Abstract
Changes in surface material characteristics can significantly affect the adhesion and overall life of coatings on wood. In order to increase the durability of transparent exterior coatings, it is possible to use the surface modification of wood with UV-stabilizing substances. In this work, selected types of surface modifications using benzotriazoles, HALS, ZnO and TiO2 nanoparticles, and their combinations were applied to oak wood (Quercus robur, L.). On such modified surfaces, the surface free energy, roughness, and contact wetting angle with three selected types of exterior transparent coatings were subsequently determined. An oil-based coating, waterborne acrylic thick layer coating, and thin-layer synthetic coating were tested and interaction with the aforementioned surface modifications was investigated after 6 weeks of accelerated artificial weathering. The results of changes in the initially measured surface characteristics of the modified oak wood were compared to the real results of degradation of coatings after artificial accelerated weathering. The positive effect of surface modification, in particular the mixture of benzotriazoles, HALS, and ZnO nanoparticles on all kinds of coatings was proven, and the best results were observed in thick-film waterborne acrylic coating. The changes in the measured surface characteristics corresponded to the observed durability of the coatings only when measured by wetting using drops of the tested coatings.
Highlights
Changes in the surface characteristics of wood can significantly affect the adhesion of coatings and their overall durability during exterior weathering [1,2,3]
The results show that modification of the underlying wood is a promising option for increasing the durability of transparent exterior coating systems [13,67]
Our work confirmed the positive effect of surface modifications using UV-stabilizing agents even on oak wood with a high content of extractives [68] and strongly inhomogeneous morphological structure [39], which lead to faster degradation of exterior coatings compared to other kinds of underlying wood [15,40]
Summary
Changes in the surface characteristics of wood can significantly affect the adhesion of coatings and their overall durability during exterior weathering [1,2,3]. More work has been devoted to the determination of wood surface characteristics, the contact angle of wetting, and surface free energy, all of which affect adsorption and the adhesion of coatings to wood [25,26,27,28] These surface characteristics are changed by the surface modification of the underlying wood species, and by its aging during exposition [4,29,30,31]. The adhesion of wood transparent coatings during exterior exposure and their total durability is strongly influenced by the decomposition of lignin and extracts due to the UV and visible (VIS) spectrum of sunlight penetrating these types of coatings [11,33,34]. For its confirmation before use in practice, it is possible to use accelerated weathering tests in UV-chambers or Xenotests, which can be confirmed by long-term multi-year tests of natural weathering in an exterior [42,43,44,45]
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