Patent GB-943350-A (GB)
Title: Improvements in or relating to thermoplastic film manufacture
Inventor: EULING TERENCE MARTIN
IPC: B29C48/90
Filed: 1961-02-10
Abstract: 943,350. Tubular films. IMPERIAL CHEMICAL INDUSTRIES Ltd. Jan. 26, 1962 [Feb. 10, 1961], No. 5014/61. Heading B5B. Tubular films of organic thermoplastic material are made by extrusion from a die in association with which are means for preventing inflation pressure from rupturing the film, means for evaporating volatile liquids from and/or cooling the film, either before or after emerging from the body of the equipment containing the extrusion die, and a stationary friction brake, which may or may not form part of the body of equipment containing the extrusion die. The tube is then inflated and hauled off at a rate greater than the feed rate to biaxially orient and stretch it. The evaporative and/or cooling means render the film sufficiently " non-sticky " to prevent it sticking to, or being marred by, the friction brake. The inflating gas is supplied via a passage in the extruding die. The friction brake has surfaces shaped to contact the tubular film and may comprise an annular or cylindrical member capable of adjustment of its diameter. The friction brake may be mounted inside or outside the tube, adapted to expand against it or contract around it. Alternatively, the friction brake may comprise a ring or cylinder of fixed diameter. Preferably, the friction brake is actuated by a device responsive to the rate of feed of the film so that increase in film speed leads to increase in the applied friction. The means for preventing rupture of the film may be e.g. an external solid support such as a tube, means to equalize the gas pressure inside and outside the tubular film or an internal baffle, isolating the inflation pressure from the pressure in the tube near the die. The baffle may comprise an internal air bearing, e.g. a cylinder of sintered metal to the interior of which air is supplied under pressure. Preferably, after passage through the friction brake, the tube is heated to orientation temperature, e.g. by infra-red radiation. The radiation is preferably supplied from at least two heaters, one of which is mounted inside the tube. The process may be applied to melt-extruded film with or without quenching, or to film extruded from a solution or dispersion in a volatile liquid which is subsequently evaporated. The process may be applied to any organic thermoplastic capable of being biaxially orientated such as polyglycol terephthalates, e.g. polyethylene terephthalate; copolymers of isophthalic and terephthalic acids with a glycol; vinylidene polymers, e.g. containing 80-95% vinylidene chloride and 20- 5% acrylonitrile and copolymers thereof with vinyl acetate or vinyl chloride; vinyl chloride; polyamides; polystyrene; cellulose esters; proteins; rubber hydrochloride and polyolefines having a degree of crystallinity (measured by X-ray diffraction) of more than 50%. Preferred polyolefines are polythene and isotactic polypropylene. In a preferred embodiment, Fig. 1, the tubular film 1 is melt extruded from die 2, passed through cooled sizing die 3 and through friction brake 13 associated with speed sensitive device 15. The tube then passes through aperture 16 in heater-housing 17 mounted on infra-red radiator 18. A rotatable internal radiator 19 is mounted on tube 20, which also carries baffle and locator 21. The inflated tube is collapsed by guide rollers 23 and flattened by nip rolls 24. To reduce the effects of convection from internal heater 19, the air between it and the locator may be replaced continuously through tube 20. Preferred types of friction brake comprise a flexible rubber sleeve held by vacuum to the tubular film (Fig. 2), a brass cylinder 32 (Fig. 3) brazed to overlapping segments of a steel cylinder 33, a segmented brass ring 38 (Fig. 4) mounted inside the tube on an expandable lay having elastic walls, and a cooled spiral Bourdon tube (Fig. 6). The process allows the melt extrusion of isotactic polypropylene at more than 10 feet per minute at 250-300 C., to form film which, when stretched to an area ratio of 40: 1 to 100: 1 may be flattened without cracking at the crease and subsequently slit down one edge only to form film of double width. Specification 820,620 is referred to.