Opaque film composites and method of preparing same

A process for preparing a film structure comprising:

• mixing a major proportion of a first thermoplastic polymeric material with a minor proportion of a second material of a higher melting point or having a higher glass transition temperature than said first material;
• heating the core mixture to a temperature of at least above the melting point of said first material;
• dispersing said second material uniformly throughout the molten first material in the form of microparticles;
• dispersing and intimately admixing therein an effective amount of a light absorbing colored pigment;
• forming a biaxially oriented core layer of said core mixture, said oriented layer having transparent thermoplastic skin film(s) adherent thereto;
• said core layer having been biaxially oriented at a temperature and to a degree to form a strata of opacifying closed cell voids of dimensions X, Y and Z wherein dimensions X and Y are major dimensions and dimension Z is a minor dimension at least generally corresponding to the diameter of said microparticles.

A process for preparing a film structure comprising:

• mixing a major proportion of polypropylene with a minor proportion of a PBT to form a core mixture;
• heating the mixture to effect melting of both materials;
• uniformly dispersing said PBT throughout the molten polypropylene in the form of microparticles;
• mixing a minor proportion of a light absorbent colored pigment with said melt and uniformly dispersing an effective amount of said light absorbent pigment throughout said melt mixture;
• reducing the temperature to below the melting point of the PBT while maintaining the polypropylene molten;
• melting a separate proportion of polypropylene;
• coextruding said core mixture with the melted polypropylene to form a core layer having skin films of polypropylene on both sides thereof;
• biaxially orienting the film structure at a temperature and to a degree to form a strata of opacifying closed cell voids in said core layer, said voids having dimensions X, Y and Z, wherein dimensions X and Y are major dimensions and dimension Z is a minor dimension at least generally corresponding to the diameter of said microparticles;
• said skin films being at least substantially void-free and of a thickness which will not manifest surface irregularities of said core layer.