Abstract:

Objective Polylactic acid (PLA) is a renewable biodegradable material while with limited mechanical properties, which can be improved by adding reinforced fibers. In this research, short carbon fiber (CF) was firstly modified with polyethylenimide (PEI) to prepare CF-PEI, and then modified with graphene oxide (GO) and MXene (MG) solution to prepare CF-PEI-MG. Finally, by using twin-screw extruder and injection molding methods, CF-PEI-MG reinforced PLA (CF-PEI-MG/PLA) composites were prepared and their tensile properties and failure modes were studied. The study provides a reference for the preparation of CF/PLA composites with high mechanical properties.

Method MXene was prepared by hydrofluoric acid etching method to further prepare MG solution, where the weight percentage of MG solution was 0.05%, 0.1% and 0.2%, respectively. CF-PEI-MG/PLA composites were prepared by the combination of twin-screw extruder and injection molding. The surface morphology of the modified fiber and the fracture cross section of the composite were characterized by scanning electron microscopy (SEM). The universal mechanical testing machine was applied to analyze tensile strength and elastic modulus of the modified composites. The influences of MG concentration on stress-strain curves, mechanical properties and failure modes of CF-PEI-MG/PLA composites were investigated.

Results After the modification of CF-PEI by MG, the uneven structure of the fiber surface was covered. In addition, the surface modification showed uniformity for CF-PEI-0.1MG(MG concentration is 0.1%). At the initial loading stage, the stress-strain curve of pure CF/PLA composites rose slowly and demonstrated the smallest slope, while that of CF-PEI/PLA and CF-PEI-MG/PLA composites rose more rapidly, with CF-PEI-0.1MG/PLA composite having the fastest rise and the largest slope. This meant that the interfacial modification of CF had a significant effect on the fracture strain of PLA. In addition, the strength of CF-PEI-0.1MG/PLA composite was the highest. This was because PEI, as a flexible chain segment, could improve the rigidity feature of binding with the matrix and reduce the stress concentration at the interface. In addition, MXene and GO also showed good compatibility with PEI by virtue of strong hydrogen bonds and electrostatic interactions. It was found that the elastic modulus of CF-PEI-0.1MG/PLA was 176.75% higher than that of CF/PLA, indicating that the addition of PEI and MXene-GO nanoparticles modified short CF had a significant effect on the stiffness of the composite. This was also because PEI, as a flexible chain segment, could improve the bonding between CF and MXene and GO. At the same time, the stiffness of CF/PLA composite was also improved by virtue of the high mechanical properties of MXene and GO. The third reason was that the uneven structure formed by MXene and GO on the fiber surface increased the anchoring effect with PLA matrix and further increased the mechanical interlocking of CF/PLA. In short, flexible PEI and rigid MXene-GO constructed a gradient interface layer, which effectively improved the tensile elastic modulus of modified CF/PLA. However, because the length of the fiber had been cut for several times, it could not play a good role in strengthening strength. Finally, the fracture surface of the modified CF/PLA composite was flat and white, showing a typical stress whitening phenomenon.

Conclusion CF-PEI-MG/PLA composites were successfully prepared by twin-screw extruder and injection molding methods. The modified CF could be evenly dispersed into PLA. However, in the process of preparing relevant composites, the length of the fiber was smaller than the critical reinforcement length, and the purpose of effectively improving the strength could not be achieved. The elastic modulus of PEI and MXene-modified CF/PLA composites had been significantly improved, among which CF-PEI-0.1MG/PLA composite had the best mechanical properties, and the elastic modulus was 176.75% higher than that of CF/PLA composite. In addition, the fractured surface of the related composites was flat, showing white stress phenomenon. Meanwhile, PLA fracture fragments could be found on the MXene-GO modified CF/PLA composite fracture surface. The study provides a reference for improving the mechanical properties of thermoplastic resin.

Key words: carbon fiber, graphene oxide, transition metal carbide/nitride, fiber surface modification, fiber reinforced composite, polylactic acid