A Mossbauer study of gamma-irradiated organotin-stabilised poly(vinylchloride).

Sn Mossbauer spectroscopy has been used to study the nuclear environment of three organotin stabilisers present in PVC, after exposure of the polymer to gamma-irradiation up to a maximum dose of 200 kGy, in order to identify the tin-containing degradation products which could migrate into foodstuffs in contact with the polymer. Chapter One of this thesis covers the aspects of the degradation and stabilisation of organotin-stabilised PVC which may influence the chemical nature of the tin-containing degradation products resulting from gamma-irradiation of the polymer. Chapter Two contains a description of the theory of the Mossbauer effect and the instrumentation and computational methods for recording and processing Mossbauer data. Chapter Three is concerned with the effect of gamma-irradiation, at doses in excess of food irradiation applications, in order to identify the terminal tin-containing degradation products of the stabilisers. In Chapter Four, evidence for the progressive degradation of organotin stabilisers upon increasing irradiation exposure is presented and intermediate degradation products are proposed. A mechanism for the degradation of stabilisers is suggested. In Chapter Five, evidence for dealkylated tin-containing degradation products in irradiated organotin-stabilised PVC is presented. Chapter Six involves the application of the Debye model of solids to variable temperature Mossbauer data for two organotin stabiliser degradation products, dibutyltin dichloride and stannic chloride, in PVC. A significant change in recoilless fraction is shown to occur when these compounds are dispersed in PVC compared with those of the pure compounds. This is attributed to changes in coordination number of the tin atom upon dispersion in PVC, and has a significant effect upon the relative sensitivity of the technique to the degradation products. Chapter Seven contains a comparison of the degradation processes occurring in thermal and gamma-irradiation degradation of organotin-stabilised PVC.It is shown that degradation.of the stabiliser during gamma-irradation is much more severe. In Chapter Eight, an appraisal of the key experimental results and their implications for the gamma-irradiation sterilisation of polymer-packaged foodstuffs is presented.