Patent ID: 8530004

Claim:
A method of manufacturing a polymeric electret film comprising steps of: mixing a suspension of polytetrafluoroethylene resin with at least one additive selected from the group consisting of titanium dioxide, silicon dioxide, carbon black, nano carbon tube, inorganic oxide and organic oxide; stretching and shaping the suspension of polytetrafluoroethylene resin to form an expanded porous polytetrafluoroethylene (ePTFE) film having a first porous layer of a porous structure, wherein the first porous structure has a pore diameter ranging between 0.01 μm and 5.0 μm and has a porosity ranging between 20% and 95%, and the expanded porous polytetrafluoroethylene film has a first surface and an opposite surface; forming a second porous layer by heating one side of a thin polytetrafluoroethylene material on the first porous layer's opposite surface to a surface temperature higher than the melting point of the thin polytetrafluoroethylene material and rapidly cooling the other side of the thin polytetrafluoroethylene material, wherein the second porous layer has a thickness which is 0.04% to 40% of the thickness of the expanded porous polytetrafluoroethylene film, a surface roughness Ra ranging between 20 nm and 165 nm and a contact angle for water ranging between 120° and 135°; forming an electrode layer on the second porous layer's opposite surface, wherein the electrode layer has a thickness ranging between 0.1 nm and 300 nm; disposing a needle electrode above the first surface of the expanded porous polytetrafluoroethylene film with a preset clearance between the needle electrode and the expanded porous polytetrafluoroethylene film, wherein the preset clearance ranges between 0.1 mm and 200 mm; charging the expanded porous polytetrafluoroethylene film via the needle electrode by a corona charging method at a preset first temperature ranging between 10° C. and 30° C. for a preset first period of time ranging between 1 second and 15 seconds, wherein the corona charging method is conducted by applying a DC bias voltage to the expanded porous polytetrafluoroethylene film; and curing the expanded porous polytetrafluoroethylene film at a preset second temperature ranging between 70° C. and 90° C. for a preset second period of time ranging between 5 hours and 10 hours, whereby the polymeric electret film having a surface potential between 0.1 V and 1000 V is formed; wherein a surface potential of the polymeric electret film decays to less than 24% of an initial value when standing for a period of time greater than 200 days from polarization at room temperature in an endurance test of the polymeric electret film.