| 【Objective】In order to reduce the initial nutrient release rate of urea formaldehyde (UF) and to increase the mid- to late-stage nutrient release rate, citric acid (CA) was used to chemically modify urea formaldehyde to improve the UF nutrient release rate and to better match it to crop growth patterns.【Method】 A new type of slow release fertilizers with excellent nitrogen nutrient retarding effect was prepared by copolymerization of citric acid and hydroxymethyl urea (MU), the reactive precursor of urea formaldehyde, using solution polycondensation with reactive extrusion process. The functional groups and molecular structure of the citric acid modified urea formaldehyde slow release fertilizer (CAUF) were characterized by FTIR and NMR, its crystallinity was characterized by XRD, its thermal stability and thermal degradation behavior were characterized by TG and DSC, and the surface morphology of CAUF were observed by SEM. The mechanical property of the fertilizer granules and the nitrogen nutrient release behavior of CAUF in still water was determined, and the effect of CAUF on the growth of rape plants was investigated by pot experiments.【Result】 The FTIR and NMR results show that the terminal -COOH of citric acid reacts with -NH2 to form amide bonds, confirming the polymerization reaction between hydroxymethyl urea and urea, and the strong hydrogen bonding is formed between them. The XRD result shows that the citric acid modification makes the crystallinity of UF significantly lower. The TG and DSC analysis shows that the maximum pyrolysis temperature of CAUF decreases from 304 ℃ to 297 ℃ , which reduces the thermal stability and makes the material easier to degrade. The compression strength test shows that CAUF has better mechanical property. After 56 d of treatment, the cumulative nitrogen release rates of citric acid modified urea formaldehyde CAUF20, CAUF30 and CAUF40 are 30.20%, 38.84% and 47.21%, respectively, which are 2.83, 11.47 and 19.84 percentage points higher than UF, while the initial N release rates of CAUF20 and CAUF30 are 1.79 and 1.32 percentage points lower than UF, respectively. The results of rapes growth trials in pots show that the fresh weights of rapes obtained from five groups of treatments: CK, UF, CAUF20, CAUF30, and CAUF40 at the same nitrogen application rate are 1.57, 1.90, 2.24, 3.25, and 3.32 g, respectively, and the dry weights are 0.10, 0.14, 0.15, 0.18, and 0.22 g. Both growth indicators, fresh weight and dry weight, increase with increasing citric acid weight fraction.【Conclusion】Citric acid modified urea formaldehyde reduces the initial release rate of nitrogen nutrients, significantly increases the mid to late-stage release rate and promotes the growth of rapes. |
