Investigation On Polyester Foamed By Supercritical Fluid

Polyethylene terephthalate (PET) is a kind of excellent thermoplastic engineering plastics. The applications of PET in the field of foam are restricted due to its linear molecular chain structure and its poor melt strength. In order to enhance melt viscoelasticlty and foaming properties of PET, the anhydride chain extender was used to extend its molecular chain in this study. The effect of sorbitol nucleating agent on PET crystallization properties and foaming properties was also investigated in this paper. The theoretical model of shear heat in the torque rheometer was summed up, which provided some reference for materials selection.Firstly, anhydride chain extender PMDA was used to modify the molecular chain of PET, HAAKE Rotational Rheometer and DSC was used to discuss the influence of the chain extender agent on rheological properties and crystallization behavior of PET. The chain extending greatly improved the melt viscoelasticity, as the result the storage modulus, loss modulus, relaxation time and complex viscosity were higher than that of unmodified PET, and the shear-thinning effect of modified PET increased significantly. Due to the chain extended, the crystallization ability of modified PET was weakened. In this study the best content of chain extender PMDA was 0.75wt%.The influence of the network formed by self-assembled of crystal nucleating agents DMDBS on the performance of PET was researched. XRD and DSC were used to dicuss the influence of DMDBS self-assembled network on crystallization properties. DMDBS played a role as the heterogeneous nucleating agent in the PET, as the result the grain size reduced, the crystallinity of the material increased, the crystallization peak temperature also increased, the melting temperature reduced, and the range of melting peak was broad. In addition, due to the self-assembled network of DMDBS, the degree of entanglement of the PET molecular chains increased, and the viscoelastic of this blends also improved.The modified PET and PET/DMDBS blends were foamed by a batch reactor. As the result, due to the chain extended by PMDA, the foaming performance of PET improved significantly. When PMDA content was 1.Owt%, the foaming properties was optimum, the minimum apparent density of the foamed specimen can reach 67Kg/m3, at the same time, the average cell diameter was 49.6um, the cell density was 2.7×10 cell/cm. With chain extended, the foaming temperature of PET widened to high temperature, when even the temperature reach 270℃, the cell structure is still complete. Computed with modified PET, the foaming performance of PET/DMDBS blends has been further improved, which have wider foaming temperature windows. Even at higher temperatures(above 280℃), the cell density of PET /DMDBS blends remained above 108 cell/cm3, and the average cell diameter was less than 55um.Based on the experiment of HAAKE Rotational Rheometer, the model of shear heating was derived, which could be used to analyze the shear heating performance of different materials.