Meaning
Polymer chain cleavage and crosslinking caused by prolonged exposure to oxygen and heat defines auto-oxidation degradation. Radical formation within the hydrocarbon backbone initiates a cascade of molecular structural shifts. This mechanism breaks covalent bonds while simultaneously creating unwanted side chains during melt processing.
Persistent radical activity continues within the solid state long after the cooling phase concludes.
Thermal Sensitivity
Moulders observe this chemistry when high barrel residence times exceed the stable operating window for a specific resin. Polypropylene remains susceptible to rapid viscosity drops as the polymer chains fragment under sustained thermal stress. Excessive heat in the nozzle zone often triggers localized yellowing or brittle failure in the final part.
Production teams avoid these issues by balancing screw speed against melt temperature to maintain steady state conditions.
Processing Impact
Degradation alters the flow characteristics of raw materials during the injection stroke. Higher melt flow index values emerge from regrind batches because the recycled polymer chains possess lower average molecular weights than virgin pellets. Extrusion lines manage this instability by monitoring back pressure fluctuations during the transition between virgin material and industrial scrap.
Inconsistent flow leads to unstable fill patterns and dimensional variation across long production runs.
Economic Consequence
Increased scrap rates during startup procedures drive up the total cost of finished goods. Moulders incur further expense when additive packages fail to stabilize the resin against secondary oxidation cycles. Replacement of oxidized inventory prevents the loss of mechanical properties in long term structural applications.
Proper management of heat history secures the integrity of the part against premature failure.
