Sulfate ester modified cellulose nanofibers and method for producing cellulose nanofibers

#This application is a National Stage of International Application No. PCT/JP2018/001070 filed Jan. 16, 2018, claiming priority based on Japanese Patent Application No. 2017-005293 filed Jan. 16, 2017, Japanese Patent Application No. 2017-102915 filed May 24, 2017, and Japanese Patent Application No. 2017-183063 filed Sep. 22, 2017.

#The present invention relates to sulfate ester modified cellulose nanofibers and a method of producing cellulose nanofibers.

#A cellulose fiber (cell wall unit) is an assembly of cellulose nanofibers (microfibrils). The cellulose nanofibers have been socially attracting close attention as a reinforcing material because the nanofibers have mechanical characteristics comparable to those of steel and have nanostructures each having a diameter of from about 5 nm to about 20 nm. The cellulose nanofibers are bound together through hydrogen bonds therebetween. Accordingly, in order that the cellulose nanofibers may be drawn out, the hydrogen bonds need to be cleaved to separate the microfibrils (hereinafter sometimes referred to as “fibrillation”). A mechanical fibrillation method involving applying an intense physical force to cellulose and a chemical fibrillation method involving releasing the hydrogen bonds through chemical action have been known as fibrillation methods.

#FIG. 1 is a photograph of aqueous dispersions (solid content concentration: 0.25 wt %) of cellulose nanofibers obtained in Examples 1, 4, and 6.

#FIG. 2 are SEM photographs of the cellulose nanofibers obtained in Example 1.

#FIG. 3 is a FT-IR spectrum of the cellulose nanofibers obtained in Example 1.

#<A. Sulfate Ester Modified Cellulose Nanofibers>

#Cellulose nanofibers of the present invention have an average fiber diameter in the range of from 1 nm to 500 nm, and have sulfuric acid-esterified hydroxyl groups on the surfaces of the cellulose nanofibers. The cellulose nanofibers may be such that all the hydroxyl groups on the surfaces of the cellulose nanofibers are subjected to sulfuric acid esterification modification, or may be such that only part of the hydroxyl groups are subjected to sulfuric acid esterification modification. A sulfate ester modified cellulose derivative has heretofore been known as sulfuric acid-esterified cellulose. However, sulfate ester modified cellulose nanofibers have not been known. The sulfate ester modified cellulose nanofibers of the present invention each have cellulose I crystalline structure intrinsic to natural cellulose. Accordingly, excellent characteristics of cellulose can be suitably maintained. Further, each of the cellulose nanofibers has high hydrophilicity, and is excellent in water retaining property and moisture retaining property. In addition, the sulfate ester modified cellulose nanofibers of the present invention are excellent in adsorbing properties for a cation, a metal, inorganic particles, and the like. Further, the sulfate ester modified cellulose nanofibers of the present invention have antiviral properties. As described later, the sulfuric acid esterification modification ratio of the sulfate ester modified cellulose nanofibers of the present invention may be adjusted within a desired range. Accordingly, the nanofibers may be turned into sulfate ester modified cellulose nanofibers having a high sulfuric acid esterification modification ratio and then used as they are in any appropriate applications. In addition, the nanofibers may be turned into sulfate ester modified cellulose nanofibers having a low sulfuric acid esterification modification ratio and then used as modified cellulose nanofibers further modified with any appropriate modification reaction agent. The sulfate ester modified cellulose nanofibers of the present invention may each have any other functional group, such as an acetyl group or a propionic group.

#The sulfuric acid esterification modification ratio of the sulfate ester modified cellulose nanofibers to be obtained in the present invention may be set to any appropriate value in accordance with applications and the like. The sulfuric acid esterification modification ratio of the sulfate ester modified cellulose nanofibers may be represented by a sulfur content (wt %) in the cellulose nanofibers. The sulfur content (wt %) in the sulfate ester modified cellulose nanofibers is preferably from 0.01 wt % to 35 wt %, more preferably from 0.1 wt % to 28 wt %, still more preferably from 0.5 wt % to 20 wt %. When the sulfur content is more than 35 wt %, the crystallinity degree and heat resistance of the nanofibers may reduce. When the sulfur content is less than 0.01 wt %, a characteristic of a sulfuric acid ester functional group may not be sufficiently obtained, and hence desired characteristics as the sulfate ester modified cellulose nanofibers cannot be exhibited in some cases. Even sulfate ester modified cellulose nanofibers having a sulfur content of less than 0.01 wt % can be suitably used in a modification step involving using a modification reaction agent to be described later.