Abstract
Most wood coating tests are done either in a short term artificial weathering chamber or long term on an outdoor rack/fence. In both cases, the coatings are exposed to both ultraviolet radiation and water. This study is focused on the influence of moisture alone on wood opaque film forming coating failures. As moisture is sorbed into the wood structure, the wood swells in proportion to the volume of water sorbed. As moisture is lost, the wood shrinks in proportion to the volume of the water lost. Moisture in the wood end grain is responsible for coating failure in, for example, window corners and end to end siding. The wood cell wall moisture can be greatly reduced by a process known as acetylation which not only reduces the moisture sorbed in the cell wall but results in high levels of dimensional stability. The reduced moisture uptake along with the stability results in less stress created between the coating wood surface interface improving the performance of the coating and increasing its useful lifetime.
Highlights
Most wood coating tests are done either in a short term artificial weathering chamber or long term on an outdoor rack/fence
In order to study the influence of moisture in coating failure it is important to understand how moisture enters the wood and how it moves within the wood [1]
Past data suggests that the sorption of moisture and the movement of moisture in the wood cell wall is through the hemicellulose polymer structure [1]
Summary
The number one enemy of exterior coated wood is moisture. In order to study the influence of moisture in coating failure it is important to understand how moisture enters the wood and how it moves within the wood [1]. It attracts moisture which bonds to the cell wall polymers through hydrogen bonding. Past data suggests that the sorption of moisture and the movement of moisture in the wood cell wall is through the hemicellulose polymer structure [1]. (a) Water molecules entering the wood cell wall;. Wall; (b) water molecules unzipping hydrophylic hemicellulose polymer chains. Thesorption sorptionofofmoisture moisture each wall polymer cellulose > lignin as is shown in Figure 2 [5]. The sorption of moisture by each cell wall polymer not onlydepends dependson onits its hygrophilic hygrophilic nature, nature, but thethe polymer’s hydroxyl only and alsoaccessibility accessibilityofofwater watertoto polymer’s hydroxyl only depends on its hygrophilic nature, and accessibility of water to the polymer’s hydroxyl groups. Sorption Isotherms for wood and cell wall polymers [5]
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