In the past, coating compositions were commonly prepared by dissolving or dispersing film forming organic polymers in volatile organic compounds. Environmental and health concerns associated with applications involving large-scale vapor emissions have led to research in the development of coating compositions wherein the emission of volatile organic compounds is minimized.
Among the various methods of reducing the vapor emissions of coating compositions, the use of reactive diluents to replace all or part of the volatile organic solvent component of a coating composition is of particular interest. As used herein, the term "reactive diluent" refers to non volatile or nearly non volatile organic solvents or dispersants having as an integral part of their structures functional groups which are reactive with a film forming polymer and/or cross linking agent.
In addition to providing a low level of volatile emissions, a coating composition should have a sufficiently low viscosity to permit easy handling and application. Other desirable properties in a coating composition are sufficient stability to ensure a commercially acceptable shelf life and the ability to provide a cured coating having suitable properties such as toughness, adhesion, gloss, uniformity, impact resistance, abrasion resistance, scratch resistance, weatherability, and resistance to attack by solvents, acids, bases and other chemicals.
Coating compositions wherein all or a portion of the volatile organic solvent component thereof is replaced by a reactive diluent are illustrated by the following patents:
U.S. Pat. No. 4,417,022, to Chang et al. discloses coating compositions, the vehicle portion of which consists essentially of from about 60 percent to about 97 percent of a curable film-forming component and from about 3 percent to about 40 percent of an organic reactive diluent capable of chemically combining with the curable film forming component. Disclosed as reactive diluents are ethers having less than five ether groups, amides, esters, urethanes, ureas, sulfur containing compounds, and mixtures thereof which have one primary or secondary hydroxyl group. The reactive diluents disclosed by Chang et al. are further characterized as having a retained solids value of greater than about 80 percent, a hydroxyl equivalent of from about 180 to about 800, and a liguid viscosity of less than about 10 poise at 60.degree. C. Preferred reactive diluents disclosed by Chang et al. are ester containing reactive diluents, with ester containing reactive diluents having allyl side chains being most preferred. Coating compositions having ester-containing reactive diluents to produce cured coatings which lack desirable adhesion, hardness and/or weatherability.
As a further example, U.S. Pat. No. 4,520,167 to Blank et al. discloses a coating compsition comprising (a) a hydroxyalkyl carbamate of the formula: ##STR1## wherein n=0 or 1, R is a C.sub.1 to C.sub.20 organic moiety which may contain one or more constituents selected from the class consisting of hetero-atoms and hydroxyl groups, and each of R.sub.1, R.sub.2, and R.sub.3 is independently H or CH.sub.3 ; (b) an aminoplast cross-linker; and (c) a polymer containing active sites which at elevated temperature are reactive with the amide-aldehyde cross-linker (b). Blank et al. exemplifying as suitable reactive diluents compounds of the formula: ##STR2## Cured coatings formed from the coating compositions containing the reactive diluents exemplified by Blank et al. are, within a range of environmentally acceptable formulations, widely variable as regards the surface properties possessed by same.
Among the compounds which have been offered for use as reactive diluents in coating compositions are dicyclopentenyl oxyethyl ethyl methacrylates, modified caprolactones, and unsaturated melamines. In general, these compounds are poor solvents, have relatively high viscosities and/or produce cured coatings having undesirable chemical and/or physical properties.
A cured coating's toughness, adhesion, impact resistance, abrasion resistance, scratch resistance, weatherability and resistance to chemical attack depend to a large extent upon the film-forming polymer and reactive diluent components of the composition used to produce same. As disclosed in U.S. patent application Ser. No. 759,172 filed on July 26, 1985, and U.S. patent application Ser. No. 807,738 filed on Dec. 11, 1985, both in the name of K. L. Hoy, et al., coating compositions containing carbamate and urea derivatives as reactive diluents are found to provide cured coatings having desirable toughness and adhesion.
The ability of a coating composition to form a defect-free film depends in part on (a) the ability of a composition to cover or "wet" a substrate and (b) the rheology of composition flow during coating application and cure.
A coating composition's ability to "wet" a substrate is related to the degree of surface energy which exists between the coating composition and the substrate surface. As the industry has moved to higher solid content compositions to satisfy ecological considerations, the molecular weight, functionality and geometry of the coating polymers have been lowered, increased and altered, respectively, to minimize the viscosity of the polymer which is reguired for application of the composition contents. In doing so, the wetting of the substrate by the coating compositions has been degraded to the point where many coatings have marginal film forming properties. Indeed, molecules having a higher reactive polar functionality content (increased cohesive character) tend to associate with themselves rather than orient towards the substrate surface. In doing so they tend to pull away from the substrate surface, especially at points of substrate imperfection. The tendency of a coating to shrink away from a substrate surface results in various types of coating defects, a more severe form of which is termed "cratering". Cratering refers to a surface defect caused by a coating composition covering patches of a substrate surface very thinly and other areas of the substrate more thickly, giving rise to the appearance of gullies or "craters" in the finished coating.
In order to reduce the severity of surface defects caused by the inability of a composition to properly wet a surface, common practice is to provide a coating composition with one or more anti-cratering additives which, typically, are surfactants. Owing to its biphilic nature, a surfactant additive is effective in reducing the surface energy at coating substrate interfaces of high chemical potential, thereby improving the ability of the coating to wet a substrate surface. These materials, however, tend to exude and migrate to the substrate and surface of the coating during curing. In the extreme, they can cause loss of adhesion, film haze (loss of gloss), and loss of corrosion protection, all of which are serious defects in a coating system.
Flow properties of a coating composition depend to some extent on the composition's viscosity. Compositions having higher viscosities generally flow onto substrate surfaces less evenly than compositions having lower viscosities. Coating compositions having poor flow ability tend to produce cured coatings having uneven or irregular surfaces. Among the imperfections attributed to poor composition flow is a surface appearance which, as a result of its resemblance to the rind of an orange, is termed "orange peel". For several applications (e.g., automotive and appliance finishes), the loss of surface uniformity and gloss resulting from orange peel is deemed a commercially unacceptable surface defect. Coatings art teaches that the addition of a high boiling but volatile "reflow" or "tail" solvent will circumvent many of these undesirable flow properties. This formulation strategy provides for the applied coating to flow and level during the early part of a curing cycle before chain extension and cross linking reactions can take place. This practice does, however, contribute to the volatile emissions and in the case of polymer systems designed for high solids finishes, results in the surface defects previously decribed.
The optimum viscosity for a given coating application depends in part on the method by which the coating is applied. For example, low pressure spray applications may reguire the use of lower viscosity compositions than high pressure spray applications. As previously noted, reactive diluents oftentimes increase the viscosities of the coating compositions into which they are incorporated. Undesirably high composition viscosities are commonly reduced by volatile organic compound addition. Owing to the environmental constraints on volatile organic compound emission levels, reactive diluent containing compositions typically have relatively narrow formulation ranges.
Accordingly, it is an object of this invention to provide a high solids coating composition suitable for use in the production of cured coatings having commercially acceptable chemical and physical properties.
It is a further object of this invention to provide a high solids coating composition capable of providing cured coatings having optimum surface appearance.
Additionally, it is an object of this invention to provide a high solids coating composition which may be formulated over a relatively broad compositional range.