Natural Kenaf Fiber Reinforced Composites as Engineered Structural Materials

Natural Kenaf Fiber Reinforced Composites as Engineered Structural Materials David B. Dittenber Constructed Facilities Center, West Virginia University Compared to most synthetic fibers, natural fibers are low-cost, easy to handle, have good strengthand stiffness-to-weight ratios, and require only around 20-40% of the production energy. Natural composites also offer the long-term potential to create large-volume, biodegradable structural components using only renewable resources. Two of the main challenges facing natural fiber reinforced polymer (NFRP) composites’ adoption as infrastructure materials are their higher moisture absorption and lower mechanical properties when compared to synthetic composites. The objective of this work was to examine these issues and to provide a comprehensive evaluation of NFRP’s ability to act as a structural material. Issues explored in this evaluation include: chemical treatment of fibers to reduce moisture absorption and improve bonding, mechanical characterization of laminates, weathering and durability, manufacture of large-scale components, theoretical modeling, and sustainability assessment. As a chemical treatment, aligned kenaf fibers were treated with sodium hydroxide (alkalization) in different concentrations and durations and then manufactured into kenaf fiber / vinyl ester composite plates. This aligned fiber composite configuration was found to possess flexural strength and stiffness over 2 times higher than for a pultruded natural fabric composite. Single fiber tensile properties and composite flexural properties, both in dry and saturated environments, were assessed. Alkalization was shown to increase single fiber tensile strength by ~35-70% and composite flexural strength by ~12-15% over untreated fibers, while stiffness was improved by as much as 50% in both single fiber tensile tests and composite flexural tests. A treatment of 5% NaOH for around 16 hours was found to produce the most promising mechanical results, although with little improvement to moisture performance. Based on ASTM standard testing, a comparison of flexural, tensile, compressive, and shear mechanical properties was also made between an untreated kenaf fiber reinforced composite, a chemically treated kenaf fiber reinforced composite, a glass fiber reinforced composite, and oriented strand board (OSB). The mechanical properties were evaluated for dry samples, samples immersed in water for 50 hours, and samples immersed in water until saturation (~2700 hours). On average, at 40% fiber volume fraction, a kenaf composite with bidirectional reinforcement had a flexural strength and stiffness of 28 ksi and 1.09 Msi, respectively, and a shear strength and stiffness of 6 ksi and 0.35 Msi, respectively. At the same volume fraction, a kenaf composite with unidirectional reinforcement had a tensile strength and stiffness of 25 ksi and 3.3 Msi, respectively, and a compressive strength and stiffness of 17 ksi and 3.35 Msi, respectively. When the mechanical properties of the kenaf composite were divided by the material density to get the specific properties, they were found to be comparable to the glass fiber composite’s specific properties. Since NFRPs are more vulnerable to environmental effects than synthetic fiber composites, a series of weathering and environmental tests were conducted on the kenaf fiber composites. The environmental conditions studied include real-time outdoor weathering, elevated temperatures, immersion in different pH solutions, and UV exposure. In all of these tests, degradation was found to be more pronounced in the NFRPs than in the glass FRPs; however, in nearly every case the degradation was less than 50% of the flexural strength or stiffness. Using a method of overlapping and meshing discontinuous fiber ends, large mats of fiber bundles were manufactured into composite facesheets for structural insulated panels (SIPs). The polyisocyanurate foam cores proved to be poorly matched to the strength and stiffness of the NFRP facesheets, leading to premature core shear or delamination failures in both flexure and compressive testing. Despite the poor performance due to the core material, theoretical calculations show that kenaf SIPs offer the potential to be 2-4 times stronger and more than 2.5 times stiffer than the OSB SIPs that are already common in the construction industry, if significant improvements can be made to the core and the core-facesheet interface. The NFRPs were found to match well with the theoretical stiffness prediction methods of classical lamination theory, finite element method, and Castigliano’s method in unidirectional tension and compression, but are less accurate for the more bond-dependent flexural and shear properties. With the acknowledged NFRP matrix bonding issues, the over-prediction of these theoretical models indicates that the flexural stiffness of the kenaf composite may be increased by up to 40% if a better bond between the fiber and matrix can be obtained. Natural FRP strength prediction using maximum stress and strain criteria was again found to be relatively accurate in unidirectional tension and compression and significantly less so in flexure and shear. The sustainability of NFRPs was examined from two perspectives: environmental and socioeconomic. The environmental sustainability was assessed by a simple comparison of five International Residential Code-based wall designs: OSB SIPs, glass fiber composite SIPs, kenaf fiber composite SIPs, stick-framing, and insulating concrete forms. The environmental impact metrics of embodied energy and contributed CO2 were examined for each of the designs. The analysis results indicated that the stick-framing and OSB SIP wall designs were the cheapest and most eco-friendly, while the composite SIPs were the least eco-friendly. While the kenaf fibers themselves possess excellent sustainability characteristics, costing less while possessing a lesser environmental impact than the glass fibers, the vinyl ester resin used in the composites is environmentally hazardous and inflated the cost and embodied energy of the composite SIPs. Consistent throughout all the designs was a correlation between the respective costs of the raw materials and the respective environmental impacts. The socioeconomic study looked at the sustainability of natural fiber reinforced composite materials as housing materials in developing countries. A literature study on the country of Bangladesh, where the fibers in this study were grown, showed that the jute and kenaf market would benefit from the introduction of a value-added product like natural fiber composites. The high rate of homeless and inadequately housed in Bangladesh, as well as in the US and throughout the rest of the world, could be somewhat alleviated if a new, affordable, and durable material were introduced. While this study found that natural fiber composites possess sufficient mechanical properties to be adopted as primary structural members, the two major remaining hurdles needing to be overcome before natural fiber composites can be adopted as housing materials are the cost and sustainability of the resin system and the moisture resistance/durability of the fibers.