Study on microstructure and physical properties of composite films based on chitosan and methylcellulose

Abstract

Biodegradable films obtained from chitosan (CH) and methylcellulose (MC) can reduce environmental problems associated with synthetic packagings. Biodegradable films still need a cost reduction to be economically profitable; the use of CH, a by-product obtained from wastes of the fishing industry would be a good alternative. The objectives of the present work were to characterize physical, mechanical and water vapor barrier properties and microstructure of films of CH, MC and their mixtures.

Solutions of 2% CH and 1% MC were mixed in different proportions. Films were obtained by casting. Film microstructure was characterized by SEM, X-ray diffraction and FTIR spectroscopy. Water vapor permeabilities for CH and MC films were 6.67±0.74×10⁻¹¹ and 7.55±0.60×10⁻¹¹ g/s m Pa, respectively; composite film permeabilities did not differ significantly from these values. CH films showed rigid characteristics (high elastic modulus and low elongation). Flexibility of composite films increased with increasing MC content, the higher elongation and lower elastic modulus of composite films indicated the relevance of hydrocolloid interactions.

Introduction

Edible films and coatings are thin films prepared from biopolymers that act as a barrier to the external elements (factors such as moisture, oil and water vapor) and thus protect the product and extend its shelf life. Biodegradable film properties depend on the material type used and obtention conditions that determine their applications (Krochta & De Mulder-Johnston, 1997). Biological materials used are polysaccharides, proteins, lipids and their derivatives. Specifically, within polysaccharides cellulose derivatives, chitosan (CH), starch, alginate, carrageenan and pectin are preferred because their high film forming capacity.

Cellulose derivatives are polysaccharides composed of linear chains of β (1–4) glucosidic units with methyl, hydroxypropyl or carboxyl substituents. Methylcellulose (MC) has excellent film-making properties, high solubility and efficient oxygen and lipid barrier properties (Donhowe & Fennema (1993a), Donhowe & Fennema (1993b); Nisperos-Carriedo, 1994; Park, Weller, Vergano, & Testin, 1993).

Chitin and its deacetylated product, CH, have received much interest for its application in agriculture, biomedicine, biotechnology and the food industry due to their biocompatibility, biodegradability and bioactivity (Kumar, 2000; Muzzarelli, et al., 1988; Tharanathan & Kittur, 2003). Among these potential applications, the use of chitin and CH as food antimicrobials and biopesticides are especially attractive (Wu, Zivanovic, Draughon, Conway, & Sams, 2005). Due to its antifungic, good mechanical and oxygen barrier properties (Caner, Vergano & Wiles, 1998; Chen, Yeh, & Chiang, 1996), CH film is a promising packaging material that can be included in the active film category (Vermeiren, Devlieghere, van Beest, de Kruijf, & Debevere, 1999).

Composite films of CH and MC may result in improved mechanical and physical properties since these two polysaccharides have compatible structures. Thus, a deep insight into component interactions would help to understand and control the physical properties of composite films. In a previous work (García, Pinotti, Martino, & Zaritzky, 2004), solutions with different CH–MC ratios were characterized rheologically; films obtained from these solutions were analyzed as well.

The objectives of the present work were: (a) to develop composite biodegradable films based on CH and MC, (b) to characterize film microstructure by SEM, X-ray diffraction and Fourier transform infrared (FTIR), (c) to evaluate mechanical properties of the films and some physical characteristics like equilibrium moisture content, film solubility in water and permeability to water vapor.