Preparation And Properties Of Citric Acid Urea-formaldehyde Biodegradable Slow Release Materials

Urea formaldehyde(UF)is a slow-release fertilizer that has been used in agriculture for a long time and has a large market share,with the presence of many incompletely reacted urea and short-chain molecules in UF,making its initial nitrogen release rate higher than the nitrogen required for early crop growth;at the same time,a large number of hydrogen bonding interactions are formed between and within the UF molecular chains,making the molecular chains well arranged and the molecular structure stable,forming crystalline regions that are difficult to degrade,resulting in a slower rate of nitrogen release in the middle and late stages of crop growth than the amount required in the middle and late stages.Therefore,how to simultaneously solve the problem of too fast release of nitrogen in the early stages of UF and slower release in the later stages becomes the key to improving the efficiency of UF nitrogen nutrient use.Citric acid(CA)is rapidly and completely biodegradable and is not harmful to the environment;in addition,it can regulate the soil microbial environment,increase the soil carbon content and promote crop growth.However,no research has been conducted on citric acid modified urea formaldehyde.In response to these issues,the main research and findings of this thesis are as follows:(1)The biodegradable slow release fertilizers were prepared by a reaction extrusion process using citric acid,a biodegradable small molecule organic acid,as a modifying material for UF.The optimum preparation process was obtained by orthogonal experiments and single factor optimization experiments.The results of Fourier transform infrared spectroscopy(FTIR),nuclear magnetic resonance spectroscopy(NMR),X-ray diffraction(XRD),thermogravimetric analyzer(TG),differential scanning calorimetry(DSC)and scanning electron microscopy(SEM)showed that: CA polymerized with hydroxymethyl urea(MU),the precursor of UF,to form the block copolymer CAUF;the introduction of CA significantly reduced the crystallinity of CAUF The introduction of CA significantly reduced the crystallinity of CAUF,which was44.39%,significantly lower than that of UF at 72.64%;effectively reduced the thermal stability of the material,with the maximum pyrolysis temperature reduced from 304 °C to 297 °C for UF;changed the microscopic morphology of UF,with a smoother surface for CAUF.In addition,the maximum compressive strength of CAUF was 9.09 MPa,which was 311% higher than that of UF at 2.21 MPa,indicating that CA could effectively improve the brittleness of UF.Hydrostatic release experiments were conducted on CAUF with different CA dosages,and the results showed that the release rate of CAUF in the middle and late stages was increased to different degrees with increasing CA addition,and the cumulative release rate of nitrogen nutrients increased from 27.37% to 47.21%,improving the slow release performance of nitrogen nutrients.(2)The effects of CAUF prepared under different citric acid addition(20,30 and 40 wt%,respectively)and different urea-formaldehyde ratios(1.3:1,1.6:1 and 1.9:1,respectively)on soil physicochemical properties(soil p H,soil conductivity,soil total nitrogen content,soil enzyme activity)and physiological indicators(fresh weight,dry weight and yield)of rapes were investigated using small oilseed rape pot experiments.The results showed that the cumulative N release rate of CAUF biodegradable slow-release materials in soil was higher than that of UF-1.3 treatment,and CA could effectively enhance the N nutrient release rate of UF.After different ratios of CAUF fertilizers were applied to the soil,the nutrients contained in the medium and short molecular chain segments were released into the soil rapidly,which enhanced the N content in the soil;secondly,by lowering the soil p H and increasing the soil conductivity in the early stage,it provided a good environment for the growth and reproduction of soil microorganisms,thus significantly increasing the fresh weight,dry weight and yield of rapes.The fresh weight and dry weight of rapes in the optimal treatment CAUF-40%-1.3 were 2.11 and 3.03 times higher than those in UF-1.3,respectively.