Heat-setting of biaxially oriented PET

The relatively low deformation temperature of biaxially oriented PET excludes its use in many hot fill applications or hot washing as part of a reuse cycle, due to the shrinkage which generally occurs on reheating towards Tg. Isothermal, isometric annealing processes (referred to as heat-setting) can cause oriented PET to maintain its dimensional stability to higher temperatures. However, to experiment on a commercial stretch blow moulding machine is a very costly and time consuming process, justifying the requirement for an accurate simulation technique. The objective of this project was, therefore, to simulate the stretch blow moulding of PET with a heat-setting phase, by biaxially drawing PET sheets on a TM Long Stretcher, then to heat-set them in a separate phase using a purpose designed jig. A heat-setting rig was designed and constructed at IPTME. Due to the size of the proposed investigation window, a factorial experimental design (FED) technique was employed to characterise the behaviour of two commercial bottle grades of PET (B90N and B95A Laser) under a variety of draws and heat-setting conditions. These sheets were then characterised in terms of shrinkage, tensile properties and resistance to creep. This data was regressed to generate predictive model equations for both grades of material and in both major and minor draw axes. Using these equations, it is now possible to define a set of processing parameters which would give a bottle with lower and more uniform shrinkage (less draw dependent) with enhanced creep properties and at least comparable mechanical properties. To develop a more fundamental understanding of the process of heat-setting in biaxially drawn PET, as well as to prove the validity of the model equations, a separate series of conventional experiments was devised to investigate the two most significant predictor variables from the FED analysis; biaxial draw ratio and heat-setting temperature (for Laser only). This material was characterised by TMA, DSC and birefringence measurements. The DSC and birefringence measurements allow the effect of heat-setting and draw on crystallinity levels and orientation to be monitored. Orientation of both phases was studied in heat-set bottles using micro polarised FTIR.