Advances in Surface Modification and Coatings of Wood and Wood Composites

Wood remains a widely utilized material across numerous industrial sectors despite several inherent drawbacks, including dimensional instability, susceptibility to biological degradation, and flammability. One promising strategy to overcome these limitations and thereby broaden the range of wood applications is wood modification. A variety of modification techniques have been developed [1] and continue to evolve, with increasing emphasis on environmentally sustainable approaches employing non-toxic chemicals. Current research directions include thermo-mechanical modification, plasma treatment, surface functionalization using novel polymers and nanomaterials, and the development of antifungal and fire-retardant surface treatments [2,3,4,5,6]. Another important field is wood finishing [7], which aims to protect wood from environmental factors, preserve its aesthetic qualities, and enhance its visual appeal. Wood finishing represents a highly complex technological process that plays a pivotal role in determining the final performance and consumer acceptance of wood products. Recent innovations focus on eco-friendly coating systems, such as coatings incorporating nanoparticles, bio-based additives derived from cellulose, lignin, or other natural polymers, and formulations providing bio-protective or fire-retardant properties [7,8,9]. Surface preparation remains a critical determinant of coating adhesion and overall performance. However, modifications that enhance certain properties may simultaneously impair others, posing a significant challenge to achieving optimal balance. Consequently, a key research question persists: Can wood surface modification methods be developed that improve performance characteristics without compromising coatability, environmental compatibility, or economic feasibility? The Special Issue, “Advances in Surface Modification and Coatings of Wood and Wood Composites,” presents recent progress in the modification and coating of wood and wood-based materials. Special attention is devoted to the interactions between coatings and modified wood surfaces, as these interfacial phenomena critically influence finishing performance and overall material functionality.

Waterborne acrylic acid (WAA) resins, as key components of environmentally friendly coating systems, have been extensively studied for performance enhancement through modification. In the study by Wu et al. [10], they developed a facile approach to prepare high-performance WAA resin coatings modified with SiO₂ nanoparticles and evaluated their applicability on wood wallboards. The modification was achieved via a simple mechanical compounding method, in which nano-SiO₂ was incorporated into the WAA resin to improve its physicochemical properties, specifically hardness, gloss, adhesion, and abrasion resistance, while maintaining desirable surface appearance. Fourier-transform infrared (FT-IR) analysis confirmed that the incorporation of nano-SiO₂ did not alter the chemical structure of the WAA resin, indicating that no new functional groups were formed and the polymer backbone remained intact. Scanning electron microscopy (SEM) further revealed that nano-SiO₂ addition modified the dispersion characteristics of the WAA matrix, which directly influenced coating performance. Overall, the study highlights the potential of nano-SiO₂-modified WAA resin coatings as promising, formaldehyde-free, waterborne systems for wood wallboard applications, providing valuable insights for the design of next-generation eco-friendly coatings.

The demand for exterior wood siding in North America has stagnated, largely due to concerns regarding limited durability and high maintenance requirements. To address these challenges, Schorr et al. [11] investigated the modification of white spruce through organosilane and aluminum-based treatments, followed by thermal processing, to enhance dimensional stability and biological durability while maintaining the natural appearance of the wood. The results demonstrated that the treatments significantly improved dimensional stability by up to 50% and reduced the number of accessible hydroxyl groups by as much as 37%. The organosilane treatment exhibited strong resistance to leaching, whereas aluminum tended to leach over time. When combined, organosilane and aluminum treatments enhanced resistance to brown rot fungi, though the addition of aluminum did not further improve dimensional stability. Optimized treatments, particularly organosilanes combined with aluminum sulfate and a subsequent thermal treatment at 140 °C, effectively improved the stability of white spruce and jack pine and provided notable fungal resistance against three brown rot strains. Contrary to initial expectations, the inclusion of aluminum sulfate as a thermal catalyst did not enhance resistance to humidity or water immersion compared with organosilane treatment alone, despite a measurable reduction in hydroxyl availability. These findings demonstrate the potential of organosilane-based systems, particularly in combination with aluminum compounds, for improving the performance of Canadian softwoods; however, further investigation is warranted to better understand the mechanisms governing their stability and durability.

Bahrami et al. [12] conducted a comprehensive comparative study on untreated and surface-treated wooden façades of buildings, evaluating their durability, environmental impact, and cost efficiency through document and literature analyses supplemented with quantitative data. The findings revealed that, from a durability standpoint, painting a wooden façade is not always advantageous, as conventional coatings require regular maintenance to prevent degradation and high long-term costs. In contrast, Yakisugi, a traditional Japanese thermal modification technique involving surface charring to form a carbonized protective layer, demonstrated superior performance. Overall, the study concluded that Yakisugi represents the most suitable surface treatment for wooden façades, combining high durability, minimal environmental impact, and cost-effectiveness. The hydrophobic carbon layer produced during the charring process provides a stable, long-lasting barrier that resists moisture and biological attack while reducing maintenance demands. Furthermore, the researchers highlighted that wider adoption of Yakisugi and other unconventional, eco-friendly surface treatments could contribute substantially to sustainable building practices and provide distinctive architectural aesthetics.

To broaden the potential applications of wood and address its inherent flammability, the development and assessment of effective fire protection strategies are of paramount importance. Kmeťová et al. [13] investigated the fire resistance of spruce wood (Picea abies (L.) H. Karst) treated with an aqueous sodium silicate solution (water glass, WG) and various types of expandable graphite (EG) flakes applied to their surfaces. Seven treatment variants were examined, including samples treated solely with WG and those treated with WG combined with EG flakes from different suppliers. All treated samples exhibited substantially improved fire performance compared with untreated controls. Notably, none of the treated specimens ignited under the test conditions, confirming the synergistic fire-retardant interaction between WG and EG. The reduced surface temperatures observed in treated samples effectively limited heat propagation to adjacent materials, thereby mitigating the risk of fire spread. Overall, the study demonstrated that WG–EG combinations form effective, environmentally benign fire-reactive coatings, enhancing wood fire resistance through a synergistic intumescent mechanism. These findings underscore the potential of WG–EG systems as sustainable, high-performance fire protection treatments for wood used in architectural and construction applications. Future research should focus on optimizing formulation parameters and processing methods to further improve their protective efficiency, durability, and practical applicability.

In another study, Rubino et al. [14] developed and evaluated a novel mono-component polyurethane (PU) coating for wood, focusing on its performance on Ayous (Triplochiton scleroxylon K. Schum), a tropical species widely used in Europe for outdoor applications. Results revealed that thermal treatment at 215 °C significantly altered the surface and mechanical properties of the wood, rendering it more brittle and less resistant to outdoor exposure. The authors suggested that applying lower thermal treatment temperatures could better preserve the mechanical integrity of the material. The polyurethane coating effectively enhanced surface protection, increased wear resistance, and imparted hydrophobic properties to both untreated and heat-treated wood surfaces. For untreated Ayous, the coating minimized color variation after artificial aging, while heat-treated samples exhibited greater color change. The coating also reduced surface roughness, with the spray-applied coatings producing a more uniform and likely thicker protective film, which contributed to increased micro-hardness and abrasion resistance. However, aging led to an overall increase in surface roughness, particularly in uncoated samples. Overall, the study demonstrated that the mono-component PU coating offers short-term protective benefits for Ayous wood, particularly when applied by spraying. These findings provide practical insights into maintenance strategies for coated wooden surfaces and highlight the importance of optimizing both thermal treatment parameters and coating formulations to ensure durable, weather-resistant wood finishes. Future work should explore alternative coating products and lower-temperature thermal modifications to balance surface protection with the mechanical stability of Ayous wood.

The particleboard industry currently faces challenges in meeting market demand, prompting the incorporation of filler materials to conserve raw resources and enhance the performance of wood-based boards. Ergun et al. [15] investigated the effects of incorporating different ratios of activated carbon (AC) on the physical, mechanical, thermal, and aesthetic properties of particleboards. Results indicated that increasing AC content raised density and reduced moisture content, reflecting improved dimensional stability and water resistance. The color of the particleboards was also affected, with higher AC levels producing a darker appearance and a total color change (ΔE) of up to 7.72%, suggesting that aesthetic considerations should guide AC use in visible applications. Mechanical properties, such as internal bond strength (IB), modulus of rupture (MOR), and modulus of elasticity (MOE), were significantly enhanced with AC addition, indicating improved bonding and overall strength. The 7.5% AC boards showed improvements in IB, MOR, and MOE of approximately 41%, 28%, and 21%, respectively, compared to control particleboards. Thermal conductivity decreased with increasing AC content, demonstrating enhanced insulation performance. Overall, the results highlight that an optimal AC concentration of 4.5% provides a balanced enhancement of physical, mechanical, and thermal properties while minimizing aesthetic alterations. Particleboards containing 4.5% AC consistently exhibited superior performance across multiple criteria, demonstrating the potential of AC as a functional additive for producing high-performance, sustainable particleboards suitable for indoor applications.

The protective and decorative performance of wood-based boards can be significantly enhanced through appropriate surface treatment techniques, extending the service life of furniture. Slabejová et al. [16] investigated the effects of various coatings and thin foils on medium-density fiberboards (MDF), including transparent and pigmented polyurethane enamel finishes, water-borne polyurethane–acrylate finishes, solvent-borne polyacrylate finishes, finishes based on natural oil and waxes, and thin foils such as PVC, PET, and lacquered acryl films applied to raw or veneered MDF. The study demonstrated that interactions between the coating or foil and the substrate strongly influenced hardness and impact resistance, while the type of surface finish substantially affected resistance to environmental and mechanical stresses. Lacquered acryl films with high gloss and matt PVC foils were recommended for mechanically stressed vertical furniture surfaces, whereas transparent finishes on veneered MDF exhibited standard mechanical resistance comparable to thin foils on raw MDF. Transparent waterborne polyurethane–acrylate and wax-based finishes showed lower resistance to cold liquids and interior molds, making them unsuitable for kitchen and dining furniture. Pigmented coatings on veneered MDF were more prone to cracking under impact compared to raw MDF, suggesting that raw MDF with pigmented finishes is preferable for children’s furniture, with matte finishes providing superior mechanical resistance compared to gloss variants. Overall, the choice of surface finish and the substrate–coating interaction critically influence the functional, protective, and decorative value of MDF boards, highlighting the importance of tailored surface treatments in furniture applications.

Kúdela et al. [17] investigated the effects of CO₂ laser irradiation on spruce wood surfaces, focusing on mass loss, surface morphology, chemical changes, and discoloration. The applied energy, expressed as the total irradiation dose (H), was shown to strongly influence wood mass loss and surface characteristics. During irradiation, immediate destruction and sublimation of thin surface layers occurred, with mass loss varying according to the density differences between earlywood and latewood. Correspondingly, surface roughness increased, with the Ra and Rz parameters rising linearly with both irradiation dose and mass loss. Maximum irradiation produced a 6-fold increase in Ra parallel to the grain and a 33.5-fold increase perpendicular to the grain, reflecting the differential density effects of wood tissues. Color analysis revealed that even at the lowest laser power and raster density, the wood surface exhibited perceptible discoloration (ΔE* ≈ 9), which intensified with increasing irradiation (ΔE* ≈ 12), resulting in a surface color distinct from the original. Chemical analysis attributed the discoloration primarily to heat-induced cleavage of carbonyl (C=O) groups in lignin and hemicellulose, which altered chromophore content responsible for the natural wood color. These findings demonstrate a clear correlation between total irradiation dose, mass loss, surface roughness, and color change, providing a basis for controlled laser processing of spruce wood. By precisely adjusting laser parameters, targeted surface discoloration and high-quality engraving patterns can be achieved, enabling the customization of aesthetic and structural surface properties in wood-based applications.

Wood is a widely used construction material, valued for its mechanical performance and aesthetic qualities, yet it is susceptible to thermal degradation and fire. Enhancing its fire resistance through the application of flame retardants is therefore an important area of research. Kmeťová et al. [18] investigated the fire-retardant performance of expandable graphite (EG) in combination with water glass (WG) as a coating for spruce wood (Picea abies (L.) H. Karst), comparing it with two commercially available retardants: Bochemit Antiflash and Bochemit Pyro. Untreated spruce wood exhibited the highest mass loss, losing 74 ± 1% more weight than samples treated with EG + WG, which demonstrated the most effective fire-retardant performance across all evaluation criteria. Additionally, treatment with flame retardants significantly reduced the burning rate, while surface temperature measurements confirmed that untreated wood reached the highest temperatures during combustion. Despite its effectiveness, the application of EG presents certain challenges. Its performance depends on particle size, decomposition temperature, and other material properties, and it requires a bonding material that is adhesive, durable, non-toxic, and economically viable. Furthermore, surfaces treated with EG may have aesthetic limitations, necessitating additional finishing for certain applications. Overall, the study demonstrates that proper selection and application of flame retardants, particularly EG in combination with WG, can substantially improve the fire resistance of spruce wood. These findings underscore the importance of protective treatments for wood to maintain its versatility and safety in construction, particularly given the high susceptibility of untreated wood to thermal degradation.