Submitted by: J. E. Kiefer 1 Checked by: A. J. Rosenthal 2
1. Procedure Wood pulp (Note 1) is activated by soaking in water for 16 h. The pulp is pressed in a Büchner funnel to remove most of the water. The remaining water is removed by displacement with glacial acetic acid on the Büchner funnel, and the final mixture is pressed until a product containing 1 part of cellulose to 2 parts of acid is obtained. In a 1 l flask equipped with a mechanical stirrer capable of mixing viscous solutions is placed 30 g of activated wood pulp wet with 60 g of glacial acetic acid. A solution of 2.1 g of 98% sulfuric acid in 390 g of glacial acetic acid is added, and the mass is mixed at 25-30o for 30 min. Acetic anhydride (Lachrymator!) (150 g) is added at the rate of 4 mL/min with mixing. The reaction mixture is maintained between 25 and 30o during addition. The mass is then warmed to 35o and mixed until a clear, fiber-free, viscous solution is obtained. The reaction normally requires about 2 h (Note 2). A solution consisting of 4 g of magnesium acetate dissolved in 40 g of water and 40 g of glacial acetic acid is added to the reaction mixture at the rate of 5 ml/min (Note 3). The cellulose acetate is precipitated by pouring into water. The product is isolated by filtration and washed with water until the water washings are neutral. It is then washed with 2 l of water containing 3 g of magnesium acetate, isolated by filtration, and dried at 60o. The white, flocculent cellulose triacetate (270 g) contains 43.5-44.5% acetate groups and is soluble in dichloromethane, chloroform, and formic acid (Note 4). The inherent viscosity of the regenerated cellulose, 0.1% in cupriethylene diamine, is 1.63 dl/g.
2. Notes 1. Special acetate cellulose (P-IB) wood pulp obtained from Rayonier, Inc., was used. Other purified grades of wood pulp, cotton linters, filter paper, or a regenerated cellulose (viscose) can be acetylated by this procedure. 2. The reaction rate varies considerably with the type of cellulose being acetylated. Regenerated cellulose is much more reactive than is filter paper, wood pulp, or cotton linters. For example, a regenerated cellulose can be acetylated in 40-60 min under these conditions, whereas cotton linters or a high molecular weight wood pulp requires 2-4 h. The completion of the reaction is best determined by microscopic examination of a drop of the solution pressed between two glass slides. Prolonging the reaction causes a reduction in the molecular weight of the product. 3. The temperature may rapidly rise about 25o as a result of hydrolysis of excess acetic anhydride. 4. Cellulose triacetate, normally called primary acetate, is insoluble in acetone. An acetone- soluble ester can be obtained by hydrolysis of the primary acetate to a secondary acetate.3
291 292 Macromolecular Syntheses, Collective Volume 1 The secondary acetates contain 35-42% acetyl. Because of their solubility characteristics and compatibility with plasticizers, the secondary acetates are preferred in most commercial applications.
3. Methods of Preparation Cellulose has been acetylated with acetic anhydride using the following catalysts: sulfuric acid,4 zinc chloride,5 perchloric acid,6 methane sulfonic acid,7 aromatic sulfonic acids,8 sulfur dioxide,9 pyridine,10 and basic salts.11 Acetyl chloride12 and ketene13 have also been used as the acetylating agents for preparing cellulose acetate.
4. Merits of the Preparation There are several advantages to using sulfuric acid as the acetylation catalyst. Mainly the sulfuric acid combines with the cellulose during the acetylation. The combined sulfate groups enhance the solubility of cellulose acetate in acetic acid (the common esterification diluent) and a more uniform acetylation results. Also, sulfuric acid is a good catalyst for hydrolysis. Consequently an acetone-soluble product can be obtained by adding water after the esterification is completed. The hydrolysis can then be carried out without isolating the cellulose triacetate.
5. References 1. Research Laboratories, Tennessee Eastman Co., Division of Eastman Kodak Co., Kingsport, TN 37662. 2. Research Laboratories, Celanese Corp., Summit, NJ 07901. 3. Bates, H.; Fisher, J. W.; Smith, J. R. U.S. Patent 2,775,585, 1956. 4. Miles, G. W. British Patent 19,330, 1905. 5. Hess, K.; Schultze, G. Ann. Chem., 1927, 455, 81. 6. Malm, C. J. U.S. Patent 1,645,915, 1927, Chem. Abst., 1929, 23, 5287. 7. Soc. des usines Chemiques Rhône-Poulenc, French Patent 705,546; Chem. Abst., 1929, 23, 5287. 8. Mark, H. S.; Little, A. D; Walker, W. H. U.S. Patent 709,922, 1902. 9. Murata, K. U.S. Patent 2,903,481, 1959; Chem. Abst., 1960, 54, 894d. 10. Hess, K.; Ljubitsch, N. Ber., 1928, 61B, 1460; Chem. Abst., 1928, 22, 4793. 11. Blume, R. C. U.S. Patent 2,632,006, 1953; Chem. Abst., 1953, 47, 5682h. 12. Malm, C. J.; Mench, J. W.; Kendall, D. L.; Hiatt, G. D. Ind. Eng. Chem., 1951, 43, 684. 13. Ketoid Co., British Patent 237,591, 1924; Chem. Abst., 1926, 20, 1522.