Preparation of hydroxy-terminated polydimethylsiloxane and nano-SiO2 hydrophobic polyurethane coated urea and investigation of controlled nitrogen release

•Castor oil-based polyurethane was hydrophobically modified by hydroxyl-terminated polydimethylsiloxane and nano-silica.•The material exhibits remarkably low water absorption, moderate porosity, and reasonable swelling.•The coated urea achieves an impressive sustained release period of 98 days ( 5 % coating amount).•The modified coating effectively reduces ammonia volatilization from urea particles.•The introduction of SiO2 may contribute to enhancing the strength of the polyurethane film material. In the present study, we successfully developed hydroxy-terminated polydimethylsiloxane (PDHT) and nano-SiO2 hydrophobic polyurethane-coated urea by applying the optimal conditions for hydrophobic polyurethane preparation. The hydrophobic polyurethane membrane material undergoes a comprehensive characterization. Infrared analysis confirmed the successful grafting of PDHT onto the polyurethane membrane material. X-ray Photoelectron Spectroscopy (XPS) and Energy Dispersive X-Ray Spectroscopy (EDX) analysis verify the even distribution of silicon on the surface of the membrane material. By Scanning Electron Microscopy (SEM) comparison, we elucidate the micro-structural disparity between the unmodified and modified castor oil-based polyurethane, accounting for the micro-level performance enhancement. The resulting modified membrane exhibited a water contact angle of 133.6°, indicative of its hydrophobic nature. The optimal ratios for PDHT and nano-SiO2 were determined to be 16 % and 2 %, respectively. Notably, hydrophobic polyurethane-coated urea demonstrated an extended-release period of 98 days (5 % coating amount), a 40 % improvement over its unmodified counterpart. In addition, the silicone-modified coated urea reduced the amount of ammonia volatilization by about 76.32 %, which plays a significant role in enhancing fertilizer utilization efficiency and is important for ecological environment protection. This research provides a straightforward and efficient way to fabricate slow-release fertilizers, with significant implications for the advancement of coated slow-release fertilizer technology.