Objectives Temperature is one of the important factors affecting crop growth and nutrient release rate of fertilizer. The nutrient release of temperature-sensitive controlled-release fertilizers could match with the crop nutrient uptake more efficiently. So we screened the temperature-sensitive compounds and studied their coating properties primarily.

Methods In this study, 18 different polyols, namely hexanedioicacid-1, 4-butanediol (PBA), polytetramethylene ether diol (PTMG), polycaprolactone diol (PCL) and polyethylene glycol (PEG) with varying molecular weights, were subjected to a one-step reaction with diphenylmethane diisocyanate (MDI) to prepare polyurethane (PU) films. Differential scanning calorimetry (DSC) was employed to assess the phase transition temperature of the films. A thermo-sensitive compound was chosen to prepare TSPU encapsulated urea, and the nutrient release period was tested.

Results The release performance of CRU prepared from different polyols descended in order of PTMG > PCL > PBA > PEG. For the same series of CRUs, the controlled-release performance decreased with the increase in the molecular weight of the polyols. DSC testing results indicated that PU films prepared from low molecular weight polyols (below 2000) did not exhibit thermo-sensitivity, but did in the high molecular weight polyols. The critical transition temperatures for PTMG2000, PTMG3000, PCL2000, PBA2000, and PCL3000 were 13℃, 22℃, 27℃, 39℃, and 43℃, respectively. PCL2000 and PCL3000 were selected as thermo-sensitive materials, and mixed with polypropylene glycol (PPG) in molar ratio of 2∶8 to prepare two TSPU-coated urea fertilizers (PUCF1 and PUCF2). Both PUCF1 and PUCF2 showed temperature-dependent nutrient release rates, with daily release rate of 11.1% and 7.1% at 35℃, which was 13.2 and 6.0 times of that at 25℃. The nutrient release period of PUCF1 and PUCF2 was as long as 30 days at 25℃.

Conclusions Polyols with high molecular weight exhibit thermos-sensitivity. Modification of them with diphenylmethane diisocyanate could improve the controlled release properties of the films made of the high molecular polyols. Under the experimental condition, the coated urea showed a certain thermo-sensitive characteristics and a controlled release period of up to 30 days. More researches are needed to design temperature-responsive controlled-release fertilizers.