Method for producing modified cellulose fibers and method for producing modified cellulose microfibers

Abstract

To provide a method for producing novel cellulose microfibers whose applications are not substantially restricted, or a method for producing cellulose fibers which serve as a raw material for the cellulose microfibers.SOLUTION: A method for producing modified cellulose fibers is characterized in that, when cellulose fibers are modified using a reaction agent comprising at least one of a polyvalent carboxylic acid and a polyvalent carboxylic acid metal salt, an alkali is added to the cellulose fibers to bring the pH to 9 or more, after which the reaction agent is added, and then an alkali is added again to set the pH to 8-14. A method for producing modified cellulose microfibers is characterized in that cellulose fibers or modified cellulose fibers are micronized to achieve an average fiber diameter of 100 nm or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for producing modified cellulose fibers and a method for producing modified cellulose microfibers.

Background Art

[0002] In recent years, nanotechnology aimed at miniaturizing substances to the nanometer level and obtaining new physical properties different from the conventional properties of substances has attracted attention. Cellulose microfibers produced from pulp, which is a cellulose-based raw material, by chemical treatment, grinding treatment, etc. are excellent in strength, elasticity, thermal stability, etc., so they are used as industrial applications such as filter materials, filter aids, substrates for ion exchangers, packing materials for chromatographic analysis instruments, fillers for compounding resins and rubbers, etc., and are expected to be used as compounding agents for cosmetics such as lipsticks, powder cosmetics, and emulsion cosmetics. In addition, since cellulose microfibers have excellent water-based dispersibility, they are expected to be used in many applications such as viscosity retainers for foods, cosmetics, paints, etc., strengtheners for food raw material fabrics, water retainers, food stabilizers, low-calorie additives, and emulsion stabilization aids.

[0003] Conventionally, in order to obtain such cellulose microfibers, a method of mechanically defibrating cellulose fibers with a high-pressure homogenizer, a high-speed rotary homogenizer, an ultrasonic homogenizer, etc. has been adopted (see, for example, Patent Document 1, etc.). However, these methods required a large amount of energy to promote defibrillation. Therefore, in order to reduce the energy required for this defibrillation, a method of defibrating by the TEMPO oxidation method has been proposed. However, this method has difficulties in terms of economy, and also has the problem of de-petroleumization. Therefore, at present, a phosphoric acid esterification method, etc. have also been proposed. However, if only this technology is relied on, it may cause problems such as marine pollution. Therefore, the proposal of other new technologies is required.

[0004] By the way, as a technology different from the above method for modifying cellulose fibers, there already exists a method for modifying cellulose fibers with citric acid (see Patent Documents 2 and 3). However, the proposal in Patent Document 2 is essentially to perform citric acid modification by steaming, resulting in a low viscosity of the dispersion, so its applications are extremely limited. In addition, the proposal in Patent Document 3 has the problem that the transparency of the dispersion is low. In this regard, especially in applications such as foods and cosmetics, high transparency of the dispersion is required, and in order to expand the applications of cellulose microfibers, the issue of increasing transparency cannot be avoided.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] The problem to be solved by the present invention is to provide a method for producing new cellulose microfibers or a method for producing cellulose fibers as a raw material for cellulose microfibers, which are not significantly limited in their applications.

Means for Solving the Problems

[0007] In order to solve the above problems, the present inventors focused on a method for modifying cellulose fibers with citric acid. This is because citric acid is a naturally derived substance, and it has various advantages such as being able to react under conditions close to neutral and at low temperature conditions. However, as described above, the conventional cellulose microfibers obtained by citric acid modification have extremely limited applications.

[0008] Therefore, as a result of various studies conducted independently by the present inventors, it has been found that when cellulose fibers are denatured with citric acid, they are more likely to crosslink, and when the cellulose fibers crosslink, the fibers become thicker, resulting in a decrease in viscosity and transparency. And based on such findings, the following means have been conceived.

[0009] Patent Document 3 states that "Cellulose nanofibers are considered to refer to those with a fiber diameter (or fiber width) of several nm to several hundred nm in terms of the literal meaning, but in this specification, they are not limited to such sizes. For example, those with a fiber diameter of up to about 3000 nm are also included." It is described that the fiber diameter of cellulose nanofibers can be freely adjusted. However, as described above, when denatured with citric acid, cellulose fibers are more likely to crosslink, making it difficult to reduce the fiber diameter. Of course, it is not impossible to reduce the fiber diameter, but for this purpose, it is necessary to make the reaction conditions severe, such as using excessive chemicals, which results in an extremely short fiber length of the cellulose fibers and ultimately a decrease in viscosity.

[0010] (The means according to claim 1) When denaturing cellulose fibers with at least one of polycarboxylic acids and metal salts of polycarboxylic acids as a reaction reagent, adding an alkali to the cellulose fibers to make the pH 9 or higher, then adding the reaction reagent, adding an alkali again to make the pH 8 - 14, A method for producing modified cellulose fibers, characterized by this.

[0011] (The means according to claim 2) Adding at least one of urea and derivatives of urea together with the addition of the reaction reagent, or before and after the addition of the reaction reagent, The method for producing modified cellulose fibers according to claim 1.

[0012] (The means according to claim 3) Using citric acid as the reactant, The method for producing modified cellulose fibers according to claim 1.

[0013] (The means according to claim 4) In obtaining modified cellulose fibers by the production method according to any one of claims 1 to 3, The cellulose fibers or modified cellulose fibers are refined to make the average fiber diameter 100 nm or less, A method for producing modified cellulose microfibers, characterized in that. [Advantages of the Invention]

[0014] According to the present invention, it becomes a method for producing new modified cellulose microfibers whose uses are not significantly restricted, or a method for producing modified cellulose fibers as raw materials for modified cellulose microfibers. [Modes for Carrying Out the Invention]

[0015] Next, modes for carrying out the present invention will be described. Note that this embodiment is an example of the present invention.

[0016] [Fiber · Dispersion Liquid] In the method for producing modified cellulose fibers of this embodiment, when modifying cellulose fibers with at least one of polycarboxylic acids and metal salts of polycarboxylic acids as reactants, an alkali is added to the cellulose fibers to make the pH 9 or more, and then the reactant is added, and an alkali is added again to make the pH 8 to 14. Further, the cellulose fibers or modified cellulose fibers are refined to make the average fiber diameter 100 nm or less, thereby obtaining cellulose microfibers. Details will be described below.

[0017] (Amount of dissociated acid) As described above, when cellulose fibers are modified with citric acid or the like, the cellulose fibers tend to crosslink, and the cellulose fibers bond to each other. Therefore, the fiber diameter tends to become thick, and the transparency of the dispersion decreases. On the other hand, cellulose fibers (citric acid) will contain only acidic groups (= first acid groups; hereinafter, also simply referred to as "strong acid groups, etc.") when crosslinking reactions occur, while when no crosslinking reaction has occurred, they will contain both strong acid groups, etc. and acidic groups with a lower acidity (= second acid groups; hereinafter, also simply referred to as "weak acid groups, etc."). Therefore, by specifying the ratio of strong acid groups, etc. and weak acid groups, etc. contained in the cellulose fibers, the degree of crosslinking can be specified, which serves as a guideline for suppressing the thickening of the fiber diameter of cellulose fibers even when they are modified with citric acid.

[0018] Incidentally, in neutralization titration, in the curve plotting pH against the amount of alkali added, there may be cases where two points where the increment (the differential value of pH with respect to the amount of alkali dropped) reaches a maximum are confirmed. Here, among these maximum values, the maximum point of the increment obtained first after starting to add alkali is called the first end point, and the maximum point of the increment obtained next is called the second end point. And the value obtained by dividing the amount of alkali required from the start of titration to the first end point by the solid content (g) in the dispersion to be titrated is equal to the first dissociation acid amount of the fibrous cellulose contained in the dispersion used for titration (the value obtained by dividing the amount of substance (mmol) of the acid ionized and neutralized up to the first stage by the solid content (g) in the dispersion), and the value obtained by dividing the amount of alkali required from the first end point to the second end point by the solid content (g) in the dispersion to be titrated is equal to the second dissociation acid amount of the fibrous cellulose contained in the dispersion used for titration (the value obtained by dividing the amount of substance (mmol) of the acid ionized and neutralized from the first stage to the second stage by the solid content (g) in the dispersion), and the value obtained by dividing the amount of alkali required from the start of titration to the second end point by the solid content (g) in the dispersion to be titrated is equal to the total dissociation acid amount of the fibrous cellulose contained in the dispersion used for titration (the value obtained by dividing the total amount of substance (mmol) of the acid ionized and neutralized up to the second stage by the solid content (g) in the dispersion). Therefore, for example, the value obtained by dividing the amount of alkali required from the start of titration to the first end point by the solid content (g) in the dispersion to be titrated is the amount of introduced strong acid group etc. (mmol / g). Similarly, the value obtained by dividing the amount of alkali required from the start of titration to the second end point by the solid content (g) in the dispersion to be titrated is the total amount of the amount of introduced acid group (mmol / g).

[0019] Note that in the measurement by neutralization titration, accurate values may not be obtained when the dropping amount of one drop of an alkali solution such as an aqueous sodium hydroxide solution is too large or when the titration interval is too short. Therefore, for example, it is desirable to titrate 0.1N aqueous sodium hydroxide solution in 10 - 50 μL portions every 5 - 30 seconds. Also, in order to eliminate the influence of carbon dioxide dissolved in the dispersion, for example, it is desirable to measure while blowing an inert gas such as nitrogen gas into the dispersion from 15 minutes before the start of titration until the end of titration.

[0020] In this form, the first dissociation acid amount (mmol / g) / the second dissociation acid amount (mmol / g) is preferably 0.25 to 1.0, more preferably 0.3 to 1.0, and particularly preferably 0.4 to 1.0. If the first dissociation acid amount / the second dissociation acid amount is less than 0.25, there is a risk that the transparency may be low because the degree of crosslinking is high. Note that the closer the first dissociation acid amount / the second dissociation acid amount is to 1, the lower the degree of crosslinking, and thus the higher the transparency.

[0021] The first dissociation acid amount is preferably 0.35 or more, more preferably 0.4 or more, and particularly preferably 0.5 or more. If the first dissociation acid amount is less than 0.35, there is a risk that the transparency may be low because the degree of esterification is insufficient.

[0022] (Raw material fiber) As the raw material fiber of the cellulose fiber, for example, plant-derived fibers (plant fibers), animal-derived fibers, microorganism-derived fibers, etc. can be used. These fibers can be used alone or in combination as necessary. However, as the raw material fiber, it is preferable to use plant fibers, and it is more preferable to use pulp fibers which are a kind of plant fibers. When the raw material fiber is pulp fiber, it is easy to adjust the physical properties of the cellulose microfiber.

[0023] As the plant fiber, for example, wood pulp made from broad-leaved trees, coniferous trees, etc., non-wood pulp made from straw, bagasse, etc., wastepaper pulp (DIP) made from recycled waste paper, waste paper, etc. can be used. These fibers can be used alone or in combination.

[0024] As the wood pulp, for example, chemical pulps such as broad-leaved tree kraft pulp (LKP), coniferous tree kraft pulp (NKP), etc., mechanical pulp (TMP), wastepaper pulp (DIP), etc. can be used. These pulps can be used alone or in combination.

[0025] The hardwood kraft pulp (LKP) may be hardwood bleached kraft pulp, hardwood unbleached kraft pulp, or hardwood semi-bleached kraft pulp. The softwood kraft pulp (NKP) may be softwood bleached kraft pulp, softwood unbleached kraft pulp, or softwood semi-bleached kraft pulp. The deinked pulp (DIP) may be magazine deinked pulp (MDIP), newspaper deinked pulp (NDIP), wastepaper pulp (WP), or other deinked pulp.

[0026] (Modified) For the cellulose fiber, a part of the hydroxy groups (-OH groups) is substituted with polyvalent carboxylic acid groups, preferably citric acid groups (polyvalent carboxylic acid modification). When polyvalent carboxylic acid modification is performed, defibrillation of the cellulose fiber becomes easier due to the electrostatic repulsion and the osmotic pressure effect caused by the coordination of sodium ions.

[0027] More preferably, a part of the hydroxy groups of the cellulose fiber is also substituted with carbamate groups (carbamate modification). When carbamate modification is performed, the transparency and viscosity of the dispersion are extremely improved. In particular, when both citric acid modification and carbamate modification are performed, the hydrogen bonds are weakened, making it easier to defibrillate the cellulose fiber.

[0028] The introduction amount of the polyvalent carboxylic acid group is a value evaluated based on the neutralization titration method of an automatic titrator. For this neutralization titration method, AUT-801 manufactured by TOA DKK is used.

[0029] The introduction amount of the carbamate group is preferably 0.01 to 0.2 mmol per 1 g of the cellulose microfibril. If the introduction amount exceeds 0.2 mmol, there is a risk that the citric acid modification may not proceed.

[0030] The calculation method of the introduction amount of the carbamate group is a value obtained by the Kjeldahl method.

[0031] (Microfibril) The average fiber diameter (width) of the cellulose microfibers obtained by fibrillation of cellulose fibers is preferably 3 to 100 nm, more preferably 4 to 20 nm. If the average fiber diameter is less than 3 nm, the cellulose may dissolve in water, and there is a risk that the physical properties as cellulose microfibers, such as strength, rigidity, and dimensional stability, will not be exhibited. On the other hand, if the average fiber diameter exceeds 100 nm, since it becomes about 1 / 10 of the wavelength of visible light, when the cellulose microfibers are dispersed in water (when made into an aqueous dispersion), refraction and scattering of visible light may occur, and there is a risk that the light transmittance will be insufficient.

[0032] The fiber diameter of the cellulose microfibers is measured as follows using an electron microscope. First, 100 ml of an aqueous dispersion of cellulose microfibers with a solid content concentration of 0.01 to 0.1 mass% is filtered through a Teflon (registered trademark) membrane filter, and the solvent is replaced once with 100 ml of ethanol and three times with 20 ml of t-butanol. Next, it is freeze-dried and osmium-coated to obtain a sample. For this sample, observation is performed by an SEM image of an electron microscope at any magnification of 5,000 times, 10,000 times, or 30,000 times according to the width of the constituent fibers. In this observation, two diagonal lines are drawn on the observation image, and three arbitrary straight lines passing through the intersection of the diagonal lines are further drawn. Then, the widths of a total of 100 fibers intersecting with these three straight lines are visually measured. The median diameter of these measurement values is taken as the fiber diameter (width).

[0033] The average fiber length of the cellulose microfibers is preferably 0.01 to 1000 μm, more preferably 0.05 to 500 μm, and particularly preferably 0.1 to 100 μm. If the average fiber length is less than 0.01 μm, it may be difficult to form a fiber network structure and the thickening effect may not be obtained. On the other hand, if the average fiber length exceeds 1000 μm, there is a risk that the cellulose microfibers will condense with each other.

[0034] The fiber length of the cellulose microfibers is the value measured by the fiber analyzer "FS5" manufactured by Barret Co., Ltd.

[0035] The axial ratio (fiber length / fiber width) of the cellulose microfiber is preferably 3 to 10,000, more preferably 10 to 1,000. When the axial ratio is less than 3, it can no longer be said to be fibrous. On the other hand, when the axial ratio exceeds 10,000, the viscosity of the dispersion may become too high.

[0036] The crystallinity of the cellulose microfiber is preferably 50 to 100%, more preferably 60 to 90%, and particularly preferably 65 to 85%. When the crystallinity is less than 50%, there is a risk that the strength and heat resistance may be insufficient.

[0037] The crystallinity can be adjusted, for example, by the selection of raw fibers, pretreatment, fibrillation, etc.

[0038] The crystallinity is a value measured by the X-ray diffraction method in accordance with the "General Rules for X-ray Diffraction Analysis" of JIS-K0131 (1996). Note that the cellulose microfiber has an amorphous part and a crystalline part, and the crystallinity means the ratio of the crystalline part in the whole cellulose microfiber.

[0039] The total light transmittance of the dispersion of cellulose microfibers (solution with a solid content concentration of 0.2%) is preferably 80% or more, more preferably 85% or more, and particularly preferably 90% or more. When the total light transmittance is less than 80%, there is a risk that the transparency may be insufficient.

[0040] The total light transmittance of the cellulose microfiber can be adjusted, for example, by the selection of pulp fibers, pretreatment, fibrillation, etc.

[0041] The total light transmittance is a value measured using a Spectrophotometer U-2910 (Hitachi, Ltd.) for the total light transmittance (transmittance of light with a wavelength of 350 to 880 nm) of a 0.2% (w / v) dispersion of cellulose microfibers.

[0042] When the concentration of the cellulose microfibrils is 1% by mass (w / w), the Brookfield viscosity of the dispersion is preferably 40,000 cP or more, more preferably 40,000 to 120,000 cP, and particularly preferably 50,000 to 100,000 cP. If the Brookfield viscosity is less than 40,000 cP, it may be impossible to handle it as a high-viscosity additive. On the other hand, if the Brookfield viscosity exceeds 120,000 cP, it may be difficult to mix with other substances when used as an additive.

[0043] The Brookfield viscosity is a value measured in accordance with the "Method for Measuring Viscosity of Liquids" of JIS-Z8803 (2011) for an aqueous dispersion of cellulose microfibrils with a solid content concentration of 1%. The Brookfield viscosity is the resistance torque when the dispersion is stirred, meaning that the higher the value, the more energy is required for stirring.

[0044] [Manufacturing Method] Next, the manufacturing method of this embodiment will be described. (Pretreatment) Prior to or after modifying the cellulose fibers with various chemicals such as citric acid, the cellulose fibers can be subjected to pretreatment such as beating as necessary. By subjecting the pulp fibers to pretreatment prior to fibrillation of the cellulose fibers, the number of fibrillation times can be significantly reduced, and the energy for fibrillation can be reduced.

[0045] The pretreatment of the cellulose fibers can be carried out by physical methods or chemical methods, preferably by physical methods and chemical methods. The pretreatment by physical methods and the pretreatment by chemical methods can be carried out simultaneously or separately.

[0046] As the pretreatment by physical methods, it is preferable to employ beating. When the cellulose fibers are beaten, the cellulose fibers are trimmed. Therefore, entanglement between the cellulose fibers is prevented (aggregation prevention). From this viewpoint, beating is preferably carried out until the freeness of the cellulose fibers becomes 700 ml or less, more preferably until it becomes 500 ml or less, and particularly preferably until it becomes 300 ml or less.

[0047] The freeness of the cellulose fiber is a value measured in accordance with JIS P8121-2 (2012). Also, beating can be performed using, for example, a refiner, a beater, or the like.

[0048] Examples of the pretreatment by chemical methods include hydrolysis of polysaccharides by an acid (acid treatment), hydrolysis of polysaccharides by an enzyme (enzyme treatment), swelling of polysaccharides by an alkali (alkali treatment), oxidation of polysaccharides by an oxidizing agent (oxidation treatment), reduction of polysaccharides by a reducing agent (reduction treatment), and the like. However, as the pretreatment by chemical methods, it is preferable to perform an enzyme treatment, and in addition, it is more preferable to perform one or more treatments selected from acid treatment, alkali treatment, and oxidation treatment. Hereinafter, the alkali treatment will be described in detail.

[0049] As a method of alkali treatment, for example, there is a method of immersing the cellulose fiber before introducing citric acid or the like into an alkali solution.

[0050] The alkali compound contained in the alkali solution may be an inorganic alkali compound or an organic alkali compound. Examples of the inorganic alkali compound include hydroxides of alkali metals or alkaline earth metals, carbonates of alkali metals or alkaline earth metals, and phosphoxoacid salts of alkali metals or alkaline earth metals. Examples of the hydroxide of an alkali metal include lithium hydroxide, sodium hydroxide, potassium hydroxide, etc. Examples of the hydroxide of an alkaline earth metal include calcium hydroxide, etc. Examples of the carbonate of an alkali metal include lithium carbonate, lithium hydrogen carbonate, potassium carbonate, potassium hydrogen carbonate, sodium carbonate, sodium hydrogen carbonate, etc. Examples of the carbonate of an alkaline earth metal include calcium carbonate, etc. Examples of the phosphoxoacid salt of an alkali metal include lithium phosphate, potassium phosphate, trisodium phosphate, disodium hydrogen phosphate, etc. Examples of the phosphate of an alkaline earth metal include calcium phosphate, calcium hydrogen phosphate, etc.

[0051] Examples of the organic alkali compound include ammonia, aliphatic amines, aromatic amines, aliphatic ammonium, aromatic ammonium, heterocyclic compounds and their hydroxides, carbonates, phosphates, etc. Specifically, for example, ammonia, hydrazine, methylamine, ethylamine, diethylamine, triethylamine, propylamine, dipropylamine, butylamine, diaminoethane, diaminopropane, diaminobutane, diaminopentane, diaminohexane, cyclohexylamine, aniline, tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, benzyltrimethylammonium hydroxide, pyridine, N,N-dimethyl-4-aminopyridine, ammonium carbonate, ammonium hydrogen carbonate, diammonium hydrogen phosphate, etc.

[0052] The solvent of the alkaline solution may be either water or an organic solvent, but is preferably a polar solvent (polar organic solvents such as water and alcohol), and more preferably an aqueous solvent containing at least water.

[0053] The pH of the alkaline solution at 25 °C is preferably 9 or more, more preferably 10 or more, and particularly preferably 11 to 14. When the pH is 9 or more, the yield of cellulose microfibrils increases. However, when the pH exceeds 14, the handleability of the alkaline solution decreases.

[0054] In the production method of this embodiment, first, an alkali is added to the aqueous dispersion of cellulose fibers to adjust the pH to 9 or more, preferably 10 or more, more preferably 11 or more. Since the cellulose fibers swell and the fibers separate from each other by the addition of the alkali, it becomes difficult to crosslink by the addition of citric acid. When the pH is less than 9, defibrillation becomes difficult due to the crosslinking reaction and it may not be possible to obtain sufficient viscosity and transparency because cellulose microfibrils cannot be formed. However, when the alkali concentration (causticity) is 18% or more, there is a risk of regenerating cellulose.

[0055] As the alkali to be added, for example, those that can adjust the dispersion of cellulose fibers such as sodium hydroxide, potassium hydroxide, aqueous ammonia, sodium carbonate, etc. to pH 9 to 14 can be used. However, since it has a high ability to swell fibers, it is preferable to use sodium hydroxide.

[0056] (Addition of various chemicals) Next, at least one of polycarboxylic acids and polycarboxylic acid metal salts (reaction solution) is added to the dispersion of cellulose fibers.

[0057] As the reaction reagent (polycarboxylic acids and polycarboxylic acid metal salts), for example, in addition to citric acid having three carboxylic acids, malic acid, aconitic acid, malonic acid, succinic acid, etc. can be used.

[0058] The addition amount of the reagent is preferably 1 to 20 mmol, more preferably 2 to 18 mmol, per 1 g of the cellulose fiber. If the addition amount is less than 1 mmol, the degree of esterification may be insufficient, resulting in insufficient transparency and viscosity. On the other hand, if the addition amount exceeds 20 mmol, the degree of esterification may reach a plateau.

[0059] The introduction amount of the reagent is measured by neutralization titration using an automatic titrator AUT-801 (Toa DKK Corporation) for a modified cellulose fiber (dispersion) with a concentration of 0.2% by mass. The automatic titration method is a measurement in which, after treating a dispersion with a concentration of 0.2% by mass with an ion exchange resin such that the mass ratio becomes 10:1, titration is performed in 0.1 mL increments using 0.05 M sodium hydroxide. The measured pH is used as a dissociation curve, and two inflection points are observed. Among them, the first obtained inflection point is defined as the first inflection point, and the next obtained inflection point is defined as the second inflection point.

[0060] Preferably, at least one of urea and urea derivatives (hereinafter also simply referred to as "urea etc.") is added either together with the addition of a reagent such as citric acid or before and after the addition of a reagent such as citric acid. When urea etc. is added, the hydrogen bonds between cellulose fibers are broken, causing swelling and the fibers to separate from each other, making it more difficult for crosslinking to occur due to the addition of citric acid. As urea etc., for example, urea, thiourea, biuret, phenylurea, etc. can be used. These ureas or urea derivatives can be used alone or in combination of multiple ones. However, it is preferable to use urea.

[0061] When urea etc. is heated, it decomposes into isocyanic acid and ammonia as shown in the following reaction formula (1). And isocyanic acid has high reactivity and modifies the hydroxyl group of cellulose into a carbamate group as shown in the following reaction formula (2). Therefore, when urea etc. is added to cellulose fibers, the introduction of carbamate groups proceeds. NH 2 -CO-NH 2 → HN=C=O+NH 3 …(1) Cell-OH + H-N=C=O → Cell-O-CO-NH 2 …(2)

[0062] In the above reaction formula (2), Cell refers to a cellulose molecule. The addition amount of urea or the like is preferably 0.1 to 5.0 mmol, more preferably 0.2 to 4.0 mmol, per 1 g of cellulose fiber. Even if the addition amount exceeds 5.0 mmol, the effect of adding urea or the like may reach a plateau.

[0063] When adding various chemicals, the cellulose fiber may be in a dry state, a wet state, or a dispersion state. Also, the various chemicals may be in a powder state or an aqueous solution state. However, from the perspective of high reaction uniformity, it is preferable to add the chemical in an aqueous solution state to the cellulose fiber in a dry state.

[0064] (Heating) The cellulose fiber added with various chemicals is heated to promote a denaturation reaction with citric acid or the like. This heating temperature is preferably 110 to 200 °C, more preferably 120 to 160 °C. If the heating temperature is less than 110 °C, the reaction between citric acid and pulp may not occur. On the other hand, if the heating temperature exceeds 200 °C, the deterioration of the cellulose fiber may proceed rapidly, which may cause coloring and a decrease in viscosity.

[0065] The pH when heating the cellulose fiber added with various chemicals such as citric acid is preferably 2.0 to 7.0, more preferably 3.0 to 6.0. In this regard, as described above, the pH of the dispersion became an alkali of 9 or more due to the addition of an alkali such as sodium hydroxide, but the pH becomes 2.0 to 7.0 due to the addition of various chemicals such as citric acid. However, if the pH is less than 2.0, there is a risk of coloring. On the other hand, if the pH exceeds 7.0, citric acid becomes trisodium citrate, and the reaction may not proceed, resulting in insufficient transparency and viscosity.

[0066] Heating of the cellulose fibers added with various chemicals is preferably carried out until the cellulose fibers are dried. Specifically, it is dried until the moisture content of the cellulose fibers is preferably 20% or less, more preferably 10% or less.

[0067] The reaction time of the cellulose fibers added with various chemicals is, for example, 1 to 3600 seconds, preferably 10 to 1000 seconds. If the reaction time is too long, the cellulose fibers may turn yellow. On the other hand, if the reaction time is too short, the modification such as citric acid may not proceed sufficiently.

[0068] As a device for heating the cellulose fibers added with various chemicals, for example, a hot air dryer, a kiln, a heating kneader, a paper making machine, a dry pulp machine, etc. can be used.

[0069] (Washing) Prior to defibrillation, it is preferable to wash the above cellulose fibers. By washing the cellulose fibers, various chemicals such as residual citric acid and alkalis such as sodium hydroxide can be washed away.

[0070] The washing of the cellulose fibers can be carried out using, for example, water, an organic solvent, etc.

[0071] (Re - addition of alkali) Next, an alkali such as sodium hydroxide is added again to the cellulose fibers to hydrolyze the cellulose fibers. By this re - addition of alkali, the cross - links of the cellulose fibers are cut and the degree of cross - linking becomes lower.

[0072] This re - addition of alkali is carried out so that the pH is 8 to 14, preferably 9 to 13. If the pH is less than 8, there may still be remaining cross - links. On the other hand, if the pH exceeds 14, there is a possibility that all citric acid groups will be desorbed.

[0073] Note that the alkali that can be used in this step is the same as the alkali added initially. Also, in the above description, the term "alkali" or "alkali treatment" is used, but the true meaning of this description is "hydrolysis". The reason is that there is a possibility that the crosslinking can be broken even by an acidic solution.

[0074] Here, a few remarks are made about the crosslinked structure and the ester structure. The structure of citric acid-modified cellulose is in a state where a crosslinked structure and an ester structure coexist before hydrolysis. And when there are many crosslinked structures, it is considered difficult to fibrillate until high transparency is achieved. On the other hand, when the crosslinked structure is destroyed by hydrolysis and the ester structure becomes more than before hydrolysis, fibrillation becomes easier. However, it is considered difficult to destroy all of the crosslinked structure and leave only the ester structure. Incidentally, if chemicals are continuously added, all of the crosslinked structure and the ester structure will be destroyed, and it will return to a state where fibrillation is difficult. Also, when the crosslinked structure increases, fibrillation is difficult, so the transparency does not increase, but the viscosity tends to increase due to the crosslinked structure. When the ester structure increases, fibrillation becomes easier, so the transparency increases, but the viscosity tends to be lower compared to modified cellulose with many crosslinked structures.

[0075] (Fibrillation) The cellulose fibers to which alkali is re-added or the like are fibrillated (refined). By this fibrillation, the cellulose fibers are microfibrillated and become cellulose microfibers (cellulose nanofibers (CNF)).

[0076] When fibrillating cellulose fibers, it is preferable to make the cellulose fibers into a slurry state. The solid content concentration of this slurry is preferably 0.1 to 20% by mass, more preferably 0.5 to 10% by mass, and particularly preferably 1.0 to 5.0% by mass. If the solid content concentration is within the above range, fibrillation can be carried out efficiently.

[0077] The fibrillation of cellulose fibers can be carried out by selecting and using one or more means from, for example, homogenizers such as high-pressure homogenizers and high-pressure homogenizing devices, mortar-type friction machines such as grinders and attritors, refiners such as conical refiners and disk refiners, various bacteria, etc. However, the fibrillation of cellulose fibers is preferably carried out using a device / method for miniaturization with water flow, particularly high-pressure water flow. According to this device / method, the dimensional uniformity and dispersion uniformity of the obtained cellulose microfibers are very high. On the other hand, for example, when using a grinder that grinds between rotating grindstones, it is difficult to uniformly miniaturize cellulose fibers, and in some cases, there is a risk that fiber lumps that cannot be disintegrated remain partially.

[0078] Examples of grinders used for the fibrillation of cellulose fibers include, for example, Mascoloider of Masuko Sangyo Co., Ltd. Examples of devices for miniaturization with high-pressure water flow include, for example, Starburst (registered trademark) of Sugino Machine Limited and Nanovater\Nanovater (registered trademark) of Yoshida Kikai Kogyo Co., Ltd. Examples of high-speed rotary homogenizers used for the fibrillation of cellulose fibers include ClearMix-11S manufactured by M-Technique Co., Ltd.

[0079] As a device for fibrillation by high-pressure water flow, it is preferable to use a high-pressure homogenizer. A high-pressure homogenizer refers to a homogenizer having the ability to eject a slurry of cellulose fibers at a pressure of, for example, 10 MPa or more, preferably 100 MPa or more. When cellulose fibers are treated with a high-pressure homogenizer, collisions between cellulose fibers, pressure differences, microcavitation, etc. act, and fibrillation of cellulose fibers occurs effectively. Therefore, the number of fibrillation treatment times can be reduced, and the production efficiency of cellulose microfibers can be increased.

[0080] The fibrillation of cellulose fibers is preferably carried out so that the average fiber width, average fiber length, crystallinity, etc. of the obtained cellulose microfibers become the desired values or evaluations described above.

Example

[0081] Next, examples of the present invention will be described. Sodium hydroxide was added to softwood kraft pulp sheet (solid content 46%) so that the molar ratio to citric acid was 1:1, and water was added so that the mass ratio to pulp solid content was 1:15, and the mixture was mixed with a mixer for 10 minutes. Then, a predetermined amount of citric acid was added, and the mixture was further mixed with a mixer for 10 minutes. After mixing, it was heated and dried and reacted at 130°C for 4 hours with a hot air dryer. After the reaction, it was washed until the filtrate became neutral to obtain modified pulp. After adding water so that the modified pulp became 1 wt%, sodium hydroxide was added so that the pH became a predetermined value, and it was hydrolyzed for 1 hour. Further, the modified pulp after hydrolysis was washed until the filtrate became neutral. The modified pulp after hydrolysis was defibrated to obtain cellulose microfibrils, and various tests were performed. Defibration was performed using a high-pressure homogenizer.

[0082] The addition amounts of citric acid, sodium hydroxide, and urea, the heating temperature and time, and the pH during hydrolysis were as shown in Table 1. The physical properties and evaluations of the obtained cellulose microfibrils are shown in Table 2. The evaluation methods for the B-type viscosity and light transmittance were as described above.

[0083]

Table 1

[0084]

Table 2

Industrial Applicability

[0085] The present invention can be used as modified cellulose fibers and modified cellulose microfibrils.

Claims

1. When modifying cellulose fibers with at least one of polycarboxylic acids and metal salts of polycarboxylic acids, an alkali is added to the cellulose fibers to make the pH 9 or higher, then the reaction reagent is added, and an alkali is added again to make the pH 8 to 14, A method for producing modified cellulose fibers, characterized by the above.

2. At least one of urea and urea derivatives is added together with the addition of the reaction reagent, or before and after the addition of the reaction reagent, The method for producing modified cellulose fibers according to Claim 1.

3. Citric acid is used as the reaction reagent, The method for producing modified cellulose fibers according to Claim 1.

4. When obtaining modified cellulose fibers by the production method according to any one of Claims 1 to 3, The cellulose fibers or modified cellulose fibers are refined to make the average fiber diameter 100 nm or less, A method for producing modified cellulose microfibers, characterized by the above.

Images