Bio-inspired Surface Structure for Slow-release of Urea Fertilizer

In this work, bio-inspired concepts, including a Self-Healing (SH) and super hydrophobic structure, were used to produce slow-release of urea fertilizer. Following a bottom-up process, an SH layer on the urea granule was produced from a combination of two natural waxes, palm and carnauba, and fabricated by a hot-melt coating process in a pan coater. Another layer for super hydrophobicity was formed by a deposition of submicron-wax and carbon black particles on the SH layer to create a micro-nanostructure during coating. After the heat treatment, a smooth coating and even deposition of waxes throughout the urea surfaces were obtained. The properties of the waxes, a healing mechanism, and releasing profiles were examined using an optical microscope. After cracking of the coated urea surface, the intrinsic self-healing behavior was stimulated by heating the samples above 45 °C, corresponding to high ambient daytime temperatures. Air-trapping behavior was observed at the interphase of the water and coated urea, creating super hydrophobic granule surfaces which act as an invisible layer for water-penetration protection. The releasing profiles of the coated urea in soil revealed that the releasing periods could be significantly extended to four-times longer than those of the uncoated urea.