Correlation of thermally stimulated current and blow molding condition in poly(ethylene terephthalate) bottles

Abstract

Thermally stimulated current (TSC) was used to study molecular relaxations in polyethylene terephthalate (PET) bottles. Unstretched PET film, which was used as a model for the bottle preform, exhibited two peaks at 77 and 90°C that correspond to the α and ρ relaxation processes, respectively. The bottles exhibited only the ρ relaxation, which is located within the temperature range for blow molding PET bottles. The α peak is associated with the main glass transition temperature (Tg) and the ρ peak may be associated with a second Tg. The second Tg is attributed to a “constrained state,” which shows dipolar behavior. Heat-shrinkage behavior was examined at 90°C. The maximum TSC (Im) of the ρ peak decreased with increasing heat set temperature, and with decreasing shrinkage. Bottles blown at 113°C showed a lower Im and shrinkage than those blown at 103°C for equivalent heat set temperatures. The higher blowing temperature allowed a higher stretch speed that produced higher crystallinity bottles with self-heat generation during rapid deformation. A relationship between the shrinkage mechanism and the dipole relaxation was proposed.