ENVIRONMENTAL AGING AND STRESS-CRACKING OF POLY(ETHYLENE-TEREPHTHALATE)

The environmental stress-cracking behavior of poly(ethylene terephthalate), PET, has been studied by exposing PET sheet samples to a stress-cracking liquid, at constant stress, for various periods of time. Samples evaluated are of equivalent molecular weights but vary in copolymer concentrations. The characterization of exposed samples includes examination of craze formation and density changes, measurement of tensile mechanical properties, and evaluation of changes in their enthalpies of relaxation by differential scanning calorimetry, DSC. Data analysis includes the correlation between the number of crazes formed as a function of exposure time and the nature of tensile failure. The relationship between measured enthalpy of relaxation and brittle vs. ductile failure is also established. Among the conclusions to be discussed are: 1) Samples with large numbers of crazes do not necessarily exhibit brittle failure during tensile evaluation; and 2) Brittle failure may be associated with crazing, but it must be caused by other factors.