| 摘要 |
A method of making free-flowing cellular pellets of a thermoplastic resin (see Group V), comprises extruding, at temperatures between 60 DEG and 120 DEG C., a mobile gel comprising an intimate mixture of a thermoplastic resin having a normally gaseous agent described therein under pressure through a constructed passageway to a zone of lower pressure, cutting the extruding plastic gel into segments before complete expansion occurs and thereafter causing the severed pieces to fall freely through the air while completing the expansion. The mobile gel may be prepared by placing the resin in a ground or powdered form in a pressure-resistant vessel, adding the gaseous agent, and maintaining the mixture under pressure till homogeneous. Suitable thermoplastic resins are polymers and copolymers of styrene, meta-ethylstyrene, meta-chlorostyrene, methylstyrene, para - isopropylstyrene, ortho - chlorostyrene and alpha-methylstyrene; polymethylmethacrylate; and copolymers of vinyl chloride and vinyl acetate or of methylmethacrylate and styrene. Suitable gaseous agents are methylchloride, methylether, ethyl ether methylethyl ether, ethyl chloride, methyl bromide, dichloro-difluoromethane, normally gaseous olefines such as propylene and butylene and cracked-oil gas fractions containing such gaseous olefines. The rate of expansion may be retarded by adding barium, calcium, lead or zinc stearate in proportions of from 0.04 to 1 per cent by weight of the resin. The extrusion pressure may be increased by mechanical forcing of the gel or by introducing to the pressure chamber carbon dioxide, air, nitrogen or the like. To aid extrusion and cutting of the gela lubricant, e.g. stearic acid, soya oil, corn oil, lard oil or butyl stearate, may be added. Pigments, fillers or binders, e.g. finely-divided carbon, aluminium powder, titanium dioxide, fuller's earth, powdered calcium silicate or clay may be added. For rapid vaporization of the gaseous agent in the extruded pellets the air through which they fall may be warmed and instead of air, nitrogen or carbon dioxide may be used. |
