Filter medium for filter, method for producing the same, and filter
Provided are a filter medium for a filter, which makes it possible to obtain a filter high in collection efficiency, low in pressure loss and long in filter lifetime, a method for producing the same, and a filter using the filter medium for a filter. A filter medium for a filter is used as a constituent member of a filter and composed of a wet type nonwoven fabric, wherein the filter medium for a filter has a multilayer structure of two or more layers, and there is no interface between the above-mentioned two layers.
1. An air filter for an internal combustion engine obtained using a filter medium for a filter, which is used as a constituent member of the filter and which comprises a wet type nonwoven fabric, wherein the filter medium for the filter has a multilayer structure of two or more layers and there is no interface between two adjacent layers of the multilayer structure, wherein short-cut nanofibers comprising a fiber-forming thermoplastic polymer, having a single fiber diameter (D) of 100 to 1,000 nm and obtained by performing cutting so that the ratio (L/D) of the length (L) to the single fiber diameter (D) is within the range of 100 to 2,500, are contained only in one layer of the multilayer structure of the filter medium for the filter, thereby providing a layer structure in which an amount of the short-cut nanofibers gradually decreases in a thickness direction, and wherein a low-density layer is disposed on a fluid inflow side, and wherein when the delamination strength between the two adjacent layers of the multilayer structure is measured 10 times, for the remaining 8 values except for the maximum value and the minimum value thereof, the ratio of the maximum value/the minimum value is 1.5 or more, and wherein core-sheath conjugate type binder fibers are contained in the filter medium for a filter in an amount of 20 to 40% by weight, and wherein the thickness thereof is 0.5 to 4.0 mm.
2. The air filter according to claim 1 , wherein the short-cut nanofibers are ones obtained by dissolving and removing a sea component from a conjugate fiber comprising an island component composed of a fiber-forming thermoplastic polymer and having an island diameter (D) of 100 to 1,000 nm and a sea component composed of a polymer more easily soluble in an alkaline aqueous solution than the fiber-forming thermoplastic polymer.
3. The air filter according to claim 2 , wherein the sea component in the conjugate fiber is polyethylene terephthalate copolymerized with 6 to 12% by mole of 5-sodium sulfoisophthalic acid and 3 to 10% by weight of polyethylene glycol having a molecular weight of 4,000 to 12,000.
4. The air filter according to claim 2 , wherein the island component in the conjugate fiber is a polyester.
5. The air filter according to claim 2 , wherein the number of islands in the conjugate fiber is 100 or more.
6. The air filter according to claim 1 , wherein in both surfaces of the filter medium for the filter, when the number of fibers on a surface on which more fibers are exposed is taken as DL and the number of fibers on the other surface on which fewer fibers are exposed is taken as DU, the DU/DL ratio is 0.8 or less.
7. The air filter according to claim 1 , wherein the basis weight thereof is within the range of 30 to 300 g/m 2 .
8. The air filter according to claim 1 , wherein a high-density layer is disposed on a fluid outflow side.