With the rapid development of engineered wood products such as laminated veneer lumber, glulam and oriented strand board (OSB), high-value utilization of fast-growing wood has obtained an important technical approach. Among them, OSB exhibits broad application prospects in building structures, furniture manufacturing and packaging materials by virtue of its advantages including light weight, high strength, good dimensional stability and accessible raw materials. However, restricted by production cost and raw material supply, OSB production in China mainly relies on fast-growing woods such as pine, Chinese fir and poplar. These fast-growing timbers generally have relatively loose texture and low mechanical strength. Even with production equipment, processes and adhesive systems similar to imported products, the overall mechanical performance of domestic OSB still has a certain gap compared with imported counterparts. Therefore, how to effectively improve the load-bearing performance of OSB without significantly increasing production costs has become one of the key issues to be urgently solved to promote the technological upgrading of China's wood-based panel industry.

In the manufacturing process of oriented strand board, the bonding performance of large wood strands is the core factor determining the final mechanical strength of the board. Wood bonding is essentially a multi-scale physicochemical process, in which effective wetting of wood surface by adhesive (i.e. close molecular-level contact formed between adhesive and wood) and improvement of chemical reactivity on wood surface are prerequisites for efficient bonding. Nevertheless, the surfaces of fast-growing wood strands tend to be passivated due to contamination by dust and oil during mechanical processing. Meanwhile, phenol-formaldehyde resin (PF) adhesive has high surface tension, which often leads to poor wettability of adhesive on strand surfaces and greatly weakens the interfacial bonding strength, ultimately limiting the overall mechanical performance of oriented strand board. To address the above problems, researchers have explored various technical methods to enhance the wood-adhesive interfacial bonding, including surface heat treatment, thermomechanical treatment, chemical modification and their combined processes. However, these treatment technologies face many practical difficulties when applied to continuously mass-produced engineered materials such as OSB. Thermomechanical treatment and heat treatment are usually carried out at high temperatures, which not only consume massive energy but also require long treatment cycles, significantly raising production costs. Chemical modification involves harsh reaction conditions and chemical reagents, which easily cause environmental pollution and are hard to adapt to continuous industrial production lines.

Plasma Treatment Technology

As an emerging green surface treatment technology, atmospheric plasma treatment provides an efficient, mild and vacuum-free route to regulate the surface and interfacial properties of wood materials. Existing studies have shown that this technology can markedly improve wood surface wettability and raise surface free energy, thereby enhancing interfacial bonding strength. Its mechanism of action lies in that high-energy active species in plasma can simultaneously trigger physical etching and chemical modification on wood surfaces, forming an active surface layer with micro-nano characteristics in situ, and this process will not damage the wood matrix structure and its overall mechanical properties.

Effect of Atmospheric Plasma Treatment Process on Adhesive Spreading

Figure 1 shows the measured initial contact angle and equilibrium contact angle of PF resin adhesive on the surfaces of untreated radiata pine large strands and strands treated under different plasma parameters. The initial contact angle of adhesive liquid on untreated strand surfaces is 136.3°, showing non-wetting characteristics. This phenomenon can be attributed to two aspects: on the one hand, strand surfaces are passivated by dust and oil contamination during mechanical processing; on the other hand, PF resin itself has high surface tension. Nevertheless, wood has porous and anisotropic features at the microscopic level. The adhesive gradually penetrates into wood during wetting, and the contact angle of untreated strands slowly decreases to an equilibrium value of 55.4° over time, presenting a certain degree of wettability. After plasma treatment, the initial contact angle and equilibrium contact angle on strand surfaces decrease to varying degrees under all treatment parameters. As treatment power and gas flow rate increase, the contact angle shows a trend of first decreasing and then rising. Under optimized parameters, the initial contact angle drops to 102.4° and the equilibrium contact angle falls to 24.7°. Compared with untreated strands, the initial contact angle and equilibrium contact angle are reduced by 33.9° and 30.7° respectively. This result confirms that the wettability of PF adhesive on strand surfaces is obviously improved after plasma treatment.

Figure 1 Effect of plasma treatment on PF adhesive contact angle on radiata pine strand surfaces

Mechanism of Plasma Action on Strand Surfaces

Figures 2a and 2b display the AFM three-dimensional topography and surface roughness statistics of untreated and plasma-treated strand fibers respectively. The surface of untreated fibers is relatively smooth, with Ra and Rq of 6.3 nm and 7.9 nm respectively. After plasma treatment, a large number of nano-scale serrated etch marks appear on fiber surfaces. Ra and Rq rise to 15.3 nm and 19.3 nm, increased by 142.9% and 144.3% compared with the untreated group. According to surface free energy test results, the dispersion component shows no significant change before and after treatment, indicating that the nano etch marks produced by plasma on strand surfaces make limited contribution to adhesive spreading. This is mainly because PF resin liquid (viscosity approx. 270 mPa·s) can hardly fully wet the nano-scale serrated grooves and thus fails to greatly increase the effective contact area. However, during hot pressing, the fluidity of adhesive improves with rising temperature. The adhesive can fully infiltrate into nano etch marks and form numerous "adhesive nail" mechanical interlocking nodes after curing, which makes an important contribution to improving interfacial bonding strength. In addition, plasma etching can expand the size of pits and other cell channels, further promoting adhesive penetration into wood.

a) Untreated; b) Plasma-treated     Figure 2 AFM images of large strand surfaces before and after plasma treatment

Figures 3a and 3b show the full XPS survey spectra of untreated and plasma-treated strands. Characteristic signal peaks at 285 eV and 533 eV correspond to C1s and O1s respectively, demonstrating that strand surfaces are mainly composed of C and O elements. Comparison reveals that after APPJ treatment, the O element content on strand surfaces increases from 26.8% to 33.8%, and the oxygen-to-carbon atomic ratio rises from 0.37 to 0.51, indicating oxidation occurs on strand surfaces upon APPJ treatment. High-resolution C1s spectra (Figures 3c, d) further reveal structural information of plasma-induced surface oxidation of strands. The characteristic peak at 284.8 eV corresponds to aliphatic carbon (C—C/C—H), while XPS signal peaks at 285.3 eV, 286.5 eV and 288.5 eV are associated with C—O, C=O/O—C—O and O—C=O respectively. Quantitative analysis of relative contents of various chemical states in C1s spectra is listed in Table 1. The results show that the relative content of aliphatic carbon (C—C/C—H) decreases by 22.6% after APPJ treatment, which indicates that high-energy particles in plasma cause obvious cleavage of C—C/C—H covalent bonds on strand surfaces. Meanwhile, the relative contents of C=O/O—C—O and O—C=O increase by 77.4% and 88.4% respectively, further verifying the remarkable oxidation effect of plasma treatment on strand surfaces. The oxygen after treatment is mainly introduced into the organic components of the strand surface layer in the forms of C=O/O—C—O and O—C=O.

Figure 3 XPS survey spectra and high-resolution C1s spectra of large strand surfaces before and after plasma treatment

Table 1 Effect of APPJ plasma treatment on relative content of functional groups on radiata pine strand surfaces

Combined with AFM and XPS analysis results, the modification mechanism of plasma treatment on strand surfaces can be summarized as the synergistic effect of physical etching and chemical modification. In terms of chemical modification, high-energy nitrogen-based and oxygen-based active species bombard the strand surfaces, breaking covalent bonds in cellulose and lignin and generating free radicals. These free radicals react with active oxygen in plasma and oxygen in the ambient environment, introducing oxygen-containing polar functional groups such as carbonyl and carboxyl groups onto strand surfaces. At the same time, bombardment by high-energy particles forms nano-scale rough structures on strand surfaces, enlarging the effective contact area between adhesive and wood, opening microscopic channels such as pits, and facilitating adhesive penetration into wood.