Porous polypropylene film, process for producing the same, and absorbent article employing the film

1. A porous polypropylene film with excellent breathability and continuous through-pores and made of a propylene-based resin composition comprising

(I) (i) a propylene-ethylene copolymer having an ethylene content of 3.0 to 7.0 wt. % and a melt flow rate (MFR, JIS K-6557) measured at 230° C. of 2.0 to 4.0 g/10 min or

(ii) a polypropylene-based resin mixture containing a propylene-ethylene copolymer, the mixture having an ethylene content of 3.0 to 7.0 wt. % and a melt flow rate (MFR, JIS K-6857) measured at 230° C. of 2.0 to 4.0 g/10 min

(II) a beta-crystal nucleating agent;

the film having a porosity of 20 to 80%, a Gurley air permeability of 5,000 sec/100 cc or lower as measured according to JIS P-8117, a water vapor permeability of 2.000 g/m 2 ·24 h or higher as measured according to JIS Z-0208 and a water pressure resistance of 74 kPa or higher as measured according to JIS L-1092 except that an aqueous surfactant solution is used instead of pure water.

2. The porous polypropylene film according to claim 1 , which has a film thickness of 5 to 50 microns and a mechanical strength of 40 MPa or higher as measured according to JIS K-7127.

3. The porous polypropylene film according to claim 1 , which is prepared by stretching an unstretched web sheet wherein the proportion of thermally stable β crystals (β1 crystals) relative to the total amount of all crystals is 65% or higher, and the β1-crystal heat of fusion per unit weight of the unstretched web sheet is 50 J/g or higher, as determined by differential scanning calorimetry (DSC).

4. The porous polypropylene film according to claim 1 , wherein at least one component of the polypropylene-based resin mixture containing a propylene-ethylene copolymer is a polypropylene resin having a weight average molecular weight (Mw) of 10 5 to 10 6 as determined by gel permeation chromatography (GPC) and a melt flow rate (MFR, JIS K-6857) measured at 230° C. of 0.5 to 10.0 g/10 min.

5. The porous polypropylene film according to claim 1 , wherein the polypropylene-based resin mixture containing a propylene-ethylene copolymer is composed of a mixture of at least two members of polypropylene-based resins selected from the following resins (A), (B) and (C):

(A) a polypropylene-based resin having an MFR of 0.1 to 2.0 g/10 min;

(B) a polypropylene-based resin having an MFR of more than 2.0 g/10 min and less than 4.0 g/10 min;

(C) a polypropylene-based resin having an MFR of 4.0 to 10.0 g/10 min;

the mixture having an ethylene content of 3.0 to 7.0 wt. % and a melt flow rate (MFR, JIS K-6857) measured at 230° C. of 2.0 to 4.0 g/10 min; and at least one component of the mixture being a propylene-ethylene copolymer.

6. The porous polypropylene film according to claim 5 , wherein the polypropylene-based resin mixture comprises 10 to 35 wt. % of resin (A), 0 to 50 wt. % of resin (B) and 25 to 80 wt % of resin (C).

7. The porous polypropylene film according to claim 1 , wherein the beta-crystal nucleating agent is an amide compound represented by formula (1)

R 2 —NHCO—R 1 —CONH—R 3   (1)

wherein R 1 is a C 1-24 saturated or unsaturated aliphatic dicarboxylic acid residue, a C 4-28 saturated or unsaturated alicyclic dicarboxylic acid residue, or a C 6-28 aromatic dicarboxylic acid residue, R 2 and R 3 are the same or different and each represents a C 3-18 cycloalkyl group, or a group of formula (a), formula (b), formula (c) or formula (d)

wherein R 4 is a hydrogen atom, a C 1-12 straight-chain or branched-chain alkyl group, a C 6-10 cycloalkyl group, or phenyl group, R 5 is a C 1-12 straight-chain or branched-chain alkyl group, and R 6 and R 7 are the same or different and each represents a C 1-4 straight-chain or branched-chain alkylene group.

8. A backsheet for absorbent articles comprising the porous polypropylene film with excellent breathability and continuous through-pores according to any one of claims 1 to 7 .

9. An absorbent article comprising a liquid-permeable topside material, a liquid-impervious backside material and an absorbing material interposed between the topside material and the backside material, wherein the backside material comprises the backsheet according to claim 8 .

10. The absorbent article according to claim 9 , wherein said absorbent article is a hygienic article.

11. The hygienic article according to claim 10 , wherein said hygienic article is a member selected from the group consisting of a disposable diaper, a pants-shaped diaper, a sanitary napkin, and an incontinence pad.

12. A process for producing the porous polypropylene film with excellent breathability and continuous through-pores according to claim 1 , wherein the process comprises the step of stretching an unstretched web sheet made of a polypropylene-based resin composition comprising

(I) (i) a propylene-ethylene copolymer having an ethylene content of 3.0 to 7.0 wt. % and a melt flow rate (MFR, JIS K-6857) measured at 230° C. of 2.0 to 4.0 g/10 min or

(ii) a polypropylene-based resin mixture containing a propylene-ethylene copolymer, the mixture having an ethylene content of 3.0 to 7.0 wt. % and a melt flow rate (MFR, JIS K-6857) measured at 230° C. of 2.0 to 4.0 g/10 min and

(II) a β-crystal nucleating agent;

the unstretched web sheet having a thermally stable β-crystal (β1-crystal) content of 65% or higher, and a β1-crystal heat of fusion per unit weight of the

unstretched web sheet of 50 J/g or higher, as determined by differential scanning calorimetry (DSC).

13. The process for producing the porous polypropylene film according to claim 12 , wherein the unstretched web sheet is made of a polypropylene-based resin composition comprising

(I) a polypropylene-based resin mixture containing a propylene-ethylene copolymer and having an ethylene content of 3-0 to 7.0 wt. % and a melt flow rate (MFR, JIS K-6857) measured at 230° C. of 2.0 to 4.0 g/10 min and

(II) a beta-crystal nucleating agent; and at least one component of the polypropylene-based resin mixture is a polypropylene-based resin having a weight average molecular weight (Mw) of 10 5 to 10 6 as determined by gel permeation chromatography (GPC) and a melt flow rate (MFR, JIS K-6857) measured at 230° C. of 0.5 to 10.0 g/10 min.

14. The process for producing the porous polypropylene film according to claim 12 , wherein the unstretched web sheet is formed by the steps of cooling and solidifying a molten sheet of the polypropylene-based resin composition extruded from a T-die, in the form of a sheet, under the following crystallization conditions:

110° C.<crystallization temperature ( T cr, ° C.)≦130° C.; and

shortest crystallization holding time ( t min (sec))≦crystallization holding time ( t cr, sec)<60 seconds,

wherein tmin (sec) is calculated by the following equations:

in the case of T cr<120° C., t min(sec)=0.1(120 −T cr) 2 +10; or

in the case of T cr≧120° C., t min(sec)=0.3(120 −T cr) 2 +10.

15. The process for producing the porous polypropylene film according to claim 14 , wherein the crystallization holding time condition is satisfied by crystallizing the molten sheet of the polypropylene-based resin composition extruded from a T-die, using a multistage chill roll having a plurality of chill-rolls or using one chill-roll in combination with a heat source other than a chill roll.

16. The process for producing the porous polypropylene film according to claim 12 , wherein the stretching step is successive biaxial stretching.

17. The process for producing the porous polypropylene film according to claim 16 , wherein the successive biaxial stretching step is carried out by conducting machine direction stretching at a ratio of 4 to 5 times at a stretching temperature of 70° C. to 90° C. and then transverse direction stretching at a ratio of 5 to 10 times at a stretching temperature of 135° C. to 155° C. and at a stretching rate of 10 to 300%/sec.