Epoxidized lignin-polyamide synergy for high-performance multifunctional adhesives

Bio-based adhesives are becoming increasingly popular due to their environmentally friendly characteristics. However, owing to the hydrophilic nature and low reactivity of biomass, it is still somewhat difficult to attain long-term water resistance. Moreover, achieving flame retardancy and mold resistance poses an even greater challenge. In this work a unique adhesive has been conceived and prepared from epoxy lignin (EL) and polyamide (PA) solving these issues. The covalent cross-linked network, generated by reaction of EL and PA respective epoxy and amino groups, has yielded an adhesive of outstanding water resistance. The plywood bonded with this adhesive met the standards of class I plywood of the Chinese national standards (GB/T 17657–2022), and the wet strength after boiling in water for 72 h reached 0.87 MPa. This adhesive did also show resistance to extreme environments, including strong acids, strong alkalis, seawater, and organic solvents. Moreover, it was shown to be capable of bonding diverse organic and inorganic substrates. Its bonding performance significantly surpassed that of commercial wood adhesives. Furthermore, owing to the formation of a carbon layer that isolated oxygen during the combustion of lignin, this adhesive exhibited excellent flame-retardant properties. Specifically, its Limiting Oxygen Index (LOI) was as high as 39.1%. Simultaneously, the EL-PA adhesive also presented remarkably good anti-mold properties. In the 40-day anti-mold experiment, no mold growth was detected on either the adhesive or its raw materials. This work is expected to offer a promising, partially eco-friendly alternative to traditional formaldehyde-based adhesives.