Study On The Scientific Problems In The Preparation Of Starch/pla/chitosan Blended Antimicrobial Materials

The notion of antimicrobial active packaging has been put forward in 1990s. The antimicrobial active packaging is prepared by adding antimicrobial agents into packaging materials. Through the release of antimicrobial agents from packaging material, it can change the condition of the packed food to extend shelf-life and to improve safety, without the change of taste quality of food. Furthermore, using antimicrobial active packaging, the antimicrobial additives in food can be reduced gradually or instead thoroughly. The antimicrobial active packaging is superior to traditional packaging, and has brilliant application and development foreground. Consequently, it attracts researchers'attentions.In this thesis, with the maintenance requirement of high-water-activity food, and based on the development direction of packaging materials, it chose water-soluble chitosan as antimicrobial agents and starch/PLA blends as substrate materials to prepare antimicrobial active packaging material. Furthermore, it studied the basic scientific problems in the preparation, which include the influence of glycerol/water mixture on the phase transition and rheological property of starch, the influence of amylose/amylopectin on the glass transition of thermoplastic starch, and the antimicrobial and releasing property of chitiosan from substrate.In this thesis, it chose glycerol as plasticizer to prepare thermoplastic starch. Former researchers have studied the effect of glycerol concentration on the gelatinisation parameters of starch before; but the results were not consistent with each other. Consequently, it studied the influence of glycerol/water mixture on the phase transition of starch with different amylose/amylopectin ratio. It found that glycerol–water solution as a whole affected the gelatinisation behaviour, and glycerol was not"anti-plasticiser". When solution has"abundant"free hydroxy, the phase transition and the enthalpy of phase transition of starch can only be affected by temperature, and is independent with glycerol content. On the other hand, when plasticized by glycerol-rich solution, the M2 phase transition peak, which was caused by the phase transition within amylose–lipid complex, disappeared. The reason for this is that glycerol can extract lipid in starch and destroy amylose-lipid complex, so M2 disappeared. These results, especially the phase transition of amylose-rich starch in glycerol/water solution with different glycerol content in high temperature (~180°C), have not been reported before, and these results can be used to guide the preparation of thermoplastic starch under the plasticization of glycerol.Glass transition temperature is the key parameter for polymer materials, and determines the application of materials. But former researchers'results about the glass transition temperature for thermoplastic starch contradicted with each other. In this thesis, it set up a new method to detect glass transition temperature of thermoplastic starch. Specifically, using high-speed DSC, which the linearity heating rate can reach 500°C/min, the signal of glass transition can be separated from crystalline melting, and the influence of water evaporation on glass transition can also be eliminated. Furthermore, the high heating rate can also enlarge thermal signal, increase sensitivity, and avoid the disturbance of baseline noise. Moreover, through an adjusting method, the temperature-lag effect that is caused by high heating rate can be solved. Consequently, high-speed DSC is the most suitable method to detect glass temperature of thermoplastic starch. After that, it also studied the influence of amlylose/amylopectin ratio on T_g, and found that T_g became higher with the increasing of amylose content. This has been explained by re-crystallization, and the phase transition model of thermoplastic starch. This is the first time to detect T_g from natural thermoplastic starch, and proofs former researcher's deduction from acetate starch material.In this thesis, it used Haake Rheometer with Mixer to study the rheological property of starch under the plasticization of glycerol/water mixture. It found that under the plasticization of glycerol-rich mixture, the loading peak was weak, but the phase transition peak was obvious. Under the plasticization of water-rich mixture, the situation was on the opposite, namely loading peak was obvious, and phase transition peak was weak. Besides, when glycerol and water ratio reached 7:3 or 8:2 in mixture, because of the interaction between glycerol and water, both the lubrication and plasticization effect of solution were impaired, so both loading peak and phase transition peak were strong. On the other hand, it also evaluated the hydrophilic property of starch/PLA blends by contact angle. It found that the hydrophilic property of PLA material was poor and the soaking rate of water into material was very slow. When added with thermoplastic starch, the hydrophilic property of blends was improved and the soaking rate of water increased with more starch content.In this thesis, the chitosan was extruded with starch/PLA blend pellets to prepare antimicrobial materials. By detecting the releasing rate of chitosan from substrate materials, it found that the releasing rate was high at the beginning, which was caused by the releasing of chitosan from the surface of materials, and then the rate became slow, which was cause by the embedded chitosan in materials. On the other, from the qualitative and quantitative detection, it found that when the concentration of chitosan in materials reached 10%, the materials can have notable antimicrobial activity, and the antimicrobial activity can last for a peirod.All in all, through studying on the basic scientific problems in the preparation of starch/PLA blend antimicrobial materials, which include the influence of glycerol/water mixture on the phase transition and rheological properties of starch, the influence of amylose/amylopectin on the glass transition of thermoplastic starch, and the antimicrobial and releasing property of chitiosan from substrate, it prepared the environment friendly antimicrobial packaging materials , which can be used for the maintenance of high-water-active food. It improves the application range of starch based and starch/PLA material.