Supercritical Fluid Treatments of Flax Fibers for Improved Mechanical Properties

Abstract

Global increases in CO2 levels and increasing concerns regarding the effects of climate change have encouraged the transportation sector to reduce the carbon footprint of its products. The automotive and aerospace sectors have focused on the replacement of monolithic materials with composites, which have the potential to reduce weight and optimize the properties of the materials they are replacing. Many structural composites contain fiberglass reinforcement, which has a high density and is energy intensive to produce. Natural fibers have been explored as a low density, renewable alternative, but high variability and low mechanical properties have historically prevented widespread use of natural fibers in structural composites. In prior work, flax fibers treated under a specific set of conditions in supercritical CO2 (scCO2) were shown to improve fiber tensile properties. In this work, the dynamic mechanical properties of the fibers treated in scCO2 were evaluated. Treatment resulted in an increase in storage modulus, loss modulus, and tan-delta, consistent with reductions in fiber porosity and increases in fiber density previously reported. Additional treatments of the fibers in supercritical nitrogen (scN2) and supercritical argon (scAr) were explored to understand how the fluid chemistry may affect fiber structure and fiber mechanical properties. Fibers treated in scAr exhibited similar crystallinity and mechanical properties to fibers treated in scCO2 under the same conditions. However, fibers treated in scN2 exhibited higher crystallinity than fibers treated in scCO2 and scAr, but no significant changes to mechanical performance. Overall, this work expands the understanding on how supercritical fluid treatments could be applied to natural fibers as a means of enhancing fiber mechanical performance for composite applications.