Process for producing oriented composite polyolefin resin films

A process for producing an oriented composite polyolefin resin film, which comprises melt-laminating a polyolefin resin sheet having fine inorganic powder dispersed therein as a paper-like layer on at least one surface of a polyolefin resin sheet monoaxially stretched in one direction as a substrate layer, and then stretching the resulting laminated sheet in a direction perpendicular to said monoaxially stretched direction thereby to generate surface cracks and minute voids on the surface and within the body of the surface layer, said polyolefin resin sheet for the substrate layer being a sheet of composite structure prepared by melt-extruding the under-defined compositions (A), (B), and (C) through a co-extrusion die in such a manner that a layer composed of the composition (A) is laminated on one surface of an intermediate layer composed of the composition (B), and a layer composed of the composition (C) is laminated on the other surface of the intermediate layer, whereby the intermediate layer composed of the composition (B) is extruded from the die with both the surfaces thereof covered with the layers composed of the compositions (A) and (C), respectively, the ratio of the thickness of the three layers composed respectively of the compositions (A), (B) and (C) after the step of stretching being about 0.005 to 0.5:1:0.005 to 0.5, said polyolefin resin sheet for the paper-like layer being composed of the under-defined composition (D), wherein:

• composition (A) is a thermoplastic resin composition containing a fine inorganic powder in a proportion of 0 to 18% by weight;
• composition (B) is a polyolefin resin composition containing a fine inorganic powder in a proportion of 20 to 68% by weight, wherein the melt index of the polyolefin resin is 0.5 to 3 g/10 minutes;
• composition (C) is a thermoplastic resin composition containing a fine inorganic powder in a proportion of 0 to 18% by weight;
• and
• composition (D) is a polyolefin resin composition containing a fine inorganic powder in a proportion of 5 to 50% by weight, wherein the melt index of the polyolefin resin is 3 to 12 g/10 minutes and is greater than the MI of the polyolefin resin used in the composition (B).