Meaning
Extensional viscosity represents the resistance of a fluid to stretching deformation. In polymer engineering, extensional viscosity characterizes the stress response when a material undergoes uniaxial elongation rather than simple shear flow. This property dictates how resins perform during blow moulding or fibre spinning where the polymer melt experiences significant tensile forces.
It remains distinct from shear viscosity because the molecular chains must orient and stretch under high strain rates in ways that shear flow does not provoke.
Processing Resistance
Melt strength depends heavily on the strain hardening behaviour exhibited during stretching. When a resin shows high extensional viscosity at high strain rates, the polymer chains resist excessive thinning during the parison formation stage of blow moulding. Weak materials with low extensional viscosity lead to uneven wall thickness or total rupture of the molten plastic before the mould closes.
Operators adjust the processing temperature to modify the rate at which chains relax, which balances the tensile resistance needed for structural integrity.
Resin Specification
Datasheet values for extensional viscosity rarely translate directly to the shop floor because molecular weight distribution changes significantly between virgin material and regrind. Processing equipment imparts high shear history that alters the entanglement density of the polymer, creating a discrepancy between lab measurements and production performance. Specifications provided by resin suppliers reflect standard conditions that ignore the specific shear history or thermal degradation that occurs in multi-cycle moulding.
Moulders calculate the effective viscosity by observing bubble stability or strand drawdown in real time to verify the batch quality.
Flow Mechanism
Molecular chains require time to disentangle and align along the axis of extension during rapid processing. Linear polymers often show thinning behaviour under tension, while long chain branching creates a strain hardening effect that prevents premature failure. Engineers use this property to predict the drawdown ratio limits of films and the stability of molten extrudates.
Controlled extensional viscosity prevents the defect of thickness variation in blown parts.
