JOURNAL OF POLYMER SCIENCE VOL. XXII, PAGES 435-448 (1956)

Factors Influencing Removal of Pentosan from Birch Wood: Evidence on the Lignin-Carbohydrate Bond*

RUSSELL NELSON and CONRAD SCHUERCH, Department of Chem- istry, College of Forestry, State University of New York, Syracuse, N m York

Wood substance is primarily composed of three kinds of polymers, pentosans, hexosans, and lignin. The major portion of the polysaccharides, although essentially linear, is reported to be not extractable from wood or to be dissolved only “with difficulty,”l--3 and almost all of the lignin fraction invariably requires a degradative chemical reaction for s01ution.~~~ The difference in solubilities of these polymers before and after isolation has been ascribed by some workers to the presence of covalent linkages either between polymer systems (lignin-carbohydrate or within each individual polymer system (lignin-lignin bonds,s cellulose cross-links6), and such linkages must be broken prior to so lu t i~n .~-~ According to other workers, the difference in solubilities before and after isolation should be ascribed to a physical restraint which the individual polymers impose on the solution of each other, and especially to the restraint which lignin imposes on the polysaccharides by interpenetration.10-12 A proper choice between these two interpretations has been a recognized problem of wood chemistry for about one hundred years, and there appears t o be no unequivocal evidence in favor of either explanation.

It is usually possible to distinguish between a chemical process involving the breaking of covalent bonds and a physical process which does not, either by establishing the identity or lack of identity of starting material and product or by a systematic study of the effect of important variables on the process. Both criteria have been applied many times to methods of dissolving lignin, and wherever available the evidence shows that a chemical process has been req~ired.~*13.14 The evidence in the case of the solution of the polysaccharides is less clear, for it is difficult to prove the identity of polysaccharides before and after extraction, and systematic studies on the solution of polysaccharides from wood are meager, although similar studies on isolated celluloses have been publ’ihed.16-18

This paper reports the results of an investigation of the important vari- ables that determine the rate and the amount of pentosans that can be extracted from white birch (Betula papyrifera Marsh.) wood meal by

* This article is abstracted from a portion of a thesis submitted by Russell Nelson in partial fulfillment of the requirements for the degree of Doctor of Philosophy and is the ninth in a series from this laboratory on the structure of lignin.

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REMOVAL OF PENTOSAN FROM BIRCH WOOD 437

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438 R. NELSON AND C . SCHUERCH

sodium hydroxide. This species was chosen because its pentosan fraction has been shown to be primarily a xylan of narrow molecular weight dis- tribution,lS and sodium hydroxide was used as a reagent since solution of the pentosan in it could conceivably involve a chemical reaction or only a physical process. Among the conclusions reached are the following: A t least 80% of the pentosan in white birch can be removed readily and rapidly by a proper choice of conditions of extraction. The inhibition of solution of pentosan observed under some conditions is due solely to a physical restriction of polysaccharide swelling and alkali cellulose forma- tion, and it is unnecessary, therefore, to postulate the existence of a lignin pentosan bond; indeed no evidence for its existence can be found. While the collected data give no direct information about the behavior and struc- ture of the small fraction of unextracted pentosan or the cellulose, the same mechanism of restraint would inhibit most effectively the removal of the most deeply imbedded pentosan and also would be expected to inhibit the extraction of the cellulose molecules which are a t least ten to fifteen times larger than those of the pentosan studied. Therefore, in

, order to explain the solubility behavior of the polymeric components of birch wood, it is not necessary to assume the existence of any network structure except that of 1ignin.m These conclusions cannot be applied indiscriminately to other lignocellulosic materials, and continuation of this study on other materials is under way.

Typical curves of the rate of extraction of pentosans under various con-

COARSE (HAMMER HIU)

MEDIUM

FINE 120-40 MESH)

(8O-lOO MESH)

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 3 6 9 12 15 18 21 24 27 30 35 36 59 42 45 48

TIME OF EXTRACTION IN HOURS

Fig. 3. Effect of wood particle size on the rate of extraction of pentoean using 3% sodium hydroxide at 60".

REMOVAL OF PENTOSAN FROM BIRCH WOOD 439

ditions of temperature, alkali concentration and wood particle size are shown in Figures 1 and 3. Each point on these and the following figures represents the result of a separate experiment in which a weighed sample of wood has been extracted for a period of time with a large excess of alkali under nitrogen, the residue isolated, and its pentosan content determined. Per cent pentosan content is calculated on the basis of the original dry weight of wood. It is clear that the rate of this process does not obey any simple kinetic law. There is an initial very short period during which the pentosan content falls extremely rapidly. This is followed by a period of diminishing rate which approaches, after a few hours or less, almost a constant value. During this period of “constant” rate only a negligible amount of pentosan Muses into solution and for all practical purposes under each set of conditions there is a maximum amount of pentosan ex- tractable from birch wood. A similar behavior has previously been observed with pulp^.^^^^^

The data in Figure 1 obtained at 80’ show also the effect of alkali concen- tration on the rate of extraction of pentosan. At higher concentrations of alkali, more pentosan is extracted in a given period of time. This effect is not due to a concentration dependence of rate characteristic of a chemical process.22 Such an interpretation of these curves would be equivalent to saying that the increase in alkali concentration causes a rupture of addi- tional covalent bonds which are, however, unattacked at the lower concen- tration. This behavior is unknown and very unlikely. A more reasonable explanation of these data is that by increasing the alkali concentration, a larger proportion of the pentosan is made accessible to the alkali and rapidly passes into solution.

A similar effect due to difference in particle size is illustrated by Figure 3. Hective surface area is substantially increased by cutting the fibers and exposing new surfaces to the extracting liquid. As a result, pentosan is removed more rapidly and completely from wood of small particle size. While it is true that covalent bonds can be broken by grinding polymers including wood, the amount of degradation only becomes significant when the particle size becomes much smaller than shown here. In our experi- ments, there was no increase in the amount of water-soluble pentosans with decrease in particle size, such as would occur if much degradation had taken place. On the other hand, Staudinger has observed the effect of particle size on the extraction of cellulose with cuprammonium solutions by grinding wood in a ball mill.23 Under such drastic conditions, molecular degradation undoubtedly does play a role in increasing cellulose solubility.

The simple relationship shown in Figure 1 between the amount of pen- tosan extracted and the concentration of the alkali used breaks down completely wben higher alkali concentrations or lower temperatures are used. To study this effect, several extractions of twenty-four hour dura- tion were made at alkali concentrations varying from 0 to 24% at tem- peratures from 0 to 80°. It is clear from the preceding C U N ~ that only little more pentosan could be extracted over a longer period, and that the

R. NELSON AND C . SCHUERCH

4 Oo

600

800

1 1 1 1 1 1 1 1 1 1 1 1 1 2 4 6 8 10 12 I4 16 I8 20 22 24

PERCENT SODIUM HYDROXIDE CONCENTRATION (W./V.)

Fig. 4. Pentosan remaining in 4060 mesh birch wood meal after 24- hour extraction with sodium hydroxide.

results plotted in Figure 4 represent nearly the maximum amount extract- able under the conditions used. Figure 4 shows that, in the low alkali concentration range and at all temperatures, a more concentrated extractant removes more pentosan than one less concentrated. However, a t some point which differs for each temperature, the concentration dependence is completely reversed. At alkali concentrations a t which cellulose swells most and at which pentosan might be expected to be most soluble, actually the amount removed reaches its lowest value. A few exploratory experi- ments under these conditions would show a marked difference in the ex- tractability of pentosans from wood before and after deligniflcation and could easily suggest the presence of a lignin-carbohydrate b ~ n d . l - ~ , ~ ~ Similarly, the temperature dependence of yield of pentosans from wood in this concentration range might also incorrectly suggest covalent bond breaking. It is clear, however, that the concept of a lignin-carbohydrate bond not only fails to explain adequately the rate data in Figures 1 and 3, but most certainly has no pertinence to the complex curves of Figure 4 which represent an investigation of a broad range of concentrations and temperatures.

On the contrary, our data show that the inhibition of pentosan solution by high alkali concentrations must be correlated with the swelling of the carbohydrate fraction and the formation of alkali cellulose within the

REMOVAL OF PENTOSAN FROM BIRCH WOOD 4'41

physical restraint of the fiber structure. The degree of solution inhibition and the extent of swelling of cellulose are both greatest at low temperatures; the maxima of both effects shift to lower alkali concentrations with lower temperatures; and the concentration range in which these maxima are found are the same for both phenomena. The similarity between alkali concentrations causing maximum swelling and solution inhibition for three temperatures is shown in Table I.

TABLE I ALKALI CONCENTRATIONS AT POINTS OF MAXIMIJM SWELLING DETERMINED

BY VARIOUS METHODS Max. water Max. vol. Max. vol. Pentman soln.

Temp., ahsorption.0 ohange.*8 change 17 inhibition. O C . % NaOH % NaOH % NaOH '% NaOH

0 and 5 9 ( S O ) 8 . 5 (0') 10 ( S O ) 9 (on) 20 11 12 13 12 40 12 - 14 . 14

Since the preceding results have been interpretable by assuming that the amount of pentosan extracted is determined by physical factors only, the interpretation of the process may be clarified by considering the molecular mechanism. Preliminary studies show that the absorption of alkali by the wood fiber is an extremely rapid process, in which a large percentage of the alkali absorbable under any speciGc conditions is imbibed almost instan- taneously. A t zero degrees, complete equilibrium is established in less than one-half hour. During this same period, most of the extractable pentosan is removed. It is almost axiomatic, however, that the solvation and swelling of the polysaccharides by alkali must precede solution and extraction of the pentosan fraction. This is undoubtedly the reason why the amount of pentosan removable is determined by the condition of the fiber swollen to equilibrium.

In the extraction of wood meal with alkali concentrations less than 1%, it is possible that the solution of the pentosans is limited by their solubility in the alkali. We have found that hemicellulose isolated from birch wood is completely soluble a t 20' in alkali of concentrations above one and under 25%. Between these concentrations, the accessibility of the pentosan in the fiber seems to be the dominant factor in its solution. At f is t higher concentrations of alkali result in a deeper penetration of the alkali into the cellulose, more swelling of the carbohydrate and more pentosan being thus made accessible for extraction. Swelling beyond a certain degree, however, within a volume restricted by the rigid fiber structure and the lignin net- work probably results in a decrease in surface available to the extracting liquid. As a result, a portion of the pentosan, which would have been leached by the more dilute alkali from a spongy surface and from the accessible regions of the cellulose, in more concentrated alkali, must Muse through a stifl? microcrystalline gel. Diffusion of a pentosan through a gel in fact scarcely occurs a t a measurable rate. This is our

442 R. NELSON AND C. SCHUERCH

explanation of the h t inflection points in the curves of Figure 4. The interpretation is essentially the same as that of Rath and Kubitizky,28 who studied the alkaline extraction of regenerated cellulose. They suggest that solution of all but the lowest molecular weight components is retarded by the peripheral formation of a highly swollen gel of limited permeability.

Since the isothermal swelling curves of cellulose approximate a family of parabolas,2K it would appear superficially that the pentosan should be least accessible at the concentration causing maximum swelling and at higher concentrations that the pentosan should again be more accessible. If that were the case, there would be a true minimum at the second inflection point in each curve of Figure 4, rather than the plateaus which are observed at the three lower temperatures. The reason for this behavior may be that the phenomena on the two concentration sides of the swelling maxima are not identical. At low concentrations alkali enters both the amorphous and crystalline cellulose; causing a partial expansion of the crystal lattice and swelling of the amorphous regi~ns.~~*~O In this range, incompletely mercerized celldlose crystallites are essentially a t equilibrium with alkali of a given concentration. On the high concentration side, these partially mercerized crystallites are not in equilibrium with alkali but tend to expand to form alkali cellulose. The decrease in volume observed on treating alkali cellulose with stronger alkali in this concentration range is not due to a reversion of the alkali cellulose crystallites to a partially mercerized condition comparable to that on the low concentration side of the swelling maximum, but is due to the removal of water from the amor- phous regions of the cellulose.a1 Now, when cellulose in a woodfiber is treated with alkali of the proper concentration to cause maximum swelling, total swelling is severely restricted by the lignin and the fiber structure, and the crystallites are only partially mercerized while unrestricted cellulose would be completely mercerized to the alkali cellulose form. Sen, Woods, and have shown that the presence of lignin has a retarding influence on the native to alkali cellulose transition. Moreover, RBnbyM has shown by x-ray diffraction that the fiber structure alone retards mer- cerization. When stronger concentrations of alkali are used on wood meal, less water is present in the swollen amorphous regions, but a larger number of crystallites can then expand into the more stable alkali cellulose con- figuration. As a result, the polysaccharides in the wood meal probably occupy about the same volume whether the alkali is of a concentration which causes maximum swelling or whether i t is substantially higher in Concentration. Since the more resistant xylan is apparently present withill the crystallites and on their surfacesa4 and therefore deeply imbedded in the swollen gel, it is not surprising that a change in the proportion of completely mercerized crystallites should not greatly increase the accessi- bility of the pentosan to the free extracting liquid, or greatly change the effective surface. It is perhaps not surprising, therefore, that no more pentosan is extracted in very high alkali concentrations than is extracted a t the point of maximum swelling.

REMOVAL OF PENTOSAN FROM BIRCH WOOD 443

Other interpretations of the details of the molecular mechanism may be possible, but a correlation between the restricted swelling of the poly- saccharides and the inhibition of pentosan extraction appears established unequivocally.

While i t has been shown that the amount of pentman extracted from wood is determined by physical factors, it is still conceivable that a lignin pentosan bond may be present and saponified so rapidly that its presence is obscured by the slower diffusion process. However, even in the low con- centration range (0.75 N> where a chemical process should be comparatively slow and where the effect of swelling is not interfering (Fig. 2), the tem- perature dependence over a sixty degree range is completely trivial. Since the data of Figure 4 are essentially single points on rate curves, this lack of significant temperature dependence is true of all low alkali concentrations and any but an extremely rapid chemical reaction can be excluded. At higher temperatures than those studied such as those encountered in commercial pulping, there is even less likelihood of a chemical process being rate determining, since the rates of chemical processes change more rapidly with temperature than the rate of removal of pentosan.

A further proof that a lignin-carbohydrate bond does not inhibit pentosan solution is the fact that the same inhibition occurs to a diminished degree even after all the lignin is removed (Fig. 5). The restraint to solution is,

20-40 MESH, 0.C

20-40 MESH, 40-

POWDER, ooc.

0 2 4 6 8 10 12 I4 16 I8 2 0 22 24 PERCENT SODIUM HYDROXIDE CONCENTRATION (W. lV.1

Fig. 5. Pentosan remaining in birch holocellulose. after %-hour extraction with sodium hydroxide.

44.4 R. NELSON AND C. SCHUERCH

therefore, in part imposed by the fiber structure itself.30 The basketweave of cellulose molecules and crystallites in the primary wall (exterior) of the fiber and the spiral of cellulose chains in the outer layer of the secondary wall limit the swelling of the fiber interior, the molecules of which lie essentially parallel to and tend to swell perpendicularly to the fiber axis.35 The inhibition of swelling is familiar in the ballooning of damaged fibers in alkali, and the inhibition of pentosan extraction is merely another facet of the same phenomenon. The restraint can be removed by grinding the holocellulose to a powder, which is the case shown in Figure 5, and appar- ently also degraded the carbohydrate fraction, since its water solubility was greatly increased. It can also be eliminated by raising the temperature to forty degrees where swelling is not sufficient to inhibit the solution from holocellulose.

In summary then, the extraction of pentosan from birchwood is not an activated process, but is largely a rapid leaching of soluble polysaccharide from the available surfaces of fibers preswollen to an equilibrium condition which is dependent on temperature, concentration of extractant, morpho- logical structure, and amount of lignin present. The process is most effective when solvent conditions are used which do not permit excessive polysaccharide swelling.

EXPERIMENTAL

Material. A log of white birch, Betula papyrifera Marsh., aged approxi- mately 40 years, was barked, cleaned, and chipped. The chips were air dried, mixed well, and ground first in a hammermill and then in a Wiley mill. The wood meal so obtained was fractionated by screening into four standard mesh sizes: 20-40, 40-60, 60-80, and 80-100. These fractions were extracted separately for 24 hours with a 1 : 2 azeotropic solution of ethanol and benzene to remove extraneous materials and subsequently air dried. They were then stirred with distilled water for another 24 hours a t 2 5 O , filtered, and air dried. The fractions were rescreened to ensure uniformity of particle size and stored in screw capped amber bottles.

Holocellulose was prepared according to a modified method of Van Beckum and RitterlS in which the wood meal was alternately chlorinated in a cold water suspension, filtered, and extracted with hot alcoholic 3% monoethanolamine. Three chlorinations of five minutes each were sufficient to reduce the lignin content to 0.6%, whereas the pentosan content was reduced from 24.6 to 24.1% based on the dry weight of the wood.

Alkali solutions of the required strength were obtained by diluting the appropriate amount of carbonate-free 50% sodium hydroxide solution with distilled water. The concentrations expressed as weight per unit volume percent were established by titrating against standard 0.1 N hydrochloric acid to the phenolphthalein end point.

100 ml. of alkali a t the extraction

Holocellulose.

Alkali Solutions.

Pentosan Extraction Procedure.

REMOVAL OF PENTOSAN FROM BIRCH WOOD 4 5

temperature was added to separate 1.0 i 0.001 g. samples of wood meal contained in screw capped bottles of 130 ml. capacity. The bottles were flushed with nitrogen, sealed tightly with plastic caps, and agitated at 80 r.p.m. for the designated time at constant temperature *0.OlQ. The bottles were then removed, the alkali liquors separated from the residues by suction through a coarse sintered glass filter stick, and 50 ml. of 12% hydrochloric acid added to the residues. The experimental error in the pentosan determination was significantly lowered by acidifying the residues directly rather than washing the residues free of the alkali liquor with distilled water. The acidified residues were then stored at -10' for no longer than 48 hours until analyzed for pentosan.

The pentosan remaining in the residues was converted to furfural by distillation with 12% hydrochloric acid, and the furfural was quantitatively determined by the bromide-bromate method in accordance with the TAPPI Standard Method T-223-m48.36 Since the hemicelluloses of white birch have been shown to be primarily xylan, the conversion factor of 0.88 representing the yield of furfural from xylose was used. No correction was made for uronic acids and the produc- tion of hydroxymethyl furfural was assumed to be 1%. Although there is a slight element of uncertainty regarding the validity of these correction factors when applied to pentosan analyses of wood, our data for original pentosan content agree closely with accepted values in the 1iterat~re.l~

Lignin in wood and holocellulose was deter- mined by hydrolyzing with 72% sulfuric acid, then by digesting with 3% sulfuric acid and finally weighing the acid-free residue in accordance with the TAPPI Standard Method T-13-m45.S6 Filtering losses were mini- mized by coating the sintered glass crucibles with a thin layer of asbestos fibers. This seems to be especially important in the analysis for hardwood lignin.

With 3% sodium hydroxide at 80°, the lignin content was observed to decrease from 18.4% for the original wood to 15.5 and 14.7% on the same basis after 24 and 72 hours' extraction, respectively. Investigations have shown that the alkaline delignification of wood has a concentration and temperance dependence approximating that of a chemical reaction, first order with respect to lignin content.14 This is in contrast to the phenomena studied here and indicates that lignin and pentosan removal by alkali are markedly different processes.

Pentosan Determination.

Lignin Determination.

This work was sponsored by the Office of Ordnance Research, U. S. Army.

References

(1) A. G. Norman, in E. Ott, H. M. Spurlin, and M. W. Graffiin, Cellulose and Cellulose Derivdives, 2nd ed., Part I, Interscience, New York-London, 1954, pp. 466-473.

(2) A. G. Norman, The Biochemistry of Cellulose, the Polyuronides, Lignin, etc., Clarendon Press, Oxford, 1937.

(3) W. Overbeck and H. F. Miiller, Svensk Papperstidn., 45,357 (1942). (4) E. Hlgglund, Chemistry of Wood, 3rd ed., Academic Press, New York, 1951, p.

196, (b) p. 48.

446 R. NELSON AND C. SCHUERCH

(5) Zbid.. p. 48. (6) C. Schuerch, J. Am. Chem. Soc., 74, 5061 (1952). (7) J. W. T. Merewether, Australian Pulp 4 Paper Ind. Tech. Assoc. Proc., 226

( 8 ) H. Grohn, Chem. Tech. (Berlin), 3, 240, 299 (1951). (9) L. E. W h , in L. E. Wise and E. C. Jahn, Wood Chemistry, 2nd ed., Vol. I,

(10) F. E. Brauns, The Chemistry of Lignin, Academic Press, New York, 1952, pp.

(11) A. Abrams, Ind. Eng. Chem., 13, 786 (1921). (12) K. Freudenberg, Tannin, Cellulose, Lignin, Berlin, 1933, pp. 139-146. (13) J. M. Calhoun, F. H. Yorston, and 0. Maass, Can. J . Reseurch, B17,121(1939). (14) G. L. Larooque and 0. Maass, Can. J. Reseurch, B19,l (1941). (15) G. A. Richter, Tappi, 38, 129 (1955). (16) A. Meller, Paper Trade J., 121, 119 (1945); 125, 57 (1947); 127, 60 (1947). (17) J. R. Schoettler, Tappi, 37,686 (1954). (18) K. E. Ohlsson, Suensk Papperstidn., 55, 347 (1952). (19) T. E. Time11 and E. C. Jahn, Suensk Papperstidn., 54,831 (1951). (20) The rather confused history of this proposal can be seen by a comparison of the

K. Freudenberg, K. Freudenberg, Fortschr. Chern. organ. Naturstoffe, 2,

F. E. Brauns, Fortschr. K. M. Meyer, Natural and

(1951).

Reinhold, New York, 1952, pp. 393-395.

14-22.

following: H. Staudinger and E. Dreher, Ber., 69, 1729 (1936). Ann. Rev. Biochem., 8,91(1939). 3 (1939). H. Hibbert, Ann. Rev. Biochem., 11, 188 (1942). Chem. organ. Naturstofe, 5, 200, 219 (1948), Reference 6. Synthetic High Polymers, 2nd ed., Interscience, New York-London, p. 422.

(21) H. F. Lewis, Paper Trade J., 95, 29 (1932). (22) S. Glasstone, K. J. Laidler, and H. Eying, The Theory of Rate Processes, Mc-

(23) H. Staudinger and E. Dreher, Ber., 69, 1099 (1936). (24) I. A. Preece, Biochem. J. (London), 25, 1304 (1931). (25) C. Beadle and H. P. Stevens, Orig. Corn. 8th Intern. Congr. Appl. Chem., 13,

(26) G. J a p e , World's Paper Trade Rev., 108, Tech. Sup. 109 (1937). (27) G. A. Richter and K. E. Glidden, Znd. Eng. Chem., 32,480 (1940). (28) H. Rath and Kubitizky, Klepzig's Teztile-Z., 40, 143 (1937), in L, E. Wise and

(29) W. A. Sisson and W. R. Saner, J. Phys. Chem., 45, 717 (1941). (30) B. G. Rbby, Acta Chem. Scand., 6, 116 (1952); Suensk Papperstidn., 10, 374

(1955). (31) J. A. Howsman and W. A. Sisson, in E. Ott, H. M. Spurlin, and M. W. G r a i n ,

Cellulose and Cellulose Deriuatioes, 2nd ed., Part I, Interscience, New York-London, 1954,

Graw-Hill, New York, 1941.

(1912).

E. C . Jahn, Wood Chemistry, 2nd ed., Vol. I, Reinhold, New York, 1952, p. 229.

pp. 317-334. (32) M. K. Sen and H. J. Woods, Biochem. ef Biophys. Acta, 3,510 (1949). (33) M. K. Sen and P. H. Hermans, Rec. Iran. chim., 68,1079 (1949). (34) W. T. Astbury, R. D. Preston, and A. G . Norman, Nature, 136,391 (1935). (35) J. E. Stone, Tappi, 38,449 (1955). (36) Testing Methods, Recommended Practices, Specifications of the Technical Associa-

tion of the Pulp and Paper Industry, 122 East 42nd Street, New York, N. Y.

Synopsis

The effect of particle size, alkali concentration, and temperature on the rate of extrac- tion of pentosans and on the amount of pentosans extractable from birch wood meal has been studied. In general, a very rapid extraction of accessible pentosans is followed by a slow, almost negligible, diffusion of inaccessible pentosans. There is, therefore, almost a limiting amount of pentosans not extractable under each given set of conditions. The

REMOVAL OF PENTOSAN FROM BIRCH WOOD 4‘47

effect of decreasing particle size is to increase the amount of pentusan extractable under any given set of conditions. The optimum sodium hydroxide concentrations at 0, 20, 40,60 and 80 degrees are about 2,4,7,10 and 12%, and under the last of these conditions at least 80% of the pentosans can be extracted rapidly from the wood meal. Pentusan is made more accessible by increasing the alkali concentration up to these levels, but thereafter the amount of accessible pentosan is decreased by an increase in alkali con- centration. The decrease in extractability has the same concentration and temperature dependence as the increase in cellulose swelling under the same conditions. Therefore, pentosan removal is inhibited by excessive swelling restricted within the rigid l i e and 6ber structure. Similar but less marked inhibition by swelling occurs even after Iignin removal.

restricted swelling occurs, neither the temperature nor alkali dependence of rate is typical of a chemical process. The factors which inhibit the removal of pentosan are therefore physical and the inhibition of solution of pentosan from birch wood gives no support to the concept of a lignin-carbohydrate bond.

Even under conditions of low alkali concentration, when no inhibition of solution by-

RBsumB

On a ktudik I’influence de la grandeur des particules, de la concentration en alcali e t de la temp6rature sur la vitesse d’extraction des pentosanes et sur la quantitk de pento- sanes extractibles aux d6pens de la poudre de bois de bouleau. En gknkral une extrac- tion tr&s rapide des pentosanes accessibles est suivie par une diffusion lente, la plus sou- vent negligeable, des pentosanes inaccessibles. I1 existe donc le plus souvent une quantitk limitke de pentosanes non-extractibles dans un systhme de conditions bien dkterminkes. L‘effet d’une diminution de grandeur de particules consiste dans I’aug- mentation de la quantitk de pentosane soluble dans des conditions bien d&termin&s. Les concentrations optimales en soude caustique h 0, 20, 40, 60 et 80’ sont environ 2, 4, 7, 10 e t 12% et dans ces dernihres conditions au mobs 80% des pentosanes peuvent dtre extraites rapidement au depart de la farine de bois. Les pentmanes sont plus accessibles en augmentant la concentration en alcali jusqu’ii ces valeurs, mais ultkrieure- ment la quantitk de pentosane accessible d k m i t par une augmentation de concentra- tion en alcali. La diminution d’extractibilitk manifeste la meme dkpendance de la concentration et de la temp6rature que I’augmentation de gonflement de cellulose dans des conditions identiques. L’klimination des pentosanes est donc freinke par un gon- flement exagkrk au sein de la lignine rigide et de la structure fibreuse. Une inhibition semblable mais moins marquee par suite de gonflement se prksente mbme apr&s avoir 61imin6 les lignines. Mbme i faibles concentrations en alcalis oh il n’y a pas d’inhibition h la dissolution par gonflement limitk, ni la dkpendance de la temp6rature ni la dkpend- ance de la concentration en alcali n’est typique d’un processus chimique. Les facteurs limitatifs de la dissolution des pentosanes dans le bois de bouleau sont donc de nature physique et ne con6rment pas le concept d’une liaison entre la lignine et les hydrates de carbone.

Zusammenfassung

Die Wirkung von Teilchengrijsse, Alkalikonzentration, und Temperatur auf die Extraktionsgeschwindigkeit von Pentosan und auf die Menge Pentosan, die aus Birken- holzmehl extrahiert werden kann, wurden untenucht. Im allgemeinen wird eine sehr schnelle Extraktion von erreichbaren Pentosanen von einer langsamen, fast vernach- llssigbaren Diffusion von unerreichbaren Pentosanen gefolgt. Daher besteht fast eine Grenzmenge von Pentosanen, die unter keiner gegebenen Reihe von Bedingungen extrahierbar ist. Die Wirkung der abnehmenden Teilchengriisse besteht in einer Erhijhung der Pentosanmenge, die unter einer gegebenen Reihe von Bedingungen extra- hierbar ist. Die optimalen Natriumhydroxyd-Konzentrationen bei 0, 20, 40, 60 und 80’ waren ungefiihr 2, 4, 7, 10 und 12%, und unter der letzten dieser Bedingungen

448 R. NELSON AND C. SCHUERCH

konnen mindestens 80% der Pentosane schnell aus Holzmehl extrahiert werden. Durch Erhohung der Alkalikonzentration bis zu diesen Niveaus wird Pentosan leichter erreichbar gemacht, aber dariiber hinaus wird die Menge an erreichbarem Pentosan durch ErhShung der Alkalikonzentration herabgesetzt. Die Abnahme der Extra- hierbarkeit hat die gleiche eh ingigkei t von Konzentration und Temperatur wie die Zunahme der Zellulosequellung under den gleichen Bedingungen. Deshalb wird die Pentosanentfernung durch ausserordentliche Quelltng inhibiert, die innerhalb der stamen Lignin- und Faserstruktur beschriinkt ist. Ahnliche aber weniger starke Inhi- bition durch Quellung tritt auch nach Ligninentfernung ad. Sogar unter Bedingungen von niedriger Alkalikonzentration, wo keine Inhibition der Liisung durch beschrlnkte Quellung auftritt, sind weder Temperatur- noch Alkaliabhhgigkeit der Geschwindigkeit typisch fiir einen chemischen Vorgang. Die Faktoren, die die Entfernung von Pentosan inhibieren, sind daher physikalisch, und die Inhibition der Pentosan-Lijsung aus Birken- hob unterstiitzt nicht die Theorie einer Lignin-Kohlenhydrat-Bindung.

Reeeived May 21, 1956