1. Field of the Invention
The present invention generally relates to a curable coating composition. More specifically, the present invention relates to a curable coating composition including a plasticizing component that forms a cured film having excellent resistance to cracking and etching.
2. Description of the Related Art
Curable coating compositions and their use in a wide variety of coating applications are known in the art. In a curable coating composition, a resin component, along with a suitable cross-linking agent, generally produces a cured film having good film properties, such as resistance to etching, excellent gloss, etc. Such properties are particularly important when the curable coating composition is to be used as a clearcoat in automotive coating applications, in which the clearcoat is the first line of defense against the environment for an underlying substrate such as a metal panel of a vehicle. In addition to excellent etch resistance, resistance to cracking is also important. Without excellent resistance to cracking and etching, the clearcoat may provide insufficient protection to prevent corrosion and damage of the underlying substrate.
In general, high crosslink density and/or high glass transition temperature, Tg, are desirable traits of the cured film for good etch resistance. Low crosslink density and/or low cured film Tg are desirable traits for good crack resistance. Although the cured films formed from the current coating compositions may include plasticizing components that provide good resistance to cracking, the plasticizing components provide poor resistance to etching of the cured film from corrosive chemicals Because most plasticizing components are materials with very low Tg (<−20° F.), and are non-crosslinkable. In many cases, relatively large amounts of the plasticizing component must be included in the curable coating compositions in order to provide acceptable resistance to etching to the cured films. The plasticizing components function to “anneal” the cured film, i.e., the plasticizing components relieve stress that builds up in the cured film. However, in doing so, they lower the overall Tg of the cured film. This leaves the system open to poor etch resistance and may allow contaminants in the atmosphere to penetrate the cured film.
An alternative way to reduce cracking is to use resins and crosslinkers that will reduce the overall crosslink density of the coating. For example, blocked hexamethylene diisocyanate and partially defunctionalized melamine have been added to curable coating compositions to increase resistance to cracking of the cured films formed from the curable coating compositions. Further, resin components including polyester moieties have also been included in the curable coating compositions to increase resistance to cracking of the cured films formed from the curable coating compositions. However, the lowering of the crosslink density has the negative effect of lowering resistance to etching of the cured film and making the cured system open to environmental degradation.
One of the problems with the current plasticizing components or with the use of resins and crosslinkers that reduce the overall crosslink density is that the additives are typically uniformly dispersed in the curable coating composition. Resistance to cracking and etching is most important adjacent an outer surface of the cured film, since crack propagation initiates at the surface of the cured film. As such, much of the plasticizing component that is not adjacent to the surface of the cured film is unnecessary, which adds cost to the curable coating compositions. Furthermore, as alluded to above, relatively large amounts of the plasticizing components must be included in the curable coating compositions to provide acceptable resistance to etching to the cured films. As such, the inclusion of the plasticizers in the coating composition and/or the reduction of crosslink density has the negative effect of decreasing resistance to etching of the cured films formed from the curable coating compositions. A combination of resistance to both cracking and etching is important in order to prevent corrosion and damage of the underlying substrate.
Due to the deficiencies of the known plasticizing components, it would be advantageous to provide a plasticizing component and a curable coating composition including the plasticizing component that is capable of forming a cured film that has resistance to both cracking and etching that, to date, has not been achieved with known plasticizing components.