Objectives The morphology and pore properties of the controlled-release film directly affect the nutrient release rate of polyethylene (PE) -coated fertilizer. The study on them will help predicting the relationship between the structure parameters of PE films and nutrient release rate of a coated fertilizer more precisely.
Methods Polyethylene coating fertilizer samples were all prepared by spraying process in the study. The surface morphology, pore structure and maximum membrane pore sizes of the obtained controlled-release fertilizers (CRFs) were quantitatively measured using scanning electron microscopy, mercury porosimeter and water immersion method.
Results The films of CRFs were smooth, uniform and loose, a few of holes were observed, which were mainly distributed in the range of 1000–50 nm. The thickness of the controlled-release film was about 60–100 μm, and the inner surface of the film was rough. The total pore volume of the film was in range of 0.4686–1.2260 mL/g, and the average pore diameter was in range of 25.1–86.8 nm, the porosity rate was in range of 33.0%–50.6%, and the structure was loose. The maximum pore size of CRFs ranged from 990 to 480 nm with a release period of 1 to 6 months.
Conclusions Comprehensively considering the results from the three measurement methods, the controlled-release PE film is generally an even and closed spherical shell, and about 50 microns in thickness. The membrane is roughly continuous and evenly compact with diameter of 3 mm. However, there are pores of average 50 nm in the partial position, and the maximum pore is about 1 μm. The maximum pores are the main pass of water and nutrient, determining the release rate of nutrients in the CRFs.
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