Resumen de Poly(ethylene terephthalate)/polycarbonate blends prepared by reactive extrusion: thermal and mechanical characterizations
The objective of the study was to determine the extent to which the mixing of bisphenol-A-polycarbonate (PC) and polyethylene terephthalate (PET), in the presence or absence of catalyst, influences the rheological, physico-chemical, dynamic mechanical, mechanical and fracture properties of the blends in the partial compatible region, when the PC is the major blend component. After careful drying, blending was carried out using a twin-screw extruder. Blends were made up at three levels (PC/PET); 90/10, 80/20, and 70/30 in the presence/absence of three based-catalysts by 0.05%w/w of the final product; Ca, Zn, and Sm, and virgin materials were also included in the study. The extrudates were then dried and sheet extrusion was carried out under certain conditions.
It was found that by decreasing the amount of PET in the non-catalyzed blends, the morphology becomes refined with smaller average particle sizes and a better dispersion. The existence of catalyst in blending processes, affects much the morphology by means of mean particle size reduction of the dispersed PET droplets, morphology refined and more uniform dispersion which contributed by a decrease in interfacial tension as a result of an enhancement in miscibility during transesterification reactions.
Thermal analysis and crystallization behavior of the blends were investigated. Glass transition (Tg) of PET tends to increase while Tg of PC tends to decrease comparable with the virgin materials when catalyst system was used. Crystallization of PET portion of the blend was shown to be impeded by PC which related to the droplet size and the dispersion of dispersed PET phase in the blends.
Dynamic mechanical thermal analysis was carried out for all systems. It was found that modification of Tg of the blend systems depends on the composition and the activity of catalyst. Sm and Zn were found very active while Ca was found less active. The secondary relaxations (Tß) of catalyzed blends increased compared with neat PC while decreased in comparison with neat PET. By the other hand, the intensity of Tß peak decreased in the presence of catalyst. Also activation energy of all materials was estimated by a simple method For tensile properties, the ductility of blends was increased by PET and the presence of catalyst. The blends exhibited improved toughness in the absence of catalyst and the properties were influenced further by the degree of transesterification. There is a significant modification on the yielding stress of catalyzed blends especially at the presence of Zn while there is no change occurred on the values of non-catalyzed blends. These observations indicate that, not only the simple interface adhesion must be considered, but also the possibility occurrence of a modification in the viscoelastic behavior of the phases as a result of transesterification reactions, as well as the morphological characteristics (size and distribution) of the products.
Fracture behavior was analyzed by EWF approach at low strain rate. There is a decrease on the essential term (we) values of non-catalyzed blends except PC90 as an indication to the weak interaction between their components while all catalyzed blends present an increment which may related to the product of the transesterification that plays like an emulsifier/compatibilizing agent to reduce the interfacial tension between the components of the blend and the favored morphological characterizations of catalyzed blends help to get a better fracture behavior. Non-catalyzed blends exhibit an increase on the non-essential term of (ßwp) by increasing the amount of PET which may due to the strain- induced crystallization and to the lowering of the yielding stress. In contrary, catalyzed systems and especially in the presence of Zn show decrease as a consequence of the restriction that occurred on the movement of PC segments during the transesterification reactions or as a decohesion of the dispersed phase during the test.
