Damage evolution and failure mechanisms of coatings on advanced wood composites

由于迫切需要使用可再生材料来减少建筑行业的环境足迹，木材的使用出现了复兴，建筑行业占全球二氧化碳排放量的30%。然而，由于木材对环境退化的抵抗力差，以及木质涂料的耐久性差，人们想要更大规模使用木材的愿望正在受到抑制。后者，以及昂贵的涂层维护，对户外使用的木材的环境足迹产生了负面影响。这个问题的明显解决方案是制造更耐用的涂层和更长的维护间隔。然而，依赖于建立涂层性能和寿命之间的关系来提高涂层耐久性的经验方法并没有达到预期的结果。需要新的方法。我们在这里提出的方法是为了阐明导致木材和木材复合材料涂层失效的基本机制。更深入地了解失效机理将使该领域超越目前的经验和特殊方法，并创造创新的机会，量身定做木材前处理、涂层微观结构和后处理，以优化和提高涂层性能。该项目将通过一系列相互关联的步骤进行。首先是开发一种准确的非破坏性技术，用于可视化涂层和木材-复合材料-涂层界面中的损伤累积。我们建议使用数字材料领域出现的X射线微计算机断层扫描(M-CT)技术，随着时间的推移准确地成像木材和木材复合材料上涂层在人工风化下的失效，并得出能够检测材料性能细微变化的差值图。由此产生的数字数据将用于确定涂层和木材复合材料中性能的空间分布，这是目前的实验室方法无法实现的。其次，我们将研究环境因素的变化以及木材基材类型和涂层性能的变化是否会导致失效机制的不同。该项目的这一实验阶段将辅之以微米和亚微米级的3D M-CT成像，即暴露于自然风化的一组样品，以交叉检查人工风化的保真度。最后，我们将使用来自M-CT的信息来开发涂层寿命的预测模型。这些模型将在项目期间改进，将评估木质复合材料和涂层的应力响应，并结合渐进失效分析，以获得更准确的木质复合材料涂层的生命周期预测。尽管该项目没有明确寻求开发新的复合材料或涂料，但它将产生的信息对加拿大大量的木材工业和建筑行业具有重要意义，因为对增强外部耐久性的木材复合材料和涂料的需求不断增加。

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.