Low-Molecular-Weight Phenolic Adhesives Tailored for Bonding Flattened Moso Bamboo (Phyllostachys edulis)

Commercial phenol-formaldehyde (PF) adhesives adapted from wood-composite applications are not specifically designed for bamboo, whose relatively high density, low porosity, poor surface wettability, and restricted transverse transport pathways hinder resin spreading and penetration. In this study, four PF adhesives with different molecular weights and viscosities were synthesized by optimizing the reaction temperature and time for bonding flattened Moso bamboo (Phyllostachys edulis). The developed adhesives showed enhanced wettability to bamboo compared with the commercial phenolic resin. The hydroxymethyl groups in the developed adhesives were determined by NMR, FTIR, and wet chemical methods to be present at high levels, demonstrating high reactivity during the hot-pressing process. When applied for laminating bamboo, the developed adhesives performed better than the commercial one from the perspective of penetration depth and bonding performance. Powered by confocal laser scanning microscopy (CLSM), the "bamboo-adhesive-bamboo" was clearly mapped, and it was clearly observed that the developed phenolic resins penetrated deeper into bamboo than the commercial one. Also, the developed phenolic adhesives contributed higher tensile shear strength to the laminated bamboo products than the commercial phenolic resin, regardless of dry or wet strength. These findings demonstrate that coordinated regulation of molecular weight and viscosity can improve resin penetration and curing reactivity, providing a bamboo-oriented PF adhesive design strategy for bamboo composites.