The use of holocellulose to study cellulose supermolecular structure

.JOURNAL OF POLYMER SCIENCE VOL. 51, PAGES 27-58 (1961)

The Use of Holocellulose to Study Cellulose Supermolecular Structure

RUSSELL NELSOK, Buckeye Cellulose. Corporation, Memphis, Tennessee

INTRODUCTION

Dissolving grade wood pulps exhibit supermolecular structural characteristics that may be correlated with the pulping process. For example, prehydrolysis sulfate fibers, in comparison with conventional acid sulfite fibers, are reported to have wider and shorter crystallites, a more heterogeneous distribution of ordered regions, a slightly higher crystallinity index, a less extensively weakened gross fiber structure, and a greater resistance to

Fiber structure characteristics such as these are believed to have an influence in the preparation6-13 and on the properties1°-13 of com- mercially important derivatives, and thus could be a primary factor in de- termining the type of pulp most suitable for a given conversion process. An understanding of how cellulose structure changes during pulping, therefore, would be of interest to both pulp manidaeturer and consumer. It is conceivable that such an understanding will lead to multiple-use pulps, better processability, and improved quality of the finished product,.

LITERATURE

Very little information has been published on cellulose structure modification as a function of pulping process. There seems to be two reasons for this. First, i t has been only within the last ten to fifteen years that cellulose structure has been recognized as an important factor influencing dissolving pulp performance, and only within the last three to six years that theory and technology have yielded suitable methods of structure analysis. Second, much research on the subject has doubtlessly been done by pulp manufacturers but has not been published because of technical investment protection. What has been published deals mostly with finished pulp structure. While this work helped to establish important differences between pulp types and thereby stimulated an interest in cellulose structure, it cannot by itself reveal direction and magnitude of change caused by processing because so little is known of the starting material, the native cellulose structiire. The term ‘hative eelhilose structiire” as used herein refers to the supermolecular organization of the cellulose as it exists in the woody plant cell wall in partnership with the lignin and hemicelluloses.

27

28 R. NELSON

A few attempts have been made to study cellulose structure directly in whole wood, mostly by x-ray methods. Some workers claim that the x-ray diffraction pattern is more perfectly resolved before delignification than after.’ 1 4 Others are of the opinion that the converse is true.15 The lack of agreement may be explained tiy difficwlties in interpretation of x-ray data (due to fiber impurities) and by mtural species variation. Still, the two claims might be considered in the light of what change the pulping opera- tion brings about in the structural properties associated with diffraction pattern resolution. Resolution may indicate the degree of orderliness or packing perfection within and on the surfaces of the crystallites. I t may also reveal the degree of development of the unit cell. Hence one or more of these properties must telid to increase during pulping if the first of the above c*laims is valid, and decrease if the second is valid.

Considerably more work has been done on the study of cellulose structure in holocellulose. Although the main advantage in working with holocellulose is that interference by ligniri is obviated, it remains a matter of un- certainty 1% hether the cellulose structure does not change as a result even of mild delignification. Thus, Preston and Allsopp l5 showed that the ch!orine dioxide removal of lignin from representative woods caused no fundamental change in the cellulose crystal lattice, but it did increase the orientation of cellulose crystallites with respect to the fiber axis. These results were interpreted as evidence that lignin docs not participate directly in the cellulose crystal structure but that it does affect the packing arrangement of the crystallites. Sometime before this, Astbury ct a1.16 noted that removal of xylan from several delignified bast fibers markedly improved x-ray diff raction pattern resolution. They concluded that xylan is incorporated into the cellulose crystallites, inferring that its removal increased crystallitt width. These observations along with chemical analyses of extracted hemicellulose fraction.; c\~entuully formed the basis of Norman’s hypothesis regarding the relationship between the polymeric components of the woody plant cell wall. According to n‘orma11’~ the lignin and easily soluble hemicelluloses or “polyuronides” occupy positions in the wood matrix ex- ternal to the cellulose crystallites, whereas the difficultly soluble hemicelluloses or ‘‘cellulosans” are closely associated with the cellulose crystallites- either adsorbed on their iurfaces or occluded within them. Some refine- ment has been necessary i n the light of subsequent advances in wood chemistry, but the basic hypothesis remains unchanged and is still widely used. Supporting evidence has been based largely on the extractability of hemicelluloses from various cellulosic materials. 17--20 S o t all evidence has been in agreement. Sen and Hermaq21 for example, were unable to detect a significant change in half width of x-ray diffraction peaks such as might be expected to occur in paswig from raw jute to holocellulose to Cross and Bevan cellulose and then to alpha cellulose. This apparent contradiction may exemplify hpecies variatioii in thc relationship between the cell wall components?? 2 3 or it may iiidicatc that thc interpretation made by Astbury of improved resolution of x-ray pattern on xylan removal was incorrect.

USE OF IIOLOCELILTLOSE 29

Ot,her work has 1)ec:n tlont: oil t lit. slruc*tjiir~: of wllulose in holocellulose. Sen and Woodsz4 demonst,rated t,he importance of removing all but the last traces of ligniii from jut,e before the swelling phenomena could be properly st,udied. When more than onlv trace amounts of lignin were present, swelling wits severely rrstrictcd. This effect has also been demonstrated in birch wood atid a t t,rihritcd to thv thrcct-dirneiisioiial and therefore unswell- a b l ~ structure of lignin. 25 111 the. drvclopmeiit of methods for characteriz- ing cellulosic. mntcrinls according to thcir mcrcerization behavior, RHnhyz6 compared an essentially lignin-free buffered-chlorite holocellulose with finished pulps. S o mention was made, however, of possible structure changes rcwilting from holocellulose preparat,ion nor of possible interference by the hemicelluloses on the methods. Therefore it is not clear how accurately the data on holocellulose apply to the native cellulose structure. In a later conimunication RBnby implies t>hat the native cellulose under- goes a greater structural change in sulfate pulping t.han in sulfite in so far as referenre is made to a “sulfate effect” t.0 explain the high mercerization resistance of sulfate pulps.2 It does not seem improbable that both pulping processes modify t.he native struct,ure hut, in different ways or different degrees or both.

Our present knowledge of cellulose structure is seen, from the preceding brief review, to be not nearly as well developed with respect to wood as it is to pulp. Indications are that the structure is more disordered in wood than in pulp owing to the intimate relationship between the cellulose and the noncellulosic components. Comparisons beyond this are difficult or impossible because of interference by lignin. Attempts have been made to circum- vent this difficulty by using holocellulose in place of wood. It is possible that these were less successful than was desired because some structural properties may change even on mild delignification. Also, the possibility of interference by hemicelluloses in analytical methods does not seem to have been considered seriously. Taking account of these factors, it should be possible in principle to follow important changes incrementally throughout the successive removal of lignin and hemicelluloses and to esti- mate the native cellulose structure by extrapolating, so to speak, back to the whole wood. By comparing parameters for cellulose in such a form with those for cellulose in pulp, it should be further possible to approximate direction and magnitude of structural change caused by the pulping process.

Deligriification by holocellulose preparative methods, and hemicellulose removal by alkaline extraction, were selected as the purification processes. There are two methods which are commonly used in preparing holocellulose : t,he chlorine-monoet,hanolamine method after Time11 and Jahn2’ and the chlorite method after Wise.28 For a given degree of delignification, the former yields products having minimum chain length reduction and maximum hemicellulose retent i~n.~’ The latter, in comparison, yields attractive products with considerably less effort involved but there is more chain length reduction and hemicellulose Recent data of Norman and

This is the approach used in the present work.

30 R. NELSON

BingerZg indicate that the supermolecular structure of the two holocelluloses might differ as a result of delignification at different pH values. Because of the possible bearing of these differences on the present work, holocelluloses were prepared by both methods. Slash pine wood was selected as the raw material, purely on the basis of its importance in dissolving pulp manufacturing.

EXPERIMENTAL

Raw Material

The No. 2 bolt from a healthy 30-40-year-old slash pine, Pinus caribaea, cut in northern Florida was barked and chipped. After hand sorting to remove inner bark and foreign material, the chips were air dried, hammer milled, and then Wiley milled through a 4-mm. screen. The 20-40 standard mesh size of fraction was retained and extracted for 24 hr. each with acetone and alcohol-benzene. The air-dried wood meal was stored in plastic bags until used.

Holocellulose Preparation

The number of treatments required to obtain holocelluloses a t the 17-19, 4 4 , and 0-2% lignin levels was found by preliminary experiment to be 1, 3, and 6, and 2, 7, and 11, with the chlorite and chlorine methods, respectively.

Chlorite Holocellulose

Wise’s method2* was used. Depending on the extent of lignin removal, the products varied in color from pale yellow to white. Technical grade (75% purity) sodium chloride and reagent-grade glacial acet,ic acid were used.

Chlorine-Monoethanolamine Holocellulose

The Van Beckum and Ritter method as modified by Time1lZ7 was used. The products varied in color from dark brown for the least delignified to off white for the most delignified. Microscopic examination of the latter showed that nearly all the color was associated with bundles of summer- wood fibers, suggesting that these thick-walled elements contained most of the residual lignin. Commercial grades of chlorine and ethanol and reagent-grade monoethanolamine were used.

Pulp Preparation

Conventional processes were used to prepare the sulfite and prehydrolysis sulfate pulps.

USE OF HOLOCELLULOSE 31

Analytical Methods

Lignin

Lignin as the acid-insoluble residue mas determined according to the method of S c h ~ a l b e . ~ ” No correction was made for acid-soluble lignin. Close agreement between hypoiodite reducing powers31 at comparable ligiiin levels given in Table I indicates that total lignin is about equal in the two holocelluloses.

TABLE I Properties of Holocelluloses

Chlorine-ethanolamine Chlorite holocellulose holocellulose

1 3 6 2 7 11 treat- treat- treat- treat- treat- treat-

Wood ment ments ments ments ments ments

Moisture Yield‘ Ash Lignin Glucose + Mannoseb Xyloseb Reducing

powerC Nitrate D.P.

Galactoseb

-18 6 5 6 8 9 2 5 8 - 93 2 78 8 66 2 103 5

0 3 1 0 1 8 1 8 0 1 25 5 16 9 4 1 0 8 1 9 1

71 65 65 71 72 22 26 26 22 20

7 9 9 7 8

63 70 79 86 71 2,340 2,260 2,130 1,870 2,310

5.8 6 . 5 85.2 72.1 0 . 2 0 . 1 6.0 2.2

71 69 21 24 8 7

77 85 2,280 2,270

8 Based on the oven dry weight of extractive free wood.

b Expressed as per cent of the neutral sugars measured. c Hypoiodite reducing value (cf. section “Experimental”).

All other values are based on the oven dry weight of holocellulose.

Carbohydrate Analyses

Carbohydrates were quantitatively estimated in duplicate according to the direct densitometric method described by Durso and P a u l s ~ n . ~ ~ Glu- cose and galactose were estimated together, since they are not resolved by the separation solvent system used; the error in glucose is small. Arabi- nose was not estimated, since its contribution to the total amount of sugars found is small, i.e., about 1-2y0 based on the wood. Glucose, mannose. and xylose as percentages of the total amount of these three sugars are reported.

Nitrate Degree of Polymerization

The nitrate was prepared with Alexander and Mitchell’s reagent a t 3°C. for 24 hr.33 Removal of soluble nitrolignin and stabilization were accomplished by prolonged extraction with fresh batches of anhydrous methanol. The polysaccharide nitrates were extracted from the stabilized residues by

32 R. NELSON

acetone, precipitated into water, and dried in uacuo over Y206 at, 45°C. to constant weight. Specific viscosities were determined in dry acetone at four concentrations with an Ubbelohde viscometer a t 25°C. Intrinsic viscosities were obtained by extrapolating reduced viscosities to infinit? dilntion ; D.P. values were calculated from the following relation, 34

DP = K [ 7 ]

where K was considered to be 100.

X-Ray Analyses

X-ray analyses were performed with a North American-Phillips diff raction unit fitted with a goniometer. A copper target x-ray tube was used and operated a t 45 kv. and 15 ma. Samples were pressed into discs having a density of ca. 17 g . / h 3 with a laboratory hand press. The random orientation of fibers in the sample obviated the need to rotate the sample holder. The diffracted radiation was monochromatized by filtration through nickel foil, counted with a Geiger tube, and translated to diffraction patterns of intensity vs. 28 with an automatic recorder. All samples were scanned a t 1/40/min. Diffracted beam intensities were corrected for air and sample holder scattering.

I. Crystallite Width. In accordance with x-ray diffraction theory, crystallite width varies inversely with the width of the reflections or peaks, assuming that strains within the crystallite are not large. Cellulose ex- hibits several characteristic peaks, but only the 002 peak is sufficiently developed and resolved in wood cellulose patterns to permit estimation of crystallite width with reasonable accuracy. Measurements were made a t half maximum intensity after correcting for amorphous scattering.

2. Crystallinity Index. Crystallinity indexes were estimated from the following equation after McCarthy and Anker-Rasch :4

115.4 Ro = 115.4 f 118.4

where IX , .~ and are diffracted beam intensities a t 20 = 15.4 and 18.4" 20, respectively, after correction for air and sample holder scattering.

3. Mercerization Resistance. Procedural details are described in the literature by RBnby2 and Parks5 Some of the principles involved are re- viewed here because of their close bearing on the present work. X-rays are diffracted from the 101 plane in the cellulose I unit cell a t an average angle of 20 equal to 14.6". When cellulose I is treated with mercerizing-strength alkali, the alkali removed by water washing and then drying, the unit cell undeFgoes changes in the spatial relationships between the cellulose chains. X-rays are diffracted from the 101 plane in this "regenerated," or cellulose 11, unit cell a t a smaller average angle of 20 equal to 12.0". On the basis of this angular shift of the 101 reflection, the following ratio of intensities a t the two angles has been devised r 4

us15 OF IIoLocIsLLIJLosI.: 33

21 12. " 2112.0 + 1 1 4 . 6

K , =

where R, is proportional to extent of mercerization. In the hypothetical c'ase of a perfect crystal of cellulosr, R, would have a value of zero before Inercerization, and unity after mercerization. In practice, however, R, has values between these limits owing to hckgrou~id scattering by amorphous cellulose and other thing,..

The per cent cellulose I1 (v ix . pc'r rent mrrwrization) may he calcrilatcd from the following equation:

x 100 (R, - RI) (RII - RI)

70 Cellulose I1 =

where RI is that value of R, a t the start of mercerization, and RII is that value of R, after completion of mercerization. Hence R, assumes values of RI and RII for 0 and 100% mercerization, respectively. Intermediate values of R, denote partial mercerization. Absolute values of R,, RI , and RII vary with the nature of the cellulose sample. In the case of high-purity wood pulps, RI was found to have a value of 0.447 and RII a value of 0.722. In the case of low-purity pulps, RI may be greater and RII lower than these respective values. The alkali cellulose transition may be illustrated by a plot of values of either of R, or per cent cellulose I1 against the concentration of alkali effecting the change in the lattice structure.

The lateral order distribution curve is obtained by plotting the differen- tial of the transition curve, either as AR,/A% XaOH or as A% cellulose I I / A % NaOH, against per cent NaOH. The width and shape of the resulting curve provide an index of the ease with which the (hative" to alkali cellulose transition occurs.

4. Experimental Error. Standard deviations of x-ray measurements wrre found to be as follows:

002 width 0.02" RO 0.01 R, 0.01 Cellulose 11, % 4

R, values represent averages of duplicate intensity readings at 12.0 and 14.6" on a single sample. Values of 002 and Ro represent single measurements on a single sample.

Alkaline Extractions

Extractions were carried out under nitrogen a t 0, 25, or 60 *O.Ol"C. for 2 hr. with continuous agitation. Aqueous sodium hydroxide solution: solid ratios were 50: 1. In the determination of alkali solubility, solutions were filtered through sintered glass, clarified by centrifugation, and analyzed for total organic content by dichromate oxidat>ion. Solubilities are

34 R. NELSON

expressed as per ceut, of the origiiial bone-dry sample. Alkali-extracted residues were washed with water of the extraction temperature, acidified with 5% acetic acid, washed with cold water, air-dried to 6 4 % moisture and stored until analyzed.

Isolation qf Hpinicelluloses

Xylan- and glucomaaiiaii-rich frac.tioiis were isolated froin the clarified extraction liquors by cooling t o 5"C., acidifying to pH 4-5 with glacial acetic acid, flooding with 4 vol. of acetone, and collecting on the centrifuge. The fractions were dialyzed against demineralized water. dehydrated with acetone, and air dried.

Slash pine glucomannan (the fraction 18-A described in reference 35) was obtained by fractional extraction of holocellulose followed by purification with Fehling's solution.

Slash pine xylan (the fraction 3-C described in reference 35) was obtained by fractional extraction of holocellulose followed by fractional precipita- tion with cetyl pyridinium bromide.

Birch xylan was obtained by extracting white birch wood meal with 12% NaOH a t 60"C., after pre-extraction with 1% SaOH, to remove soluble lignin and extraneous polysaccharides. The xylan was precipitated with Fehling's solution and the complex washed with water and decomposed with mixed acetic and dilute hydrochloric acid. The xylan was dialyzed, dehydrated with acetone, and air dried.

Electron Micrographs

Samples were slurried in water with a laboratory blender, a drop of the suspension was evaporated on a collodion membrane supported on a grid, the specimen was shadowed with chromium, and micrographs were obtained a t a magnification of 9,000.

RESULTS AND DISCUSSION

Data are presented in Table I on the composition, yield, arid degree of polymerization of the holocelluloses. According to these data, carbohydrate composition is affected very little by the method and extent of delignification. As might be expected from similar comparisons of other species,27 chain length is reduced the most during chlorite ligiiin removal.

X-ray diffraction patterns are shown in Figure 1 for the original 20-40 mesh extractive-free wood meal and a prehydrolysis sulfate slash pine pulp. In comparison with pulp, wood gives an underdeveloped and diffused pattern, and there is little tendency toward resolution of the l O l - l O i doublet such as is often observed in pulp patterns. (The difference in resolution between wood and pulp is even more pronounced when the Debye-Schemer powder diagrams are compared.) If the poorly developed pattern for wood were simply the result of diffraction from a crystalline structure im- bedded in an amorphous mass, the selective elimination of the amorphous


0 12 14 16 I8 20 2 2 24 26

2 8

Fig. 1. Diffraction pattern of slash pine wood and prehydrolysis sulfate pulp.

- 17-19% LlGNlN 4 - 6% LlGNlN 0- 2% LlGNlN

40 - ~ ____ __._

30 - I

- 17-19% LlGNlN 4 - 6% LlGNlN 0- 2% LlGNlN

40 - ~ ____ __._

I

0 0 12 14 16 18 2 0 22 2 4 26

2 8

Fig. 2. Effect of chlorite delignification and alkaline hemicellulose extraction on diffraction pattern.

material should improve the resolution to a level roughly comparable to that of the pulp. As shown by Figure 2, the stepwise removal of lignin and easily soluble hemicelluloses improves resolution, but the improvement leaves the resultant pattern far from comparable to that of pulp. There- fore crystalline cellulose seems to have a lower degree of lattice perfection, a narrower average crystallite width in wood than in pulp, or both.

It is reasonable to presume that the state of low lateral order in wood re- flects imperfections in the crystalline portion of cellulose caused by the presence of SO-SO% impurities. Raw cotton contains only 4-12% impurities, and it has an average crystallite width and degree of lattice development substantially greater than that observed in most high purity wood pulps.48

36 H. NELSON

Effect of 1,ignin Removal on the X-Ray Pattern

Values for the crystallinity index (Ro) and 002 peak width are listed in the upper part of Table I1 for both sets of holocelluloses. The proportion of crystalline material arid the width of the ordered regions or crystallites (002 varies as the reciprocal of crystallite width) are indicated as increasing with extent of lignin removal. Si1ic.e ligiiin is coiisid~red to be x-ray-amorphous, the change in 120 undoubtedly represents an increase in proportion of crystalline material. The change in 002, however, may or may not represent an increase in crystallite width. If lignin should penetrate into the crystallites in any sizable amount, the lattice planes would bc detect- ably rearranged as a result of lignin removal. In agreement with work by

TABLE I1 Effective of Successive Removal of Lignin and Hemicelluloses on Residue Structure

Sample, Wood Crpstal- % Treatments, Lignin,a removed,b 002 linity

lignin no. of and type % % width index

Lignin removed

Whole wood None 17-19 I, Chlorite" 4-6 3, Chloritec 0-2 6, Chloritec

Hemicelluloses removed

0-2 0-2

0-2

6% NaOH, 60°C., 2 hr. 6 and 8% NaOH, 60°C.,

6,8, and 10% NaOH, 60" 2 hr. each

C., 2 hr. each

Lignin removed

Whole wood None 17-19 2, Chlorine? 4-6 7, Chlorine" 0-2 11, Chlorine0

Hemicelluloses removed

0-2 0-2

0-2

6% NaOH, 6OoC., 2 hr. 6 and 8% NaOH, 60°C.,

6, 8, and 10% NaOH, 60" 2 hr. each

C., 2 hr. each

25.5 16.9 4 . 1 0 . 8

0.0 0 .0

0.0

25.5 19. I 6 .0 2 . 2

2.4 2 .4

2 . 3

0.0 6.8

21.2 33.8

51.4 53.3

54.8

0 .0

14.8 27.9

-

42.5 43.3

46.4

2.88 2.86 2.82 2.74

2.42 2.38

2.24

2.88 2.84 2.82 2.68

2.48 2.44

2.36

0.53 0.54 0.55 0.56

0.60 0.60

0.59

0.53 0.53 0.55 0.56

0.57 0.57

0.58

~

a Based on oven dry weight of residue. Based on extractive free wood after the indicated treatment. Wood removed =

cf. section "Experi- 1 0 0 ~ o yield.

mental" and Table I. a Designates number of separate delignification treatments;


TABLE I11 Lattice Spacings for Wood and Holocelluloses, Angstronis

Treatment given

Chlorite, holocellulose Chlorine, holocellulose Lignin level,

% 002 021 101 101 002 021 101 101

25 5 (wood) 3 9 4 2 5 4 6 0 3 9 4 2 5 4 6 0 17-19 3 9 4 2 5 3 6 0 3 9 4 3 5 3 G O 4-6 3 9 4 2 5 3 6 0 3 9 4 2 .i 4 6 0 0-2 3 9 4 2 5 3 6 0 3 9 4 3 5 3 6 0

Preston and Alls0pp,~5 the d spacings listed in Table I11 for whole wood and the two sets of holocelluloses show that this is not observed. Hence lignin must be extracrystalline. How, then, may the change in 002 peak width be explained? It could result from the lateral union of crystallites which were maintained as separate structures in whole wood by the intervention of lignin. It could also result from reduction of the liquid type of scattering of lignin, allowing the crystalline portion of the sample to diffract a greater share of energy during analyses. Lastly, it could result from relief of minor strains or distortions in the cellulose framework imposed by cell wall growth, the presence of lignin, or drying. A compromise with all three of these explanations does not seem unreasonable.

Effect of Hemicellulose Removal on the X-Ray Pattern

Hemicelluloses were fractionally removed from the holocelluloses containing 0-2% lignin by successive extraction of 10-g. samples with 6, 8, and 10% NaOH a t 60°C. The 002 peak width was measured after each stage and found to decrease throughout the extraction. On the basis of per cent wood removed, the increment of decrease, as shown in Table 11, is substantially greater with removal of hemicelluloses than with that of lignin. This purification effect is illustrated by the curves in Figure 3. It is not immediately clear whether the decrease which is shown by the upper portion of these curves is due to an interaction between alkali and cellulose or to removal of hemicelluloses, and whether the decrease represents a real increase in crystallite width whatever the cause. If an alkaline- induced recrystallization or perfection of crystallization is solely responsible for the change in the x-ray pattern, the change should occur independent of hemicellulose removal. Data are presented in Table IV to show, however, that an alkali-cellulose interaction and hemicellulose removal are both responsible for thr change. These data. are from an experiment in which a sample of wood meal was pre-extracted with 6% SaOH a t 60°C. for 1 hr., converted to chlorite holocellulose, and analyzed along with a control. Some decrease in 002 width resulted from the alkaline treatment even though the polysaccharide removal was negligible, but i t was less than was found on extraction of the holocellulose (i.e., 2.62 vs. 3.42") when the hemicellulose removal was large. Therefore the effect illustrated by

38 R. NELSON

0 CHL?PlNE HOLOCEkLULOSE

60 1 8 C H L F I T E HOLOCE,\LULOSE EXTRACTED WITH % NaOH SHOWN

EXTRACTED WITH % NoOH SHOWN W (0

3 s

.- 0 W > 0 H W

z z

a

P -

002 WIDTH

Fig. 3. Effect of successive removal of lignin and hemicelluloses on 002 peak width.

the upper region of the curves in Figure 3 must depend in part on an interaction between alkali and cellulose and in part on the removal of hemicelluloses. A similar narrowing effect by alkaline solution on 002 width has been observed by Mr. K. A. Zachariasen of these laboratories in working with pulps essentially free of hemicellulosic s a~cha r ides .~~ Others37 have reported on the ability of alkaline solutions to increase perfection of lateral order in various cellulosic materials, particularly in those having an initially low degree of crystallinity.

TABLE IV Properties of Chlorite Holocellulose from Alkali-Treated and Untreated Wood

Property Treated tintreated

002 peak width; degrees, 2 P Ro L i m b , % Glucose + galactose, ’% Mannose, 7’ Xylose, %

2.62 2.76 0.55 0.55 5 7

68 67 -2.5 ”6 7 -

I

a Standard deviation = f0.02.

Before offering an explanation of \vhy hemicellulosc. removal causcs 002 width to decrease, i t is informative to consider in detail the order of extraction of the hemicellulosic polysaccharides aiid the probable position of these polysaccharides in the cellulose framework. Consecutive extraction of the 0-2 (lignin level) chlorite holocellulose with 6 and 8%) SaOH at 60°C. yields two hemicellulose fractions having a glucose + galactose to mannose to xylose molar ratio of 1 : 1.5 : 3.1 and 1 :8 : 1.2, respectively. According to these data, the easily soluble or “polyuronide” fraction is enriched in xylan;

IJSE OF HOLOCELLU1,OSE

70

60

5 a 240 Z 30-

20

10

39

- -

-

- -

MANNOSE

GLUCOSE

XYLOSE

MANNOSE

XYLOSE

I , I I I . I I I I I I I I I I , , ,

0 I 2 3 4 5 6 7 8 9 10 I I 12 13 14 15 16 17 18 % NaOH

Fig. 4. Effect of alkali concentration on composition of hemicelluloses successively extracted from chlorite holocellulose a t 25”C., 3 hr., 10: 1 liquor/solid.

the difficultly soluble or “cellulosan” fraction, in glucomannan. 35 A more complete picture of the order of extraction of these two polysaccharides may be obtained from Figure 4. When Figures 3 and 4 are compared i t is interesting to note that the rate of change of 002 width with respect to per cent wood removed is less during extraction of the xylan-rich fraction than durnig subsequent extraction of the glucomannan-rich fraction.

Information on the probable position of the hemicellulosic polysaccharides may be obtained from the series of electron micrographs shown in Figure 5. These show how the physical appearance of wood changes with stepwise chlorite delignification and alkaline hemicellulose removal. The fibrillar character of the cellulose becomes increasingly apparent as more and more lignin is removed, until the 0-2% lignin level is reached, a t which the cellulose fibrils are revealed in fairly good detail. Close examination of the residue a t this point (Fig. 5d) shows the presence of a fine powderlike material deposited externally to the cellulose fibrils in regions which for- merly must also have held the lignin (Fig. 5a-d). Extraction with 6% KaOH a t 60°C. removes this material, as is shown by a comparison of Figures 5d and 5e, and allows isolation of the xylan-rich fraction from the extraction Iiquor. The fibrillar character of the residue is now revealed in even greater detail. It is from this residue that the glucomannan-rich fraction may be extracted with 8% SaOH a t 60°C. If it is assumcd that the powderlike material corresponds to the xylan-rich frncation, it appears that that fraction is located within the interstices and upon the surfaces of the cellulose framework, and for this reason it should be easily removed without effect on the cellulose structure. The glucomannan-rich fraction, in contrast, must be located deeper in the framework and, accordingly, it should be difficultly removed with an opportunity for influence on the cellulose structure.

The preceding results accord with the suggestion first offered by Norman that the “cellulosans” are more closely associated with the cellulose than the “polyuronides.” Similar extraction results have been reported for slash pine and closely related species, 38-41 and they are usually interpreted

40 I{. NKLSON

Fig. 5 . Electron photomicrographs of celluloses: (A) slash pine wood and holocelluloses, original wood, X9,OOO. ( R ) Chlorite holocellulose n-ith 16.9% lignirl, x9,OoO. ( C ) Chlorite holocellulose with 4.1% lignin, X9,OOO. (D) Chlorite holocellulose with 0.7% lignin, X9,OOO. ( B ) Chlorite holocellulose of Figure 5 0 after extraction with 6% NnOH at 60°C. for 2 hr., X9,OOO.


in terms of a hemicellulose-cellulose association. Some workers believe that this association is an important factor in commercial alkaline purification proce~ses.’~ Others discount the importance of an association in favor of the alkali-catalyzed degradation of polysaccharides. 42 While alkaline degradat.ion certainly must be influential a t t,cmperatures used in hot alkaline refining (e.g., 8O-12O0C.), it is difficult to believe that the “peeling” mechanism can account for differences in composit.ion of hemicellulose fractions removed a t the much lower t>emperatures (0-60°C.) used in this work. Since the struct,ure of the essential portion of the transition state complex for xylan would be identical to that of glucomannan (and cellulose) , one would not logically expect a degradation-controlled extraction mechanism to result in fractions enriched in either polysaccharide.

If removal of glucomannan-rich hemicelluloses narrows the 002 diffraction peak width, as the data indicate, it is then of interest to ask why the change occurs and what it represents. In regard to t.he first question, i t can be said that if glucomannan removal is to function in such a manner, the portion of the polysaccharide responsible for the change must exist in fairly close contact with the crystalline cellulose. The contact, however, must not be of the same nature as that existing between cellulose chains in the crystallite for, if it were, removal of intact glucomannan molecules would be impossible. In other words, glucomannan should have the capacit,y to associate with crystalline cellulose on the one hand and to interrupt the regularity of attractive forces between cellulose chains on the other. The glucomannan molecule, therefore, is required to have an infrequently branched structure and less recurrence symmetry (i.e., structural regularity along the chain) than cellulose. These requirements seem to be satisfied hy a pine (P. s ihesfr is) glucomannan, t’he struct,ure of which was recently report,ed by Meier and co-workers :43

3 . . . . . .

where M = mannose unit and G = glucose unit. Measurements indicate the presence of about three branch points per molecule. Recent evidence of Hamilton et al.44 suggests a slightly more branched structure in slash pine glucomannan due to the presence of galactose side groups. Such galacto- glucomannans in slash pine, however, are believed to be concentrated in hemicelliiloses which are easily removed (i.e., the xylan-rich fraction) rather thaii in those which are difficultly removed.35 If glucomannan i\ occluded within the crystallites or adsorbed 011 their surfaces, as is allowed by the linear portions of the molecule, its presence there should disorganize certain cellulose-cellulose attractive forces by virtue of structural differences between the glucomannan and the cellulose. That such structural differences aflect the crystalline form of the two polysaccharides is evident in the x-ray pon der diagrams in Figure Gal of purified cellulose I1 and gluco-

42 R. NELSON

mannan. The latter sample was acid-degraded for removal of branched material.40 It would not be surprising if extraction of the glucomannan were difficult and, once accomplished, permitted fundamental changes in the crystalline portion of the residue because of the increased opportunities for greater packing perfection.

The major portion of xylan should have little ability to associate with cellulose because of its position with respect to the fibrils and its frequently branched structure. slash pine xylan has on the average one 4-O-methyl-~-glucuronic acid branch every seven anhy- droxylose units and one arabinose branch every twelve to eighteen anhy- droxylose units :

As reported by Hamilton et

. . . x1-4x,-4x1. . . 4X1-4X1-(4X1)-4X . . . I I I 1 1 1

A GA GA Araboglucuronoxylan

where X = xylose unit, GA = -&O-methyl-D-ghcuronic acid unit, and A = arabinose unit.

Bishop4j has shown that the difficulty with which glucuronoxylans crystallize (and crystallizability certainly must be regarded as a criterion for intermolecular association) increases with the number of uronic acid branches on the xylan backbone. h xylan? such as slash pine xylan, having a uronic acid to xylose ratio of 1:7, should not be especially prone to exist in the crystalline form. As a mat,ter of fact, Y ~ n d t , ~ ~ was unable to isolate a crystalline product from slash pine xylan, but he was able to isolate such a product from numerous other xglans having either no uronic acids or a uronic acid to xylose ratio of less than 1 : 7. Figure 6b shows x-ray powder diagrams for xylans from paper birch (uronic acid to xylose =; 1 : 11) and slash pine. The samples were dried from water a t room temperature, after isolation without partial hydrolysis, to remove branched material. Data reported by Adams and Castagne4’ suggest that the number of uronic acid side groups is smaller in xylans difficultly removed than in those easily removed. Thus it. may be speculated that the uronic-acid-poor xylan fraction has greater opportunity than the “average” xylan in slash pine to associate with cellulose and, consequently, its removal should have somewhat less of an effect on 002 than glucomannan removal, owing t o the greater recurrence symmet,ry in the xylari backbone.

The question of what the decrease iii 002 peak width represents is at, present a moot one. A decrease in peak width can correspond to an increase in crystallite width, in development of the crystal lattice, or in packing perfection of the cellulose chains within and on the surfaces of the cryst.allites. In the absence of direct. means of measuring each of these, the change in 002 is best. interpreted as representing an increase in all of the above properties.

43

MANNOSE. NIL NIL

I I

MANNOSE. Ca.79% XYLOSE, Ca. 2%

0 5 10 1 5 2 0 2 5 3 0 3 5 4 0 4 5 0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 2 8 .2 e

4 % 94%

MANNOSE, XY LOSE,

2 9 2 8

Fig. 6. Photometer traces of x-ray powder diagrams: upper left, cotton linter cellulose 11; upper right, slash pine glucomannan; lower left, paper birch xylan; lower right, slash pine xylan.

For a given per cent of wood removed, 002 peak width as shown in Figure 3 increases slightly more in tJhe chlorine holocellulose than in the chlorite. This is probably due to the alkaline reaction of the monoethanolamine acting upoii the cellulose. It will be recalled (Table I1 and IV) that aqueous alkali has a similar effect. It mould appear, therefore, that there are supermolecular structure differences between the two types of holo- cellnloses, as initially siiggestrd by thr work of Korman and Ringer.29

Mercerization of Holocelluloses

The cellulose 1-11 transition curves are presented in Figures 7 and 8 for the chlorite and chlorine holocelluloses, respectively. The curve for a high purity prehydrolysis sulfate dash pine pulp has been included for comparison. The holocellulose curves are much flatter than the one for pulp,

44 H. NELSON

0.740

0720

0700

0680

0660

0640

0 620

0600 a

a580

0560

0540

0520

0500

04 80

0.460

and they shift perceptibly toward lower alkali concentrations with increasing lignin removal. Casual inspection would suggest that none of the holocelluloses is completely mercerized in the alkali concentration range shown.

-

- -

-

-

-

-

-

- -

- -

-

0.740

0.720

0.700 0 17-19% LlGNlN 0.680 C, 4- 6% LlGNlN

0 0- 2% LlGNlN 0.660 0 PREHYDROLYSIS

0.640

0.620

SULFATE PINE PULP

a" 0.600

0.580

0.560

0540

0.520

0.500

0.4 80

0.460

0440 2 4 6 8 10 12 14

% NROH

Fig 7. Cellulose 1-11 transition for chlorite holocrlluloses and prehydrolysis sulfate Pulp.

0 17-19% LlGNlN 0 4- 6% LlGNlN 0 0- 2% LlGNlN 0 PREHYDROLYSIS

SULFATE PULP

" i ' d&---&.-.-.-

6 0 10 12 14 0440 - 4-

% NROH

Fig 8. Cellulose 1-11 trarisitioii for chlorine liolocelluloses :tiid preliydrolysia sulfate pulp.

USE OF IlOLOCELT,UT,OSIS 4.5

It, might, sceni, for example, that the limiting (maxiniurn) R, valurh for t hc, holocelluloses are less than the value of 0.722 ordinarily found for such high purity pulp as the one represented and cannot, therefore, correspond to 100% mercerization. However, the relation between R, and per cent mercerization (cf. the section “Experimental”) applicable to pure pulps may not apply to samples containing appreriahle quantities of noncellulosics. This is becausc the accuracy of €2, values depends on accurate measurement of the (lO1)1 and (101)~ diffraction peak heights.

As shown earlier, the holocelluloses give poorly resolved diffraction patterns owing to a low degree of lateral order, which may be due to the amorphous lignin and hemicelluloses in the fiber and to the ordered relationship between part of the hemicelluloses and cellulose. On treatment of the holocelluloses with alkaline solutions of increasing concentration, as is required for a degree of mercerization analysis, lateral order decreases even more and so also does diffraction pattern resolution. Resolution doubtless improves as a result of hemicellulose rcmoval, although apparently not enough to compensate for its reduction on mercerization. During analysis, an alkali concentration is reached at which differences between the ( lO1)1 and (101)~ peak intensities begin to approach the magnitude of experimental error. This difficulty occurs chiefly toward the end of the transition, in the concentration region where mercerization of pulp is complete. Thus, once R, approaches its limiting value, i t is impossible to tell by the x-ray method whether treatment with more concentrated alkali would effect additional change in the cellulose crystallite structure. Ob-

0.74 - 0.72 -

0.70

0.68

0.66

0.64

0.62

- -

- -

-

0.55

0.52

0.50

0 ORIGINAL CHLORITE HOLOCELLULOSE HYDROLYZED CHLORl TE HOLOCELLULOSE

0 PREHYDROLYSIS SULFATE PULP

8 10 12 14

046

0440 2 4 6

% NaOH

Fig. 9. Cellulose 1-11 transition for chlorite holocellulose before and after partid hydrolysis and for prehydrolysis sulfate pulp.

46 I<. NELSON

viuusly, it, is lieeesaary h cieternlillc whether t,trc hiitirig fd, notled for thv holocelluloses represents 100yo mercerization, before the curves in Figures 7 and 8 can be interpreted in terms of cellulose fine structure.

The 0-2 and 4-6y0 chlorite holocelluloses have the same limiting R, of 0.645 even though the 0-2 sample is shown in Table I1 to he essentially lignin-free after alkaline extraction. (The R, of the 18-2070 holocelluloses rises to 0.618 at 20% NaOH; the ratio presumably increases to the limit of 0.635, but a t some higher alkali concentration.) Consequently, residual hemicelluloses (those not extracted by alkali during analysis) rather than lignin must be responsible for the low limiting R, value. If this were not true, the limit,ing R, would increase with extent of delignification. Also, data presented in Figure 9 and Table V show that reduction of alkali-resistant hemicelluloses by mild hydrolysis of the holocellulose prior to analysis increases the limiting R,, but this effect is not due to a lessening of mercerization resistance. The direction indicates that it is due instead to improved diffraction pattern resolution consequent upon hemicellulose removal. Acid recrystallization during hydrolysis is unimportant? for the same higher R, value is obtained by ball milling to the point where the amount of hemicelluloses resisting alkaline extraction are reduced to the same level as those in the hydrolyzed ~ample.~R

In addition to increasing the R,, the mild hydrolysis is shown, in Figure 9, to narrow the mercerization interval by 1-2y0 NaOH toward the end of the transition. This can hardly be caused by hemicellulose removal per se, for similar behavior is observed in partial hydrolysis of cotton, a fiber which is virtually devoid of hemicelluloses. According to Sisson and Saner49 and Rhby,1-26J0 moderate degradation such as used to obtain these data weakens swelling restrictions imposed by the gross fiber structure and intercrystalline chains and allows the cellulose crystallites to swell nearly independently of one another. It would seem, therefore, that hemicelluloses in holocellulose interfere more with the analytical procedure than with the mercerization reaction. The important point brought out by this discussion is that once the limiting R, is reached, as shown in Figures 7 and 8, mercerization must he essentially complete. There can be little question, therefore, of where the transition ends, even though the approach to the limiting R, may be gradual for one reason already cited and another discussed below.

TABLE V Relationship Betm-een R, and Alkaline-Resistant of Hemicelluloses

Composition of alpha cellulose, yL - 10%

Glucose NaOH Limiting + galac- Man- solubil-

Sample Rz tose nose Xylose ity, yo ~~

Before hydrolysis 0.645 83 14 3 34 After hydrolysis 0.688 95 4 1 51

USE OF IIOLOCELLULOSI.~ 47

TABLE VI hlercerization Characteristics of Holoc*elluloseh

- ~- - _ _ _ _ _ _ _ -

Average- Lignin ordera

Transition rangeb

level, chlorite, Chlorine, Chlorite, Chlorine.

17-19 l X 2 10.5+ 10 :% 11.0+ 4-6 8 .1 11 . o !).8 11.0 0-2 7.2 7 . 0 8 . 1 11.0

a Per cent NaOH at, point of 50y0 conipletion of transition. b Width of cellulose 1-11 transition, in per cent NaOH.

In contrast with the hemicelluloses, ligniii retards mercerization. This is shown by the shift of the curves in Figures 7 and 8 toward lower alkali concentrations with increasing ligniii removal. It is also shown in Table VI by the decrease in estimates of average lateral order (i.e., per cent XaOH a t 50% mercerizat.ion based on RII = 0.b45) with extent of delignification. No particular quantity of residual lignin retards mercerization more than another as reported, in the case of jute, by Sen and Woods.24 There is, however, a tendency of ligniii in slash pine to exert a greater retarding effect toward completion of the transition interval than a t the start, inasmuch as the curves representing the more lignified samples gradually fall off a t higher alkali concentrations. Thus, whether swelling restrictions are imposed by lignin or by fiber structure, their influence is most pronounced near the end of mercerization when alkali cellulose swelling is believed to be a t a maximum.

It is important at this point to ask whether cellulose smelling behavior changes on delignification and, if so, in which holocellulose the swelling behavior is most like that of the native cellulose. Since lignin interferes with mercerization, it is impossible to follow smelling behavior as a function of delignification. It is possible, however, to speculate on changes in the factors known to influence smelling behavior. As a result of the mere ab- straction of lignin from the fiber, the packing perfection of the cellulose chains within the crystallites and of the crystallites with each other probably increases. Also, as a result of side reactions during delignification, elements of the gross fiber structure and the more accessible intercrystalline chains are degraded to an extent which is dependent on the holocellulose preparative method (cf. Table I). These changes, however, probably are not large enough to alter the swelling behavior very greatly. Conse- quently, if all lignin is removed and cellulose degradation is not excessive: the swelling behavior of holocellulose in strong alkali is believed to be not grossly unlike that, of thP native cellulose.

As to which holocellulose is most representat.ive of tbe native cellulose, the difference in 002 and D.P. between the two seems to be insufficient to prevent using either in an estimation of the native cellulose swelling behavior. The difference between the transition curves shown in Figures 7 and 8 at the

48 R. NELSON

0-27; level may he explaiiied by the F/', residual and alkali-insoliihle llgnln in the chlorine sample (cf. Table 11).

Use of the chlorite holocellulose was favored in the present work only because the preparative method is simpler and complete delignification of slash pine wood is easier.

There seems to be none of the corrclatioii between the niercerixation curves showii in E'igures 7 and 8 and the alkali solubility curves shown in Figures 10 arid 11 that might haw btcii expected on the basis of the inti- macy of the cellulosaii-cellulose association. Solubility, like ease of mercerization, increases with degree of delignification by both preparative

0 17- 19% LlGNlN 0 4- 6% LlGNlN 0 0- 2 % LlGNlN

- 0

% NOOH

Pig. 10. Alkali solubility of chlorite holocclluloses; O"C., 2 hr., 50: 1 liquor/solid.

30 t 0 I 7 - 19% LlGNlN 0 4 - 6% UGNIN 0 0- 2% LlGNlN

% NaOH

Fig. 11. Alkali solubility of chlorine holocelluloses; O'C., 2 hr., 50: 1 liqnor/solid.

methods. Solubility tends to exhibit a maximum near 10% NaOH, but this concentration is considerably less than that required for 100% mercerization at 0°C. Hence solubility and swelling (i.e.. 100% mercerization) maxima are not coincident, as is sometimes thought. The difference in per cent NaOH between the two has been proposed as a measure of mercerization resistance imposed by the gross fiber s t r ~ c t u r e . ~ ~ In comparison with the sharp parabolic type of function observed for the more severely degraded materials, the flatness of the curves in Figures 10 and 1 I is impres- sive. Solubility differences between holocellulose types at each lignin level may be explained by the more rigorous attack on the whole-wood substance during chlorite delignification.


Comparison of Holocellulose and Finished Pulp Supermolecular Structures

Structural parameters are compared in Table VII for the 0-25Z0 chlorite holocellulose, a sulfite slash pine pulp, and a prehydrolysis sulfate slash pine pulp. The 002 peak width decreases as a result of pulping by either process, but it decreases slightly more during the alkaline process. As shown by values of average order given in this table, swelling resistance increases in the same order that 002 decreases. Therefore crystallites are wider and more perfectly developed in pulp than in natural wood and accordingly are more difficult to swell. The discussion in the preceding section indicates that these changes in the crystallites are connected wibh removal of the “cellulosan” hemicelluloses rather than with removal of either the “polyuronides” or the lignin. The slightly greater change in 002 peak width during the sulfate process cannot be due to greater hemicellulose removal, for carbohydrate analyses given in Table VII show that the two pulps are of comparable purity. The narrower peak width is more probably the consequence of an interact,ion bet.meen the native cellulose and the alkaline pulping liqiiors.

TABLE VII Comparison of Chlorite Holocellulose and Finished-Pulp Properties

Property

002 peak width; degrees, 28 Limiting D.P.* Average order,b yo Lateral order distribution Glucose + galactose, % Mannose, Xylose, %

0-2 % chlorite

holocellulose ~-

2.9-2.7 ca. I, 140

7.2 Wide

71 22 7

Sulfite pulp

Preh ydrolysis sulfate

PUlP

1.8-2 0 ca. 232 7.3-7.8 Narrow

97 1 2

1.7-1 8 ca. 177 8.0-8.7

Wide 98

1 1

a Values expressed as number of anhydroglucose units in long direction of average crystallite.

Per cent NaOH a t point of 507, merceri~ation.~

The concept that fundamental changes occur in crystalline cellulose during alkaline processing might be useful in explaining a number of apparently related accessibility phenomona reported in the literature. In particular, reference is made to the following: the quantity of carbonyl and carboxyl groups in certain celluloses decreases aftcr treatment with alkali ;52

acid-resistant pentosans are proportionately greater in cellulosic materials treated with alkali than in those not so treated;29 bleached sulfate and soda paper pulps have more alkali-resistant pentosans than do holocellulose or sulfite pulp from the same species;53 and more hemicelliiloses are found in pulps after treatment with alkaline solutions containing dissolved hemicelluloses than are found before.54

On the basis of limiting D.P. data, crystallites are reported to be shorter in sulfate fibers than in sulfites (1). This difference is shown by the limit-

50

0 70

068-

066-

0 64

062

060-

058-

0 56

0 54

052-

050-

048

R x

11. NELSON

-

~

-

-

-

-

0 0-2 CHLORITE HOLOCELLULOSE 0 PREHYDROLYSIS SULFATE PULP 0 SULFITE PINE PULP

I I I I

8 10 12

0 46

044 0 2 4 6

% NOOH

Fig. 12. Cellulose 1-11 transition for chlorite holocellnlose, prehydrolysis sulfate pulp, and snlfite pulp.

ing D.P. values recorded in Table VI I after Riinby.' Jn comparing the pulp and holocellulose values, it is interesting to note that, as crystallite length decreases, 002 width decreases. Thus, crystallites in the native form appear to be extraordinarily long and thin (or imperfect) structures which are modified during pulping to relatively short and thick (or perfect) structures. Such a modification is to be expected from the natural tendency of short chain molecules to crystallize more readily than long chain molecules. It is not unreasonable, however, to suggest that crystallites are modified as described, because the amount of crystalline cellulose remains constant throughout the conversion of native to pulp cellulose. The somewhat lower limiting D.P. of the sulfate pulp, in comparison with the sulfite, is believed to be due to the same alkali-cellulose interaction proposed to explain the narrower 002 peak width. In this connection, J ~ r g e n s e n ~ ~ and later RHnby' and Cho~vdhury~~ have presented evidence showing that limiting D.P. decreases with increasing intensity of alkaline treatment.

The use of limiting D.P. as a measure of crystallite length has been criticized on the grounds that hemicelluloses present within the fiber may protect the cellulose from degradation. The hemicelluloses probably do interfere with hydrolysis to some extent, just as a portion of them interferes with x-ray diffraction pattern resolution, but the differences in limiting D.P. among dissolving-grade wood pulps, in which purity should not be a problem, can be measured. Therefore the native crystallite length must be somewhat less than that shown in Table VII for holocellulose.

USE 01' IIoLocI3LI,uLosB 5 1

0 0-2 CHLORITE HOLOCELLULOSE 1 I PREHYDROLYSIS SULFATE PINE PULP

o"o SULFITE PINE PULP 0.09

0.00

0.07 I

0.0 4

0.03

0.02

0.0 I

% NoOH

Fig. 1 3 . Lateral order distrihiitioii for chlorite holocellulose, prehydrolysis sulfate pulp. :tnd sillfit,(: pulp.

Figure 12 gives the cellulosc 1-11 transition curves for the 0-2% chlorite holocellulose and for typical dissolving grade-sulfite and prehydrolysis sulfate slash pine pulps. The mercerization cont.ours or lateral order distribution curves derived from these are presented in Figure 13. If these latter curves depict the distribution of laterally ordered structures or crystallites according to width and perfection, the two pulping processes appear to affect the native distribution in different wayn. Both could be said to narrow it by increasing crystallite width and perfection, the sulfite process narrowing it more by disorganizing the larger and more perfectly developed crystallites. Such an interpretation, however, cannot be completely cor- rect, considering that the course of the mercerization reaction is influenced by factors other than the nature of the crystallites alone. As discussed earlier, intercrystalline chains and gross fiber structure, although incapable of diffracting x-rays, may influence the mercerization reaction, particularly toward the end of the transition interval.1~26~49~50 On the basis of different heterogeneous hydrolysis rates of crystalline and amorphous cellulose, the narrowing of the native mercerization contour in the upper range as a result of sulfite pulping should be expected to be caused by rupture of chains in intercrystalline and other easily accessible regions rather than by the exclusive attack on the larger crystallites.

Therefore a better interpretation of the curves in Figure 13 would be that the sulfate and sulfite processes b0t.h narrow the native mercerization contour by increasing average crystallite width and perfection, but the sulfite process narrows it more through an over-all weakening of the fiber structure. This interpretation is in agreement with conclusions by Dolmetsch et al.,6,7 Steenbe~-g,~ Bartunek,12 Treiber and Stockrnan,'3 R%nby et al.1° to the effect that sulfate fibers retain more vestiges of the original biostructure than do sulfite fibers. Of particular importance is that this interpretation does not depend on any special "sulfate effect"2 or crosslinking r e a c t i ~ n . ~ ~ , ~ ~

52 €1. NELSON

Struct iir;tl differoncw bd,wcwl the 1 wo fiber types cwi he explaiiwd in less c-omplicated terms, namely, in terms of pulping reactions wherein some high and low-energy bonds are broken, some of the same kind retained, and some low-energy bonds formed.

SUMMARY

Changes in x-ray diffraction pattern result from stepwisc delignification of slash pine wood by the chlorite and chlorine-monoethanolamine holocellulose preparative methods. These changes seem to represent. a combination of purification effects and cellulose structure modificat.ions. Evidence is presented of an alkali-cellulose interaction which causes a partial structural ‘Lcollapse” or increase in lateral order during, or as the consequence of, chlorine-monoethanolamine delignification. I n agreement with reports by other workers, lignin was found to retard the mercerization reaction. How- ever, no quantity of residual lignin was found to be particularly effective, as was reported in the case of jute. Since lignin interferes with mercerization, swelling behavior cannot be followed throughout delignification, a t least not in a manner which allows meaningful ext>rapolation back to the whole wood. It is speculated, however, that changes occurring in the character of the crystallites and of the gross fiber structure during delignification do not prevent the use of either holocellulose for est,imating the swelling behavior of the native cellulose.

Changes in x-ray diffraction pattern also result from the extraction of hemicelluloses from holocellulose with aqueous alkali. These changes seem to be due to an alkali-cellulose interaction, such as was observed in chlorine- monoethanolamine delignification, and to hemicellulose removal per se. That portion of the change in the pattern attributed to hemicellulose removal depends on the composition of the hemicelluloses removed. Ex- traction of “polyuronides” has less of an effect than extraction of “cellulosans.” This result correlates with the suspected positions of the two types of hemicelluloses relative to the cellulose fibrils, and with the molecular structures of the two component polysaccharides. The change in diffraction pattern accompanying “cellulosan” removal is considered to represent an increase in crystallite width, in development of the crystal lattice, and in packing perfection of the chains within and on the surfaces of the crystallites.

The alkali-resistant hemicelluloses (cellulosans) interfere with mercerization analyses by the x-ray method. They do not interfere with the transition itself but rather with x-ray diffraction pattern resolution.

Lignin-free chlorite holocellulose being used as a reference, the direction and the magnitude of change effected in structure by pulp processing are estimated. The average crystallite width and perfection are thus shown to increase as a result of pulping by either the sulfite or prehydrolysis sulfate process, but they increase slightly more as a result of pulping by the latter process. The wider and more perfectly developed crystallites in the sulfate

US13 OF HOLOCELLULOSE 53

pulp are ascribed to an alkali-cellulose interaction. An interesting inverse relationship is noted between crystallite length and width (and perfection), and it seems to hold from the native cellulose to the finished pulp. Both the sulfite and prehydrolysis sulfate processes appear to narrow the mercerization contour (i.e., lateral order distribution) by increasing average crystallite width and perfection, but the sulfite process narrows it more through an over-all weakening of the fiber structure. All of the essential structural differences between the two pulp types may be explained in terms which do not involve special “sulfate effects” or crosslinking reactions.

The author is grateful to Dr. 1). F. Durso and Dr. L. R. Parks for their helpful sugges- tions, to Dr. R. H. Marchessault, of the American Viscose Corporation, for the photometer traces, and to Mrs. Elizabeth Kissel Reeder for her assistance in the laboratory.

References

1. Ranby, B. G., “Fundamental Papermaking Fibers,” Brit. Paper and Board Mak-

2. Rlnby, B. G., and H. F. Mark, Svensk Papperstidn., 58, 374 (1955). 3. Marchessault, R., arid J . A. Howsmon, T e x . Research J., 27, 30 (1957). 4. McCarthy, J., and 0. A4nker-Rasch, Norsk SkogintE., 8, 320 (1957). 5. Parks, L. R., Tappi , 42,317 (1959). 6. Dolmetsch, H., E. Franz, and E. Correns, .I. nzakronLo1. Chemie, 1, 67 (1944);

7. Dolmetsch, H., Holz Izoh- u. Werks1o.f; 13, 178 (1955). 8. Haas, H., E. Bat.tenberg, and D. Teves, T a p p i , 35, 116 (1952). 9. Steenberg, B., Svensk Pappersfidn., 50, 155 (194’7).

ers’ Assoc. Proc. Tech. Sec., pp. 55-82 (1958).

Kolloid-Z., 106, 174 (1944).

10. Rbnby, B. G., H. R. Giertz, and E. Treihrr, Svensk Papperstidn., 59, 117, 205

11. Giertz, H. R., Svensk Papperstidn., 56, 893 (1953). 12. Bartunek, R., Das Papier, 2, 442 (1948); ibid., 12, No. 1/2 (1958). 13. Treiber, E., and L. Stockman, Svena Papperstidn., 59, 156 (1956). 14. Thomas, E. X., and J. Hewitt, Nature, 136, 69 (1935). 15. Preston, R. I)., and ’4. Allsopp, Biodynaniica, 53, 1 (1939). 16. Astbury, W. T., It. D. Preston, and A. G. Norman, Nature, 136,391 (1935). 17. Norman, -4. G., T h e h’iochenzistry of Cellitlose, the Polyuronides, Lignin, Etc.,

Clarendon Press, Oxford, 1937; see also, Cellitlose and Cellulose Derivatives ( H i g h Polymers, Vol. V), 2nd ed., Pt. I, E. Ott and M. H. Spurlin, Eds., Interscience, New York-London, 1954, pp. 459-479.

18. Wise, L. E., in L. E. Wise and E. C. Jahn, Wood Chemistry, 2nd ed., Vol. I, Rein- hold, New York, 1952, pp. 369-395.

19. Meller, A., Paper Trade J., 124, No. 8, 104 (1947); 20. Dymling, E., H. W. Giertz, and B. G. Rbnby, Svask Papperstidn., 58, 10 (1!).55). 21. Sen, M. K., and P. H. Hermans, Kec. trav. chinz., 68, 1079 (1949). 22. Nelson, R., and C. Schuerch, T a p p i , 40,419 (1957). 23. Booker, E., and C. Schuerch, Tappi , 41, 650 (1958). 24. Sen, M. K., ;tnd H. .I. Woo&, Biochin~ et Biophys. A d a , 3, 510 (1949). 25. Nelson, E., and C. Schuerch, J. Polymer Sci., 22,435 (1956). 26. Rbnby, B. G., Acta Chem. Scand., 6,116 (1952). 27. Timell, T. E., and E. C. Jahn, Svensk Papperstidn., 54, 831 (1951). 28. Wise, L. E., Paper Trade J., 122,2, 35 (1946). 29. Norman, A. G., and H. P. Binger, T a p p i , 39,430 (1’356); 40, 755 (1957). 30. Schwalbe, H., in E’. E. Brauns, The Chenii.dry of L i p i n , Acadrniic Prrss, Xcsw

(1956).

125, No. 11, 57 (1947).

York, 1952, pp. 162-163.

54 13. NELSON

31. Durso, D. F., unpublished results. 32. Durso, D. F., and J. C. Paulson, Anal. Chem., 30,919 (1958). 33. Timell, T. E., Tappi, 40,25 (1957). 34. RLnby, B. G., 0. W. Woltersdorf, and 0. A. Battista, Svensk Papperstidn., 60,373

35. Nelson, R., Tappi, 43,313 (1960). 36. Zachariasen, K. A.; unpublished results. 37. Howsmon, J. A., and W. A. Sisson, in Cellulose and Cellulose Derivatives, 2nd ed.,

Pt. I, (High Polymers) E. Ott and M. H. Spurlin, Eds., Interscience, New York-London,

(1957).

1954, pp. 285-286. 38. Most, D. S., Tappi, 40, 705 (1957). 39. Hamilton, J. K., E. V. Partlow, and N. S. Thompson, Tappi, 41,811 (1958). 40. Lindberg, B., and H. Meier, Svensk Papperstidn., 60, 785 (1957). 41. Hamilton, J. K., and G. R. Quimby, Tappi, 40, 781 (1957). 42. Corbett, W. M., and J. Kidd, Tappi, 41, 137 (1958). 43. Meier, H., and K. C. B. Wilkie, Holzforschzing, 13, 177 (1959). 44. Hamilton, J. K., E. V. Partlow, and N. S. Thompson, J . Am. Chem. SOC., 82, 451

45. Bishop, C. T., Can. J. Chern., 31,793 (1953). 46. Yundt, A. P., Tappi, 34,89, 91 (1951). 47. Adams, G. A., and A. E. Castagne, Can. J . Chem., 29, 109 (1951). 48. Nelson, R., unpublished results. 49. Sisson, W. A,, and W. R. Saner, J . Phys. Chetn., 45,717 (1941). 50. RLnby, B. G., Acta Chem. Scund., 6, 128 (1952). 51. Hermans, P. H., Physics and Chemistry of Cellulose Fibers, Elsevier, Xrw York,

52. Ant-Wuorinen, O., and A. Visapas, Paper i ju Ptric, 40, 313 (1958); ihid., 41, 345

53. Schoettler, J. R., Yuppi, 37, 686 (1954). 54. Yllner, S., and B. Enstrom, Svensk Papperstidn., 59, 229 (1956); ibid., 60, 440

55. Jorgensen, L., Acta Chem. Scund., 3, 780 (1949); ibid., 4, 185 (1950). 56. Chowdhury, D. K. R., Ter . Research J., 29, 394 (1959). 57. McKinney, J. W., Paper Trade J., 122, 58 (1946). 58. Haggroth, S., and B. Lindberg, Svensk Papperstidn., 59, 870 (1956).

(1960).

1949, pp. 147-155.

(1959).

(1957).

Synopsis

Holocelluloses were prepared from slash pine wood a t the 17-19, 4-6, and 0-2% lignin levels by the chlorite and chlorine-monoethanolamine methods. The products were compared with respect to carbohydrate composition, nitrate degree of polymerization, x-ray diffraction peak width, s-ray crystdlinitg index, mercerization resistance, and alkali solubility. Structural differences among holocelluloses a t each lignin level were noted. Thest. wvre attributrd to a deperidencc of x-ray diffmct,ion peak width on delignification conditioris and t,o snii~ll differences in amount of rrsitlual lignin. It was determined that alkali-resistant, heinictrlluloses inl,t.rfrrr with niercerization :tnalyses iriasrnuch :LS they tliniinish diffravt.iori pattern resolution. Tttking account of such interference, the swelling twllavior of t.lie velhilosr in ~ h ( ~ l r wood c’ari I J ~ estiiriated from either t.ype of holoc:ellulose. It. is riecessary, Ilowevrr, to mii ire t.hat :dl of’ th r li@lin is re- i~ioved aid that cellulose deyradatiou is riot excessive. The effect of st,epwise alkalinr extraction of hemicelluloses from the holocelluloses containing 0-2% lignin was followed by x-ray analyses of the extracted residues after each extraction stage. Narrowing of the 002 diffractiori peak width was observed and was found t80 depend 011 an alkali- cellulose interaction and on hemicellulose reinoval. Reinoval of “polyuronides” has less of an effect on peak width than removal of the “cellulosans.” The hemiceliulosic

r - USE OF HOLOCELLULOSE .>a

polysaccharides, araboglucuronoxylan, and glucomannan are discussed in regard to molecular structure, probable relationship to the crystalline cellulose, ease of alkaline extraction, and effect of removal on the cellulose supermolecular structure. Xarrowing of the 002 peak width, independent of cause, is considered to represent an increase in crystallite width, in development of the crystal lattice, and in packing perfection of the cellulose chains within and on the surfaces of the crystallites. Structure parameters were estimated for native slash pine cellulose, and these were compared with values found for celluloses isolated from the same species by the conventional acid sulfite and prehydrolysis sulfate processes. This comparison enabled approximation of the direction and magnitude of change effected in the native cellulose structure during pulping.

R6sum6 Des holocelluloses ont Ct6 pr6parBes h partir de copeaux de bois de pin contenant

17-19, 4-6 et 0-2 pourcent de lignine en employant les methodes au chlorite et au chlore-mono6thanolamine. Les produits sont comparks du point de vue de la composition en hydrate de carbone, au degr6 de polymkrisation, A la largeur du pic de diff’raction par les rayons X, h l’indice de cristallinitk obtenu par rayons X, h la r6sistanc.e B la mercerisa- tion et B la solubilit6 daris les alcalis. Les diffhrences de structure eiitre les holocelluloses sont notees pour chaque taux de lignine. Ces differences sont attribuees h la dkpendance de la largeur du pic de diffraction aux rayons X avec les conditions de dklignification et aux petites differences dans la quantiti: de lignine rksiduelle. Les hrmicelluloses qui rksistent aux alcalis interfkrent avec les analyses de merckrisation d’une faqori telle que cela diminue la resolution du diagranine de diffraction. En tenant conipte d’une telle interfkrence, le gonffernent de la cellulose dans l’ensemble du bois peut &re attribu6 ii l’emploi de chaque type d’holocellulose. t.oute la lignine est enlevee et yue la dkgradation de la cellulose n’est pas excessive. L’influence de l’ktape d’extraction alcaline des hemicelluloses h part,ir drs holocelluloses contenant. 0-2 pourcent de lignine a k t6 suivie par des aiialyses :tux rayons S sur les r6sidus extraits apres chaque &ape d’extraction. On a observl un r6trlcissement de la largeur du pic de diffraction 002 et on a trouve que cela depend d’une interaction alcali- cellulose et de I’extraction de l’hemicellulose. L’enlkvenient des “polyuronides” a un effet moins marque sur la largeur du pic que l’enlkvement des “cellulosanes.” 0n.discute des polysacharides h6micellulosiques, de l’arabo-glucoronoxylane et glucomannane quant h la structure molCculaire, ii la relation probable avec la cristalliniti. de la cellulose, quant B la facilit6 de l’extraction alcaline et quant h l’effet de 1’enlPvement cle la cellulose sur la structure niol6culaire. Un r6trecissement de la largeur du pic 002, indkpendam- ment de la cause, repr6sente une augmentation dans la largeur du cristallite, dans le d6veloppement du r6seau cristallin et dans la perfection de l’entassement des chalnes de cellulose a l’intkrieur et sur les surfaces des cristallites. On Bvalue les paramktres struc- turaux pour la cellulose naturelle du pin d6coup6 et ceux-ci sont compares avec les valeurs trouvkes pour les celluloses isolees h partir des m&mes espkces par les prockdes conven- tionnels au sulfite acide et h la prkhydrolyse au sulfate. Cette comparaison pormet unr :ipproxiniation de la dircction et de l’importance du changcwient qui sc produit dnns In structure de la cellulose naturelle pendant I’opCration de pulpage.

I1 est ceperidant necessaire de

Zusammenfassung Holocelluloseii wurdeii Lei Ligiiiiigehalteii voii 17-.1Y, 4-6 und 0-2% aus F’ohrenholz

nach dem Chlorit- und Chlor-Monoathanolaminverfahren dargestellt. Die Produkte wurden in bezug auf Kohlehydratzusammensetzung, Polymerisationsgrad des Kitrats, Breite der Rontgenbeugungsmaxima, Rontgenkristallinitatsindex, Mercerisierungs- hestandigkeit, und Alkaliloslichkeit verglichen. ~triikturunterschied~ znischen Holo- cellulosen mit verschiedeiiwn Lignirigehalt. \vurden festgestellt. I h s e wurden der Abhiingigkeit der Rreit der Itiiritgenbeugungsniaxima von den Iht,lignifizierungsbedin- gungen und kleinen Mengenunterschieden des residuellen Lignins sugeschrieben. Alkali-

56 1%. NELSON

bestandige Hemicellulosen storen die Mercerisationsanalysen und verringern uberdies die Auflosung der Beugungsdiagramme. Wenn diese Storung berucksichtipt wird, kann das Quellungsverhalten der Cellulose im Gesamtholz mittels jedes der Holocellulosetypen bestimmt werden. Es ist jedoch notwendig dafur zu sorgen, dass das gesamte Lignirl entfernt wird und der Celluloseabbau nicht zu stark ist. Der Einfluss der schrittweisen, alkalischen Extraktion der Hemicellulosen aus den Holocellulosen mit einem Ligninge- halt von 0-2% wurde durch Rontgenanalyse der Extraktionsruckstande nach jeder Extraktionsstufe verfolkt. Eine Verengung der Breite der 002-Beugungsmaxima wurde beobachtet und erwies sich als abhangig von einer Alkali-Cellulosewechselwirkung und von der Entfernung der Hemicellulose. Entfernung der “Polyuronide” hat geringeren Einfluss auf die Breite des Maximums als Entfernung des “Cellulosans.” Die Poly- saccharide der Hemicellulose, Arabo-glukuronxylan und Glukomannan, werden bezuglich ihrer Molekulstruktur, der wahrscheinlichen Beziehung zur kristallinen Cellulose, der Leichtigkeit der alkalischen Extraction und des Einflusses ihrer Entfernung auf die ubermolekulare Struktur der Cellulose besprochen. Verengung der Breite des 002- Maximums wird, unabhangig von der Ursache, als Zeichen fur eine Zunahme der Kris- tallitbreite, der Entwicklung des Kristallgitters und der Packungsgiite der Cellulose- kennen innerhalb und an der Oberflache der Kristallite angesehen. Strukturparameter wurden fur native Fohrenschliffcellulose bestimmt und mit den Werten verglichen, die fur Cellulose gefunden wurden, welche aus der gleichen Spezies durch die konventionellen Sulfit- und Sulfat-Prahydrolyseverfahren isoliert worden waren. Dieser Vergleich ermoglichte eine naherungsweise Bestimmung der Richtung und Griisse der in der Struktur der nativen Cellulose durch den Pulpprozess bewirkten Veranderungen.

Discussion

R. H. Marchessault (American Viscose Corporation, Marcus Hook, Pa.): Is the change of diffraction due to removal of acetyl by pretreatment with sodium hydroxide?

R. Nelson (Memphis, Tenn.): It would not be surprising if part of the change were caused by saponification of acetyl groups linked to xylan and glucomannan, perhaps even to cellulose. However, since we continue to observe marked changes in diffraction after extraction of lignin-free holocellulose under conditions (i.e., 6% NaOH, 60°C., 60 min.) which should saponify all acetyls found in the sample, additional factors must contribute to the change. As I hoped to bring out clearly, these factors are the removal of noncellulosics closely associated with the cellulose, chiefly the “rellulosa.ns,” and an interaction between the cellulose and the purification medium.

B. RBnby (ESPRZ, State University College of Forestry, Syracuse, N . Y.): Dr. Nelson has found that removal of lignin and, in particular, of hemicelluloses causes an improvement in the lattice order of the residual wood cellulose framework. My comment is that the treatments a s such, e.g., the heat treatments in aqueous acid or alkaline media referred to, can cause semicrystalline cellulose to crystallize further. This is the case with cottonseed hair cellulose which is only moderately crystalline in the immature state. The maturing process and heat treatments in aqueous media were found to improve gradually the lattice order of the cellulose, as we reported a t the First Cellulose Conference. The amount of extracted and removed material is very small in the case of cotton fibers. Imt , summer we also found a similar phenomenon for thin films of regenerated (from viscose) and native (bacterial) cellulosc, which showetl :I clrt.re:tsed accessibility (incwwsrcl ( tallinitv) t o druteriuni rxc~h:tnge whrii tlried 01’ st.eam- treated.

I mi in agreement with Or. R.%nby tlrrtt. treatments iii aqueous acidit. or alkaline media can cause improvements in lattice order. I am not in agreement that the changes repcrted here are caused only by the medium. Otherwise, how can we explain that pine wood does not show changes in diffraction patt.ern of the same magnitude as liolocellulose after both materials are extrsc,ted with :dlcali under identioal conditions. In results which I have not reportcd tod:iy, we found that the conversion

R. Nelson:


of extractive-free second cut cotton linters to holocellulose is accompanied by relatively minor changes in diffraction pattern. Also, alkaline extraction of this holocellulose caused but small changes in the pattern. In both treatments, the amount of material removed was small in relation to that removed by similar treatments of either hard- wood or softwood. The importance of these results rests in the fact that the crystal lattice in raw mature linters is already well developed, either because or in spite of the minor fraction of noncellulosics within the fiber, and accordingly it cannot develop as much percentagewise as the lattice in wood. It is interesting to note that raw cotton linters have an average crystallite width and degree of lattice development appreciably greater than most modern dissolving grade woodpulps.

Have you any data as to how the cellulose crystallinity changes with cooking temperature, especially in the case of high temperature rapid cooking? It would be even of interest to know, how the temperature affects the crystal1init.y in the cooking as the wood is heated; can an intermediak stat~e of decreased crystallinity a t high temperature be expected?

In the case of an increase in the crystallinity of cellulose during pulping, can a differen- t,iation be made between a really true recrystallization and a relative increase by a partial removal of noncrystalline noncellulosic material?

R. Nelson: Yes, we have limited data on the effect of cooking temperat re on diffraction pattern. Mr. K. A. Zachariasen of our laboratories found essentially no difference in cooking (sulfate) to the same permanganate number at 125 or a t 185°C. From these results, he concluded that the pulping temperature cannot explain structural differences between sulfate and sulfite pulps. We do not have data for lower temperatures, but I suspect the same relation is valid there. We have found no evidence for an intermediate state of decreased crystallinity during pulping. On sampling a t different times and temperatures during prehydrolysis and sulfate cooking, a gradual increase in crystallinity has been consistently observed. If an intermediate state of decreased crystallinity exists, it must be so highly unstable or transitory that it cannot be observed by our present techniques.

In regard to your second question, I would like first to say that we look upon the diffraction pattern changes described as a crystallization rather than as a recrystallization. This is an important point that we attempted to make. Namely, pulping operations are almost invariably accompanied by structural growth. Now, I would say that a differentiation can indeed be made between this growth or increase in crystallite width and/or perfection and a relative increase by removing a.morphous material. Mr. Zachariasen recognized the importance of this question and determined that diffraction peak width was essentially independent of background or amorphous scatter. This was done by measuring peak width before and after physically admixing purified wood pulp with amorphous starch.

T. E. Timell (Pulp and Paper Research Institute, Montreal): Would it not be possible tentatively to assume that any, a.nd not just one specific polysaccharide, were intimately wsociated with the cellulose microfibrils? The sequence of removal of the hemicellu- Loses could as well be explained by t,heir different, solubility characteristks with regard to the alkaline solut.ions used for cstraction. Consider, for example, the profound influence of borate on t.he effiviency of removal of g1uconi:mnans. The :iddition of borate (or boric acid) t,o :Llkali cwt,:tinly t1ot.s not increase the swelling wtiori of the solution.

T 1)elieve t h t enc.11 of t lit: Iieniicellulosic po1ysac:c:haride sociated with the cellulosr but not l o t,hc same degree. If the sequence of removal were governed only by molecular properties (e.g., solubility characteristics), how then can the x-ray results be explained? I feel that the chemical and physical evidence are complementary and together provide strong support for our interpretation. I wonder if we can be so certain that boric acid does not increase the swelling action of alkaline solutions. Hydroxides of other Iiiat,erials such :IS zinc: and beryllium do cha.nge t,he swelling action of alkaline solutions. Also, isn't the increased removal of ghcomannan

E. Treiber (Central Labolatory of the Swedish Cellulcse Industry , Stockholm) :

R. Nelson :

58 11. NELSON

by borate an indication of an increase in swelling-from limited to unlimited swelling (of glucomannan) ?

H. Meier (Swedish Forest Products Research Laboratory, Stockholm) : I am doubtful about the very close association of glucomannan and cellulose, because the glucomannan is partially acetylated in the wood.

R. Nelson: I believe it would be well to remember that acetyl analyses tell us how many acetyl groups are present on the average. Please note that I say “on the aver- a.ge.” There will, therefore, be acetyl-rich and acetyl-poor regions along the polysaccharide chain. It is in these latter regions that close association with cellulose could occur. Moreover, an occasional acetyl group should enhance the disorganizing effect of glucomannan on the regularity of attractive forces between adjacent cellulose chains.

H. Tarkow ( U . S. D. A . , Fo~est Products Laboratory, Madison, Wis.): Because of the inability of the x-ray diffraction technique to detect clearly crystalline areas belokv a certain size, is it likely that the physical differences between the two pulps are actually greater than those revealed by your measurements?

Yes, but by inference only. If we accept the concept that there exists in cellulosic fibers a spectrum or distribution of ordered arrangement of the chains with each other, from total imperfection to total perfection of order, there can be little doubt that differences between the two pulps cover a greater range than the x-ray results indicate. X-rays are diffracted from a select portion of the cellulose only-ordered arrangements above a certain level of perfection. X-rays are not diffracted from arrangements below this level. Several methods including water regain, formylation, and deuteration have been used to study structure in this so-called paracrystalline range with varying success.

H. A. Krassig (Industrial Cellulose Research, Ltd., Hawkesbury, Ont.) : In connection with this very interesting lecture, I would like to mention that the Basic Research Di- vision of our organization has performed similar studies. Preliminary results were reported as part of a lecture given by Dr. Kalish and Dr. Beazley a t the meeting of the Technical Section of the Canadian Pulp and Paper Association early this year [Pulp Paper Mag. Can., 61, T, 462-463 (1960)l. In this work we found that during the pulping process a remarkable improvement in the degree of order of the cellulose takes place and that significant differences exist between the way this improvement occurs during alkaline pulping and the way it occurs during acid pulping. These results support Dr. Nelson’s and his colleagues’ concept, and we agree with them that a clarifica- tion of the mechanism of these changes and of conditions to initiate and control them at will would be of great interest to the pulp industry.

I am aware of the work reported by Drs. Kalish and Beazley of your organization, and I am hopeful that such work coupled with the concepts put forth today will do much to improve our understanding of the mechanics by which structure is modified during pulping. It is always comfort- ing to learn that others working independently of one’s own program arrive a t similar conclusions. This is esperially t,rnc, of course, if t>hc test, of time and further experi- mentat.ion shows that, th(y :ire corrcrt, conclnsions.

R. Nelson.

R. Nelson: Thank you very much, Dr. Krassig.

The use of holocellulose to study cellulose supermolecular structure

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