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In the recent years, automakers have been striving to improve the carbon footprint of their vehicles. Sustainable composites, consisting of natural fibers, and/or recycled polymers have been developed as a way to increase the “green content” and reduce the weight of a vehicle. In addition, recent studies have found that the introduction of synthetic fibers to a traditional fiber composite such as glass filled plastics, producing a composite with multiple fillers (hybrid fibers), can result in superior mechanical properties. The objective of this work was to investigate the effect of hybrid fibers on characterization and material properties of polyamide-6 (PA6)/polypropylene (PP) blends. Cellulose and glass fibers were used as fillers and the mechanical, water absorption, and morphological properties of composites were evaluated. The addition of hybrid fibers increased the stiffness (tensile and flexural modulus) of the composites. Glass fibers reduced composite water absorption while the addition of cellulose fibers resulted in higher composite stiffness. The mechanical properties of glass and cellulose filled PA6/PP composites were optimized at loading levels of 15 wt% glass and 10 wt% cellulose, respectively.

The desire for lightweight, carbon-negative materials has been increasing in recent years, particularly as the transportation sector reduces its global carbon footprint. Natural fibers, such as flax fiber and their composites, offer a compelling combination of properties including low density, high specific strength, and carbon negativity. However, because of the low modulus and high variability in performance, natural fibers can’t compete with glass fibers as structural reinforcements in polymer composites. In this study, flax technical fibers were treated in supercritical CO 2 (scCO 2 ), and the effects of this treatment on the morphology and properties of flax fibers are reported. Treatment in scCO 2 successfully resulted in higher fiber modulus and strength by 33% and 40%, respectively. Fiber porosity was reduced by 50% and morphological changes to the fibers were observed. Specifically, fiber lumen collapsed during treatment and micro/mesoporosity was reduced by 27%. Treated flax fibers were used to create 30 vol% unidirectional flax-epoxy composites. ScCO 2 treatment raised composite modulus and strength by 33% and 25%, respectively. Because of the dependence between technical fiber size and mechanical properties, the relationship between fiber modulus and fiber size were created and applied to the rule-of-mixtures. This relationship were found to be viable representations of the fiber performance within each composite. Overall, the treatment developed in this study has the potential to significantly improve natural fiber properties, enabling their consideration for use in lightweight, semi-structural composites.

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.