A formaldehyde-reduced phenolic adhesive featuring multi-scale wood bonding: Utilizing dialdehyde xylan for enhanced performance and sustainability.

As a major carbohydrate derived from agricultural/forestry biomass and paper-making processes, xylan is a promising and environmentally benign feedstock for high-value applications. However, its use in adhesives remains limited, primarily due to its inherently low molecular weight, which results in short molecular chains offering insufficient cross-linking sites, as well as high molecular rigidity, which restricts chain flexibility and effective entanglement, thereby impeding the formation of a stable crosslinked network. In this study, xylan was selectively oxidized with sodium periodate to produce highly reactive dialdehyde xylan (DAX), which was subsequently utilized to partially substitute formaldehyde to synthesize an eco-friendly dialdehyde xylan-based phenolic resin (DAXPF). The adhesive strength of DAXPF with 30% formaldehyde substitution reached 0.97 ± 0.02 MPa, significantly surpassing that of conventional phenolic resin (PF) (0.72 ± 0.02 MPa) and unmodified xylan-based resin (0.73 ± 0.04 MPa). The formaldehyde emission of the resulting plywood was below the Chinese National Standard E0 grade (0.0019 ± 0.0011 mg/L). Moreover, DAXPF exhibited a residual carbon content of 58.9% and a 14.8% improvement in thermal stability compared to conventional PF. The enhanced thermal stability represented a key advantage for potential high-temperature applications. Fluorescence microscopic imaging further revealed that DAXPF could deeply penetrate the cell wall of wood to achieve multi-scale bonding. This strategy provided a viable pathway for the valorization of xylan and the green modification of bio-based adhesives.