Highly porous polyvinylidene difluoride membranes

• US 6,146,747 A
• Filed: 05/18/1998
• Issued: 11/14/2000
• Est. Priority Date: 01/22/1997
• Status: Expired due to Term

• Current Assignee: Pall Corporation (Danaher Corporation)
• Original Assignee: USF Filtration and Separations Group, Inc. (Siemens AG)
• Inventors: Wang, I-Fan, Zepf, Robert, Ditter, Jerome F.
• Primary Examiner(s): Copenheaver, Blaine
• Application Number:
  US09/080,713
• Time in Patent Office: 911 Days
• Field of Search: 428/310.5, 428/315.5, 428/315.7, 428/398, 210/490, 210/500.23, 210/500.27, 264/41, 264/216, 427/244, 427/245
• US Class Current: 428/310.5
• CPC Class Codes: B01D 2239/0421 Rendering the filter material hydrophilic
• CPC Class Codes: B01D 2239/0428 Rendering the filter material hydrophobic
• CPC Class Codes: B01D 2239/0622 Melt-blown
• CPC Class Codes: B01D 2239/065 More than one layer present in the filtering material apparatus incorporating such gas filtering material B01D2275/10
• CPC Class Codes: B01D 2239/0654 Support layers
• CPC Class Codes: B01D 2239/0668 The layers being joined by heat or melt-bonding
• CPC Class Codes: B01D 2239/0686 by spot-gluing
• CPC Class Codes: B01D 2239/1216 Pore size
• CPC Class Codes: B01D 2239/1291 Other parameters
• CPC Class Codes: B01D 2323/12 Specific ratios of components used
• CPC Class Codes: B01D 2323/40 in-situ membrane formation
• CPC Class Codes: B01D 2325/022 Asymmetric membranes
• CPC Class Codes: B01D 2325/20 Specific permeability or cut-off range
• CPC Class Codes: B01D 2325/36 Hydrophilic membranes
• CPC Class Codes: B01D 2325/38 Hydrophobic membranes
• CPC Class Codes: B01D 39/083 of organic material
• CPC Class Codes: B01D 39/1653 of synthetic origin
• CPC Class Codes: B01D 67/0011 Casting solutions therefor
• CPC Class Codes: B01D 67/0013 Casting processes
• CPC Class Codes: B01D 67/0016 Coagulation
• CPC Class Codes: B01D 69/02 : characterised by their properties
• CPC Class Codes: B01D 71/34 : Polyvinylidene fluoride
• CPC Class Codes: Y10T 428/249961 : With gradual property change within a component
• CPC Class Codes: Y10T 428/249978 : Voids specified as micro
• CPC Class Codes: Y10T 428/249979 : Specified thickness of void-containing component [absolute or relative] or numerical cell dimension
• CPC Class Codes: Y10T 428/2975 : Tubular or cellular

Abstract

The present invention relates to the field synthetic polymeric membrane materials formed from casting polyvinylidene difluoride (PVDF) polymer solutions and/or dispersions. Membranes formed in accordance with the present invention are all highly porous. Both internally isotropic membranes and highly asymmetric PVDF membranes are disclosed. Membranes of the invention may be hydrophobic or hydrophilic. The membranes of the invention are useful in a variety of microfiltration and ultrathin applications.

Claims

1. An internally hydrophilic polyvinylidene difluoride polymer membrane, said membrane having a first surface comprising minimum pores, said minimum pores having a first average diameter, a second surface comprising maximum pores, said maximum pores having a second average diameter, and a thickness therebetween, the thickness therebetween comprising a porous support formed from filamentous webs of polymer material, said polymer material having integrally located therein sufficient hydrophilic moieties to render said membrane inherently water wettable, said hydrophilic moieties forming a permanent part of the membrane.

2. The membrane of claim 1, wherein said membrane is an ultrafilter, capable of substantially excluding from passage therethrough molecules or particles having a molecular weight greater than or equal to about 10,000 daltons.

3. The membrane of claim 2, wherein the molecular weight is about 100,000 daltons.

4. The membrane of claim 1, wherein said minimum pores are between about 0.01 micron and about 3.0 microns in diameter.

5. The membrane of claim 1, the membrane having an integral asymmetric structure, wherein said first average diameter is less than said second average diameter, and wherein said porous support comprises openings in said filamentous web, said openings increasing gradually in size in a gradient from said first surface toward said second surface.

6. The membrane of claim 5, wherein a ratio of said second average diameter and said first average diameter is between about 5 and about 10,000.

7. The membrane of claim 1, further comprising from about 1% to about 30% by weight polyvinylnpyrrolidone.

8. The membrane of claim 7, wherein the polyvinylpyrrolidone comprises a form having an average molecular weight of about 45,000 daltons.

9. The membrane of claim 7, wherein the polyvinylpyrrolidone comprises a form having an average molecular weight of about 360,000 daltons.

10. The membrane of claim 7, wherein the polyvinylpyrrolidone comprises at least a first and a second form, wherein said first form has an average molecular weight of about 45,000 daltons and said second form has an average molecular weight of about 360,000 daltons.

11. The membrane of claim 1, further comprising a supporting material laminated thereto.

12. The membrane of claim 11, wherein the supporting material is a nonwoven polymer fabric.

13. The membrane of claim 1, wherein the membrane is a hollow fiber.

14. The membrane of claim 1, having a flow rate of water of from about 25 to about 500 cm/min at 10 psid.

15. The membrane of claim 1, having a bubble point of water of from about 0.5 to about 50 psid.

16. The membrane of claim 1, the membrane having an isotropic structure through at least about 80% of the thickness of the membrane.

17. A method of preparing a polyvinylidene difluoride membrane comprising the steps of:

providing a casting dope comprising between about 12% and 20% by weight of polyvinylidene difluoride and between about 1% and 30% by weight of a hydrophilic polymer, dissolved in a solvent;

casting the dope to form a thin film;

exposing the thin film to a humid gaseous environment;

coagulating the film in a water bath; and

recovering a formed, internally hydrophilic, polyvinylidene difluoride polymer membrane, said membrane having a first surface comprising minimum pores, said minimum pores having a first average diameter, a second surface comprising maximum pores, said maximum pores having a second average diameter, and a thickness therebetween, the thickness therebetween comprising a porous support formed from filamentous webs of polymer material, said polymer material having integrally located therein sufficient hydrophilic moieties to render said membrane inherently water wettable, said hydrophilic moieties forming a permanent part of the membrane.

18. The method of claim 17, wherein the dope has a temperature of between about 35°

C. and about 50°

C. during the casting step.

19. The method of claim 17, wherein the gaseous environment has a relative humidity of water between about 50% and 100%.

20. The method of claim 17, wherein the exposing step has a duration of about 2 seconds to about 120 seconds.

21. The method of claim 17, wherein the water bath has a temperature between about 20°

C. and 80°

C.

22. The method of claim 17, wherein the hydrophilic polymer comprises polyvinylpyrrolidone.

23. The method of claim 22, wherein the polyvinylpyrrolidone comprises a form having an average molecular weight of about 45,000 daltons.

24. The method of claim 22, wherein the polyvinylpyrrolidone comprises a form having an average molecular weight of about 360,000 daltons.

25. The method of claim 22, wherein the polyvinylpyrrolidone comprises at least a first and a second form, wherein said first form has an average molecular weight of about 45,000 daltons and said second form has an average molecular weight of about 360,000 daltons.

26. The method of claim 17, comprising the additional step of laminating to the formed membrane a support material.

27. The method of claim 26, wherein the support material is a nonwoven polymer fabric.

28. An internally isotropic, internally hydrophilic, inherently water wettable, polyvinylidene difluoride polymer membrane, said membrane having hydrophilic moieties, said hydrophilic moieties forming a permanent part of the membrane, the membrane having a first surface comprising minimum pores, the minimum pores having a first average diameter, an opposite surface comprising maximum pores, the maximum pores having a second average diameter, and a thickness therebetween, the thickness therebetween comprising a porous support formed from filamentous webs of polymer material, wherein the porous support comprises flow channels having diameters, and wherein the diameters of the flow channels substantially correspond to the first average diameter within a first region of the porous support, the first region of the porous support being adjacent the microporous surface, and wherein the diameters of the flow channels substantially correspond to the second average diameter within a second region of the porous support, the second region of the porous support being adjacent the opposite surface, and wherein the diameters of the flow channels between the first region of the porous support and the second region of the porous support are substantially constant.