This coating solution was applied o

This coating solution was applied onto porous PTFE film (void content 80%, mean pore size 0.2 μm, average thickness 30 μm) with a roll coater. The force of the roll coater was adjusted so that most of the applied solution was absorbed into the porous PTFE film, with only a scant amount remaining on the surface. The material was then dried for 5 minutes at 100° C. and heat treated for 10 minutes at 160° C. Thecoated face (i.e. surface) of the resultant composite film was imaged at 3000-10000× magnification under an electron microscope, and theelectron microscope images were examined with the naked eye. It was found that a thin coating film of polyurethane resin had formed over the entire coated face, and that in portions the polyurethane resin coating was thin enough that the contours of the porous PTFE film matrix were visible through it. Electron microscope images are shown in FIGS. 1 to 3. For purposes of comparison, electron microscope images (surface images) at the same magnifications taken of the porous PTFE film prior to coating with polyurethane resin are shown in FIGS. 4 to 6. The polyurethane resin layer on the resultant composite film was 18 μm deep in the areas of penetration thereof into the porous PTFE film. Here and in the following examples, depth of the polyurethane resin in the areas of penetration thereof into the porous PTFE film was determined by measuring average depth with the naked eye from sectional images (at 1000-3000×) made by the electron microscope, using the scale (markings indicating length) of the electron microscope images. Composite film water vapor transmission was 20,000 g/m2•24 h. Here and in the following examples, water vapor transmission is calculated by converting measurements made in accordance with JIS L 1099B-2 into 24-hour values.