Development of an accelerated test method for dirt pick-up resistance

Dirt pick-up is a long-standing issue for exterior architectural coatings. Dirt pick-up resistance (DPR) is highly sought by end users, and significant resources have been committed by paint manufacturers to improve it. The task is complicated by the multitude of regional and local factors such as seasonal weather, intensity of economic activity, or atmospheric particle burden. Typically, paint manufacturers try to control DPR via careful selection of additives and formula optimization. Testing the new formulas in real time is long and, therefore, there is a clear need for a faster and more reliable test that takes into account the key elements of the dirt pick-up mechanism: particle transport, adhesion, and release to/from the surface. Our team has developed an accelerated test that focuses on thermally driven adhesion and takes a fraction of the time necessary for outdoor exposure testing. The test correlates well with early outdoor exposure results when formulation parameters are controlled and when there are moderate-to-large performance differences among paint formulas. The test was used to improve the accelerated DPR of a paint formula, which subsequently was verified against real-time outdoor exposure in three geographic regions with different climates and airborne particulate pollution levels.