JP2019210165A - Hydrotalcite, production method thereof, heat retaining agent for agricultural film and agricultural film - Google Patents

Abstract

To provide a hydrotalcite having a large amount of silicate ions contained between layers, with its primary particles having a smaller average horizontal width and with little aggregation between the primary particles.SOLUTION: A hydrotalcite that fulfills (A) to (C) below and is represented by (Formula 1) below is provided. (M)(M)(OH)(A)mHO (Formula 1) (where Mis at least one divalent metal, Mis at least one trivalent metal, Ais a condensed silicate ion of n valence, 0.17≤x≤0.36, 0≤m≤10, and n is an integer of 1 or greater) (A) Molar ratio of Si to Mis 1.3 or greater; (B) Primary particles have an average horizontal width of 0.1 μm to 0.7 μm as determined by SEM method; (C) Monodispersion represented by the formula below is 50% or more: Monodispersion(%)=(average horizontal width of primary particles as determined by SEM method/average horizontal width of secondary particles as determined by laser diffraction method)×100.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a hydrotalcite having a large amount of silicate ions contained between layers and having a small average primary particle width and a good primary particle dispersibility, a method for producing the same, and the hydrotalcite as an active ingredient. The present invention relates to an agricultural film heat insulating agent and an agricultural film containing the hydrotalcite.

In recent years, agricultural films using synthetic resins have been widely used for farmhouses and tunnel cultivation. Agricultural films used in these systems allow sunlight to pass through the house and tunnel at high transmittance during the day, while absorbing infrared light from the ground and plants at night to absorb and reflect heat from the house and tunnel. The characteristic (heat retention) which does not release is required. As a specific heat retaining agent, the refractive index of the resin is close to 1.45 to 1.55 and is a compound composed of colorless ions in order to enhance the daytime sunlight transmittance of agricultural films, Infrared energy radiating from plants and plants and having a high infrared absorption ability in the wavelength range of 400 to 2000 cm ⁻¹ , and having good dispersibility with fine particles so as not to impair transparency when blended in agricultural films. Is required.

  Conventionally, hydrotalcite has been proposed as a heat insulating agent for agricultural films that satisfies the above requirements. For example, Patent Document 1 proposes hydrotalcite-based heat insulating particles for agricultural films that are compatible with transparency required for agricultural film heat insulating agents, dispersibility in resins, and handling at a high level. More specifically, a hydrotalcite-type particle powder is proposed in which the oil absorption is 35 ml / 100 g or less and the value of the oil absorption / plate surface diameter is 140 to 190.

In addition, various proposals have been made to introduce silicate ions between the layers of hydrotalcite to increase the infrared absorption ability in the wavelength region of 400 to 2000 cm ⁻¹ . For example, Patent Document 2 proposes an infrared absorber that has a large infrared absorption capability and is effective for agricultural films. More specifically, the surface is coated with 0.1 to 10% by weight of an anionic surfactant, and the interlayer anion is mainly occupied by condensed silicate ions and / or condensed phosphate ions (Mg and / or Or Zn) _a Al ₂ (condensed silicate ion and / or condensed phosphate ion) ^m− _{2 / m} · nH ₂ O (wherein, a is 2 ≦ a ≦ 8, m is an integer of 1 or more, and n is n A hydrotalcite-based compound represented by: ≦ 4, condensed silicate ion and / or condensed phosphate ion is Si and / or P is 2 mol or more) has been proposed. Since the hydrotalcite has a structure in which a large amount of a silicic acid component and / or a phosphoric acid component is introduced between layers, the hydrotalcite is supposed to absorb infrared rays in a wavelength region of 400 to 2000 cm ⁻¹ with high efficiency.

  Moreover, in patent document 3, the hydrotalcite type compound which shows the infrared absorption property which was excellent when it was made to contain in an agricultural film, and its light transmission method, its manufacturing method, the infrared absorber which uses this hydrotalcite type compound as an active ingredient, and Agricultural films containing the infrared absorber have been proposed. More specifically, a hydrotalcite compound in which a part and / or all of the interlayer anion holds at least one anion of silicon-based, phosphorus-based and boron-based multimer oxygenate ions and other anions. And agricultural films containing the hydrotalcite have been proposed.

In order to increase the infrared absorption ability in the wavelength region of 400 to 2000 cm ⁻¹ , it is preferable to introduce more silicate ions between the layers of hydrotalcite. However, as the amount of silicate ions contained between the layers increases, impurities such as magnesium silicate and aluminum silicate are more likely to be generated, and the primary particles of hydrotalcite are more likely to aggregate. Furthermore, the finer the hydrotalcite crystals, the easier the primary particles will aggregate. Therefore, in the conventional manufacturing method, when many silicate ions are introduced between the layers of the hydrotalcite where the primary particles are fine, the primary particles are aggregated and the transparency when blended in an agricultural film is lowered. There was a problem to do.

JP 2011-68877 A JP-A-8-217912 JP2001-2408

  An object of the present invention is to provide a hydrotalcite having a large amount of silicate ions contained between layers and having a small average lateral width of primary particles and a small amount of aggregation between the primary particles. It is to provide an agricultural film which is contained as a heat insulating agent and has excellent transparency and heat insulating property.

  As a result of intensive studies, the present inventors have discovered the cause of aggregation of the primary particles of hydrotalcite. That is, by performing acid treatment before ion exchange, a part of hydrotalcite is dissolved, and magnesium silicate or aluminum silicate is produced during ion exchange in water glass, and the primary particles of hydrotalcite are formed. It was agglomerated. The present inventors have sufficiently grown the primary particles of hydrotalcite in the aging process, treated with water glass before the acid treatment process, and protected the surface of the particles with silicic acid, so It was discovered that a hydrotalcite having a large amount of acid ions and a small average width of primary particles and a small amount of aggregation between primary particles can be produced. Furthermore, when this hydrotalcite was used as a heat retention agent for agricultural films, it was found that the transparency and heat retention of agricultural films were higher than when conventional hydrotalcite was used, leading to the present invention. .

The present invention provides a hydrotalcite represented by the following (formula 1) that satisfies the following (A) to (C).
(M ²⁺ ) _1-x (M ³⁺ ) _x (OH) ₂ (A ⁿ⁻ ) _{x / n} · mH ₂ O (Formula 1)
(Wherein in ^{M 2+} is a divalent metal of at least one or ^{more, M 3+} is at least one kind of trivalent ^{metal, A n-represents} an n-valent condensed silicate ions, 0.17 ≦ x ≦ 0 .36, 0 ≦ m ≦ 10, n is an integer of 1 or more.)
(A) The molar ratio of Si to M ³⁺ is 1.3 or more;
(B) The average lateral width of primary particles by SEM method is 0.1 μm or more and 0.7 μm or less;
(C) The monodispersity represented by the following formula is 50% or more;
Monodispersity (%) = (Average width of primary particles by SEM method / Average width of secondary particles by laser diffraction method) × 100

  The heat insulating agent for agricultural films of the present invention contains the hydrotalcite of the present invention as an active ingredient.

  The agricultural film of the present invention contains 50 to 99% by weight of a thermoplastic resin and 1 to 30% by weight of the hydrotalcite of the present invention.

The method for producing hydrotalcite of the present invention includes the following seven steps.
(Step 1) A raw material adjusting step of preparing a water-soluble composite metal salt aqueous solution containing a divalent metal and a trivalent metal, and an alkali metal hydroxide aqueous solution.
(Step 2) The water-soluble composite metal salt aqueous solution and the alkali metal hydroxide aqueous solution prepared in (Step 1) are continuously reacted at a reaction temperature of 0 ° C. to 40 ° C. and a reaction pH of 8 to 13 to include hydrotalcite. Reaction step to obtain a suspension.
(Step 3) An ion exchange step of dehydrating the suspension containing hydrotalcite after the reaction obtained in (Step 2), performing ion exchange using a carbonate ion-containing aqueous solution, and further washing with water.
(Step 4) A maturing step of stirring and holding the suspension containing the hydrotalcite after ion exchange obtained in (Step 3) at 100 ° C. to 250 ° C. for 1 hour to 60 hours.
(Step 5) Water glass primary treatment step of adding water glass to the suspension containing hydrotalcite after aging obtained in (Step 4).
(Step 6) An acid treatment step of adding nitric acid and / or hydrochloric acid to the suspension containing hydrotalcite after the water glass treatment obtained in (Step 5).
(Step 7) A water glass secondary treatment step of adding water glass to the suspension containing hydrotalcite after acid treatment obtained in (Step 6).

  The hydrotalcite of the present invention can be suitably used as a heat retaining agent for agricultural films. The agricultural film blended with the hydrotalcite of the present invention is superior in transparency and heat retention as compared with the case where the conventional hydrotalcite is blended.

It is a schematic diagram explaining a width about the primary particle of the hydrotalcite of the present invention. It is a schematic diagram explaining a width about the secondary particle of the hydrotalcite of the present invention.

Hereinafter, the present invention will be specifically described.
<Hydrotalcite>
Hydrotalcite of the present invention, chemical formula, metal type, x range, m range, interlayer anion type, Si content, average primary particle width, average secondary particle width, monodispersity, The BET specific surface area, the maximum (003) plane spacing, and the surface treatment are as follows.

(Chemical formula)
The hydrotalcite of the present invention is represented by the following (formula 1).
(M ²⁺ ) _1-x (M ³⁺ ) _x (OH) ₂ (A ⁿ⁻ ) _{x / n} · mH ₂ O (Formula 1)
(Wherein in ^{M 2+} is a divalent metal of at least one or ^{more, M 3+} is at least one kind of trivalent ^{metal, A n-represents} an n-valent condensed silicate ions, 0.17 ≦ x ≦ 0 .36, 0 ≦ m ≦ 10, n is an integer of 1 or more.)

(Metal type)
In the hydrotalcite represented by (Formula 1), M ²⁺ is at least one divalent metal and M ³⁺ is at least one trivalent metal. A preferred divalent metal is at least one selected from the group consisting of Mg and Zn, and a preferred trivalent metal is Al. This is because the safety to the living body is high and the particles are white.

(Range of x)
In the hydrotalcite represented by (Formula 1), the range of x is 0.17 ≦ x ≦ 0.36, preferably 0.21 ≦ x ≦ 0.36, more preferably 0.25 ≦ x ≦. 0.36, more preferably 0.29 ≦ x ≦ 0.36. A larger value of x is preferable because the ion exchange capacity increases and the maximum amount of condensed silicate ions contained between layers increases. However, when x exceeds 0.36, impurities such as oxides of trivalent metals are generated, which is not preferable because transparency and dispersibility when blended into a film are deteriorated.

(Range of m)
In the hydrotalcite represented by (Formula 1), the range of m is 0 ≦ m ≦ 10, preferably 0 ≦ m ≦ 6.

  Hydrotalcite causes desorption of crystal water around 180-230 ° C. as the temperature rises. Therefore, when blending hydrotalcite with a synthetic resin, if the processing temperature such as kneading or crosslinking is 200 ° C. or higher, the range of m is preferably 0 ≦ m ≦ 0.05. If m is in this range, problems such as resin foaming and silver streak due to crystallization water desorption can be prevented.

(Type of interlayer anion)
In hydrotalcite represented by formula (1), A ^n-represents an n-valent condensed silicate ion, n represents an integer of 1 or more. Specific condensed silicate ions include SiO ₃ ²⁻ , Si ₂ O ₅ ²⁻ , Si ₃ O ₇ ²⁻ , Si ₄ O ₉ ²⁻ , (HSiO ₃ ) ⁻ , (HSi ₂ O ₅ ) ^{− and the} like. Is not limited to this.

(Si content)
In hydrotalcite represented by formula (1), the molar ratio of ^{M 3+} of Si (Si ^{/ M 3+)} is 1.3 or more, is preferably 1.6 or more, more preferably 1.9 or more . The higher the molar ratio of Si to M ^{3+, the} better the infrared absorption ability when blended in an agricultural film, and the higher the heat retention.

(Definition of primary particles)
A primary particle is a particle having a clear boundary that cannot be further divided geometrically. FIG. 1 is a schematic diagram for explaining the lateral width (W ₁ ) of the primary particles used in the present invention. As shown in FIG. 1, the lateral width W ₁ of the primary particles is defined. That is, the major axis of the particle when the primary particle is a hexagonal plate surface is “lateral width W _{1 of the} primary particle”.

(Definition of secondary particles)
Secondary particles are particles in which a plurality of primary particles gather to form an aggregate. FIG. 2 is a schematic diagram for explaining the lateral width (W ₂ ) of the secondary particles used in the present invention. As shown in FIG. 2, the lateral width (W ₂ ) of the secondary particles is defined. That is, the diameter of the sphere when the secondary particles are considered to be wrapped by the sphere is the “lateral width W _{2 of the} secondary particles”.

(Average width of primary particles)
In the hydrotalcite of the present invention, the average width of primary particles by SEM method is 0.1 μm or more and 0.7 μm or less. The upper limit of the average width of the primary particles is more preferably 0.6 μm or less, further preferably 0.5 μm or less, further preferably 0.4 μm or less, and further preferably 0.3 μm or less. The more preferable lower limit of the average lateral width of the primary particles is 0.1 μm, more preferably 0.15 μm, and still more preferably 0.2 μm. If the average width of the primary particles is 0.7 μm or less, the heat retention and transparency when hydrotalcite is blended in an agricultural film can be improved. When the average width of the primary particles is less than 0.1 μm, the reactivity with the acid is strong, and part of the hydrotalcite is dissolved during the acid treatment step, and the primary particles are aggregated, which is not preferable. The average lateral width of the primary particles is obtained from the arithmetic average of the measured lateral widths of any 100 crystals in the SEM photograph by the SEM method. The lateral width of primary particles cannot be measured by laser diffraction in principle. Therefore, it confirms visually by SEM method.

(Average width of secondary particles)
In the hydrotalcite of the present invention, the average lateral width of secondary particles by laser diffraction method is 0.1 μm or more and 0.7 μm or less. The upper limit of the average width of the secondary particles is more preferably 0.6 μm or less, further preferably 0.5 μm or less, more preferably 0.4 μm or less, and further preferably 0.3 μm or less. The average lateral width of the secondary particles is more preferably 0.15 μm, still more preferably 0.2 μm. If the average width of the secondary particles is 0.7 μm or less, the heat retention and transparency when hydrotalcite is blended in an agricultural film can be improved. The average lateral width of the secondary particles is measured by a laser diffraction method. This is because it is difficult for the SEM method to accurately measure the average lateral width of the secondary particles.

(Monodispersity)
In the hydrotalcite of the present invention, the monodispersity represented by the following formula is 50% or more, more preferably 60% or more, still more preferably 70% or more, and most preferably 80% or more. If monodispersity is 50% or more, the heat retention and transparency at the time of mix | blending a hydrotalcite with the film for agriculture can be improved.
Monodispersity (%) = (Average width of primary particles by SEM method / Average width of secondary particles by laser diffraction method) × 100

(BET specific surface area)
In the hydrotalcite of the present invention, the BET specific surface area is 10 to 30 m ² / g. The upper limit with a more preferable BET method specific surface area is 28 m < ² > / g, and a more preferable upper limit is 26 m < ² > / g. The minimum with a preferable BET method specific surface area is 12 m < ² > / g, A more preferable upper limit is 14 m < ² > / g, Furthermore, a preferable upper limit is 16 m < ² > / g. If the BET method specific surface area is larger than 30 m ² / g, the dispersibility when hydrotalcite is blended in an agricultural film is not preferable. If the BET specific surface area is less than 10 m ² / g, the hydrotalcite primary particles are aggregated, which is not preferable.

(Maximum spacing between (003) faces)
In the hydrotalcite of the present invention, the maximum spacing of the (003) plane by the powder X-ray diffraction method is 10 mm or more, more preferably 10.5 mm or more, further preferably 11 mm or more, more preferably 11.5 mm or more. is there. The larger the (003) plane maximum interplanar spacing (interlayer distance) is, the easier it is to introduce silicate ions between the layers.

(surface treatment)
In the hydrotalcite of the present invention, when the particles are surface-treated, dispersibility in the resin can be enhanced when kneaded into an agricultural film. As surface treatment agents, anionic surfactants, cationic surfactants, phosphate ester treatment agents, silane coupling agents, titanate coupling agents, aluminum coupling agents, silicone treatment agents, silicic acid and water glass However, the present invention is not limited to this. Particularly preferred surface treatment agents are at least one selected from the group consisting of lauric acid, oleic acid and stearic acid. The treatment amount of the surface treatment agent is 0.01 to 20% by weight with respect to the weight of the hydrotalcite. A more preferable upper limit of the addition amount of the surface treatment agent is 15% by weight, more preferably 10% by weight, and most preferably 5% by weight. The more preferable lower limit of the addition amount of the surface treatment agent is 0.05% by weight, more preferably 0.1% by weight, and most preferably 0.5% by weight.

<Insulating film insulation film>
The heat insulating agent for agricultural films of the present invention contains the hydrotalcite of the present invention as an active ingredient. As long as it does not affect the infrared absorbing ability of hydrotalcite, other additives may be added as necessary.

<Agricultural film>
(Number of blends)
The agricultural film of the present invention contains 50 to 99% by weight of a thermoplastic resin and 1 to 30% by weight of hydrotalcite according to claim 1. The upper limit of the amount of hydrotalcite is more preferably 25% by weight, still more preferably 20% by weight, and most preferably 15% by weight. The lower limit of the amount of hydrotalcite is more preferably 2% by weight, still more preferably 3% by weight. When the blending amount is less than 1% by weight, the infrared absorption effect by hydrotalcite is not sufficiently exhibited. If the blending amount is more than 30% by weight, the transparency and mechanical strength of the agricultural film are lowered.

(Resin type)
Examples of the thermoplastic resin used in the present invention include polyvinyl chloride, polyvinyl bromide, polyvinyl fluoride, polyvinylidene chloride, chlorinated polyethylene, chlorinated polypropylene, brominated polyethylene, chlorinated rubber, and vinyl chloride-vinyl acetate. Polymer, vinyl chloride-ethylene copolymer, vinyl chloride-propylene copolymer, vinyl chloride-styrene copolymer, vinyl chloride-isobutylene copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-styrene-acrylonitrile Copolymer, vinyl chloride-butadiene copolymer, vinyl chloride-chlorinated propylene copolymer, vinyl chloride-vinylidene chloride-vinyl acetate terpolymer, vinyl chloride-maleic acid ester copolymer, vinyl chloride-methacrylic acid Acid copolymer, vinyl chloride-methacrylic acid ester copolymer, Vinyl chloride-acrylonitrile copolymer, internally plasticized polyvinyl chloride, polyethylene, copolymers of ethylene and other α-olefins, copolymers of ethylene and vinyl acetate, copolymers of ethylene and acrylate ether , Copolymers of ethylene and methyl acrylate, polypropylene, copolymers of propylene and other α-olefins, polybutene-1, poly-4-methylpentene-1, polystyrene, copolymers of styrene and acrylonitrile, Copolymer of ethylene and propylene diene rubber, copolymer of ethylene and butadiene, polyvinyl acetate, polylactic acid, polyvinyl alcohol, polyacrylate, polymethacrylate, polyurethane, polyester, polyether, polyamide, ABS, polycarbonate, polyphenylene Such as sulfide It is below, but not limited thereto.

(Other additives)
In addition to hydrotalcite, the agricultural film of the present invention has other additives such as plasticizers, lubricants, heat stabilizers, light stabilizers, antioxidants, antistatic agents, pigments, ultraviolet absorbers and antifogging agents. Alternatively, a dye or the like can be appropriately selected and blended.

(Kneading method)
There are no particular restrictions on the mixing and kneading method of the resin and the hydrotalcite of the present invention, but a method capable of uniformly mixing the two is preferred. For example, they are mixed and kneaded by a single or twin screw extruder, a roll, a Banbury mixer or the like. The agricultural composition of the present invention is obtained by processing the kneaded resin composition by a method such as inflation processing, calendar processing, or T-die processing to form a film.

<Production method of hydrotalcite>
The method for producing hydrotalcite of the present invention includes the following seven steps.

(Step 1) A raw material adjusting step of preparing a water-soluble composite metal salt aqueous solution containing a divalent metal and a trivalent metal, and an alkali metal hydroxide aqueous solution.
(Step 2) The water-soluble composite metal salt aqueous solution and the alkali metal hydroxide aqueous solution prepared in (Step 1) are continuously reacted at a reaction temperature of 0 ° C. to 40 ° C. and a reaction pH of 8 to 13 to include hydrotalcite. Reaction step to obtain a suspension.
(Step 3) An ion exchange step of dehydrating the suspension containing hydrotalcite after the reaction obtained in (Step 2), performing ion exchange using a carbonate ion-containing aqueous solution, and further washing with water.
(Step 4) A maturing step of stirring and holding the suspension containing the hydrotalcite after ion exchange obtained in (Step 3) at 100 ° C. to 250 ° C. for 1 hour to 60 hours.
(Step 5) Water glass primary treatment step of adding water glass to the suspension containing hydrotalcite after aging obtained in (Step 4).
(Step 6) An acid treatment step of adding nitric acid and / or hydrochloric acid to the suspension containing hydrotalcite after the water glass treatment obtained in (Step 5).
(Step 7) A water glass secondary treatment step of adding water glass to the suspension containing hydrotalcite after acid treatment obtained in (Step 6).

(Step 1: Raw material preparation step)
The raw materials for the hydrotalcite of the present invention are a divalent metal salt, a trivalent metal salt, and an alkali metal hydroxide salt. Examples of the divalent metal salt include water-soluble divalent metal salts such as magnesium chloride, magnesium bromide, magnesium nitrate, magnesium acetate, magnesium sulfate, zinc chloride, zinc bromide, zinc nitrate, and zinc acetate. This is not the case. Two or more divalent metal salts may be combined. Preferably, magnesium chloride and / or zinc chloride is used. Examples of the trivalent metal salt include water-soluble trivalent metal salts such as aluminum chloride, aluminum bromide, aluminum nitrate, aluminum acetate, and aluminum sulfate, but are not limited thereto. Two or more kinds of trivalent metal salts may be combined. Preferably aluminum sulfate is used. Examples of the alkali metal hydroxide include sodium hydroxide and potassium hydroxide, but are not limited thereto. Preferably, sodium hydroxide is used.

A divalent metal salt and a trivalent metal salt are dissolved in water to prepare a water-soluble composite metal salt aqueous solution. The concentration of the divalent metal in the water-soluble composite metal salt aqueous solution is 0.01 to 2 mol / L, preferably 0.1 to 1.5 mol / L. The concentration of the trivalent metal is 0.01 to 2 mol / L, preferably 0.1 to 1.5 mol / L. The density | concentration of an alkali metal hydroxide is 0.01-4 mol / L, Preferably it is 0.1-2 mol / L. The ratio of the divalent metal and the trivalent metal is 1.78 ≦ M ²⁺ / M ³⁺ ≦ 4.88, more preferably 1.78 ≦ M ²⁺ / M ³⁺ ≦ 3.77, and still more preferably 1.78. ≦ M ²⁺ / M ³⁺ ≦ 3, most preferably 1.78 ≦ M ²⁺ / M ³⁺ ≦ 2.45.

(Process 2: Reaction process)
The hydrotalcite of the present invention is prepared using a continuous reaction. In the continuous reaction, the ion concentration and pH in the solution can be kept uniform, so that hydrotalcite with less aggregation of primary particles can be produced, and productivity is better than batch reaction.

  The concentration during the reaction is 0.1 g / L or more and 100 g / L or less in terms of hydrotalcite. A more preferable upper limit of the concentration during the reaction is 90 g / L in terms of hydrotalcite, more preferably 80 g / L, and further preferably 70 g / L. The more preferable lower limit of the concentration during the reaction is 1 g / L in terms of hydrotalcite, more preferably 2 g / L, still more preferably 3 g / L. If the concentration during the reaction exceeds 100 g / L in terms of hydrotalcite, the primary particles of hydrotalcite aggregate, which is not preferable. When the concentration at the time of reaction is less than 0.1 g / L in terms of hydrotalcite, productivity is deteriorated, which is not preferable.

  The temperature during the reaction is 0 to 40 ° C. The upper limit with more preferable reaction temperature is 30 degreeC, Most preferably, it is 20 degreeC. A more preferred lower limit of the reaction temperature is 5 ° C, most preferably 10 ° C. When the reaction temperature is lower than 0 ° C., the suspension freezes, which is not preferable. When the reaction temperature is higher than 40 ° C., the hydrotalcite primary particles aggregate, which is not preferable. The pH during the reaction is 8 to 13, preferably 8.5 to 12, and more preferably 9 to 11. When the pH during the reaction is lower than 8 or higher than 13, it is not preferable because the monodispersity of hydrotalcite is lowered.

(Process 3: Ion exchange process)
After dehydrating the suspension containing hydrotalcite prepared in the reaction step, ion exchange is performed using a carbonate ion-containing aqueous solution. There are two ion exchange methods.

  In the first ion exchange method, the suspension containing hydrotalcite after the reaction is dehydrated, made into a cake, then dispersed in water and / or alcohol, an aqueous carbonate ion-containing solution is added, and the mixture is held with stirring. Is the method. At this time, the equivalent of a carbonate ion is 1-4 mol equivalent with respect to 1 mol equivalent of a trivalent metal contained in hydrotalcite, More preferably, it is 1.5-3 mol equivalent. The temperature for stirring and holding is preferably 30 to 90 ° C, more preferably 50 to 80 ° C. The concentration of hydrotalcite is preferably 0.1 to 300 g / L, more preferably 0.5 to 250 g / L, and still more preferably 1 to 200 g / L in terms of hydrotalcite.

  The second ion exchange method is a method in which a suspension containing hydrotalcite after the reaction is dehydrated to form a cake, and then a carbonate ion-containing aqueous solution is directly added. At this time, the addition amount of carbonate ions is 1 to 4 mol equivalent, preferably 1.5 to 3 mol equivalent, relative to 1 mol equivalent of trivalent metal contained in hydrotalcite.

  For example, sodium carbonate or potassium carbonate can be used as the carbonate ion-containing aqueous solution, but it is not particularly limited.

  After ion exchange, it is washed with deionized water having a weight 20 to 30 times that of hydrotalcite and resuspended in water. By passing through this step, salts such as sodium can be removed, and aggregation of primary particles during the ripening step can be prevented.

(Process 4: Aging process)
The suspension containing hydrotalcite prepared in the ion exchange step is stirred and held at 100 to 250 ° C. for 1 hour to 60 hours. By passing through this step, aggregation of the primary particles of hydrotalcite can be relaxed and a suspension in which the primary particles are sufficiently dispersed can be obtained. A more preferable upper limit of the aging temperature is 240 ° C, more preferably 230 ° C. A more preferable lower limit of the aging temperature is 110 ° C., more preferably 120 ° C. When the aging temperature is lower than 100 ° C., the primary particles of hydrotalcite are smaller than 0.1 μm, which is not preferable. When higher than 250 ° C., the primary particles of hydrotalcite are more than 0.7 μm. Is also unfavorable because it becomes large.

  The upper limit of the aging time is more preferably 50 hours, further preferably 40 hours, further preferably 30 hours, more preferably 20 hours. A more preferable lower limit of the aging time is 2 hours, more preferably 3 hours, and most preferably 4 hours. If the aging time is less than 1 hour, it is not sufficient as a time for relaxing aggregation of the primary particles of hydrotalcite. Even if the aging time is longer than 60 hours, there is no change in the aggregation state of the primary particles, which makes no sense.

(Process 5: Water glass primary treatment process)
The suspension containing hydrotalcite obtained in the aging step is stirred, and 0.1 to 5% by weight of water glass in terms of SiO ₂ is added to the solid content of hydrotalcite. The amount of water glass added is preferably 0.3 to 3% by weight, more preferably 0.5 to 2% by weight in terms of SiO ₂ with respect to the solid content of hydrotalcite. The concentration of the water glass is 0.01 to 1 mol / L in terms of Na ₂ O, preferably 0.05~0.6mol / L, more preferably 0.1~0.3mol / L. Examples of the water glass include No. 1 water glass, No. 2 water glass, and No. 3 water glass, but are not limited thereto. Processing temperature is 0-100 degreeC, Preferably it is 10-90 degreeC, More preferably, it is 20-80 degreeC, More preferably, it is 30-70 degreeC. By covering the particle surface of hydrotalcite with silicic acid and enhancing acid resistance, aggregation of primary particles during the acid treatment in step 6 can be prevented.

(Process 6: Acid treatment process)
The suspension containing hydrotalcite obtained in the water glass primary treatment step is stirred, and 1 to 1.5 mol equivalent of hydrochloric acid and / or nitric acid is added to 1 mol equivalent of the trivalent metal of hydrotalcite. The concentration of hydrochloric acid and nitric acid is 0.1 to 3 mol / L, preferably 0.2 to 2.5 mol / L, more preferably 0.3 to 2.2 mol / L. Processing temperature is 0-100 degreeC, Preferably it is 10-90 degreeC, More preferably, it is 20-80 degreeC, More preferably, it is 30-70 degreeC. By this step, the intercalation ions of hydrotalcite can be replaced from carbonate ions to nitrate ions and / or chlorine ions. When the interlayer ions are nitrate ions and / or chlorine ions, the silicate ions easily enter the hydrotalcite layer in the water glass secondary treatment step of Step 7.

(Step 7: Water glass secondary treatment step)
The suspension containing hydrotalcite obtained in the acid treatment step is stirred, and 1 to 2.5 mol equivalent of water glass in terms of SiO ₂ is added to 1 mol equivalent of trivalent metal of hydrotalcite. The amount of water glass added is preferably 1.1 to 2.4 mol equivalent, more preferably 1.2 to 2.3 equivalent in terms of SiO ₂ with respect to 1 mol equivalent of the trivalent metal of hydrotalcite. The concentration of the water glass is 0.01 to 1 mol / L in terms of Na ₂ O, preferably 0.05~0.6mol / L, more preferably 0.1~0.3mol / L. Examples of the water glass include No. 1 water glass, No. 2 water glass, and No. 3 water glass, but are not limited thereto. Processing temperature is 0-100 degreeC, Preferably it is 20-100 degreeC, More preferably, it is 40-100 degreeC, More preferably, it is 60-100 degreeC, Most preferably, it is 80-100 degreeC. In this step, intercalation ions of hydrotalcite can be replaced with silicate ions.

(Surface treatment process)
By subjecting hydrotalcite to a surface treatment after water glass secondary treatment, dispersibility in the film when blended with an agricultural film is improved. For the surface treatment, a wet method or a dry method can be used. In consideration of processing uniformity, a wet method is preferably used. The hydrotalcite after water glass secondary treatment and water washing is dispersed in water and a surface treatment agent dissolved under stirring is added. The temperature during the surface treatment is appropriately adjusted to a temperature at which the surface treatment agent is dissolved.

  Examples of the surface treatment agent include an anionic surfactant, a cationic surfactant, a phosphate ester treatment agent, a silane coupling agent, a titanate coupling agent, an aluminum coupling agent, a silicone treatment agent, and sodium silicate. At least one selected from can be used. Particularly preferred surface treatment agents are at least one selected from the group consisting of lauric acid, oleic acid and stearic acid. The addition amount of the surface treatment agent is 0.01 to 20% by weight with respect to the weight of the hydrotalcite. A more preferable upper limit of the addition amount of the surface treatment agent is 15% by weight, more preferably 10% by weight, and most preferably 5% by weight. The more preferable lower limit of the addition amount of the surface treatment agent is 0.05% by weight, more preferably 0.1% by weight, and most preferably 0.5% by weight.

(Drying process)
The suspension containing hydrotalcite after water glass secondary treatment or surface treatment is dehydrated, washed with water, and then dried to obtain the hydrotalcite of the present invention. The drying method can be hot air drying, vacuum drying, or the like, but is not particularly limited.

  In the hydrotalcite of the present invention, when the range of m is 0 ≦ m ≦ 0.05, the drying temperature is set to 150 ° C. to 350 ° C.

  EXAMPLES Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited only to these examples. In the examples, each physical property was measured by the following method.

(A) Chemical formula The content of Mg, Zn and Al in the powder sample was measured by chelate titration. The content of Si in the sample was measured by a gravimetric method. CO ₃ was measured based on JIS R 9101 with an AGK-type CO ₂ simple precision quantitative apparatus. Interlayer water was calculated from weight reduction using TG-DTA.

(B) Average width of primary particles After adding a powder sample to ethanol and performing ultrasonic treatment for 5 minutes, using a scanning electron microscope (SEM) (JSM-7600F, manufactured by JEOL Ltd.), any 100 The lateral width of the primary particles of the crystal was measured, and the arithmetic average was taken as the average lateral width of the primary particles.

(C) Average width of secondary particles After adding a powder sample to ethanol and sonicating for 5 minutes, the volume-based accumulation was performed using a laser diffraction scattering particle size distribution analyzer (MT3300, manufactured by Microtrack Bell). The 50% particle diameter (D50) was measured, and D50 was defined as the average width of the secondary particles.

(D) Monodispersity The monodispersity was calculated based on the following formula.
Monodispersity (%) = (Average width of primary particles / Average width of secondary particles) × 100

(E) BET method specific surface area Using a specific surface area measuring device (NOVA2000, Yuasa Ionics), the BET method specific surface area of the powder sample after drying was measured by a gas adsorption method.

(F) Maximum surface spacing of (003) plane Powder X-ray diffraction was performed using an X-ray diffractometer (Empirean, manufactured by Panalical), and from the obtained X-ray profile, the maximum surface spacing of (003) plane d ₀₀₃ Was calculated. As the X-ray source, Cu—Kα ray generated under conditions of 45 kV and 40 mA was used.

(G) Surface treatment amount The stearic acid treatment amount of the powder sample was measured by the ether extraction method.

(H) Thermal insulation index The thermal insulation index of the film was measured using an infrared absorption spectrum measuring apparatus (FT / IR-4100, manufactured by JASCO Corporation). The black body radiation energy (Eλ · dλ) at each wavelength is obtained by the following (Equation 2), and the black body radiation energy from 400 to 2000 cm ⁻¹ is integrated (Σ (Eλ · dλ)) to obtain the total black body radiation. Energy density was used. Next, the infrared absorption rate at each wavelength of the film is measured, and the black body radiation energy (Eλ · dλ) at each wavelength is multiplied by the infrared absorption rate at each wavelength and integrated to obtain the total absorbed energy density of the film. It was. The ratio between the total black body radiant energy density and the total absorbed energy density of the film was determined by the following (Equation 3) and used as a heat retention index. The higher the heat retention index, the higher the infrared absorption ability, that is, the higher the heat retention.
Eλ · dλ = 2πhC ^ 2 / [λ ^ 5 {e ^ (hC / λkT) -1}] · dλ (Formula 2)
(In the formula, λ represents the wavelength, h represents the Planck constant, C represents the speed of light in vacuum, k represents the Boltzmann constant, and T represents the absolute temperature.)
Thermal insulation index = (total absorbed energy density / total blackbody radiant energy density) × 100 (Equation 3)

(I) Light transmittance (total light transmittance and haze)
The total light transmittance and haze (cloudiness) of the film were measured using a haze meter (NDH-1001DP, manufactured by Nippon Denshoku Industries Co., Ltd.). The total light transmittance was measured according to JIS K 7361-1, and haze was measured according to JIS K 7105.

(J) Dispersibility in the film The presence or absence of hydrotalcite aggregates was visually confirmed to evaluate the dispersibility in the film. When there was no aggregate, it was marked with ◯, and when there was an aggregate, it was marked with ×.

(Raw material adjustment process)
Magnesium chloride hexahydrate (reagent grade 1, manufactured by Fuji Film Wako Pure Chemical Industries) and aluminum sulfate 14-18 hydrate (reagent grade 1, Fuji Film Wako Pure Chemical Industries) were dissolved in deionized water to obtain a magnesium concentration of 0. An aqueous composite metal salt solution having an aluminum concentration of 0.2 mol / L was obtained. On the other hand, sodium hydroxide (reagent grade 1, manufactured by Fujifilm Wako Pure Chemical Industries) was dissolved in deionized water to obtain a sodium hydroxide aqueous solution having a sodium concentration of 0.8 mol / L.

(Reaction process)
The composite metal salt aqueous solution and the sodium hydroxide aqueous solution were poured into the reaction vessel to continuously react. The suspension that overflowed from the reaction vessel as an overflow was collected, and the flow rate of the aqueous sodium hydroxide solution was adjusted so that the pH was 9.3 to 9.5. During the reaction, the temperature of the raw material and the reaction vessel was adjusted so that the reaction temperature was 20 ° C. During the reaction, stirring was performed at a rotational speed of 1000 rpm using a screw propeller having a diameter of 2.5 cm. The reaction concentration was 50 g / L as hydrotalcite.

(Ion exchange process)
The suspension was dehydrated by suction filtration using a circular Nutsche and a suction filter bottle to obtain a cake. Next, 2 mol equivalent of sodium carbonate aqueous solution was poured into the cake with respect to 1 mol equivalent of aluminum of hydrotalcite contained in the cake, and ion exchange was performed. Next, water washing was performed for the purpose of removing impurities such as by-products such as salts and residual sodium carbonate using deionized water 20 times as much as hydrotalcite.

(Aging process)
Using a homomixer, the hydrotalcite cake after washing with water was resuspended in deionized water to prepare a 50 g / L suspension. The obtained suspension was put into an autoclave and stirred at 130 ° C. for 15 hours for aging treatment.

(Water glass primary treatment process)
Deionized water is added to No. 3 water glass (SiO ₂ / Na ₂ O = 3.1 to 3.3) (manufactured by Fuji Chemical), and the concentration is adjusted to 0.2 mol / L as Na ₂ O concentration, and water glass treatment Liquid. The temperature of the suspension containing hydrotalcite after aging and the water glass treatment liquid were adjusted to 50 ° C. Under stirring, 0.8% by weight of water glass treatment liquid as SiO ₂ was added to the solid content of hydrotalcite, and the mixture was held for 10 minutes with stirring.

(Acid treatment process)
Deionized water was added to nitric acid (special reagent grade, manufactured by Fuji Film Wako Pure Chemical Industries), the concentration was adjusted to 2 mol / L, and a nitric acid treatment solution was obtained. The temperature of the suspension containing hydrotalcite after the primary treatment with water glass and the nitric acid treatment solution were adjusted to 50 ° C. Under stirring, 1.1 mol equivalent of nitric acid was added to 1 mol equivalent of aluminum in hydrotalcite, and the mixture was stirred for 30 minutes.

(Water glass secondary treatment process)
Deionized water is added to No. 3 water glass (SiO ₂ / Na ₂ O = 3.1 to 3.3) (manufactured by Fuji Chemical), and the concentration is adjusted to 0.2 mol / L as Na ₂ O concentration, and water glass treatment A liquid was used. The temperature of the suspension containing hydrotalcite after acid treatment and the water glass treatment solution were adjusted to 95 ° C. Under stirring, 2.25 mol equivalent of water glass treatment liquid as SiO ₂ was added to 1 mol equivalent of aluminum of hydrotalcite, and the mixture was stirred for 30 minutes. The suspension containing hydrotalcite after the water glass secondary treatment was dehydrated by suction filtration to obtain a cake. Water washing was performed using 20 mass times deionized water of the water glass added in the water glass primary treatment and the secondary treatment.

(Surface treatment process)
Deionized water was added to the cake after washing with water and dispersed with a homomixer to prepare a suspension containing hydrotalcite. On the other hand, to stearic acid (special grade reagent, manufactured by Fujifilm Wako Pure Chemical Industries), the same molar equivalent of sodium hydroxide (reagent grade 1, manufactured by Fujifilm Wako Pure Chemical Industries) is added, diluted with deionized water, and treated with stearic acid. A liquid was used. The temperature of the suspension containing hydrotalcite and the stearic acid treatment liquid was adjusted to 80 ° C. To the suspension containing hydrotalcite, a 2 wt% stearic acid treatment solution was added as stearic acid to the solid content of hydrotalcite, and the mixture was stirred and held for 20 minutes.

(Drying process)
The suspension after the surface treatment was dehydrated by suction filtration to obtain a cake. Water washing was performed using deionized water having a mass of 20 times the solid content of stearic acid added in the surface treatment step. The cake after water washing was put into a hot air dryer and dried at 105 ° C. for 12 hours. After cooling to room temperature, pulverization classification was performed to obtain hydrotalcite A of the present invention. The synthesis conditions of hydrotalcite A are shown in Table 1, and the analysis values are shown in Table 2.

In the water glass secondary treatment step of Example 1, a sample was prepared in the same manner except that the amount of water glass treated was 2 mol equivalent as SiO _{2 with} respect to 1 mol equivalent of aluminum in hydrotalcite. Talsite B was obtained. The synthesis conditions for hydrotalcite B are shown in Table 1, and the analysis values are shown in Table 2.

In the water glass secondary treatment step of Example 1, a sample was prepared in the same manner except that the treatment amount of water glass was 1.75 mol equivalent as SiO _{2 with} respect to 1 mol equivalent of aluminum of hydrotalcite, and the present invention. Of hydrotalcite C was obtained. The synthesis conditions of hydrotalcite C are shown in Table 1, and the analysis values are shown in Table 2.

  In the raw material preparation step of Example 1, a sample was prepared in the same manner except that the magnesium concentration of the aqueous composite metal salt solution was 0.6 mol / L and the aluminum concentration was 0.2 mol / L, and the hydrotalcite D of the present invention. Got. The synthesis conditions of hydrotalcite D are shown in Table 1, and each analysis value is shown in Table 2.

The hydrotalcite A obtained in Example 1 was further dried at 220 ° C. for 12 hours to obtain hydrotalcite E of the present invention. The synthesis conditions for hydrotalcite E are shown in Table 1, and the analysis values are shown in Table 2.
(Comparative Example 1)

A hydrotalcite F was obtained in the same manner as in Example 1 except that the water glass primary treatment step was omitted. The synthesis conditions for hydrotalcite F are shown in Table 1, and the analysis values are shown in Table 2.
(Comparative Example 2)

In the water glass secondary treatment step of Example 1, a sample was prepared in the same manner except that the treatment amount of water glass was 0.8 mol equivalent as SiO _{2 with} respect to 1 mol equivalent of aluminum of hydrotalcite. I got Site G. The synthesis conditions of hydrotalcite G are shown in Table 1, and the analysis values are shown in Table 2.
(Comparative Example 3)

A hydrotalcite H was obtained in the same manner as in Example 1 except that the water glass primary treatment step, the acid treatment step, and the water glass secondary treatment step were omitted. The synthesis conditions of hydrotalcite H are shown in Table 1, and the analysis values are shown in Table 2.
(Comparative Example 4)

  A hydrotalcite I was obtained in the same manner as in the aging step of Example 1, except that the aging temperature was 60 ° C. The synthesis conditions of hydrotalcite I are shown in Table 1, and each analysis value is shown in Table 2.

From Tables 1 and 2, the hydrotalcite of the present invention of Examples 1 to 5 has a high molar ratio of Si to M ³⁺ and a high monodispersity. The hydrotalcite F of Comparative Example 1 has a high Si molar ratio to M ³⁺ , but the secondary particles have a large average lateral width and a low monodispersity. Although the hydrotalcite G of Comparative Example 2 has a high monodispersity, the molar ratio of Si to M ³⁺ is low. Hydrotalcite I of Comparative Example 4 has an average primary particle width of less than 0.1 μm and a low monodispersity.

(Production of agricultural film)
Hydrotalcite A obtained in Example 1 was blended with polyethylene resin at the following blending ratio to produce an agricultural film.
(Combination)
High-pressure metallocene polyethylene (KF-282, manufactured by Nippon Polyethylene): 90% by weight
Hydrotalcite: 10% by weight

  The above blend was kneaded at 180 ° C. using a single-screw kneader, and then formed into a thickness of 100 μm by a T-die extruder at 160 ° C. to obtain an agricultural film of the present invention. Each evaluation result of the obtained agricultural film is shown in Table 3.

  Using the hydrotalcite B obtained in Example 2, an agricultural film was produced at the same mixing ratio as in Example 6. Each evaluation result of the obtained agricultural film is shown in Table 3.

  Using the hydrotalcite C obtained in Example 3, an agricultural film was produced at the same mixing ratio as in Example 6. Each evaluation result of the obtained agricultural film is shown in Table 3.

  Using the hydrotalcite D obtained in Example 4, an agricultural film was produced at the same blending ratio as in Example 6. Each evaluation result of the obtained agricultural film is shown in Table 3.

Using the hydrotalcite E obtained in Example 5, an agricultural film was produced at the same mixing ratio as in Example 6. Each evaluation result of the obtained agricultural film is shown in Table 3.
(Comparative Example 5)

Using the hydrotalcite F obtained in Comparative Example 1, an agricultural film was produced at the same blending ratio as in Example 6. Each evaluation result of the obtained agricultural film is shown in Table 3.
(Comparative Example 6)

Using the hydrotalcite G obtained in Comparative Example 2, an agricultural film was produced at the same blending ratio as in Example 6. Each evaluation result of the obtained agricultural film is shown in Table 3.
(Comparative Example 7)

Using the hydrotalcite H obtained in Comparative Example 3, an agricultural film was produced at the same blending ratio as in Example 6. Each evaluation result of the obtained agricultural film is shown in Table 3.
(Comparative Example 8)

Using the hydrotalcite I obtained in Comparative Example 4, an agricultural film was produced at the same mixing ratio as Example 6. Each evaluation result of the obtained agricultural film is shown in Table 3.
(Comparative Example 9)

  In Example 6, an agricultural film was prepared with 100% by weight of metallocene PE without adding hydrotalcite. Each evaluation result of the obtained agricultural film is shown in Table 3.

  From Table 3, the agricultural films of the present invention of Examples 6 to 10 have high heat retention index and total light transmittance, low haze, and good visual dispersibility. The agricultural film of Comparative Example 5 had a relatively high heat retention index and total light transmittance, but had a large haze, and agglomerates were visually confirmed. Since the agricultural film of Comparative Example 6 uses hydrotalcite having a low Si content, the heat retention index is low. Since the agricultural film of Comparative Example 7 uses hydrotalcite that does not contain Si, the thermal insulation index is lower than that of Comparative Example 6. Since the agricultural film of Comparative Example 8 uses hydrotalcite having a low monodispersity, the total light transmittance was low, the haze was large, and aggregates were confirmed visually.

  The hydrotalcite of the present invention can be suitably used as a heat retaining agent for agricultural films. The agricultural film blended with the hydrotalcite of the present invention is superior in transparency and heat retention as compared with the case where the conventional hydrotalcite is blended.

W ₁ ... width of primary particles W ₂ ... width of secondary particles

Claims (9)

Hydrotalcite represented by the following (formula 1) that satisfies the following (A) to (C).
(M ²⁺ ) _1-x (M ³⁺ ) _x (OH) ₂ (A ⁿ⁻ ) _{x / n} · mH ₂ O (Formula 1)
(Wherein in ^{M 2+} is a divalent metal of at least one or ^{more, M 3+} is at least one kind of trivalent ^{metal, A n-represents} an n-valent condensed silicate ions, 0.17 ≦ x ≦ 0 .36, 0 ≦ m ≦ 10, n is an integer of 1 or more.)
(A) The molar ratio of Si to M ³⁺ is 1.3 or more;
(B) The average lateral width of primary particles by SEM method is 0.1 μm or more and 0.7 μm or less;
(C) The monodispersity represented by the following formula is 50% or more;
Monodispersity (%) = (Average width of primary particles by SEM method / Average width of secondary particles by laser diffraction method) × 100 BET specific surface area is less than ^{10 m} 2 / g or more ^{30 m} 2 / g, hydrotalcite of claim 1, wherein. The hydrotalcite according to claim 1, wherein the range of m in (Formula 1) according to claim 1 is 0 ≦ m ≦ 0.05. The hydrotalcite according to claim 1, wherein the average width of secondary particles by laser diffraction method is 0.1 µm or more and 0.7 µm or less. The hydrotalcite according to claim 1, wherein the (003) plane maximum plane spacing by a powder X-ray diffraction method is 10 mm or more. Hydrotalcite surface is made of anionic surfactant, cationic surfactant, phosphate ester, silane coupling agent, titanate coupling agent, aluminum coupling agent, silicone treatment agent, silicic acid and water glass The hydrotalcite according to claim 1, wherein the hydrotalcite is surface-treated with at least one selected from the group consisting of: A heat insulating agent for agricultural films, comprising the hydrotalcite according to claim 1 as an active ingredient. An agricultural film containing 50 to 99% by weight of a thermoplastic resin and 1 to 30% by weight of hydrotalcite according to claim 1. The manufacturing method of the hydrotalcite of Claim 1 including the following seven processes.
(Step 1) A raw material adjusting step of preparing a water-soluble composite metal salt aqueous solution containing a divalent metal and a trivalent metal, and an alkali metal hydroxide aqueous solution.
(Step 2) The water-soluble composite metal salt aqueous solution and the alkali metal hydroxide aqueous solution prepared in (Step 1) are continuously reacted at a reaction temperature of 0 ° C. to 40 ° C. and a reaction pH of 8 to 13 to include hydrotalcite. Reaction step to obtain a suspension.
(Step 3) An ion exchange step of dehydrating the suspension containing hydrotalcite after the reaction obtained in (Step 2), performing ion exchange using a carbonate ion-containing aqueous solution, and further washing with water.
(Step 4) A maturing step of stirring and holding the suspension containing the hydrotalcite after ion exchange obtained in (Step 3) at 100 ° C. to 250 ° C. for 1 hour to 60 hours.
(Step 5) Water glass primary treatment step of adding water glass to the suspension containing hydrotalcite after aging obtained in (Step 4).
(Step 6) An acid treatment step of adding nitric acid and / or hydrochloric acid to the suspension containing hydrotalcite after the water glass treatment obtained in (Step 5).
(Step 7) A water glass secondary treatment step of adding water glass to the suspension containing hydrotalcite after acid treatment obtained in (Step 6).

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