kwee kok yee · 2018. 1. 9. · dmcha n,n-dimethylcyclohexylamine dmea n,n-dimethylethanolamine...

AQUEOUS POLYURETHANE DISPERSION WITH

NON-YELLOWING AND GOOD BONDING STRENGTH

FOR WATER BORNE POLYURETHANE FOOTWEAR

ADHESIVES APPLICATIONS

KWEE KOK YEE

(BSc.(Hons), Acadia University, Canada)

A THESIS SUBMITTED

FOR THE DEGREE OF MASTER OF SCIENCE

DEPARTMENT OF CHEMISTRY

NATIONAL UNIVERSITY OF SINGAPORE

2005

Acknowledgment

It is my great pleasure to express my sincere thanks to my supervisor, Professor Goh Suat

Hong for his invaluable guidance, support and enthusiastic encouragement throughout the

course of my research work.

Special thanks is also extended to Rhodia Asia Pacific Pte Ltd for the financial support of

my M.Sc. Course.

The assistance of the staff in NUS analytical laboratory and Rhodia PC&S-INCO

laboratory staff is also gratefully acknowledged.

Last but not least, I wish to express my greatest gratitude to my husband, Kenny Lim ,

my children Andrew Lim and Alfonsine Lim for their love, support, encouragement and

sacrifices which enable the completion of these studies.

i

Table of Contents

Acknowledgment i

Table of Contents ii

Summary vii

Glossary ix

List of the Tables xi

List of the Figures xiii

List of Publications xviii

Chapter 1 Introduction 1

1.1 References 3

Chapter 2 Theoretical Background 5

2.1 Introduction To Polyurethane 5

2.2 Types Of Polyurethane 6

2.2.1 Foamed Type 6

2.2.2 Solid Type 6

2.3 Polyurethane Adhesives 7

2.3.1 Types of Adhesives Technology 8

2.4 Application of Polyurethane 9

2.5 Market Trends – Rising Significance Of Aqueous Polyurethanes 10

2.6 Aqueous Polyurethane Dispersion 11

2.6.1 Various Methods Of Making Polyurethane Dispersions 14

ii

2.6.1.1 Emulsifier-Containing Dispersions 14

2.6.1.2 Ionomer Dispersions 14

2.6.1.3 Non-Ionic Dispersion 19

2.7 Ingredients For Aqueous Polyurethane Dispersions 21

2.7.1 Isocyanates crosslinkers 21

2.7.1.1 Aromatic isocyanates 21

2.7.1.2 Aliphatic isocyanates 21

2.7.1.3 Chemistry Of Isocyanates 25

2.7.2 Polyols Resins 30

2.7.2.1 Polyether Polyols 32

2.7.2.2 Polyester Polyols 32

2.7.3 Other Additives 35

2.7.3.1 Catalysts 35

2.7.3.2 Neutralizing Agents 37

2.7.3.3 Dimethylolpropionic Acid 38

2.7.3.4 Chain Extenders 39

2.8 Application Test 40

2.8.1 Strength And Adhesion 40

2.9 Introduction Of Shoe Making 42

2.9.1 Methods Of Shoe Construction 43

2.9.1.1 Method 1 : Moccasin Construction 44

2.9.1.2 Method 2: Cement Construction 44

2.9.1.3 Method 3 : Stitchdown Construction 45

iii

2.9.1.4 Method 4 : Moulded Method 45

2.9.1.5 Method 5 : Force Lasting Construction 46

2.10 References 47

Chapter 3 Experimental 48

3.1 Material 48

3.2 Preparation of Aqueous Polyurethane Dispersion 48

3.3 Preparation of Two Component (2K) Water Borne Polyurethane Footwear

Adhesives 51

3.4 Gel Permeation Chromatography (GPC) Measurement 51

3.5 Isocyanate Functionality Determination 51

3.6 Particle Size Analysis 52

3.7 FT-IR Analysis 53

3.8 Shear and Peel Strength Measurement 53

3.8.1 Shear Strength Measurement 53

3.8.2 Peel Strength Measurement 54

3.9 References 56

Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive

Bonding Strength of Aqueous Polyurethane Dispersion 57

4.1 Introduction 57

4.2 Experiment 58

4.3 Results and Discussion 62

iv

4.3.1 The Effect of NCO/OH Ratio 62

4.3.2 The Effect of DMPA Content 65

4.3.3 The Effect of TEA/DMPA Molar Ratio 67

4.4 Conclusions 71

4.5 References 72

Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous

Polyurethane Dispersion 74

5.1 Introduction 74

5.2 Experiment 75


5.3.1 Effect of Different Types of Chain Extenders 81

5.3.2 FT-IR Analysis of Aqueous Polyurethane Dispersion (PUD) 81

5.3.2.1 Formation of PUD 81

5.3.2.2 FT-IR Analysis of Residual NCO Functionality in PUD 82

5.3.3 Growth of Average Molecular Weight during the Chain Extension 86

5.3.4 Effect of the Degree of Chain Extension on the Adhesive Bonding Strength 90

5.4 Conclusions 92

5.5 References 92

Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing

and Adhesive Bonding Strength Properties in Formulating the

Footwear Adhesives 94

v

6.1 Introduction 94

6.2 Experiment 99


6.3.1 Color Appearance and Durability Comparison 101

6.3.2 Adhesive Bonding Strength Comparison 103

6.4 Conclusions 108

6.5 References 109

Chapter 7 Conclusions 110

vi

Summary

It is imperative to develop aqueous polyurethane dispersions mainly due to the

evolution of legislation towards reducing the VOC (volatile organic concentration) and

the creation of environmentally friendly products. In view of the footwear industry, the

big multi-national footwear producers like Nike, Reebok and Adidas have already

embarked on the campaign to demand their suppliers to supply water borne footwear

adhesives.

Aqueous polyurethane dispersions are binary colloidal systems having

polyurethane particles dispersed in aqueous phase, which can be classified into anionic,

cationic and nonionic systems. In this research, the polyurethane ionomers were prepared

by anionic dispersion process using polyester diol derived from caprolactone monomer

terminated by primary hydroxyl groups (CAPA®2205), isophorone diisocyanate (IPDI)

and dimethylol propionic acid (DMPA) as the potential ionic center with 1-methyl-2-

pyrrolidone (NMP) as the co-solvent. 1,6-Hexanediamine (HDA) was selected as a chain

extender for the chain extension process. The reaction parameters were NCO/OH ratio,

DMPA content, neutralization degree and chain extension. The influence of the

molecular weight and particle size of the PUD on the adhesive bonding strength i.e. peel

and shear strength have been determined.

The weight-average molecular weight (Mw) and adhesive bonding strength were

significantly affected by the NCO/OH ratio, the DMPA content, the degree of

neturalization and the level of chain extension. As the NCO/OH ratio increases, the Mw

increases and the adhesive bonding strength also increases. The lower the DMPA content

, the higher the Mw and particle size of PUD, but lower the formation of hard segments

vii

in the polyurethane main chain. Perhaps, the increase in the adhesive bonding strength

would be more influenced by the Mw than the ratio of soft and hard segments. When the

chain extension increases, the Mw increases, leading to the increase in adhesive bonding

strength. The higher the degree of neutralization, the lower the particle size, and

consequently the molecular weight and the adhesive bonding strength increased.

To obtain an aqueous polyurethane dispersion with the optimum performance, the

reaction parameters are as follows: NCO/OH ratio is 3, the DMPA content is 5% and the

degree of neutralization is 100%. The non-yellowing property of the footwear adhesive

was achieved by using aliphatic isocyanate (IPDI).

The merit of this research is that, we are able to develop an aqueous polyurethane

dispersion (PUD) with the properties fulfilling the industrial requirements such as good

compatibility, durability (non-yellowing), low VOC and excellent adhesive bonding

strength. Moreover, the performance of PUD developed in this research has also been

assessed and compared with the commercial PUD in both 1K and 2K footwear adhesives

formulations. Our PUD showed superior performance in adhesive bonding strength than

the commercial product. Therefore, the PUD developed in this research may attract

attention from footwear adhesive producers in the market.

viii

Glossary

BDMAEE Bis(N,N-Dimethylaminoethyl)ether

CAGR Compound annual growth rate

DABCO 1,4-Diazabicyclo[2,2,2]octane

DBBT Di-n-butylamine back titration

DLATGS L-alanine-dooped duterated triglycine sulfate

DMAEE 2-(2-Dimethylaminoethoxy)-ethanol

DMCHA N,N-Dimethylcyclohexylamine

DMEA N,N-Dimethylethanolamine

DMPA Dimethylol propionic acid

DMF Dimethylformamide

DMT Dimethyl terephthalate

DETDA Diethyl toluene diamine

DI Deionized water

DBTL Di-n-butyltin-di-laurate

EDA Ethylenediamine

FT-IR Fourier-tranform infrared spectroscopy

GPC Gel permeation chromatography

H12MDI 4,4’-Diisocyanatodicyclohexylmethane

HDI Hexamethylene diisocyanate

HDA 1,6-Hexanediamine

IPDI Isophorone diisocyanate

Mw Weight-average molecular weight

ix

Mn Number-average molecular weight

MDI Methylene diphenyl diisocyanate

NDI 1,5-Naphthalenediisocyanate

NMP 1-Methyl-2-pyrrolidone

PUD Polyurethane dispersion

PIR Polyisocyanurate rigid foam

PET Poly(ethylene terephthalate)

PMMA Polymethylmethacrylate

PMDETA N,N,N’,N’,N”-Pentamethyldiethylenetriamine

PS Polystryene

TDI Toluene diisocyanate

TDA Toluene diamine

TEA Triethylamine

THF Tetrahydrofurane

VOC Volatile organic concentration

W/O Water/oil

2K Two-component

1K One-component

x

List of Tables

Table 2.1. Types of polyurethane adhesives 9

Table 2.2. Characteristic features of polyurethane dispersions 20

Table 2.3. Tertiary amine catalysts and their application 36

Table 2.4. Organometallic catalysts and their application 37

Table 4.1. Formulation of aqueous polyurethane dispersion (Sequence 1) – DMPA

content is constant and NCO/OH ratio varies from 2.8 to 3.4 61

Table 4.2. Formulation of aqueous polyurethane dispersion (Sequence 2) – NCO/OH

ratio is constant and DMPA varies from 4 to 7 61


ratio and DMPA content are constant but the degree of neutralization

varies from 85% to 115% 62

Table 5.1. Characteristics and formulations of aqueous polyurethane dispersion

using HDA as chin extender with different degree of neutralization 78


using EDA as chin extender with different degree of neutralization 79


using Dytek® A Amine as chain extender with different degree of

neutralization 80

Table 5.4. Appearance of the finishing PUD product with different types of chain

extenders 81

Table 5.5. The residual NCO content of the polyurethane prepolymer by the di-n-

Butylamine back titration method 84

xi

Table 6.1. One-component (1K) water borne PU footwear adhesive

formulation 100

Table 6.2. Two-component (2K) water borne PU footwear adhesive

formulation 100

xii

List of Figures

Figure 1.1. Formation of aqueous dispersion 3

Figure 2.1. Polyurethane adhesives market segment 8

Figure 2.2. Reaction of polyol and isocyanate 12

Figure 2.3. Amine catalyst reaction mechanism 12

Figure 2.4. Anionic polyurethane dispersion with carboxylate groups 14

Figure 2.5. Non-ionic polyurethane dispersion 15

Figure 2.6. Preparation of aqueous polyurethane dispersion by acetone process 16

Figure 2.7. Preparation of aqueous polyurethane dispersion byprepolymer mixing

process 17

Figure 2.8. Preparation of aqueous polyurethane dispersion by Ketimine and ketazine

Process 18

Figure 2.9. Global split of isocyanate market in year 2000 21

Figure 2.10. The reaction rate for different types isocyanates 24

Figure 2.11. Water Reaction 26

Figure 2.12. Urea formation 27

Figure 2.13. Biuret formation and equilibria 28

Figure 2.14. Allophonate formation 29

Figure 2.15. Other isocyanates reactions 30

Figure 2.16. Polyol types used in polyurethane 31

Figure 2.17. Worldwide demand for polyester polyols by application 33

Figure 2.18. Adhesion/cohesive strength testing formats 41

Figure 2.19. Single lap joint testing 42

xiii

Figure 2.20. Parts of a shoe 43

Figure 2.21. Moccasin method 44

Figure 2.22. Stitchdown / Veldschoen method 45

Figure 2.23. Moulded method for various types of footwear 46

Figure 2.24. Slip lasting / strobel stitched method 47

Figure 3.1. Set up of apparatus for the synthesis of polyurethane prepolymer 49

Figure 3.2. Preparation of aqueous polyurethane dispersion 50

Figure 3.3. Form and dimensions of test pieces for shear tests 53

Figure 3.4. Form and dimensions of test pieces for peel strength test 54

Figure 3.5. Process for applying the adhesive 55

Figure 4.1. Preparation of aqueous polyurethane dispersion 60

Figure 4.2. Mw and Mn of PUD as a function of the NCO/OH ratio 63

Figure 4.3. Change of Mw with different NCO/OH ratio during the chain extension

reaction 64

Figure 4.4. Effect of NCO/OH ratio on the adhesive bonding strength 65

Figure 4.5. Mw and particle size of PUD as function DMPA content 66

Figure 4.6. Effect of Mw on bonding strength of the aqueous polyurethane dispersion

with different DMPA content 67

Figure 4.7. Effect of neutralization degree on the particle size of aqueous

polyurethane dispersion (PUD) 68

Figure 4.8. Particle size controlled by TEA/DMPA ratio 68

Figure 4.9. Evolution of Mw of PUD with varying the degree of neutralization from

85% to 115% during chain extension reaction 69

xiv

Figure 4.10. Mw and Mn of PUD as a function of degree of neutralization 70

Figure 4.11. Effect of neutralization on adhesive bonding strength 71

Figure 5.1. FT-IR spectra of polyol, IPDI and PUD 82

Figure 5.2. FT-IR spectra of PUD before (a) after (b) chain extension 83

Figure 5.3. Absorption FT-IR spectra of PUD of varying particle size before chain

extension : (a) 1.65µm, (b) 2.60µm, (c) 3.65µm, (d) 4.05µm (NCO/OH

ratio = 2.8) 84

Figure 5.4. Change of FT-IR spectra during preparation of PUD: (a) polyester

polyol + IPDI + DMPA, (b) after neutralization/ before dispersion,

(c) before chain extension, (d) adding 20% of chain extender

(theortically), (e) 40%, (f) 60%, (g) 80% and (h) 100% (NCO/OH = 3.0;

particle size =2.65µm) 85

Figure 5.5. Amount of residual NCO groups versus the sizes of PUD particles

in chain extension stage at average particle size 2.65µm, 2.00µm

and 1.50µm at NCO/OH =3.0 86

Figure 5.6. The change of average molecular weight in chain extension stage with

different particle sizes and different NCO/OH ratio (particle sizes varied

from 1.50µm to 2.56µm; NCO/OH ratio varied from 2.8 to 3.0) 87

Figure 5.7. Effect of particle size on the maximum value of chain extension

(CEmax) 88

Figure 5.8. The reaction of residual NCO groups 89

Figure 5.9. Two competitive reactions of residual –NCO groups on PU particle 89

xv

Figure 5.10. Effect of the degree of chain extension on adhesive bonding strength

(shear and peel strength) at NCO/OH ratio 2.8 90

Figure 5.11. Effect of the degree of chain extension on the shear strength with different

particle sizes at NCO/OH ratio 3.0 91

Figure 5.12. Effect of the degree of chain extension on the peel strength with different

Particle sizes at NCO/OH ratio 3.0 91

Figure 6.1. Total output of adhesives + sealant and PU adhesive in China 94

Figure 6.2. Segmentation of PU adhesives markets (by consumption volume, total

1666 thousands tonnes) in year 2002 96

Figure 6.3. The largest footwear producer in the world, China 97

Figure 6.4. Total footwear production in China from year 1985 to 2002 97

Figure 6.5. Comparison the color appearance of both solvent borne polyurethane

Solution and aqueous polyurethane dispersion 101

Figure 6.6. Comparison the color appearance of both solvent borne polyurethane

solution and aqueous polyurethane dispersion after storage for 6 months

at ambient temperature and humidity at 55% environment 102

Figure 6.7. Comparison of the color appearance of both 2K solvent borne and

water borne PU footwear adhesives after exposure to sunlight for

3 months 102

Figure 6.8. Comparison of initial peel strength of our 1K water borne PU footwear

adhesive versus commercial 1K water borne footwear adhesive (based

on Disperoll U54 PUD) 104

xvi

Figure 6.9. Comparison of final peel strength of our 1K water borne PU footwear



Figure 6.10. Shear strength comparison of our 1K water borne PU footwear adhesive

versus commercial product 105

Figure 6.11. Comparison of initial peel strength of 2K water borne PU footwear



Figure 6.12. Comparison of final peel strength of 2K water borne PU footwear



Figure 6.13. Shear strength comparison of our 2K water borne PU footwear

adhesive versus commercial product 107

xvii

List of Publications

1. Two Pack Water Borne Polyurethane for Furniture Coatings

K.Y. Kwee, V. Granier and C. Varron, J. Eur. Coat. 2002, 27.

2. Overview of Aliphatic Polyisocyanates Used in Polyurethane Coatings:

Chemistry and Market Trends

K.Y Kwee, E. Charriere, J. Asi. Coat. 2003, 37.

3. Fast Drying Aliphatic Polyisocyanate for 2K PU Automotive Coatings

K.Y. Kwee, Coating Manufacturing Technology for China’s Automobile Industry

2004, 18.

xviii

Chapter 1 Introduction

Chapter 1

Introduction

The term “aqueous polyurethane dispersion” refers to aqueous dispersions of

polymers containing urethane groups and optionally urea groups. Aqueous polyurethane

disperisons are well known and used in the production of a variety of useful polyurethane

products, for example, adhesives, coatings and sealants etc. Such dispersions are

produced by dispersing a water-dispersible, isocyanate-terminated polyurethane

prepolymer in an aqueous medium together with an active hydrogen-containing chain

extender, such as diamine.

It is vital to develop aqueous polyurethane dispersions mainly due to the evolution

of legislation towards reducing the VOC (volatile organic concentration) and the creation

of environmentally friendly products. Continuous increase in solvent prices, low raw

material cost and easy to clean up the reactor system made aqueous polyurethane system

more popular in the industry. Aqueous polyurethane dispersions can be classified into

anionic, cationic and nonionic systems.1,2 They can be obtained by different processes,

however, the earliest process to prepare the aqueous polyurethane dispersion is known as

acetone process. This process has remained technically important so far.3,5 Within the

last three decades several new processes have been developed such as prepolymer mixing

process, hot melt process and ketamine/ketazine process. The basic principle involved in

producing NCO-terminated polyurethane prepolymer with appropriate molecular

weights.6 Distinctly different step among several processes lies in the chain extension

step that is generally performed using diamines (-NH2 ) and /or diols (-OH).7 In the chain

extension step, it is most important to control of extremely fast reaction between NCO

1


groups and NH2 groups accompanied by the viscosity rise.2 The prepolymer mixing

process that we have used in this study has the advantage of avoiding the use of a large

amount of organic solvent. In this process, NCO-terminated polyurethane prepolymer

containing pendant acid group i.e. dimethylol or 2,2-bis(hydroxymethyl) propionic acid

(DMPA) is neutralized with base to form internal ionic emulsifier and dispersed in the

aqueous phase i.e. water to form an aqueous dispersion (see figure 1.1). The chain

extension step is accomplished by the addition of diamine to the aqueous dispersion.

Molecular weight of polyurethane dispersion increases by the formation of urea linkage

with NCO-terminate prepolymers and diamines through the chain extension step. Hence,

the most important step to determine molecular weight of polyurethane dispersion is the

chain extension step, which is the reaction between residual NCO groups and amine

groups. The chain extension is influenced by the amount of residual NCO groups and

particle diameter. The amount of residual NCO groups is determined by the molar ratio

of NCO:OH. In additional, both hydrophilic acid group contents and their degree of

neutralization can affect particle diameter.8 Consequently, the molecular weights (Mw)

can be controlled with varying these process variables. In general, the molecular weights

(Mw) of polymer materials have a significant effect on their mechanical properties.

Therefore, the control of the mean Mw can be used as a indicator to obtain the optimum

mechanical properties i.e. the adhesive bonding strength of aqueous polyurethane

dispersion.

In this research, the aqueous polyurethane dispersions had been synthesized with

different formulations by varying their NCO/OH molar ratios, DMPA contents, degrees

of neutralization, types of chain extenders and different degree of chain extension to

2


obtain the best finishing aqueous polyurethane dispersion. The footwear (PU) adhesive

was formulated using this newly developed aqueous polyurethane dispersion and then its

performances were being compared against the commercialized polyurethane dispersion.

1st Step : Preparation of polyurethane prepolymer

Soft Segment

Hard Segment

Hard Segment(-NHCOO -) or

CH2

(-NHCOOCH2CCH2COONH-)COOH or COO- NH+ (Et)3

Soft Segment(-CH2-CH2-)

2nd Step : Dispersion process

PU particle Repulsion

COOH or COO- NH+ (Et)3

Electrical double layer

Figure 1.1. Formation of aqueous dispersion 1.1 References 1. J.W. Rothause and K. Nachtkam, Advances in Urethane Science and Techology

1987,10, p.121.

2. B.K. Kim, Coll. Polym. Sci., 1996, 274, p.559.

3


3. C. Hepburn, Polyurethane Elastomers, Second ed., Elsevier, New York, 1992, p.281.

4. G. Woods, The ICI Polyurethane Book, ICI Polyurethanes, 1987, p.197.

5. G. Oertel, Polyurethane Handbook, Carl Hanser, Munich, 1985, p.31.

6. P. Thomas, Waterborne and Solvent Based Surface Coating Resins and their

Applicaions – Polyurethanes, Vol. III, SITA Technology, London, 1999, p.59.

7. H.T. Lee, Y.T. Hwang, N.S. Chang, C.C.T. Huang, H.C. Li, Waterborne, High-Solids

and Powder Coatings Symposium, New Orleans, 22-24 February, 1995, p.224.

8. H. Xiao, H.X. Xiao, K.C. Frisch, N. Malwitz, J. Appl. Polym. Sci., 1994, 54, p.1643.

9. Y.K. John, I.W. Cheong, J.H. Kim, Colloids Surfaces A Physicochem. Eng. Aspects

2001, 179 (1), p.71-78.

4

Chapter 2 Theoretical Background

Chapter 2

Theoretical Background

2.1 Introduction to Polyurethanes The reaction between isocyanate and hydroxyl compounds was originally

identified in the 19th century; the foundations of the polyurethanes industry were laid in

the late 1930s with the discovery, by Otto Bayer, of the chemistry of the polyaddition

reaction between diisocyanate and diols to form polyurethane.1

Polyurethanes are now all around us, playing a vital role in many industries –

from furniture to footwear, construction to cars. Polyurethane can appear in many

different forms, making them the most versatile of any family of plastic materials.

Commercially, polyurethanes are produced by the exothermic reaction of

molecules containing 2 or more isocyanate groups with polyol molecules containing 2 or

more hydroxyl groups. Relatively few basic isocynates and a far broader range of polyols

of different molecules weights and functionalities are used to produce the whole spectrum

of polyurethane materials. Additionally, several other chemical reactions of isocyanates

are used to modify or extend the range of isocyanate-based polymeric materials.

The unique advantage of polyurethanes lies in the wide variety of high-

performance materials that can be produced. They also differ from most other plastic

materials because the processor is able to change and control the nature and the properties

of the final product, even during the production process. It is possible because most

polyurethanes are made using reactive processing machines, which mix together the

polyurethane chemicals that then react to make the polymer required.1,2

5


2.2 Types of Polyurethane 2.2.1 Foamed Types By itself the polymerization reaction produces solid polyurethane and it is by

foaming gas bubbles in the polymerizing mixture, often refer to as ‘blowing’.

Three foam types are, in quantity terms, particularly significant: low density

flexible foams, low density rigid foams and high-density flexible foams, commonly

referred to as microcellular elastomers and integral skin foams. Low density flexible

foams have densities in the range 10 to 80 kg/m3.

Low density flexible foams have densities in the range 10 to 80 kg/m3 , made

from a lightly crossed-linked polymer with an open cell macro structure. There are no

barriers between adjacent cells, which result in a continuous path in the foam, allowing

air to flow.

Low density rigid foams are highly cross-linked polymers with an essentially

closed cell structure and a density range of 28 to 50 kg/m3. The individual cells in the

foam are isolated from each other by thin polymer walls, which effectively stop the flow

of gas through the foam.

High density flexible foams are defined as those having densities above 100

kg/m3.

2.2.2 Solid Types Solid polyurethanes are used in many diverse applications. Cast polyurethane

elastomers are simply made by mixing and pouring a degassed reactive liquid mixture

6


into a mould. These materials have good abrasion resistance, many common non-polar

solvent resistance etc. They are used in the production of printing rollers tyres and so.1,7

Polyurethane elastomeric fibres are produced by spinning from a solvent, usually

dimethylformamide (DMF), or by extrusion from an elastomer melt. The major

applications are in clothing where these fibres have effectively replaced natural rubber.

Thermoplastic polyurethane is supplied as granules or pellets for processing by

well established thermoplastic processing techniques such as injection moulding and

extrusion. By these means elastomeric mouldings having an excellent combination of

high strength with high abrasion and environmental resistance, can be mass produced to

precise dimensions. Applications include hose and cable sheathing and so on.

Polyurethanes are also used in flexible coatings or textiles and adhesives for film,

fabric laminates and footwear. Paints and coatings give the highest wear resistance to

floors and aircraft surfaces. Binders are used increasingly in the composite wood

products market for oriented strand board and laminated beams for high performance

applications.

2.3 Polyurethane Adhesives Polyurethane adhesives, which vary widely in composition, are used in many

application areas due to their outstanding properties, their simple and economical

processing and their high strength. They account for about eight percent of the global

adhesives market, at around 530,000 tonnes, excluding their use as binders for wood and

other materials. Polyurethanes are a major element in the high value reactive adhesives

category because of their versatility and moderate pricing. The market segments in which

polyurethanes find most use are construction 31 percent, flexible packaging 27 percent,

7


footwear 17 percent, woodworking 17 percent and transportation including assembly 8

percent.1,4 This can be illustrated in the figure 2.1.

ConstructionFootwear Transportation (including assembly)

8%17%

17%27%

Flexible packagingWoodworking

31%

PU Adhesives ~ 530,000 tonnes(about 8% of global adhesives market)

Figure 2.1. Polyurethane adhesives market segment

Polyurethane adhesives are normally defined as those adhesives that contain a

number of urethane groups in the molecular backbone or where such groups are formed

during use, regardless of the chemical composition of the rest of the chain. Thus, a typical

urethane adhesive may contain, in addition to urethane linkages, aliphatic and aromatic

hydrocarbons, esters, ethers, amides, urea and allophanate groups.

A common factor for all polyurethane adhesives is that they cure to produce

essentially thin films used to bond two similar or dissimilar surfaces together, if the

correct type of polymer structure for the end application.

2.3.1 Types Of Adhesives Technology Polyurethane adhesives can be divided into two main classes: non-reactive and

reactive. In both cases, the aim is to put a thin continuous layer of high molecular weight

8


polyurethane between the two surfaces to be joined. Non-reactive adhesives are based on

high molecular weight, which are applied as solvent-borne, water-borne or as hot-melts.

Film forming occurs through evaporation of the solvent or water for the first two whilst

hot-melts are applied at high temperature and films form upon cooling.

Reactive adhesives are supplied as one- or two-component systems or as hot

melts. The one-component reactive systems, based on low isocyanate prepolymers are

usually moisture-cured, whilst the prepolymer for two-component systems are reacted

with a mix of polyol and chain extenders.1,3 The types of technology are summarized in

table 2.1.

Table 2.1. Types of polyurethane adhesives

Type Form at room temperature Curing at film forming mechanismNon-reactive: Solvent-borne Water –borne Hot-melt

Liquid Liquid (dispersion) Solid

Physical evaporation of solvent Physical evaporation of water Physical cooling

Reactive: One-component Two-component Reactive hot-melt Cross-linker

Liquid Liquid Solid Liquid

Chemical cure, NCO + Moisture Chemical cure, NCO + Polyol Physical cooling + Chemical cure, NCO + moisture Chemical cure, NCO + active H

2.4 Applications of Polyurethane Footwear – some of the soles are made from synthetic material like polyurethane

to give high performances. Polyurethane adhesives are widely used in the shoe industry

and coatings are used to improve appearance and wear resistance of shoe uppers.

Automotives – applications include seating, interior padding such as steering

9


wheels and dashboards, complete soft front-ends, components for instrument assemblies

and accessories such as mirror.

Furniture – the market for cushioning materials is mainly supplied by

polyurethane flexible foam, which competes with cotton, polyester fibre etc. It is ideal

where strong, tough, but decorative integral-skinned flexible or rigid foam structures are

needed.

Thermal insulation – rigid polyurethane foam offers unrivalled technical

advantages in the thermal insulation of buildings, refrigerators and other domestic

appliances.

2.5 Market Trends - Rising Significance Of Aqueous Polyurethanes

The fact that aqueous/water borne polyurethanes have become increasingly

important commercially in recent years is due to three reasons:

1. The increasingly stringent environment legislation requires the development of

ecologically and physiologically tolerated products for which the emissions of solvents

and other volatile organic compounds (VOC) have been reduced to a minimum.

2. The use of expensive organic solvents in conventional and aqueous polyurethanes is

undesirable for economic reasons.

3. The performance of aqueous polyurethanes reaches or surpasses that of conventional

isocyanate- and/or solvent-containing polyurethanes.

Due to these reasons, companies like Nike, Rebok, New Balance etc. global based

multi-national footwear manufacturers want to be the environmental oriented companies.

10


They are making a lot of efforts in demanding their shoes adhesives producers to produce

aqueous polyurethane adhesives for their shoes.

As the environmental problems grow bigger, it is expected that other shoe

manufactures become more interested in the environmental policy, thus adopting more

water borne products in the future. Industries other than shoe industry, such as

automobile, furniture and electronic industries are expected to adopt the water borne

adhesives only when the products are supplied in a stable and consistent manner. The

overall market size for the aqueous polyurethanes will grow tremendously in the very

near future.

2.6 Aqueous Polyurethane Dispersion

Aqueous polyurethane dispersions (PUD) are fully-reacted polyurethane systems

produced as small discrete particles, 0.1 to 3.0 micron, dispersed in water to provide a

product that is both chemically and colloidally stable, which only contains minor

amounts of solvents and thus emit very little volatile organic compounds. Aqueous PUDs

are based on aliphatic – IPDI or H12MDI – or aromatic – MDI or TDI – isocyanates,

modified polyether and/or polyester polyols, chain extenders, catalysts plus additives to

modify the coalescence, flow, thickness, coagulation and defoaming properties.

Aqueous PUD is used in many application areas to coat a wide range of substrates

– for example footwear adhesives, wood lacquers for flooring and furniture, leather

finishing, vinyl upholstery topcoats, plastic coatings, printing inks and automotive base

coats.2,8-9

Aqueous PUD is produced in conventional stirred reactor fitted with distillation

equipment. The first step in the manufacture of an anionically-stabilised PUD is to

11


prepare a prepolymer from isocyanate, polyol (containing either carboxylate or

sulpfonate side chains) and chain extenders in a water-miscible solvent such as acetone.3,5

OHR + C ONR' R O

OC

HNR'

Polyol Isocyanate urethane (carbamate)

Figure 2.2. Reaction of polyol and isocyanate.

The reaction product is an isocyanate-terminated polyurethane or polyurea with

pendent carboxylate or sulpfonate groups. These groups can be converted to salts by

adding a tertiary amine compound, which, as water is added to the prepolymer/solvent

solution, disperses the prepolymer in the water.

R'OH

"R"R

R"

N

N3 R " O

+O-R '

CN-H-ROC-NR"3N R + O=CN=R-

Figure 2.3. Amine catalyst reaction mechanism

The carboxylate groups are generally neutralized before or during dispersion of the

polyurethane prepolymer into water with a tertiary amine compound (see figure 2.3). An

organic bases are less convenient, since the polyurethane will generally coagulate when

they are applied or it will provide highly water sensitive films or coatings. To prevent

coagulation it is suitable to incorporate a great number of hydrophilic polyethoxy chains

12


into the polymer system, but the coatings prepared from these dispersions will be

sensitive to water as well. The next step in the synthesis is the reaction of the remaining

isocyanate groups with more chain extender or a cross-linker or a mixture of both. The

solvent is then stripped, leaving the water-borne polyurethane dispersion with only a low

solvent content. The critical factor is achieving a fine enough particle size of the fully

reacted polyurethane so that it maintains a stable dispersion once the solvent is removed.

The final dispersions contain 35 to 50 wt% of dispersed particles.3,4

Alternatively, a low molecular weight hydrophilic prepolymer can be chain

extended at the same time as the aqueous dispersion is formed, providing that the

isocyanate reacts preferentially with the amine rather than water. In each case, the final

polymer contains a mixture of urethane and urea groups. The majority of polyurethane

dispersions are made from the slower-reacting aliphatic isocyanates.

Hybrid systems, especially urethanes-acrylates, are also increasingly used. Simple

blending of two individual dispersions can be used, but the film properties are better if a

mixed synthesis is used, giving a continuous phase on drying with final film properties

typical of polyurethane on its own.

PUD can be applied by a range of techniques- such as brush, spray, dip, curtain

and the aqueous dispersions form films by a coalescence process in which the individual

particles are forced together, as water is lost during drying, so that the particles deform

and eventually fuse together. The process is dependent on a number of parameters with a

faster rate obtained from a small particle size, low polymer glass transition temperature,

increasing temperature and the addition of a coalescing agent to achieve sufficient flow

and fusion of the particles. Cross-linkers, such as isocyanates, aziridine, carbodiimide,

13


and melamine, can be added just prior to application to improve the performance of the

coating or adhesives, but then the blends have a pot life and usually need temperature

activation in order to achieve full cure.

2.6.1 Various Methods Of Making Polyurethane Dispersions

2.6.1.1 Emulsifier-Containing Dispersions

Depending on the emulsifier used, the resulting dispersions are mainly anionic or

non-ionic, but seldom cationic. Often they contain small amounts of solvents, for example

toluene.

2.6.1.2 Ionomer Dispersions The most important dispersions are emulsifier-free ionomer dispersions. The

resulting dispersions are mainly anionic or non-ionic (see Figure 2.4 and 2.5), which are

characterized by high mechanical and chemical stability, excellent film forming

properties, good adhesion and the potential for wide variations in composition and

property level.4,5

Figure 2.4. Anionic polyurethane dispersion with carboxylate groups

14


Figure 2.5. Non-ionic polyurethane dispersion

Due to the fact that ionomers are self-dispersing, the preparation procedure does not

require emulsifiers or high shear forces. The following preparation processes have gained

technical importance:

1. The acetone process :

First a solution of high molecular weight polyurethane – especially a polyurethane

urea – ionomer is prepared in a hydrophilic organic solvent, for example acetone. The

solution is subsequently mixed with water and then the organic solvent is removed by

distillation (see Figure 2.6). An aqueous solution or dispersion of the polyurethane

ionomer is obtained. Depending on ionic group content and concentration, the dispersion

will be formed either by precipitation of the hydrophobic segments or by invasion of the

phases of a primary formed W/O emulsion. Advantages of this process are the wide

15


variety of possibilities in the molecular weight build up of the polymer and the control of

the average particle size, as well as the high quality of the final products, and the good

reproducibility from batch to batch.2,3

Figure 2.6. Preparation of aqueous polyurethane dispersion by acetone process

2. The prepolymer ionomer mix process :

Prepolymers with terminal NCO groups can be mixed with water to yield reactive

O/W emulsions; particularly when the molecular weight of the prepolymers does not

exceed approximately 8000. Prepolymers containing ionic centers or hydrophilic

polyether segments are self-emulsifiable. This means that upon mixing with water they

spontaneously form emulsions with particle sizes which decrease as hydrophilicity

increases. The reactivity of NCO groups towards water increases in the same order.

Hydrophobic NCO prepolymers necessitate the use of emulsifiers and high shear forces

16


to disperse them in water. Emulsifiers which are chemically similar to the substrate to be

dispersed are most efficient.3,5

Highly viscous prepolymers must be diluted with organic solvents, which do not

necessarily have to be miscible with water. The resulting aqueous emulsions can be

further chain-extended by the addition of di- or polyamines. When high molecular weight

polyurethanes containing hydrophilic centers or external emulsifiers are to be dispersed

with water, preferably a solution of these polymers in hydrophilic solvents is prepared,

mixed with water, and the solvent is subsequently removed. The prepolymer ionomer mix

process can be demonstrated in Figure 2.7.

Figure 2.7. Preparation of aqueous polyurethane dispersion by prepolymer mixing process 3. The ‘melt dispersion’ process with formaldehyde polycondensation :

Reaction of an NCO-terminated ionic modified prepolymer with, for example

17


ammonia or urea results in a prepolymer with terminal urea or biuret groups,

respectively. These are methylolated with formaldehyde. Before, during, or after the

reaction with formaldehyde, the hot melt is mixed with water, forming dispersion

spontaneously. Afterwards, chain-extension or cross-linking takes place through

polycondensation (lowering the pH, increasing the temperature).

4. Ketimine and ketazine process :

Diamines and especially hydrazine are reacted with ketones to yield ketimines and

ketazines, respectively. These can be mixed with NCO prepolymers containing ionic

groups without premature chain extension. These mixtures can be emulsified with water

even in the absence of co-solvents.2,3 Reactions with water liberate the diamine or

hydrazine, which then reacts with the prepolymer (see Figure 2.8).

Figure 2.8. Preparation of aqueous polyurethane dispersion by Ketimine and ketazine process

18


5. Spontaneous dispersion process for solids :

Ionic and/or non-ionic hydrophilic modified oligomers, which have an average

molecular weight of less than 8000 and which are glassy solids at room temperature, can

be dispersed in water without the need of high shear forces, emulsifiers or thermal

treatment. Due to this feature, these products can be shipped as 100% solid resin

precursors for aqueous dispersion. Once dispersed in water, cross-linkers can be added

and high molecular weight polyurethane coatings can be obtained after cure on a

substrate.

2.6.1.3 Non-Ionic Dispersion

Non-ionic dispersions can be prepared similarly to ionomer dispersions if the

ionic center is replaced by lateral or terminal hydrophilic ether chain, having a molecular

weight of approximately 600 to 1500. The preparation is the same as described in ionomer

dispersions, except that the dispersing process temperature has to be kept below 60oC.

This is because polyethylene glycol ether units lose their hydrophilicity with increasing

temperature and result in unstable dispersions. Non-ionic dispersions are stable towards

freezing, pH-changes and addition of electrolytes. Also, they can be thermally

coagulated.3,5

19


Table 2.2. Characteristic features of polyurethane dispersions

Dispersant shear force process

Acetone process

Prepolymer mixing process

Melt-dispersion process

Ketimine/ ketazine process

Solids self-dispersing process

Polyhydroxy Compound Diisocyanate Glycols

Polyether (liquid) TDI Only small amounts

Linear, variable Variable Variable

Polyethers, some polyesters TDI,IPDI, H12MDI Dimethylol propionic acid

Variable TDI,HDI, IDPI Mainly ionic

Variable Variable Variable

Variable Softening point of prepolymer >40oC MW < 8,0000

Dispersant Solvent Shear force mixer Temperature of dispersion

+ 5 to 10% toluene + ~ 20oC

- 40to70% acetone - ~ 50oC

- often 10 to 30% N-methyl pyrrolidone - 20 to 80oC

- - - 50 to 130oC

- possibly 5 to 30% acetone - 50 to 80oC

- - - 15 to 30oC

Product before dispersion Procedure after dispersion End product Solvent contents of the final dispersion Particle size (nm) Post curing temperature

Nonionic NCO pre-polymers Amine extension Poly-urethane urea 2 to 8% toluene 700 to 3,000 100oC

Poly-urethane Acetone distill. Poly-urethane Poly-urethane-urea < 0.5% 30 to 100,000 -

NCO prepolymer- ionomer Amine extension Polyurethane urea ionomer Often 5 to 15% N-methyl-pyrrolidone 100 to 500 -

Ionic-biuret-prepolymers Polyconden-sation Poly-urethane Biuret - 30 to 10,000 50 to 150oC

NCO pre-polymer + ketimine/ ketazine possibly acetone distillation Poly-urethane urea possibly < 2% acetone 30 to 10,000 50 to 150oC

Prepolymer Curing agent added Poly-urethane - 30 to 500 >120oC

20


2.7 Ingredients For Aqueous Polyurethane Dispersions

2.7.1 Isocyanates Crosslinkers

Isocyanates can be classified into the following two main types:

2.7.1.1 Aromatic isocyanates:

Methylene diphenyl diisocyanate (MDI),

Toluene diisocyanate (TDI) and

1,5-Naphthalenediisocyanate (NDI)

2.7.1.2 Aliphatic isocyanates:

Hexamethylene diisocyanate (HDI),

Isophorone diisocyanate (IPDI) and

4,4’-Diisocyanatodicyclohexylmethane (H12MDI)

Presently, the isocyanates dominating the market are the aromatic isocyantes. The

major ones are MDI and TDI. However, the second major isoyanates are from the

aliphatic group, HDI and IPDI. Below is a pie chart showing the percentage

production of various isocyanates in 2000 market.3,4

Total Market Size for Isocyanate ~ 4.4 million tonees ( in year 2000)

3.40%1.20%

MDI34.10%

TDIHDI & IPDI

61.30% Others

Figure 2.9. Global split of isocyanate market in year 2000

21


I. Methylene diphenyl diisocyanate (MDI)

Pure 4,4’-MDI is a symmetrical molecule with two aromatic isocyanate

groups of equal reactivity. Commercial products normally contain one to two percent

of the 2,4’ isomer and have hydrolysable chlorine levels below five ppm. 2,4’-MDI is

an asymmetrical molecule with two aromatic isocyanates of different reactivity. The

4-position is approximately four times more reactive than the 2-position and is of

similar reactivity to the two groups in 4,4’-MDI. It is normally commercially

available as a mixture with the 4,4’ isomer (2,4’/4’4, 55/45).typical hydrolysable

chlorine levels are less than 50 ppm. It is an aromatic isocyanate thus not light-stable

and causes yellowing appearance after exposure of sun light for a period of time.2,3

II. Toluene diisocyanate (TDI)

The isocyanate groups on 2,4-TDI have different reactivities with the 4-

position approximately four times the reactivity of the 2-position and about 50 percent

more reactive than the 4-position group in MDI, whilst for the 2,6 isomer the groups

have equal reactivity that is approximately the same as that of the 2-position in 2,4-

TDI. Due to its aromatic structure, therefore it is not light-stable and gives rise to

yellowing appearance particularly after exposure to sun light for a period of time.1,3

III. 1,5-Naphthalenediisocyanate (NDI)

NDI (1,5-naphthalenediisocyanate) is a very bulky and symmetrical molecule

with two aromatic isocyanate groups of equal reactivity, and similar to that in the 4-

position in MDI. It is normally supplied in flake form and requires melting at 130oC

or dissolving in the solvent for processing. It is not light-stable due to its aromatic

feature thus gives rise to poor durability or weatherability (yellowing appearance)

after exposure to sun light.1,3

22


IV. Hexamethylene diisocyanate (HDI),

HDI is a flexible, linear, symmetrical molecule with two primary aliphatic

isocyanate groups of equal reactivity. Their reactivity is at least two orders of

magnitude lower than that in the 4-position of MDI. Of all the commercially available

polyisocyanates, it has the highest isocyanate content. It is because it is totally

aliphatic; it gives rise to light-stable (non-yellowing) polyurethanes.3,10-12

V. Isophorone diisocyanate (IPDI)

IPDI is a bulky and a very asymmetric molecule. In fact, of all the

commercially available polyisocyanates, it is the only one with no degree of

symmetry. It is totally aliphatic, therefore giving rise to light-stable (non-yellowing)

polyurethanes. It is commercially available as a mixture of two isomeric forms (25/75

cis/trans). Because of this, it has effectively four different isocyanate groups. Two are

secondary aliphatic groups with similar reactivity, about half that in HDI. The other

two are primary groups, but both are sterically hindered, rendering them even slower,

by a factor of about five than MDI. Thus, IPDI has the slowest reactivity of all the

commercially available polyisocyanates.1,3

VI. 4,4’-Diisocyanatodicyclohexylmethane (H12MDI)

H12MDI is commercially available as a 90/10 blend of the 4,4’/2,4’isomers.

The predominant 4,4’-diisocyanatodicyclohexylmethane consists of three

conformational isomers, cis-cis, cis-trans and trans-trans. The two different

isocyanate groups, either of which can be axial or equatorial, are secondary and are of

similar reactivity to the secondary isocyanate groups in IPDI. The 10 percent of 2,4’-

diisocyanatodicyclohexylmethane, derived from the 2,4 isomer in the MDA, is made

up of four conformational isomers, cis-cis, cis-trans, trans-cis and trans-trans.

Because it is totally aliphatic it gives rise to light-stable polyurethanes.2,3

23


VII. Other Diisocyanates

All other diisocyanates are only commercially available in limited or

developmental quantities, so only have niche and specialized applications.

Figure 2.10 shows the characteristics of reaction rate K1 and K2 for different

types of isocyanates used in the aqueous polyurethane dispersion.

Figure 2.10. The reaction rate for different types isocyanates

24


2.7.1.3 Chemistry of Isocyanates In polyurethane chemistry the major focus is on the reactions of isocyanates

with compounds that contain active hydrogen groups such as hydroxyl, water, amines,

urea and urethane, but also other reactions of isocyanates needs to be considered.

I. Isocyanate Reactions With Hydroxyl Groups The most important reaction in the manufacture of polyurethanes is between

isocyanate and hydroxyl groups, Figure 2.2. The reaction product is a carbamate,

which is called a urethane in the case of high molecular weight polymers. The

reaction is exothermic and reversible going back to the isocyanate and alcohol.1,2

Aliphatic primary alcohols are the most reactive and react much faster than

secondary and tertiary alcohols due to steric reasons, but urethanes made from tertiary

alcohols do not regenerate free isocyanate instead dissociating to yield the

corresponding amine, alkene and carbon dioxide. The urethane back reaction starts at

250oC for aliphatic isocyanates, but is closer to 200oC for aromatic isocyanates.

The reaction between isocyanates and alcohols is accelerated by the addition

of catalysts such as acids, bases (most aliphatic tertiary amines) and metal complexes

(organo tin compounds). Catalysts also promote the dissociation of urethanes and so

the deblocking of blocked isocyanates can occur at lower temperature.

II. Isocyanate Reaction With Water

The reaction of isocyanates with water to produce an amine and carbon

dioxide is highly exothermic. The initial reaction product is a carbamaic acid, which

breaks down into carbon dioxide and a primary amine (Figure 2.11).

The amine will then react immediately with another isocyanate to form

symmetric urea. Due to the formation of carbon dioxide the water reaction is often

25


used as a blowing agent as the level of blow can be tailored, simply by adjusting the

amount of water in the formation.

Figure 2.11. Water Reaction

Diisocyanates having isocyanate groups of similar reactivity such as MDI,

tend to chain extend to give crystalline polymeric urea. On the other hand, 2,4-TDI

has an isocyanate group in the 2-position far less reactive than the one in the 4-

position. Consequently, urea will be formed rapidly between TDI molecules in the 4-

position leaving the 2-position unaffected, below 50oC.

Despite the highly exothermic nature, the reaction with water is generally slow

in the absence of catalyst, and one of the main reasons is that isocyanates such as

MDI and TDI are not very soluble in water.

III. Isocaynate Reaction With Amines

Isocyanates react with primary and secondary amines to produce di- and tri-

substituted urea respectively whilst tertiary amines form labile 1:1 adducts, but

generally do not react with isocyanates (Figure 2.12).

26


Figure 2.12. Urea formation

These conversions are exothermic and diamines are used as chain extending

and curing agents in polyurethane manufacture. The resulting polyureá segments

increase the potential for cross-linking.

The reaction of unhindered isocyanates with primary amines at room

temperature and in the absence of catalyst is 100 to 1000 times faster than the reaction

with primary alcohols. The reactivity of an amine increases with its basicity and

consequently, aliphatic amines are much more reactive than aromatic amines. The

reactivity of amines can be slowed down by the presence of electron withdrawing

groups. Another way is to increase the steric hindrance by branching on the carbon

next to the nitrogen or introducing substituents in the ortho position of an aromatic

amine.2,3

The kinetics of the reaction of amines with isocyanates is complicated by

strong product catalysis. Since the product urea is a much weaker base and more

hindered than the amine, its catalysis is bi-functional and based on hydrogen bonds

between urea and both amine and isocyanate.

27


IV. Isocyanate Reaction With Urea Biurets are formed from the exothermic reaction of an isocyanate with a urea.

With di-substituted urea, a biuret is formed through the active hydrogen (Figure 2.13).

Figure 2.13. Biuret formation and equilibria

This reaction is significantly faster than the allophonate reaction and occurs at

lower temperature, about 100 oC compared to 120 oC to 140 oC. In polyurethane

systems this reaction, that is reversible upon heating, is often a source for additional

cross-linking.

Another important feature of this urea-biuret equilibrium is the potential for

redistribution of the biuret across the spectrum of isocyanate species. For instance, if

polymeric MDI and a diisocyanate prepolymer are mixed together, the molecules of

the di, tri, tetra and higher species are not initially smoothly distributed across the

spectrum of derivatives – biuret, allophonate, uretonimine. However, they slowly re-

distribute through the various reversible reactions. This redistribution will be faster at

higher temperatures, resulting ultimately in a product stable in composition and

viscosity.3,10-11

V. Isocyanate Reaction With Urethanes

An allophonate group is the result of an exothermic reaction of isocyanate

with the active hydrogen on a urethane group (Figure 2.14).

28


Figure 2.14. Allophonate formation

This reaction is slow compared to biuret formation and usually takes place

uncatalysed at about 120 oC to 140 oC. The reaction is reversible at temperatures

above 150 oC and so, as with biurets, the reaction increases cross-linking in

polyurethane systems. This reverse reaction takes place at lower temperature than

with biurets so that the interchange of isocyanate homologues is faster. If the

allophanates are heated with a third equivalent of isocyanate, the cyclic

triisocyanurate or trimers can be obtained.3,6

VI. Other Reactions Of Isocyanates

There are many other reactions of isocyanates that can influence the

polyurethane process and a few special cases are illustrated in Figure 2.15.

29


Figure 2.15. Other isocyanates reactions

2.7.2 Polyols Resins The term ‘polyol’ describes compounds with hydroxyl groups that react with

isocyanates to produce polyurethane polymers. Typically ‘polyols’ contain two to

eight reactive hydroxyl groups and have average molecular weights from 200 to 8000.

The two key classes of product are polyethers and polyesters.

30


The initial polyols used were predominantly polyesters until, in the late 1950s,

it was realized that polyether polyols were particularly well suited for the manufacture

of flexible slabstock foam. They quickly became the dominant class of polyol, and

now account for 80 percent of total consumption. Total polyol use had grown from

1.75 million tonnes in 1985 to 4.5 million tonnes in 2000.

A major factor in the choice of polyol for a polyurethane application, apart

from its technical effect, is cost. A selected polyol must be competitive with other

polyols and also enables the final polyurethane product to be cost competitive with

other materials in the end application.

Figure 2.16. Polyol types used in polyurethane

31


2.7.2.1 Polyether Polyols About 90% of the polyols used in polyurethane manufacture are hydroxyl-

terminated polyethers. These are made by the addition of alkylene oxides, usually

propylene oxide, onto alcohols or amines which are usually called starters or

‘initiators’. The addition polymerization of propylene oxide occurs with either anionic

(basic) and cationic (acidic) catalysis. Polyether based on propylene oxide thus

contains predominantly secondary hydroxyl end-groups. Secondary hydroxyl end-

groups are several time less reactive with isocyanates than primary hydroxyl groups

and for some applications polyether based only on propylene oxide may have

inconveniently low reactivity. The primary hydroxyl content may be increased by a

separate reaction of the polyoxypropylene polyols with ethylene oxide to form a block

copolymer with an oxyethylene ‘tip’.

2.7.2.2 Polyester Polyols There are four main classes of polyester polyols:

• Linear or lightly branched aliphatic polyester polyols (mainly adipates) with

terminal hydroxyl groups.

• Low molecular weight aromatic polyesters for rigid foam applications.

• Polycaprolactones.

• Polycarbonate polyols.

(Aliphatic and aromatic polyester polyols will be discussed)

The worldwide demand for polyester polyols in the polyurethane industry is estimated

at around 850000 tonnes, growing at four to five percent a year and is broken down in

Figure 2.17.

Applications include the manufacture of flexible foam for textile lining, where

superior resistance to dry cleaning solvents, flame bonding performance, elongation

32


33

and tensile properties make polyester polyols the product of choice. The outstanding

abrasion resistance of polyester polyol-based polyurethanes has led to their extensive

use in surface coating and footwear applications, and the superior thermal and

oxidative stability of the aromatic polyesters is exploited in the manufacture of rigid

isocyanurate foams.3,4

Figure 2.17. Worldwide demand for polyester polyols by application

I. Linear Or Lightly Branched Aliphatic Polyester Polyols

Aliphatic polyester polyols are produced by direct esterification in a

condensation reaction. This is a reversible equilibrium reaction, with water being

removed during reaction to drive the process. As the reaction precedes

transesterification reactions also occur on the forming polymer backbone, giving rise

to a relatively wide molecular weight distribution in the final polyester polyol

Global consumption (2000): 850000 tonnes

Elastomers (footwear, cast

elastomers, TPU, fibres and

shock absorbers),

31%

Flexible slabstock foam

(textile laminates for apparel and automotive

applications), 12%

Rigid foams (aromatic polyester

polyols), 29%

Adhesives &sealants (flexible

packaging,footwear,

automotive),6%

Paints & coatings

(specialist applications

requiring high performance),

13%

Synthetic leather, 9%


(especially when compared to polyether polyols and polycaprolactones). Further,

when polyesters are made from two or more glycols, they will be incorporated into

the polymer chain in a statistical distribution irrespective of their sequence of

addition. Careful control of the ratio of the ingredients is needed to ensure the product

has the required hydroxyl, and not acid, end groups.

II. Aromatic Polyester Polyols The use of polyesters in rigid foams was traditionally very limited, with

polyether polyols being preferred. Following their introduction in the early 1980s, it

was discovered that aromatic polyester polyols offered significant advantages in

polyisocyanurate rigid foam (PIR) systems, where the highly cross-linked trimer

structure can compensate for low functionality of the polyester polyol. Based on

recycled or by-product streams, the aromatic polyester polyols are lower cost than

polyether polyols and give superior performance in fire tests. They quickly became

the polyols of choice in North America for the production of boardstock for building

insulation; in combination with polyether polyols in spray systems; and in other

applications, such as appliances and pour-in-place foam, as a diluent to cheapen the

formulation.

There are three types of aromatic polyester polyol used today:

1. Products derived from the process residues of dimethyl terephthalate (DMT)

production, commonly referred to as DMT. They are typically transesterified at 180oC

to 230oC with at least one mole of diethylene or dipropylene glycol per equivalent of

acid to produce a simple hydroxyl-ended, glycol-capped aromatic polyester polyol.

2. Products derived from the glycolysis of recycled poly(ethylene terephthalate)

(PET) bottles or magnetic tape with subsequent re-esterification with do-acids or

reaction with alkylene oxides.

34


3. Products derived by direct esterification of phthalic anhydride. The polyesters have

functionalities between two and three, typically closer to two, and hydroxyl values in

the range 200 to 330 mg KOH/g. Compatibilisers and surfactants are often added

during manufacture to reduce viscosity and to improve miscibility with blowing

agents, other polyols and isocyanates.

2.7.3 Other Additives

In addition to the basic materials needed to make polyurethanes, isocyanates

and polyols, a wide range of other chemicals can be added to modify and control both

the polyurethane chemical reaction as well as the properties of the final polymer.

2.7.3.1 Catalysts Catalysis plays a vital role in the preparation of urethane and ureathane-urea

polymers, because it not only affects the rates of the chemical reactions responsible

for chain propagation, extension, and cross-linking but also affects the ultimate

properties of the resulting polymers. Catalysts are employed whose functions are not

only to bring about faster rate of reaction but also to establish a proper balance

between the chain-propagation reaction (primarily the hydroxyl-isocyanate reaction)

and the foaming reaction.

Another important function of catalysts is to bring about completion of the

reactions resulting in an adequate ‘cure’ of the polymers.

The catalysts most commonly employed are tertiary amines and metal

catalysts, especially tin catalysts. Tertiary amines are catalysts for the isocyanate-

hydroxyl and the isocyanate-water reactions. The efficiency of tertiary amine catalysts

depend on upon their chemical structure. It generally increases as the basicity of the

amine increases and the steric shielding of the amino nitrogen decreases. Some of the

most commonly used tertiary amine catalysts are triethylenediamine, N-alkyl

35


morpholines, N,N,N’,N’-Tetramethylethylenediamine, N,N,N’,N’-Tetramethyl-1,3-

butanediamine, N,N’-substituted piperazines, and dialkylanolamines.

Organometallic catalysts are mainly seen as gelation catalysts although they

do affect the isocyanate-water blowing reaction. Organotins are the most widely used,

but organomercury and organolead catalysts are also used. The mercury catalysts are

very good for elastomers because they give a long working time with a rapid cure and

very good selectivity towards the gelation. The lead catalysts are often used in rigid

spray foams. However, both mercury and lead catalysts have unfavorable hazard

properties so alternatives are always being sought.3,4

Table 2.3. Tertiary amine catalysts and their application

Catalyst Formulae Characteristic and use

N,N-Dimethylethanolamine (DMEA)

(CH3)2NCH2CH2OH Inexpensive, used in flexible foams and in rigid foams. Acid scavenger for rigid-ester foams and fire retarded foams.

N,N-Dimethylcyclohexylamine (DMCHA)

C6H11N(CH3)2 Inexpensive, has a high odour, is used mainly in rigid foams.

Bis(N,N-Dimethylaminoethyl) ether (BDMAEE)

(CH3)2NCH2CH2O(CH3)CH2CH2N(CH3)2 Excellent blowing catalyst used in flexible, high resilience and cold moulded foams.

N,N,N’,N’,N”-Pentamethyldiethylenetriamine (PMDETA)

(CH3)2NCH2CH2N(CH3)CH2CH2N(CH3)2 Good blowing catalyst used in isocyanurate board stock and moulded rigid foams.

1,4-Diazabicyclo[2,2,2]octane (DABCO) (Also referred to as triethylenediamine (TEDA)

N(CH2CH2)3N Very good amine gelation catalyst. Used in all types of foams.

2-(2-Dimethylaminoethoxy)-ethanol (DMAEE)

(CH3)2NCH2CH2OCH2CH2OH Reactive catalyst used in low density packaging foams.

2-(2-Dimethylaminoethoxy)-ethyl methyl-amino)ethanol

(CH3)2NCH2CH2OCH2CH2N(CH3)CH2 CH2OH

Excellent reactive low odour blowing catalyst used in high resilience and flexible foams. Low vinyl staining.

1-(Bis(3-dimethylamino)-propyl)amino-2-propanol (Also referred to as N”-hydroxypropyl-N,N,N’,N’-tetramethyliminobispropyl-amine

(CH3)2N(CH2)3N(CH2CHOHCH3)(CH2)3N(CH3)2 Low odour reactive catalyst used in rigid and high resilience foams. Replaces DMCHA in spray and is low vinyl staining.

36


N,N’,N’- Tris(3-dimethylamino-propyl)hexahydrotriazine

(NRCH2)3 Where R= (CH2)3N(CH3)2

Isocyanurate catalyst that provides back end cure. Decreases demould time of appliance foams.

Dimorpholinodiethylether (DMDEE)

(O((CH2)2)2N)(CH2)2O(CH2)2(N(CH2)2)2O) Low odour catalyst used in one-component foams and sealants.

N,N-Dimethylbenzylamine C6H5CH2N(CH3)2 Characteristic smell used in polyester-based flexible foams, integral skin foams and for making prepolymers.

N,N,N’,N”,N”-Pentamethyldipropylene-triamine

(CH3)2N(CH2)3N(CH3)(CH2)3N(CH3)2 Strong ammoniacal odour used for polyether-based slabstock foams and in semi-rigid foam moulding.

N,N’-Diethylpiperazine CH3CH2N(CH2CH2)2NCH3CH2 Low odour balanced blow cure catalyst for flexible and semi-flexible systems.

Table 2.4. Organometallic catalysts and their application

Catalyst Characteristic and use Stannous octoate Slabstock polyether-based flexible foams, moulded

flexible foams. Dibutyltin dilaurate (DBTDL) Microcellular foams, elastomers, moulding system, RIM. Dibutyltin mercaptide Hydrolysis resistant catalyst for storage stable two-

component systems. Phenylmercuric propionate Delayed action catalyst for elastomers. Lead octoate Rigid spray foams. Potassium acetate/octoate (KA/KO) Isocyanurate foams. Quaternary ammonium formates (QAF) Isocyanurate foams. Ferric acetylacetonate Cast elastomers system especially those based on TDI.

2.7.3.2 Neutralizing Agents

The neutralizing component consists of one or more bases which serve for

neutralizing some or all of the carboxyl and/or sulfo groups. For example, tertiary

amines, such as N,N-Dimethylethanolamine, N-Methyldiethanolamine,

triethanolamine, N,N-Dimethylisopropanolamine, N-Methyldiisopropanolamine,

triisopropanolamine, N-Methylmorpholine, N-Ethylmorpholine, triethylamine or

ammonia, or alkali metal hydroxides, such as lithium hydroxide, sodium hydroxide,

potassium hydroxide or mixtures thereof, can be used as suitable bases. Tertiary

amines and in particular triethylamine are preferably used.

37


The neutralizing component is added in an amount such that the degree of

neutralization, based on the free carboxyl and /or sulfo groups of the polyurethane

prepolymer, is preferably 70 to 100 equivalent %, particularly preferably 80 to 90

equivalent %. During the neutralization, carboxylate and/or sulfonate groups are

formed from the carboxyl and/or sulfo groups and serve for anionic modification or

stabilization of the polyurethane dispersion.

2.7.3.3 Dimethylolpropionic Acid

Dimethyopropinonic acid is a main raw material for manufacturing water-

soluble polyurethane; presently, DMPA has been widely applied to the production of

emulsified coating agent for leather. Besides, it can be applied to the manufacturing of

polyester dope, photosynthetic substance, liquid crystal of new type, adhesive and

magnetic recording materials etc. Adding DMPA can improve the stability,

hydrophilic property, homogeneous property, and endurance property.

In a typical anionic polyurethane dispersion process, anionic groups

(carboxylic and sulfonic) are introduced along the length of the polymer chain by

using hydrophilic monomers or internal emulsifiers. DMPA improves the hydrophilic

property by serving as the potential ionic center with NMP as the co-solvent.

In polyurethane dispersions, particle size is governed by the hydrophilicity of

the polymer which in turn depends on the number of ionic centers present in it. Study

has shown that the particle size of dispersion decreases with increasing DMPA.

Therefore, increased amount of DMPA leads to more ionic centers in the PUD

backbone and thereby increasing hydrophilicity of the polymer and hence reductions

in particle size.4,12-14

38


2.7.3.4 Chain Extenders

This is a low molecular weight polyfunctional compounds, reactive with

isocyanates and are also known as curing agent. Chain extenders are difunctional

glycols, diamines or hydroxyl amines and are use in adhesives, flexible foams,

elastomers and RIM systems. The chain-extender reacts with an isocyanate to form a

polyurethane or polyurea segment in the polyurethane polymer. Through reactons

with excess isocyanate, allophonates and biuret can be formed, transforming the

chain-extender effectively into thermo-reversible cross-linker.

Simple diamines are, in general, too reactive for a high level of addition and

special amines have been developed such as aromatic amines with bulky substituents

ortho to the amino group. A widely use chain extender in RIM applications is

DETDA (diethyl toluene diamine). The steric factors are responsible for lowering the

reactivity of the amino groups as compared to TDA (toluene diamine). The reaction of

one amino group with the isocyanate introduces a urea substituent on the aromatic

ring, which lowers the reactivity of the second amino group. A more recent

development is themoreversible chain extension by hydrogen bonding through

polyols containing special end groups.

Typical chain-extending agents are as follows:

1. water

2. diethylene glycol

3. hydroquinone dihydroxyethyl ether

4. ethanolamine

5. bisphenol A bis(hydroxyethylether)

6. DETDA (diethyl toluene diamine)

39


In the chain extension step, it is most important to control the extremely fast

reaction between NCO groups and NH2 group accompanied by the viscosity rise.

Molecular weight of PUD increases by the formation of urea linkage with NCO-

terminated prepolymer and diamines through the chain extension step. Therefore, the

most important step to determine molecular weight of polyurethane dispersion is the

chain extension step, which is the reaction between residual NCO groups and amine

groups. Incidentally, chain extension is influenced by the amount of residual NCO

groups, particle size diameter and so on. Generally molecular weight affects the

mechanical properties of PUD.4,5

The efficiency of chain extension increased as total surface area of particles

increases. Increasing the NCO/OH ratio enriched residual NCO groups that react with

HDA. In chain extension urea linkage contribute to hard segments of polyurethane,

therefore the mechanical properties increased with the increase of the hard segment

and molecular weight.

2.8 Application Test

2.8.1 Strength And Adhesion

Mechanical properties are often the most significant in determining whether a

particular product can be used in a given application and there are a number of

methods for assessing the strength of adhesion to a substrate, depending on whether

the substrate is rigid or flexible. Common tests are illustrated in Figure 23-4. Lap

shear tests are used both to assess the adhesion to a substrate and the cohesive

strength of an adhesive, whilst peel and blister tests are used to measure the adhesion

to the substrate. For simple cases, the peel force is a direct measure of the test energy.

Test designed to apply a well-defined stress to the bond, and to minimize the

amount of energy that is absorbed by deformation either of the substrate or the

40


coating/adhesive, so that the strength measured is the strength of the bond formed.

The size and shapes of the bonds and joints are precisely controlled, so that stresses

are applied accurately and the results can be compared. For single lap joints, care

must be taken to ensure that the joint is correctly aligned and gripped with spacers in

the testing machines, so that the stress applied when the bond is pulled is pure shear

and does not twist the sample, Figure 2.15. Typical measurements of the strength of

the bond are normalized stress at failure, modulus and elongation.

More empirical methods are also used to test for adhesion, such as the cross-

hatch and mandrel bend test, which are, respectively, used for films on rigid and

flexible substrates. In the cross-hatch test, pressure-sensitive tape is applied and

removed over a series of cuts that have been made in film, simulating scratches or

damage to the coating. The cut may be linear, or cross- or straight-hatched. The tape

used is appropriate for the level of adhesion the coating will need in practice, and the

level of damage from repeated applications of the tape is assessed. In the mandrel

bend test, the coating on a flexible substrate is rolled around either a cylindrical or

conical mandrel of smaller and smaller size to produce tighter bends and the cracking

of the coating measured. As well as giving a measure of coating adhesion, this test

simulates what might happen to a coating when in use on a substrate that is bent.4,6

Figure 2.18. Adhesion/cohesive strength testing formats

41


Figure 2.19. Single lap joint testing 2.9 Introduction Of Shoe Making Shoes both protect feet as well as, when incompatible in size and shape,

present exciting factors in inflammatory conditions e.g. bunion. Despite the presence

of pain, people are reluctant to change their footwear styles. The main function of

modern footwear is to provide feet with protection from hard and rough surfaces, as

well as climate and environmental exposure. To the wearer the appearance of their

footgear is often more important than its function. Consumer resistance to change

style is common. Informed decisions of shoe styles are thought to occur when the

benefits of alternative shoe styles are carefully explained and footwear habits

discussed in a culturally sensitive manner. Figure 2.22 shows the different parts of a

shoe.6,15

42


Figure 2.20. Parts of a shoe 2.9.1 Methods Of Shoe Construction

There are many ways to attach the sole to the upper but commercially only a

few methods are preferred. Shoes were traditionally made by moulding leather to a

wooden last. Modern technology has introduced many new materials and mechanised

much of the manufacture. Remarkable as it may seem the manufacture of shoes

remains fairly labour intensive. No matter the type of construction the first stage in

construction is to attach the insole to the undersurface of the last. Two main

operations follow: Lasting describes the upper sections are shaped to the last and

insole. Followed by bottoming, where the sole is attached to the upper. The process of

bottoming will determine price, quality and performance of the shoe.

43


2.9.1.1 Method 1: Moccasin Construction3 Thought to be the oldest shoe construction, this consists of a single layer

section, which forms the insole, vamp and quarters. The piece is moulded upwards

from the under surface of the last. An apron is then stitched to the gathered edges of

the vamp and the sole is stitched to the base of the shoe. This method is used for

flexible fashion footwear. The imitation moccasin has a visual appearance of a

moccasin but does not have the wrap around construction of the genuine moccasin.

I. High oil content leathers

This direct stitching method allows us to use leathers that have a much higher oil

content than can be used normally. The oil keeps the leather nourished’ and supple,

and is much softer and more comfortable to wear.

II. Wax, rot proof thread

Water won’t weaken the stitching.

III. Upper fully Blake-Stitched on to Sole (or mid-sole depending on style)

Much more secure bond to the sole

Figure 2.21. Moccasin method

2.9.1.2 Method 2: Cement Construction3,4-11 Under this method the upper is stretched over the last and attached to the

inner sole. The leather is then 'roughed up' to allow the adhesive to grip, and cement

44


bonded to the sole using the best quality polyurethane cement. Only leathers with a

maximum of 12% oil/fat content can be used under this construction method.6

2.9.1.3 Method 3: Stitchdown Construction3,6-13 Here the upper is stretched over the last, folded or flanged out and glued to

the midsole. They are then stitched with a "lockstitch" machine and cement bonded to

the soles using a neoprene adhesive. A lockstitch has a top and bottom stitch which is

interstitched. This stitching will not unravel even if a stitch is removed. Stitchdown

construction shoes can use leathers of higher oil, fat content than cement construction

and therefore have a more suppler feel. When 12 cord rot-proof stitching thread is

used, the shoes will not rot like cheaper imitations. Meanwhile, Rivers shoes are

stitched using the lock-stitch method for greater security.1,6

Figure 2.22. Stitchdown / Veldschoen method

2.9.1.4 Method 4 : Moulded Method3,4-14

The lasted upper is placed in a mould and the sole formed around it by

injecting liquid synthetic soling material (PVC, urethane). Alternatively, the sole may

45


be vulcanized by converting uncured rubber into a stable compound by heat and

pressure. When the materials in the moulds cool the sole-upper bonding is complete.

These methods combine the upper permanently into the sole and such shoes cannot

therefore be repaired easily. Moulded methods can be used to make most types of

footwear.6-10

Figure 2.23. Moulded method for various types of footwear

2.9.1.5 Method 5 : Force Lasting Construction 3,6-16

Force lasting has evolved from sport shoes but is increasingly used in other

footwear. The Strobel-stitched method (or sew in sock) describes one of many force

lasting techniques. The upper is sewn directly to a sock by means of an overlooking

machine (Strobel stitcher). The upper is then pulled (force lasted) onto a last or

moulding foot. Unit soles with raised walls or moulded soles are attached to

completely cover the seam. This technique is sometimes known as the Californian

process or slip lasting.3,6

46


Figure 2.24. Slip lasting / strobel stitched method

2.9 References

1. George Woods, The ICI Polyurethanes Book, 2nd Edition, 1987, p.197.

2. J.W. Rothause, Advances in Urethane Sci. and Techology, 1987, 10, p.121.

3. Günter Oertel, Polyurethane Handbook, Hanser Publishers, 1985, p.31.

4. David Randell and Steve Lee, Polyurethane Book, 2nd Editors, John Wiley &

Sons, 2000, p.10-20.

5. B.K. Kim, Colloid Polymer Science, 1996, 274, p.599.

6. Paul F. Bruins, Polyurethane Technology, 1969, p. 1990.

7. Y. Chen and Y.L. Chen, J. Appl. Polym. Sci., 1992, 46, 435.

8. B.K. Kim and L.Y. Min, J. Appl. Polym. Sci., 1994, 54, 1809.

9. S. Ramesh and G. Radhakrishna, Polym. Sci., 1994, 1, 418.

10. K. Matsuda, H. Ohmura and T. Sakai, J. Appl. Polym. Sci., 1979, 23, 141.

11. H.A. Al-Salah and C.K. Frisch, J. Appl. Polym. Sci., 1987, 25, 2127.

12. D. Dieterich, Progr. Organic Coatings, 1981, 9, 281.

13. P.B. Jacobs and P.C. Yu, J. Coat. Tech., 1993, 65, 222.

14. J.W. Rosthauser and K.J. Nachtkamp, J. Coat. Fabrics, 1986, 16, 39.

15. C.K. Kim and H.M. Jeong, Colloid Polym. Sci., 1994, 53, 371.

16. H. Xiao and K.C. Frisch, Pure Appl. Chem., 1995, 32, 169.

47

Chapter 3 Experimental

Chapter 3

Experimental

3.1 Material

The linear polyester diol derived from caprolactone monomer, terminated by

primary hydroxyl groups (CAPA® 2205, white waxy solid, mean molecular weight

2000, hyroxyl value 56mg KOH/g, Solvay Caprolactones) was required to melt at

50°C before use. Other ingredients used for polymerisation including isophorone

diisocyanate (IPDI, Rhodia, France), 2,2-bis(hydroxymethyl) propionic acid (DMPA,

Aldrich), 1,6-hexanediamine (HDA, Aldrich), ethylenediamine (EDA, Aldrich), 2-

methylpentamethylenediamine (Dytec® A Amine), triethylamine (TEA, Merck), 1-

methyl-2-pyrrolidone (NMP, Merck) were used as received. Deionized (DI) water

was used throughout the experiment.

Reagents including hydrochloric Acid (1 mol/L), Di-n-butylamine (solution in

pure toluene about 1.25 mol/L) and Bromophenol Blue (1N), pH 3.0 – 4.6 (BPB)

were used for back titration to determine the residual NCO content. The catalyst di-n-

butyltin-di-laurate (DBTL, Air Product) was used in the experiment.

3.2 Preparation of Aqueous Polyurethane Dispersion

An aqueous polyurethane dispersion (PUD) was prepared by forming a NCO

prepolymer initially. Subsequently chain extension was performed in the aqueous

phase in the presence of a polyamine chain extenders.

The prepolymer was formed by reacting an active hydrogen containing

compound such as a linear polyester diol (CAPA® 2205) with isophorone diisocyanate

(IPDI) and 2,2-bis(hydroxymethyl) propionic acid (DMPA).

48


Three sequences of PUD were synthesized : in sequence 1, the DMPA content

was held constant, while the ratio of NCO:OH was varied from 1.2 to 3.4. In sequence

2, the ratio of NCO:OH was fixed constant and the DMPA content was varied from 4

to 7. This resulted as an increase of ionic group thus lead to the increment of hard

segment content. For sequence 3, the degree of neutralization was varied from 85 to

115 while the ratio of NCO:OH and the DMPA content were kept constant. The

details of formulations and evaluation are described in Chapter 4. The parameters

studied in this experiment also involved of the reaction of the chain extension as the

variation of residual NCO group, molecular weight and particle size of the

polyurethane during the chain extension step. Change of molecular weight and time-

dependent variation of residual NCO group were investigated by using GPC and FTIR

with different degree of chain extension and particle size of the aqueous polyurethane

dispersion (details of studies are described in Chapter 5).

Polyurethane prepolymer was synthesized in a 1-L four-neck round-bottom

glass reactor equipped with a mechanical stirrer, an electronic temperature controller,

a temperature probe, a reflux condenser and a nitrogen inlet (Figure 3.1).

Nitrogen gas

Temperature probe

Mechanical stirrer Reflux

condenser

Electronic temperature controller

Figure 3.1. Set up of apparatus for the synthesis of polyurethane

Four neck round-bottom glass reactor

prepolymer

49


Reaction was carried out in nitrogen atmosphere. Polyester diol (CAPA®

2205), IPDI and DMPA (pre-dissolved in NMP solvent) were charged into the reactor

and the mixture was stirred and heated to 80°C. The reaction proceed at 80°C until the

amount of residual NCO content reached at 20% above the theoretical value. The

amount of residual NCO (%) was checked at every hour interval using di-n-

butylamine back titration method.1,2

After the residual NCO (%) reached to end-point, the temperature was lowered

to approximately 60oC, and TEA was added whilst stirring. The prepolymer was

stirred continuously for another 10 minutes. The required amount of HDA to be added

was determined based on the formulae given below :

[% NCO/ MWt NCO X (MWt HDA/2] = required amount HDA(g)/100g prepolymer

where MWt is the molecular weight of NCO = 42 and HDA = 116.2 The required amount of DI water was poured into a metal container. The

required amount of the prepolymer was introduced slowly into the water with high

speed strring using Dispermat machine. The HDA and DI water were premixed and

then added slowly into the aqueous dispersion under stirring for about 20 minutes.

The finishing polyurethane dispersion (PUD) was then formed (Figure 3.2).

Figure 3.2. Preparation of aqueous polyurethane dispersion

50


3.3 Preparation of Two Component (2K) Water Borne Polyurethane

Footwear Adhesives

The water borne 2K footwear adhesives were formulated using the aqueous

polyurethane dispersion (PUD) which is successfully developed in this experiment,

combined with the water borne polyisocyanate crosslinker, Rhodocoat WAT-1 from

Rhodia, France. The key properties of the adhesive bonding strength (i.e. shear and

peel strength) had been evaluated using the Zwick universal shear/peel strength test

equipment.3-5 The performances of this newly developed PUD were also compared

against one commercially available PUD in the market currently. Details of the

studies are described in Chapter 6.

3.4 Gel Permeation Chromatography (GPC) Measurement

The molecular weight of the PUD was determined using a Water gel

permeation chromatograph. Polystyrene standards of known molecular weights were

used for the calibration curve for this instrument. Tetrahydrofuran (THF) was used as

an eluent. The elution was monitored using a Waters 410 differential refraction

detector which is connected to a microprocessor.

3.5 Isocyanate Functionality Determination

NCO functionality is determined by titration method. 20 ml of di-n-

butylamine solution (1.25 mol/L in pure toluene) and 2.5 g of prepolymer were added

into a 250-ml conical flask. The flask was stopped and agitated until complete

homogenization was obtained. The test solution was kept at ambient temperature for

at least 15 minutes prior titration. After that, 150 ml of acetone was added to the test

solution. Hydrochloric acid solution (1 mol/L) was used as a titratant. A few drops of

51


bromophenol blue solution (1 g/L in the acetone) was used as an indicator. Titration

was continued until the violet or yellow colouration was stable for 15 second.

In a ponderal percentage of the isocyanate functions (N=C=O) by means of the

following formula:

N=C=O % = [42.02 (Vo-V1) / 1000m] x 100 In isocyanate equivalent corresponding with the number of functions for 100g by

means of the following formula:

[(Vo-V1) x 100] / 1000m Where Vo : volume (ml) of hydrochloric acid solution used for the blank test. V1 :

volume (ml) of hydrochloric acid solution used for the sample and m : mass of the

test sample, expressed in g.

The NCO/OH ratio is determined by the following formula :

(Weight of isocyanate x NCO % x 17) / (42 x weight of polyol x OH % x solid of

polyol).

3.6 Particle Size Analysis

The particle size of the aqueous polyurethane dispersion (PUD) was

determined by the Mastersizer, Malvern MAF 5001 Mastersizer Micro Plus, based on

the principle of laser ensemble light scattering. It falls into the category of non

imaging optical systems due to the fact that sizing is accomplished without forming

an image of the particle onto a detector. Laser light scattering is an exceptionally

flexible sizing technique able, in principle to measure the size structure of the test

material. The resolution goes up to 100 size bands displayed covering a range up to

18000:1 in size capability on any single range. The test PUD samples were introduced

into the sample dispersion unit which contained of DI water. The test samples were

52


dispersed under ultrasonic condition. The particle size of the tested PUD was then

analysed based on laser light scattering.

3.7 FT-IR Analysis

FTIR spectra were recorded on a Shimadzu FTIR-8400S spectrophotometer

with resolution of 2 cm-1. FTIR-8400S uses a high sensitivity pyroelectric detector

with a DLATGS (L-alanine-dooped deuterated triglycine sulfate) element. The

detector relies upon the temperature dependent “pyroelectric effect” created on the

crystal surface by spontaneous ferroelectric polarization. As the DLATGS Curie

temperature is as low as 61°C, temperature control is required. The prepolymers

samples for FTIR analysis were prepared by casting onto KBr disk.

3.8 Shear and Peel Strength Measurement

3.8.1 Shear Strength Measurement

The test pieces were cut by a sharp cutting knife to 80± 2 mm long and (20±

0.2) mm wide with an overlap of (10± 0.2) mm, as shown in Figure 3.3.

Figure 3.3. Form and dimensions of test pieces for shear tests

The test pieces were sanded by sandpaper. After sanding, the surface of the

test pieces were cleaned with a cloth, cotton wool and a suitable solvent like acetone

53


or ethyl acetate, 1,1,1-trichlorethane or light petroleum with boiling range 80 oC to

110 oC. The test pieces were dried for about (30± 5) minutes at (23 ± 5 oC) at a

relative humidity of less than 70% in order to allow the solvent to evaporate off.

The two-component water borne polyurethance adhesive was then prepared

and applied onto the test pieces by brush. The adhesive was then dried for 10 to 15

minutes at a controlled temperature of (23 ± 5 oC) before in contact bonding between

the application of the adhesive and the assembling of the bond. The test pieces were

pressed evenly by a weight load roller for about 15 seconds. The test pieces were

further dried at the ambient temperature (23 ± 5oC, relative humidity 55%) for 4 days.

For shear strength test, the test piece was clamped in the jaws of the Zwick

universal shear/peel strength test equipment to obtain a free test length of (110 ± 2 )

mm. The test piece was then loaded at a constant rate of traverse of (25 ± 2 ) mm/min

until breakage. The maximum force in Newton (N) during this process was recorded.

The shear strength was calculated based on the following formula.

Shear strength = Maximum value of the force (N) during separation Area of overlap in mm squared (mm2)

3.8.2 Peel Strength Measurement

The test pieces were cut by a sharp cutting knife to (100± 2) mm long and

(25.4± 0.5) mm wide with an overlap of (60± 2) mm (Figure 3.4).

Figure 3.4. Form and dimensions of test pieces for peel strength test

54


The test pieces were sanded by sandpaper. After sanding, the surfaces of the

test pieces were cleaned with a cloth, cotton wool and a suitable solvent (e.g. acetone)

or cleaning agent. The test pieces were dried for about (10 ± 5) minutes at (60 ± 5 oC)

in the oven. The adhesive was then applied onto the test pieces (first coat) by brush to

obtain a uniform coating of the adhesive under test. This form and dimensions of the

test piece for peel test is illustrated in Figure 3.4.

The adhesive was dried for 4 minutes at 55 oC. The adhesive was then applied

onto the test pieces again (second coat) and dried for 5 minutes at 55 oC. After that,

the test pieces were cooled for 4-6 minutes before the two test pieces were pressed

together with pressure. The test pieces were left in contact for 5 minutes prior

subjected to peel strength test. The experiment was repeated with different contact

time of 15, 30 and 60 minutes respectively (Figure 3.5).

Surface First Coat Second Coat Treatment

OVENPressure Bond

55oC x 10 mins

A A

55oC x 4 mins

OVEN OVEN

The peel strength wa

strength test equipment. The

is measured in term of N/mm.

Apply dhesive

Figure 3.5. Process

s determined by usi

test speed of pulling i

Apply dhesive

for app

ng a

s (500

55oC x 5 mins

lying the adhe

Zwick univer

±10) m/min.

Cool for 4 mins

sive

sal shear/peel

Peel strength

55


The initial bonding strength was determined by taking the average peel

strength after the test pieces was in contact for 5, 15, 30 and 60 minutes respectively.

The 24 hours bonding strength was determined by taking the peel strength after the

test pieces was in contact for 24 hours. Peel strength is the mean peel force per unit

width, in N/mm, calculated from the trace over the course of separation as follows:

Peel Strength = Mean peel force in newton (N) during separation / Width of test piece in mm.

3.9 References :

1. C. Hepburn, Polyurethane Elastomers, 2nd Edition, Elsevier, New York, 1992,

p.281.

2. George Woods, The ICI Polyurethanes Book, 2nd Edition, ICI Polyurethanes, 1987,

p.197.

3. Paul F. Bruins, Polyurethane Technology, 1969, p1990.

4. David Randell and Steve Lee, Polyurethane Book, 2nd Editors, John Wiley & Sons,

2000, p.10-20.

5. J. W. Rothause, Advances in Urethane Sci. and Technology, 1987, 10, p.121.

56

Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion

Chapter 4

Effect of Process Variables on Molecular Weight and

Adhesive Bonding Strength of Aqueous Polyurethane Dispersion

4.1 Introduction

An aqueous polyurethane dispersion (PUD) is a binary colloidal system where

polyurethane particles are dispersed in a continuous aqueous medium. Conventional

polyurethane is insoluble in water and phase separates in large domains. To be

dispersible in water, polyurethane should contain ionic and/or non-ionic hydrophilic

segments in its structure. Particle size is governed by internal and external factors.

Among them, the most important factor is the hydrophilicity of polyurethane.

In the application of adhesives market, aqueous polyurethane has been

developed and studied1,2 in view of its unique properties and the environmental

regulations prohibiting VOC.3,4 The earliest process to prepare the polyurethane

dispersion was the acetone process, which has remained technically important so

far.5,6 Within the last three decades several new processes have been developed.

However, these processes have a common feature that is the preparation of NCO-

terminated polyurethane prepolymer with appropriate molecular weight.7,14 Distinctly

different step among several processes lies in the chain extension step that is generally

performed using diamines (-NH2) and/or diols (-OH).8-10 In the chain extension step, it

is most important to control the extremely fast reaction between NCO groups and

NH2 groups accompanied by the viscosity rise.9,14 The prepolymer mixing process that

we have used in this study has the advantage of avoiding the use of a large amount of

organic solvent. In this process, NCO-terminated polyurethane prepolymer containing

pendant acid group, such as dimethylol propionic acid (DMPA) is neutralized with

57

Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion base to form internal ionic emulsifier and dispersed in the aqueous phase to form an

aqueous dispersion. Afterwards, the chain extension step is accomplished by the

addition of diamine to the aqueous dispersion. Molecular weight of polyurethane

dispersion increases by the formation of urea linkage with NCO-terminate prepolymer

and diamines through the chain extension step. Therefore, the most important step to

determine the molecular weight of polyurethane dispersion is the chain extension step,

which is the reaction between residual NCO groups and amine groups. Incidentally,

the chain extension is influenced by the amount of residual NCO groups, particle

diameter, and so on.10,15-17 The amount of residual NCO groups is determined by the

molar ratio of NCO to OH (NCO/OH). In addition, both hydrophilic acid group

contents and their degree of neutralization can affect particle diameter.10,11

Accordingly, the molecular weight can be controlled with varying these process

variables. Generally, the molecular weights of polymeric materials have a remarkable

effect on their mechanical properties. Therefore, the control of molecular weight can

be expected to obtain the optimum mechanical properties of polyurethane dispersion.

In this experiment, several aqueous polyurethane dispersions were prepared by

varying the NCO/OH ratio, the DMPA content and the degree of neutralization. Then

their molecular weights and mechanical properties such as adhesive bonding strength

(shear and peel strength) were evaluated to study the effect of process variables and

the relationship between molecular weight and mechanical properties.

4.2 Experiment

The characteristics of all the raw materials used in this experiment such as the

linear polyester diol derived from caprolactone monomer terminated by primary

hydroxyl groups (CAPA® 2205, Solvay Caprolactones), IPDI isophorone diisocyanate

58

Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion (2,2-bis(hydroxymethyl), Rhodia France) for polymerisation, dimethylol propionic

acid (DMPA, Aldrich), 1,6-hexanediamine (HDA, Aldrich), triethylamine (TEA,

Merck), 1-methyl-2-pyrrolidone (NMP, Merck) are shown in Chapter 3, section 3.1.

Three sequences of PUD were prepared. In sequence 1, the DMPA content

was fixed, while the NCO/OH ratio was varied from 2.8 to 3.4. In sequence 2, the

NCO/OH ratio was held constant, and the DMPA content was varied. In sequence 3,

the NCO/OH ratio, the DMPA content and the total solid were held constant, and the

degree of neutralization was controlled by varing the TEA from 85% to 115% based

on DMPA content. These formulations are shown in Tables 4.1, 4.2 and 4.3.

Polyurethane prepolymer was polymerised in a 1-L round-bottom glass reactor

equipped with a mechnical stirrer, a thermometer, a reflux condenser, a temperature

controller and a nitrogen inlet. Reaction was conducted under the nitrogen

atmosphere. The linear polyester diol (CAPA® 2205) and DMPA were pre-dissolved

in NMP in the reactor flask. The mixture was heated and stirred at 80°C; IPDI were

then added to the mixture. The amount of residual NCO(%) was checked at one-hour

interval using di-n-butylamine back titration method. The reaction was allowed to

proceed until the residual NCO(%) became 20-30% above the theoretical residual

NCO(%). After the required residual NCO(%) was reached, the temperature was

lowered to approximately 60°C and TEA was then added whilst stirring to neutralize

the carboxylic acid in the DMPA. The reaction mixture was stirred continuously for

another 10 minutes. The aqueous PUD was then formed by phase inversion process.

The required reaction mixture or prepolymer was poured slowly into a metal container

which contained the required amount of DI water. The dispersion was then obtained

under high speed stirring using a Dispermat stirrer. For the chain extension, the

required amount HDA (calculated by the formulae [%NCO/MW NCO X MW

59

Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion HDA/2] = Required amount of HDA (g)/100g prepolymer) was premixed with DI

water and then added slowly into the prepared dispersion phase. The preparation of

aqueous polyurethane dispersion is illustrated in Figure 4.1.

+ HO--------------------OH

Figure 4.1. Preparation of an aqueous polyurethane dispersion

OCNNCO

POLYOL

IPDI

OH O

OO----OOCN NCONN

N NO ------- O H

OH H

CH3OH DMPAHO

COOH

OCNN

H

O

O

(

E

--

t

-

3

O

T

O

r

N

i

N

e

t

H

h

y

O

l

H

a

O

m

O

i

C

n

O

e

O

O

H

C

O

N

N

N

N

H

H

O

O

O

H

--

O-

-O

-

O

O

O

N

N

N

H

H

N

C

O

O

O

C

O

O

O

-

O

O

t

-

e

N

r

H

m

+

i

(

n

E

a

t

t

)

e

3C

d

O

N

p

N

r

e

H

p

o

O

l

y

H

m

O

e

-

r

-

O

O

N

H

N

N

C

)

N

H

W

at +

erH

H NN 1,6-HexanediamineHH

Dispersion and Chain Extension Step

60

Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion Table 4.1. Formulation of aqueous polyurethane dispersion (Sequence 1) – DMPA

content is constant and NCO/OH ratio varies from 2.8 to 3.4.

Raw Materials Weight (grams)

Polyester diol (CAPA 2205) 157.58

IPDI Variablea

DMPA 7.88

NMP 15.76

DBTL Catalyst (0.08% of prepolymer) 0.19

TEA 5.94

HDA (Theoretical) per 100g prepolymer Variablee

DI Water Variableb

Variablea = NCO/OH ratio is from 2.8, 3.0, 3.2 or 3.4 Variableb = Total solid content maintained at 50% Variablee = Based on the residual NCO content DMPA content fixed at 5% wt of polyester diol Neutralization fixed at 100%


ratio is constant and DMPA varies from 4 to 7



IPDI 52.95

DMPA Variablec

NMP 15.76


TEA 5.94


DI Water Variableb

Variablec = DMPA content is from 4, 5, 6 or 7 Variableb = Total solid content maintained at 50% Variablee = Based on the residual NCO content NCO/OH ratio fixed at 3.0 Neutralization fixed at 100%

61

Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion Table 4.3. Formulation of aqueous polyurethane dispersion (Sequence 3) –

NCO/OH ratio and DMPA content are constant but the degree of neutralization varies

from 85% to 115%.



IPDI 52.95

DMPA 7.88

NMP 15.76


TEA Variabled


DI Water Variableb

Variabled = Degree of neutralization is from 85%, 95%, 100%, 105% or 115%. Variableb = Total solid content maintained at 50% Variablee = Based on the residual NCO content DMPA content fixed at 5% wt of polyester diol

4.3 Results and Discussion

4.3.1 The Effect of NCO/OH Ratio

The weight-average molecular weight (Mw) and number-average molecular

weight (Mn) of the PUD with different NCO/OH ratio are given in Figure 4.2. The

Mw value of PUD varied from 256,350 to 475,400 with increasing the NCO/OH from

2.8 to 3.4. However, the Mn value increased gradually (with small degree of changes).

The polydisperisty index could be influenced by both Mw and Mn, as the NCO/OH

ratio increases, the polydisperisty index (Mw/Mn) becomes larger. The results showed

that the higher the NCO/OH ratio, the higher the molecular weight of PUD. In this

case, increasing the NCO/OH enriched residual NCO groups that react with HDA. In

addition, the chain extension produced urea linkages that contribute to hard segments

62

Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion of polyurethane. Therefore, the adhesive bonding strength increased with increasing

molecular weight of the hard segment.10-14

050000

100000150000200000250000300000350000400000450000500000

2.8 3 3.2 3.4

NCO/OH ratio

Molec

ular

Weig

ht (g

/mol

)

MwMn

g/m

ol

Figure 4.2. Mw and Mn of PUD as a function of the NCO/OH ratio

The change of Mw during the chain extension step is shown in Figure 4.3. The

final Mw was determined by the amount of the residual NCO groups that could react

with the HDA chain extender. These residual NCO groups increased with higher

NCO/OH ratio. However, insignificant change of the molecular weight was observed

at lower NCO/OH ratio i.e. at 2.8. This could be due to the side reaction where the

residual NCO group reacted with the water molecules instead of with the hydroxyl

functionality from the polyol. At higher NCO/OH ratio such as 3.0 and 3.2, the

molecular weight reached the optimum when about 40-50% of chain extender has

been added. This implies that the efficiency of chain extension was about 40-50% in

these formulations. This result indicates that the residual NCO groups on the

polyurethane particles did not completely react with the chain extender. The

prepolymer chains were extended to the particle surface due to the high viscosity of

63

Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion the particle at low temperature during the chain extension step. So the chain extender

required a longer time to diffuse to the particles. Hence, the efficiency of chain

extension increased as the total surface area of the particle increased.15-18

NCO/OH 3.2NCO/OH 3.0NCO/OH 2.8

Mw

(g/m

ol)

Degree of Chain Extension (%) 0 20 40 60 80 100

400000

350000

300000

250000

200000

150000

100000

50000

0

Figure 4.3. Change of Mw with different NCO/OH ratio during the chain extension reaction Figure 4.4 shows the effect of NCO/OH ratio on the adhesive bonding strength. The

shear and peel strength increased as the NCO/OH ratio increased.

64


0

5

10

15

20

25

30

35

40

2.8 3 3.2 3.4

NCO/OH ratio

Shea

r St

reng

th, N

/mm

2

0

1

2

3

4

5

6

7

8

Peel

Str

engt

h, N

/mm

Shear Strength Peel Strength

Figure 4.4. Effect of NCO/OH ratio on the adhesive bonding strength

4.3.2 The Effect of DMPA Content

The variations of Mw with DMPA content are shown in Figure 4.5. The Mw

decreased from 276,000 to 80,000 g/mol with increasing DMPA content. The

concentration of DMPA increased from 5 to 8 weight % based on total polyester

polyol used. The molecular weight of the linear polyester polyol (terminated by

primary hydroxyl group) CAPA 2205 is 2000 but that of DMPA is 134.15. If there is

no significant difference in the reactivity between the polyester polyol and DMPA,

the prepolymer chain should be shorter as the DMPA content increases at a constant

NCO/OH ratio. The average particle size as a function of DMPA content is also

shown in the Figure 4.5. In PUD, the average particle size could be controlled to some

extent by emulsification conditions such as stirring speed or dispersing temperature

which have an effect on the viscosity of prepolymer but it is mostly governed by the

65

Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion concentration of hydrophilic groups, i.e. carboxylic acids.11,21-22 The average particle

size decreased as the DMPA content increased. The decreases of the particle size with

increasing of DMPA content could be due to the stabilizing mechanism of the

ionomer. Dispersion polyurethane ionomer is stabilized as electrical double layers

formed by the ionic groups.3,19-21

0

50000

100000

150000

200000

250000

300000

5 6 7 8

DMPA Content

Mw

(g/m

ol)

0

0.5

1

1.5

2

2.5

3

Part

icle

Siz

e (µ

m)

MwParticle Size

Figure 4.5. Mw and particle size of PUD as function DMPA content

Figure 4.6 shows the impact of the molecular weight to adhesive bonding

strength. The hard segments increases as DMPA content increases at a fixed NCO/OH

ratio because DMPA molecules formed hard segments on polyurethane main chain.

Consequently, it was anticipated that the increase of hard segment content contributed

to the improvement of adhesive bonding strength such as shear and peel strength.

Contrary, the bonding strength at 5% DMPA content showed the best bonding

strength as compared to the other DMPA contents. This is due to the increase in

66

Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion molecular weight as DMPA content decreases at a constant NCO/OH ratio. This

result indicates that the bonding strength would be more influenced by the Mw than

the ratio of soft segment to hard segment.4,17-22

0

2

4

6

8

10

12

14

16

18

20

80020 100200 177340 276330Mw (g/mol)

N/m

m2

0

1

2

3

4

5

6

N/m

m

Shear Strength Peel Strength

Figure 4.6. Effect of Mw on bonding strength of the aqueous polyurethane dispersion with different DMPA content 4.3.3 The Effect of TEA/DMPA Molar Ratio

The effect of neutralization degree of carboxylic acid in DMPA on the average

particle sizes and molecular weight of the aqueous PUD is shown in Figure 4.7. The

average particle size decreased as the mole ratio of TEA to DMPA (TEA/DMPA)

increased from 0.85 to 1.00. The decrease of the particle size was due to the variation

of the number of carboxylic acid group that could be neutralized by TEA. In the

aqueous PUD, usually the greater the hydrophilicity the smaller the particle size

because the degree of dissociation depends on the degree of neutralization. When the

67

Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion degree of neutralization was over 100%, the particle size increased as the degree of

neturalization increased. Excess amount of the TEA increased the ionic strength of a

continuous phase. The electrostatic replusion can be decreased due to the contraction

of electrical double layers among the polyurethane particles as the ionic strength

increased.6,18-22 This is demonstrated in Figure 4.8.

6

5

Part

icle

Siz

e (µ

m)

4

3

2

1

085% 95% 100% 105% 115%

Degree of Neutralization

Figure 4.7. Effect of neutralization degree on the particle size of aqueous polyurethane dispersion (PUD).

Figure 4.8. Particle size controlled by TEA/DMPA ratio

68

Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion Figure 4.9 indicates the change of Mw of PUD as a function of degree of chain

extension, with varying degree of neutralization i.e. TEA/DMPA molar percentage

varied from 85% to 115% when the NCO/OH ratio fixed at 3.0. Highest Mw was

obtained when the neutralization reached 100%. This result demonstrated that the

efficiency of chain extension was closely related to the average particle size of PUD

and the diffusion-dominant reaction.

0

50000

100000

150000

200000

250000

300000

350000

0 20 40 80 100Degree of Neutralization (% )

Mw

(g/

mol

)

85%100%115%

Figure 4.9. Evoluation of Mw of PUD with varying the degree of neutralization from 85% to 115% during chain extension reaction

As shown in Figure 4.10, the Mw of PUD reached a maximum value when the degree

of the neutralization was 100%.

69


MwMn

g/m

ol

Degree of Neutralization

115%105%100%95%85%

300000

250000

200000

150000

100000

50000

0

Figure 4.10. Mw and Mn of PUD as a function of degree of neutralization

The effect of degree of neutralization on the adhesive bonding strength is

illustrated in Figure 4.11. The excess TEA remained in the polyurethane prepolymer

could react with the water which exist in the polyester polyol (even very small

quantity < 0.05% water) to form urera linkages in the prepolymer main chain. In

addition, TEA could cause NCO-terminated prepolymer to form polyfunctional

branched prepolymer chains. These branched chains may form cross-linking during

the chain extension by the reaction with HDA. As shown in Figure 4.9, the adhesive

bonding strength such as shear and peel strength increased as the degree of

neutralization increased. However, a maximum value was obtained when the

neutralization reached 100%. Above 100% neutralization, the adhesive bonding

strength decreased.17-22

70


0

5

10

15

20

25

30

85 95 100 105 115Degree of Neutralization (% )

Shea

r St

reng

th, N

/mm

2

0

1

2

3

4

5

6

7

Pee

l Str

engt

h, N

/mm

Shear StrengthPeel Strength

Figure 4.11. Effect of neutralization on adhesive bonding strength

4.4 Conclusions

The effect of NCO/OH ratio, DMPA content and degree of neutralization on

molecular weight and adhesive bonding strength for the prepared aqueous

polyurethane dispersion (PUD) have been studied.

As the NCO/OH ratio increased, the Mw increased. It was due to the chain

extension reaction which depended on the amount of residual NCO groups. The

adhesive bonding strength as shown by shear and peel strength increased

significantly, mainly due to the increases of molecule weight and hard segment

content. The molecular weight and adhesive bonding strength properties are not

seriously affected by the DMPA content, especially when the NCO/OH ratio is 2.8.

When the DMPA content decreased, polyol content increased, and consequently, the

71

Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion molecular weight of polyurethane prepolymer increased slightly. Hence, the adhesive

bonding strength, which was mainly affected by the molecular weight, was improved

slightly. The Mw increased as the neutralization degree reached 100% due to the

increase of chain extension efficiency. In addition, the mechanical properties such as

shear and peel strength reached a maximum value where the degree of neutralization

approached 100%.

4.5 References :

1. D. Dieterich, Prog. Org. Coat, 1981, 9, 281.

2. P.B. Jacobs, P.C. Yu, J. Coat. Tech. 1993, 65, 222.

3. S.H. Son, H.J. Lee, J.H. Kim, Colloids Surfaces A: Physicochem. Eng. Aspects,

1998, 133, 295.

4. D.S . Chen, M. Hsien, US Patent 5, 1994, 306,764.

5. C. Hepburn, Polyurethane Elastomers, second ed., Elsevier, New York, 1992,

p.281.

6. G. Oertel, Polyurethane Handbook, Carl Hanser, Munich, 1985, p31.

7. P. Thomas, Water Based and Solvent Based Surface Coating Resins and their

Applications – Polyurethanes, vol. III, SITA Technology, London, 1999, p59.

8. H. Xiao, H.X. Xiao, K.C. Frisch, N. Malwitz, J. Appl. Polym. Sci., 1994, 54,

1643.

9. B.K. Kim, Colloid, Polymer Sci., 1996, 274, 599.

10. Y.K. Jhon, I.W. Cheong, J.H. Kim, Colloids Surfaces A Physicochem. Eng.

Aspects, 2001, 179 (1), 71-78.

11. S.Y. Lee, J.S. Lee, B.K. Kim, Polym. Int., 1997, 42, 67.

72

Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion 12. B.K. Kim, T.K. Kim, H.M. Jeong, J. Appl. Polym. Sci., 1994, 53, 371.

13. H.T. Lee, Y.T. Hwang, N.S. Chang, C.C.T. Huang, H.C. Li, Waterborne High

Solids and Powder Coatings Symposium, New Orleans, 22-24 February, 1995,

224.

14. H.R. Allcock and F.W. Lampe, Contemporary Polymer Chemistry, New York,

1990, p.43.

15. S. Kaizerman and R.R. Aloia, US Patent 4, 1985, 198, 330.

16. R. S. Buckanin, US Patent 4, 1985, 705, 840.

17. P.H. Markusch, J.W. Posthauser and M.C. Beatty, US Patent 4, 1985, 501,852.

18. J. Weikard and E. Luhmann, US Patent 6, 2003, 541, 536.

19. C. Irle and W. Kremer, US Patent 6, 2003, 559, 225.

20. T.D. Salatin and A.M. Budde, US Patent 5, 1993, 236, 995.

21. G.A. Anderle and S.L. Lenhard, US Patent 6, 2003, 576, 702.

22. H.P. Muller and H. Gruttmann, US Patent 6, 2001, 172, 126.

73

Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion

Chapter 5

Effect of Chain Extension on Adhesive Bonding Strength of

Aqueous Polyurethane Dispersion

5.1 Introduction

During the past several decades, aqueous polyurethane dispersion has been

investigated by many researchers. However, very little systematic work has been

conducted and reported in details on the chain extension process. The chain extension

step is important and critical to the molecular weight and particle size of the aqueous

polyurethane dispersion. As a result, the chain extension step has important impact of

the physical and mechanical i.e. bonding strength properties of polyurethane. In the

chain extension step, it is most important to control the fast reaction between the

residual NCO group and amine group (NH2) as it will easily increase the viscosity.

Generally, it has been reported that residual NCO groups are measured by di-

n-butylamine back titration method (DBBT method).1,2 This method, however, can be

used to determine the NCO content in a diisocyanate intermediate or the free reactive

isocyanate available in the prepolymer. In other words, this method is not applicable

after neutralizating agent is introduced because it is impossible to determine the

residual NCO groups due to the presence of various side reaction and other base

materials such as chain extender, neutralizing agent etc. Some earlier research work

has been tried to avoid the reaction between residual NCO with the water in the

preparation of aqueous polyurethane dispersion by using blocking agent and

controlling the process temperature. However, both approaches could not stop the

reaction completely. In view of this, one must determine the concentration of NCO

group and use appropriate amount of chain extender for stoichiometric reaction

74


between the two components. Excess chain extenders may cause poor chain extension

efficiency and subsequently the deterioration of mechanical properties of the

polyurethane dispersion. In addition, it is important to know where the locus of the

chain extension reaction, particle surface or inner particle for understanding the

reaction mechanism and controlling particle morphology.3,6-9

In this experiment, the aqueous polyurethane dispersion was prepared by

prepolymer mixing and neutralization emulsification method. The parameters studied

involved of the reaction of the chain extension as the variation of residual NCO

group, molecular weight and particle size of the polyurethane during the chain

extension step. Change of molecular weight and time-dependent variation of residual

NCO group were investigated by using GPC and FTIR with different degree of chain

extension, and particle size of the aqueous polyurethane dispersion was measured

with a Mastersizer analyzer.

5.2 Experiment

The characteristics of all the raw materials used in this experiment such as the

linear polyester diol derived from caprolactone monomer terminated by primary

hydroxyl groups (CAPA® 2205, Solvay Caprolactones), isophorone diisocyanate (

2,2-Bis(hydroxymethyl), IPDI, Rhodia France), propionic acid (DMPA, Aldrich),

1,6-hexanediamine (HDA, Aldrich), ethylenediamine (EDA, Aldrich), 2-

methylpentamethylenediamine (Dytek® A Amine), triethylamine (TEA, Merck), 1-

methyl-2-pyrrolidone (NMP, Merck) are shown in Chapter 3, section 3.1.

Polyurethane prepolymer was synthesized in a 1L four-neck round-bottom

glass reactor equipped with a mechanical stirrer, an electronic temperature controller,

a temperature probe, a reflux condenser and a nitrogen inlet (see Figure 3.1).

75


Polyester diol (CAPA® 2205) and DMPA (pre-dissolved in NMP solvent)

were charged into the reactor and the mixture was stirred and heated to 80°C under

nitrogen atmosphere. The crosslinker IPDI was then added. The reaction was allowed

to proceed at 80°C until the amount of residual NCO content reached 20-30% above

the theoretical residual NCO content. The end point was hence reached and

subsequently NCO-terminated prepolymer was obtained. The amount of residual

NCO (%) was checked at every hour interval using di-n-butylamine back titration

method.3,7-11 TEA was added to neutralize the COOH groups at 60°C and

polyurethane anionomers were obtained consequently. The polyurethane anionomer

was then dispersed in DI water and chain extension reaction proceeded with the

addition of 1,6 hexanediamine.

The particle sizes of polyurethane dispersion were analyzed by the Mastersizer

which is based on the principal of laser ensemble light scattering (Malvern MAF 5001

Mastersizer Micro Plus). The relative amount of NCO groups in the polyurethane was

measured by FT-IR spectroscopy (Shimadzu FTIR-8400S). The average molecular

weight was measured by a GPC (Waters 501) equipped with refractive index detector

(Water 410). Tetrahydrofurane (THF) was used as an eluent at 1.0 mL/min flow rate

and 1 X 103 Pa pressure. One column (Polymer Laboratories gel, 1000 °A) was used

for the analysis of low molecular weight products and two columns of Millipore

microstyreagel HR 3 and HR 4 were connected for the analysis of high molecular

weight polymers. Number- and weight-average molar weights were calibrated with

PMMA (polymethyl-methacrylate, mean Mw =3000, 11800, 95100 and 1456000) and

PS (polystryene, mean Mw = 35000, 490000 and 2780000) standards.

The polyurethane prepolymer was synthesized in a 1-L round-bottom glass

reactor equipped with a mechnical stirrer, a thermometer, a reflux condenser, a

76


temperature controller and a nitrogen inlet. Reaction was conducted under a nitrogen

atmosphere. The polyester diol (CAPA® 2205) and DMPA were pre-dissolved in

NMP in the reactor flask. The aliphatic isocyanate crosslinker, IPDI, was then added

to the mixture. The mixture was heated and stirred at 80 °C. The amount of residual

NCO(%) was then checked at every one hour interval using di-n-butylamine back

titration method. The reaction was allowed to proceed until the residual NCO(%)

reached the end point (20-30% above the theoretical residual NCO content). Then the

temperature was lowered to approximately 60 °C and TEA was then added whilst

stirring to neutralize the carboxylic acid in the DMPA. The reaction mixture was

stirred continuously for another 10 minutes. The aqueous PUD was then formed by

phase inversion process. The required reaction mixture or prepolymer was poured into

a metal container and cooled to ambient temperature. The dispersion was obtained by

introducing DI water slowly into the prepolymer under high speed stirring using a

Dispermat stirrer. For the chain extension, the required amount HDA (calculated by

the formulae [%NCO/MW NCO X MW HDA/2] = Required amount of HDA(g)/100g

prepolymer) was premixed with DI water and then added slowly into the prepared

dispersion phase.

The adhesive bonding strength (i.e. shear and peel strength) was evaluated by

using the Zwick universal shear/peel strength test equipment. The details of test

method was described in Chapter 3, section 3.8.

77


Table 5.1. Characteristics and formulations of aqueous polyurethane dispersion using

HDA as chain extender with different degree of neutralization.

Characteristics F1 F2 F3 F4

NCO/OH 3 3 3 3

DMPA 5 5 5 5

Total solid % 50 50 50 50

Neutralization % 85 95 100 115

Theoretical Residual NCO % 2.80 2.79 2.78 2.78


Weight (grams)

Weight (grams)

Weight (grams)

CAPA 2205 (Polyester Diol) 157.58 157.58 157.58 157.58

IPDI 52.95 52.95 52.95 52.95

DMPA 7.88 7.88 7.88 7.88

NMP 15.76 15.76 15.76 15.76

DBTL Catalyst - 0.08% of prepolymer 0.19 0.19 0.19 0.19

TEA 5.05 5.64 5.94 6.83

HDA Chain Extender (Theoretical) per 100g prepolymer 3.87 3.86 3.85 3.85

Total 243.28 243.86 244.15 245.04

78



EDA as chain extender with different degree of neutralization.


NCO/OH 3 3 3 3

DMPA 5 5 5 5





Weight (grams)

Weight (grams)

Weight (grams)


IPDI 52.95 52.95 52.95 52.95

DMPA 7.88 7.88 7.88 7.88

NMP 15.76 15.76 15.76 15.76


TEA 5.05 5.64 5.94 6.83

EDA Chain Extender (Theoretical) per 100g prepolymer 2.00 2.00 1.99 1.99

Total 241.41 242.00 242.29 243.18

79



Dytek® A Amine as chain extender with different degree of neutralization.


NCO/OH 3 3 3 3

DMPA 5 5 5 5




Raw Materisla Weight (grams)

Weight (grams)

Weight (grams)

Weight (grams)


IPDI 52.95 52.95 52.95 52.95

DMPA 7.88 7.88 7.88 7.88

NMP 15.76 15.76 15.76 15.76


TEA 5.05 5.64 5.94 6.83Dytek chain extender (Theoretical) per 100g prepolymer 3.87 3.86 3.85 3.85

Total 243.28 243.86 244.15 245.04

80



5.3.1 Effect of Different Types of Chain Extenders

Three types of chain extenders i.e. 1,6-hexanediamine (HDA),

ethylenediamine (EDA) and 2-methylpentamethylenediamine (Dytek® A Amine)

were chosen in this research. Table 5.1 shows that using HDA as the chain extender

for preparing the PUD, the final appearance of the product is superior than using

either EDA or Dytek® A Amine chain extender.

Table 5.4. Appearance of the finishing PUD product with different types of chain extenders Types of chain extenders

1,6-hexanediamine

(HDA)

ethylenediamine (EDA)

2-Methyl-pentamethylenediamine

(Dytek® A Amine) Parameters : NCO/OH ratio Neutralization

3.0

100%

3.0

100%

3.0

100% Particle size (µm) 6.93 4.42 36.94

Appearance Desirable

Milky white liquid

Undesirable Milky white with semi-gel product

Undesirable Hazy with lots of air bubbles. Very viscous.

PUD using HDA as chain extender had a satisfactory finishing appearance

(i.e. milky white and liquid form). On the other hand, the use of the other two chain

extenders, EDA and Dytek® A Amine, gave undersirable appearance for the end

product of PUD. Hence, HDA was selected as a chain extender in further studies in

this research.

5.3.2 FT-IR Analysis of Aqueous Polyurethane Dispersion (PUD)

5.3.2.1 Formation of PUD

Figure 5.1 demonstrates the FT-IR spectra of the main component of polyester polyol

(CAPA2205), isophorone diisocyanate (IPDI) and the PUD. The 1750 -1740 cm-1

81


band indicatse the C=O stretching in polyol; the 2280 – 2260 cm-1 band is due to

N=C=O antisymmetric stretching in isocyanate. Bands at 1560 – 1530 and 1610 -

1560 cm-1 are due to N-H bending and COO- antisymmetric stretching in the PUD4,8-10

. These bands confirm the PUD structure.

6008001000120014001600180020002400280032003600400044001/cm

Abs

orba

nce

PUD

IPDI

Polyester Polyol(CAPA 2205)

Figure 5.1. FT-IR spectra of polyol, IPDI and PUD

5.3.2.2 FT-IR Analysis of Residual NCO Functionality in PUD

The absorption spectra of PUD is shown in Figure 5.2. The presence of characteristic

peaks, C=O [1733, 1703 cm-1 ] and N-H [1550 cm-1 ], confirmed the formation of

urethane group [-NHCOO-]. The absence of the N=C=O [2270 cm-1 ] stretching band

showed that all the –NCO functionalities were consumed after chain extension.

82


6 0 08 0 01 0 0 01 2 0 01 4 0 01 6 0 01 8 0 02 0 0 02 4 0 02 8 0 03 2 0 03 6 0 04 0 0 04 4 0 0

1 /cm

% T

B e fo re ch a in e x te n s io n

A fte r ch a in e x te n s io n

-N = C = O

Figure 5.2. FT-IR spectra of PUD before (a) and after (b) chain extension

Figure 5.3 shows the FT-IR spectra of the PUD with different average particle sizes at

a constant NCO/OH ratio of 2.8 before chain extension. The absence of N=C=O

stretch bands in the spectra indicated that all the residual NCO groups were consumed

completely due to the water molecules at the surface of the polyurethane particle since

only a small amount of free NCO group was present initially.

83


Abso

rban

ce

(a)

) (b

) (c

) (d

4400 4000 3600 3200 2800 2400 18002000 1/cm

1600 1400 1200 1000 800 600

Figure 5.3. Absorption FT-IR spectra of PUD of varying particle size before chain extension : (a) 1.65µm, (b) 2.60µm, (c) 3.65µm, (d) 4.05µm (NCO/OH ratio =2.8)

Table 5.5 shows that the NCO content of the polyurethane prepolymer determined by

the di-n-butylamine back titration method. The result showed that the NCO groups of

IPDI were sufficiently reacted with hydroxyl groups of polyol at the first step and

with the DMPA at the next step.

Table 5.5. The residual NCO content of the polyurethane prepolymer by the di-n-butylamine back titration method

% residual NCO NCO : OH ratio

Polyol + IPDI +DMP

Polyol + IPDI + DMP + Cataylst + 1hr

Polyol + IPDI + DMP + Cataylst After 2 hrs

2.8 3.95 3.36 3.10 3.0 4.45 3.61 3.20 3.2 4.54 4.11 4.08 3.4 5.37 4.68 4.61

84


Figure 5.4 shows the time-dependent change of NCO bands during the chain

extension process. No residual NCO peak was observed after the completion of chain

extension.

7509001050120013501500165018001950240027003000330036003900420045001/cm

0

0.05

0.1

0.15

0.2

0.25

0.3

Abs

orba

nce

[ d ]

[ c ]

[ e ]

[ f ]

[ g ]

[ h ]

[ b ]

[ a ]

Figure 5.4. Change of FT-IR spectra during preparation of PUD: (a) polyester

polyol + IPDI + DMPA, (b) after neutralization / before dispersion, (c) before chain

extension, (d) adding 20% of chain extender (theortically), (e) 40%, (f) 60%, (g) 80%

& (h) 100% (NCO/OH = 3.0; particle size = 2.65µm)

85


Figure 5.5 shows the effect of particle size on the needed amount of chain extender

for 100% extension. Relative amount of residual NCO group was calculated by taking

an alkane (-CH2 -) stretching vibration at about 2855 cm-1 as a reference because CH2

groups were not changed during the entire reaction period.2,9-12

120

Ave. Particle Size : 2.65Ave. Particle Size : 2.00100Ave. Particle Size : 1.50

Am

ount

of r

esid

ual N

CO

(%)

80

60

40

20

00 20 40 50 60 80 100

Amount of chain extender (% theoretical)

Figure 5.5. Amount of residual NCO groups versus the sizes of PUD particles in chain extension stage at average particle size 2.65µ, 2.00µ and 1.50µat NCO/OH =3.0

5.3.3 Growth of Average Molecular Weight during the Chain Extension Figure 5.6 demonstrates the change of average molecular weight during the

chain extension process with different NCO/OH ratio and particle sizes. When the

NCO/OH ratio is low such as 2.8, the molecular weight of PUD did not change

significantly during the chain extension stage. This result indicated that the side

reaction occurred between the residual NCO groups and water during and after the

dispersion stage. This phenomenon can be further verified by the absence of the

N=C=O stretch band at 2270-2280cm-1 in the FT-IR spectra as shown in Figure 5.3.

86


Particle size : 2.56µ (NCO/OH =2.8)Particle size : 2.00µ (NCO/OH =2.8)Particle size : 1.50µ (NCO/OH =2.8)Particle size : 2.56µ (NCO/OH =3.0)Particle size : 2.00µ (NCO/OH =3.0)Particle size : 1.50µ (NCO/OH =3

350000

300000M

olec

ular

wei

ght (

g/m

ol)

250000

200000

150000

100000

.0)50000

00 20 40 60 80 100


Figure 5.6. The change of average molecular weight in chain extension stage with different particle sizes and different NCO/OH ratio (particle sizes varied from 1.50µm to 2.56µm; NCO/OH ratio varied from 2.8 to 3.0)

Figure 5.6 also showed the increase in molecular weight of PUD with increasing

amount of chain extender. About half of the residual NCO groups reacted with water

molecules in the dispersion process and the rest could react with the chain extender.

In the case of 4,4’-methylenebis-phenyl isocyanate (MDI), it was reported that almost

all the residual NCO groups reacted with water during the dispersion process.7-13

Therefore, it was difficult to prepare chain extended MDI-based polyurethane

dispersion. In the IPDI-based polyurethane dispersion, the reactivity of NCO group in

IPDI is much lower than that of MDI. The relative reactivity of NCO groups in IPDI

with various functionalities can be illustrated as below :

Aliphatic NH2 > Aromatic NH2 > Primary OH > Water > Secondary OH > Tertiary

OH > Phenolic OH > COOH.12-15

87


70

Val

ue o

f cha

in e

xten

sion

(% o

f cha

in e

xten

der)

60

50

40

30

20

10

00 0.5 1 1.5 2 2.5

Number-average particle size of PUD (µm) 3 3.5

Figure 5.7. Effect of particle size on the maximum value of chain extension (CEmax)

Figure 5.7 shows the relationship between number-average particle size and

maximum value of chain extension, CEmax (50% at 2.6µm). With the decrease of the

average particle size, the value of chain extension increased. This result does not

indicate that the number of residual NCO groups locate at the surface of the particle is

proportional to the total surface area. However, possible reaction locus of the chain

extension is considered as particle surface since water-soluble chain extender was

used and CEmax was influenced by the total surface area. Moreover, available free

NCO group should be incorporated with carboxyl group near the end of prepolymer

molecule since DMPA was present between excess amount of IPDI and polyester

polyol during the preparation of prepolymer.3,14-17 This indicated that the residual

NCO-groups on the particle surface have more favourable condition than inner

particle spaces have to react with chain extender (Figure 5.8).

88


NCO

NCOH2O

H2O

H2O

H2O

-

--

-

--

-

-

-

NCONCO

NH2

H2N

H2N

NH2

NH2

H2N

NH2

H2N

Chain Extension

(A)(A)

(B)

(B)

-

-

-

- -

-

-

-

-

PU particle

Figure 5.8. The reaction of residual NCO groups.

Figure 5.9 below demonstrates the reaction scheme of residual –NCO groups, and the two competitive reactions may occur simultaneously.

2(R -N=C=O) + H 2O ---- R-NH-CO-NC-R + COUrea Linkage

---------------(1)2

2(R-N=C=O) + H 2 N (C 6 H 12 )NH 2--- R-NH-CO-NH (C6H12)NUrea Linkage

-CO -NH -R ---(2)

Figure 5.9. Two competitive reactions of residual –NCO groups on PU particle

Urea linkage is developed in both ways. The first reaction may occur in inner

particle and also on particle surface. However, the second reaction is supposed to

occur only on particle surface. Probably, polyurethane ionomer particle is swelled

with water even though swelling ratio is not so high. However, the possibility of side

reaction between residual NCO groups and water molecules inside of the particle is

related to the particle size or volume.15-17

89


5.3.4 Effect of the Degree of Chain Extension on the Adhesive Bonding

Strength

Figures 5.10, 5.11 and 5.12 show that the adhesive bonding strength (shear

and peel strength) versus the degree of chain extension with different NCO/OH ratios.

From Figure 5.3, it was found that there were no free NCO groups available due to the

small amount of NCO group and reaction with water molecules at dispersion stage.

Therefore the adhesive bonding strength decreased even though the amount of chain

extender increased. The excess chain extender became impurities and consequently

caused undesirable side effect on the bonding strength (see Figure 5.10). In the case of

higher mole ratio such as 3.0, even though the free NCO groups reacted with water

molecule, there were still some residual NCO groups remained in water phase.

Therefore, the bonding strength increased to the point of CEmax . The adhesive

bonding strength decreased again after this point, as shown in Figure 5.11 and 5.12.

930

8

25

Pee

l Str

engt

h, N

/mm

7

Shea

r St

reng

th, N

/mm

2

620

5

15

4

310

Shear Strength2Peel Strength

5

1

00 20 40 60 80 100

0


Figure 5.10. Effect of the degree of chain extension on adhesive bonding strength (shear and peel strength) at NCO/OH ratio 2.8

90


40

35

30

Shea

r st

reng

th, N

/mm

2

25

20

15

Shear Strength (particle size=1.50)

10 Shear Strength (particle size=2.00)

Shear Strength (particle size=2.56)5

00 20 40 50 60 80 100


Figure 5.11. Effect of the degree of chain extension on the shear strength with different particle sizes at NCO/OH ratio 3.0

0

30

25

Peel

Str

engt

h, N

/mm

20

15

Peel Strength (particle size=1.50)10

Peel Strength (particle size=2.00)

Peel Strength (particle size=2.56)

5

020 40 50 60 80 100


Figure 5.12. Effect of the degree of chain extension on the peel strength with different particle sizes at NCO/OH ratio 3.0

91


5.4 Conclusions

This study focused on the effect of chain extenders. The aqueous polyurethane

dispersion was prepared and the effect of the chain extension was investigated. At a

low NCO/OH ratio, no free NCO group was found due to the reaction with water

molecules from the beginning of chain extension reaction. At a NCO/OH ratio of 3.0,

about half of residual NCO groups were remained and reacted with the chain

extender. The amount of residual NCO group varied with the total surface areas or

the particle sizes at the same NCO/OH ratio. The required amounts of chain extender

for the optimal chain extension do not correspond with the theoretical residual NCO

group. With decreasing polyurethane particle size, the amounts of optimal chain

extender logarithmically increased. For larger particle, residual NCO group could not

be founded. Therefore, most chain extenders reacted with NCO groups in particle

surface more than in inner particle. This is also further verified with the results of

adhesive bonding strength. The excess amines had an unfavourable influence on the

bonding strength.

5.5 References

1. D.S. Chen, M. Hsien, US Patent 5, 1994, 306, 764.

2. Lee, H.T, Hwang, Y.T, Chang, N.S, Huang, C.C. T, Li, H.C, Water Borne,

High-Solid and Power Coatings Symposium, New Orleans, 22-24

February, 1995, p.224.

3. C. Hepburn, Polyurethane Elastomers, Second ed., Elsevier, New York,

1992.

92


4. J.B. Lambert. D.A. Lightner, H.F Shurvell, R.G. Cooks, Introduction to

Organic Spectroscopy, Macmillan, New York, 1987, p.169

5. P. Thomas, Waterborne and Solvent Based Surface Coating Resins and

their Applications in Polyurethanes, Vol III, SITA Technology Ltd London,

1999, p.59.






1990, p.43.

11. W. Koonce and F. Parks, US Patent 6, 2002, 451, 908.


13. C. Irle and W. Kremer, US Patent 6, 2003, 559, 225.



16. L.C. Hesselmans, US Patent 6, 2003, 599, 977.

17. M.A. Schafheutle and A. Artz, US Patent 6, 2002, 429, 254.

93

Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and Adhesive Bonding Strength Properties in Formulating the Footwear Adhesives

Chapter 6

The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and

Adhesive Bonding Strength Properties in Formulating the Footwear Adhesives

6.1 Introduction

Polyurethane (PU) adhesives consumption has been estimated at 216 million

lb in 1991, with a value of approximately USD 301 million. As compared to others

adhesives, PU adhesives for footwear have a great demand, especially in China,

Taiwan, Korea, Thailand and Malaysia. Overall the PU adhesives market grew at

approximately 3% per year from 1986 to 1991. Currently, the output of adhesives

(including PU adhesive) and sealants in China is roughly at 3 million tonnes in year

2002, or 7% of the global production,1,4 as shown in Figure 6.1.

Adhesive & Sealant4

PU Adhesive3.54

3.5 CAGR for adhesive& Sealant is ~ 10%

per year 33

2.61

Mill

ion

Ton

nes

2.442.5 2.272.07

2

1.5

1 CAGR for PU adhesive is ~ 14% per year 0.5

0.200.170.140.120.110.100

1998 1999 2000 2001 2002 2005

Figure 6.1. Total output of adhesives + sealant and PU adhesive in China

94


It was estimated that in year 2005, the output will reach 3.54 million tonnes.

The total adhesives + sealant grew in average about 10% per year. However, for the

PU adhesive, the average growth was estimated to be 14% per year from 1998 to

2005. Recently, the adhesive market in China has grown more aggressively than

other countries in the world, as the average growth rate is about 10% as compared to

1~1.5% in US and 2.5% globally.2,4-7

PU adhesives used to attach soles to footwear make a sizeable niche. PU

adhesives compete primarily with neoprene-based adhesives and have replaced much

of the neoprene due to improved performance.

For more than 30 years, solvent-based PU adhesives have been used in

application for attaching soles in the shoe industry. They have high initial and final

bond strengths, excellent heat resistance and the ability to be used in wet bonding or

heat reactivation application as compared to the traditional type of shoe adhesive i.e.

neoprene. Figure 6.2 shows the segmentation of the PU adhesives in various

application fields. The major market segment for PU adhesives was in footwear

industry, about 60% of the total market with consumption volume at 100 thousands

tonnes in year 2002 while the total output of PU adhesives (all applications) was 166

thousands tonnes.3,6-10

95

Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and Adhesive BoStrength Properties in Formulating the Footwear Adhesives

Footwear60%

Compound Packaging Film

12%

General18%

Sealant and Others10%

nding

96

Figure 6.2. Segmentation of PU adhesives markets (by consumption volume, total 166 thousands tonnes) in year 2002

Transportation in China depends on buses, bicycles and feet. Bicycles do not

required a lot of adhesives. The story is different for footwear. There are at least 1

billion domestic customers. Per capita, there is an annual average consumption of 1.5

pairs of footwear for Chinese and approximately five pairs of westerners depending

on gender and out door activities. Finally, it becomes clear that in China, footwear

industries play an important role among light industries because 70-80% of today's

footwear is glued together. Thus, it is natural for China to become the number one

exporter for footwear with the 1992 production of approximately 2 billion pairs. For

the period of 1991 through May 1994, Chinese footwear occupied 13.9% of the total

US import. For this, a lot of shoe manufacturer and adhesives makers have moved to

China. Figure 6.3 shows the largest footwear producer in the world, which is China,

with about 54% of global market share.2,7-12


China 54 %

Asia (excl China)21.5%

Western Europe7.3%

South America6.8%

Middle East3.0%

Eastern Europe2.8%

North and Central America2.6%

Africa1.9%

Oceania0.1%

China 54 %

Asia (excl China)21.5%

Western Europe7.3%

South America6.8%

Middle East3.0%

Eastern Europe2.8%

North and Central America2.6%

Africa1.9%

Oceania0.1%

Figure 6.3. The largest footwear producer in the world, China

Figure 6.4 illustrates the total production of footwear in China from year 1985

to 2002, from 1.6 billion pairs increased to 6.6 billion pairs of footwear.

1.6

5.76.3 6.5 6.6

0

1

2

3

4

5

6

7

1985 1995 1997 2001 2002

China Footwear Production (billion pairs)

Figure 6.4. Total footwear production in China from year 1985 to 2002.

97


In year 2002, the total production capacity for footwear in the whole Asia was

estimated to be 76% of global production, however, for China alone, it was already

about 54% of global production with 6.6 billion pairs of footwear produced.

The adhesives for footwear are mainly PU and neoprene (or named

chloroprene). China imports chloroprene and MDI, methylene diisocyanate (for PU).

The first large-scale production line, amounting to 1000 MT of PU, was installed at

the Da-Cang Factory in Jiangsu Province. Currently, there are at least 20 factories

with a total capacity of 6000 MT of PU, and there are about 2000 MT of PU solutions

available for footwear manufacturing. However, the problems of some PU solutions

have been poor stability, low initial viscosity and the yellowing of adhesives.3,13-15 In

recent years, aliphatic polyisocyanate has been chosen to replace aromatic

polyisocyanate like MDI. Aliphatic polyisocyanate has several advantages over

aromatic polyisocyanates:

1) Aliphatic polyisocyanate has better durability than aromatic polyisocyanate. This

means that aliphatic polyisocyanate has non-yellowing properties after exposure

to sunlight for long time.

2) Aliphatic polyisocyanate is a more environmentally friendly product (in term of

less toxicity) than aromatic polyisocyanate.

3) Aliphatic polyisocyanate is more stable and having longer pot-life than aromatic

polyisocyanate.

As the global trends shifting footwear production from U.S.A and Europe to

Asia, footwear industry has expanded enormously in China in the recent years. PU

adhesives are replacing the conventional type of footwear adhesives in the market due

to their high performance in adhesive bonding strength, non-yellowing and less

toxicity (using aliphatic isocyanate as a crosslinker in the footwear adhesives)

98


properties. As environmental demands on the adhesives industry have increased and

the need for adhesives with low VOC (volatile organic concentration) or no-solvent

content has developed, water borne polyurethane footwear adhesives have been

created to address these needs.7,15-17

The main component in the water borne PU adhesives is the aqueous

polyurethane dispersion (PUD). In this research, the PUDs based on aliphatic

isocyanates which were developed in the previous research work (Chapter 4 and 5)

were used in formulating the water borne footwear adhesives. The key properties such

as the appearance (colour) and adhesive bonding strength have been assessed. In

addition, the adhesion bonding strength i.e shear and peel strength of the footwear

adhesives on different shoe substrates have also been evaluated and compared with

one of the commercialized PUD, Dispercoll U54 (from Bayer, Germany).

6.2 Experiment

The water borne polyurethane footwear adhesives were prepared using the

aqueous polyurethane dispersions (PUDs) which were obtained from the previous

research described in Chapters 4 and 5. Two types of formulations known as one-

component (1K) and two-component (2K) water borne PU footwear adhesives were

prepared (see Tables 6.1 and 6.2), and the performances were then evaluated by

comparing our research developed PUD versus the commercialized PUD, Disperoll

U54.

99


Table 6.1. One-component (1K) water born PU footwear adhesive formulation

Ingredients Weight (grams)

Aqueous polyurethane dispersion i.e. research developed PUDs or Dispercoll U54

100.00

Tafigel PUR 40 thickener 0.47

Total 100.47

Table 6.2. Two-component (2K) water borne PU footwear adhesive formulation

Ingredients

Weight (grams)

Component A Aqueous polyurethane dispersion i.e. reserach developed PUDs or Dispercoll U54

100.00

Tafigel PUR 40 thickener

0.47

Component B Rhodocoat WAT-1 water borne aliphatic polyisocyanate crosslinker

3.00

Total

103.47

The appearance (color) of our research developed aqueous polyurethane

dispersion and the adhesives were compared against the commercialized solvent

borne PU footwear adhesive. The shear and peel strength of our research developed

footwear adhesives were evaluated on different substrates i.e. PVC, PU and NBR and

compared with the commercialized water borne PU footwear adhesives.

Both shear strength (based on EN1392 standard) and peel strength were

measured using the Zwick universal shear/peel strength tester. The details of both test

methods were described in Chapter 3, section 3.8.

100



6.3.1 Color appearance and durability comparison

The aqueous polyurethane dispersion which developed earlier was used to

formulate the water borne polyurethane footwear adhesive and then compared with

the commercial solvent borne polyurethane footwear adhesive. Figure 6.5 shows the

color appearance of our newly prepared product and the commercial solvent borne

polyurethane solution. The aqueous polyurethane dispersion is white and milky.

However, the commercial solvent borne polyurethane solution is yellowish.

Newly made commercial solvent borne polyurethane solution

Newly made aqueous polyurethane dispersion

Figure 6.5. Comparison the color appearance of both solvent borne polyurethane solution and aqueous polyurethane dispersion After storage both solvent borne polyurethane solution and aqueous

polyurethane dispersion for a period of times i.e. 6 months at ambient temperature and

humidity at 55% environment, the solvent borne polyurethane solution changed to

dark yellowish. However, the milky white color of aqueous polyurethane dispersion

remained unchanged (see Figure 6.6). This indicates that the durability and stability

of our aqueous polyurethane dispersion is better than the commercial solvent borne

polyurethane solution.

101


The commercial solvent borne polyurethane solution after storage for 6 months

The aqueous polyurethane dispersion after storage for 6 months

Figure 6.6. Comparison the color appearance of both solvent borne polyurethane solution and aqueous polyurethane dispersion after storage for 6 months at ambient temperature and humidity at 55% environment Figure 6.7 shows the color appearance of both two component (2K) solvent

borne and water borne polyurethane (PU) footwear adhesives. Both adhesives were

prepared by using the commercial PU solution (for solvent borne) and our PUD (for

water borne). Both prepared footwear adhesives were then applied onto a white shoe

sole base, dried and exposed to sunlight for 3 months.

Apply/brush evening on the white shoe sole base side by side and then exposure to sunlight for 3 months

A drop of 2K water borne PU adhesive (using PUD)

A drop of 2K solvent borne PU adhesive White shoe

sole base

2K water borne PU adhesive (using PUD)

2K solvent borne PU adhesive

Figure 6.7. Comparison of the color appearance of both 2K solvent borne and water borne PU footwear adhesives after exposure to sunlight for 3 months

102


After exposure the prepared footwear adhesives to sunlight for 3 months, the

commercial 2K solvent borne PU adhesive was yellowish in color whereas our 2K

water borne PU adhesive had satisfactory appearance (transparent after drying) and no

color changed after exposure to sunlight for a 3-month period. This indicates that our

2K water borne PU adhesive has a better durability property as compared to the

commercial 2K solvent borne PU adhesive. This property is particularly important for

footwear industry especially for those shoes bases which are in white or light color.

6.3.2 Adhesive bonding strength comparison

Based on the previous research studies on the effect of different process

parameters (Chapters 4 and 5), the optimal parameters for preparing the PUD have

been identified i.e. the NCO/OH ratio to be 3.0, DMPA content to be 5% and degree

of neutralization to be 100%. Our polyurethane dispersion was prepared according to

these optimal parameters and then formulated into the one-component (1K) and two-

component (2K) water borne polyurethane (PU) footwear adhesives. The adhesive

bonding strengths such as peel and shear strength of these research prepared water

borne polyurethane footwear adhesives were then compared with the commercial

products.

Figures 6.8 and 6.9 demonstrate the initial (5 minutes after bonding) and final

(24 hours after bonding) peel strengths of both our 1K water borne polyurethane

footwear adhesive (based on formulation in Table 6.1) versus the commercial 1K

water borne polyurethane footwear adhesive on different shoe substrates i.e. PVC, PU

and NBR.

103


40

Commercial product 35 Our product

30Pe

el S

tren

gth

(N/m

m)

25

20

15

10

5

0

PVC PU NBRShoe substrates

Figure 6.8. Comparison of initial peel strength of our 1K water born PU footwear adhesive versus commercial 1K water borne footwear adhesive (based on Disperoll U54 PUD)

Our product

Commercial product

Peel

Str

engt

h (N

/mm

)

Shoe substratesNBRPUPVC

100

90

80

70

60

50

40

30

20

10

0

Figure 6.9. Comparison of final peel strength of our 1K water born PU footwear adhesive versus commercial 1K water borne footwear adhesive (based on Disperoll U54 PUD)

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The initial and final peel strengths of our 1K water borne footwear adhesive

were found to be superior than the commercial product when applied on different

shoe substrates i.e. PVC, PU and NBR.

Figure 6.10 shows the shear strengths of our 1K water borne PU footwear

adhesive and the commercial product.

Our productCommercial product

Shea

r st

reng

th (N

/mm

2 )


140

120

100

80

60

40

20

0

Figure 6.10. Shear strength comparison of our 1K water borne PU footwear adhesive

versus commercial product

Once again, our 1K water borne PU footwear adhesive showed better shear

strength than the commercial product when applied onto the 3 different shoe

substarates i.e. PVC, PU and NBR.

Figures 6.11 and 6.12 show the initial and final peel strengths of our 2K water

borne PU footwear adhesives and the commercial product (based on Disperoll U54

PUD).

105


Commercial product Our product

Peel

stre

ngth

(N/m

m)

NBRPUShoe substrates

PVC

60

50

40

30

20

10

0

Figure 6.11. Comparison of initial peel strength of our 2K water borne PU footwear adhesive versus commercial 2K water borne footwear adhesive (based on Disperoll U54 PUD)

Commercial product Our product

Peel

stre

ngth

(N/m

m)

NBRPUShoe substrates

PVC

120

100

80

60

40

20

0

Figure 6.12. Comparison of final peel strength of our 2K water borne PU footwear adhesive versus commercial 2K water borne footwear adhesive (based on Disperoll U54 PUD)

106


In the comparison of initial and final peel strength for our 2K water borne PU

footwear adhesive and the commercial product, it was found that our 2K water borne

PU footwear adhesive has a superior peel strength than the commercial product.

Similarly, the shear strength of our 2K water borne PU footwear adhesive is

higher than that of the commercial product (Figure 6.9).

Our productCommercial product

Shea

r st

reng

th (N

/mm

2 )


160

140

120

100

80

60

40

20

0

Figure 6.13. Shear strength comparison of our 2K water borne PU footwear adhesive versus commercial product

The 2K water borne PU footwear adhesives provide higher adhesive bonding

strength (in term of peel and shear strength) than the 1K water borne PU footwear

adhesive. This is mainly due to the incorporation of external water borne aliphatic

isocyanate crosslinker (Rhodocoat WAT-1 from Rhodia Co.) in the 2nd part of the

formulation. With this additional crosslinker, the crosslinking network in the

polyurethane chain was increased and so enhanced the adhesive bonding strength of

the footwear adhesives between the shoe substrates. High adhesive bonding strength

is particularly needed for shoes subjected to higher degree of strain or bending.

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In the initial peel strength test, both 1K and 2K water borne PU footwear

adhesives with PVC substrate give the best result than the other two substrates.

However, in the final peel strength test, adhesive with PU substrate gives the best

result. This is because the surface energy (wetness) of the PU substrate is the highest

among all the substrates, due to its active molecular structure. With the higher surface

energy, the PUD is therefore able to create a strong covalent bond .

6.4 Conclusions

The aqueous polyurethane dispersion developed in this research provides

desirable color appearance i.e. a white milky liquid. When formulated into 1K and 2K

water borne PU footwear adhesives, the dispersion retained the same white milky

color in wet form (liquid form). However, when the dispersion was coated as the

adhesive on shoe substrates and dried, it was transparent. In addition, it offered better

durability (non-yellowing appearance after exposure to sunlight for a long period of

times) as compared to the commercial solvent borne type of footwear adhesives.

In terms of adhesive bonding strength, both 1K and 2K water borne PU

footwear adhesives developed in this research showed superior peel and shear

strengths than the commercial product regardless of shoe substrates materials used i.e.

PVC, PU and NBR.

6.5 References

1. B.S, Jackson, Industrial Adhesives and Sealants, 1995, p.10.

2. C. Hepburn, Polyurethane Elastomers, Second ed., Elsevier, New York, 1992,

p.281.

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3. P. Thomas, Waterborne and Solvent Based Surface Coating Resins and their

Applications in Polyurethanes, Vol III, SITA Technology Ltd London, 1999, p.59.

4. G. Oertel, Polyurethane Handbook, Carl Hanser, Munich, 1985, p.31.

5. George Woods, The ICI Polyurethanes Book, 2nd Edition, ICI Polyurethanes,

1987, p.197.

6. J.W. Rothause and K. Nachtkam, Advances in Urethane Sci. and Techology,

1987, 10, p.121.

7. G. Schneberger and M. Dekker, Adhesives in Manufacturing, 1983, p2.




11. W. Koonce and F. Parks, US Patent 6, 2002, 451, 908.



1990, p.43.



16. B.K. Kim and Y.M. Lee, J. Appl. Polym. Sci., 1994, 54, 1809.


.

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Chapter 7 Conclusions

Chapter 7

Conclusions

With the evolution of legislation towards reducing the VOC and the creation

of environmental friendly products, there is a great demand for the development of

water borne products for the industries.

Solvent borne adhesives are used extensively in the footwear industry for a

long time. Currently, there are several solvent borne footwear adhesives available in

the market, namely neoprene, grafted choroprene and the polyurethane based systems.

The polyurethane adhesives are generally accepted for their good bonding strength

and good resistance to water, fat, oils, chemical and solvents. Owing to the large

varieties of polyurethane adhesives systems, they are classified into one-component

(1K) and two-components (2K) systems. Two-component polyurethane adhesives are

characterized essentially by using polyisocyanates as crosslinkers and oligomeric

diols or polyols as the back-bone resin. They have the advantages of presenting no

great problems in terms of shelf life. By a skillful choice and targeted reactivity of the

monomers, it is possible to formulate systems having different pot lives, bonding

strength and chemical resistance to meet different requirements. Due to the

polyaddition reactions, these adhesives do not release any elimination products during

the crosslinking. Therefore, the two-component system is generally well accepted in

the industry.

As the regulators are implementing the policy to protect the environment

across these regions, big multi-national organizations like Nike, Reebok and Adidas

have already embarked on the campaign to demand their suppliers to supply

environmental friendly adhesives (water borne type) for their applications. This has

generated a big demand for the water borne adhesives and all suppliers are gearing

110


their R&D in this direction. The adhesives industry is therefore gearing to produce

water borne adhesives with non-yellowing property for the shoes markets especially

those with white or light based sport shoes.

In view of the fact that the trend is towards a high demand for environment

friendly products, the research on the development of an aqueous polyurethane

dispersion to form the water borne polyurethane footwear adhesives (1K and 2K

systems) with non-yellowing and good adhesive bonding strength properties were

therefore designed.

To achieve this objective, the research has focused on the following areas :

1) Formulation of an aqueous polyurethane dispersion (PUD) by forming a NCO

prepolymer initially. The chain was subsequently extended in the aqueous phase in

the presence of a polyamine chain extender.

2) The prepolymer is formed by reacting an active hydrogen containing compound

such as linear polyester diol (CAPA® 2205, white waxy solid, mean molecular weight

2000 and hyroxyl value 56mg KOH/g) with aliphatic polyisocyanate such as

isophorone diisocyanate (IPDI), 2,2-bis(hydroxymethyl) propionic acid (DMPA) and

the chain extender 1,6-hexanediamine (HDA).

3) Various formulations were designed to study the effects of process parameters such

as NCO/OH ratio, DMPA content, degree of neutralization and the degree of chain

extension. Based on these studies, it was found that the molecular weight and

adhesive bonding strength of the PUD were significantly affect by the DMPA content

and the degree of neutralization. The molecular weight of the PUD was found to

increase when the NCO/OH ratio was increased. As the particle size decreased, the

amount of chain extender needed to optimize the chain extension decreased. The non-

yellowing property could be achieved by using an aliphatic isocyanate (IPDI). From

111


the obtained results, the optimal process parameters for formulating the aqueous

polyurethane dispersion with optimum performance were therefore identified to be :

NCO/OH ratio is 3, DMPA content is 5% and degree of neutralization is 100%.

4) The aqueous polyurethane dispersion (PUD) which developed in this research was

used to formulate the 1K and 2K water borne PU footwear adhesives. Their adhesive

bonding strengths i.e. shear and peel strenghts were then assessed and compared

versus the commercial product. Our PUD gave good compatibility and outstanding

peel and shear strength than the commercial product.

In conclusion, we are able to generate a series of good and valuable data for

use to develop the aqueous polyurethane dispersion (PUD) with good compatibility,

durability (non-yellowing), low VOC (environmental friendly) and outstanding

adhesive bonding strength properties. These properties are actually the key

requirements for the footwear adhesives market now. Therefore, the present research

could bring attractivenss and added value to the adhesive producers as well as the

footwear industry.

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