The Effects of Recycling on the Chemical Properties of Pulps

J. BOUCHARD and M. DOUEK

Changes in the physical properties of pulp during recycling are sometimes re- ported to be caused by changes in chemical properties offibres. In order to determine the relationship, if any, between physical and chemical properties, a chemical characteri- zation was carried out of several recycled pulps. Measurements of carbohydrates, lig- nin, crystallinity index, and DP of cellulose were carried out as well as FTIR spectra. The results showed no direct relationship between the removal of hemicelluloses or lignin and the loss or gain of strength prop- erties during recycling. Slight cellulose de- polymerization occurred during recycling of kraf pulps. Howevel; it was largely inhibited in the presence of deinking chemicals. Also, there were no increases in cellulose crystal- linity index afer recycling.

INTRODUCTION The papermaking potential of a recy-

cled pulp is often claimed to be lower than that of a virgin pulp. As pointed out by Howard in a recent literature survey [I], there is general agreement that recycling causes a significant reduction in breaking length, burst, and fold, with a lesser reduc- tion in apparent density, and stretch. Other properties, such as tear, stiffness, and poros- ity, are usually increased. These changes have been largely ascribed to decreased swelling capacity and flexibility of the fibres which, in turn, result in a loss of bonding potential.

J. Bouchard and M. Douek JP Paprican ps 570 St. John’s Blvd. Pointe Claire, Que. H9R 3x9

It has also been speculated that, in addition, loss of bonding potential could be due to changes in surface properties during recycling. According to Eastwood and Clarke [2], a surface which is “gummy” with hemicelluloses might be particularly good for bonding. They speculated that, during recycling, hydrogen bonding is inhibited re- sulting in a loss in “surface condition”. They also reported a small solubilization of pen- tosans from a beaten semi-bleached kraft pulp after recycling three times. Changes in surface properties during recycling have also been reported by Chemaya and Bryantseva 131. Handsheets of bleached softwood sul- phite pulp were aged for six months and then repulped. They found from light microscopy and electron microscopy that the surface of the fibres was covered by a coating which was attributed to low molecular weight prod- ucts from hemicelluloses and lignin degra- dation. There was also a significant drop in the physical properties of the handsheets, which the authors attributed to the observed changes to the fibre surface without men- tioning if these changes were due to the aging or to the recycling of pulp.

In spite of such speculations about the possible effect of hemicelluloses and lignin, there are very limited data in the literature on the chemical changes which pulp constitu- ents undergo during recycling. Yamagishi and Oye [4] found a drop in EWNN solubil- ity of bleached kraft pulp after recycling and drying at 80°C. They also showed that the viscosity of pulp decreased with recycling which, in turn, corresponded to a decrease in the degree of polymerization of cellulose.

However, there was no indication from these studies whether there is a rela- tionship between such chemical changes and changes in physical properties which take

place during recycling. In addition, other types of pulps which were not examined previously, such as mechanical or CTMP pulps, may respond differently to recycling.

In a recent study on laboratory broke- type recycled pulps, it was found that the effect of recycling on physical properties varied greatly depending on pulp type and history, whether beaten or unbeaten, and whether chemical or mechanical 151. This provided us with the opportunity to charac- terize some of these pulps for their chemical properties.

It is also reported in the literature [6] that, when deinking chemicals are used dur- ing recycling, there is no loss of strength properties. In particular, the beneficial effect of sodium hydroxide on strength properties is recognized 171. However, the effect of deinking chemicals on the chemical proper- ties of pulp has not been clearly established. Accordingly, another objective of this work was to assess the effect of repulping and flotation, with and without the presence of deinking chemicals, on the chemical proper- ties of two pulp samples: a TMP, and an unbleached kraft pulp.

Several authors have also claimed that a small increase in cellulose crystallinity index occurs during recycling. For example, Yamagishi and Oye [4] have shown, using the procedure proposed by Segal et al. [8], that the increase in crystallinity index of recycled pulps ranged between 1.4 and 2.7% after five recycles. It is not clear, however, whether this was a real increase in cellulose order, or simply due to an increase of cellu- lose concentration in pulp caused by solubi- lization of hemicelluloses and/or lignin. Similarly, Bugajer [9] concluded from his work that there was an increase in cellulose crystallinity of beaten and unbeaten kraft

JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 5 MAY 1994 J131

pulp as a function of the number of recycles. His conclusion was based on the decrease of the width of the peak at a 2 0 angle of 22.6" of the X-ray diffractogram after the fifth recycle. This is an indication of an increase in crystal width but does not necessarily mean an increase of crystallinity. The num- ber of defects in the crystal must also be considered. In an attempt to settle this appar- ent controversy, the crystallinity index, cor- rected for yield, was determined on some of the samples examined in this study.

The overall objectives of this work were to determine the effect of different re- cycling and flotation procedures on the chemical properties of various types of pulp; and secondly, to assess if there is a correla- tion between chemical changes and changes in physical properties which take place dur- ing recycling.

EXPERIMENTAL Description of Samples Broke-Type Recycling

Broke-type recycled pulps used in this work were selected from those prepared by Howard and Bichard [5] for their study on the effect of recycling on physical prop- erties of the pulps. The procedure, described in detail in their paper, consisted in the preparation of pulp test handsheets followed by reslushing and handsheet making at room temperature for a total of five recycles. All handsheets were prepared according to Technical Section, CPPA Standard C.4 using a handsheet-making apparatus equipped for the recirculation of pulp fines. From the eleven commercial pulps used in their study, six were selected for chemical characteriza- tion. Details of the pulps are presented in Table I.

Repulping and Flotation Two samples were used in this work.

One of the samples was a thermomechanical pulp (TMP) obtained from an eastern Cana- dian mill, and the other was an unbleached, unbeaten, black spruce kraft pulp prepared in the Paprican pilot plant. Unprinted hand- sheets from these two samples were re- pulped in a Hobart mixer either with or without deinking chemicals. For the TMP sample, the following chemicals were added during repulping based on the 0.d. weight of fibres: NaOH (1 %), DTPA (0.4%), NazSiO3 (2S%),Hz02(1%), andsodiumoleate(l%). For the kraft pulp sample, 2% NaOH and 1 % sodium oleate were added to the pulper. Af- ter repulping, portions of each pulp were subjected to flotation in a flotation cell built at Paprican. Handsheets were prepared both after repulping and after flotation.

Analytical Procedures Monosaccharides were determined

on each pulp by gas-liquid chromatography of their alditol acetate derivatives, according to the method of Theander and Westerlund [IO]. Acid-insoluble lignin was determined using the Technical Section, CPPA Standard Method (2.9. Acid-soluble lignin was mea-

t

sured by UV spectrophotometry following TAPPI's Useful Method UM-250.

Molecular Weight Distribution of Cellulose

The molecular weight distribution (MWD) of the cellulose in pulps was deter- mined by size exclusion chromatography (SEC) of their tricarbanilate derivatives (CTC). Carbanilation was conducted fol- lowing the procedure proposed by Wood et al. [ 1 I] and scaled down for 50 mg samples. In the case of pulps having a lignin content greater than 3%, holocelluloses were pre- pared following the acid chlorite delignifica- tion procedure of Schroeder and Haig [12]. The SEC was performed using tetrahydro- furan (THF) as eluent (Curtin Matheson Scient. Inc.) and three columns of pore size of 105, IO4 and 500 A connected in series (Ultra Styragel, 300 x 7.5 mm, Waters Chrom.). One hundred microlitres of 3-6 mg/mL sample in THF (filtered on 0.45 pm filter) were injected and eluted at a flow rate of 1 .O mL/min. Detection was made using a differential refractometer (Waters 410) thermostated at 35°C. Determination of the MWD was made from universal calibration [I31 using narrow molecular weight distr- bution polystyrenes as standards (Supelco Inc.) and the Mark-Houwink coefficients ( K and a) proposed in the literature for polysty- rene [ 141 and CTC [ 151 in THE

Infrared Spectroscopy by Diffuse Reflectance (DRIFT)

Air-dried pulps were milled in a steel ball vibratory mill (Wig-L-Bug, Crescent Co.) for 30 s, mixed with potassium bromide (FUR grade, Aldrich Chem.) in order to achieve a fibre concentration of 2.00 f 0.05% in KBr and milled again for 30 s. This procedure eliminates variations due to spectral distortion [16]. Each milled sample was transferred to the diffuse reflec- tance sample cup and levelled off gently using a blade. DRIFT spectra were obtained with a 1725 Perkin Elmer lTIR spectro- photometer and 400 scans were collected

and averaged at a resolution of 4 cm-1 for the 4600400 cm-1 range. Spectra were ex- pressed as ratios to KBr background and converted to values of the Kubelka-Munk function. Baselines were linearly corrected in the 1900-800 cm-1 region and spectra were normalized to 1 K-M unit for the 1512 cm-1 band.

Crystallinity Index of Cellulose X-ray diffractograms were recorded

using a Philips diffractometer equipped with a copper source (h = 1.542 A). Spectra were recorded between 10" and 30" of 2 0 angle at a speed of 2"/min. Crystallinity indexes were determined using the procedure of Segal et al. [8] and subsequently corrected for pulp yield.

RESULTS AND DISCUSSION Effect of Recycling on Hemicelluloses and Lignin Content of Pulps

It has been known for some time that the disintegration and beating of pulps cause a partial dissolution of certain wood compo- nents in water. Sjostrom and Haglund [17] showed that between 0.3 and 0.6% by weight of bleached sulphate and sulphite hardwoods and softwoods was dissolved during disintegration and beating. Roberts and Awad El-Karim [18] showed that be- tween 1.2 and 1.6% of a softwood bleached kraft pulp can be solubilized by beating. Both groups demonstrated that carbohy- drates were the main component of the dis- solved materials and that xylose was the principal sugar making up between 70 and 100% of the hydrolyzable sugars. In most cases, arabinose was the second most abun- dant hydrolyzable sugar, accounting for up to 10% of the total.

Based on these results, the pentose content of pulp seems to be a good indicator of hemicellulose solubilization during recy- cling. From carbohydrates analyses, the ratio of [xylose + arabinose] to the total content of hemicellulosic carbohydrates in our sam- ples has been calculated. The glucose contri-

J132 JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 5 MAY 1994

bution to the hemicellulosic carbohydrates has been estimated on the basis of the ratio of 3:l for mannose:glucose proposed by Mills and Time11 [ 191 for galactoglucoman- nans. These pentosans to total hemicellu- loses ratios as well as the loss (%) of pentose sugars based on hemicellulose and on pulp are shown in Table II. Lignin contents in pulp are also given. For the repulping flota- tion experiments, values for the initial pulp, as well as for pulp after repulping and flota- tion without and with chemicals, are pre- sented.

As indicated in Table 11, the most pro- nounced decrease in pentosans level after the fifth recycle was observed with the unbeaten kraft and TMP samples (1.2 and 1.3%, based on pulp, respectively); however, the break- ing lengths (Table 11, from Ref. [5]) , as well as burst, Scott bond and fold 151 were sig- nificantly increased. Despite a significant decrease in strength properties after recy- cling of the beaten kraft pulps, the level of pentosans dropped by only 0 . 2 4 3 % (for pulps with and without fines, respectively). These latter values agree with the results reported by Sjostrom and Haglund [ 171 for the dissolution of carbohydrates during beat- ing of chemical pulps (0.3-0.6%, based on Pulp).

There was no measurable loss of pen- tosans in the CTMP sample, although the breaking length was increased by about 15% after the fifth recycle. With the 50/50 blend of TMPand bleached kraft, there was a small decrease in pentosans with no significant change in strength properties.The lignin content of all the samples examined, with the possible exception of the TMP which showed a 6% decrease after the fifth recycle, remained essentially unchanged during re- cycling.

Preliminary data on physical proper- ties of pulps from the repulping/flotation experiments suggest that there is a loss of tensile after repulping and flotation, which is partly recovered after addition of chemi- cals. As indicated in Table II, none of these effects is reflected in the changes in hemicel- luloses or lignin content.

On the basis of these results, there seems to be no direct correlation between the dissolution of pentosans and lignin during recycling and loss of strength properties. In fact, an inverse relationship was observed with the TMP and the unbeaten bleached kraft. For these types of samples, the factors proposed by Howard and Bichard [5 ] , viz fibre flattening and flexibilizing for me- chanical pulps, and curl removal for un-

JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 5 MAY 1994

beaten bleached chemical pulp, appear to be largely responsible for the recycling effects that they observed.

With beaten unbleached kraft pulps, there is also no evidence that the small de- crease in pentosans after the fifth recycle has much influence on the breaking length re- duction (17 and 25% for pulps with and without fines, respectively). This substantial loss of strength properties in beaten chemi- cal pulps has been attributed mainly to loss of fibre swelling [ 11 which, in turn, results in a reduction of bonding potential.

Effect of Rec cling on Molecular Weight Distri L ution of Cellulose

The molecular weight distribution (MWD) was determined only on the chemi- cal pulps. For the unbleached kraft pulps, the carbanilation reaction did not go to comple- tion because of interference from residual lignin and a colloidal suspension remained present. Consequently, the delignification procedure referred to in the experimental section was used to achieve a complete car- banilation of these samples. The fact that the delignified CTC samples gave higher aver- age molecular weights than the lignified

5133

samples is an indication of successful car- banilation. With high-yield mechanical pulps, TMP and CTMP, the carbanilation was ineffective even after four delignifica- tion treatments.

The different average degrees of po- lymerization calculated from the MWD of chemical pulps are presented in Table 111. Number-average DPs have been omitted be- cause the low DP part of the distribution is __ not of particular relevance. DP,, DP,, and DP,+1 better reflect the presence of relatively longer cellulose chains.

For the broke-type recycled pulps, there was no straightforward relationship be- tween the changes in DP and the changes in physical properties observed by Howard and Bichard. However, some rationalization of the results can still be made.

The unbeaten bleached kraft pulp showed some depolymerization during recy- cling. T h e m , decreased by about 200 units after five recycles, corresponding to the drop measured by viscometry by Yamagishi and Oye [4]. Moreover, in our case, the reduction in DP, in going from the initial pulp to the pulp after the fifth recycle, was constant at 19% for the three averages. In fact, the loga- rithmic DP of these samples, shown in Fig. 1, indicates a homogeneous displace- ment of the distribution toward lower mo- lecular weight with increasing levels of recycling. In this case, depolymerization ap- pears to be a random process with equal probability for the breakdown of glycosidic bonds for all the cellulose chains with a DP equal to or greater than m,. As noted pre- viously, strength properties for this pulp were increased with recycling.

In the case of the beaten, unbleached kraft pulp, the DP results are somewhat dif- ferent. Depolymerization of the high MW cellulosic chains seems to be more pro- nounced than for the unbeaten kraft. The DP, and m,+1 decreased by 20 and 25%, respec- tively. On the other hand, the shorter cellu- lose chains were considerably less affected

- -

4.0 3.5 3.0 2.5 2.0

log DP

Fig. 1. DP distribution curves for the un- beaten bleached kraft sample at various stages of recycling. 0: Initial pulp; 3: pulp after 3 stages of recycling; 5: pulp after 5 stages of recycling.

(10% drop in m,). However, we should note that, if the higher DP cellulosic chains are more frequently broken than others, the resulting generation of lower DP cellulose chains can thwart the depolymerization ef- fect in the m, measurement. This possibil- ity is reinforced by the cross-over of the cumulative DP curves shown in Fig. 2. Sig- nificant decreases in strength properties with recycling observed for this pulp could be partially attributed to the breaking of longer cellulosic chains.

The results for the bleached sulphite pulp were surprising. In spite of a consider- able drop in strength properties during recy- cling, there was no change in any of the DPs. A possible explanation is that sulphite pulp is less sensitive to mechanical action be- cause the cellulose chains are of much lower MW compared with kraft pulp. In addition, the arithmetic average fibre length of sul- phite pulp is about half that of kraft pulp [5]. The reduction in strength properties may thus be only due to physical factors.

Evidence presented suggests that

there is no direct relationship between the decrease in strength properties during recy- cling and the breaking of cellulosic ch'ains. The partial depolymerization of cellulose observed for kraft pulps may be caused by mechanical action or stress imposed on the fibres during the sequence repulping-dry- ing-repulping ... It is difficult, however, to assess the contribution, if any, from cellulose depolymerization to the decrease in strength properties.

In the pulps from the repulping-flota- tion experiments without chemicals, there was a loss of 8-13% in DP. In the presence of deinking chemicals, there is clearly inhi- bition of depolymerization for the high DP celluloses. The mz+l drops by only 1% compared with about 12% without chemi- cals. It is believed, however, that the im- provement in tensile properties in the presence of deinking chemicals is due to a partial recovery of fibre saturation point [20]. This, in turn, may result in inhibition of depolymerization by reducing the extent of mechanical stress imposed on the fibres.

,

Effect of Recycling on the DRIFT Spectra of Pulps

In an attempt to detect chemical changes from broke-type recycling and re- pulping/flotation experiments, the DRIFT spectra of all the pulp samples were re- corded. In the case of the broke-type recy- cling, the absolute amount of organic materials (lignin and hemicellulose) which has been solubilized was so small that no significant difference could be detected in the corresponding infrared spectra.

However, minor differences were ob- served in the DRIFT spectra of the re- pulping-flotation pulps before and after addition of chemicals. This is illustrated in Fig. 3 which shows superimposed DRIFT spectra of the TMP pulps between 1540 and 1800 cm-1. The three major bands in this area are related to the following functional

4.0 3.5 3.0 2.5 2.0

log DP

n

8 100-

C 0

80 5 P

.-

.- L

60 z n .-

4 0 Q, > cp .- c,

20 3 E

o E 6

Fig. 2. DP distribution curves for the beaten unbleached kraft sample (with fines). Cumulative curves are also shown. Solid line: initial pulp; dotted line: pulp after 5 stages of recycling.

J134 JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 5 MAY 1994

group vibrations: 1738 cm-l: unconjugated ketone and

carbonyl group; uronic acid carbonyl stretching carbonyl stretching of aryl- aryl ketone and quinone structures aromatic ring (in-plane) vi- bration and carbonyl stretch- ing in carboxylate ion

As indicated in Fig. 3, a decrease of intensity of the 1738 cm-l band and an in- crease of intensity of the 1593 cm-1 band was observed after flotation with chemicals. Table IV compares the ratio of these band intensities for several pulps over that of the 1512 cm-l band associated with aromatic ring vibration. There were practically no dif- ferences for TMP or CTMP before and after recycling or for TMP before and after re- pulping-flotation without chemicals. How- ever, a significant decrease of the 1738115 12 ratio and an increase of the 1593/1512 ratio were detected when flotation was performed in the presence of chemicals. This is likely due to a vibration frequency shift from 1738 cm-1 to 1593 cm-l due to the transfor- mation of a portion of the aliphatic carbox- ylic acids to their corresponding carboxylate ions in the presence of NaOH.

*

1660 cm-l:

1593 cm-l:

Effect of Recycling on the Crystallinity Index of Cellulose

Crystallinity indexes were deter- mined for three recycled pulps and also for pulps which had been repulped and floated in the presence of deinking chemicals. The results, presented in Table V, give the crys- tallinity as measured by X-ray diffraction and after correction for pulp yield. This cor- rection is based on the assumption that the small amount of material which is solu- bilized during recycling and flotation is amorphous. Consequently, the concentra- tion of crystalline cellulose is slightly in- creased in the remaining pulp.

From these results, we can conclude that, once yield corrections have been ap- plied, there is no significant increase in cel- lulose crystallinity index either during recycling or flotation. This does not agree with previous claims [4,9] which suggest that there is a small increase (up to 2%) of this index during recycling. However, these claims are based on crystallinity indexes un- corrected for yield changes.

CONCLUSIONS There appears to be no direct relation-

ship between the removal of hemicelluloses from pulp and the loss or gain of strength properties during recycling. There is also no evidence for the contribution by lignin deg- radation products to the loss of strength properties, since the concentration of lignin remains essentially unchanged at various stages of recycling. Changes in bonding po- tential are therefore likely to be largely gov- emed by physical factors, such as fibre flattening and flexibility for mechanical pulps, curl removal for unbeaten chemical pulp, and loss of fibre swelling for beaten

1660

e 0 .- Y

1800 1760 1720 1680 1640 1600 1560

Wavenumber (cm-1)

Fig. 3. Superimposed DRlFTspectra of the carbonyl group absorptions (1800-1540 cm-') of the repulping-flotation TMP pulp. 1: without deinking chemicals; 2: with deinking chemicals.

chemical pulps. There is a partial depolymerization of

cellulose during recycling of kraft pulp. The longer cellulosic chains appear to be more sensitive towards depolymerization possi- bly because of mechanical action and stress imposed on the fibres during the sequence repulping-drying-repulping ... It is difficult, however, to assess the contribution, if any, from cellulose depolymerization to the de- crease in strength properties. In the re- pulping/flotation experiments, cellulose

depolymerization is largely inhibited in the presence of oleate and NaOH. This may be due to the fact that there is less mechanical stress on the fibres because of a partial re- covery in fibre saturation point.

No significant changes in infrared spectra were evident in the pulps from broke-type recycling. However, for re- pulping/flotation with deinking chemicals, there was a frequency shift from 1738 cm-l to 1593 cm-1, which is probably due to the conversion of carboxylic acid to carboxylate

JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 5 MAY 1994 5135

ions in the presence of sodium hydroxide. Also, contrary to previous claims, there is no increase in cellulose crystallinity after recy- cling.

In conclusion, this work has shown that relatively minor changes in chemical properties occur during recycling. This sug- gests that the chemical analyses of the fibres explored in this paper cannot be used as a tool for differentiating between virgin and recycled fibres in a paper fumish.

ACKNOWLEDGEMENTS The authors wish to express their ap-

preciation to the National Sciences and En- gineering Research Council of Canada for an Industrial Post-Doctorate Fellowship to J. Bouchard. We also wish to thank Jean- FranGois Revol for the X-ray diffraction analysis, Wayne Bichard, Roger Howard and Norayr Gurnagul for the pulp samples and the latter for reviewing the manuscript.

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