Rosin-Modified Hydroxyl-Functional Waterborne Polyurethane for High-Performance Two-Component Coatings

Rosin-derived polyols offer a renewable structural platform for tailoring the properties of waterborne polyurethane coatings. Here, glyceryl monoester of fully hydrogenated rosin (GM-FHR) was synthesized and incorporated into a hydroxyl-functional waterborne polyurethane to prepare two-component waterborne polyurethane (2K-WPU) coatings with different GM-FHR contents. FTIR and 1H NMR spectroscopy confirmed the formation of GM-FHR, while DMA showed increasingly distinct segmental relaxation with increasing GM-FHR content. Increasing the GM-FHR content to 15% improved the thermal stability, surface hardness, tensile strength, hydrophobicity, water resistance, and resistance to chemical exposure. The 15% R-2K-WPU exhibited a water contact angle of 96.88°, water absorption of 8.7%, T5% of 225 °C, Tmax of 435 °C, and tensile strength of 15.45 MPa, together with improved adhesion and abrasion durability. It also showed better retention of surface hardness and hydrophobicity after repeated abrasion and exposure to concentrated acid and alkali solutions. These changes were associated with the rigid and hydrophobic fused-ring structure of GM-FHR, which restricted segmental motion and altered the organization of the polyurethane network. Excessive GM-FHR loading (20%) reduced dispersion homogeneity and caused deterioration in toughness, adhesion, and water resistance. The results show that moderate incorporation of GM-FHR provides a practical means of balancing rigidity, hydrophobicity, and mechanical integrity in rosin-modified 2K-WPU coatings.