Poly(ε-caprolactone)-g-Guar Gum and Poly(ε-caprolactone)-g-Halloysite Nanotubes as Coatings for Slow-Release DAP Fertilizer

Abstract

Biodegradable coating materials based on PCL grafted on guar gum and halloysite nanotubes have been synthetized and used to delay the nutrients release from Diammonium Phosphate (DAP) as water-soluble fertilizer granules. The combination of hydrophobic character of PCL as biodegradable polymer and the swelling behavior of guar gum (GG) and halloysite nanotubes (HNT) as natural fillers were evaluated in this study with a view of the slow release of fertilizers. Indeed, the hydroxyl groups on the guar gum (GG) and halloysite (HNTs) surfaces act as initiators for caprolactone in situ ring opening polymerization and the developing polymers are covalently grafted to the GG and HNTs. Moreover, the hydrophilic character of fillers (GG & HNT) improves the adhesion between the coating agent and the surface of the (DAP) granules and consequently DAP granules were successfully coated by the dip-coating process with fixed thickness of ~ 25 µm as revealed by the scanning electron microscopy. The evaluation of the release behavior of DAP coated by the composites (PCL-g-GG and PCL-g-HNT) demonstrates that the liberation rate could be controlled by adjusting the fillers (GG or HNT) contents in the coating materials (PCL-g-GG and PCL-g-HNT). This encapsulation expands the nitrogen and phosphorus release to more than 50 h instead of only 2 h for uncoated DAP granules.