Patent GB-1229793-A (GB)
IPC: B29C48/12
Filed: 1967-05-30
Abstract: 1,229,793. Film fibrillation. O-B. RASMUSSEN. 29 May, 1968 [30 May, 1967], No. 24837/67. Heading B5B. A process for forming a film of splitfibre forming material comprises the steps of extruding splitfibre forming material through a slot of pleated configuration to form a film of pleated cross-sectional shape, and longitudinally stretching said film, while still in fluid or semi-fluid state, into substantially flat cross-sectional shape having a width substantially smaller than the total length of the slot along the pleated configuration thereof. The width of said film after the stretching preferably does not exceed the direct span of the slot.  In a modification, the slot is circular and constructed with substantially radial pleats. Specified splitfibre forming materials include high density polyethylene, a randomly mixed dispersion of two incompatible polymers, as, for example, those disclosed in Specification 1,148,382, and a mixture of polypropylene and polystyrene, which is exemplified. There may also be used a polypropylene the major proportion of which is isotatic, but has a minor proportion of an atactic modification of the polymer. The pleating ratio, i.e. the ratio between the total length of the extruder lip along the pleated configuration of the same and the direct span of the latter, or in the case of a circular die, the average periphery, should preferably be at a value of about 2 : 1, but ratios of 3 : 1 are advantageous in most cases, and ratios as high as 4 : 1 to 7 : 1 or even higher may be used. The draw-down ratio in the melted or semimelted state, i.e. the proportion between the original thickness of the film immediately after extrusion and the thickness after stretching in the fluid or semi-fluid state, should preferably be higher than the square of the pleating ratio. Thus in the drawing, a polymer or polymer mixture is extruded from an extruder die 1 having a pleated slot 2 and hauled off by means of roller 3. Thus the film extruded from the slot will first have a pleated cross-sectional shape of substantially uniform thickness, and this film is thereafter hauled off, while still in a fluid or semi-fluid state, at a drawn-down ratio sufficient for converting the film into a state of substantial flatness, the width thereby being reduced to substantially the same value as the direct span of the slot 2 or below the said value. Air cooling is applied from both sides of the film by means of air nozzles 4. Lines 5 show the travel of those parts of the film which have been extruded at the turn points of the pleated orifice, so as to show how the film is converted from the strongly pleated to the flat shape. Hence the film, while being stretched, is subjected to a strong lateral contraction. When using a non-circular pleated slot it is advantageous to keep a relatively short distance between the slot and the haul-off roller, the said distance preferably being between 5 and 20 times as great as the depths of the pleats of the slot. In the example, the pleated slot is 0À7 mm. thick, the depth of the pleats about 18 mm., and the distance between the pleats 23 mm. The pleating ratio is 2À1 : 1. The distance from the pleated slot to the haul-off roller is about 20 cm., and the draw-down ratio about 15 : 1. Moderate air cooling is applied. The extrusion is carried out with a mixture of 85% polypropylene and 15% polystyrene, the two polymers being first mixed in the dry state and then worked into each other with a planetary screw extruder, from which the melted mixture is supplied directly to a die containing the pleated slot. From this the film is extruded at 250 C. down on to a roller at 135 C. to give a film of 78 mm. width, and 0À15 mm. thickness. The film is heat stretched between two sets of rollers having a temperature of 120 C., said rollers forming an angle of 80 degrees with one another. This asymmetric stretching subjects the film to a transverse compression, while it is at the same time extended 16 times, whereby the molecular structure is oriented in the stretching direction. The stretched film shows a high splittability, being split up into branched fibres during stretching.