Mojgan Nejad

Coatings to reduce wood preservative leaching.

The efficiency of semitransparent penetrating stains to reduce leaching of wood preservative components was evaluated. Five commercial wood deck finishes were applied to untreated and chromated copper arsenate (CCA), alkaline copper quat (ACQ), and copper azole (CA) treated wood, and leachates were collected and analyzed during 3 years of natural weathering exposure in Toronto, Canada. All stains evaluated effectively reduced the cumulative leaching of all inorganic preservative components by about 60% on average. Although most coatings showed significant film degradation starting around 12 months, the reduced leaching persisted even after 3 years. This suggests that temporary protection of wood with a coating during the early stages of use resulted in long-term reduction in preservative leaching potential. A two-week screening leaching test was able to predict the long-term leaching performance of different coatings reasonably well. Cured coating glass transition temperature (Tg) and liquid coating viscosity were the most important variables affecting a leaching prediction model. To effectively reduce leaching of preservative components from treated wood, coatings should have Tg low enough to withstand stresses caused by freezing in winter and have adequate viscosity to form a barrier film layer on the wood surface.

Exterior Wood Coatings

In addition to aesthetic appeal, coatings are designed to protect the wood from weathering degradation in outdoor conditions. This chapter gives an overview of the effects of the main coating components, coating properties, wood properties and treatments on coated wood performance in service. Understanding how different type of resins, pigments, solvents, and other major additives affect coating performance on wood, helps coating formulators to develop more durable coatings. It is beneficial for both wood scientists and coating chemists to learn which properties of coatings have the highest impact on predicting their service lives when exposed to weathering. For instance, measuring glass transition temperature (Tg) defines the degree of flexibility of a coating. Since wood swells and shrinks due to moisture uptake and subsequent drying, flexibility of a coating plays a critical role in defining its durability on wood in exterior conditions. Similarly, learning how preservative treatment or a new modification technique changes the surface properties of wood will help coating formulators to adjust properties of coatings in way that would have better adhesion and performance on that specific modified wood. Also, the effects of these factors in increasing service life of exterior wood coatings are discussed.

Protocol Comparison: Laboratory versus Natural Weathering Tests for Performance Evaluation of Coatings on Preservative-Treated Wood

Abstract Fourteen stains were tested in the laboratory to compare water uptake and leaching reduction of wood treated with chromated copper arsenate, alkaline copper quat, and copper azole. Based on results of a 2-week test, eight stains were selected to be evaluated over 3 months of accelerated weathering and five stains over 3 years of natural exposure in Toronto, Ontario, Canada. These comparisons were made in order to find a quick and reliable method for replacing natural exposure tests. Comparison of different weathering techniques showed significant correlations between leaching and water uptake results from laboratory tests and natural weathering. The cumulative percentage of inorganic elements leached from coated samples was highly correlated with the cumulative percentage leached during 3 years of natural weathering. Also, the average moisture content of treated-coated samples after 1 and 3 days of water immersion showed a relatively strong positive correlation with the average moisture content o...