The primary purposes of the polyolefin contained in the separator are to (1) provide mechanical integrity so that the separator can be enveloped at high speeds and (2) to prevent sharp grid wires or plates from puncturing the separator during battery assembly or operation. The hydrophobic polyolefin preferably has a molecular weight that provides sufficient molecular chain entanglement to form a microporous web with high stiffness and puncture resistance. The primary purpose of the hydrophilic silica is to increase the acid wettability of the separator, thereby lowering its electrical (ionic) resistivity. In the absence of silica, the sulfuric acid would not wet the hydrophobic web and ion transport would not occur, resulting in an inoperable battery. Consequently, the silica component of the separator typically accounts for between about 55% and about 70% by weight of the separator, i.e., the separator has a silica-to-polyethylene mass ratio of between about 2.0:1 and about 3.5:1.
After the calendering process, the sheet is cooled so that the oil phase separates from the polymer to form regions that will eventually become pores after solvent extraction of the oil. There is always a controlled amount of oil left in the finished separator because it has a positive impact upon the oxidation resistance of the separator. The residual oil is believed to reside within the UHMWPE fibrils that are dispersed throughout the separator as shown in Figure 2. In this case, the oil serves as a reactive species for scavenging oxygen and other oxidizing agents that can attack the long polymer chains and cause embrittlement of the separator.