GB471590A - Improvements in or relating to the polymerisation of ethylene - Google Patents

Abstract

Solid polymers of ethylene are produced (a) by subjecting ethylene, compressed to a pressure of at least 1200 ats., to a temperature of 100--300 DEG C., the temperature in any part of the gas being prevented from reaching that at which explosive decomposition occurs, or (b) by subjecting ethylene, compressed to at least 500 ats. and accompanied by a small amount of free oxygen, not exceeding 3 per cent, to an elevated temperature sufficient to induce reaction, the permissible amount of oxygen initially present being insufficient to cause explosive decomposition of the ethylene. Semi-solid or grease-like polymers are obtained by the use of pressures of 500--1200 ats. and temperatures of 200--400 DEG C., the temperature being controlled as at (a) above. In the production of the semi-solid products, the oxygen content may range up to 5 per cent subject to other operating conditions. Process (a) gives a relatively large yield, the reaction being slow. Process (b) gives a relatively low yield (5--20 per cent), the reaction being however rapid. In the rapid reaction, the oxygen is used up, and the oxygen-free ethylene may then polymerize by the slow reaction. The critical amount of oxygen present, above which explosive decomposition occurs, depends on the temperature and pressure employed, higher proportions being only usable at low temperatures and pressures. A lower limit of 0,03 per cent is specified. The production of higher molecular weight polymers is favoured by high pressure, and low temperature and oxygen content of the ethylene. Measures to prevent the temperature rising above the critical value consist in the use of diluents for the ethylene, e.g. the use of ethylene-containing industrial gases such as cracking-still gases, the use of a reaction vessel constructed of, or lined with, a metal or alloy of high thermal conductivity, e.g. copper, aluminium, alloys therof or steel, and efficient stirring of the gas during reaction. A small heating element spaced within a cold-walled reaction vessel may be used to start the reaction. In examples, (1) the effect on the speed of the reaction of 0,08 per cent oxygen is shown, the pressure and temperature employed being 1500 ats. and 200-230 DEG C. respectively; (2) the effect of the pressure on the mol. weight of the product is shown. (Ethylene containing 0,06 per cent oxygen is heated to 200 DEG C. to initiate the reaction. At 520 ats. for example, the mol. weight of the product is 2000, at 3000 ats., 12--24,000); (3) the effect of the oxygen concentration on the percentage conversion and the mol. weight of the polymer is shown; (4) ethylene containing 0,06 per cent oxygen and compressed to 1500 ats. is heated to 180 DEG C. After a sudden drop in pressure due to the quick reaction, the temperature is kept constant for 15 mins, during which time the pressure falls slowly (slow reaction), whereupon more oxygen-containing ethylene is compressed into the vessel to restore to pressure. This procedure is repeated for 3 hours; (5) ethylene containing 0,08 per cent oxygen is polymerized by passing it continuously at 1500 ats. through a reaction vessel containing an internal heating element at 230 DEG C. The reaction vessel walls are kept below 100 DEG C. by a water-jacket. The solid polymers are negligibly soluble in cold organic solvents, but are soluble in hot hydrocarbons, e.g. benzene. They melt at 110--120 DEG C., and may be moulded or cast. Decomposition occurs at about 200 DEG C. in air. Pliable elastic transparent films may be made by known methods. The films are impervious to water. Paper, fabric, &c. may be impregnated with the polymers. Threads may be formed by extrusion above the softening point. The cold drawn threads may be worked up into staple fibres which can be spun and woven. The polymers may be used for electric insulation, and for making vacuum-tight joints. Specifications 336,234, [Class 2 (iii)], and 340,513 are referred to.