Studies On SSP And Crystallization Of PET

More and more attention was paid to produce Poly(ethylene terephthalate) (PET) using Solid State Polycondensation (SSP). The temperature of SSP reaction is under the melting point of PET, which led to the difficulty of small molecule's removal and restricted reaction rate. Meanwhile, the amorphous component in PET pellet can cause adhesive effect. Therefore, it is important to investigate crystallization behavior and evolution, uncouple of reaction and crystallization in SSP process.In this thesis, we study experimentally the process of SSP reaction firstly. The reactions were carried out in an isocratic reactor and an oil bath fixed bed reactor, which are designed by us. The inherent viscosity of pellet is measured by Ubbelohde viscosity meter, and molecular weight was calculated by empirical equation. Then we can obtain the correlations between the molecular weight and the reaction temperature, the molecular weight and reaction time respectively. As reaction temperature and time increased the molecular weight decreased, the effect of temperature on the molecular weigh is more pronounced than that of reaction time.Secondly, crystal analysis of PET pellet is investigated through DSC and polarizing microscope (with heating and freezing stage). The heating rate has great influence on the peak value of crystallization temperature, when heating rate increased, the peak value of crystallization shifted toward the melting peak value. Under the heating rate of 80 °C/min, the peak value is small. The result shows that using proper method to pretreat pellet can reduce the adhesive effect. Meanwhile, the pellets pretreated with different reaction temperature and reaction time were analyzed by DSC;the results indicate that there is a melting-recrystallization-remelting process for PET at low DSC heating rates. The appearances of the crystallization and melting peak show that crystallization and melt occupy a predominant position under respective temperature range. The straight line between the peaks of crystallization and melt is due to the "dynamic balance" of the crystallization vs. melt. When the temperature exceeded 260 ℃, all crystals in the pellet melted. The crystallization structure observed by NikonE60O polarizing microscope (with heating and freezing stage) is four petals and is also can be collected timely.Finally, the simulation of temperature-rise period of PET pellet in inert gas at a temperature 400 °C was presented by FLUENT software. The result indicate that the law of thermal transmission in PET pellets is that temperature rise is rapid at initial stage, then become slowly;the temperature rise at PET pellet surface is rapid than that of PET pellet inner. Approaching to temperature equilibrium of PET pellet is taken about 15 minutes. The result shows that the control of crystallization in industrial plant is difficult using nitrogen to heat PET pellet. The method of rapidly preheating treatment of PET pellet is requested to get rid of the influence of crystallization in the process of SSP.