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TURHAN ET AL .
FIGURE 1 Schematic illustration of nitric acid steam treatment of wood fiber and the reaction mechanism between nitrogen dioxide and C C bonds in the wood structure.
between 400 and 4000 cm 1 with 64 scans at a spectral resolution of 4 cm 1 , and the data were processed in transmittance mode using Origin Pro software. The crystallinity of the modified fibers was deter- mined through X-ray diffraction (XRD) analysis using a Bruker D2 Phaser x-ray diffractometer (Bruker Corpora- tion, USA) with a Cu K α radiation source (wavelength of 1.5406 Å). The samples were positioned centrally in the holder, and then, diffraction data were collected at room temperature over a 2 θ range of 10 – 80 with a scanning rate of 1.0 /min and a sampling interval of 0.05 . Then, the data were processed using Origin Pro software. X-ray photoelectron spectroscopy (XPS) measure- ments were conducted utilizing a Thermo K-Alpha x-ray photoelectron spectrometer (Thermo Fisher Scientific, USA) under ultrahigh vacuum conditions. The binding energies in the XPS data were calibrated based on the C C bond in the C 1s region at 284.6 eV by using Avan- tage software (ver. 6.6.0, Thermo Fisher Scientific). The surface morphology of raw and nitric acid steam- modified pine needles was observed using a Zeiss Ultra Plus field emission scanning electron microscope (FESEM, Zeiss Ultra Plus, Germany). Before analysis, the samples were uniformly gold-coated for 20 s using a sput- ter coater. Imaging was conducted under high vacuum conditions with an accelerating voltage of 5 kV. The contact angle of fibers was determined using the meniscus method with an Attension Theta Lite optical tensiometer (Biolin Scientific, Finland). In this process, individual wood fibers were submerged in distilled water. As the sample was pulled out from the liquid, a meniscus
gas washing bottle containing 10 g of wood fibers, as shown in Figure 1 for different durations (15, 30, and 60 min). Then, the NO ₂ gas generated was transferred into a separate water-filled wash bottle, where it was dis- solved and converted back into nitric acid. This method utilizes nitric acid within a carefully controlled, closed system that prevents the release of NO 2 fumes, minimiz- ing environmental impact, exposure, and improved safety. Finally, the reacted wood fibers were rinsed with deionized water under vacuum filtration until the filtrate reached a neutral pH to make sure there was no nitric acid left on the fibers. The washed modified fibers were dried at 50 C for 1 day. These modified fibers were ready to be analyzed further and used in wood-panel production.
2.3 | Characterization of nitric acid steam-treated wood fibers
2.3.1 | Structural analysis
Before all measurements, samples of wood fibers were dried at 50 C overnight to ensure consistent results and remove the moisture, and each test was run three times to make sure the results were consistent and reproducible. To analyze the chemical modifications from carboxyl- ation of hydroxyl groups on the fibers, Fourier-transform infrared (FTIR) spectroscopy was used with a Thermo Scientific iS10 FTIR (Thermo Fisher Scientific, USA)
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