JP6865553B2 - Method for Producing Multiple Lyophilized Water Absorbent Polymer Particles - Google Patents

Description

The present disclosure relates to a method for producing a plurality of freeze-dried water-absorbent polymer particles.

The water-absorbent polymer particles are absorbent articles (eg, disposable diapers, sanitary napkins, etc.), agricultural and horticultural materials (eg, water retention agents, soil conditioners, etc.), industrial materials (eg, cable water stop agents, condensation). It is used in a wide range of fields such as inhibitors). Further, the water-absorbent polymer particles have been improved from various viewpoints.

For example, in Patent Document 1, for the purpose of forming a porous hydrophilic high swelling hydrogel, a hydrophilic high swelling hydrogel swollen with water is freeze-dried to produce a porous hydrophilic high swelling hydrogel. How to do it is described.

Patent Document 2 describes a method of modifying an absorbent polymer, in which an absorbent polymer having a first vortex duration is prepared, water is absorbed by the absorbent polymer, and at least a part of the absorbed water is absorbed. A method of removing from the absorbent polymer to obtain a modified absorbent polymer having a second duration, wherein the ratio of the first vortex duration to the second vortex duration is at least about 5. A method (claim 14) is described in which the polymer has a stiffness index of at least about 0.7. Further, Patent Document 2 describes a method (claim 25) characterized in that a part of the water is removed by freeze-drying.

Japanese Unexamined Patent Publication No. 10-7830 Japanese Unexamined Patent Publication No. 2006-505666

In the method described in Patent Document 1, a hydrophilic highly swelling hydrogel to be freeze-dried is produced by a solution polymerization method. Therefore, when the polymer mass polymerized by the solution polymerization method is pulverized to form a plurality of hydrophilic highly swelling hydrogel polymer particles and then freeze-dried, at least a part of them is freeze-dried. Is integrated, and a step of re-crushing the integrated portion is required. In addition, when a polymer mass polymerized by a solution polymerization method is freeze-dried and then crushed to form a porous hydrophilic highly swellable hydrogel (particles), fine powder is generated from the polymer mass that has been made porous by freeze-drying. It occurs in large quantities, and the yield of the porous hydrophilic highly swellable hydrogel tends to decrease, and the particle size distribution of the porous hydrophilic highly swellable hydrogel tends to widen.

Further, the method described in Patent Document 2 may include a step of removing water contained in the particles of the absorbent polymer by freeze-drying, but since the absorbent polymer is general, the absorbent polymer that has absorbed water When freeze-drying the particles, at least a part of them is integrated, and a step of re-grinding the integrated portion may be required.
Therefore, it is an object of the present disclosure to provide a method for producing a plurality of freeze-dried water-absorbent polymer particles having excellent production efficiency.

The present disclosures are a method for producing a plurality of freeze-dried water-absorbent polymer particles, in which (i) a polymerization composition containing an ethylene-based unsaturated monomer, a radical polymerization initiator and water is used as a plurality of droplets. , (Ii) The polymerization composition in the plurality of droplets is polymerized in the hydrophobic solvent, and a plurality of water-absorbent polymer water-containing particles are formed from the plurality of droplets. Steps, (iii) forming a plurality of water-absorbent polymer particles to be freeze-dried from the plurality of water-absorbent polymer particles, and (iv) freeze-drying the plurality of water-absorbent polymer particles to be freeze-dried. , A production method comprising the steps of forming the plurality of freeze-dried water-absorbent polymer particles, characterized in that the plurality of water-absorbent polymer particles to be freeze-dried have a gel strength of 60 mN or more has been found.

The method for producing a plurality of freeze-dried water-absorbent polymer particles of the present disclosure is excellent in production efficiency.

FIG. 1 is a schematic diagram for explaining the polymerization apparatus 1. FIG. 2 shows the water-absorbent polymer particles No. It is a particle size distribution (number basis) of 1. FIG. 3 shows the powder No. It is a particle size distribution (number basis) of 2.

Specifically, the present disclosure relates to the following aspects.
[Aspect 1]
A method for producing a plurality of freeze-dried water-absorbent polymer particles.
(I) A step of releasing a polymerization composition containing an ethylene-based unsaturated monomer, a radical polymerization initiator and water as a plurality of droplets into a hydrophobic solvent.
(Ii) A step of polymerizing the polymerization composition in the plurality of droplets in the hydrophobic solvent to form a plurality of water-absorbent polymer water-containing particles from the plurality of droplets.
(Iii) A step of forming a plurality of water-absorbent polymer particles to be freeze-dried from the plurality of water-absorbent polymer water-containing particles, and (iv) the plurality of water-absorbent polymer particles to be freeze-dried are freeze-dried. Steps to form lyophilized water-absorbent polymer particles,
Including
The plurality of water-absorbent polymer particles to be freeze-dried have a gel strength of 60 mN or more.
The above-mentioned manufacturing method.

In the above production method, since the plurality of pre-lyophilized particles formed in step (iii) have a predetermined gel strength and are excellent in gel strength, the plurality of post-lyophilized particles formed in step (iv) are formed. , It is difficult to integrate due to gel blocking, etc., and the step of crushing the integrated freeze-dried particles becomes unnecessary. Therefore, the above manufacturing method is excellent in manufacturing efficiency.

[Aspect 2]
The production method according to aspect 1, wherein step (iv) further comprises a step of loosening the block of the lyophilized water-absorbent polymer particles into individual particles of the lyophilized water-absorbent polymer particles. ..

In the above manufacturing method, in step (iv), a step of loosening a plurality of blocks of lyophilized particles into individual particles thereof is included, so that the plurality of lyophilized particles are less likely to contain those blocks, and thus the blocks are less likely to be contained. In the above production method, a plurality of freeze-dried particles having a narrow particle size distribution can be produced.

[Aspect 3]
The production method according to aspect 1 or 2, wherein in step (iii), the plurality of water-absorbent polymer water-containing particles are made to absorb water to form the plurality of water-absorbent polymer particles to be freeze-dried.

In the above production method, in step (iii), a plurality of water-absorbent polymer water-containing particles are directly absorbed to form a plurality of pre-freeze-drying particles, so that a plurality of water-absorbent polymer particles are formed once, and then a plurality of water-absorbent polymer particles are formed. The production efficiency is excellent as compared with the case where the water-absorbent polymer particles absorb water.

[Aspect 4]
In step (iii), the plurality of water-absorbent polymer particles should be dried to form the plurality of water-absorbent polymer particles, and then the plurality of water-absorbent polymer particles should be allowed to absorb water and the plurality of water-absorbent polymer particles should be freeze-dried. The production method according to aspect 1 or 2, wherein the water-absorbent polymer particles are formed.

In the above production method, since a plurality of water-absorbent polymer particles are formed in step (iii) and then a plurality of pre-lyophilized particles are formed, it becomes easy to control the water absorption of the plurality of pre-lyophilized particles. As a result, it becomes easy to control the characteristics of the plurality of lyophilized particles, for example, the porosity.

[Aspect 5]
The production method according to any one of aspects 1 to 4, wherein the plurality of water-absorbent polymer particles to be freeze-dried have a water content ratio (mass ratio) of 10 to 100 times.

In the above production method, since the water absorption of the plurality of pre-freeze-dried particles is adjusted within a predetermined range in step (iii), the post-lyophilized particles formed in step (iv) are excellent in their performance.

[Aspect 6]
The item according to any one of aspects 1 to 5, wherein the plurality of water-absorbent polymer water-containing particles have a coefficient of variation of a particle size of 0.01 to 0.20 when dried to a water content of 1% by mass or less. Manufacturing method.

In the above production method, since the plurality of water-absorbent polymer water-containing particles have a coefficient of variation of a predetermined particle size when dried to a predetermined water content, the plurality of water-absorbent polymer water-containing particles have a coefficient of variation of the same particle size. And the pre-freeze-drying particles in step (iv) are likely to have a similar coefficient of variation in particle size. As a result, in the step (iv), the number of contacts between the pre-lyophilized particles is reduced, and it becomes difficult for the plurality of post-lyophilized particles to aggregate, integrate, and the like.
Further, in the above production method, since the plurality of water-absorbent polymer water-containing particles have a coefficient of variation of a predetermined particle size when dried to a predetermined water content, the water absorption time in step (iii) and the water absorption time in step (iv) The freeze-drying time in the above can be shortened.

[Aspect 7]
The item according to any one of aspects 1 to 6, wherein the plurality of water-absorbent polymer water-containing particles have an average sphericity of 0.99 to 0.99 when dried to a water content of 1% by mass or less. Production method.

In the above production method, since the plurality of water-absorbent polymer particles have a predetermined average sphericity when dried to a predetermined water content, the plurality of water-absorbent polymer water-containing particles have the same average sphericity. It is easy, and the pre-freeze-dried particles in step (iv) are likely to have a similar coefficient of variation in particle size. As a result, in the step (iv), the number of contacts between the pre-lyophilized particles is reduced, and it becomes difficult for the plurality of post-lyophilized particles to aggregate, integrate, and the like.
Further, in the above production method, since the plurality of water-absorbent polymer water-containing particles have a predetermined average sphericity when dried to a predetermined water content, the water absorption time in step (iii) and the water absorption time in step (iv) The freeze-drying time can be shortened.

[Aspect 8]
The production method according to any one of aspects 1 to 7, wherein the plurality of water-absorbent polymer water-containing particles have a gel strength of 60 to 200 mN at a water content ratio of 1000 times.

In the above production method, since the plurality of water-absorbent polymer water-containing particles have a predetermined gel strength, the water-absorbent polymer water-containing particles are formed when the plurality of water-absorbent polymer water-containing particles formed in step (ii) collide with each other. It becomes difficult for phenomena such as the generation of fine particle fragments and the cracking of water-absorbent polymer water-containing particles. As a result, in the step (iv), the particles after freeze-drying are less likely to aggregate, integrate, etc. due to the fine particle pieces.
Further, since the water-absorbent polymer water-containing particles have the above-mentioned gel strength, when the particles before freeze-drying collide with each other in the step (iii) and the step (iv), the particles before freeze-drying become fine particle fragments. , And cracks in the water-absorbent polymer water-containing particles are less likely to occur. Further, in the step (iv), the pre-lyophilized particles are less likely to gel-block, and the plurality of post-lyophilized particles are less likely to aggregate, integrate, and the like.

[Aspect 9]
In step (i), the plurality of droplets are discharged into the hydrophobic solvent so that the plurality of droplets have a coefficient of variation of a droplet diameter of 0.01 to 0.20, according to aspects 1 to 8. The manufacturing method according to any one of the items.

In the above manufacturing method, in step (i), since the plurality of droplets have a coefficient of variation of a predetermined droplet diameter, the plurality of water-absorbent polymer particles produced have a coefficient of variation of a predetermined particle size. As a result, the above-mentioned manufacturing method tends to have the effect of the seventh aspect.

[Aspect 10]
The radical polymerization initiator is a photoradical polymerization initiator, and in the step of forming the plurality of water-absorbent polymer water-containing particles, the plurality of droplets are irradiated with light to carry out the above polymerization in the plurality of droplets. The production method according to any one of aspects 1 to 9, wherein the composition is photopolymerized.
In the above production method, since the radical polymerization initiator is a photoradical polymerization initiator, the plurality of water-absorbent polymer water-containing particles produced are likely to have the above-mentioned characteristics.

[Aspect 11]
The production method according to any one of aspects 1 to 10, wherein the temperature of the hydrophobic solvent is in the range of more than 0 ° C. and 30 ° C. or lower in the step of forming the water-absorbent polymer water-containing particles.
In the above production method, when the temperature of the hydrophobic solvent is within a predetermined range, the plurality of produced water-absorbent polymer water-containing particles tend to have the above-mentioned characteristics. In addition, the plurality of water-absorbent polymer water-absorbing particles are less likely to contain air bubbles.

[Aspect 12]
The production method according to any one of aspects 1 to 11, wherein the hydrophobic solvent is selected from the group consisting of silicone oil, oil and hydrocarbon, and any combination thereof.
In the above-mentioned production method, the plurality of produced water-absorbent polymer particles tend to have the above-mentioned characteristics.

[Definition]
・ [Water-absorbent polymer water-containing particles]
As used herein, the term "water-absorbent polymer hydrous particles" means particles formed in step (ii) of the production method of the present disclosure.
・ [Water-absorbent polymer particles]
In the present specification, the "water-absorbent polymer particles" mean particles from which the above-mentioned "water-absorbent polymer water-containing particles" have been dried to remove water.

-[Water-absorbent polymer particles to be freeze-dried]
As used herein, the "water-absorbent polymer particles to be lyophilized" are the particles formed in step (iii). The water-absorbent polymer particles conceptually mean water-absorbent polymer particles containing water, and are synonymous with "water-absorbent polymer water-containing particles" in that they contain water.
The water-absorbent polymer particles to be freeze-dried may be formed by allowing the water-absorbent polymer water-containing particles to absorb water, or may be formed by drying the water-absorbent polymer water-containing particles and then causing them to absorb water.
In the present specification, "water-absorbent polymer particles to be freeze-dried" may be referred to as "pre-freeze-dried particles".

-[Freeze-dried water-absorbent polymer particles]
As used herein, "lyophilized water-absorbent polymer particles" are particles formed in step (iv).
In the present specification, "lyophilized water-absorbent polymer particles" may be referred to as "lyophilized particles".

・ [Freeze-drying] and [Drying]
As used herein, "lyophilization" means drying performed by sublimating water. In the above freeze-drying, the frozen object is generally dried under reduced pressure while being frozen.
Further, in the present specification, when simply referred to as "drying", drying means drying performed without sublimating water. The drying can usually be carried out at 0 ° C. or higher, for example, room temperature, preferably at a high temperature, for example, 40 to 200 ° C. The drying can be carried out under 1 atm or reduced pressure.

-"Moisture content" for the polymerization composition
In the present specification, the "moisture content ratio" with respect to the polymerization composition is the mass of water in the polymerization composition, which is the total mass of components other than water contained in the polymerization composition (for example, ethylene-based unsaturated monomer and radical polymerization initiator). Calculated by dividing by. The water content ratio is a value for estimating how much water the water-absorbent polymer water-containing particles formed from the polymerization composition contain with respect to the water-absorbent polymer particles which are solid contents.

-"Water content ratio" for water-absorbent polymer water-containing particles
As used herein, the "moisture content ratio" of the water-absorbent polymer water-containing particles means the ratio (mass ratio) of water contained in the water-absorbent polymer water-containing particles to the polymer body of the water-absorbent polymer water-containing particles. Similarly, the "moisture content ratio" of the water-absorbent polymer particles to be freeze-dried is the ratio (mass ratio) of the water contained in the water-absorbent polymer particles to be freeze-dried to the polymer body of the water-absorbent polymer particles to be freeze-dried. Means. The polymer body is synonymous with the dried particles in the measurement of "moisture content" described later.

·"Moisture percentage"
In the present specification, the "moisture content" of particles (water-absorbent polymer water-containing particles, water-absorbent polymer particles to be freeze-dried, and freeze-dried water-absorbent polymer particles) is the water-absorbent polymer particles with respect to the total mass of the particles. It means the percentage (mass ratio) of water contained.

Moisture content is measured as follows.
The water-absorbent polymer particles having a mass of m ₁ (g) were dried in an oven at 110 ° C. for 10 hours to form particles after drying, and the mass of the particles after drying: m ₂ (g) was measured. Formula:
Moisture content (mass%) = (m _{1 −} m ₂ ) / m ₁
Calculated by
In the present specification, water-absorbent polymer particles or water-absorbent polymer particles to be freeze-dried, which are dried to a moisture content of 1% by mass or less, may be referred to as "water-absorbent polymer particles".

The method for producing a plurality of lyophilized water-absorbent polymer particles of the present disclosure (hereinafter, may be simply referred to as "the production method of the present disclosure") includes the following steps.
(I) A step of discharging a polymerization composition containing an ethylene-based unsaturated monomer, a radical polymerization initiator and water as a plurality of droplets into a hydrophobic solvent (hereinafter, may be referred to as a "droplet discharge step").
(Ii) A step of polymerizing the polymerization composition in the plurality of droplets in the hydrophobic solvent to form a plurality of water-absorbent polymer water-containing particles from the plurality of droplets (hereinafter, “water-absorbent polymer water-containing particles”). Sometimes referred to as "particle formation step")
(Iii) A step of forming a plurality of water-absorbent polymer particles to be freeze-dried from the plurality of water-absorbent polymer water-containing particles (hereinafter, may be referred to as “pre-freeze-drying particle formation step”).
(Iv) A step of freeze-drying a plurality of the water-absorbent polymer particles to be freeze-dried to form a plurality of freeze-dried water-absorbent polymer particles (hereinafter, may be referred to as a “particle formation step after freeze-drying”).

[Drop emission step (step (i))]
The polymerization composition in the droplet emission step contains an ethylene-based unsaturated monomer, a radical polymerization initiator, water and the like.
The ethylene-based unsaturated monomer is not particularly limited as long as it is used in the art, and examples thereof include (meth) acrylic acid-based monomers. In addition, (meth) acrylic acid means acrylic acid and / or methacrylic acid.
The polymerization composition contains the ethylene-based unsaturated monomer in a ratio of preferably 30 to 80% by mass, more preferably 35 to 70% by mass, and even more preferably 40 to 70% by mass.

Examples of the (meth) acrylic acid-based monomer include (meth) acrylic acid (that is, a non-neutralized product of (meth) acrylic acid), a neutralized product of (meth) acrylic acid, and a derivative of (meth) acrylic acid. For example, esterified products and the like can be mentioned.
Examples of the neutralized product of (meth) acrylic acid include alkali metal salts of (meth) acrylic acid. Examples of the alkali metal salt include lithium, sodium, potassium salt and the like, and sodium and potassium salts are preferable.

When the post-freeze-dried particles formed from the above polymerization composition have a monomer skeleton derived from a (meth) acrylic acid-based monomer, the post-freeze-dried particles are derived from a derivative of (meth) acrylic acid. Except for the monomer skeleton, it is preferable that 85 to 100 mol% of the acid group (carboxyl group) is neutralized.

Therefore, when 85 to 100 mol% of the acid groups (carboxyl groups) of the skeleton derived from the neutralized product of (meth) acrylic acid and (meth) acrylic acid are neutralized in the particles after freeze-drying, The polymerization composition may contain a (meth) acrylic acid-based monomer in which 85 to 100 mol% of the acid groups have been neutralized, or, for example, the acid groups contained in the water-absorbent polymer water-containing particles have been neutralized. Finally, the freeze-dried particles may contain a skeleton derived from a (meth) acrylic acid-based monomer in which 85-100 mol% of acid groups have been neutralized.

The polymerization composition preferably contains a cross-linking agent. Examples of the cross-linking agent include (i) a polymerizable cross-linking agent having at least two polymerizable unsaturated groups, (ii) a reactive cross-linking agent having at least two reactive groups capable of reacting with a carboxyl group, and (iii). ) A polymerizable reactive cross-linking agent having a polymerizable unsaturated group and a reactive group capable of reacting with a carboxyl group can be mentioned. From the viewpoint of the water absorption characteristics of the formed freeze-dried particles, the cross-linking agent is preferably an acrylate-based, allyl-based, or acrylamide-based polymerizable cross-linking agent. When the polymerization composition contains a cross-linking agent, the polymerization composition contains the cross-linking agent in a ratio of 0.02 to 0.15 mol% with respect to the ethylene-based unsaturated monomer.

Examples of the cross-linking agent include ethylene glycol di (meth) acrylate, diethylene glycol di (meth) acrylate, triethylene glycol di (meth) acrylate, polyethylene glycol di (meth) acrylate, trimethyl propantri (meth) acrylate, and penta. Elyslitol di (meth) acrylate, pentaerythritol tri (meth) acrylate, N, N'-methylenebis (meth) acrylamide, triallyl isocyanurate, (poly) ethylene glycol diglycidyl ether, glycerin polyglycidyl ether, sorbitol polyglycidyl ether, Examples thereof include pentaerythritol polyglycidyl ether and the like.

Examples of the radical polymerization initiator include a photoradical polymerization initiator and a thermal radical polymerization initiator.
From the viewpoint of the characteristics of the particles formed after freeze-drying, the radical polymerization initiator is preferably a photoradical polymerization initiator, and when the radical polymerization initiator contains a thermal radical polymerization initiator, light It is preferably used in combination with a radical polymerization initiator.
The polymerization composition contains the radical polymerization initiator in a proportion of preferably 0.001 to 20% by mass, more preferably 0.1 to 10% by mass, based on the ethylene-based unsaturated monomer.

As the photoradical polymerization initiator, those capable of generating radicals with light having a wavelength of about 200 nm to about 600 nm are preferable, and for example, benzoin, acetoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzophenone, benzyl, etc. Michler's ketone, xantone, chlorothioxanthone, isopropylthioxanthone, benzyldimethylketal, naphthol, anthraquinone, hydroxyanthracene, acetophenone diethyl ketal, α-hydroxycyclohexylphenylketone, 2-hydroxy-2-methylphenylpropane, any combination thereof, etc. Be done.

Examples of the thermal radical polymerization initiator include azobisisobutyronitrile, 4,4'-azobis (4-cyanovaleric acid), 1,1'-azobis (cyclohexanecarbonitrile), and 2,2'-azobis. (2-Methylpropionitrile), 2,2'-azobis (2-methylpropionamidine) dihydrochloride, t-butyl peroxide, cumene hydroperoxide, di-t-butyl peroxide, acetyl peroxide, lauroyl Peroxide, stearoyl peroxide, benzoyl peroxide, t-butylperoxyacetate, t-butylperoxyisobutyrate, t-butylperoxypivalate, methylethylketone peroxide, cyclohexanone peroxide, hydrogen peroxide, ammonium persulfate, Examples include potassium persulfate.

The polymerization composition preferably has a water content ratio (mass ratio) of 0.1 to 9 times, more preferably 0.2 to 6 times, and even more preferably 0.5 to 4 times. When the polymerization composition contains water so as to satisfy the water content ratio, the temperature of the droplets of the polymerization composition is less likely to rise due to the heat generated by the polymerization of the ethylene-based unsaturated monomer. The water contained in the water-absorbent polymer water-containing particles can be used in the next post-freeze-drying particle formation step while maintaining the handleability of the water-absorbent polymer water-containing particles.

The hydrophobic solvent is not particularly limited as long as it is immiscible with the polymerization composition and does not react with the components in the polymerization composition. Further, the hydrophobic solvent preferably has a specific gravity close to that of the polymerization composition.
Examples of the hydrophobic solvent include silicone oil (for example, dimethylpolysiloxane), oil (for example, triglyceride, for example, vegetable oil, for example, medium-chain fatty acid triglyceride), hydrocarbon (for example, liquid paraffin), and the like. Any combination and the like can be mentioned.

The temperature of the hydrophobic solvent is preferably in the range of more than 0 ° C. and 30 ° C. or lower. By doing so, the plurality of water-absorbent polymer water-containing particles produced are likely to have the above-mentioned properties. In addition, the plurality of water-absorbent polymer water-absorbing particles are less likely to contain air bubbles.

As long as the polymerization composition is released as droplets in a hydrophobic solvent, the release method is not particularly limited. For example, the polymerization composition can be used as droplets by using a polymerization apparatus provided with an injection nozzle. Released into a hydrophobic solvent. Specifically, with the tip of the injection nozzle immersed in a hydrophobic solvent, the polymerization composition is directly and quantitatively injected from the injection nozzle into a hydrophobic solvent to obtain a hydrophobic solvent. The above-mentioned droplets are formed within. The amount of the polymerization composition injected from the injection nozzle is, for example, 30 to 60 mL / min. As the injection nozzle, those described in JP-A-2008-11765 and the like can be used, and will be described later in relation to the polymerization apparatus 1 shown in FIG. The polymerization composition is preferably released as droplets inside the hydrophobic solvent. This is to maintain the shape of the droplet.

As a general rule, each of the plurality of droplets forms each of the plurality of water-absorbent polymer water-containing particles (plurality of water-absorbent polymer particles) by polymerization. Therefore, from the viewpoint of producing a plurality of water-absorbent polymer particles having a coefficient of variation of a predetermined particle size, the plurality of droplets preferably have a coefficient of variation of a predetermined droplet diameter. Specifically, the plurality of droplets preferably have a coefficient of variation of the particle size similar to the coefficient of variation of the particle size of the water-absorbent polymer particles, preferably 0.01 to 0.20, and more preferably 0.01 to 0.20. It has a coefficient of variation of particle size of 0.03 to 0.15, and more preferably 0.05 to 0.10.

Further, since the plurality of droplets have the above-mentioned fluctuation coefficient of the droplet diameter, each of the plurality of water-absorbent polymer water-containing particles (plurality of water-absorbent polymer particles) tends to have uniform characteristics. As a result, the plurality of water-absorbent polymer water-containing particles (plurality of water-absorbent polymer particles) are likely to have a small particle size variation coefficient, a high average sphericity, a high gel strength when water is absorbed, and the like.
The coefficient of variation of the droplet diameter can be measured in the same manner as the method for measuring the coefficient of variation of water-absorbent polymer particles, which will be described later.

[Step of forming water-absorbent polymer water-containing particles (step (ii))]
In the water-absorbent polymer water-containing particle forming step, the polymerization composition in the plurality of droplets is polymerized in the hydrophobic solvent to form a plurality of water-absorbent polymer water-containing particles.
When the radical polymerization initiator is a photoradical polymerization initiator, light from a light source such as a mercury lamp, a fluorescent lamp, a xenon lamp, a carbon arc lamp, a metal halide lamp, a UV-LED lamp, or the like, for example, from about 200 nm to Light having a wavelength of about 600 nm is applied to, for example, a hydrophobic solvent and a forming tube containing a plurality of droplets, and the polymerization composition is photopolymerized in each of the plurality of droplets, and the polymerization composition is photopolymerized from each of the plurality of droplets. Water-absorbent polymer Water-containing particles can be formed. From this point of view, the hydrophobic solvent preferably has a high transmittance with respect to the wavelength of the light emitted from the light source. Further, the forming tube preferably has a high transmittance with respect to the wavelength of the light emitted from the light source, and is formed of, for example, quartz glass.

When the radical polymerization initiator contains only a photoradical polymerization initiator, it is preferable that the polymerization composition and the hydrophobic solvent are not heated. This is because the plurality of water-absorbent polymer water-containing particles (plurality of water-absorbent polymer particles) formed are less likely to contain air bubbles.

When the radical polymerization initiator contains only the thermal radical polymerization initiator, the temperature of the hydrophobic solvent is maintained at a temperature at which the thermal radical polymerization initiator can function, for example, a temperature having a half-life temperature of 10 hours or more. Is preferable. When the radical polymerization initiator contains only the thermal radical polymerization initiator, for example, the temperature of the polymerization tank containing the hydrophobic solvent and a plurality of droplets is equal to or higher than the 10-hour half-life temperature of the thermal radical polymerization initiator. By maintaining the temperature at, the polymerization composition can be thermally polymerized in each of the plurality of droplets, and water-absorbent polymer water-containing particles can be formed from each of the plurality of droplets.

In this case, it is preferable not to heat the polymerization composition before forming the droplets. This is because the polymerization may start before the polymerization composition is released as droplets into the hydrophobic solvent, making it difficult for the polymerizable composition to form droplets.

When the radical polymerization initiator contains both a photoradical polymerization initiator and a thermal radical polymerization initiator, it is preferable not to heat the polymerization composition before forming droplets. This is because the polymerization may start before the polymerization composition is released as droplets into the hydrophobic solvent, and the polymerizable composition may be difficult to form into droplets. Also, the hydrophobic solvent can be maintained at a temperature at which the thermal radical polymerization initiator can function. Further, for example, a polymerization apparatus provided with a forming tube and a polymerization tank is prepared, the hydrophobic solvent is maintained at room temperature in the forming tube and light is irradiated from a light source to photopolymerize the polymerization composition, and then the polymerization is carried out. In the tank, the hydrophobic solvent may be maintained at a temperature at which the thermal radical polymerization initiator can function, and the polymerization composition may be thermally polymerized.

The water-absorbent polymer water-containing particles dried to a moisture content of 1% by mass or less, that is, the water-absorbent polymer particles are preferably 0.01 to 0.20, more preferably 0.03 to 0.15, and further. It preferably has a variation coefficient of particle size of 0.05 to 0.10. By doing so, the plurality of water-absorbent polymer water-containing particles are likely to have the same coefficient of variation in particle size, and the pre-lyophilized particles in step (iv) are likely to have the same coefficient of variation in particle size. .. As a result, in the step (iv), the number of contacts between the pre-lyophilized particles is reduced, and it becomes difficult for the plurality of post-lyophilized particles to aggregate, integrate, and the like.

Further, by doing so, the water absorption time in the step (iii), that is, the time for allowing the plurality of water-absorbent polymer water-containing particles to absorb water and forming the plurality of water-absorbent polymer particles to be freeze-dried can be shortened. This is because the coefficient of variation of the particle size of the plurality of water-absorbent polymer water-containing particles is reduced, so that the water absorption rates of the individual particles of the plurality of water-absorbent polymer water-containing particles are less likely to vary.

Further, by doing so, the lyophilization time in step (iv), that is, the time for lyophilizing the plurality of water-absorbent polymer particles to be lyophilized and forming the plurality of lyophilized water-absorbent polymer particles is shortened. can do. This is because the coefficient of variation of the particle size of the water-absorbent polymer particles to be freeze-dried becomes small, so that the freeze-drying rate of the individual particles of the plurality of water-absorbent polymer particles to be freeze-dried is less likely to vary.

The water-absorbent polymer water-containing particles dried to a moisture content of 1% by mass or less, that is, the water-absorbent polymer particles are preferably 0.99 to 0.99, more preferably 0.92 to 0.98, and further. It preferably has an average sphericity of 0.93 to 0.97. By doing so, the plurality of water-absorbent polymer water-containing particles are likely to have the same average sphericity, and the pre-lyophilized particles in step (iv) are likely to have the same coefficient of variation in particle size. As a result, in the step (iv), the number of contacts between the pre-lyophilized particles is reduced, and it becomes difficult for the plurality of post-lyophilized particles to aggregate, integrate, and the like.

Further, by doing so, the water absorption time in the step (iii), that is, the time for allowing the plurality of water-absorbent polymer water-containing particles to absorb water and forming the plurality of water-absorbent polymer particles to be freeze-dried can be shortened. This is because the higher average sphericity of the plurality of water-absorbent polymer water-containing particles makes it difficult for the water absorption rates of the individual particles of the plurality of water-absorbent polymer water-containing particles to vary.

Further, by doing so, the lyophilization time in step (iv), that is, the time for lyophilizing the plurality of lyophilized water-absorbent polymer particles to be lyophilized and forming the plurality of lyophilized water-absorbent polymer particles is shortened. can do. This is because the higher average sphericity of the water-absorbent polymer particles to be freeze-dried makes it difficult for the freeze-drying rate of the individual particles of the plurality of water-absorbent polymer particles to be freeze-dried to vary.

In this specification, the particle size, the average particle size, the coefficient of variation of the particle size, and the average sphericity are measured as follows.
(1) A digital microscope (trade name: VHX-1000, zoom lens: × 20 to × 200) manufactured by KEYENCE Co., Ltd. and attached to a constant temperature and humidity chamber with a temperature of 20 ± 5 ° C and a humidity of 65 ± 5% RH. Prepare the software.
(2) The particle sample is allowed to stand in the constant temperature and humidity chamber for 24 hours.
(3) Take a sample with the above digital microscope.

から算出する。
（５）平均粒径は、粒度分布（個数基準）のメジアン径（⁵⁰Ｄ_p）を意味する。具体的には、付属のソフトウェアにて、粒子１，０００個の円相当径ＣＤの粒度分布（個数基準）を作成し、当該粒度分布（個数基準）からメジアン径（⁵⁰Ｄ_p）を決定する。 (4) The particle size of each particle means the equivalent circle diameter calculated from the projected area of the projected image of each particle. Specifically, the projected area of each particle: A is measured with the attached software, and the equivalent circle diameter of each particle: CD is calculated by the following formula:

Calculate from.
(5) The average particle size means the median diameter ( ⁵⁰ D _p ) of the particle size distribution (number basis). Specifically, at the supplied software to create a particle size distribution of the particles 1,000 equivalent circle diameter CD (the number basis), to determine a median diameter ⁽⁵⁰ D _p) from the particle size distribution (number basis) ..

(6) The coefficient of variation of the particle size is calculated from the above particle size distribution (number basis) by the attached software.
(7) Average sphericity: S _p is the arithmetic mean of the particle 1,000 sphericity s _p. Specifically, the sphericity of each particle: s _p is from the outer edge of each particle, the major axis is the longest diameter and k _1, minor is the shortest diameter were measured and k _2, the following formula:
s _p = k ₂ / k ₁
Calculated by The major axis: k ₁ and minor _{axis: k 2} of each particle are automatically determined by the attached software.

The water-absorbent polymer water-containing particles have a gel strength of preferably 60 to 200 mN, more preferably 80 to 160 mN, and even more preferably 100 to 140 mN at a water content of 1000 times. By doing so, when a plurality of water-absorbent polymer water-containing particles formed in step (ii) collide with each other, fine particle pieces are generated from the water-absorbent polymer water-containing particles, and the water-absorbent polymer water-containing particles are cracked. Phenomena such as entering are less likely to occur. As a result, in the step (iv), the particles after freeze-drying are less likely to aggregate, integrate, etc. due to the fine particle pieces.

Further, since the water-absorbent polymer water-containing particles have the above-mentioned gel strength, when the particles before freeze-drying collide with each other in the step (iii) and the step (iv), the particles before freeze-drying become fine particle fragments. , And cracks in the water-absorbent polymer water-containing particles are less likely to occur. Further, in the step (iv), the pre-lyophilized particles are less likely to gel-block, and the plurality of post-lyophilized particles are less likely to aggregate, integrate, and the like.

In the present specification, the above-mentioned gel strength is measured as follows.
(1) Prepare Minebea Technograph TG-500N in a constant temperature and humidity chamber with a temperature of 20 ± 5 ° C. and a humidity of 65 ± 5% RH.
(2) The water-absorbent polymer water-containing particles are dried at 80 ° C. for 6 hours and then allowed to stand in the constant temperature and humidity chamber for 24 hours to prepare a sample.
(3) The mass of a sample of about 10 mg is accurately weighed, 10 g of ion-exchanged water is added per 10 mg of the sample, and the mixture is allowed to stand for 3 hours to swell the sample.

(4) From the swollen sample, one swollen particle is placed on the sample stage of the technograph TG-500N, and the swollen particle is compressed at a rate of 2 mm / min.
(5) Measure the load when the swollen particles break.
(6) A total of 50 measurements are performed with different swollen particles, and the arithmetic mean of the above loads for 50 times is adopted as the gel strength.

The water-absorbent polymer water-containing particles dried to a moisture content of 1% by mass or less, that is, the water-absorbent polymer particles can have an arbitrary average particle size, but are generally 100 to 1,000 μm, preferably 100 to 1,000 μm. Has an average particle size of 300-1,000 μm, and more preferably 300-800 μm.
The method for measuring the average particle size is as described above.

In the water-absorbent polymer water-containing particle forming step, it is preferable to take out the plurality of water-absorbent polymer water-containing particles from the hydrophobic solvent after forming the plurality of water-absorbent polymer water-containing particles.

[Parts of pre-lyophilization particle formation (step (iii))]
In the pre-lyophilization particle formation step, a plurality of water-absorbent polymer particles to be freeze-dried are formed from the plurality of water-absorbent polymer water-containing particles.
The pre-lyophilization particle forming step can include the following steps:
(Iii-1) A plurality of water-absorbent polymer water-containing particles are allowed to absorb water to form a plurality of water-absorbent polymer particles to be freeze-dried (hereinafter, may be referred to as “step (iii-1)”).
(Iii-2) A plurality of water-absorbent polymer particles are dried to form a plurality of water-absorbent polymer particles, and then the plurality of water-absorbent polymer particles are allowed to absorb water, and a plurality of water-absorbent polymer particles to be freeze-dried. (Hereinafter, it may be referred to as "step (iii-2)").

In step (iii-1), a plurality of water-absorbent polymer water-containing particles are made to absorb water, specifically, the plurality of water-absorbent polymer water-containing particles are swollen with water to form a plurality of water-absorbent polymer particles to be freeze-dried. .. Prior to water absorption, it is preferable to remove the hydrophobic solvent adhering to the surface of the plurality of water-absorbent polymer water-containing particles. This is because each of the plurality of pre-lyophilized particles uniformly contains water, and thus the plurality of post-lyophilized particles uniformly provide pores after lyophilization. Step (iii-1) is preferable from the viewpoint of manufacturing efficiency.

In step (iii-2), a plurality of water-absorbent polymer water-containing particles taken out from the hydrophobic solvent are dried to form a plurality of water-absorbent polymer particles, and then the water-absorbent polymer particles are allowed to absorb water, specifically. Swells a plurality of water-absorbent polymer particles with water to form a plurality of water-absorbent polymer particles to be freeze-dried. When the step (iii-2) is selected, the hydrophobic solvent adhering to the surface of the plurality of water-absorbent polymer water-containing particles is not removed before drying, and the hydrophobic solvent is volatilized during drying. You may.
The preferred conditions for drying are described in the definition section.
Step (iii-2) is preferable from the viewpoint of controlling the characteristics of the particles after lyophilization.

In step (iii-2), it is preferable that the plurality of water-absorbent polymer water-containing particles have a water content of 1% by mass or less, that is, they are dried so as to form water-absorbent polymer particles. This is to facilitate the adjustment of the water content ratio of the water-absorbent polymer particles to be freeze-dried.

In the dust generation test, the water-absorbent polymer particles are preferably 0.00 to 0.20% by mass, more preferably 0.00 to 0.15% by mass, still more preferably 0.00 to 0.10% by mass. And even more preferably, it has a dust generation rate of 0.00 to 0.05% by mass. By doing so, even when the water-absorbent polymer particles collide with each other in the step (iii), fine particle fragments are less likely to be generated from the water-absorbent polymer particles, and in the step (iv), the particles after freeze-drying are generated. , Fine particle pieces make it difficult to aggregate and integrate.

In the present specification, the above-mentioned dust generation test is carried out as follows.
(1) An Excel autohomogenizer manufactured by NISSEI is prepared in a constant temperature and humidity chamber having a temperature of 20 ± 5 ° C. and a humidity of 65 ± 5% RH.
(2) A sample of the water-absorbent polymer particles is sieved with an opening of 150 μm and allowed to stand in the constant temperature and humidity chamber for 24 hours.
(3) Approximately 10 g of sample mass: m ₁ (g) is accurately weighed, the sample is placed in a homogenizer, and the sample is stirred at a rotation speed of 1,000 rpm for 1 minute.

(4) The sample after stirring is sieved with a mesh opening of 150 μm, and the mass of fine powder passing through the sieve: m ₂ (g) is measured.
(5) Dust generation rate: D (mass%), the following formula:
D (mass%) = 100 x m ₂ / m ₁
Calculated by
(6) The test is carried out three times in total with different samples, and the arithmetic mean of the dust generation rate is adopted three times.

The water-absorbent polymer water-containing particles dried to a moisture content of 1% by mass or less, that is, the water-absorbent polymer particles are preferably 20 N or more, more preferably 30 N or more, still more preferably 40 N or more, and even more preferably 50 N. It has the above breaking strength. By doing so, even when the water-absorbent polymer particles collide with each other in the step (iii), fine particle fragments are less likely to be generated from the water-absorbent polymer particles, and in the step (iv), the particles after freeze-drying are generated. , Fine particle pieces make it difficult to aggregate and integrate.

The breaking strength was measured in the same manner as the gel strength of the water-absorbent polymer water-containing particles except that the step (3) was omitted, and the “swelled sample” and the “swelled” in the steps (4) and (5). "Particle" is read as "sample" and "particle", respectively. The upper limit of the load is 50N.

In step (iii), a plurality of pre-lyophilized particles are selected from the plurality of water-absorbent polymer hydrous particles, preferably 10 to 200 times, more preferably 20 to 170 times, and further preferably 30 to 30 to the pre-lyophilized particles. It is formed so as to have a water content ratio (mass ratio) of 150 times. By doing so, the freeze-dried particles formed in the step (iv) tend to be excellent in water absorption ratio, gel strength at the time of water absorption, and absorption rate.

The pre-lyophilized particles have a gel strength of 60 mN or more, more preferably 80 mN or more, and more preferably 100 mN or more. By doing so, in step (iii) and step (iv), when the particles before freeze-drying collide with each other, fine particle pieces are generated from the particles before freeze-drying, and the water-absorbent polymer water-containing particles are cracked. Etc. are less likely to occur. Further, in the next step (iv), it becomes difficult for the pre-lyophilized particles to gel-block each other, and it becomes difficult for the plurality of post-lyophilized particles to aggregate, integrate, and the like.

The gel strength of the particles before freeze-drying is measured as follows.
(1) Prepare Minebea Technograph TG-500N in a constant temperature and humidity chamber with a temperature of 20 ± 5 ° C. and a humidity of 65 ± 5% RH.
(2) Pre-freeze-drying The particles are stored in a closed container so that the water does not evaporate, and allowed to stand in the constant temperature and humidity chamber for 3 hours to prepare a sample.
(3) One particle from the sample is placed on the sample stage of the technograph TG-500N, and the particles are compressed at a rate of 2 mm / min.
(5) Measure the load when the particles break.
(6) A total of 50 measurements are performed with different particles, and the arithmetic mean of the above loads for 50 times is adopted as the gel strength.

[Particle formation step after freeze-drying (step (iv))]
In the post-lyophilization particle formation step, a plurality of lyophilized water-absorbent polymer particles are lyophilized to form a plurality of lyophilized water-absorbent polymer particles.
The freeze-drying can be carried out by a method known in the art.
Preferred conditions for lyophilization are described in the definition section.
In the particle formation step after freeze-drying, drying may be carried out before and after freeze-drying.

In the production method of the present disclosure, in the post-lyophilization particle formation step, a block composed of a plurality of post-lyophilized particles may be formed by agglutination of a part of the plurality of post-lyophilized particles. Therefore, the production method of the present disclosure may further include in the post-lyophilization particle forming step a step of loosening a plurality of blocks of post-lyophilized particles into their individual particles. By doing so, the particles after lyophilization are less likely to contain their agglomerates. As described above, the production method of the present disclosure is excellent in productivity because it does not require a pulverization step of pulverizing the particles after freeze-drying. When the pulverization step is added, fine powder is generally generated, and the productivity is generally inferior.

An example of a polymerization apparatus for carrying out the production method of the present disclosure is shown in FIG. In FIG. 1, the polymerization apparatus 1 includes a polymerization composition tank 3 holding the polymerization composition 103, a metering pump 5 for adjusting the transport speed of the polymerization composition 103, and a liquid feeding tube 7 for transporting the polymerization composition 103. A light irradiation chamber 9 provided with a light source 11, an injection nozzle 13 for injecting a polymerization composition 103, a rectifying perforated plate 15, a forming tube 17 for holding a hydrophobic solvent 105 and photopolymerizing droplets, and a water-absorbent polymer water-containing particle 101. A separation screen 19 that separates from the hydrophobic solvent 105, a hydrophobic solvent tank 21 that holds the hydrophobic solvent 105, a metering pump 23 that adjusts the transport speed of the hydrophobic solvent 105, and a temperature controller that adjusts the temperature of the hydrophobic solvent 105. 25, and a liquid feed tube 27 for transporting the hydrophobic solvent 105 are provided.

The hydrophobic solvent 105 circulates between the forming tube 17, the hydrophobic solvent tank 21, the metering pump 23, the temperature controller 25, the liquid feeding tube 27, and the rectifying perforated plate 15, and the circulation speed is determined by the metering pump 23. Controlled, the temperature of the hydrophobic solvent 105 is controlled by the temperature controller 25. Most of the forming tube 17 is arranged inside the light irradiation chamber 9.

The polymerization composition 103 is supplied from the polymerization composition tank 3 to the injection nozzle 13 via the metering pump 5 and the liquid feed pipe 7. The polymerization composition 103 is then ejected from the injection nozzle 13 to form a droplet 107 in the forming tube 17. Next, the droplet 107 is exposed to the light emitted from the light source 11 in the forming tube 17, and the polymerization composition 103 in the droplet 107 is photopolymerized to form the water-absorbent polymer water-containing particles 101. The water-absorbent polymer water-containing particles 101 are separated from the hydrophobic solvent 105 by the separation screen 19.

Although not shown in FIG. 1, the water-absorbent polymer water-containing particles 101 are then freeze-dried by absorbing a specified amount of water to form the water-absorbent polymer water-containing particles and then freeze-drying the water-absorbent polymer water-containing particles. Produce the water-absorbent polymer particles.

The freeze-dried water-absorbent polymer particles produced by the production method of the present disclosure include absorbent articles (for example, disposable diapers, sanitary napkins, etc.), agricultural and horticultural materials (for example, water retention agents, soil improvers, etc.). , Industrial materials (for example, water blocking agents for cables, dew condensation inhibitor, etc.) and the like.

Hereinafter, the present disclosure will be described with reference to examples, but the present disclosure is not limited to these examples.
[Example 1]
90 parts by mass of a 60% by mass potassium acrylate (manufactured by Nippon Catalyst) aqueous solution and 10 parts by mass of acrylamide (manufactured by Wako Pure Chemical Industries, Ltd.) as an ethylene-based unsaturated monomer, and 1.0 mass by mass of benzoin isobutyl ether as a photoradical polymerization initiator. The part and 0.5 part by mass of N, N'-methylenebisacrylamide as a cross-linking agent were mixed, and the polymerization composition No. I prepared 1.

Using the polymerization apparatus 1 shown in FIG. 1, the polymerization composition No. From No. 1, a plurality of water-absorbent polymer water-containing particle Nos. 1 was manufactured. Specifically, the polymerization composition No. 1 was ejected as droplets from the ejection nozzle 13 into the hydrophobic solvent of the forming tube 17 at a rate of 30 mL / sec. The forming tube 17 has a total length of about 1.0 m and an inner diameter of about 12 mm, and in the forming tube 17, about 3 L of dimethylpolysiloxane maintained at about 10 ° C. as a hydrophobic solvent. It was circulating at a speed of / minute.

Polymerization composition No. 2 ejected as droplets into the forming tube 17. No. 1 is irradiated with ultraviolet rays from a high-pressure mercury lamp (wavelength 320 to 400 nm) as a light source 11, and the polymerization composition No. 1 in the droplets is irradiated. No. 1 is polymerized to obtain a plurality of water-absorbent polymer water-containing particles No. 1 was formed.
From dimethylpolysiloxane, a plurality of water-absorbent polymer water-containing particle Nos. No. 1 was taken out and dried at 80 ° C. for 6 hours under atmospheric pressure to obtain water-absorbent polymer particles No. 1. 1 was manufactured. Water-absorbent polymer particles No. The water content of 1 was 0.8% by mass.

In the beaker, the water-absorbent polymer particles No. No. 1 and ion-exchanged water were added in a predetermined amount, and the water-absorbent polymer particles No. 1 were added. 1 was immersed in ion-exchanged water. Water-absorbent polymer particles No. The water content of 1 was followed over time. Three hours after immersion in ion-exchanged water, the pre-freeze-dried particles No. 1 having a water content ratio of 100 times. I got 1.

Particle No. before freeze-drying 1 was laminated in a container to a thickness of about 1 cm and frozen in a commercially available freezer (-25 ° C.). After 24 hours, the frozen pre-freeze-dried particles No. No. 1 was transferred to a freeze-dryer manufactured by ULVAC, Inc., and freeze-dried under a reduced pressure of 0.1 mmHg. 1 was manufactured.

[Comparative Example 2]
A water-absorbent polymer powder having a skeleton derived from an acrylic acid-based monomer (commercially available, hereinafter referred to as "powder No. 2") produced by a solution polymerization method was prepared. Powder No. No. 2 is formed by crushing and atomizing a polymer mass produced by a solution polymerization method.

By the same operation as in Example 1, the powder No. before freeze-drying having a water content ratio of 100 times. I got 2. Powder No. before freeze-drying No. 2 was freeze-dried in the same manner as in Example 1, and the integrated product formed after freeze-drying was crushed by a pulverizer, and the powder No. 2 was freeze-dried. 2 was manufactured.

Water-absorbent polymer particles No. 1 and powder No. The gel strength at the average particle size, the coefficient of variation of the particle size, the average sphericity, and the water content of 1000 times was evaluated. The measuring method is as described in the present specification. The results are shown in Table 1. In addition, the particle No. 1 and powder No. The particle size distribution (number basis) of 2 is shown in FIGS. 2 and 3. The particle size distribution shown in FIGS. 2 and 3 was measured according to the measuring method such as the average particle size of the present specification, and “400-450” of particle size / μm is “more than 400 μm and 450 μm or less”. Means.

In addition, after freeze-drying, the particle No. No. 1 and powder No. after freeze-drying The water absorption ratio and the water retention ratio of each of 2 were measured as follows.
[Water absorption ratio]
(1) The particle sample is allowed to stand in a constant temperature and humidity chamber having a temperature of 20 ± 5 ° C. and a humidity of 65 ± 5% RH for 24 hours.
(2) Mass of about 20 mg sample: m ₃ (g) was accurately weighed, and about 22 mL of ion-exchanged water (about 1,100 times the mass of the sample) was added to the weighed sample and allowed to stand for 3 hours. , Swell the sample.
(3) The sample is placed in a bag formed of a nylon mesh having an opening of 75 μm, and the bag containing the sample is dehydrated at 150 G for 90 seconds with a centrifuge.

(4) Take out the sample from the bag _{and measure its mass: m 4} (g).
(5) Sample water retention ratio (mass ratio): W _HR , the following formula:
W _HR = 100 × (m _4- m ₃ ) / m ₃
Calculated by
(6) in different samples, repeat step (1) to step (5) a total of three times, water retention ratio of three samples: the arithmetic mean of the W _HR adopted as a water retention magnification.

[Water retention ratio]
(1) The particle sample is allowed to stand in a constant temperature and humidity chamber having a temperature of 20 ± 5 ° C. and a humidity of 65 ± 5% RH for 24 hours.
(2) Mass of about 20 mg sample: m ₅ (g) was accurately weighed, and about 22 mL of ion-exchanged water (about 1,100 times the mass of the sample) was added to the weighed sample and allowed to stand for 3 hours. , Swell the sample.
(3) The sample is placed in a bag formed of a nylon mesh having an opening of 75 μm, and the bag containing the sample is dehydrated at 150 G for 90 seconds with a centrifuge.

(4) Take out the sample from the bag _{and measure its mass: m 6} (g).
(5) Sample water retention ratio (mass ratio): W _HR , the following formula:
W _HR = 100 × (m _{6 −} m ₅ ) / m ₅
Calculated by
(6) in different samples, repeat step (1) to step (5) a total of three times, water retention ratio of three samples: the arithmetic mean of the W _HR adopted as a water retention magnification.
The results are shown in Table 1.

Particle No. after freeze-drying of Example 1. No. 1 could be easily loosened when taken out from the freeze-dryer, but the powder No. 1 after freeze-drying of Comparative Example 2 was found. When No. 2 was taken out from the freeze-dryer, the whole was integrated, and it was necessary to put it in a crusher in order to return it to a powder state.
Further, in Example 1, the water absorption time and the freeze-drying time were shorter than those in Comparative Example 1.

1 Polymerization equipment 3 Polymerization composition tank 5 Metering pump 7 Liquid metering tube 9 Light irradiation room 11 Light source 13 Injection nozzle 15 Rectifying perforated board 17 Forming tube 19 Separation screen 21 Hydrophobic solvent tank 23 Metering pump 25 Temperature regulator 27 Liquid transfer tube 101 Water-absorbent polymer water-containing particles 103 Polymerization composition 105 Hydrophobic solvent 107 Droplets

Claims (9)

A method for producing a plurality of freeze-dried water-absorbent polymer particles.
(I) A step of releasing a polymerization composition containing an ethylene-based unsaturated monomer, a radical polymerization initiator and water as a plurality of droplets into a hydrophobic solvent.
(Ii) A step of polymerizing the polymerization composition in the plurality of droplets in the hydrophobic solvent to form a plurality of water-absorbent polymer water-containing particles from the plurality of droplets.
(Iii) A step of forming a plurality of water-absorbent polymer particles to be freeze-dried having a water content ratio (mass ratio) of 10 to 100 times from the plurality of water-absorbent polymer water-containing particles, and (iv) the plurality of freeze-drying. The step of lyophilizing the water-absorbent polymer particles to be formed to form the plurality of lyophilized water-absorbent polymer particles.
Including
The plurality of water-absorbent polymer particles to be freeze-dried have a gel strength of 60 mN or more at a water content ratio (mass ratio) of 1000 times.
The manufacturing method. The production according to claim 1, wherein step (iv) further comprises a step of loosening the block of the lyophilized water-absorbent polymer particles into individual particles of the lyophilized water-absorbent polymer particles. Method. The production method according to claim 1 or 2, wherein in step (iii), the plurality of water-absorbent polymer water-containing particles are made to absorb water to form the plurality of water-absorbent polymer particles to be freeze-dried. In step (iii), the plurality of water-absorbent polymer particles are dried to form the plurality of water-absorbent polymer particles, and then the plurality of water-absorbent polymer particles are allowed to absorb a predetermined amount of water. The production method according to claim 1 or 2, wherein the water-absorbent polymer particles to be freeze-dried are formed. The item according to any one of claims 1 to 4 , wherein the plurality of water-absorbent polymer water-containing particles have a coefficient of variation of a particle size of 0.01 to 0.20 when dried to a water content of 1% by mass or less. The manufacturing method described. The production method according to any one of claims 1 to 5 , wherein the plurality of water-absorbent polymer water-containing particles have a gel strength of 60 to 200 mN at a water content ratio of 1000 times. The radical polymerization initiator is a photoradical polymerization initiator, and in the step of forming the plurality of water-absorbent polymer water-containing particles, the plurality of droplets are irradiated with light to obtain the polymerization in the plurality of droplets. The production method according to any one of claims 1 to 6 , wherein the composition is photopolymerized. The production method according to any one of claims 1 to 7 , wherein the temperature of the hydrophobic solvent is in the range of more than 0 ° C. and 30 ° C. or lower in the step of forming the water-absorbent polymer water-containing particles. The production method according to any one of claims 1 to 8 , wherein the hydrophobic solvent is selected from the group consisting of silicone oil, oil and hydrocarbon, and any combination thereof.

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