PURPOSE: A film of a new material, obtained by forming polybutadiene into a thin film sheet of a foam, keeping the resultant foam under heating and reduced pressure in a vacuum furnace, expanding cells, and returning the pressure to ordinary pressure, having an ultrathin film and air permeability and water impermeability, and suitable for surgical base materials, etc.

CONSTITUTION: 1,2-Polybutadiene is mixed with a photosensitizer and thermally decomposable foaming agent, and the resultant mixture is irradiated with ultraviolet light in a short time to give a crosslinked material, which is then hot drawn to afford a thin film sheet of a foam containing many closed cells. The resultant sheet is then kept under heating and reduced pressure in a vacuum furnace to expand the above-mentioned closed cells and give ultrathin boundary walls between the respective cells and boundary walls between the front side and rear side of the sheet. The internal pressure of the furnace is returned to ordinary pressure, and the expanded cells are flattened in the surface region direction of the sheet. The sheet having 3W10 expansion ratio and 0.2W1.0mm thickness is put into the vacuum furnace, and the internal pressure is reduced to ≤500mmHg. The sheet is then heated at 70W130°C and kept at the temperature for 30min.