Continuous coating process for composite membranes

1. A process for manufacturing a composite membrane comprising:

continuously forming a first coating of fluoropolymer-containing liquid medium onto a moving elongate casting substrate, said coating having a selected thickness;

continuously contacting elongate ePTFE with said coating on said moving casting substrate to form an inchoate composite structure on said substrate, said thickness of said first coating being selected to at least partially impregnate said ePTFE and form a layer of fluoropolymer-containing liquid medium which, upon drying, forms a layer of fluoropolymer in contact with said substrate and said ePTFE;

continuously forming a final coating of fluoropolymer-containing liquid medium on said inchoate composite structure on said moving elongate casting substrate, said final coating having a selected thickness and forming on said inchoate composite structure a layer of fluoropolymer-containing liquid medium which, upon drying, forms a layer of fluoropolymer on said ePTFE;

drying said inchoate composite structure; and

removing said casting substrate.

2. The process of claim 1 wherein said thickness of said first coating and said thickness of said final coating are selected to fully impregnate said ePTFE with fluoropolymer.

3. The process of claim 2 wherein said thickness of said first coating and said thickness of said final coating are selected to form a layer of fluoropolymer in contact with said ePTFE and said substrate and a layer of fluoropolymer on said ePTFE wherein said layers are in a ratio of from about 20:80 to about 80:20.

4. The process of claim 2 wherein said thickness of said first coating and said thickness of said final coating are selected to form a layer of fluoropolymer in contact with said ePTFE and said substrate and a layer of fluoropolymer on said ePTFE wherein said layers are in a ratio of from about 25:75 to about 75:25.

5. The process of claim 2 wherein said thickness of said first coating and said thickness of said final coating are selected to form a layer of fluoropolymer in contact with said ePTFE and said substrate and a layer of fluoropolymer layer on said ePTFE wherein said layers are in a ratio of from about 35:65 to about 65:35.

6. The process of claim 1 wherein said fluoropolymer-containing liquid media of said first and final coatings have solids contents of about 1 to about 20 wt%.

7. The process of claim 1 wherein the solids content of said fluoropolymer-containing liquid medium of said first coating and the solids content of said fluoropolymer-containing liquid medium of said final coating are substantially equal and the thickness of said final coating is about 25% to about 150% of the thickness of said first coating.

8. The process of claim 1 wherein the solids content of said fluoropolymer-containing liquid medium of said first coating and the solids content of said fluoropolymer-containing liquid medium of said final coating are substantially equal and the thickness of said final coating is about 35% to about 100% of the thickness of said first coating.

9. The process of claim 1 wherein the thickness of said layer of fluoropolymer in contact with said ePTFE and said substrate is about 0.5 to about 50 μm.

10. The process of claim 1 wherein said ePTFE film has an initial thickness of about 2 to about 200 μm.

11. The process of claim 1 wherein said ePTFE film has an initial porosity of about 0.1 to about 10 μm.

12. The process of claim 1 wherein said ePTFE film in said composite membrane has a thickness of about 0.3 to about 40 μm.

13. The process of claim 1 further comprising heating said inchoate composite structure after said drying to coalesce said fluoropolymer.

14. The process of claim 1 wherein said drying of said inchoate composite structure comprises a first drying stage in which said inchoate composite structure is dried before forming said final coating of fluoropolymer-containing liquid medium.

15. The process of claim 14 further comprising winding up said inchoate composite structure after said first drying stage.

16. The process of claim 14 wherein said drying of said inchoate composite structure comprises a second drying stage after said forming said final coating of fluoropolymer-containing liquid medium.

17. The process of claim 1 wherein said fluoropolymer comprises fluorinated ion exchange polymer.

18. The process of claim 17 wherein said ion exchange polymer is perfluorinated.

19. The process of claim 18 wherein said ion exchange polymer is perfluorinated ion exchange polymer having sulfonic acid groups.

20. The process of claim 1 wherein said fluoropolymer comprises amorphous fluoropolymer.

21. The process of claim 20 wherein said fluoropolymer comprises ring-containing amorphous fluoropolymer.

22. The process of claim 21 wherein said fluoropolymer comprises a copolymer of tetrafluoroethylene and perfluoro-2,2-dimethyl-1,3-dioxole.