Modified cellulose fibers and modified cellulose microfibers

Abstract

[Problem] To provide novel cellulose microfibers that have a broad range of applications or cellulose fibers which serve as a raw material for said cellulose microfibers. [Solution] Modified cellulose fibers that are modified using a polyvalent carboxylic acid group, and that have a first dissociation acid amount / second dissociation acid amount of 0.25-1.0, and a first dissociation acid amount of at least 0.35 mmol / g. The modified cellulose microfibers are characterized by the modified cellulose fibers having an average fiber diameter of 100 nm or less.

Description

Modified cellulose fibers and modified cellulose microfibers

[0001] The present invention relates to modified cellulose fibers and modified cellulose fine fibers.

[0002] In recent years, nanotechnology, which aims to reduce the size of materials to the nanometer level and obtain new physical properties that differ from the conventional properties of materials, has attracted attention. Cellulose microfibers produced from pulp, a cellulosic raw material, by chemical treatment, pulverization, etc., have excellent strength, elasticity, thermal stability, etc., and are therefore expected to be used in industrial applications such as filter media, filter aids, base materials for ion exchangers, fillers for chromatography analysis instruments, and fillers for compounding resins and rubbers, as well as in cosmetic formulations such as lipsticks, powder cosmetics, and emulsion cosmetics. Furthermore, cellulose microfibers have excellent aqueous dispersibility, and are therefore expected to be used in many applications such as viscosity retainers for foods, cosmetics, paints, etc., strengtheners for food raw material doughs, moisture retainers, food stabilizers, low-calorie additives, and emulsion stabilization aids.

[0003] Conventionally, methods for obtaining such cellulose fine fibers have been employed in which cellulose fibers are mechanically defibrated using a high-pressure homogenizer, a high-speed rotary homogenizer, an ultrasonic homogenizer, or the like (see, for example, Patent Document 1). However, these methods require a large amount of energy to proceed with defibration. Therefore, in order to reduce the energy required for defibration, a defibration method using the TEMPO oxidation method has been proposed. However, this method has economical drawbacks, and a shift away from petroleum is an issue. Therefore, currently, methods such as phosphate esterification have been proposed. However, relying solely on this technology could cause problems such as marine pollution. Therefore, proposals for other new technologies are needed.

[0004] Meanwhile, as a technique different from the above-mentioned 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 essentially involves citric acid modification by cooking, which results in a low viscosity dispersion, significantly limiting its applications. Furthermore, the proposal in Patent Document 3 has the problem of low transparency of the dispersion. In this regard, particularly for applications such as food and cosmetics, high transparency of the dispersion is required, and the challenge of increasing transparency is unavoidable in expanding the applications of cellulose fine fibers.

[0005] JP 2010-216021 A JP 2015-140403 A JP 2019-189792 A

[0006] The problem to be solved by the present invention is to provide new cellulose fine fibers or cellulose fibers that can be used as raw materials for cellulose fine fibers, which are not significantly limited in their applications.

[0007] In order to solve the above problems, the present inventors have focused on a method of modifying cellulose fibers with citric acid. Citric acid is a naturally occurring substance and has various advantages, such as being capable of reacting under near-neutral conditions and at low temperatures. However, as described above, the applications of conventional cellulose fine fibers modified with citric acid have been significantly limited.

[0008] Therefore, the present inventors have independently conducted various studies and found that citric acid modification makes cellulose fibers more likely to be crosslinked, and that crosslinking of cellulose fibers leads to thicker fibers, resulting in reduced viscosity and transparency. Based on this finding, the present inventors have arrived at the following means.

[0009] Patent Document 3 states that "cellulose nanofibers literally refer to fibers with a fiber diameter (or fiber width) of several nanometers to several hundred nanometers, but in this specification, they are not limited to such sizes. For example, they also include fibers with a fiber diameter of approximately 3,000 nm," suggesting that the fiber diameter of cellulose nanofibers can be freely adjusted. However, as mentioned above, citric acid modification makes cellulose fibers more likely to crosslink, making it difficult to reduce the fiber diameter. Of course, it is not impossible to reduce the fiber diameter, but doing so requires harsh reaction conditions, such as the use of excessive chemicals, which results in extremely short cellulose fibers and ultimately a reduced viscosity.

[0010] (Means according to claim 1) A modified cellulose fiber characterized in that it is modified with a polycarboxylic acid group, the ratio of the amount of first dissociated acid to the amount of second dissociated acid is 0.25 to 1.0, and the amount of first dissociated acid is 0.35 mmol / g or more.

[0011] (Means according to claim 2) The modified cellulose fiber according to claim 1, which is carbamate-treated.

[0012] (Means according to claim 3) The modified cellulose fiber according to claim 1, wherein the dispersion (concentration 0.2% by mass) has a total light transmittance of 65% or more, and the dispersion (concentration 1.0% by mass) has a Brookfield viscosity of 40,000 cP or more.

[0013] (Means according to claim 4) The modified cellulose fiber according to claim 1, wherein the polycarboxylic acid is citric acid.

[0014] (Means according to claim 5) Modified cellulose fine fibers, characterized in that the average fiber diameter of the modified cellulose fibers according to any one of claims 1 to 4 is 100 nm or less.

[0015] According to the present invention, novel modified cellulose fine fibers or modified cellulose fibers that can be used as raw materials for modified cellulose fine fibers can be obtained, the uses of which are not significantly limited.

[0016] Next, an embodiment of the present invention will be described. Note that this embodiment is an example of the present invention.

[0017] [Fibers and dispersion] The modified cellulose fibers of this embodiment are modified with polycarboxylic acid (preferably citric acid) groups, and have a first dissociated acid amount / second dissociated acid amount ratio of 0.25 to 1.0, with the first dissociated acid amount being 0.35 mmol / g or more. The modified cellulose fine fibers have an average fiber diameter of 100 nm or less. This will be described in detail below.

[0018] (Amount of Dissociated Acid) As mentioned above, when cellulose fibers are modified with citric acid or the like, the cellulose fibers are more likely to crosslink, resulting in bonding between the cellulose fibers. Therefore, the fiber diameter tends to increase, and the transparency of the dispersion decreases. Meanwhile, when cellulose fibers (citric acid) undergo a crosslinking reaction, they contain only acidic groups (=first acid groups, hereinafter also referred to simply as "strong acid groups, etc."). On the other hand, when they do not undergo a crosslinking reaction, they contain both strong acid groups, etc. and acidic groups with lower acidity (=second acid groups, hereinafter also referred to simply 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 preventing the fiber diameter of the cellulose fibers from increasing even when modified with citric acid.

[0019] In neutralization titration, two maximum points of increment (differential value of pH with respect to the amount of alkali added) may be observed on a curve plotting pH against the amount of alkali added. Here, of these maximum points, the first maximum point of increment obtained after starting to add alkali is called the first endpoint, and the next maximum point of increment obtained is called the second endpoint. The value obtained by dividing the amount of alkali required from the start of titration to the first endpoint by the solid content (g) in the dispersion to be titrated is equal to the first dissociated acid amount of fibrous cellulose contained in the dispersion used for titration (the value obtained by dividing the amount of acid substance (mmol) 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 endpoint to the second endpoint by the solid content (g) in the dispersion to be titrated is equal to the first dissociated acid amount of fibrous cellulose contained in the dispersion used for titration. The value obtained by dividing the amount of alkali required from the start of titration to the second endpoint by the solid content (g) in the dispersion to be titrated is equal to the amount of second dissociated acid of the fibrous cellulose (the value obtained by dividing the amount of acid (mmol) ionized and neutralized from the first to second endpoint by the amount of solids (g) in the dispersion), and the value obtained by dividing the amount of alkali required from the start of titration to the second endpoint by the solid content (g) in the dispersion to be titrated is equal to the total amount of dissociated acid of the fibrous cellulose contained in the dispersion to be titrated (the value obtained by dividing the total amount of acid (mmol) ionized and neutralized from the second endpoint by the amount of solids (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 endpoint by the solid content (g) in the dispersion to be titrated is the amount of strong acid groups and the like introduced (mmol / g). Similarly, the value obtained by dividing the amount of alkali required from the start of titration to the second endpoint by the solid content (g) in the dispersion to be titrated is the total amount of acid groups introduced (mmol / g).

[0020] In measurements by neutralization titration, if too much alkaline solution such as aqueous sodium hydroxide solution is added or if the titration interval is too short, accurate values ​​may not be obtained. Therefore, for example, it is desirable to titrate 10 to 50 μL of 0.1 N aqueous sodium hydroxide solution every 5 to 30 seconds. In addition, to eliminate the influence of carbon dioxide dissolved in the dispersion, it is desirable to measure while blowing an inert gas such as nitrogen gas into the dispersion, for example, from 15 minutes before the start of titration until the end of titration.

[0021] In this embodiment, the ratio of the amount of first dissociated acid (mmol / g) to the amount of second dissociated acid (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 ratio of the amount of first dissociated acid to the amount of second dissociated acid is less than 0.25, the degree of crosslinking is high, and the transparency may be considered low. Note that the closer the ratio of the amount of first dissociated acid to the amount of second dissociated acid is to 1, the lower the degree of crosslinking and therefore the higher the transparency.

[0022] The amount of the first dissociated 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 dissociated acid is less than 0.35, the degree of esterification may be insufficient, resulting in low transparency.

[0023] (Raw Fiber) As the raw fiber of the cellulose fiber, for example, plant-derived fiber (plant fiber), animal-derived fiber, microbial-derived fiber, etc. can be used. These fibers can be used alone or in combination as needed. However, it is preferable to use plant fiber as the raw fiber, and it is more preferable to use pulp fiber, which is a type of plant fiber. When the raw fiber is pulp fiber, it is easy to adjust the physical properties of the cellulose fine fiber.

[0024] Examples of plant fibers that can be used include wood pulp made from broad-leaved trees, conifers, etc., non-wood pulp made from straw, bagasse, etc., and decomposed paper pulp (DIP) made from recycled waste paper, broke, etc. These fibers can be used alone or in combination.

[0025] Examples of wood pulp that can be used include chemical pulps such as hardwood kraft pulp (LKP) and softwood kraft pulp (NKP), transverse mechanical pulp (TMP), and recycled paper pulp (DIP). These pulps can be used alone or in combination.

[0026] The hardwood kraft pulp (LKP) may be bleached hardwood kraft pulp, unbleached hardwood kraft pulp, or semi-bleached hardwood kraft pulp. The softwood kraft pulp (NKP) may be bleached softwood kraft pulp, unbleached softwood kraft pulp, or semi-bleached softwood kraft pulp. The recycled paper pulp (DIP) may be magazine recycled paper pulp (MDIP), newspaper recycled paper pulp (NDIP), corrugated paper pulp (WP), or other recycled paper pulp.

[0027] (Modification) Cellulose fibers have some of their hydroxyl groups (—OH groups) substituted with polycarboxylic acid groups, preferably citric acid groups (polycarboxylic acid modification). When modified with polycarboxylic acid, the cellulose fibers are easily defibrated due to electrostatic repulsion and the osmotic pressure effect caused by the coordination of sodium ions.

[0028] More preferably, some of the hydroxyl groups of the cellulose fibers are also substituted with carbamate groups (carbamate-modified). Carbamate-modified cellulose fibers significantly improve the transparency and viscosity of the dispersion. In particular, when citric acid and carbamate-modified cellulose fibers are both modified, hydrogen bonds are weakened, making defibration of the cellulose fibers easier.

[0029] The amount of polycarboxylic acid groups introduced is a value evaluated based on neutralization titration using an automatic titrator, AUT-801 manufactured by DKK-TOA.

[0030] The amount of carbamate groups introduced is preferably 0.01 to 0.2 mmol per gram of cellulose fine fibers. If the amount introduced exceeds 0.2 mmol, the modification with citric acid may not proceed.

[0031] The amount of introduced carbamate groups was calculated by the Kjeldahl method.

[0032] (Fine Fibers) The average fiber diameter (width) of the cellulose fine fibers obtained by micronizing 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 the cellulose fine fibers may not exhibit the physical properties required for cellulose fine fibers, such as strength, rigidity, and dimensional stability. On the other hand, if the average fiber diameter exceeds 100 nm, the wavelength becomes about 1 / 10 of the wavelength of visible light. Therefore, when the cellulose fine fibers are dispersed in water (when an aqueous dispersion is formed), refraction and scattering of visible light may occur, resulting in insufficient light transmittance.

[0033] The fiber diameter of cellulose fine fibers is measured using an electron microscope as follows. First, 100 ml of an aqueous dispersion of cellulose fine fibers with a solid content concentration of 0.01 to 0.1% by 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, the sample is freeze-dried and osmium-coated to obtain a sample. This sample is observed using an electron microscope SEM image at a magnification of 5,000x, 10,000x, or 30,000x, depending on the width of the fibers constituting it. In this observation, two diagonal lines are drawn on the observed image, and three additional straight lines are arbitrarily drawn passing through the intersections of the diagonal lines. The widths of a total of 100 fibers intersecting with these three straight lines are then visually measured. The median diameter of this measurement is taken as the fiber diameter (width).

[0034] The average fiber length of the cellulose fine fibers 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, the cellulose fine fibers may aggregate with each other.

[0035] The fiber length of the cellulose fine fibers is a value measured using a fiber analyzer "FS5" manufactured by Valmet.

[0036] The axial ratio (fiber length / fiber width) of the cellulose fine fibers is preferably 3 to 10,000, more preferably 10 to 1,000. If the axial ratio is less than 3, the fibers can no longer be said to be fibrous. On the other hand, if the axial ratio exceeds 10,000, the viscosity of the dispersion may become too high.

[0037] The crystallinity of the cellulose fine fibers 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.

[0038] The crystallinity can be adjusted, for example, by selecting raw fiber, pre-treating, defibrating, etc.

[0039] The crystallinity is a value measured by X-ray diffraction in accordance with JIS-K0131 (1996) "General rules for X-ray diffraction analysis." Cellulose fine fibers have amorphous and crystalline portions, and the crystallinity refers to the proportion of the crystalline portion in the entire cellulose fine fibers.

[0040] The total light transmittance of the dispersion of cellulose fine fibers (solution with a solid content of 0.2%) is preferably 65% ​​or more, more preferably 70% or more, and particularly preferably 80% or more. If the total light transmittance is less than 65%, the transparency may be deemed insufficient.

[0041] The total light transmittance of the cellulose fine fibers can be adjusted by, for example, selecting the pulp fibers, pre-treating them, defibrating them, and the like.

[0042] The total light transmittance is a value obtained by measuring the total light transmittance (transmittance of light from 350 to 880 nm) of a 0.2% (w / v) dispersion of cellulose fine fibers using a Spectrophotometer U-2910 (Hitachi, Ltd.).

[0043] When the concentration of cellulose fine fibers is 1% by mass (w / w), the dispersion preferably has a Brookfield viscosity of 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 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.

[0044] The B-type viscosity is a value measured for an aqueous dispersion of cellulose fine fibers with a solid content of 1% in accordance with JIS-Z8803 (2011) "Method for measuring viscosity of liquids." The B-type viscosity is the resistance torque when the dispersion is stirred, and the higher the viscosity, the more energy is required for stirring.

[0045] [Manufacturing Method] Next, the manufacturing method of this embodiment will be described. (Pretreatment) Before or after modifying the cellulose fibers with various chemicals such as citric acid, the cellulose fibers can be subjected to a pretreatment such as beating, if necessary. By subjecting the pulp fibers to a pretreatment before defibrating the cellulose fibers, the number of defibration steps can be significantly reduced, and the energy required for defibration can be saved.

[0046] The pretreatment of the cellulose fibers can be carried out by a physical method or a chemical method, preferably by a physical method and a chemical method. The pretreatment by a physical method and the pretreatment by a chemical method can be carried out simultaneously or separately.

[0047] As a pretreatment by a physical method, beating is preferably employed. When cellulose fibers are beaten, the cellulose fibers are cut into uniform pieces. Therefore, entanglement of the cellulose fibers with each other (prevention of aggregation) is prevented. From this viewpoint, beating is preferably carried out 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.

[0048] The freeness of the cellulose fiber is a value measured in accordance with JIS P8121-2 (2012). Beating can be performed using, for example, a refiner, a beater, a kneader, or the like.

[0049] Examples of pretreatments by chemical methods include hydrolysis of polysaccharides with acid (acid treatment), hydrolysis of polysaccharides with enzymes (enzyme treatment), swelling of polysaccharides with alkali (alkali treatment), oxidation of polysaccharides with an oxidizing agent (oxidation treatment), and reduction of polysaccharides with a reducing agent (reduction treatment). However, as pretreatments by chemical methods, enzyme treatment is preferred, and it is more preferred to additionally carry out one or more treatments selected from acid treatment, alkali treatment, and oxidation treatment. Below, alkali treatment is described in detail.

[0050] As a method for alkali treatment, for example, there is a method in which cellulose fibers are immersed in an alkali solution before citric acid or the like is introduced.

[0051] The alkaline compound contained in the alkaline solution may be an inorganic alkaline compound or an organic alkaline compound. Examples of inorganic alkaline compounds include hydroxides of alkali metals or alkaline earth metals, carbonates of alkali metals or alkaline earth metals, and phosphate oxoacid salts 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 oxoacid salts include lithium phosphate, potassium phosphate, trisodium phosphate, and disodium hydrogen phosphate. Examples of alkaline earth metal phosphates include calcium phosphate and calcium hydrogen phosphate.

[0052] Examples of organic alkali compounds include ammonia, aliphatic amines, aromatic amines, aliphatic ammonium, aromatic ammonium, heterocyclic compounds, and hydroxides, carbonates, phosphates, etc. Specific 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 hydrogencarbonate, and diammonium hydrogenphosphate.

[0053] The solvent for the alkaline solution may be either water or an organic solvent, but is preferably a polar solvent (water, a polar organic solvent such as alcohol), and more preferably an aqueous solvent containing at least water.

[0054] 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 fine fibers increases. However, when the pH exceeds 14, the handling of the alkaline solution decreases.

[0055] In this manufacturing method, an alkali is first added to an aqueous dispersion of cellulose fibers to adjust the pH to 9 or higher, preferably 10 or higher, and more preferably 11 or higher. The addition of the alkali causes the cellulose fibers to swell and separate, making crosslinking by adding citric acid difficult. If the pH is less than 9, the crosslinking reaction makes it difficult to defibrate the fibers, preventing them from becoming fine cellulose fibers, which may result in insufficient viscosity and transparency. However, if the alkali concentration (causticity) is 18% or higher, there is a risk of regenerated cellulose.

[0056] The alkali to be added may be, for example, sodium hydroxide, potassium hydroxide, aqueous ammonia, sodium carbonate, or the like, which can adjust the pH of the cellulose fiber dispersion to 9 to 14. However, it is preferable to use sodium hydroxide because of its high ability to swell the fibers.

[0057] (Addition of Various Chemicals) Next, at least one of a polycarboxylic acid and a metal salt of a polycarboxylic acid (reaction liquid) is added to the dispersion of cellulose fibers.

[0058] As the reactant (polycarboxylic acids or metal salts of polycarboxylic acids), for example, citric acid, which has three carboxylic acids, as well as malic acid, aconitic acid, malonic acid, succinic acid, etc. can be used.

[0059] The amount of the reactant added is preferably 1 to 20 mmol, more preferably 2 to 18 mmol, per gram of cellulose fiber. If the amount added is less than 1 mmol, the degree of esterification may be insufficient, resulting in insufficient transparency and viscosity. On the other hand, if the amount added exceeds 20 mmol, the degree of esterification may plateau.

[0060] The amount of reactant introduced is measured by neutralization titration using an automatic titrator AUT-801 (DKK Toa) on a 0.2% by mass concentration modified cellulose fiber (dispersion). The automatic titration method involves treating a 0.2% by mass concentration dispersion with an ion exchange resin to a mass ratio of 10:1, followed by titration in 0.1 mL increments with 0.05 M sodium hydroxide. The measured pH is used as a dissociation curve, and two inflection points are observed. The first inflection point obtained is designated the first inflection point, and the next inflection point obtained is designated the second inflection point.

[0061] Preferably, urea and / or a urea derivative (hereinafter simply referred to as "urea, etc.") is added together with, before or after, or after the addition of a reactant such as citric acid. Addition of urea, etc. breaks the hydrogen bonds between cellulose fibers, causing the fibers to swell and separate, making crosslinking by the addition of citric acid more difficult. Examples of urea, etc. that can be used include urea, thiourea, biuret, and phenylurea. These ureas or urea derivatives can be used alone or in combination. However, the use of urea is preferred.

[0062] When heated, urea or the like is decomposed into isocyanic acid and ammonia as shown in the following reaction formula (1). Isocyanic acid is highly reactive and modifies the hydroxyl groups of cellulose into carbamate groups as shown in the following reaction formula (2). Therefore, adding urea or the like to cellulose fibers promotes the introduction of carbamate groups. NH ２ —CO—NH ２ → HN=C=O+NH ３ ...(1) Cell-OH+H-N=C=O → Cell-O-CO-NH ２ …(2)

[0063] In the above reaction formula (2), Cell refers to a cellulose molecule. The amount of urea or the like added is preferably 0.1 to 5.0 mmol, more preferably 0.2 to 4.0 mmol, per 1 g of cellulose fiber. If the amount added exceeds 5.0 mmol, the effect of adding urea or the like may plateau.

[0064] When adding various chemicals, the cellulose fibers may be in a dry state, a wet state, or a dispersion state. Furthermore, the various chemicals may be in a powder state or an aqueous solution state. However, adding chemicals in an aqueous solution state to dry cellulose fibers is preferred because it results in a high degree of reaction uniformity.

[0065] (Heating) The cellulose fibers to which various chemicals have been added are heated to promote a modification reaction with citric acid or the like. The 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 fibers may progress rapidly, which may cause discoloration or a decrease in viscosity.

[0066] The pH when heating cellulose fibers to which various chemicals such as citric acid have been added is preferably 2.0 to 7.0, more preferably 3.0 to 6.0. As mentioned above, the pH of the dispersion is made alkaline at 9 or higher by adding an alkali such as sodium hydroxide, but the pH becomes 2.0 to 7.0 by adding various chemicals such as citric acid. However, if the pH is less than 2.0, there is a risk of coloration. On the other hand, if the pH exceeds 7.0, citric acid will turn into trisodium citrate, which may prevent the reaction from proceeding and result in insufficient transparency and viscosity.

[0067] The cellulose fibers to which various chemicals have been added are preferably heated until the cellulose fibers are dried. Specifically, the cellulose fibers are dried until the moisture content of the cellulose fibers is preferably 20% or less, more preferably 10% or less.

[0068] The reaction time of the cellulose fibers to which various chemicals have been added 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 of the citric acid or the like may not proceed sufficiently.

[0069] As a device for heating the cellulose fibers to which various chemicals have been added, for example, a hot air dryer, a kiln, a heated kneader, a paper machine, a dry pulp machine, etc. can be used.

[0070] (Washing) The above cellulose fibers are preferably washed before being defibrated. By washing the cellulose fibers, it is possible to wash away various chemicals such as citric acid and alkalis such as sodium hydroxide remaining.

[0071] The cellulose fibers can be washed using, for example, water or an organic solvent.

[0072] (Addition of Alkali Again) Next, an alkali such as sodium hydroxide is added again to the cellulose fibers to hydrolyze the cellulose fibers. This addition of alkali again breaks the crosslinks in the cellulose fibers, reducing the degree of crosslinking.

[0073] The alkali is added again so that the pH becomes 8 to 14, preferably 9 to 13. If the pH is less than 8, crosslinks may still remain. On the other hand, if the pH exceeds 14, all of the citric acid groups may be eliminated.

[0074] The alkali that can be used in this step is the same as the alkali that was added initially. Although the above description uses the term "alkali" or "alkali treatment," the true meaning of this term is "hydrolysis." This is because even an acidic solution may destroy the crosslinks.

[0075] Here, some additional comments will be made regarding the crosslinked structure and ester structure. Before hydrolysis, the structure of citric acid-modified cellulose is a mixture of crosslinked structures and ester structures. If many crosslinked structures are present, it is thought that defibrating to a high level of transparency is difficult. In contrast, if the crosslinked structures are destroyed by hydrolysis and the ester structures become more numerous than before hydrolysis, defibration becomes easier. However, it is thought that it is difficult to destroy all of the crosslinked structures and leave only the ester structures. Incidentally, if chemicals are continued to be added, all of the crosslinked structures and ester structures are destroyed, returning to a state where defibration is difficult. Furthermore, if the crosslinked structures become more numerous, transparency does not increase because defibration is difficult, but viscosity tends to increase due to the crosslinked structures. If the ester structures become more numerous, transparency increases because defibration is easier, but viscosity tends to be lower compared to modified cellulose with many crosslinked structures.

[0076] (Defibrillation) The cellulose fibers to which alkali has been added again or the like are defibrillated (fine-refining treatment). Through this defibrillation, the cellulose fibers are microfibrillated into cellulose fine fibers (cellulose nanofibers (CNF)).

[0077] When defibrating cellulose fibers, it is preferable to prepare the cellulose fibers in a slurry form. The solid content 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. When the solid content is within the above range, the cellulose fibers can be defibrated efficiently.

[0078] Defibration of cellulose fibers can be carried out using one or more means selected from homogenizers such as high-pressure homogenizers and high-pressure homogenizers, millstone-type friction machines such as grinders and grinders, refiners such as conical refiners and disk refiners, and various bacteria. However, defibration of cellulose fibers is preferably carried out using an apparatus or method that uses a water flow, particularly a high-pressure water flow, to refine the fibers. This apparatus or method results in extremely uniform dimensions and uniform dispersion of the resulting cellulose fine fibers. In contrast, for example, when using a grinder that grinds the fibers between rotating grindstones, it is difficult to uniformly refine the cellulose fibers, and in some cases, there is a risk that some undisintegrated fiber clumps may remain.

[0079] Grinders used to defibrate cellulose fibers include, for example, the Masscolloider manufactured by Masuko Sangyo Co., Ltd. Devices that use high-pressure water flow to pulverize fibers include, for example, Starburst (registered trademark) manufactured by Sugino Machine Co., Ltd. and Nanovater (registered trademark) manufactured by Yoshida Kikai Kogyo Co., Ltd. High-speed rotary homogenizers used to defibrate cellulose fibers include the Clearmix-11S manufactured by M Technique Co., Ltd.

[0080] As a device for defibrating using a high-pressure water stream, it is preferable to use a high-pressure homogenizer. A high-pressure homogenizer is a homogenizer capable of ejecting a cellulose fiber slurry 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 the cellulose fibers, pressure differences, microcavitation, and the like act to effectively defibrate the cellulose fibers. Therefore, the number of defibration treatments can be reduced, and the production efficiency of cellulose fine fibers can be improved.

[0081] The cellulose fibers are preferably defibrated so that the average fiber width, average fiber length, degree of crystallinity, etc. of the resulting cellulose fine fibers have the desired values ​​or evaluations described above.

[0082] Next, examples of the present invention will be described. Sodium hydroxide was added to a softwood bleached kraft pulp sheet (46% solids) at a molar ratio of 1:1 to citric acid, and water was added at a mass ratio of 1:15 to the pulp solids, followed by mixing for 10 minutes in a mixer. A predetermined amount of citric acid was then added, and the mixture was further mixed in a mixer for 10 minutes. After mixing, the mixture was heated and dried at 130°C for 4 hours in a hot air dryer and reacted. After the reaction, the filtrate was washed until neutral to obtain modified pulp. Water was added to the modified pulp to a concentration of 1 wt %, and sodium hydroxide was added to a predetermined pH, followed by hydrolysis for 1 hour. The hydrolyzed modified pulp was further washed until the filtrate was neutral. The hydrolyzed modified pulp was defibrated to cellulose fine fibers, and various tests were performed. Defibration was performed using a high-pressure homogenizer.

[0083] The amounts of citric acid, sodium hydroxide, and urea added, the heating temperature and time, and the pH during hydrolysis were as shown in Table 1. The physical properties and evaluation of the obtained cellulose fine fibers are shown in Table 2. The methods for evaluating the Brookfield viscosity and light transmittance were as described above.

[0084]

[0085]

[0086] The present invention can be used as modified cellulose fibers and modified cellulose fine fibers.

Claims

1. A modified cellulose fiber which is modified with a polyvalent carboxylic acid group, the ratio of the amount of the first dissociated acid to the amount of the second dissociated acid is 0.25 to 1.0, and the amount of the first dissociated acid is 0.35 mmol / g or more.

2. The modified cellulose fiber according to claim 1, which is carbamate-modified.

3. The modified cellulose fiber according to claim 1, wherein the dispersion (concentration: 0.2% by mass) has a total light transmittance of 65% or more, and the dispersion (concentration: 1.0% by mass) has a B-type viscosity of 40,000 cP or more.

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

5. Modified cellulose fine fibers according to any one of claims 1 to 4, characterized in that the average fiber diameter of the modified cellulose fibers is 100 nm or less.

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