Degradation of styrene-g-cassava starch filled polystyrene plastics

Starch-g-polystyrene copolymers were prepared by the technique of simultane ous irradiation by gamma-rays from a Co-60-source. The graft copolymers wer e 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 ir radiated 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 efficien cy (62.2%) was obtained at a total dose of 10 kGy. The effects of nitric ac id inclusion for enhancing the grafting of styrene onto cassava starch were also studied. Polystyrene (PS) plastics cannot disintegrate naturally by t hemselves. The degradation of polystyrene plastics containing cassava starc h and graft copolymers was investigated by outdoor exposure, soil burial te sting, UV irradiation, and the resistance of the plastic to bacteria. All d egradation processes were followed by monitoring tensile properties, an ind ex of the extent of degradation, by carbonyl index, molecular weight, and t hermal properties of the plastic. It was found that the physical properties of graft copolymer-filled PS sheets rapidly deteriorated upon outdoor expo sure, or UV irradiation as evidenced by calculated activation energies of t he plastics to decompose. The PS plastics containing the graft copolymer ne eded less activation energy to start the decomposition process than the con trol PS plastics. In contrast, no samples significantly degraded upon indoo r exposure. All the plastics took a longer time to degrade by the soil buri al test. Bacillus coagulans 352 was used for a test of biodegradability res istance of the plastic sheets to bacteria. The composite PS sheets revealed the destroyed areas of starch, indicating that the bacteria help promote t he biodegradation of polystyrene plastics before other disintegrations take place. (C) 1999 Elsevier Science Ltd. All rights reserved.