Process for producing a multi-layer high opacity film structure of improved whiteness and machinability

Claim 1

A process for preparing a three-layer opaque, biaxially oriented polymeric film structure, comprising the steps of: (a) mixing a major proportion of a first thermoplastic polymeric material with a minor proportion of a first material of higher melting point or having a higher glass transition temperature than the first thermoplastic polymeric material to produce a core layer mixture; (b) heating the core layer mixture produced in step (a) to a temperature of at least above the melting point of the first thermoplastic polymeric material; (c) dispersing the first material of higher melting point or higher glass transition temperature of the mixture produced in step (a) uniformly throughout the molten first thermoplastic polymeric material in the form of microspheres; (d) mixing a second thermoplastic polymeric material with titanium dioxide contact pigment to produce a first skin layer mixture; (e) heating the first skin layer mixture produced in step (d) to a temperature of about the melting point of the second thermoplastic polymeric material; (f) mixing a third thermoplastic polymeric material with a finely divided, inorganic material to produce a second skin layer mixture; (g) heating the second skin layer mixture produced in step (f) to a temperature of about the melting point of the third thermoplastic polymeric material; (h) coextruding a three-layer film structure from the core layer mixture, the titanium dioxide-containing first skin layer mixture and the inorganic material-containing second skin layer mixture; and (i) biaxially orienting the three-layer film coextruded in step (h), said biaxially orienting step conducted at a temperature and to a degree to form a strata of opacifying voids within the core layer; wherein the inorganic material of the second skin layer imparts antiblocking characteristics and reduced inherent film-to-film coefficient of friction at its surface.