Constructing phenylboronic acid-tethered hierarchical kenaf fibers to develop strong soy meal adhesives with excellent water resistant, mildew proof, and flame-retardant properties

Plant protein-based adhesives are considered to be the most potential substitutes for petroleum-based adhesives in the wood panel industry, however, the poor water resistance and unsatisfactory mechanical performance limit their large-scale application. Moreover, the development of multifunctional plant protein-based adhesives with high strength, mildew proof, and flame-retardant remains a challenge. Plant fibers have been widely used as the reinforcing phase of polymers, but the smooth surface makes them easy to pull out of the matrix, which greatly reduced the strengthening effect. Herein, the phenylboronic acid-tethered hierarchical rough kenaf fibers were constructed to strengthen soy meal (SM) adhesives while imparting multifunction. The hierarchical rough structures and reactive groups on the surface of kenaf fibers made them form stronger mechanical interlocking and chemical crosslinking with SM matrix, coupled with the entanglement and interpenetration of the fibers with a high aspect ratio, which together endowed the SM adhesive with outstanding water-resistant bonding strength. The wet shear strength of the plywood prepared by the resultant SM adhesive increased by 391.7% to 1.18 MPa, which was far higher than the standard of 0.7 MPa for type II plywood (GB/T 9846-2015) in Chinese standards. Benefiting from the phenylboronic acid component, the resultant SM adhesive exhibited excellent mildew-proof and flame-retardant properties. This biomimetic design of kenaf fibers provides a facile and efficient strategy to develop high-performance and multifunctional bio-adhesives. Graphical Abstract