Synthesis of CO2-based biomass derived non-isocyanate polyurethane hybrid adhesives with excellent mechanical properties and water resistance

Abstract

The increasing focus on reducing greenhouse gas emissions and the utilization of biomass resources have driven significant interest in developing environmentally friendly, non-toxic, non-isocyanate polyurethane (NIPU) adhesives derived from CO₂ and green resources. However, the mechanical properties of NIPUs are much inferior to those of traditional isocyanate-based polyurethanes, thereby restricting their applicability in adhesives and other fields. Therefore, it is imperative to find a solution for the pressing problem of how to enhance their mechanical properties utilizing biomass resources for broadening their large-scale applications. In this work, linear NIPU prepolymers with amine-terminal groups were promptly synthesized at ambient temperature, utilizing biomass cashew phenol diglycidyl ether (602A) and bio-oil-based diamines (Priamine 1074) as raw materials. To enhance the mechanical properties of NIPU prepolymers, ethanol fractionation was utilized to extract small-molecule lignin (EFL) from enzymatic hydrolysis lignin (EHL). Subsequently, EFL was synthesized into lignin-based epoxy resin (LEP) via a solvent-free process, and then grafted onto the amine-terminated NIPUs to form a three-dimensional network structure of lignin-based epoxy resin hybrid non-isocyanate polyurethanes (LEP-NIPUs). LEP-NIPU polymers with the epoxy resin as the rigid segment and NIPUs as the flexible segment not only possess superior mechanical properties including tensile strength of up to 20.01 MPa but also have excellent hydrophobicity and corrosion resistance to seawater. These properties contributed to a wet shear strength of 2.31 MPa when applied as wood adhesives, indicating promising application prospects for the future development of underwater adhesives.