(439d) Controlled Release of Urea Composites through Formulation and Process Design
The high solubility in water and porous structure of urea applied in crop fertilization result in a 50 % loss of its content through ammonia volatilization, a greenhouse gas, and nitrate leaching. Controlled release technology has the potential to mitigate the environmental and health impact due to urea fertilizer losses. This work aims to investigate an underexplored area: the mechanistic influence of formulation and process design on the pore internal structure and the release rate of urea composites. Urea and four binders in powder form (hydroxypropyl methylcellulose grade E5- HPMC, high molecular weight hydroxypropyl cellulose- HPC, gluten from wheat, and a mixture of xanthan and konjac gums) were mixed at a fixed binders-solid ratio of 5 % (w/w). Control (urea only) and binary mixture (urea + binder) compacts of around 300 mg were produced at different compaction pressures (50 to 200 MPa) using a punch-die set of a standard machine testing system at ambient conditions. Total porosity, apparent diffusion, and dynamic contact angle profiles of the compacts were measured. The dissolution behavior relied on the Weibull model to compare the shape, b, and scale, a, parameters. The diffusion coefficients of compacts using HPMC or gums were significantly higher than the other systems at 100 and 150 MPa. For the gum formulations, this is because of the primary particle shape distributions of the powder gums used as binders, which led to distinct compaction behavior and higher void space in the internal structure of the composites. Additionally, the higher hydrophilicity of formulations with gums also explained the rapid liquid transport through the porous compacts, with contact angle hysteresis of 14.91 ± 3.36° against 26.69 ± 3.36° for the control systems at 200 MPa. The superior hydrophilicity along with the significant hydration of the compacts using gums as binders correlated to faster swelling kinetics and superior gel strength; factors necessary to ensure a better-controlled release pattern of nutrients to the solvent phase. These results will aid in the design of controlled-release fertilizer for large scale crop operations.