Research On Structure And Properties Of Thermoplastic Starch Prepared Under Volume Elongation Flow

Starch is a kind of natural renewable resource formed by photosynthesis of plants.It has many advantages,such as comprehensive sources,many varieties,high yield,low price,and,most of all,it can be degraded in various natural environments.Therefore,thermoplastic starch material（TPS）has been widely investigated by researchers and is expected to replace petroleum based plastics which are difficult to degrade.The structure and performance of TPS depend on its processing to a great extent.The traditional processing method is dominated by the shear flow field,which has a strong shear effect and is easy to cause starch degradation.Because of the shortcomings of the traditional processing method,Professor Qu Jinping of South China University of the technology proposed a new method to prepare TPS materials based on the volume elongation flow.Compared with the traditional method,the processing process is dominated by the volume elongation flow with high uniformity of heat and mass transfer,and even dispersion of plasticizer.Therefore,it can reduce the degradation of starch molecules shearing and promote the melting and plasticization of starch.The twin eccentric rotor extruder used in this paper,which our group independently developed,can realize the whole processing flow field dominated by the volume elongation flow.This paper analyzes the principle and equipment of plasticizing transportation of twin eccentric rotor and summarizes the advantages of plasticizing transportation of twin eccentric rotor compared with traditional plasticizing transportation.In this paper,TPS material was prepared by the twin eccentric rotors extruder,which was dominated by the volume elongation flow.By controlling the rotor speed to control the intensity of the elongation flow effect during the processing.The effect of different processing speeds on the structure and performance was also studied in this work.The effects of bulk stretching on glycerol dispersion and starch melt plasticization were revealed by measuring and characterizing the micromorphology,hydrogen bonding,crystal structure,rheological properties,and mechanical properties of TPS prepared at different processing speeds.The experimental results show that processing speed and glycerol content have synergistic effect on the plasticizing and melting of starch,which means the increase of the rheological effect of bulk stretching can promote the dispersion of glycerin in TPS,strengthen the hydrogen bond between glycerin and starch molecules,improve the melting degree of starch,at the same time,it has little damage to the starch molecular chain and reduces the degradation of starch molecules.On this basis,TPS and TPS/Si O₂ nanocomposites were prepared by twin eccentric rotors extruder.The effects of different contents of glycerol and Si O₂ on the structure and properties of TPS were studied at 50 rpm.The experimental results show that the hydrogen bond between the starch and the glycerin with 35 phr content can be saturated,the starch particles can reach the best melting and plasticizing state,and the best comprehensive properties can be obtained.The elongation at break reaches 226.1%,while the tensile strength is 2.82Mpa;Si O₂ can form a hydrogen bond with glycerol and starch molecules to increase the thermal stability of TPS.When the Si O₂ content is less than 4 phr,it can keep good dispersion,and act as physical entanglement node in the TPS matrix,improve the strength of TPS.When the Si O₂ content is4phr,the tensile strength of TPS increase to 5.12mpa,approximately 0.82 times better than TPS without Si O₂,and the elongation at break is 170.3%.