Notwithstanding a considerable portion of roadways being paved, both in the United States and in foreign countries, there exists a serious paint chipping problem which continues to plague motor vehicle owners, caused by gravel and other small particles on the road bed being thrown against the painted surfaces of such vehicles. The same problem plagues the owners of other painted structures such as boats, trailers, airplanes, trains, tanks, towers, and the like, although the impacting particles may be from other sources.
The chipping problem is perhaps most noted by the owners and manufacturers of automobiles, particularly those having areas wherein the body of the automobile tends to curve inwardly immediately behind the front and/or rear wheels. This situation causes particles such as gravel from streets and other roadway surfaces to be carried on the surface of the tires and impacted against the painted surface of the automobile. Since the paved roadways typically have small gravel particles and other hard particles on their surface and unpaved roads usually always contain such material, the propelling of high speed particulate material against the painted surface of the automobile is unavoidable. While means have been suggested for imposing a shield such as a mud flap or mud guard between the automobile wheel and the painted surface, this solution has not been completely satisfactory because it detracts from the vehicle's aesthetic appearance and adds yet another cost to the already high price of the automobile.
Attempts have been made to produce tougher, more chip-resistant paint for automobiles, but these have not been generally completely satisfactory. One recognized solution is to cover all or parts of the finished surface of the automobile with a protective coating.
Automobile surfaces are typically finished or covered with pigmented paints such as acrylic enamel, acrylic lacquer or nitrocellulose lacquer. These materials produce coatings which are difficult to overcoat with protective materials.
A useful protective coating composition will be chip- and abrasion-resistant, have good adhesion to the painted surface, be clear, smooth (i.e., without surface roughness) and indistinguishable when applied to the areas being protected over the painted surface. Prior to the present invenion, none of the surface protective coatings has such characteristics.
While polyurethane lacquers containing non-reactive polyurethane in solvent are preferred condidates for use as protective coatings because such polyurethanes are tough and impact resistant, they do not adhere adequately to conventional painted automobile surfaces because they lack free or available reactive groups such as isocyanate groups which would normally provide attachment sites to promote good adhesion. Thus, such coatings are prone to failure as evidenced by peeling, cracking, checkering or the like. Moreover, it is preferred to employ such a protective coating as a transparent, smooth-surface coating as some coatings tend to discolor, or provide a coated surface which is not smooth but instead rough or otherwise unattractive.
Because of its higher cost, polyurethane is typically not employed to cover the entire surface to be painted. The chip-resistant coatings are customarily added to those locations which would be more susceptible to chipping, e.g., on the automobile body immediately behind the wheels, the leading portion of the automobile such as the front of the hood and other front portions of the body. Protective coatings must therefore adhere to conventional automobile painted surfaces such as those based on acrylic resins.
Various attempts have been made to produce chip-resistant polyurethane protective coatings. One example is that disclosed by Monson (U.S. Pat. No. 4,254,168), assigned to the assignee of the present application, which discloses a pigmented polyurethane protection coating but which does not contain fully reacted polyurethane. Other examples include Levy (U.S. Pat. No. 3,875,090) which discloses a high impact and abrasion resistant coating composition comprising a mixture of a partially hydrolyzed vinyl chloride-vinyl acetate copolymer, an alkyd and an isocyanate terminated urethane prepolymer. This composition depends upon curing of the prepolymer instead of solvent evaporation.
While so-called "solution vinyls" (vinyl chloride-vinyl ester copolymers) have been reactively combined with precursor materials for producing polyurethane, the resultant polymerized material is typically not clear and is difficult to apply because of the required reaction. Such a polyblend is disclosed by Kazama et al (U.S. Pat. No. 4,081,493), teaching a resin composition having resistance to hydrolysis which comprises a polyblend of a polyvinyl chloride resin, a polyester-urethane, a lead stabilizer, and one or more polyhydric alcohols.