Flame-retardant coating agent and flame-retardant sheet

Abstract

The present invention provides a novel flame-retardant coating agent and flame-retardant sheet including anionically modified fine fibrous cellulose. A flame-retardant coating agent and a flame-retardant sheet according to the embodiments include anionically modified fine fibrous cellulose having a number-average fiber width of 2 to 1,000 nm, wherein counter ions of anionic groups in the anionically modified fine fibrous cellulose include metal ions.

Description

Flame-retardant coatings and flame-retardant sheets

[0001] The present invention relates to a flame-retardant coating agent and a flame-retardant sheet.

[0002] In recent years, from the perspective of sustainability, cellulose fibers, a naturally occurring biomass, have been attracting attention, particularly fine fibrous cellulose (also known as cellulose nanofibers) as a new form of utilization.Fine fibrous cellulose is not inherently flame-retardant, and to impart flame retardancy, flame retardants such as inorganic compounds are added.However, large amounts must be added, and uniform dispersion is difficult.

[0003] Patent Document 1 discloses a bio-based, non-toxic, flame-retardant composition containing a mixture of fibrillated cellulose nanofibers (CNF) at a concentration of 8 to 60% and a mineral component at a weight ratio of 25 to 75% (CNF / mineral component) in the form of a wet slurry or paste. Thus, Patent Document 1 describes a flame-retardant composition containing fine fibrous cellulose, but does not disclose that a metal salt of anion-modified fine fibrous cellulose has flame retardancy.

[0004] Patent No. 7179785

[0005] An object of embodiments of the present invention is to provide a novel flame-retardant coating agent and a flame-retardant sheet containing anionically modified fine fibrous cellulose.

[0006] The present inventors have found that the anionic groups of anion-modified fine fibrous cellulose can be converted into a metal salt to make the fine fibrous cellulose flame retardant, and have devised the use of such a metal salt of anion-modified fine fibrous cellulose to produce a flame retardant coating agent and a flame retardant sheet. The present invention includes the following embodiments.

[0007] [1] A flame-retardant coating agent comprising anion-modified fine fibrous cellulose having a number-average fiber width of 2 to 1,000 nm, wherein counterions of the anionic groups of the anion-modified fine fibrous cellulose comprise metal ions. [2] The flame-retardant coating agent according to [1], wherein the anion-modified fine fibrous cellulose has an anionic group content of 0.5 to 3.0 mmol / g, measured after all of the anionic groups are in the acid form. [3] The flame-retardant coating agent according to [1] or [2], wherein the metal ions comprise monovalent metal ions and polyvalent metal ions. [4] The flame-retardant coating agent according to [3], wherein the molar ratio of the monovalent metal ions to the polyvalent metal ions is 10 / 90 to 80 / 20. [5] The flame-retardant coating agent according to any one of [1] to [4], wherein the anionic groups are carboxy groups.

[0008] [6] A flame-retardant sheet comprising anion-modified fine fibrous cellulose having a number-average fiber width of 2 to 1,000 nm, wherein counterions of the anionic groups of the anion-modified fine fibrous cellulose comprise metal ions. [7] The flame-retardant sheet according to [6], wherein the anion-modified fine fibrous cellulose has an amount of anionic groups, measured after all of the anionic groups are in the acid form, of 0.5 to 3.0 mmol / g. [8] The flame-retardant sheet according to [6] or [7], wherein the metal ions comprise monovalent metal ions and polyvalent metal ions. [9] The flame-retardant sheet according to [8], wherein the molar ratio of the monovalent metal ions to the polyvalent metal ions is 10 / 90 to 80 / 20.

[0009]

[10] A flame-retardant coating layer comprising anion-modified fine fibrous cellulose having a number average fiber width of 2 to 1000 nm, wherein counter ions of the anionic groups of the anion-modified fine fibrous cellulose contain metal ions.

[0010] According to an embodiment of the present invention, a novel flame-retardant coating agent and a flame-retardant sheet containing anion-modified fine fibrous cellulose can be provided.

[0011] [Flame-Retardant Coating Agent] The flame-retardant coating agent according to this embodiment contains anion-modified fine fibrous cellulose, and the counterions of the anionic groups contain metal ions. The anion-modified fine fibrous cellulose allows metal ions to be efficiently introduced into the anionic groups, thereby making the cellulose fibers flame-retardant. Therefore, the flame-retardant coating agent can be used as a coating agent for imparting flame retardancy. Furthermore, the introduction of metal ions can improve the applicability of the coating agent and increase the strength of the coating film.

[0012] Anion-modified fine fibrous cellulose is fine fibrous cellulose into which anionic groups have been introduced. Fine fibrous cellulose is cellulose fibers refined to the nano-level and is also called cellulose nanofiber. The number-average fiber width of the fine fibrous cellulose is 2 to 1000 nm, preferably 2 to 500 nm, more preferably 2 to 300 nm, more preferably 2 to 100 nm, more preferably 3 to 50 nm, and more preferably 3 to 20 nm. The smaller the number-average fiber width, the better the application properties as a coating agent, particularly the effect of suppressing dripping after application, and the stronger the coating film.

[0013] The number average fiber length of the fine fibrous cellulose is not particularly limited and may be, for example, 500 nm to 10 μm, 700 to 5000 nm, 800 to 3000 nm, or 1000 to 2000 nm. The average aspect ratio of the fine fibrous cellulose is not particularly limited and may be, for example, 10 to 1000 or 50 to 700.

[0014] The number-average fiber width of fine fibrous cellulose can be determined by observation using an atomic force microscope (AFM). The number-average fiber width is obtained by selecting at least 120 fine fibrous cellulose fibers from an AFM image, measuring their fiber widths, and calculating the arithmetic mean. The number-average fiber length is also determined by similarly selecting at least 120 fine fibrous cellulose fibers, measuring their fiber lengths, and calculating the arithmetic mean. The average aspect ratio is determined by the ratio of the number-average fiber length to the number-average fiber width calculated in this manner.

[0015] Examples of the anionic group of the anion-modified fine fibrous cellulose include at least one selected from the group consisting of a carboxyl group, a phosphate group, a sulfate group, a sulfonate group, a nitrate group, and a borate group. Among these, at least one selected from the group consisting of a carboxyl group, a phosphate group, and a sulfate group is preferred. These anionic groups may be directly or indirectly bonded to glucose units, which are structural units of the cellulose molecule. When indirectly bonded, for example, an alkylene group having 1 to 4 carbon atoms may be present between the glucose unit and the anionic group. One or more anionic groups may be bonded to all glucose units constituting the cellulose molecule, or one or more anionic groups may be bonded to a portion of the glucose units constituting the cellulose molecule.

[0016] In one embodiment, the anionic group of the anion-modified fine fibrous cellulose is preferably a carboxy group, because the carboxy group has a more excellent effect of improving the strength of the coating film than other anionic groups.

[0017] In one embodiment, examples of anion-modified fine fibrous cellulose include oxidized cellulose nanofibers obtained by oxidizing the hydroxyl groups of glucose units in cellulose molecules, and carboxymethylated cellulose nanofibers obtained by carboxymethylating the hydroxyl groups of glucose units in cellulose molecules. Examples of oxidized cellulose nanofibers include those in which the hydroxyl group at the C6 position of the glucose unit in the cellulose molecule is selectively oxidized to a carboxyl group. Oxidized cellulose nanofibers are obtained by oxidizing natural cellulose such as wood pulp using a co-oxidant in the presence of an N-oxyl compound and subjecting the cellulose to a defibration (fine fiber) treatment. As the N-oxyl compound, a compound having a nitroxy radical, which is commonly used as an oxidation catalyst, is used, such as a piperidine nitroxyoxy radical. 2,2,6,6-tetramethylpiperidinoxy radical (TEMPO) or 4-acetamido-TEMPO is particularly preferred. In a preferred embodiment, the anion-modified fine fibrous cellulose is a TEMPO-oxidized cellulose nanofiber obtained by oxidizing using TEMPO.

[0018] The anion-modified fine fibrous cellulose preferably has an anionic group amount of 0.5 to 3.0 mmol / g, more preferably 1.0 to 2.8 mmol / g, more preferably 1.5 to 2.6 mmol / g, and even more preferably 1.7 to 2.5 mmol / g, measured after converting all anionic groups to the acid form. The greater the amount of anionic groups, the more flame retardant the cellulose can be. Because the anion-modified fine fibrous cellulose according to this embodiment is in the metal salt form, all anionic groups are converted to the acid form before measurement of the amount of anionic groups. The amount of anionic groups is the amount of anionic groups (mmol) per dry mass of the acid-form anion-modified fine fibrous cellulose, and can be measured by known methods, specifically the method described in the Examples section. In this specification, the term "dry mass" refers to the mass after drying at 140°C until the mass change rate per minute is 0.05% or less.

[0019] In this embodiment, the anion-modified fine fibrous cellulose used has a metal ion as a counter ion of the anionic group. That is, a metal ion is bonded to the anionic group to form a metal salt. In this case, it is preferable that the counter ions of all the anionic groups are metal ions, but counter ions other than metal ions may be used. For example, the anionic group may contain an onium salt together with the metal salt, and not all of the anionic groups may be in the salt form, and the anionic group may be in the acid form (the counter ion is H ＋ and is also referred to as H-type. For example, in the case of a carboxy group, it may contain —COOH). The amount of metal salt introduced is, for example, preferably 50 mol % or more of the anionic groups, more preferably 80 mol % or more, more preferably 90 mol % or more, and even more preferably 100 mol %. Here, the amount of metal salt introduced is the ratio of anionic groups forming the metal salt to 100 mol % of the anionic groups, and is calculated from the amount of the anionic groups and the metal content measured with an ICP atomic emission spectrometer.

[0020] Examples of the metal ion include monovalent metal ions such as sodium ions, lithium ions, and potassium ions, divalent metal ions such as magnesium ions and calcium ions, and trivalent metal ions such as aluminum ions. These may be used alone or in combination of two or more.

[0021] The metal ions may be monovalent metal ions alone, polyvalent metal ions such as magnesium ions, calcium ions, and aluminum ions alone, or a combination of monovalent and polyvalent metal ions. The term "combined use" means that the anionic groups of the anion-modified fine fibrous cellulose contain anionic groups bonded to monovalent metal ions and anionic groups bonded to polyvalent metal ions.

[0022] In one embodiment, it is more preferable for the metal ions to contain both monovalent and polyvalent metal ions in order to achieve both flame retardancy and coating film strength and applicability. The molar ratio of monovalent metal ions to polyvalent metal ions (monovalent metal ions / polyvalent metal ions) is preferably 10 / 90 to 80 / 20. That is, the ratio of polyvalent metal ions to 100 mol% of metal ions is preferably 20 to 90 mol%, and the ratio of monovalent metal ions is preferably 10 to 80 mol%. The molar ratio of monovalent metal ions / polyvalent metal ions is more preferably 20 / 80 to 30 / 70, and even more preferably 25 / 75 to 40 / 60. The ratio of monovalent metal ions to polyvalent metal ions can be determined by measuring the content of various metals using an ICP atomic emission spectrometer, and can be measured in detail by the method described in the Examples section.

[0023] Anion-modified fine fibrous cellulose is obtained by the steps of chemically modifying unmodified cellulose fibers and defibrating the cellulose fibers, as described above. Defibration of the cellulose fibers may be carried out after or before the introduction of anionic groups. Defibration can be carried out by treating a dispersion of cellulose fibers using, for example, a homomixer, a high-pressure homogenizer, an ultrasonic dispersion processor, a beater, a disk refiner, a conical refiner, a double-disc refiner, a grinder, or the like under high-speed rotation.

[0024] The anion-modified fine fibrous cellulose preferably has a cellulose type I crystal structure. The cellulose type I crystal structure is the crystalline form of natural cellulose, and the anion-modified fine fibrous cellulose is water-insoluble due to the cellulose type I crystal structure. The presence of a cellulose type I crystal structure can be identified by the presence of typical peaks at two positions, around 2θ = 14° to 17° and around 2θ = 22° to 23°, in a diffraction profile obtained by wide-angle X-ray diffraction image measurement.

[0025] The flame-retardant coating agent according to this embodiment is a dispersion in which the anion-modified microfibrous cellulose is dispersed in a liquid dispersion medium. Water is preferred as the dispersion medium. In this case, the dispersion medium may be water alone, or may contain a water-soluble organic solvent such as ethanol, isopropyl alcohol, or methanol together with water. When an organic solvent is contained, the amount of water in 100% by mass of the dispersion medium is preferably 30% by mass or more, more preferably 60% by mass or more. The dispersion medium may also be an aqueous solution containing an acid, alkali, or a salt thereof, as long as the effect is not impaired.

[0026] In the flame-retardant coating agent, the concentration of the anion-modified fine fibrous cellulose is not particularly limited and may be, for example, 0.01 to 10% by mass, or may be 0.1 to 1% by mass. The amount of the anion-modified fine fibrous cellulose in the solid content of the flame-retardant coating agent is also not particularly limited and, for example, is preferably 70% by mass or more, more preferably 80% by mass or more, more preferably 90% by mass or more, more preferably 95% by mass or more, relative to 100% by mass of the solid content, and may be 100% by mass.

[0027] The viscosity of the flame-retardant coating agent is not particularly limited and may be, for example, 1 to 100,000 mPa·s, or 10 to 10,000 mPa·s. Here, the viscosity of the flame-retardant coating agent is the viscosity measured with a BM-type viscometer at 25°C and a rotation speed of 3 rpm for 3 minutes.

[0028] The flame-retardant coating agent may contain additives as optional components in addition to the anion-modified fine fibrous cellulose and the dispersion medium. Examples of additives include colorants such as pigments and dyes, water-resistant agents, flame retardants, plasticizers, antioxidants, light stabilizers, fillers, and antistatic agents. Since the anion-modified fine fibrous cellulose has flame retardancy, the flame-retardant coating agent according to this embodiment does not require other flame retardants. Therefore, in one embodiment, the flame-retardant coating agent does not contain other flame retardants. The same applies to the flame-retardant coating layer described below.

[0029] The flame-retardant coating agent according to this embodiment is used to impart flame retardancy, and for example, by applying it to the surface of an object (i.e., an object) that is to be made flame-retardant, the object can be made less flammable or prevented from spreading flames, thereby imparting flame retardancy to the object.

[0030] The object to be treated with the flame-retardant coating agent is not particularly limited, and examples thereof include paper, wood, resin, rubber, etc. The method of treatment with the coating agent is not particularly limited, and examples thereof include application by spraying (atomization), application by a coating machine, application by brush, and immersion of the object in the coating agent.

[0031] In one embodiment, when an object is made flame retardant by immersion in a coating agent, the object is immersed in a coating agent containing monovalent metal salt-type anion-modified fine fibrous cellulose, thereby forming a coating layer containing anion-modified fine fibrous cellulose.The object on which the coating layer has been formed may then be immersed in an aqueous solution of polyvalent metal ions to replace at least a portion of the monovalent metal ions with polyvalent metal ions, thereby forming a coating layer containing anion-modified fine fibrous cellulose with a polyvalent metal salt.

[0032] [Flame-retardant sheet] The flame-retardant sheet according to this embodiment is a sheet containing anion-modified fine fibrous cellulose having a number-average fiber width of 2 to 1000 nm, and the counter ions of the anionic groups in the cellulose contain metal ions. Details of the anion-modified fine fibrous cellulose are as described above for the flame-retardant coating agent, and therefore will not be described here.

[0033] The flame-retardant sheet is a sheet used to impart flame retardancy, and can be used by layering it on the surface or inside of an object to be flame-retarded (i.e., an object). For example, the flame-retardant sheet can be attached to the surface of the object or sandwiched inside a laminate that is the object. More specific uses include, for example, a sheet incorporated into packaging material to make the packaging material flame-retardant, and a sheet for architectural interiors such as wallpaper. In this specification, the term "sheet" encompasses the concept of "film."

[0034] The flame-retardant sheet may be a sheet having a single layer structure consisting of a layer containing the anion-modified fine fibrous cellulose, or a sheet having a laminate structure containing a layer containing the anion-modified fine fibrous cellulose and another layer. The thickness of the flame-retardant sheet is not particularly limited and may be, for example, 0.1 to 1000 μm or 1 to 100 μm.

[0035] The flame-retardant sheet may consist solely of the anion-modified fine fibrous cellulose, or may contain additives as optional components in addition to the anion-modified fine fibrous cellulose. The amount of the anion-modified fine fibrous cellulose in the flame-retardant sheet is not particularly limited, and may be, for example, 70% by mass or more, 80% by mass or more, 90% by mass or more, 95% by mass or more, or 100% by mass.

[0036] In one embodiment, the flame-retardant sheet may contain a crosslinking agent as an additive for crosslinking the anion-modified fine fibrous cellulose, which can impart water resistance to the sheet. Examples of other additives include colorants such as pigments and dyes, water-resistant agents, flame retardants, plasticizers, antioxidants, light stabilizers, fillers, and antistatic agents. Since the anion-modified fine fibrous cellulose provides flame retardancy in the flame-retardant sheet according to this embodiment, no other flame retardants are required. Therefore, in one embodiment, the flame-retardant sheet does not contain any other flame retardants.

[0037] The flame-retardant sheet may be a fiber sheet such as paper or nonwoven fabric. When forming a fiber sheet, for example, it may be produced by applying a suspension containing anion-modified fine fibrous cellulose onto a release sheet, drying it, and then peeling it off from the release sheet.

[0038] [Flame-retardant coating layer] The flame-retardant coating layer according to this embodiment is a coating layer containing anion-modified fine fibrous cellulose having a number-average fiber width of 2 to 1,000 nm, and a counter ion of the anionic group containing a metal ion. Details of the anion-modified fine fibrous cellulose are as described above for the flame-retardant coating agent, and therefore will not be described here.

[0039] The flame-retardant coating layer is a coating layer provided to impart flame retardancy to an object to be flame-retarded, and the presence of the coating layer can impart flame retardancy to the object. The object is not particularly limited, and examples thereof include paper, wood, resin, rubber, etc.

[0040] The flame-retardant coating layer may be formed, for example, by treating an object with the flame-retardant coating agent. Therefore, in one embodiment, the flame-retardant coating layer may consist solely of anion-modified fine fibrous cellulose, or may contain additives as optional components in addition to the anion-modified fine fibrous cellulose. The amount of anion-modified fine fibrous cellulose in the flame-retardant coating layer is not particularly limited, but is preferably 70% by mass or more, more preferably 80% by mass or more, more preferably 90% by mass or more, more preferably 95% by mass or more, or even 100% by mass. Examples of additives include colorants such as pigments and dyes, water-resistant agents, flame retardants, plasticizers, antioxidants, light stabilizers, fillers, and antistatic agents.

[0041] The thickness of the flame-retardant coating layer is not particularly limited, and may be, for example, 0.01 to 100 μm, or 0.1 to 10 μm.

[0042] Examples will be described in detail below along with comparative examples, but the present invention is not limited to these examples.

[0043] The methods for measuring the various physical properties in the examples and comparative examples are as follows.

[0044] [Amount of Anionic Groups (Amount of Carboxy Groups)] A 50 mL aqueous suspension of acid-type anion-modified fine fibrous cellulose with a cellulose fiber concentration of 0.1% by mass was prepared, and the pH was adjusted to approximately 2.5 with a 0.1 mol / L aqueous hydrochloric acid solution. Next, a 0.05 mol / L aqueous sodium hydroxide solution was added dropwise to the aqueous suspension, and electrical conductivity measurements were performed until the pH reached approximately 11. The amount of carboxy groups was calculated from the amount of sodium hydroxide (V) consumed in the neutralization stage of the weak acid, where the change in electrical conductivity was gradual, according to the following formula: Amount of Carboxy Groups (mmol / g) = V (mL) × [0.05 / Mass of Acid-Type Anion-Modified Fine Fibrous Cellulose (g)]

[0045] [Amount of Anionic Groups (Amount of Phosphate Groups)] Anion-modified fine fibrous cellulose was diluted with ion-exchange water to a content of 0.2% by mass to prepare an aqueous suspension. The aqueous suspension was treated with an ion exchange resin to obtain acidic anion-modified fine fibrous cellulose, and then titrated with alkali to measure the amount. The ion exchange resin treatment was carried out by adding 1 / 10 by volume of a strongly acidic ion exchange resin (Amberjet 1024; Organo Corporation, conditioned) to the aqueous suspension, shaking for 1 hour, and then pouring the suspension onto a 90 μm mesh to separate the ion exchange resin from the aqueous suspension. The alkali titration was carried out by adding 50 μL of 0.1 mol / L aqueous sodium hydroxide solution to the aqueous suspension after the ion exchange resin treatment every 30 seconds, and measuring the change in the electrical conductivity of the aqueous suspension. The amount of phosphate groups (mmol / g) was calculated by dividing the amount of alkali (mmol) required in the region corresponding to the first region in the measurement results by the solid content (g) in the aqueous suspension to be titrated.

[0046] [Amount of Anionic Groups (Amount of Sulfate Groups)] A predetermined amount of acid-type anion-modified fine fibrous cellulose was combusted, and the sulfur content in the combustion product was measured using a combustion ion chromatograph according to a method in accordance with IEC 62321, and the amount was converted into the amount of sulfate groups.

[0047] [Measurement of Number Average Fiber Width] The number average fiber width of anion-modified fine fibrous cellulose was measured as follows. Specifically, an aqueous dispersion of anion-modified fine fibrous cellulose was prepared by high-pressure dispersion treatment to a solids content of 0.005 to 0.0001 wt %. The dispersion was cast on a mica substrate and dried to prepare a sample for observation with an atomic force microscope (AFM). Then, atomic force microscope images were observed at a magnification of 5,000x, 10,000x, or 50,000x depending on the size of the constituent fibers. In this case, an arbitrary axis of image width, either vertical or horizontal, was assumed within the obtained image, and the sample and observation conditions (magnification, etc.) were adjusted so that 20 or more fibers intersected with the axis. After obtaining an observation image satisfying these conditions, two random axes were drawn vertically and horizontally on the image, and the fiber widths of the fibers intersecting the axes were visually read. In this way, at least three non-overlapping images of the surface portion were taken with an atomic force microscope, and the fiber width values ​​of the fibers intersecting on each of the two axes were read (thus, information on the widths of at least 20 fibers x 2 x 3 = 120 fibers was obtained).The number-average fiber width was calculated from the fiber width data obtained in this way.

[0048] [Ratio of Polyvalent Metal Salts] For sheets C1 to C13 of Examples 1 to 13, 2.0 g of each sheet was sampled, subjected to Kjeldahl decomposition, and then diluted to 100 mL with ultrapure water (50-fold dilution). The contents of various metals were measured using an ICP optical emission spectrometer. From the various metal contents (ppm) thus obtained, the ratio of polyvalent metals was calculated using the following formula: Ratio of polyvalent metal (mol %) = Amount of polyvalent metal (ppm) ÷ [Amount of polyvalent metal (ppm) + Amount of monovalent metal (ppm)] × 100

[0049] [Coating agent applicability] Coating agents B1 to 17 of Examples 1 to 13 and Comparative Examples 1 to 4 were placed in a spray bottle (S-50 dispensing bottle (spray type) manufactured by Sanplatec Co., Ltd.) and sprayed onto a black plastic board attached to a wall. The coating agent applied by spraying was visually observed for dripping, and the applicability was evaluated according to the following criteria. A: Able to spray in a mist and no dripping B: Able to spray but not in a mist, or slight dripping C: Cannot be sprayed, or dripping outside the board

[0050] [Flame retardancy] Balsa wood cut into a size of 5 cm length x 1 cm width was immersed in coating agents B1 to B17 of Examples 1 to 13 and Comparative Examples 1 to 4 for 10 seconds, and then air-dried at room temperature for 24 hours. The piece was then clamped horizontally, and a gas burner flame was applied to the tip of the balsa wood. Flame retardancy was evaluated according to the following criteria: A: The flame self-extinguished within 60 seconds after contact with the balsa wood. B: The flame self-extinguished between 61 and 120 seconds after contact with the balsa wood. C: The flame self-extinguished over 121 seconds or more after contact with the balsa wood. D: The entire balsa wood was burned.

[0051] [Tensile Strength of Sheet] Test specimens were prepared by cutting sheets C1 to C15 of Examples 1 to 13 and Comparative Examples 1 and 2 into a length of 6 cm and a width of 0.5 cm. Using a tensile tester (EZ-SX, manufactured by Shimadzu Corporation), the test specimens were subjected to a tensile test under conditions of a grip distance of 3 cm, a tensile speed of 3 mm / min, 23°C, and 50% RH, to determine the tensile strength (MPa). The tensile strength is the value obtained by dividing the maximum tensile force recorded when the test specimen is pulled until it breaks by the cross-sectional area of ​​the test specimen before the test.

[0052] [Preparation of Anion-Modified Cellulose Fibers A1 to A6] Prior to producing the coating agents and sheets of the Examples and Comparative Examples, acid-type anion-modified cellulose fibers A1 to A6 to be used therein were prepared according to the following Production Examples 1 to 6.

[0053] [Manufacturing Example 1: Preparation of Anion-Modified Cellulose Fiber A1 (TEMPO-Oxidized Cellulose Fiber)] 2 g of softwood pulp was mixed with 150 mL of water, 0.25 g of sodium bromide, and 0.025 g of TEMPO, thoroughly stirred to disperse the mixture. A 13% by mass aqueous solution of sodium hypochlorite (co-oxidant) was then added to 1.0 g of pulp to achieve a sodium hypochlorite concentration of 8.0 mmol / g, initiating the reaction. As the reaction progressed, the pH decreased. A 0.5 mol / L aqueous solution of sodium hydroxide was added dropwise to maintain the pH at 10-11, and the reaction was continued until no further change in pH was observed (reaction time: 120 minutes). After completion of the reaction, 0.1 mol / L hydrochloric acid was added to adjust the pH to 2.0. The mixture was then purified by repeated filtration and washing with water to obtain cellulose fibers with oxidized fiber surfaces. This was then diluted with purified water to a cellulose fiber concentration of 4% by mass to prepare a TEMPO-oxidized cellulose fiber suspension. The pH of the slurry was then adjusted to 10 with a 24% by mass aqueous solution of sodium hydroxide, and sodium borohydride was added at 0.2 mmol / g relative to the cellulose fibers to initiate the reaction. The reaction was allowed to proceed for 2 hours for reduction treatment. After the reaction, 0.1 mol / L hydrochloric acid was added to adjust the pH to 2.0, and the resulting mixture was purified by repeated filtration and washing with water to obtain anion-modified cellulose fibers A1 in which the carboxy groups were in the acid form.

[0054] [Manufacturing Example 2: Preparation of anion-modified cellulose fiber A2 (TEMPO-oxidized cellulose fiber)] Anion-modified cellulose fiber A2 in which the carboxyl groups are in the acid form was obtained in the same manner as the preparation method for anion-modified cellulose fiber A1, except that the amount of sodium hypochlorite aqueous solution added was 4.0 mmol / g per 1.0 g of softwood pulp.

[0055] [Manufacturing Example 3: Preparation of anion-modified cellulose fiber A3 (TEMPO-oxidized cellulose fiber)] Anion-modified cellulose fiber A3 in which the carboxyl groups are in the acid form was obtained in the same manner as the preparation method for anion-modified cellulose fiber A1, except that the amount of sodium hypochlorite aqueous solution added was 6.0 mmol / g per 1.0 g of softwood pulp.

[0056] [Manufacturing Example 4: Preparation of anion-modified cellulose fiber A4 (TEMPO-oxidized cellulose fiber)] Anion-modified cellulose fiber A4, in which the carboxyl groups are in the acid form, was obtained in the same manner as the preparation method for anion-modified cellulose fiber A1, except that the amount of sodium hypochlorite aqueous solution added was 10.0 mmol / g per 1.0 g of softwood pulp.

[0057] [Production Example 5: Preparation of Anion-Modified Cellulose Fiber A5 (Phosphate-Esterified Cellulose Fiber)] A mixed aqueous solution of ammonium dihydrogen phosphate and urea was added to 100 parts by mass (bone dry mass) of softwood kraft 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 obtained chemical-impregnated pulp was then heated in a hot air dryer at 165°C for 200 seconds to introduce phosphate groups into the cellulose in the pulp, thereby obtaining a phosphate-esterified cellulose fiber. After the reaction, 0.1 mol / L hydrochloric acid was added to adjust the pH to 1.0, and the fiber was purified by repeated filtration and washing with water, resulting in anion-modified cellulose fiber A5 in which the fiber surface was phosphated and the phosphate groups were in the acid form.

[0058] [Production Example 6: Preparation of Anion-Modified Cellulose Fiber A6 (Sulfate-Esterified Cellulose Fiber)] 2 g of softwood kraft pulp, 20 g of sulfamic acid, 50 g of urea, and 100 g of ion-exchanged water were mixed and stirred for 10 minutes using a stirrer. After stirring, the slurry was suction filtered using filter paper (No. 2). Suction filtration was continued until the solution stopped dripping. After suction filtration, the pulp was peeled from the filter paper and placed in a dryer with a thermostatic chamber set to 50°C for 6 hours to react. After the reaction, 0.1 mol / L hydrochloric acid was added to adjust the pH to 1.0, and the fiber was purified by repeated filtration and washing with water to obtain anion-modified cellulose fiber A6 in which the fiber surface was sulfated and the sulfate groups were in the acid form.

[0059] [Example 1] (Preparation of Coating Agent) Anion-modified cellulose fiber A2 was diluted to 0.5% by mass with ion-exchanged water, and then neutralized with 0.5 mol / L aqueous sodium hydroxide solution to a pH (25°C) of 7.0. This was treated three times using a high-pressure homogenizer at a pressure of 100 MPa to obtain a Na salt-type anion-modified fine fibrous cellulose suspension. An aluminum chloride aqueous solution was added to this suspension to a concentration of 0.1 M, and the suspension was allowed to stand for 12 hours. Next, the obtained anion-modified fine fibrous cellulose was rinsed five times with purified water to remove excess salt, etc., and then the cellulose concentration was adjusted to 0.5% by mass with ion-exchanged water to obtain a gel-like coating agent B1 of Example 1 containing anion-modified fine fibrous cellulose having Na salt-type carboxy groups and Al salt-type carboxy groups.

[0060] (Preparation of Sheet) The obtained coating agent B1 was placed in a Teflon (registered trademark) petri dish and dried at 40° C. for 24 hours to obtain a sheet C1 of Example 1 having a thickness of 30 μm.

[0061] Example 2 A coating agent B2 and a sheet C2 of Example 2 were obtained by preparing a coating agent and a sheet in the same manner as in Example 1, except that anion-modified cellulose fiber A3 was used.

[0062] Example 3 A coating agent B3 and a sheet C3 of Example 3 were obtained by preparing a coating agent and a sheet in the same manner as in Example 1, except that anion-modified cellulose fiber A1 was used.

[0063] Example 4 A coating agent B4 and a sheet C4 of Example 4 were obtained by preparing a coating agent and a sheet in the same manner as in Example 1, except that anion-modified cellulose fiber A4 was used.

[0064] [Example 5] A coating agent and a sheet were prepared in the same manner as in Example 3, except that the concentration of aluminum chloride was set to 0.2 M when preparing the coating agent, thereby obtaining a coating agent B5 and a sheet C5 of Example 5.

[0065] [Example 6] A coating agent B6 and a sheet C6 of Example 6 were obtained by preparing a coating agent and a sheet in the same manner as in Example 3, except that the concentration of aluminum chloride was 0.05 M when preparing the coating agent.

[0066] [Example 7] The coating agent and the sheet were prepared in the same manner as in Example 3, except that, when preparing the coating agent, an aqueous magnesium chloride solution was added in place of the aqueous aluminum chloride solution so that the concentration was 0.1 M, thereby obtaining a coating agent B7 and a sheet C7 of Example 7.

[0067] [Example 8] A coating agent and a sheet were prepared in the same manner as in Example 3, except that the aluminum chloride aqueous solution was not added and allowed to stand during the preparation of the coating agent, thereby obtaining a coating agent B8 and a sheet C8 of Example 8.

[0068] [Example 9] A coating agent and a sheet were prepared in the same manner as in Example 3, except that an aqueous aluminum chloride solution was added to the coating agent to a concentration of 0.1 M and the coating agent was allowed to stand for 24 hours, thereby obtaining a coating agent B9 and a sheet C9 of Example 9.

[0069] [Example 10] The coating agent and the sheet were prepared in the same manner as in Example 3, except that the conditions for the high-pressure homogenizer treatment during the preparation of the coating agent were changed to a pressure of 100 MPa and the number of treatments was changed to one, thereby obtaining a coating agent B10 and a sheet C10 of Example 10.

[0070] [Example 11] The coating agent and the sheet were prepared in the same manner as in Example 3, except that the conditions for the high-pressure homogenizer treatment during the preparation of the coating agent were changed to a pressure of 50 MPa and the number of treatments was changed to one, thereby obtaining a coating agent B11 and a sheet C11 of Example 11.

[0071] Example 12 A coating agent B12 and a sheet C12 of Example 12 were obtained by preparing a coating agent and a sheet in the same manner as in Example 1, except that anion-modified cellulose fiber A5 was used.

[0072] Example 13 A coating agent B13 and a sheet C13 of Example 13 were obtained by preparing a coating agent and a sheet in the same manner as in Example 1, except that anion-modified cellulose fiber A6 was used.

[0073] [Comparative Example 1] A coating agent and a sheet were prepared in the same manner as in Example 3, except that hydrochloric acid was added to a concentration of 0.1 M instead of the aqueous aluminum chloride solution during the preparation of the coating agent, and the mixture was allowed to stand for 24 hours, thereby obtaining a coating agent B14 and a sheet C14 of Comparative Example 1.

[0074] [Comparative Example 2] (Preparation of Coating Agent) Anion-modified cellulose fiber A1 was diluted with ion-exchanged water to 0.5% by mass, and then neutralized with a 0.5 mol / L aqueous solution of tetrabutylammonium hydroxide (TBAH) to a pH (25°C) of 7.0. This was treated three times using a high-pressure homogenizer at a pressure of 100 MPa to obtain coating agent B15 of Comparative Example 2, which contains anion-modified fine fibrous cellulose having a TBA (tetrabutylammonium) salt-type carboxy group.

[0075] (Preparation of Sheet) The obtained coating agent B15 was placed in a Teflon (registered trademark) petri dish and dried at 40° C. for 24 hours to obtain a sheet C15 of Comparative Example 2 having a thickness of 30 μm.

[0076] Comparative Example 3 Sodium hydroxide was diluted with ion-exchanged water to a concentration of 0.1 M to obtain a coating agent B16 of Comparative Example 3.

[0077] Comparative Example 4 Aluminum chloride was diluted with ion-exchanged water to a concentration of 0.1 M to obtain a coating agent B17 of Comparative Example 4.

[0078] The ratio of polyvalent metal salt was measured for the sheets C1 to C13 of Examples 1 to 13 prepared as described above. Furthermore, the coating properties and flame retardancy of the coating agents B1 to B17 of Examples 1 to 13 and Comparative Examples 1 to 4 were evaluated. Furthermore, the tensile strength of the sheets C1 to C15 of Examples 1 to 13 and Comparative Examples 1 and 2 was evaluated. The results are shown in Table 1.

[0079]

[0080] As shown in Table 1, coating agent B13 of Comparative Example 1, which contained acid-type anion-modified fine fibrous cellulose, was unable to impart flame retardancy. Coating agent B14 of Comparative Example 2, which contained ammonium salt-type anion-modified fine fibrous cellulose, was also unable to impart flame retardancy. Comparative Examples 3 and 4, which used aqueous sodium hydroxide or aqueous aluminum chloride solutions as coating agents, were also unable to impart flame retardancy. Furthermore, because these coating agents did not contain anion-modified fine fibrous cellulose, dripping occurred in the evaluation of coatability, and coatability was also poor.

[0081] In contrast, coating agents B1 to B13 of Examples 1 to 13, which contained a metal salt of anion-modified microfibrous cellulose, were spray-applicable, suppressed dripping, and provided excellent applicability while also imparting flame retardancy to balsa wood. Furthermore, the tensile strength of the sheet was improved compared to Comparative Examples 1 and 2. A comparison of Examples 3, 12, and 13 revealed that tensile strength tended to be higher when the anionic group was a carboxyl group than when the anionic group was a phosphate group or sulfate group. Furthermore, a comparison of Examples 3, 8, and 9 revealed that the use of a combination of monovalent and polyvalent metals as counter ions was superior to the use of either a monovalent or polyvalent metal alone in achieving both flame retardancy and coating film strength and applicability.

[0082] The various numerical ranges described in this specification can be arbitrarily combined with their respective upper and lower limit values, and all such combinations are considered to be preferred numerical ranges described in this specification. Furthermore, a numerical range described as "X to Y" means from X to Y.

[0083] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their omissions, substitutions, modifications, etc. are included within the scope and spirit of the invention, as well as within the scope of the invention described in the claims and their equivalents.

Claims

1. A flame-retardant coating agent comprising an anion-modified fine fibrous cellulose having a number average fiber width of 2 to 1,000 nm, the counter ions of the anionic groups of the anion-modified fine fibrous cellulose containing metal ions.

2. The flame-retardant coating agent according to claim 1, wherein the anion-modified fine fibrous cellulose has an anionic group content of 0.5 to 3.0 mmol / g, as measured after all of the anionic groups are in an acid form.

3. The flame-retardant coating agent according to claim 1, wherein the metal ions include monovalent metal ions and polyvalent metal ions.

4. The flame-retardant coating agent according to claim 3, wherein the molar ratio of said monovalent metal ions to said polyvalent metal ions is from 10 / 90 to 80 / 20.

5. The flame-retardant coating agent according to any one of claims 1 to 4, wherein the anionic group is a carboxy group.

6. A flame-retardant sheet comprising anion-modified fine fibrous cellulose having a number average fiber width of 2 to 1,000 nm, the counter ions of the anionic groups of the anion-modified fine fibrous cellulose containing metal ions.

7. The flame-retardant sheet according to claim 6, wherein the anion-modified fine fibrous cellulose has an anionic group content of 0.5 to 3.0 mmol / g, as measured after all of the anionic groups are in an acid form.

8. The flame-retardant sheet according to claim 6 or 7, wherein the metal ions include monovalent metal ions and polyvalent metal ions.

9. The flame-retardant sheet according to claim 8, wherein the molar ratio of said monovalent metal ions to said polyvalent metal ions is from 10 / 90 to 80 / 20.

10. A flame-retardant coating layer comprising anion-modified fine fibrous cellulose having a number average fiber width of 2 to 1,000 nm, wherein the counter ions of the anionic groups of the anion-modified fine fibrous cellulose contain metal ions.

Images