Thermai treatment of cold‐formed poly(vinyl chloride)

Abstract

Cold‐drawing poly(vinyl chloride) at 24°C increased the yield strength by 25 percent, modulus by 50 percent, and the ultimate strength by 100 percent. The onset of thermal shrinkage was reduced from 80°C (Tg) to 45°C. This thermal instability can be a significant disadvantage of cold‐forming for many applications. It is shown in this study that subsequent thermal treatment at 70°C (T < Tg) re‐establishes a shrinkage onset temperature of 80°C without reducing property levels. The structural changes associated with both orientation and thermal treatment were investigated using DSC, X‐ray diffraction and birefringence. Cold‐drawing produces molecular alignment as measured by birefringence and X‐ray. Thermal treatment of unstretched samples, as well as stretched samples under constraint and stretched samples unconstrained, always leads to a small reduction in free volume as revealed by a measured increase in enthalpy at Tg. However, this free volume change, produces a thermally‐stable oriented structure only when the samples are treated under constraint. Thermal treatment does not stabilize unconstrained samples. Rather it causes almost complete molecular relaxation and a reduction of physical property levels.

Environmental stress cracking of polyvinyl chloride compounds under long‐term stress

“…Supporting evidence for increased polymer free volume and increased molecular mobility at temperatures lower than Tg was shown in the onset of strain recovery, which is reduced from 80°C to 45°C after room temperature drawing of PVC (19). Strain recov-ery in other polymers occurs at lower temperatures than Tg after compressive strain (20).…”