High bio-content polyurethane coated controlled-release fertilizer with metal coordination layer for prolonged nutrient release and enhanced degradability

Controlled-release fertilizers (CRFs) offer significant potential for reducing environmental pollution and enhancing fertilizer utilization efficiency. However, achieving improved controlled-release and degradability in high bio-content polyurethane (HB-PU) coated fertilizers remains challenging, limiting their further agricultural application. In this work, HB-PU coatings were synthesized via a one-pot process primarily using castor oil (CO), lysine diisocyanate (LDI), and polymethylene polyphenyl polyisocyanate (PAPI). Results demonstrate that the enhanced controlled-release performance of the HB-PU coating is associated with the type and concentration of metal ions within the complexation layer formed on the surface of the urea. After constructing the Cu coordination layer, the nutrient-controlled release period of castor oil-based polyurethane with bio-content over 70 % (PCU1-Cu) coated urea with 2.5 % coating percentage was extended by 20 days compared to the control. The chemical structure, thermodynamic stability, and microscopic morphology of the HB-PU with Cu coordination layer were analyzed. XPS confirms that the metal ion coordination was mainly enriched in the inner layer of the coating material. The Fenton-like reaction confirmed the presence of Cu ions in the PCU1-Cu coating and with catalytic reactivity. Degradation studies in NaOH solution demonstrated that introducing Cu ions accelerates the HB-PU degradation. The application of PCU1-Cu coated urea on Pakchoi ( Brassica rapa ssp. chinensis ) exhibited increased yield. This study prepared HB-PU controlled-release fertilizers with excellent controlled-release properties by constructing a metal coordination layer, while also providing a novel design method for supplementing essential trace metal elements for plant growth.