Research On The Preparation Of Citrate Starch By Mechanical Activation-strengthened Solid Phase Reaction And Its Properties

With the widespread use of plastic,the waste which is difficult to be degraded naturally brings harm to the ecological balance of natural environment.In order to solve the problems of "white pollution" caused by the plastic trash,biodegradable plastic has become the focus of the research.Starch is an abundant natural polymer,owing to its degradation completely in the natural environment.Thus it is expected to be a promising raw material for the development of biodegradable plastics.However,starch cannot be directly used as raw material for molding process Because its melting temperature is higher than decomposition temperature.The defects in structure and properties of starch can be improved by physical or chemical modification,such as esterification.The introduction of carbonyl function would weaken the interaction of hydrogen bonds in starch molecules evidently,which can improve the thermoplastic properties of starch.Starch Citrate(SC)is an esterification product of starch and citric acid,which is mainly used for food additives,ion exchanger in sewage processing,tablet disintegrant preparation of novel biodegradable materials,etc..In this paper,SC was prepared by mechanical activation-strengthened solid phase reaction(MASPR)in a customized stirring ball mill with using cassava starch as raw material and citric acid as esterifying agent.The effects of reaction time,reaction temperature,dosages of esterification agent and alkalizing agent on the degree of substitution(DS)of the products were investigated,respectively.On this basis,the optimization conditions were determined by means of orthogonal design test.Under optimum technological conditions of reaction time 60 min,reaction temperature 50?,dosage of citric acid 40%and dosage of alkali dosage 4%,the SC with DS of 0.147 was prepared.The molecular groups,crystal structure,thermal properties and morphology of the samples were characterized by Fourier transform infrared spectrometer(FT-IR),differential scanning calorimetry(DSC),X ray diffraction(XRD)and scanning electron microscopy(SEM),respectively.In addition water absorption and rheological properties of the samples were also analyzed and compared.The results of FTIR analysis showed that the SC was produced by MASPR successfully,and the characteristic absorption peak of carbonyl group of ester increased with the increase of DS.Thermal analysis showed that the glass transition temperature and melting temperature of starch decreased after esterification.The XRD analysis indicated that the crystalline structure of starch granules was destructed by mechanical force,leading to the decrease of crystallinity and increase of amorphous regions,which had positive effects on chemical modification of starch.SEM micrographs illustrated that the smooth surfaces of compact starch granules were destructed and corroded by mechanical force and etherification reaction.The water absorption and rheological properties of the starch samples decreased with the increasing DS of SC.Furthermore,the film properties,water absorption and mechanical properties of SC/PVA composite membrane were also studied.The thermoplastic properties of starch could be improved after modified by etherification,contributing to better film properties.The carbonyl groups in SC played a important role in internal plasticization,reinforcing the activity of molecular chains and enhancing the tensile strength and breaking elongation.The tensile strength and breaking elongation of the composite film increased with the increase of DS and the water absorption effectively reduced.SEM analysis showed that the dispersion of starch in PVA was improved by esterification,which enhanced its compatibility with PVA.