【発明の詳細な説明】[Detailed description of the invention]
産業上の利用分野
本発明は、吸水性樹脂の新規な製造法に関す
る。
従来の技術
吸水性樹脂は、生理用品、おむつ、使い捨て雑
巾等の衛生用品や保水剤等の農園芸用品として使
用されている他、汚泥の凝固、建材の結露防止、
油類の脱水等の用途にも用いられている。
この種の吸水性樹脂としては、カルボキシメチ
ルセルロース架橋物、ポリオキシエチレン架橋
物、澱粉−アクリロニトリルグラフト共重合体の
加水分解物、、澱粉−アクリル酸グラフト共重合
体、アクリル酸塩重合体架橋物、アクリル酸塩系
共重合体架橋物等が知られている。これらの内、
カルボキシメチルセルロース架橋物及びポリオキ
シエチレン架橋物では未だ満足すべき吸水能、保
水能を有するものは得られていない。また、澱粉
−アクリロニトリルグラフト共重合体の加水分解
物及び澱粉−アクル酸グラフト共重合体は比較的
高い吸水能、保水能を有するが、天然高分子であ
る澱粉を使用しているため耐熱性、腐敗分解等に
欠点があり、その製造方法も煩雑である。而し
て、アクリル酸塩重合体架橋物及びアクリル酸塩
系共重合体架橋物については吸水能、保水能及び
品質安定性等を満足し得るものである。
従来、アクリル酸塩重合体架橋物又はアクリル
酸塩系共重合体架橋物の製造法としては、水溶液
重合、逆相乳化重合、逆相懸濁重合等の各種方法
が知られているが、これらの方法の内逆乳化重
合、逆相懸濁重合等の場合は、重合工程に有機溶
剤を用いることが必須となり、この有機溶剤の使
用によれば、突発的重合や重合温度管理のミス等
の発生により、反応系の温度や圧力が異常に上昇
し、爆発、火災を招く危険性あるいは作業員に対
する環境衛生等の問題があまり好ましくない。一
方、水溶液重合の場合は、従来、反応制御が容易
な点からバツチ式で熱重合させる方法が主流であ
るが、反応終期には極端に高粘度化し、内容物の
取り出し等の作業性が劣つたり、反応の温度制御
を容易にするため比較的低温度で反応させる必要
から反応時間が長くなる等のため生産効率が低い
という欠点を有する。
本発明者は、以前よりアクリル酸塩系共重合体
架橋物の製造技術殊に水溶液重合技術について
種々研究しており、既に、比較的高濃度の単量体
水溶液をあらかじめ加温しておき、これに重合反
応開始剤を添加して外部加熱を行うことなく重合
させると共に水分を気化させることにより、エン
ドレスベルト上等で連続的に重合できしかも乾燥
工程を要しないという極めて生産効率の高い画期
的な方法を開発するのに成功した（特開昭58−
71907号）。而して、この方法においては、生産効
率が高い反面、苛酷な重合条件故重合熱による水
の蒸発のため得られる樹脂が多孔質となる傾向が
ある。そのため、保水率が低下して加圧時に一旦
吸収した水が放出されるいわゆるもどり現象を生
じる場合があるという問題点があつた。また、時
として低分子量物が多く生成するために吸水時に
べとつき感が生じることもあつた。
発明が解決しようとする問題点
本発明の目的は、吸水能、保水能、保水率等の
性能に優れる吸水性樹脂を高い生産効率で収得で
きる全く新たな製造法を提供することにある。
問題点を解決するための手段
本発明者は、前記の如き実状に鑑み、上記目的
を達成するべく、特に、アクリル酸塩系共重合体
架橋物の水溶液重合において従来行なわれたこと
のない紫外線による重合について鋭意研究を重ね
た。その結果、アクリル酸のアルカリ金属塩及び
ジビニル系化合物を含有する単量体水溶液に特定
の光増感剤を混合し、これを紫外線照射すること
により目的を達成できることを見出した。本発明
は、かかる知見に基づいて完成されたものであ
る。
即ち本発明は、アクリル酸のアルカリ金属塩及
びジビニル系化合物を含有する単量体水溶液とア
ミジノ基を有する水溶性アゾ化合物である光増感
剤との混合物に紫外線を照射して重合及び架橋せ
しめることを特徴とする吸水性樹脂の製造法に係
る。
本発明において、吸水性樹脂を構成する主要な
単量体単位は、アクリル酸アルカリ金属塩であ
る。ここでアルカリ金属塩とはナトリウム塩又は
カリウム塩をいう。これらのアルカリ金属塩は、
アクリル酸を水酸化ナトリウム、水酸化カリウム
等で中和して得られるものであり、これらは必ず
しも完全中和物であることは要せず部分中物であ
つても何らさしつかえなく、通常は中和度が50〜
100％の範囲とするのがよい。即ち、50％に満た
ない場合は、得られる吸水性樹脂の吸水能が低下
するため好ましくない。
また、本発明において併用される他の単量体で
あるジビニル系化合物は、上記アクリル酸アルカ
リ金属塩と共重合及び架橋されることにより、得
られる吸水性樹脂に架橋構造を付与するために使
用されるものである。好ましいジビニル系化合物
としては、例えばジビニルベンゼン、Ｎ，N′−
メチレンビスアクリルアミド、Ｎ，N′−メチレ
ンビスメタクリルアミド、ポリエチレングリコー
ルジアクリレート、ポリプロピレングリコールジ
アクリレート等が挙げられ、これらは単独又は組
合せて使用し得る。ジビニル系化合物の使用量
は、得られる吸水性樹脂の吸水能、ゲル強度等を
考慮して適宜決定されるが、通常は全単量体中
0.001〜5.0重量％程度好ましくは0.005〜5.0重量
％とするのが良い。即ち、0.001重量％に満たな
い場合はゲル強度が低下する傾向にあり、5.0重
量％を越える場合は吸水能が低下する傾向にある
ためいずれも好ましくない。必須の単量体である
アクリル酸アルカリ金属塩及びジビニル系化合物
に加えて、更に必要により、アクリルアミド、ア
クリルアミド−２−メチルプロパンスルホン酸
塩、低級アクリル酸エステル、メタクリル酸等を
使用することもできるが、得られる吸水性樹脂の
吸水能、保水能、ゲル強度等を考慮すれば、その
使用量は全単量体の約20重量％以下とするのが良
い。
本発明においては、特定の光増感剤、即ちアミ
ジノ基を有する水溶性アゾ化合物を用いることを
必須とする。該アゾ化合物は、上記単量体を紫外
線により水溶液重合するに際しての重合速度、ラ
ジカル発生温度、単量体水溶液への溶解性等をい
ずれも満足するものである。好ましい具体例とし
ては、２，2′−アゾビス（Ｎ，N′−ジメチレンイ
ソブチルアミジン）２塩酸塩、２，2′−アゾビス
（２−アミジノプロパン）２塩酸塩、２，2′−ア
ゾビス（Ｎ，N′−ジメチレンイソブチルアミジ
ン）等を挙げることができ、これらの少なくとも
１種を用いる。該アゾ化合物は、本発明において
は主として光増感剤の機能を有するが、熱重合開
始剤としても働く。尚一般に紫外線重合に利用さ
れる光増感剤として、ジアセチル、ベンゾイン、
ベンジル、アントラキノン、アセトフエノン、ジ
フエニルジスルフイド、ベンゾフエノン及びこれ
らの各種誘導体が挙げられるが、本発明ではこれ
ら公知の光増感剤のいずれを用いても、重合が完
結せず未反応単量体が相当量残存する結果、得ら
れる吸水性樹脂の吸水時のべとつき感が認められ
ることになり、また該光増感剤は通常人体に有害
であるため衛生材料の用途には適していない。
本発明の製造法は、より詳細には、以下のよう
にして実施される。まず第一に、アクリル酸のア
ルカリ金属塩、ジビニル系化合物及び必要に応じ
て用いることのある他の単量体成分をそれぞれ所
定量づつ水に添加して溶解させ、単量体水溶液を
調製する。該単量体水溶液は、単量体濃度が通常
25〜65重量％程度、好ましくは30〜60重量％とさ
れ、また該水溶液の液温が０〜40℃程度、好まし
くは10〜25℃となるように調製するのがよい。こ
こで、単量体濃度が25重量％に満たない場合は得
られる吸水性樹脂の重合度が低下する傾向があ
り、他方65重量％を越える場合は反応時の反応系
温度が高くなり過ぎて得られる吸水性樹脂が多孔
質になり易く保水性良好なものが得にくい傾向に
あという不利がある。また、該水溶液の液温が０
℃に満たない場合は、該水溶液が凝固することと
なり、40℃を越える場合は反応系温度が高くなり
すぎるため保水率の低い多孔質状の吸水性樹脂と
なり易い傾向にありいずれも好ましくない。
ついで、上記単量体水溶液に前記特定の光増感
剤を攪拌混合して光増感剤を溶解させる。光増感
剤の使用量は、特に限定はされないが、通常は全
単量体に対して0.001〜5.0重量％程度、好ましく
は0.01〜1.0重量％とするのが適当である。尚、
反応に際して前記光増感剤に加えて過硫酸カリウ
ム等の水溶性熱重合開始剤を併用することもでき
る。次に、この混合液を適当な反応容器に仕込ん
だ後、上記反応系に紫外線を照射して重合及び架
橋反応を開始する。本発明の重合及び架橋反応で
は、本来紫外線が十分に透過しうる限り、反応容
器形態は制限されないが、得られる吸水性樹脂の
乾燥、粉砕工程での作業性等の便宜からはエンド
レスベルトや表面積の大きい開放容器を使用する
のが好ましく、また液厚みも特に制限されないが
同じく紫外線の透過を十分にする点から通常５cm
以下程度とするのが良い。また、紫外線の光量と
しては、特に限定されないが、通常20〜3500ｍ
Joule／cm2程度とするのが良い。この範囲により
少ない場合は重合及び架橋が不充分な傾向にあ
り、またこの範囲より多い場合は一旦重合及び架
橋した後に過剰のエネルギー照射のため架橋構造
が切断されて低分子量物が生成しべとつき感を生
ずることがあるので好ましくない。好ましい光量
は、200〜2000ｍJoule／cm2程度である。また、紫
外線照射に使用する光源としては、従来公知の光
源、例えば水銀ランプ、メタルハライドランプ等
をいずれもそのまま使用でき、前記液厚み等の反
応条件を考慮して適宜決定される。照射波長も、
特に限定されないが、通常200〜450nｍの波長光
が使用できる。紫外線照射により直ちに反応が開
始する。照射時間は、上記光量になるように適宜
決定されるが、例えばエンドレスベルトを用いた
場合は、上記条件下照射箇所を通常約数秒〜数分
程度の短時間通過すれば、反応が完結する。尚、
前記単量体濃度および単量体水溶液温度に調整さ
れている限り、通常反応系内の最高到達温度が60
〜70℃に留まるため、重合等の反応熱による含有
水からの気泡の発生はおこらず、そのため多孔質
状の樹脂が得られることはない。
かくして得られた吸水性樹脂は、含水率は通常
35〜75重量％程度であり、その外観は透明ゴム状
の弾性体である。そのため、用途に応じて必要が
あれば引続いて該樹脂を乾燥、粉砕する工程を経
て粉状物、粒状物となしうる。これら工程は公知
の方法をそのまま適用すればたり、特別の操作、
装置を使用する必要はない。例えば、乾燥装置と
して熱風乾燥機、赤外線乾燥機、流動層乾燥機等
を使用でき、乾燥温度は通常70〜200℃程度とす
ればよい。得られた乾燥吸水性樹脂は、例えば振
動式粉砕装置、衝撃式粉砕装置を用いて所望の粒
度に粉砕することができる。
本発明方法で得られる吸水性樹脂の大きさ、形
状等は、特に限定されず用途に応じて適宜選択す
ることができる。例えば衛生材料として使用する
場合には、通常は粒子状であり、約10〜600メツ
シユ程度の粒径を有するものが好ましい。
本発明法により得られた吸水性樹脂は、前記各
種用途に使用することができ、殊に、生理用品、
紙おむつ等の衛生用品用として賞用し得る。該吸
水性樹脂は用途に応じて単独で又は二酸化ケイ素
粉末、酸化チタン粉末等の無機質粉末あるいはゴ
ム等の有機填料と併用して用いられる。尚、二酸
化ケイ素粉末としては、たとえばコロイダルシリ
カ、ホワイトカーボン、超微粒子状シリカなどを
例示できる。
更には、上記方法で得られた吸水性樹脂粉砕物
を必要に応じて、後改質することも可能である。
例えば、吸水性樹脂に含有されるカルボキシレー
トと水溶性ジグリシジルエーテル化合物、ハロエ
ポキシ化合物、アルデヒド化合物等の公知の架橋
剤とを反応させて吸水性樹脂粉砕物を表面改質を
することも可能であり該改質物も上記と同用途に
利用することができる。
発明の効果
本発明方法によれば、下記の如き格別顕著な効
果が得られる。
(1) 水溶液重合法において始めて紫外線重合を行
なうことが可能になつたことにより、短時間で
且つ連続的に吸水性樹脂を製造できるため生産
効率が極めて高い。
(2) 得られる吸水性樹脂は、吸水能に優れること
は勿論のこと、多孔質構造とならないため保水
能及び保水率も極めて優れており、例えば衛生
用品に用いた場合に尿や経血のもどり現象は全
く見られない。
(3) しかも、重合及び架橋反応が充分に完結して
いるため得られる吸水性樹脂が低分子量物をほ
とんど含有しておらず、吸水時のべとつき感が
ない。
実施例
以下、参考例、実施例及び比較例を挙げて本発
明方法を更に具体的に説明するが、本発明がこれ
らに限定されないことはもとよりである。
参考例 １
氷冷下、アクリル酸328ｇ及び水543.2ｇに水酸
化ナトリウム136.55ｇ（アクリル酸に対して75モ
ル％に相当）を加えて中和して後、Ｎ，N′−メ
チレンビスアクリルアミド（以下、MBAMとい
う）0.040ｇ（全モノマー中0.01重量％）及び２，
2′−アゾビス（Ｎ，N′−ジメチレンイソブチルア
ミジン）２塩酸塩（和光純薬工業(株)、商品名
「VA−044」）0.403ｇ（全モノマーに対して0.1重
量％）を加えて溶解後、窒素ガスを吹き込んで溶
存酸素を追い出し液温を20℃にして全モノマー濃
度40重量％の調製液Ａを得た。
参考例 ２
参考例１に於て、水の使用量を687.15ｇに代え
た他は同様にして全モノマー濃度35重量％の調製
液をＢを得た。
参考例 ３
参考例１において、水の使用量を472.89ｇに代
えた他は同様にして全モノマー濃度43重量％の調
製液Ｃを得た。
参考例 ４
氷冷下、アクリル酸434.58ｇ及び水321.9ｇに
水酸化カリウム243.66ｇ（アクリル酸に対して72
モル％に相当）を加えて中和して後、
MBAM0.060ｇ（全モノマー中0.01重量％）及び
「VA−044」0.300ｇ（全モノマーに対して0.05重
量％）を加えて溶解後、窒素ガスを吹き込んで溶
存酸素を追い出し液温を20℃にして全モノマー濃
度60重量％の調液Ｄを得た。
参考例 ５
参考例１において、MBAM0.161ｇ（全モノマ
ー中0.04重量％）に代えた他は同様にして調製液
Ｅを得た。
参考例 ６
参考例１において、「VA−044」0.202ｇ（全モ
ノマーに対して0.05重量％）に代えた他は同様に
して調製液Ｆを得た。
参考例 ７
参考例１において、「VA−044」1.209ｇ（全モ
ノマーに対して0.3重量％）に代えた他は同様に
して調製液Ｇを得た。
参考例 ８
参考例１において、「VA−044」を２，2′−ア
ゾビス（２−アミジノプロパン）２塩酸塩（和光
純薬工業(株)、商品名「Ｖ−50」）0.403ｇ（全モノ
マーに対して0.1重量％）に代えた他は同様にし
て調製液Ｈを得た。
参考例 ９
参考例１において、過硫酸カリウム0.202ｇ
（全モノマーに対して0.05重量％）を追加した他
は同様にして調製液Ｉを得た。
参考例 10
参考例１において、MBAMを無添加とした他
は、同様にして調製液Ｊを得た。
参考例 11
参考例１において、「VA−044」をジアセチル
（和光純薬工業(株)）1.209ｇ（全モノマーに対して
0.3重量％）に代えた他は同様にして調製液Ｋを
得た。
参考例 12
参考例１において、「VA−044」をアセトフエ
ノン（和光純薬(株)）1.209ｇ（全モノマーに対し
て0.3重量％）に代えた他は同様にして調製液Ｌ
を得た。
実施例 １
参考例１の調製液A176.7ｇを内径150mm、深さ
25mmのガラス製シヤーレに液厚み8.6mmとなるよ
う加え、コンベアー型紫外線硬化装置（アイグラ
フイツク(株)製）（高圧水銀ランプ2Kw×２灯、
80W／cm、発光波長250nｍ）にて光量900ｍ
Joule／cm2となるように速度10ｍ／min、照射時
間10.8秒で紫外線照射し、厚さ約９mmのゴム弾性
のある含水ゲル状架橋重合体（含水率約53重量
％）を得た。そして、このゲル状架橋重合体を熱
風乾燥機中140℃で２時間乾燥した後、粉砕機で
粉砕して粉体とした。この粉体を更に熱風乾燥機
中で30分乾燥し、吸水性樹脂Ａを得た。このもの
の粒度は32〜200メツシユであり、含水率は４〜
６重量％であつた。
実施例 ２
実施例１において、調製液Ａの使用量を35.3ｇ
に代えて、液厚み1.7mmとした他は同様に重合、
乾燥及び粉砕して吸水性樹脂Ｂを得た。
実施例 ３
実施例１において、調製液Ａの使用量を353.4
ｇに代え、液厚み17.2mmとした他は同様に重合、
乾燥及び粉砕して吸水性樹脂Ｃを得た。
実施例 ４
実施例１において、調製液Ａを調製液B176.7
ｇに代えた他は同様に重合、乾燥及び粉砕して吸
水性樹脂Ｄを得た。
実施例 ５
実施例１において、調製液Ａを調製液C176.7ｇ
に代えた他は同様に重合、乾燥及び粉砕して吸水
性樹脂Ｅを得た。
実施例 ６
実施例１において、調製液Ａを調製液D176.7
ｇに代えた他は同様に重合、乾燥及び粉砕して吸
水性樹脂Ｆを得た。
実施例 ７
実施例１において、調製液Ａを調製液E176.7ｇ
に代えた他は同様に重合、乾燥及び粉砕して吸水
性樹脂Ｇを得た。
実施例 ８
実施例１において、調製液Ａを調製液F176.7ｇ
に代えた他は同様に重合、乾燥及び粉砕して吸水
性樹脂Ｈを得た。
実施例 ９
実施例１において、調製液Ａを調製液G176.7
ｇに代え、かつ照射光量を300ｍJoule／cm2で照射
時間を3.6秒とした他は同様に重合、乾燥及び粉
砕して吸水性樹脂Ｉを得た。
実施例 10
実施例１において、調製液Ａを調製液H176.7
ｇに代えた他は同様に重合、乾燥及び粉砕して吸
水性樹脂Ｊを得た。
実施例 11
実施例１において、調製液Ａを調製液I176.7ｇ
に代えた他は同様に重合、乾燥及び粉砕して吸水
性樹脂Ｋを得た。
実施例 12
実施例１において、照射光量1800ｍJoule／cm2
で照射時間を21.6秒に代えた他は同様にして行い
吸水性樹脂Ｌを得た。
比較例 １
実施例１において、調製液Ａを調製液J176.7ｇ
に代えた他は同様にして重合、乾燥及び粉砕して
含水ゲル状重合物を得た。しかし、このものは外
観は含水ゲル状を呈するものの、架橋重合物では
ないため、水を添加した場合には単なる粘ちよう
水溶液となり、吸水性樹脂としての性能評価はで
きなかつた。
比較例 ２
実施例１において、調製液Ａを調製液K176.7
ｇに代えた他は同様にして重合を行つたが、末重
合物が多く含水ゲル状物を得ることはできなかつ
た。このものの重合率は72％であり、含水率は58
重量％であり、外観は粘ちよう液体であつた。そ
のため、以後の乾燥工程及び性能評価は断念し
た。
比較例 ３
実施例１において、調製液Ａを調製液L176.7ｇ
に代えた他は同様にして重合を行つたが、未重合
物が多く含水ゲル状物を得ることはできなかつ
た。このものの重合率は69％であり、含水率は58
重量％であり、外観は粘ちよう液体であつた。そ
のため、以後の乾燥工程及び性能評価は断念し
た。
次に、各実施例で得た各吸水性樹脂の性能を下
記試験方法で調べた。
吸水性樹脂性能試験方法
Γ吸水能
250メツシユナイロンネツト製テイーバツグ
に試料1.00ｇを正確にはかりとり、生理食塩水
に１時間浸漬、15分間水切り後、重量［ａ］ｇ
を測定する。さらに、試料を入れないテイーバ
ツグを用いて同様の測定を行い重量［ｂ］ｇを
求め、下式により生理食塩水吸水量を算出し
た。
生理食塩水吸水量（ｇ／ｇ）＝（［ａ］−
［ｂ］）／1.00
Γ保水能
テイーバツク法測定後、国産遠心機(株)製（Ｈ
−120A型）の遠心脱水機の目盛り6.5にて
600rpmとなるまで遠心脱水を行ない重量［ａ］
ｇを測定する。さらに、試料を入れないテイー
バツグを用いて同様の測定を行ない重量［ｂ］
ｇを求め、下式により保水量を算出した。
保水量（ｇ／ｇ）＝（［ａ］−［ｂ］）／1.00
Γ保水率
下式により保水率を算出した。
保水率（％）＝保水量／吸水量×100
結果を第１表に示す。
INDUSTRIAL APPLICATION FIELD The present invention relates to a novel method for producing a water-absorbing resin. Conventional technology Water-absorbing resins are used in sanitary products such as sanitary products, diapers, and disposable rags, as well as agricultural and gardening products such as water retention agents, as well as in coagulation of sludge, prevention of condensation on building materials,
It is also used for purposes such as dehydrating oils. Examples of this type of water-absorbing resin include carboxymethylcellulose crosslinked products, polyoxyethylene crosslinked products, starch-acrylonitrile graft copolymer hydrolysates, starch-acrylic acid graft copolymers, acrylate polymer crosslinked products, Crosslinked acrylate copolymers are known. Among these,
Carboxymethylcellulose crosslinked products and polyoxyethylene crosslinked products have not yet been found to have satisfactory water absorption and water retention abilities. In addition, the hydrolyzate of starch-acrylonitrile graft copolymer and the starch-acrylic acid graft copolymer have relatively high water absorption and water retention capacity, but because they use starch, which is a natural polymer, they have low heat resistance and It has drawbacks such as rotting and decomposition, and its manufacturing method is also complicated. Therefore, the crosslinked acrylate polymer and the crosslinked acrylate copolymer can satisfy water absorption ability, water retention ability, quality stability, etc. Conventionally, various methods such as aqueous solution polymerization, reverse-phase emulsion polymerization, and reverse-phase suspension polymerization are known as methods for producing cross-linked acrylate polymers or cross-linked acrylate copolymers. Among these methods, in the case of inverse emulsion polymerization, inverse phase suspension polymerization, etc., it is essential to use an organic solvent in the polymerization process, and the use of this organic solvent prevents sudden polymerization and errors in polymerization temperature control. When this occurs, the temperature and pressure of the reaction system rise abnormally, which poses an undesirable risk of causing an explosion or fire, as well as environmental health problems for workers. On the other hand, in the case of aqueous solution polymerization, conventionally, the mainstream method has been to thermally polymerize in batches because it is easy to control the reaction, but the viscosity becomes extremely high at the end of the reaction, making workability such as removing the contents poor. This method has drawbacks such as low production efficiency due to the need to react at a relatively low temperature in order to easily control the temperature of the reaction, resulting in a long reaction time. The present inventor has been conducting various research on manufacturing techniques, particularly aqueous solution polymerization techniques, for crosslinked acrylate copolymers, and has already heated an aqueous monomer solution with a relatively high concentration in advance. By adding a polymerization reaction initiator and polymerizing without external heating and vaporizing the water, polymerization can be carried out continuously on an endless belt, etc., and no drying process is required, a breakthrough in extremely high production efficiency. succeeded in developing a method for
No. 71907). Although this method has high production efficiency, the resulting resin tends to be porous due to evaporation of water due to heat of polymerization due to severe polymerization conditions. Therefore, there is a problem in that the water retention rate decreases and a so-called rebound phenomenon may occur in which water that has been once absorbed is released during pressurization. In addition, because a large amount of low-molecular-weight substances are sometimes produced, a sticky feeling may occur when water is absorbed. Problems to be Solved by the Invention An object of the present invention is to provide a completely new manufacturing method that can obtain a water-absorbing resin with excellent performance such as water absorption capacity, water retention capacity, and water retention rate with high production efficiency. Means for Solving the Problems In view of the above-mentioned circumstances, the present inventors, in order to achieve the above-mentioned object, particularly aimed at using ultraviolet rays, which had not been conventionally carried out in the aqueous solution polymerization of cross-linked acrylate-based copolymers. We have conducted extensive research on the polymerization of As a result, they found that the objective could be achieved by mixing a specific photosensitizer with an aqueous monomer solution containing an alkali metal salt of acrylic acid and a divinyl compound, and irradiating the mixture with ultraviolet rays. The present invention was completed based on this knowledge. That is, in the present invention, a mixture of an aqueous monomer solution containing an alkali metal salt of acrylic acid and a divinyl compound and a photosensitizer, which is a water-soluble azo compound having an amidino group, is irradiated with ultraviolet rays to cause polymerization and crosslinking. The present invention relates to a method for producing a water-absorbing resin characterized by the following. In the present invention, the main monomer unit constituting the water-absorbing resin is an alkali metal acrylate salt. Here, the alkali metal salt refers to sodium salt or potassium salt. These alkali metal salts are
It is obtained by neutralizing acrylic acid with sodium hydroxide, potassium hydroxide, etc., and these do not necessarily have to be completely neutralized, but there is no problem even if they are partially neutralized, and they are usually neutralized. Japanese degree is 50~
It is best to set the range to 100%. That is, if it is less than 50%, the water absorbing ability of the resulting water absorbent resin decreases, which is not preferable. In addition, the divinyl compound, which is another monomer used in combination in the present invention, is used to impart a crosslinked structure to the resulting water absorbent resin by copolymerizing and crosslinking with the alkali metal acrylate salt. It is something that will be done. Preferred divinyl compounds include, for example, divinylbenzene, N,N'-
Examples include methylene bisacrylamide, N,N'-methylene bismethacrylamide, polyethylene glycol diacrylate, polypropylene glycol diacrylate, and the like, and these may be used alone or in combination. The amount of divinyl compound to be used is determined appropriately taking into consideration the water absorption capacity, gel strength, etc. of the resulting water absorbent resin, but it is usually
The content is preferably about 0.001 to 5.0% by weight, preferably 0.005 to 5.0% by weight. That is, if it is less than 0.001% by weight, the gel strength tends to decrease, and if it exceeds 5.0% by weight, the water absorption capacity tends to decrease, which are both unfavorable. In addition to the essential monomers, acrylic acid alkali metal salt and divinyl compound, acrylamide, acrylamide-2-methylpropanesulfonate, lower acrylic acid ester, methacrylic acid, etc. can also be used if necessary. However, in consideration of the water absorption capacity, water retention capacity, gel strength, etc. of the resulting water absorbent resin, the amount used is preferably about 20% by weight or less based on the total monomers. In the present invention, it is essential to use a specific photosensitizer, that is, a water-soluble azo compound having an amidino group. The azo compound satisfies all of the polymerization rate, radical generation temperature, solubility in the monomer aqueous solution, etc. when the above-mentioned monomer is polymerized in aqueous solution using ultraviolet rays. Preferred specific examples include 2,2'-azobis(N,N'-dimethyleneisobutyramidine) dihydrochloride, 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis( N,N'-dimethyleneisobutyramidine), etc., and at least one of these is used. The azo compound mainly functions as a photosensitizer in the present invention, but also functions as a thermal polymerization initiator. In addition, diacetyl, benzoin,
Examples include benzyl, anthraquinone, acetophenone, diphenyl disulfide, benzophenone, and various derivatives thereof, but in the present invention, even if any of these known photosensitizers is used, polymerization will not be completed and unreacted monomers will remain. As a result, a considerable amount of the photosensitizer remains, and the resultant water-absorbing resin exhibits a sticky feeling when water is absorbed.Furthermore, the photosensitizer is usually harmful to the human body, so it is not suitable for use in sanitary materials. More specifically, the manufacturing method of the present invention is carried out as follows. First, an aqueous monomer solution is prepared by adding predetermined amounts of an alkali metal salt of acrylic acid, a divinyl compound, and other monomer components that may be used as necessary to water and dissolving them. . The monomer aqueous solution usually has a monomer concentration
The amount is about 25 to 65% by weight, preferably 30 to 60% by weight, and the temperature of the aqueous solution is preferably about 0 to 40°C, preferably 10 to 25°C. Here, if the monomer concentration is less than 25% by weight, the degree of polymerization of the resulting water absorbent resin tends to decrease, while if it exceeds 65% by weight, the reaction system temperature during the reaction becomes too high. Another disadvantage is that the resulting water absorbent resin tends to be porous, making it difficult to obtain one with good water retention. In addition, the temperature of the aqueous solution is 0.
If it is less than 40°C, the aqueous solution will solidify, and if it exceeds 40°C, the reaction system temperature will become too high, which tends to result in a porous water-absorbing resin with low water retention, both of which are not preferred. Next, the specific photosensitizer is stirred and mixed into the monomer aqueous solution to dissolve the photosensitizer. The amount of the photosensitizer to be used is not particularly limited, but it is usually about 0.001 to 5.0% by weight, preferably 0.01 to 1.0% by weight based on the total monomers. still,
In addition to the photosensitizer, a water-soluble thermal polymerization initiator such as potassium persulfate can also be used in the reaction. Next, after charging this mixed solution into a suitable reaction vessel, the reaction system is irradiated with ultraviolet rays to initiate polymerization and crosslinking reactions. In the polymerization and crosslinking reactions of the present invention, the shape of the reaction vessel is not limited as long as ultraviolet rays can pass through the reaction vessel sufficiently. It is preferable to use a large open container, and the thickness of the liquid is not particularly limited, but it is usually 5 cm from the viewpoint of ensuring sufficient penetration of ultraviolet rays.
It is best to set it to about the following. In addition, the amount of ultraviolet light is not particularly limited, but it is usually 20 to 3500 m.
It is best to set it to about Joule/ cm2 . If the amount is less than this range, polymerization and crosslinking tend to be insufficient, and if the amount is more than this range, the crosslinked structure will be cut due to excessive energy irradiation after polymerization and crosslinking, resulting in the production of low molecular weight substances and a sticky feeling. This is not preferable as it may cause A preferable amount of light is about 200 to 2000 mJoule/cm 2 . Further, as the light source used for ultraviolet irradiation, any conventionally known light source such as a mercury lamp, a metal halide lamp, etc. can be used as is, and is appropriately determined in consideration of the reaction conditions such as the liquid thickness. The irradiation wavelength is also
Although not particularly limited, light with a wavelength of 200 to 450 nm can usually be used. The reaction starts immediately upon irradiation with ultraviolet light. The irradiation time is appropriately determined so as to achieve the above-mentioned light intensity. For example, when an endless belt is used, the reaction is completed after passing through the irradiation area under the above-mentioned conditions for a short period of time, usually about several seconds to several minutes. still,
As long as the monomer concentration and monomer aqueous solution temperature are adjusted to the above, the maximum temperature in the reaction system is usually 60°C.
Since the temperature remains at ~70°C, bubbles are not generated from the water contained due to reaction heat such as polymerization, and therefore a porous resin is not obtained. The water absorbent resin thus obtained usually has a water content of
The content is approximately 35 to 75% by weight, and its appearance is that of a transparent rubber-like elastic body. Therefore, if necessary depending on the application, the resin can be subsequently dried and pulverized to form powder or granules. These steps can be carried out by applying known methods as they are, or by special operations or
No need to use equipment. For example, a hot air dryer, an infrared dryer, a fluidized bed dryer, etc. can be used as the drying device, and the drying temperature may normally be about 70 to 200°C. The obtained dry water-absorbing resin can be pulverized to a desired particle size using, for example, a vibration pulverizer or an impact pulverizer. The size, shape, etc. of the water absorbent resin obtained by the method of the present invention are not particularly limited and can be appropriately selected depending on the intended use. For example, when used as a sanitary material, it is usually in the form of particles, preferably having a particle size of about 10 to 600 mesh. The water-absorbing resin obtained by the method of the present invention can be used for the various purposes mentioned above, particularly for sanitary products,
It can be used for sanitary products such as disposable diapers. The water-absorbing resin may be used alone or in combination with an inorganic powder such as silicon dioxide powder or titanium oxide powder, or an organic filler such as rubber, depending on the purpose. Incidentally, examples of the silicon dioxide powder include colloidal silica, white carbon, and ultrafine particulate silica. Furthermore, it is also possible to post-modify the water absorbent resin pulverized product obtained by the above method, if necessary.
For example, it is also possible to surface-modify the surface of a pulverized water-absorbing resin by reacting the carboxylate contained in the water-absorbing resin with a known crosslinking agent such as a water-soluble diglycidyl ether compound, haloepoxy compound, or aldehyde compound. This modified product can also be used for the same purposes as above. Effects of the Invention According to the method of the present invention, the following particularly remarkable effects can be obtained. (1) Since it has become possible to carry out ultraviolet polymerization for the first time in an aqueous solution polymerization method, water-absorbing resin can be produced continuously in a short period of time, resulting in extremely high production efficiency. (2) The obtained water-absorbent resin not only has excellent water-absorbing ability, but also has extremely excellent water-retention ability and water-retention rate because it does not have a porous structure. No rebound phenomenon is observed at all. (3) Moreover, since the polymerization and crosslinking reactions are sufficiently completed, the resulting water-absorbing resin contains almost no low-molecular-weight substances and does not feel sticky when water is absorbed. EXAMPLES The method of the present invention will be described in more detail below with reference to Reference Examples, Examples, and Comparative Examples, but the present invention is not limited thereto. Reference Example 1 After neutralizing by adding 136.55 g of sodium hydroxide (equivalent to 75 mol% to acrylic acid) to 328 g of acrylic acid and 543.2 g of water under ice cooling, N,N'-methylenebisacrylamide ( 0.040g (0.01% by weight of all monomers) and 2,
Add 0.403 g (0.1% by weight of total monomers) of 2'-azobis(N,N'-dimethyleneisobutyramidine) dihydrochloride (Wako Pure Chemical Industries, Ltd., trade name "VA-044"). After dissolution, nitrogen gas was blown in to drive out the dissolved oxygen and the liquid temperature was raised to 20°C to obtain a prepared liquid A with a total monomer concentration of 40% by weight. Reference Example 2 A prepared liquid B having a total monomer concentration of 35% by weight was obtained in the same manner as in Reference Example 1 except that the amount of water used was changed to 687.15 g. Reference Example 3 A prepared liquid C having a total monomer concentration of 43% by weight was obtained in the same manner as in Reference Example 1 except that the amount of water used was changed to 472.89 g. Reference Example 4 Under ice-cooling, add 243.66 g of potassium hydroxide to 434.58 g of acrylic acid and 321.9 g of water (72
After neutralization by adding (equivalent to mol%),
Add 0.060 g of MBAM (0.01% by weight of all monomers) and 0.300 g of "VA-044" (0.05% by weight of all monomers) and dissolve, then blow nitrogen gas to drive out dissolved oxygen and raise the liquid temperature to 20℃. A preparation D having a total monomer concentration of 60% by weight was obtained. Reference Example 5 Preparation liquid E was obtained in the same manner as in Reference Example 1, except that 0.161 g of MBAM (0.04% by weight in total monomers) was used. Reference Example 6 Preparation liquid F was obtained in the same manner as in Reference Example 1, except that 0.202 g of "VA-044" (0.05% by weight based on the total monomers) was used. Reference Example 7 Preparation liquid G was obtained in the same manner as in Reference Example 1, except that 1.209 g of "VA-044" (0.3% by weight based on the total monomers) was used. Reference Example 8 In Reference Example 1, 0.403 g of 2,2'-azobis(2-amidinopropane) dihydrochloride (Wako Pure Chemical Industries, Ltd., trade name: A preparation solution H was obtained in the same manner except that the amount was changed to 0.1% by weight based on the monomer. Reference example 9 In reference example 1, potassium persulfate 0.202g
Preparation liquid I was obtained in the same manner except that (0.05% by weight based on the total monomers) was added. Reference Example 10 Preparation liquid J was obtained in the same manner as in Reference Example 1, except that MBAM was not added. Reference Example 11 In Reference Example 1, "VA-044" was added to 1.209 g of diacetyl (Wako Pure Chemical Industries, Ltd.) (based on all monomers).
Preparation liquid K was obtained in the same manner except that 0.3% by weight) was used. Reference Example 12 Prepare solution L in the same manner as in Reference Example 1, except that "VA-044" was replaced with 1.209 g of acetophenone (Wako Pure Chemical Industries, Ltd.) (0.3% by weight based on the total monomers).
I got it. Example 1 176.7 g of the prepared solution A of Reference Example 1 was poured into a container with an inner diameter of 150 mm and a depth of
Add the liquid to a 25 mm glass shear plate to a thickness of 8.6 mm, and add a conveyor-type ultraviolet curing device (manufactured by Eyegraphik Co., Ltd.) (high-pressure mercury lamps x 2 2Kw,
Light intensity 900m at 80W/cm, emission wavelength 250nm)
Ultraviolet rays were irradiated at a speed of 10 m/min and an irradiation time of 10.8 seconds so as to obtain a rubber elasticity of about 9 mm in thickness (water content of about 53% by weight). Then, this gel-like crosslinked polymer was dried in a hot air dryer at 140° C. for 2 hours, and then ground in a grinder to form a powder. This powder was further dried in a hot air dryer for 30 minutes to obtain water absorbent resin A. The particle size of this material is 32 to 200 mesh, and the moisture content is 4 to 200 mesh.
It was 6% by weight. Example 2 In Example 1, the amount of preparation liquid A used was 35.3g.
Polymerization was carried out in the same manner, except that the liquid thickness was 1.7 mm instead of
Water absorbent resin B was obtained by drying and pulverizing. Example 3 In Example 1, the amount of preparation liquid A used was 353.4
Polymerization was carried out in the same manner, except that the liquid thickness was 17.2 mm instead of g.
Water absorbent resin C was obtained by drying and pulverizing. Example 4 In Example 1, preparation solution A was replaced with preparation solution B176.7.
Water absorbent resin D was obtained by polymerizing, drying and pulverizing in the same manner except that g was replaced. Example 5 In Example 1, 176.7 g of Preparation Solution A was added to Preparation Solution C.
Water-absorbing resin E was obtained by polymerizing, drying, and pulverizing in the same manner except that . Example 6 In Example 1, preparation solution A was changed to preparation solution D176.7.
Water absorbent resin F was obtained by polymerizing, drying and pulverizing in the same manner except that g was replaced. Example 7 In Example 1, 176.7 g of Preparation Solution A was added to Preparation Solution E.
Water-absorbent resin G was obtained by polymerizing, drying, and pulverizing in the same manner except that . Example 8 In Example 1, 176.7 g of Preparation Solution A was added to Preparation Solution F.
Water-absorbing resin H was obtained by polymerizing, drying, and pulverizing in the same manner except that . Example 9 In Example 1, preparation liquid A was changed to preparation liquid G176.7.
Water-absorbent resin I was obtained by polymerizing, drying and pulverizing in the same manner, except that the irradiation time was changed to 300 mJoule/cm 2 and 3.6 seconds. Example 10 In Example 1, preparation solution A was replaced with preparation solution H176.7.
Water absorbent resin J was obtained by polymerizing, drying and pulverizing in the same manner except that g was replaced. Example 11 In Example 1, 176.7 g of Preparation Solution A was added to Preparation Solution I.
Water-absorbing resin K was obtained by polymerizing, drying and pulverizing in the same manner except that . Example 12 In Example 1, the irradiation light amount was 1800 mJoule/cm 2
A water-absorbing resin L was obtained in the same manner except that the irradiation time was changed to 21.6 seconds. Comparative Example 1 In Example 1, 176.7 g of Preparation Solution A was added to Preparation Solution J.
A hydrogel-like polymer was obtained by polymerizing, drying, and pulverizing in the same manner except that . However, although this product has a hydrogel-like appearance, it is not a crosslinked polymer, so when water is added, it becomes a mere sticky aqueous solution, and its performance as a water-absorbing resin cannot be evaluated. Comparative Example 2 In Example 1, Preparation Solution A was replaced with Preparation Solution K176.7.
Polymerization was carried out in the same manner except that g was used instead, but a hydrogel-like product could not be obtained because there were many end-polymerized products. The polymerization rate of this product is 72%, and the water content is 58%.
% by weight, and the appearance was a viscous liquid. Therefore, the subsequent drying process and performance evaluation were abandoned. Comparative Example 3 In Example 1, 176.7 g of Preparation Solution A was added to Preparation Solution L.
Polymerization was carried out in the same manner except that . The polymerization rate of this product is 69%, and the water content is 58%.
% by weight, and the appearance was a viscous liquid. Therefore, the subsequent drying process and performance evaluation were abandoned. Next, the performance of each water absorbent resin obtained in each example was examined using the following test method. Water-absorbing resin performance test method Γ Water-absorbing capacity Accurately weigh 1.00 g of the sample into a 250-mesh nylon net teabag, soak it in physiological saline for 1 hour, drain for 15 minutes, and then weigh [a] g.
Measure. Furthermore, similar measurements were carried out using a T-bag without a sample in it to determine the weight [b]g, and the water absorption amount of physiological saline was calculated using the following formula. Physiological saline water absorption (g/g) = ([a]-
[b]) / 1.00 ΓWater retention capacity After the tea bag method measurement,
-120A) centrifugal dehydrator scale 6.5
Centrifugal dehydration is performed until the speed reaches 600 rpm, and the weight [a]
Measure g. Furthermore, similar measurements were made using a T-bag without the sample, and the weight [b]
g was determined, and the water retention amount was calculated using the following formula. Water retention amount (g/g) = ([a]-[b])/1.00 Γ Water retention rate The water retention rate was calculated using the following formula. Water retention rate (%) = Water retention amount/Water absorption amount x 100 The results are shown in Table 1.
【表】
第１表より、本発明法により得られる吸水性樹
脂Ａ〜Ｌは、いずれも吸水能、保水能に優れ、保
水率が高いことが明らかである。また、Ａ〜Ｌ
は、いずれも吸水時のべとつき感はなかつた。[Table] It is clear from Table 1 that all of the water-absorbing resins A to L obtained by the method of the present invention have excellent water-absorbing ability and water-retaining ability, and have a high water-retaining rate. Also, A to L
There was no sticky feeling when water was absorbed.