Heterogeneous membranes from highly filled thermoplastic orientable polymers

A process for making a film having a porous core layer and nonporous outer layers is disclosed. The film is selectively permeable and suitable for controlled atmosphere packaging end uses. The film is made from a composition of certain ethylene polymers, a particulate filler which has little affinity for the ethylene polymer, and optionally an organic compound which decreases the tendency for affinity of the particulate for the polymer. The film is produced by extruding the composition through a die gap of at least 250 µm, subsequently stretching the film after cooling to a temperature below the melting temperature of the polymer, selecting the composition, die gap and stretching conditions to form a film having a substantially nonporous surface and a porous interior.