JP7566472B2 - Method for producing cellulose nanofiber dry solid and redispersion of cellulose nanofiber - Google Patents

Description

The present invention relates to a method for producing a dry cellulose nanofiber solid and a redispersion of cellulose nanofibers.

Thermoplastic resins are light and have excellent processing properties, and are therefore widely used in a wide range of applications, including automotive components, electrical and electronic components, office equipment housings, and precision components. However, resins alone often lack sufficient mechanical properties and dimensional stability, and so composites of resins and various fillers are commonly used. In recent years, the use of nanofibers such as cellulose nanofibers (CNF) as such fillers has been considered. Nanofibers such as CNF tend to aggregate in a dry state, and are therefore manufactured as a dispersion that allows for stable dispersion. When applying cellulose nanofibers to various applications, the dispersion may be dried and then redispersed in a dispersion medium to prepare a redispersion.

Patent Document 1 describes a method for redispersing a cellulose nanofiber dispersion by drying a produced cellulose nanofiber dispersion and dispersing the resulting cellulose nanofiber powder particles again in an aqueous solvent, which is characterized in that when the cellulose nanofiber powder particles are redispersed in the aqueous solvent, they are stirred and mechanical shear force is applied.

Patent document 2 describes a method for treating chemically modified fibril cellulose, characterized in that it comprises: introducing the chemically modified fibril cellulose material into a thermal drying device (20) comprising a belt (22) so that the fibril cellulose material forms at least one rod-shaped body on the belt (22); and dehydrating the chemically modified fibril cellulose material on the belt (22) using a heated air flow having a temperature of at least 40°C to concentrate and/or dry the chemically modified fibril cellulose material such that the dry solids content of the fibril cellulose material after the thermal drying device (20) is at least 10%.

Patent Document 3 describes a powdered nanofiber that is characterized in that (A) powdered nanofiber is mixed with (B) a dispersant in an amount of 1 to 40% by weight, calculated as solid content, and has a bulk density of 90 to 200 g/L.

Patent document 4 describes a method in which cellulose nanofibers are homogenized in the presence of an organic solvent, and then the organic solvent is removed to produce dry microfibers with a moisture content of 0 to 1% by mass.

International Publication No. 2017/154568 Special Publication No. 2015-512964 JP 2017-210596 A JP 2012-224960 A

When cellulose nanofibers are stored or transported in the form of a dispersion liquid, problems arise in that the cellulose nanofibers are easily deteriorated (putrefaction, etc.) because they are placed in a humid environment, and in that the dispersion medium takes up extra volume and weight, which increases storage and transportation costs. If the cellulose nanofibers are stored or transported in a dry state, the volume and weight can be reduced, and there is also the advantage that it can be easily composited with other materials depending on the application. However, once the cellulose nanofibers are dried from the dispersion liquid state, it is difficult to reproduce the good dispersion state of the cellulose nanofibers before drying, even if they are subsequently redispersed in a dispersion medium.

The present invention aims to solve the above problems and provide a method for producing a cellulose nanofiber dry solid that exhibits good redispersibility, and a redispersion liquid of cellulose nanofibers that can reproduce the good dispersion state of the cellulose nanofibers before drying when the cellulose nanofiber dry solid obtained by drying the dispersion liquid is redispersed in a dispersion medium.

As a result of intensive research to solve the above problems, the present inventors discovered that cellulose nanofibers that have been dried to a specific apparent density have excellent dispersibility in a dispersion medium, which led to the creation of the present invention. That is, the present invention encompasses the following aspects.
[1] A method for producing a redispersion of cellulose nanofibers, comprising the steps of:
A dispersing step of preparing a dispersion liquid by dispersing cellulose nanofibers in a first dispersion medium;
A drying step of drying the dispersion to obtain a dry solid of cellulose nanofibers;
A redispersion step of redispersing the cellulose nanofiber dry solid in a second dispersion medium to prepare a redispersion liquid;
Including,
The method of claim 1, wherein the viscosity at a shear rate of 100 sec ^-1 of an aqueous dispersion of a redispersion obtained by adjusting the cellulose nanofibers in the redispersion to an aqueous dispersion with a solids content of 0.5% by mass is 30% or more of the viscosity at a shear rate of 100 sec- ¹ of an aqueous dispersion of a redispersion obtained by adjusting the cellulose nanofibers in the dispersion to an aqueous dispersion with a solids content of 0.5% by mass.
[2] The method according to claim 1, wherein the cellulose nanofiber dry solid has an apparent density of 210 g/L to 1500 g/L.
[3] The method according to aspect 1 or 2, wherein the average degree of substitution (DS) of the cellulose nanofiber dry solid is 0.5 or less.
[4] The method according to any one of aspects 1 to 3, wherein the content of the dispersant in the cellulose nanofiber dry solid is less than 1 mass%.
[5] The second dispersion medium is an aprotic polar solvent,
The method according to any one of Aspects 1 to 4, wherein the redispersion is carried out by applying a shear force by stirring.
[6] The method according to any one of the above aspects 1 to 5, wherein the shear force is applied using one or more selected from the group consisting of ultrasonic waves, a bead mill, a ball mill, and a homomixer.
[7] The method according to any one of Aspects 1 to 6, wherein the aprotic polar solvent is one or more selected from the group consisting of dimethylsulfoxide, N-methylpyrrolidone, dimethylacetamide, and dimethylformamide.
[8] The method according to any one of aspects 1 to 7, further comprising chemically modifying the cellulose nanofibers during and/or after the redispersion step.
[9] A cellulose nanofiber dry solid having an apparent density of 210 g/L to 1500 g/L.
[10] The cellulose nanofiber dry solid according to the above-mentioned aspect 9, having an average degree of substitution (DS) of 0.5 or less.
[11] The cellulose nanofiber dry solid according to aspect 9 or 10, wherein the content of the dispersant is less than 1 mass%.
[12] A method for producing a cellulose nanofiber composite containing a cellulose nanofiber and an organic binder, comprising:
A step of preparing a redispersion of cellulose nanofibers by the method according to any one of aspects 1 to 8 above, and A step of mixing the redispersion or a dried form thereof with an organic binder;
A method comprising:
[13] A method for producing a resin composite containing cellulose nanofibers and a resin, comprising:
A step of preparing a redispersion of cellulose nanofibers by the method according to any one of aspects 1 to 8 above; and A mixing step of mixing the redispersion, an aqueous dispersion in which the second dispersion medium of the redispersion is replaced with water, or a dried form thereof, with a resin;
A method comprising:
[14] A method for producing a resin composition containing cellulose nanofibers and a thermoplastic resin, comprising:
A cellulose nanofiber solid forming step for obtaining a cellulose nanofiber solid, which is a dry solid of defibrated cellulose nanofibers;
a redispersion step of redispersing the cellulose nanofiber solid in a second dispersion medium to prepare a redispersion liquid; and a kneading step of kneading the redispersion liquid, an aqueous dispersion in which the second dispersion medium of the redispersion liquid is replaced with water, or a dried product thereof, with a thermoplastic resin.
Including,
The method of claim 1, wherein the viscosity at a shear rate of 100 sec ^-1 of an aqueous dispersion of a redispersion obtained by adjusting the cellulose nanofibers in the redispersion to an aqueous dispersion with a solids content of 0.5% by mass is 30% or more of the viscosity at a shear rate of 100 sec- ¹ of an aqueous dispersion of a redispersion obtained by adjusting the cellulose nanofibers in the dispersion to an aqueous dispersion with a solids content of 0.5% by mass.
[15] The method according to aspect 14, wherein in the cellulose nanofiber solid, the cellulose nanofibers are bonded to each other by hydrogen bonds of hydroxyl groups.
[16] The method according to aspect 14 or 15, wherein the rate of change (Y-X)/Y of the mass of the dry solid content (X) when the cellulose nanofiber solid is allowed to stand in water at 20°C for 2 hours and then removed from the water relative to the mass (Y) of the cellulose nanofiber solid before standing is 30% by mass or less.
[17] The method according to any one of Aspects 14 to 16, wherein the second dispersion medium is selected from the group consisting of dimethylsulfoxide, N-methylpyrrolidone, dimethylacetamide, and dimethylformamide.

According to one aspect of the present invention, there can be provided a cellulose nanofiber dry solid that exhibits good redispersibility, and a method for producing a redispersed liquid of cellulose nanofibers that can reproduce the good dispersion state of the cellulose nanofibers before drying when the cellulose nanofiber dry solid obtained by drying the dispersion liquid is redispersed in a dispersion medium.

FIG. 10 is an explanatory diagram of a method for calculating the IR index 1730 and the IR index 1030. FIG. 2 is an explanatory diagram of a method for calculating the average substitution degree of hydroxyl groups in cellulose.

The following describes exemplary embodiments of the present invention in detail, but the present invention is not limited to these embodiments.

<<Method for producing redispersed cellulose nanofiber liquid, and dried cellulose nanofiber solid>>
One aspect of the present disclosure provides a method for producing a redispersion of cellulose nanofibers, the method including a dispersion step of preparing a dispersion by dispersing cellulose nanofibers in a first dispersion medium, a drying step of drying the dispersion to obtain a dry solid of cellulose nanofibers, and a redispersion step of redispersing the dry solid of cellulose nanofibers in a second dispersion medium to prepare a redispersion.
Another aspect of the present invention provides a cellulose nanofiber dry solid having excellent redispersibility. In one aspect, the cellulose nanofiber dry solid has an apparent density of 210 g/L to 1,500 g/L.

The cellulose nanofiber refers to fine cellulose obtained by treating pulp or the like with hot water of 100°C or higher, hydrolyzing and weakening hemicellulose, and then defibrating the cellulose by a pulverizing method such as a high-pressure homogenizer, a microfluidizer, a ball mill, a disk mill, or a mixer (e.g., a homomixer). The cellulose nanofiber may be chemically modified as described below. Since cellulose nanofibers are extremely prone to aggregation in a dry state, it was thought that in a normal cellulose nanofiber dry solid, the cellulose nanofibers are firmly aggregated with each other, and even if the dry solid is redispersed in a dispersion medium, it is not easily redispersed. The present inventor unexpectedly discovered that a cellulose nanofiber dry solid having a high apparent density within a specific range can be well redispersed in a dispersion medium. A cellulose nanofiber dry solid having good redispersibility in a dispersion medium can be well dispersed in a resin, for example, when the cellulose nanofiber is composited with a resin, and therefore has an excellent effect of improving the physical properties of a composite containing a cellulose nanofiber and a resin (also referred to as a resin composite in the present disclosure). In addition, the cellulose nanofiber dry solid of the present disclosure has a high apparent density, and therefore has the advantage of reducing storage space and transportation costs. In one embodiment, the apparent density of the cellulose nanofiber dry solid is 210 g/L or more, or 300 g/L or more, or 400 g/L or more, or 500 g/L or more, and 1500 g/L or less, or 1400 g/L or less, or 1300 g/L or less, or 1200 g/L or less.
The apparent density in the present disclosure is determined by Archimedes' method, and is a value obtained using an electronic balance (for example, GR-202 electronic balance manufactured by A&D Co., Ltd.) and a specific gravity meter (for example, AD-1653 manufactured by A&D Co., Ltd.).

As the cellulose raw material, natural cellulose and regenerated cellulose can be used. As the natural cellulose, wood pulp obtained from wood species (broadleaf or coniferous trees), non-wood pulp obtained from non-wood species (cotton, bamboo, hemp, bagasse, kenaf, cotton linters, sisal, straw, etc.), and cellulose fiber aggregates produced by animals (e.g., sea squirts), algae, and microorganisms (e.g., acetic acid bacteria) can be used. As the regenerated cellulose, regenerated cellulose fibers (viscose, cupra, tencel, etc.), cellulose derivative fibers, and ultra-fine threads of regenerated cellulose or cellulose derivatives obtained by the electrospinning method can be used.

Since the cellulose raw material contains alkali-soluble and sulfuric acid-insoluble components (lignin, etc.), the alkali-soluble and sulfuric acid-insoluble components may be reduced by a purification process such as delignification by cooking and a bleaching process. On the other hand, the purification process such as delignification by cooking and the bleaching process cut the molecular chains of cellulose, changing the weight average molecular weight and number average molecular weight, so it is desirable that the purification process and bleaching process of the cellulose raw material control the weight average molecular weight of cellulose and the ratio of the weight average molecular weight to the number average molecular weight so as not to deviate from an appropriate range.

In addition, refining processes such as delignification by cooking and bleaching processes reduce the molecular weight of cellulose molecules, and there is concern that these processes will result in low molecular weight cellulose and alter the cellulose raw material, increasing the proportion of alkali-soluble matter present. Since alkali-soluble matter has poor heat resistance, it is desirable that the refining and bleaching processes of cellulose raw materials be controlled so that the amount of alkali-soluble matter contained in the cellulose raw material is within a certain range.

The cellulose raw material is obtained by a dry grinding process. Any type of grinder can be used in the dry grinding process, but a high-speed rotary impact grinder is preferable because it can produce cellulose raw material with a good shape. A high-speed rotary impact grinder is a grinder that uses pins that rotate in the grinding chamber or a rotor with a special structure to impact or shear the cellulose raw material, thereby grinding it. In addition, to suppress fiber damage during dry grinding, it is acceptable for the cellulose raw material to contain moisture to a certain extent based on its weight.

Examples of mills of this type include the Jiyuu mill, New Cosmomizer (manufactured by Nara Machinery Works, Ltd.), Victory Mill, Far In Victory Mill (manufactured by Hosokawa Micron, Ltd.), Turbo Mill (manufactured by Turbo Kogyo, Ltd.), Ultra Rotor (manufactured by W.I.R. Co., Ltd.), Makino type mill, Ultraplex (manufactured by Makino Sangyo Co., Ltd.), Fine Mill (manufactured by Nippon Pneumatic Mfg. Co., Ltd.), Impeller Mill (manufactured by Seishin Enterprise Co., Ltd.), and Disc Refiner (manufactured by Aikawa Iron Works, Ltd.).

There is no particular restriction on the method for reducing the fiber diameter in order to convert the cellulose raw material into cellulose nanofibers, but it is preferable to make the defibration processing conditions (method of applying a shear field, size of the shear field, etc.) more efficient. In addition, in order to suppress the decrease in crystallinity due to shear, wet defibration using a solvent is preferable. There is no particular restriction on the defibration solvent, but examples include water and aprotic solvents. Water is preferable because it is an inexpensive solvent and does not require explosion-proofing of the production equipment. When the cellulose raw material is immersed in an aprotic solvent, cellulose swells in a short time, and it is finely pulverized by only applying slight stirring and shear energy. In addition, by adding a cellulose modifying agent before, during, or after fineness, chemically modified cellulose nanofibers (also called chemically modified cellulose nanofibers) can be obtained directly without solvent replacement. Therefore, aprotic solvents are preferable from the viewpoint of production efficiency.

Examples of aprotic solvents include alkyl sulfoxides, alkyl amides, pyrrolidones, etc. These solvents can be used alone or in combination of two or more.

Examples of alkyl sulfoxides include di-C1-4 alkyl sulfoxides such as dimethyl sulfoxide (DMSO), methyl ethyl sulfoxide, and diethyl sulfoxide.

Examples of alkylamides include N,N-diC1-4 alkylformamides such as N,N-dimethylformamide (DMF) and N,N-diethylformamide; and N,N-diC1-4 alkylacetamides such as N,N-dimethylacetamide (DMAc) and N,N-diethylacetamide.

Examples of pyrrolidones include pyrrolidones such as 2-pyrrolidone and 3-pyrrolidone; N-C1-4 alkylpyrrolidone such as N-methyl-2-pyrrolidone (NMP); and the like.

These aprotic solvents can be used alone or in combination of two or more. Among these aprotic solvents, DMSO, DMF, DMAc, NMP, etc., and especially DMSO, can be used to more efficiently produce chemically modified cellulose nanofibers. The mechanism of action is not entirely clear, but it is presumed to be due to homogeneous micro-swelling of the cellulose raw material in the aprotic solvent.

The cellulose raw material can be refined using a device that effectively applies shear to the cellulose raw material, such as a disintegrator, beater, refiner, low-pressure homogenizer, high-pressure homogenizer, ultra-high-pressure homogenizer, emulsifier, homomixer, grinder, mass colloider, cutter mill, ball mill, bead mill, jet mill, single-screw extruder, twin-screw extruder, ultrasonic agitator, or household juicer mixer. Among these, refinement using a homomixer using an aprotic solvent is preferred because it can defibrate with low energy and enables chemical modification of cellulose nanofibers in a non-aqueous system.

In one aspect, the number average fiber diameter of the cellulose nanofibers is preferably 1 to 1000 nm from the viewpoint of obtaining a good effect of improving physical properties by the cellulose nanofibers. The number average fiber diameter of the cellulose nanofibers is more preferably 4 nm or more, or 5 nm or more, or 10 nm or more, or 15 nm or more, or 20 nm or more, and more preferably 500 nm or less, or 450 nm or less, or 400 nm or less, or 350 nm or less, or 300 nm or less, or 250 nm or less.

The average L/D of the cellulose nanofibers is preferably 50 or more, or 80 or more, or 100 or more, or 120 or more, or 150 or more, from the viewpoint of effectively improving the mechanical properties of a resin composite containing cellulose nanofibers with a small amount of cellulose nanofibers. There is no particular upper limit, but from the viewpoint of handleability, it is preferably 5,000 or less.

In the present disclosure, the length, diameter, and L/D ratio of each cellulose nanofiber are determined by diluting an aqueous dispersion of cellulose nanofibers with a water-soluble solvent (e.g., water, ethanol, tert-butanol, etc.) to 0.01 to 0.1% by mass, dispersing the nanofibers using a high-shear homogenizer (e.g., IKA, product name "Ultra Turrax T18") under processing conditions: rotation speed 25,000 rpm x 5 minutes, casting the nanofiber on mica, and air-drying the nanofibers to obtain a measurement sample, and measuring the length (L) and diameter (D) of 100 randomly selected fibrous substances in an observation field with a magnification adjusted so that at least 100 fibrous substances are observed, and the ratio (L/D) is calculated. The number average value of the length (L), the number average value of the diameter (D), and the number average value of the ratio (L/D) of the cellulose nanofibers are calculated.

The length, diameter, and L/D ratio of the cellulose nanofibers in the resin composite can be confirmed by measuring the solid resin composite as a measurement sample using the above-mentioned measurement method. Alternatively, the length, diameter, and L/D ratio of the cellulose nanofibers in the resin composite can be confirmed by dissolving the resin components in the resin composite in an organic or inorganic solvent that can dissolve the resin components of the resin composite, separating the cellulose nanofibers, thoroughly washing with the solvent, and then replacing with a water-soluble solvent (e.g., water, ethanol, tert-butanol, etc.), preparing a 0.01 to 0.1 mass% dispersion, and redispersing it with a high-shear homogenizer (e.g., IKA, product name "Ultra Turrax T18"). The redispersion can be confirmed by casting the redispersion on mica, air-drying it, and measuring it as a measurement sample using the above-mentioned measurement method. At this time, the measurement is performed on 100 or more randomly selected cellulose nanofibers.

The degree of crystallinity of the cellulose nanofiber is preferably 55% or more. When the degree of crystallinity is in this range, the mechanical properties (strength, dimensional stability) of the cellulose fiber itself are high, so that when the cellulose fiber is dispersed in a resin, the strength and dimensional stability of the resin composite tend to be high. A more preferable lower limit of the degree of crystallinity is 60%, even more preferably 70%, and most preferably 80%. There is no particular upper limit for the degree of crystallinity of the cellulose nanofiber, and the higher the better, but from a production standpoint, a preferable upper limit is 99%.

Between the microfibrils of plant-derived cellulose and between the microfibril bundles, there are alkali-soluble polysaccharides such as hemicellulose, and acid-insoluble components such as lignin. Hemicellulose is a polysaccharide composed of sugars such as mannan and xylan, and forms hydrogen bonds with cellulose to bind the microfibrils together. Lignin is also a compound with an aromatic ring, and is known to be covalently bonded to hemicellulose in the cell walls of plants. If there is a large amount of impurities such as lignin remaining in the cellulose nanofiber, discoloration may occur due to heat during processing. Therefore, from the viewpoint of suppressing discoloration of the resin composite during extrusion processing and molding processing, it is desirable to keep the crystallinity of the cellulose nanofiber within the above-mentioned range.

When the cellulose nanofiber is cellulose type I crystal (derived from natural cellulose), the degree of crystallinity is determined by the following formula using the Segal method from the diffraction pattern (2θ/deg. is 10 to 30) obtained by measuring a sample by wide-angle X-ray diffraction.
Crystallinity (%)=([diffraction intensity attributable to the (200) plane at 2θ/deg.=22.5]−[diffraction intensity attributable to amorphous at 2θ/deg.=18])/[diffraction intensity attributable to the (200) plane at 2θ/deg.=22.5]×100

Furthermore, when the cellulose nanofiber is a cellulose II type crystal (derived from regenerated cellulose), the degree of crystallinity can be calculated from the absolute peak intensity h0 at 2θ = 12.6° assigned to the (110) plane peak of the cellulose II type crystal in wide-angle X-ray diffraction and the peak intensity h1 from the baseline at this interplanar spacing, according to the following formula:
Crystallinity (%) = h1 /h0 ×100

Known crystalline forms of cellulose include types I, II, III, and IV, of which types I and II are particularly widely used, and types III and IV have been obtained on a laboratory scale but are not widely used on an industrial scale. The cellulose nanofibers disclosed herein have relatively high structural mobility, and by dispersing the cellulose nanofibers in a resin, a resin composite is obtained that has a lower linear expansion coefficient and is superior in strength and elongation during tensile and bending deformation. Therefore, cellulose fibers containing cellulose type I crystals or cellulose type II crystals are preferred, and cellulose nanofibers containing cellulose type I crystals and having a crystallinity of 55% or more are more preferred.

The degree of polymerization of the cellulose nanofiber is preferably 400 or more, more preferably 420 or more, more preferably 430 or more, more preferably 440 or more, more preferably 450 or more, and preferably 3500 or less, more preferably 3300 or less, more preferably 3200 or less, more preferably 3100 or less, more preferably 3000 or less.

From the viewpoint of processability and mechanical property expression, it is desirable that the degree of polymerization of the cellulose nanofiber is within the above-mentioned range. From the viewpoint of processability, it is preferable that the degree of polymerization is not too high, and from the viewpoint of mechanical property expression, it is desirable that the degree of polymerization is not too low.

The degree of polymerization of cellulose nanofiber refers to the average degree of polymerization measured according to the reduced specific viscosity method using a copper ethylenediamine solution as described in the Verification Test (3) of the "Japanese Pharmacopoeia Commentary, 15th Edition (published by Hirokawa Shoten)."

In one embodiment, the weight average molecular weight (Mw) of the cellulose nanofiber is 100,000 or more, more preferably 200,000 or more. The ratio (Mw/Mn) of the weight average molecular weight to the number average molecular weight (Mn) is 6 or less, preferably 5.4 or less. The larger the weight average molecular weight, the fewer the number of terminal groups of the cellulose molecule. In addition, since the ratio (Mw/Mn) of the weight average molecular weight to the number average molecular weight represents the width of the molecular weight distribution, the smaller the Mw/Mn, the fewer the number of ends of the cellulose molecule. Since the ends of the cellulose molecules are the starting points of thermal decomposition, when the cellulose molecules of the cellulose nanofibers not only have a large weight average molecular weight but also have a narrow molecular weight distribution, a particularly heat-resistant cellulose nanofiber and a resin composite containing the cellulose nanofiber and the resin can be obtained. The weight average molecular weight (Mw) of the cellulose nanofiber may be, for example, 600,000 or less, or 500,000 or less, from the viewpoint of the ease of availability of the cellulose raw material. From the viewpoint of ease of production of cellulose nanofibers, the ratio (Mw/Mn) of the weight average molecular weight to the number average molecular weight (Mn) may be, for example, 1.5 or more, or 2 or more. Mw can be controlled to the above range by selecting a cellulose raw material having an Mw appropriate for the purpose, performing physical and/or chemical treatment on the cellulose raw material appropriately within a moderate range, etc. Mw/Mn can also be controlled to the above range by selecting a cellulose raw material having an Mw/Mn appropriate for the purpose, performing physical and/or chemical treatment on the cellulose raw material appropriately within a moderate range, etc. In both the control of Mw and the control of Mw/Mn, examples of the physical treatment include dry or wet grinding using a microfluidizer, ball mill, disk mill, etc., physical treatment that applies mechanical forces such as impact, shear, shear, and friction using a crusher, homomixer, high-pressure homogenizer, ultrasonic device, etc., and examples of the chemical treatment include digestion, bleaching, acid treatment, and regenerated cellulose.

The weight-average molecular weight and number-average molecular weight of cellulose referred to here are values determined by dissolving cellulose in N,N-dimethylacetamide containing added lithium chloride and then performing gel permeation chromatography using N,N-dimethylacetamide as a solvent.

Methods for controlling the degree of polymerization (i.e., average degree of polymerization) or molecular weight of cellulose nanofibers include hydrolysis. Hydrolysis promotes depolymerization of the amorphous cellulose inside the cellulose nanofibers, decreasing the average degree of polymerization. At the same time, hydrolysis removes impurities such as hemicellulose and lignin in addition to the amorphous cellulose described above, making the inside of the fibers more porous.

The hydrolysis method is not particularly limited, and examples thereof include acid hydrolysis, alkali hydrolysis, hydrothermal decomposition, steam explosion, and microwave decomposition. These methods may be used alone or in combination of two or more. In the acid hydrolysis method, for example, α-cellulose obtained as pulp from a fibrous plant is used as a cellulose raw material, and this is dispersed in an aqueous medium, and an appropriate amount of protonic acid, carboxylic acid, Lewis acid, heteropolyacid, etc. is added and heated while stirring, so that the average degree of polymerization can be easily controlled. The reaction conditions such as temperature, pressure, and time at this time vary depending on the cellulose type, cellulose concentration, acid type, acid concentration, etc., but are appropriately adjusted so that the desired average degree of polymerization is achieved. For example, a condition in which cellulose is treated for 10 minutes or more at 100°C or higher under pressure using an aqueous mineral acid solution of 2% by mass or less can be exemplified. Under these conditions, the catalyst components such as acid penetrate into the inside of the cellulose nanofiber, promoting hydrolysis, reducing the amount of catalyst components used, and making subsequent purification easier. In addition, the dispersion of the cellulose raw material during hydrolysis may contain a small amount of an organic solvent in addition to water, as long as the effects of the present invention are not impaired.

Alkali-soluble polysaccharides that cellulose nanofibers may contain include hemicellulose, β-cellulose, and γ-cellulose. Alkali-soluble polysaccharides are understood by those skilled in the art as components obtained as the alkali-soluble portion of holocellulose obtained by solvent extraction and chlorine treatment of plants (e.g., wood) (i.e., components obtained by removing α-cellulose from holocellulose). Alkali-soluble polysaccharides are polysaccharides that contain hydroxyl groups and have poor heat resistance, and can cause inconveniences such as decomposition when exposed to heat, yellowing during thermal aging, and a decrease in the strength of cellulose fibers. Therefore, it is preferable that the content of alkali-soluble polysaccharides in cellulose nanofibers is low.

In one aspect, the average content of alkali-soluble polysaccharides in the cellulose nanofibers is preferably 20% by mass or less, 18% by mass or less, or 15% by mass or less, relative to 100% by mass of the cellulose nanofibers, from the viewpoint of obtaining good dispersibility of the cellulose nanofibers. From the viewpoint of ease of production of the cellulose nanofibers, the above content may be 1% by mass or more, 2% by mass or more, or 3% by mass or more.

The average alkali-soluble polysaccharide content can be determined by the method described in the non-patent literature (Wood Science Experiment Manual, edited by the Japan Wood Research Society, pp. 92-97, 2000), by subtracting the α-cellulose content from the holocellulose content (Wise method). This method is understood in the industry as a method for measuring the amount of hemicellulose. The alkali-soluble polysaccharide content is calculated three times for each sample, and the number average of the calculated alkali-soluble polysaccharide contents is taken as the average alkali-soluble polysaccharide content.

In one aspect, the average content of acid-insoluble components in the cellulose nanofibers is preferably 10% by mass or less, 5% by mass or less, or 3% by mass or less, relative to 100% by mass of the cellulose nanofibers, from the viewpoint of avoiding a decrease in the heat resistance of the cellulose nanofibers and the associated discoloration. From the viewpoint of ease of production of the cellulose nanofibers, the above content may be 0.1% by mass or more, 0.2% by mass or more, or 0.3% by mass or more.

The average acid-insoluble content is determined by quantifying the acid-insoluble content using the Clason method described in the non-patent literature (Wood Science Experiment Manual, edited by the Japan Wood Research Society, pp. 92-97, 2000). This method is understood in the industry as a method for measuring the amount of lignin. The sample is stirred in a sulfuric acid solution to dissolve cellulose, hemicellulose, etc., and then filtered through glass fiber filter paper. The resulting residue corresponds to the acid-insoluble components. The acid-insoluble component content is calculated from the weight of this acid-insoluble component, and the number average of the acid-insoluble component contents calculated for the three samples is taken as the average acid-insoluble component content.

The cellulose nanofibers may be chemically treated (e.g., oxidized or chemically modified using a modifying agent). As an example, fine cellulose fibers obtained by oxidizing cellulose fibers with 2,2,6,6-tetramethylpiperidine-1-oxyl radical as shown in Cellulose (1998) 5, 153-164, followed by washing and mechanical fiberization, may be used.

As a cellulose modification agent, a compound that reacts with the hydroxyl groups of cellulose can be used, and examples of such agents include esterifying agents, etherifying agents, and silylating agents. In a preferred embodiment, the chemical modification is acylation using an esterifying agent. As an esterifying agent, acid halides, acid anhydrides, vinyl carboxylates, and carboxylic acids are preferred.

The acid halide may be at least one selected from the group consisting of compounds represented by the following formula (1):
R ¹ -C(=O)-X (1)
(In the formula, ^R1 represents an alkyl group having 1 to 24 carbon atoms, an alkenyl group having 2 to 24 carbon atoms, a cycloalkyl group having 3 to 24 carbon atoms, or an aryl group having 6 to 24 carbon atoms, and X represents Cl, Br, or I.)
Specific examples of acid halides include, but are not limited to, acetyl chloride, acetyl bromide, acetyl iodide, propionyl chloride, propionyl bromide, propionyl iodide, butyryl chloride, butyryl bromide, butyryl iodide, benzoyl chloride, benzoyl bromide, and benzoyl iodide. Among them, acid chlorides can be preferably used in terms of reactivity and handling. In addition, in the reaction of acid halides, one or more alkaline compounds may be added to act as a catalyst and neutralize the by-product acidic substances. Specific examples of alkaline compounds include, but are not limited to, tertiary amine compounds such as triethylamine and trimethylamine; and nitrogen-containing aromatic compounds such as pyridine and dimethylaminopyridine.

As the acid anhydride, any suitable acid anhydride can be used. For example,
Saturated aliphatic monocarboxylic acid anhydrides, such as acetic acid, propionic acid, (iso)butyric acid, and valeric acid; unsaturated aliphatic monocarboxylic acid anhydrides, such as (meth)acrylic acid and oleic acid;
Alicyclic monocarboxylic acid anhydrides such as cyclohexanecarboxylic acid and tetrahydrobenzoic acid;
Aromatic monocarboxylic acid anhydrides such as benzoic acid and 4-methylbenzoic acid;
Examples of dibasic carboxylic acid anhydrides include saturated aliphatic dicarboxylic acid anhydrides such as succinic anhydride and adipic acid, unsaturated aliphatic dicarboxylic acid anhydrides such as maleic anhydride and itaconic anhydride, alicyclic dicarboxylic acid anhydrides such as 1-cyclohexene-1,2-dicarboxylic acid anhydride, hexahydrophthalic anhydride and methyltetrahydrophthalic anhydride, and aromatic dicarboxylic acid anhydrides such as phthalic anhydride and naphthalic anhydride;
Examples of the polybasic carboxylic acid anhydrides having three or more bases include polycarboxylic acids (anhydrides) such as trimellitic anhydride and pyromellitic anhydride.
In addition, in the reaction of an acid anhydride, one or more types of acidic compounds such as sulfuric acid, hydrochloric acid, phosphoric acid, etc., or Lewis acids (for example, Lewis acid compounds represented by MYn, where M represents a semimetal element such as B, As, Ge, etc., or a base metal element such as Al, Bi, In, etc., or a transition metal element such as Ti, Zn, Cu, etc., or a lanthanoid element, n is an integer corresponding to the atomic valence of M and represents 2 or 3, and Y represents a halogen atom, _OAc , _OCOCF3 , _ClO4 , _SbF6 , _PF6 , or _OSO2CF3 (OTf)), or alkaline compounds such as triethylamine, pyridine, etc. may be added as a catalyst.

The vinyl carboxylate may be represented by the following formula (1):
R-COO-CH=CH ₂ ...Formula (1)
{wherein R is any one of an alkyl group having 1 to 24 carbon atoms, an alkenyl group having 2 to 24 carbon atoms, a cycloalkyl group having 3 to 16 carbon atoms, and an aryl group having 6 to 24 carbon atoms.} is preferred. The vinyl carboxylate is more preferably at least one selected from the group consisting of vinyl acetate, vinyl propionate, vinyl butyrate, vinyl caproate, vinyl cyclohexanecarboxylate, vinyl caprylate, vinyl caprate, vinyl laurate, vinyl myristate, vinyl palmitate, vinyl stearate, vinyl pivalate, vinyl octylate, divinyl adipate, vinyl methacrylate, vinyl crotonate, vinyl pivalate, vinyl octylate, vinyl benzoate, and vinyl cinnamate. In the esterification reaction with a vinyl carboxylate, one or more catalysts selected from the group consisting of alkali metal hydroxides, alkaline earth metal hydroxides, alkali metal carbonates, alkaline earth metal carbonates, alkali metal hydrogencarbonates, primary to tertiary amines, quaternary ammonium salts, imidazole and derivatives thereof, pyridine and derivatives thereof, and alkoxides may be added.

Examples of alkali metal hydroxides and alkaline earth metal hydroxides include sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, barium hydroxide, etc. Examples of alkali metal carbonates, alkaline earth metal carbonates, and alkali metal hydrogen carbonates include lithium carbonate, sodium carbonate, potassium carbonate, cesium carbonate, magnesium carbonate, calcium carbonate, barium carbonate, lithium hydrogen carbonate, sodium hydrogen carbonate, potassium hydrogen carbonate, and cesium hydrogen carbonate, etc.

Primary, secondary and tertiary amines refer to primary, secondary and tertiary amines, and specific examples include ethylenediamine, diethylamine, proline, N,N,N',N'-tetramethylethylenediamine, N,N,N',N'-tetramethyl-1,3-propanediamine, N,N,N',N'-tetramethyl-1,6-hexanediamine, tris(3-dimethylaminopropyl)amine, N,N-dimethylcyclohexylamine and triethylamine.

Imidazole and its derivatives include 1-methylimidazole, 3-aminopropylimidazole, carbonyldiimidazole, etc.

Examples of pyridine and its derivatives include N,N-dimethyl-4-aminopyridine and picoline.

Examples of alkoxides include sodium methoxide, sodium ethoxide, and potassium t-butoxide.

The carboxylic acid may be at least one selected from the group consisting of compounds represented by the following formula (1).
R-COOH…(1)
(In the formula, R represents an alkyl group having 1 to 16 carbon atoms, an alkenyl group having 2 to 16 carbon atoms, a cycloalkyl group having 3 to 16 carbon atoms, or an aryl group having 6 to 16 carbon atoms.)

Specific examples of carboxylic acids include at least one selected from the group consisting of acetic acid, propionic acid, butyric acid, caproic acid, cyclohexane carboxylic acid, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, pivalic acid, methacrylic acid, crotonic acid, pivalic acid, octylic acid, benzoic acid, and cinnamic acid.

Among these carboxylic acids, at least one selected from the group consisting of acetic acid, propionic acid, and butyric acid, and particularly acetic acid, is preferred from the viewpoint of reaction efficiency.
In addition, in the reaction of carboxylic acid, one or more types of acidic compounds such as sulfuric acid, hydrochloric acid, phosphoric acid, etc., or Lewis acids (for example, Lewis acid compounds represented by MYn, where M represents a semimetal element such as B, As, Ge, etc., or a base metal element such as Al, Bi, In, etc., or a transition metal element such as Ti, Zn, Cu, etc., or a lanthanoid element, n is an integer corresponding to the atomic valence of M and represents 2 or 3, and Y represents a halogen atom, OAc, _OCOCF3 , _ClO4 , _SbF6 , _PF6, or _OSO2CF3 ( _OTf )), or alkaline compounds such as triethylamine, pyridine, etc. may be added as a catalyst.

Among these esterification reactants, at least one selected from the group consisting of acetic anhydride, propionic anhydride, butyric anhydride, vinyl acetate, vinyl propionate, vinyl butyrate, and acetic acid, and among these, acetic anhydride and vinyl acetate are preferred from the viewpoint of reaction efficiency.

When cellulose nanofibers are hydrophobized (e.g., by chemical modification such as acylation), the redispersibility of the dry solid tends to be good, but the cellulose nanofiber dry solid of the present disclosure is advantageous in that it can exhibit good redispersibility even when it is unsubstituted or has a low degree of substitution. Thus, in one embodiment, the average degree of substitution (DS) of the cellulose nanofiber dry solid (average number of substituted hydroxyl groups per glucose, which is the basic building block of cellulose) can be 0.5 or less, or 0.3 or less, or 0.1 or less, or 0. In one embodiment, the average degree of substitution (DS) can be 0 or more, or 0.1 or more, or 0.2 or more, depending on the intended use of the cellulose nanofiber.

When the modifying group of the chemically modified cellulose nanofiber is an acyl group, the degree of acyl substitution (DS) can be calculated from the reflection infrared absorption spectrum of the esterified cellulose nanofiber based on the peak intensity ratio between the peak derived from the acyl group and the peak derived from the cellulose skeleton. The peak of the absorption band of C=O based on the acyl group appears at 1730 cm ^-1 , and the peak of the absorption band of C-O based on the cellulose skeleton appears at 1030 cm ^-1 (see Figures 1 and 2). The DS of the esterified cellulose nanofiber was calculated by preparing a correlation graph between the DS obtained from the solid-state NMR measurement of the esterified cellulose nanofiber described below and the modification rate (IR index 1030) defined as the ratio of the peak intensity of the absorption band of C=O based on the acyl group to the peak intensity of the absorption band of C-O of the cellulose skeleton, and calculating the calibration curve substitution degree DS = 4.13 × IR index (1030) from the correlation graph.
It can be found by using

The DS of the esterified cellulose nanofibers can be calculated by solid-state NMR, by performing ^13C solid-state NMR measurement on the freeze-pulverized esterified cellulose nanofibers, and calculating the DS from the total area intensity (Inp) of the signals assigned to carbons C1 to C6 derived from the pyranose ring of cellulose appearing in the range from 50 ppm to 110 ppm, relative to the area intensity (Inf) of the signal assigned to one carbon atom derived from the modifying group, using the following formula:
DS=(Inf)×6/(Inp)
For example, when the modifying group is an acetyl group, the signal at 23 ppm assigned to _--CH.sub.3 may be used.
The conditions for ^{the 13} C solid state NMR measurement are, for example, as follows:
Equipment: Bruker Biospin Avance500WB
Frequency: 125.77MHz
Measurement method: DD/MAS method
Waiting time: 75 seconds
NMR sample tube: 4 mm diameter
Accumulation times: 640 times (approximately 14 hours)
MAS: 14,500Hz
Chemical shift reference: glycine (external reference: 176.03 ppm)

The DS heterogeneity ratio (DSs/DSt), defined as the ratio of the degree of modification (DSs) of the fiber surface to the degree of modification (DSt) of the entire fiber of the chemically modified cellulose nanofiber (which is synonymous with the above-mentioned degree of acyl substitution (DS)), is preferably 1.05 or more. The larger the value of the DS heterogeneity ratio, the more pronounced the heterogeneous structure of the sheath-core structure (i.e., a structure in which the fiber surface layer is highly chemically modified while the fiber center maintains the structure of cellulose close to the original unmodified cellulose structure), and while having high tensile strength and dimensional stability derived from cellulose, it is possible to improve the affinity with the resin when composited with the resin and improve the dimensional stability of the resin composition. The DS heterogeneity ratio is more preferably 1.1 or more, or 1.2 or more, or 1.3 or more, or 1.5 or more, or 2.0 or more, and from the viewpoint of ease of production of the chemically modified cellulose nanofiber, it is preferably 30 or less, or 20 or less, or 10 or less, or 6 or less, or 4 or less, or 3 or less.
The value of DSs varies depending on the degree of modification of the esterified cellulose, but for example, it is preferably 0.1 or more, or 0.2 or more, or 0.3 or more, or 0.5 or more, and preferably 3.0 or less, or 2.5 or less, or 2.0 or less, or 1.5 or less, or 1.2 or less, or 1.0 or less. The preferred range of DSt is as described above for the acyl substituent (DS).

The smaller the coefficient of variation (CV) of the DS heterogeneity ratio of the chemically modified cellulose nanofiber, the smaller the variation in various physical properties of the resin composition, and therefore the more preferable. The coefficient of variation is preferably 50% or less, or 40% or less, or 30% or less, or 20% or less. The coefficient of variation can be further reduced, for example, by a method in which the cellulose raw material is defibrated and then chemically modified to obtain chemically modified cellulose nanofibers (i.e., a sequential method), whereas it can be increased by a method in which the defibration and chemical modification of the cellulose raw material are performed simultaneously (i.e., a simultaneous method). Although the mechanism of this action is not clear, in the simultaneous method, chemical modification is more likely to proceed in the thin fibers generated in the early stages of defibration, and it is believed that defibration proceeds further when the hydrogen bonds between cellulose microfibrils are reduced by chemical modification, resulting in an increase in the coefficient of variation of the DS heterogeneity ratio.

The coefficient of variation (CV) of the DS heterogeneity ratio is calculated by taking 100 g of an aqueous dispersion of chemically modified cellulose nanofiber (solid content of 10 mass% or more) and freeze-pulverizing 10 g each to prepare measurement samples, calculating the DS heterogeneity ratio from the DSt and DSs of the 10 samples, and then using the standard deviation (σ) and arithmetic mean (μ) of the DS heterogeneity ratio among the obtained 10 samples, using the following formula.
DS heterogeneity ratio = DSs / DSt
Coefficient of variation (%) = standard deviation σ/arithmetic mean μ × 100

The DSs is calculated as follows. That is, the esterified cellulose powdered by freeze-pulverization is placed on a dish-shaped sample stage with a diameter of 2.5 mm, the surface is pressed down to make it flat, and then measurement is performed by X-ray photoelectron spectroscopy (XPS). The XPS spectrum reflects the constituent elements and chemical bond state of only the surface layer of the sample (typically about a few nm). Peak separation is performed on the obtained C1s spectrum, and DSs can be calculated by the following formula based on the integrated intensity (Ixf) of the peak attributable to one carbon atom derived from the modifying group relative to the integrated intensity (Ixp) of the peak (289 eV, C-C bond) attributable to carbons C2-C6 derived from the pyranose ring of cellulose.
DSs=(Ixf)×5/(Ixp)
For example, when the modifying group is an acetyl group, the C1s spectrum is subjected to peak separation at 285 eV, 286 eV, 288 eV, and 289 eV, and then the peak at 289 eV is used as Ixp, and the peak (286 eV) derived from the O—C═O bond of the acetyl group is used as Ixf.
The conditions for the XPS measurement are, for example, as follows:
Equipment used: ULVAC-Phi VersaProbe II
Excitation source: mono. AlKα 15 kV x 3.33 mA
Analysis size: Approximately 200 μm φ
Photoelectron extraction angle: 45°
Capture area Narrow scan: C 1s, O 1s
Pass Energy: 23.5eV

The cellulose nanofiber dry solid of the present disclosure can have good redispersibility even in the absence of a dispersant. Thus, in one embodiment, the cellulose nanofiber dry solid is substantially composed of cellulose only (i.e., does not contain a dispersant). A cellulose nanofiber dry solid that does not contain a dispersant is advantageous in that it can avoid the inconveniences (such as a decrease in physical properties) caused by the dispersant, for example, in a resin complex. In one embodiment, the dry solid may contain a small amount of dispersant (e.g., less than 1% by mass, or 0.5% by mass or less, or 0.3% by mass or less, based on 100% by mass of the total dry solid). Examples of dispersants that the cellulose nanofiber dry solid may contain include compounds having hydrophilic groups (e.g., hydroxyl groups, amino groups, etc.). In one embodiment, the dispersant is a liquid at 23°C. In one embodiment, the dispersant is a monomer having a hydrophilic group (e.g., propylene glycol, N-vinyl acetamide, etc.), and in one embodiment, a surfactant (i.e., a compound having a chemical structure in which a portion having a hydrophilic substituent and a portion having a hydrophobic substituent are covalently bonded).

As the surfactant, any of anionic surfactants, nonionic surfactants, amphoteric surfactants, and cationic surfactants can be used, but nonionic surfactants are preferred in terms of obtaining good dispersibility of cellulose nanofibers.

As the hydrophilic group of the surfactant, polyoxyethylene chain, carboxyl group, and hydroxyl group are preferred in terms of affinity with cellulose nanofibers, and polyoxyethylene chain is particularly preferred. Nonionic polyoxyethylene derivatives are particularly preferred. The polyoxyethylene chain length of the polyoxyethylene derivative may be 1 or more, or 4 or more, or 10 or more, or 15 or more. The longer the chain length, the higher the affinity for hydrophobic cellulose nanofibers, but from the viewpoint of balance with the properties of the resin composite (e.g., mechanical properties, etc.), the polyoxyethylene chain length may be 60 or less, or 50 or less, or 40 or less, or 30 or less, or 20 or less.

As the structure of the hydrophobic group of the surfactant, alkyl ether type, alkyl phenyl ether type, rosin ester type, bisphenol A type, β-naphthyl type, styrenated phenyl type, and hardened castor oil type are preferred in terms of high affinity with resin. The number of carbon atoms in the alkyl chain of the hydrophobic group (the number of carbon atoms excluding the phenyl group in the case of alkyl phenyl) is preferably 5 or more, or 10 or more, or 12 or more, or 16 or more. For example, when the resin is a polyolefin resin, the greater the carbon number of the surfactant, the higher the affinity with the resin. The carbon number may be, for example, 30 or less, or 25 or less.

Below, we will explain suitable examples of each step in the manufacturing method for the redispersion of cellulose nanofibers.

<Dispersion process>
In this step, a dispersion liquid is prepared by dispersing cellulose nanofibers in a first dispersion medium. As the first dispersion medium, a liquid medium such as water or an organic solvent can be used alone or in combination of two or more. As the organic solvent, a commonly used water-miscible organic solvent, for example, polyol (e.g., polyether polyol such as polypropylene glycol and polyethylene glycol), ethanol, methanol, dimethylformamide, dimethyl sulfoxide, N-methylpyrrolidone, dimethylacetamide, acetonitrile, acetone, acetic acid, t-butanol, etc. can be used. The dispersion step may be a fiberization step of a cellulose raw material in a cellulose nanofiber production step. The first dispersion medium may contain an additive (dispersant, etc.) in addition to the liquid medium, but does not contain an additive in one embodiment.

The concentration of cellulose nanofibers in the dispersion is preferably 0.5% by mass or more, or 1% by mass or more, or 1.5% by mass or more from the viewpoint of process efficiency in the subsequent drying process, etc., and is preferably 10% by mass or less, or 8% by mass or less, or 5% by mass or less from the viewpoint of avoiding an excessive increase in the viscosity of the dispersion and maintaining good handleability.

<Drying process>
In this step, the dispersion obtained in the dispersion step is dried to obtain a dry solid of cellulose nanofibers. An example of a drying method is drying under reduced pressure in a stirrer, and drying can be performed, for example, at a temperature of 50 to 150°C for 60 to 180 minutes under normal pressure or reduced pressure conditions. Heat press drying or spray drying may also be used. In one embodiment, a dry solid of cellulose nanofibers having the properties (particularly the apparent density) described above in the section "Dry Solid of Cellulose Nanofibers" can be obtained in this step.

<Redispersion step>
In this step, the cellulose nanofiber dry solid obtained in the drying step is redispersed in a second dispersion medium to obtain a redispersion liquid. The second dispersion medium may be the same or different from the first dispersion medium, and liquid media such as water and organic solvents can be used alone or in combination of two or more. In a preferred embodiment, the second dispersion medium is an aprotic polar solvent. As the aprotic polar solvent, one or more selected from the group consisting of dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), dimethylacetamide (DMAc), and dimethylformamide (DMF) are preferred. The second dispersion medium may contain additives (dispersants, organic binders, etc.) in addition to the liquid medium, but in one embodiment does not contain additives. Redispersion can be performed by applying a shear force by stirring (for example, a shear force by one or more selected from the group consisting of ultrasonic waves, a bead mill, a ball mill, and a homomixer).
During and/or after the redispersion step, the cellulose nanofibers may be chemically modified using the above-mentioned modifying agent.

In one embodiment, the viscosity at a shear rate of 100 sec-1 of an aqueous dispersion of the redispersion in which the cellulose nanofibers of the redispersion are adjusted to an aqueous dispersion with a solid content of 0.5% by mass (also referred to as a simulated redispersion in this disclosure) is 30% or more of the viscosity at a shear rate of 100 sec ^{-1 of} an aqueous dispersion of the cellulose nanofibers of the redispersion in which the cellulose nanofibers of the redispersion are adjusted to an aqueous dispersion with a solid content of 0.5% by mass (also referred to as a simulated dispersion in this disclosure). The viscosity is a value measured by a hysteresis loop measurement method using a double cylinder rheometer. The ratio is an index showing the extent to which the dispersibility of the cellulose nanofibers before drying can be reproduced after redispersion (i.e., redispersibility). The ratio is preferably 40% or more, 50% or more, or 60% or more. The higher the ratio, the more preferable it is, and most preferably 100%, but from the viewpoint of ease of production of the cellulose nanofibers, it may be, for example, 90% or less, or 80% or less. The simulated redispersion is an aqueous dispersion with a solid content of 0.5% by mass obtained by repeating five cycles of adding 5 times the mass of water to the redispersion, stirring, centrifugation, and discarding the supernatant, and then adding water. Similarly, the simulated dispersion is an aqueous dispersion with a solid content of 0.5% by mass obtained by repeating five cycles of adding 5 times the mass of water to the dispersion, stirring, centrifugation, and discarding the supernatant, and then adding water.

The viscosity of the simulated dispersion at a shear rate of 100 sec ^-1 may be, as an aqueous dispersion having a cellulose solids content of 0.5 mass%, preferably 10 mPa·s or more, or 13 mPa·s or more, or 15 mPa·s or more, from the viewpoint of obtaining a dispersion in which cellulose nanofibers are highly dispersed, and from the viewpoint of handleability of the dispersion, preferably 150 mPa·s or less, or 140 mPa·s or less, or 130 mPa·s or less.

The viscosity of the simulated re-dispersion liquid at a shear rate of 100 sec ^-1 may be, from the viewpoint of obtaining a re-dispersion liquid in which the cellulose nanofibers are highly dispersed, preferably 6 mPa·s or more, or 7.5 mPa·s or more, or 9 mPa·s or more, and from the viewpoint of the handleability of the re-dispersion liquid, may be, preferably 150 mPa·s or less, or 140 mPa·s or less, or 130 mPa·s or less.

The concentration of cellulose nanofibers in the redispersion liquid is preferably 1% by mass or more, or 3% by mass or more, or 5% by mass or more from the viewpoint of process efficiency when using the redispersion liquid in a cellulose nanofiber composite or resin composite, and of favorably compounding the cellulose nanofibers with other components, and is preferably 10% by mass or less, or 8% by mass or less, or 5% by mass or less from the viewpoint of ease of handling the redispersion liquid.

<Cellulose nanofiber composite>
The redispersion of cellulose nanofibers described above may be further combined with other materials in the state of the redispersion or in the state of its dried form. In one embodiment, a cellulose nanofiber composite can be produced by a method comprising a step of preparing a redispersion of cellulose nanofibers by the method of the present disclosure, and a mixing step of mixing the redispersion or its dried form with an organic binder. The dried form of the redispersion can be formed by a method of drying the redispersion under reduced pressure while stirring, or the like.

The organic binder includes a polymer having a hydrophilic group (e.g., a hydroxyl group, an amino group, etc.) (e.g., polyacrylamide, polyalkylene oxide, polyacrylic acid and its salts, polysaccharides (e.g., cellulose derivatives, starch, alginic acid and its salts (e.g., sodium alginate), guar gum, gellan gum, gelatin, etc.), polyvinyl alcohol, etc.). In one embodiment, the organic binder is a solid at 23°C.

Examples of the alkylene oxide units of the polyalkylene oxide include alkylene oxides having 2 to 4 carbon atoms, preferably ethylene oxide and propylene oxide. In a preferred embodiment, the polyalkylene oxide is composed of ethylene oxide units and/or propylene oxide units. A particularly preferred polyalkylene oxide is polyethylene oxide.

Examples of cellulose derivatives include cellulose ethers (e.g., methyl cellulose, carboxymethyl cellulose, carboxyethyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, etc.).

The weight average molecular weight of the organic binder is preferably 1,000 or more, more preferably 5,000 or more, and even more preferably 10,000 or more, and is preferably 5×10 ⁸ or less, more preferably 10 ⁸ or less, and even more preferably 5×10 ⁷ or less.

The amount of organic binder may be, per 100 parts by mass of cellulose nanofiber, preferably 1 part by mass or more, or 10 parts by mass or more, or 20 parts by mass or more, and may be preferably 50 parts by mass or less, or 40 parts by mass or less, or 30 parts by mass or less.

<Resin composite>
According to one aspect of the present invention, a resin composite (resin composition) can be produced by a method including a step of preparing a redispersion of cellulose nanofibers by the method disclosed herein, and a mixing step of mixing the redispersion or a dried form thereof with a resin. The dried form of the redispersion can be formed by a method of drying the redispersion under reduced pressure while stirring, for example. Since the aforementioned organic binder has an effect of improving the affinity between cellulose nanofibers and resin, in one aspect, it is preferable to combine the redispersion of cellulose nanofibers or a dried form thereof with the organic binder and then further combine it with a resin.

In one embodiment, the cellulose nanofiber dry solid obtained by drying the cellulose nanofiber dispersion may be a cellulose nanofiber solid. The cellulose nanofiber solid means a solid in which cellulose nanofibers are firmly bonded to each other. In a typical embodiment, the cellulose nanofiber solid can have a size (in one embodiment, the smallest dimension in the solid) of 1 mm or more, and is distinguished from powders, fibrous materials, and the like. In one embodiment, in the cellulose nanofiber solid, the cellulose nanofibers are bonded to each other by hydrogen bonds of hydroxyl groups. On the other hand, the cellulose nanofiber solid also has the property of being able to be well redispersed by a second dispersion medium to form a redispersion, and thus the redispersion or its dried body can be kneaded with a resin to form a resin composition. It is particularly preferable that the second dispersion medium applied to the cellulose nanofiber solid is selected from the group consisting of dimethyl sulfoxide (DMSO), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), dimethylacetamide (DMAc), and dimethylformamide (DMF). In a preferred embodiment, the amount of water in the second dispersion medium is 10% by mass or less, 5% by mass or less, or 0% by mass.

In one embodiment, the rate of change (Y-X)/Y of the mass of the dry solid matter (X) when the cellulose nanofiber solid matter is left to stand in water at 20°C for 2 hours and then removed from the water relative to the mass (Y) of the cellulose nanofiber solid matter before standing is preferably 30% by mass or less, or 20% by mass or less, or 10% by mass (i.e., little disintegration in water). The rate of change may be 0% by mass, but in one embodiment, it may be 0.01% by mass or more. The mass (X) is the mass of the dry solid matter obtained by removing the solid matter from the water and then drying it under vacuum at 120°C for 2 hours.

In one embodiment, the rate of change (Z-Y)/Y of the mass (Z) of the cellulose nanofiber solid when it is left to stand in water at 20°C for 2 hours and then removed from the water relative to the mass (Y) of the cellulose nanofiber solid before standing is 10% by mass or less (i.e., it absorbs almost no water). The rate of change may be 0% by mass, but in one embodiment, it may be 0.01% by mass or more. The mass (Z) is a value measured within 5 minutes after removing the solid from the water (so that it is not affected by drying) after wiping off the moisture on the surface with filter paper.

<Resin>
The resin may be a thermoplastic resin, a thermosetting resin, or a photocurable resin. The resin may be an elastomer. From the viewpoints of moldability and productivity, a thermoplastic resin is more preferable.

(Thermoplastic resin)
When the resin is a thermoplastic resin, the melting point of the thermoplastic resin may be appropriately selected depending on the application of the resin composite, etc. The melting point of the thermoplastic resin can be, for example, 150°C to 190°C or 160°C to 180°C for a resin with a relatively low melting point (e.g., a polyolefin resin), or 220°C to 350°C or 230°C to 320°C for a resin with a relatively high melting point (e.g., a polyamide resin).

The thermoplastic resin is preferably at least one selected from the group consisting of polyolefin-based resins, polyacetate-based resins, polycarbonate-based resins, polyamide-based resins, polyester-based resins, polyphenylene ether-based resins, and acrylic-based resins.

Polyolefin resins that are preferred as thermoplastic resins are polymers obtained by polymerizing olefins (e.g., α-olefins) and/or alkenes as monomer units. Specific examples of polyolefin resins include ethylene (co)polymers such as low-density polyethylene (e.g., linear low-density polyethylene), high-density polyethylene, ultra-low-density polyethylene, and ultra-high-molecular-weight polyethylene; polypropylene (co)polymers such as polypropylene, ethylene-propylene copolymer, and ethylene-propylene-diene copolymer; and copolymers of ethylene and α-olefins such as ethylene-acrylic acid copolymer, ethylene-methyl methacrylate copolymer, and ethylene-glycidyl methacrylate copolymer.

Here, the most preferred polyolefin resin is polypropylene. In particular, polypropylene having a melt mass flow rate (MFR) of 3 g/10 min or more and 30 g/10 min or less, measured at 230° C. and a load of 21.2 N in accordance with ISO 1133, is preferred. The lower limit of the MFR is more preferably 5 g/10 min, even more preferably 6 g/10 min, and most preferably 8 g/10 min. The upper limit is more preferably 25 g/10 min, even more preferably 20 g/10 min, and most preferably 18 g/10 min. From the viewpoint of improving the toughness of the resin composite, it is desirable for the MFR not to exceed the above upper limit, and from the viewpoint of the fluidity of the resin composite, it is desirable for the MFR not to exceed the above lower limit.

In addition, in order to increase the affinity with cellulose nanofibers, acid-modified polyolefin resins can also be suitably used. As the acid used for acid modification, mono- or polycarboxylic acids can be used, and examples thereof include maleic acid, fumaric acid, succinic acid, phthalic acid and their anhydrides, and citric acid. Maleic acid or its anhydride is particularly preferred because it is easy to increase the modification rate. There are no particular restrictions on the modification method, but a method in which a polyolefin resin is heated to above its melting point in the presence or absence of a peroxide and melt-kneaded is common. As the polyolefin resin to be acid-modified, all of the above-mentioned polyolefin resins can be used, but polypropylene is particularly suitable. The acid-modified polypropylene resin may be used alone, but it is more preferable to use it in combination with an unmodified polypropylene resin in order to adjust the modification rate of the resin as a whole. In this case, the ratio of the acid-modified polypropylene resin to all polypropylene resins is preferably 0.5% by mass to 50% by mass. A more preferred lower limit is 1% by mass, or 2% by mass, or 3% by mass, or 4% by mass, or 5% by mass. A more preferred upper limit is 45% by mass, or 40% by mass, or 35% by mass, or 30% by mass, or 20% by mass. In order to maintain the interfacial strength between the resin and the cellulose nanofiber, a content equal to or greater than the lower limit is preferred, and in order to maintain the ductility of the resin, a content equal to or less than the upper limit is preferred.

The melt mass flow rate (MFR) of the acid-modified polypropylene resin, measured in accordance with ISO1133 at 230°C and a load of 21.2 N, is preferably 50 g/10 min or more, 100 g/10 min or more, 150 g/10 min or more, or 200 g/10 min or more, from the viewpoint of increasing the affinity at the interface between the resin and the cellulose nanofiber. There is no particular upper limit, but it is preferably 500 g/10 min in order to maintain mechanical strength.

Preferred polyamide resins as thermoplastic resins include: polyamides obtained by polycondensation reaction of lactams (e.g., polyamide 6, polyamide 11, polyamide 12, etc.); diamines (e.g., 1,6-hexanediamine, 2-methyl-1,5-pentanediamine, 1,7-heptanediamine, 2-methyl-1-6-hexanediamine, 1,8-octanediamine, 2-methyl-1,7-heptanediamine, 1,9-nonanediamine, 2-methyl-1,8-octanediamine, 1,10-decanediamine, 1,11-undecanediamine, 1,12-dodecanediamine, m-xylylenediamine, etc.) and dicarboxylic acids (e.g., butanedioic acid, pentanedioic acid, hexanedioic acid, Polyamides obtained as copolymers with heptanedioic acid, octanedioic acid, nonanedioic acid, decanedioic acid, benzene-1,2-dicarboxylic acid, benzene-1,3-dicarboxylic acid, benzene-1,4-dicarboxylic acid, cyclohexane-1,3-dicarboxylic acid, cyclohexane-1,4-dicarboxylic acid, etc. (e.g. polyamide 6,6, polyamide 6,10, polyamide 6,11, polyamide 6,12, polyamide 6,T, polyamide 6,I, polyamide 9,T, polyamide 10,T, polyamide 2M5,T, polyamide MXD,6, polyamide 6,C, polyamide 2M5,C, etc.); and copolymers in which these are copolymerized (e.g. polyamide 6,T/6,I, etc.).

Among these polyamide resins, aliphatic polyamides such as polyamide 6, polyamide 11, polyamide 12, polyamide 6,6, polyamide 6,10, polyamide 6,11, and polyamide 6,12, and alicyclic polyamides such as polyamide 6,C and polyamide 2M5,C are more preferred.

From the viewpoint of improving the heat resistance of the resin composite, the melting point of the polyamide resin is preferably 220°C or higher, or 230°C or higher, or 240°C or higher, or 245°C or higher, or 250°C or higher, and from the viewpoint of ease of manufacturing the resin composite, the melting point is preferably 350°C or lower, or 320°C or lower, or 300°C or lower.

There is no particular restriction on the terminal carboxyl group concentration of the polyamide resin, but it is preferably 20 μmol/g or more, or 30 μmol/g or more, and preferably 150 μmol/g or less, or 100 μmol/g or less, or 80 μmol/g or less.

In the polyamide resin, the ratio of carboxyl end groups to all end groups ([COOH]/[total end groups]) is preferably 0.30 or more, or 0.35 or more, or 0.40 or more, or 0.45 or more from the viewpoint of dispersibility of the cellulose nanofiber in the resin composite, and is preferably 0.95 or less, or 0.90 or less, or 0.85 or less, or 0.80 or less from the viewpoint of the color tone of the resin composite.

The end group concentration of polyamide resins can be adjusted by known methods. One adjustment method is to add a terminal regulator (e.g., diamine compound, monoamine compound, dicarboxylic acid compound, monocarboxylic acid compound, acid anhydride, monoisocyanate, monoacid halide, monoester, monoalcohol, etc.) that reacts with the end groups during polymerization of polyamide to the polymerization liquid so as to achieve a predetermined end group concentration.

Examples of terminal regulators that react with terminal amino groups include aliphatic monocarboxylic acids such as acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, caprylic acid, lauric acid, tridecanoic acid, myristic acid, palmitic acid, stearic acid, pivalic acid, and isobutyric acid; alicyclic monocarboxylic acids such as cyclohexanecarboxylic acid; aromatic monocarboxylic acids such as benzoic acid, toluic acid, α-naphthalenecarboxylic acid, β-naphthalenecarboxylic acid, methylnaphthalenecarboxylic acid, and phenylacetic acid; and mixtures of any of the above. Among these, in terms of reactivity, stability of the blocked terminal, and price, one or more terminal regulators selected from the group consisting of acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, caprylic acid, lauric acid, tridecanoic acid, myristic acid, palmitic acid, stearic acid, and benzoic acid are preferred, and acetic acid is most preferred.

Examples of terminal regulators that react with terminal carboxyl groups include aliphatic monoamines such as methylamine, ethylamine, propylamine, butylamine, hexylamine, octylamine, decylamine, stearylamine, dimethylamine, diethylamine, dipropylamine, and dibutylamine; alicyclic monoamines such as cyclohexylamine and dicyclohexylamine; aromatic monoamines such as aniline, toluidine, diphenylamine, and naphthylamine, and any mixtures thereof. Among these, one or more terminal regulators selected from the group consisting of butylamine, hexylamine, octylamine, decylamine, stearylamine, cyclohexylamine, and aniline are preferred in terms of reactivity, boiling point, stability of the blocked terminals, price, and the like.

The concentrations of amino end groups and carboxyl end groups of a polyamide resin can be determined from the integrated values of characteristic signals corresponding to each end group by ¹ H-NMR. This method is preferred in terms of accuracy and simplicity. More specifically, it is recommended to use the method described in JP-A-7-228775, use deuterated trifluoroacetic acid as the measurement solvent, and set the number of integration scans to 300 or more.

The intrinsic viscosity [η] of a polyamide resin measured in concentrated sulfuric acid at 30°C is preferably 0.6 to 2.0 dL/g, or 0.7 to 1.4 dL/g, or 0.7 to 1.2 dL/g, or 0.7 to 1.0 dL/g, from the viewpoint of good in-mold fluidity and good appearance of the molded piece when the resin composite is, for example, injection molded. In this disclosure, "intrinsic viscosity" is synonymous with the viscosity generally called limiting viscosity. The intrinsic viscosity is determined by measuring the ηsp/c of several measurement solvents with different concentrations in 96% concentrated sulfuric acid at a temperature of 30°C, deriving the relationship between each of the ηsp/c and the concentration (c), and extrapolating the concentration to zero. This value extrapolated to zero is the intrinsic viscosity. Details of the above method are described, for example, in Polymer Process Engineering (Prentice-Hall, Inc. 1994), pages 291 to 294. From the viewpoint of accuracy, it is desirable to set the concentrations in the above-mentioned measurement solvents having different concentrations to at least four points (e.g., 0.05 g/dL, 0.1 g/dL, 0.2 g/dL, 0.4 g/dL).

Preferred polyester resins as thermoplastic resins include one or more selected from polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polybutylene terephthalate (PBT), polybutylene succinate (PBS), polybutylene succinate adipate (PBSA), polybutylene adipate terephthalate (PBAT), polyhydroxyalkanoic acid (PHA), polylactic acid (PLA), polyarylate (PAR), etc. Among these, PET, PBS, PBSA, PBT, and PEN are more preferable, and PBS, PBSA, and PBT are particularly preferable.

The end groups of the polyester resin can be changed as desired by the monomer ratio during polymerization, the presence or absence and amount of addition of a terminal stabilizer, etc. The ratio of carboxyl end groups to all end groups of the polyester resin ([COOH]/[total end groups]) is preferably 0.30 or more, or 0.35 or more, or 0.40 or more, or 0.45 or more from the viewpoint of dispersibility of cellulose nanofibers in the resin composite, and is preferably 0.95 or less, or 0.90 or less, or 0.85 or less, or 0.80 or less from the viewpoint of the color tone of the resin composite.

Preferred polyacetal resins as thermoplastic resins are homopolyacetals made from formaldehyde and copolyacetals containing trioxane as the main monomer and 1,3-dioxolane as a comonomer component. Both can be used, but copolyacetals are preferred from the viewpoint of thermal stability during processing. The amount of the structure derived from the comonomer component (e.g., 1,3-dioxolane) is preferably 0.01 mol% or more, or 0.05 mol% or more, or 0.1 mol% or more, or 0.2 mol% or more from the viewpoint of thermal stability during extrusion processing and molding processing, and is preferably 4 mol% or less, or 3.5 mol% or less, or 3.0 mol% or less, or 2.5 mol% or less, or 2.3 mol% or less from the viewpoint of mechanical strength.

(Thermosetting resin)
Examples of the thermosetting resin include bisphenol type epoxy resins such as bisphenol A type epoxy resins, bisphenol F type epoxy resins, bisphenol S type epoxy resins, bisphenol E type epoxy resins, bisphenol M type epoxy resins, bisphenol P type epoxy resins, and bisphenol Z type epoxy resins; novolac type epoxy resins such as bisphenol A novolac type epoxy resins, phenol novolac type epoxy resins, and cresol novolac epoxy resins; biphenyl type epoxy resins, biphenyl aralkyl type epoxy resins, aryl alkyl type epoxy resins, and the like. epoxy resins modified with cyclohexylmaleimide and glycidyl methacrylate; epoxy resins modified with cyclohexylmaleimide and glycidyl methacrylate; epoxy-modified polybutadiene rubber derivatives; CTBN-modified epoxy resins; trimethylolpropane polyglycidyl ether; Phenyl-1,3-diglycidyl ether, biphenyl-4,4'-diglycidyl ether, 1,6-hexanediol diglycidyl ether, diglycidyl ether of ethylene glycol or propylene glycol, sorbitol polyglycidyl ether, tris(2,3-epoxypropyl)isocyanurate, triglycidyl tris(2-hydroxyethyl)isocyanurate, novolac-type phenolic resins such as phenol novolac resin, cresol novolac resin, and bisphenol A novolac resin, unmodified resol phenolic resin, paulownia wood Examples of the resin include phenolic resins such as resol-type phenolic resins, such as oil-modified resol phenolic resins modified with oil, linseed oil, walnut oil, etc.; triazine ring-containing resins, such as phenoxy resins, urea (urea) resins, and melamine resins; unsaturated polyester resins, bismaleimide resins, diallyl phthalate resins, silicone resins, resins having a benzoxazine ring, norbornene resins, cyanate resins, isocyanate resins, urethane resins, benzocyclobutene resins, maleimide resins, bismaleimide triazine resins, polyazomethine resins, and thermosetting polyimides.

(Photocurable resin)
Examples of photocurable resins include (meth)acrylate resins, vinyl resins, and epoxy resins. These are generally classified into radical reaction types in which a monomer reacts with radicals generated by light, and cationic reaction types in which a monomer undergoes cationic polymerization, according to the reaction mechanism. Examples of radical reaction type monomers include (meth)acrylate compounds and vinyl compounds (e.g., certain vinyl ethers). Examples of cationic reaction types include epoxy compounds and certain vinyl ethers. For example, an epoxy compound that can be used as a cationic reaction type can be a monomer for both thermosetting resins and photocurable resins.

A (meth)acrylate compound is a compound that has one or more (meth)acrylate groups in the molecule. Examples of (meth)acrylate compounds include monofunctional (meth)acrylates, polyfunctional (meth)acrylates, epoxy acrylates, polyester acrylates, and urethane acrylates.

Examples of vinyl compounds include vinyl ether, styrene, and styrene derivatives. Examples of vinyl ethers include ethyl vinyl ether, propyl vinyl ether, hydroxyethyl vinyl ether, and ethylene glycol divinyl ether. Examples of styrene derivatives include methyl styrene and ethyl styrene. Other examples of vinyl compounds include triallyl isocyanurate and trimethallyl isocyanurate.

So-called reactive oligomers may be used as raw materials for photocurable resins. Examples of reactive oligomers include oligomers that have any combination selected from (meth)acrylate groups, epoxy groups, urethane bonds, and ester bonds in the same molecule, such as urethane acrylates that have (meth)acrylate groups and urethane bonds in the same molecule, polyester acrylates that have (meth)acrylate groups and ester bonds in the same molecule, and epoxy acrylates derived from epoxy resins that have epoxy groups and (meth)acrylate groups in the same molecule.

(Elastomer)
Examples of elastomers (i.e., rubber) include natural rubber (NR), butadiene rubber (BR), styrene-butadiene copolymer rubber (SBR), isoprene rubber (IR), butyl rubber (IIR), acrylonitrile-butadiene rubber (NBR), acrylonitrile-styrene-butadiene copolymer rubber, chloroprene rubber, styrene-isoprene copolymer rubber, styrene-isoprene-butadiene copolymer rubber, isoprene-butadiene copolymer rubber, chlorosulfonated polyethylene rubber, modified natural rubber (epoxidized natural rubber (ENR), hydrogenated natural rubber, deproteinized natural rubber, etc.), ethylene-propylene copolymer rubber, acrylic rubber, epichlorohydrin rubber, polysulfide rubber, silicone rubber, fluororubber, urethane rubber, and the like.

When the resin is a thermoplastic resin, a resin composite can be produced by kneading a redispersion of cellulose nanofibers or a dried product thereof (hereinafter also referred to as a cellulose nanofiber component) with a thermoplastic resin.
- A method in which a resin monomer and a cellulose nanofiber component are mixed, a polymerization reaction is carried out, the resulting resin composite is extruded into a strand shape, and cooled and solidified in a water bath to obtain a pellet-shaped molded product;
A method in which a mixture of a resin and a cellulose nanofiber component is melt-kneaded using a single-screw or twin-screw extruder, extruded into a strand shape, and cooled and solidified in a water bath to obtain a pellet-shaped molded product;
- A method in which a mixture of a resin and a cellulose nanofiber component is melt-kneaded using a single-screw or twin-screw extruder, extruded into a rod or tube, and cooled to obtain an extrusion molded product;
A method in which a mixture of a resin and a cellulose nanofiber component is melt-kneaded using a single-screw or twin-screw extruder, and extruded through a T-die to obtain a sheet or film-like molded product;
In a preferred embodiment, a mixture of a resin and a cellulose nanofiber component is melt-kneaded using a single-screw or twin-screw extruder, extruded into a strand shape, and cooled and solidified in a water bath to obtain a pellet-shaped molded product.
A specific example of a method for melt-kneading a resin and a cellulose nanofiber component is a method in which a resin and a cellulose nanofiber component that have been transported are mixed in a desired ratio, and then melt-kneaded.
When the resin is a thermoplastic resin, the minimum processing temperatures recommended by thermoplastic resin suppliers are 255 to 270°C for nylon 66, 225 to 240°C for nylon 6, 170 to 190°C for polyacetal resin, and 160 to 180°C for polypropylene. The heating setting temperature is preferably in the range of 20°C higher than these recommended minimum processing temperatures. By setting the mixing temperature within this temperature range, the cellulose nanofibers and the resin can be mixed uniformly.

Resin composites containing thermoplastic resins as resins can be provided in various shapes. Specifically, resin pellets, sheets, fibers, plates, rods, etc. can be mentioned, but the resin pellet shape is more preferable from the viewpoint of ease of post-processing and ease of transportation. Preferred pellet shapes in this case include round, elliptical, cylindrical, etc., which differ depending on the cutting method used during extrusion processing. Pellets cut by a cutting method called underwater cutting are often round, pellets cut by a cutting method called hot cutting are often round or elliptical, and pellets cut by a cutting method called strand cutting are often cylindrical. In the case of round pellets, the preferred size is a pellet diameter of 1 mm or more and 3 mm or less. In the case of cylindrical pellets, the preferred diameter is 1 mm or more and 3 mm or less, and the preferred length is 2 mm or more and 10 mm or less. It is desirable to set the above diameter and length to the lower limit or more from the viewpoint of operational stability during extrusion, and it is desirable to set them to the upper limit or less from the viewpoint of bite into the molding machine in post-processing.

Resin composites containing a thermoplastic resin as the resin can be used as various resin molded products. There are no particular limitations on the manufacturing method for the resin molded products, and any manufacturing method can be used, including injection molding, extrusion molding, blow molding, inflation molding, and foam molding. Of these, injection molding is the most preferable from the standpoint of design and cost.

When the resin is a thermosetting resin or a photocurable resin, the resin composite can be produced by, for example, a method of thoroughly dispersing the cellulose nanofiber component in a resin solution or resin powder dispersion and drying it, a method of thoroughly dispersing the cellulose nanofiber component in a resin monomer liquid and polymerizing it by heat, UV irradiation, a polymerization initiator, etc., a method of thoroughly impregnating a molded body (e.g., a sheet, a powder particle molded body, etc.) made of a dry solid of the cellulose nanofiber dispersion with a resin solution or resin powder dispersion and drying it, or a method of thoroughly impregnating a molded body made of a dry solid of the cellulose nanofiber dispersion with a resin monomer liquid and polymerizing it by heat, UV irradiation, a polymerization initiator, etc. When curing, various polymerization initiators, curing agents, curing accelerators, polymerization inhibitors, etc. can be blended.

When the resin is a thermosetting resin or a photocurable resin, a method may be used in which an uncured or semi-cured sheet called a prepreg is prepared, the prepreg is then made into a single layer or laminated, and the resin is cured and molded by applying pressure and heat. Examples of the method of applying pressure and heat include press molding, autoclave molding, bagging molding, wrapping tape method, and internal pressure molding.

When the resin is a photocurable resin, various curing methods using active energy rays can be used to produce a resin molded body.

When the resin is an elastomer, a resin composite can be produced by, for example, a method of dry kneading the dry solid of the cellulose nanofiber dispersion with raw rubber, or a method of dispersing or dissolving the cellulose nanofiber component and raw rubber in a dispersion medium, then drying and mixing. As a mixing method, a mixing method using a homogenizer is preferable because it can apply high shear force and pressure and promote dispersion, but other methods such as a propeller-type stirring device, a rotary stirring device, an electromagnetic stirring device, and manual stirring can also be used. A resin composite containing an elastomer can be molded using a desired molding method such as mold molding, injection molding, extrusion molding, hollow molding, and foam molding to obtain an unvulcanized molded product in the desired shape such as a sheet, pellet, or powder. The unvulcanized molded product can be vulcanized by heat treatment or the like as necessary to obtain a resin molded product.

A resin molded product containing a thermoplastic resin or an elastomer may be partially (e.g., several locations) melted by heat treatment and then bonded to, for example, a resin or metal substrate. The resin molded product may be a coating applied to a resin or metal substrate, or may form a laminate with the substrate. Furthermore, sheet-, film-, or fiber-shaped resin molded products may be subjected to secondary processing such as annealing, etching, corona treatment, plasma treatment, embossing, cutting, and surface polishing.

In the resin composite, the amount of cellulose nanofibers per 100 parts by mass of resin may be, from the viewpoint of the balance between processability and mechanical properties, preferably 0.001 parts by mass or more, or 0.01 parts by mass or more, or 0.1 parts by mass or more, or 1 part by mass or more, and may be preferably 100 parts by mass or less, or 80 parts by mass or less, or 70 parts by mass or less, or 50 parts by mass or less.

The present invention will be further explained based on examples, but the present invention is not limited to these examples.

<Production of cellulose nanofiber dispersion, dry solid, and redispersion thereof>
[Production Example 1]
Cotton linter pulp and water were mixed to prepare a dispersion with a solid content of 1.5% by mass, and the aqueous dispersion was beaten for 30 minutes using a SDR14 lab refiner (pressure type DISK type) manufactured by Aikawa Iron Works Co., Ltd. as a disc refiner device, with the clearance between the discs set to 1 mm. Subsequently, thorough beating was performed under conditions in which the clearance was reduced to a level close to zero, to obtain a beaten aqueous dispersion (solid content: 1.5% by mass). The viscosity of the aqueous dispersion with a solid content of 0.5% by mass obtained by diluting the obtained cellulose nanofiber slurry (as a dispersion) was 40.1 mPa·s at a shear rate of 100 sec ^-1 .

The resulting cellulose nanofiber slurry was filtered and concentrated using a papermaking machine to a solid content of approximately 10-20% by mass, and the resulting sheet-like wet cake was dried in a steam oven until the moisture content was 7% by mass or less, yielding an aggregated dried product of cellulose nanofibers (as a cellulose nanofiber dry solid). The apparent density of the resulting dry solid was 685 g/L. The cellulose nanofibers had a degree of polymerization of 2900 and an average fiber diameter of 32 nm before drying.

[Production Example 2]
Cotton linter pulp and water were mixed to prepare a dispersion with a solid content of 1.5% by mass, and the aqueous dispersion was beaten for 30 minutes using a SDR14 lab refiner (pressurized DISK type) manufactured by Aikawa Iron Works Co., Ltd. as a disc refiner device, with the clearance between the discs set to 1 mm. Subsequently, thorough beating was performed under conditions in which the clearance was reduced to a level close to zero, to obtain a beaten aqueous dispersion (solid content: 1.5% by mass). The obtained beaten aqueous dispersion was directly subjected to a high-pressure homogenizer (NSO15H manufactured by Niro Soavi Co., Ltd. (Italy)) to perform a fine treatment 10 times under an operating pressure of 100 MPa, to obtain a cellulose nanofiber slurry (as a dispersion liquid) (solid content: 1.5% by mass). The viscosity of the aqueous dispersion with a solid content of 0.5% by mass obtained by diluting the obtained cellulose nanofiber slurry at a shear rate of 100 sec ^-1 was 141.3 mPa.s.

The obtained cellulose nanofiber slurry was filtered and concentrated to a solid content of about 10 to 20% by mass using a papermaking machine, and the obtained wet sheet was sandwiched between Asahi Kasei Silky Fine (nonwoven fabric) and 10 sheets of filter paper were placed on the outside. A metal plate was placed on the outermost side and pressed at a pressure of 410 kPA for 5 minutes, and the swollen cardboard was replaced and the pressing operation was repeated to concentrate the slurry to a moisture content of 55% by mass. The obtained cellulose nanofiber wet sheet was vacuum dried at a pressure of 410 kPa and 150°C until the moisture content was 5% by mass or less, and a cellulose nanofiber dried product (as a cellulose nanofiber dry solid) was obtained. The apparent density of the obtained dry solid was 1150 g/L. The cellulose nanofiber had a degree of polymerization of 2700 and an average fiber diameter of 18 nm before drying.

[Production Example 3]
Celish KY-100G (Daicel Chemical Industries) (cellulose nanofibers) having a viscosity of 14.9 mPa·s at a shear rate of 100 sec ^-1 of a diluted water dispersion with a solid content of 0.5% by mass was dried under reduced pressure of -0.1 MPa at 307 rpm in a 60°C warm bath in an enclosed planetary mixer (Kodaira Seisakusho Co., Ltd., product name "ACM-5LVT", stirring blade is hook type) until the moisture content was 5% by mass or less, to obtain an aggregated dried product of cellulose nanofibers (as a cellulose nanofiber dry solid). The apparent density of the obtained dry solid was 291 g/L. The cellulose nanofibers had a degree of polymerization of 1450 and an average fiber diameter of 125 nm before drying.

[Redispersion Examples 1 to 6]
0.5 parts by mass of the obtained cellulose nanofiber dry solid and 24.5 parts by mass of dimethyl sulfoxide were added to KAPPA VITA HM35 manufactured by NETZSCH VAKUMIX, and the mixture was stirred for 2 hours. The cellulose nanofiber was then redispersed by treatment in a homomixer at a rotation speed of 6,500 rpm for the time shown in Table 1 to obtain a cellulose nanofiber/DMSO slurry (as redispersion liquid).
Table 1 shows the redispersion conditions, the viscosity at a shear rate of 100 sec ^-1 of an aqueous dispersion having a solid content of 0.5 mass% prepared from the resulting redispersion liquid by the method described below, and the viscosity recovery rate, which is the ratio (viscosity of the aqueous dispersion obtained from the redispersion liquid)/(viscosity of the aqueous dispersion obtained from the dispersion liquid) (%).

[Redispersion Examples 7 to 9]
0.5 parts by mass of each of the dry cellulose nanofiber solids obtained in Production Examples 1 to 3 and 24.5 parts by mass of pure water were added to a KAPPA VITA HM35 manufactured by NETZSCH VAKUMIX, and the mixture was allowed to stand for 2 hours. After that, the dry cellulose nanofiber solid was taken out.
The mass (X) of the dry solid content obtained after drying for 2 hours under vacuum conditions at 120°C was measured. The rate of change (Y-X)/Y relative to the mass (Y) of the dry solid cellulose nanofiber before standing was 6 mass% in Production Example 1, 2 mass% in Production Example 2, and 10 mass% in Production Example 3. An attempt was made to redisperse the obtained cellulose/water dispersion at a homogenizer rotation speed of 6,500 rpm, but redispersion was not achieved and the device became clogged, resulting in a halt.

[Redispersion Example 10]
(Acetylation of cellulose nanofibers after redispersion)
0.321 parts by mass of sodium bicarbonate and 1 part by mass of vinyl acetate were added to 50 parts by mass of the cellulose nanofiber/DMSO slurry (redispersion liquid) obtained in Redispersion Example 5, and the mixture was allowed to react at 60°C for 1 hour to obtain acetylated cellulose nanofibers with their surfaces acetylated.

<Production of resin composite>
[Examples 1 to 7 and Control Examples 1 to 3]
The cellulose nanofiber slurry (dispersion) obtained in each production example and the cellulose nanofiber DMSO slurry (redispersion) obtained in each redispersion example were used as shown in Table 2, and resin composites were produced by the following procedure. For the cellulose nanofiber DMSO slurry (redispersion), 5 parts by mass of water was added to 1 part by mass of the slurry, and after stirring, centrifugal separation was performed and the supernatant was discarded. The above cycle of adding 5 parts by mass of water, stirring, centrifugal separation, and discarding the supernatant was repeated five times, and then 2 parts by mass of water was added to the residue to obtain an aqueous dispersion. To each of the dispersion and the aqueous dispersion obtained from the redispersion, 43 parts by mass of PEG-PPG (GL-3000, manufactured by Sanyo Chemical Industries, Ltd.) was added per 100 parts by mass of cellulose solids, and the mixture was stirred at 70 rpm for 80 minutes at 25°C and atmospheric pressure in an enclosed planetary mixer (manufactured by Kodaira Manufacturing Co., Ltd., trade name "ACM-5LVT", stirring blade is hook type) and then dried under reduced pressure at 307 rpm for 5 hours in a 60°C hot bath under reduced pressure conditions of -0.1 MPa to obtain a cellulose/PEG-PPG dried body. The obtained cellulose/PEG-PPG dried body and a thermoplastic resin (UBE nylon 1013B manufactured by Ube Industries, Ltd.) were used in the formulations shown in Table 2, and a resin composite was produced according to the following procedure.

The obtained resin composite was circulated and kneaded for 5 minutes at 260°C and 100 rpm (shear rate 1570 (1/s)) using a small kneader (manufactured by Xplore Instruments, product name "Xplore"), and then passed through a die to obtain a resin composite strand with a diameter of 1 mm. Resin composite pellets obtained from the strands (cut into 1 cm lengths) were melted at 260°C using an attached injection molding machine to prepare dumbbell-shaped test pieces according to JIS K7127 standard, which were used for evaluation. The resin composite 1 obtained in the form of a thin film, pellet, or dumbbell-shaped test piece was used for appropriate evaluation.

Evaluation
<Preparation of measurement sample>
The cellulose nanofiber slurry obtained in each production example was filtered to adjust the solid content concentration to 10% by mass, and 4 g of the cellulose slurry was dispersed in 96 g of tert-butanol, and further dispersed in a homogenizer (manufactured by IKA, product name "Ultra Turrax T18") under the following processing conditions: rotation speed 25,000 rpm x 5 minutes until no aggregates were present (solid content concentration 0.4% by mass). 100 g of the obtained tert-butanol dispersion was filtered on filter paper (5C, Advantec, diameter 90 mm). The wet paper obtained by filtration was heated at 150 ° C. for 5 minutes with the filter paper attached, sandwiched between two sheets of 150 mm diameter filter paper, and the periphery of the wet paper was pressed down with a cylindrical weight (inner diameter 110 mm) of about 300 g to obtain a porous sheet. This porous sheet was used as a measurement sample.

<Average degree of substitution (DS) (degree of acetylation) of cellulose nanofiber>
The infrared spectrum of the porous sheet was measured by ATR-IR using a Fourier transform infrared spectrophotometer (FT/IR-6200, manufactured by JASCO Corp.) The infrared spectrum measurement was carried out under the following conditions.
Number of times: 64
Wavenumber resolution: 4cm ^-1 ,
Measurement wave number range: 4000 to 600 cm ^-1 ,
ATR crystal: Diamond,
Incident angle: 45°
From the obtained IR spectrum, the IR index was calculated according to the following formula (1):
IR index = H1730/H1030 (1)
In the formula, H1730 and H1030 are the absorbances at 1730 cm ^-1 and 1030 cm ^-1 (absorption bands of C-O stretching vibration of the cellulose backbone chain). However, the lines connecting 1900 cm ^-1 and 1500 cm ^-1 and the line connecting 800 cm ^-1 and 1500 cm ^-1 are taken as baselines, and the absorbances are calculated based on these baselines as 0 absorbance.
The average degree of substitution (DS) was calculated from the IR index according to the following formula (2).
DS = 4.13 × IR index (2)

<Degree of polymerization of cellulose nanofiber>
The viscosity was measured by the reduced specific viscosity method using a copper ethylenediamine solution as specified in the crystalline cellulose identification test (3) in the "Japanese Pharmacopoeia, 14th Edition" (published by Hirokawa Shoten).

<Crystallization degree of cellulose nanofiber>
The porous sheet was subjected to X-ray diffraction measurement, and the crystallinity was calculated from the following formula.
Crystallinity (%) = [I ₍₂₀₀₎ - I _(amorphous) ] / I ₍₂₀₀₎ × 100
I ₍₂₀₀₎ : Diffraction peak intensity due to the 200 plane (2θ = 22.5°) in cellulose I type crystals I _(amorphous) : Halo peak intensity due to amorphous in cellulose I type crystals, which is the peak intensity at an angle 4.5° lower than the diffraction angle of the 200 plane (2θ = 18.0°)

(X-ray diffraction measurement conditions)
Device: MiniFlex (manufactured by Rigaku Corporation)
Operation axis 2θ/θ
Radiation source: CuKα
Measurement method: Continuous Voltage: 40 kV
Current: 15mA
Starting angle 2θ=5°
End angle 2θ = 30°
Sampling width 0.020°
Scan speed: 2.0°/min
Sample: A porous sheet is attached onto the sample holder.

<Average fiber diameter of cellulose nanofiber>
The cellulose nanofiber slurry was dispersed using a high-shear homogenizer (manufactured by Nippon Seiki Co., Ltd., product name "Excel Auto Homogenizer ED-7", processing conditions: rotation speed 15,000 rpm × 5 minutes) to form an aqueous dispersion, which was then diluted with pure water to 0.1 to 0.5 mass %, cast onto mica, air-dried, and measured with an atomic force microscope (AFM). The measurement was performed by adjusting the magnification so that at least 100 cellulose nanofibers were observed, and the fiber diameters of 100 randomly selected cellulose nanofibers were determined, and the arithmetic average of the 100 cellulose nanofibers was calculated.

<Apparent density of cellulose nanofiber dry solid>
The apparent density of the dry solid cellulose nanofiber was determined according to the Archimedes method using an electronic balance GR-202 and a specific gravity meter AD-1653 manufactured by A&D Co., Ltd.

<Viscosity of aqueous dispersion obtained from dispersion and redispersion of cellulose nanofiber>
Water was added to the dispersion as described above to obtain an aqueous dispersion with a solid content of 0.5% by mass. In addition, the cycle of adding 5 times the amount of water by mass to the redispersion, stirring, centrifuging, and discarding the supernatant was repeated 5 times, and then water was added to obtain an aqueous dispersion with a solid content of 0.5% by mass. Viscosity measurements were performed on these aqueous dispersions. A Thermo Fisher Scientific rheometer HAAKE MARS was used for the measurement, and the viscosity was measured in a coaxial cylinder arrangement. When reading the viscosity at a shear rate of 100 sec ^-1 , in order to suppress the variation in data, a certain shear history was given and then the viscosity at a shear rate of 100 sec ^-1 was read. Specifically, a 0.5% by mass aqueous dispersion (obtained from a dispersion or redispersion) was placed in a coaxial cylinder cup, and then the shear rate was increased from 1 ^sec to 100 ^sec over 100 seconds using a rheometer, and the shear rate was then decreased from 100 ^sec to 1 ^sec over 100 seconds, and then the shear rate was increased again from 1 ^sec to 100 ^sec over 100 seconds, and the viscosity was read when the shear rate reached 100 ^sec .

<Tensile Breaking Strength of Resin Composite>
Multipurpose test pieces were molded in accordance with ISO294-3 using an injection molding machine with a maximum clamping pressure of 75 tons, and the tensile breaking strength was measured under conditions in accordance with JIS K6920-2, n = 10. Since polyamide resin changes due to moisture absorption, the pieces were stored in aluminum moisture-proof bags immediately after molding to suppress moisture absorption.

<Coefficient of linear thermal expansion (CTE) of resin composite>
The steam multipurpose test piece was cut into a size of 3 mm width x 25 mm length to be used as a measurement sample. Using a TMA6100 model SII device, the sample was heated from room temperature to 120°C at 5°C/min in a compression mode with a chuck distance of 10 mm and a load of 5 g under a nitrogen atmosphere, then cooled to 25°C at 5°C/min, and heated again from 25°C to 120°C at 5°C/min. At this time, the average linear thermal expansion coefficient between 30°C and 100°C during the second heating was measured.

According to the present invention, cellulose nanofibers can be provided as a dry solid that exhibits good redispersibility, which reduces storage and transportation costs and is extremely useful as a property improver for resin composites.

Claims (13)

A method for producing a redispersion of cellulose nanofibers, comprising the steps of:
A dispersing step of preparing a dispersion liquid by dispersing cellulose nanofibers in a first dispersion medium;
A drying step of drying the dispersion to obtain a dry solid of cellulose nanofibers;
A redispersion step of redispersing the cellulose nanofiber dry solid in a second dispersion medium to prepare a redispersion liquid;
Including,
The apparent density of the cellulose nanofiber dry solid is 210 g/L to 1500 g/L;
the second dispersion medium is an aprotic polar solvent;
The method of claim 1, wherein the viscosity at a shear rate of 100 sec ^-1 of an aqueous dispersion of a redispersion obtained by adjusting the cellulose nanofibers in the redispersion to an aqueous dispersion with a solids content of 0.5% by mass is 30% or more of the viscosity at a shear rate of 100 sec- ¹ of an aqueous dispersion of a redispersion obtained by adjusting the cellulose nanofibers in the dispersion to an aqueous dispersion with a solids content of 0.5% by mass. The method according to claim 1, wherein the average degree of substitution (DS) of the cellulose nanofiber dry solid is 0.5 or less. The method according to claim 1 or 2, wherein the content of the dispersant in the cellulose nanofiber dry solid is less than 1 mass%. The method according to any one of claims 1 to 3, wherein the redispersion is carried out by applying shear force through stirring. The method according to claim 4, wherein the shear force is applied using one or more selected from the group consisting of ultrasonic waves, a bead mill, a ball mill, and a homomixer. The method according to any one of claims 1 to 5, wherein the aprotic polar solvent is one or more selected from the group consisting of dimethylsulfoxide, N-methylpyrrolidone, dimethylacetamide, and dimethylformamide. The method according to any one of claims 1 to 6, further comprising chemically modifying the cellulose nanofibers during and/or after the redispersion step. A method for producing a cellulose nanofiber composite containing a cellulose nanofiber and an organic binder, comprising:
A step of preparing a redispersion of cellulose nanofibers by the method according to any one of claims 1 to 7, and a step of mixing the redispersion or a dried form thereof with an organic binder.
A method comprising: A method for producing a resin composite containing cellulose nanofibers and a resin, comprising:
A step of preparing a redispersion of cellulose nanofibers by the method according to any one of claims 1 to 7, and a mixing step of mixing the redispersion, an aqueous dispersion in which the second dispersion medium of the redispersion is replaced with water, or a dried form thereof, with a resin;
A method comprising: A method for producing a resin composition containing cellulose nanofibers and a thermoplastic resin, comprising:
A cellulose nanofiber solid forming step for obtaining a cellulose nanofiber solid, which is a dry solid of defibrated cellulose nanofibers;
a redispersion step of redispersing the cellulose nanofiber solid in a second dispersion medium to prepare a redispersion liquid; and a kneading step of kneading the redispersion liquid, an aqueous dispersion in which the second dispersion medium of the redispersion liquid is replaced with water, or a dried product thereof, with a thermoplastic resin.
Including,
The apparent density of the cellulose nanofiber solid body is 210 g/L to 1500 g/L,
the second dispersion medium is an aprotic polar solvent;
The method of claim 1, wherein the viscosity at a shear rate of 100 sec ^-1 of an aqueous dispersion of a redispersion obtained by adjusting the cellulose nanofibers in the redispersion to an aqueous dispersion with a solid content of 0.5% by mass is 30% or more of the viscosity at a shear rate of 100 sec- ¹ of an aqueous dispersion of a dispersion obtained by adjusting the defibrated cellulose nanofibers to an aqueous dispersion with a solid content of 0.5% by mass. The method according to claim 10 , wherein in the cellulose nanofiber solid, the cellulose nanofibers are bonded to each other by hydrogen bonds of hydroxyl groups. The method according to claim 10 or 11, wherein the rate of change (Y-X)/Y of the mass of the dry solid content (X) when the cellulose nanofiber solid is allowed to stand in water at 20°C for 2 hours and then removed from the water relative to the mass ( Y ) of the cellulose nanofiber solid before standing is 30 mass% or less. The method according to any one of claims 10 to 12 , wherein the second dispersion medium is selected from the group consisting of dimethylsulfoxide, N-methylpyrrolidone, dimethylacetamide, and dimethylformamide.

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