Sheet of microfiber assembly

1. An air-permeable sheet of microfiber assembly comprising:

microfibers, said microfibers being a mixture of water-insoluble but ethanol-soluble acetalized polyvinyl alcohol resin and water-insoluble but ethanol-soluble fluorine resin,

wherein

the microfibers comprise a matrix of the polyvinyl alcohol resin and the fluorine resin is blended into the matrix,

said air-permeable sheet of microfiber assembly is an air-permeable filter having a filtering property for oil mist,

each microfiber is obtained by electro-spinning an alcohol solution of said acetalized polyvinyl alcohol resin and said fluorine resin, wherein each microfiber has a fiber diameter in a range of 30 nm to 800 nm,

said air-permeable sheet of microfiber assembly is laminated on an air-permeable nonwoven fabric having air permeability higher than that of said air-permeable sheet of microfiber assembly,

said air-permeable nonwoven fabric contains thermoplastic synthetic fibers and has a basis weight of 20 to 50 g/m 2 , and

individual fibers or a fiber bundle of the thermoplastic synthetic fibers has a flat radial cross-sectional shape to define a smooth surface of the air-permeable nonwoven fabric, the individual fibers being aligned in one direction.

2. The sheet of microfiber assembly defined by claim 1 , wherein said microfibers contain equal to or less than 44.4% wt % of said fluorine resin.

3. The sheet of microfiber assembly defined by claim 1 , wherein said microfibers contain 22.5% wt % to 44.4% wt % of said fluorine resin.

4. The sheet of microfiber assembly defined by claim 1 , wherein said thermoplastic synthetic fibers in the fiber bundle have fiber diameters of 1 to 10 μm.