CHARACTERIZATION OF SHRINKAGE IN ORIENTED PET FILMS AND CONTAINERS BYTHERMOMECHANICAL ANALYSIS (TMA)

In this paper, we report from an extensive series of tests in which th e thermal shrinkage behaviour of poly(ethylene terephthalate) (PET) ho mopolymer and copolymer films and containers has been studied by therm omechanical analysis (TMA). Several independent variables have been in vestigated, and the accuracy of dynamic TMA scans of temperature-depen dent shrinkage has been verified by parallel measurements made isother mally. Typical shrinkage levels of biaxially oriented films, of draw r atios between 2.7 and 4.1, are of the order of 5% at 100-degrees-C. Th e magnitude of shrinkage increases with draw ratio, and any evidence t o suggest that PET copolymer is marginally more prone to shrinkage is thought to arise from differences in strain-induced crystallinity indu ced by elongational deformations during processing. Correlations betwe en measurements made on films and commercial containers are good enoug h to suggest that the simulation technique used is sufficiently valid for further experimentation on processing behaviour.