Microporous polyolefin multi layer film and preparing method thereof

A method for preparing a microporous polyolefin multilayer film which comprises steps:

• (a) melting and mixing a mixture that comprises 20-50 weight % of polyethylene having a melting temperature of 125° C. or higher and 80-50 weight % of a diluent;
• (b) melting and mixing a mixture, which does not contain diluent, comprising 20-60 weight % of polypropylene having a melting temperature of 160° C. or higher and 80-40 weight % of a heat-resistant filler having a melting temperature of 170° C. or higher, wherein the heat resistant filler is a heat-resistant resin, an inorganic material or a mixture thereof;
• (c) forming the blended melt prepared in the (a) and (b) steps to a multi-layered sheet of 2 or 3 layers;
• (d) forming a film by stretching the multi-layered sheet;
• (e) extracting diluent from the film;
• and
• (f) heat setting the film,
• wherein the pores of the polyethylene layer are micropores that are fabricated by stretching and extracting after separating the phase of polyethylene and the diluent and have an average size of 0.1 μm or smaller;
• and pores of the polypropylene and heat-resistant filler layer are macropores that are fabricated by cracking of the interface between the polypropylene and heat-resistant filler and have an average size of 1-50 μm, and the microporous polyolefin multilayer film has a thickness of 9-30 μm, a puncture strength of 0.15 N/μm or more, a permeability of 1.5×10−5 Darcy or more and a melt fracture temperature of 170° C. or higher.