Environmentally Friendly Slow-Release Urea Fertilizers Based on Waste Frying Oil for Sustained Nutrient Release

Novel biobased polyurethane (PU) was synthesized by using waste frying oil-based polyol (WFOP) and isocyanate for slow-release fertilizers (SRFs). Epoxy resin (EP) was used to modify PU to synthesize the interpenetrating network (IPN) for improving the properties of the coating film. Nine polymer-coated nitrogen (N) fertilizers were prepared from these composite coating materials. The N release characteristics of the PU-coated urea (PCU) in water and soil were determined. Degradation behavior of coatings in the soil was evaluated. Results showed that 20% EP in the coating increased the cross-linking density by 77.9%, reduced the coating porosity by 28%, and thus slowed the nutrient release from the PCU significantly. Because of its compact structure, the water absorption rate of the EP-modified PCU shell was slower than that of the unmodified ones. All the PCUs, especially the EP-modified ones, showed excellent slow N release behaviors. In addition, the biopolymers derived from waste frying oil also displayed good biodegradability in the soil environment. Findings from this work indicated that the slow release and environmentally friendly PCUs have great potential for applications in horticulture and agriculture.