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

Abstract

Disclosed is a method for producing a water-absorbing resin particle, the method comprising: preparing an aqueous solution containing a monomer including at least one (meth)acrylic acid compound selected from (meth)acrylic acid and salts thereof, protoanemonin and water; carrying out reverse phase suspension polymerization in a reaction liquid containing said aqueous solution and a dispersion medium to form a polymer of the monomer and a particulate hydrous gel polymer containing water; and removing water-absorbing resin particles containing the polymer from the reaction liquid. The aqueous liquid is prepared such that the amount of protoanemonin is 6.0 ppm by mass or less relative to the amount of the (meth)acrylic acid compound.

Description

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

[0001] The present disclosure relates to a method for producing water-absorbent resin particles and a method for controlling the particle size of water-absorbent resin particles.

[0002] Water-absorbent resins are produced in the form of particles, and are widely used in various fields, such as hygiene materials such as disposable diapers and sanitary products, gardening materials such as water retention agents and soil conditioners, and industrial materials such as water-stopping materials and anti-condensation agents.

[0003] In such water-absorbent resin particles, it is required that the proportion of particles having an excessively large particle size is small (for example, Patent Document 1).

[0004] International Publication No. 2012 / 081355

[0005] The present disclosure relates to reducing the proportion of coarse particles in the water-absorbent resin particles obtained in the production of water-absorbent resin particles by reverse-phase suspension polymerization.

[0006] The present disclosure includes the following: [1] A method for producing water-absorbent resin particles, comprising: preparing an aqueous liquid containing protoanemonin, a monomer containing at least one (meth)acrylic acid compound of (meth)acrylic acid or a salt thereof, and water; forming a particulate hydrogel polymer containing a polymer of the monomer and water by reverse-phase suspension polymerization in a reaction liquid containing the aqueous liquid and a dispersion medium; and extracting water-absorbent resin particles containing the polymer from the reaction liquid, wherein the aqueous liquid is prepared containing the protoanemonin in an amount of 6.0 ppm by mass or less relative to the amount of the (meth)acrylic acid compound, and when the aqueous liquid contains the salt of (meth)acrylic acid, the amount of the (meth)acrylic acid compound is the sum of the amount of the (meth)acrylic acid and an amount of (meth)acrylic acid corresponding to the substance amount of the salt. [2] The method according to [1], wherein the aqueous liquid is prepared containing the protoanemonin in an amount of more than 1.0 ppm by mass and not more than 4.7 ppm by mass relative to the amount of the (meth)acrylic acid compound. [3] The method according to [1] or [2], wherein the reaction solution further contains a polymerization inhibitor. [4] The method according to any one of [1] to [3], wherein a ratio of particles having a particle size of 850 μm or more in the water-absorbent resin particles is less than 20 mass% with respect to the total amount of the water-absorbent resin particles. [5] A method for controlling a particle size of water-absorbent resin particles, comprising: preparing an aqueous liquid containing protoanemonin and water, a monomer containing a (meth)acrylic acid compound of at least one of (meth)acrylic acid and a salt thereof, and forming a particulate hydrogel polymer containing a polymer of the monomer and water by reverse-phase suspension polymerization in a reaction liquid containing the aqueous liquid and a dispersion medium, and extracting water-absorbent resin particles containing the polymer from the reaction liquid, wherein the particle size of the water-absorbent resin particles is controlled by adjusting the amount of protoanemonin in the prepared aqueous liquid. [6] The method according to [5], wherein a particle size of the water-absorbent resin particles is controlled so that a ratio of particles having a particle size of 850 μm or more in the water-absorbent resin particles is less than 20 mass % with respect to a total amount of the water-absorbent resin particles.[7] The method according to [5] or [6], wherein the particle size of the water-absorbent resin particles is controlled by adjusting the amount of the protoanemonin relative to the amount of the (meth)acrylic acid compound in the prepared aqueous liquid to 6.0 ppm by mass or less, and when the aqueous liquid contains a salt of (meth)acrylic acid, the amount of the (meth)acrylic acid compound is the total amount of the amount of the (meth)acrylic acid and the amount of (meth)acrylic acid corresponding to the substance amount of the salt.

[0007] In the production of water-absorbent resin particles by reversed-phase suspension polymerization, the proportion of coarse particles in the obtained water-absorbent resin particles can be reduced.

[0008] The present invention is not limited to the following examples: In this specification, (meth)acrylic acid means acrylic acid, methacrylic acid, and combinations thereof.

[0009] An example of a method for producing water-absorbent resin particles includes preparing an aqueous liquid containing protoanemonin, a monomer containing at least one (meth)acrylic acid compound selected from (meth)acrylic acid and a salt thereof, and water; forming a particulate hydrogel polymer containing a polymer of the monomer and water by reverse phase suspension polymerization in a reaction liquid containing the aqueous liquid and a dispersion medium; and extracting water-absorbent resin particles containing the polymer from the reaction liquid.

[0010] The reaction liquid for reversed-phase suspension polymerization can be formed by stirring a mixed liquid containing an aqueous liquid and a dispersion medium. To prepare the aqueous liquid before forming the reaction liquid, the amount of protoanemonin is adjusted to 6.0 ppm by mass or less relative to the amount of the (meth)acrylic acid compound. When the amount of protoanemonin is 6.0 ppm by mass or less, water-absorbent resin particles with a small proportion of coarse particles (particles having a particle diameter of 850 μm or more formed by aggregation of the hydrogel polymer during polymerization) can be stably produced. From a similar viewpoint, the amount of protoanemonin in the prepared aqueous liquid may be 5.5 ppm by mass or less, 5.0 ppm by mass or less, 4.9 ppm by mass or less, 4.8 ppm by mass or less, 4.7 ppm by mass or less, 4.6 ppm by mass or less, 4.5 ppm by mass or less, 4.4 ppm by mass or less, 4.3 ppm by mass or less, 4.2 ppm by mass or less, 4.1 ppm by mass or less, 4.0 ppm by mass or less, 3.9 ppm by mass or less, 3.8 ppm by mass or less, 3.7 ppm by mass or less, 3.5 ppm by mass or less, 3.4 ppm by mass or less, 3.3 ppm by mass or less, 3.2 ppm by mass or less, 3.1 ppm by mass or less, or 3.0 ppm by mass or less, relative to the amount of the (meth)acrylic acid compound. The amount of protoanemonin in the prepared aqueous liquid may be more than 1.0 ppm by mass relative to the amount of the (meth)acrylic acid compound, and may be 1.1 ppm by mass or more, 1.2 ppm by mass or more, 1.3 ppm by mass or more, 1.4 ppm by mass or more, 1.5 ppm by mass or more, 1.6 ppm by mass or more, 1.7 ppm by mass or more, 1.8 ppm by mass or more, 1.9 ppm by mass or more, or 2.0 ppm by mass or more. A moderately large amount of protoanemonin can also contribute to suppressing the formation of coarse particles.

[0011] The amount of protoanemonin in the prepared aqueous liquid may be more than 1.0 ppm by mass and not more than 6.0 ppm by mass, 5.5 ppm by mass, 5.0 ppm by mass, 4.9 ppm by mass, 4.8 ppm by mass, 4.7 ppm by mass, 4.6 ppm by mass, 4.5 ppm by mass, 4.4 ppm by mass, 4.3 ppm by mass, 4.2 ppm by mass, 4.1 ppm by mass, 4.0 ppm by mass, 3.9 ppm by mass, 3.8 ppm by mass, 3.7 ppm by mass, 3.5 ppm by mass, 3.4 ppm by mass, 3.3 ppm by mass, 3.2 ppm by mass, 3.1 ppm by mass, or 3.0 ppm by mass. The amount of protoanemonin in the prepared aqueous liquid may be 1.1 ppm by mass or more and 6.0 ppm by mass or less, 5.5 ppm by mass or less, 5.0 ppm by mass or less, 4.9 ppm by mass or less, 4.8 ppm by mass or less, 4.7 ppm by mass or less, 4.6 ppm by mass or less, 4.5 ppm by mass or less, 4.4 ppm by mass or less, 4.3 ppm by mass or less, 4.2 ppm by mass or less, 4.1 ppm by mass or less, 4.0 ppm by mass or less, 3.9 ppm by mass or less, 3.8 ppm by mass or less, 3.7 ppm by mass or less, 3.5 ppm by mass or less, 3.4 ppm by mass or less, 3.3 ppm by mass or less, 3.2 ppm by mass or less, 3.1 ppm by mass or less, or 3.0 ppm by mass or less, relative to the amount of the (meth)acrylic acid compound. The amount of protoanemonin in the prepared aqueous liquid may be 1.2 ppm by mass or more and 6.0 ppm by mass or less, 5.5 ppm by mass or less, 5.0 ppm by mass or less, 4.9 ppm by mass or less, 4.8 ppm by mass or less, 4.7 ppm by mass or less, 4.6 ppm by mass or less, 4.5 ppm by mass or less, 4.4 ppm by mass or less, 4.3 ppm by mass or less, 4.2 ppm by mass or less, 4.1 ppm by mass or less, 4.0 ppm by mass or less, 3.9 ppm by mass or less, 3.8 ppm by mass or less, 3.7 ppm by mass or less, 3.5 ppm by mass or less, 3.4 ppm by mass or less, 3.3 ppm by mass or less, 3.2 ppm by mass or less, 3.1 ppm by mass or less, or 3.0 ppm by mass or less, relative to the amount of the (meth)acrylic acid compound.The amount of protoanemonin in the prepared aqueous liquid may be 1.3 ppm by mass or more and 6.0 ppm by mass or less, 5.5 ppm by mass or less, 5.0 ppm by mass or less, 4.9 ppm by mass or less, 4.8 ppm by mass or less, 4.7 ppm by mass or less, 4.6 ppm by mass or less, 4.5 ppm by mass or less, 4.4 ppm by mass or less, 4.3 ppm by mass or less, 4.2 ppm by mass or less, 4.1 ppm by mass or less, 4.0 ppm by mass or less, 3.9 ppm by mass or less, 3.8 ppm by mass or less, 3.7 ppm by mass or less, 3.5 ppm by mass or less, 3.4 ppm by mass or less, 3.3 ppm by mass or less, 3.2 ppm by mass or less, 3.1 ppm by mass or less, or 3.0 ppm by mass or less, relative to the amount of the (meth)acrylic acid compound. The amount of protoanemonin in the prepared aqueous liquid may be 1.4 ppm by mass or more and 6.0 ppm by mass or less, 5.5 ppm by mass or less, 5.0 ppm by mass or less, 4.9 ppm by mass or less, 4.8 ppm by mass or less, 4.7 ppm by mass or less, 4.6 ppm by mass or less, 4.5 ppm by mass or less, 4.4 ppm by mass or less, 4.3 ppm by mass or less, 4.2 ppm by mass or less, 4.1 ppm by mass or less, 4.0 ppm by mass or less, 3.9 ppm by mass or less, 3.8 ppm by mass or less, 3.7 ppm by mass or less, 3.5 ppm by mass or less, 3.4 ppm by mass or less, 3.3 ppm by mass or less, 3.2 ppm by mass or less, 3.1 ppm by mass or less, or 3.0 ppm by mass or less, relative to the amount of the (meth)acrylic acid compound. The amount of protoanemonin in the prepared aqueous liquid may be 1.5 ppm by mass or more and 6.0 ppm by mass or less, 5.5 ppm by mass or less, 5.0 ppm by mass or less, 4.9 ppm by mass or less, 4.8 ppm by mass or less, 4.7 ppm by mass or less, 4.6 ppm by mass or less, 4.5 ppm by mass or less, 4.4 ppm by mass or less, 4.3 ppm by mass or less, 4.2 ppm by mass or less, 4.1 ppm by mass or less, 4.0 ppm by mass or less, 3.9 ppm by mass or less, 3.8 ppm by mass or less, 3.7 ppm by mass or less, 3.5 ppm by mass or less, 3.4 ppm by mass or less, 3.3 ppm by mass or less, 3.2 ppm by mass or less, 3.1 ppm by mass or less, or 3.0 ppm by mass or less, relative to the amount of the (meth)acrylic acid compound.The amount of protoanemonin in the prepared aqueous liquid may be 1.6 ppm by mass or more and 6.0 ppm by mass or less, 5.5 ppm by mass or less, 5.0 ppm by mass or less, 4.9 ppm by mass or less, 4.8 ppm by mass or less, 4.7 ppm by mass or less, 4.6 ppm by mass or less, 4.5 ppm by mass or less, 4.4 ppm by mass or less, 4.3 ppm by mass or less, 4.2 ppm by mass or less, 4.1 ppm by mass or less, 4.0 ppm by mass or less, 3.9 ppm by mass or less, 3.8 ppm by mass or less, 3.7 ppm by mass or less, 3.5 ppm by mass or less, 3.4 ppm by mass or less, 3.3 ppm by mass or less, 3.2 ppm by mass or less, 3.1 ppm by mass or less, or 3.0 ppm by mass or less, relative to the amount of the (meth)acrylic acid compound. The amount of protoanemonin in the prepared aqueous liquid may be 1.7 ppm by mass or more and 6.0 ppm by mass or less, 5.5 ppm by mass or less, 5.0 ppm by mass or less, 4.9 ppm by mass or less, 4.8 ppm by mass or less, 4.7 ppm by mass or less, 4.6 ppm by mass or less, 4.5 ppm by mass or less, 4.4 ppm by mass or less, 4.3 ppm by mass or less, 4.2 ppm by mass or less, 4.1 ppm by mass or less, 4.0 ppm by mass or less, 3.9 ppm by mass or less, 3.8 ppm by mass or less, 3.7 ppm by mass or less, 3.5 ppm by mass or less, 3.4 ppm by mass or less, 3.3 ppm by mass or less, 3.2 ppm by mass or less, 3.1 ppm by mass or less, or 3.0 ppm by mass or less, relative to the amount of the (meth)acrylic acid compound. The amount of protoanemonin in the prepared aqueous liquid may be 1.8 ppm by mass or more and 6.0 ppm by mass or less, 5.5 ppm by mass or less, 5.0 ppm by mass or less, 4.9 ppm by mass or less, 4.8 ppm by mass or less, 4.7 ppm by mass or less, 4.6 ppm by mass or less, 4.5 ppm by mass or less, 4.4 ppm by mass or less, 4.3 ppm by mass or less, 4.2 ppm by mass or less, 4.1 ppm by mass or less, 4.0 ppm by mass or less, 3.9 ppm by mass or less, 3.8 ppm by mass or less, 3.7 ppm by mass or less, 3.5 ppm by mass or less, 3.4 ppm by mass or less, 3.3 ppm by mass or less, 3.2 ppm by mass or less, 3.1 ppm by mass or less, or 3.0 ppm by mass or less, relative to the amount of the (meth)acrylic acid compound.The amount of protoanemonin in the prepared aqueous liquid may be 1.9 ppm by mass or more and 6.0 ppm by mass or less, 5.5 ppm by mass or less, 5.0 ppm by mass or less, 4.9 ppm by mass or less, 4.8 ppm by mass or less, 4.7 ppm by mass or less, 4.6 ppm by mass or less, 4.5 ppm by mass or less, 4.4 ppm by mass or less, 4.3 ppm by mass or less, 4.2 ppm by mass or less, 4.1 ppm by mass or less, 4.0 ppm by mass or less, 3.9 ppm by mass or less, 3.8 ppm by mass or less, 3.7 ppm by mass or less, 3.5 ppm by mass or less, 3.4 ppm by mass or less, 3.3 ppm by mass or less, 3.2 ppm by mass or less, 3.1 ppm by mass or less, or 3.0 ppm by mass or less, relative to the amount of the (meth)acrylic acid compound. The amount of protoanemonin in the prepared aqueous liquid may be 2.0 ppm by mass or more and 6.0 ppm by mass or less, 5.5 ppm by mass or less, 5.0 ppm by mass or less, 4.9 ppm by mass or less, 4.8 ppm by mass or less, 4.7 ppm by mass or less, 4.6 ppm by mass or less, 4.5 ppm by mass or less, 4.4 ppm by mass or less, 4.3 ppm by mass or less, 4.2 ppm by mass or less, 4.1 ppm by mass or less, 4.0 ppm by mass or less, 3.9 ppm by mass or less, 3.8 ppm by mass or less, 3.7 ppm by mass or less, 3.5 ppm by mass or less, 3.4 ppm by mass or less, 3.3 ppm by mass or less, 3.2 ppm by mass or less, 3.1 ppm by mass or less, or 3.0 ppm by mass or less, relative to the amount of the (meth)acrylic acid compound.

[0012] When the aqueous liquid contains a salt of (meth)acrylic acid, the amount of the (meth)acrylic acid compound relative to 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 by mass of the salt of (meth)acrylic acid. The amount of (meth)acrylic acid equivalent to the amount by mass of the salt of (meth)acrylic acid compound refers to the amount of (meth)acrylic acid that is the same as the amount by mass of the salt of (meth)acrylic acid. The amount of protoanemonin can be quantified, for example, by gas chromatography. For example, a sample of a (meth)acrylic acid raw material containing (meth)acrylic acid and trace components such as protoanemonin and used to prepare the aqueous liquid, or a sample of an aqueous liquid containing a (meth)acrylic acid compound and protoanemonin, can be analyzed by gas chromatography. The amount of protoanemonin in the (meth)acrylic acid raw material may be 6.0 mass ppm or less relative to the amount of (meth)acrylic acid.

[0013] The (meth)acrylic acid raw material and the aqueous liquid prepared therefrom may contain trace components such as furfural, benzaldehyde, and acrolein in addition to protoanemonin. The amounts of these trace components within appropriate ranges can also contribute to the stable production of water-absorbent resin particles having an appropriate particle size. The amounts of furfural, benzaldehyde, and acrolein can be quantified by gas chromatography.

[0014] The amount of furfural in the (meth)acrylic acid raw material or an aqueous liquid prepared therefrom may be 3.0 ppm by mass or less, 2.5 ppm by mass or less, or 2.0 ppm by mass or less, based on the amount of the (meth)acrylic acid compound, or may be 0 ppm by mass or more, or 0.1 ppm by mass or more, or may exceed 1.0 ppm by mass. The amount of furfural in the (meth)acrylic acid raw material or an aqueous liquid prepared therefrom may be 0 ppm by mass or more and 3.0 ppm by mass or less, 2.5 ppm by mass or less, or 2.0 ppm by mass or less, based on the amount of the (meth)acrylic acid compound. The amount of furfural in the (meth)acrylic acid raw material or an aqueous liquid prepared therefrom may be 0.1 ppm by mass or more and 3.0 ppm by mass or less, 2.5 ppm by mass or less, or 2.0 ppm by mass or less, based on the amount of the (meth)acrylic acid compound.

[0015] The amount of benzaldehyde in the (meth)acrylic acid raw material or an aqueous liquid prepared therefrom may be 3.0 ppm by mass or less, 2.5 ppm by mass or less, or 2.0 ppm by mass or less, based on the amount of the (meth)acrylic acid compound, and may be 0 ppm by mass or more, or 0.1 ppm by mass or more. The amount of benzaldehyde in the (meth)acrylic acid raw material or an aqueous liquid prepared therefrom may be 0 ppm by mass or more and 3.0 ppm by mass or less, 2.5 ppm by mass or less, or 2.0 ppm by mass or less, based on the amount of the (meth)acrylic acid compound. The amount of benzaldehyde in the (meth)acrylic acid raw material or an aqueous liquid prepared therefrom may be 0.1 ppm by mass or more and 3.0 ppm by mass or less, 2.5 ppm by mass or less, or 2.0 ppm by mass or less, based on the amount of the (meth)acrylic acid compound.

[0016] The amount of acrolein in the (meth)acrylic acid raw material or an aqueous liquid prepared therefrom may be 2.0 ppm by mass or less, 1.5 ppm by mass or less, or 1.0 ppm by mass or less, based on the amount of the (meth)acrylic acid compound, or may be 0 ppm by mass or more, or 0.1 ppm by mass or more. The amount of acrolein in the (meth)acrylic acid raw material or an aqueous liquid prepared therefrom may be 0 ppm by mass or more and 2.0 ppm by mass or less, 1.5 ppm by mass or less, or 1.0 ppm by mass or less, based on the amount of the (meth)acrylic acid compound. The amount of acrolein in the (meth)acrylic acid raw material or an aqueous liquid prepared therefrom may be 0.1 ppm by mass or more and 2.0 ppm by mass or less, 1.5 ppm by mass or less, or 1.0 ppm by mass or less, based on the amount of the (meth)acrylic acid compound.

[0017] The (meth)acrylic acid raw material and the aqueous liquid prepared therefrom may further contain a polymerization inhibitor. Examples of polymerization inhibitors include methoxyphenols such as p-methoxyphenol. Adjusting the amount of polymerization inhibitor can also contribute to controlling the particle size of the resulting water-absorbent resin particles. The amount of methoxyphenol in the (meth)acrylic acid raw material or the aqueous liquid prepared therefrom may be 300 mass ppm or less, 250 mass ppm or less, or 220 mass ppm or less, based on the amount of the (meth)acrylic acid compound, and may be 0 mass ppm or more, 100 mass ppm or more, 150 mass ppm or more, or 180 mass ppm or more. The amount of methoxyphenol in the (meth)acrylic acid raw material or the aqueous liquid prepared therefrom may be 0 mass ppm or more and 300 mass ppm or less, 250 mass ppm or less, or 220 mass ppm or less, based on the amount of the (meth)acrylic acid compound. The amount of methoxyphenol in the (meth)acrylic acid raw material or an aqueous liquid prepared therefrom may be 100 ppm by mass or more and 300 ppm by mass or less, 250 ppm by mass or less, or 220 ppm by mass or less, based on the amount of the (meth)acrylic acid compound. The amount of methoxyphenol in the (meth)acrylic acid raw material or an aqueous liquid prepared therefrom may be 150 ppm by mass or more and 300 ppm by mass or less, 250 ppm by mass or less, or 220 ppm by mass or less, based on the amount of the (meth)acrylic acid compound. The amount of methoxyphenol in the (meth)acrylic acid raw material or an aqueous liquid prepared therefrom may be 180 ppm by mass or more and 300 ppm by mass or less, 250 ppm by mass or less, or 220 ppm by mass or less, based on the amount of the (meth)acrylic acid compound. The amount of methoxyphenol can be quantified by liquid chromatography.

[0018] The aqueous liquid may contain (meth)acrylic acid and a salt of (meth)acrylic acid as the (meth)acrylic acid compound. Based on the total amount of the (meth)acrylic acid compounds, the proportion of the salt of (meth)acrylic acid may be, for example, 50 mol % or more and 100 mol % or less. The salt of (meth)acrylic acid may be, for example, an alkali metal salt of (meth)acrylic acid. The alkali metal salt of (meth)acrylic acid may be a sodium salt.

[0019] The aqueous liquid containing (meth)acrylic acid and its salts can be prepared, for example, by a method including 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 a basic compound. The basic compound may be an alkali metal hydroxide (e.g., sodium hydroxide). Adjusting the iron content in the alkaline aqueous solution for neutralizing (meth)acrylic acid can also contribute to the stable production of water-absorbent resin particles having an appropriate particle size. When the iron content in the alkaline aqueous solution for neutralizing (meth)acrylic acid is less than the amount of Fe relative to the amount of the basic compound, ２ O ３ In terms of iron content, the iron content may 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. ２ O ３ Conversion is the process of converting the amount of iron atoms into the amount of Fe equivalent to that amount of substance. ２ O ３ This means converting it into the amount of

[0020] The monomers in the aqueous liquid may further include a water-soluble ethylenically unsaturated monomer other than a (meth)acrylic acid compound. The proportion of the (meth)acrylic acid compound 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, or may be substantially 100 mol%. The water-soluble ethylenically unsaturated monomer other than a (meth)acrylic acid compound may include, for example, at least one selected from the group consisting of 2-(meth)acrylamido-2-methylpropanesulfonic acid and its alkali salts, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, polyethylene glycol mono(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide.

[0021] 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 (particularly, a (meth)acrylic acid compound). The reactive functional groups may be, for example, a (meth)acryloyl group, a vinyl group, an epoxy group, a halogeno group in a haloepoxy compound, an isocyanate group, or a combination thereof.

[0022] Examples of internal crosslinking agents having two or more (meth)acryloyl groups include (meth)acrylic acid ester compounds formed from a polyol compound and (meth)acrylic acid, unsaturated polyesters formed from a polyol compound and an unsaturated carboxylic acid (maleic acid, fumaric acid, etc.), bis(meth)acrylamide compounds (N,N'-methylenebis(meth)acrylamide, etc.), (meth)acrylic acid ester compounds formed from a polyepoxide compound and (meth)acrylic acid, and (meth)acrylic acid carbamyl ester compounds formed from a polyisocyanate compound (tolylene diisocyanate, hexamethylene diisocyanate, etc.) and hydroxyethyl (meth)acrylate. The polyol compound for forming the (meth)acrylic acid ester compound or the unsaturated polyester may be, for example, ethylene glycol, propylene glycol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, polyglycerin, or a combination thereof.

[0023] The vinyl group as a reactive functional group may be a part of an allyl group. Examples of internal crosslinking agents having two or more vinyl groups (or allyl groups) include allylated starch, allylated cellulose, diallyl phthalate, N,N',N''-triallyl isocyanurate, and divinylbenzene.

[0024] Examples of the internal crosslinking agent having two or more epoxy groups include (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether.

[0025] Examples of internal crosslinking agents having two or more isocyanate groups include 2,4-tolylene diisocyanate and hexamethylene diisocyanate.

[0026] The amount of the internal crosslinking agent may be 0 mmol or more and 0.5 mmol or less, 0 mmol or more and 0.2 mmol or less, 0 mmol or more and 0.1 mmol or less, 0 mmol or more and 0.05 mmol or less, or 0 mmol or more and 0.02 mmol or less, or may be substantially 0 mmol, relative to 1 mol of the monomer.

[0027] The aqueous liquid may contain, as other components, a thickener, a hydrophilic polymer dispersant, a radical polymerization initiator, a chain transfer agent, a foaming agent, or a combination thereof.

[0028] Examples of thickeners include hydroxyalkyl celluloses such as hydroxyethyl cellulose (HEC) and hydroxypropyl cellulose (HPC), hydroxyalkyl alkyl 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. The thickeners may be used alone or in combination of two or more.

[0029] The amount of the thickener may be 0.05 parts by mass or more and 20 parts by mass or less, 0.2 parts by mass or more and 10 parts by mass or less, or 0.4 parts by mass or more and 5 parts by mass or less, relative to 100 parts by mass of the monomer.

[0030] Examples of hydrophilic polymer dispersants include polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), polypropylene glycol, polyethylene glycol-polypropylene glycol block copolymer, polyglycerin, polyoxyethylene glycerin, polyoxypropylene glycerin, polyoxyethylene-polyoxypropylene glycerin copolymer, and polyoxyethylene sorbitan fatty acid ester. The hydrophilic polymer dispersants may be used alone or in combination of two or more.

[0031] The amount of the hydrophilic polymer dispersant may be 0.001 parts by mass or more and 10 parts by mass or less, 0.005 parts by mass or more and 5 parts by mass or less, 0.01 parts by mass or more and 3 parts by mass or less, or 0.01 parts by mass or more and 1.5 parts by mass or less, relative to 100 parts by mass of the monomer.

[0032] The radical polymerization initiator may include, for example, an azo-based compound, a peroxide, or a combination thereof.

[0033] Examples of the azo compounds include 2,2'-azobis[2-(N-phenylamidino)propane]dihydrochloride, 2,2'-azobis{2-[N-(4-chlorophenyl)amidino]propane}dihydrochloride, 2,2'-azobis{2-[N-(4-hydroxyphenyl)amidino]propane}dihydrochloride, 2,2'-azobis[2-(N-benzylamidino)propane]dihydrochloride, 2,2' -Azobis[2-(N-allylamidino)propane]dihydrochloride, 2,2'-azobis(2-amidinopropane)dihydrochloride, 2,2'-azobis{2-[N-(2-hydroxyethyl)amidino]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-diazepin-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-imidazolin-2-yl]propane] dihydrochloride 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate, and 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)propionamide].

[0034] 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-t-butyl peroxide, t-butyl cumyl peroxide, t-butyl peroxyacetate, t-butyl peroxyisobutyrate, and t-butyl peroxypivalate; and hydrogen peroxide.

[0035] The amount of the radical polymerization initiator may be, for example, 0.0001 mol or more and 1 mol or less, 0.001 mol or more and 0.1 mol or less, 0.003 mol or more and 0.08 mol or less, or 0.007 mol or more and 0.05 mol or less, relative to 100 mol of the monomer.

[0036] Examples of chain transfer agents include hypophosphites, thiols, thiolic acids, secondary alcohols, and amines.

[0037] Examples of the blowing agent include inorganic blowing agents such as ammonium carbonate, sodium bicarbonate, and ammonium bicarbonate; nitroso compounds such as dinitrosopentamethylenetetramine; azo compounds such as azodicarbonamide and azobisisobutyronitrile; and organic blowing agents such as sulfonylhydrazide compounds such as 4,4'-oxybisbenzenesulfonylhydrazide and p-toluenesulfonylhydrazide.

[0038] A reaction solution for reversed-phase suspension polymerization can be prepared by stirring a mixture containing the prepared aqueous solution and an oily solution, which is a hydrophobic liquid primarily composed of a hydrophobic dispersion medium. The reaction solution contains particulate aqueous solution dispersed in the oily solution. The polymerization reaction in the reaction solution forms a particulate hydrogel polymer containing a monomer polymer and water.

[0039] The reaction solution may contain a surfactant. The surfactant may be contained in the oily liquid. The surfactant may be a nonionic surfactant or an anionic surfactant. Examples of nonionic surfactants include sorbitan fatty acid esters, polyglycerin fatty acid esters, sucrose fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylaryl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, and polyethylene glycol fatty acid esters. Examples of anionic surfactants include fatty acid salts, alkylbenzenesulfonates, alkylmethyltaurates, polyoxyethylene alkylphenyl ether sulfates, polyoxyethylene alkyl ether sulfonates, polyoxyethylene alkyl ether phosphates, and polyoxyethylene alkylallyl ether phosphates. The surfactant may be used alone or in combination of two or more.

[0040] The oily liquid in the reaction mixture is a hydrophobic liquid mainly composed of a hydrophobic dispersion medium. The dispersion medium may be a hydrocarbon dispersion medium. The oily liquid may further contain optional additives such as a hydrophobic polymer dispersant.

[0041] 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 media may be used alone or in combination of two or more.

[0042] The amount of the dispersion medium contained in the oily liquid may be 30 parts by mass or more and 1,000 parts by mass or less, 50 parts by mass or more and 650 parts by mass or less, 70 parts by mass or more and 550 parts by mass or less, or 100 parts by mass or more and 450 parts by mass or less, relative to 100 parts by mass of the monomer.

[0043] Examples of hydrophobic polymer 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 ethylhydroxyethyl cellulose. The hydrophobic polymer dispersants may be used alone or in combination of two or more.

[0044] The amount of the hydrophobic polymer dispersant may be 0.05 parts by mass or more and 10 parts by mass or less, 0.08 parts by mass or more and 5 parts by mass or less, or 0.1 parts by mass or more and 3 parts by mass or less, relative to 100 parts by mass of the monomer.

[0045] For the polymerization reaction, the reaction solution may be heated. The heating temperature for the polymerization reaction may be, for example, 40° C. or higher and 90° C. or lower. The polymerization reaction time may be, for example, 30 minutes or higher and 240 minutes or lower.

[0046] During the polymerization reaction, the reaction solution is usually stirred. When the stirring speed is low, the proportion of hydrogel polymers with large particle sizes tends to increase. However, by appropriately adjusting the amount of protoanemonin, the particle sizes of the particulate hydrogel polymers and the water-absorbent resin particles obtained therefrom can be easily controlled within an appropriate range, even at a relatively low rotation speed. The stirring speed during the polymerization reaction may be, for example, 200 rpm or more and 1000 rpm or less.

[0047] Water-absorbent resin particles containing a polymer are extracted from a reaction liquid (slurry) containing a particulate hydrogel polymer. Extracting the water-absorbent resin particles from the reaction liquid includes, for example, forming a concentrate by extracting a portion of the water from the reaction liquid by azeotropic distillation of the dispersion medium and water, and evaporating the dispersion medium, water, and trace components (protoanemonin, furfural, benzaldehyde, and acrolein) from the concentrate. After removing the dispersion medium and the like by evaporation, a powder of water-absorbent resin particles containing dried polymer particles can be obtained. Some water may remain in the dried polymer particles (water-absorbent resin particles).

[0048] After the water is removed, the polymer particles may be surface-crosslinked in a mixture containing a concentrate containing polymer particles (water-absorbent resin particles) containing a polymer and a surface-crosslinking agent.

[0049] The surface cross-linking agent may be a compound having two or more reactive functional groups, and examples thereof include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; epichlorohydrin, epibromohydrin, haloepoxy compounds such as 2,4-tolylene diisocyanate and α-methylepichlorohydrin; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; oxetane compounds such as 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. The surface cross-linking agent may include a polyglycidyl compound such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, polyglycerol polyglycidyl ether, etc. These surface cross-linking agents may be used alone or in combination of two or more.

[0050] The amount of the surface cross-linking agent may be 0.01 mmol or more and 10 mmol or less, 0.03 mmol or more and 3 mmol or less, or 0.05 mmol or more and 1 mmol or less, per mole of the monomer unit constituting the polymer in the polymer particles.

[0051] For the surface cross-linking, the mixture may be heated. The heating temperature may be, for example, 60° C. or higher and 200° C. or lower, or 80° C. or higher and 150° C. or lower. The reaction time for the surface cross-linking reaction may be, for example, 1 minute or higher and 300 minutes or lower, or 5 minutes or higher and 200 minutes or lower.

[0052] The dispersion medium can be removed from the concentrate to obtain dried polymer particles (water-absorbent resin particles), and for this purpose the concentrate may be heated.

[0053] Various additives may be further added to the dried polymer particles, examples of which include lubricants, metal chelating agents, surface modifiers, heat stabilizers, antioxidants, and antibacterial agents.

[0054] The lubricant may be, for example, amorphous silica particles. Examples of metal chelating agents include ethylenediaminetetraacetic acid and its salts (such as disodium ethylenediaminetetraacetic acid) and diethylenetriaminepentaacetic acid and its salts (such as pentasodium diethylenetriaminepentaacetic acid). Examples of surface modifiers include polyvalent metal compounds such as aluminum sulfate, potassium alum, ammonium alum, sodium alum, (poly)aluminum chloride, and hydrates thereof; and polycation compounds such as polyethyleneimine, polyvinylamine, and polyallylamine.

[0055] The additive may be attached to the surface of the polymer particles or may penetrate into the interior of the polymer particles. The additive (e.g., a lubricant) can be added to the polymer particles after drying to attach the additive to the surface of the polymer particles. The additive may be added to the liquid for polymerization or to the concentrate after water extraction. In this specification, water-absorbent resin particles mean particles containing polymer particles. The water-absorbent resin particles may contain polymer particles and an additive.

[0056] The amount of the additive (e.g., lubricant) may be, for example, 0.001 to 10 parts by mass, 0.01 to 5 parts by mass, or 0.1 to 2 parts by mass relative to 100 parts by mass of the polymer particles.

[0057] In the dried powder of polymer particles (water-absorbent resin particles), the ratio of particles having a particle diameter of 850 μm or more may be less than 20% by mass, 15% by mass or less, 10% by mass or less, or 5% by mass or less, based on the total amount of water-absorbent resin particles. The ratio here means the ratio after drying and before the powder is classified by sieving or the like. The method for determining the ratio of particles having a particle diameter of 850 μm or more is to calculate the mass W of the water-absorbent resin particles. ０ g of powder is sieved through a sieve with an opening of 850 μm, and the mass W of the particles remaining on the sieve １ g and the ratio of particles having a particle diameter of 850 μm or more = (W １ / W ０ ) × 100 to calculate the proportion of particles having a particle diameter of 850 μm or more. Particles having a particle diameter of 850 μm or more may be excluded from the product by classification. A small proportion of particles having a particle diameter of 850 μm or more in the powder after polymerization and before classification is advantageous in terms of effective utilization of raw materials for producing (meth)acrylic acid, etc.

[0058] The present invention is not limited to the following examples. 1. Acrylic Acid Raw Material Acrylic acid raw materials (1) to (5) containing acrylic acid as a main component were prepared. Table 1 shows the pure content (acrylic acid concentration) of each acrylic acid raw material and the contents of trace components, protoanemonin, furfural, benzaldehyde, acrolein, and p-methoxyphenol. The contents of trace components shown in the table are percentages (ppm by mass) based on the mass of acrylic acid.

[0059]

[0060] 2. Production of Water-Absorbent Resin Particles Example 1a A round-bottomed, cylindrical, separable 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 gas inlet tube, and a stirrer. A stirring blade with two stages of four inclined paddle blades with a blade diameter of 5 cm was attached to the stirrer. 313 g of n-heptane as a hydrocarbon dispersion medium and 0.920 g of maleic anhydride-modified ethylene-propylene copolymer (Hiwax 1105A, Mitsui Chemicals, Inc.) as a polymer dispersant were placed in the prepared separable flask. The mixture in the separable flask was heated to 80°C while stirring to form an n-heptane solution containing maleic anhydride-modified ethylene-propylene copolymer. After stirring was completed, the n-heptane solution was cooled to 56°C.

[0061] 73.60 g (1.02 mol) of acrylic acid raw material (1) was placed in a 300 mL beaker. While the beaker was cooled with ice water, 102.1 g of a 30% by weight aqueous sodium hydroxide 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 radical polymerization agent, and 0.0083 g (0.048 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved in the beaker to prepare an aqueous solution containing acrylic acid and sodium acrylate.

[0062] The prepared aqueous solution was added to the n-heptane solution containing the maleic anhydride-modified ethylene-propylene copolymer in a separable flask. The mixture in the separable flask was stirred for 10 minutes. Next, a surfactant solution containing 0.920 g of sucrose stearate (Ryoto Sugar Ester S-370, HLB: 3, Mitsubishi Chemical Foods Corporation) and 8.28 g of n-heptane was added to the mixture. The reaction solution formed in the separable flask was stirred with a stirrer at 450 rpm while the atmosphere in the separable flask was thoroughly purged with nitrogen. After the nitrogen purge, the separable flask was immersed in a 70°C water bath to raise the temperature of the reaction solution and initiate the polymerization reaction. The polymerization reaction was allowed to proceed for 60 minutes with continued stirring, resulting in the formation of a slurry containing a particulate hydrogel polymer.

[0063] The separable flask containing the slurry was immersed in an oil bath at 125°C. While refluxing the n-heptane contained in the slurry in the separable flask, 126.8 g of water contained in the slurry was extracted to the outside of the separable flask by azeotropic distillation of n-heptane and water. Next, the separable flask was heated in an oil bath at 125°C to evaporate the n-heptane and the like, thereby obtaining 95.3 g of dried water absorbent resin particles.

[0064] Examples 2a, 2b, 3a, 4a and Comparative Example 1a Water-absorbent resin particles were obtained in the same procedure as in Example 1a, except that the acrylic acid raw material shown in Table 2 was used and the rotation speed of the stirrer during the polymerization reaction was set as shown in Table 2.

[0065] 3. Evaluation of Water-Absorbent Resin Particles The powder of water-absorbent resin particles obtained in Examples or Comparative Examples was sieved with a sieve having an opening of 850 μm. The mass of water-absorbent resin particles remaining on the sieve and having a particle diameter of 850 μm or more was measured. The proportion (mass%) of particles having a particle diameter of 850 μm or more to the total amount of water-absorbent resin particles was calculated.

[0066]

[0067] 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 resulting water-absorbent resin particles could be controlled so as to reduce the proportion of particles with a particle size of 850 μm or more.

Claims

1. A method for producing water-absorbent resin particles, comprising: preparing an aqueous liquid containing a monomer containing at least one (meth)acrylic acid compound of (meth)acrylic acid or a salt thereof, protoanemonin, and water; forming a particulate hydrogel polymer containing a polymer of the monomer and water by reverse phase suspension polymerization in a reaction liquid containing the aqueous liquid and a dispersion medium; and extracting water-absorbent resin particles containing the polymer from the reaction liquid, wherein 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, and when the aqueous liquid contains a 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 substance amount of the salt.

2. The method according to claim 1, wherein the aqueous liquid is prepared containing the protoanemonin in an amount of more than 1.0 ppm by mass and not more 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 further contains a polymerization inhibitor.

4. The method according to claim 1, wherein the proportion of particles having a particle diameter of 850 μm or more in the water-absorbent resin particles is less than 20 mass % based on the total amount of the water-absorbent resin particles.

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

6. The method according to claim 5, wherein the particle size of the water-absorbent resin particles is controlled so that the ratio of particles having a particle size of 850 μm or more in the water-absorbent resin particles is less than 20 mass % relative to the total amount of the water-absorbent resin particles.

7. The method according to claim 5 or 6, wherein the particle size of the water-absorbent resin particles is controlled by adjusting the amount of protoanemonin relative to the amount of the (meth)acrylic acid compound in the prepared aqueous liquid to 6.0 mass ppm or less, and when the aqueous liquid contains a salt of (meth)acrylic acid, the amount of the (meth)acrylic acid compound is the total amount of the amount of the (meth)acrylic acid and the amount of (meth)acrylic acid equivalent to the substance amount of the salt.

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