Carbon Fiber Reinforced Thermoplastics (CFRTP) have attracted attention in the automotive industry for their productivity and high specific strength and modulus. To guarantee the durability of CFRTP, it is important to understand the mechanical properties of CFRTP under practical service temperature. Since the mechanical properties of CFRTP are affected not only by reinforcement fibers and matrix resins but also by the fiber matrix interface, to reveal the fiber/matrix interface properties is one of the important issues to be solved. Among thermoplastic resins, polyamide (PA) is expected to be used for the matrix of CFRTP, owing to good interfacial adhesion to the carbon fiber, and good moldability. Although tensile tests of CFRTP under the high temperature environment were performed, the effects of temperature on the fiber/matrix interfacial shear strength have not been fully clarified. In this study, in order to evaluate the fiber/matrix interfacial properties at 25 °C, 40 °C and 80 °C, single fiber pull-out tests, measurement of the thermal expansion coefficient of the resin, and analysis of the thermal expansion of the resin were performed. As temperature rose, resin expanded and interfacial shear strength of the CF/PA model composites decreased. This result suggests that the decrease of the interfacial shear strength by temperature rise is caused by the expansion of the resin.

To enhance the application of Carbon Fiber Reinforced Thermoplastics (CFRTP) for automobile, it is important to understand the mechanical properties of CFRTP under service environmental conditions. Since the vehicle roof surface is exposed to high temperature, to clarify mechanical properties of CFRTP under high temperature environment is important for reliability assessment. Since the mechanical properties of CFRTP under high temperature environment depend on matrix resin, the highly heat resistant resin is preferred for the matrix of CFRTP. Among highly heat resistant resin, polynonamethyleneterephthalamide (PA9T) is expected to be used for the matrix of CFRTP due to its properties such as high heat-moisture resistance. Although the mechanical properties of neat PA9T resin at high temperature were evaluated and showing better heat resistance property than PA46 and PA6T, the potential of PA9T as the matrix of CFRTP have not been clarified yet. Since the mechanical properties of CFRTP are affected by not only the properties of reinforcing fibers and the matrix resin but also the properties of fiber/matrix interface, it is important to evaluate the effects of temperature on the fiber/matrix interfacial properties of CF/PA9T. In this study, single fiber pull-out tests at R.T., 40 °C and 80 °C, measurement of specific volume and softening point of the resin were conducted to evaluate the fiber/matrix interfacial properties of CF/PA9T model composites and the results were compared with these of CF/PA6 and CF/PA12 model composites. The resin expanded and interfacial shear strength of the CF/PA9T model composites decreased as temperature rose, and the decrease rate of interfacial shear strength of the CF/PA9T model composites was smaller than these of CF/PA6 and CF/PA12 model composites. Instead of chemical deterioration and softening of the resin, the reduction of residual stress at high temperature under 80 °C decreases the fiber matrix interfacial shear strength.

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