Starch and castor oil mutually modified, cross-linked polyurethane for improving the controlled release of urea

[Display omitted] •The bio-based polymer film was prepared using starch-based polyol and castor oil.•The starch-based polyol generated micropores in the film, regulated early N release.•Mutual modification regulated nutrient release more accurately.•The bio-based coating material was degradable and has no toxic effect to seeds. Due to the poor controlled release ability, bio-based materials are difficult for large scale application on controlled release fertilizers (CRFs). Starch-based polyol (SP) and castor oil (CO) were mutually modified, and a cross-linked polymer film was formed on the surface of urea by in-situ reaction, which improved the slow release ability of the bio-based material. The results showed that increasing the CO ratio reduced porosity of coating and prolonged the nitrogen (N) release period, while the SP changed the high-temperature wrinkle characteristics and regulated the early N release rate. The mutual modification achieved an ultra-long release period of bio-based CRUs for 7 months. The degradation rate during nine months of bio-based coatings (5.05 %) was significantly higher than that of petroleum-based (3.74 %), and the coating was non-toxic to rice seeds. Mutual modification provided a safe and effective solution for the preparation of bio-based CRFs with long-term controlled release capability.