Biaxially stretched polypropylene composite film and electrical insulating film made therefrom

A biaxially stretched, composite polypropylene film suitable for use as an electrical insulating film, comprising

• a first layer comprising a first propylene polymer and having a maximum roughness height R.sub.z of its exposed surface of about 0.15 .mu.m, and
• a second layer contiguous to said first layer comprising a second propylene polymer having a composition different from said first propylene polymer and having a roughness height R.sub.z on its exposed surface ranging between about 0.2 .mu.m and 0.4 .mu.m,
• said composite film being biaxially stretched and having a modulus of elasticity of at least about 2000 N/mm.sup.2 in all directions of the plane of the film and a DC dielectric strength of at least about 650 V/.mu.m, said composite film having been produced by the steps consisting essentially of joining said first and second layers together and subjecting the resulting composite film to biaxial stretching, and wherein the differences in said first and second propylene polymers are sufficient to produce said different roughness heights during said biaxial stretching.