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TURHAN ET AL .
3.3 | Physical properties of modified fiberboards
free water occupies micropores and macropores within cell walls, filling voids and being retained by capillary forces. When water vapor permeates and absorbs into cell walls and middle lamella, the cell swells. As a result, bound water saturates the cell wall and middle lamella until the water saturation point of plant cells, typically around 20% – 40% for wood cells, is reached. Subse- quently, cellular cavities (lumens and porosities) achieve complete saturation because they are filled by free water. 36,45 Thus, the closure of the pits in the modified fibers prevents bulk water from diffusing into the interior of the fiber and the panels. Another reason for lower water uptake and TS can be the carboxylic acid groups on the modified wood sur- faces, which introduce extra reactive sites through the Fischer esterification. 46 The formation of covalent bonds between the fibers and urea-formaldehyde resin results in stronger binding with the resin. This results in a slower water uptake in the panels produced from the 5% and 10% CA'ed fibers, whereas in unmodified fibers, the WA is fast and possibly reaches closer to the saturation
Our one-step method of modification showed that fibers with carboxylic acid groups resulted in a stronger struc- ture and more resilient bonds compared with untreated fibers. The panels were made from three types of fiber mixtures: 100% unmodified fibers, 5% CA'ed fibers mixed with 95% unmodified fibers, and 10% CA'ed fibers mixed with 90% unmodified fibers, which were tested in terms of their dimensional stability and water uptake. We chose 15-min treated fibers to mix with raw fibers since they are effectively carboxylated without surface damage (as in 30-min treatment) or disintegration (as in 60-min treatment), and the pits are partially closed (see the above sections for details). The process of composite panel production with UF resin is given in Section 2 and Data S1. To assess the impact of the modification on the performance of wood products, the density, IBS, TS, and WA were measured over periods of 2 and 24 h. The 2-h and 24-h WA and TS test results from the panels composed of 5% and 10% CA'ed fibers (Figure 6A,B) show significant reduction when modified fibers are used. The improved dimensional stability can be related to the closed pits of fibers after modification 36 (as schematically shown in the reaction mechanism in Figure 7). WA in the wood fibers unfolds in two stages: Water first binds to the cell walls and middle lamella via hydrogen bonds with hydroxyl groups ( OH), and then
FIGURE 7 The reaction mechanism between UF and modified fibers indicates a crosslinking mechanism in both UF resin and UF resin with carboxylated wood fiber. As a result of this reaction, denser and more compact structures of the panels were obtained, further decreasing water penetration and swelling. Furthermore, it can be attributed to their enhanced interaction with the surrounding urea formaldehyde matrix during the curing process.
FIGURE 6 The comparison of wood panel (A), 2 and 24 h water absorption (WA) of raw, 5% CA'ed and 10% CA'ed fibers, (B) 2 and 24 h thickness swelling (TS) of raw, 5% CA'ed and 10% CA'ed fibers, (C) density of raw, 5% CA'ed and 10% CA'ed and 100% CA'ed fibers, and (D) internal bond strength (IBS) of panels made of raw, 5% CA'ed and 10% CA'ed fibers.
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