Meaning
Dynamic flow properties define the resistance of a polymer melt to stretching deformation over a specific time interval. The measurement of transient extensional viscosity is particularly relevant in processes like film blowing, blow molding, and thermoforming where the melt is subjected to rapid extension. This property determines how well the polymer can resist localized thinning.
Extensional Behavior
Linear polymers typically exhibit strain thinning, where the resistance to stretching decreases as the deformation rate increases. By contrast, branched polymers show strain hardening, which causes the transient extensional viscosity to rise rapidly during extension. This hardening behavior stabilizes the polymer melt, preventing the bubble or sheet from rupturing.
Processing Defect
Low resistance to stretching leads to melt instability and defects such as draw resonance or non-uniform thickness. Monitoring the transient extensional viscosity helps predict these issues before the resin is run on the production line. When the viscosity drops too low during the stretching phase, the polymer cannot support its own weight or the applied tension.
This results in cosmetic lines, thin spots, or complete failure of the continuous web. By selecting a resin with the appropriate strain-hardening characteristics, processors can avoid these defects and maintain a stable production run.
Part Quality
Consistent thickness distribution in the finished part relies on the strain-hardening behavior of the molten polymer. High values of transient extensional viscosity ensure that the material stretches uniformly across the mold surface. This uniformity improves the barrier properties and impact resistance of the final package.
