The effects of environmental conditioning on tensile properties of high performance aramid fibers at near-ambient temperatures

Abstract

Aramid and aramid copolymer fibers are used in a wide variety of military and civilian applications; however, the long-term effects of environmental exposure on tensile properties are still not well understood. The current effort investigates the effect of hygrothermal conditioning on the tensile properties of Kevlar^® KM2 ^®, Twaron^®, and the newly available Russian copolymer, Armos^® high performance fibers. Moisture uptake studies show that at room temperature, water diffuses more slowly into the copolymer Armos ^® (D = 8.7 × 10^-13 cm²/s) compared to the Kevlar^® KM2^® and Twaron^® homopolymers (D = 2.16 × 10^-12 cm²/s and D = 1.8 × 10 ^-12 cm²/s, respectively). Tensile properties have been measured for these aramid fibers that have been conditioned in water at 40°C, 60°C, 80°C, and 100°C for periods of 17 and 34 days. For both aramid and aramid copolymer fibers, hygrothermal conditioning did not significantly change fiber tensile properties below 80°C. At the most extreme condition of 100°C, 34 days, aramid fibers showed significant loss of tensile strength (58% for KM2 and 34% for Twaron^®), while a reduction in tensile strength of 13% (Armos^®) was observed for aramid copolymer (Armos^®) fibers. Conditioned fibers exhibited no significant change in mass as a result of the conditioning procedure and FTIR spectroscopy results did not indicate signs of chemical or thermo-oxidative change due to hygrothermal conditioning. These results suggest that in aramid fibers, the primary mechanism of degradation at temperatures between 80°C and 100 °C is due to the ingress and egress of moisture in the highly ordered core structure of the fiber. The presence of water in the intercrystalline regions of the fiber core enable segmental chain motion that can relax tie molecules, alter crystal orientation, and change apparent crystallite size. Because of differences in molecular architecture and phase morphology, the aramid copolymer, Armos^®, is less susceptible to degradation of tensile properties under these conditions.