Effect of exposure conditions on dirt pickup resistance (DPR)

Discoloration over time is a common cause for dissatisfaction of exterior architectural (décor) paints in much of the world. As such, it is an area of active research interest for producers of these paints and their raw material suppliers. The rate of discoloration of any particular paint depends on the interplay between certain physical properties of that paint and the environmental exposure conditions. Such an interplay can be established by exposing a series of paints to a variety of real-world environmental conditions. In this paper, we report the dirt pickup resistance results for 26 commercial exterior paints exposed at four different locations and the results of a subsequent study of ten commercial paints at two exposure locations. Repeatability and reproducibility of these tests were determined by exposing the same two paints, with replicates, at different exposure sites and at different dates. We believe this is the first time such information has been reported. Within-series repeatability was good, but series-to-series reproducibility was poor, stressing the need to compare performance only among paints exposed at the same time and location. In addition to testing dirt pickup, we investigated the degree to which microbial growth can contribute to discoloration and found a strong correlation between mildew infestation and decrease in L * under conditions that favor mildew growth. Finally, we investigated the extent to which panel orientation affects the decrease in film brightness over time and found that change in L * was partially dependent on orientation, and that early results at 45° south-facing were accurate predictors of long-term results for vertical orientations for the paints tested.