JP3871445B2 - Production method of super absorbent resin - Google Patents

Description

【００１２】
上述した本発明における重合開始剤の添加方法は、具体的には、予めモノマー又はその水溶液中に、重合開始剤又はその水溶液を加えることによる方法、モノマー又はその水溶液を後述の分散媒中に添加し、その際同時に並行して重合開始剤又はその水溶液を分散媒中に添加する方法、及び予め所定量の重合開始剤又はその水溶液を分散媒中に共存させておき、ここへモノマー又はその水溶液を添加する方法等によって行うことができ、これにより本発明の目的は達成される。
【００１３】
モノマーを水溶液として用いる場合のモノマー水溶液の濃度は、好ましくは１０〜９０重量％、更に好ましくは３０〜６０重量％である。また、重合開始剤を水溶液として用いる場合の重合開始剤水溶液の濃度は、好ましくは０．０５〜９０重量％、更に好ましくは１〜５０重量％である。
【００１４】
本発明に用いられる重合開始剤としては、アゾ系重合開始剤及び酸化性重合開始剤等が挙げられる。
アゾ系重合開始剤としては、具体的には、特開平８−３３７７２６号公報の第４頁第５欄第４〜１９行に記載のものを例示することができる。これらは１種以上を使用することができる。
これらの中でも、２，２’−アゾビス（２−アミジノプロパン）ジヒドロハライド、２，２’−アゾビス［２−（２−イミダゾリン−２−イル）プロパン］ジヒドロハライド及び４，４’−アゾビス−４−シアノバレリックアシッドからなる群より選択される１種以上が本発明の目的を達成する為に好ましい。また、上記の化合物において、ハライドはクロリドであることが経済面より好ましい。
【００１５】
また、酸化性重合開始剤としては、具体的には、特開平８−３３７７２６号公報の第３頁第４欄第４３行〜第４頁第５欄第３行に記載のものや、過酸化水素／第１鉄塩、過硫酸塩／亜硫酸塩、クメンヒドロパーオキシド／第１鉄塩、過酸化水素／Ｌ−アスコルビン酸等のレドックス系重合開始剤等を例示することができる。これらは１種以上を使用することができる。これらの中でも、過硫酸塩が本発明の目的を達成する為に好ましい。
【００１６】
また、アゾ系重合開始剤と酸化性重合開始剤とを１種以上ずつ併用することも可能である。
【００１７】
本発明の高吸水性樹脂の製法において、モノマーの重合法は特に限定されないが、好ましくは、モノマーを重合反応の進行に従って重合反応系へ供給する重合法により行われる。
更に好ましくは、得られる高吸水性樹脂の粒径を制御する点で、モノマー又はその水溶液を疎水性有機溶媒を含有する分散媒中へ供給する逆相懸濁重合法により行われる。
上記疎水性有機溶媒としては、例えば、ｎ−ペンタン、シクロペンタン、ｎ−ヘキサン、シクロヘキサン、ｎ−ヘプタン、メチルシクロヘキサン等の脂肪族炭化水素、ベンゼン、トルエン等の芳香族炭化水素、ｎ−ブチルアルコール、ｎ−アミルアルコール等の炭素数４〜６の脂肪族アルコール、メチルエチルケトン等の脂肪族ケトン、酢酸エチル等の脂肪族エステル類等を例示することができる。これらの疎水性有機溶媒は、１種以上を用いることができる。
また、上記疎水性有機溶媒の使用量は、モノマー１００重量部に対して、好ましくは１００重量部以上、更に好ましくは２００〜１０００重量部である。
【００１８】
また、上記分散媒には、上記疎水性有機溶媒以外に、両親媒性の溶剤を加えてもよい。該両親媒性の溶剤としては、メタノール、エタノール、プロパノール、及び２−プロパノール等のアルコール類、アセトン等のケトン類、及びテトラヒドロフラン及びジオキサン等のエーテル類が挙げられる。該両親媒性の溶剤の使用量は、該疎水性有機溶媒との合計量で、モノマー１００重量部に対し５００重量部までの量であることが好ましい。
【００１９】
また、モノマーの重合を行う際に、分散剤を用いることができる。
分散剤としては、例えば、ソルビタンモノステアレート、ソルビタンモノラウレート及びポリオキシメチレンソルビタンモノオレート等のソルビタン脂肪酸エステル、トリメチルステアリルアンモニムクロリド及びカルボキシメチルジメチルセチルアンモニウム等の陽イオン性及び両性の界面活性剤、ポリオキシエチレンドデシルエーテル硫酸エステルナトリウム塩及びドデシルエーテル硫酸エステルナトリウム塩等の陰イオン性界面活性剤、アルキルグルコシド等のグリコシド化合物、エチルセルロース及びベンジルセルロース等のセルロースエーテル、セルロースアセテート、セルロースブチレート及びセルロースアセテートブチレート等のセルロースエステル、マレイン化ポリブタジエン、マレイン化ポリエチレン、マレイン化α−オレフィン、スチレン−ジメチルアミノエチルメタクリレート４級塩及びイソプロピルメタクリレート−ジメチルアミノエチルメタクリレート４級塩等の高分子分散剤を例示することができる。これらの分散剤は１種以上を用いることができる。
分散剤の使用量は、モノマー１００重量部に対して、好ましくは０．０１〜５重量部である。
特に、モノマーの逆相懸濁重合を行う場合には、モノマーの分散剤としてイオン性界面活性剤を用いることが、高吸水性樹脂粒子の凝集を防ぐという点で好ましい。
【００２０】
モノマーを重合する際の重合温度は、好ましくは２０〜１２０℃、更に好ましくは４０〜１００℃である。重合温度がこの範囲の場合、好ましい重合速度が達成される。
【００２１】
また、本発明の高吸水性樹脂の製法の実施にあたり、重合前、重合時、重合後又は乾燥時等において、架橋剤を添加することができる。該架橋剤としては、例えば、ポリアリル化合物、ポリビニル化合物、ポリグリシジルエーテル、ハロエポキシ化合物、ポリアルデヒド、ポリオール、ポリアミン、ヒドロキシビニル化合物、またカルシウム、マグネシウム、亜鉛及びアルミニウム等の多価イオンを生じる無機塩又は有機金属塩等を例示することができる。
【００２２】
また、モノマーの重合（好ましくは逆相懸濁重合）を終了した後、必要に応じ通常の後処理、例えば、共沸脱水、乾燥等を行うことにより、所望の高吸水性樹脂を得ることができる。
【００２３】
【実施例】
実施例及び比較例における「％」は、「重量％」を表す。試験方法は、下記の通りである。
【００２４】
〔生理食塩水の保持量の測定法〕
高吸水性樹脂１ｇを生理食塩水（０．９％ＮａＣｌ水溶液、大塚製薬製）１５０ｍｌで３０分間膨潤させた後、不織布袋に入れ、遠心分離機にて１４３Ｇで１０分間脱水し、脱水後の総重量（全体重量）を測定し、下記式（２）に従って、遠心脱水後の保持量を測定した。
【００２５】

【００２６】
〔実施例１〕
撹拌機、還流冷却器、滴下ロート、窒素ガス導入管を付した５０００ｍｌの４つ口フラスコに、疎水性有機溶媒としてシクロヘキサン１６００ｍｌ、分散剤としてポリオキシエチレンラウリルエーテル硫酸塩（平均エチレンオキシド付加モル数＝２）の２５％水溶液０．８２ｇを仕込み、窒素ガスを吹き込んで溶存酸素を追い出し、７５℃まで昇温した。別のフラスコにて、８０％アクリル酸水溶液５１０ｇを外部より冷却しつつ、３０％水酸化ナトリウム水溶液で中和度７０％まで中和して、アクリル酸ナトリウム水溶液を得た。次いで、重合開始剤として過硫酸カリウム（以下、「ＫＰＳ」という）１．４２８ｇをポリオキシエチレンラウリルエーテル硫酸塩（平均エチレンオキシド付加モル数＝２）の２５％水溶液２．０４ｇと共にイオン交換水３５ｇに溶解させて、重合開始剤水溶液を得た。
この重合開始剤水溶液の３／１４を取り出し、上記のアクリル酸ナトリウム水溶液の１／４と混合した（モノマー溶液Ａ）。更に、重合開始剤水溶液の２／１４を取り出し、アクリル酸ナトリウム水溶液の１／４と混合した（モノマー溶液Ｂ）。残りの重合開始剤水溶液（全重合開始剤水溶液の９／１４）は、残りのアクリル酸ナトリウム水溶液（全アクリル酸ナトリウム水溶液の１／２）と混合した（モノマー溶液Ｃ）。その後、窒素ガスを吹き込み水溶液内に残存する酸素を除去した。
上記４つ口フラスコに、まず、モノマー溶液Ａを１５分間かけて滴下し、続いて、モノマー溶液Ｂを１５分間かけて滴下し、更にモノマー溶液Ｃを３０分間かけて滴下し重合した。
重合終了後、脱水管を用いて共沸脱水を行い、高吸水性樹脂の含水量を高吸水性樹脂１００重量部に対して４０重量部に調整した。その後、架橋剤としてポリグリセロールポリグリシジルエーテル（ナガセ化成工業（株）製、商品名デナコールＥＸ−５１２）０．４ｇを水１０ｇに溶解したものを添加し、７５〜８０℃で２時間反応させた。その後、更に共沸脱水を行い、高吸水性樹脂の含水量を高吸水性樹脂１００重量部に対して３０重量部に調整した。冷却後、シクロヘキサンをデカンテーションで除き、８０〜１００℃、６．６６６ｋＰａの条件で乾燥させることにより高吸水性樹脂を得た。得られた高吸水性樹脂の生理食塩水の保持量を測定した。その結果を表１に示す。
【００２７】
〔実施例２〜４及び比較例１〕
重合開始剤として使用するＫＰＳの添加量を表１に示すようにすること以外は、実施例１と同様の操作を行い、高吸水性樹脂を得た。得られた高吸水性樹脂の生理食塩水の保持量を測定した。その結果を表１に示す。
【００２８】
【表１】

【００２９】
【発明の効果】
本発明の高吸水性樹脂の製法によれば、高吸水保持能力を有し、しかも、含水状態での経時安定性が良好な吸水性樹脂を、添加剤等を加えることなく簡便に且つ安価に、再現良く得ることができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a superabsorbent resin used for hygiene products and the like.
[0002]
[Prior art and problems to be solved by the invention]
Superabsorbent resins are used as hygroscopic substances in absorbent articles such as disposable diapers (disposable diapers) for infants, adults or incontinent persons, or sanitary napkins for women in the sanitary products field.
At present, polyacrylic acid-based superabsorbent resins dominate as superabsorbent resins.
[0003]
In the process of producing a highly water-absorbent resin performed by polymerization of water-soluble vinyl monomers, especially acrylic acid monomers, a redox system is generally used as a polymerization initiator, which is a combination of persulfate, peroxide, azo compound, peroxide and reducing agent. However, when industrial production is actually considered, peroxides are not preferred because they often require handling because of the danger of explosion. Persulfate is inexpensive and highly safe, and is most commonly used, but it is known that self-crosslinking occurs together with the ability to initiate polymerization. When obtaining a water-absorbing resin having a high water-absorbing ability, it is required to lower the degree of crosslinking. However, when a persulfate is used, a resin having a sufficient water-absorbing ability cannot be obtained because self-crosslinking occurs. Although reducing the amount of persulfate used in order to suppress self-crosslinking is disclosed in JP-A-6-287233, the reduction of persulfate substantially reduces the initiator, so the polymerization rate And increase in the amount of unreacted monomer and destabilization of polymerization. If an azo compound is used as an initiator, self-crosslinking can be suppressed, but the polymerization rate is low, the amount of unreacted monomer is large, and the unit price is economically disadvantageous at present.
[0004]
Further, a resin having a low degree of crosslinking tends to be insufficient in strength in the state of gel containing water, that is, in shape retention, water absorption capacity, stability over time, and the like in a pressurized state. Recently, for the purpose of improving the durability and the like of this water-containing gel, there is a method of incorporating a specific titanium compound (Japanese Patent Laid-Open Nos. 6-306118 and 7-62252). In the actual production, the method of adding the agent causes an increase in addition equipment and a complicated process, which is economically disadvantageous.
[0005]
Therefore, an object of the present invention is to produce a water-absorbing resin having a high water-absorbing retention capability and having good stability over time in a water-containing state, easily and inexpensively and with good reproducibility without adding additives or the like. It is to provide a method.
[0006]
[Means for Solving the Problems]
In the present invention, when a highly water-absorbent resin is obtained by polymerizing a water-soluble monomer (hereinafter referred to as a monomer) in the presence of a polymerization initiator, the total amount of the polymerization initiator used is 0.01 parts by weight based on 100 parts by weight of the monomer. 10 parts by weight or more, and while the monomer conversion is up to 50%, the polymerization initiator is added to 1/4 or more of the total amount used, and the rate of addition of the polymerization initiator to the polymerization system is started. Provided is a method for producing a superabsorbent resin that is changed at least once during the period from the start to the end (excluding the polymerization start and end points).
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Hereafter, the manufacturing method of the superabsorbent resin of this invention is demonstrated per the preferable embodiment. Examples of the monomer used in the present invention include various vinyl monomers that are water-soluble and have a polymerizable unsaturated group, and specifically include olefinic unsaturated carboxylic acids or salts thereof, olefinic unsaturated carboxylic acids. Esters, olefinically unsaturated sulfonic acids or their salts, olefinically unsaturated phosphoric acids or their salts, olefinically unsaturated phosphates, olefinically unsaturated amines, olefinically unsaturated ammonium, olefinically unsaturated amides, etc. Examples thereof include vinyl monomers having a polymerizable unsaturated group.
Of these, olefinically unsaturated carboxylic acids and salts thereof are preferably used, more preferably acrylic acid, methacrylic acid, and alkali metal salts and ammonium salts thereof, and more preferably acrylic acid and acrylic acid. Acid alkali metal salts (sodium salt, potassium salt, etc.) and ammonium acrylate are used.
One or more of these monomers can be used.
[0008]
The monomer can also be used in combination with a water-insoluble vinyl monomer that can be copolymerized therewith. Examples of the water-insoluble vinyl monomer include unsaturated carboxylic acid ester monomers such as acrylic acid, methacrylic acid, maleic acid and fumaric acid having an alkyl group having 1 to 18 carbon atoms.
In this case, the monomer is preferably contained in the total vinyl monomer in an amount of 50% by weight or more, particularly 70% by weight or more.
[0009]
In this invention, the total usage-amount of a polymerization initiator is 0.01-10 weight part with respect to 100 weight part of monomers, Preferably it is 0.02-5 weight part. When the amount of the polymerization initiator is within this range, it is possible to obtain a water-absorbing resin in which polymerization proceeds smoothly, has a high water absorption retention capability, and has good stability over time in a water-containing state.
[0010]
Moreover, the addition method of the polymerization initiator in this invention is as follows.
That is, while the monomer conversion is up to 50%, the polymerization initiator is added to 1/4 or more, preferably 3/10 to 9/10 of the total amount used, and the polymerization initiator is added to the polymerization system. The speed is changed at least once, preferably twice or more, from the start to the end of the polymerization (except for the start and end points of the polymerization).
According to such an addition method, it is possible to obtain a highly water-absorbent resin having good temporal stability in a desired water-containing state in the present invention. When the rate of addition of the polymerization initiator to the polymerization system is constant from the start to the end of the polymerization, the desired highly water-absorbent resin in the present invention cannot be obtained.
[0011]
Here, the monomer conversion rate (%) is represented by the following formula (1).

[0012]
The method for adding a polymerization initiator in the present invention described above is specifically a method by adding a polymerization initiator or an aqueous solution thereof in advance to a monomer or an aqueous solution thereof, and the monomer or an aqueous solution thereof is added to a dispersion medium described later. At the same time, a method of simultaneously adding a polymerization initiator or an aqueous solution thereof into the dispersion medium, and a predetermined amount of the polymerization initiator or an aqueous solution thereof previously coexisted in the dispersion medium, and here the monomer or the aqueous solution thereof Thus, the object of the present invention is achieved.
[0013]
When the monomer is used as an aqueous solution, the concentration of the aqueous monomer solution is preferably 10 to 90% by weight, more preferably 30 to 60% by weight. Further, the concentration of the polymerization initiator aqueous solution when the polymerization initiator is used as an aqueous solution is preferably 0.05 to 90% by weight, more preferably 1 to 50% by weight.
[0014]
Examples of the polymerization initiator used in the present invention include an azo polymerization initiator and an oxidative polymerization initiator.
Specific examples of the azo polymerization initiator include those described in JP-A-8-337726, page 4, column 5, lines 4-19. These can use 1 or more types.
Among these, 2,2′-azobis (2-amidinopropane) dihydrohalide, 2,2′-azobis [2- (2-imidazolin-2-yl) propane] dihydrohalide and 4,4′-azobis-4 -1 or more types selected from the group which consists of cyanovaleric acid are preferable in order to achieve the objective of this invention. In the above compound, the halide is preferably chloride from the economical viewpoint.
[0015]
Specific examples of the oxidative polymerization initiator include those described in JP-A-8-337726, page 3, column 4, line 43 to page 4, column 5, line 3; Examples include hydrogen / ferrous salt, persulfate / sulfite, cumene hydroperoxide / ferrous salt, and redox polymerization initiators such as hydrogen peroxide / L-ascorbic acid. These can use 1 or more types. Among these, persulfate is preferable for achieving the object of the present invention.
[0016]
Further, one or more azo polymerization initiators and oxidative polymerization initiators can be used in combination.
[0017]
In the method for producing the superabsorbent resin of the present invention, the monomer polymerization method is not particularly limited, but is preferably performed by a polymerization method in which the monomer is supplied to the polymerization reaction system as the polymerization reaction proceeds.
More preferably, from the viewpoint of controlling the particle size of the resulting superabsorbent resin, it is carried out by a reverse phase suspension polymerization method in which a monomer or an aqueous solution thereof is fed into a dispersion medium containing a hydrophobic organic solvent.
Examples of the hydrophobic organic solvent include aliphatic hydrocarbons such as n-pentane, cyclopentane, n-hexane, cyclohexane, n-heptane, and methylcyclohexane, aromatic hydrocarbons such as benzene and toluene, and n-butyl alcohol. And aliphatic alcohols having 4 to 6 carbon atoms such as n-amyl alcohol, aliphatic ketones such as methyl ethyl ketone, and aliphatic esters such as ethyl acetate. One or more of these hydrophobic organic solvents can be used.
The amount of the hydrophobic organic solvent used is preferably 100 parts by weight or more, and more preferably 200 to 1000 parts by weight with respect to 100 parts by weight of the monomer.
[0018]
In addition to the hydrophobic organic solvent, an amphiphilic solvent may be added to the dispersion medium. Examples of the amphiphilic solvent include alcohols such as methanol, ethanol, propanol, and 2-propanol, ketones such as acetone, and ethers such as tetrahydrofuran and dioxane. The amount of the amphiphilic solvent used is preferably a total amount with the hydrophobic organic solvent and up to 500 parts by weight per 100 parts by weight of the monomer.
[0019]
Moreover, a dispersing agent can be used when polymerizing a monomer.
Examples of the dispersant include cationic and amphoteric surfactants such as sorbitan monostearate, sorbitan monolaurate and sorbitan fatty acid esters such as polyoxymethylene sorbitan monooleate, trimethylstearyl ammonium chloride and carboxymethyldimethylcetylammonium. Agents, anionic surfactants such as polyoxyethylene dodecyl ether sulfate sodium salt and dodecyl ether sulfate sodium salt, glycoside compounds such as alkyl glucoside, cellulose ethers such as ethyl cellulose and benzyl cellulose, cellulose acetate, cellulose butyrate and Cellulose esters such as cellulose acetate butyrate, maleated polybutadiene, maleated polyethylene, maleated α-o Fin, styrene - dimethylaminoethyl methacrylate quaternary salt and isopropyl methacrylate - can be exemplified a polymer dispersant such as dimethylaminoethyl methacrylate quaternary salt. One or more of these dispersants can be used.
The amount of the dispersant used is preferably 0.01 to 5 parts by weight with respect to 100 parts by weight of the monomer.
In particular, when reverse phase suspension polymerization of a monomer is performed, it is preferable to use an ionic surfactant as a dispersant for the monomer in terms of preventing aggregation of the superabsorbent resin particles.
[0020]
The polymerization temperature for polymerizing the monomer is preferably 20 to 120 ° C, more preferably 40 to 100 ° C. When the polymerization temperature is within this range, a preferable polymerization rate is achieved.
[0021]
In carrying out the method for producing the superabsorbent resin of the present invention, a crosslinking agent can be added before polymerization, at the time of polymerization, after polymerization, or at the time of drying. Examples of the crosslinking agent include polyallyl compounds, polyvinyl compounds, polyglycidyl ethers, haloepoxy compounds, polyaldehydes, polyols, polyamines, hydroxyvinyl compounds, and inorganic salts that generate polyvalent ions such as calcium, magnesium, zinc, and aluminum, or An organic metal salt etc. can be illustrated.
[0022]
In addition, after the polymerization of the monomer (preferably reversed phase suspension polymerization) is completed, a desired superabsorbent resin can be obtained by performing usual post-treatment, for example, azeotropic dehydration, drying, etc., if necessary. it can.
[0023]
【Example】
“%” In Examples and Comparative Examples represents “% by weight”. The test method is as follows.
[0024]
[Measurement method of physiological saline retention]
1 g of superabsorbent resin was swollen with 150 ml of physiological saline (0.9% NaCl aqueous solution, manufactured by Otsuka Pharmaceutical) for 30 minutes, then placed in a non-woven bag, dehydrated with a centrifuge at 143 G for 10 minutes, The total weight (total weight) was measured, and the retained amount after centrifugal dehydration was measured according to the following formula (2).
[0025]

[0026]
[Example 1]
In a 5000 ml four-necked flask equipped with a stirrer, reflux condenser, dropping funnel and nitrogen gas inlet tube, 1600 ml of cyclohexane as a hydrophobic organic solvent and polyoxyethylene lauryl ether sulfate (average number of moles of added ethylene oxide = 2) 0.82 g of 25% aqueous solution was charged, nitrogen gas was blown in to expel dissolved oxygen, and the temperature was raised to 75 ° C. In another flask, while cooling 510 g of 80% aqueous acrylic acid solution from the outside, it was neutralized with a 30% aqueous sodium hydroxide solution to a neutralization degree of 70% to obtain an aqueous sodium acrylate solution. Next, 1.428 g of potassium persulfate (hereinafter referred to as “KPS”) as a polymerization initiator was added to 2.04 g of 25% aqueous solution of polyoxyethylene lauryl ether sulfate (average number of moles of added ethylene oxide = 2) to 35 g of ion-exchanged water. It was dissolved to obtain a polymerization initiator aqueous solution.
3/14 of this aqueous polymerization initiator solution was taken out and mixed with 1/4 of the aqueous sodium acrylate solution (monomer solution A). Further, 2/14 of the polymerization initiator aqueous solution was taken out and mixed with 1/4 of the sodium acrylate aqueous solution (monomer solution B). The remaining aqueous polymerization initiator solution (9/14 of the total aqueous polymerization initiator solution) was mixed with the remaining aqueous sodium acrylate solution (1/2 of the total aqueous sodium acrylate solution) (monomer solution C). Thereafter, nitrogen gas was blown to remove oxygen remaining in the aqueous solution.
First, the monomer solution A was dropped into the four-necked flask over 15 minutes, then the monomer solution B was dropped over 15 minutes, and then the monomer solution C was dropped over 30 minutes for polymerization.
After completion of the polymerization, azeotropic dehydration was performed using a dehydrating tube, and the water content of the superabsorbent resin was adjusted to 40 parts by weight with respect to 100 parts by weight of the superabsorbent resin. Thereafter, a solution obtained by dissolving 0.4 g of polyglycerol polyglycidyl ether (manufactured by Nagase Kasei Kogyo Co., Ltd., trade name Denacol EX-512) in 10 g of water as a crosslinking agent was added and reacted at 75-80 ° C. for 2 hours. . Thereafter, azeotropic dehydration was further performed, and the water content of the superabsorbent resin was adjusted to 30 parts by weight with respect to 100 parts by weight of the superabsorbent resin. After cooling, cyclohexane was removed by decantation and dried under conditions of 80 to 100 ° C. and 6.666 kPa to obtain a highly water-absorbent resin. The amount of physiological saline retained in the obtained superabsorbent resin was measured. The results are shown in Table 1.
[0027]
[Examples 2 to 4 and Comparative Example 1]
Except for making the addition amount of KPS used as a polymerization initiator as shown in Table 1, the same operation as in Example 1 was performed to obtain a highly water-absorbent resin. The amount of physiological saline retained in the obtained superabsorbent resin was measured. The results are shown in Table 1.
[0028]
[Table 1]

[0029]
【The invention's effect】
According to the method for producing a highly water-absorbent resin of the present invention, a water-absorbent resin having a high water-absorbing retention ability and good stability over time in a water-containing state can be easily and inexpensively added without adding an additive or the like. Can be obtained with good reproducibility.

Claims (3)

When a highly water-absorbent resin is obtained by polymerizing a water-soluble monomer (hereinafter referred to as a monomer) in the presence of a polymerization initiator,
The total amount of the polymerization initiator is 0.01 to 10 parts by weight with respect to 100 parts by weight of the monomer, and the polymerization initiator is not less than 1/4 of the total amount used while the conversion rate of the monomer is up to 50%. A method for producing a superabsorbent resin, which is added and the rate of addition of the polymerization initiator to the polymerization system is changed at least once during the period from the start to the end of the polymerization (except for the start and end points of the polymerization). The method for producing a superabsorbent resin according to claim 1, wherein the monomer or an aqueous solution thereof is supplied into a dispersion medium containing a hydrophobic organic solvent and subjected to reverse phase suspension polymerization. 3. The process for producing a superabsorbent resin according to claim 2, wherein reverse phase suspension polymerization is carried out in the presence of an ionic surfactant.