Composition having PTFE as main component, mixed powder, material for molding, filtering medium for filter, air filter unit, and a method for manufacturing a porous membrane

1. A filtering medium for an air filter, the filtering medium comprising:

one or more porous membranes, each porous membrane including polytetrafluoroethylene that can be fibrillated,

a non-hot melt processable component that is not fibrillated, and

a hot melt processable component with a melting point lower than 320° C. that is not fibrillated; and

at least one air permeable support supporting the one or more porous membranes and arranged at least as an outermost layer,

a pressure loss is less than 200 Pa when air is passed through the filtering medium at a flow rate of 5.3 cm/sec,

a PF value is 17 or higher when air containing NaCl particles with a particle diameter of 0.3 μm is passed through the filtering medium at a flow rate of 5.3 cm/sec,

a dust holding capacity of polyalphaolefin particles is 20 g/m 2 or more when air containing polyalphaolefin particles with a count median diameter of 0.25 μm is continuously passed through the filtering medium at a flow rate of 5.3 cm/sec and the pressure loss is increased to 250 Pa,

a membrane thickness of one porous membrane is 30 μm or more, and

the PF value=−log [(Transmittance (%)/100]/Pressure loss (Pa)×1000, with Transmittance=100−Collection efficiency (%).

2. The filtering medium for an air filter according to claim 1 , wherein

collection efficiency of the NaCl particles is 99.97% or more when air including the NaCl particles with a particle diameter of 0.3 μm is passed through the filtering medium at a flow rate of 5.3 cm/second.

3. The filtering medium for an air filter according to claim 1 , wherein

a coefficient of variation obtained by dividing standard deviation of pressure loss distribution by an average value of pressure loss distribution is 5% or less.

4. The filtering medium for an air filter according to claim 1 , wherein

the non-hot melt processable component is low molecular weight polytetrafluoroethylene, a thermosetting resin, or an inorganic filler.

5. The filtering medium for an air filter according to claim 1 , wherein

the hot melt processable component is a fluororesin.

6. The filtering medium for an air filter according to claim 5 , wherein

the fluororesin is a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, or a mixture thereof.

7. The filtering medium for an air filter according to claim 1 , wherein

the filtering medium has a plurality of porous membranes.

8. The filtering medium for an air filter according to claim 7 , wherein

the at least one the air permeable support includes a plurality of air permeable supports, with one of the air permeable supports inserted between two adjacent porous membranes.

9. The filtering medium for an air filter according to claim 1 , wherein

the air permeable support is a non-woven fabric.

10. The filtering medium for an air filter according to claim 9 , wherein

a fiber forming the non-woven fabric is a composite synthetic fiber with a sheath-core structure in which a core component has a higher melting point than a sheath component.

11. An air filter unit including the filtering medium for an air filter according to claim 1 , the air filter unit further comprising:

a frame body holding the filtering medium.

12. A method of manufacturing a porous membrane usable as a filtering medium for a filter to capture fine particles in fluid, the method comprising

mixing

an aqueous dispersion of a hot melt processable component with a melting point lower than 320° C. that is not fibrillated, an aqueous dispersion of polytetrafluoroethylene that can be fibrillated, and

an aqueous dispersion of a non-hot melt processable component that is not fibrillated such that the non-hot melt processable component is contained at equal to or more than 0.1% by weight but less than 20% by weight of a total weight of the total components which form the porous membrane, and performing co-coagulation;

paste-extruding a material obtained from the mixing into a sheet shape extrudate by using a T die;

rolling the extrudate obtained from the extruding into a rolled sheet; and

obtaining a monoaxially drawn product by

drawing the rolled sheet obtained from the rolling 4 or more times but less than 10 times in a length direction of the rolled sheet, at a temperature which is equal to or higher than a melting temperature of the hot melt processable component but equal to or lower than a decomposition temperature of each component forming the porous membrane, to obtain a first resultant product, and

subsequently drawing the first resultant product more than 10 times but less than 25 times in a width direction perpendicular to a length direction of the monoaxially drawn product so that a second resultant product is obtained that is drawn 40 times or more but less than 150 times in elongation area ratio.