Understanding indentation, scratch and wear behavior of UV-cured wood finishing products

In the wood furniture and flooring industry, the protective and aesthetic properties of the final product often rely, at least partially, on the coatings applied to the wood surface. For flat surfaces, UV-cured coatings are often preferred due to their many advantages, such as fast curing times, low volatile organic compound (VOC) emissions, low energy consumption and high crosslinking densities. To increase the durability of interior wood products, the behavior of the protective coatings while subjected to wear and deterioration has to be understood in order to be enhanced. Depending on the type of solicitation, mar (i.e. shallow defects), scratches or indentations can be formed, which impact the coating's appearance and can shatter the perception of the whole product. In this work, formulations based on different monomer-oligomer couples were prepared and photo polymerized to investigate their performances. First, the glass transition temperature and the crosslinking density were determined to understand the contribution of both components in the polymeric network formed upon UV-curing. Then, hardness, and abrasion, scratch and wear resistances were studied. The results showcased the importance of the monomer and oligomer structure, functionality and main physical properties. In scratch experiments, the hard and brittle coatings tend to display failures at lower loads than soft and ductile ones. The friction experiments also caused subsurface tearing and fissuring in the soft coatings and generated several fractures in the harder ones. Furthermore, the best overall mechanical resistance was obtained for the couples with high crosslinking density, and a correlation was found between the crosslinking density and the hardness of UV-cured coatings. Interestingly, the investigation of tracks after scratch and wear experiments revealed a significant amount of information about the coatings' behavior under various mechanical loads.