Weathering degradation of a polyurethane coating

The degradation of polyurethane topcoat over a chromate pigmented epoxy pri mer was examined by atomic force microscopy (AFM), scanning electronic micr oscopy (SEM), X-ray photo-electron spectroscopy (XPS) and Fourier transform infra-red spectroscopy (FTIR) after the coated panels were exposed in a QU V chamber. It was found that upon QUV exposure, blisters formed on the topc oat surfaces during the initial phase of coating degradation. As a result o f blister formation and subsequent breakage, coating gloss was lost before any pigments were exposed in the coating. XPS and FTIR analysis indicated t hat urea and urethane group concentrations increased in the coating upon ex posure. It is suggested that the build-up of hydrophilic groups in the coat ing during coating degradation promoted water absorption into the coating s ystem, then the alternating dry and wet environment caused the formation of osmotic cells and thus blisters on the coating surfaces. (C) 2001 Elsevier Science Ltd. All rights reserved.