BIAXIALLY ORIENTED MICROPOROUS MEMBRANE

Abstract

A microporous membrane is made by a dry-stretch process and has substantially round shaped pores and a ratio of machine direction tensile strength to transverse direction tensile strength in the range of 0.5 to 5.0. The method of making the foregoing microporous membrane includes the steps of: extruding a polymer into a nonporous precursor, and biaxially stretching the nonporous precursor, the biaxial stretching including a machine direction stretching and a transverse direction stretching, the transverse direction including a simultaneous controlled machine direction relax.

Description

DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there is shown in the drawings a form that is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

FIG. 1 is a photograph of one embodiment of the instant invention (single ply membrane).

FIG. 2 is a photograph of another embodiment of the instant invention (multi-ply membrane, plies laminated together then stretched).

FIG. 3 is a photograph of another embodiment of the instant invention (multi-ply membrane, plies coextruded then stretched).

FIG. 4 is a photograph of a prior art dry-stretched membrane (single ply membrane).

FIG. 5 is a photograph of a prior art dry-stretched membrane (multi-ply membrane, plies laminated then stretched).

Claims

1. A method of making a microporous membrane comprising the steps of: extruding a polymer into a nonporous precursor, andbiaxially stretching the nonporous precursor, the biaxial stretching including a machine direction stretching and a transverse direction stretching, the transverse direction including a simultaneous controlled machine direction relax.

2. The method of claim 1 wherein the polymer excludes any oils for subsequent removal to form pores or any pore-forming materials to facilitate pore formation.

3. The method of claim 1 wherein the polymer being a semi-crystalline polymer.

4. The method of claim 1 wherein the polymer being selected from the group consisting of polyolefins, fluorocarbons, polyamides, polyesters, polyacetals (or polyoxymethylenes), polysulfides, polyvinyl alcohols, co-polymers thereof, and combinations thereof.

5. The method of claim 1 further comprising the step of: annealing the non-porous precursor after extruding and before biaxially stretching.

6. The method of claim 5 wherein annealing being conducted at a temperature in the range of Tm−80° C. to Tm−10° C.

7. The method of claim 1 wherein biaxially stretching comprising the steps of: machine direction stretching, andthereafter transverse direction stretching including a simultaneous machine direction relax.

8. The method of claim 7 wherein machine direction stretching being conducted either hot or cold or both.

9. The method of claim 8 wherein cold machine direction stretching being conducted at a temperature <Tm−50° C.

10. The method of claim 8 wherein hot machine direction stretching being conducted at a temperature <Tm−10° C.

11. The method of claim of claim 1 wherein the total machine direction stretch being in the range of 50-500%.

12. The method of claim 1 wherein the total transverse direction stretch being in the range of 100-1200%.

13. The method of claim 1 wherein the machine direction relax being in the range of 5-80%.

14. A membrane comprising: a microporous polymer film made by a dry-stretch process and having substantially round shaped pore and a ratio of machine direction tensile strength to transverse direction tensile strength in the range of 0.5 to 5.0.

15. The membrane according to claim 14 wherein said polymer being a semi-crystalline polymer.

16. The membrane according to claim 14 wherein said polymer being selected from the group consisting of polyolefins, fluorocarbons, polyamides, polyesters, polyacetals (or polyoxymethylenes), polysulfides, polyvinyl alcohols, co-polymers thereof, and combinations thereof.

17. The membrane according to claim 14 wherein an average pore size of said microporous polymer film being in the range of 0.03 to 0.30 microns.

18. The membrane according to claim 14 wherein said microporous polymer film having a porosity in the range of 20-80%.

19. The membrane according to claim 14 wherein said substantially round shaped pores having an aspect ratio in the range of 0.75 to 1.25.

20. The membrane according to claim 14 wherein said transverse tensile strength being ≧250 Kg/cm2.

21. A battery separator comprising the membrane of claim 14.

22. A multi-layered membrane structure comprising the membrane of claim 14.

23. The membrane according to claim 14 wherein said dry-stretched process excludes the use of oils for subsequent removal to form pores or pore-forming materials to facilitate pore formation.