Lignin nanoparticles prepared via lactic acid-based deep eutectic solvents as an emulsifier for waterborne polyurethane

Lignin nanoparticles (LNPs) have been extensively studied as emulsifiers, where the particle size and hydrophilic lipophilic balance value (HLB) are pivotal to the emulsion's stability. This investigation leveraged a range of lactic-based deep eutectic solvents (DES) for the nanoscale modification of lignin. Subsequently, the size characteristics of LNPs were characterized by measuring particle size and size distribution. Combined with the detachment energy theory and the maximum capillary pressure theory, the mechanism of how the size of Pickering emulsifier particles affects emulsion stability was thoroughly analyzed, thereby screening the preparation process for LNPs with suitable particle sizes. Meanwhile, the composition and structure of the LNPs were analyzed using NMR spectroscopy. The HLB was calculated based on NMR information and further refined the preparation process to achieve LNPs with appropriate hydrophilicity/lipophilicity. The research results indicate that LNPs prepared using the LA/AL DES at 100 °C for 2 h are most suitable for the preparation of waterborne polyurethane, with an average particle size of 204 nm and an HLB value of 9.14. When the waterborne polyurethane derived from this LNP was applied to plywood, the resulting adhesive demonstrates wet shear strengths of 1.03 MPa and 0.74 MPa under warm and boiling water conditions, respectively.