Improvement of Natural Fiber Mechanical Properties for Structural Polymer Composites

Abstract

Transportation of people and goods accounts for 25% of global energy consumption, with personal transportation accounting for more energy consumption than all forms of freight combined. As global CO2 levels have increased in recent years, the transportation sector has increased its focus on the development of low carbon footprint products. Automakers have focused on the replacement of monolithic materials with composites, which can be stronger and lighter than the materials they are replacing. However, many structural polymer composites contain fiberglass reinforcement, which has high density and is energy intensive to produce. Prior work has found that replacement of glass fiber with natural fibers as reinforcing agents in polymer composites can reduce component weight by 25-30% and CO2 emissions by >8 kg/ vehicle. However, the widespread use of natural fibers as a replacement for glass in structural polymer composites has been limited by the lower intrinsic mechanical properties of natural fibers in comparison to glass. Research efforts to improve natural fiber composite properties have been mainly focused on improving fiber-matrix adhesion via chemical and physical treatments, with some treatments known to compromise the mechanical performance of the fiber itself. This work focused on the development of a treatment for natural fibers capable of improving natural fiber stiffness to enable the widespread use of natural fibers in structural composites. Treatment of flax fibers in supercritical fluids in the presence of nanomaterials was explored to attempt to improve flax fiber mechanical properties. Treatment of flax fiber in supercritical CO2 (scCO2) in the presence of titanium dioxide (TiO2) nanoparticles resulted in a 71% and 80% increase in fiber tensile modulus and ultimate tensile strength, respectively. No evidence of incorporation of TiO2 nanoparticles within flax fibers was observed. Treatment resulted in changes to fiber morphology and structure. Prior work has shown that smaller cross-sectional area fibers exhibit higher strength and modulus. Treatment in scCO2 with TiO2 resulted a reduction in fiber cross-sectional area, suggesting that treatment resulted in fiber fibrillation. Additionally, after treatment, a 70% reduction in fiber porosity was observed, including collapse of the lumen (an internal closed pore within each cell in a fiber) and closure of micro/meso pores. The crystallinity of the fibers was increased by 11%, as determined via x-ray diffraction. In addition, treatment resulted in surface smoothing, as a 98% reduction in fiber surface area was observed. Two mechanisms for changes to the fibers were proposed: 1) fiber fibrillation: in which low-crystallinity, high porosity components of each fiber were removed via repeated impact with nanoparticles during treatment, resulting in a fiber with higher crystallinity, low porosity, and smaller cross-sectional area, and 2) shot peening: in which repeated impact of the fiber surface with nanomaterials under high pressure resulted in local plastic deformation of the fiber causing cellulose crystallization, surface smoothing, and pore closure. Formation of 30 vol% epoxy composites containing flax fibers treated in scCO2 with TiO2 nanoparticles resulted in composites with 43% and 37% higher modulus and strength than composites containing untreated fiber. New models for the prediction of composite modulus were created, considering fiber size as a non-negligible factor contributing to fiber modulus. Overall, this dissertation laid the groundwork for development of a cost-effective, optimized method for improving the mechanical properties of flax fibers and their resulting polymer composites.