Patent GB-1420082-A (GB)
Title: Filter membrane
IPC: C08G16/00
Filed: 1972-12-15
Abstract: 1420082 Coated materials; reverse osmosis SATORIUS-MEMBRANFILTER GmbH 15 Dec 1972 [28 Dec 1971] 58100/72 Headings B1X and B2E A multiple-layer filter membrane, comprises (a) an osmotic skin comprising at least one unbroken film of a selectively permeable high molecular weight polymer and (b) a porous supporting base comprising a film of gel which swells but does not dissolve in water, backed by a sponge layer of a cellulose derivative containing filler, the sponge layer being impregnated on the side remote from the gel film with an impregnating composition to minimize or prevent a reduction in the percolation rate of the sponge layer after it has dried during formation thereof, and the unbroken film being applied to the gel film. The membrane may be produced by continuously pouring a castable solution containing 8-20% by weight of cellulose acetate having a 31.5% acetyl content, 2-7% by weight filler material such as titanium dioxide, quartz, alumina, powdered pumice, bentonite, graphite, talc, glass fibres or pyrogenic highly dispersed silica, 10-13% by weight of water and 60-80% by weight of a solvent consisting of acetonitrile, ethanol, acetone, ethyl lactate or ethyl glycol or a mixture thereof from a stationary pourer having an elevatiomally adjustable weir on to a moving endless receiving surface to form a layer, exposing the surface of the poured layer to air for 5-15 seconds before being detached from the receiving surface and immersed in water for 5-15 minutes, securing the edges of the supporting base thus formed, the base having a side exhibiting a sponge structure and an opposite side exhibiting a film of gel, tempering the base for 1-10 minutes in a water bath at 70- 90C, impregnating the side of said supporting base at 40-60C for 1-5 minutes, to minimize or prevent a reduction in the percolation rate of the sponge structure after it has been dried, drying the base in air at 30-100C for 1-5 minutes, and after the prior application of any supplementary layers coating said opposite side of the supporting base with at least one unbroken film of a selectively permeable inelastic, elastic or elastified high molecular weight polymer. The watered supporting base of cellulose acetate may be hydrolysed to a hydrated cellulose by treating with an alcoholic potassium hydroxide solution at room temperature for 1-10 minutes. In a modification of this method a cellulose acetate supporting base consisting exclusively of a sponge layer with a superimposed gel film consisting of a chemically different polymer is produced by at once immersing the solution for forming the supporting base having been poured without exposure to air, the resultant supporting base then being impregnated on one side, dried and coated with a gel substance consisting of an agar-agar/formaldehyde reaction product and  when the gel has been dried in warm air at 50C for 2 minutes applying an elastic layer of natural rubber, butadiene-acrylonitrile rubber or silicone rubber and drying the resultant selectively permeable high polymer film. Suitable inelastic high polymers for the selectively permeable membrane layer are phenoxy resins, polyvinyl chloride, polyvinyl fluoride, polysulphones, cellulose acetate, polyacrylates, polyamides. Suitable elastic high polymers are silicone rubber, natural rubber, butadiene-acrylonitrile rubber. Suitable agents for elastifying inelastic high polymers are polyurethane, polymeric plasticizers, silicone rubber, natural or synthetic rubber. The supporting base may be reinforced with a fabric of glass, synthetic fibres, graphite fibres or stainless metal fibres. Suitable impregnants are glycerine, polyvinylmethyl ether, polyacrylic acid.