Hierarchical Functionalized Lignin Interfaces Regulate Water Transport in Low-Coating-Content Bio-Based Polyurethane-Coated Urea

Abstract

Bio-based polyurethane coatings are promising for controlled-release fertilizers, but their release performance at low coating contents is often limited by rapid surface wetting and water transport through the coating membrane. Here, we constructed a hierarchical lignin-based interface on castor-oil-derived polyurethane-coated urea to regulate water–coating interactions. Lignin micro/nanospheres were sequentially functionalized with polydopamine and octadecyltrichlorosilane and introduced at the outer polyurethane interface during the final coating stage. Thermodynamic calculations and spectroscopic analyses were consistent with interfacial chemical coupling between polydopamine-associated functional groups and residual isocyanate groups in the polyurethane coating. The resulting interface increased the apparent water contact angle from 87.4° to 157.2°, weakened the calculated adsorption of water, and reduced water-vapor permeability through the coating. Under a nominal total coating-material input of approximately 3 wt %, the cumulative nitrogen release within 10 days decreased from 56% for bio-based polyurethane-coated urea (BCU) to 20% for OTS-LMNPs@PDA-modified bio-based controlled-release urea (SBCU), while the time required to reach 80% nitrogen release increased from 20 to 49 days. This enhanced release performance was associated with reduced surface wetting, lower water affinity, and slower water transport across the modified coating, which collectively delayed water penetration, urea dissolution, and subsequent nitrogen diffusion. These results demonstrate that lignin-based interfacial modification provides an effective strategy for regulating water transport and improving the controlled-release performance of low-coating-content bio-based polyurethane fertilizers.