Mechanism of carbon-carbon double bond regulating and controlling properties of vegetable oil-based fertilizer coating materials

Objectives Carbon-carbon double bonds in vegetable oils are the precursor to form hydroxyl group. This study investigated the effect of carbon-carbon double bonds in different vegetable oils on the physicochemical properties of polyols, and characterized the structures and properties of the polyurethane coatings derived from these polyols. The structure-property relationships among carbon-carbon double bonds, polyol characteristics, and the performances of the polyurethane coatings were elucidated, to reveal the regulatory mechanism of the carbon-carbon double bonds on the performance of the coatings.

Methods Four vegetable oils (linseed oil, soybean oil, olive oil, and palm oil) were used as feedstock, and the epoxidation/ring-opening method was used to synthesize four vegetable oil polyols, denoted as LOP, SOP, OOP, and POP. Then the four polyols were reacted with isocyanate to prepare coated urea LPCU, SPCU, OPCU, and PPCU within coating machine through in-situ reaction technology, respectively. The hydroxyl value, acid value, and viscosity of vegetable oil-based polyols were analyzed, the microstructures of the vegetable oil polyols and the coatings were characterized by Fourier Transform Infrared Spectroscopy (FTIR), ¹H Nuclear Magnetic Resonance Hydrogen Spectroscopy (¹H NMR), and X-ray Photoelectron Spectroscopy (XPS). The cumulative N release periods of the coated urea samples were tested using water dissolving and soil incubation methods. Finally, LPCU was used by a pot experiment.

Results The C=C bond contents in LO, SO, OO, and PO were 5.71, 4.26, 4.29, and 1.97, respectively. After modification, FTIR spectra of all polyols showed O-H stretching vibration peak near 3500 cm⁻¹, and ¹H NMR methylene proton peak adjacent to hydroxyl group at 3.60 ppm, indicating the successful synthesis of four types of vegetable oil polyols. The hydroxyl values of LOP, SOP, OOP, and POP were 194, 137, 141, and 132 mg KOH/g, the functionalities were 4.4, 3.6, 3.8, and 2.6, and the viscosities were 1431, 2016, 2858, and 746 mPa·s, respectively. Polyurethane coatings derived from these polyols showed significant differences in the C-O stretching vibration (1250−1000 cm⁻¹), with the fitted peak area percentages in LPU, SPU, OPU, and PPU of 18.15%, 15.43%, 11.03%, and 10.70%, respectively, and the C=O fitting peak area percentages were 4.20%, 4.15%, 3.53%, and 3.46%, respectively. The initial nitrogen release rates (<2.5%) of LPCU, SPCU, OPCU, and PPCU were 1.7%, 1.9%, 1.0%, and 2.4%, demonstrating the integrity of the coatings. The N release periods were 56, 35, 35, and 14 days, with only PPCU failing to meet controlled-release criteria. The pot experiment showed that although LPCU was reduced by 30%, the crop yield did not decrease.

Conclusions The content and position of carbon-carbon double bonds in vegetable oils critically affected the physicochemical properties of modified vegetable oil polyols and the structure and function of polyurethane coatings. Vegetable oils with high conjugated trienes and dienes exhibited higher hydroxyl values after modification, leading to higher crosslinking densities in the controlled-release coatings and improved controlled-release performances of the coated urea samples. However, the minimum number of carbon-carbon double bonds required in vegetable oils to form high-quality coatings, as well as the lower limit of double bonds at different positions, still need further research. Additionally, the process for increasing the hydroxyl values of modified vegetable oils also requires further exploration.