Fundamental Aspects on the Re-use of Wood Based Fibres

- Porous Structure of Fibres and Ink Detachment

Jennie Forsström

Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan i Stockholm framlägges till offentlig granskning för avläggande av teknologie doktorsexamen onsdagen den 15 december 2004 kl. 13.00 i SCA salen, Mitthögskolan, Holmgatan 10, Sundsvall. Avhandlingen försvaras på svenska.

Fundamental Aspects on the Re-use of Wood Based Fibres

- Porous Structure of Fibres and Ink Detachment

This thesis resulted from work jointly carried out in the Fibre Science and Communication Network at the Mid Sweden Univeristy, Sundsvall, Sweden and the Fibre Technology group at The Royal Institute of Technology, Stockholm, Sweden

© 2004 Jennie Forsström Trita-FPT-Report 2004:37 ISSN 1652-2443 ISRN/KTH/FTP/R-2004/37-SE

ABSTRACT In this work, different aspects on the re-use of wood based fibres have been studied, focusing on ink detachment

of flexographic ink from model cellulose surfaces and changes in porous structure of kraft fibres following

different treatments. New model systems for evaluation of ink detachment and ink-cellulose interactions were

used. Ink detachment was studied using Impinging jet cell equipment, taking into consideration the influence of

storage conditions, surface roughness and surface energy of the cellulose substrate. A micro adhesion

measurement apparatus (MAMA) was used to directly study ink-cellulose interactions, from which the adhesive

properties between ink and cellulose, having various surface energies, could be derived. UV-light, elevated

temperatures, longer storage time, decreased surface energy, i.e. making the cellulose surface more hydrophobic,

and high surface roughness all negatively affected ink detachment. Attenuated total reflectance – fourier

transform infra red (ATR-FTIR) and atomic force microscopy (AFM) was used to evaluate structural and

chemical changes of ink and cellulose upon storage at elevated temperature or under UV-light. After storage at

elevated temperatures, ATR-FTIR spectra indicated that a hydrolysis or an oxidative reaction took place as a

peak at 1710 cm-1 appeared. AFM revealed that storage at elevated temperatures caused the latex particles

present in the ink to form a film, most likely due to annealing. Less ink detached from hydrophobic cellulose

surfaces. Ink detachment decreased for rougher cellulose substrates due to an increased molecular contact area.

Fibre pore structure and water retaining ability influenced fibre/fibre joint strength and different paper strength

properties. Investigations took into account the effect of pulp yield, counter-ion types, pH, salt, hornification and

strength enhancing additives. Nuclear magnetic resonance relaxation (NMR), inverse size exclusion

chromatography (ISEC) and water retention value (WRV) measured the changes that occur in the fibre wall

upon varying the conditions. Each different measuring technique contained unique information such that a

combination of the techniques was necessary to give as complete a picture as possible over the changes that

occurred in the fibre wall upon varying the conditions for the fibre. A correlation between fibre pore radius and

sheet strength properties was found, suggesting that fibres with larger pores allow for a larger molecular contact

area between fibres to be formed during drying and consolidation of the paper. Fibre/fibre joint strength, fibre

flexibility, and the number of efficient fibre/fibre contacts also controlled sheet strength. The effect of different

strength enhancing additives on fibre pore structure and paper strength was investigated. Larger pores in the

fibres allowed for additives to penetrate into the fibre wall. Additives with low molecular mass (Mw) penetrated

into the fibre wall to a larger extent than additives with a high Mw, causing an embrittlement of the fibre.

However, low Mw additives gave higher sheet tensile strength despite a leveling out in strength at high additions,

indicating that the fibre wall can only adsorb a limited amount of chemical. Polyallylamine hydrochloride (PAH)

and polyelectrolyte complexes (PEC) of PAH and polyacrylic acid (PAA) were added separately to the pulp.

PEC significantly improved both tensile strength and Z-strength, whereas PAH alone did not increase the

strength properties to the same extent unless the sheets were heated to 150°C for 10 minutes. The results

suggested that the effect of PEC was dominated by an improvement in fibre/fibre joint strength, whereas the

effect of PAH was significantly affected by an improvement of the intra-fibre bond strength.

PREFACE

Paper has played a vital role in the cultural development of mankind. It still has a key role in

communication and is needed in several different areas of society. Products from the pulp and

paper industry have been, and still are, one of the most important sources of income for this

country. For a long period of time, pulp and paper were produced and sold as relatively cheap

bulk products. However, in recent years the companies involved have been forced to realise

the importance of refinement alongside improvements in production efficiency. Continuous

product development is necessary in order to meet the ever-increasing demands on paper

performance and to maintain competitiveness in comparison with other raw materials, such as

plastics.

Environmental awareness, legislations, the strong demand of today’s environmental policy

and the increased use of recycled paper as a raw material requires deeper knowledge about the

recycling process. It is important to study in-detail all parts of the recycling process to find the

critical factors in each step. Such studies can be challenging due to the presence of different

fibres, additives and seasonal variations of the raw material. It is necessary to find reliable

model systems that reflect each part of the recycling process while minimising unwanted or

unknown sources of variations. Different fibre properties, the effect of different treatments

and the correlation between fibre and paper properties must be studied in order to optimise the

treatment of the fibres to best utilise the inherent properties of certain pulps. This type of

study is best conducted using a well-characterised fibre material to gain a better understanding

and a clearer view of how different fibre and paper properties are correlated.

This thesis is the result of work jointly carried out at the Mid Sweden University in Sundsvall

and The Royal Institute of Technology in Stockholm. In Papers I and II, different aspects

regarding ink detachment and ink-cellulose interactions are considered. Papers III – VII focus

on fibre properties and the effect of different treatments, e.g. salt, pH and addition of strength

enhancing additives. Different methods to evaluate fibre properties are investigated, relating

the fibre properties to fibre/fibre joint strength and sheet strength.

Jennie Forsström, Stockholm, December 2004

ACKNOWLEDGEMENT

Throughout these four years I have worked together with a lot of skilled and inspiring people.

Together you have all made the atmosphere special and when I look back on this work I see

friends and colleagues more than scientific results. I would like to take this opportunity to

thank everyone who has crossed my path during this time.

I would especially like to thank my supervisor Lars Wågberg for introducing me to the

exciting world of pulp fibres, for all the guidance and support along the way, and for the

scientific discussions that really made me grow. Thanks are also due to Bo Andreasson and

Bengt Wikman at SCA research. You have both been excellent co-supervisors, but foremost

very good friends throughout the years.

I would also like to thank my colleagues and friends at the Department of Natural and

Environmental Science at Mid Sweden University, the Department of Fibre and Polymer

Technology at KTH, and SCA Research. Special thanks are due to Linda Gärdlund and

Annsofie Torgnysdotter for being excellent co-writers, colleagues and, last but not least,

really good friends. Malin Eriksson, Andrew Horvath and Shannon Notley are also thanked

for welcoming me to KTH. This thesis would not have been written without all the fun we

had at work and in our spare time. Andrew, you are also thanked for the linguistic revision of

this thesis. Anna Haeggström, Siw Stenlund and Brita Gidlund are thanked for taking care of

all the practicalities that needed to be arranged. To all of you not mentioned by name, thank

you for all your support, big and small favors, and for making my PhD time interesting and

fun.

The Fibre and Science Communication Network together with SCA Research AB are

gratefully acknowledged for financial support.

Finally, I would like to thank my family and all of my friends. They are the ones who have

been around for all the good and bad times. Samuel, I would neither have started nor finished

my PhD if it were not for you. You have always believed in me and encouraged me to do

more than I ever thought possible. You light up my days and you always make me feel good

about myself.

LIST OF PAPERS

This thesis is a summary of the following papers, which are referred to in the text by their

roman numerals. The papers are appended at the end of the thesis.

I Influence of different storage conditions on deinking efficiency of waterbased

flexographic ink from model cellulose surfaces and sheets. Forsström J. and Wågberg L. Nordic Pulp and Paper Research Journal, 2004, 19(2), 250-256.

II A new technique for evaluating ink-cellulose interactions: Initial studies of the

influence of surface energy and surface roughness. Forsström J., Eriksson M. and Wågberg L. Submitted to Journal of Adhesion Science and Technology.

III The porous structure of pulp fibres with different yields and its influence on

paper strength. Andreasson B., Forsström J. and Wågberg L. Cellulose, 2003, 10(2), 111-123.

IV Influence of polyelectrolyte complexes on the strength properties of papers from

unbleached kraft pulps with different yields. Gärdlund L., Forsström J., Andreasson B. And Wågberg L Accepted for publication in Nordic Pulp and Paper Research Journal.

V Determination of fibre pore structure: Influence of salt, pH and conventional wet

strength resins. Andreasson B., Forsström J. and Wågberg L. Accepted for publication in Cellulose.

VI Influence of pore structure and water retaining ability of fibres on the strength of

papers from unbleached kraft fibres. Forsström J., Andreasson B. and Wågberg L. Submitted to Nordic Pulp and Paper Research Journal.

VII Influence of fibre/fibre joint strength and fibre flexibility on the strength of

papers from unbleached kraft fibres. Forsström J., Torgnysdotter A. and Wågberg L. Submitted to Nordic Pulp and Paper Research Journal.

RELATED MATERIAL Forsström J. and Wågberg L. (2004): Aging of flexographic printed model cellulose surfaces and determination of the mechanisms behind aging. 7th Research Forum on Recycling, Quebec, QC, Canada, 21-25. Gärdlund L., Forsström J., Andreasson B. and Wågberg L. (2003): Influence of polyelectrolyte complexes on strength properties of papers made from unbleached chemical pulps. 5th International Paper and Coating Chemistry Symposium, Montreal, QC, Canada, 233-238.

11

TABLE OF CONTENTS

1 INTRODUCTION............................................................................................................... 13

1.1 Raw materials for papermaking ..................................................................................... 13

1.2 Deinking ......................................................................................................................... 14

1.3 The individual fibre ........................................................................................................ 22

1.4 Utilisation of fibres in the fibre network ........................................................................ 29

1.5 The effect of hornification on fibre and paper properties .............................................. 33

2 RESULTS AND DISCUSSION.......................................................................................... 35

2.1 Evaluating the use of the impinging jet cell setup for studying ink detachment from model cellulose surfaces (Papers I and II)............................................................................ 35

2.2 Influence of surface roughness and surface energy on ink detachment (Paper II)......... 39

2.3 Interfacial interactions between model substrates and ink – Influence of surface energy and surface roughness (Paper II). ......................................................................................... 41

2.4 Comparison between different methods to measure the water retaining ability of fibres (Papers III, V and VI)........................................................................................................... 45

2.5 Correlation between fibre and sheet properties (Papers III, VI and VII) ....................... 50

2.6 Influence of different strength enhancing additives on fibre and sheet properties (Papers IV and V)................................................................................................................. 55

3 CONCLUDING REMARKS.............................................................................................. 61

4 REFERENCES .................................................................................................................... 62

APPENDIX 1 .......................................................................................................................... 72

Equations relevant for calculating interfacial energies from contact angle measurements and the Lifshitz-van der Waals/acid-base approach............................................................. 72

APPENDIX 2 .......................................................................................................................... 73

Equations relevant to the JKR theory as applied in the thesis ............................................. 73

12

13

1 INTRODUCTION

1.1 Raw materials for papermaking

The basic constituent of paper is the individual fibre. The property of the individual fibre

depends on the wood raw material, the pulping process and the treatment history. The

treatment history of the fibre is important since recycled paper constitutes one third of the

fibrous raw material used in the paper making process today. For many years, the main part of

the collected wastepaper had been used for making unbleached packaging paper and

paperboard. However, a smaller but continuously increasing portion is now being treated for

the removal of contaminants, such as inks, stickies and resins, so that it can be used for tissue

products and higher grades of paper including printing and writing papers [1, 2]. Processing

recovered paper is a challenging task due to the wide variety of paper types, ink types and

printing techniques. Some types of papers are relatively easy to recycle, while others can

create problems. Recycling plants are expected to produce a pulp of consistently high quality

from a mixture of recovered papers, preferably using as low quality of the recycled raw

material as possible in order to lower the cost of the paper production. Unfortunately, this

ambitious goal is often difficult to achieve. When discussing the properties of a paper product,

it is necessary to bear in mind the properties of the individual fibre. Before entering the

interesting world of single fibre properties and their influence on the mechanics of the formed

paper network, different aspects regarding deinking, which is one of the most important steps

in the recycling process, will first be considered.

14

1.2 Deinking

Deinked fibres are a cost effective component of paper furnishes, traditionally being used

successfully in the manufacture of several paper grades, e.g. newsprint and tissue. The

development of new printing techniques and the complex ink formulations in use today have

made thorough ink removal a challenging task since non-dispersed ink particles remaining in

the deinked fibres appear as specs in the finished paper. This is unacceptable, especially for

printing and writing grades.

The efficiency of the deinking process depends on several factors, such as ink characteristics

and paper surface properties. Ink properties, e.g. pigment size and type of solvent, strongly

influence deinkability [3-7]. Paper and fibre surface properties are also important as they

affect the ease of ink detachment. Inks printed on coated surfaces detach easier than inks

printed directly on uncoated paper surfaces [8]. Printing conditions are other factors that

affect the deinking efficiency [5, 6, 8, 9]. The principles during recycling of paper have not

changed during the last 20 years. The stock preparation process for recovered paper

production generally consists of three main steps. The first step is re-pulping of paper, in

which the paper is disaggregated into individual fibres under severe agitation, moderately

high temperatures and fairly high pH. Secondly, different contaminants are removed after re-

pulping by means of conventional separation steps. Examples of contaminants are: metal

objects, adhesives and glues, printing ink, and other non-paper components. Separating

printing ink from fibres uses technical principles based on physical and chemical differences

between the materials, e.g. density, particle size and hydrophobicity. Bleaching is the third

step. Depending on the paper grade that is produced, bleaching is applied to various extents in

order to obtain the right optical properties of the pulp. The main focus in this part of the thesis

has been detachment of ink from the pulp.

1.2.1 Detachment and separation of ink from pulp

Detachment is one of the key steps in the deinking process. Efficient ink release from

cellulose fibres is essential to achieve selective ink removal and a high yield in the deinking

process. To fully understand the mechanisms of detachment, it is important to know the

chemical and mechanical properties of the paper surface, the chemistry of the ink, how the ink

was applied to the paper, printing techniques and printing conditions, and aging of the ink.

15

Mechanical, chemical and thermal forces, mainly during the pulping stage, are utilised to

detach ink from the fibres. The basic chemical detachment recipe used in the detachment

consists of alkali, surfactants and dispersing agents.

Alkali, primarily applied as sodium hydroxide, has two main effects. First, it is used to

increase the pH to promote fibre swelling. Fibre swelling loosens ink particles and coatings,

facilitating their detachment from cellulose. Second, sodium hydroxide promotes ester

hydrolysis (saponification) [5]. Excess alkali can lead to lignin yellowing by formation of

chromophores [10, 11]. Therefore, it is a careful balancing act keeping the alkalinity

sufficiently high for fibre swelling and good saponification, but not so high as to risk forming

chromophores. The use of sodium hydroxide in the reuse of wood containing fibres also leads

to delignification that can be compared to what is achieved in the cold soda process. This

decrease in lignin yield leads to an increase in paper strength for mechanical pulps.

Surfactants [5, 10, 11] are surface-active chemicals containing hydrophobic ends that

associate with ink, oil and dirt, and hydrophilic ends that remain in water, forming micelles

and different types of self assembled structures [12]. Surfactants can alter the wetting ability

of the cellulose surface, thereby promoting ink detachment. Some surfactants are used to wet

the released pigment by adsorbing at the ink-water interface, thus facilitating dispersion into

the water phase by lowering the surface tension.

Dispersants are chemicals whose function is to prevent re-agglomeration and re-deposition of

ink onto the fibres, so that the ink can be removed during a washing or thickening stage [13].

The ideal dispersant combines wetting, emulsification and dispersing features. Flotation

deinking uses surfactants to some extent to render larger ink particles. Calcium soaps of fatty

acids are the most commonly used surfactant system in the flotation deinking process.

Calcium soaps act as collectors and enhance ink removal by precipitating as particles on the

ink particles. Following this process, the ink particles become hydrophobic, agglomerate and

attach more efficiently to the air bubbles [10-14].

Once the ink particles have been detached from the fibres, they need to be separated from the

pulp suspension. This separation is highly dependent on the ink particle properties in the

liquid phase, e.g. size, surface activity, gravity, etc. [5, 15, 16]. The two most common

methods to separate detached ink particles from fibres are flotation and washing. Both are

16

carried out to scavenge and remove ink particles, although their operation principles are

entirely different [5]. The washing process removes ink particles smaller than 10 µm and

requires the ink particles to remain in the aqueous phase so that they can be removed along

with the water. Washing is accomplished by an aggressive multi-stage washing sequence in

which water is added and drained several times. The loss in yield associated with such

washing stages is often economically unacceptable. Froth flotation removes particles in the

size range 10 - 100 µm. The process relies on the capture of ink particles by air bubbles rising

to the surface, where a foam is formed that can be removed [17-19]. The optimum size range

for the different unit operations is schematically shown in Fig. 1.

Figure 1: The optimum size range for effective removal of ink particles [6].

1.2.2 Ink-cellulose interactions

With the rapid development of progressively accurate model surfaces for fibres, it is

becoming easier to study interfacial interactions between cellulose and other substrates.

Interactions between ink and cellulose, as illustrated in Fig. 2, are very important for ink

detachment.

Washing Flotation Cleaning Screening

Particle size (µm)

Rem

oval

effi

cien

cy (%

)

2 10 30 100 300

Washing Flotation Cleaning Screening

Particle size (µm)

Rem

oval

effi

cien

cy (%

)

2 10 30 100 300

17

Figure 2: Schematic representation of the mechanisms behind ink detachment from cellulose in water. Wcwi = total energy change associated with the separation of ink from cellulose in water.

Interactions between cellulose and ink can be described to be physical, purely adhesive or a

mixture of both. Ink detachment is rendered more difficult if, for example, the surface

roughness of the cellulosic substrate increases, creating a larger molecular contact area

between the two substrates. A larger molecular contact area increases the probability of

chemical reactions, such as oxidative reactions, between the cellulose substrate and the ink.

Ink detachment is also affected if the interfacial tension between ink and cellulose changes,

which is the case with, for example, coated paper compared to conventional paper. The total

energy change, cwiW , associated with the separation of cellulose (c) and ink (i) in water (w), as

illustrated in Fig. 2, can thermodynamically be described by:

( )iwcwwciciiwcwiwcwwwcicwi CosCosWWWWWW Θ+Θ−=−+=−−+= γγγγ (1)

where W represents the various work of adhesion or cohesion, γ is the interfacial or surface

energy and Θ is the contact angle between water and the respective components. If the work

of adhesion and cohesion are the only determining factors for ink release, cwiW < 0 would

indicate spontaneous ink release from cellulose. From Eq. (1), it becomes obvious that Wcwi

can be calculated from interfacial energies (γ12) between any two materials (1 and 2). These

interfacial energies can be determined from contact angle measurements using three reference

liquids with known surface properties. The dispersive and polar parts of the surface energy are

then calculated by applying the Lifshitz-van der Waals/acid-base approach [20, 21], see

Appendix 1 for equations. The total energy change can also be calculated from the work of

adhesion between ink and cellulose combined with contact angles of water on cellulose and

• Physical interaction (e.g. mechanical interlocking)

• Adhesive interaction (specific, e.g. hydrogen bonding, or non-specific, e.g van der Waalsinteractions)

Cellulose

Ink

Detachment

• Spontaneous ink release (Wcwi < 0 )

• Physical interaction (e.g. mechanical interlocking)

• Adhesive interaction (specific, e.g. hydrogen bonding, or non-specific, e.g van der Waalsinteractions)

Cellulose

Ink

Detachment

• Physical interaction (e.g. mechanical interlocking)

• Adhesive interaction (specific, e.g. hydrogen bonding, or non-specific, e.g van der Waalsinteractions)

• Physical interaction (e.g. mechanical interlocking)

• Adhesive interaction (specific, e.g. hydrogen bonding, or non-specific, e.g van der Waalsinteractions)

Cellulose

Ink

Detachment

Cellulose

Ink

Detachment

• Spontaneous ink release (Wcwi < 0 )

18

ink. Today’s existing experimental techniques make it possible to measure the adhesive

properties between two materials using the JKR methodology, outlined by Chaudhury and

Whitesides [22], applied to cellulose, by Rundlöf et al. [23].

In the JKR type of experiments, two surfaces are brought into contact and pulled apart again.

At least one of the surfaces must be elastic, such that the elastic surface deforms upon contact.

The area between the surfaces increases due to the adhesive interaction between the

interacting bodies. In a typical experiment, the surfaces are slowly brought together and the

changes in area are recorded as a function of applied load (F). This is realised in a so-called

micro adhesion measurement apparatus (MAMA), seen in Fig. 3. An optical microscope,

connected to a computer via a CCD camera, monitors the area change and the applied load is

recorded using an analytical balance.

Figure 3: The experimental set up of a load-controlled MAMA equipment. A magnification of the contact area is also shown to the right [24].

The JKR theory of contact mechanics relates the radius of the deformed zone to the Dupré

work of adhesion and the applied load, see Appendix 2 for the theory. Any further

information regarding the JKR methodology when measuring molecular adhesion can be

found elsewhere and is not be described here [22, 24-26].

An extremely smooth surface, required to ensure molecular contact, and a well-defined

surface, in terms of chemical compositions, are necessary for determining the thermodynamic

work of adhesion using the MAMA. The most commonly used surfaces are silica wafers,

mica and glass [22, 26-30]. Modifications of these surfaces have been numerous, e.g. Rundlöf

2a

F

19

et al. [23, 24] coated a mica surface with cellulose. Crosslinked PDMS (poly - dimethyl-

siloxane) caps, developed by Chaudhury and Whitesides [22], are the most commonly used

hemispheres. Recently, modified caps have been developed [31] in which the cap is coated

with a polymer. This gives an unique possibility to study the adhesive properties between

almost any two substances as long as they both fulfil requirements such as smoothness,

elasticity, cleanliness, etc.

1.2.3 Influence of aging

Ink detachment and separation become more difficult if the printed furnishes are stored for a

period of time before being recycled. Different furnishes inevitably undergo a natural aging

process during storage that accelerates if the furnishes are exposed to UV-light or elevated

temperatures. Deinking of aged paper has been reported to be difficult for various reasons.

Aging differs between paper qualities and it uniquely affects deinkability for each ink type.

For example, aging is more prominent with newspaper quality paper than it is with magazine

quality, due to the low mechanical pulp content and the additional protection provided by the

coated surface in the latter type of furnish [32]. Furnish from offset printed newsprint

decreases in brightness after aging [8], whereas xerographic printed waste paper is unaffected

by natural aging [33]. Deinkability of aged offset-printed newspaper has been investigated in

numerous studies, with the outcome seeming to be that deinkability of offset printed

newspaper decreases rapidly with the age of the print and that artificially aged samples are

extremely difficult to deink [32, 34-36].

The poor deinkability of aged offset ink has been explained to be due to an oxidative process

by which the chemical interaction between ink and paper increases, negatively affecting the

deinkability [32, 35-38]. The presence of, for example, aldehyde resins in the ink causes

crosslinking that with time might induce covalent bonds between ink and cellulose via

oxidative polymerisation [39]. The presence of acidic groups, both in the fibres and in the ink

system might increase the chances of forming effective hydrogen bonds and acid-base

interactions between inks and fibres [32]. The existence of these types of interactions between

the crosslinked ink and the cellulose substrate are therefore of utmost importance when

developing practical strategies for successful deinking of aged prints.

20

Aging of flexographic ink has not been that extensively studied. Sain and Daneault [40] aged

flexographic ink and binders at 105°C, finding that the presence of univalent metal ions in

sulphopolyester-based flexographic ink inhibits aging (crosslinking) of acrylic binders. The

fact that several flexographic inks contain polystyrene acrylic compounds has a large impact

on their deinkability, as it has been shown that poly(butyl methacrylate) latex films [41] and

polystyrene latex films [42] subjected to temperatures 20 to 30°C above their glass transition

temperature exhibit polymer diffusion across the inter-particle interface that anneals the films.

If polystyrene acrylic-based flexographic printed papers are subjected to elevated

temperatures and/or UV-light, annealing of the latex might occur and this will obviously

reduce the deinkability.

1.2.4 The need of model systems in deinking studies

Deinking studies are challenging due to the mixture of different fibres, additives and seasonal

variations of the raw material existing in secondary fibres. In order to develop the most

efficient deinking system, it is important to find a reliable model system that represents each

part in the deinking process and is well controlled without any unwanted or unknown

variations.

Numerous model systems, regarding both cellulose surfaces [43-47] and ink removal studies

[18, 35, 48], have been developed over the years. Several different model surfaces for

cellulose exist, although very few were actually used for deinking studies [2, 44]. Rao and

Stenius [44] used cellophane and compared the results with deinking from sheets. Andreasson

and Wågberg [2] employed the model cellulose surface developed by Gunnars et al. [45] to

study ink detachment of flexographic ink and offset ink. Ink detachment and separation

studies have usually involved the use of a flotation or washing system. These systems have

lacked in their ability to study the actual molecular mechanisms behind ink detachment. In

order to identify the molecular mechanisms responsible for ink detachment, other techniques

such as the Impinging jet cell, seen in Fig. 4, and 1H-NMR imaging have therefore been

developed.

21

Figure 4: Schematic description of the Impinging jet cell equipment [2]. A magnification of the Impinging jet cell is also shown.

The Impinging jet cell technique rapidly evaluates different deinking systems. With this

technique printed model surfaces are mounted in the liquid-filled Impinging jet cell and

impinged with release chemicals. The ink detachment process is then monitored with a

microscope equipped with a CCD-camera. 1H-NMR imaging provides a rapid screening tool

for predicting the effectiveness of deinking surfactants and ink detachment in situ [49].

However, NMR imaging equipment is expensive and still rather complicated for effective use.

Printed surface

Release chemicals

Printed surface

Release chemicals

22

1.3 The individual fibre

1.3.1 Fibre structure

The cell wall of a softwood fibre is considered as a gel composed of cellulose fibrils,

hemicellulose and lignin [50]. These three wood components build up several concentric

lamellae surrounding the lumen, shown in Fig. 5. The layers are mainly distinguished by the

different orientation of the fibrils.

Figure 5: Schematic illustration of a typical softwood fibre. M = middle lamella, P = primary layer, S = secondary layer and L = lumen [51].

Cellulose, hemicellulose and lignin are distributed differently in the cell wall and play

different roles in the structure of the native softwood fibre. Cellulose molecules are ordered

into fibril aggregates, whose different orientation gives the fibre its tensile stiffness, strength

and flexibility. The role of lignin is to support the slender cellulose fibrils and prevent them

from buckling, thereby giving the fibre high compressive stiffness and strength. In a

simplified way, lignin can be thought of as the glue in the fibre matrix. The function of

hemicellulose is less evident, although it has been suggested that hemicellulose serves as a

coupling agent or as an intermediate between the cellulose and the lignin [52].

M (0,1 – 1µm)P (0,1 – 0,3 µm)

S1 (0,1 – 0,2 µm)

S2 (1 – 5 µm)S3 (0,1 µm)

L

M (0,1 – 1µm)P (0,1 – 0,3 µm)

S1 (0,1 – 0,2 µm)

S2 (1 – 5 µm)S3 (0,1 µm)

L

23

1.3.2 Imbibition of water – swelling

During pulping, a highly porous fibrillar structure of the fibre wall, shown in Fig. 6, is created

as chemical and mechanical treatments constantly modify the gel and remove material such as

lignin and hemicellulose from the different layers within the fibre cell wall [53].

Figure 6: High resolution-cryo-field emission-scanning electron microscopy micrograph illustrating the surface ultrastructure of a frozen hydrated kraft pulp fibre subjected to deep freezing [54].

The void volume in the fibre wall of native fibres is around 0.2 cm3/g. As the fibres are

liberated from wood by chemical treatment, the void volume at a pulp yield of 47 % (kraft

pulping) increases to around 0.6 cm3/g, as determined by nitrogen adsorption of solvent

exchanged pulps [55]. The void volume naturally depends on the degree of delignifiction.

Specifically, removing lignin increases the void volume while decreasing the pulp yield. A

nanoporous structure is created because imbibition of water into the fibres de-bonds and

separates the lamellar structure in the fibre wall. In this respect, the macroscopic softness, i.e.

the transverse modulus, of the fibre is affected, having a profound effect on the ability of the

fibres to form strong fibre/fibre joints during pressing and drying of the paper [53]. A soft

fibre wall, i.e. a fibre wall with a low transverse modulus, also allows for a better contact

between the fibres since low modulus fibres yield to a larger extent under the influence of

capillary forces exerted during drying and consolidation. As seen in Fig. 7, the macroscopic

softness of a never dried fibre decreases with decreasing pulp yield [56, 57]. The decrease is

pronounced for high yield pulps.

24

Never-driedDried & rewet

12

10

8

6

4

2

0100 80 60 40

PULP YIELD, %

ELA

STI

C M

OD

ULU

S, M

Pa

Never-driedDried & rewet

12

10

8

6

4

2

0100 80 60 40

PULP YIELD, %

ELA

STI

C M

OD

ULU

S, M

Pa

Never-driedDried & rewet

12

10

8

6

4

2

0100 80 60 40

PULP YIELD, %

ELA

STI

C M

OD

ULU

S, M

Pa

Figure 7. The elastic modulus of a never-dried and once-dried and rewetted kraft pulp with different yield [56].

Fibre wall expansion during water imbibition also has a direct impact on the flexibility of the

fibres [58-61]. Flexibility is very important for the crowding factor of the fibres [62-64]

during paper formation and for the fibres ability to form contacts with other fibres, which is

essential for the possibility to form efficient fibre/fibre joints during drying [65]. The

imbibition of water, i.e. swelling, has been extensively studied [50, 66-76], sharing in

common that the swelling ability varies with pulp yield, as shown in Fig. 8.

Figure 8: Fibre saturation point, a measure of the fibres swelling ability, as a function of yield for defibrated suphite pulp and kraft pulp [70].

0.4

1.2

1.6

2.0

100 80 60 40

Yield, %

Fibre Saturation

Pointml./g.

Sulphite

Kraft

0.4

1.2

1.6

2.0

100 80 60 40

Yield, %

Fibre Saturation

Pointml./g.

0.4

1.2

1.6

2.0

100 80 60 400.4

1.2

1.6

2.0

100 80 60 40100 80 60 40100 80 60 40

Yield, %

Fibre Saturation

Pointml./g.

Sulphite

Kraft

25

As the pulp yield first decreases from 100 %, the swelling ability, measured as the fibre

saturation point, increases until it reaches a maximum. At a critical yield, which differs

between pulps and chemical environment, the swelling diminishes as the pulp yield is further

decreased. The maximum occurs in swelling behaviour, and also the porous structure of

fibres, due to the fact that removal of lignin and hemicellulose not only creates a nanoporous

structure but also lowers the amount of ionisable groups in the fibre wall. Hence, two

counteracting forces affect the swelling ability; the charge density, creating osmotic pressure

and electrostatic repulsion, and the 3-dimensional fibrillar structure, restricting expansion of

the network.

1.3.3 The effect of pH, electrolytes and counter-ion valency on the swelling of

chemical pulps

The degree of swelling is not only a function of the number of ionisable groups within the gel,

but also of the degree to which they are dissociated. Dissociated acidic groups in the fibre, as

is the case when pH is clearly above the average pKa-value for the charged groups, create an

electrostatic repulsion that acts to expand the fibre wall. If the acidic groups are protonated, as

is the case at pH values clearly below the pKa-value, less electrostatic repulsion is available

and the only thing, apart from hydration forces, that strives to maintain an open porous

structure is the presence of a matrix material such as lignin [77]. An alternative approach is to

consider the swelling behaviour in terms of a Donnan equilibrium, see Fig. 9.

Figure 9: Schematic drawing of a fibre wall, gel phase, in contact with an external solution. Bound anionic charges and mobile anionic charges are shown.

Cl-

Na+

H+

OH-

COO-

COOH

Gel phase Solution phase

Virtual membrane

Cl-

Na+

H+

OH-

COO-

COOH

Cl-

Na+

H+

OH-

COO-

COOH

Gel phase Solution phase

Virtual membrane

26

The Donnan equilibrium is based on concentration gradients, which give rise to an osmotic

pressure between the gel phase and the external solution. Bound charged groups inside the gel

that are fully dissociated induce a higher concentration of mobile ions inside the fibre wall

than in the external solution, giving rise to an osmotic pressure that in turn causes the fibre

wall to swell.

Both approaches describe the swelling of the fibre and how it is affected by the valency of the

counter-ion, pH inside and outside the gel, and the presence of electrolytes. Electrolytes and

pH have a large effect on the swelling of different fibres [78-82]. It can be seen in Fig. 10 that

the swelling ability in deionised water is highly affected by pH. Increasing the salt

concentration stabilises the swelling behaviour over a wider pH range. It can also be seen that

further increasing the salt concentration, to levels above 0.01 M NaCl, decreases the swelling

ability.

Figure 10: Theoretic summary of E-values (the excess concentration of mobile ions within the gel, taken as being proportional to the degree of swelling) as a function of pH and external salt concentration for a gel containing weak acidic groups [78].

This behaviour eminates from the fact that in deionised water the proton concentration inside

the fibre wall is higher than the concentration outside the fibre wall. Hence, the swelling is

extremely low at a solution pH below 9 due to fully associated acidic groups or, as described

by the Donnan equilibrium, due to a low concentration of mobile ions inside the fibre wall. In

the presence of low salt concentrations the proton concentration inside the fibre wall equals

the concentration outside the fibre wall and the swelling ability is almost as great as when no

0

0,02

0,04

0,06

0,08

0,1

0 2 4 6 8 10 12 14

Water

0.001M NaCl

0.01M NaCl

0.033M NaCl

0.14M NaCl

SOLUTION pH

E, e

quiv

alen

ts/lit

re

0

0,02

0,04

0,06

0,08

0,1

0 2 4 6 8 10 12 14

Water

0.001M NaCl

0.01M NaCl

0.033M NaCl

0.14M NaCl

0

0,02

0,04

0,06

0,08

0,1

0 2 4 6 8 10 12 14

0

0,02

0,04

0,06

0,08

0,1

0 2 4 6 8 10 12 14

0

0,02

0,04

0,06

0,08

0,1

0 2 4 6 8 10 12 14

Water

0.001M NaCl

0.01M NaCl

0.033M NaCl

0.14M NaCl

SOLUTION pH

E, e

quiv

alen

ts/lit

re

27

salt is present. If more salt is added, the swelling ability decreases as the repulsive forces in

the fibre wall between the charged acidic groups become shielded.

Differences in counter-ion are also important for the swelling effect of fibres [79, 81].

Typically, the swelling ability increases in the following order [79]:

Al3+ < H+ < Mg2+ < Ca2+ < Li+ < Na+

The difference in swelling ability due to ionic form can be traced back to differences in the

degree of dissociation of carboxyl groups and the valency of the counter-ions [79]. The degree

of dissociation decreases at higher valency. Although the molecular origin to this is not clear,

it has been detected as a lower activity coefficient of the counter-ion [83] which could be due

to condensation of the counter-ion onto the carboxyl groups.

1.3.4 Measuring fibre properties

The pore structure and swelling ability of the fibre cell wall has received a good deal of

attention in recent years. The approach one uses to characterize a particular porous system

depends on the type of material, the available analytical techniques and the information most

relevant to the given situation [66, 84]. Over the years, several different methods have been

developed to study the porous structure of fibres and the fibres swelling ability. A completely

reliable method is not yet available for analysing the porous structure of swollen fibres and

various techniques measure differing pore sizes in the fibre wall. A measure that is often used

to characterise fibre swelling is the water retention value (WRV) [85]. In the WRV test, a

pulp pad is centrifuged under conditions that are assumed to remove water between the fibres

and in the fibre lumen. The moisture content of the pad after centrifuging is a measure of the

fibre swelling. The WRV is an empirical test that depends on the specific test conditions [86].

The swelling behaviour is determined by calculating the amount of water in the fibre wall of

the pulp from the WRV [87]. Solute exclusion chromatography (SEC) has shown to correlate

well with the WRV [88, 89]. The underlying principle is to mix wet pulp fibres with a non-

interacting probe solution so that the change in probe solution concentration can be used to

calculate the inaccessible amount of water [90, 91]. It is possible with the SEC to get

information about the apparent pore size distribution, the average pore radius and the fibre

saturation point (FSP). Using pulp as the stationary phase, as with the inverse size exclusion

28

chromatography (ISEC), the pore volume of the fibres can be obtained by measuring the

difference of the “dead” volume in a column for probe molecules having different radii of

gyration [92]. As pointed out by Lindström [66, 74], it is not possible to determine the pore

size distribution with SEC or ISEC without knowing the geometry of the pores even if the

fibre wall is fully accessible to all molecules. The limiting size in SEC and ISEC is the

openings in the fibre wall, meaning that the volume within the wall is ascribed the smaller

pore radius in the external layers of the fibre wall if the pores within the fibre wall are larger

than the opening in the external fibre wall layer.

The NMR relaxation method has been shown to be a useful method for determining pore size

distributions and the average pore radius in various materials [93-95]. The basis of NMR

relaxation is that the molecular dynamics of the liquid molecules near the surface differ from

that of the bulk liquid. Measurements of the relaxations times, T1 and T2, for molecules in the

pores determine the pore size distribution since they are directly related to the pore volume to

surface area ratio, according to the two-fraction fast exchange model [94, 96, 97]. In this

model, it is assumed that the exchange of probe liquid molecules between bound sites and free

sites is fast, such that the relaxation profile is exponential. A saturated sample with a discrete

pore size distribution should therefore give rise to a distribution of NMR relaxation times.

The pore size distribution determined with this technique naturally depends on the

geometrical model chosen for the pores in the fibre [93].

29

1.4 Utilisation of fibres in the fibre network

Inducing fibre swelling either mechanically, i.e. by beating, or chemically, i.e. by changing

the carboxyl groups counter-ion, impacts the transverse modulus of the fibres and hence the

flexibility. The fibres ability to conform towards each other, forming fibre/fibre joints during

sheet consolidation, and the utilised sheet properties are therefore affected. It has been shown

that the swelling ability, as determined by FSP, is highly correlated to the breaking length [79]

and the tensile strength of the obtained paper [79, 82, 98, 99]. It has also been shown that the

location of the charges in the fibre wall greatly influences paper strength properties. Charges

located at the fibre surface have a larger impact on paper strength properties than bulk charges

[100-103], as a highly swollen and soft external fibre surface allows for a better mixing of

fibrils on the surface of adjacent fibres during drying and consolidation. This results in a

much stronger joint between the fibres because the mixing of fibrils creates a fibril reinforced

pseudo-composite in the joining zone between two fibres.

It has been debated for at long time whether the joint strength between the fibres and the

surrounding matrix or the single fibre strength itself is most important for the final strength

properties of the paper product [104-106]. It is obvious that these two entities are responsible

for different strength development in the formed paper network [105, 107]. Fundamental

knowledge has been collected around the impact of single fibre strength and the fibre/fibre

joint strength on the obtained paper properties [104-106, 108-111]. Unfortunately, there are

still no methods available to quantify the relative importance of these two important entities.

There is no doubt that both parameters are important and it is also clear that they are

interdependent, i.e. it is not possible to utilise the fibre strength unless the joint strength is

sufficiently high.

1.4.1 The fibre/fibre joint

Fibre/fibre joints develop during the consolidation and drying process as highly swollen fibre

surfaces are pushed together by the capillary forces formed between fibres during water

removal. Fibre/fibre joint formation is highly influenced by fibre flexibility. Specifically, a

more flexible fibre enables more fibre/fibre joints both in the pulp suspension and in the

finished paper [100, 112]. A more flexible fibre also produces a denser sheet, although the

enabling of more fibre/fibre joints and a denser sheet does not necessarily give stronger paper.

30

The strength of the single fibre and the strength of the created fibre/fibre joint also influences

the sheet properties [100]. Fig. 11 seeks to illustrate a cross-section of a fibre/fibre joint. The

molecular contact area in the contact zone, the number of contact points in the formed joint,

the intermolecular forces, mechanical entanglement and covalent linkages are the most

important factors that determine the strength of the bonded joint.

Figure 11: Illustration of a fibre/fibre joint. A magnification of the contact zone between two fibres is also shown.

The fibre/fibre joint strength is usually evaluated for paper by application of Page’s equation

[106], where the so-called relative bonded area (RBA) is estimated by measuring the light

scattering of papers made from the same type of fibres and pressed to different densities. As

this method only provides an estimation, more direct methods to measure the joint strength

are needed. This is a challenging and difficult task, and very few studies exist that actually

measure the strength of a single fibre/fibre joint. Davison [104] measured the fibre/fibre joint

strength by pulling single fibres out from a sheet. Stratton and Colson [113] and

Torgnysdotter and Wågberg [100] measured the joint strength directly in a perpendicular

cross between two single fibres, correlating the joint strength to the strength of the formed

paper.

1.4.2 Addition of wet and dry strength additives

As previously stated, the main constituents to paper strength are the fibre/fibre joint strength

and the strength of the individual fibre. In certain applications, though, it is desirable to

increase the fibre network strength even further. This is of utmost importance for recycled

fibres. Refining the fibres is one way of increasing the papers strength properties, as shown by

Page [114]. Refining increases flexibility, activates the fibre wall, creates a porous fibre wall,

Contact zoneContact zone

31

straightens the fibres and produces fines [114]. All the above factors permit a larger molecular

contact area and help create a stronger joint. However, refining of fibres also gives a higher

swelling ability [72, 90], hence the dewatering on the paper machine becomes more difficult.

Another way to improve strength properties is to add different wet and dry strength additives.

Wågberg [115] has given an extensive overview of the mechanisms behind the action of dry

and wet strength additives, and therefore only a short summary shall be given here.

The creation of wet and dry strength paper can be achieved by: (1) strengthening the fibre

wall, as achieved by crosslinking, (2) creating new bonds in the fibre/fibre joint or (3)

increasing the strength of already existing bonds. Each approach is based on different

molecular mechanisms. When adding different wet and dry strength additives, it is obvious

that both the porosity of the fibre wall and the amount of charged groups of the fibre material

are of importance [116]. The porosity of the fibre wall influences the amount of additive that

adsorbs to the pulp [71, 117]. More additives adsorb to a highly porous pulp than to a low

porosity pulp, provided the molecular mass of the additive is low enough to enter the pores

within the fibre wall. Together with the ionic strength of the solution, the charge of both the

fibres and the additive also control the amount of additive adsorbed [116].

To create paper with high relative wet strength, it is necessary to either crosslink the fibre

wall, creating an insoluble network around and through the fibre/fibre contacts, or create

covalent bonds between the additive and the cellulose [107, 118-125]. In order to crosslink

the fibre wall, the additives need to be small enough to enter the pores of the fibre wall, as is

the case with butane tetra carboxylic acid (BTCA) [117].

The mode of action is not fully understood for dry strength additives. It has been suggested

that the weak link in paper strength is the fibre/fibre joint [104]. In order to improve paper

strength, the already existing joints must be reinforced [104, 126, 127] or new joints must be

created [124].

Instead of using wet and dry strength additives independently close to the paper formation, it

is possible to pre-treat the fibres with polyelectrolyte multilayers or pre-formed

polyelectrolyte complexes. Multilayers are built-up by consecutively treating a fibre surface

with alternating additions of cationic and anionic polyelectrolytes [128]. Wågberg et al. [129]

reported that the tensile index increased more than 100 % for fibres treated with layer of

32

cationic polyallylamine hydrchloride (PAH) and anionic polyacrylic acid (PAA) before sheet

preparation. They also showed that the multilayer technique alters the surface properties of

wood pulp and that the sheet properties vary depending on which polymer is fixed to the outer

layer [129]. Another approach is to pre-form polyelectrolyte complexes before addition to the

pulp [130]. This has proven to be a very useful technique to enhance paper strength properties

[131]. Polyelectrolyte complexes are in some cases easier to use since both polyelectrolytes

are added at the same time. The size and charge distribution of the complexes can be readily

controlled and the constituents can be varied to ensure the desired properties.

33

1.5 The effect of hornification on fibre and paper properties

The fibre is exposed to a drying and rewetting cycle during recycling, altering the fibre

properties as well as the properties of the obtained paper. Drying causes hornification and is

associated with a reduction in the swelling ability of the fibre and a stiffening of the fibres,

thus reducing their ability to form a high molecular contact area in the fibre/fibre joints [132].

Other physical properties of the fibre, e.g. E-modulus, wet fibre flexibility and fibre average

length, are also affected upon drying [56, 133, 134]. Hornification is mainly a feature of low

yield pulps, as shown in Fig. 12. It is primarily brought about by the removal of water from

fibre walls, rather than any associated heat treatment [132].

Figure 12: The levels of swelling, measured as fibre saturation point, for never-dried and once-dried and rewetted kraft pulps with different yields [132].

It has been proposed that hornification results from an increased degree of crosslinking

between fibrils, due to the formation of hydrogen bonds upon drying. The presence of lignin

and hemicelluloses between the fibrils in high yield pulps prevent this bonding. The absence

of these materials in low yield pulps permits hornification to occur. During complete drying

of a low yield pulp, hydrogen bonds are formed that cannot be broken upon rewetting. Hence,

after drying and rewetting the fibre is less swollen and contains smaller pores than that of

never-dried fibres [132]. It has been suggested that the pores on the outer surface of the fibres

are irreversibly closed since water is first removed from the surface of the fibres, whereas

pores in the interior of the fibre could be re-opened upon re-slushing [135].

0.4

1.2

1.6

100 80 60 40

PULP YIELD %

FIBR

E SA

TUR

ATIO

N P

OIN

T. g

/g

0.8

Never-dried

Dried & rewet0.4

1.2

1.6

100 80 60 40

PULP YIELD %

FIBR

E SA

TUR

ATIO

N P

OIN

T. g

/g

0.8

Never-dried

Dried & rewet

34

Not only do the physical properties, such as swelling ability and porosity, of the fibres change

upon hornification, but the properties of the formed network are also affected. It has, for

example, been shown that recycling of chemical fibres results in a paper with lower strength

properties, lower density, etc. [133, 136-140]. Recycled mechanical fibres, on the other hand,

give a paper with improved tensile properties [134, 141, 142].

As hornification of fibres occurs, finding ways to recover the papermaking potential of the

fibres is necessary. This can be achieved mechanically by, for example, beating or chemically

by, for example, carboxymethylation [132]. Beating reverses hornification to some extent, but

it also creates dewatering problems and increased consumption of paper chemicals [132, 143].

Chemical treatments such as carboxymethylation have proven successful, but they are too

expensive and not of practical use [132] with today’s techniques. In the present thesis, only

fundamental aspects regarding fibre properties before and after drying are investigated.

Beating or carboxymethylation are not considered.

35

2 RESULTS AND DISCUSSION

2.1 Evaluating the use of the impinging jet cell setup for studying ink

detachment from model cellulose surfaces (Papers I and II).

Studies of the deinking process are challenging. In order to develop the most efficient

deinking system, it is important to study in-detail each part of the deinking process to find the

critical factors in each step. The Impinging jet cell setup has previously been used to study ink

detachment from model cellulose surfaces [2]. In this study, ink detachment of flexographic

ink from printed model cellulose surfaces, stored under different conditions, has been

investigated. Ink removal from sheets is compared to model cellulose surfaces, printed and

stored under the same conditions. Ink detachment from model cellulose surfaces is plotted

against storage time in Fig. 13.

Figure 13: The influence of different storage conditions on ink removal from model cellulose surfaces. Surfaces stored at different temperatures were stored under dark conditions. The lines are merely a guideline to the eye.

Storage under UV-light, elevated storage temperatures and longer storage times makes ink

detachment efficiency more difficult. These results are intriguing and important, as aging of

flexographic ink has not been extensively studied, although they are of little use unless they

can be correlated to ink detachment from sheets. The results from ink detachment from fibres

are shown in Fig. 14.

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14 16

Storage time (days)

Ink

deta

chm

ent

(100

- R

esid

ual i

nk (%

))

UV15°C25°C55°C

36

Figure 14: The effect of storage time, UV-light and temperature on the brightness value of sheets made from reslushed, deinked printed sheets. Sheets stored at different temperatures were stored under dark conditions.

It is obvious from Figs. 13 and 14 that elevated temperatures, UV-light, and storage time

negatively affect ink detachment from both model cellulose surfaces and sheets, although the

effects are slightly different. Ink detachment is hardly possible for printed model cellulose

surfaces stored under UV-light or at a storage temperature of 55°C. For printed sheets, UV-

light and elevated temperatures decrease ink detachment, detected as lower sheet brightness,

although not to the same extent as for the model cellulose surfaces. This difference can be

traced back to the different physical structure of the surfaces and also to the differences in ink

removal process. Model cellulose surfaces have been deinked using the Impinging jet cell

equipment, shown in Fig. 4, where the printed cellulose film is exposed to a stream of alkaline

water at almost no shear. Furthermore, the stream always hits the surface at the same place at

the same impact angel. The sheets, on the other hand, are reslushed and hyper-washed, which

means that each fibre is exposed to a high shear and the alkaline water hits the fibre from

numerous angles.

The results shown in Figs. 13 and 14 indicate that different storage conditions affect ink

detachment. It has previously been shown that deinking efficiency of offset ink is affected by

storage conditions [32] and that the ink-cellulose interactions change as covalent bonds

become possible between the carboxylic groups present in the ink and the fibre [32, 35, 38]. It

is therefore important to investigate if the detected changes in ink detachment from the flexo

printed cellulose surfaces could be traced back to any chemical and/or physical changes in the

40

50

60

70

80

90

100

0 1 2 3 4 5Storage time (days)

Brig

htne

ss (%

)

ReferenceUV25°C55°C85°C105°C

37

ink and cellulose. In Fig. 15, ATR-FTIR spectra for non-treated and heat-treated cellulose are

shown.

Figure 15: ATR-FTIR spectra of reference cellulose and heat-treated (105°C) cellulose.

No chemical changes can be seen in the ATR-FTIR spectra after storing cellulose at elevated

temperatures. Physical changes in cellulose, due to an elevated temperature of the cellulose,

have previously been shown by Notley (personal communication). No chemical changes are

detected when storing cellulose surfaces under UV-light. The surface roughness decreases

upon heat treating the cellulose. ATR-FTIR spectra for ink varnish (non-treated and heat-

treated) are shown in Fig. 16.

Figure 16: ATR-FTIR spectra of the reference ink varnish and heat-treated (105°C) ink varnish.

0

0,05

0,1

0,15

0,2

80012001600200024002800320036004000

Cellulose 105°C treated Cellulose

Abs

orba

nce

Wavelength (cm-1)

0

0,1

0,2

0,3

0,4

0,5

0,6

80012001600200024002800320036004000

Reference Varnish105°C Treated Varnish

Abs

orba

nce

Wavelength (cm-1)

38

Upon heat treating the ink, a peak occurred at approximately 1710 cm-1 in the spectra,

indicating that hydrolysis or an oxidative reaction in the ink has taken place. No chemical

changes are detected when storing the ink varnish under UV-light. Due to the lack of

chemical information regarding the ink composition, final conclusions cannot be drawn

regarding the chemical reactions that occur in the films of ink and cellulose. In Fig. 17, AFM

phase images are shown of non-treated and heat-treated ink varnish spin-coated onto silicon

wafers.

Figure 17: Phase images (1×1 µm) in tapping mode of non-treated (left image) and heat-treated (right image) ink varnish spin coated onto silicon wafers (unpublished data).

In non-treated ink varnish, spherical structures can be observed that most likely are latex

particles present in the ink. The spherical structures disappear upon heat-treating the ink

varnish, most likely due to an annealing that consequently forms a film of the latex binder.

39

2.2 Influence of surface roughness and surface energy on ink detachment

(Paper II)

Preparing cellulose surfaces from solutions of different concentrations influences the surface

roughness of the model cellulose surface. The out-of-plane surface roughness of the cellulose

surfaces has been studied with AFM height images. In Fig. 18, the effect of surface roughness

on ink detachment is shown.

Figure 18: The effect of surface roughness on ink detachment efficiency from model cellulose surfaces. The line is merely a guideline to the eye.

It is obvious that ink detachment is more difficult from a rough surface. Increasing the out-of-

plane roughness from 3 nm to 13 nm lowers ink detachment approximately 33 %. Cellulose

surfaces with a roughness of 4 nm and 10 nm are shown in Fig. 19.

Figure 19: Height images (1×1 µm) in tapping mode of model cellulose surfaces with surface roughness of 4 nm (left image) and 10 nm (right image), Z-scale is 50 nm is both images.

0

20

40

60

80

100

120

0 2 4 6 8 10 12 14Surface roughness (nm)

Ink

deta

chm

ent

(100

- R

esid

ual i

nk (%

))

40

As can be seen when comparing the images, increasing the surface roughness creates larger

holes in the cellulose surface. The images in Figs. 17 and 19 show that the spherical latex

particles are in a size range that fit into the holes present in the cellulose surface. Rougher

cellulose surfaces have larger holes; hence, it seems safe to conclude that a larger molecular

contact area between ink and cellulose can explain the decrease in ink detachment. A larger

molecular contact area facilitates molecular adhesion between ink and cellulose, inter-

diffusion of chemical components across the interface, physical entanglement, and increasing

the development of chemical bonding across the interface.

The surface energy of the model cellulose surfaces has also been changed before printing and

ink detachment studies. As seen in Fig. 20, making the cellulose surface more hydrophobic

significantly lowers ink detachment. The deinking efficiency decreases almost 65 % at a

surface roughness of 4 nm.

Figure 20: The effect of surface roughness on ink detachment efficiency from model cellulose surfaces. The lines are merely a guideline to the eye.

Increasing the surface roughness also lowers the ink detachment of the hydrophobic cellulose

surfaces. Determining the mechanisms behind the decrease in ink detachment for the

hydrophobic cellulose surface necessitates the study of interfacial interactions between

cellulose and ink.

0

20

40

60

80

100

120

0 2 4 6 8 10 12 14Surface roughness (nm)

Ink

deta

chm

ent

(100

- R

esid

ual i

nk (%

))

CelluloseHydrophobic cellulose

41

2.3 Interfacial interactions between model substrates and ink – Influence of

surface energy and surface roughness (Paper II).

From a thermodynamic point of view, ink detachment from cellulose can be described as a

change in total energy, Wcwi, associated with the liberation of ink from cellulose in water. Wcwi

can be calculated according to Eq. (2) from interfacial energies between cellulose and ink

( ciγ ), cellulose and water ( cwγ ), and ink and water ( iwγ ).

ciiwcwcwiW γγγ −+= (2)

The interfacial energies are determined from contact angle measurements performed using

three reference liquids with known surface properties. The Lifshitz-van der Waals

contribution (γLW), an acid (γ+) contribution and a base (γ−) contribution have been calculated

according to the equations presented in Appendix 1, with the results shown in Table 1.

Table 1: Surface energy (mJ/m2) of solid surfaces (Paper II).

From these entities and the surface energy properties of water, the interfacial energies (γ12)

between two materials (1 and 2) and the change in total energy (Wcwi) when separating ink (i)

and cellulose (c) in water (w) have been calculated using Eq. (2), see Table 2.

Table 2: Interfacial energies (γ12) between two materials (material 1 and 2) and the total energy change (Wcwi) (Paper II).

Making the cellulose surface more hydrophobic increases both the interfacial energy between

cellulose and water (γcw) and between ink and cellulose (γci), by drastically lowering the acid-

base parts of the cellulose substrate, as seen in Table 1. A higher interfacial energy between

ink and cellulose, as obtained for the hydrophobic cellulose, indicates that less energy is

required to separate ink from cellulose. However, the total energy change indicates that ink

Substrate γLW γ− γ+ Cellulose 40,06 44,47 1,94 Hydrophobic cellulose 29,44 3,91 0,08 Flexographic ink 45,97 64,16 12,21

Substrate γci (mN/m) γcw (mN/m) γiw (mN/m) Wcwi (mJ/m2) Cellulose 6 -9 -5 -20 Hydrophobic cellulose 40 30 -5 -15

42

release from a hydrophobic cellulose surface should be less spontaneous, as the Wcwi value

became less negative. This can be explained by the fact that the interfacial energy between

cellulose and water also increases.

The total energy, Wcwi, can also be calculated according to Eq. (3) from contact angle

measurements in water and the direct adhesion measurements between ink and cellulose in

air.

( )iwcwwcicwi CosCosWW Θ+Θ−= γ (3)

The contact angles in water are 22° for cellulose, 94° for hydrophobic cellulose and 60° for

flexographic ink. The JKR methodology, see Appendix 2 for mathematical relationships,

outlined by Chaudhury and Whitesides [22] and applied to cellulose by Rundlöf [24]

determines the adhesive properties between ink and cellulose. The loading and unloading

results between an ink coated PDMS cap and the different cellulose surfaces are shown in

Figs. 21 and 22.

Figure 21: The cube of the contact radius (a3) as a function of the applied load for adhesion between a PDMS cap coated with ink varnish and a model cellulose surface. The loading data was fitted to the JKR theory presented in Appendix II.

0

0,0005

0,001

0,0015

0,002

-300 -200 -100 0 100 200 300

Cellulose/Ink varnish

a3 loada3 unload

a3 (mm

3 )

Load (mg)

43

Figure 22: The cube of the contact radius (a3) as a function of the applied load for adhesion between a PDMS cap coated with ink varnish and a model cellulose surface with covalently bonded C4-tails (making the surface more hydrophobic). The loading data was fitted to the JKR theory presented in Appendix II.

The work of adhesion (Wloading) upon loading, the adhesion energy at minimum load (Wmin),

i.e. at pull-off, and the total energy change connected with the separation of ink and cellulose

in water (Wcwi) have been calculated using the equations in Appendix 2. The results are shown

in Table 3.

Table 3: Work of adhesion (W) and total energy change (Wcwi) for different model systems, * = not measured (Paper II).

Making the cellulose surface more hydrophobic decreases both the work of adhesion upon

loading and the adhesive energy at minimum load, i.e. at pull-off. Rundlöf [24] has previously

noted this behavior in adhesive properties between hydrophobic cellulose and PDMS. A

probable explanation to the lower adhesive properties of hydrophobic cellulose surfaces is

that the amount of OH-groups on the cellulose is lower, diminishing the possibility for

hydrogen bonding between cellulose and ink. An alternative explanation can be a diminished

contact induced rearrangement across the interface, as previously suggested by Mangipudi et

al. [31], whereby less migration of components across the interface between ink and cellulose

contribute to a lower adhesion between the surfaces.

0

0,0005

0,001

0,0015

0,002

-300 -200 -100 0 100 200 300

Hydrophobic cellulose/Ink varnish

a3 loada3 unload

a3 (mm

3 )

Load (mg)

Substrate W loading (mJ/m2) W min (mJ/m2) Wcwi (mJ/m2) Cellulose/PDMS 43 400 * Cellulose/ink varnish 34 465 -69 Hydrophobic cellulose/ink varnish 27 383 -4

44

The total energy change indicates a less spontaneous ink release from a hydrophobic cellulose

surface, i.e. Wcwi becomes less negative, whereas the adhesive properties between ink and

cellulose are lower for hydrophobic cellulose surfaces. In conclusion, the adhesive properties

between ink and cellulose decrease for hydrophobic cellulose surfaces but at the same time

water wets the cellulose surface to a lesser extent, as the larger contact angles indicate. Hence,

ink detachment in water becomes more difficult from a hydrophobic cellulose surface than

from a native cellulose surface.

45

2.4 Comparison between different methods to measure the water retaining

ability of fibres (Papers III, V and VI)

Swelling ability and porous structure are two, out of several, fibre properties that change

significantly during pulping. Over the years, they have received a great deal of attention and

several different methods have been developed to study these properties. As of yet, no

completely reliable method is available for analysing the porous structure of swollen fibres, as

different techniques measure different pore sizes in the fibre wall. Inverse size exclusion

chromatography (ISEC) is used to obtain information about the apparent pore size distribution

of fibres, the average pore radius and the fibre saturation point (FSP). However, the technique

has its limitations due to the fact that the accessibility of the polymers to the fibre wall is

limited by the openings in the fibre wall. Hence, ISEC actually measures a distribution of the

pore size openings in the external parts of the fibre wall and should be used with care.

However, a correct value of FSP can be obtained nevertheless. Nuclear Magnetic Resonance

(NMR) relaxation is another technique that provides a possibility to measure pore size

distribution and average pore radius. The pore size distribution, as determined with this

technique, naturally depends on the geometrical model chosen to describe the pores in the

fibres. The two different methods are compared and, as can be seen in Fig. 23, the results

clearly differ.

Figure 23: Cumulative pore volume, normalized to the fibre saturation point for the NMR measurements, versus pore radius for unbleached and unbeaten kraft pulp with kappa number 60 as determined with ISEC and NMR. The influence of salt and pH is shown.

0

0,2

0,4

0,6

0,8

1

1,2

1,4

0 5 10 15 20 25Pore radius (nm)

Cum

. Por

e vo

lum

e (m

l/g)

NMR 1mM NaCl, pH 8

NMR 100 mM NaCl, pH 8

NMR 1mM NaCl, pH 2

ISEC 1mM, pH 8

ISEC 100 mM, pH 8

ISEC, 1 mM, pH 2

Fibre saturation point (ml/g)

46

NMR measurements result in significantly higher average pore radius values than ISEC. The

two methods also give different results when changing pH or ionic strength of the pulp

suspension. ISEC is very sensitive towards changes in salt concentration and pH, detected as a

6 % decrease in fibre saturation point (FSP). NMR, on the other hand, is unaffected by

changes in salt, but lowering pH drastically decreases the average pore radius.

The effect of drying on FSP and NMR data has also been investigated. In Fig. 24, the average

pore radius, as detected with NMR, decreases significantly upon drying.

Figure 24: Average pore radius, as determined with NMR, versus pulp yield for never-dried and once-dried and rewetted unbleached and unbeaten kraft pulp.

It becomes clear when comparing Fig. 24 with Fig. 23 that the decrease in average pore

radius, as a result of drying, is comparable to the decrease in average pore radius when

lowering the pH to 2 (for a kappa number 60 pulp (yield 53 %)). The same behavior is also

seen for FSP, see Fig. 25. For a kappa 60 pulp, drying lowers the FSP 5 %. This is definitely

comparable to the 6 % decrease in FSP, seen in Fig. 23, when lowering the pH to 2.

8

9

10

11

12

13

14

15

16

17

18

40 45 50 55 60 65Yield (%)

Ave

rage

por

e ra

dius

(nm

)

Never-driedOnce-dried

47

Figure 25: Fibre saturation point, as determined with SEC, versus pulp yield for never-dried and once-dried and rewetted unbleached and unbeaten kraft pulp. The lines are merely a guideline to the eye.

Besides FSP, another extensively used technique to estimate of the amount of water held by the fibre wall is the water retention value (WRV). Therefore, a comparison between FSP and WRV needs to be made, see Fig. 26.

Figure 26: Comparison between FSP and WRV measurements for never-dried and once-dried and rewetted unbleached kraft pulp. The variable in this figure is the yield of the fibres. The lines are merely a guideline to the eye.

It is obvious that a unique relationship does not exist between FSP and WRV for never-dried

and once-dried and rewetted kraft fibres. Drying has a large effect on the amount of water

held in the fibre wall as detected by WRV, whereas only a minor decrease is detected in FSP.

All this taken together indicates that different methods are sensitive towards changes in

different parts of the fibre wall. Changes detected with NMR and WRV are significantly

0,8

1

1,2

1,4

1,6

1,8

2

40 45 50 55 60 65Yield (%)

FSP

(ml/g

)

Never-driedOnce-dried

1

1,2

1,4

1,6

1,8

2

1 1,2 1,4 1,6 1,8 2WRV (ml/g)

FSP

(ml/g

)

Never-driedOnce-dried

High Yield

High Yield

Low Yield

Low Yield

48

larger than changes in FSP. The reason for the detected changes can be explained by what

these methods actually measure. From the collected results, a rather unified view is proposed

describing what happens to the swelling and structure of the fibre wall during drying and also

when changing pH or electrolytic concentration of the pulp suspension.

The interior of the fibre wall is a rigid structure consisting of concentric lamellae of cellulose

fibrils. Random contacts between adjacent fibrillar lamellae in the fibre wall can be formed,

as shown in Fig. 27, due to a diminished electrostatic repulsion, as a consequence of lower

pH, salt addition or drying. In order to explain the current data, the formation of these local

contacts must change the surface area available to water without drastically changing the

overall fibre wall volume. A situation where adjacent lamellae, with basically two to three

layers of adsorbed water each, are joined together expelling the bound water would give a

large reduction in surface area and only a minor reduction in the fibre wall volume. At the

same time, a situation where adjacent lamellae further apart are brought together would give a

minor reduction in both surface area and volume. In conclusion, all lamellae are gradually

brought together giving an overall minor decrease in fibre wall volume. Meanwhile, lamellae

originally very close together are completely closed, giving a large reduction in surface area

without affecting the fibre wall volume to a large extent. Hence, the formation of these

random contacts only lowers the FSP to a minor extent, whereas the ratio between surface and

bulk water present in the fibre, measured in NMR, is affected to a larger extent.

Figure 27: Modes of closure of pores within the fibre wall as suggested by Stone and Scallan [135].

49

The drastic changes occurring in WRV, contrary to the small changes noticed in FSP, can be

due to the fact that WRV is highly sensitive towards changes in the properties of the fibre

surface. It has previously been shown that WRV is affected by variations in surface charge

[101]. In fact, it has been found that changes in surface charge have a larger impact on WRV

than changes in bulk charge.

50

2.5 Correlation between fibre and sheet properties (Papers III, VI and VII)

Measuring fibre properties is of little use unless it can be correlated to the obtained paper

properties. Paper properties are controlled by how well the fibres conform towards each other,

the strength of the fibre/fibre joint and also the strength of the single fibre. Density is usually

taken as a measure of how well the fibres have conformed towards each other during pressing

and drying, and the water retaining ability is said to correlate well to sheet strength properties.

It is therefore of interest to check how the water retaining ability correlates to density. As can

be seen in Fig. 28, a straight correlation does not exist.

Figure 28. Paper density as a function of fibre wall volume, measured either as WRV or FSP, for never-dried unbleached and unbeaten kraft fibres with different pulp yield and with their charged carboxyl groups in different ionic forms.

The imbibition of water is therefore not the single reason for how well the fibres conform

towards each other. Another difference that might be of importance for how the fibres are

brought together is the pore size inside the fibre wall. It is likely that larger pores create a

more conformable fibre wall. Indeed, this seems to be the case when comparing fibres where

the properties have not been drastically changed, as shown in Fig. 29.

520

540

560

580

600

620

640

660

1,3 1,4 1,5 1,6 1,7 1,8 1,9Volume (ml/g)

Den

sity

(kg/

m3 )

WRV Never-dried NaWRV Never-dried CaWRV Never-dried HFSP Never-dried Na

51

Figure 29: Paper density as a function of average pore radius, as determined with NMR, for never-dried unbleached and unbeaten kraft fibres with different pulp yield and in different ionic forms.

For fibres with the same counter-ion, larger pores give a denser sheet; hence, larger pores

create a fibre wall that conforms more than a fibre wall containing smaller pores. However,

when comparing fibres with different counter-ions, the results did not cohere. The sheet

tensile strength has further been investigated as function of average pore radius for all the

different investigated pulps in order to clarify if any correlation exists between these entities.

The results are shown in Fig. 30.

Figure 30: Average pore radius versus sheet tensile index for never-dried and once-dried and rewetted unbleached and unbeaten kraft pulps with different pulp yield and different ionic forms.

520

540

560

580

600

620

640

10 11 12 13 14 15 16 17 18Average pore radius (nm)

Den

sity

(kg/

m3 )

Never-dried NaNever-dried CaNever-dried H

0

10

20

30

40

50

60

70

80

90

10 11 12 13 14 15 16 17 18Average pore radius (nm)

Shee

t Ten

sile

Inde

x (k

Nm

/kg)

Never-dried NaNever-dried CaNever-dried HOnce-dried Na

52

A definite correlation exists between pore size and tensile strength, however, the correlation is

only realised when comparing all the different fibres and not for a single type of fibre (fibres

with the same counter-ion) at different yield.

It is suggested that larger pores create a more conformable fibre wall. However, the average

fibre pore radius, as determined with NMR, is a bulk property and not the only determining

factor for the utilised paper strength. It has previously been shown that fibre surface charge is

of importance for sheet tensile index [144], and this is verified by the results in Fig. 31.

Figure 31: Surface charge versus sheet tensile index for never-dried and once-dried and rewetted unbleached and unbeaten kraft pulp with different pulp yield. The lines are merely a guideline to the eye.

Fibres having a lower surface charge create weaker sheets. However, this relationship is no

longer valid when comparing fibres where the properties have been drastically altered by, for

example, drying. The decrease in tensile index upon drying does not accompany a similar

decrease in fibre surface charge. A previous study for regenerated cellulose fibres has shown

that an increased surface charge creates stronger joints and that fibre surface softness is of

primary importance for the development of fibre/fibre joint strength [100]. It was also found

in the same study that fibre/fibre joint strength strongly correlates with sheet strength of

papers made from the same type of fibres. It therefore became of interest to investigate

whether a correlation between fibre/fibre joint strength and sheet tensile index could be found

also for kraft fibres, as used in the present work. The results are shown in Fig. 32.

0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5 6

Surface charge (µeq/g)

Tens

ile in

dex

(kN

m/k

g)

Never-dried NaOnce-dried Na

53

Figure 32: Fibre/fibre joint strength versus sheet tensile index for unbleached kraft pulp with different yield. The effect of drying and rewetting is also shown.

The sheet tensile index increases with joint strength and the high yield fibres in their Na-form

produce the strongest sheets. However, the decrease in joint strength is not followed by an

equal decrease in tensile strength. This can be explained by the fact that the low yield pulps

are more flexible than the high yield pulps (see Fig. 33), allowing a more efficient packing of

the fibres, which results in sheets with a higher density.

Figure 33: Fibre flexibility as a function of pulp yield and counter-ion for unbleached and unbeaten kraft pulp. The effect of drying is also shown for the Na form. The lines are merely a guideline to they eye.

The decrease in joint strength is to some extent counteracted by a sheet densification and

therefore the sheet tensile index is only slightly lowered. This is probably due to changes in

0

10

20

30

40

50

60

70

80

90

0,5 1,0 1,5 2,0 2,5

Joint strength (Nm)

Teni

sle

inde

x (k

Nm

/kg)

Never-dried NaNever-dried CaNever-dried HOnce-dried Na

High yield

High yield

Low yield

Low yield

5,00

6,00

7,00

8,00

9,00

40 45 50 55 60 65Yield (%)

Wet

fibr

e fle

xibi

lity

Never-dried NaNever-dried CaNever-dried HOnce-dried Na

54

electrostatic repulsion during consolidation of the fibres. Pulps with different yields behave

differently upon drying. For high yield pulps, the large decrease in sheet tensile index can be

explained by the almost equally large decrease in fibre/fibre joint strength. For the low yield

pulps, the large decrease in tensile index cannot only be caused by a decrease in joint strength.

Once again, changes in fibre flexibility are the explanation. For low yield pulps, the flexibility

decreases slightly after drying, as does the sheet density. It can also be seen in Fig. 33 that a

change of counter-ion from Na to Ca or H weakens fibre/fibre joints 50%, whereas the sheet

tensile index only decreases 15 – 20 %. This can be due to a loss in surface swelling of the

fibres when changing counter-ion to Ca or H. The relatively smaller decrease in tensile index

is suggested to be due to a lower degree of dissociation of the carboxyl groups, giving denser

sheets. In conclusion, fibre flexibility, fibre/fibre joint strength and the number of efficient

contacts in the fibre/fibre joint control how well the fibre properties can be utilised in the final

sheet.

55

2.6 Influence of different strength enhancing additives on fibre and sheet

properties (Papers IV and V)

In order to create papers with a high wet stiffness and a high relative wet strength, it has been

found that it is necessary to both join the fibres with a covalent linkage and to crosslink the

fibre wall. Fibre/fibre joint strength is also of importance when creating paper that requires

strong out-of-plane properties. When adding different strength enhancing additives, it is

obvious that both the porosity of the fibre wall and the charge of the fibre material are of

importance. The porosity, as well as the size of the openings in the fibre wall, influences the

amount of additive that adsorbs to the pulp. It is therefore vital to establish a relationship

between the size of the additive and the size of pores within the fibre wall. The porosity and

the pore size of the fibre wall after treatment with different strength enhancing additives have

therefore been investigated using conventional wet strength agents of varying molecular mass

and using polymeric complexes. The fibre properties are eventually linked to the obtained

sheet properties.

It has previously been shown [76, 145] that poly – 3, 6 –ionene, a polyelectrolyte with a

average molecular mass of 5900 g/mol, can reach all charged groups in the fibre wall of an

unbleached kraft pulp and a carboxymethylated bleached softwood kraft pulp. The data in Fig.

34 represent the effect of adding different molecular mass (Mw) additives to a TCF-bleached

(total chlorine free) pulp.

Figure 34: Pore size distribution determined with NMR for a reference TCF pulp and a TCF pulp treated with poly-isobutylene maleic anhydride (PIMA, Mw 6500) and poly-maleic anhydride (PMA, Mw 700).

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

0,45

0 5 10 15 20 25

Pore radius (nm)

Prob

abili

ty d

ensi

ty (n

m-1

)

Refined TCFRefined TCF treated with PIMARefined TCF treated with PMA

56

The average pore radii of these pulps have been estimated to be 15.5, 12.3 and 8.5 nm for the

reference pulp, PIMA and PMA treated pulp, respectively. The clear trend in these results is

that the smaller crosslinking agents yield smaller measured pore size for the treated fibres.

Thus, the molecular mass of the additive is important for the penetration of the additive into

the fibre wall. Lower molecular mass additives can penetrate the fibre wall, whereas high

molecular mass additives are limited to the outer parts of the fibre. This, together with the

type of additive, influences the strength properties of the sheet made from these fibres.

It has previously been shown that additives that crosslink the fibre wall cause an

embrittlement of the sheet [120]. Paper V shows that additives with low Mw penetrate the

fibre wall to a higher extent than additives with a higher Mw. Therefore, a low Mw additive

should decrease the fibre strength due to an embrittlement. As seen in Fig. 35, this is in fact

the case.

Figure 35: Zero-span tensile index as a function of average pore radius for refined TCF pulp and refined TCF pulp treated with PIMA (Mw 6500) and PMA (Mw 700).

The lower molecular mass PMA decreases the fibre strength, in terms of zero-span tensile

index, from 146 kNm/kg to 119 kNm/kg, whereas the higher molecular mass PIMA only

decreases the zero-span tensile index to 131 kNm/kg. Further investigations have been made

on how different molecular mass additives affect the tensile indexes of handseets treated with

PMA, PIMA and also BTCA. The results are shown in Fig. 36.

110

115

120

125

130

135

140

145

150

6 8 10 12 14 16 18Average pore radius (nm)

Zero

-spa

n te

nsile

inde

x (k

Nm

/kg)

Refined TCFRefined TCF treated with PIMARefined TCF treated with PMA

57

Figure 36: Tensile index as a function of added polymer concentration for refined TCF pulp and refined TCF pulp treated with PIMA (Mw 6500), PMA (Mw 700) and BTCA (Mw 234).

The results in Fig. 35 indirectly show that a low Mw additive penetrates the fibre wall,

whereas a high Mw additive is limited to the more exterior parts of the fibre. It can be seen in

Fig. 36 that lower molecular mass additives, BTCA and PMA, obtain the highest tensile

index, whereas the highest molecular mass additive, PIMA, shows no increase in tensile index

compared with the reference TCF pulp. This is most probably caused by a lack of availability

to the interior of the dry fibre wall for the latter polyelectrolyte. The result also indicates that

PIMA could not strengthen the fibre/fibre joint. The effect of added amount is shown and, as

expected, a larger amount of additive further increases the tensile index. However, the

increasing trend in tensile index levels out at higher additions, indicating that the fibre wall

can only adsorb a limited amount of chemical.

Not only the effect of molecular mass is important for additives penetrating into the fibre

wall, the size of the fibres pores is also of interest. Therefore, unbeaten kraft pulps with

different yield have been treated with polyallylamine hydrochloride (PAH) and polymer

complexes (PEC) of PAH and polyacrylic acid (PAA). In Fig. 37, the results are plotted as

average pore radius against pulp yield.

110

115

120

125

130

135

140

145

150

6 8 10 12 14 16 18Average pore radius (nm)

Zero

-spa

n te

nsile

inde

x (k

Nm

/kg)

Refined TCFRefined TCF treated with PIMARefined TCF treated with PMA

58

Figure 37: Average pore radius, as determined with NMR, versus pulp yield of a once-dried and rewetted kraft pulp, untreated and treated with PAH (Mw 15000) and PEC. The lines are merely a guideline to the eye.

As can be seen for a pulp with a pulp yield of 57 %, the average pore radius does not change

after treatment with either PAH or PEC. However, for a pulp with yield 47 %, the average

pore radius decreases from 10 nm to 8.7 nm, indicating that a fraction of the strength

enhancing additives has penetrated the fibre wall. In conclusion, the pore size in the fibre wall

is of importance for penetration of chemicals into the fibre wall. If the pores are too small, the

chemical additive cannot penetrate the fibre wall and is hence limited to the outer parts of the

fibre, whereas a fibre wall with larger pores can adsorb a higher amount of the chemical

additive. This influences the strength properties obtained from the sheets made from these

fibres. It is shown in Fig. 36 that the higher molecular mass additive, PIMA, does not increase

sheet tensile strength, concluding that the additive is not able to enhance the fibre/fibre joint

strength. PAH and PEC are even larger in molecular mass, although their mechanism of

action differs from that of PIMA. Therefore, investigating whether these additives could

increase sheet tensile index is of interest. In Fig. 38, the tensile index is plotted against pulp

yield for untreated kraft pulp as well as PAH and PEC treated kraft pulp.

7

7,5

8

8,5

9

9,5

10

10,5

11

11,5

12

46 48 50 52 54 56 58Yield (%)

Ave

rage

por

e ra

dius

(nm

)

Dried referenceDried PEC treatedDried PAH treated

59

Figure 38: Sheet tensile index versus pulp yield for sheets made from unbleached kraft pulp, non-treated pulp and pulp treated with PAH or PEC. The lines are merely a guideline to the eye.

PEC enhance the sheet tensile index for all pulp yields. PAH also increases the sheet strength,

although not to the same extent. When comparing Fig. 37 with Fig. 38, it can be suggested

that PEC enhance the sheet tensile index by improving the fibre/fibre joint strength. On the

other hand, PAH does not affect the fibre/fibre joint strength to the same extent and the

increase in sheet tensile index is due to a combination of crosslinking, due to penetration, and

an increase in fibre/fibre joint strength. This hypothesis is further strengthened when

investigating the obtained Z-strength properties of the sheets prepared with different

additives. The Z-strength is plotted in Fig. 39 against pulp yield for untreated kraft pulp, PAH

treated, and PEC treated kraft pulp.

Figure 39: Z-strength versus pulp yield for sheets made from unbleached kraft pulp, non-treated pulp and pulp treated with PAH or PEC. The lines are merely a guideline to the eye.

40

50

60

70

80

90

100

110

120

130

40 45 50 55 60 65

Yield (%)

Tens

ile in

dex

(kN

m/k

g)

ReferencePAH treatedPEC treated

100

150

200

250

300

350

400

450

500

40 45 50 55 60 65

Yield (%)

Z-st

reng

th (k

N/m

2)

ReferencePAH treatedPEC treated

60

It is clear that PAH did not enhance Z-strength to the same extent as PEC. This strengthens

the hypothesis that the effect of PEC is dominated by an improvement in the strength of the

fibre/fibre joint, whereas PAH improves the intra-fibre bond strength more efficiently.

61

3 CONCLUDING REMARKS

Aspects behind the re-use of wood based fibres have been presented in this thesis. The

Impinging jet cell equipment has proven to be an easy and accurate tool for studying the

mechanisms behind ink detachment from model cellulose surfaces. The MAMA provides a

possibility to evaluate the adhesive properties between ink and cellulose. It has been shown

that the difficulties of ink detachment correlate with the age of the flexographic print. Longer

storage times, storage at elevated temperature, or storage under UV-light result in ink that is

more difficult to remove. Ink detachment also depends on the properties of the surface it is

attached to. Ink detachment is made more difficult by a higher surface roughness and a lower

surface energy, i.e. a more hydrophobic surface. The latex particles present in ink anneal at

elevated temperatures, causing film formation. Elevated temperatures also cause an oxidative

reaction or a hydrolysis of the ink. All this taken together explains why aged flexographic ink

and flexographic ink on hydrophobic paper surfaces is more difficult to deink.

The porous structure and water retaining ability of fibres has been measured, proving that

NMR, FSP and WRV provide different information. Combining the methods is necessary to

give as complete a picture as possible of the changes that occur in the fibre wall. It has been

concluded that fibre flexibility, fibre/fibre joint strength and the number of efficient fibre/fibre

contacts control the sheet strength. Adsorption of strength enhancing additives is strongly

influenced by the porous structure of the fibres and the molecular mass of the additive. Low

molecular mass additives penetrate the fibre wall, and while they cause an embrittlement of

the fibre strength they also enhance the sheet tensile index. Polyelectrolyte complexes only

penetrate the fibre wall to a smaller extent. The large strength improvements gained from

these additives is dominated by an improvement of the fibre/fibre joint strength.

62

4 REFERENCES

1. Dey A., Sarkar D. and Basgupta B. K. 1995. The current state of paper recycling - A global review. IPPTA. 7(4):1. 2. Andreasson U. and Wågberg L. 2001. Ink release from printed surfaces - New methodology and initial insights to the true mechanism behind ink detachment. C. F. Baker, ed., The Science of papermaking - 12th Fund. Res. Symp., Oxford, UK, p. 339. 3. Rangamannar G., Grube G. and Karneth A. M. 1992. Behaviour of water-based flexographic inks in newsprint deinking. Tappi Pulping Conf., Boston, p. 933. 4. Wasilewski O. 1987. Composition and chemistry of novel ink used in the newspaper industry and deinking. Tappi Pulping Conf., Washington, p. 25. 5. Shrinath A., Szewczak J. T. and Bowen I. J. 1991. A review of ink-removal techniques in current deinking technology. Tappi J. 74(7):85. 6. Borchardt J. K. 1995. Ink types: the role of ink in deinking. Prog. Paper Rec. 5(1):81. 7. Larsson A., Stenius P. and Ödberg L. 1984. Surface chemistry in flotation deinking. Part 2. The importance of ink particle size. Svensk Papperstidning. 87(18):R165. 8. Sjöström L. and Calmell A. 1997. Detachment of printing ink from different types of fibers. J. Pulp. Paper Sci. 23(2):J67. 9. Miller J. D. and Xiansheng N. 1997. The effect of ink types and printing processes on flotation deinking. Rec. Symp. Tappi proc., p. 131. 10. Ferguson L. D. 1992. Deinking chemistry. Part 1. Tappi J. 75(7):75. 11. Ferguson L. D. 1992. Deinking chemistry. Part 2. Tappi J. 75(8):49. 12. Jönsson B., Lindman B., Holmberg K. and Kronberg B. 1991. Surfactants and polymers in aqueous solution, John Wiley & Sons Inc., New York. 13. 2000. Recycled Fibre and Deinking, Fapet Oy, Helsinki. 14. Rutland M. and Pugh R. J. 1997. Calcium soaps in flotation deinking; fundamental studies using surface force and coagulation techniques. Coll. Surf., A: Physicochem. Eng. Aspects. 125(1):33. 15. Paraskevas S. 1989. Ink removal - various methods and their effectiveness. Seminar on Contaminant problems and strategies in waste paper recycling, Tappi Press, Atlanta, p. 41. 16. Larsson A. 1987. Surface chemistry in flotation deinking. Paper Techn. Industry. 28(1):388.

63

17. Ferguson L. D. 1995. Flotation deinking technology. Tappi Press, Atlanta, p. 235. 18. Heindel T. J. 1999. Fundamentals of flotation deinking. Tappi J. 82(3):115. 19. Borchardt J. K. 1994. Mechanistic Insights into De-Inking. Coll. Surf., A: Physicochem. Engineering Asp. 88(1):13. 20. Good R. J. 1992. Contact angle, wetting, and adhesion: a critical review. J. Adhesion Sci. Technol. 6(12):1269. 21. Van Oss C. J., Chaudhury M. K. and Good R. J. 1988. Interfacial Lifshitz-Van der Waals and Polar Interactions in Macroscopic Systems. Chem. Rev. 88(6):927. 22. Chaudhury M. K. and Whitesides G. M. 1991. Direct measurement of interfacial interactions between semispherical lenses and flat sheets of poly(dimethylsiloxane) and their chemical derivatives. Langmuir. 7(5):1013. 23. Rundlöf M., Karlsson M., Wågberg L., Poptoshev E., Rutland M. and Claesson P. 2000. Application of the JKR Method to the Measurement of Adhesion to Langmuir-Blodgett Cellulose Surfaces. J. Coll. Interface Sci. 230(2):441. 24. Rundlöf M., Interaction of dissolved and colloidal substances with fines of mechanical pulp - Influence on sheet properties and basic aspects of adhesion, Fibre and Polymer Technology, PhD thesis, Royal Institute of Technology, Stockholm (2002). 25. Falsafi A., Deprez P., Bates F. S. and Tirrell M. 1997. Direct measurement of adhesion between viscoelastic polymers: A contact mechanical approach. J. Rheology. 41(6):1349. 26. Johnson K. L., Kendall K. and Roberts A. D. 1971. Surface energy and the contact of elastic solids. Proc. Royal Soc. Series A: Math., Phys. Engineering Sci. 324(1558):301. 27. Vigil G., Xu Z. H., Steinberg S. and Israelachvili J. 1994. Interactions of Silica Surfaces. J. Coll. Interface Sci. 165(2):367. 28. Kim S. J., Choi G. Y., Ulman A. and Fleischer C. 1997. Effect of chemical functionality on adhesion hysteresis. Langmuir. 13(25):6850. 29. Deruelle M., Leger L. and Tirrell M. 1995. Adhesion at the Solid-Elastomer Interface - Influence of the Interfacial Chains. Macromolecules. 28(22):7419. 30. Chen Y. L., Helm C. A. and Israelachvili J. N. 1991. Molecular Mechanisms Associated with Adhesion and Contact-Angle Hysteresis of Monolayer Surfaces. J. Phys. Chem. 95(26):10736. 31. Mangipudi V. S., Huang E., Tirrell M. and Pocius A. V. 1996. Measurement of interfacial adhesion between glassy polymers using the JKR method. Macromolecular Symp. 102(131. 32. Rao R. and Kuys K. 1995. Deinkability of aged paper. Appita - 49th Annual General Conf. Proc., p. 601.

64

33. Cathie K., Fabris J. and Fullick C. Influence of ageing on the deinkability of different non-impact printed grades. Wochenbl. Papierfabr. 121(22):952. 34. Frank E. 1991. Influence of different printing colour compositions on the result of deinking. Wochenbl. Papierfabr. 119(20):819. 35. Rosinski J. 1995. Aging and deinking of soy printed newsprint. Prog. Pap. Recycling. 4(3):55. 36. Larsson L. O. and Busk H. 1985. Must it be difficult to remove printing ink from waste newsprint? Wochenbl. Papierfabr. 113(16):573. 37. Blechschmidt J., Knittel A. and Strunz A. M. 1993. Ink removal levels obtained for newsprint. Wochenbl. Papierfabr. 121(19):783. 38. Castro C., Daneault C. and Dorris G. M. 1996. Analysis of the accelerated thermal aging of oil ink vehicles using isothermal thermogravimetry. J. Pulp. Paper Sci. 22(10):J365. 39. Johansson K., Ström G. and Stenius S. 1991. The interactions between alkyd resin binders and kaolinite. Coll. Surf. 56(313. 40. Sain M. M. and Daneault C. 1996. Thermo-oxidative behaviour of water-borne ink binders in the presence of monovalent metal ion. Polymer Degradation. Stab. 51(1):67. 41. Goh M. C., Juhue D., Leung O. M., Wang Y. C. and Winnik M. A. 1993. Annealing Effects on the Surface-Structure of Latex Films Studied by Atomic Force Microscopy. Langmuir. 9(5):1319. 42. Goudy A., Gee M. L., Biggs S. and Underwood S. 1995. Atomic-Force Microscopy Study of Polystyrene Latex Film Morphology - Effects of Aging and Annealing. Langmuir. 11(11):4454. 43. Buchholz V., Wegner G., Stemme S. and Ödberg L. 1996. Regeneration, derivatization, and utilization of cellulose in ultrathin films. Adv. Materials. 8(5):399. 44. Rao R. N. and Stenius P. 1998. Mechanisms of ink release from model surfaces and fiber. J. Pulp. Paper Sci. 24(6):183. 45. Gunnars S., Wågberg L. and Cohen Stuart M. A. 2002. Model films of cellulose: I. Method development and initial results. Cellulose. 9(3/4):239. 46. Neuman R. D., Berg J. M. and Claesson P. M. 1993. Direct measurement of surface forces in papermaking and paper coating systems. Nordic Pulp. Paper Res. J. 8(1):96. 47. Schaub M., Wenz G., Wegner G., Stein A. and Klemm D. 1993. Ultrathin films of cellulose on silicon wafers. Adv. Materials. 5(12):919. 48. Chabot B., Daneault C. and Dorris G. M. 1995. Entrapment of water-based inks in fiber mats. J. Pulp. Paper Sci. 21(9):J296.

65

49. Borchardt J. K., Tutunjian P. and Prieto N. E. 1993. Novel Methods for laboratory evaluation of deinking surfactants. Tappi J. 76(7):104. 50. Stone J. E. and Scallan A. M. 1967. Effect of component removal upon the porous structure of the cell walls of wood. II. Swelling in water and the fiber saturation point. Tappi. 50(10):496. 51. Fellers C. and Norman B. 1996. Pappersteknik, TABS - Tryckeri AB i Småland, Stockholm. 52. Kolseth P. and de Ruvo A. 1986. The cell wall components of wood pulp fibres. J. A. Bristow and P. Kolseth, eds., Paper structure and properties. Marcel Dekker Inc., New York, p. 3. 53. Stone J. E. and Scallan A. M. 1965. A study of cell wall structure by nitrogen adsorption. Pulp. Pap. Mag. Canada. 66(8):T407. 54. Duchesne I. and Daniel G. 1999. The ultrastructure of wood fibre surfaces as shown by a variety of microscopical methods - a review. Nordic Pulp. Paper Res. J. 14(2):129. 55. Stone J. E. and Scallan A. M. 1965. Effect of component removal upon the porous structure of the cell wall of wood. J. Polymer Sci., Part C: Polymer Symp. 11(1):13. 56. Scallan A. M. and Tigerström A. C. 1992. Swelling and elasticity of the cell walls of pulp fibers. J. Pulp. Paper Sci. 18(5):188. 57. Nilsson B., Wågberg L. and Gray D. 2001. Conformability of wet pulp fibres at small length scales. C. F. Baker, ed., The Science of Paper making - Trans. 12th Fund. Res. Symp, Oxford, p. 211. 58. Doo P. A. T. and Kerekes R. J. 1982. The Flexibility of Wet Pulp Fibers. Pulp. Paper Canada. 83(2):46. 59. Mohlin U. B. 1975. Cellulose fiber bonding. Part 5. Conformability of pulp fibers. Svensk Papperstidning. 78(11):412. 60. Paavilainen L. 1993. Conformability-flexibility and collapsibility-of sulfate pulp fibers. Paperi Ja Puu. 75(9-10):689. 61. Steadman R. and Luner P. 1985. The effect of wet fibre flexibility of sheet apparent density. V. Punton, ed., Papermaking raw materials Trans. - 8th Fund. Res. Symp., Oxford, UK, p. 311. 62. Kerekes R. J., Soszynski R. M. and Tam Doo P. A. 1985. The flocculation of pulp fibres. V. Punton, ed., Papermaking raw materials Trans. - 8th Fund. Res. Symp., Oxford, UK, p. 265. 63. Martinez D. M., Buckley K., Jivan S., Lindström A., Thiruvengadaswamy R., Olson J. A., Ruth T. J. and Kerekes R. J. 2001. Characterizing the mobility of papermaking fibres during

66

sedementation. C. F. Baker, ed., The science of paper making Trans. - 12th Fund. Res. Symp., Oxford, UK., p. 225. 64. Mason S. G. 1950. The motion of fibers in flowing liquids. Pulp. Paper Mag. Canada. 51(5):93. 65. Torgnysdotter A. and Wågberg L. 2004. Influence of electrostatic on fibre/fibre joint and paper strength. Nordic Pulp. Paper Res. J. 19(4):in press. 66. Lindström T. 1986. The porous lamellar structure of the cell wall. J. A. Bristow and P. Kolseth, eds., Paper structure and properties. Marcel Dekker Inc., New York, p. 99. 67. Carlsson G., Lindström T. and Söremark C. 1978. Expression of water from cellulosic fibers under compressive loading. Fibre-Water Interact. Paper-Making - Trans. Symp., Oxford, UK, p. 389. 68. Alince B. and Van de Ven T. G. M. 1997. Porosity of swollen pulp fibers evaluated by polymer adsorption. C. F. Baker, ed., Fund. Papermaking Mat. - 11th Fund. Res. Symp., Cambridge, UK, p. 771. 69. Salmen L. and Berthold J. 1997. The swelling ability of pulp fibers. C. F. Baker, ed., Fund. Papermaking Material - 11th Fund. Res. Symp., Cambridge, UK, p. 683. 70. Stone J. E. and Scallan A. M. 1968. Effect of component removal upon the porous structure of the cell wall of wood. III. Comparison between the sulfite and kraft processes. Pulp. Paper Mag. Canada. 69(12):69. 71. Stone J. E. and Scallan A. M. 1968. Structural model for the cell wall of water-swollen wood pulp fibers based on their accessibility to macromolecules. Cell. Chem. Techn. 2(3):343. 72. Scallan A. M. 1978. The accommodation of water within pulp fibers. Fibre-Water Interact. Paper-Making - Trans. Symp., Oxford, UK, p. 9. 73. Scallan A. M. 1987. Non-solvent water in cellulosic fibers as determined by salt exclusion. Cellulose Chem. Techn. 21(3):215. 74. Lindström T. 1986. The concept and measurement of fibre swelling. J. A. Bristow and P. Kolseth, eds., Paper structure and properties. Marcel Dekker Inc., New York, p. 75. 75. Maloney T. C. and Paulapuro H. 1999. The formation of pores in the cell wall. J. Pulp. Paper Sci. 25(12):430. 76. Swerin A. and Wågberg L. 1994. Size-exclusion chromatography for characterization of cationic polyelectrolytes used in papermaking. Nordic Pulp. Paper Res. J. 9(1):18. 77. Flory P. J. 1953. Principles of Polymer Chemistry, Cornell University Press, Ithaca, N. Y. 78. Grignon J. and Scallan A. M. 1980. Effect of pH and neutral salts upon the swelling of cellulose gels. J. Appl. Polymer Sci. 25(12):2829.

67

79. Scallan A. M. and Grignon J. 1979. The effect of cations on pulp and paper properties. Svensk Papperstidning. 82(2):40. 80. Lindström T. 1980. Effect of chemical factors on fiber swelling and paper strength. Papier. 34(12):561. 81. Lindström T. and Carlsson G. 1982. The Effect of Chemical Environment on Fiber Swelling. Svensk Papperstidning. 85(3):R14. 82. Lindström T. and Kolman M. 1982. The Effect of Ph and Electrolyte Concentration During Beating and Sheet Forming on Paper Strength. Svensk Papperstidning. 85(15):R140. 83. Ampulski R. S. 1989. The equilibrium for the sodium/calcium ion-exchange reaction on bleached northern softwood kraft pulp. Nordic Pulp. Pap. Res. J. 4(1):38. 84. Maloney T. C., On the pore structure and dewatering properties of the pulp fiber cell wall, Acta Polytechn. Scandinavica, Chem. Techn. Series, PhD thesis, Helsinki University of Technology, Helsinki (2000). 85. Jayme G. 1958. Properties of wood celluloses. II. Determination and significance of water-retention value. Tappi. 41(11):180A. 86. Abson D. and Gilbert R. D. 1980. Observations on water retention values. Tappi. 63(9):146. 87. Häggkvist M., Li T.-Q. and Ödberg L. 1998. Effects of drying and pressing on the pore structure in the cellulose fiber wall studied by proton and deuteron NMR relaxation. Cellulose. 5(1):33. 88. Scallan A. M. and Carles J. E. 1972. Correlation of the water retention value with the fiber saturation point. Svensk Papperstidning. 75(17):699. 89. Maloney T. C., Laine J. E. and Paulapuro H. 1999. Comments on the measurement of cell wall water. Tappi J. 82(9):125. 90. Stone J. E., Scallan A. M. and Abrahamson B. 1968. Influence of beating on cell wall swelling and internal fibrillation. Svensk Papperstidning. 71(19):687. 91. Stone J. E. and Scallan A. M. 1968. Effect of component removal upon the porous structure of the cell wall of wood. III. Comparison between the sulfite and kraft processes. Pulp & Paper Magazine of Canada. 69(12):69. 92. Berthold J. and Salmen L. 1997. Inverse size exclusion chromatography (ISEC) for determining the relative pore size distribution of wood pulps. Holzforschung. 51(4):361. 93. Li T.-Q., Interaction between water and cellulose fibres, Ph. D Thesis, Stockholm (1991). 94. Gallegos D. P. and Smith D. M. 1988. An NMR technique for the analysis of pore structure: determination of continous pore size distribution. J. Coll. Int. Sci. 122(1):143.

68

95. Glaves C. L. and Smith D. M. 1989. Membrane pore structure analysis via NMR spin-lattice relaxation experiments. J. Membrane Sci. 46(2+3):167. 96. Gallegos D. P., Munn K., Smith D. M. and Stermer D. L. 1987. A NMR technique for the analysis of pore structure: Application to materials with well defined pore structure. J. Coll. Int. Sci. 119(127. 97. Li T.-Q., Henriksson U. and Ödberg L. 1993. Determination of pore sizes in wood cellulose fibres by 2H and 1H NMR. Nordic Pulp. Pap. Res. J. 8(3):326. 98. Thode E. F. and Ingmanson W. L. 1959. Factors contributing to the strength of a sheet of paper. I. External specific surface and swollen specific volume. Tappi. 42(1):74. 99. Robertson A. A. 1958. Permeability method for determining surface development and swelling during beating and refining. F. Bolam, ed., Fund. Paper-making Fibres, Trans. Symp., Cambridge, UK, p. 411. 100. Torgnysdotter A. and Wågberg L. 2003. Study of the joint strength between regenerated cellulose fibres and its influence on the sheet strength. Nordic Pulp. Paper Res. J. 18(4):455. 101. Laine J., Lindström T., Glad-Nordmark G. and Risinger G. 2002. Studies on topochemical modification of cellulosic fibres - Part 2. The effect of carboxymethyl cellulose attachment on fibre swelling and paper strength. Nordic Pulp. Paper Res. J. 17(1):50. 102. Barzyk D. and Page D. H. 1997. Carboxylic acidic groups and fibre bonding. C. F. Baker, ed., Fund. Papermaking Material - 11th Fund. Res. Symp., Cambridge, UK, p. 893. 103. Barzyk D., Page D. H. and Ragauskas A. 1997. Acidic group topochemistry and fibre-to-fibre specific bond strength. J. Pulp. Pap. Sci. 23(2):J59. 104. Davison R. W. 1972. Weak link in paper dry strength. Tappi. 55(4):567. 105. Davison R. W. 1980. Theory of dry strength development. W. F. Reynolds, ed., Dry strength additives. p. 1. 106. Page D. H. 1969. Theory for the tensile strength of paper. Tappi. 52(4):674. 107. Taylor D. L. 1968. Mechanism of wet tensile failure. Tappi. 51(9):410. 108. Stratton R. A. and Colson N. L. 1993. Fiber wall damage during bond failure. Nordic Pulp. Paper Res. J. 8(2):245. 109. McKenzie A. W. 1984. The structure and properties of paper. Part XXI: The diffusion theory of adhesion applied to interfiber bonding. Appita. 37(7):580. 110. Pelton R., Zhang J., Wågberg L. and Rundlöf M. 2000. The role of surface polymer compatibility in the formation of fiber/fiber bonds in paper. Nordic Pulp. Paper Res. J. 15(5):400.

69

111. Zhang J., Pelton R., Wågberg L. and Rundlöf M. 2000. The effect of charge density and hydrophobic modification on dextran-based paper strength enhancing polymers. Nordic Pulp. Paper Res. J. 15(5):440. 112. Clark J. d. A. 1985. Pulp technology and treatment for paper, Miller Freeman inc, San Fransisco. 113. Stratton R. A. and Colson N. L. 1990. Dependence of fiber/fiber bonding on some papermaking variables. Mat. Res. Soc. Symp. Proc., p. 173. 114. Page D. H. 1985. The mechanism of strength development of dried pulps by beating. Svensk Papperstidning. 88(3):R30. 115. Wågberg L. 2003. On the mechanisms behind the action of dry and wet strength agents. Preprints - 5th Int. Paper. Coating Chem. Symp., Montreal, Canada, p. 281. 116. Wågberg L. 2000. Polyelectrolyte adsorption onto cellulose fibers - a review. Nordic Pulp. Paper Res. J. 15(5):586. 117. Häggkvist M., Solberg D., Wågberg L. and Ödberg L. 1998. The influence of two wet strength agents on pore size and swelling of pulp fibers and on tensile strength properties. Nordic Pulp. Paper Res. J. 13(4):292. 118. Dunlop-Jones N. 1991. Wet-strength chemistry. J. C. Roberts, ed., Paper Chemistry. Blackie Academic & Professional, Glasgow, p. 76. 119. Caulfield D. F. 1994. Ester crosslinking to improve wet performance of paper using multifunctional carboxylic acids, butanetetracarboxylic and citric acid. Tappi J. 77(3):205. 120. Caulfield D. F. and Weatherwax R. C. 1976. Cross-link wet-stiffening of paper: The mechanism. Tappi. 59(7):114. 121. Weatherwax R. C. and Caulfield D. F. 1978. The pore structure of papers wet stiffened by formaldehyde crosslinking. I. Results from the water isotherm. J. Coll. Interface Sci. 67(3):498. 122. Weatherwax R. C. and Caulfield D. F. 1978. The pore structure of papers wet stiffened by formaldehyde crosslinking. II. Results from nitrogen sorption. J. Coll. Interface Sci. 67(3):506. 123. Zhou Y. J., Luner P. and Caluwe P. 1995. Mechanism of crosslinking of papers with polyfunctional carboxylic acids. J. Appl. Polymer Sci. 58(9):1523. 124. Xu Y. F., Yang C. Q. and Chen C. M. 1999. Wet reinforcement of paper with high-molecular-weight multifunctional carboxylic acid. Tappi J. 82(8):150. 125. Espy H. H. 1995. The mechanism of wet-strength development in paper: a review. Tappi J. 78(4):90.

70

126. Howard R. C. and Jowsey C. J. 1989. The effect of cationic starch on the tensile strength of paper. J. Pulp. Paper Sci. 15(6):J225. 127. Stratton R. A. 1989. Dependence of sheet properties on the location of adsorbed polymer. Nordic Pulp. Paper Res. J. 4(2):104. 128. Decher G. 1997. Fuzzy nanoassemblies: toward layered polymeric multicomposites. Science. 277(5330):1232. 129. Wågberg L., Forsberg S., Johansson A. and Juntti P. 2002. Engineering of fibre surface properties by application of the polyelectrolyte multilayer concept. Part I. Modification of paper strength. J. Pulp. Paper Sci. 28(7):222. 130. Dautzenberg H. 1997. Polyelectrolyte Complex Formation in Highly Aggregating Systems. 1. Effect of Salt: Polyelectrolyte Complex Formation in the Presence of NaCl. Macromolecules. 30(25):7810. 131. Gärdlund L., Wågberg L. and Gernandt R. 2003. Polyelectrolyte complexes for surface modification of wood fibres. II. Influence of complexes on wet and dry strength of paper. Coll. Surf. A: Physicochem. Eng. Aspects. 218(1-3):137. 132. Laivins G. V. and Scallan A. M. 1993. The mechanism of hornification of wood pulps. C. F. Baker, ed., Trans. 10th Fund. Res. symp., Oxford, UK, p. 1235. 133. Howard R. C. and Bichard W. 1992. The basic effects of recycling on pulp properties. J. Pulp. Paper Sci. 18(4):J151. 134. Chatterjee A., Lee J., Roy D. N. and Whiting P. 1991. Effect of Recycling on Surface Characteristics of Paper. International Paper Physics Conference, Atlanta, GA, p. 129. 135. Stone J. E. and Scallan A. M. 1965. Influence of drying on the porous structures of the cell wall. F. Bolam, ed., Consolidation of the paper web - Trans. 3rd Fund. Res. Symp., Cambridge, UK, p. 145. 136. Howard R. C. 1990. The effects of recycling on paper quality. J. Pulp. Paper Sci. 16(5):J143. 137. McKee R. C. 1971. Effect of repulping on sheet properties and fiber characteristics. Paper Trade J. 155(21):34. 138. Eastwood F. G. and Clarke B. 1978. Handsheet and pilot machine recycling degradation mechanisms. Fibre-Water Interact. Pap.-Making, Trans. Symp., Oxford, UK, p. 835. 139. Chatterjee A., Roy D. N., Teki S. and Whiting P. 1991. Effect of Recycling on Tear and Fracture Behaviour of Paper. Int. Paper Phys. Conf., Kona, Hawai, p. 143. 140. Bovin A., Hartler N. and Teder A. 1973. Changes in Pulp Quality due to Repeated Papermaking. Paper Techn. 14(5):261.

71

141. Ferguson L. D. 1991. Multiple Recycling: The Impact of Chemical Addition on Optical and Physical Properties. 1st Res. Forum. Rec., Toronto, Ontario, p. 57. 142. Ferguson L. D. 1992. Effects of recycling on strength properties. Paper Techn. 33(10):14. 143. Wang X., Maloney T. C. and Paulapuro H. 2003. Internal fibrillation in never-dried and once-dried chemical pulps. Appita J. 56(6):455. 144. Laine J., Lindström T., Bremberg C. and Glad-Nordmark G. 2003. Studies on topochemical modification of cellulosic fibers part 5. Comparison of the effects of surface and bulk chemical modification and beating of pulp on paper properties. Nordic Pulp. Paper Res. J. 18(3):325. 145. Wågberg L., Ödberg L. and Glad-Nordmark G. 1989. Charge determination of porous substrates by polyelectrolyte adsorption. Part 1. Carboxymethylted, bleached cellulosic fibres. Nordic Pulp. Pap. Res. J. 4(2):71.

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Appendix 1

Equations relevant for calculating interfacial energies from contact angle

measurements and the Lifshitz-van der Waals/acid-base approach

Adhesion between two surfaces of dissimilar materials can be described according to the

following equation:

slssl lW γγγ −+= (4)

where sγ and lγ are the surface energies of a solid (s) and a liquid (l), and slγ is the

interfacial energy between the solid and liquid. Interfacial energies can be determined from

contact angle measurements using Young’s equation and the Lifshitz-van der Waals/acid-base

approach according to the equations given below:

sllssl Θ−= cosγγγ (5)

where slΘ is the contact angle between the solid and the liquid. Lifshitz-van der Waals/acid-

base method uses the following equation when estimating the Lifshitz-van der Waals ( LWγ )

contribution and the acid ( +γ ) and the base ( −γ ) contribution of a solid material:

( ) 2/12 +− ⋅+= ssLWss γγγγ (6)

The interfacial energy between two materials (1 and 2) can then be calculated with the

following equation:

γ12 = (( γ1LW )1/2 - (γ2

LW)1/2)2 + 2((γ1+ · γ1

-)1/2 + (γ2+ · γ2

-)1/2 - ( γ1+ · γ2

- )1/2 - ( γ1- · γ2

+ )1/2 ) (7)

73

Appendix 2

Equations relevant to the JKR theory as applied in the thesis

The underlying equations of the JKR theory are given below as they are described in [24]. For

two elastic solids of radii having curvature R1 and R2, Young’s moduli E1 and E2, and Poisson

ratios v1 and v2, the work of adhesion, W, can be estimated from the relationship between the

cube of the contact radius, a, and the applied load F:

( ) ⎥⎦⎤

⎢⎣⎡ +++= 23 363 WRWRFWRF

KRa πππ (8)

where the elastic constant K can be determined by:

1

2

22

1

21 11

34

−

⎥⎦

⎤⎢⎣

⎡ −+

−=

Ev

EvK (9)

and the equivalent radius R of the non-deformed spheres can be determined by:

21

21

RRRRR

+= (10)

Upon pulling the surfaces apart, a separation (pull-off) finally occurs. The load required to

separate the surface from contact is called the pull-off force, Fs, and it is determined by:

min23 RWFs π= (11)

where Wmin is the adhesive energy at minimum load.