Patent ID: 8349236
Filing Date: 2013-01-08
Classification: H01M,Y02E

Abstract:
1. A method of preparing a polyethylene microporous film for a rechargeable battery separator comprising the steps of: melt-mixing 20 to about 40 wt % of a polyethylene with 60 to about 80 wt % of an aliphatic hydrocarbon solvent at an extrusion temperature of 160-220° C. and a screw rotation rate of 150-250 rpm using a first intermeshing corotating twin-screw extruder to form an A layer, which is 1 to about 8 μm thick and has a porosity ranging from about 33 to 55%; melt-mixing 45 to about 65 wt % of a polyethylene with 35 to about 55 wt % of an aliphatic hydrocarbon solvent at an extrusion temperature of 160-220° C. and a screw rotation rate of 150-250 rpm using a second intermeshing corotating twin-screw extruder to form B layers, and each B layer is 1 to about 4 μm thick and has a porosity ranging from about 23 to 41%; coextruding the A layer and B layers to laminate B layers on both surfaces of the A layer, and then cooling the coextruded layers, to form a gel composition having a laminated structure comprising B layer/A layer/B layer, the laminated structure having an overall porosity ranging from about 28 to about 46% with the A layer having a greater thickness and a higher porosity that either of the B layers, and the laminated structure has a thickness of 3 to about 16 μm; biaxially stretching the gel composition, to prepare a film; extracting the aliphatic hydrocarbon solvent from the A layer and B layers of the film using an organic solvent to remove the aliphatic hydrocarbon solvent, to prepare a microporous film; and heat-treating the microporous film at a temperature not greater than a melting temperature of the polyethylene.