Removal mechanisms of nanosecond pulsed laser cleaning of blue and red polyurethane paint

In this study, laser cleaning of blue and red polyurethane paint on aluminum alloy surface were investigated by using a nanosecond pulsed laser at 1.8, 3.2 and 8.0 J/cm2. The surface topography after laser cleaning was analyzed by confocal laser scanning microscope (CLSM) and field emission scanning electron microscopy (SEM), and the paint removal process was recorded by high-speed camera (HSC) and spectrophotometer. It was found that the cleaning results and phenomena of the two paints were significantly different. After cleaned at 1.8 J/cm2, the blue residual paint is a uniform and flat paint layer while the red residual paint presents a loose mesh morphology. Although both red paint and blue paint could be removed at 3.2 and 8.0 J/cm2, the surface of the substrate shows different topography. It could be observed by HSC that the blue paint detached from the substrate by being vaporized and the red paint was disintegrated into pieces and ejected from the substrate. This phenomenon was caused by the difference in the absorption coefficient of the two paints to the laser. To illustrate this, an energy distribution model was established considering Beer-Lambert’s law and the temperature distribution of paint and thermal stress of substrate were calculated. Under laser irradiation, blue paint was more likely to be ablated and vaporized, while red paint was subject to greater thermal stress. For blue paint removal, vaporization, spallation and combustion are the dominated mechanisms and the red paint was mainly erupted by thermal stress of substrate.