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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. 

Polymer composites are becoming an important class of materials for a diversified range of industrial applications due to their unique characteristics and natural and synthetic reinforcements. Traditional methods of polymer composite fabrication require machining, manual labor, and increased costs. Therefore, 3D printing technologies have come to the forefront of scientific, industrial, and public attention for customized manufacturing of composite parts having a high degree of control over design, processing parameters, and time. However, poor interfacial adhesion between 3D printed layers can lead to material failure, and therefore, researchers are trying to improve material functionality and extend material lifetime with the addition of reinforcements and self-healing capability. This review provides insights on different materials used for 3D printing of polymer composites to enhance mechanical properties and improve service life of polymer materials. Moreover, 3D printing of flexible energy-storage devices (FESD), including batteries, supercapacitors, and soft robotics using soft materials (polymers), is discussed as well as the application of 3D printing as a platform for bioengineering and earth science applications by using a variety of polymer materials, all of which have great potential for improving future conditions for humanity and planet Earth.