控释肥料膜层孔隙结构量化及养分释放的微孔扩散模型研究

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. This project is based on our previous observation that there exist some local micropores in the controlled release film. The objects of this research are the controlled release membranes which are produced by spray coated and reaction film preparation techniques. Our proposed experiments are, firstly, to quantitatively determine the membrane structure parameters with bubble point and mercury porosimetry methods, and then to furtherly study the membrane structure characteristics using some other analysis experiments such as water dissolution, water vapor permeability, urea permeability, and SEM testing techniques, etc. Based on the above experimental observations, we will analyze the dynamical variation of membrane pore structure under different immersion conditions, as well as the relationship between controlled release properties and the pore structure parameters. Our final goal is to explore the controlled nutrient release mechanism and the determinant factors of the nutrient release rate based on our proposed micropore-flow release model for nutrient diffusion though the membrane and valuable experimental data. These determinant factors mainly include size, number and length of the membrane pores, etc.. The investigation of the nutrient release mechanism through membrane micropores will guide the production of coated fertilizer, regulate controlled release film preparation process and help us improve the quality of coated controlled release fertilizers.

控释肥料膜层孔隙结构是影响养分渗透扩散行为和控释性能优劣的核心因素之一。在观测到控释膜层局部存在微孔的研究基础上，本项目以喷雾包膜和反应成膜两种工艺制备的控释肥料膜层为研究对象，首先建立直接量化测定膜层结构参数的压泡法和压汞法；然后结合水浸泡法、水汽渗透、尿素渗透和扫描电镜等测试技术，定量化研究不同控释膜层的物理孔隙结构特征，以及膜层受浸泡条件影响下的动态变化规律，明确孔隙参数与释放性能的相关关系；最后，根据膜层孔隙特征构建微孔释放的传质模型，并结合试验测定值验证模型计算结果的准确性。以此揭示微孔大小、数量和长度等孔隙特征参数对养分释放速率快慢的制约关系，探明膜层孔隙结构对养分释放速率的影响机制，为调控包膜肥料膜层制备过程、指导控释肥料生产和提高控释肥料产品质量提供理论依据。

控释肥料膜层孔隙结构是影响养分渗透扩散行为和控释性能优劣的核心因素之一。本项目以控释肥料膜层为研究对象，通过对聚合物包膜肥料膜层结构参数定量化研究，明确释放性能与肥膜层孔隙物理结构参数的相关关系，建立孔流扩散的释放模型。研究发现控释膜层表面连续平整，整体光滑，但在局部存在少量孔隙。孔径分布在800~3nm之间，中值孔径为4.5~5.3nm；以25nm以下的小孔为主；决定养分释放速率的是25nm以上为数不多的大孔，不同控释膜之间的主要区别是大孔的数量和尺度存在差异。随释放期延长，膜孔隙率从51%到33%减小，孔体积从0.17mL/g到0.08mL/g减小。孔道是肥芯养分的主要通道，本项目发明了聚烯烃包膜肥料膜层的最大孔的测定方法，采用塑料管端封装包膜肥膜层的装置测定最大孔隙结构。释放期为199~46天的四种包膜肥料的平均最大孔径值分别为0.41~1.93μm，最大孔径随着释放期的缩短而增大，随着微分溶出率的增加而增大，与释放性能存在密切联系。渗透系数的结果表明，尿素以稳定的渗透系数透膜释放，释放期为1-4个月聚乙烯控释尿素膜层渗透系数处于7.2~18.7 E-15m2/s的范围内，渗透系数随着释放期的延长而减小，试验测定的控释尿素释放期和膜层渗透系数呈反比例关系，符合养分释放模型中渗透量与时间的数量关系。聚乙烯致密膜的尿素渗透系数约为7.11E-18m2/s，约为聚乙烯膜层尿素渗透系数的两千倍，可认为养分主要从膜层上的孔隙结构释放。根据养分扩散规律，膜孔和厚度决定了释放快慢，根据理论计算，大颗粒尿素的膜孔孔径为微米级，实测结果也比较接近。综合来看，项目研究的结果对于理解养分释放途径，控释膜层孔结构与释放的关系均有重要的科学意义；在应用上，我们可以根据微孔扩散模型，构建包膜尿素的生产中所需要的孔隙结构，开发施肥装置，使其具有控释功能。