One-step preparation of castor oil based polyurethane coated urea by paraffin modification and its application

Food security has been threatened by the increase in industrialization, urbanization, and desertification. The fertilizer can be applied to grow, while the fertilizer utilization rate is only 40% in China. The annual loss of nitrogen has reached up to 120 Tg, due to the ammonia evaporation volume and nitrogen oxide emissions. The unabsorbed nutrients are lost during leaching and runoff pathways, leading to soil acidification, the eutrophication of the water bodies, and greenhouse gas environmental issues. Therefore, it is an urgent need to improve the utilization rate of the fertilizer, in order to alleviate the agricultural surface pollution due to the high production in the food industry. In this study, a one-step coating was proposed to prepare the castor oil-based polyurethane-coated urea (PCU), in order to solve the complex procedure and the unfriendly environment of the conventional petroleum-based polyurethane-coated urea. The castor oil (CO) and Polymethylene polyphenyl isocyanate (PM-200) were used as the raw materials, while the paraffin was introduced as a modifier. A systematic investigation was then implemented to determine the water absorption, porosity, swelling capacity, and water contact angle of the liquid paraffin-modified polyurethane membranes before and after 7 days of immersion. The membrane with the 0.5% liquid paraffin also exhibited the lowest porosity (1.01%) and swelling capacity (1.67). Notably, the water contact angle of the 0.5% liquid paraffin-modified membrane reached 102.8°, indicating a 4.35° increase, compared with the unmodified polyurethane membrane. The optimal slow-release ratios were explored using a UV spectrophotometer, Kjeldahl nitrogen tester, and scanning electron microscope. The slow-release performance, nitrogen loss, and microstructure were determined to be the influencing factors on the castor oil content, paraffin wax addition, coating temperature, and rotational speed. The soil column drenching and field tests were carried out to validate the application. Paraffin tests showed that the slow-release period of PCU with 5% coating reached 66 days, which was 34.7% longer than that of the unmodified system, under the optimal conditions of 60% castor oil, 0.5% paraffin addition, 60 ℃ coating temperature, and 20 r/min rotational speed; Paraffin was significantly improved the hydrophobicity, where the pores of the film layer were filled to reduce the surface energy. As a result, the cumulative ammonia evaporation volume of PCU with 5%-9% coating decreased from 1 200 mg/kg (normal urea) to 170-340 mg/kg within 30 days, indicating a reduction of 71.67%-85.83%. Additionally, the 30-day leaching loss rate of the total cumulative nitrogen decreased by 77.94%-91.01%, and the losses of ammonium nitrogen, nitrate nitrogen, and amide nitrogen dropped to 119.6, 151.4, and 50.2 mg/L, respectively, indicating a decrease of 82%-93%. A field experiment was conducted in Xinxiang City, Henan Province, China. Furthermore, the maize plant height, stem thickness, chlorophyll content (SPAD value) and dry matter accumulation of the homemade film-wrapped urea-treated group were 73.13 cm, 1.883 cm, 61.20, and 322 g, respectively, under the condition of 20% yield reduction, compared with the ordinary urea-treated group, which were 11.07%, 2.3%, 9.76%, and 10.30% higher, respectively. The yield reached 11 914.35 kg/hm², which was 22.1% higher than the normal urea treatment group. The optimal raw material ratio and paraffin hydrophobic modification can be expected to overcome the technical limitations of the conventional multi-step modification of the vegetable oil-based polyols, which was 22.1% higher than that of the control. The finding can provide a promising potential approach for high-efficiency slow-release fertilizers, in order to prevent and control agricultural surface pollution.