Surface Modification Of Carbon Fibers Via Diazonium Reaction And Interfacial Properties Of Their Composites

On account of their high specific strength and stiffness, carbon fiber reinforced polymer matrix composites have been extensively used in aerospace, automobile industry, wind power generation, fuel cell, marine, deep sea drilling platform and so on. As one of the most important parts of a composite, the interface can influence the way stress transmit and the failure mode of the entire composite, determine the main mechanical properties of a composite to a great extent. However, the interfacial adhesion strength of carbon fiber with the matrix is not strong due to the smooth and inert surface of of carbon fiber. So many physical and chemical methods have been used to modify the of carbon fiber surface in order to improve the interfacial adhesion of the resulting composite through enhancing surface roughness, surface energy and chemical bonding and so on. Nonetheless, a lot of modification methods are used to improve the interfacial performance at the expense of tensile strength for base carbon fibers, environment contamination and tedious procedures. In this paper, from the perspective of the intrinsic properties of carbon fiber and epoxy resin and the interface microstructure of the composite, aminobenzene grafted carbon fibers, benzoic acid grafted carbon fibers, and carbon nanotube/carbon fiber(CNT/CF) hierarchical reinforcement were designed and prepared by virtue of the flexibility of diazonium reaction to improve the interfacial adhesion strength of carbon fiber reinforced composites, and their micro- and macro-properties, toughening and strengthening mechanisms were characterized and synthesized, under the premise of ensuring the intrinsic strength of carbon fibers and reducing environment pollution.A single variety of oxygenic functional benzoic acid groups was rapidly introduced into the carbon fiber surface by diazonium grafting method using molten urea as reaction medium. The introduction of benzoic acid groups improved the surface roughness and surface energy of carbon fibers, which made it possible to take place interfacial chemical reaction for carboxyl and resin matrix. The interfacial adhesion of the composite was greatly improved by the chemical bonding at the interface. The interface property of phenylamine grafted surface energy of carbon fiber reinforced composites was distinctly improved after the molten diazonium grafting. Compared to the untreated carbon fiber reinforced composite, The interfacial shear strength(IFSS), interlaminar shear strength(ILSS) and impact-resistance strength of the composite increased by 66.4%, 46.7% and 30.2%, respectively, while the single fiber tensile strength of benzoic acid grated carbon fibers remained realistic intrinsic strength and the tensile strength had no obvious change.In order to further improve the modification effect of carbon fibers, phenylamine groups were introduced into the interface of carbon fiber/epoxy resin composites via employing diazonium grafting technique to modify carbon fibers “on water”. The effects of reaction proportion on surface microstructure and chemical composition of carbon fibers and interfacial performance of composites were investigated. The introduction of phenylamine groups greatly improved the surface polarity, roughness and wettability of carbon fibers, enhanced the contact area between carbon fiber and resin and mechanical interlocking, which improved the interfacial adhesion between the fibers and resin. carbon fiber, p-phenylenediamine and isopropyl nitrite in a mole ratio of 1:4:2 and reaction time of 12 h can be determined as the optimal chemical composition and morphology of carbon fiber surfaces and interfacial performance of composites. The IFSS and ILSS of phenylamine grafted carbon fiber composites can separately enhance from 46.8 MPa(untreated) to 81.1 MPa and from 41.5 MPa(untreated) to 62.5 MPa, with the increase amplitude of 73.3% and 50.6%, respectively. Moreover, impact-resistance strength also has shown an increase of 34.9%. Meanwhile, the tensile strength of a single fiber can be controlled below 3%.Using amidogen obtained from phenylamine grafted carbon fiber surface with better modification effect as the secondary active centre of diazonium reaction and taking advantage of the high suitability of diazonium grafting reaction, a kind of carbon nanotube/carbon fiber(CNT/CF) hierarchical reinforcement were prepared by the grafting of untreated multiwall carbon nanotube onto the carbon fiber surface in the form of chemical bonding via secondary diazonium reaction. It was shown that carbon nanotubes evenly distributed the carbon fiber surface in different angles and no aggregation occurred. Compared to untreated carbon fiber/epoxy composite, ILSS, IFSS and impact-resistance strength of the composite increased by 79.0%, 104.7% and 49.7%, respectively. The especially big length-diameter ratio structure and a large number of amidogen groups remained at the surface after the second diazonium reaction improved the surface polarity, roughness and wettability of the fiber surface, and the fiber and the resin matrix were firmly connected to each other through chemical bonding and mechanical interlocking, which greatly enhanced the integral interface adhesion of the composite. Meanwhile, the single fiber tensile strength of carbon fiber was increased by 8.7% due to the mild preparation technology adopted by this research and the modification of carbon nanotube on the surface defect of carbon fibers.