TURHAN ET AL .
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unmodified wood panel to 0.89 MPa in the wood panel treated with 5% CA, highlighting a 3.3-times increase com- pared with the untreated panel and other similar studies. In terms of water uptake over 2 h, the unmodified wood fiber panel absorbed 30.38%, while the modified panels treated with 5% CA and 10% CA absorbed only 3.46% and 5.48%, respectively. Most importantly, the curing tempera- ture of the resin can be decreased by ≈ 50 C without sacrificing the mechanical performance of the composites, which can lead to significant energy savings. Furthermore, the modification of wood fibers also contributed to a reduction in formaldehyde emissions. Panels made from unmodified fibers exhibited a formaldehyde emission of 11.40 ± 0.57 mg/100 g, while panels incorporating 10% CA-modified fibers showed a slightly reduced formalde- hyde emission of 9.71 ± 0.021 mg/100 g, corresponding to a reduction of approximately 14.82%. The unique properties achieved through our carboxylation process, alongside the promising performance results, suggest that this modified wood fiber approach holds strong potential for enhancing IBS and dimensional stability in wood composites.
pentahydropyridine oxide (TEMPO) approach has been widely used to selectively oxidize the primary hydroxyl groups ( CH 2 OH) on the cellulose chains to carboxylic acid groups ( COOH). The methods target primarily the cellulose components with a small effect on lignin and hemicellulose, requiring many pretreatment procedures for cellulose extraction from untreated biomass. 25 – 28 TEMPO is costly and poses economic challenges for large-scale industrial use due to its multistep process and reliance on radical-generating chemicals like TEMPO reagents, sodium hypochlorite, and bromide, involved in the process. Additionally, the excess chemical use gener- ates significant amounts of waste byproducts, thus limit- ing its sustainability. In this work, we focus on the surface oxidation of wood fibers through a steam oxidative treatment, particu- larly employing nitric acid, to enhance the adhesion and interfacial properties within wood fiber-reinforced resin composites. Unlike the traditional methods, this approach offers a practical and cost-effective solution to improve interfacial wettability without compromising the mechanical integrity of wood fibers. The nitro-oxidation process (NOP) has been used to isolate cellulose nanofibers (CNF) from various biomass sources, simplifying wood pulping and oxidative modifi- cation of cellulose by using only nitric acid and sodium nitrite. 29 – 31 A gas-phase NO ₂ treatment further oxidizes delignified fibers, achieving high carboxylic acid group concentrations (1.31 – 1.45 mmol/g), comparable to TEMPO-mediated oxidation. 32 This process enables notably oxygen-containing functional groups such as hydroxyl, carbonyl, ketones, and carboxylic acids. The key distinction of our study from other carboxylation pro- cesses lies in achieving comparable and enhanced wood composite properties, such as increased internal bonding strength (IBS) and dimensional stability. This was achieved through a straightforward nitric acid modifica- tion method applied directly to wood fibers. This approach also reduces curing temperature and energy demands, set- ting our method apart from traditional, more complex, and multistep oxidation techniques. Using multiscale char- acterization techniques, our study shows that using the straightforward and one-step nitric acid steam method can be an alternative to conventional oxidation methods by directly forming carboxylated (CA'ed) wood fibers and reducing environmental impact by utilizing a controlled, closed system that captures NO ₂ fumes, thereby minimiz- ing environmental and exposure risks. The treatment influenced the effect of treatment on the morphology, crystallinity, and surface features of individual wood fibers, which reflect on the bulk mechanical properties of the wood composites. The internal bond strength (IBS) of the wood was significantly improved from 0.27 MPa in the
2 | EXPERIMENTAL METHODS
2.1 | Materials
The red pine wood fibers used in this study were supplied by Kastamonu Integrated Wood Industry (KEAS, _ Istanbul, Turkey). The experimental chemicals included nitric acid (HNO 3 , 65 wt%), (EMSURE ® analytical grade) were purchased from (Sigma-Aldrich Chemie, Germany) for chemical modifications. The UF resins (1.22 M) widely used in particleboard production as binding agents were provided by KEAS ( _ Istanbul, Turkey). The preparation of laboratory-scale particleboards, as well as testing for mechanical properties and formaldehyde emis- sions, was carried out using the advanced facilities avail- able at KEAS ( _ Istanbul, Turkey). For formaldehyde emission tests, toluene ( ≥ 99.9% purity, GC, EMSURE ® ), acetylacetone ( ≥ 99.0% purity, EMSURE ® analytical grade) and ammonium acetate ( ≥ 98.0%, ACS reagent grade, Sigma-Aldrich) were used as reagents.
2.2 | Chemical treatment of wood fibers
Carboxylation process was carried out on wood fibers using the following procedure. Initially, 200 mL of nitric acid underwent thermal decomposition at a tem- perature of 130 C in a round bottom flask placed in an oil bath, and then the formed gas was passed through the
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