Gerhard Grüll

Accurate prediction of moisture distributions in wood is among the most critical challenges in timber engineering. Achieving this requires a well-coordinated comparison of experimental methods and simulation tools. While significant progress has been made in developing simulation tools in recent years, a lack of experience with and trust in these tools continues to hinder broader implementation, especially when it comes to free water and its absorption. Investigations and model advancements have allowed for the simulation of increasingly complex cases, including one-dimensional moisture transport above the fiber saturation point (FSP) in coated boards and below FSP in coated glued laminated timber (GLT). However, free water flow in coated GLT beams has not yet been addressed, which can become problematic in case of extreme scenarios, such as water infiltration. In this study, we demonstrate that the multi-Fickian free water transport model developed by some of the authors can successfully simulate three-dimensional coated cases. Uncoated and coated boards and GLT members were subjected to cyclic wetting and drying, both experimentally and numerically. To simplify the calibration process of the mass transfer coefficient of free water—identified as the most significant parameter for the simulation of free water transport—experiments previously conducted by some of the authors were simulated. Based on the simulation results, approaches for an initial estimation of the mass transfer coefficient were developed. If the water uptake of coated specimens is measured three days after continuous soaking in water and the result exceeds a specific limit, the coefficient can be sufficiently predicted. The simulation and experimental results show a good agreement.

Due to their bio-based character, oil-based coatings become more and more prevalent in wood surface finishing. These coatings impart appealing optical and haptic properties to the wood surface, but lack sufficient protection against water and mechanical influences. The present study reports a simple green route to improve the performance of linseed oil coating by the addition of nanofibrillated cellulose (NFC). In order to achieve surface chemical compatibility with linseed oil, NFC was chemically modified with acetic anhydride and (2-dodecen-1-yl)succinic anhydride, respectively, using propylene carbonate as a solvent. NFC/linseed oil formulations were prepared and applied to wood substrates. The wear resistance of oil-coated wood surfaces was assessed by a newly developed test combining abrasive loading with subsequent contact angle measurement. As revealed by infrared and nuclear magnetic resonance (NMR) spectroscopy, as well as X-ray diffraction (XRD), NFC has been successfully modified without significantly affecting the structure of cellulose. In abrasion tests, all NFC-modified oil coatings performed better than the original oil. Interestingly, NFC only suspended in propylene carbonate, i.e., without chemical modification, had the strongest improvement effect on the coating’s wear resistance. This was primarily attributed to the loose network structure of this NFC variant which effectively prevents the oil from penetration into the wood surface, thus forming a protective NFC/oil composite layer on the wood surface.

Wood moisture content of coated panels of Scots pine sapwood was recorded during 24 months natural weathering in Vienna by logging electric resistance and temperature near the surface. Eight coating systems with various film thicknesses were used, including three solvent borne alkyd stains, three water borne acrylic stains and two water borne acrylic paints. At five sites in Europe wood moisture content of panels coated with three solvent borne alkyd stains, a brown acrylic stain and a white opaque acrylic paint was recorded weekly by changes in panel mass. Fluctuations in wood moisture content were influenced by the film thickness, water permeability and colour of the coating systems used. Degradation phenomena led to decreasing moisture protection of less durable coating systems over time of exposure. The exposure site clearly affected wood moisture content in the panels.