Method for producing water-absorbent resin particles and method for controlling particle size of water-absorbent resin particles

Abstract

Disclosed is a method for producing water-absorbent resin particles, which comprises: a step for preparing an aqueous liquid that contains a monomer of a (meth) acrylic acid compound containing at least one of (meth) acrylic acid or a salt thereof, protoanemonin, and water; a step for forming a particulate water-containing gel polymer containing a polymer of a monomer and water by reversed-phase suspension polymerization in a reaction liquid containing an aqueous liquid and a dispersion medium; and a step for extracting the water-absorbent resin particles containing the polymer from the reaction solution. The aqueous liquid containing protoanemonin in an amount of 6.0 ppm by mass or less relative to the amount of the (meth) acrylic acid compound is prepared.

Description

Technical Field

[0001] This disclosure relates to a method for manufacturing water-absorbing resin particles and a method for controlling the particle size of water-absorbing resin particles. Background Technology

[0002] Absorbent polymers are manufactured in particle form and are widely used in various fields such as hygiene materials (diapers, sanitary products, etc.), water-retaining agents, horticultural materials (soil conditioners, etc.), waterproofing materials, and industrial materials (anti-fogging agents, etc.).

[0003] In such absorbent resin particles, the proportion of particles with excessively large particle sizes is required to be small (e.g., Patent Document 1).

[0004] Existing technical documents Patent documents Patent Document 1: International Publication No. 2012 / 081355 Summary of the Invention

[0005] The technical problem to be solved by the invention This disclosure relates to a method for reducing the proportion of coarse particles in the obtained superabsorbent resin particles during the manufacture of superabsorbent resin particles based on reverse suspension polymerization.

[0006] means for solving technical problems This disclosure includes the following. [1] A method for manufacturing absorbent resin particles, comprising: The process of preparing an aqueous solution, wherein the aqueous solution contains a monomer of a (meth)acrylic acid compound comprising at least one of (meth)acrylic acid or a salt thereof, protoanemonin, and water; The process of forming a particulate hydrogel polymer containing the monomer and water through reverse suspension polymerization in a reaction solution containing the aqueous liquid and dispersion medium; and The process of removing the water-absorbing resin particles containing the polymer from the reaction solution. Prepare an aqueous solution containing less than 6.0 ppm by mass of the protoanemonin relative to the amount of the (meth)acrylic acid compound. When the aqueous solution contains the salt of the (meth)acrylic acid, the amount of the (meth)acrylic acid compound is the sum of the amount of the (meth)acrylic acid and the amount of (meth)acrylic acid equivalent to the amount of the salt. [2] According to the method described in [1], wherein, Prepare an aqueous solution of the protoanemonin containing more than 1.0 ppm by mass and less than 4.7 ppm by mass relative to the amount of the (meth)acrylic acid compound. [3] According to the method described in [1] or [2], wherein, The reaction solution also contains a polymerization inhibitor. [4] According to any one of [1] to [3], wherein, The proportion of particles with a diameter of 850 μm or larger in the water-absorbing resin particles is less than 20% by mass relative to the total amount of water-absorbing resin particles. [5] A method for controlling the particle size of absorbent resin particles, comprising: The process of preparing an aqueous solution, wherein the aqueous solution contains a monomer of a (meth)acrylic acid compound comprising at least one of (meth)acrylic acid or a salt thereof, protoanemonin, and water; The process of forming a particulate hydrogel polymer containing the monomer and water through reverse suspension polymerization in a reaction solution containing the aqueous liquid and dispersion medium; and The process of removing the water-absorbing resin particles containing the polymer from the reaction solution. The particle size of the water-absorbing resin particles is controlled by adjusting the amount of protoanemonin in the prepared aqueous solution. [6] According to the method described in [5], wherein, The particle size of the absorbent resin particles is controlled such that the proportion of particles with a diameter of 850 μm or more in the total amount of absorbent resin particles is less than 20% by mass. [7] According to the method described in [5] or [6], wherein, By adjusting the amount of the original anthracisin relative to the amount of the (meth)acrylic acid compound in the prepared aqueous solution to below 6.0 ppm by mass, the particle size of the water-absorbing resin particles is controlled. When the aqueous solution contains the salt of the (meth)acrylic acid, the amount of the (meth)acrylic acid compound is the sum of the amount of the (meth)acrylic acid and the amount of (meth)acrylic acid equivalent to the amount of the salt.

[0014] Invention Effects In the manufacture of superabsorbent polymer particles based on reverse suspension polymerization, it is possible to reduce the proportion of coarse particles in the obtained superabsorbent polymer particles. Detailed Implementation

[0015] This invention is not limited to the following examples. In this specification, (meth)acrylic acid means acrylic acid, methacrylic acid, and combinations thereof.

[0016] An example of a method for manufacturing absorbent resin particles includes: a step of preparing an aqueous liquid containing a monomer of a (meth)acrylic acid compound comprising at least one of (meth)acrylic acid or a salt thereof, protoanemonin, and water; a step of forming a particulate hydrogel polymer containing the monomer and water by reverse suspension polymerization in a reaction liquid containing the aqueous liquid and a dispersion medium; and a step of removing the absorbent resin particles containing the polymer from the reaction liquid.

[0017] The reaction solution for reverse suspension polymerization can be formed by stirring a mixture containing an aqueous solution and a dispersion medium. To prepare the aqueous solution before forming the reaction solution, the amount of protoanemonin relative to the amount of (meth)acrylic acid compound is adjusted to be 6.0 ppm by mass or less. When the amount of protoanemonin is 6.0 ppm by mass or less, it is possible to stably produce water-absorbing resin particles with a small proportion of coarse particles (particles with a particle size of 850 μm or more formed by the aggregation of hydrogel polymers during polymerization). From the same perspective, the amount of protoanemonin in the prepared aqueous solution relative to the amount of (meth)acrylic acid compound can be less than 5.5 ppm by mass, less than 5.0 ppm by mass, less than 4.9 ppm by mass, less than 4.8 ppm by mass, less than 4.7 ppm by mass, less than 4.6 ppm by mass, less than 4.5 ppm by mass, less than 4.4 ppm by mass, less than 4.3 ppm by mass, less than 4.2 ppm by mass, less than 4.1 ppm by mass, less than 4.0 ppm by mass, less than 3.9 ppm by mass, less than 3.8 ppm by mass, less than 3.7 ppm by mass, less than 3.5 ppm by mass, less than 3.4 ppm by mass, less than 3.3 ppm by mass, less than 3.2 ppm by mass, less than 3.1 ppm by mass, or less than 3.0 ppm by mass. The amount of protoanemonin in the prepared aqueous solution relative to the amount of (meth)acrylic acid compound can exceed 1.0 ppm by mass, or can be 1.1 ppm by mass or higher, 1.2 ppm by mass or higher, 1.3 ppm by mass or higher, 1.4 ppm by mass or higher, 1.5 ppm by mass or higher, 1.6 ppm by mass or higher, 1.7 ppm by mass or higher, 1.8 ppm by mass or higher, 1.9 ppm by mass or higher, or 2.0 ppm by mass or higher. A moderately high amount of protoanemonin can also help inhibit the formation of coarse particles.

[0018] The amount of protoanemonin in the prepared aqueous solution relative to the amount of (meth)acrylic acid compound can be greater than 1.0 ppm by mass and less than 6.0 ppm by mass, less than 5.5 ppm by mass, less than 5.0 ppm by mass, less than 4.9 ppm by mass, less than 4.8 ppm by mass, less than 4.7 ppm by mass, less than 4.6 ppm by mass, less than 4.5 ppm by mass, less than 4.4 ppm by mass, less than 4.3 ppm by mass, less than 4.2 ppm by mass, less than 4.1 ppm by mass, less than 4.0 ppm by mass, less than 3.9 ppm by mass, less than 3.8 ppm by mass, less than 3.7 ppm by mass, less than 3.5 ppm by mass, less than 3.4 ppm by mass, less than 3.3 ppm by mass, less than 3.2 ppm by mass, less than 3.1 ppm by mass, or less than 3.0 ppm by mass.

[0019] The amount of protoanemonin in the prepared aqueous solution relative to the amount of (meth)acrylic acid compound can be above 1.1 ppm by mass and below 6.0 ppm by mass, below 5.5 ppm by mass, below 5.0 ppm by mass, below 4.9 ppm by mass, below 4.8 ppm by mass, below 4.7 ppm by mass, below 4.6 ppm by mass, below 4.5 ppm by mass, below 4.4 ppm by mass, below 4.3 ppm by mass, below 4.2 ppm by mass, below 4.1 ppm by mass, below 4.0 ppm by mass, below 3.9 ppm by mass, below 3.8 ppm by mass, below 3.7 ppm by mass, below 3.5 ppm by mass, below 3.4 ppm by mass, below 3.3 ppm by mass, below 3.2 ppm by mass, below 3.1 ppm by mass, or below 3.0 ppm by mass.

[0020] The amount of protoanemonin in the prepared aqueous solution relative to the amount of (meth)acrylic acid compound can be above 1.2 ppm by mass and below 6.0 ppm by mass, below 5.5 ppm by mass, below 5.0 ppm by mass, below 4.9 ppm by mass, below 4.8 ppm by mass, below 4.7 ppm by mass, below 4.6 ppm by mass, below 4.5 ppm by mass, below 4.4 ppm by mass, below 4.3 ppm by mass, below 4.2 ppm by mass, below 4.1 ppm by mass, below 4.0 ppm by mass, below 3.9 ppm by mass, below 3.8 ppm by mass, below 3.7 ppm by mass, below 3.5 ppm by mass, below 3.4 ppm by mass, below 3.3 ppm by mass, below 3.2 ppm by mass, below 3.1 ppm by mass, or below 3.0 ppm by mass.

[0021] The amount of protoanemonin in the prepared aqueous solution relative to the amount of (meth)acrylic acid compound can be above 1.3 ppm by mass and below 6.0 ppm by mass, below 5.5 ppm by mass, below 5.0 ppm by mass, below 4.9 ppm by mass, below 4.8 ppm by mass, below 4.7 ppm by mass, below 4.6 ppm by mass, below 4.5 ppm by mass, below 4.4 ppm by mass, below 4.3 ppm by mass, below 4.2 ppm by mass, below 4.1 ppm by mass, below 4.0 ppm by mass, below 3.9 ppm by mass, below 3.8 ppm by mass, below 3.7 ppm by mass, below 3.5 ppm by mass, below 3.4 ppm by mass, below 3.3 ppm by mass, below 3.2 ppm by mass, below 3.1 ppm by mass, or below 3.0 ppm by mass.

[0022] The amount of protoanemonin in the prepared aqueous solution relative to the amount of (meth)acrylic acid compound can be above 1.4 ppm by mass and below 6.0 ppm by mass, below 5.5 ppm by mass, below 5.0 ppm by mass, below 4.9 ppm by mass, below 4.8 ppm by mass, below 4.7 ppm by mass, below 4.6 ppm by mass, below 4.5 ppm by mass, below 4.4 ppm by mass, below 4.3 ppm by mass, below 4.2 ppm by mass, below 4.1 ppm by mass, below 4.0 ppm by mass, below 3.9 ppm by mass, below 3.8 ppm by mass, below 3.7 ppm by mass, below 3.5 ppm by mass, below 3.4 ppm by mass, below 3.3 ppm by mass, below 3.2 ppm by mass, below 3.1 ppm by mass, or below 3.0 ppm by mass.

[0023] The amount of protoanemonin in the prepared aqueous solution relative to the amount of (meth)acrylic acid compound can be above 1.5 ppm by mass and below 6.0 ppm by mass, below 5.5 ppm by mass, below 5.0 ppm by mass, below 4.9 ppm by mass, below 4.8 ppm by mass, below 4.7 ppm by mass, below 4.6 ppm by mass, below 4.5 ppm by mass, below 4.4 ppm by mass, below 4.3 ppm by mass, below 4.2 ppm by mass, below 4.1 ppm by mass, below 4.0 ppm by mass, below 3.9 ppm by mass, below 3.8 ppm by mass, below 3.7 ppm by mass, below 3.5 ppm by mass, below 3.4 ppm by mass, below 3.3 ppm by mass, below 3.2 ppm by mass, below 3.1 ppm by mass, or below 3.0 ppm by mass.

[0024] The amount of protoanemonin in the prepared aqueous solution relative to the amount of (meth)acrylic acid compound can be above 1.6 ppm by mass and below 6.0 ppm by mass, below 5.5 ppm by mass, below 5.0 ppm by mass, below 4.9 ppm by mass, below 4.8 ppm by mass, below 4.7 ppm by mass, below 4.6 ppm by mass, below 4.5 ppm by mass, below 4.4 ppm by mass, below 4.3 ppm by mass, below 4.2 ppm by mass, below 4.1 ppm by mass, below 4.0 ppm by mass, below 3.9 ppm by mass, below 3.8 ppm by mass, below 3.7 ppm by mass, below 3.5 ppm by mass, below 3.4 ppm by mass, below 3.3 ppm by mass, below 3.2 ppm by mass, below 3.1 ppm by mass, or below 3.0 ppm by mass.

[0025] The amount of protoanemonin in the prepared aqueous solution relative to the amount of (meth)acrylic acid compound can be above 1.7 ppm by mass and below 6.0 ppm by mass, below 5.5 ppm by mass, below 5.0 ppm by mass, below 4.9 ppm by mass, below 4.8 ppm by mass, below 4.7 ppm by mass, below 4.6 ppm by mass, below 4.5 ppm by mass, below 4.4 ppm by mass, below 4.3 ppm by mass, below 4.2 ppm by mass, below 4.1 ppm by mass, below 4.0 ppm by mass, below 3.9 ppm by mass, below 3.8 ppm by mass, below 3.7 ppm by mass, below 3.5 ppm by mass, below 3.4 ppm by mass, below 3.3 ppm by mass, below 3.2 ppm by mass, below 3.1 ppm by mass, or below 3.0 ppm by mass.

[0026] The amount of protoanemonin in the prepared aqueous solution relative to the amount of (meth)acrylic acid compound can be above 1.8 ppm by mass and below 6.0 ppm by mass, below 5.5 ppm by mass, below 5.0 ppm by mass, below 4.9 ppm by mass, below 4.8 ppm by mass, below 4.7 ppm by mass, below 4.6 ppm by mass, below 4.5 ppm by mass, below 4.4 ppm by mass, below 4.3 ppm by mass, below 4.2 ppm by mass, below 4.1 ppm by mass, below 4.0 ppm by mass, below 3.9 ppm by mass, below 3.8 ppm by mass, below 3.7 ppm by mass, below 3.5 ppm by mass, below 3.4 ppm by mass, below 3.3 ppm by mass, below 3.2 ppm by mass, below 3.1 ppm by mass, or below 3.0 ppm by mass.

[0027] The amount of protoanemonin in the prepared aqueous solution relative to the amount of (meth)acrylic acid compound can be above 1.9 ppm by mass and below 6.0 ppm by mass, below 5.5 ppm by mass, below 5.0 ppm by mass, below 4.9 ppm by mass, below 4.8 ppm by mass, below 4.7 ppm by mass, below 4.6 ppm by mass, below 4.5 ppm by mass, below 4.4 ppm by mass, below 4.3 ppm by mass, below 4.2 ppm by mass, below 4.1 ppm by mass, below 4.0 ppm by mass, below 3.9 ppm by mass, below 3.8 ppm by mass, below 3.7 ppm by mass, below 3.5 ppm by mass, below 3.4 ppm by mass, below 3.3 ppm by mass, below 3.2 ppm by mass, below 3.1 ppm by mass, or below 3.0 ppm by mass.

[0028] The amount of protoanemonin in the prepared aqueous solution relative to the amount of (meth)acrylic acid compound can be above 2.0 ppm by mass and below 6.0 ppm by mass, below 5.5 ppm by mass, below 5.0 ppm by mass, below 4.9 ppm by mass, below 4.8 ppm by mass, below 4.7 ppm by mass, below 4.6 ppm by mass, below 4.5 ppm by mass, below 4.4 ppm by mass, below 4.3 ppm by mass, below 4.2 ppm by mass, below 4.1 ppm by mass, below 4.0 ppm by mass, below 3.9 ppm by mass, below 3.8 ppm by mass, below 3.7 ppm by mass, below 3.5 ppm by mass, below 3.4 ppm by mass, below 3.3 ppm by mass, below 3.2 ppm by mass, below 3.1 ppm by mass, or below 3.0 ppm by mass.

[0029] When an aqueous solution contains a salt of (meth)acrylic acid, the amount of (meth)acrylic acid compound used as a basis for the amount of trace components such as protoanemonin is the sum of the amount of (meth)acrylic acid and the amount of (meth)acrylic acid equivalent to the amount of the salt of (meth)acrylic acid. The amount of (meth)acrylic acid equivalent to the amount of the salt of (meth)acrylic acid compound refers to the amount of (meth)acrylic acid that is the same as the amount of the salt of (meth)acrylic acid. The amount of protoanemonin can be quantified, for example, by gas chromatography. For example, gas chromatography can be used to analyze (meth)acrylic acid raw materials containing trace components such as (meth)acrylic acid and protoanemonin, or samples of aqueous solutions containing (meth)acrylic acid compounds and protoanemonin. The amount of protoanemonin in the (meth)acrylic acid raw material, based on the amount of (meth)acrylic acid, can be less than 6.0 ppm by mass.

[0030] Besides protoanemonin, (meth)acrylic acid raw materials and aqueous solutions prepared from it may also contain trace amounts of furfural, benzaldehyde, and acrolein. When these trace amounts are within appropriate ranges, they can contribute to the stable production of water-absorbing resin particles with suitable particle sizes. The amounts of furfural, benzaldehyde, and acrolein can be quantified by gas chromatography.

[0031] The amount of furfural in (meth)acrylic acid raw materials or aqueous solutions prepared therefrom, based on the amount of (meth)acrylic acid compound, can be less than 3.0 ppm by mass, less than 2.5 ppm by mass, or less than 2.0 ppm by mass; it can also be more than 0 ppm by mass, more than 0.1 ppm by mass, or more than 1.0 ppm by mass. The amount of furfural in (meth)acrylic acid raw materials or aqueous solutions prepared therefrom, based on the amount of (meth)acrylic acid compound, can be more than 0 ppm by mass and less than 3.0 ppm by mass, less than 2.5 ppm by mass, or less than 2.0 ppm by mass. The amount of furfural in (meth)acrylic acid raw materials or aqueous solutions prepared therefrom, based on the amount of (meth)acrylic acid compound, can be more than 0.1 ppm by mass and less than 3.0 ppm by mass, less than 2.5 ppm by mass, or less than 2.0 ppm by mass.

[0032] The amount of benzaldehyde in (meth)acrylic acid raw materials or aqueous solutions prepared therefrom, based on the amount of (meth)acrylic acid compound, can be less than 3.0 ppm by mass, less than 2.5 ppm by mass, or less than 2.0 ppm by mass, or more than 0 ppm by mass or more than 0.1 ppm by mass. The amount of benzaldehyde in (meth)acrylic acid raw materials or aqueous solutions prepared therefrom, based on the amount of (meth)acrylic acid compound, can be more than 0 ppm by mass and less than 3.0 ppm by mass, less than 2.5 ppm by mass, or less than 2.0 ppm by mass. The amount of benzaldehyde in (meth)acrylic acid raw materials or aqueous solutions prepared therefrom, based on the amount of (meth)acrylic acid compound, can be more than 0.1 ppm by mass and less than 3.0 ppm by mass, less than 2.5 ppm by mass, or less than 2.0 ppm by mass.

[0033] The amount of acrolein in (meth)acrylic acid raw materials or aqueous solutions prepared therefrom, based on the amount of (meth)acrylic acid compound, can be less than 2.0 ppm by mass, less than 1.5 ppm by mass, or less than 1.0 ppm by mass, or more than 0 ppm by mass or more than 0.1 ppm by mass. The amount of acrolein in (meth)acrylic acid raw materials or aqueous solutions prepared therefrom, based on the amount of (meth)acrylic acid compound, can be more than 0 ppm by mass and less than 2.0 ppm by mass, less than 1.5 ppm by mass, or less than 1.0 ppm by mass. The amount of acrolein in (meth)acrylic acid raw materials or aqueous solutions prepared therefrom, based on the amount of (meth)acrylic acid compound, can be more than 0.1 ppm by mass and less than 2.0 ppm by mass, less than 1.5 ppm by mass, or less than 1.0 ppm by mass.

[0034] (Meth)acrylic acid raw materials and aqueous solutions prepared therefrom may also contain polymerization inhibitors. Examples of polymerization inhibitors include methoxyphenols such as p-methoxyphenol. Adjusting the amount of polymerization inhibitor can also contribute to the control of the particle size of the obtained superabsorbent resin particles. The amount of methoxyphenol in (meth)acrylic acid raw materials or aqueous solutions prepared therefrom, based on the amount of (meth)acrylic acid compound, can be less than 300 ppm by mass, less than 250 ppm by mass, or less than 220 ppm by mass, or it can be more than 0 ppm by mass, more than 100 ppm by mass, more than 150 ppm by mass, or more than 180 ppm by mass. The amount of methoxyphenol in (meth)acrylic acid raw materials or aqueous solutions prepared therefrom, based on the amount of (meth)acrylic acid compound, can be more than 0 ppm by mass and less than 300 ppm by mass, less than 250 ppm by mass, or less than 220 ppm by mass. The amount of methoxyphenol in (meth)acrylic acid raw materials or aqueous solutions prepared therefrom, based on the amount of (meth)acrylic acid compound, can be above 100 ppm by mass and below 300 ppm by mass, below 250 ppm by mass, or below 220 ppm by mass. The amount of methoxyphenol in (meth)acrylic acid raw materials or aqueous solutions prepared therefrom, based on the amount of (meth)acrylic acid compound, can be above 150 ppm by mass and below 300 ppm by mass, below 250 ppm by mass, or below 220 ppm by mass. The amount of methoxyphenol in (meth)acrylic acid raw materials or aqueous solutions prepared therefrom, based on the amount of (meth)acrylic acid compound, can be above 180 ppm by mass and below 300 ppm by mass, below 250 ppm by mass, or below 220 ppm by mass. The amount of methoxyphenol can be quantified by liquid chromatography.

[0035] The aqueous solution may contain (meth)acrylic acid and its salts as (meth)acrylic acid compounds. Based on the total amount of the (meth)acrylic acid compounds, the proportion of the (meth)acrylic acid salts may, for example, be 50 mol% or more and 100 mol% or less. The (meth)acrylic acid salts may, for example, be alkali metal salts of (meth)acrylic acid. The alkali metal salts of (meth)acrylic acid may be sodium salts.

[0036] Aqueous solutions containing (meth)acrylic acid and its salts can be prepared, for example, by a method comprising the following steps: preparing a (meth)acrylic acid raw material containing (meth)acrylic acid and protoanemonin; and neutralizing a portion of the (meth)acrylic acid in the (meth)acrylic acid raw material with an alkaline aqueous solution containing an alkaline compound. The alkaline compound can be an alkali metal hydroxide (e.g., sodium hydroxide). Adjusting the iron content in the alkaline aqueous solution used to neutralize (meth)acrylic acid can also help to stably manufacture water-absorbing resin particles with appropriate particle size. The iron content in the alkaline aqueous solution used to neutralize (meth)acrylic acid, relative to the amount of the alkaline compound, can be less than 4.0 ppm by mass, less than 3.0 ppm by mass, less than 2.0 ppm by mass, less than 1.0 ppm by mass, less than 0.5 ppm by mass, or less than 0.2 ppm by mass, converted to Fe2O3. Fe2O3 conversion refers to converting the amount of iron atoms into the amount of Fe2O3 equivalent to their molar amount.

[0037] The monomers in the aqueous solution may also contain water-soluble olefinic unsaturated monomers other than (meth)acrylic acid compounds. The proportion of (meth)acrylic acid compounds relative to the total amount of monomers may be 60 mol% or more, 70 mol% or more, 80 mol% or more, 90 mol% or more, or 95 mol% or more, and may substantially be 100 mol%. The water-soluble olefinic unsaturated monomers other than (meth)acrylic acid compounds may include, for example, at least one selected from the group consisting of 2-(meth)acrylamide-2-methylpropanesulfonic acid and its base salt, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-hydroxymethyl(meth)acrylamide, polyethylene glycol mono(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide.

[0038] The aqueous liquid may contain an internal crosslinking agent that crosslinks the polymer of the monomer formed by the polymerization reaction. The internal crosslinking agent may be a compound having two or more reactive functional groups that are reactive with the monomer (especially (meth)acrylic acid compounds). The reactive functional groups may be, for example, (meth)acryloyl, vinyl, epoxy, halogenated groups in halogenated epoxy compounds, isocyanate groups, or combinations thereof.

[0039] Examples of internal crosslinking agents having two or more (meth)acryloyl groups include (meth)acrylate compounds formed from polyol compounds and (meth)acrylic acid, unsaturated polyesters formed from polyol compounds and unsaturated carboxylic acids (maleic acid, fumaric acid, etc.), bis(meth)acrylamide compounds (N,N'-methylenebis(meth)acrylamide, etc.), (meth)acrylate compounds formed from polyepoxide compounds and (meth)acrylic acid, and (meth)acrylate carbamoyl ester compounds formed from polyisocyanate compounds (toluene diisocyanate, hexamethylene diisocyanate, etc.) and hydroxyethyl (meth)acrylate. The polyol compounds used to form (meth)acrylate compounds or unsaturated polyesters may, for example, be ethylene glycol, propylene glycol, trimethylolpropane, glycerol, polyoxyethylene glycol, polyoxypropylene glycol, polyglycerol, or combinations thereof.

[0040] Vinyl groups, as reactive functional groups, can be part of allyl groups. Examples of internal crosslinking agents having two or more vinyl (or allyl) groups include allylated starch, allylated cellulose, diallyl phthalate, N,N',N''-triallyl isocyanurate, and divinylbenzene.

[0041] Examples of internal crosslinking agents having two or more epoxy groups include (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerol diglycidyl ether, (poly)glycerol triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether.

[0042] Examples of internal crosslinking agents having two or more isocyanate groups include 2,4-toluene diisocyanate and hexamethylene diisocyanate.

[0043] The amount of the internal crosslinking agent relative to 1 mole of monomer can be more than 0 mmol and less than 0.5 mmol, more than 0 mmol and less than 0.2 mmol, more than 0 mmol and less than 0.1 mmol, more than 0 mmol and less than 0.05 mmol, or more than 0 mmol and less than 0.02 mmol, and can also be practically 0 mmol.

[0044] Aqueous liquids may contain thickeners, hydrophilic polymeric dispersants, free radical polymerization initiators, chain transfer agents, foaming agents, or combinations thereof as other components.

[0045] Examples of thickeners include hydroxyalkyl celluloses such as hydroxyethyl cellulose (HEC) and hydroxypropyl cellulose (HPC); hydroxyalkyl celluloses such as hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose, and hydroxyethyl ethyl cellulose; carboxyalkyl celluloses such as carboxymethyl cellulose; and carboxyalkyl hydroxyalkyl celluloses such as carboxymethyl hydroxyethyl cellulose. Thickeners can be a single agent or a combination of two or more agents.

[0046] The amount of thickener relative to 100 parts by mass of monomer can be more than 0.05 parts by mass and less than 20 parts by mass, more than 0.2 parts by mass and less than 10 parts by mass, or more than 0.4 parts by mass and less than 5 parts by mass.

[0047] Examples of hydrophilic polymeric dispersants include polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), polypropylene glycol, polyethylene glycol-polypropylene glycol block copolymers, glycerol, polyoxyethylene glycerol, polyoxypropylene glycerol, polyoxyethylene-polyoxypropylene glycerol copolymers, and polyoxyethylene dehydrated sorbitol fatty acid esters. Hydrophilic polymeric dispersants can be a single type or a combination of two or more.

[0048] The amount of hydrophilic polymeric dispersant relative to 100 parts by mass can be more than 0.001 parts by mass and less than 10 parts by mass, more than 0.005 parts by mass and less than 5 parts by mass, more than 0.01 parts by mass and less than 3 parts by mass, or more than 0.01 parts by mass and less than 1.5 parts by mass.

[0049] Free radical polymerization initiators may include, for example, azo compounds, peroxides, or combinations thereof.

[0050] Examples of azo compounds include 2,2'-azobis[2-(N-phenylamidinyl)propane] dihydrochloride, 2,2'-azobis{2-[N-(4-chlorophenyl)amidinyl]propane} dihydrochloride, 2,2'-azobis{2-[N-(4-hydroxyphenyl)amidinyl]propane} dihydrochloride, and 2,2'-azobis[2-(N-benzylamidinyl)propane] dihydrochloride. 2,2'-Azobis[2-(N-allylamidinyl)propane] dihydrochloride, 2,2'-Azobis(2-amidinylpropane) dihydrochloride, 2,2'-Azobis{2-[N-(2-hydroxyethyl)amidinyl]propane} dihydrochloride, 2,2'-Azobis[2-(5-methyl-2-imidazolin-2-yl)propane] dihydrochloride, 2,2'-Azobis[2-(2- [Imidazolin-2-yl)propane] dihydrochloride, 2,2'-azobis[2-(4,5,6,7-tetrahydro-1H-1,3-diazaphen-2-yl)propane] dihydrochloride, 2,2'-azobis[2-(5-hydroxy-3,4,5,6-tetrahydropyrimidin-2-yl)propane] dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolinyl] [2,2'-azoline-2-yl]propane] dihydrochloride, 2,2'-azobis(2-methylpropamid) dihydrochloride, 2,2'-azobis[2-(2-imidazoline-2-yl)propane] disulfate dihydrate, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropamidine] tetrahydrate and 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)propamid].

[0051] Examples of peroxides include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; organic peroxides such as methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-tert-butyl peroxide, tert-butyl cumene peroxide, tert-butyl peracetate, tert-butyl perisobutyrate, and tert-butyl perpentyl peroxide; and hydrogen peroxide.

[0052] The amount of free radical polymerization initiator relative to 100 moles of monomer can be, for example, more than 0.0001 moles and less than 1 mole, more than 0.001 moles and less than 0.1 moles, more than 0.003 moles and less than 0.08 moles, or more than 0.007 moles and less than 0.05 moles.

[0053] Examples of chain transfer agents include hypophosphites, thiols, thiolic acids, secondary alcohols, and amines.

[0054] Examples of foaming agents include inorganic foaming agents such as ammonium carbonate, sodium bicarbonate, and ammonium bicarbonate; nitroso compounds such as dinitrospentamethylenetetramine; azo compounds such as azodicarbonamide and azobisisobutyronitrile; and organic foaming agents such as sulfonyl hydrazides such as 4,4'-oxobisbenzenesulfonyl hydrazide and p-toluenesulfonyl hydrazide.

[0055] By stirring a mixture containing a prepared aqueous liquid and an oily liquid, which is mainly composed of a hydrophobic dispersion medium, a reaction solution for reverse suspension polymerization can be formed. The reaction solution contains particulate aqueous liquid dispersed in the oily liquid. Through polymerization in the reaction solution, a particulate hydrogel polymer containing monomers and water is formed.

[0056] The reaction solution may contain surfactants. Surfactants may be contained in oily liquids. Surfactants may be nonionic or anionic. Examples of nonionic surfactants include sorbitan fatty acid esters, polyglycerol fatty acid esters, sucrose fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerol fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene cured castor oil, alkylallyl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropylene alkyl ethers, and polyethylene glycol fatty acid esters. Examples of anionic surfactants include fatty acid salts, alkylbenzene sulfonates, alkyl methyl taurate, polyoxyethylene alkylphenyl ether sulfates, polyoxyethylene alkyl ether sulfonates, polyoxyethylene alkyl ether phosphates, and polyoxyethylene alkylallyl ether phosphates. Surfactants may be a single type or a combination of two or more types.

[0057] The oily liquid in the reaction solution is a hydrophobic liquid mainly composed of a hydrophobic dispersion medium. The dispersion medium can be a hydrocarbon dispersion medium. The oily liquid may also contain any additives such as hydrophobic polymeric dispersants.

[0058] Examples of hydrocarbon dispersion media include chain aliphatic hydrocarbons such as n-hexane, n-heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 3-ethylpentane, and n-octane; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, trans-1,2-dimethylcyclopentane, cis-1,3-dimethylcyclopentane, and trans-1,3-dimethylcyclopentane; and aromatic hydrocarbons such as benzene, toluene, and xylene. The hydrocarbon dispersion medium can be a single type or a combination of two or more types.

[0059] The amount of dispersion medium contained in the oily liquid relative to 100 parts by mass can be more than 30 parts by mass and less than 1000 parts by mass, more than 50 parts by mass and less than 650 parts by mass, more than 70 parts by mass and less than 550 parts by mass, or more than 100 parts by mass and less than 450 parts by mass.

[0060] Examples of hydrophobic polymeric dispersants include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified EPDM (ethylene-propylene-diene-terpolymer), maleic anhydride-modified polybutadiene, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, maleic anhydride-butadiene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethyl cellulose, and ethyl hydroxyethyl cellulose. Hydrophobic polymeric dispersants can be a single type or a combination of two or more types.

[0061] The amount of hydrophobic polymeric dispersant relative to 100 parts by mass of monomer can be more than 0.05 parts by mass and less than 10 parts by mass, more than 0.08 parts by mass and less than 5 parts by mass, or more than 0.1 parts by mass and less than 3 parts by mass.

[0062] For the polymerization reaction to proceed, the reaction solution can be heated. The heating temperature for the polymerization reaction can be, for example, above 40°C and below 90°C. The polymerization reaction time can be, for example, above 30 minutes and below 240 minutes.

[0063] During the polymerization reaction, the reaction liquid is typically stirred. If the stirring speed is low, the proportion of hydrogel-like polymer particles with large particle sizes tends to increase. However, by appropriately adjusting the amount of protoanemonin, even at relatively low stirring speeds, the particle size of the particulate hydrogel polymer and the resulting water-absorbing resin particles can be easily controlled within an appropriate range. The stirring speed during the polymerization reaction can, for example, be above 200 rpm and below 1000 rpm.

[0064] The process involves removing water-absorbing resin particles containing a polymer from a reaction solution (slurry) containing a particulate hydrogel polymer. Methods for removing the water-absorbing resin particles from the reaction solution include, for example, a step of extracting a portion of the water from the reaction solution by azeotropic distillation of the dispersion medium and water to form a concentrate; and a step of evaporating the dispersion medium, water, and trace components (protoanemonin, furfural, benzaldehyde, and acrolein) from the concentrate. After removing the dispersion medium by evaporation, a powder containing dried polymer particles of water-absorbing resin particles can be obtained. A certain amount of water may remain in the dried polymer particles (water-absorbing resin particles).

[0065] After water is extracted, polymer particles can be surface crosslinked in a mixture containing a concentrate and a surface crosslinking agent, wherein the concentrate contains polymer particles (hygroscopic resin particles).

[0066] Surface crosslinking agents can be compounds with two or more reactive functional groups, examples of which include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerol, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerol; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerol diglycidyl ether, (poly)glycerol triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; halogenated epoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepicochlorohydrin; 2, Isocyanate compounds such as 4-toluene diisocyanate and hexamethylene diisocyanate; oxetane compounds such as 3-methyl-3-oxetane methanol, 3-ethyl-3-oxetane methanol, 3-butyl-3-oxetane methanol, 3-methyl-3-oxetane ethanol, 3-ethyl-3-oxetane ethanol, and 3-butyl-3-oxetane ethanol; oxazoline compounds such as 1,2-ethylene bisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]hexamethylenediamide. Surface crosslinking agents may include polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerol diglycidyl ether, (poly)glycerol triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether. These surface crosslinking agents can be used alone or in combination of two or more.

[0067] The amount of surface crosslinking agent relative to each mole of the monomer unit constituting the polymer in the polymer particles can be more than 0.01 mmol and less than 10 mmol, more than 0.03 mmol and less than 3 mmol, or more than 0.05 mmol and less than 1 mmol.

[0068] To achieve surface crosslinking, the mixture can be heated. The heating temperature can be, for example, above 60°C and below 200°C, or above 80°C and below 150°C. The reaction time for the surface crosslinking reaction can be, for example, above 1 minute and below 300 minutes, or above 5 minutes and below 200 minutes.

[0069] The dispersion medium can be removed from the concentrate to obtain dried polymer particles (hygroscopic resin particles). Therefore, the concentrate can be heated.

[0070] Various additives can be further added to the dried polymer particles. Examples of additives include lubricants, metal chelating agents, surface modifiers, heat stabilizers, antioxidants, and antibacterial agents.

[0071] Lubricants can be, for example, amorphous silica particles. Examples of metal chelating agents include ethylenediaminetetraacetic acid (EDTA) and its salts (such as disodium EDTA), and diethylenetriaminepentaacetic acid (DTA) and its salts (such as pentasodium DTA). Examples of surface modifiers include polyvalent metal compounds such as aluminum sulfate, potassium sulfate, ammonium sulfate, sodium sulfate, (poly)aluminum chloride, and their hydrates; and polycationic compounds such as polyethyleneimine, polyethyleneamine, and polyallylamine.

[0072] Additives can adhere to the surface of polymer particles or penetrate into the interior of polymer particles. Additives (e.g., lubricants) can be added to the surface of the polymer particles after they have been dried. Additives can also be added to the liquid used for polymerization or to the concentrate after water extraction. In this specification, water-absorbing resin particles refer to particles containing polymer particles. Water-absorbing resin particles may contain both polymer particles and additives.

[0073] The amount of additives (e.g., lubricants) relative to 100 parts by mass of polymer particles can be, for example, more than 0.001 parts by mass and less than 10 parts by mass, more than 0.01 parts by mass and less than 5 parts by mass, or more than 0.1 parts by mass and less than 2 parts by mass.

[0074] In dried polymer particle (hygroscopic resin particle) powder, the proportion of particles with a diameter of 850 μm or larger relative to the total amount of hygroscopic resin particles can be less than 20% by mass, 15% by mass or less, 10% by mass or less, or 5% by mass or less. This proportion refers to the proportion before the powder is classified by sieving or other methods after drying. Methods for determining the proportion of particles with a diameter of 850 μm or larger include: a step of sieving a powder with a mass W0 g of hygroscopic resin particles using a sieve with a mesh size of 850 μm; a step of measuring the mass W1 g of particles remaining on the sieve; and a step of calculating the proportion of particles with a diameter of 850 μm or larger using the formula: Proportion of particles with a diameter of 850 μm or larger = (W1 / W0) × 100. Particles with a diameter of 850 μm or larger are sometimes excluded from the product through classification. From the perspective of effectively utilizing manufacturing raw materials such as (meth)acrylic acid, it is also advantageous to have a small proportion of particles with a particle size of 850 μm or larger in the powder after polymerization and before classification.

[0075] Example The present invention is not limited to the following embodiments.

[0076] 1. Acrylic acid raw materials Acrylic acid raw materials (1) to (5) containing acrylic acid as the main component were prepared. Table 1 shows the purity (concentration of acrylic acid) and the contents of trace components such as protoanemonin, furfural, benzaldehyde, acrolein, and p-methoxyphenol in each acrylic acid raw material. The contents of trace components shown in the table are proportions based on the mass of acrylic acid (mass ppm).

[0077] [Table 1]

[0078] 2. Manufacturing of water-absorbing resin particles Example 1a A round-bottomed cylindrical flask with an inner diameter of 11 cm and an internal volume of 2 L was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen inlet tube, and a stirrer. The stirrer was fitted with four inclined blades, each with a diameter of 5 cm and two sections. 313 g of n-heptane was added to the flask as a hydrocarbon dispersion medium, and 0.920 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc., HIWAX 1105A) was added as a polymeric dispersant. The mixture in the flask was stirred while the temperature was raised to 80 °C, forming a n-heptane solution containing the maleic anhydride-modified ethylene-propylene copolymer. After stirring, the n-heptane solution was cooled to 56 °C.

[0079] 73.60 g (1.02 mol) of acrylic acid raw material (1) was added to a 300 mL beaker. While cooling the beaker with ice water, 102.1 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to prepare a 75 mol% neutralized product of acrylic acid (1). Next, 68.6 g of water, 0.1104 g (0.408 mmol) of potassium persulfate as a free radical polymerizing agent, and 0.0083 g (0.048 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added to the beaker and dissolved to prepare an aqueous solution containing acrylic acid and sodium acrylate.

[0080] The prepared aqueous solution was added to a heptane solution containing maleic anhydride-modified ethylene-propylene copolymer in a removable flask. The mixture in the removable flask was stirred for 10 minutes. Next, a surfactant solution containing 0.920 g of sucrose stearate (Mitsubishi Chemical Foods Corporation, RYOTO SUGAR ESTER S-370, HLB: 3) and 8.28 g of heptane was added to the mixture. The reaction mixture formed in the removable flask was stirred with a stirrer at 450 rpm, and the flask was fully purged with nitrogen. After nitrogen purging, the removable flask was immersed in a 70°C water bath to raise the temperature of the reaction mixture, thereby initiating the polymerization reaction. The polymerization reaction was continued with stirring for 60 minutes, resulting in a slurry containing a particulate hydrogel polymer.

[0081] A detachable flask containing the slurry was immersed in an oil bath at 125°C. The n-heptane in the slurry was refluxed, and 126.8 g of water was extracted from the slurry to the outside of the detachable flask via azeotropic distillation of n-heptane and water. Next, the detachable flask was heated in an oil bath at 125°C to evaporate the n-heptane, thereby obtaining 95.3 g of dried absorbent resin particles.

[0082] Examples 2a, 2b, 3a, 4a and Comparative Example 1a Using the acrylic raw materials shown in Table 2, the stirring speed of the mixer in the polymerization reaction was set as shown in Table 2, and except that, water-absorbing resin particles were obtained in the same order as in Example 1a.

[0083] 3. Evaluation of water-absorbing resin particles The powder of superabsorbent resin particles obtained in the examples or comparative examples was sieved using a sieve with a mesh size of 850 μm. The mass of superabsorbent resin particles with a particle size of 850 μm or larger remaining on the sieve was determined. The proportion (mass %) of particles with a particle size of 850 μm or larger relative to the total amount of superabsorbent resin particles was calculated.

[0084] [Table 2]

[0085] The evaluation results are shown in Table 2. It was confirmed that by adjusting the amount of protoanemonin relative to the amount of acrylic acid, the particle size of the obtained superabsorbent resin particles could be controlled to reduce the proportion of particles with a diameter of 850 μm or larger.

Claims

1. A method for manufacturing water-absorbing resin particles, comprising: The process of preparing an aqueous solution, wherein the aqueous solution contains a monomer of a (meth)acrylic acid compound comprising at least one of (meth)acrylic acid or a salt thereof, protoanemonin, and water; The process of forming a particulate hydrogel polymer containing the monomer and water through reverse suspension polymerization in a reaction solution containing the aqueous liquid and dispersion medium; and The process of removing the water-absorbing resin particles containing the polymer from the reaction solution. Prepare an aqueous solution containing less than 6.0 ppm by mass of the protoanemonin relative to the amount of the (meth)acrylic acid compound. When the aqueous solution contains the salt of the (meth)acrylic acid, the amount of the (meth)acrylic acid compound is the sum of the amount of the (meth)acrylic acid and the amount of (meth)acrylic acid equivalent to the amount of the salt.

2. The method according to claim 1, wherein, Prepare an aqueous solution of the protoanemonin containing more than 1.0 ppm by mass and less than 4.7 ppm by mass relative to the amount of the (meth)acrylic acid compound.

3. The method according to claim 1 or 2, wherein, The reaction solution also contains a polymerization inhibitor.

4. The method according to claim 1, wherein, The proportion of particles with a diameter of 850 μm or larger in the water-absorbing resin particles is less than 20% by mass relative to the total amount of water-absorbing resin particles.

5. A method for controlling the particle size of absorbent resin particles, comprising: The process of preparing an aqueous solution, wherein the aqueous solution contains a monomer of a (meth)acrylic acid compound comprising at least one of (meth)acrylic acid or a salt thereof, protoanemonin, and water; The process of forming a particulate hydrogel polymer containing the monomer and water through reverse suspension polymerization in a reaction solution containing the aqueous liquid and dispersion medium; and The process of removing the water-absorbing resin particles containing the polymer from the reaction solution. The particle size of the water-absorbing resin particles is controlled by adjusting the amount of protoanemonin in the prepared aqueous solution.

6. The method according to claim 5, wherein, The particle size of the absorbent resin particles is controlled such that the proportion of particles with a diameter of 850 μm or more in the total amount of absorbent resin particles is less than 20% by mass.

7. The method according to claim 5 or 6, wherein, By adjusting the amount of the original anthracisin relative to the amount of the (meth)acrylic acid compound in the prepared aqueous solution to below 6.0 ppm by mass, the particle size of the water-absorbing resin particles is controlled. When the aqueous solution contains the salt of the (meth)acrylic acid, the amount of the (meth)acrylic acid compound is the sum of the amount of the (meth)acrylic acid and the amount of (meth)acrylic acid equivalent to the amount of the salt.

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