Meaning
The self-accelerating chemical reaction sequence initiated by the reaction of carbon-centered polymer radicals with oxygen is a primary pathway of thermo-oxidative degradation during melt processing. A peroxy radical cascade begins when shear or thermal energy breaks polymer chains, generating highly reactive radical sites that rapidly combine with dissolved oxygen. This creates peroxy radicals that abstract hydrogen atoms from adjacent polymer chains, generating new carbon radicals and hydroperoxides.
The reaction continues in a loop that quickly consumes the polymer’s molecular weight, destroying its physical properties and leading to embrittlement.
Chemical Pathway
Each loop of the degradation cycle multiplies the number of active radical sites within the polymer matrix. During the peroxy radical cascade, unstable hydroperoxides decompose into alkoxy and hydroxy radicals, both of which initiate their own abstraction cycles. This exponential growth in radical concentration causes rapid chain scission and cross-linking.
The melt flow index of the resin rises or falls dramatically, depending on whether scission or cross-linking dominates.
Material Degradation
Degraded polymer exhibits a severe loss of impact strength and optical clarity. Because a peroxy radical cascade generates carbonyl groups, the polymer undergoes yellowing. Without protection, the material becomes too brittle for high-stress applications.
Stabilizer Action
Prevention of this auto-catalytic breakdown requires the inclusion of primary antioxidant additives like hindered phenols. These stabilizers donate hydrogen atoms to the peroxy radicals, converting them into stable, non-reactive species and interrupting the peroxy radical cascade before it can propagate. Secondary antioxidants then decompose the hydroperoxides into non-radical byproducts.
This combination maintains the resin’s molecular weight during high-temperature extrusion.
