A process for preparing a porous film or sheet, which process comprises melt-forming a composition into a film or sheet and then stretching the thus-formed film or sheet at a temperature in the range of 30.degree. to 110.degree. C., said composition containing:...http://www.google.es/patents/US4793956?utm_source=gb-gplus-sharePatente US4793956 - Process for preparing porous film or sheet
