Cross-link variation in polyacrylic acid polymers for coating fertilizer pellets, promoting controlled release
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Polyacrylic acid (PAA) cross-linkers have been prepared by the esterification of acrylic acid (AA) with linear diols and glycerol using an acid exchange resin catalyst (Amberlyst B-23) and Dean-Stark water removal to promote di-ester formation. Fourier-transform infrared spectroscopy (FTIR), atmospheric pressure chemical ionised mass spectroscopy (APCI-MS) and nuclear magnetic resonance spectroscopy (NMR) served to verify the formation of ester products qualitatively. The esterification reaction optimization was carried out to determine the ideal conditions for di-ester product formation without needing to use pre-reaction reagent activation steps. Following the post-synthesis workup and purification, NMR was employed to quantify the mono-ester and di-ester components of the ester mixtures by using an internal biphenyl standard and ester-specific signal identification. Ester formation favoured the mono-ester, and the mono-ester-to-di-ester ratios exceeded 3:1. The ester mixtures were investigated as PAA cross-linkers for controlled-release fertilizer coatings. Potassium nitrate (KNO₃) and monoammonium phosphate (MAP) were obtained from Omnia and coated with single-layer and multiple-layer 5 weight-to-weight percentage (w/w%) coatings containing 5% cross-linker. Cross-linkers identified for multiple-layered coatings were determined by thermogravimetric analysis (TGA), and the application of 5% cross-linked PAA followed a 2 + 2 + 1 w/w% layered coating process. Cross-linking effect on PAA was determined by TGA of polymer samples formed under coating conditions. The coating efficiency and effect on controlled-release of the coating polymer on KNO₃ and MAP were determined by scanning electron microscopy (SEM) and moisture absorbance and solution rate analyses. The effect of the cross-linked PAA had a minor effect on the thermal degradation, characteristic of PAA, suggesting limited di-ester formation and thus the presence of cross-linking. From the moisture absorbance and solution rate testing decreased solution rates were observed, however, only minimal, with the 1,5-pentanediol esterification ester mixture resulting in the best controlled-release effects. SEM analysis of the coating efficiency and coating thickness showed ineffective coating with fertilizer surface areas remaining exposed, thus explaining the limited effect of imparted controlled-release characteristics.