Fiber level catalyst‐free oxidative carboxylation enhances physical properties of wood polymer composites

Abstract The widespread use of environmentally friendly medium‐density fiberboard (MDF) panels as a wood composites is driven by their versatility, affordability, and durability. However, reliance on traditional wood preservatives and modifications raises significant environmental, health, and cost concerns due to harmful chemicals. To address this, we present a one‐step nitric acid steam oxidative modification on wood fibers to directly introduce carboxylic acid groups on the surface and eliminate the need for catalysts, organic solvents, or complex multistep procedures often used in traditional methods, such as TEMPO‐mediated oxidation. Our multiscale characterization techniques revealed significant changes in the morphology, crystallinity, and surface features of the treated wood fibers, which directly translated to enhanced bulk mechanical properties of the wood composites. Remarkably, the internal bond strength (IBS) of the wood panels increased from 0.27 MPa in untreated panels to 0.89 MPa in panels treated with 5% carboxylated (CA) fibers, suggesting a 3.3‐fold enhancement. Additionally, the water uptake of the modified panels was dramatically reduced, with 5% CA‐treated panels absorbing only 3.46% compared with 30.38% in unmodified panels, signifying dimensional stability. Furthermore, the curing temperature of the adhesive with CA‐treated fibers was lowered by 50°C without reducing composite strength, highlighting significant energy savings. Also, formaldehyde emissions from the 10% CA‐modified panels were reduced by 14.82% compared with unmodified panels, aligning with regulatory standards. These findings demonstrate that catalyst‐free oxidation enhances adhesive bonding and mechanical performance in wood composites while providing an eco‐friendly method for lignocellulosic fiber modification. Highlights Catalyst‐free, one‐step oxidation of wood fibers with nitric acid steam. Carboxylation greatly enhanced fiber bonding with adhesives. Modified fibers improved panel strength and reduced water absorption. Lower curing temperature offers energy savings in production.