Meaning
Chemical degradation mechanisms cleave polymer backbone chains specifically at tertiary carbon atoms located at chain branch points during thermal or shear stress. Exposure to elevated melt temperatures during extrusion triggers branch scission, reducing high-molecular-weight fractions and altering melt elasticity. The reaction governs structural degradation in branched polyethylenes and polypropylene copolymers during melt compounding.
It stops applying once polymer chains degrade into volatile oligomers or when non-branched linear chains undergo uniform random chain scission.
Rheological Shift
Viscosity drop under high shear rates reflects the shortening of polymer chains and loss of molecular entanglements. Melt flow index values increase sharply after repeated processing passes, signaling irreversible molecular weight reduction. Measurement of branch scission allows moulders to monitor virgin resin degradation when processing scrap regrind.
Blending degraded regrind with virgin pellets lowers melt strength, causing parison sag during blow moulding and wall thickness variation in finished parts. Moulding datasheets list initial melt flow rates for virgin polymer, but shear-induced degradation lowers viscosity inside the injection barrel. Excess shear heating in tight runner systems accelerates this degradation, producing flash on parting lines due to sudden flowability spikes.
Melt Elasticity
Die swell behavior during profile extrusion depends heavily on long-chain branching architectures. Loss of long-chain branches through branch scission impairs the strain hardening response of polymer melts during biaxial stretching operations. Lower melt elasticity results in bubble instability during film blowing, leading to gauge variation and reduced tensile performance.
Extrusion temperatures exceeding recommended limits accelerate tertiary carbon cleavage, forcing processors to lower line speeds to maintain thermal control.
Structural Integrity
Mechanical properties in moulded components decline when average molecular weight falls below critical thresholds. Impact resistance drops dramatically as chain entanglements vanish in degraded resin domains. Tensile failure shifts from ductile yielding to brittle fracture under dynamic load conditions.
Monitoring branch scission provides early indication of thermal abuse during compounding, enabling resin suppliers to adjust antioxidant package formulations.
