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.

Carbon Fiber Composites (CFCs) recycling has received increasing interest by the composites industry, but it is still in its early stages as an industry. There are two primary challenges that need to be addressed in order to achieve full property retention during CFC recycling: (1) the ability to recover the fiber content without property loss; and (2) conversion of the recycled, short fiber material back into high-performance CFC structures. The ability to manufacture high fiber volume fraction CFCs with end-of-life products would provide an opportunity to reduce material cost and lifetime-embodied energy. In this paper, recycled, short carbon fibers are processed via solvolysis and converted into high-performance CFCs with fiber volume fraction of ~50% and excellent composite property retention. This is enabled through alignment of the discontinuous, recycled fiber feedstock using the Tailorable universal Feedstock for Forming (TuFF) process. The paper introduces the necessary steps to process the fibers in the wet-laid process and explores the resulting mechanical and microstructural properties. The importance of incoming fiber surface quality and the effect of surface contamination from residue left by the recycling process on both the TuFF process and final composite properties is discussed in detail. A pyrolytic process has been adopted to remove the residue that is a by-product of the recycling process from the incoming recycled fiber surface. The approach presents a promising pathway for the recycling of high-performance CFCs.

Recycling of carbon fibers enables a sustainable feedstock for industrial applications of high-performance composite materials. This allows light weighting with recycled carbon fibers due to their superior mechanical properties while reducing the high embodied energy and cost of virgin carbon fiber composites. This study optimizes a pyrolysis cycle for fiber recovery of an aerospace-grade thermoset prepreg and a cleaning (oxidation) step to minimize fiber degradation and left-over resin residue, enabling dispersion and alignment of the recycled, discontinuous fibers in the Tailorable Universal Feedstock for Forming alignment process. The study balances the influence of the optimized thermal cycle (pyrolysis + oxidation step) on recycled carbon fiber strength retention with the ability to disperse at the filament level to create aligned, recycled carbon fiber composite samples with high fiber volume fraction. The optimized thermal cycle for efficient fiber recovery applied a pyrolysis step at 500 °C for 4 h in an inert gas environment and an additional oxidation step at the same temperature for 100 min. This resulted in ~20% strength degradation of the fiber compared to the virgin fiber. The processed recycled composite achieved 44% fiber volume fraction with full modulus translation (~128 GPa) compared to the virgin continuous composite with strength translation (~870 MPa), reaching ~50%.