Influence of Coating Additives on Water Vapour Absorption and Desorption in Norway Spruce

Abstract

Summary The growing concern for the environment and occupational health aspects have led to the development and use of waterborne coatings for architectural applications. These coatings can be considered as a two-phase system with the binder emulsified or dispersed in the water phase. During film formation, phase separation occurs and coating additives can be mobilised and transported into wood, thus changing wood moisture sorption characteristics. In the present study, wood samples were treated with various coating additives, normally used in waterborne coatings, and exposed to various humid conditions. The moisture adsorption and desorption in the treated wood samples was compared to untreated wood samples. It is shown that the presence of substances common in waterborne coatings changes the moisture sorption characteristics of wood. The presence of a surfactant, nonylphenol ethoxyphenoxylate adduct, and the presence of a rheology modifier, hydroxyethyl cellulose, lower the rate of moisture absorption and, to a great extent, delay the release of moisture during desorption. Due to the fact that these substances occur in waterborne coatings, may be mobilised during and after film formation and accumulate at the coating/substrate interface, there is a great probability that these substances change the moisture sorption characteristics of the wood substrate in an unfavourable way and create unexpected moisture conditions at the coating/wood interface.