GB1410979A - Process for the dispersion of blowing agents in thermoplastics - Google Patents

Abstract

1410979 Extruding foam plastics IMPERIAL CHEMICAL INDUSTRIES Ltd 23 March 1973 [14 April 1972] 17248/72 Heading B5A [Also in Divisions B1 and C3] A foamed synthetic thermoplastic polymer is made by extruding a mixture of the polymer and 30-120% by wt. of blowing agent from a zone of high to lower pressure using a screw extruder. The blowing agent is injected by metering as a liquid into the extruder at a position where the polymer is molten and divided into a plurality of separate streams. The blowing agent, which is chemically inert towards and is completely miscible with the molten polymer at the high pressure within the extruder and has a b.p. at the high pressure which is above the extrusion temperature and a b.p. at the lower pressure which is below the extrusion temperature. The polymer mixture is free of any solid or liquid that decomposes to form a gas at the extrusion temperature and of any material having a critical temperature below the extrusion temperature. In the examples foamed polyethylene film was formed from polyethylene (optionally a low and high density mixture) and optionally titanium dioxide or silica using as blowing agent pentane or 1,2 - dichloro - 1,1,2,2 - tetrafluoroethane; the polymer mixture was fed into an extruder (see Fig. 1) via a feed hopper (not shown), where it was compressed and melted in an extruder, 1, which is heated externally (not shown), and has a single screw, 2, and compression section, 3; the melt is forced to divide into a plurality of streams in section 4, which is fluted and has a series of axial grooves 5, which are rigidly mounted on the screw (see Fig. 2). The blowing agent is fed to port 6 via metering pump 7 from a reservoir (not shown), and the polymer/blowing agent mixture is forced into a turbine mixing section, 8, having a series of spaced toothed rotors, 9 to 16, which may be staggered (see Fig. 3), and the polymer mixture is extruded through a die (not shown), in a conventional screw section 18, into a zone of lower pressure.