| 摘要 |
A porous membrane formed of a thermoplastic resin having a critical surface tension of not higher than 35 dyn/cm and having a three-dimensional network structure is provided. Communicating pores having a porosity of at least 60% and an average pore diameter of 0.1 to 2.0 microns are formed in the porous membrane. The strength factor F defined by the following formula is at least 1 as measured in any direction: F=A/[B(1- alpha )], wherein A is the tensile strength at break of the membrane, B is the tensile strength at break of a non-porous film formed of the same resin and alpha is the porosity fraction of the membrane. The thickness of the porous membrane is 5 to 500 microns. The porous membrane is prepared by a process wherein a porous membrane formed of a thermoplastic resin having a critical surface tension of not higher than 35 dyn/cm and having a three-dimensional network structure including communicating pores having an average pore diameter of 0.05 to 1 micron with a porosity of 30 to 70%, is subjected to a space drawing operation in at least one direction at a temperature between 50 DEG C. lower than the melting or softening point of the thermoplastic resin and 5 DEG C. lower than the melting or softening point of the thermoplastic resin. |
