PROCESS FOR PREPARING A COPOLYMER

摘要

According to this invention, a process is provided for preparing an ethylene copolymer having a melt index of 0.1 to 10 and a density of 0.900 to 0.945. The process comprises copolymerizing (1) ethylene, (2) at least one of propylene and butene-1 and (3) an ?Y-olefin having 6 to 12 carbon atoms while maintaining the total amount of the components (2) and (3) in the range of from 1 to 40 mol% of the component (1) and the molar ratio of the component (2) to the component(3) in the range of from 0.01 : 0.99 to 0.90 : 0.10, in a substantially solvent-free vapor phase and in the presence of a catalyst comprising a solid substance and an organoaluminum compound, the solid substance containing magnesium and at least one of titanium and vanadium. The reaction can be carried out in a very stable manner and in an extremely high polymerization activity, with decreased production of coarse or ultra-fine particles, improved particle properties and bulk density, and with little adhesion to the reactor and agglomeration of polymer particles. The copolymer formed has a high melt index and superior physical properties, e.g., transparency, and can be easily and inexpensively obtained. The copolymer has an extremely high transparency, good appearance and gloss, and excellent flexibility and elasticity even at a low temperature, not to mention at room temperature. It also exhibits a strength equal to or even higher than that of ordinary polyolefins. The copolymer contains very few unsaturated bonds and very little residual catalyst or other impurities, so it is very superior in weathering and chemicals resistance and also in electrical characteristics, e.g., dielectric loss, break-down voltage and resistivity. It further exhibits a very high resistance to impact and to environmental stress cracking. Consequently, the copolymer can be formed into various products, e.g., films, sheets, hollow containers and electric wires, by the existing molding methods such as extrusion molding, blow molding, injection molding, press forming and vacuum forming. In the field of films, the copolymer exhibits its features because it is superior in strength, elongation, transparency, anti-blocking property, heat-sealing property and flexibility. The copolymer which, when extracted with hexane, only gives an extremely small amount of extract and thus satisfied the "U.S. Food Medicines Administration Standard on Extracts to be Contacted with Foods" (n-hexane extract not more than 5.5% by weight at 50.degree.C.). The copolymer can be used safely as a food packaging film. It is also a suitable resin for blow molding because of its superior transparency, stiffness and resistance to environmental stress cracking. It is further superior in electrical characteristics and is easy to be extrusion-molded, which allow the copolymer in question to be used suitably for electric wires.