Biobased Polyurethane Derived from Lignin as a Coating Material for Urea Fertilizer to Achieve Controlled Release of Nitrogen

Agriculture plays a vital role in sustaining the global food supply and improving nitrogen (N) use efficiency is essential to balance crop productivity with environmental sustainability. Controlled-release fertilizers (CRFs) have gained increasing attention for their ability to regulate nutrient release through polymeric coatings applied to fertilizer granules. However, most conventional coatings are derived from petroleum-based polymers, which raise environmental concerns due to their persistence. In this study, biobased polyurethanes (BPUs) were synthesized and evaluated as sustainable coating materials for urea granules. The BPU networks were synthesized by reacting polymeric methylene diphenyl diisocyanate (pMDI) with lignin (L) as the biopolyol, along with polytetramethylene ether glycol (DIOL) and 2-ethyl-2-(hydroxymethyl)-1,3-propanediol (TRIOL). A Design of Experiments (DoE) approach was used to optimize polyol composition and ensure a stoichiometric 1:1 balance between isocyanate (NCO) and hydroxyl (OH) groups. The optimized formulation, 0.25-L_0.25-D_0.5-T with a 9% coating, exhibited a crushing strength of 2.39 ± 0.59 MPa, corresponding to an 82% improvement over uncoated urea and indicating excellent mechanical integrity. The urea release behavior of this formulation showed a gradual and sustained release, reaching approximately 88% cumulative release after 28 days in water. In soil, N release was maintained for up to 14 weeks, confirming the coating’s ability to effectively regulate nutrient diffusion over extended periods. Greenhouse studies further demonstrate that BPU-coated fertilizers supported ryegrass growth comparable to commercial CRFs, validating the performance of the biobased polyurethane system as a sustainable alternative to conventional coatings.