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Separator Manufacturing

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 °C below the polymer melting point such that crystallite size and orientation are controlled. Next, the sheet is rapidly stretched in the machine direction to achieve slit-like pores or voids at 35-45% porosity. A PP/PE/PP trilayer separator can also be produced in this fashion.

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.