THERMOOXIDATIVE DEGRADATION OF PVC IN SOLUTION IN THE PRESENCE OF METAL STEARATE STABILIZERS

Detailed research has been carried out to determine the effect of metal stearates (PbSt2, CdSt2, BaSt2, CaSt2, and ZnSt2) on the thermooxidative degradation of PVC in inert dilute solution. The induction periods of free HCl evolution increase in the presence of PbSt2, CdSt2, CdSt2, BaSt2, and CaSt2, while ZnSt2, due to ZnCl2 formation, leads to fast HCl loss without induction period. The induction periods are much shorter than those obtained during thermal degradation, but the relative order of effectiveness of these stabilizers is the same as in inert atmosphere: PbSt2 > CdSt2 > BaSt2 > CaSt2 > ZnSt2. After the induction period, the initial rates of free HCl evolution decrease as metal stearate concentrations increase. Surprisingly, this dependence is independent of the kind of metal. At higher stabilizer concentrations, the initial rates of free HCl loss during the thermooxidative degradation approach the initial dehydrochlorination rate of thermally degraded PVC. As UV and visible spectra indicate, there is a sharp change in the normalized integral absorption versus time plots at the end of the induction periods in the presence of PbSt2, CdSt2, and BaSt2. Based on these experimental results, it has been concluded that ffective metal stearates have manifold roles in the course of PVC stabilization under thermooxidative conditions. The main role of these compounds is not the replacement of labile structures and HCl-scavenging, but the blocking of the rapid zip-elimination is the most important event of the stabilization process. However, this blocking is reversible, which becomes effective mainly right after the consumption of the metal soaps (reversible blocking mechanism). Another important effect of these stabilizers is that they destroy peroxides and/or peroxy radicals (antioxidant effect).