Development of High-Performance Biocomposites from Kenaf, Bagasse, Hemp, and Softwood: Effects of Fiber pH Modification and Adhesive Selection on Structural Properties

This study aims to develop high-performance biocomposites for structural applications using Kenaf, Bagasse, Hemp fibers and Softwood bonded with phenol-formaldehyde (PF) and phenol-urea-formaldehyde (PUF) resins commonly used in the industry for the production of the particleboard. A simple, low-cost fiber treatment was performed by adjusting the fiber pH from 5–6 to 11 and 13 using a 33% NaOH solution, following a standard protocol to improve fiber adhesion. PF and PUF adhesives were applied at a loading level of 13% (w/w), while polymeric diphenylmethane diisocyanate (pMDI) was used as a control adhesive at 5% (w/w) for untreated fibers. The fabricated panels were evaluated for mechanical properties, including modulus of elasticity (MOE), modulus of rupture (MOR), and internal bond strength (IB), as well as physical properties such as thickness swelling (TS) and water absorption (WA) after 24 hours of immersion. Bagasse at pH 11 exhibited the highest IB values with both PF and PUF, followed by Kenaf at pH 13, exceeding the EN 312:6 (2010) standard for heavy-duty load-bearing panels in dry conditions. The highest MOE and MOR values were obtained with Kenaf fibers at pH 11, meeting the EN 312:4 (2010) standard for load-bearing panels in dry conditions, followed by Bagasse, whereas Softwood and Hemp showed lower performance. In terms of thickness swelling, Bagasse demonstrated the best performance at all pH levels with both adhesives, followed by Kenaf and Hemp, outperforming the pMDI-based composites. The findings suggest that the high pH of the fibers creates an alkaline environment that boosts the reactivity of PF and PUF resins by enhancing the nucleophilic character of the phenolic rings during polymerization. Kenaf and Bagasse fibers can serve as viable alternatives to industrial softwood particles in EU panel production for structural applications. Furthermore, PF and PUF adhesives offer cost-effective alternatives to the more expensive and toxic pMDI. The observed performance variations among fibers are attributed to differences fiber structure (aspect ratio), and intrinsic properties, which influence their interactions with adhesives under varying pH conditions. The study suggested that Kenaf and Bagasse fibers could serve as promising raw materials for the production of medium-density homogeneous particleboards, with potential for structural application.