| Controlled-release pesticide microcapsule(MCP)can greatly improve the utilization of pesticide and reducing the environmental pollution by slow-release pesticide,which can realize the sustainable development of agriculture.The MCP with urea formaldehyde resin(UF)shell cannot only prolong the action time of pesticide,but also be used as a nitrogen source for plant after the degradation.However,UF has the disadvantages of high brittleness,low degradation rate and low water solubility;it is not suitable to be used as a single wall material to prepare microcapsule.In this paper,a kind of natural amino polysaccharide polymer material,chitosan is used to improve the degradation properties of UF.Chitosan has the properties of easy-degradation,high water solubility and biocompatibility,so it can modify the degradation rate of UF.However,chitosan has a large molecular weight and steric hindrance,which is difficult to be cross-linked into UF shell directly.In order to solve the above problems,this paper presents a two-step method for the preparation of CS/UF resin composite shell MCP.The first step is pre polymerization of urea and formaldehyde,and then transferred to the polycondensation stage;after adding CS solution and formaldehyde,formaldehyde and chitosan begin to polymerize and the prepolymer reacts with methylol rapidly,forming a spatial network structure,which coat on the surface of the core material of the droplet.Then MCP is prepared.In this paper,the preparation methods of CS/UF resin microcapsules and the influencing factors of the controlled release properties were studied:(1)The uniform and stable preparation method of emulsion.Because of the amino group in chitosan,it is positively charged.Anionic emulsifier should not be used,so as not to charge neutrality and lose the emulsification.If we choose cationic emulsifier,then surface of emulsion droplet will be covered by positive charge,which will exclude chitosan to precipitate at the droplet surface,affecting the introduction of chitosan into shell.(2)CS has a large steric hindrance at the process of crosslinking,and it can only be dissolved in acidic solution,so the introduction of CS should be conducted during the process of polycondensation.Study showed that if the crosslinking agent-formaldehyde was introduced in Prepolymerization stage and polymerization stage individually,the efficiency of CS crosslinking into the shell would improve effectively,further improving the microencapsulation effect and controlled-release effect.MCP-4 is prepared by twice introduction of formaldehyde into the reaction.The average particle size of MCP-4 is 7.14 ?m and the encapsulation efficiency is about 70.39%,and it has good slow release property and which make it an ideal sustained-release microcapsule.(3)The content of Uron ring also affects the release property of microcapsules.Urea and formaldehyde can directly produce Uron ring in acidic solution.The Study on the effect of the introduction of urea on the properties of MCP showed that reducing the content of urea in the prepolymerization stage,turning to the introduction of urea in the polycondensation stage,can effectively improve the content of Uron in shell.In addition,due to the reduction of the pre stage U/F,there are more formaldehyde transferred from prepolymerization phase into the polycondensation stage so that the volume of the formaldehyde at polycondensation stage increased,so to some extent,the crosslinking efficiency of CS into the shell is improved.These two effects can improve slow-release effect of MCP.The average particle size of MCP-3 is 11.7?m and the encapsulation efficiency is about 48.97%,and it has good slow release property.(4)In order to compare the effect of CS and Uron in the shell on the encapsulation efficiency and sustained release effect of MCP,the amount of urea and formaldehyde is reduced in the prepolymerization stage and increased in the polycondensation stage.The results showed that the release rate of MCP improved by increasing the content of Uron ring,but the increase of the content of Uron ring also affected the encapsulation rate of MCP.Therefore,it is necessary to introduce some formaldehyde in the polycondensation stage to ensure the crosslinking efficiency of CS into the shell,and then to ensure the encapsulation rate of the MCP;it was also necessary to introduce some urea to improve the content of Uron ring in the shell,and further improve the release efficiency of MCP.After optimization,the average particle size of MCP-7 was 6.44?m,the encapsulation rate was up to 62.26%,and the release performance was the best. |
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