Damage evolution and failure mechanisms of coatings on advanced wood composites

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There is a renaissance in use of wood due to the pressing need to use renewable materials to reduce the environmental footprint of the building sector, which responsible for 30% of global emissions of CO2. However, the desire to use wood on an even larger scale is being curtailed by its poor resistance to environmental degradation and the poor longevity of wood coatings. The latter, and costly maintenance of coatings has negative effects on the environmental footprint of wood used outdoors. The obvious solution to this problem is to make more durable coatings with longer maintenance intervals. Empirical approaches to improving coating durability that rely on establishing relationships between coating properties and longevity, however, have not delivered the desired outcomes. Fresh approaches are needed. The approach we propose here is to elucidate the fundamental mechanisms responsible for coating failure on wood and wood composites. A much deeper understanding of failure mechanisms will allow the field to move beyond current empirical and ad hoc approaches, and create innovative opportunities to tailor wood pre-treatments, coating microstructure and post-treatments to optimize and enhance coating performance. The project will be carried out via a series of inter-linked steps. The first is to develop an accurate non-destructive technique for visualizing damage accumulation in coatings and the wood-composite-coating interface. We propose to use X-ray microcomputed tomography (M-CT) technologies emerging from the digital materials field to accurately image over time the failure of coatings on wood and wood composites subjected to artificial weathering, and derive difference maps that can detect subtle changes in material properties. The resulting digital data will be used to determine spatial distribution of properties in coating and wood composite, which is not possible using current laboratory methods. Secondly, we will examine if failure mechanisms differ due to changes in environmental factors, and variation in wood substrate type and coating properties. This experimental phase of the project will be complemented by 3D M-CT imaging at the micron and sub-micron scale of a sub-set of samples exposed to natural weathering to cross-check the fidelity of artificial weathering. Finally, we will use information from M-CT to develop predictive models for coating longevity. These models, which will be refined during the project will estimate stress responses of wood composites and coatings, coupled with progressive failure analysis to obtain more accurate life-cycle predictions of coatings on wood composites. Although the project does not explicitly seek to develop new composites or coatings the information it will yield holds great significance to Canada's substantial wood industry and the construction sector because of the increasing demand for wood composites and coatings with enhanced exterior durability.