Alkyd coatings are used in a wide variety of applications. A typical alkyd has high gloss, good drying, wetting, and adhesion properties, and is compatible with other coating systems such as epoxies. All alkyds have a specific composition of either oil or fatty acid. The amount used in each composition gives the alkyd its distinct characteristics.
Various types of alkyds may be applied as primers on aluminum substrates, but they are most often applied as topcoats. Alkyd topcoats may be used in different service locations based on the paint’s functional properties, such as ultraviolet (UV) radiation protection, antifouling, and anti-staining.
Modified alkyd coatings are formulated in a similar manner to traditional alkyds. However, modifiers can be used to alter an alkyd in order to add or enhance certain properties and characteristics of the paint. Alkyds that use a phenolic resin as the modifier are corrosion resistant and exhibit better adhesion. Monobasic acids increase the flexibility in the coating, and maleic anhydride increases resistance to water, reduces cure time, and improves the color quality.
TT-P-45 is the federal specification for Formula 84 alkyd paints. This formula has good adhesion and anticorrosive properties and is used by the Navy as the primer coat for interior compartments, unheated piping, fittings, valves, and electrical equipment.
MIL-DTL-24607, Coating Compounds, Non-flaming, Fire Resistant is the detail specification for chlorinated alkyd resin enamel the Navy uses as the topcoat in interior compartments and above the bilge in machinery spaces on surface ships.
Silicones and Silicone Alkyd Coatings
Silicone is used in a wide range of products including resins, oil, rubbers and waxes. In general, silicone coatings have excellent resistance to the sun’s ultraviolet (UV) radiation and provide extended gloss durability. They come in a variety of forms and compounds, such as high-solids silicone, waterborne silicone, silicone alkyds, siliconized polyesters, and siliconized acrylics.
High-solids and waterborne silicone coatings were introduced to comply with VOC regulations. Both coatings have little or no solvent. They can be used on substrates that operate at high temperatures and can prevent corrosion as well.
Silicone coatings are used as the antifouling component of the underwater hull coating system. They are also known as foul-release coatings, due to their ability to release organisms from the hull without causing damage to the surrounding environment. These provide a non-toxic, low surface energy coating that is VOC compliant.
Another type of coating combines two coatings groups together: silicones and alkyds. Silicone alkyds provide very good adhesion to steel, wood, tanks, bridges, and other architectural structures. This coating type provides improved gloss retention and durability against chalking and weathering.
The Navy’s governing performance specification for silicone alkyds is MIL-PRF-24635, Coating Systems, Weather-Resistant, Exterior Use. A coating qualified to MIL-PRF-24635 may contain additional pigments to reduce heat absorption by the substrate and diminish the appearance of rust staining. Coating specifications are available for download from the ASSIST Quick Search website.
Low solar absorption (LSA) coatings are designed to reduce the heat load and infrared signature of the ship. Interior compartments with one or more bulkheads forming a section of the superstructure stay cooler during warm weather, which benefits both sailors and equipment. LSA topcoats contain additional pigments that increase reflectance without sacrificing color or gloss.
Anti-stain coatings are designed to reduce running rust on topside shipboard surfaces by using chelating agents to turn the rust into a transparent film. This improves the overall topside appearance of the ship, provides an easy-to-clean surface, and reduces maintenance efforts required by ship’s force.
“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.