Dielectric properties of high performance fibers

Kevlar^® (poly p-phenylene terephthalamide, PPTA) and PBI (poly benzimidazole) are two high-performance fibers with excellent thermal stability, a feature that makes them suitable for applications in aerospace, automotive and firefighting protective clothing. In a previous study, it was found that the tensile breaking force of yarns pulled from Kevlar/PBI fabric decreased to 50% of its original value after 2 weeks of continuous exposure at 190°C, a temperature which supposedly should allow continuous operation of both Kevlar^® and PBI fibers without any significant loss of mechanical properties. In order to investigate the mechanisms leading to the extensive drop in mechanical tensile performance observed in this blend after thermal aging, the dielectric properties were measured on fabric samples using a frequency-domain dielectric spectrometer from 10^-2 to 10⁵ Hz at temperatures from room temperature to 400°C. The results from the dielectric response of these high performance fibers are presented in this work. These are new data since there has not been many, if any, dielectric results published in the literature on these materials, mostly due to their intractable nature. Changes in the crystal structure of the PPTA fibers were found to cause remarkable changes in the dielectric response of the fabrics.