Objectives The physicochemical properties of vegetable oil polyols influence the microstructure and macroscopic properties of polyurethane materials. We changed the methanol dosage to adjust the hydroxyl value, viscosity and other indexes of palm oil-based polyols, to increase the properties of palm oil-based polyurethane coatings. The research was also designed to clarify the relationship between the physicochemical properties of polyols, polyurethane properties and the controlled release properties of fertilizers, to provide theoretical basis for optimizing coating quality and reducing cost of vegetable oil-based degradable coating fertilizer.

Methods Palm oil with high saturation was used as feedstock, and the epoxidation/ring-opening method was used to synthesize four palm oil-based polyol samples by adjusting the adding ratios of methanol, denoted as POP1, POP2, POP3, and POP4. Then the four polyol samples were used to make controlled release urea PCU1, PCU2, PCU3, and PCU4 through in-situ reaction technology, respectively. The hydroxyl value, acid value, viscosity, molecular weight and functionality of palm oil-based polyols were measured, the microstructures of the palm oil-based polyols and the formed coating films were observed by FTIR, ¹H NMR, SEM, and TG methods, respectively. The water absorption rate and cumulative N release rate of the coated urea samples were tested using weighing and water dissolving methods. The safety of the coating materials was evaluated by germination test.

Results After adjusting methanol dosage, the hydroxyl values of the synthesized four POPs were 101−132 mgKOH/g, viscosities were 365−618 mPa·s, and the functionalities were 1.89−2.73. FTIR and ¹H NMR showed that the POPs belonged to polyether ester polyols. With the increase of methanol ratio, the number of hydroxyl groups increased, but the molecular structure did not change. When the mass ratio of methanol and epoxidized palm oil was 0.8∶1−1∶1, the hydroxyl value of POP was about 130 mgKOH/g, the viscosity was less than 700 mPa·s, and functionality was more than 2. The resultant coating layers showed good thermal stability and water resistance. At the coating rate 3% (no accessory ingredient addition), the initial N release rates of PCU3 and PCU4 were about 7%, and the N release period lasted more than 20 days. The germination and safety experiment results showed that the coating materials did not cause side-effect on seed germination and crop growth.

Conclusions The optimal mass ratio of methanol and epoxidized palm oil is in range of 0.8∶1 to 1∶1. Within the range, the synthesized palm oil-based polyols have hydroxyl value about 130 mgKOH/g, functionality more than 2, and viscosity less than 700 mPa·s. The resultant palm oil-based degradable coating materials show good controlled release performance, and the palm oil proportion in the coating materials could be as high as 65%.