The Preparation,macrostructures And Characterization Of PLA Self-reinforced Composites

Poly(lactic acid)(PLA) became the fastest developing biodegradable polymer due to its sustainable feedstock, biodegradablity and biocompatibility. PLA has been widely used in packaging, textile, agriculture, and biomedical applications. However, poor mechanical properties(low elongation rate and resistance) and low soften temperature limited its use in durabal packaging, heat-resisting packaging and other areas. To improve the mechanical and thermal properties of PLA and maintain its biodegaradable and bicompatibility at the same time, using poly(L-lactic acid)(PLLA) and poly(L,D-lactic acid)(PDLLA) as materials, self-reinforced PLA(SR-PLA) composities was developed according to the physical differences between PLLA and PDLLA.Self-reinforced composites was developed by film stacking which used PDLLA as matrix material and PLLA fiber as reinforcement material based on their thermal difference. The effects of the thermal treatment on the properties of interface and PLLA fiber and the relationship between interface, fiber and composites were studied as well. The results showd that the crystallinity of PLLA fiber increased first and then kept constant with the thermal treatment temperature increasing. Its glass transition temperature and damping property decreased with the thermal treatment temperature increasing. The tensile strength and elongation of PLLA fiber increased with the thermal treatment temperature increasing; The good interface between PLLA and PDLLA formed under 120°C because of their good miscibility and low soften temperature of PDLLA, the inferface strength increased with the processing temperature increasing; The modulus and tensile strength of SR-PLA composites improved compared with that of PDLLA sheets. The relationship between thermal processing temperature and thermal properties of SR-PLA composites depended on the properties of PLLA fiber.It was the first time to use wet fabrication to develop SR-PLA composites. This method is based on the solubility difference between PDLLA and PLLA towards ethyl acetate, the PDLLA was dissolved with ethyl acetate as matrix and partial dissolvable PLLA fiber was used as reinforcement, uniaxial SR-PLA composite was developed using PLLA fiber continuously rolling onto a roller after impregnating PDLLA solvent and short fiber SR-PLA composite was made by casing the PDLLA solvent with distributed short PLLA fiber. The results showed that ethyl acetate dissolved part of PLLA fiber and induced PLLA molecular at the fiber surface to crystallize, and the orientation in interval of PLLA fiber did not change. The fiber surface became rough which improved the impregnation of PDLLA chains and promoted PDLLA to crystallize at the fiber surface. The modulus, tensile strength and elongation of the uniaxial SR-PLA composite improved 7.35%, 206.04%, 923.17%, respectively, compared with those of matrix. However, the tensile properties of the short fiber SR-PLA composite decreased with the increase of the percentage of PLLA fiber, and damping property increased slightly. It is likely because the shear deformation easily appeared inside the transcrystalline phase or at the interface between fiber and transcrystalline phase, and the area of the interface increased with the increases of the fiber concentration. When the fiber concentration is 25%, the modulus and tensile strength improved. It is likely because the thickness of transcrystalline decreased with the increases of the fiber concentration when the amount of the matrix kept constant.To investigate the possibility of thermal processing with shear force to fabricate SR-PLA composites, PBSA was used as matrix and PLLA short fiber was used as reinforcement, the extrusion was introduced to fabricate PBSA-based composites. The work studied the relationship among structure, processing and properties. The results showed that PLLA fiber kept morphology and contributed to the mechanical properties of the composites under shear force and high temperature which below the melting temperature of PLLA fiber 30~40°CThis work focused on the key problem in the processing of SR-PLA composites, the effects of thermal processing tempreature on PLLA fiber and interface were studied, the relationship between the structure and properties of SR-PLA composites, and processing conditions was created, which provided some theory reference for semi-crystalline thermal plastics based self-reinforced composites. Wet fabrigcation was sucessfully used to fabricate SR-PLA composites which provided a novel method for developing semi-crystalline thermal plastics based self-reinforced composites.