926
INDUSTRIAL A S D E,VGINEERING CHEMISTRY
1-01. 20,
KO.9
Action of Alkalies on Cellulose' Galo W. Blanco DU
PowT RAYONCOMPANY, BUFFALO, N. Y .
cellulose, and degradation of the cellulose molecule. Mercer (1844) believed in the formation of a chemical compound HE swelling of cellulose in caustic soda parallels the between cellulose and caustic, and this same opinion has been solubility of the easily soluble cellulose present',* and, held by Gladstone,16 Thiele.lGVieweg," Lindemann,'8 Norand others of the more recent inas Collins and Williams2point out, depends on the length man,'g Traube,20 and thickness of the fiber and increases very rapidly with in- vestigators. The views of these investigators have been opcreasing concentration, falling off a t the higher concentra- posed by MillerfZ2J ~ y n e rand , ~ ~L e i g h t ~ nwho , ~ ~ view alkalitions. These authors3 do not fully agree with Heuser4 that cellulose from a colloidal chemistry standpoint as an adthe ability of alkalies to produce swelling is greater with greater sorption phenomenon. conductivity of the alkali. Heusers looks upon the rule on With very few exceptions the work has been done on cotton hydration of ions as a means to explain the decrease in swelling cellulose,* there being little published information on wood in going from lithium to caesium hydroxide and also in con- cellulose. Since the property of hydration exhibited by cellucentrated alkali solutions. Lorens6 holds similar views. lose is largely influenced by the condition or kind of cellulose," It is generally admitted it is necessary to select a uniform product of high that swelling is greatest 1, when the concentration of purity with a low ash and This paper summarizes a portion of the work perthe electrolyte is equivalent low oxidized cellulose conformed during the last eighty years on the action of to the combining capacity tent. This selection is paralkalies on cellulose. The review has been limited of the cellulo~e.~Fausts ticularly desirable when chiefly to the mercerizing effects of caustic soda solufound maximum swelling comparing the behavior of tions on cotton cellulose. Reference is also made to the with 9.5 t o 10 per cent different celluloses, since more recent investigations using other alkalies and NaOH by weight, irrespecthe concentration of sodium cellulose materials. The published data have aimed tive of the temperature. hydroxide a t which quanto show that cellulose and caustic react to form a It appears that during swelltitative formation of alkalichemical compound or that the combination of the ing a contraction of the cellulose takes place, at two is merely a physical one. thread along the long axis which the x-ray diagram takes place which may be 1' I of cellulose disaDDearsrand explained by assuming that a t which maximim swelling the crystallites of the cellulose fibers which lie in rod shape is obtained. is different for different celluloses.2~ Chemists generally agree that an alkali-cellulose is formed. are "liquefied or softened and tend to assume a spherical shape.9 Swelling breaks apart the cellulose crystals and a t The majority have used only chemical means, which today least partially destroys the material which held them together appear inadequate in view of the more intimate knowledge and far-reaching observations gained through x-ray spectrogin their natural state. Hydration may be a condition of intermolecular disten- raphy. The methods of Gladstone and Vieweg have been tion, whereby the surface reactions are largely increased and discussed in the literature and are now being supplemented the absorptive property developed. The cellulose hydrate by x-ray analyses as set forth by Katz,26 Herzog,*' and obtained by absorption of alkalilo is formed in such propor- Clark.2s Of the chemical methods that of Vieweg appears tions which increase with the degree of hydration of the cellu- to be more satisfactory. It has been carefully studied who found the smallest per cent lose. The hydration of cuprammonium cellulose was meas- and modified by ured by Beadle and Stevensll by immersing skeins in sodium error in titration when using 1 part of cellulose and 10 parts hydroxide solutions and weighing after removal of surface liq- of alkali by weight. uor. The weight of alkali determined by titration sub- Effect of Time, Temperature, and Concentration of Alkali tracted from the total increase in weight of the skeins gave The rate of absorption of caustic by cellulose is extremely the hydration figure. rapid with caustic of mercerizing con~entration.~OThe time Bovard12 favors the colloidal theory of hydration, in which cellulose is assumed to possess a sponge-like structure con- of immersion beyond 10 minutes does not appear to influence Dehnert,32 ~ Karrer,33and L i e p a t ~ f have f ~ ~ shown sisting of small particles held together by surface tension or a b ~ o r p t i o n . ~ capillary forces, water forming a continuous phase between that adsorption is completed in a few minutes. Beadle and Stevens1' have shown (Figure 1) that for any the particles. Hydration depends upon the adsorption of given temperature between 5" and 40" C. maximum hydrathe hydroxyl ion by cellulose from caustic solutions. The chemical theory of hydration as it applies to the process tion takes place, these maxima being greater the lower the of beating13is supported by Cross in England and by Schwalbe temperature. The maxima for hydration and sodium hydroxide absorpin Germany. The former suggests a chemical reaction betion take place a t greater concentration as the temperature tween the hydroxyl groupings in cellulose and the water. rises. (Figures 2 and 3) The extent to which these two curves General differ from one another indicates the change in composition Metallic hydroxidesl4 principally caustic alkali, exert on due to absorption. The results indicate that the mothercellulose four possible influences-swelling and hydration, liquor concentration point of maximum hydration for any adsorption, chemical combination of metallic hydroxide with given temperature corresponds somewhat with the point a t which there is osmotic equilibrium-that is, the point a t 1 Presented before Division of Cellulose Chemistry at the 75th Meeting which no difference exists between the composition of the of the American Chemical Society, St. Louis, Mo., April 16 t o 19, 1928. absorbed liquor and the solution which surrounds it. * Numbers in text refer to bibliography a t end of article. Swelling and Hydration
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I-YD CSTRIAL AND ENGINEERING CHEMISTRY
September, 1928
927
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the affinity of cellulose for caustic soda is greater for the higher concentrations of NaOH solutions, with the ultimate formation of (CGH1006)z.NaOH between 16 and 24 per cent NaOH solutions and of (C~HloOs)z(NaOH)z at 40 per cent NaOH has been repeatedly investigated. RassowG6found a steady increase of NaOH absorption with increasing IiaOH concentrations. However, comparing the work of Heuser,3' K ~ r r e r , ~Vieweg,I4 8 and Koenig,I4 it may be concluded that with concentrations of over 16 per cent NaOH the absorption does not in~rease.1~"The absorption corresponds to (C&O&.NaOH containing 12.34 grams NaOH per 100 grams cellulose. Figure 5 indicates that with the lower concentrations of caustic both caustic and water are absorbed rapidly. With the higher concentrations (about 14 per cent) the cellulose continues to take up NaOH somewhat, less rapidly while there is a distinct decrease in the absorption of water.39 Leighbon2*in Figure 6 shows a marked difference in absorption obtained by analyzing the solution and by analyzing the centrifuged cotton.
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The difference in absorption of sodium hydroxide a t difEffect of Neutral Salts ferent temperatures is not so great in cellulose of low solubility in caustic s0lution.~J5 The absorption of caustic by cellulose is increased by the If the temperature of mercerization is kept constant, the addition of sodium salts,4o the carbonate being most proform of the curve obtained (Figure 4) will differ considerably nounced and the sulfate least. Kolthoff obtained no absorpfrom that when the natural rise in temperature takes pla~e.~O tion with sodium carbonate and potassium carbonate up to Hubner and Pope30 h a r e studied the action of hot alkalies half normal. The bicarbonate and phosphate cause a deon cellulose. Other references on this phase are omitted, crease in the absorption. The degree of absorption is not since we are not now concerned with elevated temperatures. affected by sodium chloride or sodium carbonate when using The familiar curve of Vieweg (Figure 4), indicating that caustic concentrations lom-er than 12 per cent.' (Figure 7)
INDC;XTRIAL AND ENGINEERING CHEMISTRY
928
The salt concentration is not affected by the presence of cellulose. Differences of opinion appear to exist as to the exact effect of salts for Hubnere41 Knecht, Ermen, Miller,42 and others have not confirmed the above findings. Liepatoff34 and JoynerZ3studied the effect of potassium salts. The latter concludes that potassium chloride is more powerful than sodium chloride, probably owing to its slightly increas-
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ing the OH concentration and also to its raising the potential of OH ions, or to specific action on the cellulose. Effect of Alcohol
The absorption of sodium hydroxide by cellulose a t the same temperature is much stronger in alcohol solution than in water solution and increases with increase in concentration of alcohol.lJO The flat portion of the absorption curve between 16 and 24 per cent NaOH is obtained using small quantities of alcohol. With the higher alcohol concentrations only the first part of the absorption curve is reached and the flat portion takes place a t higher concentrations of NaOH-i. e., the flat portion is retarded. (Figure 8) The trend of Vieweg's curves (Figures 9 and 10) shows that in the presence of alcohol there is no chemical reaction between cellulose and caustic, and that the NaOH divides itself between cellulose and aqueous alcohol, which may be represented
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proportionality between the amount of alkali taken up and the concentration of the solution. Contrary to Hess, the swelling action of NaOH, although reduced, is not prevented by alcoh01.~' Exothermic Reaction
Barrat and Lewis44 determined the heat of reaction between cotton and NaOH of different concentrations. The curve shows a strong rise with 10 to 15 per cent NaOH, then climbs less and almost on a straight line to 30 per cent NaOH when it rounds off. Increased liberation of heat during constant absorption may be explained by the fact that concentrations of 15 to 30 per cent NaOH are accompanied by an enlargement of the cellulose grating, which points to a breaking up of the molecular structure. It is possible that this intermolecular swelling may be the cause of strong heat development. V i g n ~ showed n ~ ~ the thermal effects produced hy immersing silk, wool, and cotton in acids, alkalies, and salts. Alkalies Other than Sodium Hydroxide
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Vol. 20, No. 9
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Little work has been $3J done on the action of potassium hydroxide on cellulose. (C8HlO- i! 05)2.KOH is formed a ~ . ~ with 35 per cent KOH solution.46 T a u s and ~ ~ ~~ Z O Weber4* believe cellu- $lose is attacked more 2 ' 5 by KOH than by KaOH, but Hoppe- $'' Seyler believe the con- 9 5 t r a r ~ . ~ VThe curve for KOH has the same 5 /o /5 I0 25 30 general character as ~ auo Contcoyspcr x /odrc //co/mhc Jdutisn that for NaOH, as F/$ 9. pointed out by Liepat0ff.34 The amount of barium hydroxide absorbed by cellulose is greater than the amount of either NaOH or KOH absorbed.34 Barium hydroxide is taken up in proportion to the square root of the final concentration.50 The critical concentrations of lithium, potassium, and rubidium hydroxide for alkali-cellulose formation are 9, 35, and 38 per cent, re~pectively.5~Figures 11 and 12 show the type of curves obtained with LiOH, NaOH, KOH, RbOH, and CsOH. The last two form (C&o05)s.RbOH and (CC H~OO~)&SOH.~ Dehnert,52 in his experiments using tetramethyl ammonium hydroxide, etc., obtained mercerizing action on cellulose and apparently compounds of the form ( C ~ H ~ O O ~[(CH&OH]. )~.N Dry ammonia is absorbed by dry cell~lose.5~There is co reaction between cellulose and ammonia of 22 per cent concentration. However, with liquid ammonia a t -33" or -35" C. there is strong swelling which furnishes a case of swelling without ionic reaction.54
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What Is Alkali-Cellulose?
by an increasing straight line relationship in proportion to the alcohol content. Kitad3 has indicated that the influence of alcohol on NaOH solution is negligible for NaOH concentrations corresponding to the flat portion of the curve. Katz,26 in his work on the x-ray structure of mercerized cellulose, used aqueous alcoholic NaOH, since then the level portion of Vieweg's absorption curve disappears and there is complete
The mechanism of the reaction between cellulose and caustic soda is not yet well explained or understood, especially since the composition of the mixture formed is not constant and depends on the temperature and type of cellulose as well as on substances such as alcohol and salts present in the caustic solution. Figure 13b5 shows the trend of curves typifying the reaction, which may be one of adsorption, dis-
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IXDUSTRIAL AND ESGINEERI,VG CHEMISTRY
September, 1928
tribution of the caustic between cellulose and water, or the formation of a chemical compound. L e i g h t , ~ n obtained *~ a smooth absorption curve and believes that Vieweg's curve is theoretically impossible. The removal oi' OH ions by cellulose from KaOH solutions is not due to absorption, since all the alkali can be readily washed outsS6 From the theoretical standpoint of the phase rule, the horizontal part of the curve does not indicate stoichiometric combination and the other portions of the Fg./O. curve should be concave toward t h e NaOH axis instead of convex.' L i e p a t ~ f f ~considers ~ the absorption of KaOH by cellulose as purely chemical absorption and as a hydrolytic chemical reaction which can best be explained by Henry's theory of solutions C J C , = K . The hysteresis found by Coward and Spencer39 in the absorption of NaOH solutions must be due to the retarded response of the structural elements of the cellulose fiber to change under the influence of variable alkali concentrations. From the work of Ermen and Jenkins30 it may be concluded t,hat the vertical portion of the absorption curve may be interpreted as a constant partition of caustic between cellulose and water. The esterification of alkali-cellulose with carbon bisulfide I ,OH
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929
cellulose. It does not take place as would be expected from the alcoholic character of the cellulose-i. e., not always so that only one hydroxyl group is released.29b Gebhard58 supposes that under the influence of alkalies an ether linkage of the cellulose is absorbed with resolution into a keto group which through enolic formation passes into a hydroxyl group. A part of the oxygen of the molecule would first pass over to ketonic form, then upon action of more concentrated solution into enolic form. Using hydroxylamine Gebhard showed that with weak alkali solutions the ketonic form is more marked than with a strong alkali solution. The conception of a chemical compound of cellulose with alkali has been somewhat verified by x-ray analysi?. With the hydroxide of lithium, sodium, and potassium the "sray point" coincides with the break in Viemeg's curve.j9 This fact, in conjunction with that discovered by ,Heuser that the quantity of alkali absorbed in the flat portions of the curve is equimolecular for the three alkalies. forms a basis for the assumption that alkali-cellu- ji I lose is formed a s changedcellulose. I n any case the experimental fact remains $ that the cellulose spectrum by swelling in these three alkalies disappears by the a b s o r p t i o n of a l most equimolecular a m o u n t s whereby Co"
