Meaning
Autoxidative degradation reactions cleave polymer backbones when thermal decomposition of oxygenated intermediates generates unstable alkoxy radicals. During high-temperature melt processing, hydroperoxide radical scission reduces polypropylene molecular weight by fragmenting tertiary carbon centers along the polymer chain. This radical reaction mechanism governs degradation rates in polyolefins exposed to heat and mechanical shear during compounding or moulding.
The process stops when radical concentration drops through termination reactions or when primary antioxidant additives quench peroxyl radicals.
Thermal Oxidation
Heat and oxygen react with polymer alkyl radicals to form unstable hydroperoxides during melt processing. Subsequent homolytic cleavage triggers hydroperoxide radical scission, producing reactive alkoxy radicals that undergo beta-scission. Polypropylene suffers rapid backbone breakage because tertiary carbon radicals formed during scission possess high thermodynamic stability.
Secondary Reaction
Alkoxy radicals generated by hydroperoxide breakdown capture hydrogen atoms from adjacent polymer chains. This hydrogen abstraction propagates hydroperoxide radical scission across neighboring macromolecular chains, compounding mechanical degradation. Hindered phenol antioxidants arrest this cycle by donating sterically hindered hydrogen atoms to neutralizing radical species.
Melt Index
Macromolecular chain shortening increases polymer melt flow rate while impairing mechanical strength. Uncontrolled hydroperoxide radical scission elevates melt index, turning high-viscosity extrusion grades into runny melt streams unsuited for blow moulding. Monitoring melt index changes serves as a primary quality test for thermal degradation in reprocessed polyolefins.
