Advanced Technologies in Physical and Mechanical Wood Modification

In recent years, research on wood modification, wood composites, and the use of renewable raw materials and plant industry by-products in materials engineering has grown dynamically. Sustainable development, the need to reduce chemical emissions, and the improvement of the functional properties of wood and wood-based materials are key challenges facing modern science and the wood industry.

The publications collected in this “Advanced Technologies in Physical and Mechanical Wood Modification” Special Issue address these challenges comprehensively and in an interdisciplinary manner. They contain the results of experimental and modeling studies on the impact of chemical, physical, and thermomechanical treatment on the properties of solid wood, composite materials, and biopolymers. Particular attention is paid to issues related to the following:

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Modification of wood using organic compounds such as furfuryl alcohol, which at low concentrations (10%) reduced total water vapour sorption by approx. 28% in poplar wood and 35% in Chinese fir [1]. In turn, the use of silane-modified linseed oil contributed to a 38.6% increase in dimensional stability and a reduction in water swelling of over 30% [2], which, according to the authors, should translate into increased resistance of wood to colonising and decomposing fungi.

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Waste biomass, such as tree bark or beet pulp, used to produce wood-based materials. It has been shown that in the case of plywood with 10% bark biomass added, an increase in shear strength at the glue line of approximately 18% was observed [3]. In the case of particleboard, beet pulp content of up to 25% may be acceptable for producing lower-density boards that meet the requirements of EN 312 for general-purpose P2 boards (used in dry conditions) [4].

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Improving biological durability and resistance to fungi. Betlej et al. [5] researched the effectiveness of ethanol extracts and essential oils obtained from various varieties of mint (Mentha sp.) in limiting the growth of mould fungi and wood discolouration. Ethanol extracts used at a dose of ≥40 g/m² showed vigorous fungistatic activity—mycelium growth inhibition reached up to 90% on agar medium. Although essential oils were more active (especially against Chaetomium globosum), they did not show a full biocidal effect on wood. The authors emphasise that the chemical composition of the extracts, mainly the presence of oxygenated monoterpenoids and monoterpenes, determines the strength of the biological effect, and the use of natural plant-based agents can be an ecological supplement to traditional wood protection methods, especially in the early stages of their use.

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Analysis of wood’s mechanical, sorption, and structural properties after modification, e.g., with phenol–formaldehyde resins [6,7,8]. Lang et al. [6] investigated the effect of the molecular weight of phenol–formaldehyde (PF) resin oligomers on the mechanical properties and dimensional stability of beech wood impregnated with these resins. The results showed that resins with a lower molecular weight (approx. 237–305 g/mol) provide better penetration and more uniform filling of cell walls. This translates into significantly increased dimensional stability (less shrinkage and swelling) and greater weight gain (WPG), reaching 24.7% at a 20% solution concentration. The authors showed that the observed decrease in modulus of elasticity (MOE) and impact resistance (IBS)—by as much as 60–64% compared to unmodified wood—is a consequence of the formation of a rigid, crystalline PF structure in the cell wall, which limits the plasticity of the material. Microscopic studies confirmed that low-molecular-weight resins penetrate cell walls better, while those with a higher molecular weight are mainly deposited in the lumen of the vessels. In another publication [7], the same authors showed that thanks to PF wood modification, water vapour sorption kinetics decreased by 25–30%.

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The use of thermomechanical modification. It has been shown [9] that thermomechanical treatment of pine wood significantly increases its density (35%), stiffness, bending strength (47%), and resistance to cracking, making it a more valuable construction material. In addition, long-term deformation (creep) is significantly reduced, improving structural applications’ durability and reliability. Furthermore, by controlling the process parameters (pressure, temperature, and degree of compression), the characteristics of the final product can be adjusted. In summary, the authors state that TMD is an effective way to modify low-grade wood, making it suitable for more demanding engineering applications.

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Innovative structural solutions, such as the use of metal dampers in tenon and mortise joints, which increased the shear strength of the joints by 20–25% and improved the behaviour of the structure under cyclic dynamic loads [10]. The authors researched using these innovative dampers in traditional tenon and mortise joints in wooden structures. The results of quasi-static experiments showed that using dampers made of Q235 steel resulted in a significant increase in stiffness, load-bearing capacity, and energy absorption by the joints. The reinforced joints showed more stable and symmetrical hysteresis curves and reduced residual displacements, which means less play and better behaviour under cyclic loads. As the researchers point out, this technology can be used to reinforce new wooden structures and protect historic buildings without compromising their aesthetic and structural integrity. The dampers are discreetly mounted on the side surfaces.

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Using numerical and spectroscopic methods. Molecular dynamics simulations have shown that the oxygen content in the cellulose environment has a significant impact on hydrogen bonding with water, which may affect the hygroscopicity and stability of cellulose materials [11]. Guo et al. [11] conducted molecular dynamics (MD) studies to evaluate the effect of oxygen concentration on the mechanical properties and structure of cellulose. The study analysed models with 0–10% oxygen content, reflecting the conditions of steam heat treatment of wood. It was shown that at an oxygen concentration of about 2%, the highest values of Young’s and shear modulus are obtained, indicating greater stiffness of cellulose. Higher concentrations led to the degradation of internal hydrogen bonds, increased water diffusion, and greater mobility of cellulose chains, resulting in reduced mechanical properties. The study results provide theoretical support for the design of thermal–steam treatment processes at the molecular level, with the possibility of optimising conditions to achieve better wood stiffness and material stability.

One of the essential aspects of the research is its environmental benefits. Modifying wood using natural agents—such as vegetable oils, bioadhesives, or natural compounds—reduces the use of toxic chemicals, including traditional preservatives and formaldehyde-based adhesives [2,12]. Research on the use of bio-based adhesives, based on natural raw materials, especially starch, in the production of HDF boards has shown that they can provide parameters comparable to conventional synthetic adhesives [12]. In addition, using agricultural and food industry by-products as additives to wood-based materials supports the circular economy by reducing waste and improving resource efficiency [3,4]. Using such solutions promotes the creation of materials with a smaller carbon footprint and greater durability, which in the long term leads to a reduction in the exploitation of forest resources.

A key result of the research published in this SI is a significant increase in the durability of wood and wood-based materials. The use of appropriate modification methods—such as chemical impregnation, heat treatment, or the use of bioactive additives—contributes to improving the resistance of wood to biological factors (fungi, insects), moisture, and changing weather conditions [1,5,13]. The increases the service life of the materials, affecting their suitability for structural and outdoor applications and reducing the need for frequent replacement or maintenance, thereby reducing costs and the environmental footprint of use.

The practical applications of the developed solutions cover a wide range of industries. In construction, modified wood is used as a structural element, as a facade element, or in window and door joinery [9,14], characterised by increased durability and resistance to external conditions. In the furniture and finishing industries, improving wood’s mechanical and aesthetic properties allows for the production of durable and environmentally friendly products [12]. In the packaging and transport sector, biodegradable wood composites and materials reinforced with natural fibres and bioadhesives are being developed [15]. Finally, using agricultural waste as additives to wood-based materials opens up new opportunities for agriculture and the food industry, promoting local processing and regional innovation [4].

It is also worth noting that wood used in the timber industry increasingly comes from trees weakened or dying due to environmental stress, disease, or climate change. As shown by research on Scots pine (Pinus sylvestris L.) wood originating from deadwood [16], the process of biological dieback significantly affects its mechanical properties. The reduction in the strength parameters of such raw material may hinder its direct use, creating the need for appropriate physical, chemical, or structural modification before such wood is approved for structural or composite use. Wood modification may therefore prove to be not only a way to improve the quality of the material in the future, but also a requirement for the effective use of raw material with reduced technical value.

Modern wood modification methods also include advanced artificial intelligence algorithms that allow for precise modeling of changes in wood properties, e.g., colour due to heat treatment [17]. The high accuracy of such models indicates their potential in predicting the effects of technological processes and optimising production parameters. In turn, biological approaches using fungal inoculation and controlled mechanical damage allow for the activation of trees’ natural defence mechanisms and the stimulation of the production of desired secondary metabolites, such as sesquiterpenes [18]. Such approaches are critical in producing high-value aromatic raw materials like agarwood. These types of approaches are focused on the intentional design of wood properties.

The research presented in this SI deepens knowledge in the field of wood technology and materials science and responds to the real needs of the wood, furniture, and construction industries. The results can be applied both in developing new material production technologies and in designing modern, durable, and environmentally friendly wood products.