Porous stretched article of polypropylene-based resin and process for its preparation

A process for producing a porous stretched article of polypropylene-based resin, the process comprising the steps of crystallizing a melt of a polypropylene-based resin composition, wherein the polypropylene-based resin is selected from the group consisting of polylene homopolymers, random and block copolymers of propylene as a predominant comonomer and at least one comonomer selected from the group consisting of ethylene and at least one 1-alkene, propylene-ethylene copolymers which further contain other comonomer(s) selected from the group consisting of 5-ethylidene-2-nornornene, 5-methylene-2-norbornene and 1,4-hexadiene, propylene-styrene copolymers, and a blend of polypropylene and a small quantity of a thermoplastic resin selected from the group consisting of high-density polyethylene, polybutene-1 and poly-4-methylpentene-1, and wherein the polypropylene-based resin has a melt flow rate of about 0.1 to 100 g/10 min, at a temperature of 15.degree. to 140.degree. C. to form a solidified product containing .beta.-form crystals and stretching the resulting solidified product at a temperature which is higher than 20.degree. C. but lower than the melting point of the .beta.-form crystals in said solidified product, the polypropylene-based resin composition consisting essentially of the polypropylene-based resin and 0.0001 to 5 parts by weight, per 100 parts by weight of the polypropylene-based resin, of at least one amide compound selected from the group consisting of:

• (1) an amide compound of the formula
• R.sup.2 --NHCO--R.sup.1 --CONH--R.sup.3 ( 1)
• wherein R.sup.1 is a residue formed by elimination of the two carboxyl groups of a C.sub.3-26 saturated or unsaturated aliphatic dicarboxylic acid, a C.sub.6-30 saturated or unsaturated alicyclic dicarboxylic acid or a C.sub.8-30 aromatic dicarboxylic acid;
• R.sup.2 and R.sup.3 are the same or different and each represents a C.sub.3-18 cycloalkyl group, a C.sub.3-12 cycloalkenyl group, or a group of the formula ##STR11## wherein R.sup.4 is a hydrogen atom, a C.sub.1-12 straight- or branched-chain alkyl group, a C.sub.2-12 straight- or branched-chain alkenyl group, a C.sub.6-10 cycloalkyl group or a phenyl group;
• R.sup.5 is a C.sub.1-12 straight- or branched-chain alkyl group or a C.sub.2-12 straight- or branched-chain alkenyl group;
• and R.sup.6 and R.sup.7 each represents a C.sub.1-4 straight- or branched-chain alkylene group;
• (2) an amide compound of the formula
• R.sup.9 --CONH--R.sup.8 --NHCO--R.sup.10 ( 2)
• wherein R.sup.8 is a residue formed by elimination of the two amino groups of a C.sub.1-24 saturated or unsaturated aliphatic diamine, a C.sub.4-28 alicyclic diamine, a C.sub.4-14 heterocyclic diamine or a C.sub.6-28 aromatic diamine;
• R.sup.9 and R.sup.10 are the same or different and each represents a C.sub.3-12 cycloalkyl group, a C.sub.3-12 cycloalkenyl group, or a group of the formula ##STR12## wherein R.sup.11 is a hydrogen atom, a C.sub.1-12 straight- or branched-chain alkyl group, a C.sub.2-12 straight- or branched-chain alkenyl group, a C.sub.6-10 cycloalkyl group or a phenyl group;
• R.sup.12 is a C.sub.1-12 straight- or branched-chain alkyl group, a C.sub.2-12 alkenyl group, a C.sub.6-10 cycloalkyl group or a phenyl group;
• and R.sup.13 and R.sup.14 each represents a C.sub.1-4 straight- or branched-chain alkylene group;
• and
• (3) an amide compound of the formula
• R.sup.16 --CONH--R.sup.15 --CONH--R.sup.17 ( 3)
• wherein R.sup.15 is a residue formed by elimination of one amino group and one carboxyl group from of a C.sub.2-29 saturated or unsaturated aliphatic amino acid, C.sub.7-13 saturated or unsaturated alicyclic amino acid or C.sub.7-15 aromatic amino acid;
• R.sup.16 and R.sup.17 are the same or different and R.sup.16 has the same meaning as R.sup.9 or R.sup.10 in the formula (2) and R.sup.17 has the same meaning as R.sup.2 or R.sup.3 in the formula (1).