Process for preparing an asymmetric permselective membrane
AssigneeThe United States of America as represented by the Secretary of the Interior
InventorRonald L. Fox
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Abstract
Asymmetric reverse osmosis membranes are made from synthetic aromatic polyimides having a relatively high predominance of polar groups in the polymer backbone. A polar/non-polar group ratio of from 2:2 to 2:7 per imidized nitrogen link is required. Such membranes can be fabricated using conventional casting equipment by providing a solution of the polyimide in a good solvent and a poor solvent, the poor solvent being compatible with solution components at poor solvent/good solvent ratios of at least 1:1 up to about 7:1, casting a film of such solution, and partially evaporating the exposed surface of the cast film for 100.+-.30 seconds prior to gelation.
Claims4
A process for preparing an asymmetric permselective membrane consisting essentially of a synthetic aromatic polyimide derived from the co-condensation of benzophenone3,3',4,4'-tetracarboxylic acid dianhydride and a mixture of either (1) di(4-aminophenyl)methane and toluene diamine or (2) 4,4'-methylenebis (phenyl isocyanate) and toluene diisocyanate, said process comprising:
- (a) providing a solution of about 18 to 25 weight percent of said polyimide in about 20 to 10 weight percent of N,N'-dimethylformamide and about 60 to 70 weight percent of dioxane, said formamide being a good solvent for said polyimide;
- (b) casting a film of said polyimide solution onto a suitable surface;
- (c) partially evaporating the solvent system from the exposed film surface for 100.+-.30 seconds;
- and
- (d) gelling the polyimide film by immersion in water maintained at 15.degree. to 32.degree. C., whereby a thin, dense permselective polyimide skin is formed over a relatively thick, porous polyimide layer.
Dependencies are inferred from the claim text.