Elongational rheometry of thermoplastic PET in the rubbery region

Recent advances in processing have witnessed the development of products shaped from amorphous poly(ethylene terephthalate) (PET) by controlled stretching in the temperature region immediately above the glass transition. This paper describes how deformation and rheological data relevant to these processes may be derived from a uniaxial specimen-end-separation Elongational Rheometer, which was originally developed to study the tensile flow characteristics of polymer melts. The operating principles and use of the rheometer are presented, together with details of the various testing modes which are feasible. Some typical results are shown, illustrating test output data for a PET resin of intrinsic viscosity 0·73 dl/g. The steady uniaxial extension data illustrate the sensitivity of various viscoelastic properties (and therefore processing characteristics) to temperature, strain and rate effects, in the processing range 80–110°C.