Heat-Press Technique for Coating Ammonium Nitrate Granules with Biodegradable Hydrophobic Polymer Blend of Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) and Poly(ε-caprolactone)

The use of coated nitrogen fertilizers increases nitrogen use efficiency by reducing the nutrient’s loss to the environment through ammonia volatilization, leaching, and denitrification. Biodegradable polymers are the preferred coating materials because they are consumed and eventually mineralized by microbes, which minimizes pollution. In this study, two biodegradable hydrophobic polymers, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) and poly(ε-caprolactone) (PCL), were used to coat ammonium nitrate (NH4NO3) granules using a heat-press technique that, for the first time, eliminated the need for organic solvents at the coating stage. With a polymer coating weight content of 8.6%, the release of nitrate ions (NO3–) into water from PCL-coated fertilizers was very fast (approximately 90% in 1 week), whereas there was no release of NO3– from the PHBH-coated fertilizers until after 10 weeks. The NO3– release rate, the water vapor permeability, and the Young’s modulus of the polymer coatings were regulated by blending PHBH and PCL (80:20 weight ratio). The controlled release of NO3– was also maintained in biologically active soil; biodegradation of the coatings resulted in no burst release of NO3– over the time period tested. These results demonstrated that PHBH, PCL, or their blend, when applied to the surface of NH4NO3 granules using a heat-press technique, could effectively control nutrient release rate.