Effects Of Nanosilica On Crystallization And Thermal Ageing Behaviors Of Polyethylene Terephthalate

Polyethylene terephthalate (PET) is widely used in many fields. However, thermal ageing at high temperatures limits its applications in the field of engineering plastics. In this work, nanosilica (SiO2) modified by γ-methacryloxypropyl trimethoxysilane (γ-MPS) was employed to investigate its effect on crystallization and ageing behavior of PET. The relations among SiO2 nanoparticle, crystallization and ageing behavior of PET at high temperature were also studied.In this work, crystallization behavior of PET/SiO2 nanocomposites were investigated via differential scanning calorimetry (DSC) and polarized microscopy (POM). The crystallization kinetics were studied through Avrami and Jeziorny equations. The ageing behaviors were researched by intrinsic viscosity, carboxyl content measurements, tensile test and scanning electron microscope (SEM).The PET/SiO2 nanocomposites were prepared via solution blending method. The crystallinity of PET declined considerably with the addition of a small content of SiO2, but the crystallization rate increased with further addition of SiO2 attibuted to agglomeration of SiO2.The PET/SiO2 nanocomposites were also fabricated via melt blending method. The crystallization rate of PET increased considerably with increasing content of SiO2 due to a large number of nucleation sites while the relative crystallinity of the nanocomposites had little differences with that of neat PET. According to POM observation, the nucleation of PET became faster and the nucleation density increased significantly with increasing SiO2 content. For PET and its nanocomposites thermally aged at 190℃, the degradation rate was reduced with the addition of a small content of SiO2 but t increased with further addition of SiO2, owing to the two-faced effect of SiO2 on PET degradation.Although the addition of SiO2 reduced degradation rate of PET, cold crystallization at high temperatures led greatly decreased toughness. In order to improve the toughness, diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HMDI), phthalic anhydride (PA), polybenzoxazole (PBO) were used as chain extenders to react with PET. As a result, the introduction of MDI, HMDI or PBO increased molecular weight of PET, reducing the crystallization rate and crystallinity. Meanwhile, the ageing resistance of PET improved with the addition of a small content of chain extender. However, PA reduced PET molecular weight and promoted crystallization and degradation of PET.PET/SiO2/chain extender nanocomposites were prepared via melt blending method. Addition HMDI and PBO hindered the crystallization of PET, improving mechanical properties after ageing. On the other hand, SiO2 hindered the crystallization of PET/HMDI films and enhanced its mechanical properties after ageing. In comparison, SiO2 reduced mechanical properties of aged PET/PBO films. Besides, the unaged nanocomposite films exhibited high transparency while the light transmittance reduced greatly after ageing.