Bio-based alkyd urethane formulations: Advancing sustainable agriculture and environmental protection through slow-controlled release of NPK fertilizers

Abstract

Global population growth and climate change demand sustainable food production through integrated soil fertility management. However, the use of inorganic fertilizers contributes to environmental issues, making controlled-release fertilizers a promising solution for sustained, eco-friendly crop nourishment. In this study, we present the development of Polyalkyd-urethan (PAUs) formulations based on alkyd resin to serve as coating films for NPK (Nitrogen-Phosphorus-Potassium) granules, creating Slow-Controlled Release Fertilizers (SCRF). Firstly, we thoroughly investigated the synthesis and characterization of alkyd resins from rapeseed oil and a mixture of phthalic and maleic anhydride. These alkyds were then reacted with Hexamethylene di-isocyanate (HMDI) to produce polyalkyd-urethane (PAU) films. The films were characterized using various methods including SEM, FTIR, DSC, TGA, contact angle, water uptake, and mechanical tests. The developed PAUs formulations were applied as coating films for NPK granules, and the release of Phosphorus (P), Nitrogen (N), and Potassium (K) nutrients in water was investigated following EN 13266 standards. The coated fertilizers displayed cohesive films on the NPK granules, and those coated with 6% polyalkyd-urethane formulation prepared with 100% phthalic anhydride exhibited excellent slow-release properties. After three months of incubation in water, these granules achieved a nutrient release percentage of 92.28%, 86.14%, and 82.34% for N, P, and K, respectively. Furthermore, the coated films significantly improved the hardness of the NPK granules, reaching a maximum value of 10.75 Kgf, representing a 300% improvement compared to uncoated NPK granules. These experimental results underscore the dual advantages of the biobased polyalkyd-urethane formulations, enhancing both the physical quality of fertilizers and control over nutrient release, offering economic and environmental benefits.