Comparative Analysis of Kevlar-, PBO-, and Carbon Fiber-Reinforced Polymer Composites

Fiber-reinforced composites are in increasing demand in the automotive, aerospace, and defense industries due to their robust mechanical and thermal properties. For many applications, rigid-rod high-performance polymers are ideal candidates for reinforcing fibers because of their exceptional strength to weight ratios, but their widespread use is limited by poor compressive properties. Polyacrylonitrile (PAN)-based carbon fibers (CFs) offer an alternative with improved compressive properties and much better electrical and thermal conductivity but are difficult to work. Furthermore, such fibers are extremely expensive to produce, resulting from the lengthy and costly oxidative stabilization process required to convert the PAN fibers to a thermally stable form that will not melt. While already suitable as high-performance composite reinforcements, poly(p-phenylene terephthalamide) (Kevlar) and poly(p-phenylene-2,6-benzobisoxazole) (PBO) can be converted to CFs via rapid carbonization processes without oxidative stabilization due to their pre-existing aromatic backbone. In this study, a comprehensive test of the flexural strength of polymer fiber- and carbon fiber-reinforced composites was conducted using a commercial epoxy-amine thermosetting matrix. Composites with neat Kevlar and PBO fibers were compared to those made with thermally treated Kevlar, Kevlar carbonized at 1200 °C, and commercial PAN-based CF. The successful creation of all five composites via resin transfer molding and subsequent density tests show the potential to use these fibers in composites with good fiber-matrix wettability. Flexural tests show that Kevlar-reinforced composites outperform PBO-based composites in flexural strength without any significant compromise in flexural modulus. However, PBO shows significantly higher char yields than Kevlar by thermogravimetric analysis. The structure of the PBO monomer is also more conducive to producing highly graphitizable carbon networks upon single-step carbonization, as previous literature has reported. For these reasons, we expect to produce much more robust composites in the future from carbonized PBO than were produced from carbonized Kevlar. Kevlar and PBO fibers each have unique advantages that make them attractive candidates for fiber-reinforced composite applications, and our results herein provide evidence that such fibers have the potential to address the fabrication cost of CF composites on a commercial scale. Also, their workability while still a CF precursor could allow for the fabrication of complex shaped composites that can be carbonized in a single step.