FRACTIONATION AND MOLECULAR MAGNITUDES SYDNEY COPPICK AND EDWIX C. JAHN New York State College of Forestry at Syracuse University, Syracuse, N . Y.
structure by hydrolytic or oxidatke effects on certain groups and by cleavage of chain length; decreased molecular magnitude is the result. Lignin appears to be particularly sensitive to chemical changes which m r g with the method of isolation used, and cellulose is susceptible to degradation of chain length. For example, treatment of cellulose with dilute chlorine lvater or calcium hypochlorite is reported to decrease greatly the degree of polymerization (6) unless the bleaching treatments are carefully controlled as in optimum commercial practice. It is probable that any of the methods for isolating the wood components would also cleave the weak linkages which are postulated by several investigators to exist between lignin and the hemicelluloses. A previous paper ( 2 ) showed that nitrated wood is almost completely soluble in acetone and other simple solvents, and that the nitrated wood lends itself to fractionation on the basis of solubilities in acetone and water. This offers a new
HE exact nature of the chemical components of wood and their relation to one another as they exist in the woody tissue is unknown. F o o d is not a uniform mixture of cellulose, hemicelluloses, and lignin, since these substances are distributed in varying proportions in the different morphological parts of the tissue. Physical means of separating the cell wall components, such as extraction or solution and fractionation, are not possible. Chemical methods of isolating any of the constituents of 15-ood are all drastic, and i t is apparent that they result in isolated compounds which are altered from their original condition in the tissue. Chemical changes may be brought about in the
T
0 The photograph shows molding powders, films, lacquers, molded products, lacquer-finished articlea, eto., prepared from nitrated sawdust.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
approach to the study of the chemical relation of the woody constituents to one another and to the study of the molecular magnitude of the components and chemical complexesin wood.
2.(
d
MATERIAL AND METHODS
Various woody tissues were nitrated as previously described (2). These included groundwood, mechanical pulp, raw sulfate pulps, and bleached sulfate pulps. For viscosity studies the nitrated products were purified by hot water extraction, alcohol extraction, reprecipitation from acetone with water, or bleaching with calcium hypochlorite or sodium chlorite (7). Viscosity was determined in values designated as “effective viscosities”. This term indicates the viscosity at a given concentration, neglecting the small amount of material insoluble in the solvent. The nitrated products were fractionated by a combination of solution and precipitation methods (3) as shown in Figure 6. Chlorine consumptions during bleaching were measured by titrating the excess bleach with standard sodium thiosulfate. The “brightness” or light reflectance for blue light of the various products were compared on the Higgins reflection meter (8). The ester nitrogen analysis was conducted by‘the Schults-Tieman method (1). VISCOSITY AND MOLECULAR MAGNITUDE
The viscosities of the nitrated lignocelluloses examined do not differ greatly (Figure l), despite the fact that considerable amounts of lignin have been removed in the case of the very raw sulfate pulps. On the basis of Staudinger’s theory (4),
I.?
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Y
!1.c I-
I W
V
-
z
c8 0.5 v)
?!! >
0
0.5
1.0
1.5
CONCENTRATION
2.0
8.4
3
I N GRAMS PER LITER
Figure 1. Relation of Viscosity to Concentration in Acetone Solution for Nitrated Lignocelluloses
Equation 1may be written: when qsp. = specific viscosity of a long-chain polymer cp. d. *. = concentration, primary moles/liter K , = a constant, M = molecular weight of long chain
D. P. = limit c
+o
[y]l/lOK,
where D. P. = degree of polymerization c = concentration of polymer, grams/100 cc.
If the constant K , = 10 X lo-’ is substitutedl, Equation 2 becomes: Because of its solubility in acetone and other simple organic solvents, nitrated wood provides a new approach to the chemistry of woody constituents in s i t u . Nitrated spruce and white pine contain considerable amounts of ligneous material; most of it may be removed by alcohol extraction to yield a product which appears to contain 5 per cent lignin and has a high viscosity. The average molecular size of nitrated spruce and white pine woods is equivalent to that of the nitrate from moderately bleached cotton linters. However, the distribution of molecular sizes seem to be much wider than that of nitrated cotton linters or pulp. Evidence points to the existence of large amounts of material in wood, having exceptionally large molecular size. I t appears that nitration does not reduce the chain length of the cellulose in the wood to the extent that cooking and bleaching processes for isolating cellulose do. Evidence is obtained for the existence in nitrated wood of easily severed linkages; the result, upon fission, is a decrease in viscosity. The behavior of nitrated wood to bleaching and to hot water and hot alcohol extraction suggests that these are lignin-carbohydrate linkages. However, pending further evidence, the possibility that these easily cleaved linkages connect other residues or are of some other type, such as ester cross linkages or glucosidic linkages connecting furanose rings in hemicelluloses, cannot be excluded.
D. P. = limit c 4 0
p?]
(3)
From which M = D. P. X m, where m is the weight of the repeating unit. Similarly, Kraemer’s intrinsic viscosity, [q] =
limit
=
limit c+o
PI]
where ultracentrifuge measurements have calibrated the viscosity equation such that D. P. = 270 [TI. Since nitrated wood is not a purified chemical compound, the above theory cannot be applied to its viscosity in acetone solutions without the following restriction: The molecular weight calculated will not be the average molecular weight of the compounds in solution, but will represent that average molecular weight of purified cellulose nitrate which will give the same viscosity relation-i. e., the “equivalent molecular weight”. By plotting the reduced viscosity (qssp./c) against c (concentration in grams per 100 cc.) and extrapolating the straight line obtained to infinite dilution, the value of the function, limit is determined. These values for the different materials investigated, as well as the calculated values for equivalent degrees of polymerization and equivalent molecular weights, are shown in Table I. The equivalent molecular weights for the different nitrated lignocelluloses are of the same order of magnitude.
-, py],
3 More recent work by Staudinger a n d co-workers (6) gives a Km value of 11 X 10-4 for cellulose nitrate. However, since the exact value of R m is still questioned, t h e original value is used here a n d is satisfaotory for comparative purposes.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 35, No. 8
TABLEI. EQCIVALENT MOLECULAR SIZEOF CRUDENITRATED WOOD Lignin Content, Nitrated Material Eastern spruce mechanical pulp Eastern sprnce sawdust Eastern white pine sulfate Pulp Eastern white pine sulfate Pulp
%
27.1 27.6
Limit C+o
[?I 6.2 4.0
Equivalent Degree of Polymerization, D. P. Staudinger Kraemer 620 400
1670
1080
Nitrogen, % ’
NOS, %
Wt. of ~~~~~~i~~ Unit, rn
Equivalent. Mol. Wt., A4 Staudinger Kraemer
10.0 10.3
32.8 33.8
238 242
147,500 96,700
398.000 262,000
23.74
5.8
580
1560
11.6
37.9
258
149,000
4 08,000
16.68
6.1
610
1640
12.1
39.7
266
162,000
4 38,onn
These values are in the same range as those obtained by Staudinger (4, 6) for nitrated bleached cotton linters. The values he found for the nitrates from slightly bleached, moderately bleached, and strongly bleached cotton linters mere 443,000, 176,000, and 81,000, respectively. Y e will show later that the crude nitrated wood contains considerable amounts of low-molecular-size material (probably oxidation products of lignin in the main); this is also evident from the relatively high per cent hot water solubilities, permanganate numbers, and losses on bleaching previously reported (2). However, the nitrated wood has an average equivalent molecular weight comparable with that of bleached cotton linters. It appears, then, that nitration does not reduce the chain length of the cellulose in the wood to the extent that cooking and bleaching processes for isolating cellulose do; also, nitrated lignocelluloses seem to have a much wider distribution of molecular sizes than do nitrated cotton linters, and must therefore contain larger quantities of material of relatively high molecular size than do bleached cotton linters. The results indicate that little difference in yield or degree of esterification is obtained by varying the particle size of the spruce sawdust nitrated. Viscosities are fairly constant, but some abnormal values are obtained for the very fine mesh sizes. Extracting and bleaching the nitrated wood result in higher nitrogen content and viscosity, which was t o be expected since previous evidence indicated that the extractives are composed of material of small molecular dimensions (2). Precipitation of acetone solutions of nitrated spruce wood in water and then bleaching shorn that a fairly good yield of very white material can be obtained. The bleach con-
sumptions are low. Precipitation from acetone followed by bleaching or by stabilization by boiling in water results in products of much higher viscosities than the original crude material; this again confirms the previous data that a considerable low-molecular-weight fraction is removed. Although purification of nitrated wood by precipitation from acetone followed by bleaching gives a bright white material, bleaching alone fails to remove all color; the products show a shade of yellow. For example, a consumption of 11.3 per cent chlorine resulted in a brightness of 22.5, and a consumption of 22.6 per cent chlorine gave a product with a brightness of 38.5. Similar results were obtained by bleaching nitrated spruce wood which had previously been stabilized by hot water extraction. The per cent of original wood lost by bleaching the crude wood nitrate increases with increased bleach consumption; there is about 6 per cent loss with 2.8 per cent chlorine consumed and 10 per cent loss n i t h 22.6 per cent chlorine consumed (Figure 4). However, by varying the chlorine consumed in bleaching the stabilized wood nitrate from 6.0 to 22.7 per cent, no significant difference in yield was obtained; the bleaching loss was about 3 per cent, in all cases. The viscosity of the nitrated products uniformly decreases in all cases with increased chlorine consumption within the limits examined-namely, 0 to 23 per cent chlorine consumed (Figures 2 and 3). Extraction with hot w t e r does not affect the viscosity of the crude nitrate appreciably, but lowers the viscosity of the bleached nitrate somewhat (Figures 2 and 4), presumably due t o some hydrolysis of the oxidized product. But extraction with alcohol increases the viscosity of both the crude and bleached nitrate t o a considerable extent (Figures 2,3, and 4). The alcohol-ex t r a c t e d residues are also susceptible t o the hydrolytic action of boiling water, sincc their viscosities are less after hot water extraction (Figure 2). The action of alcohol extraction in removing 1o w-m o 1e c u 1a r weight material is again demonstrated in the case of nitrated spruce T.T ood which u a s subjected to EXTRACTION MEDl s t a b i l i z i n g in boiling water prior to bleaching. Here again the v i s c o s i t i e s are higher (Figure 3). 5 IO I5 20 2: 5 These phenomena are ?6 C H L O R I N E C O N S U M E D summarized in Figure 4; it is apparent that, reFigure 2. Changes i n Viscosity of Nitrated Figure 3. Changes in Viscosity of Stabigardless of the order Spruce Wood Caused by Bleaching and lized Nitrated Spruce Caused by BleachExtraction ing and Alcohol Extraction followed in purification Viscosity a t 15’ C. i n 1 alcohol-2 benzene-7.5 acetone, 2.85 grams per liter of crude wood nitrate
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August, 1943
P -
ALCOH
f 0
>
c
901
PURIFICATION OF WOOD
NITRATED
’ 5
TlON
1s LOSS
IN
80 YIELD,
25
46
30 OF
35
ORIGINAL
40
J
WOOD
Figure 4.. Relation of Viscosity of Nitrated Spruce Wood with Loss in Yield by Purification Treatments
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the viscosity remains constant at a value equivalent to that of a high alpha-cellulose nitrate pulp (Figure 5). This initial decrease in viscosity with increased bleach consumption, followed by a leveling off a t such an appreciable viscosity, is significant and would appear to be due t o the breaking of weaker linkages than the normal glucosidic linkages in cellulose chains, perhaps lignin-carbohydrate linkages. An analysis of the data supports this conclusion. The conditions of bleaching (short time, very weak acid medium in the case of sodium chlorite) are such that the cellulose chains should undergo very little degradation. It has been shown that well controlled hypochlorite bleaching causes no serious degradation (6) and that weakly acid sodium chlorite solutions have no effect on the viscosity of cellulose ( 7 ) . Hence, it must be concluded that the bleaching treatments employed have, a t most, only a slight degradative effect on the cellulose chains, but that bleaching cleaves some of the more easily split linkages in large molecules and brings about a drop in viscosity which remains constant once these easily broken bonds are severed.
by bleaching, stabilizing, and alcohol extracting, approximately the same values for viscosity and yield are obtained. Since hot water extraction appears t o remove considerable material of low molecular weight and yet decreases the visTABLE 11. DELIO-NIFICATION OF ALCOHOL-EXTRACTED NITRSTED cosity, it must also cause some hydrolysis of that portion of SPRUCE WOOD the nitrate which constitutes the high-molecular-weight Loes Effective fraction. On the contrary, alcohol extraction apparently during Sp. Viscosity Treatment, (Calcd. from removes material of small molecular sizes without appreciable Treatment % 2 . 8 5 G./L.) action on the long chains. Bleaching gives a product of None 0 10.3 lower molecular size, owing either to a degradative action Alcohol-benzene and water extns. 2.74 8.90 on the long chains of the nitrated polysaccharides or to the 10 chlori,nations and NaBOs digestions 4.92 2.6 decomposition of a possible lignin-carbohydrate compound. 10 chlorinations and cold Alcohol extraction removes most of the ligneous material dil. NHdOH extns. 2.22 1.45 6 chlorinations and hot dil. from the crude nitrated wood, and the amount remaining in NHrOH digestions 6.47, 0.2 the extracted residue is evidently low, since delignification of the residues by the Cross and Bevan method removes only 5-6 per cent of material. Furthermore, the alcohol-benzene The behavior of nitrated wood to hot alcohol and hot and water extractions prior to chlorination remove 2-3 per water extraction lends further support to the above probcent of material, so that the amount of lignin in the alcoholability (Figure 2). Although hot water extraction removes extracted nitrates may be as low as 2-3 per cent. Degradaover 20 per cent of low-molecular-size material, it does not tion of chain length by the chlorination procedures is severe, raise the viscosity of the residue; in fact, the latter may even as indicated by the large decrease in viscosity (Table 11). R e m o v a l by bleaching of the small amount of 12r I 1 ligneous material t present in the alcohol-extracted IO Q nitrated spruce HYPOC H L O R I T C SODIUM C H L O R I T E wood is accomELE A C H l N G panied by a decided drop i n viscosity. However, on stronger n b l e a c h i n g with w either calcium LL w hypochlorite a t I 10 20 30 40 SO ) O 2 4 6 8 10 12 pH 11.3 for 0.5 CHLORINE CONSUMED .b SODIUM CHLORITE C O N S U M E D Y to 3 hours or with sodium Figure 5. Effect of Bleaching with Calcium Hypochlorite and Sodium Chlorite on the Viscosity chlorite a t pH of Alcohol-Extracted Nitrated Spruce Wood 4.5 for 1 hour, Viscosity at 1 5 O C. in 1 alcohol-2 benzene7.5 acetone, 2.85 grams per liter
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voi. 35, N ~ a.
OF KITRATED EASTERN SPRUCEMECHANIXLPULP FIGURE 6. FRACTIONATION
and gave nearly colorless solutions. The methods of fractionation, as well as the yields and effective specific viscosities of the fractions, are shown in Figure 6, The alcohol extracts of the nitrated wood contain materials which tion; 35. SY,,1 0 . 5 qsp. r P p t d . wit,h water; 5.1’%, Crude nitratehave little effect on the viscosity of 0 . 0 2 %p. the solvent and are evidently of -Pptd. on concentrating; 2.55 Tap. low molecular weight. However, Remaining in final s o h . Alcohol-sol -. 4.6%, 0.005~ e p . -Residue on evaporating the alcohol-extracted nitrated mood to dryness; 10.5%, behaves as a cellulose derivative in 0.005 qsp. that fractionation results in a series of products with a wide range in viscosity. A plot of the per cent yields of the fractions drop slightly. This is significant in that hot water stabilizaagainst their effective viscosities would lead to the conclution of cellulose nitrates causes no appreciable degradation. sion that the distribution of molecular sizes in nitrated wood On the other hand, hot alcohol extraction of wood nitrate, is best represented by two curves, each component being a which removes about the same amount of low-molecularseparate colony. weight material as does hot water, greatly increases the Apparently nitrated wood is composed of two different viscosity of the residue. Both the alcohol-extracted and the types or groups of materials. One type is of large average bleached residues are still somewhat susceptible to the molecular size comparable to nitrated cotton linters and is hydrolytic action of hot water. Apparently the very mild probably mainly carbohydrate in nature. This group has a hydrolytic effect of hot water is sufficient to bring about the wide distribution of molecular weights. The second type cleavage of some large-molecular-weight material. The has a narrow distribution of molecular sizes which are not viscosities of the alcohol-extracted nitrated spruce mood and in the form of long-chain molecules. This group is probably mechanical pulp are much higher than that of the highest a mixture, for the most part, of various degradation and grade of cellulose pulp examined (Table 111). The const,ant nitration products of lignin and other noncellulosic materials. viscosity which alcohol-extracted nitrated spruce maintains on excessive bleaching is equivalent to that of a cellulose LITERATURE CITED nitrate prepared from pulp having a cuprammonium viscosity of about 20 centipoises, which is a high-grade alpha-cellulose (1) Doree, Charles, “Methods of Cellulose Chemistry”, p. 231, New York, D. Van Xostrand Co., 1933. Pulp (8). T h e first number after the fraction is the per cent yield of the crude nitrate fractionated; the second number is the specific viscosit,y calculated from the effective viscosity in 1 alcohol-2 benzene-7.5 acetone solvent a t 15’ C. which is 2 . 8 5 grams per liter. rAcetone-insol.; 0.91% McohoI-inso1.I rPptd. with alcohol; LAcetone-sol. 1 4 . 0 % , 18.Oq.p.
~
I
TABLE 111. VISCOSITIESOF CELLULOSE AND WOOD KITRATW CUPRAMRIONIUM AKD I N
Material Nitrated Bleached kraft Bleached kraft Bleached k r a f t Bleached k r a f t Bleached k r a f t Eastern apruce mechanical Pulp Eastern spruce sawdust
1 ALCOHOL-2
SOLVENT
IN
BENZENE-^.^ ACT.TONE
Pulp VisoosityQ in Cuprammonium (8), Centipoises 5.07
Effective
Sp.Viscosity (Calcd. from
2.85 G./L.)
11.83 15.6 17.74
28.0
... I . .
1.67
2.98 4.39 4.15
7.38 10.86
(2) Jahn, E. C., and Coppick, Sydney, IND. E m . CHEM.,33, 678-82 (1941). (3) Mark, H., Tech. Assoc. of Pulp and Paper Ind., New York, Feb. 17, 1941. (4) Staudinger, H., Trans. Faraday Soc., 29, 18-32 (1933). (5) Staudinger, H., John, R., Haas, H., and Feuerstein, K., Be?., 70B, 2296-309 (1937). (6) Staudinger, H., and Jurisch, J., Papiei-Fabr., 35, Tech. TI. 462-9 (1937). (7) Taylor, &I. C., White, J. F., and Vincent, G. P., Tech. Assoc. Papers, 23, 251-6 (1940). (8) Tech. Assoc. of Pulp and Paper Ind., Testing Methods-Recommended Practices-Specifications, T442 m-40, Official Standard, Jan. 15, 1940; Van den Akker, J. d.,Tech. Assoc. Papers, 23, 480-9; Paper Trade J., 111, 142-51 (1940).
10.3
a.Determined by C. P. Donofrio, New York S t a t e College of Forestry.
FRACTIONATION O F NITRATED WOOD
Fractionation of alcohol-extracted nitrated spruce mechanical pulp by slow precipitation from acetone m-ith alcohol and water indicates that this material contains a much greater proportion of large molecules than does nitrated alpha-cellulose sulfate pulp. Only about 5 per cent of the alcohol-extracted nitrated wood is composed of the shorter chains remaining in solution (3) after the second precipitation, whereas these shorter chains are present in the nitrated cellulose to the extent of about 80 per cent. Viscosities of these fractions are indicative of their relative average chain lengths. The total crude nitrated spruce groundwood was separated into eight distinct fractions. Very slow precipitation was obtained for some of the fractions; Mark (3) showed that this technique yields fractions of more uniform and narrower range of chain length. The products obtained from the alcohol extracts were orange or brown powders which gave dark solution. The fractions precipitated from the acetone solution of the alcohol-extracted residues were light in color
Hauling Old Growth Spruce
