An In-Depth Study on the Enhanced Controlled-Release Performance of Coated Urea via the Construction of a Multiscale SiO ₂ -Based Superhydrophobic Coating

The water barrier capacity of controlled-release urea (CRU) coatings plays a critical role in determining their overall performance. While enhancing coating hydrophobicity through the incorporation of nanomaterials presents a promising approach, the influence of nanomaterial addition methods and their properties on the coating microstructure and nutrient release behavior remains poorly understood. This study first systematically evaluated the effects of three nano-SiO₂ addition methodsbulk blending, outer-layer blending, and surface coatingon the physicochemical properties of the coating and the controlled release performance of CRU, with the aim of identifying optimal formulation parameters. Subsequently, the influence of SiO₂ particles with different particle sizes on the surface roughness, hydrophobicity, and controlled-release performance of CRU was studied. The results demonstrated that the surface coating method was the most effective addition process. Coating roughness initially increased and then decreased as the SiO₂ particle size varied from 20 nm to 3 μm. Notably, CRU coated with 20 nm nano-SiO₂ exhibited the best controlled release performance, promoting the formation of a dense and superhydrophobic coating structure with a water contact angle (WCA) of 165.44°, extending the controlled-release period by 23 days compared to polyurethane-coated urea (PCU). This work provides a facile strategy for fabricating high-performance CRU, thereby enhancing agricultural sustainability through improved nitrogen use efficiency.