Biaxially oriented polypropylene film with improved migration resistance

摘要

Biaxially oriented polypropylene (BOPP) film with at least one base layer contg. polypropylene (PP) is claimed, in which (a) the isotactic block length between two structural defects is more than 40 (statistical average), (b) the n-heptane-insol. component of the PP has an isotacticity (by <13>C-NMR spectrometry) of at least 95%, (c) the mol. wt. ratio Mw/Mn is less than 4, (d) the PP contains less than 1% n-heptane-soluble component, and (e) the overall migration of the film (measured as described in DIN EN V 1186-3) is less than 5. Also claimed is a process for the prodn. of this BOPP film, by heating, compressing and melting the polymers for the different layers in extruders, combining the different melts, extruding through a sheet die, cooling and compacting the film on a take-off roll, and orienting the film by stretching in the machine direction by a factor of (3:1)-(9:1) and at right angles by a factor of (4:1)-(12:1).