The science behind how coating systems protect the water you drink
December 29, 2020 •Corrosion CONTROLLED, Infrastructure, Water
Many people take it for granted that the water coming from the various faucets in our homes is safe for cleaning and drinking. That water usually comes from huge community water tanks. The coatings inside those tanks require complex chemistry to protect the tank, but they are also completely safe for use in potable water storage situations. How is this achieved?
Part 1: Moisture-cure urethane primers for water tank re-coat
The recoating of a potable water tank in Lancaster, PA (read more here) used a three-coat lining system: a zinc-rich urethane primer; a brush-applied “seam coat” of epoxy applied to all weld seams, angles, and edges; and a ceramic epoxy topcoat. Each coating that was used is NSF International/American National Standards Institute (ANSI) 61 certified, meaning that the materials are safe and will not produce negative health effects. The following outlines how this system protects the inside of a steel tank full of potable water.
A moisture-cure urethane primer offers a number of technical and applicator-friendly attributes. First, urethanes are widely acknowledged to have excellent protective and barrier properties with extremely low permeability. Second, a good moisture-cure coating system provides quick-to-handle properties as well as the ability to apply a subsequent coating in a timely fashion. Last, while it exhibits fast cure properties, it also has a long recoat window (at least a year). This unique combination of properties makes this an attractive primer system.
Part 2: Ceramic epoxies for water tank topcoat
The zinc-rich primer is coated with a high-performance epoxy system, providing a layer of protection and serving as an effective surface for the ceramic epoxy topcoat. While epoxies are well known for their excellent mechanical and chemical properties, ceramic epoxies add two levels of protection.
First, well-designed ceramic epoxy systems employ hollow ceramic spheres with a tailored distribution of sizes. This creates an extra level of efficiency in barrier properties to moisture. Epoxies provide tightly cross-linked barriers, but any moisture that does make its way through this layer of protection then has to maneuver around the gauntlet of all of the ceramic spheres in the system. This makes the distance the moisture has to traverse much longer, in effect providing the protection of a significantly thicker layer. The result is superior protection while also providing a cost-efficient coatings package, since a 15-mil (381.0 microns) ceramic epoxy coating can deliver the protection of a 25- or 30-mil (635.0–762.0 microns) standard epoxy coating.
Ceramic epoxies also provide a second level of protection. Epoxies are well known as tightly cross-linked, durable systems. But if corrosion or any other type of stress is applied to the system, normally that stress would lead to a failure in the epoxy network. Nature is all about energy management, and systems always seek to minimize energy. In a standard epoxy system, stress can build until it creates a critical path length failure, which can propagate throughout the epoxy network. Ceramic epoxies provide an alternate mechanism for energy reduction by localizing the stress absorption in a manner that never allows the stress energy to build up to that required for critical path length propagation.
Protecting the water in the tank
As stated earlier, all layers in this particular coating system are NSF/ANSI 61 certified. For a coating to receive this certification, it must go through a rigorous testing process by a qualified third party. This process is, in many ways, more stringent than U.S. Food and Drug Administration (FDA) certifications for food contact systems; many of those rely on a list of good and bad chemicals, and certification requires only that coatings don’t contain anything on the “bad” list. NSF/ANSI 61, on the other hand, requires actual testing of the coating system with the test specimens from plant production batches. Detailed formulation information is reviewed by an expert team of toxicologists. The system is tested via water immersion tests, with leeching results analyzed at parts per billion levels. The certifying body also performs on-site plant audits and sampling. If “NSF/ANSI 61 certified” is noted on a product, one can be confident that it has been subjected to demanding inspection and testing processes.
Water safety confidence through coatings
While it may, on the surface, seem fairly straightforward to prepare and paint a potable water tank, there is an enormous amount of complex science (including the NSF/ANSI 61 certification process) that goes into ensuring the protection of the tank and the water inside.
For more information on protecting assets in the water/treatment industry, sign up for a free subscription to NACE’s WaterCorr News, a triannual, digital publication covering this industry’s technologies and best practices.
Source: Originally appeared in CoatingsPro Magazine, May 2020, by Jeff Lackey, Ph.D., Technical Director for Induron Protective Coatings.
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