Macromolecular Structure of Sunn Hemp Fiber and its Influence on Properties of the Fiber Reinforced Epoxy Composites

Abstract

The present dissertation entitled “Macromolecular Structure of Sunn Hemp Fiber and its Influence on Properties of the Fiber Reinforced Epoxy Composites” is the amalgamation of the investigation aimed at the structural modification of sunn hemp fiber and using those modified fibers as a reinforcement for composites fabrication. Various characterization tools have been adopted for both the fiber and composites. The whole thesis has been classified into seven chapters. Due to high resistance to root-knot nematodes and wide application in cordage, fishing nets, ropes and canvas industries, sunn Hemp holds a potential emerging fiber crop. Unsheathing from the stem region, sunn Hemp exhibits 70-78% of cellulose, about 12.5 MPa flexural strength, low density (1.5 g/ cm3) with a high aspect ratio (450-600). All these properties make sunn Hemp fiber a target material for automobiles, domestic appliances, sports industries, etc. However, the inherent hydrophilic character of sunn hemp fiber due to intramolecular hydrogen bonds causes it to be incompatible with hydrophobic matrix resin. In such a series, various modification techniques are proposed to enhance the functionalization process between fiber-matrix, aiming to improve the various physical properties of the end-use composite products. In the recent study, treatment strategies, including the dewaxing process, potassium hydroxide (KOH) and microwave irradiation treatment, are addressed to modify the sunn hemp fiber structurally. Before any treatment, the fiber loading is optimized for composites fabrication. The hand layup technique is adopted to process composite specimens. Among 5, 10, 15, 20, 25, and 30 vol% of loading, 20 vol% loading fiber in composites yields better mechanical strength and a low dielectric loss and is subsequently optimized for further composite fabrication. The hydrophilic sunn hemp fiber is functionalized using the dewaxing process, where fiber is soaked in an ethanol and benzene solution in a ratio of 1:2, followed by 12 hours of consecutive heating and cooling. The second chemical way of treating the fiber is the alkalization process, where fiber is immersed in 5, 10, 15 and 20wt% of KOH solution for 2, 4,6, 8 and 10h. In physical treatment, the fiber is exposed to microwave irradiation at 160, 320, 480, 640 and 800 watts for 2, 6 and 10 min exposure time. The effect of treatment on the fiber as well as on the composites is investigated. The fine structure of untreated and treated fiber is studied by SAXS and XRD analysis. The macromolecular parameters like average periodicity to layers, volume fraction in the matter and void phase, range of inhomogeneity, heterogeneity, etc., are computed and found to be enhanced in all treated fiber for a specific combination of concentration-soaking and irradiation power-exposure time. The XRD study revealed the crystallographic parameters like crystalline index and cellulose crystallites and observed to be higher in dewaxed fiber, fiber treated with 10% alkali concentration for 6 h (10K6) and fiber irradiated at 320 watts for 2 min. FTIR peaks show specific bond removal in treated fiber, which justifies the removal of non-cellulosic components from fiber structure. The increased roughness and fibrillation are visible on the treated fiber surface, but beyond optimization, it shows the degradation as confirmed by FESEM images. The single fiber tensile strength of the sunn hemp fiber is evaluated using Weibull analysis and a statistical average on the fiber strength is reported. The effect of pre-treatment on the fibers can be legitimate as the composites's overall mechanical and dielectric properties are compared to the untreated ones. The crosslinking density of the composites is determined by the Flory-Rehner equilibrium theory and correlated with the mechanical properties of composites. The 3-point bending and tensile test of the composites are higher in dewaxed, 10K6 and 800W6M fiber. The improvement in mechanical behavior indicates an excellent interfacial adhesion between fiber-matrix. However, the decrease in mechanical properties is the consequence of weak adhesion and compatibility between fiber and matrix. Such a scene may also arise due to low crosslinking between fiber and matrix resulting from the void formation at the interface region. The dielectric study of the composites is carried out by an LCR impedance meter and notices that the treated fiber results in a lower dielectric constant (𝜖′) and lower dielectric loss (tanδ). However, after an optimal controlling parameter, the 𝜖′ and tanδ are observed to be increasing. It can be attributed to the loss of the polar hydroxyl (-OH) group from the fiber structure upon treatment, which led to a decrease 𝜖′ and tanδ. Other electrical parameters like ac conductivity, modulus and impedance of the composites are also studied and reported in the subsequent chapters.