Patent GB-989855-A (GB)
Title: Extrusion process and apparatus
IPC: B29C48/29
Filed: 1962-06-01
Abstract: A process for preparing a dispersion of a liquid in a thermoplastic resin by feeding thermoplastic resin to a single screw extruder, melting and forwarding the thermoplastic resin through the extruder, injecting a liquid into the melted resin, and discharging the liquid containing resin from a die, comprises the steps of (1) heating the resin in a first extruder zone to a temperature above its melting point, (2) subjecting the melted resin to a substantial pressure in said first zone, (3) passing the melted resin to a second extruder zone, (4) maintaining the melted resin at substantially the same temperature in the second zone as in the first, (5) injecting a liquid into the melted resin in the second zone, (6) subjecting the melted resin to substantial forces transverse to the screw and little or no force axial to the screw in the second zone, (7) passing the mixture of melted resin and liquid to a third extruder zone, (8) increasing the pressure on the mixture in the third zone, and (9) passing the mixture to the die.  In the preparation of a dispersion of a foaming agent in a thermoplastic resin for extrusion and blowing to form foamed resin film, thermoplastic resin particles are fed through a hopper 11, Fig. 1, to an extruder zone A where it is heated by a heating jacket 20 so that it melts and has a viscosity of less than 1.5 x 104 poises.  The melted resin passes to a zone B where substantial forces transverse to the screw are applied to the resin by scraper plates 26, 28, 30 &c. In the zone B liquid foaming agent is injected through injectors 34, and the mixture of resin and foaming agent passes to an extruder zone C to be further heated by a heating jacket 38 to ensure uniform dispersion of the foaming agent. Its temperature is then lowered by a cooling jacket 38a whilst the pressure is increased by the restrictive action of the die 40. Through the latter the mix is extruded as a tube 45 which is delivered to pinch rolls (not shown), and air is blown through a conduit 44 into the pinched tube to form a blown film of foamed resin.  The thermoplastic resins which may be employed include cellulose ethers and esters; polycarbonates; polyamides; polyesters; polyformaldehyde; homopolymers and interpolymers of vinyl halides; olefines; vinyl esters of carboxylic acids, vinyl ethers, unsaturated carboxylic acids and derivatives thereof, e.g. methyl methacrylate; vinyl aromatic compounds, e.g. styrene, o-chlorostyrene; and interpolymers of vinylidene monomers and alpha, beta-unsaturated polycarboxylic acids and derivatives thereof, e.g. maleic anhydride; <PICT:0989855/C3/1> and blends of two or more resins, e.g. blends of polystyrene with natural rubber, butadiene-styrene, or butadiene-acrylonitrile; and styrene graft polymers.  The foaming agents specified include aliphatic hydrocarbons such as butane, halogenated aliphatic hydrocarbons such as ethyl chloride and particularly perchlorofluorocarbons such as dichloro difluoromethane; aliphatic amines such as ethylamine; aliphatic ethers such as diethyl ethers; and acetaldehyde. The additive may comprise 70 to 98% by weight of foaming agent and 30 to 2% of an organic compound having a solvent action on the resin, e.g. acetone, methylene chloride &c.