Strategies for improving kraftliner pulp properties

Stefan Antonsson

Doctoral Thesis

Royal Institute of Technology School of Chemical Science and Engineering Department of Fibre and Polymer Technology Division of Wood Chemistry and Pulp Technology

Stockholm 2008

Strategies for improving kraftliner pulp properties

Supervisor: Professor Mikael E. Lindström

AKADEMISK AVHANDLING

Som med tillstånd av Kungliga Tekniska Högskolan framläggs till offentlig granskning för avläggande av teknologie doktorsexamen, fredagen den 19 december 2008 kl. 14.00 i Sal E2, KTH, Lindstedtsvägen 3, Stockholm. Avhandlingen försvaras på svenska.

TRITA-CHE-Report 2008:70 ISSN 1654-1081 ISBN 978-91-7415-175-6

©Stefan Antonsson 2008

The following papers are reprinted with permission: Paper I © Elsevier 2008 Paper II © Springer 2008

Abstract

A large part of the world paper manufacturing consists of production of corrugated board components, kraftliner and fluting, that are used in many different types of corrugated boxes. Because these boxes are stored and transported, they are often subjected to changes in relative humidity. These changes together with mechanical loads will increase the deformation of the boxes compared to the case where the same loads are applied in a static environment. This enlarged creep due to the changes in relative humidity is called mechano-sorptive or accelerated creep. Mechano-sorptive creep forces producers to use high safety factors when designing boxes, and therefore, this is one of the key properties of kraftliner boards.

Different strategies to decrease mechano-sorptive creep, and to simultaneously gain more knowledge about the causes for this phenomenon in paper, are the aim of this work. Derivatised and underivatised black liquor lignins, a by-product produced in pulp mills in large quantities, have been used together with biomimetic methods, to modify the properties of kraftliner pulp. Furthermore, the properties of kraftliner pulp have been compared to other pulps in order to evaluate the influence of fibre morphological factors, such as fibre width and shape factor, on the mechano-sorptive creep. In addition the influence of the chemical composition of the kraftliner pulp has been evaluated both by means of treating a kraftliner pulp with chlorite and xylanase and by producing pulps with different chemical composition.

By using lignin and biomimetic methods, to create radical coupling reactions, it has been shown that it is possible to increase the wet strength of kraftliner pulp sheets. This method of treating the pulp showed, however, no significant effects on the mechano-sorptive creep. The addition of an apolar suberin-like lignin derivative, which has been shown to be possible to produce from natural resources, did show a positive effect on mechano-sorptive creep properties, but at the expense of stiffness properties in constant climate. Different pulps were compared with a kraftliner pulp and it was observed that the ratio between tensile stiffness and hygroexpansion can be used to estimate the mechano-sorptive creep properties. The hardwood kraft pulps investigated had lower hygroexpansion, probably due to more slender and straighter fibres, and higher tensile stiffness, probably due to lower lignin content. As the lignin content was varied by different methods in kraft pulps, it was observed that increased lignin content gives an increased hygroexpansion and decreased tensile stiffness as well as an increased mechano-sorptive creep. There were also indications of increased mechano-sorptive creep due to higher xylan content.

Keywords: lignin, xylan, kraftliner pulp, mechano-sorptive creep, hygroexpansion, stiffness, fibre shape, fibre width

Sammanfattning

En stor del av världens papperstillverkning utgörs av produktion av wellpappkomponenter, kraftliner och fluting, som används i en uppsjö av olika wellpapplådor. När dessa lådor lagras och transporteras utsätts de ofta för förändringar i relativa luftfuktigheten. Dessa förändringar tillsammans med mekanisk belastning ökar lådornas deformation jämfört med om samma belastning skulle ha applicerats vid ett statiskt klimat. Denna förhöjda krypning på grund av förändringarna i relativ luftfuktighet kallas mekanosorptiv- eller accelererad krypning. Mekanosorptiv krypning tvingar producenterna att ha höga säkerhetsmarginaler vid dimensioneringar av lådor och är därför en av nyckelegenskaperna för kraftliner.

Olika strategier för att minska denna effekt, och på samma gång erövra mer kunskap om orsakerna till detta fenomen, har varit syftet med arbetet. Derivatiserade och oderivatiserade svartlutslignin, en biprodukt möjlig att få ut i stora kvantiteter från massabruk, har används tillsammans med biomimetriska metoder, för att modifiera kraftlinermassas egenskaper. Dessutom har kraftlinermassans egenskaper jämförts med andra massors egenskaper för att utvärdera inverkan av fibermorfologiska faktorer, såsom fiberbredd och fibreform på det mekanosorptiva krypet. Också inverkan av den kemiska sammansättningen av kraftliner massan har undersökts både genom behandling med klorit och xylanas och genom att producera massor med olika kemiska sammansättningar.

Genom att använda lignin och biomimetriska metoder för att skapa radikal- kopplingsreaktioner har det visats på möjligheten att öka våtstyrkan i massa-ark. Det här sättet att behandla massa visade dessvärre inga signifikanta effekter på det mekanosorptiva krypet. Tillsatts av ett apolärt suberin-liknande ligninderivat, som visats möjligt att producera ur naturliga råmaterial, visade en positiv effekt på det mekanosorptiva krypegenskaperna även om det var på bekostnad av styvheten vid konstant klimat. Olika massor jämfördes med en kraftlinermassa och det observerades att relationen mellan dragstyvhet och hygroexpansion kan användas för att uppskatta de mekanosorptiva krypegenskaperna. Lövvedssulfatmassorna som undersöktes hade lägre hygroexpansion, antagligen beroende på smalare och rakare fibrer, och högre dragstyvhet, troligen beroende på en lägre ligninhalt. När ligninhalten varierades i sulfatmassor med olika metoder observerades att ökad ligninhalt ger en ökad hygroexpansion och minskad dragstyvhet liksom en ökad mekanosorptiv krypning. Dessutom fanns indikationer på en ökad mekanosorptiv krypning till följd av högre xylaninnehåll.

Nyckelord: lignin, xylan, kraftlinermassa, mekanosorptivt kryp, hygroexpansion, styvhet, fiberform, fiberbredd

List of Publications

This thesis is based on the following papers that are appended at the end. They will be referred to with Roman numerals:

I. Low MW-lignin fractions together with vegetable oils as available oligomers for novel paper-coating applications as hydrophobic barrier. Stefan Antonsson, Gunnar Henriksson, Mats Johansson and Mikael E. Lindström Industrial Crops and Products 27: 98 − 103, 2008

II. Laccase initiated cross-linking of lignocellulose fibres using a ultra-filtered lignin isolated from kraft black liquor Graziano Elegir, Daniele Bussini, Stefan Antonsson, Mikael E. Lindström and Luca Zoia. Applied Microbiology and Biotechnology 77(4): 809 − 817, 2007

III. Adding lignin derivatives to decrease the effect of mechano-sorptive creep in linerboard Stefan Antonsson, Gunnar Henriksson and Mikael E. Lindström. Appita Journal 61(6): 468 − 471, 2008

IV. Comparison of the physical properties between hardwood and softwood pulps Stefan Antonsson, Petri Mäkelä, Christer Fellers and Mikael E. Lindström Manuscript

V. The relationship between hygroexpansion, tensile stiffness, and mechano– sorptive creep in bleached hardwood kraft pulps Stefan Antonsson, Iiro Pulkkinen, Juha Fiskari, and Mikael E. Lindström Manuscript

VI. The influence of lignin and xylan on some kraftliner pulp properties Stefan Antonsson, Gunnar Henriksson and Mikael E. Lindström Manuscript

VII. Applying a novel cooking technique to produce high kappa number pulps - the effect on physical properties Stefan Antonsson, Katarina Karlström and Mikael E. Lindström Manuscript

Author’s contribution to appended papers:

I. Principal author, performed the experimental work.

II. Contributed as co-author to discussion part and to relevant pieces of experimental part, performed the preparation of the ultra-filtered lignin and kraftliner pulp.

III. Principal author, performed the experimental work.

IV. Principal author, performed all the experimental work except the isocyclic and isochronous creep measurements.

V. Principal author, performed the hygroexpansion and isocyclic creep stiffness measurements

VI. Principal author, performed the experimental work.

VII. Contributed to roughly half the writing and experimental work.

Results from some of the above publications have been presented on the following occasions:

13th ISWFPC: International Symposium on Wood, Fibre and Pulping Chemistry (incorporated with the 59th Appita Annual Conference and Exhibition), May 16-19 2005, Auckland, New Zealand, (2005) Antonsson, S., Henriksson, G., Johansson, M. and Lindström, M.E., Biomimetic synthesis of suberin for new biomaterials, Vol 2, p. 561 − 564

6th International Paper and Coating Chemistry Symposium, Stockholm, (2006) Antonsson, S. Henriksson, G. and Lindström, M.E. The Utilization of Lignin Derivatives and Radical Coupling reactions to Increase wet strength of Kraftliner, p. 55

1st Workshop on Chemical Pulping Processes, Karlstad, Sweden, (2006) Antonsson, S. and M. E. Lindström The influence on the mechanical properties of some different pulping raw materials - a comparison between some different hardwoods and softwoods. p. 22 − 26

2nd Workshop on Chemical Pulping Processes, Karlstad, Sweden, (2008) Antonsson, S. Henriksson, G. and Lindström, M.E. The influence of cellulose, hemicellulose and lignin on kraft pulp properties

1. Introduction ...... 1 1.1 Background...... 1 1.1.1 The structure and composition of wood...... 1 1.1.2 Chemical pulping...... 3 1.1.3 Technical lignins ...... 4 1.1.4 Biomimetics involving lignin derivatives...... 4 1.1.5 Use of lignin derivatives in papermaking...... 7 1.1.6 Corrugated board boxes and kraftliner...... 7 1.1.7 Mechano-sorptive creep...... 8 1.1.8 Tensile stiffness...... 10 1.1.9 Hygroexpansion...... 11 1.1.10 Possible strategies to affect mechano-sorptive creep...... 12

1.2 Purpose of the work ...... 12

2. Experimental...... 13 2.1 Materials...... 13 2.1.1 Black liquor...... 13 2.1.2 Linseed oil ...... 13 2.1.3 Kraftliner pulps...... 13 2.1.4 Hardwood pulps...... 14

2.2 Methods...... 14 2.2.1 Fibre analysis equipment...... 14 2.2.2 Chlorite delignification...... 15 2.2.3 Extended Impregnation Cooking...... 15 2.2.4 Cross-flow filtration...... 15 2.2.5 Isolation of lignin from black liquor...... 16 2.2.6 Lignin characterisation methods...... 16 2.2.7 Derivatisation and oxidative coupling ...... 17 2.2.8 Physical and mechanical evaluation methods...... 18

3. Results and Discussion ...... 20 3.1 Influence of lignin derivatives ...... 20 3.1.1 Lignin fractionation and isolation ...... 20 3.1.2 Suberin-like lignin-derivative ...... 21 3.1.3 Effect of treatments with lignin derivatives ...... 22

3.2 Influence of fibre morphology...... 25

3.3 Influence of chemical composition...... 30

4. Conclusions...... 36

5. Acknowledgements...... 37

6. References...... 38

1. Introduction

Ever since the 19th century when F.G. Keller invented the stone ground wood process, H. Burgees and C. Watt the soda process, C.F. Dahl the kraft cooking, and C.D. Ekman and R. Mitscherlich commercialised sulphite pulping invented by B.C. Tilghman (Rydholm 1965a), the main raw material for paper pulp production has been wood. The tremendous development of the field of pulp and paper since then, with huge increases in production volume, has made paper to an economically beneficial material to use in a number of applications. Although paper pulp with wood as the raw material has been produced and developed for a long time there are still many aspects in need for further developments, especially since new areas of use and shift in use put new demands on the products.

1.1 Background

1.1.1 The structure and composition of wood

Wood is a complex composite material consisting mainly of different polymers, morphologically ordered on different hierarchical levels. These polymers are , furthermore, all covalently bonded to each other (Lawoko et al. 2006). Cellulose, in general the dominant constituent in wood, is a linear polymer usually consisting of on average approximately 9 000−10 000 glucose (D-glucopyranoside) residues (Goring and Timell 1962) connected by β 1−4 linkages. Cellulose is ordered and structured in parallel β-sheets building up crystalline fibrils bundled together into fibril aggregates (Fengel and Wegener 1984). These fibril aggregates create the skeleton of the different cell walls, i.e., the primary and three sub walls of the secondary wall, of which the wood fibres consist (Kerr and Bailey 1934). Hemicelluloses can be defined* as the other polysaccharides, besides cellulose and pectin, in wood. This includes e.g. different glucomannans and xylans. The average size or degree of polymerisation of these hemicelluloses is on the order of 100 − 200 units (Jacobs and Dahlman 2001), i.e., about 100 times smaller than cellulose. Hemicellulose together with lignin is located between the fibril aggregates. Lignin is also to a large extent located in the middle lamella, which has the function of linking different fibres together. Lignin is built up by different propylphenols linked together by various carbon-oxygen and carbon-carbon bonds created by radical coupling reactions (Freudenberg 1959). This results in an amorphous three dimensional network polymer. In additions to these polymers, there is also a wide variety of different organic and inorganic compounds in the wood, but usually in minor amounts.

The structure and content of the different polymers as well as the type of other chemical substances present in the wood vary between different wood species and especially between those belonging to the coniferous gymnosperms (softwoods) and those belonging to the eudicotyledones angiosperms (hardwoods). Furthermore, there are huge variations within wood species and individual trees. An example of these variations is the differences in composition of the monomeric units of lignin. The hardwoods contain three different units, as shown in Figure 1; p-coumaryl-, coniferyl- and sinapyl alcohol, whereas the softwoods generally only contain detectable amounts of the first two units. The content of p-coumaryl alcohol units is usually small in both softwoods and hardwoods (Nimz et al. 1981).

*Hemicellulose is a term originally based on a theory that these compounds are precursors to cellulose (Schulze 1891) and several definitions of this term exist.

1 HO HO HO

O O O OH OH OH

(I) (II) (III)

Figure 1: The three monomeric units of which lignin consists; p-coumaryl- (I), coniferyl- (II) and sinapyl alcohol (III). Sinapyl alcohol units are normally not present in coniferous gymnosperms.

Furthermore, the composition of the lignin is determined not only by the wood species but also by whether it is reaction wood (Hägglund and Ljunggren 1933) or juvenile wood (Freudenberg and Lehmann 1963), and by which part of the tree (Bland 1966), which type of cell (Fergus and Goring 1970; Hardell et al. 1980a; Hardell et al. 1980b) and where in the cell wall or middle lamella the lignin originates from (Fergus and Goring 1970; Christiernin 2006).

The composition of and amount of different hemicelluloses is also drastically different between different wood species. Generally, softwoods contain very much, typical on the order of 5−10 times more glucomannans, and considerable lower xylan content, typical on the order of one half to two thirds, compared to hardwoods (Sjöström 1993a). Even though the chemical composition of the cellulose is the same in all wood species, the relative amount is dissimilar. The cellulose content is lower in softwood reaction wood (Hägglund and Ljunggren 1933) while it is higher in hardwood reaction wood (Norberg and Meier 1966). Furthermore, the angle at which the fibril aggregates are ordered relative the fibre axes in the S2 wall, i.e., the so called micro fibril angle, differs depending on the location in the tree, being higher lower down and closer to the pith in the steam (Bonham and Barnett 2001; Sarén et al. 2004).

On a higher hierarchical level, differences in terms of cell wall thickness, fibre width and fibre length can be observed. Furthermore, softwood and hardwoods are composed of different cell types. Softwoods have tracheids and parenchyma cells ,while hardwoods have libriform fibres, fibre tracheids, parenchyma cells and vessels (Sjöström 1993b). The proportions of different cells can differ considerable between different wood species. The formation of the cells is also affected by the growing conditions of the tree, and in temperate forests the woods have parts with shorter and wider cells created during spring, termed earlywood, and parts with longer and narrower cells created during autumn, termed latewood. The amount of earlywood and latewood in the trees is affected by the growth conditions of the tree, such as climate.

Wood is hence a non-homogeneous material, and this is one of the most important parameters affecting pulp properties.

2 1.1.2 Chemical pulping

In order to make a pulp, the raw-material for paper and board, different cells have to be liberated from each other. There are two main ways to achieve this: either pulling them apart with force, i.e., mechanical pulping, or by means of chemical reactions to remove lignin from the wood, and in particular the middle lamella, to an extent where no significant force is needed to separate them, i.e., chemical pulping.

There are several different chemical pulping methods. Sulphite pulping, with sulphite and hydrogen sulphite ions as active chemicals, has advantages such as high brightness and easy bleaching of the pulp. Due to the higher sensitivity to raw material and difficult regeneration of cooking chemicals or even sometimes, depending on the counter ion used, the lack of any possible regeneration method for the pulping chemicals, it is not as widely used nowadays. However, sulphite pulping with magnesium as the counter ion has a more simple regeneration of cooking chemicals and is less sensitive to raw material. Hence, there are also other reasons for the decline of sulphite pulping, such as the decline of traditional areas of use for these pulps.

The predominant chemical pulping method nowadays is kraft pulping. About 90% of the produced chemical pulp produced in the world is produced via this method (FAO 2008). In kraft pulping, sodium hydroxide and hydrogen sulphide ions are used at elevated temperatures to degrade and dissolve lignin. Dependent on the temperature and the amount of hydroxide, sulphide and sodium present during the cook, different chemical compositions of the pulp can be obtained (Aurell and Hartler 1965; Paavilainen 1989). The spent cooking liquor, black liquor, is burnt at the mill in a recovery boiler to regenerate the cooking chemicals and produce steam, which can be used in the mill and in municipal heating networks. The steam can also be used to generate electricity.

The equipment for regenerating cooking liquors and burning the organic substances in the black liquor is extensive. The recovery boiler is about 20% of the total investment cost of a new greenfield mill producing dried, fully bleached pulp. The investment cost for a new mill is about 800 MEUR*. However, in Europe and North America few greenfield mills are built and many mills have successively increased their pulp productions by investments removing bottlenecks in the production. This is done due to the fact that every extra ton pulp produced in a mill without any major investments is profitable as investments in equipment, together with acquiring of the wood raw material are the dominating costs of chemical pulp production.

It is expensive to increase the capacity of the recovery boiler by building a new one or retrofitting an existing one. A new recovery boiler with a larger capacity is generally not built until the old one is in need for replacement. Hence, the bottleneck for further increase in pulp production rate is often the amount of organic matter in the black liquor, mainly originating from the lignin and hemicelluloses that contribute to the production of heat in the recovery boiler (Kirkman et al. 1986). Removal of a part of the organic substances from these recovery boilers would therefore be desired, especially if it is possible to make valuable by-products.

*The estimation is based on the 170 MEUR recovery boiler at SCA Östrand, Sweden (SCA 2006) and the 830 MEUR Metsä Botnia greenfield mill at Fray Bentos, Uruguay (Finnfacts 2005).

3 1.1.3 Technical lignins

At the end of the 19th century and beginning of the 20th century, spent sulphite cooking liquors, originating from cheap cooking chemicals, were generally wasted in the nearest recipient of the mills in Sweden (Davidsson et al. 1975). As this uneconomical use of raw material was questioned and opinions against the environmental problems this procedure caused were raised, a way of using these liquors became the subject of research (Sundin 1981). Even though visionary plans existed involving the creation of a large chemical industry with sulphite pulping liquors as raw material, similar to the German one based on cokes (Sundin 1981), a more simple use, dust binding liquor on gravel roads, was commercialised as one of the first large products based on lignosulphonates (Davidsson et al. 1975).

As time went by, methods of separating and modifying lignosulphonates have been developed, and a wide range of applications, such as concrete additives, feed binders and surface modification additive in lead acid batteries have been found (Gargulak and Lebo 2000). The developments, e.g., in separation and modification methods to yield products for different applications, have been made predominantly by the lignosulphonate producers. If the process knowledge were commonly available, many mills would have the opportunity to produce high quality lignosulphonate. Hence, knowledge in this area is not widespread. However, processing steps such as sugar degradation and cross-flow filtration are used for many applications (Gargulak and Lebo 2000).

Even though lignosulphonates are by far the most important commercial group of lignin products from spent pulping liquors, there are also commercial products from kraft lignin, lignin fragments obtained from the kraft cooking black liquor, in addition to post-sulphonated ones (Gargulak and Lebo 2000). A new technique to obtain a solid, dry lignin from black liquor, called Lignoboost, has recently been developed (Theliander 2007). This lignin has so far mainly been used as fuel but may in future be used in other applications.

Kraft lignin has, however, very different characteristics than lignosulphonates and has for instance almost no solubility in water. Hence the products are also different and more advanced modifications are sometimes needed to make attractive products, as when asphalt emulsifier is produced via reaction with glycidylamin (Gargulak and Lebo 2000). Although kraft lignin possesses solubility properties, it is still quite polar, as confirmed by its solubility in ethylene glycol but not in solvents such as hexane or diethyl ether (Schuerch 1952).

1.1.4 Biomimetics involving lignin derivatives

There are examples of applications where more hydrophobic lignin derivatives would be desired. To use lignin derivatives as compatibility mediators in reinforcement pulp fibres in plastic composites or to use lignin derivatives as antioxidants in plastics could possibly be achievable with a more hydrophobic lignin derivative, which would have better interactions with plastics. Furthermore, such a lignin derivative could also be used to give new properties to fibreboards and papers.

In nature, suberin is an example of such a compound. Simplified, suberin is a lignin-type polymer that also contains numerous fatty acids, anchored through, among others, ester bonds (Kolattukudy 2002). It is arranged in polyphenolic and polyaliphatic lamellas or domains. The polyaliphatic part can incorporate a variety of different fatty acids depending on source of the

4 polymer (Bernards 2002). However, a tentative structure influenced by that made of Bernads (2002) of a possible suberin structure is presented in Figure 2. (Bernards 2002)

O Suberin O O

O O O HO O O HO O O O O O OH O OH O HO O O HO O O O HO O O O Suberin O O O O OH O O O O O OH O O O HO O O O O O HO O O O O O Suberin O HO O O O O HO O O HO OH O O O O OH HO O O HO

O O O Lignin O Suberin Lignin O O O

Figure 2: A tentative structure of a part of a suberinpolymer, freely drawn with inspiration from the structure presented by Bernads (2002).

Suberin is the biopolymer giving, for example, cork-oak bark and birch bark their hydrophobic and resistant characteristics. Even if it is hard to estimate the content of suberin because of the complex polymeric structure and similarities to lignin and cutin, an ester inter- linked polyaliphatic waxy tissue, contents of over 50% in for example birch, Betula pendula, and cork oak, Quercus suber, barks have been reported (Gandini et al. 2006). Suberin acts, however, as a diffusion barrier in all barks, roots and other parts of plants that need protection against apoplastic water transportation and pathogens (Kolattukudy 2002).

Another group of hydrophobic phenolic substances from nature is urushiols, which are present together with, among other things, laccase in urushi, the sap from the Japanese lacquer tree, Rhus vernicifera. In Japan, urushi is used as lacquer for handcrafts (Ikeda et al. 2001). Urushiols are catechol derivatives with saturated or more often unsaturated hydrocarbon chains (Majima 1909; Majima 1922). After they have been applied on a surface, the urushiols can react by oxidative cross-linking reactions, initiated by the laccase and the oxygen in the air, involving the phenolic part, and autoxidation of the unsaturated hydrocarbon part (Ikeda et al. 2001), similar to the biosynthesis of lignin and drying of polyunsaturated vegetable oils, (e.g., linseed oil) respectively.

Biosynthesis of lignin is accomplished through enzymatic initiated oxidative coupling reactions of lignin monomers (Erdtman 1933; Freudenberg 1959). For this to proceed, the free phenolic group of the lignin monomer has to be oxidised to a phenoxyl radical that is resonance stabilised. These radicals will couple, creating typical carbon-carbon and carbon- oxygen lignin bonds eventually resulting in a lignin polymer, as shown in Figure 3.

5 HO

O OH

-H+, -e-

HO HO HO HO HO . . . . O O O O O O O O. O O

OH HO OH HO O HO O H3CO OH OCH O OH 3 HO O H CO OH 3 OCH3 OH HO O OCH3 O HO O OCH3 O HO OH H3CO OH OCH O HO 3 OCH3 OH HO HO O H CO O 3 OCH3 HO HO OH HO O O HO OH OH H3CO H3CO OH O OCH3 HO OO O OCH3 OH O HO OH OH O OH HO H CO H3CO 3 O HO OCH3 OCH3 O OOCH3 O OCH 3 OH HO O HO O H3CO OH OH HO OCH3 O O H3CO O HO OH

OH HO OH HO OH

Figure 3: Oxidation of the free phenolic group of a coniferyl alcohol to a phenoxyl radical that is resonance stabilised. Radical coupling, creating typical carbon-carbon and carbon-oxygen lignin bonds, eventually results in a softwood lignin polymer of which the lower part of the figure is a tentative model (with content of p- coumaryl alcohol).

This biochemical reaction was biomimetically performed in the laboratory with iso-eugenol, similar in structure to coniferyl alcohol, and mushroom oxidase enzymes already in 1908 (Cousin and Herissey 1908). However, it is not certain that the enzymes directly oxidise the monolignols. A redox-shuttle mechanism, where the enzyme oxidises manganese(II) to manganese(III), which in turn initiates radicals in lignin monomers, has been suggested as a possible route for the biosynthesis of lignin (Önnerud et al. 2002). It has also been demonstrated that transition metal salts of iron, cobalt, manganese and copper, may be used on their own as radical initiator (Landucci 1995; Landucci et al. 1995). These metal ions may also be used in a technical process. In addition, manganese(II) could be regenerated to manganese(III) by oxygen in a solution with an excess of citrate at pH 7-9 (Klewicki and Morgan 1998) and hence also work as a catalyst without enzymes.

However, there are many other possibilities for initiating radicals in free phenolic groups. Persulphate as well as hexacyano ferrate and ferric chloride have been demonstrated as potential coupling agents for phenols and lignocellulosic fibres (Allan et al. 1971). Less expensive and more environmentally friendly compounds could also be used, such as

6 Fenton’s reaction, which with peroxide and transition metal ions, generates hydroxyl radicals that also react with non-phenolic lignin. Radicals can also be initiated by physical means such as gamma irradiation (Seifert 1964), UV-irradiation (Brunow and Sivonen 1975), ultrasonic waves (Yoshioka et al. 2000) and corona discharges (Chen et al. 2004). It is, however, also possible to heavily degrade the lignin with some of these methods.

The same type of reactions that take place in nature when urushi and linseed oil are drying or when lignin is biosynthesised could be utilised in lignin derivative systems as well. To enhance the radical coupling reactions it is then an advantage to have a high free phenolic content in the lignin derivative used. As lignin is degraded in kraft cooks the content of free phenolic groups increase both in the residual and in the black liquor lignin (Gellerstedt and Lindfors 1984; Robert et al. 1984). By means of cross-flow filtration, it has also been demonstrated that it is not only possible to obtain a lignin with lower molecular weight but also a higher free phenolic content compared to the non-filtered black liquor lignin (Keyoumu et al. 2004). This is not surprising in view of the fact that lower molecular weight lignins have been shown to be higher in free phenolic content (Griggs et al. 1985).

1.1.5 Use of lignin derivatives in papermaking

There are other demands on the lignin derivatives besides possessing the ability to polymerise and to be hydrophobic, if one aims to modify paper properties. It is necessary that the substance can be applied somewhere in the paper mill without problems with scaling, fouling, biological growth and a large increase of biological and chemical oxygen demands in waste water. Except changes of colour, smell and taste of the paper, of which the later two in particular are important for use in applications with different degrees of contact with food, there are many mechanical properties that should be taken into account.

When considering lignin derivatives most of these demands can be fulfilled for at least certain lignin derivatives. However, even if the colour of the lignin derivatives can be very different, they will more or less change the colour and decrease the brightness of the paper. This is not always a problem; in production of testliner, pigments are sometimes added to give the impression of a brown quality board similar, with regards to mechanical properties, to kraftliner. Nevertheless, the use of lignin derivatives as paper chemicals should probably be limited to unbleached paper qualities, such as corrugated board material. About 25 – 30% of the world paper production is corrugated board material (FAO 2008).

1.1.6 Corrugated board boxes and kraftliner

Corrugated board and boxes was developed in several steps during the second half of the 19th century in England and United States. At the beginning of the 20th century, corrugated boxes started to replace wooden boxes for the transportation of goods (Hunter 1978). The demands on the corrugated box have diversified since then. This is due in part to the development of several different uses and the subsequent invention of different types of corrugated boards, as well as the development in the handling of these boxes. The corrugated board is composed by corrugated fluting paper sandwiched between linerboards. The principal assignment of the fluting paper is to separate the linerboards in order to give high bending stiffness to the corrugated board, while the linerboards are expected to carry the in-plane loading. Some properties are important for many types of corrugated boxes; stacking performance, buckling

7 resistance, torsion resistance and resistance to drops, as well as die cutting performance, are some examples. These parameters measured on boxes have been found to correlate with properties of the components of the corrugated board i.e., the kraftliner, testliner and fluting (Henriksson et al. 2007). This means that it is of interest to develop, e.g., the kraftliner towards an improved property composition in order to obtain mechanically improved corrugated boxes. This can be done both by making changes in the papermaking process and in the production of the kraftliner pulp.

1.1.7 Mechano-sorptive creep

One of the most critical parameters of kraftliner is the so called mechano-sorptive or accelerated creep. It was discovered in paper (corrugated board) by Byrd (1972), but has been known in other materials, such as concrete (Pickett 1942), wool (Mackay and Downes 1959) and the main raw material for paper pulp production, wood (Armstrong and Kingston 1960), for a longer time. The stiffness of a paper is decreased as the moisture content of the paper is increased (Salmén et al. 1984) at higher relative humidity. When a paper is simultaneously subjected to a load and changes in relative humidity it will, however, creep or deform, more than if the same load were to be applied at a high constant relative humidity, as described in Figure 4. This is called mechano-sorptive creep. (Byrd 1972)

6 Constant 90%RH Cyclic 90-35%RH 5 Failure

4

3

Creep strain [%] strain Creep 2

1

0 10 100 1 000 10 000 Time [min]

Figure 4: When the same load (57% of critical stress) is applied on a paper (corrugated board) at cyclic relative humidity it will creep more and fail more quikly than if the same load were to be put at constant but high relative humidity as this reprinted data from Byrd shows (Byrd 1972).

Different theories about the reasons behind this behaviour in paper have been put forward. One theory says that the changes in humidity make the free volume of the amorphous polymers in the paper change, and therefore the paper will becomes more sensitive to creep (Padanyi 1991). Another theory explains the phenomenon with moisture gradients in the z- direction of the paper, creating different stresses at different locations in this direction, as illustrated in Figure 5 (Habeger and Coffin 2000).

8 High %RH Low %RH High %RH

Moisture profile in z- direction

Stress profile in z- direction

Tensile load

a) b) c) d) e)

Figure 5: One of the theories put forward to explain the mechano-sorptive creep in paper is based on the hypothesis that moisture gradients in paper result in a stress distribution in the z-direction of the paper. This can be illustrated by the following steps: a) the moisture is uniformly distributed and so is the stress; b) as the air becomes drier the moisture at the surfaces decreases and to obtain equal strain across the z-direction the stress at the surfaces will be higher; c) when the moisture is uniform again the stress level will be equal; d) as the air becomes more moisturised a lower level of stress will be obtained at the surface; e) as equilibrium is reached the levels are uniform again (Habeger and Coffin 2000).

A third theory explains the mechano-sorptive creep with stresses that are built up due to anisotropic hygroexpansion in fibre-fibre joints at changes in humidity (Alfthan et al. 2002) as illustrated in Figure 6. Hygroexpansion in the transverse direction of fibres is considerable higher than in the longitudinal direction.

Low %RH High %RH

High stresses

Figure 6: Building up of stresses due to anisotropic hygroexpansion in fibre-fibre joints at changes in relative humidity is one of the theories explaining the mechano-sorptive creep effect in paper (Alfthan et al. 2002).

It is still not clear which of the theories or combinations of theories best explain the mechano- sorptive effect. However, different studies investigating the influence of various parameters on the mechano-sorptive creep have been conducted.

Morphological factors, such as fibre length, width, cell wall thickness, coarseness and fibril angle, may influence the mechano-sorptive creep. It has been found that the choice of pulping raw material may affect the creep rate in cyclic humidity in a different manner than it affects the creep rate at constant humidity (Byrd 1984). It has also been shown that pulps with higher yield and consequently higher lignin content (i.e. a kappa number higher than 100) have a higher creep rate in changing relative humidity (Byrd and Koning 1978; Byrd 1984)

Furthermore, there appears to be a connection between wet strength and mechano-sorptive creep. By increasing the wet strength of paper by cross-linking with multi-functional carboxylic acids or formaldehyde it has been reported that mechano-sorptive creep decreases

9 (Caulfield 1994). Increasing the hydrophobicity of paper also appears to lower mechano- sorptive creep; wax-dipped paper that doubtless is more hydrophobic has been reported to creep less than untreated paper. This implies that the hydrophobicity of the material can also be of importance for mechano-sorptive creep (Caulfield 1994). Processing operations that improve tensile stiffness by straightening fibres or improving bonding between them, such as drying under restrain, pressing and beating are also reportedly important for lowering mechano–sorptive creep (Panek et al. 2005). It is, however, clear that also individual wood fibres show mechano-sorptive creep (Olsson et al. 2007).

1.1.8 Tensile stiffness

Tensile stiffness is the slope of the first linear part of the stress strain curve as described in Figure 7. The tensile stiffness, or elastic modulus, is a key parameter for many papers, and a parameter most likely linked to the stiffness at changing relative humidity, i.e., the mechano- sorptive creep behaviour. Tensile stiffness has been stated to be a result of the elastic modulus of the fibres, the degree of bonding in the sheet and the presence of curl, kinks, crimps and microcompressions in the fibres (Page et al. 1979).

Figure 7: Schematic figure of a tensile stress-strain curve of paper describing the different parameters.

It has indeed been shown that the stiffness is increased as the density of the sheet is increased by pressing or beating the pulp (Brezinski 1956; Onogi and Sasaguri 1961). Lower coarseness of the fibres is also thought to give a higher contact area between fibres in the sheet (Paavilainen 1994; Seth 1996) and therefore a higher tensile stiffness index. Longer and narrower fibres are, moreover, believed to give higher stiffness (Page and Seth 1980a). The stiffness of individual fibres, which most likely is important for the stiffness of the sheet, has been reported to be affected by the fibril aggregate angle in the dominating S2 wall (Watson and Dadswell 1964). Data of single wood pulp fibres with low fibril angle give steep substantially linear stress-strain curves (Page and El-Hosseiny 1983). The effect of the fibril angle on the single fibres has also been shown to be valid for a sheet. Sheets of pulps with a lower average fibril angle show higher tensile stiffness (Courchene et al. 2006).

The influence of the chemical composition on the stiffness of the pulp fibres is not completely clear. The stiffness has been reported to be optimal for kraft pulps at a kappa number of about 30-50 (Neagu et al. 2006), corresponding to a lignin content of about 6%. A decreased amount of hemicellulose, achieved by making the pulp with the prehydrolysis kraft method instead of the ordinary kraft method, reportedly increases the stiffness of individual fibres at comparable kappa numbers (Neagu et al. 2006). However, to some extent contradictory to this result, it has been stated that the stiffness of the individual fibres is approximately the

10 same at different hemicellulose contents, as compared between holocellulose and kraft pulp fibres of a 45% yield (Page et al. 1977). For sheets, it has been shown that a higher content of hemicelluloses in softwood (Norway Spruce, Picea abies) pulp sheets with the same density increases the tensile stiffness (Molin and Teder 2002). Furthermore, the addition of a xylan- rich black liquor to a softwood (Norway Spruce, Picea abies) kraft cook can significantly increase the tensile stiffness, at the same density of the sheets made from the pulp (Danielsson and Lindström 2005). Bleached pulps seem to have lower tensile stiffness compared to corresponding non-bleached pulps. This could be due to the introduction of micro- compressions in the fibres (De Grâce and Page 1976) and fibre deformations (Page and Seth 1980b) rather than a decreased lignin content. Chemical modifications of pulps by means of cationic polyallylamine (Gimåker et al. 2007) and carboxymethyl cellulose, CMC, (Duker et al. 2007) have been shown as a possible rout of increasing the sheet stiffness.

1.1.9 Hygroexpansion

The stiffness of the sheet in constant humidity probably influences the mechano-sorptive creep performance of sheets, and the movement of the sheet structure due to changing relative humidity, i.e., hygroexpansion, may have an influence. Furthermore, hygroexpansion is important for printing and converting operations of corrugated board. It has been suggested that sheets of lower density and higher porosity might be expected to exhibit smaller changes in external dimensions with changes in humidity because of the greater opportunity for fibre expansion and contraction within the voids of the sheet (Swanson 1950). However, higher densities due to more even fibre wall thickness distributions have recently been suggested to give less hygroexpansion (Pulkkinen et al. manuscript). How the density of the sheet is modified is likely important and can possibly determine the extent of hygroexpansion.

Increasing sheet density by beating (Swanson 1950; Klipper 1952; Brecht et al. 1956; Laptew and Kraft 1967) and adding fines (Salmén et al. 1987) to the pulp increases hygroexpansion. Beating followed by fines removal, however, reportedly does not significantly affect the hygroexpansion in restrained dried sheets (Salmén et al. 1987). This finding, suggesting that the fines that fill in the macroscopic pores of the sheets increase hygroexpansion, somewhat contradicts the theory that increased conformability and bonding area together with decreased free fibre length would cause increased hygroexpansion when pulps are beaten (Gallay 1973).

An increase in hemicellulose content (Brecht et al. 1974) or pentosane (i.e. roughly xylan) content (George 1958) has been claimed to give an increased hygroexpansion. However, additions of guar gum (Swanson 1950) or cationic starch (Laurell Lyne 1994), have not been shown to significantly influence the hygroexpansion. In addition lignin can affect hygroexpansion. A study where NSSC-pulps of spruce were made by cooking to different yields followed by refining to 55°SR shows that the hygroexpansion is highest at a yield of about 55% corresponding in this case to a lignin content of 22% (Brecht and Hildenbrand 1960) and lower above and below this yield. This corresponds, however, not only to a high lignin content but also high values of fibre saturation point (FSP) (Scallan and Tigerström 1992; Andreasson et al. 2003), water retention value (WRV) (Ohta et al. 1986; Andreasson et al. 2003) and pore size distribution (Andreasson et al. 2003). Additionally, pulp from the NSSC process will contain a sulphonated lignin that may affect the hygroexpansion in another way than non-sulphonated kraft pulp lignin. Morphological factors, such as fibre length, fibre width, and cell wall thickness, can also influence hygroexpansion. Increased fibre length, altered by choosing different hardwoods as pulping raw material or by the Bauer-McNett

11 fractionation of a softwood pulp, reportedly reduces hygroexpansion (Kijima and Yamakawa 1979; Uesaka and Moss 1997). Increased cell wall thickness, studied in various hardwood pulps, has been suggested to give a lower hygroexpansion coefficient (Uesaka and Moss 1997; Pulkkinen et al. manuscript). Furthermore, if the angle of orientation of the fibril aggregate in relation to the fibre axis is lower, it has been shown to reduce both the hygroexpansion coefficient and hygroexpansion (Uesaka and Moss 1997; Courchene et al. 2006).

1.1.10 Possible strategies to affect mechano-sorptive creep in paper

There are many parameters that could affect the mechano-sorptive creep in paper, and there are many possibilities in varying these parameters. The wet strength of kraftliner has been reported to be increased by treatments with laccase and lignin model compounds (Lund and Felby 2001; Chandra et al. 2004), and hence cross-flow filtrated black liquor lignin derivatives, with high free phenolic content, together with a radical initiator may also lead to increased wet strength in kraftliner. A hydrophobic lignin derivative could also be added to the pulp. These treatments might possibly affect mechano-sorptive properties. Furthermore, different morphological factors of the pulp may be of importance. A way to investigate the influence of different fibre geometries is to compare pulps made from different wood species. The influence of the chemical composition of pulps on the mechano-sorptive creep can be studied by means of different cooking strategies as well as utilizing more selective delignification methods and enzymes to remove carbohydrates. Hence, several possibilities for gaining more knowledge about how different parameters connected to the paper pulp affect the mechano-sorptive creep need to be investigated.

1.1 Purpose of the work

The purpose of this work is mainly to find strategies to decrease mechano-sorptive creep in a common paper grade used as top and bottom layer in corrugated boxes, that is, kraftliner. This has been done with the aim of avoiding negatively influencing on other mechanical and physical properties. Possible benefits of using kraft lignin derivatives, using different pulping raw materials, having different chemical composition as well as practising a novel cooking concept have been evaluated as possible strategies. In doing this gaining better understanding of the mechano-sorptive creep phenomenon has also been a goal of this work.

12 2. Experimental This part is an overview of the material and methods used. The details concerning the experimental work are presented in papers I-VII.

2.1 Materials

2.1.1 Black liquor (I-III)

Black liquor is a common name of the spent pulping liquor after kraft cooks. There are often large differences between industrial and laboratory liquors; for example industrial black liquor contains minor amounts of sulphate and carbonate as residues from the recovery and more degraded lignin due to several re-uses of the black liquors in impregnations of chips to recover heat. There are also differences between different industrial black liquors due to different cooking strategies and variations in equipment as well as raw material. The black liquor used in this study originated from the industrial cooking of softwood, almost exclusively Scots Pine, Pinus sylvestris and Norway spruce, Picea abies. The liquor was kindly supplied from the Korsnäs pulp mill, Gävle, Sweden, and withdrawn from a continuous cooking resulting in a pulp with kappa number 25, i.e., a lignin content of about 4%.

2.1.2 Linseed oil (I)

Linseed oil is widely used for the surface coating of wood. In this study a linseed oil with trade name Purolin was used and kindly supplied from the Swedish Farmers Supply and Crop Marketing Organization, Sweden. A detailed description of this linseed oil variety can be found elsewhere (Stenberg et al. 2005) but the main difference of this linseed oil is a high linoleic acid content, 74.2%, compared to conventional linseed oils, which normally are rich in linolenic acid. Linoleic acid is a fatty acid with two unsaturated bonds whereas the linolenic acid possesses three unsaturated bonds, as shown in Figure 8.

O O OH OH

Figure 8: Linoleic acid (left) and linolenic acid (right).

This pattern of fatty acid in the oil results in a more even and complete drying when used in the coating of wood (Stenberg et al. 2005).

2.1.3 Kraftliner pulps (II-IV, VI-VII)

An advantage when modifying pulp by means of oxidative radical coupling reactions is a high content of residual lignin and pulp manufactured with the kraft process possesses a high abundance of free phenolic groups in the residual lignin compared to native lignin (Gellerstedt and Lindfors 1984). A kraftliner pulp with a kappa number of about 76 was chosen for the study. It was kindly supplied by Smurfit Kappa Kraftliner, Piteå, Sweden, and was made of

13 almost exclusively Scots Pine, Pinus sylvestris and Norway Spruce, Picea abies. The pulp was received never-dried and carefully washed with de-ionised water to a conductivity below –1 10 μS cm . It was Escher-Wyss beaten to about 30 M°SR (corresponding to about 16 °SR) before use. This pulp was also used as reference in studies not involving the lignin derivative treatments. However, two other kraftliner pulps made of same wood species, washed in a similar way, were also used. One was from SCA Munksund, Sweden, and had a kappa number of about 73 and the other was from Smurfit Kappa Kraftliner, Piteå, Sweden, and had a kappa number of 67.

2.1.4 Hardwood pulps (IV-VI)

In order to compare the influence of fibre morphology on the physical properties of a sheet different hardwood pulps were selected. Unbleached never-dried kraft pulps of birch (Betula pendula/B. pubescens) and eucalyptus (Eucalyptus urograndis as well as E. globulus) as well as a Neutral Sulphite Semi Chemical, NSSC pulp of birch (Betula pendula/B. pubescens) were used. Also several bleached hardwood, birch (Betula pendula/B. pubescens), eucalyptus (Eucalyptus grandis/E. dunnii; E. globulus; E. urograndis; E. urograndis/E. grandis; E. grandis /E. saligna) and acacia (Acacia magnium), kraft pulps received as dry sheets were used.

2.2 Methods

2.2.1 Fibre analysis equipment (IV-V)

The determination of average pulp fibre geometrics was a tedious work involving microscopy until the 1980-ties. Equipment for these determinations, based on a flow of a much diluted pulp suspension trough a cell, the recording of pictures of the pulp constituents by 2D CCD- camera and picture analysis software, was then developed. In this work two different apparatuses have been used to determine pulp fibre properties: STFI Fibermaster3, STFI- Packforsk, and FiberLab, Metso Automation. Approximately 10 000 fibres were measured in each sample, and the mean fibre length, fibre width, coarseness, cell wall thickness and shape factor (curl) are examples of properties that can be measured or calculated. Coarseness is the average weight per length unit fibre, determined by the sum of fibre lengths and the total weight of the sample.

L Shape Factor = l l l Curl = −1 L L

Figure 9: Illustration of the definition of curl and shape factor.

Curl is calculated as the relationship between the fibre contour length and the longest dimension of the fibre subtracted by 1 (Robertson et al. 1999). Shape factor is calculated as the relationship between the longest dimension and fibre contour length, as illustrated in Figure 9.

14 2.2.2 Chlorite delignification (VI)

Wood is a complex material, and in order to analyse the influence of different constituents it is important to be able to remove part of the wood as selectively as possible without damaging other material. To remove lignin more selectively than during traditional chemical pulping, delignification with chlorite is a common method (Ahlgren and Goring 1970). However, despite its high selectivity, the average degree of polymerisation of cellulose is somewhat lowered by chlorite delignification (Timell 1959).

2.2.3 Extended Impregnation Cooking (VII)

A novel kraft cooking technique has recently been developed by Metso and goes under the marketing name CompactCooking G2 concept. This Extended Impregnation Cook technique, EIC, involves longer impregnation time at lower temperature in order to facilitate diffusion over the consumption of active cooking chemicals and thus resulting in a more homogenous delignification of the wood chips and less reject. By applying this technique and cooking at as low a temperature as 135oC, compared to conventional kraft cooks often conducted at 160oC, it has been shown that softwood kraft pulp with a kappa number in the region of 90 to 60 can be made with a low reject content without inline refining (Karlström and Lindström Manuscript). Such pulps have been used to compare the influence of lignin content on different mechanical properties.

2.2.4 Cross-flow filtration (I-III)

Cross-flow filtration was used to separate black liquor lignin in order to obtain specified lignin fractions. Cross-flow filtration is a pressure-driven membrane separation process operating according to the principle shown in Figure 10 (Coulson et al. 1999). Depending on the size of the materials that are possible to separate by the membranes used, the filtration can be classified as microfiltration (>0.1μm), ultrafiltration (0.1μm-2nm), nanofiltration (<2nm) and reversed osmosis (selective movement of solvent molecules against its osmotic pressure difference) (Koros et al. 1996).

Permeate Feed flow Retentate

Permeate

Figure 10: The principle of cross-flow filtration.

Membranes used for cross-flow filtration are most commonly made by polymeric materials, such as polyamide, polysulphone and polycarbonate. However, membranes of inorganic oxides materials are also available from micro- down to nanofiltration. The advantage of the inorganic membranes is high stability over a wide range of pH and temperatures as well as increased resistance to fouling (Coulson et al. 1999). These membranes allow the use of cross-flow filtration on black liquor with high pH and, if necessary, high temperatures. The equipment used in this study consisted of a mixing tank, a gear pump and a cross-flow ™ ™ filtration unit (Kerasep K01B Module) with a Kerasep , ZrO2 coated ceramic membrane

15 from Orelis, France. The membrane cut-off was 1 000 Da, the total filter area 816 cm2 and the total volume 0.76 dm3. The filtration was made at ambient temperature with a pressure drop over the membrane of about 1 bar and with recirculation of the retentate black liquor.

2.2.5 Isolation of lignin from black liquor (I-III)

In black liquor free phenolic and carboxylic groups of the dissolved lignin are deprotonised. Even though it might be possible to extract some lignin from the liquor using less polar solvents such as dichloromethane or ethyl acetate this would yield very small amounts of lignin and would not be an industrially feasible method. Instead, precipitation of lignin by acidification of the black liquor and protonation of the lignin functional groups, followed by filtration, is a more efficient and industrially applicable way of isolating lignin. This could be done by carbon dioxide or different kinds of mineral acids, such as sulphuric acid (Rydholm 1965b; Lin 1992); the latter was used in this study. Both sulphuric acid and carbon dioxide result in residual liquor that can possibly be regenerated in the recovery system.

Due to the fact that the solubility of different kraft lignin molecules at a certain lignin concentration, pH and ionic strength are dependent on the different pKa values of the functional groups (that also though could vary depending on neighbouring substituents (Ragnar et al. 2000)), as well as size of the kraft lignin molecules, it is also possible to separate different fractions by stepwise precipitation of lignin between different pH-intervals (Lin 1992). This was partly applied in isolation of lignin derivatives in this study.

To separate inorganic salts and other contaminants from the precipitated kraft lignin a washing step could be conducted by making a slurry of the precipitated lignin in diluted mineral acid (Lin 1992). Washing was practised for the stepwise precipitated lignins.

2.2.6 Lignin characterisation methods (I-III)

A standard method to determine the molecular weight distributions of polymers involves different kinds of size exclusion chromatography, SEC (sometimes also referred to as gel permeation chromatography, GPC). In this study, such a system was used, consisting of a Waters 515 HPLC-pump, three Ultrastyragel columns, with pore sizes of 10 nm, 50 nm and 100 nm respectively, and a Waters 2487 UV-detector measuring at 280 nm. Tetrahydrofuran, –1 THF, was used as a mobile phase, and the flow rate was 0.8 ml min . Linear polystyrene –1 –1 standards with molecular weights between 162 g mol and 115 000 g mol were used as calibration enabling the estimation of the molecular weights of the samples.

The advantage with this system is its robustness and ease of use, but the drawbacks are that kraft lignin has to be acetylated before injections, that the absorptivity for kraft lignin molecules of different sizes is not necessarily the same, and that the linear polystyrene standards are quite different in shape compared to the three dimensional network of the kraft lignin molecules. Hence, the results of the molecular weight distribution of the lignin derivatives should be seen in relation to other lignin fractions rather than as precise values.

However, there are many properties other than the molecular weight distribution that are important for the kraft lignin characteristics. The free phenolic content in the lignin is an important parameter. It affects the ability of lignin to crosslink by oxidative coupling reaction,

16 as discussed earlier, as well as the ability to act as a radical scavenger in for instance plastics, due to the fact that radicals usually are introduced on these groups in lignin.

There are several methods for the determination of the abundance of this functional group. An easy and quick analysis of the free phenolic content could be done by UV-Vis spectroscopy using the fact that free phenols differ considerably in absorption in alkaline and neutral or acidic conditions (Aulin-Erdtman 1954; Goldschmid 1954).

–3 By dissolving lignin in 1:1 dioxane: 0.2 mol dm sodium hydroxide solution and diluting it –3 with a pH 6 buffer and a 0.2 mol dm sodium hydroxide solution respectively, a difference spectrum could be measured by a double beam UV-Vis-spectrophotometer. Based on an empirical relationship between free phenolic content determined by aminolysis and the UV- Vis method proposed by Gärtner et al. (1999), the content could be determined using –1 Equation 1, where [ϕ-OH] is the free phenolic content in mmol g , A is the absorbance, c is the concentration and l is the length of the vial containing the solution. (Gärtner et al. 1999)

1 []ϕ − OH = {}0.250× A + 0.107× A × (Eq. 1) 300nm 350nm c×l

Although this is a rapid and easy method, it is only applicable to typical phenolic structures in lignin and should not be used in highly modified lignins or lignins of very exotic origin (Gärtner et al. 1999). A more accurate way of determine the free phenolic content is 1H-, 13C- or 31P-NMR, nuclear magnetic resonance spectroscopy determinations.

Carboxylic content is also an interesting functional group in the lignin to characterise due to the fact that it also could also be a site for chemical modification of the lignin. The carboxylic content could be determined by NMR-methods or conductometric titration but also easily semi-quantitativly by means of Fourier-transform infrared, FTIR, spectroscopy, as carbonyl groups have strong absorbance in the infrared region. In this study FTIR was used to determine carboxylic content.

2.2.7 Derivatisation and oxidative coupling (I-III)

In order to obtain a more hydrophobic lignin derivative, kraft lignin could be derivatised with, for instance, fatty acids or triglycerides by esterification resulting in structures similar to units in suberin. The free phenolic groups are not as strong nucleophiles when conjugated with carbonyl or unsaturated moieties on the propane side chain (Ishikawa et al. 1961) and hence less reactive in esterfications than carboxylic and aliphatic hydroxyl groups. However, aliphatic hydroxyl groups are not very abundant in kraft lignin, especially not with low molecular weight (Robert et al. 1984; Griggs et al. 1985), while carboxylic groups could be more abundant in fractions isolated at lower pH. Hence, a black liquor lignin isolated by precipitation between pH 5 and pH 3 was used together with linseed oil in acetone and with sulphuric acid catalyst to create a suberin-like lignin-derivative. The reaction was performed over a nitrogen-flow to avoid reactions with the unconjugated cis-unsaturations in the linseed oil, i.e., drying of it.

The hydrophobicity of the suberin-like lignin derivative was measured by dissolving some of it in acetone; applying it to filter paper and measuring the contact angle in a FIBRO DAT 1100 system.

17 There are a large number of possible radical initiating systems to use. However, one laccase, Trametes pubescens, and a manganese(III) acetate-citrate system were chosen. These systems were used to initiate radicals in lignin present both as residual lignin in fibres and as added kraft lignin isolated from cross-flow filtrated black liquor.

2.2.8 Physical and mechanical evaluation methods (III-VII)

Except for ISO and SCAN standard methods used to prepare Rapid Köthen sheets and evaluate properties as dry- and wet strength, non-standardised methods and/or apparatus have also been used when standard methods/apparatus not have been available. Hygroexpansion was measured according to ISO 8226-1 using paper-strips with 15 mm width and a span of 100 mm between the clamps. The measurements were performed on the STFI HygroexpansivityTester, an instrument developed at STFI-Packforsk, schematically presented in Figure 11.

Weight

Rigid Clamp Movable Clamp

Chamber Test piece Detector

Figure 11: Schematic figure of the hygroexpansion measurement apparatus used.

It consists of pairs of rigid and freely movable clamps with a gap between the clamps of 100 mm. Thirty paper strips can be measured independently in a horizontal position. A weight was placed upon the strips to eliminate any effects of buckling during registration of lengths. These clamps were placed in a chamber with regulated humidity. Movement of the movable clamps was measured by a detector. The hygroexpansion coefficient, β%RH, was calculated by dividing the hygroexpansion with the differences in relative humidity. Another common way of presenting a hygroexpansion coefficient is the hygroexpansion divided by the differences in the moisture content of the test piece.

The creep measurements at both constant humidity and with variations in relative humidity were performed on an apparatus developed by Haraldsson et al. (1994). It allows measurements with a load, both in tension and compression, during any period of time and with any humidity profile to be conducted; and the principle construction of the apparatus is shown schematically in Figure 12. This apparatus is placed in a climate chamber that can generate climates between 10 and 90% RH at 23°C. (Haraldsson et al. 1994)

The test pieces used in the mechano-sorptive creep, isochronous creep and hygroexpansion measurements were cycled without load between 50 and 90% RH at least four times prior to testing to release dried in stresses. Mechano-sorptive creep tests under tension were performed by ramping the relative humidity between 50 % RH and 90% RH. The cycle time was 7 h, and three cycles, starting and ending at 50% RH were performed.

18 Rigid Possible Clamp load application Test piece

Movable Clamp Columns

Figure 12: Schematic figure of the creep apparatus used (Haraldsson et al. 1994). The function of the columns is to stabilise the test piece in compression mode measurements, hindering macroscopic buckling of the material. Strain gages measure the strain obtained between the clamps.

By using the strain values after three cycles and the corresponding stress values, isocyclic plots were constructed and the isocyclic creep stiffness determined, as schematically illustrated in Figure 13. It has previously been reported that the strain value should be limited to a maximum strain of 0.2% to ensure to measure within a linear region (Panek et al. 2004). However, the experiments in that study were comparing a range of papers involving recycled qualities. As the error of the slope determination is increased the closer the origin the measurement is performed, it is important not to be unnecessarily close to origin. Hence the measurements were made as long as no nonlinearity was observed.

0.9 90%RH 90%RH 6.0 0.8 90%RH 50%RH 5.0 50%RH 50%RH 0.7 Slope = Isocyclic creep stiffness 4.0

0.6 Stress=5.5 3.0 0.5

Stress [Nm/g] Stress 2.0 0.4 Strain [%] Strain

1.0 0.3 Stress=2.5

0.0 0.2 0 0.10.20.30.40.50.6 Strain [%] 0.1

0 0 200 400 600 800 1000 1200 1400 Time [min]

Figure 13: Schematic illustration of how isocyclic creep stiffness can be determined from the measured data.

Isochronous creep was determined by measurements of the strain value after 100 s with a certain applied stress at constant 90% RH and in the same manner as in the case of isocyclic creep stiffness; the value was plotted against the applied stress to determine the slope, i.e., the isochronous creep stiffness after 100 s.

19 3. Result and Discussion

Mechano-sorptive creep is a key property for corrugated board boxes, and hence the summary of the results will focus on this property, even though other properties are important as well. The influence of additions of lignin derivatives, the fibre morphology in different pulps and the influence of the chemical compositions of the pulps have been examined.

3.1 Influence of lignin derivatives (I-III)

3.1.1 Lignin fractionation and isolation (I)

In order to use a kraft lignin derivative, either to make a suberin-like lignin derivative or to enhance oxidative radical coupling reactions, it is important to have a suitable lignin material. In accordance with earlier studies (Keyoumu et al. 2004) it has been demonstrated that the molecular size are lower and free phenolic content higher of the cross-flow-filtrated black liquor lignin fractions, compared to the non-filtered lignin. Additionally, the pH-fractionation of the nano-filtered lignin, giving fractions denoted pH 10.5, pH 9.0, pH 7.0, pH 5.0 and pH 3.0 lignin, resulted in even more specified properties, as seen in Table 1. The pH 9.0 lignin may still, after a dioxane:water carbohydrate removal stage, contain some carbohydrates, perhaps linked to the lignin, which would explain the higher molecular weight and lower free phenolic content of this fraction.

Table 1: Characteristics of the lignin fractions obtained by cross-flow filtration and pH-fractionation, as well as the characteristics of the non-fractionated lignin.

Lignin MW Free phenolic content Fraction of total mass –1 mmol, g %

pH 10.5 3 200 2.1 24 pH 9.0 4 100 1.9 24 pH 7.0 2 800 2.2 13 pH 5.0 3 200 2.5 23 pH 3.0 2 400 2.2 16

Filtrated 1kDa 2 700 2.1 - Non-fractionated 52 000 1.7 -

In particular, the pH 3.0 fraction has a low average molecular weight. This fraction also has a relatively large amount of carboxylic acid groups, as can be seen in IR-spectrum of the fractions, in Figure 14. Consequently, this fraction was chosen for lignin derivatisation with the triglyceride. For the oxidative coupling reactions on the other hand, further separation by pH-fractionation was not applied.

20

Figure 14: FTIR spectra of the lignin fractions; a) pH 3.0 b) pH 5.0 c) 1 kDa cross-flow-filtered d) pH 7.0 e) pH 9.0 f) pH 10.5. The content of carboxylic groups at 1700 cm–1 is highest in the pH 3.0-lignin. All spectra are normalised to the same absorbance at 1590 cm–1 where the aryl carbon-hydrogen vibrations appear.

3.1.2 Suberin-like lignin-derivative (I)

The synthesis performed to demonstrate the possibility of creating a suberin-like lignin derivative indicates that esters between the carboxylic groups of the lignin and the glycerol hydroxyl groups of the linseed oil were created. When studying the FTIR spectra in Figure 15, it is clear that functional groups from both the linseed oil and lignin are present in the product: observe the evidence of aromatic rings at 1590 cm–1 and aliphatic carbon at 2925 cm –1. The exact structure of the linking between the oil and the lignin is more difficult to assess since the wide carbonyl peak also contains peaks from the original compounds. Esters between phenols and fatty acids at 1760 cm–1 are only seen as a small shoulder on the ester peak of the aliphatic constituents at 1740 cm–1. Esters between aromatic carboxylic groups and diglycerides, monoglycerides and glycerol are present at 1700 cm–1 and could not be separated from peaks originating from the pure lignin. The FTIR-spectra furthermore show that the peak at 3010 cm–1, representing carbon-hydrogen vibrations of the hydrogen atoms on the unconjugated cis-unsaturations, remains after the reactions. This indicates that reactions between phenols and unconjugated cis-unsaturations, have not taken place to any significant extent.

21 a)

A A b)

c)

4000.0 3000 2000 1500 1000 600.0 4000 3000 2000 cm-1 1500 1000 600 cm-1

Figure 15: FT-IR spectra of the lignin-linseed oil reaction product and the reactants; a) pH 3.0-lignin b) lignin- linseed oil derivative c) linseed oil.

To evaluate the ability of the lignin derivative to act as a hydrophobic barrier, possibly also affecting the mechano-sorptive creep properties, on paper the suberin-like material was applied on filter paper surfaces. This showed that this material has the ability to make paper surfaces hydrophobic. After the suberin-like lignin derivative was applied on filter paper, the contact angle of the paper after 10 min was 120° while the contact angle for the filter paper treated with lignin was not measurable after 10 min; all water was absorbed by the filter paper. The contact angle of linseed oil itself showed about the same contact angle as the lignin-linseed oil derivative.

3.1.3 Effect of treatments with lignin derivatives (II, III)

When the cross-flow filtrated lignin and the kraftliner pulp is treated with laccase, the molecular weight of lignin increases significantly as seen in Table 2, indicating that radicals have been initiated and that the lignin molecules have coupled.

Table 2: Molecular weight distribution of 1 kDa lignin and acidolysis isolated lignin from unbleached kraftliner pulp before and after laccase treatment.

1 kDa lignin Lignin isolated from kraftliner pulp

Untreated Laccase Untreated Laccase

Mw 3 000 13 500 11 300 35 600

Mn 1 400 3 800 3 600 12 800 M /M 2.2 3.6 3.2 2.8 w n

22 By utilizing the fact that laccase could initiate radicals in lignin and thereby couple lignin together, resulting in new bonds between lignin molecules or monomers, one could create wet strength in lignin-rich papers, as shown earlier (Lund and Felby 2001; Chandra et al. 2004). A problem with using these enzymes commercially for lignin reactions is that toxic and expensive mediators, such as 2,2’ azinobis-(3-ethylbenzthiazoline-6-sulphonate acid), ABTS also need to be added (Rochefort et al. 2004). However, as seen by the results in Table 3, the addition of cross-flow filtrated lignin together with the laccase, but without mediators, also has a large effect on the wet strength and without lowering the dry strength, on the contrast to the laccase ABTS system.

Table 3: Effect of laccase/1kDa lignin versus laccase/ABTS system on kraftliner properties.

Dry tensile Wet tensile Wet strength index strength strength Nm/g Nm/g % kraftliner reference pulp 93 2.8 3.0 + 1kDa lignin 94 2.8 3.0 + laccase 96 4.5 4.7 + laccase +1kDa lignin 99 6.0 6.1 + laccase +ABTS 89 7.7 8.7

The cross-flow-filtered lignin was used together with manganese(III) to create covalent bonds in the kraftliner pulp sheets, and the wet strength improved significantly to about 5% of the dry strength as seen in Figure 16. This may be seen as a proof that oxidative radical coupling reactions have covalently bonded fibres together in the sheet.

90.0 9.0 Dry strength 80.0 Wet strength 8.0 70.0 7.0 60.0 6.0 50.0 5.0 40.0 4.0 30.0 3.0 Dry strength [Nm/g]Dry strength 20.0 2.0 Wet strength [Nm/g] 10.0 1.0 0.0 0.0 Kraftliner reference Manganese (III) + Suberin-like lignin lignin derivative

Figure 16: Treatment of a kraftliner pulp with manganese(III) and cross-flow-filtrated lignin significantly improves wet strength compared to that of the reference kraftliner pulp. The addition of a suberin-like lignin- derivative does not change the wet strength but the dry strength is decreased.

Wet strength seems not to increase to more than about 5 – 6% of the dry-strength with either of these systems, even if the reaction time or concentration of the radical initiator are increased. A paper with a wet strength above 10 – 15% of the dry-strength is considered a wet

23 strength paper (Neal 1988), and hence these are not remarkable numbers. However, a smaller increase in the wet strength may also be beneficial for the mechano-sorptive creep properties, and at the same time it is as problematic for the recycling of the paper, as it is for more heavily wet strengthened paper.

When a suberin-like lignin derivative is added to the pulp, the dry strength is lowered as seen in Figure 16. This is a disadvantage when comparing possible treatments and additives because the dry strength is of course also a parameter of great importance when producing corrugated boxes. If it affects the mechano-sorptive creep properties in a positive way it may, however, still be interesting.

It can be assumed that an increase in wet strength and hydrophobicity of papers also influences the mechano-sorptive creep properties in a positive direction. The pulp with the addition of the suberin-like lignin-derivative has statistically higher isocyclic creep stiffness at a 95% confidence level, while the effect of the manganese (III) cross-linking is more uncertain; see Table 4.

Table 4: Isocyclic creep stiffness measured in tension after three humidity cycles between 50% RH and 90% RH

Isocyclic creep stiffness CI (95%) kNm/g kNm/g Kraftliner reference 0.89 0.04 Manganese(III) + lignin 0.91 0.11 Suberin-like lignin-derivative 1.08 0.12

Even though the addition of the suberin-like lignin derivative yields a sheet with lower tensile index, the creep in cyclic humidity is lowered. The mechanism behind this result is not obvious. One explanation is that the uptake of water in the sheet is decreased by the addition of a hydrophobic compound and that this results in a decrease in the hygroexpansion. Hygroexpansion was measured, and it is also possible to see, in Figure 17, that it is lowered as the suberin-like lignin derivative is added to the pulp.

0.300%

0.290%

0.280%

0.270%

0.260% 0.286% 0.250% 0.274%

Hygroexpansion [%] Hygroexpansion 0.255% 0.240%

0.230% Kraftliner reference Manganese (III) + Suberin-like lignin lignin derivative

Figure 17: The hygroexpansion of the kraftliner pulp sheets with the different treatments as the relative humidity is increased from 33% RH to 66% RH.

24 However, when comparing the isochronous creep measured at 90% RH of the reference pulp and the suberin-like lignin-derivative pulp, it shows that the strain is higher for the suberin- like lignin-derivative treated pulp at the same stress level, as seen in Figure 18.

10.0 9.0 8.0 7.0 6.0 5.0 4.0

Stress [Nm/g] 3.0 2.0 Kraftliner reference 1.0 Suberin-like lignin derivative 0.0 0 0.1 0.2 0.3 0.4 0.5 Strain [%]

Figure 18: Isochronous creep after 100 s preformed at 90% RH with the reference kraftliner and the suberin treated pulp sheet.

This would mean that if a paper with higher hygroexpansion is put under load in an environment of changing relative humidity, the effect of hygroexpansion will be exaggerated. This will then play an important role for the mechano-sorptive creep. While at constant humidity, the hygroexpansion naturally has no effect on the creep, but the well known fact that hydrophobic compounds lower the tensile strengths will be observable, as found by Brandal and Lindheim (1966). (Brandal and Lindheim 1966)

3.2 Influence of fibre morphology (IV, V)

Lignin derivatives can affect the physical properties of a paper, but other factors are important as well. Morphological factors, such as fibre length, width and coarseness and the flexibility of the fibre wall have large influence on pulp and paper properties. These parameters are to a large extent determined by the pulping raw material used. Pulps from different pulping raw materials and processes were compared, and differences in properties was also observed.

It has been reported that morphological factors can influence the hygroexpansion. Pieces of handsheets of pulps from different wood raw materials and different processes were tested, and quite different hygroexpansion was observed, as seen in Figure 19. It is also possible to see that the hygroexpansion is clearly increased with increases density, affected by beating.

25 0.30

0.25

0.20

0.15

0.10 SW KL

Hygroexpansion [%] Hygroexpansion Euc U 0.05 Euc G Birch NSSC 0.00 500 600 700 800 900 Density [kg/m3]

Figure 19: The effect of sheet density, varied through beating, on hygroexpansion. Hygroexpansion is significant higher for the softwood pulp at comparable density while the birch pulp shows the lowest deformation due to changes in relative humidity. The error bars indicate the 95% confidence interval. The dotted lines are guides to the eye.

The softwood kraftliner pulp (SW KL) has the highest hygroexpansion at a given density and the birch kraft pulp the lowest among the pulps investigated. This could be due to several different causes. Fibre shape factor, or curl, has previously been reported (Salmén et al. 1987) to be an important factor for the hygroexpansion. As seen in Figure 20, more curled fibres lead to higher hygroexpansion. The SW KL pulp with highest hygroexpansion has the lowest shape factor and the birch fibres are much straighter.

0.25%

0.20%

0.15%

0.10%

Hygroexpansion [%] 0.05%

0.00% 89.0 90.0 91.0 92.0 93.0 94.0 95.0 Mean Shape Factor [%]

Figure 20: Shape factor of pulps compared at a drainage resistance of 16°SR. The hygroexpansion is decreased by an increase in fibre straightness of the fibres in the pulps. The error bars indicate the 95% confidence interval. The dotted line is a guide to the eye. R2 = 0.89

Another factor of importance might be the fibre width. An increased fibre width correlates with an increase in the hygroexpansion as seen in Figure 21. No previous studies investigating this parameter have been found and even though the data indicate a dependence of hygroexpansion on fibre width, this factor must be more closely investigated with more data and less variation in other properties.

26 0.25%

0.20%

0.15%

0.10%

Hygroexpansion [%] Hygroexpansion 0.05%

0.00% 15.0 20.0 25.0 30.0 35.0 40.0 Fibre Width [μm]

Figure 21: Fibre width of pulps compared at a drainage resistance of 16°SR. The hygroexpansion is decreased with a decrease in the width of the fibres in the pulps. The error bars indicate the 95% confidence interval. The dotted line is a guide to the eye. R2 = 0.93.

As indicated in the evaluation of the effect of lignin derivative addition, hygroexpansion may be an important factor in finding causes for differences in isocyclic creep stiffness, a parameter describing the mechano-sorptive creep property of a paper. Sheets were made and measured on various beaten bleached hardwood pulps, and once dried, it was indeed the case that the hygroexpansion tends to correlate rather well with the isocyclic creep stiffness, as seen in Figure 22. The tensile stiffness of these beaten pulps was, however, very similar, 7.6 ± 0.5 kNm/g for all investigated pulps.

1.40 1.30 1.20 1.10 1.00 0.90 0.80 0.70 0.60 0.50 Isocyclic Creep [kNm/g]Stiffness 0.40 60 65 70 75 80 85 β [μm/(m,%RH)] %RH

Figure 22: The effect of hygroexpansion coefficient (hygroexpansion devided by the difference in relative humidity) on isocyclic creep stiffness. As the tensile stiffness is more or less constant, there is a clear trend between the hygroexpansion coefficient and the mechano–sorptive creep stiffness in tension. Error bars indicate the 95% confidence interval. R2 = 0.75.

Even when the tensile stiffness is more varied, as in the case where unbleached pulps were studied in Figure 23, it is possible to see a trend between hygroexpansion and isocyclic creep stiffness. The trend is, however, not as clear, but the tensile stiffness, varying between about 8

27 and 11 kNm/g for these pulps, is also potentially of importance, as well as other variations that are also more pronounced among these pulps.

1.40

1.20

1.00

0.80

0.60 SW KL 0.40 Euc U Euc G 0.20 Birch NSSC Isocyclic Creep Stiffness [kNm/g] [kNm/g] Stiffness Creep Isocyclic 0.00 0.00 0.10 0.20 0.30 0.40 Hygroexpansion [%]

Figure 23: The effect of hygroexpansion on the isocyclic creep of unbleached pulps. The isocyclic creep stiffness is higher at lower hygroexpansion. The error bars indicate the 95% confidence interval. The dotted lines are guides to the eye. R2 = 0.30.

That the stiffness of the paper, neither in 90% RH nor in 50% RH, is the only parameter affecting the mechano-sorptive creep is, on the other hand, also obvious from Figure 24, showing the relationship between the stiffness and isocyclic creep stiffness for different kinds of unbleached pulps. This is in line with the results from the addition of the suberin-like lignin derivative.

1.40

1.20

1.00

0.80

0.60 SW KL Isochronous Euc U 0.40 creep stiffness Tensile Euc G Stiffness 0.20 Birch NSSC Isocyclic Creep Stiffness [kNm/g] [kNm/g] Stiffness Creep Isocyclic 0.00 0.0 2.0 4.0 6.0 8.0 10.0 12.0 Stiffness [kNm/g]

Figure 24: Isochronous creep stiffness (90%RH) and tensile stiffness respectively, show no correlation with the isocyclic creep stiffness. The error bars indicate the 95% confidence interval. R2 (IchCS) = 0.16; R2 (Tensile Stiffness) = 0.10.

The tensile stiffness is clearly dependent on density, as seen in Figure 25, which is expected due to the fact that the degree of bonding in the sheet has been shown to affect the tensile stiffness (Page et al. 1979). Furthermore, it is possible to see that the high kappa number pulps show a lower tensile stiffness.

28 12.0

10.0

8.0

6.0

4.0 SW KL Euc U

Tensile Stiffness [kNm/g] [kNm/g] Stiffness Tensile 2.0 Euc G Birch NSSC 0.0 500 600 700 800 900 Density [kg/m3]

Figure 25: The effect of density on the tensile stiffness. Tensile stiffness is, as expected, dependent on the sheet density, and the high kappa number pulps have lower tensile stiffness at comparable density. The error bars indicate the 95% confidence interval. The dotted lines are guides to the eye. R2 = 0.68 (for all points).

The tensile stiffness ought to have some influence on the stiffness in cyclic humidity. If the relationship between tensile stiffness and hygroexpansion is compared with the isocyclic creep stiffness for the unbleached pulps, it is also evident that this gives a clearer correlation than stiffness or hygroexpansion gives alone, as shown in Figure 26. This means that the tensile stiffness to hygroexpansion ratio can be used to roughly estimate the mechano-sorptive creep behaviour. Since isocyclic creep measurements are time consuming and variation in papers demand a fairly high number of measurements to obtain precise results, this relationship might be valuable.

1.40

1.20

1.00

0.80

0.60

0.40 SW KL Euc U Euc G 0.20 Birch NSSC Isocyclic Creep Stiffness [kNm/g] [kNm/g] Stiffness Creep Isocyclic 0.00 20 30 40 50 60 70 80 Tensile Stiffness Index [kNm/g] : Hygroexpansion [%] Ratio

Figure 26: The ratio between tensile stiffness and hygroexpansion shows a correlation with the isocyclic creep stiffness. The error bars indicate the 95% confidence interval. The dotted line is a guide to the eye. R2 = 0.60.

Since both hygroexpansion (Figure 19) and tensile stiffness (Figure 25) are increased with density, it is not obvious how isocyclic creep stiffness is affected by increased density. In Figure 27, one can see how the density, increased by beating, influences the isocyclic creep stiffness of the unbleached pulps.

29 1.40

1.20

1.00

0.80

0.60

0.40 SW KL Euc U Euc G 0.20 Birch NSSC Isocyclic Creep Stiffness [kNm/g] [kNm/g] Stiffness Creep Isocyclic 0.00 500 600 700 800 900 Density [kg/m3]

Figure 27: Unexpectedly the isocyclic creep stiffness is not clearly dependent on the sheet density. The error bars indicate the 95% confidence interval. R2 = 0.01

It is clear that the isocyclic creep stiffness, in contrast to the stiffness in constant climate, is generally not increased by an increased density accomplished by beating. It can also be observed that the pulps with higher lignin content (i.e., softwood kraftliner pulp, SW KL, and birch NSSC pulp, NSSC) may have somewhat lower isocyclic creep stiffness.

3.3 Influence of chemical composition (VI, VII)

In order to more carefully evaluate the effect of chemical composition of the pulp on the physical properties, a high kappa number softwood pulp was treated with chlorite and xylanase. In addition to this industrial pulps and laboratory pulps from similar raw materials but with different lignin contents were evaluated as well. From the chlorite and xylanase treatment, pulps with lignin content between 3 – 14 %, and xylan content between 4 – 7% were obtained while the densities were still comparable (760 ± 50 kg/m3). Physical properties for these pulps were tested, and it is possible to see that the tensile stiffness is clearly positively affected by a decreased lignin content, as seen in Figure 28. 10.5

10.0

9.5

9.0

8.5

8.0

7.5

7.0

6.5 Tensile Stiffness Index [kNm/g] Tensile 6.0 0.0% 2.0% 4.0% 6.0% 8.0% 10.0% 12.0% 14.0% 16.0% Klason Lignin [%]

Figure 28: Effect of Klason lignin, varied by chlorite delignification, on tensile stiffness. The tensile stiffness increases with lowered lignin content. The error bars indicate the 95% confidence interval. The dotted line is a guide to the eye. The unfilled circle represents the untreated kraftliner reference pulp. R2=0.89.

30 Laboratory pulps with similar chemical compositions, except for variations in lignin and cellulose content, were evaluated as well over a span of sheet densities, varied through beating. In line with the chlorite delignification results it is possible to see, in Figure 29, that the pulp with kappa number 75 (i.e., a lignin content of about 11%) has a lower tensile stiffness than the pulp with a kappa number of 60 (i.e., a lignin content of about 9%). The increase in tensile stiffness due to a decrease in lignin content may be due to an increased degree of bonding between fibres at a certain sheet density, as well as an increased stiffness of the fibres as the relative amount of cellulose is increased. 10.0

9.0

8.0

7.0

6.0 EIC 60 EIC 75 Tensile Tensile Stiffness Index [kNm/g] 5.0 550 600 650 700 750 800 850 Density [kg/m3]

Figure 29: The tensile stiffness versus the density of two laboratory-cooked high kappa number pulps with different lignin contents. Tensile stiffness is lower for the pulp with higher kappa number. The error bars indicate the 95% confidence interval. EIC stands for Extended Impregnation Cooking and the figures after the abbreviation represents the kappa number.

The xylan content, varied by means of xylanase treatments, tends, among the pulps investigated in the chlorite and xylanase treated pulps, not to affect the tensile stiffness in any certain direction, as shown in Figure 30. This is not in accordance with previous studies showing the benefits of hemicelluloses on the pulp handsheet stiffness (Molin and Teder 2002). However, kraftliner pulp cooks are usually terminated before the xylan substituents are cleaved off enough for xylan to precipitate back on fibre surfaces, and the effect of the xylan as a strength enhancer might have to do with its charge density and other parameters as well.

10.5

10.0

9.5

9.0 Beaten

8.5

8.0

7.5 12-14% Lignin 7.0 7-8% Lignin 6.5 3-4% Lignin Tensile Stiffness Index [kNm/g] Index Stiffness Tensile 6.0 3.0% 3.5% 4.0% 4.5% 5.0% 5.5% 6.0% 6.5% 7.0% Xylan [%]

Figure 30: Effect of xylan content on tensile stiffness. No effect of the xylan content on the tensile stiffness is, surprisingly, seen among these pulps. The sheet of the beaten pulp has a different density than the others with lignin content between 12 and 14%. The dotted lines are guides to the eye. R2 = 0.09 (for all points)

31

The hygroexpansion is negatively influenced by higher lignin content. The different chlorite delignified pulps are compared and it is possible to see that a decrease in lignin content decrease the hygroexpansion, shown in Figure 31. The effects are, however, not as pronounced as the effect of decreased lignin content on the tensile stiffness.

0.25%

0.24%

0.23%

0.22%

0.21%

0.20% Hygroexpansion [%] 0.19%

0.18% 0.0% 2.0% 4.0% 6.0% 8.0% 10.0% 12.0% 14.0% 16.0% Klason Lignin [%]

Figure 31: Effect of Klason lignin, varied by chlorite delignification, on hygroexpansion. Decreased Klason lignin content gives lower hygroexpansion. The dotted line is a guide to the eye. The error bars indicate the 95% confidence interval. The unfilled circle represents the untreated kraftliner reference pulp. R2 = 0.83

That decreased lignin content gives a decreased hygroexpansion is also seen in Figure 32, showing the hygroexpansion coefficient of the laboratory cooked pulps, with different lignin contents, at different densities. However, the effect of higher lignin content tends to be smaller at higher degrees of beating.

0.0080%

0.0075%

0.0070%

0.0065%

0.0060% [%/%RH]

β 0.0055%

0.0050% EIC 75 EIC 60 0.0045%

0.0040% 550 600 650 700 750 800 850 Density [kg/m3]

Figure 32: The effect of density on hygroexpansion coefficient (hygroexpansion divided by difference in % RH) for two pulps of different lignin content. Hygroexpansion coefficient is decreased as lignin content is decreased from 11% (kappa no. 75) to 9% (kappa no. 60), but the differences become comparably small and is decreasing as the degree of beating is increased. EIC stands for Extended Impregnation Cooking and the figures after the abbreviation represents the kappa number. The error bars indicate the 95% confidence interval.

The decrease in lignin content is accompanied with a raised relative amount of cellulose, and this might be the cause of the decrease in hygroexpansion. Due to the strength of its ordered fibrillar structure, cellulose might help to fixate the pulp fibre network.

32

The effect of xylan content on hygroexpansion, shown in Figure 33, is not as clear as the effect of lignin content. Even though the effects are not statistically significant, there might be a tendency of increased hygroexpansion with increasing xylan content.

0.25% Beaten 0.24%

0.23%

0.22%

0.21%

0.20% 12-14% Lignin Hygroexpansion [%] Hygroexpansion 7-8% Lignin 0.19% 3-4% Lignin 0.18% 3.0% 3.5% 4.0% 4.5% 5.0% 5.5% 6.0% 6.5% 7.0% Xylan [%]

Figure 33: Effect of xylan content on hygroexpansion. Hygroexpansion is decreased as xylan content is decreased at certain lignin contents, but the differences are comparably small and not statistically significant. The sheet of the beaten pulp has a higher density while the other sheets had similar densities in the same lignin content range. The dotted lines are guides to the eye. The unfilled circle represents the untreated kraftliner reference pulp.

There can be several different explanations for the differences in hygroexpansions due to differences in chemical composition. Differences in moisture uptake are one possible explanation to the differences. However, it is not possible to see any effect of lignin content on the moisture uptake in the pulp in Figure 34. It has also been reported that beating pulps does not change the moisture uptake to any significant extent (Brecht and Hildenbrand 1960).

4.5% 4.0% 3.5% 3.0% 2.5% 2.0%

Moisture Content Moisture 1.5% Δ (33% RH – 66% RH) 66%(33% RH – 1.0% 0.5% 0.0% 0.0% 5.0% 10.0% 15.0% Klason Lignin [%]

Figure 34: Difference in percentage units, of the moisture content of the pulp handsheet pieces, from 33% RH to 66% RH. The lignin content has no obvious effect on the moisture uptake of the sheets. R2 = 0.01.

33 Lignin is often referred to as a hydrophobic compound, and it is hence somewhat surprising that the lignin does not affect moisture uptake more. It is also surprising that the xylan content does not affect the moisture uptake, as seen in Figure 35.

4.5% 4.0% 3.5% 3.0% 2.5% 2.0%

Moisture Content 1.5% (33% RH – 66% RH) Δ 1.0% 0.5% 0.0% 3.0% 4.0% 5.0% 6.0% 7.0% Xylan [%]

Figure 35: The difference in percentage units, of the moisture content of the pulp handsheet pieces from 33% RH to 66% RH. The xylan content has no obvious effect on the moisture uptake of the sheets. The dotted line is guide to the eye. R2 = 0.05.

From the study with different pulping raw materials, a relationship between tensile stiffness and hygroexpansion hinted at the mechano-sorptive creep performance. As decreased lignin content gives an increased tensile stiffness and a decreased hygroexpansion, it can be hypothesised that increased lignin content will give lower isocyclic creep stiffness. In a similar manner, decreased xylan content, which might decrease the hygroexpansion somewhat but not affect the tensile stiffness, should give somewhat increased isocyclic creep stiffness. As the isocyclic creep stiffness for these pulps is measured, it is also possible to see, as shown in Figure 36, that both an decreased lignin content and a decreased xylan content tends to give an increase in isocyclic creep stiffness.

1.70

1.50

1.30

1.10

6.5 – 5.5 % Xylan 0.90 5.5 – 4.5 % Xylan 0.70 4.5 – 3.5 % Xylan Isocyclic Creep Stiffness [kNm/g] Stiffness Creep Isocyclic 0.50 0.0% 2.0% 4.0% 6.0% 8.0% 10.0% 12.0% 14.0% 16.0% Klason Lignin [%]

Figure 36: The effect of xylan and lignin content on isocyclic creep stiffness, in tension after 3 humidity cycles. The isocyclic creep stiffness is slightly decreased by increasing the lignin content and increased by increasing the xylan content. Between 11 and 6 measurements were performed on each sample. Error bars indicate a 95% confidence interval and the unfilled circle is the untreated kraftliner reference pulp. The dotted lines are guides to the eye. The unfilled circle represents the untreated kraftliner reference pulp. R2 = 0.25 (for all points).

34

When the pulps cooked to different lignin contents are compared it is also possible to see that decreased lignin content gives increased isocyclic creep stiffness, as presented in Table 5. However, the number of measurements is small (4 – 5 on each sample) and the deviations high and hence these results should only be seen as an indication. It is also possible to observe that the conventional kraft (CK) mill pulps have lower isocyclic creep stiffness compared to the laboratory EIC-pulps. This might be attributed to more curly fibres of the mill pulps. Since the laboratory cooked pulps are made with the EIC-technique, they do not require in-line refining to be defibrated and might hence have less curly fibres.

Table 5: The effect of lignin content on the isocyclic creep stiffness compared between two industrial pulps and two laboratory cook pulps. Decreased lignin content tends, as expected, to give increased isocyclic creep stiffness. However, the number of measurements is small and the deviations are high and the results should be seen as an indication. The differences between laboratory cooked pulps and mill pulps might be attributed to more curly fibres in the mill pulps.

Isocyclic creep stiffness [kNm/g] EIC 60 1.26 EIC 75 1.22 CK 67 1.14 CK 73 1.03

These conclusions are supported by the relationship between the tensile stiffness and the hygroexpansion coefficient shown in Figure 37. The pulps having higher lignin content have higher hygroexpansion at a given tensile stiffness. Moreover, it is possible to see that the conventional kraft mill pulps have a higher hygroexpansion at a given tensile stiffness. It is known that PFI-beating results in straighter fibres (Page 1985) and that both tensile stiffness (Page et al. 1979) and hygroexpansion (Salmén et al. 1987) is dependent on the fibre form. Hence, the decrease in hygroexpansion coefficient with an increase in tensile stiffness shown for the CK 73 pulp can be explained by a straightening of the fibres in the pulp, indicating a high initial curl.

0.0090% 0.0085% 0.0080% 0.0075% 0.0070% 0.0065% [%/%RH]

0.0060%

%RH CK 73 β 0.0055% CK 67 0.0050% EIC 75 0.0045% EIC 60 0.0040% 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 Tensile Stiffness Index [kNm/g]

Figure 37: The relationship between tensile stiffness and the hygroexpansion coefficient suggest that mill pulps and the pulps with higher kappa number will perform worse in isocyclic creep stiffness. The pulps beaten with 3000 revolutions are marked with larger dots. Error bars indicate the 95% confidence interval.

35 4. Conclusions

To gain a better understanding of mechano-sorptive creep phenomenon in kraftliner and to find strategies to decrease this creep was the aim of this work. This was made by investigating the use of lignin derivatives and the effects of morphology and different chemical compositions.

Separating black liquor lignin in an industrial feasible way into fractions with different molecular size and amount of functional groups was shown to be possible. Furthermore, by using this lignin and biomimetic methods, to create radical coupling reactions, it was possible to enhance the wet strength of kraftliner pulp sheets. This way of treating the pulp gave, however, no significant effects on the mechano-sorptive creep.

It was also demonstrated that it is possible to make an apolar suberin-like lignin derivative that could possibly be used in several applications. By adding this lignin derivative to pulp it was possible to decrease the mechano-sorptive creep, but at the expense of stiffness properties in constant climate.

Mechano-sorptive creep was greatly influenced by the pulping raw material and processing. Unbleached hardwood kraft pulps and especially a birch kraft pulp, showed considerably higher isocyclic creep stiffness compared to softwood kraftliner pulp, probably due to more slender and straighter fibres.

For a variety of pulps, it has been observed that the ratio between tensile stiffness and hygroexpansion can be used to estimate the mechano-sorptive creep properties. As mechano- sorptive creep measurements are time consuming, this relationship might be a useful tool.

The pulp raw material and processing affect hygroexpansion considerable. Increased fibre width, lignin content and possibly xylan content as well as decreased fibre shape gives increased hygroexpansion. Decreased lignin content gives an increased tensile stiffness. Hence, it was observed that increased lignin content gives an increased mechano-sorptive creep. There were also indications of a negative effect of xylan on the mechano-sorptive creep.

36 5. Acknowledgements

First I would like to give my thanks to my supervisor Mikael E. Lindström for giving me the opportunity to do this work and for valuable scientific guidance.

I am also grateful to Gunnar Henriksson for always having time for valuable discussions and comments. Mats Johansson, Petri Mäkelä, David Stenman and Martin Ragnar are acknowledged for giving me guidance and valuable ideas throughout the work.

I wish to thank all my co-authors; Mikael E. Lindström, Gunnar Henriksson, Mats Johansson, Graziano Elegir, Daniele Bussini, Luca Zoia, Christer Fellers, Petri Mäkelä, Iiro Pulkkinen, Juha Fiskari, and Katarina Karlström for good co-operation.

Special thanks are also given to Magnus Gimåker, Lars Wågberg, Petri Mäkelä, Christer Fellers and Anne-Mari Olsson for good co-operation within the SustainPack program and the Paper Mechanics Cluster at STFI-Packforsk.

Ants Teder, Katarina Karlström, Elisabet Brännvall, Lars-Erik Enarsson and Helena Wedin are acknowledged for reading and giving comments on the thesis before printing.

Mona Johansson, keeping the order in laboratories, as well as Inga Persson and Brita Gidlund keeping order on all practical matters, are gratefully acknowledged not only for there skilful work but also for there way of cheering up the atmosphere at the department. However, also past and present colleges at the department have contributed much to the good atmosphere and I thank them for that and for good co-operation.

Finally I would like to thank my family and friends for all support and understanding.

This work was carried out with the financial support from SustainPack, a European Union 6th Framework Programme; Vinnova, The Swedish Agency for Innovation Systems, (Project No. P23947-1A) and WPCRN, Wood and Pulping Chemistry Research Network, which are gratefully acknowledged.

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