Modified cellulose fiber and modified cellulose microfiber

Abstract

Provided is a novel cellulose microfiber which is not significantly restricted in use, or a cellulose fiber which is a raw material for cellulose microfibers. A modified cellulose fiber characterized by being modified with a polycarboxylic acid group and having a first dissociation acid amount / second dissociation acid amount of 0.25-1.0 and a first dissociation acid amount of 0.35 mmol / g or more. A modified cellulose microfiber characterized in that the average fiber diameter of the modified cellulose fiber is 100 nm or less.

Description

Technical Field

[0001] This invention relates to modified cellulose fibers and modified cellulose microfibers. Background Technology

[0002] In recent years, nanotechnology, which aims to miniaturize materials to the nanoscale to obtain novel properties different from the existing properties of materials, has attracted attention. Cellulose microfibers, produced from cellulose pulp through chemical processing and pulverization, exhibit superior strength, elasticity, and thermal stability. Therefore, they are expected to be used in industrial applications such as filter materials, filter aids, substrates for ion exchangers, fillers for chromatographic instruments, and fillers for resin and rubber blends, as well as in cosmetic formulations such as lipsticks, powder cosmetics, and emulsion cosmetics. Furthermore, cellulose microfibers exhibit excellent aqueous dispersibility, making them promising applications in various fields such as viscosity retention agents in food, cosmetics, and coatings; reinforcing agents for food raw materials; moisture retention agents; food stabilizers; low-calorie additives; and emulsion stabilizing agents.

[0003] Previously, to obtain such fine cellulose fibers, mechanical defibrillation methods were employed, such as using high-pressure homogenizers, high-speed rotary homogenizers, and ultrasonic homogenizers (see, for example, Patent Document 1). However, these methods consume a large amount of energy during defibrillation. Therefore, to reduce the energy required for defibrillation, a method using TEMPO oxidation was proposed. However, this method faces economic challenges, and depetrochemicalization remains a technical issue. Therefore, methods such as phosphate esterification have also been proposed. However, relying solely on this technology could potentially contribute to problems such as marine pollution. Therefore, proposals for other new technologies are needed.

[0004] However, as a technique different from the aforementioned methods for modifying cellulose fibers, methods for modifying cellulose fibers with citric acid already exist (see Patent Documents 2 and 3). However, the proposal in Patent Document 2, in short, involves citric acid modification through cooking, resulting in a low viscosity dispersion, thus significantly limiting its applications. Furthermore, the proposal in Patent Document 3 suffers from low transparency of the dispersion. This is particularly problematic in applications such as food and cosmetics, where high transparency of the dispersion is required. Improving transparency is an unavoidable technical challenge in expanding the applications of cellulose microfibers.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2010-216021

[0008] Patent Document 2: Japanese Patent Application Publication No. 2015-140403

[0009] Patent Document 3: Japanese Patent Application Publication No. 2019-189792 Summary of the Invention

[0010] The technical problem that the invention aims to solve

[0011] The technical problem to be solved by the present invention is to provide a novel cellulose microfiber or cellulose fiber as a raw material for cellulose microfiber whose uses are not significantly limited.

[0012] Technical means for solving problems

[0013] To address the aforementioned technical problems, the inventors have focused on a method for modifying cellulose fibers with citric acid. This is because citric acid is a naturally occurring substance and possesses several advantages, such as the ability to react under near-neutral conditions and even at relatively low temperatures. However, as mentioned earlier, the applications of existing citric acid-modified cellulose microfibers are significantly limited.

[0014] Therefore, the inventors independently and repeatedly conducted various studies and found that cellulose fibers are prone to cross-linking after modification with citric acid. Once cross-linking occurs, the fibers become coarser, leading to a decrease in viscosity and transparency. Based on this insight, the following method was devised.

[0015] It is important to note that Patent Document 3 states: "The term 'cellulose nanofiber,' literally, refers to fibers with a diameter (or width) of several nanometers to hundreds of nanometers, but in this specification, it is not limited to this size range. For example, it also includes fibers with a diameter of approximately 3000 nm." The document also states that the fiber diameter of cellulose nanofibers can be freely adjusted. However, as mentioned earlier, after modification with citric acid, cellulose fibers are prone to cross-linking, making it difficult to reduce the fiber diameter. Of course, it is not entirely impossible to reduce the fiber diameter, but this requires stringent reaction conditions (such as excessive use of chemicals), which in turn leads to extremely short cellulose fiber lengths, ultimately resulting in a decrease in viscosity.

[0016] (The means described in Scheme 1)

[0017] A modified cellulose fiber, characterized in that it is modified with a polycarboxylic acid group, and the ratio of a first dissociative acid to a second dissociative acid is 0.25 to 1.0, and the first dissociative acid is 0.35 mmol / g or more.

[0018] (The means described in Scheme 2)

[0019] The modified cellulose fiber according to Scheme 1 is urethane-esterified.

[0020] (The means described in Scheme 3)

[0021] According to Scheme 1, the total transmittance of the dispersion of the modified cellulose fiber with a concentration of 0.2% by mass is 65% or higher, and the B-type viscosity of the dispersion with a concentration of 1.0% by mass is 40,000 cP or higher.

[0022] (The means described in Scheme 4)

[0023] According to the modified cellulose fiber described in Scheme 1, the aforementioned polycarboxylic acid is citric acid.

[0024] (The means described in Scheme 5)

[0025] A modified cellulose microfiber, characterized in that the average fiber diameter of the modified cellulose fiber in any one of Schemes 1 to 4 is 100 nm or less.

[0026] Invention Effects

[0027] According to the present invention, it is possible to obtain a novel modified cellulose microfiber or a modified cellulose fiber as a raw material for the modified cellulose microfiber, the use of which is not significantly limited. Detailed Implementation

[0028] Next, the method for implementing the present invention will be described. It should be noted that this embodiment is an example of the present invention.

[0029] [fiber [Dispersion]

[0030] The modified cellulose fibers of this method are modified with polycarboxylic acid (preferably citric acid) groups, and the ratio of the first dissociation acid content to the second dissociation acid content is 0.25 to 1.0, with the first dissociation acid content being 0.35 mmol / g or higher. Furthermore, the average fiber diameter of the modified cellulose microfibers is 100 nm or less. Detailed explanation follows.

[0031] (Dissociation acidity)

[0032] As mentioned above, after cellulose fibers are modified with citric acid or the like, they are more prone to cross-linking, leading to the fibers binding together. This tends to increase fiber diameter and reduce the transparency of the dispersion. On the other hand, if cellulose fibers (after citric acid modification) undergo cross-linking, they contain only acidic groups (i.e., primary acid groups, hereinafter referred to as "strong acid groups, etc."); while if no cross-linking occurs, they contain both strong acid groups and acidic groups with lower acidity (i.e., secondary acid groups, hereinafter referred to as "weak acid groups, etc."). Therefore, by controlling the ratio of strong acid groups and weak acid groups in a specific cellulose fiber, the degree of cross-linking can be controlled, providing a strategy to suppress fiber diameter thickening even in the case of citric acid modification.

[0033] However, in neutralization titration, when plotting the pH curve for the amount of alkali added, sometimes two points are identified where the increments (the differential values ​​of pH relative to the amount of alkali added) reach their maxima. Here, among these maxima, the first maximum point of the increment obtained after the addition of alkali is called the first endpoint, and the subsequent maximum point of the increment is called the second endpoint. Furthermore, the amount of alkali required from the start of titration to the first endpoint, divided by the solid content (g) in the dispersion to be titrated, is equal to the first dissociation acid amount of fibrous cellulose contained in the dispersion used for titration (the amount of acid (mmol) ionized and neutralized up to the first stage divided by the amount of solid content (g) in the dispersion); the amount of alkali required from the first endpoint to the second endpoint, divided by the solid content (g) in the dispersion to be titrated, is equal to the second dissociation acid amount of fibrous cellulose contained in the dispersion used for titration (the amount of acid (mmol) ionized and neutralized from the first stage to the second stage divided by the amount of solid content (g) in the dispersion); and the amount of alkali required from the start of titration to the second endpoint, divided by the solid content (g) in the dispersion to be titrated, is equal to the total dissociation acid amount of fibrous cellulose contained in the dispersion used for titration (the total amount of acid (mmol) ionized and neutralized up to the second stage divided by the amount of solid content (g) in the dispersion). Therefore, for example, the amount of alkali required from the start of titration to the first endpoint, divided by the solid content (g) in the dispersion to be titrated, is the amount of strong acid groups introduced (mmol / g). Similarly, the amount of alkali required from the start of titration to the second endpoint, divided by the solid content (g) in the dispersion to be titrated, is the total amount of acid groups introduced (mmol / g).

[0034] It is important to note that in determinations using neutralization titration, if the single drop volume of an alkaline solution such as sodium hydroxide aqueous solution is too large, or the titration interval is too short, accurate values ​​may not be obtained. Therefore, it is desirable, for example, to titrate 10-50 μL of 0.1N sodium hydroxide aqueous solution every 5-30 seconds. Furthermore, to eliminate the influence of carbon dioxide dissolved in the dispersion, it is desirable, for example, to conduct the determination while blowing an inert gas such as nitrogen into the dispersion from 15 minutes before the start of the titration until the end of the titration.

[0035] In this method, the ratio of the first dissociative acid content (mmol / g) to the second dissociative acid content (mmol / g) is preferably 0.25~1.0, more preferably 0.3~1.0, and particularly preferably 0.4~1.0. If the ratio of the first dissociative acid content to the second dissociative acid content is less than 0.25, the degree of cross-linking is high, which may result in lower transparency. It should be noted that the closer the ratio of the first dissociative acid content to the second dissociative acid content is to 1, the lower the degree of cross-linking, and therefore the higher the transparency.

[0036] The amount of the first dissociative acid is preferably 0.35 or more, more preferably 0.4 or more, and particularly preferably 0.5 or more. If the amount of the first dissociative acid is less than 0.35, it may result in insufficient esterification, leading to low transparency.

[0037] (Raw fiber)

[0038] Raw material fibers for cellulose fibers can include, for example, plant-derived fibers (plant fibers), animal-derived fibers, and microbial-derived fibers. These fibers can be used alone or in combination as needed. Among these, plant fibers are preferred as raw material fibers, and pulp fibers, which are a type of plant fiber, are more preferred. When the raw material fiber is pulp fiber, it is easier to adjust the physical properties of the cellulose microfibers.

[0039] As plant fibers, materials such as wood pulp made from broad-leaved trees and coniferous trees, non-wood pulp made from rice straw and bagasse, and recycled paper pulp (DIP) made from recycled paper and waste paper can be used. These fibers can be used alone or in combination.

[0040] As wood pulp, chemical pulps such as hardwood kraft pulp (LKP) and softwood kraft pulp (NKP), mechanical pulp (TMP), and recycled paper pulp (DIP) can be used. These pulps can be used alone or in combination.

[0041] Hardwood kraft pulp (LKP) can be bleached hardwood kraft pulp, unbleached hardwood kraft pulp, or semi-bleached hardwood kraft pulp. Coniferous kraft pulp (NKP) can be bleached coniferous kraft pulp, unbleached coniferous kraft pulp, or semi-bleached coniferous kraft pulp. Discarded paper pulp (DIP) can be magazine pulp (MDIP), newspaper pulp (NDIP), cardboard pulp (WP), or other types of discarded paper pulp.

[0042] (modified)

[0043] Some of the hydroxyl groups (-OH groups) in cellulose fibers are replaced by polycarboxylic acid groups, preferably citric acid groups (polycarboxylic acid modification). After polycarboxylic acid modification, the disintegration of cellulose fibers becomes easier due to the osmotic pressure effect generated by electrostatic repulsion and sodium ion coordination.

[0044] More preferably, some of the hydroxyl groups in the cellulose fibers are also replaced with urethane groups (urethane modification). After urethane modification, the transparency and viscosity of the dispersion are significantly improved. In particular, if both citric acid modification and urethane modification are performed, hydrogen bonds are weakened, thus making the disintegration of cellulose fibers easier.

[0045] The amount of polycarboxylic acid groups introduced was evaluated based on a neutralization titration method using an automated titration apparatus. This neutralization titration method used was AUT-801 manufactured by Toa DKK Corporation.

[0046] The amount of carbamate groups introduced is preferably 0.01 mmol to 0.2 mmol relative to 1 g of cellulose microfibers. If the amount introduced exceeds 0.2 mmol, citric acid modification may not be possible.

[0047] The amount of carbamate groups introduced was calculated using the Kjeldahl method.

[0048] (microfibers)

[0049] The average fiber diameter (width) of the cellulose microfibers obtained by micronizing cellulose fibers is preferably 3 nm to 100 nm, more preferably 4 nm to 20 nm. If the average fiber diameter is less than 3 nm, the cellulose may not exhibit the physical properties of cellulose microfibers, such as strength, rigidity, and dimensional stability, when dissolved in water. If the average fiber diameter exceeds 100 nm, it reaches about 1 / 10 of the wavelength of visible light. Therefore, when the cellulose microfibers are dispersed in water (in the case of forming an aqueous dispersion), refraction and scattering of visible light occur, and the light transmittance may be insufficient.

[0050] The diameter of the cellulose microfibers was measured using an electron microscope as follows.

[0051] First, 100 ml of an aqueous dispersion of cellulose microfibers with a solid content of 0.01%–0.1% by mass was filtered through a Teflon (registered trademark) membrane filter. The dispersion was then subjected to one solvent exchange with 100 ml of ethanol and three solvent exchanges with 20 ml of tert-butanol. Following freeze-drying, the dispersion was coated with osmium to prepare a sample. This sample was then observed using SEM images at magnifications of 5000x, 10000x, or 30000x, depending on the width of the fibers. During this observation, two diagonal lines were drawn on the image, and then three lines were arbitrarily drawn passing through the intersection of these diagonal lines. The width of the total 100 fibers intersecting these three lines was then visually measured. The median of these measurements was taken as the fiber diameter (width).

[0052] The average fiber length of the cellulose microfibers is preferably 0.01 μm to 1000 μm, more preferably 0.05 μm to 500 μm, and particularly preferably 0.1 μm to 100 μm. If the average fiber length is less than 0.01 μm, it is difficult to form a fiber network structure, and the thickening effect may not be achieved. If the average fiber length exceeds 1000 μm, the cellulose microfibers may agglomerate together.

[0053] The fiber length of the cellulose microfibers was determined using a Valmet KK fiber analyzer "FS5".

[0054] The axial ratio (fiber length / fiber width) of the cellulose microfibers is preferably 3 to 10,000, more preferably 10 to 1,000. If the axial ratio is less than 3, it can no longer be described as fibrous. If the axial ratio exceeds 10,000, the viscosity of the dispersion may become too high.

[0055] The crystallinity of the cellulose microfibers is preferably 50% to 100%, more preferably 60% to 90%, and particularly preferably 65% ​​to 85%. If the crystallinity is less than 50%, the strength and heat resistance may be insufficient.

[0056] Crystallinity can be adjusted, for example, by selecting raw material fibers, pretreatment, and defiberization.

[0057] Crystallinity was determined using X-ray diffraction according to JIS-K0131 (1996) "General Rules for X-ray Diffraction Analysis". It should be noted that cellulose microfibers have both amorphous and crystalline components; crystallinity refers to the proportion of the crystalline component in the overall cellulose microfiber.

[0058] The total transmittance of the dispersion of cellulose microfibers (in a solution with a solid component concentration of 0.2%) is preferably 65% ​​or more, more preferably 70% or more, and particularly preferably 80% or more. If the total transmittance is less than 65%, the transparency may be insufficient.

[0059] The total light transmittance of cellulose microfibers can be adjusted, for example, by selecting pulp fibers, pretreatment, and defiberization.

[0060] The total transmittance was obtained by measuring the total transmittance (transmittance of light from 350 nm to 880 nm) of a 0.2% (w / v) cellulose microfiber dispersion using a Spectrophotometer U-2910 (Hitachi, Ltd.).

[0061] When the concentration of cellulose microfibers is 1% by mass (w / w), the B-type viscosity of the dispersion is preferably 40,000 cP or higher, more preferably 40,000 cP to 120,000 cP, and particularly preferably 50,000 cP to 100,000 cP. If the B-type viscosity is lower than 40,000 cP, it may not be suitable for use as a high-viscosity additive. If the B-type viscosity exceeds 120,000 cP, it may be difficult to mix with other substances when used as an additive.

[0062] Type B viscosity is a value obtained by measuring the viscosity of an aqueous dispersion of cellulose microfibers with a solid content of 1% according to JIS-Z8803 (2011) "Method for Determination of Viscosity of Liquids". Type B viscosity is the resistance torque when the dispersion is stirred. The higher the viscosity, the more energy is required for stirring.

[0063] [Manufacturing Method]

[0064] Next, the manufacturing method of this method will be explained.

[0065] (Preprocessing)

[0066] Before or after modifying cellulose fibers with various chemicals such as citric acid, pretreatment such as pulping can be performed on the cellulose fibers as needed. By pretreating the pulp fibers before defiberization, the number of defiberization steps can be significantly reduced, thus saving energy.

[0067] Pretreatment of cellulose fibers can be carried out using physical or chemical methods, preferably a combination of both. Physical and chemical pretreatment methods can be performed simultaneously or separately.

[0068] As a pretreatment using physical methods, pulping is preferred. When cellulose fibers are pulped, they are cut evenly. This prevents the cellulose fibers from tangling together (prevents aggregation). From this perspective, pulping is preferably performed until the freeness of the cellulose fibers is 700 ml or less, more preferably 500 ml or less, and particularly preferably 300 ml or less.

[0069] The degree of freeness of cellulose fibers was determined according to JIS P8121-2 (2012). Furthermore, pulping can be performed using equipment such as a refiner, mill, or kneader.

[0070] Examples of pretreatment methods using chemical methods include hydrolysis of polysaccharides using acids (acid treatment), hydrolysis of polysaccharides using enzymes (enzyme treatment), swelling of polysaccharides using alkalis (alkali treatment), oxidation of polysaccharides using oxidizing agents (oxidation treatment), and reduction of polysaccharides using reducing agents (reduction treatment). Among these, enzyme treatment is preferred as a pretreatment using chemical methods, and more preferably, one or more treatments selected from acid treatment, alkali treatment, and oxidation treatment are also performed. The alkali treatment will be described in detail below.

[0071] As a method of alkali treatment, one method is to impregnate cellulose fibers that have not yet been introduced with citric acid or the like in an alkaline solution.

[0072] The alkaline compounds contained in an alkaline solution can be either inorganic or organic. Examples of inorganic alkaline compounds include hydroxides of alkali metals or alkaline earth metals, carbonates of alkali metals or alkaline earth metals, and phosphate oxyacids of alkali metals or alkaline earth metals. Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, and potassium hydroxide. Examples of alkaline earth metal hydroxides include calcium hydroxide. Examples of alkali metal carbonates include lithium carbonate, lithium bicarbonate, potassium carbonate, potassium bicarbonate, sodium carbonate, and sodium bicarbonate. Examples of alkaline earth metal carbonates include calcium carbonate. Examples of alkali metal phosphate oxyacids include lithium phosphate, potassium phosphate, sodium phosphate, and disodium hydrogen phosphate. Examples of alkaline earth metal phosphates include calcium phosphate and calcium hydrogen phosphate.

[0073] Examples of organic base compounds include ammonia, aliphatic amines, aromatic amines, aliphatic ammonium compounds, aromatic ammonium compounds, heterocyclic compounds and their hydroxides, carbonates, and phosphates. Specifically, examples include 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 bicarbonate, and diammonium hydrogen phosphate.

[0074] The solvent for the alkaline solution can be either water or an organic solvent, preferably a polar solvent (water, alcohol, or other polar organic solvents), and more preferably an aqueous solvent containing at least water.

[0075] The pH of the alkaline solution at 25°C is preferably 9 or higher, more preferably 10 or higher, and particularly preferably 11 to 14. When the pH is 9 or higher, the yield of cellulose microfibers increases. However, if the pH exceeds 14, the treatability of the alkaline solution decreases.

[0076] In this manufacturing method, firstly, an alkali is added to an aqueous dispersion of cellulose fibers to adjust the pH to 9 or higher, preferably to 10 or higher, and more preferably to 11 or higher. By adding the alkali, the cellulose fibers swell and separate from each other, thus making it difficult for cross-linking caused by the addition of citric acid to occur. If the pH is below 9, the cross-linking reaction makes defibrillation difficult, preventing the formation of fine cellulose fibers and resulting in insufficient viscosity and transparency. However, if the alkali concentration (alkalinity) is 18% or higher, regenerated cellulose formation may occur.

[0077] As the added alkali, substances such as sodium hydroxide, potassium hydroxide, ammonia, and sodium carbonate, which can adjust the pH of the cellulose fiber dispersion to 9-14, can be used. However, since sodium hydroxide has a strong swelling capacity for fibers, it is preferred to use sodium hydroxide.

[0078] (The addition of various chemicals)

[0079] Next, at least one of the polycarboxylic acids and polycarboxylic acid metal salts is added to the dispersion of cellulose fibers (reaction solution).

[0080] As reactants (polycarboxylic acids, polycarboxylic acid metal salts), in addition to citric acid which has three carboxyl groups, malic acid, aconitic acid, malonic acid, succinic acid, etc. can also be used.

[0081] The amount of reactant added, relative to 1 g of cellulose fiber, is preferably 1 mmol to 20 mmol, more preferably 2 mmol to 18 mmol. If the amount added is less than 1 mmol, insufficient esterification may result in insufficient transparency and viscosity. If the amount added exceeds 20 mmol, the degree of esterification may reach its limit.

[0082] The amount of reactant introduced was determined using an automated titration apparatus AUT-801 (DKK East Asia) via neutralization titration on a 0.2% (w / w) concentration of modified cellulose fiber (dispersion). The automated titration method involves treating the 0.2% (w / w) dispersion with an ion exchange resin at a mass ratio of 10:1, followed by titration with 0.05 M sodium hydroxide, in increments of 0.1 mL. The resulting pH value was used as the dissociation curve, and two inflection points were observed. The first inflection point was taken as the first inflection point, and the second inflection point as the second inflection point.

[0083] Preferably, at least one of urea and urea derivatives (hereinafter also referred to as "urea, etc.") is added simultaneously with or before and after the addition of a reactant such as citric acid. When urea, etc., is added, the hydrogen bonds between cellulose fibers break, causing swelling and separation of the fibers, thus making cross-linking caused by the addition of citric acid less likely to occur. Examples of urea, such as urea, thiourea, biuret, and phenylurea, can be used as urea, etc. These urea or urea derivatives can be used individually or in combination. Urea is preferred.

[0084] When heated, urea and the like decompose into isocyanate and ammonia as shown in reaction formula (1) below. Furthermore, isocyanate is highly reactive and, as shown in reaction formula (2) below, modifies the hydroxyl groups of cellulose into urethane groups. Therefore, adding urea or the like to cellulose fibers promotes the introduction of urethane groups.

[0085] NH2-CO-NH2 → HN=C=O+NH3…(1)

[0086] Cell-OH+HN=C=O → Cell-O-CO-NH2 …(2)

[0087] In the above reaction formula (2), Cell refers to cellulose molecules.

[0088] The amount of urea or the like added is preferably 0.1 mmol to 5.0 mmol, more preferably 0.2 mmol to 4.0 mmol, relative to 1 g of cellulose fiber. Even if the amount added exceeds 5.0 mmol, the effect of adding urea or the like may have reached its limit.

[0089] It should be noted that when adding various chemicals, the cellulose fibers can be in a dry state, a wet state, or a dispersion state. Furthermore, the chemicals can be in a powder state or an aqueous solution state. However, from the viewpoint of high reaction uniformity, it is preferable to add chemicals in an aqueous solution state to the cellulose fibers in a dry state.

[0090] (heating)

[0091] Cellulose fibers with added chemicals are heated to promote modification reactions initiated by citric acid, etc. The preferred heating temperature is 110–200°C, more preferably 120–160°C. If the heating temperature is below 110°C, citric acid may not react with the pulp. However, if the heating temperature exceeds 200°C, the cellulose fibers deteriorate rapidly, potentially becoming a major cause of color change and viscosity reduction.

[0092] The preferred pH for heating cellulose fibers containing citric acid and other chemicals is 2.0–7.0, more preferably 3.0–6.0. ​​As mentioned earlier, the pH of the dispersion can be adjusted to 9 or higher (alkaline) by adding alkalis such as sodium hydroxide, but the pH can be adjusted to 2.0–7.0 by adding citric acid and other chemicals. However, if the pH is below 2.0, discoloration may occur. If the pH exceeds 7.0, citric acid will convert to trisodium citrate, preventing the reaction from proceeding and potentially resulting in insufficient transparency and viscosity.

[0093] When heating cellulose fibers containing various chemicals, it is preferable to heat them until they are dry. Specifically, the cellulose fibers are dried until the moisture content is preferably reduced to 20% or less, more preferably to 10% or less.

[0094] The reaction time for cellulose fibers with added chemicals is, for example, 1 to 3600 seconds, preferably 10 to 1000 seconds. If the reaction time is too long, the cellulose fibers may yellow. If the reaction time is too short, modification with citric acid, etc., may not be fully achieved.

[0095] As a device for heating cellulose fibers with various chemicals added, such as hot air dryers, baking ovens, heated mixers, paper machines, and pulp dryers can be used.

[0096] (Cleaning)

[0097] The cellulose fibers mentioned above are preferably washed before desiccation. Washing the cellulose fibers removes residual citric acid and other chemicals, as well as alkalis such as sodium hydroxide.

[0098] Cellulose fibers can be cleaned using, for example, water or organic solvents.

[0099] (Add more alkali)

[0100] Next, an alkali such as sodium hydroxide is added to the cellulose fibers again to hydrolyze them. This second addition of alkali breaks the cross-linking of the cellulose fibers, reducing the degree of cross-linking.

[0101] The pH should be adjusted to 8-14, preferably 9-13, when adding alkali again. If the pH is below 8, cross-linking may still remain. If the pH is above 14, the citric acid groups may be completely removed.

[0102] It is important to note that the alkali used in this process is the same as the alkali initially added. Furthermore, although the text above uses the terms "alkali" or "alkali treatment," it actually means "hydrolysis." The reason is that even acidic solutions can potentially disrupt cross-linking.

[0103] Here, we would like to provide some additional explanation regarding the cross-linked structure and the ester structure.

[0104] Before hydrolysis, citric acid-modified cellulose exists in a state where cross-linked and ester structures coexist. Furthermore, the presence of a large amount of cross-linked structures is considered difficult to de-fibrillate, thus hindering the achievement of high transparency. Conversely, by hydrolyzing and disrupting the cross-linked structures, the ester structure increases compared to before hydrolysis, making de-fibrillation easier. However, destroying all cross-linked structures while retaining only the ester structure is considered difficult. Incidentally, if chemicals are added further, both cross-linked and ester structures are destroyed, returning to a state of difficult de-fibrillation. Moreover, while increased cross-linked structures do not necessarily increase transparency due to de-fibrillation difficulty, they tend to increase viscosity. Increased ester structures facilitate de-fibrillation, thus increasing transparency, but compared to modified cellulose with more cross-linked structures, viscosity tends to decrease.

[0105] (Fiber unraveling)

[0106] Cellulose fibers that have undergone alkali re-addition and other processes are defibrilated (microfibrilization treatment). Through this defibrilation, the cellulose fibers are microfibrilized, becoming cellulose microfibrils (cellulose nanofibrils (CNF)).

[0107] When defiberizing cellulose fibers, it is preferable to prepare the cellulose fibers into a slurry beforehand. The solid content concentration of the 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, defiberization can be performed effectively.

[0108] Cellulose fiber defibrillation can be achieved using one or more methods, such as high-pressure homogenizers, high-pressure homogenizing devices, grinding mills, stone mortar and pestle mills, conical grinding mills, disc grinding mills, and various bacteria. Among these methods, defibrillation of cellulose fibers is preferably performed using devices and methods that utilize water flow, particularly high-pressure water flow, for micronization. Using this method, the resulting cellulose microfibrils exhibit very high uniformity in size and dispersion. In contrast, if a grinding mill is used, for example, in a rotating grinding stone chamber, it is difficult to uniformly micronize the cellulose fibers, and depending on the situation, some undefibrillated fiber clumps may remain.

[0109] Milling machines used in the defiberization of cellulose fibers include, for example, the MASSCOLLOIDER from Masukuni Sangyo Co., Ltd. Additionally, devices that utilize high-pressure water jets for micronization include, for example, the Star Burst (registered trademark) from SuginoMachine Co., Ltd., and the Nanovater (registered trademark) from Yoshida Machinery Kogyo Co., Ltd. Furthermore, high-speed rotary homogenizers used in the defiberization of cellulose fibers include, for example, the Clearmix-11S from M-Technique Co., Ltd.

[0110] As a device for defiberization using high-pressure water flow, a high-pressure homogenizer is preferred. A high-pressure homogenizer is a homogenizer capable of ejecting cellulose fiber slurry at a pressure of, for example, 10 MPa or higher, preferably 100 MPa or higher. When processing cellulose fibers using a high-pressure homogenizer, the collision of cellulose fibers, pressure differences, and microcavitation all contribute to the efficient defiberization of the cellulose fibers. Therefore, the number of defiberization processes can be reduced, and the manufacturing efficiency of cellulose microfibers can be improved.

[0111] Cellulose fiber defibrillation is preferably carried out in a manner that ensures the average fiber width, average fiber length, and crystallinity of the obtained cellulose microfibers reach the aforementioned desired values ​​or evaluations.

[0112] Example

[0113] Next, embodiments of the present invention will be described.

[0114] To bleached coniferous kraft pulp flakes (46% solids), sodium hydroxide (at a molar ratio of 1:1 to citric acid) and water (at a mass ratio of 1:15 to the pulp's solids) were added, and the mixture was stirred for 10 minutes. Then, a specified amount of citric acid was added, and the mixture was stirred for another 10 minutes. After mixing, the pulp was dried in a hot air dryer at 130°C for 4 hours. After the reaction, the pulp was washed until the filtrate was neutral, yielding modified pulp. Water was added to bring the modified pulp to 1 wt%, and sodium hydroxide was added to adjust the pH to a specified value. Hydrolysis was then carried out for 1 hour. Further, the hydrolyzed modified pulp was washed until the filtrate was neutral. The hydrolyzed modified pulp was defibrinated into cellulose microfibers, and various tests were conducted. Defibrination was performed using a high-pressure homogenizer.

[0115] The amounts of citric acid, sodium hydroxide, and urea added, the heating temperature and time, and the pH during hydrolysis are shown in Table 1. The physical properties and evaluation of the obtained cellulose microfibers are shown in Table 2. The evaluation methods for type B viscosity and transmittance are as described above.

[0116] [Table 1]

[0117] [Table 2]

[0118] Industrial applicability

[0119] This invention can be used as modified cellulose fibers and modified cellulose microfibers.

Claims

1. A modified cellulose fiber, characterized in that, Modified with polycarboxylic acid groups, and the ratio of first dissociated acid to second dissociated acid is 0.25~1.0, and the first dissociated acid is 0.35 mmol / g or higher.

2. The modified cellulose fiber according to claim 1, wherein it is urethane-esterified.

3. The modified cellulose fiber according to claim 1, wherein the total transmittance of the dispersion with a concentration of 0.2% by mass is 65% or higher, and the type B viscosity of the dispersion with a concentration of 1.0% by mass is 40,000 cP or higher.

4. The modified cellulose fiber according to claim 1, wherein, The polycarboxylic acid is citric acid.

5. A modified cellulose microfiber, characterized in that, The modified cellulose fiber according to any one of claims 1 to 4 has an average fiber diameter of 100 nm or less.

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