Patent GB-1334062-A (GB)
Title: Producing resin sheet
IPC: B29C44/28
Filed: 1971-09-20
Abstract: 1334062 Foamed thermoplastic resin sheets MONSANTO CHEMICALS Ltd 20 Sept 1971 [3 Nov 1970 18 Feb 1971] 52268/70 and 4900/71 Heading C3C [Also in Division B5] An expanded thermoplastic resin sheet is produced by extrusion of the thermoplastic, from a linear slit die orifice 1, into a zone in which the thermoplastic expands and is cooled at least partially and which is defined by a pair of opposed surfaces cooled to below the temperature of the orifice 1 and which, in the direction of extrusion, are respectively of concave and convex curvature and of increasing spacing apart, the convex surface being defined by a member continuously moving throughout the length of the zone, away from the orifice and along a fixed convex path. The convex surface may be defined by an endless belt movable around a roller or other movable or fixed convex surface. The concave surface may be stationary, or may comprise an endless belt movable over a fixed concave surface against which it is urged by the pressure of the expanding resin. The concave and convex surfaces may be parabolic, but are preferably cylindrical as in the illustrated embodiment wherein the concave surface 10 is defined by a member 9 (or a sequential plurality thereof defining surfaces 10 of increasing radius) which is fixed to the upper die lip 4 adjacent heat insulation 14, and the convex surface is defined by a roller 8 which may be freely rotatable by the extruding thermoplastic, or rotated by a variable speed drive and may effect embossing of the sheet, the radius of the surface 10 preferably being less than that of the roller 8, which is positionally adjustable with respect to the die orifice. The spacing between the concave and convex surface may decrease towards the end of the expansion zone, and side pieces may bound the side of the zone. Channels 12 are provided in the dip lips for the circulation of heating or cooling fluid, and the member 9 is cooled by a fluid channel 13 or by cooling fins assisted by a fan. The roller 8 may be cooled internally via a central fluid channel, or externally by a cold air stream. The foamed sheet passes around a roller 16 on being drawn from the roller 8, e.g. by opposed endless belts, or a roller nip or nips, on which roller it may additionally be cooled by an air knife. The surface speed of the roller 8 may be less, but is preferably the same, or higher than that of extrusion. The expanded sheet comprises spherical closed cells of a diameter of 0À001 to 0À12 inch, and the cells may be elongated normal to the sheet surface which comprises a higher density skin. The thermoplastic resin may comprise a polymer of ethylene, propylene, butadiene, styrene, chlorostyrene, vinyl toluene, α- methylstyrene, acrylonitrile, vinyl or vinylidene chloride, vinyl acetate, methyl acrylate or methacrylate, and/or ethyl acrylate, or it may comprise natural or synthetic rubber modified polystyrene. Preferred thermoplastics comprise high and low density polyethylene, polypropylene, and polystyrene or styrene, butadiene, acrylonitrile copolymer. The expanding agent may comprise methane, ethane, ethylene, propane, butane, butylene, pentane, methyl chloride, trichloromethane, dichlorodifluoromethane, 1,2-dichlorotetrafluoroethane, or acetone, or carbon dioxide or nitrogen, or isobutylene or a mixture thereof with a chlorofluorocarbon. The blowing agent may also comprise sodium or ammonium bicarbonate, or dinitrosopentamethylenediamene, or barium azodicarboxylate. The thermoplastic preferably also contains a nucleating agent such as silica, talc, or alumina, e.g. in conjunction with zinc stearate, or sodium bicarbonate, e.g. with tartaric or citric acid, or boric acid. The foamed sheet may be used as thermal insulation, wall or ceiling panelling, or be further formed into a body panel or part shell of an automobile, boat, a container, or a furniture article. The foamed sheet may be calendered and shaped by vacuum, or between matched dies and may be embossed or laminated with paper or a glossy film. The article made from the sheet may include tray containers for packaging food, or drinking cups for automatic vending. In the examples, polypropylene or acrylonitrile butadiene/styrene polymers are plasticized in one extruder from which they are fed for extrusion to another extruder in which the blowing agent is injected into the polymer and the melt cooled prior to extrusion. The polypropylene contains talc nucleating agent and is expanded by isobutylene, and the acrylonitrile / butadiene / styrene contains talc nucleating agent and is expanded by sequential injection of dichlorodifluoromethane and either carbon dioxide or isobutylene.