Regulation Of The Structure And Properties Of PP/EPR In-reactor Alloy By Multi-stage Sequential Polymerization

Synthesis of polypropylene in-reactor alloys by multi-stage sequential polymerization with superactive spherical Ti based Ziegler-Natta catalyst was investigated and the PP/EPR alloys were characterized.The effect of the multiple hydrogen addition on the propylene homopolymerization was also studied.The polymerization process consists of three stages: the first stage is pre-polymerization in slurry at 1 atm, the second stage is the propylene homopolymerization in slurry at 6 atm, and the third stage is gas phase copolymerization of propylene and ethylene or homopolymerization of propylene in different ways.The purpose of this study is to simulate the propylene polymerization process based on multi-zone circulation reactor (MZCR). In such a process the polymer-catalyst granules rapidly circulate between a homopolymerization zone and a copolymerization zone. In this work, frequent switchings between homopolymerization and copolymerization in the gas-phase polymerization stage have been adopted to study the effect of circulation speed on the polymer structure and properties.According to previous studies on pre-polymerization, slurry polymerization and gas phase polymerization, the conditions of gas phase copolymerization were optimized. The polymer samples were synthesized directly in reactor under these conditions. The catalyst efficiency increase obviously while increasing hydrogen amount in the propylene/hydrogen mixture, but it is hardly effected by the switching frequency. Influences of the switching frequency and the partial pressure of H₂ on the molecular weight, MWD, melt flow index and crystalline morphology of PP were studied by GPC, DSC, PLM. When keeping the total polymerization time unchanged, increasing the switching frequency caused little influence on the polymerization activity, the molecular weight and MWD, but the spherulites of PP became smaller. Increasing the partial pressure of H₂ resulted in broadening of MWD and decrease of spherulite size. Double melting peaks were found in DSC curves of PP synthesized in the presence of H2.For the PP/EPR samples which synthesized by sequential homopolymerization and copolymerization, the catalyst efficiency increased with the copolymerizationtime to some extent and then decreased. For the multi-stage samples which synthesized in a constant time, the catalyst efficiency increased with the switch frequency. Supported by the pore-size distribution data of the PP/EPR particles, it is proposed that the diffusion barrier in the particles plays an important role in the gas-phase polymerization.The mechanical properties of the PP/EPR alloys were measured. The impact strength increased with increasing copolymerization time and the flexural modules decreased. For samples synthesized by multi-stage sequential polymerization process, it was found that increasing the switch frequency has a positive effect on both the impact strength and flexural modulus.The structure and properties of PP/EPR alloys were also characterized by IR, DSC, SEM, NMR and TREE The results indicated that there are mainly three components in the PP/EPR alloys: PP homopolymer, EPR random copolymer and EP block copolymer. The results of TREF, NMR and DSC analysis show that the PP/EPR alloys prepared at different switching frequency have similar composition distribution. But the phase structure of PP/EPR alloys revealed by SEM showed that the particle size of elastomer in the PP matrix is smaller than 0.5 u , which is an ideal size for toughening. As the switching frequency increased, the EP elastomer phase became smaller and the distribution of elastomer in the PP matrix became more homogeneous. So the distribution of EPR elastomer in PP matrix and the size of elastomer phase play an important role to the mechanical properties of PP/EPR alloysIt has been proved that the structure and property of PP alloy can be efficiently regulated by multi-stage sequential polymerization. These studies have not only great theory value, but also have important applications in the polyolefin industry.