Process for making a wettable polyolefin battery separator

The process of forming a wettable battery separator from a polyolefin which comprises extruding a heated polyolefin through multiple die openings into a gas stream which attenuates the polyolefin into fibers having an average diameter between 1 to about 10 microns, collecting said fibers as a self-supporting mat on a collecting device, compacting the thus formed mat to a thickness between 10 and 40 mils and pore sizes less than 40 microns, contacting the mat with an aqueous solution of a water-soluble, peroxy compound at a temperature below 70.degree. C. without the introduction of oxygen, rinsing the thus contacted mat in a water bath and thereafter at a temperature below 70.degree. C. immersing the thus rinsed mat in an aqueous solution of a hydrophilic vinyl monomer of the formula ##STR3## wherein at least one of R and R.sub.1 are hydrophilic radicals, said solution containing a redox catalyst, thereby causing a graft polymerization of said hydrophilic vinyl monomer onto said polyolefin mat to form a wettable polyolefin battery separator.