Patent GB-1227961-A (GB)
IPC: C02F1/44
Filed: 1968-06-05
Abstract: 1,227,961. Semi-permeable membranes. AMERICAN STANDARD Inc. 5 June, 1968, No. 26725/68. Heading B5B. [Also in Division B1] A process for preparing a tubular osmotic membrane comprises in combination the steps of preparing a freshly formed osmotic or semipermeable film, partially drying said film, soaking said film in cold water, imparting a tubular configuration to said film, and subsequently annealing said film by soaking the same in hot water while maintaining said tubular configuration. Preferably the film forming material consists essentially of cellulose acetate and may be made by casting from a solution of cellulose acetate, acetone, magnesium perchlorate and water, especially a solution comprising cellulose acetate 15 to 25%, magnesium perchlorate 1 to 2%, acetone from 60 to 80%, and water from 1 to 20% all percentages being by weight. The solution may also comprise hydrochloric acid in proportion between 0À1 and 1% by weight. Casting is advantageously performed with apparatus and solution precooled to a temperature between - 15 and 5 C., and the freshlyformed film is suitably cast to an initial thickness between 10 and 25 mils. The film may then be partially dried in air for not more than 5 minutes, and the partially dried film suitably soaked in cold water at a temperature of 1 C. for approximately one hour. The tubular configuration may be imparted to the film by wrapping the said film about a mandrel, either longitudinally (Fig. 6) or spirally (Fig. 5). The film may then be annealed by soaking in water at a temperature between 60 and 85 C. for not less than 15 minutes. The opposed edges of the annealed film may be overlapped to form a lap seam, the overlapping edges united by interposing therebetween an adhesive material, inserting the film in a tubular support member, and applying an internal pressure to said film of sufficient magnitude to force it into close relationship with the internal walls of said tubular support member whilst substantially avoiding stretching said film. Such internal pressure is suitably applied by inserting an inflatable member into the tubular support member. The choice of the adhesive is critical, as it should permit the overlapping portions of the membrane to slide one over the other without breaking the seal. This may be effected by using an adhesive which will not permanently bond during either its application or the seating of the membrane against the porous tube, but will prevent the solution under pressure from seeping between the layers of the overlap. An example of such an adhesive is a viscous solution of aqueous polyvinyl alcohol. Alternatively an adhesive forming a permanent bond to the membrane before it is put under pressure may be used, if the mechanical properties of the adhesive are such that it will stretch under the applied pressure, in preference to any strain undergone by the membrane itself. Low modulus elastomers may be so used. Alternatively, adhesives which bond permanently before the brine pressure is applied may also be used if in seating the membrane against the porous tube, a pressure is applied which is equal to greater than the brine pressure to be subsequently used. If this is done, the overlap will open up sufficiently during the seating process so that there will be no further strain either on it or on the membrane when the brine pressure is applied. An example of such an adhesive is " Super Cement ". Films so made are suitable for use in a reverse osmosis purification system. For details see Division B1.