Process for preparing membranes for separation of substances

#This invention relates to a process for preparing a semipermeable membrane by crosslinking with a plasma the surface of a shaped article comprising a mixture of a water-insoluble high polymer and a water-soluble high polymer and/or a water-soluble surfactant. The resulting membrane can be used for separation and concentration of substances according to reverse osmosis, ultrafiltration and the like methods.

#The semipermeable membrane currently being commercialized is a cellulose acetate membrane produced according to the Loeb and Souriaranjan method (U.S. Pat. Nos. 3,133,132 and 3,133,137) or a similar method. In spite of its good performances regarding the flux of water and the rejection of salt, however, the said cellulose acetate membrane is undesirably limited in life for such reasons that it is deteriorated in quality due to susceptibility of cellulose acetate to hydrolysis or is made higher in density due to high pressure applied thereto with the result that the salt rejection or the flux of permeated water is decreased with lapse of time.

#In order to overcome the above-mentioned disadvantages of the conventional cellulose acetate membrane and to prepare membranes that are much higher in water permeability, many attempts have been made to develop reverse osmosis membranes utilizing ultrathin membranes. For example, a cellulose acetate membrane of less than 1 micron has been prepared according to a casting method using as the substrate a porous filter of cellulose ester or polysulfone. So far as cellulose acetate is used as the membrane material, however, such disadvantages of the resulting membrane as susceptibility to hydrolysis and the like cannot be neglected.

#The accompanying drawing is a schematic diagram of a plasma irradiating apparatus for use in the process of the present invention.

#In the drawing, 1 and 7 are valves, 2 a neon transformer, 3 and 4 electrodes, 5 a vessel, and 6 a sample.

#According to the process of the present invention, it is possible to produce a novel synthetic high polymer membrane not susceptible to such deterioration due to hydrolysis or attack of bacteria which deterioration is the disadvantage of the conventional cellulose acetate membrane. Thus, the present invention can provide a membrane utilizable for separation and concentration of substances by reverse osmosis, ultrafiltration or the like which is more excellent in properties than the cellulose acetate membrane of Loeb.

#The shaped articles comprising a water-insoluble high polymer, a water-soluble high polymer and/or a water-soluble surfactant which are used in the process of the present invention may be in various forms, in practice, and particularly preferred forms are film, sheet, filament and tube. These articles are made into crosslinked membranes by exposure to a plasma, and then put into practical use as modules of suitable forms. For example, a cast solution comprising a water-insoluble high polymer, a water-soluble high polymer and/or a water-soluble surfactant and a co-solvent therefor is cast on a glass plate or the like, and the resulting membrane is exposed to a plasma to crosslink only the surface thereof and may then be used as a flat or spiral-wound module. On the other hand, a membrane obtained by casting the said cast solution on such substrate as a tubular or rod-shaped glass may be used as an inner or outer pressure type tubular module. Further, a hollow fiber membrane can be obtained by spinning a hollow filament from the above-mentioned cast solution, and a hollow fiber membrane reinforced with a filament can be obtained by immersing a filamentary fiber in the cast solution, followed by drying.