Thesis (M.Sc.)--Chulalongkorn University, 1997

Starch-g-polystyrene copolymers were prepared by a simultaneous irradiation technique of y-rays from a ⁶⁰Co-source. The graft copolymers were used as part of the styrene-based polymer for a study of the degradation of the plastic. Starch (4 g) and styrene monomer (4 g) were blended with 1 cm3 water and 1.5 cm3 of methanol and the mixture 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 percent add-on. The highest % 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 plastics connot disintegrate naturally by itself. The degradations of polystyrene plastics containing cassava starch and graft copolymers were investigated by outdoor exposure, soil burial test, gamma radiation, UV irradiation, and the resistance of the plastic to bacteria. all degradation processes were followed by monitoring tensile properties, the extent of degradation by carbonyl index, molecular weights and distribution, and thermal property of the plastic. It was found that the graft copolymer-filled PS sheet lost their physical properties rapidly by outdoor exposure, similarly with gamma and UV irradiations evidenced by calculating activation energies of the plastics. The PS plastics containing the graft copolymer need less activation energy to start a decomposition process than that of the control PS plastics. The indoor exposure test gave the opposite result. All the plastics took a longer time to degrade by 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