Improvements in and relating to the manufacture of films

摘要

A molten film-forming polymer is extruded on to a moving support maintained at a temperature below the melting point of the polymer through a slot orifice positioned a distance of more than one quarter of an inch from the support and the extruded molten film is supported along its edges and kept extended during its passage from the slot orifice to the moving support. The polymer may be a synthetic linear polyamide or a polymer of ethylene. As shown in Fig. 1, the molten polymer is extruded from a hopper comprising two lips 1, 2, channel 4, and slot opening 3. The molten film falls vertically for more than one quarter of an inch on to a rotating drum 7. The film solidifies on the drum and is passed over roll 14 to a winding-drum 15. At each end of the slot 3 there is located a downwardly extending guide 6 as shown in Fig. 3. The guides 6 may be formed in one piece as shown in Fig. 4 and they may be made of spring steel. The molten film is supported between the inner edges of the two guides by surface tension. The guides are heated by means of the electric resistance wires 9 to prevent premature setting of the polymer. The lower ends of the guides extend into peripheral grooves 11 in the casting drum. The drum is internally cooled by water. An alternative method of supporting the edges of the molten film is shown in Fig. 5. Each edge of the film is supported by a thread 20 from a spool 21. One of the hopper lips is provided at each end of the extrusion slot 3 with a guide hook 22 over which the threads are drawn by the drum p 7 which is provided with grooves 24 to accommodate the threads. The molten polymer is preferably filtered through sand before use and the molten film is cooled as rapidly as possible from the temperature at which it first begins to solidify to a temperature at least 60 DEG C. below its melting point. Preferably, the film is cooled to such a temperature within one second of striking the moving support. To avoid condensation of moisture, the temperature of the drum is preferably maintained slightly above the dew point of the surrounding air. The casting is preferably vertically downward with the plane of casting displaced towards the centre of the drum a distance of less than one inch from the parallel vertical plane tangent to the casting drum on the downward moving side. To improve the contact between drum and film and thus promote better heat transfer there may be applied to the drum liquids such as tricresyl phosphate, chlorinated diphenyls, mineral oils, kerosene, xylene, or ethyl alcohol. In examples, films are formed from an interpolymer of hexamethylene diamine, adipic acid and sebacic acid, from an interpolymer of hexamethylene diammonium adipate and hexamethylene diammonium sebacate and from a polymer of ethylene. The casting drum may have a higher peripheral speed than the speed of extrusion. Films of polyesters, ethers, acetals, urethanes, thioureas, or ester-amides, may be obtained. The polymer may contain plasticisers, dyes, pigments, anti-oxidants, stabilizers, and resins such as phenol-formaldehyde. The surface of the drum or belt may be plain and smooth or it may be engraved with any desired pattern. The casting zone may be surrounded with an inert atmosphere such as carbon dioxide, or nitrogen. Heat may be supplied to the molten polymer between the hopper and the drum by means of infra-red lamps. The films may be used as photographic film base, wrapping material, artificial leather, coatings, diaphragms for fuel pumps, wall coverings, window shades, playing cards, lamp shades, currency or policy paper, battery plate separators, membranes for musical drums, strips for millinery or furniture seats, glass substitutes, or interlayers for safety glass. With reinforced edges, the films may be used for dress belts, suspenders, or watch straps. Specifications 461,236, 461,237, 471,590 and 542,509 are referred to.