Photo-crosslinkable biodegradable polymer coating to control fertilizer release

Abstract

A photo-crosslinkable biodegradable polymer based on a poly(ɛ-caprolactone) backbone was herein developed to enhance the efficiency of nitrogen uptake from fertilizers. To this end, ammonium and calcium nitrate fertilizers were coated with photo-crosslinkable acrylate-endcapped urethane-based poly(ɛ-caprolactone) through fluidized bed technology. We hypothesized that the coating would enable to control the release time of nitrogen species from the granules for 1-3 months. Fourier transform infrared spectroscopy was used to monitor the reaction progress while chemical structure verification of the acrylate-endcapped urethane-based poly(ɛ-caprolactone) was executed via 1H-NMR spectroscopy. The physical properties and crosslinking efficiency of the coating material were determined through rheological measurements, gel fraction determination and thermogravimetric analysis before and after crosslinking. The data showed that the optimal fluidized bed processing temperature was 29°C. Scanning electron microscopy allowed to evaluate both the coating application and final coating thickness onto the polymer coated fertilizer granules. Three different coating thicknesses were applied onto each granule type. The polymer coated fertilizers were evaluated for their nitrate and ammonium release properties in sand substrate columns on laboratory scale. Osmocote bloom was used as reference throughout the experiments. The resulting cumulative release curves were fitted to a normalized non-linear regression with variable slope and the release was found to be based on a diffusion mechanism. Nitrogen release dynamics were primarily controlled by the thickness of the coating layer. The longest release times were observed for the ammonium nitrate and calcium nitrate granules with thickest coating layers (105 ± 10 μm and 148 ± 12 μm, respectively); 75 % nitrate was released after 75 - 77 and 74 - 75 days, respectively. The release was shown to be in line with the target release time of a commercial fertilizer included as benchmark throughout the release study. The herein developed polymer can be considered promising to serve as coating material for controlled release fertilizer development because of its benefits as compared to Osmocote (i.e., single coating application possible, biodegradable coating, no additional toxic components or solvents and room temperature processing).