P–N Covalent Organic Framework Significantly Enhances the Flame-Retardant Properties and Mechanical Strength of Lactose-Based Biomass Adhesives

In this study, a P–N covalent organic skeleton (COFPN) was introduced for the first time to improve the flame-retardant properties and mechanical strength of lactose-based biomass adhesives (LHC). Through the synthesis and characterization of COFPN, it was found that its unique covalent backbone structure exhibited significant effects in improving the thermal stability and fire resistance of LHC adhesives. Specific experimental results showed that the addition of COFPN increased the limiting oxygen index (LOI) of the LHC adhesive from 23.2 to 42.0, the vertical combustion test (UL-94) reached class V-1, and its static flexural strength and modulus of elasticity increased by 27 and 22%, respectively. These improvements were attributed to the homogeneous dispersion of COFPN in the LHC adhesive and formation of a dense carbonized layer. This study not only provides a new way to develop high-performance biomass adhesives but also lays a theoretical foundation for the design of future flame-retardant materials.