Biaxial hot-drawing of poly(ethylene terephthalate): An experimental study spanning the processing range

An experimental study was made of hot-drawing of PET films under biaxial stress, relevant to industrial orientation processing. The aim was to generate a data set more comprehensive than previously available, to support the development of an improved constitutive model for PET, suitable for process modelling. Isotropic, almost-amorphous films were prepared by extrusion from two grades of PET, with differing molecular weight. The ranges of strain-rate and temperature encompassed were 0.2–16 s−1 and 81–116 °C. Constant width (CW) and equal-biaxial (EB) straining was employed. The material response varied dramatically across the range, from glass-like with post-yield strain-softening at the highest rates and lowest temperatures, to elasto-viscous fluid-like flow at the lowest rates and highest temperatures. Effects of differing molecular weight were seen at the highest temperatures, with the higher molecular weight polymer showing more pronounced strain-stiffening. The results were interrogated so as to identify and quantify agreement and disagreement with the constitutive model proposed previously (Adams AM, Buckley CP. Polymer 2000; 42: 771–786), thereby informing revision of the model.