Recently developed cellulose nanofibers are plant-derived, natural-ingredient nanofillers and have drawn much attention as low-relative-density, high-strength materials for forming composites with resins. It is known that adding small amounts of cellulose nanofibers to resins has a reinforcing effect and increases mechanical strength such as toughness at break and bending strength.
Known examples of the method for obtaining cellulose nanofibers by miniaturizing cellulose having many hydroxyl groups to a nanometer order include a method that involves fibrillating cellulose in water or a hydrophilic solvent (refer to PTL 1 and PTL 2) and a method that involves fibrillating cellulose in a resin (refer to PTL 3).
All of cellulose nanofibers produced by the above-mentioned methods undergo agglomeration and sedimentation if they are mixed with organic solvents other than those, such as ethanol, which are highly hydrophilic. This is because these cellulose nanofibers are highly hydrophilic. Thus, it has been extremely difficult to disperse cellulose nanofibers in resins that contain such organic solvents.
Several proposals have been made to improve the dispersion state of cellulose nanofibers in a composition, the proposals including a method that involves providing modified cellulose nanofibers obtained by causing hydroxyl groups in cellulose nanofibers to react with an acid anhydride so as to half-esterify the cellulose nanofibers and improve the dispersion state in the composition (refer to PTL 4, PTL 5, and PTL 6). However, these methods do not improve dispersing of cellulose nanofibers in resins containing organic solvents.
A method for modifying cellulose nanofibers with a resin has been reported, which involves living radical polymerization (refer to PTL 7). However, this method has many limitations and difficulties such as difficulty of introducing an initiating group into cellulose nanofibers and inability to perform living radical polymerization in the presence of oxygen.