Patent GB-664638-A (GB)
Title: Improved method of making film tubes from normally crystalline vinylidene chloride polymers or copolymers
IPC: B29D7/01
Filed: 1949-06-08
Abstract: <PICT:0664638/IV (a)/1> Tubular films are made from a normally crystalline vinylidene chloride polymer (or from a copolymer of vinylidene chloride with another polymerizable substance, the copolymer being normally crystalline, as determined by X-ray diffraction technique) by extruding a tube of the fused polymer downwards into a bath of cold water to supercool it while maintaining a column of inert lubricating liquid in that portion of the freshly extruded tube which passes vertically downward into the bath, conveying the tube vertically upward between two spaced apart pairs of pinch rolls, and inflating the tube to effect stretching and recrystallization thereof between the pairs of rolls by introducing air therein under pressure until the introduction of more air results in no further radial expansion of the tube and causes only elongation of the inflated portion of the tube toward the lower pinch rolls, the process being characterized by maintaining within the tube above the lower pinch rolls a body of finely powdered crystalline vinylidene chloride polymer (or copolymer) of composition similar to that of the tube, in effective dusting contact with the inner walls of the tube, thereby to overcome the normal tendency of those walls to stick to one another when the tube is flattened in passing through the upper pinch rolls.  The polymer powder may be in the form of a loose pile carried within the tube above the nip of the lower set of rolls, or, preferably, it may be contained within a dusting bag carried at the same position within the tube and made of any cloth sufficiently porous to exude or sift out the fine polymer powder continuously as the tube moves upward past and around the bag.  The powder is preferably sufficiently fine to pass a 100-mesh screen, U.S. sieve series, and particularly of particle size of a few microns or finer. As shown, the fused polymer or copolymer is extruded downwardly from the orifice of an extruder 10 and the freshly formed tube 11 thus produced falls directly into a bath of cold water 12 wherein it is supercooled. It is passed through a submerged set of pinch rolls 13 and a column of oil 14 is carried in the portion of the tube 11 above the rolls 13.  The flattened and super-cooled tube is led around a guide roll 15 and thence vertically out of the bath 12 and between two sets of pinch rolls 16 and 17 located one set above the other. A dusting bag 18 containing finely divided crystalline polymer rests inside the tube on the shoulders of the inflated portion thereof immediately above the rolls 16.  The portion of the tube between the pairs of pinch rolls 16 and 17 is inflated with air to from 3 to 5 times the supercooled diameter until the addition of more air results merely in migration of the stretching zone "A" downwards towards rolls 16.  The fully stretched tube is forwarded in a flattened condition by rolls 17 which operate at a peripheral speed of from 2-4 times that of rolls 13, 15, and 16, and is taken up on a storage reel 19.  Specification 656,735 is referred to.