Flame-retardant coating agent and flame-retardant sheet

Abstract

To provide a novel flame-retardant coating agent and a flame-retardant sheet including anionically modified fine fibrous cellulose.SOLUTION: A flame-retardant coating agent and a flame-retardant sheet according to embodiments include anionically modified fine fibrous cellulose having a number-average fiber width of 2 to 1,000 nm, wherein the counter ions of the anionic groups in the anionically modified fine fibrous cellulose include metal ions.SELECTED DRAWING: None

Description

Technical Field

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

Background Art

[0002] In recent years, from the perspective of sustainability, cellulose fibers, which are biomass abundantly present in nature, especially microfibrillated cellulose (also referred to as cellulose nanofibers) as a new form of its utilization, have attracted attention. Microfibrillated cellulose inherently has no flame retardancy, and in order to impart flame retardancy, a flame retardant such as an inorganic compound is added, but it is necessary to add a large amount, and it is not easy to disperse it uniformly.

[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 microfibrillated cellulose, but it does not disclose that the metal salt of anionic modified microfibrillated cellulose has flame retardancy.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] An embodiment of the present invention aims to provide a novel flame retardant coating agent and a flame retardant sheet containing anionic modified microfibrillated cellulose.

Means for Solving the Problems

[0006] The inventors have found that by using a metal salt as the anionic group of anionic-modified microfibrillar cellulose, the microfibrillar cellulose is rendered flame-retardant, and have devised a flame-retardant coating agent and a flame-retardant sheet using such a metal salt of anionic-modified microfibrillar cellulose. The present invention includes the embodiments shown below.

[0007] [1] A flame-retardant coating agent comprising anionic-modified microfibrillar cellulose having a number-average fiber width of 2 to 1000 nm, wherein the counter ion of the anionic group of the anionic-modified microfibrillar cellulose contains a metal ion. [2] The flame-retardant coating agent according to [1], wherein the anionic-modified microfibrillar cellulose has an amount of anionic groups measured with all the anionic groups in the acid form of 0.5 to 3.0 mmol / g. [3] The flame-retardant coating agent according to [1] or [2], wherein the metal ion contains a monovalent metal ion and a polyvalent metal ion. [4] The flame-retardant coating agent according to [3], wherein the molar ratio of the monovalent metal ion to the polyvalent metal ion is 10 / 90 to 80 / 20. [5] The flame-retardant coating agent according to any one of [1] to [4], wherein the anionic group is a carboxy group.

[0008] [6] A flame-retardant sheet comprising anionic-modified microfibrillar cellulose having a number-average fiber width of 2 to 1000 nm, wherein the counter ion of the anionic group of the anionic-modified microfibrillar cellulose contains a metal ion. [7] The flame-retardant sheet according to [6], wherein the anionic-modified microfibrillar cellulose has an amount of anionic groups measured with all the anionic groups 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 ion contains a monovalent metal ion and a polyvalent metal ion. [9] The flame-retardant sheet according to [8], wherein the molar ratio of the monovalent metal ion to the polyvalent metal ion is 20 / 80 to 80 / 20.

[0009]

[10] Flame-retardant coating layer containing anion-modified microfibrillar cellulose with a number average fiber width of 2 to 1000 nm, wherein the counter ion of the anionic group of the anion-modified microfibrillar cellulose contains a metal ion.

Advantages of the Invention

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

Modes for Carrying Out the Invention

[0011] [Flame-retardant coating agent] The flame-retardant coating agent according to this embodiment contains anion-modified microfibrillar cellulose, and the counter ion of its anionic group contains a metal ion. As for anion-modified microfibrillar cellulose, metal ions can be efficiently introduced into the anionic group, and the cellulose fibers can be made flame-retardant. Therefore, it can be used as a coating agent for imparting flame retardancy. Further, by introducing metal ions, the coatability of the coating agent can be improved, and the strength of the coating film can be increased.

[0012] Anion-modified microfibrillar cellulose is microfibrillar cellulose into which an anionic group has been introduced. Microfibrillar cellulose is obtained by refining cellulose fibers to the nanolevel and is also referred to as cellulose nanofiber. The number average fiber width of the microfibrillar 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, more preferably 3 to 20 nm. The smaller the number average fiber width, the higher the coatability as a coating agent, particularly the effect of suppressing dripping after coating, and the strength of the coating film can also be increased.

[0013] The number average fiber length of the microfibrillar 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 microfibrillar cellulose is not particularly limited, and may be, for example, 10 to 1000, or 50 to 700.

[0014] The number average fiber width of the microfibrillar cellulose can be determined by observing using an atomic force microscope (AFM). From the AFM image, at least 120 microfibrillar celluloses are selected to measure the fiber width, and the number average fiber width is obtained by calculating the arithmetic mean. For the number average fiber length as well, at least 120 microfibrillar celluloses are similarly selected to measure the fiber length, and it is determined by 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 way.

[0015] Examples of the anionic group of the anionic modified microfibrillar cellulose include at least one selected from the group consisting of a carboxy group, a phosphate group, a sulfate group, a sulfonic acid group, a nitrate group, and a borate group. Among these, at least one selected from the group consisting of a carboxy group, a phosphate group, and a sulfate group is preferable. These anionic groups may be directly bonded or indirectly bonded to the glucose unit which is a constituent unit of the cellulose molecule. When indirectly bonded, an alkylene group having 1 to 4 carbon atoms may be present between the glucose unit and the anionic group. The anionic group may be bonded to one or more of all the glucose units constituting the cellulose molecule, or may be bonded to one or more of some of the glucose units constituting the cellulose molecule.

[0016] In one embodiment, the anionic group of the anionic modified microfibrillar cellulose is preferably a carboxy group. When it is a carboxy group, the effect of improving the strength of the coating film is more excellent than other anionic groups.

[0017] In one embodiment, examples of the anionic modified microfibrillar 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 the 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 and modified to a carboxyl group. The oxidized cellulose nanofibers can be obtained by oxidizing natural cellulose such as wood pulp in the presence of an N-oxyl compound using a co-oxidizing agent and then performing a defibrillation (fibrillation) treatment. As the N-oxyl compound, a compound having a nitroxyl radical generally used as an oxidation catalyst is used. For example, it is a piperidine nitroxyl radical, and particularly 2,2,6,6-tetramethylpiperidinooxyl radical (TEMPO) or 4-acetamido-TEMPO is preferable. The anionic modified microfibrillar cellulose according to a preferred embodiment is TEMPO-oxidized cellulose nanofibers oxidized using TEMPO.

[0018] The anionic modified microfibrillar cellulose preferably has an amount of anionic groups measured with all anionic groups in the acid form of 0.5 to 3.0 mmol / g, more preferably 1.0 to 2.8 mmol / g, still more preferably 1.5 to 2.6 mmol / g, and even more preferably 1.7 to 2.5 mmol / g. The greater the amount of anionic groups, the higher the flame retardancy can be enhanced. Since the anionic modified microfibrillar cellulose according to this embodiment is in the metal salt form, when measuring the amount of anionic groups, all anionic groups are converted to the acid form before measurement. The amount of anionic groups is the amount of substance (mmol) of anionic groups per dry mass of the acid-form anionic modified microfibrillar cellulose and can be measured by a known method. Specifically, it can be measured by the method described in the Examples section. In this specification, the "dry mass" refers to the mass after drying at 140°C until the mass change rate per minute becomes 0.05% or less.

[0019] In this embodiment, as the anionic modified microfibrillar cellulose, those having a metal ion as a counter ion of the anionic group are used. That is, a metal ion is bonded to the anionic group to form a metal salt. In that case, it is preferable that the counter ions of all the anionic groups are metal ions, but it may have counter ions other than metal ions. For example, the anionic group may contain an onium salt together with the metal salt, and not all may be in the salt form. It may contain an acid form (the counter ion is H + and is also referred to as the H form. For example, in the case of a carboxy group, it is -COOH). As the introduction amount of the metal salt, for example, it is preferably 50 mol% or more of the anionic group, more preferably 80 mol% or more, still more preferably 90 mol% or more, and even more preferably 100 mol%. Here, the introduction amount of the metal salt is the ratio of the anionic group forming the metal salt to 100 mol% of the anionic group, and is calculated from the amount of the above anionic group and the metal content measured by an ICP emission spectroscopic analyzer.

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

[0021] The metal ion may be only a monovalent metal ion, only a polyvalent metal ion such as magnesium ion, calcium ion, and aluminum ion, or a combination of a monovalent metal ion and a polyvalent metal ion. The combined use means that among the anionic groups of the anionic modified microfibrillar cellulose, an anionic group bonded to a monovalent metal ion and an anionic group bonded to a polyvalent metal ion are included.

[0022] In one embodiment, it is more preferable for the metal ions to contain both monovalent metal ions and polyvalent metal ions in order to achieve both flame retardancy and the strength and coatability of the coating film. The ratio between the two is preferably a molar ratio of monovalent metal ions to polyvalent metal ions (monovalent metal ions / polyvalent metal ions) of 10 / 90 to 80 / 20. That is, it is preferable that the ratio of polyvalent metal ions to 100 mol% of the metal ions is 20 to 90 mol%, and the ratio of monovalent metal ions is 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. Here, the ratio of monovalent metal ions to polyvalent metal ions is determined by measuring various metal contents using an ICP emission spectroscopic analyzer, and specifically, it can be measured by the method described in the Examples section.

[0023] As described above, the anionic modified microfibrous cellulose is obtained by a step of chemically modifying unmodified cellulose fibers and a step of defibrating the cellulose fibers. The defibrating of the cellulose fibers may be carried out after introducing an anionic group or before the introduction. The defibrating treatment can be carried out, for example, by treating a dispersion of cellulose fibers using a homomixer under high-speed rotation, a high-pressure homogenizer, an ultrasonic dispersion processor, a beater, a disk-type refiner, a conical-type refiner, a double-disk-type refiner, a grinder, or the like.

[0024] The anionic modified microfibrous cellulose preferably has a cellulose I crystal structure. The cellulose I crystal structure is the crystal form of natural cellulose, and by having the I crystal structure, the anionic modified microfibrous cellulose has water insolubility. Having a cellulose I crystal structure can be identified from the fact that typical peaks are present at two positions near 2θ = 14° to 17° and near 2θ = 22° to 23° in the 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 above-mentioned anionic modified microfibrous cellulose is dispersed in a liquid dispersion medium. Water is preferably used 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 included, 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. In addition, the dispersion medium may be an aqueous solution containing an acid, an alkali, or a salt thereof as long as the effect is not impaired.

[0026] In the flame-retardant coating agent, the concentration of the anionic modified microfibrous cellulose is not particularly limited. For example, it may be 0.01 to 10% by mass, or may be 0.1 to 1% by mass. The amount of the anionic modified microfibrous cellulose in the solid content of the flame-retardant coating agent is also not particularly limited. For example, it 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 be 100% by mass with respect to 100% by mass of the solid content.

[0027] The viscosity of the flame-retardant coating agent is not particularly limited. For example, it may be 1 to 100,000 mPa·s, or may be 10 to 10,000 mPa·s. Here, the viscosity of the flame-retardant coating agent is the viscosity measured by 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 an additive as an optional component together with the above-mentioned anionic modified microfibrous cellulose and dispersion medium. Examples of the additive include colorants such as pigments and dyes, water resistance agents, flame retardants, plasticizers, antioxidants, light stabilizers, fillers, antistatic agents, and the like. Since the anionic modified microfibrous cellulose in the flame-retardant coating agent according to this embodiment has flame retardancy, other flame retardants are not required. 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 later.

[0029] The flame retardant coating agent according to this embodiment is used to impart flame retardancy. For example, by treating the surface of an object to be flame retarded (i.e., the target object), the object can be made less flammable or the spread of flames can be prevented, and flame retardancy can be imparted to the object.

[0030] The object to be treated with the flame retardant coating agent is not particularly limited, and examples include paper, wood, resin, rubber, etc. The treatment method of the coating agent is not particularly limited, and examples include coating by spraying, coating by a coating machine, brushing, dipping the object in the coating agent, etc.

[0031] In one embodiment, when making an object flame retardant by dipping it in the coating agent, the object is dipped using a coating agent containing monovalent metal salt type anion-modified fine fibrous cellulose, thereby forming a coat layer containing anion-modified fine fibrous cellulose. Next, the object with the coat layer formed thereon may be dipped in an aqueous solution of polyvalent metal ions, and at least a part of the monovalent metal ions is replaced with polyvalent metal ions to form a coat layer containing anion-modified fine fibrous cellulose having 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 with a number average fiber width of 2 to 1000 nm, and the counter ion of the anionic group contains metal ions. The details of the anion-modified fine fibrous cellulose are as described above for the flame retardant coating agent, and the description is omitted.

[0033] The flame-retardant sheet is a sheet used to impart flame retardancy. By overlaying it on the surface or inside of an object to be flame-retarded (i.e., the target object), flame retardancy can be imparted to the target object. For example, the flame-retardant sheet can be used by attaching it to the surface of the target object or sandwiching it inside a laminate that is the target object. More specific applications include, for example, a sheet for incorporating into a packaging material to make the packaging material flame-retardant, and a sheet for building interior decoration such as wallpaper. In this specification, "sheet" is a concept that includes "film".

[0034] The flame-retardant sheet may be a single-layer sheet composed of a layer containing the anionic modified microfibrous cellulose, or may be a laminated sheet containing a layer containing the anionic modified microfibrous cellulose and other layers. The thickness of the flame-retardant sheet is not particularly limited, and may be, for example, 0.1 to 1000 μm, or may be 1 to 100 μm.

[0035] The flame-retardant sheet may consist only of the anionic modified microfibrous cellulose, or may contain an additive as an optional component together with the anionic modified microfibrous cellulose. The amount of the anionic modified microfibrous 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, a crosslinking agent for crosslinking the anionic modified microfibrous cellulose may be added as an additive to the flame-retardant sheet, and water resistance can be imparted to the sheet by crosslinking. Other additives include, for example, colorants such as pigments and dyes, water resistance agents, flame retardants, plasticizers, antioxidants, light stabilizers, fillers, antistatic agents, etc. Note that since the anionic modified microfibrous cellulose in the flame-retardant sheet according to this embodiment has flame retardancy, other flame retardants are not necessary. Therefore, in one embodiment, the flame-retardant sheet does not contain other flame retardants.

[0037] The flame-retardant sheet may be a fiber sheet such as paper or non-woven fabric. When forming the fiber sheet, for example, a suspension containing anionic modified microfibrous cellulose may be applied onto a release sheet, dried, and then peeled off from the release sheet to be produced.

[0038] [Flame-retardant coating layer] The flame-retardant coating layer according to this embodiment is a coating layer containing anionic modified microfibrous cellulose with a number-average fiber width of 2 to 1000 nm, and the counter ion of the anionic group thereof contains a metal ion. The details of the anionic modified microfibrous cellulose are as described above for the flame-retardant coating agent, and the description is omitted.

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

[0040] The flame-retardant coating layer may be formed, for example, by treating the object with the above-mentioned flame-retardant coating agent. Therefore, in one embodiment, the flame-retardant coating layer may consist only of anionic modified microfibrous cellulose, or may contain an additive as an optional component together with the anionic modified microfibrous cellulose. The amount of the anionic modified microfibrous cellulose in the flame-retardant coating layer is not particularly limited, but for example, it is preferably 70% by mass or more, more preferably 80% by mass or more, still more preferably 90% by mass or more, still more preferably 95% by mass or more, and may be 100% by mass. Examples of the additive include colorants such as pigments and dyes, water resistance agents, flame retardants, plasticizers, antioxidants, light stabilizers, fillers, antistatic agents, and the like.

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

Examples

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

[0043] The measurement methods of each physical property in the examples and comparative examples are as follows.

[0044] [Amount of anionic group (amount of carboxy group)] A 50 mL aqueous suspension of acid-type anionic modified microfibrillar cellulose with a cellulose fiber concentration of 0.1% by mass was prepared, and the pH was adjusted to about 2.5 with a 0.1 mol / L hydrochloric acid aqueous solution. Next, a 0.05 mol / L sodium hydroxide aqueous solution was added dropwise to the aqueous suspension, and conductivity measurement was carried out until the pH reached about 11. From the amount of sodium hydroxide (V) consumed in the stage of neutralization of the weak acid where the change in conductivity was gentle, the amount of carboxy group was calculated according to the following formula. Amount of carboxy group (mmol / g) = V (mL) × [0.05 / mass of acid-type anionic modified microfibrillar cellulose (g)]

[0045] [Amount of anionic group (amount of phosphate group)] The aqueous suspension prepared by diluting anionic modified microfibrillar cellulose with ion-exchanged water to a content of 0.2% by mass was treated with an ion-exchange resin to obtain acid-type anionic modified microfibrillar cellulose, and then titration with an alkali was carried out for measurement. The treatment with the ion-exchange resin was carried out by adding a strongly acidic ion-exchange resin (Amberjet 1024; Organo Corporation, conditioned) with a volume of 1 / 10 to the aqueous suspension, shaking for 1 hour, and then pouring it onto a mesh with an opening of 90 μm to separate the ion-exchange resin from the aqueous suspension. Also, the titration with an alkali was carried out by measuring the change in the value of the conductivity shown by the aqueous suspension while adding 50 μL of a 0.1 mol / L sodium hydroxide aqueous solution to the aqueous suspension after the treatment with the ion-exchange resin once every 30 seconds. The amount of phosphate group (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 anionic modified microfibrillar cellulose was burned, and the sulfur content contained in the combustion product was measured by using combustion ion chromatography in accordance with the method specified in IEC 62321 and converted to the amount of sulfate groups for calculation.

[0047] [Measurement of number-average fiber width] The number-average fiber width of the anionic modified microfibrillar cellulose was measured as follows. That is, an anionic modified microfibrillar cellulose aqueous dispersion that had been subjected to high-pressure dispersion treatment at a solid content ratio of 0.005 to 0.0001% by weight was prepared, and the dispersion was cast on a mica substrate and dried to obtain a sample for observation with an atomic force microscope (AFM). Then, observation was carried out using an atomic force microscope image at any magnification of 5000 times, 10000 times, or 50000 times according to the size of the constituent fibers. At that time, an axis with an arbitrary image width in the vertical and horizontal directions was assumed in the obtained image, and the sample and observation conditions (magnification, etc.) were adjusted so that 20 or more fibers intersected the axis. After obtaining an observation image that satisfied this condition, two random axes in the vertical and horizontal directions were drawn for each 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 values of the fiber widths of the fibers intersecting the two axes were read for each (therefore, at least 20 × 2 × 3 = 120 pieces of fiber width information were obtained). The number-average fiber width was calculated from the fiber width data obtained in this way.

[0048] [Ratio of polyvalent metal salts] Regarding sheets C1 to C13 of Examples 1 to 13, 2.0 g of each sheet was collected, subjected to Kjeldahl decomposition, and then made up to 100 mL with ultrapure water (50-fold dilution). The contents of various metals were measured using an ICP emission spectroscopic analyzer. From the contents of various metals (ppm) obtained in this way, the ratio of polyvalent metals was calculated using the following formula. Ratio of polyvalent metals (mol%) = amount of polyvalent metals (ppm) ÷ [amount of polyvalent metals (ppm) + amount of monovalent metals (ppm)] × 100

[0049] [Coating property of coating agent] The coating agents B1 to 17 of Examples 1 to 13 and Comparative Examples 1 to 4 were placed in a spray container (dispensing bottle (spray type) S-50 manufactured by Sanplatec Co., Ltd.) and sprayed onto a black plastic board pasted on the wall. The dripping of the coating agent applied by spraying was visually observed, and the coating property was evaluated according to the following criteria. A: Those that can be sprayed in a mist form and have no dripping B: Those that can be sprayed but are not in a mist form, or those that have slight dripping C: Those that cannot be sprayed, or those that have dripping outside the board

[0050] [Flame retardancy] Balsa wood cut into 5 cm in length and 1 cm in width was immersed in the coating agents B1 to 17 of Examples 1 to 13 and Comparative Examples 1 to 4 for 10 seconds, and then air-dried at room temperature for 24 hours. It was fixed horizontally with a clamp, and the tip of the balsa wood was contacted with a gas burner flame. The flame retardancy was evaluated according to the following criteria. A: Those that self-extinguished within 60 seconds after the gas burner flame contacted the balsa wood B: Those that self-extinguished within 61 seconds or more and within 120 seconds after the gas burner flame contacted the balsa wood C: Those that self-extinguished over 121 seconds after the gas burner flame contacted the balsa wood D: Those in which the entire balsa wood burned

[0051] [Tensile strength of sheet] The sheets C1 to 15 of Examples 1 to 13 and Comparative Examples 1 to 2 were cut into 6 cm in length and 0.5 cm in width to prepare test pieces. Using a tensile tester (EZ-SX, manufactured by Shimadzu Corporation), the test pieces were subjected to a tensile test under the conditions of a grip distance of 3 cm, a tensile speed of 3 mm / min, 23 °C, and 50% RH, and the tensile strength (MPa) was determined. The tensile strength is the value obtained by dividing the maximum tensile force recorded when the test piece was pulled until it broke by the cross-sectional area of the test piece before the test.

[0052] [Preparation of anionic modified cellulose fibers A1 to A6] Prior to the preparation of the coating agents and sheets of the examples and comparative examples, the acid-type anionic modified cellulose fibers A1 to A6 used therein were prepared according to the following Production Examples 1 to 6.

[0053] [Production Example 1: Preparation of Anionic Modified Cellulose Fiber A1 (TEMPO-Oxidized Cellulose Fiber)] To 2 g of softwood pulp, 150 mL of water, 0.25 g of sodium bromide, and 0.025 g of TEMPO were added, and after thoroughly stirring and dispersing, an aqueous sodium hypochlorite solution (co-oxidizing agent) at 13% by mass was added such that the amount of sodium hypochlorite was 8.0 mmol / g with respect to 1.0 g of the above pulp, and the reaction was started. Since the pH decreased as the reaction proceeded, a 0.5 mol / L aqueous sodium hydroxide solution was added dropwise while maintaining the pH at 10 to 11, and the reaction was carried out until no 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, followed by repeated filtration and washing with water for purification to obtain cellulose fibers with an oxidized fiber surface. Pure water was added thereto and diluted to a cellulose fiber concentration of 4% by mass to prepare a TEMPO-oxidized cellulose fiber suspension. Thereafter, after adjusting the pH of the slurry to 10 with a 24% by mass aqueous sodium hydroxide solution, sodium borohydride was added at 0.2 mmol / g with respect to the cellulose fibers, and the reaction was started. The reaction was carried out for 2 hours for reduction treatment. After the reaction, 0.1 mol / L hydrochloric acid was added to adjust the pH to 2.0, followed by repeated filtration and washing with water for purification to obtain anionic modified cellulose fiber A1 in which the carboxy groups were in the acid form.

[0054] [Production Example 2: Preparation of Anionic Modified Cellulose Fiber A2 (TEMPO-Oxidized Cellulose Fiber)] Anionic modified cellulose fiber A2 in which the carboxy groups were in the acid form was obtained in the same manner as the preparation method of anionic modified cellulose fiber A1, except that the addition amount of the aqueous sodium hypochlorite solution was 4.0 mmol / g with respect to 1.0 g of softwood pulp.

[0055] [Production Example 3: Preparation of Anionic Modified Cellulose Fiber A3 (TEMPO-Oxidized Cellulose Fiber)] An anionic modified cellulose fiber A3 with carboxyl groups in acid form was obtained in the same manner as the preparation method of the anionic modified cellulose fiber A1, except that the addition amount of the aqueous sodium hypochlorite solution was 6.0 mmol / g with respect to 1.0 g of softwood pulp.

[0056] [Production Example 4: Preparation of Anionic Modified Cellulose Fiber A4 (TEMPO-Oxidized Cellulose Fiber)] An anionic modified cellulose fiber A4 with carboxyl groups in acid form was obtained in the same manner as the preparation method of the anionic modified cellulose fiber A1, except that the addition amount of the aqueous sodium hypochlorite solution was 10.0 mmol / g with respect to 1.0 g of softwood pulp.

[0057] [Production Example 5: Preparation of Anionic Modified Cellulose Fiber A5 (Phosphate-Esterified Cellulose Fiber)] To 100 parts by mass (dry mass) of softwood kraft pulp, an aqueous mixed solution of ammonium dihydrogen phosphate and urea was added and adjusted to contain 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. Subsequently, the obtained chemical-impregnated pulp was heated in a hot air dryer at 165 °C for 200 seconds to introduce phosphate groups into the cellulose in the pulp, and a phosphate-esterified cellulose fiber was obtained. 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 for purification to obtain an anionic modified cellulose fiber A5 in which the fiber surface was phosphate-esterified and the phosphate groups were in acid form.

[0058] [Production Example 6: Preparation of Anionic 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 using a stirrer for 10 minutes. After stirring, the slurry was suction-filtered using filter paper (No. 2). The suction filtration was carried out until the solution stopped dripping. After suction filtration, the pulp was peeled off from the filter paper, and the pulp was placed in a dryer with the temperature of the thermostat set at 50 °C and reacted for 6 hours. 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 for purification, whereby an anionic modified cellulose fiber A6 with a sulfated fiber surface and an acid-type sulfate group was obtained.

[0059] [Example 1] (Preparation of coating agent) Anionic modified cellulose fiber A2 was diluted to 0.5 mass% with ion-exchanged water and then neutralized with a 0.5 mol / L aqueous sodium hydroxide solution to adjust the pH (25 °C) to 7.0. This was treated 3 times at a pressure of 100 MPa using a high-pressure homogenizer to obtain a suspension of anionic modified microfibrillar cellulose in the Na salt form. An aqueous aluminum chloride solution was added to this suspension to a concentration of 0.1 M and allowed to stand for 12 hours. Next, the obtained anionic modified microfibrillar cellulose was rinsed 5 times with purified water to wash away excess salts and the like, and then adjusted to a cellulose concentration of 0.5 mass% with ion-exchanged water, whereby a gel-like coating agent B1 of Example 1 containing anionic modified microfibrillar cellulose having a carboxy group in the Na salt form and a carboxy group in the Al salt form was obtained.

[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 with a thickness of 30 μm.

[0061] [Example 2] The coating agent and the sheet were prepared in the same manner as in Example 1 except that anionic modified cellulose fiber A3 was used, and a coating agent B2 and a sheet C2 of Example 2 were obtained.

[0062] [Example 3] Except for using anionic modified cellulose fiber A1, the coating agent B3 and sheet C3 of Example 3 were obtained by preparing the coating agent and the sheet in the same manner as in Example 1.

[0063] [Example 4] Except for using anionic modified cellulose fiber A4, the coating agent B4 and sheet C4 of Example 4 were obtained by preparing the coating agent and the sheet in the same manner as in Example 1.

[0064] [Example 5] Except that the concentration of aluminum chloride was set to 0.2 M during the preparation of the coating agent, the coating agent B5 and sheet C5 of Example 5 were obtained by preparing the coating agent and the sheet in the same manner as in Example 3.

[0065] [Example 6] Except that the concentration of aluminum chloride was set to 0.05 M during the preparation of the coating agent, the coating agent B6 and sheet C6 of Example 6 were obtained by preparing the coating agent and the sheet in the same manner as in Example 3.

[0066] [Example 7] During the preparation of the coating agent, an aqueous magnesium chloride solution was added so that the concentration was 0.1 M instead of the aqueous aluminum chloride solution, and the other procedures were the same as in Example 3 to obtain the coating agent B7 and sheet C7 of Example 7.

[0067] [Example 8] Except that the aqueous aluminum chloride solution was added during the preparation of the coating agent and not allowed to stand, the coating agent B8 and sheet C8 of Example 8 were obtained by preparing the coating agent and the sheet in the same manner as in Example 3.

[0068] [Example 9] The coating agent B9 and the sheet C9 of Example 9 were obtained by preparing the coating agent and the sheet in the same manner as in Example 3, except that an aqueous aluminum chloride solution was added so that the concentration became 0.1 M during the preparation of the coating agent and left standing for 24 hours.

[0069] [Example 10] The coating agent B10 and the sheet C10 of Example 10 were obtained by preparing the coating agent and the sheet in the same manner as in Example 3, except that the conditions of the high-pressure homogenizer treatment during the preparation of the coating agent were changed to a pressure of 100 MPa and a treatment number of 1 time.

[0070] [Example 11] The coating agent B11 and the sheet C11 of Example 11 were obtained by preparing the coating agent and the sheet in the same manner as in Example 3, except that the conditions of the high-pressure homogenizer treatment during the preparation of the coating agent were changed to a pressure of 50 MPa and a treatment number of 1 time.

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

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

[0073] [Comparative Example 1] The coating agent B14 and the sheet C14 of Comparative Example 1 were obtained by preparing the coating agent and the sheet in the same manner as in Example 3, except that hydrochloric acid was added so that the concentration became 0.1 M instead of the aqueous aluminum chloride solution during the preparation of the coating agent and left standing for 24 hours.

[0074] [Comparative Example 2] (Preparation of Coating Agent) After diluting anionic modified cellulose fiber A1 to 0.5% by mass with ion-exchanged water, it was neutralized with a 0.5 mol / L aqueous solution of tetrabutylammonium hydroxide (TBA) to adjust the pH (25 °C) to 7.0. This was then treated 3 times at a pressure of 100 MPa using a high-pressure homogenizer to obtain coating agent B15 of Comparative Example 2 containing anionic modified microfibrillar cellulose having a carboxy group in the form of a TBA (tetrabutylammonium) salt.

[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 sheet C15 of Comparative Example 2 with a thickness of 30 μm.

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

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

[0078] For sheets C1 to 13 of Examples 1 to 13 prepared above, the ratio of the polyvalent metal salt was measured. Also, for coating agents B1 to 17 of Examples 1 to 13 and Comparative Examples 1 to 4, the coatability and flame retardancy were evaluated. Further, for sheets C1 to 15 of Examples 1 to 13 and Comparative Examples 1 to 2, the tensile strength was evaluated. The results are shown in Table 1.

[0079]

Table 1

[0080] As shown in Table 1, the coating agent B13 of Comparative Example 1 containing acid-type anionic modified microfibrillar cellulose could not impart flame retardancy. The coating agent B14 of Comparative Example 2 containing ammonium salt-type anionic modified microfibrillar cellulose also could not impart flame retardancy. In Comparative Examples 3 and 4 using an aqueous sodium hydroxide solution or an aqueous aluminum chloride solution as the coating agent, flame retardancy could not be imparted, and since these do not contain anionic modified microfibrillar cellulose, dripping occurred in the evaluation of coatability, and they were also inferior in coatability.

[0081] On the other hand, for the coating agents B1 to B13 of Examples 1 to 13 containing metal salts of anionic modified microfibrillar cellulose, spray coating was possible, dripping could be suppressed, the coatability was excellent, and flame retardancy could be imparted to the balsa wood material. Also, the tensile strength of the sheet was improved compared to Comparative Examples 1 and 2. From the comparison of Examples 3, 12, and 13, it was found that the tensile strength tended to be higher for the carboxy group than for the phosphate group or sulfate group in the case of anionic groups. Also, from the comparison of Examples 3, 8, and 9, regarding the metal ions as counterions, using both a monovalent metal and a polyvalent metal together was superior in the combined effect of achieving both flame retardancy and the strength and coatability of the coating film compared to using either the monovalent metal or the polyvalent metal alone.

[0082] Note that the various numerical ranges described in the specification can be arbitrarily combined with their upper and lower limit values, and all of these combinations are described in the specification as preferred numerical ranges. Also, the description of the numerical range "X to Y" means X or more and Y or less.

[0083] As described above, some embodiments of the present invention have been explained, but 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, replacements, and changes can be made without departing from the gist of the invention. These embodiments and their omissions, replacements, changes, etc. are included in the scope and gist of the invention, and are also included in the invention described in the claims and its equivalent scope.

Claims

1. A flame retardant coating agent containing an anion-modified microfibrillar cellulose having a number average fiber width of 2 to 1000 nm, wherein the counter ion of the anionic group of the anion-modified microfibrillar cellulose contains a metal ion.

2. The flame retardant coating agent according to Claim 1, wherein the amount of the anionic group measured with all the anionic groups of the anion-modified microfibrillar cellulose in the acid form is 0.5 to 3.0 mmol / g.

3. The flame retardant coating agent according to Claim 1, wherein the metal ion contains a monovalent metal ion and a polyvalent metal ion.

4. The flame retardant coating agent according to Claim 3, wherein the molar ratio of the monovalent metal ion / the polyvalent metal ion is 20 / 80 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 containing an anion-modified microfibrillar cellulose having a number average fiber width of 2 to 1000 nm, wherein the counter ion of the anionic group of the anion-modified microfibrillar cellulose contains a metal ion.

7. The flame retardant sheet according to Claim 6, wherein the amount of the anionic group measured with all the anionic groups of the anion-modified microfibrillar cellulose in the acid form is 0.5 to 3.0 mmol / g.

8. The flame retardant sheet according to Claim 6 or 7, wherein the metal ion contains a monovalent metal ion and a polyvalent metal ion.

9. The flame retardant sheet according to Claim 8, wherein the molar ratio of the monovalent metal ion / the polyvalent metal ion is 10 / 90 to 80 / 20.

10. A flame retardant coat layer containing an anion-modified microfibrillar cellulose having a number average fiber width of 2 to 1000 nm, wherein the counter ion of the anionic group of the anion-modified microfibrillar cellulose contains a metal ion.

Images