Thermal stability, degradation, and stabilization mechanisms of poly(vinyl chloride)

This survey concerns the major features of stability, thermal degradation, and stabilization of poly(vinyl chloride) (PVC). The effect of microstructure and chain defects of PVC on the stability of the resin is analyzed and reviewed in the light of recent findings. New experimental results of thermal degradation of PVC in dilute solution in the presence of stabilizers are also summarized These results indicate that the most important role of PVC stabilizers is not preventing initiation by labile chlorine substitution but blocking the fast zip-elimination of HCl during degradation. After stabilizer consumption, the blocked structures undergo reinitiation (reversible blocking mechanism). The possible reaction of blocking is the attachment of ester or thioglycolate groups to the propagating polyenes. Reinitiation is explained by the rapid acid-catalyzed thermolysis of these structures by the appearance of free HCl after consumption of stabilizers. Comparison of experimental findings with degradation kinetics expected on the basis of the Frye-Horst, the Minsker, and the Michell stabilization mechanisms indicates that none of these mechanisms is able to explain stabilization of PVC.