Characterization of Carbon Mat Thermoplastic Composites: Flow and Mechanical Properties

Carbon mat thermoplastics (CMT) consisting of 12.7 mm or 25.4 mm long, 7.2 micrometer diameter, chopped carbon fibers in a polypropylene (PP) or poly(ethylene terephthalate) (PET) thermoplastic matrix were manufactured using the wetlay technique. This produces a porous mat with the carbon fibers well dispersed and randomly oriented in a plane. CMT composites offer substantial cost and weight savings over typical steel construction in new automotive applications. In production vehicles, automotive manufacturers have already begun to use glass mat thermoplastic (GMT) materials that use glass fiber as the reinforcement and polypropylene as the matrix. GMT parts have limitations due to the maximum achievable strength and stiffness of the material. In this study the glass fibers of traditional GMT are replaced with higher strength and higher stiffness carbon fibers.

The tensile strength and modulus and the flexural strength and modulus of the CMT materials were calculated for fiber volume fractions of 10-25%. Additionally, the length of the fiber (12.7 mm or 25.4 mm) was varied and four different fiber treatments designed to improve the bond between the fiber and the matrix were tested. It was found that the fiber length had no effect on the mechanical properties of the material since these lengths are above the critical fiber length. The tensile and the flexural moduli of the CMTs were found to increase linearly with the FVF up to 25% FVF for some treatments of the fibers. For the other treatments the linearly increasing trend was valid up to 20% FVF, then stiffness either stayed constant or decreased as the FVF was increased from 20% to 25% . The strength versus FVF curves showed trends similar to those of the modulus versus FVF curves. It is shown that choosing an appropriate sizing can extend the usable FVF range of the CMT by at least 5%. Published micromechanical relations over-predicted the tensile modulus of the composite by 20-60%. An empirical fiber efficiency relation was fit to the experimental data for the tensile modulus and the tensile strength giving excellent agreement with the experimental results.

Flow tests simulating the compression molding process were conducted on the CMT to determine what factors affect the flow viscosity of the CMT. The melt viscosity of the neat PP was measured using cone and plate rheometry at temperatures between 180°C–210°C and was fit with the Carreau relation. The through thickness packing stress of the CMT mat was measured for FVFs of 8-40% and was found to follow a power law behavior based on the local bending of fibers up to a FVF of 20.9%. Above this FVF the power law exponent decreases, and this is attributed to fracture of some of the fibers. Heated platens were used to isothermally squeeze the CMT at axial strain rates of 0.02-6 s^-1. The plot of the load-displacement behavior for the 10% FVF CMT was similar in shape to that for a fluid with a yield stress. For FVFs of 15-25% the load-displacement curves showed a load spike at the beginning of the flow, then followed the curve for a fluid with a yield stress. The matrix was burned off the squeezed samples, and the remaining carbon mat was dissected and visually inspected. It was found that fiber breakage increased and fiber length decreased as the FVF of the sample was increased.