Objectives The microstructure and hydrophilicity of coating materials are important factors that affect the nutrient release characteristics of coated urea. We investigated the variations in water contact angle and water absorption rate of the coating material with prolonged immersion duration, establishing correlations between hydrophilicity characteristic parameters and nutrient release patterns. This study provides a theoretical basis for precise regulation of nutrient release in coated urea formulations.

Methods Three kinds of polyurethane membranes (COPU1, COPU2 and ESOPU) with different hydrophilicity were selected as coating materials. Based on this, three coated urea samples were prepared with the coating materials with an addition amount of 3%. The functional group characteristics of different membrane materials were characterized by Fourier infrared spectroscopy (FTIR). The microstructure, water contact angle, water absorption rate of coating materials and the nitrogen accumulation and release rate of coated urea samples were observed after being immersed in non-ionized water, with Scanning Electron Microscopy (SEM), contact angle measuring instrument and conventional methods, respectively, and thus the relationships between microstructure and hydrophilic and hydrophobic characteristics and urea nutrient release of coated urea samples were analyzed.

Results The water contact angle of three coating materials decreased first and then increased with the soaking time. When the film was not soaked, the order of water contact angle was ESOPU>COPU1>COPU2, which was opposite to the initial release rate of coated urea. The water absorption rate of coating materials had the trend of ESOPU>COPU2>COPU1, in line with the controlled-release performance of coated urea. There was a significant negative correlation between the nitrogen cumulative release rate and the water contact angle of the coating materials before the latter reached the minimum. However, a significant positive correlation existed between the nitrogen cumulative release rate and the water absorption rate of coating materials before the latter reached the saturation. There was a significant linear correlation between the release periods of coated urea and the time when the water absorption of coating materials reached saturation (y=1.94x, R²=0.987). Compared with COPU1 and COPU2, the ESOPU had more types of polar groups, and the pore size and surface porosity of coating materials were coincided with the water absorption rate and nutrient release characteristics, but the surface porosity changed inversely with the water contact angle with the extension of soaking time.

Conclusions The water absorption rate of coating materials could be used as an indicator to evaluate the controlled-release performance of coated urea, the time when it reached saturation could predict the release period of coated urea, and the chemical composition and porosity of coating materials were essential reasons for affecting the release of nutrients in coated urea.