Solvent-borne acrylic coatings made the first significant entry into the protective coatings market as color and gloss retentive topcoats for epoxy intermediate coats. The newer waterborne acrylics represent the forefront of this technology. The best of these appear to be blends of acrylate and methacrylate resins.
Some of the newer two-component versions contain both the waterborne acrylic and a cross-linking material, such as an epoxy or urethane hardener.
Waterborne latexes have been around for a long time, but their limited resistance to aggressive chemicals and corrosive conditions have restricted their use mostly to aesthetic or architectural purposes.
Film formation of waterborne acrylics depends on the fusion of the tiny particles of polymer (acrylic or latex) once the water has evaporated from the applied film. Thus, all waterborne paints and coatings are susceptible to water damage during their early stages of cure.
In reality, many waterborne acrylics contain small amounts of coalescing agents added to the formula to improve the ability of the coating to wet the surface and to speed the fusion (melting together) of the acrylic/latex particles. Residual amounts of this coalescing solvent often remain in the film as plasticizers, giving the protective coating good flexibility with age.
Single-component, self cross-linking systems for waterborne acrylic coatings are achieved by adding materials that cure by air oxidation. These cross-linking systems significantly improve chemical and solvent resistance.
The major shortcoming of single-component waterborne acrylics is their inability to cure at low temperatures, which drastically restricts the painting season in cold and humid climates. Careful choice of coalescing solvents can speed the cure at low temperatures and increase long-term hardness of the coating film.
Advantages of Waterborne Acrylic Coatings
- Ease of application by brush, roller, or spray;
- Relatively inexpensive;
- Excellent color and gloss retention;
- Good long-term flexibility;
- Good adhesion to galvanized surfaces.
Disadvantages of Waterborne Acrylic Coatings
- Early flash rusting when applied in temperatures approaching freezing;
- Susceptible to washing off when subjected to rain within a few hours after application (during the curing process);
- Surface preparation can be demanding; surface must be free of all residual oil or grease;
- These coatings do not wet the substrate very well;
- Slow cure during high relative humidity conditions that retard evaporation of water.
This article is adapted from The Protective Coating User’s Handbook, Second Edition, Louis D. Vincent (Houston, TX: NACE International, 2010), pp. 28–29.