Fully bio-based polyurethane coating for environmentally friendly controlled release fertilizer: Construction, degradation mechanism and effect on plant growth

The polyurethane coatings of controlled-release fertilizers (CRFs) have problems with low biomass content and poor degradability. The residual coatings tend to be present in the soil as microplastics, which limits their environmentally sustainable application. This study prepared a fully bio-based polyurethane CO-LTI mainly from castor oil (CO) and ethyl ester L-lysine triisocyanate (LTI) using the one-pot method. The CO-LTI coatings have nutrient-controlled release properties. As the coating percentage increased from 3 % to 6 %, the minimum cross-sectional thickness of the coating measured by scanning electron microscopy (SEM) increased from 19 μm to 42 μm, which prolonged the time of controlled release under the effect of physical barrier. The CO-LTI coatings have good biodegradability in soil, with a remarkable biodegradation rate of 27 % within 12 months. The degradation properties of CO-LTI coatings were characterized using various techniques, including SEM, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and atomic force microscopy (AFM). The biodegradation mechanism of the CO-LTI coating in soil was elucidated, and it was confirmed that the degradation was carried out under the action of chemical degradation related to microorganisms. The effects of coating residues on plant growth and soil bacterial communities were simulated by applying CRFs with different concentration gradients. They were confirmed to be related to the coating's concentration gradient and degradation process. This study provides a new approach for preparing environmentally friendly controlled-release fertilizers and valuable insights into the potential mechanisms of biodegradation and its impact on ecological environments.