Meaning
Thermal and mechanical energy input breaks polymer backbone bonds during melt processing, generating highly reactive unpaired electrons known as radical chain scission. Extrusion and injection moulding machines subject molten thermoplastics to high shear rates and residence times, driving this molecular degradation pathway. Excessive barrel temperatures accelerate bond homolysis, reducing the average molecular weight of the polymer melt before the material enters the mould cavity.
Degradation Kinetics
Shear stress gradients inside the screw channel govern the rate of molecular weight reduction during plastication. Unstabilized homopolymers undergo rapid backbone cleavage when residence times exceed critical thresholds at elevated processing temperatures. High molecular weight fractions experience preferential chain rupture because longer polymer coils intercept more mechanical energy from rotating screw flights.
Thermal Stability
Antioxidant packages scavenge reactive intermediates before the degradation cascade spreads through the polymer melt. Virgin resin formulations contain hindered phenols and phosphites designed to terminate radical propagation reactions during multiple thermal cycles. Introducing regrind material into the feed throat dilutes the active stabilizer concentration, causing viscosity drops that compromise part dimensional repeatability.
Material Specification
Datasheet melt flow rate values reflect virgin polymer characteristics measured under standardized laboratory conditions rather than the thermal history experienced on shop floors. Compounders establish processing windows by tracking intrinsic viscosity retention across simulated moulding passes. Shear induced degradation alters crystallization kinetics, shifting shrinkage rates and leaving moulded components vulnerable to premature mechanical failure under load.
