Method for producing cellulose ester

Abstract

To provide a method for producing a cellulose ester.SOLUTION: A cellulose ester having an esterified uronic acid residue in the molecule is produced by a process including (step A) treating cellulose with an oxidizer to obtain an oxycellulose having a uronic acid salt residue; then, (step B) dispersing the oxycellulose in an aprotic polar solvent; and (step C) reacting the oxycellulose with an alkylation agent in the aprotic polar solvent.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing a cellulose ester, and more particularly to a method for producing a cellulose ester having an esterified uronic acid residue in the molecule. [Background technology]

[0002] In recent years, the production of various derivatives and composites with resins using cellulose as a raw material has been studied. Cellulose has hydroxyl groups and is highly hydrophilic, so it has low compatibility with resins. Therefore, in order to improve compatibility with resins, various modifications to make cellulose hydrophobic have been studied.

[0003] For example, a method for hydrophobizing cellulose has been disclosed in which the hydroxy group at the C6 position of the glucose residue of natural cellulose is oxidized to obtain a reaction product fiber, which is dispersed in a solvent to obtain fine cellulose fibers, which are then treated with an organic onium compound and further reacted with an alkylating agent to carry out esterification, thereby producing fine cellulose ester fibers (Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2010-59571 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, the method of Patent Document 1 requires the oxidized cellulose fiber to be treated with an organic onium compound, and also requires a step of removing the organic onium compound by washing before esterification, which complicates the process and increases costs. The object of the present invention is to provide a method for easily producing cellulose ester. [Means for solving the problem]

[0006] As a result of extensive research to achieve this object, the present inventors discovered that cellulose esters can be produced simply and easily without using organic onium compounds by reacting oxidized cellulose with an alkylating agent in an aprotic polar solvent, thereby completing the present invention.

[0007] The present invention includes, but is not limited to, the following: [1] The following steps: A step (A) of reacting cellulose with an oxidizing agent to obtain oxidized cellulose having uronate residues; (B) dispersing the oxidized cellulose having uronic acid salt residues obtained in (A) in an aprotic polar solvent; and After step (B), a step (C) of reacting the oxidized cellulose having uronic acid salt residues with an alkylating agent in an aprotic polar solvent. The present invention relates to a method for producing a cellulose ester having an esterified uronic acid residue in the molecule, the method comprising the steps of: [2] The method according to [1], comprising using an alkyl halide as the alkylating agent. [3] The method according to [1] or [2], wherein no condensation agent is used in step (C). Effect of the Invention

[0008] In the method of the present invention, various groups can be easily introduced into the uronic acid sodium salt residue in the oxidized cellulose via an ester bond without necessarily using a condensing agent or an organic onium compound. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The present invention relates to a method for producing a cellulose ester having an esterified uronic acid residue in the molecule. The cellulose ester obtained by the production method of the present invention has an esterified uronic acid residue in its molecule. The uronic acid (salt) residue refers to a repeating unit having a structure in which only the 6-position carbon in the pyranose ring is oxidized and converted to a carboxyl group, as exemplified by the following formula (1). In formula (1), when X is H, it is a uronic acid residue, and when X is a metal ion such as Na, it is a uronic acid salt residue.

[0010] [ka]

[0011] The esterified uronic acid residue refers to a repeating unit represented by the following formula (2).

[0012] [ka]

[0013] In the above formula (2), R is an alkyl group, benzyl group, phenethyl group, cholesteryl group, or cholestaryl group having 1 to 50 carbon atoms, preferably 1 to 30 carbon atoms, and more preferably 1 to 10 carbon atoms. When R is a benzyl group, it is desirable from the viewpoint of reactivity that the carbon atom at the benzyl position be covalently bonded to the oxygen atom of the ester bond.

[0014] Next, the method for producing the cellulose ester of the present invention will be described below. The method for producing the cellulose ester of the present invention includes the following steps (A), (B), and (C). Step (A): reacting cellulose with an oxidizing agent to obtain oxidized cellulose having uronate residues; Step (B): dispersing the oxidized cellulose having uronic acid salt residues obtained in step (A) in an aprotic polar solvent; and Step (C): After step (B), the oxidized cellulose having uronate residues is reacted with an alkylating agent in an aprotic polar solvent.

[0015] (Process (A)) Step (A) is a step in which an oxidizing agent is allowed to act on cellulose to obtain oxidized cellulose having uronic acid salt residues. This step can be carried out, for example, as disclosed in JP 2008-1728 A, by oxidizing cellulose in water using an N-oxyl compound as an oxidation catalyst and allowing a co-oxidizing agent to act on it (oxidation treatment). The resulting reaction product (oxidized cellulose having uronic acid residues) may be subjected to a purification treatment to remove impurities. Each treatment will be described below.

[0016] (Oxidation treatment) The type of cellulose to be subjected to the oxidation treatment is not particularly limited, and examples of the cellulose that can be used include cellulose originating from plants (e.g., wood, bamboo, hemp, jute, kenaf, agricultural waste, cloth, pulp (softwood unbleached kraft pulp (NUKP), softwood bleached kraft pulp (NBKP), hardwood unbleached kraft pulp (LUKP), hardwood bleached kraft pulp (LBKP), softwood unbleached sulfite pulp (NUSP), softwood bleached sulfite pulp (NBSP), thermomechanical pulp (TMP), recycled pulp, waste paper, etc.), animals (e.g., ascidians), algae, microorganisms (e.g., acetic acid bacteria (Acetobacter)), microbial products, etc., and regenerated cellulose. It is preferable to use cellulose originating from plants or microorganisms. More preferable is cellulose derived from plants. Regenerated cellulose is also preferable because it can be completely dissolved in the reaction liquid in step (A) and can be reacted in a homogeneous system.

[0017] The dispersion medium of the cellulose dispersion during the oxidation treatment is water, and the cellulose concentration in the dispersion can be any concentration that allows sufficient diffusion of the reagent, but is usually about 5% by mass or less relative to the mass of the dispersion.

[0018] The oxidation treatment produces oxidized cellulose having uronate residues, i.e., carboxylated cellulose. Carboxylation refers to a reaction in which the hydroxyl groups on the pyranose ring are oxidized to carboxyl groups (-COOH (acid type) or -COOX (metal salt type) (wherein X is a metal ion)). In this specification, oxidized cellulose having uronate residues may be referred to as carboxylated cellulose or simply as oxidized cellulose.

[0019] Carboxylated cellulose can be obtained by carboxylating (oxidizing) the above-mentioned cellulose by a known method such as that disclosed in JP-A-2008-1728. The amount of carboxyl groups in the carboxylated cellulose is not particularly limited, but is preferably adjusted to 0.6 to 3.0 mmol / g, more preferably 1.0 to 2.0 mmol / g, based on the bone dry mass of the carboxylated cellulose. The amount of carboxyl groups can be adjusted by controlling the type and amount of the oxidizing agent, the temperature and time during the oxidation reaction, etc.

[0020] The amount of carboxy groups in carboxylated cellulose can be measured by the following method: 60 ml of a 0.5% by mass slurry of carboxylated cellulose (medium: water) is prepared, and a 0.1 M aqueous hydrochloric acid solution is added to adjust the pH to 2.5. A 0.05 N aqueous sodium hydroxide solution is then added dropwise to measure the electrical conductivity until the pH reaches 11. The electrical conductivity is calculated using the following formula from the amount of sodium hydroxide (a) consumed in the neutralization stage of the weak acid, where the change in electrical conductivity is gradual: Amount of carboxyl group [mmol / g carboxylated cellulose] = a [ml] × 0.05 / mass of carboxylated cellulose [g].

[0021] An example of a carboxylation (oxidation treatment) method is a method in which cellulose is oxidized in water using an oxidizing agent in the presence of an N-oxyl compound and a compound selected from the group consisting of bromides, iodides, or a mixture thereof. This oxidation treatment selectively oxidizes the primary hydroxyl group at the C6 position of the glucopyranose ring on the cellulose surface, and cellulose (carboxylated cellulose) having an aldehyde group and a carboxy group on the surface can be obtained. The concentration of cellulose in water during the reaction is not particularly limited, but is preferably 5% by mass or less.

[0022] N-oxyl compounds are oxidation catalysts for cellulose, and many N-oxyl compounds that can be used in the above-mentioned oxidation treatment have been reported (see, for example, "Catalytic oxidation of cellulose using TEMPO derivatives: HPSEC and NMR analysis of oxidation products" by I. Shibata et al., "Cellulose", Vol. 10, 2003, pp. 335-341). In particular, 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), 4-acetamido-TEMPO, 4-carboxy-TEMPO, and 4-phosphonooxy-TEMPO are preferred in terms of reaction rate in water at room temperature. The addition of these N-oxyl compounds is sufficient in catalytic amounts, and is preferably added to the reaction solution in the range of 0.1 to 4 mmol / L, more preferably 0.2 to 2 mmol / L.

[0023] As the co-oxidizing agent, a hypohalous acid or its salt, a halo acid or its salt, a perhalogen acid or its salt, hydrogen peroxide, a perorganic acid, etc. can be used. An alkali metal hypohalite, such as sodium hypochlorite or sodium hypobromite, is preferred. When sodium hypochlorite is used, it is preferable in terms of reaction rate to proceed with the reaction in the presence of an alkali metal bromide, such as sodium bromide. The amount of the alkali metal bromide added is about 1 to 40 times the molar amount, preferably about 10 to 20 times the molar amount of the N-oxyl compound.

[0024] The pH of the reaction solution during the oxidation treatment is preferably maintained in the range of about 8 to 11. The temperature may be in the range of about 4 to 40° C., and the treatment can also be performed at room temperature; no particular control of the temperature is required.

[0025] In the case of natural cellulose, the reaction end point can be any point, but in the case of regenerated cellulose, the end point is when the solution becomes a transparent aqueous solution. The medium in the reaction solution is preferably water, since it is easy to handle and unlikely to cause side reactions.

[0026] The reaction time in the oxidation treatment can be appropriately set according to the degree of progress of the oxidation, and is usually from 0.5 to 6 hours, for example, about 0.5 to 4 hours. The oxidation treatment may be carried out in two stages. For example, the oxidized cellulose obtained by filtration after the completion of the first-stage reaction is oxidized again under the same or different reaction conditions, allowing the oxidation to proceed efficiently without being inhibited by table salt produced as a by-product in the first-stage reaction.

[0027] Another example of the carboxylation (oxidation) method is a method in which cellulose is oxidized by contacting it with an ozone-containing gas. This oxidation treatment oxidizes at least the hydroxyl groups at positions 2 and 6 of the glucopyranose ring to carboxy groups, and decomposes the cellulose chain. The ozone concentration in the ozone-containing gas is 50 to 250 g / m 3 It is preferable that the thickness is 50 to 220 g / m 3It is more preferable that the ozone is added to the cellulose in an amount of 0.1 to 30 parts by mass, and more preferably 5 to 30 parts by mass, based on 100 parts by mass of the solid content of the cellulose. The temperature during the ozone treatment is preferably 0 to 50°C, and more preferably 20 to 50°C. The ozone treatment time is not particularly limited, but is about 1 to 360 minutes, and preferably about 30 to 360 minutes. When the ozone treatment conditions are within these ranges, it is possible to prevent the cellulose from being excessively oxidized and decomposed, and the yield of the oxidized cellulose is good. After the ozone treatment, an additional oxidation treatment may be performed using an oxidizing agent. The oxidizing agent used in the additional oxidation treatment is not particularly limited, but examples thereof include chlorine-based compounds such as chlorine dioxide and sodium chlorite, oxygen, hydrogen peroxide, persulfuric acid, and peracetic acid. For example, these oxidizing agents are dissolved in a polar organic solvent such as water or alcohol to prepare an oxidizing agent solution, and the additional oxidation treatment can be performed by immersing the cellulose in the solution.

[0028] (Refining process) In the purification process, compounds other than the reaction product and water contained in the reaction slurry, such as unreacted hypochlorous acid and various by-products, are removed from the system. A conventional purification method can be used for the purification process, and for example, a high-purity oxidized cellulose aqueous dispersion can be obtained by repeating washing with water and filtration. For example, a method using centrifugal dehydration (for example, a continuous decanter) may be used. The concentration of oxidized cellulose in the aqueous dispersion after the purification process is not particularly limited, but may be in the range of approximately 10% to 50% by mass as the solid content (cellulose) concentration. In the subsequent process, if the solid content concentration is higher than 50% by mass, it is not preferable because extremely high energy is required for dispersion. When regenerated cellulose is used as the raw material, the reaction product (oxidized cellulose) after the oxidation process is completely dissolved, but the reaction product can be insolubilized and emerged by dropping the reaction solution into an excess of poor solvent (reprecipitation). The insolubilized reaction product can be obtained by filtering.

[0029] (Process (B)) Step (B) is a step of dispersing the oxidized cellulose having uronic acid salt residues obtained in step (A) in an aprotic polar solvent. Examples of aprotic polar solvents include acetonitrile, dimethylacetamide, N-methylpyrrolidone, dimethylsulfoxide, and dimethylformamide. If the reaction rate is to be improved by increasing the reaction temperature in the subsequent step (C), it is preferable to use a high-boiling point solvent, such as dimethylsulfoxide or dimethylformamide, as the aprotic polar solvent. The aprotic polar solvent may be a single type or a mixture of two or more types. The amount of the aprotic polar solvent used can be appropriately changed depending on various conditions such as the amount of oxidized cellulose to be added and the amount of alkylating agent to be added in the subsequent step (C), and is usually an amount such that the concentration of oxidized cellulose is 0.01 to 50% by mass, preferably 0.5 to 20% by mass, and more preferably 1.0 to 10% by mass.

[0030] The device used for dispersion in step (B) is not particularly limited, and a common dispersing machine or stirrer can be used. (Process (C)) In step (C), the oxidized cellulose having uronic acid salt residues dispersed in an aprotic polar solvent in step (B) is reacted with an alkylating agent in the same solvent. In this step, the uronic acid salt residues are esterified to obtain a cellulose ester. Representative alkylating agents that can be used in step (C) include alkyl halides, alkyl sulfates (dimethyl sulfate, diethyl sulfate, etc.), alkyl tosylates (methyl p-toluenesulfonate, etc.), alkyl mesylates (methyl methanesulfonate, etc.), and alkyl triflates (methyl trifluoromethanesulfonate, etc.). In terms of cost, etc., alkyl halides are preferred. Specific examples of alkyl halides include methyl chloride, methyl bromide, methyl iodide, ethyl chloride, ethyl bromide, ethyl iodide, propyl chloride, propyl bromide, propyl iodide, butyl chloride, butyl bromide, butyl iodide, hexyl chloride, hexyl bromide, hexyl iodide, decyl chloride, decyl bromide, decyl iodide, hexadecyl chloride, hexadecyl bromide, hexadecyl iodide, octadecyl chloride, octadecyl bromide, octadecyl iodide, cholesteryl chloride, Examples of the alkylating agent include cholesteryl bromide, cholesteryl iodide, cholestaryl chloride, cholestaryl bromide, cholestaryl iodide, benzyl chloride, benzyl bromide, benzyl iodide, phenethyl chloride, phenethyl bromide, phenethyl iodide, C1-C50 polyfluoroalkyl chloride, C1-C50 polyfluoroalkyl bromide, C1-C50 polyfluoroalkyl iodide, etc., and among the above, bromides are particularly preferred in terms of reactivity and handling. When chlorides or bromides are used as alkylating agents, sodium iodide may be added as a catalyst.

[0031] The reaction temperature in step (C) (esterification reaction) may be appropriately changed depending on the boiling point of the solvent and the stability of the oxidized cellulose, and can usually be carried out in the range of about room temperature to 200°C. It is preferably in the range of 50°C to 150°C, and most preferably 70°C to 120°C. The reaction time is preferably 0.1 to 48 hours, more preferably 1 to 24 hours, even more preferably 8 to 18 hours, and even more preferably about 10 to 14 hours. The equivalent of the alkylating agent is preferably 1 or more per unit of uronic acid (salt) residue, and the higher the equivalent, the higher the reaction efficiency.

[0032] In the present invention, condensation agents such as carbodiimide-based condensation agents such as 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide (EDC) and N,N'-dicyclohexylcarbodiimide (DCC), imidazole-based condensation agents such as N,N'-carbonyldiimidazole (CDI), triazine-based condensation agents such as 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMT-MM), or phosphonium-based condensation agents such as 1H-benzotriazol-1-yloxytris(dimethylamino)phosphonium hexafluorophosphate, which are generally used in esterification, are not necessarily required. In addition, organic onium compounds such as those listed in Patent Document 1 (JP Patent Publication 2010-59571 A) are not necessarily required. By using the oxidized cellulose having uronate residues (e.g., oxidized cellulose having a metal salt type carboxy group such as -COONa) obtained in step (A), it becomes possible to react the oxidized cellulose with an alkylating agent in step (C).

[0033] Furthermore, in general, in order to reduce the hydrophilicity of oxidized cellulose, oxidized cellulose having carboxy groups in a metal salt form obtained by oxidation treatment such as step (A) is contacted with an acid to convert the metal salt form to an acid form (-COOH); however, the present invention does not require such a treatment for converting the metal salt form of the carboxy groups into the acid form.

[0034] (Defibrilization) The cellulose ester obtained by the present invention may be in a defibrated form such as cellulose nanofiber or microfibrillated cellulose, if necessary. When defibration is performed, it may be performed after step (C) or before step (C) (for example, before step (A), between steps (A) and (B), or between steps (B) and (C)).

[0035] The device used for defibration is not particularly limited, but it is preferable to use a device capable of applying a strong shear force, such as a high-speed rotation type, colloid mill type, high-pressure type, roll mill type, or ultrasonic type. In particular, for efficient defibration, it is preferable to use a wet-type high-pressure or ultra-high-pressure homogenizer that can apply a pressure of 50 MPa or more and a strong shear force. The pressure is more preferably 100 MPa or more, and even more preferably 140 MPa or more. Furthermore, prior to defibration with a high-pressure homogenizer, a pretreatment may be performed, if necessary, using a known mixing, stirring, emulsifying, or dispersing device such as a high-speed shear mixer. The number of treatments (passes) with the defibration device may be one or two or more, and two or more are preferred.

[0036] (Application) The cellulose ester obtained by the present invention is considered to be usable in various fields.For example, but not limited to, it can be used as thickener, gelling agent, paste, excipient, paint additive, adhesive additive, abrasive, rubber / plastic compounding material, water retention agent, shape retention agent, viscosity adjuster, emulsion stabilizer, foam stabilizer, dispersion stabilizer, muddy water adjuster, filter aid, overflow prevention agent, etc. in cosmetics, medicine, various chemical products, paint, ink, spray, agricultural chemicals, glaze, civil engineering, construction, electronic materials, flame retardant, household goods, adhesive, detergent, fragrance, lubricant composition, etc. EXAMPLES

[0037] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples. In the following description, unless otherwise specified, "parts" and "%" mean "parts by mass" and "% by mass", respectively.

[0038] [Example 1] 9.57 g of cellulose (crystalline cellulose, trade name: Avicel (registered trademark)) was stirred in 21.5% NaOH aqueous solution (43 g NaOH / 157 mL water) for 24 hours, and then neutralized with 4% acetic acid. 20 minutes after the start of neutralization, the insoluble portion was filtered off and washed with distilled water (100 mL x 3). The filtered material was then freeze-dried to obtain regenerated cellulose (9.29 g). 1.97 g of the obtained regenerated cellulose, 189 mg (0.15 eq) of sodium bromide, and 21.6 mg (0.01 eq) of TEMPO were added to 100 mL of distilled water and stirred until TEMPO was completely dissolved. Then, 20.8 mL (2.2 eq) of sodium hypochlorite solution was added. During the reaction, the pH was maintained at 10 using 1N NaOH aqueous solution. When the yellow color of the reaction suspension disappeared, sodium hypochlorite was added each time. When the solution became almost transparent, ethanol (5 mL) was added. The pH was adjusted to 7 with 1N hydrochloric acid. Then, reprecipitation was repeatedly performed using ethanol. Finally, the obtained precipitate was freeze-dried to obtain 2.2776 g of oxidized cellulose having uronic acid residues in the sodium salt form.

[0039] The obtained oxidized cellulose (148.3 mg) was added to dimethylformamide and stirred at 90°C. After 15 minutes, 1.5 mL of benzyl bromide was added dropwise, followed by the addition of 20 mg of sodium iodide. After 26 hours, the reaction was quenched by reprecipitation with ethanol. The reaction was then washed three times with ethanol. The resulting precipitate was dried in vacuum to obtain the product. NMR analysis of the product confirmed that it was the benzyl ester of oxidized cellulose (44.6 mg).

[0040] The NMR measurement was performed by dissolving the sample (the above product) in DMSO-d6. The equipment used was a Varian 500MHz NMR (Agilent Technologies). Spectral assignment was performed using COSY, HSQC, HMBC, and TOCSY.

[0041] [Example 2] The phenethyl ester of oxidized cellulose was obtained in the same manner as in Example 1, except that benzyl bromide was replaced with phenethyl bromide.

[0042] [Example 3] A phenethyl ester of oxidized cellulose was obtained in the same manner as in Example 2, except that the reaction time was changed. The reaction conditions are shown in Table 1.

[0043] The degree of phenethyl substitution in the phenethyl ester of the obtained oxidized cellulose was measured by the following method: The product was prepared in aqueous solution, and the UV absorption (258 nm) due to the aromatic nucleus of the phenethyl group was measured. The equipment used was a JASCO UV-Visible Spectrophotometer (V-560). A calibration curve was prepared using an aqueous solution of benzyl alcohol.

[0044] The measurement results of the phenethyl substitution degree are shown in Table 1. It can be seen from Table 1 that the longer the reaction time, the higher the phenethyl substitution degree.

[0045] [Table 1]

[0046] [Example 4] A phenethyl ester of oxidized cellulose was obtained in the same manner as in Example 2, except that the equivalent amount of the alkylating agent (phenethyl bromide) was changed. The conditions are shown in Table 2.

[0047] The degree of phenethyl substitution in the phenethyl ester of the obtained oxidized cellulose was measured by the method described in Example 3. The results are shown in Table 2. The results in Table 2 show that the degree of phenethyl substitution increases as the alkylating agent equivalent is increased. Note that from about 12 equivalents, esterification proceeded further and the cellulose became insoluble in water, making it impossible to measure the degree of substitution in the same manner.

[0048] [Table 2]

Claims

1. The process is as follows: A step (A) of reacting cellulose with an oxidizing agent to obtain oxidized cellulose having uronate residues; A step (B) of dispersing the oxidized cellulose having uronic acid salt residues obtained in the step (A) in an aprotic polar solvent; and After step (B), a step (C) of reacting the oxidized cellulose having uronic acid salt residues with an alkyl halide in an aprotic polar solvent. A method for producing a cellulose ester having an esterified uronic acid residue in the molecule, comprising: The above production method does not use an organic onium compound.

2. The method according to claim 1, wherein no condensing agent is used in step (C).

Images