Epoxy resins are a widely used common polymer due to their excellent mechanical properties. On the other hand, cellulose nanofiber (CNF) is one of the new generation of fibers, and recent test results show that CNF reinforced polymers have high mechanical properties. It has also been reported that an extremely low CNF addition increases the mechanical properties of the matrix resin. In this study, we prepared extremely-low CNF (~1 wt.%) reinforced epoxy resin matrix (epoxy-CNF) composites, and tried to understand the strengthening mechanism of the epoxy-CNF composite through the three-point flexural test, finite element analysis (FEA), and discussion based on organic chemistry. The flexural modulus and strength were significantly increased by the extremely low CNF addition (less than 0.2 wt.%), although the theories for short-fiber-reinforced composites cannot explain the strengthening mechanism of the epoxy-CNF composite. Hence, we propose the possibility that CNF behaves as an auxiliary agent to enhance the structure of the epoxy molecule, and not as a reinforcing fiber in the epoxy resin matrix.

Cellulose nanofibers (CNF) have recently attracted attention as one of the reinforcements for composite materials. However, the same as other nanofibers, CNFs can be easily agglomerated and the nano-level defibration is necessary to obtain their outstanding properties in polymer matrices. To overcome this issue, two major defibration methods, chemical and mechanical defibration have been considered. The chemically-defibrated CNF is expensive and so prevents the practical realization of CNF as general-purpose products. Therefore, the mechanical defibration method by a clean low-cost water jet was focused on. Mechanically-defibrated CNF reinforced epoxy resin matrix (Epoxy-CNF) composites was fabricated via mechanical mixing and evaluated the variation of their tensile and flexural properties with different CNF volume fractions. The tensile and flexural modulus of the epoxy composites were increased by CNF addition, while the fracture elongation was decreased. The calculated ultimate tensile strength (UTS) and ultimate flexural strength (UFS), using the aspect ratio of the agglomerated CNF clusters, indicated the validity of the random dispersion model considering the agglomerated CNF as one whole fiber. It seems that epoxy resin is enhanced by the mechanical interaction when the CNF volume fraction is higher than 0.37 vol.%.

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