Patent ID: 8034541

Claim:
A process for producing a non-transparent microvoided self-supporting film comprising the steps of: i) mixing at least one linear polyester having together monomer units consisting essentially of terephthalate, isophthalate and aliphatic dimethylene with a molar ratio of isophthalate monomer units to terephthalate monomer units being at least 0.02 to provide a linear polyester matrix; at least one amorphous high polymer with a higher glass transition temperature than a glass transition temperature of said linear polyester matrix and/or at least one crystalline high polymer having a higher melting point than the glass transition temperature of said linear polyester matrix; and optionally at least one ingredient selected from the group consisting of inorganic opacifying pigments, whitening agents, UV-absorbers, light stabilizers, antioxidants, flame retardants and colorants in a kneader or an extruder; ii) forming the mixture produced in step i) into a thick film followed by quenching; iii) longitudinally stretching the thick film at a stretching force of >4 N/mm 2 to at least twice the initial length; and iv) transversely stretching the longitudinally stretched film from step (iii) to at least twice its initial width.