Laminated porous film and separator for battery

1. A laminated porous film comprising:

at least two porous layers consisting of a first layer and a second layer laminated thereon,

wherein said first layer consists of a composition containing a β crystal nucleating agent and a mixed resin comprising a polypropylene resin and a resin whose crystal fusion peak temperature is lower than that of said polypropylene resin and is from 100° C. to 150° C. in a mixing mass ratio of propylene resin to resin whose crystal fusion peak temperature is lower than that of said polypropylene resin of 20:80 to 80:20; and

said second layer is a heat-resistant layer consisting of a resin composition whose crystal fusion peak temperature is higher than that of a resin composition of said first layer and not less than 200° C.;

said laminated porous film has a β activity and/or a β crystal generation power;

a shut down temperature of the laminated porous film at which pores of the porous layer are closed is set to a range of from 100° C. to 160° C.,

a break down temperature of the laminated porous film at which the laminated porous film breaks is not less than 200° C., and

a pin puncture strength of the laminated porous film is not less than 1.5 N.

2. The laminated porous film according to claim 1 , wherein said resin whose crystal fusion peak temperature is not less than 100° C. nor more than 150° C. is polyethylene resin.

3. The laminated porous film according to claim 1 , further comprising a third layer consisting of a composition whose crystal fusion peak temperature is lower than that of said composition of said first layer.

4. The laminated porous film according to claim 3 , wherein said third layer contains a thermoplastic resin whose crystal fusion peak temperature is not less than 100° C. nor more than 150° C. and is formed as a shut-down layer having a function of closing pores at not less than 100° C. nor more than 150° C.

5. The laminated porous film according to claim 4 , wherein said thermoplastic resin, of said third layer, whose crystal fusion peak temperature is not less than 100° C. nor more than 150° C. is polyethylene resin.

6. The laminated porous film according to claim 1 , wherein said β crystal nucleating agent is mixed with said polypropylene resin at a ratio of 0.0001 to 5.0 parts by mass to 100 parts by mass of said polypropylene resin.

7. The laminated porous film according to claim 1 , wherein said thermoplastic resin whose crystal fusion peak temperature is not less than 200° C. is at least one kind of a resin selected from a group consisting of a polyester resin, a polystyrene resin, a fluorine resin, and a polymethylpentene resin.

8. The laminated porous film according to claim 1 , wherein an air permeability resistance is 1 to 10000 seconds/100 ml.

9. A separator for a battery comprising a laminated porous film as defined in claim 1 and having an air permeability resistance of 5 to 3000 seconds/100 ml.

10. The battery in which a separator for a battery as defined in claim 9 is incorporated.