Developments in Engineered Fibres Yunqiao Pu, Dongcheng Zhang and Arthur J. Ragauskas

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1 Developments in Engineered Fibres Yunqiao Pu, Dongcheng Zhang and Arthur J. Ragauskas Published by Pira International Ltd Cleeve Road, Leatherhead Surrey kt22 7ru UK T +44 (0) F +44 (0) E publications@pira-international.com W

2 The facts set out in this publication are obtained from sources which we believe to be reliable. However, we accept no legal liability of any kind for the publication contents, nor any information contained therein nor conclusions drawn by any party from it. Pira International Ltd acknowledges product, service and company names referred to in this report, many of which are trade names, service marks, trademarks or registered trademarks. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without the prior permission of the Copyright owner. ISBN Copyright Pira International Ltd 2007 Head of publications and events Philip Swinden philip.swinden@pira-international.com Publisher Rav Lally rav.lally@pira-international.com Head of editorial Adam Page adam.page@pira-international.com Global editor Nick Waite nick.waite@pira-international.com Head of US publishing Charles E. Spear, Jr. chuck@intertechusa.com Assistant editor Claire Jones claire.jones@pira-international.com Customer services manager Denise Davidson publications@pira-international.com T +44 (0) Typeset in the UK by Jeff Porter, Deeping St James, Peterborough, Lincs jeffp@publishink.plus.com

3 Contents List of tables iv List of figures v 1Introduction 1 2Overview 3 3Pulp and fibre charge 9 Kraft pulp 9 Pulp fibre charge 9 Effect of kraft pulping on pulp fibre charge 11 Fibre charge characterisation and enhancement during oxygen delignification 15 Effect of bleaching chemicals and sequence on fibre charge 20 Bleaching chemicals 21 Bleaching sequences 23 4Fibre modification 27 Enzymatic fibre modification 27 Cellulase 27 Hemicellulase 29 Laccase 31 Plasma treatment 39 Surface properties 40 Chemical/physical properties 44 Water absorption 47 Fibre carboxyl and cationic enrichment 48 Carboxyl enrichment with additives 48 Xylan absorption 50 Cationic enrichment 52 Alternative surface fibre grafting 53 Enzymatic grafting 53 Polyelectrolyte multilayers grafting 53 Corona discharge-initiated grafting 55 5Applications and conclusion 59 Applications 59 Conclusion 61 Bibliography 63 Page iii Copyright Pira International Ltd 2007

4 List of tables 2.1 Some key fibre properties that can be modified by fibre engineering Effects of charges on paper machine operations and paper properties Typical modification technologies for fibre acidic group enhancement Carboxylic acid content in different pulps Effect of pulping conditions on fibre charge of conventional batch kraft pulps Charged group content before and after oxygen delignification of softwood kraft pulps ESCA atomic composition and functional groups present on the surface of oxygen delignified softwood kraft pulps Pulp physical and chemical properties after oxygen delignification with the addition of catalyst Fibre charge: effects of bleached kraft pulps Fibre charge affected by different bleaching sequences Total fibre carboxyl group content of fully bleached softwood kraft pulps Strength results for unrefined pulp fibres treated with laccase and gallic acid ESCA carbon atom classification Effect of CMC fibre attachment on fibre swelling and physical properties of paper made of unbeaten bleached softwood kraft pulps (Na-Form) Grafting bleached thermomechanical pulp with 5% Ce(NH 4 ) 2 (NO 3 ) 6 and methyl acrylic acid Grafting bleached softwood stone-groundwood with chloroacetic acid Physical properties of ECF bleached softwood kraft pulp before and after xylan absorption Physical properties of ECF bleached softwood kraft pulp before and after xylan absorption at varying temperatures of application Physical properties of cationised softwood stone groundwood fibres Physical properties of handsheets prepared from cationised softwood stone groundwood (SGW) fibres and unreacted SGW fibres Physical properties of handsheets prepared from cationised linerboard softwood kraft pulp Selected key end-use physical properties of paper influenced by the fibre charge of the pulp 59 Page iv Copyright Pira International Ltd 2007

5 List of figures 3.1 Comparison between carboxylate content and kappa number of loblolly pine kraft pulps from conventional and low solids pulping Profile of fibre carboxyl group content in total fibre and polysaccharide fraction of oxygen delignified kraft pulps Carboxylic acid content in total fibre of high kappa softwood kraft pulps and the corresponding oxygen delignified pulps Bulk acid group contents of highyield kraft pulp fibres treated with laccase and vanillic, syringic and 4-hydroxybenzoic acids Surface acid group content (percentage of total carbon on surface) of high-yield kraft pulp fibres treated with laccase and 4-hydroxybenzoic acid Bulk acid group content of highkappa pulps treated with laccase, tyrosine, 4-hydroxyphenylacetic acid and guaiacol sulphonate Water retention value of high-kappa pulps treated with laccase, tyrosine, 4-hydroxyphenylacetic acid and guaiacol sulphonate Kappa number and bulk carboxylic group content of unrefined high-kappa pulp reacted with laccase and gallic acid Tensile index and z-direction tensile of high-kappa pulps treated with laccase and vanillic, syringic and 4-hydroxybenzoic acids Burst index and tensile index of high kappa pulps treated with laccase celestine blue Schematic for a dielectric-barrier discharge treatment Contact angles for dielectric-barrier discharge treated thermomechanical pulp fibres Dispersive surface energies of dielectric-barrier discharge treated bleach kraft pulp and thermomechanical pulp fibres determined via inverse gas chromatography O/C ratio changes for dielectricbarrier discharge treated lignocellulosic fibres determined via ESCA Surface roughness of dielectric-barrier discharge treated bleached kraft (BKP) and thermomechanical pulp (TMP) fibres at various treatment power levels Surface carboxylic acid group content on bleached kraft (BKP) and thermomechanical pulp (TMP) fibres at various dielectric-barrier discharge treatment levels Coefficient of friction of bleached kraft (BKP) and thermomechanical pulp (TMP) fibres at various dielectric-barrier discharge treatment levels Wet tensile index of dielectric-barrier discharge treated softwood stone groundwood bleached kraft (BKP) and unbleached thermomechanical pulp (TMP) fibres Total acid groups content after grafting maleic acid on to bleached kraft fibres at various dielectricbarrier discharge treatment levels 56 Page v Copyright Pira International Ltd 2007

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7 Introduction 1 Engineered fibres are defined as fibres that possess specific improved properties or new functionality developed through engineered chemical, biological or mechanical processes. One of the growing research themes in the pulp and paper industry since the late 1990s has been fibre engineering, directed at tailoring the topochemical-physical properties of fibres to yield optimal cost performance of fibres for a given product platform. In its full breadth, this initiative covers as many aspects of paper as does the application of paper itself, including water absorbency, printability, composites, physical strength and optical and electromagnetic properties. Despite the fact that pulp fibres are the foundation of the paper industry and its assorted paper products, many different grades of paper with different end-use requirements are often made from almost the same basic resources. There is no doubt that basic wood properties and papermaking operations play an important role in determining the final properties of fibres and paper products. However, the most readily available fibre resources often do not yield the ideal pulp for some particular grades and certain process operations also result in unwanted fibre structure changes. Given these considerations, fibre engineering is considered one of the key pulp and paper manufacturing technologies of the new millennium that will dramatically enhance the performance, value, quality and versatility of wood fibres. Fibre modification/engineering has received extensive research attention since the late 1990s, as today s pulp and paper industry focuses on developing new and improved products. Recent years have seen dramatic advances in our knowledge of and ability to control the surface and bulk chemical properties of pulp fibres. Furthermore, research efforts have demonstrated that at the intersection of fibre chemistry and paper physics, unique opportunities exist to develop new and improved paper properties. The full spectrum of fibre engineering is undoubtedly too broad to cover fully in just one review, hence this report examines new and rewarding developments that focus primarily on fibre modification through enzymatic and chemical treatments of chemical and mechanical pulp fibres after the wood furnish is delivered to the pulp mill. The scope of this report includes: Chapter 2 Overview: a brief introduction of the concept of fibre engineering and the importance of fibre engineering for new cellulosic product platform development; Chapter 3 Pulp and fibre charge: defines the relationship between pulp fibres and fibre charge and the effect that modern pulping and bleaching has on fibre charge; Chapter 4 Fibre modification: reviews technology developments in pulp fibre modification, including enzymatic and plasma treatment, charge enrichment, polymer adsorption and grafting; Chapter 5 Applications and conclusion: briefly highlights the fibre engineering applications. Page 1 Copyright Pira International Ltd 2007

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9 Overview 2 According to Baum s report, Ten-year Forecast of Disruptive Technologies in the Paper Supply Chain, the pulp and paper industry needs to develop new products and process technologies to remain viable in today s world of accelerating change and competing technologies. One of the greatest challenges is to tailor pulp fibre properties for specific applications and enhanced performances. For example, several grades of softwood fibres have excessive bonding capacity when used for fluff pulp applications that could be addressed by changes in the structure of these fibres. The coarseness values of kraft softwood fibres have also been noted as limiting for select tissue and top-coat linerboard applications. The need to increase bulk and stiffness remains one of the most urgent needs for bleached kraft pulps. Enhanced bond strength for mechanical pulps is also required. Fibre engineering has demonstrated great potential in achieving desired fibre product properties, developing new functionalities for specific usage and improving operation process performances in the pulp and paper industry. Practitioners of fibre engineering refuse to accept the current compromise in pulp properties for many product platforms. Instead, advances in fibre engineering are being developed that will tailor physical chemical fibre properties to specific applications. It represents a new pulp paradigm, mass customisation of pulp fibres, that will provide new and improved grades for printing, packaging, hygiene composites and other grades. These rapidly developing technologies will be married with advances in the one-man paper machine, to usher in a new generation in pulping, bleaching and papermaking technologies. In the long term this change in pulp and paper manufacturing is expected to have a huge impact. A variety of fibre engineering approaches are being developed to yield a better pulp fibre for papermaking and to tailor pulp fibre strength and surface properties for specific end uses. Long-term efforts in this direction are to bio-engineer the right plant genotypes and then breed trees possessing these desired properties. Alternatively, existing fibre resources can be modified by mechanical, chemical and enzymatic techniques. Mechanical modifications are typically accomplished by refining, which is more or less random with respect to individual fibres. Hence, subsequent paper properties will always be something of a compromise. Modern chemical and enzymatic fibre modifications are directed at altering specific components of fibre chemistry, morphology or topography. It is easy to envisage that fibre engineering will evolve into a marriage of plant genetics and process engineering. For example, plant genetics will enhance fibre properties such as fibre charge, and pulping, bleaching and refining operations will be optimised to maximise fibre charge retention. Indeed, a general challenge in fibre engineering is the need to engineer fibre properties that maximise the performance/ value of fibres for their final product applications. This challenge includes the need to optimise interactions of fibres not only with other fibres, but also with wet-end chemicals, fillers and composites. Although the potential of this opportunity is beginning to be appreciated, it is also clear that much research and development needs to be accomplished before this vision is realised. Page 3 Copyright Pira International Ltd 2007

10 Developments in Engineered Fibres Overview Obviously, fibre engineering has great potential to improve almost all grades of paper and papermaking process performance. Recent advances in nanotechnology, biotechnology, plant genomics, process engineering and the integration of fibre chemistry and material science are making these technologies viable on a short timeline. These technical advances, coupled with society s demand for sustainable technologies and products, suggest that tomorrow s winners will be those products and organisations that integrate these technologies into synergistic products and quickly bring them to commercial application. Table 2.1 lists some of the key fibre properties that can be modified by fibre engineering. TABLE 2.1 Some key fibre properties that can be modified by fibre engineering Engineered fibre properties Fibre fibre bonding strength Fibre water adsorption/retention Fibre compatibility Fibre lignin content and structure Methodology Fibre charge enhancement Fibre surface modification through chemical or enzymatic treatment/adsorption Surface fibrillation Deformability Refining Fibre charge enhancement Fibre grafting Pore size Biological treatment Fibre grafting Compatibility additives Molecular genetics and genomics of forest trees Pulping and bleaching Source: Pira International Ltd Based on the recent Forest, Wood and Paper Industry Technology summit and Pira s International Fibre Engineering for Papermakers Conference, fibre engineering can be viewed as the modification of pulp fibres that results in specific fibre attributes required for welldefined end-use performance parameters. The general goals of fibre engineering include: Innovative ways of enhancing fibre-fibre bonding to yield enhanced sheet strength properties with fewer fibres; New technologies for improving pulp properties for paper grades where strength is not a key parameter; Development of new grades of paper; Development of novel fibre-based composites. In general, these engineered fibre attributes can be accomplished by four different technology platforms: Mechanical refining Genetic modification of the basic wood resource Chemical modification of fibres Enzymatic fibre modification. Page 4 Copyright Pira International Ltd 2007

11 Developments in Engineered Fibres Overview 2 Important advances have already been made in each of these areas. Mechanical refining primarily involves the mechanical treatment of fibres during the refining and beating processes. The theories in describing the action of beaters and refiners were recently reviewed by Genco. Refining is considered a critical unit operation that influences all pulp properties, as well as the operation of the paper machine. The technologies for decreasing refiner specific energy consumption and increasing refiner production rate have been a primary research topic for mechanical fibre modification in the past few decades. The advances in mechanical pulping technologies and mechanisms in refining have been highlighted in several reviews (Cannell 1999; Salmen 1999; Aoshima 2003). Genetic fibre engineering research has been conducted on several fronts, including: Expression and modification of molecular genetics and genomics of forest tree species, such as pine species (Sederhoff 1998; Sykes 2003), populus species and eucalyptus species (Taylor 2002; Grattapaglia 2004; Poke 2005; Poupin 2005; Sato 2006) that are key wood resources for the paper industry in need of improved wood quality. Lignin structure modification or lignin reduction in trees through genetic modification of lignin biosynthesis has been actively investigated (Tzfira 1998; Merkle 2000; Tuskan 1999; McCord 2004; Wallinger 2004) and recently reviewed (Boerjan 2003). Wood quality. Biomimetics studies to design biocomposites (Zhou 2005; Vincent 2006; Zhou 2006b; Teeri 2007) and advancements in biomimetic engineering of cellulose-based materials to develop novel biomaterials (Teeri 2007). Apart from these advances in genetic engineering and mechanical refining, enzymatic and chemical fibre modification is being extensively investigated to change fibre functionalities and papermaking properties. Among the assorted fibre chemistry properties that can be engineered, fibre charge has become a dominant research theme for improved processing and final product properties. Surface fibre charge affects sizing, retention of papermaking additives, wet strength of the paper, refining, formation and final paper strength properties. Bulk fibre charge affects fibre swelling, retention, refining, sheet formation and final strength properties (Lindstrom 1989; Barzyk 1997a). The importance of surface and bulk fibre charges on refining, paper machine operations and final paper properties are summarised in Table 2.2. Table 2.2 Effects of charges on paper machine operations and paper properties Operations or paper properties Surface charges Bulk charges Tensile strength Very positive Very positive Wet-strength Very positive Weak Absorbency Positive Very positive Refining Positive Weak Retention Positive Weak Sizing Very positive Weak Sources: Laine 2003; Vander Wielen 2004a; Zhang 2006a; Horvath 2006; Lindstrom 2006 Page 5 Copyright Pira International Ltd 2007

12 Developments in Engineered Fibres Overview Early studies by Lindstrom et al. and Scallan et al. established that pulp fibres in water behaved as a swollen polyelectrolytic gel when containing sufficient charge. A polyelectrolytic gel has the ability to swell because of the electrostatic repulsions between existing like charges, and the maximum swelling effect occurs with charged groups ionised in low ionic strength solutions. The ionisable groups in cellulosic fibre may be carboxylic acids, sulphonic acids, phenols or hydroxyl groups, depending on different pulping processes and the ph involved. Under typical papermaking process conditions, the main groups that are ionised and contribute to fibre charge are carboxyl groups and sulphonate groups. The effect caused by these bulk anionic groups in a fibre and accompanying counter cation groups leads to the generation of osmotic pressure that causes additional water to enter the fibre wall. The resultant fibre swelling increases fibre conformability and fibre fibre bonding. Therefore, fibre charge/acidic groups of pulps are of great importance for fibre behaviours in pulping and papermaking processes, as well as for the properties of final paper products. A series of fibre modification technologies relying on introducing additional acidic groups into fibres have been explored for mechanical and chemical pulps, as shown in Table 2.3. Table 2.3 Typical modification technologies for fibre acidic group enhancement Fibre modification References Carboxylmethylation Gellerstedt 1999; Lindstrom Grafted polymerisation Barzyk 1997a; Barzyk 1997c; Barzyk 1997d; Fors Chemical adsorption/precipitation Laine 2001; Laine 2002a; Vander Wielen 2004a; Lindstrom Chemical oxidation Dang 2007; Zhang 2007a. Enzymatic treatment Chandra Source: Pira International Ltd Ragauskas et al developed a fibre charge database for US mill-produced fully bleached kraft pulps. The bleaching chemicals used for the production of these pulps included: oxygen, chlorine dioxide and hydrogen peroxide. The fibre charge of these pulps was found to be in the range of 16 70µmol/g. The only significant factor contributing to fibre charge for these pulps was the fibre resources; although it was clear that different manufacturing processes had an impact on final fibre charge properties. The acid group content for mechanical pulp and sulphite pulp is much higher (83 320μmol/g) when sulphonation occurs during the pulping process (Fors 2000; Horvath 2006). Over the last few years, research studies have highlighted how kraft pulping and bleaching can be used to control and enhance pulp fibre charge (Dang 2007; Laine 2001; Toven 1999; Zhang 2007a). In addition, new technologies for chemo-enzymatic fibre charge grafting have developed rapidly, in addition to other treatments, such as oxoreductase enzymes and plasma treatment. Extensive research efforts on surface and bulk fibre chemical modifications to achieve improved fibre properties have been conducted, including carboxyl enrichment, cationic charge enrichment, carboxyl methylcellulose absorption and polyelectrolyte multilayers grafting (Laine 2001; Laine 2002a; Laine 2002b; Laine 2003; Page 6 Copyright Pira International Ltd 2007

13 Developments in Engineered Fibres Overview 2 Lindstrom 2005; Horvath 2006; Eriksson 2005a; Lu 2007; Schneider 2007). In the light of these recent technical accomplishments, this report covers fundamental research and technological developments in fibre charge control that can be accomplished via: Kraft pulping and bleaching Enzymatic and plasma treatment Charge enrichment via additives Chemical grafting of cellulosic pulp fibres. Page 7 Copyright Pira International Ltd 2007

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15 Pulp and fibre charge 3 Individual wood pulp fibres are natural filament-wound reinforced composites, which are liberated through chemical or mechanical treatments of the cell wall. The predominant pulping process for chemical pulps is kraft pulping. This chapter reviews the status of fibre charge development for modern kraft pulps and its impact on strength properties. Also, the relationship between kraft pulping and bleaching and fibre charge is examined. Kraft pulp Kraft pulping is still the world s dominant chemical pulp manufacturing process. In a conventional kraft cook, wood chemical components react with sodium hydroxide and sodium sulphide from cooking liquor in a large pressure vessel at a temperature of approximately 170 C for about two hours (Smook 1994). During the cook, hydroxide and hydrosulphide anions react with the lignin. As a result, the lignin macromolecules degrade into smaller aqueous-alkali soluble fragments, which are removed (Gierer 1980; Gellerstedt 1989). Accompanying these reactions, wood carbohydrates also experience some reactions, such as deacetylation of hemicelluloses, peeling and stopping reactions. Typically, there are three distinct phases of delignification during kraft cooking: initial, bulk and the final phase. The initial phase of delignification starts at a temperature of about 150 C and is diffusion-limited (Smook 1994). The bulk phase occurs from 150 to 170 C. Most of the lignin removal takes place during kraft pulping in this phase and the rate of delignification is controlled by chemical reactions. The final phase begins when the rate of delignification significantly decreases until about 90% of the lignin has been removed, which marks the end of the cook. The selectivity in the final phase is poor and extension of pulping could result in significant degradation of carbohydrate. The remaining or residual lignin, typically 4% to 5% by weight at the end of a conventional softwood kraft cook, is removed via subsequent bleaching treatments (Grace 1987; Smook 1994). Pulp fibre charge Fibre charge, a chemical property of fibres, is associated with charged functional groups in pulp. Typically, pulp fibres have a negative charge in aqueous solution, which is caused by ionisation of the acid groups. Regarding acidic functional groups within wood fibres, they may consist of carboxyl groups ( ~ pka 4.5), phenolic hydroxyl groups ( ~ pka 10.2) and weakly acidic hydroxyl groups present in polysaccharides ( ~ pka 13.7) (Pu 1989). With the exception of pulps that contain significant amounts of sulphonate groups, carboxylic acids are usually the only functional groups that are responsible for charged site formation on the pulp fibres under typical papermaking conditions. Fibre charge has been shown to be one of the primary factors influencing final paper sheet physical properties and has therefore attracted considerable research interest. Total charge, surface charge and surface composition are parameters that impact sheet consolidation and fibre fibre bonding. The primary source of fibre charge for kraft pulps under typical papermaking conditions is carboxyl groups. The proposed explanation for the impact of bulk fibre carboxyl groups on paper properties is based on Donnan equilibrium and has been developed by Scallan et al. If pulp fibre carboxyl groups are deprotonated under certain conditions, the ionised anionic groups will be fixed anionic Page 9 Copyright Pira International Ltd 2007

16 Developments in Engineered Fibres Pulp and fibre charge sites. According to Donnan theory, fixed anionic sites in a gel surrounded by a semi-permeable membrane can cause swelling because of the generated osmotic pressure across the membrane due to the condition of electric neutrality. Fixed anionic sites must have cationic counter ions. As a result, the concentration of cations inside the membrane is greater than that in the bulk solution. The resultant concentration gradient across the membrane induces an osmotic pressure that causes the gel to swell to equalise the concentration of metal ions on both sides of the membrane. Scallan s work has shown that wood pulp fibres behave as swollen gels in water surrounded by semi-permeable membranes. If the amount of charged groups is sufficiently high, the pulp fibres behave as polyelectrolytic gels. The basic factors controlling fibre swelling have been reported to include cationic species associated with anionic fibre sites, the degree of dissociation and the ionic strength of the solution (Grignon 1980; Scallan 1983, 1992). It has been reported that there is a linear correlation between the tensile strength of handsheets made from chemical pulp fibres and the degree of fibre swelling with the charged groups in different ionic forms (Scallan 1979). If the concentration of fibre carboxyl groups increases and all other factors are equal, fibre swelling increases as well. A more swollen fibre is more conformable in the wet state and results in a denser sheet with greater bonded area. Therefore, enhanced fibre carboxyl groups can influence fibre bonding by improving fibre swelling ability, which increases fibre flexibility and promotes conformability, allowing fibres to form more fibre fibre contacts (i.e., relative bonded area) (Laine 1997b). As a result, paper sheet strength increases. Several studies by Scallan et al demonstrated that the strength increase of mechanical pulps after treatment with caustic soda was caused by an increase of acidic group content in the pulps. A subsequent investigation by Engstrand et al. showed that charged groups were increased in mechanical pulps from 90µmol/g to 250µmol/g after treatment with an alkaline 4% hydrogen peroxide. Such an increase in fibre charge resulted in a 177.7% increase in paper tensile index and 117.6% increase in specific elastic modulus. Ampulski conducted another study to investigate the impact of the surface and bulk charge of chemithermomechanical pulp (CTMP) fibres on tensile strength. It was found that paper tensile strength increased with increasing pulp charged group content; the surface fibre charge contributed more strength improvements than bulk charge; and carboxyl groups showed a greater effect than sulphonates. To understand further how surface fibre charge affects paper properties, Barzyk et al. prepared a series of carboxylate-enriched softwood bleached kraft pulp samples. Employing well-controlled chloroacetylation reaction conditions, the researchers applied an OZE (O oxygen delignification; Z ozone; E alkaline extraction) treated pulp with a fibre charge of 72 meq acid groups/gr to prepared pulp with 144 meq acid groups/gr with (i) a uniform distribution of carboxyl groups across the fibre cell wall and (ii) carboxyl groups located primarily on the fibre surface. It was found that the tensile strength was Page 10 Copyright Pira International Ltd 2007

17 Developments in Engineered Fibres Pulp and fibre charge 3 improved by 44% when the surface acid group content increased from 12meq/kg to 400meq/kg and specific bond strength was enhanced by ~ 50%. These strength benefits were attributed to surface acid groups on the fibre and this was subsequently validated by a series of studies (Laine 2001; Laine 2002a; Laine 2002b; Laine 2003; Lindstrom 2005; Horvath 2006). Employing carboxyl methylcellulose (CMC) surface grafting of kraft pulp fibres, a two- to tenfold increase in surface charge resulted in a more than threefold improvement in strength development in respect of rupture properties such as tensile index, strain to failure and tensile energy absorption; elastic properties were less affected. Since neither the sheet density nor the light scattering coefficient was impacted by this treatment, the data suggest that these benefits were due to improved specific bond strength, which is consistent with the earlier results by Barzyk. Although the benefits of increased surface charge provide enhancements in fibre fibre specific bond strength, in general, mechanical and kraft fibres exhibit improvements in a variety of important physical properties as surface/bulk charges increase. For example, Zhang et al. reported that dry paper tensile stiffness could be improved by 4.4% to 11.0% with 17.4% to 22.3% fibre carboxyl group enhancement through peroxide oxidation of fully bleached kraft pulps. In addition, fibre charge has also been shown to impact pulp refining. Hiltunen s study demonstrated that unbleached softwood kraft pulp with a high fibre charge suffered less damage during refining than pulps with a low fibre charge. Effect of kraft pulping on pulp fibre charge Although some carboxyl groups such as 4-O-methylglucuronic acid (MeGlcA) exist in native wood (Sjostrom 1989; Sjostrom 1993; Buchert 1995; Fors 2000), the amount of these groups is relatively low (7 15mmol/100g). During kraft cooking and pulp bleaching processes, the constituents of the fibre wall undergo profound chemical changes that impact the overall fibre carboxyl group content. In general, the bleaching process decreases the total fibre charge of chemical pulps due to the dissolution of lignin and hemicelluloses as shown in Table 3.1. TABLE 3.1 Carboxylic acid content in different pulps Pulp samples Sulphonic acid Total acidic group 1 groups (µeq/g) content (µeq/g) Unbleached TMP Peroxide bleached TMP Unbleached CTMP Peroxide bleached CTMP Unbleached softwood kraft pulp (KN 25.9) 85 ECF (ODEDED) softwood kraft pulp 32 TCF (OOQQPO) softwood kraft pulp 70 Unbleached hardwood kraft pulp (KN 18.2) 125 ECF (DE OP DD) hardwood kraft pulp 55 TCF (OOQPO) hardwood kraft pulp 120 Note: TMP: Thermomechanical pulp; CTMP: Chemithermomechanical pulp ECF: Elemental chlorine free; TCF: Totally chlorine free; O: oxygen delignification; D: chlorine dioxide; E: alkaline extraction; Eop: Oxygen and peroxide reinforced alkaline extraction; Q: chelating agent; P: peroxide; KN: kappa number 1 Total acidic groups (carboxylic + sulphonic groups); Source: Zhang 1994; Buchert 1995; Fors 2000 Page 11 Copyright Pira International Ltd 2007

18 Developments in Engineered Fibres Pulp and fibre charge During the kraft pulping process, new carboxyl groups are created in the carbohydrate component due to the peeling and stopping reactions (Johansson 1974). However, the amount of these carboxyl acid groups generated is naturally dependent on the pulping conditions employed and the size of fragmented molecule chains that undergo dissolution. Residual lignin in kraft pulps also contains aliphatic carboxyl groups that are enriched during pulping and/or formed by disproportionation reactions (Fors 2000). Up to 20% of the phenylpropane units can be transformed to carboxylic acids in this manner (Sjostrom 1989). In addition, research studies have shown that the amount of 4-O-methylglucuronic acid (MeGlcA) xylan side chains decreases during kraft pulping, being converted, in part, to hexenuronic acids (HexA) (Buchert 1995; Chakar 2000). HexA is an elimination product of MeGlcA and accounts for most of the carboxyl groups present in the xylan after the cook, although some HexA degradation occurs during cooking. Therefore, the main chemical components that contribute to fibre charge of unbleached kraft pulps are residual lignin and polysaccharides that consist of uronic acids, oxidised reducing ends and HexA. As reported by Chai et al., the total amount of charged groups in kraft pulps decreases during the conventional kraft pulping processes, due to the loss of lignin. Since the late 1980s, kraft pulping technology has experienced significant development towards achieving extended delignification, enhanced pulp yield, lower chemical usage, less energy consumption and increased production rates employing batch and continuous digesters (Marcoccia 1996). Low solids pulping is a typical example of a modern continuous pulping technology, which involves the extraction of dissolved organic substances before bulk delignification. Bhardwaj et al. investigated the fibre charge of low solids kraft pulps by conductometric titration and found that the fibre charge was in the range of 6 12mmol/100g for the pulps, with kappa number from 40 to 128. Although no linear relationship between fibre charge and kappa number of continuous kraft pulps was found in Bhardwaj s study, fibre charge was found to be positively proportional to kappa number, which is widely accepted in a number of studies of kraft pulps (Buchert 2001; Chai 2003; Bhardwaj 2004; Zhang 2005; Liu 2004). A recent study by Liu et al. reported the effect of green liquor pretreatment of US southern pine wood chips prior to kraft pulping on carboxyl groups of pulps. It was found that the carboxyl groups of linerboard pulps with a green liquor pretreatment gave higher fibre charge proportional to the degree of green liquor pretreatment (Liu 2004). Studies by Buchert et al. demonstrated that the carboxyl groups in residual lignin of Pinus sylvestris kraft pulp accounted for 32% of the total acids in the conventional pulp; however, the ratio was estimated to be 45% in the superbatch pulp due to the nearly complete degradation of HexA during cooking (Buchert 2001). Dang et al. conducted a detailed study on how fibre charge was influenced by varying kraft pulping conditions, as well as the effect of the low solids continuous kraft pulping versus conventional kraft pulping technology on fibre charge. Table 3.2 summarises some of these results. The data indicate that the higher the effective alkali (EA) for a batch kraft cook, the lower the carboxylic acid content of the resulting pulp when pulping is accomplished at the same H-factor and pulping temperature (170 C). Page 12 Copyright Pira International Ltd 2007

19 Developments in Engineered Fibres Pulp and fibre charge 3 TABLE 3.2 Effect of pulping conditions on fibre charge of conventional batch kraft pulps EA (% on oven-dried wood) Kappa number Sulphidity (%) Intrinsic viscosity (ml/g) Total fibre charge (mmol/100g) Fibre charge in holocellose (mmol/100g) Surface fibre charge (mmol/100g) Note: Maximum cooking temperature: 170 C; H-factor: 1,000; L/W: 4 Source: Dang 2006 Zhang et al. found that when cooking southern pine wood chips with high active alkaline (HAA, 19% on oven-dried wood) and low active alkaline (LAA, 15% on oven-dried wood) to a kappa number of ~ 49 at 25% sulphidity and at 170 C, the fibre charge in HAA cooked softwood kraft pulp (111.9µmol/g) was 21.1% higher than in LAA cooked softwood kraft pulp (92.4µmol/g). It is recognised that higher alkalinity in extended pulping increases lignin removal, minimises the reprecipitation of lignin and hemicellulose on to the fibres surface at the end of cook (Wagberg 1997) and most likely extracts acidic fragments, which results in the low fibre charge of kraft pulp. In contrast, pulping sulphidity did not appear to have an effect on the carboxyl group content of fibres. The data in Table 3.2 also suggest the effect of EA on surface and bulk fibre charge is comparable. To determine the extent of the fibre charge on pulp carbohydrates, a sample of each pulp in Table 3.2 was holocellulose pulped, which provides a pulp sample practically free of lignin and hexenuronic acid groups. Total fibre charge analysis of these pulps is summarised in Table 3.2 (Dang 2006). This data indicates that there is a difference in carboxyl group content between kraft pulps and the corresponding holocellulose pulps. In elemental chlorine free (ECF) fully bleached chemical pulps, fibre charge originates primarily from carboxyl groups attached to saturated polysaccharides. It is now well established that most hexenuronic acids in pulp are oxidatively removed during ECF bleaching via chlorine dioxide (Laine 1997a). The difference in carboxyl groups between bulk kraft pulp fibres and holocellulose pulps could be mainly attributed to the charge associated with lignin and hexenuronic acid groups present in the kraft pulps. These results also demonstrate that approximately 56% to 86% of the carboxyl groups of the kraft pulps can be attributed to pulp carbohydrates. The key pulping parameters influencing fibre charge on polysaccharides was found to be effective alkali and pulping temperature. Although holocellulose pulping is not employed industrially, it involves chlorine dioxide delignification, which is relevant to ECF bleaching protocols. The results in Table 3.2 also show that the kraft pulps with higher fibre charge end up with higher fibre charge in holocellulose, indicating the possibility of carryover of fibre carboxyl groups produced during the early pulping process. Dang, Elder and Ragauskas also examined the effect of conventional batch and continuous low solids kraft pulping on fibre charge. It was found that over the kappa number range of , there is a linear relationship between kappa number and fibre Page 13 Copyright Pira International Ltd 2007

20 Developments in Engineered Fibres Pulp and fibre charge charge for a softwood kraft pulp furnish. The two trend lines intersect at a kappa number of 18.2 (see Figure 3.1). Low solids cooked pulps were found to have a higher fibre charge than conventional kraft pulps when cooked to a kappa number greater than This data indicates that low solids pulps should not be pulped to a high H-factor if the desired effect is to produce high fibre charge pulps. This study demonstrates that fibre charge can be influenced by the type of pulping technology employed. FIGURE 3.1 Comparison between carboxylate content and kappa number of loblolly pine kraft pulps from conventional and low solids pulping Source: Dang 2006 To further investigate the effect of acidic groups on fully bleached paper physical strength, Dang examined the strength properties of holocellulose pulps with fibre charge of 5.58 and 8.35mmol/100g oven-dried pulp respectively. Since it is known that curl and kink detrimentally impact physical strength properties, the holocellulose pulps were PFI refined at 2,000 revolutions to obtain similar curl ( ) and kink values ( mm 1 ). The results demonstrate that a 50% increase in fibre charge of holocellulose fibres enhances tensile and burst index by 7% and 8% respectively, indicating that higher fibre charge corresponds to enhanced tensile strength and burst strength of paper prepared from holocellulose fibres with similar curl and kink values. In conclusion, kraft cooking conditions and pulping technologies provide viable approaches to enhancing fibre charge not only on the fibre, but also on the carbohydrate component. This field of study has not been fully explored and additional research needs to be undertaken to define fully the role of pulping and pulping additives in controlling fibre charge. Page 14 Copyright Pira International Ltd 2007