White, biaxially oriented film made from a crystallizable thermoplastic having high degree of whiteness

摘要

White film with a thickness of 10-500 microns based mainly on a crystallizable thermoplastic contains titanium dioxide (rutile type) as white pigment and an optical brightener, either or both of which are introduced as a masterbatch during production of the film. An Independent claim is also included for an extrusion process for the production of white film by melting the thermoplastic in an extruder, extruding through a slot die, quenching on a cooling roller, reheating and stretching (a) in the machine (MD) and cross (CD) directions, (b) in the MD and in the CD, or (c) in MD, in CD and again in MD and/or CD, using a stretching temperature of Tg + 10K to Tg + 60K and stretch ratios of 2-6 (preferably 2.5-4.5) for MD and 2-5 (preferably 3-4.5) for CD, with a stretch ratio of 1.1-3 for any second stretching in the MD.