Separators for the lithium battery market are usually manufactured via a “wet” or “dry” process. In the “dry” process, polypropylene (PP) or polyethylene (PE) is extruded into a thin sheet and subjected to rapid drawdown. The sheet is then annealed at 10-25
Polyolefin separators based upon UHMWPE are usually produced in a “wet” or “gel” process involving extrusion of a plasticizer/polymer mixture at elevated temperature, followed by phase separation, biaxial stretching, and extraction of the pore former (i.e., plasticizer). The resultant separators have elliptical or spherical pores and porosity in the 40-50% range. Because of the biaxial orientation, good mechanical properties are achieved in both the machine and transverse directions. The separators also have strong chemical resistance, abrasion resistance, and good wettability with organic solvents. As such, UHMWPE separators have found wide use in lithium batteries.