Hydrophobic modification of castor oil-based polyurethane coated fertilizer to improve the controlled release of nutrient with polysiloxane and halloysite

Castor oil has been developed rapidly as a bio-based coating material for controlled-release fertilizers. However, the release characteristics of coated fertilizers are limited because the coating materials are hydrophilic with many micropores. To improve the surface hydrophobicity of coating materials, superhydrophobic halloysite nanotubes (SHTs) were fabricated by hydrolytic condensation of siloxane on the surface of halloysite nanotubes (HNTs). SHTs were used to modify the castor oil-based polyurethane (SHPU) to enhance the surface nanoscale roughness and reduce surface energy. Compared with a PU-coated fertilizer (PUC), the nitrogen-release properties of the hydrophobic PU-coated fertilizer (SHPUC) were improved significantly. This improvement was because the nanoscale surface roughness and hydrophobicity of SHPU increased the water contact angle, and the micropores on the surface were blocked by nanotubes and hindered water entrance into the coating. This method of hydrophobic modified coating materials could provide a new idea for development of high efficiency and environmentally friendly bio-based coated controlled-release fertilizer and sustainable development of agriculture.