Method of making asymmetric cellulose triacetate membranes

DETAILED DESCRIPTION What is claimed is: 1.
A method for making an asymmetric cellulose triacetate semipermeable membrane, which method comprises forming a solution of cellulose triacetate having an acetyl content of 43.
5 percent, by dissolving said cellulose triacetate in a solvent mixture of acetone and dioxane wherein said casting solution contains a ratio of dioxane to acetone of about 3.
8 to 1 and also contains a swelling agent consisting of a mixture of methanol and maleic acid, casting said solution to form a film, evaporating solvent from one surface of said film for sufficient time to form a dense gel layer at said surface, and then treating said film with water to form a gel, and thereafter annealing said gelled film by immersion in water at a temperature of about 85.
degree.
C.
, to thereby provide an asymmetric semipermeable membrane having a high salt-rejection capability and good flux.
2.
A method for making an asymmetric semipermeable membrane in accordance with claim 1 wherein said evaporation is carried out for between about 25 and about 90 seconds.
3.
A method for making a semipermeable membrane in accordance with claim 2 wherein said evaporation is carried out at a temperature between about 15.
degree.
C.
and about 35.
degree.
C.
4.
A method for making a semipermeable membrane in accordance with claim 3 wherein said solution being cast is maintained at a temperature of between about 0.
degree.
C.
and about 30.
degree.
C.
5.
A method for making an asymmetric semipermeable membrane in accordance with claim 1 wherein said solution contains cellulose triacetate in an amount between about 11 and about 18 weight percent.
6.
A method for making a semipermeable membrane in accordance with claim 1 wherein said solution contains cellulose triacetate in an amount between about 15.
5 and about 16.
5 weight percent and has a viscosity between about 200 and about 2000 poises at the temperature at which casting takes place.




Description:
BACKGROUND OF THE INVENTION Asymmetric semipermeable membranes suitable for use in reverse osmosis processes for producing potable water from brackish or saline water have been known for sometime