The effect of carbon nanotubes on the thermal expansion isotropy of injection molded carbon fiber reinforced thermoplastics

Plastics have a weight advantage over metals but they lack their strength, stiffness, dimensional stability, and electrical conduction. Fiber reinforced plastics have been used to bridge this properties gap, specifically engineering thermoplastics including carbon fibers (CFs). These composites are light, strong and stiff, electrostatically dissipating, and relatively easy to process by injection molding. However, the high aspect ratio and rigidity of the CFs cause orientation and anisotropy, especially when injection molded, thus reducing dimensional stability on the final product and limiting its use in precision components. In this study, polymer composite formulations of polyether imide containing CFs and carbon nanotubes (CNTs) were compounded and injection‐molded following a design of experiments (DOE) methodology. The coefficient of thermal expansion (CTE) in the polymer flow direction and perpendicular to the flow direction was used to evaluate thermal expansion isotropy. The electrical resistivity, impact strength, and morphological structure were also investigated. It was found that the addition of CNT caused a significant reduction in the thermal expansion anisotropy of the parts without compromising the impact strength. Also, it was found that CNTs are significantly more efficient than CFs for reducing the electrical resistivity. The conclusions of this study can be used to fine‐tune polymer composites with high dimensional thermal stability, electrostatic dissipation, and good mechanical properties, suitable for high‐performance devices. POLYM. COMPOS. 34:1367–1374, 2013. © 2013 Society of Plastics Engineers