Biobased Polyurethane, Epoxy Resin, and Polyolefin Wax Composite Coating for Controlled-Release Fertilizer

Reducing the use of petrochemical products in coated controlled-release fertilizers while regulating the release rate is a popular research topic in the field of controlled-release fertilizers. In this study, a novel biobased polyurethane (BPU), epoxy resin (ER), and polyolefin wax (PW) composite coating method for the controlled release of urea was successfully established. The method involved: (1) the use of PW as a modified inner coating, which improved fertilizer surface performance and reduced urea surface roughness; (2) the degradable BPU film was synthesized with liquefied starch (LS) as the outer coating material; and (3) epoxy resin is a protective layer, which improved the hydrophobicity of the coated urea for controlled release. The chemical structure, thermostability and microscopic morphology of composite-coated urea (CCU) were examined by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and scanning electron microscopy (SEM), respectively. A central composite design of response surface methodology was used to examine the effects of different film percentage, PW contents, and BPU/ER ratios on nutrient release behavior. The results showed that PW optimized the fluidity, thermal insulation properties, and microscopic surface of the particles and improved the uniformity of the heating of urea. When the same amount of ER was used, the CCU showed a 3-fold increase in the release period compared to that of the cross-linked interpenetrating coated urea. Polynomial mathematical models were established for CCU preparation and could be an effective tool for manufacturing CCUs with specific nutrient release characteristics that could meet the nutrient requirements of crops in different cropping systems. The new coating method introduced in this study could guide the development of a new generation of biobased controlled-release fertilizers.