Synthesis Of Polyarylate-polysiloxane Block Copolymers And Low-temperature Toughening Polycarbonate

Polycarbonate(PC) is one of the five engineering plastics. It possesses high mechanical strength, high heat resistance, electrical insulation, good dimensional stability and many other excellent performances, has been widely applied in the fields of machinery, aviation, transportation, optics, electronics, agriculture, textiles and medicine, and so on. However, some disadvantages of PC increasingly expose in the practical application, including the poor notched impact strength at low temperature which severely limits the application of PC in cold region or low temperature environment, the relative low fire-retardant level(UL-94 V-2) which could not meet the requirement in certain areas, and the difficulty of preparing the large thin-wall injection molding items for its high melt viscosity. Overcoming the above-mentioned shortcomings is the fundamental way to solve the practical problems of PC application, and also is the target of this work. Compared to PC, polydimethylsiloxane(PDMS) has the complementary characteristics of low temperature flexibility, low surface energy and fire-retardancy. It is expected that the block copolymer containing both PDMS and polyarylate structure(PARSi) could improve compatibility of PDMS and PC, then simultaneously elevating PC properties of the low-temperature toughness, the melt fluidity and flame retardancy by the characteristics of PDMS.Acyl chloride terminated polyarylate oligomer was synthesized through the interfacial polycondensation of bisphenol A and m-phthaloyl chloride in the presence of phase transfer catalyst of benzyl triethylammonium chloride.Three kinds of amino-terminated PDMS with the number average molecular weight of 2500, 5000, 27000 were respectively further reacted with acyl chloride terminated polyarylate oligomer to obtain three types of polyarylate-polysiloxane block copolymers(PARSi2.5, PARSi5, PARSi27). The chemical structure of block copolymers was characterizated by nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy and X- ray photoelectron spectroscopy, and the thermal stability of copolymers was also investigated by the thermal gravimetric analysis.PC and PARSi blends(PC/PARSi) were prepared through melt blending methods. It is focused on the influences of PARSi loadings and the number average molecular weight of PDMS blocks on the properties of low-temperature toughness, melt fluidity, flame resistance and the relative mechanism. Meantime, the tensile, flexural, thermal stability and water contact angle of PC blends were also measured and analysed.The results showed that PARSi could significantly improve the low-temperature toughness of PC at loading of 5 wt% PARSi27, and the corresponding PC blend possessed the high impact strength(52.2 kJ/m2) at-50 ℃ and this impact strength value increased 3.7 times comparing to that of PC. Increasing the molecular weight of the polysiloxane block or appropriate content of PARSi could both effectively improve the low-temperature toughness of PC. From the impact section morphology of PC blends, the main mechanism of enhancing low-temperature toughness was as described followings, the micron-sized holes formed via the interface debonding between PC and PARSi absorbed the great energy, and those holes caused the high elastic deformation of PC matrix. In improvement in melt fluidity, PDMS with flexibility and low surface energy from PARSi acted as a lubricant, which weakened the friction between PC melt flow and the wall or inner PC melt flow, hence, PC melt viscosity and flow rate were obviously reduced and increased, respectively. For example, the shear viscosity at a shear rate of 3686 s-1 and flow rate of the PC blend with 5 wt% PARSi27 could decrease to 138 Pa?s and increased to 29.1 g/10 min, respectively, correspondingly reduced by 37% and increased 58% comparing with those of PC. In addition, the melt fluidity of PC blends increased by increasing amounts of PARSi and PDMS block molecular weight. The effect of PARSi27 loadings on the flame retardancy was unlike to that on the low-temperature toughness and melt fluidity of PC blends. Only PC blends with relative low contents of PARSi displayed the good flame retardancy. PC blend containing 2 wt% of PARSi could obtained a high retardant level of UL-94 V-0@1.6 mm, and its peak heat release rate(196 kW/m2) decreased to 57% of that of pure PC. The closed bubbles were formed on the inner layer of residual carbon, the polysiloxane chains migrated to the outer surface and the structure of-Si-O-,-Si-C- was formed on the outer layer of residual carbon during the combustion process. It is concluded the main flame-retarded mechanism of PC blends in the condensed phase.The hydrophobicity of PC/PARSi27 was also enhanced to a certain extent contributed by the low surface energy of PDMS blocks. Furthermore, the tensile and flexural properties, as well as thermal stability of PC/PARSi27 varied within narrow limits. When PC/PARSi27-5 possessed the maximal low-temperature toughness, the tensile strength and flexural strength decreased by 9% and 13% compared to those of PC, and the 5% weight loss temperature fell from 496 ℃ of pure PC to 445 ℃. PC/PARSi blends prepared in this work have a good overall performance to meet the requirements of low-temperature, flame-retardance application, and are expected to be applied in large thin-wall injection molding items.