Failure of Polyethylene Terephthalate Assemblies

Cracking occurred within the plastic jacket (injection molded from an impact-modified, 15% glass-fiber-reinforced PET resin.) of several assemblies used in a transportation application during an engineering testing regimen which involved cyclic thermal shock (exposing the parts to alternating temperatures of -40 and 180 deg C (-40 and 360 deg F)). Prior to molding, the resin had reportedly been dried at 135 deg C (275 deg F). The drying process usually lasted 6 h, but occasionally, the material was dried overnight. Comparison investigation (visual inspection, 20x SEM views, micro-FTIR, and analysis using DSC and TGA) with non-failed parts supported the conclusion that that the failure was via brittle fracture associated with the exertion of stresses that exceeded the strength of the resin as-molded caused by the disparity in the CTEs of the PET jacket and the mating steel sleeve. The drying process had exposed the resin to relatively high temperatures, which caused substantial molecular degradation, thus limiting the part's ability to withstand the stresses. The drying temperature was found to be significantly higher than the recommendation for the PET resin, and the testing itself exposed the parts to temperatures above the recognized limits for PET.