Properties Of Flame Retardant Thermotropic Liquid Crystal Copolyesters (LCPs) And PET/LCP Blends

A kind of thermotropic liquid crystal copolyesters (LCPs) containing phosphorus derived from 2-(6-oxide-6H-dibenz〈c, e〉〈1, 2〉oxa phosphorin-6-yl)-1,4-dihydroxyphenylene, 4-hydroxybenzoic acid, terephthalic acid and isophthalic acid, were synthesized by melting polycondensation. The structures and properties, such as liquid crystallinity, thermal behavior, crystallinity, and thermal stability were investigated by FT-IR, hot-stage polarizing optical microscope, differential scanning calorimetry (DSC), wide-angle X-ray diffractometry (WAXD), and thermogravimetry analysis (TGA). The results showed that the copolyesters were thermotropic. The schlieren textures were observed when heated above 240℃. The crystallinity of LCPs was very low. The Tg values of LCPs were above 190℃ and the softening temperature above 240℃. The LCPs possessed good thermal stability and higher char yield (34.2%).The LCPs were blended with PET and the flame retardancy, the crystallinity, thermal oxidative degradation behaviour of PET/LCP blends were investigated via different methods such as LOI values and UL-94 rating, different scanning calorimetry (DSC) and thermogravimatric analysis (TG). LCPs improved dramatically the flame retardancy and the melt dripping behavior of PET. DSC showed that LCP accelerate the crystallization rate of PET. Some kinetic parameters of thermal oxidative degradation were evaluated. It was proved that the PET/LCP blends had good thermal stability and that the addition of LCP did not greatly influence the degradation of PET. The morphology and mechanical properties of PET/LCP blends were investigated. Spherical particles were observed by SEM in extruded strands and elliptoid particles in standard mechanical samples of the blends. When the the content of LCP (intrinsic viscosity 2.01dL/g) was 10 weight percent, the tensile strength of the PET/LCP blends increased by 1.44 times than that of PET. The rheology properties of PET and LCPs were also investigated. The result showed that both PET and LCPs exhibited a shear-thinning phenomenon and the apparent viscosity of melt of PET and LCPs were especially sensitive to temperature and decreased sharply with the increasing of temperature.