IMPROVED PENETRATION OF PULPING REAGENTS INTO WOOD

Transcription

1 IMPROVED PENETRATION OF PULPING REAGENTS INTO WOOD JAMES L. MINOR, Reserch Chemist EDWARD L. SPRINGER, Chemicl Engineer USDA Forest Service 1 Forest Products Lbortory Mdison, WI U.S.A. In: Proceedings, 6th Interntionl symposium on wood nd pulping chemistry; 1991 April 30-My4; Melbourne, Victori, Austrli. Melbourne, Victori, Austrli: Austrlin Pulp nd Pper Industry Technology Assocition; 1991: Vol. 1. ABSTRACT Pretretment of hrdwood chips or sticks with lkli incresed the permebility of the wood nd incresed the rte of diffusion of wter-soluble mterils. Commercil-size hrdwood (spen, red lder, nd red ok) chips were pulped with peroxymonosulfuric cid solutions to give bright pulps with very low lignin content, high viscosity, nd krft qulity strength. The enhnced penetrtion effect ws demonstrted with uniformly cut pieces of spen wood. The homogeneity of delignifiction through the wood structure ws nlyzed using the cetyl bromide procedure for lignin. Penetrtion nd delignifiction were much more uniform for the pretreted specimens thn for wter-soked controls. The mechnism of enhnced penetrtion is considered from the stndpoint of chemistry nd the effects on wood microstructure. The primry chemicl ction is sponifiction of esters with preferentil wekening of the intercellulr region. 1. INTRODUCTION The use of peroxymonosulfuric cid s delignifying regent hs been studied recently t the Forest Products Lbortory [l]. This initil work ws performed using ground spen wood s the substrte. Subsequently, chemimechnicl (cold sod) pulp, hightemperture therml-refiner mechnicl pulp (hrdbord fiber), nd destructured spen wood were used s substrtes [2]. These mterils were selected to minimize diffusion problems while minimizing fiber dmge. The pulps prepred from these substrtes hd properties tht would mke them suitble for 1 The Forest Products Lbortory is mintined in coopertion with the University of Wisconsin. This rticle ws written nd prepred by U.S. Government employees on officil time, nd it is therefore in the public domin nd not subject to copyright. mny fiber products, but their strengths were somewht less thn those of krft pulps from chips of the sme wood species. In chemicl delignifiction of wood chips, the following sequence of events occurs: (1) the pulping regent diffuses to the rection site within the wood chip, (2) the regent rects with the lignin, nd (3) the rection products diffuse out of the wood structure into the bulk liquid. If the rection within the wood structure is rte controlling, the pulping is homogeneous nd uniform pulp is obtined. With most oxidtive regents, the diffusion of the oxidnt to the rection site is rte controlling. The slow wter-solution mss trnsfer rtes re even more hindered by the rpid consumption of ctive oxidnt in the outer regions of the chip. Thus, the outer fibers re extensively delignified before the inner core hs hd n opportunity to rect. If wood chips re pretreted with lkli, the subsequent rte of diffusion into nd out of the internl wood structure is gretly incresed. A much more uniform pulping is thereby chieved. Strting with chips, pulps of krft strength re obtined with very low lignin contents nd high yields. These pulps cn be esily bleched without use of chlorine-contining regents. This pper reports the results of pulping lkli-pretreted wood with peroxymonosulfuric cid nd other cidic oxidtive pulping regents. Experiments were performed to elucidte the mechnism of the lkli pretretment. 2. RESULTS AND DISCUSSION 2.1 Pretretment of Wood Sticks The reltive importnce of rection nd diffusion rtes ws studied by using mtchstick-size pieces of wood tht hd cross-sectionl thickness bout equivlent to hlf-inch chip (3 mm) nd length (40 mm) such tht the more rpid longitudinl diffusion would not dominte. The sticks were pretreted with lkline solutions, wshed with wter, nd then pulped with 6th ISWPC/389

2 cidified peroxymonosulfte solutions. The pulped sticks were extrcted with 1% sodium hydroxide to remove degrded lignin products. Wter-soked sticks were used s pretretment controls. The pulped fibers were seprted from the incompletely pulped core by suspending the stick in wter nd pressing nd stirring with glss rod. This test for extent of fiberiztion ws subjective, but it ws dequte for the purposes of this study becuse the difference between the lkli-pretreted nd untreted hrdwood sticks ws so lrge. The liberted fibers (referred to s fiber in the tbles) nd the unfiberized cores were ech nlyzed for their residul lignin content. Pretreting spen (Populus tremuloides) sticks with lkli significntly improved the speed nd uniformity of the peroxymonosulfte pulping rection compred to tht for untreted wood (Tble I). The pretreted sticks were completely fiberized in <39 h t room temperture. At 24 h, even though the pulped stick could be seprted into fiber nd n unfiberized core, the difference in lignin contents ws not lrge. By contrst, if the spen sticks were soked in wter, the pulped sticks were only 20% fiberized fter 48 h nd lrge grdient of lignin concentrtion existed between the outer fibers nd the inner core. Other wood species were lso exmined (Tble II). With red ok (Quercus rubr), the positive effect of the pretretment ws clerly shown even though the pulping of the pretreted wood ws somewht slower thn tht of pretreted spen. After 48 h t room temperture, the pretreted ok ws completely fiberized nd hd lignin content of 7.1%. After the sme period, the untreted ok ws only 23% fiberized, nd the lignin content ws 9.4% in the fiber nd 20% in the core. After 188 h, the untreted ok sticks still hd n unfiberized core. The pretretment effect ws lso observed with honey locust (Gleditsi tricnthos), ring-porous hrdwood. Although the honey locust smples were different size thn those of the other wood species, for the sme-size sticks of honey locust, pretreted smples could be pulped in 24 h to lignin content of 7.5%, wheres untreted smples were essentilly unfiberized nd hd lignin content of 16.7% fter the sme pulping tretment. The effect ws not s cler for the coniferous wood, southern yellow pine (Pinus elliottii). The springwood of this species is more esily penetrted thn the dense summerwood, nd core is not redily identified. In ddition, the vrince between different southern yellow pine smples ws lrger thn the difference in the effect between pretreted nd untreted smples. Solvent extrction of the resins with ethnol-toluene zeotrope before pretretment did not chnge the sitution. Aspen sticks were used to investigte the effects of time of tretment, lkli concentrtion, solvent extrc 390/6th ISWPC tion, nd reversibility s pplicble to the pretretment (Tble III). None of the vribles produced significnt difference in the pulping results compred with no pretretment. Not much difference ws observed in the results of chnging sodium hydroxide concentrtion over the rnge of 1% to 10%; however, 1% my be ner the minimum necessry. We used 5% solution for most of our studies. One hour of pretretment ppered to be sufficient for the 5% concentrtion. Preextrction of the spen sticks with methnol nd cetone did not hve significnt effect on the peroxymonosulfte pulping of either the lkli-pretreted or untreted smples. Becuse peroxymonosulfuric cid is strong oxidizing gent, the temperture must be kept low for diffusion to compete with the delignifiction rections. However, if the temperture is too low, the pulping times re excessively long. Tble IV shows the results of pulping pretreted spen sticks t 35 C nd 50 C. At 50 C, the pulping rection ws too fst for diffusion, nd the outer fibers becme pulped while the inner fibers remined lignified. Pulping ws more uniform t 21 C, but it required over 24 h (Tble I). The optimum temperture to blnce diffusion with rection rte ppers to be ner 35 C. After 9 h t tht temperture, the stick ws soft nd it could be fiberized, though not esily. The core ws not clerly defined, but the inner fibers were slightly more yellow thn the core nd hd slightly higher lignin content. The fiberiztion point occurred t the sme stge of delignifiction tht hs been observed for other chemicl pulping processes (6% to 7% residul lignin). The lkline pretretment should effectively improve the penetrtion nd thereby the pulping of other cidic oxidtive pulping regents. As Tble V shows, lkline pretretment decidedly improved the rte nd uniformity of cidic sodium chlorite delignifiction of spen sticks. Alkli-pretreted white birch swdust hs been shown to pulp fster by the cid chlorite process thn untreted white birch [3]. 2.2 Chip Pulping Becuse of the incresed penetrtion chievble with n lkline pretretment, direct pulping of wood chips seemed fesible. The detils of chip pulping re given in the Experimentl section, nd the results re shown in Tble VI. In 3 dys t room temperture, lklipretreted 19-mm (0.75-in.) spen chips could be pulped with cidic peroxymonosulfte to kpp number of 2.5 with 2.3% screenings nd totl yield of 58.2%. Hndsheet ter-tensile indices were comprble to those of spen krft pulps from chips. Highstrength pulps were lso prepred from red ok nd red lder (Alnus rubr) chips. The low-kpp spen pulp ws very esily bleched with 0.5% lkline hydrogen peroxide

3 Tble I. Effect of Pretretment on Peroxymonosulfte Pulping of Aspen. Pulping Extent time fiberized Lignin content Yield Pretretment (h) (%) (%) (%) NOH b 15 0 > (core) (fiber) Wter sok (inner core) (outer core) 1.1 (fiber) Temperture, 21 C. b Ambient temperture, 24 h; 5% concentrtion. Tble II. Effect of Pretretment on Peroxymonosulfuric Acid Pulping of Vrious Species. Wter sok (%) NOH pretretment (%) Pulping Lignin content time Extent Lignin Extent Species (h) Yield fiberized content Yield fiberized Core Fiber Red ok Honey locust b ' Pine Ambient temperture, 24 h; 5% concentrtion. b 1.5 by 1.25 by 50 mm sticks. c White outer fibers; most of the stick ws hrd nd drk. Tble III. Effect of Vrying Pretretment Conditions on Pulping of Aspen. Pretret- Fiber NOH ment Extent lignin concentrtion time Solvent Acid fiberized content (%) (h) extrction wshed (%) (%) 1 72 No No b No No b No No b No No No No No Yes Yes No Yes No No No Ambient temperture, 48 h. b Extensively wshed with wter. 6th ISWPC/391

4 Tble IV. Effect of Temperture on Peroxymonosulfte Pulping of Pretreted Aspen. Extent Temperture Time Yield fiberized Lignin content C (h) (%) (%) (%) (100) b 6.7 c d (fiber) 9.3 (core) Ambient temperture, 24 h; 5% NOH. b Not esily fiberized (see text). c Inner fibers. d untreted specimens hd 5% fiberiztion. Tble V. Effect of Pretretment on Acidic Sodium Chlorite Pulping of Aspen. Lignin content Extent Yield fiberized Pretretment (%) (%) Fiber Core Wter sok NOH NA Ambient temperture, 24 h; 5% concentrtion. Tble VI. Results of Chip Pulping" With Peroxymonosulfuric Acid. Cndin Totl Lignin stndrd Ter Tensile Brightyield Screenings Viscosity content freeness index index ness Species (%) (%) (mp-s) (%) (ml) (mn-m 2 /g) (N-m/g) (%) Aspen A Aspen B Aspen krft b Red ok Red Alder Bleched spen B See Experimentl for pulping conditions. Chips for krft pulping tken from sme smple s chips for peroxymonosulfuric cid pulping. b 392/6th ISWPC

5 2.3 Mechnism of Alkli Pretretment Although swelling of the wood structure is undoubtedly importnt to the improved penetrtion of wterborne regents, we did not know whether the residul sodium hydroxide or the sodium counterions plyed role in drwing the cidic pulping regent into the wood. The swelling of hrdwoods my be reversed prtilly but not completely by decresing the ph from 13 to 3 [4]. The superswollen stte is gin obtined by rising the ph. The positive effect of the pretretment could not be reversed by extensive wshing with wter or by wshing with cid to remove bound sodium ions (Tble 111). Thus, the sodium ions do not ply significnt role in the penetrtion of the cid regent. The Donnn effect hs been evoked to explin the initil swelling in lkli [5]. The lck of sodium ion effect plus the high proton nd slt concentrtion of the pulping solution suggest tht the Donnn effect is not mjor contributor during pulping regent penetrtion. The incresed permebility of wood chips cused by lkline pretretment might be due to (1) incresed ccess to the individul wood fibers through the gross wood structure or (2) incresed permebility of the individul fiber wlls; incresed permebility my be due to both of these fctors s well. To test this question, the delignifiction of fiberized spen wood pretreted with lkli ws compred with the effect of identicl pulping conditions on untreted fiberized wood. In this experiment, the gross wood structure hd been destroyed nd the chnge in permebility of the individul fiber wlls s result of lkli tretment ws studied. Aspen hrdbord fiber [2] ws used in this experiment. The results of the comprison re shown in Tble VII. Both the residue yield nd the lignin content of the lkli-pretreted fiber were substntilly lower thn tht of the untreted fiber. This indictes tht lklipretretment incresed the permebility of the individul fiber wlls. Alkli tretment of hrdwoods ws studied in connection with improved digestibility by ruminnts [4]. The results showed tht the diffusion rte of wter-soluble mteril through the cell wlls is incresed severlfold by sodium hydroxide tretment. Also, the permebility of wter into ir-dried pretreted hrdwoods ws considerbly greter thn tht into corresponding untreted wood. The uthors ttributed these observtions nd the superswelling effect of lkli to sponifiction of cross-linking uronic cid esters of the 4-O-methylglucuronoxylns. Direct evidence for glucuronoxyln involvement ws not obtined. The identity of the cell wll polymer tht contributed the lcohol function to the ester ws not determined, but it ws postulted to be lignin or nother xyln. The Tble VII. Effect of Alkli Pretretment on Delignifiction of Aspen Hrdbord Fiber. Residue yield Lignin content Pretretment (%) (%) None NOH See Experimentl for conditions; 1% concentrtion. importnce of breking the ester bond ws supported by the fct tht mmonolysis with liquid mmoni ws lso effective in incresing the swelling cpcity of the wood. Diffuse reflectnce Fourier trnsform infrred (DRIFT) spectr my be obtined directly from the surfce of spen sticks (Fig. 1). The interior surfce my be exmined by simply splitting the sticks longitudinlly through the center. In ll exmples reported, the spectr from the interior surfces were the sme s those from the exterior surfce. A strong bnd t 1740 cm 1 ws observed in the spectrum from untreted wood. This bnd ws ssigned to ester crbonyl stretching nd my hve lrge contribution from cette groups. The bnd disppered upon lkli tretment nd ws replced by incresed infrred bsorption in the 1550 to 1700 cm 1 region nd decresed bsorption t 1260 cm 1, which could be ester C-O stretching. The incresed bsorption is in the region chrcteristic of ionized crboxyls. Tretment of the sponified wood with cid should result in n increse in the bsorption frequency. Indeed, curve c ws obtined fter cid wshing of the lkli-treted wood (curve b). In ddition, the cid wsh pprently produced some reesterifiction or lctoniztion. Tretment of spen wood with liquid mmoni under pressure lso essentilly eliminted the 1740 cm 1 bnd (curve e). A strong bnd centered t 1670 cm 1 ppered in the spectrum. The mide function my contribute to this new bnd, but the 1670 cm 1 bnd ws lso present in the spectrum of spen treted with gseous mmoni under conditions tht would not be expected to produce mny mide functionl groups (curve d). Liquid penetrtion into wood occurs on scle tht might be observed by electron microscopy. The min mode of liquid trnsport into hrdwoods is through the vessels nd then into other wood cells through the pits. A thin pit membrne covers the opening between the pits of two djcent cells. By scnning electron microscopy, we were unble to observe ny disruption of the pit membrnes cused by lkline pretretment. However, fter ir drying, the membrnes of the treted wood ppered more wrinkled thn those of ir-dried untreted wood. A more drmtic effect ws noted by splitting ir-dried sticks in 6th lswpc/393

6 oxymonosulfte pulping regent ws dded without ny wshing. The results were essentilly the sme s those for sodium hydroxide pretretment (100% fiberized, 2% lignin remining fter peroxymonosulfte pulping for 48 h t 21 C). The DRIFT spectrum showed tht the 1740 cm 1 bnd hd been eliminted nd tht pronounced bnd ppered t 1670 cm 1 (Fig. 1, curve d). The bnd t 1260 cm 1 ws lso reduced compred with tht of untreted wood. Although some limited movement of xyln polymers could hve occurred during the mmoni gs tretment, it ppers tht clevge of the ester bonds is more importnt thn removl of xyln to the improvement in penetrtion. 3. CONCLUSIONS Pretretment of hrdwood chips or sticks with lkli incresed the permebility of the wood nd incresed the rte of diffusion of wter-soluble mterils. This tretment cn be used before pulping with cidic, oxidtive regents such s peroxymonosulfuric cid to increse the homogeneity nd overll rte of pulping. Penetrtion into the wood mcrostructure nd through the micropores of the fiber cell wll re both improved. With this pretretment, preliminry fiberiztion step is not necessry, nd strong pulps cn be obtined directly from wood chips. Figure 1. DRIFT spectr of spen wood. the xil direction. If the wood ws not pretreted, the splitting occurred with tering of the vessel wlls. If the wood ws pretreted with lkli, the splitting occurred primrily between cell wlls. This is probbly relted to the observed decrese in the reltive torsionl modulus of birch upon tretment with 10 g/l NOH [6]. These observtions imply tht the opening of the wood structure tkes plce in the middle lmell s well s in the fiber wll. Therefore, sponifiction of pectic substnce uronic cid esters nd possibly lignin-derived cid esters is probbly importnt s well s sponifiction of xyln uronic cid esters. An exmintion of the liquors from the pretretment stge showed tht most of the dissolved mteril ws xyln. Removl of xyln ws considered to be the reson for the increses in medin pore width nd pulping rte when lkli-pretreted white birch swdust ws delignified with cid chlorite [3]. We sought to determine if xyln removl ws the explntion for improved penetrtion of cidic pulping regents into lrger wood prticles. Aspen sticks were treted with mmoni gs by exposing them to the vpors over mmonium hydroxide for three dys. After tretment, excess gs ws removed with vcuum nd per 394/6th ISWPC It is probble tht the mechnism of improved pulping regent penetrtion is clevge of ester bonds s proposed by Trkow nd Feist [4]. The 4-0-methyl glucuronic cids of hrdwood xylns my be forming ester cross-links with other wood polymers, which, when broken, llow the wood structure to swell beyond the wter-swollen stte. Penetrtion into the wood mcrostructure nd through the micropores of the fiber cell wlls re both improved. The observtion tht the middle lmell structure seems to be wekened by lkli pretretment implictes the glcturonic cids of pectin nd other pectic polyscchrides tht re present in those intercellulr regions s possible cross-linking cids. Some of the crboxylic cid portion of the esters my lso be in the lignin polymer. 4. EXPERIMENTAL 4.1 Stick Penetrtion Mtchstick-shped smples (40 by 3 by 3 mm) were cut from the spwood in the bole of mture trees. The length ws in the longitudinl xis direction. After determintion of moisture content, the remining smples were stored frozen until used. After thwing, the sticks were individully weighed nd plced in individully lbeled test tubes. Pretretment solutions (10 ml) ws dded to completely cover the sticks. Excess ir in the wood ws removed with vcuum so tht

7 the stick ws submerged during pretretment. Unless stted otherwise, the sticks were rinsed with distilled wter until the wter ws neutrl fter 5-min sok. The wter-logged sticks were then treted in the next stge. The pulping liquor ws composed of 56 g Oxone 1 (2 KHSO 5 KHSO 4 K 2SO 4), 20 ml concentrted H 2SO 4, nd 500 ml wter. The pulping stge ws performed t controlled tempertures. The other stges were performed t mbient tempertures. The degrded lignin ws extrcted with 1% NOH solution for 24 h. The pulped nd extrcted sticks were plced in 100 ml beker with 50 ml of wter. The loosened fibers were dispersed with glss rod. The fiber nd core frctions were seprtely solvent-exchnged with cetone nd dried in vcuum oven for the determintion of yield, extent of fiberiztion, nd lignin content. The perchloric cid ctlyzed cetyl bromide method [7] ws used to mesure residul lignin. 4.2 Chip Pulping Mixed 19-mm spen chips (screened only to remove fines, lrge knots, nd oversized pieces) (400-g ovendry bsis, 34% moisture) were soked in 350 ml of 5% NOH solution overnight. The lkli ws removed by filtrtion nd the chips were wshed twice with distilled wter before treting with solution composed of 896 g Oxone, 320 ml concentrted H 2SO 4, nd 5 L wter. After 50 h t room temperture, the cidic pulping liquors were removed, nd the chips were wshed twice with wter nd extrcted for 24 h t mbient temperture with 4 L of 1% NOH solution. The lkli ws removed by filtrtion, nd the softened chips were wshed with hot wter nd fiberized with lrge excess of hot wter in vigorously stirred mixing tnk. Red ok nd red lder chips were similrly pulped. nd clculted on volume bsis. Viscosities were determined by TAPPI Test Method T230 om Hrdbord Fiber Comprison Aspen hrdbord fiber used in previous studies [2] ws soked for 16 h in 1.0% solution of sodium hydroxide t 10% consistency nd then drined. It ws then delignified t room temperture (22 C) for 3 dys using 2.7% solution of peroxymonosulfuric cid cidified to ph 0.30 with sulfuric cid t 10% consistency. An untreted smple ws lso delignified under identicl conditions. The sodium hydroxide left in the drined fibers ws neutrlized by dding n equivlent mount of sulfuric cid to the peroxymonosulfuric cid treting solution. To remove the degrded lignin, both smples were extrcted with 1.0% NOH t 50 C for 3 h. 4.4 DRIFT Spectr The diffuse reflectnce Fourier trnsform infrred (DRIFT) spectr were obtined using Nicolet Instrument corportion (Mdison, Wisconsin) series 6000 Fourier trnsform spectrometer with Hrrick (Ossining, New York) prying mntis diffuse reflectnce ttchment. Spectr were recorded from externl or internl surfces of ir-dried stick smples. There ws no dditionl smple preprtion. 5. ACKNOWLEDGMENTS The uthors thnk A. Cnnon, G. Vldez, nd J. Mc- Sweeny for experimentl ssistnce. We lso thnk M. Wesolowski nd N. Ross-Sutherlnd for nlyticl ssistnce nd pper testing, respectively. REFERENCES Aspen peroxymonosulfte pulp with residul lignin content of 0.4% nd brightness of 74% (reltive to mgnesium crbonte) ws bleched with hydrogen peroxide. The pulp ws first treted with 0.5% diethylenetriminepentcetic cid. It ws then bleched with 0.5% H 2O 2 on pulp t 11% consistency with stndrd mgnesium nd silicte buffer. After 45 min t 75 C, the pulp ws rinsed with dilute bisulfite-thiosulfte solution followed by distilled wter. The pulp brightness ws 79%. A second tretment with 0.28% H 2O 2 on pulp rised the brightness to 85%. Hndsheets were formed nd tested by TAPPI Test Methods T205 om-88 nd T220 om-88. Tensile indices were performed on necked-down specimens 1 The use of trde or firm nmes in this publiction is for reder informtion nd does not imply endorsement by the U.S. Deprtment of Agriculture of ny product or service. Printed on recycled pper 6th ISWPC/395