Abstract: A battery separator formed into an envelope and having improved oxidation resistance is disclosed. The sealed edge along the bottom of the envelope and preferably also along the vertical side edges of the envelope has a greater amount of a processing aid, such that the treated area has from a 5 to 100%, preferably 5 to 20% and more preferably 10 to 100% greater amount of processing aid than the average of the rest of the envelope. One embodiment is to impregnate the treated area with additional processing aid. One embodiment is to impregnate the treated area with additional processing aid. A further embodiment is impregnate a porous layer with additional processing aid and locate that porous layer adjacent and preferably in intimate contact with the treated edge. A third embodiment is to add an amount of processing aid into the battery box before inserting the enveloped plates and allowing the envelopes to absorb the aid.