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Urea particle coating for controlled release by using DCPD modified sulfur Yong-Hui Liu, Ting-Jie Wang ?, Liang Qin, Yong Jin Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Available online

23 November

2007 Abstract The shell of sulfur coated urea was easily cracked due to sulfur being friable.

Sulfur was modified with dicyclopentadiene (DCPD) to increase its strength and abrasion resistance. SEM images showed that the micro-structure of modified sulfur was denser and more uniform than pure sulfur. The strength of modified sulfur increased with the DCPD/S ratio. Experiments of urea particle coating with sulfur and modified sulfur were carried out in a fluidized bed coater. The shell of coated urea particles with modified sulfur was more compact than that with pure sulfur. The modification retarded the sulfur phase transformation from monoclinic to orthorhombic, avoiding the crack formation in the coating shell of sulfur. The modified sulfur coated urea particles can be produced with thinner shell and higher strength, and had better controlled release properties. ?

2007 Elsevier B.V. All rights reserved. Keywords: Coating;

Urea;

Sulfur;

Modification;

Control release 1. Introduction Sulfur coated urea (SCU) has been produced for about

40 years. Many works have reported the improvement of the coating quality [1,2]. Due to that sulfur is friable, the coated shell cracks easily, and the shell is even peeled from the particle surface during transportation, stockpile storage, and fertilization. Wax and wax- like materials were usually coated on the outer surface of the sulfur coated urea to seal the flaws to reduce the release rate. However, these have a high cost, and the wax coating made the product adhesive so that additional treatments have to be set in the flowsheet for improving the fluidity of SCU particles. Sulfur was plasticized by adding modifiers in sulfur containing composites for road repair, road-making material and concrete in building construction [3C6]. Most modifiers reported in literature were polymeric polysulfides or, alternatively, substances which react with sulfur, such as mercaptan and unsaturated hydrocarbon, to give in situ formation of polymeric polysulfides. Blight et al. [7] studied modifying sulfur with dicyclopentadiene (DCPD) and styrene, and showed that the modifiers can stop or reduce sulfur embrittlement. Bordoloi and Pearce [8] researched the viscosity of sulfur-DCPD solutions varied with reaction time and composi- tions, and showed that the viscosity increased exponentially as the time and DCPD quantity increased. A kind of sulfur-plasticizing reagent with the formula ACRCSxCRCA was reported [9], where each R is a hydrocarbon radical having up to about

10 carbon atoms, at least one A is hydroxyl or carboxyl and x is an integer from

2 to 5. This polysulfide modifier was prepared by the reaction of sulfur with a mercapto compound selected from mercapto acids and mercapto alcohol in the presence of basic catalysts such as amines and ammonia. The viscosity of modified sulfur could be reduced by adding persulfides with ACRCSxCRC A structure. In addition, a slow release particulate fertilizer product in which the fertilizer particles were encapsulated with a plasticized sulfur coating, where the plasticized sulfur was prepared by adding a certain amount of a substituted symmetrical dialkyl polysulfide plasticizer to sulfur has been reported [10]. In this paper, DCPD, a product from petroleum cracking, was used as the modifier to prepare modified sulfur as the coating material for producing controlled release urea. The properties of the modified sulfur and the sulfur coated urea particles were investigated. 2. Experimental 2.1. Materials Commercial urea particles with size in the range of 2?4 mm and