Aqueous suspension containing layered silicate

Abstract

To provide an aqueous suspension of a layered silicate mineral, capable of imparting high barrier properties against air, water vapor, or the like to a coating surface of a sheet such as base paper.SOLUTION: An aqueous suspension contains a layered silicate mineral having a thickness of 0.04 μm or less calculated by the following equation (1) and a dispersant of 0.25 mass% or more to the layered silicate mineral: thickness (μm)=(2*r) / (a*r*b-4) (1). In the equation, r is a value of D50 by laser diffraction method (μm), a is particle absolute specific gravity (g / cm3), and b is BET specific surface area (m2 / g).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an aqueous suspension that can be applied to a sheet of paper or the like to impart barrier properties against gases such as water vapor, making it usable as a packaging material for food, etc. [Background technology]

[0002] Conventionally, to provide a sheet with barrier properties against gases and liquids, an organic binder has been applied to the sheet alone, or a layered silicate mineral with a large aspect ratio (long diameter / thickness) has been applied to the surface together with a resin binder. Patent Document 1 describes that by coating paper with a composite of a layered inorganic compound with a high aspect ratio and an organic binder, oil penetration is prevented and oil resistance is improved. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2015-21200 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, while layered silicates with a high aspect ratio have a large major axis, it is difficult to make them very thin while maintaining a large major axis. Therefore, the thickness of layered silicates with a high aspect ratio is not particularly thin compared to general layered silicates with a low aspect ratio. When layered silicates with a high aspect ratio are coated on paper or the like, the thickness is not sufficiently thin, so when the layered silicates overlap on the coated surface, the layered silicates are arranged in a raised position due to their thickness, allowing gases and liquids to pass through the gaps, thereby impairing their barrier properties. Therefore, an object of the present invention is to provide an aqueous suspension of a layered silicate mineral that can impart high barrier properties against gases and liquids to a coated surface. Another object of the present invention is to provide a coated sheet produced using the aqueous suspension. Another object of the present invention is to provide a method for producing a coated sheet using the aqueous suspension. [Means for solving the problem]

[0005] The present inventors have found that by reducing the thickness of the layered silicate mineral without being concerned about the aspect ratio, and by using an aqueous suspension containing the layered silicate mineral, a sheet with high barrier properties can be obtained that does not create large gaps when the layered silicate minerals are stacked. 1. A layered silicate mineral having a thickness calculated by the following formula (1) of 0.04 μm or less, and 0.25% by mass or more of a dispersant relative to the layered silicate mineral; A water suspension comprising: Thickness (μm) = (2 × r) ÷ (a × r × b - 4) (1) (wherein r = D50 value (μm) by laser diffraction method, a=particle true specific gravity (g / cm 3 ), b=BET specific surface area (m 2 / g)) 2. The aqueous suspension according to 1 above, wherein the dispersant is at least one selected from the group consisting of sodium polyacrylate, sodium hexametaphosphate, sodium tripolyphosphate, potassium tripolyphosphate, 2-amino-2-methyl-1-propanol, monoisopropanolamine, and poly(diallyldimethylammonium chloride). 3. The aqueous suspension according to 1 above, wherein the dispersant contains sodium polyacrylate in an amount greater than 0% by mass and not more than 0.25% by mass relative to the layered silicate mineral. 4. The aqueous suspension according to 1 above, wherein the dispersant contains sodium hexametaphosphate in an amount greater than 0% by mass and not greater than 5% by mass relative to the layered silicate mineral. 5. The aqueous suspension according to any one of 1 to 4 above, wherein the solid content of the aqueous suspension is 45 mass % or more. 6. The aqueous suspension according to any one of 1 to 5 above, wherein the layered silicate mineral is kaolin. 7. A coated sheet having a layer containing a layered silicate mineral, the thickness of which is calculated from the following formula (1) being 0.04 μm or less, provided on at least one side of the sheet to be coated. Thickness (μm) = (2 × r) ÷ (a × r × b - 4) (1) (wherein r = D50 value (μm) by laser diffraction method, a=particle true specific gravity (g / cm 3 ), b=BET specific surface area (m 2 / g)) 8. A method for producing a coated sheet, comprising the step of coating a sheet with the aqueous suspension according to any one of 1 to 6 above. [Effects of the Invention]

[0006] According to the present invention, by using a layered silicate that is as thin as possible, regardless of the length of the major axis or the aspect ratio, it is possible to reduce defects in the coating film formed after coating on a sheet such as paper (for example, steps formed when the layered silicate mineral is layered and the resulting gaps), thereby providing a layered silicate suspension that can be applied without impairing barrier properties. The coated sheet of the present invention has high barrier properties against gases and liquids. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a scanning electron microscope photograph of the surface of the coated paper obtained in Example 1. [Figure 2] 1 is a scanning electron microscope photograph of the surface of the coated paper obtained in Example 2. [Figure 3] 1 is a scanning electron microscope photograph of the surface of the coated paper obtained in Example 3. [Figure 4] 1 is a scanning electron microscope photograph of the surface of the coated paper obtained in Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0008] [Layered silicate] The layered silicate mineral used in the present invention has a thickness calculated by formula (1) of 0.04 μm or less. Thickness (μm) = (2 × r) ÷ (a × r × b - 4) (1) (wherein r = D50 value (μm) by laser diffraction method, a=particle true specific gravity (g / cm 3 ), b=BET specific surface area (m 2 / g)) where: D50 means the median diameter, and is a value measured in a wet state using a laser diffraction particle size distribution analyzer (Mastersizer 3000 manufactured by Spectris) under conditions of a refractive index of 1.56 and an absorptivity of 0.01. The true particle density is the value for the relevant mineral listed in the Encyclopedia of Earth Sciences (Heibonsha). The BET specific surface area is a value measured by the BET three-point method (N2 gas method) using a specific surface area / pore distribution measuring device.

[0009] Traditionally, the thickness of layered particles has been measured by image analysis using SEM observation or visual measurement. In this case, it is difficult to maintain a consistent particle state during SEM imaging, making it difficult to measure a large number of particles. Furthermore, visual measurement makes objective evaluation difficult. Therefore, to calculate the thickness using an analytical method that provides stable measurement results and takes into account the influence of all particles, we have developed Equation (1). Equation (1) is calculated assuming that each layered particle is cylindrical. The diameter of the base of the cylinder corresponds to the long diameter of the layered particle, and the height of the cylinder corresponds to the thickness. Laser diffraction particle size analyzers, which use visible light, have significantly lower sensitivity for measuring particles smaller than 0.1 μm. Therefore, layered silicates with thicknesses generally smaller than 0.1 μm produce measurement results that depend on the long diameter. In Equation (1), the long diameter (r, base diameter) is used as the D50 value determined by laser diffraction. The total surface area of particles per gram was calculated using the BET specific surface area (b). The area of the front and back of the base is "π×(r / 2)^2×2" (μm 2 ), and the side area is "π × r × thickness" (μm 2) and the sum of these was the surface area per particle. The total volume per 1g of particle was divided by the particle's true specific gravity (a) to obtain "1 / a x 10^12" (μm 3 ) and divide this value by the volume of one particle, "π × (r / 2)^2 × thickness", to obtain the number of particles per 1g. The unit of BET specific surface area is μm 2 The thickness was calculated by simplifying the equation by equating "b x 10^12" converted to / g with "surface area per particle" x "number of particles per 1g", and the equation (1) was obtained.

[0010] The thickness of the layered silicate mineral before pulverization is usually 0.08 μm or more. The layered silicate mineral of the present invention can be obtained by wet-pulverizing it with strong energy to a thickness of 0.04 μm or less. The layered silicate mineral of the present invention is thinner than general kaolin and has a smaller D50 measured by laser diffraction, allowing for the formation of a dense coating layer. The layered silicate mineral of the present invention is smoother and less three-dimensional than general kaolin, allowing for significantly smaller passageways for gases such as air and liquids. This effect generally increases as the thickness of the layered silicate mineral decreases. A thickness of 0.03 μm or less is preferred. From the standpoint of handleability, a thickness of 0.01 μm or more is preferred. The D50 measured by laser diffraction, which depends on the major axis of the layered silicate mineral of the present invention, is not particularly limited, but if it is large, even a slight tilt will cause large voids when stacked, so D50 measured by laser diffraction is preferably 15 μm or less, more preferably 5 μm or less. If D50 is too small, the viscosity of the aqueous suspension will increase too much, making it difficult to handle, so the lower limit of D50 is preferably 0.1 μm or more, more preferably 0.12 μm or more.

[0011] Examples of layered silicate minerals that can be used in the present invention include kaolin, smectite, montmorillonite, bentonite, hectorite, etc. Kaolin is preferred from the viewpoint of good fluidity in aqueous suspensions. The dispersion medium of the aqueous suspension of the present invention is water, but other dispersion media, such as monohydric alcohols such as methyl alcohol, ethyl alcohol, and isopropyl alcohol, and polyhydric alcohols such as ethylene glycol, propylene glycol, and glycerin, may be contained to the extent that the effects of the present invention are not impaired. In the present invention, the layered silicate mineral is in the form of an aqueous suspension, and once it is made into an aqueous suspension, it does not undergo a drying process, so it does not aggregate and lose the thinness and fineness of the micro-delaminated kaolin.

[0012] [Dispersant] The aqueous suspension of the present invention contains a dispersant, which can prevent the layered silicate mineral of the present invention, which is much thinner than conventional layered silicate minerals, from agglomerating in the aqueous suspension. The dispersant that can be used in the present invention is not particularly limited, but examples include sodium polyacrylate, sodium hexametaphosphate, sodium tripolyphosphate, potassium tripolyphosphate, 2-amino-2-methyl-1-propanol, monoisopropanolamine, and poly(diallyldimethylammonium chloride). Sodium polyacrylate is listed in FDA Sec. 176.170 "Components of paper and paperboard in contact with aqueous and fatty foods" as a component that can be safely used in packaging materials that come into contact with food, and therefore can be safely used when the coated product of the present invention is used as a food packaging material. The dispersant used in the present invention is preferably at least one selected from the group consisting of sodium polyacrylate and sodium hexametaphosphate.

[0013] 〔ratio〕 The content of the layered silicate mineral in the aqueous suspension is preferably 38% by mass or 73% by mass, more preferably 4870% by mass. If the solid content of the layered silicate mineral is in this range, it is advantageous in terms of drying costs and transportation costs after coating. The content of the dispersant in the aqueous suspension is 0.2 to 10 mass %, preferably 0.7 to 7 mass %, and more preferably 1 to 5 mass %, relative to the layered silicate mineral. When the content of the dispersant is within this range, sufficient fluidity of the aqueous suspension can be ensured. To prepare a highly concentrated dispersion of a thin layered silicate mineral with a high specific surface area, a relatively large amount of dispersant must be added to prevent aggregation of the layered silicate mineral and maintain the viscosity of the aqueous suspension. Conventional layered silicates containing dispersants are prepared by first preparing an aqueous suspension using a dispersant and then drying it using a spray dryer or similar device. However, even though the dispersant is contained in the dried layered silicate, additional dispersant addition is required when preparing the aqueous suspension for use. However, in the case of the layered silicate mineral of the present invention, since the aqueous suspension does not require a drying process after dispersant addition, only one dispersant addition is required, eliminating the need for a large amount of dispersant. For example, in the case of coated sheets that come into direct contact with food, the dispersant content of the aqueous suspension that forms the coating film varies depending on government regulations. To comply with FDA Sec. 176.170, the sodium polyacrylate addition rate must be 0.25% by mass or less based on the solid content of the layered silicate. Layered silicate minerals that are finely ground to provide barrier properties normally require a large amount of dispersant. However, even in the case of aqueous suspensions for producing coated sheets for food packaging, the dispersant content of the present invention is within the range specified by the FDA, preventing aggregation and maintaining low slurry viscosity. For cost reasons, the amount of sodium hexametaphosphate is preferably 5% by mass or less relative to the layered silicate mineral. The total amount of sodium polyacrylate and sodium hexametaphosphate is preferably 5% by mass or less relative to the layered silicate mineral. The concentration of the aqueous suspension varies depending on the amounts of the layered silicate mineral, dispersant, and optional additives, but is, for example, 40 to 75 mass %, preferably 45 to 73 mass %, and more preferably 50 to 72 mass % in terms of solid content. A solid content in this range is advantageous in terms of the cost of drying after coating and the cost of transporting the aqueous suspension.

[0014] [Additives] The aqueous suspension of the present invention may further contain additives depending on the intended use. For example, when a coated sheet is produced from the aqueous suspension of the present invention, the aqueous suspension may contain an organic binder, a sizing agent, a preservative, a pH adjuster, etc. Examples of organic binders include polyvinyl alcohol, styrene-butadiene latex, acrylic latex, starch, etc. From the viewpoint of high air barrier properties, fully saponified polyvinyl alcohol is preferred. Examples of sizing agents include oxidized starch, styrene-acrylic copolymer, and styrene-methacrylic copolymer. Examples of preservatives include bronopol and isothiazolinone preservatives. Examples of pH adjusters include sodium hydroxide, sodium carbonate, calcium hydroxide, etc. From the viewpoint of fixing the layered silicate to the sheet, it is preferable to contain an organic binder. The amount of additive in the aqueous suspension of the present invention can be determined appropriately by those skilled in the art, but for example, in the case of an organic binder, it is preferable to add it in a mass ratio of solids to organic binder of 1:1 to 10:1 relative to the solids content of the aqueous suspension, which allows the layered silicate mineral to be fixed on the sheet.

[0015] [Preparation of aqueous suspension] The aqueous suspension of the present invention can be prepared, for example, by suspending the layered silicate mineral and the dispersant in water, or by mixing an aqueous dispersion of the layered silicate mineral with an aqueous suspension of the dispersant.

[0016] [Coated material] The aqueous suspension of the present invention can be applied to paper to produce coated paper for food packaging and the like, or to woven fabrics or nonwoven fabrics made of natural or synthetic fibers to produce leak-proof textile products such as diapers and other personal care products. The type of paper or fiber is not particularly limited, but for example, when coated paper is used as a food packaging material, fine paper, medium-quality paper, coated paper, kraft paper, glassine paper, paperboard, white paperboard, liner, etc. can be used. Considering society-wide efforts to achieve a decarbonized society, it is recommended to apply the suspension to paper and natural fiber products. The properties of the base paper that constitutes the coated paper are not particularly limited, but for example, a paper with a basis weight of 30 to 400 g / m 2 When this type of coating is used, it can be used for packaging paper and containers for food, etc. The coating weight after drying is 2 to 25 g / m 2 When the aqueous suspension of the present invention is applied so that the concentration is about 100% or more, it is easy to coat the paper and also provides barrier properties. When applying the aqueous suspension of the present invention to paper or nonwoven fabric, a wire bar, blade coater, roll coater, curtain coater, spray gun, brush, or the like can be used. The aqueous suspension of the present invention may further contain additives depending on the intended use of the object to which the aqueous suspension of the present invention is applied. For example, when the aqueous suspension of the present invention is applied to a sheet to be used as a food packaging material, 10 to 100% by mass of an organic binder or the like may be contained relative to the layered silicate. The coated article obtained from the aqueous suspension of the present invention has excellent barrier properties against gases such as water vapor and air, and liquids. Specifically, the air resistance, which can be measured using an Oken testing machine method in accordance with JIS P8117:2009, is usually 50,000 seconds or more. [Example]

[0017] In the following examples, "%" represents "% by mass" unless otherwise specified. Example 1 [Adjustment of coating liquid] The D50 measured using a Spectris Mastersizer 3000 under the conditions of a refractive index of 1.56 and an absorptivity of 0.01 was 3.3 μm, and the specific surface area measured using the BET 3-point method (N2 gas method, Quantachrome NOVA1000) was 21.8 m 2 / g, and the thickness of the kaolin calculated by formula (1) is 0.036 μm (the true specific gravity is 2.61 g / cm, which is the value of kaolinite listed in the Earth Science Dictionary (Heibonsha)). 3 A 53% aqueous suspension containing 0.20% sodium polyacrylate and 1.0% sodium hexametaphosphate as dispersants relative to the kaolin was prepared. This suspension was mixed with a solution of Poval PVA117 (fully saponified polyvinyl alcohol, manufactured by Kuraray Co., Ltd.) that had been dissolved in distilled water to a 10% concentration in advance, at a solids ratio (mass ratio of dry kaolin + dispersant to dry Poval, the same applies below) of 1:1, excluding water, to adjust the concentration of the aqueous suspension to 15%. [Paper coating] Basis weight 64g / m 2 The coating weight after drying was 5g / m on one side of Mitsubishi Paper Mills PPC paper. 2 The coated paper was then dried at 110°C for 1 minute to obtain a coated paper. [Evaluation of coated paper] Air resistance (Oken) was measured and air permeability was evaluated in accordance with JIS P8117:2009 using an Oken-type smoothness and air permeability tester (digital type) manufactured by Kumagai Riki Kogyo Co., Ltd. The surface of the coated paper was also observed using a Hitachi High-Technologies Corporation field emission scanning electron microscope (SU8220).

[0018] <Example 2> The aqueous kaolin suspension used in Example 1 [Preparation of Coating Liquid] had a D50 of 0.75 μm measured using a Spectris Mastersizer 3000 under conditions of a refractive index of 1.56 and an absorptivity of 0.01, and a specific surface area of 33.6 m measured using the BET 3-point method (N2 gas method, Quantachrome NOVA1000). 2 / g, and the thickness was calculated to be 0.024 μm by calculation formula (1). 3) and a concentration of 53%, containing 0.20% sodium polyacrylate and 1.8% sodium hexametaphosphate as dispersants relative to the kaolin, was used, and coated paper was prepared and evaluated in the same manner as in Example 1, except that the concentration was 53%.

[0019] Example 3 The aqueous kaolin suspension used in Example 1 [Preparation of Coating Liquid] had a D50 of 0.15 μm measured using a Spectris Mastersizer 3000 under conditions of a refractive index of 1.56 and an absorptivity of 0.01, and a specific surface area of 35.3 m measured using the BET 3-point method (N2 gas method, Quantachrome NOVA1000). 2 / g, and the thickness of the kaolin calculated by formula (1) is 0.030 μm (the true specific gravity is 2.61 g / cm, which is the value of kaolinite listed in the Earth Science Dictionary (Heibonsha)). 3 Coated paper was prepared in the same manner as in Example 1, except that the dispersant contained sodium polyacrylate at a concentration of 0.25% relative to the kaolin and sodium hexametaphosphate at a concentration of 4.5% relative to the kaolin, for a total concentration of 53%, and then evaluated.

[0020] <Comparative Example 1> [Adjustment of coating liquid] The kaolin used in Example 1 [Preparation of Coating Liquid] had a D50 of 8.2 μm measured using a Malvern Mastersizer 3000 under conditions of a refractive index of 1.56 and an absorptivity of 0.01, and a specific surface area of 12.8 m measured using the BET 3-point method (N2 gas method, Nova1000 manufactured by Quantachrome). 2 / g, and the thickness of the kaolin calculated by formula (1) is 0.061 μm (the true specific gravity is 2.61 g / cm, which is the value of kaolinite listed in the Earth Science Dictionary (Heibonsha)). 3 Coated paper was prepared and evaluated in the same manner as in Example 1, except that the concentration of the coating material was 53% of Barisurf HX (manufactured by Imerys Minerals Japan Co., Ltd.).

[0021] [Table 1]

[0022] As shown in Table 1, Examples 1 to 3, in which the kaolin thickness is 0.04 μm or less, have higher air resistance (Oken method) values than Comparative Example 1, in which the thickness is 0.061 μm, demonstrating low air permeability and high barrier properties. In the SEM photograph of Comparative Example 1 in Figure 4, portions in which the kaolin particles appear to be raised due to the thickness were observed, but such portions were not observed in Examples 1 to 3 in Figures 1 to 3. Experiments have revealed that by reducing the thickness to less than 0.04 μm, as in Examples 1 to 3, defects in the coating layer are reduced and the barrier properties for air, etc. are improved.

Claims

1. A layered silicate mineral having a thickness calculated by the following formula (1) of 0.04 μm or less, and 0.25% by mass or more of a dispersant relative to the layered silicate mineral; A water suspension comprising: Thickness (μm) = (2 × r) ÷ (a × r × b − 4) (1) (wherein r = D50 value (μm) measured by laser diffraction method) a = true specific gravity of particles (g / cm 3 )、 b = BET specific surface area (m 2 / g))

2. 2. The aqueous suspension according to claim 1, wherein the dispersant is at least one selected from the group consisting of sodium polyacrylate, sodium hexametaphosphate, sodium tripolyphosphate, potassium tripolyphosphate, 2-amino-2-methyl-1-propanol, monoisopropanolamine, and poly(diallyldimethylammonium chloride).

3. 2. The aqueous suspension according to claim 1, wherein the dispersant comprises sodium polyacrylate in an amount of more than 0% by mass and not more than 0.25% by mass relative to the layered silicate mineral.

4. 2. The aqueous suspension according to claim 1, wherein the dispersant comprises sodium hexametaphosphate in an amount of more than 0% by mass and not more than 5% by mass relative to the layered silicate mineral.

5. The aqueous suspension according to any one of claims 1 to 4, wherein the solid content of the aqueous suspension is 40 to 75 mass%.

6. 6. The aqueous suspension according to any one of claims 1 to 5, wherein the layered silicate mineral is kaolin.

7. A coated sheet comprising a layer containing a layered silicate mineral and having a thickness of 0.04 μm or less as calculated by the following formula (1), provided on at least one surface of the sheet to be coated. Thickness (μm) = (2 × r) ÷ (a × r × b − 4) (1) (wherein r = D50 value (μm) measured by laser diffraction method) a = true specific gravity of particles (g / cm 3 )、 b = BET specific surface area (m 2 / g))

8. A method for producing a coated sheet, comprising the step of coating a sheet with the aqueous suspension according to any one of claims 1 to 6.

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