Method for producing water-absorbing resin particles

Abstract

Provided is a production method by which the amount of a heating medium to be used in a process for producing water-absorbing resin particles can be reduced and the water-absorbing resin particles can be efficiently produced.　This method for producing water-absorbing resin particles comprises a polymerization step, a concentration step, and a drying step in this order. In the polymerization step, a reaction liquid comprising a polymer of a water-soluble, ethylenically unsaturated monomer, water, and a dispersion medium is produced by reversed-phase suspension polymerization. In the concentration step, water is removed by azeotropic distillation while the dispersion medium of the reaction liquid is being refluxed to the reaction liquid, thereby reducing the water content of the reaction liquid to produce a concentrate. In the drying step, the concentrate is dried while being exposed to air, thereby producing a dried object. The content of residual water in the concentrate to be dried in the drying step is 72 mass% or greater with respect to the total mass of the polymer contained in the concentrate.

Description

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[0001] The present invention relates to a method for producing water-absorbing resin particles.

[0002] Since water-absorbing resin particles have excellent water absorption ability, they are widely used in fields such as sanitary materials such as paper diapers, sanitary napkins, and incontinence pads. In addition, since water-absorbing resin particles exhibit a water-stopping effect, they are also used as water-stopping materials for communication cables such as optical cables and power cables.

[0003] Water-absorbing resin particles are typically known as crosslinked polymers of partially neutralized salts of acrylic acid. For example, a method for producing water-absorbing resin particles by the inverse phase suspension polymerization method using a monomer containing acrylic acid is generally known. For example, Patent Document 1 discloses a method for producing water-absorbing resin particles using a device equipped with a polymerization vessel, a concentrator, a dryer, and a heat exchanger. In the production method using such a device, the reaction solution obtained by the inverse phase suspension polymerization method is concentrated in a concentrator to prepare a concentrated solution, and the obtained concentrated solution is dried in a dryer, whereby the target water-absorbing resin particles are obtained.

[0004] International Publication No. 2021 / 200031

[0005] In recent years, in addition to producing water-absorbing resin particles at a lower cost, further improvement in the efficiency of the production method of water-absorbing resin particles has been strongly demanded from the viewpoint of reducing environmental impact through energy saving. In this regard, in the production method of water-absorbing resin particles using the above-described device, a large amount of heat medium (for example, steam) is used for the concentration treatment and the drying treatment. Therefore, it can be said that reducing the amount of such heat medium used is extremely effective for the efficient production of water-absorbing resin particles.

[0006] The present invention has been made in view of the above, and an object thereof is to provide a production method capable of reducing the amount of heat medium used in the production process of water-absorbing resin particles and efficiently producing water-absorbing resin particles.

[0007] As a result of diligent research to achieve the above objective, the inventors of the present invention have discovered that the above objective can be achieved by adjusting the residual water content of the concentrate subjected to the drying step to a predetermined range in a method for producing water-absorbent resin particles, which includes a polymerization step, a concentration step, and a drying step, and have completed the present invention.

[0008] In other words, the present invention encompasses, for example, the subject matter described in the following sections. Section 1 A method for producing superabsorbent polymer particles comprising a polymerization step, a concentration step, and a drying step in this order, wherein in the polymerization step, a reaction solution containing a polymer of water-soluble ethylenically unsaturated monomers, water, and a dispersion medium is prepared by reverse-phase suspension polymerization; in the concentration step, a concentrate is prepared by reducing the amount of water in the reaction solution by reducing the amount of water in the reaction solution by azeotropic distillation while refluxing the dispersion medium from the reaction solution back into the reaction solution and removing the water; in the drying step, a dried product is prepared by drying the concentrate while exposing it to air; and the residual water content of the concentrate dried in the drying step is 72% by mass or more based on the total mass of the polymer in the concentrate. Section 2 The method for producing superabsorbent polymer particles according to Section 1, wherein the drying step includes step A of adding an inorganic reducing agent to the concentrate. Section 3 The method for producing superabsorbent polymer particles according to Section 2, wherein in step A, the addition of the inorganic reducing agent is performed so that the residual water content in the concentrate is 70% by mass or less based on the total mass of the polymer. Item 4 The method for producing water-absorbing resin particles according to item 2 or 3, wherein the drying step further comprises step B of adding a surface crosslinking agent to the concentrate after step A, and the residual water content in the concentrate when the inorganic reducing agent is added is X by mass %, and the residual water content in the concentrate when the surface crosslinking agent is added is Y by mass %, and the following formula (1) 10 ≤ X - Y ≤ 70 (1) is satisfied. Item 5 The method for producing water-absorbing resin particles according to any one of items 2 to 4, wherein the addition of the inorganic reducing agent in step A is performed when the residual water content in the concentrate is 30% by mass or more.

[0009] According to the method for producing water-absorbent resin particles of the present invention, the amount of heat transfer medium used in the production process of water-absorbent resin particles can be reduced, and water-absorbent resin particles can be produced efficiently.

[0010] Embodiments of the present invention will be described in detail below. In this specification, the expressions "containing" and "including" include the concepts of "containing," "including," "substantially consisting of," and "consisting only of."

[0011] In the numerical ranges described stepwise in this specification, the upper or lower limit of a numerical range in one step can be arbitrarily combined with the upper or lower limit of a numerical range in another step. In the numerical ranges described in this specification, the upper or lower limit of that numerical range may be replaced with the value shown in the example or a value that can be uniquely derived from the example. Also, in this specification, numbers connected by "~" mean a numerical range that includes the numbers before and after "~" as the lower and upper limits. The concentration and amount of each component described in this specification are based on the total amount of multiple substances if there are multiple substances corresponding to each component, unless otherwise specified. "Physiological saline" means a 0.9% by mass sodium chloride aqueous solution.

[0012] The present invention provides a method for producing water-absorbent resin particles, comprising a polymerization step, a concentration step, and a drying step in that order.

[0013] In the polymerization step, a reaction solution containing a water-soluble ethylenically unsaturated monomer polymer, water, and a dispersion medium is prepared by reverse-phase suspension polymerization. In the concentration step, a concentrate is prepared by reducing the water content in the reaction solution by azeotropic distillation while refluxing the dispersion medium back into the reaction solution and removing the water. In the drying step, a dried product is prepared by drying the concentrate while exposing it to air. The residual water content of the concentrate dried in the drying step is 72% by mass or more, based on the total mass of the polymer in the concentrate.

[0014] According to the present invention's method for producing superabsorbent resin particles, the amount of heat transfer medium used in the manufacturing process of superabsorbent resin particles can be reduced, and superabsorbent resin particles can be produced efficiently. In other words, according to the present invention's method for producing superabsorbent resin particles, it is possible to produce superabsorbent resin particles at a lower cost and with a smaller environmental impact than conventional methods.

[0015] (Polymerization Process) The polymerization process is a process for preparing a reaction solution containing a water-soluble ethylenically unsaturated monomer polymer, water, and a dispersion medium by reverse-phase suspension polymerization. Specifically, the water-soluble ethylenically unsaturated monomer polymer referred to here is polymer particles.

[0016] The method for preparing the reaction solution by reverse-phase suspension polymerization is not particularly limited, and known reverse-phase suspension polymerization methods used to produce water-absorbent resin particles can be widely employed in the present invention. For example, a reaction solution containing the polymer, water, and dispersion medium can be prepared by suspending an aqueous monomer aqueous solution containing a water-soluble ethylenically unsaturated monomer in a dispersion medium and carrying out a polymerization reaction.

[0017] As water-soluble ethylenically unsaturated monomers, a wide range of known monomers that can be used to synthesize general absorbent resin particles can be applied. Examples of water-soluble ethylenically unsaturated monomers include (meth)acrylic acid (hereinafter, "acry" and "methacry" are collectively referred to as "(meth)acry"; the same applies hereinafter) and its salts; 2-(meth)acrylamido-2-methylpropanesulfonic acid and its salts; nonionic monomers such as (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, polyethylene glycol mono(meth)acrylate; amino group-containing unsaturated monomers such as N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, diethylaminopropyl(meth)acrylamide, and their quaternary derivatives. These water-soluble ethylenically unsaturated monomers may be used individually or in combination (copolymerized) of two or more types. Among these, (meth)acrylic acid and its salts, (meth)acrylamide, and N,N-dimethylacrylamide are preferred, and (meth)acrylic acid and its salts are more preferred, due to their ease of industrial availability.

[0018] When acrylic acid and its salts are used as water-soluble ethylenically unsaturated monomers, the acrylic acid and its salts may be used as the main water-soluble ethylenically unsaturated monomers, and it is preferable that they be used in an amount of 70 to 100 mol% relative to the total number of moles of water-soluble ethylenically unsaturated monomers.

[0019] The above-mentioned water-soluble ethylenically unsaturated monomer may be used as an aqueous solution to increase the dispersion efficiency in the dispersion medium (particularly the hydrocarbon dispersion medium described later) when reverse-phase suspension polymerization is carried out. The concentration of the above-mentioned monomer in such an aqueous solution may be 20% by mass or more and below the saturation concentration, 25 to 90% by mass and below the saturation concentration, 30 to 75% by mass and below the saturation concentration, or 30 to 60% by mass and below the saturation concentration.

[0020] When the water-soluble ethylenically unsaturated monomer has an acidic group, such as (meth)acrylic acid or 2-(meth)acrylamide-2-methylpropanesulfonic acid, it is possible to use a solution in which the acidic group has been neutralized in advance with an alkaline neutralizing agent. Examples of such alkaline neutralizing agents include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, and potassium carbonate; and ammonia. In particular, these alkaline neutralizing agents may be used in aqueous solution form to simplify the neutralization operation. The above-mentioned alkaline neutralizing agents may be used individually or in combination of two or more. When neutralizing with an alkaline neutralizing agent, the degree of neutralization for all acidic groups of the water-soluble ethylenically unsaturated monomer may be adjusted to 50 to 100 mol%.

[0021] As the dispersion medium used in reverse-phase suspension polymerization, for example, the dispersion medium used in conventional reverse-phase suspension polymerization can be widely adopted in the present invention. Hydrocarbon dispersion media can be used as the dispersion medium. Examples of hydrocarbon dispersion media include aliphatic hydrocarbons such as n-hexane, n-heptane, n-octane, and ligroin; alicyclic hydrocarbons such as cyclopentane, methylcyclopentane, cyclohexane, and methylcyclohexane; and aromatic hydrocarbons such as benzene, toluene, and xylene. Among these dispersion media, n-hexane, n-heptane, and cyclohexane are preferably used because they are readily available industrially, have stable quality, and are inexpensive. These dispersion media may be used individually or in combination of two or more. As dispersion media, for example, Exxsol Heptane (manufactured by ExxonMobil: heptane and its isomer hydrocarbons) and Nappar 6 (manufactured by ExxonMobil: cyclohexane and its isomer hydrocarbons), which are known as mixed solvents, can be used.

[0022] Inverse-phase suspension polymerization, as is well known, may be a multi-step polymerization in which monomers are polymerized in multiple stages, for example, a two-step polymerization. In multi-step polymerization, the first polymerization reaction is called the first stage, and thereafter, monomers added stepwise after the first stage polymerization are sequentially referred to as the second stage, third stage, and so on.

[0023] In reverse-phase suspension polymerization, a dispersion stabilizer can be added to the dispersion medium as needed. Examples of dispersion stabilizers include polymeric dispersants and / or surfactants.

[0024] Examples of 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, ethylcellulose, and ethyl hydroxyethylcellulose. In particular, from the viewpoint of monomer dispersion stability, maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, and oxidized ethylene-propylene copolymer are preferred. These polymer-based dispersants may be used alone or in combination of two or more types.

[0025] When using a polymeric dispersant in reverse-phase suspension polymerization, the amount of polymeric dispersant used may be 0.1 to 30 parts by mass, preferably 0.3 to 20 parts by mass, more preferably 0.3 to 10 parts by mass, and even more preferably 0.4 to 5% by mass, per 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer, in order to maintain a good dispersion state of the water-soluble ethylenically unsaturated monomer in the dispersion medium and to obtain a dispersion effect commensurate with the amount used.

[0026] Examples of surfactants that can be used include sucrose fatty acid esters, polyglycerin fatty acid esters, sorbitan 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, alkylallylformaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkylgluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, and phosphate esters of polyoxyethylene alkylallyl ethers. Among these, sorbitan fatty acid esters, polyglycerin fatty acid esters, and sucrose fatty acid esters are preferred in terms of monomer dispersion stability. These surfactants may be used individually or in combination of two or more.

[0027] The amount of surfactant used is 0.1 to 30 parts by mass, preferably 0.3 to 20 parts by mass, more preferably 0.3 to 10 parts by mass, and even more preferably 0.4 to 5% by mass, per 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer, in order to maintain a good dispersion state of the water-soluble ethylenically unsaturated monomer in the hydrocarbon dispersion medium and to obtain a dispersion effect commensurate with the amount used.

[0028] In reverse-phase suspension polymerization, a thickener can be included in the monomer aqueous solution and / or dispersion medium as needed. Examples of thickeners that can be used include hydroxyethylcellulose, hydroxypropylcellulose, methylcellulose, carboxymethylcellulose, polyacrylic acid, partially neutralized polyacrylic acid, polyethylene glycol, polyacrylamide, polyethyleneimine, dextrin, sodium alginate, polyvinyl alcohol, polyvinylpyrrolidone, polyethylene oxide, and the like.

[0029] In reverse-phase suspension polymerization, known polymerization initiators, particularly radical polymerization initiators, can be widely used. Examples of radical polymerization initiators include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; peroxides such as methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butylcumyl peroxide, and hydrogen peroxide; and azo compounds such as 2,2'-azobis(2-methylpropionamidine) dihydrochloride, 2,2'-azobis[2-(N-phenylamidino)propane] dihydrochloride, 2,2'-azobis[2-(N-allylamidino)propane] dihydrochloride, and 4,4'-azobis(4-cyanovaleric acid). Radical polymerization initiators can also be used in combination with reducing agents such as sodium sulfite, sodium bisulfite, ferrous sulfate, and L-ascorbic acid to form redox polymerization initiators.

[0030] From the viewpoint of polymerization stability, the lower limit of the amount of polymerization initiator used in reverse-phase suspension polymerization may be 0.01 mmol or more, preferably 0.05 mmol or more, and more preferably 0.10 mmol or more, per mole of water-soluble ethylenically unsaturated monomer used in each stage. Furthermore, from the viewpoint of polymerization stability, the upper limit of the polymerization initiator may be 20 mmol or less, preferably 10 mmol or less, and more preferably 5 mmol or less, per mole of water-soluble ethylenically unsaturated monomer used in each stage.

[0031] In reverse-phase suspension polymerization, chain transfer agents can be used as needed. Examples of chain transfer agents include hypophosphates, thiols, secondary alcohols, and amines.

[0032] In reverse-phase suspension polymerization, an internal crosslinking agent may be used. This may result in polymer particles having a structure in which the interior is crosslinked by the internal crosslinking agent. In this specification, the crosslinking agent used for crosslinking the interior of a polymer is referred to as an internal crosslinking agent to distinguish it from the surface crosslinking agent (also called a post-crosslinking agent) described later.

[0033] Examples of internal crosslinking agents include compounds or precursors thereof having two or more functional groups that react with the functional groups of a water-soluble ethylenically unsaturated monomer, or compounds having two or more polymerizable unsaturated groups. Specific examples include carbonate compounds such as ethylene carbonate; glycidyl group-containing compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether; and (poly)ethylene glycol, (poly)propylene glycol, (poly)glycerin, pentaerythritol, ethylenediamine, polyethyleneimine, glycidyl (meth)acrylate, etc. Other internal crosslinking agents include trialyl compounds; sucrose allyl ethers; di or tri(meth)acrylic acid esters of polyols such as (poly)ethylene glycol, (poly)propylene glycol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and (poly)glycerin; unsaturated polyesters obtained by reacting the above polyols with unsaturated acids such as maleic acid and fumaric acid; bisacrylamides such as N,N'-methylenebis(meth)acrylamide; di or tri(meth)acrylic acid esters obtained by reacting polyepoxides with (meth)acrylic acid; di(meth)acrylic acid carbamyl esters obtained by reacting polyisocyanates such as tolylene diisocyanate and hexamethylene diisocyanate with hydroxyethyl (meth)acrylate; allyl starch; allyl cellulose; diallyl phthalate; N,N',N''-trialyl isocyanurate; and divinylbenzene. These internal crosslinking agents may be used individually or in combination of two or more. In this specification, for example, "polyethylene glycol" and "ethylene glycol" are collectively referred to as "(poly)ethylene glycol."

[0034] The amount of internal crosslinking agent used may be 0.001 mmol or more, preferably 0.005 mmol or more, more preferably 0.01 mmol or more, and even more preferably 0.03 mmol or more, per mole of water-soluble ethylenically unsaturated monomer used in each stage. Furthermore, the amount of internal crosslinking agent used may be 50 mmol or less, preferably 10 mmol or less, more preferably 5 mmol or less, and even more preferably 1 mmol or less, per mole of water-soluble ethylenically unsaturated monomer used in each stage.

[0035] In reverse-phase suspension polymerization, the polymerization reaction temperature can be appropriately set according to the type and amount of water-soluble ethylenically unsaturated monomers, polymerization initiators, etc., and may be between 20 and 110°C. The reaction time may be set between 0.1 hours and 4 hours.

[0036] In reverse-phase suspension polymerization, a water-soluble ethylenically unsaturated monomer, water, and a dispersion medium are used to create a water-in-oil type reverse-phase suspension state for polymerization. For example, this can be done by adding an aqueous solution containing a water-soluble ethylenically unsaturated monomer, a thickener, a radical polymerization initiator, and an internal crosslinking agent to a dispersion medium in which a polymer dispersion stabilizer is dissolved, and then adding a surfactant to create a suspension state. Through this reverse-phase suspension polymerization, the water-soluble ethylenically unsaturated monomer polymerizes to produce polymer particles. Therefore, reverse-phase suspension polymerization in the polymerization process prepares a reaction solution containing a water-soluble ethylenically unsaturated monomer polymer (particles), water, and a dispersion medium. When an internal crosslinking agent is used in reverse-phase suspension polymerization, the resulting polymer has a structure crosslinked by the internal crosslinking agent.

[0037] If the reversed-phase suspension polymerization is a multi-step polymerization as described above, the first stage of reversed-phase suspension polymerization can be performed using the method described above. Then, the reaction mixture obtained in the first stage of polymerization can be mixed with a water-soluble ethylenically unsaturated monomer, and the second and subsequent stages of reversed-phase suspension polymerization can be performed using the same method as the first stage. In the reversed-phase suspension polymerization of each stage from the second stage onward, in addition to the water-soluble ethylenically unsaturated monomer, a polymerization initiator and an internal crosslinking agent added as needed can be added within the range of the molar ratio of each component to the water-soluble ethylenically unsaturated monomer described above, based on the amount of water-soluble ethylenically unsaturated monomer added in the reversed-phase suspension polymerization of each stage from the second stage onward, and the reversed-phase suspension polymerization can be performed under the same conditions as described above. When reversed-phase suspension polymerization is performed in multiple stages, from the viewpoint of facilitating the production of the desired superabsorbent resin, it is preferable that the total amount of polymerization initiator and the total amount of internal crosslinking agent used as needed per mole of water-soluble ethylenically unsaturated monomer used in reversed-phase suspension polymerization be within the range described above.

[0038] As described above, the reaction solution obtained by reverse-phase suspension polymerization in the polymerization step is subjected to the next concentration step. Furthermore, the residual water content in the reaction solution is not particularly limited, but from the viewpoint of making it easier to reduce the amount of energy required for heating in the concentration step, it may be 200% by mass or less, preferably 180% by mass or less, and more preferably 150% by mass or less, relative to the polymer of water-soluble ethylenically unsaturated monomers.

[0039] (Concentration step) In the concentration step, the dispersion medium contained in the reaction solution prepared in polymerization step 1 is refluxed back into the reaction solution by azeotropic distillation while removing water to prepare a concentrate in which the amount of water in the reaction solution is reduced.

[0040] In the concentration process, azeotropic distillation can be performed by applying external energy, such as heat, to the reactor. This causes the dispersion medium in the reaction solution to reflux back into the reaction solution, while the water contained in the reaction solution is expelled from the system. In this invention, steam can be used as the heat transfer medium for azeotropic distillation. The method of azeotropic distillation is not particularly limited, and for example, known azeotropic distillation methods can be widely employed.

[0041] The temperature of azeotropic distillation can be set within an appropriate range according to the type of dispersion medium, the ratio of water to the dispersion medium, and the pressure within the system. For example, when the dispersion medium is a hydrocarbon dispersion medium, azeotropic distillation can be carried out at a temperature of 40 to 110 °C.

[0042] By azeotropic distillation carried out in the concentration step, a concentrate with the amount of water in the reaction solution reduced to a predetermined amount is obtained. Specifically, in the concentration step, it is preferable to perform azeotropic distillation so that the residual water ratio in the obtained concentrate is 72% by mass or more based on the total mass of the polymer. In this case, the residual water ratio in the concentrate to be subjected to the drying step described later is easily adjusted to 72% by mass or more based on the total mass of the polymer. Incidentally, just to be on the safe side, in this specification, the residual water ratio in the concentrate is based on the total mass of the polymer. In the present invention, the residual water ratio can be calculated by the following formula (2). Residual water ratio (% by mass) = W1 / W2 × 100 (2) In formula (2), W1 means the amount of water (parts by mass) in the concentrate, and W2 means the mass (parts by mass) of the water-soluble ethylenically unsaturated monomer used in inverse phase suspension polymerization. When the polymerization reaction is carried out in multiple stages, W2 is based on the total amount of the masses of the water-soluble ethylenically unsaturated monomers used in all the polymerization reactions.

[0043] The residual water ratio in the concentrate obtained by azeotropic distillation may be 150% by mass or less, and from the viewpoint that aggregation of the concentrate in the drying step is unlikely to occur, it is preferably 130% by mass or less, more preferably 120% by mass or less, still more preferably 110% by mass or less, and particularly preferably 100% by mass or less.

[0044] In the concentration step, an inorganic reducing agent can also be added to the concentrate as needed. By adding an inorganic reducing agent to the concentrate, the content of residual monomers in the polymer (in the water-absorbing resin particles), that is, the unreacted water-soluble ethylenically unsaturated monomers remaining in the water-absorbing resin particles, can be reduced. Further, since the inorganic reducing agent is contained in the water-absorbing resin particles, the stability of the water-absorbing resin particles in the gel state is likely to increase, and the water-absorbing resin particles are likely to exhibit good water absorption performance.

[0045] The type of the inorganic reducing agent is not particularly limited, and for example, known inorganic reducing agents added to the water-absorbing resin particles can be widely exemplified. Among them, in terms of being likely to enhance the performance of the water-absorbing resin particles, the inorganic reducing agent is preferably a sulfite compound.

[0046] Examples of the sulfite compound include sodium sulfite, potassium sulfite, calcium sulfite, sodium bisulfite, potassium bisulfite, ammonium bisulfite, sodium pyrosulfite, and potassium pyrosulfite. The sulfite compound may be only one type or two or more types.

[0047] The addition amount of the inorganic reducing agent is preferably 0.001 part by mass or more, more preferably 0.01 part by mass or more, still more preferably 0.03 part by mass or more, and preferably 3.0 parts by mass or less, more preferably 1.0 part by mass or less, still more preferably 0.50 part by mass or less, particularly preferably 0.25 part by mass or less, based on 100 parts by mass of the polymer contained in the concentrate (synonymous with the total mass of the water-soluble ethylenically unsaturated monomer used in inverse phase suspension polymerization). Preferred ranges are 0.001 to 3.0 parts by mass, 0.01 to 1.0 part by mass, etc. The inorganic reducing agent can be added to the concentrate in a solid or aqueous solution state.

[0048] When adding the inorganic reducing agent in the concentration step, it is preferable to add the inorganic reducing agent to a concentrate having a residual water ratio of 72% by mass or more based on the total mass of the polymer.

[0049] In the concentration step, the inorganic reducing agent can be added as described above, but in terms of efficiently reducing the content of the residual monomer, the addition amount of the inorganic reducing agent in the subsequent drying step may be larger than the addition amount of the inorganic reducing agent in the concentration step, or the inorganic reducing agent may not be added in the concentration step and added in the drying step.

[0050] In the concentration step, a chelating agent can also be added to the concentrate as needed. When the chelating agent is contained in the water-absorbing resin particles, the stability of the water-absorbing resin particles in the gel state is likely to be enhanced, and the water-absorbing resin particles are likely to exhibit good water absorption performance.

[0051] The type of chelating agent is not particularly limited, and a wide range of known chelating agents added to water-absorbent resin particles can be used. Among these, ethylenediamine-N,N'-disuccinic acid, diethylenetriaminepentaacetic acid, glycol etherdiaminetetraacetic acid, ethylenediaminetetramethylenephosphonic acid, and diethylenetriaminepentamethylenephosphonic acid are preferably used as chelating agents because they easily improve the performance of the water-absorbent resin particles. The chelating agent may be added in the form of a salt, or it may be present in the water-absorbent resin particles in the form of a salt. Examples of salts include sodium salts and potassium salts. There may be only one type of chelating agent, or there may be two or more types.

[0052] The amount of chelating agent added is preferably 0.001 parts by mass or more, more preferably 0.005 parts by mass or more, even more preferably 0.01 parts by mass or more, and preferably 2.0 parts by mass or less, more preferably 1.0 part by mass or less, even more preferably 0.5 parts by mass or less, and particularly preferably 0.3 parts by mass or less, per 100 parts by mass of polymer contained in the concentrate.

[0053] When adding a chelating agent during the concentration process, it is preferable to add the chelating agent to a concentrate in which the residual water content is 72% by mass or more based on the total mass of the polymer. The chelating agent can be added to the concentrate in solid or aqueous solution form.

[0054] When adding inorganic reducing agents and chelating agents during the concentration process, there are no particular restrictions on the order in which they are added. For example, the order can be to add the inorganic reducing agent first, then the chelating agent, or vice versa. The inorganic reducing agent and chelating agent can also be added simultaneously.

[0055] In this invention, when an inorganic reducing agent and / or chelating agent is added to a concentrate in the form of an aqueous solution, water is also added to the concentrate. However, the residual water content in the concentrate is based on the residual water content immediately before adding the aqueous solution of the inorganic reducing agent and / or chelating agent (i.e., the water in the aqueous solution being added is not considered in the calculation of the residual water content).

[0056] As described above, the concentrate obtained by azeotropic distillation in the concentration process is subjected to the next drying process.

[0057] (Drying process) The drying process is a process for preparing a dried product by drying the concentrate while exposing it to air. In this drying process, the dispersion medium and any remaining moisture in the concentrate can be removed, thereby obtaining a solid polymer as water-absorbing resin particles.

[0058] In the drying process, the residual water content of the concentrate to be dried (specifically, the residual water content in the concentrate immediately before drying) is set to 72% by mass or more, based on the total mass of the polymer in the concentrate. This shortens the time required for the concentration process and reduces the amount of heat transfer medium (e.g., steam) used in the concentration process. When the time required for the concentration process is shortened, the time required for the drying process increases, resulting in a larger amount of heat transfer medium being used in the drying process. On the other hand, since the amount of heat transfer medium used per unit weight of water removed is greater in the concentration process than in the drying process, the overall amount of heat transfer medium used in the manufacturing method is significantly reduced.

[0059] In the drying process, the concentrate obtained in the concentration process is dried while being exposed to air. The drying method is not particularly limited, and a wide range of known drying methods can be employed.

[0060] Drying can be carried out using known dryers, such as various drying devices that can supply thermal energy from an external source. Examples of drying devices include grooved agitation dryers equipped with spray nozzles.

[0061] In the present invention, steam can be used as the heat transfer medium in the drying process. The drying process may be carried out under either atmospheric pressure or reduced pressure, and may also be carried out under a stream of gas such as nitrogen to improve drying efficiency. When drying is carried out under atmospheric pressure, the drying temperature may be 70 to 150°C, preferably 75 to 130°C, and more preferably 80 to 120°C. When drying is carried out under reduced pressure, the drying temperature may be 50 to 150°C, and more preferably 70 to 120°C.

[0062] The drying step preferably includes step A, in which an inorganic reducing agent is added to the concentrate. In this case, the content of residual monomers in the water-absorbing resin particles is more easily reduced. The type of inorganic reducing agent is the same as described above.

[0063] Furthermore, the amount of inorganic reducing agent added is the same as described above. That is, the amount of inorganic reducing agent added is preferably 0.001 parts by mass or more, more preferably 0.01 parts by mass or more, even more preferably 0.03 parts by mass or more, and preferably 3.0 parts by mass or less, more preferably 1.0 part by mass or less, even more preferably 0.50 parts by mass or less, and particularly preferably 0.25 parts by mass or less, with preferred ranges being 0.001 to 3.0 parts by mass, 0.01 to 1.0 parts by mass, etc. In the drying process, the inorganic reducing agent can also be added to the concentrate in solid or aqueous solution form.

[0064] In step A, it is preferable that the inorganic reducing agent is added so that the residual water content in the concentrate is 70% by mass or less based on the total mass of the polymer. This makes it easier to further reduce the content of residual monomer in the water-absorbing resin particles. In step A, it is more preferable that the inorganic reducing agent is added so that the residual water content in the concentrate is 65% by mass or less based on the total mass of the polymer, even more preferably 60% by mass or less, even more preferably 55% by mass or less, and most preferably 50% by mass or less.

[0065] As described above, in the method for producing water-absorbent resin particles of the present invention, it is preferable to add an inorganic reducing agent in the drying step (i.e., to include step A) from the viewpoint of making it easier to reduce residual monomers, and it is even more preferable to add a larger amount of inorganic reducing agent in the drying step than in the concentration step from the viewpoint of making it easier to efficiently reduce residual monomers even with a smaller amount of inorganic reducing agent added, and it is even more preferable not to add an inorganic reducing agent in the concentration step and to add an inorganic reducing agent in the drying step.

[0066] The drying process may also preferably include step B, in which a surface crosslinking agent is added to the concentrate during drying. This allows the surface vicinity of the resulting water-absorbing resin particles to have a crosslinked structure, which further improves the performance of the water-absorbing resin particles.

[0067] Surface crosslinking agents, sometimes referred to as post-crosslinking agents, can increase the crosslinking density near the surface of water-absorbent resin particles. The type of surface crosslinking agent is not particularly limited, and various crosslinking agents can be applied; similar types of crosslinking agents as those used for internal crosslinking agents can be used. Among these, compounds or precursors thereof having two or more functional groups that react with the functional groups of water-soluble ethylenically unsaturated monomers are preferably used as surface crosslinking agents. Surface crosslinking agents may be used alone or in combination of two or more types. They may also be used without dilution, but it is preferable to use them as aqueous solutions for easier and more uniform dispersion.

[0068] The amount of surface crosslinking agent used may be 0.01 mmol or more and 10 mmol or less per mole of water-soluble ethylenically unsaturated monomer used in reverse-phase suspension polymerization. From the viewpoint of easily obtaining water-absorbing resin particles with excellent water absorption properties, it is preferably 0.03 mmol or more, more preferably 0.05 mmol or more, even more preferably 0.1 mmol or more, and also preferably 5 mmol or less, more preferably 3 mmol or less, and even more preferably 1 mmol or less.

[0069] The temperature of the concentrate when adding the surface crosslinking agent can be appropriately adjusted according to the reaction temperature of the surface crosslinking agent, and may be 50 to 220°C, 60 to 200°C, or 70 to 150°C. The reaction time for the surface crosslinking agent (i.e., the time for treating the polymer with the surface crosslinking agent at the above temperature) is, for example, 1 to 300 minutes, preferably 5 to 200 minutes. A surface crosslinking step in which the surface crosslinking agent is reacted may be included, but the drying step may substantially serve as the surface crosslinking step.

[0070] Step B may be performed before, after, or simultaneously with Step A, but it is preferable that Step B be performed after Step A. This makes it easier to further reduce the amount of residual monomer in the resulting water-absorbing resin particles.

[0071] If the drying process includes a step B after step A, it is preferable that the inorganic reducing agent is added in step A when the residual water content in the concentrate is 30% by mass or more. Here, the residual water content refers to the percentage of water content based on the total mass of the polymer, as described above, and means the residual water content when the inorganic reducing agent is added. It is more preferable that the inorganic reducing agent is added in step A when the residual water content in the concentrate is 35% by mass or more, even more preferable that it is 40% by mass or more, and particularly preferable that it is 45% by mass or more.

[0072] In particular, as a preferred embodiment of the method for producing water-absorbent resin particles of the present invention, the drying step further comprises step B of adding a surface crosslinking agent to the concentrate after step A, wherein the residual water content in the concentrate when the inorganic reducing agent is added is X by mass%, and the residual water content in the concentrate when the surface crosslinking agent is added is Y by mass%, and the following formula (1) 10 ≤ X - Y ≤ 70 (1) is satisfied.

[0073] When the X-Y value is 10 or greater, the inorganic reducing agent and the surface crosslinking agent become less reactive, and the content of residual monomers in the water-absorbing resin particles can be particularly reduced. The X-Y value may be 65 or less, 60 or less, 55 or less, 50 or less, or 45 or less. Furthermore, from the viewpoint of making it easier to reduce the amount of residual monomers, the X-Y value is preferably 13 or greater, more preferably 18 or greater, and even more preferably 23 or greater.

[0074] Furthermore, when the inorganic reducing agent is added in the concentration step rather than the drying step, it is also preferable that the residual water content in the concentrate when the inorganic reducing agent is added is Z by mass%, and that 10 ≤ Z - Y ≤ 70 is satisfied. The value of Z - Y is preferably 30 or more, more preferably 40 or more, and even more preferably 45 or more.

[0075] The above drying process can yield a dried material. This dried material can be obtained as water-absorbing resin particles. Alternatively, the dried material can be subjected to appropriate treatment to obtain water-absorbing resin particles. Additives such as silica can also be added to the dried material.

[0076] The residual water content of the water-absorbing resin particles obtained as described above is, for example, 30% by mass or less, preferably 25% by mass or less, and more preferably 20% by mass or less.

[0077] The method for producing water-absorbent resin particles of the present invention may include other steps, as long as it comprises a polymerization step, a concentration step, and a drying step in this order. The polymerization step, concentration step, and drying step may include other steps between them, or they may not include other steps.

[0078] In the manufacturing method of the present invention, various manufacturing apparatuses used in the production of water-absorbent resin particles can be used, for example, the manufacturing apparatus shown in Figure 1 disclosed in Patent Document 1 can be used. Preferably, a manufacturing apparatus is used that independently comprises a polymerizer for carrying out a polymerization process (reverse-phase suspension polymerization), a concentrator for carrying out a concentration process (azeotropic distillation), and a dryer for carrying out a drying process. Of course, these facilities do not have to be independent of each other; for example, the polymerization process to the drying process may be carried out in the same facility (chamber), and the concentrator for carrying out the concentration process (azeotropic distillation) and the dryer for carrying out the drying process may be the same facility. Preferably, the polymerizer, concentrator and dryer are connected to each other by transfer pipes such as piping so that processing can be carried out continuously. The manufacturing apparatus may be equipped with other equipment as needed, such as a heat exchanger, a washing machine, etc.

[0079] The superabsorbent resin particles obtained by the method for producing superabsorbent resin particles of the present invention are widely used in fields such as sanitary materials like disposable diapers, sanitary napkins, and incontinence pads. Furthermore, by utilizing the water-stopping effect of the superabsorbent resin particles, they can also be used as a water-stopping material for communication cables such as optical cables and power cables.

[0080] In specifying the inventions contained herein, the components (properties, structures, functions, etc.) described in each embodiment of this disclosure may be combined in any way. That is, this disclosure encompasses all subject matter consisting of any combination of the combinatable components described herein.

[0081] The present invention will be described more specifically below with reference to examples, but the present invention is not limited to the embodiments of these examples.

[0082] (Comparative Example 1) Polymerization Process (Preparation of Reaction Solution by Reverse-Phase Suspension Polymerization) In the polymerization process, a reaction solution containing a polymer of a water-soluble ethylenically unsaturated monomer, water, and a dispersion medium was prepared by reverse-phase suspension polymerization, which comprises a two-stage polymerization reaction as described below. Specifically, a two-stage reverse-phase suspension polymerization method was carried out using a radical polymerization initiator in a hydrocarbon dispersion medium in the presence of a dispersion stabilizer, with the water-soluble ethylenically unsaturated monomer. First, 9033 kg of n-heptane was placed in the reactor body as the hydrocarbon dispersion medium, and 207 kg of a 10% by mass n-heptane solution of maleic anhydride-ethylene-propylene copolymer was placed in the reactor body as a dispersion stabilizer, and these were stirred using a stirring device installed in the polymerizer. The internal temperature at this time was 58 to 65°C.

[0083] Meanwhile, in a separate container, 2383 kg of a 99.9% by mass aqueous solution of acrylic acid was added as a water-soluble ethylenically unsaturated monomer. While cooling, 3184 kg of a 31.5% by mass aqueous solution of sodium hydroxide was added dropwise as an alkaline neutralizing agent to neutralize the mixture to a degree of neutralization of 76 mol% of the acid groups of the water-soluble ethylenically unsaturated monomer. Next, 101.2 kg of a 2% by mass aqueous solution of potassium persulfate was added as a radical polymerization initiator, 20.2 kg of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was added as an internal crosslinking agent, and 2029 kg of water was added and dissolved to prepare the first stage monomer aqueous solution, which was maintained at a temperature of 18-19°C. The entire amount of this first stage monomer aqueous solution was added to the reactor body, and while heating the contents, 214 kg of a 10% by mass n-heptane solution of sucrose fatty acid ester was added to the reactor body as a dispersant, and then the inside of the reactor body was purged with nitrogen by blowing nitrogen into it. Next, while stirring the contents of the reactor, the contents were heated to 63°C to initiate the polymerization reaction. After polymerization began, the heat of polymerization caused the contents of the reactor to rise, reaching a maximum temperature of 84°C. Cooling was started when the internal temperature began to decrease, and the first stage reaction mixture was obtained.

[0084] Furthermore, a second monomer aqueous solution was prepared in a separate container. Specifically, 3360 kg of a 99.9% by mass acrylic acid aqueous solution was added as a water-soluble ethylenically unsaturated monomer, and while cooling, 4490 kg of a 31.5% by mass sodium hydroxide aqueous solution was added dropwise as an alkaline neutralizing agent to neutralize the mixture to a degree of neutralization of 76 mol% of the acid groups of the water-soluble ethylenically unsaturated monomer. Next, 134.2 kg of a 2% by mass potassium persulfate aqueous solution was added as a radical polymerization initiator, 18.5 kg of a 2% by mass ethylene glycol diglycidyl ether aqueous solution was added as an internal crosslinking agent, and 1396 kg of water was added and dissolved to prepare the second monomer aqueous solution, which was maintained at a temperature of 12-13°C. The entire amount of the second monomer aqueous solution thus obtained was added to the reactor body containing the first-stage reaction mixture, and the system was thoroughly purged with nitrogen. Next, while stirring the reactor body, the contents of the reactor body were heated to 60°C to start polymerization. After polymerization began, the contents of the reactor body were heated by the heat of polymerization, reaching a maximum temperature of 84°C. Cooling was started when the internal temperature began to decrease, thereby obtaining the second stage reaction mixture. This second stage reaction mixture was obtained as the target reaction solution. This reaction solution was named "Reaction Solution 1".

[0085] Concentration Process (Preparation of Concentrate) The reaction solution 1 obtained in the polymerization process described above was transferred to a concentrator for concentration. Specifically, azeotropic distillation of n-heptane and water was performed by heating the reaction solution 1 to 85°C while stirring it with a stirring means provided in the concentrator. Steam was used as the heat transfer medium in the concentration process, and in azeotropic distillation, the dispersion medium (n-heptane) in the reaction solution 1 was refluxed into the concentrator while a predetermined amount of water was removed from the system to obtain the concentrate. When the residual water content in the concentrate reached 70.5% by mass based on the total mass of the polymer, 43 kg of a 10% by mass sodium bisulfite aqueous solution was added as an inorganic reducing agent. Then, when the residual water content reached 70% by mass, 14.3 kg of a 40% by mass diethylenetriaminepentaacetic acid / pentasodium aqueous solution was added to the concentrator as a chelating agent, and azeotropic distillation was completed. The residual water content in the concentrate obtained was 70% by mass based on the total mass of the polymer. The obtained concentrate was named concentrate 1a.

[0086] Drying Process (Preparation of Dried Product) The concentrate 1a was transferred to a grooved stirring dryer equipped with a spray nozzle (Nara Machine Works, single-axis paddle dryer), and while stirring inside the dryer, it was heated to a temperature of 85-115°C, and the dispersion medium (n-heptane) and water in the concentrate 1a were expelled from the system under reduced pressure. Steam was used as the heat transfer medium in the drying process. Next, when the residual water content in the concentrate 1a reached 26% by mass based on the total mass of the polymer, 123.3 kg of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was added to the concentrate during drying as a surface crosslinking agent, and the drying process was carried out until the residual water content reached 18% by mass. After that, the contents of the dryer were discharged to obtain superabsorbent polymer particles. These superabsorbent polymer particles were named superabsorbent polymer particles 1a. The water retention capacity of the physiological saline solution and the amount of residual monomer of the superabsorbent polymer particles 1a were measured.

[0087] (Example 1) Polymerization process (Preparation of reaction solution by reverse-phase suspension polymerization) Reaction solution 1 was obtained by carrying out a polymerization process in the same manner as the polymerization process carried out in Comparative Example 1. Concentration process (Preparation of concentrate) The reaction solution 1 was transferred to a concentrator and a concentration process was carried out. Specifically, azeotropic distillation of n-heptane and water was performed by heating the reaction solution 1 to 85°C while stirring the reaction solution 1 in the concentrator with a stirring means provided in the concentrator. Steam was used as the heat transfer medium in the concentration process, and in azeotropic distillation, the dispersion medium (n-heptane) in the reaction solution 1 was refluxed into the concentrator while a predetermined amount of water was extracted from the system to obtain a concentrate. When the residual water content in the concentrate reached 75.5% by mass relative to the total mass of the polymer, 43 kg of a 10% by mass sodium bisulfite aqueous solution was added as an inorganic reducing agent. Then, when the residual water content reached 75.0% by mass, 14.3 kg of a 40% by mass diethylenetriaminepentaacetic acid aqueous solution was added to the concentrator as a chelating agent, and azeotropic distillation was completed. The residual water content in the resulting concentrate was 75% by mass relative to the total mass of the polymer. The obtained concentrate was named Concentrate 1A.

[0088] Drying Process (Preparation of Dried Product) Absorbent resin particles were obtained by carrying out the drying process in the same manner as in Comparative Example 1, except that concentrate 1A was used instead of concentrate 1a. These absorbent resin particles were named absorbent resin particles 1A. The saline water retention capacity and residual monomer amount of absorbent resin particles 1A were measured.

[0089] (Example 2) Polymerization process (Preparation of reaction solution by reverse-phase suspension polymerization) Reaction solution 1 was obtained by carrying out the polymerization process in the same manner as the polymerization process carried out in Example 1. Concentration process (Preparation of concentrate) The reaction solution 1 was transferred to a concentrator and the concentration process was carried out. Specifically, azeotropic distillation of n-heptane and water was carried out by heating the reaction solution 1 to 90°C while stirring the reaction solution 1 in the concentrator with a stirring means provided in the concentrator. Steam was used as the heat transfer medium in the concentration process, and in azeotropic distillation, the dispersion medium (n-heptane) in the reaction solution 1 was refluxed into the concentrator while a predetermined amount of water was removed from the system to obtain the concentrate. When the residual water content in the concentrate reached 75% by mass based on the total mass of the polymer, 14.3 kg of 40% by mass diethylenetriaminepentaacetic acid aqueous solution was added to the concentrator as a chelating agent, and the azeotropic distillation was terminated. The residual water content in the concentrate obtained was 75% by mass based on the total mass of the polymer. The resulting concentrate was named concentrate 1B.

[0090] Drying Process (Preparation of Dried Product) The concentrate 1B was transferred to a grooved agitator dryer equipped with a spray nozzle (Nara Machine Works, single-axis paddle dryer), and while stirring inside the dryer, it was heated to a temperature of 85-115°C, and the dispersion medium (n-heptane) and water in the concentrate 1B were expelled from the system under reduced pressure. Steam was used as the heat transfer medium in the drying process. Next, when the residual water content in concentrate 1B reached 50% by mass based on the total mass of the polymer, 43 kg of a 10% by mass sodium bisulfite aqueous solution was added to the concentrate during drying as an inorganic reducing agent (Step A). ​​Then, when the residual water content reached 26% by mass, 123.3 kg of a 2% by mass ethylene glycol diglycidyl ether aqueous solution was added to the concentrate during drying as a surface crosslinking agent (Step B), and the drying process was carried out until the residual water content reached 18% by mass. After that, the contents of the dryer were discharged to obtain superabsorbent polymer particles. These superabsorbent polymer particles were named superabsorbent polymer particles 1B. The water retention capacity of the water-absorbing resin particle 1B in physiological saline solution and the amount of residual monomer were measured.

[0091] (Example 3) Polymerization process (Preparation of reaction solution by reverse-phase suspension polymerization) Reaction solution 1 was obtained by carrying out the polymerization process in the same manner as the polymerization process carried out in Example 2. Concentration process (Preparation of concentrate) Reaction solution 1 was obtained by carrying out the concentration process in the same manner as the concentration process carried out in Example 2. A concentrate with a residual water content of 75% by mass was obtained. The obtained concentrate was named concentrate 1C.

[0092] Drying Process (Preparation of Dried Product) Absorbent polymer particles were obtained by carrying out the drying process in the same manner as in Example 2, except that the inorganic reducing agent (aqueous solution of sodium bisulfite) was added when the residual water content in concentrate 1C reached 45% by mass based on the total mass of the polymer. These absorbent polymer particles were named absorbent polymer particles 1C. The saline water retention capacity and residual monomer content of absorbent polymer particles 1C were measured.

[0093] (Example 4) Polymerization process (Preparation of reaction solution by reverse-phase suspension polymerization) Reaction solution 1 was obtained by carrying out the polymerization process in the same manner as the polymerization process carried out in Example 2. Concentration process (Preparation of concentrate) Reaction solution 1 was obtained by carrying out the concentration process in the same manner as the concentration process carried out in Example 2. A concentrate with a residual water content of 75% by mass was obtained. The obtained concentrate was named concentrate 1D.

[0094] Drying Process (Preparation of Dried Product) Absorbent polymer particles were obtained by carrying out the drying process in the same manner as in Example 2, except that the inorganic reducing agent (aqueous solution of sodium bisulfite) was added when the residual water content in concentrate 1D reached 40% by mass based on the total mass of the polymer. These absorbent polymer particles were named absorbent polymer particles 1D. The saline water retention capacity and residual monomer content of absorbent polymer particles 1D were measured.

[0095] <Calculation of Residual Water Rate> The residual water rate was calculated using the following formula (2). Residual water rate (mass%) = (W1 / W2) × 100 (2) In formula (2), W1 is the amount of water in the concentrate (parts by mass), and W2 is the mass of the water-soluble ethylenically unsaturated monomer used in the polymerization reaction (parts by mass). When the polymerization reaction was carried out in multiple stages, W2 was based on the total mass of the water-soluble ethylenically unsaturated monomer used in all polymerization reactions. Therefore, for example, in Example 1, it was the total amount of acrylic acid and sodium acrylate used in the first and second stages of polymerization.

[0096] The residual water content in the concentrate obtained during the concentration process was measured by appropriately sampling the concentrate as it was being transferred from the concentrator to the dryer. This was because the residual water content of the concentrate during transfer could be considered to be the same as the residual water content of the concentrate immediately after being subjected to the drying process.

[0097] <Water retention capacity of superabsorbent polymer particles> The water retention capacity test was conducted under conditions of 25°C and 50% RH. A cotton bag (membrane no. 60, 100 mm wide x 200 mm long) containing 2,000 g of superabsorbent polymer particles was placed in a 500 mL beaker. 200 mL of 25°C physiological saline was poured into the cotton bag containing the superabsorbent polymer particles in one go, taking care not to let it spill. The top of the cotton bag was tied with a rubber band and left to stand for 30 minutes to allow the superabsorbent polymer particles to swell. After 30 minutes, the cotton bag was dehydrated for 1 minute using a dehydrator (manufactured by Kokusan Co., Ltd., model number: H-122) set to a centrifugal force of 167 G. The mass Wa (g) of the cotton bag containing the swollen gel after dehydration was measured. The same procedure was performed without adding superabsorbent polymer particles, and the empty mass Wb (g) of the cotton bag when wet was measured. The water retention capacity of physiological saline was calculated from the following formula (3): Water retention capacity of physiological saline [g / g] = (Wa - Wb) / 2.000 (3).

[0098] <Residual Monomer Amount> The residual monomer test was conducted under conditions of 25°C and 50% RH humidity. 500 g of physiological saline solution at 25°C was placed in a 500 mL beaker, and 2,000 g of superabsorbent polymer particles were added and stirred for 60 minutes. The contents of the beaker were then filtered through a JIS standard sieve with a mesh size of 75 μm, and then through filter paper (ADVANTEC, filter paper No. 3) to separate the absorbent gel from the extract. The monomer content dissolved in the obtained extract was measured by high-performance liquid chromatography. The measured values ​​were converted to values ​​per mass of superabsorbent polymer particles to obtain the residual monomer content (mass ppm). The conditions for the high-performance liquid chromatography used are as follows. • Equipment used: Shimadzu Corporation "SCL-10AVP + CTO-10A + LC-10AD + DGU-4A + SIL-10A" • Detector: Shimadzu Corporation "SPD-10A (UV wavelength 210 nm)" • Column: Showa Denko Corporation "Shodex KC-811" • Column temperature: 45°C • Carrier: Distilled water adjusted to pH 2 with phosphoric acid • Substances to be measured: Acrylic acid and sodium acrylate

[0099] Table 1 shows the steps taken when an inorganic reducing agent and a surface crosslinking agent were added in the method for producing superabsorbent polymer particles in each example and comparative example, and the residual water content in the concentrate at that time. Table 1 also shows the reduction in the amount of steam used in the total method for producing superabsorbent polymer particles in each example, as well as the measurement results of the residual monomers and water retention capacity (saline solution water retention capacity) of the superabsorbent polymer particles obtained in each example and comparative example. The amount of steam reduction is based on the amount of steam used in the method for Comparative Example 1, i.e., the reduction relative to the amount of steam used in the method for Comparative Example 1, and is expressed as the reduction per ton of superabsorbent polymer particle production.

[0100] Table 1 shows that the manufacturing methods used in Examples 1 to 4 reduced the amount of steam used compared to Comparative Example 1. Therefore, it has been demonstrated that by setting the residual water content of the concentrate dried in the drying process to 72% by mass or more, the total amount of steam used in the entire manufacturing process can be significantly reduced.

[0101] Furthermore, as can be seen from Table 1, when the inorganic reducing agent was not added in the concentration step but was added in the drying step, as in Examples 2 to 4, water-absorbing resin particles with a lower amount of residual monomer (amount of unreacted acrylic acid and sodium acrylate) were obtained than in Example 1. While we do not necessarily want a restrictive interpretation, the following can be inferred as to the reason. The inorganic reducing agent can inactivate water-soluble ethylenically unsaturated monomers by reducing the carbon-carbon double bond to a single bond. Also, the amount of residual monomer tends to decrease as the concentration progresses. In Example 1, the concentration step was shortened, so the inorganic reducing agent was added to a concentrate with a high residual water content where a large amount of residual monomer was present, and the effect of reducing residual monomer was not sufficient. In contrast, in Examples 2 to 4, by including a step of adding the inorganic reducing agent in the drying step, the inorganic reducing agent could be added to a system with a relatively small amount of residual monomer, and it is presumed that water-absorbing resin particles with an even lower amount of residual monomer were obtained.

[0102]

Claims

1. A method for producing superabsorbent polymer particles, comprising a polymerization step, a concentration step, and a drying step in this order, wherein in the polymerization step, a reaction solution containing a polymer of a water-soluble ethylenically unsaturated monomer, water, and a dispersion medium is prepared by reverse-phase suspension polymerization; in the concentration step, a concentrate is prepared by reducing the amount of water in the reaction solution by removing water while refluxing the dispersion medium from the reaction solution to the reaction solution by azeotropic distillation; in the drying step, a dried product is prepared by drying the concentrate while exposing it to air; and the residual water content of the concentrate dried in the drying step is 72% by mass or more based on the total mass of the polymer in the concentrate.

2. The method for producing water-absorbent resin particles according to claim 1, wherein the drying step includes step A of adding an inorganic reducing agent to the concentrate.

3. The method for producing water-absorbing resin particles according to claim 2, wherein in step A, the addition of the inorganic reducing agent is carried out in a manner in which the residual water content in the concentrate is 70% by mass or less based on the total mass of the polymer.

4. The method for producing water-absorbing resin particles according to claim 2 or 3, wherein the drying step further comprises step B of adding a surface crosslinking agent to the concentrate after step A, and the residual water content in the concentrate when the inorganic reducing agent is added is X by mass%, and the residual water content in the concentrate when the surface crosslinking agent is added is Y by mass%, and the following formula (1) 10 ≤ X - Y ≤ 70 (1) 5. The method for producing water-absorbing resin particles according to claim 2 or 3, wherein the addition of the inorganic reducing agent in step A is performed in a manner that results in a residual water content of 30% by mass or more in the concentrate.

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