Extrusion Shear Induced Degradation Ratios in Recycled Polyethylene Manifold Systems
Maintain manifold shear rates below 300 s⁻¹ and specify 25-minute minimum OIT to prevent catastrophic molecular weight loss in recycled polyethylene extrusion.
Thermal Stabilizers
Meaning
Chemical additives introduced to polymer formulations suppress degradation reactions driven by high temperatures during compounding and processing. Thermal stabilizers retard dehydrochlorination in polyvinyl chloride and intercept free radicals generated by shear and heat in polyolefins. Exceeding processing thresholds causes polymer chain scission or crosslinking, destroying mechanical performance and altering melt viscosity.
The additive system acts through scavenging hydrochloric acid, absorbing ultraviolet energy, or decomposing peroxides before molecular weight breakdown occurs.
Degradation Kinetics
Polymer chains undergo thermal scission when barrel temperatures exceed recommended processing windows for extended residence times. Thermal stabilizers interrupt chain propagation by donating hydrogen atoms to peroxy radicals or absorbing free chlorine ions liberated during processing. Unstabilized formulations exhibit rapid yellowing, molecular weight reduction, and loss of tensile strength during single screw extrusion.
Moulders balance stabilizer concentration against plate-out tendencies and cost constraints inherent in virgin resin purchasing.
Resin Economics
Virgin polymer pricing incorporates additive packages calibrated for baseline processing parameters without secondary modification. Regrind incorporation introduces cumulative heat histories that deplete the original stabilizer package, necessitating booster masterbatch additions to prevent gel formation and embrittlement. Virgin resin sheets provide predictable melt flow indices, whereas high regrind ratios shift viscosity profiles unpredictably across identical machine settings.
Moulders quantify financial losses from degraded parts against the marginal expense of dosing additional stabilizer concentrates during compounding.
Moulding Variance
Datasheet values represent laboratory conditions that rarely reflect thermal environments inside production injection moulding barrels. Screw design, back pressure, and cycle time variations subject polymer melts to localized overheating that outpaces baseline stabilizer capacity. Excessive residence time in hot runner systems induces localized polymer degradation even when barrel zone temperatures remain within nominal limits.
Finished parts display surface blemishes, reduced impact resistance, and dimensional instability when thermal stabilizers fail to arrest degradation under extreme shear forces.