EPOXY and HIGH-SOLIDS COATINGS
Epoxy paints consist of two components that react with each other to form a hard, inert coating. Part A typically consists of a resin, also known as the binder, along with pigments and additives. Part B is the curing agent, often called the hardener. Epoxy paints are typically referred to as anticorrosive barrier coatings. The excellent adhesion of epoxy coatings is due to the strong polar bonds it forms with the surface. The cross-linking reaction of epoxies, for the most part, is independent of the surrounding environment except with regards to temperature.
The Navy uses different epoxy formulations in various areas. Coatings qualified to the MIL-DTL-24441 specification are epoxy-polyamide coatings. NAVSEA Standard Item (NSI) 009-32 specifies this type of epoxy as the primer and/or intermediate coat in areas such as aircraft carrier potable water tanks, reserve feedwater and fresh water drain collecting tanks, and exterior surfaces of submarines. If this coating is used topside, it must be overcoated to prevent degradation from ultraviolet radiation.
Coatings qualified to the MIL-PRF-23236 specification are epoxy-polyamine coatings. These high-solids coatings are chemically resistant and have edge-retentive properties that make them a good choice for areas with a lot of structural edges, such as chemical holding tanks (CHT) and well deck overheads.
Traditional coating systems consist of a high concentration of solvents. It was not uncommon for coatings to contain 60% to 85% solvents. High-solids coatings were developed to comply with more stringent volatile organic compound (VOC) regulations. In comparison to traditional coating systems, high-solids coatings have a higher ratio of solids, usually around 65% to 80% solid content. Ultra-high-solids coatings have as much as 100% solid content and no solvent evaporation. Due to the higher percentage of solids and the reduction of solvents, these coatings have much higher viscosities, up to three to four times thicker than traditional coatings.
High-solids coatings offer several advantages over traditional coatings. These coatings not only emit less VOCs, they also provide increased corrosion resistance, coating thickness, and edge retention. They provide a higher level of performance, require less surface preparation, and have the ability to self-prime. Another advantage of high-solids coatings is that they require fewer coats, thereby reducing the cost per application. The Navy uses MIL-PRF-23236 two-part epoxy high-solids coatings on tanks, topside surfaces, and well deck overheads
Due to the higher viscosity, application of a high-solids coating is more challenging. The reduction of solvents results in a very short pot life. Depending on ambient temperatures, it is not uncommon for the pot life to be less than 10 minutes. Fortunately, coating application equipment has evolved to meet the demands of this type of coating. Plural component spray equipment uses a high-pressure pump to apply the coating system. There is a hopper for each paint component. Depending on the manufacturer’s instructions, each paint component may be heated to help ensure proper mixing and atomization at the spray nozzle. Some paints require that the spray hoses be heated as well. The proportioning unit controls the mixing ratio of each component. The components do not come in contact with each other until they reach the mixing unit, or in some cases, at the spray gun itself.
“Coatings”, NAVSEA Basic Paint Inspector Training Student Guide. Version 13, Naval Sea Systems Command and Naval Surface Warfare Center, Carderock Division, Code 614, October 2016, sec 10, pp. 10-1 – 10-12.
“Paint Application Guidance and Requirements” in Naval Ships Technical Manual: Chapter 631, Preservation of Ships in Service – General, Rev 3, sec 6, pp. 631-6-10 – 631-6-11.
Frank W.G. Palmer, “Application of Industrial Coatings” in Good Painting Practice, SSPC Painting Manual Volume 1, 4th ed. Pittsburgh: The Society of Protective Coatings, 2002, ch 5.1, pp. 251-265.