This study evaluates the potential of feather keratin hydrolysates, obtained through ultrasound-assisted alkaline hydrolysis at low and high temperatures, as sustainable binders for high-density fiberboards (HDFs). The performance of keratin-based adhesives was compared with that of current binders, namely urea-formaldehyde (UF) and soy protein isolate (SPI). Adhesives were applied at a content of 12%. Additionally, a second formulation using 15% keratin hydrolysate extracted at high temperature (KHT 15) was tested to assess the influence of binder loading. The mechanical and physical properties of the HDF panels—including modulus of rupture, modulus of elasticity, internal bond strength, screw withdrawal resistance, thickness swelling, water absorption, and surface wettability—were systematically evaluated. Both keratin formulations outperformed SPI and matched or surpassed UF in key performance indicators. Among them, keratin extracted at high temperature exhibited superior mechanical strength and moisture resistance, which may be related to the formation of stronger disulfide bonds. In particular, KHT 15 increased internal bond strength by 74% compared to UF and 96% compared to SPI, and reduced 24 h water absorption by 36% and 58% respectively. Keratin-based panels also retained higher water contact angles after 60 s, demonstrating improved surface hydrophobicity. Although keratin extracted at high temperature at 12% fulfilled the EN 622-5 standard for dry-use panels, increasing the content to 15% enabled compliance with the requirements for humid conditions, with TS below 30%. These results position keratin hydrolysates, particularly at high temperature, as viable, bio-based alternatives to synthetic and food-derived adhesives in engineered wood composites aligned with circular bioeconomy principles.
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