Chalking of Heat-Resistant Paint

Question:

I have a customer who manufactures and paints fire rings, which are used at parks and campsites around the country. They use a heat resistant paint rated to 500F. In our testing, we found that the coating can go at least as high as 750F, however there is some “chalking” of the coating. We’re wondering if this chalking is normal for high-temperature applications. Thanks for your help. J.J.

Answer:

When an organic coating, paint or insulating varnish, is given a temperature rating, that means it will withstand prolonged exposure to that temperature. Withstand usually means that it will continue to provide protection to the product at that temperature. The resin (polymer) in the coating will not degrade.

Chalking is caused by degradation of the resin. In outdoor exposure conditions, what we call chalking is degraded resin that has lost its gloss after exposure to ultraviolet radiation in sunlight. Oftentimes, the degraded resin will wash away, exposing pigments and fillers. Clear coatings can also chalk in outdoor exposure conditions.

Thermal exposure can also cause chalking because of polymer degradation. At extremely high temperatures, the resin can pyrollyze, leaving a powdery film composed of only pigments and fillers.