TURHAN ET AL .
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emissions within the E2 range (>10 mg/100 g ≤ 30 mg/100 g). Modification with CA fibers resulted in a reduction in formaldehyde emissions, bringing the panels closer to the E1 standard ( ≤ 10 mg/100 g), 68 while enhancing their mechanical and dimensional stability.
carboxylated fibers. The contrasting trends highlight the influence of fiber composition and curing conditions on the IBS and density of the panels. Figure 6 shows a posi- tive correlation between density and IBS, with crosslink- ing occurring not only between the resin matrix and the fibers but also between unmodified fibers (rich in hydroxyl groups) and carboxylated fibers (rich in car- boxylic acid group). This enhanced crosslinking in mix- tures leads to higher IBS values and greater density due to the more compact structure formed, improving mechanical strength. 64,65 In contrast, Figure 9 shows data for panels made entirely from 100% carboxylated fibers, where reduced hydroxyl groups limit crosslinking potential. The cross- linking primarily occurs between carboxylic acid groups on the fibers and the resin matrix, leading to a similar packing structure without significantly changing the den- sity. The reduced curing temperature of 150 C for the 100% carboxylated fibers also impairs the bonding effi- ciency, leading to lower IBS despite similar density. 46,66 In summary, while the curing temperature and fiber sur- face chemistry influence the IBS, panels with modified fibers maintain comparable IBS to unmodified fibers, offering improved water resistance and dimensional sta- bility. Furthermore, lowering the curing temperature in panel production without sacrificing the mechanical properties offers advantages in terms of energy consump- tion needed to heat the manufacturing equipment to the desired temperature as well as to cool the final products. Thus, not only does the catalyst-free modification process itself contribute toward sustainability but also the effects on interaction with the adhesive help minimize green- house gas emissions associated with energy production. In addition to these mechanical and dimensional improvements, the formaldehyde emission results reveal further benefits of the modifications. Panels made from unmodified fibers exhibit a formaldehyde emission of 11.40 ± 0.57 mg/100 g, while panels incorporating 10% CA-ed fibers show a slightly reduced formaldehyde emis- sion of 9.71 ± 0.021 mg/100 g, corresponding to a reduc- tion of approximately 14.82%. The reduction can be explained by the fact that CA modification may introduce functional groups, such as carboxylic groups, 56 which react with free formaldehyde and methylol groups, form- ing stable bonds due to extra crosslinking 57 and further reducing its release. 47,67 Additionally, the modified panels play a role in restricting the diffusion of formalde- hyde gas because a tighter fiber network can reduce free formaldehyde gas by effectively trapping it within the modified structure. These results are consistent with find- ings from previous investigations into UF resins and par- ticleboard production. 5,68 Typically, using a 1.22 molar ratio UF resin results in panels with formaldehyde
4 | CONCLUSIONS
In conclusion, our investigation into the carboxylic acid formation process through nitric acid gas treatment in a closed system minimizing environmental impact and health risks and then subsequent modification of wood fibers has provided valuable insights into the formation of robust covalent bonds between carboxylic acid- containing wood fibers and UF resin. This highlights the method's potential in enhancing the overall strength of wood fibers, with IBS increasing 3.3-fold (from 0.27 to 0.89 MPa) in panels treated with 5% car- boxylated (CA) fibers. Dimensional stability improved significantly, with WA decreasing from 30.38% in untreated panels to 3.46% in treated ones. Additionally, the curing temperature of the adhesive was lowered by 50 C without compromising composite strength, highlighting the practical benefits of this approach. The carboxylation process not only enhances the strength of the fibers but also introduces hydrophilic characteristics, so this provides an improvement in adhesion and also offers a solution to challenges asso- ciated with cost and energy consumption. The preci- sion of this novel method in targeting the surface chemistry of wood fibers contributes to improved adhe- sion and compatibility with synthetic resins. Addition- ally, the formaldehyde emissions from modified fiber panels (9.71 ± 0.021 mg/100 g) were lower than those from unmodified panels (11.40 ± 0.57 mg/100 g), highlighting the effectiveness of the modification pro- cess in reducing emissions and meeting emission standards. Overall, these modification techniques initiate new ways for further exploration and optimization of carbox- ylation processes. This approach not only aligns with sus- tainability goals by potentially reducing environmental impact but also advances the field of wood-polymer inter- actions, leading to improved performance and new uses for wood-based materials. ACKNOWLEDGMENTS This research is funded by TÜB _ ITAK under the 2244 Pro- gram (grant no: 119C160). The authors express their grat- itude to Dr. Bar ı s¸ Ya gc ı for his assistance with the SEM imaging. All authors have reviewed and approved the final manuscript.
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