Controlled release fertilizer with temperature-responsive behavior coated using polyether polyol (PPG) / polycaprolactone (PCL) blend-based polyurethane performs smart nutrient release

The development of the temperature-responsive controlled release fertilizers (CRFs) can tune the nutrient release matching plant demands even when temperature fluctuates. Herein, polycaprolactone (PCL) with different molecular weights (PCL500, PCL1000, PCL2000 and PCL3000) were used as raw materials and six types of PU films as coating materials for CRFs were synthesized by reacting the polyether polyol (PPG)/PCL blends and methylene diphenyl diisocyanate (MDI). Furthermore, the CRFs coated using PU films (PUCFs) were prepared in a fluidized bed device by in-situ reaction method, and the nitrogen (N) release rate of PUCFs was measured by water immersion experiment. The temperature sensitivity and compositions of PU films were evaluated with differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD). Results showed that PUCFs based on PCL500 satisfied the CRF ISO standard, but the corresponding PU film had no temperature-responsive property. However, PUCFs based on PCL2000 and the corresponding PU film showed the opposite results. FTIR and XRD results showed that the PPG/PCL2000 blends could copolymerize with MDI to format the block copolymer with the two-phase structure of crystalline phase/amorphous phase. The block copolymer comprising of PPG and PCL2000 at a molar ratio of 8 to 2 was indicated to have shape memory characteristics and the CRFs coated by this copolymer had excellent N release characteristics with a controlled release period of more than 30 days. Moreover, these CRFs showed significant temperature-switch at 33 °C. In conclusion, the production of temperature-responsive PUCF which is suitable for the application into agriculture will improve nutrient availability, indicating that the stimuli-responsive system has potential application in sustainable and green development of agriculture.