Influence of internal stress and extensibility on the exterior durability of wood coatings

Different types of coatings were characterised for: swelling by moisture, moisture permeability, elastic modulus and extensibility. These data were used to calculate maximum internal stress. Internal stress was also measured directly whilst changing temperature and humidity. The stress data were compared to cracking and flaking results in both natural and accelerated weathering tests. Results showed that high levels of stress occur if there is a difference in swelling between coating and wood. In most cases stress relaxation took place rapidly. Comparing maximum stress, extensibility and cracking, a good correlation between these data was found. The ratio between maximum stress and extensibility appeared to be a very good predictor for crack formation.