| With the development of the plastic industry to a certain extent, the white pollution of environmental problem and social problems in worldwide was appeared. Develop biodegradable plastic as an effective way to governance white pollution has been paid more and more attention. Polylactic acid (PLA) and starch as two green biodegradable polymer with the most promising, so study on starch/PLA biodegradable composites have attracted much attention in recent years. Starch/PLA composites combine the excellent properties of PLA and low cost of starch, not only can partly replace traditional petroleum based plastics, but also to expand the use of starch in non-food, is a new biodegradable plastic with great development prospects. However, there are poor interfacial compatibility problems when the hydrophilic starch directly blends with hydrophobic PLA. The purpose of this thesis is to improve the interfacial compatibility between starch and PLA, prepare a biodegradable composite with good performance.In this paper, starch was modified by plasticization and dry method esterification, and then the modified starch was blended with PLA by melt extrusion, reveal the improve mechanism of compatibility and toughness of starch/PLA composites. Systematic study of the influence of MAH content in esterified starch and esterified starch content on the properties of composites, so as to optimization formulation for preparing composite materials. The UV aging properties and soil burial degradation properties of composites has been characterized, revealing the aging and biological degradation mechanism of starch/PLA biodegradable composites. The main research contents are as follows:(1) Using ethylene glycol, glycerol and sorbitol plasticized treatment on corn starch respectively, and plasticizing effect has been characterized. The results showed that glycerol has best plasticizing effect for starch, both the tensile strength and elongation at break of glycerol plasticized thermoplastic starch (TPS) were better. The results of the influence of plasticizing treatment time on the plasticizing effect of starch and the properties of TPS showed that the degree of plasticizing of starch was improved with the increasing of plasticizing treatment time. The effect of different plasticizers on the properties of starch/PLA composites was studied. The experiment results showed that the composites plasticized with glycerol had the best compatibility, and melt flow property, mechanical property and water resistance were also better. Considering the plasticizing effect of TPS and the properties of composites, glycerol was select as plasticizer and18h as plasticizing time for subsequent experiments. The influence of glycerol content on the interface compatibility, mechanical properties, water absorption, thermal properties and melting properties of starch/PLA composites has been studied. The results show that with the increasing of glycerol content, the compatibility, tensile strength and flexibility of composites were increased gradually. At the same time, the increase of glycerol content was also help to improved melt processing property and water resistance of composites. Composites have best performance when glycerol content was15%.(2) Maleic anhydride esterified corn starch was prepared by dry method with maleic anhydride (MAH) as esterification agent. Changes of crystalline structure and crystallization of esterified starch were characterized. The results showed that the crystalline structure of starch was destroyed by esterification reaction, led to gelatinization temperature and thermal decomposition temperature of esterified starch was lower than native starch. Study on granule morphology of esterified starch was found that esterification reaction mainly occurs in the surface of starch, the starch granules have a certain degree of corrosion, and the particle size became large. The synthesis process of dry method were studied, the results showed that with the increasing of reaction temperature and reaction time, both the degree of substitution and reaction efficiency of esterified starch were gradually increased, and extended to stabilize when the temperature reached80℃and the time extended to2h. In the case of starch excess, the degree of substitution was increased with the increasing of MAH amount, and reaction efficiency was exceeding90%.(3) Compared with native starch/PLA, native starch/MAH/PLA and esterified starch/PL A composites, analyzed the compatibility of three composites. The results showed that the esterification modification of starch had the best enhancement effect to the compatibility of composites. At the same time, esterification modification can significantly improve the processing performance and water resistance of composites. The influence of MAH content in esterified starch on the properties of composites was studied. The results showed that with the increasing of MAH content, the degree of crystallinity of composites was decreased, and the compatibility between esterified starch and PL A was increased. With the effect of crystallinity decreased and compatibility increased, the glass transition temperature (Tg) and water absorption rate of composites were decreased, and melt flow property was improved. When MAH content was increased from0%to1%, the tensile strength, bending strength, storage modulus and complex viscosity were increased, but those properties were decreased when MAH content was increased to1.5%and2%. Therefore, composites have the best performance when MAH content was1%.(4) Considering the cost of production and properties of composites, that should minimize production costs, but also to ensure the performance of material, so discussed on the mixing ratio of esterified starch and PLA. The results showed that with the decreasing of esterified starch/PLA mixing ratio, the compatibility of two-phase in composites was increased, the Tg and Crystalline melting temperature (Tm) of PLA in composites were increased, but the cold crystallization temperature (Tc) showed a trend of decrease, the degree of crystallinity and mechanical strength were increased, water absorption ratio was decreased. The rheological properties of composites was closely related with the compatibility, with the decreasing of mixing ratio, the strain threshold of storage modulus began to decline was increased, but the storage modulus and complex viscosity were decreased.(5) The influence of esterification modification on the UV aging properties of composites was studied. The results showed that the anti-aging capability of esterified starch/PLA was better than native starch/PLA. When the aging time was same, the damage degree of esterified starch/PLA was less than native starch/PLA. With the prolonging of aging time, the damage extent of composites was increased. In the UV aging process, the hydrolysis rate of starch was greater than PLA, and the amorphous region of PLA was preferentially hydrolyzed. The change of crystallinity was impacted the thermal properties and mechanical properties of composites. When the aging time was same, the thermal stability and mechanical strength of esterified starch/PLA were better than those of native starch/PLA. With the prolonging of aging time, the mechanical strength of native starch/PLA and esterified starch/PLA were decreased.(6)Composites were natural degradation by soil burial method, so as to study the influence of dry method esterification modified on the degradation properties of composites. The results showed that, the degradation of first30days was mainly starch degradation, and the degradation rate of esterified starch/PLA was slower than native starch/PLA. Therefore, the damage degree of esterified starch/PLA on the surface and inside was smaller than native starch/PLA, and the degradation degree of macromolecular chain was smaller than native starch/PLA. With the increasing of soil burial time, the damage degree to native starch/PLA and esterified starch/PLA on the surface and inside was gradually increased. Test results of crystal structure showed that the non-crystalline regions of PLA was degraded first, followed by degradation of crystalline regions. When soil burial time was same, the diffraction peak intensity of PLA in esterified starch/PLA was stronger than that of native starch/PLA. Affected by the crystal structure and degradation degree, To and T; and the residual rate of thermal decomposition of composites were increased in first60days, and then gradually reduced. To and Ti and the residual rate of native starch/PLA were larger than those for esterified starch/PLA. The tensile strength and bending strength of composites were decreased gradually with prolonged soil burial time. Both the tensile strength and bending strength of esterified starch/PLA were stronger than those of native starch/PLA. |
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