BATTERY SEPARATORS

There is described a battery separator comprising: a porous membrane (preferably of a cellulose film) having a mean thickness less than about 25 microns (preferably 15 microns) and optionally with one or more of the following properties: a) a tensile strength when dry (measured in the transverse direction of the membrane web) of greater than about 120 Nmm; b) a wet ionic resistance of less than about 20 mOmega.cm ; c) a membrane degree of polymerisation (D.P.) of greater than 350 chain units after being immersed for four hours in a bath of sodium hydroxide and hydrogen peroxide; and/or a) a tensile strength when dry (measured in the machine direction of the membrane web) of greater than about 200 Nmm. Preferred membranes comprise biopolymeric films, such as films derived from cellulose and/or polylactic acid, more preferably a regenerated cellulosic film made using NMMO as regeneration liquid.