Mechanical Activation Of Esterification Crosslinking Starch Preparation And Its Application On Slow-release Fertilizer

With the rapid growth of population in recent years,the food crisis has become an issue of widespread concern.In order to increase grain production,the use of chemical fertilizer has played an irreplaceable role in modern agricultural production.However,long-term use of fertilizer will lead to the loss of excess nutrients to the soil,resulting in soil degradation,groundwater pollution,serious loss of farmland resources and other problems.In order to solve the risk of food security and environmental problems caused by the excessive use of chemical fertilizer,slow-release fertilizer has become an important direction of research and development.Starch is a kind of natural polysaccharide polymer with a wide range of sources.Because of its excellent biodegradability,starch has become one of the ideal materials for the preparation of slow-release fertilizer.However,the industrial application of starch is limited by its structural characteristics and performance defects.At present,most scholars adopt the method of physical or chemical modification of starch to make it have ideal properties.In this paper,cassava starch was modified by mechanical activation and citric acid esterification.Then the properties and structure of modified starch were studied.Modified starch was used as raw material to prepare a new type of large particle slow-release fertilizer,and the nutrient release behavior of slow-release fertilizer was studied.The main research contents and results are as follows:（1）Cassava starch was pretreated by mechanical activation.We used starch with different activation time as raw material and citric acid as esterification reagent,and citrate starch was prepared by dry process.The degree of substitution and the content of resistant starch are used as evaluation indexes.The effects of mechanical activation time,amount of esterifying agent,reaction time and reaction temperature on the esterification reaction were investigated.The results show that the citric acid 30%,reaction temperature 130℃,reaction time of 5 h,15 min under the conditions of mechanical activation pretreatment time,can get a degree of substitution 0.225 and resistant starch content of 64.61 g/100 g.（2）The crystal structure,molecular groups,thermal properties and surface morphology of starch samples were characterized by XRD,FTIR,SEM and DSC.The results of X-ray diffraction analysis showed that the crystal structure of starch was destroyed and the relative crystallinity decreased.After esterification with citric acid,the diffraction peak of starch decreased obviously.The infrared spectrum showed that the starch pretreated by mechanical activation did not produce new chemical groups,and the esterified starch had an obvious characteristic peak near 1734cm^-1,which proved the occurrence of esterification.The scanning electron microscope showed that the morphology of starch particles gradually changed from smooth round to rough irregular shape under the action of mechanical force,and the esterified starch was ring-shaped.The results of differential scanning calorimetry analysis showed that mechanical activation pretreatment could reduce the gelatinization temperature of starch,and there was no gelatinization peak of esterified starch.（3）Citrate starch ester-based large-particle slow-release fertilizer was prepared by hot pressing method using citrate starch and PVA as slow-release matrix,ammonium dihydrogen phosphate as fertilizer nutrient and glycerin as plasticizer.The results show that the slow-release performance and mechanical properties of slow-release fertilizer decrease with the increase of ammonium dihydrogen phosphate content and increase with the increase of PVA content.The slow-release curve of slow-release fertilizer meets the skeleton dissolution mechanism.Thermal analysis showed that the esterified starch had lower glass transition temperature and melting temperature,and had good thermoplasticity.The mechanical properties and slow-release properties of the slow-release fertilizer prepared by mechanical ball milling are better than that of the slow-release fertilizer prepared by mechanical mixing.SEM analysis shows that the ball milling mixture is more beneficial to the dispersion of fertilizer in the slow-release matrix,and its slow-release performance is better.