Patent GB-942095-A (GB)
Title: Improvements in or relating to oriented formaldehyde polymer films
Inventor: BUTEUX RICHARD HAROLD BARCLAY
IPC: B29C48/10
Filed: 1961-07-10
Abstract: 942,095. Formaldehyde polymer films. IMPERIAL CHEMICAL INDUSTRIES Ltd. June 29, 1962 [July 10, 1961], No. 24857/61. Heading B5B. A substantially uniformly thick, biaxially oriented film of a linear high molecular weight formaldehyde polymer has a tensile break strength in any direction in its plane of at least 15,000 p.s.i preferably at least 30,000 p.s.i., measured at 20 C. Desirably, the ratio of tensile break strengths in any two directions at right-angles in the plane of the film is not greater than 1.3:1. The polymer may contain between 4 and 8 mole per cent of comonomer units each containing 2 or more adjacent carbon atoms in the polymer chain, said copolymer being capable of being copolymerized with formaldehyde, or a low molecular weight polymer of formaldehyde, e.g. trioxane to form high molecular weight formaldehyde polymers. Specified co-monomers include ethylene oxide, 1,3-dioxolane, butadiene, paraldehyde, butadiene mono-oxide, styrene oxide, indene, coumarone, Y-propanolactone, ethylene carbonate, 1,3,5-trioxepane, 1,3-dioxane, trimethylene oxide, 2,2-di(chloro-methyl) oxide, 1,2-butylene-oxide, 1,3-butylene oxide, 2,2- di(chloro-methyl)-1,3-propylene oxide, diethylene glycol mono-formal, pentamethylene formal, neopentyl formal, pentaerythritol diformal, 1,3 - dioxep - 5 - ene, 2 - isopropyl - 1,3 - dioxep - 5 - ene, 2,2 - dimethyl - 1,3 - dioxep - 5 - ene, methacrolein dimer, xylylene glycol formal, 1,3 - dioxa - 6 - thiocyclo - octane and the cyclic formal of bis(-hydroxyethyl) sulphone. The film may be made by cooling the polymer melt, or allowing it to cool, to a temperature below but near to the optical melting point of the polymer, suitably not more than 8 C. below the optical melting point, and then biaxially stretching the film simultaneously in two perpendicular directions in the plane of the desired oriented film as soon as possible so that the film is stretched before it has fully crystallized. The applied stretch ratio in any direction is suitably 3: 1 to 7: 1. Alternatively the polymer may be cooled so that it crystallizes and it is then heated to a temperature near to but below the optical melting point of the polymer, suitably 22  to 8  C. below the optical melting point, and biaxially stretched simultaneously in two perpendicular directions in the plane of the desired oriented film. The applied stretch ratio in any direction in this case is suitably 3: 1 to 5: 1. In either method, the polymer may have a melt viscosity of 10<SP>3</SP>-10<SP>6</SP> poises, especially 10<SP>4</SP> to 5 X 10<SP>5</SP> poises, measured at 190 C. at low shear stress, the polymer being melt extruded as a tube and the tube expanded to cause biaxial stretching thereof. In the apparatus of Fig. 1, polymer of melting point, e.g. 168 C., is extruded from die 2, in close proximity to a sandblasted, chromium-plated sizing die 3 in which the tube is cooled to a suitable temperature, e.g. 155 C., so that it can be gripped by the caterpillar belts 5, the inner surface of the tube being cooled meanwhile by air at about 140 C. issuing from an air bearing 4 within the tube. The caterpillar belts 5 are of V-section and surfaced with silicone rubber, and the belts and tubing are maintained at e.g. 155 C. in a housing 6. The tube is then heated to e.g. 157- 158 C. by infra-red heaters 7 and stretched and hauled off by nip rollers 8. Air for inflation of the tube is supplied through a pipe 9 through the air bearing 4 which is itself supplied with heated air by the pipe 10. In the apparatus of Fig. 2, molten polymer is supplied under pressure through a pipe 11 to a chamber 12 and then between die lands 13, surrounded by chambers 15, 151 in which a boiling liquid maintains a constant temperature, to orifice 14. Coolant liquid can be supplied through annular passages 16, 16<SP>1</SP>. The die is raised to the required operating temperature by electric heaters 17, 17<SP>1</SP>, and heat is retained within the die by lagging 18. Polymer is extruded at e.g. 157-158 C. and immediately inflated to form a stretched tube. In a modification, the lower parts of the die lands are widened by reducing the diameter of its inner and outer surfaces and inserting a porous metal member in the recess formed in its outer surface through which member air under pressure is supplied to form an air bearing between the outer surface and the polymer, and so control the friction between the polymer and the lands. To extrude film at e.g. 160 C., the liquid used in the die may be one boiling between 130 and 160 C., fine adjustment being effected by controlling the pressure over the liquid. Suitable liquids include amyl acetate, anisole, propylbenzene, isobutyl butyrate and propyl butyrate. Using the apparatus of Fig. 1, the polymer must not be brought into contact with the caterpillars at too high a temperature or the polymer may be deformed by the belts. In that of Fig. 2, sticking of the polymer in the die is overcome by the force exerted by the extrusion press forcing polymer into it, and hence the polymer may be extruded and stretched at temperatures close to the optical melting point. In Fig. 3, a tube 21 of polymer is melt extruded from a die 22 in close proximity to a sand blasted chromium-plated sizing die 23 which is cooled to solidify the tube. The tube is passed, at a temperature below, e.g. 140 C., through nip rollers 24 and into a radiant heating zone where it may be heated to about 155 C. by infra-red heaters 26. The tube is then stretched and after being collapsed by guide rollers 27 is hauled off by nip rollers 28. A probe 25 supplies the inflating gas between the nip rolls 24 which have sponge rubber grooves to receive the probe. Specifications 746,386 and 787,479 are referred to.