Meaning
Rheological metric measuring a fluid’s resistance to stretching or elongational flow determines how a material behaves when subjected to tensile forces rather than shear. This extensional viscosity governs processes such as fibre spinning, blow moulding, and film blowing where polymer melts are drawn out. It represents a fundamental material property that diverges from shear viscosity in non-Newtonian fluids.
Deformation Behavior
Industrial polymer processing relies on the response of the melt to rapid stretching, which can cause either strain hardening or strain thinning. When a fluid exhibits high extensional viscosity under tension, it resists breaking and necking, ensuring uniform thickness in the produced film or fibre. Measuring this response requires specialised extensional rheometers that isolate the tensile deformation from any wall-slip or shear effects.
Process Optimization
Selecting materials with the appropriate extensional viscosity prevents premature failure during high-speed extrusion lines. For example, in film blowing, low resistance to extension causes bubble instability and frequent tearing, halting the entire production run. Conversely, excessive resistance demands higher motor torque and increases energy consumption across the plant.
Molecular Structure
The presence of long-chain branching in the polymer architecture directly increases this resistance. Linear polymers slide past one another easily, whereas branched molecules entangle under tension.
