Method for producing cellulose nanofibers
Provided is a method which is capable of producing a cellulose nanofiber dispersion liquid that has a low viscosity and excellent fluidity even at a high concentration, while exhibiting excellent transparency. In a method for producing cellulose nanofibers, wherein a cellulosic starting material is oxidized in water using an oxidant in the presence of an N-oxyl compound and a compound that is selected from the group consisting of a bromide, an iodide and a mixture thereof and the thus obtained oxidized cellulose is defibrated and dispersed, pulp which is obtained by carrying out kraft cooking after a hydrolysis process is used as the cellulosic starting material.
1. A method for producing cellulose nanofibers, the method comprising the steps of:
(A) subjecting pulp to hydrolysis treatment and subsequent kraft cooking to prepare dissolving kraft pulp;
(B) oxidizing the dissolving kraft pulp in water using an oxidizing agent in the presence of (1) an N-oxyl compound and (2) a compound selected from the group consisting of a bromide, an iodide and a mixture thereof, to produce oxidized cellulose;
(C) defibrating and dispersing the oxidized cellulose to produce cellulose nanofibers, wherein acid hydrolysis treatment is not carried out between steps (A) and (B); and
(D) performing a viscosity-reducing treatment of oxidized cellulose, between the steps (B) and (C), wherein the viscosity-reducing treatment is selected from the group consisting of ultraviolet irradiation, oxidative decomposition using hydrogen peroxide and ozone, hydrolysis with acid, hydrolysis with alkali, and combinations thereof.
2. The method of claim 1 , wherein a concentration of the oxidized cellulose at the defibration and dispersion in the step (C) is 1 to 10% (w/v).
3. The method of claim 1 , further comprising a step (E) of purifying the dissolving kraft pulp, between the steps (A) and (B), wherein the purification is selected from the group consisting of enzyme treatment, alkali treatment, treatment with oxidizing chemical, and combinations thereof.
4. The method of claim 1 , wherein an amount of carboxyl groups in the oxidized cellulose is 1.0 mmol/g or greater based on a bone dry mass of the oxidized cellulose.
5. The method of claim 1 , wherein the defibration and dispersion in the step (C) are performed under a pressure condition of 50 MPa or higher.