinking in the industry, to continue building a body of scientificunderstanding, aimed at enhancing the sustainability of an industry thatproduces a unique group of materials, derived from a natural source.This book is the culmination of a 40 year career in chemistry and the leather

industry. My love of chemistry was fuelled at Ackworth School, Pontefract,UK, by Phillips Harris, a genius of a teacher. I obtained an HNC in chemistryat Newcastle Polytechnic (now the University of Northumbria), followed bythe Graduateship of the Royal Institute of Chemistry (now the Royal Society ofChemistry) at Teesside Polytechnic (now the University of Teesside). Thisprovided me with a firm basis in general chemistry, with a strong emphasis onpractical skills – mine was the last year the GRIC involved four days ofpracticals as part of finals! My chemistry experience was enhanced in mydoctoral studies in physical organic chemistry at Stirling University under theinvaluable supervision of Professor R. P. Bell FRS, followed by post-doctoralresearch in physical chemistry with Professor Arthur Covington DSc (coin-cidence, no relation) at the University of Newcastle upon Tyne – the city of mybirth.In 1976 I joined the British Leather Manufacturers’ Association (later to

become BLC The Leather Technology Centre of Northampton, UK), where Ispent 18 years engaged in research and development, industrial consultancyand problem solving, mostly under Director Dr Robert Sykes OBE. The valueof this period was in my exposure to a wide range of aspects of the globalleather industry, particularly the opportunity to work in different types of

Tanning Chemistry: The Science of Leather

By Tony Covingtonr Tony Covington 2009

Published by the Royal Society of Chemistry, www.rsc.org

v

tanneries around the world. At this time, I was initiated into doctoral super-vision, in collaboration with Dr Richard Hancock of the Royal Hollowaycampus of the University of London: the student was Ioannis Ioannidis, later tobecome a leading light in major European projects.In 1995 I was appointed by Nene College, later to be The University of

Northampton, to teach leather science in the British School of Leather Tech-nology. It is this latter period that proved to be the most fruitful in thedevelopment of my understanding of the subject, stimulated in the main by thequestioning of my students. It has been said that to understand a subjectfundamentally, one should try teaching it: I can vouch for the truth of thatobservation. Only when I was confronted with classes of (mostly overseas)students, to whom I was trying to convey a logical approach to thinking aboutthe subject, did I become aware of the inconsistencies and inaccuracies ofreceived wisdom, but with which I had worked without critical review for twodecades. Since few scientists in the last quarter of a century have been workingon the principles of leather making, these problems in the body of knowledgehad to be addressed by my research group.Contrary to the belief expressed in some factions in higher education, leather

technology and leather science are real subjects: they are full of technical andscientific rigour and difficulty, typically at the overlapping regions betweenconventional disciplines, because they are interdisciplinary subjects. Leathertechnology is fully deserving of recognition and respect in the field of theapplied sciences. Those who beg to differ are challenged to examine the coursesand examinations my students must face; dealing as they do with chemistry,biochemistry, biology, physics and materials science, not to mention manage-ment and legislation, and where additionally the topics are embedded with newconcepts from emerging research.In science and technology, we all stand on the shoulders of giants. I have

been privileged to meet, befriend and work with the best. There are too many tomention them all, but here are a few, in no particular order of precedence:Eckhart Heidemann, Betty Haines, Gunther Reich, Bi Shi, David Bailey, SteveFeairheller, Richard Daniels, Jaume Cot, Roy Thomson, Alberto Sofia, SjefLangerwerf, Gabi Renner and Jakov Buljan. I must also include my old friendShri Dr T. Ramasami, former Director of the Central Leather ResearchInstitute, Chennai, India, to whom I am related – not by blood, but by aca-demic ties – his PhD supervisor was a PhD student of my PhD supervisor.For any academic, his or her relationship with their postgraduate students

creates an unusual bond, quite unlike any other relationship. I have been for-tunate in having had many highly gifted students, whose contribution to mythinking and understanding has been immense. Nor do I forget the contribu-tions of my colleagues, both at BLC and in the British School of LeatherTechnology.As I reflect on a lifetime in science, my only regret is that the future of leather

science as a subject is uncertain. There are very few of us leather scientistsaround. Moreover, such is the current disinclination towards financing science,especially applied science, more especially fundamental applied science,

vi Preface

whether in industry or in higher education, it seems unlikely that leather sciencewill command the attention of many future practitioners in the field. Never-theless, there will always be a need in the global industry for highly qualifiedpractitioner leather technologists and it to them that I direct this text.My undergraduate students have often distinguished a clear demarcation

between leather technology and leather science, on the basis that leathertechnology is the easier option, because it is mostly concerned with the prac-ticalities of making leather; but leather science is perceived as more difficult,perhaps just because it is called science. I have always shared Pasteur’s viewthat there is no real boundary between science and technology and especiallyleather technology and leather science; the former is merely the application ofthe latter, they are but two sides of the same coin. Indeed, my personal view isthat leather science is easier, because it is entirely logical. But then I would saythat, wouldn’t I?

Tony Covington

viiPreface

Dedication

This book is dedicated to my wife, Gill. To my favourite stepdaughter, Kate,and my stepsons, Tom and Nick, and especially to our grandsons, Jack andCallum. To Dave and Michelle Waterston and Peter, Joan, Simon and SamArmstrong in the North East. To my sisters Anna and Christine and theirrespective families, brother-in-law Chris and nephew Christopher Dobson,brother-in-law John and nephew Ross Covington Wilson. To my treasuredNorthampton friends, Rowena, Derrick, Jacob and Molly Simpson, Brian andRosemary Gotch, Viv and Bill Jeffcoate, Vikki and Glen Addy and Jeff Gorrellin the USA. To my students, past and present, undergraduate, master anddoctoral, to whom I owe a great debt of gratitude for stimulating my thinkingand for making teaching the most rewarding of jobs. Finally, I dedicate it to thetanners of the world; historically at the bottom of the social scale, doing a jobno-one else wants to do, but making products everyone else wants to own.

Acknowledgements

The author acknowledges the support and contributions of the following:Mrs Ann Tate, Vice Chancellor of The University of Northampton, Commo-dore Jonathan Cooke, Clerk to the Worshipful Company of Leathersellers,Mr Malcolm Leafe, Editor, Journal of the Society of Leather Technologistsand Chemists, Mr Robert White, Editor, Journal of the American LeatherChemists Association, Dr Vikki Addy, Technical Director, BLC The LeatherTechnology Centre, Mr John Basford, Dr Mark Wilkinson and Mr JohnSinclair, Associate Deans, School of Applied Sciences, The University ofNorthampton, Professor Geoff. Attenburrow, Dr Paula Antunes, Mr PaulStroud, Professor Matthew Collins, Dr Hannah Koon, Mr Garen Nalbandian,Mr Murat Aldemir, Mr Arthur Onyuka and Ms Amanda Michel.

viii

Contents

Chapter 1 Collagen and Skin Structure 1

1.1 Introduction 11.2 Hierarchy of Collagen Structure 4

1.2.1 Amino Acid Sequence 41.2.2 The a-Helix 61.2.3 The Triple Helix 6

1.3 Isoelectric Point 81.4 Collagen and Water 101.5 Quarter Stagger Array 141.6 Fibrils 171.7 Fibril Bundles 201.8 Fibres 201.9 Other Collagens 20

1.9.1 Type III Collagen 201.9.2 Type IV Collagen 211.9.3 Type VII Collagen 21

1.10 Chemistry of Collagen 211.11 Hydrothermal Stability 23References 27

Chapter 2 Skin and its Components 29

2.1 Introduction 292.1.1 Epidermis 292.1.2 Grain 302.1.3 Junction 332.1.4 Corium 35


By Tony Covington

r Tony Covington 2009


ix

2.1.5 Flesh Layer 352.1.6 Flesh 35

2.2 Skin Features and Components 362.2.1 Hair or Wool 362.2.2 Follicles 362.2.3 Erector Pili Muscle 362.2.4 Sweat Glands 362.2.5 Veins and Arteries 372.2.6 Elastin 38

2.3 Non-structural Components of Skin 422.3.1 Glycosaminoglycans (GAGs) 422.3.2 Hyaluronic Acid 422.3.3 Dermatan Sulfate 442.3.4 Chondroitin Sulfate A 442.3.5 Chondroitin Sulfate C 452.3.6 Melanin 45

2.4 Skin 482.4.1 Area of Skin or Leather 51

2.5 Processing 542.5.1 Splitting 542.5.2 Grain–Corium Thickness Ratio 552.5.3 Fleshing 562.5.4 Dung 56

2.6 Variations in Skin Structure due to Species 652.6.1 Hereford Cattle and Vertical Fibre 652.6.2 Sheepskin 672.6.3 Cutaneous Fat 67

References 69

Chapter 3 Curing and Preservation of Hides and Skins 72

3.1 Introduction 723.2 Drying 763.3 Salting 79

3.3.1 Stacking 803.3.2 Drum Curing 813.3.3 Types of Salt 823.3.4 Additives for Salt 853.3.5 Dry Salting 853.3.6 Brining 85

3.4 Alternative Osmolytes 863.5 pH Control 873.6 Temperature Control 89

3.6.1 Chilling 893.6.2 Icing 903.6.3 Biocide Ice 90

x Contents

3.6.4 Freezing 903.7 Biocides 913.8 Radiation Curing 913.9 Fresh Stock 92References 93

Chapter 4 Soaking 95

4.1 Introduction to Beamhouse Processing 954.2 Soaking Process 96

4.2.1 Rehydration 994.2.2 Removal of Salt 994.2.3 Cleaning the Pelt 1014.2.4 Removal of Non-structural Proteins 1014.2.5 Removal of Dung 1014.2.6 Removal of Hyaluronic Acid 102

4.3 Conditions in Soaking 1024.3.1 Long Float 1024.3.2 Change of Float 1034.3.3 Temperature 1034.3.4 pH 1034.3.5 Time 1034.3.6 Mechanical Action 103

4.4 Components of Soaking Solutions 1044.4.1 Water 1044.4.2 Detergents 1054.4.3 Soaking Enzymes 1064.4.4 Biocides 107

4.5 Role of the Erector Pili Muscle 109References 111

Chapter 5 Unhairing 112

5.1 Introduction 1125.2 Keratin and the Structure of Hair 1125.3 Hair Burning 118

5.3.1 Role of Swelling 1225.3.2 Chemical Variations 123

5.4 Immunisation 1245.5 Hair Saving 1255.6 Variations in Unhairing Technologies 126

5.6.1 Heidemann’s Darmstadt Process 1265.6.2 Oxidative Unhairing 1275.6.3 Reductive Unhairing 1285.6.4 Acid Unhairing 128

5.7 Enzymes in Unhairing 128

xiContents

5.7.1 Enzyme-assisted Chemical Unhairing 1285.7.2 Chemical-assisted Enzyme Unhairing 1295.7.3 Enzyme Hair Saving 1295.7.4 Keratinase 130

5.8 Painting 1305.9 Role of Shaving in Unhairing 131References 132

Chapter 6 Liming 134

6.1 Introduction 1346.1.1 Float 1346.1.2 pH 1356.1.3 Temperature 1366.1.4 Time 136

6.2 Purposes of Liming 1366.2.1 Removal of Non-collagenous Components

of the Skin 1386.2.2 Splitting the Fibre Structure at the Level

of the Fibril Bundles 1396.2.3 To Swell the Pelt 1396.2.4 Hydrolysis of Peptide Bonds 1436.2.5 Hydrolysis of Amide Sidechains 1436.2.6 Hydrolysis of Guanidino Sidechains 1486.2.7 Removal of Dermatan Sulfate 1486.2.8 Fat Hydrolysis 149

6.3 Variations in Liming 1496.3.1 Chemical Variations 1496.3.2 pH Variations 1506.3.3 Biochemical Variations 150

6.4 Limeblast 151References 153

Chapter 7 Deliming 154

7.1 Introduction 1547.2 Deliming Agents 155

7.2.1 Water 1567.2.2 Strong Acids 1567.2.3 Weak Acids 1577.2.4 Acidic Salts 1577.2.5 Ammonium Salts 1587.2.6 Alternative Buffers 1597.2.7 Hydroxyl ‘Sinks’ 1607.2.8 Carbon Dioxide 161

7.3 Melanin 164

xii Contents

7.4 Limeblast 165References 165

Chapter 8 Bating 166

8.1 Introduction 1668.2 Factors Affecting Enzyme Catalysis 169

8.2.1 Temperature 1698.2.2 pH 1708.2.3 Concentration: Bate Formulation 1718.2.4 Time 1728.2.5 Origin 172

8.3 Monitoring Bating 1738.4 Enzyme Assays 174References 176

Chapter 9 Pickling 177

9.1 Introduction 1779.2 Processing Conditions 179

9.2.1 Float 1799.2.2 Salt 179

9.3 Lyotropic Swelling 1849.4 Sulfuric Acid 1859.5 Hydrochloric Acid 1869.6 Formic Acid 1869.7 Colour 1889.8 Non-swelling Acids 1909.9 Pickle Formulations 192

9.10 Implications for Chrome Tanning 193References 194

Chapter 10 Tanning 195

10.1 Introduction 19510.2 Hydrothermal Stability 196

10.2.1 Shrinkage Temperature (Ts) 19810.2.2 Boil Test 19910.2.3 Differential Scanning Calorimetry 20010.2.4 Hydrothermal Isometric Tension 201

References 202

Chapter 11 Mineral Tanning: Chromium(III) 204

11.1 Introduction 20411.2 Preparation of Chrome Tanning Salts 205

xiiiContents

11.3 Brief Review of the Development

of Chrome Tanning 20811.4 Chromium(III) Chemistry 20911.5 Chrome Tanning Reaction 21311.6 Basification 221

11.6.1 Soluble Alkaline Salts 22311.6.2 Carboxylate Salts 22411.6.3 Other Basic Salts 22511.6.4 Self-basifying Salts 225

11.7 Avoiding Basification 22811.8 Reactivity at High Basicity 22811.9 Role of Temperature 230

11.10 Relative Effects of pH and Temperature 23111.11 Masking 237

11.11.1 Polycarboxylates 24311.12 Stability of Chrome Tanned Leather 24611.13 Role of Sulfate in the Chrome

Tanning Mechanism 24711.14 Role of the Counterion in Chrome Tanning 25011.15 Role of the Solvent 25211.16 Zero Float Processing 25211.17 Non-aqueous Floats 25311.18 Role of Ethanolamine 25511.19 Nature and State of the Substrate 255

11.19.1 Modifying the Substrate 255References 256

Chapter 12 Mineral Tanning 259

12.1 Introduction 25912.1.1 Blocks and Groups of the

Periodic Table 26012.2 Experimental Tanning Reviews 26512.3 Aluminium in Leather Making 267

12.3.1 Alum Pickle 26912.3.2 Suede 26912.3.3 Furskins 26912.3.4 Semi-alum Tannage 27012.3.5 Chrome Uptake 270

12.4 Titanium Tanning 27412.5 Zirconium Tanning 27512.6 Iron Tanning 27612.7 Mixed Mineral Tannages 27712.8 Overview 278References 278

xiv Contents

Chapter 13 Vegetable Tanning 281

13.1 Introduction 28113.2 Vegetable Tannin Classification 284

13.2.1 Hydrolysable Tannins 28413.2.2 Condensed Tannins 28813.2.3 Complex Tannins 293

13.3 General Properties of Vegetable Tannins 29313.4 Practical Vegetable Tanning 29413.5 Modern Pit Tanning 295

13.5.1 Counter-current Pit Tanning 29513.6 Other Vegetable Tanning Technologies 29613.7 Combination Tanning 296

13.7.1 Semi-metal Tanning 29613.7.2 General Properties of Semi-metal

Leathers 30013.7.3 Semi-chrome Tanning 301

13.8 Condensed Tannins and Aldehydic

Crosslinkers 30313.9 Biodegradability 310

13.10 Effluent Treatment 313References 313

Chapter 14 Other Tannages 315

14.1 Oil Tanning 31514.2 Sulfonyl Chloride 31714.3 Syntans 318

14.3.1 Auxiliary Syntans 32314.3.2 Retans 32414.3.3 Replacement Syntans 32414.3.4 Other Syntans 324

14.4 Resins 32614.5 Aldehydes and Aldehydic Tanning Agents 328

14.5.1 Introduction 32814.5.2 Formaldehyde 32914.5.3 Glutaraldehyde 33114.5.4 Other Aliphatic Aldehydes 332

14.6 Aldehydic Tanning Agents 33414.6.1 Oxazolidines 33414.6.2 Phosphonium Salts 337

14.7 Other Tanning Applications: Wet White 33814.7.1 Wet White Production 33914.7.2 Use of Part Processed Materials 342

14.8 Miscellaneous Tannages 34214.8.1 Epoxide Tanning 342

xvContents

14.8.2 Isocyanate Tannage 34314.8.3 Multi-functional Reagents 345

References 345

Chapter 15 Post Tanning 348

15.1 Definition 34815.2 Relationship between Tanning and Post Tanning 34915.3 Chrome Retanning 35115.4 Sequence of Post Tanning Steps 35215.5 Principles of Post Tanning 353

15.5.1 Mechanisms of Post Tanning 35315.5.2 Role of the Isoelectric Point 35615.5.3 Role of the Peptide Link 35915.5.4 Role of the Sulfonate Group 35915.5.5 Coordinating Post Tanning Processes 360

15.6 Compact Processing 36215.6.1 Neutralise and Retan 36215.6.2 Retan and Dye 36215.6.3 Retan and Fatliquor 36315.6.4 Dye and Fatliquor 36315.6.5 Retan, Dye and Fatliquor 363

15.7 Role of Processing on Leather Properties:

Dynamic Mechanical Thermal Analysis 364References 368

Chapter 16 Dyeing 370

16.1 Introduction 37016.2 Acid Dyes 37316.3 Basic Dyes 37316.4 Direct Dyes 37416.5 Mordant Dyes 37516.6 Premetallised Dyes 377

16.6.1 1 : 1 Premetallised Dyes 37716.6.2 1 : 2 Premetallised Dyes 378

16.7 Reactive Dyes 37916.8 Sulfur Dyes 38216.9 Dye Reactivity and Fixation 383

16.10 Role of the Substrate 38416.10.1 Chrome Tanned Leather 38416.10.2 Vegetable Tanned Leather 38516.10.3 Other Tannages 385

16.11 Dyeing Auxiliaries: Levelling and Penetrating

Agents 385

xvi Contents

16.11.1 Anionic Auxiliaries 38516.11.2 Auxiliaries that Complex with Dyestuff 38716.11.3 Auxiliaries that have Affinity

for both Leather and Dye 38716.11.4 Intensifying Agents 38716.11.5 Cationic Tannages 38816.11.6 Cationic Auxiliaries 388

16.12 Alternative Colouring Methods 389References 391

Chapter 17 Fatliquoring 392

17.1 Introduction 39217.2 Anionic Fatliquors 401

17.2.1 Sulfated Fatliquors 40117.2.2 Sulfited Oils 404

17.3 Soap Fatliquors 40517.4 Cationic Fatliquors 40517.5 Nonionic Fatliquors 406

17.5.1 Alkyl Ethylene Oxide Condensates 40617.5.2 Protein Emulsifiers 407

17.6 Multi-charged Fatliquors 40717.7 Amphoteric Fatliquors 40717.8 Solvent Fatliquors 40717.9 Complexing Fatliquors/Water Resistance

Treatments 40817.10 Water Resistance 409

17.10.1 Introduction 40917.10.2 Principles of Conferring Water Resistance 41117.10.3 Chemistries of Water Resistance

Treatments 41617.10.4 Chrome Tanned Leather 41717.10.5 Non-chrome Tanned Leather 419

References 420

Chapter 18 Drying 421

18.1 Two-stage Drying 42718.2 Conclusion 430References 430

Chapter 19 Theory of Tanning: Concept of Link-Lock 432

19.1 Introduction 43219.2 Discussion 435

xviiContents

19.3 Conclusions 441References 441

Chapter 20 The Future of Leather Processing 444

20.1 Future of Chrome Tanning 44420.2 Other Mineral Tanning Options 44520.3 Non-chrome Tanning for ‘Chrome Free’ Leather 44620.4 Single Tanning Options 44720.5 Tanning Combinations 44820.6 Leather Properties 45020.7 Organic Tanning Options 451

20.7.1 Polyphenol Chemistry 45120.7.2 Polymer and Crosslinker 453

20.8 Natural Tanning Agents 45420.8.1 Carbohydrates 45420.8.2 ‘Bog Body’ Chemistry 45520.8.3 Nor-dihydroguaiaretic Acid (NDGA) 45620.8.4 Genepin 45920.8.5 Oleuropein 459

20.9 Other Reagents 46120.10 Compact Tanning 46220.11 Alternative Technologies 46220.12 Overview 46420.13 Conclusions 464References 465

Subject Index 467

xviii Contents

Introduction

This treatise is a discussion of the leather making process from the point of viewof the scientific principles involved. The approach adopted here is to build anunderstanding of the steps involved, particularly the chemistry of tanning, bystarting at the beginning of the industrial process and going on to the end. Thismeans starting with an understanding of the nature, structure and properties ofthe material with which we make leather, then considering what to do with it,how to do it and what the outcomes of the transformations may be.Leather is made from (usually) the hides and skins of animals – large animals

such as cattle have hides, small animals such as sheep have skins. The skin ofany animal is largely composed of the protein collagen, so it is the chemistry ofthis fibrous protein and the properties it confers to the skin with which thetanner is most concerned. In addition, other components of the skin impact onprocessing, impact on the chemistry of the material and impact on the prop-erties of the product, leather. Therefore, it is useful to understand the rela-tionships between skin structure at the molecular and macro levels, the changesimposed by modifying the chemistry of the material and the eventual propertiesof the leather. Consequently, these aspects of the subject will be addressed inthe following order: collagen chemistry, collagen structure, skin structure,processing to prepare for tanning, the tanning processes and processing aftertanning.Leather making is a traditional industry, after all it has been in existence

since time immemorial, certainly over 5000 years, because the industrywas established at the time of the Hammurabi Code (1795–1750 BC), whenArticle 274 laid down the wages for tanners and curriers.1 Indeed, the use ofanimal skins is one of man’s older technologies, perhaps only predated by toolmaking. In the modern world, the global leather industry exists because meateating exists. Hence, most leather made around the world comes from cattle,sheep, pigs and goats. One of the byproducts of the meat industry is the hides




xix

and skins: considering that the annual kill of cattle alone is of the order of 300million, such a byproduct, amounting to 10–20 million tonnes in weight, wouldpose a significant environmental impact if it were not used by tanners. Leathermakers around the world perform the ultimate example of adding value: anunwanted, discarded byproduct from one industry is turned into one of themost desired and widely applied materials in the world. Unfortunately, onetonne of hide is converted into only 200 kg of leather: the difference is waste,pollutants or byproducts, depending on your point of view. So, the problemswith which tanners must contend become apparent.Here, notably, there is a difference between the leather industry and the fur

industry. In general, tanners process the skins of animals killed for the con-sumption of their carcases: in the fur trade, the skin is the important part of theanimal and the carcase may be the byproduct, albeit perhaps turned into fer-tiliser. The processes for preparing and stabilising furskins have similaritieswith tanning, but the chemistries can be significantly different. However, theindustries have traditionally been separate. Within the leather trade, some skinsare processed from animals sustainably farmed in part for their skins, e.g.ostrich and alligator. It is important to emphasise that any animals specified byCITES (Convention on International Trade in Endangered Species) are notpart of the ethical global leather industry.It might appear anachronistic that such an old established technology should

still be relevant today, particularly because some of those traditional technol-ogies for making specific leathers survive in the modern industry. However, it isworth considering the properties and performance of modern leathers, incomparison to other modern materials. The suitability of leather for shoemanufacture is based upon the twin abilities of being able to exclude water, butallow air and water vapour to pass through the cross section of the upper: this isthe basis of foot comfort when shod. The properties are so important thatattempts have been made to mimic them in synthetic materials, the so-calledporomerics. To date, none has been successful: witness the failure of Corfam inthe 1960s, documented by Kanigel.2 Similarly, the reconstituting of leatherfibres into a polymer matrix creates materials that cannot exhibit all theproperties of leather, it merely creates other versions of poromerics. Strictly, allsuch materials cannot carry names connoting leather, since the definition ofleather is based on the requirement that it must be made from the intact hide orskin of an animal.3

The properties of leather do not limit its use only as shoe upper material. Thenatural structure of leather, founded in the properties of the protein collagenwith chemical modification, confers the unique ability to function within thetemperature range at least �100 to +130 1C, unmatched by any other material:synthetic materials either soften at high temperature or become brittle at lowtemperature. However, the continuing popularity of leather is based on the wayit feels, the way it looks, the way it interacts with and moulds to the body, theway it wears in use, even the way it smells (for information – ‘essence of leather’is plant polyphenol extract plus a little fish oil). The aesthetics of leather ensureit is likely to remain the preferred material of choice for many modern

xx Introduction

applications: shoes, clothing, upholstery, belts, handbags, etc. Importantly, thechoice is supported by its performance. For example, the high hydrothermalstability of modern chromium(III) tanned leather allows it to be used in rapidmass production processes, in which shoe uppers are moulded directly ontomelted polymeric soles; other leathers can be used to make foundry workers’safety gaiters because they can resist splashing from molten metal streams, evenliquid steel, clothing leathers can be steam pressed, etc. The best material formotorcyclists is leather, because, in the event of the rider coming off the bike,the frictional heat of the skidding action over the road surface will not cause thematerial of the clothing to melt and consequently it is not fused into the rider’ssoft tissues.The properties of leather depend on the origin of the raw materials, how the

pelt is prepared for chemical modification, how that modification is conferredchemically, how the leather is lubricated and finally how the surfaces are pre-pared. Leather can be made as stiff and as tough as wood, as soft and flexible ascloth and anything in between. It is the traditional art and craft of the leathertechnologist to control the parameters and variables of processing to makeleathers with defined, desired or required properties. It is from the creativity ofthe leather scientist that the range of leathers that can be made is continuallywidening.As a traditional craft, leather making has its own jargon, built up over

centuries, but much of which has no clear origin. Some of the terms are set outin Glossary below and more can be found in the British Standard Glossary ofLeather Terms.3

Leather scientists and technologists do not restrict their attention to leatheralone. Leather is merely one set of examples within a range of collagenic bio-materials; there are many more already used in industry and many more can bedeveloped: that should become clear by reading this text. Any application ofproteinaceous materials, particularly collagen, and especially where the proteinis modified in some way, is exploiting a biomaterial. For example, fixing oforganic specimens in histology for microscopy study is using tanning tech-nology, as is the stabilisation of medical sutures, e.g. in chromic catgut, whenthe rate of dissolution in the body is controlled by the degree of modification ofthe tissue by chrome tanning, the techniques of increasing the ‘weight’ of silkuse tanning chemistry and so on. Therefore, clearly, the transfer of scientificand technological principles from the leather industry into other sectors ispractical, feasible, desirable and full of potential.It will become apparent from this text that the leather field has borrowed

from other areas of science, often just using the principles rather than followingdevelopments in these subjects. In the past, tanners exploited technology andbiotechnology long before the science was completely understood: so no changethere! Hence, it is clear that some aspects of the development of the technologyof the leather industry and progress in that technology have been slow, indeedThomson has argued that the industrial revolution in leather making took placeone hundred years after it began to transform the rest of industry.4 The slowrate of change was compounded by the inherent conservatism of tanners, aware

xxiIntroduction

that change in processes and procedures can be detrimental to leather qualityand performance; hence avoidance of change was the lesser of the twin evils oflack of technical progress or risking loss of money.The continuing importance of leather production in the world cannot be

under estimated. As a labour intensive operation, it is especially applicable tothe developing economies. Moreover, in those countries, the industry offerssignificant employment opportunities for women: it is recognised that givingwomen the power to control the economics of the family has a potent stabi-lising effect on society. It has been argued that that movement of the globalindustry to the east and to the south has taken place to exploit cheaper labourand less stringent environmental legislation. The former is certainly the case,although the cheaper labour is becoming noticeably less cheap. The latter is nottrue: consent limits for discharges to the environment are surprisingly con-sistent throughout the world;5 nowhere is the legislation on pollution sig-nificantly lax, it is merely the enforcement that varies. The perception that theleather industry is a dirty polluter is largely out of date. However, this does notmean that it can be assumed to be generally clean – unfortunately, there is still along way to go. It is an important function of leather science to provide thetechnologists with sufficient understanding of the principles of the chemistryand biochemistry of processing for them to be able to make improvingchanges in the operations. Table I.1 characterises this progression towards zeroenvironmental impact.Developments in this direction should ideally be economically attractive

at the same time, including using waste streams, by adding value to thebyproducts. Since we are dealing here with an applied, industrial science, theeconomics of the operation should never be far from the analytical mind.Making money is a big incentive for change.The text of this book is designed to be scientific, to define as much as possible

the principles underpinning the complex technology of leather making. Hence,it is part of the function of this text to point the way forward, to contribute tothe sustainability of the global industry, always based on a fundamentalunderstanding of those principles.

There are not applied sciences . . . there are only applications of thescience and this is a very different matter. The study of the applicationsof science is very easy to anyone who is the master of the theory of it.Louis Pasteur.6

Table I.1 Progression towards zero environmental impact.

CURRENT - CLEANER - CLEAN

CATNAP BATNEEC BATCurrently applied technology,narrowly avoiding prosecution

Best available technology, notentailing excessive cost

Best availabletechnology

xxii Introduction

REFERENCES

1. R. Reed, Ancient Skins, Parchments and Leathers, Seminar Press, 1972.2. R. Kanigel, Faux Real, Joseph Henry Press, Washington DC, 2007.3. British Standard Glossary of Leather Terms, British Standards Institution,

BS 2780:1983.4. R.S. Thomson, Trans. Newcoman Soc., 1981/2, 53, 139.5. J. Buljan and M. Bosnic, World Leather, Nov., 1994, 7(6), 54.6. E. Haslam, J. Soc. Leather Technol. Chem, 1988, 72(2), 45.

xxiiiIntroduction

Glossary of Terms

Alum General term for aluminium(III) complexes used intanning.

Aniline dyed Leather dyed with transparent colouring; traditionallydyes prepared with aniline as the precursor: theimportance is the lack of opaque colouring on theleather surface, which would obscure the natural grainpattern.Semi-aniline has some colour opacity, to hide minorblemishes.

Barkometer Also Baume, Twaddle: scales of specific gravity, usedto indicate concentrations of solutes, e.g. sodiumchloride, vegetable tanning agents, etc.

Bating A generic term for the use of proteolytic enzymes, todegrade non-structural proteins in skin. Sources maybe pancreatic material or bacterial fermentation. Lesscommon terms are puering (dog dung), mastering(pigeon guano) and bran drenching, referring to otheranimal or vegetable sources of enzymes, applied to peltas warm aqueous infusions.

Basification Used in the context of mineral tanning, when the pHof the system is gradually raised, to increase the ioni-sation of the carboxyl sidechains of the collagen andthereby drive reaction/interaction with the cationicmetal complex ions.

Beamhouse The early stages of processing, to prepare the pelt fortanning, up to the point of initiating tanning.




xxiv

BLC The British Leather Confederation, formerly theBritish Leather Manufacturers’ Research Association(BLMRA), latterly BLC The Leather TechnologyCentre, Northampton, UK.

Bloom Insoluble residues from when hydrolysable tannins(ester-based plant polyphenols) are hydrolysed duringtanning.

Boil test Measuring the resistance of leather to boiling water:the period of the test varies from factory to factory(1–5min) and reflects how far the conventionallymeasured shrinkage or denaturation temperature ofthe leather lies above 100 1C.

Bovine Applied to ox or cow. Ovine (sheep), caprine (goats),porcine (pigs) and cervine (deer) are terms also used,but less commonly.

Buffing Applying sandpaper/glasspaper to a leather surface:buffing the grain surface removes material and flattensit in preparation for correcting or for making nubucksuede, buffing the split or flesh surfaces creates a suedenap.

Butt Central portion of a hide or skin: the remainder,bellies and neck are self-explanatory.

Chamois Leather tanned with oil (typically, polymerisable codliver oil), usually applied to sheepskin flesh splits.

Chrome Refers to chromium salts, usually Cr(III), but mayinclude Cr(VI), depending on the context.

Chrome oxide Analysis of the chromium content of leather or othermaterials is usually expressed in the form of the tri-valent oxide, Cr2O3, regardless of whether the oxidehas anything to do with the chromium speciesanalysed.

Chrome retan Chrome tanned leather retanned with vegetabletannin(s).

Chrome tanning Tanning with basic chromium(III) salts, mostly thesulfate.

Clearing Sequestration of iron, when it causes staining onvegetable tanned leather.

CLRI Central Leather Research Institute, Chennai, India.Condensed tan Vegetable tannin with flavonoid structure – also called

catechol tannin.Corrected grain The grain layer is partially removed by buffing and

replaced by polymer coatings, often with a new grainpattern impressed into it. The new grain pattern maynot necessarily be the same as the original.

Crock Particulate matter removed from leather by rubbing: itmay be leather fibre or loose dye.

xxvGlossary of Terms

Crust leather Leather processed to a point when it can be dried:this is usually after post tanning, but may apply toleather given a primary tanning, possible includ-ing some lubricant to prevent fibre sticking duringdrying.

Curing Generic term for preserving hides and skins.Currier A term no longer in use: one who took tanned hides

and completed the processing by fatliquoring andcrusting them.

Deliming The step after liming, when the pH is lowered inreadiness for the subsequent step, bating at pH8–9.The commonest reagent is ammonium salt; chloride ispreferred to sulfate, but carbon dioxide is increasinglyused.

Denaturation Damaging the natural structure of collagen: it may beachieved chemically, but usually means heat damageof wet collagen or leather, when the observed effect isshrinking.

Detanning Reversing the effect of tanning, usually chemically,lowering the shrinkage temperature to that of raw pelt.

DMTA Dynamic mechanical thermal analysis: a technique forgathering data about molecular structure by stressingmaterials in a cyclical fashion while varying thetemperature.

Double face Sheepskin tanned for clothing with the wool stillattached: the wooled surface and the suede surface areequally important to the quality of the leather.

Drenching Bating with fermenting bran (less common).DSC Differential scanning calorimetry: a technique for

measuring the thermodynamic parameters associatedwith a reaction or transition, by determining energyabsorption or energy produced as the temperaturerises at a specific rate.

Enamel Grain enamel: the outermost surface of the skin afterthe hair or wool has been removed.

EXAFS Extended X-ray analysis fine structure: a technique inX-ray spectroscopy, used to quantify the structure of amaterial in terms of the nearest atomic neighbours toan atom of interest.

Fatliquoring Lubrication of the leather, often using partially sul-fated or sulfonated oils, which may be animal, vege-table or mineral in origin. The partially sulf(on)atedsulfo fraction is the emulsifying agent used to carry theunreacted, neutral oil into the leather: it is the neutraloil that performs the lubricating action.

xxvi Glossary of Terms

Fellmonger(ing) Literally a dealer in fells (sheep) skins, typicallyapplied to processing raw sheepskins to the dewooled,pickled condition. Also used as a verb, referring par-ticularly to the process of removing the wool: a thix-otropic lime suspension with sulfide in solution ispainted onto the flesh surface of the skin, allowing thesulfide to penetrate to the base of the follicle andattack the keratin of the prekeratinised zone of thewool fibre.

Fixation A general term referring to the chemical binding of atanning agent to collagen.

Flesh The muscle and fat of a carcase adhering to the peltafter flaying.

Flesh layer The part of the corium structure of hide or skin closestto the carcase.

Flesh split The layer closest to the carcase, produced by splittinghide or skin through the cross section: the other part isthe grain split.

Float The water used as process medium: the quantitiesrelative to the amount of pelt may be characterised as‘short’ or ‘long’.

Grain (layer) Outer layer of the hide or skin, the more valuable partconsisting of the corium minor and the enamel outersurface.

Grain pattern Pattern of the follicles on the surface of the skin orleather: it characterises the leather and it is a prizedfeature.

Green Refers to rawstock that is fresh, i.e. without pre-servation: may be used as the basis for chemical offers.

Ground substance Non-collagenous components of unprocessed pelt,consisting of non-structural proteins, glycosami-noglycans, fats, minerals.

Gums The high molecular weight fraction of vegetabletannins, 43 000Da: complexes of carbohydrateswith polyphenols. They are undesirable componentsbecause of their reaction on the pelt surface.

Hair burn Method of removing the hair by dissolving it, usuallyusing alkali and a reducing nucleophile: the conven-tional reagents are lime and sodium sulfide at pH 12.5.

Hair save Method of unhairing by attacking the hair at the baseof the follicle, to allow the hair to be removed intact.

Handle General term referring to the sum total of the per-ceived properties of the leather when handled, flexed,etc.

Hide Pelt of a large animal, e.g. cattle.

xxviiGlossary of Terms

HIT Hydrothermal isometric tension: a technique formeasuring the forces developed when a collagenicmaterial is heated in a solvent through the temperatureat which the shrinking transition occurs.

Hide substance Collagen content of part or fully processed pelt.Hydrolysable tan Vegetable tannin based on saccharide esterified by

polyphenol – also called pyrogallol tannin.JALCA Journal of the American Leather Chemists AssociationJapanese leather Thin but strong leather, made by removing as much as

possible of the ground substance, to allow the collagenstructure to collapse.

JSLTC Journal of the Society of Leather Technologists andChemists (UK)

Leather ‘Hide or skin with its original fibrous structure moreor less intact, tanned to be imputrescible’ (BritishStandard Glossary of Leather Terms, British Stan-dards Institution, BS 2780:1983).The definition excludes any materials made fromwaste tanned byproducts, such as shavings, recon-stituted in a polymer matrix: these materials are oftendesignated ‘leather board’ or by other name connotingleather.

Leathering Stabilisation of collagen by tanning-type reactions,but the product does not possess all the expectedattributes of leather, although it does appear leather-like.

Lime/liming Generic term for alkaline treatment, to cause swellingand hydrolysis in pelt – usually, but not exclusively,achieved with slaked lime, Ca(OH)2.

Masking Altering the properties of a mineral tanning agent bychanging the ligand field of the metal ion complex,usually involving carboxylates.

Mastering Bating with a warm infusion of bird guano (lesscommon).

Metal oxide It is conventional to express the metal content of amaterial in the form of the oxide, regardless of thechemical nature of the metal, whether as the elementor any of its compounds.

Metal retan Leather made by mineral tanning followed by vege-table tanning.

Mineral tanning Use of metal salts for tanning.Neutralisation Process of raising the pH after main tanning, prior

to initiating post tanning reactions, to adjust thecharge on the leather. Despite the term, the final pHmay be significantly below conventional neutrality(pH7).

xxviii Glossary of Terms

Non-tans Low molecular weight fraction of vegetable tanninextracts, o500 Da. They are unreactive as tanningagents, but useful in the tanning process for solubi-lising the tannins.

Nubuck Very fine suede created by buffing the grain surface ofchrome tanned leather.

Offer The amount of a chemical introduced into the processvessel. Offers in the early stages may be based on theraw (green), salted or limed weight of the pelt. Aftertanning, offers are usually based on damp, shavedleather weight.

Organic tanning Tanning without metal salts.Paste drying Leathers are struck to glass plates with paste and dried

in warm air. The paste is designed to allow release ofthe dry leather from the plate. Used to produce leatherwith a very flat grain surface.

Pelt General term for part processed hide or skin, beforetanning.

Pickling Acidification of pelt in brine for preservation orpreparation prior to tanning.

Post tanning Usually applies to chrome tanning: comprises retan-ning, dyeing and fatliquoring.

Puering Bating with a warm infusion of dog dung: used to bethe industry standard but now uncommon.

Rawstock Hides and skins, fresh or preserved.Reds Phlobaphene aggregates in solutions of condensed

tannin extracts.Retanning Following the first and main tanning process, usually

chrome tanning, the leather is tanned again, typicallyto modify the handle properties. Any tanning agentmay be used for this purpose, depending on the desiredleather properties.

Samm(y)ing Squeezing leather through felt covered rollers, toremove moisture.

Scud(ding) Residual epidermis, hair debris and melanin, left afterbeamhouse operations. The verb refers to the removalof scud, which might be by mechanical means or bythe use of biochemicals.

Semi-metal Tanning process involving vegetable tanning, usuallya hydrolysable tannin, followed by retanning with ametal salt. Options include: semi-chrome using Cr(III),semi-alum using Al(III), semi-titan using Ti(IV), semi-zircon using Zr(IV).

Setting Squeezing (samming) the pelt, but with a spreadingaction over the grain surface from the rollers – toflatten the leather and remove creases.

xxixGlossary of Terms

Shaving Usually follows splitting: mechanically cuttingthrough the surface to produce accurate and con-sistent thickness.

Skin Pelt of a small animal, e.g. sheep, goat or similar.Soaking First stage in processing, primarily to rehydrate pre-

served pelt. Usually involves a preliminary dirt soak,one or more main soaks and rinsing.

Splitting, split Act of cutting a hide layerwise, laterally through thecross section. Also used to describe the layers aftersplitting: grain split, flesh split, sometimes middle split.

Spue Spue usually refers to ‘fatty acid spue’, which is themigration of low molecular weight fatty acids to the(grain) surface of the leather, giving a white waxyappearance. ‘Salt spue’ is the migration of neutralelectrolyte to the (grain) surface, often observed aftershoes have been wetted and dried.

Staling Delaying between flaying and curing, allowing bac-terial proliferation: the damaging effect depends on theextent of the delay.

Striking out Use of a blunt blade on a fast rotating roller to spreadand flatten tanned skins: used to remove creases.

Suede Leather that has been rubbed with glasspaper orsandpaper to raise the nap on the surface. A nap onthe flesh surface makes conventional suede, a nap onthe grain surface makes nubuck. Note, the suede fromthe split surface of hide or shin is usually too coarse tobe of value.

Syntan Synthetic tanning agent: polymerised aromatichydroxyl compounds, designed to perform variousfunctions, depending on the structure, includingassisting chemical processes and acting as solo tanningagents, like vegetable tannins.

Tanning Conversion of putrescible organic material into astable material capable of resisting biochemical attack.

Tans Usually applied to vegetable tannins: the molecularweight fraction 500–3000 Da of vegetable tanninextracts, distinguished from the ‘non-tans’ and the‘gums’.

Toggle drying Leathers are clipped to a perforated plate with‘toggles’ and dried in warm air.

Ts Shrinkage temperature, the most common measure-ment of hydrothermal stability: an increase in Ts

indicates a consequence of tanning.

Sometimes this term is used interchangeably with‘denaturation temperature’ and occasionally with‘melting temperature’.

xxx Glossary of Terms

Unhairing Removal of hair, usually by chemical means, butbiochemical treatments are known. In this context, thegeneral term unhairing can include removal of wool orbristles.

Vegetable tan(nin) Polyphenolic mixtures of plant origin: may be in theform of ground plant material, but now more usuallyrefers to dried extracts of plant material.

Wet blue Chrome tanned pelt: it is wet because it not usuallydried before progressing to post tanning and it is bluebecause of the nature of the chrome complexes boundto the collagen.

Wet salting Commonest form of rawstock preservation: the termindicates the treatment is applied to pelt in its naturalmoist state and the reagent is sodium chloride.

Wet white Non-chrome tanned leather or chemically stabilisedpelt, capable of being split and shaved.

Wool/hair slip Loosening or detaching of wool or hair from the baseof the follicle: an indicator of putrefaction.

xxxiGlossary of Terms

CHAPTER 1

Collagen and Skin Structure

1.1 INTRODUCTION

At the heart of the leather making process is the raw material, hides and skins.As the largest organ of the body of mammals, the skin is a complex structure,providing protection against the environment and affording temperature con-trol, but it is also strong enough to retain, for example, the insides of a onetonne cow. Skin is primarily composed of the protein collagen and it is theproperties and potential for chemical modification of this protein that offerthe tanner the opportunity to make a desirable product from an unappealingstarting material. It is part of the tanner’s job and skill to simplify or purify thisstarting material, allowing it to be converted into a product that is bothdesirable and useful in modern life.Collagen is a generic name for a family of at least 28 distinct collagen types,

each serving different functions in animals, importantly as connective tissues.1–4

The major component of skin is type I collagen: so, unless otherwise specified,the term ‘collagen’ will always refer to type I collagen. Other collagens dofeature in leather making and their roles are defined later.Collagens are proteins, i.e. they are made up of amino acids. They can be

separated into a-amino acids and b-amino acids (Figure 1.1). Each one featuresa terminal amino group and a terminal carboxyl group, which become involvedin the peptide link (see below), and a sidechain attached to the methylene groupin the centre of the molecule. When the amino acids are linked together to formproteins, they create an axis or ‘backbone’ to the polymer, from which thesidechains extend. It is the content and distribution of the sidechains thatdetermine most of the properties of any protein. In the case of collagen, it is thesidechains that largely define its reactivity and its ability to be modified by thestabilising reactions of tanning, when leather is made. In addition, the chem-istry of the backbone, defined by the peptide links, offers different reaction sitesthat can be exploited in some tanning processes.




1

All the common amino acids are found in skin or skin components. There aretwo notable aspects of the amino acid content of collagen. Hydroxyproline(Figure 1.1) is almost uniquely present in collagen compared to other proteinsand, therefore, offers the basis of measuring the collagen content in any skin orskin derivative. Tryptophan (Figure 1.2) is absent, therefore making collagendeficient as a foodstuff.In terms of leather making, some amino acids are more important than

others, since they play defined roles (Table 1.1): the roles of importance areeither in creating the fibrous structure or involvement in the processing reac-tions for protein modification. Other amino acids, not included in Table 1.1, areimportant in defining the properties of the collagen, but play less defined rolesin the leather making processes.Amino acids create macromolecules, proteins such as collagen, by reacting

via a condensation process: the amide or peptide link is in bold:

H2N-CHR-CO2HþH2N-CHR-CO2H

ÐH2N-CHR-CO-HN-CHR-CO2HþH2O

The condensation reaction can be reversed by hydrolysis, by adding theelements of water. Clearly, hydrolysis as set out in this chemical equation

H2N CH CO2H

R

HN CH CO2H

H or OH

CHalpha amino acid

proline or hydroxyprolinebeta amino acid

Figure 1.1 Amino acid structures.

H2N CH C

CH2

OH

O

HN

Figure 1.2 Tryptophan.

2 Chapter 1

cannot be fast, nor does the equilibrium lie to the left, otherwise the proteinwould be unstable and useless as the basis of life. In contrast, the hydrolysisreaction is catalysed by general acid and general base – importantly for leathermaking, it is catalysed by H1 and OH�. The impact on processing can beindicated as follows.In the earliest stage of processing, hair is usually removed and at the same

time the skin is given a prolonged alkali treatment, typically conducted overabout 18 hours for cattle hides, often in aqueous lime solution, Ca(OH)2; longer

Table 1.1 Amino acids of importance to leather making.

Name Symbol Type Sidechain (R)Importance inleather making

Glycine Gly a, neutral –H Collagenstructure

Alanine Ala a, neutral –CH3 Hydrophobicbonding

Valine Val a, neutral –CH(CH3)2 Hydrophobicbonding

Leucine Leu a, neutral –CH2CH(CH3)2 Hydrophobicbonding

Isoleucine Ileu a, neutral CH3CH2CH(CH3) Hydrophobicbonding

Phenyl-Alanine Phe a, neutral –CH2C6H5 Hydrophobicbonding

Serine Ser a, neutral –CH2OH UnhairingCysteine CySH a, neutral, S

containing–CH2SH Unhairing

Cystine CyS-SCy a, neutral, Scontaining

–CH2SSCH2– Unhairing

Aspartic Acid Asp a, acidic –CH2CO2H Isoelectricpoint (IEP), a

mineraltanning

Asparagine Asn a, neutral –CH2CONH2 IEPGlutamic Acid Glu a, acidic –(CH2)2CO2H IEP, mineral

tanningGlutamine Gln a, neutral –(CH2)2CONH2 IEPArginine Arg a, basic –(CH2)3NHC(NH)NH2 IEPLysine Lys a, basic –(CH2)4NH2 IEP, aldehydic

tanning, dye-ing,lubrication

Histidine His a, basic–CH2-CH=CH-NH

N=CHAldehydic tan-ning, dyeing,lubrication

Proline Pro b, neutral See Figure 1.1 Collagenstructure

Hydroxy-proline

Hypro b, neutral See Figure 1.1 Collagen struc-ture, hydro-gen bonding

aIn biology, referred to as pI.

3Collagen and Skin Structure

treatment may result in detectable damage to the fibre structure. In saturatedlime solution, [OH�]¼B10�2 molar. Conversely, pickling in brine solutionwith acid is routinely used as a preservation technique for more vulnerablesheep skins, enabling them to be transported across the world between Europeand Australia and New Zealand over a period of several months: here[H1]¼B10�2 molar. Hence, by this practical comparison, in which pickledpelt remains undamaged for months, but limed pelt shows damage after a fewhours, hydroxyl ion produces a much faster reaction than hydrogen ion – atleast 100� faster.An important feature of the peptide link is that it is partially charged.

The link can be expressed in two forms. The charged structure makeschemical sense, but nature does not favour charge separation in this way.However, the electronegativity difference between the oxygen and the nitrogenmeans that the structure can be set out in a slightly different way, shown inFigure 1.3.The two parts of the peptide link each carry only a partial charge, but this

still allows the peptide link to play significant roles in the interaction betweenthe protein and water and in the fixation of reagents in the leather makingprocesses, most important in post tanning, when the leather is dyed andlubricated (Chapters 16 and 17).

1.2 HIERARCHY OF COLLAGEN STRUCTURE

It is a feature of the properties of collagen that it has ‘layers’ of structure,collectively known as the ‘hierarchy’ of collagen structure, which combine toallow the formation of fibres. These can be defined as follows, starting with themost fundamental element of structure.

1.2.1 Amino Acid Sequence

Collagens are characterised by a repeating triplet of amino acids: -(Gly-X-Y)n-.Therefore, one-third of the amino acid residues in collagen are glycine. Fur-thermore, X is often proline and Y is often hydroxyproline: 12% of the triplets

O

NH− NH−

O−

−C

−C

−C

NH

Oδ-

δ+ _

+

Figure 1.3 Electronic structure of the peptide link.

4 Chapter 1

are -Gly-Pro-Hypro-, 44% are -Gly-Pro-Y- or Gly-X-Hypro- and 44% are-Gly-X-Y- where X and Y are not defined. In this way, the helical shape of themolecule is determined (see below).The amino acid compositions of bovine skin proteins are compared in Table 1.7.

In leather making terms, the amino acid sequence plays only a minor role; indeed,the technologies for making leather are essentially the same for all animal skins:variations in technologies are much more dependent on the macro-structure of theskin and its age rather than the details of the chemistry of the protein. The aminoacid content determines the reactivity of the protein towards reagents such astanning compounds and the sequence influences the formation of electrostaticlinks,5 which is important for protein stability but which is also exploited in leathermaking: the amino acid content determines the isoelectric point (see below), whichtogether with pH controls the charge on the protein. Therefore, these features ofthe protein influence its affinity for different tanning reagents, but there is littledifference between animal species in terms of the outcomes of stabilising reactions.Here, stability or more commonly the hydrothermal stability (resistance to wetheat) is conventionally measured by the temperature at which the protein loses itsnatural structure: for collagen this is referred to as the ‘shrinkage temperature’, Ts,or sometimes as the melting or denaturation temperature.The only discernible difference in hydrothermal stabilities of collagens is

observed in extreme variations in skin source, where there can be big diffe-rences in shrinkage temperature, depending on the natural environment of theanimals concerned (see Table 1.4 below). When there are differences inshrinkage temperatures between collagen sources, the difference is also reflectedin the shrinkage temperatures acquired from stabilising chemistries (tanningoptions).

N

C

C

H

O

R

RO

H

C

C

N

RO

H

C

C

N

HH

H

C

N

H

C

H R

O

O

RH

C

H

N

C

H

H

N

C

C

H

OR

N

C

C

H

O

R

O

H

C

C

N

H2C

CH2

CH2

Figure 1.4 The peptide link: free rotation for a-amino acid chains, but lockedconformation when b-amino acids are in the chain.


1.2.2 The a-Helix

The presence of a high content of b-amino acid causes the chain of aminoacids to twist, due to the fixed tetrahedral angles, locking the twist in place(Figure 1.4). Notably, the twist is left-handed, i.e. anti clockwise, because of thenatural L-conformation of the molecules.

1.2.3 The Triple Helix

The notion of the triple helix structure of collagen was first proposed byRamachandran.6 In type I collagen, the monomeric molecule, protocollagen,contains three chains, designated aI(I) and aI(2): there are two aI(I) chains andone aI(2) chain, which differ only in the details of the amino acid sequence.These three chains twist about each other in a right-handed or clockwise triplehelix: this is only possible because of the high glycine content, which has the

Figure 1.5 The triple helix. (Reproduced by permission of Edward Arnold (Pub-lishers) Ltd: from Collagen. Anatomy of a Protein by J. Woodhead-Galloway. r John Woodhead-Galloway, 1980.)

6 Chapter 1

smallest a-carbon sidechain, a hydrogen atom, so that glycine is always situatedin the centre of the triple helix (Figure 1.5).Each a-chain is about 1050 amino acids long, so the triple helix takes

the form of a rod about 300 nm long, with a diameter of 1.5 nm: this is themonomeric unit from which the polymeric fibrous structure is created. Inthe production of extracellular matrix from the fibroblast cells of the skin, thetriple helix is synthesised as the monomer procollagen and then the monomersself assemble into a fibrous form. Before it becomes fibrous, soluble collagencan be isolated as triple helices and it is in this form it can be extracted fromimmature skin under acid conditions.In the triple helix there is an inside and an outside of the structure. The

minimum size of the glycine sidechain and its occurrence every third residue inthe alpha helix allows it to fit into the inside part of the structure. If the side-chain was bigger, the triple helix could not form.At each end of the triple helices there are regions that are not helical, con-

sisting of about 20 amino acids, called the telopeptide regions. If soluble col-lagen is isolated with the aid of pepsin, a proteolytic enzyme, triple helices areobtained without the associated telopeptide regions.7 In the case of acidextraction of soluble collagen, individual triple helices are obtained with thetelopeptide regions intact (Figure 1.6).In the context of leather making, the importance of the telopeptide regions lies

in their role in bonding, to hold the collagen macromolecule together: thecovalent bonds holding the triple helices together link the helical region of onetriple helix to the non helical region of another triple helix, indicated in Figure 1.6.Modelling studies have shown that, because of their flexibility, the telopeptidechains are capable of their own interactions between a-helices and triple helices.9

In tanning terms, the telopeptide regions probably do not play a significant role inthe preparative processes or in the stabilising reactions: in the latter case, thestability arises from the structure created between the triple helices, based on thehigh degree of structure already in place in the helical region, but which is notpresent in the more random telopeptide chains.

Acid soluble collagen Pepsin soluble collagen

Figure 1.6 Extraction of soluble collagen (taken from Zeugolis8).


1.3 ISOELECTRIC POINT

Importantly, the triple helix is also held together by electrostatic bonds, asfollows:

P-CO�2 � � �þ H3N-P

These are the so-called ‘salt links’, formed by electrostatic reaction betweenacidic and basic sidechains of the protein, which together determine the iso-electric point of collagen. The IEP is an important parameter, because it con-trols the charge on the protein at any given pH: since the reactions in leathermaking are usually dependent on charge, they are in turn dependent on the IEPand the way the value of the IEP varies during processing. The isoelectric pointis defined in several ways, but the most fundamental definition is that it refers tothe point on the pH scale at which the net charge on the protein is zero, illu-strated as follows:

P-CO2H ðacidicÞ þH3Nþ-P

ÐP-CO�2 � � �H3N

þ-P ðprotein at IEP; charge zeroÞÐP-CO�

2 ðalkaline; anionicÞ þH2N-P

Because the charge on the protein can be adjusted with acid or alkali, tomake the protein positively or negatively charged, respectively, there must be apoint on the pH scale when the net charge passes through zero. Therefore, theIEP depends on the relative availability of groups that can participate inreactions with acid and alkali, the carboxyl and amino groups. So IEP can bedefined as follows:

IEP ¼ f1½NH2�Tf2½CO2H�T

¼ f1ð½NH2� þ ½NHþ3 �Þ

f2ð½CO2H� þ ½CO�2 �Þ

where: f indicates some function of concentration and T indicates totalconcentration.The second, expanded form of the equation indicates that it is only the total

availability of a species that is important, its form is unimportant, i.e. the IEPdoes not change with pH.More strictly, the definition should incorporate the role of the pKa or pKb of

these groups, which would require that each specific pH active group would betreated separately, exemplified as follows:

IEP ¼P

i fi½NH2�iPj fj½CO2H�j

In this way, all i and j species provide individual contributions to the iso-electric point. Unfortunately, the nature of the functions f is not known, so

8 Chapter 1

calculation of the precise value of the IEP is not yet possible. However, this isclearly not an intractable problem.The equations are usefully presented in this form, with the amino function as

numerator and the carboxyl function as denominator, because they then pre-dict how varying the functions influences the IEP, in terms of the direction ofchange (Table 1.2).At the isoelectric point, the following phenomena occur, each of which might

be used as the definition, as a whole or in part.

� net charge is zero� content of intramolecular salt links is maximised� swelling is at a minimum� shrinkage temperature (and other aspects of hydrothermal stability) is

maximum

There are two points relating to isoelectric point and its relevance to leathermaking that are very important and worth emphasising:

1. IEP is a point on the pH scale, so it does not change with changing pH ofthe system. The IEP of collagen is the same whether is in the alkaline,limed state or in the acidic, pickled state.The importance of this point is that the isoelectric point can only be

changed if there is a chemical change that alters the availability of activegroups; this can occur in the beamhouse processes, in the tanning pro-cesses and in the post tanning processes.

2. The charge on collagen is determined by the relative values of the IEP andthe pH. If the pH is higher than the IEP, the collagen is negatively charged,and if the pH is lower than the IEP, the collagen is positively charged.Moreover, the further the pH is from the IEP, the greater is the charge,although it is limited by the availability of amino and carboxyl groups.The importance of this concept relates to the application of charged

reagents, particularly post tanning reagents and their interaction with thecharged leather substrate.

It can be assumed that at the starting point of processing for collagen, priorto raw pelt going into liming (alkaline treatment for unhairing and modificationof the skin’s fibre structure), the IEP is at physiological pH, about 7.4.

Table 1.2 Effect on IEP of changing the content of active groups.

Amino function Carboxyl function

Increase content Higher LowerDecrease content Lower Higher


1.4 COLLAGEN AND WATER

An important part of the structure of collagen is the role of water, which is anintegral part of the structure of collagen and hence of its chemically modifiedderivatives (Table 1.3).10

Gustavson11 observed that the shrinkage temperature of raw skin depends onthe pyrrolidine content, i.e. proline and hydroxyproline (Table 1.4); thisobservation was extended by Privalov,12 who demonstrated that the relation-ship relies more on the hydroxyproline content than on the proline content. Theentropy loss for the secondary amino acids in the denatured state is less than forother residues, because the ring component of their structures restricts stericconformations; therefore, this could contribute to the relationship between thehigher stability and higher proline and hydroxyproline content of the protein.Clearly, the hydrothermal stability of the collagen is also a reflection of theenvironment in which the animal lives.Berg and Prockop13 extracted protocollagen, a non-hydroxylated version of

collagen, and demonstrated that its structure is the same as collagen, based onthe dimensions of the polypeptides and optical rotatory properties. It wasfound that the shrinkage temperature is 15 1C lower than hydroxylated

Table 1.3 Collagen and water.

Water con-tent (g g�1)

Freezingpoint (K)

Desorptionenergy (Jmol�1)

Absorption energy(Jmol�1) Structural function

0–0.07 Notfreezable

71 71 Bound to collagenby two H-bonds

0.07–0.25 o180 50 59 Bound to collagenby one H-bond

0.25–0.50 265 38 38 Bound to side-chains and pep-tide links

0.50–42.0 265 Mechanicallyremovable

Swelling water,interstitial withinthe fibrestructure

Table 1.4 Dependence of shrinkage temperature on imino acid content ofcollagens.

Collagen sourceNumber of Pro and Hyproresidues per 1000 Shrinkage temperature (1C)

Calf 232 65Carp 197 54Cod 155 40

10 Chapter 1

collagen, indicating the importance of hydroxyproline to the stability ofcollagen.Privalov believed that the hydrogen bonding by water at hydroxyproline is

important in stabilising collagen,14 but thought that the Ramachandranmodel15 could not solely explain the high denaturation energy – rather thestabilisation probably included wider layers of water. He stated:12

having in mind the tendency of water molecules to cooperate with theirneighbours, it does not seem improbable that the hydroxyprolyl can serveas an initiator to an extensive network of hydrogen bonds. This envelopesthe collagen molecule and might be responsible for the exceptionalthermodynamic properties of collagen.

In this way, he regarded the water bound to the triple helix as constituting amatrix structure. Model studies have confirmed that the triple helix is indeedsurrounded by such a water structure, nucleated at hydroxyproline, as indi-cated in Figure 1.7.16 The crystal structure gives experimental proof of theexistence of water bridges, whereby the triple helices are surrounded by

Figure 1.7 Supramolecular water around the triple helix: water molecules in blueare bound to hydroxyproline, creating nuclei for additional water mole-cules to form a solvating sheath. (Reprinted from J. Bella, B. Brodsky,H.M. Berman, Structure, 3(9), Hydration structure of a collagen peptide,893–906, r (1995), with permission from Elsevier.)


a cylinder of hydration and the hydroxyproline residues appear to act as the‘keystones’, connecting the water molecules to the polypeptide. It can be seenthat the grooves in the triple helix are filled with solvent molecules directlybonded to the anchoring groups on the peptide or connected to those waters inthe first hydrogen bonded shell. The third shell of water molecules completesthe cylinder of hydration, which is supramolecular solvation. While it wasrealised that without the hydroxyproline residues there were only localisedregions of water structure, Berman later suggested17 that the hydroxyprolineacts as a nucleus for water bridges, extending the network far beyond whatmight be expected and that is critical for the lateral assembly and the super-molecular structure of collagen. Engel et al.18 demonstrated that, comparedwith peptides of the same length, those containing Hypro residues had higherthermal stability than those without Hypro.The triple helices are linked through hydrogen bonding (Figure 1.8), in which

water may form part of the hydrogen bonding system.15 In this way, water is anintegral part of collagen structure. The figure merely models the effect,emphasising that water is involved directly in the interactions between proteinchains, linking parts by hydrogen bonding, but the bound water is itself linkedto water molecules. It is the build up of layers of water structure, starting ornucleating at hydroxyproline, that creates the supramolecular solvent sheath.This is an important feature of the stabilisation of native collagen, but it is alsothe basis for chemical stabilisation, the tanning reactions, because the waterillustrated in Figure 1.7 occupies the region in the collagen molecule where thetanning or stabilising reactions occur.

CHR

OO

O

O

OO

O

O O

H

H

H

H

H

H

H

H

H

H

HCH2H2C

CH

HH

N N N

OOO

O

C C CCHRCHR

HH HH

H

HO

H

CHR

CHN N NC C CCHRCHR CHR

Figure 1.8 Model of hydrogen bonding in collagen, illustrating the way hydrogenbonding can link elements of the protein structure.

12 Chapter 1

Privalov12 has calculated that the contribution of hydroxyproline to theentropy of water bound to the helix is 98 JK�1mol�1. In comparison to bulkwater freezing, this represents the equivalent of freezing four molecules of waterper hydroxyproline residue, illustrating the degree of association of water at theHypro sidechain.The involvement of water in structure is an important feature of collagen,

because it influences the relationship between drying and subsequent leatherproperties. If the drying conditions are severe enough to remove water close tothe triple helix, the fibre structure can approach close enough to allow theformation of additional chemical bonds19 (Chapter 18). This adversely affectsthe strength of leather by embrittling the fibre structure and the handle or feelof the leather is stiffened.An alternative view of the role of solvent is based on inductive effects.

Cooper20 thought that structural stabilisation might originate in solventorganisation around the triple helix, but he also suggested that the severerotational restrictions placed on the polypeptide chain by the presence ofpyrrolidone residues act to stabilise the triple helical structure by reducing thetotal conformational entropy change in the helix to non helix transition.Raines21 agreed that the hydroxy group on proline helps to stabilise the col-lagen structure, but suggested that the important effect is due to induction. Hisgroup synthesised the polypeptide (Pro-Flp-Gly)10, where Flp is a 4-(R)-fluoroproline residue: fluorine is used because it is the most electronegativeelement and because it is a poor hydrogen bond acceptor. Therefore, anystabilisation due to the fluorine must come from a source other than hydrogenbonding. By comparing circular dichroism spectra, they showed that thestructure is similar to the comparative polypeptides in which the place of Flp istaken by Pro or Hypro. However, the difference in residues content did affectthe unfolding transition temperature (Table 1.5), indicating the stabilisinginfluence of an inductive effect.It is quite possible that both views are correct and the net effect of the presence

of the hydroxyproline residue is a combination of direct inductive and hydrogenbonding effects. Whichever is right, this view of the structure of the fundamentalunit of structure also indicates the way in which the structure–reactivity rela-tionships of collagen can be addressed. In native collagen or in modified col-lagen prior to chemical stabilisation (tanning), the triple helices are held in anarray, in which the interstices are occupied by water, which interacts with thesidechains and backbone amide links of the protein. Chemical stabilisation of

Table 1.5 Effect of polypeptide structure on the unfolding transitiontemperature.

Polymer Unfolding transition (1C)

(Pro-Pro-Gly)10 41� 1(Pro-Hypro-Gly)10 69� 1(Pro-Flp-Gly)10 91� 1


the protein can be regarded as the result of reactions occurring in this zone,acting at the surfaces of the triple helices. This model will be become clear as thereactions of leather making are addressed. Importantly, the model will allow thedevelopment of a general theory of tanning (Chapter 19).

1.5 QUARTER STAGGER ARRAY

The pattern of the amino acid sequences is characterised by the clustering ofacidic and basic amino acids, to create a pattern that is repeated four timeswithin the triple helix, known as the D-period. Heidemann has mapped thepattern of charged sidechains in the a1 and a2 chains5 and demonstrated that thetheoretical interactions between the sidechains are predominantly attractive,with few regions of charge repulsion. To allow the maximum formation ofthe salt links, there are two possible types of interaction (Figure 1.9): the side byside interaction, called segment long spacing, and the alignment of the D-periods,resulting in staggering by a quarter of the length of the triple helices. It is possibleto create the ribbon-like segment long spacing structure, by precipitatingcollagen from weak acid solution, and this structure may play some part infibrillogenesis,22 but it is the quarter stagger array that is the basis of fibre for-mation.23 Note the so-called gap region between the longitudinal arrangement ofthe triple helices. Not illustrated here is that there is a superhelical, left-handedtwist to the array – at every level of structure there is a change to the directionof twist, a form of structure long exploited by man for making twine or rope.

segment long spacing quarter stagger

Figure 1.9 Models of possible interactions between triple helices.

14 Chapter 1

The triple helices are held together by salt links, hydrogen bonding, hydro-phobic bonding (Figure 1.10) and covalent bonding. The salt links, betweencationic and anionic sidechains, occur along the alpha helices, as do hydrogenbonds, involving water and charge–charge interactions. Similarly, less impor-tant hydrophobic interactions involve the electrically neutral sidechains, by theexclusion of water and induced charge attractions.The covalent links are formed at the ends of the triple helices, between the

telopeptide region of one triple helix and the helical region of another. In brief,the covalent bonding begins with formation of a Schiff base, from the reactionbetween a lysine sidechain and an allysine sidechain, created by enzymaticoxidation of lysine by lysyl oxidase:24

P-ðCH2Þ4-NH2ðlysineÞ þOHC-ðCH2Þ3-PðallysineÞÐP-ðCH2Þ4-N¼HC-ðCH2Þ3-PðSchiff baseÞ

Schiff base formation is the first step in the covalent bonding between triplehelices and is characteristic of immature skin, particularly apparent in foetalskins. As the animal matures, the Schiff bases are reduced: the reaction iscomplex, but can be modelled as follows:

P-ðCH2Þ4-N¼HC-ðCH2Þ3-PÐP-ðCH2Þ4-NH-CH2-ðCH2Þ3-P

The actual reaction in nature is complicated by the involvement of a histidinesidechain becoming attached to the allysine side of the new bond1 (Figure 1.11).

Protein-(CH2)3-CH-NH-CH-(CH2)3-Protein

OH

N

HC C-CH2-Protein

CHN

Figure 1.11 Maturation of Schiff base bonding between a-helices.

P CH

CH3

CH3

H3C P

valine alanine

Figure 1.10 Illustration of hydrophobic bonding.


This difference between immature and mature skins is important to thetanner, because they react differently during alkali treatment in the early stagesof processing. Schiff bases are vulnerable to hydrolysis: if the bonds are brokenthen the triple helices can become detached from each other, i.e. the collagen issolubilised. Therefore, immature skins must be treated carefully when under-going processing under alkaline hydrolytic conditions. On the other hand, themature, reduced crosslinks are not hydrolysable, so can resist hydrolytic con-ditions. Immature skins can be artificially aged, by applying reducing agents,such as sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4)(BLC The Leather Technology Centre, UK, unpublished results). However,with those reagents the process is expensive and dangerous, due to the evolu-tion of hydrogen under the reaction conditions, so it is typically not used inindustry. Nonetheless, the principle has been proved and there could be eco-nomic benefits.The quarter stagger model of collagen set out here so far has not defined the

array of triple helices in the third dimension. There have been two modelsproposed (Figure 1.12). Hexagonal closest packing (HCP) is the closest circularsections can approach – this is common in crystal structures. (Try puttingtogether seven coins of the same denomination – six will fit exactly around acentral seventh.) The pentafibril model assumes that the triple helices arearranged in units of five, as a regular pentagon in section. However, both viewsare probably right, because the pentafibrils can form a sort of distorted HCP, iftheir regular structure is slightly flattened (Figure 1.12).25

The way in which the triple helices are packed together constitutes a con-tribution to the stability of the protein, referred to as a ‘polymer-in-a-box’ byMiles and Ghelashvili:26 in their model, the confining of the triple helices withinthe fibre structure reduces the entropy of the thermally labile domain when itundergoes the transition to random coil and this stabilises the intact helixconformation.

hexagonal closest packing (HCP) pentafibril or microfibril

distorted hexagonal closest packing

Figure 1.12 Packing of triple helices.

16 Chapter 1

The molecular structure at the triple helix level and interactions between thesecomponents of collagen have been investigated by molecular modelling.27–32

Ultimately, this approach is likely to be the most fruitful in defining the detailsof structure, conformations and interactions with stabilising (tanning) species.Indeed, it may be the only reliable way to confirm the validity of theoreticalviews of tanning.Cot33 has recently reviewed the current understanding of the structure of

collagen. There, he includes a discussion of the music that can be generated byassuming an amino acid scale of frequencies/notes: for example, the hydrophilicamino acids are assigned higher pitch, the hydrophobic amino acids areassigned lower pitch.34,35 This use of the amino acid sequence generates musicwith recurring themes: true electronic music or music of the small spheres!

1.6 FIBRILS

The triple helices are bound together in bundles called fibrils (Figure 1.13). Therepeating pattern of charges in the D-period banding becomes apparent in themacro structure of the fibrils: it can be visualised in raw collagen by reacting thecharged sidechains with heavy metal salts, to create a corresponding pattern ofhigh electron beam density. This is commonly done with, for example, anionicphosphotungstate to react with positive basic groups and counter staining with,for example, cationic uranyl ion to react with the carboxyl groups of acidic

Figure 1.13 Fibrils shown by scanning electron microscopy, exhibiting the char-acteristic banding pattern of the D-period.


amino acids. Tanning with less heavy chromium(III) salts can suffice to show thepattern of charge distribution.Fibrils are the smallest units of collagen structure that are visible under the

electron microscope. That is, unless the structure is disrupted chemically, e.g.by the effect of acid swelling (Figure 1.14).36 The figure shows that there is somesub-structure present in fibrils. However, the nature of the sub-units is notknown: it might be thought that the structures are pentafibrils, but the thick-ness of the units visible in the figure is much too large.Moreover, in Figure 1.14 there appear to be layers of spiral structure so they

must represent a discrete layer in the hierarchy of structure. It is possible that whatis visible in the electron photomicrograph is unravelled surface. This would be inagreement with Koon’s observations of apparent hollow structure in collagenfibrils recovered from bone37 (Figure 1.15). Here the demineralisation conditionsare too mild to cause significant damage to protein, so the structure observed bytransmission electron microscopy must be a reflection of the actual structure.This clearly begs the question: what is the structure of fibrils? The question is

complicated by the observation that sectioning through fibrils reveals nointernal structure (Figure 1.16).5,37

Kronick has reported that the cores of fibrils can be melted at a lowertemperature than denaturation of the surrounding sheaths38 and that the innermaterial of the fibrils can be degraded by trypsin to leave hollow fibrils,39 butthis reaction is prevented if the fibrils are first stabilised with glutaraldehyde.40

Other workers have suggested that the core material might be glycosami-noglycans41 or glycoproteins.42 Therefore, it is uncertain what lies within fibrils,if anything, but the latter observations are not inconsistent.

Figure 1.14 Sub-structure in fibrils revealed by acid swelling.

18 Chapter 1

Figure 1.15 Fibrils recovered from bone by demineralisation with 0.6 M HCl.(Courtesy of H.E.C. Koon.)

Figure 1.16 Cross section through fibrils in chrome tanned bovine hide, visualised bytransmission electron microscopy.


1.7 FIBRIL BUNDLES

Fibrils are arranged in the form referred to here as fibril bundles (Figure 1.17)in which the fibril bundle are a sub-structure or the constructing units of fibres.This level of the hierarchy of collagen structure is important with respect to

opening up of the fibre structure, in preparation for the tanning process. Splittingopen the fibre structure at this level, at the junctions between fibril bundles, isimportant for creating softness and strength in the final leather, particularly inregard to the deposition of lubricating agents (discussed further in Chapter 17).

1.8 FIBRES

Fibril bundles come together to create fibres, as clearly seen under low reso-lution light microscopy (Chapter 2). The fibres characteristically divide andrejoin with other fibres throughout the corium structure. It is this variation incrosslinking or linking that provides the strength to the material. Some workershave used the term ‘fibre bundle’, but this merely reflects the observation thatthe fibres have a range of diameters, largely dependent on the position throughthe skin cross section (Chapter 2, Figure 2.3). Therefore, strictly this does notconstitute another level of structure in the hierarchy.

1.9 OTHER COLLAGENS

1.9.1 Type III Collagen

Also called reticulin, type III collagen conforms to the typical characteristic ofcollagens: it is one-third glycine and has a high content of b-amino acids and,

Figure 1.17 Scanning electron photomicrograph of fibril bundles in collagen fibres.

20 Chapter 1

therefore, it has the triple helical structure. It differs from type I in two respects:there are differences in the amino acid composition and the triple helix consists ofthree aI(III) chains. As a consequence, it forms finer fibres, although it can formmixed fibres with type I collagen,24 about 40% in foetal skin, reducing to about15% in mature skin, when the requirement for flexibility of the fibre structureduring growth becomes less important.43 It can be seen as fine fibres within thecoarser fibre structure of the corium. It is understood to be a major component ofthe grain or coriumminor enamel, which is the most valuable part of grain leather.

1.9.2 Type IV Collagen

Type IV collagen is found in the basement membrane, where it forms part ofthe structure by which the epidermis is fixed to the grain layer or corium minor.It is not a fibrous collagen, but instead has a network structure,24 as indicated inFigure 1.18. Type IV collagen can be selectively attacked by an enzyme mixturecalled dispase, which offers an option of unhairing mechanism, because thebasement membrane extends down the hair follicles (Chapter 3).

1.9.3 Type VII Collagen

Type VII collagen is also not a fibre forming protein, but has a simple dimerictype of structure.24 It acts as the link between the type IV collagen that is boundto the epidermis and the grain layer (Figure 1.18).

1.10 CHEMISTRY OF COLLAGEN

Figure 1.19 presents the titration curves for native and alkali treated collagen.44

Table 1.6 sets out the regions of the curve, comparing native collagen, in itsnatural state, and collagen that has received the typical hydrolytic alkalitreatment at the early stage of processing, called liming.

Epidermis

Type IV collagen basementmembrane

Type VII collagen

Corium minor

Corium major

Figure 1.18 Illustration of the structure of the basement membrane.


It is useful to make comparisons between the amino acid content of collagenand other proteins associated with skin (Table 1.7).Compare the amino acids contents of collagen types I and III. There are

differences, but not to the extent that significant differences in chemical prop-erties might be expected: the carboxy amino acid content is similar, so thereaction with chromium(III) in tanning will be the same. Therefore,

Figure 1.19 Titration curve for type I bovine dermal collagen. (Reproduced withpermission from Bowes and Kenten, Biochem. J., 1948, 43(3), 358–365.r The Biochemical Society)

Table 1.6 Regions of interest in the titration curve for type 1 collagen, nativeor limed, i.e. prolonged treatment in saturated calcium hydroxidesolution at pH 12.6.

pH range Reactions pKa

Amount of acid/alkali(mmol per g-collagen)

Native Limed

1.5–5.5 Carboxyl sidechains Asp, 3.85 0.87 0.96Glu, 4.25

5.5–7.0 Uncharged 0.03 0.177.0–9.5 Histidine His, 6.5 0.04 0.10

a-Amino terminal 8.58.5–12.5 Lysine amino Lys, 10.5 0.34 0.3610.5–14.0 Arginine guanidine Arg, 12.5 0.49 0.47

22 Chapter 1

technologically it is typically assumed that processing reactions will affect thecollagen types equally and equivalently. For comparison, the amino acidcompositions of elastin and keratin are included: these are proteins of impor-tance in leather making, the former for the physical properties of skin andleather, the latter for hair and epidermis removal, which are contributors toleather quality. These proteins are discussed further in Chapter 2.

1.11 HYDROTHERMAL STABILITY

One property that is routinely used to characterise collagen, whether native,structurally modified or chemically modified, is the hydrothermal stability: thisis defined as the effect of wet heat on the integrity of the material, usually interms of the denaturation transition. The value of this parameter, dependent onprocessing, preoccupies tanners and leather scientists, as will become apparentin this text. Typically the discerned initiation of the transition is referred to asthe shrinkage temperature, reflecting the observation that collagenic materialsrespond to wet heat by shrinking: in the case of native collagen this is at 65–70 1C, depending on the conditions of the test. Chapters 10 and 19 discuss thisfurther.

Table 1.7 Amino acid contents of skin proteins: residues per 1000.

Type Amino acidCollagentype I

Collagentype III Elastin Keratin

Non polar Glycine 330 309 355 81Alanine 110 97 178 50Valine 22 27 175 51Leucine 26 36 59 69Isoleucine 12 18 19 28

Hydroxy Serine 34 43 4 102Threonine 18 22 4 65Tyrosine 5 3 12 42

Carboxy Aspartic acid+asparagine

47 53 2 60

Glutamicacid+glutamine

73 76 12 121

Basic Lysine 38 22 5 23Hydroxylysine 6 12 0 0Arginine 48 45 5 72Histidine 4 6 0 7Tryptophan 0 0 0 12

Imino Proline 126 97 125 75Hydroxyproline 93 108 23 0

Sulfur Cystine 0 0 1 112Methionine 4 8 0 5

Aromatic Phenylalanine 14 18 22 25Tyrosine 5 3 12 42


Collagen has been described in terms of a block copolymer, incorporatinghard and soft monomeric units,45 in which there are crystalline regions, asso-ciated with the presence of hydroxyproline,46 where the protein structure issupported by the supramolecular water. Where there is deficiency in Hypro, thecollagen is thermally labile, so that hydrothermal breakdown of the structure isinitiated at those sites in the molecule.47 Interestingly, type III collagen appearsto be less stable than type I collagen: this may explain the suggestion fromleather makers that the grain enamel is more vulnerable to hydrothermaldamage than the rest of the skin structures. Degradation of collagen by wetheat causes the transition:

helixÐ random coil

when the product is well known in the form of gelatine. The gelling property ofgelatine depends on the degree to which the helical structure is unravelled: themore this happens, the less strength the gel has, but if some helical structure isretained the protein interacts in a state intermediate between solubilised andsolid. Although the transition is expressed here as an equilibrium, it is anirreversible rate process, only exhibiting reversibility in the early stages of thereaction.48–50 This is understandable when considering the progress of thereaction. In the early stages, when the triple helices have only unravelled alittle, the chains can re-register back into place. However, there comes a pointwhen so much of the helical structure is lost that the randomised structurecannot spontaneously recreate the highly structured helical form. From a lea-ther making standpoint, the reaction is the same for raw collagen or leather; thehydrothermal reaction is seen as a shrinking of the fibre structure and is inpractice an irreversible and highly damaging effect. The process can be sum-marised as:

intactÐshrinking ! shrunk ! gelatinisation ! solubilisation

Reversibility only applies in the first step. Subsequent steps constituteincreasing heat damage to the collagen, which can occur at any stage in theleather making process. The potential for the reactions to go to completiondepends on the state of the collagen. In its raw state, collagen can be solubilised.Moreover, the conditions causing the shrinking do result in different inter-mediate states. Wet heat damage under alkaline conditions causes rapidhydrolysis and accelerated solubilisation, but before that happens the fibresexhibit a visible crimping effect (Figure 1.20).36 Alternatively, under acidconditions, crimping is less obvious because the hydrolysis reaction is slower, sothe damaged fibre structure is characterised by the presence of broken collagenchains and collagen fragments: the outcome in dried pelt is a cementing of thefibre structure causing the material to be embrittled. When collagen is in atanned state, depending on the chemistry of the process, the reaction typicallystops at the shrunk stage.

24 Chapter 1

The phenomenon of partial reversibility of shrinking in wet heat is reflectedin the properties of some tanned collagen materials. When the stabilisingreaction involves the creation of an interpenetrating network within the col-lagen network, shrinking can be partially reversed. This is called the Ewald

Figure 1.20 Crimp in corium fibres, caused by (A) alkali+heat damage (obviouscrimp) and (B) acid+heat damage (minor crimp and gelatinisation).

a

b

mW

-2

0

2

4

6

8

10

12

14

min

˚C40 60 80 100 120 140 160 180

0 5 10 15 20 25 30

^endo

Figure 1.21 DSC thermograms of turkey tendon: (A) acid demineralised and (B) aciddemineralised and denatured with LiBr.


effect; it is observed in oil tanned leather51 (Chapter 13) or if butadiene ispolymerised within the fibre structure. Here the re-registering of the chaininteraction is assisted by the scaffolding effect of the polymerised oil, shaped tothe structure of the collagen.The impact of hydrothermal damage reflects the nature of the starting

material, whether raw or tanned and, in turn, how the collagen has been tan-ned. Untanned collagen is eventually liquefied, which is the fate of collagentanned by plant extracts, vegetable tannins, but collagen tanned with chro-mium(III) salts does not readily liquefy. The difference lies in the nature of thestabilising chemistry: in the former reaction, stabilisation comes from hydrogenbonding, while in the latter reaction, stabilisation comes from covalent com-plexation. The impact of tanning chemistry on outcome is developed further inChapters 11–14.Only recently, it has been demonstrated that the shrinking transition is not the

only hydrothermal transition, because there is another, higher temperature tran-sition observable by differential scanning calorimetry52,53 (Figures 1.21 and 1.22).In Figure 1.21, the native collagen of turkey tendon exhibits the expected lowtemperature transition at about 60 1C, but has another transition at about 130 1C.The first transition is eliminated if the collagen is treated with lithium bromide, awell-known hydrogen bond breaker, but the higher temperature transitionremains unaffected. Figure 1.22 shows the expected effect of the chrome tanningprocess; the shrinking transition is moved to higher temperature, but the samehappens to the higher temperature transition.

a

b

mW

6

8

10

12

14

16

18

°C40 60 80 100 120 140 160 180

^endo

Figure 1.22 DSC thermograms of bovine hide collagen: (a) untanned hide powderand (b) chromium(III) tanned hide powder.

26 Chapter 1

It is not clear what reaction or reactions are responsible for the highertemperature transition: it may be a break down of other higher energy struc-tures associated with the sidechains of the protein or decomposition of theamide links of the peptide chain.

REFERENCES

1. A. J. Bailey and R. G. Paul, J. Soc. Leather Technol. Chem., 1998, 82(3),104.

2. W. D. Comper, Extracellular Matrix, 2, Harwood Academic Publishers,Melbourne, 1996.

3. C. M. Kiely, et al., Connective Tissue and its Heritable Disorders. Mole-cular, Genetic and Medical Aspects, Wiley-Liss, New York, 1993.

4. K. E. Kadler, et al., J. Cell Science, 2007, 120(12), 1955.5. E. Heidemann, J. Soc. Leather Technol. Chem., 1982, 66(2), 21.6. G. N. Ramachandran, J. Amer. Leather Chem. Assoc., 1968, 63(3), 160.7. N. D. Light, Methods in Skin Research, John Wiley and Sons Ltd, 1995.8. D. Zeugolis, PhD Thesis, The University of Northampton, February 2006.9. E. M. Brown, J. Amer. Leather Chem. Assoc., 2004, 99(9), 376.

10. K. J. Bienkiewicz, J. Amer. Leather Chem. Assoc., 1990, 85(9), 305.11. K. H. Gustavson, The Chemistry and Reactivity of Collagen, Academic

Press, New York, 1956.12. P. L. Privalov, Advances in Protein Chemistry, Vol. 35, Academic Press,

1982.13. R. A. Berg and D. J. Prockop, Biochem. and Biophys. Res. Comm., 1973,

52(1), 115.14. P. L. Privalov and E. I. Tiktopulo, Biopolymers, 1970, 9, 127.15. N. Ramachandran and C. Ramakrishnan, Biochemistry of Collagen,

Plenum Press, 1976.16. H. M. Berman, J. Bella and B. Brodsky, Structure, 1995, 3(9), 893.17. H. M. Berman, et al., J. Mol. Biol., 1998, 260, 632.18. J. Engel, H.-T. Chen and D. J. Prockop, Biopolymers, 1977, 16, 601.19. M. Komanowsky, J. Amer. Leather Chem. Assoc., 1991, 86(5), 269.20. A. Cooper, J. Mol. Biol., 1971, 55, 123.21. R. T. Raines, et al., Nature, 1998, 392, 666.22. A. G. Ward, J. Soc. Leather Technol. Chem., 1978, 62(1), 1.23. D. J. S. Hulmes, A. Miller and D. A. D. Parry, et al., J. Mol. Biol., 1973, 79,

137.24. A. J. Bailey, J. Soc. Leather Technol. Chem., 1992, 76(4), 111.25. J. P. R. O. Orgel, A. Miller, T. C. Irving, R. F. Fischetti, A. P. Hammersley

and T. J. Wess, Structure, 2001, 9, 1061.26. C. A. Miles and M. Ghelashvili, Biophysical J., 1999, 76, 3243.27. E. M. Brown, J. M. Chen and G. King, Protein Engineering, 1996, 9(1), 43.28. E. M. Brown and G. King, J. Amer. Leather Chem. Assoc., 1996, 91(6),

161.


29. E. M. Brown and G. King, J. Amer. Leather Chem. Assoc., 1997, 92(1), 1.30. D. Buttar, R. Docherty and R. M. Swart, J. Amer. Leather Chem. Assoc.,

1997, 92(8), 185.31. J. Fennen, J. Amer. Leather Chem. Assoc., 1998, 82(1), 5.32. L. Siggel and F. Molnar, J. Amer. Leather Chem. Assoc., 2006, 101(5), 179.33. J. Cot, J. Amer. Leather Chem. Assoc., 2004, 99(8), 322.34. http://www.whozoo.org/mac/music/collagen.htm. Accessed Feb. 10, 200935. http://algoart.com/forum. Accessed Feb. 10, 200936. K. T. W. Alexander, A. D. Covington, R. J. Garwood, A. M. Stanley,

Annexe to Proc., IULTCS Congress, Porto Alegre, Brazil, Nov. 1993.37. H. E. C. Koon, PhD Thesis, University of York, October 2006.38. P. L. Kronick, B. Maleeff and R. Carroll, Conn. Tissue Res., 1988, 18, 123.39. P. L. Kronick and B. Maleeff, J. Amer. Leather Chem. Assoc., 1990, 85(4),

122.40. P. L. Kronick, B. Maleeff and P. Cooke, N.Y. Acad. Sci., 1990, 580, 448.41. N. Nakao and R. Bashey, Exptl. Molec. Path., 1972, 17, 6.42. S. Franc, J. Submicrosc. Cytol. Path., 1993, 29, 85.43. E. M. Epstein and N. H. Munderloh, J. Biol. Chem., 1978, 253, 1336.44. J. H. Bowes and R. H. Kenten, Biochem. J., 1948, 43(3), 358.45. A. C. T. North, P. M. Cowan and J. T. Randall, Nature, 1954, 174, 1142.46. T. J. Wess, A. P. Hammersley, L. Wess and A. Miller, J. Mol. Biol., 1998,

275, 255.47. C. E. Weir, J. Amer. Leather Chem. Assoc., 1949, 44(3), 108.48. C. A. Miles, Int. J. Biol. Macromol., 1993, 15, 265.49. C. A. Miles, T. V. Burjanadze and A. J. Bailey, J. Mol. Biol., 1995, 245,

437.50. A. D. Covington, R. A. Hancock and I. A. Ioannidis, J. Soc. Leather

Technol. Chem., 1889, 73(1), 1.51. J. H. Sharphouse, J. Soc. Leather Technol. Chem., 1985, 69(2), 29.52. T. Green, PhD thesis, The University of Northampton, 2004.53. T. Green, A. D. Covington, J. Ding, M. J. Collins, Tecnologie Conciare,

93, December 2003.

28 Chapter 1

CHAPTER 2

Skin and its Components

2.1 INTRODUCTION

It is important to understand the nature of hide and skin, in order to rationalisethe structure–function, structure–reactivity and structure–property relation-ships.

� What roles are played by the various structural features observed in skin?� What roles are played by any other components naturally occurring in

skin?� What can be understood and rationalised by considering the structure of

skin, in terms of the impact the structure has on determining the way skinis processed into leather?

� How does the structure of skin influence the properties and performance ofleather?

� How do modifications to the natural structure influence the properties andperformance of leather?

Figure 2.1 illustrates diagrammatically the structure of skin:1 it represents thegeneric structure of most types of animal skin – human, cattle, pig, goat andsheep skin, although there are minor differences between those main materialsused in the global leather industry (see below). Figure 2.2 presents an actualphotomicrograph.

2.1.1 Epidermis

This is the outermost layer of the raw skin, the barrier between the animal andits environment: it is composed of so-called ‘soft keratin’, characterised by arelatively low content of cystine compared to cysteine, i.e. less oxidation of thethiol groups to the crosslinking disulfide group (Chapter 5).2




29

In the early stages of processing, when hair or wool is removed from the skin,particularly by chemical dissolving techniques, the epidermis is also removed.Efficiency in this process is paramount for leather quality: the chemical dif-ferences in structure between keratin and collagen (Table 2.1) mean that theyreact differently in dyeing.Even though the contents of basic sidechains are the same, the overall che-

mical reactivities are different (Chapter 3). Hence, the presence of residualepidermis may lead to areas of dye resist and a dull appearance. Its presence iseasily diagnosed by inspection under a microscope, seen as a rippled materialon the relatively flat grain surface (Figure 2.3).

2.1.2 Grain

The uppermost layer in unhaired or dewooled pelt, the corium minor, is alsoreferred to in the jargon as the grain layer. The structure is fibrous, but the

Figure 2.1 Illustration of the structure of skin. (Courtesy of BLC.)

30 Chapter 2

Grain layer (coriumminor), exhibitingempty hair follicles

Grain-corium junction

Corium (major)Note, the averageangle of weave isabout 45°

Flesh layer (part of thecorium major)Note the angle of weavein comparison to therest of the corium

Figure 2.2 Photomicrograph of the cross section of cattle hide.

Table 2.1 Amino acid contents of collagen and keratin: residues per 1000.

Amino acid type Collagen Keratin

Non-polar 500 279Acidic 120 181Basic 95 114Hydroxy 57 209Cystine 0 112Proline+hydroxyproline 219 75

31Skin and its Components

fibres are so fine the appearance is more like a solid. The lack of fibre inter-action, in comparison with lower layer of the skin, makes the grain weak.3,4 Themacro-structure is a convoluted sheet, because the grain layer is larger in areathan the lower layers, so it has to be folded. This conformation is held in placeby the presence of elastin (illustrated diagrammatically in Figure 2.4).There is a layer of collagen on the grain surface, known in the jargon as the

grain enamel: the structure may be based on type III collagen since it is knownto concentrate in the grain.5 The surface is shown in the photomicrograph(Figure 2.5) as an interwoven fibrous structure: the resolution is high, so thesurface that is visible in the photomicrograph is not the convolutions. This isthe most valuable part of the skin, because it provides the desired appearance of

grain

corium

Figure 2.4 Relationship between the grain and the corium.

Figure 2.3 Epidermis on skin under the microscope. (Courtesy of A. Michel,LeatherWise UK.)

32 Chapter 2

the grain: in particular, the enamel confers the appearance to the naked eye of acontinuous, reflective surface.Damage to the grain enamel reveals the underlying fine fibres of the grain,

which scatter light more than the enamel, therefore altering the perceivedcolour and appearing dull. Breaks in the enamel may create paler or darkerareas, because coloured molecules either penetrate more easily through thesurface or coloured particles lodge in the exposed fibre structure. This can occurwith dyes: the water soluble, hydrophilic dyes penetrate more easily to confer apaler appearance, the less water soluble or insoluble dyes cause colour build upin the faults, to appear darker.Chemically, the grain is the same as the more obviously fibrous corium, so

the impact of chemical modification is the same. However, because the grainhas a more solid structure, filling it with stabilising chemicals can embrittle it,allowing it to crack when stressed, especially if it is not adequately lubricated.

2.1.3 Junction

The grain–corium junction is the transition zone between the very fine fibres ofthe grain and the much larger fibres of the corium. It is an open structure,consisting of relatively small fibres and carrying other structural components ofthe skin: these include the veinous system and, in the case of sheepskins,lipocytes, which are the cells that contain triglyceride fat (Figure 2.6).6

The junction is vulnerable to breaking by flexing through mechanical actionin the process vessel. The consequent defect is called looseness, in which thedetachment of the layer becomes visible when the pelt or leather is flexed. Theeffect can occur in any skin or leather, but is facilitated by the presence, and

Figure 2.5 Surface of chrome tanned bovine hide: high-resolution scanning electronphotomicrograph (30 000�).


particularly the removal, of the fat cells from sheepskin. The fault affects the‘break’ of the leather: this is the rippling in the grain surface observed when theleather is bent with the grain uppermost. Coarse rippling, coarse break, is a signof inferior quality; fine break is desirable and is a feature of high quality leather.Figure 2.7 illustrates an extreme case, where the junction has been damaged,causing separation between the grain and the corium: this is known in thejargon as ‘double hiding’ or ‘double skinning’.

Figure 2.6 Fat layer of lipocytes within the junction in sheepskin. (Courtesy of BLC.)

Figure 2.7 Looseness in leather. (Courtesy of A. Michel, LeatherWise UK.)

34 Chapter 2

2.1.4 Corium

The main part of the skin is the obviously fibrous structure called the corium orthe corium major. The fibre structure varies through the cross section of hide orskin: the fibres increase in size, reaching a maximum fibre diameter in the centreof the corium and then decreasing a little as they approach the next lower layer.The network of fibres, often referred to as the weave, consists of fibres

dividing and recombining with other fibres.7 This makes the corium strong,able to resist stresses placed on it: the distribution of a stress imposed on thefibre structure from the point of stress over the surrounding area is reflected inresistance to the stress, observed as strength. An example of the consequence ofthe effect occurs when sewing leather. The use of a conventional needle, whichacts by piercing the material, is likely to result in breaking the needle, becauseof the high resistance of the fibre structure to being torn apart as the needlepasses through the cross section. Therefore, a specialist needle is required forleather, where the sharp end takes the form of a blade, to cut the fibres.An important feature of the corium structure is the angle of weave: experi-

enced observers of corium structure can estimate the average angle – themagnitude of the angle can provide useful information regarding the processhistory of the pelt. The average angle of weave in raw skin is about 451; a lowervalue indicates greater depletion or relaxation of the corium and a higher valueindicates a degree of swelling (Figure 2.2).

2.1.5 Flesh Layer

The so-called flesh layer is the layer of the skin closest to the flesh of the animal:although it has a distinct fibre structure, it is still part of the corium. Itsstructure is characterised by the low angle of weave, always lower than thecorium angle of weave. Consequences of the lower angle and finer nature of thefibres are as follows:

1. There is a lesser ability for the flesh structure to relax, to spread out tocreate greater area, than in the corium: hence, the flesh layer has a con-trolling effect on the area of the skin or leather.

2. The flesh layer is stronger than the corium.3. The smaller fibres can be abraded to a fine nap for making suede leather,

unlike the coarse fibres of the centre of the corium.

2.1.6 Flesh

Hides and skins are inevitably presented to the tanner with adhering flesh(muscle) and fat. This must be removed at the earliest stage possible in theprocessing programme, because it creates a barrier to the uniform penetrationof chemicals, which would cause non-uniformity of leather properties.The traditional method of fleshing was to place the hide or skin over a

wooden beam, inclined at about 451, then manually scrape off the flesh and fatwith a sharp, double handled knife. This is the derivation of the term


‘beamhouse processes’, referring to the process steps conducted on the beam,leading to the tanning step, illustrated in Chapter 4. In modern tanneries thisprocess is done by machine, using a blade fixed to a rotating cylinder.

2.2 SKIN FEATURES AND COMPONENTS

2.2.1 Hair or Wool

The hair or wool is situated in follicles. It is common in the industry to removethe hair or wool: this can be achieved in various ways (Chapter 5). In somespecialist applications, the hair or wool may be left in situ, e.g. wool sheepskinsfor winter coats (referred to as double face) or for rugs.

2.2.2 Follicles

The hair follicles provide the grain pattern that is so much a feature of the valueof the leather. The follicle angle reflects the angle of weave of the corium: it istypically 451 in raw pelt, but rises if the corium angle increases and decreases ifthe corium angle decreases, so that the angle of weave is a reflection of changein area. It is important that the follicle angle should be low in the final leather,because it determines the fineness of the grain pattern. The higher the angle ofthe follicle, the more the mouth of the follicle appears open, referred to as‘coarse, open or grinning grain’, which is undesirable.In most animals the grain pattern is uniform, in terms of both regularity and

distribution over the pelt. In some animals the pattern/appearance are dis-tinctive, such as the arc pattern of pig bristles, in some it is the major feature ofimportance, such as the quill markings of ostrich.

2.2.3 Erector Pili Muscle

Also known as the arrector pili muscle (Figure 2.8), this is the mechanism bywhich animals resist the cold, by raising the angle of the follicle, to trap a layer ofair within the hairs: by contracting, the muscle pulls across the upper part of theskin. In humans this creates the phenomenon called ‘goose flesh’ or ‘goose bumps’.It has been suggested8 that starting processing under cold conditions in the

preliminary soaking step may result in coarse grain (Chapter 4). The muscle isdegraded during liming, so thereafter its action is no longer available toinfluence follicle angle and hence the grain quality.

2.2.4 Sweat Glands

Sweat glands do not play any role in leather processing: they are componentsthat are degraded in the early alkali based process and their removal does notinfluence leather properties or quality.Dog skin is noteworthy because it does not contain any sweat glands. Lea-

ther from this source of raw material is now uncommon, although it was oncethe traditionally preferred material for riding gloves.

36 Chapter 2

2.2.5 Veins and Arteries

The veinous system of the skin is located in the grain–corium junction, wherethe main vessels run parallel to the skin surface, fed by smaller vessels runningvertically down into the body of the animal.At slaughter, efficient bleeding is required, to empty the blood vessels, to allow

them to collapse: if this is achieved, they will typically not affect the quality ofthe leather. However, if it is not achieved or the blood vessels are degradedduring processing, a defect called ‘veininess’ may be observed (Figure 2.9): theunmistakable appearance is a reflection of the blood vessel network on the split(corium) surface, but occasionally in severe cases the pattern may be observedon the grain surface.The appearance of a pattern of blood vessels may arise from two different

mechanisms. Inadequate bleeding in the abattoir leaves the vessels expandedbecause they still contain blood. Although the blood is degraded and removedduring liming, the veinous vessels do not collapse completely, creating voids inthe hide or skin. Alternatively, because the veinous system is largely composedof elastin, due the requirements of flexing as the heart pumps blood around it,the use of elastolytic enzymes in processing will dissolve the blood vesselsthemselves, also leaving voids in the pelt cross section.

Figure 2.8 The erector (arrector) pili muscle. (Courtesy of A. Michel, LeatherWiseUK.)


The fault is developed during process steps that employ pressure, because thevoids are compressed. Since the corium is more compressible than the grain, theeffect is most commonly seen on the flesh side of the leather. Because the grainis less compressible, it is only in severe cases that the effect is seen on the grainsurface (Figure 2.10).The defect cannot be treated by filling the voids, since that cannot be

accomplished without over filling the rest of the fibre structure and the holes aretoo big on the molecular level to fill with tanning agent (even polymers). Theonly solutions are either to avoid or minimise pressure on the leather or tomake the leather less compressible: the latter can be achieved by using fillingretannages, such as vegetable tannins or polymeric resins (Chapters 13 and 14).

2.2.6 Elastin

The second most important protein in skin after collagen is elastin (Table 2.2).It contributes greatly to the physical properties of skin and leather, because itcontrols the elasticity of the grain layer. The material of the grain is weak, so itcannot stretch to accommodate stresses in the skin when, for example, a joint isflexed. Hence it adopts a convoluted, rippled form, that can flatten as thecorium stretches. The mechanism by which it returns to its convoluted state

grain side suede side

Figure 2.9 Pattern of veins on leather, shown as a fault. (Courtesy A. Michel, Lea-therWise UK.)

before pressure typical effecton flesh side

severe effect on bothflesh and grain sides

Figure 2.10 Representation of the effect of veins on leather appearance.

38 Chapter 2

when a stress is removed is through the action of elastin fibres, which extendwhen the skin is stretched, then contract to the resting position when the stressis removed. The elastin fibres are centred on the follicles, with coarse fibresrunning parallel to the skin surface and finer fibres running at right angles tothe skin surface (Figure 2.11).9

Several observations can be made concerning the numbers given in Table 2.2:

1. The proteins have similar glycine content – by comparison with collagen,this indicates that the structure of elastin could be helical, which it is.

2. Elastin has more apolar amino acids – therefore, the protein is morehydrophobic than collagen.

3. There are more acid and basic amino acids in collagen than in elastin –making collagen relatively hydrophilic.

4. The proteins have similar proline contents, which supports the suggestionof helical structure in elastin.

5. There is less hydroxyproline in elastin – so the structure is less reliant onhydrogen bonding than collagen.

6. The lack of basic residues in elastin means there is little lysine and very littlehistidine: covalent crosslinking of the type found in collagen is not possible.

In addition, the following deductions can be made:

1. Although we know that collagen structure depends on covalent bonding,to hold the triple helix units together, there is not enough informationwithin the amino acid composition alone to comment on the presence ofcovalent bonding in elastin.

2. Collagen structure depends on electrostatic, salt links from the chargedsidechains, but they are not important in elastin.

3. Collagen structure depends on hydrogen bonding, based on the highHypro content – since the Hypro content is low in elastin, its structurerelies less on H-bonding. This, too, supports the notion of the hydro-phobic character of elastin.

4. Bonding in elastin is dependent on hydrophobic interactions, due to thehigh content of apolar sidechains. Such bonding is relatively unimportantin collagen.

Table 2.2 Comparison of elastin and collagen: residues per 1000.

Amino acid type Elastin Collagen

Glycine 355 330Apolar 431 170Acidic 14 120Basic 10 96Hydroxy 20 57Proline 125 126Hydroxyproline 23 93


Figure 2.11 Light photomicrographs of the structure of elastin in the grain, showing(a) intact elastin in the cross section of the grain of wool sheepskin; (b)elastin fibres in a lateral section through the grain of wool sheepskin; and(c) degraded elastin fibres around a hair follicle. (Courtesy M. Aldemir)

5. The differences in amino acid sidechains affect the chemical reactivity ofthe proteins: reactions dependent on the presence of charged sidechainswill be less important in elastin than in collagen. Conversely, reactionsdependent only on the backbone chemistry of the proteins will work thesame in elastin and collagen. This difference can be exploited in thecontext of area yield (see below).

These deductions, based only on the amino acid composition, can be tabu-lated in terms of the relative importance of the bonding types (Table 2.3).Elastin is an unusually stable protein, with a chain structure in the form of a

single protein strand, twisted into a double helix: it can resist boiling water, whichoffers a way of isolating it from collagen, because the less stable protein is de-natured, degraded and solubilised by boiling water. The macro-structure of elastinis a crosslinked network, which adopts a random, disordered conformation,creating regions of hydrophobically bonded groups, by excluding water, whetherin its native state or in leather, which still contains some water.10 When elastin isextended, it has a more ordered structure, when the hydrophobic protein interactswith water (Figure 2.12). The protein acts just like more familiar elastomers, such

Table 2.3 Relative importance of bonding types in elastin and collagen: morestars means a greater role in protein structure.

Type of bonding Elastin Collagen

Covalent ** ***Electrostatic * ***Hydrogen bonding * ***Hydrophobic bonding *** *

o

o

o

o

o

oooo

o

hydrophobic groupsexclude water

hydrophobic groups mustinteract with water

elastin stretched

Figure 2.12 Mechanism of elastin elasticity.


as rubber, because the driving force to contract after stretching is entropic: theordered nature of the fibres is high energy and there is energy released when therandom structure is regained. In addition, the high energy interaction of hydro-phobic regions with water, in nature as well as in leather, will require the networkto re-establish the energetically favoured hydrophobic bonding: this is anenthalpic driver, operating in the same direction as the entropic effect.11

The network is crosslinked by desmosine and iso-desmosine groups(Figure 2.13):12 in each case the aromatic crosslink is created from the con-densation of three allysine sidechains and one lysine sidechain, apparent fromthe numbers of carbon atoms in the chains and in the ring. Isolation of thesecrosslinks provides the basis for assaying elastin content in a material.In conventional processing, pancreatic enzymes may be employed in the so-

called ‘bating’ step, see below, intended to break down the non-structuralproteins of skin: under these conditions elastin is not degraded, nor does thisoccur under the alkaline hydrolytic conditions of liming (see below). However,elastin is readily degraded and dissolved by elastolytic enzymes, which are oftenfound in bacterial fermentation formulations also used for bating. Theseenzymes can be used to break down the elastin structure, thereby allowing thegrain to relax and flatten, so increasing the area of the skin (see below).

2.3 NON-STRUCTURAL COMPONENTS OF SKIN

2.3.1 Glycosaminoglycans (GAGs)

Glycosaminoglycan is a generic term for a class of minor but significantcomponents of skin, important in the leather making process. They can take theform of free polysaccharides, called glycosaminoglycans, but may also have aprotein component, when they are called proteoglycans. Four GAGs should beconsidered during processing.13,14

2.3.2 Hyaluronic Acid

In terms of leather production, hyaluronic acid (HA) is probably the mostimportant of this class of compounds in skin. The structure is a chain of

N

(CH2)2

(CH2)4

(CH2)3

(CH2)2 (CH2)2

(CH2)3

(CH2)4

(CH2)2

N

(CH2)3CHO

(CH2)4NH2

+

3

iso-desmosine desmosine

Figure 2.13 Structures of linking groups in elastin, desmosine and iso-desmosine.

42 Chapter 2

alternating D-glucuronic acid and D-N-acetyl-glucosamine, joined by b-1,3 glyco-sidic links, with molecular weight of the order of 106, corresponding to 3–19000disaccharide units: Figure 2.14 illustrates the chemical structure in two formats.With regard to technological impact, HA can be considered to be a polymeric

chain of aliphatic carboxylate groups with pKa B4, all ionised at physiologicalpH7.4. Since these will function as sites for reaction with chromium(III) tanningsalts or other mineral tanning agents, fixing HA within the fibre structureduring tanning will stiffen the leather; consequently it must be removed prior totanning. It is important that HA is removed prior to the alkali treatmentconducted with lime, Ca(OH)2, because calcium ions will react with the car-boxyl groups, precipitating it within the skin structure, thus preventing itsremoval: thereafter it can be reactivated as the free carboxylate in subsequentacid processing, to cause problems to the tanner.HA functions in skin by providing resiliency, caused by the repulsive forces

from the negative charges, which force the molecule to adopt an extendedconformation, thereby creating a large exclusion volume in the form of a gel.15 Itis this effect that makes it hard to rinse out of native hide or skin, although it canbe removed efficiently, albeit with difficulty, requiring prolonged washing,because it is not bound to the fibre structure, merely entangled in it. However,removal is not a problem if the rawstock has been salted, because the ionicnature of the electrolyte causes the gel structure to collapse. This is illustrated inFigure 2.15, in which the attractions of ions for opposite charge effectively‘smear out’ or weaken the magnitude of the full negative charge on the carboxylgroups. The effect is to allow the charged carboxyl groups to approach muchcloser, reducing their exclusion volume, while also reducing the ionic interactionwith the protein of the pelt, and so it is easily rinsed from the fibre structure.The role of salt in the mechanism of removing HA is a useful spin off from

the commonest method of preserving hides; the use of sodium chloride is called‘salting’ (Chapter 3). In modern processing, it is becoming less common forrawstock to be preserved in this way: because of the undesirability of neutral

D-glucuronicacid

OO

HOH2CO

OHOH

O-O2C

HOO

COCH3

O

O

O

OH

HO

-O2C

HO

CH2OH

NH

O O

n

D-N-acetylglucosamine

Figure 2.14 Structure of hyaluronic acid.


electrolyte in effluent, the industry is turning to other methods of preservation,including no preservation at all. If salt is not present within the rawstock at thestart of wet processing, the removal of HA is more difficult. Options are to usechondroitinase enzymes to degrade the molecule into smaller pieces for ease ofrinsing out, but this is not established biotechnology, or to include someelectrolyte in the soaking bath, which is a perverse contradiction to the reasonfor processing fresh rawstock in the first place.

2.3.3 Dermatan Sulfate

Dermatan sulfate (DS) proteoglycan or chondroitin sulfate B has a protein coreand 2–3 sidechains of 35–90 repeating units, containing one molecule of L-iduronicacid and one molecule of N-acetyl-D-galactosamine-4-sulfate (Figure 2.16). Themolecular weight is about 105, of which 60% is protein and 40% is polysaccharide.DS proteoglycan is bound to the surface of the fibrils in the hierarchy of

collagen structure by protein links and electrostatic interactions (Figure 2.17).The role of DS in skin in vivo is concerned with hydrating the fibrils, but its

role and importance in leather processing are unclear. However, it is unlikely toplay a major role, since the opening up reactions of the beamhouse applyprimarily at the level of the fibril bundles. Nevertheless, since DS is bound tothe collagen via peptide links, it is hydrolysed from collagen during liming:hence, its removal has been used to measure the progress of hydrolytic reactionsand thereby provide a marker for the extent of hydrolytic opening up.A minor component of skin is decorin, which is a low molecular weight type

of DS, also bound to collagen fibrils:15 it has a protein core of 329 amino acidresidues and a single glycosaminoglycan sidechain. Its fate in processing is thesame as the more prevalent dermatan sulfate.

2.3.4 Chondroitin Sulfate A

Chondroitin sulfate A is a repeating polymer of D-glucuronic acid and N-acetyl-D-galactosamine-4-sulfate (Figure 2.18). The molecular weight is of the order of

CO2- δ+ δ-

CO2-

bulk solution

CO2-

CO2- δ+ δ -

Figure 2.15 Smearing of charge by an electrolyte.

44 Chapter 2

106, consisting of a globular protein with 60–100 GAG chains, each with10–100 disaccharide units (molecular weight 5000 to 50 000).This is a minor component, a detached proteoglycan not bound to the fibre

structure. As a sulfate derivative of hyaluronic acid, its properties will be thesame, except for greater hydrophilicity and solubility, but its fate in processingwill be the same as hyaluronic acid itself.

2.3.5 Chondroitin Sulfate C

Chondroitin sulfate C is a repeating polymer of D-glucuronic acid andN-acetyl-D-glucuronic acid-6-sulfate (Figure 2.19). Like chondroitin A, it is a minor com-ponent of skin, which has no significant impact of the leather making process.

2.3.6 Melanin

Melanins are the pigmenting materials that provide much of the colour tomammals and other species. They are chemically inert pigments, based onoxidative polymerisation of phenols and naphthols. Mammalian melanins maybe classified into the brown to black eumelanins and the pheomelanins whichare yellow to red-brown. Figures 2.20 and 2.21 outline the chemistries.

O

O

O

OH

HO

- O2C

OSO3-

CH2OH

COCH3

O O

n

N-acetyl-D-galactosamine-4-sulfate

L-iduronicacid

NH

protein core

I is L-iduronic acidS is N-acetyl-D-galactosamine-4-sulfate

(-I-S-I-S-I-S-I-S-)n(-I-S-I-S-I-S-I-S-)n

(-I-S-I-S-I-S-I-S-)n

Figure 2.16 Structure of dermatan sulfate.


Figure 2.20 represents the Raper–Mason scheme of eumelanogenesis,16–20

involving the hydroxylation of tyrosine to DOPA and subsequent oxidationto dopaquinone. Cyclisation creates leuco-dopachrome, leading to indolederivatives and polymerisation to the black pigment. Figure 2.21 showsthe scheme for the synthesis of the pheomelanins, starting with cysteinereacting with dopaquinone, leading to cyclisation and the formation of

n

OO

COCH3

CH2OH

OSO3-

-O2C

HO

OH

O

O

O

NH

Figure 2.18 Structure of chondroitin A.

Figure 2.17 Relationship between dermatan sulfate and the fibril structure. (Takenfrom ref. 13, courtesy of JALCA.)

46 Chapter 2

1,4-benzothiazines.21–24 Like lignin, discussed below, the melanins have com-plex structures; so, although the pathways to their synthesis have beenelucidated and the precursors are known, the actual structures are not known.From a tanning viewpoint, the pigments occur in the epidermis, in the cortex

of the hair and around the hair follicles in the form of granular melanocytecells: in Figure 2.22, granular melanocytes are visible in the black haired hide,but not in the white haired hide.25 Much of the skin melanin is lost duringliming, as the epidermis is solubilised, but it can be trapped within the grainwhen the pH of the pelt is lowered from the alkali swollen state: this is discussedin the section devoted to deliming (Chapter 7). The presence of residual melaninis a quality issue, since any discoloration of the pelt is likely to be apparent afterdyeing. The inertness of the pigments means they cannot be chemicallydegraded without causing damage to the leather; Rao et al. at CLRI havedemonstrated that it can be accomplished by xylanase treatment, a reactionknown to degrade hemicellulose and used to bleach wood products, but it has

n

OO

COCH3

- O3SOH2C

OH

-O2C

HO

OH

O

O

O

NH

Figure 2.19 Structure of chondroitin C.

HONH2

CO2H

HO

O

O

NH

O

CO2H

NH2HO

HO

NH2

CO2H

HO

dihydroxyindole-2-carboxylic acid

CO2H CO2H

HON

tyrosine dihydroxyphenylalanine(DOPA)

CO2H

HONH

HO

ONH

O

CO2H melanochrome eumelanins

indole-5,6-quinone

dopaquinone

leuco dopachrome dopachrome

Figure 2.20 Synthesis of eumelanins.


not yet been developed industrially for the leather sector.26 The biochemistry ofxylan is discussed in detail below.The role of melanin in leather making is limited to its influence on hair

degradation (Chapter 4) and in the subsequent process of deliming.

2.4 SKIN

As a natural material, required to perform different functions, to deal withdifferent stresses over its area, skin is anisotropic: its structure and properties

tyrosine

O

O

H2N CO2H

S

NH2

CO2H

dopaquinone cysteinyl dopa

CO2H

NH2

HS

CO2HH2N

OH

O

O

N

H2N CO2H

CO2H

S S

CO2H

CO2HH2N

N

OH

pheomelanins

1,4-benzothiazine

cysteine

1,4-benzothiazone

Figure 2.21 Synthesis of pheomelanins.

Black haired hide White haired hide

Figure 2.22 Presence of melanin in hair and skin at the follicle mouth and the melaninassociated with the hair follicles.

48 Chapter 2

vary over its area. The parts of a hide or skin can be defined in terms of the‘butt’, the ‘belly’ and the ‘neck’. The butt is defined by the region up to half wayfrom the backbone to the belly edge and two-thirds of the way from the root ofthe tail to the neck edge: within this region the fibre structure is relativelyconsistent and hence the physical properties of the skin or leather are con-sistent. The remaining regions to the side of the butts are called the bellies andthe remaining region beyond the butt towards the head is the neck. This isillustrated in Figure 2.23.The regions can be characterised as follows. The butt has a tight fibre

structure, making the skin relatively firm and stiff: it is thick compared to thebelly, but thin compared to the neck. (In pigskin, the butt contains the shellarea, which is particularly hard. Interestingly, pig collagen is the closest tohuman collagen of the commonly available collagens and hence is often used tomake wound dressings.) The bellies of all skins are the thinnest parts, with anopen structure, making them relatively weak. The neck is the thickest part, alsowith a relatively open structure. It is an important aspect of the technology ofleather making to try to make the non-uniform skin structure as uniform aspossible in the final leather product.The butt region contains the Official Sampling Position: because of the ani-

sotropic nature of skin, to accommodate its physiological functions, it isnecessary to adopt the idea of an OSP, to make best comparisons of leatherproperties. It is also important to note how the anisotropy varies over the wholeskin (Figure 2.24).26 For this reason, sampling for physical testing is routinelydone in two ways, parallel and perpendicular to the backbone. The region of thebackbone extends from tail to neck and a few centimetres either side of that line.It is thicker than the butt and must be shaved to the thickness of the butt region.

neck

backbone

belly belly

shell

Region of the OfficialSampling Position (OSP)

shell (pigskins only)

butt butt

Figure 2.23 Defined regions within the hide: butt, belly, neck.


The direction of the fibre weave, indicated in Figure 2.24, demonstrates theinfluence of the direction of physical testing. Because the skin has to stretch toaccommodate stomach filling, skin and its derivatives exhibit greater strengthperpendicular to the backbone than parallel to the backbone. Another notablefeature is the symmetry of features in the skin about the backbone: it can beassumed that the two sides of a hide or skin are symmetrically the same. This isuseful in experiments or testing, when samples are matched from the two sidesof the same hide. For example, control samples might be taken from the leftside and the experimental samples would then be taken from the same positionson the right side or vice versa. In contrast, comparability between any two skinscannot be assumed, even if the same area of the skin is sampled, because ani-mals vary for many reasons: breed, sex, age, diet, husbandry, curing, storagehistory, etc. Therefore, measuring the effect of a treatment must be done sta-tistically, with a large enough number of representative skins.The appearance of the grain layer is generally consistent over the skin, with

the exception of the neck region, where ‘growthiness’ may be encountered. Theincidence and degree of this effect depends strongly on species and breed, beingmost apparent on bull cattle hides and merino sheepskins. It takes the form ofripples in the skin, across the neck region, held in place by the elastin content.

Figure 2.24 Anisotropy of fibre structure over the hide or skin.27 (Courtesy of BLC.)

50 Chapter 2

The effect can largely be removed by degrading the elastin (see below). In thecase of merino sheep, the effect can extend over the whole skin and the mag-nitude of these extreme growth ripples is illustrated in Figure 2.25: this naturalfeature was thought in the past to detract from the value of the pelt, hence theseskins were only used for making chamois (oil tanned) leather, when the grain isdiscarded prior to tanning. Nowadays, the feature of growthiness is regarded asa selling point for finished articles, such as ethnic jackets.

2.4.1 Area of Skin or Leather

Most leathers are sold by area, so the area yield from processing is a criticalfeature. (Note, vegetable tanned sole leather and other so-called ‘heavy’ lea-thers are sold by weight.) To put this into context, some actual values can bequoted, although they are clearly subject to change and the model is rough: thenumbers apply at the beginning of the twenty-first century:An extra 1% area gain is worth: d2.5m to the UK, h35m to Europe and

$500m to the global industry.

Figure 2.25 Characteristic feature of ‘‘growthiness’’ in a merino pelt. (Courtesy ofA. Michel, LeatherWise UK.)


The area of a piece of hide, skin or leather depends primarily on the angle ofweave of the corium: high angle of weave means low area, low angle of weavecreates larger area (Figure 2.26) – the same effect can be observed in trellis fencing.The angle of weave is fixed within the structure by two controlling elements

of the structure, the flesh layer and the grain. The area of the flesh layer candecrease, because the angle of weave can increase. In contrast, its ability toreduce its angle of weave is limited by its already low angle. Therefore, it has alimiting effect on allowing area increase if it is present, as it usually is in leathermade from skins, which are relatively thin. In the case of cattle hides, this isusually not the case for most applications, since the thickness of the naturalhide necessitates splitting through the cross section, to produce usable thick-ness. The products are the grain split, wherein lies the primary value of the hide,and the flesh split, typically used for making suede leather. When the flesh layeris separated from the grain–corium split, its limiting effect on area is removedand the corium may be able to relax its angle of weave, to give greater areayield. This is particularly apparent when pelt is split at an early stage in theprocess, such as in the limed condition.Features in the structure of skin, such as the angle of weave, are maintained

or ‘tanned in’ by the tanning process, so it is useful to compare the effects ofsplitting after liming and after chrome tanning. In the former case, the opennature of the split surface allows easy access to process chemicals, resulting inmore extensive opening up, yielding softer leather, suitable for upholstery. Ifthe structure remains intact through tanning, opening up is less extensive andthe leather is firmer, more suited for shoe uppers.The area of the grain is determined by its convoluted nature, maintained by

the presence of elastin. If the elastin is removed, then the grain can relax intogreater area, allowing the corium to follow, by lowering its angle of weave. Thiscan be achieved by the application of elastolytic enzymes, which may be used inliming, in bating or at the wet blue (chrome tanned) stage. The use of alkaliactive enzymes in unhairing usually means there is elastase activity associatedwith the bacterial proteases. However, the extended proteolytic effect typicallyresults in looseness. This is also the case in bating with bacterial enzymeformulations. In each case, the problem could be avoided by using pure elas-tase. However, it is rare for a microorganism to express elastase alone and theseparation of enzymes is an expensive operation, making that option eco-nomically non-viable for the leather industry.The economic option is to apply a protease/elastase mixture to wet blue:28

this is commercially available as NovoCor AX. The argument goes as follows.

Figure 2.26 Representation of the influence of fibre angle of weave on area.

52 Chapter 2

Elastin can be degraded by elastase enzyme at any early stage in processing,but using purified enzyme is uneconomically expensive for an industrialapplication such as this. However, elastase can be obtained cheaply in com-bination with protease: the unfortunate consequence is that while elastase isdegrading the elastin, the protease is degrading the collagen. The result inevi-tably is loose, weakened leather with reduced value, despite the added valueof increased area. Following the beamhouse steps, chemical modification ofprotein alters its properties, including its ability to interact with the reactionsites of enzymes. In this way, chrome tanning protects the collagen from bio-chemical attack, which is part of the definition of tanning. So, the protease partof the enzyme formulation has no damaging effect on wet blue leather andhence no adverse effect on the physical properties of the fibre structure. Con-versely, because elastin contains fewer acidic amino acid residues, as shownabove, the chrome tanning process causes little chemical change to elastin, so itremains vulnerable to degradation by elastase. Therefore, applying a mixture ofproteolytic and elastolytic enzymes to wet blue will result in the degradation ofelastin, but no change in the modified collagen. Under these circumstances, thepresence of the protease is actually of positive benefit to the process, becauseit can accelerate the degradation of the elastin damaged by elastase. Theremoval of the elastin allows the grain to relax and spread, in turn allowingthe corium angle of weave to be lowered. In this way up to 9% extra areayield can be obtained, although 4–5% is typical; but because the collagen isunaffected, the physical properties of the leather are unaffected, as is thequality of the leather. Even greater increases in yield may be obtainedfrom pigskins, because of the naturally high content of elastin controllingthe area.The argument applies only to chrome tanned leather, because the tanning

reaction applies to the carboxyl bearing sidechains, with very little reaction withthe protein backbone. Any alternative or additional tanning reactions that relyon reacting with the backbone of the protein or via hydrophobic interactionswill render the elastin as resistant as collagen to enzymatic degradation and thebiotechnology will not work.In this context, it is worth reiterating that tanning means resistance to

microbiological attack. Bating enzymes have long been sought to modify theproperties of wet blue, to create greater uniformity of properties between lea-thers obtained from different sources (common practice in large tanneriesaround the world) and claims have been made for opening up and softening ofwet blue enzymatically.29,30 However, such claims are doubtful: studies of theeffects of protease on wet blue demonstrated that there is no observable effectunless the concentration of enzyme is abnormally high, the pH is close to theoptimum, around 9, and the temperature of the reaction bath is close to themaximum tolerated by the enzyme, about 50 1C. These latter two conditionscombine to produce incipient hydrolytic damage to the leather, reversal oftannage, after prolonged contact, at least 1 hour. Under these conditions noeffects are observed up to the point when there is sudden, catastrophic, fast andcomplete degradation of the leather.31


Area can also be controlled by physical processing. Applying stretchingduring drying increases area yield. However, the gain is not all permanent andthe physical properties are adversely affected: the leathers are stiffened, becausethe effect relies on fibre sticking to retain area gain.32,33

2.5 PROCESSING

Some process steps should be discussed with reference to the influence of thecomponents of skin structure on the operations and their outcomes.

2.5.1 Splitting

Most cattle hides must be split at some stage of processing, because they areusually too thick for typical end use applications. Here, notably, tanners in thisfield expect their economics to be determined by grain leather production – ifany money can be made from the flesh split, it is a bonus.It is usual for shoe upper leather to be split after chrome tanning: this is

primarily because it is necessary to retain spring and solidity in the leather,which might be lost if the opening up processes were too effective. The hide isprocessed to this point as full thickness rawstock, which means that all reagentsmust penetrate through the intact cross section. This has attendant problems ofuniformity of reaction, compounded by the variability of structure over thehide. The products of the operation are chrome tanned grain split and chrometanned flesh split. Since the market for the suede leather is variable and thealternative is to make cheap coated (artificial grain) leather, the value of theflesh split is likely to be limited.Notably, suede is leather too, even though it is often colloquially referred to

as an alternative to leather. The suede split will typically go through all theprocessing applied to the grain split, the opening up, tanning and post tanning.At the end, the suede nap is prepared by ‘buffing’ the surface with glass paper:as is the case with sanding a wood surface, the fineness of the fibre nap iscontrolled by the fineness of the grit on the paper. The nap is usually created onthe flesh side of the leather, rather than the split surface, because the fibres ofthe corium produce a coarse nap, i.e. a rough suede compared to the fibres ofthe flesh layer. Alternatively, suede leather can be created from the grain split,by buffing the grain surface to create a very fine nap: this product is called‘nubuck’.The alternative to splitting after tanning is to split at the end of the liming

step. The hide is thick and relatively stiff, because it is in the swollen state, socutting is precise and easy. Less easy is handling the very slippery and heavypelt. Nevertheless, lime splitting is routinely conducted for making upholsteryleather. The benefit is the greater ease with which the reagent can penetratethrough the cross section, through the more open structure of the split coriumsurface, causing a greater degree of opening up. When this is combined with theloss of the flesh layer, area yield and softness are enhanced, although usually atthe cost of looseness and often grain damage. Since the leather is required to be

54 Chapter 2

cloth-like, for its end use, and it is commonly pigment finished, these are notdrawbacks. The products of the operation are limed grain split and limed fleshsplit. In an untanned state, the limed split may be used in other sectors: animportant application is to use the collagen for making sausage and salamicasings by comminuting the skin and reconstituting it as a film.The remaining option for splitting is in the raw state, either as uncured or as

soaked salt cured hide. This is uncommon, primarily because it is more difficultto split the thinner and more floppy material. However, another limiting factoris the presence of dung: if the hair side is not flat, the splitting knife will cut intothe hide, causing area loss and hence loss in value. If the dung can be removedin the soaking step, though, then green splitting is an environmentally andeconomically sound operation (see below). The products of the operation areraw grain split and raw flesh split. The thin grain split is chemically processedmore efficiently and effectively than if it is split later. The collagen of the fleshsplit is undegraded by highly alkaline treatment conditions and therefore hashigh value as a source of intact collagen.

2.5.2 Grain–Corium Thickness Ratio

The act of splitting thick hide, to create the valuable grain split, must take intoaccount the nature of the starting material. Consider the production of shoeupper leather from a pack of tanned leather, in which the rawstock was non-uniform in weight. Since raw hide is traded by weight, hides are selected inweight ranges, but there is no guarantee that the weights are entirely consistent.The situation shown in Figure 2.27 could arise, in which hides of differentstarting thickness are split to the same thickness for the same end use. It isgenerally the case that heavier hides are thicker, although they will usually alsobe larger in area.The outcome is splits of the same thickness, but variations in the grain to

corium thickness ratio. Because the strength of the leather depends only on the

heavy hide

split split

flesh

flesh

graingrain

light hide

Figure 2.27 Representation of the effect of hide thickness and grain to corium ratiobefore and after splitting to the same thickness.


corium, the strength is determined by the grain to corium ratio: the lower theratio, the stronger the leather will be.3,4 The lesson to be learned from thisanalysis is that rawstock and resulting process packs should ideally be selectedon the basis of area rather than weight and, strictly, by the grain to corium ratioin the raw material. In this way, the product would be more consistent instrength.

2.5.3 Fleshing

As stated above, the adhering flesh on raw hides and skins must be removed.The earlier this can be done, the more process steps will be conducted on cleanpelt, so that penetration of chemicals and biochemical reagents will be uniform.There are two options open to the tanner:Lime fleshing is the commonest option, conducted after the hair has been

removed and the hide has been subjected to many hours of treatment in asaturated lime solution at pH12.5. The disadvantage is the limed state of thefleshings. If the fleshings are regarded as waste, there is an additional cost fordisposing of the alkaline material to landfill. Alternatively, if the fleshings aretreated to recover the tallow content, their value is diminished by liming,because alkali-catalysed hydrolysis reduces the tallow content and the contentof free fatty acid is increased, which downgrades the quality of the tallow.Green fleshing, after soaking, produces fleshings with high tallow content

and low free fatty acid in the product: this improves the economics of theprocess. However, green fleshing can only be conducted if the hair side ofthe hide is free from dung: its presence causes the fleshing knife to cut into thehide and area is lost. Recently developed technology allows dung removal in thedirt soak, making this process step practicable.

2.5.4 Dung

Dung is not a skin component, but it is commonly associated with skin(Table 2.4). It may appear to be an integral part of the structure, because it canform a composite material with the hair, which has strength and water resis-tance. Indeed composites of dung, clay and hair are the daub component of thetraditional building material ‘wattle and daub’, which is still used in the

Table 2.4 Composition of UK Friesian cattle dung.

Component Content (% on dry wt.)

Cellulose 30Hemicellulose (xylan) 28Lignin 21Protein 6Ether soluble 10Cold water solubles 6Hot water solubles 4

56 Chapter 2

developing economies. It is important to remove the dung as early as possible inthe process, to allow greatest flexibility in processing and opportunities formaking financial savings in the operation. Figure 2.28 illustrates the interactionbetween dung and the skin. Clearly, there is no direct contact between the dungand the surface of the skin. This is an important feature of the interaction,because any treatment of the live animal to remove dung, e.g. prior to

Dung ball

region betweendung and hide

hide crosssection

Figure 2.28 Photomicrograph of dung on a cattle hide. (Courtesy of Journal of theSociety of Leather Technologists and Chemists.)


slaughter, would not be an invasive procedure and hence would not causedistress to the animal. The following discussion shows that dung can beremoved from hides in the tannery by the action of specific enzymes: theprinciple of treating live animals has also been demonstrated. Moreover, it hasbeen shown by rabbit eye testing that the enzyme mixture for dung removaldescribed below has no irritating effect on mucous membranes, even at tentimes the required concentration, which raises the prospect of spraying ormisting cattle in the lairage prior to slaughter, depending on the national reg-ulations for the state of animals presented for slaughter (A.D. Covington andC.S. Evans, Klenzyme Ltd., unpublished results).Analysis of typical dung reveals that almost 80% of the material is lig-

nocellulosic: there is a little protein and some fatty components, indicated bythe presence of ether solubles.34 The composition reflects the diet of the animal,because the lignocellulosic content is similar to the composition of plant cellwalls, with a slight excess of less easily digested lignin. The low levels of proteinand fat explain the lack of efficacy of removal by proteases and lipases. The lowsolubility in water is shown by the levels of water solubles content. Conse-quently, the removal of dung clearly depends on degrading the main compo-nents, which must be the targets for specific enzymes: cellulase, xylanase andligninase. The need for all three enzymes is indicated in the representation oflignocellulose in Figure 2.29.35 Lignin is degraded by ligninase to allow accessto the crosslinking xylan, whose degradation by xylanase releases the cellulosefibres, which are then solubilised by cellulose: the effect can be accomplishedjust by cellulose with xylanase, but the effectiveness is reduced. Effectiveremoval of dung by using enzyme technology can only be successful if thecomponents of the dung, which constitute the adhesive mechanism, are tar-geted by enzymes.Cellulose is the major polysaccharide of the cell walls of higher plants and is

the most abundant polymer in the biosphere. It is a linear polymer made ofglucose subunits linked by b-1,4 bonds (Figure 2.30). Each glucose residue isrotated by 1801 relative to its neighbours, so that the basic repeating unit is infact cellobiose.The degree of polymerisation of cellulose is of the order of 10 000. Cellulose

chains form numerous intra- and intermolecular hydrogen bonds, whichaccount for the formation of rigid, insoluble microfibrils. The resistance ofcellulose to hydrolysis is caused by both the nature of b-1,4 bonds and the

Figure 2.29 Representation of lignocellulose: cellulose fibres are linked by xylan, andwrapped around by lignin.

58 Chapter 2

crystalline state; however, b-1,4 linkages can be hydrolysed by cellulases,enzymes produced only by certain bacteria and fungi.36 Cellulase is a multi-component enzyme system, generally considered to be composed of three maincomponents: endoglucanase (endo-1,4-b-D-glucan 4-glucanohydrolase, EC3.2.1.4), exoglucanase (cellobiohydrolase, EC 3.2.1.91) and b-glucosidase (EC3.2.1.21). Fungi tend to produce extracellular cellulase; those from Trichodermareesei, T. viride, T. koningii and Fusarium solanii are the best characterised.36,37

The endoglucanases attack cellulose in a random fashion, resulting in a rapidfall in the degree of polymerisation and produce oligosaccharide. They havelittle apparent capacity to hydrolyse crystalline cellulose. The cellobiohy-drolases (exogluconases) are considered to degrade cellulose by removing cel-lobiose from the non-reducing end of the cellulose chain. b-Glucosidaseshydrolyse cellobiose and some soluble cello-oligosaccharides to glucose.36

Figure 2.31 shows the generalised scheme for cellulolysis. Cellulose is initiallyattacked in amorphous zones by endoglucanases, thus generating multiple sitesfor attack by cellobiohydrolase. The continued cooperative action between exo-and endo-splitting polysaccharidases continues, combined with the terminalaction of cellobiase to yield glucose. The cooperative action of cellobiohy-drolase and endogluconase is synergistic. Most individual enzymes do notpromote effective hydrolysis, although certain cellobiohydrolysis reactions willcompletely degrade crystalline cellulose. Exo- and endopolysaccharidases caneach occur in multiple forms and can exhibit different specificities in relation tothe degree of polymerisation of the substrate. Two forms of each endogluca-nase and cellobiohydrolase are theoretically possible, owing to the two ste-reoconfigurations of the cellobiosyl unit in the chain.36

Hemicellulose is a general term applied to a group of chemically hetero-geneous carbohydrates found in the cell walls of plants.36,39 They are closelyassociated with cellulose, cementing the microfibrils together, as well as cova-lently bound to lignin to give a lignocarbohydrate complex. The hemicelluloses,like cellulose, are polymers of anhydro sugar units linked by glycosidic bonds.Unlike cellulose molecules, hemicellulose molecules are much shorter, and arebranched and substituted, and as a result they are usually non crystalline.The classification of hemicelluloses is according to their chemical structure

and three predominant types are recognised: xylans, glucans and mannans. Thexylans are the predominant type of hemicellulose found in plants. After

O

cellobiose glucose

O

O

O

CH2OH

CH2OH

H

H

OH

OH

OH

OH

HH H

HH

H

H

HH

H

H

HH

HHH

OH

OH

OH

OH

H

H

CH2OH

CH2OH

O

O

O

Figure 2.30 Structure of cellulose.


cellulose, it is the most abundant polysaccharide in nature. The basic structureof xylans is a main chain of b-1,4 linked D-xylose residues carrying shortsidechains (Figure 2.32).Enzymes capable of degrading xylans include b-1,4 xylanases (1,4-b-D-xylan

xylohydrolyses, EC 3.2.1.8) and b-xylosidases (1,4-b-D-xylan xylohydrolases,EC 3.2.1.37).40 In general terms, the xylanases attack internal xylosidic linkageson the backbone and the b-xylosidases release xylosyl residues by endwiseattack of xylooligosaccharides (Figure 2.33).40,41

Lignin is the second most abundant organic compound on Earth; it is closelyassociated with the cellulose fibres and chemically bonded to the hemi-cellulose.42 Chemically, lignin is an amorphous, three-dimensional aromaticpolymer, formed at the sites of lignification in plants by enzyme-mediated

crystallineregions

amorphousregions

endoglucanases

cellobiohydrolase

OLIGOMERS

exoglucohydrolase

cellobiase

GLUCOSE

CELLULOSE

CELLOBIOSE

endoglucanases

cellobiohydrolase+

+

+

Figure 2.31 Generalised scheme for cellulolysis (modified from Eveleigh38).

60 Chapter 2

polymerisation of three substituted cinnamyl alcohols: p-coumaryl, coniferyland sinapyl alcohols (Figure 2.34).43

Certain fungi, mostly basidiomycetes, are the only organisms able to bio-degrade lignin extensively; white-rot fungi can completely mineralise lignin,

O

O

HOH2C

H

H

OH

OH

H

HH

HH

OH

OH

H

H

CH3O

O

H H

H H

H OH

H

H

O

OH

H H

H

OHH

H

O

OH

H H

H

OHH

H

O

OH

H H

HOH

H

H

OH

O

H

H

OHH

HO

OAc

OAcOO

CO2H

Figure 2.32 Structure of xylan.

-4Xyl β1-4Xyl β1-4Xyl β1-4Xyl β1-4Xyl β1-4Xyl β1-4Xyl β1-4Xyl β1-4Xyl β1-4Xyl β1-

3-α−araf3-Ac

2-Ac2-α-MeGlcA 2-α-MeGlcA

4Xyl β1-4Xyl β1-4Xyl β1-

α-glucuronidase (EC 3.2.1.8)

acetyl (xylan) esterase (EC 3.1.1.6)

α-L-arabinofuranosidase (EC 3.2.1.55)

endo-1,4-β-xylanase

β-xylosidase

Figure 2.33 Attack on xylan by xylanolytic enzymes (modified from Bajpai41).

CH=CH-CH2OH

OH

CH3O OCH3

OH

CH=CH-CH2OH

OH

CH=CH-CH2OH

CH3O

p-coumarylalcohol

sinapyl alcoholconiferylalcohol

Figure 2.34 Structures of the main constituents of lignins.


whereas brown-rot fungi merely modify lignin while removing the carbohy-drates from the plant. The major enzymes isolated from ligninolytic fungi toaffect lignin and lignin-model compounds are laccases (polyphenol oxidases),lignin-peroxidases (LiP) and manganese-dependent peroxidases (MnP).44

White-rot basidiomycetes such as Coriolus versicolor, Phanerochaete chrysos-porium and Phebia radiata have been found to secrete typical lignin degradingenzymes.45 Using model compounds, it has been established that both ligninperoxidase and laccase catalyse one-electron oxidations of either phenolic ornon-phenolic compounds.46,47 Lignin peroxidase converts both phenolic andnon phenolic moieties into their corresponding phenoxy radical and aryl radicalcation intermediates, whereas laccase only catalyses phenoxy radical formationfrom phenolic substrates.46 The aryl radical cations then undergo nucleophilicattack to generate aryl radical intermediates. Such reactive species, as well asthe phenoxy radicals can then react with dioxygen radicals, leading to thecleavage of sidechain and aromatic ring opening of the compounds or alter-natively they can undergo free radical coupling reactions.46,47 Thus, twocompeting reactions can occur during lignin biodegradation: reaction withdioxygen radicals resulting in lignin degradation and free radical couplingleading to repolymerisation.46

Figure 2.35 gives a structural representation of lignin. The breakdown oflignin is of interest to the leather scientist because many attempts have beenmade to use the byproducts of the Kraft paper process in creating useful tan-ning agents (Chapter 14). Although none have been successful so far, theavailability of such large quantities of cheap materials that bear a resemblanceto tanning agents is an incentive to keep trying.Solid state NMR studies35 have shown that treating dung with either indi-

vidual enzymes or mixtures of the three enzymes degrades the dung compositionas a whole, so that the effect is to solubilise the material rather than degradeselected components. In this way, the enzyme mixture effectively attacks theadhering mechanism, causing the dung to be removed from the hide intact. Thisis illustrated in Figure 2.36, in which a salted Irish hide is totally cleaned afteronly one hour in the dirt soak treated with the three enzyme mixture.The presence of dung has important implications for the economics of leather

production, because it affects the timing of the preliminary handling steps inprocessing. Table 2.5 summarises the impact of efficient dung removal, pre-senting some of the consequences of soaking technologies.If green fleshing is not possible, fleshing must be conducted on limed hide,

when the dung is removed at the same time as the hair is dissolved; however,opening up reactions can be adversely affected by the presence of flesh and fat,preventing uniform penetration of chemicals. Even if hair saving is attempted,the keratin byproduct will be highly contaminated by the degraded dung,reducing its disposal routes. If fleshing is conducted on the limed hide, thebyproduct is low value, because the options are as follows: either it is disposedof to landfill or, if tallow recovery is employed, the yield will be low and containa high level of free fatty acid, due to the prolonged hydrolytic conditions,thereby reducing the quality.

62 Chapter 2

In an independent cost analysis in 2002,49 a tannery processing 10 000 hidesper week calculated the benefits of routinely introducing green fleshing,assuming it would be possible to ensure dung removal in the dirt soak:

1. Throughput: The presence of dung cladding adds to the hide weight,which must be included when determining the pack weight. If the tannery

CHOH

CH3O

CH3O

OCH3

OCH3

CH2OH

HC

CH

CH

HC

CH2OH

CHOH

CH

HCOH

CH2OH

C=O

H2C

CH2OH

CHOH

CH2OH

CHOH

HC

CH2OH

CH

HC

HOH2C

CH

HC

CH2OH

CH

HCOH

CH2OH

O

O

HC O

O

O

O

O

O

O

O

CH

H2C

CHOH

O

OCH3

HC

CH

CH3O

HC

O

CH2OH

HC

CH3O

HO

OCH3

CH3O

OH

OCH3

CH3O

CH3O

CHOH

HCO-(HEMICELLULOSE)

CH2OH

H2C

O

OH

HOCH2-CH-CHO

OCH3

O

CH2

H2C

OH

Figure 2.35 Schematic structural formula of lignin (modified from Adler48).


could increase throughput by 15%, as a consequence of removing its highlevel of dung cladding, the following argument applies:Annual current throughput ¼ 10 000 hides per week

¼ 5 � 105 hides per yearIncrease in throughput ¼ 7.5� 103 hides per yearIncrease in area yield ¼ 3� 105 ft2 per year

In this case, the additional income was estimated at h500 000 per year.2. Chemical costs: The presence of 15% dung and the associated flesh on the

hides is estimated to use up an additional 15% of chemical costs up to wetblue: this is put at h100 000 per year.

3. Tallow recovery: The average production of fleshings can be assumed tobe about 5 kg per hide, either green or limed. The recovery of tallow fromlimed fleshings is about 15% and it has reduced value, because of the highfree fatty acid content. The recovery of tallow from green fleshings ishigher at about 70% and the quality is high, because of the low free fattyacid content. The value of tallow is variable, rising to h2000 per tonne,

Figure 2.36 Dung clad hide, before (a) and after (b) enzymatic dirt soak.

64 Chapter 2

dependent on quality. In this case, the annual benefit from improvedtallow sales was estimated to be h400 000 per year.

4. Fleshings disposal: In a total cost analysis of the effects of dung removal,the cost of disposing of limed fleshings should be included. The disposalcost to landfill was typically about h50 per tonne: for an annual pro-duction of 500 tonnes, this represents a cost of h25 000 per year.

Therefore, successful dung removal offers the potential for additional incomeof h2 per hide, representing 2–5% saving in rawstock costs. A specificallyfermented reagent containing a mixture of all three enzymes was successfullytrialled as a soaking auxiliary by Klenzyme Ltd, illustrated in Figure 2.36, but itis no longer available.

2.6 VARIATIONS IN SKIN STRUCTURE DUE TO SPECIES

2.6.1 Hereford Cattle and Vertical Fibre

There are many breeds of cattle used in the global leather industry: in any packof hides there could be several breeds represented, but the distribution is not

Table 2.5 Comparison of soaking options for dung clad hides.

Process step Conventional processing Enzyme soak

Soaking No dung removal Complete dung removalFleshing Not possible Possible:

� High value byproduct, yield-ing high quantity of highquality tallow

� Savings in liming chemicals� Facilitates hair saving

Green splitting Not possible Option available:

� Savings in liming chemicals� Green split as protein source

Unhairing Possibly limited to hair burning:

� Environmental impact ofCOD

� Contaminated hair, if hairsaving

Options available:

� Hair burn� Hair save – added value, clean

byproduct� Hair shaving

Liming � Conventional, applied to fullthickness

� May be non-uniform, due topresence of flesh

� Uniform reaction� Better opening up� May be applied only to grain

split, if desired

Fleshing Low value limed fleshings Not needed

Splitting Produces limed flesh split, lim-iting options of disposal

Conventional production


normally taken into account in the processing. There are subtle variations inhide and consequent leather properties that originate with breed or breedcrosses.50 One breed dependent fault that must be taken into account is called‘vertical fibre’ (Figure 2.37). This is a genetic fault, associated primarily withHereford cattle and crosses.51 The fault can be readily diagnosed by theappearance on the flesh side of the pelt as a pattern that looks like the surface ofa cauliflower: this originates from the enhanced ease of reagents to penetratethrough the fibre structure and consequent increased opening up. Because the

Figure 2.37 Vertical fibre in Hereford cattle. (courtesy of Journal of the AmericanLeather Chemists Association.)

66 Chapter 2

fibres are much less interwoven than in other breeds, the effect is to cause areaof weakness: there is no remedial action possible.

2.6.2 Sheepskin

A feature of sheepskin is the open, loose nature of the fibre structure. Breeddifferences are clearest in the degree to which the fibre structure is loose and thepresence of cutaneous grease in the grain–corium junction. The combinationcan lead to weakness at the junction, even double skinning separation of thelayers.The properties of sheepskin can be compared with goatskin (Figure 2.38),

because there is a spectrum of species, with sheep and goat at the extremes, andwithin the range of species there is hair sheep.Clearly, from Figure 2.38, the preferred qualities in the skin relate inversely

to the preferred qualities in the fibre: at one extreme there are merino sheep andcrosses, which are primarily farmed for the fine wool, but the skins havelimited use, and at the other end of the sheep scale is hair sheep, which havehair of no value, but skin that makes the finest gloving and clothing leather.In the same way, goats range from those with fine hair such as angora,where the fibre is the valued part of the animal, to those which have hairof no value, but have very tough skin for making hard wearing leather.Globally, the most important source of hair sheep is Ethiopia, providing thepreferred raw material for gloving leather: the skin is characterised by the tightfibre structure, more like goatskin than sheepskin, but with the softness ofsheepskin.Note: some breeds of sheep and goats can look remarkably similar and the

way to tell the difference between them is allegedly as follows – sheep hold theirtails down, goats hold their tails up.

2.6.3 Cutaneous Fat

Wool sheepskins, such as UK domestic, New Zealand or Australian, have a fatlayer in the grain–corium junction, shown in the photomicrograph presented inFigure 2.39:52 the structure is in the form of large lipocytes, closely associated ina mass, which can act as a barrier to the penetration of aqueous reagents. Also

goat coarse woolsheep

Increasing tightness of fibre structure Decreasing fat layer of lipocytes Increasing strength Increasing fineness of grain pattern

fine hairgoat

fine woolsheep

hairsheep

Figure 2.38 Properties of sheep and goats.


visible from the red staining in Figure 2.6 is the sebaceous grease in the woolfollicles. The amount of grease is variable, breed-dependent, and may rangefrom 1 to 30% of the skin weight.It is important that the grease should be removed from all hides and skin in

which it occurs, not only to allow uniform reactions through the skincross section, but also to avoid odour problems if the fat turns rancid. Forsheepskins, the traditional method is to treat pickled pelt with warm paraffin,then rinse with brine. This efficient method of degreasing has fallen into dis-repute, due to the environmental impact of using an organic solvent.An alternative is to use aqueous detergents. However, since the grease has amelting point of about 42 1C, processing at a temperature when the grease ismolten is too dangerous, due to the risk of heat damage to the collagen, whichhas a denaturation temperature of about 60 1C under these conditions. Ingeneral, grease dispersal below its melting point is inefficient and so aqueousdegreasing of pickled pelt has not yet reached a universally satisfactory stage ofdevelopment.An alternative option is to use a lipolytic enzyme, lipase, to degrade and

self-emulsify the grease by partially hydrolysing it.6 Unfortunately, there is

Figure 2.39 Scanning electron photomicrograph of sheepskin lipocytes. (Courtesy ofJournal of the Society of Leather Technologists and Chemists.)

68 Chapter 2

a difficulty for the enzyme to reach its substrate, because of the barrier formedby the lipocytes’ cell walls. This can be overcome by using a formulation oflipase with protease, so that the protease can break down the protein compo-nent of the cell wall structure, to release the contents. The disadvantage of thisbiotechnology is that the prolonged treatment with protease causes too muchdamage, mostly to the corium, resulting in looseness, but also possibly indamage to the grain.A newer and more appropriate approach is to formulate the lipase with

phospholipase, to target the other component of the cell wall structure, phos-pholipids.52 The technology has been demonstrated, but the phospholipaseenzymes are not available in industrial quantity and at a suitable price foreconomic application. Note, this situation may not last, because an economicproduction of phospholipase A2 has been developed recently, using a newtechnology of infecting alfalfa grass and then harvesting the enzyme from thisnew vegetable source.53

REFERENCES

1. J. H. Sharphouse, Leather Technician’s Handbook, Leather producersAssociation, Northampton, 1971.

2. K. Bienkievicz, Physical Chemistry of Leather Making, Krieger, Florida,1983.

3. G. E. Attenburrow, A. D. Covington and A. J. Long, J. Soc. LeatherTechnol. Chem., 1999, 83(3), 167.

4. G. E. Attenburrow, A. D. Covington and A. J. Long, J. Amer. LeatherChem. Assoc., 1998, 93(10), 316.

5. J. A. M. Ramshaw, Conn. Tissue Res., 1986, 14(4), 307.6. V. L. Addy, A. D. Covington, D. A. Langridge and A. Watts, J. Soc.

Leather Technol. Chem., 2001, 85(1), 6.7. G. E. Attenburrow, J. Soc. Leather Technol. Chem., 1993, 77(4), 107.8. G. D. McLaughlin, et al., J. Amer. Leather Chem. Assoc., 1929,

24(7), 339.9. M. Aldemir, MSc Thesis, The University of Northampton, 1999.

10. S. P. Robins, Bailliere’s Clinical Rheumatology, Vol. 2, no. 1, 1998.11. J. M. Golsline and J. Rosenbloom, Elastin, In: Extracellular Matrix

Biochemistry, Eds. K. A. Piz and A. H. Reddi, Elsevier, Amsterdam,1988.

12. G. Francis, R. John and J. Thomas, Biochem. J., 1973, 136, 45.13. P. D. Kemp and G. Stainsby, J. Soc. Leather Technol. Chem., 1981,

65(5), 85.14. K. T. W. Alexander, J. Amer. Leather Chem. Assoc., 1988, 83(9), 278.15. P. L. Kronick and S. K. Iondola, J. Amer. Leather Chem. Assoc., 1998,

93(5), 139.16. H. S. Raper, Physiol. Rev., 1928, 8, 245.17. H. S. Mason, J. Biol. Chem., 1948, 172, 83.


18. S. Ito, Biochemistry and Physiology of Melanin, In: Pigmentationand Pigmentary Disorders, Ed. N. Levine, CRC Press, Boca Raton,1993.

19. V. Hearing and M. Jimenez, Int, J. Biochem., 1987, 19, 1141.20. A. Thompson, et al., Biochim. Biophys. Acta., 1985, 843, 49.21. G. Prota, Structure and Biogenesis of Phaeomelanin, in: Pigmentation:

Its Genesis and Control, ed. V. Riley, Appleton-Century-Crofts, New York,1972.

22. R. H. Thompson, Angew. Chem. Int. Ed. Eng., 1974, 13, 305.23. G. Prota, Melanins and Melanogenesis, Academic Press, San Diego,

192.24. G. Prota, et al., J. Heterocyclic Chem 1970, 7, 1143.25. G. Nalbandian, BSc Thesis, The University of Northampton, 2001.26. J. Rao, et al., J. Amer. Leather Chem. Assoc., 2008, 103(7), 203.27. Progress in Leather Science: 1920-45, British Leather Manufacturers’

Research Association, 1948.28. A. D. Covington, UK Patent Application GBA 0110695.4, May 2001.29. M. Siegler, et al., J. Amer. Leather Chem. Assoc., 1994, 89(11), 352.30. J. Mitchell and D. G. Ouellette, J. Amer. Leather Chem. Assoc., 1998,

93(8), 255.31. K. Filiz, MSc Thesis, The University of Northampton, 1999.32. G. E. Attenburrow, J. Soc. Leather Technol. Chem., 1993, 77(4), 107.33. G. E. Attenburrow and D. M. Wright, J. Amer. Leather Chem. Assoc.,

1994, 89(12), 391.34. A. D. Covington, et al., J. Soc. Leather Technol. Chem., 1999, 83(4),

215.35. A. D. Covington, C. S. Evans and M. Tozan, J. Amer. Leather Chem.

Assoc., 2002, 97(5), 178.36. G. O. Aspinall, Polysaccharides, Pergamon, Oxford, 1970.37. T. M. Wood, Biochem. Soc. Trans., 1985, 13, 407.38. D. E. Eveleigh, Phil. Trans. Roy. Soc. London, 1987, A321, 425.39. W. B. Betts, et al., in: Biodegradation Natural and Synthetic Materials, ed.

W. B. Betts, Springer-Verlag, London, 1991, 139,.40. K. K. Y. Wong, L. U. L. Tan and J. N. Saddlar, Microbiol. Rev., 1988,

52, 305.41. P. Bajpai, Adv. App. Microbiol., 1997, 43, 141.42. R. L. Crawford, Lignin Biodegradation and Transformation, Wiley-

Interscience, New York, 1981.43. C. S. Evans, in: Biodegradation Natural and Synthetic Materials, ed. W.B.

Betts, Springer-Verlag, London, 1991, 139.44. C. S. Evans, et al., FEMS Microbiology Reviews, 1994, 13, 235.45. U. Tuor, K. Winterhalten and A. Fiechter, J. Biotech., 1985, 41, 1.46. T. Higuchi, ACS Symposium Series, 1989, 399, 482.47. T. Higuchi, J. Biotech., 1993, 30, 1.48. E. Adler, Wood, Sci. Technol., 1971, 11, 169.

70 Chapter 2

49. A. D Covington, C. S. Evans, M. Tozan, Proc. IULTCS Congress,Cancun, Mexico, May, 2003 pp. 59–64.

50. B. M. Haines, J. Soc. Leather Technol. Chem., 1981, 65(4), 70.51. C. Bottema, et al., J. Amer. Leather Chem. Assoc., 2000, 95(3), 85.52. V. L. Addy, et al., J. Soc. Leather Technol. Chem., 2001, 85(1), 6.53. Phytatec Ltd., Unstead Lane Guildford GU5 0BT, UK.


CHAPTER 3

Curing and Preservation of Hides andSkins

3.1 INTRODUCTION

Preservation of rawstock has the objective of rendering the flayed pelt resistantto putrefaction to allow transport and storage. Preservation is often referred toas ‘curing’, but this implies some form of chemical treatment, although theterms are often used interchangeably. Preservation is accomplished either bydestroying active bacteria, by preventing bacterial activity or by preventingbacterial contamination. Ideally, any treatment applied to pelt should bereversible, without altering the properties of the pelt. This clearly eliminates anysort of tanning process as a curing option.The mechanism of putrefaction is the production of proteolytic enzymes by

bacteria. The source may be the natural saprophytic bacteria within the pelt,present in the living animal, where the purpose is to degrade and remove deadtissue. Following the death of the animal, these bacteria cause autolysis (self-hydrolysis) of the collagen. This breaks down the protein to amino acids, whichfurther break down to produce ammonia. Alternatively, opportunistic bacteriafrom the environment may contribute to putrefaction. Other reactions includethe hydrolysis of triglyceride fat to free fatty acids and glycerol by lipases andthe breakdown of carbohydrates into sugars by carbohydrase enzymes.It important to recognise the effect of ‘staling’, defined as the delay between

flay and cure, when bacterial activity may proceed. Ideal practice in the abattoirwould be to spread the newly flayed pelt onto concrete, to remove the body heatas rapidly as possible, then to put it into cure or into processing as soon aspossible. It has been shown that a delay of as little as 24 hours will guaranteesome grain damage.1 This can be appreciated when it is known that the bacteriaon hides can double in numbers in less than 4 hours at 25 1C.2 In reality, hidesand skins are typically merely held in piles prior to curing or processing, so they




72

are effectively kept warm – this is a reflection of the abattoirs’ view thathides and skins are only a byproduct of the meat industry, with little associatedvalue to them compared to the value of the carcases. Consequently, becausesome incipient damage is often caused, whatever curing system is used, theprocess can only halt bacterial action and the damage caused after flayingremains.The following conditions favour putrefaction, so each may offer options for

approaches to preservation:

� pH6–10, preferably pH7–9.Note, physiological pH is generally accepted to be 7.4, where enzymes in

the body will function optimally. Altering the pH of the pelt might appearto be a sound option for preservation. However, acidification can result inacid swelling if there is no electrolyte present. In the case of wet salting, seebelow, acidic additives may be included. Alkali treatment is limited, becauseat pH 10 and above there is a risk of immunising the hair (Chapter 4) andadversely affecting hair removal. Some alkaline additives for salt areknown: it is not uncommon for soaking to be conducted at pH9–10, partlyto aid rehydration, but also in the expectation of reducing bacterial andenzyme activity. This cannot be relied upon.

� The presence of nutrients: protein, fat, sugar, blood, dung.The only way of reducing the availability of nutrients is to process

immediately, i.e. to eliminate storing, by processing as soon as possibleafter flaying.

� Moisture content 425%.Fresh hide contains 60–70% moisture. The reduction of moisture is an

important feature of preservation methods, playing a role in salting, butalso when drying itself is used for preservation.

� Temperature 420 1C, particularly 30–40 1C.Controlling the temperature of the storage environment is an important

feature of preservation methods.� Calcium and magnesium ions in curing salt favour the growth of halophilic

bacteria (see below).

Figure 3.1 shows that the exponential growth phase occurs when nutrients areplentiful, which is likely to be the usual scenario for hides and skins in storageprior to processing: the stationary phase refers to diminishing nutrients and thiswould only apply when the pelt is nearing complete degradation. Therefore, ifthe conditions of pH, temperature, moisture and lack of protecting chemicalsare right for bacteria, they will soon reproduce at an exponential rate, therebyrapidly creating enough enzyme to cause noticeable damage.Both fresh and salted hides have been shown to be contaminated with

Bacillus sp., Micrococcus sp. and Staphylococcus sp., but the greater danger ofdamage seems to be from the Bacillus sp.3 The observation of bacteria withinthe pelt is the clearest sign of pelt status and quality: this can be done by fixing asample of the pelt in formaldehyde or some other suitable agent, sectioning it

73Curing and Preservation of Hides and Skins

for visualising the bacteria with Alcian Blue and observing by light microscope(Figure 3.2). Debris in the pelt can also take up the stain and may be confusedwith bacteria; however, identification is aided by the knowledge that bacterialgrowth occurs from the flesh side and tends often but not always to be seen aslike rows of beads along the fibres. If there are bacteria only at the flesh surface,this is normal and not threatening. Bacteria within the corium are a dangersign. If they have reached the grain–corium junction, grain damage in the finalleather is inevitable. Here, it should be noted that the presence of bacteria is nota problem in itself, it is the proteolytic enzymes the bacteria excrete; hence thedanger associated with bacteria extends beyond the immediate vicinity of thecells themselves.The presence of the danger can be demonstrated by the Gelatine Film Test.4,5

It is based on obtaining a sample of liquid from the pelt (the pressure of a vicemay be required to squeeze enough moisture out of the relatively dry pelt), thenapplying it to commercial monochrome photographic film (although this is notlikely to continue to be a cheaply available testing material). After incubatingthe treated film at about 37 1C for about an hour, the film is washed with warmwater to remove any degraded gelatine, which is easily seen. The test is notcompletely reliable, as it may yield false negative results. However, it can beuseful in the factory and may even offer insight into degrees of contamination,particularly if multilayered film is used.

time

lag phase stationary phase

deathexponential growth

log (viable organisms/ml)

Figure 3.1 Bacterial growth phases: slow growth, followed by exponential rate ofgrowth, a steady state of viable bacteria and then death.

74 Chapter 3

The effects of inadequate preservation are as follows:

� Loss of structural fibre, shown by soluble nitrogen analysis (BLC,unpublished results, see also ref. 18.).As bacterial enzymes attack the skin components, particularly the protein,

there is an increase in both volatile and soluble nitrogen: the former isessentially ammoniacal nitrogen and the latter is degraded protein. Table 3.1provides some guideline values.If the measured levels are significantly higher than these values, there is

evidence that the pelt has deteriorated, although the tests may not be ableto discriminate between pre- or post-cure damage, so other clues need to besought. The damage to the protein of the pelt is also apparent by thelooseness of the fibre structure, particularly apparent in the looser flanks.Chapter 7 refers to a method for monitoring the progress of enzymes

through the pelt, using the biuret reaction: this colour test has been tried

Figure 3.2 Bacteria in skin fibre structure. (Courtesy A. Michel, LeatherWise UK.)

Table 3.1 Analytical results for salted hides.

Analysis Immediately after salting After typical storage

Volatile N per total N (%) 0.4 0.75Soluble N per total N (%) 2.0 3.0


on stored and staled rawstock, but the method is too insensitive to indicatebacterial damage.

� Damage to grain occurs if the bacteria reach the grain–corium junction,because the enzymes can extend through the grain to the enamel–epidermisjunction. Hence, the signs of damage include: lifting of the epidermis,loosening of the hair and loss of enamel, only visible after unhairing.

� Increase in free fatty acid, seen as yellowing of the fat. Fat breakdown hasbeen proposed by Koppenhoeffer as a measurement of pelt quality. Within30 days, the phospholipid is completely hydrolysed, in 30 days there is15% loss of triglyceride and in six months there is 67% loss of triglyceride.The degree of hydrolysis of the triglyceride fat may give an indicationof the history of the pelt, but does not yield information about the status ofthe protein. Oxidative rancidity does not become apparent until after6 months storage.6

� Prominent vein marking can result from loss of hide substance, whichmakes the intact veinous system more apparent at the surfaces.

Here, notably, the industry distinguishes between long- and short-term pre-servation. The former is exemplified by drying and salting, when the qualities ofthe rawstock can be retained over many months, depending on the conditionsof storage. Short-term preservation is usually thought of as a period of one tofour weeks without deterioration, suitable for transporting the rawstock, forexample, across Europe.

3.2 DRYING

Drying hides and skins to preserve them was once commonly done and theproducts were traded globally; however, the practice has become less common,abandoned in favour of salting, which is easier and therefore more reliable. Theuse of drying is limited to those countries where the climate is conducive todrying, i.e. the tropics, in countries where salt is either scarce or expensive.Temperate climate is typically not warm enough to cause rapid drying. There isa balance to be struck between the rate of drying and the likely rate of bacterialgrowth, which can be expressed as follows.If the rate of evaporation of moisture from the surface is ke and the rate of

diffusion of moisture from the centre of the pelt to the surface is kd, there arethree possible situations and a fourth related relationship:

1. keokdThe rate of evaporation is very slow, so that the equilibration between

the moisture contents in the centre and at the surfaces is maintained. Thismeans that the overall process is slow, so the likelihood of bacterialproliferation and consequent damage is high.

2. ke¼ kdRates of evaporation and diffusion are the same, which means that the

rate and efficient removal of moisture is optimised. The hides or skins may

76 Chapter 3

be dried on the ground (flint drying) or suspended on wires, ropes orpoles; alternatively, they may be loosely fixed (nailed) to wooden frames.In all cases, they must be dried in open sided sheds, for good ventilation,but protected from direct sunshine and rain. The reason for shade dryingis that fast drying must be avoided.

3. ke 4 kdIf the rate of evaporation is faster than the rate of diffusion the outer

layers may dry completely, causing fibre structure sticking, thereby pre-venting moisture from escaping at the surfaces, referred to as an exampleof ‘case hardening’. In turn, this means that the inner layer remains wetand the combination of moisture and warmth favours bacterial growth.The combination of bacterial degradation of the skin and the highambient temperature may even cause gelatinisation in the centre of thehide or skin: this is usually not obvious during the preserving process, butbecomes apparent when the pelt is rehydrated. When such hides arerehydrated in soaking, they can delaminate, separating into two outerlayers, ‘double hiding’ or ‘double skinning’. The damaged inner surfacesmust be removed mechanically before tanning, because on drying thedegraded collagen would ‘glue’ the surface, making it brittle and cracky.

4. ke 6o kpThis is the requirement that the rate of evaporation is faster than the

rate of bacterial proliferation (kp), which in this case refers to the rateat which bacteria can grow from the flesh surface to the grain layer –bacterial growth the other way is hindered or prevented by the presence ofthe epidermis.

This apparently simple and ‘low tech.’ approach to preservation is actuallydifficult to control. The rate of evaporation of moisture from the surfacedepends on the surface characteristics, the air temperature, the surface tem-perature and the rate of air flow over the surface. The surface characteristics arecontrolled by the animal and the flaying operation. The air temperature is aweather feature. The surface temperature is moderated by the conditions –hence the requirement for drying in the shade. The air flow is a weather feature,but may be controlled by the design of the facility and by the use of fans: thelatter clearly has implications for capital and energy cost. In this way, processcontrol can only be exerted indirectly and there is no simple way of checkingsuccessful progress (Table 3.2).Dried hides or skins must be folded while damp, to avoid cracking the dried

pelt. Similarly, it is important to handle and process dried hides or skins withcare: the first part of the soak must be conducted statically, to avoid creasing orcracking, then the remainder must be done for a period long enough to rehy-drate to equilibrium. In both parts of the soak, the pelt must be protected bybactericide. The horny appearance of dried hide does not at first sight indicatethe easy reversibility of the preservation method. However, it is reversiblebecause of the presence of the ground substance: this is a term used collectivelyfor all the non-collagenous components of skin, the non-structural proteins, the


glycosaminoglycans and the fats, which are attacked and removed in thebeamhouse operations. These components protect the collapsed fibre structurefrom sticking, acting as a rehydratable barrier between the elements of thehierarchical structure. In contrast, if partially processed pelt is allowed to dry tothis degree, the triple helices can approach close enough for new chemicalbonds to form, making the structure resistant to rehydration.7 If the dryingprocess has been conducted well and the rawstock is adequately stored, par-ticularly with regard to keeping it dry, it can be kept for many months. Thismeans the method can confer long-term preservation.8

There are variations in the dehydrating approach to preservation, based onpartial dehydration:

1. Sawdust can act as a dehydrating agent, but the weak effect limits itsconsequence to preservation for only a few days, before bacterial activitycauses damage, with or without added biocide. If the process is com-pounded by the presence of electrolyte, the preserving power is extendedto a month. The Liricure process (developed by the Leather IndustriesResearch Institute of South Africa, since closed) was a formulation of35% sawdust, 40% sodium chloride and 25% tetrasodium EDTA,applied at the rate of 150 g per sheepskin.9 Here, the sawdust acts as adrying agent, the salt acts as a drying and osmotic agent and the EDTAacts as a sequestrant for the metal component of metalloproteinases,enzymes commonly associated with putrefaction and which rely on thepresence of a metal ion for their activity. If the metal ion is removed by asequestering agent, the enzyme cannot operate in the same way, losing itscatalytic function.

2. Silica gel can be used, as proposed by Indian workers.10 The reagent,familiar as the laboratory desiccant, was used at up to 5%, together with5% salt on pelt weight, with or without para-chloro-meta-cresol. Duringthat time, the moisture content of goat skins dropped from 65–70% toabout 35%, to confer preservation for two weeks.

Table 3.2 Advantages and disadvantages of air drying as a preservationmethod.

Advantages Disadvantages

Low capital cost – simple equip-ment and housing

Drying problems, e.g. during the monsoon

Low running cost – free source ofenergy

Growth of bacteria, causing putrefaction,especially if drying is too fast at hightemperature

Lower freight charges – moisturecontent 10–14%

Susceptibility to damage by insects, rats, etc.

Difficult to rehydrate – must be done slowlyDifficult to assess quality of cureStoring in dry conditions required

78 Chapter 3

3. Superabsorbent acrylate polymer, the type of polymer used in babies’nappies/diapers can be used in a similar way to silica gel. The reagent iseffective at 3% on pelt weigh, when the drying effect is similar to the effectof 45% sodium chloride. This treatment is suitable for one or two weeksstorage, with easy rehydration at the end, but higher offers of polymertend to dehydrate too much for a short-term preservation process.11

In each case, the effect is suitable only for short-term preservation, becausethe dehydration is partial, only targeting preservation for a few days, to allowtransport of the raw material.

3.3 SALTING

The use of salt for preserving foodstuffs is ancient: globally it is still the mostcommon way of preserving hides and skins. In the context of clean technology,it is useful to calculate the amount of salt used in the global leather industry. Ifthe global annual kill of cattle is 300 million and each hide requires an averageof 10 kg of salt, the annual use of fresh salt is 3 million tonnes. If other rawstockis added the calculation, at least another million tonnes is required (skinsrequire an average of 1 kg of salt).‘Wet salting’ is easy and convenient: the term refers to the application of dry

sodium chloride to the freshly flayed hide or skin, which is wet with the naturalmoisture content. The only requirements for successful treatment are threefold:enough salt, of the right kind, distributed evenly over the pelt. Thereafter,storage is simple, merely requiring that the salt is not washed off in the rain.However, all the advantages are outweighed by the disadvantage of the pro-blem of disposal of the salt when the pelt is processed (Table 3.3. Consider theextent of the problem of neutral electrolyte and the options for its disposal.

Table 3.3 Disposal of preserving salt.

Option Pros Cons

Shake off excess salt fromhides

Saves B10% Salt is dirty, reuse iscounterproductive.

Disposal?Solar pans for soakliquors

Separates salt from soakwater by evaporation

Salt is even dirtier.

Cannot be reused.Disposal to where?

Reverse osmosis Allows recovery for reuse Expensive – contamina-tion of filtrationmaterials

Landfill: solid or sludge Simple, inexpensive Water table contaminatedDisposal to sea Simple, inexpensive Marine life destroyedTreat and reuse Ecologically sound

(apparently)Expensive.

Use of energy, chemicalsfor sterilisation.


The problem of salt stems from the consequences of allowing it to con-taminate the land: most plants cannot tolerate even brackish water, certainlynot most food crops. Moreover, contaminated land becomes infertile, becausethe sodium ion will strip out the mineral micronutrients. A quick assessment ofthe simple options as set out in Table 3.3 reveals that there is no ecologicallysound method of treating salt or disposing of waste salt. Couple that with thesituation that salt is inexpensive, of the order of $100 per tonne, and there is noeconomic incentive to recover salt for reuse. Current technologies includelagooning of soak liquor, to allow solar evaporation of the water content, butthat just leaves extremely dirty, unusable salt. Excess salt can be removed frompelts prior to soaking, using a perforated drum (Figure 3.3): the product is stillcontaminated salt, with limited use if it is untreated and no more ecologicallydisposable than clean salt. Therefore, waste salt is a material that has trulynegative value. Regardless of all the other perceived environmental issues, thisis possible the biggest problem facing the global leather industry.Wet salting is typically conducted by distributing salt over static pelt.

Spreading it by kicking a pile across the pelt on the ground is the commonpractice, but a more automated approach is to present the pelt by conveyor to acurtain of fluidised solid salt. In each case the mechanism is essentially thesame. The osmotic effect of high concentration of salt on the outside of the pelt,with the surface acting as a semi-permeable membrane, causes the water in thepelt to migrate to the outside and the salt to migrate to the inside. The removalof water reduces the viability of bacterial activity and the high concentration ofsalt within the pelt creates an osmotic effect at the cell wall of the bacteria,reducing its viability further. It is necessary to achieve at least 90% saturation(i.e. 432% w/v in the moisture in the pelt, about 14% on hide weight) withinthe pelt, to ensure there is no bacterial activity. Note, the effect of salt is not tokill the bacteria, it does not act as a bactericide, but it does act as a bacteristat.Collagenolytic activity is still rapid at 7% salt on hide weight, but low at 10%,although the bacteria still function.12 This means that the bacteria are still alive,still viable and can be reactivated when the pelt is rehydrated.If the salting is done well, with regard to enough salt of the right quality

distributed over the whole surface area, and storage avoids wetting the pelt, therawstock can be kept for several months.

3.3.1 Stacking

This is the commonest method of wet salting. The freshly flayed hides or skinsare spread out on the ground, flesh side up, then salt is sprinkled or spread outover the whole of the surface. Alternatively, the salt may be applied auto-matically, using a conveyor and automatic dosing. In both cases, as a guide,hides are treated with about 25–30% salt, typically about 10 kg; skins aretreated with up to 40–50% of salt, typically about 1 kg. The salted pelts arestacked in piles 1.5–2.5 metres high, often folded into quarters, with additionalsalt in between each layer and on the top of the pile. The piling helps to squeezebrine out of the pelts, leaving 40–50% moisture. The distribution of salt is close

80 Chapter 3

to equilibrium through the cross section after 24 hours and equilibrium iseffectively reached in 48 hours.12

3.3.2 Drum Curing

The freshly flayed hides or skins are tumbled in a closed drum with 30–50%salt, which may contain an additive, so distribution is guaranteed. The rota-tional speed is kept low and the temperature is limited to 25 1C. The process

Figure 3.3 Perforated drum for desalting preserved rawstock: demonstrated byDr S. Rajamani (centre) of CLRI.


lasts 6 hours for hides, 3 hours for skins. It is less common to conduct salting ina drum:13 this produces more rapid saturation of pelt moisture than stacking,effectively complete in 4 hours, compared to a required period of one of twodays for the static process.

3.3.3 Types of Salt

Regardless of the type of salt used, there are necessary limits to the impuritycontent of the salt: calcium and magnesium 0.1%, iron 0.01%. Both Ca21 andMg21 can react with phosphate and carbonate in the pelt, fusing the fibrestructure and creating a fault known as ‘hard spot’, more usually found incalfskins.

3.3.3.1 Vacuum Salt. This is a high purity (99.9% NaCl) and therefore arelatively high cost product, most commonly used as a reagent in leathermaking, e.g. for making brine solution. It is characterised by its small crystalsize, o1mm, and free flowing nature. The crystal size makes vacuum saltunsuitable for preservation, because in the presence of moisture the crystalsfuse and grow. This effect occurs to reduce the surface free energy: the resultof this mechanism is that the salt can create a fused mass, when the area ofcontact between the salt and the pelt surface is reduced, actually slowing thepreserving process and therefore risking bacterial growth.

3.3.3.2 Granular Salt. Granular salt is also high purity, but the crystal sizeis larger, 2–4mm, and it is cheaper than vacuum salt. The crystal size meansthe rate of crystal growth is slower and contact with the surface is main-tained as it is dissolved into the pelt’s moisture. The main use for this gradeof salt is rawstock preservation.

3.3.3.3 Rock Salt. This grade of salt is mined and crushed to the requiredcrystal dimensions. There may be appreciable concentrations of calcium andmagnesium salts. The coarse crystals tend to have sharp edges, so indentationand cutting may occur during storage, making this grade of salt unsuitablefor curing purposes.

3.3.3.4 Marine Salt. This grade of salt is prepared by evaporating seawater, a common source of salt for many purposes; consequently, it is highlycontaminated with calcium, magnesium and other salts. Unlike sea salt pre-pared for the food industry, which is sterilised by the boiling and evaporatingprocesses, the salt destined for industrial applications is evaporated by solaraction and is likely to be contaminated with bacteria: these are halophilic(and halotolerant) bacteria, archaebacteria that evolved before mammals,examples of extremophiles, requiring extreme conditions for survival. Halo-philic bacteria require saturated salt solution for growth, 5 molar, 30%

82 Chapter 3

solution.14 They are pink, red or purple, having in their constitution rhodop-sin (Figure 3.4), a molecule capable of switching between yellow and purpleconformations. This molecule is also involved in vision, capable of changingcolour in 10�15 s, the cause of ‘red eye’ in flash photography.The colour gives rise to the effect known as ‘red heat’, although the effect has

nothing to do with a rise in temperature. Proliferation of these bacteria in theiraerobic phase causes a red colouration over the flesh side. This in itself does notnecessarily create a problem, because it is not a rapidly damaging phenomenon,since the temperature of storage is usually sub-optimum. However, itsappearance is a warning of a problem coming, especially damage to the grain.15

If the bacteria within piled hides change to the anaerobic phase, the colourchanges to purple, referred to as ‘violet heat’. At this point the damage happensmuch faster, so that by the time it is observed, often in the centre of piles ofhides or skins, severe damage has already been done, effectively rendering therawstock useless and valueless. Halotolerant bacteria are colourless, capable ofgrowing in 20% salt solution, and so they are more likely to thrive when thecure is incomplete, for whatever reason. Therefore, marine salt is strictlyunsuitable for preserving rawstock, but it is better than no curing at all.The pelts shown in Figure 3.5 are distinctly red. If they were unaffected the

flesh side should be cream coloured. This is a badly affected pack. The like-lihood of violet heat being present within this pallet load is high.

3.3.3.5 Khari Salt. Strictly this material cannot be considered as a gradeof salt. It is the efflorescence found on the surface of soil after rain, especiallyafter the monsoon. It is caused by ground/soil salts dissolving in the rainwater and migrating to the surface, where it is left after the water has evapo-rated. It is very impure, composed of variable amounts of sodium, magne-sium, chloride, sulfate, bicarbonate and carbonate ions, typically a mixtureof sodium and magnesium sulfates.16 It is also contaminated with halophilicand halotolerant bacteria. The presence of alkaline impurities causes proteinhydrolysis under warm conditions. These characteristics make this grade ofsalt generally unsuitable for rawstock preservation; however, it is still used inthe Indian sub-continent, where it is probably better than no salt at all.At the first sign of red heat, the problem can be effectively eliminated by

starting the normal processing. By initiating the soaking step, the salt

H3C CH3CH3

CH3 H3C

CH3

O

O

CH3

CH3CH3H3C

Figure 3.4 Isomers of rhodopsin.


concentration around the rawstock is diluted below the critical concentrationand the cells are killed. If required, the rawstock can be resalted safely withgranular salt and preservation is restored.The status of the salt cure can be assessed easily by measuring the quantity of

salt in the pelt: by extracting a sample of pelt into 0.2 M sodium acetate solution,then titrating the solution with 0.1 M silver nitrate, using sodium chromateindicator. By determining the moisture content, the salt concentration in thepelt moisture can be calculated. The salt should be not less than 90% satura-tion. If there is excess salt on the pelt, it should be removed as much as possible,before conducting the analysis. Excess salt may indicate an adequate cure, but

Figure 3.5 Hides infected with ‘red heat’.

84 Chapter 3

it is no guarantee that the curing was done properly, nor that the pelt was ingood condition at the time of curing.

3.3.4 Additives for Salt

To enhance or guarantee the action of salting or to discourage damage byinsects, additives may be included in the salt for preservation. Some additivesthat have been used in the past include:16

� sodium carbonate� sodium pentachlorophenate� sodium silicofluoride� boric acid� zinc sulfate

Naphthalene has also been used, often formulated with the salts in the listabove.In modern processing, most of those traditional additives are no longer used,

due to toxicity or environmental impact. Modern additives include sodiummetabisulfite and bactericides (see below).

3.3.5 Dry Salting

This is a combination of any type of wet salting and sun drying, used in hotcountries where salt is plentiful and cheap. Here the moisture content is typi-cally reduced to 20–25%. It has the advantages of reduced weight for trans-porting and the preserved rawstock is easier to rehydrate than rawstock thathas only been dried. On the other hand, the common use of impure salt canproduce red heat and drying may lead to rancid fat and alkaline hydrolysis.However, dry salting is not commonly practiced as a method of rawstockpreservation.

3.3.6 Brining

An alternative to wet salting is to apply the salt in the form of a solution(Table 3.4). This is a method almost exclusively used in North America. Hidesare tumbled in a paddle with saturated brine (36% w/v at 15 1C) for 12 hours.After draining, the hides may have additional salt, with or without additive,applied to the flesh side: the latter is a strategy particularly used if there is to beprolonged storage or transportation.Brined hides are wetter than wet salted hides, but the salt concentration in the

associated moisture is about the same: typically brined hides contain 85%saturated salt solution. The preservation is comparable to wet salting and allthe problems of halophilic bacterial contamination also apply to brined hides,particularly if the temperature reaches 37 1C.15 It is not obvious where suchspecies come from, but infection in premises handling salted stock is common


and surprisingly difficult to eliminate. Bailey19 has shown that halophilic bac-teria in North American brining solutions can be controlled by bactericides,including a mixture of sodium bisulfite and acetic acid (see below).

3.4 ALTERNATIVE OSMOLYTES

In all the discussion about the use of salt, it is understood that the salt is sodiumchloride. However, any material that can exert an osmotic pressure might be acandidate for curing. The requirements are cheapness, availability and nodamaging effects, e.g. lyotropic swelling. The obvious alternative options areother electrolytes.As the damaging part of salt is the sodium ions, clearly any alternative

electrolyte cannot be a sodium salt. In contrast, plants require potassium ionsas a nutrient, so the use of potassium chloride as a curing agent would not onlysolve the environmental impact problem but also allow the waste stream to bedisposed of to agricultural or horticultural land. Bailey has demonstrated thefeasibility of using this technology.19,20 The main drawback is higher cost.From a North American perspective, an additional factor to take into accountis the temperature dependence of the solubility in water (Table 3.5). The aqu-eous solubility of NaCl does not display a marked temperature dependence, sobrining operations are not affected by seasonal changes in the ambient tem-perature. However, the lower solubility of KCl at lower temperature wouldrequire adjusting the brining solution in cold weather, to avoid under-curing.Potassium chloride is not immune from contamination with extremophiles.

An opportunistic Staphylococcus sp. was found on preserved hides containing4 M KCl:18 this organism is proteolytic, so there is potential danger of hidedamage during storage.More speculative osmotic options in this application might include sac-

charides (sugars). These agents would not present an effluent problem, because

Table 3.4 Effects of brining on the composition of hide.17

Fresh (%) Brined (%)

Hide substance 36–40 36–40Water 60–64 34–38Salt 10

Table 3.5 Effect of temperature on the solubilityof potassium chloride.

Temperature (1C) Solubility (molal)

0 3.610 4.020 4.430 4.8

86 Chapter 3

they will easily break down in a tannery effluent treatment plant. However, theproblem of mould growth would have to be addressed, but the addition of afungicide should solve any problems. More important is the comparison withsalt. Here the critical parameter is the water activity, defined as the ratioof the vapour pressure of a solution to the pure solvent: Figure 3.6 gives acomparison.21

It can be seen that salt is more effective on a weight for weight basis, althoughboth are used as dehydrating agents in food preservation: sugar is used in jamand preserve making, honey is an effective bacteristat in wound management.Table 3.6 illustrates the impact of water activity on preventing bacterial growth.22

Recently, the use of sodium silicate has been proposed: sodium silicate(wasserglass, water glass) solution is neutralised with sulfuric and formic acids,then dried.23 It is claimed to be a viable complete substitute for salt and thesoak solution from processing the preserved stock can be applied to growingplants without any adverse effects.

3.5 PH CONTROL

Treatment of rawstock for high pH storage is risky, mainly due to the rapid rateof base-catalysed hydrolysis of the collagen. Therefore, the only pH basedsystem in common usage is storage pickling (Table 3.7), which is commonly used

Figure 3.6 Water activities of salt (sodium chloride, NaCl) and sugar (sucrose)solutions. (Courtesy of Journal of the Society of Leather Technologists andChemists.)


for woolskins in UK, Europe, Australia and New Zealand. The earliest storagepickling systems involved 12–14% salt and 1% sulfuric acid, corresponding topHB0.7. However, the high concentration of protons produced too rapid a rateof acid-catalysed hydrolysis. The modern version relies much less on acid. Peltsare processed conventionally to the point of pickling: soak, drain, samm(squeeze) or centrifuge, paint the flesh side with lime/sulfide, pile, pull the wool,relime, delime, bate and wash. Pickling is conducted with the following solution,processing in a drum or paddle: 12–15% salt on water volume at 15 1C, 0.7–0.8% sulfuric acid, corresponding to pHB1.0, plus antimould.Note, the concentration of salt is relatively low compared to wet salting, but

is more than enough to resist acid swelling (Chapter 8).Although storage pickling is conventionally used for sheepskins, it is not

used for cattle hides. However, there is no good reason why this should nothappen. A formulation of 100% float, 10–15% salt and 2% sulfuric acid willconfer pH2.5–2.7 to hides, which is effective preservation for a year.24 It isprobably only tradition that prevents this application of acid preservation forlarger pelts.A salt-free pickling preserving system has been proposed by Bailey

and demonstrated to work, using acetic acid and sodium sulfite.25,26 Themethod relies on both acidity, pHo4, to reduce bacterial activity, and thebiocidal action of sulfur dioxide to sterilise the hide. Trials indicated thatthe method is suitable for short-term preservation, and that it has the potentialto provide long-term preservation, but this approach has not been developedcommercially.

Table 3.6 Limiting water activities for growth of some bacteria.

Species Limiting water activity below which no growth occurs

Bacterium mycoides 0.99Escherichia coli 0.932Bacillus subtilis 0.949Micrococcus roseus 0.905Pseudomonas aeruginosa 0.945Staphylococcus aureus 0.86–0.88

Table 3.7 Advantages and disadvantages of storage pickling.

Advantages Disadvantages

Savings on beamhouse processing:economic and environmentalimpact

Processing faults by fellmonger: weak grain,scud/pigment, mottle, drawn grain, mouldgrowth

Ease of sorting for qualityimmediately

Freight charges – B45% moisture content

Pelts sensitive to elevated temperature: store ato15 1C, rapid hydrolysis at 30 1C

88 Chapter 3

3.6 TEMPERATURE CONTROL

Rawstock can be preserved by storing it at temperatures that slow bacterialactivity. The Arrhenius equation is often used to estimate the effect on reactionsof lowering the temperature:

k ¼ Ae�E=RT or ln k ¼ ln A� ðE=RTÞ

where k is the rate, A is the pre-exponential factor (a constant), E is the acti-vation energy, R is the gas constant and T is the temperature (K).Because the relationship is logarithmic, a rough guide is that for every 10 1C

fall in temperature the rate is halved. The converse is true: for a rise of 10 1C therate is doubled.The limit to holding untreated pelt is about 24 hours at ambient tempe-

rature, if that is assumed to be 20 1C. The period is relatively short, becauseit is affected by the way in which the rawstock is usually treated: theanalysis also assumes that post flay treatment is not ideal, i.e. the body heat isnot removed immediately and efficiently from the pelt, compounded byholding the pelts in the abattoir in piles. The Arrhenius equation clearlyindicates the limits to lowering temperature for preservation and thereforepredicts that any method based on this change must be considered to be short-term preservation.

3.6.1 Chilling

Hides can be chilled to 3 1C within half an hour by blowing air at �1 1Cat a rate of 100mmin�1. There is a limit to how cold the air can be – forexample, using air at –9 1C actually causes parts of the hides to freeze, which isinconvenient in this context,27 as discussed below. It is important to use moistair, otherwise the rawstock will dry, which could create problems when thehides are put into work. If the hides are stored at 2–3 1C, they will remainfresh for up to three weeks. This is enough time for transport within a con-tinent, e.g. hides are routinely moved from north to south Europe in thisstate, using refrigerated lorries. There are alternative cooling systems inoperation. In Australia, eutectic or chilling plates have been used to coolsheepskins rapidly after flay. Other coolants have been shown to work, e.g.carbon dioxide snow (�80 1C), which has been used to chill carcases: its via-bility depends on availability and cost. Liquid nitrogen (�100 1C) has been usedin abattoirs for rapidly chilling carcasses, but it is actually less effective on peltsthan higher temperature coolants, because the rapid boiling of the liquid createsa gas layer between the coolant and the pelt surface, reducing heat transfer outof the pelt.The cooling methods generally have low environmental impact, but there is

typically considerable capital cost involved: cooling plant and subsequent coldstorage.


3.6.2 Icing

Ice can be applied to rawstock in the same way as salt is applied. There aresimilarities between the methods, especially with regard to the size of the iceparticles. If the crystals are small, the ice will melt too rapidly, so the chillingperiod is limited; too large and the contact with the pelt is limited. Icing canachieve rapid cooling, but storage cannot prevent the ice melting. Therefore, thisis a short-term method of preservation, unreliable for more than a few days –five days maximum at 15 1C. The best results have been achieved with wool-skins, which provide an insulating effect for the centre of the pile. However, theedges of piles are soon unprotected. Ice is relatively cheap, but the costing mustinclude not only ice generating plant, but also storage facilities.An alternative form of ice is flo-ice, which is a slush of ice in a solution of salt,

sugar or alcohol in water. Cooling is rapid, but so is melting. It is worth notingthat the wetting effect of cold water for cooling the pelt or the creation ofmelting ice may be counterproductive, because either condition can promotebacterial growth in the wetted pelt. In such cases, it may actually be better notto treat at all, i.e. the drier pelt containing the natural level of moisture is aworse medium for bacterial growth than wetted pelt.

3.6.3 Biocide Ice

To avoid the problems of rehydration by melting ice, the concept of usingbiocide ice was developed by BLC in UK. The idea is to create ice from a biocidesolution, so that melting releases the biocide, providing an additional element ofprotection. One difficulty is the availability of a suitable biocide. The biocide mustbe effective as a bactericide and it must be soluble in water, to allow ice to bemade. Most bactericides currently used in the industry are hydrophobic chemi-cals, formulated to form emulsions with water: in these cases, freezing merelyseparates the biocide from the water. The biocide shown to be effective is 1,3-dihydroxy-2-bromo-2-nitropropane, also known as bronopol or Myacide AS(Table 3.7).The effective period of preservation still depends on the conditions of storing

– the lower the ambient temperature the longer the preservation period.The application rate is similar to salting, with regard to the amount to beapplied, and the costs have been calculated to be comparable. The majordrawback of this technology is run off from melting ice, which contains thebiocide. This means that the piles of rawstock must be held in such a way thatthe run off is contained; subsequently, the run off must be treated as an effluentstream.

3.6.4 Freezing

Within the options for using temperature control, it might appear that theobvious extreme is to freeze the rawstock, by analogy with food preservation.This is certainly a practical option, since this is technology already common-place in the food industry. However, there are drawbacks.

90 Chapter 3

Rawstock must be frozen quickly, to avoid the formation of large ice crystals,which can disrupt the fibre structure: this is not a simple procedure for largeitems like hides. Also the faster the pelts are frozen, the less likely staling is tooccur.

� The storage period is limited, because of the growth of ice crystals (astypically observed in domestic freezers).

� The pelts must kept at o0 1C, i.e. in a very large freezer, rather than coldstorage.

� Thawing prior to processing is the defining problem. If a pallet of hides isfrozen and stored at �30 1C, thawing at ambient temperature takes 48hours; if the hides are stored at �10 1C thawing at 10 1C also takes 48hours.27 The thawing operation is likely to allow time for bacterial growthand consequent damage. Thawing in the processing vessel is impractical.Loading the vessel with a stack of frozen hides is likely to cause damage tothe vessel. Once loaded into the vessel, the absorption of the requiredlatent heat is enough to freeze the float, even if it is in large excess. Themass of pelt and float then needs to be thawed – a non-trivial practicalproblem.

3.7 BIOCIDES

Short-term preservation can be achieved by spraying hides on both sides with asolution of a bactericide. Agents that have been used include phenols, 2-(cya-nomethylthio)benzothiazole (TCMTB), methylene bis(thiocyanate) (MBT),1,2-benzisothiazolin-3-one (BIT), biguanide, sodium chlorite, boric acid28 and1,3-dihydroxy-2-bromo-2-nitropropane (bronopol) (Figure 3.7).29 Preservationperiods may extend to about two weeks, depending on the agent and theconcentration used, but one week is a safer period to assume, depending on thetemperature of storage. In each case, there are problems associated withapplying the solutions safely and disposal of the biocide after soaking.An important factor to consider is the mode of action of the biocide. Some

biocides work by reacting with the protein of the microorganisms, in effecttanning it: this is the case for aldehydic and triazine biocides. In the context ofhides and skins preservation this can be counterproductive. If it is the intentionto remove the hair or wool, particularly by hair saving methods, tanning thepelt stabilises the proteins at the base of the follicle, preventing the detachingmethods from working.

3.8 RADIATION CURING

In a comprehensive review of modern preservation of rawstock, Bailey18

included his groundbreaking work on the use of radiation. Gamma radiationprovided complete sterilisation at a dose of just over 20 kGy (Gy¼ gray),providing protection for 6 weeks. The radiation is capable of damaging the pelt,reducing the tear strength, but damage was not apparent below 30 kGy.30


Electron beam radiation was also investigated: the procedure was to rinse hideswith biocide, seal them in polythene bags and irradiate with 1.4Mrad at10MeV, which provided protection for 3 weeks.31 Both of these radiationtechniques have been approved in the USA for eliminating Escherichia colifrom hamburger beef. Issues arising from these technologies are the cost of theplant for treatment and keeping the hides sterile, i.e. avoiding recontamination.

3.9 FRESH STOCK

The use of chilling notwithstanding, the alternative to salt curing is not to cureat all. This is an increasingly common approach to processing, particularly in

S

N

C-SCH2SCN

CH2(SCN)2

N-CnH2n+1

S

C

O

R-NH-C

S

S-

H2N-C-NH -C-NH2

NH NH

CN

NNC

C

HOCH2 CH2OHC

Br NO2

2-(thiocyanomethylthio)benzothiazole (TCMTB)

methylene bis (thiocyanate)(MBT)

1,2-benzisothiazolin-3-one

dithiocarbamate

biganide

triazine

bronopol

Figure 3.7 Structures of some biocides that may be applicable to rawstockpreservation.

92 Chapter 3

those countries where there is a direct link between the abattoir and the tan-nery. It is non-trivial to point out that the difference is the presence or absenceof salt: since salt has a profound influence on the fibre structure of pelt by thedehydrating effect, this may have a lasting impact on the leather quality,compared to rawstock which does not undergo such a treatment. Haines32

made the comparison and found no difference between salted and fresh raw-stock, with regard to physical properties and area yield of the leather. She did,however, observe that leather made from fresh hide exhibited slightly coarserbreak. The only apparently related feature observed in the fibre structure wasthat the junction region of leather from fresh hides was more finely split, i.e.more opened up, making the region looser, which is a cause of coarse break,because it allows the grain to ripple (Chapter 5). The reason for this differencein opening up is not clear: it appears that the salting somehow confers pro-tection to the junction, perhaps related to the depleting effect.

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577.5. R. R. Schmitt and C. Deasy, J. Amer. Leather Chem. Assoc., 1964, 59(6),

361.6. R. M. Koppenhoeffer, J. Amer. Leather Chem. Assoc., 1937, 32(4), 152.7. M. Komanowsky, J. Amer. Leather Chem. Assoc., 1989, 89(12), 369.8. P. J. Water, L. J. Stephens, C. Surridge, Leather, October 1999.9. A. E. Russell, H. Tandt and R. Kohl, J. Soc. Leather Technol. Chem., 1997,

81(4), 137.10. N. K. Chandra Babu, J. Amer. Leather Chem. Assoc., 2000, 95(10), 368.11. A. H. Quadery, MSc Thesis, University College Northampton, 1999.12. D. R. Cooper, J. Soc. Leather Technol. Chem., 1973, 57(1), 19.13. G. W. Vivian and M. B. Rands, J. Soc. Leather Technol. Chem., 1976,

60(6), 149.14. D. G. Bailey, M. Birbir, A. Ilgaz and W. Kallenberger, J. Soc. Leather

Technol. Chem., 1996, 80(3), 87.15. D. G. Bailey and M. Birbir, J. Amer. Leather Chem. Assoc., 1996, 91(2), 47.16. Progress in Leather Science: 1920-45, British Leather Manufacturers’

Association, 1948.17. D. G. Bailey, J. Amer. Leather Chem. Assoc., 2003, 98(8), 308.18. D. G. Bailey and M. Birbir, J. Soc. Leather Technol. Chem., 2000, 84(5),

201.19. D. G. Bailey, J. Amer. Leather Chem. Assoc., 1995, 90(1), 13.20. D. G. Bailey, J. Amer. Leather Chem. Assoc., 1996, 91(12), 317.


21. A. E. Russell and A. C. Galloway, J. Soc. Leather Technol. Chem., 1980,64(1), 1.

22. Hugo. (Ed.) Inhibition and Destruction of the Microbial Cell, AcademicPress, London, 1971.

23. K. H. Munz, J. Amer. Leather Chem. Assoc., 2007, 102(1), 16.24. B. M. Haines and R. L. Sykes, J. Soc. Leather Technol. Chem., 1973, 57(6),

153.25. D. G. Bailey and W. J. Hopkins, J. Amer. Leather Chem. Assoc., 1877,

72(9), 334.26. M. A. Haffner and B. M. Haines, J. Soc. Leather Technol. Chem., 1973,

59(4), 114.27. B. M. Haines, J. Soc. Leather Technol. Chem., 1981, 65(3), 41.28. R. Hughes, J. Soc. Leather Technol. Chem., 1974, 58(4), 100.29. The Biocides Business, Eds, D. J. Knight and M. Cooke, Wiley-VCH, 2003.30. D. G. Bailey, J. Amer. Leather Chem. Assoc., 1999, 94(7), 259.31. D. G. Bailey, G. L. DiMaio, A. G. Gehring and G. D. Ross, J. Amer.

Leather Chem. Assoc., 2001, 96(10), 382.32. B. M. Haines, J. Soc. Leather Technol. Chem., 1981, 65(3), 41.

94 Chapter 3

CHAPTER 4

Soaking

4.1 INTRODUCTION TO BEAMHOUSE PROCESSING

Soaking is the first of the ‘beamhouse’ processes, so-called because they tra-ditionally included operations that were conducted over a wooden beam. Priorto the introduction of machinery, hides or skins would be draped over anangled wooden beam, so that they could be hand fleshed, hand dehaired(scudded) and hand shaved to thickness (Figure 4.1).Nowadays, the term is used to encompass all the processes conducted in the

tannery leading to the tanning step. The purpose of the beamhouse is to preparethe pelt for tanning. Another way of putting that would be to say the beam-house is for purifying the pelt or ‘opening up’ the pelt structure. Opening up is ageneric term that has two components:

1. The removal of non-collagenous skin components: the hyaluronic acidand other glycosaminoglycans, the non-structural proteins, the fats. Thisis not done to completion (except in the special case of Japanese leather,when removal of non-collagenous components is conducted to extreme),so the processes in a tannery must be geared to the degree of removingthese materials, required to produce the desired properties of the finalleather.

2. Splitting the fibre structure at the level of the fibril bundles, to separatethem.

These effects are the result of the complex reactions under the conditions inthe beamhouse and the degree to which they happen depends on the preciseconditions adopted throughout these preparative process steps.Table 4.1 summarises the process steps in the beamhouse: the indication of

amounts of water on raw weight basis and times merely represent typicalindustrial conditions, which can vary greatly.




95

It has been said that leather is made or marred in the beamhouse.What this means is that the fundamental properties and performance para-

meters of the final leather are conferred to the pelt during the beamhouseprocessing. Importantly, each step is important, insofar as subsequent processsteps can never compensate for or overcome deficiencies in any given priorprocess step – at least not without compromising leather quality. This is anexample of the importance of the axiom ‘get it right first time’.

4.2 SOAKING PROCESS

Soaking is the first process applied to the rawstock. This and all subse-quent chemical steps are most commonly conducted in drums as shown in

Figure 4.1 Traditional use of beams for hand fleshing and other tasks. (Courtesy ofJ. Basford.)

Table 4.1 Indicative process conditions for bovine hides.

Process stepWater (% on hideweight) pH Time (hours)

Dirt soak 200 6–10 1–2Main soak (often more than a singlestep, n¼ 1–4)

n� 200 6–10 4–8

Unhair (lime/sulfide) with liming 200 12–13 15–20Rinse 200 12–13 1Delime (ammonium salt) 100 8–9 1–2Bate (proteolytic enzyme) 100 8–9 1–2Rinse 100 8–9 1Pickle (sulfuric+formic acids) 50–100 50–100 1–2

96 Chapter 4

Figures 4.2 and 4.3, illustrating that the mechanics of chemical processing havenot changed markedly in a 100 years, except for the drive mechanisms. Alter-natively, early steps may be conducted in open vessels, where the pelts aremoved in the solution by a paddle.

Figure 4.2 Processing in drums in the first half of the twentieth century. (Courtesy ofJ. Basford.)

Figure 4.3 Modern drum processing – could be anywhere in the world.

97Soaking

Several parameters should be considered in the soaking process. In reviewingthe variables, McLaughlin and Theis considered the following aspects ofsoaking:1

� history of the pelt and the type and degree of curing – at that time, saltingwas the primary curing process;

� water content of the rawstock;� exchange of substances between the soak liquor and the pelt;� nature of the protein solubilised in the soak;� character of the soak water;� concentration of salt in the soak liquor and the rate of diffusion from the

pelt into the solution;� pH of the soak;� temperature;� float to pelt ratio;� time in soak;� effect of changing the soak float;� impact of biocide in the soak and the chemistry of the reagent.

Table 4.2 presents selected data from their paper.The effectiveness of the soak (here longer than typical processing) depends on

the amount of water used: the efficiency of salt removal depends on the amountof water, since the salt is effectively partitioned between the two phases and thesolubilisation rate depends on the difference in concentration between the soakliquor and the rehydrated pelt. The salt removal figures can be used to estimatethe effects of multiple soaking processes. Assume the total salt content is 15%and the efficiency of salt removal is defined by the values in Table 4.2, theaccumulation of salt as a percentage of the total content in the pelt can becorrelated with the number of separate soaking operations and the totalamount of water used in those operations. Table 4.3 presents the calculations.

Table 4.2 Effects of float to goods ratio and temperature on the outcome ofsoaking salted hide for 24 hours.1

Water to hideratio

Salt conc. insoak (%)

Salt in soak(% on hide weight)

Swelling (%)

4 1C 20 1C 37 1C

1 : 1 6.77 6.81 : 2 4.75 9.5 31 32 281 : 3 3.50 10.5 28 33 281 : 4 2.63 10.5 40 33 271 : 5 2.34 11.7 36 47 421 : 6 1.95 11.7 48 48 381 : 7 1.79 12.5 48 42 321 : 8 1.64 13.1 48 51 301 : 9 1.36 12.2 41 41 341 : 10 1.35 13.5 31 31 26

98 Chapter 4

Assuming an acceptable level of salt removal in the soaking step is 95% ofthe total content, Table 4.3 shows that can be achieved in few steps if theamount of float per soak is higher, but the total amount of fresh water isgreater. These results indicate the optimum float is 200%, but the actual valuewill depend on individual circumstances.In each of the variables in soaking there are several contributors: in no

category are the parameters given in order of importance here, the hierarchymust be judged in the context of the actual process and the specific require-ments of the particular rawstock.

4.2.1 Rehydration

The function of rehydration is to fill up the fibre structure with water, ensuringthat the all elements of the hierarchy of structure are wetted to equilibrium andbeyond, with the purpose of facilitating movement of dissolved chemicalreagents through the pelt cross section. In the case of dried or dry salted hides,this is the dominant purpose of soaking, but it applies equally to wet saltedstock. Even fresh hide requires filling with water, which is a faster reaction thanany rehydrating process. Only when the pelt is completely wetted and filled withwater can chemicals be transported throughout the structure, particularly downthe hierarchy of the fibre structure. If this does not happen, chemical processingwill be non-uniform and the effect will be reflected in the final leather, givingrise to non-uniform properties.

4.2.2 Removal of Salt

All salt curing causes dehydration, which means the fibre structure collapses,which consequently hinders movement of solution within the pelt. The removalof salt occurs at the same time as rehydration happens. Therefore, the saltbecomes increasingly diluted, reducing the osmotic effect, so the fibre structureis plumped up, eventually beyond the situation in fresh pelt. This process hasbeen modelled by Kolomaznik,2 assuming a diffusion controlled system of salttransport, while neglecting adsorption of salt onto the fibre structure. Furthermodelling by Kolomaznik of the economics of salt removal in soakingdemonstrated the potential for damage to the fibre structure by ‘shock’

Table 4.3 Calculated salt removal by multiple soaks.

Float togoods ratio

Accumulated salt removal (%) and volume of water used (%)

Soak 1 Soak 2 Soak 3 Soak 4 Soak 5

1 : 1 45 100 70 200 83 300 91 400 95 5001 : 2 63 200 86 400 95 6001 : 3 70 300 91 600 97 9001 : 4 70 400 91 800 97 12001 : 5 78 500 95 1000

99Soaking

desalination,3 although it is not clear exactly what the consequences are fromthis alleged phenomenon. However, most tanners around the world take noaccount of this, simply applying fresh water to their salted stock.The removal of salt can require a lot of fresh water, depending on how the

process is conducted. There are three options:

1. Running washing, when water is continuously pumped into a rotatingdrum fitted with a lattice door. The rinse water is continuously dischargedthrough the door to waste. The advantage is that the pelt is in contact withsolution containing very little salt, so the rates of transfer of water into thepelt and salt out of the pelt are maximised. The disadvantage is that thereis very high use of fresh water.

2. Batch washing, when the pelt is washed with several changes of freshwater. This is common, when there is a short first or dirt soak, followed byone or more prolonged main washes, possibly with rinsing. The effec-tiveness of the partitioning of salt between the pelt and the solutiondepends on the ratio of water to pelt, the degree of mechanical action andthe time allowed to move towards equilibrium, and the effectiveness of theprocess also depends on the number of washes. The requirement for freshwater depends on the size and number of washes.

3. Counter-current washing is an option for reducing the requirement forfresh water. Here, the only discharged solution is the dirt soak, thesolution most contaminated with dirt and containing the highest con-centration of salt. Figure 4.4 illustrates the principle, showing a system offive washes. The first line of washes is for the first pack. The second packuses the second soak liquor from the first pack as its dirt soak solution, its

Figure 4.4 Counter-current reuse of wash water.

100 Chapter 4

third soak is conducted with the previous second soak and so on, until it isfinally rinsed with fresh water. The third pack continues the pattern ofreusing solutions and the system can continue indefinitely. It is calledcounter-current because the water moves from fresh to increasinglycontaminated, and the hides move in the opposite direction, from heavilycontaminated solution to fresh water. In this way, salt and dirt isaccumulated in the small volume of water that is discharged to waste andthe consumption of fresh water is minimised. Clearly, this does notremove any problems associated with salt, but it can mitigate the requiredtreatment, e.g. using reverse osmosis or membrane technology to isolatethe salt.

The efficiency of salt removal still depends on the ratio of solvent to pelt, thedegree of mechanical action, the time of contact and the number of treatments.As the number of packs of pelt treated increases, the concentration of salt inany given recycled soak solution will increase. However, the pattern is likely toreflect another unrelated example of recycling also when accumulated elec-trolyte is in equilibrium with pelt, in which it was found that a steady state wasreached after five cycles.4 The procedure may require more time than theconventional process, because it involves transferring solution from the processvessel into holding vessels, via a screen. Moreover, the rate of salt diffusiondepends on the difference in the concentration between the bulk solution andthe solution within the pelt: this difference is always going to be smaller than asoaking procedure involving only fresh water.

4.2.3 Cleaning the Pelt

Cleaning means the removal of dirt, primarily from the surfaces, i.e. dis-colouring or abrading agents, such as mud or soil and grit. Cleaning includesremoval of blood, urine, body fluids from both surfaces and within the pelt.

4.2.4 Removal of Non-structural Proteins

This may occur,2 indeed McLaughlin and Theis quantified the effect, deter-mining that the extent depended on the number of changes made to the soakliquor.1 However, unless the process is prolonged, and if it is conducted withoutthe assistance of chemical or enzymatic breakdown, it is unlikely to be a majorcontributor to opening up. In the absence of appropriate enzymes, it is assumedby tanners that the soaking step has little effect on the fibre structure and itscomponents, other than rehydration.

4.2.5 Removal of Dung

Under conventional condition of soaking, including the use of typical auxiliariessuch as alkali and surfactants, dried on dung is not removed. Only in the presenceof lignocellulosic enzymes is dung removed quickly and efficiently (Chapter 2).

101Soaking

4.2.6 Removal of Hyaluronic Acid

The structure of hyaluronic acid is discussed in Chapter 2 (Figure 2.14): it iseffectively a chain of carboxylate groups at physiological pH. The chargerepulsion effect gives the molecule a large exclusion volume, making it difficultto remove it from the fibre structure, even though it is not bound to the col-lagen. The presence of salt (neutral electrolyte) effectively smears out thecharges, allowing the chain structure to contract in volume, causing themolecules to be washed out of the pelt.For fresh hide, this mechanism is not available. The options are as follows:

1. Add salt to the soak solutions, which is ecologically counterproductive,but done in practice.

2. Use prolonged soaking, which has implications for damage by bacterialproliferation and the likely requirement of biocide addition.

3. Apply chondroitinase to break the chains into fragments for easierremoval, but this has not been developed industrially.

The reference to options in processing fresh hide assume the necessity forremoving the hyaluronic acid (HA). The argument is based on specious logic:HA is removed very efficiently from salted hide and the outcome is leather ofaccepted and acceptable quality, therefore HA removal is necessary. Theargument is supported by the notion that HA can react with chromium(III),because of the high carboxyl content: this should result in polymeric cross-linking throughout the collagen structure, causing a reduction in mobility,which should be observed as reduced softness. Since the most common raw-stock of recent years has been wet salted hide, the argument has not been tested.However, a lesser raw material used to be dried hides and skins: because thepelt was soaked with little mechanical action, it is reasonable to suppose thatthe HA remained throughout the beamhouse processing and took part in thechrome tanning process. It is the experience of tanners of dried material thatthe product is always firmer than the same leather made from wet salted hide.5

The difference was always assumed to originate in the collapsing of the fibrestructure during drying and the inability of rehydration to return the peltcompletely to its natural state. However, it is also likely that the leather owes itshandle, at least in part, to the extra element of structure, in a way analogous tothe tanning of residual non-structural protein. It is not known how the HAcontributes to the physical and chemical properties of the leather, but for someapplications it could be a benefit.

4.3 CONDITIONS IN SOAKING

4.3.1 Long Float

Soaking float is typically 200–300% on salted pelt weight, depending on theraw material and its state of cleanliness. The float length tends to be consistent,

102 Chapter 4

even when multiple soaking steps are employed. The removal of salt is a dif-fusion and osmotic pressure controlled process, so the concentration of salt insolution determines the rate of salt removal, and hence changes of float arenecessary.

4.3.2 Change of Float

The options for changing float have already been addressed. One advantage ofusing only fresh water is that the requirement to include a bactericide may bereduced.

4.3.3 Temperature

Since the pelt is raw, with a denaturation/shrinkage temperature (Ts) ofB65 1Cat around neutral pH, the temperature must be limited to o30 1C: this appliesto all pH conditions. It has been suggested that the arrector pili muscle can stillcontract when the raw pelt is placed in cold water (Chapter 2). Hence, it may bebetter to use water at 20–30 1C rather than at 10–20 1C, to avoid raising theangle of the follicle and thereby risk coarsening the grain.

4.3.4 pH

Typically, soaking is conducted without pH change: due to the combination ofpelt at physiological pH8 and water at pH6 due to absorbed carbon dioxidethe bath is typically around pH7. However, some tanners add alkali to thebath, usually sodium carbonate, to raise the pH to about 10 to assist the wettingaction.

4.3.5 Time

In conventional batch soaking, salted hides usually require 6 hours or more toremove enough salt to ensure that the pelt is completely rehydrated in the centreof the cross section and down the hierarchy of structure. Dried skins and hidesrequire 24–48 hours or more.

4.3.6 Mechanical Action

For salted stock, the degree of mechanical action is not critical for leatherquality. However, violent mechanical action is not recommended in the earlieststages of processing, when the hides or skins are in a relatively stiff condition. Asin any process involving the transfer of solution into and out of the pelt, effi-ciency and effectiveness increases with the degree of mechanical action. This isbecause the mechanism of solution penetration and solute transfer rely onalternately compressing and relaxing the pelt. In the case of dried hides or skins,it is essential that the first stage of rehydrating is conducted in a static bath,probably for at least 24 hours, depending on the pelt thickness, when rehydrationshould only occur through diffusion. Only when the pelt has softened can theprocess be conducted with any mechanical action, to begin to accelerate the rate.

103Soaking

The role of mechanical action with regard to the energy applied to the load isnot easy to define: power input can be measured, but needs special electricalequipment. Mitton addressed the problem of drumming theoretically in 1953,6

when he analysed the way in which hides tumble in a drum fitted with shelves orpegs. He concluded that, in the absence of internal structure and under typicalconditions, pelts are never carried higher than the drum axle. But it was notuntil twenty years later that the subject was addressed practically and reviewedby Longstaff.7 One of the problems faced by tanners is how to compare theaction in one drum to that in another, particularly when the drums are not thesame size: this is the difficulty of achieving realistic results in small experimentaldrums. Lhuede8 showed that there was dynamic similarity of action if thefollowing relationship applied:

N1D1=21 ¼ N2D

1=22

where N is the drum speed in revolutions per minute and D is the drum dia-meter in feet. This also assumes that the loading is similar, i.e. the drum is filledto the same degree and the float to goods ratio is the same. The optimum valuefor the term ND

12 was found to be 47–55, corresponding to the typical condi-

tions for chrome tanning of 17 rpm for a drum 8 ft in diameter. Lhuede8

observed:

the expenditure of equal quantities of energy by batches of falling skins ofequal mass does not necessarily imply equally effective pounding.

Therefore, the assumption that all the components of a pack of goods in thedrum receive the same treatment may not necessarily be the case.Hinsch9 showed that the optimum speed for maximum mechanical action is

about 66% of the centrifugal speed, i.e. when ND1/2B 28: this refers to thespeed at which the pelts are carried round the drum by centrifugal force to thehighest point when they drop back down into the load. None of this takes intoaccount the effects of any internal structure to the drum. All tannery drumshelp the tumbling action by having either horizontal shelves or inward pointingpegs: if neither are present, the hides or skins merely roll around causing themto tangle.10 Longstaff experimented with peg design and demonstrated thatlonger pegs were more effective in causing mixing action.7 Corning has conti-nued investigation into the role of mechanical action in processing on the smallscale, but the work has not been openly published.11 This is an area of researchthat needs to be developed.

4.4 COMPONENTS OF SOAKING SOLUTIONS

4.4.1 Water

The quality of the water, in terms of its bacterial status, is an issue that needs tobe addressed (see below).

104 Chapter 4

4.4.2 Detergents

These agents accelerate rehydration by wetting the fibre structure. Removinggrease is a less effect, since the temperature of soaking is typically low andtherefore the grease is not mobilised. Triglyceride grease is associated with thefibre structure, present in varying amounts, dependent on the species of animal:it is effectively removed by alkyl poly(ethylene oxide) type nonionic detergents:

CnH2nþ1O½CH2CH2O�mH

The melting point of triglyceride grease is 40–45 1C, so the actions ofdegreasing agents are greatly reduced at lower temperatures. The advantage ofusing this class of detergents is that they do not interact strongly with collagen;hence they are washed out and cannot interfere with subsequent processes suchas dyeing. Note, detergents that are derivatives of nonyl phenol are no longerused, since they have been shown to be endocrine disruptors.Sebaceous grease is better removed by anionic detergents that have, or are of

similar type to, the structure shown in Figure 4.5. Currently available anionicdetergents can bind to collagen and survive into post tanning, when they canadversely affect dyeing and water resistance. This ionic interaction is importantin post tanning (Chapter 15).Sebaceous grease compositions vary considerably between species (Table 4.4).

CnH2n+1

SO3- Na+

CnH2n+1O(CH2CH2O)mSO3- Na+

Figure 4.5 Examples of anionic detergents.

Table 4.4 Composition of examples of sebaceous grease (weight%).

Component Sheep12 Bovine a13 Human14

Squalene 0 15Sterols 12 1Sterol esters 46 2Wax esters 10 11 25Diesters 21 0Glyceryl esters 0 0Triacyl esters 0 53 41Phospholipids 21Free acids 0 10 16Free alcohols 11 0aFleshed hides fat content, i.e. sebaceous grease with some cutaneous lipids.

105Soaking

Sheep or wool grease is high in sterols and waxes, which are useful for manyapplications as lanolin. Bovine sebum is lower in sterols and higher in trigly-ceride lipids, therefore it is more vulnerable to degradation by lipases.15

4.4.3 Soaking Enzymes

4.4.3.1 Proteases. Soaking proteases are essentially diluted bating enzymes.They are included in soaking, not only to break down protein, particularly non-structural protein, to enhance the rehydration process, but also to contribute tothe opening up of the fibre structure. Care must be taken not to loosen the peltfibre structure by over bating during the soaking steps.

4.4.3.2 Lipases. These enzymes hydrolyse triglyceride grease, thereby redu-cing the hydrophilicity of the fibre structure and aiding wetting. It has beenclaimed that attacking the sebaceous grease on the grain surface of cattle hidesfacilitates subsequent chemical processing, because the reagents are not hin-dered by the natural protection that the epidermis confers.15

4.4.3.3 Amylases. These enzymes are marketed for the soaking step. How-ever, because enzymes are specific in their action on substrates, it is difficult tosee how they can be effective: amylases (named from amylum, an old name forstarch) break down starch and starch is not an animal carbohydrate, it is aplant carbohydrate. The specificity means that they do not have a spectrum ofactivity for other carbohydrates.16,17

4.4.3.4 Lignocellulosic Enzymes. Dung contains 80% lignocellulose, withlittle protein or fat. Therefore, it is only broken down by a mixture of enzymesthat attack the primary composition: ligninase, cellulase and xylanase(addressed in Chapter 2).

4.4.3.5 Other Enzymes. There are many other enzymes that might be appliedat this stage of processing, although the options are speculative, since they havenot reached industrial acceptance.

4.4.3.6 Chondroitinases. These enzymes might be used to break down theglycosaminoglycans, the hyaluronic acid and dermatan sulfate: in the formercase, this would be a useful addition to the technology of processing fresh orchilled hides, when salt is not available to facilitate the removal of hyaluronicacid. The value of degrading dermatan sulfate is not certain, considering itsfunction in skin (Chapter 2).

4.4.3.7 Amidases. Whether or not liming/unhairing technology involvesprolonged alkali treatment, amide sidechain hydrolysis can be achieved andenhanced by the inclusion of amidases to prepare the pelt for chrome tanning.

106 Chapter 4

The enzymes can specifically target asparagine or glutamine or both: sincechrome tanning may preferentially occur at aspartic acid sidechains, see below,asparaginase might be preferred, but a cheaper option is a general amidase.

4.4.3.8 Phospholipase. The biotechnology of using phospholipase indegreasing,18 to target lipocyte cell walls, has been mentioned and it may beappropriate to consider starting the process in the soak.

4.4.4 Biocides

At this stage of processing, the major risk is from bacterial activity; fungaldamage is not a problem until the point of holding in the wet tanned state.Some tanners allow a degree of bacterial activity during soaking, as a con-tributor to opening up, by running a warm soak, in which bacteria are allowedto proliferate. However, this is a high risk strategy, because the conditions varybetween packs and therefore the effect is inconsistent. Inconsistency in openingup at any stage of processing means that the product becomes inconsistent.Bacterial activity can be controlled by the use of biocides. Note, there is usually

no need for a fungicide at this stage of production, although some bactericides dopossess fungicidal activity. The need for bactericide in soaking depends on thetotality of all the prevailing conditions and circumstances – the more conditionsthat would contribute to danger, the greater is the imperative for a bactericide tobe used. Bactericide types and comments are set out in Table 4.5. The choice ofbiocide is determined by the requirements of the raw material and the product tobe made (e.g. those biocides that have a tanning mechanism are unsuitable forprocesses in which hair or wool is to be removed by hair save techniques) andcompatibility with other agents present, e.g. enzymes.In deciding whether the use of a biocide in soaking is advisable, the prevailing

conditions must be assessed. The following conditions might each indicate thata bactericide is necessary, but do not necessarily constitute a list of definitivethreshold conditions that apply individually, nor are they listed according topriority.

4.4.4.1 pH. This is not strictly part of the problem, since pH can beadjusted to any value. Even soaking at physiological pH does not indicatethe necessity for a bactericide, because other factors are overriding.

4.4.4.2 Temperature. The higher the temperature, the faster bacteria willgrow and the faster their enzymes will react. Soaking at temperatures above20 1C constitutes a risky condition.

4.4.4.3 Time. The longer the rawstock is held in the soaking solutions, thegreater the risk of damage. Above 6 hours is regarded as likely to incurdamage.

107Soaking

Table 4.5 Biocides that might be used in soaking.

Biocide type Comments

Phenolic � Can be substantive to collagen and causedyeing problems

� Inhibitory to enzymes� Variable effectiveness, but typically not sui-

table for soaking

Oxidising, e.g. hypochlorite, chlorite � May damage components of pelt� Requires high concentration to be effective� Likely to be inhibitory to enzymes

Isothiazolone, e.g. octyl � Very good fungicide� Less effective bactericide

2-(Thiocyanomethylthio)benzothia-zole (TCMTB)

� Good substantivity to skin� Relatively low bactericidal activity� Currently the industry standard fungicide

Thiocyanate, e.g. bis isothiocyanate(BIT)

� Effective bactericide� Degraded at pH 4 8� Frequently found in formulations with

TCMTB

Quaternary ammonium � Relatively ineffectual

Biguanide � Useful, but inhibitory to pancreatic types ofenzymes

Thione/thiadiazine � Good activity in the presence of organicmatter

� May adversely affect leather� Can have unpleasant odour

Aldehyde � Exhibits tanning effect, therefore unsuitablefor applications when the hair or wool is to beremoved

� Not suitable for soaking

Triazine � Very good biocidal activity in the presence oforganic matter

� Has a tanning action, cf. chemistry of reactivedyes (Chapter 15)

Phosphonium, e.g. tetra-kis(hydroxymethyl)phosphoniumsulfate (THPS)

� Good biocidal activity� Has an aldehydic tanning action

Bronopol, i.e. 1,3-dihydroxy-2-bromo-2-nitropropane

� Effective bactericide� Good health and safety record

108 Chapter 4

4.4.4.4 Fresh Hides or Skins. Salted hides produce a brine solution duringsoaking: in an analogy to salting for curing, the presence of salt in solutionconfers some bacteriostatic effect. In the case of fresh hides or skins, therecan be no bacteriostatic effect, so they are at greater risk of bacterial damage.

4.4.4.5 Dried Hides. Clearly, these need bactericide during their long firstsoak, which is likely to last at least 24 hours. Also, the conditions of dryingare likely to have resulted in higher levels of bacterial than salt cured stock.

4.4.4.6 Dirty Rawstock. The presence of contamination, particularly dung,provides additional bacterial loading and will heighten the risk of bacterialdamage.

4.4.4.7 Putrefied Rawstock. If it is known or suspected that the rawstockhas already been damaged, by staling (delay between flay and cure) or byinadequate curing or preservation or by inadequate storing conditions, theuse of biocide is mandatory.Note, the presence of red heat does not constitute a danger warning in this

context. This problem can be dealt with by simply reducing the salt con-centration in a conventional soaking process.

4.4.4.8 Nature of the Cure. Some salt curing processes include biocide orbiocidal additives, e.g. sodium metabisulfite: in these circumstances, addi-tional bactericide is probably not needed.

4.4.4.9 Water Quality. If the water is significantly loaded with bacteria, asmay be the case with directly abstracted ground water, unlike processedtown water, the likelihood of damage is increased.

4.4.4.10 Use of Enzymes. Proteolytic enzymes in the soak provide a con-trolled way to begin the opening up process: the presence of additional,uncontrolled numbers of bacteria is likely to result in more enzyme actionthan is desirable and thereby cause excessive action leading to damage, parti-cularly on the grain surface.

4.4.4.11 Soaking Procedure. The cleaner the water is at any stage of soak-ing, the less likely bacterial damage can occur. This will depend in part onthe number of changes of water during the soaking procedure. Recyclingsoak liquors would be high risk in this regard.

4.5 ROLE OF THE ERECTOR PILI MUSCLE

Chapter 2 mentions the influence of the erector (arrector) pili muscle on fibrestructure, as a possible effect of soaking conditions. Tables 4.6 and 4.7

109Soaking

demonstrate that there is indeed an effect of temperature and calcium con-centration on the muscle post-mortem:19 an increase in angle correlates withmuscle contraction, elevating the angle of the follicle and thereby opening themouth of the follicle, producing undesirable coarsened grain pattern.Relaxation of the erector pili muscle depends both on temperature and time:

cold temperatures do produce residual contraction, although the effect isrelatively small. Similarly, the presence of calcium ion, important in the musclecontracting mechanism, indicates that the muscle can still react, exhibitingmaximum relaxation at 10�4–10�5 molar, corresponding to 0.4 to 4 ppm (partsper million) calcium ion: this effect is also small.Table 4.8 shows the corresponding effects of soaking on the corium angle of

weave and the follicle angle, indicating the effects of rehydration. The followinginferences can be drawn:

1. The effects of hydration outweigh the effects of the action of the erectorpili muscle.

2. The rate of rehydration of the corium is faster than the rate of rehydrationof the grain layer. This is presumably a consequence of the difference inphysical structure.

It may be concluded that the soaking process itself will affect the follicleangle, regardless of conditions: the only parameter that has a large effect is the

Table 4.6 Effect of temperature and time in soaking on the angle of the erectorpili muscle. Errors are standard deviations of ten measurements.

Angle (1)

Time (h) 5 1C 10 1C 15 1C 20 1C 30 1C

1 8� 1 9� 1 10� 1 11� 1 12� 22 10� 1 14� 2 13� 2 15� 1 16� 13 12� 2 16� 2 17� 1 17� 2 19� 15 13� 2 17� 2 17� 1 18� 2 20� 28 14� 2 18� 1 18� 1 19� 1 20� 2

Table 4.7 Effect of calcium(II) concentration in soaking on the angle of theerector pili muscle. Errors are standard deviations of tenmeasurements.

Angle (1)

Time (h) 10�7M 10�6

M 10�5M 10�4

M 10�3M

1 12� 2 16� 1 20� 2 16� 2 12� 22 14� 2 18� 1 23� 2 20� 2 16� 23 16� 2 20� 2 24� 2 22� 2 18� 25 18� 2 21� 2 24� 3 24� 2 20� 18 18� 2 21� 2 25� 3 24� 2 20� 2

110 Chapter 4

time over which soaking is conducted. Therefore, since the erector pili muscle isdegraded in liming, the continued rehydration and swelling during that processwill dominate the grain quality, so the impact of the soaking step in that regardcan be ignored.

REFERENCES

1. G. D. McLaughlin and E. R. Theis, J. Amer. Leather Chem. Assoc., 1923,18(7), 324.

2. K. Kolomaznik and A. Blaha, J. Soc. Leather Technol. Chem., 1989, 73(5),136.

3. K. Kolomaznik, Z. Prokopova, V. Vasek and D. G. Bailey, J. Amer.Leather Chem. Assoc., 2006, 101(9), 309.

4. J. G. Scroggie, et al., J. Soc. Leather Technol. Chem., 1976, 60(4), 106.5. R. P. Daniels, personal communication.6. R. G. Mitton, J. Soc. Leather Trades Chem., 1953, 37(4), 109.7. E. Longstaff, J. Soc. Leather Technol. Chem., 1974, 58(2), 41.8. E. P. Lhuede, J. Amer. Leather Chem. Assoc., 1969, 64(4), 164.9. H. Hinsch, Leder und Hautemarkt, 1969, 21, 278.

10. C. Pillard and D. Vial, Technicuir, 1969, 3, 169.11. D. R. Corning, BLC, restricted circulation.12. D. T. Downing, in Mammalian Waxes, Ed. P.E. Kolluttukudy, Elsevier,

Amsterdam, 1976.13. J. Christner, et al., World Leather, 2008, 21(5), 31.14. J. J. Leyden, J. Amer. Acad. Derm., 1995, 32, 815.15. J. Christner, J. Amer. Leather Chem. Assoc., 1992, 87(4), 128.16. A. D. Covington, et al., J. Soc. Leather Technol. Chem., 1999, 83(4), 215.17. A. D. Covington, C. S. Evans and M. Tozan, J. Amer. Leather Chem.

Assoc., 2002, 97(5), 178.18. V. L. Addy, A. D. Covington, D. Langridge and A. Watts, J. Soc. Leather

Technol. Chem., 2001, 85(2), 52.19. L. Ma, MSc Thesis, The University of Northampton, 2008.

Table 4.8 Effect of time in soaking at 20 1C on the angle of the hair follicle andthe angle of weave in the corium. Errors are standard deviations often measurements.

Angle (1)

Time (h) Follicle Corium weave

1 16� 2 19� 32 22� 2 28� 23 28� 2 39� 25 33� 3 57� 38 41� 2 57� 2

111Soaking

CHAPTER 5

Unhairing

5.1 INTRODUCTION

Unhairing and liming are often linked because the traditional processes of hairdissolving and alkaline hydrolysis combine the process steps in one. However,strictly they ought to be thought of as separate processes and, in modernprocessing, the steps are increasingly commonly conducted separately.As the term implies, unhairing is the process of removing the hair from the

pelt. It is traditionally one of the dirtier aspects of leather processing, from thepoint of view of the odour created (typically from the sulfide employed andthe decomposed protein) and the polluting load generated.1 The traditionalmethod of dissolving the hair is to dissolve it, called ‘hair burning’: this is anexample of ‘low tech.’ processing, so-called because it has the benefit ofapplying to closed drums and does not need any active process control as thereactions progress. It is technically more difficult to undertake the process ofhair removal in alternative ways, ‘hair saving’, keeping the hair intact whileremoving it; each technology requires a different degree of process control.However, in the future, tanners will certainly be universally required to adoptsome form of hair saving technology. Table 5.1 compares the two approaches.Frendrup2 has reviewed the technologies of hair saving and makes the fol-

lowing comparisons of pollutant production (Table 5.2).

5.2 KERATIN AND THE STRUCTURE OF HAIR

Keratin differs from the other main proteins in skin in its elemental composi-tion (Table 5.3).3

Keratins vary in structure and composition, depending on the application.The amino acid composition varies: keratins contain less glycine and prolinethan collagen and no hydroxylysine, but the contents of cystine and tyrosine arehigh. The feature of importance here is the ease with which cysteine can be




112

converted into cystine by oxidation (Figure 5.1): this is the reaction that takesplace in the prekeratinised zone during the growth of hair and which is anequilibrium that can be exploited in the removal of hair from hides and skins.Keratin is usually encountered in the b-form, which is a stretched helix, in the

Table 5.1 Comparison between hair burning and hair saving.

Hair burn Hair save

Technology/processcontrol

Simple technology More complicated,technology

Minimum process control. More process control nee-ded. Reagents may besafer

Usually relies on undesir-able chemicals

Hair removal Incomplete – staple remainswithin the follicle

Complete – allows betterdyeing of cleaner grain

Environmental impact High, due to COD/BODfrom dissolved proteinand process chemicals

Big reduction in BOD/COD(biochemical/chemicaloxygen demand)

Environmentalperception

Not clean Cleaner/clean

Advantages � Reliable� Capable of improvement,

e.g. prior hair shaving� Combined with liming

� Cleaner� Better grain quality, e.g.

for dyeing� Recovered hair has value

Disadvantages � Dirty process� Residual hair can reduce

leather quality

� Residual hair must bedissolved by hair burningtechnology

� Liming is a separateprocess step

Table 5.2 Comparisons with hair burning of hair saving on pollutantproduction.

PollutantDischarge from hair saving(kg per tonne raw hide)

Reduction (%) comparedto hair burn

Total solids 60 30Chemical oxygen demand 50 50Total Kjeldahl nitrogen 2.5 55Sulfide 0.6–1.2 50–60

Table 5.3 Elemental composition of skin proteins.

Protein Total nitrogen (%) Total sulfur (%)

Collagen 17.8 0.2Elastin 16.8 0.3Keratin 16.5 3.9

113Unhairing

way collagen’s fundamental structure is helical. It forms fibrils created fromprotein chains linked via disulfide bonds, which are the target of many of theunhairing techniques.The structure of hair or wool is illustrated in Figure 5.2, using human hair as

an example (originally published in Gray’s Anatomy). The figure also

Figure 5.1 Interconversion of cysteine and cystine.

Dermis

Sebaceousgland

Cortex of hairVessel

Dermic coat

Inner root sheath

Outer root sheath

Bulb of hair

Papilla of hair

Stratum corneum

Stratum lucidumStratum granulosumStratum mucosum

Stratum germinativum

Arrector pili muscle

Dermic coat

Medulla of hair

Figure 5.2 Structure of hair.

114 Chapter 5

demonstrates the relationship between the hair, with its accompanying struc-tures, and the epidermis:

� The bulb – containing the papilla, this is the structure from where the hairgrows. Only in the case of young, papillary hair is the hair still attached tothe base of the follicle: in the case of bovine hair, the mature or club hairsare shed two or three times a year.1 The structure of the bulb is protein, butnot keratin. It can be used as the point of attack in hair saving technologies.

� The prekeratinised zone – this is the region in the growing hair where thekeratin is laid down and disulfide links are created; it is situated above thebulb and extends partway up the follicle. It is protein, with some of thecharacteristics of keratin. It also may be the point of attack in hair savingtechnologies.

� The cuticle – this is the outer surface structure of the hair staple (Figure 5.3).It is made of hard keratin, i.e. contains a high concentration of disulfidelinks, which is not fibrous in texture. The cuticle is composed of sheet-likecells that overlay each other, making it relatively chemically inert. In thetime scale of hair burning, it is not degraded, merely broken off fromthe dissolved cortex, visible in solution as suspended solids.

The hair surface is shown in Figure 5.3(B), illustrating the scales that formthe outer surface. The degree to which they lie flat determines the ability orotherwise to lock the shafts together, seen in the tangling of long human hair,usually a relatively mild effect, or the creation of felt from fur or sheep fibre, anextreme effect. In the latter case, the cuticle scales can be chemically lifted for

Figure 5.3 Cuticle of a black bovine hair.

115Unhairing

great tangling effect, when the traditional material for hats is felt made frombeaver or hare fur. The cuticle has an inner structure: layers are the epicuticle,the exocuticle and the endocuticle. The structure is illustrated in Figures 5.4 and5.10 (below): in the former, alkaline degradation of the whole of the cuticleleaves the cortex exposed, in the latter, sulfide and enzyme degradation leavesthe cuticle layer, from which the scales can flake away.

� The cortex – this is the inner structure of the hair, making up most of thecross section. Its structure is soft keratin, containing less sulfur thanthe cuticle and it is fibrous. In hair burning, this is the main target of thekeratin degrading chemistry (often sulfide). This is where the hair pigmentmelanin is situated.4 Figure 5.5 illustrates the cortex: its fibrous structure isnot immediately apparent because of its compact nature.

� The medulla – this is the central structure of the hair (Figure 5.5). It isprotein, but not keratin, since it does not contain any sulfur. In very finehair or wool, e.g. merino or merino cross wool, the medulla may bemissing, but it is typically present in cattle hair. In hair burning themedulla is not attacked, except for some alkaline hydrolysis, but it breaksup due to the mechanical action on the weak structure.

The structure of keratin does not impact greatly on leather making, for threereasons:

1. The hair may be left intact and chemically not altered, except when it mightbe dyed, e.g. in woolskins. In the case of wool dyeing, the substrate is intactcuticle: the physical and chemical structure of the cuticle means that it is

Figure 5.4 Alkali degraded cuticle of pig bristle (mag. 200�). (Courtesy A. Onyuka)

116 Chapter 5

resistant to the dyes most commonly used for colouring collagen, the acidand premetallised dyes. Therefore, the chemistry of the reactive dyes isrequired (Chapter 16). Alternatively, keratin may be ‘immunised’, i.e. stabi-lised as part of a hair removal technology (discussed below), so the chemistryof keratin becomes important, but the physical structure is not exploited.

2. The hair is often dissolved, in which case the structure is lost and the onlyrequirement is to understand the chemistry of its destruction.

3. The hair may be removed intact, when the reaction is usually focussed onthe structure within the follicle, rather than on the hair structure itself.

At the same time as hair is attacked in hair burning, the epidermis is alsoattacked. Since it is primarily composed of keratin, the structure is degradedunder the conditions of hair burning, using a reducing nucleophile in alkali. It isworth considering that not all hair saving processes include epidermis removal,but this must be undertaken at some time before tanning is initiated. Theepidermis consists of five layers of structure (Figure 5.2).Stratum basale (stratum germinativum): the closest layer to the grain surface.

It is a single layer of cylindrical cells that is the source of cells to create the otherlayers in the epidermis. Melanin is found in this layer, although it can also befound within the grain.Stratum spinosum (stratum mucosum): the layer above the stratum basale,

consists of several layers of polyhedral cells, where keratin synthesis occurs.Stratum granulosum: the layer above the stratum spinosum. It is one to three

layers of cells that participate in the keratinisation process.

Figure 5.5 Cross section of bovine hair (mag. 900�). (Courtesy A. Onyuka)

117Unhairing

Stratum lucidum: the layer above the stratum granulosum. This is two to threelayers of cells that have no nuclei: they are characterised by exhibiting high gloss.Stratum corneum: this is the outermost layer. The cells are flattened and in

the uppermost part of the layer they can flake off.The keratinous nature of the epidermis and its physical structure mean that

in the production of grain leather it must be removed (Figure 5.6). If epidermisis present, the difference in chemistry between keratin and collagen results inareas of dye resist. The flaking nature of the epidermal surface means that thesurface is not reflective, as is the case of the grain enamel, so the scattering oflight by epidermis means that the surface appears dull.The epidermis is attached to the grain by the basement membrane, consisting

of collagens types IV and VII (Chapter 2): this is also a potential site of attack forhair saving unhairing. In hair burning, the combination of alkali and reducingnucleophile is usually enough to degrade and displace the epidermis efficiently.

5.3 HAIR BURNING

Hair burning refers to the practice of dissolving the hair, usually with sulfide inthe presence of lime. The mechanism can be summarised as follows:5

1. Hair burning occurs from the tip down to the surface and beyond. Thisusually occurs faster than penetration of the reagents through the peltfrom the flesh side. However, once the reagents encounter the hair at thebase of the follicle, they begin to dissolve the prekeratinised zone. The net

Figure 5.6 Epidermis lifting from a hide surface. (Courtesy of A. Michel, Leather-Wise UK.)

118 Chapter 5

result is that a plug of hair remains in the follicle: although not attached, itis held in place by friction with the follicle wall. This is commonlyobserved by the thumbnail test: applying pressure with a thumbnail acrossthe grain surface squeezes the residual hair from the follicle. This can bedone mechanically, to clean the grain surface, but as a separate operation‘scudding’ is no longer common.

2. Hair burning chemistry acts on the softer keratins of the cortex and epi-dermis. The presence of melanin in the cortex gives rise to the observedhair colour, which is alleged to influence the rate of degradation:6 there areindications that there may some truth in the suggestion that black hairsdissolve more slowly than white hairs.7 However, the melanin is releasedinto the solution and remains as a component of the ‘scud’, residual pig-mented solids: scud can affect the grain quality by being driven into thesurfaces of the pelt by mechanical action, causing uneven discoloration.

3. The cuticle is typically not dissolved under the normal sulfide con-centration and time conditions of unhairing.8 The scales flake off from thedegraded cortex, to remain as ‘scud’, suspended in solution.

4. The medulla is not attacked by the unhairing chemicals (but it is attackedby proteolytic enzymes if they are present), so it merely breaks off, wea-kened by the hydrolytic effect of high pH.

Figure 5.7 presents the basis of the chemical mechanism, which can besummarised as follows:9

� The reducing nucleophilic sulfide ion attacks the disulfide bond of cystine.� The disulfide link is broken, creating a cysteine moiety and an anionic

cystine disulfide moiety.� The disulfide moiety is attacked by sulfide and is reduced to cysteine.� The sulfide is converted into polysulfide.

The polymerisation of sulfur will not progress much further, because thebisulfide ion is a weaker nucleophilic than sulfide and therefore is less effectivein attacking the disulfide link. This can be seen in Table 5.4, which sets out theredox potentials.10

The presence of lime in the reaction is critical: its function is to buffer thesystem, so that the equilibrium between the sulfide species is controlled infavour of sulfide over hydrosulfide (Figure 5.8) since hydrosulfide does notunhair.11 Calcium hydroxide buffers by virtue of its limited solubility. At 20 1Cthe solubility is about 1.6 g l�1, so the hydroxyl concentration is 0.043M, cor-responding to pH12.6.While sulfide is still used in hair burning technology, it is very important for the

tanner to be aware of these equilibria, because they determine what is in solutionat any given pH; in particular, which species are present at pH values of sig-nificance in the beamhouse processes. Within these considerations, the conditionsthat might lead to the production of hydrogen sulfide gas are critical for safety.Note, H2S attacks the nervous system – it is more toxic than hydrogen cyanide,

119Unhairing

HCN. If the pH is allowed to fall, there is loss of sulfide in creating hydrosulfideand the unhairing reaction will slow down and ultimately stop.The theoretically required amount of sulfide can be calculated as follows:

Cystine content of hair ¼ 50mmol per 100 g

CO

CH

NH

SS

NH

CH

CO

NH

CH

CO

NH

CH

CO

NH

CH

CO

CH2 CH2 CH2 SH

NH

CH

CO

[H]

[O]

CH2CH2

CO

CH

NH

S

NH

CH

CO

S2-

nucleophilicattack

CH2 S

S2-

-S

-S

CH2 S- + S22-

polysulfide

cystine cysteine

S

Figure 5.7 Chemical mechanism of hair burning.

Table 5.4 Redox potentials for sulfide species.

Species Redox potential, E1 (mV)

H2S"S+2H1+2e� �0.142HS�+OH�"S+H2O+2e� +0.478S2

2�" 2S+2e� +0.428S2�"S+2e� +0.476

120 Chapter 5

If the requirement for hair burn is an equimolar offer of sulfide this equals50mmol S2� per 100 g of hair¼ 3.9 gNa2S per 100 g of hair.If the hair is 10% of the hide weight, then the sulfide requirement is 0.4%

Na2S� 0.6% sulfide flake, where ‘flake’ is industrial sodium sulfide, 70% Na2S.The typical industrial offer is 2–3% flake, i.e. 4–5�more than theoretically

required.The rationale for the excess can be expressed as follows:

1. Loss from oxidation: sulfide species are oxidised in the presence of atmo-spheric oxygen, although the rate is not fast enough to use up a significantproportion of the total offer within the timescale of typical unhairing.

2. Errors in calculation, particularly the assumption of weight of hair, whichcan vary seasonally.12

3. The rate of reaction depends on the sulfide concentration and the extent ofthe reaction depends on rate multiplied by time: the available time is fixed,so the extent of the hair dissolving reaction is controlled by the rate ofreaction. Since the available time is controlled by production conditions,the outcome must be controlled by the chemical conditions.

It is worth reflecting on the contention that using sulfide is the techno-logically safest of the traditional hair burning processes. Unhairing can be

Figure 5.8 Relationship between sulfide species as a function of pH.

121Unhairing

achieved with alkali alone (see below) but it was recognised long ago that therate of hair dissolution could be increased by the addition of so-called ‘shar-pening’ agents, of which sodium sulfide was one. In the Victorian leatherindustry it was not uncommon to use other reagents, such as sodium cyanideand arsenic sulfides.1

It is common practice in the industry to use a mixture of sodium hydrosulfideand sodium sulfide. The technological reason is not clear, so it is useful tocalculate the effects of varying the composition of the unhairing solution interms of the ionic content (Table 5.5). In Table 5.5 it is assumed that there is asimple titration of the hydrosulfide by the lime, to create the sulfide tohydrosulfide ration corresponding to the lime buffer pH.The simplistic use of weights to calculate the offers means there is a difference

in the total sulfide content, which could easily be adjusted for parity of thesulfide concentration. However, two points emerge from the analysis:

1. Equal weight substitution of sulfide by hydrosulfide results in a lowerconcentration of sodium ions.

2. Substituting sulfide by hydrosulfide results in a much higher concentrationof calcium ions in solution. Since the lyotropic effect of calcium ion isgreater than that of sodium ion, it can be deduced that the sulfide/hydrosulfide mixture will contribute to enhanced opening up of the fibrestructure due to greater damage to the hydrogen bonding in the pelt.

5.3.1 Role of Swelling

In hair burning, the pelt will be swollen by the alkaline conditions (Chapter 6).This can contribute to a problem if the rate of swelling is faster than the rate ofhair degradation. Consider the following sequence of events:

1. The pelt swells quickly. The outcome is that an additional portion of thehair shaft becomes hidden by the follicle in the swollen grain layer.

2. Degradation of the hair proceeds, from the tip down to the surface. At thesurface, the rate slows because of the reduced surface area, so the hair is

Table 5.5 Ionic content of unhairing liquors.

ProcessConcentration in 200% float (molar)

Ca21 Na1 S2� HS� [S]total

2% lime a

2% sulfide b 0.022 0.26 0.19 0.064 0.13

2% lime1% sulfide1% hydrosulfide c 0.055 0.22 0.12 0.038 0.089aSolubility is 1.6 g l�1.bRatio [S2�] : [HS�] taken as 3 : 1.cOffers are typically based on convenient weight basis rather than molar ratios.

122 Chapter 5

degraded only a little way into the follicle. Also, by this time, the rate ofkeratin degradation has slowed as sulfide is used up. The net effect is thatthe amount of hair shaft degraded is less than it would be if the grain hadnot swelled as quickly.

3. In deliming, when the pH is lowered, the pelt is depleted. Once thedepletion is complete, some of that hair trapped by the swelling nowprotrudes from the mouth of the follicle. If it is visible, as it would be if itis coloured, there is a loss of value, because the grain may have to becorrected (buffed to remove faults).

4. If the result of a mismatch of swelling and hair burning rates is that hair isleft at or below the mouth of the follicle, then dyeing will be affected. Thesurface of the leather has effectively become a combination material, partcollagen and part keratin. The dyeing characteristics of these two proteinsare different. Under the usual conditions of chrome leather dyeing, keratinis less reactive than collagen, so the colour struck is typically paler than itshould be.

The problems associated with swelling in hair burning can be overcome bythe so-called Herfeld drum painting method.13 The process can be summarisedas follows:

� soak� drain� 0.75–1.0% sodium hydrosulfide flake (70% NaHS)� 0.25% surfactant� run 15min, stand 15min, to give pH 11.0� add 2.5% sodium sulfide flake� run 150 min intermittently, to give pH12.5� check hair pulping� add 250% water at 25–28 1C� add 3% lime� run conventionally for about 15 hours.

The key to this technology is that the hair burning is conducted in the mini-mum amount of water, therefore the pelt cannot swell: swelling can only occur inthe presence of water to plump up the structure, regardless of the chemical statusof the fibre structure. Hair burning proceeds below the grain surface, part wayinside the follicle. If the pelt is not swollen, then the residual hair cannot protrudefrom the mouth of the follicle, as it might if swelling occurred before hair pulpingis completed. In this way, the grain quality is not compromised by visible hair.However, tumbling hides in a drum without water can cause grain abrasion,particularly under alkaline conditions, so this process is rarely used.

5.3.2 Chemical Variations

5.3.2.1 Lime-free Hair Burning. The calcium hydroxide in conventional hairburning can be substituted by sodium hydroxide, to maintain the equilibrium

123Unhairing

of the sulfide species and the rates of alkaline hydrolysis reactions. However, asa strong base, the pH will tend to be high unless controlled by dosing, therebyrunning the risk of over swelling the pelt, i.e. creating irreversible swelling(Chapter 6). It will not have a buffering effect, like lime. Unhairing can beconducted with sodium hydroxide alone, so-called ‘caustic soda unhairing’.This has been used commercially, but the requirement for pH 4 13 means therisk of over swelling is very great.

5.3.2.2 Sulfide-free Hair Burning. Other reducing agents have been proposedand a comparison of the reducing power of candidate reagents is presented inTable 5.6.10

One commercial alternative is to use amines. They have been used success-fully,14 but then fell into disuse when it was found that they could be convertedinto nitrosamines, which are potent carcinogens.15 Thioglycolate is a viableoption;16 although not established in the leather industry, it is commonly usedin cosmetic depilatory formulations.

5.4 IMMUNISATION

Under conditions of moderate alkalinity, pH 11 and above, e.g. in lime solu-tion, another reaction can take place with keratin. Unlike the degradationreaction, which commences with sulfide attack at the disulfide bond, this isbased on hydroxyl ion attack at the methylene group on the cystine sidechain.The result is the creation of cysteine and serine: the latter loses water to formdehydroalanine. The products can then react to form a new crosslink,methionine.16,17 The overall effect is for the disulfide link to become a singlesulfur or thioether link: since the thioether link is much less susceptible toattack and scission by reducing nucleophiles, the keratin becomes resistant toconventional hair burning and is said to be immunised against degradation.Figure 5.9 shows the mechanism.An accompanying reaction is between dehydroalanine and lysine, to create

lysinoalanine:

R2C ¼ CH2 þH2N-ðCH2Þ4-RÐR2CH-NH-ðCH2Þ4-R

Table 5.6 Redox potentials of some unhairing agents.

Chemical FormulaE1 (mV) atpH12

Thiourea dioxide (sulfinic acid) (H2N)2C¼SO2 610Sodium sulfide Na2S 550Thioglycolic acid HSCH2CO2H"�SCH2CO2

� 410Cysteine H2N-CH(CH2SH)-CO2H"

H2N-CH(CH2S�)-CO2

�400

Sodium dithionite Na2S2O4 220

124 Chapter 5

The chemistry of immunisation is exploited in some hair saving processes (seebelow). Tanners may experience the implications of immunisation if they makethe mistake of adding the lime before the sulfide. The result is difficulty orinability to remove the hair by conventional hair burning: using high con-centrations of unhairing agents is likely to damage the grain. The only optionopen is to shave the hair and burn the remainder conventionally (see below).

5.5 HAIR SAVING

As the term hair saving indicates, there is the possibility of removing the hairintact, i.e. saving the hair structure. All these processes target the structure ofthe hair within the follicle, either the bulb or the prekeratinised zone.2 The effectis to ensure detachment of the hair from the base of the follicle: the hair may bepartially degraded, reducing the frictional forces holding the fibres within thefollicles. Some technologies rely on immunisation to protect the hair staplefrom degradation. Probably the best known method in the industry is theSirolime process, developed at CSIRO in Australia.17 The process can besummarised as follows, with comments in square brackets:

� soak: impregnate with sodium hydrosulfide for 2 hours; the pH falls from11.2 to 8.5 [NaHS is not an unhairing agent, so there is no keratin damageas the reagent penetrates the cross section, to reach the base of the follicle.]

� drain [Now the only sulfide in the system is within the pelt.]

CO

CH

NH

SS

NH

CH

CO

NH

CH

CO

NH

CH

CO

CH2 CH2

OH

CH2CH2

CO

CH

NH

S

NH

CH

CO

CH2-S

S-

CH2S

-OHnucleophilicattack

CH2

CO

CH

NH

CO

CH

NH

serine

lanthionine

loss ofwater

loss of sulfur

cysteinedehydroalanine

Figure 5.9 Mechanism of immunisation.

125Unhairing

� add calcium hypochlorite, run 5 min [Any residual sulfide on the outside ofthe pelt is oxidised, to prevent reaction on the hair when the unhairingagent is activated.]

� add lime, raise pH to 12.5 [At this point the hair may be immunised: this isnot strictly required, but it is not counterproductive in the process. Rapidpenetration by hydroxide ion from the flesh side – the epidermis remains abarrier – causes the hydrosulfide to be converted into sulfide, so attackoccurs at the prekeratinised zone to sever the link between the hair and thefollicle. The hair is loosened after an hour.]

� run to remove the hair and filter the float [There is no mechanism by whichthe hair will come out of the follicle of its own accord – it is held in place byfriction. Therefore, the rubbing action of the pack is needed to pull thehairs out. This is an effect that depends on the hair staple remaining intact:it does not apply to residual hair in a hair burning process.]

� relime [There are always short hairs remaining after hair save processing –it is a measure of the success of a technology how much reliming, typicallywith sulfide, is required after hair saving to dissolve residual hair.]

The importance of accurate pH control at the stage of applying the hydro-sulfide was emphasised during a demonstration of the technology in NewJersey, USA in 1989, attended by the present author. The process had beenhalted at this point for inspection, when the technologists and scientist presentbecame aware of a peculiar and unpleasant odour, which no-one could identify.Then one of the observers became temporarily incapacitated by breathing ahigh concentration of the gas at the open door of the processing vessel. It wasassumed that the culprit must have been hydrogen sulfide, even though it isnoteworthy that the smell was not one with which the experienced practitionerswere familiar. Nor was it the odour associated with the beginnings of anaes-thetisation of the olfactory nerves, a smell much more sour than the sweeterodour of hydrogen sulfide at low concentration. A curious incident which, ifnothing else, emphasises the dangerous nature of hydrogen sulfide.

5.6 VARIATIONS IN UNHAIRING TECHNOLOGIES

5.6.1 Heidemann’s Darmstadt Process

This technology was developed as part of a rapid processing approach totanning: the objective was to produce wet blue in 6 hours from raw.18 Themethod does work, although the quality of the product is not ideal, as may bejudged by the absence of process steps that would allow transport of unwantedcomponents out of the pelt. The process can be summarised as follows, withcomments in square brackets:

� soak/wash [In this technology it is essential to remove the dung: here, thisis a more important step than green fleshing, since dung can protect thehair from chemical degradation.]

126 Chapter 5

� green flesh [Necessary to ensure sulfide penetration, as part of the openingup process.]

� hang hides over bars� submerge in 10% sodium sulfide solution or spray sulfide solution on the

hair side [Time in sulfide must be limited, otherwise hair accumulates in thesolution. Some swelling occurs – 4–6% on hide weight.]

� mechanically press to remove excess sulfide� hang for 10min� scrape off epidermis and hair� hang in 10% sodium peroxide solution or spray on the solution [Con-

ventional times are: heavy ox 60min, calfskin 35min, goatskin 25min.There is an exothermic reaction that removes residual hair and epidermis,has a bleaching effect, opens up the fibre structure. Recycling is limited toabout 50 times, due to build up of sodium sulfate.]

� mechanically press to remove excess sodium peroxide� neutralise surface to pH5.0 over 5min with 10% solution of ammonium

sulfate and hydrochloric acid� clean and split� complete neutralisation� pickle conventionally for 1 hour� chrome tan in concentrated 50% basic chromium(III) sulfate� hot age [Boil fast leather is achievable.]

5.6.2 Oxidative Unhairing

This is a technology that has been used commercially, based on the productsodium chlorite, sold as Imprapell CO. It is conducted under acid conditions, asfollows.Note, this reaction involves the changing of an oxidation state to both higher

and lower oxidation states, known as disproportionation:

4ClðIIIÞO�2 þ 2HþÐ2ClðIVÞO2 þ ClðVÞO�

3 þ Clð�IÞ� þH2O

4R-S-S-Rþ 10ClO2 þ 4H2O Ð 8R-SO3� þ 8Hþ þ 5Cl2

In terms of dangerous chemistry, this is about as bad as it gets. The fire riskof solid sodium chlorite is exchanged for the toxicity of chlorine dioxide gas,which is the actual unhairing reagent, and then one of the products of thereaction is chlorine, used in the past as a chemical weapon. Because of thestrong oxidising chemicals involved, the process cannot be conducted inwooden vessels, so stainless steel vessels have to be employed.Other oxidising agents that have been proposed are hydrogen, sodium and

calcium peroxides and peracids, e.g. peracetic acid, perborate, hydrogen per-oxide with potassium cyanate or urea.19–21 In the latter case, the oxidising agentis assisted by the action of hydrogen bond breakers. Although options fre-quently appear in the literature, and claims of safe operation and technologicallyfavourable outcomes have been made, none has obtained industrial acceptance.

127Unhairing

5.6.3 Reductive Unhairing

This chemistry of reduction can be used in principle, but has not found com-mercial acceptance. The mechanism can be summarised as follows:

BH�4 þ 3H2OÐ4H2 þ BOðOHÞ�2

R-S-S-RþH2 Ð 2R-SH

The reaction may be undertaken with sodium borohydride or lithiumaluminium hydride (BLC The Leather Technology Centre, unpublishedresults). In either case, the reactants are pyrophoric, which means they willspontaneously combust in air, thereby posing a fire hazard. As with artificialageing of pelt (Chapter 2), the principle is proved, but the technology isundesirable.

5.6.4 Acid Unhairing

The technology of exploiting the lyotropic effect for unhairing was developed inAustralia for fellmongering sheepskins.1,22,23 Concentrated acetic acid and salt isapplied on the flesh side of fresh skins (the process does not work on dried orsalted pelts): overnight the wool is loosened and the epidermis separated fromthe grain by the combined actions of the acetic acid and the autolytic enzymeswithin the pelt. Heidemann reported that the same reaction could be achieved bythe in situ fermentation of lactobacillus for cattle hides, his so-called ‘sauerkraut’process: here the active agent is lactic acid, which loosens the hair and destroysthe basement membrane, allowing the epidermis to be removed almost intact.24

5.7 ENZYMES IN UNHAIRING

5.7.1 Enzyme-assisted Chemical Unhairing

Conventional hair burning can be accelerated by the presence of so-called‘alkali stable’ proteolytic enzymes, obtained from bacterial fermentation.In fact, such enzymes are no more stable in alkali than any other, but theiractivity profile indicates they have significant activity at high pH, so they can beused in liming.25 When protein has been chemically damaged, proteasewill accelerate the breakdown. This means keratin breakdown is faster, as isthe degradation of the non-structural proteins as well as the collagen itself.Hence, there is a danger of loosening the corium structure if the reactions gotoo far. This makes the process high risk. The outcomes of this process are asfollows:

� cleaner grain, better for dyeing;� more opening up, so the leather is softer;� allows a reduction in sulfide offer or a reduction in process time;� removes the need for a bating step.

128 Chapter 5

A typical feature of these bacterial enzymes is that they exhibit elastolyticactivity, i.e. they degrade elastin. This changes the characteristic of the leatherand produces additional outcomes:

� growth (ripples in the skin, particularly in the neck) is removed, so theleather is more uniformly flat;

� the skin is thinner, with greater surface area, because the elastin isdegraded;

� strength per unit thickness is increased;� leather is less stretchy.

These changes are clearly only suitable for some leathers, such as upholsteryor clothing, where softness and drape are useful properties. Leathers that requirestiffness, stretch or resilience, referred to in the jargon as ‘stand’, such as shoeupper leather for structured shoes, would not benefit from this biotechnology.

5.7.2 Chemical-assisted Enzyme Unhairing

This a more speculative application of enzyme technology, developed byBLC,22 a version of hair saving, which has not been used in industry to date.The argument is that the shortest way to the prekeratinised zone is through thegrain. However, the epidermis of the pelt, as in life, is a barrier to the pene-tration of chemicals, and microorganisms and their products. Therefore, theapproach here is to damage the epidermis sufficiently to allow a proteolyticenzyme to penetrate to the base of the follicles. The process can be summarisedas follows (comment in square brackets):

� conventional soak� disrupt the epidermis with conventional unhairing agent, e.g. sulfide [This

is a short process, taking only a few minutes, that exploits the reactivitydifference between the epidermis and the hair. If the time is short, the hairis relatively unaffected.]

� drain the solution for recycling� apply alkali stable enzyme� run and filter loosed hair� relime for short hairs.

This too is a high risk process, which requires a high level of process control.It may also cause looseness in the leather because of the use of alkali stableenzymes with their elastolytic action.

5.7.3 Enzyme Hair Saving

An alternative approach to the use of enzymes is to attack another feature ofthe hair structure in the follicle, namely the epidermis. The argument is that bydegrading the structure by which the epidermis is attached to the dermis (grainenamel), the basement membrane, the hair will also be removed. One of the

129Unhairing

components in the basement membrane is collagen type VII, which can bedegraded by dispase, a mixture of proteolytic enzymes. Trials showed that thehair could be loosened and removed without causing associated damage tothe fibre structure. Dispase also has elastolytic activity, hence the elastincontent of the pelt was also degraded.26

5.7.4 Keratinase

Since the hair is largely composed of keratin, it might seem obvious to attack itwith keratinase enzymes. However, there has been a traditional reluctance touse these reagents in industry because of the potential danger to operatives,associated with possible degradation of the epidermis. Recent work hasdemonstrated the feasibility of this biotechnology for removing residual hairfrom wet blue leather,27 but the idea has not been pursued.

5.8 PAINTING

For the special case of woolskins, the technology of painting has been devel-oped. The idea is to drive the unhairing, more strictly called fellmongering,from the flesh side of the pelt. In this way, the chemicals do not make contactwith the wool and devalue it. In these circumstances, although prices fluctuate,it is often the case that the wool is more valuable than the pelt. Table 5.7summarises paint compositions.28

The paint is mixed and, importantly, left overnight to thicken – this is afunction of the lime content. (Note, if lime is mixed with a limited amount ofwater, it will thicken to a paste, which can be used as a cement for building –known as lime mortar, used for centuries before the advent of modern cement,which is based on calcium sulfate hemihydrate.) The following day, the paint isapplied to the flesh side of the pelt: this may be done by hand or mechanically.The pelts are then stacked, flesh to flesh for several hours, usually overnight.The following day the pelts are ‘pulled’: this means the wool is removed, eitherby hand or by machine. If the reaction has been successful, the wool is wiped offthe pelt with ease. Processing continues with a conventional liming andbeamhouse processing.

� The principles underpinning this technology are as follows:� The paint is thixotropic: it can be sprayed onto the pelt, but does not run

off. A Newtonian liquid is one in which the resistance to a shear force(stirring) is proportional to the force, i.e. the faster you stir, the harder the

Table 5.7 Paint compositions.

Typical (g l�1) New Zealand Quikpul (g l�1)

Sodium sulfide flake (64% Na2S) 100–150 185Calcium hydroxide 400–600 200Sodium hydroxide 30

130 Chapter 5

work becomes. Alternatively, a thixotropic liquid is non-Newtonian, i.e.the resistance to a shear force is inversely proportional to the force. Thatmeans if the liquid is semi-solid, a gel, then stirring it makes it runny, butas soon as the stirring stops it becomes a gel again: this is the principle ofnon-drip paint.

� The concentration of sulfide ion is high enough to penetrate the pelt bydiffusion alone in the time allowed, because there is no mechanical actionto drive penetration.

� Sulfide reacts at the prekeratinised zone of the wool shaft, to degrade thekeratin and detach the wool in a hair saving process.

� The pH of the system is maintained by the buffering action of the lime.Therefore, the sulfide to hydrosulfide ratio is maintained for maximumunhairing rate and alkali-catalysed hydrolytic reaction can occur withinthe pelt, for opening up.

� The concentration of water is low, hence swelling is minimised.

The paint composition can vary as follows:

1. Replace the sulfide with another unhairing reagent, e.g. thioglycolate, �S-CH2-CO2

�. Thioglycolate is marginally less effective than sulfide, in partbecause in trials the workers did not trust it because it had a different smell(A.D. Covington, unpublished results).

2. Replace the lime with another thickener, e.g. synthetic polymer, togetherwith another alkaline compound, e.g. sodium hydroxide.

The use of unhairing paint is not confined to woolskins: it can be used for anythin rawstock, which allows the reagents to penetrate through the cross sectionin the available time. Hence, it has been used commercially for immature skinssuch as calf, so that the liming reaction is controlled. Another useful applica-tion is for pigskins, in which the shell region of the skin is particularly tough.However, if the pelt were to be conventionally limed enough to soften thisregion, the rest of the pelt would become over limed. The solution is to treat theflesh side of the shell areas with a lime-sulfide paint, leave overnight, then limein the normal way. In this way, only the hard shell is doubly limed and henceselectively opened up and softened.

5.9 ROLE OF SHAVING IN UNHAIRING

The simple act of cutting the hair completely changes the technology of hairremoval. El Baba et al.5,8 showed that removing most of the hair from cattlehide with electric clippers (therefore not a particularly close shave) was enoughto reduce the COD and suspended solids to nearly half the levels obtained byconventionally hair burning the full staple. The hair cutting allows a reductionin the sulfide used, which can be compounded by including alkali-stableenzyme, to accelerate the breakdown of degraded hair. Because the hairburning is initiated closer to the pelt surface, the reaction proceeds further

131Unhairing

down the hair shaft than in the conventional process, resulting in less hair closeto the surface. Closer examination of the outcome of hair burning cut hairrevealed a new mechanism of unhairing. Figure 5.10 illustrates the outcome ofhair burning technology applied to cut hair: the reaction is concentrated in thecortex, effectively hollowing out the hair, and is accelerated by the presence ofproteolytic enzyme, which targets not only the degraded keratin but also theintact medulla.The intact cuticle is not strong enough to resist collapse, which reduces the

friction with the follicle wall. Since the hydrolytic attack reaches the base of thefollicle, there is no anchoring mechanism and the residue of the hair can besqueezed out by normal mechanical action. In this way, the outcome is grainquality normally associated with hair saving technologies, but obtained withsimple hair burning technology.

REFERENCES

1. C. A. Money, J. Soc. Leather Technol. Chem., 1996, 80(6), 175.2. W. Frendrup and J. Buljan, UNIDO, US/RAS/92/120, 1998.3. K. Bienkiewicz, Physical Chemistry of Leather Making, Krieger, Florida,

1983.4. P. Hallegat, R. Peteranderl and C. Lechene, J. Invest. Derm., 2004, 122,

381.

Figure 5.10 Residual hair, showing the loss of cortex and medulla, but remainingcuticle. (Courtesy of Journal of the Society Leather Technologists andchemists.)

132 Chapter 5

5. H. A. M. el Baba, A. D. Covington and D. Davighi, J. Soc. LeatherTechnol. Chem., 1999, 83(4), 200.

6. F. O’Flaherty and W. T. Roddy, J. Amer. Leather Chem. Assoc., 1932,27(4), 126.

7. A. Onyuka, PhD Thesis, The University of Northampton, 2009.8. H. A. M. el Baba, A. D. Covington and D. Davighi, J. Soc. Leather

Technol. Chem., 2000, 84(1), 347.9. S. H. Feairheller, et al., J. Amer. Leather Chem. Assoc., 1972, 67(3), 98.

10. Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 1987.11. M. Kaye and R. M. Marriott, Int. J. Soc. Leather Trades Chem., 1925, 591.12. R. M. Hayman, in: Biology of the Skin and Hair Growth, eds. A. G. Lyne,

B. F. Short, Angus and Robertson, 1965.13. H. Herfeld and B. Schubert, Das Leder, 1963, 14, 77.14. I. C. Sommerville, et al., J. Amer. Leather Chem. Assoc., 1963, 58(5), 254.15. D. G. Bailey, et al., J. Amer. Leather Chem. Assoc., 1982, 77(10), 476.16. C. S. Cantera, J. Soc. Leather Technol. Chem., 2001, 85(2), 47.17. R. W. Cranston, M. H. Davies and J. G. Scroggie, J. Amer. Leather Chem.

Assoc., 1986, 81(11), 347.18. E. Heidemann, Fundamentals of Leather Manufacturing, Eduard Roether,

Darmstadt, 1993.19. S. Vitolo, et al., J. Amer. Leather Chem. Assoc., 2005, 100(2), 45.20. W. N. Marmer and R. L. Dudley, J. Amer. Leather Chem. Assoc., 2004,

99(9), 386.21. K. Vehara, et al., J. Amer. Leather Chem. Assoc., 1986, 81(10), 331.22. J. R. Yates, Aust. J. Biol. Sci., 1968, 21, 1249.23. C. A. Money and J. G. Scroggie, J. Soc. Leather Trades Chem., 1971,

55(10), 333.24. L. Schlosser and E. Heidemann, Das Leder, 1986, 37, 33.25. K. T. W. Alexander, J. Amer. Leather Chem. Assoc., 1988, 83(9), 287.26. R. G. Paul, I. Mohamed, D. Davighi, V. L. Addy and A. D. Covington,

J. Amer. Leather Chem. Assoc., 2001, 96(5), 180.27. A. D. Covington, C. S. Evans, F. Foroughi UK Patent Appl. GB

0519172.1, Sept. 2005.28. M. Dempsey, et al., J. Soc. Leather Technol. Chem., 1978, 62(5), 108.

133Unhairing

CHAPTER 6

Liming

6.1 INTRODUCTION

In typical processing, when the hair is removed by dissolving it with a solution ofcalcium hydroxide and sodium sulfide, the liming step is not separated fromunhairing. The unhairing step is dominated by the reaction of sulfide and couldbe accomplished in the absence of lime: typically, unhairing commences withsulfide alone to avoid the possibility of immunising the keratin, but lime is soonadded and so the combination of steps merely extends the period of treatmentwith the two reagents. The hair is usually dissolved completely within a fewhours, often 2–4 hours, then the liming is continued usually to a total of 18 hours.The value of using lime in this process step is that the pH is controlled only by thesolubility of the sparingly soluble alkali: excess is routinely used, to allow forvariations in conditions, so the solution and hence the process are self-regulating.A feature of the conventional liming process is the accompanying swelling of

the pelt, due to the high pH (Figure 6.4 below). It should be recognised that theswelling typically achieved in industrial processing is less than the swelling thatmight be achieved if the pelt were allowed to swell to equilibrium. If the pelt isactually allowed to swell to the maximum extent that the conditions allow,there would be structural damage and the swelling would not be completelyreversible. The extent of swelling in conventional liming is restricted by con-ducting the process under the conditions set out below.

6.1.1 Float

Typically the float length is 200% on raw weight, sometimes more but rarelyless. In this way, the pelts will tumble over and around each other, withoutcreating the pressure/squeezing type of mechanical action characteristic ofshorter float processing. Also, using a relatively long float minimised the pos-sibility of abrasion damage to the grain. The minimum mechanical action is




134

provided by the process vessel, either by processing in paddle driven vats or byrunning conventional drums slowly and usually intermittently.

6.1.2 pH

Amide (peptide) hydrolysis is general base catalysed1 and to have a sufficientlyfast reaction for industrial processing conditions the pH must be 12–13. If thepH is lower, the unhairing chemistry does not work, because the equilibriumbetween the non-unhairing hydrosulfide ion and the unhairing sulfide ion isunfavourable. Moreover, the liming rate would be slow because the hydroxylion concentration is reduced: a reduction of one pH unit means a tenfolddecrease in hydroxyl ion concentration. At higher pH, the rate is faster, but thedanger of over-swelling is greater, since swelling is pH dependent.The theoretically required amount of alkali can be calculated as follows. To

start with it is necessary to refer to the titration curve for collagen, given inFigure 1.19 in Chapter 1:2

100 kg of hide contains about 40 kg of collagen and 60 kg of water. FromFigure 1.19, collagen at neutrality requires about 0.4 moles NaOH foradjustment to pH12.5. Therefore, if the hide contains 40 kg of collagen, itrequires 0.64 kg NaOH.At pH 12.5, [OH�]¼ 0.03M¼ 1.26 kgNaOH per litre.Therefore, 60 kg of moisture requires 0.08 kg NaOH.For 100 kg of pelt, a 200% process float requires 0.25 kg NaOH.For the sulfide contribution, from Figure 5.8, the following equilibrium applies:

Na2SþH2Oð3ÞÐNaHSð1Þ þNaOHð1Þ

where the numbers in brackets refer to molar ratios.If the minimum sulfide offer is 0.4% Na2S, as discussed above, this is 0.4 kg

Na2S per 100 kg of pelt (¼ 5moles).If, at pH 12.5, [S2�] : [HS�]¼ 3 : 1, the amount of NaOH in the system is

1.25moles (¼ 0.05 kgNaOH).From the ratios, the total amount of sulfide in the system is 3 moles of sulfide

plus one mole of hydrosulfide, i.e. the starting quantity of sulfide is 4 moles not3 moles.This amount of alkali is added by the system, not required by the system, so

this is an amount that partially offsets the overall alkali requirement.Total amount of alkali¼0.64+0.08+0.25 – 0.05 kg NaOH¼0.92 kg NaOH�

0.85% Ca(OH)2 on hide weight.The technological offer is usually 2% on hide weight in 200% float, some-

times more; therefore, the offer is always in excess of the calculated amount.The calculation makes assumptions that may have an associated error. Also,additional requirements, such as the presence of dung, are not included. It isessential for consistent buffering that the solution should be saturated, whichmeans there must be excess lime suspended in the solution: the amount inexcess varies, but larger offers have no effect on the conditions in solution.

135Liming

For example, the Official Method (IUF 151)3 for making standard chrometanned leather (wet blue) specifies the use of 10% lime in 500% float: from theanalysis set out above, this represents an excess of about 90% of the offer. Anyexcess is a cost that creates no benefit, but merely contributes to the suspendedsolids component of the effluent and can act as a damaging abrasive.

6.1.3 Temperature

The temperature of the liming solution does not influence swelling, exceptinsofar as it affects the solubility of the lime and hence may alter the pH of thesolution, but it will control the kinetics of hydrolysis. Since the hydroxyl ion is avery effective catalyst, any acceleration of the rate would increase the sensitivityof the process to cause damage. A feature of this high pH process step is thatthe shrinkage temperature is lowered to 50 1C or less (Figure 6.1).4 Therefore, itis important to control the process temperature to o30 1C, so that it does notencroach within 25 1C of the denaturation temperature.Features to note from Figure 6.1 are as follows:

1. Studies of this kind always measure effects at equilibrium. Practicaltechnology avoids reaching equilibrium, so the actual situation is slightlydifferent from that presented in the graphs – the variations in industrydepend on the specific conditions used.

2. The hydrothermal stability of collagen is very sensitive to extremes of pH.3. The presence of salt influences both swelling and hydrothermal stability,

discussed below and in Chapter 9.

6.1.4 Time

The longer the process goes on, the greater the degree of hydrolysis and hencethe greater is the amount of damage to the protein components of the hide orskin. Therefore, limits are typically set to the time allowed for this step. Formedium ox hide, the normal liming period is 18 hours, with commensuratelyshorter periods for more vulnerable skins. Thicker or tougher rawstock mayneed longer, e.g. bull hides and goatskins.The swelling reaction itself is a rate process, but typically rapidly reaches a

kind of steady state, which is dependent on the conditions, particularly thedegree of mechanical action. Therefore, this is strictly not a true equilibrium,because an increase in the level of mechanical action would change the swelling.Therefore, the swelling can be assumed to be complete during the usual periodof liming, restricted by controls imposed by the liming conditions.

6.2 PURPOSES OF LIMING

The purposes of liming are set out below, which together constitute controlleddamage to the pelt and to the collagen in particular. This is illustrated in Table 6.1,showing the time-dependent changes in bovine pelt upon alkali treatment.5

136 Chapter 6

70

60

50

40

30

80

60

40

20

0

20 2 4 6 8 10 12

A

A

BB

A

B%

Sw

ellin

gS

hrin

k Te

mpe

ratu

re °

C.

pH

Figure 6.1 Effect of pH on hydrothermal stability: (A) without salt, (B) with 2M

sodium chloride. (Courtesy of the Journal of the American Leather Che-mists Association.)

Table 6.1 Analysis of bovine hide, treated with lime alone at ambient tem-perature (see text for details).

Time in lime (h) 0 6 18 336Non-collagenous protein, fat, salt (%) 14 5 6 5Amide N : collagen ( relative %) 100 64 68 49Isometric tension: To (1C) 67 62 61 61Tm (1C) 96 89 91 84Fm (gmm�2) 3.5 2.1 1.8 0.6

137Liming

From Table 6.1, there is relatively rapid amide hydrolysis during the first6 hours of liming. From isometric tension measurements6 (Figure 6.2), in whichthe specimen is heated while clamped under tension, the hydrolysis is accom-panied by a rapid destabilisation of the collagen, as suggested by the loweringof the shrinking onset temperature, To, which is related to the conventionalshrinkage temperature. This destabilisation is reflected by the drop in thetemperature at which maximum force in generated during shrinking, Tm,associated with a decrease in the energy of the crosslinks, and by the maximumforce generated during shrinking, Fm, related to the number of crosslinks.Notably, most changes to pelt that relate to structure appear to be achieved inthe first six hours of liming.In reviewing the totality of the beamhouse processes it will be clear that the

properties desired in the final product are largely conferred by the beamhouseoperations in general and by the liming process in particular. The effects are asfollows.

6.2.1 Removal of Non-collagenous Components of the Skin

The hydrolytic action is aimed mainly at the non-structural proteins, thealbumins and globulins within the fibre structure. The degrading effect of theliming step on the non-collagenous protein is so effective that it dominatesthe opening up processes. However, notably, removal of the fragments barelytakes place during liming. This is because the trend of water movement is fromthe bulk solution into the pelt. The usual rinsing mechanism of squeezing andrelaxing the pelt is not available during liming, because that is also themechanism of swelling. Furthermore, the swollen nature of the pelt makes itless likely that the degraded protein would diffuse through the thickened crosssection. Other affected components include keratin, in the form of residual hair

17

15

13

11

9

7

5

3

120 45 70 95 120

Temperature (°C)

Tens

ion

(mv)

Figure 6.2 Hydrothermal isometric tension measurements on bovine hide.

138 Chapter 6

and epidermis: any triglyceride grease present will be degraded in a saponifi-cation reaction (see below).

6.2.2 Splitting the Fibre Structure at the Level of the Fibril Bundles

A feature of the liming process is splitting of the fibres at the level of the fibrilbundles (Figure 6.3). Under the microscope, the splitting can be characterisedas coarse or fine, depending on the degree of the effect.6 Fibre splitting allowsbetter penetration of reagents, for more effective reaction. This is especially truein the case of lubrication (fatliquoring), when fibre splitting is a prerequisite forsoft, strong leather.

6.2.3 To Swell the Pelt

It is received wisdom that a necessary contributor to the opening up process isswelling, when (unspecified) features of the fibre structure are damaged and

A. Not opened up.

If the structure is nottreated with alkali forsplitting at the level ofthe fibril bundles, thefibres remain in a solidform.The effects on theleather are stiffnessand weakness, withthe probability oflooseness in thecorium.

B. Opened up.

After the treatmentwith alkali, the fibresare split at thehierarchical level of thefibril bundles.This allows optimumtanning and posttanning process steps,which should optimisethe physical properties of the leather.

Figure 6.3 Scanning electron photomicrographs of the effects of liming on fibresplitting. (Courtesy of the Journal of the American Leather ChemistsAssociation.)

139Liming

chemical bonds are broken. This view is not unreasonable, since the effect ofwater is to push the structure apart, a splitting effect.Pelt can be swollen by three mechanisms:

1. Charge effects, based on breaking salt links and creating charge in theprotein structure.

2. Osmotic swelling, caused by the imbalance between the ionic concentra-tion outside the pelt and inside the pelt.

3. Lyotropic swelling, caused by disruption to the structure by species thatcan insert into the hydrogen bonding.

In the case of alkali swelling, the mechanisms operating are charge effects andlyotropy. The effect of alkali on collagen is to break the natural salt links, tomake the protein anionic:

�CO�2 � � �H3N

þ �ÐOH�

� CO�2 þH2N�

The repelling effect of the anionic centres causes the structure to open up,allowing water in, which is observed as swelling. In breaking the salt links thereis no electrolyte bound by the collagen: there is no imbalance in electrolyteconcentration between the inside and the outside of the pelt, therefore, theosmotic mechanism does not operate in the same way as it does under acidconditions (Chapter 8). Consequently, salt does not reverse the swelling effectunder alkaline conditions. However, the swelling curve (Figure 6.4) shows thatsalt does have a small effect on the degree of swelling. This is the same effect

800

700

600

500

400

300

200

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Final pH

3% salt

No saltNo salt

3.0% salt

Water uptake g per 100g collagen

Figure 6.4 Dependence of swelling on pH and the effect of added neutral electrolyte,2 M sodium hydroxide.

140 Chapter 6

that salt has on hyaluronic acid removal: the ions effectively smear out anddiffuse the charges, reducing their repelling interactions. In this way, some ofthe swelling is prevented, but the mechanism is not powerful enough to reverseit completely.It is frequently assumed in the technology that large changes in pH carry a

danger for the quality of the pelt and the subsequent leather: moreover, it isassumed that the danger and deleterious effect is exacerbated if the pH changeis rapid. In the case of liming, there is a suspicion that the fast change fromsoaking at neutrality to liming at pH 12.6 cannot be a good thing for the peltintegrity. To test that view, trials were conducted in which the pH was elevatedfrom soaking to liming in stages:

1. Soak at pH 6;2. Add sodium carbonate, to continue soaking at pH8;3. Refloat with sodium hydrosulfide, raising the pelt to pH10;4. Add sodium sulfide, to initiate unhairing at pH13, falling to pH11;5. Add calcium hydroxide, to buffer at pH 12.6.

The trials incorporated all the combinations of rate of pH change, so the ratevaried from as slow as the technology allowed to the maximum rate of change ofthe conventional process. No statistical difference in the properties of the lea-thers was found (A.D. Covington, unpublished results). Whilst there was noevidence from these small-scale trials that rapid pH change made any differenceto the outcome, the intuitive assumption of cause and effect is worth confirming.Some tanners assert that liming can only be conducted in the presence of

lime. In effect, they are saying that the calcium ion is a necessary component ofa liming process. The effect can be modelled by considering the reactionbetween a calcium ion and a hydrogen bond:

4C¼Od� � � �dþ H�Noþ Ca2þÐ4C¼Od� � � �Ca2þ $dþ H�No

Here, the attractive force between the slightly charged negative and positivecentres, which forms the hydrogen bond, is turned into a repulsive forcebetween the positively charged centres. This lyotropic reaction will contributeto the whole swelling effect, which results in the opening up of the collagenstructure.The lyotropic effect of cations can be modelled in the following way:

�C¼Od��dþH�NoÐ� C ¼ Od��Aþ $dþ H�No

Similarly, anions can be inserted into the structure:

�C ¼ Od��dþH�NoÐ� C ¼ Od� $ B��dþH�No

Lyotropy does not always depend on charge, any other species capable ofhydrogen bonding can also act in this way, including electrolytes with

141Liming

significant covalent character, e.g. lithium bromide; Figure 6.5 illustrates theuse of an aliphatic carboxylic acid. The figure demonstrates that unionisedcarboxylic acids can pose a danger to collagen of causing disruption to thestructure, ultimately solubilising the protein.The effectiveness of electrolytes in this reaction depends on the charge to

radius ratio, the charge density or polarising power: the more intense the chargedensity, i.e. smaller ions or higher charge, the better the ion can interact with acharged centre in the protein. The outcome is a solubilising or ‘peptising’ effecton the protein. The effect is analogous to the phenomenon of ‘salting out’,where the solubility of a gas is less in an aqueous solution of an electrolyte thanit is in pure water: this a due to the effect of polarising power on the structure ofthe solvent, when it may be preferentially organised around the electrolyte ions:

log S ¼ 1=2 log So � KI

where the solubility S of a protein in a salt solution is related to the solubility inwater (So) and the ionic strength (I). K is the salting out constant.The ionic strength, I¼1/2

Pcizi

2¼1/2[c1z12+c2z2

2+c3z32+ . . . cizi

2], where:Pis the continued sum, ci is the concentration of the ith ion, zi is the charge on

the ith ion:

ci ¼ aigi

where a is the activity and g is the activity coefficient of the ith ion.The activity coefficients of dilute solutions of strong electrolyte ions are the

same when the ionic strength of the solutions is the same. In practical leatherscience the concentrations of solutions are generally high, usually in the range10�3�1 molal, so that thermodynamic treatments or analysis are not possibleor strictly valid.The relative effects of polarising power are set out in the Hofmeister or

lyotropic series.7 Measurement of the effects of ions can be variable in thiscontext, so the following series represent examples of Hofmeister series, wherethe order is the increasing polarising power, dependent on atomic weight andionic charge:

CsþoRbþoNHþ4 oKþoNaþoLiþoBa2þoSr2þoCa2þoMg2þ

R

C

O H N<COO>C Hδ− δ+ δ− δ+

Figure 6.5 Model of the lyotropic effect with an electrically neutral hydrogen-bondbreaker.

142 Chapter 6

citrateotartrateoSO2�4 ¼ S2O

2�3 oCH3CO

�2 oCl�oNO�

3 oBr�

¼ ClO�3 oI�oCNS�

Combinations of ions can be viewed in the same way, e.g.:

Na2SO4oNaCloCaCl2

Hence, ammonium chloride deliming should produce softer leather thanammonium sulfate, due to the effect of the chloride ion (see also Chapter 7).From Figure 6.4, swelling is pH dependent, with a positive correlation in the

alkaline region of the pH scale. It was shown earlier that the primary cause ofswelling is the repulsive effect of the negative charges generated by the neu-tralising of the basic groups. The pKa of lysine sidechain amino groups inproteins is 9.4–10.6 and the pKa of the sidechain guanidino groups of argininein proteins is 11.6–12.6; therefore, the charge continues to increase up to theend of the pH scale, as indicated in the titration curve.The effect of neutral electrolyte is to reduce slightly the swelling at moderate

alkaline pH values by a charge diffusion effect. However, at higher pH values,the effect of neutral electrolyte is to enhance swelling by the lyotropic influenceof the ions in solution. It is not uncommon for tanners to include salt in the limebath, in the hope of controlling swelling. It can be seen that this practice mayactually be counterproductive, producing the opposite effect to the desiredoutcome. Alternatively, any positive effect is likely to be small. Heidemann hasasserted that swelling may not be a prerequisite of opening up.8 Since thedisruption of the natural structure of pelt is the result of different types ofreaction, it may be a practical proposition, although it has not yet been testedby developing an alternative biochemical process.

6.2.4 Hydrolysis of Peptide Bonds

The liming step is dominated by the breaking of peptide links, i.e. liming doesdamage to protein, but in a controlled way. There is a fundamental difference inthe effect of hydrolysis on collagen and on the non-structural proteins: it is thelatter that must be degraded and removed (to the required degree) by the action ofalkaline hydrolysis. These proteins are linear, although folded, so when a peptidelink is broken the chain is severed into two fragments – continued degradationproduces more and more, smaller and smaller fragments, making their removaleasier and easier. In contrast, if a peptide link in collagen is broken, the effect isnegligible, because the structure is hardly affected, due to the hierarchy of struc-tural elements. This means that many peptide links must be broken before signi-ficant damage is observed. In this way, collagen can resist the effect of hydrolysis,so the non-structural proteins are damaged relatively much more quickly.

6.2.5 Hydrolysis of Amide Sidechains

Amide sidechain hydrolysis occurs in the same way as hydrolysis of peptidelinks; indeed peptide bonds are often called amide bonds, to emphasise the

143Liming

similarity to amide groups. The reaction is as follows:

P-ðCH2ÞnCONH2 þH2OþOH�ÐP-ðCH2ÞnCO�2 þNH3

where, n¼ 1 for asparagine, converted into aspartic acid, and n¼ 2 for gluta-mine, converted into glutamic acid.A byproduct of the reaction is ammonia, which is readily perceived at the end

of the reaction by its smell. This reaction converts the neutral amide groups intocarboxyl groups, which has a major implication for the isoelectric point (IEP).The IEP is defined by the relative numbers of pH active basic and acidic groups.Therefore, the dependency does not include amide groups, because they are elec-trically neutral, but does include carboxyl groups. Therefore, the degree to whichhydrolysis occurs will determine the effect on the IEP. In conventional processing,lasting about 18 hours, around half of the available amide sidechains are hydro-lysed. The deamidation reaction causes the relative proportion of the total carbo-xyl groups with respect to amino groups to increase; this shifts the IEP of collagenfrom physiological pH7.4 to pH5–6, depending on the degree of hydrolysis.In proteins, the hydrolytic reaction of amide sidechains has two possible

mechanisms: direct hydrolysis by hydroxyl catalysis or intramolecular cata-lysed hydrolysis.9,10 Heidemann11 found that prolonged liming changes theconformation of L-amino acids into their D-isomers. In the case of asparaginyland aspartyl residues in peptides and proteins, the mechanism for this race-misation is via a succinimide ring, formed from an asparaginyl sidechain andthe adjacent peptide bond nitrogen in the collagen backbone12 (Figure 6.6).After ring closure, the racemisation at the chiral carbon (indicated by anasterisk) occurs by general base catalysis, removing and replacing the proton.This mechanism, however, is highly dependent upon the rigidity or intactness ofthe collagen backbone: the more degraded the collagen is, and hence moreflexible, the easier it is for the reaction to occur.10 Using molecular dynamics, ithas been estimated that the rate of asparagine deamidation by intramolecularcatalysis at 37 1C would be 104 times slower in an extended a-chain than ina random coil.13 The accompanying reaction, forming the racemised iso-aspartate residues, is likely to lead to localised disruption of the triple helix.10

Total ammonia production results from hydrolysis of asparagine and gluta-mine amide groups and the guanidine groups of arginine: all these reactions cantake place by direct catalysis with hydroxyl ion, at similar rates (Figure 6.7).13

Production of racemised aspartic acid itself can take place by the same intra-molecular mechanism, set out in Figure 6.6, but it is at least an order of mag-nitude slower than the reaction of asparagine hydrolysis, based on peptidestudies.12 Because the formation of a six-membered ring is less favoured than theformation of the five-membered succinimide ring in this reaction, the racemi-sation of aspartic acid is faster than the racemisation of glutamic acid.14,15

In practice, liming typically takes up to 24 hours; it is evident that prolongedliming causes some changes in the collagen structure, but it is still not known towhat extent conformational changes occur within collagen’s three-dimensionalstructure during conventional liming periods.

144 Chapter 6

From Figure 6.7,4 ammonia production conforms to (pseudo) first-orderkinetics during the reaction period of 120 hours and the data fit a rate constantof 8.7� 10�6 s�1 (r2¼ 0.98). The calculated infinity value of 4.7mg% NH3

compares well with the observed yield at 120 hours and with the theoretical

Figure 6.6 Intramolecular-catalysed amide hydrolysis by succinimide ring formation.

145Liming

ammonia content of collagen (4.5mg% on dry weight) calculated from theknown amino acid sequence of type I bovine collagen; the difference inthe values for ammonia content is accounted for by the additional, non-collagenous protein content of the hide. The kinetics predict a half-life of22 hours; this confirms that, during a conventional liming process lasting18 hours, typically about half of the available amide groups are converted intocarboxyl groups.From Figure 6.8.4 the percentage of D-aspartic acid in the limed samples

initially increases and then decreases; this may be explained by all of the non-structural, non-collagenous aspartic acid changing its molecular conformationwithin a period of 18 hours. The rate of racemisation is low during the earlypart of the liming process, increasing only after 24 hours. Some racemisation byacid hydrolysis always occurs during the processing of the samples, and thisexplains why, in Figure 6.8, there is a slightly positive measurement for theuntreated collagen at time zero. From the literature, deamination of sidechainamide groups by the intramolecular mechanism also follows first-order kinetics:in synthetic polypeptide the reaction displays a rate constant of 1.0� 10�6 s�1

at 25 1C, but the observed rate in collagen decreases rapidly below the meltingtemperature,16 and is calculated to be over one thousand times slower.12

Therefore, the observed rate of ammonia production from the liming reactionof intact hide is much faster, so it probably originates from an intermolecularmechanism.Conventional liming processes do not cause a marked increase in racemisa-

tion of aspartic acid, so it is concluded that hydrolysis does not cause majordamage to the high order of collagen structure during the first 24 hours ofliming. This means that the collagen fibres remain intact, with no relaxation ofthe triple helix structure. If there was significant conformational damage of the

0

1

2

3

4

5

0 20 40 60 80 100

Time in lime ( hours )

Ammonia ( mg % )

120

Figure 6.7 Ammonia production during white liming. (Courtesy of the Journal of theSociety of Leather Technologists and Chemists.)

146 Chapter 6

a-chains, then the formation of succinimide rings within the peptide chainswould be easier and therefore faster. That kind of destabilisation of the col-lagen structure is only obtained by prolonged liming. Therefore, the mechanismof amide hydrolysis during conventional liming is by a direct base-catalysedmechanism, but prolonged liming introduces the intramolecular catalysismechanism. The inference is that the collagen helical structure remains effec-tively intact for up to 24 hours in lime, but then starts to denature significantly.This observation has implications for enzyme-assisted unhairing and openingup. If enzymes are to be used in liming, their application should be restricted tothe earlier part of the process. Intact collagen is resistant to attack by generalproteases, because of its high degree of structural order, but chemicallydamaged collagen can be attacked by those same enzymes.17–19 So, if enzymicaction is allowed to continue to the end of the normal liming period or beyond,there is a likelihood of enzymolysis damage, which would rapidly adverselyaffect leather integrity and strength.Interestingly, there is a lack of intramolecular contribution to the hydrolysis

mechanism; this means that there is little discrimination between asparagineand glutamine in the reaction, i.e. no preferential reaction at asparagine. Sincethere is evidence to show that the chrome tanning reaction may be favoured ataspartic acid carboxyl groups, compared to glutamic acid carboxyl groups,20

the liming reaction cannot be expected to change the chrome tanning reaction.Although liming increases the efficiency of chrome uptake, by increasing thenumber of fixation sites, the effectiveness of the reaction, in terms of the rise inshrinkage temperature per unit bound chrome, is unlikely to be different when

0

0.05

0.1

0.15

0.2

0.25

0

Time in lime (hours)

D-aspartic acid (%)

20 40 60 80 100 120

Figure 6.8 Rate of racemisation of L-aspartic acid in collagen during white liming.(Courtesy of the Journal of the Society of Leather Technologists andChemists.)

147Liming

typical liming periods are used. Therefore, the consequence of abnormallyprolonged liming will be a more efficient but not more effective tannage, withaccompanying loss of leather performance due to reduced strength.

6.2.6 Hydrolysis of Guanidino Sidechains

The guanidino groups on the sidechains of arginine residues can also undergohydrolysis, but in a stepwise manner, when the byproducts are urea, ammoniaand carbon dioxide:

P-ðCH2Þ3-NH-Cð¼ NHÞ-NH2ðarginineÞ þH2O

ÐP-ðCH2Þ3-NH-Cð¼ OÞ-NH2ðcitrullineÞÐP-ðCH2Þ3-NH2ðornithineÞ

The arginine group can participate in the IEP determination, because it canact as a basic group:

P-ðCH2Þ3-NH-Cð¼NHÞNH2 þHþ

ÐP-ðCH2Þ3-NH-CþðNH2Þ2$ P-ðCH2Þ3-NHþ ¼ CðNH2Þ2

During liming, the guanidino group is hydrolysed to an amino group (seeabove). Therefore, the conversion of one type of basic group into anothermight, as a first-order effect, not markedly alter the IEP. However, the pKb

values are different (9.0 for free arginine and 8.7 for free ornithine), so there willbe a small second-order effect on the IEP.

6.2.7 Removal of Dermatan Sulfate

Section 2.3 (concerned with collagen and skin structure) describes the pre-sence of dermatan sulfate proteoglycan and its structure. Because the proteincore of the compound is bound to the surface of the fibrils, it is hydrolysed andreleased during liming, accelerated by the presence of alkali stable enzyme(Table 6.2).21

The compound can be easily quantitatively analysed and hence has beenused a marker or measure of the extent and progress of liming. However, its

Table 6.2 Removal of dermatan sulfate inliming.

Liming conditions Removal of DS (%)

6 hours 226 hours+enzyme 5318 hours (conventional) 50

148 Chapter 6

removal must not be regarded as an important part of the opening reaction,since that operates at the level of the fibril bundles; there is no evidence thatopening up or splitting at the level of the fibrils contributes significantly tothe leather making operation. The function of glycosaminoglycans in tissue isto control hydration by their hydrophilic anionic chains. Since the function ofdermatan sulfate in vivo is therefore concerned with hydration of the fibrils, thisraises the question as to what its effect might be if it were to remain in theleather.

6.2.8 Fat Hydrolysis

Triglyceride fat, both cutaneous and sub-cutaneous, is likely to be present inthe pelt at the beginning of liming, particularly if the pelts have not previouslybeen fleshed. The fat is hydrolysed (saponified, i.e. converted into soap andglycerol) during liming (Figure 6.9).The odour of soap is invariably apparent during and at the end of liming.

6.3 VARIATIONS IN LIMING

The variations in liming that might be used depend to a great extent on theunhairing process. If there is a departure from conventional lime/sulfide hairburning, then other reactions may be used to achieve the limed outcome.

6.3.1 Chemical Variations

Since the reactions required for opening up are hydrolytic, they could beconducted using any other alkaline agent capable of reaching the required pHrange. The use of strong alkali, such as sodium hydroxide, is possible butcontrol would have to be imposed, so that the pH does not reach too highvalues. Since the alkali may be used up in achieving equilibrium, it is necessaryto keep adding it, to maintain the required pH, because there is no bufferingeffect with strong alkali chemistry. The danger of over-swelling is increased,even though mechanical action may be minimised. However, it is possible tomonitor and control the pH using conventional pH-stat technology, although

CH2O2CR

R′′CO2-

RCO2-

R′CO2-CHO2CR′

CH2O2CR′′ CH2OH

CHOH

CH2OH

triglyceride glycerol soap

Figure 6.9 Hydrolysis of triglyceride.

149Liming

that presumes the process vessel includes recycling into a so-called ‘lab box’,where chemical dosing can be conducted.

6.3.2 pH Variations

Higher pH can be achieved with buffering, using the limited solubility of lime,but influencing the solubility with a sequestrant:

½CaðOHÞ2�solidÐ½CaðOHÞ2�solutionÐCa2þ þ 2OH�

The solubility product, Kp¼ [Ca21][OH�]2¼ 4.7� 10�6

If the equilibrium between dissolved lime and its ions is changed, bysequestering calcium ion, more lime must be dissolved to maintain the solu-bility product. In this way the hydroxyl ion concentration can be increased,raising the pH. This can achieved in several ways, but the technologicallyaccepted way is to add sugar. Monosaccharide (glucose) or disaccharide(sucrose) can complex with calcium ion via their hydroxyl groups, causing thepH to rise, when the equilibrium pH depends on the quantity of sequestrant.The effect of raising the pH is to accelerate the hydrolysis reactions, but stillhave lime buffering, together with the lyotropic effect of calcium ion. Clearly,the potential danger of over-swelling arises if pH values are elevated aboveconventional conditions.

6.3.3 Biochemical Variations

Hydrolytic reactions in proteins can be achieved enzymatically, using proteasesand, therefore, it is theoretically possible to conduct liming biochemically.Three points must be considered in this regard:

1. Chemical liming is non-specific, all reaction sites are vulnerable to attackand may be attacked equally. Enzyme reactions are relatively much morespecific; hence, the biochemical approach presumes that the requiredreactions are known and targeted by appropriate choice of enzymes. Sincethis is not the case, because the precise outcome of chemical liming is notknown, the use of enzymes as the only liming agents is not currentlyachievable.

2. Proteolytic enzymes are used industrially in liming. They are so-calledalkali stable enzymes: actually they are not stable in alkali, because theyare degraded as rapidly as any other enzyme, but they do have enoughreactivity at high pH for them to operate sufficiently to contribute to theprocess reaction.21 Typically, they are used to enhance keratin degrada-tion rate, but clearly can contribute to hydrolysis reactions, as evidencedby the faster rate of dermatan sulfate removal.

3. It might be thought possible to select for the degradation of non-struc-tural proteins by using non-collagenolytic proteases, since intact collagen

150 Chapter 6

is only damaged by collagenase. It is true that the highly structured natureof collagen requires a specific enzyme to attack it. However, the damagingeffect of liming does make collagen increasingly vulnerable to generalprotease attack.

6.4 LIMEBLAST

An important reaction associated with limed pelt is the damaging fault called‘limeblast’. The reaction is as follows:

½CaðOHÞ2�solid þ ½CO2�gasÐ½CaCO3�solid þH2O

The heterogeneous reaction occurs between solid lime, which has been driveninto the grain surface of the pelt (the flesh surface is not important in thiscontext), and gaseous carbon dioxide in the atmosphere, resulting in the

Figure 6.10 Effect of limeblast on the surface of pelt or leather.

Figure 6.11 Light photomicrograph of limeblast. (Courtesy of A. Michel, Leather-Wise UK.)

151Liming

formation of solid crystals of calcium carbonate. The fault arises if the limedpelts are removed from the liming drum, e.g. for fleshing or splitting, when theyare usually held in piles. Since the concentration of carbon dioxide in air is low,the reaction can only proceed if there is a constant supply of air, i.e. if the pilesare uncovered, in a draught. Therefore, the fault is prevented if the piles arecovered, e.g. with polythene sheeting.The damaging effect is caused by the distortion of the surface by the growing

crystals and that effect remains on the grain surface, even if the crystals aresubsequently removed by solubilisation in the deliming or pickling processes.The fault can be seen as dull areas on limed hide, but becomes more apparentwhen the resulting leather is dyed and dried. The change in surface char-acteristics is represented in Figure 6.10 and illustrated in Figures 6.11 and 6.12.

Figure 6.12 X-ray backscatter electron photomicrograph of a section through lime-blasted pelt, showing the presence of calcium in false colour. (Courtesy ofA. Michel, LeatherWise UK.)

152 Chapter 6

The intact grain acts like a smooth surface, reflecting light uniformly, but thedistorted grain causes light scatting from the surface, observed as dullness.

REFERENCES

1. W. P. Jencks, Catalysis in Chemistry and Enzymology, McGraw-Hill, 1969.2. J. H. Bowes and R. H. Kenten, Biochem. J., 1948, 43(3), 358.3. Official Methods of Analysis, Society of Leather Technologists and

Chemists, UK, 1996.4. E. R. Theis and R. G. Steinhardt, J. Amer. Leather Chem. Assoc., 1942,

37(9), 433.5. B. M. Haines and S. G. Shirley, J. Soc. Leather Tech. Chem., 1988, 72(5),

165.6. A. D. Covington and K. T. W. Alexander, J. Amer. Leather Chem. Assoc.,

1993, 88(12), 241.7. D. Rabinovich, World Leather, May 2008.8. E. Heidemann, J. Sagala and A. Hein, Das Leder, 1987, 38, 48.9. O. Menderes, A. D. Covington, E. R. Waite and M. J. Collins, J. Soc.

Leather Technol. Chem., 1999, 83(2), 107.10. M. J. Collins, E. R. Waite and A. C. T. van Duin, Phil. Trans. Roy. Soc.,

1999, 354, 1379.11. E. Heidemann and M. Deselnicu, Das Leder, 1972, 23, 17.12. T. Geiger and S. Clarke, J. Biol. Chem., 1987, 262(2), 785.13. A. C. T. van Duin and M. J. Collins, Org. Geochem., 1998, 29(5), 1227.14. H. T. Wright, Crit. Rev. in Biochem. Mol. Biol., 1991, 26(1), 1.15. A. B. Robinson and C. J. Rudd, Current Top. Cell. Regul., 1974, 8, 247.16. F. M. Sinex, J. Gerontol., 1960, 15, 15.17. J. W. Giffee, et al., J. Amer. Leather Chem. Assoc., 1965, 60(6), 264.18. R. G. Donovan, et al., J. Amer. Leather Chem. Assoc., 1967, 62(8), 564.19. A. L. Rubin, et al., Biochemistry, 1965, 4(2), 181.20. A. D. Covington, J. Amer. Leather Chem. Assoc., 1998, 93(6), 168.21. K. T. W. Alexander, J. Amer. Leather Chem. Assoc., 1988, 83(9), 287.

153Liming

CHAPTER 7

Deliming

7.1 INTRODUCTION

The process steps of deliming and bating are traditionally considered as inex-tricably intertwined, because the former is thought of as only preparing the peltfor the latter. Indeed, it is common for bating to be conducted in the delimingsolution. However, from an analysis of the specific functions of the steps, it isuseful to regard them as separate.The functions of the deliming step can be summarised as follows:

1. Lowering the pH, in preparation for bating: in preparing the pelt forchrome tanning, the pH will be adjusted from 12.6 (in liming) to 2.5–3.0(in pickling); between those stages, the pelt is adjusted to pH8.5–9.0,suitable for the biochemical reaction of protease.

2. Removing the lime: it is an often quoted feature of deliming that one of itsmain purposes is to remove lime. It is clearly part of the reaction oflowering the pH to neutralise residual alkali from the liming process.

CaðOHÞ2 þ 2HþÐCa2þ þ 2H2O

It is less clear what happens to the calcium ions that are associated withthe ionised carboxyls or are engaged with the hydrogen bonding system asa lyotropic effect during liming. It is certainly the case that the boundcalcium is largely eliminated by the time the pelt is pickled.

3. Depleting the pelt, partially reversing the swelling: From the swellingcurve, reproduced in Figure 7.1, the change from pH12.5 to 8.5 isaccompanied by a significant degree of deswelling or depletion. Eventhough the swelling may not have been conducted to equilibrium, thechange is marked.




154

There are two features of importance in the depleting:

1. The pelt appearance becomes less translucent, more opaque. As the wateris lost, the fibre colour reappears.

2. There is a net transfer of water from the inside of the pelt to the outside.This may seem a trivial restatement of the effect of the process step, that isthe reversing of swelling, but it is of profound importance to opening up,because this is a rare occasion in the leather making process that such anet flow occurs.

Consider any rinsing or washing operation: it relies on transferring waterinto the pelt, dissolving the component of interest, then squeezing the solutionfrom the inside out into the bulk solution. This mechanism therefore includestransporting solutes not only out of the pelt, but also back in again. In the caseof deliming, the transport is effectively one way, so the removal of the degradedprotein and other products from the liming reactions is conducted efficiently.Therefore, deliming is not merely adjusting the pH, it is one of the moreimportant steps in opening up.The process of depletion also occurs in pickling, but it can be seen from

Figure 7.1 that the effect is much smaller from pH 8.5–9.0 to 2.5–3.0 than frompH 12.6 to 8.5–9.0. In addition, the impact of depletion will also depend on therate of the process: the faster the water flows out of the pelt, the smaller isthe reversing effect of the conventional washing mechanism on the removal ofdebris from within the pelt.

7.2 DELIMING AGENTS

Deliming can be conducted using any chemical capable of reducing the pH tothe required range. The options are as follows.

800

700

600

500

400

300

200

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Final pH

3% salt

No saltNo salt

3.0% salt


Figure 7.1 Effect of pH on collagen swelling.

155Deliming

7.2.1 Water

Continued washing with water will lower the pH by two mechanisms. The firstmechanism is merely diluting the hydroxyl content and the second mechanismis neutralising the hydroxyl with the bicarbonate content in the water, as shownin Figure 7.2, illustrating the difference between the inside and the outside ofthe pelt, with the surface acting as a semi-permeable membrane.The dilution effect is a factor of 103, therefore a lot of water is needed to

reach equilibrium between the inside and the outside of the pelt: the slow effectof diffusion will contribute to the need for even more water. The concentrationsof protons and bicarbonate are too low to provide a driving chemical potentialfor diffusion from the outside to the inside.There are two other good reasons why deliming by washing is undesirable or

risky:

1. Fresh water is a resource that should be conserved: this applies more insome countries than in others, but it is generally the case that fresh wateris neither so cheap nor so abundant that it can be used wastefully.

2. It must be remembered that deliming applies to limed hide that has notreached swelling equilibrium, therefore it is capable of additional,damaging swelling. Mechanical action, particularly at the beginning ofdeliming in fresh water, runs the risk of over-swelling the grain. Since peltis most vulnerable to abrasion damage in an alkali swollen state, there isan extra risk of grain damage.

7.2.2 Strong Acids

Any strong acid might be a candidate for acting as a deliming agent, as long asthe counterion does not react with calcium ion. The undesirable consequencewould be the creation of an insoluble salt, which can disrupt the grain structureby precipitating within the grain layer, in the same way as the problem oflimeblast is caused. This effectively rules out sulfuric and phosphoric acids.More importantly, the use of strong acids might lead to pH values in solutionlow enough to cause acid swelling: this type of damage could be avoided by theaddition of neutral electrolyte, but that would be counterproductive in terms ofeconomics and environmental impact. The pH achieved in solution depends onthe concentration of acid added into the system: if the total offer were added in

Inside Outside

Start [OH]- = 10-2 molar [OH]- = 10-8 molar [H+] = 10-6 molar

End [OH-] = 10-5 molar[HCO3

-] = 10-5 molar

Figure 7.2 Deliming with water.

156 Chapter 7

one aliquot, acid swelling would be highly likely. An offer of 1% sulfuric acid in100% float gives a concentration of 10 g l�1, equivalent to 0.1 molar, so that thehydrogen ion concentration is 0.2 molar, equivalent to pH 0.7. Even thoughthis concentration falls into the region of reduced swelling (Figure 7.1), owingto the electrolyte effect (discussed in Chapter 9), consumption of acid will raisethe pH into and through the zone of maximum swelling at pH 2.The alternative is to add the acid in aliquots small enough to maintain the pH

not lower than 5, to stay within the zone of minimum swelling (Figure 7.1).However, this strategy means that the concentration of hydrogen ions insolution is always low and, therefore, the rate of deliming will be slow. It is thiseffect on the kinetics that usually rules out the routine use of strong acids assolo deliming agents, although they may be included in a mixture of agents.

7.2.3 Weak Acids

It might be assumed that weak acids would be safer than strong acids, becausethey would not cause the pH to drop low enough to cause acid swelling.However, that assumption cannot be relied upon, because even weak acids cancause acid swelling:1% HCO2H in a 100% float is 10 g l�1¼ 0.2M; the pKa of formic acid is 3.75:

Ka ¼½Hþ�½HCO�

2 �½HCO2H� ¼ ½Hþ�2

½HCO2H� ¼ 10�4M

For a weak acid, assume the concentration of unionised acid is the same asthe total concentration. Then, [H1]2¼2 � 10–5, i.e. [H1]¼4.5 � 10–3 M � pH2.3. Therefore, the solution pH is close to the pH of maximum swelling.In particular, unionised carboxylic acids can cause lyotropic swelling, as

discussed earlier. In addition, carboxylate ions often react with calcium ion toproduce an insoluble salt. An exception is lactic acid, CH3CH(OH)CO2H,which produces a soluble calcium salt: it has been used in industry as a delimingagent.The application of aliquots of weak acids leads to the problem of too slow a

rate of reaction, in the same way that it applies to strong acids.

7.2.4 Acidic Salts

Many acidic salts might be used for deliming. Sodium bicarbonate, NaHCO3,has been proposed,1 but it has not been applied industrially, probably becauseof cost and low solubility (see carbon dioxide deliming, Section 7.2.8). Sodiummetabisulfite, Na2S2O5, is often included in ammonium salt deliming, but itsfunction there is primarily to scavenge residual sulfide after lime/sulfideunhairing. The reaction is complicated, yielding several sulfur species, e.g.elemental sulfur, thiosulfate, S2O3

2� and other sulfur oxyanions.

157Deliming

At pH 9, during deliming, the dominant sulfide form is HS�:

S2O2�5 ÐSO2�

3 þ SO2

So, under these conditions the dominant form would be HSO3�:

2HS� þHSO�3 Ð3Sþ 3OH�

7.2.5 Ammonium Salts

Ammonium salts are preferred because they readily yield the conditionsrequired for conventional pancreatic bating enzymes to operate ideally. Asammonium salts are weak, pKa B9, they buffer around pH9:

pKa ¼ pHþ logf½NH4�þ=½NH3�g

A significant advantage of the buffering effect is that any error in the offerwill have only a small effect on the pH. The effect of, for example, a 20% errorcan be calculated, assuming here the pKa of NH4

1¼ 9.0.Assume half the ammonium salt is used up in the reaction, i.e. 1% ammo-

nium chloride reacts with the residual lime:

NHþ4 þOH�ÐNH3 þH2O

Then, normally, [NH41]¼ [NH3]¼B0.2M, giving pH9.0.

If 20% too much is added, [NH3]¼ 0.20M and [NH41]¼ 0.28M, and so

[NH41]/[NH3]¼ 1.4 and pH¼ 8.85.

If 20% too little is added, [NH3]¼ 0.20 M and [NH41]¼ 0.12M, and so

[NH41]/[NH3]¼ 0.6 and pH¼ 9.22.

The deliming reaction is very fast, because the whole offer of ammonium saltcan be added at the beginning of the process step and the pH cannot fall belowthe value fixed by the equilibrium condition of the reaction products. Also,ammonium salts are very soluble in water, so the offer may even be added to theprocess vessel after draining the lime liquor or any wash liquor from thatprocess, without adding any fresh float. This creates an additional contributionto the fast diffusion rate, driven by the osmotic effect of the concentratedsolution. The water released from the alkali swollen pelt by the pH change canfunction as the float for the deliming step.In industry it is common to use either ammonium sulfate or ammonium

chloride. It is important to know that they are not equivalent reagents: forexample, a 1% offer of ammonium chloride contains 0.34% NH4

1, but thesame offer of the sulfate contains 0.55% NH4

1. Technologically, the maindifference is the effect of the counterion on the residue of the liming step. In thepresence of sulfate, calcium ions can form the sparingly soluble salt:

CaðOHÞ2 þ SO2�4 ÐCaSO4 þ 2OH�

158 Chapter 7

This may be a problem, because crystals can grow within the fibre structureand, more importantly, below the grain surface. The disrupting effect on thesurface distorts the surface by stretching it, so that even if the cause of the faultis removed by solubilising the crystals, the effect remains and can adverselyaffect uniformity of dyeing. The greater danger is the conversion of limeblastcrystals of calcium carbonate into a more insoluble salt, which may survivepickling and be present in tanned leather:

CaCO3 þ SO2�4 ÐCaSO4 þ CO2�

3

The occurrence of this reaction depends on the conditions in the vessel withregard to concentration. Three possibilities determine the outcome (Figure 7.3).In long float, the solubility of the sparingly soluble salt is exceeded, at short

float the high concentration of ammonia in solution allows the formation of thecalcium complex, which keeps it in solution in the presence of sulfate ion. Onlyin the intermediate situation does calcium sulfate precipitate – which is thetypical industrial condition for deliming with ammonium sulfate. In the case ofammonium chloride this danger does not exist, because chloride ion does notform an insoluble salt with calcium ion. The lyotropic effect of chloride ion isgreater than that of sulfate ion, so the outcome will be enhanced opening up;although the difference in effects may not be significant enough to be discernibleafter the relatively brief period of contact.There are two problems associated with using ammonium salts. First, the

release of ammonia gas into the working environment: whilst it does not createa serious toxicity hazard, it does create a nuisance, causing discomfort toworkers. Second, the discharge of ammoniacal nitrogen in the effluent iscommonly subject to stringent consent limits, because ammonia is poisonous tofish and other aquatic life. Therefore, there is an imperative to develop analternative technology.

7.2.6 Alternative Buffers

One option, suggested by Koopman, is to use Epsom salt, magnesium sulfate,which can operate in the following way:

MgSO4 þ 2OH�ÐMgðOHÞ2 þ SO2�4 ÐMg2þ þ 2OH�

long float

CaCO3 + SO42- CaSO4 precipitates

CaSO4 dissolves – as [Ca(NH3)2]2+

CaSO4 dissolves – solubility 2g/l

short float

medium float

Figure 7.3 Formation of calcium sulfate.

159Deliming

The reaction product between the salt and hydroxyl ions is the sparinglysoluble magnesium hydroxide. As with lime, calcium hydroxide, the pH isdefined by the solubility; in this case the system is at equilibrium at about pH10. Although this value is a little high for many enzymes that might be used inbating, it still offers conditions that could be used as the basis for preparing forbating.The technology was developed in The Netherlands, and at the instigation of

the USA’s Environmental Protection Agency (EPA) trials were conducted bythe American Leather Chemists Association (ALCA).2 The process was to use50% float at 35 1C, 7% magnesium sulfate and 10% sulfuric acid added in 10%float, resulting in pH6.4 in solution. The author, Constantin, reported a resi-dual high pH profile through the cross section, based on the use of phe-nolphthalein indicator. The trials were not successful, because he could not getthe chrome tanning salt to penetrate the hide and so he reported that he wasobliged to terminate the trials. Consequently, the originator of the technology,Koopman, responded that the trials were flawed, because the indicator shouldhave been thymolphthalein, which would have shown that the centre of the peltwould be at pH4 10.5 and therefore should have been more carefully acidifiedin pickling prior to chrome tanning, i.e. the trials could not succeed because ofthe way in which they were conducted.3 This drew a tart response from Con-stantin and the upshot was that the technology has never been pursued. It isprobable that the Epsom salt technology is feasible, if the underlying science isunderstood.A combination technology has been suggested, using magnesium lactate

as deliming agent:4 the reagent was available from waste whey from thedairy industry. It is reported that it can be applied as an offer of 3% for30min, to give solution pH9.7, when the bate can be added directly into thesolution.

7.2.7 Hydroxyl ‘Sinks’

Any chemical system that absorbs hydroxyl ions has the effect of lowering thepH: this has been exploited in the chemistry of ester hydrolysis for deliming.The reaction principle is a follows:

R1CO2R2 þOH�ÐR1CO�

2 þR2OH

Hydrolysis is the addition of the elements of water, to recreate the organicacid and alcohol, but under alkaline conditions the product is the acid anion.The R groups have to be selected to ensure solubility of the reactant and theproducts. The preferred reagents are methyl lactate and ethylene glycol mono-or diformate, when the claimed effect is 70% deliming in 30 min and a cutcolourless to phenolphthalein after 2 hours, giving solution pH 6.4.5 Thispatented technology has not been taken up by industry, presumably because ofissues of cost and rate of reaction.

160 Chapter 7

7.2.8 Carbon Dioxide

In the latter quarter of the twentieth century, the use of carbon dioxide as adeliming agent was developed. It works as follows:

CO2 þH2OÐHCO�3 þHþÐ

OH�

CO2�3 þH2O

The gas is sparingly soluble in water, producing acid that can neutralise thealkalinity of the limed pelt. However, a limiting factor is the solubility, whichmeans the availability of acid is limited and hence the reaction rate is slow:typically, skins take an hour to delime and a 25 kg ox hide can take severalhours to delime completely. This applies even under the conditions of supplyingexcess gas as quickly as possible into the system.Technologically, the application of CO2 deliming is simple (Figure 7.4): the

gas is stored under pressure in liquid form and is led by pipe through the hollowaxle of the drum. It is not important to introduce complexity with regard toleading the gas into the float, merely supplying it to the headspace is sufficient.However, in industrial processing, it is important to lead the gas from the

storage vessel through a small heater, because the release of pressure causesthe gas to cool down. The consequence is that the gas will solidify and block thepipe, causing it to burst under the pressure. (NB this effect of gas cooling isobserved when CO2 fire extinguishers are discharged, when the gas stream isaccompanied by some CO2 ‘snow’.) The feed rate is dependent only on the rateat which the gas dissolves in the float. The concentration achieved depends ontemperature and pressure (Table 7.1).

Figure 7.4 Development trials on the technology of carbon dioxide deliming.

161Deliming

Although the use of carbon dioxide for deliming is increasingly common, theuse of pressurised vessels is rare, so the solubilities of interest correspond toambient pressure. Additionally, it is common to conduct deliming at 30–40 1C,in preparation for the bating step. Whilst there is a reduction in solubilitybecause the water is warm, the rate of deliming through the cross section isfaster than processing with colder float.Apart from the rate difference, the biggest difference between ammonium salt

deliming and carbon dioxide deliming is the pH effect. There is a bufferingeffect with CO2, but it is at a much lower pH than ammonium salts achieve;CO2 deliming can lower the pH to 5 (Figure 7.5).6

This clearly has implications for bating, because most conventional bateshave no activity at such a low pH. Therefore, the tanner faces two options todeal with this problem:

1. Change the bate. This is possible, because there are acid acting bateformulations on the market. However, there is no doubt that the bio-chemistry will be different, so the outcome may not be the same as the

time

pH

NH4+

CO2

Figure 7.5 Relative effects on pH in deliming by ammonium salt and carbon dioxide.

Table 7.1 Solubility of carbon dioxide in water.

Temperature (1C)Solubility (g l�1)

1 atmosphere 5 atmospheres

0 3 1716 2.5 1032 2 749 1 3

162 Chapter 7

more usual pancreatic bating. That problem notwithstanding, there is theproblem of adjusting the offer and conditions for the new process tomatch the broad effects of the conventional process. This can be done bymatching the actions through assaying (Chapter 8).

2. Control the acidification. In essence, this means limiting the supply of gas,so the pH is not allowed to fall below pH 9 or making sure the reactionfinishes at that pH. However, this is only achieved by reducing theavailability if the gas in solution, limiting the concentration, with theconsequence of slowing the rate of reaction, which is already slower thandesirable.

The relative rates of deliming using ammonium chloride and carbon dioxidecan be estimated from the solubility in solution, as follows.Assume 2% NH4Cl is added to drained pelt, 10% residual float. [NH4Cl]¼

200 g l�1¼B4 molar. The solubility of CO2¼ 2.5 g l�1, which at ambienttemperature and 1 atm pressure¼B0.06 molar. The calculation indicates thedifference is almost two orders of magnitude.Recent work offers a novel solution to the problem: carbonic anhydrase

accelerates the solubilisation of carbon dioxide in water in the pH range 4–10,optimally at pH 7.5–8.0, therefore speeding up the deliming reaction.7 Theproblem of slow rate of deliming may be understood by considering the reac-tions involved (Figure 7.6).From Reaction (7.1) (Figure 7.6), the availability of acid is via hydrated carbon

dioxide and reaction with or ionisation of the unstable (transition) carbonic acidspecies. Therefore, the available acidity depends on the solubility. Alternatively,from Reaction (7.2), there may be direct reaction between the gas molecule andhydroxyl ion. However, this assumes that the gas can be transported to the requiredsites in the substrate or that the alkalinity diffuses out of the pelt: such equilibrationis slow. Enlisting the aid of enzymology, using carbonic anhydrase, the enzymeresponsible for rapid equilibration of carbon dioxide and water in organisms, therate of CO2 deliming can approach the rate of ammonium salt deliming.There is a ‘low tech’ solution to this problem, based on delivering all the gas

required for the reaction in a single offer, rather than feed the gas into thesystem as the reaction is conducted. The technique is to sparge the processvessel of air, filling the headspace with carbon dioxide.8 In this way the rate ofreaction is maximised and the process time can match the reaction withammonium salt. However, the technology assumes modifications to the processvessel, to prevent loss of gas during sparging: this is not always feasible withmost conventional processing drums.

CO2 + H2O [CO2]hydrated [H2CO3] HCO3- + H+ (7.1)

HCO3- (7.2)OH-

Figure 7.6 Reaction of carbon dioxide with water.

163Deliming

A remaining problem with carbon dioxide deliming is the effect of the lowsolution pH on residual sulfide from liming: at the typical values, pH5–6, anysulfide in solution is in the form of hydrogen sulfide. Although the amount ofsulfide left in the pelt is relatively small, the toxicity of the gas constitutes a deadlyrisk to workers. However, sulfide in solution can be scavenged efficiently andeffectively by oxidation. The application of an offer of hydrogen peroxide as lowas 0.1% easily maintains the concentration of H2S in the headspace at a safe level,below the level of detection by smell, i.e. below 1ppm (part per million). Impor-tantly, it should be recalled that swollen pelt is particularly vulnerable to damageby oxidising agents: therefore, the sequence of events should be as follows.

� Begin CO2 deliming: continue for 5 min – this is sufficient to delime onlythe surface, avoiding leaching out much of the sulfide held within the pelt.

� Add the hydrogen peroxide: the sulfide in solution is oxidised, but thegrain is protected against oxidation damage because the pH is lowered bythe initial deliming.

� Resume deliming – any sulfide leached into solution is scavenged by theperoxide.

S2� þ 4H2O2ÐSO2�4 þ 4H2O

7.3 MELANIN

A feature of the CO2 deliming process is the effect it has on the melanin contentof pelt. The skin pigment, melanin, is present in cells called melanocytes, mainlyin the epidermis, but also within the grain, associated with the hair follicles, asdiscussed in Chapter 2. In conventional processing with ammonium salts, thepH is rapidly lowered to about 8.5–9.0, where there is still some swelling in thepelt, with consequent elevation of the follicle angle. The effect of this residualswelling is to allow the melanocytes to be removed from the pelt by thedepleting action, resulting in relatively pigment free grain. However, in carbondioxide deliming, when the pH is relatively slowly lowered to 5, it hasbeen observed that the melanin remains, clearly visible as coloration on thegrain (A.D. Covington, unpublished results). The residual melanin retainsthe pattern of pigmentation in black hides, particularly apparent in Friesianhides. Microscopic examination has revealed that the effect is due to the pre-sence of the melanin within the grain, caused by the depleting effect of the lowerpH, not as an effect of rate of depletion. Furthermore, it was found that thecritical pH for the effect to be manifested is 7: deliming to no lower than pH7results in clean grain, but deliming below pH7 results in melanin remainingwithin the grain. This creates a dilemma for the tanner: to maintain pH47 insolution, the availability of carbon dioxide should be restricted and there is aconsequent slowing of the deliming rate, which is already too slow.The solution for tanners of melanin pigmented hides is to delime as rapidly as

possible, with or without carbonic anhydrase, but then to degrade any residual

164 Chapter 7

melanin. Chemical methods would be too violent, but a biochemical option isavailable (Chapter 2). Rao et al. have reported that a solution of 0.5% xylanasecan completely remove melanin from hide within a few hours.9 It is suggestedthat the mechanism is based on attack by the enzyme at the structures bindingthe melanin to the protein: the reaction is assisted by the presence of a-amylase,used in the opening up processes in their trials. Other biochemistries may be ofassistance, such as polyphenol oxidases, capable of degrading the pigment.

7.4 LIMEBLAST

Chapter 6 describes how the action of CO2 in the air can cause the problem oflimeblast, the growth of calcium carbonate crystals below the grain surface. How-ever, in CO2 deliming, limeblast is not observed. Therefore, this begs the question:does CO2 deliming cause limeblast? The reactions of importance are as follows:

CaðOHÞ2 þ CO2ÐCaCO3 þH2O

At high pH, insoluble calcium carbonate is formed, potentially creating limeblast:

CaCO3 þH2Oþ CO2ÐCaðHCO3Þ2At lower pH, with continued addition of carbon dioxide, soluble calcium

bicarbonate is formed: this is the consequence of CO2 deliming, lowering thesolution pH to 5–7. Therefore, CO2 deliming is unlikely to cause limeblast. Thisis the basis of the traditional school experiment, when students blow through astraw into lime water, a clear solution of calcium hydroxide: in this way, CO2 inthe breath is bubbled through the solution. First a precipitate forms, seen ascloudiness in solution, then the precipitate redissolves and the solution goes clear.

REFERENCES

1. E. Heidemann, A. Hein and P. Herrera, Das Leder, 1988, 39, 141.2. J. M. Constantin, J. Amer. Leather Chem. Assoc., 1981, 76(1), 40.3. R. C. Koopman, J. Amer. Leather Chem. Assoc., 1982, 77(7), 356.4. K. Kolomaznik, et al., J. Amer. Leather Chem. Assoc., 1996, 91(1), 18.5. E. Hahn, B. Magerkurth, D. Lach, L. Wuertele, UK Pat. Appl., 2026538A,

June 1979.6. M. J. Klaase, J. Amer. Leather Chem. Assoc., 1990, 85(11), 431.7. A.D. Covington, K.B. Flowers, A. Onyuka, UK Pat. Appl.,0606317.6,

March 2006.8. K.B. Flowers and C.A. Jackson-Moss, Proc. IULTCS Congress, Capetown,

South Africa, March 2001.9. J. Rao, et al., J. Amer. Leather Chem. Assoc., 2008, 103(7), 203.

165Deliming

CHAPTER 8

Bating

8.1 INTRODUCTION

Bating is a generic term that refers to the use of enzymes in an early stage ofleather making. Its purpose is to break down specific skin components: usuallythe non-structural proteins are the target. These proteins can be degraded bygeneral proteases, because they do not have highly defined structure, even thoughthey may be folded specifically. Their lack of structure means that scission of thepeptide chain creates smaller molecules, making them more easily removable byrinsing. Intact collagen is resistant to such attack, because of its highly structurednature, and therefore requires specific collagenases to break it down. However,notably, alkali-damaged collagen is vulnerable to attack by general proteases, andso limed collagen can be significantly degraded during conventional bating. Theusual effect of this type of damage is grain enamel loss and/or corium loosening.Bating has been performed in different ways over the centuries. Traditional

sources of enzymes have typically been animal faeces, where the requiredcomponent was the pancreatic or digestive enzymes: using bird guano wascalled ‘mastering’, using dog faeces was called ‘puering’. There was also abotanical version, called ‘bran drenching’, where the origin of the batingmaterial is apparent. Clearly, using warm infusions of such materials must havecontributed greatly to the tanners’ reputation of dealing with unpleasant pro-ducts and making smells! However, shortly after the beginning of the twentiethcentury, the first enzymes isolated from animal sources were made available forleather making purposes by Otto Rohm, although it was about 50 years beforethe use of puer was discontinued, at least in Europe. More recently, batingenzymes from bacterial fermentations have become available.Alexander1 has reviewed the use of enzymes in the beamhouse, signalling the

new era of biochemistry based processing, which will characterise the leatherindustry of the twenty-first century. Enzymes are biochemical catalysts: theyare the products of microorganisms, but they are not microorganisms




166

themselves; they are not alive because they cannot reproduce. Their function isto catalyse organic reactions, which they can do very effectively: reactions maybe accelerated up to 1020 times faster than the uncatalysed reaction. The defi-nition of a catalyst is a reagent that can accelerate a reaction to equilibrium, butis chemically unchanged in the process, although it may be physically changed.Note, a catalyst cannot change or displace the point of equilibrium, it can onlyaffect the rates of a reaction, both forward and back.In enzyme catalysis, there are two equations that define the process:

1. Eþ SÐk1

k�1

ES

2. ES �!k2 Eþ P

where E is the concentration of the enzyme, S is the concentration of thesubstrate or reactant, ES is the concentration of the intermediate complexbetween the enzyme, the Michaelis–Menten complex, and the substrate and P isthe concentration of the product.2 The rate equations are as follows:

�d½S�dt

¼ d½P�dt

¼ k1½E�½S� � k�1½ES� ¼ k2½ES� ð8:1Þ

d½ES�dt

¼ k1½E�½S� � k�1½ES� � k2½ES� ð8:2Þ

The concentration of the complex reaches a steady state when:

d½ES�dt

¼ 0

Applying the condition to Equation (8.2):

½ES� ¼ k1½E�½S�k�1 þ k2

ð8:3Þ

The initial conditions are as follows:

½E0� ¼ ½E� þ ½ES� ð8:4Þ

Substituting [E] in Equation (8.4) into Equation (8.3):

½ES� ¼ k1½E0�½S�k�1 þ k2 þ k1½S�

ð8:5Þ

From Equation (8.1), the rate of formation of the product becomes:

d½P�dt

¼ k2k1½E0�½S�k�1 þ k2 þ k1½S�

ð8:6Þ

167Bating

The combination of rate constants is called the Michaelis–Menten constant(Km):

Km ¼ k�1 þ k2

k1ð8:7Þ

� d½S�dt

¼ k2½E0�½S�Km þ ½S� ð8:8Þ

If the substrate concentration is high, the rate equation becomes:

v ¼ k2½E0� ð8:9Þ

If the enzyme concentration is low, the rate equation becomes:

v ¼ k2

Km½E0�½S� ð8:10Þ

v ¼ k1k2½E0�½S�k�1 þ k2

ð8:11Þ

so the kinetics are first order in substrate concentration.Alternatively, if [S] 44 Km then the kinetics are zero order.If k2 is much bigger than k�1:

v ¼ k1½E0�½S� ð8:12Þ

then the kinetics refer to the formation of the enzyme–substrate complex.If k�1 is much bigger than k2:

v ¼ k1k2

k�1½E0�½S� ð8:13Þ

then the activation energy calculated from a plot of log v against 1/T will referto the activation energies for the two reactions set out at the beginning of thisargument:

E ¼ E1 þ E2 � E�1

From Equation (8.10), the rate of the enzyme-catalysed reaction is a functionof the substrate concentration and the Michaelis–Menten constant: the lowerthe value forKm, the greater is the affinity between the enzyme and the substrate.Hence, measuring the kinetics of the reaction provides information regardingthe way the reaction is conducted. An example of such data is presented inChapter 13, concerning an enzyme-catalysed polyphenol polymerisation.The traditional view of the mechanism of enzyme catalysis is expressed as the

‘lock and key’ theory. The reaction site of the enzyme is designed to receive the

168 Chapter 8

specific chemical structure of the transition state of the intermediate betweenreactant/substrate and the product, as a key is shaped to fit into a lock.However, there are two ways of looking at the model. The mechanism ofenzyme catalysis has been argued in terms of the way in which the reactant isconverted via a transition state into the product. This has been expressed asstabilisation of the transition state by the electric field of the active site ofthe enzyme relative to the reactant, i.e. the enzyme is altered to accommodatethe reactant or the enzyme has a greater affinity for the transition state thanfor the reactants. Alternatively, the reactant may be arranged by the enzyme inthe active site, leading to the transition state. Figure 8.1 summarises themechanisms, relating the free energies of the interaction or binding betweenthe enzyme and the Michaelis–Menten complex (MC) and the transitionstate (TS).3

Marti et al.3 have argued that these apparently contradictory views can bereconciled in a single mechanism. The enzyme favours progress in the reaction,because it has a reaction site that is preorganised to accept the substrate. Theenzyme is likely to have a predisposition to interact with those substrate con-formations that resemble the transition state more closely than others, therebyincurring a lesser energy penalty because deformation of the substrate and/orthe enzyme’s reaction site is not required.

8.2 FACTORS AFFECTING ENZYME CATALYSIS

Enzyme catalysis can be highly specific regarding the substrates with which anenzyme will or will not interact; moreover, the conditions under which enzymesfunction are typically also critical.

8.2.1 Temperature

The rate of the reaction depends on the origin of the organism that pro-duced the enzyme. Since most bates are produced from bacteria, the optimumtemperature relates to the environment of the host, i.e. body temperature(Figure 8.2).The shape of the profile is asymmetric. On the low temperature side of the

maximum there is a rapid increase in the rate of reaction as the temperature rises

∆G uncat

E + S

∆GMCbind ∆GTSbind

∆G catESMC

E + S

ES

Figure 8.1 Relationship between the roles of the Michaelis–Menten complex and thetransition state in enzyme catalysis.

169Bating

to reach the optimum value, which is typically close to the natural environmentof the organism that produced the enzyme. Therefore, the optimum temperatureis usually 35–40 1C, although enzymes are known that operate at much lower ormuch higher temperatures, the so-called extremophiles. However, enzymes usedin leather making are typically conventionally reactive.On the high side of the temperature maximum, there is a steep fall in activity.

This is because the reaction site of the enzyme is highly sensitive to temperatureand, indeed, the protein may be denatured at a temperature slightly higher thanthe optimum temperature. In either case, the enzyme ceases to function. Thesteepness of the activity curve as temperature rises is indicative of the sensitivityof the enzyme to temperature and hence the action of the enzyme in bating.A small deviation of the temperature from the optimum value means a big lossin activity and therefore a loss of value from the product.

8.2.2 pH

Figure 8.3 illustrates the pH–activity profile of an enzyme.Notably, real profiles may differ from the model in Figure 8.3 in the

following aspects:

� the profile may or may not be symmetrical;� the profile may be more narrow or more broad;� the maximum activity may not be at pH 8–9.

activity

0 10 20 30 40 50

Temperature (°C)

Figure 8.2 Illustration of the rate of an enzyme-catalysed reaction as a function oftemperature.

170 Chapter 8

Unlike the temperature dependence of activity, there is likely to be significantactivity below and above the optimum pH, without the same catastrophic lossof activity due to damage to the reactive site on the enzyme: in this case, theeffect of pH is to change the charge on the protein, causing a change to theelectrical field of the active site, with roughly similar changes either side of thepH optimum.

8.2.3 Concentration: Bate Formulation

The activity of enzymes is a function of concentration, because they are cata-lysts, as presented in the kinetic equations above. Therefore, offers in leathermaking are not made on the usual weight fraction basis on pelt weight, as withother reagents. Offers depend on the amount of water being used.Commercial bate products are not pure enzymes. They are not pure in terms

of being a single enzyme: proteolytic enzymes from any source are likely to be amixture of enzymes, so that the precise reactions they catalyse may not beknown. They are also not pure in terms of the formulation of the product. Sincethe activities are so high, the amount of pure, isolated enzyme that would beneeded in a bating process would be very small. Therefore, there could be alarge error in the weighing and hence there would be large variations in theconsistency of the effect on the pelt. Consequently, bates are supplied in adiluent, which can have an effect on the process.

8.2.3.1 Ammonium Salt. If the deliming step is conducted with ammoniumsalt, the presence of more ammonium salt in the bate formulation will contri-bute to the buffering effect. Because the salt is very soluble in water, theenzyme is immediately available for reaction.

8.2.3.2 Wood Flour. An alternative diluent is wood flour, which is ligno-cellulose. It has the advantage of not contributing to the ammoniacal

activity

5 6 7 8 9 10 11 12

pH

Figure 8.3 Profile of enzyme activity as a function of pH.

171Bating

nitrogen in the effluent, but it may contribute to the suspended solids loadingof the effluent. Because there is no great affinity between the diluent and theenzyme, the enzyme is immediately available for reaction.

8.2.3.3 Kaolin. The use of kaolin, a form of clay, as a bate diluent is lesscommon, but it is useful to know there are two consequences if it is part ofthe formulation. First, the diluent will contribute to the suspended solidsin the liquid effluent and/or the sludge from mixed effluent streams. Second,there is strong affinity between protein and clay, so there will be a delay afteradding the bate to the solution before the enzyme is desorbed from the dilu-ent surface. Therefore, the reaction is not immediate and so allowance mustbe made for the delay.

8.2.4 Time

The extent of the bating effect depends on the time allocated to the process step.This is typically about 1 hour. It is important to know that enzymes will usuallypenetrate through only about 1mm of pelt in an hour. For skins, this meansthat the bating effect will probably extend through the cross section, causingdamage to the non-collagenous proteins and to the collagen. Importantly, theavailable time is insufficient for the enzymes to penetrate through hide com-pletely, even in the case of light ox or cow. Therefore, in most processes, thebating effect is confined to the grain layer and the outer part of the flesh side.This is usually deliberate: it avoids the risk of a loosening effect in the corium,but takes advantage of the potent enzymatic reactions to clean the grain ofresidual epidermis and damaged hair/keratin.In reviewing industrial practices, it is not uncommon to find that bating is not

conducted under optimum conditions for enzyme activity, pH and temperature.It is as though tanners recognise the inherent danger of the efficiency of enzymecatalysis and consequently opt for lesser effects than bates are capable ofdelivering. A cheaper option, of course, would be a lower offer at optimumconditions.

8.2.5 Origin

The origin of the bating enzymes will determine the actual reaction applied tothe proteins in the pelt. These reactions are many and varied: some are set outin Table 8.1.4

The other important aspect of origin is the difference between pancreatic andbacterial bates. The critical difference lies in their ability to degrade elastin.Pancreatic or digestive enzymes do not degrade elastin at all, at least not withinthe timescale of conventional liming and bating, but bacterial bates often con-tain elastolytic enzymes. Therefore, a comparison of these bates would revealthe impact on leather character of degrading the elastin. The effect is important,because it results in very different leather, in terms of area, but more importantlywith regard to the physical properties and performance (Chapter 2).

172 Chapter 8

Note, the appearance of bacterial bates to replace the more common pan-creatic bates coincided with changes to the production of insulin for diabetics.The traditional source of insulin was extraction from bovine pancreas, so thebyproduct was pancreas tissue and the pancreatic enzymes. However, morerecently, human insulin can be produced by gene engineering of bacteria, sobovine pancreatic tissue is no longer a cheap byproduct of another industry. Theobvious cheap source of proteolytic enzymes is by fermentation of bacteria –hence the trend of change in formulation of the product.

8.3 MONITORING BATING

There are two traditional, qualitative ways of monitoring the progress of thebating process:

1. The thumbprint method for hides or skins. This relies on the effect ofbating to remove the resiliency of the pelt when the interfibrillary proteinsare degraded. The pressure of a finger, usually the thumb, squeezes thebated, opened up fibre structure, which does not spring back, leaving athumbprint that recovers only slowly. This is a pass–fail test, with nodegrees of bating easily discernible.

2. The air permeability method for skins. This test relies on the effect ofopening up on the ability of the pelt to allow the passage of air through it.

Table 8.1 Reaction mechanisms of some bating enzymes.

Enzyme Site of attack

Trypsin Pancreatic serine protease. High specificityCleaves the peptide link on the carbonyl side of basic aminoacids, lysine and arginine. Only applies to the telopeptideregion, not the helical region

Chymotrypsin Cleaves the peptide link on the carbonyl side of amino acids witharomatic sidechains, i.e. phenylalanine, tyrosine, tryptophan(note, there is no tryptophan in collagen)

Optimum pH7–9Nagarse Isolated from Bacillus subtilis

Non-specific cleavage. Preferably active against smallpolypeptides

Subtilisin As for nagarsePapainase Sulfhydryl protease from papaya fruit

Active against long polypeptides containing Phe-X-YLeucineaminopeptidase

a-Aminoacyl exopeptidase, active towards peptides containingfree a-amino group

Metal activated, 3–4mM Mn(II), Mg(II)Prolidase Isolated from pig kidney

Metal activated, cleaves peptide bonds containing proline andhydroxyproline

Pronase Non-specific enzyme, isolated from Streptomyces griseusVery broad reactivity

173Bating

The tanner folds the pelt to create a bubble by trapping air. Holding thepelt tightly, the bubble is gradually made smaller, but the amount of air ismaintained. This creates pressure within the bubble, forcing air from theinside to the outside, but only if transmission is allowed by the degree towhich the fibre structure is opened up. This is also pass–fail test, with nodegrees of bating easily discernible.

The progress of the bating reaction can be followed more accurately using thebiuret test, which is a test for protein, using basic copper(II) sulfate.5,6 Theargument is that at the site of enzyme action there will be degraded protein, sothere will be more reaction with the reagent than in unaffected regions. Thereagent, which can be made in situ, is applied to a freshly cut edge: a drop ofconcentrated copper(II) sulfate solution is put onto the cut surface, followed bya drop of concentrated sodium hydroxide. Where there is enzyme activity, thereis a purple colour reaction. Even though the colour developed in this way isagainst a blue background it can be easily discerned.

8.4 ENZYME ASSAYS

Enzymes can be assayed in the tannery with ease. The analysis is based onenzymes attacking a solid, insoluble protein substrate that is dyed with areactive, covalently bound dye (Chapter 16): the rate of reaction is measured bythe rate at which the substrate is solubilised, measured by the rate at whichcolour goes into solution. Some commonly used substrates are set out below:

� Undyed casein: the Lohlein Volhard test, conducted at pH 8.2, for proteaseactivity.

� Azo albumin: blue substrate made from bovine serum albumin, for generalprotease activity. Since the preparation of standard hide powder includesliming,4 the collagen is not pristine and hence the assay is not specific.

� Undyed hide powder, for collagenase activity.� Hide powder azure, denatured or undenatured: blue acid dyed collagen,

for general protease activity or collagenase activity. The substrate may giveproblems of too high a background colour in solution. Denaturing isachieved if the substrate is washed hot, to remove excess or loosely bounddye. An alternative or additional washing treatment is with a hydrogenbond breaking solvent, such as dimethyl sulfoxide, which also causesdenaturation of the collagen. Even if the HPA is allegedly undenatured,the argument about the specificity of enzyme action on non-pristine col-lagen also applies.

� Hide powder black, denatured or undenatured: collagen dyed black withreactive dye having two sites for binding, so gives less colour in controls. Itis used for general protease activity or collagenase activity.

� Azocoll: blue substrate for general protease activity.� Elastin red: red substrate for elastase activity.� Keratin azure: blue substrate for keratinase activity.

174 Chapter 8

The principles are as follows. Prepare a solution at the pH for testing, pre-ferably using a buffer. This is a test tube type of volume.Add the substrate: coloured material makes the test more sensitive.

The amount is theoretically unimportant, because it is not in solution andwill not be completely used up. However, in practice, because dye may bereleased by the control, it is preferable to use consistent amounts of thesubstrate.Agitate the solution at the temperature for testing for a specified length

of time: this can be determined experimentally, but typically might bean hour.At the end of the specified time, rapidly filter the solution, preferably through

inert glass wool, to remove residual substrate.The solution contains the products of the degradation reaction. Measure the

amount, either by spectrophotometer or some other device for measuring thecolour or optical density.The test is set up so it can be assumed that the rate of reaction is relatively

constant as the initial rate, when it can be assumed the kinetics are zero order,as shown in Figure 8.4, where the dotted line indicates a suitable time for theassay to run.In this way, the relative activities of enzymes can be quantified. The method

can be used to detect the presence of enzymes. For example, using elastin redand typical conditions, such as 35 1C and pH8.5 for one hour, the presence ofred colour in the filtered solution demonstrates the presence of elastase in thebate, depending on how much colour is generated in the control. Notably, thestatus of the protein can lead to false positive results in the assay for col-lagenase: denaturation or degradation of pristine collagen is difficult to avoid,so it is necessary to view the effects of proteases on (allegedly) intact collagenwith scepticism.All these assays can be automated: the solution can be circulated through the

cuvette of a spectrophotometer and the optical density can be recorded con-stantly. All that is required is to isolate the suspended substrate from thecuvette, which can be accomplished with a sintered glass barrier.7 If required,the quantitative kinetics of the reaction can be calculated.

colour

Initial rate

Time

Figure 8.4 Typical shape of enzyme kinetics.

175Bating

REFERENCES

1. K. T. W. Alexander, J. Amer. Leather Chem. Assoc., 1988, 83(9), 287.2. L. Michaelis and M. L. Menten, Biochem. Z., 1913, 49, 333.3. S. Marti, et al., Chem. Soc. Rev., 2004, 33, 98.4. O. Menderes, PhD Thesis, The University of Leicester, 2002.5. O. Glezer, BSc Thesis, Nene-University College, 1997.6. A. Onyuka, BSc Thesis, The University of Northampton, 2004.7. R.G. Stosic, PhD Thesis, University of London, 1995.

176 Chapter 8

CHAPTER 9

Pickling

9.1 INTRODUCTION

The pickling process is primarily conducted to adjust the collagen to the con-ditions required by the chrome (or any other) tanning reaction. The traditionalrecipe for pickling is as follows, based on limed weight of pelt:

� 100% float� 10% salt� 1% sulfuric acid

The timing of the pickling step is variable, depending on circumstances, notleast of which is the thickness of the pelt. Typically, it is conducted for about anhour before adding the chrome tanning salt.The order of addition of the components is as follows:

1. To drained pelt, add the salt offer and run to allow the salt to be absorbedinto the pelt as concentrated solution – to avoid acid swelling.

2. Add most of the float, nominally 90%.3. Add the acid, previously diluted 10 : 1 in water and cooled – to avoid

localised high concentrations of (hot) acid, which can cause hydrolyticdamage (see below).

4. The offer of acid in pickling is determined by the pH required for thechrome tanning process: this varies, but for chrome tanning typically liesin the range pH2.5–3.0, since a solution of 33% basic chrome tanning saltis at pH2.7–2.8.

The function of the acid is to acidify the collagen, to protonate the carboxylgroups: in this way the reactivity is modified, because the chrome tanningreaction only involves ionised carboxyl groups. The effect of acidification




177

can be calculated from the Henderson equation, applied to the ionisation of aweak acid.

HA!Hþ þA�

The dissociation constant Ka is given by:

Ka ¼½Hþ�½A��½HA�

Taking logarithms and multiplying by �1:

� logKa ¼ � log½Hþ� � log½A�� þ log½HA�

pKa ¼ pHþ log½HA�½A�� ðHenderson equationÞ

Since the pKa values for aspartic and glutamic acids are 3.8 and 4.2, respec-tively, the overall pKa for collagenic carboxyl groups must lie between thesevalues, assuming that the reaction with chromium(III) applies to both types ofsidechain carboxyl groups. Therefore, for calculation purposes, the pKa forcollagen carboxyls can be assumed to be 4.0 (Table 9.1).Notably, another aspect of the pickling reaction is the contribution to

opening up. In the same way as hydrolysis of peptide bonds is accelerated byhydroxyl catalysis, the same reaction is catalysed by hydrogen ion:

� CO-NH�þH2OÐOH�

�CO�2 þH2N-

� CO-NH�þH2OÐHþ

�CO2HþH2Nþ-

For most processes, in conventional rapid pickling, this reaction is unim-portant because it is relatively slow. However, there are two situations when thereaction can be important:

1. Storage pickling, as a preservation technique.2. As an alternative to alkali opening up for woolskins, when the wool is

vulnerable to detachment if the process goes too far too rapidly. Under

Table 9.1 Calculation of the degree of carboxyl ionisation in collagen.

pH log([HA]/[A�]) ([HA]/[A�]) [A–] (%)

1.5 2.5 316 0.32.0 2.0 100 12.5 1.5 31.6 33.0 1.0 10.0 93.5 0.5 3.2 244.0 0.0 1.0 504.5 –0.5 0.32 76

178 Chapter 9

conditions of alkalinity or proteolytic enzyme action, the wool on wool-skins can become loosened or even detached, referred to as ‘woolslip’.This is clearly undesirable if pelt quality depends on the presence of wool,e.g. for rugs or for ‘double face’. In this case, woolskins can be stored forseveral weeks in the conventionally pickled condition, to allow proteinhydrolysis to take place, making the resulting leather softer. Under theseconditions, the opening up reactions are much slower than alkalineopening up, and hence are more easily monitored/controlled for delicaterawstock.

An additional function of the pickling process is to complete the depleting ofthe pelt, following alkaline swelling. From the swelling curve reproduced inFigure 9.1, the pelt goes through minimum swelling, the isoelectric point, andthe depletion is maintained by the effect of neutral electrolyte. In this way,damage to the pelt structure is avoided and the subsequently applied reagentshave the minimum thickness of pelt to penetrate (without collapsing thestructure). The problem of cationic collapse is most commonly observed in theform of ‘pickle creasing’, caused by adhesions occurring as the pelt dries out,particularly along folds or creases during storage. Light adhesions can bereversed by tumbling the pelt in warm brine with non-ionic detergent:intractable creases may be removed by tumbling drained pelt first in paraffin,then in warm brine and non-ionic detergent. Some creases may not be remo-vable at all if new bonds are formed by the fibre structure collapse, as describedby Komanowsky.1

9.2 PROCESSING CONDITIONS

9.2.1 Float

The overall float length is not critical to the efficiency and effectiveness of thepickling process. The traditional and typical amount of water reflects morethe requirement of the subsequent process, in which the concentration of thechrome tanning reagent exerts some control over the efficiency of the reaction.

9.2.2 Salt

To avoid acid swelling, the concentration of sodium chloride in the whole of theavailable solution (added water+water carried over from the previous step+water within the pelt) should not be less than 1.0 molar, corresponding to 6%.The relationship between salt and acid swelling is shown again in Figure 9.1.The relationship assumes that the swelling is prevented by the presence of thesalt, rather than the swelling being reversed by the electrolyte. There is a dangerthat acid swelling may cause disruptive damage that cannot be reversed by theaddition of electrolyte: hence there is the requirement for the specified order ofaddition of pickling components.The swelling curve is not symmetrical over the pH scale: the shape of the

curve must be rationalised, in order to understand the role of conditions in

179Pickling

leather making and their effects on collagen. We can begin by considering thechanges that occur within collagen when the pH is changed, as indicated inFigure 9.2, in which P represents protein.The development of charge and its effect on swelling must be the same at

both ends of the pH scale. However, clearly acid swelling has some differentfeatures because:

� The curve exhibits a turning point� The effect of salt is to reverse or prevent acid swelling. The extent of the

reversal depends on the salt concentration – the higher the salt con-centration the less swelling is observed.

From the Henderson equation, the ratio of acid to basic form of thecarboxyl groups can be calculated at any pH, assuming the pKa of collagen is4.0 (Table 9.1). At pH2 the titration is effectively complete, so there is maxi-mum charge, corresponding to maximum swelling.

800

700

600

500

400

300

200

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Final pH

3% salt

No saltNo salt

3.0% salt


Figure 9.1 Relationship between pH and acid swelling and the role of neutralelectrolyte.

P-NH2 -O2C-P

OH-

P-NH3+-----O2C-P

At the isoelectric point H+

P-NH3+ HO2C-P

Figure 9.2 Effect of pH on the charge status of protein (P¼protein).

180 Chapter 9

The observation of the effect of salt on acid swelling clearly indicates thatthere is another influence beyond merely smearing out the charge on the colla-gen. Consider again the effect on the charge of the protein of changing the pH(Figure 9.3).When collagen is acidified, the acid effectively binds electrolyte to the protein:

therefore, in an acidification reaction in water, there is a net increase in elec-trolyte within the pelt, compared to outside the pelt. If the surface of the peltcan be regarded as a semi-permeable membrane, across which electrolyte andwater can pass, an osmotic pressure will be developed. Therefore, water willpass into the pelt, to reduce the chemical potential within the pelt, because theelectrolyte concentration is higher than the concentration in the externalsolution. This is observed as swelling.The phenomenon of diffusible and non-diffusible salts separated by a semi-

permeable membrane was analysed by Donnan.2 In the case of pelt in acid, theelectrolyte inside and outside the pelt is separated by the pelt surface and theinternal structure of the pelt constitutes the non-diffusible electrolyte, R�. Whensalt alone is present, the situation at equilibrium can be summarised as follows:

outside : insideNaþ Cl� : Naþ R� Cl�

ðc1 � xÞ ðc1 � xÞ ðc2 � xÞ c2 x

The system must be electrically neutral on both sides of the surface and thechemical potentials must be equal:

mNaClð1Þ ¼ mNaClð2Þ

The chemical potential of an electrolyte is the sum of the potentials of the ions:

m�NaþþRT ln aNaþð1Þ þ m�Cl� þ RT ln aCl�ð1Þ

¼ m�Naþ þ RT ln aNaþð2Þ þ m�Naþ þ RT ln aNaþð2Þ

where a is the activity.Therefore:

aNaþð1Þ � aCl�ð1Þ ¼ aNaþð2Þ � aCl�ð2Þ

P-NH2 -O2C-P M+

MOH

P-NH3+-----O2C-P

At the isoelectric point HA

P-NH3+ A- HO2C-P

Figure 9.3 Effect of acidification and basification on protein.

181Pickling

Under dilute conditions, the activities can be substituted for concentrations.Therefore:

ðc1 � xÞðc1 � xÞ ¼ ðc2 � xÞx

If we now consider the situation of a protein in acid, as is the case in pickling,the acid creates cations at the sites of the amino groups. Note, although thecationic nature strictly arises because of the breakdown of the salt links,the reaction can be regarded as fixation of acid on the protein. Referring to theprotein as P, the equilibrium can be represented as follows.

outside : insideHþ Cl� : Hþ Cl� PHþ Px x y ðyþ zÞ b a

If the protein has n basic groups, then b¼ z/n.Therefore, as before:

x2 ¼ yðyþ zÞ

or

z ¼ ðx2 � y2Þy

where z is the excess acid bound to the protein.In pickling, swelling must be avoided, so the Donnan equilibrium condi-

tion has to be met, otherwise there would be a net osmotic diffusion of waterinto the pelt. The solution is to have an excess of electrolyte, which can equi-librate across the semi-permeable membrane. This changes the situation asfollows:

outside : insideNaþ Hþ Cl� : Naþ Hþ Cl� PHþ PA x xþ A A y ðAþ yþ zÞ b a

ðAþ xÞðxþ AÞ ¼ ðAþ yÞðAþ yþ zÞ

If A is large compared to x, y and z, then the equation is effectively balancedand the Donnan equilibrium condition is met. Typical pickling conditionsinclude 6% offer of salt and the float is 100%.Assume the concentration of added salt, inside and outside¼ 1.0 M, i.e.

[Na1]¼ [Cl�]¼ 1.0M.

The concentration of pickling acid¼ 10�2M¼ [H1]¼ [Cl�], applying to both

inside and outside.

182 Chapter 9

If we assume all amino groups on collagen are protonated, the counterion(Cl�) concentration in solution balances the amino content of 0.6 moles per kgdry collagen.Assuming the float is 100% for collagen containing 75% moisture, the anion

concentration¼ 0.15M. Therefore, the relationship with regard to the Donnansituation can be calculated.In the absence of added electrolyte:

Outside : ð0:01Þ � ð0:01Þ ¼ 1:0� 10�4 M

Inside : ð0:01þ 0:15Þ � ð0:01Þ ¼ 1:6� 10�3 M

A difference of 16 times or 1600%.In the presence of added electrolyte:

Outside : ð1:0þ 0:01Þ � ð1:0þ 0:01Þ ¼ 1:02M

Inside : ð1:0þ 0:01þ 0:15Þ � ð1:0þ 0:01Þ ¼ 1:17 M

A difference of only 16%.The Donnan equilibrium condition can be achieved using any electrolyte,

although the side effects of lyotropic swelling must be borne in mind. For any 1:1monovalent salt, the calculated amount is based on molar activities, which can besimplified to concentration. In the case of multivalent ions, the concentration canbe corrected for activity coefficients from the calculation of ionic strength (I):

I ¼ 0:5Sciz2i ¼ 0:5½c1z21 þ c2z22 þ c3z

23 þ . . .�

where: ci is the molar concentration of ion i and z is its charge.Among the alternative neutral electrolytes that might be used in place of

sodium chloride, the most likely candidate is sodium sulfate. There are asso-ciated environmental impact problems with its use, because sulfate ion canleach calcium ions from concrete and hence weaken it. The effect on seweragesystems means that consent limits for sulfate discharge in effluent are likely tobecome increasingly stringent. Sulfate can interact with chromium(III) species,affecting hydrolysis rate and influencing the nature of the species in solutionand hence the tanning effect (Chapter 11).The concentration of salt in pickling must be subject to control, because too

much will constitute a problem. In the same way as salt causes dehydration incuring, if the concentration in pickle is higher than the minimum requirementto meet the Donnan equilibrium condition, the semi-permeable pelt system willallow water to move from the pelt into solution, causing the fibre structure tocollapse, as it does in wet salting curing. The lower moisture content of thepelt will restrict the free movement of solubilised reagents, possibly resulting innon-uniform chrome tanning through the pelt cross section.

183Pickling

Strictly, the Donnan condition cannot be met at equilibrium in pickling, butthe application of the electrolyte before the acid ensures the equilibrium isapproached from the side corresponding to depletion rather than the sidecorresponding to swelling. Equilibrium is not established in the industrialprocess, so in this way swelling is avoided.The remaining issue is a rationale for the turning point in the swelling curve:

swelling reaches a maximum at about pH2, but additional acid causes it todiminish. The pelt cannot swell any more at pH values below 2 because thenegative charge is at a maximum. If acid continues to be added, to lower the pH,this is equivalent to adding electrolyte, even though one of the ions is H1. AtpH1.0, the concentration of acid is molar, comparable to the situation calculatedabove for the addition of the conventional amount of salt in pickling, and so theswelling curves with and without salt converge at low pH.

9.3 LYOTROPIC SWELLING

There is another contributor to swelling, namely the lyotropic effect. Here,species are inserted into hydrogen bonds, thereby opening up the structure, asintroduced in the discussion of liming, and indicated in Figure 9.4. The effectdoes not depend on pH, so the effects of ions and electrically neutral com-pounds apply equally under acid and alkaline conditions.In the case of the lyotropic ions, the insertion of the ion converts the

attractive interaction into a repulsion interaction, as indicated by the doubleheaded arrows in Figure 9.4. Alternatively, a neutral compound capable ofinvolvement in hydrogen bonding can insert itself into the protein hydrogenbonding. In each case, the natural structure is disrupted and the protein mayeven be solubilised. Here, the effect of unionised weak acids must be taken intoaccount. For example, comparing formic and acetic acids under conditions ofpickling for chrome tanning: starting at pH 2.75 and ending at pH 3.75, forcalculation purposes (Table 9.2).

C

H

OC

OC

OC

NH

NH

NH

M+ M+

A- A-

OO

C

R

H

RCO2H

O

N

Figure 9.4 Lyotropic interactions.

184 Chapter 9

Under either condition, acetic acid equilibrates with a higher proportion ofunionised, lyotropic form, thereby constituting a greater lyotropic swellingdanger.

9.4 SULFURIC ACID

The commonest acid used in industry is sulfuric, because of its ready avail-ability and low cost. It is a feature of the properties of concentrated sulfuricacid that it releases a lot of energy when it is diluted, indeed the solution canoften boil. It is imperative that the acid is diluted and cooled prior to appli-cation in this step: failure to cool the solution will result in rapid, acid-catalysedhydrolytic damage, particularly to the grain. This type of damage is unde-tectable in wet pelt or leather; it is only manifested in crusted leather, when thefault appears as ‘cracky’ grain, a weakness in the grain manifested as crackingopen if it is stressed. The damage is variable and may only be observed by adouble fold: the appearance of cracky grain is almost certainly due to hot aciddamage.Here, a salutary and true tale can be told. A tanner was regularly having

problems of cracky grain in his leather, so he turned to a consultant. Theconsultant reviewed the process, convinced from the outset that the cause washot acid damage. According to the procedure in the tannery, which was alwaysfollowed, the sulfuric acid was added to water (the right order of addition) andthe warm solution was allowed to cool overnight, before it was used forpickling the following day. It appeared the tanner was doing everything right,but still the problem persisted. However, with a few questions, the consultantdiagnosed the source of the problem: The solution was not stirred properly.When the acid was added to the water, the dense acid flowed to the base of

the preparation vessel, through the dilution water. Mixing between the liquidswas minimal, so the heat of dilution only warmed the water; most of the heat ofdilution was not liberated. True mixing and diluting only happened when thewater and undiluted acid were pumped into the pickling float. When completemixing of acid and water occurred, the liberated heat then caused damage.The effect was not very different to adding concentrated acid directly to thepickle float.The solution was simple: install a motorised mixer to mix the acid with the

water as it was added. The problem disappeared.

Table 9.2 Ionisation of formic acid (pKa 3.75)and acetic acid (pKa 4.75).

pH[HA] : [A�]

Formic Acetic

2.75 10 1003.75 1 10

185Pickling

9.5 HYDROCHLORIC ACID

Of all the available alternative strong acids, only hydrochloric acid has foundindustrial application. For all other commonly available acids, issues of cost,damage to the collagen or interference with the chrome tanning reaction rulethem out.As is the case for sulfuric acid, hydrochloric acid also must be diluted before

adding it to the pickle float. There is no heat of dilution, because the acid isalready in aqueous solution, but the damaging effect of localised high hydrogenion concentration must be avoided, because of its catalytic effect on proteinhydrolysis.It is claimed that hydrochloric acid pickling makes flatter leather: this may be a

reflection of the relative lyotropic effects of the chloride and sulfate ions. How-ever, a review of ionic concentrations indicates that such an argument is weak,because the pickling period is short, then the addition of chrome tan powderintroduces 50% of its weight as sodium sulfate, not to mention the contributionof the sulfate as counterion for the chromium(III) (Table 9.3). The short period ofpickling (usually 1 hour) is followed by a longer period of low reactivity chrometanning, when the differences between the pickling systems are effectively elimi-nated. In each case, it does not make chemical sense to argue whether the con-ditions refer to hydrochloric acid+sodium sulfate or to sulfuric acid+sodiumchloride, because the acids are strong electrolytes and so are the salts, thereforethey only exist in solution as separated ions. This argument can apply, becausethe pickling process is rarely conducted to equilibrium prior to starting chrometanning. If it is necessary to consider the ionic strength of the solution before andafter chrome tanning, the only difference arises from the concentration differencein sulfate from sulfuric acid in comparison to hydrochloric acid.

9.6 FORMIC ACID

It is common for the acid in the pickle formulation to be split between sulfuricand formic acids. Since formic acid is more expensive than sulfuric, there has tobe a distinct technological advantage for its inclusion.As for sulfuric acid, formic acid must be diluted before adding it to the pelt.

However, in this case the reason is not concerned with heat damage, becausethere is little heat of dilution. Nor is the danger concerned with localised highconcentration of hydrogen ion for hydrolysis catalysis, even though the acid isweak. Here the danger is associated with the lyotropic effect of the molecule,which can cause swelling damage by hydrogen bond disruption. The acid canbe added as the concentrated acid or in the form of sodium formate, which canbe converted into the acid by the addition of extra sulfuric acid, according tothe following equation:

HCO2NaþH2SO4!HCO2Hþ SO2�4 þNaþ þHþ

It is often said that: ‘formic acid penetrates faster than sulfuric acid’.

186 Chapter 9

Table9.3

Ioniccontentofpickleform

ulations(m

olar).Assume:100%

float,6%

NaCl,0.2

molar[H

1],chrometanpowder

offer

of8%.a

HClpickle

H2SO

4pickle

H1

Na1

Cl�

SO

4�

Cr(

III)

H1

Na1

Cl�

SO

4�

Cr(

III)

Salt

–1.0

1.0

––

–1.0

1.0

––

Acid

0.2

–0.2

––

0.2

––

0.1

–NaCl

–0.5

–0.25

––

0.5

–0.25

–Cr(OH)SO

4–

––

0.25

0.25

––

–0.25

0.25

Totalmolarity

0.2

1.5

1.2

0.5

0.25

0.2

1.5

1.0

0.6

0.25

Ionic

strength:before

chrome

1.95

2.05

Ionic

strength:after

chrome

2.6

2.7

aA

33%

basicchromepowder

contains25%

Cr 2O

3,em

piricallyasCr(OH)SO

4,and50%

sodium

sulfate.

187Pickling

Technologically, what this means is that a pickle formulation containingformic acid will acidify through the pelt cross section faster than a formulationcontaining sulfuric acid alone. The assertion is not strictly true. Whilst it is truethat formic acid penetrates, strictly sulfuric acid does not penetrate. In the caseof any strong acid, the penetrating species is hydrated hydrogen ion, thehydronium ion, H3O

1. The penetration rate is hindered by the ion carrying afull charge, because it can interact with the protein substrate. In contrast, theweak formic acid penetrates predominantly in the form of an electrically neu-tral molecule and therefore can move through the protein relatively unhin-dered. In the production of hides, the penetration rate benefit of includingformic acid may be reason alone to use it.It is a common misconception that the inclusion of formate in the pickle

constitutes a masked chrome tannage. The term ‘masking’ is defined as thecreation of a complex between a ligand moiety, such as between formate, and ametal tanning species, such as chromium(III): this traditionally is part of tanningtechnology, because the modification to chrome reactivity is designed to reducethe ability of the chrome species to complex further and thereby to reduce therate of fixation with respect to the rate of penetration through the pelt. However,there are two problems associated with that view of chromium(III) complexchemistry:

1. The rate of reaction between chromium(III) and formate is the same as therate of reaction between chromium(III) and collagen, they are both car-boxylate complexation.3 This has been demonstrated experimentally usingacetate as the masking ligand. Therefore, the chromium must beunmasked at the beginning of the reaction and become masked as thetanning reaction proceeds: this runs counter to the requirement forincreasing the chrome reactivity as the reaction proceeds.4 If the tanningprocess calls for true formate masking, the complexation reaction must beundertaken prior to initiating the tanning reaction or an appropriatelypremasked tanning product must be purchased (Chapter 11).

2. The assumption of reactivity reduction is flawed, since it is possible,indeed probable, that masking can increase the reactivity of chromium(III)complexes, depending on the chemistry of the masking agent and themasking ratio, defined as the ratio of the number of moles of appliedligand to the number of gram atoms of metal5 (Chapter 11).

9.7 COLOUR

The colour of chrome tanned pelt, called ‘wet blue’, depends on how the tan-ning reaction is conducted: it is determined by the nature of the chromium(III)complexes created during the process and the complexation of the tanningagents in situ. The colour of transition metal complexes depends on theligand field around the central metal ion: in the case of chromium(III) thereare six ligands in the octahedral complex. In a tanning context, the ligands are

188 Chapter 9

likely to be water, oxy bridges and carboxy compounds, including collagen(Chapter 11).The ligands in the octahedral ligand field of chromium(III) split the five

degenerate 3d orbitals into non-degenerate orbitals and the energy differencebetween them determines the colour (Figure 9.5).If the chrome tannage is conducted in a sulfuric-acid-only pickle liquor, the

leather is coloured green-blue, because the ligand field is modified by sub-stituting an aquo ligand for a carboxylate group attached to protein. Theadditional inclusion of formate in the ligand field changes the colour to brightpale blue. The paler colouring is much better for dyeing, which is influenced bythe base colour, especially pastel shades. In many cases, this is actually the solevalid reason for including formate in the system.The addition of masking salt from the pickling process into the complex

strongly influences the colour of the chrome complexes and hence modifies thecolour of the leather (Table 9.4).Some processes employ a sulfuric acid pickle and introduce sodium formate

either during the tannage or as part of the basifying system towards the end ofthe chrome tanning process (formate is a mildly alkaline salt, see calculationbelow, where the mass terms are given as concentrations although they shouldstrictly be activities). These approaches both result in a masked chrome process,yielding pale blue leather:

HCO�2 þH2O!HCO2HþOH�

dx2-y2, dz2

3d degenerateorbitals Δenergy = colour

dxy, dxz, dyz

Figure 9.5 Effect of the ligand field on the energy of the d orbitals of chromium(III).

Table 9.4 Effect of complexation on the colour of chromium(III) ions.

Masking salt/ligandCr(III) complexcolour

Leather colour – paler thanthe Cr complex itself

Water only Violet Pale blue/greenHydroxyl Dark green Pale blue/greenPhosphate Apple green Apple greenFormate Pale blue Pale blueAcetate Royal blue Royal blueEDTA Purple Purple

189Pickling

The hydrolysis constant is:

Kh ¼ ½HCO2H�½OH��½HCO�

2 �

The dissociation constant is:

Ka ¼½HCO2H�½Hþ�

½HCO�2 �

Kh ¼ ½Hþ�½OH��Ka

The ionic product of water, Kw¼ [H1][OH�].

Kh ¼ Kw

Ka¼ 10�14

10�4¼ ½OH��2

½HCO�2 �

If it is assumed that [HCO2�]C [HCO2

�]total, since the salt is a relatively strongelectrolyte, and that the total concentration of formate is 0.1 molar: [OH�]2¼10�11. Therefore, pOH¼ 5.5, i.e. pH¼ 8.5.The colour of the chrome tanned leather can be modified by the use of

reagents capable of complexing in post tanning steps, such as ‘neutralisation’,when the pH of the leather is raised by basic compounds. Although the colourof the chrome tanned leather can be altered by the choice of masking agents,the effect on dried or ‘crust’ leather colour is slight and insufficiently deep inshade to constitute a colouring process. The influence of tanning on colouringis limited to modifying the colour struck in the conventional dyeing process.

9.8 NON-SWELLING ACIDS

So-called ‘non-swelling acids’ can be used in place of conventional picklingacids: they can be regarded as auxiliary syntans (synthetic tanning agents) in theacid form (Chapter 14). These reagents may be characterised as possessing thefollowing features:

� aromatic, typically phenyl or naphthyl, which may be substituted, e.g. withalkyl or other groups;

� low content of phenolic hydroxyl groups, including none at all;� high content of sulfonic acid groups.

They have low tanning ability, due primarily on their lack of phenolic hydroxylcontent, but they function as strong acids because of their sulfonic acid groupcontent.Table 9.5 contains data for some examples of non-swelling acids: the range of

possible structures is wide, as may be seen in Heidemann’s book.6

190 Chapter 9

Notably, the pH will influence the shrinkage temperature of the collagen;in the case of acidification to the values shown in Table 9.5, the shrinkagetemperature might be as low as 50 1C.The properties of the non-swelling acids can be summarised as follows:

1. At similar pH values, the swelling effect is inversely related to the increasein shrinkage temperature.

2. The tanning effect, as measured by the shrinkage temperature, is directlyrelated to the complexity of chemical structure.

3. Complexity of structure is a function of the presence of phenolic hydroxylgroups (primarily) and sulfonic groups (secondarily).

Non-swelling acids perform two functions:

1. The sulfonic acid group is strong and is therefore effectively completelyionised in solution: R-SO3H!RSO3

�+H1.The situation can be understood by analogy with sulfuric acid:

(HO)2SO2! [O2SO2]2�+2H1.

Therefore, the sulfonic acid equilibrium could be rewritten in the formRS(OH)O2!RSO3

�+H�.This view of the equilibrium highlights the properties of sulfonate/sulfonic

acid chemistry, which is a common feature of reagents in leather makingchemistry. In aspects other than non-swelling acids, the group is in theneutralised form, as the sulfonate group, and typically undergoes fixationunder acid conditions (Chapters 15 and 16). Just as sulfate ion is not conver-ted into sulfuric acid by mild acidification (the reaction must be conductedunder extremely acidic solution, when the solvent is effectively no longerwater), sulfonate groups are not protonated. The reaction involves electro-static interaction with liberated charged amino groups on the collagen.

2. The compounds interact with collagen: the degree to which they inter-act and how they interact creates a tanning effect. Therefore, the term

Table 9.5 Effects of some non-swelling acids.

pH Ts (1C)Swelling(%)

Tanningeffect a

Water 6.3 66 100 0Sulfuric acid 1.2 43 235 –Benzene sulfonic acid 1.8 49 198 –4-Hydroxybenzene sulfonic acid 1.8 58 193 –Naphthalene-2,6-disulfonic acid 1.8 60 114 0Naphthalene-2-sulfonic acid 2.3 61 101 03,4-Dicarboxybenzene sulfonic acid 1.7 62 112 0Naphthalene-2-hydroxy-3,6-dis-ulfonic acid

1.7 63 87 +

4-Hydroxybenzyl sulfonic acid 2.1 65 62 ++Naphthalene-2-hydroxy-3-sulfonicacid

2.1 68 63 ++

aScored semi-quantitatively positively or negatively.

191Pickling

‘non-swelling acid’ is relative, since not all members of the group canovercome the swelling effect of low pH. The degree of effectiveness dependson the extent to which the agent acts as a syntan, since conventionaltanning confers resistance to osmotic swelling. Note, the only differencebetween a non-swelling acid and a syntan lies in the stage of production.This type of syntan is conventionally made by sulfonating an aromaticcompound – the resulting compound is a sulfonic acid, a non-swelling acid.After neutralisation to the sulfonate salt, the auxiliary syntan is produced.

Notably, the use of a non-swelling acid implies a degree of stabilisation of thepickled pelt, which can also be viewed as a pretannage. Therefore, it should beexpected that the more effective a non-swelling acid is, the more the character ofthe leather will be altered by the pretanning process. However, the moreeffective the tanning is, the safer it is to conduct a salt-free pickling process.7

9.9 PICKLE FORMULATIONS

Clearly, from the information set out above, there is an optimum formulation forpickling. This implies that deviation from the optimum conditions will createproblems. This is summarised in Figure 9.6, which applies both to conventionaltannery pickling and curing by storage pickling. The diagram, modified fromJordan Lloyd,8 indicates that insufficient or excess components of the pickle

Salt Excesssalt

Saltdeficiency

Excessiveacid effects

Hydrolytic acid damage

Excessivehydrolytic acid

Optimum acid, optimum salt

NO DEFECTS

Bacterial growth+ acid swelling

Depleted pelt= resistance toreagent penetration

Acid + bacterialdamage

Bacterial growth= putrefaction

Depletedpelt

Excess acid

Acid deficiency

Figure 9.6 Effects of variations in pickling formulation compositions (modified fromref. 8).

192 Chapter 9

solution can be equally counterproductive and result in problems in processing orin leather performance.

9.10 IMPLICATIONS FOR CHROME TANNING

Because tanners routinely deal with a substrate of finite thickness this implies thatconditions may vary through the cross section. Figure 9.7 shows examples of the

Limed Pickled Chrome tanned

completely delimed uniform

one third delimed

surface delimed

pH 2.5-3.5 pH 3.5-4.5 pH>4.5

Increasing chromecontent

Figure 9.7 Implications of variations in pH profiles through hide cross section –modelling the effects of notional pH ranges over the pelt cross section.

193Pickling

effect of different pH profiles, illustrating how three variations of liming practicecan yield (notionally) five patterns of pickling profile. Since chrome fixation isdependent on the pH conditions met by the chrome as it penetrates, the pattern ofchrome fixation through the cross section can also vary in those notional ways.The practice of partial deliming used to be common, although less so

nowadays, probably because of issues of inconsistency. Processing closer toequilibrium, at least to apparent pH uniformity over the cross section, will yieldmore consistent results in subsequent steps, although the outcome may not beideal, as discussed below.Pickling to equilibrium will cause lower fixation in the centre than at the

surfaces, because the continuing generating of acid during the tanning reactionwill reduce fixation as penetration proceeds and there is always a higher con-centration of chrome outside the pelt than inside. Pickling close to equilibriumwill result in more uniform fixation, with the pattern dependent on the degree ofpickling. As the pickling becomes more non-uniform, with a distinctly lessacidified central region, the chrome fixation in the centre can be higher than atthe surfaces. In a more extreme case, high chrome fixation at the surfacescreates a barrier to chrome penetration, resulting in much lower fixation in thecentre, even leaving it untanned.The outcome of non-equilibrium deliming and pickling, as illustrated in the

figure is that more chrome should be deposited in the centre than at the sur-faces. This has two benefits:

1. Following tanning and splitting (of bovine leather), the split surface ismore likely to be strongly and uniformly coloured, reducing the require-ment for chrome retanning for cosmetic purposes (Chapter 15).

2. Whatever the reasons for chrome retanning may be, the process typicallytends to tan the surfaces more than the cross section, because of theconditions used. This means the grain is tanned more than necessary andmay result in lesser affinity for acid dyes (Chapter 11).

REFERENCES

1. M. Komanowsky, J. Amer. Leather Chem. Assoc., 1989, 84(12), 369.2. F. G. Donnan, Kolloid-Z., 1910, 7, 208: J. Chem. Soc., 1911, 99, 1554.3. S. G. Shuttleworth and A. E. Russell, J. Soc., Leather Trades Chem., 1965,

49(6), 221.4. A. D. Covington, J. Amer. Leather Chem. Assoc., 2001, 96(12), 467.5. A. D. Covington, J. Amer. Leather Chem. Assoc., 2008, 103(1), 7.6. E. Heidemann, Fundamentals of Leather Manufacturing, Eduard Roether,

Darmstadt, 1993.7. P. M. Pojer, C. P. Huynh, Leather, April 1999.8. D. Jordan Lloyd in Progress in Leather Science: 1920–45, British Leather

Manufacturers’ Research Association, p. 151, 1948.

194 Chapter 9

CHAPTER 10

Tanning

10.1 INTRODUCTION

Before reviewing the options for tanning, it is first necessary to consider whatconstitutes a tanning reaction. More specifically, what is the definition oftanning? Most practitioners will offer some or all of the following features oftanning as the expected outcomes:

1. Appearance: dried raw pelt is translucent, horny, but tanned leather driesopaque or and may change in colour, e.g. chrome tanning;

2. Handle: some degree of softness in comparison to dried raw pelt;3. Smell: some tanning agents will introduce smell, e.g. cod oil for chamois

leather, extracts of plant materials for vegetable tanned leather or alde-hydic compounds;

4. Rise in denaturation temperature;5. Resistance to putrefaction by microorganisms;6. A degree of permanence to the changes.

In fact, the only strict definition of tanning is the conversion of a putrescibleorganic material into a stable material that resists putrefaction by spoilagebacteria. Tanners do expect other changes and the first three changes in the list,appearance, handle and smell, will typically accompany a tanning process.Most tanners, however, additionally expect a rise in hydrothermal stability,typically observed as a rise in shrinkage/denaturation temperature, DTs. Fur-thermore, there is usually an expectation that the change is not transitory, buthas some degree of permanence in conventional use. Consider the examples ofmodified collagen presented in Table 10.1.

In each case given in Table 10.1, the material looks and feels like leather but,in the absence of further information or access to some testing facilities,it would not be possible to demonstrate that it is actually otherwise. However,




195

it can be seen from the table that the materials do not conform to the expectedrequirements of the definition of tanning, although these materials will notputrefy as long as they are kept dry. This begs the question: are the materialsreferred to in Table 10.1 really leather, even though they may have serviceableproperties?The solvent dried pelt is not truly stabilised, except when dry, and remains

raw: if it is wetted it reverts to its previous raw, vulnerable state. But the alu-minium tawed material may also not be considered to be leather, hence thereaction is called tawing (Chapter 12), because the reaction is reversible andthere is no increase in hydrothermal stability. Nevertheless, for hundreds ofyears this was the preferred process for making gloving leather. In contrast, oiltanning creates chamois leather (Chapter 14), which is well known as a surfacecleaning material, particularly for glass and automobile bodies; it does meet allthe criteria for tanning that might be expected, except for the lack of increase inhydrothermal stability.Therefore, it is possible to distinguish two types of stabilising processes:

tanning and leathering. Tanning achieves all the expected changes to theproperties of collagen. Leathering confers enough stability for the material tobe useful, but it may not exhibit all the changes expected from a tanningreaction, so there may be some limitations to the range of uses.

10.2 HYDROTHERMAL STABILITY

Hydrothermal stability is the measurement of the resistance of a material to wetheat. In the case of collagenic materials, pelt or leather, it is the effect of heat onwater saturated material. Note, the thermal properties of collagenic materialsare dependent on water content,1,2 hence when they are tested in this contextthey are required to be wetted to equilibrium and therefore they are presentedin a reproducible condition for testing.3 The shrinkage temperature has beenassumed to be the breaking of hydrogen bonds in the triple helices, whichwould apply equally to raw and chemically modified (tanned) collagen,allowing the protein to undergo the transition from helix to random coil.4,5

Chapter 19 develops this point.

Table 10.1 Some observations on modified collagen.

Material AppearanceDTs

(1C) a Effect of water

Solvent dried pelt White, opaque, soft 0 Reverts to rawAluminiumtawed b

White, opaque, soft 0 Al(III) washed out,reverts to raw

Oil tanned Beige, opaque, soft 0 No change, absorbsand holds water

aQuoted here as zero, but the reactions may stabilise collagen by a few degrees only, depending onthe conditions of processing.bThe term tawing is traditional, used in contrast to the term tanning, because it is relativelyreversible.

196 Chapter 10

Weir5 has demonstrated that, in the early stages of the transition between theintact helix and random coil structures of collagen, shrinking is a rate processand the rate is influenced by the moisture content: shrinking is independent ofthe soaking period once the collagen is hydrated to equilibrium, but drierspecimens shrink more slowly. The rate therefore affects the value assigned tothe shrinkage temperature: the slower the rate of shrinking, the apparentlyhigher is the quoted shrinkage temperature, as discussed in Chapter 1. Thekinetics of shrinking are affected by pH: as demonstrated in Chapter 1,the value drops at pH extremes and this is reflected in the thermodynamics ofthe reaction (Table 10.2).From Table 10.2, it is apparent that the thermodynamic parameters change,

depending on the relationship of the sample’s condition relative to the iso-electric point: the further away from the point of highest stability, the lower theactivation parameter values, when the free energy is controlled by both theenthalpy and entropy terms: the former indicating a breakdown of structureand the latter indicating the change in the state of disorder. From investigationsof collagen in the form of tendon, it is more apparent than in skin that theshrinking reaction is not uniform: it initiates at certain points in the specimen,then propagates until the whole specimen is affected. This is consistent with theconcept of the effect of zones in the protein that have fewer secondary aminoacids (Chapter 1).Weir has shown that when collagen is stabilised chemically the thermo-

dynamic parameters change (Table 10.3).5

Treatments that reduce the entropy term or increase the enthalpy termincrease the free energy of activation and consequently increase the shrinkagetemperature. Chromium(III) tans principally by the latter method, i.e. byincreasing the structural component: this applies to offers up to 1% Cr2O3

(corresponding to half the usual industrial offer), but at higher offers the freeenergy is still controlled by the enthalpy rather than the entropy. All othertannages represented in Table 10.3 are controlled by the entropy term: this maybe pictured as resulting from an increased shrinkage temperature by sterichindrance to the shrinking process, when the uncoiling structure collapses intothe interstices between the triple helixes. The amount of steric hindrance may beassumed to be proportional to the chemical impact of the tannin on collagen

Table 10.2 Thermodynamics of collagen shrinking at 60 1C.

pH DHz (kJmol�1) TDSz (kJ 1C�1mol�1) DGz (kJmol�1)

1.8 280 210 733.0 330 243 863.6 590 506 834.2 610 512 946.8 700 600 1037.2 635 532 10410.8 550 455 9812.5 320 246 77

197Tanning

structure and its consequent ability to interfere with the collapsing process.The traditional way of thinking about tanning is based on the idea that thetanning agent confers stability to the collagen by changing the structurethrough crosslinking and thereby preventing the helices from unravelling: hereis the first indication that a new concept of tanning is required, in whichthe function of the tanning agent is to prevent shrinking occurring by alteringthe thermodynamics of the process. This view of tanning is developed further inChapter 19.Hydrothermal stability can be measured in the following ways.

10.2.1 Shrinkage Temperature (Ts)

The shrinkage temperature of pelt or leather is the most commonly quotedmeasurement of hydrothermal stability, certainly in technical publications andspecifications. The principle of the method is to suspend the test piece in water,in the form of a strip (the dimensions are immaterial since the property does notdepend on sample size), then to heat the water at a rate of 2 1Cmin�1, accordingto the Official Method.3 The shrinkage temperature is noted when the samplevisibly shrinks.The extent of any reaction is a product of the rate and the duration of the

reaction period. In this case, shrinking is observed when the rate becomessufficiently fast to see. Since the observed effect at any point in the test is theproduct of temperature and time, it must also be dependent on the rate ofheating: the faster the heating rate the higher the temperature of shrinkingappears to be and, conversely, the slower the rate of heating the lower thetemperature of shrinking appears to be. Therefore, the test is somewhat arbi-trary, but it is reproducible if the heating rate is consistent. However, cases maybe encountered when the nature of the material significantly alters the kinetics,making comparisons less reliable.A feature of the Official Method3 is that it specifies the use of water as the

heating medium. This is not surprising, because the sample must be wetted withwater before testing. However, the limiting factor is the boiling point of thesolvent. Leathers with shrinkage temperature above 100 1C, e.g. chrome tanned

Table 10.3 Kinetics of shrinking for stabilised collagen at 60 1C.

TannageOffer(% Cr2O3)

DHz

(kJmol�1)DSz

(J 1C�1mol�1) DGz (kJmol�1)

None 670 1705 104Formaldehyde 470 1055 121Zirconium(IV) 570 1285 147Aluminium(III) 345 720 104Chromium(III) 0.06 670 1690 111

0.13 850 2180 1190.57 1350 3600 1511.02 1630 4350 1841.98 1360 3515 189

198 Chapter 10

leathers, cannot be accurately tested. Two simple methods have been com-monly used to get around the problem. The use of high boiling paraffin can beused as the heating medium, but the results cannot be relied upon above 105 1C,because the water in the pelt boils off during the test, effectively raising theshrinkage temperature of the piece under test. It is doubtful that the waterwithin the pelt becomes so superheated that measurement of the temperature ofwet material remains accurate. Nevertheless, it is common for values of4110 1C obtained in this way to be quoted in the technical literature. Analternative is to use mixtures of water and glycerol as the heating medium: theAmerican Leather Chemists Association suggested a 75% glycerol–watermixture.6 However, the same criticism applies, because although it might beargued that the pelt does become saturated with the higher boiling aqueoussolvent, the shrinkage temperature is dependent on the amount of water in thesolvent, rising with decreasing water content, as might be expected.

10.2.2 Boil Test

Tanneries rarely possess shrinkage temperature testing equipment: conventionaltesting would be of arguable value in a chrome tanning context because of thehigh shrinkage temperatures above 100 1C, and typically they would not have aDSC instrument (see below); therefore, an alternative testing approach is needed.In most cases, tanners test chrome tanned leather at the end of the process, priorto unloading the tanning vessel. The procedure is to cut a piece of leather andnote the area, by drawing around it roughly on paper. The test piece is thendropped into boiling water and left for a specified period: this might be 1–5min,sometimes longer, but commonly 2min. After the required time, the piece isretrieved, quickly cooled in cold water and the area is compared with the startingshape. The criterion is pass or fail: if there is no discernible change the leather haspassed, but if any shrinkage can be seen, the leather has failed. Often, in the caseof failure, other changes can be seen in the texture of the leather, such as rubberyfeel and wrinkling of the grain. These observations can yield informationregarding the uniformity of the tannage, particularly through the cross section.The rationale for the boil test can be expressed as follows. At any temperature,

the rate of shrinking or the development of apparent thermal damage dependson the time, t, for which the sample is held at that temperature: at any tem-perature below the conventionally measured shrinkage temperature, where thedifference is DT, the rate of shrinking is slower than the discernibly fast rate atthe shrinkage temperature, so the accumulation of damage depends on theproduct of t and the reciprocal of DT. In the boil test, the temperature is fixed at100 1C, when the rate is fixed at k100 for a particular leather. Therefore, shrinkingdepends on the product of rate multiplied by time (k100� t) – so the longer theboil test period lasts, the more likely the pelt is to exhibit discernible shrinkage.If the shrinkage temperature (Ts) is greater than 100 1C, then the difference

between the point at which shrinking is observably fast and the test conditionsis Ts – 100¼DT4100.Therefore, k100p 1/DT4100

199Tanning

The maximum period that the piece can resist the boil test without discernibleshrinking, tmax, is inversely proportional to k100, so:

tmax / DT

That is, the longer a leather can resist boiling water, the higher the shrinkagetemperature is above 100 1C. However, the correlation between resistance timeand Ts is typically not used commercially; the test time is normally fixed to givea pass–fail result that corresponds to the hydrothermal requirements/specifi-cations of the product. Because the kinetics of the shrinking reaction may vary,depending on how the tanning reaction is conducted, comparisons are lessreliable than shrinkage temperature measurements. However, the results shouldbe consistent for any given process.

10.2.3 Differential Scanning Calorimetry

This is an instrumental technique, in which the sample is held in a metal cap-sule, which may be open or closed, and heated electrically. The amount ofenergy absorbed by the sample and capsule is compared to a control emptycapsule and displayed as change in energy as a function of temperature. As inthe shrinkage temperature test, the heating rate must be controlled and defined:here, a rate of 5 1Cmin�1 is more common. Figure 10.1 presents an example ofa thermograph. The advantage of differential scanning calorimetry (DSC) is

5mW

min

°C0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Figure 10.1 DSC thermograph of wet bovine collagen.

200 Chapter 10

that wet samples can be sealed in the capsule, so the shrinking transition can beobtained under true hydrothermal conditions.It can be seen that thermal transitions can be characterised by an onset

temperature (the peak maximum is less commonly quoted): this is generallyassumed to be close to the conventionally measured shrinkage temperature. Afeature of this technique is that it yields the enthalpy associated with thethermal transition: this is conventionally normalised to energy per unit dryweight of the material, but in this context it ideally should be adjusted to energyper unit weight of dry collagen. In the case of collagenic biomaterials, includingleather, additional information can be obtained. The value of the parametersobtained can be summarised as follows:

� The shrinkage temperature is a reflection of the chemical status of the col-lagen. Chemical modifications alter the shrinkage temperature: stabilisationby tanning reactions increases Ts, detanning of tanned collagen lowers Ts.

� The enthalpy of shrinking/denaturation reflects the intactness of the col-lagen structure. In general, the energy of shrinking is relatively constantregardless of the tanning chemistry, but is lower than the energy associatedwith native collagen. Any process that results in damage to the collagenwill lower the enthalpy of shrinking: the effect of damage is to decrease theamount of energy required to convert the intact helical structure intorandom coil.

� A change in either Ts or DH can be independent of the other parameter. Inany case, a shrinkage temperature rise is caused by chemical stabilisationof the structure (tanning) or measurements taken on dried material, but ashrinkage temperature fall is due to destabilisation of the newly stabilisedcollagen structure (detanning) or a damaging effect that might be describedas a negative tanning effect, such as is observed when collagen is treatedwith hydrogen bond breakers, lyotropic agents. The enthalpy of shrinkingis only likely to increase because the sample is drier than it should be or anerror has been made in the moisture content measurement. The enthalpycan be decreased by damage to the collagen structure that corresponds to acontribution to shrinking/denaturing the protein.

� The entropy of shrinking can be calculated, but this is a less frequentlyused parameter.

10.2.4 Hydrothermal Isometric Tension

This is a much less common technique, borrowed from polymer science.7 Themethod is similar to the Official Method of measuring shrinkage temperature,except here the piece is constrained from shrinking and changes in tension arerecorded. Additional information regarding the molecular status of the piece isobtained, but there are significant differences between the temperature values oftransition onsets.The traces can be divided into three parts, illustrated in Figure 10.2 (as in

Figure 6.2): the first is the tension increasing process from zero to maximum

201Tanning

tension, followed by a relatively constant tension process, if present; finally, thetension is either constant or a relaxation process occurs, due to the gradualdestruction of collagen structure or rupture of some crosslinking bonds. Theslope of the curve in the tension increasing process accounts for the collagenfibre rigidity, caused by crosslinks: the steeper the slope of the contractioncurve, the more crosslinks should be present in the collagen materials.Relaxation represents the stability of these connecting elements (crosslinkingbonds): the steeper the rate of the relaxation curve, the more unstable is thecrosslinking.To date, no mathematical or physical model has been proposed for a full

analysis of hydrothermal isometric tension (HIT) curves obtained under linearheating conditions.8 In 1987, J. Kopp and M. Bonnet proposed a tentativemodel for the development of isometric tension in collagen,9 but these equa-tions are probably only suited to very limited conditions (medium, pH, ionicstrength, etc.). However, in the context of collagenic materials, the shapes of theHIT curves can yield useful information about the relative crosslink densityand stability in comparable specimens.

REFERENCES

1. S. S. Kremen and R. M. Lollar, J. Amer. Leather Chem. Assoc., 1951,46(1), 34.

2. C. A. Miles, et al., J. Mol. Biol., 2005, 346, 551.3. Official Methods of Analysis, Society of Leather Technologists and

Chemists, 1996.4. A. D. Covington, R. A. Hancock and I. A. Ioannidis, J. Soc. Leather

Technol. Chem., 1989, 73(1), 1.5. C. Weir, J. Amer. Leather Chem. Assoc., 1949, 44(3), 108.

1

3

5

7

9

11

13

15

17

12095704520

Ten

sion

(m

V)

Temperature (°C)

Figure 10.2 Hydrothermal isometric tension trace for bovine hide.

202 Chapter 10

6. J. Amer, Leather Chem. Assoc., 1941, 36(12), 682.7. B. M. Haines and S. G. Shirley, J. Soc. Leather Technol. Chem., 1988,

72(5), 165.8. A. J. Bailey and N. D. Light, Connective Tissue in Meat and Meat Products,

Elsevier, 1989.9. J. Kopp and M. Bonnet, Advances in Meat Research, vol. 4, Collagen as

Food, 1987.

203Tanning

CHAPTER 11

Mineral Tanning: Chromium(III)

11.1 INTRODUCTION

The use of chromium(III) salts is currently the commonest method of tanning:perhaps 90% of the world’s output of leather in tanned in this way. Up to theend of the nineteenth century, virtually all leather was made by ‘vegetable’tanning, i.e. using extracts of plant materials (Chapter 13). The development ofchrome tanning can be traced back to Knapp’s treatise on tanning of 1858,1 inwhich he described the use of chrome alum: this is referred to as the ‘singlebath’ process, because the steps of infusion and fixing of the chromium(III)species are conducted as consecutive procedures in the same vessel. It is usuallyaccepted that chrome tanning started commercially in 1884, with the newprocess patented by Schultz:2 this was the ‘two bath’ process, in which chromicacid was the chemical infused through the hides or skins, conducted in onebath, the pelt was then removed to allow equilibration (but no fixation), thenthe chrome was simultaneously reduced and fixed in the second bath. The storyis told that the invention was a solution to the problem of corset construction:iron bracing strips (an alternative to whale bone) in the garments wouldinteract with the vegetable (plant polyphenol) tanned leather, under the con-ditions of moist heat around the body. Dampness caused the iron to rust andthe liberated iron reacted with the plant polyphenols of the vegetable tannins toproduce dark colours. This reaction is similar to the basis of the chemistry ofwriting inks and hence was an unacceptable change to the garment. Theintroduction of the new chrome tanning process eliminated this reaction.However, chrome tanning may have a longer history than that, as proposed byThomson,3 but the clearly observed change in tanning technology occurred atthe beginning of the twentieth century. It was soon recognised in the industrythat the new reaction was faster and more reliable than the then currentvegetable tanning reactions, and so after several thousands of years using




204

vegetable tannins, the global leather industry was converted to chrome tanningover a period of less than 50 years.The basis of the chrome tanning reaction is the matching of the reactivity of

the chromium(III) salt with the reactivity of the collagen. Chapter 9 has shownthat the availability of ionised carboxyls varies over the range pH2–6. This isthe reactivity range of collagen, since the metal salt only reacts with ionisedcarboxyls: the rate of reaction between chromium(III) and unionised carboxylsis so slow it can be neglected.4

Chromium(III) salts are stable in the range pH2–4, where the basicitychanges, but at higher values they will precipitate. This can be modelled in thefollowing way, using empirical formulae:

Cr3þÐOH�

½CrðOHÞ�2þÐOH�

½CrðOHÞ2�þÐ

OH�

CrðOHÞ3

The parameter Schorlemmer ‘basicity’ (named after the chemist who defined it)is used a great deal to express the status of tanning salts basicity: it is a way ofexpressing the degree to which a metal salt has been basified. The percentagebasicity is defined by the following expression:

total number of hydroxylsð Þ � 100

total number of metal atomsð Þ � maximum number possible per metal ionð Þ

Table 11.1 illustrates the calculation of basicities, assuming the polymerisedcomplex ions are linear (Figure 11.1).Interestingly, the calculation indicates that the ultimate basic compound has

the empirical formula Cr(OH)21, rather than the Cr(OH)3 that might be

expected from the valency.

11.2 PREPARATION OF CHROME TANNING SALTS

Chromium(III) salts are typically prepared from chromium(VI) compounds,which are commercially derived from chromite ore:

CrðOHÞ3 þOH�ÐCrO�2 ðchromite saltÞ þH2O

Table 11.1 Calculated basicity of linear chromium(III) aquo complexes.

No. ofCr atoms

No. ofhydroxyl groups

Maximumno. of hydroxylgroups

Basicity(%)

Approximate pH ofsolution of complexes

1 0 0 0 o22 2 6 33 2.73 4 9 42 3.34 6 12 50 3.9N 2n–2 6n–6 67 45

205Mineral Tanning: Chromium(III)

Chromite ore is roasted in rotary kilns at 1200 1C, in the presence of alkaliand oxygen, to convert it into dichromate:

Cr2O3 þNa2CO3 þ 1=2O2 Ð Na2Cr2O7 þ CO2

Acidification converts the dichromate into chromic acid:

Na2Cr2O7 þH2SO4 þH2OÐ 2H2CrO4ðchromic acidÞ þNa2SO4

The chemistry of chromite processing also highlights the potential dangers inburning or incinerating waste products from chrome tanning, such as leatherdust and trimmings: the fly ash will contain Cr(VI) and therefore constitute anenvironmental danger. Cases have been reported from developing economies ofpeople using chrome leather shavings as fuel for cooking and consequentlydeveloping lung cancers, because they had breathed in fly ash from the open fires.

Cr

H2O

OH2

SO4

H2O

H2O

OH2

Cr

H2O

OH

SO4

H2O

H2O

OH2

O

H2OH2O

H2O

S

OCr Cr

O

OH2

OH2

OH2

O O

O

CrCr

H2OH2O

H2OH2O H2O

H2O OH2

OH2

OH2

OH2

O

O

OH2

OH2

Cr Cr

+

+ H+

hydrolysis orbasification

2+

hexaquo-µ-dihydroxy-µ-sulfato dichromium (III)

basification

50% basic chromium (III)

olation

4+

H

H

H

HO

OH

HO

OH

H

Figure 11.1 Structures of basic chromium(III) complexes.

206 Chapter 11

Reduction converts the chromic acid or chromate or dichromate into a basicchromium(III) salt. Reduction is illustrated in the following equation by themost commonly used reducing agent, sulfur dioxide:

2Hþ þ 2CrO2�4 þ 3SO2 Ð 2CrðOHÞSO4 þ SO2�

4 ð33% basicÞ

The outcome of sulfur dioxide reduction is basic chromium(III) sulfate,accompanied by free sulfate salt: commercial tanning salt contains roughly50% sodium sulfate. Almost any reducing agent can be used as an alternativeand all the obvious organic materials have been used at some time, e.g. starch,cellulose, sawdust, etc. The commonest organic reductant was and still is wastemolasses or glucose, producing so-called sugar-reduced basic chromium(III)tanning salt. Table 11.2 sets out the relationship between the ratios of reactantsand the outcomes, where 133 – (% basicity required)¼ kg acid per 100 kgdichromate.Other reducing agents were also used industrially, particularly sulfite,

metabisulfite and thiosulfate. The difference between organic reduction andinorganic reduction is the presence of organic salts as residues of the break-down of the organic molecules. These salts can act as complexing ligands,referred to as ‘masking agents’ (see below), which change the composition ofthe ligand field around the chromium(III) and hence lead to the followingperceived or assumed effects on the leather:

� The leather is more blue in colour, compared to the distinctly greenercolour from SO2 reduced chrome tanning salt.

� The leather is softer and more full in handle.� The tanning reaction is assumed to be less astringent, i.e. slower, so

penetration is assumed to be better than with SO2 reduced salt.� The leather is more susceptible to mould growth in storage.

Sugar-reduced chrome salts are now relatively less common in the modernindustry than they used to be, but may be found in those tanneries that stillprepare their own chrome tanning salts. In these cases, it is crucial to ensurethat the reducing reaction is complete, so that the leather is not contaminatedby chromium(VI). Residual chromium(VI) is difficult to analyse unequivocally in

Table 11.2 Preparation of basic chromium(III) tanning salts.a

Reactant offers (parts by weight)Reactant Zero basic 33% basic 42% basic 50% basic

Dichromate 100 100 100 100Acid 133 100 91 83Glucose 25 25 25 25a4 moles acid+1 mole dichromate¼ 0% basicity; 3 moles acid+1 mole dichromate¼ 33% basicity;2 moles acid+1 mole dichromate¼ 67% basicity.


the presence of chromium(III): the currently accepted test is set out in theOfficial Methods of the Society of Leather Technologists and Chemists, UK.Notably, there is continuing controversy regarding the validity of the test for

Cr(VI), especially as a contaminant formed in chrome leather by ambient oxi-dation of Cr(III).5 It is claimed that conditions in the leather, such as theavailability of oxidisable lubricating oils, can lead to the presence of Cr(VI).6

Criticisms of this conclusion are based on the possibility of obtaining falsepositive results from the wet chemical procedures involved. The global leatherindustry contends that chromium(III) salts are non-toxic and therefore theyhave little environmental impact, particularly because the complexes cannotpass through cell walls and interfere with the DNA in the nucleus. Indeed theworld has had a century of experience of people wearing chrome tanned leathernext to the skin, with incidences of allergic response, e.g. chrome eczema, beingrare. However, the industry does agree with the regulatory authorities thatchromium(VI) is toxic and carcinogenic, because its compounds can passthrough the cell wall, whereupon it is reduced and able to react with the DNA.Hence the validity of testing is paramount, but currently unresolved. Oneoption for an alternative to wet chemistry might be the use of synchrotronX-ray spectroscopy. Here only the innermost electrons are excited, at far higherenergies than those associated with the outer electrons, which determine theoxidation state. Hence, the redox status would not be perturbed.

11.3 BRIEF REVIEW OF THE DEVELOPMENT OF CHROME

TANNING

The development of modern chrome tanning went through three distinctphases:

1. Single bath process: The original process used chrome alum, Cr2(SO4)3.K2SO4.24H2O, applied as the acidic salt, typically giving pHB 2 insolution. Following penetration at that pH, when the collagen isunreactive, the system is basified to pHB 4, with alkalis such as sodiumhydroxide or sodium carbonate to fix the chrome to the collagen.

2. Two bath process: The first commercial application was an alternativeapproach to the single bath process: it was recognised that a moreastringent, and consequently more efficient, tannage could be achievedif the technology of making chromium(III) tanning salts was conductedin situ. This means that the process was conducted in two steps. The pelt issaturated by chromic acid in the first bath, then it is removed, usually tostand overnight. At this time there is no reaction, because Cr(VI) salts donot complex with protein. Next, the pelt is immersed in a second bath,containing a solution of a reducing agent and enough alkali to ensure thefinal pH reaches at least 4. At the same time, processes were also devisedthat combined both valencies of chromium, exemplified by the Ochs’process.7 However, the dangers of using chromium(VI) drove change backto the single bath process. Not least of these considerations was the

208 Chapter 11

incidence of damage to workers by chromium(VI) compounds: the highlyoxidising nature of the reagents typically caused ulceration to the nasalseptum. Notices warning of the dangers of chromium(VI) to health can stillbe found in UK tanneries, even though these compounds have long sinceceased to be used.

3. Single bath process: With the development of masking (see below) to modifythe reactivity of the chromium(III) salt and hence its reactivity in tanning, theglobal industry universally reverted to versions of the single bath process.

There have been attempts to reintroduce versions of the two bath process.Notably, the Gf process was developed towards the end of the twentieth cen-tury in Copenhagen. However, European tanners were reluctant to handlechromium(VI), regardless of the advised measures to ensure complete reduction.For this reason, the method was never generally adopted.

11.4 CHROMIUM(III) CHEMISTRY

Chromium is a 3d44s2 element, so chromium(III) compounds have the electronicconfiguration 3d3, forming octahedral compounds (Figure 11.1). The hexaquoion is acidic, ionising as a weak acid or may be made basic by adding alkali. Thehydroxy species is unstable and dimerises, by creating bridging hydroxy com-pounds, because the oxygen of the hydroxyl can form a dative bond via a lonepair, as shown below. This process is called olation. It is a rapid, but notimmediate reaction.In Figure 11.1, the chromium complexes are presented as linear species: the

shape of basic chromium(III) species is discussed in detail below.A question that might arise is: how do we know that there are hydroxy

bridges in these complex molecules? The answer is provided by the work ofBjerrum.8 He titrated chromium(III) salts with alkali, until they precipitated,then immediately back titrated with acid. As illustrated in Figure 11.2, the pH

pH Titration with base, followed byimmediate back titration with acid

Observed effectof ageing

Back titration afterageing basified salt

acid/base

Figure 11.2 Model titration curve for chromium(III) salt.


trace for the alkaline reaction is retraced as acid is added. However, if there wasan overnight delay between the alkali titration and the subsequent acid titra-tion, the traces do not overlay. The difference indicates that some of thehydroxyls created by the alkali reaction are not available to react with acid, i.e.they have been changed in nature by their bridging function.In 1974 Irving reviewed the chemistry of chromium(III) complexes from the

point of view of the leather industry.9 Some of his more important observationscan be summarised as follows:

1. The half-life of water exchange in the Cr(III) ligand field, determined byisotopic exchange of 18O, is 54 hours at 27 1C: this is an associativeinteraction. This can be compared to other metals, including Al(III), whichhave half-lives of the order of 10–2 s: these are dissociative interactions.This difference between associative and dissociative complexation hasimplications not only for the stability of Cr(III) complexes, but also for therole of Al(III) in tanning technology.

2. The diol complex, constructed from the two hydroxyl bridges, wasassumed to be the preferred form of the chromium dimer. It was alsoargued that the trimer is the linear version of the bridged structure (seebelow).

3. The stability of transition metal complexes can be discussed in terms ofthermodynamic stability and kinetic stability. Chromium(III) complexeswith carboxylates are thermodynamically less stable than some othercomplexes, such as ammines, but they are kinetically more stable.

4. The mechanism of exchange between ligands into an octahedral complexdepends on the stability of the intermediate crystal field, either five-coordinate square pyramidal (SN1 mechanism) or seven-coordinate pen-tagonal bipyramid (SN2 mechanism). From calculations of the crystalfield activation energies, both mechanisms exhibit high values, with ahigher value for the mechanism involving seven-coordination. Whicheveris the actual dominating mechanism, the high activation energies explainwhy the complexes are kinetically stable.

5. The stability of complexes between Cr(III) and carboxylates is inverselyproportional to the dissociation constant of the carboxylic acid. This wasfirst proposed by Shuttleworth.10 A plot of log KCrL against log KLH doesindicate a good correlation (Figure 11.3).9,11–13

6. The formation of chelate complexes is favoured compared to complexeswith monobasic carboxylates (Table 11.3).Whilst there is not a simple stepwise relationship between the successive

stability constants, particularly for the monobasic carboxylates, therelative values do show the enhanced stability of chelates. Interestingly,the results for chelation do not clearly reflect the well-known stability offive- and six-membered rings in organic chemistry, although there is apreference for those ring sizes.16

7. Olation occurs at the trans positions (Figure 11.4) because the rate ofionisation of the aquo ligand is faster.15

210 Chapter 11

This suggests that the polymerisation of basic chromium(III) complexesis linear (structure I in Figure 11.5). However, there is an alternativestructure for polymers larger than the dimer, the three-dimensional Orgelstructures17 (II and III in Figure 11.5).

Figure 11.3 Relationship between chromium(III) complex stability and carboxylateligand dissociation constant, mainly at 25 1C and I¼ 0.1M: 1, thiocya-nate; 2, nitrite; 3, formate; 4, azide; 5, fluoride; 6, acetate; 7, propionate;8, oxalate; 9, maleate; 10, phthalate; 11, succinate; 12, malonate; 13,citrate; 14, asparagine; 15, arginine; 16, serine; 17, lysine; 18, methionine;19, valine; 20, glycine; 21, alanine; 22, leucine; 23, sulfosalicylate; 24,hydroxide; 25, nitrilotriacetate (NTA); 26, EDTA. The points for EDTAand NTA fall outside the limits of the figure: KCrL is 1023.4 and 4 1010

respectively. (Courtesy of Journal of the Society Leather Technologistsand Chemists).

Table 11.3 Stability constants for carboxylate complexes with chromium(III):25 1C, 0.1MKClO4 for dibasic acids (DA),11 0.3 M HClO4 formonobasic acids (MA).14,15

Ligand log K1 log K2 log K3

Oxalate (DA) 5.3 5.2 4.9Malonate (DA) 7.1 5.8 3.3Succinate (DA) 6.4 4.6 2.9Maleate (DA) 5.4 3.0 1.9Formate (MA) 1.9 0.7 1.3Acetate (MA) 4.6 2.4 2.5Propionate (MA) 4.7 2.4 2.7


Several studies have elucidated the kinetics of formation of the Orgelstructures and their stabilities;18–20 it has been suggested that the three-dimensional structures are the preferred conformations and the trimer isthe fundamental unit of structure of basic chromium(III) oligomers. Thestructures as represented in Figure 11.5 are controversial because of thethree-centre bonding,21 but the unusual lability of the of molecules mayrequire unusual structural features.Ramasami22 has reported that structures similar to the Orgel structures

are found accumulated in recycled tanning solutions, indicating they areformed as minor polymer components, but also that they have poorertanning power than the linear complexes. In contrast, Montgomery et al.

H2O

OH2

OH2

OH2

OH2

O

RCO2

H2O

O

H2O

Cr Cr

cis

trans

trans

trans

trans

cis cis

cis

H

H

Figure 11.4 Stereochemistry of chromium(III) complexes.

II

Cr

HO OHCr

Cr

O

O

H2O

H2O

H2O

H2O

H2O H2O

OH2

OH2

OH2

H2O

OH2

OH2

OH2

OH2

H2O

H2O

H2O

H2O

H2O

H2O

H2O

H2O

H2O H2O

H2O OH2 OH2

OH2

OH2

OH2OH2

O

O

CrCrCrCr

I

O

O

O

O

Cr Cr

O

O

Cr Cr

HO

HO OH

OH

H H H

H

H

HH

III

Figure 11.5 Polymeric chromium(III) aquo structures: linear (I) and nonlinear (II andIII) (Orgel).

212 Chapter 11

has reported that the Orgel trimer is a better tanning agent that the dimer,particularly in the presence of sulfate.23 These and other studies of theeffect of structure on tanning power and ability are based on investigationof species in solution, rather than species actually engaged in the tanningprocess on collagen. This is discussed in more detail below.

8. Irving9 has addressed the relationship between sulfate and chromium(III),the rate of exchange between ionic and coordinated sulfate. Infraredspectroscopy indicated that sulfate is unidentate in the pentaaquo-chro-mium(III) ion,24 but in the dichromium species the sulfate is bidentate.Using 35S-labelled sulfate, measurement of the stability of the mono-sulfatopentaaquo complex gave a t1/2 of 20 hours at 20 1C, but that thebidentate complex has t1/2¼ 30 hours at 25 1C.25

However, questions remain regarding the stability of the sulfato complex ofpolymeric chromium(III) under tanning conditions and in a tanning reaction,particularly whether the sulfate acts as a bridging ligand, as indicated in Figures11.1 and 11.6. In the latter case, the model is the reaction mechanism assumedover the last half century or more to summarise the chrome tanning reaction.26,27

The implications of this model and its accuracy are discussed in detail below.

11.5 CHROME TANNING REACTION

Although the modern process is conventionally referred to as ‘chrome tanning’,it is important to remember that the reaction is most commonly conducted withbasic chromium(III) sulfate, as the commonest reagent used in the global leatherindustry. The importance of that caveat lies in understanding the roles of everycomponent of the salt and in reviewing the alternative options.The reasons for the popularity of the process are clear, when the features of

the process are compared with vegetable tanning (Chapter 13):

� The process time for the chrome tanning reaction itself is typically less than24 hours: the vegetable tanning reaction takes several weeks, even in amodern process.

O2C(CH2)3

OH2

OH2

O

OO

O

H2O

O

(CH2)2CO2

H2O

O

Cr Cr

aspartic acid glutamic acid

H

S

H

Figure 11.6 Bridging sulfate in a dichromium aquo complex.


� Chrome tanning confers high hydrothermal stability; a shrinkage tempera-ture of 110 1C is easily attainable. This opens up new applications, comparedwith vegetable tanned leather, where the maximum achievable shrinkagetemperature is 85 1C, depending on which vegetable tannin type is used.

� Chrome tanning alters the structure of the collagen in only a small way: theusual chrome content of fully tanned leather is 4% Cr2O3, whereas vege-table tanned leather may contain up to 30% tannin and hence the handleand physical properties are inevitably modified, restricting applications ofthe leather.

� Vegetable tanning creates hydrophilic leather, because of the chemicalnature of the plant polyphenols that constitute the tanning material, butchrome tanning makes collagen more hydrophobic, so the tannage allowswater resistance to be built into the leather.

� Chromium(III) can act as a mordant (fixing agent for dyes) and its palecolour allows bright deep and pastel shades (even though the base colourof the leather is pale blue). Tanning with plant polyphenols has the effect ofmaking the dyeing effect dull, whichever vegetable tannin or dye types areused – the leather is said to be ‘saddened’.

� Vegetable tanned leather may exhibit poor light fastness, depending on thetype of vegetable tannin, but chrome tanned leather is lightfast. Hence,dyed chrome tanned leather will retain its colour better.

In summary, chrome tanning is faster, better in all sorts of ways and offersmore versatility to the tanner, with regard to the leather that can be made fromwet blue, the name given to leather after the chrome tanning reaction is complete.Versatility is a key characteristic of the process. It is theoretically possible tocreate any type of leather from any wet blue hide: men’s weight or ladies’ weightshoe upper, combat upper, soling, clothing, gloving, upholstery, etc. It is true thatthe leathers would not be ideal, since in this example the wet blue is not createdspecifically for every application, but the principle of the general assertion is true.In essence, the chrome tanning reaction is the creation of covalent complexes

between collagen carboxyl groups, specifically the ionised carboxylate groupsand the chromium(III) molecular ions. In this way, the reaction is no different tomaking any other carboxylate complex, such as acetate or oxalate, althoughtanners tend to think of the reaction as fixation of chrome onto collagen. Eitherway, the reaction is the same, since the way of thinking does not alter theproduct. The one difference between simple complexation, such as acetate andchromium(III), and the typical tanning reaction is the ability of the reactants tocome together. For a simple reaction, the reactants are in solution and cancome together without hindrance, limited only by diffusion. In the case of thetanning reaction, the substrate has finite thickness, so the additional parameterof penetration through the cross section comes into play. The tanner mustbalance competing process rates, so there are three possible conditions:

1. Rate of penetration4 rate of reaction2. Rate of penetration¼ rate of reaction3. Rate of penetrationo rate of reaction.

214 Chapter 11

In the first case, the rate of penetration is faster than the rate of fixation, sothe chrome salt is likely to colour all the way through the pelt, but may not befixed, depending on the degree to which the system is made unreactive by thepH conditions in pickling. This is the usual condition created at the beginningof practical tanning.The second case is the ideal situation: following colouring through the cross

section, the rates should be the same or similar, because that will lead to themost uniform fixation through the pelt, although the actual concentrationprofile through the cross section will depend on the initial pH profile (Chapter9). Completely uniform fixation is clearly difficult to control, but is only one ofthe processing problems facing the tanner.In the third case, the system would be too reactive (astringent), resulting in

excessive fixation on or at the surfaces, potentially leading to a raw centre in thepelt. This is the usual condition aimed for at the end of practical tanning, whenthe remaining chrome in solution becomes less of a damaging threat even iffixation occurs at the surface.These rates are controlled in large part by the pH and temperature profiles,

so the conditions at the beginning of the reaction and the end of the reaction areadjusted accordingly (Table 11.4).The term ambient as applied to temperature is, of course, relative: in con-

sidering initial conditions of tanning account must be taken of local conditions,when the temperature might be lower or more likely higher than the rangequoted here. For example, in the height of summer in some parts of the world,the ambient and municipal water temperature may be significantly higher than25 1C, whence tanners sometimes resort to the addition of ice before tanningcan start. Kinetically, the reaction must be accelerated during its progress,because the rate will fall as the chrome is fixed. The rate follows a pseudo-firstorder path (Figure 11.7). The tanner cannot afford to wait until the reactionreaches equilibrium, so the conditions of reaction are constantly varied, so thatthe reaction fits into the available process time. Notably, attempts to define thekinetics of the chrome tanning reaction have met with little success, primarilybecause the rate is dependent on the finite thickness of the substrate. Therefore,only empirical equations have been developed, of the following kinds:28

½Cr��n ¼ ktþ 1 or ½Cr� ¼ ktm

Such expressions have limited use, since they do not eliminate the effect of thesolid substrate; they merely emphasise that the rate depends on the chromium

Table 11.4 Conditions at the beginning and end of the chrome tanningprocess.

pH Temperature Reaction rate Penetration rate

Start 2.5–3.0 Ambient, e.g. 15-25 1C Slow FastFinish 3.5–4.0 Elevated, e.g. 40-60 1C Fast Slow


concentration. The solution kinetics of carboxylate complexation with chro-mium(III) have been analysed according to the scheme set out in Figure 11.8.29

The rate equation for the rate determining step leading to species E is asfollows:

d½Crt�dt

¼ �k3½D�

where [Crt]¼ [A]+[C]+[D].If:

½C�½A�½B� ¼

k1

k�1¼ K1

and:

½D�½Hþ�½C� ¼ k2

k�2¼ K2

then:

d½Cr�dt

¼ k3K2½A��½Cr�f½Hþ� þ ð½Hþ� þ K2ÞK1½A��g

From the rate equation developed from the proposed mechanism, someassumptions can be made regarding the parameters of the reaction, to simplifythe kinetics, to be able to draw some conclusions about their reliance onsolution conditions. The argument is as follows.

If [H1] 4 K2- lower pH

d½Cr�dt

¼ � k1½A��½Cr�½Hþ�ð1þ K1½A��Þ

Log [conc]

product

reactant

Time

Figure 11.7 Representation of the kinetic plot for a first-order reaction: the rate atany point is a tangent to the curve.

216 Chapter 11

If K1[A�]o1

d½Cr�dt

¼ �k1½A��½Cr�½Hþ�

If K1[A�] 4 1

d½Cr�dt

¼ �k2½Cr�½Hþ�

¼ �k2½Cr�½A��Ka½HA�T

¼ �K5½Cr�½A��

[Cr(H2O)6]3+ + RCO2−

k1

k-1

(H2O)

(H2O)3

Cr2+

H

H

O H

C R

O H O

O

O H

C R

O H O

O

O H

C R

O H O

O

A B

C

D

E

O

O

R

C

H+O

H

Cr

(H2O)3

(H2O)

(H2O)2

(H2O)

rapid

H

H

slow

k3

H

H

(H2O)3

(HO)

k-2

k2 + H+

+ H2O

Cr+

Cr+

Figure 11.8 Reaction mechanism of carboxylate complexation with chromium(III).


At lower pH, rate depends on chrome concentration and number/con-centration of available reaction sites.

If [H1] o K2- higher pH

d½Cr�dt

¼ � K3½A��½Cr�K2K1½A��

¼ K4½Cr�At higher pH, rate depends only on chrome concentration.

The relative quantities of the reactants can be calculated (Table 11.5). In thecase of the collagen, the total content of carboxyl groups is 1 mole per kg dryweight, from which can be assumed 0.2 kg wet weight during the tanningreaction and that the pKa of the carboxyls is 4. The chrome offer in 100% floatis 2% Cr2O3 and the fixation rate can be assumed to terminate at 90%. Tocorrect the amount of carboxylate available for reaction, it can be assumedeach carboxylate reacts with a Cr4 species.

30

At the beginning of the reaction, the chrome is in excess, so the rate dependson the substrate’s reactant concentration: by the end of the reaction the sub-strate reactant is in excess and the rate depends on the chromiumconcentration.There is another factor that may be taken into account, which is the

mechanics of the process. The steps involved in any general reaction in which areagent is fixed onto a substrate may be defined as follows (see also Chapter 15):

1. Transfer of the reagent from solution into the substrate;2. Hydrophobic interaction between the reagent and the substrate;3. Electrostatic interaction between the reagent and the substrate;4. Covalent reaction between the reagent and the substrate.

Consideration of step 1 proposed here is analogous to the measurable freeenergy of transfer between two solvents;31 in this case, the solvating environ-ment for the reacting species is changed from the conventionally understoodaquation in water to an environment that reflects the chemistry of the substrate,typically more hydrophobic than the aqueous solvent itself. Alternatively, thisis analogous to the partitioning on a solute between two solvents, a conceptused in solvent extractions. However, an empirical view of the relative

Table 11.5 Conditions during conventional chrome tanning.

pHBuffer ratio[HA] : [A�] [CO2

�] (M)Chromeuptake (%) [Cr] (M) [CO2

�]corr (M) [Cr] : [CO2]

2.5 0.032 0.006 0 0.26 0.006 433.0 0.10 0.018 40 0.16 0.016 103.5 0.32 0.048 70 0.08 0.037 24.0 1.0 0.100 90 0.03 0.071 0.4

218 Chapter 11

properties of the solvent and the substrate is a powerful way of understandingand predicting heterogeneous reaction of the type routinely used in leathermaking.The role of step two depends on the chemistry involved: hydrophobic

interactions are thought to be important in the initial stages of vegetable tan-ning32 and may play a part in other organic tanning reactions. Step 2 might beregarded as a special case of step 3, which is the start of most reactions, aninitial charge interaction: however, it might also be considered to be a reflectionof step 1, when transfer is driven by affinity. The extent to which step 4 appliesdepends entirely on the chemistry of the reaction and may not even apply at all.In the case of chrome tanning, the transfer reaction is very important. Thecontrolling factors can be expressed as follows:

½reagent�solvated þ ½substrate�solvatedÐ½substrate� reagent�solvated

The position of the equilibrium depends on the relative affinities of thereagent for the solvent and the environment within the substrate. In the case ofaqueous solvent, typical for tannery processing, the equilibrium is controlled bythe hydrophilicity or hydrophobicity of the reagent. That is, the relationshipbetween the reagent and the solvent water, i.e. the equilibrium in the followingequation:

½reagent� þ waterÐ½reagent�hydrated

A hydrophilic reagent will tend to remain in solution, because it interactsfavourably with water. In contrast, a hydrophobic reagent will not be as solublein water and will tend to move into a more hydrophobic environment, into thesubstrate.With chrome tanning, all the steps of the reaction mechanism operate.

Transfer depends on the relationship between the chrome species and the solvent,in this case water: the more hydrophobic the species, the faster the transfer willbe. This effect operates when chromium(III) is basified, when the chrome speciesare polymerised and become less soluble in water. Ultimately, if the chromiumsalt is on the point of precipitation, reaction will be very fast on the surfaces, theorigin of ‘chrome staining’, the appearance of green marks on the leather. This isa serious defect, because the chrome confers colour and also takes up reactionsites on the leather required for dyeing: each of these will adversely affect thecolour of the dyed leather. The electrostatic interaction depends on the charge onthe chrome complex and this can be adjusted by altering the ligand field: this isthe effect of ‘masking’, addressed in detail below. Clearly, the charge will alsocontribute to the partitioning of the solute between the water and the morehydrophobic environment. Ultimately, covalent complexes are formed betweenthe chromium species and the collagen carboxyl groups.Sykes33 has demonstrated that the primary chrome tanning reaction occurs

at the carboxyl groups and this is the reaction responsible for raising theshrinkage temperature of collagen. Minor reactions also occur at the sidechain


amino groups and the peptide links, where the interaction is hydrogen bondingbetween the charged sites on the collagen and the aquo ligands and hydroxylbridged of the chromium(III) molecular ions: these reactions account for only afew percent of the total amount of fixed chrome, but they have no effect onraising the shrinkage temperature of the leather. This provides us with anunderstanding of the consequences of chrome staining, which ultimately cause‘dye resist’, when the dyeing reaction cannot take place where the precipitatedchrome species is fixed to the leather surface. The highly basic chrome fixes veryrapidly to the surface by hydrogen bonding – a much faster reaction thancomplexation. Therefore, the hydrogen bondable sites on the collagen are nolonger available for reaction with acid dyes, so no colouring is possible and thesurface remains stained green.In defining the chemistry of the chrome tanning reaction, it was consequently

tacitly assumed that it occurred at both aspartic and glutamic acid carboxyls(Figure 11.6). An analysis of the availability of reaction sites attempted tocalculate the distribution of crosslinks, on the assumption that mere availabilityensured the possibility of reaction (Table 11.6).34,35 The analysis is based on theproximity of carboxyl groups and random ionisation and the assumption thattanning depends on crosslink formation between carboxyl sidechains.An incidental inference from the calculations is that the deamidation reaction

(amide sidechain hydrolysis in liming) can have little influence on the funda-mental mechanism of chrome tanning, as was also concluded in Chapter 6.Subsequent experimental work has cast doubt on that analysis as the basis of

tanning at the molecular level. Tanning studies by Ioannidis et al.,35 usingaluminium(III) as a model for the mineral reaction and NMR spectrometryanalysis of its interaction with polyaspartate and polyglutamate, have shownthat the former reaction is stronger than the latter. Although this reactiondiffers from chromium(III), because its nature is much more electrostatic, thedifference between the sidechains was attributed to the additional entropypenalty of reaction at the longer sidechain of glutamate. In extended instru-mental analysis of the chrome tanning reaction, Brown concluded that aspar-tate is favoured over glutamate:36 the indication from Table 11.6 is thatglutamic acid should be favoured because of its greater availability, althoughaspartic acid has a slight advantage with regard to its dissociation constant. Inan attempt to isolate chrome crosslinks on collagen and obtain direct evidencefor the chemistry of the reaction, Covington andMenderes produced somewhat

Table 11.6 Calculated theoretical crosslinking between triple helices inchrome tanned collagen and the effect of deamidation.

Crosslink Calculated relative occurrenceRelative effect ofdeamidation

Native collagen Deamidated collagenAsp-asp 1.0 1.0 1.5Asp-glu 3.4 4.2 1.8Glu-glu 4.0 4.6 1.6

220 Chapter 11

inconclusive evidence, although it was not inconsistent with preferential reac-tion at aspartate.37

The relative amounts of aspartic and glutamic acids in collagen are about 26and 45 per 1000 residues, respectively, which increase to about 35 aspartic acidsand about 60 glutamic acids per 1000 residues after liming to the typicalcommercial extent, due to hydrolysis of asparagine and glutamine. The pKas ofthe aspartic and glutamic sidechain carboxyls are 3.8 and 4.2, respectively.Hence, the availability of the carboxylate at any pH can be readily estimated(Table 11.7), taking into account that reaction via the unionised carboxylic acidgroup is so slow it can be neglected.4

The analysis in Table 11.7 indicates that the availability of reaction sites issimilar in numbers for aspartate and glutamate, although the trend with pH isclear. But what is not clear is whether that trend might be exploited in thechrome tanning reaction.It is both implicit and explicit in publications in tanning science that the

fundamental element of the mechanism of collagen stabilisation is crosslinking– as may appear to be the case in the foregoing exposition. Without pre-empting the argument set out in Chapter 19 regarding the general mechanismof tanning, in which a theoretical model of tanning is derived to explain alltanning reactions, the term ‘crosslinking’ will be significant by its absence fromthis treatise. Once that theory has been developed, it might be useful to returnto the data presented here and in the literature, to review the significance of theinformation in the light of newer fundamental thinking.

11.6 BASIFICATION

The figures quoted in Table 11.7 show that raising the pH of the solution hasthe effect of ionising the carboxyl groups on the collagen: this is the mechanismby which the reactivity of the system is increased. At the same time, two thingshappen to the other reactant, the chromium(III) salt:

1. The basicity is increased, so the molecules are polymerised by olation. Asthe molecular weight increases, the hydrophobicity of the species

Table 11.7 Calculation of the relative availability of chrome complexationreaction sites on collagen.

PH [CO2H] : [CO2�] Anions per 1000 residues

Anionic ratio[asp] : [glu]

Asp Glu Asp Glu

2.5 20 50 1.7 1.2 1.43.0 6.3 16 4.8 3.6 1.33.5 2.0 5.0 12 10 1.24.0 0.6 1.7 21 23 0.94.5 0.3 0.7 27 35 0.8


increases: this makes the species more reactive towards the collagen. Thisis part of the explanation of the tanners’ ‘rule of thumb’: the tanningpower of a metal is greatest just before the point of precipitation.

2. The polymerisation reaction increases the number of chromium atomsper molecular ion, therefore if the reaction rate remains the same asmeasured by the reaction of the carboxyl groups, the fixation rate of thechromium is increased. This means that for every complexation reactionbetween a chrome species and a carboxyl group, the number of chro-mium atoms fixed to the collagen increases. Since the reaction kineticsinvolve complexation at the trans positions, Figures 11.3 and 11.5illustrate that the kinetics of linear polymeric ions will be unaffectedwhen the reaction pH is increased. This is additional rationale for theabove ‘rule of thumb’.

Any compound that can raise the pH of the tanning solution to a valuehigher than 4.0 can be considered as a candidate to be a basifying agent.Table 11.8 sets out examples of basifying agents. In addition to the basi-fying effect, the other effect that must be considered is reaction betweenthe basifying species and the chromium species, because any complexationreaction will influence the reactivity of the chrome to tanning complexation(see below).In all cases, the basifying reaction must be related to the pH in solution at

any time during the basifying process. Above the typical end point of basi-fication, usually pH 3.8–4.0, there is a danger zone, consisting of the max-imum allowed pH, leading to the precipitation point. The danger arises fromthe olation reaction causing polymerisation and hence elevated reactivity ofthe chrome species: this reaction is fast and so is the ionisation of the carboxylgroups on the collagen, so even a localised high pH can create conditions forenhanced reaction in the local vicinity. Here, it is not necessary to havecomplexation to cause fast reaction – hydrogen bonding is all that in needed.The mechanism of allowing chrome to fix on the collagen can be regarded as aone-way reaction, since the bonding is effectively irreversible. A mild effect

Table 11.8 Approximate pH values of solutions of industrial basifying agents,as 10% solutions or maximum values for less soluble salts.

Basifier Concentration (%) pH

Sodium carbonate 10 11Sodium aluminium silicate 1 10Magnesium oxide (magnesia) Maximum (9�10�5 g l�1) 10Sodium tetraborate (borax) 10 9.2Sodium formate 10 8.5–8.7Sodium bicarbonate 10 8.0Sodium sulfite 10 7.8–8.0Ammonium bicarbonate 10 7.5–7.8Dicarboxylates:mono disodium salts 10 6–8

8–11

222 Chapter 11

would be enhanced reaction on the surface, leading to chrome ‘shadowing’, amore serious effect would be clearly uneven colouring of the surface; both ofthese would lead to uneven colouring in dyeing, but the ultimate problemof chrome precipitation would cause dye resist. In all cases, the solutionto the defect may involve pigment finishing, resulting in loss of added valuefor the final leather. Therefore, any basification procedure that involvesapproaching or exceeding the maximum allowed pH value must causeconcern. It is for this reason that basifying chrome tannage is often regardedas the most dangerous step in leather making, because the effect on uniformityof dyeing can be so great. Any adverse effect on dyeing the leather is ofultimate importance, since the appearance of the surface is the first perceptionof a (potential) purchaser.

11.6.1 Soluble Alkaline Salts

Examples of soluble basifying agents include sodium carbonate, sodiumbicarbonate and sodium hydroxide. The only difference between these salts isthe degree of their solubility in water and their ability to buffer, giving rise todifferent pH values in solution. In all cases, the type of effect of their use inbasification is the same (Figure 11.9 below). In the figure, it is assumed thatthe danger point is not far above the ideal final value, when precipitationof the chrome in solution is likely to occur. In practice the critical value may bejust above pH4, but certainly under typical tanning conditions precipitationwill occur at pHo5.

pH

Fast basifying agent

Maximum allowed value

Required end value

Optimum basification profile

Time

Figure 11.9 Basification curves for chrome tannage.


It is worth recalling that the following relationship between bicarbonate andcarbonate is temperature dependent:

2NaHCO3ÐNa2CO3 þ CO2 þH2O

Above 40 1C, which may apply in practical tanning processes, bicarbonateloses carbon dioxide, thereby becoming a more alkaline salt and hence this canaffect the basification process in terms of the extent and outcome.Figure 11.9 contains an idealised basification curve and a representation of

the impact of adding all the required base at once. The effect of a single additionof base will be the same, the only difference will be the maximum valueachieved, dependent on the solubility and buffering ability. The initial rate ofincrease in pH depends on the solubility, so it will be higher if the base is solubleand strong.Figure 11.10 shows the effect of adding the base in aliquots, where the ideal

curve is tracked by multiple additions, but the critical point is never exceeded.The number of aliquots to be added is determined by the maximum con-centration allowed to remain below the critical pH value; the minimum numberis usually arrived at by experience.

11.6.2 Carboxylate Salts

It is not common for carboxylates, such as sodium formate, to be used alone asbasifying agents, mainly due to cost and their side reaction of masking thechrome in solution. However, as part of a basifying system, they can contributea buffering effect that is useful in controlling the pH during the reaction. Forexample, formate will buffer around its pKa value of 3.75, which is ideal forchrome tanning. Other options are succinate, glutarate, adipate and phthalate,of which only phthalate is commonly used in industry.

pH

Maximum allowed value Aliquot additions

Required end value

Optimum basification profile

Time

Figure 11.10 Basification by aliquots of base.

224 Chapter 11

The side reaction of masking is a critical feature of the chrome tanningreaction, since the effects are not always as expected. The effect of a maskingsalt on the chrome tanning reaction depends on the chemistry of the carbo-xylate, which can have effects beyond merely occupying reaction sites on thechromium species: the structure of the carboxylate can have a marked influenceon the solubility of the complex.38 This in turn can impose limitations on theamount that can be used in practice, best illustrated by phthalate, which willprecipitate chrome at low masking ratios. The situation is analysed in detail inthe section on masking (Section 11.11).A less common outcome is the formation of chrome soaps, ionic compounds

of chromium(III) and carboxylate: such compounds may form, but do notnecessarily result in any particular effect, neither desirable nor undesirable.The exceptions are chrome soaps of higher molecular weight carboxylates,classified as anions of free fatty acids: these soaps sometimes form in wet blue,observed as solvent soluble pink colouration. The mechanism of their forma-tion is not obvious, because it is actually difficult to make them in thelaboratory.

11.6.3 Other Basic Salts

There are not many other reagents commonly available and used in the globalindustry. Among the less common reagents for basification is sodium alumi-nium silicate, which is promoted as a solo agent or in combination with sodiumglutarate for masking, when it is claimed the modification can lead to animprovement in chrome uptake.

11.6.4 Self-basifying Salts

The commonest drawback to basification by the aliquot method is the need tostop the tanning drum to add the base – unless automatic or external dosing ispossible. Therefore, the concept of self-basification was developed. Here, thebasifying agent is a water-insoluble compound, capable of reacting with acid:since the reaction cannot occur in solution, it must happen on the surface ofthe solid, so the rate of reaction depends not only on the chemistry but also onthe available surface area, i.e. the finer the particle dimensions the faster thereaction with acid.

11.6.4.1 Dolomite. Dolomite is an equimolar mixed complex of calcium andmagnesium carbonates, available naturally as dolomite rock and patented bythe Bayer Company as a basifying agent for chrome tannage. The term self-basifying in this context means that the powdered dolomite is mixed with the33% basic chrome tanning powder, so that at the end of the reaction the pH isa little below 4.0 (Figure 11.11), i.e. there is no need for the tanner to add baseseparately, because the system basifies itself. The reactions are as follows:

MCO3ðsolidÞ þHþðsolutionÞÐM2þðsolutionÞ þHCO�3 ðsolutionÞ


In the Bayer product, the molar ratio CaCO3 : MgCO3 is typically 1 : 1 orsmaller, i.e. the calcium component never dominates the compound. However,other commercial versions of dolomite exist, in which the molar ratio is 41 : 1.It might be tempting to use such products, since they are typically offeredat a lower price, but there is a potential problem associated with the cheaperproducts that is not found with the Bayer product.Consider the progress of the reaction with acid. The magnesium carbonate in

the complex is more reactive than the calcium carbonate, so it is preferentiallyleached from the particle, from the surface inwards. If the structure is equi-molar in Mg(II) and Ca(II), as the magnesium component is removed there isinsufficient residual calcium carbonate for the structure to remain intact as asolid, so it rapidly crumbles and the calcium salt continues to react as a veryfinely divided solid. Conversely, if the compound contains a bigger molar ratioof calcium carbonate to magnesium carbonate, the reaction of the magnesiumcomponent leaves a strong structure of calcium carbonate. The particle isintact, but the surface is deeply pitted, leaving sharp edges that can cut into thesurface of the leather. In this way, the cheaper products may abrade the grainsurface, which would constitute a downgrading damage, with an associatedeconomic penalty.

11.6.4.2 Calcium Carbonate. Several products are available in the marketthat consist solely of powdered calcium carbonate. These products are actu-ally safer than the high CaCO3 : MgCO3 ratio products because the saltdecomposes uniformly in acid. The rate at which they react depends on theparticle size: the faster the reaction, the higher the pH reached in solution. Inmany cases, the solution pH is high enough to require the product beingadded in aliquots – defeating the idea of self-basification. The effect of theseproducts on the pH profile is the same as for strong base (Figure 11.10). Inaddition, the high concentration of calcium ions in solution may lead to theformation of calcium sulfate, by reaction with the sulfate present from thechrome tanning salt and (perhaps) from the pickling acid. This may result in

pH

End value

Time

Figure 11.11 Illustration of the basifying reaction of dolomite.

226 Chapter 11

the formation of sparingly soluble crystals within the grain surface, givingrise to an effect analogous to limeblast.

11.6.4.3 Magnesium Oxide (Magnesia). Laboratory grade magnesia isavailable in different particle sizes, often distinguished as ‘light’ or ‘heavy’.These chemicals are essentially insoluble in water, but they can rehydrate onthe surface of the solid, giving rise to the hydroxide, also a very low solubi-lity compound, so that reaction with acid occurs on the surface. In this way,they function in precisely the same way as the calcium carbonate products,giving rise to the same effects, i.e. too high a pH if a high reactivity species isadded as a single dose (Figure 11.9).

MgOþH2O!MgðOHÞ2

MgðOHÞ2 þ 2Hþ!Mg2þ þ 2H2O

Industrial versions of this compound can be obtained in lightly or heavilycalcined (heated) forms. The effect of heating is to drive off water from thehydroxide (magnesium hydroxide is obtained by precipitation from sea water)to greater or lesser degrees and thereby reduce the chemical reactivity to greateror lesser degrees. The lightly calcined products are offered in different particlesize distributions and are no better for chrome tannage basification thanlaboratory grade reagents. In contrast, heavily calcined magnesia is rendered sounreactive it follows an idealised basification trace (Figure 11.12).The first product based on magnesium oxide available to the industry was

Tanbase, ex ICI, in which the particle size distribution is uniform, to ensureuniform reaction.39 It is safe to add this product at the beginning of tanning,when the offer can be calculated as 5% of the offer of 33% basic chrometanning salt (25% Cr2O3 content). This will typically produce a final pH ofabout 3.8, but the precise value will depend on the specific conditions ofpickling.

pH

End value

Time

Figure 11.12 Illustration of the basifying effect of magnesium oxide (Tanbase).


There is one caveat for users of this technology. The reaction must be heatedat the end of basification, in other words the temperature at the end of thetanning+basification processes must have reached at least 40 1C, preferably50 1C, and been held at the elevated temperature for at least two hours. Thereason is to ensure complete reaction and dissolution of the magnesia particles.If the reaction is not completed, e.g. because the process was conducted atlower temperatures, such as ambient conditions, minute particles of magnesiamay remain, invisible to the naked eye, but large enough to continue to reactwith the available acid in the pelt. The result can be chrome stains, createdduring the period of piling after unloading from the tanning vessel.

11.7 AVOIDING BASIFICATION

It is recognised in the industry that completing chrome tanning by basificationis a high risk operation. Therefore, an alternative to the automating of basifi-cation by self-basification is the avoidance of applying basifying agents. Thiscan be achieved by controlling the pickling step, to leave enough residual alkaliin the pelt to act as the basifying mechanism. This approach is old and wasoriginally called the ‘chaser’ pickle process, when the offer of acid on bated peltwas only designed to acidify the surface, then the chrome salt ‘chased’ the mildpickle through the cross section. Cooper suggested an alternative approach,applying a formaldehyde pretannage to limed pelt, squeezing out much of themoisture and then treating with chrome liquor: the technology combined rapiduptake of the tanning liquor and rapid fixation by the alkaline pelt.40 A decadeor so ago, Das Gupta redeveloped the chase pickle technology into the‘ThruBlu’ process:41 the required elements are:

1. Pickle to about pH72. Use chrome tanning salt with much of the associated sodium sulfate

removed, to reduce the effect of the sulfate ion on creating non-ionicchrome complexes.

3. Include polyamide in the tannage, for unspecified contribution to themechanism.

4. Apply to sheepskins or lime split hide.

The reaction involves elevated astringency, which increases the chromeuptake to nearly 99%, so the process may be limited to thin and open struc-tured substrates.

11.8 REACTIVITY AT HIGH BASICITY

It has been assumed that the use of high basicity chromium salts means that thesalt is more reactive than lower basicity salts. The assumption is based on thefollowing generalised view of the relationship between process rates, as appliedto the binding of a reagent to a substrate of finite thickness:

228 Chapter 11

� High reactivity means fast reaction, which also means slower penetration,leading to surface reaction, with the consequence of non-uniform reaction.

� Low reactivity means slow reaction, which also means faster penetration,leading to reaction through the cross section, with the consequence of evensurface reaction.

However, although high reactivity causes surface reaction, the observation ofsurface reaction does not necessarily mean high reactivity. The latter can be duemerely to the molecular size of the reactant, a known effect of basification, since:

� Higher molecular weight means bigger molecules, leading to slowerpenetration, causing more surface reaction.

In common with every other fixation reaction, the first step of the chrometanning reaction is the transfer from the solution environment into the sub-strate environment. The thermodynamics of the equilibrium are determined bythe solvating power of the solvent on the solute, when greater affinity betweenthe solvent and a solute results in a tendency for the solute to remain in thesolvent. Basification makes the chrome species larger and hence more hydro-phobic, therefore facilitating or promoting transfer of the species from thesolvent into the more hydrophobic environment of the substrate, which willindeed have a positive effect on chrome uptake. However, these are second-order effects, relative to the role of the complexation in the reactivity, i.e. theeffect of basicity on available reaction sites for complexation by carboxylgroups on collagen. It is typically assumed that polymerisation in basificationresults in the creation of more reaction sites within the larger molecular weightchromium ion. However, that assumption is based on the further assumptionthat all the available reaction sites on the chromium atoms within the mole-cular ion are equivalent with regard to complexation with carboxylate species.

OH2

OH2

Cr

H2O

O

H2O

H2O

O

Cr

OH2

OH2

H2O

O

H2OH2O

O

Cr

OH2

OH2

Cr

O

H2O

H2O

O

H2O

O

O

H2O

CrCr

OH2 OH2

OH2 OH2

∗

∗

∗

∗

∗

∗

∗

∗

33% basic

50% basic

H

H

H

H H

H

H

H

Figure 11.13 Reaction sites (asterisked) in chromium(III) tanning complexes.


The literature evidence indicates that this is not true, because reaction is pre-ferred at the trans positions, i.e. in the plane of the hydroxy bridges (Figure11.13).42 These positions are favoured by the presence of both aquo ligands andthe bridging hydroxyl groups.Figure 11.13 demonstrates that, if only the trans positions are reactive for

complexation, the polymerisation process does not increase the availability ofreaction sites. Therefore, the more basic chrome salt is essentially no morereactive than the less basic salt. However, the buffering effect of the more basicsalt will affect the state of ionisation of the collagen, which will, accordingly,make the chemical system more reactive.Importantly, the rate of olation, i.e. rate of polymerisation of basic chrome,

is fast but the reverse reaction is slow, an observation well known in thechemistry of chrome recovery.43 This means that the addition of high basicitychrome salt into a tanning bath whose pH corresponds to lower basicity willresult in equilibration of pH. But the distribution of chrome species sizes willremain unaltered. The effect of these variables on the chrome tanning reactionis summarised semi-quantitatively in Table 11.9.

11.9 ROLE OF TEMPERATURE

The rates of most reactions are controlled by temperature: usually the rateincreases with temperature. The relationship can be expressed in the form ofthe Arrhenius equation (as discussed in Chapter 3, Section 3.6). The rate ofionisation of collagen carboxyls is very fast and therefore hardly affected bytemperature. The speciation of chromium(III) compounds is also relativelyfast and hence little affected by temperature change. Therefore, raising thetemperature of chrome tannage does not alter the system, it only increases thereaction between the chrome and the collagen.Although is possible to control the temperature in a tanning vessel by cir-

culating the float through a so-called ‘lab. box’, it is common for tanners to relyon the development of friction energy within the tanning vessel to raise thetemperature. This is consistent for a given vessel containing similar loads andwater to rawstock ratios, but tends to be inconsistent between vessels. Anoption is to inject steam or add hot water. The latter is referred to as ‘hot water

Table 11.9 Influences of chromium(III) salt basicity and solution pH on thechrome tanning reaction.

System conditions Consequences

Solution pHCr(III) saltbasicity

Size of Cr(III)species

Reaction rateas Cr uptake

CrReaction Ts

Low Low Small Low Penetrating LowLow High Large Low Penetrating LowHigh Low Increasing High Penetrating HighHigh High Large High Surface Low

230 Chapter 11

ageing’, which is often used in conjunction with basification and hence some-times regarded as a basification process itself. That is only strictly true when thesolution is diluted to the extent that hydrolysis occurs, and is likely to result inchrome precipitation.

11.10 RELATIVE EFFECTS OF pH AND TEMPERATURE

The chrome tanning reaction is characterised by the type of curve shown inFigure 11.14, which can represent the effect of time on chrome uptake and theeffect of chrome content on shrinkage temperature. It also is the shape of theeffect of pH or temperature on chrome uptake or shrinkage temperature. Ofcourse, the lines of these dependencies are quite different. However, this raisesthe question as to whether pH and temperature have equivalent effects onchrome uptake and the resulting shrinkage temperature.At this point, it is important to recognise that there are two aspects to the

chrome tanning reaction. Fixation is important, in terms of the economics ofthe process and its environmental impact. But more important is the intro-duction of hydrothermal stability, which reflects and determines the propertiesof the leather in terms of the physical, chemical and biochemical resistance toexternal conditions. The tanner’s function strictly is to make stable leather, notmerely to absorb chromium(III) from solution: in other words, the parameter ofactual importance is the effectiveness of the reaction, which can be defined asthe rise in shrinkage temperature per unit of bound chrome. This can easily becalculated as the difference between the tanned collagen shrinkage temperatureminus the shrinkage temperature of the collagen before tanning (normalised tothe same pH) divided by the percentage of chrome in the leather, as Cr2O3 ondry weight. The influence of pH and temperature on tanning effectiveness ispresented in Figure 11.15, based on model studies in which the tanning con-ditions were held constant.44

Clearly, chrome fixation is dominated by temperature, whilst a rise inshrinkage temperature is dominated by pH. This can be understood with

Ts or Cr2O3

Time, pH, temperature or Cr2O3

Figure 11.14 Generalized effect of controlling parameters in chrome tanning.


reference to the direct effects of each parameter. The effect of increased tem-perature is to drive the reaction under the particular pH conditions that applyat any point in the process: in that way, the constitutions of the reactants areunaffected, only their interaction is speeded up. In the case of increasing pH,both reactants are affected. The collagen is most affected by increasing pH,which causes ionisation of carboxyls, creating reaction sites for chrome com-plexation, resulting in faster reaction, but there is no obvious link between thekinetics of the reaction and the effect of the reaction. The chromium species areincreased in size by olation, although it has been argued that this process doesnot influence the reactivity. However, the change in molecular dimensions

Figure 11.15 Relative effects of pH and temperature on the effectiveness of chrometanning under constant conditions. (Courtesy of Journal of the Amer-ican Leather Chemists Association.)

232 Chapter 11

introduces a change to the outcome with regard to nature of the product andthis is the reason why pH affects the shrinkage temperature in a different way totemperature change. This idea is developed further in Chapter 19, with regardto the basis of hydrothermal stability.The typical chrome tanning reaction is not conducted under constant con-

ditions, but requires dynamic changes of pH and temperature. This begs thequestion: if the starting and finishing values of pH and temperature are known,does the route taken affect the outcome? Figure 11.16 presents a model of theoptions.45

The conditions have their equivalents in industrial processing. Heating atthe start is similar to stating at elevated temperature, which is common inhotter countries. Early heating and late heating could be the result of efficientand inefficient frictional heating, respectively. Heating at the end is accom-plished by so-called hot water ageing, when the drum is floated up with hotwater just prior to unloading. Early basification applies when the tannage isbasified right at the start of the process: the solution of the basifying agent isadded immediately after adding the chrome in solution. Intermediate andcontinuous basification refers to variations in self-basification. Late basifica-tion means basifying after prolonged drumming to allow the chrome topenetrate.A detailed study of hydrothermal stability conducted on chrome tanned

leather with high chrome content and high shrinkage temperature, 4115 1C,revealed some inconsistent behaviour when the leathers were subjected torelatively prolonged boil testing (Table 11.10).46 Purely on the basis of theshrinkage temperature, which is consistent with the high chrome content in theleathers, it would be assumed that the leathers would pass the conventional wet

Heating pH

a b c d a b c d

a heat at startb early heatingc late heatingd heat at end

a early basificationb intermediate basificationc continuous basificationd late basification

Figure 11.16 Dynamic changes in pH and temperature.


boil test and there would be no reason to suppose that the leathers would failthe same test but starting from the dry condition.From Table 11.10, the way in which pH and temperature changes are

managed produces an anomaly in the hydrothermal stability, in which per-formance in boil testing is apparently dependent on the way in which theshrinkage temperature was produced, but not on the value of the shrinkagetemperature. Although it is still not clear why the extremes of processingconditions should influence the hydrothermal stability of wet leather, anexplanation for the difference between boil testing wet leather and boil testingdry leather has been offered.46 If the dry leather is hydrophilic, then the wettingprocess at the beginning of the test would be rapid, thereby releasing the heat ofhydration quickly: this energy will contribute to the heating of the leather andcause faster shrinking. Therefore, it appears that the relatively extreme pro-cessing conditions give rise to the anomalous results from the dry boil test,because they create more hydrophilic leather. This is not a large effect inpractice because the test is unusually stringent, but it is worth considering thatit may have some influence on the production of water resistant leather.One approach to making the best use of chrome salts in the tannery is to

achieve extreme conditions of pH and temperature. Figure 11.17 presents somedata taken from the industrial development work of Daniels:47 although theobservations were made under scientifically uncontrolled conditions, they dorepresent the final conditions in industrial processes.It can be seen that the efficiency tends to 100% at pH 5 and 70 1C. It is indeed

possible to basify to pH 6, but only under the following conditions:

1. The tannage must be highly efficient before raising the pH above theconventional finishing point, pHo4.0. The chrome concentration insolution needs to be low for two reasons.

(i) The rate of reaction should be as low as possible. This isnecessary to control the relative rates of penetration and fixation.

(ii) The likelihood is for the residual chrome to fix more on thesurface than within the pelt. The lower the amount of residual

Table 11.10 Effects of dynamic pH and temperature changes on hydrothermalstability: 5-min boil tests conducted on wet samples (designated‘wet boil test’) or on dry, conditioned samples (designated ‘dryboil test’). 0 means no shrinking in either test, X means shrinkingfailure in the dry boil test, XX means shrinking failure in bothtests.

Basification Heat at start Early heating Late heating Heat at end

Early X a X X XIntermediate 0 0 0 0Continuous 0 0 0 0Late 0 0 XX XXaThe only anomaly to remain after ageing at 60 1C.

234 Chapter 11

Figure 11.17 Combined effects of pH and temperature on chrome fixation efficiency.(Courtesy of World Leather.47)


chrome in solution, the less likely surface fixation is to cause aproblem, particularly in subsequent dyeing.

2. The rate of pH increase must be as slow as possible. As indicated in point1(i), the chrome must be allowed to penetrate as much as possible duringthe process of enhancing the reactivity of the pelt.

3. The basification should not proceed beyond pH6, because then the reactionbegins to show signs of reversal (A.D. Covington, unpublished results.). Thereason for this is not clear, because reversal by hydrolysis to chromite mightnot be expected to apply much below pH9. However, polymerisation of thechrome species and interference with the establishment of the tanningmatrix may be a contributor to the high pH effect. High pH basification wasproposed by Gauglhofer,48 to achieve very high chrome uptake efficiency;however, he was obliged to admit that the leathers could not be dyed.

High temperature tanning is technologically possible, feasible and highly desir-able. In conventional processing, a final temperature of 40–60 1C should be the tar-get. The value achieved depends on the method of heating, usually obtained solelyby frictional energy within the process vessel, which limits the extent of heating.Higher temperatures can be achieved by heating the recirculated float or by inject-ing steam into the vessel. There are limitations to this approach, which relate to thekinetics of shrinking. Consider the dynamic conditions in the tanning process:

� As the reaction proceeds: temperature rises, chrome fixation increases,shrinkage temperature rises.

� Shrinking, either as gross denaturation or incipient heat damage, is a kineticprocess. The closer the process temperature approaches the shrinkagetemperature, the faster damage occurs, i.e. the sooner damage occurs.

From this analysis, the shrinkage temperature must always be higher than thetemperature in the vessel and preferably much higher. In the case of chrome tan-ning, assuming the temperature rise is not fast in the early stages of tanning, thatcondition will be met. If we consider reaching the extreme condition indicated inFigure 11.17, is it feasible? The benefit is clear, but what are the contra-indications?

1. Under conventional industrial conditions, the shrinkage temperature ofthe chrome tanned leather is not likely to exceed 120 1C, so the rule aboutthe temperature of the solution not approaching closer than 30 1C lowerthan the shrinkage temperature might be broken if the processing tem-perature is abnormally high. Therefore, the safety margin will probablyapply, but equally may not under extreme conditions.

2. The energy cost may be prohibitive.3. Workers will not be able to handle the leather when it is dropped from the

process vessel, because it will be too hot. This may not seem to be aproblem, since they merely have to wait for it to cool, but that in itselfcreates additional problems:

(i) Cooling will take a long time, whether within the process vesselor after unloading, so delaying production.

236 Chapter 11

(ii) Reaction between the residual chrome and the pelt will con-tinue. This can cause shadowing on the surfaces, depending onthe amount of residual chrome in the float.

(iii) The leather, whether in or out of the process vessel is alwayscreased: if further tanning reaction occurs, the creases in theleather can be made permanent.

11.11 MASKING

Masking is conventionally defined as:

the modification of metal complexes, by replacing aquo ligands with otherless labile ligands – the purpose is to render the complex less susceptible toadditional complexing reactions, including precipitation.

In this way, it has been assumed that the function of masking is to aidchrome penetration at the start of tanning, by reducing the reactivity of thecomplexes. Although such masking can in principle be achieved with any ligandcapable of creating a complex, in the case of chrome tanning the commonestpractical masking agent is formate, usually derived from the pickling for-mulation, but it may be added after a sulfuric acid pickle or the maskingreaction might be conducted prior to the tanning reaction.The traditionally accepted purpose of masking is threefold.

1. Since the chrome tanning reaction depends on creating carboxyl com-plexes with collagen carboxyl groups, modifying the complexes byincluding other carboxyls as ligands reduces the number of reaction sitesavailable to the collagen, so the reactivity of the chromium(III) is reduced.This is the rationale for using masking at the beginning of the tanningprocess, when the masking reaction is completed before tanning starts: todecrease the rate of fixation relative to the rate of penetration.The ratio of the number of moles of carboxylic masking agent to the

number of gram atoms of chromium is known as the masking ratio.Typical masking ratios for industrial processes lie in the range 0.5–1.0(Figure 11.18, where R might be either collagen or a group from an ali-phatic carboxylate salt).

3+OH2

OH2

OH2

OH2H2O

H2O

RCO2RCO2

Cr Cr

H2O

O

O O

O

Cr Cr

OH2

OH2

OH2

O2CR

2+

H2O

H2O

H2OH

(a) (b)

H H

H

Figure 11.18 Masking ratios 0.5 (a) and 1.0 (b).


Since the masking agents are typically anionic, they reduce the chargeon the complex, thereby reducing the initial affinity of the chrome com-plex for collagen carboxyls. In the limit, the chrome complex can be madeanionic, e.g. in the trioxalato complex: this does not necessarily preventfurther reaction, since ligands can be exchanged, but clearly the affinity foradditional complexation with anions would be markedly adverselyaffected. The reactive sites for complexation are the trans positions(Figure 11.3): it is assumed that multiple substitutions will minimise stericinteractions.

2. An additional consequence of reducing the number of complexationreaction sites is that the concentration of hydroxyl ions required to pre-cipitate the salt is increased: in other words, the pH of precipitation israised. This means that the system can be made more reactive by raisingthe pH, to increase the availability of reaction sites on collagen, avoidingprecipitating the chrome. Although the argument seems reasonable, theeffect is not often used, because the precipitation point is only raisedsignificantly by high masking ratios. In this way, the tanner sets up acompetition between the increase in substrate astringency caused byincreasing the reactivity of the collagen and the decrease in the astringencyof the chromium caused by the loss of reaction sites.

3. The colour of the chrome complex is altered by the ligands in the moleculeand this affects the colour struck in the wet blue leather, as discussedearlier.

It should be recalled that the complexation reaction between collagen andchromium(III) is the same as the rate of any other carboxylate complexationreaction, i.e. the masking rate is the same as the chrome fixation rate, as shownby Shuttleworth in Figure 11.19.28,49 Therefore, it is important to recognisethat, if a masked tannage is required, the masking reaction needs to be com-pleted prior to commencing the tanning reaction.However, it is difficult to obtain a truly accurate comparison of collagen

complexation versus carboxylate salt complexation. This is because of diffusioneffects with the protein substrate and uncertainties in the pKa of the reactionsites on collagen. Nevertheless, a simple calculation is useful.If the offer of formic acid in the pickle is 1% on limed weight, this corre-

sponds to about 5% on dry weight, which is the same as 1 mole of formate perkg of dry protein.Dry limed collagen contains about 1 mole of carboxylic acid groups.The pKa of formic acid is 3.75: the pKa values for the carboxylic acid groups

on collagen lie in the range 3.8–4.2.Therefore, allowing for errors in numbers of carboxyl groups, differences in

pKa and sites of reaction on collagen, the concentrations of formate and col-lagen carboxylate groups are certainly within the same order of magnitude.By that argument, the two sources of complexing carboxylate groups com-

pete on roughly equal terms. Therefore, if the masking and tanning kinetics arecomparable, fixation of formate and collagen ligands proceeds at the same rate.

238 Chapter 11

This means that the tannage is unmasked at the start, but masked to somedegree by the end. It is not known if the chrome in solution and the chromebound to collagen are equally reactive. If they are, then the masking ratio is lessthan assumed and masking competes with collagen complexation. If they arenot, this would mean that chrome in solution is more reactive and therefore themasking ratio for unbound chrome will increase during the process, making thechrome increasingly unreactive. It is reasonable to assume that the boundchrome can react with more masking agent, although the reaction will behindered by the reduction in available reaction sites on the chrome species: if achromium(III) molecular ion is complexed with collagen at a trans site, theadjacent trans site will be sterically hindered and the only available sites foreasy access are situated at the other end of the molecular ion.There is considerable uncertainty over the way masking operates under

typical reaction conditions. If certainty of the masking effect is required, thechrome must be masked to equilibrium prior to starting the tanning processand to that end commercial products are available, otherwise the reaction mustbe conducted in the tannery. In most commercial tanning processes, when

Figure 11.19 Comparison of reaction rates for chromium(III) with collagen and withacetate: reactions were for [Cr] : [CO2H]total ratios 1 : 1 to 1 : 8 andresults presented refer to comparable reaction times. (Courtesy ofJournal of the Society of Leather Technologists and Chemists)


masking is taking place at the same time as chrome is being fixed onto collagen,it is probable that the dynamic masking effect plays little part in the tanningprocess, e.g. in its role of affecting chrome penetration.The technology of masking must be reconsidered, because there is much

misunderstanding regarding its effects on chrome tanning. Introducing a ligandinto the chrome complex can be considered to have two competing outcomes:reduction in reactivity due to the occupation of complexation reactions sites,but, alternatively, the reactivity of the masked chrome species can be enhanced,because the complex becomes more hydrophobic by the introduction of thecarboxylate into the ligand sphere. It is typically the case that all carboxylatesare more hydrophobic than an aquo ligand – at least those commonly used inthe art. An exception might be sugar-reduced chrome salt, but that is a differenttanning technology.Table 11.11 shows the effects of masking with formate and other masking

salts, which can react in different ways. Here the masking ratio is 1 mole permole Cr2O3: the effect of the number of equivalents of carboxyl per mole shouldbe taken into account. In each case, the effect of the masking on the reactivityof the chrome species is designated, based on the known chemistry of themasking agent and the observation of the relative extent of reaction.The designation of ‘hydrophobic masking’ indicates the influence of the

masking agent on the properties of the complex and hence the element ofthe reaction mechanism that is affected: increasing the hydrophobicity of thecomplex encourages transfer from aqueous solution.The designation of ‘complexation masking’ indicates that the net effect is to

modify the ability of the complex to react further, i.e. this conforms more to theconventional view of masking. Comparing the effects of formate and oxalatemasking (Figure 11.20), the notional structures indicate that formate is likely tobe less able to interact with water than an aquo ligand, thereby conferring adegree of hydrophobicity. In contrast, oxalate has the potential for effectivehydrogen bonding, because it offers only carbonyl groups for interaction with

Table 11.11 Effects of masking agents on relative chrome uptake.

Masking saltChelate ringsize

Complexreaction

Relative chromecontent50 Masking effect

None – – 1.00 NoneFormate – – 1.06 HydrophobicAcetate – – 1.18 HydrophobicOxalate 5 Chelate 0.97 ComplexationMalonate 6 Chelate 1.05 HydrophobicMaleate 7 Chelate 1.40 HydrophobicSuccinate 7 Chelate/

crosslinking1.85 Hydrophobic/

molecular sizePhthalate 7 Chelate 1.93 HydrophobicAdipate 11 Crosslinking 2.03 Hydrophobic/

molecular size

240 Chapter 11

the solvent, so the net effect is a reduction in the number of available reactionsites and hence reduced affinity for collagen carboxyls, driven by the chelatingmechanism.Table 11.11 illustrates the following important points:

1. If we assume that chrome content in pelt is a reflection of the overallreaction rate or reactivity, there is no indication that masking with for-mate at this level reduces the reactivity of the chrome salt; indeed, there isan increase in reactivity. This runs contrary to conventional thinkingabout the masking effect of formate. It must be recognised that the relativeeffects of masking are dependent on the masking ratio. Those apparentlycontradictory effects are: the reduction in reactivity by using up reactionsites and the increase in reactivity by increasing the hydrophobicity.Taking the example of formate, as the simplest and commonest maskingagent, at higher masking ratios, [RCO2] : [Cr2O3]41, the reactivityreducing effect will begin to dominate, typically resulting in a reductionin tanning efficiency.50 At lower masking ratios, which is more typi-cally related to industrial practice (the masking ratio of one moleof formate per mole of chrome oxide corresponds to a formic acid offer of0.6% if the chrome offer is 2% Cr2O3), the outcome is an enhancement ofchrome reactivity.The effect of incorporating formate has the same hydrophobic effect as

masking with malonate; the latter has a more predictable impact on thecomplex properties, because of the presence of the methylene group, butthe outcome is similar for formate and malonate masking.

2. In recognising the role of masking ratio, the combination of the chemicaloutcome with the kinetics of the process must also be taken into account.Unless the chrome is masked by conducting the reaction separately, priorto starting the tanning process, the masking ratio will necessarily be low atthe beginning of the chrome tanning reaction and then increase as thetanning reaction proceeds: the progress and change of masking ratio willdepend on the chrome fixation kinetics, controlled by the pH profile, andon the offers of chrome and masking agent.The influence of masking ratio on chrome reactivity is modelled in

Figure 11.21, where the ideal masking situation is presented in terms of

Cr

OH2

OH2

OH2

OH2

Cr

H2O

H2O

H2O

OO

O

C

O

OH

O

C

O

C

H

H

(a) (b)

Cr

OH2

OH2

OH2

OH2

Cr

H2O

H2O

OO

O

H

H

Figure 11.20 Masking with formate (a) and oxalate (b).


reduced reactivity at the beginning of the reaction, gradually increasing bythe end of the reaction. Clearly, this bears no resemblance to any realsituation. The assumed effect is to reduce the reactivity by eliminatingreaction sites, but the actual effect is to include some enhancement ofreactivity by increasing the hydrophobicity or reducing the hydrophilicityof the ion. In this way, the actual effect of masking is directly opposite tothe effect required. If more control is required, then the chrome must bepremasked by prior complexation and the effect will be the same as theassumed effect presented in Figure 11.21, in terms of the effect of maskingratio: at low masking ratios the chrome will be made more reactive andonly at unusually high ratios will the classical masking effect come intoplay. In Figure 11.21, the effect of premasking is modelled as a function ofprogress in tannage: the reactivity of the chrome complex is fixed by themasking ratio, so the reactivity will not change. The position of the pre-masked line on the reactivity axis will depend on the masking ratio andtherefore may even lie above the value of the unmasked chrome, i.e. lowratios of premasking can produce chrome salt of enhanced reactivity.The observation that low masking ratios will usually increase rather

than decrease chrome astringency is interesting, because it means that theimpact of hydrophobicity of the complex can be more powerful thanthe effect of reduction in charge. In other words, it is confirmation of thestepwise mechanism of reaction, where transfer from solution precedeselectrostatic interaction of solute with substrate.The premasked effect is assumed here to be constant, if there is no

additional masking agent available. What is predictable is that the reac-tivity will not decrease by reversal of the masking reaction as the tannageproceeds: the contrary is another unaccountable assumption by some

Metal complex ion reactivity

Masking ratio orprogress in tannage

actual effect

assumed effect

ideal effect

premasked effect

unmasked effect

Figure 11.21 Modelled effects of masking on metal ion reactivity to complexationwith collagen; tanning in the presence of masking agent, e.g. formate.

242 Chapter 11

tanners. The stability of chrome tanned collagen is well known and appliesequally to masking complexation.In Figure 11.21 the relative effects of the two contributing parameters of

hydrophobicity and reaction site occupation will depend strongly on thechemistry of the masking agent. The role of masking in practical tanningdepends on the tanner understanding, even if only qualitatively, the nat-ure of the actual effect.

3. From the analysis of typical tanning conditions presented above it is likelythat conventional, industrial formate masking will only increase thereactivity of chrome, since the masking ratio is unlikely to exceed thecondition represented at the beginning of the ‘actual’ curve in Figure11.21. Therefore, in practical tanning, the conventional notion of maskingdoes not apply: even in premasked chrome tannage, the masking ratio willnot be high enough to overcome the hydrophobic effect.It is useful to reconsider the case of sugar-reduced chrome tanning salts.

The properties can be rationalised in terms of classical masking: themasking ratio is high (assumed, not specified) and the masking agents arebreakdown products of carbohydrate, i.e. polyhydroxy polycarboxylates,so they are relatively hydrophilic. In this way, the salts have reducedastringency, but enhanced filling power.

4. The role of masking as a general aspect of chrome tanning is an importantfeature, which can be technologically exploited. The use of specificmasking agents, which gradually increase the astringency of the chromespecies, by increasing their tendency to transfer from solution to substrate,is an aspect of the reaction that has not received scientific attention. Here,the requirement is to match the rate of diminishing concentration ofchrome in solution with the rate of masking complexation, to maintain orincrease the rate of chrome uptake. Since the complexation interactioncannot easily be reversed, changing the reactivity in the way indicated bythe ideal masking trace in Figure 11.21 would require changes within themasking ligand, altering the ligand from hydrophilic to hydrophobicwithin the time scale of conventional tannage and under the restrictions ofthe typical chrome tanning conditions. Potential exploitable commercialoptions are not difficult to imagine, such as the following:

(i) hydrolysis of esters(ii) reaction at keto groups(iii) aldehydic reactions(iv) solvent modification (see below).

11.11.1 Polycarboxylates

The use of dicarboxylates can lead to different kinds of masking: chelation,when the dicarboxylate compound reacts with a single metal ion, or cross-linking, when the dicarboxylate reacts with two metal ions (Figure 11.22). Thereaction adopted depends on the size of the ring formed if the salt can act as achelate.


The relationship between ring size and chelate or chain forming masking isset out in Table 11.11.The formation of the ring type of complex depends on thesteric distortion in the ring and the loss of entropy that results from the twoends of the chelating molecule being constrained on the same atom. The loss isallowable only if the ring is five- or six-membered: smaller rings are highlystrained, so less favoured, larger rings lose too much entropy and are notformed in favour of a crosslinking structure, which can adopt any conforma-tion. Therefore, for 5–7-membered rings chelation is generally favoured, but forlarger ring sizes crosslinking is generally preferred.51

Although crosslinking increases the size of the molecular ion, the number ofreaction sites is effectively not increased. In the case of chelation, the reactivityof the species is significantly reduced, in comparison with monodentatemasking, since the model in Figure 11.21 indicates blocking of potentialcrosslinking power. Table 11.12 presents some effects of masking, as the relativechrome uptake and the shrinkage temperature of aluminium tanned collagen,using decomposed zeolite.52

(b)

(CH2)n (a)CO

OC

CO

CO(CH2)2

O

OCr

O

OCr

OH2

OH2H2O

H2OH2O

H2O

H

H

O

OCr

O

OH2

Cr

OH2

OH2H2O

H2OH2O

H2O

H

H

O

OCr

OH2

OH2

Cr

OH2

OH2H2O

H2OH2O

O

H

H

Figure 11.22 Reactions of dicarboxylate masking agents: (a) chelation and (b)crosslinking.

Table 11.12 Effects of bidentate masking agents on chromium(III) andaluminium(III) tanning.

Masking salt Relative chrome uptake Ts of Al(III) tanned collagen (1C)

None (sulfate) 1.00 60 (estimated)Fumarate – 65Maleate 1.40 80Phthalate 1.93 92

244 Chapter 11

Table 11.12 introduces an interesting example of commercial masking tech-nology, the use of phthalate (benzene-1,2-dicarboxylate). The use of this salt iswell known, typically applied late in the process, to increase the reactivity of thechrome. It is widely assumed that the reactivity increase stems from cross-linking, and Vernali has calculated the energetics of crosslinking by molecularmodelling.53 However, crosslinking is sterically less likely, so the maskingreaction is more likely to be chelation, like the reaction with maleate,creating a seven-membered ring: here, only fumarate is sterically capable ofcrosslinking. If that analysis is correct, the effect of masking must be to alter thenature of the interaction with the solvent, i.e. the hydrophilic/hydrophobicnature of the solute. From considerations of the structures of the maskingsalts, the relative effects on the hydrophobicity of the salts can be understood(Figure 11.23).The extent of increasing the reactivity of the masked chrome is predictable,

based on the evident chemical nature of the masking ligand, i.e. the CH¼CHp-bond of the maleate can interact with water more than the saturated linkedmethylene groups of succinate, which are in turn less hydrophobic than thelarge benzene ring of phthalate. The relative effects of the ligands are based ontheir interaction with the solvent. This is clearly a mechanism for manipulatingchrome reactivity that can be exploited. Table 11.13 compares the relativeeffects of the chelating and crosslinking reactions.Crosslinking masking depends on the calculated chelate ring size exceeding

seven: for aliphatic dicarboxylates this starts with glutarate and the effect of thefour methylene groups in the chain of adipate is clear in the reactivity illustratedin Table 11.11. Therefore, all crosslinking masking agents are likely to increasethe reactivity of chrome: the effect is not concerned with the inherent reactivity

OH2

Cr CrO

O

OH2H2O

H2OH2O

H2O OH2

CH

CH

CH

CH

O2C

fumarate

maleate phthalate

H

H

OH2

Cr CrO

O

OH2H2O

H2OH2O

H2O

O2C

O2CH

HOH2

Cr CrO

O

OH2H2O

H2OH2O

H2O

O2C

O2CH

H

OH2

Cr CrO

O

OH2H2O

H2OH2O

OH2

OH2

CO2 H

H

Figure 11.23 Bidentate masking of chromium(III).


towards complex formation, but is dependent on the enhancement of fixationrate by polymerisation.The relative affinities of potential masking agents for reacting in the chrome

complex sphere are as follows:

hydroxide4oxalate4citrate4lactate4malonate4maleate

4phthalate4glycolate4tartrate4succinate4adipate4acetate

4formate4sulfite4sulfate4chloride4nitrate4chlorate:

This list is not exhaustive, but in practical terms only formate and phthalateare conventionally important.There is another effect of masking, which is to change to colour of the chro-

mium complex (discussed in Chapter 9). As indicated in Table 9.4, the colour ofthe complex and hence the colour of the tanned leather depends on chromecomplexation. If chrome tanning is conducted in the presence of only sulfate andchloride as counterions, the tannage is unmasked. Therefore, the only complexesformed are with collagen carboxyls: the result is wet blue that has a distinctlygreen cast. If formate is present, whether truly acting as a masking agent ormerely present in the system, the resultant wet blue is bright pale blue in colour.

11.12 STABILITY OF CHROME TANNED LEATHER

Figure 11.24 illustrates the range and limits of stability of chrome tannedleather over the pH scale.

Table 11.13 Comparison of chelating and chain forming (crosslinking)masking reactions.

Chelate Chain

No change in size ofchrome species

Polymerisation of chrome species: more surface reactive, notmore active towards complex formation

Strong masking effect Weaker masking effectLess effect on solubi-lity of complex

Complexes less soluble: more surface reactive

0 2 4 6 8 10 12 14

solubilisation asCr(III) ion

hydrolysis of complex,solubilisation as chromiteion, CrO2

−

region of stability

Figure 11.24 Guideline limits of chrome tanned leather stability within the pH scale.

246 Chapter 11

The chromium complex can be broken by the presence of other complexingagents. The following conditions are combined in the accelerated artificial per-spiration test, which is capable of stripping all chrome out of leather overnight:

� high concentration of alternative complexing ligand, e.g. 8% sodium lactate;� elevated pH, at least pH 8, to assist in the hydrolysis reaction;� elevated temperature, e.g. 50 1C, to accelerate the rate of reaction.

11.13 ROLE OF SULFATE IN THE CHROME TANNING

MECHANISM

In the literature it is suggested that sulfate participates in the chrome tanningprocess because it is included in the reactive chromium(III) crosslinking species.The frequently presented model of the chrome tanning crosslink has sulfateacting as a bidentate ligand in a dichromium species (Figure 11.6). Conven-tional thinking in the latter half of the twentieth century has the sulfate actingin the following roles:

� as a counterion (or gegenion) for cationic Cr(III);� as a counterion for protonated amino groups on collagen;� as a ligand in the Cr(III) complex.

This is the model used by Gustavson26,27 in his analysis of the chrome tanningreaction, in which he used wet chemistry to distinguish between those differentroles of sulfate. From an analysis of wet blue, he concluded that the dominantchrome species was of the form indicated in Figure 11.6 i.e. a dichromiumcomplex bridged by sulfate: the important conclusion from his chemical argu-ment is that only 10% of the bound chrome is acting as a crosslinker.This argument has been challenged in the following way.54 The simple

association of the anion with the cationic centres in the collagen would notappear to offer a contribution to the stabilising effect. It is also unlikely that acomplexation reaction would be part of the stabilising effect: indeed, because itis a weak ligand, it might be expected to be hydrolysed out of the complexrapidly in solution chemistry, although the studies in solution tend to disagree.7

Gustavson55 found that adding excess sulfate to chrome tannage maximised theshrinkage temperature, to indicate that it does play some role in the tanningmechanism, and it has been shown that sulfate does exchange between thecoordinate chrome and the solution in tanning.56 On the other hand, Spicciahas demonstrated that the m-sulfato dichromium species loses the sulfate duringreaction with collagen carboxyl groups.57

The relationship between sulfate and the chrome complexes bound to col-lagen was directly addressed in the first published study of the species usingextended X-ray analysis fine structure (EXAFS) by Covington et al.54 Thisadvanced analytical technique is based on the absorption of a spectrum of X-rays from a synchrotron source by the target atom, in this case chromium,


A: typical sloping baselineB: feature associated with theoxidation state of chromiumC: region of structuralinformation

A

B

C

-15

-10

-5

0

5

10

15

3 4 65 87 9 10 12 13

Fou

rier

tran

sfor

m a

mpl

itude

k/Å−1

0.0

0.5

1.0

1.5

2.0

0 1 2 3 4 5 6

k3χ(k)

Radial distance/Å

0.55

0.35

0.37

0.39

0.41

0.43

0.45

0.47

0.49

0.51

0.6 0.65

AbsorptionEdge

X-ray energy (eV)

X-r

ay a

bsor

ptio

n

0.7

Above edgeBelow edge

(a)

(b)

(c)

Figure 11.25 Basis of chromium(III) EXAFS in tanned collagen: experimental dataare solid lines, best fit lines are broken. (a) K-edge absorption of X-raysby chromium in leather. (b) Cr K-edge spectrum for hide powdertanned with 33% basic chromium(III) sulfate, basified to pH4.0.(c) Corresponding Fourier transform, corrected with the phase shift ofthe first shell. (From ref. 54, courtesy of Elsevier Science.)

which has also been described in the similar study by Reich.58 The way in whichenergy quanta are reflected within the molecule is seen as the absorption data(Figure 11.25).The analysis depends on predicting the structure of the molecule, in terms of

the nearest neighbours and next-nearest neighbours and so on: a computerprogramme generates an absorption spectrum, which is compared with theacquired data and coincidence confirms the predicted structure. The EXAFSstudy yielded data for the ratios of neighbouring atoms and the experimentalresults can be compared with the theoretical values for numbers of nearestneighbours (Tables 11.14 and 11.15).Comparison of the calculated ratios from the nonlinear structures with the

linear structure, given in Figure 11.5, supports the linear nature of the averagechrome species bound to collagen and confirms that the average molecular ioncontains four chromium atoms.It has been suggested that sulfate ion plays a part in the polymerisation

mechanism during basification.54 This is plausible because the transientbidentate complexing effect of sulfate ion at the cis positions in the chromiu-m(III) species (Figure 11.4) causes a labilising of all the other positions,including the cis positions and facilitates exchange reactions by an associativeinterchange, Ia, reaction mechanism.59 This would promote the formation ofnonlinear species (Figure 11.26).

Table 11.15 Calculated values for nearest neighbours in linear and nonlinearchrome species (structures given in Figure 11.5).

Chrome speciesNo. of first shelloxygens

No. of second shellchromiums

No. of third shelloxygens

Nonlinear Cr3 (II) 6.0 2.0 4.0Nonlinear Cr4 (III) 6.0 2.5 3.25Linear Cr4 (I) 6.0 1.5 6.0Linear Cr4 (I)measured

6.5 1.4 6.2

Table 11.14 EXAFS data for nearest neighbours in chrome species bound tocollagen.

Basicity (%) Shell Coordination number (error in brackets)

33 Oxygen 6.29 (0.14)Chromium 1.45 (0.30)Oxygen 7.43 (1.37)




These data lead to the conclusion that the polymerisation mechanism forchromium(III) may be different in solution to in situ on collagen: the formerfavouring three-dimensional species, the latter favouring linear species andhence contributing to the effectiveness of the primary interaction with collagen.If this is truly the case, it should be possible to influence the formation of basicchrome species by manipulating the tanning and basifying procedures, tooptimise the formation of reactive linear species.In the EXAFS analysis of chrome tanned collagen, when a sulfur atom was

introduced into the complex it produced a theoretical spectrum totally unlikethe experimental data. Therefore, it was concluded that sulfate was notdetectably close to the chromium atoms in the ionic complexes. This means thatthe structure of the tanning species is likely to be as illustrated in Figure 11.27,in which the sulfate ion can link chrome species via the aquo ligands, but doesnot approach closer.

11.14 ROLE OF THE COUNTERION IN CHROME TANNING

In the conventional understanding of the typical range of tanning chemistries, itis frequently asserted that chromium tanning is unique, because it is the onlysolo tannage capable of raising the shrinkage temperature of collagen above100 1C. However, if we examine some observations of the effects of chromiu-m(III) salts in this regard that assertion must be modified. If collagen is tannedwith basic chromium(III) chloride60 or perchlorate61 in the conventional way,the shrinkage temperature of the leather only reaches 80–85 1C (Table 11.16).

OH2

CrCr

H2O

O

H2O O

H2O

H2O OH2

OH2H2O

H2O

OH2O

OCr Cr

OH2

H2O ∗

H2O ∗

H2O ∗

fast

slow

H

H

H

OH2

CrCrO

H2O O

H2O

H2O OH2

H

H

H

Figure 11.26 Kinetics of water exchange at chromium(III).

250 Chapter 11

Figure 11.27 Possible interaction between chromium(III) species and sulfate, whichmay involve more water molecules between the aquo ligand and thesulfate ion.

Table 11.16 Effect of anions on the shrinkage tem-perature of leather tanned with basicchromium(III) perchlorate (Ts¼ 80 1C).

Anion DTs (1C)

Sulfate 32Formate 24Acetate 16Oxalate 30Maleate 32Fumarate 37Phthalate (1,2-dicarboxybenzene) 29Terephthalate (1,4-dicarboxybenzene) 32


In the commonest version of chrome tanning, the shrinkage temperaturereaches as high as 120 1C, which is the typically expected outcome. Further-more, rigorous washing of leather made from chromium(III) sulfate in the usualway will reduce the high shrinkage temperature to the lower range.62 In eachcase, simply adding sodium sulfate to the solution restores the expected highshrinkage temperature: interestingly, orthophosphate does not have the sameeffect, indeed it actually destabilises the tannage.Therefore, it can be concluded that the chromium(III) molecular ions them-

selves have only a moderate effect on the shrinkage temperature. This obser-vation means that chromium(III) is not unique, but apparently conforms to thepattern of performance of other mineral tannages, which typically conferlimited hydrothermal stability. However, unlike the other mineral tannages, theintroduction of the weak reaction of sulfate ion and other anions produces asynergistic effect on the shrinkage temperature, giving rise to the familiar highhydrothermal stability. Therefore, the conventional chrome tanning effectshould not be regarded as a solo tanning reaction: it is, in fact, a synergisticcombination of the effects of the chromium(III) molecular ions and theircounterions. The conclusion raises the possibility of improving the effect ofchrome tanning by manipulating the type and amount of reactive counterions.In solution chemistry, sulfate acts as a structure maker and it has a positiveeffect on the stability of raw collagen, in a way that might be interpreted as aweak tanning effect.63 A component of the stability of raw and processedcollagen is the presence of a sheath of supramolecular water surrounding thetriple helix, centred on the hydroxyproline.64 It was therefore postulated thatthe role of sulfate in stabilising collagen and in also in chrome tanning is tocontribute to the stabilising effect of the supramolecular water. This may workby direct incorporation within that layer or by acting as a link between thesolvent layer and the collagen or the chromium or by all three mechanisms.65

This view of tanning is further developed in the discussion of tanning theory inChapter 19.

11.15 ROLE OF THE SOLVENT

The mechanism of reaction between a reagent and a substrate was introducedearlier. There the primary interest was in the interaction between the reagentand the aqueous solvent; however, discussion of the role of the solvent can bewidened.

11.16 ZERO FLOAT PROCESSING

Although the term ‘zero float’ is well known in the industry, tanners are awarethat the term is meaningless. In practice, it refers to processing after draining asmuch of the float as possible. Since the degree of draining is variable, across therange of process vessels and indeed within factories, all that can be said of thisapproach is that it is low float processing, usually 10–30% of the load. Thepresence of residual water in the process vessel distinguishes this condition from

252 Chapter 11

zero water in non-aqueous float (discussed below). The residual free waterexerts some control over the transfer step of the fixation reaction by solvation,so that transfer remains incomplete. Therefore, low float alone cannot producecompletely efficient chrome uptake. That can only be achieved in aqueoussolution by adopting extreme conditions by which all the available water isabsorbed with the chrome or involving high temperature and pH. Tanners haverecognised that reducing the float, i.e. increasing the concentration of thetanning agent, results in faster reaction, which is of course a consequence ofexploiting the kinetic equation. Note, although the kinetics of tanning aredifficult to model, the kinetics of complexation are clear.

11.17 NON-AQUEOUS FLOATS

The notion of tanning in a non-aqueous solvent is not new. The literaturecontains many examples of technologies based on the use of non-aqueoussolvents for leather processing. They fall into two categories:

1. The pelt is solvent dehydrated, then processing continues in the chosenorganic solvent.

2. The non-aqueous solvent is applied to the pelt, but the solvent system hadto include at least one component that would allow the solvent to bemiscible with the water in the pelt from the previous process.

The primary motivation in these processes was to save energy when dryingthe leather. All manner of conventional organic solvents have been proposed,from alcohols to chlorofluorocarbons: each suffered from the same funda-mental drawback of flammability. Hence, solvent tanning has never been usedcommercially. More recently, Wei66 proposed a process based on a non-mis-cible organic solvent, in this case paraffin; this has been developed further bySilvestre et al.,67,68 in which an alternative solvent is used only as the tumblingand heat transfer medium, but the substrate is conventionally processed wetpelt. This approach has the advantage that the organic solvent has lowflammability, the efficiency of reagent uptake is practically complete, but thesavings in drying energy are relatively small.Using an organic solvent tanning medium and water wet pelt is an extreme

case, strictly an example of low float tanning, but it is still covered by the pro-posed four-step mechanism of fixation of a reagent onto a substrate. Previously,it was assumed that the interaction between the solvent and the reagent/solutewas finite. However, by choosing a highly hydrophobic solvent, the affinity for ahydrophilic chromium ion will be negligible. Now, consider the situation whenwet pickled pelt is tumbling in liquid paraffin and chrome tan powder is addedinto the system. It is reasonable to suppose that the chrome salt is insoluble in thesolvent, but water is available in the form of wet pelt. Therefore, the solute iseffectively partitioned between an inert solvent and the water within the pelt.Consequently, uptake of the chrome is very rapid, controlled by the rate ofdissolution in the pickle liquor in the pelt: uptake is total and is achieved in a


matter of minutes. Uptake does not of course mean fixation: the kinetics ofcomplexation are controlled by the conditions in the aqueous solution, not howthe solution was created. Therefore, the rest of the reaction proceeds at thetypical tanning pace. The way in which the chrome binds is also governed by itsability to diffuse through the substrate. In ordinary tanning, diffusion throughthe cross section of the pelt dominates the process; this is regarded in terms ofpenetration – lateral diffusion is relatively unimportant because diffusion occursevenly across the pelt. However, the non-aqueous solvent case is different.Chrome salt particles/crystals will dissolve where they contact the wet pelt andpenetration through the cross section will occur locally. The process is fast andtherefore is necessarily uneven over the surface. The rate of fixation will dependon the net pH conditions at the site of dissolution. The higher the pH, the fasteris the fixation, the slower is the lateral diffusion and hence the more uneven thereaction becomes. Clearly, the solution lies in controlling the relative rates in thereaction: the rate of diffusion versus the rate of fixation, which is analogous to thecircumstances in conventional tanning.The reaction must be controlled by controlling the net pH. Usual basification

cannot be employed, since the problem of reaction at the site of contact will bethe same as for the chrome salt, resulting in greater unevenness in reaction andhence greater non-uniformity of colouring/Ts. This applies to any form ofbasification, so the preferred approach would be an analogue of the ThruBluprocess (Section 11.7), when the alkalinity within the pelt provides the basifyingfunction. This is illustrated in Table 11.17.48

The tanning reaction can be driven by heating the pelt, when the outcome isindependent of the variations in chrome basicity and initial pelt pH, but ishighly dependent on chrome offer (Figure 11.14). It is possible to basify thetannage, when it is necessary to employ an agent that will not react on contact;however, there is no advantage over self-basification by the pelt.Any suggestion that non-aqueous solvent might be employed in the leather

industry would appear to run counter to the ridding all tannery operations ofreliance on organic solvents. However, many sectors of industry use organicsolvents successfully in an environmentally sound manner, e.g. dry cleaning

Table 11.17 Chrome tanning of wet pickled pelt in paraffin: conventional saltsoffered, tumbled for up to 6 hours, then aged overnight at 50 1C.(A.D. Covington, unpublished results).

Process Time in process (h)

Chromeoffer(%Cr2O3)

Chromebasicity(%)

PeltpH 0.5 1.5 2.5 3.5 4.5 6.0 Overnight

1.0 33 4.0 79 87 95 93 95 97 982.0 33 4.0 94 103 113 115 117 125 1232.0 a 33 3.0 94 96 98 107 105 1232.0 50 5.0 93 101 103 101 105 124a+0.4% MgO.

254 Chapter 11

(although they too are moving away from organic solvents to super criticalfluids). Therefore, it is not unreasonable to imagine processing in an alternativesolvent, although that would presuppose a fundamental change in operatingvessels. The use of supercritical carbon dioxide has received some attention as aprocessing medium, but to date the potential of the technology has not beenfully reviewed.

11.18 ROLE OF ETHANOLAMINE

Rohm and Haas Co. supplied ethanolamine hydrochloride as an auxiliaryagent for pickling, to improve chrome tanning efficiency.69 It had been pos-tulated that the effect of ethanolamine on chrome uptake is the organic ana-logue of aluminium(III) catalysis.70 However, the effect was also observed usingethylene glycol and ethylenediamine, with only marginal superiority withethanolamine, in terms of additional hydrothermal stability.48 Therefore, amore likely rationalisation is that the effect is based on the changes to thesolvent system. The controlling parameter for the solute–solvent interaction isthe ability of the solvent to solvate. For hydration reactions, a parameter thatreflects the electrostatic nature of the solvent is the dielectric constant (alsoknown as the relative permittivity): the values for water, ethylene glycol,ethanolamine and ethylenediamine are 78, 38, 34 and 14, respectively. There-fore, the addition of even small amounts of these solvents will markedly changethe properties of water, reducing the solvating effect of the primarily aqueousmixed solvent. Indeed, an acceleration was observed when the offer of organicsolvent was as little as 3� 10�4 mole fraction, corresponding to 1% w/v.48

Many other solvents also exhibit this effect. However, it is not known to whatextent solvation itself influences the effect. For example, dimethyl sulfoxide inthe aqueous float markedly increases chrome penetration rate,46 but here it isknown that in such solvent mixtures, surprisingly, the chromium(III) ions arepreferentially solvated by DMSO (A.D. Covington and A.K. Covington,unpublished results).This is a general principle: a change in the solvating power of the reaction

solvent will alter the affinity of any solute for the solvent. Examples are knownof reactions in the leather making process that already exploit the effect,although often without recognition of the scientific principles involved. There isclearly scope for further exploitation, whether to accelerate solute uptake ontothe substrate, by reducing the solvating energy, or to promote penetrationthrough the substrate, by increasing the solvating energy.

11.19 NATURE AND STATE OF THE SUBSTRATE

11.19.1 Modifying the Substrate

The reaction rate between chrome and collagen depends on the availabilityof reaction sites on the collagen, at least at the beginning of the process, so asimple method of enhancing the chrome tanning reaction, in terms of the


fixation efficiency, is to increase the availability of reaction sites. Whilst thiscan be achieved by increasing pH, to change the degree of ionisation of thenatural carboxylic acid groups, there is a limit to the use of this approach,because of the effect on the chrome species. The alternative is to introduce morecarboxyl groups into the collagen. One way is to hydrolyse more side-chain amide groups in liming; however, extending the liming period is likely tobe counterproductive with regard to the physical properties of the leather.The use of amidases in this context might be a future practical biotechnology.The remaining option is to attach carboxyl bearing groups to the collagen,usually at the amino groups of lysine. The reaction is exemplified by the use ofglycine attached via formaldehyde, shown in the reaction below; this is atechnology developed by Joan Bowes71 that has been used as an industrialprocess in UK:

Coll-NH2þHCHO þH2NCH2CO2H

ÐColl-NH-CH2-HNCH2CO2H

This principle has been adapted and extended by Feairheller et al. to includethe Mannich72 and Michael73 reactions (P¼protein):

P-NH2 þHCHOþCHðCO2HÞ2ðmalonic acidÞ! P-NH-CH2-CHðCO2HÞ2

P-NH2þCH2 ¼ CH-CO2CH2CH2CO�2 ðb-carboxyethyl acrylateÞ

! P-NH-CH2CH2CO2CH2CH2CO2H

! P-N-ðCH2CH2CO2CH2CH2CO2HÞ2

Holmes74 has demonstrated the technology of improving complex metal ionfixation by linking carboxylate moieties to collagen via halogenated azo het-erocycles, in reactions analogous to reactive dyeing (Chapter 16). This mod-ification to collagen has more important application to tannages weaker thanchromium(III) (discussed in Chapter 12).

REFERENCES

1. F. Knapp,Nature und Wesen der Gerberei and des Leders, J. G. Cotta, Jena,1858.

2. A. Schultz, US Patent, 291,784 (1884).3. R. S. Thomson, J. Soc. Leather Technol. Chem., 1985, 69(4), 93.4. J. Geher-Glucklich and M. T. Beck, Acta. Chimica. Acad. Sci. Hung., 1971,

70(3), 235.5. A. Marsal, et al., J. Soc. Leather Technol. Chem., 1999, 83(6), 300.6. K. Nakagawa, Hikaku Kagaku, 1999, 45(3), 210.7. E. E. Ochs, et al., J. Amer. Leather Chem. Assoc., 1953, 48(12), 730.8. N. Bjerrum, Z. Phys. Chem., 1910, 73, 724.

256 Chapter 11

9. H. M. N. H. Irving, J. Soc. Leather Technol. Chem., 1974, 58(3), 51.10. S. G. Shuttleworth, J. Soc. Leather Trades Chem., 1954, 38, 419.11. Stability Constants of Metal Ion Complexes, Chem. Soc. Special publica-

tion No. 17 (1964) and supplement No. 25 (1971) see ref. 9.12. R. Tsuchiya, et al., Bull. Chem. Soc. Japan, 1964, 37, 692.13. R. Tsuchiya, et al., Bull. Chem. Soc. Japan, 1965, 38, 1059.14. Tedesco, de Russi and Gonzalez Quintana, J. Inorg. Nuclear Chem., 1973,

35, 2856.15. H. M. N. H. Irving and Williams, J. Chem. Soc., 1953, 3192.16. R. P. Bell and M. A. D. Fluendy, Trans. Faraday Soc., 1963, 59, 1623.17. L. E. Orgel, Nature, 1960, 187, 504.18. H. Stunzi and W. Marty, Inorg. Chem., 1983, 22, 2145.19. H. Stunzi, W. Marty and F. P. Rotzinger, Inorg. Chem., 1984, 23, 2160.20. L. Spiccia, W. Marty and R. Giovanoli, Inorg. Chem., 1988, 27, 2660.21. D. A. House, Advances in Inorg. Chem., 1997, 44, 341.22. T. Ramasami, et al., J. Soc. Leather Technol. Chem., 1997, 81(6), 234.23. T. Gotsis, L. Spiccia and K. C. Montgomery, J. Soc. Leather Technol.

Chem., 1992, 76(6), 195.24. J. E. Finholt, et al., Inorg. Chem., 1965, 4, 43.25. A. Kawamura and K. Wada, J. Amer. Leather Chem. Assoc., 1967,

62(9), 612.26. K. H. Gustavson, J. Amer. Leather Chem. Assoc., 1953, 48(9), 559.27. K. H. Gustavson, J. Soc. Leather Trades Chem., 1962, 46(2), 46.28. S. G. Shuttleworth and A. E. Russell, J. Soc. Leather Trades Chem., 1965,

49(6), 221.29. R. E. Hamm, et al., J. Amer. Chem. Soc., 1958, 80, 4469.30. A. D. Covington, J. Amer. Leather Chem. Assoc., 2001, 96(12), 461.31. A. K. Covington and K. E. Newman, Pure and Appl. Chem., 1979,

51, 2041.32. E. Haslam, J. Soc. Leather Technol. Chem., 1988, 72(2), 45.33. R. L. Sykes, J. Amer. Leather Chem. Assoc., 1956, 51(5), 235.34. A. D. Covington, et al., J. Soc. Leather Technol. Chem., 1986, 70(2), 33.35. A. D. Covington, R. A. Hancock and I. A. Ioannidis, J. Soc. Leather

Technol. Chem., 1989, 73(1), 1.36. E. M. Brown, R. L. Dudley and A. R. Elsetinow, J. Amer. Leather Chem.

Assoc., 1997, 92(2), 225.37. A. D. Covington, O. Menderes, et al., Proc. IULTCS Congress, Capetown

South Africa, March 2001.38. A. D. Covington, J. Amer. Leather Chem. Assoc., 2008, 103(1), 7.39. A. D. Covington and R. D. Allen, J. Soc. Leather Technol. Chem., 1982,

66(1), 1.40. D. R. Cooper and R. J. Davidson, J. Soc. Leather Technol. Chem., 1969,

53(11), 428.41. S. Das Gupta, J. Soc. Leather Technol. Chem., 1998, 82(1), 15.42. T. W. Swaddle, et al., Inorg. Chem., 1994, 33, 465.43. A. D. Covington, et al., J. Soc. Leather Technol. Chem., 1986, 67(1), 5.


44. A. D. Covington, J. Amer. Leather Chem. Assoc., 1991, 86(10), 376.45. A. D. Covington, J. Amer. Leather Chem. Assoc., 1991, 86(11), 456.46. A. D. Covington, J. Soc. Leather Technol. Chem., 1988, 72(1), 30.47. R. P. Daniels, World Leather, 1994, 7(2), 73.48. J. Gauglhofer, J. Soc. Leather Technol. Chem., 1991, 75(3), 103.49. D. L. Huizenga and H. H. Patterson, Anal. Chim. Acta, 1988, 206, 263.50. H. C. Holland, J. Int. Leather Trades Chem., 1940, 24(5), 152.51. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, John Wiley

and Son, 1988.52. A. D. Covington and N. Costantini, J. Amer. Leather Chem. Assoc., 2000,

95(4), 125.53. T. Vernali and A. Amado, J. Amer. Leather Chem. Assoc., 2002, 97(5), 167.54. A. D. Covington, et al., Polyhedron, 2001, 20, 461.55. K. H. Gustavson, J. Amer. Leather Chem. Assoc., 1950, 45(8), 536.56. A. Kawamura and K. Wada, J. Amer. Leather Chem. Assoc., 1967,

62(9), 612.57. L. Spiccia and M. R. Grace, Inorg. Chim. Acta, 1993, 213, 103.58. G. Reich, et al., J. Amer. Leather Chem. Assoc., 2001, 96(4), 133.59. T. W. Swaddle, Coordination Chemistry Reviews, 1974, 14, 217.60. K. Shirai, et al., Hikaku Kagaku, 1975, 21(3), 128.61. K. Shirai, K. Takahashi and K. Wada, Hikaku Kagaku, 1984, 30(2), 91.62. D. E. Peters, J. Amer. Leather Chem. Assoc., 1978, 73(7), 333.63. W. P. Jencks, Catalysis in Chemistry and Enzymology, McGraw-Hill, 1969.64. H. M. Berman, J. Bella and B. Brodsky, Structure, 1995, 3(9), 893.65. A. D. Covington, J. Soc. Leather Technol. Chem., 2001, 85(1), 24.66. Q.-Y. Wei, J. Soc. Leather Technol. Chem., 1987, 71(6), 195.67. F. Silvestre, et al., J. Amer. Leather Chem. Assoc., 1993, 88(12), 426.68. F. Silvestre, et al., J. Amer. Leather Chem. Assoc., 1993, 88(12), 440.69. W. E. Prentiss and I. V. Prasad, J. Amer. Leather Chem. Assoc., 1981,

76(10), 395.70. A. D. Covington, J. Soc. Leather Technol. Chem., 1986, 70(2), 33.71. J. H. Bowes and R. G. H. Elliott, J. Amer. Leather Chem. Assoc., 1962,

57(8), 374.72. S. H. Feairheller, M. M. Taylor and E. M. Filachione, J. Amer. Leather

Chem. Assoc., 1967, 62(6), 408.73. S. H. Feairheller, M. M. Taylor and E. H. Harris, J. Amer. Leather Chem.

Assoc., 1988, 83(11), 363.74. J. M. Holmes, J. Soc. Leather Technol. Chem., 1996, 80(5), 133.

258 Chapter 11

CHAPTER 12

Mineral Tanning

12.1 INTRODUCTION

The recent history of chrome tanning has been centred on its alleged envir-onmental impact. Therefore attention has been turned to the possibility offinding an alternative tanning metal for the global leather industry, ideally toconfer the same properties as chrome. The availability of candidates to bealternative mineral tanning agents can be assessed by reviewing the periodictable, in terms of the known chemistries of the elements and their compoundsand their likely reactivity towards collagen. The following criteria should beapplied:

1. Does the reaction work?It is assumed that an alternative to chromium(III) tanning should be

able to confer high shrinkage temperature to collagen. For this to happen,the metal ion must be able to complex with carboxyl groups, ideallycovalently. Using the analogy of chromium(III), it is desirable that thereactions of an alternative should be similar, to have a similar outcome. Itis implied in Chapter 11 that the reaction may not rely on the mechanismof crosslinking, but the defined and understood elements of chrome tan-ning should be present. Kuntzel and Droscher1 concluded that a satis-factory mineral tannage involved aggregation or polymerisation, to allowmultiple interactions with collagen, ideally complexation reactions. Therole of the counterion was recognised and Gustavson2 has summarised thecontribution of the counterion in the following order: NO3

�oClO4�

oCl�oSO42�.

2. Are there toxicity or other hazard implications in the use of the metalsalts?Could the reagent be used as a bulk industrial chemical and what would

be the environmental impact of the industrial use of the element?




259

3. Is the tanning reagent available in sufficient quantity to be practical forthe global leather industry?A chemical is only industrially useful if it is available in sufficient

quantity for the price to be economical with respect to the value of thefinal product.

4. Does it produce coloured leather?Given a choice, it would be preferable to have a colourless tannage, to

assist dyeing.

12.1.1 Blocks and Groups of the Periodic Table

12.1.1.1 s Block Elements. This block consists of groups Ia to IIIa. Theelements are metallic, with chemistries dominated by cation formation. Theyhave little complex chemistry, in particular they do not form complexes withcarboxylic or amino groups, hence are unlikely to react covalently with proteinsidechains.

12.1.1.2 p Block Elements. This block consists of groups IIIb to VIIb andgroup 0. These elements range from metals to semi-metals to non-metals, goingtowards the higher numbered groups and going from high to low atomicweight. Only the elements of high atomic weight and low group number havechemistries that could apply in this context. At higher atomic weight the loweroxidation state becomes more stable, e.g. Tl(I) compared to Tl(III) or Pb(II)compared to Pb(IV).

12.1.1.3 d Block Elements. These are the transition metals, all of whichexhibit some degree of complex formation. Hence they are candidates astanning agents.

12.1.1.4 f Block Elements. The lanthanides have a complex chemistry, hencethey might work.The actinides exhibit radioactivity, toxicity or they may even be synthetic.

Therefore, they are unlikely to be of use in this context.

12.1.1.5 Group Ia. This group is made up of alkali metals: (hydrogen),lithium, sodium, potassium, caesium and francium (radioactive).These form monovalent, highly electrovalent, cationic monomeric species:

there is no useful complex chemistry with carboxylate groups.

12.1.1.6 Group IIa. This group contains the alkaline earth metals: beryllium,magnesium, calcium, strontium, barium and radium (radioactive).These metals form divalent, cationic monomeric species. The lighter elements

form oxyions: beryllium in the form of the sulfate or the phosphate can raisethe shrinkage temperature to 93 1C,3 but the salts are toxic.

260 Chapter 12

The heavier metals can create precipitates with collagen by electrostaticinteraction, but this is not tanning. Indeed, magnesium, calcium and bariumchlorides have a destabilising effect due to lyotropy.4

12.1.1.7 Group IIIb. This group consists of boron, aluminium, gallium,indium and thallium. These elements are on the borderline between the metalsand the non-metals: they form trivalent species.Boron is toxic and forms oxy complexes. Aluminium forms cationic basic

ions, capable of reacting with collagen (see below).Gallium and indium do not have a complex chemistry. Additionally, they are

expensive.Thallium has very limited complex chemistry in the 1+ or the 3+ states. It is

toxic.

12.1.1.8 Group IVb. Carbon, silicon, germanium, tin and lead comprise thisgroup. Carbon creates organic chemistry, which is quite another topic in leathermaking.Silicon, as a non-metal does not form cations. It has limited use in the

industry in the form of silicates,5 where it is claimed to be a useful substitute forlime in opening up. Perhaps its best application is as a pretreatment prior totanning: this is discussed further in Chapter 14 (Section 14.7 on wet whiteprocessing). However, clearly, tanning is not a likely option: although theoxyanions can interact with collagen, via hydrogen bonds and electrostaticattraction, this has no effect on hydrothermal stability. However, as silicates oras silicic acid (pKaB10) they can function as auxiliaries, when they have a minorapplication as a pretreatment for vegetable tannage.6 They can occupy reactionsites on collagen and hence slow the reaction rate between the polyphenols andcollagen: this is similar to the function of polyphosphates (see below).Germanium, tin and lead have chemistries with oxygen, so form complex

anions. In the lower valence state they tend to hydrolyse in solution and canform complex cations. However, they are expensive, toxic and have limitedtanning power.7

12.1.1.9 Group Vb. This group contains nitrogen, phosphorus, arsenic, anti-mony and bismuth. Nitrogen has a chemistry that is not applicable to this situation.Phosphorus chemistry is dominated by oxy compounds. The use of phos-

phates is limited to a pretreatment for vegetable tanning. In this context,phosphate in the form of polyphosphate or pyrophosphate can interact viahydrogen bonds and electrostatic interactions, to mask reaction sites, makingcollagen less reactive towards vegetable tannins, promoting penetration.6

Arsenic, antimony and bismuth are semi-metals, with limited chemistries ascomplex anions, so they have no useful chemistry for tanning.

12.1.1.10 Group VIb. Oxygen, sulfur, selenium, tellurium and polonium(radioactive) comprise this group. These elements form anions and oxyanions,

261Mineral Tanning

which have no tanning use. Furthermore, as the atomic weight increases thecompounds become more toxic and much more smelly, in comparison withsulfur compounds.The concept of ‘sulfur tannage’ is well known: this involves the precipitation

of elemental sulfur by, for example, acidifying thiosulfate in situ:8

S2O2�3 þ 2Hþ!Sþ SO2 þH2O

The reaction typically also occurred with the two-bath chrome tanning process,when thiosulfate was the reducing agent. The benefits were claimed to beenhanced strength in the product, although with no effect on hydrothermalstability. The interaction is almost certainly one of preventing fibre sticking, theusual function of fatliquoring/lubrication: here, the effect is to maximise theability of the fibre structure to resist tearing, but at the same not to causeweakening by disrupting the natural bonding when an emulsion of oil isdeposited (Chapter 17). The effect of coating the fibre structure is supported bythe observation that the softening effect is reversed when the dry material iswetted then dried. Also, the sulfur can be extracted with carbon disulfide,indicating that there is no chemical bonding between the sulfur and the protein.The chemistry does offer an alternative approach to the lubrication of leather,which may confer additionally useful properties.

12.1.1.11 Group VIIb. This group is made up of the halides: (hydrogen),fluorine, chlorine, bromine, iodine and astatine (radioactive).The chemistry is dominated by anion formation, which has no tanning value.

12.1.1.12 Group 0. This group consists of the noble or inert elements:helium, neon, argon, krypton, xenon and radon (radioactive).These elements are certainly inert in a tanning context.

12.1.1.13 Group IIIa. This group contains the transition elements scandium,yttrium and lanthanum.Scandium(III) tends to behave like a main group element, with similarity to

the lanthanides.Yttrium can be considered with the lanthanides (see below).Lanthanum is the first in the lanthanide series (see below).

12.1.1.14 Group IVa. This group contains the transition elements titanium,zirconium and hafnium.Titanium forms oxy complexes, as linear chains, in the 4+ oxidation state.

The 3+ oxidation state is metastable: it does form complexes, but they spon-taneously revert to the higher oxidation state in air.9 Ti(IV) salts have long beenused in the leather industry (discussed below).Zirconium(IV) has a well-known function in tanning (see below).Hafnium is too expensive and rare to consider.

262 Chapter 12

12.1.1.15 Group Va. This group contains the transition elements vanadium,niobium and tantalum.Vanadium has several oxidation states, all capable of forming some com-

plexes, e.g. the 3+ oxidation state can form covalent carboxy complexes,creating lemon yellow leather with a shrinkage temperature of 69 1C.10 How-ever, it is an expensive and relatively rare metal.Niobium and tantalum are stable in the 5+ oxidation state. They tend to

form complex anions, resembling the chemistry of phosphorus or arsenic,therefore behaving like non-metals.

12.1.1.16 Group VIa. This group contains the transition elements chro-mium, molybdenum and tungsten.Both Mo and W have some history of involvement with the leather industry

in Victorian times. However, this was confined to the use of the oxy com-pounds, molybdates, phosphomolybdates, tungstates, silicotungstates andphosphotungstates: Casaburi and Simoncini11 demonstrated that these com-plex anions have no effect on hydrothermal stability, but might have someapplication in combination with vegetable tannins. Their use is retained inhistological staining, which relies on electrostatic interaction with protein andother biological materials: they are particularly useful for incorporating heavyatoms into specimens for electron microscopy.

12.1.1.17 Group VIIa. This group contains the transition elements manga-nese, technetium and rhenium.The most stable oxidation state of manganese, Mn(II), forms some relatively

unstable complexes. Other oxidation states are unstable.Technetium and rhenium have little cationic aqueous chemistry: cost and

availability also rule them out.

12.1.1.18 Group VIIIa. This group contains the transition elements iron,cobalt and nickel, ruthenium, rhodium and palladium, osmium, iridium andplatinum.Iron forms complexes in the +2 and +3 oxidation states: Fe(II) forms

octahedral complexes with a tendency to oxidise in air. Fe(III) also formsoctahedral complexes, which tend to hydrolyse. In this way, Fe(III) functionsmore like Al(III) than Cr(III). However, iron has a history of application in theleather industry (see below).Cobalt salts are most stable in the 3+ oxidation state, which has an extensive

complex chemistry. The octahedral complexes are stable, with low lability.There is a preference for nitrogen donors, but oxy ligands may be reactive. Costand availability are limitations.Nickel salts are effectively only available in the 2+ oxidation state. It can form

oxy complexes, but considerations of cost, allergenicity and toxicity rule it out.Ruthenium(III) and osmium(III) have limited complex chemistries, dominated

by ammines. Osmium is particularly toxic, so even though some compounds

263Mineral Tanning

(e.g. osmium tetroxide) are highly reactive in organic reactions, they areunsuitable for tanning applications.Rhodium(III) and iridium(III) form kinetically stable cationic complexes.Pt has some interesting complex chemistry, but application is confined to

high value products, e.g. anticancer drugs such as cisplatin [cis-diamminedi-chloroplatinum(II)].The elements Ru, Rh, Pa, Ir and Pt are expensive; they are used as jewellery

metals, so are ruled out of a leather application.

12.1.1.19 Group Ib. Copper, silver and gold make up this group. Copper(II)has a complex chemistry with oxy ligands, but with a tendency to form complexanions, which are not useful for tanning.Silver and gold do not have a suitable complex chemistry and the metals are

more commonly used in jewellery or high value applications.

12.1.1.20 Group IIb. Zinc, cadmium and mercury comprise this group. Zincand cadmium have chemistries similar to magnesium, but do exhibit somecomplex formation. They are toxic.Mercury can form complexes: linear two-coordinate, tetrahedral four-coor-

dinate and octahedral six-coordinate. The bonding can have significant cova-lent character, so it does have some tanning power:7 acetate bufferedmercury(II) acetate can raise the shrinkage temperature by 11 1C.12 Cost andtoxicity rule out its use as a tanning agent.

12.1.1.21 Lanthanides or Rare Earth Elements. The elements lanthanum,cerium, praseodymium, neodymium, promethium, samarium, europium, gado-linium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lute-tium make up this group.These elements have very similar properties, so they are often encountered

in nature as mixtures. An exception in chemical applications is cerium,which is routinely used as an analytical oxidising agent in the Ce(IV) oxidationstate.All elements function usually as trivalent compounds. They do not seem to

have a useful complex chemistry for tanning, although it was claimed in 191013

that a mixture of the chlorides is a better tanning agent than aluminium(III).There has been information in the technical literature regarding the applicationof lanthanides in the leather industry, mostly by the Chinese, who have sub-stantial amounts of lanthanides in their geology. However, with the possibleexception of semi-metal tanning as retanning agents for vegetable tanned lea-ther (Chapter 13) their use is limited, because they do not have a large enougheffect on raising the shrinkage temperature of collagen: typically the shrinkagetemperature outcome is B80 1C.

12.1.1.22 Actinides. Thorium, protactinium, uranium and the transuranicelements make up the actinides.

264 Chapter 12

This group strictly only includes thorium, protactinium and uranium, sincethe transuranic elements are synthetic. 238U, the non-fissile isotope, is leastradioactive, but it is toxic. Its use is confined to salts as the uranyl ion, UO2

21,when it used as a histological stain in the form of the acetate.

12.2 EXPERIMENTAL TANNING REVIEWS

Clearly, from the brief survey presented above, the number of mineral optionsopen to the tanner is limited. The remaining question is whether any of thoselimited options can rival chromium(III) as an industrial tanning agent. A fewwide ranging reviews of tanning abilities of inorganic species have beenundertaken, all characterised by being somewhat selective, i.e. they were notcompletely comprehensive nor were the salts all related.In a study of the thermodynamics of tanning, Weir and Carter4 made several

leathers (Table 12.1).In 1957, Borasky14 reviewed mineral tanning, including plutonium nitrate

(Table 12.2).The most comprehensive study to date was undertaken by Chakravorty and

Nursten in 1958:7 Table 12.3 presents some of their results.Whilst Tables 12.1–12.3 do not cover all the options, it is apparent that there

has not been much success in finding an alternative to chromium. Indeed, asdiscussed in Chapter 19, there can be no alternative. The only generalisationthat comes out of reviewing tanning effects is that all metals exhibit theirgreatest tanning power at the point of precipitation.7 This is understandable

Table 12.1 Outcomes of some unrelated mineral tannages from Weir andCarter.4

Salt pH of tannage Maximum Ts (1C)

Diammine mercury(II)chloride[Hg(II)(NH3)2Cl2]

7.5 67

Lead(II) nitrate[Pb(NO3)2]

7 68

Thorium(IV) sulfate[Th(SO4)2]

4 68

Magnesium sulfate(MgSO4)

7.5 68.5

Titanium(IV) chloride(TiCl4)

6 70

Beryllium(II) sulfate(BeSO4)

5.5 71

Copper(II) sulfate(CuSO4)

5.5 73

Mercury(II) acetate[Hg(II)(O2CCH3)2]

a7 91

aHg(II) removed by washing.

265Mineral Tanning

because at that point the following conditions apply:

1. Collagen is at its most reactive for that metal. The solution pH is at thehighest value, which is consistent with still keeping the metal ions in solution.

2. The molecular metal ions are in their most highly polymerised state, whichis consistent with remaining solubilised. This gives the species maximumhydrophobicity and hence greatest affinity for the environment of thecollagen substrate.

Table 12.2 Outcomes of some further unrelated mineral tannages.14


None 63Strontium(II) chloride (SrCl2) 6.0 59Silver(I) nitrate (AgNO3) 6.0 63Barium(II) chloride (BaCl2) 6.0 63Sodium tungstate (Na2WO4) 6.0 63Mercury(II) chloride (HgCl2) 4.2 63Lead(II) acetate [Pb(O2CCH3)2] 6.0 60Lead(II) nitrate [Pb(NO3)2] 5.3 46Uranyl nitrate [UO2(NO3)2] 3.8 43Chrome alum 4.0 98Phosphotungstic acid (H3PW12O40) 3.0 63Osmium tetroxide (OsO4) 4.9 78Uranyl acetate [UO2(O2CCH3)2] 4.6 73Plutonium(IV) nitrate [Pu(NO3)4] 1.0 78

Table 12.3 Outcomes of some of the unrelated mineral tannages ofChakravorty and Nursten.7


Tin(II) chloride (SnCl2) 5 60Manganese(II) chloride (MnCl2) 7.5 62Yttrium(III) chloride (YCl3) 7 62.5Silver(I) nitrate (AgNO3) 8 63Cerium(III) chloride (CeCl3) 7.5 63Thorium(IV) chloride (ThCl4) 4.5 63Lead(II) nitrate [Pb(NO3)2] 7 63Lanthanum(III) nitrate [La(NO3)3] 8.5 63.5Cuprammonium hydroxide{[Cu(NH3)4(H2O)2](OH)2}

11 63.5

Thorium(IV) nitrate [Th(NO3)4] 4.5 64Potassium titanyl oxalate [KTiO(O2C)2] 5.5 64Zinc(II) sulfate (ZnSO4) 7 65Neodymium(III) nitrate [Nd(NO3)3] 7.5 65Cuprammonium sulfate{[Cu(NH3)4(H2O)2]SO4}

10.5 65.5

Lithium chloride (LiCl) 8 66Tin(IV) chloride (SnCl4) 6.5 66Cadmium(II) sulfate (CdSO4) 6.5 67

266 Chapter 12

The only practical options as alternatives to chrome, which have some his-tory of use in the leather industry, are aluminium(III), titanium(IV) and zirco-nium(IV) [and to a lesser extent iron(II)] (see below): Table 12.4 summarisessome of their properties in the context of tanning.15

Generalising the information in Table 12.4, aluminium, titanium and zirco-nium cannot match the tanning power of chromium. The tanning power oftitanium(IV) falls between aluminium(III) and zirconium(IV). From their posi-tions in the periodic table in group IVa, similarities between titanium andzirconium chemistry would be expected. If aluminium in group IIIa is viewed asa pre-transition element then any similarities between aluminium and titaniumconstitute an example of a diagonal relationship, more commonly associatedwith the first and second periods. These relationships depend on the polarisingpower of the ions (charge to radius ratio, i.e. charge density), which determinesthe nature of the bonding in compounds.

12.3 ALUMINIUM IN LEATHER MAKING

Potash alum, a mixed salt of aluminium and potassium sulfate, occurs native,i.e. it can be dug out of the ground. It has been used for thousands of years inleather making: this is known because written recipes exist.16 In combinationwith traditional vegetable tanning, it makes semi-alum leather (Chapter 13):this combination of reagents yields unusually high hydrothermal stability, butthat was almost certainly unknown to the ancient Egyptians and earlier

Table 12.4 Comparisons between chromium(III), aluminium(III), titanium(IV)and zirconium(IV).

Tanning metal ion Cr(III) Al(III) Ti(IV) Zr(IV)Electronic configuration [A]3d3 [Ne] [A]3d0 [Kr] 4d0

Species in aqueous acid [Cr(H2O)6]31 [Al(H2O)6]

31 [TiO]n2n1 a [ZrO]m

2m1a

Species in aqueous alkali Chromite Aluminate Titanate InsolubleCoordination number 6 6 6 6–8Effect of complexation: b

oxy complexes ** **** *** *carboxy complexes **** ** ** ***amino complexes ** * ** **Metal oxide Cr2O3 Al2O3 TiO2 ZrO2

Equivalent basicity fortanning (%)

33 33 50 50

Equivalent weight ofoxide

38 25 40 61

Relative equivalent tan-ning offer (% metaloxide)

6 4 6 10

Maximum Ts (1C)c 4120 90 96 97

a n 4 m.bMore stars means more tanning effect.cExtreme conditions of astringency and magnitude of offers.

267Mineral Tanning

civilisations. The earliest mention of this effect was from Procter,17 who pointedout that heating linseed oil treated vegetable tanned leather, to make patentleather, could be conducted at higher temperatures if the leather had beentreated with an aluminium salt.A more common reason for including aluminium(III) in a tanning formula-

tion is for colouring. In traditional dyeing processes, the dye is better fixed ifthere is a mordant present in the leather, because natural dyes have a greateraffinity for metal mordants than for collagen (Chapter 16). Another possibleuse of aluminium salts is to precipitate the salt as the hydroxide (hydroxide isspontaneously turned into hydrated oxide), which allows dye to be adsorbedonto the oxide. This creates a densely coloured sort of pigment, called a ‘lake’,providing colour to cover the leather surface.A role for aluminium salt, originally potash alum, used to be for making

white gloving leather. This process is strictly not tanning, but is known as‘tawing’. Traditionally, this process was operated in England by the ‘whi-tawyers’, in times when leather making operations were separated and con-trolled by the London Guilds in the earlier part of the last millennium.The formulation has been described as a ‘Yorkshire pudding’ mix, with thefollowing ingredients (Yorkshire pudding only omits the alum):6

For 100 kg of pelt:50–60 litres of water at 35 1C, to create a paste.6–8 kg of potash alum, to react with the collagen.8–12 kg of flour, to provide starch, which acts as a masking agent for the

Al(III).1–2 kg of salt, to prevent swelling under the acidic condition.8–15 kg of egg yolk, to act as a lubricant, due to the content of phospholipids.

The paste is introduced into pelt, which is then dried and softened. The whiteleather is functional, but the effect may be reversed if it gets wet.Aluminium(III), [Al(H2O)6]

31, is an acidic complex, with a greater tendencyto hydrolyse to basic salts than chromium(III), especially at low concentration.It also has a lesser tendency to olate and thereby to form discrete basic salts: itreadily polymerises to an Al13 species and precipitates.18 Sulfate does notcomplex with Al(III) in the dry salt, as it does in basic chromium(III) sulfate; theinteraction is much more electrostatic. Aluminium(III) is stable at pH 4, wherethe basicity is 30–35%, helped by the presence of a masking agent, otherwise itis likely to precipitate at pHo4. Masking effects can be summarised by thefollowing series:19–23

glucoheptonate4gluconate4oxalate4citrate4malonate4lactate

4tartrate4succinate4acetate4glycolate4formate4sulfate:

Masking may be applied to aluminium(III) sulfate, to allow the creation of amore reactive salt at pH approaching 4. Alternatively, basic chloride salts are

268 Chapter 12

available. They can be more basic because chloride ion is one of the few ligandsthat can create true covalent complexes. For this reason, the chloro complexesare typically less reactive than those based on the sulfate, and additional sulfatein the tannage is beneficial.24

Al(III) interacts with collagen carboxyls, but in a reaction that is much moreelectrostatic than covalent. The consequence is that the leather retains a sig-nificantly cationic character: in this state the fibre structure is vulnerable tocollapsing and sticking together, producing thin, hard leather or, in the case ofpickled pelt, a tendency to form irreversible adhesions.25 The cationic characteralso poses difficulties for applying anionic reagents, such as fatliquors, but thiscan be overcome to a large degree by treating the leather with polyphosphate,to sequester the charge.The weak interaction between aluminium(III) and collagen means that the

shrinkage temperature from a tawing process is barely raised. By the use ofmasking, the aluminium tanning process can be conducted at pH 4, yieldingleather with a shrinkage temperature of about 80 1C. Despite the obviousinadequacies of aluminium(III) as a tanning agent, there are some modern usesof aluminium(III) salts (summarised below).

12.3.1 Alum Pickle

The presence of aluminium in the tannage, whether alone or in conjunctionwith chromium(III), confers firmness, as mentioned above. For some applica-tions, this is desirable: a well-known example is for light weight leathers forshoe uppers, such as ladies weight calf leather, which is stiff leather capable ofbeing shaped on the shoemaker’s last. One way of incorporating Al(III) into thetannage is to include an aluminium(III) salt such as the sulfate in the pickleformulation. In this way, the acidity of the salt contributes to the picklingprocess and the conditions of chrome tannage are sufficient to fix some alu-minium into the pelt.

12.3.2 Suede

The stiffening effect of aluminium(III) is also useful for making suede leather.Here, one of the technological problems is to cut the fibres cleanly to obtain afine suede nap. If the fibres are not cut cleanly they may be pulled from thesurface, creating a raggy suede. If the fibres are stiffened they are cut by thesueding paper more easily. The aluminium(III) may be introduced in the pickle,but more usually is applied as part of the retannage of chrome leather.

12.3.3 Furskins

Furskin processing is more concerned with stabilising the pelt than conferringhigh stability to elevated temperature, i.e. the processing can be characterised asleathering rather than tanning. Aluminium(III) salts play a role in thistechnology.

269Mineral Tanning

12.3.4 Semi-alum Tannage

The relationship between plant polyphenols and metal salts, to create semi-metal tanned leather, is addressed in Chapter 13. In that context, aluminium(III)is the preferred metal salt, because it has greatest affinity for phenolic hydroxyls.

12.3.5 Chrome Uptake

The effect of the presence of aluminium(III) in chrome tanning has been shownto enhance the rate of chrome fixation, by an effect analogous to catalysis.23

Figure 12.1 present the results of laboratory studies.It can be seen that the pretreatment with aluminium has a positive effect on

both chrome content and shrinkage temperature, maximised at up to 0.25%Al2O3 offer: at higher offers the aluminium seems to compete with the chromeand the benefit is diminished. Part of the overall reaction is the displacement offixed aluminium(III) by incoming chromium(III) (Figure 12.2).26

The aluminium effect appears to arise from the difference between alumi-nium(III) complexation and chromium complexation reactions: in the com-parison presented below in Table 12.5, the rate of complexation byaluminium(III) is favoured by the entropy of activation. Therefore, it can bepostulated that the reaction between collagen carboxyls and aluminium(III) isfast and results in loose complex formation. So, the sidechains can adopt asuitable conformation for reaction with metal ions without a high energypenalty and the consequent reaction with chromium(III) is energeticallyfavoured.It is useful to compare the stability of aluminium tanned leather with that of

chrome tanned leather; Table 12.5 sets out some of the properties of the

Figure 12.1 Effect of aluminium and chromium offers on (a) the shrinkage tem-perature and (b) chrome content (% on dry weight) of bovine leather.

270 Chapter 12

Figure 12.2 Displacement of aluminium(III) by chromium(III) at pH3.8–4.0.

Table 12.5 Comparisons of the chemistries of chromium(III) andaluminium(III).

Cr(III) Al(III)

Hydration number:by NMR 6 6by conductivity 12 11Solvation of M31 ion at 298K:DH1 (kJmol�1) –4400 –4660DS1 (JK�1mol�1) –405 +155Transfer from crystal lattice to solution:TDSs (kJmol�1)

–109 –113

Transfer from unit molality to dilute solution:DStr (JK

�1mol�1)–405 –426

Partial molar hydration volume, relative tozero for H1 at 298K (cm3mol�1)

–40 –42

[M(H2O)6]31! [M(H2O)5OH]21+H1

pK1 (I¼ 0) 3.9 5.0DH1 (kJmol�1) 38 79qM31+pH2O!Mq(OH)p

(3q–p)1 +pH1

log10b2,2 –3.4 –7.3Cr(III) Al(III)

Solvent exchange aH2O DMSO DMF H2O DMSO DMF

pK (298K) 6.3 7.5 7.3 0.8 12 0.8DHz (kJmol�1) 109 97 100 113 84 75DSz (JK�1mol�1) 0 –50 –42 +118 +17 +21Complex formation Associative DissociativeaDMSO¼dimethyl sulfoxide, CH3SOCH3

DMF¼dimethyl formamide, H2CON(CH3)2

271Mineral Tanning

chemical species.27–29 Tables 12.4 and 12.5 reveal some superficial similaritiesbetween aluminium and chromium chemistry, due to their preferred valencystate and the amphoteric nature of the hydroxides (capable of acting as acids orbases). However, a closer examination of the properties of ions in solutionreveals they have very different relationships with solvating species: this can beused as a reflection of the ability of the ions to complex with collagen. Thereactions may be summarised as associative for chromium and dissociative foraluminium: the rates of exchange of solvate species at aluminium(III) are 105–106 times faster than exchange at chromium(III).The basic ions in solution have similar structures and interact with the same

sites on collagen, but the outcomes of tanning reactions are very differentbecause the bonding is very different. This difference is addressed further inChapter 19.Figure 12.3 compares, diagrammatically, the tanning powers of aluminium and

chromium. The nature of the bonding confers the range of stability: the covalentlybound chromium(III) is stable over eight pH units, but the electrostatic bonding ofaluminium(III) is stable over only two pH units. In contrast, the semi-alum reactionproduces covalent bonding and the semi-alum bond is actually more stable thanchrome tanning bonding at the alkali end of the scale.

It has been shown by 27Al NMR that the magnetic environment ofthe aluminium(III) nucleus is not changed during the shrinking reaction.18

Figure 12.4 models the possible interactions between aluminium(III) andcollagen.

0 2 4 6 8 10 12 14

region of chrome leather stability

solubilisation as Cr(III) ion

region of alum leather stability

solubilisation as hydrolysis of Al(III) ion Al(III) complex

solubilisation as aluminate

region of semi alum leather stability

hydrolysis of complex,solubilisation as chromite ion, CrO2

-

Figure 12.3 Guideline limits of tanned leather stability within the pH scale.

272 Chapter 12

The (basic) aquo ion can interact electrostatically via a water ligand or acomplex might be formed, which would be more electrovalent than covalent.Since it is known that aluminium(III) does not form stable complexes, it may besurmised that the former case is more likely. Therefore, the bonding betweenthe collagen and the matrix could break down hydrothermally, but the envir-onment of the aluminium nucleus would not be substantially altered. There-fore, it can be concluded that the aluminium-based matrix involves water thatcan break down, to allow shrinking, i.e. the electrostatic interaction with col-lagen carboxyls is sufficiently distant to allow this to happen, but the alumi-nium(III) nucleus experiences no change in its magnetic field. In contrast,covalent complexation between collagen carboxyls and chromium(III) is a directinteraction, which cannot break down under the conditions of shrinking.Therefore, the shrinking transition cannot involve breakdown of the metal–collagen bonds, so the process must involve the breaking of the hydrogenbonding in the triple helices, causing them to unravel, and in the associatedmatrix around the triple helices.Holmes has proposed an alternative approach,30 in which chelating

groups were bound to lysine sidechains using dihalogenated azo hetero-cycles, i.e. exploiting the chemistry used in reactive dyeing (Chapter 16). Thereis an additional side effect to metal fixation: the shrinkage temperature ishigher than expected. The effect, which applies to all metal tannages, chro-mium(III) and less effective reactions, demonstrates that the tanning effect ofmetals depends in part on the way in which the metal species interactsdirectly with the collagen. Here, the firmer interaction of chelate bondingis sufficient to raise the shrinkage temperature of aluminium tannedleather (Table 12.6). This aspect of the tanning reaction is developed further inChapter 19.An incidental outcome of these modifications of collagen is the change in

shrinkage temperature, which is mostly higher, due to the tanning effect,although such reactions can have a destabilising effect. The difference relates tothe way in which the tanning molecule interacts with the triple helix and thesupramolecular matrix. However, it can be seen that the effect of chelation is

Figure 12.4 Interactions between collagen and aluminium.

273Mineral Tanning

relatively consistent and in all cases shows an improvement over the controltannage.

12.4 TITANIUM TANNING

Titanium(IV) salts have a similar affinity for collagen as aluminium(III), in partdue to some similarities in properties,31 such as the acidity of the ion and thetendency to hydrolyse and to precipitate as the pH is raised above 3. Theinteraction with collagen carboxyls is similarly electrovalent, rather thancovalent. However, one difference is the greater filling effect of Ti(IV) salts, duethe polymeric nature of the salts, which produces softer leather,. The empiricalformula of the cation, the titanyl ion, is TiO21: in reality, the ion is a chain(-Ti-O-Ti-O-)n with an octahedral ligand field. The weak chemical interactionwith collagen results in shrinkage temperatures of 75–80 1C. The resultantleather is initially colourless, although it tends to go pale yellow on ageing,owing to the creation of charge transfer bonding; this is a change fromhydrogen bonding to electrostatic bonding.The only industrial widespread use of Ti(IV) salts was the traditional process

for gentlemen’s hat banding leather: this is the leather that is sewn on the insideof the rim, the area in contact with the head. The process was semi-titan:starting with vegetable tanned leather, it was retanned with potassium titanyloxalate, to give a very hydrothermally stable leather, which is also capable ofresisting the effects of perspiration because the elements of the combinationtannage are rendered non-labile.An application was suggested by Russian workers32 in which sole leather is

made from large offers of ammonium titanyl sulfate, (NH4)2TiO(SO4)2 (a saltthat gives pH4 in solution), basified with hexamethylenetetramine. This has notmet with commercial success. Using excessive offers of ammonium titanylsulfate (30% salt, 6%TiO2) on sheepskin and basifying with sodium sulfite andHMT, shrinkage temperatures up to 96 1C could be achieved and acceptableleather was made, although clearly not like chrome tanned leather.

Table 12.6 Effect of chelation on the shrinkage temperature of metal tannedcollagen.

Chelating groupTs (1C)

No Al(III) With Al(III) DTs�Al

None 63 79 16Salicylate1-hydroxy-2-carboxybenzene 70 90 20MandelateC6H6CH(OH)CO2H 72 94 22Phthalate1,2-dicarboxybenzene 74 101 27Iso-phthalate1,3-dicarboxybenzene 55 77 22

274 Chapter 12

More recently, Chinese workers reviewed the effects of masking on titaniumtannage, when they tried 13 hydroxyl carboxylates and 22 other agents: theycould only achieve shrinkage temperatures up to 92 1C.33,34

It is possible to made metastable salts of titanium(III): these violet salts can beeasily made by dissolving titanium metal in sulfuric acid and can be held formany days in this valence state, but only if air is excluded. Attempts to tan withthese salts produced no additional benefits: although it can be initially bound asTi(III), the tanning agent rapidly reverts in situ to the 4+ oxidation state in theleather.9

12.5 ZIRCONIUM TANNING

Zirconium(IV) salts are expected to perform in a similar way to Ti(IV), becauseof its position in the periodic table. They have been available commerciallyfrom the middle of the twentieth century: Hock has comprehensively reviewedthe chemistry and tanning properties.35 The tanning reaction is similar to Ti(IV),yielding slightly higher shrinkage temperatures, but the colour is stable andmakes white leather with a full handle.The chemistry of the complexes is more like chromium(III) than titanium(IV)

(Figure 12.5). It can be seen that the 50% basic ion is a tetrameric species,where the metal is eight-coordinated, with m-hydroxy bridges between metalions.Like titanium(IV), the unmasked zirconyl ion is acidic and unstable to

hydrolysis. The conventional way of applying it is to use as little water aspossible for the penetration stage, then add water (float up, in the jargon) tocause polymerisation by hydrolysis, in the same way as chromium(III) poly-merizes (Figure 12.6).The increase in molecular weight increases the astringency of the species,

driving it into the substrate, which results in fixation. It has been claimed, bydeactivating the carboxyl groups and amino groups of collagen, that zirco-nium(IV) complexes do not interact with the carboxyls and that they interactsignificantly with the amino groups.36 However, the mechanism is unclear,since zirconium(IV) can coexist as cationic, neutral and anionic species, by

Zr

HO

HO HO

HO

O

O

O

Zr(H2O)3

O

OH

OH

OH OHH

H

H

H

Zr Zr(H2O)3(H2O)3

(H2O)3

4+

Figure 12.5 Structure of the 50% basic zirconium(IV) ion.

275Mineral Tanning

interaction with counterions, particularly sulfate and masking agents such ascitrate. Significantly, in reacting with collagen, counterions are displaced fromthe zirconium complexes.Zirconium(IV) tanning has been referred to as the mineral equivalent of

vegetable tanning, because of the presence of so many hydrogen bonding siteson the complex. This is a significant contributor to the binding mechanism intanning. The large size of the complexes produces a filling tannage (a char-acteristic of vegetable tanning) and the commonest use for this chemistry is as aretannage, particularly to stiffen (tighten) the grain. Like vegetable tanning,zirconium(IV) tanned leather is hydrophilic because of the hydrophilic nature ofthe tanning species.

12.6 IRON TANNING

This mineral tannage does not have a long history: initial developmentstook place in Germany in the mid part of the twentieth century, when theavailability of chromium was restricted. Tanning with iron(II) salts is preferredto iron(III), primarily due to the oxidising power of Fe(III). The applications arelimited, because the lack of true complexation with collagen carboxyls meansthat the resulting shrinkage temperature is much the same as aluminium(III).A disadvantage for workers is that they suffer from a metallic taste in themouth, due to absorption of the metal salt through the skin when they handlewet leather.Iron(II) sulfate complexed with ethylenediamine and masked with dicar-

boxylates can produce a shrinkage temperature of 76 1C,37 but tartrate maskingalone can give a shrinkage temperature of 85 1C.38

Iron salts, especially Fe(III), have high affinity for plant polyphenols,although this may be a disadvantage in tanning because the resulting colour istypically intensely blue-black. Hence, these reactions have been exploited forthousands of years for making inks. The effect is often observed in factories

OH2

OH2

OH2

OH2

8+

SO4

SO4

6+

6+ 12+

12+H

HH

H

OH

OH2

OH2

OH

O

O

O

O

OSO3-

Figure 12.6 Polymerisation of zirconium(IV) species.

276 Chapter 12

using vegetable tanned leather, when the leather becomes affected by blackspots from the action of sparks from, for example, the continuous sharpeningof splitting band knives. The treatment or ‘clearing’ action is to use oxalic acidor EDTA (ethylenediamine tetraacetic acid disodium salt) to complex the ironand solubilise it.In a leather context, in combination with phenolic compounds, there is a

progressive weakening, due to the catalytic oxidative effect from the ironspecies.

12.7 MIXED MINERAL TANNAGES

All metal salts can be used together in all proportions. The benefits of intro-ducing another metal salt can be summarised as follows:

� The efficiency of chrome uptake is improved by reducing the offer.39

� The rate of chrome uptake can be improved by the presence of anotherweaker mineral tanning agent, applied as a pretreatment.23

The contra-indications can be summarised as:

� In no case is the shrinkage temperature positively affected, indeed theopposite applies. If it is required that the leather should be boilfast, thechrome offer must be 6o0.75% Cr2O3.

� The properties of the leather are modified, changed by the presenceof another mineral tanning agent, maximised if the substitution istotal.

� The handle or feel of the leather may be changed, in terms of collapsing [aswith aluminium(III) salts)] or filling [as with titanium(IV) or zirconium(IV)salts] the fibre structure.

� The leather will become more cationic and therefore more surface reactivetowards anionic reagents.

� The colour may be changed (as with iron salts). At all proportionsin a mixture with a colourless substitute, chrome tanned leather remainsdistinctly blue, even at very low chrome offers, i.e. the effect of blue towhiten, as applied to fabric washing, does not apply to chrome tannedleather.

� The hydrothermal stability is adversely affected, in terms of the pH win-dow within which the mineral-collagen bonding remains intact.

Aluminium(III) with chromium(III) has been discussed above, in the contextof assisting the chrome tanning reaction.26 Iron(II) masked with tartrate orcitrate has been suggested as a suitable accompaniment to chrome tanning,40

although there appears to be no obvious advantage.Aluminium(III) with titanium(IV) has been suggested as a suitable mix-

ture for colourless tannage for sheepskin rugs.15 The mixture of aluminium(III)sulfate and ammonium titanyl sulfate was masked with polyhydroxy

277Mineral Tanning

carboxylate [HO(CHOH)nCO2�] preferably gluconate where n¼ 5 or gluco-

heptonate where n¼ 6. The tannage was adequately fast to washing. Aformulation of tanning salts and masking salt was briefly commercialised byICI as Synektan TAL, but it could never achieve the aspirations of the producerto replace chromium(III).Development work has been conducted on combination tannages of chro-

mium(III) with the aluminium(III)+titanium(III) complex.15 Tannages withoffers as little 0.75% Cr2O3 with 1.3% total metal oxide as Al2O3+TiO2 couldproduce a shrinkage temperature of 106 1C. In addition, the leathers were bothsofter and more full and less cationic than similar leathers tanned with chromeand aluminium.

12.8 OVERVIEW

. . . the periodic table holds out little hope of the discovery of newcommercially useful tanning agents, based on elements not previouslyconsidered.Nursten.7

The value of a tanning process depends on two main parameters: the reversibilityof the tannage and the shrinkage temperature of the leather. For those salts thatmight be candidates to compete with chromium(III) on cost and availability, boththese requirements are impossible because of the nature of the bonding betweenthe metal ions and the collagen carboxyls. The stability of mineral tanned leathercan be enhanced if the approach demonstrated by Holmes is adopted, in whichhe grafted multifunctional complexing sites onto collagen.30 In this way, thelinking part of the tanning mechanism is improved, fixing the mineral matrix tothe collagen more firmly. This is a chemical modification to the substrate thatcould be exploited commercially, but it clearly complicates the tanning operationand is likely to be impractical. However, for some specialised applications, itcould be appropriate.Consequently, further searching for a viable single mineral tanning alter-

native to chromium(III) for the global leather industry is futile. The future oftanning remains with chromium(III) – either in its present form or modified inways discussed in this treatise.

REFERENCES

1. A. Kuntzel and T. Droscher, Collegium, 1940, 839, 106.2. K. H. Gustavson, The Chemistry of the Tanning Process, Academic Press

Inc., New York, 1956.3. via Rev. Tech. Ind. Cuir, 1947, 39, 238.4. C. E. Weir and J. Carter, J. Amer. Leather Chem. Assoc., 1950, 44(7), 421.

278 Chapter 12

5. K. H. Munz and R. Sonnleitner, J. Amer. Leather Chem. Assoc., 2005,100(2), 45.

6. P. Chambard, in: The Chemistry and Technology of Leather, Eds.F. O’Flaherty, W. T. Roddy, R. M. Lollar, Reinhold Publishing Corp.,New York, 1958.

7. H. P. Chakravorty and H. E. Nursten, J. Soc. Leather Technol. Chem.,1958, 42(1), 2.

8. J. A. Wilson, The Chemistry of Leather Manufacture, Chemical CatalogCo. Inc., New York, 1929.

9. G. S. Lampard, PhD Thesis, University of Leicester, 2000.10. A. Kuntzel and H. Erdmann, Collegium, 1938, 824, 630.11. V. Casaburi and E. Simoncini, J. Int. Soc. Leather Trades Chem., 1936,

20(1), 2.12. K. H. Jacobsen and R. M. Lollar, J. Amer. Leather Chem. Assoc., 1951,

46(1), 7.13. M. Parenzo, Collegium, 1910, 403, 121.14. R. Borasky, J. Amer. Leather Chem. Assoc., 1957, 52(11), 596.15. A. D. Covington, J. Amer. Leather Chem. Assoc., 1987, 82(1), 1.16. R. Reed, Ancient Skins, Parchments and Leathers, Seminar Press, 1972.17. H. R. Procter, Textbook of Tanning, Spon, London, 1885.18. A. D. Covington, R. A. Hancock and I. A. Ioannidis, J. Soc. Leather

Technol. Chem., 1989, 73(1), 1.19. A. D. Covington and R. L. Sykes, J. Amer. Leather Chem. Assoc., 1984,

79(3), 72.20. A. Kuntzel, et al., Collegium, 1935, 786(10), 484.21. T. C. Thorstensen and E. R. Theis, J. Int. Soc. Leather Trades Chem., 1950,

34(6), 230.22. A. Kuntzel and S. Rizk, Das Leder, 1962, 13(5), 101.23. A. Simoncini, et al., Cuoio, 1978, 54(4), 439.24. C. Pal and B. M. Das, J. Leather Tech. Assoc. (India), 1955, 3, 67.25. M. Komanowsky, J. Amer. Leather Chem. Assoc., 1989, 84(12), 369.26. A. D. Covington, J. Soc. Leather Technol. Chem., 1986, 70(2), 33.27. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Wiley,

1962.28. J. Burgess, Metal Ions in Solution, John Wiley & Sons, 1978.29. C. S. G. Phillips and R. J. P. Williams, Inorganic Chemistry, vol.2, Oxford

University Press, 1966.30. J. M. Holmes, J. Soc. Leather Technol. Chem., 1996, 80(5), 133.31. M. P. Swamy, et al., Leather Science, 1983, 30(10), 291.32. Motov, et al., USSR Patent 234,598, 1972.33. B. Shi, et al., J. Amer. Leather Chem. Assoc., 2007, 102(9), 261.34. B. Shi et al., J. Amer. Leather Chem. Assoc., 2007, 102(10), 297.35. A. L. Hock, J. Soc. Leather Technol. Chem., 1975, 59(6), 181.36. T. S. Raganathan and R. Reed, J. Soc. Leather Trades Chem., 1958,

42(2), 59.

279Mineral Tanning

37. S. Balasubramanian and R. Gayathri, J. Amer. Leather Chem. Assoc.,1997, 92(2), 218.

38. E. L. Tavani and N. A. Lacour, J. Soc. Leather Technol. Chem., 1994,78(2), 50.

39. Future Tanning Chemistries, UNIDO Workshop, Casablanca, Sept. 2000.40. N. K. Chandra Babu, et al., J. Soc. Leather Technol. Chem., 2007,

102(12), 383.

280 Chapter 12

CHAPTER 13

Vegetable Tanning

13.1 INTRODUCTION

Man’s relationship with plant polyphenols is ancient, since they are compo-nents of plant materials and hence are a traditional feature of our diet. Moderninterest usually centres on the anti-oxidant properties, which means they canscavenge carcinogenic and mutagenic oxygen free radicals in the body. How-ever, polyphenols are also associated with other physiological reactions ofimportance, such as accelerating blood clotting, reducing blood pressure andlowering blood serum lipid concentration.1

The oldest exploitation of plant polyphenols in technology is their ability tostabilise collagen in skin against putrefaction. It is not difficult to understandhow vegetable tanning may have come about: prehistoric man perhapsobserved changes in hide or skin after it had lain in a puddle with plantmaterial. The plant polyphenols that can be leached out of vegetable matter orplant parts have the power to react with the collagen, rendering it resistant tobiochemical degradation, but more importantly remaining soft after a wetting–drying cycle. All manner of plant parts may contain tannins. They are presentas a defence mechanism against insects, because they impart a stringent taste tothe plant material. An example is unripe fruit, such as green apples, which candry up the inside of the mouth, because the polyphenols react with protein inthe saliva: this effect is designed to deter consumption until the seeds aremature. The reactivity of polyphenols is referred to as ‘astringency’, becausethey can react with protein, but also implying that the reactivity is high. Thetraditional test for astringency is to precipitate gelatine from solution with theactive agent under investigation, in a tanning type of reaction.Tannins are extracted from plant materials that contain commercially viable

concentrations: this may be done with water or with organic solvents. The




281

extracts typically contain three fractions of materials:

1. Non-tans are the low molecular weight fraction, with a molecularweighto500.2

These compounds are designated non-tans to indicate that they havelow tanning power; however, their presence in the extract is useful,because they contribute to solubilising the tans and hence assist pene-tration into the pelt.

2. Tans are the medium molecular weight fraction, with mol. wt. 500–3 000.2

The tanning function depends primarily on the phenolic hydroxylcontent, to provide the astringency. Lower molecular weight compoundslack astringency, a higher molecular weight impedes penetration into thepelt.

3. Gums are the high molecular weight fraction, with mol. wt. 43000.These compounds are tans and higher molecular weight polyphenols

complexed with carbohydrate species. The astringency and high molecularweight prevent penetration into pelt, causing surface reaction only. Hence,their presence is undesirable. It is a common experience that higher yieldsof extract can be obtained by using more aggressive conditions, such ashigher temperatures: this is actually counterproductive, because theextract contains higher amounts of gums rather than tannins and so theproduct can be damaging to the quality of the leather.

The mechanism of fixation of tannins onto collagen follows the stepwisemechanism introduced in Chapter 11. It has been proposed that the preliminaryinteraction between polyphenols and collagen starts with hydrophobic inter-actions, because these molecules are relatively insoluble in water, solubilised bythe presence of the non-tans. Reaction then proceeds to hydrogen bonding, asthe electrostatic element of the process. The details of the mechanism andstructure–reactivity relationships of the vegetable tannins have been compre-hensively reviewed by Haslam:3 his treatise contains the following importantdiscussions:

1. Other than the leather industry, there was no major industry exploitingvegetable tannins, hence the development of the understanding of thestructural chemistry of these secondary metabolites was slow in com-parison with other groups of natural products.

2. The roles of the components of plant extracts were not well understood,particularly the effects of the low molecular weight species on the solutionproperties of the higher molecular weight species.

3. The hydrolysable tannins are freely soluble components of plant tissueand thereby have dual roles of protection by astringency and contributionto taste.

4. The condensed tannins are chemically associated with the plant tissue. Thestructural relationships between the procyanidins indicate there are pre-ferred conformations, although the significance is not known (see below).

282 Chapter 13

5. The role of hydrophobicity in tanning is illustrated by the observationthat more tannin is required to precipitate a dilute solution of protein thana concentrated one. This is because the tannin creates a hydrophobiclayer over the surface of concentrated protein, causing precipitation. Inthe case of dilute protein, the tannin reacts with multiple sites, making itless hydrophilic, but more tannin is required to make it drop out ofsolution.

6. Polyphenols inhibit enzymes due to interaction with their protein struc-ture. This has important implications in leather processing, particularlyleather area, as discussed in Chapter 2, and in leather recycling, discussedbelow. In the former case, it is worth recalling that the biotechnology ofarea gain by elastin degradation in chrome leather by elastase does notwork in the presence of vegetable tannins. They interact with protein viahydrophobic bonding; therefore, they can effectively tan elastin, renderingit resistant to elastolytic action, in the same way chrome stabilises collagento microbiological attack.

7. Polyphenols react with proteins via multiple bonding and in this way theyare better suited to reaction with structured proteins such as collagen,rather than unstructured species such as the globular proteins.

The family of plant polyphenols is characterised by the presence of phenolichydroxyls, capable of reacting with collagen, at the basic groups on sidechainsand at the partially charged peptide links via hydrogen bonding (Figure 13.1).4

Haslam4 has proposed that a reaction of importance occurs in the gapregions of collagen, between the ends of the triple helices. Whilst the apparentability of galloylated glucose molecules, the hydrolysable gallotannins, to fitinto the available spaces might indicate tannage can take place there, molecularmodelling could not confirm affinity between catechin and sidechains in the gapzone.5 Therefore, in the absence of contradictory information, it may beassumed that reaction can take place throughout the collagen structure.

N

H

C

O

CH

R

R

CH

O

C

H

N

R

CH

O

C

H

N

R

CH

O

C

H

NN

H

C

O

CH

R

H

OO

HHO OH δ+

δ− δ−

δ−δ−

δ+

Figure 13.1 Model of the interaction between plant polyphenol and collagen.

283Vegetable Tanning

13.2 VEGETABLE TANNIN CLASSIFICATION

Plant polyphenols can be classified into the following three groups, based ontheir structural characteristics: (i) hydrolysable tannins, with subgroups gallo-tannins and ellagitannins, (ii) condensed tannins and (iii) complextannins.6 Table 13.1 gives some examples of commercial tannins.

13.2.1 Hydrolysable Tannins

The so-called ‘hydrolysable’ or ‘pyrogallol’ tannins are saccharide-basedcompounds, in which the aliphatic hydroxyls are esterified by carboxylatespecies carrying the pyrogallol group (1,2,3-trihydroxybenzene). The tanninsare typically based on glucose, but there are variations in which derivatives ofglucose can be the central moiety, such as sucrose.6 They can be separated intotwo groups: the gallotannins and the ellagitannins. In each case, the propertiesare dominated by the ease of hydrolysis of the ester linkages and the reactivityof the phenolic hydroxyl groups.

13.2.1.1 Gallotannins. The smaller group within the hydrolysable tannins,i.e. less common polyphenols, is the gallotannins, in which the glucose core isesterified only with gallic acid. In addition, the bound gallate groups can beesterified themselves, at their phenolic hydroxyls: this is called depside esteri-fication. Figure 13.2 illustrates the structures.Variations in structures come from the degree of esterification of the glucose

centre and the degree of depside esterification. The astringency of any poly-phenol depends on the effective concentration of reaction sites within themolecule. This is demonstrated in Table 13.2, in which the direct interactionbetween protein and polyphenol can be quantified: the more negative the freeenergy of transfer, the greater the interaction.

13.2.1.2 Ellagitannins. In these compounds, the esterifying moieties includegallic acid, ellagic acid and chebulic acid (Figure 13.3); examples of combina-tion esterification are given in Figure 13.4.

Table 13.1 Some sources of tannins and the composition of the tannin source.

PlantSource oftannin Tannin class

Tannin content(%)

Non-tanscontent (%)

Chestnut Wood Hydrolysable 5–15 1–2Myrobalan Nuts Hydrolysable 25–48 14–17Sumac Leaves Hydrolysable 22–35 14–15Oak Wood Hydrolysable 4–10 1–2Oak Bark Condensed 6–17 5–6Quebracho Wood Condensed 14–26 1–2Mimosa Bark Condensed 22–48 7–8Mangrove Bark Condensed 16–50 9–15

284 Chapter 13

Some of the properties of the hydrolysable tannins can be discerned fromtheir structures:

1. They are highly astringent, due to the large number of phenolic hydroxylsgroups so close together: in this regard, the gallotannins are moreastringent than the ellagitannins. A contributor to the reactivity of thehydrolysable tannins is the acidity due to the presence of carboxylic acidgroups, e.g. pH values of solutions of valonia and sumach and chestnut(Figure 13.5) are 3.2, 3.7–4.2, 2.6–2.8, respectively; the less acidic tanninsare therefore self-buffering.

OG

O

OG

OGGO

GOG

HO

HO

C

O

OH

OC

HO

HO

O

C

HO

HO

G = G G =

O

G

HO

Figure 13.2 Structures of the gallotannins, hydrolysable plant polyphenols, illustratedby Chinese gallotannin.

Table 13.2 Astringency of model hydrolysable tannins, measured by the freeenergy of transfer of the protein bovine serum albumin fromaqueous solution to an aqueous solution containing thepolyphenol.7

Polyphenol: D-glucose esters Molecular weight DG (kJmol�1)

b-1,2,6-tri-O-galloyl 636 –0.9b-1,2,3,6-tetra-O-galloyl 788 –9.1b-1,2,3,4,6-penta-O-galloyl 940 –26.9


2. Reaction with collagen is exclusively via hydrogen bonding. This meansthe reaction can be reversed by the action of H-bond breaking com-pounds, e.g. urea, calcium chloride, etc. or organic solvent; in this waycomplete detanning is possible.

3. The shrinkage temperature conferred by these tannins is 75–80 1C: this ischaracteristic, i.e. if a leather is known to be fully tanned and the shrinkagetemperature is o80 1C it is probably tanned with hydrolysable tannin(s).

4. The ability of phenols to discolour (in the case of vegetable tannins, thedarkening effect is to redden) depends on the formation of phenyl radicalsby the loss of hydrogen to atmospheric oxygen. The free radical formationcauses bond shifts and oxidative coupling, which means polymerisation:if this results in the creation or the linking of chromophoric groups(Chapter 16) then colour is developed. In the case of the hydrolysabletannins, the chromophores, the benzene rings of the ester moieties, are notlinkable because they are too far apart in the molecule: in this way they areresistant to reddening, referred to as lightfast. Tara, a gallotannin, is oftenquoted as being the most lightfast vegetable tannin and makes very palecoloured leather.

5. From their name, these tannins can be hydrolysed like any other ester andthis can occur both within the tan bath and within the leather. The formercauses a loss of tanning material and silts up the tanning pits, the latter isno bad thing, because the deposited carboxylic acid moieties contribute tothe water resistance and wear properties of the leather. In addition, thecarboxylates have a buffering effect, conferring resistance to pH change:this is an important property in archival leathers, such as bookbinding,

HO

HO

C O

O C

O

OH

OH

O

OH

HOOC

COOH

HO OH

OH

HOOC

O

O

HO

HO

Chebulic acid

OH

OHHO

COOH

HOOC

Ellagic acid

Figure 13.3 Structures of ellagic and chebulic acids.

286 Chapter 13

where leathers are at risk of long-term damage by the action of acidicatmospheric oxides of sulfur and nitrogen.

6. The presence of pyrogallol groups in these tannins means they canundergo semi-metal reactions (see Section 13.7.1).

7. They are generally unreactive towards aldehydic reaction at the aromaticnuclei, because of the inductive effect of the carboxyl groups (see below).

HO

HO

HO

COO

CH

HC

CH

HC

HC

CH2

CO

OOC OHCOOH

COO

HO

HO

HO

COO

HO

HO

HO

OOC

O

OH

OH

OH

OH

O

OOC

HO

HO

HO

COO

HO

HO

HO

COO

COOHOHOOC

CO

CH2

HC

HC

CH

HC

CH

COO

HO

HO

HO O

O

Chebulinic acid(myrabolan tannn)

Chebulagic acid

Figure 13.4 Structures of chebulinic and chebulagic acids.


13.2.2 Condensed Tannins

The so-called ‘condensed’ or ‘catechol’ tannins have the flavonoid ring structure(Figure 13.6): the more common structures have a catechol group (3,4-dihy-droxy benzene) as the B-ring. Rings A and B are aromatic and ring C is alicyclic.The differences between tannins lie in the pattern of hydroxyls: position 3

in the C-ring is always occupied, in the A-ring position 7 is always occupied,

H2C

HO

HO OH HO OH

OH

C

OO

C

O O

OO

O

O

O

OC

C

HOHO

OH

HO

OH

OHHO OHOH

R1

R2

C

R1 = H, R2 = OH, castalaginR1 = OH, R2 = H, vescalagin

Figure 13.5 Major components of chestnut tannin.

O

3

5

7A

B

C C

B

A

5′

4′3′

5′

4′3′

7

5

3

O

hydroxylation positions polymerisation positions

Figure 13.6 Flavonoid ring system, showing the positions for hydroxyls and thepositions for polymerisation.

288 Chapter 13

but position 5 may or may not be occupied, in the B-ring positions 30 and40 are always occupied, but position 50 may or may not be occupied(Figure 13.7).The hydroxyl in the 3-position in the central C-ring may be cis or trans with

the B-ring, as exemplified by gallocatechin (Figure 13.8).Because the linking point for polymerisation is the alicyclic C-ring, there is a

variation in conformation (Figure 13.9):3 B-1–B-4 are the principal con-formations, B-5–B-8 are known but less common.The hydroxylation pattern determines the shape of the polymers of

flavonoids; because the presence of a 5-hydroxyl restricts the ability of thering system to react at the 6-position, these tannins tend to be linear.

O

OH

OH

OH

OH

HO

OH

HO HO

OH

HO

OH

OH

O

OH

HO

OH

OH

O

OH

HO

OH

OH

O

procyanidin prodelphinidin

profisetinidin prorobinetinidin

Figure 13.7 Variations in hydroxyl patterns in the flavonoid polyphenols.

O

OH

OH

OHHO

OH

HO HO

OH

HO

OH

OH

O

(+)-gallocatechin (-)-epigallocatechin

OH

Figure 13.8 Stereochemistry in the flavonoid ring system.


OH

OH

OH

HO

HO O

OHO

HO

OH

OH

OH

OHO

HO

OH

OH

OH

OHO

HO

OH

OH

OH

B-1 B-2

OHO

HO

OH

OH

OH

OHO

HO

OH

OH

OH

OHO

HO

OH

OH

OH

OHO

HO

OH

OH

OH

B-3 B-4

B-5 B-6

OH

OH

OH

HO

HOO

OHO

HO

OH

OH

OH OH

OH

OH

HO

HO O

OHO

HO

OH

OH

OH

OH

OH

OH

HO

HO O

OHO

HO

OH

OH

OH

OH

OH

OH

HO

HO O

OHO

HO

OH

OH

OH

B-7 B-8

Figure 13.9 Conformations of procyanidins.

290 Chapter 13

If the 5-hydroxyl is absent, the ring system can involve reaction at the6-position; therefore, these tannins can be branched. Figure 13.10 illustratesthe variation.The procyanidins and prodelphinidins are linear polyphenols. The profise-

tinidins and prorobinetinidins are branched polyphenols. The effects of poly-meric structure on tanning reactions have not been investigated. However, itcan be postulated that the shape of the molecule will probably not have mucheffect on conventional vegetable tanning, when the pelt structure is filled withexcess tannin. However, shape is more likely to have some influence on com-bination tannages (see below).There is no evidence available to indicate whether these stereochemistries

impact on the reactivity of these isomers in a tanning context. In conventionalvegetable tanning it is probable that any effect will be masked by the highcontent and multiple weak interactions in collagen, so subtle differences will belost. However, any tannages that rely on more precise interactions betweenflavonoid molecules and collagen and between flavonoid molecule and flavo-noid molecule, as discussed below in the context of combination tannages, mayexhibit some preferential interactions based on conformation.

HO

HO

OH

OH

OH

OH

O

O

OH

OH

OH

OH

HO

HO

O

OH

OH

OH

OH

HO

HO

n

HO

HO

OH

OH

OH

OH

O

O

HO

HO

HO

HOOH

O

OH

OH

HO

HO

HO

HO

HO

HO

OHO

HO

OH

OH OH

O

Figure 13.10 Structures of linear and branched polyflavonoid tannins.


Some of the properties of the condensed tannins can be discerned from theirstructures:

1. Although the condensed tannins may contain as many aromatic hydroxylgroups as the hydrolysable tannins, they are dispersed over bigger mole-cules and therefore the flavonoid compounds are typically less astringent.The molecular weight of catechin is 290, so the lower limit for inclusion inthe tans fraction is the dimer.

2. Like the hydrolysable tannins, the condensed tannins react with collagenvia hydrogen bonds. In addition, they can react at the 5- and 7-positionsvia quinoid species, resulting in covalent reaction at the collagen lysineamino groups;8 the reaction is modelled in Figure 13.11. Because of thiscontribution to the fixation reaction, 5–10% of the tannin is not remo-vable by hydrogen bond breakers or organic solvent, so the pelt does notrevert to raw after detanning.9

3. The shrinkage temperature of leather tanned with condensed tannin(s) is80–85 1C: as with hydrolysable tannins, the Ts is characteristic, i.e. anobserved shrinkage temperature of 480 1C is a strong indication thatcondensed tannins have been used. The difference between the outcomesof tanning with the different types of tannins is interesting: it must comefrom the difference in reaction type, i.e. the contribution of covalency tothe similar reactions makes a measurable difference to the shrinkagetemperature. This is explained in Chapter 19.

4. The proximity of the aromatic nuclei in the flavonoid structure means thatthe free radical oxidative bond rearrangements can take place easily.Therefore, these tannins redden, creating a rapid colour change on theleather surface: mimosa tanned leather exhibits this effect, darkeningvisibly after only minutes of exposure to sunlight in air.

5. The flat structure of these tannin molecules means that they can aggregatetogether as layers, linked by van der Waal’s forces and electrostaticinteractions: the aggregates are referred to as ‘reds’. The formation of redsoccurs particularly at lower temperatures, resulting in the insoluble par-ticles reacting on the pelt surface because they are astringent but too big topenetrate into the pelt. The attractive forces are weak and are readilyreversed by warming the solution or including a solubilising agent in

OH

OH

R-NH2

NH-R

O

O OH

OH

OH

O•

O2

Figure 13.11 Quinoid reaction with collagen, modelled by hydroquinone.

292 Chapter 13

solution, such as an auxiliary syntan (Chapter 14). The formation of redsis so important in practical tanning that the tendency is quoted in productspecifications. Under acid conditions, reds may take the form of phlo-baphenes, insoluble red pigments, that will function like reds in theirtanning action, but are not solubilised merely by warming the solution.

6. Condensed tannins typically do not engage in semi-metal reactions, exceptfor the less common prodelphinidins and prorobinetinidins, which have apyrogallol B-ring. However, they can undergo analogous polymerisationreactions with aldehydic reagents (Section 13.7).

13.2.3 Complex Tannins

These are mixtures of tannin types, in which a hydrolysable gallotannin orellagitannin moiety is bound glycosidically to a condensed tannin moiety.2 Suchtannins demonstrate the properties of both types of polyphenol, illustrated byacutissimin A (Figure 13.12). Such tannins are not uncommon in nature, butthey are traditionally uncommon in industrial vegetable tanning.

13.3 GENERAL PROPERTIES OF VEGETABLE TANNINS

Vegetable tannins exhibit two general properties:

1. The reaction of plant polyphenols at the basic sidechains lowers the iso-electric point of collagen by 1–2 pH units.9 The effect is to reduce the need

HO

HO

HO

HO

HO

HO

HOHO

HO

OH

OH

OHOH

OH

OH

OH

OH

OH

OH

O

HOO2C

H

O

H

H

H

CH2

O2C

CO2

CO2

OCO

H

Figure 13.12 Structure of a complex tannin, acutissimin A.


to neutralise before post tanning, in contrast to the necessity of neu-tralising chrome tanned leather, in which the isoelectric point is raised bya similar amount (Chapter 15).

2. The reactivity of vegetable tannins can be modified in two ways. First, thepH can be raised, e.g. in the case of chestnut extract it is said to be‘sweetened’ when the pH is adjusted from about 2.7 to 3.5–4.5. Second, thewater solubility can be increased by sulfonation using sodium bisulfite: thisis more commonly applied to the condensed tannins, which are less watersoluble naturally and readily undergo such reactions at their A-rings.

13.4 PRACTICAL VEGETABLE TANNING

The history of vegetable tanning concerns the development of both the tanningagents employed and the equipment used for the process. In the former,changes involved the nature of the agent, gradually changing from the use ofthe plant material itself, to the availability of extracts. In the latter, pits havebeen the preferred vessel, but the way in which they were used changed. His-torically, tanning was conducted in pits in a process referred to as ‘layering’. Alayer of the appropriate plant material, e.g. oak bark, was placed in the bottomof the pit, followed by a layer of hides or skins. Another layer of plant materialwas placed on the hides, followed by another layer of plant material: thealternate layering continued until the pit was full (Figure 13.13). Then the pitwas filled with water.The water leached the polyphenols out of the plant material and the dilute

tannin diffused into the hides, converting them into leather. The dilute nature ofthe solution limits the reactivity of the tannins, allowing it to penetrate throughthe pelt cross section. The reaction is slow, in part due to the static nature of theprocess, so an early form of quality assurance applied more than half a

Pit filled with water

layers of plant material

Layers of hide or skin• • • • • • • • • • • • • • • • • • • •

• • • • • • • • • • • • • • • • • • • •

• • • • • • • • • • • • • • • • • • • •

• • • • • • • • • • • • • • • • • • • •

• • • • • • • • • • • • • • • • • • • •

• • • • • • • • • • • • • • • • • • • •

• • • • • • • • • • • • • • • • • • • •

• • • • • • • • • • • • • • • • • • • •

Figure 13.13 Principle of layering as a tanning technique.

294 Chapter 13

millennium ago in England was the requirement for hides to stay in the pit for ‘ayear and a day’.

13.5 MODERN PIT TANNING

Once the principles of extracting and concentrating vegetable tannins had beenestablished, their application meant that the process could be accelerated toreaction times of weeks. This was made achieved by introducing heating to thepits, so-called ‘hot pitting’. However, the use of extracts created a problem initself. The astringency of vegetable tannins means that there is a strong like-lihood of overtanning the surfaces. This is an example of ‘case hardening’,when the structure at the surface becomes so overtanned or filled with tanninthat a barrier to tannin penetration is created and the tanning reaction withinthe cross section stops. To avoid this, corrective action must be taken. It wasrecognised that one approach to mitigate the effects of tannin astringency is toreduce the reaction rate and the easiest way to control the rate is to control theconcentration of the tannin: the lower the concentration, the slower is the rateof reaction. This is another example of the requirement from the tanner ofbalancing the rate of penetration, kp, and the rate of fixation, kf.If kp 4 kf the tannin colours through the pelt, but does not react – this is the

requirement at the outset of the reaction.If kp¼ kf the tannin reacts uniformly through the cross section of the pelt –

this is the desired effect during most of the tanning process.If kpokf the tannin will tend to react more on the surfaces than within the pelt –

this is an acceptable situation in chrome tanning, when the concentration of thetanning agent diminishes towards the end of the reaction, but this does not applyin vegetable tanning and hence this particular balance of rates is undesirable.

13.5.1 Counter-current Pit Tanning

Figure 13.14 illustrates the modern counter-current system of vegetable tan-ning: four pits are used in the illustration, but in reality more might be used.

Leather out Raw hides inleather leather leather

WasteliquorTannin in liquor liquor liquor

Figure 13.14 Counter-current system of pit tanning.


The method is called counter-current, because the tannin solutions move oneway and the hides move the other way, i.e. counter to the current of tanninmovement.9

The hides start in the first pit, where the tannin concentration is low.Therefore, the reaction is slow and the tannin can penetrate through the crosssection. After a few days, the hides are ready to go into a more concentratedsolution: faster reaction is allowed, because tannin fixation from the first bathhas reduced the reactivity of the substrate somewhat. Some of the depletedsolution is discharged to waste and the concentration is made back up withsome of the solution from the second pit, ready to receive the second pack ofrawstock. The process is repeated: the hides are moved to a third pit, and thefirst and second pits are ‘mended’ with higher concentration tannin solutions,the second pack is moved to the second pit and a third pack is introduced intothe first pit. This process is continued until the leather is fully tanned and takenfrom the pits.

13.6 OTHER VEGETABLE TANNING TECHNOLOGIES

Vegetable tanning can be conducted in drums, although it is uncommon. Tofacilitate rapid penetration, higher offers of pretanning agents are used: thisincludes non-swelling acids (Chapter 9), syntans (Chapter 14) or otherhydrogen bonding auxiliaries such as polyphosphates. The reaction is typicallyconducted at about pH 3.5 and up to 38 1C, but in zero float.10

A more radical suggestion is to tan in apolar solvent.11 This technology isdiscussed in Chapter 11: the principle is the same as applied to chrome tanning,but in the work reported on vegetable tanning the solvents were dichlor-omethane, 1,1,1-trichloroethane, 1,1,2-trichloroethene and 1,1,2,2-tetra-chloroethene. Experiments on 20% offers of chestnut, mimosa and sulfitedquebracho extracts in 300% volume of solvent for sheepskins showed that 99–100% uptake of tannin was achieved in 1–2 hours.

13.7 COMBINATION TANNING

Strictly, the term combination just refers to the use of more than one tanningagent. The retanning of chrome tanned leather could be viewed as a combi-nation tannage, but there is no assumption of interaction between the com-ponents. A useful basis for several combination tannages in which thecomponents interact positively is the polyphenols and some of those optionsare discussed here. Chapter 20 presents a more general analysis of combinationtanning.

13.7.1 Semi-metal Tanning

The combination of vegetable tannins with metal salts has been used forthousands of years. What was not known for most of that time is how the tworeagents combine. The shrinkage temperature of semi-metal tanned leather is

296 Chapter 13

much higher than expected. This is an example of synergy, when the total effectis greater than the sum of the parts. If the two components of the tannage canbe separated for the purposes of this argument, there are three possibilities:

1. DTs (observed)o [DTs(vegetable tannage)+DTs(aluminium tannage)]2. DTs (observed)¼ [DTs(vegetable tannage)+DTs(aluminium tannage)]3. DTs (observed)4 [DTs(vegetable tannage)+DTs(aluminium tannage)]

If the first case applies, the observed change in stability is less than the cal-culated value: therefore the tannages are antagonistic, creating unexpecteddestabilisation. If the second case applies, the effects of the two tannages aresimply additive, so it would indicate independent action. If the third caseapplies, the observed change in stability is greater than the calculated value:therefore the tannages are synergistic, creating a new stabilising reaction. Thedegree of difference, from the equation, is an indication of the extent of thepotency of the new reaction. We can put some actual numbers to the equation,where the Ts of raw pelt is typically 65 1C:

DTsðobservedÞ4½DTsðvegetable tannageÞ þ DTsðaluminium tannageÞ� :DTsð�CÞ 115� 65 ¼ 50; 75� 65 ¼ 10; 75� 65 ¼ 10:

More than half a century after Procter’s observation (ref. 17 Chapter 12),Frey and Beebe demonstrated the useful properties of semi-alum leather12–14

and a quarter of a century later, Rao and Nayudamma studied the reactionusing citrate masked aluminium(III) sulfate (Table 13.3).15,16 Here individualtannages were compared with semi-alum, when aluminium(III) followed vege-table tannin, and vegetable retan, when the reverse order was used.Divi-divi and myrobalan are typical hydrolysable tannins, producing high

hydrothermal stability in the semi-alum reaction. However, when the vegetabletannin is applied after the aluminium, the outcome appears to be more like anadditive combination. This can be understood in terms of the high affinity ofthe interaction: in the semi-metal reaction, the metal creates a network bylinking the polyphenol molecules together, but the vegetable retan reactionmerely creates metal complexes at the collagen reaction sites, so no network isformed. The procyanidin molecules of quebracho do not complex with metals

Table 13.3 Semi-alum tannages.

TanninVeg tannage Ts

(1C)Al tannage Ts

(1C)Semi-alum Ts

(1C)Veg retan Ts

(1C)

None 80Divi-divi 68 115 85Myrobalan 68 120 84Quebracho 76 88 85Wattle 78 98 102


to any significant degree, unlike the prorobinetinidin component of wattle(mimosa). Interestingly, the semi-metal and vegetable retan reactions arealmost the same: the lesser reactivity of the wattle even allows the combinationreaction to take place if the reagents are offered together.17 Kallenberger andHernandez have reviewed to scope of the semi-metal reaction using vegetabletanned leather (the composition is not disclosed, but is likely to have includedmimosa), indicated in Table 13.418 and Table 13.5.19

It can be seen that the outcome is variable and that masking is more likely toreduce the reactivity of transition metal salts in this reaction. The authors con-cluded that the preferred metal salts are of aluminium(III)20 and that the reactionis based on polymerisation of the polyphenol by the metal, moreover suggestingthat the carboxyls of collagen play no significant part in the reaction. In this way,they proposed the formation of a stiff network within the collagen fibre structure.Sykes and Cater21 proposed that the critical requirements of the semi-metal

reaction are the presence of a polyhydroxy aryl ring and more than one suchmoiety in the molecule. Orszulik et al.22 demonstrated more specifically that thereaction requires the presence of pyrogallol moieties in the polyphenol com-pounds (Table 13.6).

Table 13.4 Semi-metal tannages.

Metal saltTs (1C)

Unmasked Citrate masked

None 86Co(II) chloride 112 95Pb(II) thiocyanate 90 85Mn(II) sulfate 90 90Ni(II) sulfate 107 89Fe(III) chloride 101 111

Table 13.5 Semi-titan tannages, using citrate maskedTi(IV).

Vegetable tannin Ts (1C)

None 84Quebracho 99Chestnut 90Wattle 106

Table 13.6 Model vegetable tannin species for thesemi-alum tanning reaction.

Hexahydroxy diphenyl sulfone (Ph-SO2-Ph) Ts (1C)

2,20,3,30,4,40 982,3,30,4,40,5 1043,30,4,40,5,50 105

298 Chapter 13

The presence of the third hydroxyl group in the pyrogallol system lowersthe second ionisation constant compared to the dihydroxyphenyl system(Table 13.7). Since the complexation reaction occurs at pH values much lowerthan the range in which the phenol groups are ionised, the reaction involvesdative complexation (Figure 13.15). Therefore, the inductive effect of the thirdhydroxyl weakens the O–H bonds, increasing the ability of the other hydroxylsto engage in complexation with the aluminium(III) species.Semi-metal tannage can be conducted with a range of metals salts, including

zinc(II)23 and lanthanides(III),24 but it has been proposed that semi-metal tan-nage with aluminium(III) could be an alternative to chrome tanning.25 Using apretannage of 1% glutaraldehyde or 12% syntan, then tannage with 20% taraextract and 8.5% Al2O3 as citrate masked sulfate, the respective shrinkagetemperatures were 113 and 108 1C. Interestingly, the enthalpies of denaturationshow an inverse relationship between Ts and DH, indicating there is moredisruption to the natural collagen structure by the more effective tanningprocess. The quantities of reagents employed in this study are unlikely to bepractical, with regard to a commercially viable and versatile product.In a modern review of semi-metal tanning, Covington and Lampard26

retanned myrobalan tanned pelt with vanadium(IV) oxysulfate or basic tita-nium(IV) sulfate, isoelectronic d1 transition metal salts: the respective shrinkage

OH

HO

polyphenol

polyphenol

H

O

O

O

O

H H

H

M+

Figure 13.15 Basis of the semi-metal tanning reaction.

Table 13.7 Properties of model polyphenol compounds.

Ionisation Catechol Pyrogallol

3,30,4,40-Tetra-hydroxydiphenylsulfone

3,30,4,40,5,50-Hexa-hydroxydiphenylsulfone

pKa1 9.5 9.2 7.8 7.5pKa2 12.5 11.6 12.1 10.4pKa3 12.7 12.6

Metal–ligandequilibrium

pK1 6.6 6.0 5.0 4.7pK2 9.6 8.6 7.2 6.6pK3 14.3 14.6 10.7 9.6


temperatures were 104 and 100 1C. After storing the leathers for one year underambient conditions, the shrinkage temperature of the semi-vanadyl leather hadfallen to 60 1C, equivalent to raw pelt. (A similar observation was made byKallenberger and Hernandez in a semi-molybdenum leather.18) At the sametime the tear strength fell to a third of the original value. Since the fall inhydrothermal stability includes a complete loss of the tanning effect of thevegetable tannin, the mechanism must be based on the degradative redox effectof the metal salt. This is an uncommon example of reversal of tannage underambient conditions, extending to the collagen structure itself, with the seeds ofreversal built into the leather. In the context of a ‘cradle to cradle’ approach tomaterials (the reuse of materials rather than disposal at the end of the workinglife – ‘cradle to grave’), the reactions are interesting.Measurement of the rate of shrinking in the denaturation transition shows an

order of magnitude difference between the ellagitannin myrobalan and theprofisetinidin/prorobinetinidin mimosa (Table 13.8): the indication is that thepyrogallol and gallic acid groups are not equivalent, so a more stable complex isformed by the pyrogallol group when it is unaffected by the inductive influenceof the carboxyl group.25

13.7.2 General Properties of Semi-metal Leathers

The general properties of semi-metal leathers can be summarised as follows.

1. The most marked property is the elevated shrinkage temperature: likeother tannages, the value of the shrinkage temperature depends on thenature of the complexation reaction (Chapter 19).

2. The vegetable tannin is immobilised because it is crosslinked. This meansthat the tendency for the polyphenol to migrate when the leather is wettedis practically eliminated. Therefore the semi-metal tanned leather is usefulin applications where the leather is in contact with skin, e.g. wristwatchstraps.

3. The complexation reaction reduces the cationic nature of the metal spe-cies, the charge on the cation is dispersed to a degree over the polyphenolcomplex but not completely, because the bonding is not made with ionisedhydroxyls. Although the problem of interaction with anionic species is noteliminated, complexation in situ, e.g. sequestration by polyphosphate, iseffective in neutralising the cationic nature.

Table 13.8 Properties of some semi-metal leathers.

Vegetable tannin Metal salt retannage Rate of shrinking (102 s�1) Ts (1C)

Mimosa Al(III) 4.06 94Mimosa Zr(IV) 3.65 91Myrobalan Ti(III) 0.51 98Myrobalan V(IV) 0.57 95

300 Chapter 13

4. The leathers are significantly fuller and more hydrophilic than, forexample, chrome tanned leather, i.e. they have the characteristics ofvegetable tanned leather, because they conform to the tanners’ ‘rule ofthumb’ that asserts that the dominating character of the leather comesfrom the tannage applied first.

13.7.3 Semi-chrome Tanning

In practical semi-metal tanning, the semi-chrome process appears to beanomalous, because it is observed to be less easily accomplished than thecorresponding tannage with aluminium(III) or other successful metal salts.Table 13.9 summarises generalised observations of semi-metal tanning,revealing important differences between chromium(III) reactions and thoseinvolving other metal salts, exemplified by aluminium(III) (A.D. Covington,unpublished results).The reaction mechanism of semi-metal tanning has been explained in terms

of the metal species crosslinking the pyrogallol moieties, to create a matrix thatinteracts with the collagen in a concerted manner;27 so it has been assumed thatthe less effective reaction involving chromium(III) is due to the lack of affinitybetween chromium(III) and phenolic hydroxyls, causing slow reaction andhence a slow rise in shrinkage temperature.A powerful piece of evidence regarding the nature of the reaction comes from

dynamic mechanical thermal analysis, in which the sample is mechanicallystressed whilst being heated.28 The response of the sample (Figure 13.16)provides information about the viscoelastic properties of the material: in thiscase, the properties are affected by the chemistry of the tanning processes. Thetechnique is based on measuring the way in which the response or strain of thematerial lags behind a deforming stress (see also Chapter 15). Collagen hasbeen likened to a copolymer, made up of so-called soft and hard blocks.29 InFigure 13.16,30 the region studied corresponds to the b or soft block transition,which has been shown to be dependent on shrinkage temperature and isassumed to relate to the mobility of the a-amino acids, as opposed to the iminoacids, which determine the restricted motion of the a or hard blocks, the morestructured regions in collagen.

Table 13.9 Observed differences between semi-chrome and semi-alumtannages.

Reaction parameter Semi-chrome Semi-alum

Rates of reaction:metal salt uptake Slow Fastrise in Ts Slow FastEffect of heating tannage Apparent increase in both

rates of reactionNo apparent increase ineither rate of reaction

Ts achieved (1C) o110 Up to 120


It can be seen that all tannages affect the trace, with respect to untanned skin.It is apparent that tara (hydrolysable gallotannin) and chromium(III) havemarkedly different effects. However, there are similarities between the traces forsemi-alum(III) and semi-titan(III) tannages, as might be expected. What is notexpected is that the graph for semi-chrome tanned leather is unlike the graphsfor the other two semi-metal tannages and is more like a combination of the

Figure 13.16 Dynamic mechanical thermal analysis (DMTA) of leathers.

302 Chapter 13

tara tanned result and the chromium tanned result. This observation leads tothe conclusion that the semi-chrome tanning reaction is a combination of thetwo separate reactions; so the high shrinkage temperature is due to chrometanning, although modified by the interfering or blocking effect of the poly-phenol. This does not rule out the possibility of some complexation betweenchromium(III) and the polyphenol hydroxyl groups, which is established inother semi-metal reactions,21 but these latest observations and conclusions areconsistent with the qualitative observations set out in Table 13.9.

13.8 CONDENSED TANNINS AND ALDEHYDIC CROSSLINKERS

The interaction between condensed tannins and formaldehyde is well knownand has been used as the basis for making materials analogous to plastic longbefore the chemistry of polymerisation was developed:31 the reaction might bemodelled as shown in Figure 13.17.The application of a combination of condensed tannin and aldehyde to

tanning was first proposed by Das Gupta:32 the specific requirements ofthe flavonoid ring system in the reaction were clarified in Covington and

HO

HO

OHCHO

OH

OH CH2OH

OH

OH

OHO

HO

HO

OH

OH

OH

O

HO

HO

OH

OH

OH

O

HO

HO

HO

CH2

Figure 13.17 Reaction between catechin and formaldehyde.


Ma’s patent of 1996, in which the parameters of the reaction were specified(Tables 13.10 and 13.11).33 In Table 13.10, the tanning method for East Indianleather is typically not specified – whatever comes to hand is used – but in thiscase reactive condensed tannin was clearly at least one component of thetanning mixture.The reaction conditions to fix condensed tannin and aldehyde are mutually

exclusive: vegetable tannin is fixed at relatively low pH, 3–5, but aldehydiccompounds are fixed at relatively high pH, 7–9. Therefore, the best conditionsfor the combination process is to conduct the crosslinking retannage of vege-table tanned leather at the intermediate condition pH 6 and drive the reactionwith elevated temperature. The reaction does not apply to all aldehydic agents;here, the use of THPS or glutaraldehyde derivative merely makes an additivetannage, but the oxazolidine creates a synergistic tannage.Aldehydic retannage is only additive following hydrolysable polyphenol

tannage. The comparison between the condensed tannins suggests that thepresence of a pyrogallol B-ring is advantageous in this reaction. The reactionwas further studied by Covington and Shi,34 using the crosslinker oxazolidine(Figure 13.18): it is an alicyclic ring system, capable of opening to create two N-methylol reactive sites.Table 13.12 shows the dependency of the reaction on different tannins and

the crosslinker offer. The reaction is almost independent of the offer of thealdehydic retanning agent, indicating that the synergy only requires a few

Table 13.10 Effect on shrinkage temperature (1C) of retanning East Indianbuffalo calf leather with 10% of aldehydic agents at differenttemperatures.

Aldehydic crosslinker aReaction temperature (1C)

20 40 60

None 84Phosphonium salt b 94 93 93Glutaraldehyde derivative 90 91 92Oxazolidine 100 105 110aSee Chapter 14.bTetrakis(hydroxymethyl)phosphonium sulfate (THPS).

Table 13.11 Shrinkage temperature (1C) from aldehydic combination tannagewith different vegetable tannins: 5% syntan pretan, 20% vege-table tannage, 5% oxazolidine retannage.

Vegetable tannage Vegetable tanned only After oxazolidine

Condensed Mimosa 84 111Quebracho 84 105

Hydrolysable Chestnut 73 87Tara 75 86

304 Chapter 13

linkages between the polyphenol molecules to create the effective network. Inthe case of the hydrolysable tannins, there is evidence of antagonism at higherretan offers.The synergy of the reactions can be easily calculated from the relationship:

DTs-synergy ¼ DTs-observed� DTs-vegtan� DTs-oxaz

For comparison, Table 13.13 shows the results of changing the order oftanning reactions. The differences between the alternative combinations is inpart a measure of the synergy of the oxazolidine retanning process, but ismodified by the antagonistic contribution.33

Tables 13.14 and 13.15 confirm the dependency of Ts on the polyphenoloffer.34 It is apparent that the hydroxylation pattern of the condensed tanninplays an important role in the eventual hydrothermal stability conferred by thecombination tannage. An NMR study of the interaction between catechin andoxazolidine demonstrated that reaction occurs at the 6- and 8-positions n theA-ring, but that the B-ring remains unchanged.35 Molecular modelling ofcatechin and gallocatechin and their reaction with formaldehyde has indicatedthat the 6- and 8-positions in catechin are equally reactive towards aldehydes,but that the 8-position is more reactive than the 6-position in gallocatechin.

H2C C CH2

OO N

H2C CH2

C2H5 C2H5

CH2OHHOH2C

NHO OH

CH2CH2C

5-ethyl-1-aza-3,7-dioxabicyclo (3.3.0)octane

1,3-dihydroxy-2-ethyl-2-N,N-di(hydroxymethyl)propane

Figure 13.18 Structure of oxazolidine II.

Table 13.12 Shrinkage temperatures (1C) of leathers tanned with 20% vege-table tannin and retanned with oxazolidine.

Oxazolidine offer (%) 0 2 4 6 8 10 12

No veg tan 65 76 78 80 80 79 79Mimosa 84 108 114 114 114 114 113Quebracho 84 95 101 101 101 101 101Gambier 80 104 103 101 101 101 101Chestnut 78 86 85 84 85 84 84Valonia 75 88 88 87 82 82 82Sumach 75 91 90 88 90 89 89Myrobalan 72 88 88 86 86 86 84


Also, the B-ring of gallocatechin is more reactive than the B-ring of catechin(Table 13.16).36

In the reaction of condensed tannins and oxazolidine, the preferred poly-phenols are prodelphinidin or prorobinetinidin, but they are uncommon interms of availability: pecan and Myrica esculenta are less well known com-mercial options (Table 13.17).37

The relative reactivity of the components in the flavonoids can be gaugedfrom the effect of polymerisation of the individual components (Table 13.18):38

interactions may be in the form of positive synergy or negative synergy, wherethe latter indicates antagonism between the components of the process.

Table 13.13 Shrinkage temperatures (1C) of leathers tanned with oxazolidineand retanned with 20% vegetable tannin: DTs is the differencebetween the reactions with polyphenol then oxazolidine and withoxazolidine then polyphenol.

Oxazolidine offer (%)

2 4 6 8

Ts DTs Ts DTs Ts DTs Ts DTs

Mimosa 96 12 97 14 100 14 100 14Quebracho 91 4 92 9 93 8 96 5Chestnut 81 5 81 4 83 1 82 3Myrobalan 78 10 78 10 81 5 80 6

Table 13.14 Relative contributions of mimosa and oxazolidine offer to theoutcome of the reaction, shrinkage temperature (1C).

Mimosa offer (%)


2 4 6 8 10 12

20 108 114 114 114 114 11315 101 102 112 112 112 11210 99 100 103 103 103 1037.5 94 96 98 99 99 1005 92 95 95 95 95 95

Table 13.15 Calculated synergy (1C) of mimosa-oxazolidine combinationtannages.

Mimosa offer (%)


2 4 6 8 10 12

20 13 17 15 15 16 1515 6 5 13 13 14 1410 7 6 7 7 8 87.5 5 5 6 5 7 85 6 6 5 5 6 6

306 Chapter 13

Table 13.16 Calculated charges associated with positions within the flavonoidring system (see Figure 13.6 for ring numbering).

Position (+)-Catechin (+)-Gallocatechin

C-6 –0.163 –0.162C-8 –0.197 –0.193C-20 –0.042 –0.074C-50 –0.155 –C-60 –0.133 –0.161

Table 13.17 Characteristics of some condensed tannins: PC¼ procyanidin,PD¼ prodelphinidin, PF¼ profisetinidin and PR¼ prorobine-tinidin.

Tannin

Ratios of main componentsMolecularweight

Degree ofpolymerisationPC PD PF PR

Pecan shellpith

16 84 3000 10

Myricaesculenta

100 2500 7–8

Quebracho 100 1230 4–5Mimosa 5 25 70 1250 4–5Gambier 100 570 2

Table 13.18 Tanning effects of polyhydroxy benzene compounds (0.18 mol%offer, equivalent to 20% dihydroxybenzene) crosslinked withoxazolidine (equimolar offer).

Hydroxy benzeneTs

(1C) aDTs1

(1C) bDTs112

(1C) cPredictedTs (1C)

MeasuredTs (1C)

Synergy(1C)

Hydroquinone(1,4-dihydroxy)

69 16 43 96 79 –17

Catechol(1,2-dihydroxy)

60 7 34 87 77 –10

Phloroglucinol(1,3,5-trihydroxy)

63 10 37 90 97 +7

Pyrogallol(1,2,3-trihydroxy)

58 5 32 85 96 +11

Resorcinol(1,3-dihydroxy)

56 3 30 83 115 +32

aEffect of the polyphenol alone.bCompared with raw pelt, Ts¼ 53 1C.cDTs�oxaz¼ 27 1C.


The effects of different aldehydic crosslinkers on polyphenols are not thesame (Table 13.19).37

The flavonoids themselves can be modelled by mixtures of polyphenols(Table 13.20).37

The reaction rates of resorcinol and pyrogallol are consistent, whether theyare alone or mixed with another polyphenol. Catechol is consistently slow,indicating the lack of participation in crosslinking reactions of this type.Pyrogallol is reactive, so its presence in the flavonoid system contributes aneffect of about an additional quarter of an A-ring: this explains the enhancedeffect in combination tanning processes, because the pyrogallol B-ring providesa measurable added facility to the creation of a crosslinked network.

Table 13.19 Rates of reaction (M�1 s�1) at pH 6.0 and 20 1C for equimolarconcentrations of polyphenol and crosslinker: rates are notcorrected for the availability of reaction sites on the polyphenols.

Crosslinker

Polyphenol Oxazolidine Formaldehyde

Phloroglucinol 0.433 0.234Resorcinol 0.024 0.008Pyrogallol 0.018 –Catechol o0.001 o0.001

Table 13.20 Modelling the crosslinking reactions between flavonoid tanninsand oxazolidine: total phenol to oxazolidine molar ratio 1:1,pH 6.0, 20 1C.

Phenols Reaction rate (M�1 s�2)Crosslinkingreaction

Resorcinol 0.048 Re-4 to Re-4Phloroglucinol 0.433 Ph-2 to Ph-2Pyrogallol 0.036 Py-4 to Py-4Phloroglucinol+catechol (modelprocyanidin)

Phloroglucinol: 0.396 Cate-chol: o0.001

Ph-2 to Ph-2

Catechin (procyanidin) A-ring: 0.217 A to AB-ring: o0.001

Phloroglucinol+pyrogallol (modelprodelphinidin)

Phloroglucinol: 0.393 Ph-2 to Ph-2

Pyrogallol: 0.056 Ph-2 to Ph-4Gallocatechin (prodelphinidin) A-ring: 0.217 A to A

B-ring: 0.058 A to BB to B

Resorcinol+catechol (modelprofisetinidin

Resorcinol: 0.048 Re-4 to Re-4

Catechol: o0.001Resorcinol+pyrogallol (modelprorobinetinidin

Resorcinol: 0.046 Re-4 to Re-4

Pyrogallol: 0.050 Re-4 to Py-5

308 Chapter 13

Table 13.21 presents the role of molecular weight on combination tannages,37

comparing conventional plant extracts with phloroglucinol and green teaextract (the latter contains four monomeric flavonoids: catechin and galloca-techin, half not esterified and half esterified with gallic acid at the alicyclic3-position39).The low molecular weight green tea extract is relatively effective in this

combination tannage: this is discussed further in Chapter 20. The componentsof green tea are monomeric catechin, epicatechin, gallocatechin and epigallo-catechin, all as the 3-gallate:39 the effect of isomeric structures presented inFigure 13.9 on the rate of reaction with oxazolidine is shown in Table 13.22.The following conclusions may be drawn from the rates of reaction in

Table 13.22:

� There is a clear indication that the stereochemistry of the polyphenol caninfluence the reactivity, at least as far as the locking aspect of tanning isconcerned. This has not been investigated scientifically.

� In the only comparison possible from these results, the reactions forcatechin and epicatechin, in which the locking reaction only takes place atthe A-ring, the cis isomer favours the aldehydic crosslinking reaction.

� The presence of a pyrogallol B-ring increases the reactivity of the poly-phenol to the aldehydic locking reaction; effectively it adds about 10% tothe rate of reaction.

� The presence of a gallate group (esterified at the hydroxyl group of theC-ring) also increases the rate of reaction.

Unlike the semi-metal reaction, in which the crosslinking agent has littleaffinity for the collagen but high affinity for the polyphenol, the aldehydic

Table 13.21 Effects of crosslinking polyphenol tannages and the effect ofwashing with acetone (H-bond breaker).

Tannin offer Ts phenol alone (1C) Ts crosslinked (1C) DTs washed (1C)

8% Oxazolidine 83 05% Phloroglucinol 60 95 –25% Green tea extract 68 101 –520% Pecan 83 112 –820% Myrica 85 113 –820% Mimosa 82 110 –6

Table 13.22 Rates of reaction between flavonoid species and oxazolidine.

Flavonoid Rate (M�1 s�1)

Catechin 0.18Epicatechin 0.20Epigallocatechin 0.22Epigallocatechin gallate 0.24


crosslinker has high affinity for the collagen as well as the polyphenol. Theresult is the crosslinked matrix of polyphenol is not only hydrogen bonded(with some covalent bonding) to the collagen, but also it is covalently linked tothe collagen via oxazolidine crosslinks. This confers more resistance to rever-sing the tanning effect and extracting the tanning species.39–41 This is discussedfurther in Chapter 19.

13.9 BIODEGRADABILITY

The disposal of leather waste or leather products at the end of their useful life isan issue that must be addressed, since both are contributors to the generalproblem of landfilling. In the case of chrome tanned leather, one perceivedproblem is the chromium content, with regard to leaching and possible oxidationin situ. The current trend is to favour non-chrome tanning methods, on the basisthat it is better to use renewable tanning agents, particularly vegetable tannins,assumed to be more recyclable, i.e. readily returned to the earth. Controlleddegradation of waste using microorganisms is not straightforward for leatherwaste, because by definition it is non-putrescible: however, the approach wouldbecome more feasible if a simple method can be found to reverse the effects oftanning or if leathers can be produced that have an inbuilt weakness towardsbiodegradation. Model laboratory studies were conducted on the use of anae-robic microorganisms that can degrade organic matter to produce biogas; theamount produced is used as a measure of susceptibility to degradation.42

Biogas is a mixture of methane and carbon dioxide, resulting from theanaerobic decomposition of organic waste.43 The conversion of complex organicmatter into methane requires a consortium of microorganisms, consisting ofseveral interacting metabolic groups of anaerobic microorganisms.44 Biogasformation occurs at four overlapping stages of reaction: hydrolysis, acidifica-tion, acetification and methanogenesis. Methanogenesis is the last step in thecomplete anaerobic decomposition of organic compounds and therefore playsan essential role in the mineralisation of organic material.45 There are two dis-tinct routes by which the methane is produced, which are determined by thetypes of methanogenic bacteria present.46 The acetoclastic methanogens convertacetic acid into methane and carbon dioxide (Equation 13.1): this route accountsfor approximately 70% of the methane formed during anaerobic degradation.The second route involves hydrogenotrophic methanogens that utilise carbondioxide and hydrogen to form methane and water (Equation 13.2). The action ofthese methanogenic bacteria is critical to achieve complete degradation, as theirsubstrates have an inhibitory effect on earlier phases of degradation.

CH3CO2H ! CH4 þ CO2 ð13:1Þ

CO2 þ 4H2 ! CH4 þ 2H2O ð13:2Þ

Figure 13.19 shows the rate of biogas produced from untanned collagen,chrome tanned collagen and mimosa tanned collagen. It is apparent that the

310 Chapter 13

tanning effect prevents the microorganisms breaking down the proteinstructure.Figure 13.20 shows the effect of heat denaturing the tanned collagen prior to

treating it with the anaerobic bacteria. The chrome tanned collagen is as

Figure 13.19 Biogas production from leather powder.

Figure 13.20 Biogas production from denatured leather powder.


degradable as the raw collagen, but the mimosa tanned collagen remainsresistant to degradation.It is clear why those results were observed. The reactivity of chrome is

spent by the tanning reaction, so the denatured collagen behaves just like anyother protein, but containing some inert chromium(III). On the other hand,the vegetable tannins react more weakly, retaining their reactivity towardsprotein and the denaturing reaction does not alter their chemistry, so thepolyphenols remain reactive to protein. The outcome is that the degradedprotein is protected against biodegradation and the polyphenols retainreactivity towards the bacterial proteins, causing them to malfunction.This is in agreement with previous studies that have shown condensed tanninsto be toxic to various microorganisms47 and it is the vegetable tannins thatform the least biodegradable constituents of tannery effluent.48 This toxicity isthought be due to enzyme inhibition, substrate deprivation, action on mem-branes and metal ion deprivation.49 This runs counter to the perceived rever-sibility of tannages and the biodegradability of differently tanned collagenicmaterials. Therefore, if you want to recycle leather to the earth, chrome tanningis better.Using starch as a reference biogas producer, the condensed vegetable tannins

were added at concentrations ranging from 500 to 300mgL�1: Table 13.23 setsout the lowest concentrations causing almost complete inhibition to biogasgeneration.The inhibitory effect of the condensed vegetable tannins may be explained in

part by the pattern of hydroxylation found in the tannin monomers. Quebrachoand mimosa are constructed from epifisetinidol and epirobinetinidol mono-mers, respectively, both of which are only hydroxylated on the A ring atposition 7. The monomers of the procyanidin polyphenol gambier and theprodelphinidin polyphenol myrica, (epi)catechin and (epi)gallocatechin,respectively, are hydroxylated on the A-ring at positions 5 and 7. A trihydroxyB-ring makes those species more reactive in terms of crosslinking and quinoidtanning reactions, by reaction at both the A- and the B-rings.50 That impliedreactivity towards protein is not reflected in an ability to inhibit microbialattack. But it is clear that any inhibitory effect is dependent upon the structure–reactivity relationship leading to the hydrogen bonding capability of thepolyphenols.

Table 13.23 Effect of condensed tannins on inhibition of anaerobic bacteria.

Veg tan Min. inhibitory conc (g l�1)

Hydroxylation pattern

A-ring B-ring

Myrica 3 5,7 30,40,50

Gambier 3 5,7 30,40

Mimosa 1 7 30,40,50

Quebracho 1.5 7 30,40

312 Chapter 13

13.10 EFFLUENT TREATMENT

It is useful to know that vegetable tannins are incompatible with non-ionicdetergents. This is because most detergents of this type are based on ethyleneoxide condensates (Figure 13.21).It can be see that the oxygens are lined up, albeit in an apparently staggered

pattern, although there is free rotation about all bonds. This allows plantpolyphenols to engage in multiple hydrogen bonding, so the polyphenolsbecome coagulated, resulting in insolubility.

REFERENCES

1. K.-T. Chung, et al., Critical Reviews in Food Science and Nutrition, 1998,38(6), 421.

2. T. White, The Chemistry of Vegetable Tannins, Soc. Leather TradesChem., 1956.

3. E. Haslam, J. Soc. Leather Technol. Chem., 1988, 72(2), 45.4. E. Haslam, J. Soc. Leather Technol. Chem., 1997, 81(2), 45.5. E. Brown and P. Qi, J. Amer. Leather Chem. Assoc., 2008, 103(9), 290.6. K. Khanbabaee and T. van Ree, Nat. Prod. Rep., 2001, 18, 841.7. E. Haslam, et al., Phytochemistry, 1986, 25, 2629.8. E. Heidemann, Fundamentals of Leather Manufacturing, Eduard Roether,

Darmstadt, 1993.9. K. H. Gustavson, J. Soc. Leather Technol. Chem., 1966, 50(12), 845.

10. C. Jones, World Leather, April, 2006.11. F. Silvestre, et al., J. Soc. Leather Technol. Chem., 1993, 77(6), 174.12. R. W. Frey and C. W. Beebe, J. Amer. Leather Chem. Assoc., 1940,

35(7), 440.13. R. W. Frey and C. W. Beebe, J. Amer. Leather Chem. Assoc., 1942,

37(10), 478.14. R. W. Frey and C. W. Beebe, J. Amer. Leather Chem. Assoc., 1942,

37(11), 539.15. K. P. Rao and Y. Nayudamma, Leather Sci., 1963, 10, 433.16. K. P. Rao and Y. Nayudamma, Leather Sci., 1964, 11, 6, 39, 84, 89.17. E. Heidemann and B. Balatsos, Das Leder, 1984, 35, 186.18. W. E. Kallenberger and J. F. Hernandez, J. Amer. Leather Chem. Assoc.,

1983, 78(8), 217.19. W. E. Kallenberger, et al., J. Amer. Leather Chem. Assoc., 1985,

80(10), 237.

CnH2n+1

O

O

O

O

OH

CH2

CH2 CH2

CH2 CH2

CH2 CH2

CH2

Figure 13.21 Alkyl alcohol condensed with four molecules of ethylene oxide.


20. W. E. Kallenberger and J. F. Hernandez, J. Amer. Leather Chem. Assoc.,1984, 79(5), 182.

21. R. L. Sykes and C. W. Cater, J. Soc. Leather Technol. Chem., 1980,64(2), 29.

22. S. T. Orszulik, R. A. Hancock and R. L. Sykes, J. Soc. Leather Technol.,Chem., 1980, 64(2), 32.

23. J. M. Morera, et al., J. Soc. Leather Technol. Chem., 2004, 80(4), 120.24. Z. Shan and G. Wang, J. Soc. Leather Technol. Chem., 2004, 8(2), 72.25. S. Vitolo, et al., J. Amer. Leather Chem. Assoc., 2003, 98(4), 123.26. A. D. Covington and G. S. Lampard, J. Amer. Leather Chem. Assoc., 2004,

99(12), 502.27. A. D. Covington, J. Soc. Leather Technol. Chem., 2001, 85(1), 24.28. S. Jeyapalina, PhD Thesis, The University of Northampton, 2004.29. V. Yannis, J. Macromol. Sci. – Revs. Macromol. Chem., 1972.30. G. E. Attenburrow, A. D. Covington and S. Jeyapalina, Proc. IULTCS

Congress, Cape Town, South Africa, 2001.31. A. Pizzi, et al., J. Polymer Sci. Polymer Chem. Ed., 1980, 18, 323.32. S. Das Gupta, J. Soc. Leather Technol. Chem., 1977, 61(5), 97.33. A. D. Covington and S. Ma, UK Patent 2,287,953, June 1996.34. A. D. Covington and B. Shi, J. Soc. Leather Technol. Chem., 1998,

82(2), 64.35. A. D. Covington and B. Shi, et al., J. Soc. Leather Technol. Chem., 1999,

83(1), 8.36. B. Shi, et al., J. Amer. Leather Chem. Assoc., 2005, 100(11), 432.37. A. D. Covington, C. S. Evans, T. H. Lilley and L. Song, J. Amer. Leather

Chem. Assoc., 2005, 100(9), 325.38. S. Phillips, MSc Thesis, The University of Northampton, 1998.39. L. Song, PhD Thesis, The University of Northampton, 2003.40. A.D. Covington and L. Song, Proc. IULTCS Congress, Cancun, Mexico,

May 2003, pp. 88–94.41. B. Shi, et al., J. Soc. Leather Technol. Chem., 2003, 87(5), 173.42. S. Yagoub, PhD Thesis, The University of Leicester, 2006.43. P. N. Levett, Anaerobic Bacteria, 1990, 95-115.44. J. G. Ferry, Critical Reviews in Biochemistry and Molecular Biology, 1992,

27(6), 473.45. M. Blaut, Metabolism of Methanogens, Antonie van Leeuwenhoek, 1994,

66, 187.46. M. Aloy, et al., J. Amer. Leather Chem. Assoc., 1989, 84(4), 97.47. T. K. Bhat, et al., Biodegradation, 1998, 9, 343.48. A. Folachier, et al., Das Leder, 1978, 29(4), 49.49. J. D. Reed, J. Animal Sci., 1995, 73, 1516.50. A. D. Covington, Chem. Soc. Rev., 1997, 26(2), 111.

314 Chapter 13

CHAPTER 14

Other Tannages

14.1 OIL TANNING

Oil tanning is the method for making chamois leather: the seminal reviewis by Sharphouse.1 This leather is best known for its properties of holdingwater, which is useful for cleaning and drying washed surfaces, such as win-dows. It is commonly conducted by in situ oxidation of unsaturated oil, suchas cod liver oil. More traditionally, it was the tannage used for making buck-skin clothing leather. It is an example of a leathering type of processbecause, although it resists microbial attack, the shrinkage temperature isnot raised significantly above the value of raw pelt. In essence, the processinvolves filling wet pelt with unsaturated oil, then polymerising the oil in situ byoxidation.The principal steps are as follows, with comments in square brackets:

1. Start with pickled sheepskin pelt [The pelt needs to be acidic to allowacid swelling.]

2. Wash to swell. [Use fresh water – swelling of the pelt only occurs if wateris available and if the swelling is not suppressed by neutral electrolyte.The effect of swelling will also extend the opening up or loosening of thefibre structure, which will assist the penetration of the oil.]

3. Split, to remove the grain and leave the flesh split [This minimises thedifficulties of getting oil to penetrate into pelt wet with water.]

4. Repickle, using 10% salt solution [Acidification in the presence ofsalt depletes the pelt, to optimise oil penetration, preferably by adjustingthe pH to the isoelectric point, about 5.0, where swelling is minimum.]

5. Optionally apply aldehydic pretannage, traditionally formaldehyde,2

but now glutaraldehyde is most common: other aldehydic agents willbecome more common, e.g. phosphonium salt (see below). [The change




315

in the HHB of the pelt aids oil penetration and confers ‘dry soft’properties. The tanning process is conducted under acid conditions,under which aldehyde reaction is very slow – this indicates the lackof reliance on fixing the polymerised oil to the collagen via aldehydicreactions.]

6. Squeeze [Minimising the water content to allow oil penetration.]7. Add cod liver oil [The requirement is for unsaturated oil, which must be

forced into the pelt, e.g. by drumming – the traditional equipment is‘fulling stocks’ in which the oil was effectively hammered into the pelt bythe action of a bank of large wooden mallets.]

8. Oxidise the oil with air [Blow in hot humid air or use frictional heat, toraise the temperature to 40–50 1C to initiate auto-oxidation, thereaftercool to prevent hydrothermal damage by the rapidly rising temperature:the shrinkage temperature rises as moisture content is lowered, startingat 65–70 1C for wet pelt/leather, but the reaction temperature must notget within 30 1C of the shrinkage temperature.]

9. Degrease with sodium carbonate solution or solvent [The residualreacted oil extracted at this point is called ‘degras’ or ‘moellon’ and isused to make lubricants for other leathers.]

10. Buff the flesh surface, also the split surface if necessary [This, for cos-metic reasons, uses a rotating gritted wheel or automatic lightningbuffer.]

11. Condition, typically at 20 1C and 65% RH [The area of any leatherdepends on moisture content, but this applies to chamois more than mostleathers.]

12. Measure area [The area is highly dependent on measuring method,particularly if the leather is stretched during area measuring, as it may begoing through the mechanical pinwheel instrument.]

13. Grading for area, faults, handle and function [The guideline property ofwater retention is that it must be able to hold 800% water.]

The reactions in the oil tanning process are not completely clear. The activeagent is unsaturated oil, preferably cod oil, which can be modelled by linoleicacid, CH3(CH2)4CH¼CHCH2CH¼CH(CH2)7CO2H, which is known topolymerise but not to form epoxides. It has been generally accepted that thereaction is based on the formation of aldehydic compounds, particularly sincethe process is accompanied by the release of acrolein, CH2¼CHCHO, whichhas been used as an element of quality control.2 However, acrolein alone doesnot make acceptable chamois leather.1

Sharphouse1 summarised the tannage in these terms:

fixation of oily or resinous auto-oxidation products to the protein fibre insome intimate sheath-like form. These may be in polymer form and resistremoval by alkaline wash waters and common solvents. It is presumed thatthey account for the differences between aldehyde tannages and ‘‘full oil’’chamois tannage.

316 Chapter 14

In this way, he defined the outcome of the tannage as a polymer matrixwithin the collagen matrix: there is no certainty of reaction between the poly-mer and the collagen, unlike the product of aldehydic tanning. Therefore, thesystem can be pictured as a matrix of polymerised hydrocarbon chains, holdingthe collagen fibre structure apart, as an extreme form of lubrication to preventthe fibre structure coming together and sticking (Chapter 17). This model does,however, provide a rationale to explain the three most important features of oiltanned leather:

1. Hydrothermal stability: The shrinkage temperature of oil tanned leather isonly a few degrees higher than raw pelt. Therefore, the conventional viewof tanning does not hold, i.e. there is little interaction between the tanningagent and the collagen. This is an example of the tanning agent havingmore affinity for itself than for the substrate. This aspect of collagenstabilisation is discussed further in Chapter 19.

2. Water retention: The effect of keeping the fibre structure apart means thatthe collagen can be hydrated and hold excess water within the hydro-phobic polymerised oil matrix. In this way there is the apparently con-tradictory situation of a hydrophobic tanning chemistry producing themost hydrophilic leather.

3. The ‘Ewald’ effect: Oil tanned leather is one of a few cases where theleather exhibits a reversibility of hydrothermal shrinking. If the leather isheld in hot water, at or above the shrinkage temperature, it will shrinkas expected; however, if the leather is immediately placed in coldwater it regains about 90% of its original area. The phenomenon can beused to mould the leather into shapes, a shrink fitting process called‘tucking’. When leather shrinks under hydrothermal conditions, the triplehelix structure begins to unravel, hence reversibility only applies in theearliest stages, when the structure can reregister by itself. However, theoil matrix provides a scaffold that mirrors the collagen structure, sothe denatured protein can regain much of its original structure, but notquite all of it.

14.2 SULFONYL CHLORIDE

Alkyl sulfonyl chloride, available in the early part of the twentieth centuryunder the trade name Immergan, has been used as a sort of synthetic oil tan-ning, an alternative chamoising process.1–3 The chemistry of this reaction canbe summarised as follows:

Prot-NH2 þ C16H33-SO2-ClÐProt-NH-SO2-C16H33

The leather is less coloured and the shrinkage temperature rises to80 1C, giving similar properties to oil tanning. However, the reaction raises the

317Other Tannages

following points of interest:

� The tanning agent cannot create a matrix, so the effect relates more tocovering the fibre structure to produce a hydrophobic surface, like con-ventional lubrication (Chapter 17).

� The reagent can only react at the sulfone group via a single bond, i.e. itcannot crosslink the collagen, yet the shrinkage temperature is elevated.This is discussed further in Chapter 19, where an explanation is offered.

14.3 SYNTANS

The term syntan refers to the range of synthetic tanning agents.2,3 Their historystarts with the work of Stiasny, who patented the synthesis of polymeric speciescapable of tanning.4 This class of reagents is wide ranging, but typically theyare aromatic, as hydroxy and sulfonate derivatives, so some general principlesregarding their functions can be derived. This group does not strictly includeother resins and polymers, which might also be used in leather making, i.e.acrylates, urethanes, etc. are excluded (see below).Usually, there are two steps in the synthesis of syntans: sulfonation and

polymerisation. These steps can be conducted in either order: these are theNerodol and Novolak (Novolac) syntheses, typically leading to differentmolecular constitutions and hence properties. The reactions are represented inFigures 14.1 and 14.2, using naphthalene or phenol and monomeric for-maldehyde as models. When the sulfonation step comes before the poly-merisation step, typically the sulfonate content is relatively high. This meansthe solubility in water is increased, which reduces the astringency. These effectscan be balanced by the choice of precursor, to make the syntan more or lessreactive. In contrast, when the sulfonation step comes after the polymerisationstep, the syntan typically contains less sulfonate, making the molecule lesssoluble in water and hence more reactive towards the substrate The overallproperties of the syntans, with regard to the affinity for collagen or leather,depend on the choices of aromatic precursor and crosslinker, order of reactionsin synthesis, the degree to which the reaction steps are applied, the solubility of

HCHOSO3H

SO3H

CH2

SO3H

H2SO4

Figure 14.1 Nerodol synthesis: sulfonation then polymerisation.

318 Chapter 14

the syntan and its molecular weight. Although the reaction conditions may beprecisely controlled, the constitution of syntans, their chemical structures, arerelatively non-specific and are rarely analysed in detail.5

The properties of the syntan depend on the nature of the monomeric pre-cursors, some of which are presented in Figure 14.3. In addition, the crosslinkercan have an effect on the properties, particularly the lightfastness, whichdepends on the ability of the aromatic moieties to interact to create colour

OH

HCHO CH2

OH

OH OHOH

CH2

SO3HSO3H

OHOH

CH2

n

n

H2SO4

Figure 14.2 Novolak synthesis: polymerisation then sulfonation.

OH

CH3 CH3 CH3

OH

OH

CH3

OH

OH

OH

OH

S

O

O

OHHO

phenol toluene 2-methylphenol(ortho-cresol)

3-methylphenol(meta-cresol)

4-methylphenol(para-cresol)

2 naphthol resorcinol 4,4′-dihydroxydiphenyl sulfone

Figure 14.3 Some syntan precursors.

319Other Tannages

(Chapter 13): Figure 14.4 presents some of the crosslinkers used to makecommercial syntans.The range of syntan properties and functions can be summarised in the

notional model of trends in reactivity and properties (Figure 14.5). Here, thereactivity of the syntans depends primarily on the availability of phenolichydroxyl groups for hydrogen bonding, like the plant polyphenols. A sec-ondary reaction is fixation via the sulfonate groups (Chapter 15). However, theimpact of the sulfonate groups is more concerned with its effect on HHB valueof the syntan and the consequent effect on the solubility in water, acting againstits astringency properties. The range of syntans is typically separated intogroups of syntans, with names reflecting their properties, but the ranges areneither specific nor defined, so they overlap. This model does not includeallowance for variations in the crosslinker, which is an additional variable insyntan constitution.The size of the syntan molecules and hence their properties depend on the

ratio of crosslinker to aromatic nuclei (Table 14.1) according to Thorstensen.6

The effects of the syntans with regard to their tanning power depend pri-marily on the combination of molecular weight and availability of reactive sites,phenolic hydroxyls and sulfonate groups, together with the secondary influenceof the crosslinking chemistry (Figure 14.6).The positions within the aromatic nuclei and the relationship between the

phenolic hydroxyl groups and the sulfonate groups can influence the tanningpower due to inductive effects: syntans based on b-naphthols are better tanningagents than syntans based on a-naphthols.8 The relative meta positions of theoxygens in xanthene species allow more effective tanning than an ortho or paraarrangement (Figure 14.7), analogous to the arrangement in the condensedtannins.9

The effects of relative positions of phenolic hydroxyl and sulfonate groupsrely more on the combination of inductive effects; here, the inductiveeffect of sulfonate adjacent to phenolic hydroxyl is to reduce its reactivity byweakening the O–H bond and thereby weakening its ability to engagein hydrogen bonding. The relative tanning powers of the species shown inFigure 14.8 are attributed to the ability of the tanning species to interact via onehydrogen bonded hydroxyl and one sulfonate ionic link, but the non-tanningspecies only interacts via sulfonate links. The effect of the HHB (hydrophobic–hydrophilic balance) difference will contribute to the observed difference inproperties.

HCHO SO2Cl2 CH3COCH3 CH2CH2

O

formaldehyde sulfonylchloride

acetone ethyleneoxide

Figure 14.4 Crosslinkers for syntans.

320 Chapter 14

retans

auxiliary syntans replacement syntans

non swelling acids

nontanning

solotanning

Range of syntan constitution and properties

Increasing molecular weight

Increasing molecular complexity

Increasing phenolic hydroxyl content

Increasing sulfonate content (solubility)

Increasing [OH−]:[SO3−] ratio

Decreasing solubility in water (hydrophobicity)

Increasing solubility in water (hydrophilicity)

Increasing tanning power = increase in Ts

Increasing hydrophilicity of leather

Trend of Nerodol to Novolac synthesis

Increasing effectiveness of non swelling acidsin pickling, see Chapter 9

Figure 14.5 Trends in syntan properties.

Table 14.1 Effect of crosslinking ratio on molecular weight.

Ratio (number of mols. ofcrosslinker to no. of mols.of aromatic nuclei)

Average number of aro-matic nuclei per molecule Average molecular weight

0.50 2 300–3500.66 3 450–5000.75 4 600–7000.80 5 750–9000.90 10 1500–200041.0 Theoretically infinite Theoretically infinite

321Other Tannages

From Figures 14.6–14.8, the following conclusions can be drawn:

� The effectiveness of the syntan depends on its ability to engage in hydrogenbonding, from both the phenolic hydroxyls and the crosslinks.

� The positions of the hydrogen bonding groups determines the tanningeffect.

CH2

OHHO

SO2

OH

HO

OH HO

CH2 OH

CH2

CH2

∆Ts 0°C ∆Ts + 3°C

∆Ts + 6.5°C∆Ts + 8°C

∆Ts + 4°C∆Ts + 2°C

∆Ts + 6°C∆Ts + 1°C

∆Ts + 8°C∆Ts + 10°C

SO2 OH HO

CH2

OH

HO

C OHHO C

HOOH

HO

CH2

CH3CH3

CH3CH3

OH

Figure 14.6 Effect of simple syntan structures on tanning properties.7

322 Chapter 14

� The tanning effect depends on a combination of inductive effects from allthe substituents involved.

� The steric effects of additional groups can influence the tanning ability byreducing the free rotation and hence introducing some degree of fixedgeometry.

14.3.1 Auxiliary Syntans

As the name implies, these low molecular weight syntans are relativelyunreactive (in a tanning sense), so their performance functions are limited toaiding other reactions in the following ways:

� Solubilising higher molecular weight syntans or vegetable tannins, to allowmore uniform surface reaction and promoting penetration (Chapter 13).

� Dispersing reds and dyes, to allow more level tannin fixation and colouring(Chapter 16).

� Non-swelling acids fall into this category, since they rely on a mild tanningeffect after giving up their acid content to collagen in the pickling step.

� So-called ‘bleaching’ syntans may fall into this or the next group. Whenapplied to chrome tanned leather, the reaction is to strip some of the

H

O OHHO

CHC OHHO

O

3,6-dihydroxy-9-phenylxanthene∆Ts 25°C

2,7-dihydroxy-9-phenylxanthene∆Ts 5°C

Figure 14.7 Effect of relative positions of phenolic hydroxyls on tanning power.9

OH OHHOHO

CH2

CH3CH3CH3CH3

CH2

SO3−−O3S−O3S

does not tantans

Figure 14.8 Effect of relative positions of hydroxyl and sulfonate groups on theability to react with protein.

323Other Tannages

chrome from the leather and hence reduce the colouring on the surface, adetanning effect.

� ‘Neutralising’ syntans serve the function indicated by the name: thealkalinity in the product neutralises chrome tanned leather, raising the pHfrom 3–4 to 5–7, depending on the post tanning requirements with regardto charge on the leather (Chapter 15). At the same time they provide aretanning effect that may be useful for the production of certain kinds ofleathers: the tanning/filling effect required is determined by the requiredproperties/handle of the leather. Therefore, these syntans may also fall intoany of the other categories.

14.3.2 Retans

As the name implies, the medium molecular weight, mildly astringent syntansare typically used in post tanning as retanning agents. They will confer somefilling effect, limited by the molecular weight, which will also limit the degree ofhydrophilicity conferred to the leather.They also aid vegetable tanning and dyeing, if they are reacted with the

collagen or leather before applying vegetable tannin or dye, thereby reducingthe reactivity of the substrate. In this way, tanning and dyeing is hindered at thesurface, not only causing even reaction on the surface but also promotingpenetration through the cross section.

14.3.3 Replacement Syntans

As the name implies, the high molecular weight, astringent syntans are designedto replace vegetable tannins and act as solo tanning agents. They can match theastringency of vegetable tannins and provide the same degree of filling. How-ever, their structures can be created to minimise colour change under theinfluence of light, so they can offer improved lightfastness compared to thecondensed tannins.The effect of a syntan on collagen is dependent on its structure. This is

demonstrated in Figure 14.5, where the effect of structure on the shrinkagetemperature rise of collagen depends on both the crosslinker and the precursor,illustrated by dimeric syntan species.8

14.3.4 Other Syntans

The ‘Supra’ syntans may not contain any aromatic hydroxyl groups; they relyfor their effect on hydrogen bonding reaction via their amino sulfonyl groups, -NHSO2- (Figure 14.9), which also indicates the importance of the inductiveeffects.The filling power of syntans depends on the molecular weight and the

chemistry of the polymer: to increase the filling power, syntans have been madewith additional chemical species, notably lignosulfonic acid, a byproduct fromthe paper industry. These chemical species do not possess tanning power, as

324 Chapter 14

may be deduced from the structures indicated in Chapter 2 (Figure 2.34) andthe breakdown products shown in Figures 14.10 and 14.11.Approximate relative yields of degradation products are as follows:10 vanillin

to vanillic acid to 4-hydroxybenzaldehyde to 2-methoxyphenol is 1 : 1 : 5 : 10.Indeed, polymerisation of these breakdown products does not create effectivesyntans.11 Therefore, it is understandable that syntans based on byproductsfrom the paper manufacturing industry have never been a major success in theleather manufacturing marketplace.

NHSO2CH3−O3SCH2

NHSO2 CH3−O3S

tans

does not tan

Figure 14.9 Functionality of Supra syntan structures.

CHCHO

CH2OH

SO3Na

CH3OSO3Na

SO3Na

CHCHCH2OH

CH2OH

CH2CHO

CH3OCH3O

CH2OH

CHCHO

SO3Na

CH2OH

CH2CHO

CH2OH

CHCHO

SO3Na

NaO3S

CH3O

Figure 14.10 Intermediate breakdown product of Kraft lignin, lignosulfonic acid.

OH

OCH3

CHO

OH

OCH3

OH

OCH3

CO2H

OH

CHO

vanillin vanillicacid

4-hydroxybenzaldehyde

2-methoxyphenol

Figure 14.11 Ultimate breakdown products of Kraft lignin.

325Other Tannages

All the syntans discussed so far are anionic (or neutral in the case of theSupra syntans). However, by altering the chemistry of the precursor it is pos-sible to create amphoteric syntans, containing both acid and basic groups.These syntans can be useful, because they are less reliant on pH variation forfixation. The same would be true for extreme nerodol syntans with no sulfonategroups, but these would be relatively insoluble in water.It is apparent that any tanning reaction exhibited by natural polyphenols can

be matched by appropriate choices of syntan precursor, crosslinker and pro-duction conditions. However, with the exception of Reich’s work,7,9 there islimited information regarding the structural parameters contributing to syntanreactivity: these are necessary to allow the leather scientist to model therequirements for new tannins. Nevertheless, informed choices should allowimprovements in tanning technology. In making those choices, reference shouldbe made to the analyses presented in Chapters 19 and 20, which will form thebasis of developments in this field.

14.4 RESINS

The term resin refers to synthetic polymers, some of which have applicationsin leather making. The chemistries of some examples are presented inFigure 14.12(a) and (b).Clearly, from the chemical structures, the resin species have low astringency,

because the opportunity for fixing to collagen or leather is limited to somehydrogen bonding – some can contribute to the hydrothermal stability,12 but apositive effect is uncommon, weak and not relied upon by tanners. An excep-tion is the melamine-formaldehyde resins (Figure 14.12b), which have reactivityanalogous to phenolic syntans, because they can hydrogen bond to collagen viatheir triazinyl amino groups. This functionality can be used in combinationreactions with aldehydic crosslinkers (Table 14.2).13

The reaction is analogous to the combination of condensed polyphenol andaldehydic crosslinker (Chapter 13):

� The reaction is driven by elevated temperature.� The outcome depends more on the amount of the polymer than it does on

the amount of the crosslinker.� The aldehydic agents are not equally good at achieving high shrinkage

temperature: the preferred crosslinker is THPS [tetrakis(hydroxymethyl)-phosphonium sulfate].

� In this case, the reaction does not work with every commercial melamineresin: there appears to be a dimensional requirement; the preferred meanparticle size is 80 nm.

Typically, the resins do not function in combination reactions, due to thelimited reactivity they possess. One exception is reaction of carboxylate poly-mers with aluminium(III)12 and chromium(III). This has been exploited inreagents for high water resistance, when acrylate polymer is partially esterified

326 Chapter 14

(a)urea-formaldehyde

dicyandiamide-formaldehyde

polyurethane

styrene-maleic (anhydride) acid

polyacrylate

O C

NH2

NH2 H2N NH

CH2

NH2

C

O

NC-NH-C-NH2

NH

CH2OH

OCN-(CH2)x-NCO+ HO-(CH2)y-OH

NH2

NH-CH2OH

CO C

O

NH NH

NC-NH-C-NH-CH2OH

HCHO

HCHOHCHO

NC-N-C-NH-CH2OH

N-CH2OH

[-O-C-(CH2)x-NH-C-O-(CH2)y-O-]n

O

O

CH=CH2CH

CO2H

HC

OC CO

O

[-CH-CH2-CH-CH-]npolymerise

hydrolyse

R-CH=CH-CO2H [-CH-CH-]n

R

CO2H

+

HO2C

(b)

H2N

NH2

NH-CH2OH

NH-CH2-NH

H2N

N

N

N

NN

N NH2

NH2

H2N

C C

C

NH2

NN

N NH2H2N

C C

C

HCHO

N

N

N

Figure 14.12 (a) Chemistries of resins; (b) melamine-formaldehyde structure.

327Other Tannages

with long-chain alcohols (Chapter 17).14 Because of their ability to complex viatheir carboxylate groups, polyacrylates have been marketed as chromium(III)fixing agents.15

Resins have limited value as substantive reagents, but they are used asstructure or property modifiers for leather: where they have value is in filling thefibre structure, to tighten and even up the properties over the pelt, particularlyin loose bellies.

14.5 ALDEHYDES AND ALDEHYDIC TANNING AGENTS

14.5.1 Introduction

The reaction between an aldehyde and protein can be expressed in the form setout in Figure 14.13. There is nucleophilic attack at the carbonyl carbon by thelone pair of electron on the amino nitrogen, to form an N-methylol derivative:the reaction requires the amino group to be uncharged, so reaction is slow atpHo7 and fast at around pH9. The reaction is dominated by the involvementof lysine amino groups,16 with little involvement of other sidechains with activehydrogens. This hydroxy compound can lose water, to form the correspondingunsaturated Schiff base.It is commonly held that the tanning reaction mechanism involves further

reaction between the N-methylol derivative and another basic group in theprotein system, as illustrated in the following model chemical equations, usingmonomeric formaldehyde as the example (P¼ protein):

P-NH2 þHCHOÐP-NH-CH2OHðN-methylol derivativeÞ

ÐP-NH2

P-NH-CH2-HN-P

The presence of the nitrogen in the N-methylol group with its lone pairmakes the hydroxyl a good leaving group, i.e. it is reactive to groups with anactive hydrogen. This can be illustrated in the extremes of structure as follows:

P-NH-CH2-OH Ð P-NþH¼CH2 � � �OH�

Table 14.2 Combination tannages of 10% commercial melamine–formalde-hyde resin and aldehydic crosslinkers, applied to sheepskin.

Crosslinker offer Process conditionsShrinkage temperature(1C)

Initial pelt pH Overnight temp.(1C)

2% Glyoxal 7 50 1065% Oxazolidine 6 50 1064% Phosphoniumsalt (THPS)

5 50 112

328 Chapter 14

Clearly, crosslinking reactions can happen and their role in tanning will bediscussed further in Chapter 19. What is not in doubt is that the reactionproduct is covalently bonded; hence reversal can only be accomplished byhydrolysis, general acid and general base catalysed.

14.5.2 Formaldehyde

Formaldehyde is a useful example of aldehyde tanning, because its structure isthe simplest. As illustrated in Figure 14.14, it is more than likely to be present ina polymerised form, depending on the history of the sample. It is common tosee a white precipitate in laboratory bottles of aqueous formalin solution.Apart from the hydrate (II in Figure 14.14), formaldehyde species are

dominated by paraformaldehyde (IV). Some insight has been gained into thereactions of formaldehyde using NMR spectroscopy.17 Reaction with gelatineshowed that lysine was the preferred site and then arginine. Studies with modelamino acids showed that they are difficult to crosslink. In studies using poly-meric amino acids, no crosslinking could be achieved in either polylysine orpolyarginine, although links between the polymers could be formed. Formationof the methylol derivative and Schiff base has been demonstrated in the reac-tions between formaldehyde and protein, but the subsequent reactions arecomplicated.18 Preferred reaction occurs at lysine, arginine, histidine andcysteine. Schiff base adducts react preferentially with arginine and tyrosine: inthe former, reaction is with the terminal amino groups; in the latter, reaction iswith the aromatic nucleus of the sidechain (see below). From subsequentreactions, the crosslinking chains can be variable in length. Therefore, althoughcrosslinking of protein is theoretically possible and sensible paper chemistrycan illustrate the reactions, it seems to be the case that simple crosslinking is not

R C

O

H

H2N-P R C

O−

H

N+

H

H

P

R-CH=N-P

OH−

P

H

N

H

OH

CRloss of water

nucleophilic attack

Schiff base

H2O

• •

Figure 14.13 General aldehyde reaction with protein amino group (P¼ protein).

329Other Tannages

necessarily the natural consequence of these reactions and the overall reactionis complicated.Formaldehyde has long held a place in leather making technology, in the

following applications:

� Non-mineral tannages, e.g. woolskins for nursing purposes.� Applications where high perspiration resistance is required, e.g. gloving

leather.� Wool treatment in woolskins, i.e. straightening, in a reaction analogous to

permanent waving of straight hair or fixing relaxed naturally kinked hair.

Tanning with formaldehyde produces white leather, which is characterised byplumpness and hydrophilicity. These latter features are the effect of the poly-meric nature of the tanning agent, pushing the fibre structure apart, and thepresence of hydrogen bondable groups in the polymer. The tannage yieldsleather with shrinkage temperature up to 80 1C.Notably, because of its toxicity hazard, the restrictions on the allowable

concentration in the atmosphere of the workplace have effectively created a ban

H C

H

OH

n

CO

HH

C

H2C CH2

CH2

O O

OH

HO-(CH2-O)n-H

weak base

I formose

II formaldehydehydrate

III metaformaldehyde(trioxane)

IV paraformaldehyde

OH

C

H

HO

C OH

H

H

O

Figure 14.14 Formaldehyde structures.

330 Chapter 14

on its use as an industrial reagent for leather making. Its use in preservingbiological specimens remains: although formaldehyde is effective as a bacter-icide, its mode of preservation for specimens in prolonged contact with it is totan the protein. It is useful to recognise that such biological specimens havebeen transformed chemically and artefacts can be introduced because of thefilling effect of polymerised formaldehyde.The toxicity and tanning properties of formaldehyde call into question the

benefits of its continued use in biology and histology. Some of the otheraldehydic agents discussed below may be better options, because they are lesstoxic and react as monomers.

14.5.3 Glutaraldehyde

As the use of formaldehyde declined, the use of glutaraldehyde as a replacementgrew. Figure 14.15 gives the structure of this aliphatic dialdehyde in solution.Consideration of the structure of the monomer might lead to the conclusion

that the tanning reaction involves the creation of a four-centre crosslink, sinceeach end can theoretically link two amino groups. Intuitively, this is doubtful,because the coincidence of four adjacent amino groups is unlikely and theentropy penalty of such a bond would be very high. However, it is not necessaryto invoke a mechanism that is chemically unsound: the polymer structure(Figure 14.15), based on condensation of the hydrate, is a linking structurebetween two reacted glutaraldehyde molecules (Figure 14.16).19

Damink et al.19 conclude the following:

1. Within a complex scheme of reactions, glutaraldehyde forms Schiff baseswith protein and these are stabilised by other glutaraldehyde molecules.

2. There is no evidence that crosslinks are formed.3. Three glutaraldehyde molecules are fixed per lysine amino group: there is

no evidence for a polymerised matrix.

O OHO OH

OHO OHO On

HO OH

OO

+H2O

-H2O

Figure 14.15 Polymerisation of glutaraldehyde hydrate.

331Other Tannages

Therefore, although crosslinking is clearly possible, when notionally up tofour amino groups might be linked by one glutaraldehyde molecule, there is noevidence that the tanning reaction relies on crosslinking (Chapter 19).Like formaldehyde, the leather tanned with glutaraldehyde is plump and

hydrophilic, for the same reasons. The shrinkage temperature is similar.However, the colour is different, having a distinct yellow cast, which can appearquite orange. The reason for the colour development is unclear, but hassomething to do with changes in structure upon reaction with collagen. Thecolour of normally tanned glutaraldehyde creates a problem in dyeing suchleathers and is generally unacceptable in leathers designed to be white. Con-sequently, derivatives of glutaraldehyde have been offered to the industry. Theoriginal option was Relugan GTW, the bisulfite addition derivative. Thisproduces paler leather, but there is always a degree of yellow in the leather,since the addition compound must be reversed for the reaction with collagen tooccur. Other derivatives have included glyptal compounds with alcohols andaldol addition and condensates with other aldehydes, including formaldehyde.Unsurprisingly, none of these has successfully overcome the problem.Glutaraldehyde is currently widely used. However, it is probable that it will

experience the same fate as formaldehyde and eventually its use will have to bediscontinued.

14.5.4 Other Aliphatic Aldehydes

Other aldehydes are not commonly used in the industry. This is due in partbecause there is no environmental advantage, but mainly because they are noteffective tanning agents. Other aldehydes in the aliphatic homologous series,R-CHO, can tan, but they are not effective and they are not cheap. Glyoxal,(CHO)2, the lowest dialdehyde in the homologous series, is an industrial che-mical, but not widely used in this context.

Coll-NH2 +

O O Coll-N O

N

coll

HO OHOHO

coll

N OH

coll

NO On

stable Schiff base

Figure 14.16 Reaction between glutaraldehyde and protein.

332 Chapter 14

Acrolein, CH2¼CH-CHO, has both aldehyde and unsaturation function-ality, allowing polymerisation. It is a toxic chemical, with consequent asso-ciated problems for industrial use. Its roles in leather making are incidental:

1. It is generated as a byproduct of oil oxidation in oil tanning for chamoisleather production (see above).

2. It is generated as a byproduct of wood burning and is an active ingredientin the traditional tanning technology of plains dwellers, e.g. NorthAmerican Indians and the Mongols. The first step exploits the lubricatingeffect of the phospholipids in partially cooked brains, when the paste isworked into the split surface from which the grain has been cut away,allowing the leather to dry soft, without fibre sticking: this is the manu-facture of buckskin. However, like other leathering procedures, which donot create permanence in the material, wetting causes the fibre structure tostick and revert to a horny, untanned appearance. Permanence to wettingcan be introduced by holding the buckskin over a smoking fire, to obtainthe effect of acrolein tanning. The Sioux of North America have a say-ing:20 ‘each animal has enough brains to tan itself’.

A more commercial agent is dialdehyde starch (or starch dialdehyde), madeby the oxidation of starch with periodic acid (HIO4) (Figure 14.17). The valueof n, the degree of depolymerisation, depends on the ratio of starch to periodicacid.21

The tanning power is relatively weak, but it has potential value as a com-ponent in combination organic tanning. It too produces white, hydrophilic,plump leather, where the latter property depends on the size of the molecule.Dialdehyde formation is not confined to starch: any other carbohydrate may

be used, e.g. dextrin, a lower molecular weight version of the starch structure.22

Alginates can be oxidised to dialdehydes in the same way: the structure isindicated in Figure 14.18, where the polymer contains blocks of either man-nuronic acid (M) or guluronic acid (G) or alternating G and M or random Mand G. Alginate dialdehydes have been shown to be effective tanning agents,although the tensile strength of the leathers is inferior to glutaraldehyde tannedleather.23

C

H

C

CH2OH

O

C

HO

n

CH

OH

HC

H

OH

HIO4

O OOHC

H

CH

CHO

C

n

OH

O

CH2OH

C

Figure 14.17 Preparation of starch dialdehyde.

333Other Tannages

14.6 ALDEHYDIC TANNING AGENTS

Aldehydic agents are distinguished from conventional aldehydes because theyare not aldehydes themselves, but either they undergo some aspect of thetypical aldehyde reaction or they exhibit analogous reactions.

14.6.1 Oxazolidines

Oxazolidines were introduced to the leather industry by Das Gupta (Figure14.19).24 The chemistry of their reaction with protein is clearly aldehydic,although they are not themselves aldehydes.The aldehydic character can be seen from the way in which they are syn-

thesised, creating an aliphatic heterocyclic ring system, which can open to formN-methylol groups. Since they can reproduce the intermediate in formaldehydetanning it might be assumed that they function in the same way. Das Guptaet al.25 have proposed that the reaction is concerned in part with the productionof a simple Schiff adduct, analogous to the formaldehyde reaction; Figure 14.20illustrates the electronic changes. Reaction scheme A is the route by which theoxazolidine is bound to the protein. Reaction scheme B is the route by whichthe oxazolidine could break down to create the Schiff base. The latter reactionappears less plausible, but there is evidence that the reaction can work in thisway, adding a methylene group to amino sidechains.18 However, consideringthe mechanistic options, it is not certain whether the reaction works because ofScheme B or because there is formaldehyde in solution from the breakdown ofthe oxazolidine back to the components from which it was made.Figure 14.21 illustrates the reaction between the Schiff base and the tyrosine

sidechain. This reaction is analogous to the reaction with polyphenol deriva-tives already discussed. Similarly, the Schiff base can react with other groupsthat contain an active hydrogen, such as the amino groups in arginine andlysine.However, because oxazolidines are known to react as monomers, the leather

is less filled than aldehyde tanned leather. Since they contain no chromophores,they produce white leather. They produce a typical aldehyde tanned shrinkagetemperature, up to 85 1C: there is a difference between the reactions dependingon whether the oxazolidine is monocyclic or bicyclic, as shown in Figure 14.19and illustrated in Table 14.3.24

O

OO

OHO

CO2H

OH HO2C

HO

OH

m

n

O

β-D-mannuronic acid α-L-guluronic acid

Figure 14.18 Structural features of alginates.

334 Chapter 14

H3C

CH3

NH2

CH2

HN O

CH2

HO

OH

C C

OH

CH2

CH3

H3C H3C

CH3

CH2C

OHHN

CH2OH

C

OH

CH2

NH2

CH2CH3

H2C H2C

CH2CH3

CH2C

ON

CH2 HOH2C

HOO

H2C CH2OH

N

C CH2

CH2CH3

H2C

HCHO

2 HCHO

Figure 14.19 Examples of structures of mono- and bicyclic oxazolidines.

H3C

CH3

C

HN O

CH2

Prot NH

H

H

H2C

C

H H

NH Prot

CH2

HN

C

CH3

H3COH

OH

OH−

OH

H3C

CH3

C

H2N

CH2

ProtOH−

OH−

H2C

HO

HH

C

H

H

NHProt

CH2

OHN

C

CH3

H3C

N

H3C

CH3

C

H2NO

CH2

ProtN

H

H

OH

H2C

A

B B

• •

• •

Figure 14.20 Reaction between oxazolidines and protein sidechain amino groups.

335Other Tannages

The following points of interest arise from the results in Table 14.3:

� The outcome of the reaction, in terms of the hydrothermal stability, is notimproved by offering excess reagent.

� The monocyclic oxazolidine confers higher shrinkage temperature.� The monocyclic oxazolidine reacts independently of pH, unlike the bicyclic

oxazolidine, which reacts like a conventional aldehyde.

Das Gupta et al.25 have proposed that the main tanning reaction of theoxazolidines is by the formation of the methylene derivative of the substrateamino groups. This would suggest that the outcome of reaction by themonocyclic and bicyclic derivatives would be the same. It is a contraryobservation that the monocyclic derivative yields shrinkage temperatures below

Prot

H2C =N

H

Prot

OH

OH+

Prot

H-

H

OH−

OH+

H OH−

NH

OH

OH

Prot

ProtCH2

OH

-

Prot

ProtH2C-NH

Figure 14.21 Reaction between a Schiff base or methylol derivative and an argininesidechain.

Table 14.3 Comparison of the tanning reactions of monocyclic and bicyclicoxazolidines.

Oxazolidine offer (%) Initial pHShrinkage temperature (1C)

Monocyclic Bicyclic

5 2.0 84 655 4.0 85 725 7.4 84 82

10 2.0 84 6910 4.0 85 7410 7.4 84 82

336 Chapter 14

100 1C when retanning flavonoid polyphenol tanned leather, but the bicyclicderivative yields shrinkage temperatures above 100 1C: in the former case, theoutcome is additive tanning, in the latter case there s synergistic crosslinking.Clearly, from the chemistry of the oxazolidines, the bicyclo species can

theoretically form crosslinks with protein, but the monocyclic species cannot:therefore, the tanning abilities of the single components become divorced fromthe necessity of crosslinking (Chapter 19).The monocyclic oxazolidine reacts too quickly for a commercial tanning

process: the bicyclic oxazolidine in Figure 14.19 is available commercially asNeosyn TX or Mirosan E.

14.6.2 Phosphonium Salts

Methylol phosphonium salts have been known for many years, but it isonly recently that the sulfate [tetrakis(hydroxymethyl)phosphonium sulfate –THPS] has become available commercially.26 Before their potency as tanningsalts became known, they were used as biocides. Figure 14.22 presents thechemistry.The reaction mechanism appears aldehydic, with the notable difference that

it depends on the reactivity of the P-methylol group: considering the position ofphosphorus relative to nitrogen in the period table, the analogy to N-methylolchemistry is apparent. However, the nature of the mechanism of reaction withprotein is not clear: it has not been extensively investigated, so it is stillsomewhat conjectural. It has been assumed that reaction occurs with at leastone protein amino group – it is unlikely that a four-centre bond would becreated – but it is known each molecule reacts with 2.2 amino groups.26 It hasbeen argued that subsequent oxidation is advantageous, presumably to convertthe residual methylol groups into an oxy moiety.It is known that the phosphonium salt is hydrolysed at pH 4 8 in the

following way, to create the oxide, which is unreactive as a tanning

HOCH2

CH2OH

CH2OH

CH2OH

P+ P+

CH2OH

CH2NH-Prot

CH2OH

Prot-NHCH2

Prot-NHCH2

O

CH2NH-ProtP+

[O]

Prot-NH2

Figure 14.22 Reactions of phosphonium salts.

337Other Tannages

agent (P¼phosphorus):27

ðHOCH2Þ4PþÐOH�

ðHOCH2Þ4POH

ÐðHOCH2Þ3PþH2OþHCHO

Ð½O�

ðHOCH2Þ3PO

THPS is a potent tanning agent and is likely to be a useful addition not onlyto the tanning industry but also to all other sectors that rely on the chemicalstabilisation of protein by covalent organic means.

14.7 OTHER TANNING APPLICATIONS: WET WHITE

The most common intermediate state of modern leather is wet blue: the pelt isconverted into leather by chrome tanning and in this state it is ready to have thefinal properties imposed by the post tanning processes. At this stage it istypically subjected to splitting and shaving, to adjust the thickness of the splitsto the requirements of the final product. It is the splitting and more particularlythe shaving that produce a substantial quantity of the solid tanned waste thatmust be disposed of by the tanner. Discharge to landfill is costly and envir-onmentally undesirable. Therefore, it would be preferable if the thickness of thepelt could be adjusted at a different stage of manufacture, thereby creating abyproduct with some value and which would not constitute a disposal problem.It is possible to split raw material – by a band knife – and the technology ofsplitting either before or after soaking, whether cured or not, has been appliedcommercially.28 However, it is not possible to shave raw material, because therotating blade on the cylinder cuts into the surface and creates frictional heat,which gelatinises and therefore damages the pelt surface. Splitting withoutprior shaving does not provide enough accuracy to the thickness to remove theneed for further thickness adjustment.At the same time, consideration ought to be given to the international trade

hides and skins. The natural desire of exporting countries to give added value totheir production has led to an increasing world trade in part processed (asopposed to raw) materials. An example of this trend is the rise of wet blue as acommodity in place of dried hides or pickled skins that used to be morecommon. Wet blue has several advantages to the purchaser, among them beingthe transfer of the beamhouse effluent and some of the chrome pollution to theexporting country. It also allows preliminary grading of supplies and thepotential to meet greater demand for finished leather than could be producedfrom raw. However, wet blue is not an ideal intermediate for the followingreasons:

1. Inflexibility: the range of products that can be made from a given wet blueis limited; the grain and flesh have already received the same basic tannageand the structure of the pelt is irreversibly fixed.

338 Chapter 14

2. Waste: approximately 30–40% of the chrome applied to full thicknesshide is lost as tanned waste and the chrome content restricts its use andmay raise problems of disposal.

3. Variability: wet blue bought on the open market varies in its degree oftannage, colour and the effects of storage; this also applies to wet bluepurchased from tanneries where the processing is known or controlled bythe purchaser.

4. Drying: dried wet blue is difficult to hydrate satisfactorily.

The ideal material to meet the requirements of this situation would retain theadvantages of wet blue, whilst avoiding its problems. Since the new technologysets out to avoid the use of chromium salts, the alternative to wet blue is termed‘wet white’. The requirements of an alternative part processed commodity areas follows:

1. Preservation: Hides or skins should be preserved/stabilised in a mannerthat precludes as few end uses as possible. This means the stock should bestable for more than six months and the mechanism for preservationshould be reversible. The preservation technology should be achievablewith readily available products, giving consistent results. Since con-sistency in production cannot be guaranteed worldwide, reversing theprocess should be possible to a high degree. Since the purchaser willdischarge the process chemicals, they should have minimum environ-mental impact.

2. Splitting and shaving: the preserved material should be capable of spittingand shaving, to have maximum flexibility of end use and ease of bypro-duct disposal.

3. Versatility: The treatment should retain the versatility of chrome tanning,not excluding any end uses and applications. It must be chrome free, toallow easier disposal of treated byproducts.

4. Wettability: it must be easily rewetted, in case of drying out during storageand to introduce the option of ‘dry white’.

14.7.1 Wet White Production

Wet white can be produced in many ways, using tanning materials such asaldehydes, syntans, zirconium(IV) salts, titanium(IV) salts. The followingmethods have received most attention in recent years.

14.7.1.1 Aluminium Sulfate, the Heidemann Method 29. In this treatment,there is a high offer of aluminium(III) in the form of the sulfate, giving a wetwhite with 2–5% Al2O3. The high mineral content produces a material thatis easier to shave and squeeze, having less variability over the area of thehide. The wet white can be efficiently stripped of its aluminium content atpHo4. If the wet white is chrome tanned without prior treatments to remove

339Other Tannages

the aluminium, the chrome uptake is high, due to the effect of the bound alu-minium on the fixation rate (Chapter 12) but the leather is firm. The firmnesscan be overcome by treating the wet white with glutaraldehyde and fatliquorbefore chroming.

14.7.1.2 Aluminium Polyacrylate, the Rohm and Haas Method .30 The pro-duct was designated Chromesaver A-31: aluminium(III) is masked with a lowmolecular weight acrylate polymer, claimed to avoid problems of prematurereversal, but fully removable by acid treatment. The process results in ashrinkage temperature of 71 1C and the consequent chrome tanned leather issaid to be softer than the matched controls.

14.7.1.3 Aluminium(III) Nitrate+Sorbitol, the Centre Technique du Cuirmethod .31–33 The method is based on treating the hide of skin with thehydrophilic polyol sorbitol (2R,3S,4S,5S)-hexane-hexol, HOCH2(CH.OH)4-CH2OH), followed by aluminium(III). The process is conducted as follows:acidify the pelt in 10% brine to pH3.2 with sulfuric/formic acids;add sorbitol and run for 12 hours;add aluminium(III) nitrate nonahydrate, basify to pH4.5 with sodiumbicarbonate;rinse and (if required) dry.The order of adding the reagents is important, because it affects the outcomeof the reaction (Table 14.4).

14.7.1.4 Sodium Aluminium Silicate, the Henkel Method. This reagentcombines aluminium(III) tanning with some ‘silica tanning’:34 the product is asynthetic zeolite with formula (Na2O.Al2O3.2SiO2)1227H2O. The aluminiumtanning reaction is more effective than reactions with the simple basic alumi-nium(III) salts such as the sulfate: the action of acid at pHB3.5 on the struc-ture creates chains of aluminium(III) silicate, so the metal is in a highermolecular weight complex, allowing multiple interactions. The improved tan-ning effect is likely to mean that the reversibility of the reaction is dimin-ished: in the published trials, the wet white was merely washed with waterand then processed in various ways. It is claimed that the pretreatment con-ferred improvements to the physical properties of the leathers, particularly

Table 14.4 Wet white made from aluminium(III) andsorbitol.

Order of additionAluminium content of wet white(% Al on dry weight)

Sorbitol then aluminium 0.85Aluminium with sorbitol 0.45Aluminium then sorbitol 0.28

340 Chapter 14

the softness. Importantly, low environmental impact of the mineral contentmeant that the byproducts could be used as fertiliser.

14.7.1.5 Syntan Pretanning. Various syntans might be used in this contextand some have been claimed to be useful.35,36 The reversibility of the treat-ment depends on the reactivity of the syntan, based on its functionality andmolecular weight: if the stabilization of the collagen requires the syntan to betoo reactive, reversing the fixation may be damaging to the collagen. Thealternative approach is to accept that the syntan will have some influence onthe properties and character of the leather: the conferring of hydrophilicitymay be undesirable in the case of water-resistant leather production.

14.7.1.6 Aldehydes, Aldehyde Derivatives, Aldehydic Agents. Any aldehydicreagent may be used as a pretanning treatment prior to chrome tanning,37,38

but they all have the same drawback of irreversibility because of their cova-lent bonding to collagen. In addition, the pelt may not be white, notably byreaction with glutaraldehyde and its derivatives. As with syntans, the leathercharacter conferred by aldehydic reagents will be affected; in particular, thespongy handle and hydrophilicity may limit their application.

14.7.1.7 Silica. Silica, in the form of colloidal silica, hydrogels or silicicacid, has applications in leather processing,39–41 but it is perhaps in the con-text of wet white that it will be most useful. The reagent binds to collagenvia hydrogen bonding, so it is not unexpected that there is no increase inshrinkage temperature: the effect cannot be called tanning or even tawing.However, the presence of silica within the fibre structure has the effect ofreducing the ability of the fibre structure to slip over itself when subjected todistorting pressure. When the shaving blade applies pressure to untreatedpelt, the surface can ripple ahead of the blade, until the pressure gets toomuch and the blade slips over the surface, generating frictional heat. It thepelt is treated with silica it is stiffened, so the shaving blade cuts rather thandistorts the surface. The treated waste/byproduct can be used as a fertiliser,instead of being disposed of to landfill.

14.7.1.8 Low Chrome Wet Blue. The option of trading in low chromeleather has been suggested as an alternative approach to the problem oftanned waste: here, a chrome syntan complex was used to give a chrome con-tent of 0.5% Cr2O3 on conditioned weight (20 1C, 65% RH, typically produ-cing 14% moisture in the leather).42 The leather is stable over six months, iftreated with a suitable bactericide/fungicide, and can be split and shaved.Clearly, this treatment does not conform to the requirements of wet white,not least of which is the lack of whiteness because the pelt has an obviousblue cast. (Note, even at chrome levels five times lower, the pelt is still pale

341Other Tannages

blue.) Moreover, the low level of chrome may still be too high for disposalpurposes.

14.7.1.9 Pickled Pelt. Chapter 9 discussed the possibility of trading inpickled hide. Storage pickles can preserve hide for a year, which satisfies oneof the requirements of wet white. However, the danger of irreversible dryingduring storage is a limitation and a problem. Pickled pelt can be split, but itis not known whether it is feasible to shave conventionally pickled pelt. Oneoption might be to employ non-swelling acids for pickling and gain an addi-tional benefit from the weak tanning action.

14.7.2 Use of Part Processed Materials

Finally, an aspect of wet white not addressed so far is the place it holds in thehierarchy of part processed materials that might be used as the starting pointfor leather production. Tanners face problems associated with the environ-mental impact of their production, so the closer the starting point of produc-tion is to the end the less pollution is caused, with an associated reduction incost. The time it takes to convert raw hide into finished leather is an issue: if thereceipt of an order triggers the start of production, conventional processingfrom wet salted stock to delivery of the order may take up to a month. Reactiontime in this context is limited more by the starting point than the chemicalprocessing. If a ‘Just in Time’ approach is required, when the supplier candeliver within a short time of getting the order, the commodity must be alreadyavailable in the warehouse close to completion. In the case of leather produc-tion, the closer the received raw material is to finished leather, the less versa-tility the tanner can achieve. In this case, versatility can include ranges ofapplications (footwear, apparel, upholstery, leathergoods) texture, softness,strength, water resistance, colour, surface features, finishes, etc. This is acommercial decision: Table 14.5 contains some of the major points to beconsidered when taking that decision.

14.8 MISCELLANEOUS TANNAGES

It was demonstrated above that polymeric resins can be applied to collagen orleather to confer physical properties, such as filling up the fibre structure, butthe chemistry does not allow high energy bonding. Consequently, they cannotbe considered to be tanning agents of any importance. The alternativeapproach is to create the polymeric species in situ.

14.8.1 Epoxide Tanning

Epoxides are currently used in the leather industry as crosslinking agents forpolymeric finishes on the surface of grain leather: because of their instability tohydrolysis they were typically applied in solvents. However, because of the

342 Chapter 14

requirement for aqueous processing, some epoxides have been developedthat are water soluble and capable of being applied to wet collagen(Figure 14.23).43,44

Epoxides can be used as tanning agents, but the shrinkage temperature istypically limited to 82 1C, with a maximum of 85 1C under extreme conditions, soin this way they function like any other single component tannage, withoutoffering any advantage. They have been compared with aldehydic reagents (glu-taraldehyde, oxazolidine, THPS), but in all cases exhibited weaker tanning effects.

14.8.2 Isocyanate Tannage

Traubel has suggested that isocyanates and derivatives provide an alternativeapproach to organic tanning.45 They work by reacting with an active hydrogenin the following way:

R-NCOðisocyanateÞ þNaSO3

! R-NH-CO-SO�3 Naþðcarbamoyl sulfonateÞ

�!Protein-HR-NH-CO-Protein

Diisocyanates can be used, to attempt crosslinking (Figure 14.24).Although the compounds can react with several types of sites on the protein

and their functionality indicates the potential for crosslinking, the shrinkagetemperature of the products is limited to 82 1C, just like the epoxides.

Table 14.5 Comparison of the properties of part processed materials.

Starting material Advantages a Disadvantages b

Dry hides Cost of transport; long-termpreservation

Wetting back; damage dur-ing drying

Salted hides Ease of reverting to originalstatus; long-term preservation

Salt pollution; curing faults

Pickled hides No pollution from beamhouseprocesses; still versatile

Damage in storage

Wet white Lack of tanned waste Reversibility; polluting pre-servative; character ofleather

Dry white As for wet white; reducedtransport cost

As for wet white; wettingback

Wet blue Stability; reduced pollutionfrom tanning

Character is fixed; drying instorage; variation in colour

Undyed crust Little pollution; fast response toorders

Leather properties are fixed;wetting back to dye

Dyed crust No pollution from wet proces-sing; no solid waste

Leather properties are fixed;dyed colour is fixed; colouronly by pigment

aPollution from production decreases on moving down the column.bDecreasing versatility in product range on moving down the column.

343Other Tannages

CH2

CH-CH2-O-CH2 H2C-O-H2C-HC

O-H2C-HC

H2C-O-H2C-HC

CH-CH2-O-CH2-CH-CH-CH-CH-CH2-O-CH2-HC

CH-CH2-O

H2C

H2C

H2C

O

CH2

O

H2C

H2C

O

CH-CH2-O-(CH2CH2O)n-CH2-CHpolyethyleneglycol diglycidyl ether

CH2-O-CH2-HC

CH-CH2-O-CH2

CH2

O

CH-O-CH2-HC

CH2

O

CH2-O-CH2-HC

glycerol triglycidyl ether

C pentaerythritol glycidyl ether

OHHO

sorbitol tetraglycidyl ether

CH2

O

CH2

O

CH2

O

CH2

OO

O

O

O

Figure 14.23 Examples of epoxides for tanning that are water soluble and can beapplied to wet collagen.

OCN-(CH2)6-NCO OCN-(CH2)11-NCO

OCN-CH2-CH2-CH(CH3)-CH 2C(CH3)2-CH2-NCO

CH2OCN NCO OCN NCO

OCN-CH2

CH2-NCO

NCO

H3C

H3C

CH3

CH2-NCO

Figure 14.24 Diisocyanates for crosslinking tanning.

344 Chapter 14

14.8.3 Multi-functional Reagents

Some reagents can offer various functions: one example is the reaction productof salicylic acid and formaldehyde, reported by Montgomery46 (Thiotan GDC7, ex Sandoz) (Figure 14.25). The product has hydrogen bonding and metalcomplexation potential: by itself it only raises the shrinkage temperature of peltby 7 1C, so its function as a tanning auxiliary is more important (Table 14.6)(A.D. Covington, unpublished results).A useful byproduct of the employment of this auxiliary is that the heat

resistance is increased: in trials (A.D. Covington, unpublished results), com-mercially chrome tanned bovine leather with a shrinkage temperature of 110 1Cexhibited 13% linear shrinkage after being boiled in water for 15min, but thesame leather treated with 5% of the SAF product relaxed by 2% in the sameboil test.

REFERENCES

1. J. H. Sharphouse, J. Soc. Leather Technol. Chem., 1985, 69(2), 29.2. Progress in Leather Science: 1920-45, British Leather Manufacturers’

Research Association, 1948.3. J. A. Wilson, The Chemistry of Leather Manufacture, American Chemical

Society, The Chemical Catalog Co. Inc, New York, 1929.4. E. Stiasny, Austrian Patent 58, 405, 1913.

CO2H

OHHCHO

CH2

HOOH CO2HHO2C

Figure 14.25 Salicylic acid polymerised by formaldehyde.

Table 14.6 Tanning processes with 5% salicylic acid–formaldehyde polymeras pretreatment.

Mineral offer Mineral saltShrinkage temperature(1C)

2% Cr2O3, 33% Basic chromium(III)sulfate

121� 7

5% Al2O3 Basic formate maskedsulfate at pH4

81� 2

5% TiO2 Titanyl ammonium sul-fate, solution at pH4

79� 2

3% Al2O3+1.5% TiO2 Basic gluconate maskedmixed sulfate, pH4

89� 2

345Other Tannages

5. J. Guthrie, et al., J. Soc. Leather Technol. Chem., 1982, 66(5), 107.6. T. C. Thorstensen, Practical Leather Technology, Van Nostrand Reinhold

Co., New York, 1969.7. G. Reich, Das Leder, 1958, 9, 6.8. K. Bienkiewicz, Physical Chemistry of Leather Making, Robert E. Krieger

Publishing Co., Malabar, Florida, 1983.9. G. Reich, Ges. Abh. Inst. Freiberg, 1960, 16, 40.

10. O. Suparno, PhD Thesis, The University of Northampton, 2005.11. O. Suparno, et al., Resources, Conservation and Recycling, 2005, 45, 114.12. A. D. Covington and R. L. Sykes, J. Amer. Leather Chem. Assoc., 1981,

65(2), 21.13. A. D. Covington and M. Song, UK Patent Application GB 2,287,953A,

March 1994.14. A. G. el Amma, European Patent 0372746A2, 1989.15. R. Schmidt, Das Leder, 1989, 40, 41.16. J. H. Bowes and R. G. H. Elliott, J. Amer. Leather Chem. Assoc., 1962,

57(8), 374.17. S. K. Taylor, F. Davidson and D. W. Overall, Photo. Sci. and Eng., 1978,

22(3), 134.18. B. Metz, et al., J. Biol. Chem. 2004, 279(8), 6235.19. L. H. H. O. Damink, et al., J. Mat. Sci.: Mats. In Medicine, 1995, 6, 460.20. L. Belitz, Pine Ridge Reservation, Star Route, Box 176, Hot Springs, South

Dakota, 1973.21. E. M. Filchione, et al., J. Amer. Leather Chem. Assoc., 1958, 53(2), 77.22. R. Sugiyama, Y. Chonan and H. Okamura, Hikaku Kagaku, 1997,

43(1), 55.23. J. R. Rao, S. V. Kanth and B. U. Nair, J. Soc. Leather Technol. Chem.,

2008, 92(2), 65.24. S. Das Gupta, J. Soc. Leather Technol. Chem., 1977, 61(5), 97.25. S. Das Gupta, et al., Int. J. Biol. Macromol., 2007, 40(4), 351.26. Y. Li, et al., J. Soc. Leather Technol. Chem., 2006, 90(5), 214.27. Y. Wu and Q. Wu, J. Soc. Print Dye., 1990, 54(4), 11.28. A. D. Covington and C. S. Evans, Proc. IULTCS Congress, Cancun,

Mexico, 2003, pp. 59–65.29. E. Heidemann and L. Tonigold, Das Leder, 1985, 36(10), 170.30. W. C. Prentiss, et al., J. Amer. Leather Chem. Assoc., 1982, 77(2), 84.31. B. Vulliermet and G. Gavend, Proc. IULTCS Congress, Melbourne,

Australia, 1987, pp. 73–84.32. R. Haran and M. Gervais-Lugan, Rev. Tech. Ind. Cuir, 1989, 8901, 28.33. G. J. Ward, J. Amer. Leather Chem. Assoc., 1995, 95(5), 142.34. R. Zauns and P. Kuhm, J. Amer. Leather Chem. Assoc., 1995, 90(6), 177.35. M. Siegler, J. Amer. Leather Chem. Assoc., 1987, 82(5), 117.36. A. Puntner, J. Amer. Leather Chem. Assoc., 1995, 90(7), 206.37. S. Wren and M. Saddington, J. Amer. Leather Chem. Assoc., 1995,

90(5), 146.38. A. D. Covington and S. Ma, UK Patent, 2,287,953, June 1995.

346 Chapter 14

39. K. H. Fuchs, et al., J. Amer. Leather Chem. Assoc., 1995, 90(6), 164.40. K. H. Munz, J. Amer. Leather Chem. Assoc., 2003, 98(5), 159.41. K. H. Munz and R. Sonnleitner, J. Amer. Leather Chem. Assoc., 2005,

100(2), 66.42. N. F. Lee and C. R. Jacklin, Proc. IULTCS Congress, Melbourne, Aus-

tralia, 1987, pp. 384–386.43. S. Clara, et al., J. Soc. Leather Technol. Chem., 2005, 89(5), 186.44. S. Clara, et al., J. Soc. Leather Technol., Chem., 2006, 90(3), 93.45. H. Traubel, J. Amer. Leather Chem. Assoc., 2005, 100(7), 304.46. N. A. Evans and K. C. Montgomery, Proc. IULTCS Congress, Melbourne,

Australia, 1987, pp. 292–301.

347Other Tannages

CHAPTER 15

Post Tanning

15.1 DEFINITION

The term ‘post tanning’ refers to the wet processing steps that follow theprimary tanning reaction. This might refer to following tannage with chro-mium(III), as is usually the case in industry, but equally it applies to vegetabletanning or indeed any other tannage used to confer the primary stabilisation topelt. The combination of post tanning processes may not always be the samefor all tannages: the choice of post tanning processes depends on the primarytannage and the type of leather the tanner is attempting to make. In all cases,post tanning can be separated into three generic processes:

1. Retanning: This may be a single chemical process or may be a combi-nation of reactions applied together or more usually consecutively. Thepurpose is to modify the properties and performance of the leather. Thesechanges include the handle, the chemical and hydrothermal stability orthe appearance of the leather. The effects are dependent on both theprimary tanning chemistry and the retanning reactions.Retanning can involve many different types of chemical reactions.

These include mineral tanning with metal salts [including chromium(III)applied to chrome tanned leather], aldehydic reagents, hydrogen bondablepolymers, electrostatic reactions with polymers or resins or any other typeof synthetic tanning agent (syntan).

2. Dyeing: This is the colouring step. Almost any colour can be struck onany type of leather, despite the background colour, although the finaleffect is influenced by the previous processes.Colouring almost invariably means dyeing. Applying dye in solution or

pigment, to confer dense, opaque colour, can be performed in the drum orcolouring agents may be sprayed or spread by hand (padding) onto thesurface of the leather.




348

3. Fatliquoring: This step is primarily applied to prevent fibre sticking whenthe leather is dried after completion of the wet processes. A secondaryeffect is to control the degree of softness conferred to the leather. One ofthe consequences of lubrication is an effect on the strength of the leather(Chapter 17).Fatliquoring is usually conducted with self-emulsifying, partially sul-

fated or sulfonated (sulfited) oils, which might be animal, vegetable,mineral or synthetic. This step might also include processing to confer tothe leather a required degree of water resistance.

15.2 RELATIONSHIP BETWEEN TANNING AND POST TANNING

In the whole of leather making, each step in the process affects all subsequentsteps and none more so than the impact of tanning on the post tanning reac-tions. Table 15.1 illustrates the nature of tanning reactions and their possibleeffects by the following types of reaction.Modification of collagen by the chemistry of the tanning agent(s) affects the

following features of the properties of the new material:

1. The hydrophilic–hydrophobic balance of the leather is markedly affectedby the chemistry of the tanning agent, because it is likely to change therelationship between the leather and the solvent, which in turn will affectthe equilibrium of any reagent between the solvent and the substrate.

2. The site of reaction between the reagent and the collagen will affect theisoelectric point of the collagen and consequently there will be a differentrelationship between pH and charge on the leather. The lower the iso-electric point, the more anionic or less cationic the charge on the pelt willbe at any pH value: the higher the isoelectric point, the more cationic orless anionic the charge on the pelt will be at any pH value.

3. The relative reactions at the sidechains and the backbone of the proteinwill determine the type of reaction and hence the degree of stability of thetannage: the fastness of the reagent will influence the interaction betweenreagents and the substrate. Here, the important factors are the effects offree tanning agent and charge on the leather: reactions by covalency,electrostatic interaction, hydrophobic interaction and hydrogen bondingwill all have effects on leather properties.

The nature of the tanning agent will impact differently on the differentaspects of post tanning:

1. Retanning may involve interaction with the first tanning agent or it maybe independent, because the chemistries are incompatible or the reactionssites may be different.

2. Dyeing outcome depends on the chemistry of the dye and the chemistry ofthe substrate. They may work together, as is the case of metal mordanting,or colouring may be adversely affected by similarity of chemistry, such as

349Post Tanning

Table

15.1

Categorisationoftanningreactions.

Tannage

IEP

HHB

Charge

Reactionsites

Bonding

Tanningagent

Leather

Mineral

Higher

Hydrophilic

Hydrophilic

Cationic

CO

2�

Electrovalent

Chromium( III)

Higher

Hydrophilic

Hydrophobic

Cationic

CO

2�

Covalent

Aldehydic

Lower

Hydrophilic

Hydrophilic

Neutral

NH

31

Covalent

Syntan

Lower

Hydrophilic/H

ydrophobic

Hydrophilic

Neutral

NH

31,amide

Electrovalent,H-bonding

Vegetable

Lower

Hydrophobic

Hydrophilic

Neutral

NH

31,amide

H-bonding,hydrophobic

Oil

Nochange

Hydrophobic

Hydrophilic

Nochange

None

None,

hydrophobic

350 Chapter 15

syntan tanning, or the colour may be ‘saddened’. The HHB (hydrophilic–hydrophobic balance) properties of the substrate will influence the uptakeof dyes, because the dyes can exhibit the full range of HHB properties.

3. Fatliquoring has two aspects that can be influenced by the substrate.First, there is the question of depositing the neutral oil by damaging theemulsification mechanism. This is clearly affected by charge, but also bythe availability of charged sites on the leather. Second, there is the degreeof compatibility of the neutral oil with the HHB properties of the leather,modified by the particular chemistries of the previous process steps.

15.3 CHROME RETANNING

A common retannage for chrome tanned leather is more chrome tanning. Thisraises the question: why? It is not obvious why the leather should be rechromed,when the reaction could have been completed during primary tanning. Possiblereasons are as follows:

1. To increase the shrinkage temperature.Since rechroming is typically conducted with an offer of about 1% Cr2O3

(4% chrome tan powder) or less, i.e. half or less than the original offer,usually over a period of about an hour, i.e. an order of magnitude shorterthan the original tanning process, it can be understood that the effect ofretanning is going to be much less than the original reaction. Moreover, thetime allotted is unlikely to result in an effect through the cross section.Therefore, the retannage is more to do with an effect on the surfaces.It is a common experience that rechroming has little or no effect on the

shrinkage temperature of the final leather. This is understandable, becauseof the conditions and the resulting change in chrome content contributeslittle to the tanning effectiveness.

2. To increase the chromium content.For many applications of chrome tanned leather, the specification

typically includes the requirement for 4% Cr2O3 on dry weight, e.g.defence specifications. Note, this is a sort of quality assurance specifica-tion, because at that level of chrome the tannage is likely to be completeand the shrinkage temperature is likely to be 4100 1C. Note, too, there isno guarantee that those properties will be met – it is possible to fix thatamount of chrome and still obtain a shrinkage temperature considerablylower than 100 1C.From the conditions of retannage set out above, the fixation of

chrome is not going to be as effective as the primary reaction. Indeed, it ispossible to discharge more chrome in the effluent stream than is offered inthe retannage, by not fixing much chrome and by stripping chrome fromthe wet blue during retanning!

3. To even up the colour.When bovine wet blue is split, it is common to observe variations in

colour over the split surface. Whilst this may not adversely influence the

351Post Tanning

overall properties and performance of the leather, cosmetically it looksbetter to make the colour more even or uniform.Rechroming is often used if wet blue is purchased from different

sources, in an attempt to make the colour more uniform between batchesof leather.

4. To change the reactivity of the leather.Incorporating fresh chrome into aged wet blue has the effect of creating

new cationic sites, which might be useful in fixing anionic reagents later inthe post tanning process. More fundamentally, the isoelectric point (IEP)will be moved to a higher value, so that at any pH the charge on theleather is either less anionic (negative) or more cationic (positive) than theunretanned leather. This too will influence the reactivity towards the posttanning reagents, which are often anionic.Note, the effect of IEP is more important than the introduction of

cationic charges, because they can be discharged chemically, whereas theinfluence of IEP with pH on the charge of the leather is a permanent effect.

5. To modify the properties of the leather.Fixing chrome under conditions at the limits of basicity in solution

means that polymeric chrome is bound to collagen (Chapter 11). In thisway, a degree of filling and softening is achieved, without using reagentsthat might adversely affect light fastness or water resistance, etc. However,considering the amount of chrome likely to be fixed, the effect will besmall.

15.4 SEQUENCE OF POST TANNING STEPS

Whilst there are very many variations on the procedures within post tanningprocesses, in general they tend to conform to the following sequence: retanningthen dyeing then fatliquoring.The rationale can be expressed as follows:

� Retanning may not only modify the properties of the leather, but willtypically also modify the reactivity of the pelt towards other reagents.Even if the processes do not include specific reactions to assist uniformcolouring, it is possible to achieve such a side effect as a bonus. Alter-natively, if the retannage includes mineral reagents, they may also have amordanting effect on the dyeing step, to achieve modified colour or betterfixation (Chapter 16).

� Dyeing comes next, to take advantage of the changes to the reactivity ofthe pelt conferred by additional tanning. The relationship between dyeingand fatliquoring is controlled by the requirements of the dyeing com-pounds for reactivity towards the substrate. This reactivity is partlydependent on the HHB characteristics of the substrate, which can bepositively or negatively modified by the presence of hydrophobic speciessuch as fatliquors.

352 Chapter 15

� Fatliquoring usually comes last, in order not to interfere with the colouringreaction. In particular, if the lubrication step includes reactions to conferwater resistance, this can create a barrier to reaction of aqueous reagentswith the collagen, so it must be done just prior to drying.

15.5 PRINCIPLES OF POST TANNING

15.5.1 Mechanisms of Post Tanning

All the post tanning process steps involve the fixation of a solute in solutiononto a solid substrate. The reactions are made more complicated by the factthat the tanner is dealing with a substrate that has finite thickness. Therefore, tocontrol the outcomes of these steps, it is necessary to understand the para-meters that come into play. First, it is important to understand that there is ageneral mechanism that must be taken into account, which is the steps by whichfixation occurs; this was introduced in Chapter 11. The steps involved in anygeneral reaction in which a reagent in solution is fixed onto a solid substrate,e.g. in the heterogeneous system of post tanning, may be defined as follows:1,2

1. Transfer of the reagent from solution into the substrate;2. Hydrophobic bonding;3. Electrostatic interaction between the reagent and the substrate;4. Covalent reaction between the reagent and the substrate.

The first controlling factor can be expressed as follows:

reagentþ solventÐ ½reagent�solvated ð15:1Þ

½reagent�solvated þ ½substrate�solvated Ð ½substrate-reagent�solvated ð15:2Þ

favoured disfavoured

disfavoured! favoured

The position of the equilibrium for Equation (15.1) depends on the affinity ofthe reagent for the solvent. This is analogous to partitioning a solute betweentwo immiscible solvents, where the equilibrium constant of transfer is analo-gous to the partition coefficient (see below). In the case of water as the solvent,the equilibrium is defined by the degree of hydrophilicity or hydrophobicity ofthe solute/reagent. This has been defined as the hydrophilic–hydrophobic(HHB) or hydrophilic–lipophilic (HLB) balance. This property is con-ventionally measured by chromatography in various solvents, ranging fromwater to petroleum ether (also known as ‘light petroleum’), i.e. highly polar tohighly non-polar. Figure 15.1 shows the principle of liquid chromatography, inwhich the solvent travels upwards, taking the reagent with it. The fraction ofthe way from the initial position of the reagent spotted on the paper or thinlayer plate to the solvent front is referred to as the Rf value and this is a measure

353Post Tanning

of the affinity of the reagent for the solvent. The closer the Rf value approaches1.0, the higher the affinity of the reagent for the solvent.Transfer from the solvent to the substrate will depend on whether the reagent

is charged and whether the substrate is charged. This will affect the interactionwith the solvent, particularly if the solvent is polar, such as water. Therefore,the relative charging of the reactants will influence the relative affinities of thesolute for the solvent and for the substrate. This can be distinguished fromthe role of electrostatic attraction or repulsion, which determines the rate offixation, following transfer from solution into the substrate.It must follow that the equilibrium for Equation (15.2) can be altered by

changes to each component of the equilibrium:

1. Changing the HHB/HLB value of the reagent will alter its relative affinityfor the solvent versus the substrate. This might be done using the chem-istry of the reagent itself or by changing the reagent for one with a moreappropriate HHB/HLB value.

2. Changing the chemistry of the substrate will alter the relative affinity ofthe solute for the substrate versus the solvent. This might be achieved bymanipulating the chemistry of the substrate: possibilities include chemicalmodification and charge change. Alternatively, the HHB/HLB value ofthe substrate may be changed by applying hydrophilic or hydrophobicreagents, e.g. water resistance fatliquor.

3. Changing the solvent will alter the position of the first equilibrium, witha consequent change to the relative affinity of the solute for the substrate.This change can be in either direction, depending on the nature ofthe change made to the solvent. Water can be made more or less polar by

Rf = xY

Solvent front

Final position of reagent

y x

Initial position of reagent

solvent

Closed chromatography jar

Figure 15.1 Representation of the liquid chromatography method for measuring thehydrophilic–lipophilic balance.

354 Chapter 15

the presence of neutral electrolyte. Similarly, any other solvent can bemodified by the presence of other components, such as solutes or othersolvents. Ultimately, the solvent itself can be substituted.

While reviewing the principles of tanning and advanced aspects of tanning,and especially post tanning, examples of all these possibilities will be encoun-tered. An understanding of the variables in these reactions is a powerful tool forfuture developments in leather technology.The equilibrium for Equation (15.2) depends on the relative affinities of the

reagent/solute for the solvent and the environment within the substrate. That is,a hydrophilic reagent will tend to remain in solution, because it interactsfavourably with water. In contrast, a hydrophobic reagent will not be as solublein water and will tend to move into a more hydrophobic environment, intothe substrate. The degree to which this happens and the rate of transfer dependon the magnitude of the HHB/HLB value of the solute and the properties of thesolvent.The concept of transferring a solute is familiar to chemists in the form of

partitioning between two solvents in the technique of solvent extraction. Thenotion can be extended to the free energy of transfer between two solvents,which determines the degree of preferential solvation when a solute is dissolvedin a binary solvent.3

The step of hydrophobic bonding is essentially a special case for aqueoussolution. If the driving force for transfer is dominated by the hydrophilic natureof the reagent, the first mechanism of reaction may be hydrophobic bonding.This has been suggested as the route of plant polyphenol fixation (Chapter 13).Such bonding is likely to change to hydrogen bonding as its equivalent of theelectrostatic interaction.The concept of transfer from solvent to substrate should not be confused

with the notion of penetration through the cross section of the substrate.Tanners are constantly dealing with heterogeneous systems and balancing therate of penetration with the rate of fixation. This is consistent with the model ofstepwise reaction between a reagent and a substrate. Penetration is a con-sequence of favourable reagent–solvent interaction. The more favourable thisis, or the more soluble the reagent is in the solvent, the less favoured is transfer,so the reagent can penetrate through the substrate cross section. Fixation is aconsequence of favourable transfer into the environment of the substrate. Sincethe rate of transfer of a reagent is affected by the HHB/HLB value, the reagentproperties also affect the nature of the reaction. All reactions are dependentinitially on some form of charge-charge interaction. So, because electrostaticinteractions are fast, reaction between the substrate and the reagent is initiatedby the rate-determining step of transfer. Therefore, fast uptake means surfaceinteraction, rather than penetration into the cross section of the substrate. Thishas many implications in different processes in leather making, not only for thescience of reaction efficiency, but also for the technological outcome.The extent to which covalent reaction applies depends entirely on the chemistry

of the reaction and may not apply at all. All chemical reactions result in bonding

355Post Tanning

that lies somewhere on the scale between pure electrostatic and pure covalent. Fewlie at the extremes, but it is clear that most reactions can readily be designated asone or the other.

15.5.2 Role of the Isoelectric Point

The concept of an isoelectric point in proteins was introduced in Chapter 1,where it was shown that it could be defined as follows:

IEP ¼P

i fi½NH2�iPj fj½CO2H�j

There are two points, relating to isoelectric point and its particular relevanceto post tanning, that are very important and worth emphasising:

1. IEP is a point on the pH scale, so it does not change with changing pH ofthe system. The IEP of collagen is the same whether it is in the limed stateor in the pickled state. The importance of this point is that the isoelectricpoint can only be changed if there is a chemical change that alters theavailability of active groups.In the context of charge on leather, care must be taken in ascribing the

influence of charged reagents. For example, treating collagen with cationicchromium(III) species may be regarded as altering the IEP, because it mayintroduce positive charge. Whilst the introduction of charge is true,notably, the charge can be lost by treatments other than pH change. Fromthis point of view, the charge is immaterial as far as the IEP is concerned,although the binding of chromium(III) clearly impacts on the IEP, byremoving some carboxyl function from the IEP determining ratio.In the context of charge as a function of pH, the definition of IEP must

be applied correctly. The introduction of charged species into the sub-strate clearly can influence the overall charge and hence modify thereactivity of the substrate towards specified reagents. An example ispolycationic species as dye intensifying agents (Chapter 16).

2. The charge on collagen is determined by the relative values of the IEP andthe pH. If the pH is higher than the IEP, the collagen is negativelycharged, and if the pH is lower than the IEP, the collagen is positivelycharged. Moreover, the further the pH is from the IEP, the greater thecharge. The clear consequence is the effect on the affinity of reagents thatrely on charge for fixation to the leather. This can be modelled as shownin Figure 15.2, which also shows the effect of moving the isoelectric point.

From the model in Figure 15.2, it can be seen that the effects of moving theisoelectric point are as follows:

� Change to higher IEP: at any pH value the charge on the protein/leatheris either less negative or more positive, depending on which side of the IEPis considered.

356 Chapter 15

� Change to lower IEP: at any pH value the charge on the protein/leather iseither less positive or more negative, depending on which side of the IEP isconsidered.

The effects of charge on an electrostatic reaction can be understood from thesimple relationship:

rate / ½reagent charge�a½substrate charge�b

where the rate refers to the rate of the fixation reaction, which will be controlledby the magnitudes of the charges, because they determine the degree ofattraction. Clearly, if the charges are different, the affinity is favoured: thisresults in fast uptake, with a limitation on the degree to which the reagent canpenetrate the substrate. In contrast, if the charges are the same, the affinity isdisfavoured: the reaction is slowed, because of the availability of reaction sites,so penetration is favoured. The measurement of the magnitude of the charge onleather is not easy. Attempts have been made to use charged dyes to determinecharge interactions, but not with much success. In the context of processing, thebest we can do currently is to estimate the overall effect of the accumulation ofchange. In native collagen, charge as a function of pH will reflect the swellingcurve, since there is some cause and effect. The swelling is also influenced byosmosis and lyotropy, and may even be controlled by those mechanisms,depending on the specific conditions in solution.The direction of change in IEP is known for most leather-making process

steps or can be judged from the reaction if conflicting reactions occur, such as indyeing with premetallised dyes. However, in the absence of direct data on thechange in IEP, the change may be estimated from the nature of the reaction andbearing in mind the typical offers of the reagents. The starting point is the IEP

increasingly zero charge increasingly positive charge negative charge

0 14

lower IEP IEP higher IEP pH scale

increasing affinity ofanionic reagents anionic reagents = more efficient uptake = less efficient uptake = more surface reaction = better penetration = less surface uniformity = more surface uniformity

decreasing affinity of increasing affinity of cationic reagents cationic reagents = less efficient uptake = more efficient uptake = better penetration = more surface reaction = more surface uniformity = less surface uniformity

decreasing affinity of

Figure 15.2 Relationship between pH and charge, governed by the isoelectric point.

357Post Tanning

of raw collagen, the accepted change by liming and Gustavson’s measurementof the effect of chrome tanning.4 Table 15.2 gives estimates of the effects of posttanning. The numbers have to be estimates, because, although it can beaccepted that covalent reaction will have a certain effect on the value of IEP, itis not known how electrostatic reaction influences the value. This is because itmust be assumed that a pH change can have a reversing effect on the interactionand hence the group on the protein may still be counted in the calculation of theIEP ratio. In addition, the influence of introducing charge is also not known.Using the model of Table 15.2, the changes in isoelectric point might be esti-mated as shown in Table 15.3, starting with chrome tanned leather with IEP7.0, retanning with only one reagent, dyeing with only one type of dye, thenfatliquoring. Furthermore, the changes to IEP will depend on the amounts ofreagents bound to the leather: here the estimates are generalised, assumingtypical processing. It is assumed that the effects of reagents on IEP areaccumulative.The likely scenario in post tanning is that the IEP will gradually move down,

from pH7 to pH5–6. Therefore, at any given pH, the leather will be lesspositive or more negatively charged than it was before. In other words, theleather has less and less affinity for anionic reagents, so penetration of thesereagents is favoured. However, variations in this pattern must be recognised,especially when using premetallised dyes or if large offers are made in theprocess step.

Table 15.2 Known and estimated changes in the isoelectric point of type Icollagen due to leather making processing.

Process ReactionEstimated effect onIEP

None, native collagen Physiological pH7.4Liming Amide hydrolysis 5.0 to 6.0Chrome tanning Reaction with carboxylate 6.5 to 7.5Retanning reactions:Aldehyde Covalent bonding Estimate DIEP¼�1.0Vegetable tannin Electrostatic bonding Estimate DIEP¼�0.5Syntan Electrostatic bonding Estimate DIEP¼�0.5Resin No effect Estimate DIEP¼0Dye reactions:Acid dye Electrostatic bonding with

NH31

Estimate DIEP¼�0.5

1 : 1 Premetallised dye Metal complexation at car-boxyl, sulfonate reaction atamino

Estimate DIEP¼+0.5

Reactive dye Covalent reaction with NH2

Electrostatic reaction withNH3

1Estimate DIEP¼�1.0

Fatliquor, sulfo reaction atamino

Electrostatic bonding Estimate DIEP¼�0.5

358 Chapter 15

15.5.3 Role of the Peptide Link

When considering the properties of collagen, reactions at the peptide link areimportant, since the link is the most common feature of protein structure.Hence, its susceptibility to hydrolysis is a feature of beamhouse processing. Inthe case of post tanning processing, its role depends on its ability to engage infixation reactions. As described in Chapter 1, the bond can be drawn in twoways, showing that charge is separated between the amino and carbonylgroups. This means that the peptide link can engage in two types of bondingwith incoming reagents (Figure 15.3):

� Hydrogen bonding: the presence of a negative charge on the oxygen allowshydrogen binding with an active hydrogen bearing group.

� Electrostatic bonding: the presence of positive charge on the nitrogenallows electrostatic bonding with anionic species.

The versatility of the reactivity of proteins originates from the partiallycharged nature of the peptide group. Despite the limitations of the partial charge,the peptide links have a powerful effect, because their concentration is higherthan the other obvious reaction centres on the sidechains. The contents ofcarboxyl groups and amino groups in dry collagen are 1.0 and 0.6moles per kgdry weight and may be compared with the amount of peptide links as follows.The molecular weight of the triple helix is 300 000, containing 3000 amino

acids. Consequently, collagen contains ten amino acids per kilogram, equiva-lent to nine peptide links. Therefore, there is an order of magnitude morepeptide links than carboxyl groups and 15 times more than amine sidechains.

15.5.4 Role of the Sulfonate Group

A chemical theme that runs through the whole of the post tanning reactionsis the role of the sulfonate group. This theme is expanded in the individualsections below. Reagents that typically carry sulfonate groups include:

� syntans;� modified vegetable tannins;

Table 15.3 Estimated changes in isoelectric point, starting with chrome tan-ned leather at 7.0, then retanning, dyeing (1 : 1 premetallised dye,acid dye or reactive dye) and fatliquoring.

RetanAldehyde Syntan Vegetable Resin

6.0 6.5 6.5 7.0Dye1 : 1 Acid Reac. 1 : 1 Acid Reac. 1 : 1 Acid Reac. 1 : 1 Acid Reac.

6.5 5.5 5.0 7.0 6.0 6.5 7.0 6.0 5.5 7.5 6.5 6.0Fatliquor

6.0 5.0 4.5 6.5 5.5 6.0 6.5 5.5 5.0 7.0 6.0 5.5

359Post Tanning

� dyes: acid, basic, direct, premetallised, reactive;� fatliquors: sulfated fatliquors included.

In each case, the mechanism of fixation is the same. An electrostatic reactioncan take place between the anionic sulfonate group and the protonated aminogroup. However, this can only happen when the collagen is acidified, to createthe cationic group. Above the isoelectric point, there are no cationic centresfor reaction. At the isoelectric point, the cationic groups are locked togetherwith the anionic carboxyl groups: it is only when the salt links are broken byacidification that reaction with sulfonate can occur. The carboxyl groups arenot in competition with the sulfonate groups, because the weak carboxylicacid groups become protonated, as the mechanism of breaking the salt link.This is the reason that acid dyes are so-called, because they are fixed by acid.Figure 15.4 sets out the general reaction mechanism.The mechanism of sulfonate fixation has sometimes been wrongly rationa-

lised in terms of protonation of the sulfonate group, particularly as themechanism for lowering the emulsifying power of the sulfo fraction of fatli-quors (Chapter 17). However, it is important to recognise that the mechanismactually does not include protonation of the sulfonate group: it is much toostrong for that to happen under the aqueous conditions of leather making(Chapter 9). A practical demonstration of the strength of sulfonate group isto try to create a non-swelling acid from an auxiliary syntan by acidification,i.e. the reverse of the reaction to create a syntan from a sulfonated compound.The reaction can only be made to work in concentrated sulfuric acid, a solventthat cannot be regarded as aqueous.

15.5.5 Coordinating Post Tanning Processes

Creating a process for post tanning is not a case of merely putting together thesteps for applying the required reagents in the right order. The followingconsiderations must be addressed, applying to every reagent that the tannerintends to fix onto the leather:

1. Surface reaction or penetration?2. The required degree of penetration down the hierarchy of structure.3. The requirement for uniformity of reaction: through the cross section and

over the surfaces.

C

H

O

O R

NH-OSO3

R

NH

O

C

δ+

δ−δ−

δ+δ−

δ+

Figure 15.3 Illustrations of bonding at the peptide link.

360 Chapter 15

4. The amount and suitability of the reagent necessary to achieve therequirement.

5. The rate of reaction, i.e. the process time yielding optimum efficiencyand hence cost effectiveness.

6. The affinity of the reagent for the substrate and consequent fastnessproperties.

The specific nature of each reagent and its function must be considered, in thelight of the following features of post tanning processing:

1. Is this the best reagent for the purpose, using any criteria – cost, efficiency,safety, environmental impact, etc.?

2. Are there any options for ‘compact processing’, i.e. combining more thanone process step into a single step (Chapter 20)?

3. Do the process steps interact to create synergy, i.e. obtaining effectsgreater than the sum of the parts, e.g. one reagent influencing the fastnessof another or together producing a new interacting species on the sub-strate that contributes to the leather stability in some way?

The following variables must be considered, to achieve the required outcome:

1. Compatibility of the reagent for the substrate, i.e. the status of the sub-strate at any point in the process: it is not enough to consider the tannedstarting material, it is important to consider how previous processes havemodified the leather.

2. Compatibility of the reagent not only for the leather modified by previousprocesses, but also for subsequent reagents.

3. Role of isoelectric point: although the relationship between processingand quantitative change to IEP is not known with any precision, thedirection of change due to reagent reaction can be judged qualitatively.

4. Role of pH: The relationship between pH and the isoelectric pointdetermines the charge on the leather. This controls transfer of the reagentonto the leather and the initial electrostatic interaction.

P-CO2- H2N-P

H+no direct route

+ H3N-PP-CO2-

+H3N-PP-CO2H +H3N-P

OH-

RSO3-RSO3-

Figure 15.4 Mechanism of sulfonate fixation on collagen (P¼ protein).

361Post Tanning

5. Role of HHB/HLB value: The value for the leather will change as posttanning progresses. Whilst value changes may not be available, qualitativejudgements about the directions of change can be made.

15.6 COMPACT PROCESSING

The conventional components of post tanning are neutralisation, retanning,dyeing and fatliquoring: in each case, an industrial process will use one or moreagents of that type. However, in post tanning it is possible to combine processsteps, to create ‘compact’ processing.5 Here the principles of compact proces-sing will focus on simple combinations of agents. The permutations andcombinations are as follows, limited to a degree by the order in which theymight have to be applied:

� neutralise+retan� retan+dye� retan+fatliquor� dye+fatliquor� retan+dye+fatliquor

15.6.1 Neutralise and Retan

This is conventional technology, in which neutralising syntan can achieve bothrequired outcomes at the same time. The mechanism relies on enough basifyingpower to raise the pH to the required level and sufficient tanning power to lowerthe isoelectric point enough to ensure the pelt is adjusted to be anionic.

15.6.2 Retan and Dye

The concept for this process is to colour the leather, whilst conferring aretanning effect. Here, it might be assumed that the neutralising process will beseparate, as the chemistries of the processes are likely to be different. In posttanning the retanning process is usually applied to modify the leather proper-ties, especially the handle and the uniformity of handle over the hide or skin.However, the consequence for hydrothermal stability is typically relativelyunimportant, because that is taken care of by the chrome tanning.A commercial option available is the use of vegetable tannin, modified by

fixing dye to the polyphenol. This has been done by Forestal Quebracho. Therange of colours is limited and the presence of the vegetable tannins inevitablysaddens the colour struck on the leather. An alternative approach is to use thechemistry of melanin formation:6 using model reactant polyphenols and the aidof polyphenol oxidase, it is possible to achieve a tanning reaction and developcolour at the same time (Chapter 16).The chemistry of dyeing itself might be considered to be a retanning

reaction, depending on the type of dye, which determines the nature of thebinding interaction with leather, and the offer. The use of reactive dyes would

362 Chapter 15

clearly constitute a form of retanning, because they bond to basic sidechainsvia one or two covalent links. The effect will depend on the amount of reactivedye and the ability of the structure to interact with the aqueous supramolecularmatrix.

15.6.3 Retan and Fatliquor

The concept in this combination is for the retanning reaction to lubricatethe fibre structure, to prevent fibre resticking during drying and to soften theleather. To a degree this can be considered conventional technology, becausepolymeric tanning agents often confer softness, e.g. vegetable tannins, repla-cement syntans, resins and acrylate polymers. However, in some cases thetanning effect is weak: synthetic resins and polymers can bind to the collagen ina useful way, but may contribute nothing to the stability of the leather. Somespecialist polymers, e.g. the water resistance acrylic esters, certainly confersoftness and their complexing reaction with bound chromium(III) might beconsidered to be a version of retanning.

15.6.4 Dye and Fatliquor

The concept of this combination is to colour the leather at the same time asintroducing lubrication. To date, there is no industrial product available to dothis. However, such a concept is not impossible. It might be accomplished byusing highly hydrophobic dyes (which do exist) and delivering them throughthe cross section by an appropriate emulsification system. The drawback is theuse of expensive dyestuff deep within the fibre structure, where its presence doesnot contribute to the perception of colour. However, such a combination mightbe useful in the contexts of grain layer or suede leather lubrication.If this type of combination is desirable, using conventional dyes and fatliquors

together, the compatibility of the components must be considered. One way oflooking at the problem is to consider the impact of one reagent on the other:

current operation: ! compact processing: ! alternative processing :dye then fatliquor! dye with fatliquor! fatliquor then dye

Looking at the problem in this way highlights the effect of hydrophilic–hydrophobic balance (HHB) on the affinity of the dye for the substrate. Anychange from current processing will tend to make the substrate more hydro-phobic than before, therefore changing the outcome of dyeing. Therefore, ifcompacting means merely combining the two steps of dyeing and fatliquoring,it is clear that the dyes will have to be changed to meet the new requirements ofthe substrate.

15.6.5 Retan, Dye and Fatliquor

The concept of this combination is to conduct the whole of post tanning in asingle step. It is possible that all these requirements could be met in a single

363Post Tanning

chemical reagent. As a beginning, Gaidau et al.7 have described a compatiblemixture for aqueous processing.However, the concept does raise the question of the role of the solvent. If the

tanner could use a more exotic solvent, to solubilise simultaneously a widerrange of reagents than is possible in water, it might be feasible to apply all ofthem at once. A candidate solvent is liquid (supercritical) carbon dioxide.According to the choice of solvent, there may be no need for neutralisation.Alternatively, neutralisation may have to be retained as an aqueous process, toachieve the charge required by the new mixed process.

15.7 ROLE OF PROCESSING ON LEATHER PROPERTIES:

DYNAMIC MECHANICAL THERMAL ANALYSIS

Although the basic properties of leather are created by the earlier processes inthe beamhouse and in tannage, the properties are considerably modified by thelatter processes of retanning and fatliquoring. Collagenous materials are vis-coelastic:8 that means they exhibit viscous properties, when they resist shearforces, and elastic properties, when they return to the original state when astress is released. These properties can be measured by dynamic mechanicalthermal analysis (DMTA) (Figure 15.5), which is introduced in Chapter 13.

Figure 15.5 Working head of DMTA apparatus, showing the sample locked at eachend and the oscillating mechanism operating in the middle.

364 Chapter 15

The technique is based on measuring the way in which the response or strainof a material lags behind a deforming stress: the modulus of the material, l, ismeasured as the conditions change:

l ¼ stress=strain ¼ ðforce per unit areaÞ=ðratio of change to original stateÞ

When the stress in sinusoidal, the lag phase, d, will lie between 01 for a perfectlyelastic material and 901 for a perfectly viscous material. The time lag betweenthe displacement and response, the damping effect, is measured as tan d. If theamplitude of the applied stress is s0 and the amplitude of the subsequent strainis e0, the storage modulus, E 0, is calculated as follows:

E0 ¼ ðs0=e0Þ cos d

The loss modulus, E00, a measure of the irrecoverable energy due to internalmolecular motion is calculated as follows:

E00 ¼ ðs0=e0Þ sin d

tan d ¼ E00=E0

Proteinaceous materials can exhibit glass transitions, when there is a changein physical properties at a particular temperature: above the Tg, the materialhas liquid/rubbery properties and consequently the molecules have highmobility; below Tg, the material has amorphous/solid, glassy properties andconsequently the molecules have low mobility. Glass transitions can be detectedby a shift in the baseline of differential scanning calorimetry, when endothermicand exothermic changes in a sample can be measured as a function of tem-perature or heating rate (Figure 15.6).Collagen has been regarded as a semi-crystalline material due to the presence

of disordered regions, revealed by X-ray diffraction.9,10 For this reason, leathermay be expected to show thermal properties associated with both crystallineand amorphous materials. Some researchers regard the shrinkage temperatureas melting of the crystalline regions, whereas the glass transition temperatureis associated with amorphous region of leather fibres.11–13 Recently, Cot et al.have reported the presence of a glass transition in leather tanned with chro-mium(III) salt: it was found to be approximately 45 1C, for leather conditionedat 65% relative humidity at room temperature.14,15 Odlyha et al. have reportedtwo transitional events below the shrinkage temperature and they associatedthese transitions with relaxation of the polypeptide chains of collagen, con-sidering collagen as a block copolymer.16,17 The viscoelastic transition tem-peratures of leather, which may give practical information regarding physicalperformance, have typically not been measured and characterised.Figure 15.7 displays a typical DMTA response of a leather conditioned

at 0% RH and shows three tan d peaks of interest, labelled a, b and g,associated with energy dissipation during stressing. Here, the b peak is assignedto the glass transition and the a peak is assigned to the shrinking relatedtransition.18

365Post Tanning

When a and b peak temperatures are plotted against wet shrinkage tem-perature obtained by DSC only the a peak temperatures show a correlationwith shrinking transition (Figure 15.8).Figure 15.9 presents the dynamic mechanical properties of chrome and

vegetable tanned calfskin leathers, compared with acetone-dried untanned

Onset 32°C

Midpoint 37°C

Acetone dried skin conditioned at 65% RH

mW

10

°C-30 -20 -10 0 10 20 50 60 70 80

^exothermic

Onset 50°C

Midpoint 56°C Acetone dried skin conditioned at 35% RH

30 40

Figure 15.6 DSC thermograms of acetone-dried skins, conditioned at 35% and 65%RH, heated at 20 1Cmin�1, showing the base line shift indicating theglass transition temperatures.

0.01

0.1

1

Tan

δ

Temperature (°C)

γ transition

β transition

α transition

-100 -50 0 50 100 150 200 250 300

Figure 15.7 Typical DMTA thermogram of a leather tanned with vegetable tannin,conditioned at 0% RH.

366 Chapter 15

calfskin. The progressive decrease of storage modulus from –100 to +100 1Cindicates a broad viscoelastic transitional region for all samples. However, themagnitude of the decrease in modulus is dependent on the tanning chemistryand corresponds to the regions where the b transition is observed. The tannedleathers show the b/glass transitions at a lower temperature than that of

R = 0.85

200

250

300α

−α

−tra

nsi

tio

n te

mp

erat

ure

(°C

)

Shrinkage temperature (°C)

60 70 80 90 100 110 120 130

Figure 15.8 Relationship between wet shrinkage temperature measured by DSCand the a-transition temperature measured by DMTA (samplesconditioned at 65% RH) for various leathers with different shrinkagetemperatures.

Figure 15.9 DMTA thermograms of chrome tanned skin, vegetable tanned skin anduntanned skin, all preconditioned at 0% RH.

367Post Tanning

untanned skin. This demonstrates that tanning molecules are acting as aplasticiser:

1. From Table 15.4, hydroxyproline analysis of leathers conditioned at 0%RH shows that the chrome tanned leather has 89% collagen content,whereas vegetable tanned leather contains 67% collagen. A greater pre-sence of vegetable tannins inside the collagen structure leads to greaterdepression of Tg.

2. Vegetable tannins are high molecular weight polyphenols and will givemultipoint reactive sites for hydrogen bonding and hydrophobic inter-action with the fibre. Thus, they are effective in reducing the chain rigidityof collagen by intermolecular hydrogen bonds.

3. Chromium(III) binds to carboxyl groups, which occur along the surface oftropocollagen molecules: as a result, the temperature at which segmentmotion of collagen molecules becomes thermally activated is increased.However, chromium complexes that form an interpenetrating networkbetween the collagen molecules act as ‘spacers’ and thus Tg is depressed tolower temperature.

Tanning agents are conventionally understood to preserve collagen byintroducing crosslinking between collagen molecules. Viscoelastic transitionswould then be expected to move to higher temperatures. Because this was notobserved for any of the leathers examined, it suggests that the mechanism oftanning is matrix or interpenetrating network formation around the collagenmolecules. This is in line with the latest thinking of the tanning mechanism,presented in Chapter 19.The identification of the glass transition temperatures of leathers has several

practical implications. This is especially so for post tanning, where it may besuggested that processing above the glass transition temperature will be bene-ficial. This is because fibres are much more flexible and hence the uptake andfixation of dyes, fatliquor and retanning chemicals will be optimised.

REFERENCES

1. A. D. Covington, J. Amer. Leather Chem. Assoc., 2001, 96(12), 467.2. A. D. Covington, J. Amer. Leather Chem. Assoc., 2008, 103(1), 7.

Table 15.4 Transition temperatures of tanned and untanned skin, conditionedat 0% RH.

TannageCollagen con-tent (%)

g Transitiontemperature(1C)

b Transition/glasstransition tempera-ture (1C)

b0 Transitiontemperature(1C)

None 93 �53� 7 81� 5 �Chromium(III) 89 �66� 4 38� 8 65� 3Vegetable 67 �78� 5 20� 3 �

368 Chapter 15

3. A. K. Covington and K. E. Newman, Pure and Appl. Chem., 1979, 51,2041.

4. K. H. Gustavson, J. Amer. Leather Chem. Assoc., 1952, 47(6), 425.5. A. G. Puntner, J. Amer. Leather Chem. Assoc., 1999, 94(3), 96.6. A. D. Covington, C. S. Evans, T. H. Lilley and O. Suparno, J. Amer.

Leather Chem. Assoc., 2005, 100(9), 336.7. C. Gaidau et al., Proc. IULTCS Conf., Istanbul, Turkey, 2006. p. 23.8. S. Jeyapalina, G. E. Attenburrow and A. D. Covington, J. Soc. Leather

Technol. Chem., 2007, 91(6), 236.9. T. J. Wess, A. P. Hammersley, L. Wess and A. Miller, J. Mol. Biol., 1998,

275, 255.10. D. J. S. Hulmes, T. J. Wess, D. J. Prockop and P. Fratzl, Biophys. J., 1995,

68, 1661.11. C. A. Miles and T. V. Burjanadze, Biophys J., 2001, 80, 1480.12. A. D. Covington, G. S. Lampard, R. A. Hancock and I. A. Ioannidis,

J. Amer. Leather Chem. Assoc., 1998, 93(4), 107.13. M. Komanowsky, J. Amer. Leather Chem. Assoc., 1991, 86(7), 269–279.14. J. Cot, et al., J. Amer. Leather Chem. Assoc., 2002, 97(11), 441.15. J. Cot, et al., J. Amer. Leather Chem. Assoc., 2003, 98(7), 279.16. M. Odlyha, et al., ICOM Committee for Conservation, 1999, 2, 702.17. M. Odlyha, et al., J. Therm. Anal. Cal., 2003, 71, 939.18. S. Jeyapalina, PhD Thesis, The University of Leicester, 2004.

369Post Tanning

CHAPTER 16

Dyeing

16.1 INTRODUCTION

Dyeing is one of the more important steps in leather making as it is usually thefirst property of the leather to be assessed by the consumer or customer. He orshe will make judgements in a glance: colour, depth of shade, uniformity.Therefore, it is critical that the science and hence the technology of colourationis well understood. The origin of colour and its perception was reviewed byMcLaren in a Procter Memorial Lecture for the SLTC.1 Randall has exploredcolour as it applied to leather.2 There, he reviewed the measurement of colourfrom surfaces, pointing out the difference between reflection from a relativelycontinuous surface, as is the case for grain leather, and the effects of reflectionfrom the tips or the fibre of suede, resulting in a directional effect of colourperception. The standard system for measuring colour defines the coordinatesin the CIELAB colour space: a* is the red-green component, b* is the blue-yellow component and L* is the black-white component from which theother parameters can be calculated, such as the hue angle and the chroma.These measurements are useful in comparing colour and for colour matching.Tysoe has reviewed the classification of dyes in the Colour Index and intro-duced the problem of colour matching for leather.3 Colour match prediction isrelatively straightforward when dealing with pigments, but when the dyestuffconfers transparent colour to leather, the outcome is dependent on thefollowing parameters:

� dyestuff chemistry and mechanism of fixation;� relative affinities of the dyestuff and the substrate;� nature of the substrate, including its colour;� illumination of the leather: the perceived colour depends on the light

source, an effect called ‘metamerism’.




370

The modern synthetic dyestuffs industry was initiated by the development ofmauveine by Perkin in 1856: the primary markets were textiles, but the leatherindustry eventually took advantage half a century later, especially when it wasrealised that bright deep shades could be achieved with the new tannage withchromium(III).All dyes work on the same principle. The colour is generated by the creation

of a system of delocalised electrons, made possible by chemically synthesisingmolecules that contain a system of conjugated double bonds, i.e. alternatedouble and single bonds (Figure 16.1).What this actually means is the p orbitals of the double bonds create a

molecular orbital over the whole carbon chain. The system can include otherdelocalised groups, such as benzene rings or naphthalene rings, etc., wheremolecular orbitals are already present above and below the plane of the ring.Note, dye structures are often presented in the form of the Kekule structures ofthe aromatic rings, merely to emphasise the concept of conjugation over themolecule.The groups with the delocalised electrons are called ‘chromophores’, because

they are the primary sources of colour. Often the chromophores are linked bythe azo group, N¼N, which can contribute to the delocalisation of electronsover the chromophore system. The diazotisation reaction to create theazo group is conducted as follows. Acidification of sodium nitrite producesnitrous acid:

NaNO2 þHCl Ð HO-N ¼ O

An aryl amine is reacted with nitrous acid under cold conditions, preferablyclose to 0 1C, to yield the diazonium salt:

Ar-NH2 þHONO Ð Ar-N ¼ Nþ Cl�

The diazonium salt then reacts with an active hydrogen on another arylcompound, to link the molecules via the diazo group:

Ar-N ¼ Nþ þAr-H Ð Ar-N ¼ N-ArþHþ

Many aromatic amines have been used to make the so-called azo dyes,but are now known to be toxic, particularly carcinogenic. Because there is apossibility of regenerating the amine from the azo derivative they are nowexcluded from this application, notably benzidine (Figure 16.2) and its

-C=C-C=C-C=C-C=C- ↔ =C-C=C-C=C-C=C-C ↔ C-C-C-C-C-C-C-C-

Figure 16.1 Formation of a molecular orbital from adjacent p orbitals.

371Dyeing

derivatives. Other banned reagents include 4-aminodiphenyl, toluidine,2-naphthylamine, dianisidine, cresidine, etc.The delocalisation of electrons can be modified by the presence of sub-

stituents on the aromatic chromophores: these are called ‘auxochromes’. Theyalter the distribution of electrons by electronegativity effects or hyperconju-gation. By changing the energy of the molecular orbital, the energy to excitean electron is changed and consequently the energy of the quantum releasedwhen the electron falls back to its ground state is changed. If the effect ofthe structural change is to cause the colour to move to longer wavelength itis called a ‘bathochromic shift’. Change to shorter wavelength is called a‘hypsochromic shift’.Groups that donate electrons to the aromatic ring system (nucleophiles)

include OH (phenolic), CH3, OCH3, NH2, NHR and NR2.Groups that accept electrons (nucleophilic) include NO2, COOH and SO3

�.Groups that contribute to the solubility of the dye include SO3

�, CO2�

and NH31.

In a leather context, the auxochromes react with the collagenic substrate invarious ways, to confer colour addition. In addition, the reaction can be con-sidered a form of tanning: any chemical species that binds to the substrate altersthe properties of the substrate, including contributing to the hydrothermalstability. This is not usually taken into account when considering the eventualoutcome of wet processing.Dyestuffs as supplied are typically not pure compounds. Owing to variations

in synthesis, they may be formulated with other dyes to remain on shade. Thiscan lead to problems in colour matching (see below). They are also typicallymarketed with a diluent. These can include sodium chloride, sodium carbonate,dextrin, sulfite cellulose, naphthalene sulfonate, sodium sulfite, etc. The reasonis technologically sound: to allow the tannery operative to weigh quantitiesin kilograms, with little associated error. If the undiluted dyestuff were to besupplied, the quantities required for a pack would be grams; such weighingswould have a large associated error, with consequent variations in dyeingconsistency. It is the same argument that applied to formulations of enzymes,discussed in Chapter 8.The following sections illustrate the structures of the different, commonly

used dye types. Each has its name, usually indicating the dye chemistry, thecolour and a designated number: each has a Chemical Index number, where it iscategorised by its structure. Not all the dyes given as examples are designatedparticularly for leather applications: the structures are illustrative of the

H2N NH2

benzidine

Figure 16.2 Benzidine.

372 Chapter 16

principles involved in creating dyes with different reaction characteristics anddifferent degrees of applicability to leather.4

16.2 ACID DYES

Acid dyes are so called because they are fixed under acid conditions (Chapter15). This class of dyes is the most commonly used in the leather industry,particularly for chrome tanned leather. Figure 16.3 illustrates the type ofstructure.The properties can be summarised as follows:

1. Relatively small, typically hydrophilic molecules;2. Used for penetrating dyeing, producing level shades;3. Anionically charged, therefore high affinity for cationic leather;4. Fixed by acidification, due to the presence of sulfonate groups (Chapter

15);5. They react predominantly through electrostatic reaction between their

sulfonate groups and the protonated amino groups of lysine;6. Secondary reaction is via hydrogen bonding through auxochrome groups;7. Some dyes may react with the bound chrome, using it as a mordant (see

below);8. Good fastness properties: less complex molecules offer fewer opportu-

nities for structural changes by free radical mechanisms, as discussed inthe context of condensed vegetable tannins;

9. Wide range of colours, offering bright deep shades.

16.3 BASIC DYES

The structures of basic dyes are essentially the same as the acid dyes, exceptthey carry a net positive charge from the cationically charged amino sub-stituents, even though they may also have anionic sites (Figure 16.4). Qua-ternary amino groups are less hydrophilic than sulfonate or carboxylategroups, so they are typically less water soluble than acid dyes.

NaSO3 N=N

HO

SO3NaNaSO3

HO NHCOCH3

N=N

Acid Orange 7 (CI 15510) Acid Red 1 (CI 18050)

Figure 16.3 Mono azo acid dyes.

373Dyeing

The properties can be summarised as follows:

1. They produce strong, brilliant colours (red, orange, yellow, green, blue,indigo, violet and black);

2. They tend to be relatively hydrophobic, because they contain fewersolubilizing groups than the acid dyes; consequently, they are oftensoluble in oils and non-aqueous solvents.

3. They tend to bronze, i.e. produce a metallic sheen. This is due to surfacereaction, when the dye molecules lie on top of one another, attracted byvan der Waal’s dispersion forces, allowing light to be reflected from thelayered structure. This is a consequence of hydrophobicity, when thedyes have greater affinity for themselves than for the aqueous solution orthe substrate.

4. Poor light fastness;5. Good perspiration fastness, because they are not displaced by elevated

pH;6. High affinity for anionic leather, e.g. vegetable tanned, anionic retanned,

acid dyed leathers. The latter property is exploited in ‘sandwich dyeing’:acid dye, then basic dye, possibly topped with more acid dye. The elec-trostatic attraction between the charged species creates deep shades, withgood rub fastness.

7. Precipitated by hard water and anionic reagents;8. Applied by mixing with acetic acid, then diluting with hot water;9. They react electrostatically through their protonated amino groups and

ionized carboxyl groups on collagen;10. Secondary reaction is by hydrogen bonding;11. Since they are typically more hydrophobic than acid dyes, some reaction

will be via hydrophobic bonding.

16.4 DIRECT DYES

Direct dyes have the same sort of structural features as the acid and basic dyes,but with higher molecular weight. Figure 16.5 shows that direct dyes haveobvious similarities to acid and basic dyes. There is an additional category ofdirect dyes, the developed direct dyes, which can be diazotised in situ andadditional aromatic chromophores added to the dye as a route to black.

H2N

H2N

NH2

NH2

N=NN=N

Figure 16.4 Disazo basic dye: Manchester Brown or Bismarck Brown.

374 Chapter 16

The properties of this type of dye can be summarised as follows:

1. They are larger molecules than typical acid or basic dyes.2. Used for surface dyeing, with consequent likelihood of uneven colouring.3. Acid is not needed for fixation, because they are more reactive, due to the

higher number of reactive sites on the molecule and the hydrophobicity ofthe overall structure.

4. Fastness properties average to good;5. Usually dark colours;6. Have the same sort of structures as acid and basic dyes, although typically

with lower charge, resulting in lesser importance of electrostatic bonding.7. High molecular weight means more direct reaction, not requiring fixation

by pH adjustment.8. Relies more on hydrogen bonding from larger number of auxochromes

per molecule, similar to the relative astringencies of vegetable tanningagents, and more emphasis on hydrophobic bonding.

16.5 MORDANT DYES

The original mordant dyes were typically plant extracts, which producedrelatively dull and pale shades and fixed poorly to textiles when used alone: tofix the colouring agent to a substrate, it was necessary to provide an additionalfixing mechanism, by applying metal salt either before or with the dye. Modern

NaO3s

NH2

N=N

OH

Direct Red 118 (CI 17780)

N=NN=N

NaO3S

OH

NHPh

OMe

MeSO3Na

Me

Me

Direct Violet 51 (CI 27905)

NHCO

Figure 16.5 Direct dyes.

375Dyeing

mordant dyes are similar to acid dyes, but usually with less anionic charge(Figure 16.6). They generally have poor affinity for collagen, but rely on thepresence of a metal ion with which they can complex, so that the metal acts asthe link between the leather and the mordant dye.Suitable metal salts include chromium(III), which of course may already be

bound to collagen. Other metals that can be used are aluminium(III) andiron(III), etc. The formation of metal complexes means that the final colourstruck by the dye depends on the metal mordant. This is illustrated by alizarin(Figure 16.7 and Table 16.1).The technology is to apply the mordant to the leather, then treat with the

dye. A common option used to be to treat the leather with a chromium(VI) salt,usually as dichromate, and reduce it to chromium(III) in situ, in a process

N=NN= N

OHOHHO

OH

NaO3S

H2 N

NH2

ClNaO3S SO3Na

Mordant Brown 13 Mordant Blue 13

.. ..

.. .. ..

..

.. .... ..

Figure 16.6 Mordant dye structures: lone pair sites for possible complexation areindicated.

O

O

OH

OH

Figure 16.7 Structure of alizarin, mordant dye.

Table 16.1 Effect of metal mordant on thecolouring of alizarin dyeing.

Metal ion Colour

Al(III) RedSn(IV) PinkFe(III) BrownCr(III) Puce-brownCu(II) Yellow-brown

376 Chapter 16

analogous to two-bath chrome tanning. This is no longer used in the leatherindustry for two reasons:

1. The environmental impact of Cr(VI) is too high and discharges are strictlyregulated.

2. The availability of premetallised dyes has simplified the process (seebelow).

The mechanisms of fixation are the same as for acid dyes, with the additionalmechanism of covalent complexation:

� They have low affinity for collagen, typically having fewer sulfonategroups, particularly if they are natural dyes, and few hydrogen bondinggroups.

� Reliance on creating complexes with metal ions (mordants) previouslyfixed to collagen.

� Binding to mordant metals will vary in the degree of electrostatic andcovalent character, depending on the metal, e.g. Al(III), Cr(III).

16.6 PREMETALLISED DYES

16.6.1 1 : 1 Premetallised Dyes

The concept of premetallising dyes is to avoid the two-step process ofmordanting then dyeing, by preparing the complex of dye and metal salt inadvance. In this way, this is an example of a compact process, a conceptintroduced in Chapter 14. Figure 16.8 illustrates this principle, with a chro-mium(III) complexed dye, showing how the availability of lone pairs of elec-trons can be used to create dative bonds in a complex. Other metal ions that areused in premetallised dyes include cobalt, iron and copper.

-O3S

O

N= N

O SO3-

CrH2O

H2O

OH2

-

Na+

Figure 16.8 A 1 : 1 chromium(III) premetallised dye, Acid Blue 158 (CI 14880).

377Dyeing

The mechanisms of fixation are the same as summarised for mordant dyes.The dyes exhibit the following properties:

1. They have lower anionic charge than the corresponding anionic uncom-plexed dye.

2. Penetration and levelness of colouring are good;3. They include pale, dull and pastel shades.4. Fastness properties are good to very good;5. They tend to be expensive, so are used for premium leathers, e.g. gloving,

clothing, suede, nubuck, aniline.6. They may be formed from either mordant dyes or conventional acid dyes

precomplexed to a mordant metal ion, in the ratio of one dye molecule toone atom of metal.

7. Primary fixation mechanism is through reaction between the metal ionand collagen carboxyl groups, which can vary in covalent character.

8. Secondary reactions may be electrostatic and hydrogen bonding,depending on whether the dye is derived from mordant or acid dyeprecursor.

16.6.2 1 : 2 Premetallised Dyes

From Figure 16.8, further complexation can, obviously, occur in 1 : 1complexes, because there are complexing sites remaining on the metal ion(Figure 16.9). Note, the second complexing dye species does not have to bethe same as the first. They are referred to either as 1 : 2 or as 2 : 1 premetalliseddyes.

O

N=N

O

Co

-

Na+O2N

NO2

O

N=N

O

Figure 16.9 A 1 : 2 premetallised dye: Perlon Fast Violet BT (CI 12196).

378 Chapter 16

The difference between 1 : 1 and 1 : 2 premetallised dyes is that the fullycoordinated dyes are more anionic and can exhibit a wider range of reactionproperties. For leather applications, there are three designated types. Thesedesignations and their consequences for reaction can be understood from theHHB/HLB [hydrophilic–hydrophobic (HHB) or hydrophilic–lipophilic (HLB)balance] mechanism of reactions, set out above.Type 1: Water insoluble, soluble in alcohols, glycols, etc. Used for spray

dyeing and finishing aniline and semi-aniline leathers. High covering power,with high tinctoral power. Good fastness properties to water spotting, wet rub,light.Type 2: Low water solubility, more soluble in non-aqueous solvents. Fastness

properties inferior to Type 1.Type 3: Higher water solubility. Used for dyeing woolskins; to colour the

leather but not the wool.In addition:

� Composition is similar to the 1 : 1 complexes, except for the ratio of dye tometal, therefore reaction between the metal and collagen is not part of thefixation mechanism.

� Fixation is less dependent on electrostatic and hydrogen bonding than aciddyes and 1 : 1 complexes.

� The fixation mechanism is similar to direct dyes, therefore there arehydrophobic interactions.

� They may function more like pigments than dyes, with high coveringpower.

16.7 REACTIVE DYES

Reactive dyes are typically acid dyes that have been covalently bound to areactive group, capable of reacting covalently with collagen or leather. Theprinciple behind this technology is that a covalently bound dye will be fast tochemical removal. This is especially useful in applications when the leather maybe subjected to washing, dry cleaning or perspiration damage, e.g. clothing andespecially gloving leathers. The original dyes in this class were called Procion,based on triazine chemistry, developed by ICI, and others followed, illustratedin Figure 16.10, where the originating company is indicated. In Figure 16.10shows other examples of nitrogen heterocycle chemistries, but the principles arethe same. Note, the nitrogen heterocycle may contain a second dye species thatis not identical to the first:

1. The heterocyclic ring system can react with available amino groups oncollagen or leather.

2. The reaction is conducted under alkaline conditions, to ensure the avail-ability of uncharged amino groups, by reacting with the acid produced bythe reaction.

379Dyeing

3. The reaction is driven by the addition of sodium chloride. This is anexample of exploiting the stepwise mechanism of binding a solute to asubstrate. Changing the affinity of the solute for the solvent, by makingthe solvent more hydrophilic due to the charged ionic content, decreasesits affinity for the dye molecules and drives them into a more hydrophobicsubstrate.

4. There is competition between the reaction sites on the substrate and thehydrolysing effect of the solvent. The reactive dye is hydrolysed by waterto the hydroxy derivative: the degree to which this happens depends onthe specific chemistry of the reactive dye, but it always happens to someextent. In circumstances like this, it is important to remember that waterusually has a competitive advantage over other reactants, because it ispresent at a concentration of 55.5 molal. Because the resulting aromatichydroxy group is much less labile than the chloride, the reactive part ofthe molecule essentially becomes deactivated. In effect, hydrolysis turnsthe reactive dye back to a conventional acid dye.

5. The tanner is then faced with a dilemma. Fixing the acid dye under typicalconditions for that type of dye will maximise the depth of shade from thedyeing, but the fastness properties will be compromised. Removing theacid dye will reduce the depth of shade, but the fastness properties willbe maximised. This is a commercial decision.

An alternative approach was later developed by Hoechst, namely, the Remazolseries of dyes based on vinyl sulfone chemistry (Figure 16.11).

The principle of the mechanism of fixation is the production of the vinylsulfone group in situ, because sulfate is a good leaving group. The presence ofthe sulfone group also activates the vinyl group, allowing it to react with other

N

NN

Dye

2,4,5-trichloropyrimidyl 2,3-dichloroquinoxaline(ex Ciba-Geigy, Sandoz) (Levafix ex Bayer)

ClCl

F

F

Cl

Dye-CO

Cl Cl

Cl

Dye

N

N Dye

N

N

Cl

Cl

Dye

N

NCl

2,4-dichloropyrimidyl 2,4-difluoropyrimidylDichlorotriazine(Procion ex ICI) (ex ICI) (Drimalan F ex Sandoz)

N

N

Figure 16.10 Reactive dye chemistry based on aromatic nitrogen heterocycles.

380 Chapter 16

groups that have an active hydrogen, e.g. hydroxyl, amino, etc. Figure 16.11also shows another approach – the formation of an ethylene imine derivative:the heterocycle ring is readily opened to react with groups with an activehydrogen. In all cases, hydrolysis is still a competitive reaction.Figure 16.12 illustrates an alternative approach, using the acryloyl group.

The presence of the carbonyl group activates the system in the same way as thesulfone group does in the same position in the molecule. The acryloyl groupmay be present from the start or it can be created from the sulfate derivative(Figure 16.12).Within the variations of chemistries of reactive dyes, some properties are

common:

� very good fastness to washing, dry cleaning, perspiration;� good light fastness;� limited range of colours: pale and medium shades;� expensive;� health hazard, due to their reactivity towards organic substrates, therefore

stricter COSHH regulations (Control of Substances Hazardous to Health)apply to their use.

Dye-SO2CH2CH2-OSO3Na Dye-SO2CH=CH2

Remazol ex Hoechst

RH Dye-SO2CH2CH2-R

Dye-SO2CH2CH2-N-CH2CH2OSO3Na Dye-SO2CH=CH2

Dye-SO2-NH-CH2CH2OSO3Na Dye-SO2-NCH2

CH2

N-methyl taurine ethylsulfone

heat, pH5

Levafix ex Bayer

CH3

CH3

Figure 16.11 Reactive dye chemistry based on vinyl sulfones.

N N

N

CH2=CHCO COCH=CH2

COCH=CH2

Dye-NHCOC=CH2

Br

triacryloylhexahydro-s-triazine α-bromo-acrylamide

Dye-NHCOCH2CH2OSO3Na Dye-NHCOCH=CH2

NaOH

Primazin ex BASF

Figure 16.12 Reactive dye chemistry based on acryloyl derivatives.

381Dyeing

It is not necessary to rely on a single chemistry in reactive dyes. Figure 16.13gives examples of mixed fixing systems.

16.8 SULFUR DYES

Clearly from the descriptions of the dye types presented above, the dyes dis-cussed so far are all chemically related. Sulfur dyes originate from 1893; theyform a class of colourants that is different in chemistry and fixation mechanism.Like syntans, the structures of sulfur dyes are so complicated, because of theway they are prepared, that the structures are largely unknown. Consistency ofcolour therefore depends on consistency of production conditions.Sulfur dyes are made by heating together aromatic compounds containing

amino and hydroxy groups with a source of sulfur, in the following ways:

� Sulfur bake: the organic compound is heated with elemental sulfur to160–320 1C.

� As for sulfur bake, but using polysulfide salt.� Polysulfide melt: the reagents are heated with water, under reflux or in a

pressure vessel.� Solvent thionation or melt: as for polysulfide melt, but the solvent is

butanol, dioxitol, etc.

Excess sodium bisulfite may be used to incorporate thiosulfate groups,e.g. -S2O3

�, to enhance water solubility. In this way the preparation is cheap,although somewhat variable. The product often has low water insolubility, dueto its structure: Figure 16.14 illustrates a few of the identified chromophores,typically linked by disulfide bonds, as already encountered in cystine.

Cl

Dye

N

N

N

Dye

N N

N

F

NH- -SO2CH2CH2OSO3Na

NH-(CH2)nSO2CH=CH2

Sumifix Supra

Cibacron C

Figure 16.13 Reactive dye chemistry based on combinations of fixing mechanisms.

382 Chapter 16

In the technology of applying these dyes, they must be solubilised. This canbe achieved using the chemistry already familiar in sulfide-based hair burning.The effect of sulfide ion, a reducing nucleophile, on the insoluble dye is to breakthe disulfide links, creating the soluble, leuco (white) form. Subsequent oxi-dation reforms the insoluble dye on and in the substrate leather:

Dye-S-S-DyeðinsolubleÞÐS2�

½O�2Dye-S�ðsoluble; leuco formÞ

The properties of sulfur dyes can be summarised as follows:

� They are suitable only for leathers that can resist the high pH necessaryfor dye reaction, e.g. aldehyde or oil tanned. Note, chromium(III) andvegetable tanned leathers are severely damaged at pH 12–13: in each casethe tanning is reversed.

� Good perspiration and wash fastness; insoluble in dry cleaning solvents.� Generally dull shades, with a limited range of colours – a true red shade is

not available.� They have little or no affinity for wool.� They exhibit little reactivity of the kinds set out for the other dyes;

therefore, there is primary reliance on hydrophobic interactions.

16.9 DYE REACTIVITY AND FIXATION

Dye fixation is subject to the principles of reaction from solution onto a sub-strate, as set out in Chapter 15. The reactions are dependent on the followingconditions:

1. Status of the substrate, including the impact of prior processes;2. pH of the substrate and solution;3. HHB/HLB values of the dyes.

The trichromatic theory of colour is based on the observation that all colours oflight can be created by a mixture of red, blue and yellow lights. By analogy, it iscommon for tanners to use a mixture of dyes, particularly for brown. However,if there are differences in the chemical properties of the components of the

N CH

HC S

CH

N

S O

S

S

thiazole thiazone thianthrene

Figure 16.14 Chromophores in sulfur dyes.

383Dyeing

mixture, they will react independently. This often happens if the HHB/HLBvalues are not matched, so that the surface reactions and penetration will not bethe same, so the cross section may be a different colour to the surface, leading toproblems if the leather is scuffed or cut surfaces are visible in leather articles.This problem has been addressed in the Sellaset dyes, where the HHB values ofthe dyes are matched and the mixture behaves as though it were a single dye.For comparison, Table 16.2 summarises dye fixation mechanisms.

16.10 ROLE OF THE SUBSTRATE

16.10.1 Chrome Tanned Leather

One of the many advantages of chrome tanning is the minimum change to thecollagen structure, particularly with regard to occupation of the sidechainsand lack of inhibition of consequent reactions. There is a degree of cationiccharacter, which is useful in fixing acid dyes. Also, the colour conferred to theleather is pale, hence there is little background colour to interfere with thecolour struck by dyes.The one area in which chrome has a major impact on dyeing is when there is

too much surface reaction. Chapter 11 shows that chrome fixes to a smalldegree at the peptide links. Ordinarily, the small amount and uniform dis-tribution do not affect dyeing, but at high concentration it does interfere. Thiscan be at two levels of effect:

1. If the effect is shadowing, causing uneven colour, dyeing is likely to beuneven, due to the variation in base colour of the substrate.

2. If the effect is staining, reaction sites for dye fixation can be all taken up bythe chrome, resulting in dye resist. Therefore, the fault of staining cannotbe covered up by dyeing.

In each case, the fault cannot be remedied. The commonly used tannery optionof ‘put it into black’ may work for chrome shadowing, but does not work forchrome staining. There the only solution is pigment colouring in finishing.

Table 16.2 Dye fixation mechanisms: ||= primary mechanism, |=sec-ondary mechanism.

Dye type Electrostatic Covalent H-bonded Hydrophobic

Acid || |Basic | | |Mordant | |Direct | ||1 : 1 Premetallized | ||1 : 2 Premetallized | | |Reactive | || |Sulfur ||

384 Chapter 16

Chrome tanning is very lightfast, so any effect of light on the leather is due tothe light fastness of the dyes themselves.

16.10.2 Vegetable Tanned Leather

The chemical basis of vegetable tanning is completely different to chrometanning. The tanning agent is usually present at much higher concentrationwithin the leather, it interacts with both charged sidechains and peptide linksand confers colour. In this way, all vegetable tannins block most of the dyefixation sites and produce colour themselves, which can be intense.The consequences are as follows:

� The dye will have to bind to the polyphenol molecules, at least in part, sothe colour will be affected.

� The base colour will usually affect the overall colour of the dyes leather:this can vary from very pale cream, in the case of tara, to the very darkbrown, in the case of chestnut.

� The overall outcome is a marked dulling of the shade in comparison withthe effect on chrome tanned leather: this is referred to as ‘saddening’ thecolour.

� Plant polyphenols may be susceptible to light damage, particularly thecondensed tannins, which are likely to redden, although occasionallybleaching is observed with hydrolysable tannins such as tara. Change inbase colour after dyeing will affect the perceived colour of the dyed leather.

16.10.3 Other Tannages

Syntan tannage will have a similar effect to vegetable tanning, especially if areplacement syntan is used. Aldehydic tannage deprives the substrate of sitesfor reaction by dyes relying on sulfonate fixation.

16.11 DYEING AUXILIARIES: LEVELLING AND PENETRATING

AGENTS

16.11.1 Anionic Auxiliaries

This class of levelling agents is based on low molecular weight, aromaticsulfonate condensates (Chapter 14): some structures are illustrated inFigure 16.13. They function by competing with dyestuff for binding sites oroccupy those sites by prior treatment. In either case, if the kinetics might beexpressed as follows:

Rate ¼ k½dye�a½collagen sites�b

Therefore, any reduction in availability of reaction sites reduces the rate ofinteraction with substrate, making it more uniform. Note, the presence of theauxiliary may also modify the HHB/HLB and the charge of the substrate,

385Dyeing

to contribute to the effect. Controlling dye concentration is an option, likevegetable tanning, but it is not used as a technological strategy.In Figure 16.15, structure 1 is a naphthalene-2-sulfonate condensate: for

levelling, the degree of polymerisation may be 2–10. Modifying the structure ofthe condensate by using an alternative crosslinker, such as a sulfone derivative,improves the light fastness of the auxiliary, as illustrated in structure 3.An alternative approach is to use a byproduct of the Kraft process for paper

making, as introduced in Chapter 14. Since the structure of lignin, which isthe basis of the material, is a non-uniform polymer, the structure can only beillustrated in the notional form presented in Figure 16.16. The similarity to the

CH2

SO3Na

SO3Na

CH2

SO3Na

CH2

CH2SO3Na

CH2SO3Na

CH2SO3Na

SO3Na

CH2

CH2

NaO3S

SO3Na

OHHO

CH2

CH2

HO

SO2

HO

CH2

CH2

OH

1. Naphthalene sulfonatecondensate

2. Condensate of naphthalene with formaldehyde and sodium sulfite

3. Condensate of naphthalene-2-sulfonic acid and 4,4'-dihydroxy sulfone with formaldehyde

4. Condensate of naphthol-2-sulfonic acid and cresol with formaldehyde

Figure 16.15 Structures of some dye levelling agents.

386 Chapter 16

other synthetic reagents is clear. Such byproducts have only weak affinity forcollagen or leather (Chapter 14): in this case, there is no disadvantage, sincewhat is required is for the reagent to interfere with dye fixation, to slow downthe reaction, not necessarily to prevent it.In all cases, if the auxiliary is added before the dye, the colour intensity may

be reduced by up to one-third of what would be obtained on untreated leather.If it is added with the dye, there is less effect on the colour, but the levellingeffect is slightly reduced. Conditions that favour penetration of the levellingagent, such as short float, increase surface colour intensity and reduce levelness.The effect of the auxiliary is enhanced at higher temperature, e.g. 70 1C is betterthan 20 1C.

16.11.2 Auxiliaries that Complex with Dyestuff

These auxiliaries change the HHB/HLB properties and reduce the anioniccharge. Examples include polyglycol ethers with weakly cationic amino groups.These will form strong complexes with high affinity dyes, to improve penetra-tion and levelness. They form weak complexes with low affinity dyes – conse-quently with little useful effect.

16.11.3 Auxiliaries that have Affinity for both Leather and Dye

Examples include poly(ethylene oxide) chains with strong cationic groups.These agents cannot be used with premetallised dyes containing anionicdispersing agents. They tend to produce very pale colours when offered at lowlevels. At higher offers, colour intensity can be increased or enhanced.

16.11.4 Intensifying Agents

The purpose of colour intensifying agents is to allow dyes to accumulate athigher concentration on the surface of the leather. In this way, deeper shadesare obtained, but the fastness properties of the leather, particularly to rub orcrock (see below), are not compromised as they would be by merely

CHCHO

CH2OH

SO3Na

CH3OSO3Na

SO3Na

SO3Na

CHCHCH2OHCH2OH

CH2CHO

CH3OCH3O

CH2OH

CHCHO

CH2OH

CHCHO

CH2CHO

CH2OH

SO3Na

NaO3S

CH3O

Figure 16.16 Illustrative structure of lignin sulfonic acid.

387Dyeing

precipitating the dye on the leather surface by pH manipulation. Theseauxiliaries may also be referred to as dye fixing agents, since their function is tocreate additional sites at which dye molecules can be chemically bound to theleather, rather than physically bound to it.

16.11.5 Cationic Tannages

The presence of cationic centres from the tanning reaction provides additionsites with which anionic dye can react. Such tannages include Al(III) chloride orsulfate, Zr(IV) sulfate, polyurethane and dicyandiamide retanning resins.

16.11.6 Cationic Auxiliaries

This class of auxiliaries includes cationic polyamine compounds with nonionicethylene oxide chains (Figure 16.17). Short chains of this type increase dyeingintensity, but levelness is reduced. The opposite applies to longer chains.Other fixing agents are based on polyamines, such as dicyandiamide deri-

vatives (Figure 16.18).

R1-N-CH2CH2-(N-CH2CH2)n-N-R5

CH3 CH3 CH3(n+2)+

R2 R3 R4

R1 is higher alkyl radicalR2-5 are -(OCH2CH2)nOH, where n=10-20

Figure 16.17 Cationic polyamines.

H2N-C-NH-CN + H2NCONH2

NH NH

H2N-C-NH- C-NH-CONH2

NH

NH NH NHNH

CH3CO2- -O2CCH3

dicyandiamide urea

formaldehyde

acetic acid

quaternised fixing agent

H2N-C-NH-C-NH-CO-NH-CH2HNOC-HN-C--NH-C-NH-CN

+H3N-C-NH-C-NH-CO-NH-CH2-HN-OC-HN-C--NH-C-NH3+

NH NHNHNH

Figure 16.18 Dicyandiamide quaternary fixing agents.

388 Chapter 16

Typically this type of treatment is applied in a separate short float at lowtemperature before dyeing. It is followed by thorough washing, otherwisedyeing is likely to be unlevel. Leather treated in this way usually has slightlypoorer rub fastness properties, compared to untreated leather.

16.12 ALTERNATIVE COLOURING METHODS

Chapter 2 introduces the chemistry and biochemistry of the mammalian pig-ments, the melanins (Section 2.3.6). A potential route to colouring leather is toharness those reactions, to create colour in situ and possibly obtain some sta-bilisation of the collagen through the formation of quinoid species by poly-merising phenols with enzymes. Laccase (benzenediol:oxygen oxidoreductase,E.C. 1.10.3.2) is a copper-containing polyphenol oxidase that catalyses theoxidation of electron-rich substrates such as phenols.5–7 It is produced by somefungi basidiomycetes that cause white rot of wood8 and certain reaction pro-ducts of laccase-mediated oxidation and polymerisation of phenolic com-pounds can give coloured products, such as melanins.9,10 Besides producingcolour, polymerisation products of phenolic compounds may produce tanningproperties, causing the coloured products to stabilise the collagen whilst at thesame time binding covalently and thereby making the colour fast to washing.11

Some phenolic compounds (Figure 16.19) give tanning effects with laccase(Table 16.3). The effects are relatively small, certainly in comparison withsimilar chemical reactions,12,13 although the outcome is not unexpected.Table 16.4 gives the kinetic parameters, KM and Vmax, from the Michaelis–

Menten equation. Based on the KM values, the order of affinity of laccase

OH

OH

OH

OH

HO

OH

OH

HO

HO

NH2OH

O

NH2HO

HO

catechol hydroquinone pyrogallol

3-hydroxytyramine L-DOPA3,4-dihydroxy-L-phenylalanine

Figure 16.19 Phenols used with laccase in colouring studies.

389Dyeing

with phenolic compounds is pyrogallol 4 hydroquinone 4 catechol 43-hydroxytyramine 4 L-DOPA.Figure 16.20 shows the effects of laccase-mediated polymerisation in the

presence of chrome tanned pelt. L-DOPA is oxidised to L-dopaquinone by

Table 16.3 Shrinkage temperature (1C) of hide powder dyedwith phenolic compounds and laccase.

Phenol Phenol+laccase Phenol alone

Catechol 71 61Hydroquinone 74 61Pyrogallol 59 60L-DOPA 59 573-Hydroxytyramine 57 56

Table 16.4 Kinetic parameters for laccase reaction withphenols at 30 1C and pH6.0.

Substrate KM (mM) Vmax (Umin�1ml�1)

Catechol 0.50 76.9Hydroquinone 0.21 100.0Pyrogallol 0.19 70.9L-DOPA 0.89 91.73-Hydroxytyramine 0.82 73.5

Figure 16.20 Colours created on chrome tanned leather by phenol oxidation by laccase.

390 Chapter 16

laccase in an oxygen environment and in vivo is polymerised to produce the finalproduct melanin.9,10 Hydroquinone is oxidised to para-benzoquinone by lac-case under aerobic conditions14 and the outcome is a weak a purple colourconferred to the pelt. Catechol is oxidised by laccase to ortho-benzoquinoneand this results in the deepest shade achieved in these tests, a dark brown-blackcolour. In this context, the colour struck on the leather is a reflection of theinteraction between the phenol and the enzyme: the deeper shades resulted fromhigher affinity, but not the degree of blackness.The principle of colouring without dyes and achieving simultaneous covalent

tanning appears to be feasible, although clearly it is not easy to transfer thebiochemistry of melanin production into a biomimetic technology. The targetin these trials was deep jet black, but the best colour achieved was a darkbrown-black. However, although the principle was not proved completely,the approach to new compact processing is sufficiently interesting to warrantfurther study.

REFERENCES

1. K. McLaren, J. Soc. Leather Technol. Chem., 1990, 74(3), 67.2. D. L. Randall, J. Amer. Leather Chem. Assoc., 1994, 89(10), 309.3. C. S. Tysoe, J. Soc. Leather Technol. Chem., 1995, 79(3), 67.4. R. L. M. Allen, Colour Chemistry, Thomas Nelson and Sons Ltd, 1971.5. C. S. Evans, Enzymes of lignin degradation, in: Biodegradation: natural

and synthetic materials, ed. W. B. Bett, Springer-Verlag, London, 1991,175–184.

6. N. Duran and E. Esposito, Applied Catalysis B: Environment, 2000, 28, 83.7. E. Srebotnik and K. E. Hammel, J. Biotech., 2000, 8, 179.8. A. Hattaka, FEMS Microbiology Reviews, 1994, 13, 125.9. S. Chaskes and R. L. Tyndall, J. Clinical Microbiology, 1975, 1, 509.

10. H. Rorsman, G. Agrup, C. Hansson and E. Rosengren, Biochemicalrecorders of malignant melanoma, in: Malignant Melanoma Advances of aDecade, ed. R M. McKie, Karger AG, Basel, 1983.

11. A. D. Covington, Chem. Soc. Revs., 1997, 26(2), 111.12. O. Suparno, PhD Thesis, The University of Northampton, 2005.13. A. D. Covington, C. S. Evans and O. Suparno, Pubscie AEIF, 2003, 3(1),

18.14. A. Sanchez-Amat and F. Solano, Biochem. Biophys. Res. Commun., 1997,

240, 787.

391Dyeing

CHAPTER 17

Fatliquoring

17.1 INTRODUCTION

Contrary to popular belief, the primary function of fatliquoring is not to softenthe leather: this is only the secondary function. The primary function of fatli-quoring is to prevent the fibre structure resticking during drying. As the leatherdries, the interfibrillary water is removed, allowing elements of the fibrestructure to come close together, which consequently allows interactions tooccur. In the limit, these interactions become strong, because they are createdby the Maillard reaction. Therefore, it is essential for leather quality to preventthis happening.Figure 17.1 shows the structure of a wet blue fibre.1 The photomicrograph

is the result of cryo-scanning electron microscopy: the sample is snap frozen ina slush of liquid and solid nitrogen at about –200 1C, then the temperature isallowed to rise a few degrees, so that the water can sublime away under vacuumto leave intact structure.There are several aspects of the photomicrograph to note:

1. Collagenic materials are at their softest when they are soaking wet.2. The photomicrograph is a representation of wet leather, i.e. no artefacts of

drying are present.3. The fine structure elements are fibril bundles: they indicate a high

degree of opening up and the important level of the higher ofcollagen structure at which opening up occurs is at the level of the fibrilbundles.

4. Taking points 1 and 3 together, the sites where lubrication is required arethe fibril bundles: this implies that the conditions under which the lubri-cant penetrates the fibre structure are important.




392

Figure 17.2 shows a wet blue fibril bundle at higher resolution;2 there are twoaspects of this photomicrograph to note:

1. The structural elements are fibrils;2. At the surface, where the sample is wettest, the fibrils are separated, but

where evaporation of water can begin to take place it can be seen that thefibrils are much closer together.

The opening up processes apply also to the fibril bundles themselves, although it isprobable, because of the close association of the fibrils, that the physical propertiesare derived from the ability of the fibril bundles to distort and slip when stress isapplied. Those properties are the elements of handle, including softness, andstrength. Handle is a complex concept, because it relates to the way the leatherfeels when it is manipulated: it is an algorithm that combines density, softness,compressibility, stiffness, smoothness, springiness and stretchiness, easily solved inthe brain of the experienced tanner, but much less easy to quantify objectively.Strength is the ability of a material to resist breaking or tearing stress: in the caseof leather, it is the ability of the material to dissipate stress over its volume bymovement of the fibre structure. To be able to do this, two criteria must be met:

1. The fibre structure must not be stuck together by the adhesions createdduring drying.

2. The fibre structure must be lubricated to allow the elements to slide overone another.

Figure 17.1 Cryo-scanning electron photomicrograph of wet blue leather.

393Fatliquoring

It is the purpose of the fatliquoring step to satisfy those criteria: it preventsfibre sticking during drying by providing an oil surface to the fibre structure,which then gives it the required lubrication. The effectiveness of the processstep then depends on the degree to which the lubricant penetrates down thehierarchy of structure and its ability to allow contacting surfaces to slide. Thephotomicrograph in Figure 17.3 illustrates this, showing the effect of dryingwithout lubricating; whereby the well opened up fibre structure is stucktogether at the level of the fibril bundles.Typically, the leather industry employs partially sulfated or sulfited oils,

which might be animal vegetable, synthetic or, less commonly, mineral. Mostcommonly, the neutral oil is in the form of triglyceride (see below). Here, theso-called sulfo fraction is the emulsifying agent, keeping the neutral oil sus-pended in solution and thereby transporting it into the leather: it is the neutraloil that is the lubricant (see below).Oil in water emulsions are created by the formation of particles, consisting of

a small drop of oil, surrounded by an emulsifier/detergent/surfactant/tenside.

Figure 17.2 Cryo-scanning electron photomicrograph of a wet blue fibril bundle.

394 Chapter 17

The emulsifying agent has a hydrophobic part, which is dissolved in the oil, anda charged, hydrophilic part that interacts with the solvent (water), to keep theparticle suspended (Figure 17.4).The emulsion particles are prevented from coagulating or coalescing because

they are held apart by the repulsing effect of the high charges on the surface.Any chemical reaction that reduces the charge on the particle surface willallow them to come together, allowing the neutral oil particles to coalesce.High temperature can drive the particles together, breaking the emulsion.An important mechanism to make this happen, which is exploited in leathermaking, is the ionic interaction between the sulfate/sulfonate group and theprotein, in the way discussed in Chapter 15.In the 1986 SLTC Conference the theme was fatliquoring: Waite reviewed

the state of understanding at that time and summarised the important con-tributors to the mechanism, as follows:3

1. The charge on the leather (neutralisation dependent) can affect fatli-quoring effectiveness;

Figure 17.3 Chrome tanned leather: well opened up in the beamhouse, but driedwithout lubricant after tanning.

o

o

o ooo

oo

oo o

ohydrophobic tailhydrophilic (charged) head o

o

o

o

Figure 17.4 Model of an anionic oil-in-water emulsion particle.

395Fatliquoring

2. Penetrating power depends on emulsion stability;3. Particle size distribution an emulsion is a variable that influences stability

and penetration;4. Sulfonation (to make partially sulfited oils) imparts more stability than

sulfation (to make partially sulfated oils);5. Softness depends on the ratio of emulsifier fraction to neutral oil fraction;6. Softening is influenced by the viscosity and interfacial tension of oils.

Points 1 and 2 emphasise the importance of pH in this process step: if theemulsion is anionic (which it usually is), the charge on the leather should alsobe anionic to allow penetration. Neutralisation of the substrate not onlymeans pH adjustment through the cross section but also down the hierarchy ofstructure. The mechanism of fatliquoring can be expressed in simple terms asfollows:

1. The neutral oil is transported into the pelt as an oil-in-water emulsion.2. The emulsifying agent interacts with the leather, reducing or eliminating

its emulsifying power.3. The neutral oil is deposited over the fibre structure – the level of the

hierarchy of structure depends on the degree of penetration.4. The water is removed by drying, allowing the neutral oil to flow over the

fibre structure.

The distribution of neutral oil determines the degree to which fibre sticking isprevented, which in turn depends on the depth of penetration of the emulsion,which in turn depends on the pH of the system and the particle size ofthe emulsion. Additional factors are the nature of the sulfo fraction carrier,sulfated or sulfited, the mechanism of distributing the oil, where the oil isdistributed within the hierarchy of the fibre structure and the nature of the oil.It is this latter parameter that contributes to the softness of the leather: ingeneral, the higher the viscosity of the neutral oil, the better are the lubricatingproperties. However, there is a limiting effect, which is the ability of the oil tobe emulsified and hence carried into the leather: at high viscosities, the oil tendsto deposit on the surface, leaving the leather empty.4

Clearly, from the analysis of the mechanism, the application of a fatliquormust incorporate the notion of creating as small an emulsion particle size asthe formulation will allow. This is accomplished by applying the followingconditions:

� High temperature of the diluting water: ideally heating the fatliquor to thesame temperature as the water; this allows the oil to break up into smalldrops due to the decreased viscosity. Note, there is a limit to the tem-perature, since heating is a powerful mechanism of coagulation: a practicallimit is 60 1C. This would be a maximum value to avoid damaging vege-table tanned leather, although chrome tanned leather is typically capableof withstanding much higher temperatures.

396 Chapter 17

� Mechanical action: for mixing the fatliquor into water, mechanical actionmust be maximised, because this is the way the oil is dispersed into drops.Ideally, a motorised stirrer should be used, with the fatliquor added intothe vortex. The common practice of adding cold fatliquor to hot water in ablue barrel, stirring with a broom handle, is not optimum and thereforebest use will not be made of the fatliquor.

� Some tanners have favoured adding water to the fatliquor. In this way, awater-in-oil emulsion is formed. Continued additions of hot water reversethe emulsion to oil-in-water. There is no clear evidence to indicate that thisa better way of optimising the eventual emulsion.

The particle size of the emulsion is determined by the degree to which the oilis converted into the sulfo derivative: the bigger the sulfo to neutral oil ratio themore solubilised the oil becomes and the particle size is reduced. Most fatliquorformulations contain about 50% neutral oil, 25% sulfo fraction and 25%water: this will produce emulsion particles of about 30 nm. Higher sulfo frac-tions can create micro-emulsions, where the oil is on the boundary of suspen-sion and solubilisation, when the particle size is about 5 nm. Figure 17.5 showsthe influence of particle size, comparing the typical emulsion with a micro-emulsion: the oil particles can be seen as black dots, visualised in transmissionelectron microscopy by osmium tetroxide.1

The relative effects of the neutral oil and the sulfo fraction were distinguishedby separating the components of synthetic fatliquors and applying them indi-vidually.1 For comparison of equal quantities of reagent, 20% fatliquor wascompared with 5% sulfo fraction and 10% fatliquor was compared with 5%neutral oil. To allow the neutral oil to penetrate, it was formulated with either5% non-ionic surfactant or acetone. Table 17.1 gives the results.From Table 17.1:

� softness is conferred by the neutral oil, with no contribution from thesulfo fraction.

� the alternative ways of delivering the neutral oil appear more effective thanthe original fatliquor.

Figure 17.5 Penetration of oil emulsions, shown by transmission electron micro-scopy: 25 nm reaches the fibril bundles, 5 nm reaches the fibrils.

397Fatliquoring

It is clearly not feasible to use organic solvents in fatliquoring, but alternativeemulsifying agents are of interest (Figure 17.6). An obvious feature of theemulsion of neutral oil and non-ionic surfactant is the instability of the emul-sion: in attempting to measure the particle size by laser light scattering it wasapparent that the rate of coagulation was faster than the rate of measuringparticle size dimension. This raised the issue as to why the fatliquoring effectwas so good.The wet leather fatliquored with sulfated oil has the appearance of droplets

over the surface, which would be expected if the oil was deposited as emulsifieddrops. However, the leather fatliquored with neutral oil emulsified with non-ionic surfactant looks completely different: the surface is coated with whatappears to be a water-in-oil emulsion. In both cases the dry leathers do notshow any difference in structure, so the oil is assumed to be uniformly dis-tributed in the regions where it was deposited. Therefore, it can be inferred thatthe unstable emulsion exhibits a different way of delivering the oil. Once astable emulsion particle hits the fibre structure it is likely that the sulfo fractionwill interact with it, depending on the charge conditions, causing the emulsionto lose its emulsifier and the neutral oil will be deposited: there is no mechanismfor the neutral oil to move within the aqueous medium, so it is static until thewater is removed in drying and it can flow over the leather surface. However, ifthere is residual emulsifier in solution, because of the weak emulsifying effect,continuous interaction can occur between the deposited oil and the solventwater, creating a water-in-oil system that allows the oil to flow over the surfacein the presence of water.Sulfated oils and sulfited oils can be compared. Sulfated oils are less stable

and hence less penetrating because the sulfo fraction reacts readily with theleather, so the emulsions break easily over the leather. In contrast, sulfited oilare less reactive towards the substrate, so the emulsions are apparently morestable in the environment of the leather and hence the emulsion particlespenetrate more deeply into the hierarchy of structure. For this reason thetechnological approach to fatliquoring is to use a mixture of the two types, to

Table 17.1 Relative effects of neutral oil and sulfo fraction. The softness ofthe leather is measured by the loop test, where a lower weight/magnitude means softer leather.

Offers on wet weight Sulfited oil Sulfated oil

20% Fatliquor 44 5610% Fatliquor 127 1235% Sulfo fraction in water 450 5905% Neutral oil in acetone 69 665% Neutral oil+5% non-ionic surfactant 70 53Controls:5% Non-ionic surfactant 164Acetone then rehydrate 781Water 1220

398 Chapter 17

get both surface and internal lubrication. But there is another consequence ofthe lower affinity of sulfited oil for the fibre surface: the sulfo fraction remainsavailable to interact with oil deposited on the leather surface. This is illustratedin Figure 17.7, where it is apparent that the neutral oil is deposited in a globular

Figure 17.6 Leathers treated with sulfated synthetic oil fatliquor and with neutralsynthetic oil emulsified with non-ionic surfactant (from Table 17.1).

399Fatliquoring

manner by sulfated emulsifier, but in both a globular and a smeared manner bythe sulfited emulsifier.The mechanism of fatliquoring can be summarised by the interaction of the

contributing parameters (Figure 17.8).1

The development of softness in leather is intimately associated withmechanical action, because the process of drying causes adhesions to occur,weak adhesion in the case of well fatliquored leather, which must be broken tosoften the leather. This is achieved by the process of staking, in which the

Figure 17.7 Effect of sulfation or sulfonation on the mode of deposition of neutral oilon leather.

degree of neutralisation

degree of sulfation or sulfonation

emulsionstability

mechanical action

solution pH penetration throughcross section

particlesize

solubility penetration down hierarchy

residual sulfo component

flow softness

Figure 17.8 Model of the fatliquoring mechanism.

400 Chapter 17

leather is mechanically stressed: in industry the process used to involve bendingthe leather by hand through an acute angle over a blunt blade, but nowmachines pummel the leather automatically and with precisely controlled force.A crucial feature of the staking process is the water content of the leather: morethan equilibrium moisture content is required, 15–20%, to provide additionallubrication of the fibre structure.Figure 17.9 demonstrates the relationship between softness and strength.5

In Figure 17.9(a), there is positive correlation between softness and strength,indicating that higher strength is obtained from more effective fatliquoring.There will be a maximum value of the strength for any leather, which isdependent on the chemical processing up to the point of the fatliquoring: itis the fatliquoring process that determines whether the optimum strength isdisplayed or less. Figure 17.9(b) shows a negative correlation, demonstratingthat as the stressing continues the fibre structure begins to break down. Thiscan be achieved by too much mechanical action, either too much energy appliedto the leather or lesser energy applied for too long, such as multiple passesthrough the staking machine. Alternatively and additionally, if the moisturecontent is too low, the fibre structure is less flexible, the components slip lesseasily, so the brittle structure tends to loosen and break, which is seen asincreasing weakness.

17.2 ANIONIC FATLIQUORS

17.2.1 Sulfated Fatliquors

For sulfating, the oil must be unsaturated, with a minimum iodine value of 70:the iodine value is the defined as the number of grams of iodine absorbed by100 grams of oil or fat. Oils that have been used in this regard are castor,

(b)

softness softness

Developing chemical softness

= stress softening

(a)

strength strength

Overstaking fibre structure = stress weakening

Figure 17.9 (a) Chemical softness; (b) stress softness.

401Fatliquoring

neatsfoot, soya, groundnut, cod. Note, after 1980, sperm whale oil wasvoluntarily abandoned by the global leather industry.The chemistry of the preparation of a sulfated oil is in three steps:

1. Preparation: 10–20% Concentrated sulfuric acid on the weight of theoil is added slowly to the oil, with constant stirring. The temperatureof the exothermic reaction must be controlled to o28 1C, otherwise theoil can char, causing darkening, and the triglyceride oil may be hydrolysedto release free fatty acids. This latter effect can give rise to the problemof ‘spue’, when the longer chain carboxylic acids can migrate from theinternal structure of the leather to the grain surface, visible as a whiteefflorescence. Diagnosis is confirmed when the efflorescence melts bythe effect of a match. Problems associated by overheating can be catalysedby the presence of iron salts. An alternative is to use a mixture ofsulfuric and phosphoric acids (usually in the ratio 0.8 : 1.0). Thisremoves the effect of iron (by reaction with the phosphoric acid), thereaction is less exothermic, so the acid mixture can be added faster. Theprocess is more expensive and results in a mixture of sulfated andphosphated oils.

2. Brine wash: Excess free acid is removed by washing the partially sulfate oilwith brine, which also separates the oil fraction from the aqueous fraction.Brine is used to avoid creating an emulsion, which would happen if wateralone were to be used. Alternatively, sodium sulfate, ammonium chlorideor sulfate could be used. Some hydrolysis of bound sulfate may take place.Additionally, there may be lactonisation of adjacent hydroxyl groups withcarboxyl groups (Figure 17.10).

3. Neutralisation: bound and free acid groups are neutralised with alkali:

-OSO3H ! -OSO�3 Naþ or NHþ

4 ; etc:

-COOHðfree fatty acidÞ ! -COO�Naþ or NHþ4 etc: ðwater soluble soapÞ

Also, free sulfuric acid is converted into salts: 2Na1SO42� or 2NH4

1SO42�,

etc. Any lactones will be hydrolysed.

Bound SO3 may range from 2%, considered a low level, to 3–4%, consideredmedium level, to 6–8%, considered to be high level: fatliquors may therefore bedesignated low, medium or high sulfated. Figure 17.11 shows some sulfatingreactions.

CH

OH

COOH CH

O

H2O +CO

Figure 17.10 Lactonisation.

402 Chapter 17

As the level of sulfation increases:

� anionic charge increases, hence greater affinity for cationic leather;� lubricating effect decreases, due to the lower concentration of neutral oil;� emulsion particle size decreases, ultimately to the point of forming a

microemulsion (o5 nm) or even actually dissolving in water;� stability of the emulsion to coagulation by acid or metal salts

increases;� at high levels, the oil functions more like a wetting agent than a lubricant,

hence the leather becomes more hydrophilic;� the leather becomes looser, in terms of break, possibly due to the damaging

effect of the sulfate species on collagen;� the likelihood of hydrolysing the oil to create free fatty acids increases,

thereby creating the possibility of chrome soaps, fatty acid spue, poorwetting back, uneven dyeing and poor finish adhesion.

Sulfated oils are used as follows:

� Low level of sulfation: Low stability of the emulsion to coagulating(cracking) by acids or metal salts. Typically used to lubricate the outersurfaces, particularly the grain. May be used for drum oiling of vegetabletanned leather.

1. Combination of sulfuric acid with double bonds.

2. Reaction of sulfuric acid with hydroxyl groups eg ricinoleic acid in castor oil.

3. Hydrolysis of triglycerides and reaction of sulfuric acid with a hydroxyl of a glycerol derivative.

-CH=CH- -CH2-CH-

-CH-CH2-CH=CH- -CH-CH2-CH=CH-

CH2CO2R

CH2CO2R

CHCO2R

+ RCO2H

HOSO3H

OSO3H

OSO3H

HOSO3H

CHCO2R

CH2OH

CH2CO2R

CHCO2R

CH2OSO3H

CH2CO2RHOSO3H

OH

Figure 17.11 Sulfating reactions.

403Fatliquoring

� Medium level of sulfation: More stable to coagulation, therefore greaterpotential for penetration. Used for surface neutralised chrome leather.

� High level of sulfation: used for complete penetration, e.g. throughneutralised chrome leather for gloving, clothing, softee leathers.

17.2.2 Sulfited Oils

The requirement for the oil is unsaturation, as for sulfating. Options includecod oil, neatsfoot oil, etc. Preparation is conducted in two steps:

1. Sulfitation/oxidation: Using the example of cod oil, air is blown through amixture of 100 parts of oil and 50 parts of 401 Baume sodium bisulfitesolution, with stirring at 60–80 1C. Alternatively, hydrogen peroxide maybe used instead of air. Figure 17.12 illustrates the chemistry involved.

2. Brine wash: washing with brine removes excess sodium bisulfite.

Note, there is no pH adjustment required, due to the use of bisulfite ratherthan sulfur dioxide or sulfurous acid.Compared with sulfated oils, sulfited oils exhibit the following properties:

� no charring or darkening;� higher emulsion stability to acids, hard water salts, metal ions, e.g. Al(III),

Cr(III), due to the presence of sulfonate and hydroxysulfonate groups andthe low level of free fatty acids or soaps, because there is little hydrolysis oftriglyceride during synthesis.

1. Reaction between bisulfite and double bonds.

2. Oxidation of double bonds, then reaction with bisulfite.

3. Reaction between carbonyl groups and bisulfite.

-CH=CH- -CH2-CH-

-CH=CH- -CH-CH-

HO

O

O

HO

NaHSO3

SO3H

SO3Na

NaHSO3

-C- -C-NaHSO3

SO3H

-CH-CH-

Figure 17.12 Sulfitation and oxidation.

404 Chapter 17

The fatliquor may be formulated with non-ionic detergent, to increaseemulsion stability, to promote better penetration, to make the leather softerand fuller. There is a danger of giving the leather loose break.Sulfited oils are used as follows:

� softness and strength (see below) for all leathers by deep penetration;� woolskins and furskins in mineral tanning baths;� in shrunken grain production, to minimise loss of tensile strength in the

acidic tanning bath.

17.3 SOAP FATLIQUORS

Raw oil, e.g. neatsfoot (100 parts), is emulsified with soft soap, e.g. potassiumoleate (30 parts). Note, stearate is likely to cause fatty acid spue. The emulsionshave a large particle size, due to their tendency to have low stability to waterhardness and acid. The formulation typically has pHB8. At pHo6, the soapis increasingly converted into free fatty acid (Figure 17.13), which does not actas an emulsifier, and so the emulsion coagulates.The uses of soap fatliquors are limited, due to the low emulsion stability.

They have traditionally been used for surface fatliquoring calfskins for shoeuppers and formaldehyde tanned sheepskin for gloving leather. There is anAmerican practice of improving the emulsion stability by formulating a soapfatliquor with a small amount of low sulfated oil (up to 0.5% SO3 on moisturefree basis), together with a stabilising colloid, e.g. starch or natural gum.

17.4 CATIONIC FATLIQUORS

Raw oil is emulsified with a cationic agent, exemplified by the structures inFigure 17.14, where the hydrophilic group is typically straight chain, aliphaticC10 to C18. The following points apply to such fatliquors:

� low affinity for cationic charged leathers, e.g. chrome tanned;� high affinity for anionic charged leathers, e.g. vegetable tanned, for

lubricating the outer layers;� incompatible with anionic reagents, e.g. anionic dyes, fatliquors, retans;� high stability to acid, but unstable to alkali;

C17H35COOH H+ water insoluble fatty acid

C17H35COO- Na+

water soluble sodium soap

(C17H35COO)2Ca water insoluble calcium soap

Ca2+

Figure 17.13 Soap reactions.

405Fatliquoring

� good stability to metal ions and salts;� may have poor shelf-life;� emulsion stability can be improved by formulating with non-ionic deter-

gents, e.g. alkyl ethylene oxide condensates;� as a second fatliquor after anionic fatliquor (cf. ‘sandwich dyeing’).

It is well known in the detergent industry that a combination of anionicand cationic surfactants that creates a total carbon chain length of 424 willproduce a water-insoluble precipitate. This can be easily observed by mixingany domestic anionic liquid detergent with any domestic cationic fabricsoftener:

� on chrome retanned leather to increase surface oil and aid paste drying;� on vegetable tanned leather for bags or cases, to lubricate the grain layer

and it make it more pliable;� for oiling vegetable tanned leather;� in combination with basic aluminium(III) chloride, for retanning chrome

leather for suede; this will increase the cationic charge, which gives higheraffinity for anionic dyestuff and produces a greasy nap;

� for woolskins in the mineral tanning bath;� for white gloving leather, which might be made with a combination of

aluminium(III) and sulfonyl chloride.

17.5 NONIONIC FATLIQUORS

17.5.1 Alkyl Ethylene Oxide Condensates

These fatliquors are emulsified with compounds made by condensing ethyleneoxide in the presence of an aliphatic alcohol:

CnH2nþ1-ðOCH2CH2ÞxOH

The properties of the emulsifying agent depend on the value of n, thealiphatic carbon chain length and the value of x, the degree of polymerisationof ethylene oxide. These compounds are not very good emulsifying agentsbecause the hydrophilic end, the ethylene oxide chain, does not have a highaffinity for water, relying only on hydrogen bonding via the ether oxygens.

N+ C16H33-[N(CH3)3]+ Br-Br-C16H33

cetylpyridinium bromide alkyl(trimethyl)ammonium bromide

Figure 17.14 Examples of cationic emulsifying agents.

406 Chapter 17

The following points apply to these fatliquors:

� high stability to metal ions, salts, hard water and wide pH tolerance;� miscible with cationic and anionic reagents;� little or no affinity for anionic or cationic charged leathers;� the non-ionic emulsifier increases the hydrophilicity of leather;� used for fatliquoring zirconium(IV) or aluminium(III) tanned leathers,

which are highly cationic charged;� as a crusting fatliquor for suede splits, i.e.merely to prevent fibre resticking

on drying, to aid rewetting;� may be formulated with anionic or cationic fatliquors, to improve stability.

17.5.2 Protein Emulsifiers

Uncharged proteins, at their isoelectric point, can act as emulsifiers. Globulinproteins, e.g. albumin, produce a ‘mayonnaise’ type of fatliquor. These pro-ducts are limited to a narrow pH range, close to the isoelectric point, otherwisethey become significantly charged and function as anionic or cationicformulations.

17.6 MULTI-CHARGED FATLIQUORS

These fatliquors are formulations of non-ionic, anionic and cationic fatliquors,in which the presence of the non-ionic species prevents precipitation of theanionic and cationic species. They are more stable to a wider pH range thansingly charged fatliquors and hence more stable to variations in leather charge.The proportions of the constituents can be varied, depending on the leatherproperties required, i.e. depth of penetration, surface lubrication and ease ofremoval from paste drying plates (where it would be an alternative to two-stagefatliquoring).

17.7 AMPHOTERIC FATLIQUORS

Raw oil is emulsified with an amphoteric reagent, i.e. one containing bothacidic and basic groups, e.g. as shown in Figure 17.15.The point of neutrality, the isoelectric point, depends on the numbers of

acidic and basic groups, as discussed for proteins. A typical IEP for this type ofstructure is pH5. At pH 4 5 the emulsifier is negatively charged, at pHo5 theemulsifier is positively charged. The choice of pH of the fatliquor depends onthe charge of the leather and the requirement of surface reaction or penetration.

17.8 SOLVENT FATLIQUORS

These are typically anionic fatliquors, containing a high boiling point, polarpetroleum solvent with minimum flash point 60 1C and preferably odourless.

407Fatliquoring

The function of the solvent is to replace the water as it is removedduring drying. With lesser affinity for the leather than the polar triglycerideoils, it can spread further and deeper. Therefore, these products are usedfor making soft leather. However, the current attitude to the use of solventsin leather making means that these products are used less now than inthe past.

17.9 COMPLEXING FATLIQUORS/WATER RESISTANCE

TREATMENTS

Water resistance is reviewed comprehensively below, discussing the che-mistries of modern approaches. Before they were developed, a simplerapproach to making leather hydrophobic was adopted, which was alsopart of the lubricating step in leather manufacture. There are two tradi-tional forms of these reagents, designed to be applied to chrome tannedleather:

1. An old option was to use a long-chain fatty acid dimer, e.g. oleic aciddimer, applied as a soap at pH 8.6,7 These were used as a method ofintroducing water resistance, when the carboxylic groups are complexedwith bound chromium(III), leaving the hydrophobic moieties exposed overthe fibre structure. More often than not, the technology did not work inindustrial practice.

2. Another traditional option was to use a dibasic or tribasic acid esters, suchas succinic, phthalic, citric or phosphoric acid, bound to an aliphaticchain with 15–24 carbon atoms (Figure 17.16). These too were unreliablereagents for making water resistant leather.

N-(CH2)n-CO2-

R1

R2

R2

R1

N-(CH2)n-CO2H

R2

R1

N-(CH2)n-CO2-H

H N-(CH2)n-CO2H

R1

R2

OH-

H+

+

+

Figure 17.15 Amphoteric emulsifier.

408 Chapter 17

17.10 WATER RESISTANCE

17.10.1 Introduction

The general function of post tanning operations is to confer specific perfor-mance properties to the leather. One of the important features of that part ofleather making is to make the leather water resistant. This is typically appliedonly to shoes and boots, with the possible inclusion of clothing leathers. Suchprocesses are usually applied to chrome tanned leather, since other methodsof tanning make the leather too hydrophilic to allow high levels of waterresistance to be conferred.It is common to refer to treatments for leather with hydrophobic reagents as

waterproofing. This implies that water is completely excluded from the crosssection of the leather, so that no transmission of moisture occurs. In practice,the traditional methods of achieving this were to fill the structure with grease,so-called ‘stuffing’, or to apply a water impermeable finish coating. In the firstcase, the treatment is not actually completely effective, since moisture can stillfind its way through the fibre structure. In the second case, the product is morelike plastic than leather. The role of finishing in creating water resistance isrecognised in the Official Methods of testing, where a requirement is to removethe finish by buffing prior to testing. It is important to recognise that leather isexpected to ‘breathe’, i.e. its water vapour permeability must not be sig-nificantly reduced by some treatments. Therefore, some of the techniques usedhitherto fail by this criterion. It is also useful to bear in mind that the success ofthe possible treatments is relative, ranging from small to large effect; hence theapplication of hydrophobing treatments is preferably referred to as conferringwater resistance: the term waterproof is typically not used in a leather context.In considering ways to confer water resistance, it is useful to consider how a

leather can fail a water resistance test or fail in use. There are three mechanisms:

1. Water may flow through the fibre structure, because of the voids in thematerial.

2. Water may wet the surfaces of the fibre structure and flow across the solidsurface.

3. Water may travel through the fibre structure, in the way wax flowsthrough a wick in a candle: this mechanism is referred to as ‘wicking’.

The first mechanism is resisted by the barrier approaches. The secondmechanism is resisted by making the fibres less wettable. The third mechanism

CH2-CO2R

CH2-CO2R

HO-CH-CO2R CH2-CO2R

R-CH-CO2R

O=P

OH

OH

OR

Figure 17.16 Complexing lubricants.

409Fatliquoring

is a special example of the second mechanism, since wicking depends on theinner elements of the fibre structure being unaffected by the water resistancetreatment. Therefore, the second mechanism is addressed by a relativelyunsophisticated hydrophobing treatment, which is required to penetratethrough the cross section, but may not penetrate down the hierarchy ofstructure. The third mechanism can only be countered if the treatment pene-trates down the hierarchy of structure: it is relatively easy to reach to the levelof the fibril bundles, since that is achievable in well conducted fatliquoring,so this may confer a good degree of water resistance. Consequently, it maybe assumed that the highest degree of water resistance is conferred by treatingthe fibre structure at the level of the fibrils. From fatliquoring science, it isknown that penetration to this level requires the reagent to be solubilised or, atthe limit, to be in the form of micro-emulsions.The water resistance of a material like leather depends on the way in which

water (or any other solvent) interacts with the surface. In the case of leather,this means not only the actual surfaces, but also the surface of the fibreswithin the fibre structure and lower down the hierarchy. This is illustrated inFigure 17.17.The surface tension of the solvent and the surface determine the wetting of

the surface by the solvent. Table 17.2 presents some values.

contact angle obtuse- no wetting - surface wetting

θθ

contact angle acute

Figure 17.17 Interaction between a liquid and a surface: the contact angle (y).

Table 17.2 Surface tension (g) of some liquids and the critical surface tension(gc) of some solid surfaces.

Surface/liquid ChemistrySurface tension, g orgc (mNm�1)

Water H2O 74Ethanol CH3CH2OH 23Acetone CH3COCH3 24n-Octane (liquid) C8H18 22Leather Collagen 470Polyurethane [-CO2(CH2)x-NH-CO2-(CH2)y-O-]n 33Silicone polymer [(CH3)SiHO]n 24Fluorinatedpolyacrylate

[CH2CH(CO2CnF2n11)]m 11

410 Chapter 17

The critical surface tension of a solid, gc, is equivalent to the surface tensionof a liquid, g, if the contact angle is zero. Liquids that have g values greater thangc will not wet the surface, but if gogc the surface will be wetted. The purpose ofmodern water resistance treatments is to change the critical surface tension ofthe available fibre surface sufficiently to prevent water wetting it. Note, thepresence of neutral electrolyte in aqueous solution does not change the surfacetension markedly.

17.10.2 Principles of Conferring Water Resistance

The ability of a material such as leather to resist the absorption and trans-mission of water can be expressed in terms of weak resistance and strongresistance. Weak resistance can be regarded in clothing terms as ‘showerresistance’: this means a garment does not immediately absorb rain water, butprolonged exposure will result in both rapid absorption of water and itstransmission through the cross section. This applies to both clothing and shoes:in the case of the latter, almost any wet circumstances will result in wet feet.Shower resistance may be enough to prevent water spotting of double face orsuede clothing leathers: this is the effect of wetting causing the surface fibres tostick, giving rise to a visible non-uniformity, typically in the form of spots fromthe raindrops. Strong resistance means that there is a significant delay betweenexposure to water and its transmission across the cross section: the accom-panying amount of water absorption reflects the movement of water within thematerial, although the relationship between wetting and transmission dependson the nature of the material, including methods of leather processing. Whilstthe resistance of leather to wetting can be high, notably it cannot strictly becalled ‘waterproof’, because that term must be reserved for materials thatconstitute a continuous and complete barrier to water, e.g. a plastic sheet. Theimportance of water resistance was demonstrated when the American LeatherChemists Association (ALCA) held a symposium in 1995, when it was theonly topic.8,9

The water resistance specifications for defence leathers vary from country tocountry, but in no case might the requirement be regarded as ‘high perfor-mance’: e.g. in the UK, resistance to water penetration in the Bally Penet-rometer test (IUP 1010) (Figure 17.18) is required to be only 3 hours.11 For themodern soldier in the field, in a war theatre for periods of 24 hours or more,such performance is unsatisfactory. However, it is possible to raise the waterresistance to over 120 hours on the Bally Penetrometer or 4500 000 flexes onthe more stringent, but similar Maeser tester. It is likely that lack of failure aftera week on these tests means that the leather is unlikely to fail the test at all,although the fibre structure may fail. See below for technologies to exceed thatspecification.There are many types of treatment traditionally offered to the leather

industry and some more modern (see below). However, it has been a feature ofwater resistance treatments that there are two shortcomings, which for many

411Fatliquoring

years resulted in the technologies being kept a closely guarded secret in eachtannery making such leather:

� The effects were often moderate, conferring relatively poor water resistance.� The effects were often inconsistent, sometimes working, sometimes not.

In the modern leather industry, such inconsistency and poor performanceare easily overcome, if the tanner is aware of the factors that contribute to theoutcome of modern treatments. Very high degrees of water resistance canbe achieved by careful control of appropriate chemical treatments, even thosenot known for conferring high performance do have the potential to workbetter than might be expected. The chemistries are specifically designed toretain the ability of the leather to allow transmission of water vapour, the‘breathability’ property. Although the treatments are technically feasible andrelatively simple, the outcome of the treatments depends on many factorsthat can either prevent the developing of water resistance or contribute to itscreation.Some workers have addressed the factors that influence water resistance:12

for any chemical treatment, the factors that influence the developing of waterresistance can be listed and rationalised as follows.

17.10.2.1 Opening Up. The opening up is critical, to allow the reagent topenetrate deeply down the hierarchy of structure. Here, the important effect

Figure 17.18 Bally Penetrometer for measuring water resistance: bent leather is flexedin a bath of water until penetration through the cross section occurs.

412 Chapter 17

is to split the fibre structure finely, separating the fibres at the fibril bundlelevel, to allow the penetration of reagents. In the case of reagents that mustbe presented in the form of an emulsion, the particle size requirement maymean taking particular steps to solubilise materials that would ordinarily beapplied as an emulsion, thereby contributing to the performance of thereagent. This may be accomplished simply, depending on the chemistry ofthe reagent, e.g. adjusting the pH to a higher value than specified or as sup-plied. Alternatively, if the hydrophobic agent can only be solubilised bymeans of detergents, this is clearly counter-productive (see below).The degree of opening up is important, because unsplit fibres can act as a

wick, to transport water through the leather cross section. The more finely splitthe fibres are, the less easily it can be wetted and the wicking mechanism isreduced. Clearly, if the reagent penetrates to the level of the fibril surfaces, thisis the best treatment that may be achieved.

17.10.2.2 Looseness. If opening up goes too far or too much mechanicalaction is applied, particularly in the alkaline swollen condition, the coriumstructure will become loosened. The open structure will facilitate water trans-mission, because it can occur without wetting the fibres. Similarly, it is moredifficult to confer high water resistance to leathers with a naturally openstructure, e.g. sheepskin.

17.10.2.3 Substrate. The primary tanning chemistry can influence the abil-ity of the leather to achieve high water resistance: in essence, there are twopractical possibilities in this regard:

1. Chrome tannage: The hydrophilicity/hydrophobicity of chrome tannedleather can be controlled by the way in which the tannage is conducted. Inthis case the parameters can be set to obtain the maximum hydrophobicnature, i.e. slowest rate of change of pH and temperature, within theconstraints of the processing time.

2. Hydrophilic tannage: The use of vegetable tannins (plant polyphenols),resins or syntans, either for low stability tannage or in combination forhigh hydrothermal stability tannage, depends on the formation ofhydrogen bonds between the tanning agent and the collagen. This reactionnecessarily produces hydrophilic leather – in the case of vegetable tannins,hydrolysable tannins are worse than condensed tannins in this regard.These tannages make it more difficult to make water resistant leather: thisis mostly due to the hydrophilicity, but also relates to the lack of metalspecies in the leather, which is essential for some powerful water resistancetreatments.

17.10.2.4 Reagent Chemistry. There are many strategies for incorporatinghydrophobic groups into the leather structure (see below). These range fromthe so-called hydrophobic fatliquors through to specific agents. Modern

413Fatliquoring

high-performance agents are based on partial esters of acrylate polymers:these can react covalently with bound chromium(III) in chrome tannedleather. If there is no metal ‘mordant’ for this function, other reagentsmay have to be used, e.g. metal complexes of fluorochemicals or siloxanederivatives.

17.10.2.5 Reagent Offer. The degree of water resistance conferred typicallycorrelates with the amount of the reagent used, although there is an effectiveupper limit to the amount that is necessary to achieve high performance.This can be understood in terms of the extent of the treatment over the avail-able fibre structure surface. If the whole surface is inadequately treated, thereis a mechanism for water to pass through the wetted regions.

17.10.2.6 Neutralisation. Most modern high-performance, water resistancereagents are anionic compounds and hence rely for their effect on penetrationinto the fibre structure, which in turn depends on the degree of neutralisationof the leather. There are three components to this step that must beconsidered:

1. The isoelectric point of the leather determines the nature and magnitudeof the charge on the leather at any point on the pH scale. In the case ofacrylate reagents with pKaB 4, the leather must be neutral or anionic atpH6. Any processing that raises the IEP significantly above pH6 wouldbe counter-productive in this regard: this therefore rules out tannages thattarget the basic amino sidechains.

2. The pH must be high enough to ensure that there is minimum interactionbetween the anionic reagent and the leather, i.e. cationic centres: theseinclude both protonated basic groups and residual cationic character inthe bound metal ions. In the case of applying a cationic agent, such as afluorocarbon complex of chromium(III), the pH must lie below the iso-electric point.

3. The process must be prolonged, to allow neutralisation to take place downthe hierarchy of fibre structure. The conventional neutralising period isrelatively short, because for most post tanning processes neutralisation isless critical in this regard.

17.10.2.7 Temperature. The temperature at which the reagent is preparedcan be important, whether the condition refers to emulsification or solubilisa-tion. When making emulsions, it is critical that the solvent and the productshould be heated, to reduce the viscosity of the hydrophobic compound, so itcan break up into as small particles as the chemistry allows. It is not possibleto define an optimum temperature for this operation. This is because hightemperature is an effective way of destabilising emulsions, so there is a practi-cal upper limit to the temperature used, usually 60–70 1C. Destabilisationoccurs when the temperature is high enough to make the particles collide

414 Chapter 17

with sufficient energy to overcome the charge repulsion and hence theycoalesce. Also, elevated temperature can help the reagent to penetrate and,depending on its chemistry, to fix.

17.10.2.8 Neutral Electrolyte. The wettability of a leather is the startingpoint for water resistance: the easier it is to wet the fibres, the easier it is forwater to be transmitted through the leather. The presence of neutral electro-lyte makes the fibres easier to wet, because ions attract water into their solva-tion shell, providing nuclei for additional water fixation: the more watersoluble the salt is, the worse the effect on water resistance. Therefore, it isimportant to minimise the amount of salts in the leather, first by ensuringthere is minimum neutral salt in all post tanning processing and then bywashing prior to applying the water resistance treatment and washing after-wards, before drying the leather.

17.10.2.9 Surfactants. It is common to include surfactants of various typesin leather making processes, particularly in the earlier steps of beamhouseprocessing. Since their function is to facilitate wetting, their presence inleather is incompatible with water resistance. Cationic detergents are unusualand typically not used in the leather industry. Anionic detergents are themost common industrial and domestic agents, because they are the mosteffective wetting and cleaning agents. It is particularly important to excludethese agents from processing if water resistant leather is the required product.Sulfates and sulfonates have high affinity for leather and they can remainwithin the structure long after they have been applied. If a wetting agent isneeded in the beamhouse, non-ionic surfactants should be used. Althoughthey are less effective at wetting and cleaning, they have little affinity forcollagen and therefore do not persist in the leather.

17.10.2.10 Post Tanning. It is usual to apply a water resistance treatmentat the end of the post tanning sequence of operations, because of the likelyinterference it would have on other processes if the fibre structure becomeshydrophobic. So it is important to recognise the influence of the other posttanning reagents on the final performance of the leather as a water resistantmaterial:

� Retanning: just as the primary tanning chemistry can influence waterresistance, the retanning can control it. If a hydrophilic retanning agent isused, it will confer wettability to the leather. A less hydrophilic retannage,such as chromium(III) is more compatible with water resistance and thismay be further controlled by appropriate masking.

� Dyeing: the choice of dyes can influence the wettability of leather, becausetheir chemistries can vary, even within a group of the same or similarcolour. Indeed, the concept of hydrophilic/hydrophobic balance (HHB) as

415Fatliquoring

a quantifiable parameter is already established for dyestuffs and used tocontrol the degree of surface reaction or penetration. The presence ofhydrophilic dyes in the leather will act as a wetting agent, allowing water tobe transmitted through the leather.

� Fatliquoring: the extent of reliance upon lubricants other than the waterresistance agent depends on the nature of the water resistance agent, sincesome can operate effectively as the lubricant. For others it is necessary toinclude some conventional fatliquoring agents. Fatliquor formulationstypically contain partially sulfated or sulfited (sulfonated) oil, as theemulsifying agent for the neutral oil. Therefore, they act like surfactants,possessing hydrophilic and hydrophobic functionality; consequently, theyalso have the ability to wet surfaces. So, despite the oily nature of thefatliquor, it can have the same effect as an anionic detergent treatment.The choice of fatliquor should be confined to the so-called ‘water resistant

fatliquors’: they typically have a weak effect on water resistance, but willoffer the least adverse effect on a more powerful water resistance treatment.

17.10.2.11 Capping. The acrylic based water resistance treatments includea final step called ‘capping’, in which the residual anionic character ofthe leather is neutralised by applying a cationic agent. If this step is omitted,the water resistance treatment will merely confer a weak effect, e.g. resistanceto water penetration for a few minutes or an hour on the Bally Penetrometer.Effective capping can extend the period of resistance to many hours. It ispossible to use various cationic agents. Those that merely interact electrosta-tically, such as quaternary ammonium compounds, are not very effective.Similarly, metals that react electrostatically with the carboxyl groups of theacrylic polymer, e.g. Al(III), Ti(IV) and Zr(IV), are not optimally effective. Thepreferred reagent is chromium(III), which forms covalent complexes: this hasless charge separation in the bonding and, thereby, does not exhibit theequivalent effect of the presence of neutral electrolyte.

17.10.2.12 Solvent. Water resistance agents may be formulated with non-aqueous solvent, such as cellusolve, as part of the manufacturing process oras an aid to emulsification. Its presence in leather can also facilitate wetting,since such solvents are water miscible, and the consequence may be observedas time dependent water resistance following crusting. That is, if the leatheris tested immediately after crusting and conditioning, it may exhibit poorwater resistance because the solvent remains in the leather. However, ifthe leather is stored for several days, e.g. in the conditioning room, furthertesting would show an improvement in water resistance, because the storageperiod allows residual solvent to evaporate.

17.10.3 Chemistries of Water Resistance Treatments

In the time when leather was made predominantly from vegetable tanning, itwas difficult to confer a high degree of water resistance. However, filling the

416 Chapter 17

fibre structure of the leather with hydrophobic, greasy material was the onlytraditional option in this regard. For example, Tim Severin’s successfulexperiment to reproduce the transatlantic voyage of St Brendan from Ireland toAmerica in a leather boat was achieved by constructing the boat from oaktanned hides, stuffed for water resistance with wool grease.13 However, clearly,this approach to creating water resistance from a hydrophilic substrate offerslimited wider applications.

17.10.4 Chrome Tanned Leather

Chrome leather is naturally relatively hydrophobic, at least in comparison withuntanned collagen. Hence, it provides a good start for making water resistantleather.Compared to the simplistic barrier approaches to water resistance of the

past, what is needed in the modern industry is chemical treatments that donot adversely affect the handle and ‘breathability’ properties of the leather.Figures 17.19 and 17.20 briefly present some such treatments, where is can beseen that they tend to rely on the presence of bound chromium(III) species,acting as a type of mordant for the hydrophobic agent.The silicone reaction is readily understood: the chain of Si–O moieties pro-

vides many oxygen atoms that can interact electrostatically with collagen or

Cr

O

HO

HO2CCH2C7F15

R1R2SiCl2 R1R2Si(OH)2 Si

O

R1 R2

Fluorochemical

Silicone

Alkenyl succinic acid(Bavon 2)

Bavon D

H13C6

H13C6 (CH2)7CO2-

R1C=CH-CH

R2 CO2H

CH2CO2H

Cr

H2O -H2O

R2R1

O

Si

R2R1

O

Si

n

CH=CH-(CH2)7CO2-

Figure 17.19 Chemistries of some water resistance treatments.

417Fatliquoring

leather. In this way, the fibre structure becomes coated with a surface of waterrepellent alkyl groups. This is an effective type of treatment, although it is morecommonly used in conjunction with other chemistries, which facilitate thedelivery of the silicone.The Bavon process was used commercially for some time, but is the arche-

typal inconsistent process. The principle lies in a stepwise mechanism:

1. Reaction between the alkyl succinyl dicarboxylate and bound chromium(III);2. Creation of a second layer of the reagent, when the alkyl groups interact

in a hydrophobic bonding arrangement;3. When the dry leather is wetted, a water-in-oil emulsion structure is formed

at the carboxylate surface, preventing the passage of water through to thefibre surface.

Si

O

R R RR

O

Si

RR

O

Si

RR

O

Si

RR

O

Si

leatherfibre

leatherfibre

Figure 17.20 Modes of action of silicone and the Bavon processes.

418 Chapter 17

At least, that is the theory! Sometimes it worked, but success could never beguaranteed. Hence, for many years, tanners closely guarded the details ofprocessing for water resistant leather.The current industry standard for making water resistant leather from

chrome tanned stock is partially esterified acrylic polymers, exemplified by theLubritan series of agents from Rohm and Haas Co. The process is simple, if theconditions set out above are adhered to. Here the mechanism involves com-plexing available carboxyl group in the polymer with bound chromium(III),therefore presenting the surface of the fibre structure as covered with alkylgroups. These groups also act as spacers within the fibre structure, functioninglike a lubricant. In this way, processing is made more compact (see below).The role of silicones in this context is interesting, because there is added

versatility due to the potential for introducing additional functionality into themolecule, such as amine, amide, carboxyl and epoxide groups, which canenhance the use of these reagents in retanning.14 The reactivity of silicones isassisted by the unusually flexible nature of the molecule: the rotational energyfor the Si–O bond is close to zero, compared to 14 and 20 kJmol�1 forpolyethylene and polytetrafluoroethylene, respectively. In addition, poly-dimethylsiloxane exhibits the lowest known glass transition temperature(Table 17.3), making it the most flexible polymer known.

17.10.5 Non-chrome Tanned Leather

The alternative substrate for water resistance is leather tanned with reagentsother than chromium(III): this means all those organic tannages, such asaldehyde, syntan and vegetable. These tannages have the common feature ofmaking the leather highly hydrophilic. This makes the task of introducingwater resistance much more difficult than for chrome leather. The optionsavailable are:

� Silicones, which can bind to the hydrophilic groups, but are not veryeffective as solo agents.

� Fluorocarboxylate complexes of chromium(III). The 3M company offerssuch a product. However, if the purpose of the tannage is to make mineral-free, specifically chrome-free, leather this is counter-productive.

Table 17.3 Glass transition temperatures(Tg) of some polymers.

Polymer Tg (1C)

Polydimethylsiloxane �127Polyethylene �125Polythiofluoromethylene �118Polytetrafluoroethylene �113Polyisobutylene �73

419Fatliquoring

REFERENCES

1. A. D. Covington et al., Annexe to Proc. IULTCS Congress, Porto Alegre,Brazil, 1993.

2. A. D. Covington and K. T. W. Alexander, J. Amer. Leather Chem. Assoc.,1993, 88(12), 241.

3. T. Waite Proc., Conf., Soc. Leather Technol. Chem., 1986, p 3, 1–6.4. R. M. Koppenhoeffer, J, Amer. Leather Chem. Assoc., 1952, 47(4), 280.5. A. D. Covington and K. T. W. Alexander, J. Amer. Leather Chem. Assoc.,

1993, 88(12), 252.6. P. S. Briggs, J. Soc. Leather Trades Chem., 1968, 52(8), 296.7. P. S. Briggs, J. Soc. Leather Trades Chem., 1969, 53(8), 302.8. J. Amer. Leather Chem. Assoc., 1995, 90(4).9. J. Amer. Leather Chem. Assoc., 1995, 90(5).

10. Official Methods of Analysis, Soc. Leather Technol. Chem., UK, 1996.11. Defence Standard, UK/SC5611 Issue 1, 1998.12. R. Palop and A. Marsal, J. Soc. Leather Technol. Chem., 2000, 84(4), 62.13. R. L. Sykes, et al., J. Soc. Leather Technol. Chem., 1978, 62(3), 55.14. D. Narula, J. Amer. Leather Chem. Assoc., 1995, 90(3), 93.

420 Chapter 17

CHAPTER 18

Drying

From Chapter 1, water clearly plays a major part in leather making: both interms of structural features and the part it plays in processing; as expressed byBienkiewicz, leather and water are a ‘system’.1 In her studies of the effects onthe viscoelastic properties of collagenic biomaterials of moisture content andheat, Jeyapalina et al.2 approached the system from a materials science point ofview. In this chapter the discussion of the effect of drying regime is taken fromJeyapalina’s work (S. Jeyapalina, G E Attenburrow, A D Covington, unpub-lished results)3 in which important implications of the water–leather relation-ship for leather properties are defined, in particular the effect of drying onleather softness.Drying of leather has been intensively studied and reviewed.4–16 The process

has been explained in terms of different phases of drying applying to differentstates of water that are found within leather, bulk water and different degrees ofbound water.1 In its natural state, skin contains 150–190% of water on a dryweight basis. Chemical processing changes the fibre structure at the molecularlevel and alters the relationship between structure and moisture content inthe pelt, causing variation in the capillary water. The water in collagen canbe divided into three main groups: structural water, bound water andbulk water.17 Bulk water has a liquid-like character and can form ice crystalsat 0 1C. Bound water exhibits a structure between solid and liquid,19 so itdoes not freeze at 0 1C. Structural water molecules are part of the fibrestructure and behave like a solid. Figure 18.1 illustrates the observed progressof drying.In the earliest stage of drying, there may be sufficient bulk water on the

surface of a leather for it to act like a liquid water surface.1 Evaporation occursat a constant rate, which is directly proportional to the surface area of leather(A), mass transfer coefficient (Kg) and to the difference between the vapourpressure of the water at the surface temperature (Ps) and the partial pressure of




421

the water vapour in the air (Pa):

dW=dt ¼ AKgðPs � PaÞ

This indicates that the rate of drying is dependent on the temperature and therelative humidity of the air. In theory, fast drying conditions can be used duringthis phase of drying, as long as the losses of the bound and structural watermolecules are prevented. After this constant rate of drying period is completed,the rate of drying is controlled by the diffusion of moisture/vapour from thehigher concentration in the centre of the leather to the lower concentration atthe surface. During this period the rate of drying is entirely dependent on theease of diffusion of water vapour through the structure. Consequently, thefactors that affect the diffusion rate will control the drying rate.Friedrich attributed the constant rate period, a, to the removal of water from

between fibres, the first falling rate period, b, to the water that is situatedbetween the fibrils and the second falling rate period, w, to the water locatedwithin the fibrils.18 He also reported that the constant drying rate and the firstfalling rate cease at the critical points of 50% and 30% moisture contentsrespectively18 confirmed by Elamir.19 Leather is a viscoelastic material and, as aresult, the mechanical response to loading involves both a viscous component,associated with energy dissipation, and an elastic component, associated withenergy storage. Water acts as a plasticiser in leather and hence introducing orremoving this plasticiser will inevitably change the viscoelastic properties. Itfollows that, during drying, the viscoelastic properties of leather change con-tinuously with the removal of water. Therefore, the measurement of thedynamic mechanical behaviour of leather during drying can give vital infor-mation on changes occurring to the internal fibre structure.2

Figure 18.2 shows the dynamic mechanical thermal analysis (DMTA)drying curve for partially processed chrome tanned leather at constant air

Rate of drying

Moisture in leather

X

β

α

Figure 18.1 Rate of drying as a function of moisture content.

422 Chapter 18

temperature 70 1C, where the modulus and tan d are expressed as a function oftime. The bend modulus E0, indicating the stiffness of the leather, shows anoverall increase as the leather changes from a wet state to a dry state. Thegeneral shape of the curve is the same regardless of the drying temperature – theonly difference is the point at which inflections are observed.If the profile of the DMTA drying curve is related to the molecular motions

associated with leather fibres, then at the second inflection point, c, tan d isexpected to give rise to a peak. None of the results show such behaviour andhence suggest the changes observed in modulus must be related to the moisturecontent of the leather.Figures 18.3 and 18.4 show the DMTA drying curves for chrome tanned

leather at 20–70 1C. The moisture contents at the inflection points b and c ofthe modulus curve remain constant regardless of the drying temperature: theinflection point b corresponds toB60% moisture content and inflection point ccorresponds to B30% moisture content.Figure 18.5 shows the drying profile of vegetable (tara) tanned leather. The

general shape of the curve is same as that obtained for chrome tanned leather,but the moisture contents at the inflection points differ: the critical moisturecontents are 42% at inflection point b and 27% moisture at inflection point c.However, when the moisture contents at the inflection points are calculated

on dry collagen weight, the chrome and vegetable tanned leathers show similarvalues: B60% at inflection point b, B34% at inflection point c. The obser-vations from Figures 18.3–18.5 suggest that the inflection points on the dryingcurve must relate to the collagen–water relationship, rather than be a reflectionof the chemistry of the leather type. The changes in the modulus during thedehydration process of a leather must be related to the different states of water

6.0

6.5

7.0

7.5

8.0

Time (min)

E′ (

MP

a)

-0.02

0.02

0.06

0.10

0.14

0.18

0.22

0.26

0.30

Tan

◊δ

E′ (MPa): 1Hz

E′ (MPa): 10Hz

tan δ: 1 Hz

tan δ: 10Hz

a

b

c

d

20 400 60 80 100 120 140

Figure 18.2 DMTA drying trace of partially processed wet blue at 70 1C: the initialand the final moisture contents of the sample were 162% and 7%,respectively, on dry weight basis. (Courtesy of Journal of the Society ofLeather Technologists and Chemists.)

423Drying

6.0

6.2

6.4

6.6

6.8

7.0

7.2

7.4

7.6

7.8

8.0

120

Time (min)

E′(M

Pa)

0

20

40

60

80

100

120

140

160

% M

ois

ture

co

nte

nt

(dry

wei

gh

t b

asis

)

Modulus changes during drying at 70°C

% Moisture content during drying at 50°CModulus changes during drying at 50°C% Moisture content during drying at 70°C

bb

c

c

0 20 40 60 80 100

Figure 18.3 Changes in storage modulus (E0) and moisture content as a functionof time during drying of wet blue leather at 50 and 70 1C at 65%relative humidity. (Courtesy of Journal of the Society of Leather Tech-nologists and Chemists.)

6.2

6.4

6.6

6.8

7.0

7.2

7.4

Time (min)

E′ (

MP

a)

0

20

40

60

80

100

120

140

160

180

% M

ois

ture

co

nte

nt

(dry

wei

gh

t b

asis

)

Modulus changes during drying at 20°C% Moisture content during drying at 20°CModulus changes during drying at 30°C% Moisture content during drying at 30°C

0 500 1000 1500 2000 2500 3000 3500

Figure 18.4 Changes in storage modulus (E0) and moisture content during drying ofwet blue at 20 and 30 1C at 65% relative humidity.

424 Chapter 18

and, therefore, a generic master curve for drying of leather can be compiled(Figure 18.6).From differential scanning calorimetry (DSC) analysis, it is possible to

measure the amount of freezable water in the leather and this is plotted as afunction of total moisture content in Figure 18.7.

0

5

10

15

20

25

Time (min)

E′ (

MP

a)

0

20

40

60

80

100

120

140

160

% M

ois

ture

co

nte

nts

(d

ry w

t b

asis

)


% Moisture content during drying at 50°C


% Moisture content during drying at 25°C

c

c

b

200 400 600 800 1000 1200 1400 16000

Figure 18.5 Changes in storage modulus (E0) and moisture content during dryingof partially processed vegetable (tara) tanned leather at 25 and 50 1C at65% relative humidity. (Courtesy of Journal of the Society of LeatherTechnologists and Chemists.)

Time

Mo

du

lus

a

b

c

d

Moisture content ~ 60% (dry collagen weight basis)

Moisture content ~30% (dry collagen weight basis)

Figure 18.6 Generic curve of changes in bend modulus during drying of leather.

425Drying

From Figure 18.7, the non-freezable portions of water in chrome tannedleather and vegetable tanned leather are approximately 61% and 42%,respectively. When these values are normalised to the weight of collagen in theleathers, both leathers contain 60� 3% moisture on dry weight. Therefore,the initial increase in bend modulus from a to b in Figure 18.6 is attributed tothe removal of freezable water from the leather.Heidemann and Keller20 have analysed the equatorial X-ray diffraction

peaks of hide at various moisture contents: they observed that the intensity ofthe diffraction goes through a maximum with the water content. This impliesthat the crystallinity/orderly packing of collagen molecules has a high depen-dency on the moisture content of the sample and the optimum order is when thesample contains approximately 33% moisture on wet weight or 49% moistureon dry weight basis. Hence, as the drying proceeds, the storage modulus ofthe sample is expected to increase with decreasing moisture content, until theoptimum molecular order is reached at 49% moisture content. The initialincrease in storage modulus from a to b in Figure 18.6 is therefore attributed toan increase in the crystallinity of the collagen.There are two possible explanations for the decrease in modulus from b to c

in Figure 18.6. First, it comes from decreasing crystallinity of the collagenmolecules with removal of water. Second, it might be due to changes in fibrildimensions. Komanowsky4 has demonstrated that, as water is removed fromthe space between fibrils, a rapid, longitudinal and lateral shrinking of fibrilstakes place; this process occurs as the average moisture content drops fromabout 50% to about 27% on dry weight. These dimensional changes wouldallow more freedom of movement to the fibre network and, hence, a decrease inbend modulus with decreasing fibre size would be expected.Several workers have suggested that, below 23% moisture on dry weight,

water content is associated with the collagen structure. Pineri and co-workers21

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4W

eig

ht

of

free

zab

le w

ater

per

un

it o

f le

ath

er w

eig

ht

(mg

/g)

Moisture content ( % on dry weight basis)

Vegetable tanned leather

Chrome tanned leather

Disappearance of freezable water

0 20 40 60 80 100 120 140 160 180

Figure 18.7 Freezable or free water plotted against total water content of the leathersample.

426 Chapter 18

have argued that a water concentration between 11% and 23% corresponds tothe association of water molecules between triple helices. Removal of suchassociated water will decrease the free volume of the collagen molecules andthus an increase in modulus is expected.Below point c, the equatorial spacing changes are almost directly proportional

to moisture content.22 This indicates these water molecules may be directlybound to the sidechains of the collagen helix or microfibrils, so removal of thistype of bound water molecules creates voids between the fibrils. To fill the void,fibres are drawn together and the alignment of sidechains may form covalentcrosslinks: a molecular dynamic simulation by Mogilner and co-workers23

illustrates this possibility. They have concluded that the absence of associatedwater produces a distortion of molecular conformation and also introducesan increased number of hydrogen bonds. This will restrict molecular mobility ofthe collagen and will result in an increase in bend modulus. Once all thewater that can be removed from the structure of collagen has been removed,the modulus will plateau. Therefore, an increase in modulus between c and d(Figure 18.6) is attributed to a decrease in free volume, as well as an increase incrosslinking between collagen molecules. The DMTA drying profile of leathersunder various isothermal conditions indicates that differences in storage modulusare only found between c and d. The implication is that the stiffness of leather isonly governed by the final phase of drying.It can be concluded that leather may be dried at high temperature to give a

softer product, as long as the humidity is maintained to stop the structuralwater molecules being lost to cause stiffness. In practice, controlling thehumidity of the drying environment at high temperatures presents problems.The implication is that there would be a benefit in having a two-stage dryingprocess: during the first stage, high temperature may used to remove moistureuntil approximately 30% moisture on dry weight is achieved, after whichleather may be dried in a conditioned environment to give a final residualmoisture content of 14%.

18.1 TWO-STAGE DRYING

To examine the concept of drying in phases, Jeyapalina et al.2 (andS. Jeyapalina, G E Attenburrow, A D Covington, unpublished results) driedchrome tanned leathers at various temperatures for 48 hours, but in anatmosphere controlled at 85% relative humidity, when the moisture content fellto 35%, after which they were conditioned at 65% relative humidity and 20 1C,so the moisture was equilibrated at 14%. Control chrome tanned leathers weredried at constant temperature for 48 hours, in an atmosphere without humiditycontrol, after which they were conditioned in the same way. Figure 18.8 showsthe effect of drying regime on the bend modulus of the leathers.When leather is dried at a high temperature, the stiffness is generally found to

be high. Therefore, it is commonly believed that stiffness has a high dependencyon the drying temperature. Conversely, Figure 18.7 shows that the two-stagedrying process can produce softer leather. This confirms that the water–leather

427Drying

relationship during drying plays an important part in determining the softnessproperties of leather.Drying of leather at elevated temperature is a diffusion-controlled process.5,6

The rate of drying is entirely dependent on the concentration gradient: whenthe evaporation of water is faster than the diffusion of water from interfibrillaryregions to the surface, because the temperature of drying is too high, then theouter surfaces may become irreversibly crosslinked, as the structural water ofleather fibres is lost. Once fibre sticking is introduced into the collagenousmatrix, rehumidifying will not replace the lost structural water to the sameextent as before, resulting in the high bend modulus observed for the leathersamples dried at high temperature. When leather is dried with controlledhumidity, the rate of drying will still be a diffusion-controlled process at hightemperature and the gradient of water vapour pressure between the leather andthe external atmosphere falls with drying time. Thus, when the equilibriummoisture content is approached, the rate of drying slows down. If the relativehumidity is high enough to stop any structural water being replaced, thendrying the leather at a high temperature will be faster, with no undesirable effecton stiffness.Komanowsky4 has explained why high degree of shrinkage of fibres occurs

when moisture contents drops from about 50% to 27% (on a dry weight basis).During the initial stage of drying the water lost from smaller capillaries isquickly replaced by capillary ‘suction’ from larger capillaries. Larger capillaryregions vary in size and hydrostatic pressure is reported to be lower than the

0

20

40

60

80

100

120

140

90Temperature (°C)

Ben

d m

od

ulu

s (M

Pa)

Control set

Experimental set

0 10 20 30 40 50 60 70 80

Figure 18.8 Effect of drying temperature on bend modulus: higher modulus meansstiffer leather. Experimental leathers were dried to 35% moisture contentat various temperatures and then conditioned to 14% moisture contentat 65% RH and 20 1C. Control leathers were dried at various tempera-tures, after which they were conditioned at 65% RH and 20 1C.

428 Chapter 18

3.5 atm osmotic swelling pressure. The hydrostatic pressure generated duringthe evaporation of water molecules will not therefore exceed the osmoticswelling pressure and hence no shrinkage of fibre dimensions is expected.Therefore, removal of water at a higher rate at high temperature will be withoutundesirable consequence. After the larger capillaries have lost all the water,during the second stage of drying, water from the inter-fibril region is removed.The hydrostatic pressures of these regions are reported to be �18.2 and�78 atm, respectively.4 These values are greater than the osmotic swellingpressure and hence a noticeable shrinking of fibres is expected. However,employing high temperature to remove moisture from this region will not causeadverse effects because closely bound structural water is not lost. In practice, itis not easy to predict the precise end of this drying period and prolonging thesedrying conditions into the next stage of drying may cause damage at the sur-face. During the final stage of drying, water molecules that are removed mustbe from the intra-fibril region, the bound structural water.17 If the leather is tobe dried at elevated temperature during this phase of drying, while fibreshrinking is also proceeding due to higher hydrostatic pressure, fibrils have agreater possibility of coming close enough to form intermolecular crosslinksand hence fibre sticking. High temperature also facilitates permanent cross-linking between collagen molecules by supplying the activation energy.Therefore, to produce softer leather, more moderate drying conditions shouldbe employed during this final stage of drying.Figure 18.9 shows the effect of drying regime on the strength of chrome

tanned leather. Drying by the two-stage process was shown to produce softer

0

5

10

15

20

25

30

Temperature (°C)

Ten

sile

str

eng

th (

MP

a)

Experimental setControl set

80 70 60 50 40 30 20

Figure 18.9 Effect of drying temperature on the tensile strength of wet-blue dried by atwo-stage procedure. Experimental samples were dried to 30% moisturecontent at various temperatures and then conditioned to 14% moistureat 65% RH and 20 1C. Controlled samples were dried at various tem-peratures, after which they were conditioned to 65% RH at 20 1C.

429Drying

leather than the conventional drying approach, but not at the expense ofadversely altering its tensile strength.In the context of drying, Corning (D.R. Corning, BLC The Leather

Technology Centre, unpublished results) demonstrated that leather area iscontrolled only by the moisture content and not by the rate of drying, which iscontrolled by the temperature of drying. Therefore, if the drying regimeinvolves introducing structural changes that cannot be reversed by rehydration,area is lost.

18.2 CONCLUSION

The commercial alternatives for drying have traditionally been as follows:

1. Dry completely to moisture content 410%, then recondition (‘season’)with sprayed on water and allow equilibration, but rarely under condi-tions of a temperature and humidity controlled environment.

2. Dry down the required moisture content.

The former option is easy to implement: the leather is dried at elevatedtemperature to equilibrium and then rehydrated with a limited quantity ofwater. The ease of operation is balanced with a loss of quality in terms of thephysical properties. The latter option has always been viewed as impractical,because it would involve sophisticated process control. However, now it hasbeen shown that this option is actually practical, because the only change toconventional hard drying technology is the addition of humidity control inthe drier.By adopting the use of high air temperature the initial stages of drying to

achieve rapid drying down to 30%, followed by a slower rate of drying forthe removal of the remainder of the moisture from the leather, the quality of theleather is improved, without major alteration to the operational aspects ofprocessing. In addition, the two-stage drying process does not result in irre-coverable area loss, which is reason enough to consider adopting this devel-opment in technology.

REFERENCES

1. K. Bienkiewicz, J. Amer. Leather Chem. Assoc., 1990, 85(9), 305.2. S. Jeyapalina, PhD Thesis, The University of Northampton, 2004.3. S. Jeyapalina, G. E. Attenburrow and A. D. Covington, J. Soc. Leather

Technol. Chem., 2007, 91(3), 102.4. M. Komanowsky, J. Amer. Leather Chem. Assoc., 1990, 85(1), 6.5. J. Lamb, J. Soc. Leather Tech. Chem., 1982, 66(1), 8.6. K. Bienkiewicz, Physical Chemistry of Leather Making, Krieger Publishing

Co., Malabar, Florida, 1983.

430 Chapter 18

7. L. Buck, in: The Chemistry and Technology of Leather, Eds. F. O’Flaherty,W. T. Roddy and R. M. Lollar, Reinhold Publ. Corp., New York, 1962.

8. K. T. W. Alexander, D. R. Corning and A. D. Covington, J. Amer. LeatherChem. Assoc., 1993, 88(12), 252.

9. E. Heidemann, Fundamentals of Leather Manufacturing, Eduard RoetherKG, Darmstadt, 1993.

10. A. W. Landmann, J. Soc. Leather Technol. Chem., 1994, 78(2), 44.11. M. Komanowsky, J. Amer. Leather Chem. Assoc., 1990, 85(5), 131.12. S. A. J. Shivas and A. Choquette, J. Amer. Leather Chem. Assoc., 1985,

80(5), 129.13. B. Abu el hassan, A. G. Ward and S. Wolstenholme, J. Soc. Leather

Technol. Chem., 1984, 68(5), 159.14. J. Monzo-Cabrera, et al., J. Soc. Leather Technol. Chem., 2000, 84(1), 38.15. E. P. Lhuede, J. Amer. Leather Chem. Assoc., 1969, 64(8), 375.16. K. Liu, N. P. Latona and J. Lee, J. Amer. Leather Chem. Assoc., 2004,

99(5), 205.17. A. R. Haly and J. W. Snaith, Bioploymers, 1971, 10, 1681.18. E. Friedrich, in: Handbuch der Gerbereichemie und Lederfabrikation, Ed.

W. Grassmann, Springer-Verlag, 1955.19. F. M. Elamir, Ph.D Thesis, Leeds University (1975).20. E. Heidemann and H. Keller, J. Amer. Leather Chem. Assoc., 1970, 65(11),

512.21. M. H. Pineri, M. Escoubes and G. Roche, Bioploymers, 1978, 17, 2799.22. M. A. Rougvie and R. Bear, J. Amer. Leather Chem. Assoc., 1953, 48(12),

735.23. G. I. Mogilner, G. Ruderman and R. Grigera, J. Molecular Graphic

Modelling, 2002, 21(3), 209.

431Drying

CHAPTER 19

Theory of Tanning:Concept of Link-Lock

19.1 INTRODUCTION

A review of the literature indicates that the theoretical basis of leather making,particularly the impact of tanning technologies, had received little attentionprior to the work of Gustavson.1 The suggestion that the mechanism of tanningdepends on a crosslinking effect had been made and the breaking of the arti-ficial bridges claimed to be the basis of the shrinking transition.2 This viewreceived some criticism because some stabilising reactions actually weakencollagen, but later it was demonstrated that tanning reactions may eitherweaken or strengthen collagen.3 In fact there is no direct correlation betweenhydrothermal stability and physical stability. The concept of crosslinking seemsto have been generally accepted by the scientific and technological community,although Gustavson himself remarked:1

It must be admitted that the concept of crosslinking has been appliedin many instances without sufficient experimental support, even purelyspeculatively. Critical appraisal of this concept is therefore appropriate.

In his studies on chrome tanning, Gustavson thought he had provided theproof required, that the proportion of crosslinks in bound chrome couldbe measured by wet chemistry analysis. It was shown in Chapter 11 that theassumptions and conclusions of that analysis were flawed; consequently, theonly evidence that remained to support the concept is indirect.Chemically unmodified collagen has received much attention, regarding the

origin of its high hydrothermal stability relative to other proteins. Thestability of intact, native collagen can be attributed in the first instance to itshierarchy of structure, within which hydrogen bonding4 and inductive effects5




432

are likely to operate. In addition, it has been postulated that the quinarystructure, namely the packing of the triple helices into fibrils, constitutes a‘polymer-in-a-box’ model,6 providing a further level of stabilisation. Recentstudies on unmodified and chemically modified collagen confirmed that theobserved hydrothermal stability is dependent on the moisture content: reducingthe water content causes the fibres to approach more closely, preventing themfrom collapsing into the interstices and this effect is correlated with elevateddenaturation temperature.7 Therefore, a reduced ability to shrink is the same asincreased hydrothermal stability. Consequently, chemical modification mayreduce the ability of collagen to shrink, which results in higher observeddenaturation temperature.The triple helix of collagen is surrounded by a supramolecular water sheath

nucleated at the hydroxyproline sidechains,8 in which the water moleculesbinding the sheath to the triple helix are labile but constitute a stable structure.9

In studies of the effects of basic chromium(III) sulfate and tara extract(hydrolysable plant polyphenol) on skin collagen, dynamic mechanical thermalanalysis of leather during drying indicates that the amount of structural water isindependent of the nature of the stabilising chemistry.10 When collagen is wet,the matrix can be degraded at elevated temperature, at the same time hydrogenbonds in the triple helix are broken, allowing the alpha helices to becomedetached from the triple helix structure, observed as shrinking, leading togelatinisation. If the supramolecular structure is only made of water, the pro-cess is facilitated to the greatest degree, causing the lowest denaturation tem-perature. However, if the supramolecular water sheath is chemically modified,resistance to shrinking is introduced, which is observed as an elevation of thedenaturation temperature, i.e. higher hydrothermal stability.The hydrothermal stability of collagen can be altered by many different

chemical reactions, well known in the fields of histology, leather tanning andother industrial applications of collagen.11 Table 19.1 summarises the effects of

Table 19.1 Typically observed effects of some chemical modifications ondenaturation temperature ranges of collagen.

Chemical modificationDenaturation tempera-ture (1C)

None 65Metal salts: e.g. Al(III), Ti(IV), Zr(IV), etc. 70–85Plant polyphenol: gallotannin or ellagitannin 75–80Plant polyphenol: flavonoid 80–85Synthetic tanning agent: polymerised phenols 75–85Aldehyde: formaldehyde or glutaraldehyde 80–85Aldehydic: phosphonium salt or oxazolidine 80–85Basic chromium(III) sulfate 105–115Semi-metal: hydrolysable polyphenol+(for example) Al(III) 110–120Synthetic polymer+aldehydic reagent, e.g. melamine–formaldehyde+oxazolidine

105–115

433Theory of Tanning: Concept of Link-Lock

some of these chemical modifications; the denaturation temperature is typicallymeasured by the perceptible onset of shrinking.From these selected data, well known in the art of leather making, it

appears that the stabilising effects fall into two groups: moderate increaseor large increase in hydrothermal stability. This has been rationalised interms of the entropic and enthalpic contributions to the modified collagenstructure,12 when the shrinking/denaturation kinetics are controlled by theenthalpy of activation, moderated by the entropy of activation.11 In thisway, most chemical modifications confer only the moderate shrinkage tem-perature rise observed in most cases, because they contribute only a minorenhancement to the structure. Here, the assumed basis of hydrothermalstability was the assumption that unravelling of the triple helix wascontrolled by the supramolecular effect of crosslinking the triple helices,primarily by interacting with the amino acid sidechains. Using that con-ventional view of the tanning mechanism, it was possible to constructa theoretical model of the tanning mechanism, in which hydrothermalstability was correlated with the degree to which the collagen could beheld together by stiff crosslinks, which were bound to the triple helices byhigh energy interactions.11,13,14 This view seemed to explain the observedeffects of known tanning systems, although it did not explain the relationshipbetween the chemistry of tanning and the mechanism of hydrothermalshrinking.In his John Arthur Wilson Memorial Lecture, Ramasami reviewed the

history of tanning theories over the last century in an attempt to create aunified theory.15 He too accepted the notion of crosslinking, but focussedhis thinking on the effect of tanning compounds to alter the structure ofcollagen at different levels of the hierarchy of collagen structure. Sincethe definition of tanning is based on resistance to biochemical attack, heconsidered the effects of tanning reagents on the reaction between collagenaseand collagen. He concluded that there is a correlation between hydrothermalstability and conformational changes at the fibrillar-triple helix level ofcollagen structure and the consequent effect on resistance to enzymaticdegradation.There is a clear discrimination between those tanning reactions that confer

moderate hydrothermal stability and those that confer high hydrothermalstability to collagen. This begs the question: if most chemical modifications canonly raise the denaturation temperature by up to 20 1C, what is the mechanismby which collagen can achieve high hydrothermal stability, when the dena-turation temperature is raised by over 60 1C? To put this into context, assumingan activation energy for the shrinking/denaturation reaction of 518 kJmol�1,16

elevating the shrinkage temperature from 65 to 85 1C enhances the hydro-thermal stability by four orders of magnitude, but the rise from 85 to 110 1Crepresents a further five.In this chapter, an answer to that question is proposed, by synthesising an

explanation, a mechanism of tanning, based on recent thinking in leathermaking science, the observation of known tanning reactions and experimental

434 Chapter 19

investigations of novel reactions for stabilising collagen. The requirements ofany useful theory are threefold:

1. It must explain all known stabilising reactions, without exception;2. It must be capable of predicting the requirements for new reactions;3. It must allow accurate predictions of the outcomes of new reactions.

19.2 DISCUSSION

The combination reactions in Table 19.1 have some features in common:

� The primary reaction between hydrolysable plant polyphenols, the gallo-tannins and ellagitannins, and collagen is through multiple hydrogenbonding. The subsequent reaction is to crosslink the tannin moleculestogether, via the pyrogallol moieties locked together by the complexationreaction with metal ions.17,18

� There is a similar primary reaction between collagen and the flavonoidtannins, which is weaker in terms of the availability of phenolic hydroxylreaction sites on the tannin, but which also includes some covalentbonding between the lysine sidechains of the collagen and the aromaticA-ring of the flavonoid, via quinoid reactions. The next reaction involvescrosslinking the tannin molecules by oxazolidine, an aldehydic reactant:this occurs at the 6- and 8-positions on the A-rings of procyanidin orprorobinetinidin polyphenols, and additionally at the 20- and 60-positionsof the B-rings of prodelphinidin or profisetinidin polyphenols,19–21 whenthe latter polyphenolic reactions yield higher denaturation temperatures.

� Similar high stability combination reactions have been observed withmelamine-formaldehyde polymer as primary reactant, with tetra-kis(hydroxymethyl)phosphonium sulfate as the aldehydic crosslinker22

and with low molecular weight phenolic compounds crosslinked withaldehydic compounds or laccase, polyphenol oxidase.23

In each case, the synergistic combination reactions create matrices ofcrosslinked polyphenolic or synthetic polymeric species, which means they actin concert, effectively working as a single chemical moiety. An important fea-ture of the flavonoid combination stabilisation reaction is the linking of thepolyphenol species to collagen via the aldehydic crosslinking reaction.15 Thesehigh hydrothermal stability reactions also exhibit a strong reaction with col-lagen: in some cases multiple hydrogen bonding and in others there is somecovalent reaction. In this way, the matrix is firmly bound to the collagenstructure. It has been proposed that this aspect of the combination reaction isan important element in the overall requirements for matrix stabilisation ofcollagen that leads to high shrinkage temperature.23 The work of Holmessupports the part played by fixation of the matrix to collagen:24 he showed thatmodifying collagen by binding chelating pairs of carboxyl groups to the amino


groups of lysine can enhance the weak stabilising effect of aluminium(III) salts,to the extent of increasing the denaturation temperature from 75 to 95 1C.In the group of high stability reactions, the process involving chromium(III)

salt might appear to be a chemical exception, having little in common with theorganic combination reactions. This is deceptive. The chromium(III) species areknown to be covalently bound at carboxyl sidechains. Extended X-rayabsorption fine structure (EXAFS) studies25 of chromium(III) bound to col-lagen showed that the dominant bound species are linear tetrachromiumcompounds, but the counterion, in this case sulfate, is not directly bound to thechromium as a ligand. Furthermore, if the counterion is different, e.g. chlorideor perchlorate,26 the effect of the stabilisation is only moderate, producingdenaturation temperatures of about 85 1C. So, the chromium(III) species andthe counterion must create a matrix with the supramolecular water structure, ina system analogous to the combination reactions. In the case of the inorganicreaction, the effect is a combination of the metal ions and the counterions andthe water. The overall synergistic nature of the combined reactions depends onthe functioning of the counterion as a structure maker in water: sulfate acts inthis way, but chloride is a water structure breaker,27 shown by circulardichroism to be capable of destabilising the collagen structure.28

A useful method for gaining insight into the structure of modified collagen ishydrothermal isometric tension (HIT), in which samples are constrainedagainst shrinking and the forces generated during transitions are recorded.Figures 19.1–19.3 show the HIT curves of unmodified and chemically modifiedcollagen.29

The traces can be divided into three parts: the first is the tension increasingprocess from zero to maximum tension, followed by a relatively constant ten-sion process, if present; finally, the tension will be constant or a relaxationprocess will occur, due to the gradual destruction of collagen structure orrupture of some crosslinking bonds. The slope of the curve in the tension

1

3

5

7

9

11

13

15

17

20 45 70 95 120Temperature (°C)

Collagen

Oxazolidine

Ten

sio

n (

mV

)

Tea polyphenol

Mimosa

Figure 19.1 Hydrothermal isometric tension tests on skin treated with single organicstabilising agents.

436 Chapter 19

increasing process accounts for the collagen fibre rigidity, caused by crosslinks:the steeper the slope of the contraction curve the more crosslinks should bepresent in the collagen materials. Relaxation represents the stability of theseconnecting elements (crosslinking bonds): the steeper the rate of the relaxationcurve the more unstable is the crosslinking. In the case of collagen, the cross-linking does not refer to direct links between triple helices but to the structureof the intervening matrix. Although we cannot quantify crosslinking, westill can get some useful information about the relative crosslink density andstability from the shapes of the HIT curves. Table 19.2 shows the calculatedresults.29

Stabilisation by polyphenol reactions yields results that can be understood interms of the availability of hydrogen bonding and subsequent localised

1

3

5

7

9

11

13

15

17

20 45 70 95 120Temperature (°C)

Cr(III) -A

Al(III) -B

Ti(IV)

Collagen

Ten

sio

n (

mV

)

Mimosa + Al(III)B

A

Figure 19.2 Hydrothermal isometric tension tests on skin treated with mineralstabilising agents.

1

3

5

7

9

11

13

15

17

20 45 70 95 120Temperature (°C)

Collagen

Tea polyphenol+ oxazolidine Mimosa + oxazolidine

Ten

sio

n (

mV

)

Mimosa + Al(III)

Figure 19.3 Hydrothermal isometric tension tests on skin treated with combinationsof stabilising agents.


crosslinking. However, the rate of tension increase during the shrinking tran-sition of chromium(III) stabilised collagen exhibits a result similar to that of rawcollagen. The natural conclusion is that chromium does not crosslink collagenby joining adjacent sidechains. This runs counter to the traditional view ofcollagen stabilisation, in which the sidechains of the collagen structure aresupposed to be linked by single species, such as chromium(III) molecular ions;however, the only quantitative analysis in the literature leading to such aconclusion, the Gustavson model,1 is flawed.30 Therefore, the traditional viewof crosslinking does not have to be invoked in explaining chemical stabilisationreactions. The explanation lies in the creation of a chemical matrix, securelybound to the collagen.Bone is an example of a situation where the stability of collagen clearly comes

from the presence of a matrix. Here, the matrix is polymeric hydroxyapatite,not thought of as a typical crosslinker, but which can raise the shrinkagetemperature of the constrained collagen to upwards of 155 1C.6,31 In terms ofthe ‘polymer-in-a-box’,6 this is a very strong ‘box’, an extreme case andprobably the upper limit to collagen stabilisation, without dehydrating theprotein. Indeed, it is not yet clear that the mechanism of denaturation of col-lagen in bone is the same as for free collagen, because at this elevated tem-perature the melting of the triple helix will be competing with hydrolysis of thepeptide backbone, which has a lower activation energy (173 kJmol�1).32 Thiswould explain the surprisingly low activation energies (B350 kJmol�1)observed by Trebacz and Wojtowicz for the denaturation of mineralisedcollagen.33

The stability of unmodified or chemically modified collagen must depend onits ability to collapse or shrink by unravelling its chains into the availablespace between the chains. The ease with which this can happen depends onthe constraints applied to the chains or the ease with which the interveningmolecules can be displaced as the collagen shrinks. If the molecules are water,this matrix can be broken down and displaced relatively easily, since thereaction is the breaking of the hydrogen bonds in bulk water. If the matrix is

Table 19.2 Relative rates of increase in tension after shrinking transition isinitiated and relative rates of decrease in tension after theshrinking transition.

Stabilising agent Slope of contraction Slope of relaxation

None (raw collagen) 0.25 �0.13Aluminium(III) 0.27 �0.20Oxazolidine 0.28 0.00Chromium(III) 0.29 Assumed zeroGreen tea polyphenol 0.37 �0.13Titanium(IV) 0.50 �0.52Mimosa 0.67 �0.20Mimosa+oxazolidine 0.91 Assumed zeroMimosa+aluminium(III) 1.23 �0.05

438 Chapter 19

stabilised by the inclusion of species bound to collagen, also by substitutingsome of the supramolecular water and interacting with the remaining water, thematrix is less easily displaced, which is observed as a rise in denaturationtemperature. If the notion of direct crosslinking between triple helices is dis-missed as the source of stabilisation, the matrix displacement model explainsthe similarity of outcome for the stabilising reactions already known. Some ofthe more compelling evidence comes from Das Gupta,34 in his studies ofoxazolidine tanning. In comparing a monofunctional with a bifunctionalderivative, it was shown that the outcome of solo tanning of sheepskin was thesame, the shrinkage temperature was about 80 1C: the potential ability tocrosslink did not affect the linking reaction. However, when used to retanvegetable tanned leather, the monofunctional oxazolidine conferred only asmall increase in Ts, but the bifunctional derivative produced a big increasein Ts, indicating that the bifunctional derivative is capable of crosslinking, butdoes not necessarily do it in this context.Therefore, the maximum effect of a single reagent, even with the potential to

crosslink, is due to interference in the shrinking process, i.e. the bound speciesmerely hinder the process by getting in the way. This view is confirmed by theHIT data and the specific case of chromium(III): if the crosslinking reaction isparamount, the stabilising effect of chromium(III) salts would be independent ofthe counterion – but it is demonstrably true that the shrinkage/denaturationtemperature is highly dependent on the nature of the counterion.30,35

Even the relationship between the linking agent and the supramolecularwater does not control the outcome of the stabilisation reaction. Table 19.1indicates the range of stabilising reactions that yield only moderate hydro-thermal stability: all have different relationships with water, in terms of theirhydrophilicity or hydrophobicity, but the results are the same. The small var-iations in hydrothermal stability achievable by linking reagents appear to berelated to the nature of their primary interaction with collagen: the weaker theinteraction, the lower the denaturation temperature. The order of increasinghydrothermal stability depends on the chemistry of the bonding: electro-staticohydrogen bondedocovalent, exemplified by the following selection ofreactions:

AlðIIIÞohydrolysable polyphenoloaldehyde:

It has been assumed that the shrinking reaction is independent of the sta-bilising reaction, because the enthalpy of denaturation is independent of thestabilising chemistry:36 the assumption was that the process is controlled onlyby the breaking of hydrogen bonds within the triple helix. If collagen stabili-sation is modelled in these terms,37 the number of crosslinks required to elevatethe melting temperature is unacceptably high. However, it is more satisfactoryto regard the shrinking process of collagen, unmodified or modified, as con-trolled by the breaking of hydrogen bonds of the water in the matrix. This isclear, since the relationship between denaturation temperature and moisturecontent is broadly similar (although different in detail) for unmodified collagen


and highly stabilised, chromium(III) tanned collagen,38 where the differencein the denaturation temperatures of the wet materials is about 40 1C, butthe denaturation/decomposition temperature for both the dry materials isabout 200 1C.The water associated with the collagen structure is practically independent of

the tanning chemistry: the amount is the same, about 23%, whether the col-lagen is stabilised with chromium(III), formaldehyde or high levels of vegetabletannin, hydrolysable or condensed.10,38 It can be further postulated that theremust be some of this structural water remaining in the matrix modified bythe tanning chemistry. Therefore, shrinking involves the unravelling of thetriple helix, aided by the breakdown of the residual structural water associatedwith the triple helix. That unravelling is only possible if the intervening matrixcan be rearranged around the new protein conformation, which presupposesthat the hydrothermal event can also affect the matrix. In the case of a watermatrix, the least hydrothermally stable, labile water can hydrate the unravelledchains and therefore there is no need to postulate a displacement of water orany other part of the supramolecular matrix out of the material.As an approach to stabilising collagen, merely loading the structure with

molecular species is not sufficient to confer high hydrothermal stability. Forexample, plant polyphenols confer only moderate stability, limited to about85 1C, even when present at 30% on dry weight of collagen. Like most otherstabilising reactions, the chemical reactions are limited to linking elements ofthe collagen structure to a relatively unstable matrix: in this case, a packedarray of unlinked molecules, but with some water still in the interstices. In thecase of high offers of condensed polyphenols, the excess molecules can interactwith bound molecules, forming ‘reds’ in situ. However, the interactions areweak and easily broken under hydrothermal conditions; therefore, the inter-actions do not constitute a stabilising of the matrix, so the hydrothermal sta-bility remains unchanged. The difference between the hydrolysable tannins andthe condensed tannins can now be understood in terms of their abilities tofunction as linking agents: an element of covalency in the reactions of con-densed tannins with collagen contributes to the stability of the matrix, resultingin shrinkage temperatures higher by 5–10 1C than the effects of a similarreaction based on hydrogen bonding alone.In those chemical processes that result in high hydrothermal stability, the

linking step must be combined with an additional step that locks the compo-nents of the matrix together. In this way, the matrix acts more like a singlechemical compound, which is much less easily displaced. The higher energyrequired to achieve breakdown of the structure is observed as a higher tem-perature transition. It is an important aspect of the matrix stabilisingmechanism that the matrix should be bound to the collagen in a stable way, sothat displacement of the interaction, which might lead to allowing shrinking, isprevented. This element of the high hydrothermal stability requirements may beconferred during the linking reaction or it may be conferred as a side effect, anadditional aspect of the locking reaction. Notably, it is reported to be impos-sible to create boilfast leather if the pelt is in the swollen condition,39 when it

440 Chapter 19

was assumed that the reason was the enhanced separation between the reactionsites. However, the observation can be rationalised in terms of the separationbetween the chromium(III) linking agents and the consequently reduced effec-tiveness of the locking reaction.Returning to the conventional concept of crosslinking between sidechains, it

should not be concluded that such crosslinking does not occur. If the chemistryof a tanning system allows bonds to be created between two sites on adjacenttriple helices, then crosslinks may be formed. Indeed it can be shown that thishappens, e.g. with 3,30-difluoro-4,4 0-dinitrodiphenyl sulfone reaction at lysinesidechains.40 It is also worth recalling though that glutaraldehyde, a reagentapparently capable of crosslinking to a high degree, does not exhibit anysuch reaction.41 The conclusion from the link-lock view of tanning is that thehydrothermal stability does not depend on creating crosslinks of this type,because by themselves they are incapable of conferring high shrinkagetemperature.

19.3 CONCLUSIONS

All reactions that stabilise collagen conform to the link-lock model:

1. Single-component tannages are restricted by the thermodynamics of theshrinking reaction to confer only moderate shrinkage temperature.

2. High shrinkage temperature can only be achieved by tanning with at leasttwo conventional components.

3. High shrinkage temperature is achievable if there is strong primarychemical bonding between the tanning agent and the collagen and if theprimary tanning agent is locked in place by a secondary tanning agent.The chemistry of the reactions is less important than conforming to therequirements. Therefore, a high shrinkage temperature is achievable bymany more routes than are currently known.

The current view of chemical stabilisation of collagen is derived from specificlinking reactions by single chemical species, between specific sidechains on thecollagen triple helices. This new explanation of the stabilisation of collagen issimpler and more powerful than the currently accepted view. There is no evi-dence in the literature to support the old assumed view, but much evidence toindicate the new, matrix-based view is right.

REFERENCES

1. K. H. Gustavson, J. Amer. Leather Chem. Assoc., 1953, 48(9), 559.2. K. H. Gustavson, J. Amer. Leather Chem. Assoc., 1946, 41(2), 47.3. K. H. Jacobsen and R. M. Lollar, J. Amer. Leather Chem. Assoc., 1951,

46(1), 7.4. L. Pauling, J. Amer. Chem. Soc., 1940, 62(10), 2643.


5. R. T. Raines, et al., Nature, 1998, 392, 666.6. C. A. Miles and M. Ghelashvili, Biophysical J., 1999, 76, 3243.7. C. A. Miles, C. A. Avery, V. V. Rodin and A. J. Bailey, J. Mol. Biol., 2005,

346, 551.8. H. M. Berman, J. Bella and B. Brodsky, Structure, 1995, 3(9), 893.9. M. Melacini, A. J. J. Bonvin, M. Goodman, R. Boelens and R. Kaptein,

J. Mol. Biol., 2000, 300, 1041.10. S. Jeyapalina, G. E. Attenburrow and A. D. Covington, J. Soc. Leather

Technol. Chem., 2007, 91(3), 102.11. A. D. Covington, J. Soc. Leather Technol. Chem., 2001, 85(1), 24.12. C. E. Weir, J. Amer. Leather Chem. Assoc., 1949, 44(3), 108.13. A. D. Covington, et al., J. Amer. Leather Chem. Assoc., 1998, 93(4), 107.14. A. D. Covington, J. Amer. Leather Chem. Assoc., 1998, 93(6), 168.15. T. Ramasami, J. Amer. Leather Chem. Assoc., 2001, 96(8), 290.16. C. A. Miles, T. V. Burjanadze and A. J. Bailey, J. Mol. Biol., 1995,

245, 437.17. J. F. Hernandez and W. Kallenberger, J. Amer. Leather Chem. Assoc.,

1984, 79(5), 182.18. R. A. Hancock, S. T. Orszulik and R. L. Sykes, J. Soc. Leather Technol.

Chem., 1980, 64(2), 32.19. A. D. Covington and B. Shi, J. Soc. Leather Technol. Chem., 1998,

82(2), 64.20. A. D. Covington, B. Shi, Y. He, H. Fan, S. Zeng and G. E. Attenburrow,

J. Soc. Leather Technol. Chem., 1999, 83(1), 8.21. A. D. Covington, C. S. Evans, T. H. Lilley and L. Song, J. Amer. Leather

Chem. Assoc., 2005, 100(9), 325.22. A.D. Covington and M. Song, Proc., IULTCS Congress, London,

September, 1997.23. A. D. Covington, C. S. Evans, T. H. Lilley and O. Suparno, J. Amer.

Leather Chem. Assoc., 2005, 100(9), 336.24. J. M. Holmes, J. Soc. Leather Technol. Chem., 1996, 80(5), 133.25. A. D. Covington, G. S. Lampard, O. Menderes, A. V. Chadwick,

G. Rafeletos and P. O’Brien, Polyhedron, 2001, 20, 461.26. K. Shirai, K. Takahashi and K. Wada, Hikaku Kagaku, 1975, 21(3), 128.27. A. Cooper, Biochem. J., 1970, 118, 355.28. E. M. Brown, H. M. Farrell and R. J. Wildermuth, J. Prot. Chem., 2000,

19(2), 85.29. A. D. Covington and L. Song, Proc. IULTCS Congress, Cancun Mexico,

May, 2003. pp 84–87.30. A. D. Covington, J. Amer. Leather Chem. Assoc., 2001, 96(12), 461.31. P. L. Kronick and P. Cooke, Connective Tissue Res., 1996, 33(4), 275.32. K. M. Holmes, K. A. Robson-Brown, W. P. Oates and M. J. Collins,

J. Arch. Sci., 2005, 32, 157.33. H. Trebacz and K. Wojtowicz, Int. J. Biol. Macromol., 2005, 37, 257.34. S. Das Gupta, J. Soc. Leather Technol. Chem., 1977, 61(5), 97.35. K. Shirai, K. Takahashi and K. Wada, Hikaku Kagaku, 1984, 30(2), 91.

442 Chapter 19

36. A. D. Covington, R. A. Hancock and I. A. Ioannidis, J. Soc. LeatherTechnol. Chem., 1989, 73(1), 1.

37. M. J. Collins, M. S. Riley, A. M. Child and G. Turner-Walker, J. Arch.Sci., 1995, 22(2), 175.

38. M. Komanowsky, J. Amer. Leather Chem. Assoc., 1991, 86(5), 269.39. K. H. Gustavson, Stiasny Festschrift, 1937, 117.40. H. Zahn and D. Wegerle, Das Leder, 1954, 5(6), 121.41. L. H. H. O. Damink, et al., J. Mats. Sci.: Mats. in Medicine, 1995, 6, 460.


CHAPTER 20

The Future of Leather Processing

20.1 FUTURE OF CHROME TANNING

The environmental impact of chromium(III) is low: as a reagent, basic chro-mium salts are safe to use industrially and can be managed efficiently, to theextent that discharges from tanneries can routinely be as low as a few parts permillion. Therefore, clearly, the industry should be able to meet all the futurerequirements of environmental impact. Consequently, we can reasonablyassume that the future of tanning will include a major role for chrome.Nevertheless, the technology can be improved: efficiency of use can beimproved, as can the outcome of the reaction, in terms of the performance ofthe leather (shrinkage temperature) and the effectiveness of the reaction(shrinkage temperature rise per unit bound chrome).1

The role of masking in chrome tanning is an important feature, which can betechnologically exploited. The use of specific masking agents, which graduallyincrease the astringency of the chrome species, by increasing their tendency totransfer from solution to substrate, is an aspect of the reaction that has notreceived scientific attention. Here, the requirement is to match the rate ofdiminishing concentration of chrome in solution with the rate of maskingcomplexation, to maintain or increase the rate of chrome uptake. This type ofreaction is already technologically exploited by the use of disodium phthalate,although the degree of hydrophobicity conferred by even a very low maskingratio can cause an undesirably fast surface reaction. Other hydrophobicmasking agents could be developed, to give a more controllable increase inastringency.It is already recognised that the chrome tanning reaction is controlled

by the effect of pH on the reactivity of the collagen substrate: a second-ordereffect is the increasing of the hydrophobicity of the chrome species bypolymerisation. In addition, the rate of reaction is controlled by temperature.However, the efficiency of the process is limited by the role of the solvent, in




444

retaining the reactant in solution by solvation: this is typically only counteredby applying extreme conditions of pH, which is likely to create problems ofsurface fixation, causing staining and resistance to dyeing.1

The role of the counterion in chrome tanning offers potential for change.The chrome tanning reaction is controlled by the particular counterionpresent.2–4 It was fortunate for the leather industry that chrome alum[potassium chromium(III) sulfate hydrate] was the most readily availablesalt for the original trials of tanning ability: sulfate ion is highly effectivein creating a stable supramolecular matrix, because it is a structure makerin water.5,6 The effect is very different if other salts are used, e.g. chlorideor perchlorate, when the outcome is only moderate hydrothermal stability.However, even if these salts are used, the high hydrothermal stabilitycan be acquired by treating the leather with another counterion. Since theeffect is independent of a complexing reaction, the process of modifyingthe moderate tanning effect is fast. This opens up the chrome tanningreaction to modifications that exploit the separation of the link and lockreactions:

� The environmentally damaging sulfate ion might be replaced by other lessdamaging counterions, such as nitrate.

� Reactive counterions can then be applied; options include using thestoichiometric quantity of sulfate or organic anions.

� The counterion might be replaced with polymeric agents, includingpolyacrylates with the right steric properties.

20.2 OTHER MINERAL TANNING OPTIONS

It has been suggested7 that the environmental impact of chromium can bealleviated by substituting all or part of the offer by other metal tanning salts;the following options are the likeliest candidates:

AlðIIIÞ; TiðIIIÞ=ðIVÞ; FeðIIÞ=ðIIIÞ; ZrðIVÞ; lanthanideðIIIÞ

The list of available options is limited to the few presented above by con-siderations of cost, availability, toxicity and reactivity towards carboxylgroups. All metals salts are mixable in all proportions in this context. However,the following general truism should be noted:The damaging effect of any (alleged) pollutant is not eliminated and is barely

significantly mitigated by reducing the degree to which it is used.The effects of substituting other metals into the chrome tanning process or

even completely substituting it have been discussed, and are briefly reviewedbelow. There are circumstances when other mineral tanning systems mightbe appropriate, but it should be clear that there is no viable alternative forgeneral applications and particularly for those applications that require highhydrothermal stability.

445The Future of Leather Processing

20.3 NON-CHROME TANNING FOR ‘CHROME FREE’ LEATHER

In the current climate of emphasis on ecologically friendly processing andproducts, attention has been directed to the environmental impact of chro-mium(III) in tanning and leather. Therefore, this situation has fuelled themovement to produce so-called ‘natural’ leathers, perceived to be and marketedas ‘chemical free’ or using other appellations that are designed to indicate theirecological credentials. The perception of the abuse of resources in leathermaking has been exploited in the marketing of leather that is not mineraltanned. Hence, it is typically assumed that:

chrome free ¼ organic:

Consequently, leathers and leather articles are offered to the market as non-chrome. The use of the term ‘chrome free’ leather implies that there is a pro-blem underlying the inclusion of chromium(III) salt in the production of leather.This concept has its origins in the (alleged) environmental impact of chro-mium(III) waste streams, liquid and solid, and the mobilisation and viability ofchromium(III) ions from tanned leather waste. However, latterly the accusationthat some chrome leathers are contaminated with chromium(VI) or that Cr(VI)can be generated during the use of the leather article has heightened the notionthat chrome tanned leather may be undesirable.8

The decision to produce leather by means other than chrome tanningdepends on elements of technology and elements of economics: some of theseaspects will clearly overlap. In each case, there are points for change and pointsagainst. If production methods are changed at the tanning stage, it must beaccepted that every subsequent process step will be altered; some will benefit,others will not and additional technological problems will have to be overcome.Technological aspects:

� environmental impact of waste streams;� ‘just in time’ quick response, wet white technologies;� colour, may be counterproductive if vegetable tannins are used;� changed properties, e.g. HHB (hydrophobic–hydrophilic balance), IEP

(isoelectric point);� process timings may or may not benefit;� resource management will change, e.g. the continued programme of con-

ventional retanning, dyeing then fatliquoring.

Economic aspects:

� waste treatment and cost of treatments, adding value and use ofbyproducts;

� ‘recyclability’ of leather, which traditionally means compostability, i.e.returning the leather into the environment without adverse impact –although this must involve at least partial denaturation of the leather,either by pH or heat treatment or both;

446 Chapter 20

� thinking beyond ‘cradle to grave’, extending to ‘cradle to cradle’;� marketing of ‘natural’ leather;� public perception of leather making – the use of ‘chemicals’, renewable

reagents;� eco-labelling, as a marketing tool for developed economies or just access

for developing economies to developed markets.

It is important to retain a clear idea of what is required in the alternativeleather. Typically what is required is ‘the same, but different’. Here the obviousdifference is the absence of chromium(III) salt, but which elements or aspectsof chrome leather character/performance/properties should or need to beretained is less simple. Here, the tanner needs to distinguish between thespecific properties required in the leather and the more general property of‘mineral character’: the former may be relatively easy to reproduce, at leastwith regard to individual properties/performance, but the latter is moredifficult to reproduce, since mineral character usually means chrome tannedcharacter.Clearly, no other tanning system will be able to reproduce all the features of

chrome leather. Therefore, it is unlikely that a generic leather making processwith the versatility of chromium(III) will be developed – any new leather makingprocess must be tailored so that the resulting leather can meet the needs of therequired application, to be ‘fit for purpose’.

20.4 SINGLE TANNING OPTIONS

� Other metals:For example, Al(III), Ti(IV), Zr(IV), possibly Fe(III), lanthanides(III). In eachcase, the leather is more cationic, creating problems with anionic reagents;the leather can be collapsed or over-filled depending on the metal. TheHHB of leather depends on the bonding between the metal salt and thecollagen. In all cases Ts is lower than Cr(III).

� Other inorganic reagents:This category includes complex reagents, e.g. sodium aluminosilicate,silicates, colloidal silica, polyphosphates, sulfur, complex anions suchas tungstates, phosphotungstates, molybdates, phosphomolybdates anduranyl salts, i.e. those that have application in histology.

� Aldehydic agents:This group includes glutaraldehyde and derivatives, polysaccharide(starch, dextrin, alginate, etc.) derivatives and analogous derivatives of, forexample, hyaluronic acid, oxazolidine and phosphonium salts. Thesereagents are not equivalent: they react in different polymerised states andtheir affinities are different, even though the chemistry of their reactivitiesis the same, similar or analogous. In each case, the leather is plumpedup and made hydrophilic. The IEP is raised. The colour of the leatherdepends on the tanning agent – glutaraldehyde and derivatives confer


colour, others do not. Oxazolidine and phosphonium salts may produceformaldehyde in the leather or there is a perception based on odour (whichmay be the original reagent itself).

� Plant polyphenols, vegetable tannins – hydrolysable, condensed:Reactivity is a problem for a drum process, so the reaction may requiresyntan assistance. Filling effects and high hydrophilicity characterise theseleathers. Colour saddening and light fastness may be a problem, particu-larly with the condensed tannins. Migration of tannin can occur when theleather is wet. Ts is only moderate, not more than 80 1C for hydrolysable,not more than 85 1C for condensed.

� Syntans, i.e. retans or replacement syntans:As for vegetable tannins, although reactivity is more controllable by thechoice of structure. Light fastness is similarly controllable.

� Polymers and resins:This category includes: melamine-formaldehyde, acrylates, styrene maleicanhydride, urethanes, etc.

� Miscellaneous processes, e.g. oil tanning, in situ polymerisation, etc.

20.5 TANNING COMBINATIONS

Clearly, from the brief account of the individual tanning options set out above,the number of available combinations is very large. Hence, it is important tounderstand the principles of combination tanning. Analysis can commence withthe effects of the components on the shrinkage temperature, as an indicator ofthe combined chemistries:

1. The components may react individually, i.e. they do not interact, pri-marily because they react with collagen via different mechanisms and thereis no chemical affinity between the reagents.Here the contributions to the overall shrinkage temperature can be

treated additively. Also, the properties of the leather will be a combinationof the two tannages, although the first tannage applied will tend todominate the leather character.In Table 20.1 this is indicated by I¼ Individual.

2. The components may interact antagonistically, i.e. they interfere witheach other. This may be due to competition for reaction sites or onecomponent simply blocks the availability of reaction sites for the othercomponent. It can be assumed that the two components either do notinteract or that one component will react with the other in preference toreacting with collagen. Under these circumstances, the interaction doesnot constitute the creation of a useful new tanning species, i.e. the reactionof the second component does not involve crosslinking the first compo-nent. The outcome of the combined reaction is likely to be domination ofthe properties by the reagent that is in excess or has greater affinity forcollagen.

448 Chapter 20

Here, the hydrothermal stability of the combination will be less than theanticipated sum of the effects of the individual tanning reactions. Theeffect of destabilising the leather is likely to be undesirable.In Table 20.1 this is indicated by A¼Antagonism.

3. The components may react synergistically, i.e. they interact to create anew species, which adds more than expected to the overall hydrothermalstability. Since a new tanning chemical species is created, the properties ofthe leather are likely to reflect the sum of the individual tanning effects,but to differ significantly from a simple additive outcome.The additional elevation of Ts may constitute a high stability process, as

rationalised in the link-lock theory5 (discussed in Chapter 19).In Table 20.1 this is indicated by S¼ Synergistic.

In making choices of combinations, it is important to consider how thereagents combine to create the leather properties other than hydrothermalstability: these could include handle (softness, stiffness, fullness, etc.), hydro-philic–hydrophobic balance, strength (tensile, tear, extensibility, etc.) andso on. If the components react individually, it might be expected that theproperties of each would be conserved in the leather, dependent on the relativeamounts fixed. If the components react antagonistically, the overall propertieswill be determined by the dominant reactant, although they may be modified bythe presence of the second component, unless the antagonism means the secondreagent is not fixed. Synergistic reaction means the tanning reaction is changed,so it can be assumed that the outcome will not be the same as the sum ofthe individual properties, although it can also be assumed that the leather willstill reflect the individual properties.In each case that might be considered, it is important to recognise that the

order of addition of the reagents can make a significant difference to the out-come. The nature of the interaction in two-component combination tannagescan be analysed as shown in Table 20.1, where italics indicate a lack of clarity inthe effects or there may be exceptions. Some general points can be made:

Table 20.1 Indicative/possible interactions in combination tanning.a

Second reagent

First reagentMetalsalts Inorganic Aldehydic

Veg tan:hydrol.

Veg tan:cond. Syntan Resin

Metal salts I I I A I A IInorganic I I I A I A IAldehydic I I A I S S SVeg tan:hydrol

S S I A A A A

Veg tan: cond I I S A A A ASyntan S S S A A A AResin I I S A A A AaI¼ individual, A¼ antagonism and S¼ synergistic. Italics indicate a lack of clarity in the effects orthere may be exceptions.


� The concept of the link-lock mechanism is useful in this context, because itoffers a model of the tanning reaction that can be visualised and hence usedin the analysis of reaction and outcome. The matrix model is easier to usethan the previous model of specific interaction with the collagensidechains.

� All metals are mixable in all proportions for tanning and all metals reactprimarily at carboxyl groups, typically electrostatically. It is unlikely thatthere will be any positive impact on the tanning reaction from mixedspeciation. The filling/collapsing effects can be controlled by mixing themetal offers. There is no benefit with respect to Ts or cationic character.

� Any combination in which the components rely on hydrogen bonding forfixation is likely to be antagonistic.

� Hydrolysable vegetable tannins and metals: This is established technology.The product is full, hydrophilic leather. Covalent fixation of metal bypolyphenol complexation modifies the properties of the metal ions, butthere is still cationic character.

� Condensed tannin with aldehydic reagent: Prodelphinidin and profiseti-nidin tannins or non-tans are preferred. Not all aldehydic reagents work:oxazolidine is preferred. The result can be high Ts.

� Syntan with aldehydic reagent: The industry standard is to use unspecifiedsyntan with glutaraldehyde. There is apparently no attempt to define thesyntan in terms of reactivity to create synergy; in general, reliance isprobably placed on simple additive effects, so the rationale is not easy todiscern.The technology could be improved by analogy with the condensed

tannins, by applying the structure–reactivity criteria known in thatcontext.

� Polymer with aldehydic reagent: Melamine-formaldehyde polymer withphosphonium salt can be a synergistic combination, depending on thestructure of the resin, especially the particle size.There will be other polymeric reagents capable of creating synergistic

tannages.� Reliance on syntans and/or resins may carry the additional problem

of formaldehyde in the leather, which is subject to limits in specifications.It is possible to avoid this problem by scavenging the formaldehydeby the presence of condensed tanning, preferably prodelphinidin orprofisetinidin.9

20.6 LEATHER PROPERTIES

In deciding what alternative tanning system to adopt, the following features ofthe new leather must be considered:

� The appearance of the leather, for example the presence of draw fromsurface reaction, the fineness of the grain pattern (largely dependent on theclosing of the follicle mouths by relaxing the fibre structure).

450 Chapter 20

� The colour of the leather is usually predictable, but occasionally it is not,e.g. the combination of glyoxal and tara produces a plum coloured leather(A.D. Covington, unpublished results).

� Area yield: the more the leather is filled or the higher the astringency of thetannage, the higher the angle of weave in the corium and hence the lowerthe area yield.

� Stability fastness of the tanning agents: the response to conditions of use.� Hydrothermal stability: the requirements of the conditions of end use by

the customer, e.g. footwear manufacture.� Post tanning reactions: affinity of retans, dyes and fatliquors for the new

substrate. In particular, the colour struck by dyeing systems may beaffected.

� The effect of the new substrate on properties and performance after posttanning: strength and handle.

� The response of the substrate to finishing: the effect on hardness, adhesion,rub fastness, etc.

Much of this information is predictable from leather science and fromexperience in leather technology.

20.7 ORGANIC TANNING OPTIONS

20.7.1 Polyphenol Chemistry

The best known example of plant polyphenol exploitation for high hydro-thermal stability tanning is the semi-metal reaction. Here, the requirement is forpyrogallol chemistry to create the covalent complex between the linkingpolyphenol and the locking metal ion:10 in practice, this means using thehydrolysable tannins, but alternatively some condensed tannins can be used(see Figure 13.8): the prodelphinidins (e.g. Myrica esculenta, pecan andgreen tea)11 and prorobinetinidins (e.g. mimosa) each have the requiredstructure in the B-ring. Many metal salts are capable of reacting in this way, sothere may be useful applications for the future.The condensed tannins can confer high hydrothermal stability by acting as the

linking agent, with aldehydic crosslinker acting as the locking agent; othercovalent crosslinkers may also have application in this context. The reactionapplies to all flavonoid polyphenols, when reaction always occurs at the A-ring.In the case of the prodelphinidins and profisetinidins, additional reaction can takeplace at the B-ring.12 The effect is to increase the ease of attaining high hydro-thermal stability. It is useful to recall that the combination tannage does not relyon conventional vegetable tannins, because it can work with the low molecularweight non-tans: here, there is advantage in terms of reducing or eliminatingproblems of achieving penetration by the primary tanning component.The link-lock mechanism can be exploited in other ways, even using

reagents that at first do not appear to be tanning agents, e.g. naphthalene diols(Figure 20.1).13


The behaviour of naphthalene diols in the tanning process is highly depen-dent on the structure of the isomer (Table 20.2).Clearly, the presence of a hydroxyl in the 2-position activates the naphtha-

lene nucleus: the 1-position does not work, shown by comparing the 1,5 withthe 1,6 (2,5) diol. When there are two groups in the 2,6-positions, they acttogether. When the hydroxyls are in the 2,7-positions they act against eachother. The basis is the inductive effect of the hydroxyl on the aromatic ring,activating the ortho positions to electrophilic attack or allowing those positionsto engage in nucleophilic attack at the methylene group of the N-methylolgroup of the oxazolidine (Figure 20.2). The results in Table 20.2 also illustratethe principle that the linking agent may only exhibit a very weak effect intanning terms, but successful locking of the linking species, combined with theability to link the matrix to collagen in the locking reaction, can result in highhydrothermal stability.Non-chemical polymerisation is less effective. Applying laccase (phenol

oxidase enzyme) to hide powder treated with 2,6-dihydroxynaphthalene

HO

OH

HO

OH

HO

OH HO OH

1,5- 1,6- 2,6- 2,7-

Figure 20.1 Structures of some naphthalene diols.

Table 20.2 Shrinkage temperatures (1C) of hide powder treated with dihy-droxynaphthols (DHNs) and oxazolidine.

Dihydroxynaphthol DHN alone DHN+oxazolidine

None 57 751,5 56 851,6 (2,5) 64 902,6 62 1102,7 62 79

OH +OH

-

-

+OHNH-

CH2

OH

Figure 20.2 Reaction between the phenol and N-methylol moieties.

452 Chapter 20

produced the highest rise in shrinkage temperature for the range of this type oflinking agents tested (by 26 1C, to 85 1C).14 Clearly, the locking reaction is moreeasily and effectively accomplished by applying a second reagent, rather thanrelying on direct reactions between linking molecules.

20.7.2 Polymer and Crosslinker

Perhaps surprisingly, it is less easy to create high stability tannage with poly-mers than it is using oligomers or monomers, depending on the polymericcompound. High hydrothermal stability can be achieved using melamine resincrosslinked with tetrakis(hydroxymethyl)phosphonium salt. Several conclu-sions were drawn from these studies:15

� Not all melamine linking resins work. Therefore, the requirements formatrix formation are likely to be more important than possessing specificchemical reactivity.

� Not all aldehydic locking agents work. The locking function does dependon creating stable bonding between the linking molecules and forming arigid species capable of resisting the collapsing triple helices.

� The linking reaction is dependent on physical parameters; particle size maybe critical. It is not sufficient to provide space filling.

� The ability to form the basis of a supramolecular matrix must depend onthe stereochemistry of the linking agent. This requirement is more easilysatisfied with lower molecular weight species.

This reaction provided an interesting aspect of the matrix theory of tanning,when an attempt was made to accumulate hydrothermal stability by adding amatrix to a matrix. Here, the sequence of reagent additions was as follows:melamine resin, phosphonium salt, condensed tannin (mimosa), oxazolidine.The observation was the achievement of high hydrothermal stability from themelamine resin and phosphonium salt, added to by the condensed tannin,reaching a shrinkage temperature of 129 1C, thereby matching the maximumshrinkage temperature achieved by chromium(III) in the presence of pyr-omellitate (1,2,4,5-tetracarboxybenzene).16 The melamine and phosphoniumsalt create a matrix in which the melamine polymer reacts with the collagen viahydrogen bonds, the phosphonium salt crosslinks the polymer, whilst probablylinking the matrix to the collagen. The introduction of condensed polyphenolraises the shrinkage temperature by a small additive effect, since it is appliedafter the matrix formation, it has no affinity for the resin, but has limitedaffinity for the phosphonium salt. The subsequent addition of oxazolidine iscapable of forming a synergistic matrix with the polyphenol, but the reactioncauses the shrinkage temperature to drop significantly (A.D. Covington,unpublished results). The clear inference is that the new reaction is antagonisticto the established matrix: by analogy with other antagonistic combination,17

there is competition with the mechanism that binds the first matrix to thecollagen, effectively loosening the binding between the matrix and the collagen,


allowing shrinking to occur, with the consequence that the hydrothermal sta-bility is lowered. It is difficult to rationalise the observation by any mechanismother than the formation of matrices.

20.8 NATURAL TANNING AGENTS

Recently, new tanning chemistries have come to light: they have the char-acteristic of being biomimetic, using natural reactions in a new context. Suchorganic tanning reactions are of interest from three points of view. First, theyoffer new methods of making leather, to yield new products, which maycontribute to lessening the environmental impact of tanning. Second, theyoffer new opportunities for high hydrothermal stability tanning, by actingas new linking agents, then allowing manipulation of the chemistry of thelocking step. Third, they may involve the novel use of enzymes in tanning,operating as catalysing activating agents, so that the rate of reaction is highlycontrollable.

20.8.1 Carbohydrates

Derivatives of carbohydrates are well known in the form of dialdehydes,obtained by the oxidative effects of periodic acid as discussed in Chapter 14.However, there can be direct reactions between carbohydrate and proteins, i.e.glycosylation-type reactions. Komanovsky effectively generalised these reac-tions in his studies of the reactions within collagen at low moisture contents.18

The basis of the formation of permanent bonds is the Maillard reaction, whichis responsible for many of the transformations during cooking of proteinaceousfood. The reaction depends on reducing the moisture content, in order for thegroups to approach close enough to react: it works better at higher temperatureand lower pH, in contrast to the high pH requirements of most aldehydes,although not all aldehydic agents.13,19

Figure 20.3 illustrates the stabilising reaction: it is assumed that the ketoa-mine product can react further with collagen, to form a crosslink (of unknownstructure), a reaction that becomes more feasible when the interacting chainsare close, unlike the situation in wet collagen and leather.The natural products that are responsible for the cooking reaction include

sugars and the glycosaminoglycans, hyaluronic acid, dermatan sulfate andchondroitin. These compounds or their derivatives might be applied as reagentsas part of a two-step organic tannage.Hyaluronic acid and dermatan sulfate offer the potential for useful new

derivatives: they are byproducts of the leather making process and are, asindicated by their structures given in Chapter 2 (Figures 2.14 and 2.16), ana-logous to carbohydrate structure discussed in Chapter 14; thus it would appearlikely that aldehydic derivatives could be made. Although the reactivity of thealdehyde function would not show any advantage, the overall reactive func-tionality is enhanced by the greater variety of chemical moieties in the molecule,such as carboxyl groups. Since the molecular basis of the tanning reaction

454 Chapter 20

would be a natural product, a component of skin, it might be expected that theleather would be hypoallergenic – useful for applications when metal content isto be avoided because of sensitisation.

20.8.2 ‘Bog Body’ Chemistry

A related stabilising process for protein that has not previously been reviewedin a leather context is the preservation of so-called ‘bog bodies’,20 illustrated by‘Tollund Man’ in Figure 20.4. Perhaps surprisingly, the preservation mechan-ism is not an example of vegetable tanning. The environment in which thetissues of the body are preserved is typically a sphagnum peat bog: the effect isthought to be due to a Maillard reaction between free amino groups in theproteins and reactive carbonyl groups in a soluble glucuronoglycan, sphagnan,containing residues of D-lyxo-5-hexosulopyranuronic acid (Figure 20.5). It canform the furan derivative, which in turn can be converted into a species likeascorbic acid, which can be oxidised to form a species like dehydroascorbicacid, which resembles ninhydrin, the well-known colour generating reactant foramino acid analysis.Sphagnan is a pectin-like chemical, bound covalently to cellulosic and

amyloid chains in sphagnum moss: it is liberated by hydrolysis, as the mossturns into peat. As in periodic acid oxidised carbohydrate derivatives, the chainlength of the sphagnan products may influence the tanning potential, but thishas not been investigated. In nature the reaction is slow, due to the low con-centration of the active species in solution, but this is not a limiting factor forthe leather scientist.The chemistry of plant cell walls may offer a new source of chemicals from

which tanning agents might be synthesised. The main breakdown product,ferulic acid, is polymerisable by laccase and can be converted by fungalspecies into other compounds that may be of interest to the leather scientist(Figure 20.6).21

CHO

HCOH

HOCH

HCOH

HCOH

CH2OH

HC = N-(CH2)4-protein H2C-NH-(CH2)4-protein

glucose (reducing sugar)

aldimine ketoamine(Schiff base)

R-(CH2)4-NH2 Amadori

lysine

CH2OH

HCOH

HCOH

HOCH

HCOH

CH2OH

HCOH

HCOH

HOCH

HCOH

rearrangement

Figure 20.3 Maillard reaction.


Other sources of potential tanning agents might be exploited. Henna is a dyeextracted from the leaves of Lawsonia inermis: lawsone and isoplumbagin areextracted from the leaves and bark respectively (Figure 20.7).22

The reactivity of quinone itself and derivatives is well known in tanning,23 sothe dual functionality of quinone and phenol reactivity is of interest in thecontext of combination processes.

20.8.3 Nor-dihydroguaiaretic Acid (NDGA)

NDGA is a naturally occurring polyphenol, isolated from the creosote bush;it has been proposed as a stabilising reagent for collagen, when it exhibitsthe unusual phenomenon of actually increasing the strength of the fibre,24

rather than the more common effect of lessening the weakening effect ofprocessing. Figure 20.8 presents the mechanism of polymerisation of NDGA:the polymer is thought to align with the protein chains, constituting asupporting and strengthening structure. Clearly, the polymeric species offersopportunities for additional reactions. To date, the physical and thermo-dynamic properties of leathers have never been correlated, since they areregarded as properties based on different aspects of collagen structure; theformer dependent on the macro-structure at the microscopic and visiblelevels, the latter dependent on the molecular level. Since these propertieshave now been shown to be dependent on collagen modification, perhaps auseful relationship does exist.The strengthening aspect of the reaction is an interesting outcome of the

introduction of this particular matrix into the collagen structure. This offers the

Figure 20.4 Tollund man. (Source: Wikipedia, www. en.wikipedia.org/wiki/Tollund_Man accessed 13.2.09.)

456 Chapter 20

HO

H

H

H3N+

N

H

C N

O O

O

H

C

C

OO

N

C

N

NH3+

HO

5

5-positionin D-lyxo-5-hexosulopyranuronic acid

guanidinogroup in arginine

glycosylamine

5

HO2C OH

O

HO

OH

RO

OH

O

D-lyxo-5-hexosulopyranuronic acid D-lyxo-5-hexosulofuranuronic acid

analogue of ascorbic acid analogue of dehydroascorbic acid (resembles ninhydrin)

OH

RO

OH

HO

OC

CO2H

O

HO

HO

O

OR

CHO

[O]

CHO

OR

O

O

O

O

Figure 20.5 Reactions of decomposition products from sphagnum moss.

CO2H

OCH3

OH

CO2H

OCH3

OH

OHOH

OCH3

A. nigerPycnoporusP. cinnabarinus

ferulic acid vanillic acid methoxy hydroquinone

Figure 20.6 Breakdown products from ferulic acid.


O

O

OH

HO

CH3

lawsone isoplumbagin

O

O

Figure 20.7 Secondary metabolites from Lawsonia inermis.

HO

HO

OH

OH

CH3

H3C

OH

OH

HO

HO O

O

O

O

dicatechol

oxidation

diquinone

oxidativecoupling

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

Figure 20.8 Reactions of NDGA, leading to the stabilisation of collagen.

458 Chapter 20

opportunity of tailoring the tanning matrix for improvements in the physicalproperties.25

20.8.4 Genepin

Genepin is an iridoid derivative (Figure 20.9) isolated from the fruits ofGardenia jasminoides. Its reaction with protein is characterised by the genera-tion of deep blue colouration under the alkaline conditions required forfixing.The reaction mechanism by which protein is stabilised is not clearly under-

stood, but is thought to derive from opening of the oxy heterocyclic ring,causing polymerisation, which is the source of the blue colour. The shrinkagetemperature of hide powder can be raised to 85 1C,26 characteristic of a singletanning reagent. That in itself is not important, but the incorporation ofnew reactive groups into collagen by a linking reaction using genepin mayprovide new sites for reaction in a locking step. Alternatively, genepinmay provide another approach to the locking step in a combination tannage.There are many other iridoids, nearly 600 are known,27 characterised bythe presence or absence of glucose moieties and the presence or absence of thecyclopentane ring.

20.8.5 Oleuropein

Oleuropein is a natural product, a secoiridoid glycoside, found in privet andolive vegetation, where its function is part of the self-defence mechanismagainst infections and herbivores: it contains glucose, but not the cyclopentanering in its structure. Figure 20.10 presents the mechanism of reaction withprotein: it is thought to involve the activation of oleuropein to the aglyconform, which reacts with lysine residues on proteins.27

From the work of Antunes et al., the stabilising effect can be comparedwith that of glutaraldehyde (Table 20.3), in which the collagen was recast as afilm from solubilised collagen.28

1

34

56

7

89

HOH2C OR

CO2CH3 HO

OGlc

O O

HOH2C

R = H genepin R = Glc geneposide

aucubin

Figure 20.9 Structures of some iridoids.


As with genepin, the chemistry of oleuropein offers the potential for acting aseither a linking agent or a locking agent, using an appropriate complementarycombination reagent.These biomimetic approaches to tanning constitute a new, potentially

powerful aspect to applications of biotechnology in leather making. However,it is equally clear that they do not offer the opportunity for a single step, highhydrothermal stability tannage. The advantages lie in the new, covalent binding

HO

HO

O O

C CO2CH3

OGlc

β-glucosidase

protein lysine NH2

oleuropein aglycone(oxidised)

O

protein

proteinprotein

protein

protein

O

CO2CH3C

OO

O

O

O

O O

C CO2CH3

N

NN

N

N

oleuropein(secoiridoid glycoside)

Figure 20.10 Schematic representation of the oleuropein reaction with protein.

Table 20.3 Stabilisation of collagen film.

Stabilisation Shrinkage temperature (1C)

Control 50Oleuropein 67Glutaraldehyde 68

460 Chapter 20

reactions, which create the linking part of the supramolecular matrix, which isfirmly bound to the triple helix, and then is capable of undergoing variouslocking reactions.

20.9 OTHER REAGENTS

Some sugars are known to have a stabilising effect on proteins, including apreserving effect on the activity of enzymes, whether in solution or in the freeze-dried state. This aspect of chemistry has not been exploited in the leatherindustry. The effect is thought to be due to the interaction between the car-bohydrate and water associated with the protein structure. Of the sugarsstudies, the most effective in this regard is trehalose.29 Figure 20.11 shows thecomparative structures of sucrose and trehalose.Clearly, there is a possibility of using low molecular weight carbohydrates as

curing agents (as introduced in Chapter 3): trehalose can be obtained fromstarch, so this technology might be cost effective. However, the more interestingapplication might be as part of a tanning combination.There is a remaining class of reagents that might be classified as either

linkers or lockers; the requirement for new components for tanning is thatthey should have the ability to react covalently under the typical conditions oftanning.6 They are options already known in the leather industry in the form ofreactive dye chemistries (already discussed in Chapter 16); it is merely a matterof widening the exploitation. Those chemistries could be exploited in the formof multiple reactive groups in the same molecule. This notion has already beeninitiated by BASF in the form of their product (dye) Fixing Agent P, triacryoyltriazine, but there is clearly potential for developing the approach further.

O

HO

OH

CH2OH

HOH2C

OOH

OHO

CH2OH CH2OH

OH

HO

O

OH

O

CH2OH

OH

HO

HO

O

HO

sucrose = glucose + fructose

trehalose (diglucose)

Figure 20.11 Structures of sucrose and trehalose.


Similarly, the epoxide and dicyandiamide chemistries might prove to be useful,but only if they are applied according to the requirements of link-lock.

20.10 COMPACT TANNING

The concept of ‘compact processing’ is the condensing or shortening of pro-cessing by combining two or more reactions into a single process step. In thisway, time is saved and consistency is created in the interaction between theprocess steps involved, because only one reaction takes place, instead of two ormore. This is a powerful contribution to innovation in the tannery. The idea ofcompact processing can be applied at any stage in processing, although it ismore commonly applied in the later stages of wet processing. Therefore,thinking about this concept should not be limited in scope. If it is accepted thathigh stability can only be achieved by a two-stage tanning action, then there areopportunities to exploit such an approach to tanning in compact processing.The use of a non-swelling acid incorporates the notion of pretanning. It may

be possible to substitute pretanning for retanning. However, it should berecognised that the first tanning agent controls the character of the leather.In the case of prime tanning being conducted with vegetable tannins, therequirement for further tanning is uncommon. An exception would be organictanning, which might be based on polyphenols, when there would be arequirement for applying the second component of a combination process forhigh hydrothermal stability. Such an approach is feasible for the combinationof gallocatechin-type condensed tannins with oxazolidine, when the reaction isactivated by elevated temperature.11,17,25,30

The case of chromium(III) tanning is less clear. No technologies have beenoffered to the industry in which a retanning agent has been combined withconventional chrome tanning salt. The closest to that situation are productsreferred to as ‘chrome syntans’, which are well known to the industry. However,these are primarily offered as filling versions of chrome retanning, rather than anew approach to prime tanning. Because chromium(III) has little affinity forphenolic hydroxide as a ligand,31 chrome syntans are either mixtures of syntanand chrome salt or the syntan may be capable of complexation with chrome,by having some carboxyl functionality in its structure. In the latter case, thefunction of retanning might be accomplished with tanning. Similarly, anextended application of masking, to confer additional features, may be useful.The application of compact processing to post tanning is discussed in

Chapter 15. Clearly, there is potential for further development, using creativechemistry.

20.11 ALTERNATIVE TECHNOLOGIES

It is conventionally assumed that tanning has to be conducted in aqueoussolution. However, there are options available for the future. Wei hasdemonstrated the possibility of processing water wet pelt in a tumbling mediumof water-immiscible solvent.32 The technology is feasible, but brings problems

462 Chapter 20

of its own, notably the requirement for diffusion across the pelt, unlike theconventional requirement for diffusion through the cross section. However,such difficulties are solvable, if the willingness is there. The alternative is anessentially non-aqueous process, as indicated in the context of compact pro-cessing: this approach to processing, including the use of liquid carbon dioxide,has been discussed for some steps,33 but industrial development has not yetfollowed. The advantage of such an approach to one-step post tanning is thatthe substrate is dry (at least to the touch), so the moisture in the leather wouldhave a limited influence on the process. It is less clear that such an approachwould be feasible for primary tanning, in whatever way that might be donechemically. If chrome tanning were to be developed in that direction, it is notdifficult to imagine the HHB properties of the chrome complex being tailor-made for the solvent by appropriate masking.It is useful to speculate on the roles that enzymes might play in tanning.

Clearly, biotechnology has an increasingly important part to play in thebeamhouse, but it is less clear if it can contribute to collagen stabilisation.From the matrix theory of tanning and collagen stabilisation, the inability oftransglutaminase to increase the hydrothermal stability is predictable andunderstandable, even though it is clearly capable of introducing crosslinkingin the conventional sense.34 Similar reactions, which can introduce crosslinksinto collagen, are unlikely to function as useful tanning agents, beyond alteringthe texture of the protein.At the other end of the processing procedures, the role of drying remains to

be exploited to advantage by industry (as discussed in Chapter 18). Recentstudies have linked the properties of leather, including area yield, to the pro-gramme of drying conditions:35 softness does not depend on the rate of drying,only the moisture content. Using conventional plant, it is possible to modify thedrying programme into two stages, to obtain advantage from the relationshipbetween the viscoelastic properties of leather and its water content, so that areagain of intact leather does not have to be at the expense of softness.In considering alternative technologies, it is useful to examine whether there

is an alternative mechanism to link-lock, i.e. whether there could be exceptions.If the mechanism of shrinking, as set out here, is right, then the impact of thetanning reaction on hydrothermal shrinking is right. Consequently, the use ofconventional, penetrating reactants, in the way also outlined above, is alsoright. Therefore, the only exception to the link-lock mechanism would be asingle step reaction that combines both features. The requirement would be theformation of the stabilising matrix from a single species polymerisation reac-tion, which would have to include reaction with the collagen. In this regard,polymerisation of cod oil is the closest reaction known in the art. However, inthis case, despite the complexity of the chemistry of polymerisation by oxida-tion, there appears to be little direct interaction between the polymer and thecollagen, despite the possibility of creating aldehyde groups.36 Hence, the lea-thering reaction does not elevate the shrinkage temperature significantly.Examples of chemistries apparently capable of multiple bonding with collagendo not react in that manner, because there is no evidence of an outcome that


might be expected from such tanning reactions (as discussed in Chapter 14).Nevertheless, it is not inconceivable that such a polymerising tannage might bedeveloped, particularly if an extended range of practically useful solvents ismade available to the industry.

20.12 OVERVIEW

The range of chemistries available to the tanner is widening. By considering themolecular basis of the tanning mechanism, especially those requirements toconfer high hydrothermal stability, the options open to the tanner are widened.If high stability organic tannages are desired, they can be created just byobserving the following rules:

1. Apply the first reagent, linking to the collagen with high stability bonding,preferably covalently. The reagent must offer the potential for a secondreaction, by possessing usefully reactive groups, but need not have highmolecular weight.

2. Apply the second reagent, to react with the first reagent by locking themolecules together; therefore, the second reagent must be multifunctional.Contributing to linking the matrix to the collagen is useful.

The properties of the resulting leather can be controlled by the choices ofreagents: this applies to both mineral and organic tanning options. In theabsence of a polymerising tannage capable of meeting the stabilising matrixcriteria, we are limited to the two-step process. But the two-step process doesnot have to extend processing times: this approach can contribute to compactprocessing, moreover it can offer specifically required properties and thereforeoffers the basis of the production of bio-vulnerable or so-called recyclableleathers. The latter category has not been adequately explored. The definitionof tanning refers to the resistance to biodegradation of a previously putrescibleprotein material: here the resistance refers to proteolytic attack. Therefore, it isfeasible to consider tanning processes that incorporate a degree of vulner-ability. In this way, high hydrothermally stable leather could be chemically orbiochemically destabilised, to allow denaturation of the collagen at moderatetemperatures, so that proteolytic degradation can be achieved. Targeting spe-cific groups in the matrix may be sufficient to degrade its effectiveness: optionsare hydrolases, oxidases, reductases, etc., depending on the chemistry of thetannage. Alternatively, tannages can be devised in which the stability istemporary, such as semi-metal chemistry that is vulnerable to redox effects(as introduced in Chapter 13). Other approaches are possible and should notbe beyond the wit of the leather scientist.

20.13 CONCLUSIONS

The expression of the link-lock mechanism has made it possible to takequantum steps forward in developments in tanning technology. This new

464 Chapter 20

theory is a simpler and more powerful view of collagen stabilisation than theolder model of direct crosslinking between adjacent sidechains; it is a moreelegant view. It is no longer worth pursuing the single reagent alternative tochrome tanning, because it does not exist. Indeed, why should we seek analternative to chrome tanning? It works well, it can be made to work even betterand, anyway, by all reasonable judgements it causes little environmentalimpact. Nevertheless, organic options provide potential for new products fromthe leather industry. Now we can develop those options using sound logic,based on sound theory. The literature is littered with examples of multi-component tanning systems:37–41 the outcome of each is entirely predictable ifwe apply our new understanding of the theoretical basis of tanning.The continuation of developing tanning and leather technology depends on

constant reappraisal of all aspects of the subject. This is the role of leatherscience. Conventional, received wisdom should not be relied upon withoutcritically reviewing exactly what it means, what it contributes to processing andproducts and what the wider implications are for the practical tanner. It isimportant to recognise that the scrutiny of current technology will oftenidentify inconsistencies and misunderstanding of principles: the technologymay work, but the science may not. However, this is not always a bad thing,because it can lead to new thinking, new developments and more profitabilityin an environmentally sound, sustainable industry.

REFERENCES

1. A. D. Covington, J. Amer. Leather Chem. Assoc., 2001, 96(12), 461.2. A. D. Covington, G. S. Lampard, O. Menderes, A. V. Chadwick,

G. Rafeletos and P. O’Brien, Polyhedron, 2001, 20, 461.3. K. Shirai, K. Takahashi and K. Wada, Hikaku Kagaku, 1975, 21(3),

128.4. K. Shirai, K. Takahashi and K. Wada, Hikaku Kagaku, 1984, 30(2), 91.5. A. D. Covington, M. J. Collins, H. E. C. Koon, L. Song and O. Suparno,

J. Soc. Leather Technol. Chem., 2008, 92(1), 1.6. A. D. Covington, Proc. UNIDO Workshop, Casablanca, Morocco, Sept.

2000.7. A. D. Covington, J. Soc. Leather Technol. Chem., 1986, 70(2), 33.8. A. Marsal, et al., J. Soc. Leather Technol. Chem., 1999, 83(6), 300.9. A. J. Escobedo, MSc Thesis, University College Northampton, 2002.

10. R. A. Hancock, S. T. Orszulik and R. L. Sykes, J. Soc. Leather Technol.Chem., 1980, 64(2), 32.

11. A. D. Covington, C. S. Evans, T. H. Lilley and L. Song, J. Amer. LeatherChem. Assoc., 2005, 100(9), 325.

12. L. Song, PhD Thesis, The University of Northampton, 2003.13. A. D. Covington, C. S. Evans and O. Suparno, J. Soc. Leather Technol.

Chem., 2007, 91(5), 188.14. O. Suparno, PhD Thesis, The University of Northampton, 2005.


15. A. D. Covington and M. Song, Proc., IULTCS Congress, London, Sept.1997. pp. 565–570.

16. S. Das Gupta, J. Soc. Leather Technol. Chem., 1979, 63(4), 69.17. A. D. Covington and B. Shi, J. Soc. Leather Technol. Chem., 1998, 82(2),

64.18. M. Komanovsky, J. Amer. Leather Chem. Assoc., 1989, 84(12), 369.19. S. Das Gupta, J. Soc. Leather Technol. Chem., 1977, 61(5), 97.20. T. J. Painter, Carbohydrate Polymers, 1991, 15, 123.21. C. Faulds, B. Clark and G. Williamson, Chemistry in Britain, 2000, 36(5),

48.22. A. E. Musa, et al., J. Amer. Leather Chem. Assoc., 2008, 103(6), 167.23. E. Heidemann, Fundamentals of Leather Manufacturing, Eduard Roether,

Darmstadt, 1993.24. T. J. Koob and J. Hernandez, Biomaterials, 2003, 24, 1285.25. A. D. Covington, C. S. Evans, T. H. Lilley and O. Suparno, J. Amer.

Leather Chem. Assoc., 2005, 100(9), 336.26. K. Ding, M. M. Taylor and E. M. Brown, J. Amer. Leather Chem. Assoc.,

2006, 101(10), 30.27. K. Konno, C. Hirayama, H. Yasui and M. Nakamura, Proc. Natl. Acad.

Sci. USA, 1999, 96, 9159.28. A. P. M. Antunes, G. E. Attenburrow, A. D. Covington, J. Ding, J. Soc.

Leather Technol., Chem., Proc., IULTCS Conf., Istanbul, Turkey, 2006.p. 12.

29. J. K. Kaushik and R. Bhat, J. Biol. Chem., 2003, 278(29), 26458.30. A. D. Covington and L. Song., Proc., IULTCS Cogress, Cancun, Mexico,

2003. pp. 88–94.31. S. Jeyapalina, A. D. Covington, G. E. Attenburrow, Proc. IULTCS

Congress, Cape Town, South Africa, 2001.32. Q.-Y. Wei, J. Soc. Leather Technol. Chem., 1987, 71(6), 195.33. G. Gavend, Industrie du Cuir, Aug/Sept. 1995.34. R. J. Collighan, S. Clara, S. Li, J. Parry and M. Griffin, J. Amer. Leather

Chem. Assoc., 2004, 99(7), 293.35. S. Jeyapalina, G. E. Attenburrow and A. D. Covington, J. Soc. Leather

Technol. Chem., 2007, 91(3), 102.36. J. H. Sharphouse, J. Soc. Leather Technol. Chem., 1985, 69(2), 29.37. J. R. Rao, et al., J. Amer. Leather Chem. Assoc., 2004, 99(2), 73.38. J. R. Rao, et al., J. Amer. Leather Chem. Assoc., 2006, 101(2), 58.39. J. R. Rao, et al., J. Amer. Leather Chem. Assoc., 2006, 101(5), 161.40. K. Ding, M. M. Taylor and E. M. Brown, J. Amer. Leather Chem. Assoc.,

2007, 102(5), 164.41. K. Ding, M. M. Taylor and E. M. Brown, J. Amer. Leather Chem. Assoc.,

2007, 102(6), 198.

466 Chapter 20

Subject Index

Page numbers in bold refer to entries in tables. Page numbers in italics refer to entries in figures.

acetic acid 128, 185 acid dyes 373 acid swelling 18, 156, 157

and salt 179–81 acid unhairing 128 acidic salts, in deliming 157–8 acrolein 316, 333 acrylolys 381 actinides 264 activation energies 168 activity coefficients 142, 183 acutissimin A 293 air permeability method 173 aldehydes 108, 328–33, 334

dialdehydes 333, 454 glutaraldehyde 331–2 other aliphatic 332–3, 334 in wet white production 341 see also formaldehyde

aldehydic crosslinkers 453 condensed tannins and 303–10 and resins 326, 328

aldehydic tanning agents 334, 335, 336–8 future prospects 447–8 phosphonium salts 337–8, 453 see also oxazolidines

alginates 333, 334 alizarin 376 alkali metals 260 alkaline earth metals 260–1

alkaline salts, soluble 223–4 alkyl ethylene oxide condensates 406–7 α-amino acids 1, 2, 3, 5 aluminium 261 aluminium nitrate 340 aluminium polyacrylate 340 aluminium sulfate

and vegetable tanning 297, 298, 299 in wet white production 339–40

aluminium tanning 267–74 alum pickle 269 chrome uptake 270–4 furskins 269 masking in 244, 268 semi-alum leather 267–8, 270 suede 269

aluminium(III) salts 267, 271, 272 ambient temperature 215 amidases 106–7, 256 amide, hydrolysis 137, 138, 143–8 amino acids 1

in collagen 23, 31, 39 in elastin 23, 39 in keratin 23, 31, 112–13 in leather making 3 sequence 4–5 in skin proteins 22–3 structures 2 see also triple helix

ammonia 144–6, 159

468 Subject Index

ammonium chloride 158, 159, 163 ammonium salts 158–9, 171 ammonium sulfate 158–9 amphoteric fatliquors 407, 408 amphoteric syntans 326 amylases 106 anaerobic bacteria 310–12 angle of the follicle 36, 110, 111 angle of weave 31, 35, 36

and area 52, 53 soaking and 110, 111

aniline dyed xxiv anionic auxiliaries 385–7 anionic detergents 105, 415 anionic fatliquors 401–5 anisotropy 48, 49, 50 antagonistic interactions 297, 448–9, 453 apolar solvents 296 area, skin or leather 51–4 arginine group 148, 336 Arrhenius equation 89, 230 arteries 37–8 asparaginase 107 asparagine hydrolysis 144, 145 aspartic acid 144, 145, 146, 147

in chrome tanning 220–1 astringency 281, 284, 285, 444 auxiliaries, dyeing 385–9

anionic 385–7 cationic 388–9 cationic tannages 388 dyestuff complexes 387 intensifying agents 387–8 leather/dye affinity and 387

auxiliary syntans 190, 192, 323–4 auxochromes 372 azo albumin 174 azo dyes 371, 373, 374 azocoll 174 bacteria

anaerobic 310–12 halophilic 82–3, 85–6, 170 putrefaction 72, 73 in skin fibre structure 75

bacterial bating enzymes 166, 169–70, 172–3

bacterial growth and drying 76–7 phases 74 water activities and 88

Bally Penetrometer test 411, 412, 416 basement membrane 21, 118, 129–30 basic dyes 373–4 basicity

chromium(III) complexes 205 high basicity reactivity 228–30

basification xxiv, 221–8 basification curves 223, 224 basifying agents 222

carboxylate salts 224–5 self-basifying salts 225–8 soluble alkaline salts 223–4

batch washing 100 bate formulations 171–2 bathochromic shift 372 bating xxiv, 42, 166–75

monitoring 173–4 vs deliming 154

bating enzymes 42, 106, 166 concentration and 171–2 origin of 172–3 pH and 170–1 temperature and 169–70 time and 172

Bavon processes 417, 418–19 beamhouse processing xxiv, 95–6 belly region 49 bend modulus 427, 428 benzidine 371, 372 β-amino acids 1, 2, 3, 5 bicyclic oxazolidines 335, 336, 337 biguanide 108 biochemical variations, in liming 150–1 biocide ice 90 biocides 91, 92

in soaking 107, 108, 109 biodegradability 310–12 biogas 310, 311, 312 bis isothiocyanate 108 biuret reaction 75, 174 ‘bleaching’ syntans 323–4 bleeding, at slaughter 37 bloom xxv

Subject Index 469

‘bog body’ 455–6 boil test xxv, 199–200, 233–4 bone 438

demineralisation 18, 19 boron 261 bovine hides see cattle hides ‘bran drenching’ 166 ‘breathability’ 412 brining 4, 85–6 bronopol (Myacide AS) 90, 108 buckskin clothing 315, 333 buffing xxv, 54, 409 bulb (hair) 115, 125 butt region xxv, 49 cadmium 264 calcium bicarbonate 165 calcium carbonate 152, 159, 165, 225–7 calcium hypochlorite 126 calcium ions

erector pili muscle and 110 in liming 141, 150 removal in deliming 154 in unhairing liquors 122

calcium sulfate 159 capping 416 carbohydrates 59, 106, 454–5 carbon dioxide 161–4

supercritical 255, 364 carbon dioxide snow 89, 161 carbonic anhydrase 163 carboxyl ionisation 178 carboxylates

in chrome tanning 224–5 complexation 188, 217, 218 polycarboxylates 243–6 stability constants 211

case hardening 77, 295 catalysis see enzyme catalysis catechin 292, 303, 305–6, 307, 308, 309 catechol 307, 308, 390, 391 cationic auxiliaries 388–9 cationic fatliquors 405–6 cationic tannages 388 cattle hides 31, 33

dung-clad 56, 57, 65 liming 137, 138

processing 96 sebaceous grease 105, 106 soaking of 65, 98, 99 splitting 54 variation in species 65–7

caustic soda unhairing 124 cellulase 59, 106 cellulolysis 59, 60 cellulose 58–9, 60 Centre Technique du Cuir method 340 cerium 264 chamois leather xxv, 315–16 charge effects 140, 142 chase pickle process 228 chebulagic acid 287 chebulic acid 286 chebulinic acid 287 chelation

in masking agents 245, 246 in metal tanned collagen 274

chemical-assisted enzyme unhairing 129 chemical potential 181 chestnut tannin 288, 294, 304, 305, 306 chilling 89 chlorine dioxide 127 chondroitin sulfates 44–5, 46, 47 chondroitinases 44, 102, 106 chrome alum 208, 445 ‘chrome free’ leather 446–7 chrome oxide xxv, 241 chrome reactivity 241–3 chrome retanning 351–2 chrome shadowing 223 chrome soaps 225 chrome staining 219, 220 chrome syntans 341, 462 chrome tanned leather

drying of 422–3, 424, 426 in dyeing 384–5, 390 lubrication 408–9 stability of 246–7 water resistance 409, 413, 417–19

chrome tanning 18, 19, 26, 204–5 advantages of 213–14 aluminium in 270–4 basification in 221–8 and colour 188–90

470 Subject Index

counterion role in 250, 251, 252, 445 development of 208–9 ethanolamine, role of 255 future of 444–5 high basicity reactivity 228–30 masking in 237–46, 444 non-aqueous floats 253–5 pH and temperature, relative effects

231–7, 444–5 and pickling 193, 194 semi-chrome tanning 301–3 substrate and 255–6 sulfate role in 213, 247, 248, 249–50, 251 temperature, role of 230–1 zero float processing 252–3

chrome tanning reaction 213–21 chromic acid 206–7 chromite ore 205–6 chromium(III) complexes 188–9, 206

basicity 205 chemistry of 209–13 and sulfate 251

chromium(III) salts carboxylate complexation of 217, 218 chemistry of 267, 271 in dyes 376–7 preparation of 205–8 tanning power of 272

chromium(III) sulfate 207, 213 chromium(VI) complexes, toxicity of

208, 209 chromophores 371–2, 383 chymotrypsin 173 CIELAB colour space 370 cleaning, pelt 101 clearing xxv, 277 coarse break 34 cobalt 263 cod liver oil 315, 316 collagen 1–2

amino acids in 23, 31, 39 bonding types in 41 carboxyl ionisation in 178, 219 chemistry of 21–3 chrome tanned 220, 221 elemental composition of 113 extraction of soluble 7

hierarchy of structure 4–7 hydrothermal stability 5, 23–7, 432–4 isoelectric point 8–9, 180, 358 in the liming process 146–7 modified 196 and non-swelling acids 191–2 other types of 20–1 in pickling 180–3 plant polyphenol interaction 282–3,

293–4, 435 quarter stagger array 14–17 stabilisation of 11, 12–13, 435–9,

456, 458, 460 titration curves 22, 135 and water 10–14, 421, 426–7 see also fibrils

colour and chrome retanning 351–2 matching 370 and pickling 188–90 see also dyes

Colour Index 370 combination tanning 296–303, 448–50

semi-chrome tanning 301–3 semi-metal properties 300–1 semi-metal tanning 296–300

compact processing 362–4 future prospects 462 premetallised dyes 377

complex tannins 293 complexation masking 240 concentration, and enzyme catalysis 171–2 condensation reaction 2 condensed tannins 284, 288–93, 451

and aldehydic crosslinkers 303–10 biodegradability of 312

contamination see pollution corium 30, 31, 32, 35

grain–corium junction 30, 31, 33–4 grain–corium thickness ratio 55–6 see also angle of weave

corium minor see grain corrected grain xxv cortex 116, 117

in hair burning 119 and shaving 131–2

counter-current pit tanning 295–6

Subject Index 471

counter-current washing 100–1 counterions

in chrome tanning 250, 251, 252, 445 in mineral tanning 259

covalent bonding 15, 41 cracky grain 185 crimping 24, 25 crock xxv, 387 crosslinkers

for condensed tannins 303–10 and polymers 453–4 for syntans 320

crosslinking 432, 434, 435, 437–9, 441 in chrome tanned collagen 220 in masking agents 245–6 see also link-lock

crosslinking ratio 321 crust leather xxvi, 190 cryo-scanning electron microscopy

392, 393, 394 curing see preservation cutaneous fat 67–9, 76 cuticle 115–16, 117


cysteine 112–13, 114, 120, 124–5 d block elements 260 D-period 14, 17 daub 56 deamidation 144 decorin 44 dehydroalanine 124–5 deliming xxvi, 154–65

functions of 154–5 limeblast in 165 melanin and 164–5

deliming agents 155–65 acidic salts 157–8 alternative buffers 159–60 ammonium salts 158–9 carbon dioxide 161–4 hydroxyl ‘sinks’ 160 strong acids 156–7 water 156 weak acids 157

delocalised electrons 371, 372 demineralisation

bone 18, 19 tendon 25, 26

denaturation temperature see shrinkage temperature

depletion (deswelling) in liming 154–5 in pickling 179

depside esterification 284 dermatan sulfate 44, 45, 46, 106, 454

removal in liming 148–9 desmosine 42 detanning xxvi, 201, 324 detergents

in soaking 105–6 in water resistance 415

dialdehydes 333, 454 dicarboxylates 243–5 dichromate 206–7, 213 dielectric constant 255 differential scanning calorimetry xxvi,

25, 26, 200–1, 425–6 diffusion, rate of 76–7 diisocyanates 344 dimethyl sulfoxide 255 dipase 21 direct dyes 374–5 discolouration 286 disodium phthalate 444 dispase 130 disproportionation 127 divi-divi 297 dolomite 225–6 Donnan equilibrium condition 181,

182–4 double face xxvi, 36, 179, 411 double hiding/skinning 34, 77 dried hides, soaking 109 drums

curing in 81–2 Herfeld drum painting method 123 soaking in 97, 104 vegetable tanning in 296

dry boil test 234 dry salting 85

472 Subject Index

drying 76–9, 421–30 advantages and disadvantages of 78 future prospects 463 partial dehydration 78–9 rate of 422–5 two-stage 427–30

dung 56–65 and biocides 109 composition of 56, 58 enzymatic soaking 64, 65 green fleshing benefits 63–5 removal of 101

dye resist 220 dyeing 348, 349, 351, 352, 370–91

auxiliaries 385–9 colouring methods, alternative 389–91 and fatliquoring 363 substrate, role in 384–5 water resistance and 415–16

dyes acid 373 basic 373–4 direct 374–5 mordant 268, 375–7, 376 premetallised 377–9 reactive 379–82, 461 reactivity and fixation 383–4 sulfur 382–3 see also colour

dynamic mechanical thermal analysis xxvi, 301, 302, 364–8, 427

effluent treatment 313 elastase 52–3 elastin 32, 38–9, 40, 41–2

amino acids in 23, 39 degradation 53, 129, 172 elemental composition of 113

elastin red 174, 175 elastolytic enzymes 42, 129 electrolytes

in liming 140–3 neutral 415 in pickling 179–84 in preservation 86

electron beam radiation 92

electrostatic bonds 8, 41 ellagic acid 286 ellagitannins 284, 285, 286, 287 emulsifying agents 394–5, 396, 398 endoglucanases 59, 60 enthalpy of shrinkage/denaturation

197, 201, 434, 439 entropy of shrinkage/denaturation

197, 201, 434 environment see biodegradability;

pollution; toxicity enzyme catalysis

factors affecting 169–73 mechanism of 166–9

enzymes 463 assays 174–5 proteolytic 128, 150 in soaking 106–7 in unhairing 128–30 see also bating enzymes

epidermis 29–30, 32, 117–18 in enzyme unhairing 129 in hair burning 119

epoxide tanning 342–3, 344 erector (arrector) pili muscle 36, 37

in soaking 109–11 ester hydrolysis 160 ethanolamine 255 ethylene glycol 255 ethylenediamine 255 eumelanins 45–6, 47 evaporation, rate of 76–7 Ewald effect 25–6, 317 exogluconases 59, 60 extended X-ray analysis fine structure

(EXAFS) xxvi, 247, 248, 249–50 extremophiles see halophilic bacteria f block elements 260 faeces 166

see also dung fatliquoring xxvi, 349, 351, 353, 392–419

complexing lubricants 408–9 conditions of 396–7 and dyeing 363 mechanisms of 395–6, 400 water resistance 409–19

Subject Index 473

fatliquors amphoteric 407, 408 anionic 401–5 cationic 405–6 multi-charged 407 nonionic 399, 406–7 soap 405 solvent 407–8 see also sulfated fatliquors; sulfited

fatliquors fellmongering xxvii, 130 ferulic acid 455, 457 fibre weave 35, 50

see also angle of weave fibres 20, 139 fibrils 17–18, 19, 20

and dermatan sulfate 46 during liming 139

filling power, syntans 324 fine break 34 finishing 409 fixation xxvii

in chrome tanning 231, 235 in dyeing 383–4 in post tanning 353, 360, 361 in vegetable tanning 282–3

fixation rate 214–15, 237, 355 Fixing Agent P 461 flavonoids 288–9, 291, 292, 435 flesh xxvii, 35–6 flesh layer xxvii, 30, 31, 35, 52 flesh split xxvii, 52, 54–5 fleshing 56, 62–5, 96 flo-ice 90 float xxvii

change of 103 length of 134–5 non-aqueous 253–5 in pickling 177, 179 zero float processing 252–3

float to goods ratio 98, 99 follicles 36, 115

angle of 36, 110, 111 hair saving and 125–6 melanin in 47, 48

formaldehyde 308, 329–31 and catechin 303 and salicylic acid 345

formate masking 240–1 formic acid 185, 238

in pickling 186, 188, 189–90 freezing 90–1 fresh stock 92–3, 109 frictional heating 230, 233, 236 fungi, ligninolytic 61–2 furskins 269 gallocatechin 305–6, 307, 308 gallotannins 284, 285 gamma radiation 91 Gelatine Film Test 74 gelatinisation 24, 25 genepin 459 glass transitions 365, 366, 367, 368, 419 glucose 60, 150, 207 glutamic acid 220–1 glutaraldehyde 331–2 glycine 4–5, 256

in the triple helix 6–7 glycosaminoglycans 42, 106, 149, 454 glyoxal 332 goatskin 67 grain xxvii, 30, 31, 32–3

and angle of weave 52 consistency of appearance 50 corrected xxv damage to 76 elastin in 40

grain enamel 32–3, 129 grain leather 54, 118 grain pattern xxvii, 36 grain–corium junction 30, 31, 33–4 grain–corium thickness ratio 55–6 granular salt 82 green fleshing 56, 62, 63–5 green splitting 55, 65 green tea extract 309 ground substance xxvii, 77–8 growthiness 50, 51 guanidino 148 gums xxvii, 282 hair 36, 114, 115–16 hair burning xxvii, 112, 118–24

chemical mechanism of 120 chemical variations 123–4

474 Subject Index

epidermis in 117 and shaving 131–2 and swelling 122–3 vs hair saving 113

hair saving xxvii, 112, 125–6 enzyme 129–30 point of attack in 115, 125 vs hair burning 113

halophilic bacteria 82–3, 85–6, 170 handle xxvii, 393 ‘hard spot’ 82 Heidemann method 339–40 Heidemann’s Darmstadt Process 126–7 hemicelluloses 59–60 Henderson equation 178, 180 Henkel method 340–1 Herfeld drum painting method 123 hide powder azure 174 hide powder black 174 hide thickness 55 hides xxvii, 49

soaking 109 see also cattle hides; skin

high reactivity 229 Hofmeister (lyotropic) series 142–3 hot water ageing 230–1, 233 hyaluronic acid 42–4, 454

removal in soaking 102, 106 hydrochloric acid 186, 187 hydrogen bonding 15, 41

hydroxyproline 11–13 in liming 141–2 in lyotropic swelling 184

hydrogen peroxide 164 hydrogen sulfide 119–20, 121, 126, 164 hydrolysable tannins 284–8 hydrolysis 2–4, 24

amide sidechains 137, 138, 143–8 ester 160 during liming 136, 138 triglycerides 149

hydrophilic reagents 219 hydrophilic tannage 413 hydrophilic–hydrophobic balance 353,

354–5 in dyes 379 in tanning 350, 351

hydrophilic–lipophilic balance 353, 354–5, 379

hydrophobic bonding 15, 283 in elastin and collagen 41, 42 in post tanning 355

hydrophobic masking 240, 241–3 hydrophobic reagents 219 hydroquinone 390, 391 hydrothermal isometric tension xxviii,

201–2, 436–7 hydrothermal stability 5, 23–7, 432–4

in chrome tanning 231, 233–4, 445 oil tanned leather 317 and pH 137 in tanning 196–202 see also shrinkage temperature

hydroxyl ‘sinks’ 160 hydroxyproline 2, 4–5

hydrogen bonding 11–13 and shrinkage temperature 10

hypsochromic shift 372 icing 90 immature skins, Schiff base formation

15–16 Immergan 317 immunisation 124–5 in situ oxidation 315–16 individual interactions 297, 448, 449 inductive effects, solvent and 13 insulin 173 intensifying agents 387–8 intramolecular catalysed hydrolysis

144, 145 ionic strength 142 iridoids 459 iron 263 iron tanning 276–7 iso-desmosine 42 isocyanate tannage 343, 344 isoelectric point 8–9, 144, 148, 179, 180

and post tanning 352, 356–8, 359 isometric tension measurements 138 isothiazolone 108 Japanese leather xxviii, 95 just in time approach 342

Subject Index 475

kaolin 172 keratin 29, 112–18

amino acids in 23, 31, 112–13 elemental composition of 113 in hair burning 119 immunisation 124–5 and leather making 116–17

keratin azure 174 keratinase 130 khari salt 83–5 kinetics, chrome tanning reaction

215–16, 217, 218 Kraft lignin 62, 325, 386, 387 L-DOPA 390, 391 laccase 62, 389–91 lactic acid 128, 157 ‘lake’ 268 lanthanides 264 Lawsonia inermis 456, 458 layering 294 leather xxviii

area of 51–4 chamois leather xxv, 315–16 ‘chrome free’ 446–7 grain leather 54, 118 Japanese xxviii, 95 lubrication of 408–9 pattern of veins on 38 softness of 398, 400–1 white leather 268, 330, 333, 334 see also chrome tanned leather;

semi-alum leather; semi-titan leather; vegetable tanned leather

leather processing alternative technologies 462–4 beamhouse processing 95–6 cattle hides 96 in drums 97 future of 444–65 keratin and 116–17 and properties 364–8, 450–1 skin 54–65 technology and economics of 446–7 see also individual processes

leather properties 364–8, 450–1 leathering xxviii, 196 leucine aminopeptidase 173 ligand field 188–9 lightfastness 286, 319 lignin 60–2, 63

Kraft lignin 62, 325, 386, 387 lignin peroxidase 62 ligninase 58, 106 lignocellulose 58, 106, 171–2 lignosulfonic acid 325 lime fleshing 56, 62, 64 lime-free hair burning 123–4 lime splitting 54–5 limeblast 151–3, 165 liming xxviii, 3–4, 134–53

amide sidechain hydrolysis 137, 138, 143–8

chemical variations in 149–51 dermatan sulfate removal 148–9 float length in 134–5 in hair burning 119, 122 in hair saving 126 hyaluronic acid removal before 43 non-structural protein removal 138–9 and painting 131 pH in 135–6, 137, 140, 141, 143, 150 purposes of 136–49 swelling the pelt 139–43 temperature during 136, 137 titration curves 22, 135

link-lock 432–41, 450, 451–3, 463 lipases 68–9, 106 lipocytes 33, 34, 67–9 liquid chromatography 353–4 liquid nitrogen 89 liquid (supercritical) carbon dioxide

255, 364 Liricure process 78 lithium aluminium hydride 16, 128 lithium bromide 25, 26 ‘lock and key’ theory 168–9 long float 102–3, 134–5 loop test 398 looseness 33–4, 413 loss modulus 365

476 Subject Index

low chrome wet blue 341–2 low float processing 252–3 low reactivity 229 lubrication, leather 408–9 Lubritan 419 lyotropic swelling 140, 141–2, 143, 157

in pickling 184–5

Maeser tester 411 magnesium carbonate 225–6 magnesium lactate 160 magnesium oxide 227–8 magnesium sulfate 159–60 Maillard reaction 454, 455 manganese 263 marine salt 82–3 masking xxviii, 237–46, 268, 444

definition 237 purpose of 237–8

masking agents 188, 189, 207 in chrome tanning 225 and chrome uptake 240 polycarboxylates 243–6

masking ratio 237, 238, 239, 240, 241–3 ‘mastering’ 166 matrix model 450, 453, 463 medulla 116, 117


melamine resins 326, 327, 328, 453 melanin 45–8, 116, 117, 389, 391

in deliming 164–5 in hair burning 119

melting temperature see shrinkage temperature

mercury 264 metal mordants 376 metal oxide xxviii metamerism 370 methanogenesis 310 Michaelis–Menten complex 167, 169 Michaelis–Menten constant 168 mimosa 297, 298, 300, 304, 305, 306, 453

biodegradability 311, 312 mineral tanning 259–78

experimental reviews on 265–7 future of 445

iron tanning 276–7 mixed mineral tannages 277–8 titanium tanning 267, 274–5, 298 zirconium tanning 275–6 see also aluminium tanning; chrome

tanning mixed mineral tannages 277–8 modulus of a material 365 moisture, and preservation 73 molecular modelling 17 molecular weight, and crosslinking

ratio 321 molybdenum 263 monocyclic oxazolidines 335, 336, 337 mordant dyes 268, 375–7, 376 multi-charged fatliquors 407 multi-functional reagents 345 multiple soaking process 98, 99, 100–1 myrobalan 297, 299–300, 305, 306 N-methylol groups 328, 334, 336, 452 nagarse 173 naphthalene diols 452 naphthalene sulfonate condensate 386 natural tanning agents 454–61 NDGA (nor-dihydroguaiaretic acid)

456, 458, 459 neck region 49, 50 Nerodol synthesis 318 neutral electrolyte 415 neutralisation xxviii, 190, 414 neutralise and retan 362 ‘neutralising’ syntans 324 Newtonian liquid 130–1 nickel 263 niobium 263 nitrogen heterocycles 379, 380 non-aqueous floats 253–5 non-chrome tanning 310, 446–7 non-structural components 42–8

in bating 166 removal in liming 138–9 removal in soaking 95, 101

non-swelling acids 190–2 non-tans xxix, 282 nonionic detergents 105, 415

Subject Index 477

nonionic fatliquors 399, 406–7 NovoCor AX 52 Novolak synthesis 318, 319 nubuck xxix, 54 nutrients, and preservation 73 offer xxix Official Methods of Analysis 136, 198,

201, 208, 409 Official Sampling Position 49 oil-in-water emulsions 394–5 oil tanning 315–17 olation 209, 230 oleuropein 459–61 opening up 95, 412–13

biocides and 107 in deliming 155 in liming 139 in pickling 178–9 in splitting 54

organic tanning xxix, 451–4 see also plant polyphenols; vegetable

tanning Orgel structures 211–13 osmium 263–4 osmolytes, alternative 86–7 osmotic swelling 140 oxalate masking 240–1 oxazolidines 304–9, 334–7, 452, 453

structures 335 tanning reactions 336

oxidative unhairing 127 p block elements 260 packing, triple helices 16 paint compositions 130, 131 painting 130–1 pancreatic bating enzymes 42, 158, 166,

172–3 papainase 173 papilla 115 paraffin 68, 254 part processed materials, wet white

and 342, 343 partial dehydration 78–9 paste drying xxix

pelts xxix see also hides; skin

penetration rate 214–15, 237, 355 peptide bonds 4, 5

hydrolysis 143 in post tanning 359, 360

Periodic Table 260–5 pH

and acid swelling 180–1 and biocides 107 in chrome tanning 215, 218, 221–3,

231–7, 444–5 in deliming 154, 155, 162–3 and enzyme catalysis 170–1 in hair burning 119–21 and isoelectric point 8, 9, 356–8 in liming 135–6, 137, 140, 141, 143, 150 in pickling 193, 194 and preservation 73, 87–8 during soaking 103

phenolic biocides 108 phenols 389, 390–1 pheomelanins 45, 46, 48 phlobaphenes 293 phloroglucinol 307, 309 phosphates 261 phospholipase 69, 107 phosphonium salts 337–8, 453 phthalate masking 245, 444 physical testing, direction of 49, 50 ‘pickle creasing’ 179 pickle formulations 187, 192–3 pickled pelt 342 pickling xxix, 177–94

chase pickle process 228 and chrome tanning 193, 194 colour and 188–90 formic acid in 186, 188, 189–90 hydrochloric acid in 186, 187 lyotropic swelling in 184–5 in non-swelling acids 190–2 processing conditions 179–84 storage pickling 87–8 sulfuric acid in 177, 185, 187, 189

pigskin 49 pit tanning 295–6

478 Subject Index

plant polyphenols 281, 299 and aldehydic crosslinkers 308 collagen interaction 282–3, 293–4, 435 future prospects 448, 451–3

point of precipitation 219, 265–6 pollution

in chrome tanning 259, 444, 446 hair burning/saving and 113 from salting 80 from sodium sulfate 183 and wet blue 338 and wet white 342 see also biodegradability; toxicity

polyamines, cationic 388 polycarboxylates 243–6 polyhydroxy benzene compounds 307 ‘polymer-in-a-box’ 16, 433, 438 polymerisation, syntans 318, 319 polymers, and crosslinker 453–4 polyphenols see plant polyphenols polysulfide melt 382 post tanning xxix

chrome retanning 351–2 coordinating processes of 361–2 definition 348–9 isoelectric point and 352, 356–8, 359 leather properties, processing role on

364–8 mechanisms of 353–6 peptide link role in 359, 360 sequence of steps 352–3 sulfonate group in 359–60, 361 and tanning 349–51 water resistance in 415–16 see also compact processing; drying;

dyeing; fatliquoring potash alum 267, 268 potassium chloride 86 precursors, syntans 319 prekeratinised zone 115, 125 premasked effect 242 premetallised dyes 377–9 preservation 72–93

alternative osmolytes 86–7 biocides 91 drying 76–9 effects of inadequate 75–6

fresh stock 92–3 long- and short-term 76 pH control 73, 87–8 radiation curing 91–2 salting 79–86 and temperature control 73, 89–91 wet white 339

pretanning, syntan 341 Procion 379, 380 procyanidins 290, 291, 307 prodelphinidins 289, 291, 307, 451 profisetinidins 289, 291, 307, 451 prolidase 173 proline 2, 4–5, 10 pronase 173 prorobinetinidins 289, 291, 307, 451 proteases 106

in bating 171–2 and biocides 109 in liming 150 protease/elastase 52–3 protease/lipase 69 in unhairing 128, 129

protein emulsifiers 407 proteins 454, 455, 459, 460

aldehyde reaction with 328, 329, 332 oxazolidine reaction with 335 see also amino acids; collagen;

elastin; enzymes; keratin; skin proteins

proteoglycans 42 proteolytic degradation 464 proteolytic enzymes 128, 150 protocollagen 6, 7, 10 ‘puering’ 166 putrefaction 72, 73, 109 pyrogallol 298–9, 300, 307, 308, 390, 451 quarter stagger array 14–17 quebracho 297, 304, 305, 306 quinoids 292 quinone 456 racemisation 144–7 radiation curing 91–2 Raper–Mason eumelanogenesis 46, 47 rare earth elements 264

Subject Index 479

rate equations 167–8 reaction sites, in chrome tanning 229 reactive dyes 379–82, 461 reagent chemistry, in water resistance

413–14 reagent offer 414 ‘red heat’ 83, 84, 109 redox potential

sulfides 119, 120 unhairing agents 124

‘reds’ xxix, 292–3 reducing agents, in chrome tanning 207 reductive unhairing 128 rehydration 99, 103, 110 relaxation 437, 438 Relugan GTW 332 Remazol 380, 381 replacement syntans 324 resins 326, 327, 328 resorcinol 307, 308 retan, dye and and fatliquor 363–4 retan and dye 362–3 retan and fatliquor 363 retanning xxix, 348, 349, 352, 415

chrome retanning 351–2 retans 324 reticulin 20–1 reversibility, shrinking 24, 25 rhodopsin 83 rock salt 82 Rohm and Haas method 340 running washing 100 ruthenium 263 s block elements 260 saccharides 86–7 ‘saddened’ leather 214 salicylic acid 345 salt

additives for 85 disposal of 79 in pickling 177, 179–84 removal in soaking 98–101 types of 82–5

salt-free pickling 88 salt links 8, 15

breaking 140

salting xxxi, 79–86 brining 85–6 drum curing 81–2 dry salting 85 hyaluronic acid removal 43 stacking 80–1

‘salting out’ 142 sammying xxix sandwich dyeing 374 saponification 149 sawdust 78 scandium 262 Schiff base formation 15–16, 334, 336 Schorlemmer ‘basicity’ 205 scudding xxix, 119 sebaceous grease 105, 106 segment long spacing 14 self-basifying salts 225–8 semi-alum leather 267–8, 270, 297,

298, 301 semi-chrome tanning 301–3 semi-metal leathers xxix, 300–1 semi-metal tanning 296–300, 451 semi-titan leather 274, 298, 302 sequestration 150 serine 124–5 setting xxix sharpening agents 122 shaving xxx, 131–2

and tan wastage 338 sheepskin 51, 67

elastin in the grain 40 lipocytes 67–9 looseness in 33–4 sebaceous grease 105, 106 storage pickling 88

shell area 49 shower resistance 411 shrinkage temperature xxx, 5, 23,

103, 433–4, 439–40 and aldehydic crosslinkers 304, 305,

306 and chrome retanning 351 in chrome tanning 236, 251, 252, 444 condensed tannins 292 during liming 136, 137, 138 in non-swelling acids 191

480 Subject Index

in post tanning 365–6, 367 and pyrrolidine content 10 in semi-metal tanning 296–7 in tanning 196–7, 198–9, 201 see also hydrothermal stability

shrinking, reversibility 24, 25 silica 341 silica gel 78 silicates 261 silicic acid 261 silicon 261 silicone reaction 417–18, 419 single bath process 204, 208, 209 Sirolime process 125–6 skin xxx, 1, 30, 48–54

area of 51–4 features and components 36–42 layers of 29–36 non-structural components of see

non-structural components processing 54–65 structural fibres loss 75–6 variations in structure with species 65–9 see also epidermis; follicles; hide;

sheepskin skin proteins

amino acids in 22–3 elemental composition of 113 see also collagen; elastin; keratin

soaking xxx, 95–111 dung clad hides 65 erector pili muscle in 109–11 non-structural protein removal 95, 101

soaking conditions change of float 103 long float 102–3 mechanical action 103–4 pH 103 temperature 103 time 103

soaking process 96–102 cleaning the pelt 101 dung removal 101 hyaluronic acid removal 102 parameters considered 98–9 rehydration 99 salt removal 98–9, 99–101

soaking solutions biocides 107, 108, 109 detergents in 105–6 enzymes 106–7 water 104

soap fatliquors 405 sodium aluminium silicate 340–1 sodium bicarbonate 157, 223, 224 sodium borohydride 16, 128 sodium carbonate 223, 224 sodium chlorite 127 sodium formate 224 sodium hydrogensulfide 122, 125, 135 sodium hydroxide

in chrome tanning 223 in hair burning 123–4 in liming 135, 149

sodium metabisulfite 109, 157–8 sodium peroxide 127 sodium silicate 87 sodium sulfate 183, 186 sodium sulfide 121, 122, 124, 135 ‘soft keratin’ 29 softness, leather 398, 400–1 solubilisation, collagen 24 solvents

apolar 296 fatliquors 407–8 inductive effects of 13 thionation 382

sorbitol 340 sphagnum moss 455, 457 splitting xxx, 54–5, 65, 338 spue xxx, 402, 405 stabilisation, collagen 11, 12–13, 435–9,

456, 458, 460 stability

chrome tanned leather 246–7 chromium(III) complexes 211 type III collagen 24

stacking 80–1 staking 400–1 staling xxx, 72, 109 Staphylococcus sp. 86 sterols 105, 106 storage modulus 365, 367, 423, 424,

425, 426

Subject Index 481

storage pickling 87–8 stratum basale/germinativum 117 stratum corneum 118 stratum granulosum 117 stratum lucidum 118 stratum spinosum/mucosum 117 strength 35, 393

and softness in leather 401 striking out xxx strong acids, in deliming 156–7 strong resistance 411 ‘stuffing’ 409 substrate

in chrome tanning 255–6 reagent interaction 218–19 in water resistance 413

subtilisin 173 succinimide ring formation 144, 145 sucrose 150, 461 suede xxx, 38, 52, 54, 269, 406, 411 sulfated fatliquors 394, 397–400, 401–4

levels of sulfation 402–4 preparation 402

sulfation, in chrome tanning 213, 247, 248, 249–50, 251

sulfide-free hair burning 124 sulfides

in deliming 164 in hair burning 119–22 in liming 135 in painting 130, 131

sulfited fatliquors 398–400, 404–5 sulfonate group 191

in post tanning 359–60, 361 in syntans 320

sulfonation in fatliquoring 400 syntans 318, 319

sulfonic acid group 191 sulfonyl chloride 317–18 sulfur bake 382 sulfur dioxide 207 sulfur dyes 382–3 sulfur tannage 262 sulfuric acid, in pickling 177, 185, 187, 189 superabsorbent acrylate polymer 79 supercritical carbon dioxide 255, 364

Supra syntans 324, 325 supramolecular water sheath 11, 12 surface reaction 229 surface tension 410, 411 surfactants see detergents sweat glands 36 swelling

and hair burning 122–3 in liming 134, 136, 137, 139–43 in non-swelling acids 191 in pickling 179–80, 184 see also acid swelling; lyotropic

swelling swelling curve 140, 154, 179, 184 synchrotron X-ray spectroscopy 208 synergy 297, 304–5, 306, 449 syntans xxx, 318–26

auxiliary 190, 192, 323–4 chrome 341, 462 future prospects 448 pretanning 341 properties and functions 320, 321 replacement syntans 324 retans 324

tallow recovery 64–5 tan wastage 338–9 Tanbase 227 tanning xxx, 195–202

combination options 448–50 definition 195 hydrothermal stability in 196–202 link-lock concept 432–41 natural tanning agents 454–61 in non-swelling acids 191 and post tanning 349–51 single tanning options 447–8 see also chrome tanning; mineral

tanning; oil tanning; vegetable tanning

tanning power, syntans 320, 322, 323 tanning reactions 53, 195

categorisation of 350 chrome 213–21 oxazolidines 336 in pickling 188

tans xxx, 282

482 Subject Index

tantalum 263 tara 302, 304 tawing 196, 268 TCMTB 92, 108 telopeptide regions 7 temperature

ambient 215 and biocides 107 in chrome tanning 215, 230–1, 231–7,

444–5 and enzyme catalysis 169–70 erector pili muscle and 110 during liming 136, 137 and preservation 73, 89–91 during soaking 103 in water resistance 414–15 see also hydrothermal stability;

shrinkage temperature tendon, demineralisation 25, 26 thawing 91 thermodynamics, of collagen shrinkage

197–8 thiadiazine 108 thioglycolate 124, 131 thione 108 thiosulfate 262 thixotropy 130–1 THPS 108, 326, 337–8, 453 ThruBlu process 228, 254 thumbnail test 119 thumbprint method 173 time

angle of hair follicle/weave and 111 and biocides 107 and enzyme catalysis 172 erector pili muscle and 110 liming and 136 and soaking conditions 103

titanium 262 in tanning 267, 274–5, 298

titration curves, collagen 22, 135 toggle drying xxx toxicity

acrolein 333 chromium(VI) 208, 209 formaldehyde 330–1 see also pollution

transition elements 262–4 transition state 169 trehalose 461 triazine 92, 108 triglycerides 106, 394

hydrolysis 149 loss of 76 removal of 105

triple helix 6–7 electrostatic bonds 8 fibrils 17–18, 19 quarter stagger array 14–17 water structure 11–13

trypsin 18, 173 tryptophan 2 tucking 317 tungsten 263 two bath process 204, 208–9 two-stage drying 427–30 type I collagen see collagen type III collagen 20–1, 23, 24 type IV collagen 21 type VII collagen 21 undyed casein 174 undyed hide powder 174 unfolding transition temperature 13 unhairing xxxi, 65, 112–32

enzymes in 128–30 immunisation 124–5 painting 130–1 shaving and 131–2 variations in technologies 126–8

unhairing agents 124 unhairing liquors 122 vacuum salt 82 vanadium 263 vegetable tanned leather

drying of 423, 425, 426 in dyeing 385 semi-alum 267–8, 270 semi-titan 274

vegetable tanning 281–313 biodegradability and 310–12 combination tanning 296–303 effluent treatment 313

Subject Index 483

modern pit tanning 295–6 other technologies 296 practical techniques 294–5 vs chrome tanning 213–14 and zirconium tanning 276

vegetable tannins xxxi in compact processing 362 complex 293 condensed 284, 288–93 condensed and aldehydic crosslinkers

303–10 extraction 281 future prospects 448 general properties of 293–4 hydrolysable 284–8

‘veininess’ 37, 76 veins 37–8 vertical fibre 65–7 vinyl sulfones 380, 381 ‘violet heat’ 83 viscoelasticity 301, 364–5, 367–8, 422 water

carbon dioxide solubility in 162, 163 and collagen 10–14, 421, 426–7 deliming with 156 in soaking 98, 104, 109

water activities 87, 88 water resistance 409–19

chemistries of treatments 416–17 chrome tanned leather 409, 413, 417–19 principles of conferring 411–4166

water retention 317

waterproofing 409 waxes 105, 106 weak acids

in deliming 157 in pickling 178, 184, 185

weak resistance 411, 416 weave see angle of weave; fibre weave wet blue xxxi, 188, 214, 338–9, 392,

393, 394 drying curves 423, 424 low chrome 341–2

wet boil test 234 wet salting xxxi, 79, 80–1 wet white xxxi, 338–42

part processed materials and 342, 343

production 339–42 white leather 268, 330, 333, 334 wicking 409–10 wood flour 171–2 wool 36 woolskins 130–1, 178–9 woolslip xxxi, 179 xylanase 47, 58, 60, 61, 106, 165 xylans 59–60, 61 yttrium 262 zero float processing 252–3 zinc 264 zirconium 262

in tanning 267, 275–6

tanning chemistry: the science of leather tanning chemistry

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