Microporous polyolefin film with improved meltdown property and preparing method thereof

Abstract

The present invention is related to microporous polyolefin films that may be used for battery separators and the methods of manufacturing the same. These microporous polyolefin films are characterized by being manufactured in a method comprising the steps of melt-extruding a composition, comprised of 20-50 weight % of a resin composition, comprised of 90-98 weight % of polyethylene (Component I) having a weight average molecular weight of 2×10˜4×10and less than 5 weight % of molecules of which molecular weight is less than 1×10and less than 5 weight % of molecules of which molecular weight is greater than 1×10, and 2-10 weight % of polypropylene (Component II) of which weight average molecular weight is 3.0×10˜8.0×10and the peak of the melting point is higher than 145° C., and 80-50 weight % of a diluent (Component III), to mold in the form of sheets; stretching the above sheets to the form of films; extracting the diluent from the above films; and heat-setting the above films. They are also characterized by having a puncture strength of greater than 0.14 N/μm, Darcy's permeability constant of greater than 1.5×10Darcy, shut-down temperature of microporous films of lower than 140° C., and melt-down temperature of higher than 160° C. They can enhance the performance and stability of batteries using them as well as the productivity of microporous films owing to their high thermal stability and superior extrusion compoundability and physical properties.