Surface free energy based filled polymer microporous films
A microporous membrane may be manufactured using a polymer binder and a filler material using a liquid pore forming agent having a surface free energy that is lower than that of the filler and higher than that of the polymer. The repulsion of the pore forming agent to the polymer may form the pores of the membrane, while the attraction of the polymer to the filler may encapsulate the filler into the structure of the membrane. The filler may be particles that are on the order of or smaller than the wall thickness of the microporous structure.
1. A method for producing a microporous membrane, the microporous membrane comprising a plurality of pores and walls, the walls defining the pores, said method comprising:
mixing a polymer with a pore forming agent and a filler material to form a pre-formed mixture; and
solidifying said polymer in the presence of said pore forming agent and said filler material to form said microporous membrane where said filler material is incorporated into said microporous membrane and bound with said polymer;
wherein said microporous membrane comprising at least 2 parts of said filler material to 3 parts of said polymer by volume; said microporous membrane comprising pores having a diameter of 0.1 to 1 micron;
said polymer having a surface free energy lower than said pore forming agent;
said polymer wetting said filler material; and
the ratio of said wall thickness to particle size of said filler material being in a range of 1:1 to 20:1.
2. The method of claim 1 , said pore forming agent having a surface free energy lower than said filler material.
3. The method of claim 1 , said pore forming agent having a surface free energy higher than said filler.
4. The method of claim 1 , said filler material having an average particle size less than an average wall thickness of said microporous membrane.
5. The method of claim 4 , said filler material having an average particle size less than 0.1 times said average wall thickness.
6. The method of claim 1 , said polymer being dissolved in a solvent in said pre-formed mixture.
7. The method of claim 6 , said solvent having a lower boiling point than said pore forming agent.
8. The method of claim 7 , said solvent being removed during said solidifying.
9. The method of claim 8 , said solvent being removed by evaporation.
10. The method of claim 1 , said polymer being melted in said pre-formed state.
11. The method of claim 10 , said pore forming agent being an oil.
12. The method of claim 11 , said solidifying being performed by cooling said mixture.
13. The method of claim 12 , said pore forming agent being removed by rinsing after said solidifying.
14. The method of claim 1 , said polymer comprising PVDF.
15. The method of claim 1 , said polymer being one of a group composed of:
polyethylene, polypropylene, PET, acrylic, PVC, and amide.
16. The method of claim 1 , said polymer being one of a group composed of: polyethylene, polypropylene, PET, acrylic, PVC, PVDF, and amide.