Tiffany Nelson-Williams, Ph.D. (2010)
Dissertation Topic: Development of Bio-based, Hazardous Air
Pollutant-Free Sheet Molding Compounds
Major Professor: Dr. Mahesh Hosur, Professor of Materials
Science & Engineering
M.S.: Materials Science, University of
B.S.: Chemistry, Tuskegee
Employment: NASA-Lewis, Cleveland, OH
The synthesis of kenaf reinforced soybean oil composites has been investigated for potential use as sheet molding compounds in automotive applications. This study was carried out by first, alkali treating kenaf fibers in hopes of enhancing the mechanical properties; synthesizing and characterizing a non-VOC causing fatty acid reactive diluent for potential replacement in styrene-containing resins; synthesizing and characterizing a plant oil-based monomer to help in reducing petroleum dependency from using synthetic unsaturated polyester resins. It has been observed that chemical treatment of natural cellulosic fibers can result in an increase in fiber fibrillation, fiber crystallinity, thermal stability, and mechanical properties. However, effectiveness of treatment is dependent upon chemical concentration, temperature, and treatment time. Bast kenaf fibers (Hibiscus Cannibinus, L.) were treated with an alkali solution to determine the effect of treatment and treatment duration on the physical, thermal, and mechanical properties. Fibers were characterized to study the surface, chemical structure, crystallinity, and mechanical properties of fiber bundles as a function of alkali treatment time. From the results, it was determined that room temperature alkali treatment was successful with removing impurities and unwanted components from kenaf fiber surfaces. X-ray diffraction data showed that there was a gradual increase in crystallinity as the treatment times increased. Additionally, it was determined that the mechanical properties were significantly increased as a result of treating kenaf fibers for 16 hours. Also, results from XPS and ATR-IR spectroscopy showed that there were very little changes in surface composition after 8 hours of treatment, which may have been an indication that obtaining an optimum treatment time was achieved.
Next, maleinized soybean oil prepolymers and methacrylated fatty acid based monomers and co-monomers were synthesized for use as a bio-based, thermosetting resin. The soybean oil prepolymer was synthesized by reacting maleic anhydride with AESO triglycerides. The addition of maleate groups to AESO was needed to further increase the reactivity of soybean oil. The resins were later cured and consisted of ~67wt% monomer to 33 wt% reactive diluents. Initially, MAESO with three different crosslinking systems were studied. When MAESO was crosslinked with styrene, MOct, or a blend of styrene + MOct, it was observed that the resins did not display high mechanical or thermo-mechanical properties that would be suitable for use as an SMC resin. Therefore, a commercial unsaturated polyester resin was added to these systems to aid in enhancing the thermal and mechanical stability. After the addition of the commercial UPE, a 12.5% increase in storage modulus and 29% increase in Tg were observed in bio-based resins.
Finally, the biocomposites for commercial UPE, biopolyester, and fatty acid biopolyester systems were fabricated and characterized to study the effect of alkali treated kenaf fibers on biocomposites properties. Overall, it was observed that alkali treatment decreased or had little influence on the flexural properties of biocomposites. On the other hand, alkali treated biocomposites showed a 28-30% improvement in compressive behavior. This was more than likely due to the reduced internal friction in the bonding of composites, which allowed for better energy absorption from compressive loading. It was also determined that alkali treated biocomposites showed up to a 30% reduction in moisture uptake compared to untreated biocomposites. It was assumed that the lower moisture absorption was caused by increased crystallinity in alkali treated kenaf. The change in crystallinity from alkali treated kenaf fibers was also believed to be the result of lower flexural properties and increased compressive properties.