| abstract |
A method of making a microporous sheet is provided which comprises the steps of melt blending crystallizable thermoplastic polymer with a compound which is miscible with the thermoplastic polymer at the polymer melting temperature but immiscible on cooling below the polymer melting temperature, forming a sheet of the melt blend, cooling the sheet to a temperature at which the compound becomes immiscible with the polymer to cause phase separation to occur between the thermoplastic polymer and the compound to provide a sheet comprising a first phase comprising particles of thermoplastic polymer in a second phase of said compound, orienting the film in at least one direction to provide a network of interconnected micropores throughout the sheet. The microporous sheet comprises about 30 to 80 parts by weight crystallizable thermoplastic polymer and correspondingly about 70 to 20 pans by weight of the compound. The oriented sheet has a microporous structure characterized by a multiplicity of spaced randomly dispersed, equiaxed, non-uniform shaped, non-porous particles of the thermoplastic polymer which are coated with the compound. Adjacent thermoplastic particles within the sheet are connected to each other by a plurality of fibrils consisting of the thermoplastic polymer. The fibrils radiate in three dimensions from each particle. The compound may be removed from the sheet. e.g., by solvent extraction. |