Patent GB-1277519-A (GB)
Title: Method for the production of biaxially oriented polypropylene film
Inventor: GOSPER DAVID LEE
IPC: B29C55/14
Filed: 1970-08-21
Abstract: 1277519 Polypropylene films MOBIL OIL CORP 21 Aug 1970 40330/70 Heading B5B [Also in Division D1] Process for producing biaxially oriented polypropylene films comprises the steps of extruding a molten polypropylene film through the nip of a pair of rollers; quenching the film; preheating the quenched film to at least 143‹ C. stretching the film, preferably by at least 800%, in a transverse direction while continuously increasing the temperature of the film within the range 143-166‹ C. and stretching the transversely stretched film in a longitudinal direction, preferably by at least 400%, while maintaining the film above 149‹ C. and restraining the film edges against transverse shrinkage. Preferably the film is transversely stretched by between 800 and 900% and longitudinally stretched by between 500 and 650%. Suitably the film has a longitudinal bead along each of its edges, these beads being restrained during transverse and longitudinal stretching when the edge portions are preferably maintained at a lower temperature than the remainder of the film. After the process of the invention the polypropylene film may be annealed at a temperature of from 143‹ to 177‹ C. in order to render the product dimensionally stable at elevated temperatures. Apparatus (Fig. 2) usable in the process of the invention comprises an extruder 12 from which a beaded edge film is extruded through the nip of rollers 14, 14a into quench tank 16. The quenched film 9 passes through pre-heat tunnel 19 and into transverse orientation zone 20 where the beads 8 are gripped by a series of retaining clips 21 mounted on a continuously moving endless chain 22. In zone I no substantial stretching of the polypropylene film occurs. In zone II, where the edge bead retaining clips diverge and the temperature of the film continuously rises, the film is transversely stretched and then passes into zone III where again no substantial transverse stretching occurs. Longitudinal stretching is accomplished in the region 27 by passing the film between a bank of radiant heaters 24 into a set of nip rollers 26 which are rotated at a higher speed than the speed of entry of the film into the orienter 27. Figs. 3 and 4 (not shown) illustrate an extruder die 13 and nip rolls 14, 14a for use in the apparatus. The mode of action of the retaining or tenter clips 21 are illustrated in Figs. 8-11. In Fig. 9a clip 21 is provided along its leading edge with an upwardly extending arm 71 integrally joined to the rear of the leading edge of a clip shoe member 72. Arm 71 functions to ensure positive release of the edge bead 8 of the film 9 from the clip 21 as it is opened at the end of the transverse orienter 20. Fig. 5 illustrates a pair of wheels 25 which are used to restrain the edge 8 of the film 9 while it passes through the longitudinal stretching zone 27. Wheels 66 and 67 freely rotate on shafts 70<SP>1</SP> and 70, lower wheel 67 being spring-mounted on spring 69 to ensure gripping action on the bead edge 8 of the film. Shafts 70 and 70<SP>1</SP> are mounted on hollow block member (61) through which is circulated heat exchanger fluid to cool the wheels 25 and hence the edge of the film (Fig. 6, not shown).