Cellulose Nanocrystals-Stabilized Bio-Based Waterborne Polyhydroxyurethane Nanocomposites with Enhanced Adhesive Performance

Polyurethanes are widely used in adhesive applications, but their conventional synthesis relies on hazardous isocyanates and often solvent-based formulations. In this work, waterborne polyhydroxyurethanes (PHUs) were synthesized from 1,6-hexanediol bis-(cyclic carbonate) and bio-based Priamine 1075 via catalyst-free suspension polymerization in water. Pristine cellulose nanocrystals (CNCs) acted as the sole stabilizers, eliminating the need for petroleum-derived surfactants while simultaneously serving as reinforcing nanofillers. Stable monomer-in-water emulsions were obtained with CNC loadings up to 200 mg mL^-1 per monomer, corresponding to ∼17 wt % CNCs in the final dried nanocomposites. In the latex state, CNCs were located at the particle surfaces, ensuring colloidal stability, while in the dried PHU/CNC nanocomposites they were uniformly distributed throughout the matrix, yielding adhesives with markedly enhanced performance. The nanocomposites exhibited up to 680% and 340% increases in probe tack adhesion strength and lap-shear strength, respectively, compared with surfactant Tween 80-stabilized waterborne PHUs, reaching performance levels comparable to commercial pressure-sensitive adhesives. These findings demonstrate that combining bio-based monomers with CNC stabilization offers a robust strategy for producing sustainable, high-performance PHU adhesives consistent with green chemistry principles.