Processes of making a porous membrane material from polyvinylidene fluoride, and products

• US 4,203,848 A
• Filed: 05/25/1977
• Issued: 05/20/1980
• Est. Priority Date: 05/25/1977
• Status: Expired due to Term

• Current Assignee: Millipore Investment Holdings, Ltd.
• Original Assignee: Millipore Corporation
• Inventors: Grandine, Joseph D. II
• Primary Examiner(s): Anderson, Philip
• Application Number:
  US05/800,574
• Time in Patent Office: 1,091 Days
• Field of Search: 264/41, 264/49, 264/331, 264/180, 264/38, 264/216, 260/2.5 M, 260/29.2 TN, 260/30.8 DS, 210/500 M, 210/490
• US Class Current: 210/490
• CPC Class Codes: B01D 67/0011 Casting solutions therefor
• CPC Class Codes: B01D 71/34 Polyvinylidene fluoride
• CPC Class Codes: B29C 67/20 for porous or cellular articles, e.g. of foam plastics, coarse-pored chemical aspects of working up macro-molecular substances to porous or cellular articles C08J9/00

Abstract

Covers processes for making microporous membranes and molecular filtration membranes from vinylidene fluoride polymers, and the products so produced. A continuous version of the process consists of applying a layer of a solution of a polyvinylidene fluoride polymer to a rigidly supported surface of a backing belt to form a film, then passing the belt through a formation bath, in which the membrane is formed. To form the polymer solution, the polymer is dissolved in a liquid vehicle that will dissolve up to about 25% by weight of the polymer at a temperature of at least 50° C. but that dissolves substantially less of the polymer at about 20° C. (room temperature). The polymer solution is formed at 50° C. or higher, and the formation bath is maintained at about room temperature. The formation bath is a mixture of the liquid vehicle solvent with a second liquid that is miscible with the solvent but that is not itself a solvent for the polymer. The film is maintained immersed in the formation bath until it has been converted to a porous membrane whose pore structure is essentially fully formed, by leaching of the solvent from the film and its replacement with the non-solvent liquid. Thereafter the residual solvent is extracted from the porous membrane, and after separation of the membrane from the backing belt, the membrane is dried. The preferred solvent - non-solvent system for the formation bath is acetone-water. A preferred formation bath composition is from about 70% to about 80% acetone by volume.

Claims

1. A continuous process for forming a microporous membrane from a polymer that is at least 90% by weight of polyvinylidene fluoride, the membrane having at least 60% of its volume taken up by intercommunicating capillary pores of average size below about 14 micrometers, comprising:

applying a layer of a solution consisting essentially of the polymer in acetone at a polymer concentration of from about 15% to about 25% by weight of the solution as a thin layer to a rigidly supported surface of a travelling backing belt,

immersing the belt carrying the layer of polymer solution in a formation bath consisting essentially of a mixture of acetone and water, with the acetone forming from about 55% to about 85% by volume of the bath, the balance being water,

maintaining the layer immersed in and passing it through an elongate path in the formation bath between the point of entry of the belt and layer into the formation bath and the point of departure of the belt and layer from the bath, to permit replacement of the acetone in the layer with the formation bath liquid, until the layer has been converted to a microporous membrane having an average pore size in the range from about 25 nanometers to about 14 micrometers, wherein the pores intercommunicate, form at least 60% by volume of the membrane, and are of substantially uniform size, and the porous structure is essentially fully formed,

continuously withdrawing formation bath liquid from the bath adjacent the entry point of the belt into the bath,

continuously monitoring the composition of the withdrawn formation bath liquid, and adding the necessary amount of water to maintain the liquid at a composition within the specified range of composition for the bath, so that it is recirculable liquid,

returning the recirculable liquid to the formation bath adjacent the departure point of the belt from the bath,

separating the formed membrane from the belt,

washing the membrane with water, apart from the formation bath, to extract any residual acetone or formation bath liquid from the membrane, and then

drying the membrane to remove water.

2. The process of claim 1 wherein the layer of polymer solution is applied to the backing belt from a supply reservoir, including the step of maintaining a substantially constant head on the solution in the supply reservoir as it is applied to the belt, to deposit the layer at a uniform thickness.

3. The process of claim 1 wherein the formation bath is a shallow, elongate bath, and including the step of continuously recirculating the formation bath liquid through the bath in a direction opposite to the direction of travel of the belt.

4. The process of claim 1 wherein the polymer concentration in the polymer solution is in the range from about 16% by weight of polymer to about 19% by weight of polymer, and wherein the acetone in the formation bath forms from about 70% to about 80% of the bath by volume.

5. The process of claim 1 wherein the entry speed of the belt carrying the applied layer of polymer solution, into the formation bath, is in excess of about 10 cm./second.

6. A microporous membrane of polyvinylidene fluoride prepared in accordance with claim 1.

7. The process of claim 4 wherein the polymer concentration in the polymer solution is in the range from about 17.5% to about 18.5% by weight of the solution, and wherein the acetone in the formation bath forms from about 72% to about 73% by volume of the bath.

8. A microporous membrane of polyvinylidene fluoride prepared in accordance with the process of claim 4.

9. A continuous process for forming a microporous membrane from a homopolymer of vinylidene fluoride, the membrane having at least 60% of its volume taken up by intercommunicating capillary pores of average size below about 14 micrometers, comprising:

applying a layer of a solution consisting essentially of the polymer in acetone at a polymer concentration of from about 16% to about 19% by weight of the solution as a thin layer to a rigidly supported surface of a travelling backing belt from a supply reservoir, while maintaining a substantially constant head on the solution in the supply reservoir as it is applied, to deposit the layer on the belt at a uniform thickness,

immersing the belt carrying the layer of polymer solution in, and carrying it through a shallow, elongate formation bath that consists essentially of a mixture of acetone and water, with the acetone forming from about 70% to about 80% by volume of the bath, the balance being water,

maintaining the layer immersed in the formation bath to permit replacement of the acetone in the layer with formation bath liquid, until the layer has been converted to a microporous membrane having an average pore size in the range from about 25 nanometers to about 14 micrometers, wherein the pores intercommunicate, form at least 60% by volume of the membrane, and are of substantially uniform size, and the porous structure is essentially fully formed,

continuously withdrawing formation bath liquid from the bath adjacent the entry point of the belt into the bath,

continuously monitoring the composition of the withdrawn formation bath liquid and adding the necessary amount of water to maintain the liquid at a composition within the specified range of composition for the bath, so that it is recirculable liquid,

returning the recirculable liquid to the formation bath adjacent the departure point of the belt from the bath,

separating the formed membrane from the belt,

washing the membrane with water apart from the formation bath to extract any residual formation bath liquid from the membrane, and then

drying the membrane to remove water.

10. A microporous membrane of polyvinylidene flouride prepared in accordance with the process of claim 9.

11. The process of claim 9 wherein the concentration of polymer in the polymer solution is from about 17.5% to about 18.5% by weight of the solution, and wherein the acetone in the formation bath forms from about 72% to about 73% by volume of the bath.

12. The process of claim 11 wherein the speed of entry of the belt carrying the layer of polymer solution, into the formation bath, is in excess of 10 cm./second.

13. In a process for making either a microporous or an asymmetric membrane that has at least 60% of its volume taken up by pores, from a polymer of vinylidene fluoride, by casting a thin layer of a 15% to 25% by weight solution of the polymer on a substrate, then passing the substrate into a formation bath comprising a liquid that is not a solvent for the polymer but that is miscible with the solvent for the polymer to form a porous membrane, removing the solvent and the non-solvent from the porous membrane, and drying, the improvement wherein the casting solution is formed by dissolving the polymer in hot acetone above 50°

C., and including the steps of;

immersing the cast layer in a formation bath consisting essentially of a mixture of acetone and water, with the acetone forming at least a major portion, up to 80% by volume, of the bath, the balance being water,

maintaining the layer immersed in the formation bath until the layer has been converted to a porous membrane whose pore structure is essentially fully formed,

separating the porous membrane from its substrate, then

washing the membrane with water to remove residual liquid from the membrane.

14. A process in accordance with claim 13 in which the process steps are practised for continuous production of the membrane, and wherein the substrate is a travelling backing belt, including:

passing the immersed layer and belt through an elongate path in the formation bath, between the point of entry of the belt and layer into the formation bath and the point of departure of the belt and layer from the bath,

withdrawing formation bath liquid from the bath adjacent the entry point of the belt into the bath,

monitoring the composition of the withdrawn formation bath liquid and adding any necessary water to maintain the liquid at a composition within the specified range of composition for the bath so that it is recirculable liquid, then

returning the recirculable liquid to the formation bath adjacent the departure point of the belt from the bath.

15. The process of claim 13 for making an asymmetric membrane, wherein the acetone is present in the formation bath at a concentration approaching 50% by volume, sufficient that an essentially asymmetric membrane is formed.

16. The process of claim 14 for making a microporous membrane, wherein the acetone is present in the formation bath at a concentration approaching 80% by volume, sufficient that an essentially microporous membrane is formed.

17. The process of claim 14 wherein the rate of withdrawal of formation bath liquid and the rate of return of the recirculable liquid respectively are such that an equilibrium is established and the deviation from the equilibrium composition is not more than about 2% at most.

18. The process of claim 14 for making a skinned membrane, wherein the water is present in the formation bath at a concentration approaching 50%, by volume, sufficient to cause skin formation.

19. A skinned molecular filtration membrane of a vinylidene fluoride polymer prepared in accordance with the process of claim 18.

20. The process of claim 16 wherein the casting solution has a polymer concentration from 17.5% to 18.5% by weight based on the solution weight, and where the formation bath has a composition of 72% to 73% acetone by volume.

21. A microporous membrane of a vinylidene fluoride polymer prepared in accordance with the process of claim 16.