Degradation of styrene-g-cassava starch filled polystyrene plastics

Starch-g-polystyrene copolymers were prepared by the technique of simultaneous irradiation by gamma-rays from a Co-60-source. The graft copolymers were used as part of the styrene-based polymer for studies of the degradation of the plastic. The mixture of starch, styrene and methanol solution was irradiated by gamma rays to various total doses ranging from 2 to 16 kGy at a fixed dose rate of 2.5x10(-3) kGy s(-1). The copolymers were characterized in terms of the homopolymer content, grafting efficiency, grafting ratio, conversion, and percentage add-on. The highest percentage grafting efficiency (62.2%) was obtained at a total dose of 10 kGy. The effects of nitric acid inclusion for enhancing the grafting of styrene onto cassava starch were also studied. Polystyrene (PS) plastics cannot disintegrate naturally by themselves. The degradation of polystyrene plastics containing cassava starch and graft copolymers was investigated by outdoor exposure, soil burial testing, UV irradiation, and the resistance of the plastic to bacteria. All degradation processes were followed by monitoring tensile properties, an index of the extent of degradation, by carbonyl index, molecular weight, and thermal properties of the plastic. It was found that the physical properties of graft copolymer-filled PS sheets rapidly deteriorated upon outdoor exposure, or UV irradiation as evidenced by calculated activation energies of the plastics to decompose. The PS plastics containing the graft copolymer needed less activation energy to start the decomposition process than the control PS plastics. In contrast, no samples significantly degraded upon indoor exposure. All the plastics took a longer time to degrade by the soil burial test. Bacillus coagulans 352 was used for a test of biodegradability resistance of the plastic sheets to bacteria. The composite PS sheets revealed the destroyed areas of starch, indicating that the bacteria help promote the biodegradation of polystyrene plastics before other disintegrations take place. (C) 1999 Elsevier Science Ltd. All rights reserved.