Objectives The molecular structure of the polyurethane soft segment significantly influences the properties of polyurethane. This study systematically investigated the effects of polyols with different chain lengths on the molecular structure, properties, and nutrient-controlled release behavior of film materials prepared primarily from vegetable oil−polyether polyurethane. The aim is to provide theoretical foundation and technical support for developing efficient and environmentally friendly controlled-release fertilizers.

Methods A series of polyurethane-coated urea samples (labeled PPCU1−3) were prepared via in-situ curing and film-forming technology, using castor oil blended with poly(tetrahydrofuran) glycols of different chain lengths (PTMEG-1000, 650, 250) as the soft segment and polymeric methylene diphenyl diisocyanate (PAPI) as the hard segment. The chemical structure and micro-morphology of the coating layers were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The thermal stability, mechanical properties, and wettability of the films were evaluated by thermogravimetric analysis (TGA), mechanical testing, and water contact angle (WCA) measurements. Nutrient release rates were determined via a water immersion method, and pot experiments were conducted to verify the application effects.

Results As the PTMEG chain length decreased, the surface elemental composition of the coating changed significantly: the atomic percentage of C decreased continuously from 96.31% in PPU1 to 75.30% in PPU3, while N and O increased substantially from 1.92% and 1.77% in PPU1 to 13.79% and 10.90% in PPU3, respectively. The coating surface showed reduced wrinkle density, with the PTMEG-250 film becoming smoother and interfacial gaps gradually diminishing. The crosslink density and hard-segment ratio of the film materials increased significantly. With the gradual shortening of the polyol chain length, the material exhibited a regular evolution: tensile strength increased from 2.6 MPa (PPU1) to 6.4 MPa (PPU3), a rise of 146%; the elastic modulus first decreased from 7.4 MPa (PPU1) to 7.0 MPa (PPU2) and then surged to 111.9 MPa in PPU3; meanwhile, the elongation at break peaked at 71.4% in PPU2 before dropping sharply to 13.9% in PPU3. The short-chain PTMEG-250 substantially enhanced the compactness of the coating and reduced interfacial gaps. Under a 3% coating rate, the release period of PPCU3 reached 70 days, which is 100% longer than that of pure castor oil-based coating. Even at a low coating rate of 2%, a release period of 49 days was maintained. Pot experiments demonstrated that the optimized coated fertilizer increased nitrogen use efficiency by 36.01% and maintained yield levels comparable to conventional fertilization while reducing nitrogen application by 30%.

Conclusions The PTMEG chain length is a key factor regulating the performance of coating materials. Using short-chain PTMEG-250 significantly increases the crosslink density and hard-segment ratio of the system, thereby enhancing the compactness of the coating. This enables the material to maintain high mechanical strength while achieving excellent long-term controlled-release functionality. Based on this principle, the precisely designed coated urea PPCU3 achieved a nutrient release period of 70 days at a 3% coating rate, and still maintained a release period of 49 days even at a low 2% coating rate, demonstrating promising controlled-release stability and potential for application rate reduction. Therefore, precise regulation of polyol chain length to optimize the crosslinked network structure offers a viable pathway for the bio-based substitution of high-performance coating materials.