This microporous film has an approximately uniform configuration in which the intervals between slender fibrils extend approximately two-dimensionally which are connected to each other between adjacent unstretched planar flat portions to define fine through-pores, and the distribution of pore size is...http://www.google.ca/patents/US4994335?utm_source=gb-gplus-sharePatent US4994335 - Microporous film, battery separator employing the same, and method of producing them
