Non-transparent microvoided axially stretched film, production process therefor and process for obtaining a transparent pattern therewith

A process for preparing a non-transparent microvoided axially 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 polyester matrix, ii) forming the mixture produced in step i) in a thick film followed by quenching;
• iii) stretching said thick film at a stretching tension of >2.5 N/mm2 at a temperature between the glass transition temperature of said SAN-polymer and the glass transition temperature of said linear polyester to at least twice the initial length, 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 the initial length at a stretching tension of >2.5 N/mm2 and at a temperature of 90° C. or below, wherein the weight ratio of said polyester matrix to said non-crosslinked random SAN-polymer is in the range of 2.0:1 to 19.0:1 and wherein one of said at least one aromatic dicarboxylate monomer units is isophthalate and said isophthalate is present in said polyester matrix in a concentration of 15 mole % or less of all the dicarboxylate monomer units in said linear polyester matrix.