Char formation in polyvinyl chloride. III. Mechanistic aspects of isothermal degradation of PVC containing some dehydrochlorination/charring agents

We pyrolyzed PVC with and without the char-promoters iron (III) pyromellitate, zinc pyromellitate, and MoO3 (1 phr) under isothermal conditions (207°C, nitrogen) and analyzed the soluble and insoluble residues to characterize the reactions important to charring. Both pyromellitates, apparently acting as their metal chlorides, catalyze dehydrochlorination and crosslinking that could lead to rapid charring; however, the zinc additive also catalyzes undesirable fragmentation. MoO3, possibly acting as an oxychloride, has only a small effect on PVC degradation under the conditions used; however, it does suppress fragmentation. The relative rates of crosslinking to fragmentation are important in determining char yields.