BIAXIAL ORIENTATION OF POLY(ETHYLENE-TEREPHTHALATE) .2. THE STRAIN-HARDENING PARAMETER

Results of a study of the orientation characteristics of extruded poly(ethylene terephthalate) (PET) (homopolymer) sheets are discussed with respect to stretching conditions. PET exhibits strain-induced crystallization upon being stretched to large extension ratios. This may lead to strain-hardening, which is accompanied by an upswing in the stress-strain curve recorded during the process of extension. A parameter pertaining to the extension ratio, at which the onset of strain-hardening occurs during stretching of the specimen, is defined. This is called the strain-hardening parameter (SHP). The dependence of the SHP on the mode of stretching, the temperature of stretching, the strain rate, and the limiting extension ratio in the first stretch direction, are studied. It is seen that the SHP in both simultaneous and sequential modes decreases with increasing strain rate and the limiting extension ratio in the first stretch direction, while it increases with increasing temperature of stretching. For specimens stretched in the simultaneous mode at 90-degrees-C, SHP versus extension rate shows a minimum. This could be due to viscous dissipation at high rates of extension, Also studied is the relationship of strain-hardening and the SHP with strain-induced crystallization. It is seen that for specimens stretched in the uniaxial constrained mode, the rapid increase in properties with extension occurs much before the upswing in the stress-strain curve. The upswing occurs at the extension ratio at which the increase in properties once again becomes gradual. This means that the upswing in the stress-strain curve indicates significant completion of strain-induced crystallization and the establishment of structural order within the specimen.