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.
Peroxyl Radical
Meaning
Oxygen-centered reactive intermediates formed when carbon radicals react with dissolved oxygen drive the propagation stage of polymer auto-oxidation. The presence of a peroxyl radical accelerates molecular weight loss during extrusion and heat exposure of polyolefin resins. These short-lived species abstract hydrogen atoms from adjacent polymer chains to form hydroperoxides and new alkyl radicals.
This autocatalytic cycle damages un-stabilized thermoplastics during processing and long-term outdoor exposure, but primary antioxidants interrupt the pathway by donating active hydrogen atoms.
Chain Propagation
Rapid reaction between alkyl radicals and molecular oxygen generates reactive oxygen species inside polymer melt streams. Each peroxyl radical abstracts labile hydrogen from tertiary carbon sites, propagating thermal degradation through polyolefin matrices. Propagation rates increase exponentially at elevated barrel temperatures inside injection molding machinery.
Insufficient oxygen diffusion limits propagation speed in thick molded parts compared to thin extruded films.
Oxidative Scission
Hydroperoxides generated during radical propagation decompose into alkoxy and hydroxyl radicals that cleave polymer backbones. Accumulation of peroxyl radical species leads to chain scission in polypropylene and crosslinking in polyethylene matrices. Scission causes severe loss of impact strength and elongation at break in finished molded components.
Processors observe unexpected melt flow index changes when melt processing degraded regrind streams.
Stabilizer Interception
Sterically hindered phenolic antioxidants scavenge oxygen-centered radicals before hydrogen abstraction occurs. Intercepting the peroxyl radical stops auto-oxidation cascades and preserves mechanical performance properties. Phosphite co-stabilizers decompose resultant hydroperoxides into inert alcohol species.
Overloading stabilizer packages causes bloom without providing proportional thermal protection gains.