Meaning
Thermal analysis profiles occasionally display double-peak oxidation during the degradation testing of semicrystalline polymers. Differential scanning calorimetry traces reveal this split exotherm when distinct crystalline lamellae melt at different temperatures while simultaneous oxidative chain cleavage proceeds. Regrind material containing prior thermal history frequently exhibits this anomalous double peak due to heterogeneous molecular weight distributions created by previous processing cycles.
Part specifications governed by high service temperature requirements demand careful monitoring of this oxidation behavior to prevent premature embrittlement during prolonged thermal exposure.
Thermal Kinetics
Molecular mobility changes during the melting transition directly influence oxygen diffusion rates within the polymer matrix. Lower melting crystallites melt first and create amorphous fractions where oxygen dissolves rapidly and reacts with free radicals generated during scission. Subsequent melting of higher stability lamellae releases additional amorphous material, which triggers the second distinct oxidation rate maximum on the thermogram.
Virgin resin processing stability relies on uniform molecular weights that suppress this splitting phenomenon entirely.
Resin Degradation
Additive packages added during compounding inhibit radical propagation and alter the relative heights of both oxidation peaks. Excessive shear heating inside the extruder barrel accelerates polymer chain scission and promotes the formation of low molecular weight fractions that oxidise prematurely. Moulding machine temperature profiles must avoid localized hotspots that initiate polymer degradation before the melt reaches the nozzle.
Uncontrolled moisture levels during processing hydrolyze condensation polymers and worsen the split exotherm severity by increasing terminal hydroxyl groups.
Quality Variance
Datasheet values measured on virgin resin pellets rarely predict the oxidation behaviour of moulded components after multiple melt cycles. Processing parameters such as back pressure and screw speed alter residence time distribution and shift the magnitude of both peaks during subsequent thermal scans. Component failures in service correlate directly with the presence of the lower temperature oxidation peak because those fractions degrade fastest under mechanical stress.
Tight control over melt temperature and regrind addition rates prevents the compositional drift that causes these anomalous thermal signatures in production parts.
