Multilayer, microporous polyolefin membrane and battery separator

Abstract

A multi-layer, microporous polyolefin membrane having at least three layers, which comprises first microporous layers made of a polyethylene resin for constituting at least both surface layers, and at least one second microporous layer comprising a polyethylene resin and polypropylene and disposed between both surface layers, the heat of fusion (ΔH m ) of the polypropylene measured by differential scanning calorimetry being 90 J/g or more, and the polypropylene content in the second microporous layer being 50% or less by mass based on 100% by mass of the total of the polyethylene resin and the polypropylene.

What is claimed is: 
     
         1 . A multi-layer, microporous polyolefin membrane having at least three layers, which comprises first microporous layers comprising a polyolefin resin for constituting at least both surface layers, and at least one second microporous layer comprising a polyethylene resin and polypropylene and disposed between both surface layers, the heat of fusion (ΔH m ) of the polypropylene measured by differential scanning calorimetry being 90 J/g or more, and the polypropylene content in the second microporous layer being 50% by mass or less based on 100% by mass of the total of the polyethylene resin and the polypropylene, said first microporous layers containing no polypropylene, and said multi-layer, microporous polyolefin membrane having a shutdown speed of 10 seconds or less. 
     
     
         2 . A multi-layer, microporous polyolefin membrane having at least three layers according to  claim 1 , wherein a shutdown temperature difference between said first microporous layer and said second microporous layer is within 10° C., and said multi-layer, microporous polyolefin membrane has a meltdown temperature of 175° C. or higher. 
     
     
         3 . The multi-layer, microporous polyolefin membrane having at least three layers according to  claim 1 , wherein said polyolefin resin in said first microporous layer contains ultra-high-molecular-weight polyethylene. 
     
     
         4 . The multi-layer, microporous polyolefin membrane having at least three layers according to  claim 2 , wherein said polyolefin resin in said first microporous layer contains ultra-high-molecular-weight polyethylene. 
     
     
         5 . A battery separator formed by the multi-layer, microporous polyolefin membrane having at least three layers of  claim 1 , which comprises first microporous layers comprising a polyolefin resin for constituting at least both surface layers, and at least one second microporous layer comprising a polyethylene resin and polypropylene and disposed between both surface layers, the heat of fusion (ΔH m ) of the polypropylene measured by differential scanning calorimetry being 90 J/g or more, and the polypropylene content in the second microporous layer being 50% by mass or less based on 100% by mass of the total of the polyethylene resin and the polypropylene, said first microporous layers containing no polypropylene, and said multi-layer, microporous polyolefin membrane having a shutdown speed of 10 seconds or less. 
     
     
         6 . The battery separator according to  claim 5 , wherein a difference between shutdown temperatures of said first microporous layer and said second microporous layer is 10° C. or less, and said multi-layer, microporous polyolefin membrane has a meltdown temperature of 175° C. or higher. 
     
     
         7 . The battery separator according to  claim 5 , wherein said polyolefin resin in said first microporous layer contains ultra-high-molecular-weight polyethylene. 
     
     
         8 . The battery separator according to  claim 6 , wherein said polyolefin resin in said first microporous layer contains ultra-high-molecular-weight polyethylene.