Multilayered composites are among the most ubiquitous and ancient of synthetic structural materials in society that combine the properties of dissimilar substances to produce a single product with chemical and/or physical capabilities not otherwise readily obtainable. Certainly among the oldest method of preparing a structural composite is to bond layers of dissimilar substances together using an adhesive. Plastic films and fiber, however, may be coextruded or heat laminated to form a multilayered composite and electrodeposition of polymers can be used to coat metal film. But overall, adhesive bonding prevails as the means to prepare composites from dissimilar substances, particularly composites of plastic and metal film.
Physical chemical experts in plastic/metal film adhesion hold that if a perfect union of plastic and metal substrates were obtained, i.e., one where the two surfaces mate completely within van der Waal's radii, the strength of the adhesive bond would be the algebraic sum of the substrate bond strengths and, therefore, the adhesive bond strength would exceed that of the lower (plastic) substrate tensile strength. The impediments to achieving this plausible ideal are the ability of the plastic material, or precursor, to wet the metal surface and the rheological properties of the plastic that may inhibit its flow into and full contact with the craters and valleys comprising the metal surface-and do all this while providing a composite plastic substrate having good tensile strength. Now, if the mundane but overriding considerations of production cost and composition cost are added to the equation, the complete challenge to researchers in the science of adhesion chemistry seeking to improve the art of metal/plastic adhesion is defined.
Of course, there are many adhesives known in the art for metal/plastic adhesion, especially aluminum/plastic adhesion. Their large number is indicative of the vagaries of the applications in which the adhesive compositions and composites are used, requiring different plastic or metal properties and, in turn, different adhesive properties and compositions. Compositions may be applied as solutions or dispersions of oligomers, simple polymers, copolymers or graft copolymers and heat sealed to form the adhesive bond. For aluminum-plastic adhesion, polyolefins are often used, despite the fact that adhesion to polyolefin surfaces is often difficult to achieve. But polyolefins are inexpensive and, therefore, extensive development has been conducted to formulate effective adhesives for dissimilar substrates, at least one of which is a polyolefin.
U.S. Pat. No. 4,753,708 discloses a film forming dispersion for heat sealing of dissimilar substrates such as polyolefins and aluminum wherein the dispersion comprises at least two different types of polymers, particularly olefin polymers or random copolymers and acrylic resins, in an organic solvent system. The two polymer types are essentially incompatible and have different adhesive affinities for the substrates. The two polymers have an acid number ranging from 0 to 160 mg KOH per gram of polymer. The dispersion further contains a graft polymer containing components corresponding to the two different polymer types. As a rule, the graft copolymer is prepared by making a dispersion of the olefin polymer/copolymer, with the aid of an appropriate emulsifier, and grafting acrylic monomers onto it.
The method for preparing the adhesive dispersion of the foregoing '708 patent comprises a three step process which requires about forty hours to complete. A substantial portion of the lengthy preparation process is dictated by the need to form the dispersion of an ethylene/propylene copolymer in organic solvent in contact with emulsifier. The dispersion step requires long agitation of the mixture under shearing conditions and a long period of heating in organic solvent in order to swell the olefin polymer or random copolymer particles and allow grafting or reacting the polymer with acrylates.
The object of the present invention is to provide an improved adhesive composition for bonding dissimilar substrates at a high peel strength. The preferred substrates are aluminum and polyolefin film.
A particular objective of the invention is to provide the foregoing improved adhesive composition prepared by a method simpler and substantially faster than methods employed in the relevant art heretofore.