Preparation And Research Of Long Polyimide Fiber Reinforced Nylon 6 Composites

Long fiber reinforced thermoplastics (LFT) technologies have been achieved a rapid and constant development in recent years. Nowadays, the fiber that commonly used to enhance the thermoplastics is inorganic fibers such as glass fiber, carbon fiber, etc. But since one is organic, the other is inorganic, it is often faced with the problem of poor compatibility. This work has conducted an extensive investigation on the effect of discontinuous long polyimide (PI) fiber that has a high elastic modulus of 115 GPa and tensile strength of 3.1 GPa on the mechanical properties, fracture morphology and crystallization behaviors of polyamide-6 (PA6) matrix composites. A series of PA6 matrix composites with different content of PI fiber were prepared through a standard melt-pultrusion process.The resulting composite specimens not only achieved a prominent reinforcement but also obtain a significant improvement in impact toughness. It is highlighted that the Izod impact strength of the composites increases by a factor of five at a fiber content of 12 wt%. From the three point bending test, we found that the fracture toughness of the material that is adding fiber has improved significantly, and showed a same trends with impact strength. The mechanical properties could be well predicted by the Cox-Krenchel model. The morphologic observation of fracture surface indicated that fiber pullout was a major mechanism for tensile failure as a result of long PI fiber-reinforcing effectiveness, and it was also the predominant energy absorption mechanism for the impact fracture of composite specimens. Meanwhile, the coarse fiber surface also indicated a good interfacial adhesion between the thermoplastic matrix and fibers, so when the fiber debonding occurs, it could spread part of energy to enhance the mechanical properties of the composites. The samples after injection is immersed into the concentrated formic acid solution to remove matrix. Then when we analysis the length of the residual fiber, we can see that polyimide fibers loss less of fiber length in the process compared to carbon fiber and glass fiber. And the fiber show a long and curved shape. The fiber have a greater length than critical fiber length which means it could take full advantage of its strength. The crystallization behavior of the materials was analyzed by DSC, and the results were fitted with Avrami model. The results showed that the addition of fiber played a role of nucleating agent, which improved both the crystallinity and the crystal ability of PA6. The specimen was first culturing in the hot stage, and then using polarized light microscopy to observe the influence of fiber to matrix. In this process, we observed a full process of the formation of transcrystallization, which can improve the Interface property between fiber and the matrix.