Transient temperatures of wet and dry polymeric film coated specimens experiencing outdoor exposure

A mathematical model based on an “aggregated-capacity” temperature analysis is presented and validated for predicting the temperature of thin polymeric films bonded to substrate materials. This model is applicable to coatings on metallic or other substrate materials when the ratio of the thermal resistance across the substrate to the resistance across the coating to ambient is less than approximately 0.1. An analytical rather than a numerical method, was employed to circumvent formulation difficulties and calculation constraints associated with the latter approach for extremely thin coatings. Periodic measurements of ambient dry-bulb and dew-point temperature, wind speed (to estimate convection coefficients), sky temperature, precipitation, and solar irradiation are inputs for determining heat exchange at coating surfaces. In addition to convection and solar irradiation, the model accounts for the effects of infrared exchange with the surrounding sky, dew formation on coating surfaces, and precipitation along with insulation and ambient convection at the back surface of the substrate. Based on comparisons with National Institute of Standards and Technology data on specimen temperature and simultaneous weather data compiled between 2006 and 2008, the analytical model correlated well to measured temperatures, and provided a tractable, computationally efficient, and validated solution for predicting long term transient temperatures in thin-coated specimens.