Catalyst Polymerization Kinetics and Polyethylene Particle Morphology

6 Catalyst Polymerization Kinetics and Polyethylene Particle Morphology 

The polyethylene particle morphology (particle shape, size and density) is an important property for high-activity Ziegler catalysts used in the gas-phase, fluidized-bed process. As discussed previously in patent description IV, U.S. Patent 4,302,565 issued to Union Carbide Corporation on November 24, 1981, magnesium/titanium complexes discovered in the 1970s (patents described in II and III above) were impregnated into silica in order to provide a catalyst system that could be injected into the gas-phase process as a solid material. 

The silica provided a suitable particle size for the catalyst so that the catalyst could be easily fed into the reactor. In this case, the silica is used primarily as a host material to deposit the Mg/Ti/Cl/THF compound into the silica as a template to grow polyethylene particles. The polymerization characteristics of the solid Mg/Ti/Cl/THF complex are similar to the silica-supported material, suggesting that the role of silica after the complex has been deposited into the silica pore structure is to provide finished polyethylene catalyst particles that produce polyethylene particles with acceptable morphology. 

The removal of the heat of polymerization from the polymer particle to the fluidization gas is an important factor in forming polymer particles with good morphology Therefore, the initial polymerization rate of the catalyst particle during the first few minutes after the catalyst particle is introduced into the reactor determines how well the silica particle is fragmented during the early stages of the polymerization process. 

Therefore, the overall catalyst productivity of commercial catalysts in the gas-phase process is limited by constraints to only produce granular 

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