Patent GB-751351-A (GB)
Title: Improvements in or relating to method and apparatus for producing and treating filmsof thermoplastic material and the resulting product
IPC: B29C55/02
Filed: 1952-02-07
Abstract: <PICT:0751351/IV(a)/1> <PICT:0751351/IV(a)/2> "Distended orientation" of molecules in the continuous production of crystalline films of high molecular weight thermoplastic material is prevented by passing a non-crystalline film of the material into a crystallization zone without subjecting the film to any longitudinal or lateral stretching forces, maintaining the temperature of the zone at the crystallization temperature of the material and regulating the rate of passage of the film to maintain the film in the zone for a period sufficient to effect substantially complete crystallization of the film.  As shown, the thermoplastic material is fed into a hopper 24 of a screw-type extruder 20 in which it is heated in the absence of air and from which it is extruded through a die 25 and delivered into the nip 21 formed between a paper-carrying roll 22 rotating downwardly towards the nip and a substantially stationary doctor roll 23.  The rolls 22, 23 are hollow and provided with conduits 26, 27 to supply heated oil to the interiors of the rolls to maintain them at the desired temperature.  When the material is that sold under the Registered Trade Mark "Saran," it is extruded from the die 25 at 335-340 DEG  F., the doctor roll 23 is maintained at an average temperature of 295-305 DEG  F. and the paper-carrying roll 22 is maintained at 330-335 DEG  F.  The material forms a "floating" rotating roll 26 in the nip 21 and is subjected to extreme working, so that its temperature rises above that of the rolls 22, 23 and may be about 345-350 DEG  F.  The paper web 12 is unwound from the roll 11 rotatably supported on the bearings 10, over the idler and tensioning rollers generally indicated at 13, a pair of preheat rollers 14, 15 heated to 320 DEG  F. by means of steam supplied through the lines 16, 17 and a third preheat roller 18 heated by the steam line 19 to about 345 DEG  F. so as to heat the paper prior to the coating or laminating operation to about 320 DEG  F.  The paper passes through the nip 21 of the rolls 22, 23 and is thereby coated with a thin film, the thickness of which is determined by the adjustable pressure between the rolls 22, 23.  From the nip 21 the coated web is led over a heater 30, e.g. an infra-red gas flame, to apply to the exterior surface of the film a sudden intense heat to raise its temperature to over 400 DEG  F., preferably 440-540 DEG  F. The sudden application of this heat serves to evaporate the plasticiser from the exterior surface of the film and to eliminate surface defects in the coating.  The speed of the coated web is sufficiently high to prevent injury and, when the coating is Saran, may be 170 ft./min.  Immediately after the passage of the web over the heater a first water spray 31 is applied to the paper side of the web at a rate sufficient to cause the water to form a pool 32 at the point when the web meets a guide roller 33, the excess water draining off from the sides of the web.  A second water spray 34 is applied to the web immediately before it enters the crystallization oven 35. The water sprays are at atmospheric temperature and their purpose is to saturate the web and thus render it capable of changing its dimensions without setting up internal stresses between the paper and the coating due to the latter shrinking.  Any tendency to curl is also avoided.  The coated web 12 enters the oven 35 through the slot 43 in the end wall 41, passes underneath the lower row of guide rolls 44 and around a roll 45 which reverses the direction of travel of the web in the oven.  The paper side, and not the coated side, of the web contacts the rolls 44, 45.  The web emerges from the oven and passes over two chill rolls 46, 47 provided with coolant such as tap water through lines 48, 49.  The coated side of the web contacts rolls 46, 47 but is prevented from sticking to them by a thin film of moisture which condenses on the rolls.  After passing around the rolls 46, 47, the web re-enters the oven and passes along the idler rolls 44 and out through a slot 50 in the far end wall of the oven, again with the paper side and not the coated side of the web in contact with the rolls 44. The coating is maintained at its optimum crystallization temperature as it passes through the oven 35 by means of three hot air ducts 51 extending longitudinally through the oven and provided with a number of slot-like openings 52 through which hot air is directed against the coated face of the web.  An air pump 53 draws air from the oven through an opening 56 and delivers it to an air heater 54.  The air, heated to 220-250 DEG  F., is led into each of the ducts 51 through a series of connecting ducts 55 and thereby maintains the temperature of the oven at 180-200 DEG  F.  Any vapours given off from the coating escape from the oven through a vent 57. The length and rate of travel of the coated web through the oven are such as to allow any given particle of the coating to remain in the oven for sufficient time to effect complete crystallization.  For Saran this time is about 22 seconds and for a rate of web travel of 150 ft./min. the oven is about 20 feet long.  From the oven the web is led around a guide roll 60, a chill roll 61 chilled by tap water from the line 62 and thence through the winding apparatus 63.  This includes a take-up reel 64, guide rolls 65, 66 and a pair of keeper rolls 67, 68 to enable the take-up reel 64 to be changed when full without the passage of the web through the oven being interrupted. An additional spray 69 may be provided to rewet the web as it leaves the oven to bring the web to any desired moisture content before it is wound.  The means for driving the rolls are not shown.  The substantially stationary roll 23 may be permitted by a ratchet or other device to rotate very slowly, and means may be provided to clean the back surface of the roll constantly to present a cleaned surface to the nip 21.  The plasticizer vapours evaporated from the surface of the film by the heater 30 are drawn off through a hood 70 connected to an exhaust system.