Hydrothermal Liquefied Bio-Oil from Municipal Sewage Sludge as a Reactive Filler in Polymeric Diphenylmethane Diisocyanate (p-MDI) Wood Adhesives

The growing environmental concerns associated with petrochemical-based adhesives have driven interest in sustainable alternatives. This study investigates the use of bio-oil, derived from municipal sewage sludge (MSS) through hydrothermal liquefaction (HTL), as a reactive filler in polymeric methylene diphenyl diisocyanate (pMDI) wood adhesives. The bio-oil, rich in hydroxyl and carbonyl functional groups, was characterized using FTIR (Fourier transform infrared spectroscopy), elemental analysis, and NMR (nuclear magnetic resonance). These functional groups interact with the isocyanate groups of pMDI, enabling crosslinking and enhancing adhesive performance. Various MSS bio-oil and pMDI formulations were evaluated for tensile shear strength on Southern yellow pine veneers under dry and wet conditions. The formulation with a 1:4 bio-oil to pMDI weight ratio exhibited the best performance, achieving tensile shear strengths of 1.96 MPa (dry) and 1.66 MPa (wet). Higher bio-oil content led to decreased adhesive strength, attributed to reduced crosslinking and increased moisture sensitivity. This study demonstrates the potential of MSS-derived bio-oil as a sustainable additive in pMDI adhesives, offering environmental benefits without significantly compromising adhesive performance and marking a step toward greener wood adhesive solutions.