Development of a Gel Coat With Improved UV Stability

The ability of composite materials to penetrate into business sectors dominated by other construction materials has been limited by a cost-effective means of achieving acceptable performance characteristics with current gel coat technologies. In many cases, the limiting factor has been the gel coats ability to retain its original color and gloss upon continual ultraviolet exposure as compared to the performance of alternate coating technologies. The upcoming MACT requirements regulating the composites industry will also have a detrimental effect on the ability to use many of the current techniques in formulating the high performance gel coats currently on the market. This paper will describe the development of a gel coat that has been developed to optimize the coating’s color and gloss retention upon ultraviolet light exposure. The development process will be outlined in detail and the weathering attributes will be illustrated with Xenon Arc and Q-UV weather test results. These results will compare the new coating’s benefits over existing conventional and MACT compliant products. Physical property and blister resistance differences will also be examined. INTRODUCTION Gel coat performance characteristics can be separated into two fundamental areas; The application characteristics of the product during the composite molding process and the in-service performance of the coating during the parts life. During the application process the gel coat must spray and cure such that no cosmetic defects such as porosity, prerelease, color separation and resin tear take place. The final part must have appealing cosmetics, gloss and richness of color. In-service performance characteristics include resistance to cracking, water, thermal shock and weathering. The resistance to water, cracking and thermal shock depends on choices made in the entire laminate structure whereas the resistance to weathering is almost exclusively dependent on the gel coat chemistry. Proper construction techniques are also essential to obtaining the optimal in-service properties of the composite part. Poor workmanship can generate loss of water, cracking, thermal shock and UV resistance. Environmental exposure conditions contribute to the surface degradation of the gel coat film. Exposures to light, heat, and water all lead to eventual change in color and gloss of the gel coat. The irradiation of ultraviolet light in the gel coat film causes free radical formation and the development of yellow color bodies. Previous studies have shown that the presence of aromatics in the gel coat greatly enhances the formation of these free radicals and subsequent yellowing [1]. These aromatic structures come from monomers used in the gel coat as well as in the polymer backbone. Eventual surface erosion of the resin-rich area of the gel coat film causes a loss of gloss. The yellowing characteristic is most offensive to the end consumer in lighter colors where the yellowing is visible whereas the loss of gloss is more visually offensive in darker, more chromatic colors. Current gel coat technologies have not been able to compete in markets with extreme UV durability requirements such as the automotive, heavy truck and architectural segments. These markets have been dominated by other coatings and construction materials with superior resistance to weathering. Many of the high performance gel coats previously commercialized will no longer be available once compliance to the EPA MACT requirements is necessary. This pointed to the need for further gel coat performance enhancements in a MACT compliant product. DEVELOPMENT A development project was undertaken to determine a means for improving the weathering characteristics of polyester gel coats. The criteria of the development included the following characteristics.