It. L. M IT C H E L L
R a y o n ie r I n c o r p o r a t e d , S h e l t o n , W a sh .
T h e cellu losic con stitu en ts o f wood have been separated as com pletely soluble nitrates in a less degraded state than has heretofore been accom plish ed. T he isolated cellulose nitrates have been fraction ated as an in d ica tion o f the distribution o f chain lengths that m ay exist in th e original wood. T h e m eth od s used fo r nitration o f w ood, for fra c
tionation o f the cellu losic com p on en ts, and for ca lcu la tion o f the degree o f polym erization are described in detail.
Graphs are presented to show the relative p u rifying and degrading a ction s exerted o n the w ood cellulose by tlie m ajor steps o f pulp m a n u fa ctu re and viscose p rodu ction .
I
N THE process of making “ dissolving pulps” from wood, it is often desirable to know the length and uniformity in length of the cellulose chains which constitute the cellulosic portion of a particular wood. In many research problems it is im
portant to study the chain length of the cellulose as it exists in the wood, as it is changed in the various phases of pulp manufac
ture, and as it is modified in processing industries such as the pro
duction of rayon.
Viscometric measurements offer one of the most convenient means known for obtaining useful information on the relative chain lengths of celluloses. However, reliable viscosity meas
urements are possible only when the cellulosic material is pure. Separating pure cellulose from wood in an undegraded state is difficult. Even in the preparation of holocellulose, from which lignin is removed by a treatment far less drastic than either the kraft or sulfite pulping processes, some degradation undoubt
edly takes place.
Wood nitration has been studied with the object of separating cellulose in the form of the nitrate. Jahn and Coppick {IS) and Coppick (5), in nitrating wood with a nitrating mixture contain
ing sulfuric acid, obtained high yields of soluble wood nitrates;
the cellulosic component could readily be separated from the nitrates but it probably existed in a considerably degraded state1, according to Berl and Rueff {2), Staudingor and Mohr {15), and our data (Table I). Friese, Furst, and Ludecke (7, 8) and Hil- pert, Kruger, and Heckler {10) nitrated wood with strong nitric acid and with nitration mixtures containing either phosphoric anhydride or acetic anhydride; but they reported that the products obtained were only slightly soluble. Graldn and R&nby (5) nitrated wood with a phosphoric anhydride nitrating mixture and obtained a product which was only 40% soluble in acetone.
A nitration procedure has been developed which offers a means for separating the nitrated cellulosic constituents of wood in a completely soluble and, at the same time, a less degraded state than has heretofore been possible. The principle is to nitrate the cellulose instantaneously and completely prior to elimina
tion of the lignin component; then, by continued action of the nitrating mixture. With this mixture the cellulosic constituents of the wood are nitrated rapidly, then become comparatively stable toward degradation during further prolonged action of the nitrating acid. With this mixture also the ligneous constituents of the wood arc slowly modified by the nitrating acid so that, after a suitable time, they can be removed effectively by washing with methanol or by boiling with water.
N I T R A T I O N P R O C E D U R E
Pr e p a r a t i o n o f Wo o d. A selected block of wood (1 X 1 X
1.5 inches, grain long way), which had never been allowed to dry, was cut on a microtome along the grain into thin (40-mlcron) shavings, 1 X 1.5 inches in area {18). The wet shavings were extracted successively at room temperature with water, ethyl alcohol, and benzene, and then dried from benzene at 50° C..
in a dry atmosphere.
Other forms of wood, such as ground wood or sawdust, may be used. Extraction of the undricd wood to preserve an open struc
ture is desirable when a highly uniform product is needed for chain length studies.
Pr e p a r a t i o n o f Ni t r a t i n g Mi x t u r e. For a standard batch of nitrating acid, 1000 grams of 90% nitric acid were weighed into a 2-liter flask and placed in an ice-cold bath. A 404-gram portion of phosphoric anhydride was added slowly while the acid was cooled and swirled. This gave a mixture with the following com
position; 64% H N 0 3, 26% H3PO4, and 10% P20 6 (Table II). swirled vigorously during addition to prevent the formatidn of oxides of nitrogen. It was kept cool and shaken until solution with P20 6 was complete; then it was filtered through glass wool.
The acid was stored in a dark cool place and used within 2 or 3 quickly and stirred into the mixture. The nitration was allowed to proceed-for approximately 24 hours at 20.0° C. At the end of the reaction period the nitrated shavings were centrifuged to free them from excess acid and were drowned in a large volume of cold water. A centrifuge basket and drain made of special high-chromium low-nickel alloy were used in centrifuging the acid mixture.
The shavings turned dark brown when they were first added to the nitrating acid, but after a few hours their color began to lighten noticeably. After about 24 hours they were not much darker than the original wood shavings. The nitration time should be varied to give optimum solubility for various woods.
843
Figure 1. Types of Nonbirefringent Ligneous Skinlike Residues from Nitrated Wood Fibers after Extraction with Ethyl Acetate
(X 100)
A . T h i n u n r u p t u r c d s e c t i o n o f l i g n e o u s s h e a t h ( f i b e r - t i p ) B . S h e a t h b r o k e n i n e x p a n d e d s p i r a l p a t t e r n
C . F r a g m e n t e d s h e a t h
R E M O V A L O F L I G N I N R E S I D U E S
The wood nitrate still retained the greater part of its original lignin content when it was removed from the. nitrating acid. To remove this lignin, the centri
fuged shavings were thoroughly washed with cold water over a period of several days and then boiled with repeated changes of water until no more colored matter was extracted. The shavings were washed with methyl alcohol and dried at 50° C. in a dry atmosphere.
Loss of lignin occurred not in the cold washing but during boiling. Ligneous material could be removed with methyl alcohol after the cold wash, but boiling was still considered desirable to improve the stability of the nitrate. Ligneous material recovered by evapora
tion of the extraction liquids was dark brown and quite brittle and, when fired, gave little flash and much smoke. The extracted shavings were light yellow- brown; they retained their original form but were easily broken down into single fibers. The wood nitrate contained about 13.8% nitrogen and was 9 9 + % soluble in ethyl acetate; when fired it gave a bright flash with no smoke or residue. It was quite stable; samples showed no sign of decomposition after several years of storage in the dry state. The yield depended on the type of wood but was in the neighborhood of 120 to 130% as compared with about 180% for pure cellulose.
(This indirectly permits calculation of the cellulosic content of the wood, which for western hemlock is about 70%.)
If further purification was desired, the boiled wood nitrate could be bleached to a very light color by use of sodium chlorite, chlorine, or sodium hypochlorite alternated with mild alkaline extractions. These treatments, however, resulted in some additional degradation.
F R A C T I O N A T I O N O F C E L L U L O S E N I T R A T E
According to a scheme similar to that suggested by Schieber (14) for the fractionation of cellulose nitrate, the nitrated wood cellulose was separated into a number of fractions by stepwise solution with successively richer solvent mixtures. The series of solvent mixtures was prepared by mixing 95%
ethyl alcohol and 100% ethyl acetate in the following proportions by volume:
9 5 % 1 0 0 % 9 5 % 1 0 0 %
S o lv e n t E th y l E t h y l S o lv e n t E t h y l E th y ! M ix t. A lc o h o l A c e t a t e M ix t . A lc o h o l e e ta te
I 1 0 0 % 0 % V I I 3 7 % 0 3 ' o
r i 70 3 0 V I I I 35 05
h i 60 40 I X 33 67
IV 50 50 X 31 69
v 45 55 X I 29 7 1 , e tc.
V I 4 0 6 0
A 10-gram sample of wood nitrate was weighed and placed in a ground-glass-stoppcred bottle of about SOO-ml. capacity. A 400-ml. portion of sol
vent mixture I was added to the nitrate and allowed to stand overnight. The mixture was then poured off through a 200-mcsh nickel screen and replaced with a 100-ml. portion of solvent mixture I. After standing for a few hours, the mixture was again poured off. The nitrate was
A u g u s t , 1 9 4 6 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 8 4 5
Occasional gentle shaking or swirling was desirable soon after addition of fresh solvent; too much shaking, however, had to be avoided as it promoted fiber breakdown and gelatinization.
When the solvent was poured off, the nitrate was pressed against the screen with a flattened stirring rod and could be squeezed to free it from excess liquid.
The nitrate from which the first fraction had been extracted was then returned to the bottle and similarly extracted with suc
cessively richer solvent mixtures to obtain further fractions.
For high viscosity fractions it was often desirable to increase the amount of solvent used in the extraction above the 400 ml.
specified for early fractions; the purpose was to have sufficient solvent to avoid a highly viscous extract and thereby help effect a clean-cut separation.
The dissolved nitrate was recovered by evaporating the sol
vent from the combined filtrates containing the particular ex
tracted nitrate fraction and was weighed. It often happened that a few cellulose nitrate fragments passed the screen; it was important that these be removed before the fraction was re
covered. This was accomplished by allowing the solution to stand for a few hours and taking an aliquot from the clear super
natant solution. Instead of this procedure, it was possible to centrifuge or filter the solutions. Evaporation was carried out on a hot water bath and fractions were washed into weighing bot
tles just prior to dryness. At the end of the evaporation, prefer
ably when the nitrate was just dry, a small quantity of water was added to help remove the last traces of solvent and to loosen the film from the weighing bottle. The high viscosity fractions gave films that were sparkling clear and exceedingly tough. The fractions were allowed to stand over P20 6 before weighing.
Solutions were made of each
ments were taken. Because the viscosity of the nitrate solutions (especially the more dilute ones) showed a slight tendency to decrease with age, it was desirable to effect rapid solution of the nitrate and measure viscosities without delay, to avoid standing periods of more than 24 hours even for 1% solutions.
Viscosities were determined at 20° =*= 0.10 C. in Cannon-Fenske (4) type viscometers at concentrations of 0.1, 0.05, and 0.025 gram per 100 ml. of solvent. Most of the viscosity measure
ments were originally made with a Cannon-Fenske routine type viscometer which had an efflux time of 26.4 seconds (8.8 second per ml.) for ethyl acetate and therefore required a rather high kinetic energy correction. Using cellulose nitrates covering a wide range of specific viscosities, this viscometer was later cali
brated against a Cannon-Fenske viscometer for nonviscous liq
uids which had an efflux time of 210 sec./ml. for ethyl acetate.
It was found that the relation between specific viscosities for the two viscometers was linear, and that corrected specific viscosities for the fast viscometer could be obtained by multiplying the original specific viscosity by 1.31.
C A L C U L A T I O N O F D E G R E E O F P O L Y M E R I Z A T I O N
The following formula gives values for D P which are consistent with the experimental viscosity and concentration data over the whole range of values shown for cotton, wood, wood pulp, and
concentration of cellulose nitrate, g./lOO ml. solvent 100
0.35
The value 0.35 for k ' was determined according to Huggins {11) by plotting i},P/c vs. vp for selected cellulose nitrates, covering a wide range in i/, at concentrations of 0.025, 0.05, and 0.10 g ./
.100 ml. ethyl acetate. This family of curves was found to be a
series of straight lines having a ratio of slope to intercept falling with no apparent order within the range 0.335 to 0.365.
The value 100 for K has been used because it yields D P values of the same order of magnitude as D P values calculated by Staud-Figurc 2. Effect of Nitration Time on Solubility of Nitrated Wood Fibers in Ethyl Acetate
A B C
N i t r a t i o n t i m a a t 2 0 * C . i n 6 4 - 2 6 - 1 0 m i x t u r e , h o a r s 1 6 2 4
S o l u b i l i t y i n e t h y l a o e t a t e , % 5 30 99 . f
S o l u b i l i t y i n E . A . a f t e r d e l i g n i f i o a t i o n , % 9 9 + 100 100
D ir e c t Illu m in a t io n C r o e s e d N ic o l a
inger; his formula uses Am = 11 X 10” 4 from viscosities obtained at suitably low concentrations. Formula 1 is preferred to that of Staudinger, mainly because it eliminates the concentration ef
fect.
D I S C U S S I O N O F M E T H O D
Iii the wood, particularly at fiber surfaces, lignin seems to be associated with cellulose through some type of cellulose-lignin linkage at hydroxyl groups. These linkages seem to be dis
rupted by the action of nitrating acid in the substitution of the hydroxyl group by the nitrate group without appreciable cleavage of the main cellulose chain, provided the nitration mixture is capable of quickly substituting all available positions.
For chain length studies on wood, the nitration method em
ploying phosphoric anhydride-nitric acid was found to be by far the most successful (Tables I, II, and III). The ability of mix
tures similar to 64% H N 0 3-2 6 % H3P 0 1-1 0 % PjCL to give a high degree of nitration is substantiated by the data of Bouchonnet, Trombe, and Petitipas (8) and of Friedrich and Dubien (6).
The work of Berl and Rueff (2) and of Staudinger and Mohr (IB) further indicates that degradation is comparatively slight in the nitration of cellulose with such mixtures.
The use of acetic anhydride was found to result in a highly ni
trated cellulose with, possibly, slightly less degradation than re
sulted from the use of phosphoric anhydride; but acetic anhy
dride caused relatively little destruction of lignin (Table I).
The nitrated wood product had low solubility in ethyl acetate or acetone.
When sulfuric acid was used as the dehydrating agent, a solu
ble wood nitrate was obtained in a relatively short time but the cellulose component was severely degraded (Table I) and con
tained only about 12% nitrogen. The degrading action of sul
furic acid nitration mixtures may be due to the fact that all available positions along the cellulose chain are not rapidly ni
trated and protected; therefore, oxidation and cleavage may possibly occur rather easily at points along the chain where cellulose-lignin linkages are broken without accompanying in
stantaneous esterification. In other words, a partially nitrated cellulose is probably susceptible to attack at positions that are not substituted, particularly at early stages of the nitration reac
tion.
In previous work (7-10) on the nitration of wood with mixtures containing phosphoric anhydride, the very poor solubility of wood nitrate was undoubtedly due to the presence of residual
lignin which had been insufficiently attacked to ensure effective removal.
The successful performance of. the present nitrating method using phosphoric anhydride is due to: (a) the proper preparation of the wood to give a structure that can bo quickly and com
pletely penetrated, (6) the use of a suitable nitrating acid com
position that gives a high degree of nitration, and (c) the use of a long nitration time. Factors (a) and (b) ensufe rapid and rela
tively complete nitration of the cellulosic constituents with a minimum of degradation. Factor (c) ensures good solubility by facilitating subsequent' lignin removal. Unless the lignin is modified so that it can be largely removed, a wood nitrate cannot be dissolved even though nitration of the cellulosic constituents is relatively complete. Although ligneous residues on the sur
face of the fibers permit passage of the solvent into the fibers, they effectively retard the passage of the highly swollen cellulose nitrate out through the skinlike casing which envelopes the fiber.
Increase in nitration time reduces the effectiveness of the skin in barring outward passage of the cellulose nitrate, but the skin may still exert a slight hindrance even at optimum time. Suit
able nitration time must be determined experimentally for each particular wood.
Figure 4. Swelling of Nitrated Wood Fibers by Increasingly Rich Solvent Mixtures (X 100)
A B C
S o l v e n t m i x t u r e , %
E t h y l a l c o h o l 50 3 0 0
E t h y l a c e t a t e 5 0 7 0 100 P e r c e n t d i s s o l v e d 3 0 7 0 9 9 +
The small amount of lignin remaining in the wood nitrate after stabilization seems to exist largely on the fiber surfaces. Whereas the greater part of the lignin is readily removed with the hemicellu- lose fraction during fractionation, a small part seems to be firmly associated with the highly oriented cellulose in the primary wall; in many instances it remains intact • throughout the fractionation and tends to preserve the fiber form even after all the cellulose nitrate has been extracted. These skin
like ligneous residues remaining at the end of the fractionation have the appearance under the micro
scope of empty distended sausage casings, spirally fractured (Figures 1 to 4).
Bleaching of wood nitrate {12) completely re
moves residual lignin in the primary wall but initially lowers the viscosity (Table III); a second bleaching treatment causes no further lowering. This is probably due to the presence of a few unnitrated cellulose-lignin linkages in the highly oriented por
tions of the fiber; such linkages are subject to oxidation by the bleaching agent immediately after removal of the lignin by bleaching. A nitrated pure cellulose with all hydroxyl positions substituted appears to be almost completely immune to the action of bleach
ing liquor (Table III).
Bleaching with hypochlorite effectively eliminates insolubility due to skin substance but seems to im
part a slight haze to solutions of the nitrate; it ap
parently makes some small portions of the gamma- cellulose fraction insoluble. The haziness is pos
sibly due to certain pentosans which apparently are completely soluble only when they are unbleached, or are held undispersed in the undissolved portion of the unbleached primary wall.
It is probably better to nitrate wood for a long time until a substantially lignin-free and soluble product is obtained than to nitrate for a shorter time and rely on bleaching for removal of large quanti
ties of lignin. In spite of the fact, that the lignin- cellulose linkages are probably broken at an early stage of nitration, a prolonged action of the acid is needed to solubilize the lignin. Apparently lignin cannot be removed if it is attacked only mildly, and serves mechanically to prevent solution of the cellu
lose nitrate fiber. If nitration is carried to the optimum point, the amount of unremovable lignin is small and the insoluble portion of the nitrate remaining as skin fragments amounts to less than 1%. This small amount should not impair the ac
curacy of chain length distribution curves.
The fractional solution method does not give absolute chain length distributions. There is con
siderable difference of opinion as to the relative merits of solution and precipitation methods of fractionation; each has advantages and disad
vantages. Admittedly, no existing method of frac
tionation is capable of giving absolutely clean frac
tions; each fraction in itself represents an average in chain length. However, the described solution technique appears to give fairly complete resolution of fractions with good reproducibility and is useful for comparative purposes.
The method of solubilizing wood with a phosphoric anhydride-nitric acid mixture is primarily of interest because it produces a soluble, relatively undegraded cellulose nitrate; but it may also be important as a means for isolating nitrated lignins. The
CELLULOSENITRATEDISSOLVED - CUMULATIVEPERCENT-
CUMULATIVEPERCENT-August, 1946 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 849
S O L V E N T M IXTU RE U SE D
Figure 5. Solubility of Nitrate from a 'Western Hemlock Wood
possibility of separating lignins of relatively high molecular weight and varying degrees of solubility should be of considerable interest.
A P P L I C A T I O N S
The usefulness of this nitration method for evaluating relative chain lengths of cellulose is illustrated in Figures 5, 6, and 7 which show, respectively, solubility, integral chain length dis
tribution, and differential chain length distribution curves for the carbohydrate fraction of a representative sample of western hem
lock. A series of differential chain length distribution curves (Figure 8) shows the changes in cellulose chain length of southern pine which occur during the major steps in sulfite pulp manu
facture and viscose processing. Atchinson (I) obtained distri
bution curves for holoeellulose and for various pulps at certain stages of manufacture. However, he delignified the wood prior
bution curves for holoeellulose and for various pulps at certain stages of manufacture. However, he delignified the wood prior