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EXPERIMENTAL EVALUATION OF TENSILE PROPERTIES OF EPOXY COMPOSITES WITH ADDED CELLULOSE NANOFIBER SLURRY H. Kurita,1 R. Ishigami, C. Wu,

UDC 539.4

and F. Narita Cellulose nanofiber (CNF) is one of natural fibers, and its Young modulus and tensile strength have been estimated close to 140 GPa and at least 2–3 GPa, respectively. As the homogeneous dispersion method of CNF in polymer matrix, the chemical modification of the CNF surface or solvent exchange process are often used. However, the environmental load of these processes is large, and the chemically modified CNF is expensive. In this study, mechanically defibrillated CNF reinforced epoxy resin matrix (Epoxy-CNF) composites with various CNF volume fraction were fabricated. Their tensile modulus and ultimate strength of the epoxy composites were deteriorated by the CNF slurry addition, while the fracture elongation was increased. This can be attributed to the interaction of epoxy and water, concentration of microvoids, and CNF agglomeration. Thus, the reduced water content in Epoxy-CNF composites improves their tensile properties. Keywords: cellulose nanofiber, epoxy resin, tensile properties, composite. Introduction. Cellulose nanofiber (CNF) is a string-like material with a large aspect ratio, which is highly crystalline and consists of long cellulose molecules linked by strong hydrogen bonds. It is known that the long cellulose molecules are incorporated into the cell walls of higher plants such as hemicellulose and lignin [1, 2]. Depending on how it is produced, CNF takes various forms: cellulose microfibrils of width 4 nm, which are the most basic units; cellulose microfibril bundles of width 10–20 nm, loose bundles of several CNFs that occur as basic units in the cell wall; and microfibrillar cellulose (MFC) in which microfibril bundles are further combined in bundles of several tens to several hundred nanometers to form a web network [3]. Although the strength of cellulose microfibrils or microfibril bundles has never been measured directly, an elastic modulus of about 100 GPa and strength of 1.7 GPa have been obtained in tensile tests on Kraft pulp, which is an aggregate of cellulose microfibrils [4, 5]. Considering that 70–80% of microfibrils are oriented in the fiber axial direction in pulp, the modulus of the microfibrils should be close to 140 GPa, and the strength may be estimated as at least 2–3 GPa. The elastic modulus remained unchanged in the range -200 to 200°C. A result of 0.17 ppm/K, close to the limit of measurement, was obtained for the linear coefficient of thermal expansion in all-cellulose fiber materials. This rivals the coefficient for quartz glass and is about 1/50 of the value for E-glass. Furthermore, CNF was found to have a thermal conductivity of the same order as glass [6]. Epoxy resins are widely used in adhesives, coatings, electronics, sporting goods, and aerospace applications, owing to their excellent properties of being a polymeric material with high strength, stiffness, temperature resistance, dimensional stability, and f