Analytical Series Principles of Accelerated Weathering – Evaluations of Coatings

By Mark Nichols

One of paint’s most important attributes is its ability to main­tain performance for an extended period of time in its intended service environment. That environment may be an interior wall in a single family home, the outside of an underground pipe­ line, or the exterior of a vehicle. The service environment is different in all three cases, but the ability to maintain the paint’s functions-aesthetics, adhesion, corrosion protection, and mechanical performance-is still required.

Objects that predominantly reside outdoors are subjected to one of the most challenging service environments for coatings. Exposure to solar radiation, temperature fluctuations, rain, snow, and environmental fallout (acid rain), challenge the performance of most coatings. Examples of painted objects that are exposed to such environments include automobiles, aircraft, infrastructure (bridges and roads), houses, and buildings. To achieve long-term performance, exterior coatings require resis­tance to degradation by UV radiation, resistance to hydrolysis, and resistance to erosion by rain and snow.

The durability of a coating is typically assessed by expos­ing it at selected outdoor locations to quantify the coating’s real-world performance. While natural exposure outdoors is a reliable method of assessment, natural exposure provides little acceleration. A coating that performs acceptably after five years of Florida exposure means that the coating will survive five years of exposure in Florida, but this says nothing about its performance after 5.5 years or 10 years. It does mean that the coating may survive longer than five years in a less harsh environment, but the failure mode may change, as environmen­tal loads can vary dramatically from region to region.’ Thus, it is impractical to develop coatings using natural outdoor exposure as a method to assess their long-term durability, as product development time cycles are not compatible with test methods that take five-plus years to perform.

Coating formulators, therefore, rely heavily on accelerated weathering tests to develop and optimize coating formu­lations. Accelerated weathering tests attempt to degrade a coating at a faster rate than that which occurs during natural exposure. However, to be reliable and useful, the increased rate of degradation must not sacrifice the accuracy of the results, meaning the correlation between accelerated weather­ing results and natural weathering results must be quite high. Unreliable results produced quickly are not useful, and can potentially be quite damaging to a company’s reputation and bottom line. The bulk of this article will discuss the science behind paint degradation and the various methods used to assess paint weatherability.