High Shear Rheology Calibration Thresholds for Recovered Polyolefin Industrial Blends
Calibrating high-shear capillary rheology via Bagley and Weissenberg-Rabinowitsch corrections prevents off-spec polyolefin lot processing failures.
Flow Activation Energy
Meaning
Thermodynamic parameters describe the sensitivity of polymer melt viscosity to temperature changes. In polymer processing, the flow activation energy governs how much the viscosity drops when the processing temperature is raised. Resins with high activation energy values exhibit sharp viscosity changes in response to small adjustments in the heater bands.
Material Sensitivity
High values of the flow activation energy are typical for polymers with rigid backbones or bulky side groups, such as polycarbonate and polystyrene. In contrast, polyolefins like polyethylene have low activation energy, meaning their viscosity remains relatively stable even when temperatures change by several degrees. This difference determines how easily a moulder can adjust flow behavior during a production run.
Linear chains slide past each other with less energy required to overcome the flow barrier, making polyolefins highly tolerant of minor thermal variations across the barrel zones.
Process Stability
Tight control of barrel temperatures is essential for resins with high activation energy to prevent shot-to-shot variation. If the temperature fluctuates by even a small margin, the resulting viscosity change alters the flow front speed, causing part weight variations and surface defects. This thermal sensitivity makes it difficult to maintain tight dimensional tolerances across long production shifts.
Moulding Tradeoff
Increasing the melt temperature reduces the viscosity but accelerates thermal degradation of the polymer. For materials with high flow activation energy, a modest temperature rise can lower the injection pressure enough to fill thin sections. This tactic must be balanced against the risk of polymer chain scission, which degrades the mechanical strength of the finished part.