Study On The Crystallization Behavior And Thermal Stability Of Modified Film-use Polyester

Biaxially oriented polyester film (BOPET) with excellent performance shows a wide range of applications from packaging to electron, optics, image, magnetic, etc. But BOPET film still has some intrinsic defects, such as slow crystallization rate, low heat resistance level, especially as electrical insulating materials. To solve these problems, PET was modified by adding the third monomers during its polymerization process to produce copolyesters which could improve the crystallization and thermal stability of BOPET films.Modified copolyesters were prepared from TPA and EG, with Blycerol (GL) and 4,4'-bibenzoic acid (BPDA) as the third monomor respectively. Molecular structure and composition of the copolyesters were confirmed by NMR and FT-IR, and molecular weight of the copolyesters were measured by GPC and intrinsic viscosity. The results demonstrated that not only GL and BPDA successfully linked to PET macromolecular chain by copolymerization respectively, but also high molecular weight polymers were obtained.Non-isothermal crystallization processes of GL-PET copolyesters were investigated by differential scanning calorimetry (DSC) under different heating rates, and the crystallization kinetics of copolymers were analyzed through Avrami Ozawa and combined Avrami with Ozawa equations respectively. The results indicated that the crystallization rates of GL-PET were higher and the crystallizabilities of them were lower than that of pure PET. But the crystallization rates and kinetic crystallizabilities of GL-PET were not linear correlation with contents of GL. When content of GL was 900ppm, the copolyester diplayed the highest crystallization rate constant and kinetic crystallizability. By comparing different methods, the modified Avrami and combined Avrami with Ozawa equations were found to analyze the non-isothermal crystallization processes reasonably.The DSC analysis of BPDA-PET copolyester showed that with the increasing content of BPDA, the glass transition temperature (Tg) of BPDA-PET was enhanced and the melting point (Tm) decreased. Combining XRD and DMA results, it was showed that crystallization in BPDA-PET was mainly composed of crystal structure of PET, but when content of BPDA was 25%, the copolyester exhibited a few crystalline characteristic of PEBB. Thus most of uncrystallized PEBB sequences which were also incompatible with PET retained in amorphous regions, so BPDA-PET appeared two glass transitions.The thermal stabilities of BPDA-PET copolyester in nitrogen and air atmosphere were tested via thermogravimetry analysis (TGA), and the results showed that PET and BPDA-PET copolyester had only one weight-loss stage. The thermal stability of BPDA-PET was enhanced with the increasing content of BPDA unit. In air, all the samples would show two weight-loss stages (the second was weak), and the degradation behavior during the first stage was similar with that in nitrogen although the onset degradation temperature decreased nearly 30℃, still BPDA-PET possessed higher onset degradation temperature than PET. It was deduced that the oxygen in air acted as a catalyzer in the first weight-loss stage and the second stage should be explained to the further oxidative of the first stage. The Friedman and Chang methods were used to calculate the decomposition activation energy and reaction order of different heating rates under nitrogen and air atmosphere, which indicated that the decomposition activation energy of BPDA-PET enhanced and the reaction order n increased too.