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fibers, was applied through spraying. The bonded fibers were manually formed into mats by placing them within a wooden frame (200 200 8mm 3 ). This process was repeated three times for each treatment. In the final step, the pine fiber mats were subjected to hot pressing at a pressure of 80 bar and a temperature of 200 C for 4min. This process was aimed at promoting the curing of the resin, ensuring the fibers were firmly bonded. After press- ing, the panels were allowed to cool gradually at room temperature for 24 h. The physical properties of the wood panels were assessed by measuring the IBS under dry conditions with a ZwickRoell testing machine (Model Z010 ZwickRoell, Germany) equipped with a 10 kN load cell as well as thickness swelling (TS). After 2 h and water absorption (WA) after 24 h were based on the European Standard (EN) methods. The formaldehyde content in modified wood panels, specifically those treated with nitric acid, was determined according to the European Norm (EN 12460 – 5) 33 using the perforator method. In this process, the panels were first cut into small pieces of approximately 110 ± 0.01 g with the size of 25 mm 25 mm for uniform extraction. They were immersed in 600 mL of toluene and boiled at 200 C for 2 h in reflux, with a rate of 70 – 90 drops per minute, in a round-bottom flask attached to a perforator setup, which included 1000 mL of distilled water. During the extraction, the formaldehyde gas released from the panels was directed through the condenser and collected in the gas absorption bulb, which was filled with 100 mL of distilled water. After the 2-h extraction was complete, the water was cooled to room temperature. The water with extract collected in the absorption bulb of the gas absorption equipment was transferred into the flask, and distilled water was added to bring the total volume of the solution to 2000 mL. The formaldehyde content was mea- sured spectrophotometrically using the acetylacetone method based on the EN ISO 12460-5 standard. First, 4 mL of acetylacetone was diluted in 1000 mL of distilled water, and 200 g of ammonium acetate was dissolved in 1000 mL of distilled water separately. 33 Then, 10 mL of the extract from the 2000 mL solution was mixed with 10 mL each of the acetylacetone and ammonium acetate solutions in a flask. The mixture was shaken, heated to 60 C, and then cooled to room temperature for 1 h, while protecting it from sunlight. The solution color turned yel- lowish. The solution was then transferred to a formalde- hyde test cuvette (HACH LCK 325, UK), and the formaldehyde concentration was measured using a DR 2800 spectrophotometer (HACH, UK) at a wavelength of 412 nm. The results were repeated two times and are reported as mg of formaldehyde per 100 g of oven-dry board.
took shape, and the angles were recorded from the right and left angles. Deionized water, with a surface tension of 72.6 ± 0.1 mN/m, was utilized in all experiments at a temperature of approximately 22 C to ensure accurate and reproducible results. UV – visible spectra of the carboxylated wood fibers were recorded in the wavelength range of 200 – 700 nm using a Shimadzu UV-3600 UV – VIS – NIR spectrophotom- eter (Shimadzu Corporation, Japan) in reflectance mode at room temperature with a sampling interval of 1 nm.
2.3.2 | Thermal analysis
Thermogravimetric analysis (TGA/DTG) was performed on the samples using a (Mettler Toledo, Turkey) under a nitrogen atmosphere with a flow rate of (30 mL/min) from 25 to 600 C at a heating rate of 10 C/min, followed by an additional heating step from 600 to 800 Cunder an oxygen atmosphere at 10 C/min. The DSC 1 instrument (Mettler Toledo DSC 3 + , USA) was utilized to investigate the curing processes through the differential scanning calorimetry (DSC) method. The wood fibers were initially mixed with urea- formaldehyde adhesives, with the same binder-to-fiber ratio of 11%, which matched the quantity required for panel production. The prepared samples were then care- fully positioned between an aluminum cup and lid to securely contain them and prevent deformation under pressure. The samples were sealed tightly to keep water inside during the testing process, preventing any loss of water content. Then, the sealed container was placed into a specialized machine, and nitrogen gas with a flow rate of (30 mL/min) was used to increase the pressure inside to 1.6 MPa. This helped control the temperature range for water vapor and prevent water from evaporating while the samples were heated. The samples were gradu- ally heated at a rate of 10 C/min, ranging from 20 to 200 C.
2.4 | MDF production
Figure S4 illustrates the production parameters and pro- cess for MDF. Panels including (i) unmodified fibers, (ii) 5 wt% carboxylated fibers mixed with raw fibers, (iii) 10 wt% carboxylated fibers mixed with raw fibers, and lastly (iv) 100% carboxylated fibers (not mixed with raw fibers) were separately prepared to investigate the modification effect on MDF. The solid content of the urea-formaldehyde resin is 65% by weight without using any paraffin or other water-repellent chemicals. The resin solution, at a concentration of 11% by weight of
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