Studies On Homo-and Co-polymerizations Of Propylene For Preparing Polyolefin Reactor Alloy

To develop high performance polyolefin alloys by the multistage polymerization, laboratory reactor system with a novel on-line monitor and control technology for monomer concentrations has been established. Spherical propylene homopolymer and multiphase multicomponent polyolefin alloys were produced. Kinetics and particle morphology for each polymerization stages along with the polymer properties were investigated.In the laboratory reactor system, the monomer compositions in gas phase were on-line monitored and controlled by combination of a differential pressure volumetric flow meter and a thermal mass flow meter with a short response time (0.01-3 s). The system was especially useful to study the kinetics of binary gas-phase copolymerization of olefin.The kinetics of propylene gas phase homo-polymerization was studied at elevated pressures using a spherical TiCl₄/MgCl₂ catalyst in semi-batch mode. It was found that the reaction rate had a linear relationship with the monomer concentration. The time evolution of polymerization rate under all conditions followed a same pattern, i.e. fast decay in the early stage and then relatively slow decay in the later stage. An nth decay model was developed to describe the dynamic changes of the polymerization rate. The decay order n was found to be 2.5 and the lumped propagation activation energy was estimated to be 77.1 kJ/mol.Comparing to the gas phase polymerization process, the lumped propagation activation energy in liquid phase was estimated to be 72.9 kJ/mol, similar to that in the gas phase process. While the decay order in liquid phase was found to be 1.7, lower than that in the gas phase process, which implies that less thermal runaway was achieved in the liquid process. The kinetic models for gas and liquid phase processes fitted the experiments well.The kinetics of the second stage polymerization, i.e. ethylene/propylene gas phase copolymerization in the presence of spherical porous polypropylene particles produced by liquid phase polymerization, was studied in a semi-batch mode with constant gas phase composition. It was found that the instantaneous reaction rate changes for both monomers followed the same pattern, similar to that in the homopolymerization stage. An nth decaymodel was developed to describe the kinetics in the second stage. The lower limit of the decay order was estimated to be 2.1, still higher than that in the liquid phase propylene homopolymerization, which implied that thermal runaway might occur in this stage.Morphology of the propylene homopolymer particles was examined for both gas and liquid phase processes. The effects of polymerization conditions, such as monomer and hydrogen concentrations, prepolymerization temperature and time, homopolymerization temperature and time, on morphology of the propylene homopolymer particles were discussed in detail. It was found that spherical porous polypropylene particles could be obtained via a spherical catalyst. Each polymer particle was made up of numerous microparticles with diameter of about one micrometer. The so called intraparticle volume were found to be divided into pores and cracks, where pores were formed by pile of the microparticles with diameter of less than one micrometer, while cracks were formed by faultage and interstices with size of about l⁵0um. Most of the pores and cracks were open to the outside and the ratio of closed intraparticle volume tended to be zero. It was found that process conditions affected only the volume of the cracks but not the micro pores which were determined only by structure of the supported catalyst.A method of calculating the theoretical diameter and fragmentation ratio of the spherical polypropylene particles was presented. Effects of polymerization conditions on particle diameter were investigated. The results showed that the heat transfer resistance and the initial reaction rate of the polymerization system had large effect on particle fragmentation ratio. The larger the heat transfer resistance or the initial reaction rate the higher the fragmentation ratio.Polypropylene/ethylene-propylene rubber (PP/EPR) in situ alloy were produced by a multistage polymerization process. It was found that the morphology replication still exist during the polymerization. There were large cracks in the alloy particles even at high EPR content, which were plugged up by the EPR phase. The specific volume of the closed cracks was affected mainly by mass transfer resistance in the polymerization process. The value increased with increasing particle diameter since long path meant high mass transfer resistance. The EPR phase had a distribution on particle radius and particles with different diameters for the same reason. The closer the position to the particle surface, the higher theratio of EPR phase. The larger the particle diameter, the lower the ratio of EPR phase. The dynamic mechanical properties of the alloy were also affected by the EPR phase distribution. The miscibility of the EPR phase and the PP matrix decreased with increasing particle diameterThe mechanical properties of the alloy were also investigated. The results showed that the in-situ PP/EPR alloy had an outstanding low-temperature impact resistance even without hydrogen as a molecular regulating agent.