Preparation Of Heterophasic Polypropylene Copolymer With High Performance By A Novel Atmosphere-Switching Polymerization Process And Its Modeling

Multistage and multiphase polymerization for preparing heterophasic polypropylene copolymer (HPC) in reactor provides an important route to develop the general polymer materials with special and high performance, which currently is a research and development frontier. However, the composition and structure of HPC exhibit special complexity and multicomponent features. The effect of polymerization process on the composition, chain structure, molecular weight, and phase structure of HPC is still unclear now. Renovating polymerization process and establishing the relationship of the characteristic of process and the above-mentioned variables, and then to prepare HPC with controllable composition, chain structure, and phase structure, are not only challenging, but also have important theoretical meaning and value of industrial applications.This thesis presents a novel atmosphere-switching polymerization process (ASPP) for preparing HPC with controllabe structure and properties. Two different polymer species, that are impact polypropylene copolymer (PC) and flexible polypropylene, were prepared by designing monomers feeding policies, respectively. The composition, chain structure, and phase morphology of HPC were characterized by solvent fractionation combining with 13C NMR, GPC, DSC, SEM et al. Then the relationship of the characteristic of polymerization process and the above properties were established. On the basis of the proposed mechanism, a mathematica model of ASPP was developed. This thesis achieved some results as follows:1. A steady-state particle size distribution (PSD) model was developed to predict the size distribution of polypropylene particles in multistage industrial horizontal stirred bed reactors (HSBRs) of BP-Amoco process. The model takes into account both the single polymer particle growth and the catalyst residence time distribution (RTD). It is validated by data generated from industrial processes. The effects of the mass- and heat-transfer in paricle scale, and the RTD in reactor scale on polymer PSD were both analyzed through the simulation. The temporal-spatial evolution of monomer concentration and temperature in a single polymer particle are predicted by the polymer multilayer model (PMLM). It is shown that, as the initial catalyst size increases, the monomer concentration in the particle decreases. As a result, large radial concentration gradients can arise inside the particle during the early stages of polymerization. No remarkable temperature gradients arise inside the particle. A decrease in the effective diffusion coefficient, and an increase in polymer crystallinity lead to a decrease in the particle growth rate. Consequently, the PSD shifts to a smaller particle size.In the case of a uniform size catalyst feed, the PSDs predicted from three flow models for HSBR are very different from each other. The fact that the polymer particles produced by Amoco process has a narrow PSD, which is in favor of the dispersion of ethylene-propylene rubber, was revealed through the simulation.2. The composition and structure of IPC prepared by sequential polymerization could be regulated by gas phase pressure, copolymerization time and the sequence of polymerization. It is shown that an increase in the gas phase pressure and copolymerization time lead to an increase in the content of ethylene-propylene random copolymer (EPR) and ethylene-propylene segmented copolymer (EPS). The mass fraction of ethylene in IPC is also increased correspondingly. The impact strength at low temperature is enhanced, but the flexural modulus and flexural strength are decreased. The phenomenon of powder sticking becomes serious. The catalyst efficiency and the content of EPR were both increased using the strategy of copolymerization-homopolymerization in comparison with the strategy of homopolymerization-copolymerization. The impact strength at low temperature of IPC is improved, but the phenomenon of powder sticking appears remarkably.3. A novel ASPP set-up was designed and established innovatively. Firstly, IPC was prepared by using a periodic monomers feeding policy. The effect of the characteristic parameter of the process, that is switch frequency, on the structure and properites of IPC was revealed. Secondly, flexible polypropylene material with high content of EPR was prepared by designing two feeding policies.1) It is found that the content of the EPS is increased whereas that of the EPR decreased with increasing switch frequency or faster circulation between the homopolymerization of propylene and copolymerization of ethylene and propylene. As a result, the size of the EPR dispersed phase domains decreased and the interfacial adhesion between EPR and PP improved, providing a new evidence of EPS as a in situ compatibilizer. The periodic operation provides an efficient way to control the structure and properties of IPC. The relationship of the characteristic of multizone circulating reactor technology and the composition, and the structure of the produced IPC was illustrated.2) Flexible polypropylene with the content of EPR more than 40 wt%, and good parcile morphology and transparency was synthesized directly in reactor using two feeding polices. It is shown that, as the flexrual modulus maintains 600 MPa, the impact strength at low temperature (-20 ℃) of flexible polypropylene prepared by Spherizone process is as high as 50 kJ/m2 (Izod), which is a dramatical increase compared with that of IPC. The phase structure shows that an increase in switch frequency leads to the appearance of multiphase morphology. The flexible polypropylene prepared by periodic ASPP, of which the time ratio of ethylene-propylene copolymerization to propylene polymerization is 3, has excellent impact resistance at low temperature (Izod,60 kJ/m2,-20 ℃). As the content of EPR is close to 50%, the phase morphology of the flexible polypropylene shows obvious co-continuous structure.4. The mechanism of periodic ASPP catalyzed by Ziegler-Natta catalyst was proposed, and a method of deconvolution combining with a variety of experimental characterization techniques was developed to analyze the chain structure of IPC in detail. Successive self-nucleation annealing (SSA) was used to fractionate EPS. The sequence length and distribution of the crystallizable ethylene sequence (average sequence length>30) in EPS were obtained. Comparing with the results of 13C NMR, the average sequence length by SSA is 4 times that of the NMR results, revealing that there is still a large amount of non-crystalline random ethylene-propylene segments in the chain of EPS.5. Based on the proposed mechanism, a mathematical model for describing ASPP was developed, which could predict the content of EPR and EPS, the molecular weight and its distribution, the copolymer composition and its distribution well. The effect of the characteristic of process, that is switch frequency, on the above properties was simulated and also validated by experimental data. The effect of the non-ideal behavior of the reactor on the composition of IPC was also revealed, and the operation criterion was presented. The model could provide an important reference for controlling the microstructure and properties of IPC in industrial continuous polymerization reactors.