Xylan deposition onto eucalypt pulp fibers during oxygen delignification

Holzforschung, Vol. 65, pp. 605–612, 2011 • Copyright � by Walter de Gruyter • Berlin • Boston. DOI 10.1515/HF.2011.070

2010/241

Article in press - uncorrected proof

Xylan deposition onto eucalypt pulp fibers during oxygen

delignification

11th EWLP, Hamburg, Germany, August 16–19, 2010

Marcelo Coelho dos Santos Muguet*, CristianePedrazzi and Jorge Luiz Colodette

Pulp and Paper Laboratory (LCP), Federal University ofVicosa (UFV), Minas Gerais, Brazil¸

*Corresponding author.Pulp and Paper Laboratory,Federal University of Vicosa,Minas Gerais, 36570-000, BrazilE-mail: [email protected]

Abstract

A novel alternative was investigated for elevating the xylancontent of eucalyptus pulp through xylan deposition in thecourse of the oxygen delignification stage. The pH in therange of 10–13 was the only variable evaluated, while theother variables were kept constant in a range similar to indus-trial practice. Xylans were obtained from unbleached andbleached eucalyptus pulps by cold caustic extraction (CCE),giving rise to brown xylan liquor (BXL) and white xylanliquor (WXL), respectively. The liquors were added to acommercial brown pulp during oxygen delignification. Thexylan-enriched pulps were subsequently bleached to 90%ISO with the D(EP)D sequence, beaten in a PFI mill andevaluated for their physical and mechanical properties. Xylandeposition occurred at variable degrees depending upon thepH. Pulp bleachability was not impaired by WXL xylan dep-osition but was slightly negatively affected by BXL xylan.Pulp beatability was improved by both WXL and BXL xylandeposition. The deposited xylan was more stable acrossbleaching and beating with the WXL xylan than the BXLxylan deposits. At low energy consumption, the depositedxylan improved pulp physical and mechanical properties.Xylan extraction by CCE with subsequent deposition in theoxygen delignification is an interesting technique to manu-facture eucalypt pulps of high xylan content. The potentialfor industrial application is high as no significant modifica-tion of current pulp mill installations is needed.

Keywords: eucalyptus; fiberline yield gains; hemicelluloses;kraft pulp; oxygen delignification; xylan; xylan deposition;xylan extraction.

Introduction

Eucalypt fibers are largely used for printing and writing(P&W) paper grades. Pulps for P&W papers require signif-icant beating and can benefit from a high xylan content,

which is supposed to facilitate this operation. High-qualityP&W paper grades require pulps of high tensile strengthbecause they are manufactured in high-speed machines.Hemicelluloses contribute significantly to pulp fiber bondingstrength (Leopold and McIntosh 1961; Pettersson and Ryd-holm 1961; Spiegelberg 1966; Kettunen et al. 1982). Highxylan content in the pulp fibers increases mainly the tensilestrength of papers, but does not affect the bonding strengthat a level of the same density (Molin and Teder 2002). Thequantity of hemicelluloses is more important for paper prop-erties than the chemical nature of hemicelluloses (Annergrenet al. 1962). Xylan deposited in kraft pulps result in increasedtensile and bonding strengths (Sihtola and Blomberg 1975;Schonberg et al. 2001; Sjoberg et al. 2002). New carboxylicgroups are introduced into fibers via xylan and these play animportant role. Fibers with higher acid group content swellmore, exposing larger surface areas with more potentiallyreactive sites available for interaction (Eriksson and Sjostrom1968). The increased flexibility and conformability increasesfiber bonding. The hemicelluloses are present in the wholefiber wall, both on the surface and in the innermost layers(Mobarak et al. 1973; Bachner et al. 1993; Schonberg et al.2001). Dahlman et al. (2003a) evaluated their molecularproperties in hardwood and softwood pulps. The concentra-tion of hemicelluloses, their number-average molar mass(Mn), and weight-average molar mass (Mw) are higher in thesurfaces layers. Thus, these parameters could serve as a sen-sitive indicator for predicting the potential surface propertiesand bonding ability of pulp fibers. Hemicelluloses may actas an effective stress-transfer matrix (Kersavage 1973; Liitiaet al. 2005).

Many attempts have been made to increase xylan contentin pulp fibers, especially during the cooking process (Yllnerand Enstrom 1956, 1957; Aurell 1965; Hansson and Hartler1969; Danielsson and Lindstrom 2009). Xylans are solubleand easily removed in the kraft process owing to their lowmolecular mass and amorphous structure. The partial deg-radation and dissolution of hardwood xylan during alkalinepulping consumes reagents and decreases pulp yield (Clark1985). The dissolved xylan in the black liquor is thought tobe precipitated onto the fibers (Yllner and Enstrom 1956)and this process improves both yield and tensile strength(Clark 1985; Dahlman et al. 2003b; Danielsson and Lind-strom 2005). Danielsson and Lindstrom (2009) studied xylandeposition during kraft pulping and observed that pulps con-taining precipitated xylan of low and high Mw were strongerthan the reference without xylan.

Other workers have tried to increase pulp xylan contenton a laboratory scale through xylan addition on bleachedpulp (Kohnke and Gatenholm 2007). Xylan is deposited onto

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606 M.C.S. Muguet et al.


Figure 1 Flow diagram of the experimental concept.

Table 1 Characterization of the pulp samples.

Original pulps Pulps after CCE stage

Characteristics A B C A C

Kappa number 14.0 13.9 0.9 4.5 0.1Viscosity (dm3 kg-1) 1106 1318 859 – –Brightness (% ISO) 39.5 42.9 90.0 – –Xylan content (%)a 16.5 14.0 16.5 2.7 0.9HexA (mmol kg-1) 52.2 51.9 9.3 10.3 0.9Xylan extraction yield (%) – – – 85.3 95.3aBased on pulp. CCE, cold caustic extraction.

bleached pulps on both external surfaces and inner layers ofthe fiber (Mitikka-Eklund 1996; Schonberg et al. 2001). So,it is reasonable to assume that xylan deposited on never-driedbleached pulp would act as ‘‘native xylan’’ upon drying andcould have a function as a stabilizer of the cellulose fibrils;they may also prevent aggregation and hornification (Kohnkeand Gatenholm 2007). This method is, however, difficult torealize on an industrial scale because it would require highlypurified and expensive xylan.

In the present study, a novel approach for xylan depositiononto eucalypt pulp fibers is described. The deposition shouldoccur during the oxygen delignification stage, without spe-cific purification requirements after extraction from an exter-nal source. Both the xylan extraction from an external sourceand the deposition of xylan onto kraft pulp during theO-stage will be discussed, along with the influence of xylanon bleachability and paper strength properties.

Materials and methods

Two unbleached (A and B) and one O/O-D(EP)DP bleached (C)eucalypt kraft pulps from industrial sources were assessed. SamplesA and C were the sources of xylans obtained by extraction. SampleB was the base material for the xylan deposition (Figure 1). Kappanumber, viscosity, and brightness were determined according toTAPPI standard procedures. Xylan contents were analyzed by high-performance liquid chromatography combined with pulsed ampe-rometric detection (HPLC-PAD, Dionex, Sao Paulo, Brazil) afteracid hydrolysis following the procedure described by Wallis et al.(1996). Pulp hexenuronic acid (HexA) content was measuredaccording to Vuorinen et al. (1999). The carbohydrate compositionof the extracts was evaluated according to Wallis et al. (1996) withslight modification: instead of 300 mg solid material 10 ml of theextract was analyzed. The xylans were isolated from the extractsaccording to Teleman et al. (1995) and the sugar monomers wereevaluated according to Wallis et al. (1996) and molecular weightaccording to Pinto et al. (2005). All analyses were repeated twice(for results see Table 1).

Xylan extraction from pulps

The xylans were extracted via cold caustic extraction (CCE) fromthe bleached and unbleached eucalypt pulp, giving rise to the whitexylan liquor (WXL) and brown xylan liquor (BXL) extracts, respec-tively. A charge of 240 g of NaOH was applied to 300 g oven-dried(o.d.) pulp, at 10% consistency (30 min, at 258C). Following this,the liquor was separated from pulp fibers by centrifugation and the

liquid phase was collected (Figure 1). (A Thermo electro corpora-tion, IEC CL 10 centrifuge from Datamed, Belo Horizonte, Brazilwas used.)

Xylan deposition during oxygen delignification

Oxygen delignification conditions were: 60 min at 1058C 20 kg O2 t-1

(based on o.d. pulp), and 500 kPa, according to Muguet et al.(2010). The alkali charges were varied so that pH values between10 and 13 were reached. A volume of 1500 ml of each xylan liquorwas prepared with xylan concentrations between 15.8 g l-1 (BXL)and 17.5 g l-1 (WXL) (Table 2). Based on these liquors, solutionswere prepared with pH of 10.0"0.1, 12.0"0.1, 12.5"0.1, and13.0"0.1 via neutralization with concentrated sulfuric acid. Theinitial NaOH charges measured by titration were: pH 10s0.15 kgt-1, pH 12s25 kg t-1, pH 12.5s70 kg t-1, and pH 13s150 kg t-1

(tonnes based on o.d. basis). The xylan liquors were added to 250 go.d. of sample B in a mixer/reactor Mark V (Quantum Technologies,Twinsburg, OH, EUA), reaching a consistency of 10% (Figure 1).The reference sample was prepared under the same conditions,except that the initial pH of NaOH solution was not changed.


Xylan deposition during oxygen delignification 607


Table 2 Composition of the liquors as determined by total hydrol-ysis and HPLC of BXL (prepared from unbleached pulp) and WXL(prepared from bleached pulp) and their pH.

Glucans Xylansa Lignin(g l-1) (g l-1) (g l-1) pH

WXL ND 17.5 0.00 13.7BXL ND 15.8 0.92 13.6aConsidering only the xylose backbone. HPLC, high-performanceliquid chromatography; BXL, brown xylan liquor; WXL, whitexylan liquor; ND, not detected.

Table 3 Xylan fractions obtained from BXL (prepared from unbleached pulp) and WXL (preparedfrom bleached pulp) and their characterization.

Carbohydrates Bound lignin Properties

Glucans Xylansa UA Klason L Soluble L UA/Xylan Mw

Sample (%) (%) (%) (%) (%) ratio (kDa)

BXL 1.00 78.4 14.2 2.25 4.10 0.181 19.09WXL 1.76 92.2 5.68 0.00 0.33 0.062 15.13

aConsidering only the xylose backbone. BXL, brown xylan liquor; WXL, white xylan liquor; L, lignin;UA, uronic acid.

Scanning electron microscopy (SEM) images were acquired with ascanning electron microscopy model 1430VP (LEO ElectronMicroscopy, Oberkochen, Germany), operating at 11 kV.

Elemental chlorine-free bleaching, evaluation of

physical and mechanical properties of paper

Two selected pulp samples containing deposited xylan and theirreference were bleached to 90% ISO brightness, with the D(EP)Dsequence. Physical and mechanical paper testing was performedaccording to TAPPI test methods.

Results and discussion

Xylan extraction and characterization

Table 1 shows that the CCE treatment was more effective inthe case of the bleached pulp (sample C, 95.3% xylan remov-al) than in that of the unbleached pulp (sample A, 85.3%xylan removal). This result is consistent with the higherxylan concentration (Table 2) in the liquor extracted fromthe bleached pulp (WXL) than in that extracted from theunbleached pulp (BXL). This can be explained by lignin-carbohydrate complexes (LCCs) existing in the unbleachedpulp. While separated xylan and lignin are alkali soluble,LCCs are less soluble. Thus, removal of xylan linked to lig-nin is a challenge. The CCE treatment removed significantamounts of lignin from the unbleached pulp. The value of0.75% lignin on pulp found in the BXL is equivalent to akappa removal of five units (Table 2). The pulp sample lost9.5 kappa units across CCE, from 14 to 4.5. Since only fiveunits came from lignin, the remaining kappa number is attrib-utable to HexA and LCC removal. Approximately four unitsderived from HexA which dropped from 52.2 mmol kg-1

(5.2 kappa units) to 10.3 (one kappa unit), assuming that

10 mmol kg-1 HexA is equivalent to one kappa unit (Vuo-rinen et al. 1999). Thus, the remaining kappa unit is likelyderived from LCCs.

The xylan isolated from the WXL had lower Mw, lessbound lignin and less uronic acids than the one isolated fromthe BXL (Table 3). The lower Mw of xylan extracted frombleached pulp is explained by oxidation effects occurringduring bleaching, particularly in the oxygen delignificationstage. The lower lignin content of the WXL is obvious asthe lignin content of original pulp, from which the xylan wasextracted, was previously diminished by bleaching. The low-er content of uronic acids (UA) was also expected, as somebleaching stages effectively remove them. In contrast, xylanisolated from the BXL was only modified by the pulpingprocess. The decrease in UA across bleaching was also foundby Dahlman et al. (2003a) in hardwood pulps.

Xylan deposition during oxygen delignification

Xylan deposition occurred in all cases to different extents(Figure 2a). The samples BXL and WXL deposited xylanwith different patterns. Many studies suggest that hemicel-luloses deposition onto fibers is more efficient at low pHvalues (Yllner and Enstrom 1957; Aurell 1963, 1965; Hans-son and Hartler 1969; Danielsson and Lindstrom 2009; Ribeet al. 2010). At low electrolyte concentration, the adsorptionis favored if the chemical affinity of the polymer to the sur-face is high (Osterberg et al. 2001). This behavior was seenfor the BXL deposition. However, a pH up to 12 does notaffect WXL xylan deposition. The presence of lignin in theBXL liquor seemingly disturbs xylan deposition in the higherpH range ()12). This theory is supported by Westbye et al.(2007), who claimed that xylan can interact with soluble lig-nin by phenolic groups bound on its backbone. The fact thatlignin was found in the extracted xylan preparation (Table2) supports this hypothesis. With decreasing pH, the xylanand soluble lignin tend to assemble aggregates, which thenbegin to precipitate onto the fibers. Since there is no ligninbound to the xylan present in the WXL, xylan tends to self-assemble, regardless of the pH value, in the range between10 and 13.

The UA/xylan ratio seemingly affects xylan deposition.Linder et al. (2003) proposed a mechanism whereby uronicacids, which are negatively charged, repulse one xylan chainfrom another, preventing the formation of aggregates. It canbe seen that xylan with a lower uronic acid content (WXL)deposited to a greater extent. These results are in accordance




Figure 2 Effect of starting pH of oxygen delignification on xylandeposition onto unbleached pulp (sample B) for processes with addi-tion of brown xylan liquor (BXL), white xylan liquor (WXL), andreference sample (REF, no xylan deposition).

Figure 3 Effect of initial reaction pH of oxygen delignification on (a) end pH, (b) kappa number, (c) brightness, and (d) viscosity, forprocesses with addition of brown xylan liquor (BXL), white xylan liquor (WXL), and reference sample (REF, no xylan deposition).

with those of Danielsson and Lindstrom (2009), who foundthat eucalypt xylan with lower uronic acid substitution tendsto form bigger aggregates and deposits onto fibers in largerquantities.

Molecules with lower Mw tend to deposit more efficientlyonto fibers owing to their high entropy. Smaller moleculesare more flexible and may attach better to each other andthus will form bigger aggregates and will be deposited to begreater extent. The xylan isolated from the WXL had lowerMw and thus deposited more effectively. The results foundfor BXL and WXL isolated xylan corroborate the results ofDahlman et al. (2003a), who showed that during bleachingMw decreases with decreasing uronic acid content.

The quantitative deposition values are presented in Figure2b. The WXL deposition occurred to a higher extent regard-less of pH value. The higher xylan concentration and favor-able chemical structure of WXL extracts compared to BXLextract (Tables 2 and 3) explain this observation.

Impact of xylan deposition on oxygen delignification

performance

Figure 3a–d illustrates the impact of xylan deposition on thefinal pH, kappa number, brightness, and viscosity as a func-tion of the initial pH of the oxygen delignification. The finalpH correlates well with the initial pH, as anticipated. Nosignificant effect of the WXL and BXL xylan deposition onthe final pH was observed (Figure 3a). This is an indicationthat reactions are negligible between xylan and delignifica-tion chemicals (NaOH and O2). However, the presence oflignin in the xylan extract affects oxygen delignification effi-ciency, as the pulps treated with the BXL showed higherkappa numbers (Figure 3b) and lower brightnesses (Figure3c) than those treated with WXL and the reference. Thexylan deposition with both BXL and WXL extracts resultedin decreased pulp viscosity owing to the low molecularweight of these hemicelluloses in relation to cellulose (Figure3d). The xylan deposition increases the pulp xylan/celluloseratio, thus diminishing overall viscosity. The increase in theinitial pH had the expected effect of decreasing pulp kappanumber and viscosity and increasing brightness. The pulpviscosity is particularly deteriorated at pH 13, where alkalinehydrolysis of cellulose chains becomes severe, for both treat-ments without and with BXL and WXL added xylans.

The theory that lignin-xylan is assembling before precip-itation (Westbye et al. 2007) was confirmed in this study.The sample with BXL addition at pH 10 resulted in a pulpwith lower brightness and higher kappa number when com-pared to the starting unbleached pulp (B), indicating thatlignin precipitation did occur.




Figure 4 SEM images of the reference (REF) at pH 12: (a); and with brown xylan liquor (BXL) addition: (b) pH 10, (c) pH 12, (d) pH12.5, (e) pH 13; and white xylan liquor (WXL) addition: (f) pH 10, (g) pH 12, (h) pH 12.5, (i) pH 13.

Figure 5 Energy consumption vs. xylans content at a level of 60 Nm g-1 tensile index for processes with addition of brown xylan liquor(BXL), white xylan liquor (WXL), and reference sample (REF, noxylan deposition).

SEM images of thin handsheets (50 g m-2) prepared withreference pulp and pulps treated with BXL and WXLextracts are presented in Figure 4. The xylan deposition ontopulp fibers is visible in all images obtained with the pulpstreated with BXL and WXL, regardless of pH, whereasuntreated pulps do not show such a pattern. The formation ofxylan aggregates is visible, as particularly seen in Figure 4c.

All images show that deposition occurred to a greaterextent with the WXL extract than with the BXL extract. Thistrend was confirmed through the carbohydrate analysis of thepulps by HPLC (Figure 2a). In images of BXL-treated pulps,slight differences are seen among samples prepared at dif-ferent pH values. In Figure 4b, the xylan is not visible asaggregate, but it appears like a fiber coating, turning thesurface smoother. In almost all images, xylan is aggregatedand forms a web, thus helping interfiber bonding. This sup-ports the theory that xylan can act as a stress transfer matrix(Kersavage 1973; Liitia et al. 2005).

Elemental chlorine-free bleaching

The pulps treated with WXL and BXL at pH 12.5 were cho-sen to be bleached to a final brightness of 90% ISO with theD(EP)D sequence. The amount of reagents needed to bleach

the reference and WXL-treated pulp samples to 90% ISOwere almost the same, with slightly less chlorine dioxideneeded for the WXL pulp. The BXL-treated pulp consumedlarger amounts of active chlorine to reach 90% ISO bright-ness than the other two samples. This result was anticipatedsince the BXL extract contained sizeable amounts of lignin,which contaminated the pulp during the oxygen delignifi-




Figure 6 Energy consumption vs. (a) density, (b) tensile index, (c) tensile energy absorption, (d) tear index, (e) specific elastic modulus,and (f) internal bonding strength properties for processes with addition of brown xylan liquor (BXL), white xylan liquor (WXL) andreference sample (REF, no xylan deposition).

Figure 7 Deposited xylans stability after pulping, oxygen delig-nification, bleaching and beating of pulps with addition of brownxylan liquor (BXL), white xylan liquor (WXL), and reference sam-ple (REF, no xylan deposition).

cation stage to a point that the post-oxygen kappa numberwas one unit higher for this sample than for the other two(Figure 3b). For a fixed amount of bleaching chemicals, thefully bleached pulps showed the following order of finalbrightness: WXL)reference)BXL, with the inverse orderfor chemical consumption. The deposited xylans were quitestable during bleaching, as can be seen from the values pre-sented in Figure 5.

Pulp physical and mechanical properties

The role of xylan during beating and for paper properties hasbeen widely studied and their importance is confirmed forP&W paper grades (Bhaduri et al. 1995; Anjos et al. 2005;Molina et al. 2008). This subject has not yet been studied inthe context of oxygen delignification. Figure 5 shows thatpulps with higher xylan content (evaluated after bleaching)need much less energy to reach the same tensile index (60 Nm g-1).

Xylan improves beatability, leading to less beating energydemand and also improvements in the mechanical and bond-ing properties of the paper. Figure 6 shows a summary ofthe properties evaluated. The xylan-treated pulps have goodproperties at low beating energy levels. This leads to signif-icant economical savings, taking into account that pulp beat-ing is one of the most costly operations of the paper manu-facturing process.

Deposited xylan stability across the process

The xylan content was evaluated in the bleached pulp andpulp sheets refined to 358SR, which is typical of eucalypt

P&W paper-grade pulp. The smaller backbone and less sub-stituted xylan seems to attach better to fibers (WXL), and istherefore more resistant to bleaching operations and themechanical forces during beating (Figure 7).

The double line concept

Pulps with high xylan content tend to have high density, hightensile and bonding strength, and low air resistance, and thusare well suited for P&W paper grades. On the other hand,pulps with low xylan content possess high bulk and softness(leading to high liquid absorption by capillarity), and thusare advantageous for tissue paper grades. Accordingly, thetechnique described here (xylan extraction and redeposition)




Figure 8 (a) Proposed white xylan liquor (WXL) double line concept and (b) brown xylan liquor (BXL) double line concept to createhigh-quality final products.

Figure 9 Proposed biorefinery concept based on agricultural wastes, grasses, and mill fibrous rejects as a cheap source of xylans forcreation of high-quality final products.

allows for implementation of the so-called double line con-cept (Figure 8), whereby the xylan-rich extraction liquorfrom a tissue production line can be used in a parallel P&Wproduction line where it is redeposited over the fibers; in thiscase, a low caustic make-up is required. Note that the alkalirequirements in the CCE stage, in case of the double lineconcept, can be largely minimized after proper optimization;this is under investigation in the authors’ laboratories.

Figure 9 is a flow diagram of the potential application ofCCE of agricultural wastes and the integration of hemicel-luloses obtained into the oxygen delignification process ofpulp production. Such a process would be in agreement withthe ideals of the biorefinery concept, which intends to min-imize the environmental burden and maximize the economicadvantages of a process.

Conclusions

Xylan deposition during oxygen delignification seems to betechnically feasible for improving the fiberline yield withoutsignificant modification of existing equipment. Xylan extrac-tion was more efficient in bleached pulps than in unbleachedpulps. Deposition of xylan extracted in the first case was notaffected by pH, whereas deposition of xylan extracted fromunbleached pulps was effective only below pH 12. Xylanswith fewer side-chain groups and lower Mw have a morepronounced tendency for deposition. Oxygen delignificationperformance and pulp bleachability were not influenced byWXL xylan deposition, while the performance was slightlydeteriorated by BXL xylan deposition. Pulp beatability was

improved by both WXL and BXL xylan deposition. Thedeposited xylans were quite stable across bleaching and beat-ing operations, with the WXL xylan being more stable thanthe BXL xylan. At low energy consumption, the depositedxylan improved the physical and mechanical properties ofthe pulp.

Acknowledgements

The authors would like to acknowledge the LCP technicians Oldairde Paula for support during the running of the bleaching experi-ments, Maurıcio Lino for the pulp carbohydrates analyses, and LuizOtavio for support during the beating experiments. The authorswould also like to acknowledge the Nucleo de Microscopia eMicroanalise (NMM) of UFV, for the SEM imaging.

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Received November 26, 2010. Accepted March 2, 2011.Previously published online May 4, 2011.


Xylan deposition onto eucalypt pulp fibers during oxygen delignification

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