Bio-based polyurethane (PU)-coated controlled release fertilizers are attracting a lot of attention; however, generally they have poor properties, so it is difficult for them to meet the agricultural needs. Herein, γ-aminopropyl triethoxy silane (KH550) was first used with nanosilica (NS) to prepare bio-based PU nanocomposite-coated urea (KSPCU). The coating microstructures and nutrient controlled release behaviors of KSPCU were investigated and compared with those of unmodified NS containing PU nanocomposite-coated urea (SPCU) and bio-based PU-coated urea (PCU). The KSPCU with KH550 exhibited an excellent controlled release performance. Its nutrient release longevity exceeded 105 d, which was nearly 6 times greater than that of PCU and 2 times more than that of SPCU, and it was much longer than that of PCU reported in previous research at a coating rate of 3 wt %. A series of characterization methods combined with water resistance capacity and porosity measurements confirmed that a hydrogen bond was formed by the reaction between the nanoparticle and PM200, the nanoparticle was bonded on the macromolecular chain, and KH550 in the coating increased the cross-linking degree, which were beneficial to slowing down the nutrient release of the KSPCU. The innovative application of KH550 on bio-based PU-coated fertilizers will provide a new coating technology for improving their controlled release property.

To manufacture high bio-content degradable polyurethane-coated fertilizer, the co-liquefaction of corn straw and starch was carried out to convert more biomass into bio-polyol so as to substitute petroleum-based polyol. The effect of the corn straw to starch ratio on liquefaction behavior was mainly investigated by monitoring acid value, hydroxyl value, and liquefaction rate. Both chemical structures and properties of bio-polyols and their coatings were characterized by Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), etc. The results indicated that adding a certain amount of starch to corn straw for co-liquefaction could contribute to a higher hydroxyl content of bio-polyol than that from sole biomass. Compared to molecular weight, the hydroxyl value was a more significant index to ensure coating quality. The incorporation of starch segments to bio-polyol led to good thermal stability, limited residual small molecules, and high crosslinking density of the coating. The controlled-release ability of coated urea prepared by co-liquefied bio-polyol was better than that of corn-straw-based bio-polyol, and the optimal ratio of corn straw to starch was 3:1. Overall, this study provides a new route to fabricate corn straw as a cost-effective and degradable coating with good controlled-release properties, which benefits solid biomass recycling.

Vegetable-oil-based polyurethane has become a promising sustainable candidate for controlled-release fertilizer based on green chemistry. The purpose of this study was to prepare a series of coatings from selective feedstocks including five vegetable oils with a high saturation degree, mono-unsaturation degree, or poly-unsaturation degree, considering that vegetable oil fatty acids played a key role in the synthesis of polyol and polyurethane. The effect of the type and proportion of fatty acids on the physicochemical properties, microstructure, and macro-properties of vegetable-oil-derived polyols and their resulting coatings was characterized and discussed. The position and number of the hydroxy groups were determined by the type and proportion of fatty acid, and polyol from linseed oil with a high poly-unsaturation degree and three carbon–carbon double bonds had a high hydroxyl value and functionality, whereas polyol from palm oil with a high saturation degree possessed the lowest hydroxyl value and functionality. The resultant coating from linseed-oil-based polyol had a good cross-linking density, and the nitrogen release longevity of coated urea was 56 days at a coating percentage of 3%, and its nitrogen use efficiency was increased by 27.15% compared with conventional urea. Although the palm-oil-based coating had good hydrophobicity, its coated urea was not ideal. Overall, this study has enriched theories of bio-based polyurethane coatings for controlled-release fertilizers; using vegetable oil with a poly-unsaturation degree, it is easy to obtain an excellent coating for controlled-release fertilizer, and this will help provide economic and environmental benefits.

Polymer coated fertilizers (PCFs) play a crucial role in the sustainable development of precision agriculture. However, conventional petroleum-based coating materials for PCFs are non-degradable and depend on non-renewable resources. Thus, biobased coatings, particularly those derived from vegetable oils, are promising alternatives. This review presents an innovative classification of vegetable oils based on their chemical structure, specifically whether they contain natural hydroxyl groups or require hydroxylation modification. By leveraging data from the Web of Science database, we systematically analyze the research progress of vegetable oil-based polyurethane coated urea from 2005 to 2024. Hydroxyl modification strategies for vegetable oils lacking natural hydroxyl groups are explored. Preparation processes and controlled-release properties of polyurethane coated urea made from both types of vegetable oils are included. Furthermore, aiming at potential coating defects, various modification strategies and their practical effects are evaluated in detail. These insights provide a systematic reference for raw material selection and formulation optimization of vegetable oil-based coated urea. While summarizing the key research achievements, this review comprehensively describes the future challenges faced by the practical application of vegetable oil-based polyurethanes in this field, and proposes potential breakthrough paths based on material innovation and process optimization.