Hydrophobicity Improvement on Wood for a Better Application of This Bio-Based Material

The over-exploitation of fossil fuels and increasing global industrialization has precipitated the release of greenhouse gases, resulting in an increase in global temperature [1]. Rapid urbanization in China has caused more than 25% of the world’s carbon dioxide emissions. China bears a great responsibility to limit global warming to 2 °C, together with the USA and the EU. Therefore, the Chinese government has launched several plans to achieve carbon neutrality before 2060 [2]. The high-value utilization of bio-based materials is an important and promising method of reaching this target. It avoids the over-exploitation of fossil resources and does not cause environmental issues [3,4,5]. As a sustainable, bio-based material, wood in particular is widely used in daily life, such as in outdoor construction and interior decoration. However, natural wood adsorbs moisture or water more easily because of its hydrophilic property. Alongside changes in environmental humidity and temperature, wood shows anisotropic swelling or shrinkage. These changes can lead to deformation, warping and biological degradation [6,7,8]. The modification of wood can effectively help overcome these defects and endow wood with high performances and good qualities. As a result, the use of this technology could further expand the application of bio-based materials.

Traditional modification technologies with physical or chemical strategies can clearly improve the dimensional stability and hydrophobicity of wood [9,10]. Among these approaches, paraffin emulsion impregnation technology is widely studied and has attracts much attention. Paraffin emulsion is a dispersion prepared by dispersing hydrophobic paraffin in water with nano-micelles. The impregnation of paraffin in wood can significantly reduce water absorption. Additionally, a higher content level of paraffin retained in treated wood can result in better hydrophobicity and dimensional stability. Moreover, its decay resistance and photostability could be improved accordingly [11,12]. Most importantly, only 2.5 wt. % paraffin emulsion impregnation can clearly improve the hydrophobicity of wood [13]. In previous studies, it was shown that mechanical properties can also be remarkably enhanced [14]. In addition to paraffin, other waxes are applied to deposit hydrophobic surfaces in wood, including beeswax and carnauba wax. However, owing its higher melting point compared to others, it is difficult to commercially produce carnauba wax. Carnauba wax is used in leathers, cars and floor polishing by depositing stiff coatings. Among all types of commercially available wax, paraffin is most commonly used as a waterproofing agent in wood industries. However, its chemical inertness means that natural paraffin cannot produce chemical reactions to form fully hydrophobic coatings in wood and avoid the negative effects of water. Physical adhesion in porous wood was determined to provide hydrophobicity [15,16].

Polyacrylate emulsion is widely used in adhesives, impregnated papers and wood industries [17]. As a chemical agent for wood modification, it is produced via monomer polymerization, such as esters of acrylic or methacrylic acid. Radical polymerization produces various polymers that can prevent water penetration in porous wood. Multifunctional monomers’ impregnation can form cross-linked networks in wood to decrease the concentration of hydroxyl groups, resulting in improved hydrophobicity, decay resistance and mechanical properties [18]. For instance, the introduction of methyl methacrylate and styrene into wood and subsequent catalyst thermal treatment could improve dimensional stability and decay resistance [19]. In wood impregnated with methyl methacrylate monomers, improvements in water repellency and resistance against biodegradation could be detected [20,21]. In addition, polyacrylate emulsion could be combined with nano-alumina dispersion to deposit hydrophobic coatings in wood. By coupling polyacrylate emulsion and a 1.5 wt. % nanoparticle dispersion for wood treatment, the hydrophobic surface showed was significantly enhanced in terms of abrasion resistance and hardness [22].

Nevertheless, most chemical strategies show potential threats to humans and the environment owing to their toxicity and harmful effects. Sustainable approaches are needed for wood modification to keep up with the social demand [23]. Due to the nontoxicity of the process, thermal modification has been extensively studied to improve wood properties. Dating back to 1915, it was reported that wood hygroscopicity could be reduced after being heated in superheated steam at 150 °C [24]. In the 1970s, 1980s and 1990s, research and renewed interest promoted the commercialization of thermally treated wood in France, Finland, Germany and the Netherlands [25,26,27]. The commercial production of thermally treated wood is usually carried out at 160–240 °C [28]. Improvements in dimensional stability, biological durability and anti-weathering were then obtained, along with changes in physical and chemical characteristics [29]. Interestingly, paraffin or wax impregnation in thermally treated wood could further improve the properties of hydrophobicity and anti-weathering. Additionally, surface hydrophobicity and mechanical properties could be improved by introducing melamine–urea–formaldehyde polymer in thermally treated wood [30]. However, this contradicts its eco-friendly applications to some extent. Silification, as another eco-friendly approach for wood modification, is inspired by petrified wood naturally formed in burial wood over millions of years. With the help of sol–gel technology, silica-sol-impregnated wood showed satisfactory properties, such as improved dimensional stability, stiff surface hardness, enhanced thermal stability and decay resistance [31]. Improvements in the abovementioned properties are mainly attributed to the penetration of nano-sized sol into hierarchical pores of wood, which is very important for the preparation of high-performance wood. Additionally, the stiff cross-linked networks formed on inner surfaces, or even the formation of chemical reactions within cell walls, could remarkably improve the hydrophobicity, thermal stability, dimensional stability and mechanical strength of wood [32].

Last but not least, moisture or water have significant roles in the performance of wood during applications. This is because moisture affects material properties, such as dimensional stability, decomposition resistance and mechanical strength. The question of how to design modification techniques and processes to produce high-performance wood or wood-based composites for better applications in a sustainable way must be discussed. On the other hand, a good understanding of the interaction between moisture and wood, as well as the reaction in the wood matrix, could provide novel insight into how to further improve the performance of moisture-induced materials by altering the swelling of cell walls. Namely, it is necessary to understand the mechanism of modification processes to know how to control and predict the moisture-induced deformation of wood.