Method for manufacturing filter medium for air filter

1. A method for manufacturing a filter medium for an air filter, the method comprising:

causing a mixed solution containing cellulose nanofibers and a dispersion medium to adhere to an air-permeable support; and

subsequently freeze-drying the resulting product,

wherein the cellulose nanofibers have an average fiber diameter that is in a range of 1 to 50 nm,

wherein the dispersion medium is a mixed dispersion medium of water and a water-soluble organic solvent, and

wherein the cellulose nanofibers in the mixed solution have a solid content concentration in a range of 0.001 to 0.150% by mass

wherein the cellulose nanofibers have an adhesion amount ratio with respect to the air-permeable support in a range of 0.001 to 0.200% by mass.

2. The method for manufacturing a filter medium for an air filter according to claim 1 , the method further comprising:

setting an ambient temperature of the mixed solution adhered to the air-permeable support to a temperature equal to or lower than a melting point of the mixed solution in the freeze-drying.

3. The method for manufacturing a filter medium for an air filter according to claim 1 , wherein the filter medium has a PF value that is higher than a PF value of a support by 0.5 or more when a surface wind speed is 5.3 cm/sec and a target particle diameter is 0.10 to 0.15 μm.