Using Infrared Thermography to Image the Drying of Polymer Surfaces
Fike, Gregory Michael
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During the drying of a surface, the liquid evaporation acts to keep the temperature relatively constant, due to evaporative cooling. As the drying nears completion the liquid film begins to break, exposing areas that are no longer cooled through evaporation, which begin to heat. Although this heating can be measured with an Infrared (IR) camera, the sensitivity is often not sufficient to recognize the point at which the film breaks. Complicating the measurement is the changing emissivity that commonly occurs as objects dry. The sensitivity and emissivity issues can be addressed by analyzing the temperature in the area of interest and computing the coefficient of variance (COV) of the temperature. This technique is compared to temperature and standard deviation measurements made with an IR camera and the COV technique is shown to be superior for determining when the liquid film breaks. The film breakage point is found to vary with temperature and material roughness in two industrially significant applications: the drying of wood flakes and the drying of polymer films. Film breakage in wood flakes is related to detrimental finished quality problems and also to emission problems. The rate at which an adhesive dries affects the roughness of the polymer film and subsequently, the bond strength. The COV technique is used to predict the roughness of the finished polymer film. Use of the COV technique allows the drying of a liquid film to be visualized in a way that has been previously unreported.