Polymer Rheology
Viscoelastic characterization defines the response of molten materials under transient uniaxial elongation. Extensional stress growth identifies the specific evolution of tensile resistance during the start-up of steady flow. Higher molecular weight polymers exhibit a rapid increase in stress until reaching a steady state, whereas materials with broader distribution or long chain branching show an pronounced overshoot before settling.
This material property limits the speeds allowed on film blowing towers and high speed coating lines because excessive resistance ruptures the melt curtain before it achieves the final gauge.
Dynamic Resistance
Melt strength depends on the internal arrangement of polymer chains as they align along the machine direction during cooling. Extensional stress growth remains sensitive to the presence of entanglements that resist immediate orientation. Converting machines operating at high tension require consistent polymers that display predictable stress profiles to prevent thickness variations across the web.
Variations in raw resin batches alter the timing of this growth, which leads to unstable necking or localized thinning in thin film extrusion.
Performance Constraint
Precise monitoring of this physical interaction provides processors with a predictive tool for evaluating runnability on high speed lines. High extensional stress growth creates a substantial demand on mechanical haul-off systems, requiring increased motor torque to maintain constant web velocity. Low values during trial runs indicate a high probability of melt fracture or curtain instability at production speeds.
Optimal calibration of processing temperature minimizes the variance in this growth across a single production lot.
