| The pore structure in the membrane of polymer coated fertilizer is one of the key factors which determine the nutrient diffusion behavior and controlled release quality.At present,there are few studies on the relationship between the properties of the film itself,the pore structure and the permeability coefficient and the nutrient release.In this study,polyethylene-coated controlled-release urea was used as the object of study.The permeability coefficient of polyethylene controlled-release membrane was determined and analysis of its nutrient release pathway;the structural characteristics of the polyethylene controlled-release film were characterized by scanning electron microscopy and mercury intrusion;the method of determination of the maximum pore size of controlled release film was improved by bubble pressure method;the maximum pore size of the polyethylene controlled-release film was determined by the improved bubble pressure method,and the relationship between membrane pore and nutrient release was investigated.The main findings are as follows:(1)Research indicated that urea was constantly released through PE controlled-release film and the permeability coefficient remained constant.The permeability coefficient for PE controlled-release film coating on urea with 1-4 months release time was in the range of 7.17~18.7E-15 m~2/s,conforming to the inversely proportional relationship between permeation amount and time in the nutrient release model.It is investigated that the measured values are close to the theoretical values and can be used in model calculation.The urea permeability coefficient of PE dense membrane was 7.11E-18 m~2/s,which is 1,000~2,600 times smaller than that of the PE controlled-release film.The contribution of permeability of polymer material itself is negligible.It can be concluded that PE controlled-release film is not a dense membrane but porous and that nutrient release is mainly determined by pore configuration of the film.(2)The results of scanning electron microscopy of polyethylene controlled-release film showed that the surface of the polyethylene controlled-release film was continuous and smooth,with no macropore morphology,but there was a small amount of porous structure.The pore volume of the controlled-release film is between 0.4686 and 1.2260ml/g,the average pore size is in the range of 25.1 to 86.8 nm,and the porosity is between33.0%and 50.6%,and there are more of the pores,the structure of loose;10nm below the hole accounted for more than 80%of the total pore area,2000nm~50nm within the distribution of a small amount of large holes,the macro porous contribution to the largest nutrient release and are the main channel for nutrient dissolution.(3)The improved bubble pressure method is stable and the pressure increases at about 0.25kpa/s;the minimum detection limit of pore size of polyethylene controlled film is 0.13?m by this device.The film maximum hole position was determined by immersion for 3 days at 25?,GQ16 as wetting agent,the bubble detection rate reached more than89%,the detection precision in 0.12%~0.38%.The improved bubble method shortens the time of sample pretreatment,simplifies the method,and the data is accurate.(4)The effect of soaking treatment on the deformation and the maximum pore size of the controlled release film is small;the maximum pore size of controlled release fertilizer was 1.69?m,0.73?m,0.70?m,0.59?m,0.48?m,and 0.42?m,respectively,for a period of 29 days,59days,75days,95days,116days and 137days;with the extension of the release period,the maximum pore size of polyolefin film was reduced,and there is a correlation between the maximum pore size and nutrient release;there is a linear relationship between the release period and the square of the maximum pore size,and the correlation coefficient was 0.985,which was significantly correlated with the Hagen-Poisseuille equation. |
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