Microfibril cellulose, rubber composition, and crosslinked rubber composition

Abstract

The present invention addresses the problem of providing: a microfibril cellulose which has excellent dispersibility in a rubber composition; and a rubber composition which comprises the microfibril cellulose and a rubber component. The present invention also addresses the problem of providing a crosslinked rubber composition which is obtained by crosslinking the rubber composition.　The microfibril cellulose of the present invention has an average fiber width of 1 μm to 100 μm inclusive and a polymerization degree of 50 to 500 inclusive.

Description

Microfibril cellulose, rubber composition, and crosslinked rubber composition

[0001] The present invention relates to microfibril cellulose, a rubber composition, and a crosslinked rubber composition.

[0002] In recent years, materials using renewable natural fibers have been attracting attention due to the need to replace petroleum resources and growing environmental awareness. Among natural fibers, fibrous cellulose with a fiber diameter of 10 μm to 50 μm, particularly wood-derived fibrous cellulose (pulp), has been widely used mainly for paper products.

[0003] Cited Document 1 discloses a method for producing a rubber composition, the method comprising: (a) treating pulp with an acid; (b) mechanically defibrating the acid-treated pulp to prepare microfibrillated plant fibers having an average fiber length of 1 to 20 μm, an average fiber diameter of 10 μm or less, and an aspect ratio of 2 to 1000; (c) mixing the microfibrillated plant fibers obtained in step (b) with rubber latex to disperse the microfibrillated plant fibers in the rubber latex; and (d) drying the dispersion obtained in step (c), with the aim of providing a rubber composition in which microfibrillated plant fibers are well dispersed in the rubber component and have improved breaking properties.

[0004] JP 2015-025141 A

[0005] Patent Document 1 discloses a method for producing a rubber composition containing microfibrillated plant fibers and a rubber component. However, there is a demand for microfibril cellulose with better dispersibility in rubber compositions. The present invention aims to provide microfibril cellulose with better dispersibility in rubber compositions and a rubber composition containing the microfibril cellulose and a rubber component. Another aim is to provide a crosslinked rubber composition obtained by crosslinking the rubber composition.

[0006] The present inventors have found that the above-mentioned problems can be solved by microfibril cellulose having a specific fiber width and degree of polymerization. The present invention relates to the following items <1> to <8>. <1> Microfibril cellulose having an average fiber width of 1 μm or more and 100 μm or less and a degree of polymerization of 50 or more and 500 or less. <2> Microfibril cellulose according to <1>, which has anionic groups. <3> Microfibril cellulose according to <2>, in which the anionic groups are selected from the group consisting of phosphorus oxoacid groups, sulfur oxoacid groups, and carboxy groups. <4> Microfibril cellulose according to <2> or <3>, in which the content of the anionic groups is 0.50 mmol / g or more and 2.50 mmol / g or less. <5> A rubber composition comprising the microfibril cellulose according to any one of items <1> to <4> and a rubber component. <6> The rubber composition according to <5>, wherein the content of microfibril cellulose per 100 parts by mass of the rubber component is 5 parts by mass or more and 100 parts by mass or less. <7> The rubber composition according to <5> or <6>, wherein, when G'(0.1%) is the storage modulus measured at a frequency of 10 Hz and 70°C at a strain of 0.1%, and G'(650%) is the storage modulus measured at a frequency of 10 Hz and 70°C at a strain of 650%, ΔG', which is expressed by the following formula (1), is 1,800 Pa or less: ΔG'=G'(0.1%)-G'(650%) (1) <8> A crosslinked rubber composition obtained by crosslinking a rubber compound containing the rubber composition according to any one of <5> to <7> and a crosslinking agent.

[0007] According to the present invention, there are provided microfibril cellulose having excellent dispersibility in a rubber composition, a rubber composition containing the microfibril cellulose and a rubber component, and a crosslinked rubber composition obtained by crosslinking the rubber composition.

[0008] 1 is a graph showing the relationship between the amount of NaOH dropped onto a slurry containing fibrous cellulose having a phosphorus oxo acid group and pH. 2 is a graph showing the relationship between the amount of NaOH dropped onto a slurry containing fibrous cellulose having a carboxy group and pH. 3 is a side cross-sectional schematic diagram of a double drum dryer, which is an example of a heated cylindrical dryer. 4 is a side cross-sectional schematic diagram of a double drum dryer, which is an example of a heated cylindrical dryer.

[0009] [Microfibril Cellulose] The microfibril cellulose of this embodiment has an average fiber width of 1 μm or more and 100 μm or less and a degree of polymerization of 50 or more and 500 or less. The microfibril cellulose of this embodiment has excellent dispersibility in a rubber composition containing the microfibril cellulose and a rubber component. Furthermore, by crosslinking a rubber compound containing the rubber composition and a crosslinking agent, a crosslinked rubber composition having excellent elongation at break (hereinafter also referred to as breaking elongation) and high tensile energy can be obtained. Furthermore, in dynamic viscoelasticity measurements, a crosslinked rubber composition having an excellent balance between the loss tangent at 0°C (tan δ (0°C)) and the loss tangent at 60°C (tan δ (60°C)) can be obtained. Although the detailed mechanism by which the above effects are achieved is unknown, carbon black has traditionally been used as a modifier for rubber components. However, crosslinked rubber compositions obtained using rubber compositions containing carbon black have a high tensile strength at break (hereinafter also referred to as breaking strength), but suffer from the problem of reduced breaking elongation (elongation at break). Microfibril cellulose having an average fiber width and degree of polymerization within a specific range has excellent dispersibility in a rubber component, and while maintaining the function of improving breaking strength, the reduced degree of polymerization results in shorter fibers, which is thought to suppress the decrease in breaking elongation. Note that the mechanism by which the effects of the present invention are obtained is not limited to the above. In the following description, a crosslinked rubber composition having excellent physical properties means a crosslinked rubber composition having a high tensile energy, which is the product of breaking elongation and breaking strength, and a high ratio of tanδ(0°C) to tanδ(60°C) (tanδ(0°C) / tanδ(60°C)) in dynamic viscoelasticity measurement.

[0010] Hereinafter, embodiments of the present invention will be described in detail. The following description of the constituent elements may be based on representative embodiments or specific examples, but the present invention is not limited to such embodiments. In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits. When a numerical range is written in stages, the upper and lower limits of each numerical range can be combined in any way.

[0011] The average fiber width of the microfibril cellulose of this embodiment is 1 μm or more and 100 μm or less. From the viewpoint of obtaining a crosslinked rubber composition that has excellent dispersibility in a rubber composition and excellent physical properties, the average fiber width of the microfibril cellulose is preferably 3 μm or more, more preferably 5 μm or more, even more preferably 10 μm or more, and preferably 75 μm or less, more preferably 50 μm or less, even more preferably 40 μm or less, still more preferably 30 μm or less, and even more preferably 20 μm or less. The average fiber width of the microfibril cellulose can be adjusted by the defibration treatment conditions in the defibration treatment step, such as the degree of defibration, the defibration treatment time, the number of defibration treatments, etc. For example, when a disc refiner is used as the defibration treatment device, the degree of defibration can be adjusted by changing the clearance or the number of treatments, and when a homogenizer is used, the degree of defibration can be adjusted by changing the pressure. When the average fiber width of the microfibril cellulose is 1 μm or more, the breaking elongation of a crosslinked rubber composition containing the microfibril cellulose is high, and when the average fiber width is 100 μm or less, the dispersibility in the rubber composition is excellent. The average fiber width of the microfibril cellulose is measured by the method described in the Examples.

[0012] The degree of polymerization of the microfibril cellulose of this embodiment is from 50 to 500. From the viewpoint of obtaining a crosslinked rubber composition that is excellent in dispersibility in a rubber composition and excellent in physical properties, the degree of polymerization of the microfibril cellulose is preferably 100 or more, more preferably 150 or more, even more preferably 180 or more, still more preferably 200 or more, and preferably 450 or less, more preferably 400 or less, even more preferably 350 or less, and still more preferably 320 or less. The degree of polymerization of the microfibril cellulose is adjusted by the presence or absence of a polymerization degree reduction treatment, which will be described later, the treatment conditions, etc.

[0013] The microfibril cellulose of this embodiment preferably has a type I crystal structure. The presence of type I crystal structure in microfibril cellulose can be identified by a diffraction profile obtained from a wide-angle X-ray diffraction photograph using CuKα (λ=1.5418 Å) monochromated with graphite. Specifically, it can be identified by the presence of two typical peaks at two positions: near 2θ=14° to 17° and near 2θ=22° to 23°. The proportion of type I crystal structure in the microfibril cellulose is, for example, preferably 30% or more, more preferably 40% or more, and even more preferably 50% or more. The degree of crystallinity can be determined by measuring the X-ray diffraction profile and using the pattern in a conventional manner (Seagal et al., Textile Research Journal, Vol. 29, p. 786, 1959).

[0014] In this embodiment, the microfibril cellulose has, for example, both crystalline regions and amorphous regions. Microfibril cellulose having both crystalline regions and amorphous regions and having an axial ratio within the above range is realized by the method for producing microfibril cellulose described below.

[0015] In this embodiment, the microfibril cellulose preferably has an ionic substituent. The ionic substituent may include, for example, either or both of an anionic group and a cationic group. In this embodiment, it is particularly preferable that the ionic substituent is an anionic group. Furthermore, the ionic substituent is preferably a group that is introduced into the microfibril cellulose via an ester bond or an ether bond, and more preferably a group that is introduced into the microfibril cellulose via an ester bond. In this case, the ester bond is preferably formed by dehydration condensation between the microfibril cellulose and a compound that becomes the ionic substituent.

[0016] Examples of anionic groups as ionic substituents include phosphorus oxoacid groups or substituents derived from phosphorus oxoacid groups (sometimes simply referred to as phosphorus oxoacid groups), carboxy groups or substituents derived from carboxy groups (sometimes simply referred to as carboxy groups), sulfur oxoacid groups or substituents derived from sulfur oxoacid groups (sometimes simply referred to as sulfur oxoacid groups), xanthate groups or substituents derived from xanthate groups (sometimes simply referred to as xanthate groups), phosphonic groups or substituents derived from phosphonic groups, phosphine groups or substituents derived from phosphine groups, sulfonic groups or substituents derived from sulfonic groups, carboxyalkyl groups, etc. Among these, the anionic group is preferably at least one selected from the group consisting of phosphorus oxoacid groups, substituents derived from phosphorus oxoacid groups, carboxy groups, sulfur oxoacid groups, substituents derived from sulfur oxoacid groups, carboxymethyl groups, carboxyethyl groups, and sulfonic groups, and more preferably at least one selected from the group consisting of phosphorus oxoacid groups, substituents derived from phosphorus oxoacid groups, carboxy groups, sulfur oxoacid groups, and substituents derived from sulfur oxoacid groups. That is, the anionic group is more preferably selected from the group consisting of a phosphorus oxo acid group (a phosphorus oxo acid group or a substituent derived from a phosphorus oxo acid group), a sulfur oxo acid group (a sulfur oxo acid group or a substituent derived from a sulfur oxo acid group), and a carboxy group, and is even more preferably a phosphorus oxo acid group. By introducing a phosphorus oxo acid group as the anionic group, the dispersibility of microfibril cellulose can be further improved, for example, even under alkaline or acidic conditions, and as a result, a crosslinked rubber composition excellent in strength and elongation at break can be more easily obtained. Examples of cationic groups as ionic substituents include ammonium groups, phosphonium groups, and sulfonium groups. Of these, the cationic group is preferably an ammonium group.

[0017] The phosphorus oxo acid group or the substituent derived from the phosphorus oxo acid group is, for example, a substituent represented by the following formula (1). A plurality of substituents represented by the following formula (1) may be introduced into each microfibril cellulose. In this case, the plurality of introduced substituents represented by the following formula (1) may be the same or different.

[0018] In formula (1), a, b, and n are natural numbers, and m is an arbitrary number (where a=b×m). At least one of n α and α′ is O. － and the rest are R or OR. Note that all of α and α' are O － The n α's may all be the same or may be different. ｂ＋ is a cation of one or more valences consisting of organic or inorganic substances.

[0019] R is a hydrogen atom, a saturated linear hydrocarbon group, a saturated branched hydrocarbon group, a saturated cyclic hydrocarbon group, an unsaturated linear hydrocarbon group, an unsaturated branched hydrocarbon group, an unsaturated cyclic hydrocarbon group, an aromatic group, or a group derived therefrom. In formula (1), n ​​is preferably 1.

[0020] Examples of saturated linear hydrocarbon groups include, but are not limited to, methyl, ethyl, n-propyl, or n-butyl groups. Examples of saturated branched hydrocarbon groups include, but are not limited to, i-propyl or t-butyl groups. Examples of saturated cyclic hydrocarbon groups include, but are not limited to, cyclopentyl or cyclohexyl groups. Examples of unsaturated linear hydrocarbon groups include, but are not limited to, vinyl or allyl groups. Examples of unsaturated branched hydrocarbon groups include, but are not limited to, i-propenyl or 3-butenyl groups. Examples of unsaturated cyclic hydrocarbon groups include, but are not limited to, cyclopentenyl or cyclohexenyl groups. Examples of aromatic groups include, but are not limited to, phenyl or naphthyl groups.

[0021] In addition, the derivative group in R may be a carboxy group, a carboxylate group (-COO － ), functional groups to which at least one functional group selected from functional groups such as a hydroxy group, an amino group, and an ammonium group has been added or substituted, but are not particularly limited. The number of carbon atoms constituting the main chain of R is not particularly limited, but is preferably 20 or less, more preferably 10 or less. By setting the number of carbon atoms constituting the main chain of R within the above range, the molecular weight of the phosphorus oxoacid group can be set within an appropriate range, facilitating penetration into the fiber raw material and increasing the yield of microfibril cellulose. When multiple R groups are present in formula (1) or when multiple types of substituents represented by formula (1) are introduced into microfibril cellulose, the multiple R groups may be the same or different.

[0022] β ｂ＋ is a monovalent or higher cation made of an organic or inorganic substance. Examples of the monovalent or higher cation made of an organic substance include organic onium ions. Examples of the organic onium ions include organic ammonium ions and organic onium ions. Examples of the organic ammonium ions include aliphatic ammonium ions and aromatic ammonium ions, and examples of the organic onium ions include aliphatic phosphonium ions and aromatic phosphonium ions. Examples of the monovalent or higher cation made of an inorganic substance include ions of alkali metals such as sodium, potassium, or lithium, ions of divalent metals such as calcium or magnesium, hydrogen ions, ammonium ions, etc. It should be noted that in formula (1), β ｂ＋ When a plurality of β ｂ＋ may be the same or different. The monovalent or higher cations made of organic or inorganic substances include β ｂ＋ Sodium or potassium ions are preferred because they are less likely to yellow when the fiber raw material containing the cation is heated and are easy to use industrially, but there is no particular limitation.

[0023] More specifically, the phosphorus oxo acid group or a substituent derived from a phosphorus oxo acid group is a phosphate group (-PO ３ H ２ ), salts of phosphate groups, phosphite groups (phosphonic acid groups) (-PO ２ H ２ and salts of a phosphorous acid group (phosphonic acid group). The phosphorus oxo acid group or the substituent derived from a phosphorus oxo acid group may be a group in which a phosphoric acid group is condensed (e.g., a pyrophosphate group), a group in which a phosphonic acid is condensed (e.g., a polyphosphonic acid group), a phosphate ester group (e.g., a monomethyl phosphate group, a polyoxyethylene alkyl phosphate group), an alkyl phosphonic acid group (e.g., a methyl phosphonic acid group), or the like.

[0024] The sulfur oxoacid group (a sulfur oxoacid group or a substituent derived from a sulfur oxoacid group) is, for example, a substituent represented by the following formula (2). A plurality of types of substituents represented by the following formula (2) may be introduced into each microfibril cellulose. In this case, the plurality of introduced substituents represented by the following formula (2) may be the same or different.

[0025]

[0026] In formula (2), b and n are natural numbers, p is 0 or 1, and m is an arbitrary number (where 1 = b × m). When n is 2 or more, multiple p's may be the same number or different numbers. In formula (2), β ｂ＋is a monovalent or higher cation composed of an organic or inorganic substance. Examples of the monovalent or higher cation composed of an organic substance include organic onium ions. Examples of the organic onium ions include organic ammonium ions and organic onium ions. Examples of the organic ammonium ions include aliphatic ammonium ions and aromatic ammonium ions, and examples of the organic onium ions include aliphatic phosphonium ions and aromatic phosphonium ions. Examples of the monovalent or higher cation composed of an inorganic substance include ions of alkali metals such as sodium, potassium, or lithium, ions of divalent metals such as calcium or magnesium, hydrogen ions, ammonium ions, etc. Note that when multiple types of substituents represented by formula (2) are introduced into microfibril cellulose, the multiple β ｂ＋ may be the same or different. The monovalent or higher cations made of organic or inorganic substances include β ｂ＋ Sodium or potassium ions are preferred because they are less likely to yellow when the fiber raw material containing the cation is heated and are easy to use industrially, but there is no particular limitation.

[0027] The amount of ionic substituents, preferably anionic groups, introduced into microfibril cellulose is, for example, per 1 g (mass) of microfibril cellulose, preferably 0.10 mmol / g or more and 5.20 mmol / g or less, more preferably 0.20 mmol / g or more, even more preferably 0.40 mmol / g or more, still more preferably 0.50 mmol / g or more, even more preferably 0.60 mmol / g or more, more preferably 3.65 mmol / g or less, even more preferably 3.00 mmol / g or less, still more preferably 2.50 mmol / g or less, and even more preferably 2.00 mmol / g or less. Here, the denominator in the unit mmol / g is calculated based on the number of counter ions of the ionic substituents, i.e., hydrogen ions (H ＋ By setting the amount of ionic substituent introduced within the above range, it is possible to easily refine the fiber raw material and to increase the stability of the microfibril cellulose.

[0028] The amount of ionic substituents introduced into microfibril cellulose can be measured, for example, by neutralization titration after defibrating cellulose fibers. In measurement by neutralization titration, the amount introduced is measured by determining the change in pH while adding an alkali such as an aqueous sodium hydroxide solution to a slurry containing the obtained microfibril cellulose. Note that, since the change in the amount of ionic substituents introduced by defibrating treatment is slight, the amount of ionic substituents introduced into microfibril cellulose can also be determined by measuring the amount of ionic substituents introduced into cellulose fibers before defibrating treatment, into which the ionic substituents have been introduced.

[0029] FIG. 1 is a graph showing the relationship between the amount of NaOH added dropwise to a slurry containing fibrous cellulose having phosphorus oxo acid groups and pH. The amount of phosphorus oxo acid groups introduced into fibrous cellulose is measured, for example, as follows. First, ion-exchanged water is added to the target fibrous cellulose (cellulose fiber) to prepare a slurry with a solids concentration of 0.2% by mass. The fibrous cellulose dispersion is then treated with a strongly acidic ion-exchange resin. Next, the pH change is observed while adding aqueous sodium hydroxide solution, and a titration curve such as that shown in the upper part of FIG. 1 is obtained. The titration curve shown in the upper part of FIG. 1 plots the measured pH against the amount of alkali added, while the titration curve shown in the lower part of FIG. 1 plots the pH increment (differential value) (1 / mmol) against the amount of alkali added. In this neutralization titration, two points at which the increment (differential value of pH with respect to the amount of alkali added) is maximized are confirmed on the curve plotting the measured pH against the amount of alkali added. Of these, the first maximum point of the increment obtained after starting the addition of alkali is called the first endpoint, and the next maximum point of the increment is called the second endpoint. The amount of alkali required from the start of titration to the first endpoint is equal to the amount of first dissociated acid of the fibrous cellulose contained in the slurry used for titration, the amount of alkali required from the first endpoint to the second endpoint is equal to the amount of second dissociated acid of the fibrous cellulose contained in the slurry used for titration, and the amount of alkali required from the start of titration to the second endpoint is equal to the total amount of dissociated acid of the fibrous cellulose contained in the slurry used for titration. The value obtained by dividing the amount of alkali required from the start of titration to the first endpoint by the solids content (g) in the slurry to be titrated is the amount of phosphorus oxo acid group introduced (mmol / g). Note that when simply referring to the amount of phosphorus oxo acid group introduced (or amount of phosphorus oxo acid group), it refers to the amount of first dissociated acid. 1, the region from the start of titration to the first endpoint is referred to as Region 1, and the region from the first endpoint to the second endpoint is referred to as Region 2. For example, when the phosphorus oxoacid group is a phosphate group and this phosphate group undergoes condensation, the amount of weak acid groups in the phosphorus oxoacid group (also referred to herein as the second dissociated acid amount) appears to decrease, and the amount of alkali required in Region 2 becomes smaller than the amount of alkali required in Region 1.On the other hand, the amount of strongly acidic groups in the phosphorus oxoacid group (also referred to herein as the first dissociated acid amount) is equal to the amount of phosphorus atoms, regardless of whether condensation occurs. Furthermore, when the phosphorus oxoacid group is a phosphorous acid group, the phosphorus oxoacid group does not contain any weakly acidic groups, so the amount of alkali required in the second region may be reduced or even zero. In this case, the titration curve will have only one point at which the pH increment is maximized.

[0030] The above-mentioned amount of introduced phosphorus oxoacid groups (mmol / g) indicates the amount of phosphorus oxoacid groups in the acid-form fibrous cellulose (hereinafter referred to as the amount of phosphorus oxoacid groups (acid form)), since the denominator indicates the mass of the acid-form fibrous cellulose. On the other hand, when the counter ions of the phosphorus oxoacid groups are substituted with an arbitrary cation C so as to be the charge equivalent, the amount of phosphorus oxoacid groups in the cellulose with the cation C as the counter ion (hereinafter referred to as the amount of phosphorus oxoacid groups (C form)) can be determined by converting the denominator to the mass of the fibrous cellulose when the cation C is the counter ion. That is, it is calculated using the following formula: Amount of phosphorus oxoacid groups (C type) = Amount of phosphorus oxoacid groups (acid type) / {1 + (W - 1) × P / 1000}, P [mmol / g]: total amount of anions derived from phosphorus oxoacid groups in the fibrous cellulose (total amount of dissociated acid of phosphorus oxoacid groups), W: formula weight per monovalent of cation C (for example, 23 for Na and 9 for Al).

[0031] FIG. 2 is a graph showing the relationship between the amount of NaOH added dropwise to a fibrous cellulose dispersion having carboxy groups as ionic substituents and pH. The amount of carboxy groups introduced into the fibrous cellulose is measured, for example, as follows: First, ion-exchanged water is added to the target fibrous cellulose (cellulose fiber) to prepare a slurry with a solids concentration of 0.2% by mass. The fibrous cellulose dispersion is then treated with a strongly acidic ion-exchange resin. Next, the pH change is observed while adding aqueous sodium hydroxide solution, and a titration curve such as that shown in the upper part of FIG. 2 is obtained. The titration curve shown in the upper part of FIG. 2 plots the measured pH against the amount of alkali added, while the titration curve shown in the lower part of FIG. 2 plots the pH increment (differential value) (1 / mmol) against the amount of alkali added. In this neutralization titration, a single point where the increment (differential value of pH with respect to the amount of alkali added) is maximized is identified on the curve plotting the measured pH against the amount of alkali added. This maximum point is referred to as the first endpoint. Here, the region from the start of titration to the first endpoint in Figure 2 is referred to as the first region. The amount of alkali required in the first region is equal to the amount of carboxy groups in the dispersion used for titration. The amount of carboxy groups introduced (mmol / g) is calculated by dividing the amount of alkali (mmol) required in the first region of the titration curve by the solid content (g) in the dispersion containing the fibrous cellulose to be titrated.

[0032] The above-mentioned amount of carboxy groups introduced (mmol / g) indicates the amount of carboxy groups possessed by the acid-form fibrous cellulose (hereinafter referred to as the amount of carboxy groups (acid form)), since the denominator is the mass of the acid-form fibrous cellulose. On the other hand, when the counter ions of the carboxy groups are substituted with any cation C so as to be charge equivalent, the amount of carboxy groups possessed by the fibrous cellulose in which the cation C is the counter ion (hereinafter referred to as the amount of carboxy groups (C form)) can be determined by converting the denominator to the mass of the fibrous cellulose when the cation C is the counter ion. That is, it is calculated using the following formula: Amount of carboxy groups (C form) = Amount of carboxy groups (acid form) / {1 + (W - 1) × (Amount of carboxy groups (acid form)) / 1000} W: Formula weight per valence of cation C (for example, 23 for Na, 9 for Al)

[0033] When measuring the amount of ionic substituents by titration, if too much sodium hydroxide aqueous solution is added or if the titration interval is too short, the amount of ionic substituents may be lower than expected, and an accurate value may not be obtained. An appropriate amount of addition and titration interval, for example, is preferably a 0.1 N sodium hydroxide aqueous solution titrated at 10 to 50 μL intervals over 5 to 30 seconds. Furthermore, to eliminate the influence of carbon dioxide dissolved in the fibrous cellulose dispersion, it is preferable to perform the measurement while blowing an inert gas such as nitrogen gas into the slurry, for example, from 15 minutes before the start of titration until the end of titration.

[0034] The amount of sulfate ester groups and sulfonic acid groups introduced into microfibril cellulose is determined by wet ashing the obtained microfibril cellulose using perchloric acid and concentrated nitric acid, diluting the microfibril cellulose at an appropriate ratio, and measuring the amount of sulfur by ICP atomic emission spectrometry. The amount of sulfur is divided by the bone dry mass of the microfibril cellulose tested, and this value is taken as the amount of sulfur oxoacid groups and sulfonic acid groups (unit: mmol / g).

[0035] In order to obtain microfibril cellulose having an ionic substituent introduced thereinto, it is preferable to have an ionic substituent introduction step for introducing an ionic substituent into a fiber raw material containing cellulose, a washing step, an alkali treatment step (neutralization step), and a defibration treatment step in this order, and an acid treatment step may be carried out instead of or in addition to the washing step. ４ ＋ ), the counter ion may be NH ４ ＋ Examples of the ionic substituent introduction step include a phosphorus oxo acid group introduction step, a carboxy group introduction step, a sulfur oxo acid group introduction step, a xanthate group introduction step, a phosphonic or phosphine group introduction step, a sulfonic group introduction step, and a cationic group introduction step. Each of these steps will be described below.

[0036] <Fiber Raw Materials> The fiber raw materials are fiber raw materials containing cellulose. Examples of the fiber raw materials include, but are not limited to, wood pulp, non-wood pulp, and deinked pulp. Examples of wood pulp include, but are not limited to, chemical pulps such as hardwood kraft pulp (LBKP), softwood kraft pulp (NBKP), sulfite pulp (SP), dissolving pulp (DP), soda pulp (AP), unbleached kraft pulp (UKP), and oxygen bleached kraft pulp (OKP), semi-chemical pulps such as semi-chemical pulp (SCP) and chemi-ground wood pulp (CGP), and mechanical pulps such as groundwood pulp (GP) and thermomechanical pulp (TMP, BCTMP). Examples of non-wood pulp include, but are not limited to, cotton-based pulps such as cotton linters and cotton lint, and non-wood pulps such as hemp, straw, bamboo, and bagasse. The deinked pulp is not particularly limited, but examples thereof include deinked pulp made from waste paper. The pulp raw material of this embodiment may be one of the above, or a mixture of two or more. Among the above pulps, wood pulp and deinked pulp are preferred from the viewpoint of ease of availability. Furthermore, among wood pulps, chemical pulp is more preferred, and kraft pulp and sulfite pulp are even more preferred, from the viewpoints of a high cellulose ratio, a high yield of microfibril cellulose during defibration treatment, and the fact that decomposition of cellulose in the pulp is small and long-fiber microfibril cellulose with a large axial ratio can be obtained. Note that the viscosity tends to increase when long-fiber microfibril cellulose with a large axial ratio is used.

[0037] <Phosphorus Oxo Acid Group Introduction Step> When obtaining cellulose fibers having ionic substituents, it is preferable to provide an ionic substituent introduction step before the defibration treatment step. Examples of the ionic substituent introduction step include a phosphorus oxo acid group introduction step. The phosphorus oxo acid group introduction step is a step of reacting a cellulose-containing fiber raw material with at least one compound (hereinafter also referred to as "compound A") selected from compounds capable of introducing phosphorus oxo acid groups by reacting with hydroxyl groups possessed by the cellulose-containing fiber raw material. This step results in the production of cellulose fibers having phosphorus oxo acid groups.

[0038] In the phosphorus oxo acid group introduction step according to this embodiment, the reaction of the cellulose-containing fiber raw material with compound A may be carried out in the presence of at least one selected from urea and its derivatives (hereinafter also referred to as "compound B"). Alternatively, the cellulose-containing fiber raw material may be reacted with compound A in the absence of compound B.

[0039] An example of a method for reacting compound A with a fiber raw material in the presence of compound B is a method in which compound A and compound B are mixed with a fiber raw material in a dry, wet, or slurry state. Among these, using a fiber raw material in a dry or wet state is preferred because of the high uniformity of the reaction, and using a fiber raw material in a dry state is particularly preferred. The form of the fiber raw material is not particularly limited, but is preferably, for example, in a cotton-like or thin sheet form. Examples include methods in which compound A and compound B are added to the fiber raw material in the form of a powder, a solution dissolved in a solvent, or a melted state obtained by heating to or above their melting point. Among these, adding compound A and compound B in the form of a solution dissolved in a solvent, particularly an aqueous solution, is preferred because of the high uniformity of the reaction. Compound A and compound B may be added to the fiber raw material simultaneously, separately, or as a mixture. The method for adding compound A and compound B is not particularly limited. When compound A and compound B are in the form of a solution, the fiber raw material may be immersed in the solution to absorb the liquid and then removed, or the solution may be added dropwise to the fiber raw material. Alternatively, the required amounts of compound A and compound B may be added to the fiber raw material, or excess amounts of compound A and compound B may be added to the fiber raw material, and then the excess compound A and compound B may be removed by squeezing or filtration.

[0040] The compound A used in this embodiment may be any compound that has a phosphorus atom and can form an ester bond with cellulose, including, but not limited to, phosphoric acid or a salt thereof, phosphorous acid or a salt thereof, dehydrated condensed phosphoric acid or a salt thereof, and phosphoric anhydride (diphosphorus pentoxide). Phosphoric acid can be used with various purities, such as 100% phosphoric acid (orthophosphoric acid) or 85% phosphoric acid. Phosphorous acid can be 99% phosphorous acid (phosphonic acid). Dehydrated condensed phosphoric acid is formed by the condensation of two or more molecules of phosphoric acid through a dehydration reaction, and examples thereof include pyrophosphoric acid and polyphosphoric acid. Phosphates, phosphites, and dehydrated condensed phosphates include lithium salts, sodium salts, potassium salts, and ammonium salts of phosphoric acid, phosphorous acid, or dehydrated condensed phosphoric acid, which can be neutralized to various degrees. Among these, from the viewpoints of high efficiency of introduction of phosphate groups, ease of further improving defibration efficiency in the defibration treatment step described below, low cost, and ease of industrial application, phosphoric acid, sodium salt of phosphoric acid, potassium salt of phosphoric acid, ammonium salt of phosphoric acid, or phosphorous acid, sodium salt of phosphorous acid, potassium salt of phosphorous acid, ammonium salt of phosphorous acid are preferred, and phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, or phosphorous acid, sodium phosphite are more preferred.

[0041] The amount of compound A added to the fiber raw material is not particularly limited, but for example, when the amount of compound A added is converted into the amount of phosphorus atoms, the amount of phosphorus atoms added to the fiber raw material (bone dry mass) is preferably 0.5% by mass or more and 100% by mass or less, more preferably 1% by mass or more and 50% by mass or less, and even more preferably 2% by mass or more and 30% by mass or less. By setting the amount of phosphorus atoms added to the fiber raw material within the above range, the yield of microfibril cellulose can be further improved. On the other hand, by setting the amount of phosphorus atoms added to the fiber raw material to the above upper limit or less, a balance can be achieved between the yield improvement effect and costs.

[0042] As described above, compound B used in this embodiment is at least one selected from urea and its derivatives. Examples of compound B include urea, biuret, 1-phenylurea, 1-benzylurea, 1-methylurea, and 1-ethylurea. From the viewpoint of improving the uniformity of the reaction, compound B is preferably used as an aqueous solution. Furthermore, from the viewpoint of further improving the uniformity of the reaction, it is preferable to use an aqueous solution in which both compound A and compound B are dissolved.

[0043] The amount of compound B added relative to the fiber raw material (bone dry mass) is not particularly limited, but is, for example, preferably 1 mass% or more and 500 mass% or less, more preferably 10 mass% or more and 400 mass% or less, and even more preferably 100 mass% or more and 350 mass% or less.

[0044] In the reaction of a fiber raw material containing cellulose with compound A, the reaction system may contain, in addition to compound B, for example, amides or amines. Examples of amides include formamide, dimethylformamide, acetamide, and dimethylacetamide. Examples of amines include methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, and hexamethylenediamine. Among these, triethylamine is known to function as a particularly good reaction catalyst.

[0045] In the phosphorus oxo acid group introduction step, it is preferable to add or mix compound A or the like to or with the fiber raw material, and then heat-treat the fiber raw material. The heat treatment temperature is preferably selected so that the phosphorus oxo acid group can be efficiently introduced while suppressing thermal decomposition and hydrolysis of the fiber. The heat treatment temperature is, for example, preferably 50°C or higher and 300°C or lower, more preferably 100°C or higher and 250°C or lower, and even more preferably 130°C or higher and 200°C or lower. Various types of equipment having heat transfer media can be used for the heat treatment, including, for example, a stirring dryer, a rotary dryer, a disk dryer, a roll-type heater, a plate-type heater, a fluidized-bed dryer, a band-type dryer, a filtration dryer, a vibration fluidized-bed dryer, a flash dryer, a reduced-pressure dryer, a hot-air dryer, an infrared heater, a far-infrared heater, a microwave heater, and a high-frequency dryer.

[0046] In this embodiment, the heat treatment can be carried out by, for example, adding compound A to a thin sheet-like fiber raw material by a method such as impregnation, followed by heating, or by heating while kneading or stirring the fiber raw material and compound A in a kneader or the like. This can suppress unevenness in the concentration of compound A in the fiber raw material, making it possible to more uniformly introduce phosphorus oxoacid groups onto the surface of the cellulose fibers contained in the fiber raw material. This is thought to be due to the fact that, when water molecules move to the surface of the fiber raw material as it is dried, dissolved compound A can be prevented from being attracted to the water molecules by surface tension and similarly moving to the surface of the fiber raw material (i.e., causing unevenness in the concentration of compound A).

[0047] Furthermore, the heating device used for the heat treatment is preferably one that can constantly discharge, to the outside of the device system, for example, the moisture retained in the slurry and the moisture generated in the dehydration condensation (phosphorylation) reaction between compound A and hydroxyl groups contained in cellulose or the like in the fiber raw material. Examples of such heating devices include an oven using a blower system. Constantly discharging moisture from the device system can suppress the hydrolysis reaction of phosphate ester bonds, which is the reverse reaction of phosphate esterification, and also suppress acid hydrolysis of sugar chains in the fibers. This makes it possible to obtain microfibril cellulose with a high axial ratio.

[0048] The heat treatment time is, for example, from when moisture is substantially removed from the fiber raw material, preferably from 1 second to 300 minutes, more preferably from 1 second to 1,000 seconds, and even more preferably from 10 seconds to 800 seconds. In this embodiment, by setting the heating temperature and heating time within appropriate ranges, the amount of phosphorus oxo acid groups introduced can be kept within a preferred range.

[0049] The phosphorus oxo acid group introduction step may be carried out at least once, but may also be carried out twice or more times. By carrying out the phosphorus oxo acid group introduction step twice or more, a large number of phosphorus oxo acid groups can be introduced into the fiber raw material.

[0050] The amount of phosphorus oxoacid groups introduced into the fiber raw material is, for example, preferably 0.10 mmol / g or more and 5.20 mmol / g or less, more preferably 0.20 mmol / g or more, even more preferably 0.50 mmol / g or more, still more preferably 0.60 mmol / g or more, and more preferably 3.65 mmol / g or less, even more preferably 3.00 mmol / g or less, still more preferably 2.50 mmol / g or less, and even more preferably 2.00 mmol / g or less, per gram (mass) of cellulose fiber. By keeping the amount of phosphorus oxoacid groups introduced within the above ranges, it is possible to facilitate the fine pulverization of cellulose fibers in the defibration treatment step and to increase the stability of microfibril cellulose.

[0051] <Carboxy group introduction step> The ionic substituent introduction step may include a carboxy group introduction step, which is carried out by subjecting a fiber raw material containing cellulose to an oxidation treatment such as ozone oxidation, oxidation by the Fenton method, or TEMPO oxidation treatment, or by treating the fiber raw material with a compound having a carboxylic acid-derived group or a derivative thereof, or an acid anhydride of a compound having a carboxylic acid-derived group or a derivative thereof.

[0052] The compound having a group derived from carboxylic acid is not particularly limited, but examples thereof include dicarboxylic acid compounds such as maleic acid, succinic acid, phthalic acid, fumaric acid, glutaric acid, adipic acid, and itaconic acid, and tricarboxylic acid compounds such as citric acid and aconitic acid. Furthermore, the derivative of the compound having a group derived from carboxylic acid is not particularly limited, but examples thereof include imidized products of acid anhydrides of compounds having carboxy groups, and derivatives of acid anhydrides of compounds having carboxy groups. The imidized products of acid anhydrides of compounds having carboxy groups are not particularly limited, but examples thereof include imidized products of dicarboxylic acid compounds such as maleimide, succinimide, and phthalimide.

[0053] The acid anhydride of a compound having a group derived from carboxylic acid is not particularly limited, and examples thereof include acid anhydrides of dicarboxylic acid compounds such as maleic anhydride, succinic anhydride, phthalic anhydride, glutaric anhydride, adipic anhydride, itaconic anhydride, etc. Furthermore, the derivative of an acid anhydride of a compound having a group derived from carboxylic acid is not particularly limited, and examples thereof include acid anhydrides of compounds having carboxy groups such as dimethyl maleic anhydride, diethyl maleic anhydride, diphenyl maleic anhydride, etc., in which at least some of the hydrogen atoms have been substituted with a substituent such as an alkyl group or a phenyl group.

[0054] When TEMPO oxidation treatment is performed in the carboxyl group introduction step, it is preferable to perform the treatment under conditions of, for example, a pH of 6 or higher and 8 or lower. This type of treatment is also referred to as neutral TEMPO oxidation treatment. Neutral TEMPO oxidation treatment can be performed, for example, by adding pulp as the fiber raw material, a nitroxy radical such as TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) as a catalyst, and sodium hypochlorite as a sacrificial reagent to a sodium phosphate buffer solution (pH = 6.8). Furthermore, by adding sodium chlorite, aldehydes generated during the oxidation process can be efficiently oxidized to carboxy groups. The TEMPO oxidation treatment may also be performed under conditions of a pH of 10 or higher and 11 or lower. This type of treatment is also referred to as alkaline TEMPO oxidation treatment. The alkaline TEMPO oxidation treatment can be performed, for example, by adding a nitroxy radical such as TEMPO as a catalyst, sodium bromide as a co-catalyst, and sodium hypochlorite as an oxidizing agent to pulp as the fiber raw material.

[0055] The amount of carboxy groups introduced into cellulose fibers varies depending on the type of substituent. For example, when carboxy groups are introduced by TEMPO oxidation, the amount per gram (mass) of cellulose fibers is preferably 0.10 mmol / g or more and 3.65 mmol / g or less, more preferably 0.20 mmol / g or more, even more preferably 0.50 mmol / g or more, still more preferably 0.60 mmol / g or more, and more preferably 3.00 mmol / g or less, even more preferably 2.50 mmol / g or less, and still more preferably 2.00 mmol / g or less. Alternatively, when the substituent is a carboxymethyl group, the amount of carboxy groups introduced may be 5.8 mmol / g or less per gram (mass) of cellulose fibers. By setting the amount of carboxy groups introduced within the above range, it is possible to facilitate the fine pulverization of cellulose fibers in the defibration treatment step and to enhance the stability of microfibrillated cellulose.

[0056] <Sulfonic acid group introduction step> The ionic substituent introduction step may include a sulfonic acid group introduction step, in which a hydroxyl group in a fiber raw material containing cellulose reacts with a sulfur oxoacid to obtain a cellulose fiber having a sulfonic acid group (a sulfonic acid group-introduced fiber).

[0057] In the sulfonic acid group introduction step, instead of compound A in the above-described <Phosphorus Oxo Acid Group Introduction Step>, at least one compound (hereinafter also referred to as "compound C") selected from compounds capable of introducing sulfonic acid groups by reacting with hydroxyl groups in cellulose-containing fiber raw materials is used. Compound C may be any compound having a sulfur atom and capable of forming an ester bond with cellulose, including, but not limited to, sulfuric acid or its salts, sulfurous acid or its salts, and sulfuric acid amides. Sulfuric acid of various purities can be used, for example, 96% sulfuric acid (concentrated sulfuric acid). Examples of sulfurous acid include 5% aqueous sulfurous acid. Examples of sulfates or sulfites include lithium, sodium, potassium, and ammonium salts of sulfates or sulfites, which can be neutralized to various degrees. Examples of sulfuric acid amides include sulfamic acid. In the sulfonic acid group introduction step, it is preferable to use compound B in the above-described <Phosphorus Oxo Acid Group Introduction Step> in the same manner.

[0058] In the sulfonic acid introduction step, the cellulose raw material is preferably mixed with an aqueous solution containing a sulfur oxoacid and urea and / or a urea derivative, and then the cellulose raw material is subjected to a heat treatment. The heat treatment temperature is preferably selected so that sulfonic acid groups can be efficiently introduced while suppressing thermal decomposition and hydrolysis of the fiber. The heat treatment temperature is preferably 100°C or higher and 300°C or lower, more preferably 120°C or higher, even more preferably 150°C or higher, and more preferably 250°C or lower, even more preferably 200°C or lower.

[0059] In the heat treatment step, heating is preferably performed until substantially no moisture is present. Therefore, the heat treatment time varies depending on the amount of moisture contained in the cellulose raw material and the amount of aqueous solution containing sulfur oxoacid and urea and / or a urea derivative added, but is preferably 10 seconds to 10,000 seconds. For the heat treatment, various devices having a heat medium can be used, such as a hot air dryer, a stirring dryer, a rotary dryer, a disk dryer, a roll-type heater, a plate-type heater, a fluidized bed dryer, a band-type dryer, a filtration dryer, a vibration fluidized dryer, a flash dryer, a reduced-pressure dryer, an infrared heater, a far-infrared heater, a microwave heater, or a high-frequency dryer.

[0060] The amount of sulfonic groups introduced into the cellulose raw material is preferably 0.05 mmol / g or more and 5.00 mmol / g or less, more preferably 0.10 mmol / g or more, even more preferably 0.20 mmol / g or more, still more preferably 0.40 mmol / g or more, even more preferably 0.50 mmol / g or more, and more preferably 3.00 mmol / g or less, and even more preferably 2.50 mmol / g or less. By setting the amount of sulfonic groups introduced within the above range, it is possible to facilitate the fine pulverization of cellulose fibers in the defibration treatment step and to increase the stability of microfibril cellulose.

[0061] <Oxidation step using a chlorine-based oxidizing agent (second carboxyl group introduction step)> The ionic substituent introduction step may include an oxidation step using a chlorine-based oxidizing agent. In the oxidation step using a chlorine-based oxidizing agent, a chlorine-based oxidizing agent is added to a wet or dry fiber raw material having a hydroxyl group to cause a reaction, thereby introducing a carboxyl group into the fiber raw material.

[0062] Examples of chlorine-based oxidizing agents include hypochlorous acid, hypochlorites, chlorous acid, chlorites, chloric acid, chlorates, perchloric acid, perchlorates, and chlorine dioxide. From the viewpoints of the efficiency of introducing substituents, and therefore the defibration efficiency, cost, and ease of handling, the chlorine-based oxidizing agent is preferably sodium hypochlorite, sodium chlorite, or chlorine dioxide. When adding a chlorine-based oxidizing agent, it may be added to the fiber raw material as a reagent (solid or liquid) as is, or may be dissolved in an appropriate solvent and added.

[0063] The concentration of the chlorine-based oxidizing agent in the solution in the oxidation step using the chlorine-based oxidizing agent is, for example, converted into an effective chlorine concentration, preferably from 1 to 1,000% by mass, more preferably from 5 to 500% by mass, and even more preferably from 10 to 100% by mass. The amount of the chlorine-based oxidizing agent added per 100 parts by mass of the fiber raw material is preferably from 1 to 100,000 parts by mass, more preferably from 10 to 10,000 parts by mass, and even more preferably from 100 to 5,000 parts by mass.

[0064] The reaction time with the chlorine-based oxidizing agent in the oxidation step using the chlorine-based oxidizing agent may vary depending on the reaction temperature, but is, for example, preferably 1 minute to 1,000 minutes, more preferably 10 minutes to 500 minutes, and even more preferably 20 minutes to 400 minutes. The pH during the reaction is preferably 5 to 15, more preferably 7 to 14, and even more preferably 9 to 13. At the start of the reaction, the pH during the reaction is preferably maintained constant (for example, pH 11) by appropriately adding hydrochloric acid or sodium hydroxide. After the reaction, excess reaction reagents, by-products, etc. may be washed and removed with water by filtration or the like.

[0065] <Xanthate Group Introduction Step> The microfibril cellulose production process may include a xanthate group introduction step as an ionic substituent introduction step. In the xanthate group introduction step, hydroxyl groups in a fiber raw material containing cellulose are substituted with xanthate groups represented by the following formula (3), thereby obtaining cellulose fibers having xanthate groups (xanthate group-introduced fibers). -OCSS － M ＋ ... (3) Here, M＋ is at least one selected from the group consisting of hydrogen ions, monovalent metal ions, ammonium ions, and aliphatic or aromatic ammonium ions.

[0066] In the xanthate group introduction process, first, the cellulose-containing fiber raw material is treated with an alkaline solution to obtain alkali cellulose. Examples of alkaline solutions include an aqueous alkali metal hydroxide solution and an aqueous alkaline earth metal hydroxide solution. Among these, the alkaline solution is preferably an aqueous alkali metal hydroxide solution such as sodium hydroxide or potassium hydroxide, and more preferably an aqueous sodium hydroxide solution. When the alkaline solution is an aqueous alkali metal hydroxide solution, the alkali metal hydroxide concentration in the aqueous alkali metal hydroxide solution is preferably 4% by mass or more to 9% by mass or less, more preferably 5% by mass or more. By setting the alkali metal hydroxide concentration at or above the lower limit, the cellulose mercerization can be sufficiently promoted, the amount of by-products generated during the subsequent xanthation can be reduced, and as a result, the yield of xanthate group-introduced fiber can be increased. This allows the defibration process described below to be performed more effectively. Furthermore, by setting the alkali metal hydroxide concentration to the above upper limit or less, it is possible to prevent the aqueous alkali metal hydroxide solution from penetrating into the crystalline regions of cellulose while allowing mercerization to proceed, which makes it easier to maintain the cellulose type I crystal structure and further increases the yield of microfibril cellulose.

[0067] The alkali treatment time is preferably 30 minutes to 6 hours, more preferably 1 hour to 5 hours. By setting the alkali treatment time within the above range, the final yield can be increased, and productivity can be improved.

[0068] The alkali cellulose obtained by the above-mentioned alkali treatment is preferably subjected to solid-liquid separation afterwards to remove as much aqueous solution as possible.This can reduce the water content during the subsequent xanthate treatment, and can promote the reaction.As a method of solid-liquid separation, for example, a general dehydration method such as centrifugation or filtration can be used.In addition, the concentration of alkali metal hydroxide contained in the alkali cellulose after solid-liquid separation is preferably 3% by mass or more and 8% by mass or less with respect to the total mass of the alkali cellulose after solid-liquid separation.

[0069] In the xanthate group introduction step, a xanthate formation treatment step is carried out after alkali treatment. In the xanthate formation treatment step, carbon disulfide (CS) is added to alkali cellulose. ２ ) is reacted to form (-O － Na ＋ ) group to (-OCSS － Na ＋ ) group to obtain xanthate group-introduced fibers. In the above, the metal ions introduced into the alkali cellulose are typically Na ＋ However, similar reactions occur with other alkali metal ions.

[0070] In the xanthation treatment, it is preferable to supply 10% by mass or more of carbon disulfide relative to the bone dry mass of cellulose in the alkali cellulose.In addition, in the xanthation treatment, the time for which carbon disulfide and alkali cellulose are in contact is preferably 30 minutes or more, more preferably 1 hour or more.Although the xanthation proceeds quickly when carbon disulfide contacts alkali cellulose, it takes time for carbon disulfide to penetrate into the inside of the alkali cellulose, so it is preferable to set the reaction time within the above range.On the other hand, the time for which carbon disulfide and alkali cellulose are in contact can be 6 hours or less, which allows sufficient penetration into the alkali cellulose mass after dehydration, and can almost complete the reactive xanthation.

[0071] The reaction temperature in the xanthate treatment is preferably 46 ° C or less. By making the reaction temperature within the above range, it is easy to suppress the decomposition of alkali cellulose. In addition, by making the reaction temperature within the above range, it is easy to react uniformly, so it can suppress the generation of by-products, and furthermore, it can also suppress the removal of the generated xanthate group.

[0072] The amount of xanthate group introduced in the xanthate group introduction step is preferably 0.50 mmol / g or more and 5.00 mmol / g or less per 1 g (mass) of fiber raw material, more preferably 0.70 mmol / g or more, even more preferably 0.80 mmol / g or more, even more preferably 1.00 mmol / g or more, even more preferably 1.20 mmol / g or more, and more preferably 3.00 mmol / g or less, even more preferably 2.50 mmol / g or less. By setting the amount of xanthate group introduction within the above range, it is possible to facilitate the micronization of the fiber raw material and improve the stability of the microfibril cellulose.

[0073] <Phosphonic or Phosphine Group Introduction Step (Phosphoalkylation Step)> The ionic substituent introduction step may include a phosphonic or phosphine group introduction step (phosphoalkylation step). In the phosphoalkylation step, a compound having a reactive group and a phospho group or a phosphine group (compound E) is used as an essential component. Ａ ), and an optional component, an alkali compound, and a compound B selected from the above-mentioned urea and its derivatives are added to a wet or dry fiber raw material having a hydroxyl group, and the reaction is carried out to introduce a phosphonic group or a phosphine group into the fiber raw material.

[0074] Examples of the reactive group include a halogenated alkyl group, a vinyl group, and an epoxy group (glycidyl group). Ａ Examples of the compound E include vinylphosphonic acid, phenylvinylphosphonic acid, and phenylvinylphosphinic acid. In terms of the efficiency of introducing substituents, and therefore the defibration efficiency, cost, and ease of handling, compound E Ａis preferably vinylphosphonic acid. Furthermore, as an optional component, it is also preferable to use the compound B in the above-mentioned <Phosphorus oxo acid group introduction step> in the same manner, and the amount added is also preferably as described above.

[0075] Compound E Ａ When adding, the reagent (solid or liquid) may be added directly to the fiber raw material, or may be dissolved in an appropriate solvent and added. The fiber raw material is preferably converted into alkali cellulose in advance or simultaneously with the reaction. The method for converting into alkali cellulose is as described above.

[0076] The reaction temperature is, for example, preferably 50°C or higher and 300°C or lower, more preferably 100°C or higher and 250°C or lower, and even more preferably 130°C or higher and 200°C or lower.

[0077] Compound E Ａ The amount of the additive per 100 parts by mass of the fiber raw material is preferably 1 part by mass or more and 100,000 parts by mass or less, more preferably 2 parts by mass or more and 10,000 parts by mass or less, and even more preferably 5 parts by mass or more and 1,000 parts by mass or less.

[0078] The reaction time may vary depending on the reaction temperature, but is, for example, preferably from 1 minute to 1,000 minutes, more preferably from 10 minutes to 500 minutes, and even more preferably from 20 minutes to 400 minutes. After the reaction, excess reaction reagents, by-products, etc. may be washed and removed with water by filtration or the like.

[0079] <Sulfone Group Introduction Step (Sulfoalkylation Step) (Second Sulfone Group Introduction Step)> The ionic substituent introduction step may include a sulfone group introduction step (sulfoalkylation step). In the sulfoalkylation, a compound having a reactive group and a sulfone group (compound E) is used as an essential component. Ｂ ) and, as an optional component, an alkali compound and a compound B selected from the above-mentioned urea and its derivatives are added to a wet or dry fiber raw material having a hydroxyl group and reacted to introduce a sulfonic acid group into the fiber raw material.

[0080] Examples of the reactive group include a halogenated alkyl group, a vinyl group, and an epoxy group (glycidyl group). Ｂ Examples of suitable acrylic acid esters include sodium 2-chloroethanesulfonate, sodium vinylsulfonate, sodium p-styrenesulfonate, and 2-acrylamido-2-methylpropanesulfonic acid. Among these, compound E is particularly preferred in terms of the efficiency of introducing substituents, and therefore the defibration efficiency, cost, and ease of handling. Ｂ is preferably sodium vinyl sulfonate. Furthermore, as an optional component, it is also preferable to use the compound B in the above-mentioned <Phosphorus oxo acid group introduction step> in the same manner, and the amount added is also preferably as described above.

[0081] Compound E Ｂ When adding, the reagent (solid or liquid) may be added directly to the fiber raw material, or may be dissolved in an appropriate solvent and added. The fiber raw material is preferably converted into alkali cellulose in advance or simultaneously with the reaction. The method for converting into alkali cellulose is as described above.

[0082] The reaction temperature is, for example, preferably 50°C or higher and 300°C or lower, more preferably 100°C or higher and 250°C or lower, and even more preferably 130°C or higher and 200°C or lower.

[0083] Compound E Ｂ The amount of the additive per 100 parts by mass of the fiber raw material is preferably 1 part by mass or more and 100,000 parts by mass or less, more preferably 2 parts by mass or more and 10,000 parts by mass or less, and even more preferably 5 parts by mass or more and 1,000 parts by mass or less.

[0084] The reaction time may vary depending on the reaction temperature, but is, for example, preferably from 1 minute to 1,000 minutes, more preferably from 10 minutes to 500 minutes, and even more preferably from 15 minutes to 400 minutes. After the reaction, excess reaction reagents, by-products, etc. may be washed and removed with water by filtration or the like.

[0085] <Carboxyalkylation Step (Third Carboxy Group Introduction Step)> The ionic substituent introduction step may include a carboxyalkylation step. As an essential component, a compound having a reactive group and a carboxy group (compound E Ｃ ), and an optional component, an alkaline compound, and a compound B selected from the above-mentioned urea and its derivatives are added to a wet or dry fiber raw material having a hydroxyl group and reacted to introduce a carboxyl group into the fiber raw material.

[0086] Examples of the reactive group include a halogenated alkyl group, a vinyl group, and an epoxy group (glycidyl group). Ｃ As the compound, monochloroacetic acid, sodium monochloroacetate, 2-chloropropionic acid, 3-chloropropionic acid, sodium 2-chloropropionate, and sodium 3-chloropropionate are preferred from the viewpoints of the efficiency of introducing the substituent, and therefore the defibration efficiency, cost, and ease of handling. Furthermore, it is also preferred to use, as an optional component, the compound B in the above-mentioned <Phosphorus oxoacid group introduction step> in the same manner, and the amount added is also preferably as described above.

[0087] Compound E Ｃ When adding, the reagent (solid or liquid) may be added directly to the fiber raw material, or may be dissolved in an appropriate solvent and added. The fiber raw material is preferably converted into alkali cellulose in advance or simultaneously with the reaction. The method for converting into alkali cellulose is as described above.

[0088] The reaction temperature is, for example, preferably 50°C or higher and 300°C or lower, more preferably 100°C or higher and 250°C or lower, and even more preferably 130°C or higher and 200°C or lower.

[0089] Compound E Ｃ The amount of the additive per 100 parts by mass of the fiber raw material is preferably 1 part by mass or more and 100,000 parts by mass or less, more preferably 2 parts by mass or more and 10,000 parts by mass or less, and even more preferably 5 parts by mass or more and 1,000 parts by mass or less.

[0090] The reaction time may vary depending on the reaction temperature, but is, for example, preferably from 1 minute to 1,000 minutes, more preferably from 3 minutes to 500 minutes, and even more preferably from 5 minutes to 400 minutes. After the reaction, excess reaction reagents, by-products, etc. may be washed and removed with water by filtration or the like.

[0091] <Cationic Group Introduction Step (Cationization Step)> As an essential component, a compound having a reactive group and a cationic group (compound E Ｄ ), and an optional component, an alkaline compound, and a compound B selected from the above-mentioned urea and its derivatives are added to a wet or dry fiber raw material having a hydroxyl group and reacted to introduce a cationic group into the fiber raw material.

[0092] Examples of reactive groups include halogenated alkyl groups, vinyl groups, and epoxy groups (glycidyl groups). Examples of cationic groups include ammonium groups, phosphonium groups, and sulfonium groups. Among these, the cationic group is preferably an ammonium group. Compound E Ｄ As the compound, glycidyl trimethyl ammonium chloride, 3-chloro-2-hydroxypropyl trimethyl ammonium chloride, etc. are preferred from the viewpoints of the efficiency of introducing the substituent, and therefore the defibration efficiency, cost, and ease of handling. Furthermore, it is also preferable to use, as an optional component, the compound B in the above-mentioned <Phosphorus oxo acid group introduction step> in the same manner. The amount added is also preferably as described above.

[0093] Compound E Ｄ When adding, the reagent (solid or liquid) may be added directly to the fiber raw material, or may be dissolved in an appropriate solvent and added. The fiber raw material is preferably converted into alkali cellulose in advance or simultaneously with the reaction. The method for converting into alkali cellulose is as described above.

[0094] The reaction temperature is, for example, preferably 50°C or higher and 300°C or lower, more preferably 100°C or higher and 250°C or lower, and even more preferably 130°C or higher and 200°C or lower.

[0095] Compound E ＤThe amount of the additive per 100 parts by mass of the fiber raw material is preferably 1 part by mass or more and 100,000 parts by mass or less, more preferably 2 parts by mass or more and 10,000 parts by mass or less, and even more preferably 5 parts by mass or more and 1,000 parts by mass or less.

[0096] The reaction time may vary depending on the reaction temperature, but is, for example, preferably from 1 minute to 1,000 minutes, more preferably from 5 minutes to 500 minutes, and even more preferably from 10 minutes to 400 minutes. After the reaction, excess reaction reagents, by-products, etc. may be washed and removed with water by filtration or the like.

[0097] <Washing step> In the step of obtaining cellulose fibers having ionic substituents, a washing step can be carried out on the fibers having ionic substituents introduced therein, if necessary. The washing step is carried out by washing the fibers having ionic substituents introduced therein with, for example, water or an organic solvent. Furthermore, the washing step may be carried out after each step described below, and the number of washing steps carried out in each washing step is not particularly limited.

[0098] <Alkali Treatment Step> In the step of obtaining cellulose fibers having ionic substituents, an alkali treatment step may be provided between the ionic substituent introduction step and the defibration treatment step. The alkali treatment method is not particularly limited, but examples thereof include a method of immersing the ionic substituent-introduced fibers in an alkali solution.

[0099] The alkaline compound contained in the alkaline solution is not particularly limited and may be an inorganic alkaline compound or an organic alkaline compound. In this embodiment, it is preferable to use, for example, sodium hydroxide or potassium hydroxide as the alkaline compound because of their high versatility. The solvent contained in the alkaline solution may be either water or an organic solvent. Among these, the solvent contained in the alkaline solution is preferably water or a polar solvent including a polar organic solvent such as an alcohol, and more preferably an aqueous solvent including at least water. As the alkaline solution, for example, an aqueous sodium hydroxide solution or an aqueous potassium hydroxide solution is preferable because of their high versatility.

[0100] The temperature of the alkaline solution in the alkaline treatment step is not particularly limited, but is, for example, preferably from 5° C. to 80° C., more preferably from 10° C. to 60° C. The immersion time of the ionic substituent-introduced fiber in the alkaline solution in the alkaline treatment step is not particularly limited, but is, for example, preferably from 5 minutes to 30 minutes, more preferably from 10 minutes to 20 minutes. The amount of the alkaline solution used in the alkaline treatment is not particularly limited, but is, for example, preferably from 100% by mass to 100,000% by mass, more preferably from 1,000% by mass to 10,000% by mass, based on the absolute dry mass of the ionic substituent-introduced fiber.

[0101] In order to reduce the amount of alkaline solution used in the alkali treatment step, the ionic substituent-introduced fiber may be washed with water or an organic solvent after the ionic substituent-introducing step and before the alkali treatment step. From the viewpoint of improving handleability, it is preferable to wash the alkali-treated ionic substituent-introduced fiber with water or an organic solvent after the alkali treatment step and before the defibrating step.

[0102] <Counter ion is NH ４ ＋ In the present embodiment, when the microfibril cellulose has an anionic group, the counter ion is an ammonium ion (NH ４ ＋ When an anionic group is introduced into cellulose fibers by esterification using the above-mentioned compound B, the counter ion of the introduced anionic group may be NH ４ ＋ Therefore, a step of substituting the counter ion is not required. Therefore, the above-mentioned alkali treatment step is also not required. On the other hand, when an anionic group is introduced by TEMPO oxidation treatment, hypochlorous acid oxidation treatment, carboxymethylation treatment, etc., the counter ion of the anionic group can be substituted with NH ４ ＋ In this case, the anionic group-introduced cellulose fiber is subjected to an acid treatment process and a counter ion is replaced with NH ４ ＋ It is preferable to carry out a step of replacing the counter ion with NH.４ ＋ The step of replacing the microfibril cellulose with the cellulose syrup may be carried out before or after the defibration treatment step described below, but from the viewpoint of ease of production, it is preferably carried out before the defibration treatment step. Since the microfibril cellulose after the defibration treatment step has high water absorption and the slurry has high viscosity, the load on the washing step increases.

[0103] Counterion is NH ４ ＋ The method for substitution is not particularly limited, but an example is a method in which the anionic group-introduced fiber that has been subjected to the acid treatment step described above is immersed in aqueous ammonia. The concentration of the aqueous ammonia used is not particularly limited, but is preferably 5N or more, more preferably 10N or more, and even more preferably 14N or more.

[0104] The temperature of the ammonia water is not particularly limited, but is, for example, preferably from 5° C. to 100° C., more preferably from 20° C. to 90° C. The immersion time in the ammonia water is not particularly limited, but is, for example, preferably from 5 minutes to 120 minutes, more preferably from 10 minutes to 90 minutes.

[0105] <Acid Treatment Step> In the step of obtaining cellulose fibers having an ionic substituent, an acid treatment step may be provided between the ionic substituent introduction step and the defibration treatment step. For example, the ionic substituent introduction step, acid treatment, alkali treatment, and defibration treatment may be performed in this order.

[0106] The acid treatment method is not particularly limited, but examples include a method of immersing the fiber raw material in an acid-containing acid solution. The concentration of the acid solution used is not particularly limited, but is preferably 10% by mass or less, more preferably 5% by mass or less. The pH of the acid solution used is also not particularly limited, but is preferably 0 to 4, more preferably 1 to 3. Examples of the acid contained in the acid solution include inorganic acids, sulfonic acids, and carboxylic acids. Examples of inorganic acids include sulfuric acid, nitric acid, hydrobromic acid, hydroiodic acid, hypochlorous acid, chlorous acid, chloric acid, perchloric acid, phosphoric acid, and boric acid. Examples of sulfonic acids include methanesulfonic acid, ethanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid, and trifluoromethanesulfonic acid. Examples of carboxylic acids include formic acid, acetic acid, citric acid, gluconic acid, lactic acid, oxalic acid, and tartaric acid. Among these, hydrochloric acid or sulfuric acid is particularly preferred.

[0107] The temperature of the acid solution in the acid treatment is not particularly limited, but is, for example, preferably from 5° C. to 100° C., more preferably from 20° C. to 90° C. The immersion time in the acid solution in the acid treatment is not particularly limited, but is, for example, preferably from 5 minutes to 120 minutes, more preferably from 10 minutes to 60 minutes. The amount of the acid solution used in the acid treatment is not particularly limited, but is, for example, preferably from 100% by mass to 100,000% by mass, more preferably from 1,000% by mass to 10,000% by mass, based on the absolute dry mass of the fiber raw material.

[0108] <Defibril treatment step> Microfibril cellulose is obtained by defibrating raw fiber material or fibers into which ionic substituents have been introduced in the defibration treatment step. In the defibration treatment step, for example, a defibration treatment device can be used. Examples of defibration treatment devices that can be used include high-speed defibrators, grinders (stone mill-type grinders), high-pressure homogenizers, ultra-high-pressure homogenizers, high-pressure collision grinders, ball mills, bead mills, disk-type refiners, conical refiners, twin-screw kneaders, vibration mills, homomixers under high-speed rotation, ultrasonic dispersers, and beaters. Among the above-mentioned defibration treatment devices, from the viewpoint of obtaining microfibril cellulose having a desired fiber width, refiners are preferred, disk-type refiners are more preferred, and single-disc refiners are even more preferred.

[0109] In the defibration process, for example, the fiber raw material or the fiber having an ionic substituent introduced therein is preferably diluted with a dispersion medium to form a slurry. The dispersion medium can be one or more selected from water and organic solvents such as polar organic solvents. The polar organic solvent is not particularly limited, but examples thereof include alcohols, polyhydric alcohols, ketones, ethers, esters, and aprotic polar solvents. Examples of alcohols include methanol, ethanol, isopropanol, n-butanol, and isobutyl alcohol. Examples of polyhydric alcohols include ethylene glycol, propylene glycol, and glycerin. Examples of ketones include acetone and methyl ethyl ketone (MEK). Examples of ethers include diethyl ether, tetrahydrofuran, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol mono-n-butyl ether, and propylene glycol monomethyl ether. Examples of esters include ethyl acetate and butyl acetate. Examples of aprotic polar solvents include dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), and N-methyl-2-pyrrolidinone (NMP).

[0110] The solid content concentration of the microfibril cellulose during the defibration treatment can be appropriately set. The slurry obtained by dispersing the ionic substituent-introduced fibers in a dispersion medium may contain solids other than the ionic substituent-introduced fibers, such as urea having hydrogen bonding properties.

[0111] <Polymerization Reduction Treatment> In addition to the steps described above, the method for producing microfibril cellulose according to this embodiment preferably further comprises a step of performing a polymerization reduction treatment to obtain microfibril cellulose with a desired degree of polymerization. Specifically, as described above, the method preferably comprises a step of subjecting cellulose fibers into which ionic substituents, preferably anionic groups, have been introduced to a defibration treatment to obtain microfibril cellulose with a fiber width of 1 μm to 100 μm, and a step of subjecting the microfibril cellulose to a polymerization reduction treatment. Note that the order of the defibration treatment and the polymerization reduction treatment may be either first, but it is preferable to perform the polymerization reduction treatment after the defibration treatment. That is, the method for producing microfibril cellulose according to this embodiment preferably comprises, for example, a step of subjecting cellulose fibers into which anionic groups have been introduced to a defibration treatment, followed by a polymerization reduction treatment. Furthermore, since the amount of ionic substituents (preferably the amount of anionic groups) in microfibril cellulose hardly changes even after the polymerization reduction treatment, the amount of ionic substituents (preferably the amount of anionic groups) in microfibril cellulose before the polymerization reduction treatment can be approximated as the amount of ionic substituents (preferably the amount of anionic groups) in microfibril cellulose after the polymerization reduction treatment.

[0112] In this specification, the step of carrying out a polymerization degree reduction treatment is a step of carrying out a treatment to reduce the degree of polymerization of a dispersion containing microfibril cellulose to an appropriate range. Specifically, the polymerization degree reduction treatment is a step of reducing the degree of polymerization of microfibril cellulose having a fiber width of 1 μm or more and 100 μm or less to 50 or more and 500 or less.

[0113] Examples of the process for carrying out the polymerization reduction treatment include an ozone treatment process, an enzyme treatment process, a hypochlorous acid treatment process, a subcritical water treatment process, and a radiation irradiation treatment process. The process for carrying out the polymerization reduction treatment is preferably at least one selected from an ozone treatment process, an enzyme treatment process, a hypochlorous acid treatment process, a subcritical water treatment process, and a radiation irradiation treatment process, and more preferably at least one selected from an enzyme treatment process and an ozone treatment process. Note that the ozone treatment process, the enzyme treatment process, the hypochlorous acid treatment process, the subcritical water treatment process, and the radiation irradiation treatment process all have in common the reduction of the polymerization degree.

[0114] In the ozone treatment step, ozone is added to a dispersion (slurry) of cellulose fibers (preferably cellulose fibers into which anionic groups have been introduced) or a dispersion (slurry) of microfibril cellulose. When adding ozone, it is preferable to add it as an ozone / oxygen mixed gas, for example. In this case, the ozone addition rate per 1 g of cellulose fibers or microfibril cellulose contained in the dispersion (slurry) is 1.0 × 10 －４ g or more, and 1.0 × 10 －３ The ozone addition rate per 1 g of cellulose fiber or microfibril cellulose is preferably 1.0 × 10 １ g or less, and 1.0 × 10 ０ g or less, and more preferably 1.0 × 10 －１ g or less, and more preferably 3.0 × 10 －２ g or less, and more preferably 1.5 × 10 －２ g or less, and more preferably 1.0 × 10 －２ g or less, and more preferably 6.0 × 10 －３ After adding ozone to a cellulose fiber (preferably cellulose fiber having an anionic group introduced therein) dispersion (slurry) or a microfibril cellulose dispersion (slurry), the mixture is preferably stirred at a temperature of 10°C to 50°C for 10 seconds to 10 minutes, and then allowed to stand for 1 minute to 100 minutes.

[0115] In the enzyme treatment process, an enzyme is added to a dispersion (slurry) of cellulose fibers (preferably cellulose fibers into which anionic groups have been introduced) or a dispersion (slurry) of microfibril cellulose. The enzyme used here is preferably a cellulase enzyme. Cellulase enzymes are classified into the carbohydrate hydrolase family based on the higher-order structure of the catalytic domain that functions to hydrolyze cellulose. Cellulase enzymes are broadly classified into endo-glucanases and cellobiohydrolases based on their cellulose degradation properties. Endo-glucanases have high hydrolytic activity against the amorphous portion of cellulose, soluble cellooligosaccharides, and cellulose derivatives such as carboxymethylcellulose, randomly cleaving their molecular chains from the inside and reducing the degree of polymerization. In contrast, cellobiohydrolases decompose the crystalline portion of cellulose to produce cellobiose. Cellobiohydrolase hydrolyzes cellulose molecules from their terminals and is also called an exo- or processive enzyme. The enzyme used in the enzymatic treatment step is not particularly limited, but it is preferable to use an endo-glucanase.

[0116] In the enzyme treatment step, the enzyme addition rate is preferably 10 nkat or more per gram of cellulose fiber (preferably cellulose fiber into which anionic groups have been introduced) or microfibril cellulose, more preferably 50 nkat or more, even more preferably 100 nkat or more, and even more preferably 1,000 nkat or more. The enzyme addition rate is preferably 20,000 nkat or less per gram of cellulose fiber (preferably cellulose fiber into which anionic groups have been introduced) or microfibril cellulose. After adding the enzyme to the cellulose fiber (preferably cellulose fiber into which anionic groups have been introduced) dispersion (slurry) or microfibril cellulose dispersion (slurry), the mixture is stirred at 30°C to 70°C for 1 minute to 10 hours, and then placed at 80°C or higher. The enzyme is preferably inactivated by adding sodium hypochlorite, for example.

[0117] In the hypochlorite treatment step, sodium hypochlorite is added to a cellulose fiber (preferably cellulose fiber having an anionic group introduced therein) dispersion (slurry) or a microfibril cellulose dispersion (slurry). The sodium hypochlorite addition rate is 1.0 × 10 per 1 g of cellulose fiber (preferably cellulose fiber having an anionic group introduced therein) or microfibril cellulose. －４ g or more, and preferably 1.0 × 10 －３ g or more, and more preferably 1.0 × 10 －２ The sodium hypochlorite addition rate is preferably 1.0 × 10 per 1 g of cellulose fiber (preferably cellulose fiber into which an anionic group has been introduced) or microfibril cellulose. ２ g or less, and preferably 1.0 × 10 １ g or less, and more preferably 3×10 ０ After adding sodium hypochlorite to a cellulose fiber (preferably anionic group-introduced cellulose fiber) dispersion (slurry) or a microfibril cellulose dispersion (slurry), the mixture is preferably stirred at a temperature of 10°C to 50°C for 1 minute to 10 hours.

[0118] In the subcritical water treatment process, a cellulose fiber (preferably cellulose fiber having anionic groups introduced therein) dispersion (slurry) or a microfibril cellulose dispersion (slurry) is subjected to high-temperature and high-pressure treatment to create a subcritical state. The cellulose fiber (preferably cellulose fiber having anionic groups introduced therein) or microfibril cellulose is hydrolyzed in the subcritical state. Specifically, the cellulose fiber (preferably cellulose fiber having anionic groups introduced therein) dispersion (slurry) or microfibril cellulose dispersion (slurry) is placed in a reaction vessel, and the temperature is increased to 150°C to 500°C, preferably 150°C to 350°C, and the pressure inside the reaction vessel is increased to 10 MPa to 80 MPa, preferably 10 MPa to 20 MPa. The heating and pressurizing time is preferably 0.1 seconds to 100 seconds, more preferably 0.3 seconds to 50 seconds.

[0119] After the above-described polymerization degree reduction treatment, a second defibration treatment step may be further carried out. The second defibration treatment step may be the same as the above-described defibration treatment step for obtaining microfibril cellulose.

[0120] <Substituent Removal Treatment> The method for producing microfibril cellulose may include a substituent removal treatment step of removing at least a portion of the substituents from microfibril cellulose that has substituents, has a fiber width of 1 μm or more and 100 μm or less, and has a degree of polymerization of 50 or more and 500 or less. In this specification, the step of removing at least a portion of the substituents from the microfibril cellulose obtained by the above-mentioned step is also referred to as a substituent removal treatment step.

[0121] Examples of the substituent removal treatment step include a step of heat treating, enzyme treating, acid treating, alkali treating, etc., microfibril cellulose having substituents, a fiber width of 1 μm or more and 100 μm or less, and a degree of polymerization of 50 or more and 500 or less. These may be performed alone or in combination. Among these, the substituent removal treatment step is preferably a heat treating step or an enzyme treating step. By undergoing the above treatment steps, at least a portion of the substituents can be removed, and microfibril cellulose having an introduced substituent amount of less than 0.5 mmol / g can be obtained.

[0122] The substituent removal treatment step is preferably carried out in the form of a slurry. That is, the substituent removal treatment step is preferably a step of subjecting a slurry containing microfibril cellulose having substituents, a fiber width of 1 μm or more and 100 μm or less, and a degree of polymerization of 50 or more and 500 or less, to a heat treatment, an enzyme treatment, an acid treatment, an alkali treatment, or the like. By carrying out the substituent removal treatment step in the form of a slurry, it is possible to prevent the residue of colored substances generated by heating or the like during the substituent removal treatment, as well as acids, alkalis, salts, etc. that are added or generated. This makes it possible to suppress the coloration of the microfibril cellulose. Furthermore, when a treatment is carried out to remove salts derived from the substituents removed after the substituent removal treatment, it is also possible to increase the efficiency of salt removal.

[0123] When a slurry containing microfibril cellulose is subjected to a substituent removal treatment, the concentration of microfibril cellulose in the slurry is preferably 0.05% by mass or more and 20% by mass or less, more preferably 0.1% by mass or more, even more preferably 0.2% by mass or more, and more preferably 15% by mass or less, and even more preferably 10% by mass or less. By controlling the concentration of microfibril cellulose in the slurry within the above range, the substituent removal treatment can be carried out more efficiently. Furthermore, by controlling the concentration of microfibril cellulose in the slurry within the above range, it is possible to prevent the residue of colored substances caused by heating or the like during the substituent removal treatment, as well as acids, alkalis, salts, etc. that are added or generated. This makes it possible to suppress coloration of the resulting microfibril cellulose. Furthermore, when a treatment is carried out to remove salts derived from the removed substituents after the substituent removal treatment, it is also possible to increase the efficiency of salt removal.

[0124] When the substituent removal treatment step is a step of heat-treating microfibril cellulose having a substituent, a fiber width of 1 μm or more and 100 μm or less, and a degree of polymerization of 50 or more and 500 or less, the heating temperature in the heat treatment step is preferably 40° C. or more and 250° C. or less, more preferably 50° C. or more, even more preferably 60° C. or more, more preferably 230° C. or less, and even more preferably 200° C. or less. In particular, when the substituent in the microfibril cellulose to be subjected to the substituent removal treatment step is a phosphorus oxo acid group or a sulfone group, the heating temperature in the heat treatment step is preferably 80° C. or more, more preferably 100° C. or more, and even more preferably 120° C. or more.

[0125] When the substituent removal treatment step is a heat treatment step, the heating device that can be used in the heat treatment step is not particularly limited, and examples that can be used include a hot air heater, a steam heater, an electric heater, a hydrothermal heater, a thermal heater, an infrared heater, a far-infrared heater, a microwave heater, a high-frequency heater, a stirring dryer, a rotary dryer, a disk dryer, a roll-type heater, a plate-type heater, a fluidized bed dryer, a band-type dryer, a filtration dryer, a vibration fluidized dryer, a flash dryer, and a reduced-pressure dryer. From the viewpoint of preventing evaporation, the heating is preferably carried out in a closed system, and from the viewpoint of further increasing the heating temperature, it is preferably carried out in a pressure-resistant device or container. The heat treatment may be a batch process, a batch continuous process, or a continuous process.

[0126] When the substituent removal treatment step is a step of enzymatically treating microfibril cellulose having a substituent, a fiber width of 1 μm or more and 100 μm or less, and a degree of polymerization of 50 or more and 500 or less, the enzyme treatment step preferably uses a phosphate ester hydrolase, a sulfate ester hydrolase, or the like. In the enzyme treatment step, the enzyme is preferably added so that the enzymatic activity per 1 g of microfibril cellulose is 0.1 nkat or more and 100,000 nkat or less, more preferably 1.0 nkat or more, even more preferably 10 nkat or more, and more preferably 50,000 nkat or less, even more preferably 10,000 nkat or less. After adding the enzyme to the microfibril cellulose dispersion (slurry), treatment is preferably carried out under conditions of 0°C or more and less than 50°C for 1 minute or more and 100 hours or less.

[0127] After the enzymatic reaction, a step of deactivating the enzyme may be performed. Examples of the method for deactivating the enzyme include a method of adding an acid component or an alkali component to the enzymatically treated slurry to deactivate the enzyme, and a method of increasing the temperature of the enzymatically treated slurry to 90°C or higher to deactivate the enzyme.

[0128] When the substituent removal treatment step is a step of acid-treating microfibril cellulose having a substituent, a fiber width of 1 μm or more and 100 μm or less, and a degree of polymerization of 50 or more and 500 or less, in the acid treatment step, it is preferable to add an acid compound that can be used in the acid treatment step described above to the slurry.

[0129] When the substituent removal treatment step is a step of alkali-treating microfibril cellulose having a substituent, a fiber width of 1 μm or more and 100 μm or less, and a degree of polymerization of 50 or more and 500 or less, in the alkali treatment step, it is preferable to add an alkali compound that can be used in the alkali treatment step described above to the slurry.

[0130] In the substituent removal treatment step, it is preferable that the substituent removal reaction proceeds uniformly. To proceed with the reaction uniformly, for example, the slurry containing microfibril cellulose may be stirred or the specific surface area of ​​the slurry may be increased. As a method for stirring the slurry, external mechanical shear may be applied, or self-stirring may be promoted by increasing the slurry feeding speed during the reaction.

[0131] In the substituent removal treatment step, spacer molecules may be added. The spacer molecules enter between adjacent microfibril celluloses, thereby acting as spacers for creating minute spaces between the microfibril celluloses. Adding such spacer molecules in the substituent removal treatment step can suppress aggregation of the microfibril cellulose after the substituent removal treatment. This can more effectively improve the designability and tensile properties of rubber compositions and crosslinked rubber compositions containing microfibril cellulose.

[0132] The spacer molecule is preferably a water-soluble organic compound. Examples of water-soluble organic compounds include sugars, water-soluble polymers, and urea. Specific examples include trehalose, urea, polyethylene glycol (PEG), polyethylene oxide (PEO), carboxymethyl cellulose, and polyvinyl alcohol (PVA). Furthermore, examples of water-soluble organic compounds that can be used include alkyl methacrylate-acrylic acid copolymer, polyvinylpyrrolidone, sodium polyacrylate, propylene glycol, dipropylene glycol, polypropylene glycol, isoprene glycol, hexylene glycol, 1,3-butylene glycol, polyacrylamide, xanthan gum, guar gum, tamarind gum, carrageenan, locust bean gum, quince seed, alginic acid, pullulan, carrageenan, pectin, cationized starch, raw starch, oxidized starch, etherified starch, esterified starch, and starches such as amylose; glycerin, diglycerin, polyglycerin, hyaluronic acid, and metal salts of hyaluronic acid.

[0133] Also, known pigments can be used as spacer molecules, such as kaolin (containing clay), calcium carbonate, titanium oxide, zinc oxide, amorphous silica (containing colloidal silica), aluminum oxide, zeolite, sepiolite, smectite, synthetic smectite, magnesium silicate, magnesium carbonate, magnesium oxide, diatomaceous earth, styrene-based plastic pigments, hydrotalcite, urea resin-based plastic pigments, and benzoguanamine-based plastic pigments.

[0134] <pH Adjustment Step> When the substituent removal treatment step is carried out in the form of a slurry, a step of adjusting the pH of the slurry containing microfibril cellulose may be carried out before the substituent removal treatment step. For example, anionic groups are introduced into cellulose fibers, and the counter ions of the anionic groups are Na ＋In this case, the slurry containing defibrated microfibril cellulose exhibits weak alkalinity. If heating is performed in this state, monosaccharides, which are one of the causes of coloration, may be generated due to decomposition of cellulose, so the pH of the slurry is preferably adjusted to 8 or less, more preferably adjusted to 6 or less. Similarly, monosaccharides may be generated under acidic conditions, so the pH of the slurry is preferably adjusted to 3 or more, more preferably adjusted to 4 or more.

[0135] Furthermore, when the microfibril cellulose having a substituent is a microfibril cellulose having a phosphate group, it is preferable that the phosphorus of the phosphate group is in a state susceptible to nucleophilic attack, from the viewpoint of improving the efficiency of removing the substituent. The phosphorus susceptible to nucleophilic attack is cellulose-O-P(=O)(-O － H ＋ ) (-O － Na ＋ To achieve this state, the pH of the slurry is adjusted to preferably 3 or more and 8 or less, more preferably 4 or more and 6 or less.

[0136] The means for adjusting the pH is not particularly limited, and for example, an acid component or an alkali component may be added to a slurry containing microfibril cellulose. The acid component may be either an inorganic acid or an organic acid. Examples of inorganic acids include sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid. Examples of organic acids include formic acid, acetic acid, citric acid, malic acid, lactic acid, adipic acid, sebacic acid, stearic acid, maleic acid, succinic acid, tartaric acid, fumaric acid, and gluconic acid. The alkali component may be an inorganic alkali compound or an organic alkali compound. Examples of inorganic alkali compounds include lithium hydroxide, sodium hydroxide, potassium hydroxide, lithium carbonate, lithium bicarbonate, potassium carbonate, potassium bicarbonate, sodium carbonate, and sodium bicarbonate. Examples of the organic alkali compound 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, and N,N-dimethyl-4-aminopyridine.

[0137] In addition, in the pH adjustment step, an ion exchange treatment may be performed to adjust the pH. In the ion exchange treatment, a strongly acidic cation exchange resin or a weakly acidic ion exchange resin can be used. By treating with an appropriate amount of cation exchange resin for a sufficient time, a slurry containing microfibril cellulose having a desired pH can be obtained. Furthermore, in the pH adjustment step, the addition of an acid component or an alkali component may be combined with the ion exchange treatment.

[0138] <Salt Removal Treatment> After the substituent removal treatment step, it is preferable to carry out a treatment to remove salts derived from the removed substituents. Removing the salts derived from the substituents makes it easier to obtain microfibril cellulose that can suppress coloration. The means for removing the salts derived from the substituents is not particularly limited, and examples thereof include a washing treatment. The washing treatment is carried out, for example, by washing the microfibril cellulose that has aggregated in the substituent removal treatment with water or an organic solvent. From the viewpoint of more effectively suppressing yellowing, it is preferable to carry out the washing treatment by filtration dehydration, centrifugal dehydration, or centrifugation.

[0139] <Uniform Dispersion Treatment> The method for producing microfibril cellulose may include a substituent removal treatment step of removing at least a part of the substituents from microfibril cellulose having a substituent, a fiber width of 1 μm or more and 100 μm or less, and a degree of polymerization of 50 or more and 500 or less, and a uniform dispersion treatment step after the substituent removal treatment. The uniform dispersion treatment step is a step of uniformly dispersing the microfibril cellulose obtained through the substituent removal treatment in the substituent removal treatment step.

[0140] In the uniform dispersion treatment step, for example, a high-speed defibrator, a grinder (stone mill type grinder), a high-pressure homogenizer, a high-pressure collision type grinder, a ball mill, a bead mill, a disk type refiner, a conical refiner, a twin-screw kneader, a vibration mill, a homomixer under high-speed rotation, an ultrasonic disperser or a beater can be used. Among the above-mentioned uniform dispersion treatment devices, a refiner is preferred, a disk refiner is more preferred, and a single disk refiner is even more preferred.

[0141] In the uniform dispersion treatment step, the above-mentioned spacer molecules may be newly added. By adding such spacer molecules in the uniform dispersion treatment step, the uniform dispersion of the microfibril cellulose can be carried out more smoothly.

[0142] [Rubber Composition] The rubber composition of the present embodiment contains the microfibril cellulose of the present embodiment and a rubber component. The rubber composition of the present embodiment has excellent dispersibility of the microfibril cellulose.

[0143] [Rubber Component] The rubber composition of this embodiment contains a rubber component. Examples of the rubber component that can be used include natural rubber (NR) and synthetic rubber. Examples of synthetic rubber include styrene-butadiene rubber (SBR), nitrile rubber (NBR), chloroprene rubber (CR), ethylene propylene rubber (EPDM), butyl rubber (IIR), chlorobutyl rubber (CIIR), acrylic rubber (ACM), silicone rubber (Q), fluororubber (FKM), butadiene rubber (BR), epoxidized butadiene rubber (EBR), epichlorohydrin rubber (CO, CEO), urethane rubber (U), and polysulfide rubber (T). Examples of nitrile rubber include modified nitrile rubbers such as hydrogenated nitrile rubber, carboxyl-modified nitrile rubber, silicone-modified nitrile rubber, maleic acid-modified nitrile rubber, and hydroxyl-modified nitrile rubber, as well as hydrogenated versions of these; and acrylonitrile-butadiene-isoprene copolymers in which part of the butadiene is replaced with isoprene. Hydrogenated nitrile rubber (H-NBR) is sometimes called hydrogenated nitrile rubber or hydrogenated acrylonitrile-butadiene rubber. Hydrogenated nitrile rubber can be obtained by hydrogenating the double bonds contained in nitrile rubber. In addition to natural rubber (NR), examples of natural rubber include epoxidized natural rubber (ENR), modified natural rubber such as methyl methacrylate (MMA) graft-polymerized natural rubber, hydrogenated natural rubber, and deproteinized natural rubber. These rubber components may be used alone or in combination of two or more. Furthermore, these rubber components may be pre-crosslinked raw materials that do not have a crosslinked structure, or may have a crosslinked structure.

[0144] Among these, the rubber component is preferably a diene rubber, and is preferably at least one selected from natural rubber, nitrile rubber, butadiene rubber, and styrene-butadiene rubber, more preferably at least one selected from natural rubber, nitrile rubber, and styrene-butadiene rubber, and even more preferably at least one selected from natural rubber, carboxylated nitrile rubber, and styrene-butadiene rubber. The rubber component may also be a pre-crosslinking raw material. The rubber component is preferably a latex of these rubber components. Use of a rubber component latex improves the dispersibility of microfibril cellulose, making it easier to obtain a composite material with excellent tensile properties. When using a rubber component latex, a solid rubber component may be further mixed. The solid rubber component may be added when masticating the rubber composition or when preparing a rubber compound, but is preferably added when preparing a rubber compound.

[0145] In the rubber composition of this embodiment, from the viewpoint of ease of production of the rubber composition and obtaining a crosslinked rubber composition with superior physical properties, the content of microfibril cellulose per 100 parts by mass of the rubber component is preferably 1 part by mass or more and 200 parts by mass or less, more preferably 2 parts by mass or more, even more preferably 5 parts by mass or more, still more preferably 10 parts by mass or more, even more preferably 15 parts by mass or more, and preferably 150 parts by mass or less, more preferably 100 parts by mass or less, even more preferably 80 parts by mass or less, still more preferably 60 parts by mass or less, even more preferably 40 parts by mass or less, and even more preferably 30 parts by mass or less. Note that the content of microfibril cellulose per 100 parts by mass of the rubber composition of this embodiment may be different from the content of microfibril cellulose per 100 parts by mass of the rubber component when the crosslinked rubber composition is obtained. Specifically, the content of microfibril cellulose may be increased in the rubber composition, and then a rubber component may be added when preparing a rubber compound described below to adjust the content of microfibril cellulose per rubber component.

[0146] The rubber composition of the present embodiment may contain other components such as zinc oxide, a vulcanization accelerator, an antioxidant, and a reinforcing agent, which may be added at any stage in the production process.

[0147] In the rubber composition of this embodiment, when the storage modulus measured at a frequency of 10 Hz, 70°C, and a strain of 0.1% is G'(0.1%), and the storage modulus measured at a frequency of 10 Hz, 70°C, and a strain of 650% is G'(650%), the difference between G'(0.1%) and G'(650%), ΔG', expressed by the following formula (1), is preferably 1,800 Pa or less: ΔG'=G'(0.1%)-G'(650%) (1) ΔG' is the difference between the storage modulus at low strain (G'(0.1%)) and the storage modulus at high strain (G'(650%)), and is used as an index of the Payne effect. In other words, a large ΔG' indicates low dispersibility. Therefore, ΔG' is more preferably 1,500 Pa or less, even more preferably 1,200 Pa or less, even more preferably 900 Pa or less, and even more preferably 600 Pa or less. The lower limit of ΔG' is not particularly limited, but is preferably 100 Pa or more from the viewpoint of ease of production. When the dispersibility of the microfibril cellulose in the rubber composition is good, ΔG' in the above-mentioned range can be obtained. The microfibril cellulose having the specific fiber diameter and degree of polymerization of this embodiment has excellent dispersibility in the rubber composition, and therefore the above-mentioned ΔG' can be obtained.

[0148] <Method for Producing Rubber Composition> The method for producing a rubber composition is not particularly limited, but it is preferable to produce a rubber composition by preparing a mixed liquid containing at least a rubber component and microfibril cellulose and removing the solvent from the mixed liquid. In the above-mentioned production method, it is preferable to first prepare a mixed liquid containing the rubber component and microfibril cellulose. That is, the method for producing a rubber composition of this embodiment preferably includes the following steps (I) and (II): (I) a step of preparing a mixed liquid containing a rubber component and microfibril cellulose; and (II) a step of drying the mixed liquid to obtain a rubber composition.

[0149] Specifically, in step (I), the rubber component and microfibril cellulose are mixed and dispersed in an aqueous medium to obtain a dispersion of the mixture. Alternatively, a mixed liquid can be obtained by mixing an aqueous dispersion of the rubber component (rubber latex) with an aqueous dispersion of microfibril cellulose. Among these, it is preferable to mix an aqueous dispersion of the rubber component (rubber latex) with a dispersion of microfibril cellulose to obtain a dispersion containing the rubber component and microfibril cellulose (mixed liquid A). Mixing can be performed using known devices such as a disperser, a three-one motor, a clearmix, a homomixer, a homogenizer, or a propeller agitator (e.g., a tornado agitator). The mixing temperature is not limited, but room temperature (20 to 30°C) is preferred. The mixing time can also be adjusted appropriately. When preparing the dispersion of the mixture, thorough mixing of the rubber component and microfibril cellulose tends to improve the physical properties after crosslinking.

[0150] The rubber latex is preferably a dispersion of a rubber component in an aqueous medium. The aqueous medium contains water as a main component, and the water content relative to the entire aqueous medium is preferably 50% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and even more preferably 95% by mass or more. The upper limit is not particularly limited, but is 100% by mass or less. In addition to water, the aqueous medium may contain a known organic solvent or the like to the extent that the effects of the present disclosure are not impaired.

[0151] The solids concentration (by mass) of the mixed solution is preferably 0.5% by mass or more and 50% by mass or less, more preferably 1% by mass or more, even more preferably 3% by mass or more, and more preferably 40% by mass or less, and even more preferably 30% by mass or less. When the solids concentration of the mixed solution is within the above range, the amount of energy required to remove solvents such as water can be reduced, and further, aggregation of microfibril cellulose particles in the resulting rubber composition is less likely to occur, resulting in excellent kneadability of the resulting rubber composition and excellent properties of the crosslinked rubber composition.

[0152] The solid content concentration of the mixture dispersion can be calculated by the following formula (I) from the mass of the dried product obtained by drying a predetermined amount of the dispersion in a dryer at 105°C until it reaches a constant weight and the mass of the mixture dispersion subjected to drying: solid content concentration (mass %) of the mixture dispersion = mass of dried product [g] / mass of the mixture dispersion subjected to drying [g] × 100 (I)

[0153] The rubber composition is obtained by heating and drying the resulting mixed liquid, as shown in step (II) above. The heating and drying method is not particularly limited, and examples include a method of coating the mixed liquid on a substrate and a method of drying using a drum dryer. In the coating method, for example, a mixed liquid containing a rubber component and microfibril cellulose is coated on a substrate. Furthermore, sheets can be continuously produced by using a coating device and a long substrate. The material of the substrate used in the coating method is not particularly limited, but a substrate with high wettability with the mixed liquid can suppress shrinkage of the sheet during drying, and it is preferable to select a substrate that allows easy peeling of the formed sheet after drying. Among these, resin films and plates or metal films and plates are preferred, but are not particularly limited. For example, resin films and plates such as acrylic, polyethylene terephthalate, vinyl chloride, polystyrene, polypropylene, polycarbonate, and polyvinylidene chloride, metal films and plates such as aluminum, zinc, copper, and iron plates, and their surfaces with oxidation treatment, stainless steel films and plates, brass films and plates, etc. can be used. In the coating method, if the viscosity of the mixture dispersion is low and it spreads on the substrate, a damming frame may be fixed to the substrate to obtain a sheet of the desired thickness and basis weight. Alternatively, a substrate with a batt-shaped damming frame may be used. The damming frame is not particularly limited, but it is preferable to select one that allows the edge of the sheet to be easily peeled off after drying. From this perspective, molded resin or metal plates are more preferable. In this embodiment, for example, resin plates such as acrylic plates, polyethylene terephthalate plates, vinyl chloride plates, polystyrene plates, polypropylene plates, polycarbonate plates, and polyvinylidene chloride plates, metal plates such as aluminum plates, zinc plates, copper plates, and iron plates, and metal plates with their surfaces oxidized, stainless steel plates, brass plates, etc., can be used. The coating machine used to coat the mixture onto the substrate is not particularly limited, but examples include roll coaters, gravure coaters, die coaters, curtain coaters, and air doctor coaters.A die coater, curtain coater, or spray coater is particularly preferred since it can make the thickness of the sheet more uniform.

[0154] The temperature of the mixed liquid and the ambient temperature when applying the mixed liquid to the substrate (hereinafter, the temperature of the mixed liquid and the ambient temperature are collectively referred to as the "coating temperature") are not particularly limited, but are preferably, for example, 5°C or higher and 80°C or lower, more preferably 10°C or higher and 60°C or lower, even more preferably 15°C or higher and 50°C or lower, and particularly preferably 20°C or higher and 50°C or lower. If the coating temperature is above the above lower limit, the dispersion of the mixture can be applied more easily. If the coating temperature is below the above upper limit, evaporation of the dispersion medium during coating can be suppressed. The drying device used is not particularly limited, but examples include a hot air dryer and an explosion-proof dryer. In the coating process, it is preferable to apply the dispersion of the mixture to the substrate so that the finished basis weight and thickness of the sheet are within the above-mentioned preferred ranges.

[0155] The mixed liquid may be heated and dried in a heated cylindrical dryer to obtain a rubber composition. FIG. 3 is a schematic side cross-sectional view of a double drum dryer 10, which is an example of a heated cylindrical dryer, and FIG. 4 is a schematic side view of the double drum dryer 10. First, a heat transfer medium (generally steam) is introduced into the interior of a rotating cylinder 1 (drum), and the mixed liquid is introduced into a feed section 2. The introduced mixed liquid adheres to the surface of the heated cylinder 1 as a sheet-like mixture 3, and is quickly heated and dried. As the cylinder 1 rotates, the sheet-like mixture 3 is scraped off by a fixed scraper 4, and a rubber composition 5 (heat-dried product) can be obtained.

[0156] The lower limit of the surface temperature of the heated cylindrical dryer is preferably 80°C or higher, more preferably 90°C or higher. The upper limit of the surface temperature is preferably 250°C or lower, more preferably 200°C or lower. The surface temperature of the heated cylindrical dryer refers to the temperature of the cylinder surface that comes into contact with the mixed liquid. The surface temperature of the heated cylindrical dryer may be, for example, 80 to 250°C, 90 to 200°C, or 95 to 180°C. When the surface temperature of the heated cylindrical dryer is within the above range, production efficiency is good and a rubber composition that has excellent physical properties after crosslinking can be obtained.

[0157] The lower limit of the heat-drying time in a heated cylindrical dryer is preferably 2 seconds or more, more preferably 4 seconds or more. The upper limit of the heat-drying time is preferably 1,800 seconds or less, more preferably 600 seconds or less. The heat-drying time [sec] in a heated cylindrical dryer refers to the time [sec] during which the mixture is in contact with the surface of the cylinder, and the heat-drying time may be, for example, 2 to 1,800 seconds, 4 to 600 seconds, or 6 to 400 seconds. By keeping the heat-drying time within the above range, a dried product that is neither underdried nor overdried can be obtained, and the product has excellent releasability from the drum.

[0158] The lower limit of the cylinder width w of the heating cylinder dryer is preferably 0.1 m or more, more preferably 0.2 m or more. The upper limit of the cylinder width w is not particularly limited, but is usually 7 m or less. The cylinder width w may be, for example, 0.1 to 7 m, or 0.2 to 5 m. By keeping the cylinder width w within the above range, a dried product with a uniform thickness can be obtained.

[0159] The lower limit of the cylinder rotation speed v of the heated cylinder dryer is preferably 0.001 [m / sec] or more, more preferably 0.002 [m / sec]. The upper limit of the cylinder rotation speed v is not particularly limited, but is usually 35 [m / sec] or less. The cylinder rotation speed v may be, for example, 0.001 to 3.5 [m / sec], or 0.002 to 0.2 [m / sec]. By keeping the cylinder rotation speed v within the above range, the time the mixture is in contact with the heated drum can be kept within an appropriate range, and a dried product with excellent kneading properties and post-crosslinking physical properties can be obtained.

[0160] The heated cylindrical dryer used in the method for producing a rubber composition may be a conduction heating dryer in which a heat medium is introduced into the interior of a cylinder and the mixture is brought into contact with the surface of the heated cylinder to heat and dry. For example, a drum dryer such as a double drum dryer, a single drum dryer, or a twin drum dryer may be used, and a cylinder dryer, Yankee dryer, etc. may also be used. Among these, from the viewpoint of heat and drying efficiency, the heated cylindrical dryer is preferably a double drum dryer or a cylinder dryer.

[0161] [Rubber Compound] In this embodiment, the rubber compound refers to a composition obtained by kneading a rubber composition and before crosslinking. The rubber compound preferably contains at least a crosslinking agent, and may contain, in addition to the crosslinking agent, additives usable in the rubber field, such as zinc oxide, a vulcanization accelerator, a filler, a softener, a fatty acid, an antioxidant, a peptizer, a colorant, a pH adjuster, and a cured resin. Furthermore, a new solid rubber may be added in addition to the rubber components used in preparing the rubber composition. When a new solid rubber is added, the solid rubber may be the same as those exemplified as the rubber component, but it may be added in a solid state (solid rubber) rather than in a latex form. Examples of the crosslinking agent include sulfur-based crosslinking agents (e.g., powdered sulfur, sulfur flowers, precipitated sulfur, colloidal sulfur, surface-treated sulfur, insoluble sulfur, and other sulfur; sulfur-containing compounds such as amine disulfide, polymer polysulfide, sulfur olefin adducts, sulfur chloride, and sulfur dichloride; and insoluble polymeric sulfur), peroxide-based crosslinking agents (e.g., dicumyl peroxide, dichlorobenzoyl peroxide, benzoyl peroxide, 1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane, n-butyl-4,4-bis(t-butylperoxy)valerate, di-t-butylperoxy-di-isopropylbenzene, and 2,5-dimethyl-2,5-di(t-butylperoxy)hexane), and quinoid-based crosslinking agents (e.g., p-quinonedioxime and p,p'-dibenzoylquinonedioxime), and among these, sulfur-based crosslinking agents or peroxide-based crosslinking agents are preferred. The amount of the crosslinking agent added is not particularly limited, but is preferably 0.1 part by mass or more and 10 parts by mass or less, more preferably 0.5 part by mass or more, even more preferably 1.0 part by mass or less, and is preferably 5 parts by mass or less, more preferably 3 parts by mass or less, per 100 parts by mass of the rubber component.

[0162] Furthermore, a crosslinking accelerator having the effect of accelerating crosslinking by a crosslinking agent may be contained, and when a sulfur-based crosslinking agent is used, it is preferable to contain a crosslinking accelerator (vulcanization accelerator). When a sulfur-based crosslinking agent (vulcanizing agent) is used as the crosslinking agent, it is preferable to use a sulfenamide-based vulcanization accelerator, a guanidine-based vulcanization accelerator, a thiazole-based vulcanization accelerator, a thiuram-based vulcanization accelerator, a dithiocarbamate-based vulcanization accelerator, or the like as the crosslinking accelerator (vulcanization accelerator).

[0163] Examples of the filler include carbon black and silica. These fillers may be used alone or in combination of two or more. The content of the filler is not particularly limited and is preferably 10 parts by mass or more and 150 parts by mass or less, more preferably 20 parts by mass or more and 100 parts by mass or less, per 100 parts by mass of the rubber component.

[0164] Examples of the softener include aromatic oils, paraffin oils, naphthenic oils, vegetable oils other than castor oil, low PCA oils such as MES, TDAE, and SRAE, and heavy naphthenic oils. Suitable low PCA oils include various plant-derived oils harvested from vegetables, nuts, and seeds. Examples of plant-derived oils include soybean oil, sunflower oil, safflower oil, corn oil, linseed oil, cottonseed oil, rapeseed oil, cashew oil, sesame oil, camellia oil, jojoba oil, macadamia nut oil, coconut oil, and palm oil. The content of the softener is preferably 1 part by mass or more and 35 parts by mass or less, more preferably 5 parts by mass or more, even more preferably 10 parts by mass or more, and more preferably 30 parts by mass or less, and even more preferably 25 parts by mass or less, per 100 parts by mass of the rubber component.

[0165] Examples of the fatty acid include stearic acid, palmitic acid, arachidic acid, oleic acid, linoleic acid, and arachidonic acid. Among these, stearic acid is preferred. The content of the fatty acid is not particularly limited, and is preferably 0.1 parts by mass or more and 5 parts by mass or less, more preferably 1 part by mass or more and 4 parts by mass or less, per 100 parts by mass of the rubber component.

[0166] Examples of the antioxidant include N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), 2,2,4-trimethyl-1,2-dihydroquinoline polymer (TMDQ), 6-ethoxy-2,2,4-trimethyl-1,2-dihydroquinoline (AW), N,N'-diphenyl-p-phenylenediamine (DPPD), and 2-mercaptobenzimidazole (MBI). One type of antioxidant may be used alone, or two or more types may be used in combination. The content of the antioxidant is not particularly limited, and is preferably 0.1 parts by mass or more and 5 parts by mass or less, more preferably 1 part by mass or more and 3 parts by mass or less, per 100 parts by mass of the rubber component. The total content of the antioxidants is preferably 0.2 parts by mass or more and 10 parts by mass or less, more preferably 1 part by mass or more and 5 parts by mass or less, per 100 parts by mass of the rubber component.

[0167] The amount of zinc oxide (zinc white) is not particularly limited and is preferably 1 part by mass or more and 10 parts by mass or less, and more preferably 1.5 parts by mass or more and 8 parts by mass or less, per 100 parts by mass of the rubber component.

[0168] [Method for Producing Rubber Compound] The method for producing the rubber compound is not particularly limited, but preferably includes at least a kneading step of kneading the rubber composition of the present embodiment. In the kneading step, it is preferable to masticate the rubber composition of the present embodiment, and then add at least a crosslinking agent and knead it. In addition to the crosslinking agent, the above-mentioned additives may be compounded. Furthermore, in the kneading step, a solid rubber may be compounded in addition to the rubber component contained in the rubber composition. In the present embodiment, in the rubber compound and the crosslinked rubber composition described below, the content of microfibril cellulose per 100 parts by mass of the rubber component is preferably 0.1 parts by mass or more and 50 parts by mass or less, more preferably 1 part by mass or more, even more preferably 3 parts by mass or more, and more preferably 40 parts by mass or less, even more preferably 30 parts by mass or less, from the viewpoint of improving the physical properties of the crosslinked rubber composition.

[0169] The method for producing the rubber compound is not particularly limited, but preferably includes a step of masticating the rubber composition of this embodiment. Mastication improves the dispersibility of the crosslinking agent and other components to be subsequently blended. Mastication may be performed by a conventional method, but may be low-temperature kneading using a roll machine such as an open roll, or high-temperature kneading using a Banbury mixer or the like. Among these, low-temperature kneading is preferred from the viewpoint of the dispersibility of microfibril cellulose in the rubber composition. The temperature for low-temperature kneading is preferably 15°C or higher and 70°C or lower, more preferably 20°C or higher and 65°C or lower, and even more preferably 25°C or higher and 60°C or lower. The temperature for high-temperature kneading is preferably 80°C or higher and 200°C or lower. Furthermore, a rubber compound is produced by adding additives such as a crosslinking agent to the masticated rubber composition and kneading them.

[0170] Kneading is a process of uniformly dispersing a crosslinking agent and other compounding ingredients in a rubber composition (preferably a masticated rubber composition). Kneading may be carried out in a known manner, for example, using a Banbury mixer, kneader, open roll, or the like. Examples of crosslinking agents include sulfur and peroxides. Examples of other compounding ingredients include the above-mentioned additives and solid rubbers.

[0171] [Crosslinked Rubber Composition and Manufacturing Method Thereof] The crosslinked rubber composition of this embodiment is obtained by crosslinking a rubber compound containing a crosslinking agent. Molding may also be performed during crosslinking. That is, the manufacturing method of the crosslinked rubber composition preferably includes a step of crosslinking the rubber composition and the rubber compound containing the crosslinking agent, and a crosslinking and molding step. The manufacturing method of the crosslinked rubber composition of the present invention may further include a step of molding the rubber compound obtained by the kneading step into an intended shape (molding step). Molding in this molding step can be performed by various molding methods using an extruder, a calendar roll, a press, an injection molding machine, a transfer molding machine, a blow molding machine, a foam molding machine, or the like. The molding method may be appropriately selected depending on the shape, application, and molding method of the final product. The kneading step and the molding step may be performed separately or consecutively.

[0172] Regarding crosslinking, there are no particular limitations on the temperature as long as the conditions are such that the crosslinking reaction proceeds, but generally, a crosslinked rubber composition is obtained by heating an uncrosslinked rubber compound obtained by kneading to crosslink (also called vulcanization when sulfur is contained). The heating temperature is preferably 140°C or higher, and preferably 200°C or lower, and more preferably 180°C or lower. Therefore, the heating temperature is preferably about 140 to 200°C, and more preferably about 140 to 180°C. For crosslinking, for example, a vulcanization device that performs mold vulcanization, can vulcanization, continuous vulcanization, etc. can be used.

[0173] In this embodiment, the breaking elongation (elongation at break) of the crosslinked rubber composition having the formulation described in the examples, measured in accordance with JIS K 6251:2017, is preferably 250% or more, more preferably 300% or more, even more preferably 350% or more, even more preferably 385% or more, and even more preferably 400% or more. The upper limit is not particularly limited, but from the viewpoint of ease of production, it is preferably 600% or less, more preferably 550% or less. Conventionally, rubber additives such as carbon black have been used to improve the breaking strength (breaking strength) of crosslinked rubber compositions. While such additives are highly effective in improving breaking strength, they tend to reduce the breaking elongation of the crosslinked rubber composition. The crosslinked rubber composition obtained by crosslinking the rubber composition containing the microfibril cellulose of this embodiment maintains the effect of improving breaking strength while suppressing the reduction in breaking strength.

[0174] When the breaking elongation of a crosslinked rubber composition obtained by crosslinking a rubber compound to which no reinforcing material such as carbon black or microfibril cellulose has been added is A%, and the breaking elongation of a crosslinked rubber composition obtained by crosslinking a rubber compound to which a reinforcing material has been added is B%, the rate of decrease in breaking elongation can be obtained by the following formula: Breaking elongation decrease rate (%) = (A - B) / A x 100 The breaking elongation decrease rate is preferably 50% or less, more preferably 35% or less, even more preferably 30% or less, still more preferably 28% or less, and even more preferably 25% or less, and the lower limit is not particularly limited, but is preferably -20% or more.

[0175] In this embodiment, the breaking strength (tensile strength at break) of the crosslinked rubber composition having the formulation described in the examples, measured in accordance with JIS K 6251:2017, is preferably 5 MPa or more, more preferably 10 MPa or more, even more preferably 12 MPa or more, still more preferably 15 MPa or more, and even more preferably 17 MPa or more. The upper limit is not particularly limited, but from the viewpoint of ease of production, it is preferably 50 MPa or less, more preferably 40 MPa or less, and even more preferably 30 MPa or less. Note that when the breaking strength of the crosslinked rubber composition obtained by crosslinking a rubber compound to which no reinforcing material has been added is C (MPa) and the breaking strength of the crosslinked rubber composition obtained by crosslinking a rubber compound to which a reinforcing material has been added is D (MPa), the increase rate of elongation at break can be obtained by the following formula: Increase rate of breaking strength (%)=(D−C)÷C×100 The increase rate of breaking strength is preferably 50% or more, more preferably 100% or more, even more preferably 150% or more, still more preferably 200% or more, and even more preferably 250% or more. There is no particular upper limit, but from the viewpoint of ease of production, it is preferably 700% or less, more preferably 600% or less, and even more preferably 500% or less.

[0176] The tensile energy is a value obtained by multiplying the breaking elongation (%) by the breaking strength (MPa) (breaking elongation (%) × breaking strength (MPa)) in accordance with JIS K 6251:2017, and a high tensile energy means excellent tensile properties. In this embodiment, the tensile energy of the crosslinked rubber composition is preferably 6,000 or more, more preferably 6,500 or more, even more preferably 7,000 or more, and even more preferably 7,500 or more. The upper limit is not particularly limited, but from the viewpoint of ease of production, it is 20,000 or less, more preferably 15,000 or less.

[0177] In the present embodiment, when the loss tangent of the crosslinked rubber composition measured in accordance with JIS K 6394:2007 at 0°C and a frequency of 1 Hz is defined as tan δ(0°C), and the loss tangent at 60°C and a frequency of 1 Hz is defined as tan δ(60°C), the ratio of tan δ(0°C) to tan δ(60°C) (tan δ(0°C) / tan δ(60°C)) is preferably 1.7 or more, more preferably 1.8 or more, and even more preferably 1.9 or more. The upper limit is not particularly limited, but from the viewpoint of ease of production, it is preferably 3.5 or less, more preferably 3.0 or less, and even more preferably 2.5 or less. For example, when used in tires, the wet performance generally depends greatly on the tan δ(0°C) of the crosslinked rubber composition, and the larger the tan δ(0°C), the better the wet performance. On the other hand, rolling resistance is largely dependent on the tan δ (60°C) of the crosslinked rubber composition, and the smaller the tan δ (60°C), the better the rolling resistance of the tire (lower rolling resistance). The crosslinked rubber composition containing the microfibril cellulose of this embodiment has a good balance between tan δ (0°C) and tan δ (60°C), and when the crosslinked rubber composition of this embodiment is used for a tire, it is excellent in both wet performance and rolling resistance. Tan δ (0°C) and tan δ (60°C) are measured by the methods described in the examples.

[0178] The loss tangent (tanδ(0°C)) of the crosslinked rubber composition at 0°C and a frequency of 1 Hz, measured in accordance with JIS K 6394:2007, is preferably 0.060 or more, more preferably 0.075 or more, and although the upper limit is not particularly limited, is preferably 0.120 or less. The loss tangent (tanδ(60°C)) of the crosslinked rubber composition at 60°C and a frequency of 1 Hz, measured in accordance with JIS K 6394:2007, is preferably 0.080 or less, more preferably 0.070 or less, even more preferably 0.060 or less, still more preferably 0.050 or less, and even more preferably 0.045 or less, and although the lower limit is not particularly limited, is preferably 0.020 or more.

[0179] <Applications> The crosslinked rubber composition of the present embodiment can be used for various applications, and is not particularly limited. For example, it is preferably used for sealing materials, hoses, shoe soles, tire components, anti-vibration rubber, etc. Among these, the crosslinked rubber composition is preferably used for tires.

[0180] The features of the present invention will be explained in more detail below with reference to examples and comparative examples. The materials, amounts used, ratios, treatment contents, treatment procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the specific examples shown below.

[0181] <Production of Microfibril Cellulose Fiber> (Production Example A) As a raw material pulp, softwood kraft pulp (solid content 93% by mass, basis weight 245 g / m) manufactured by Oji Paper Co., Ltd. was used. ２ A sheet-form pulp (disintegrated, with a Canadian Standard Freeness (CSF) of 700 mL as measured in accordance with JIS P 8121-2:2012) was used. This raw pulp was subjected to a phosphorus oxo-oxidation treatment as follows. First, a mixed aqueous solution of ammonium dihydrogen phosphate and urea was added to 100 parts by mass (bone dry mass) of the raw pulp to adjust the total weight to 45 parts by mass of ammonium dihydrogen phosphate, 120 parts by mass of urea, and 150 parts by mass of water, to obtain a chemical-impregnated pulp. The resulting chemical-impregnated pulp was then heated in a hot air dryer at 165°C for 250 seconds to introduce phosphate groups into the cellulose in the pulp, thereby obtaining a phosphorylated pulp. The resulting phosphorylated pulp was then subjected to a washing treatment. The washing treatment was carried out by repeatedly adding 10 L of ion-exchange water to 100 g (bone dry mass) of phosphorylated pulp to obtain a pulp dispersion, stirring the resulting dispersion to uniformly disperse the pulp, followed by filtration and dewatering. The washing was terminated when the electrical conductivity of the filtrate reached 100 μS / cm or less. The washed phosphorylated pulp was then neutralized as follows: First, the washed phosphorylated pulp was diluted with 10 L of ion-exchanged water, and then a 1 N aqueous solution of sodium hydroxide was added little by little while stirring to obtain a phosphorylated pulp slurry having a pH of 12 to 13. The phosphorylated pulp slurry was then dehydrated and washed to obtain a neutralized phosphorylated pulp.

[0182] The obtained phosphorylated pulp was subjected to infrared absorption spectroscopy using FT-IR. －１ Absorption due to the P=O of the phosphate group was observed near the peak, confirming that the phosphate group had been added to the pulp. Furthermore, when the obtained phosphorylated pulp was tested and analyzed using an X-ray diffractometer, typical peaks were observed at two positions, around 2θ = 14° to 17° and around 2θ = 22° to 23°, confirming the presence of cellulose type I crystals. The amount of phosphate groups (amount of first dissociated acid) measured by the measurement method described below was 1.45 mmol / g. The total amount of dissociated acid was 2.45 mmol / g.

[0183] Ion-exchanged water was added to the obtained phosphorylated pulp to prepare a slurry with a solids concentration of 2% by mass. This slurry was processed 30 times in a single-disc refiner with a clearance set to 600 μm for defibration treatment, to obtain a microfibril cellulose dispersion (1). The fiber width measured by the measurement method described below was 13.7 μm, and the degree of polymerization was 814.

[0184] (Production Example B) An enzyme-containing solution (ECOPULP, endo-1,4-glucanase, manufactured by AB Enzyme) was added to the microfibril cellulose dispersion (1) obtained in Production Example A so that the enzyme activity per 1 g of microfibril cellulose was 9,333 nkat, and the mixture was subjected to an enzyme treatment at a temperature of 50°C for 100 minutes. 350 ppm of sodium hypochlorite solution was added to the dispersion to inactivate the enzyme, thereby obtaining microfibril cellulose dispersion (2) (MFC dispersion (2) in the table). The fiber width measured by the measurement method described below was 13.1 µm, and the degree of polymerization was 238.

[0185] (Production Example C) The phosphorylated pulp obtained in Production Example A was subjected to refiner treatment and enzyme treatment in the same manner as in Production Example B, except that the number of times of treatment with the single disc refiner was set to 20, to obtain a microfibril cellulose dispersion (3) (MFC dispersion (3) in the table). The fiber width measured by the measurement method described below was 16.6 μm, and the degree of polymerization was 239.

[0186] (Production Example D) Phosphated pulp was obtained in the same manner as in Production Example A, except that the raw material pulp was hardwood dissolving pulp (dry sheet) manufactured by Oji Paper Co., Ltd. In this case, the amount of phosphate groups (amount of first dissociated acid) was 1.42 mol / g, and the total amount of dissociated acid was 2.41 mmol / g. Next, refiner treatment and enzyme treatment were carried out in the same manner as in Production Example B, to obtain microfibril cellulose dispersion (4) (MFC dispersion (4) in the table). The fiber width measured by the measurement method described below was 17.5 μm, and the degree of polymerization was 310.

[0187] (Production Example E) Phosphated pulp was obtained in the same manner as in Production Example A, except that the drying time of the chemical-impregnated pulp was 140°C and 200 seconds. At this time, the amount of phosphate groups (amount of first dissociated acid) was 0.80 mmol / g, and the total amount of dissociated acid was 1.40 mmol / g. Next, refiner treatment and enzyme treatment were carried out in the same manner as in Production Example B, to obtain microfibril cellulose dispersion (5) (MFC dispersion (5) in the table). The fiber width measured by the measurement method described below was 18.3 μm, and the degree of polymerization was 450.

[0188] (Production Example F) A phosphorylated pulp was obtained in the same manner as in Production Example A, except that the washed phosphorylated pulp before the neutralization treatment was further subjected to the above-mentioned phosphorus oxo-oxidation treatment and the above-mentioned washing treatment once each. At this time, the amount of phosphate groups (amount of first dissociated acid) was 2.00 mmol / g, and the total amount of dissociated acid was 3.30 mmol / g. Next, refiner treatment and enzyme treatment were carried out in the same manner as in Production Example B, to obtain a microfibril cellulose dispersion (6) (MFC dispersion (6) in the table). The fiber width measured by the measurement method described below was 15.1 μm, and the degree of polymerization was 227.

[0189] (Production Example G) A phosphited pulp was obtained in the same manner as in Production Example A, except that 33 parts by mass of phosphorous acid (phosphonic acid) was used instead of ammonium dihydrogen phosphate. The infrared absorption spectrum of the obtained phosphited pulp was measured using FT-IR. As a result, －１Absorption due to P=O of the phosphonic acid group, which is a tautomer of the phosphorous acid group, was observed near the peak, confirming that a phosphorous acid group (phosphonic acid group) had been added to the pulp. Furthermore, when the obtained phosphorous-oxidized pulp was analyzed using an X-ray diffractometer, typical peaks were observed at two positions, around 2θ = 14° to 17° and around 2θ = 22° to 23°, confirming the presence of cellulose type I crystals. The amount of phosphorous acid (first dissociated acid amount) measured by the measurement method described below was 1.51 mmol / g. The total amount of dissociated acid was 1.54 mmol / g.

[0190] Next, refiner treatment and enzyme treatment were carried out in the same manner as in Production Example B to obtain microfibril cellulose dispersion (7) (MFC dispersion (7) in the table). The fiber width measured by the measurement method described below was 17.2 μm, and the degree of polymerization was 246.

[0191] (Production Example H) Softwood kraft pulp (undried) manufactured by Oji Paper Co., Ltd. was used as the raw material pulp. This raw material pulp was subjected to an alkaline TEMPO oxidation treatment as follows. First, 100 parts by mass of the raw material pulp (dry mass equivalent), 1.6 parts by mass of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), and 10 parts by mass of sodium bromide were dispersed in 10,000 parts by mass of water. Next, a 13% by mass aqueous solution of sodium hypochlorite was added to 10 mmol per 1.0 g of pulp to initiate the reaction. During the reaction, a 0.5 M aqueous solution of sodium hydroxide was added dropwise to maintain the pH at 10 to 10.5, and the reaction was considered complete when no further change in pH was observed. Next, the resulting TEMPO-oxidized pulp was washed. The washing treatment was carried out by dehydrating the pulp slurry after TEMPO oxidation to obtain a dehydrated sheet, pouring 5,000 parts by mass of ion-exchanged water into the sheet, stirring to uniformly disperse the pulp, and then filtering and dehydrating the sheet. The washing was completed when the electrical conductivity of the filtrate reached 100 μS / cm or less.

[0192] The remaining aldehyde groups in this dehydrated sheet were further oxidized as follows. 100 parts by weight of the dehydrated sheet, equivalent to the dry weight, was dispersed in 10,000 parts by weight of 0.1 mol / L acetate buffer (pH 4.8). Next, 113 parts by weight of 80% by weight sodium chlorite was added, and the container was immediately sealed. The reaction was allowed to proceed at room temperature for 48 hours while stirring at 500 rpm using a magnetic stirrer, yielding a pulp slurry. The resulting TEMPO-oxidized pulp was then washed. The washing was performed by dehydrating the pulp slurry after further oxidation to obtain a dehydrated sheet, pouring 5,000 parts by weight of ion-exchanged water into the slurry, stirring to uniformly disperse the pulp, and repeatedly filtering and dehydrating the pulp. The washing endpoint was reached when the electrical conductivity of the filtrate reached 100 μS / cm or less.

[0193] The carboxyl group content of the resulting TEMPO-oxidized pulp, as measured by the method described below, was 1.80 mmol / g. Furthermore, when the resulting TEMPO-oxidized pulp was analyzed using an X-ray diffractometer, typical peaks were observed at two positions, near 2θ = 14° to 17° and near 2θ = 22° to 23°, confirming the presence of cellulose type I crystals. Subsequently, refiner treatment and enzyme treatment were carried out in the same manner as in Production Example B to obtain microfibril cellulose dispersion (8) (MFC dispersion (8) in the table). The fiber width, as measured by the method described below, was 16.9 μm, and the degree of polymerization was 241.

[0194] (Production Example I) Ion-exchanged water was added to the phosphorylated pulp obtained in Production Example A to prepare a slurry with a solids concentration of 2% by mass. This slurry was treated five times with a single-disc refiner and once with a wet pulverizer (Starburst, manufactured by Sugino Machine Co., Ltd.) at a pressure of 150 MPa to obtain a microfibril cellulose fiber dispersion. This microfibril cellulose dispersion was subjected to defibration treatment and enzyme treatment in the same manner as in Production Example B to obtain microfibril cellulose dispersion (9) (MFC dispersion (9) in the table). The fiber width measured by the measurement method described below was 12.8 μm, and the degree of polymerization was 230.

[0195] <Production of Fine Fibrous Cellulose> (Production Example J) Ion-exchanged water was added to the phosphorylated pulp obtained in Production Example A to prepare a slurry with a solids concentration of 2% by mass. This slurry was treated four times in a wet pulverizer at a pressure of 200 MPa to obtain a fine fibrous cellulose dispersion (1) (CNF dispersion (1) in the table). X-ray diffraction confirmed that the obtained fine fibrous cellulose maintained cellulose type I crystals. Furthermore, the fiber width measured by the measurement method described below was 3 to 5 nm, and the degree of polymerization was 514.

[0196] <Cellulose Fiber Evaluation Method> (Measurement of Phosphorus Oxo Acid Group Amount) To measure the phosphorus oxo acid group amount (phosphate group amount or phosphite group amount) of cellulose fibers, ion-exchanged water was first added to the cellulose fibers to prepare a slurry with a solids concentration of 0.2% by mass. The resulting cellulose fiber dispersion was treated with an ion exchange resin, followed by titration with an alkali. The ion exchange resin treatment was performed by adding 1 / 10 by volume of a strongly acidic ion exchange resin (Amberjet 1024; manufactured by Organo Corporation, conditioned) to the cellulose dispersion, shaking for 1 hour, and then pouring the mixture onto a 90 μm mesh to separate the resin and slurry. The alkali titration was performed by adding 10 μL of 0.1 N sodium hydroxide aqueous solution to the cellulose fiber-containing slurry after the ion exchange resin treatment at 10 μL every 5 seconds, while measuring the change in the pH of the slurry. Nitrogen gas was blown into the slurry 15 minutes before the start of the titration. In this neutralization titration, two maximum points of increment (the differential value of pH with respect to the amount of alkali added) are observed on a curve plotting measured pH against the amount of alkali added. Of these, the first maximum point of increment obtained after starting alkali addition is called the first endpoint, and the next maximum point of increment obtained is called the second endpoint (Figure 1). The amount of alkali required from the start of titration to the first endpoint is equal to the amount of first dissociated acid in the slurry used for titration. Furthermore, the amount of alkali required from the start of titration to the second endpoint is equal to the total amount of dissociated acid in the slurry used for titration. The amount of alkali required from the start of titration to the first endpoint (mmol) divided by the solids content (g) in the slurry to be titrated was defined as the amount of phosphorus oxo acid group (amount of first dissociated acid) (mmol / g). The amount of alkali (mmol) required from the start of titration to the second end point was divided by the solid content (g) in the slurry to be titrated, and the resulting value was taken as the total amount of dissociated acid (mmol / g).

[0197] <Measurement of Carboxy Group Content> The carboxy group content of cellulose fibers was measured by adding ion-exchange water to the cellulose fibers to adjust the content to 0.2% by mass, treating the fibers with an ion-exchange resin, and then titrating the fibers with an alkali. The ion-exchange resin treatment was performed by adding 1 / 10 by volume of a strongly acidic ion-exchange resin (Amberjet 1024; manufactured by Organo Corporation, conditioned) to a 0.2% by mass cellulose fiber dispersion, shaking for 1 hour, and then pouring the mixture onto a 90 μm mesh to separate the resin from the slurry. The alkali titration was performed by adding 0.1 N aqueous sodium hydroxide to the cellulose fiber-containing slurry after the ion-exchange resin treatment and measuring the change in pH of the slurry. Observing the change in pH while adding aqueous sodium hydroxide yielded a titration curve as shown in Figure 2. As shown in Figure 2, in this neutralization titration, a single point was observed where the increment (the differential value of pH with respect to the amount of alkali added) was maximized on the curve plotting the measured pH against the amount of alkali added. The maximum point of this increment is called the first endpoint. Here, the region from the start of titration to the first endpoint in Figure 2 is called the first region. The amount of alkali required in the first region is equal to the amount of carboxy groups in the slurry used for titration. The amount of alkali (mmol) required in the first region of the titration curve was then divided by the solid content (g) in the cellulose fiber-containing slurry to be titrated to calculate the amount of carboxy groups introduced (mmol / g).

[0198] (Measurement of fiber width) The fiber width of microfibril cellulose is the number average fiber width measured using a fiber length measuring instrument (FS-5, manufactured by Valmet Co., Ltd.) The fiber width of fine fibrous cellulose is the number average fiber width measured using a transmission electron microscope.

[0199] (Measurement of Degree of Polymerization) The specific viscosity and degree of polymerization of cellulose fibers were measured according to Tappi T230. That is, the viscosity (referred to as η1) of the cellulose fibers to be measured dispersed in a dispersion medium and the blank viscosity (referred to as η0) measured using only the dispersion medium were measured, and then the specific viscosity (ηsp) and intrinsic viscosity ([η]) were measured according to the following formula: ηsp = (η1 / η0) - 1 [η] = ηsp / (c(1+0.28×ηsp)) where c in the formula represents the concentration of cellulose fibers at the time of viscosity measurement. Furthermore, the degree of polymerization (DP) of the cellulose fibers was calculated according to the following formula: DP = 1.75×[η] Because this degree of polymerization is the average degree of polymerization measured by the viscosity method, it is sometimes referred to as the "viscosity-average degree of polymerization."

[0200] <Production of Rubber Composition and Crosslinked Rubber Composition> (Example 1) Microfibril cellulose dispersion (2) was diluted with ion-exchanged water to a solids concentration of 1% by mass. For dilution, a tornado agitator (general-purpose high-speed agitator, PM-202, manufactured by AS ONE Corporation) and an agitator blade with a diameter of 6 inches were used, and the mixture was stirred at 1,000 rpm for 5 minutes.

[0201] To the resulting 1% by mass microfibril cellulose dispersion, an aqueous dispersion of natural rubber latex (ULACOL, solids concentration 61% by mass, manufactured by Resitex Co., Ltd.) was added so that the solids content of the rubber component was 100 parts by mass and the solids content of the microfibril cellulose was 20 parts by mass, and the mixture was stirred at 1,000 rpm for 10 minutes. Next, the mixture of microfibril cellulose and natural rubber latex was poured into a tray and dried in a hot air dryer at 40°C for approximately 48 hours to remove the aqueous solvent, yielding a rubber composition. The Payne effect of this rubber composition was measured using the measurement method described below.

[0202] The obtained rubber composition was wound around two mixing rolls (DY6-15, roll diameter 6 inches, manufactured by Daihan Co., Ltd.) and kneaded for approximately 10 minutes. Next, according to the formulation shown in Table 2, each additive was added in small amounts and kneaded for approximately 10 minutes. The amounts of additives added were 2 parts by mass of crosslinking agent (1) (dicumyl peroxide (Percumyl D, manufactured by NOF Corporation)), 3 parts by mass of zinc oxide (zinc oxide type 2, manufactured by Seido Chemical Industry Co., Ltd.), 1.5 parts by mass of antioxidant (1) (2-mercaptobenzimidazole (ANTAGE MB, manufactured by Kawaguchi Kogyo Co., Ltd.)), and 1.5 parts by mass of antioxidant (2) (2,2,4-trimethyl-1,2-dihydroquinoline polymer (ANTAGE RD, manufactured by Kawaguchi Kogyo Co., Ltd.)) per 100 parts by mass of rubber. The roll temperature during kneading was room temperature, and the roll speed was 26 / 30 rpm. The obtained rubber compound was placed in a mold and press-heated at 165° C. for 20 minutes to prepare a crosslinked rubber composition in a sheet shape with a thickness of 2 mm. The tensile properties, compression set, and dynamic viscoelasticity of this crosslinked rubber composition were measured using the measurement methods described below.

[0203] Example 2 A rubber composition and a crosslinked rubber composition were obtained in the same manner as in Example 1, except that the microfibril cellulose dispersion (3) was used.

[0204] Example 3 A rubber composition and a crosslinked rubber composition were obtained in the same manner as in Example 1, except that the microfibril cellulose dispersion (4) was used.

[0205] Example 4 A rubber composition and a crosslinked rubber composition were obtained in the same manner as in Example 1, except that the microfibril cellulose dispersion (5) was used.

[0206] Example 5 A rubber composition and a crosslinked rubber composition were obtained in the same manner as in Example 1, except that the microfibril cellulose dispersion (6) was used.

[0207] Example 6 A rubber composition and a crosslinked rubber composition were obtained in the same manner as in Example 1, except that the microfibril cellulose dispersion (7) was used.

[0208] Example 7 A rubber composition and a crosslinked rubber composition were obtained in the same manner as in Example 1, except that the microfibril cellulose dispersion (8) was used.

[0209] Example 8 A rubber composition and a crosslinked rubber composition were obtained in the same manner as in Example 1, except that microfibril cellulose dispersion (9) was used.

[0210] Example 9 A crosslinked rubber composition was obtained in the same manner as in Example 2, except that the amounts of additives added to the rubber composition of Example 2 were 1.5 parts by mass of crosslinking agent (2) (insoluble sulfur (Mucron OT-20, manufactured by Shikoku Chemical Industry Co., Ltd.)), 2 parts by mass of zinc oxide (zinc oxide type 2, manufactured by Seido Chemical Industry Co., Ltd.), 2 parts by mass of crosslinking accelerator (N-(t-butyl)-2-benzothiazole sulfenamide (TBBS, manufactured by Sanshin Chemical Industry Co., Ltd.)), 2 parts by mass of stearic acid (stearic acid 50S, manufactured by New Japan Chemical Co., Ltd.), and 2 parts by mass of antioxidant (3) (N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD, manufactured by Ouchi Shinko Chemical Industry Co., Ltd.)) per 100 parts by mass of rubber.

[0211] Comparative Example 1 A rubber composition and a crosslinked rubber composition were obtained in the same manner as in Example 1, except that the microfibril cellulose dispersion (1) was used.

[0212] Comparative Example 2 A rubber composition and a crosslinked rubber composition were obtained in the same manner as in Example 1, except that the fine fibrous cellulose dispersion (1) was used.

[0213] Comparative Example 3 Natural rubber (RSS#3, manufactured by Koshigaya Rubber Industry Co., Ltd.) was wound around two mixing rolls, and 60 parts by mass of carbon black (Seast 9H (carbon black grade: SAF-HS, particle size 18 nm), manufactured by Tokai Carbon Co., Ltd.) and each additive were added to 100 parts by mass of the rubber component according to the compounding recipe shown in Table 1, and the mixture was kneaded for about 10 minutes to obtain a rubber composition. The Payne effect of this rubber composition was measured using the measurement method described later. Furthermore, a crosslinked rubber composition was obtained from the obtained rubber composition in the same manner as in Example 1. The tensile properties, compression set, and dynamic viscoelasticity of this crosslinked rubber composition were measured using the measurement methods described later.

[0214] Comparative Example 4 A rubber composition and a crosslinked rubber composition were obtained in the same manner as in Example 1, except that the cellulose fiber was not used and natural rubber latex alone was dried.

[0215] Comparative Example 5 A crosslinked rubber composition was obtained in the same manner as in Comparative Example 1, except that the amounts of additives added to the rubber composition of Comparative Example 1 were changed to 1.5 parts by mass of crosslinking agent (2) (insoluble sulfur (Mucron OT-20, manufactured by Shikoku Chemical Industry Co., Ltd.)), 2 parts by mass of zinc oxide (zinc oxide type 2, manufactured by Seido Chemical Industry Co., Ltd.), 2 parts by mass of crosslinking accelerator (N-(t-butyl)-2-benzothiazole sulfenamide (TBBS, manufactured by Sanshin Chemical Industry Co., Ltd.)), 2 parts by mass of stearic acid (stearic acid 50S, manufactured by New Japan Chemical Co., Ltd.), and 2 parts by mass of antioxidant (3) (N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD, manufactured by Ouchi Shinko Chemical Industry Co., Ltd.)) per 100 parts by mass of rubber.

[0216] Comparative Example 6 A crosslinked rubber composition was obtained in the same manner as in Comparative Example 2, except that the amounts of additives added to the rubber composition of Comparative Example 2 were 1.5 parts by mass of crosslinking agent (2) (insoluble sulfur (Mucron OT-20, manufactured by Shikoku Chemical Industry Co., Ltd.)), 2 parts by mass of zinc oxide (zinc oxide type 2, manufactured by Seido Chemical Industry Co., Ltd.), 2 parts by mass of crosslinking accelerator (N-(t-butyl)-2-benzothiazole sulfenamide (TBBS, manufactured by Sanshin Chemical Industry Co., Ltd.)), 2 parts by mass of stearic acid (stearic acid 50S, manufactured by New Japan Chemical Co., Ltd.), and 2 parts by mass of antioxidant (3) (N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD, manufactured by Ouchi Shinko Chemical Industry Co., Ltd.)) per 100 parts by mass of rubber.

[0217] Comparative Example 7 A crosslinked rubber composition was obtained in the same manner as in Comparative Example 3, except that the amounts of additives added to the rubber composition of Comparative Example 3 were 1.5 parts by mass of crosslinking agent (2) (insoluble sulfur (Mucron OT-20, manufactured by Shikoku Chemical Industry Co., Ltd.)), 2 parts by mass of zinc oxide (zinc oxide type 2, manufactured by Seido Chemical Industry Co., Ltd.), 2 parts by mass of crosslinking accelerator (N-(t-butyl)-2-benzothiazole sulfenamide (TBBS, manufactured by Sanshin Chemical Industry Co., Ltd.)), 2 parts by mass of stearic acid (stearic acid 50S, manufactured by New Japan Chemical Co., Ltd.), and 2 parts by mass of antioxidant (3) (N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD, manufactured by Ouchi Shinko Chemical Industry Co., Ltd.)) per 100 parts by mass of rubber.

[0218] Comparative Example 8 A crosslinked rubber composition was obtained in the same manner as in Comparative Example 4, except that the amounts of additives added to the rubber composition of Comparative Example 4 were 1.5 parts by mass of crosslinking agent (2) (insoluble sulfur (Mucron OT-20, manufactured by Shikoku Chemical Industry Co., Ltd.)), 2 parts by mass of zinc oxide (zinc oxide type 2, manufactured by Seido Chemical Industry Co., Ltd.), 2 parts by mass of crosslinking accelerator (N-(t-butyl)-2-benzothiazole sulfenamide (TBBS, manufactured by Sanshin Chemical Industry Co., Ltd.)), 2 parts by mass of stearic acid (stearic acid 50S, manufactured by New Japan Chemical Co., Ltd.), and 2 parts by mass of antioxidant (3) (N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD, manufactured by Ouchi Shinko Chemical Industry Co., Ltd.)) per 100 parts by mass of rubber.

[0219] <Method for Evaluating Rubber Composition> (Measurement of Payne Effect) The rubber composition was wound around two mixing rolls to obtain a 3 mm thick sheet sample. After preheating at 100°C for 1 minute, the dynamic shear storage modulus (G' (Pa)) was measured using a rubber analyzer (RPA 2000, manufactured by Alpha Technologies). The Payne effect ΔG' (G' (0.5%) - G' (650%)) was calculated from G' at dynamic strains of 0.5% and 650%. The test conditions were a temperature of 70°C, a measurement time of 10 minutes, a frequency of 0.5 Hz, and a dynamic strain of 0.07 to 650%.

[0220] <Method for evaluating crosslinked rubber composition> (Measurement of tensile properties) For the crosslinked rubber composition, test pieces were punched out with a dumbbell (dumbbell No. 6) described in JIS K 6251:2017, and the breaking strength (MPa) and breaking elongation (%) were measured using a tensile tester (TENSILON RTG-1310, manufactured by A&D Co., Ltd.) based on JIS K 6251:2017, and the tensile energy (breaking strength (MPa) × breaking elongation (%)) was calculated. The test conditions were a test temperature of 23°C, a gauge length of 20 mm, and a pulling speed of 500 mm / min. A larger tensile energy is preferred.

[0221] (Measurement of Compression Set) For the crosslinked rubber composition, six test pieces punched to a diameter of 29 mm were stacked, and the compression set was measured based on JIS K 6262 Method A: 2013. The test conditions were a test temperature of 90°C, a compression rate of 25%, and a test period of 1 day.

[0222] (Measurement of Dynamic Viscoelasticity) For the crosslinked rubber composition, test pieces were punched out to a width of 4 mm and the loss tangent (tan δ) was measured using a dynamic viscoelasticity measuring device (NEXTA DMA200: manufactured by Hitachi High-Tech Corporation), and the ratio of tan δ at 0°C and 60°C (tan δ(0°C) / tan δ(60°C)) was calculated. The test conditions were a test temperature of -100 to +200°C (temperature change rate of 3°C / min) and a frequency of 1 Hz. Generally, when a crosslinked rubber composition is used as a tire, the larger the tan δ at 0°C, the more improved wet grip performance can be expected, and the smaller the tan δ at 60°C, the more improved rolling resistance (low rolling resistance) can be expected. Therefore, it was determined that the larger the tan δ(0°C) / tan δ(60°C), the better the crosslinked rubber composition obtained.

[0223]

[0224]

[0225]

[0226]

[0227] The results in Table 1 indicate that the rubber compositions obtained in the Examples had small ΔG' values, and that the microfibril cellulose of the present embodiment had excellent dispersibility in the rubber compositions. On the other hand, the rubber compositions of Comparative Examples 1 and 5, which had degrees of polymerization outside the range of the present invention, and the rubber compositions of Comparative Examples 3 and 7, which contained carbon black, had large ΔG' values ​​and were therefore poor in dispersibility. Furthermore, the rubber composition of Comparative Example 2, which contained fine fibrous cellulose with a fiber width of less than 1 μm, had better dispersibility than Comparative Examples 1 and 3, but was slightly poorer in dispersibility than Examples 1 to 8. Furthermore, the results in Table 2 indicate that the crosslinked rubber compositions obtained in the Examples suppressed the decrease in breaking elongation due to the addition of a reinforcing material. On the other hand, the crosslinked rubber products of Comparative Examples 3 and 7, which contained added carbon black, showed a significant decrease in breaking elongation. In Comparative Examples 1 and 2, the decrease in breaking elongation was suppressed compared to Comparative Example 3, but did not achieve sufficient breaking elongation compared to Examples 1 to 8. Furthermore, compared to Comparative Example 4, the crosslinked rubber compositions of Examples 1 to 8 had higher breaking strengths, confirming the effect of microfibril cellulose in improving breaking strength. Similarly, in Comparative Examples 5 and 6, the decrease in breaking elongation was suppressed compared to Comparative Example 7, but sufficient breaking elongation was not obtained compared to Example 9. Furthermore, compared to Comparative Example 8, the crosslinked rubber composition of Example 9 had higher breaking strengths, confirming the effect of microfibril cellulose in improving breaking strength. Furthermore, the crosslinked rubber compositions of Examples 1 to 8 had tan δ(0°C) / tan δ(60°C) of 1.7 or more, and when used in tires, they are expected to achieve both good wet performance and rolling resistance, and improved fuel economy is expected. On the other hand, the tan δ(0°C) / tan δ(60°C) of the crosslinked rubber compositions of Comparative Examples 1 to 3 was inferior to the examples. Similarly, the crosslinked rubber composition of Example 9 had a tan δ(0°C) / tan δ(60°C) of 1.7 or more, and when used in a tire, it is expected to achieve both good wet performance and rolling resistance, and to improve fuel economy. On the other hand, the tan δ(0°C) / tan δ(60°C) of the crosslinked rubber compositions of Comparative Examples 5 to 7 was inferior to that of Example 9.

[0228] 10... double drum dryer, 1... cylinder, 2... feed section, 3... sheet-like mixture, 4... scraper, 5... composite material

Claims

1. Microfibril cellulose having an average fiber width of 1 μm or more and 100 μm or less and a degree of polymerization of 50 or more and 500 or less.

2. The microfibril cellulose according to claim 1, which has anionic groups.

3. The microfibril cellulose according to claim 2, wherein the anionic groups are selected from the group consisting of phosphorus oxoacid groups, sulfur oxoacid groups, and carboxy groups.

4. The microfibril cellulose according to claim 2, wherein the content of the anionic groups is 0.50 mmol / g or more and 2.50 mmol / g or less.

5. A rubber composition comprising the microfibril cellulose according to any one of claims 1 to 4 and a rubber component.

6. The rubber composition according to claim 5, wherein the content of the microfibril cellulose is 5 parts by mass or more and 100 parts by mass or less per 100 parts by mass of the rubber component.

7. The rubber composition according to claim 5, wherein, when the storage modulus measured at a frequency of 10 Hz, 70°C, and a strain of 0.1% is G'(0.1%), and the storage modulus measured at a frequency of 10 Hz, 70°C, and a strain of 650% is G'(650%), ΔG', which is the difference between G'(0.1%) and G'(650%) and is expressed by the following formula (1), is 1,800 Pa or less. ΔG'=G'(0.1%)-G'(650%) (1) 8. A crosslinked rubber composition obtained by crosslinking a rubber compound containing the rubber composition according to claim 5 and a crosslinking agent.

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