Formaldehyde-free wood bio-adhesives from sewage sludge: Role of ammonium polyphosphate

A novel formaldehyde-free wood bio-adhesive with remarkable flame resistance has been developed using hydrochar derived from sewage sludge (SS). The extracellular polymeric substances in SS are further broken down during hydrothermal carbonization via catalysis by layered double oxides (LDO). This process leads to more organic matter transferring into the aqueous phase and the formation of low-molecule-weight compounds. Simultaneously, LDO is converted into layered double hydroxides (LDH) as a flame-retardant. However, due to the formation of low-molecule-weight compounds, the resulting bio-adhesive exhibited poor wet shear strength (0.37 MPa). To address this issue, ammonium polyphosphate (APP) was introduced into the formulation, significantly improving the bio-adhesive's wet shear strength to 1.00 MPa, which meets the Chinese national standard GB/T 9846–2015 (≥0.7 MPa). The enhanced performance is attributed to the reaction between APP and the organic matter on the surface of hydrochar to form macromolecular compounds. The combination of LDH and APP imparts excellent flame-retardant properties to the bio-adhesive, making it a significant advancement in the valorization of hydrochar from SS. This research presents a novel and sustainable approach to converting waste materials into valuable products. © 2025 Elsevier B.V.