Patent GB-1221804-A (GB)
Title: Porous polymeric sheet material
IPC: C08J9/28
Filed: 1968-02-13
Abstract: 1,221,804. Vapour-permeable films. E. I. DU PONT DE NEMOURS & CO. 13 Feb., 1968[16 Feb., 1967; 26 Jan., 1968], No. 7120/68. Heading B5B. [Also in Divisions B2, C3, D1 and G1] A vapour-permeable sheet material, preferably in the form of an unsupported film, has a permeability value of over 750 and comprises a layer of microporous polymeric material, said layer having a total pore volume of from 20 to 35%. From 65 to 100% of said pore volume may consist of pores having a cross-section diameter of up to 2 microns and from 0 to 35% of pores having a cross-section diameter of greater than 2 microns. Up to 30% of the pore volume may consist of pores having a crosssection diameter of from 2À01 to 20 microns and up to 15% of pores having a cross-section diameter of from 20À01 to 100 microns. Preferably, the upper half of said layer has a pore volume of up to 28% and the lower half of said layer a pore volume of from 20 to 43%. Desirably, (a) in the upper half of said layer, from 70 to 100% of the pore volume consists of pores having a crosssection diameter of up to 2 microns and from 0 to 30% of pores having a cross-section diameter of greater than 2 microns, (b) in the lower half of said layer, from 63 to 95% of the pore volume may consist of pores having a cross-section diameter of up to 2 microns and from 5 to 37% of pores having a cross-section diameter of greater than 2 microns, and (c) at least 50% of the total pore volume may be in the lower half of said layer. Suitably, at least 51% by weight of said polymeric material consists of a polyurethane elastomer and up to 49% by weight of a vinyl chloride polymer. Pigments and other additives may be added if desired. In a preferred method, there is (a) applied to a suitable substrate a layer of a liquid polymeric composition comprised of a polymer dispersed in a liquid comprising an organic solvent (e.g., one capable of absorbing water), which layer is capable of being solidified in situ to form a microporous structure when applied to the substrate, and (b) said layer of liquid composition is then exposed to an atmosphere adapted to initiate coagulation of the layer; e.g., by exposing it to a humid air zone having suitable relative humidity (e.g., 40%) and temperature (e.g., 26 C.) until said layer has absorbed enough water to cause at least the uppermost portion of the layer to coagulate, and said layer of composition is then (c) bathed with a bathing liquid e.g. water, adapted to extract the organic solvent from said composition until substantially all of said solvent is extracted; (d) the resulting sheet material is then dried, and (e) pressed while at least the upper portion thereof is at an elevated temperature until the microporous structure is consolidated sufficiently to provide the structure described. By these means the composition is caused to solidify in situ and become a microporous moisture-permeable structure. The sheet material may thus be in the form of an unsupported film, the film consisting essentially of said layer of microporous polymeric material. In various preferred embodiments, the liquid polymeric composition to be applied to the substrate is prepared by adding water to the polymer solution in an amount sufficient to provide the composition with the water content of about 3À7 to 4À5% based on the total weight of the resulting composition. The bathing liquid is preferably water or a mixture of water and a water-miscible organic liquid, e.g. dimethylformamide. The bathing liquid employed in at least the last portion of step (C) is preferably volatile in the sense that it can be evaporated from the sheet material under drying conditions which cause no undue damage to the fibre and polymer components. The bathing liquid also is a coagulant for the liquid polymeric composition employed in preparing the sheet material; thus the bathing liquid is adapted to coagulate any of the polymeric composition which remains uncoagulated after step (b) when the bathing liquid comes in contact with the composition. The abrasion-resistant vapour-permeable unsupported microporous polymeric films are useful as e.g. filters and surgical bandages. In example 2, an unsupported polymeric film useful in filter and bandage application is prepared by repeating example 1 (for which see B2 abridgment) except (a) the non-woven fabric is treated so that the microporous film can be removed, this being done by passing the fabric into a tank filled with water at 24 C. and passing the immersed fabric out of the tank and then between a pair of squeeze rolls whereby the water content of the fabric is reduced to 75% based on the combined weight of fabric and water; (b) steps 6-9 are omitted; (c) after the hot-press consolidation step, the microporous polymeric layer or film is removed from the substrate by merely pulling the film away from the substrate. Such a film has a permeability value of 5000. Its other properties are similar to those for the product of example 1, which has a permeability value of 2,000 and a Schiltknecht flex durability value of over 20 million. The microporous polymeric layer has a total pore volume of 27À5%, and 78% of the pore volume consisting of pores having various cross-section diameters within the range of about 0À07 to 2 microns; about 19% of the pore volume consists of pores having various cross-section diameters within the range of about 2À01 to 20 microns; and about 3% of the pore volume consists of pores having various cross-section diameters within the range of about 20À01 to 100 microns. The upper half of the microporous layer has a pore volume of about 26%, and the lower half has a pore volume of about 32%. The surface of the microporous polymeric material has an impact scuff resistance rating of 2, and in the Taber Abrasion tests the sheet material undergoes a weight lose of 0À2 grams.