ELONGATIONAL DEFORMATION AND STRETCH BLOW MOLDING OF POLY(ETHYLENE-TEREPHTHALATE)

A purpose-designed elongational rheometer has been used to study the f ree-surface viscoelastic stretching characteristics of a grade of PET homopolymer, in the so-called rubbery region above the glass transitio n. Rheological data are presented according to the technique developed by Meissner, whereby a stress growth function is plotted against expe rimental time. The effects of temperature and elongational strain rate have been determined, and the results show how the data for PET, at e ach temperature between 80 and 110-degrees-C, superpose at small times , then deviate according to the experimental strecthing rate, i.e. at a critical elongational strain. Parallel experimentation has shown tha t the position of baseline deviation corresponds to the onset of defor mation-induced crystallinity in PET. Practical injection stretch blow moulding (ISBM) trials have been carried out, in order to verify the r ate-dependent stretching behaviour predicted from rheometric analysis. It is shown that the development of strain-induced crystallinity is a ccelerated at higher inflation pressure, and is a desirable property w hich imparts uniform thickness distribution and enhanced physical prop erties to moulded bottles. It is anticipated that the rheological tech nique described can be used as a predictive tool for the purposes of n ew material development, and also for process optimisation and product design.