Nanofiber sheet, process for producing the same, and fiber-reinforced composite material

Abstract

Provided is a nanofiber sheet that sufficiently refined by fibrillation and has high crystallinity of cellulose fiber and can realize a fiber-reinforced composite material exhibiting high transparency, a high elastic modulus, a low coefficient of linear thermal expansion, and high heat resistance and being high in flatness and smoothness. This nanofiber sheet includes crystalline cellulose as the main component and a lignin in an amount of from 10 ppm to 10 wt %. When a fiber/resin composite material obtained by impregnating the nanofiber sheet with tricyclodecane dimethacrylate, subjecting the impregnated product to UV-curing at 20 J/cm, and heating the cured product in vacuum at 160° C. for two hours includes 60 wt % of the cured tricyclodecane dimethacrylate and 40 wt % of nanofiber, the following physical characteristics (i) to (iii) are satisfied: (i) the parallel light transmittance of light of a wavelength of 600 nm at a sheet thickness of 100 μm is 70% or more; (ii) the Young's modulus is 5.0 GPa or more; and (iii) the coefficient of linear thermal expansion is 20 ppm/K or less.