Water-Soluble Cellulose Ethers A New Method of Preparation and Theory of Solubility
A new method of alkylating cellulose has been developed, which consists of treating a solution of cellulose in a quaternary ammonium hydroxide with an alkyl halide or sulfate. The reaction proceeds smoothly at room temperature and is particularly useful in preparing watersoluble derivatives. There is a striking difference between the degree of alkylation of these cellulose ethers and of the water-soluble cellulose ethers prepared by the use of alkali cellulose. By the new method, ethers containing only 0.6 to 0.7 methyl or ethyl group are soluble in water whereas the i n troduction of 1.2 to 1.6 groups is necessary by the alkali cellulose process. The explanation for this difference is that a more even distribution of alkyl groups is obtained when cellulose is alkylated in solution.
L. H. BOCK Rohm & Haas Company, Philadelphia, Pa. agents. In 1930 a patent was issued to Lilienfeld (9) for a process of preparing cellulose solutions by dissolving cellulose in a quaternary ammonium hydroxide. He gave as examples tetramethyl ammonium hydroxide, tetraethyl ammonium hydroxide, and phenyltrimethyl ammonium hydroxide. An extensive study of the quaternary ammonium hydroxides as cellulose solvents has been made in this laboratory. In general, it has been found that those quaternary ammonium hydroxides which contain unsubstituted hydrocarbon groups will dissolve cellulose. Strangely enough, one of the most important factors contributing to the solvent power of these bases seems to be their concentration in aqueous solution. A solution with a concentration of 35 to 50 per cent is usually a good cellulose solvent, almost without regard to the size of the alkyl or aryl groups. For this reason tetramethyl ammonium hydroxide is not a useful solvent since a saturated solution a t room temperature contains only about 20 per cent of the base. The normality does not appear to be important since a 40 per cent solution of trimethylethyl ammonium hydroxide is about as good as a 40 per cent solution of trimethylbenzyl ammonium hydroxide, although their normalities are 4.3 and 2.5, respectively.
ATER-SOLUBLE methylcellulose is already familiar in industry where it has a variety of uses as a thickening and emulsifying agent. It has been recommended as an emulsifying agent for salad dressings, pharmaceutical preparations, and even as a foaming agent for beer. Water-soluble cellulose ethers form viscous solutions which are useful as thickening agents to replace starch and casein and have the advantage that they are not susceptible to bacterial or fungal deterioration. I n common with the solvent-soluble cellulose ethers and esters, they have the valuable property of forming films of good tensile strength and pliability. A new method of preparing cellulose ethers has been found which makes it possible to make water-soluble products with a lower degree of alkylation than is achieved by the methods previously described. I n brief, this new process consists in the use of certain quaternary ammonium hydroxides as the alkaline condensing agent in the reaction of cellulose with an alkyl halide or alkyl sulfate.
TABLE I. MERCERIZINQ BASES Mercerizing Agents (CHa)aNOH
(CHa)aSOH (NHdzCNH
Quaternary Ammonium Hydroxides as Cellulose Solvents
Normality
Per Cent
2.41
1.89
21 9 28 9
0.39
11.2
5.4 6.35
50.7 48.9
The importance of the percentage concentration on the solvent power was recently stated in another way by Lieser and Lechzych (7') who showed that a straight-line curve was obtained by plotting the normality of base necessary to dissolve cellulose against the molecular weight of the base. Most of the quaternary ammonium hydroxides are quite stable at ordinary temperatures, and certain of them are
One of the earliest references describing the effect of quaternary ammonium hydroxides on cellulose is an article in 1913 by Knecht and Harrison (8) who reported that solutions of tetramethyl ammonium hydroxide caused considerable shrinkage in cotton. I n 1925 Dehnert and Konig (2) studied the bases listed in Table I and found them to be good mercerizing 985
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stable up to about 100" C. in 40 per cent concentration. The phenyl quaternary ammonium hydroxides are exceptions, since they are unstable at this concentration even a t room temperature. The benzyl ammonium hydroxides have been found to be the best bases for cellulose work in view of the fact that they are more stable than the pure aliphatic derivatives and are readily prepared from cheap raw materials. The use of these bases is disclosed in patents (11).
Preparation of Cellulose Ethers Water-soluble methyl, ethyl, and hydroxyethyl celluloses were prepared by the use of solutions of cellulose in benzylsubstituted quaternary ammonium hydroxides. As an example, ethyl cellulose is prepared as follows: A solution of cellulose is prepared by mixing 150 grams of Alpha Flock, a high grade of a-cellulose, with 1 liter of 35 per cent trimethylbenzyl ammonium hydroxide. The mixing is done in a Werner-Pfleiderer mixer for 1 hour at room temperature. A viscous solution results. One hundred and fifty grams of diethyl sulfate are then added with mixing over a period of 1 hour. After 2 hours of additional mixing at room temperature, the product is completely soluble in water. The pure cellulose ether may be obtained by neutralizing the excess quaternary ammonium hydroxide with acid and precipitating with acetone. A more convenient method of purification is t o heat the solution to 80" C. whereupon the cellulose ether separates in the form of curds which can be washed with hot water. It can then be dried to a crumblike solid. It dissolves slowly in water to form a clear, viscous solution. Analysis by the Zeisel method gave 15 per cent ethoxyl which corresponds to 0.6 ethyl group to each C6HI0O6 unit of cellulose.
I n the same way water-soluble methylcellulose containing unit has been prepared. only 0.7 methyl group to each CLH,OOS By using more alkylating agent, a higher degree of alkylation can be obtained. However, if 1.2 ethyl groups are introduced, the product is not water-soluble.
Degree of Alkylation A striking difference is found in the degree of etherification r
of cellulose ethers prepared by this process compared with commercial cellulose ethers and those described in the literature. Samples of water-soluble methylcellulose available on the market contain 1.25 to 1.65 methyl groups. Trail1 (IS), in a review of the cellulose ethers, reported that conimercial water-soluble methylcellulose always contained 22 to 26 per cent methoxyl which corresponds to 1.27 to 1.54 methyl groups. He also reported that ethylcellulose containing 1.15 ethyl groups is soluble in water. In general, all cellulose ethers previously described which contained less ethyl or methyl groups than these figures were not water-soluble unless these ethers were made from highly degraded cellulose.
Literature Review Denhnm and Woodhouse (3) prepared methylcellulose by treating cotton cloth, previously beaten under water for several days, with 1.6 moles of dimethyl sulfate a t about 50" C. in the presence of 15 per cent sodium hydroxide. The product was not soluble in water. The methylation was repeated five times to give a product which contained 1.5 methyl groups and was partially soluble in water. Heuser and Neuenstein (5) examined methylcellulose prepared by the method of Denham and Woodhouse and separated it into water-soluble and water-insoluble fractions. They found that the water-soluble portion always contained 29-33 per cent methoxyl, which corresponds to 1.74-2.0 methyl groups. Berl and Schupp (1) tested the water Rolubility of methylcellulose prepared by the method of Denham and Woodhouse and found monomethylcellulose to be partially (13.5 per cent) soluble in water. Maximum solubility was found with 1.5
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methyl groups. Ethers containing more than 2.1 methyl groups were completely insoluble. Hess (4) used cellulose which was prepared by treating cotton with acetyl chloride and hydrochloric acid to form cellulose acetate. This was then hydrolyzed to give a regenerated cellulose which was soluble in 8 per cent sodium hydroxide and which he called cellulose A. He treated this material with barium hydroxide in which it was insoluble, and 11 moles of dimethyl sulfate were added at 80" to 90" C. i n two steps over a period of 2 hours. The mixture was then stirred at 90" C. for 1 hour. The product contained two methyl groups and was soluble in water. Further methylation in the presence of sodium hydroxide with 50 moles of dimethyl sulfate gave a product containing 2.8 methyl groups, which was also soluble in water. Hess considered that no degradation had taken place. However, his treatment with acetyl chloride and hydrochloric acid must have resulted in some splitting of the cellulose chain. This is indicated by the fact that his regenerated cellulose was soluble in alkali. Lilienfeld's original patent (8) on cellulose ethers describes the preparation of ethylcellulose. As raw material he used cellulose which had been pretreated by xanthation, by dissolving in cuprammonium, or by heating with 30 to 50 per cent sodium hydroxide. The regenerated cellulose was in each case soluble in sodium hydroxide solution and was undoubtedly degraded. A solution of this regenerated cellulose in 15 per cent sodium hydroxide was treated with 2 moles of diethyl sulfate a t about 50" C. for an undetermined time. The product was claimed to be soluble in water. Berl and Schupp (1) reported that, in carrying out Lilienfeld's process with 16 moles of diethyl sulfate, they obtained a water-soluble product containing 1.3 ethyl groups. In other trials products cpntaining only 0.8 ethyl group were obtained which were not soluble in water. A comparison of these methods of preparing water-soluble cellulose ethers is given in Table 11. I n reviewing the literature on methyl- and ethylcellulose, it is evident that degradation of the cellulose is an important factor in water solubility. I n alkylating cellulose, a homogeneous product is never obtained. Analysis of the product gives only an average value, as has been brought out by the work of Heuser and Neuenstein. TABLE11. SUMMARY OF METHYLAND ETHYLCELLULOSE PREPARATIONS rllkylating Author Denham and Woodhouse Hess Lilienfeld Present urocess Present brocess
Agent MezSO4 -vE2SO4
EtiSOa hlezS04 EtzSO4
Moles Used
CsgkOs
Alkylation Degree,of
Unit 8,O 11.0 16 0 1.1 1.1
1.5 2.0 1.3 0.7 0.6
Theory of Formation of Water-Soluble Cellulose Ethers The fact that a water-soluble cellulose ether containing only 0.6 to 0.7 alkyl group can be prepared using a quaternary ammonium hydroxide while 1.2 to 1.6 groups are necessary using alkali cellulose, requires an explanation. One possible reason may be that the process of dissolving cellulose in a quaternary ammonium hydroxide results in its degradation with a resulting chain length only a fraction of the original. This idea was eliminated by determining the cuprammonium viscosities of cellulose regenerated from solution in a quaternary ammonium hydroxide and comparing it with that regenerated from alkali cellulose such as is used in the prepara-
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SEPTEMBER, 1937
tion of cellulose ethers. Cellulose regenerated from the quaternary ammonium hydroxide had a viscosity of 4.6 seconds compared with 3.0 seconds for alkali cellulose aged 20 hours. Naturally both viscosities were considerably lower than that .of the original cellulose, but at least it showed that the cellulose was no more degraded by the quaternary ammonium hydroxide than by alkali. I n addition, the cellulose regenerated from the quaternary ammonium hydroxide solution is not soluble in sodium hydroxide. The reason for this increased solubility of low alkylated cellulose ethers made by the new process is believed to be due to the fact that the reaction product is more homogeneous. In alkylating alkali cellulose the reaction takes place in a heterogeneous system. The alkali cellulose itself is not homogeneous and is not soluble in the reaction medium. The resulting product, as shown by Heuser and Neuenstein, is a mixture of alkyl celluloses of varying degrees of alkylation. I1
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for hydration. The fact that a quaternary ammonium hydroxide can carry this swelling a step farther and actually put cellulose into solution may indicate that the larger molecules of the quaternary ammonium hydroxide pry apart the in’dividual cellulose chains and bring the hydroxyl groups into *positions more readily accessible for hydration. Sisson (19) obtained evidence to support this theory in preliminary x-ray measurements of the crystal lattice of cellulose fibers swollen by quaternary ammonium hydroxides. His investigation is being continued. I n the case of the water solubility of cellulose ethers it is suggested that the individual chains of cellulose are held apart by methoxyl or ethoxyl groups so that the hydroxyl groups are available for hydration. According to this theory the essential group for water solubility is the hydroxyl group which is normally present in cellulose but which is usually not available for hydration because of steric relationships or possibly secondary valence. This steric effect is illustrated in Figure 1, which is an attempt to show that the normal, closely packed structure of cellulose would be disturbed by the introduction of relatively few ether groups if they are properly distributed. A certain number of hydroxyl groups is necessary for water solubility as is shown by the fact that, if the ethylation is carried beyond a certain point, the product is no longer soluble.
Literature Cited b
FIGURE 1
-
It is reasonable to suggest that, when cellulose is alkylated in a homogeneous solution such as is obtained in a quaternary ammonium hydroxide, there will be more even distribution of alkyl groups in the resulting product. The result here is visualized as a long chain with alkyl groups attached a t regular intervals. As an analogy we might compare it to a fisherman’s net with floats a t regular intervals so that the entire net is suspended near the surface. If the floats are bunched a t a few points, a large part of the net will hang far under water. It is interesting to note that a similar viewpoint was independently suggested in a publication of Traube, Piwonka, and Funk (14) who prepared water-soluble methylcellulose containing 0.8 to 0.9 methyl group, using a cellulose-copper hydroxide-sodium hydroxide complex. Their explanation was that, in carrying out the alkylation according to this method, a more homogeneous product was obtained. By splitting the product with alcoholic hydrogen chloride and fractionating the methyl glucosides so obtained, they showed that the material contained only monomethylglucose units. Although the above theory suggests why it is possible to prepare water-soluble cellulose ethers with a lower degree of alkylation when the reaction is carried out in a quaternary ammonium hydroxide solution, no one has offered a satisfactory explanation of why cellulose ethers are soluble in water a t all. Ordinarily a hydroxyl group has greater influence on water solubility than a methoxyl or ethoxyl group. There is some connection between the swelling of cellulose and its ultimate solution. Sodium hydroxide is capable of swelling cellulose but not of dissolving it. Neale ( I O ) postulated that the swelling of cellulose in an aqueous soIution is a manifestation that the hydroxyl groups are becoming more accessible
Berl, E., and Schupp, H., Celluloschem., 10, 41 (1929). Dehnert, F., and Konig, W., Ibid.. 6, 1 (1925). Denham, W. C., and Woodhouse, H., J. Chem. SOC.,103, 1735 (1913). Hess, K., Weltaien, W., and Messmer, E., Ann., 435, 76 (1923). Heuser, E., and Neuenstein, W., Celluloschem. 3, 89 (1922). Knecht, E., and Harrison, W., J. SOC.Dyers Colourists, 29, 224 (1913). Lieser, T., and Leohzych, E., Ann., 522, 56 (1936). Lilienfeld, Leon, U. S. Patent 1,188,376 (1916). Ibid., 1,771,462 (1930). Neale, S. M., S. TextileInst., 15, T157 (1924). Powers, D. H., Bock, L. H., and Houk, A. L., U. S. Patent 2,009,015 (1935) ; Rohm & Haas Co., British Patent 455,253 (1937). Sisson, W. A., private communication. Traill, D., J. SOC.Chem. Ind., 53, 337T (1934). Traube, W., Piwonka, R., and Funk, A., Ber., 69B,1483 (1936). RECFJVEDJune 4, 1937. Presented before the Division of Cellulose Chemistry at the 93rd Meeting of the American Chemical Society, Chapel Hill, N. C., April 12 to 15, 1937.
Corrections In the article on “Viscosity-Concentration Relations in Ethylcellulose Solutions,” which appeared in the July, 1937, number of INDUSTRIAL AND ENGINEERING CHEMISTRY, the conversion factor from viscosities of ethylcellulose determined by the modified Ostwald viscometer to seconds by the A. S. T. M. falling ball method has been found to be 410 instead of 310 as previously reported. The right-hand scale of the ethylcellulose viscosityconcentration chart (Figure 3, page 802) should be corrected accordingly. T. A. KAUPPIAND S. L. BASS THE Dow CHEMICAL COMPANY, MIDLAND,MICA. July 21, 1937
..... In the article on “Textiles Go Chemical” by Joseph F. X. Harold which appeared in the July, 1937, number of INDUSTRIAL AND ENGINEERING CHEMISTRY, credit for the photographs on pages 743 and 744 was omitted through inadvertence. These pictures were reproduced through the courtesy of the Tennessee Eastman Corporation.
