Research On Continuous Carbon Fiber Reinforced Polylactic Acid Composites

Polylactic acid?PLA?is a completely biodegradable thermoplastic.Compared with other degradable resins,it has strength that matches engineering plastics and excellent processability.However,it has the disadvantages of poor toughness and high cost,which severely limits its wide application.Carbon fiber reinforced thermoplastic?CFRTP?composites have the characteristics of light weight and high strength.Carbon fibers?CF?have a very high specific strength and specific stiffness.Using CF as a reinforcing material can significantly improve the comprehensive properties of the material and expand the application range of the material.Therefore,it is of great significance to study continuous carbon fiber reinforced polylactic acid?CCF/PLA?composites.In this paper,CCF/PLA unidirectional prepreg tape was prepared by melt-impregnation method,and PLA was used as the resin matrix and CCF as the reinforcement.The effects of processing conditions such as heat treatment temperature,processing temperature,tension force,and traction speed on the mechanical properties of composite were studied,and the optimal process conditions were determined.The tensile properties,dynamic mechanical behavior,thermal decomposition process,crystallization and melting behavior,and crystalline morphology of CCF/PLA composites were characterized by universal material electronic testing machine,dynamic mechanical analysis?DMA?,thermogravimetric analysis?TGA?,differential scanning calorimetry?DSC?,and polarized light microscope?POM?.Through the study of the preparation process of CCF/PLA unidirectional prepreg tapes,the best preparation conditions are that the heat treatment temperature is 200°C,the processing temperature is 220°C,the tension force is 11 N,and the traction speed is 4r/min.The great improvement in the mechanical properties of prepreg tapes is due to the appropriate heat treatment temperature.The mechanism of CF heat treatment is that the sizing agent on the fiber surface undergoes a thermal oxidation reaction,which increases the oxygen-containing functional groups on the CF surface,thereby enhancing the interfacial bonding strength of the composites.As the skeleton of CCF/PLA composites,the content of CF has an important effect on the tensile properties of the composites.The tensile strength of composites increases with the fiber content increased.The stress-strain curves of pure PLA and CCF/PLA composites were piecewise fitted.The stress-strain behavior of composites is mainly divided into two phases.The stress-strain curve of the material in the first phase rises rapidly,and the tensile modulus of the composites with the fiber content of 30 wt%and 35 wt%reaches 14.54GPa and 17.40 GPa;in the second stage,most of the fibers in the prepreg tapes are broken,only a small part of the fibers can withstand the load,and the tensile modulus is reduced to10.14 GPa and 16.31 GPa,respectively,but it is also much higher than that of pure PLA.The piecewise equations obtained by fitting were used to calculate the toughness modulus of pure PLA and the composites with 30 wt%and 35 wt%fiber content were 2.5947 N/m²,28.715 N/m² and 47.488 N/m²,respectively.The results of DMA show that CF was the main bearer of the external load in the composites.When the external force was applied,the resin in the composites will transmit the alternating force to the CF,which greatly improves the E?of the composites.The initial E?of 30 wt%and 35 wt%fiber content composites reached 45.14 GPa and 54.75 GPa,respectively,which were 15.78 and 19.35 times higher than the E??2.69 GPa?of pure PLA resin,respectively.In addition,the cold crystallization of the composites occurs during the heating process,and E?rises without falling.Compared with the pure PLA,the internal friction peak of the composites is significantly reduced.The excellent heat resistance of CF and the obstruction of the molecular chain movement of PLA resin make the thermal decomposition temperature of the material increase and the heat resistance becomes stronger.And the higher the CF contents in the composites,the higher the initial thermal decomposition temperature of the materials,and the better the heat resistance.Analysis of the crystallization and melting behavior,crystalline morphology of the CCF/PLA unidirectional prepreg tapes showed that the addition of CF played a certain role in hindering the movement of PLA molecular chains.In addition,CF promoted the formation of crystal nuclei in the composites,so that glass transition temperature?T_g?of the composites were moved to a high temperature direction,the cold crystallization temperature(T_cc)were moved to a low temperature direction,melting point?T_m?were moved to a high temperature direction,and the crystallinity of the composites were improved.