Orthotropic Thermal Conductivities of Carbon Fiber Reinforced Thermoplastic Composites

Carbon fiber reinforced thermoplastic polymer composites possess several advantages over conventional thermosetting polymer composites, including relatively rapid consolidation rate, reformability, amenability to welding and repair, and no material out-time limitation. In addition, thermoplastics have excellent toughness and low gas permeability for aerospace structural applications. The National Aeronautics and Space Administration (NASA) Hi-Rate Composite Aircraft Manufacturing (HiCAM) project is developing new technologies for the rapid production of composite aircraft to meet increasing global industrial demand for lightweight, high performance and lower cost commercial single-aisle aircraft. NASA has evaluated semi-crystalline polyaryletherketone (PAEK) resins to be used in multiple high-rate manufacturing processes. Candidate PAEK resins include polyetheretherketone (PEEK), polyetherketoneketone (PEKK) and low melt-polyaryletherketone (LM-PAEK). The material temperature, tool temperature, and placement speed need to be optimized by both experiment and thermal analysis for better composite consolidation quality. Carbon fiber reinforced composites exhibit intrinsically independent thermal properties in three orthogonal directions, and it is critical to understand orthotropic thermal properties for both the manufacturing process and thermal model analysis. Specific heat capacity, orthotropic thermal expansion, thermal diffusivity and thermal conductivity of PAEK composites fabricated with unidirectional and quasi-isotropic layup configurations of carbon fibers were evaluated by experimental tests. The thermal conductivity in the carbon fiber axial direction was about 800 to 900% higher than in the perpendicular or in-plane directions at 25°C. The orthotropic thermal conductance difference increased at elevated temperature, showing about 1100 to 1300% higher thermal conductivity in the axial direction at 300°C. In addition to these results, the effect of semi-crystalline resins and carbon fiber layup configuration on the orthotropic thermal conductance will be discussed.