Behavior of Residual Lignin in Kraft Pulp During Bleaching

Home , Lignin, Pulp (paper)

25. LI, J. and van HEININGEN, A.R.P., Kinetics dium Sulfate-Potassium Sulfate Eutectic: Re- Thesis, McGill Univ. (1991). of Gasification of Black Liquor Char by actions of Some Sulfur Compounds, Inorg. 28. WYNNYCKYJ, J.R. and RSUKIN, W.J., An Steam, IEC Research 30(7):1594 (1991). Chem. 22(22):3243 (1983). Intrinsic Transport Model for Solid-Solid Re- 26. DEARNALEY, R.I., KERRIDGE, D.H. and 27. ZOU, X., Recovery of Kraft Black Liquor actions Involving a Gaseous Intermediate, ROGERS, D.J., Molten Lithium Sulfate-So- Including Direct Causticization, Ph.D. Metallurgical Trans. B 19B(2):73 (1988).

Behaviour of Residual Lignin in Kraft Pulp During Bleaching

D. LACHENAL, J.C. FERNANDES and P. FROMENT

The behaviour of residual lignin in tive has been to replace chlorine, hypo- The approach described in this paper softwood kraft pulp during bleaching has chlorite and, if necessary, chlorine dioxide consists of analyzing the actual effect of the been investigated. Chlorine, chlorine diox- by oxygen-based reagents (oxygen, hydro- bleaching chemicals on the residual lignin. ide, oxygen, hydrogen peroxide and ozone gen peroxide, ozone, peracids), it is of prime Basically two factors strongly influence lig- treatments have been carried out on un- importance to have a better understanding of nin solubilization during bleaching. The first bleached pulp and the residual lignin has the reasons why the chlorine-containing one is the amount of hydrophilic groups been extracted by enzymatic hydrolysis of chemicals are such efficient bleaching (mainly phenolic and carboxylic) and the the carbohydrates. The main functional agents. The role of chlorine during bleaching second one is the size of the degraded lignin groups have been measured by I9F NMR was re-examined recently and a new mecha- macromolecules. Several experimental pro- spectroscopy and the molecular weight dis- nism has been found to explain its outstand- cedures had to be developed. First the resid- tribution has been studied. New information ing reactivity on lignin [2]. Chlorine dioxide ual lignin was extracted in an almost on the chemistry of the bleaching agents has bleaching has also been under investigation quantitative way after each bleaching stage. been obtained, which explains some of the in many places during recent years [3-61, Enzymatic hydrolysis of the carbohydrate differences observed in their bleaching ef- primarily to try to inhibit the side reactions. fraction was achieved [14-15] by using a fect. For example, chlorine has the capabil- Simultaneously, oxygen, peroxide very powerful enzyme mixture [IO]. Then a ity to depolymerize the residual lignin while and ozone bleaching chemistry has been the new technique based on the analysis of some forming new free phenolic groups, which subject of considerable research activity [7- fluoro derivatives by I9F NMR was applied makes it without any equivalent among the IO]. The merit of these studies has been to for the determination of the main functional bleaching agents. On the contrary, ozone explain why chlorine-free bleaching is fea- groups in the lignin structure [16-171. The does not cleave the residual lignin very effi- sible, where the problems are coming from molecular weight distribution of lignin was ciently. Consequently, it must be associated and how some of them could be solved. also measured by gel permeation chroma- with another chemical which has this capa- However, the understanding is still tography. bility, like oxygen or chlorine dioxide. fragmented and does not give satisfactory With these tools in hand there was no answers to many questions, especially those obstacle to obtaining new and informative !NTROD!JCTION which relatc to thc optimum order of appli- rzsults cjn the effect of the bleaching chenii- The recent concern [I] about the fate cation during bleaching, and to the respec- cals (chlorine, chlorine dioxide, oxygen, of chlorinated organic compounds down- tive delignification power of the various hydrogen peroxide and ozone) on residual stream from a kraft mill has generated reagents. The reason for this lack of informa- lignin and to improve our understanding of unprecedented research activity in the tion comes partly from the fact that the actual the bleaching process. bleaching area. Although the major objec- modifications brought about to the residual lignin during the bleaching stages have very D. Lachenal, J.C. Fernandes and P. Froment seldom been investigated [ 11-1 31. The reac- EXPERIMENTAL rS Ecole Franqaise de Papeterie tions of lignin model compounds or of kraft Bleaching stages B.P. 65 lignin cannot fully represent the behaviour An unbleached softwood (roughly 38402 Saint Martin d'Heres of the insoluble and relatively unreactive 50% Picea abies, 50% Pinus sylvestris) kraft Cedex residual lignin embedded in a cellulosic ma- pulp of kappa number 30 was used in this France trix. study.

JOURNAL OF PULP AND PAPER SCIENCE: VOL. 21 NO. 5 MAY 1995 J173 The bleaching stages were carried out COOH groups in lignin was performed by covered by precipitation with water, washed under the following conditions: reaction with the 4-fluorophenyldiazo- with water and dried. I9F NMR spectra were - Chlorine bleaching (C): room tempera- methane according to: performed as before. Precision of the data is ture, 3.5% consistency, 60 min, 2 and 4% around 10%. C12 on pulp. After treatment the kappa RCOOH + F - C&,CHN2 numbers were 14.2 and 7.4, respectively. Molecular Weight Distribution - Chlorine dioxide bleaching (D): 70°C, --> RCO - O - CH,C,H,F of Lignin 10% consistency, 60 min, 2 and 5% ac- The lignin samples were dissolved in tive Cl2 on pulp. After treatment, the The 4-fluorophenyldiazomethane was DMF and their elution curves obtained by kappa numbers were 21.1 and 12.0, re- obtained from 4-fluorobenzylamine follow- HPLC (polystyrene gel: PL gel mixed D, spectively. ing the procedure described by Overberger 2 columns) with a UV detector (flow rate - Oxygen bleaching (0):1 OO°C, 10%con- and Anselme [ 181. 0.5 mL/min, 60°C). sistency, 60 min, 3% NaOH on pulp, The 4-fluorophenyldiazomethane was 0.5 MgS04, 7H20 on pulp, 5 bars oxy- recovered as a 0.5 moIL solution in ether Analysis of Carbohydrates gen. After treatment the kappa number and was used just after its preparation. The Lignin (100 mg) was treated with was 14.1. lignin previously treated with 4-fluoroben- 20 mL of a 2 mol/L solution of trifluoro- - Hydrogen peroxide bleaching (P): 90°C, zoic anhydride (400 mg) was dissolved in acetic acid at boiling temperature for 4 h. 10% consistency, 120 min, 2% NaOH on 60 mL of dioxane and 20 mL of the solution The hydrolysate was extracted twice with pulp, 3% H202 on pulp. After treatment of 4-fluorophenyldiazomethane. After 24 h ethyl ether and evaporated to dryness. The the kappa number was 20.0. at 40°C, the volume was reduced to 5 mL by residue was redissolved in 2 mLof water and - Ozone bleaching (Z): room temperature, evaporation under reduced pressure. The analyzed by HPLC (Column: Polysphere 35% consistency, 1.2% ozone consumed derivatized lignin was recovered by precipi- OH- Pb, Merck; solvent: water; flow rate: on pulp. Before treatment the pulp was tation with ether, washed with ether and 0.4 mL/min; temperature: 80°C). acidified to pH 2.5 with sulphuric acid. dried. After treatment the kappa number was 19F NMR spectra of the derivatives RESULTS AND DISCUSSION 15.0. were recorded at 188.226 MHz on a Bruker Characterization of AC 200 spectrometer as already described Residual Lignin Extraction of Residual Lignin [16-17]. The phenolic and alcoholic OH Extraction of lignin by the enzymatic Enzymatic hydrolysis was performed groups were measured after the first esterifi- treatment gave preliminary information on on the unbleached and on each treated pulp cation and the carboxyl groups after the sec- its solubility at pH 4.6. Table I shows the with a commercial cellulase-hemicellulase ond one. Precision of the data is around 10%. respective weight (in percent of the theoreti- preparation (ONOZUKA R-10, Yakult Phar- cal content of lignin in pulp) of the insoluble maceutical Industries, Tokyo Japan). Hy- Determination of and soluble fractions. Most of the residual drolysis was performed at pH 4.6 (acetic Carbonyl Groups lignin went into solution during enzymatic acid-sodium acetate buffer) and 37°C. Five The carbonyl groups present in lignin hydrolysis after oxygen, chlorine dioxide successive treatments of 72 h each were were reacted with the 4-fluorophenylhy- and hydrogen peroxide bleaching, indicat- carried out. The residue was dissolved in drazine according to: ing that these bleaching stages caused some dioxane-water (9: 1) and after evaporation depolymerization and created new hydro- \ solubilized in DMF and then precipitated / C = 0 + NH2 - NH - C,H,F philic groups. Solubilization was less after with ether. chlorination and very low after ozonation, \ Part of the lignin went into solution -> C = N - NH - C6H4F indicating that chlorine and ozone differ during the enzymatic hydrolysis. It was re- radically from the other chemicals in their covered by acidification to pH 2 with HCI, Lignin (100 mg) was dissolved in action on lignin. dissolution in DMF and precipitation with 5 mLofdioxane/DMF(l:l)andmixed with Table I1 gives the amount of the major ether. a solution of 4-fluorophenylhydrazine functional groups in residual lignin after The yield of lignin represented at least (100 mg of 4-fluorophenylhydrazine in each bleaching stage. The quantity is ex- 80% of the theoretical content in pulp calcu- 2 mL of DMF and 0.5 mL of orthophos- pressed as the number of groups per 200 g lated according to the formula: lignin con- phoric acid). The mixture was allowed to of lignin. The results show substantial differ- tent in pulp (in per cent) = 0.15* kappa react for three days in the dark at ambient ences between the bleaching chemicals. The number). temperature. The derivatized lignin was re- only one causing an increase in the amount The carbohydrate impurities in the extracted lignin represented 4 to 6%. Resid- ual proteins were of the order of 5% (average N content around 0.8 %). Each lignin sample was given the name of the treatment carried out just before extraction. Determination of OH Groups The lignin samples were esterified with 4-fluorobenzoic anhydride following the procedure described in [ 171. Alcoholic and phenolic OH groups are reacting according to the reaction:

R - OH + FC6H4 - CO - O - CO - C6H4F

-> R - O - CO - C&F + FC6HdCOOH

In a second step, esterification of

J174 JOURNAL OF PULP AND PAPER SCIENCE: VOL. 21 NO. 5 MAY 1995 of free phenolic groups was chlorine. The [IO]. A significant but smaller amount was not show any large change, suggesting that others led to a significant decrease. A con- observed after oxygen, chlorine dioxide and the side chains remained unaffected during siderable amount of new carboxyl groups hydrogen peroxide treatments. Chlorination, the bleaching stages. Carbonyl groups were was created during ozonation, which con- on the contrary, introduced very few car- increased in the case of oxygen bleaching firms the results obtained with I3C NMR boxyl groups. The alcoholic OH groups did and to a lesser extent in chlorine dioxide bleaching. Demethoxylation was observed in all cases except ozonation. Figures 1 to 4 give the elution curve of the residual lignin before and after treatment with the bleaching chemicals (the high molecular weights are on the left part of the curves). Again, ozonation showed a peculiar feature since no depolymerization was observed in contrast to what happened after the other treatments. The greatest effect was produced by chlorination. Lignin depolymerization followed the order: C (4%C12) > 0 > D (5% active Cl2) = P > Z. Relevance to the Bleaching of Kraft Pulp These results will help to understand better the role of the bleaching chemicals. Chlorination of the aromatic rings in lignin cannot explain the extensive lignin removal during chlorine bleaching. It is clearly shown here that the lignin macromolecules are cleaved, with leads to the for-

E 0.07 1 I I I I I c E 0.06 - -Unbl. + - c 2% 0.05 ...._____ - - -c 4% g 0.04 - ' 0.03 - 0.02

- 0.01

I I I I 1 I I 1 -0.02 i -0.01 12 14 4 6 8 10 12 14 4 6 8 10 Elution volume (ml) Elution volume (ml)

Fig. 1. Elution curve of residual lignin in unbleached pulp (Unbl) Fig. 2. Elution curve of residual lignin in unbleached pulp (Unbl) and in chlorine-treated pulps (C). and in chlorine dioxide-treated pulp (D).

I I I 0.07 , I I I r E 0.07 - m - -

0.05 - c P - 2 0.03 -

- 0.01 -

I I I I I I I 1 I -0.01 -0.01 ' ' 4 6 8 10 12 4 6 8 10 12 14 Elution volume (ml) Elutlon volume (mi)

Fig. 3. Elution curve of residual lignin in unbleached pulp (Unbl), Fig. 4. Comparison of the effect of the bleaching agents on the and in pulps treated with oxygen (0)or hydrogen peroxide (P). elution curves of residual lignin. (C - chlorine; D - chlorine dioxide; 0 -oxygen; Z - ozone)

JOURNAL OF PULP AND PAPER SCIENCE: VOL. 21 NO. 5 MAY 1995 J175 mation of new phenolic groups. This is con- ide and oxygen bleaching is also very simi- exhibits unique features which makes it sistent with the mechanism recently de- lar. This would suggest that hydrogen perox- without any equivalent among the known scribed [2] where chlorine is shown to cause ide delignification proceeds like oxygen bleaching chemicals. Indeed none of the the hydrolysis of ether groups (OCH3 and delignification. The species responsible for others has the capability to depolymerize the remaining ether linkages). The substantial lignin degradation during peroxide treat- residual lignin while forming new free phe- depolymerization observed in this study in- ment are not well known. The perhydroxyl nolic groups. A real substitute for chlorine dicates that a fair amount of interunit ether anion HOO- cannot degrade the phenolic must possess this property. linkages survive the kraft pulping process. groups [2O]. It has been claimed that the Ozone, at the opposite extreme, does For the first time, the highly phenolic char- decomposition products of hydrogen perox- not depolymerize the residual lignin very acter of the residual lignin after chlorination ide (OHO, 020-, 02) are the actual lignin-de- efficiently. Consequently it must be associ- is demonstrated. This explains why chlori- grading species [21]. The superoxide anion ated with another chemical which has this nation activates a subsequent oxygen 020- was recently suggested as the key spe- capability. Apart from chlorine, oxygen is bleaching stage, as shown in several studies cies in oxygen bleaching [22]. The results the best one. This suggests that OZ is a good [ 191. Oxygen, being a radical, reacts primar- obtained in this study suggest that it would sequence. Unfortunately, delignification ily with the free phenolic groups. The very be the same in peroxide bleaching. The role must be limited in order to avoid severe low content of carboxyl groups after chlo- of the perhydroxyl anion would be restricted cellulose degradation. Then a final stage is rine treatment indicates that oxidation of the to the elimination of carbonyl groups and required. From the results obtained in this phenol or catechol groups does not take thereby the destruction of colored groups. study, chlorine dioxide and hydrogen perox- place to a great extent. This explains why brightness development ide should be equally good candidates. How- The situation differs completely with is better in a P stage than in an 0 stage. The ever, some recent studies [IO] indicate that chlorine dioxide. Instead of creating new lower carbonyl content after P is in agree- H202 causes cellulose degradation after an free phenolic groups, C102 reacts with them ment with this hypothesis. Consequently the ozone stage. and reduces their number. As aconsequence, only advantage of adding H202 in an 0 stage chlorine dioxide is not an activating agent is the higher level of brightness reached. before oxygen bleaching. Depolymerization Another consequence is that a two-stage OP REFERENCES is less than with chlorine. Lignin dissolution process should not be an efficient process in I. REEVE, D.W. and EARL, P.F., “Chlorinated during the D stage may be caused by the terms of delignification. Organic Matter in Bleached Chemical Pulp large number of carboxyl groups which are Production: Part I, Environmental Impact and introduced. This confirms the fact the C102, CONCLUSION Regulation of Effluent”, Pulp Paper Can. 90(4):65-70 (I 989). as a radical, reacts mainly with the free phe- This study presents new information 2. NI, Y.and Van HEININGEN, A.R.P., “A New nolic groups and opens the aromatic ring to on the effect of the bleaching chemicals on Mechanism for Pulp Delignification during generate muconic acid derivatives [4]. This residual lignin in kraft pulp. This informa- Chlorination”, .I.Pulp Paper Sci. 16( 1):J13- major reaction does not give rise to depo- tion is particularly useful in explaining some J19 (1990). lymerization. previous results and for defining optimum 3. KOLAR, J.J., LINDGREN, B.O. and The behaviour of residual lignin dur- bleaching sequences. It appears that chlorine PETTERSSON, B., “Chemical Reactions in ing ozonation is quite unexpected. Accord- ing to the molecular weight distribution, no depolymerization seems to take place. This is consistent with the small lignin fraction which passes into solution during the enzymatic hydrolysis. After ozonation the residual lignin looks like the lignin in unbleached pulp in many respects, except its considerable content in carboxyl groups. Since the number of OH alcoholic groups is not changed, the origin of the carboxyl groups must be the oxidation of aromatic rings, including the non-phenolic ones. It is suggested that the aromatic rings would be de- stroyed one by one, thereby introducing the carboxyl groups. The absence of depolymerization limits the removal of lignin, which would mean that ozonation must be com- bined with other stages. On the positive side, the high carboxyl content makes the final bleaching easier. Since no free phenolic groups are created during ozonation, ozone does not activate the lignin prior to oxygen bleaching. Many similarities exist between oxygen and chlorine dioxide bleaching. They both react with phenolic groups, generate new carboxyl groups and cause some depolymerization. Some carbonyl groups are introduced too. Oxygen and chlorine dioxide will therefore not complement each other very effectively in a bleaching sequence, FT PULPS, BLEACH, contrary to chlorine and chlorine dioxide in IDE, OZONE, CHEM- a CED process. Residual lignin after hydrogen perox-

J176 JOURNAL OF PULP AND PAPER SCIENCE: VOL. 21 NO. 5 MAY 1995 ’* Chlorine Dioxide Stages of Pulp Bleaching”, Bleaching of Kraft Pulps”, Holzforschung schung 47(3):261-267 (1993). Wood Sci. Technol. 17: 1 17-1 28 ( 1983). 48 (Suppl.):133-139 (1994). 17. BARRELLE, M., FERNANDES, J.C., 4. GELLERSTEDT, G., LINDFORS, E.L., 11 GELLERSTEDT, G., GUSTAFSSON, K. FROMENT, P. and LACHENAL, D., “An PETTERSSON, M., SJOHOLM, E. and and LINDFORS, E.L., “Structural Changes Approach to the Determination of Functional ROBERT, D., “Chemical Aspects on Chlo- in Lignin During Oxygen Bleaching”, Nordic Groups in Oxidized Lignins by I9F NMR’, J. rine Dioxide as a Bleaching Agent for Chemi- Pulp PaperRes. J. l1(3):1417 (1986). Wood Chem. Technol. 12(4):4 13424 ( 1992). cal Pulps’’, Proc. 6th Intl. Symp. Wood 12 GELLERSTEDT, G., LJUNGGREN, S. and 18. OVERBERGER, C.G. and ANSELME, J.P., Pulping Chem., Melbourne, 331-336 (1991). PETTERSSON, M., “Chemical Aspects of “A Convenient Synthesis of Phenyldia- 5. WARTIOVAARA, I., “Reaction Mechanism the Degradation of Lignin During Oxygen zomethane”, J. Org. Chem. 28:592- - of Effective Chlorine Dioxide Bleaching”, Bleaching”, Proc. 6th Intl. Symp. Wood Pulp- 593 (1963). Tuppi J. 69(2):82-85 (1986). ing Chem., Melbourne, 229-236 (1991). 19. LACHENAL, D., BOURSON, L., MU- 6. NI, Y.,KUBES, G.J. and Van HEININGEN, 13. BACKMAN, L. and GELLERSTEDT, G., GUET, M. and CHAUVET, A,, “Lignin Acti- A.R.P., “Mechanism of Chlorate Formation “Reactions of Kraft Pulp Lignin with Alka- vation Improves Oxygen and Peroxide During Bleaching of Kraft Pulp with Chlorine line Peroxide”, Proc. 7th Intl. Symp. Wood Delignification”, Cellulose Chem. Technol.

Dioxide”, J. Pulp Paper Sci. 19(1):JI-J6 Pulping Chem., Beijing, (1):223-229(1993). 24593-601 (1990).,, (1 993). 14 LACHENAL D. and PAPADOPOULOS, J., 20. GIERER, J. and IMSGARD, F., “The Reac- 7. EK, M., GIERER, J. and JANSBO, K., “Improvement of Hydrogen Peroxide Delig- tions of Lignin with Oxygen and Hydrogen “Study on the Selectivity of Bleaching with nification”, Cell. Chem. Technol. (22):537- Peroxide in Alkaline Media”, Svensk Papper- Oxygen Containing Species”, Hol7,forschung 546 (1988). sfidn. 80(16):510-517 (1977). 43(6):391-396 (1989). 15. HORTLING, B., TURUNEN, E. and 21. AGNEMO, R. and GELLERSTEDT, G., 8. ERIKSSON, T. and GIERER, J., “Studies on SUNDQUIST, J., “Isolation of Residual Lig- “The Reactions of Lignin with Alkaline Hy- the Ozonation of Structural Elements in Re- nin from Enzymatically Hydrolysed Pulps by drogen Peroxide. Part 11. Factors Influencing sidual Kraft Lignins”, J. Wood Chem. Tech- Dissolution in Aprotic (DMAC) and Protic the Decomposition of Phenolic Structures”, nol. 5(1):53-84 (1985). (Alkali) Solutions. Proc. 6th Intl. Symp. Acta Chem. Scand. B 33(5):337-342 (1979). 9. GIERER, J., “Basic Principles of Bleaching”, Wood Pulping Chem., Melbourne, 323- 22. GIERER, J., YANG, E. and REITBERGER, Parts 1 and 2, Holzforschung 44(5):387-394, 330 (1991). T., “The Role of Superoxide Anion Radicals 395400 ( 1990). 16. BARRELLE, M., “A New Method for the in Delignification”, Proc. 7th Intl. Symp. IO. CHIRAT, C. and LACHENAL, D., “Effect of Quantitative I9F NMR Spectroscopic Analy- Wood Pulping Chem., Beijing, (1):24O- Ozone on Pulp Components. Application to sis of Hydroxyl Groups in Lignins”, Holzjor- 247 (1993).

MICROGRAPHS NEEDED FOR THE FRONT COVERS OF JPPS

Do you have any interesting micrographs which would be suitable for publication on the front covers of the Journal of Pulp and Paper Science? Preference is given to micrographs with esthetic appeal that contain useful scientific information. Coloured micrographs would be preferred, but black and white ones which could be printed with a coloured tint would also be considered. The micrographs should be 4” x 4“ and submitted with a caption, including magnification, and your name, company affiliation and address, to:

Dr. Derek Page Scientific Editor, Journal of Pulp and Paper Science c/o Paprican, 570 St. John’s Blvd. Pointe Claire, QC, Canada H9R 359

Photo credits will be published for all micrographs used.

JOURNAL OF PULP AND PAPER SCIENCE: VOL. 21 NO. 5 MAY 1995 J177 Fibre S H.A. CISNEROS, G.it . WILLIAMS and J.V. HATTO-”

A fundamental study into the break- this work was to analyze a large number of down of aspen and birch fibres was under- hardwood refiner pulp fibres in cross-sec- taken to provide a better understanding of tion. the peeling and fibrillation processes that The primary objective of this work, occur during chip refining of hardwoods. therefore, was to determine the effects of Fibres of TMe CTMP and CMP pulps were refiner mechanical pulping conditions on the analyzed in cross-section. The degrees to surface development and cell wall damage which the compound middle lamella and outer s the structure of a of hardwood fibres; a secondary objective secondary wall (SI layer) were retained, and was to attempt to clarify the relationships the S2 layer exposed, were recorded. Higher between fibre surface development and pulp proportions of S2 layers were exposed in properties. This detailed microscopical TMPfibres compared with CTMP and CMP study is expected to contribute to a better fibres. Alkaline sulphite pretreatment of understanding of the behaviour of hardwood chips did not improve the degree of S2 layer fibres during chip refining. exposure or the extent of fibrillation of the .fibres. These treated fibres retained most of their compound middle lamellae. Pulp due to its carbohydrate-rich strength was not related to the degree of Further, exvosure of the Y presumes the formation of fibrillar, lose-rich fines with improved bonding a il- ity. In addition, the fibres become mo flexible as the surface layers are removed [9]. The exposure of the S2 layer-\ is particularly desirable for hardwood fibres. Since hardwoods have a higher concentra- tion of carbohydrates in the S2 layer than do INTRODUCTION 1 softwood tracheids [lo], a high surface Mechanical pulps from bardwoods bonding capacity would be expected for fi- are important members of the large family of bres from which the SI layers have been mechanical pulps available today and it is removed and the S2 layers exposed. A anticipated that their use will continue to chemical treatment applied to the wood increase, particularly in printing papers chips could cause the S 1 layer to peel off in [1,2i. The stiefigth of these pulps is fiat sat- a mamer similar tc that fer spruce TMP isfactory for their inclusion in printing pa- fibres [6]. This is known as the “rolling- pers unless some type of chemical treatment sleeve” mechanism or skinning, since the is applied during the pulping process [3,4]. fibre outer layers are rolled back to expose The initial fibre length is not considered to the S2 layer. be an important limiting factor for strength There is, however, no quantitative evi- dence that hardwood refiner pulp fibres from chemically treated chips would expose more H.A. Cisneros, G.J. Williams Fig. 1. Model of the cell wall structure of and J.V. Hatton S2 layer material than those produced from JP untreated chips. The longitudinal examina- softwood tracheids and hardwood libri- Paprican form fibres. ML = middle lamella, P = pri- ps 3800 Wesbrook Mall tion of tibres provides insufficient informa- mary wall, S1 = secondary wall 1, S2 = Vancouver, BC tion about their surface or the pattem of secondarv wall 2. T = tertiarv wall. W = V6S 2L9 removal of the outer layers. Our approach in warty layer.

5178 JOURNAL OF PULP AND PAPER SCIENCE: VOL. 21 NO. 5 MAY 1995