Sustainable Shellac Coatings for Controlled Release Fertilizers: Experimental and Modeled Performance
Biodegradable shellac coatings were applied to urea fertilizer granules to create controlled-release fertilizers (CRFs) with tunable release properties. Coating thicknesses of 29, 60, and 92 μm achieved complete release in water after 11, 20, and 30 days, respectively, exhibiting desirable sigmoidal release profiles. The determined urea diffusion coefficient through the shellac coating, 7.01 × 10–10 cm2/s at 20 °C, compares favorably to other biodegradable CRF coatings reported in the literature. Additional kinetic studies under varying temperatures (10–30 °C) and pH levels (4–10) revealed that both higher temperatures and pH levels accelerate nutrient release. To simulate release performance under real-world conditions and guide informed coating design, a mathematical model was developed to translate the results of water-based release data to soil environments. For a 60 μm shellac coating with a release duration of 60 days in a field capacity wetted soil, the model showed robust performance, achieving RMSE values less than 6%, accurately reflecting soil release behavior. Beyond release performance, mechanical testing revealed that shellac coating can withstand 3.5 times the force of uncoated urea without undergoing breakage, ensuring resilience against rough handling in the fertilizer industry. To address brittleness concerns, two modified shellacs were also examined. Blending shellac with a hydrophilic plasticizer increased coating elasticity by 444% but also increased urea release rates by 855% due to heightened water permeability. In contrast, aging the shellac enhanced elasticity by 429% while simultaneously decreasing release rates by 17%, attributed to the formation of a denser coating. Overall, the sigmoidal and tunable release rates of shellac-coated urea show promise for sustainable agriculture by better aligning nutrient availability with plant demand, helping to minimize negative environmental impacts, and improving nutrient utilization efficiency.
