At about the same time as the colamination was being practiced, coextrusion was becoming a widely available option for sheet and film production. Because coextrusion is inherently a continuous extrusion process the overall materials and uptimes of the OPP lines were improved over the colamination process allowing lower manufacturing costs. Therefore coextrusion of low melting skin layers on OPP became the standard process for film production. With the eventual proliferation of new copolymer PP resins this became an ideal way to improve film properties, especially in relationship to seal range and improved adhesion.
Homopolymer PP is difficult to treat well and this could lead to many adhesion problems. The ethylene copolymers were found to be easy to treat well and to enhance adhesion, and if not used for heat sealing were often termed “adhesion promoting layers” and used in improved coating products and to improve ink adhesion in print webs. Adhesion in laminating and metallized films was also improved when the copolymer layers were laminated and metallized.
Today, OPP films seldom have a plain homopolymer surface and skin layers of many different polymers are possible. It is common to find OPP films with ethylene or butylene copolymers, Ethylene butylene terpolymers, HDPE, MDPE and even EVOH skin layers to enhance many film surface properties and final product properties. Heat sealable surfaces also have benefited from the use of the many different copolymers and terpolymers. Today minimum temperature heat sealing films are possible and can challenge the seal rang of coated products, but cannot add the flavor and aroma or oxygen barrier enhancements of the coated products.