Tunable thermo-physical performance of castor oil-based polyurethanes with tailored release of coated fertilizers

In this work, castor oil-based polyurethanes were prepared through rotating-drum coating technology as encapsulating coating materials for slow-release fertilizers. The physico-chemical properties and microstructure of the polyurethanes were tailored by selection of diisocyanates, control of the hydroxyl number of castor oil used, and controlling the hydroxyl/isocyanate molar ratio. The effect of the microstructures and the composition of the resulting polyurethanes on the controlled release behavior of coated urea granules were evaluated by measuring the N release characteristics of the system in water under laboratory conditions. In addition, the critical mechanisms underlying the polyurethanes’ structures and properties are clarified and discussed. The results show that isophorone diisocyanate, and 2,6-diisocyanatotoluene as the hard segment are not suitable for production of slow-release fertilizers while polymethylene diisocyanate does. The N release longevity of SRFs coated with only 5.0 wt% of polyurethane can be tuned from 20 days to 140 days, constituting the widest range reported in the literature to date. The high hydroxyl number of castor oil and high content of NCO in the polyurethanes prepared lead to a high crosslink density, hydrophobicity, and compact micro-structure, resulting in a material that exhibits low water and nutrients permeabilities, and therefore slow release. This work offers a simple and effective method to create slow release fertilizers that is cost-effective and environmentally friendly.