Synthesis Of High Molecular Weight PET And Optimization Of SSP Process

Poly(Ethylene Terephthalate) (PET), in particular high molecular weight polyester (HMPET), is widely used due to its excellent performance, with solid state polycondensation (SSP) as primary production process. However, the SSP process still has some disadvantages, such as slow reaction rate and long reaction time, which are resulted from the complexity and unclear knowledge about SSP diffusion and reaction. In view of the deficiency of research on SSP diffusion and crystallization, the new approaches to enhanced diffusion for production of HMPET have been investigated and a new idea for novel process and an optimization SSP process have been put forward. The main contributions of this paper include:The solid state polymerization (SSP) of PET with a wide range of powder size was studied experimentally. It shows that the SSP reaction rate increases with the reduction of particle sizes and PET powder samples perform accelerative effect of reaction rate comparing to normal size chip. For powder with particle sizes in 0.283mm-0.795mm range, it takes only eight hours for the intrinsic viscosity (Ⅳ) to increase from 0.65 to 1.0 dL/g, which is an effective method to produce HMPET. It is also confirmed that the polymerization and degradation reactions coexist and compete with each other during the SSP process. Therefore, there exists a maximum intrinsic viscosity. The polymerization reaction is dominant at the initial stage and the IV increases with the SSP process going on. In the later period of the SSP process, the degradation reaction becomes dominated while the IV decreases. The degradation rate is more evident with high reaction temperature and small size of powders.Based on experimental SSP data of PET powder, a comprehensive model involving polycondensation, degradation and polycondensation of vinyl end groups together with diffusion was set up and the corresponding parameters of model have been gained by using Crank-Nicolson method and fitting the experimental data. The activation energy of polycondensation, degradation and polycondensation of vinyl end groups are 26.09 kcal/mol,52.33 kcal/mol and 39.34 kcal/mol, respectively. EG activation energy for diffusion is 42.91 kcal/mol.SSP process model analysis shows that temperature is a sensitive factor affecting the reaction rate, and should be increased as high as possible, but under the premise of no adhesion occur, in order to shorten the reaction time. In addition, alkenyl end concentration decreases sensitively with decreasing temperature. Reasonable temperature range from 200 to 230℃, preferably 210～220℃. Particle size is another important factor affecting reaction rate. The reaction rate increases with reduction of particle size and reaction time. The threshold value of particle size is about 0.8mm, preferably in the 0.8～1.6 mm range. The combination of low reaction temperature and small particle size will help to reduce the carboxyl end group concentration and improve HMPET quality.The swollen state polymerization (SwSP) of PET in tetradecane has been experimental studied, showing greater reaction rate than SSP. According to the simulation results, the rate constants of polycondensation of transesterification and vinyl end group polycondensation for SwSP are enhanced, while degradation reactions are inhibited. The diffusivity of SwSP is greater than that of SSP due to the honeycomb like structure. The combined effect of the two factors makes the reaction rate for SwSP increased. PET samples by SwSP show two crystalline melting peaks, the first one is resulted from the melting of crystal formed in SwSP process, and increased with the polymerization temperature; the second one is due to the melting of crystals formed in heating process of DSC.The dusts generated during SSP process affect the quality of PET chip. The crystallization behaviors of chips and dusts have been experimentally studied. The dusts show faster cooling crystallization ability than chip, and the dusts coming from pre-crystallizer and crystallizer show fast cooling crystallization rate than those from other stages of SSP process. The high impurity content and high temperature history are the main reasons for the dusts with capacity of high crystallinity.The crystallization and melting adhesion behavior for PET have been studied. The crystallization and reaction coexist and interact during the SSP process of PET. The initial and peak temperatures of the first crystallization peak increase with the reaction temperature and time. The method of gradient elevation of the temperature can avoid the sticking in SSP process, thereby HMPET preparation time can be shortened by half.Based on the investigation of PET non-isothermal crystallization process with DSC, a model to simulate the heat-transfer and non-isothermal crystallization behavior for medium viscosity PET melt pelletizing in hot water has been set up. It shows that the residual temperature of PET particles increases with the increase of particle size and the reduction of pelletizing time. For medium PET particle size, its crystallinity is larger than others due to long duration in the rapid crystallization temperature range. About 54% heat of PET melt can be retained for normal PET particle with pelletizing time of 1second.A new SSP process has been proposed based on the melt pelletizing method for PET in hot water. The hot dust-free spherical particles can be obtained by controlling particle size in the fast reaction range (0.8-1.6mm) and maintaining the pelletizing temperature about 90℃for 1 second. These particles are used to produce UMPET through pre-crystallization, crystallization and solid state polymerization. The new process has some advantages, such as low energy consumption, less dust and short reaction time.