Patent GB-949655-A (GB)
Title: Process for producing foamed film of synthetic organic thermoplastic polymeric material
IPC: C08J9/12
Filed: 1962-01-17
Abstract: 949,655. Foamed films of organic material; extruding foamed synthetic resins. SUN CHEMICAL CORPORATION. Jan. 17, 1962 [Aug. 14, 1961; Oct. 13, 1961], No. 1773/62 Headings B5A and B5B. A foamed film having a closed cell structure and consisting of a synthetic organic thermoplastic material is produced by heat plasticizing the material, mixing it with a blowing agent, allowing the mixture to expand into an impermeable foam in an expansion zone at a lower pressure and temperature through a radially outwardly facing annular die orifice of circular or polygonal cross-section, collapsing and drawing off the impermeable tubular film so formed by means of a pair of nip rolls at a linear drawing speed greater than the linear speed of extrusion to stretch the tubular film longitudinally and injecting a gas under pressure into the tubular film to inflate it to a diameter greater than the diameter of the die to stretch the tubular film radially as it cools and solidifies, but before it sets into a non-deformable condition. As shown in Fig. 1, polystyrene homogeneously mixed with a " cell size control additive," e.g. 0.5% of sodium bicarbonate and 0.4% of citric acid, is fed into the hopper 1 of an extruder having a cylindrical barrel 2 and a screw 3 rotatably supported in a bearing 4 and provided with a drive sprocket 5. A " blowing agent," e.g. 4% of pentane under a pressure of 11 kg/cm.<SP>2</SP>, is continuously injected into the extruder through the tube 6 and well mixed with the polymer which is raised to a temperature of, e.g. 350‹ F., in the part of the extruder between the hopper 1 and the tube 6 by heating bands 7. In the part of the extruder between the tube 6 and an extrusion head 9 the temperature of the mixture of polymer, cell size control additive and blowing agent is reduced to the extrusion temperature, e.g. to about 250‹ to 260‹ F., by means of a heat transfer fluid passed through a jacket 8. The pressure acting on the mixture in its passage through the part of the extruder following the tube 6 is reduced owing to the screw 3 having a lower root diameter in this zone. The relationship between the temperature and the pressure within this part of the extruder is carefully controlled to cause the polymer to foam as it leaves the extrusion head 9 and to prevent it from foaming within the die head or within the extruder. The extrusion head 9 is cylindrical and comprises a duct 10 of annular cross-section communicating with the interior of the extruder and with a circumferential extrusion orifice 11 in the lateral wall of the extrusion head 9. The central core 14 of the die is connected to the rest of the die by metal bridges or spiders (not shown) extending across the duct 10. The portion of the duct 10 extending in a general radial direction towards the orifice 11 is tapered in that direction (Figs. 2 and 3), to prevent the polymer foaming while it is still in the extrusion head 9. As the polymer leaves the head 9 it foams and is formed into a continuously radially-extending annular sheet which is then drawn into the shape of a bubble which is inflated to a predetermined amount by air at superatmospheric pressure introduced through the central duct 15 in the die core 14. As shown in Fig. 4, the extruder may comprise two separate offset barrels in series, each provided with a separately driven screw, the barrels communicating with one another through an adapter 12. This construction has the advantage that the stresses in the screw 3 are reduced and that more efficient heat transfer can be effected in the cooling and mixing zone of the extruder by circulating a heat transfer liquid internally through the screw in the second portion of the extruder by means of the piping 13. The adapter 12 is provided with a pressure gauge P which may be used as a guide in manually regulating the respective speeds of the screws in the two parts of the extruder. In addition, the pressure sensed within the adapter 12 may be employed for automatically regulating the speed at which the screw 3 in the first portion of the extruder is driven. The blowing agents may include aliphatic hydrocarbons boiling between 80‹ and 200‹ F. The " cell size control additives " include the alkali metal and alkaline earth metal carbonates and bicarbonates mixed with a solid organic acid such as tartaric or citric acid or a solid inorganic acid such as boric acid. It is preferable to extrude the thermoplastic material from the lateral surface of the extrusion head in a plane perpendicular to the longitudinal axis of the die head, i.e. the direction of takeoff, as shown at S in Figs. 1 and 4, but the material may also be extruded from the extrusion head in the directions shown at S in Fig. 2 or at S in Fig. 3. If liquid hydrocarbons are used as blowing agents they function as plasticizers as well which permits of desirable lowering of the extrusion temperature.