JP6727531B2 - Method for producing dry solid of cellulose nanofiber - Google Patents

Description

The present invention relates to a method for producing a dry solid product of cellulose nanofibers.

Cellulose nanofiber (CNF) is a fine fiber having a size of about 4 to several hundreds nm, which is excellent in water-based dispersibility, and retains the viscosity of foods, cosmetics, medical products or paints, and strengthens food material dough. It is expected to be used as a water retention, food stability improvement, low calorie additive or emulsion stabilization aid.

The solid matter obtained by drying CNF in a state of being dispersed in water (wet state) has hydrogen bonds formed between fine cellulose fibers. Various properties such as solubility, dispersibility, sedimentation degree, and viscosity of state) are not restored. Therefore, CNF is usually produced in a state of being dispersed in water (wet state), and is usually used for various purposes in a wet state without being dried.

However, in order to stabilize this CNF in a wet state, several to several hundred times the weight of water is required to the CNF, which causes various problems such as securing storage space, increasing storage and transportation costs. There is.

As means for solving this problem, in addition to the freeze-drying method, the critical point drying method, and the like, a method of performing substitution treatment with an organic solvent and then drying (Patent Document 1) and the like have been proposed.

JP-A-6-233691

However, when CNF is freeze-dried, a huge amount of energy is required, and depending on the conditions, when water between the fine fibers of CNF is frozen, the growth of ice crystals larger than the voids between the fine cellulose fibers is caused. Then, problems such as occurrence of association between fine fibers of CNF occur.

Further, since the voids between the fine fibers of CNF are very small and a large amount of water is hydrated on the surface of the fine cellulose fibers, it is necessary to use a large amount of solvent and time for drying by solvent replacement. Will be needed. Furthermore, since water that cannot be replaced with the solvent is present internally, the surfaces of the fine CNF cellulose fibers are firmly bonded to each other by hydrogen bonds during the drying process of the solvent. Therefore, it is difficult to restore the original CNF state.

Therefore, in the present invention, physical properties such as viscosity and transparency of a CNF dispersion obtained by redispersing a CNF dry solid obtained by drying a stably dispersed CNF dispersion in water are compared with those of a CNF dispersion before drying. It is an object of the present invention to provide a method for producing a dry solid of CNF, which has characteristics (redispersibility) with little change.

The present invention provides the following [1] to [2].
[1] A method for producing a dry solid of cellulose nanofibers, which comprises dehydrating and drying after adjusting the pH of an aqueous suspension of cellulose nanofibers to 9 to 11.
[2] A dried solid product of cellulose nanofibers produced by the method according to [1].

According to the present invention, physical properties such as viscosity and transparency of a CNF dispersion obtained by redispersing a CNF dry solid obtained by drying a stably dispersed CNF dispersion in water are compared with those of a CNF dispersion before drying. Thus, it is possible to provide a method for producing a dry solid product of CNF which has a characteristic of little change (redispersibility).

According to the present invention, the dry solid matter of cellulose nanofibers obtained by dehydrating and drying after adjusting the pH of the aqueous suspension of cellulose nanofibers to 9 to 11 is a wet solid obtained by dispersing the solid matter in water. The CNF in the state has a characteristic (redispersibility) in which changes in various properties such as solubility, dispersibility, sedimentation degree and viscosity of the CNF in a wet state before drying are small.

It is not clear why the dried solid product of CNF of the present invention exhibits excellent redispersibility, but by adjusting to the alkaline region (pH 9 to 11) of the aqueous dispersion of CNF before drying, acid by It is presumed that deterioration and aggregation of CNF are suppressed. However, if the pH is too high, there is a risk of deterioration due to alkali and a change in the crystal structure of CNF.

In the present invention, the dry solid matter of cellulose nanofibers means the cellulose nanofibers dehydrated and dried so that the water content is 12% by weight or less.

(PH adjuster)
In the present invention, the chemicals used to adjust the pH of the aqueous suspension of cellulose nanofibers to 9 to 11 are not particularly limited, and include sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide, A basic inorganic compound selected from magnesium hydroxide, ammonia, copper hydroxide, aluminum hydroxide, iron hydroxide, ammonium hydroxide, sodium hydrogen carbonate, potassium hydrogen carbonate, sodium carbonate, calcium carbonate, magnesium carbonate and magnesium oxide, Alternatively, a basic organic compound selected from arginine, lysine, histidine and ornithine can be exemplified.

(Cellulose nanofiber)
In the present invention, the cellulose nanofiber (CNF) is a fine fiber having a fiber width of about 4 to 500 nm and an aspect ratio of 100 or more, and can be obtained by defibrating a cellulose raw material such as pulp.

(Cellulose raw material)
In the present invention, as the cellulose raw material, plants (for example, wood, bamboo, hemp, jute, kenaf, agricultural land waste, cloth, pulp (softwood unbleached kraft pulp (NUKP), softwood bleached kraft pulp (NBKP), broadleaf wood) Bleached kraft pulp (LUKP), hardwood bleached kraft pulp (LBKP), softwood unbleached sulfite pulp (NUSP), softwood bleached sulfite pulp (NBSP) thermomechanical pulp (TMP), recycled pulp, waste paper, etc., animals (eg Those originating from ascidians), algae, microorganisms (eg acetic acid bacteria (acetobacter)), microbial products, etc. are known, and any of them can be used in the present invention. And more preferably plant-derived cellulose fibers.

(Defibration)
In the present invention, the device for defibration is not particularly limited, but a strong shearing force is applied to the water dispersion using a device such as a high-speed rotation type, colloid mill type, high pressure type, roll mill type, or ultrasonic type. Is preferred. In particular, for efficient defibration, it is preferable to use a wet high-pressure or ultra-high-pressure homogenizer that can apply a pressure of 50 MPa or more and a strong shearing force to the water dispersion. The pressure is more preferably 100 MPa or more, further preferably 140 MPa or more. If necessary, prior to the defibration/dispersion treatment with a high-pressure homogenizer, the above CNF can be pretreated by using a known mixing, stirring, emulsifying, or dispersing device such as a high-speed shear mixer. Is.

(Drying method)
In the method of the present invention, the dehydration/drying method may be any conventionally known method, and examples thereof include spray drying, pressing, air drying, hot air drying, and vacuum drying. Examples of the drying device specifically used in the method of the present invention are as follows. That is, a continuous tunnel dryer, a band dryer, a vertical dryer, a vertical turbo dryer, a multi-stage disc dryer, an aeration dryer, a rotary dryer, an air flow dryer, a spray dryer dryer, a spray dryer. , Cylindrical dryer, drum dryer, screw conveyor dryer, rotary dryer with heating tube, vibration transport dryer, batch-type box dryer, aeration dryer, vacuum box dryer, stirring dryer, etc. These drying devices can be used alone or in combination of two or more.

Among these, it is preferable to use the drum dryer from the viewpoint of energy efficiency because the heat energy is directly supplied directly to the material to be dried. In addition, in the case of water-containing cellulose nanofibers before drying, in order to improve the efficiency of the drying process, a pre-concentration process is usually performed to reduce the water content as much as possible. Becomes an obstacle. On the other hand, by forming a thin film on the drum with a blade, a die or the like and drying the film, the drying process can be performed more efficiently and uniformly in a short time. Furthermore, the drum dryer is preferable in that the dried product can be immediately recovered without applying excessive heat.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited thereto.
<Manufacture of CNF>
Bleached unbeaten kraft pulp from softwood (85% whiteness) is adjusted to 1.0% (w/v) with water and treated with an ultra-high pressure homogenizer (20°C, 150Mpa) 50 times to give cellulose nanofibers. A dispersion was obtained. The obtained fiber had an average fiber diameter of 60 nm and an aspect ratio of 110.

(Measuring method of average fiber diameter and aspect ratio)
The average fiber diameter and the average fiber length of the cellulose fibers were analyzed for 200 randomly selected fibers by using a field emission scanning electron microscope (FE-SEM). The aspect ratio was calculated by the following formula:
Aspect ratio = average fiber length / average fiber diameter

<Example 1>
A 0.7 mass% aqueous suspension of the above CNF (average fiber diameter: 60 nm, aspect ratio: 110) was stirred for 60 minutes with a TK homomixer (12,000 rpm). To this aqueous suspension, 0.5% aqueous sodium hydroxide solution was added to adjust the pH to 9, and then the vapor pressure was 0.5 MPa. G was dried with a drum dryer D0303 (Katsuragi Industry Co., Ltd.) having a drum rotation speed of 2 rpm to obtain a dry solid of CNF having a water content of 5% by weight.

Next, water was added to the dry solid obtained above so as to form a 0.7 mass% aqueous suspension, and the mixture was stirred for 60 minutes using a TK homomixer (12,000 rpm) to redisperse CNF. An aqueous suspension was obtained.

(B-type viscosity measurement)
The B type viscosity of CNF (solid content 0.7%, 25° C.) was measured. The measurement conditions for the B-type viscosity were a rotation speed of 30 rpm and 3 minutes.
(Measurement of transparency)
The transparency (transmittance of 660 nm light) of the CNF dispersion liquid (solid content 0.1%) was measured using a UV spectrophotometer U-3000 (Hitachi High-Tech).

(Evaluation of restoration rate)
The B-type viscosity and the restoration rate of transparency were calculated by the following formulas.
Recovery rate (%)=(viscosity or transparency before drying)/(viscosity or transparency after redispersion)×100

<Example 2>
The same procedure as in Example 1 was carried out except that the pH was adjusted to 11.

<Comparative Example 1>
The procedure was performed in the same manner as in Example 1 except that the pH was not adjusted, but it was not possible to evaluate because the precipitation of CNF aggregates remarkably occurred in the aqueous suspension.

<Comparative example 2>
The procedure was performed in the same manner as in Example 1 except that the pH was adjusted to 8. However, it was not possible to evaluate because the precipitation of aggregates of CNFs significantly occurred in the aqueous suspension.

Claims (1)

After adjusting the pH of an aqueous suspension of cellulose nanofibers (excluding anion-modified cellulose nanofibers) having a fiber width of 4 to 500 nm and an aspect ratio of 100 or more to 9 to 11, dehydration and drying are performed without washing. A method for producing a dry solid product of cellulose nanofibers, comprising: