(377h) Reaction Models Describing Antioxidant Depletion in Polyethylene, Polypropylene and Polyvinyl-Chloride Caused By Thermal Degradation
During high temperature processing of polymers, the free radical generation is expected to start by breakage of chemical bond with the lowest bond dissociation energy. In PE, the weakest are those C-C bonds that are in allylic positions, and, for a PP monomeric unit, the bond between primary Carbon and tertiary Carbon is the weakest which can break to give rise to free radicals. The free radicals can then get oxidized with dissolved oxygen (even at ambient temperature) and produce peroxide radicals that can abstract hydrogen from PE/PP to generate hydroperoxide and other free radical groups. The regenerated free radical continues the degradation of PE/PP in a cyclic manner. Antioxidants present in the polymer deactivate these peroxide radicals and interrupt the oxidative degradation cycle by converting them to hydroperoxide groups (which are much stable compared to the free radicals). This enhances the period of stability of PE/PP parts. However, the accumulated hydroperoxide groups can slowly degrade to generate free radicals that eventually depletes all the antioxidants. Once the antioxidants are depleted, the cyclic degradation continues at an unchecked rate to quickly deteriorate the mechanical properties.
Similar to polyethylene and polypropylene, polyvinyl-chloride also degrades via oxidation of free radicals. In addition to this, dehydrochlorination of PVC enhances the degradation reactions. Degradation of PVC begins with an initiation step which generates alkyl free radicals and chlorine radicals. These free radicals then react through propagation, chain scission, termination, and stabilization reactions. Some of these reactions are fast, some are slow, while some others are insignificant in presence of active antioxidants.
Mathematical models and computer simulations of PE, PP and PVC degradation reactions helps to predict antioxidant depletion features and give insight to identify significance of various degradation and stabilization reactions for each polymer. The models can give predictions of antioxidant depletion in few seconds/minutes, the experiments for which would require several years.