Capillary Rheometry Shear Rate Correction Protocols for Bimodal High Density Polyethylene
Correcting capillary rheometry data via Bagley and Rabinowitsch protocols is mandatory to prevent up to 50 percent viscosity errors in bimodal HDPE die design.
Extensional Flow
Meaning
A mode of polymer deformation occurs when a melt is subjected to stretching forces along the direction of flow rather than shearing against a solid wall. During this movement, the polymer chains align parallel to the flow direction, which alters the rheological behaviour of the melt. This extensional flow dominates in processes like film blowing and blow moulding where the material is drawn or stretched.
It is governed by extensional viscosity and can lead to melt fracture if the tensile limits of the melt are exceeded.
Tensile Behavior
Measuring the resistance of a polymer to stretching is described by its extensional viscosity, which behaves differently from shear viscosity. Under high strain rates, some polymers exhibit strain hardening where the resistance to stretching increases rapidly, helping to prevent localized thinning. This behavior depends on the branching of the polymer molecules and the overall molecular weight distribution.
Processing Area
Stretching occurs predominantly at the die exit during extrusion and in the free-boundary zones of blow moulding. In these regions, the absence of wall friction allows the melt to elongate freely as it is pulled by downstream equipment. This stretching determines the final wall thickness and molecular orientation of the extruded film, ensuring the finished part has uniform properties throughout its structure.
Melt Fracture
When the tensile stress applied during stretching exceeds the melt strength of the polymer, the material undergoes structural failure. This failure results in surface irregularities or complete tearing of the extrudate. To avoid this defect, processors must limit line speeds or modify the polymer structure to improve melt elasticity.