Tuning soft segment composition enables ultra-thin bio-based polyurethane coatings for controlled-release fertilizers

Polymer-coated fertilizers offer a promising strategy to address low nutrient use efficiency (NUE) in agriculture, but conventional bio-based polyurethane (BPU) coatings exhibit poor controlled-release performance, which necessitates thick coatings and thus increases cost. To overcome this bottleneck, this study systematically explores how soft segment components affect the molecular structure, macroscopic performances, and nutrient release behavior of BPU coatings. A series of coatings were designed by blending castor oil (CO) with a short-chain linear polyether polyol, polytetrahydrofuran diol (PTMEG, Mn = 250 g mol⁻¹), at varying molar ratios to establish a structure–property–release relationship, aiming to develop ultra-thin, high-performance BPU coatings. The chemical structure, morphology, thermal stability, water resistance, mechanical properties, and nutrient release behavior were systematically characterized. The results demonstrate that the CO/PTMEG ratio critically governs the hydrogen bonding density, crosslinking network, and PAPI-derived hard segment content, thereby enabling precise tuning of the coating’s macro-properties. The optimal formulation (PU3, CO/PTMEG = 1:1) exhibits a unique balance of moderate swelling behavior, enhanced rigidity, and relatively improved water resistance. At a coating rate of 3%, PCU3 achieves a nitrogen release duration of 98 d, while at an ultra-low coating rate of 2%, the release duration remains as high as 45 d. Pot experiments further confirm that PCU3 significantly enhances crop growth and NUE. Utilizing the synergistic interactions generated from polyol mixing, the present research establishes an innovative and feasible route to fabricate low-cost and high-efficiency bio-based controlled-release fertilizers.