The wood bonding interface significantly affects the stress/strain transmission of wood engineering materials, therefore, modifying the wood bonding interface can effectively enhance the macroscopic mechanical properties of wood engineering materials. In recent years, efficient and controllable low-temperature plasma has been considered highly suitable for enhancing the mechanical properties of wood bonding interfaces and wood engineering materials. Low temperature plasma can be excited in air and contains various high-energy particles such as electrons, ions, atoms, and photons, which can cause the breaking or recombination of chemical bonds on the surface of wood. In addition, as listed in Table 1.1, compared with traditional wood surface modification techniques such as sanding and thermal modification, low-temperature plasma modification has the advantages of fast modification speed, high efficiency, and environmentally friendly modification process, which is very in line with the demand for green and sustainable development in society. Research has shown that low-temperature plasma modification can cause surface etching and oxidation of wood, both of which can significantly increase the surface free energy of wood and improve the wettability and permeability of coatings or adhesives on its surface.

Etching of Wood by Low Temperature Plasma

The etching effect of low-temperature plasma on the surface of wood can remove weak boundary layers, improve surface roughness, increase the specific surface area of wood, and improve the wettability of adhesives and coatings on wood surfaces. As shown in Figure 1.11, researchers found that the etching effect of low-temperature plasma modification can reduce the thickness of wood cell walls. After modification, pits of different sizes will appear on the secondary walls of wood cell walls. This is due to the degradation of aromatic and fatty polymers in the wood cell walls during the low-temperature plasma modification process. The reduction of cell wall thickness and the appearance of concave pores are beneficial for promoting the penetration and filling of adhesives in the cell wall, and forming cross-linked structures with macromolecules in the cell wall, enhancing the mechanical engagement between adhesives and wood, thereby successfully improving the mechanical properties of the cell wall within the bonding interface. In addition, the research results indicate that almost all low-temperature plasma modification methods can form etching on the surface of wood, increase the contact area of the adhesive wood surface, and successfully enhance the bonding strength of the interface.

Figure 1.11: Cell walls of early wood (a, c) and late wood (b, d) before and after low-temperature plasma modification under scanning electron microscopy

Oxidation of Wood by Low Temperature Plasma

The quantity and type of oxygen-containing functional groups on the surface of wooden veneer affect its surface wettability, which is of great significance for the bonding performance of wood engineering materials. Research has shown that active oxygen-containing groups (mainly including hydroxyl, carboxyl, and carbonyl groups) form hydrogen bonds with water molecules, promoting the wetting of water and water dispersed macromolecules on the material surface. As shown in Figure 1.12, after low-temperature plasma modification, a large number of negatively charged oxygen-containing groups (including hydroxyl groups, ether bonds, and C=O) appeared on the surface of wheat straw, thereby increasing the polarity force of the veneer surface and promoting the spreading of adhesive on its surface. In addition, the oxygen-containing groups successfully introduced through low-temperature plasma modification may also form covalent bonds with the large molecules of the adhesive, which is beneficial for promoting the wettability of the adhesive on its surface and improving the bonding strength. The study on the surface mechanism of low-temperature plasma oxidation of wood materials confirms that high-energy particles inside the low-temperature plasma can generate a large amount of free radicals on the surface of wood, including highly reactive oxygen-containing free radicals. The above research results confirm that the oxidation of low-temperature plasma modified wood materials is mainly completed through free radical reactions, and the relevant reaction sites mainly appear in weaker chemical bonds within the wood macromolecule structure, such as cellulose β - glycosidic bonds and lignin β - O-4 structure C β - O covalent bonds.

Figure 1.12 Measurement scans and high-resolution XPS spectra of untreated wheat straw and plasma treated wheat straw

Low temperature plasma modification can oxidize and etch the surface of wood, increasing the relative content of polar oxygen-containing functional groups (including hydroxyl, carbonyl, and carboxyl groups) on the modified wood surface. This can promote the penetration of adhesives into the wood cell cavity and cell wall, construct a more robust interpenetrating polymer network structure, and enhance the static properties of the cell wall at the bonding interface of modified wood.