| Polylactic acid (PLA) as a rapid development of bio-degradable polymer materials has broad prospects in recent years. But its poor thermal stability and flame retardant properties is restricted on wide application in aerospace, electronics, automotive and other fields. So it is important to improve thermal stability and flame resistance of PLA. In recent years, nanotechnology as a member of multi-walled carbon nanotube (MWCNTs) has been widely used in the material, while it is also an ideal polymer modifiers and paid great attention to in the field of materials.Because of its excellent mechanical properties, electrical properties and thermal properties. Firstly, the MWCNTs was processed by the way of soaking and oxidation of mixing concentrated sulfuric acid and concentrated nitric acid oxidation and became oxidized MWCNTs (f-MWCNTs); The reaction of f-MWCNTs with thionyl chloride (SOCl2) could obtain acid chloride of MWCNTs (COCl-MWCNTs); The graft reaction of COCl-MWCNTs with 9,10-dihydro-9-oxa-10-phospha phenanthrene-10-oxide (DOPO) derivative could get surface functionalized MWCNTs (f-MWCNTs-DH) in this article. Polylactic acid (PLA) as matrix, unmodified MWCNTs (p-MWCNTs), f-MWCNTs and f-MWCNTs-DH as modifier respectively, a series of PLA/ MWCNTs nanocomposites was prepared by melt blending.The structure and morphology of modified and unmodified MWCNTs, the estimation of the relative grafting rate derivatives DOPO on the surface of MWCNTs and the study on the particle size and dispersion stability in ethanol of modified MWCNTs before and after, as well as the effects of modified MWCNTs on the mechanical properties, thermal stability, flame retardancy and rheological properties of PLA were characterized by using infrared spectroscopy (FITR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermal gravimetric analysis (TGA), UV-visible spectrophotometer, laser nano-particle size analyzer, electronic universal testing machine, rotational rheometer and micro-combustion calorimeter (MCC), et al. Halpin-Tsai equation was used to evaluate the mechanical properties of PLA nanocomposites.The results showed that:(1) After the oxidation of mixed strong acids, carboxyl appears in the surface of MWCNTs; the relative graft rate of DOPO derivatives on MWCNTs is 51%; The particle sizes of p-MWCNTs, f-MWCNTs and f-MWCNTs-DH are 12147.6 nm,176.1 nm and 352.3 nm respectively, MWCNTs modified has a good dispersion stability in ethanol.(2) MWCNTs modified can significantly improve the mechanical properties of polylactic acid, of all f-MWCNTs-DH is best; When the f-MWCNTs-DH dosage is 0.1%, the mechanical properties of the composites is best. Notched impact strength of the composites is 16.9 kJ/m2, with an improvement of 32.6% than pure PLA.(3) Halpin-Tsai model can be used to evaluate dispersion of f-MWCNTs-DH in PLA matrix and predict elasticity modulus of the composite material. The elastic modulus of the composite material could be fitted better and the optimum amount of the composite of f-MWCNTs-DH could be predicted by efficiency coefficient and the direction coefficient correction, which could provide a theoretical basis for optimizing formulation and the composition composite material, combined with thermal stability and combustion performance of the composite.(4) f-MWCNTs-DH can significantly enhance the thermal stability of PLA,the limiting oxygen index of composite material increased by 22%. Reduce the heat release rate during the combustion, thereby improve the flame retardancy properties. With the f-MWCNTs-DH content increasing, the thermal stability of the composites improves, when the content of f-MWCNTs-DH is0.1%,the heat release rate is lowest, indicating that PLA matrix forms a good network structure, which can suppress combustion and improve flame retardancy. |
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