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TURHAN ET AL .
TABLE 2 The comparison of T onset , T endset , and T peak of UF resin and its mixtures with unmodified and 15, 30, and 60 min modified fibers.
30min modified fiber-UF
15min modified fiber-UF
60min modified fiber-UF
UF only
Raw fiber-UF
T onset
132.84 110.09
63.28
49.89
46.43
186.35 179.86
127.47
118.23
111.13
T endset
T peak
165.98 147.21
100.67
89.31
75.97
initiation. This change can be related to the presence of carboxylic acid groups on the surface of the wood fibers, which can facilitate the crosslinking of the resin mole- cules due to the pH of the reaction environment. How- ever, excessive acidity can cause the bonds to break down over time due to moisture. 55 This breakdown mainly occurs at the interface between the adhesive and wood fibers, resulting from decomposition, swelling, or shrink- ing. Consequently, the methylene linkages within the cured resins begin to degrade. This is another reason (apart from the structural breakdown and reopening of the pits) for not using 30-min and 60-min treated fibers in the panel production. Another way of having cross- linking is the reaction between the carboxylic acid groups of the fibers and the OH groups of the resin to form ester bonds 47,56,57 (see Figure 7). When carboxylated fibers are incorporated into a resin matrix containing hydroxyl-functionalized components, such as UF resins, esterification occurs. This chemical reaction involves the condensation of COOH and OH groups to form ester bonds ( COO ), which act as crosslinks within the com- posite. 58,59 This reaction enhances mechanical strength and composite properties. However, excessive carboxyla- tion leading to excessive acidity, as observed in 60-min treated fibers, results in a significant reduction in the cur- ing onset temperature, as confirmed by DSC analysis. 60 – 63 This reduction increases the risk of precuring during manufacturing, disrupting the production process. To address the issue of precuring, 63 it is essential to carefully control the modification conditions, particularly the treatment time. Based on DSC analysis, 15-min treated fibers achieved an optimal balance between effective car- boxylation and preserving fiber integrity. The partial pit closure observed in these fibers, without surface damage, enhances their interaction with UF resin while maintain- ing desirable curing conditions. Tests on density, IBS, TS, and WA confirmed that the 15-min treatment provides both enhanced mechanical strength and optimal physical properties in the final product. Overall, pH influences the kinetics and energetics of the curing reactions in UF resins, impacting the tempera- ture range, reaction rates, and overall efficiency of the curing process. Understanding and controlling the pH of the system is essential for achieving the desired curing
characteristics and properties in the final cured resin product. The impact of lower curing temperatures was assessed on the panels fabricated from 100% unmodified and 100% carboxylated fibers (15-min treated) using dif- ferent curing temperatures. Specifically, the panels made from 100% carboxylated fibers were cured at 150 C ( ≈ 50 C higher than their peak temperature) while those from unmodified fibers underwent standard curing at 200 C (also ≈ 50 C higher than its peak temperature) using the same panel production methods detailed in the materials and methods sections. Despite lower press tem- peratures, the panels with modified fibers exhibit signifi- cantly lower water uptake (Figure 9A), decreased TS (Figure 9B), enhanced density (Figure 9C), and similar IBS values (Figure 9D). There is an opposite trend between Figures 6 and 9 in terms of IBS and density. This can be explained by the fact that Figure 6 shows samples containing a mixture of unmodified and carboxylated fibers, while Figure 9 pre- sents data for panels made entirely from 100% FIGURE 9 The comparison of wood panel (A) 2 and 24 h water absorption (WA) of raw and 100% CA'ed fibers, (B) 2 and 24 h thickness swelling (TS) of raw and 100% CA'ed fibers, (C) density of raw and 100% CA'ed fibers, and (D) internal bond strength (IBS) of raw and 100% CA'ed fibers.
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