Effects of polyether polyol hydroxyl equivalent weight on controlled release polyurethane coatings of urea (46-0-0)

Abstract

Layer-by-layer polyurethane coatings incorporating wax additives were applied to urea granules (46-0-0) to investigate the influence of poly(propylene oxide) triol molecular weight on controlled-release fertilizer (CRF) performance. Triols ranging from 260–1000 g mol⁻¹ were evaluated at constant coating weight, triethanolamine active catalyst inclusion, temperature, and isocyanate index to modulate hydroxyl equivalent weight (HEW), calculated crosslink density, and measured glass transition temperature (T_g) for the coating formulations. Castor oil-derived polyurethane served as a benchmark to validate the chosen coating process variables. Lower-HEW triols required increased polyisocyanate content to maintain stoichiometry, resulting in higher crosslink densities and enhanced nutrient release control. In contrast, the glycerol-based coating, despite a high calculated crosslink density and measured T_g failed to form uniform films, leading to negligible release. These results establish a structure–property relationship linking poly(propylene oxide) triol molecular weight to CRF performance, enabling tunable release profiles through precise polyurethane formulation.