GB1328362A - Dispersion of materials in thermoplastics - Google Patents

Abstract

1328362 Extruding foam plastics IMPERIAL CHEMICAL INDUSTRIES Ltd 4 Oct 1971 [19 Oct 1970] 49447/70 Heading B5A [Also in Division C3] A foamed thermoplastic polymer is formed in an apparatus comprising a screw extruder provided with two inlet ports, downstream of the melting zone, through which a liquid and a gaseous blowing agent respectively may be injected, the inlet port for the gaseous blowing agent being connected to a constant pressure supply of gaseous blowing agent via a flow restricting device across which a pressure drop of at least 100 p.s.i. can be maintained while the inlet port for the liquid blowing agent is connected to a metering device delivering a given quantity of liquid blowing agent per unit time. The flow restricting device may be a length of capillary tube. The extruder screw may have a section, rotating with the screw and opposite at least the injection port for the liquid blowing agent, wherein the molten thermoplastic is divided into a plurality of streams. This division may be achieved by providing a fluted section in the screw comprising a cylinder having several (at least 3) axial channels formed on its outer surface. It is preferred that the cylinder has a length at least equal to the internal diameter of the extruder barrel, e.g. 3-6 times the diameter. The fluted section may be followed by a turbine section which both shears and further subdivides the mixture. The turbine mixing section preferably comprises a plurality of toothed rotors mounted on the screw to rotate therewith, the teeth of alternate rotors being staggered. After this section there is preferably a further mixing section in which a cylindrical member provided with inlet grooves not extending along the whole length of the member is used. In Fig. 1 a screw rotates in a barrel 1 and the thermoplastic polymer is compressed and melted in a compression section 3 of the screw. The polymer then passes to section 4 of the screw which is fluted having a series of axial grooves (Fig. 2) (not shown) which divides it into a plurality of streams. The first blowing agent is fed in at port 6 and the second injected via a capillary tube 9 at 8. The polymer then passes to turbine mixing section 10 containing a series of spaced toothed rotors (Fig. 3) (not shown) 11, 12, 13, 14 mounted on the screw so that they rotate therewith. The polymer is finally passed through a shearing section 18 consisting of a cylinder 19 provided with axial grooves 20 (Fig. 4) (not shown). The process for forming the foamed polymer is described (see Division C3), the first blowing agent comprising 20-75% by weight of the thermoplastic and the second up to 10% by weight of the thermoplastic. The polymer may be polyethylene.