Uni‐ and biaxial stretching of chlorinated pvc sheets. A fundamental study of thermoformability

Abstract Chlorinated PVC is superior to unmodified PVC as a thermoplastic for use in thermoforming, especially if improved heat resistance and dimensional stability are required. In the present report, results tire given of a fundamental experimental study on the thermoformability of CPVC sheets obtained by calendering various formulations based on CPVC resins with at least 65 percent chlorine content. Extensibility as well as the relationship between stress and strain in uni‐ and biaxial stretching have been determined as a function of temperature and rate of stretching by means of specially devised, highly instrumented laboratory equipment. Stress‐strain relations under isothermal conditions and at constant strain rate are compared for the two modes of stretching, and the difference in behavior between PVC and CPVC, particularly with regard to the effect of temperature, is emphasized. Internal stresses frozen in during cooling, following rapid stretching at appropriate thermoforming temperatures, have been determined by means of a detailed analysis of retractive force measurements. The relationship between internal stress and molecular orientation is discussed as well as the effect of the latter parameters on various properties of technological interest: dimensional stability, impact resistance, and gas permeability.