The Role of the Processing Operation on the UV Degradation of PVC

Abstract

In this work, the thermal degradation of a commercial poly (vinyl chloride) has been studied during the mixing in a processing machine which simulates a commercial operation (Rapra Torque Rheometer). Processing was carried out without additives and also with dibutyltin maleate-based stabilizer Irgastab T290 and lubricants Ca-stearate and Wax E. Photodegradation of PVC processed with both additives and without additives have also been studied. Apart from thermal energy, the factors which influenced the thermal degradation of unstabilized PVC were found to be the presence of air and mechanical work (shear forces). Peroxide was formed in the initial stage of processing. The concentration of peroxide rapidly decreased due to the occurrence of hydrogen chloride-induced decomposition. The presence of a lubricant decreased the shear forces, and the contribution of a pure mechanical process to the overall thermomechanical degradation was smaller. On prolonged processing, crosslinking and chain scission occurred, and chemical analysis showed an increase in peroxide concentration. Infrared spectroscopy showed the formation of carbonyl and conjugated carbonyl groups. The chain scission was shown to be of an oxidative character. The lower molecular weight species were formed as a result of rearrangement of intermediate alkoxy radicals formed by the decomposition of both peroxide and peroxy-crosslinks. The exact mechanism of the crosslinking reaction is not yet clear, but the occurrence of oxidation suggested the participation of intermediate alkoxy, hydroxy, and alkylperoxy radicals. The study of the processing of PVC with dibutyltin maleate-based stabilizer (Irgastab 1290) has shown the occurrence of the exchange reaction between the carboxylate moiety of the stabilizer and the labile chlorine atoms (probably allylic). The formation of anhydride, dibutyltin chloride, and the occurrence of a Diels-Alder type reaction was demonstrated. The role of Ca-stearate in the presence of dibutyltin maleate stabilizer was shown to be predominantly that of a lubricant. Photodegradation of processed unstabilized PVC in the presence of air by the ultraviolet radiation of wavelength greater than 280nm has resulted in extensive photo-oxidation. The photo-oxidation proceeded in two stages: an initial fast-rate stage followed by the second slower-rate stage. Initiation of photo-oxidation by the peroxide formed during processing has been demonstrated. The unsaturation generated by the thermomechanical treatment was considered to be important as a sensitizer of photo-oxidation. The formation of polymeric ketone and polymeric alcohol has been shown, and it was suggested that they were the products of photolysis of intermediate hydroperoxide. Retardation of photo-oxidation was shown to be the result of a slower diffusion rate of air through the crosslinked structure. It has been demonstrated that during photodegradation, the thermal stabilizer dibutyltin maleate, which remained in the polymer after processing, acted as a UV-stabilizer.