TURHAN ET AL .
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point in 2 h. Still, the WA of the panels with modified fibers was reduced by half after 24-h soaking. Therefore, not only do the slow kinetics but also the closure of the pits in the modified fibers play a crucial role. The reduc- tion in the number of open pores or capillary channels in the modified wood fibers limits the pathways through which water molecules can penetrate the fiber structure, thus lowering the overall water uptake. The effect of modification on the WA is more appar- ent in a short time, whereas the effect on TS becomes more significant for the TS at long soaking times. The panel with raw fibers shows about 30% swelling, while the swelling remains less than 10% in the modified fiber, suggesting their enhanced dimensional stability. 47 Since the swelling occurs across the panel thickness (opposite to the press direction) by repulsion of the individual wood fibers upon hydration, the reduced TS in the com- posites with the modified fibers implies an improvement in the internal bonding. The IBS of the panels increased from ≈ 0.27 MPa in panels with raw fibers to ≈ 0.89MPa in panels with 5% modified fibers due to the presence of carboxylic acid groups on the wood surface, leading to stronger interactions with the resin. Increasing the frac- tion of modified fibers from 5% to 10% caused a slight decrease in IBS (on average) likely due to inhomoge- neous mixing with the raw fibers. This also explains the minor deterioration of the WA and TS performance observed for these panels. Nevertheless, ≈ a 3.3-fold increase in IBS (Figure 6C) by incorporating just 5% mod- ified fibers is remarkable. IBS also promotes the densifi- cation of the panels (Figure 6D), resulting in a more tightly packed structure due to enhanced interaction with the UF resin. 48,49 The improved density observed in the MDF panels made from modified fibers in Figure 6D can be attributed to several factors. The nitric acid steam treatment introduces carboxylic acid groups onto the fibers, enhancing their hydrophilicity and improving their interaction with the adhesive. This results in better adhesive penetration, stronger bonding, and more effi- cient fiber alignment during pressing, which reduces void spaces and increases compaction, contributing to higher panel density. Additionally, the chemical modification reduces the fiber diameter, making them thinner and less rigid, which allows for greater compressibility under pressure. This reduces the volume occupied by the fibers due to reduced thickness, leading to denser MDF panels (see Figure S5). Together, these effects improved adhesive interaction, greater compressibility, and more efficient fiber packing. 36,49 Consequently, this denser structure offers more contact points between the fibers and the resin matrix, and effective load transfer between the fibers, thus resulting in greater resistance to deforma- tion and better overall mechanical performance. 48,49
Additionally, the modified wood panels demonstrate good performance compared with the requirements spec- ified in the EN 622-5:2006 (E) standard 50 for interior- grade particleboards. According to this standard, the IBS should reach a minimum of 0.65 N/mm 2 , and the accept- able TS after 24 h is 17%. Our modified fiber panels, how- ever, achieve an IBS close to 1 N/mm 2 and a TS of approximately %8, thus satisfying the standard require- ments for enhanced durability and stability in interior applications. Compared to values reported in the literature, 51 – 53 where panels made solely from pine fibers exhibit higher IBS and a reduction in both swelling and WA. These improvements highlight the effectiveness of our modifications in enhancing panel stability and resis- tance to moisture. The physical performance of the composite panels is influenced by the curing behavior of the UF resin. Figure 8 displays the characteristic DSC curves of UF resins, acquired at a heating rate of 10 C/min within the temperature range of 25 – 200 C. UF resins underwent an exothermic curing reaction attributed to the heat gener- ated from the polycondensation reaction involving the primary amino groups of free urea and the hydroxy- methyl groups ( CH 2 OH). 54 In the presence of raw fibers, the peak temperature of the curing of UF resin was decreased from 165.98 to 147.21 C. The decrease of the curing temperature continues further in the presence of the modified fibers and progressively decreases to 100.67, 89.31, and 75.97 C for 15-min, 30-min, and 60-min treated fibers, respectively (see Table 2). These findings suggest that the initiation of polymerization occurred more rapidly in the UF resin with modified fibers compared with unmodified fibers with two cross- linking mechanisms. One is that acidic pH will accelerate the condensation reaction (crosslinking of OH groups to form ether bridges in the UF resin). The addition of modified fibers to the UF resin accelerates the curing
FIGURE 8 Differential scanning calorimetry thermograms of UF resin and its mixtures with unmodified and 15-min, 30-min, and 60-min modified fibers. The arrows show the location of the peak points.
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