Thesis Advisor: Assoc. Prof. Dr. Ali KILIÇ
Thesis co-advisor: -
Student Name/SURNAME: Tolera Aderie NEGAWO
Supporting company/Project accomplished: -
Summary: Composite materials are developed from two or more components having dissimilar properties to create the desired materials' physical, chemical, and mechanical properties. The main constituents of composite materials formulations include matrices, fibers, and additives or fillers. When composite matrices are from polymers, it is considered polymer composite. Fibrous materials from different sources classified as synthetic and natural fibers are used to reinforce the polymeric matrices. The composites from renewable raw material sources either bio-based polymers or natural fibers have been given attention by recent research works. Due to increasing environmental awareness and concern about sustainability, the academic and industrial sectors have started to focus on greener technologies, a major part of which is played by material development.
This thesis aims to develop ensete fiber-based composite materials by characterizing their properties. Ensete fiber is extracted by decortication of pseudostem and leaf parts of the Ensete plant as a by-product. Ensete plant known as ensete ventricosum (scientific name) is a perennial plant that has leaves, a large underground corm, pseudostem, and one of the main food plants grown for traditional diet in East Africa especially in Ethiopia. Besides its abundance, Ensete fiber has a tensile strength of 513 MPa, fracture strain of 3.2%, fineness of 8-16tex, crystallinity index of 64.9%, and moisture content of 12.2%. Ensete fiber chemical composition is from 56% of cellulose, 24% of hemicellulose, 2.2% of lignin, and 17.8 % of other extractives, wax, and ashes. These properties of ensete fiber indicate its competitive candidate to make natural fiber-based biocomposites.
The novelty of this research contributed to academic literature through its first extensive work on the effects of ensete fiber surface modification and grafting of compatibilizers to polymers on developed composites properties. Additionally, the effects of hybridization and stacking sequences on composite structures and properties; the extraction of micro cellulose crystals from ensete fibers were investigated. The nanocomposite developed from nano cellulose fibrils filled thermoset resin was also characterized for its thermal stability improvements and dynamic mechanical properties. The outcomes of this study have been given in the form of research articles published in high-impact factor journals such as Composite Structures and Composite Science and Technology.
The methodology used to develop ensete fiber-based composites includes manufacturing techniques such as vacuum-assisted resin transfer moulding (VARTM), carding of fiber webs, twin-screw melt compounding and granulating, hot press moulding, and liquid cast moulding of composites. The design of experiments was prepared for each research article to address specific objectives and to contribute to the purpose of the thesis in general. Surface modification of ensete fiber by varying alkali concentration was investigated for its effects on the fiber properties and ensete fiber-based unsaturated polyester (UP) composite physical, mechanical, dynamic mechanical, and morphological properties. Mechanical test results revealed that 5.0 wt% alkali treated Ensete fibers and unsaturated polyester composites showed 14.5% and 43.5% increase in flexural strength and Young's modulus respectively when compared with untreated fiber composite. A positive shift in glass transition temperature (Tg) of composites after alkali treatment and tensile fracture surface morphology and roughness of ensete fiber SEM images indicates better interfacial interaction in treated ensete fibers and UP composites.
The hybrid composites developed from carded ensete fiber webs and woven glass fiber fabrics reinforcing unsaturated polyester were characterized for the effects of stacking sequences. A hybrid composite GGEE and GEEG showed improvement in tensile properties when compared to polyester composites from pure ensete fiber in its carded web form. The composites stacked as glass-ensete-glass (GEEG) showed higher storage modulus as compared to glass-ensete (GGEE) composites whereas the loss modulus of the composites reinforced with glass fiber exhibited the maximum value of 407 MPa and the height of the damping curve decreased in the GEEG composite. As observed from fracture surfaces, a more extensive fiber pullout was observed for the GEEG sample compared to the only ensete fiber composite sample. Tg of composites was increased for ensete-glass hybrid composites which might be related to more restrictions and a higher degree of reinforcements in hybrid composites. The ensete/glass fiber hybrid polyester composites can be used as load-bearing structures and components where high resistance to deformations and thermal stability is necessary.
Ensete fiber and high-density polyethylene (HDPE) composites were developed in the presence of maleic anhydride grafted polyethylene (Ma-g-PE) as a compatibilizer. The grafting process was done by optimized maleic anhydride concentration of 1.5% by weight fraction and 0.5% of reaction initiator called dicumyl peroxide (DCP) and the rest 98% of HDPE polymer. The formulated and premixed composite constituents (chopped ensete fiber, MA-g-PE, and HDPE) were melt compounded by twin-screw extrusion, granulated, and then composite plates were molded using a hot press machine. Increasing the ensete fiber loading from 15 wt.% to 30 wt.% has resulted in the composites being stiffer and harder leading to a decrease in elongation at the break of the composites. The physical properties such as density and water absorption % increased with fiber loading increments while melt flow index reduced. The addition of 5wt% compatibilizer into 25wt% ensete fiber-filled HDPE improved the fiber-matrix adhesion. Its tensile strength, flexural strength, and impact absorption energy increased by nearly 43%, 46%, and 56% respectively when compared to composites with the same fiber loading and without compatibilizer. Morphological analysis on micrograph images taken by SEM confirmed the failure mechanism of the composites. The results of the study show that ensete fiber-HDPE composite could be commercialized in the industry for construction and building, low-density furniture, and moldable structures in need of design flexibility.
The micro or nanometer cellulose fillers extracted from green material sources such as ensete fiber are needed to be utilized to develop biocomposites. The study focused on the extraction of micro cellulose crystals from lignocellulosic ensete fiber and additions of nano cellulose fibrils to epoxy resin were investigated. The process of isolating micro cellulose includes chemical and mechanical methods. Lignin and hemicellulose of ensete fibers were removed by soaking chopped short fibers in an alkali solution (17.5 wt% NaOH concentration) for 4 hr. at room temperature. The alkali treated fibers were washed with deionized water several times to keep PH value at 7; filtered to remove lignin and hemicellulose, and dried at 80 ℃ for 24 hr. Acid hydrolysis by 1M hydrochloric acid was done at 80 ± 5 ℃ for 2 hr. to remove hemicelluloses, pectin, and some of the suspended cellulose amorphous, and then fibrils of cellulose were mechanically ball milled to micro cellulose crystals. The analysis from FTIR measurements, x-ray diffraction, optic microscope, and SEM images revealed the removal of lignin, hemicelluloses, pectin, wax, and also hydrolysis of amorphous cellulose parts during the chemical process. The diameters of extracted MCC were ranging from 1-10µm.
The epoxy resin filled with varying weight fractions of cellulose nanofibrils was moulded and cured by liquid cast moulding techniques. The effects of incorporating nano cellulose fibrils filler into epoxy at different loading (1wt%, 3wt%, and 5wt%) were investigated. The results of thermal properties from DSC, TGA, and DMA discussed thermal stability and stiffness of epoxy nanocomposites. From DMA test results, the reduction in tan delta peak height of nanocomposites with the increase in NCF loading shows increments of stiffness imparted by fillers. The glass transition temperature of nanocomposites shifted to a higher temperature as the filler loading and uniform dispersion were attained. The thermal stability of CNF filled nanocomposites can be compared from TGA results. The temperature where maximum weight loss happened is shifted to a higher temperature when compared with pure epoxy polymer. CNF fillers acted as a thermal insulating barrier to the epoxy polymer and resulted in less thermal degradation of nanocomposites relatively. From overall test results, 3wt% CNF is the optimized cellulose nanofibrils filler loading for better thermal properties and modulus of the epoxy-based nanocomposites.
In conclusion, based on currently available technologies the utilization of an ensete fiber which is from renewable sources to be used as reinforcements of polymeric matrices and alternative new green composite products were able to be developed for desired engineering applications as recommended in each specific study done.