Blends of Poly(ethylene terephthlate) with bisphenol-A polycarbonate
Blends of Poly(ethylene Terephthalate) with Bisphenol-A-Polycarbonate The objective of the study was to determine the extent to which bisphenol-Apolycarbonate (PC) influences the rheology, processing behaviour and subsequent crystallinity and mechanical properties of poly(ethylene terephthalate) (PETP) in the 'compatible' region, when the polycarbonate is the minor blend component. Also, further characterisation of this blend system, in terms of miscibility and micro- structure development was required. After careful drying in a desiccant-type hopper drier melt-phase, blending was carried out using a twin-screw extruder with a purpose-designed screw configuration. Blends were made up at three levels (PETP/PC); 85/15,80/20 and 75/25 using three different molecular weights of PC, and virgid-materials were also included in the study. The extrudates were then dried and injection moulding was carried out under various conditions, which were chosen to modify the degree of order in the crystallisable phase. The effects of PC on the shear-flow behaviour of the blends was examined, and in general the PETP/PC 80/20 blends demonstrated lower shear viscosities than expected from additivity. Otherwise the shear flow behaviour was generally consistent with blend composition. Thermal analysis and crystallisation behaviour of the blends were investigated to determine the effects of PC on the crystallisability of the blend and the PETP T. Solid state isothermal crystallisation behaviour was studied using a modified thermal analysis technique. Crystallisation of the PETP portion of the blend was shown to be impeded by PC. A specific and rapid technique has been developed to determine depth-dependent orientation distributions Three dimensional analyses of birefringence were obtained for moulded plaques of various PETP/PC blends. The orientation distribution was in accordance with the flow pattern during processing and was noticeably planar in nature. However, the level of orientation in the mouldings investigated was very low. Thermal analysis and microscopy techniques indicate there is no evidence for miscibility in the blends. The effect of PC molecular weight and content on mechanical properties of the blend was investigated. Generally, the PETP/PC blends exhibited improved toughness, in terms of total energy absorbed, and the properties were influenced further by the degree of crystallinity. It has been shown that mechanical properties of the blend deteriorate rapidly when samples are stored for extended periods in water at 70'C. Due to the PET? portion in the blends crystallising, and hydrolysis, the samples pass into the brittle mode. The addition of PC to PETP was found to modify the thermoelastic behaviour. Addition of PC permits thermoelastic processing of PETP/PC blends over a wider temperature range than would be suitable for PETR Also, the addition of PC appears to accelerate the onset of strain-induced crystallisation in the PETP.