聚乙烯包膜肥料控释膜层结构特征研究

【目的】包膜肥料控释膜层结构和孔隙性质直接影响其养分释放速率。研究包膜肥料膜层结构特征，可以明确膜层结构参数与养分释放速率的关系，揭示包膜肥料控释机制，为建立养分释放数理模型提供理论依据。【方法】以聚乙烯包膜肥料控释膜层作为研究对象，量化研究了聚乙烯喷涂控释膜层的结构特征参数。利用扫描电镜，观测了在不同放大倍数下，采用喷涂工艺制备的聚乙烯包膜肥料膜层的外表层、横断面和内表层特征；以压汞仪测定了膜上孔隙的大小和分布；采用泡点法研究了大孔隙的最大孔径。【结果】不同放大倍数下的扫描电镜观测结果表明，喷涂法制备的包膜肥料控释膜层外表面整体上光滑、平整、均匀、疏松，但局部存在少量孔隙，孔径主要分布在1000～50 nm的范围内；在放大倍数很高的情况下，整体上膜层无细微孔隙结构。控释膜层厚度约为60～100 μm，断面形貌疏松无孔。膜层内表面粗糙，高低起伏不平、犬牙交错。膜壳材料堆密度为0.4～0.8 g/mL，低于聚乙烯密度，属于疏松结构。孔隙结构分析结果表明，聚乙烯控释膜层的中值孔径为4.5～5.3 nm，与对比的聚乙烯薄膜基本一致，说明两种膜分子链间的细微结构没有差异；但是聚乙烯控释膜层中存在占比18%的直径约为1000～50 nm的较大孔，孔径小于10 nm的间隙占82%，进一步说明占比少的大孔影响控释膜层释放性能。喷涂控释膜层总孔体积在0.4686～1.2260 mL/g，平均孔径在25.1～86.8 nm范围内，孔隙率为33.0%～50.6%，显著高于拉伸工艺制备的聚乙烯薄膜。释放期在1～6个月的包膜控释肥料，最大孔径在990～480 nm的范围，随包膜肥料释放期的增加，膜孔直径逐步减小，说明包膜控释肥料养分释放速率与其最大孔径存在内在联系。【结论】综合3种方法的测定结果，聚乙烯控释膜层可以看作是膜层均匀致密且局部有孔隙，膜壳直径3 mm，膜层厚度约为50 μm，最大孔径为1 μm，平均孔径为50 nm的密闭球形壳体。最大孔是水分和养分进出膜层的主要通道，决定了包膜肥料养分释放速率的快慢。

Morphological structure and pore property of polyethylene controlled-release film sprayed on urea

【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.