Accelerated weathering of wood coatings in conjunction with moisture modelling

The development of new types of water borne paints with ever lower VOC contents and improved
application properties continues. Their use in the building industry is also increasing. New environmental and material developments are stilt being introduced into the market. It is necessary to have tools to predict the in-service behaviour of coating systems over the long term.
In this research two accelerated weathering methods were applied: Weather-Ometer; and, outdoor exposure. A new Weather-Ometer weathering cycle was developed for coated wood substrates. As in other accelerated weathering test cycles, the new test cycle was not suitable for the investigation of the effects of dirt retention, mildew growth, hail impact and mechanical fatigue. The importance of leaching water soluble components during the test was confirmed. Characteristics of the coating-wood system from the outdoor exposure were compared with the chamber tests.
A newly developed wet test cycle shows good correlation with outdoor exposure tests, particularly with
respect to changes in the water barrier function and visco-elastic properties of the paint systems. Acrylic based water borne paints have a relatively good UV resistance and the strong decrease in total
water uptake found for some paint systems may be caused by leaching of water soluble components. The determined characteristics of the coating-wood system were used to model the moisture transfer in, for example, wooden window frames. Different cases were investigated with respect to Dutch climate circumstances. From the model it would appear that large differences in water permeability between paint films don not greatly influence moisture fluctuation within the wood in practical situation