Chemical and physical changes in paints or painted wood due to ageing: Dissertation

Repainting exterior cladding has been estimated to be one of the highest costs in the maintenance of small wooden houses over a long-term period (80 years) because of the relatively short interval of painting. Reportedrot damage in wooden exterior cladding in Norway and Sweden raised the question of the part played by paints in the damage. Consequently, many studies on the problem of the durability and performance of painted wood have been started in Nordic laboratories and institutes. The aim of this study was to analyze chemical and physical changes in paint and painted wood due to ageing. The paints were experimental products based on alkyd, acrylate and oil binders, which are the most widely used in coatings for wooden substrate. Chemical changes were analyzed by Fourier transform infrared spectroscopy. In addition to the chemistry of ageing, the flexibility of the paints, the condition and moisture transport properties of the painted wood and the adhesion between the paints and the substrate were monitored vs. ageing. The painted specimens were subjected to both artificial exposure in a weathering device and to natural exposure. Only the influence of relatively short-term artificial ageing periods (250 h, 500 h and 1000 h) and natural ageing for 1 - 2 years were included in this study. The unpigmented stains failed during the longest exposure. The ultraviolet stabilizer in the stain did not give any longer-term protection against light. The alkyd and oil paints underwent various chemical changes due to their susceptibility to auto-oxidation reactions of carbon-carbon double bonds along the fatty acid chain. The drier containing zirconium accelerated crosslinking reactions more than the lead drier in the alkyd paints. The acrylate paints were relatively inert to external stresses. The changes in moisture transport properties of the acrylate emulsion paints were probably due to the migration of smaller molecules. The cracking tendency of the combination paints based on the alkyd and acrylate binders was assessed to result from their relatively high water permeation properties, which caused cracking in the wood substrate and thus internal stresses in the paint film itself. The cracking of the solvent-borne paints was suggested to be due to the very low flexibility of the paint.