Characterisation of shrinkage in oriented PET films and containers by thermomechanical analysis (TMA)

In this paper, we report from an extensive series of tests in which the thermal shrinkage behaviour of poly(ethylene terephthalate) (PET) homopolymer and copolymer films and containers has been studied by thermomechanical analysis (TMA). Several independent variables have been investigated, and the accuracy of dynamic TMA scans of temperature‐dependent shrinkage has been verified by parallel measurements made isothermally. Typical shrinkage levels of biaxially oriented films, of draw ratios between 2.7 and 4.1, are of the order of 5% at 100°C. The magnitude of shrinkage increases with draw ratio, and any evidence to suggest that PET copolymer is marginally more prone to shrinkage is thought to arise from differences in strain‐induced crystallinity induced by elongational deformations during processing. Correlations between measurements made on films and commercial containers are good enough to suggest that the simulation technique used is sufficiently valid for further experimentation on processing behaviour.