Rheological Identification of Thermal History in Recycled Polyolefin Compounds
Dynamic low-frequency rheometry identifies thermal degradation in recycled polyolefins that single-point melt flow tests miss entirely.
Shear Thinning Index
Meaning
A mathematical descriptor of pseudoplastic behaviour quantifies how rapidly a polymer melt loses viscosity under increasing shear stress during extrusion or injection moulding. Molecular weight distribution and branching determine the shear thinning index of a thermoplastic resin, setting the baseline for how easily chains orient and slip past each other inside a runner system. This dimensionless ratio governs die swell and melt fracture during high speed screw advancement, dictating whether feedstock fills thin walls cleanly or stalls under pressure.
Below critical activation rates, the material behaves as a Newtonian fluid where molecular entanglement resists flow entirely, rendering the index inapplicable at rest.
Viscosity Profile
Polymer characterisation laboratories extract this metric from rotational rheometer sweeps across ascending frequency ranges. High density polyethylene compounds exhibit steep power law slopes because flexible backbones align rapidly within processing channels. Polycarbonate formulations maintain flatter profiles due to rigid aromatic rings resisting conformational change under mechanical force.
Moulders rely on raw material datasheets for incoming resin verification, yet actual machine parameters often alter the effective melt behaviour inside the barrel. Shear heating reduces apparent viscosity locally, meaning thermal history shifts the operational curve away from certified laboratory values.
Injection Mechanics
Screw design and nozzle geometry dictate the mechanical force applied to the melt during plastication and injection phases. Low index compounds require elevated barrel temperatures to achieve adequate mold filling without excessive clamp tonnage. High index grades tolerate faster injection speeds because molecular orientation lowers flow resistance abruptly as the polymer enters the gate.
Gate land dimensions must accommodate this rapid viscosity drop to prevent jetting and surface blemishes on the finished part. Pressure spikes occur immediately if the moulding machine attempts to force a stiff material through narrow orifices too quickly.
Part Defect
Molecular orientation frozen during rapid cooling creates residual stress that warps moulded components upon ejection from the tooling. Insufficient shear thinning during high speed filling leads to incomplete shots and visible weld lines where meeting flow fronts fail to knit properly. Virgin feedstock maintains predictable pseudoplastic responses, whereas excessive regrind incorporation alters molecular weight distribution unpredictably.
Processors compensate for batch variation by adjusting injection velocity profiles to maintain uniform part weight and dimensional stability across long production runs.
Dynamic Rheological Frequency Sweeps for High Molecular Weight Fraction Detection in Bimodal Polyolefins
Dynamic rheological frequency sweeps detect ultra-high molecular weight tails in bimodal polyolefins where standard melt flow rates fail.