Objectives Starch based polyurethane, a type of coating materials for controlled-release fertilizer production, is easy to degradation after entering soil, so results in low environmental risk. However, its strong hydrophilicity, high impurities and poor densification reduce the controlled release performance of nutrients. We optimized the composition ratio of starch based polyurethane with other additives from the chemical structure and physical properties of the coating materials.

Methods The composite coating materials (NBPU) were composed of starch-based polyurethane (BPU) and nano-TiO₂-modified polyester polyurethane (PPU). The coating procession included three steps, i.e. polyolefin wax as the base coating, BPU as the internal control layer, and PPU as the external control layer. Total 20 combination schemes of BPU, PPU and TiO₂ were designed using the response surface method, and the response models of initial nutrient release rate and release period to the BPU, PPU and TiO₂ ratio were established. Electron microscope scanning, atomic force microscopy, Fourier infrared spectroscopy, and GCMS were used to investigate the morphology and chemical structures of the composite coating materials. Thermogravimetric, Zeta potential and hydrostatic extraction method were used to evaluate the stability and nutrient release kinetics. Soil embedding bag test was used to evaluate the degradation performance of coating materials.

Results 1) Compared with BPU, the absolute Zeta potential of designed NBPU increased by 34.8% under neutral condition, and that was further increased by 26.5%−64.6% through addition of nano-TiO₂, indicating the improvement in physical stability of the NBPU After 365 days of natural degradation in soil, the weight loss rate of polyester polyurethane with 1.00% TiO₂ increased to 4.92%, which was 3.3 times higher than that without 1.00% TiO₂. 2) The surface of the composite coating layer was smooth and flat, with few pores, and the section surface was compact and uniform. The particle hardness of PPU was 85.2 N, which was 37.0% higher than that of BPU, the surface roughness was reduced by 71.3%, and the fertilizer mechanics and wear resistance were improved. BPU played a significant role in regulating the early stage of nutrient release. When the added amount of PPU was less than 3%, the nutrient release period was gradually extended with the increase of the added amount. Nano TiO₂ had a significant effect on the nutrient release rate of 3−28 days. 3) Response surface method was used to establish quadratic polynomial mathematical models of nutrient release period, initial release rate and addition amount of BPU, PPU and TiO₂ (P<0.01), the model could be used to analyze and predict the corresponding indicators. The contribution of design variables to nutrient release period was determined as PPU>BPU>TiO₂, the contribution to the initial release rate was TiO₂>BPU>PPU.

Conclusion Nano TiO₂ can improve the microphase uniformity of polyurethane coating materials and reduce the initial release rate of fertilizer nutrients, and its photocatalysis can improve the degradation rate of residual coating materials in soil after nutrient release. The composite coating can improve the structural stability of the coating materials, and the nutrient release period can be controlled by adjusting the addition ratio of PPU, BPU and TiO₂. The characteristic response model of fertilizer nutrient release based on curved surface center design can provide technical reference for practical production.