Membrane microstructures and mechanism of thermoplastic coated urea

The nutrient release characteristic of Thermoplastic Coated Urea (PCU) is one of the most important indexes, indicating its quality. The aim of this study was to explore the membrane microstructures and mechanism of thermoplastic coated urea using electron scanning microscope (SEM), water dissolution, vapor pressure in saturated salt solution and soil extraction method. In addition mathematical models on nutrient release characteristics were developed based on the experimental results. The results indicated that the surface of thermoplastic coated urea was slick with small cracks and pores, but no large pores that permit a free circulation of the solution between the interior of grains and the exterior was observed. There were lots of fiber-network-like porosity in coating with the thickness of 53.8–65.2μm. The basic structure of coats was irregular sponginess and the pores were distributed randomly and interlaced, which forms the channels by which the water and nutrient can come into. In water dissolution method, nitrogen release rate increased with increasing incubation temperature and the nitrogen release time of PCU was 96, 80, 12, 8 and 4 d at the temperature of 25, 30, 40, 50 and 60℃, respectively. The higher the temperature, the faster the nutrients release rates. The dynamics of nitrogen release rate could be quantitatively described by three equations: the first-order kinetics equation (Nt=No (1-e-kt)), Elovich equation (Nt=a+blnt) and parabola equation (Nt=a+bt0.5). However, the imitation of the first-order kinetics equation was the best at 25 and 40℃. Nitrogen release rate of coated urea increased with the increasing of differential water vapor pressure, and decreased with the increasing of soil water contents. The differential water vapor pressure between the inside and outside of PCU was the domain factor controlling the nutrient release.