process for preparing a white non-transparent microvoided biaxially stretched film

A process for preparing a white non-transparent microvoided biaxially stretched film comprising the steps of:

• i) mixing at least one linear polyester having monomer components consisting essentially of at least one aromatic dicarboxylic acid, at least one aliphatic diol and optionally at least one aliphatic dicarboxylic acid, a non-crosslinked random styrene acrylonitrile polymer (SAN-polymer) and at least one ingredient selected from the group consisting of inorganic opacifying pigments, whitening agents, colorants, UV-absorbers, light stabilizers, antioxidants and flame retardants in a kneader or an extruder to produce a mixture comprising said non-crosslinked random SAN-polymer in a linear polyester matrix,
• ii) forming the mixture produced in step i) into a film with a thickness between 10 to 6000 μm followed by quenching;
• iii) stretching said film at a stretching tension between 4.0 to 8.0 N/mm2 at a temperature between 80 and 90° C.;
• and
• iv) subjecting the film obtained in (iii) to a further stretching process at an angle substantially 90° to the first stretching process to at least twice its initial length at a stretching tension between 4.0 to 8.0 N/mm2 and a temperature between 80 and 90° C.;
• wherein the weight ratio of said linear polyester matrix to said non-crosslinked random SAN-polymer is in the range of 2.0:1 to 9.0:1 and wherein the monomer component of one of said at least one aromatic dicarboxylic acid is isophthalate and said isophthalate is present in said linear polyester matrix in a concentration of 15 mole % or less of all dicarboxylic acid monomer units in said linear polyester matrix.