Determining Antioxidant Depletion Thresholds in Recycled Polypropylene Extrusion Processing
Oxidation induction time testing at 200 °C establishes true additive depletion in recycled polypropylene before melt processing causes chain scission.
Tertiary Carbon Degradation
Meaning
Thermo-oxidative cleavage targeting substituted carbon atoms along a polymer backbone reduces molecular weight and mechanical toughness in branched polyolefins. Susceptibility to tertiary carbon degradation causes rapid loss of tensile strength and impact resistance in polypropylene components during heat exposure. The weak tertiary carbon-hydrogen bond exhibits lower dissociation energy compared to secondary methyl carbons, making it prone to radical abstraction.
This degradation path affects polypropylene and branched polyethylene grades, whereas linear polyethylene exhibits greater resistance to thermal oxidation.
Chemical Mechanism
Oxygen attack at tertiary carbon sites forms unstable hydroperoxide intermediates that undergo homolytic cleavage. Experiencing tertiary carbon degradation generates alkoxy radicals that cause beta-scission along the main polymer chain backbone. Chain cleavage decreases average molecular weight and broadens molecular weight distribution profiles.
Higher processing temperatures accelerate hydroperoxide decomposition and chain scission rates.
Viscosity Drop
Chain scission reduces polymer chain length and melt entanglement density during melt conversion processes. Occurrence of tertiary carbon degradation leads to significant increases in melt flow rate during injection molding. Lower melt viscosity causes mold flash, sink marks and unstable cavity filling pressures.
Regrind processed through multiple thermal cycles exhibits severe viscosity reduction and brittle part failure.
Stabilization Route
Hindered phenolic antioxidants donate hydrogen atoms to neutralize free radicals before backbone cleavage occurs. Mitigating tertiary carbon degradation requires combining radical scavengers with secondary phosphites to decompose hydroperoxides into stable alcohols. Proper antioxidant dosing preserves mechanical properties across multiple processing passes.
Molders track melt flow index changes to evaluate stabilizer residual content in reprocessed materials.