(511l) Microstructure Design and Release Kinetics of Layer-Wise Agglomerated Granules

Internal density plays a critical role in differentiating the capillary and disintegration kinetics in granulated systems. Pore size distributions, pore channel tortuosity, and mechanical properties dictate the final granule microstructure. Moreover, formulation functionality may also influence the structural evolution underwater. Consequently, process and formulation design significantly affect liquid penetration and dissolution onset. Studies have shown marked changes in the release profile of active ingredients attributable to optimizing composite density arrays. One way to promote material consolidation and tailor product quality attributes is through layer-wise agglomeration techniques that produce core-shell structures with a core denser than the layer. This work investigates the influence of formulation and process design on the internal structure and nutrient release of layer-wise agglomerated urea for the controlled release of fertilizers. Controlled release technology, which consists of granulating urea with polymers, is an alternative to mitigate the environmental problems derived from urea byproduct leaching in soil. Yet, there are still challenges in controlling nutrient release rates when incorporating biopolymers in the fertilizer granules. In this study, urea and a binder, xanthan/konjac gum, in powder form were mixed at a fixed binder-solid ratio to produce highly compacted composites. The composites were milled to adjusted particle sizes and subjected to a layer-agglomeration process to produce core-shell granules. The final product was dried at 80 ºC for 3 hours. The layer consisted of a mixture of urea and binder that promoted further densification of the granule structure. The core and layered granules were analyzed for their total and internal pore structure, dissolution, and microscopic behavior underwater and compared to control groups. Preliminary results for the water dissolution studies indicate that core-shell granules reduce nutrient release up to 77 % compared to core granules for the first 60 minutes of analysis. The core and layered granules containing the binder can extend the release profile in water dissolution studies for a time factor of 6 compared to the control group (with only urea). The synergism between granule density and binder functionality maintains the composite integrity while enabling binder migration and gelation at the solid-liquid interface. The gelation reduces the composite's solvation and dissolution and delays mass transport of the nutrient to the solvent phase. This study intends to unveil the role of granule internal structure in the liquid penetration, disintegration, and dissolution kinetics for a better design of controlled-release fertilizers.