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Reaction between Chitosan and Cellulose on Biodegradable Composite Film Formation Jun Hosokawa,*JMasashi Nishiyama,?Kazutoshi Yoshihara,?Takamasa Kubo,?and Akira Terabe* Government Industrial Research Institute Shikoku, 2-3-3 Hananomiya-cho, Takamatsu 761,Japan, and Aicello Chemical Company Limited, Ishimakihon-machi, Toyohashi 441 -11, J a p a n
It was found that the film formation of composite films is accompanied by cross-linking between cellulose and chitosan, the composite films obtained thus becoming insoluble in water. In this paper the cross-linking reaction is discussed mainly from the point of view of biodegradability, swelling, and strength of the composite film. An increase in the carbonyl groups of cellulose enhances cross-linkings in the composite film formation, while an increase in carboxyl groups does not. Though only trace amounts of carbonyl groups exist in cellulose, they evidently play an important role in cross-linking to chitosan. Further, the free amine form of the primary amino group of chitosan reacts more easily with cellulose than does the amine salt form. These facts suggest that one of the cross-linking structures between cellulose and chitosan in the stage of film formation originates from Schiff base formation between carbonyl groups of cellulose and primary amino groups of chitosan. Plastics made from petroleum have come into wide use throughout the world. With increased applications, the disposal of waste plastics has become a serious problem due to the lack of landfill dump sites and harmful smoke problems when they are burned in incinerators. Therefore, the development of new plastics that can be degraded by microorganisms in soil and seawater has recently been undertaken (Otey et al., 1980; Stepto and Tomka, 1987; Kunioka et al., 1989). We reported a novel and biodegradable composite film derived from chitosan and homogenized cellulose that has a high oxygen-gas barrier capacity and is hydrophilic but insoluble in water (Hosokawa et al., 1990). The period of its biodegradation can be controlled by adjusting the conditions of film formation. We and collaborating companies are now planning to apply this biodegradable material as a binder in nonwoven fabrics, films, sponge type moldings, and so on (Hosokawa, 1990). We wrote in the previous report that the biodegradability of the composite film can be controlled by adjusting the degree of oxidation of the cellulosic material and other Government Industrial Research Institute Shikoku.
* Aicello Chemical Company Limited.
conditions of film formation. We proposed that the biodegradability of the composite film is related to the cross-linking structure between chitosan and homogenized cellulose. However, little is known about the mechanism of reaction between chitosan and cellulose in the step of the composite film formation. We shall, therefore, discuss the mechanism of the cross-linking in this report.
Experimental Section A. Materials. Commercial-grade chitosan (chitosan 10B, Katokichi Co.; origin, prawn; degree of deacetylation, 99.8%; viscosity, 200 CPat 0.5% concentration in 0.5% acetic acid at 25 "C) was used to prepare the composite films used in this study. Fine cellulosic fiber (homogenized cellulose) was obtained by Daicel Chemical Industries, Ltd. The homogenized cellulose named Micro-fibril-cellulose from Daicel was made from bleached pulp and was offered as a 4 % suspension in water. Micro-fibril-cellulosehas a diameter of
