Process for preparing microporous polyvinylidene fluoride membranes

• US 4,806,291 A
• Filed: 06/13/1988
• Issued: 02/21/1989
• Est. Priority Date: 02/22/1988
• Status: Expired due to Fees

• Current Assignee: Ionics Incorporated
• Original Assignee: Ionics Incorporated
• Inventors: Susa, Thomas J.
• Primary Examiner(s): Foelak, Morton
• Application Number:
  US07/205,691
• Time in Patent Office: 253 Days
• Field of Search: 264/41, 264/49, 521/61, 521/145, 521/64, 521/94, 521/95, 210/500.42
• US Class Current: 264/41
• CPC Class Codes: B01D 67/0011 Casting solutions therefor
• CPC Class Codes: B01D 71/34 Polyvinylidene fluoride
• CPC Class Codes: C08J 2327/16 Homopolymers or copolymers of vinylidene fluoride
• CPC Class Codes: C08J 9/26 by elimination of a solid phase from a macromolecular composition or article, e.g. leaching out
• CPC Class Codes: C08J 9/28 by elimination of a liquid phase from a macromolecular composition or article, e.g. drying of coagulum

Abstract

A process for preparing assymetric polyvinylidene fluoride microporous membranes using novel pore forming agents of an ammonium salt which is a non-solvent for the polymeric casting solution; such agents for example being ammonium hydroxide, ammonium chloride and the like.

Claims

1. In a process for preparing an assymetric, microporous polymeric membrane, which process comprises preparing a membrane-casting solution, casting a thin film of the casting solution on a support base and contacting the cast film with a leaching liquid to provide the microporous membrane, the improvement which comprises:

providing a casting solution comprising a vinylidene fluoride polymer, a polar solvent capable of dissolving said polymer and a pore forming agent of an ammonium salt which is a nonsolvent for the casting solution and which does not precipitate out in said casting solution.

2. The process of claim 1 wherein the pore forming agent is selected from the group consisting of ammonium hydroxide, ammonium acetate, ammonium bifluoride, ammonium carbonate, ammonium chloride and ammonium thiocyanate.

3. The process of claim 1 wherein said casting solution is comprised of between about 14-21 wt. % of vinylidene fluoride polymer, between about 0.05 to 500 parts per million (ppm) of the pore forming agent with the remainder being substantially said polar solvent.

4. The process of claim 1 wherein the polar solvent is selected from the group consisting of dimethyl acetamide, dimethyl formamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, diethyl acetamide, triethyl phosphate, hexamethyl phosphoramide, propylene carbonate, tetramethylurea, and acetone.

5. The process of claim 1 wherein the temperature of the leaching liquid is between 20°

C. to 80°

C. and the leaching liquid is methanol.

6. The process of claim 1 wherein the leaching liquid comprises a mixture of methanol and water and the leaching temperature is from about 20°

to 80°

C.

7. The process of claim 1 wherein the casting solution is cast in a thin film onto a porous or fibrous support base.

8. The process of claim 7 wherein the support base is selected from the group consisting of polypropylene, polyethylene and epoxy coated cloth.

9. The process of claim 7 wherein the film is cast in a thickness of about 0.5 to 125 mills on a porous support base.

10. In a process for preparing an assymetric, microporous polymeric membrane, which process comprises preparing a membrane-casting solution, casting a thin film of the casting solution on a support base and contacting the cast film with a leaching liquid to provide the microporous membrane, the improvement which comprises:

providing a casting solution comprised of between about 14-21 weight percent of a polyvinylidene fluoride polymer, 79-86 weight percent of N-methyl-2-pyrrolidone and 0.5-500 ppm of a pore forming agent selected from the group consisting of ammonium hydroxide, ammonium acetate, ammonium bifluoride, ammonium carbonate, ammonium chloride and ammonium thiocyanate, casting a film of a thickness of about 0.5-125 mils on a porous support base and thereafter coagulating said film by contacting the same with a methanol or methanol-water mixture leach bath at a temperature between 20°

-80°

C. to provide a membrane having a high permeation rate with controlled and distinct particle size cut-off properties.

11. The process of claim 10 wherein the film is immediately coagulated by contacting with said leach bath with substantially no prior exposure to air.