The effects of roll drawing on the structure and properties of oriented poly(ethylene terephthalate)

The effect of processing conditions on the structure and properties of roll drawn poly(ethylene terephthalate) (PET) was examined. It was found that, when roll drawing amorphous PET at temperatures just above the glass transi tion, only very low draw ratios were obtained. This is probably because the re were no crystallites to lock in the applied extension. Roll drawing at h igh temperatures, above 130 degrees C, where there was significant thermal crystallization, produced film of high strength. At temperatures between 13 0 degrees C and 190 degrees C, the properties were almost independent of pr ocessing temperature. Mechanical tests performed on roll drawn samples, pro cessed in this temperature range, showed that the initial modulus and the y ield stress increased linearly with draw ratio. The yield strain decreased with draw ratio up to lambda = 4.0, and then became almost constant. The pr ocessing temperature that produced samples with the greatest strength was 1 70 degrees C. This was because the highest draw ratios were obtained at thi s temperature while maintaining constant width deformation. At low draw rat ios, the crystallinity increased with the processing, whereas at higher dra w ratios, it was independent of temperature. This constant level of crystal line fraction may have produced the constant failure strain that was observ ed at high draw ratios. The orientation functions were similarly unaffected by the processing temperature, although birefringence measurements did sug gest that lower processing temperatures may have produced higher levels of orientation. The orientation of the trans conformers was independent of the temperature, but the overall content was increased at higher processing te mperatures.