TURHAN ET AL .
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TABLE 1
Surface chemistry analysis of wood fibers by x-ray photoelectron spectroscopy.
% C component (binding energy, eV)
Sample
C1
C2
C3
C4
O/Cratio
Untreated
35.13 (284.5)
21.53 (285.79)
16.07 (287.13)
4.38 (288.91)
0.29
Modified
15min
30.85 (284.5)
22.95 (286.06)
7.58 (287.2)
5.8 (288.55)
0.48
30min
25.51 (284.5)
22.86 (285.82)
10.5 (287.11)
7.09 (288.95)
0.51
60min
21.93 (284.5)
28.17 (286.12)
8.21 (287.46)
4.23 (288.75)
0.59
Note : Carbon components belong to C1: C H or C C; C2: C O or C O; C3: C O C ; C4: O C O groups.
FIGURE 5 (A) Thermogravimetric analysis of wood fibers with raw, 15-min, 30-min, and 60-min CA'ed wood fibers, (B) DTGA analysis of wood fibers with raw, 15-min, 30-min, and 60-min CA'ed wood fibers, and (C) Meniscus angles of raw, 15-min, 30-min, and 60-min CA'ed fibers, respectively.
900 C). 44 After the carboxylation of the wood fiber, the weight loss of unmodified fiber decreased from 65.43 wt% to 59.31% (for 15 min), 59.96% (for 30 min), and 62.04% (for 60 min) in the temperature range of 200 – 450 C. Interestingly, the 15-min treated sample resulted in less weight loss, possibly due to less surface damage and clo- sure of the pits preventing heat and mass transport toward the fibers by forming a continuous char layer. Increasing treatment time reverses the effect, and the result for the 60-min treatment approaches the value obtained for the unmodified fibers. Additionally, the onset temperature of the 60 min treated started at a lower temperature compared with the unmodified samples, due to the removal of the lignin component promoting the breakdown of cellulose and hemicellulose components. After 600 C, the gas was switched to oxygen to complete the burning process. The derivative weight loss data for this region show the rapid oxidative degradation of the 60-min treated sample due to its thinner fiber morphol- ogy and higher cellulose content. The interaction of individual fibers with water is also important as the adhesives are water-based. Figure 5C and Figure S3 display the meniscus angle analysis. The wetting angles with water demonstrated a notable range, averaging around 81.65 for raw fiber, 75.69 for 15min modified fiber, 68.11 for 30 min modified fiber, and 41.74 for 60 min modified fiber. The large meniscus angle for the untreated wood fibers indicates a weak attraction to water, suggesting their less hydrophilic nature due to lignin content. On the other extreme,
60-min CA'ed wood fibers, with their lowest meniscus angle, display a notable hydrophilic character due to car- boxylation as well as exposure of cellulose on the surface. The meniscus angle of 15 and 30 min modified fibers dis- plays an intermediate response. Besides the immediate effect on the mechanical properties of the composite panels we discuss in the next section, incorporating hydro- philic fibers can help control humidity levels by absorbing excess moisture from the surroundings, contributing to a more stable and comfortable indoor environment. In summary, we detailed the modification involving an oxidative process to introduce carboxylic acid groups to fibers through nitric acid treatment and subsequent reaction of the modified fibers with UF resin. The esterifi- cation process through the reaction between the carbox- ylic acid groups of the fibers and the hydroxyl groups of the resin precisely altered the fibers, reducing open pits and hydroxyl content. The resulting changes in the nitric acid-treated fibers established stronger covalent bonds between the fibers and the resin, exceeding the strength of hydrogen bonding. This suggests that the covalent attachment of nitric acid-treated fibers containing car- boxylic acid groups to the resin serves as a modification method, enhancing strength and introducing hydro- philic characteristics. This not only improves adhesion but also addresses challenges related to cost, energy con- sumption, and potential fiber strength loss. This innova- tive approach precisely targets the surface chemistry of wood fibers, promoting enhanced adhesion and compat- ibility with synthetic resins.
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