Document Type : Research Paper
Authors
Department of Wood and Paper Science and Technology, Faculty of Natural Resources, College of Agriculture and Natural Resources, University of Tehran, Karaj, Iran
Abstract
stability, and resistance to accelerated aging.Background and Objectives: Surface coating plays a crucial role in enhancing the durability, dimensional stability, and environmental resistance of wood. In this context, water-based polyurethane (WBPU) coatings have attracted increasing attention as sustainable alternatives to solvent-based systems due to their lower VOC emissions and satisfactory mechanical and aesthetic performance. However, the functional efficiency of these coatings is strongly influenced by the application method and, in particular, by coating thickness. Despite its significance, systematic investigations on the effect of coating thickness especially when applied using advanced techniques such as spin coating under aging conditions remain limited. The present study introduces spin coating as a novel approach for wood finishing and evaluates the influence of different WBPU coating thicknesses on the surface and mechanical performance of beech wood (Fagus orientalis). The main objective is to determine an optimal coating thickness that provides a balanced performance in terms of hydrophobicity, adhesion, visual stability, and resistance to accelerated aging.
Materials and Methods: Beech wood specimens were coated using a commercial acrylic-based WBPU applied via spin coating at four different application levels (3, 6, 9, and 12 coating cycles). Each cycle involved a fixed spin speed of 2000 rpm and a duration of 30 seconds. The thickness of the resulting films was measured through stereomicroscopy and image analysis. A comprehensive set of physical and surface tests was conducted both before and after artificial aging, including water absorption (ASTM D5795), surface roughness (SJ-201P profilometer), dynamic water contact angle (ASTM D5946), color variation (CIE-Lab, ΔE), tensile pull-off adhesion strength (ASTM D4541), and scratch adhesion (ASTM D3359). The accelerated aging process simulated environmental degradation through a humidity-temperature chamber, allowing for the assessment of coating durability over time. In addition, multi-objective optimization using Minitab’s Response Optimizer was employed to identify the film thickness that offered the best overall functional performance.
Results: Results showed a nearly linear increase in film thickness from 75 µm (3 cycles) to 286 µm (12 cycles). Thicker coatings demonstrated lower water absorption and higher contact angles exceeding 90° indicating enhanced hydrophobic properties. After aging, thicker films also showed reduced color change (ΔE), with the 9-cycle treatment presenting the best balance of aesthetic and mechanical attributes. This treatment exhibited the highest tensile adhesion strength (2.99 MPa) before aging, and it retained significant adhesion after aging, suggesting strong interfacial bonding and structural integrity. In contrast, the 12-cycle treatment, while superior in terms of water resistance, showed a noticeable drop in adhesion, potentially due to internal stresses or micro-cracks caused by excessive film thickness. Scratch adhesion remained high (5B) in all treatments before aging, but minor deterioration was observed in thicker coatings after aging.
The multi-criteria optimization revealed that a film thickness of approximately 206 µm corresponding closely to the 9-cycle treatment provided the highest composite desirability score (0.94) across all evaluated parameters. This result confirms that increasing thickness beyond an optimal range does not necessarily improve overall performance and may instead compromise adhesion or visual consistency due to mechanical stress accumulation during drying or exposure.
Conclusion: In conclusion, spin coating is demonstrated to be an effective and innovative method for applying uniform, controllable WBPU coatings on wooden substrates. This study confirms that coating thickness is a pivotal factor influencing water resistance, surface quality, color stability, and adhesion. An intermediate thickness (210 µm) offers the most favorable combination of performance attributes, making it suitable for practical use in interior wood products, furniture, and protective applications. The research supports the integration of spin coating into sustainable wood finishing systems and provides a foundation for further exploration of advanced coating technologies in the wood science field.
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