71. Biodegradable Composites Using Starch as Matrix

Starch is inherently non-thermoplastic but is made thermoplastic using plasticizers and/or chemical modifications, and the modified starch has been used as matrix for composites. In one such study, starch was reinforced with bacterial cellulose, and the tensile properties, resistance to biodegradation, and moisture absorption were studied [09Wan]. Starch was plasticized with 30 % glycerol and made into 10–20 % solutions. Bacterial cellulose sheets cultured from Acetobacter xylinum X-2 were added into the solution and made into composite sheets with an average thickness of 0.5 mm. The amounts of fibers in the starch were 7.8, 15.1, and 22 wt%. Tensile properties of the BC-reinforced starch fiber composites are shown in Table 71.1 [09Wan]. Morphological analysis of the fractured surface of a starch composite containing 22 % bacterial cellulose showed that the BC fibers were present in a layered fashion as seen in Fig. 71.1. Such a layered structure was typical of bacterial cellulose. Pullout length of fibers from the matrix was low suggesting good fiber–matrix interaction [09Wan]. The presence of bacterial cellulose also increased the resistance of the fibers to moisture absorption. Degradation by soil burial tests showed that the weight loss of the composites was similar to that of unreinforced starch, and about 30 % weight loss had occurred after 30 days of burial. However, the bacterial cellulose-reinforced composites had slightly higher strength retention than the starch films. In a similar study, bacterial cellulose containing nanofibrils with diameters between 10 and 100 nm was mixed (1 or 5 %) with starch containing 30 % glycerol. Later, the mixture was heated at 120 °C for 20–30 min and later injection molded into composites in the form of tensile bars [09Mar]. More than six times increase in strength and modulus were obtained for composites containing 5 % nanocellulose compared to the thermoplastic starch [09Mar].