Cristalização a frio do PET e das blendas PET/PS e PET/SAN.

Abstract

The cold crystallization of PET and the blends with polystyrene (PS) or styrene-co-acrylonitrile (SAN) was investigated by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMA). Kinetics parameters of isothermal crystallization of PET and blends PET/PS and PET/SAN were determined by Avrami theory, while Ozawa and Mo theories were used to non isothermal crystallization. Morphology of PET and blends were analyzed by optical microscopy (OM) and scanning electron microscopy (SEM). PET/PS and PET/SAN blends form separate phases, as confirmed by DMA and SEM, showing distinct phases and two glass transition temperatures. On the other hand, the determination of the equilibrium melting temperature ( om T ) of PET indicated that this parameter decreased when PS or SAN were added, suggesting that a limited solubility shall exist. The presence of non crystalizable molecules like polystyrene or poly (styrene-coacrylonitrile), partially soluble in the PET cristalizable phase, reduces the driving force for crystallization. In the current study this effect was observed as a shift of the cold crystallization DSC peaks to higher temperatures and also by a reduction in the rate of cold crystallization. Crystallization occurring in two stages was observed by kinetics theories. The rate constants K, (T) K andK'(T) , determined by Avrami, Ozawa and Mo approaches, respectively, decreased significantly when PS or SAN were present. The blends containing only 1 wt% of PS or SAN had the same magnitude of reduction in the crystallization rate as the blends with higher PS and SAN contents. This behaviour has a high practical significance since in low concentrations of PS or SAN the mechanical properties of PET are not affected, as also shown in this study. Cold crystallization double peaks were observed in DSC scans of PET/PS and PET/SAN blends, due two stages crystallization. The existence of different molecule species with different crystallinity degrees, might also have contributed to the crystallization behaviour.