Meaning
Chemical degradation pathways break covalent backbone bonds within macromolecular chains, reducing average molecular weight and polymer melt viscosity. Extrusion overheating and excessive shear induce polymer scission, compromising the mechanical properties of reprocessed feedstock. Resin datasheets specify maximum thermal exposure limits and shear rates to prevent chain breakage during processing.
Excessive scission shifts molecular weight distribution towards shorter chains, causing drastic drops in impact strength and environmental stress crack resistance.
Molecular Breakdown
Thermal energy, mechanical shear forces, and residual moisture split polymer chains via free radical or hydrolytic mechanisms. Free radicals generated along carbon backbones react with dissolved oxygen, forming peroxides that accelerate further chain breakdown. Higher melt temperatures combined with high screw speed increase shear stresses, tearing high molecular weight entanglements apart.
Moisture present during polyester or polyamide extrusion causes rapid ester or amide linkage cleavage.
Material Degradation
Regrind materials experience multiple heat histories, accumulating scission damage that elevates melt flow index relative to virgin resin. Processing degraded resin leads to flash formation, drooling at the nozzle, and erratic cavity pressure profiles. Thermal stabilizers added during compounding neutralize free radicals to delay chain degradation.
Material specifications set upper limits on allowable regrind blend ratios to preserve baseline tensile performance.
Mechanical Consequence
Chain length reduction degrades long-term creep performance and structural toughness in finished plastic parts. Parts molded from over-sheared resin exhibit premature brittle cracking under environmental stress. Preventing thermal degradation maintains mechanical integrity and extends material reprocessability.
