Patent GB-1092635-A (GB)
Title: Stretched tubular films
IPC: B29C48/10
Filed: 1964-04-04
Abstract: 1,092,635. Tubular films. B. SAMWAYS. March 29, 1965 [April 4, 1964], No. 13989/64. Heading B5B. Stretched oriented tubular films of polymeric materials are made by extruding a film-forming composition through a die into a bath of setting liquid (which is defined as including a fluidized bed) to form a tube, drawing the tube upwardly through the setting liquid by a drawing means, setting the tube to a stretch-orientable state within the bath, maintaining a gaseous medium under pressure within at least the uppermost portion of the tube passing through the bath at a pressure sufficient to inflate and stretch the tube in the stretch-orientable state and maintaining the setting liquid in the bath at such a level that the effective inflating pressure, being the differential pressure between the internal pressure exerted within the tube and the external pressure exerted by the setting liquid, acting upon the tube in the freshly extruded unset state is insufficient to rupture the tube. The tube may be entirely filled with gaseous medium under pressure or in a modification may be partly filled with setting liquid having a level lower than that of the setting liquid in the bath, the pressure exerted by the setting liquid in the bath upon the tube at the level of the liquid in the tube being substantially equal to the pressure of the gaseous medium within the tube. The orientable polymeric material may be a regenerated cellulose or a thermoplastic polymer, e.g. polyethylene, polypropylene, polybutene-1, polyvinyl chloride, cellulose acetate, or polyethylene terephthalate, and the cooling liquid may be a liquid metal, e.g. mercury, Wood's metal, Rose's metal or Lipowitz's alloy. Where a fluidized bed is used, spherical particles of glass or stainless steel are preferred. The stretched oriented tubular film may be heat-set by heating to a temperature between that of inflation and stretching and the melting or softening point of the polymer, while maintaining within the tube in the heat-setting zone a pressure sufficient to restrain shrinkage of the film. If the optimum pressure to restrain shrinkage is the same as that maintained by the gaseous medium in the tube, heat-setting may be effected on the stretched oriented tube before collapsing of the tube above the bath. If, however, said optimum pressure differs from the gaseous pressure within the tube, the tube may be reinflated to the required pressure after the collapsing rollers. Alternatively the stretched oriented tube may be heat-set in the slit opened-out state while held in a stenter. Where the film-forming composition is a coagulable liquid, the setting liquid is a coagulation liquid, e.g. viscose or alkaline hydroxy-ethyl cellulose solutions may be extruded into an acid coagulation bath. To prevent the formation of gauge bands, means may be provided for rotating or reciprocating the die or drawing means about an axis coincident with the axis of the tube. In Fig. 1, a die 1 with annular extrusion orifice 2 is fitted to the base of a receptacle 3 having an annular cut-away portion 4 through which the extrudate passes upwardly through 3 to nip rollers 5. Collapsing rollers 51 are mounted between 3 and 5. Base 6 of receptacle 3 in contact with the die is made of ceramic or other heat-insulating material. A closed tank 7 is connected to the bottom of 3 by pipe 9. 8 and 8<1> are heat-exchangers. Air-pipe 10, supplied via valve 11, is connected to the top of tank 7 and also by way of a connector 12, to an inlet pipe 13 which passes through the centre of the die 1, and base 6 of receptacle 3 and terminates near the top of receptacle 3. In operation, tank 7 is nearly filled with a liquid metal alloy 14 held at a temperature within the temperature range at which the thermoplastic material is orientable, by a fluid pumped through 8. Tank 7 is so positioned that the level of alloy 14 is that of line 15. Extrusion is then started and the flaccid tube 16 is drawn up by and flattened between nip rollers 5. Valve 11 is then opened admitting air under pressure through pipe 10, connector 12 and inlet pipe 13 to the tube 16, causing it to become inflated, while at the same time air is admitted through air-pipe 10 to the top of tank 7 causing the alloy 14 to be forced into the receptacle 3 around tube 16. Tube 16 rapidly solidifies as the alloy temperature is lower than the solidification point of the thermoplastic material. When the desired degree of inflation and stretching of the tube wall is reached, valve 11 is closed (see Fig. 2). The capacity of tank 7 is such that the level of alloy 14 falls to that of line 17, slightly below that of line 15. Thus the pressure of alloy 14 exerted on tube 16 at the bottom of receptacle 3 is substantially equal to the air pressure maintained within tube 16 since it is the air pressure in pipe 10 which maintains the difference in level between the alloy 14 in the receptacle 3 and the tank 7. As the tube 16 passes through receptacle 3 the internal air pressure remains constant but the pressure exerted on the tube by the alloy falls, so that the effective inflating pressure steadily rises during the passage of the tube through the alloy 14, thus effecting gentle inflation and thinning of the tube as solidification proceeds, followed by stretching and orientation. Fluctuation of air pressure is immaterial as the system is self-balancing. In Fig. 4 the tank 7 is mounted on a platform 18 capable of vertical movement by the action of hydraulic jacks 19, 20, and tank 7 is connected to the receptacle 3 by a pivotable pipe 91, and air pipe 10 is connected to the inlet pipe 13 by a flexible connector 12<1>. By these means, the level of alloy 14 in tank 7 can be adjusted in relation to the bottom of receptacle 3 (as indicated by line 15) to adjust the effective inflating pressure acting on tube 16, particularly at the point of entry into receptacle 3. In Fig. 5 the level of alloy 14 in receptacle 3 is maintained at any desired level by header tank 23 connected to receptacle 3 by a movable jointed feed pipe 24 and capable of being lifted and lowered by hydraulic jacks 25, 26. Alloy 14 is also introduced into the interior of tube 16 by means of a further feed pipe 29 connected to pipe 24 and passing through the centre of die 1, and the base 6 of receptacle 3. The alloy 14 within tube 16 facilitates the cooling of the extrudate and since there is no effective inflating pressure within the portion of tube 16 occupied by alloy 14 it is possible to adjust the position at which the effective inflating pressure first becomes operative by adjusting the level of alloy 14 inside tube 16, either by adjusting the air pressure in tube 16 or by adjusting the height of the header tank 23. By lifting tank 23 to such a degree that alloy 14 in tube 16, when air pressure has been applied to the tube 16, is on a level with the bottom of receptacle 3, the apparatus of Fig. 5 may be used in the manner of that of Figs. 1 and 2. Fig. 6 shows an alternative tube-forming die 32 which has a circular orifice 33 opening from the side of die 32. This has the advantage that the flow of material from the orifice is substantially in the same direction as the freshlyformed tube 16 is caused to follow. The body of die 32 is thermally insulated from receptacle 3 by an insulating plug 34. In Fig. 7 the setting medium in receptacle 3 consists of a fluidized bed 35 kept agitated by air injected into the base of 3 at openings 36 from supply channel 37. The level of the bed may be controlled either by adding particles 38 from reservoir 39 or by removing particles 38 by outlet pipe 40. In addition, the level of bed 35 may be varied over close limits by varying the rate of injection of compressed air into bed 35, when the volume of bed 35 is varied.