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TURHAN ET AL .
controlling how much adhesive can get into. This locking mechanism plays a crucial role in determining the physi- cal properties of wood composite panels. As the treat- ment progresses toward 30 min, NO 2 gas continues to react with the fibers, potentially penetrating deeper into the material. This resulted in more pronounced surface roughness as compared with the 15-min stage (Figure 3C). More importantly, the pits become more apparent, and the fiber surface exhibits signs of partially damaged areas or etching. Increased chemical activity might lead to the removal of more surface layers, expos- ing the internal structure of the fibers. The overall trend is toward increased surface damage and roughness with prolonged nitric acid steam treatment. While the carbox- ylation is increased in 30-min treated fibers and can favor interaction with the adhesive at the surface, the reopened pores allow adhesive to fill in the fibers, thus, reducing the crosslinking density between the fibers. For 60-min treated fiber, the surface damage was notably greater, and the inner layers of the cell wall are even noticeable through the alignment of the cellulose microfibrils (Figure 3D). The size reductions became more evident as shown in Figure S2, leading to highly brittle fibers, accompanied by a reduction in overall fiber dimensions. Consequently, considering the partial closure of the pits after a short treatment time with little surface damage, the panels were produced with fibers modified for 15 min as discussed in Section 3.3. In addition to surface morphology, we investigated the surface chemistry characteristics of the unmodified and modified wood fibers using XPS (see in Figure 4). The binding energy observed in the C1s* peak can be divided into four distinct carbon species, denoted as C1, C2, C3, and C4. C1 represents carbon atoms attached to hydrogen or carbon atoms (C H or C C), typically observed at 284.5 eV. C2 corresponds to carbon atoms bonded to one oxygen atom, such as in C O or C O groups at 285.79 eV, C3 is associated with carbon atoms connected to two noncarbonyl oxygen atoms, as in ester or ether linkages ( C O C ) at 287.13 eV. Finally, C4 corresponds to carbon atoms linked to both carbonyl and noncarbonyl oxygen atoms (O C O ) at 288.91 eV. When comparing modified fiber with unmodified wood fibers, there was a decrease in C1. C1 is found mainly in lignin and extractives, so the decrease in C1 was likely caused by breaking the bond in the C C bond in the structure of lignin and the formation of oxygen- containing structures like carboxyl structures because both oxygen content increased in the carbon structures C2 ( C O) and C4 (O C O ). While for 60 min modi- fication, C4 (O C O ) decreased, the formation of C2 ( C O) or ( C O) groups increased. This change can be related to the degradation of lignin and extractives
FIGURE 4
High-resolution x-ray photoelectron spectroscopy
spectra of (A) wood fibers, (B) 15 min CA'ed wood fibers, (C) 30 min CA'ed wood fibers, and (D) 60 min CA'ed wood fibers. Carbon components: C1: C H or C C; C2: C O or C O; C3: C O C ; C4: O C O groups.
after a longer modification time, so the formed carboxylic acid groups can be decreased. Another reason can be due to the existing high oxidation state; additional oxidation causes the removal of the carboxyl carbon in the form of carbon dioxide. Depending on the reaction parameters, the oxidation state of the remaining organic structure may decrease or remain unchanged. Additionally, these findings align with the results of the FTIR analysis. 40 – 43 Furthermore, it was found that the O/C ratio of unmodi- fied wood fibers was 0.29, which increased to 0.48, 0.51, and 0.59 for 15, 30, and 60 min modified fibers, respec- tively (Table 1). This confirmed that oxygen-containing structures on the surface were increased after modification. The change in surface chemistry reflects also on the thermal stability of the CA'ed wood fibers as well as hydrophilicity. Thermogravimetric analysis (TGA) and differential thermogravimetry (DTG) curves are pre- sented in Figure 5A,B. The essential parameters of the pyrolysis process, including onset degradation tempera- ture ( T onset ), maximum degradation temperature ( T max ), weight loss, and char residues, were characterized and presented in Table S1. The results of TGA showed that the decomposition of hemicellulose, cellulose, and lignin occurs over different temperature ranges; generally, hemicellulose decomposes at a lower temperature range (220 – 315 C) than cellulose (300 – 400 C), while lignin decomposes over a broad range of temperatures (150 –
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