Flame-retardant material and flame-retardant sheet

Abstract

The present invention provides a novel flame-retardant material and flame-retardant sheet including anionically modified pulp. A flame-retardant material and a flame-retardant sheet according to the embodiments include anionically modified pulp in which the counter ions of anionic groups include metal ions, wherein the anionically modified pulp contains anionic groups in the amount of 1.1-3.0 mmol / g as measured after converting all anionic groups to acid type.

Description

Flame-retardant materials and flame-retardant sheets

[0001] The present invention relates to a flame-retardant material and a flame-retardant sheet containing anionically modified pulp.

[0002] Pulp is not inherently flame-retardant, and neither is paper made from pulp. To make it flame-retardant, flame retardants such as inorganic compounds are added, but they must be added in large quantities and are not easy to disperse uniformly.

[0003] Patent Document 1 discloses a flame-retardant molded article containing composite fibers, such as cellulose fibers, whose surfaces are coated with inorganic particles, and a water-insoluble flame retardant, and discloses that the molded article has a sheet shape and that the composite fiber is prepared by synthesizing inorganic particles in a liquid containing the fibers. Patent Document 1 also discloses anion-modified cellulose fibers, such as TEMPO-oxidized CNF, as the fiber, but does not disclose that metal salts of anion-modified pulp have flame retardancy.

[0004] Japanese Patent Application Laid-Open No. 2021-25003

[0005] SUMMARY OF THE INVENTION An object of embodiments of the present invention is to provide novel flame-retardant materials and flame-retardant sheets that include anionically modified pulp.

[0006] The present inventors have discovered that pulp can be made flame retardant by introducing anionic groups into the pulp and converting the anionic groups into metal salts, and have devised the use of metal salts of such anion-modified pulp to produce flame retardant materials and flame retardant sheets. The present invention includes the following embodiments.

[0007] [1] A flame-retardant material comprising an anion-modified pulp in which counter ions of anionic groups contain metal ions, the anion-modified pulp having an amount of anionic groups measured with all of the anionic groups in an acid form of 1.1 to 3.0 mmol / g. [2] The flame-retardant material according to [1], in which the metal ions contain polyvalent metal ions. [3] The flame-retardant material according to [1] or [2], in which the anionic groups are carboxy groups.

[0008] [4] A flame-retardant sheet comprising anion-modified pulp in which counter ions of anionic groups contain metal ions, wherein the anion-modified pulp has an amount of anionic groups measured with all of the anionic groups in the acid form of 1.1 to 3.0 mmol / g. [5] The flame-retardant sheet according to [4], in which the metal ions contain polyvalent metal ions. [6] The flame-retardant sheet according to [4] or [5], in which the anionic groups are carboxy groups. [7] The flame-retardant sheet according to any one of [4] to [6], further comprising unmodified pulp.

[0009] According to an embodiment of the present invention, a novel flame-retardant material and a flame-retardant sheet containing anionically modified pulp can be provided.

[0010] [Flame-Retardant Material] The flame-retardant material according to this embodiment includes anion-modified pulp in which the counter ions of the anionic groups contain metal ions. Anion-modified pulp allows metal ions to be efficiently introduced into the anionic groups, making the pulp flame-retardant. Therefore, the anion-modified pulp can be used as a material for imparting flame retardancy. Furthermore, by introducing metal ions, for example, when the pulp is formed into a sheet, strength such as tensile strength can be increased, thereby achieving both strength and flame retardancy.

[0011] Anionically modified pulp is pulp into which anionic groups have been introduced and is obtained by chemically modifying unmodified pulp. "Pulp" refers to cellulose fibers extracted by mechanically and / or chemically treating plants such as wood. The anionic groups are preferably introduced at least onto the fiber surface of the pulp.

[0012] Examples of plant-derived pulps include unbleached softwood kraft pulp (NUKP), bleached softwood kraft pulp (NBKP), unbleached hardwood kraft pulp (LUKP), bleached hardwood kraft pulp (LBKP), unbleached softwood sulfite pulp (NUSP), bleached softwood sulfite pulp (NBSP), thermomechanical pulp (TMP), recycled pulp, and recycled paper pulp. Any of these may be used alone or in combination of two or more.

[0013] Examples of anionic groups 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 cellulose molecules. 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.

[0014] In one embodiment, the anionic group of the anion-modified pulp is preferably a carboxy group, which has a more excellent effect of improving the strength of a sheet made from the pulp than other anionic groups.

[0015] In one embodiment, examples of anion-modified pulp having carboxy groups include oxidized cellulose fibers obtained by oxidizing the hydroxyl groups of glucose units in cellulose molecules, and carboxymethylated cellulose fibers obtained by carboxymethylating the hydroxyl groups of glucose units in cellulose molecules. Examples of oxidized cellulose fibers 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 fibers are obtained by oxidizing natural cellulose, such as wood pulp, using a co-oxidant in the presence of an N-oxyl compound. Examples of N-oxyl compounds include compounds containing a nitroxy radical, which are commonly used as oxidation catalysts. Examples of N-oxyl compounds include piperidine nitroxyoxy radicals, with 2,2,6,6-tetramethylpiperidinoxy radical (TEMPO) or 4-acetamido-TEMPO being particularly preferred. Anion-modified cellulose fibers according to a preferred embodiment are TEMPO-oxidized cellulose fibers oxidized using TEMPO.

[0016] In this embodiment, the anion-modified pulp has an anionic group content of 1.1 to 3.0 mmol / g, measured after converting all anionic groups to the acid form. Having an anionic group content of 1.1 mmol / g or more can impart flame retardancy. The anionic group content is preferably 1.3 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. Because the anion-modified pulp according to this embodiment is a metal salt-type anion-modified pulp, the content of anionic groups is measured after all anionic groups are converted to the acid form. The content of anionic groups is the amount of anionic groups (mmol) per dry mass of the acid-type anion-modified pulp and can be measured by known methods, specifically the method described in the Examples section. In this specification, "dry mass" refers to the mass after drying at 140°C until the mass change rate per minute is 0.05% or less.

[0017] In this embodiment, the anion-modified pulp 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 also be used. For example, the anionic group may contain an onium salt together with a 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) together with the salt form containing a metal salt. ＋ 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.

[0018] 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.

[0019] 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 in the anion-modified pulp include anionic groups bonded to monovalent metal ions and anionic groups bonded to polyvalent metal ions.

[0020] In one embodiment, the metal ions preferably contain polyvalent metal ions from the viewpoint of flame retardancy and strength. That is, the metal ions as counter ions may be polyvalent metal ions alone, or may be a combination of polyvalent metal ions and monovalent metal ions. The ratio of polyvalent metal ions to 100 mol% of metal ions is not particularly limited, but is preferably, for example, 30 mol% or more, more preferably 50 mol% or more, even more preferably 70 mol% or more, even more preferably 90 mol% or more, and particularly preferably 100 mol%. Here, the ratio of polyvalent metal ions can be determined by measuring the content of various metals using an ICP atomic emission spectrometer, and in detail, can be measured by the method described in the Examples section.

[0021] As described above, anionically modified pulp is obtained by chemically modifying unmodified pulp and is not defibrated. In this respect, it is distinguished from cellulose nanofibers, which are made by defibrating pulp. The fiber diameter of anionically modified pulp is equivalent to the fiber diameter of untreated pulp and, although it varies depending on the raw material pulp, is usually several tens of μm. Specifically, the number average fiber width of anionically modified pulp is preferably 5 to 100 μm, more preferably 10 to 60 μm, and may be 20 to 40 μm.

[0022] The number average fiber width of anion-modified pulp is measured as follows: an aqueous suspension of anion-modified pulp diluted to 0.01% by mass is photographed using an optical microscope, 10 images are taken, 25 fibers are selected from the images, the fiber widths (diameters) are measured, and the arithmetic mean is calculated.

[0023] The flame-retardant material according to this embodiment may consist solely of the anion-modified pulp, or may contain additives as optional components in addition to the anion-modified pulp. The amount of the anion-modified pulp in the flame-retardant material 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, and may even be 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. Since the anion-modified pulp provides flame retardancy, the flame-retardant material according to this embodiment does not require additional flame retardants. Therefore, in one embodiment, the flame-retardant material does not contain additional flame retardants.

[0024] The form of the flame-retardant material is not particularly limited, and may be, for example, fibrous. "Fibrous" refers to a material having the form of fibers, and may be an aggregate of fibers, such as cotton, or a fiber sheet such as paper or nonwoven fabric.

[0025] The flame-retardant material according to this embodiment is used to impart flame retardancy. For example, by adding or blending the flame-retardant material to a flammable material, the material can be made less flammable or the flame can be prevented from spreading, thereby imparting flame retardancy to the material.

[0026] For example, when producing paper products such as paper, a flame-retardant paper product can be obtained by adding a flame-retardant material containing the anion-modified pulp to unmodified pulp and mixing them together to produce a paper product according to a conventional method. Alternatively, a flame-retardant resin product can be obtained by adding the flame-retardant material to a resin and mixing them together to produce a resin product according to a conventional method. Paper products are not particularly limited, and examples include architectural interior materials such as wallpaper, filters, honeycomb materials, and packaging materials.

[0027] [Flame-retardant sheet] The flame-retardant sheet according to this embodiment is a sheet containing anion-modified pulp in which the counter ions of the anionic groups contain metal ions, and the amount of anionic groups in the anion-modified pulp, measured after all the anionic groups are in the acid form, is 1.1 to 3.0 mmol / g. Details of the anion-modified pulp are as described above for the flame-retardant material, and therefore will not be described here.

[0028] 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-retardant (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, architectural interior sheets such as wallpaper, filters, honeycomb materials, and packaging materials. In this specification, the term "sheet" encompasses the concept of "film."

[0029] The flame-retardant sheet may be a sheet having a single layer structure consisting of a layer containing the anion-modified pulp, or a sheet having a laminate structure including a layer containing the anion-modified pulp and another layer. The thickness of the flame-retardant sheet is not particularly limited and may be, for example, 0.001 to 50 mm, or 0.01 to 5 mm.

[0030] The flame-retardant sheet may consist solely of the anion-modified pulp, or may contain additives as optional components in addition to the anion-modified pulp. 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 pulp provides flame retardancy, the flame-retardant sheet according to this embodiment does not require additional flame retardants. Therefore, in one embodiment, the flame-retardant sheet does not contain additional flame retardants.

[0031] The flame-retardant sheet may be a fiber sheet such as paper or nonwoven fabric. When forming a fiber sheet, for example, paper may be produced by papermaking using a suspension containing anion-modified pulp as a paper stock. Papermaking is a process in which the paper stock is dehydrated by filtration to form a sheet, which is then pressed and dried to produce paper. For papermaking, known papermaking machines such as a Fourdrinier wet paper machine, a twin-wire paper machine, a Yankee paper machine, a cylinder paper machine, or a cylinder-short wire combination paper machine may be used. Alternatively, for example, a suspension containing anion-modified pulp may be filtered under reduced pressure to form a sheet, which is then dried and pressed to produce a fiber sheet.

[0032] When the metal ions include polyvalent metal ions, the method for preparing the anion-modified pulp is not particularly limited. For example, the anion-modified pulp may be prepared by forming a sheet using a monovalent metal salt of the anion-modified pulp, and then immersing the sheet in an aqueous solution of the polyvalent metal salt to replace the monovalent metal ions with polyvalent metal ions.

[0033] In one embodiment, the flame-retardant sheet may contain unmodified pulp together with the anion-modified pulp, or may be a composite sheet in which anion-modified pulp and unmodified pulp are combined. Such a composite sheet may be prepared, for example, by using a suspension containing both the anion-modified pulp and the unmodified pulp as a stock in the papermaking process, or by filtering a suspension containing both the anion-modified pulp and the unmodified pulp under reduced pressure in the production of the fiber sheet.

[0034] In the composite sheet, the ratio of anion-modified pulp to unmodified pulp is not particularly limited, and may be, for example, a mass ratio of anion-modified pulp / unmodified pulp of 10 / 90 to 90 / 10, 20 / 80 to 80 / 20, or 30 / 70 to 70 / 30.

[0035] Examples will be described in detail below along with comparative examples, but the present invention is not limited to these examples.

[0036] The methods for measuring the various physical properties in the examples and comparative examples are as follows.

[0037] [Amount of Anionic Groups (Amount of Carboxy Groups)] A 50 mL aqueous suspension of acid-type anion-modified pulp with a pulp 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 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 Pulp (g)]

[0038] Anionic group content (phosphate group content) was measured by diluting anion-modified pulp with ion-exchange water to a content of 0.2% by mass, treating the resulting aqueous suspension with ion-exchange resin to obtain an acidic anion-modified pulp, and then titrating the aqueous suspension with alkali. The ion-exchange resin treatment was performed 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 performed by adding 50 μL of 0.1 mol / L sodium hydroxide solution to the aqueous suspension after the ion-exchange resin treatment every 30 seconds, while 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.

[0039] [Amount of Anionic Groups (Amount of Sulfate Groups)] A predetermined amount of acid-type anion-modified pulp 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.

[0040] [Ratio of Polyvalent Metal Salts] 2.0 g of each of the sheets C1 to C9 of Examples 1 to 9 and the sheet C15 of Comparative Example 5 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. The ratio of polyvalent metals was calculated from the thus obtained contents of various metals (ppm) 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

[0041] [Flame retardancy of sheets] Test specimens were prepared by cutting sheets C1 to C9 of Examples 1 to 9 and sheets C11 to C13 and C15 of Comparative Examples 1 to 3 and 5 into 5 cm length x 1 cm width. The test specimens were fixed horizontally with clamps, and a gas burner flame was applied to the tip of the test specimen. Flame retardancy was evaluated according to the following criteria. A: Self-extinguishing within 60 seconds after contact between the gas burner flame and the test specimen. B: Self-extinguishing over 60 seconds or more after contact between the gas burner flame and the test specimen. C: The entire test specimen was burned.

[0042] [Flame retardancy of composite sheets] Composite sheets D1 to D9 of Examples 1 to 9 and composite sheets D11 to D15 of Comparative Examples 1 to 5 were cut into 5 cm length x 1 cm width to prepare test specimens. The test specimens were fixed horizontally with clamps, and a gas burner flame was applied to the tip of the test specimen. Flame retardancy was evaluated according to the following criteria. A: The test specimen self-extinguished within 60 seconds after contact with the gas burner flame. B: The test specimen self-extinguished over 61 seconds or more after contact with the gas burner flame. C: The entire test specimen was burned.

[0043] [Tensile Strength of Sheets] Test specimens were prepared by cutting sheets C1 to C9 of Examples 1 to 9 and sheets C11 to C13, C15 of Comparative Examples 1 to 3, C16 to C18 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 gripping 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 maximum tensile force recorded when the test specimen is pulled until it breaks, divided by the cross-sectional area of ​​the test specimen before the test.

[0044] [Preparation of Anion-Modified Pulps A1 to A5] Prior to producing the sheets of the Examples and Comparative Examples, acid-type anion-modified pulps A1 to A5 used therein were prepared according to the following Production Examples 1 to 5.

[0045] [Production Example 1: Preparation of Anion-Modified Pulp 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 the pulp to achieve a sodium hypochlorite concentration of 8.0 mmol / g, thereby 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 mixture was purified by repeated filtration and washing with water to obtain anion-modified pulp A1 in which the carboxyl groups were in the acid form.

[0046] [Manufacturing Example 2: Preparation of anion-modified pulp A2 (TEMPO-oxidized cellulose fiber)] Anion-modified pulp A2 in which the carboxyl groups are in the acid form was obtained in the same manner as the preparation method for anion-modified pulp A1, except that the amount of sodium hypochlorite aqueous solution added was 6.0 mmol / g per 1.0 g of softwood pulp.

[0047] [Manufacturing Example 3: Preparation of anion-modified pulp A3 (TEMPO-oxidized cellulose fiber)] Anion-modified pulp A3, in which the carboxyl groups are in the acid form, was obtained in the same manner as the preparation method for anion-modified pulp A1, except that the amount of sodium hypochlorite aqueous solution added was 10.0 mmol / g per 1.0 g of softwood pulp.

[0048] [Manufacturing Example 4: Preparation of anion-modified pulp A4 (TEMPO-oxidized cellulose fiber)] Anion-modified pulp A4, in which the carboxyl groups are in the acid form, was obtained in the same manner as the preparation method for anion-modified pulp A1, except that the amount of sodium hypochlorite aqueous solution added was 4.0 mmol / g per 1.0 g of softwood pulp.

[0049] [Production Example 5: Preparation of Anion-Modified Pulp 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 phosphoric acid-esterified cellulose fiber. After the reaction, 0.1 mol / L hydrochloric acid was added to adjust the pH to 1.0, and the fiber surface was purified by repeated filtration and washing with water to obtain anion-modified pulp A5 in which the phosphate groups were in the acid form.

[0050] [Production Example 6: Preparation of Anion-Modified Pulp 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 bath 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 then filtration and washing with water were repeated to purify the fiber surface, yielding anion-modified pulp A6 in which the sulfate groups were in the acid form.

[0051] [Example 1] (Neutralization step) Anion-modified pulp A2 was diluted to 0.2% by mass with ion-exchanged water and then neutralized with a 0.5 mol / L aqueous sodium hydroxide solution to a pH (25°C) of 7.0, thereby obtaining a suspension B1 of anion-modified pulp having Na salt-type carboxy groups.

[0052] (Preparation of Sheet) The suspension B1 was filtered under reduced pressure through a nylon mesh filter with a mesh size of 59 μm to obtain a sheet-like wet deposit. After air-drying at room temperature, the sheet was sandwiched between polyimide films and heated at 105°C for 5 minutes at 0.4 MPa using a heat press to obtain a sheet having Na salt-type carboxy groups. The sheet having Na salt-type carboxy groups was immersed in a 0.1 M aqueous aluminum chloride solution for 24 hours and then washed five times with purified water. After air-drying, the sheet was sandwiched between polyimide films and heated at 105°C for 5 minutes at 0.4 MPa using a heat press to obtain a 100 μm thick sheet C1 of Example 1 made of an anion-modified pulp having Al salt-type carboxy groups.

[0053] (Preparation of Composite Sheet) Unmodified softwood pulp was diluted with ion-exchanged water to 0.2% by mass and disintegrated using a pulper at 3,000 rpm for 10 minutes to prepare a pulp suspension. This pulp suspension was mixed with suspension B1 of anion-modified pulp having Na-salt-type carboxyl groups obtained in the neutralization step in a 50:50 mass ratio. This mixture was filtered under reduced pressure through a nylon mesh filter with a mesh size of 59 μm to obtain a sheet-like wet deposit. After air-drying at room temperature, the mixture was sandwiched between polyimide films and heated at 105°C for 5 minutes at 0.4 MPa using a heat press to obtain a composite sheet of anion-modified pulp having Na-salt-type carboxyl groups and unmodified pulp. This composite sheet was immersed in a 0.1 M aluminum chloride aqueous solution for 24 hours and then washed five times with purified water. After air-drying, the sheet was sandwiched between polyimide films and heated at 105°C for 5 minutes at 0.4 MPa using a heat press to obtain a composite sheet D1 of anion-modified pulp having Al salt-type carboxyl groups and unmodified pulp.

[0054] [Example 2] The neutralization step, sheet preparation, and composite sheet preparation were carried out in the same manner as in Example 1, except that anion-modified pulp A1 was used, to obtain sheet C2 and composite sheet D2 of Example 2.

[0055] [Example 3] The neutralization step, sheet preparation, and composite sheet preparation were carried out in the same manner as in Example 1, except that anion-modified pulp A3 was used, to obtain sheet C3 and composite sheet D3 of Example 3.

[0056] [Example 4] The neutralization step, sheet preparation, and composite sheet preparation were carried out in the same manner as in Example 2, except that in the preparation of the sheet and the preparation of the composite sheet, immersion was performed in a 0.05 M aqueous aluminum chloride solution instead of a 0.1 M aqueous aluminum chloride solution, thereby obtaining a sheet C4 and a composite sheet D4 of Example 4.

[0057] [Example 5] The neutralization step, sheet preparation, and composite sheet preparation were carried out in the same manner as in Example 2, except that the sheet preparation and composite sheet preparation were not immersed in a 0.1 M aluminum chloride aqueous solution, thereby obtaining sheet C5 and composite sheet D5 of Example 5.

[0058] [Example 6] The neutralization step, sheet preparation, and composite sheet preparation were carried out in the same manner as in Example 2, except that in the preparation of the sheet and the preparation of the composite sheet, the sheet was immersed in a 0.1 M aqueous magnesium chloride solution instead of a 0.1 M aqueous aluminum chloride solution, to obtain a sheet C6 and a composite sheet D6 of Example 6.

[0059] [Example 7] The neutralization step, sheet preparation, and composite sheet preparation were carried out in the same manner as in Example 2, except that in the preparation of the sheet and the preparation of the composite sheet, immersion was performed in a 0.1 M aqueous calcium chloride solution instead of a 0.1 M aqueous aluminum chloride solution, to obtain a sheet C7 and a composite sheet D7 of Example 7.

[0060] [Example 8] The neutralization step, sheet preparation, and composite sheet preparation were carried out in the same manner as in Example 1, except that anion-modified pulp A5 was used, to obtain sheet C8 and composite sheet D8 of Example 8.

[0061] [Example 9] The neutralization step, sheet preparation, and composite sheet preparation were carried out in the same manner as in Example 1, except that anion-modified pulp A6 was used, to obtain sheet C9 and composite sheet D9 of Example 9.

[0062] [Comparative Example 1] Unmodified softwood pulp was diluted with ion-exchanged water to 0.2% by mass and then disintegrated using a pulper at 3,000 rpm for 10 minutes to prepare a pulp suspension. The resulting suspension was filtered under reduced pressure through a nylon mesh filter with a mesh size of 59 μm to obtain a sheet-like wet deposit. After air-drying at room temperature, the pulp was sandwiched between polyimide films and heated at 105°C for 5 minutes at 0.4 MPa using a heat press to obtain a 100 μm-thick sheet C11 of Comparative Example 1.

[0063] [Comparative Example 2] (Preparation of acid-type anion-modified pulp suspension) Anion-modified pulp A1 was diluted to 0.2% by mass with ion-exchanged water, and then disintegrated using a pulper at 3000 rpm for 10 minutes to obtain suspension B12 of anion-modified pulp having acid-type carboxy groups.

[0064] (Preparation of Sheet) The suspension B12 was filtered under reduced pressure through a nylon mesh filter with a mesh size of 59 μm to obtain a sheet-like wet deposit. After air-drying at room temperature, the sheet was sandwiched between polyimide films and heated at 105° C. for 5 minutes at 0.4 MPa using a heat press to obtain a 100 μm-thick sheet C12 of Comparative Example 2, which was made of an anion-modified pulp having an acid-type carboxyl group.

[0065] (Preparation of Composite Sheet) Unmodified softwood pulp was diluted with ion-exchanged water to 0.2% by mass and disintegrated using a pulper at 3,000 rpm for 10 minutes to prepare a pulp suspension. This pulp suspension was mixed with the above-mentioned suspension B12 of anion-modified pulp having acid-type carboxy groups in a 50:50 mass ratio. This was filtered under reduced pressure using a nylon mesh filter with a mesh size of 59 μm to obtain a sheet-like wet deposit. After air-drying at room temperature, the mixture was sandwiched between polyimide films and heated at 105°C for 5 minutes at 0.4 MPa using a heat press to obtain composite sheet D12 of anion-modified pulp having acid-type carboxy groups and unmodified pulp.

[0066] [Comparative Example 3] (Neutralization step) Anion-modified pulp A1 was diluted to 0.2% by mass with ion-exchanged water and then neutralized with a 0.5 mol / L aqueous solution of tetrabutylammonium hydroxide (TBAH) to a pH (25°C) of 7.0, thereby obtaining a suspension B13 of anion-modified pulp having TBA salt-type carboxy groups.

[0067] (Preparation of Sheet) The suspension B13 was filtered under reduced pressure through a nylon mesh filter with a mesh size of 59 μm to obtain a sheet-like wet deposit. After air-drying at room temperature, the sheet was sandwiched between polyimide films and heated at 105° C. for 5 minutes at 0.4 MPa using a heat press to obtain a 100 μm-thick sheet C13 of Comparative Example 3, which was made of an anion-modified pulp having a TBA salt-type carboxyl group.

[0068] (Preparation of Composite Sheet) Unmodified softwood pulp was diluted with ion-exchanged water to 0.2% by mass and disintegrated using a pulper at 3,000 rpm for 10 minutes to prepare a pulp suspension. This pulp suspension was mixed with the anion-modified pulp suspension B13 having TBA salt-type carboxy groups obtained in the neutralization step in a 50:50 mass ratio. This mixture was filtered under reduced pressure through a nylon mesh filter with a mesh size of 59 μm to obtain a sheet-like wet deposit. After air-drying at room temperature, the mixture was sandwiched between polyimide films and heated at 105°C for 5 minutes at 0.4 MPa using a heat press to obtain a composite sheet D13 of anion-modified pulp having TBA (tetrabutylammonium) salt-type carboxy groups and unmodified pulp.

[0069] [Comparative Example 4] (Preparation of Sheet) Aluminum hydroxide was suspended in ion-exchanged water to a concentration of 2% by mass, and the resulting suspension was filtered under reduced pressure through a nylon mesh filter with a mesh size of 59 μm to obtain a wet deposit. However, the resulting deposit could not be formed into a sheet, and therefore, the sheet was not evaluated.

[0070] (Preparation of Composite Sheet) Unmodified softwood pulp was diluted with ion-exchanged water to 0.2% by mass and pulped using a pulper at 3,000 rpm for 10 minutes to prepare a pulp suspension. The pulp suspension was mixed with a 0.2% by mass aqueous aluminum hydroxide suspension in a 50:50 mass ratio. This mixture was filtered under reduced pressure using a nylon mesh filter with a mesh size of 59 μm to obtain a sheet-like wet deposit. After air-drying at room temperature, the mixture was sandwiched between polyimide films and heated at 105°C for 5 minutes at 0.4 MPa using a heat press to obtain a composite sheet D14 of unmodified pulp and aluminum hydroxide.

[0071] [Comparative Example 5] The neutralization step, sheet preparation, and composite sheet preparation were carried out in the same manner as in Example 1, except that anion-modified pulp A4 was used, to obtain sheet C15 and composite sheet D15 of Comparative Example 5.

[0072] The ratio of polyvalent metal salt was measured for the sheets C1 to C9 of Examples 1 to 9 and sheet C15 of Comparative Example 5 prepared as described above. Furthermore, the flame retardancy and tensile strength were evaluated for these sheets C1 to C9 and C15, as well as sheets C11 to C13 of Comparative Examples 1 to 3. Furthermore, the flame retardancy was evaluated for the composite sheets D1 to D9 of Examples 1 to 9 and composite sheets D11 to D15 of Comparative Examples 1 to 5. The results are shown in Table 1.

[0073]

[0074] As shown in Table 1, Comparative Example 1, which was a sheet made of unmodified pulp, as well as Comparative Examples 2 and 3, in which the counter ions of the anion-modified pulp were not metal ions, had no flame retardancy.

[0075] In contrast, in Examples 1 to 9, where the counter ion of the anion-modified pulp was a metal ion and the amount of anionic groups was equal to or greater than the specified amount, flame retardancy was achieved not only in sheets made of the anion-modified pulp alone, but also in composite sheets made of the anion-modified pulp and unmodified pulp. Furthermore, in Examples 1 to 9, the tensile strength was improved compared to sheets made of unmodified pulp or anion-modified pulp other than metal salts. Tensile strength tended to be higher when the anionic group was a carboxyl group than when it was a phosphate group or sulfate group. Furthermore, with regard to metal ions used as counter ions, polyvalent metals were superior to monovalent metals in flame retardancy and tensile strength.

[0076] 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.

[0077] 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 material comprising an anion-modified pulp in which the counter ion of the anionic group contains a metal ion, the anion-modified pulp having an amount of anionic groups, measured after all of the anionic groups are in the acid form, of 1.1 to 3.0 mmol / g.

2. The flame retardant material of claim 1, wherein the metal ions include polyvalent metal ions.

3. The flame-retardant material according to claim 1 or 2, wherein the anionic group is a carboxy group.

4. A flame-retardant sheet comprising an anion-modified pulp in which the counter ions of the anionic groups contain metal ions, the anion-modified pulp having an amount of anionic groups, measured after all of the anionic groups are converted to an acid form, of 1.1 to 3.0 mmol / g.

5. The flame-retardant sheet according to claim 4, wherein the metal ions include polyvalent metal ions.

6. The flame-retardant sheet according to claim 4 or 5, wherein the anionic group is a carboxy group.

7. The flame-retardant sheet according to claim 4 or 5, further comprising unmodified pulp.

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