Lead acid battery separator with improved electrical and mechanical properties
An oxidation resistant, microporous polyolefin web exhibiting high-strength mechanical and low electrical resistance properties is a solid matrix that includes an ultrahigh molecular weight polyolefin component and a friable precipitated silica component. The ultrahigh molecular weight polyolefin component provides high-strength mechanical properties to the web. The friable precipitated silica component is broken down into predominately discrete silica aggregates dispersed throughout the microporous web to maintain a low electrical resistance in the presence of an electrolyte. In an alternative preferred embodiment, a polyethylene web includes an antioxidant coating that is applied to the separator web following extraction of the majority of the process oils and drying of the extraction fluid from the separator pores. The resultant “sheath” of antioxidant protects the mechanical integrity of the polyethylene separator by suppressing polyethylene degradation.
1. An oxidation resistant, microporous polyolefin web having high-strength mechanical and low electrical resistance properties, comprising:
a solid matrix including an ultrahigh molecular weight polyolefin component and a friable, highly dispersing, precipitated silica component;
wherein the ultrahigh molecular weight polyolefin component has a molecular weight that provides sufficient molecular chain entanglement to form a microporous web with high-strength mechanical properties; and
wherein the silica component is comprised of weakly bonded silica aggregates wherein the bonds between the silica aggregates are broken down in response to shear forces imparted during material processing to form predominately discrete individual porous silica aggregates that are mutually separate from one another and dispersed throughout the microporous web to maintain a low operational electrical resistance in the presence of an electrolyte.
2. The polyolefin web of claim 1 , in which the discrete individual porous silica aggregates are substantially uniformly dispersed throughout the microporous web.
3. The polyolefin web of claim 1 , in which the precipitated silica and ultrahigh molecular weight polyolefin components define a compositional weight ratio of between about 1.8:1 and about 2.7:1.
4. The polyolefin web of claim 1 , in which the ultrahigh molecular weight polyolefin includes ultrahigh molecular weight polyethylene.
5. The polyolefin web of claim 1 , further comprising a residual process oil in an amount of between about 12% and about 18%.