Patent GB-922611-A (GB)
Title: Improvements in and relating to cast self-supporting porous sheets of films of organic polymeric material
Inventor: PATCHELL ALBERT GEORGE
IPC: B29C48/91
Filed: 1959-03-23
Abstract: A self-supporting composite film comprises two integral portions one of which consists of a porous cast film of a plasticized and gelled polyvinyl chloride or other organic polymeric material, while the other is a non-porous reticular film consisting of a gelled plasticized polyvinyl chloride or other organic polymer compatible with the material of the porous <PICT:0922611/IV(a)/1> portion of the film, the reticular portion having an openwork pattern of fine lines and functioning as a reinforcing structure for the main or porous portion of the film.  The film may be made by laying a paste comprising a dispersion of the P.V.C. or other polymeric material in a non-volatile plasticizer (or mixture of plasticizers in which the polymer is substantially insoluble at ordinary temperatures) in the form of fine-line openwork or reticular pattern 20 (Figs. 1 and 2) on to an endless travelling belt 10 passing around guide rollers 11 to 13 and driven in the direction of the arrows by the driving roller 14.  The upper run of the belt is supported by the fixed rigid table 15 and the material is extruded on to the belt from the extrusion head 16 provided with a row of equally spaced fine apertured nozzles and mounted to reciprocate transversely over the belt upon fixed parallel guides 18 in a uniform manner by a connecting rod 19. The reticular film 20 thus formed may be carried on the belt 10 through heating means 21 to 23 which first evaporate off any volatile solvent which may be present in the film and then gel the film.  The gelled film may then be carried through the nip of rolls 24 and 25 and beneath a hopper 26 containing a second paste 27 comprising a dispersion of P.V.C. (or of a polymeric material compatible with that of the first paste) and a particulate pore-forming agent dispersed in a plasticizer or mixture of plasticizers in which the polymer is insoluble at ordinary temperatures.  The second paste 27 is discharged from the gap at the lower part of the wall 29 of the hopper 26 so that a film 30 of the paste 27 is cast on the reticular film 20 to form a composite film which is passed through the heaters 31 to 33 to remove any thinning solvent from the composite film and to gel the paste 27 component of the film.  The film may be heated in this zone to a temperature sufficiently high to decompose the pore-forming agent if this is a "blowing agent" which decomposes at or below the gelling temperature with evolution of a gas.  The film is thereafter stripped from the belt 10 before or after being cooled, and if the pore-forming agent used is of the water-soluble type, it is leached out of the film with water.  The second paste covers the reticular film and enters and wholly or partially fills all the spaces of the reticular film.  The thickness of the layer of the second paste may be equal to or greater than the thickness of the reticular film.  The reticular film may be deposited on the travelling belt by a grooved roller instead of the extrusion head 16, the patterned grooves of the roller being fed with the first paste which is then transferred to the travelling belt by rotating the roller in pressure contact therewith.  A film of the second paste containing the poreforming agent may be cast on the belt first and the reticular film of the first paste cast on top of this. Furthermore, a second film of the paste containing the pore-forming agent may be cast on the reticular film, which in this case therefore constitutes the intermediate layer of a three layer composite porous film.  The composite film may be laminated with a porous textile fabric by pressing the fabric lightly on to the exposed surface of the film before the latter is gelled so that the fabric penetrates slightly into the film.