Developing polysulfide castor oil–based polymer coating for controlled-release urea fertilizers

Urea coated with only sulfur is prone to cracking and exhibits poor controlled-release characteristics. Conversely, employing polymers as the sole coating material is expensive and not easily biodegradable. To address these challenges, polysulfide castor oil–based polyols with varying sulfur contents (PSOPs-30 %, PSOPs-40 %, PSOPs-50 %, and PSOPs-60 %) were successfully synthesized using a simple chemical process. This process utilized castor oil and elemental sulfur as primary raw materials. Subsequently, PSOPs-30 % and PSOPs-40 % were cured to form films (P-30 % and P-40 %) and coated urea (PSCUF-30 % and PSCUF-40 %). The synthesis process, mechanism, and physical and chemical properties of PSOPs revealed that the amount of sulfur involved in polymerization was not proportional to the initial amount of sulfur added. The structure and thermal stability of PSOPs were characterized using Fourier-transform infrared spectroscopy (FTIR), 1H nuclear magnetic resonance (1H NMR), and thermogravimetric analysis (TGA). Minor variations in crosslinking degree and sulfur content between P-30 % and P-40 % caused some differences in mechanical properties, shore hardness, water absorption, and water contact angles. TG-FTIR analysis of P-30 % and P-40 % indicated the generation of carbonyl sulfide throughout the process, predominantly in the early stage and very few in the later stage. The cumulative nitrogen release times for PSCUF-40 %–7.0 % and PSCUF-30 %–7.0 % were 50 and 60 days, respectively. In terms of degradability, whether in distilled water or NaOH solution with pH 8, the degradation rate of P-40 % was higher than that of P-30 %, reaching 6.55 % and 6.92 % over 70 days. In addition, methyl ricinoleic (MCO) was utilized as a polyol to prepare PCUF-MCO for comparative analysis.