JP3544372B2 - Acrylate-based polymer and sanitary material using the same - Google Patents

Description

【００２９】
本願発明は、（３） 該ポリマーの表面近傍が多価アルコールで架橋されたものである、上記（１）または（２）に記載のアクリル酸塩系ポリマーに関するものである。
【００３０】
本願発明は、（４） 残存モノマー量が１６０ｐｐｍ以下であるアクリル酸塩系ポリマーであって、かつ重合体中で部分中和または完全中和のアクリル酸塩５０〜１００モル％およびその他０〜５０モル％の親水性不飽和単量体および／または疎水性不飽和単量体の繰り返し単位を有する中和率３０〜１００モル％のアクリル酸塩系ポリマーであり、該ポリマーが架橋構造を有する乾燥した平均粒子径１００〜１０００μｍの吸水性樹脂であり、該ポリマーの表面近傍が多価アルコールでさらに架橋され、β−ヒドロキシプロピオン酸およびその塩の合計含有量が１０００ｐｐｍ以下であるアクリル酸塩系ポリマーに関するものである。
【００３１】
本願発明は、（５） 前記アクリル酸塩が、アクリル酸ナトリウム塩またはアクリル酸カリウム塩であることを特徴とする上記（１）〜（４）のいずれか１つに記載のアクリル酸塩系ポリマーに関するものである。
【００３２】
本願発明は、（６） 吸水性樹脂が平均粒子径３００〜６００μｍの粉末である上記（１）〜（５）のいずれか１つに記載のアクリル酸塩系ポリマーに関するものである。
【００３３】
本願発明は、（７） 上記（１）〜（６）のいずれか１つに記載のアクリル酸塩系ポリマーを用いた衛生材料に関するものである。
【００５０】
以下、本発明をさらに詳しく説明する。
【００５１】
本発明において、重合に用いられる水溶性不飽和単量体中のアクリル酸塩の割合は、５０〜１００モル％であることが必須である。（尚、本発明でアクリル酸塩とは、未中和アクリル酸および中和アクリル酸塩の合計（すなわち部分中和または完全中和のアクリル酸塩）を指す）。本発明において残存モノマー低減効果がより多く発現するため、本発明のより好ましいアクリル酸塩の割合は７０〜１００モル％、更には９０〜１００モル％である。アクリル酸塩の含有量が５０モル％未満の場合、性能の低下やコストアップとなるのみならず、本発明の効果が現れ難く、場合によっては、かえって残存モノマーが増加する場合があるので好ましくない。
【００５２】
また、水溶性不飽和単量体に用いられるアクリル酸塩の中和率は、３０〜１００モル％が必須である。中和率がこの範囲から外れると、諸物性の低下や残存モノマーの増加が見られる場合もある。（尚、本発明でいうアクリル酸塩の中和率は、水溶性不飽和単量体の調製に用いられるアクリル酸塩の中和率、または、必要に応じて他の単量体と混合された後の水溶性不飽和単量体中でのアクリル酸塩の中和率を指す）。更に、本発明の水溶性不飽和単量体を用いる場合、従来法に比べより顕著に残存モノマーの低減されるため、より好ましい中和率は４０〜９５モル％、更には５０〜８０モル％である。
【００５３】
本発明に用いられる水溶性不飽和単量体は、中和率３０〜１００モル％のアクリル酸塩を５０〜１００モル％含むものであるが、その他０〜５０モル％の親水性不飽和単量体および／または疎水性不飽和単量体を用いてもよい。
【００５４】
必要により０〜５０モル％の範囲で用いることのできる親水性不飽和単量体として、例えば、メタクリル酸、マレイン酸、無水マレイン酸、フマール酸、クロトン酸、イタコン酸、ビニルスルフォン酸、スチレンスルフォン酸、２−（メタ）アクリルアミド−２−メチルプロパンスルフォン酸、２−（メタ）アクリロイルエタンスルフォン酸、２−（メタ）アクリロイルプロパンスルフォン酸などの酸基含有の親水性不飽和単量体およびその塩；アクリルアミド、メタアクリルアミド、Ｎ−エチル（メタ）アクリルアミド、Ｎ−ｎ−プロピル（メタ）アクリルアミド、Ｎ−イソプロピル（メタ）アクリルアミド、Ｎ，Ｎ−ジメチル（メタ）アクリルアミド、２−ヒドロキシエチル（メタ）アクリレート、２−ヒドロキシプロピル（メタ）アクリレート、メトキシポリエチレングリコール（メタ）アクリレート、ポリエチレングリコールモノ（メタ）アクリレート、ビニルピリジン、Ｎ−ビニルピロリドン、Ｎ−アクリロイルピペリジン、Ｎ−アクリロイルピロリジンなどのノニオン性の親水性不飽和単量体；Ｎ，Ｎ−ジメチルアミノエチル（メタ）アクリレート、Ｎ，Ｎ−ジエチルアミノエチル（メタ）アクリレート、Ｎ，Ｎ−ジメチルアミノプロピル（メタ）アクリレート、Ｎ，Ｎ−ジメチルアミノプロピル（メタ）アクリルアミドおよびその四級塩などのカチオン性の親水性不飽和単量体などを挙げることができ、これらの群から選ばれる１種あるいは２種以上を使用できる。また、親水性不飽和単量体として、メチル（メタ）アクリレート、エチル（メタ）アクリレート、酢酸ビニルなどの様に、官能基の加水分解によって、親水性樹脂を形成する不飽和単量体を用いてもよい。
【００５５】
これらの中でも、本発明では０〜５０モル％用いられる親水性不飽和単量体として、メタクリル酸（塩）、２−（メタ）アクリロイルエタンスルフォン酸（塩）、２−（メタ）アクリルアミド−２−メチルプロパンスルフォン酸（塩）、メトキシポリエチレングリコール（メタ）アクリレート、Ｎ，Ｎ−ジメチルアミノエチル（メタ）アクリレート、アクリルアミドが好ましい。
【００５６】
また、必要により０〜５０モル％の範囲で用いることのできる疎水性不飽和単量体としては、例えば、スチレン、塩化ビニル、ブタジエン、イソブテン、エチレン、プロピレン、ステアリル（メタ）アクリレート、ラウリル（メタ）アクリレートなどが挙げられる。これらの疎水性不飽和単量体は、全単量体中で０〜５０モル％、好ましくは０〜２０モル％、更に好ましくは０〜１０モル％使用される。
【００５７】
なお、中和率３０〜１００モル％のアクリル酸塩を５０〜１００モル％含む水溶性不飽和単量体の中和は、該水溶性不飽和単量体の調製時にだけで行ってもよいし、前記範囲に調整後の該水溶性単量体を重合する際に重合中や重合後に更に中和してもよい。
【００５８】
本発明では、上記したような中和率３０〜１００モル％のアクリル酸塩を５０〜１００モル％含む水溶性不飽和単量体を調製するにあたり、水溶性不飽和単量体あたりのβ−ヒドロキシプロピオン酸およびその塩（以下、両者を併せて、単にβ−ヒドロキシプロピオン酸と呼ぶ。）の含有量が、１０００ｐｐｍ以下の水溶性不飽和単量体を調製することが必須である。
【００５９】
本発明において、水溶性不飽和単量体中のβ−ヒドロキシプロピオン酸の含有量は、固形分あたり１０００ｐｐｍ以下であることが必須であり、好ましくは５００ｐｐｍ以下、更に好ましくは３００ｐｐｍ以下、最も好ましくは１００ｐｐｍ以下である。β−ヒドロキシプロピオン酸の含有量が１０００ｐｐｍを越える場合、得られたアクリル酸塩系ポリマーは残存モノマーが多く、しかも、その後の製造工程や高温下や長時間の使用で、更に残存モノマーが発生し増加するという現象を示すのみならず、アクリル酸塩系ポリマーの諸物性が不充分となる場合もあり好ましくない。
【００６０】
本発明では、重合に用いる水溶性不飽和単量体中のβ−ヒドロキシプロピオン酸の含有量が１０００ｐｐｍ以下の量に制限されていればよく、この様な水溶性不飽和単量体を調製する手段は特に制限されるものではないが、好ましい具体的実施態様の一例は下記の如くである。
【００６１】
アクリル酸を蒸溜精製後、できる限り短時間、例えば２４時間、好ましくは１２時間、更に好ましくは６時間以内に中和や水溶性不飽和単量体の調整に用いる。また、中和工程では、少なくとも一時期は中和率が１００モル％を越える状態を経過させる。次いで、該アクリル酸や得られたアクリル酸塩、その他必要に応じて他の単量体を用いて水溶性不飽和単量体を調製する。最後に、得られた水溶性不飽和単量体はできる限り短時間、例えば、水溶性不飽和単量体調製終了後２４時間、好ましくは１２時間、更に好ましくは６時間、特に好ましくは２時間以内に重合すれば良い。また、蒸留精製後のアクリル酸は中和や水溶性不飽和単量体の調製に用いるまで、可能な限り低温、例えば３０℃以下、好ましくは凝固点〜２５℃に保たれるのが好ましい。中和工程は低温で短時間、好ましくは４時間以内に行われることが好ましい。蒸留後のアクリル酸が比較的長時間保たれる場合は、非水状態にしておくのが良い。更に調製終了後の水溶性不飽和単量体は、その凝固点以上〜４０℃以下、好ましくは０〜３０℃での保存されるべきである。尚これらの条件を外れると、β−ヒドロキシプロピオン酸や残存モノマーが増加する傾向があり注意を要する。
【００６２】
アクリル酸はアクリル酸製造所で最終的な蒸留後、貯蔵、出荷されるため、ユーザーで工業的に実使用されるまでには短くて４〜５日、通常数１０日〜数ヶ月経過しているのが現状である。また、水溶性不飽和単量体は工場で多量に調製および貯蔵されているため、調製終了後から実使用されるまでには、平均３日以上が経過し、しかも、重合に際して、水溶性不飽和単量体の脱気時間や温度調製時間にも更に数時間要しているのが実状である。しかし、本発明者は、アクリル酸蒸留精製後の時間が長い程、および、水溶性不飽和単量体の調製終了後から重合されるまでの時間が長い程、β−ヒドロキシプロピオン酸や残存モノマーが増加することを見いだし、よって、本発明では、なるべく短時間に重合に供するのがよい。
【００６３】
アクリル酸の中和に用いられる塩基性物質としては、例えば、炭酸（水素）塩、アルカリ金属の水酸化物、アンモニア、有機アミンなどが公知のものが挙げられ、これらを併用してもかまわない。しかし、これら塩基性物質の中では、諸物性に優れ、より残存モノマーの低減されたアクリル酸塩系ポリマーを得るためには、水酸化ナトリウムや水酸化カリウムなどの強塩基性物質を少なくとも用いることが好ましく、水酸化ナトリウムが特に好ましい。
【００６４】
少なくとも中和工程中の一時期は、アクリル酸の中和率が１００モル％を越える状態を経過する条件下に中和する方法のうち最も簡便な方法は、（１）中和反応系を冷却しながら、一定量の塩基性物質に徐々にアクリル酸を加えていく方法がある。また、他の方法として、特開平２−２０９９０６号公報に開示された、（２）中和開始時から中和系でのアクリル酸塩の中和率が１００モル％未満の状態を保ち、次いで、中和の途中では中和率が１００モル％を越える状態を経た後、最終的に中和率３０〜１００モル％に調整する方法も挙げられる。
【００６５】
これら中和工程でのアクリル酸塩溶液の温度は、冷却によって０〜５０℃に保つことが好ましい。β−ヒドロキシプロピオン酸の含有量の少ないアクリル酸塩を得るために、（１）の中和方法では、従来法より低温、好ましくは１０℃以下での中和が好ましいが、従来より大幅な生産性の低下を招く。しかし、（２）の中和方法では、上記アクリル酸を用いる限り、β−ヒドロキシプロピオン酸の含有量１００ｐｐｍ以下への高度の低減が迅速に行いやすい。
【００６６】
尚、本発明において、β−ヒドロキシプロピオン酸の含有量は少ない方が好ましいが、その含有量が１００ｐｐｍ以下に低減されておれば、充分に本発明が目的とするアクリル酸塩系ポリマーが得られる上に、その含有量が１０ｐｐｍ未満、特に１ｐｐｍ程度まで低減しても、残存モノマーなどの諸物性に与える顕著な影響は見られない。しかも、β−ヒドロキシプロピオン酸の含有量が１ｐｐｍ未満や検出不能とするには、極めて困難な精製や製造工程が必要で一般的には不経済であるため、本発明において、１〜１０ｐｐｍ程度の含有は差障りない。
【００６７】
本発明では上記して得られたβ−ヒドロキシプロピオン酸の含有量の少ない水溶性不飽和単量体を用いることを必須とするが、重合に際して、架橋構造をもたらしてアクリル酸塩系吸水性樹脂とすることができる。残存モノマーの低減が困難な吸水性樹脂に関して、本発明のアクリル酸塩系ポリマーの製造方法は好適に用いられる。
【００６８】
用いられる架橋方法としては特に制限はなく、例えば、（Ａ）本発明の水溶性不飽和単量体を重合させることで水溶性樹脂を得た後、更に重合中や重合後に架橋剤を添加して後架橋する方法、（Ｂ）ラジカル重合開始剤によるラジカル架橋、（Ｃ）電子線などによる放射線架橋する方法なども挙げられるが、性能の優れた吸水性樹脂を生産性良く得るには、（Ｄ）予め所定量の内部架橋剤を水溶性不飽和単量体に添加して重合を行ない、重合と同時または重合後に架橋反応させることが好ましい。
【００６９】
かかる手法（Ｄ）で用いられる内部架橋剤としては、例えば、Ｎ，Ｎ’−メチレンビスアクリルアミド、（ポリ）エチレングリコールジ（メタ）アクリレート、（ポリ）プロピレングリコールジ（メタ）アクリレート、トリメチロールプロパントリ（メタ）アクリレート、トリメチロールプロパンジ（メタ）アクリレート、（ポリ）エチレングリコールジ（β−アクリロイルオキシプロピオネート）、トリメチロールプロパントリ（β−アクリロイルオキシプロピオネート）、ポリ（メタ）アリロキシアルカン、（ポリ）エチレングリコールジグリシジルエーテル、エチレングリコール、ポリエチレングリコール、グリセリン、ペンタエリスリトール、エチレンジアミン、ポリエチレンイミンなどの内部架橋剤の１種または２種以上が用いられ、その使用量は水溶性不飽和単量体に対して、通常、０．００５〜５モル％、より好ましくは０．０１〜１モル％である。なお、これら内部架橋剤の中では、得られる吸水性樹脂の耐久性や吸水特性、そして製造時の含水ゲルの取扱性などから、分子内に２個以上の重合性不飽和基を有する重合性内部架橋剤を必須に用いることが好ましい。
【００７０】
なお、重合に際して、本発明の水溶性不飽和単量体はそのままバルク重合や沈澱重合してもよいが、性能面や重合の制御の容易さから、溶液として重合を行うことが好ましい。重合系溶媒としては、水溶性不飽和単量体が溶解する液体ならば特に制限がなく、例えば、水、メタノール、エタノール、アセトン、ジメチルホルムアミド、ジメチルスルホキシド等が例示されるが、水または水性液が特に好ましい。また、水溶性不飽和単量体を溶液として重合する際、その濃度は特に制限はなく飽和濃度を越えてもかまわないが、諸物性や残存モノマー低減の面から、通常、２０重量％〜飽和濃度、好ましくは２５〜５０重量％の範囲である。また、重合に際して、水溶性不飽和単量体に、次亜燐酸（塩）、チオール類、チオール酸（塩）類などの水溶性連鎖移動剤や、澱粉、セルロースやそれらの誘導体、ポリビニルアルコール、ポリアクリル酸、ポリアクリル酸塩架橋体などの親水性高分子を添加してもよい。それらの使用量は通常、前者は５重量部以内、後者は５０重量部以内である。
【００７１】
上記して得られたβ−ヒドロキシプロピオン酸の含有量が１０００ｐｐｍ以下の水溶性不飽和単量体は次に重合される。なお、本発明において、調製終了後の水溶性不飽和単量体は、好ましくはなるべく短時間に重合することは前述の通りである。
【００７２】
本発明で用いられる重合方法としては、例えば、（ａ）ラジカル重合開始剤による重合、（ｂ）放射線重合、（ｃ）電子線重合、（ｄ）光増感剤による紫外線重合などの公知の手法を挙げることが出来るが、性能の優れたアクリル酸塩系ポリマーを得るためには、（ａ）ラジカル重合開始剤による重合が好ましい。
【００７３】
かかる重合法（ａ）としては、例えば、型枠の中で行う注型重合（特公昭４８−４２４６６号）、ベルトコンベアー上での重合（特開昭５８−４９７１４号）、含水ゲル状重合体を細分化しながら行う重合（特開昭５７−３４１０１号）、加圧下での重合（特開平２−１２９２０７号）などの各種水溶液重合；逆相懸濁重合（特公昭５９−３７００３号）、逆相乳化重合（特開昭６３−９０５１０号、特開昭６３−９０５３７号）、沈澱重合（特開昭５８−８４８１９号，特開平１−１７１０号，特開平１−２０４９１０号）、バルク重合などの公知の重合方法が例示できるが、逆相懸濁重合または水溶液重合が特に好ましい。尚、重合の際、連続重合、半回分式重合、回分式重合の区別や減圧、加圧、常圧の区別は特に問わないし、更に、重合時に繊維基材などを共存させ吸水性複合体としてもよい（特開平２−２４２９７５号）。
【００７４】
かかる重合に用いられるラジカル重合開始剤としては、例えば、過硫酸カリウム、過硫酸アンモニウム、過硫酸ナトリウム等の過硫酸塩；ｔ−ブチルハイドロパーオキサイド、クメンハイドロパーオキサイド等の有機過酸化物；過酸化水素；２，２’−アゾビス（２−アミジノプロパン）二塩酸塩等のアゾ化合物；その他、亜塩素酸塩、次亜塩素酸塩、第二セリウム塩、過マンガン酸塩など公知の重合開始剤が挙げられるが、これらの中でも、過硫酸塩、過酸化水素、アゾ化合物よりなる群から選ばれる１種或は２種以上が好ましい。また、酸化性ラジカル重合開始剤を用いる場合、亜硫酸（水素）塩やＬ−アスコルビン酸などの還元剤を併用してもよいし、アゾ系重合開始剤などを用いる場合は紫外線を併用してよい。尚、これらラジカル重合開始剤などは重合系に一括添加してもよいし、逐次添加してもよいが、その使用量は水溶性不飽和単量体に対して、通常０．００１〜２モル％、好ましくは０．０１〜１モル％である。なお、本発明の水溶性不飽和単量体を用いることで、通常より少ないラジカル重合開始剤の使用量でも残存モノマーを低減できるため、重合の制御も容易で、且つ得られるアクリル酸塩系ポリマーの諸物性も更に良好なものとなり好ましい。
【００７５】
水溶性不飽和単量体を溶液として重合を行う場合、重合後のアクリル酸塩系ポリマーはそのまま用いてもよいが、取り扱い性の向上や残存モノマー低減の為、乾燥することが好ましい。
【００７６】
本発明で用いられる乾燥方法としては、公知の手段が挙げられ、例えば、高濃度で重合させることで、重合熱により乾燥と重合を同時に行う方法（特開昭５８−７１９０７号，特開平２−３４６０７号）を行っても良いし、重合後の固形分によっては、得られたゲル状重合体を更に乾燥してもよい。更に乾燥を行う場合には、例えば、高湿条件下での乾燥（特開平１−２６６０４号）、有機溶剤中での共沸脱水、マイクロ波乾燥、所定温度に加熱されたベルトまたはドラムドライヤー乾燥、高速ロータを備えたシリンダー中での乾燥（特開平２−２４０１１２号）、その他、強制通風炉、赤外線、減圧乾燥機などを用いて乾燥することができる。
【００７７】
本発明における乾燥温度は通常、７０〜２３０℃の範囲であり、好ましくは１１０〜２２０℃、より好ましくは１５０〜２１０℃である。この温度範囲を外れると本発明の水溶性不飽和単量体を用いた効果が現れ難く、場合によっては、残存モノマーの増加、諸物性や生産性の低下を招く場合がある。なお、乾燥時間はゲル状重合体の含水率や粒子径、乾燥温度などによって、適宜決定される。
【００７８】
以上、今回、本発明者は乾燥途中での残存モノマーの増加を見いだし、本発明の水溶性不飽和単量体を用いることによって初めて、如何なる重合法や乾燥温度や乾燥時間でも、残存モノマーの少ない優れた物性のアクリル酸塩系ポリマーが得られる様になった。
【００７９】
この様にして得られた本発明のアクリル酸塩系ポリマーは、このままの状態で使用してもよいが、更に粉砕や造粒を行って粒度を調整してもよい。目的とする粒度はアクリル酸塩系ポリマーの種類や使用目的によっても異なるが、粉末状の吸水性樹脂の場合は通常、平均粒子径１０〜２０００μｍ、更に好ましくは１００〜１０００μｍ、最も好ましくは３００〜６００μｍ程度である。また、得られたアクリル酸塩系ポリマーに各種の無機粉末、酸化剤、還元剤、香料、肥料、バインダー、水などを更に添加してもよい。
【００８０】
更に本発明が提供する表面近傍が架橋されたアクリル酸塩系ポリマーの製造方法は、上記の方法で得られたアクリル酸塩系ポリマーの表面近傍を架橋することによって達成される。
【００８１】
表面近傍を架橋するアクリル酸塩系ポリマーとして、β−ヒドロキシプロピオン酸の含有量が１０００ｐｐｍを越える水溶性不飽和単量体より得られたアクリル酸塩系ポリマーを用いると、表面架橋によって残存モノマーの増加が起こり安全性に悪影響を与えるのみならず、諸物性の低下をも起こす場合もあり好ましくない。
【００８２】
用いられるアクリル酸塩系ポリマーの含水率としては、重合後のアクリル酸塩系ポリマーをそのまま用いてもよいが、表面架橋効果などから、通常は含水率４０％以下、より好ましくは含水率３０％以下、更に好ましくは含水率１０％以下まで乾燥されたアクリル酸塩系ポリマーが好ましい。更に、用いられるアクリル酸塩系ポリマーの粒度としては、平均粒子径１０〜２０００μｍ、更に好ましくは１００〜１０００μｍ、最も好ましくは３００〜６００μｍ程度であり、その粒度分布は狭い方が好ましい。また、用いられるアクリル酸塩系ポリマーは水溶性樹脂であってもかまわないが、表面架橋による諸物性の改善効果はアクリル酸塩系吸水性樹脂に対して、より明確に発現する。
【００８３】
本発明でアクリル酸塩系ポリマーの表面近傍の架橋には、放射線などによる架橋を用いてもよいが、通常、表面近傍に表面架橋剤を添加して行われる。用いられる表面架橋剤としては、公知のものが特に制限なく例示され、例えば、エチレングリコール、プロピレングリコール、ポリエチレングリコール、プロピレングリコール、グリセリン、ポリグリセリン、１，６−ヘキサンジオール、トリメチロールプロパン、ジエタノールアミン、トリエタノールアミン、ポリオキシプロピレン、オキシエチレンオキシプロピレンブロック共重合体、ペンタエリスリトール、ソルビトールなどの各種多価アルコール類；エチレングリコールジグリシジルエーテル、ポリエチレングリコールジグリシジルエーテルなどの各種多価エポキシ化合物；エチレンジアミン、ポリエチレンイミンなどの各種多価アミン化合物；２，２−ビスヒドロキシメチルブタノール−トリス（３−（１−アジリジニル）プロピオネート）などの多価アジリジン化合物；１，３−ジオキソラン−２−オン、４−メチル−１．３−ジオキソラン−２−オン、４，６−ジメチル−１，３−ジオキサン−２−オンなどの各種アルキレンカーボネート化合物；グリオキサールなどの各種多価アルデヒド化合物；２，４−トリレンジイソシアネートなどの多価イソシアネート化合物；１，２−エチレンビスオキサゾリンなどの多価オキサゾリン化合物；エピクロロヒドリンなどのハロエポキシ化合物；アルミニウム、鉄、ジルコニウム等の水酸化物及び塩化物などの多価金属塩；その他、これらの官能基を合わせ持った化合物も例示することができる。
【００８４】
これら表面架橋剤の中から本発明では、多価アルコール類、多価グリシジル化合物類、多価アミン類、アルキレンカーボネートからなる群より選ばれた１種または２種以上を用いることがより好ましく、本発明の効果において、多価アルコールが最も好ましい。なお、これら表面架橋剤の使用量は用いる架橋条件などによっても異なるが、通常、アクリル酸塩系ポリマーの固形分１００重量部に対して、０．０１〜２０重量部、好ましくは０．０５〜１０重量部の範囲である。
【００８５】
アクリル酸塩系ポリマーに表面架橋剤を添加する方法は公知の方法が用いられ、例えば、アクリル酸塩系ポリマーに直接表面架橋剤を添加する方法や、溶媒に分散させたアクリル酸塩系ポリマーに表面架橋剤を添加する方法などが挙げられる。前者の方法としては、ＤＥ４０２０７８０などが例示され、この場合、均一な表面架橋剤の添加のために、酸化珪素微粉末などの無機化合物（ＵＳＰ−４５８７３０８）や界面活性剤を更に併用してもよいし、水の混合を水蒸気として行っても良い（特開平１−２９７４３０号）。また、後者の方法としては、アクリル酸塩系ポリマーを親水性有機溶媒に分散させる方法（特開昭５７−４４６２７号）、アクリル酸塩系ポリマーを疎水性溶剤に分散させる方法（特開昭５９−６２６６５号）などが例示され、この場合にも、特定量の水を共存させることが好ましい（特開昭５８−１１７２２２号）。
【００８６】
なお、表面架橋剤をアクリル酸塩系ポリマーに添加する際、表面架橋剤を溶液や分散液として添加してもよい。用いられる溶媒としては、例えば、メチルアルコール、エチルアルコール、ｎ−プロピルアルコール、ｉｓｏ−プロピルアルコール、アセトン、テトラヒドロフランなどの親水性有機溶剤や水が好ましい。その使用量はアクリル酸塩系ポリマーの固形分１００重量部に対して、通常０〜２０重量部、好ましくは０〜８重量部の範囲である。
【００８７】
本発明において表面架橋を行う際、水が存在していることが表面処理効果から好ましく、よって、実質乾燥したアクリル酸塩系ポリマーを用いる場合、水も添加することが好ましい。水の添加は表面架橋剤と同時に行っても良いし、別途行っても良いが、その量はアクリル酸塩系ポリマーの固形分１００重量部に対して、通常２０重量部以下、好ましくは０．５〜１０重量部の範囲である。
【００８８】
上記手法に従って、アクリル酸塩系ポリマーと表面架橋剤とを混合した後、本発明の表面架橋方法では、諸物性の面や残存モノマーの面などから、好ましくは加熱処理が行われる。
【００８９】
本発明のアクリル酸塩系ポリマーを用いる場合、加熱処理温度は通常７５℃以上、好ましくは、１００〜３００℃、より好ましくは１２０〜２５０℃、更により好ましくは１５０〜２３０℃の範囲である。かかる温度範囲を外れると、諸物性の低下が見られ、しかも、本発明のアクリル酸塩系ポリマーを用いた効果が現れ難い場合があり好ましくない。なお、加熱時間は目的とする表面架橋効果や加熱温度などによって適宜決定されるが、通常、１分から１０時間の範囲である。
【００９０】
本発明において、加熱処理を行う方法としては公知の手段が制限なく用いられ、例えば、（１）アクリル酸塩系ポリマーに直接、表面架橋剤やその溶液を添加した後そのまま加熱処理する方法や、（２）溶媒に分散させたアクリル酸塩系ポリマーに表面架橋剤を添加した後分散させたまま加熱処理する方法や、（３）分散媒から濾過して加熱処理する方法などが挙げられるが、加熱処理の容易さ、安全性、コストや生産性などから、（１）の方法が好ましい。なお、加熱処理装置については特に制限はなく、熱風乾燥機、流動層乾燥機、ナウター式乾燥機などの公知の装置が用いられる。
【００９１】
以上、今回、本発明者は表面架橋での残存モノマーの増加を見いだし、本発明のアクリル酸塩系ポリマーを用いることによって初めて、如何なる表面架橋方法でも残存モノマーの少ない優れた物性のアクリル酸塩系ポリマーが得られる様になった。
【００９２】
以上の様にして得られた表面架橋前または表面架橋後の本発明のアクリル酸塩系ポリマーは、残存モノマーが１００ｐｐｍ以下と少ないのみならず、β−ヒドロキシプロピオン酸の含有量をも低減された新規なアクリル酸塩系ポリマーである。本発明のβ−ヒドロキシプロピオン酸の含有量は１〜１０００ｐｐｍ、好ましくは１〜５００ｐｐｍ、更には１〜１００ｐｐｍである。
【００９３】
即ち、本発明のアクリル酸塩系ポリマーは残存モノマーが少ないのみならず、β−ヒドロキシプロピオン酸の含有量をも少ないため、残存モノマーの発生や増加が殆どなく、極めて安全なアクリル酸塩系ポリマーである。従来のアクリル酸塩系ポリマーでは低残存モノマーであっても、加熱や経時変化によって大幅な残存モノマーの発生増加が見られたが、本発明のアクリル酸塩系ポリマーにおいて、初めてかかる問題も解決された。
【００９４】
なお、水溶性不飽和単量体中やアクリル酸塩系ポリマー中のβ−ヒドロキシプロピオン酸が残存モノマーの増加や発生に結び付く原因は不明である。敢えて推定すると、アクリル酸塩系ポリマーを加熱すると、β−ヒドロキシプロピオン酸ナトリウムの融点（１４２℃）以下の加熱でも、残存モノマーの増加や発生が見られることから、アクリル酸系ポリマー中ではβ−ヒドロキシプロピオン酸が極めて分解し易いか、あるいは、β−ヒドロキシプロピオン酸がアクリル酸塩系ポリマーの解重合を促進させるとも推定される。更には、かかる残存モノマーの発生や増加、ポリマーの解重合などに伴って、アクリル酸塩系ポリマーの諸物性を低下させていたとも推定される。
【００９５】
【実施例】
以下、実施例によって本発明を説明するが、本発明の範囲がこれらの実施例にのみ限定されるものではない。尚、実施例に記載のアクリル酸塩系ポリマーの諸物性は下記の試験方法によって測定した値を示す。
【００９６】
また、ここで潜在的残存モノマーとは、アクリル酸塩系ポリマー中に通常室温で測定される残存モノマー以外に、長時間の使用や高温下で発生・増加する残存モノマーのことである。たとえ見かけの残存モノマーが少なくても、この潜在的残存モノマーが多くては、安全上好ましくないことは云うまでもない。
【００９７】
（１）吸水倍率
アクリル酸塩系ポリマー０．２ｇを不織布製のティーバッグ式袋（４０×１５０ｍｍ）に均一に入れ、０．９重量％塩化ナトリウム水溶液中に浸漬した。３０分後にティーバッグ式袋を引き上げ、一定時間水切りを行った後、ティーバッグ式袋の重量を測定し、以下の式で吸水倍率を算出した。
【００９８】
【数３】

【００９９】
（２）残存モノマー
アクリル酸塩系ポリマー０．５ｇを脱イオン水１０００ｍｌ中で２時間攪拌した後濾過して、濾液中の残存モノマーを高速液体クロマトグラフィーを用いて測定した。
【０１００】
（３）潜在的残存モノマー
潜在的残存モノマーの分析を簡便化するため、得られたアクリル酸塩系ポリマーを更に１８０℃で３時間加熱した後、（２）の方法で残存モノマーを測定し、加熱によって増加した残存モノマーを潜在的残存モノマーと定義した。
【０１０１】
【数４】

【０１０２】
（５）吸引力
ティシュペーパーを置いたシャーレに人工尿（組成；尿素、ＮａＣｌ、ＣａＣｌ_２、ＭｇＳＯ_４を各々１．９重量％、０．９重量％、０．１重量％、０．１重量％）を２０ｍｌ注いだ後、更に１ｇの表面近傍が架橋されたアクリル酸塩系ポリマーをシャーレの中心部に落とし、ティシュペーパーを通して人工尿を吸収させる。１０分経過後、膨潤したゲルの重量を測定して、アクリル酸塩系ポリマーの吸引力とする。
【０１０３】
（使用されるアクリル酸塩）
以下、実施例及び比較例で使用されるアクリル酸塩は下記の手法によって得たアクリル酸塩である。また、β−ヒドロキシプロピオン酸の含有量は液体クロマトグラフィーによって定量した固形分あたりの値である。
【０１０４】
なお、蒸溜精製後のアクリル酸は中和に使用される迄、常温を仮定した２０〜４０℃の一定温度の暗室に保存した。通常、製造現場で使用されているアクリル酸はアクリル酸メーカーで蒸溜精製されてから数１０日経過しているのが実情である。また、製造現場では、凝固点の高く凍結しない８０％程度のアクリル酸水溶液として取り扱われ、保存されることが多い。
【０１０５】
（製造例１）
アクリル酸製造現場より得たアクリル酸を蒸留精製した。この蒸留精製後のアクリル酸を温度３０℃で３時間保存し該アクリル酸を以下に示す特開平２−２０９９０６号の実施例１の手法に従って中和した。 攪拌機を備えた蒸留フラスコにイオン交換水２７４４ｇを仕込んだ。フラスコ内の中和反応系の温度を２０〜４０℃に保ちながら、アクリル酸１３９０ｇおよび４８重量％水酸化ナトリウム１４８０ｇを、水酸化ナトリウム／アクリル酸＝０．９〜０．９５の滴下比にて、１００分間かけて該フラスコ内に同時に滴下した。滴下終了後、更に４８重量％の水酸化ナトリウム水溶液１６０ｇを供給して、フラスコ内の中和反応系の中和率を１０２モル％した。そして、中和反応系の温度を４０℃に調製し、３０分間の熟成を行った。熟成終了後、中和反応系にアクリル酸４９９ｇを１０分間にわたって供給することで、中和率７５モル％で濃度３７％のアクリル酸塩（Ｉ）を得た。アクリル塩（Ｉ）中の固形分あたりのβ−ヒドロキシプロピオン酸の含有量は４０ｐｐｍであった。
【０１０６】
（製造例２，３）
製造例１において、中和に用いるアクリル酸として、蒸留精製後に温度３０℃で保存し、その保存時間を１０時間，２４時間としたアクリル酸を用いる以外は製造例１と同様に行うことで、アクリル酸塩（ＩＩ），（ＩＩＩ）を得た。アクリル塩（ＩＩ），（ＩＩＩ）中の固形分あたりのβ−ヒドロキシプロピオン酸の含有量はそれぞれ９０ｐｐｍ、１９０ｐｐｍであった。
【０１０７】
（製造例４）
製造例１において、中和に用いるアクリル酸として、蒸留精製後に温度２５℃で保存し、その保存時間を２４時間としたアクリル酸を用いる以外は製造例１と同様に行うことで、アクリル酸塩（ＩＶ）を得た。アクリル塩（ＩＶ）中の固形分あたりのβ−ヒドロキシプロピオン酸の含有量は１００ｐｐｍであった。
【０１０８】
（製造例５〜７）
製造例１において、中和に用いるアクリル酸として、蒸留精製後に温度２０℃で保存し、その保存時間を１０時間，２４時間，４８時間としたアクリル酸を用いる以外は同様に行うことで、アクリル酸塩（Ｖ）〜（ＶＩＩ）を得た。アクリル塩（Ｖ）〜（ＶＩＩ）中の固形分あたりのβ−ヒドロキシプロピオン酸の含有量はそれぞれ、５０ｐｐｍ、８０ｐｐｍ、１３０ｐｐｍであった。
【０１０９】
（製造例８）
市販のアクリル酸製造（和光純薬特級）を蒸留精製した。この蒸留精製後のアクリル酸を温度３０℃で３時間保存し、該アクリル酸を以下に示す特開平２−２０９９０６号の比較例２の手法に従って中和した。
【０１１０】
攪拌機を備えた蒸留フラスコにイオン交換水２７４４ｇおよび４８重量％水酸化ナトリウム１６４０ｇを仕込んだ。次いで、中和反応系の温度を２０〜４０℃に保ちながら、該フラスコにアクリル酸１８８９ｇを１２０分に渡って供給することで中和率７５モル％で濃度３７％のアクリル酸塩（ＶＩＩＩ）を得た。アクリル塩（ＶＩＩＩ）中の固形分あたりのβ−ヒドロキシプロピオン酸の含有量は、２３０ｐｐｍであった。
【０１１１】
（製造例９，１０）
製造例８において、中和に用いるアクリル酸として、蒸留精製後に温度３０℃で保存し、その保存時間を１０時間，２４時間としたアクリル酸を用いる以外は製造例８と同様に行うことで、アクリル酸塩（ＩＸ），（Ｘ）を得た。アクリル塩（ＩＸ），（Ｘ）中の固形分あたりのβ−ヒドロキシプロピオン酸の含有量はそれぞれ、２９０ｐｐｍ、３９０ｐｐｍであった。
【０１１２】
（製造例１１）
市販のアクリル酸製造（和光純薬特級）を蒸留精製した。この蒸留精製後のアクリル酸を、凝固点が高く取り扱いが容易な８０％水溶液とした後温度３０℃で３時間保存し、製造例８に類似した以下の手法に従って中和した。
【０１１３】
攪拌機を備えた蒸留フラスコにイオン交換水２２７２ｇおよび４８重量％水酸化ナトリウム１６４０ｇを仕込んだ。次いで、中和反応系の温度を２０〜４０℃に保ちながら、該フラスコに８０％アクリル酸水溶液２３６１ｇを１２０分に渡って供給することで、中和率７５モル％で濃度３７％のアクリル酸塩（ＸＩ）を得た。アクリル塩（ＸＩ）中の固形分あたりのβ−ヒドロキシプロピオン酸の含有量は、２９０ｐｐｍであった。
【０１１４】
（製造例１２）
製造例８において、中和温度を１０℃に下げる以外は同様に中和を行い、アクリル酸塩（ＸＩＩ）を得た。但し、中和温度を下げたため、中和に要る時間は製造例８の３倍の６時間を必要とした。アクリル塩（ＸＩＩ）中の固形分ありのβ−ヒドロキシプロピオン酸の含有量は、７０ｐｐｍであった。
【０１１５】
（製造例１３）
製造例１において、中和率１０２モル％での熟成終了後、中和反応系に添加するアクリル酸の量を、アクリル酸４９９ｇから１４４６ｇに変更する以外は製造例１と同様に行い、中和率５０モル％で濃度４５％のアクリル酸塩（ＸＩＩＩ）を得た。アクリル塩（ＸＩＩＩ）中の固形分あたりのβ−ヒドロキシプロピオン酸の含有量は、３０ｐｐｍであった。
【０１１６】
（製造例１４）
製造例１において、中和率１０２モル％での熟成終了後、中和反応系に添加するアクリル酸の量を、アクリル酸４９９ｇから１８４ｇに変更する以外は製造例１と同様に行い、中和率９０モル％で濃度３４％のアクリル酸塩（ＸＩＶ）を得た。アクリル塩（ＸＩＶ）中の固形分あたりのβ−ヒドロキシプロピオン酸の含有量は、５０ｐｐｍであった。
【０１１７】
（製造例１５）
製造例８において、水の量を２７７４ｇから１９４２ｇに変更し、また中和反応系に適下するアクリル酸の量を、１８８９ｇから１４１８ｇに変更する以外は製造例１と同様に行い、中和率１００モル％で濃度３７％のアクリル酸塩（ＸＶ）を得た。アクリル塩（ＸＶ）中の固形分あたりのβ−ヒドロキシプロピオン酸の含有量は、２９０ｐｐｍであった。
【０１１８】
（比較製造例１）
製造例１において、中和に用いるアクリル酸として、蒸留精製後に２５℃で２０日間保存したアクリル酸を用いる以外は製造例１と同様に行い、比較アクリル酸塩（Ｉ）を得た。比較アクリル塩（Ｉ）中の固形分あたりのβ−ヒドロキシプロピオン酸の含有量は、２５００ｐｐｍであった。
【０１１９】
（比較製造例２）
製造例１において、中和に用いるアクリル酸として、市販のアクリル酸（関東化学・特級）を蒸留精製することなく、そのまま用いる以外は製造例１と同様に行い、比較アクリル酸塩（ＩＩ）を得た。比較アクリル塩（ＩＩ）中の固形分あたりのβ−ヒドロキシプロピオン酸の含有量は、４２００ｐｐｍであった。
【０１２０】
（比較製造例３）
製造例１において、中和に用いるアクリル酸として、市販のアクリル酸（和光純薬・特級）を蒸留精製することなく、そのまま用いる以外は製造例１と同様に行い、比較アクリル酸塩（ＩＩＩ）を得た。比較アクリル塩（ＩＩＩ）中の固形分あたりのβ−ヒドロキシプロピオン酸の含有量は、８２００ｐｐｍであった。
【０１２１】
（比較製造例４，５）
製造例１において、中和に用いるアクリル酸として、蒸留精製後に１２０時間，２４０時間保存したアクリル酸を用いる以外は製造例１と同様に行うことで、比較アクリル酸塩（ＩＶ），（Ｖ）を得た。比較アクリル塩（ＩＶ）、（Ｖ）中の固形分あたりのβ−ヒドロキシプロピオン酸の含有量は、それぞれ１１００ｐｐｍ、１９００ｐｐｍであった。
【０１２２】
（比較製造例６）
製造例１において、中和に用いるアクリル酸として、蒸留精製後に温度４０℃で保存し、その保存時間を４８時間としたアクリル酸を用いる以外は製造例１と同様に行うことで、比較アクリル酸塩（ＶＩ）を得た。比較アクリル塩（ＶＩ）中の固形分あたりのβ−ヒドロキシプロピオン酸の含有量は１３００ｐｐｍであった。
【０１２３】
（比較製造例７，８）
製造例８において、中和に用いるアクリル酸として、蒸留精製後に１２０時間，２４０時間保存したアクリル酸を用いる以外は製造例１と同様に行うことで、比較アクリル酸塩（ＶＩＩ），（ＶＩＩＩ）を得た。比較アクリル塩（ＶＩＩ），（ＶＩＩＩ）中の固形分あたりのβ−ヒドロキシプロピオン酸の含有量は、それぞれ１３００ｐｐｍ、２１００ｐｐｍであった。
【０１２４】
（比較製造例９，１０）
製造例１１において、中和に用いる８０％アクリル酸水溶液として、蒸留精製後に２４０時間，４８０時間保存したアクリル酸水溶液を用いる以外は製造例１と同様に行うことで、比較アクリル酸塩（ＩＸ），（Ｘ）を得た。比較アクリル塩（ＩＸ），（Ｘ）中の固形分あたりのβ−ヒドロキシプロピオン酸の含有量は、それぞれ３３００ｐｐｍ、６７００ｐｐｍであった。
【０１２５】
（比較製造例１１）
製造例８において、中和時間の短縮のために、中和系の温度を２０〜４０℃から５０〜６０℃に上昇させた所、中和に要する時間は１２０分から４０分に短縮された。しかし、得られたアクリル酸塩中のβ−ヒドロキシプロピオン酸の含有量は、２１００ｐｐｍと大幅に増加した。以下、このアクリル酸塩を比較アクリル酸塩（ＸＩ）と呼ぶ。
【０１２６】
（比較製造例１２）
製造例１３において、中和に用いるアクリル酸として、比較製造例１で用いたアクリル酸を用いる以外は製造例１３と同様に行い、β−ヒドロキシプロピオン酸の含有量が１８００ｐｐｍの中和率５０モル％で濃度４５％の比較アクリル酸塩（ＸＩＩ）を得た。
【０１２７】
（比較製造例１３）
製造例１５において、中和に用いるアクリル酸として、比較製造例１で用いたアクリル酸を用いる以外は製造例１５と同様に行い、β−ヒドロキシプロピオン酸の含有量が３２００ｐｐｍで中和率１００モル％の比較アクリル酸塩（ＸＩＩＩ）を得た。
【０１２８】
（実験例１）
製造例１で得られたアクリル酸塩（Ｉ）５５００ｇに、内部架橋剤としてＮ，Ｎ’−メチレンビスアクリルアミド１．８ｇを溶解させることで、β−ヒドロキシプロピオン酸の含有量が４０ｐｐｍの水溶性不飽和単量体（Ｉ）を得た。（濃度３７％，中和率７５モル％） 窒素ガスで３０分脱気後の水溶性不飽和単量体を内容積１０Ｌでシグマ型羽根を２本有するジャケット付きステンレス製双腕型ねつか機（ニーダー）に蓋をつけた反応器に供給し、水溶性不飽和単量体（Ｉ）を３０℃の温度に保ち反応系を窒素置した。次いで、３５℃の温水を通じて加熱しながら、重合開始剤として、硫酸アンモニウム０．３モル％および亜硫酸水素ナトリウム０．０３モル％を添加した。なお、上記操作によって、水溶性不飽和単量体（Ｉ）を調製してから、重合開始剤を投入し重合を開始するまで２時間を要した。
【０１２９】
重合開始剤を添加して１分後に重合が開始し、１６分後には含水ゲル重合体は約５ｍｍの径に細分化され、更に攪拌を続け重合を開始して６０分後に含水ゲル重合体を取り出した。次いで、得られた含水ゲル状重合体の細粒化物を５０メッシュの金網上に広げ、１３０℃で９０分間熱風乾燥した。乾燥物を振動ミルを用いて粉砕し、更に２０メッシュで分級し、アクリル酸系塩吸水性樹脂（１）を得た。
【０１３０】
（実験例２〜４）
実験例１において、水溶性不飽和単量体（Ｉ）を所定時間放置することで、調製してから重合を始めるまでの時間を６，１２時間，２４時間に延長する以外は、実験例１と同様の操作を繰り返すことで、アクリル酸塩系吸水性樹脂（２）〜（４）を得た。
【０１３１】
（実験例５〜７）
実験例１において、水溶性不飽和単量体の調製に用いられるアクリル酸塩として、アクリル酸塩（Ｉ）に代えて、アクリル酸塩（ＩＩ）〜（ＩＶ）する以外は実験例１と同様にして、β−ヒドロキシプロピオン酸の含有量がそれぞれ９０ｐｐｍ，１９０ｐｐｍ，１００ｐｐｍの水溶性不飽和単量体（ＩＩ）〜（ＩＶ）を得た。次いで、これら水溶性不飽和単量体（ＩＩ）〜（ＩＶ）について、調製後２時間で実験例１と同様に重合を行い、以下、実験例１との同様の操作を繰り返すことでアクリル酸塩系吸水性樹脂（５）〜（７）を得た。
【０１３２】
（実験例８）
実験例１において、水溶性不飽和単量体の調製に用いられるアクリル酸塩として、アクリル酸塩（Ｉ）に代えて、アクリル酸塩（Ｖ）に変更し、同時に内部架橋剤もＮ，Ｎ’−メチレンビスアクリルアミドに代えて、ポリエチレングリコールジアクリレート（平均ｎ数は８）２．２ｇ（０．０２モル％，対モノマー）とする以外は実験例１と同様にして、β−ヒドロキシプロピオン酸の含有量が５０ｐｐｍの水溶性不飽和単量体（Ｖ）を得た。
【０１３３】
次いで、水溶性不飽和単量体（Ｖ）について、調製後６時間で実験例１と同様に重合を行った。こうして得られたゲル状重合体を１５０℃で７５分乾燥し、以下、実験例１と同様に粉砕・分級を行うことで、アクリル酸塩系吸水性樹脂（８）を得た。
【０１３４】
（実験例９，１０）
実験例８において、水溶性不飽和単量体の調製に用いられるアクリル酸塩として、アクリル酸塩（Ｖ）に代えて、アクリル酸塩（ＶＩ），（ＶＩＩ）にする以外は実験例８と同様にして、β−ヒドロキシプロピオン酸の含有量がそれぞれ８０ｐｐｍ，１３０ｐｐｍの水溶性不飽和単量体（ＶＩ），（ＶＩＩ）を得た。次いで、これら水溶性不飽和単量体（ＶＩ），（ＶＩＩ）について、実験例８と同様に行うことで、アクリル酸塩系吸水性樹脂（９），（１０）を得た。
【０１３５】
（実験例１１）
実験例１において、水溶性不飽和単量体の調製に用いられるアクリル酸塩として、アクリル酸塩（Ｉ）に代えてアクリル酸塩（ＶＩＩＩ）に変更し、同時に内部架橋剤もＮ，Ｎ’−メチレンビスアクリルアミドに代えて、トリメチロールプロパントリアクリレート１３．６ｇ（０．２モル％，対モノマー）とする以外は実験例１と同様にして、β−ヒドロキシプロピオン酸の含有量が２３０ｐｐｍの水溶性不飽和単量体（ＶＩＩＩ）を得た。
【０１３６】
以下、水溶性不飽和単量体（ＶＩＩＩ）について、調製後２時間で実験例１と同様に重合を行い、得られたゲル状重合体を１８０℃で６０分間乾燥し、以下、実験例１と同様に行うことで、アクリル酸塩系吸水性樹脂（１１）を得た。
【０１３７】
（実験例１２，１３）
実験例１１において、水溶性不飽和単量体の調製に用いられるアクリル酸塩として、アクリル酸塩（ＶＩＩＩ）に代えて、アクリル酸塩（ＩＸ），（Ｘ）にする以外は同様にして、β−ヒドロキシプロピオン酸の含有量がそれぞれ２９０ｐｐｍ，３９０ｐｐｍの水溶性不飽和単量体（ＩＸ），（Ｘ）を得た。次いで、これら水溶性不飽和単量体（ＩＸ），（Ｘ）について、実験例１１と同様に行うことで、アクリル酸塩系吸水性樹脂（１２），（１３）を得た。
【０１３８】
（実験例１４）
攪拌棒、窒素吹き込み管、温度計を備え付けた反応容器内で、３０ｇのトウモロコシ澱粉を６００ｇの水に溶解させ、更に、アクリル酸塩（ＸＩ）を７１８ｇ（３モル）およびアクリルアミド７１ｇ（１モル）、内部架橋剤としてトリメチロールプロパントリアクリレート０．１２ｇ（対モノマー０．０１モル％を澱粉水溶液に溶解させることで、β−ヒドロキシプロピオン酸の含有量が２３０ｐｐｍの水溶性不飽和単量体（ＸＩ）を調製した。
【０１３９】
次いで、水溶性不飽和単量体（ＸＩ）を３０℃に２時間保った後、窒素ガスを１時間吹き込んで溶存酸素を追い出した。更に、この水溶性不飽和単量体に、重合開始剤として過硫酸ナトリウム０．１モル％、Ｌ−アスコルビン酸０．０５モル％を加え、得られた含水ゲルを３時間重合せしめた。更に、このゲル状重合体を表面温度１５０℃のダブルドラムドライヤーにて乾燥させ後、粉砕し２０メシュ通過物のアクリル酸塩系吸水性樹脂（１４）を得た。
【０１４０】
（実験例１５）
アクリル酸塩（ＸＩＩ）１０００ｇに内部架橋剤としてテトラエレングリコールジアクリレート３．４ｇ（０．１モル％，対モノマー）を添加し溶解させ、β−ヒドロキシプロピオン酸の含有量が７０ｐｐｍの水溶性不飽和単量体（ＸＩＩ）を調製した。
【０１４１】
水溶性不飽和単量体（ＸＩＩ）を調整終了後に５０℃に保ち、窒素ガスを吹き込んで溶存酸素を追い出した。そして、水溶性不飽和単量体を窒素雰囲気下に厚さ５ｍｍに層状流化延長させた後、重合開始剤として０．２モル％の２，２’−アゾビス（２−アミジノプロパン）二塩酸を添加した。なお、水溶性不飽和単量体を調製後、重合を開始するまでには２時間を要している。即座に重合は開始し、１０分後にゲル状重合体を取り出し、細分化した後、１５０℃で６０分間乾燥させた。次いで、乾燥物を実験例１と同様に粉砕・分級することで、アクリル酸塩系吸水性樹脂（１５）を得た。
【０１４２】
（実験例１６，１７）
実験例１５において、調製終了後３０℃で１２時間，２４時間保存した水溶性不飽和単量体（ＸＩＩ）を用いて重合を行う以外は、実験例１と同様に行うことでアクリル酸塩系吸水性樹脂（１６），（１７）を得た。
【０１４３】
（実験例１８）
アクリル酸塩（ＸＩＩ）８３．４ｇおよび内部架橋剤として、Ｎ，Ｎ’−メチレンビスアクリルアミド０．００４ｇ（対モノマー０．００６５モル％、イオン交換水１７．７７ｇを用いて、濃度３５％、中和率７５％のβ−ヒドロキシプロピオン酸の含有量が７０ｐｐｍの水溶性不飽和単量体（ＸＩＩ’）を調製した。水性不飽和単量体（ＸＩＩ）を調整終了後、４時間後に窒素ガスを１時間吹き込んで溶存酸素を追い出した。
【０１４４】
別に反応容器として、攪拌機、還流冷却器、温度計、窒素ガス導入管および滴下漏斗を付けた５００ｍｌの四つ口セパラブルフラスコにシクロヘキサン２５０ｍｌを取り、分散剤としてソルビタンモノステアレート（ＨＬＢ４．７）２．０ｇを加えて溶解させ、窒素ガスを吹き込んで溶存酸素を追い出した。１時間脱気後の水溶性不飽和単量体に重合開始剤として過硫酸カリウム０．０６モル％を添加し、浴温を６０℃に昇温して重合反応を開始させた。なお、水溶性不飽和単量体を調製してから、重合を行うまで、４時間を要している。２時間重合後、共沸脱水を行うことでポリマーの含水率１０％以下になってから濾過し、更に１３０℃のオーブンで乾燥させることで、アクリル酸塩系吸水性樹脂（１８）を得た。
【０１４５】
（実験例１９）
実験例１において、水溶性不飽和単量体の調製に用いられるアクリル酸塩として、アクリル酸塩（Ｉ）に代えて、アクリル酸塩（ＸＩＩＩ）に変更することで、β−ヒドロキシプロピオン酸の含有量が３０ｐｐｍで中和率５０％，濃度４５％の水溶性不飽和単量体（ＸＩＩＩ）を得た。
【０１４６】
次いで、この水溶性不飽和単量体（ＸＩＩＩ）について、調製後４時間で実験例１と同様に重合を開始して、更に実験例１との同様の操作を繰り返すことで、アクリル酸塩系吸水性樹脂（１９）を得た。
【０１４７】
（実験例２０）
実験例１において、水溶性不飽和単量体の調製に用いられるアクリル酸塩として、アクリル酸塩（Ｉ）に代えて、アクリル酸塩（ＸＩＶ）に変更することで、β−ヒドロキシプロピオン酸の含有量が５０ｐｐｍで中和率９０％，濃度３４％の水溶性不飽和単量体（ＸＩＶ）を得た。
【０１４８】
次いで、この水溶性不飽和単量体（ＸＩＶ）について、調製後２４時間で実験例１と同様に重合を開始して、更に実験例１との同様の操作を繰り返すことで、アクリル酸塩系吸水性樹脂（２０）を得た。
【０１４９】
（実験例２１）
アクリル酸塩（ＸＶ）４０００ｇをそのままβ−ヒドロキシプロピオン酸の含有量が２９０ｐｐｍの水溶性不飽和単量体（ＸＶ）として用い、内面を４弗化エチレン樹脂でライニングしたＳＵＳ３１６製で、３００ｍｍ×３００ｍｍ×５０ｍｍの内容積を持つ開閉可能な注型重合装置に入れて窒素置換し、３０℃のウオーターバスにつけた。
【０１５０】
さらに調製終了後２４時間経過後、過硫酸アンモニウムを０．０５モル％及び亜硫酸水素ナトリウムを０．０２モル％を加えて重合を行い、重合開始から５時間後に注型重合装置より含水ゲル状重合体を取り出し、これをミートチョパーで紐状にした後、実験例１と同様に乾燥、粉砕を行いアクリル酸塩系水溶性樹脂（２１）を得た。
【０１５１】
（実施例１）
実験例１で得られた吸水性樹脂（１）１００部に、グリセリン１部、水２部、エチルアルコール２部を混合した後、得られた混合物を１９０℃で２０分間加熱処理することで、表面近傍が架橋された吸水性樹脂（２２）を得た。
【０１５２】
（実施例２）
吸水性樹脂（５）１００重量部にエチレングリコールジグリシジルエーテル０．１部、水５部、イソプロピルアルコール１部を混合した後、得られた混合物を１８０℃で３０分間加熱処理することで、表面近傍が架橋された吸水性樹脂（２３）を得た。
【０１５３】
（実施例３）
吸水性樹脂（６）１００部に、ジエチレングリコール０．５部、水２部、イソプロピルアルコール１部を混合した後、得られた混合物を１５０℃で３時間加熱処理することで、表面近傍が架橋された吸水性樹脂（２４）を得た。
【０１５４】
（実施例４）
吸水性樹脂（８）１００重量部をメタノール３００部および水３０部からなる混合溶媒に分散させ、更にエチレングリコールジグリジルエーテル０．１部を混合した後、得られた混合物を１６０℃で１時間加熱処理し蒸発乾固することにより、表面近傍が架橋された吸水性樹脂（２５）を得た。
【０１５５】
（実施例５）
吸水性樹脂（９）１００部に、エチレンカーボネート２．５部、水２．５部、アセトン２．５部を加えた後、１８０℃で１時間加熱処理を行うことで、表面近傍が架橋された吸水性樹脂（２６）を得た。
【０１５６】
（実施例６）
吸水性樹脂（１１）１００重量部に珪素無機微粉末（アエロジル）を１部添加し、更にエチレングリコールジグリジルエーテル０．１部および水１０部を混合した後、得られた混合物を１５０℃で１時間加熱処理することにより表面近傍が架橋された吸水性樹脂（２７）を得た。
【０１５７】
（実施例７）
吸水性樹脂（１２）１００重量部に硫酸アルミニウム１部、グリセリン１部、水８部を混合した後、得られた混合物を１８０℃で３０分間加熱処理することで、表面近傍が架橋された吸水性樹脂（２８）を得た。
【０１５８】
（実施例８）
吸水性樹脂（１８）をシクロヘキサン２５０ｍｌに分散剤させた。別途、フラスコにシクロヘキサン５０ｇに界面活性剤としてソルビタンモノラウレート（花王株式会社製、レオドールＳＰ−１０、ＨＬＢ＝８．６）０．５ｇを溶解させ、そこへエチレングリコールジグリシジルエーテル０．０４ｇを水２ｍｌに溶解させた水溶液を強い攪拌下に加えて、エチレングリコールジグリシジルエーテルの分散液（平均液滴径３ミクロン）を得た。次いで、この分散液を吸水性樹脂（１８）の懸濁液に攪拌下で混合したのち、系の温度を７５℃に３時間保持し、更に濾過、減圧乾燥することにより、表面近傍が架橋された吸水性樹脂（２９）を得た。
【０１５９】
（比較例１〜３）
実験例１において、水溶性不飽和単量体の調製に用いられるアクリル酸塩として、アクリル酸塩（Ｉ）に代えて、比較アクリル酸塩（Ｉ）〜（ＩＩＩ）を用いる以外は実験例１と同様にして、β−ヒドロキシプロピオン酸の含有量がそれぞれ２５００ｐｐｍ，４２００ｐｐｍ，８２００ｐｐｍの比較水溶性不飽和単量体（Ｉ）〜（ＩＩＩ）を得た。次いで、これら比較水溶性不飽和単量体（Ｉ）〜（ＩＩＩ）について、以下、実験例１との同様の操作を繰り返すことで、比較アクリル酸塩系吸水性樹脂（１）〜（３）を得た。
【０１６０】
（比較例４〜６）
比較例１において、比較水溶性不飽和単量体（Ｉ）を調製終了後、重合を始めるまでの時間を１２時間，２４時間および２４０時間に延長する以外は、比較例１と同様の操作を繰り返すことで、比較アクリル酸塩系吸水性樹脂（４）〜（６）を得た。
【０１６１】
（比較例７〜９）
実験例８において、水溶性不飽和単量体の調製に用いられるアクリル酸塩として、アクリル酸塩（Ｖ）に代えて、比較アクリル酸塩（ＩＶ）〜（ＶＩ）にする以外は実験例８と同様にして、それぞれβ−ヒドロキシプロピオン酸の含有量が１１００ｐｐｍ，１９００ｐｐｍ，１３００ｐｐｍの比較水溶性不飽和単量体（ＩＶ）〜（ＶＩ）を得た。次いで、これら比較水溶性不飽和単量体（ＩＶ）〜（ＶＩ）について、以下、実験例８と同様の同様の操作を繰り返すことで、比較アクリル酸塩系吸水性樹脂（７）〜（９）を得た。
【０１６２】
（比較例１０〜１２）
実験例１１において、水溶性不飽和単量体の調製に用いられるアクリル酸塩として、アクリル酸塩（ＶＩＩＩ）に代えて、比較アクリル酸塩（ＶＩＩ）〜（ＩＸ）にする以外は実験例８と同様にして、それぞれβ−ヒドロキシプロピン酸の含有量が１３００ｐｐｍ，２１００ｐｐｍ，３３００ｐｐｍの比較水溶性不飽和単量体（ＶＩＩ）〜（ＩＸ）を得た。次いで、これら比較水溶性不飽和単量体（ＶＩＩ）〜（ＩＸ）について、以下、実験例１１と同様の同様の操作を繰り返すことで、比較アクリル酸塩系吸水性樹脂（１０）〜（１２）を得た。
【０１６３】
（比較例１３）
実験例１４において、水溶性不飽和単量体の調製に用いられるアクリル酸塩として、アクリル酸塩（ＸＩ）に代えて、比較アクリル酸塩（Ｘ）にする以外は実験例１４と同様にして、β−ヒドロキシプロピオン酸の含有量が６７００ｐｐｍの比較水溶性不飽和単量体（Ｘ）を得た。次いで、この比較水溶性不飽和単量（Ｘ）について、実験例１４と同様の同様の操作を繰り返すことで、比較アクリル塩系吸水性樹脂（１３）を得た。
【０１６４】
（比較例１４）
実験例１５において、水溶性不飽和単量体の調製に用いられるアクリル酸塩として、アクリル酸塩（ＸＩＩ）に代えて、比較アクリル酸塩（ＸＩ）にする以外は実験例１５と同様にして、β−ヒドロキシプロピオン酸の含有量が２１００ｐｐｍの比較水溶性不飽和単量体（ＸＩ）を得た。次いで、この比較水溶性不飽和単量体（ＸＩ）について、以下、実験例１５と同様の操作を繰り返すことで、比較アクリル酸塩系吸水性樹脂（１４）を得た。
【０１６５】
（比較例１５，１６）
比較例１４において、調製終了後３０℃で２４時間，１２０時間保存した水溶性不飽和単量体を用いて重合を行う以外は実験例１３と同様に行うことで、アクリル酸塩系吸水性樹脂（１５），（１６）を得た。
【０１６６】
（比較例１７）
実験例１９において、水溶性不飽和単量体の調製に用いられるアクリル酸塩として、アクリル酸塩（ＸＩＩＩ）に代えて、比較アクリル酸塩（ＸＩＩ）に変更することで、β−ヒドロキシプロピオン酸の含有量が１８００ｐｐｍの比較水溶性不飽和単量体（ＸＩＩ）を得た。次いで、この水溶性不飽和単量体（ＸＩＩ）について、以下、実験例１９と同様の操作を繰り返すことで、比較アクリル酸塩系吸水性樹脂（１７）を得た。
【０１６７】
（比較例１８）
実験例２１において、水溶性不飽和単量体の調製に用いられるアクリル酸塩として、アクリル酸塩（ＸＶ）に代えて、比較アクリル酸塩（ＸＩＩＩ）に変更することで、β−ヒドロキシプロピオン酸の含有量が３２００ｐｐｍ比較水溶性不飽和単量体（ＸＩＩＩ）を得た。次いで、この水溶性不飽和単量体（ＸＩＩＩ）について、以下、実験例２１と同様の操作を繰り返すことで、比較アクリル酸塩系水溶性樹脂（１８）を得た。
【０１６８】
（比較例１９）
実験例２１において、アクリル酸塩（ＸＶ）に別途β−ヒドロキシプロピオン酸を２５９０ｐｐｍ添加することで、β−ヒドロキシプロピオン酸の含有量が２８００ｐｐｍ比較水溶性水溶性不飽和単量体（ＸＩＶ）を得た。次いで、この比較水溶性不飽和単量体（ＸＩＶ）について、以下、実験例２１と同様の操作を繰り返すことで、比較アクリル酸塩系水溶性樹脂（１９）を得た。
【０１６９】
（比較例２０〜２２）
実施例１〜３において、表面近傍を架橋される吸水性樹脂として、吸水性樹脂（１），（５），（６）に代えて、比較吸水性樹脂（１）〜（３）とする以外は全く同様に行い、比較吸水性樹脂（２０）〜（２２）を得た。
【０１７０】
（比較例２３，２４）
実施例４，５において、表面近傍を架橋される吸水性樹脂として、吸水性樹脂（８），（９）に代えて、比較吸水性樹脂（７），（８）とする以外は全く同様に行い、表面近傍が架橋された比較吸水性樹脂（２３），（２４）を得た。
【０１７１】
（比較例２５，２６）
実施例６，７において、表面近傍を架橋される吸水性樹脂として、吸水性樹脂（１１），（１２）に代えて、比較吸水性樹脂（１１）（１２）とする以外は全く同様に行い、表面近傍が架橋された比較吸水性樹脂（２５），（２６）を得た。
【０１７２】
以下、実験例１〜２１と比較例１〜２９のアクリル酸塩系ポリマーの分析結果をそれぞれ表１及び表２に示す。また、実施例１〜８および比較例２０〜２６の表面近傍が架橋されたアクリル酸塩系ポリマーの分析結果を表３に示す。
【０１７３】
なお、実施例で得られたアクリル酸塩系ポリマー中のβ−ヒドロキシプロピオン酸は何れも５００ｐｐｍ以下、特に、β−ヒドロキシプロピオン酸含有量１００ｐｐｍ以下の水溶性不飽和単量体から得られたものは、何れも１００ｐｐｍ未満であったのに対して、比較例では１０００ｐｐｍを超えていた。
【０１７４】
【表１】

【０１７５】
【表２】

【０１７６】
【表３】

【０１７７】
【発明の効果】
本発明の製造方法は下記（１）〜（５）などの優れた特長を有している。この優れたアクリル酸塩系ポリマーは、衛生材料、食品用、土木、農業などの分野に幅広く利用できる。
【０１７８】
（１）従来、添加剤の添加など複雑な工程を経て、アクリル酸塩系ポリマーの性能、生産性、コストや安全性などを犠牲にして低減させていた残存モノマーが高度かつ簡便に低減できる。
【０１７９】
（２）製造工程に高温を用いても、製造途中での残存モノマーの増加が少ないため高い生産性で製造できる。しかも、高温での反応を行えるため、より優れた幅広い諸物性の改善が行える。
【０１８０】
（３）その後の高温下や長時間の使用中での残存モノマーの発生・増加量が極めて少ないため、農園芸などの長期間の使用や熱水などでの高温下での使用などいかなる使用条件下でも安全性が高い。
【０１８１】
（４）重合性も向上する上に、少ない重合開始剤や穏和な重合条件でも残存モノマーが低減でき、しかも、優れた物性のアクリル酸塩系ポリマーが得られる。
【０１８２】
（５）表面架橋時の残存モノマーの増加が殆ど見られない上に、表面処理効果も向上する。[0001]
[Industrial applications]
The present invention relates to an acrylate polymer and a sanitary material using the same. More specifically, the present invention relates to a partially neutralized or completely neutralized acrylate-based polymer in which the amount of residual monomer is small and little generation or increase of the residual monomer is observed even under various use conditions, and a sanitary material using the same. Things.
[0002]
The acrylate-based polymer of the present invention can be easily and inexpensively produced, and is excellent in performance and safety, so that it can be used in a wide range of fields as a water-absorbing resin or a water-soluble resin.
[0003]
[Prior art]
Hydrophilic resins are generally classified into two types, water-soluble resins and water-absorbent resins, depending on their solubility in water.
[0004]
The water-soluble resin is a hydrophilic resin that dissolves in water and is used as a coagulant for water treatment, an oil drilling additive, a food additive, a thickener, and the like. For example, sodium polyacrylate (Japanese Patent Publication No. 48-46666). , Japanese Patent Publication No. 42-9656), polyacrylic acid, polyacrylamide (Japanese Patent Application Laid-Open Nos. 54-145782 and 57-18652), and a polymer of 2-acrylamido-2-methylpropanesulfonic acid (Japanese Patent Application Laid-Open No. -173108), partially hydrolyzed polyacrylamide (JP-A-52-137483), acrylic acid-acrylamide copolymer (JP-A-59-15417), (meth) acrylic acid-itaconic acid copolymer ( JP-A-58-91709) and polyvinyl alcohol are known.
[0005]
Further, the water-absorbing resin is a water-insoluble hydrophilic resin that absorbs water and gels, and is widely used in the field of sanitary materials such as disposable diapers and sanitary napkins, agriculture and forestry, and civil engineering. Cross-linked polyacrylic acid partially neutralized products (JP-A-55-84304, JP-A-55-108407, JP-A-55-133413), and a hydrolyzate of a starch-acrylonitrile graft polymer (JP-A-55-84413). 46-43995), neutralized starch-acrylic acid graft polymer (JP-A-51-125468), saponified vinyl acetate-acrylate copolymer (JP-A-52-14689), acrylonitrile A copolymer or a hydrolyzate of an acrylamide copolymer (JP-A-53-15959) or a cross-linked product thereof, or a cross-linked product of a cationic monomer (JP-A-58-159) No. 54709, has been known, such as JP-A-58-154710).
[0006]
As described above, many monomers have been proposed as monomers for obtaining these hydrophilic resins. However, due to their performance and cost, acrylates that are currently partially or completely neutralized (hereinafter referred to as acrylates) ) Is mainly used, and acrylate-based polymers containing acrylate as a main component are produced today in large quantities in both water-absorbent resins and water-soluble resins, and are used for sanitary materials, Widely used in the food field. However, in these acrylate-based polymers, usually about 500 to 3000 ppm of residual monomers such as unpolymerized acrylic acid (salt) remain, and it is presently required to reduce the residual monomers. In addition, among these hydrophilic resins, the water-absorbing resin is used for sanitary materials and the like. Therefore, it is particularly required to reduce the residual monomer, and in recent years, it has been required to reduce the residual monomer to 100 ppm or less. However, since the water-absorbing resin has an essential cross-linked structure, it is difficult to perform uniform polymerization and uniform mixing of additives and organic solvents for reducing residual monomers after polymerization, and acrylates have a medium pH. Since the polymerization rate becomes slower as the property approaches, it is particularly difficult to reduce the residual monomer among water-absorbent resins that require a neutral or cross-linked structure among these hydrophilic resins.
[0007]
Conventionally, attempts to reduce the residual monomer of hydrophilic resins have been studied for a long time in the field of polymer flocculants and the like, and even today, many of the above-described hydrophilic resins including acrylate polymers are used. With regard to, studies have been made to reduce residual monomers.
[0008]
These known techniques are roughly classified into the following six (a) to (f).
[0009]
(A) Methods for improving the polymerization rate of the polymer itself and reducing the residual monomer include, for example, increasing the amount of a polymerization initiator, using a composite initiator (JP-A-50-96689), increasing the reaction temperature, and increasing the polymerization concentration. , Extension of polymerization time, specification of aging conditions (JP-A-53-145895), two-stage addition of a polymerization initiator (JP-A-56-72005), and irradiation after polymerization (JP-A-63-19955). 43930) and ultraviolet irradiation after polymerization (JP-A-63-260906).
[0010]
(B) As a method of adding an additive for reducing the residual monomer after polymerization, for example, a method of post-adding a primary or secondary amine (JP-A-50-40689), a method of post-adding sulfur dioxide ( USP-3780006), and a method of post-adding an alkali metabisulfite (JP-A-55-135110).
[0011]
(C) Examples of the method of extracting the residual monomer include a method of extracting with a hydrophilic organic solvent (JP-A-1-292003) and a method of supercritical extraction with carbon dioxide.
[0012]
(D) As a method of adding a microorganism that degrades the residual monomer, for example, a method of decomposing the residual acrylamide by the microorganism (Japanese Patent Publication No. 60-29523) can be mentioned.
[0013]
(E) As a method of volatilizing the residual monomer at a high temperature, for example, a method of volatilizing the residual acrylonitrile at a high temperature (JP-A-54-119588) can be mentioned.
[0014]
(F) A method focusing on the water-soluble unsaturated monomer before polymerization has been proposed in recent years. For example, a method of polymerizing using an acrylate obtained by a specific neutralization method (Japanese Patent Laid-Open No. 2-209906) No.) and a method of polymerizing using a monomer having a small heavy metal (JP-A-3-31306).
[0015]
However, in the method (a), the effect of reducing the residual monomer is insufficient. Generally, not only 0.03% or more remains but also self-crosslinking of the hydrophilic resin due to severe polymerization and post-treatment conditions and basic molecular weight. Inevitably, the physical properties of the target hydrophilic resin were reduced, such as an increase in the water-soluble content of the water-absorbent resin and a decrease in the gel strength. Moreover, the addition of the polymerization initiator in two stages or the use of a large amount of the polymerization initiator may leave the polymerization initiator, which is not preferable in terms of safety.
[0016]
In the methods (b) and (c), the possibility of reducing the residual monomer to 0.03% or less is also described, but the process is complicated and the additives and the organic solvent used are not sufficient. There was a possibility of remaining in the hydrophilic resin. Moreover, since the acrylate does not dissolve in an organic solvent such as methanol, the effect of reducing the residual monomer by the method (c) is limited.
[0017]
Furthermore, in the method (d), not only is it industrially difficult to use a microorganism, but also such a microorganism is not preferable in terms of safety. In the method (e), various physical properties may be reduced due to the high temperature, and the acrylate does not volatilize even at the high temperature, so that the effect of reducing the residual monomer can hardly be expected. Was. Furthermore, even when the method (f) was used, the effect of reducing the residual monomer was still insufficient.
[0018]
As described above, even if these methods are used, the effects are not only insufficient, but also complicated steps are required, and the reality is that productivity and various physical properties are reduced and cost is significantly increased. . Moreover, even with these methods, the residual monomers of the acrylate-based polymer were merely reduced apparently, and it was impossible at all to suppress the increase in the residual monomers after the polymerization described below.
[0019]
That is, the present inventors have now found a phenomenon in which several tens to several thousand ppm of residual monomers are generated and increased in the acrylate-based polymer during or after the production process of the acrylate-based polymer. . That is, in the conventional acrylate-based polymer, it was also found that a large amount of the residual monomer actually increased with time even if the residual monomer was apparently reduced to about several hundred ppm. This increase over time is noticeable when heat is applied. Therefore, these conventional acrylate-based polymers have at least residual monomers, for example, when a further heating step is required, or when used for a long time in agriculture and horticulture or at a high temperature such as hot water. Needless to say, this is not preferable in terms of safety because the amount of residual monomers increases.
[0020]
In addition, regarding the production of acrylate polymers, in order to further improve their physical properties after polymerization, cross-linking near the surface of acrylate polymers is widely performed, especially in water-absorbing resins, which improves the basic physical properties. At present, many improvements are still being proposed.
[0021]
For example, as a surface cross-linking method using a specific surface cross-linking agent, a method using a polyhydric alcohol (JP-A-60-17328, JP-A-61-16903), a method using an alkylene carbonate (DE-4020780C), glyoxal (JP-A-52-117393), a method using a polyvalent metal (JP-A-51-136588, JP-A-61-257235, JP-A-62-7745), a method using a silane coupling agent. The methods used (JP-A-61-211305, JP-A-61-252212, JP-A-61-264006) are known. Examples of a method of surface crosslinking under specific reaction conditions include a method of crosslinking by dispersing a water-absorbent resin in a mixed solvent of water and a hydrophilic organic solvent (Japanese Patent Laid-Open No. 57-44627), A method in which a water-absorbent resin is dispersed in an inert solvent and crosslinked in the coexistence (JP-A-58-117222), a method in which inorganic powder and water coexist and crosslink (USP-4587308), and irradiation with electromagnetic radiation. A method (JP-A-63-43930) and the like are known.
[0022]
However, in the course of studying the improvement of the surface cross-linking of the acrylate-based polymer, the present inventors have found that the residual monomer in the acrylate-based polymer, which has not been known at all, has been reduced to several tens by the surface cross-linking. It was found that the amount of the residual monomer increased by about 100 ppm, and that the increase in the residual monomer due to the surface crosslinking accounted for a large proportion of the residual monomer in the final product.
[0023]
[Problems to be solved by the invention]
The present invention has been made in view of the above situation. Therefore, an object of the present invention is to reduce a residual monomer, and further, even after polymerization, a unique phenomenon that a residual monomer is further generated and increased in a manufacturing process and during use is found. An object of the present invention is to provide an acrylate polymer showing excellent physical properties, having a small amount of residual monomers, hardly generating or increasing residual monomers, and a sanitary material using the same. Another object of the present invention is to improve the various physical properties of the acrylate-based polymer by cross-linking the surface of the acrylate-based polymer, exhibiting excellent physical properties, having a small amount of residual monomers, and showing little occurrence or increase of residual monomers. An object of the present invention is to provide a crosslinked acrylate polymer and a sanitary material using the same.
[0024]
Means and Action for Solving the Problems
As a result of intensive studies to achieve the above object, the present inventors have found that a conventional acrylate polymer having a large amount of residual monomer and a conventional acrylate polymer having a large amount of residual monomer generated The fact that β-hydroxypropionic acid (salt) is contained in the order of several thousand ppm to about 1% as a minor component other than the residual monomer, and the correlation between the content of β-hydroxypropionic acid (salt) and the residual monomer I found a fact.
[0025]
Therefore, the present inventors have paid attention to the importance of β-hydroxypropionic acid (salt) contained in a small amount in an acrylate-based monomer, which has not been noticed at all, as a means for solving the above problems. The above problem was solved by controlling the content thereof, and the present invention was completed.
[0026]
That is, the present invention provides (1) Remaining An acrylate-based polymer having a monomer content of 160 ppm or less, Partial or complete neutralization Acrylate having a neutralization ratio of 30 to 100 mol%, having a repeating unit of 50 to 100 mol% of acrylate and another 0 to 50 mol% of a hydrophilic unsaturated monomer and / or a hydrophobic unsaturated monomer. Acrylate having a crosslinked structure, wherein the polymer is further crosslinked in the vicinity of the surface thereof, and the total content of β-hydroxypropionic acid and a salt thereof is 1,000 ppm or less. System polymer To It is about.
[0027]
The present invention provides (2) an acrylate polymer having a residual monomer content of 160 ppm or less, Partial or complete neutralization Acrylate having a neutralization ratio of 30 to 100 mol%, having a repeating unit of 50 to 100 mol% of acrylate and another 0 to 50 mol% of a hydrophilic unsaturated monomer and / or a hydrophobic unsaturated monomer. The polymer is a water-absorbent resin having a cross-linked structure, the vicinity of the surface of the polymer is further cross-linked, Socks The present invention relates to an acrylate-based polymer characterized by having a latent residual monomer amount defined by the above formula of 30 ppm or less.
[0028]
(Equation 2)

[0029]
The present invention relates to (3) the acrylate-based polymer according to the above (1) or (2), wherein the vicinity of the surface of the polymer is crosslinked with a polyhydric alcohol.
[0030]
The present invention relates to (4) an acrylate polymer having a residual monomer content of 160 ppm or less, and Partial or complete neutralization Acrylate having a neutralization ratio of 30 to 100 mol%, having a repeating unit of 50 to 100 mol% of acrylate and another 0 to 50 mol% of a hydrophilic unsaturated monomer and / or a hydrophobic unsaturated monomer. Is a dry polymer having a crosslinked structure and a dry average particle diameter of 100 to 1000 μm, and the vicinity of the surface of the polymer is further crosslinked with a polyhydric alcohol to form β-hydroxypropionic acid and a salt thereof. The present invention relates to an acrylate polymer having a total content of 1000 ppm or less.
[0031]
The present invention provides (5) the acrylate-based polymer according to any one of the above (1) to (4), wherein the acrylate is a sodium acrylate or a potassium acrylate. It is about.
[0032]
In the present invention, (6) the water-absorbent resin according to any one of the above (1) to (5), wherein the water-absorbent resin is a powder having an average particle diameter of 300 to 600 μm. Acrylate type It is about a polymer.
[0033]
The present invention relates to (7) a sanitary material using the acrylate-based polymer according to any one of the above (1) to (6).
[0050]
Hereinafter, the present invention will be described in more detail.
[0051]
In the present invention, the proportion of the acrylate in the water-soluble unsaturated monomer used for the polymerization is essential to be 50 to 100 mol%. (In the present invention, the term "acrylate" refers to the sum of unneutralized acrylic acid and neutralized acrylate (that is, partially or fully neutralized acrylate). In the present invention, since the residual monomer reduction effect is more manifested, the ratio of the acrylate salt of the present invention is more preferably 70 to 100 mol%, further more preferably 90 to 100 mol%. When the content of the acrylate is less than 50 mol%, not only the performance is reduced and the cost is increased, but also the effect of the present invention is hardly exhibited, and in some cases, the residual monomer may be increased, which is not preferable. .
[0052]
In addition, the neutralization ratio of the acrylate used for the water-soluble unsaturated monomer must be 30 to 100 mol%. If the neutralization ratio is out of this range, various physical properties may be reduced and residual monomers may be increased. (Note that the neutralization rate of the acrylate in the present invention may be the neutralization rate of the acrylate used for preparing the water-soluble unsaturated monomer, or may be mixed with another monomer as necessary. After neutralization of acrylate in water-soluble unsaturated monomer). Further, when the water-soluble unsaturated monomer of the present invention is used, the residual monomer is more remarkably reduced as compared with the conventional method, so that a more preferable neutralization ratio is 40 to 95 mol%, furthermore 50 to 80 mol%. It is.
[0053]
The water-soluble unsaturated monomer used in the present invention contains an acrylate having a neutralization ratio of 30 to 100 mol% in an amount of 50 to 100 mol%, and other 0 to 50 mol% of a hydrophilic unsaturated monomer. And / or hydrophobic unsaturated monomers may be used.
[0054]
Examples of the hydrophilic unsaturated monomer which can be used in the range of 0 to 50 mol% as necessary include, for example, methacrylic acid, maleic acid, maleic anhydride, fumaric acid, crotonic acid, itaconic acid, vinylsulfonic acid, and styrene sulfone. Acid-containing hydrophilic unsaturated monomers such as acid, 2- (meth) acrylamido-2-methylpropanesulfonic acid, 2- (meth) acryloylethanesulfonic acid, 2- (meth) acryloylpropanesulfonic acid, and the like. Salt; acrylamide, methacrylamide, N-ethyl (meth) acrylamide, Nn-propyl (meth) acrylamide, N-isopropyl (meth) acrylamide, N, N-dimethyl (meth) acrylamide, 2-hydroxyethyl (meth) Acrylate, 2-hydroxypropyl (meth) ac Nonionic hydrophilic unsaturated monomers such as methacrylate, methoxypolyethylene glycol (meth) acrylate, polyethylene glycol mono (meth) acrylate, vinylpyridine, N-vinylpyrrolidone, N-acryloylpiperidine, N-acryloylpyrrolidine; N-dimethylaminoethyl (meth) acrylate, N, N-diethylaminoethyl (meth) acrylate, N, N-dimethylaminopropyl (meth) acrylate, N, N-dimethylaminopropyl (meth) acrylamide and quaternary salts thereof And one or more selected from these groups can be used. As the hydrophilic unsaturated monomer, an unsaturated monomer which forms a hydrophilic resin by hydrolysis of a functional group, such as methyl (meth) acrylate, ethyl (meth) acrylate, or vinyl acetate, is used. You may.
[0055]
Among these, methacrylic acid (salt), 2- (meth) acryloylethanesulfonic acid (salt), and 2- (meth) acrylamide-2 are used as hydrophilic unsaturated monomers used in the present invention in an amount of 0 to 50 mol%. -Methylpropanesulfonic acid (salt), methoxypolyethylene glycol (meth) acrylate, N, N-dimethylaminoethyl (meth) acrylate, and acrylamide are preferred.
[0056]
Examples of the hydrophobic unsaturated monomer which can be used in the range of 0 to 50 mol% as needed include, for example, styrene, vinyl chloride, butadiene, isobutene, ethylene, propylene, stearyl (meth) acrylate, lauryl (meth) A) acrylate and the like. These hydrophobic unsaturated monomers are used in an amount of 0 to 50 mol%, preferably 0 to 20 mol%, more preferably 0 to 10 mol%, based on all the monomers.
[0057]
The neutralization of a water-soluble unsaturated monomer containing 50 to 100 mol% of an acrylate having a neutralization ratio of 30 to 100 mol% may be performed only when preparing the water-soluble unsaturated monomer. However, when the water-soluble monomer adjusted to the above range is polymerized, it may be further neutralized during or after the polymerization.
[0058]
In the present invention, in preparing a water-soluble unsaturated monomer containing 50 to 100 mol% of an acrylate having a neutralization ratio of 30 to 100 mol% as described above, β- It is essential to prepare a water-soluble unsaturated monomer having a content of hydroxypropionic acid and a salt thereof (hereinafter, both of which are simply referred to as β-hydroxypropionic acid) of 1000 ppm or less.
[0059]
In the present invention, the content of β-hydroxypropionic acid in the water-soluble unsaturated monomer is essential to be 1000 ppm or less per solid, preferably 500 ppm or less, more preferably 300 ppm or less, most preferably It is 100 ppm or less. When the content of β-hydroxypropionic acid exceeds 1000 ppm, the obtained acrylate-based polymer has a large amount of residual monomers, and further residual monomers are generated in the subsequent production process, high temperature, or long-term use. In addition to the phenomenon of increase, the physical properties of the acrylate polymer may be insufficient, which is not preferable.
[0060]
In the present invention, the content of β-hydroxypropionic acid in the water-soluble unsaturated monomer used for the polymerization may be limited to 1000 ppm or less, and such a water-soluble unsaturated monomer is prepared. The means is not particularly limited, but one example of a preferred specific embodiment is as follows.
[0061]
Acrylic acid is used for neutralization and preparation of a water-soluble unsaturated monomer within as short a time as possible after distillation purification, for example, within 24 hours, preferably 12 hours, more preferably within 6 hours. In the neutralization step, a state where the neutralization ratio exceeds 100 mol% is passed at least for one period. Next, a water-soluble unsaturated monomer is prepared using the acrylic acid, the obtained acrylate, and other monomers as required. Finally, the obtained water-soluble unsaturated monomer is used in a time as short as possible, for example, 24 hours, preferably 12 hours, more preferably 6 hours, particularly preferably 2 hours after completion of preparation of the water-soluble unsaturated monomer. It may be polymerized within the range. Further, it is preferable that the acrylic acid after the distillation and purification is kept at a temperature as low as possible, for example, 30 ° C. or lower, preferably a freezing point to 25 ° C., until it is used for neutralization or preparation of a water-soluble unsaturated monomer. The neutralization step is preferably performed at low temperature for a short time, preferably within 4 hours. If the acrylic acid after distillation is kept for a relatively long time, it is preferable to keep it in a non-aqueous state. Further, the water-soluble unsaturated monomer after completion of the preparation should be stored at a temperature of from its freezing point to 40 ° C, preferably 0 to 30 ° C. If these conditions are not satisfied, β-hydroxypropionic acid and residual monomers tend to increase, and thus care must be taken.
[0062]
Acrylic acid is stored and shipped after the final distillation at the acrylic acid factory, so it takes 4-5 days, usually several tens days to several months, before it is actually used industrially by the user. That is the current situation. Further, since a large amount of the water-soluble unsaturated monomer is prepared and stored in the factory, it takes an average of 3 days or more from the completion of the preparation to the actual use thereof. In reality, it takes several hours to deaerate the saturated monomer and to adjust the temperature. However, the present inventor believes that the longer the time after the purification by distillation of acrylic acid and the longer the time from the completion of the preparation of the water-soluble unsaturated monomer to the polymerization, the more the β-hydroxypropionic acid and the remaining monomer Is found to increase, and therefore, in the present invention, the polymerization is preferably performed in a short time.
[0063]
Examples of the basic substance used for neutralizing acrylic acid include known substances such as a carbonate (hydrogen) salt, an alkali metal hydroxide, ammonia, and an organic amine, and these may be used in combination. . However, among these basic substances, in order to obtain an acrylate polymer having excellent physical properties and a reduced residual monomer, it is necessary to use at least a strong basic substance such as sodium hydroxide or potassium hydroxide. Is preferred, and sodium hydroxide is particularly preferred.
[0064]
At least during the neutralization step, the simplest method of neutralizing the acrylic acid under conditions in which the neutralization rate of acrylic acid exceeds 100 mol% is (1) cooling the neutralization reaction system. Meanwhile, there is a method of gradually adding acrylic acid to a certain amount of a basic substance. Further, as another method, disclosed in JP-A-2-209906, (2) the state where the neutralization ratio of the acrylate in the neutralization system is less than 100 mol% from the start of neutralization, Also, there is a method in which the neutralization rate is adjusted to 30 to 100 mol% after the neutralization rate exceeds 100 mol% during the neutralization.
[0065]
The temperature of the acrylate solution in these neutralization steps is preferably kept at 0 to 50 ° C. by cooling. In order to obtain an acrylate having a low content of β-hydroxypropionic acid, the neutralization method (1) is preferably carried out at a lower temperature than that of the conventional method, preferably at a temperature of 10 ° C. or lower. This leads to a decrease in sex. However, in the neutralization method of (2), as long as the above-mentioned acrylic acid is used, the content of β-hydroxypropionic acid can be rapidly reduced to a high level of 100 ppm or less quickly.
[0066]
In the present invention, the content of β-hydroxypropionic acid is preferably small, but if the content is reduced to 100 ppm or less, the acrylate-based polymer intended by the present invention can be sufficiently obtained. Furthermore, even if the content is reduced to less than 10 ppm, particularly to about 1 ppm, no remarkable influence on various physical properties such as residual monomers is observed. In addition, in order to make the content of β-hydroxypropionic acid less than 1 ppm or undetectable, extremely difficult purification and production steps are required, which is generally uneconomical. The content is not harmful.
[0067]
In the present invention, it is essential to use a water-soluble unsaturated monomer having a low content of β-hydroxypropionic acid obtained as described above. It can be. The method for producing an acrylate polymer of the present invention is suitably used for a water-absorbent resin in which it is difficult to reduce the residual monomer.
[0068]
The crosslinking method used is not particularly limited. For example, (A) a water-soluble resin is obtained by polymerizing the water-soluble unsaturated monomer of the present invention, and then a crosslinking agent is added during or after polymerization. And post-crosslinking, (B) radical crosslinking with a radical polymerization initiator, (C) radiation crosslinking with an electron beam and the like. In order to obtain a water absorbent resin with excellent performance with good productivity, D) It is preferable to add a predetermined amount of an internal crosslinking agent to the water-soluble unsaturated monomer in advance to carry out polymerization, and to carry out a crosslinking reaction simultaneously with or after the polymerization.
[0069]
Examples of the internal crosslinking agent used in the technique (D) include N, N′-methylenebisacrylamide, (poly) ethylene glycol di (meth) acrylate, (poly) propylene glycol di (meth) acrylate, and trimethylolpropane. Tri (meth) acrylate, trimethylolpropane di (meth) acrylate, (poly) ethylene glycol di (β-acryloyloxypropionate), trimethylolpropane tri (β-acryloyloxypropionate), poly (meth) ali One or more internal crosslinking agents such as roxyalkane, (poly) ethylene glycol diglycidyl ether, ethylene glycol, polyethylene glycol, glycerin, pentaerythritol, ethylenediamine, and polyethyleneimine are used. The amount for the water-soluble unsaturated monomer, usually, 0.005 to 5 mol%, more preferably from 0.01 to 1 mol%. Among these internal cross-linking agents, polymerizable polymers having two or more polymerizable unsaturated groups in the molecule due to the durability and water absorption properties of the resulting water-absorbent resin, and the handling properties of the hydrogel during production. It is preferable to use an internal crosslinking agent indispensably.
[0070]
At the time of polymerization, the water-soluble unsaturated monomer of the present invention may be subjected to bulk polymerization or precipitation polymerization as it is, but it is preferable to carry out polymerization as a solution from the viewpoint of performance and easy control of polymerization. The polymerization solvent is not particularly limited as long as it is a liquid in which the water-soluble unsaturated monomer is dissolved, and examples thereof include water, methanol, ethanol, acetone, dimethylformamide, and dimethyl sulfoxide. Is particularly preferred. When the water-soluble unsaturated monomer is polymerized as a solution, its concentration is not particularly limited and may be higher than the saturation concentration. Concentration, preferably in the range of 25 to 50% by weight. In addition, at the time of polymerization, water-soluble unsaturated monomers may be added to water-soluble chain transfer agents such as hypophosphorous acid (salt), thiols, thiolic acid (salt), starch, cellulose and derivatives thereof, polyvinyl alcohol, A hydrophilic polymer such as polyacrylic acid or a crosslinked polyacrylate may be added. Their usage is usually less than 5 parts by weight for the former and less than 50 parts by weight for the latter.
[0071]
The water-soluble unsaturated monomer having a β-hydroxypropionic acid content of 1000 ppm or less obtained above is then polymerized. In the present invention, as described above, the water-soluble unsaturated monomer after preparation is preferably polymerized in as short a time as possible.
[0072]
Examples of the polymerization method used in the present invention include known methods such as (a) polymerization using a radical polymerization initiator, (b) radiation polymerization, (c) electron beam polymerization, and (d) ultraviolet polymerization using a photosensitizer. In order to obtain an acrylate-based polymer having excellent performance, (a) polymerization with a radical polymerization initiator is preferable.
[0073]
Examples of the polymerization method (a) include casting polymerization performed in a mold (JP-B-48-42466), polymerization on a belt conveyor (JP-A-58-49714), and a hydrogel polymer. (Japanese Patent Application Laid-Open No. 57-34101), various aqueous solution polymerizations such as polymerization under pressure (Japanese Patent Application Laid-Open No. 2-129207); reverse-phase suspension polymerization (Japanese Patent Publication No. 59-37003); Phase emulsion polymerization (JP-A-63-90510, JP-A-63-90537), precipitation polymerization (JP-A-58-84819, JP-A-1-17110, JP-A-1-204910), bulk polymerization, etc. Can be exemplified, but reverse phase suspension polymerization or aqueous solution polymerization is particularly preferred. In the case of polymerization, continuous polymerization, semi-batch polymerization, batch polymerization and reduced pressure, pressurization, normal pressure is not particularly limited, and further, a fiber substrate and the like coexist during polymerization as a water-absorbing composite. (JP-A-2-242975).
[0074]
Examples of the radical polymerization initiator used for such polymerization include persulfates such as potassium persulfate, ammonium persulfate and sodium persulfate; organic peroxides such as t-butyl hydroperoxide and cumene hydroperoxide; Hydrogen; azo compounds such as 2,2'-azobis (2-amidinopropane) dihydrochloride; and other known polymerization initiators such as chlorite, hypochlorite, ceric salt, and permanganate Among them, one or more selected from the group consisting of persulfate, hydrogen peroxide, and azo compound are preferable. When an oxidizing radical polymerization initiator is used, a reducing agent such as a sulfurous acid (hydrogen) salt or L-ascorbic acid may be used in combination, and when an azo polymerization initiator or the like is used, ultraviolet light may be used in combination. . Incidentally, these radical polymerization initiators and the like may be added to the polymerization system all at once or may be added sequentially, but the amount of use is usually 0.001 to 2 mol based on the water-soluble unsaturated monomer. %, Preferably 0.01 to 1 mol%. In addition, by using the water-soluble unsaturated monomer of the present invention, the amount of the residual monomer can be reduced even with the use amount of the radical polymerization initiator smaller than usual, so that the polymerization can be easily controlled and the obtained acrylate-based polymer can be obtained. Are also preferable because the various physical properties are further improved.
[0075]
When the polymerization is carried out using a water-soluble unsaturated monomer as a solution, the acrylate-based polymer after the polymerization may be used as it is, but it is preferable to dry it in order to improve the handleability and reduce the residual monomer.
[0076]
Examples of the drying method used in the present invention include known means. For example, a method in which polymerization is carried out at a high concentration to simultaneously carry out drying and polymerization by the heat of polymerization (JP-A-58-71907, JP-A-Hei. No. 34607), or the obtained gel polymer may be further dried depending on the solid content after polymerization. In the case of further drying, for example, drying under high humidity conditions (JP-A-1-26604), azeotropic dehydration in an organic solvent, microwave drying, belt or drum dryer heated to a predetermined temperature Drying in a cylinder equipped with a high-speed rotor (Japanese Patent Application Laid-Open No. 2-240112), and drying using a forced draft oven, infrared rays, a reduced-pressure dryer, and the like are also possible.
[0077]
The drying temperature in the present invention is usually in the range of 70 to 230C, preferably 110 to 220C, more preferably 150 to 210C. If the temperature is out of this range, the effect of using the water-soluble unsaturated monomer of the present invention is less likely to be exhibited, and in some cases, the residual monomer may increase, and various physical properties and productivity may decrease. The drying time is appropriately determined depending on the water content, the particle size, the drying temperature and the like of the gel polymer.
[0078]
As described above, this time, the present inventors have found an increase in residual monomers during drying, and for the first time by using the water-soluble unsaturated monomer of the present invention, any polymerization method, drying temperature or drying time, less residual monomer. An acrylate polymer having excellent physical properties can be obtained.
[0079]
The acrylate polymer of the present invention thus obtained may be used as it is, but may be further crushed or granulated to adjust the particle size. The target particle size varies depending on the type and purpose of use of the acrylate polymer, but in the case of a powdery water-absorbent resin, the average particle size is usually 10 to 2000 μm, more preferably 100 to 1000 μm, and most preferably 300 to 1000 μm. It is about 600 μm. Further, various inorganic powders, oxidizing agents, reducing agents, fragrances, fertilizers, binders, water, and the like may be further added to the obtained acrylate polymer.
[0080]
Further, the method of the present invention for producing an acrylate-based polymer having a crosslinked surface is provided by crosslinking the vicinity of the surface of the acrylate-based polymer obtained by the above method.
[0081]
When an acrylate polymer obtained by using a water-soluble unsaturated monomer having a β-hydroxypropionic acid content of more than 1000 ppm is used as the acrylate polymer that crosslinks the vicinity of the surface, the residual monomer is removed by surface cross-linking. It is not preferable because not only does the increase cause adverse effects on safety, but also may deteriorate various physical properties.
[0082]
As the water content of the acrylate polymer used, the acrylate polymer after polymerization may be used as it is. However, from the viewpoint of the surface crosslinking effect, the water content is usually 40% or less, more preferably 30%. Hereinafter, an acrylate polymer dried to a water content of 10% or less is more preferable. Further, the particle size of the acrylate polymer used is an average particle size of about 10 to 2000 μm, more preferably about 100 to 1000 μm, and most preferably about 300 to 600 μm, and it is preferable that the particle size distribution is narrow. The acrylate-based polymer used may be a water-soluble resin, but the effect of improving various physical properties by surface crosslinking is more clearly exhibited with respect to the acrylate-based water-absorbent resin.
[0083]
In the present invention, the crosslink near the surface of the acrylate polymer may be crosslinked by radiation or the like, but is usually performed by adding a surface crosslinker near the surface. As the surface cross-linking agent to be used, known ones are exemplified without particular limitation, for example, ethylene glycol, propylene glycol, polyethylene glycol, propylene glycol, glycerin, polyglycerin, 1,6-hexanediol, trimethylolpropane, diethanolamine, Various polyhydric alcohols such as triethanolamine, polyoxypropylene, oxyethyleneoxypropylene block copolymer, pentaerythritol, sorbitol; various polyhydric epoxy compounds such as ethylene glycol diglycidyl ether and polyethylene glycol diglycidyl ether; ethylene diamine; Various polyamine compounds such as polyethyleneimine; 2,2-bishydroxymethylbutanol-tris (3- (1-aziridinyl) propyl Polyvalent aziridine compounds such as 1,3-dioxolan-2-one, 4-methyl-1.3-dioxolan-2-one, and 4,6-dimethyl-1,3-dioxan-2-one; Various alkylene carbonate compounds; various polyhydric aldehyde compounds such as glyoxal; polyvalent isocyanate compounds such as 2,4-tolylene diisocyanate; polyvalent oxazoline compounds such as 1,2-ethylenebisoxazoline; haloepoxy compounds such as epichlorohydrin Polyhydric metal salts such as hydroxides and chlorides of aluminum, iron, zirconium and the like; and compounds having these functional groups in combination.
[0084]
Among these surface cross-linking agents, in the present invention, it is more preferable to use one or more selected from the group consisting of polyhydric alcohols, polyhydric glycidyl compounds, polyamines, and alkylene carbonates. In the effects of the present invention, polyhydric alcohols are most preferred. The use amount of these surface cross-linking agents varies depending on the cross-linking conditions to be used and the like. It is in the range of 10 parts by weight.
[0085]
A known method is used for adding the surface cross-linking agent to the acrylate-based polymer, for example, a method of directly adding the surface cross-linking agent to the acrylate-based polymer or a method for dispersing the acrylate-based polymer in a solvent. A method of adding a surface cross-linking agent and the like can be mentioned. Examples of the former method include DE4020780 and the like. In this case, an inorganic compound (USP-4587308) such as silicon oxide fine powder or a surfactant may be further used in order to uniformly add a surface crosslinking agent. Alternatively, water may be mixed as steam (Japanese Patent Laid-Open No. 1-297430). As the latter method, there are a method of dispersing an acrylate polymer in a hydrophilic organic solvent (JP-A-57-44627) and a method of dispersing an acrylate polymer in a hydrophobic solvent (JP-A-59-44659). In this case, it is preferable that a specific amount of water coexists (Japanese Patent Application Laid-Open No. 58-117222).
[0086]
When the surface crosslinking agent is added to the acrylate polymer, the surface crosslinking agent may be added as a solution or a dispersion. As the solvent used, for example, hydrophilic organic solvents such as methyl alcohol, ethyl alcohol, n-propyl alcohol, iso-propyl alcohol, acetone, and tetrahydrofuran, and water are preferable. The amount used is usually in the range of 0 to 20 parts by weight, preferably 0 to 8 parts by weight, based on 100 parts by weight of the solid content of the acrylate polymer.
[0087]
In performing surface cross-linking in the present invention, the presence of water is preferable from the viewpoint of the surface treatment effect. Therefore, when a substantially dried acrylate-based polymer is used, it is preferable to add water. Water may be added at the same time as the surface cross-linking agent or separately, but the amount is usually 20 parts by weight or less, preferably 0.1 part by weight, per 100 parts by weight of the solid content of the acrylate polymer. It is in the range of 5 to 10 parts by weight.
[0088]
After mixing the acrylate polymer and the surface cross-linking agent according to the above method, the surface cross-linking method of the present invention is preferably subjected to a heat treatment from the viewpoint of various physical properties and the remaining monomer.
[0089]
When the acrylate polymer of the present invention is used, the heat treatment temperature is usually 75 ° C. or higher, preferably 100 to 300 ° C., more preferably 120 to 250 ° C., and still more preferably 150 to 230 ° C. If the temperature is out of the above range, various physical properties are reduced, and the effect of using the acrylate polymer of the present invention may not be easily exhibited, which is not preferable. The heating time is appropriately determined depending on the desired surface crosslinking effect, heating temperature, and the like, but is usually in the range of 1 minute to 10 hours.
[0090]
In the present invention, as a method of performing the heat treatment, known means can be used without limitation. For example, (1) a method of directly adding a surface cross-linking agent or a solution thereof to an acrylate polymer and then performing a heat treatment as it is, (2) a method of adding a surface cross-linking agent to an acrylate polymer dispersed in a solvent and then heat-treating the dispersion while dispersing, or (3) a method of performing heat treatment by filtering from a dispersion medium, and the like. The method (1) is preferred from the viewpoint of easiness of heat treatment, safety, cost and productivity. In addition, there is no restriction | limiting in particular about a heat processing apparatus, A well-known apparatus, such as a hot-air dryer, a fluidized-bed dryer, and a Nauta type dryer, is used.
[0091]
As described above, this time, the present inventors have found an increase in residual monomers due to surface cross-linking, and for the first time by using the acrylate-based polymer of the present invention, the acrylate-based polymer having excellent physical properties with little residual monomer in any surface cross-linking method. A polymer was obtained.
[0092]
The acrylate polymer of the present invention before or after surface crosslinking obtained as described above has not only a low residual monomer content of 100 ppm or less, but also a reduced content of β-hydroxypropionic acid. It is a novel acrylate polymer. The content of β-hydroxypropionic acid of the present invention is 1 to 1000 ppm, preferably 1 to 500 ppm, and more preferably 1 to 100 ppm.
[0093]
That is, the acrylate-based polymer of the present invention not only has a small amount of residual monomer, but also has a low content of β-hydroxypropionic acid. It is. In conventional acrylate-based polymers, even with low residual monomers, a large increase in the generation of residual monomers was observed due to heating and aging, but this problem was solved for the first time in the acrylate-based polymer of the present invention. Was.
[0094]
It is unknown how β-hydroxypropionic acid in the water-soluble unsaturated monomer or the acrylate polymer leads to an increase or generation of residual monomers. It is presumed that, when the acrylate polymer is heated, the residual monomer increases or is generated even when the temperature is lower than the melting point (142 ° C.) of sodium β-hydroxypropionate. It is estimated that hydroxypropionic acid is extremely easily decomposed, or that β-hydroxypropionic acid promotes depolymerization of the acrylate polymer. Further, it is also presumed that various physical properties of the acrylate-based polymer were reduced due to the generation and increase of the residual monomer and the depolymerization of the polymer.
[0095]
【Example】
Hereinafter, the present invention will be described with reference to examples, but the scope of the present invention is not limited only to these examples. The physical properties of the acrylate polymers described in the examples are values measured by the following test methods.
[0096]
The term "potential residual monomer" used herein refers to a residual monomer that is generated or increased in a long-time use or at a high temperature, in addition to the residual monomer usually measured at room temperature in an acrylate polymer. Even if the apparent residual monomer is small, it is needless to say that if the potential residual monomer is large, it is not preferable for safety.
[0097]
(1) Water absorption ratio
0.2 g of an acrylate polymer was uniformly placed in a nonwoven fabric bag (40 × 150 mm) and immersed in a 0.9% by weight aqueous sodium chloride solution. Thirty minutes later, the tea bag-type bag was pulled up and drained for a certain period of time. Then, the weight of the tea bag-type bag was measured, and the water absorption capacity was calculated by the following equation.
[0098]
[Equation 3]

[0099]
(2) Residual monomer
0.5 g of the acrylate polymer was stirred in 1000 ml of deionized water for 2 hours and then filtered, and the residual monomer in the filtrate was measured by using high performance liquid chromatography.
[0100]
(3) Potential residual monomer
In order to simplify the analysis of potential residual monomers, the obtained acrylate-based polymer was further heated at 180 ° C. for 3 hours, and then the residual monomers measured by the method (2) were used. Defined as potential residual monomer.
[0101]
(Equation 4)

[0102]
(5) Suction force
Artificial urine (composition: urea, NaCl, CaCl) in a Petri dish with tissue paper ₂ , MgSO ₄ 1.9 wt%, 0.9 wt%, 0.1 wt%, 0.1 wt% each) and then 1 g of the acrylate-based polymer whose surface was crosslinked near the surface was further added to the center of the dish. And let artificial urine be absorbed through tissue paper. After a lapse of 10 minutes, the weight of the swollen gel is measured and used as the suction force of the acrylate polymer.
[0103]
(Acrylate used)
Hereinafter, the acrylate used in Examples and Comparative Examples is an acrylate obtained by the following method. In addition, the content of β-hydroxypropionic acid is a value per solid content determined by liquid chromatography.
[0104]
Until the acrylic acid after distillation purification was used for neutralization, it was stored in a dark room at a constant temperature of 20 to 40 ° C. assuming normal temperature. Usually, acrylic acid used in a production site has passed several tens of days after being distilled and refined by an acrylic acid maker. In addition, at the manufacturing site, it is often handled and stored as an aqueous 80% acrylic acid solution having a high freezing point and not freezing.
[0105]
(Production Example 1)
Acrylic acid obtained from the acrylic acid production site was purified by distillation. The acrylic acid after the distillation purification was stored at a temperature of 30 ° C. for 3 hours, and the acrylic acid was neutralized according to the method of Example 1 of JP-A-2-209906 shown below. 2744 g of ion-exchanged water was charged into a distillation flask equipped with a stirrer. While maintaining the temperature of the neutralization reaction system in the flask at 20 to 40 ° C., 1390 g of acrylic acid and 1480 g of 48% by weight sodium hydroxide were added at a drop ratio of sodium hydroxide / acrylic acid = 0.9 to 0.95. Over 100 minutes. After the completion of the dropwise addition, 160 g of a 48% by weight aqueous sodium hydroxide solution was further supplied to make the neutralization rate of the neutralization reaction system in the flask 102 mol%. Then, the temperature of the neutralization reaction system was adjusted to 40 ° C., and aging was performed for 30 minutes. After the ripening, 499 g of acrylic acid was supplied to the neutralization reaction system over 10 minutes to obtain acrylate (I) having a neutralization rate of 75 mol% and a concentration of 37%. The content of β-hydroxypropionic acid per solid content in the acrylic salt (I) was 40 ppm.
[0106]
(Production Examples 2 and 3)
In Production Example 1, the same procedure as in Production Example 1 was carried out except that acrylic acid used for neutralization was stored at a temperature of 30 ° C. after distillation and purification, and the storage time was 10 hours and 24 hours. Acrylates (II) and (III) were obtained. The contents of β-hydroxypropionic acid per solid content in the acrylic salts (II) and (III) were 90 ppm and 190 ppm, respectively.
[0107]
(Production Example 4)
In Preparation Example 1, acrylate was prepared in the same manner as in Preparation Example 1 except that acrylic acid used for neutralization was stored at a temperature of 25 ° C. after distillation and purification, and the storage time was set to 24 hours. (IV) was obtained. The content of β-hydroxypropionic acid per solid content in the acrylic salt (IV) was 100 ppm.
[0108]
(Production Examples 5 to 7)
The same procedure as in Production Example 1 was carried out except that acrylic acid used for neutralization was stored at a temperature of 20 ° C. after distillation and purification, and the storage time was changed to 10, 24, and 48 hours. The acid salts (V) to (VII) were obtained. The contents of β-hydroxypropionic acid per solid content in the acrylic salts (V) to (VII) were 50 ppm, 80 ppm, and 130 ppm, respectively.
[0109]
(Production Example 8)
Commercially available acrylic acid production (Wako Pure Chemicals special grade) was purified by distillation. The acrylic acid after the distillation purification was stored at a temperature of 30 ° C. for 3 hours, and the acrylic acid was neutralized according to the method of Comparative Example 2 of JP-A-2-209906 shown below.
[0110]
A distillation flask equipped with a stirrer was charged with 2744 g of ion-exchanged water and 1640 g of 48% by weight sodium hydroxide. Next, while maintaining the temperature of the neutralization reaction system at 20 to 40 ° C., 1889 g of acrylic acid was supplied to the flask over 120 minutes, whereby acrylate (VIII) having a neutralization ratio of 75 mol% and a concentration of 37% was supplied. Got. The content of β-hydroxypropionic acid per solid content in the acrylic salt (VIII) was 230 ppm.
[0111]
(Production Examples 9 and 10)
In Production Example 8, the same procedure as in Production Example 8 was carried out except that acrylic acid used for neutralization was stored at a temperature of 30 ° C. after distillation and purification, and the storage time was changed to 10 hours and 24 hours. Acrylates (IX) and (X) were obtained. The contents of β-hydroxypropionic acid per solid content in the acrylic salts (IX) and (X) were 290 ppm and 390 ppm, respectively.
[0112]
(Production Example 11)
Commercially available acrylic acid production (Wako Pure Chemicals special grade) was purified by distillation. The acrylic acid after the distillation purification was converted into an 80% aqueous solution having a high freezing point and easy to handle, then stored at 30 ° C. for 3 hours, and neutralized according to the following method similar to that of Production Example 8.
[0113]
A distillation flask equipped with a stirrer was charged with 2272 g of ion-exchanged water and 1640 g of 48% by weight sodium hydroxide. Then, while maintaining the temperature of the neutralization reaction system at 20 to 40 ° C., 2361 g of an 80% aqueous solution of acrylic acid was supplied to the flask over 120 minutes, whereby acrylic acid having a neutralization rate of 75 mol% and a concentration of 37% was supplied. The salt (XI) was obtained. The content of β-hydroxypropionic acid per solid content in the acrylic salt (XI) was 290 ppm.
[0114]
(Production Example 12)
Neutralization was performed in the same manner as in Production Example 8 except that the neutralization temperature was lowered to 10 ° C., to obtain an acrylate (XII). However, since the neutralization temperature was lowered, the time required for the neutralization was 6 times, which is three times that of Production Example 8. The content of β-hydroxypropionic acid with a solid content in the acrylic salt (XII) was 70 ppm.
[0115]
(Production Example 13)
In Production Example 1, neutralization was performed in the same manner as in Production Example 1 except that after aging at a neutralization ratio of 102 mol%, the amount of acrylic acid added to the neutralization reaction system was changed from 499 g of acrylic acid to 1446 g. An acrylic acid salt (XIII) having a concentration of 50% and a concentration of 45% was obtained. The content of β-hydroxypropionic acid per solid content in the acrylic salt (XIII) was 30 ppm.
[0116]
(Production Example 14)
In Production Example 1, neutralization was performed in the same manner as in Production Example 1 except that after aging at a neutralization rate of 102 mol%, the amount of acrylic acid added to the neutralization reaction system was changed from 499 g of acrylic acid to 184 g. An acrylic acid salt (XIV) having a concentration of 90% and a concentration of 34% was obtained. The content of β-hydroxypropionic acid per solid content in the acrylic salt (XIV) was 50 ppm.
[0117]
(Production Example 15)
Production Example 8 was carried out in the same manner as in Production Example 1 except that the amount of water was changed from 2774 g to 1942 g, and the amount of acrylic acid suitable for the neutralization reaction system was changed from 1889 g to 1418 g. An acrylic acid salt (XV) with a concentration of 37% at 100 mol% was obtained. The content of β-hydroxypropionic acid per solid content in the acrylic salt (XV) was 290 ppm.
[0118]
(Comparative Production Example 1)
Comparative Example 1 was obtained in the same manner as in Production Example 1, except that acrylic acid used for neutralization was stored at 25 ° C. for 20 days after purification by distillation. The content of β-hydroxypropionic acid per solid content in the comparative acrylic salt (I) was 2500 ppm.
[0119]
(Comparative Production Example 2)
In Comparative Example 1, a commercially available acrylic acid (Kanto Chemical, special grade) was used as the acrylic acid to be used for neutralization in the same manner as in Comparative Example 1 except that the acrylic acid was used without purification by distillation. Obtained. The content of β-hydroxypropionic acid per solid content in the comparative acrylic salt (II) was 4200 ppm.
[0120]
(Comparative Production Example 3)
Comparative acrylic acid salt (III) was prepared in the same manner as in Production Example 1 except that commercially available acrylic acid (Wako Pure Chemical Industries, Ltd., special grade) was used as the acrylic acid for neutralization without distillation purification. Got. The content of β-hydroxypropionic acid per solid content in the comparative acrylic salt (III) was 8,200 ppm.
[0121]
(Comparative Production Examples 4 and 5)
Comparative acrylic acid salts (IV) and (V) were prepared in the same manner as in Production Example 1 except that acrylic acid used for neutralization was stored for 120 hours and 240 hours after distillation purification. Got. The content of β-hydroxypropionic acid per solid content in the comparative acrylic salts (IV) and (V) was 1100 ppm and 1900 ppm, respectively.
[0122]
(Comparative Production Example 6)
Comparative acrylic acid was prepared in the same manner as in Production Example 1 except that acrylic acid used for neutralization was stored at a temperature of 40 ° C. after distillation and purification, and the storage time was 48 hours. The salt (VI) was obtained. The content of β-hydroxypropionic acid per solid content in the comparative acrylic salt (VI) was 1,300 ppm.
[0123]
(Comparative Production Examples 7 and 8)
Comparative Acrylates (VII) and (VIII) were prepared in the same manner as in Production Example 1 except that acrylic acid used for neutralization was stored for 120 hours and 240 hours after distillation purification. Got. The content of β-hydroxypropionic acid per solid content in the comparative acrylic salts (VII) and (VIII) was 1300 ppm and 2100 ppm, respectively.
[0124]
(Comparative Production Examples 9 and 10)
Comparative acrylic acid salt (IX) was prepared in the same manner as in Production Example 11 except that an aqueous acrylic acid solution preserved for 240 hours and 480 hours after distillation purification was used as the 80% aqueous acrylic acid solution used for neutralization in Production Example 11. , (X). The contents of β-hydroxypropionic acid per solid content in the comparative acrylic salts (IX) and (X) were 3300 ppm and 6700 ppm, respectively.
[0125]
(Comparative Production Example 11)
In Production Example 8, when the temperature of the neutralization system was increased from 20 to 40 ° C. to 50 to 60 ° C. to shorten the neutralization time, the time required for neutralization was reduced from 120 minutes to 40 minutes. However, the content of β-hydroxypropionic acid in the obtained acrylate significantly increased to 2100 ppm. Hereinafter, this acrylate is referred to as comparative acrylate (XI).
[0126]
(Comparative Production Example 12)
In Production Example 13, the same procedure as in Production Example 13 was carried out except that the acrylic acid used in Comparative Production Example 1 was used as the acrylic acid used for neutralization, and the content of β-hydroxypropionic acid was 1800 ppm, and the neutralization ratio was 50 mol. % Of comparative acrylate (XII) having a concentration of 45%.
[0127]
(Comparative Production Example 13)
In Production Example 15, the same procedure as in Production Example 15 was carried out except that the acrylic acid used in Comparative Production Example 1 was used as the acrylic acid used for the neutralization, the content of β-hydroxypropionic acid was 3200 ppm, and the neutralization rate was 100 mol. % Of the comparative acrylate (XIII) was obtained.
[0128]
(Experimental example 1)
By dissolving 1.8 g of N, N'-methylenebisacrylamide as an internal cross-linking agent in 5500 g of the acrylate (I) obtained in Production Example 1, the aqueous solution having a β-hydroxypropionic acid content of 40 ppm An unsaturated monomer (I) was obtained. (Concentration: 37%, neutralization rate: 75 mol%) A jacketed stainless steel double-armed cutter having 10 L of water-soluble unsaturated monomer and two sigma-type blades after degassing with nitrogen gas for 30 minutes. (Kneader) The reactor was supplied with a lid, and the water-soluble unsaturated monomer (I) was kept at a temperature of 30 ° C. and the reaction system was placed in nitrogen. Then, while heating through warm water at 35 ° C., 0.3 mol% of ammonium sulfate and 0.03 mol% of sodium hydrogen sulfite were added as polymerization initiators. In addition, it took 2 hours from the preparation of the water-soluble unsaturated monomer (I) by the above operation, to the addition of the polymerization initiator to the initiation of the polymerization.
[0129]
One minute after the addition of the polymerization initiator, polymerization starts, and after 16 minutes, the hydrogel polymer is subdivided into a diameter of about 5 mm, and stirring is further continued to start polymerization. I took it out. Next, the obtained granulated product of the hydrogel polymer was spread on a 50-mesh wire net and dried with hot air at 130 ° C. for 90 minutes. The dried product was pulverized using a vibration mill, and further classified with 20 mesh to obtain an acrylic acid salt water-absorbent resin (1).
[0130]
(Experimental Examples 2 to 4)
Experimental Example 1 was conducted in the same manner as in Experimental Example 1 except that the time from preparation to the start of polymerization was extended to 6, 12, and 24 hours by leaving the water-soluble unsaturated monomer (I) for a predetermined time. By repeating the same operation as above, acrylate-based water-absorbing resins (2) to (4) were obtained.
[0131]
(Experimental Examples 5 to 7)
In Experimental Example 1, the same as Experimental Example 1 except that acrylates (II) to (IV) are used instead of acrylate (I) as the acrylate used for preparing the water-soluble unsaturated monomer. Thus, water-soluble unsaturated monomers (II) to (IV) having β-hydroxypropionic acid contents of 90 ppm, 190 ppm and 100 ppm, respectively, were obtained. Next, these water-soluble unsaturated monomers (II) to (IV) are polymerized 2 hours after preparation in the same manner as in Experimental Example 1, and thereafter, the same operation as in Experimental Example 1 is repeated to obtain acrylic acid. The salt-based water-absorbing resins (5) to (7) were obtained.
[0132]
(Experimental example 8)
In Experimental Example 1, the acrylate used for preparing the water-soluble unsaturated monomer was changed to acrylate (V) instead of acrylate (I), and the internal crosslinking agent was also N, N Β-Hydroxypropionic acid was prepared in the same manner as in Experimental Example 1 except that 2.2 g (0.02 mol%, based on monomer) of polyethylene glycol diacrylate (average n number: 8) was used instead of '-methylenebisacrylamide. Of a water-soluble unsaturated monomer (V) having a content of 50 ppm.
[0133]
Next, the water-soluble unsaturated monomer (V) was polymerized 6 hours after preparation in the same manner as in Experimental Example 1. The gel polymer thus obtained was dried at 150 ° C. for 75 minutes, and then crushed and classified in the same manner as in Experimental Example 1 to obtain an acrylate water-absorbent resin (8).
[0134]
(Experimental Examples 9 and 10)
Experimental Example 8 was the same as Experimental Example 8 except that acrylates (VI) and (VII) were used instead of acrylate (V) as the acrylate used for preparing the water-soluble unsaturated monomer. Similarly, water-soluble unsaturated monomers (VI) and (VII) having β-hydroxypropionic acid contents of 80 ppm and 130 ppm, respectively, were obtained. Next, acrylate-based water-absorbing resins (9) and (10) were obtained in the same manner as in Experimental Example 8 for these water-soluble unsaturated monomers (VI) and (VII).
[0135]
(Experimental example 11)
In Experimental Example 1, the acrylate used in the preparation of the water-soluble unsaturated monomer was changed to acrylate (VIII) instead of acrylate (I), and at the same time, the internal crosslinking agent was N, N ′. In the same manner as in Experimental Example 1, except that 13.6 g (0.2 mol%, based on monomer) of trimethylolpropane triacrylate was used instead of methylenebisacrylamide, a water solution containing 230 ppm of β-hydroxypropionic acid was used. The unsaturated monomer (VIII) was obtained.
[0136]
Hereinafter, the water-soluble unsaturated monomer (VIII) was polymerized 2 hours after preparation in the same manner as in Experimental Example 1, and the obtained gel polymer was dried at 180 ° C. for 60 minutes. In the same manner as in the above, acrylate-based water-absorbent resin (11) was obtained.
[0137]
(Experimental Examples 12 and 13)
In Experimental Example 11, the acrylate used in the preparation of the water-soluble unsaturated monomer was changed to acrylate (IX), (X) instead of acrylate (VIII) in the same manner. Water-soluble unsaturated monomers (IX) and (X) having β-hydroxypropionic acid content of 290 ppm and 390 ppm, respectively, were obtained. Next, these water-soluble unsaturated monomers (IX) and (X) were treated in the same manner as in Experimental Example 11 to obtain acrylate-based water-absorbing resins (12) and (13).
[0138]
(Experimental example 14)
In a reaction vessel equipped with a stir bar, a nitrogen blowing tube and a thermometer, 30 g of corn starch was dissolved in 600 g of water, and 718 g (3 mol) of acrylate (XI) and 71 g (1 mol) of acrylamide were further dissolved. By dissolving 0.12 g of trimethylolpropane triacrylate as an internal crosslinking agent (0.01 mol% based on a monomer in an aqueous starch solution, a water-soluble unsaturated monomer having a content of β-hydroxypropionic acid of 230 ppm (XI ) Was prepared.
[0139]
Next, after keeping the water-soluble unsaturated monomer (XI) at 30 ° C. for 2 hours, nitrogen gas was blown for 1 hour to drive off dissolved oxygen. Further, 0.1 mol% of sodium persulfate and 0.05 mol% of L-ascorbic acid were added to the water-soluble unsaturated monomer as a polymerization initiator, and the resulting hydrogel was polymerized for 3 hours. Further, this gel polymer was dried with a double drum dryer having a surface temperature of 150 ° C. and then pulverized to obtain an acrylate-based water-absorbent resin (14) which passed through 20 mesh.
[0140]
(Experimental example 15)
To 1000 g of acrylate (XII) was added 3.4 g (0.1 mol%, based on monomer) of tetraelene glycol diacrylate as an internal cross-linking agent and dissolved, and a water-soluble non-soluble compound having a β-hydroxypropionic acid content of 70 ppm was used. A saturated monomer (XII) was prepared.
[0141]
After the adjustment of the water-soluble unsaturated monomer (XII) was completed, the temperature was maintained at 50 ° C, and nitrogen gas was blown in to expel dissolved oxygen. Then, the water-soluble unsaturated monomer was layered and flow-extended to a thickness of 5 mm in a nitrogen atmosphere, and then 0.2 mol% of 2,2′-azobis (2-amidinopropane) dihydrochloride was used as a polymerization initiator. Was added. In addition, it takes 2 hours from the preparation of the water-soluble unsaturated monomer to the start of polymerization. Polymerization was started immediately, and after 10 minutes, the gel polymer was taken out, fragmented, and dried at 150 ° C. for 60 minutes. Next, the dried product was pulverized and classified in the same manner as in Experimental Example 1 to obtain an acrylate-based water-absorbent resin (15).
[0142]
(Experimental Examples 16 and 17)
An acrylic acid-based compound was prepared in the same manner as in Experimental Example 1 except that polymerization was performed using the water-soluble unsaturated monomer (XII) stored at 30 ° C. for 12 hours and 24 hours after completion of the preparation. Water absorbing resins (16) and (17) were obtained.
[0143]
(Experimental example 18)
83.4 g of acrylate (XII) and 0.004 g of N, N'-methylenebisacrylamide as an internal crosslinking agent (0.0065 mol% based on monomer, 17.77 g of ion-exchanged water, 35% concentration, A water-soluble unsaturated monomer (XII ') having a content of β-hydroxypropionic acid of 70% and a content of 70 ppm was prepared, and nitrogen gas was added 4 hours after the preparation of the aqueous unsaturated monomer (XII) was completed. Was blown for one hour to drive off dissolved oxygen.
[0144]
Separately, 250 ml of cyclohexane was placed in a 500 ml four-neck separable flask equipped with a stirrer, a reflux condenser, a thermometer, a nitrogen gas inlet tube and a dropping funnel, and sorbitan monostearate (HLB 4.7) was used as a dispersant. 2.0 g was added for dissolution, and nitrogen gas was blown in to expel dissolved oxygen. After degassing for 1 hour, 0.06 mol% of potassium persulfate was added as a polymerization initiator to the water-soluble unsaturated monomer, and the bath temperature was raised to 60 ° C. to start the polymerization reaction. It takes 4 hours from the preparation of the water-soluble unsaturated monomer to the polymerization. After polymerization for 2 hours, azeotropic dehydration was performed to reduce the water content of the polymer to 10% or less, followed by filtration, and further drying in an oven at 130 ° C. to obtain an acrylate water-absorbent resin (18). .
[0145]
(Experimental example 19)
In Experimental Example 1, the acrylate used for the preparation of the water-soluble unsaturated monomer was changed to acrylate (XIII) instead of acrylate (I), so that β-hydroxypropionic acid was A water-soluble unsaturated monomer (XIII) having a content of 30 ppm, a neutralization ratio of 50% and a concentration of 45% was obtained.
[0146]
Then, about 4 hours after the preparation, the polymerization of the water-soluble unsaturated monomer (XIII) was started in the same manner as in Experimental Example 1, and the same operation as in Experimental Example 1 was repeated. Water absorbent resin (19) was obtained.
[0147]
(Experimental example 20)
In Experimental Example 1, the acrylate used for the preparation of the water-soluble unsaturated monomer was changed to acrylate (XIV) instead of acrylate (I), whereby β-hydroxypropionic acid was A water-soluble unsaturated monomer (XIV) having a content of 50 ppm, a neutralization ratio of 90% and a concentration of 34% was obtained.
[0148]
Then, about 24 hours after the preparation, polymerization of the water-soluble unsaturated monomer (XIV) was started in the same manner as in Experimental Example 1, and the same operation as in Experimental Example 1 was repeated. Water absorbent resin (20) was obtained.
[0149]
(Experimental example 21)
Acrylate (XV) 4000 g was used as it is as a water-soluble unsaturated monomer (XV) having a content of β-hydroxypropionic acid of 290 ppm, and the inner surface was made of SUS316 lined with tetrafluoroethylene resin, 300 mm × 300 mm It was placed in an openable and closable casting polymerization apparatus having an internal volume of × 50 mm, and was replaced with nitrogen.
[0150]
Twenty-four hours after the completion of preparation, polymerization was carried out by adding 0.05 mol% of ammonium persulfate and 0.02 mol% of sodium hydrogen sulfite, and 5 hours after the start of the polymerization, a hydrogel polymer was cast from a casting polymerization apparatus. Was taken out and formed into a string with a meat chopper, and then dried and pulverized in the same manner as in Experimental Example 1 to obtain an acrylate-based water-soluble resin (21).
[0151]
(Example 1)
After mixing 1 part of glycerin, 2 parts of water, and 2 parts of ethyl alcohol with 100 parts of the water-absorbent resin (1) obtained in Experimental Example 1, the resulting mixture is heated at 190 ° C. for 20 minutes. A water-absorbing resin (22) having a crosslinked surface was obtained.
[0152]
(Example 2)
After mixing 0.1 part of ethylene glycol diglycidyl ether, 5 parts of water and 1 part of isopropyl alcohol with 100 parts by weight of the water-absorbent resin (5), the resulting mixture is heated at 180 ° C. for 30 minutes to obtain a surface. A water-absorbing resin (23) having a crosslinked area was obtained.
[0153]
(Example 3)
After mixing 0.5 part of diethylene glycol, 2 parts of water, and 1 part of isopropyl alcohol with 100 parts of the water-absorbent resin (6), the resulting mixture is subjected to heat treatment at 150 ° C. for 3 hours, whereby the vicinity of the surface is crosslinked. Water absorbent resin (24) was obtained.
[0154]
(Example 4)
After 100 parts by weight of the water-absorbent resin (8) is dispersed in a mixed solvent consisting of 300 parts of methanol and 30 parts of water, and 0.1 part of ethylene glycol diglycidyl ether is further mixed, the resulting mixture is heated at 160 ° C. for 1 hour. By heat-treating and evaporating to dryness, a water-absorbing resin (25) having a cross-linked surface was obtained.
[0155]
(Example 5)
After adding 2.5 parts of ethylene carbonate, 2.5 parts of water, and 2.5 parts of acetone to 100 parts of the water-absorbent resin (9), heat treatment is performed at 180 ° C. for 1 hour, whereby the vicinity of the surface is crosslinked. Water absorbent resin (26) was obtained.
[0156]
(Example 6)
One part by weight of silicon inorganic fine powder (Aerosil) is added to 100 parts by weight of the water absorbent resin (11), and 0.1 part of ethylene glycol diglycidyl ether and 10 parts of water are further mixed. Heat treatment was performed for one hour to obtain a water-absorbent resin (27) in which the surface was crosslinked.
[0157]
(Example 7)
After mixing 1 part of aluminum sulfate, 1 part of glycerin and 8 parts of water with 100 parts by weight of the water-absorbent resin (12), the resulting mixture is heat-treated at 180 ° C. for 30 minutes to obtain a crosslinked water absorbing material in the vicinity of the surface. A resin (28) was obtained.
[0158]
(Example 8)
The water absorbing resin (18) was dispersed in 250 ml of cyclohexane. Separately, 0.5 g of sorbitan monolaurate (manufactured by Kao Corporation, Rheodol SP-10, HLB = 8.6) as a surfactant was dissolved in 50 g of cyclohexane in a flask, and 0.04 g of ethylene glycol diglycidyl ether was added thereto. An aqueous solution dissolved in 2 ml of water was added under vigorous stirring to obtain a dispersion of ethylene glycol diglycidyl ether (average droplet diameter: 3 μm). Next, this dispersion is mixed with a suspension of the water-absorbent resin (18) under stirring, the system is kept at 75 ° C. for 3 hours, and further filtered and dried under reduced pressure to crosslink the surface vicinity. Water-absorbent resin (29) was obtained.
[0159]
(Comparative Examples 1 to 3)
Experimental example 1 except that comparative acrylates (I) to (III) were used instead of acrylate (I) as the acrylate used in the preparation of the water-soluble unsaturated monomer in Experimental Example 1. In the same manner as in the above, comparative water-soluble unsaturated monomers (I) to (III) having β-hydroxypropionic acid contents of 2500 ppm, 4200 ppm and 8200 ppm, respectively, were obtained. Next, the comparative acrylate-based water-absorbent resins (1) to (3) are repeated for the comparative water-soluble unsaturated monomers (I) to (III) by repeating the same operation as in Experimental Example 1. Got.
[0160]
(Comparative Examples 4 to 6)
In Comparative Example 1, the same operation as in Comparative Example 1 was carried out except that the time from the completion of preparation of the comparative water-soluble unsaturated monomer (I) to the start of polymerization was extended to 12, 24 and 240 hours. By repeating this, comparative acrylate-based water-absorbing resins (4) to (6) were obtained.
[0161]
(Comparative Examples 7 to 9)
Experimental example 8 except that comparative acrylates (IV) to (VI) were used instead of acrylate (V) as the acrylate used in the preparation of the water-soluble unsaturated monomer in Experimental Example 8. In the same manner as in the above, comparative water-soluble unsaturated monomers (IV) to (VI) having β-hydroxypropionic acid contents of 1100 ppm, 1900 ppm and 1300 ppm, respectively, were obtained. Next, the comparative water-soluble unsaturated monomers (IV) to (VI) are subjected to the same operation as in Experimental Example 8 to repeat the comparative acrylate-based water-absorbing resins (7) to (9). ) Got.
[0162]
(Comparative Examples 10 to 12)
Experimental example 8 except that comparative acrylates (VII) to (IX) were used instead of acrylate (VIII) as the acrylate used in the preparation of the water-soluble unsaturated monomer in Experimental example 11 In the same manner as in the above, comparative water-soluble unsaturated monomers (VII) to (IX) having β-hydroxypropinic acid contents of 1300 ppm, 2100 ppm and 3300 ppm, respectively, were obtained. Subsequently, the comparative acrylate-based water-absorbent resins (10) to (12) are repeated for these comparative water-soluble unsaturated monomers (VII) to (IX) by repeating the same operation as in Experimental Example 11. ) Got.
[0163]
(Comparative Example 13)
In Experimental Example 14, in the same manner as in Experimental Example 14 except that a comparative acrylate (X) was used instead of the acrylate (XI) as the acrylate used for preparing the water-soluble unsaturated monomer. And a comparative water-soluble unsaturated monomer (X) having a β-hydroxypropionic acid content of 6700 ppm. Next, the same operation as in Experimental Example 14 was repeated for this comparative water-soluble unsaturated monomer (X) to obtain a comparative acrylic salt-based water-absorbent resin (13).
[0164]
(Comparative Example 14)
In Experimental Example 15, in the same manner as in Experimental Example 15 except that a comparative acrylate (XI) was used instead of the acrylate (XII) as the acrylate used for preparing the water-soluble unsaturated monomer. And a comparative water-soluble unsaturated monomer (XI) having a β-hydroxypropionic acid content of 2100 ppm. Next, the comparative acrylate-based water-absorbent resin (14) was obtained by repeating the same operation as in Experimental Example 15 for the comparative water-soluble unsaturated monomer (XI).
[0165]
(Comparative Examples 15, 16)
In Comparative Example 14, except that polymerization was performed using a water-soluble unsaturated monomer stored at 30 ° C for 24 hours and 120 hours after completion of preparation, the acrylate-based water-absorbing resin was obtained in the same manner as in Experimental Example 13. (15) and (16) were obtained.
[0166]
(Comparative Example 17)
In Experimental Example 19, the acrylate used for the preparation of the water-soluble unsaturated monomer was changed to the comparative acrylate (XII) instead of the acrylate (XIII), whereby β-hydroxypropionic acid was obtained. Of a water soluble unsaturated monomer (XII) having a content of 1800 ppm. Next, the same operation as in Experimental Example 19 was repeated for the water-soluble unsaturated monomer (XII) to obtain a comparative acrylate-based water-absorbent resin (17).
[0167]
(Comparative Example 18)
In Experimental Example 21, β-hydroxypropionic acid was obtained by replacing the acrylate (XV) with the comparative acrylate (XIII) as the acrylate used for preparing the water-soluble unsaturated monomer. Of a water soluble unsaturated monomer (XIII) having a content of 3200 ppm. Next, the same operation as in Experimental Example 21 was repeated for the water-soluble unsaturated monomer (XIII) to obtain a comparative acrylate-based water-soluble resin (18).
[0168]
(Comparative Example 19)
In Experimental Example 21, by adding 2590 ppm of β-hydroxypropionic acid separately to the acrylate (XV), the content of β-hydroxypropionic acid was 2800 ppm, and the comparative water-soluble water-soluble unsaturated monomer (XIV) was obtained. Was. Next, the same operation as in Experimental Example 21 was repeated for this comparative water-soluble unsaturated monomer (XIV) to obtain a comparative acrylate-based water-soluble resin (19).
[0169]
(Comparative Examples 20 to 22)
In Examples 1 to 3, the comparative water absorbent resins (1) to (3) are used instead of the water absorbent resins (1), (5), and (6) as the water absorbent resin whose surface vicinity is crosslinked. Was performed in exactly the same manner to obtain comparative water absorbent resins (20) to (22).
[0170]
(Comparative Examples 23 and 24)
In Examples 4 and 5, the water absorbent resin whose surfaces are crosslinked was replaced with comparative water absorbent resins (7) and (8) in place of the water absorbent resins (8) and (9). Then, comparative water-absorbent resins (23) and (24) having a crosslinked surface area were obtained.
[0171]
(Comparative Examples 25 and 26)
Examples 6 and 7 were carried out in exactly the same manner as in Examples 6 and 7, except that comparative water-absorbent resins (11) and (12) were used instead of the water-absorbent resins (11) and (12) as the water-absorbent resins whose surfaces were crosslinked. Thus, comparative water-absorbent resins (25) and (26) having a crosslinked surface area were obtained.
[0172]
Hereinafter, the analysis results of the acrylate polymers of Experimental Examples 1 to 21 and Comparative Examples 1 to 29 are shown in Tables 1 and 2, respectively. Table 3 shows the analysis results of the acrylate-based polymers in Examples 1 to 8 and Comparative Examples 20 to 26 in which the vicinity of the surface was crosslinked.
[0173]
Incidentally, the β-hydroxypropionic acid in the acrylate-based polymer obtained in the examples was all 500 ppm or less, particularly those obtained from a water-soluble unsaturated monomer having a β-hydroxypropionic acid content of 100 ppm or less. Was less than 100 ppm in each case, whereas in Comparative Examples, it exceeded 1000 ppm.
[0174]
[Table 1]

[0175]
[Table 2]

[0176]
[Table 3]

[0177]
【The invention's effect】
The production method of the present invention has excellent features such as the following (1) to (5). The excellent acrylate polymer can be widely used in fields such as sanitary materials, food, civil engineering, and agriculture.
[0178]
(1) Residual monomers which have conventionally been reduced at the expense of performance, productivity, cost, safety and the like of an acrylate polymer through complicated processes such as addition of additives can be reduced in an advanced and simple manner.
[0179]
(2) Even if a high temperature is used in the production process, the production can be performed with high productivity because the increase in the residual monomer during the production is small. In addition, since the reaction can be performed at a high temperature, various excellent physical properties can be improved.
[0180]
(3) Since the amount of residual monomer generated and increased during subsequent use at high temperature or for a long time is extremely small, any use conditions such as long-term use in agricultural and horticultural use or use under high temperature in hot water etc. High safety even underneath.
[0181]
(4) In addition to improving the polymerizability, the amount of residual monomers can be reduced even with a small amount of a polymerization initiator or mild polymerization conditions, and an acrylate polymer having excellent physical properties can be obtained.
[0182]
(5) Almost no increase in residual monomer during surface crosslinking is observed, and the surface treatment effect is also improved.

Claims (11)

An acrylate-based polymer having a residual monomer content of 100 ppm or less, wherein 50 to 100 mol% of acrylate and 0 to 50 mol% of other hydrophilic unsaturated monomers and / or hydrophobic unsaturated monomers in the polymer. An acrylic acid-based polymer having a repeating unit of a saturated monomer and having a neutralization ratio of acrylic acid of 30 to 100 mol%, wherein the polymer is a water-absorbing resin having a crosslinked structure, and the vicinity of the surface of the polymer is An acrylate polymer which is further crosslinked and has a total content of β-hydroxypropionic acid and a salt thereof of 1 to 1,000 ppm. An acrylate polymer having a residual monomer content of 100 ppm or less, wherein 90 to 100 mol % of acrylate and 0 to 10 mol % of other hydrophilic unsaturated monomer and / or hydrophobic unsaturated monomer in the polymer. An acrylic acid-based polymer having a repeating unit of a saturated monomer and having a neutralization ratio of acrylic acid of 30 to 100 mol%, wherein the polymer is a water-absorbing resin having a crosslinked structure, and the vicinity of the surface of the polymer is The acrylate polymer according to claim 1, further crosslinked, wherein the total content of β-hydroxypropionic acid and a salt thereof is 1 to 1,000 ppm . The polymer according to claim 1 or 2 , wherein the water-absorbent resin is a powder having an average particle diameter of 300 to 600 µm. After preparing a water-soluble unsaturated monomer containing 90 to 100 mol% of an acrylate having a neutralization ratio of 30 to 100 mol%, polymerization is carried out using water or an aqueous liquid as a polymerization solvent, and further, if necessary, drying. Acrylate-based polymer having an average particle diameter of 10 to 2000 μm, wherein the polymer is selected from a water-soluble resin and / or a water-absorbent resin, and the total of β-hydroxypropionic acid and its salt is 1 to 1,000 ppm ( Acrylate-based polymer, characterized in that a water-soluble unsaturated monomer contained therein is used for polymerization of the polymer. The acrylate polymer according to claim 4, wherein the polymer is a water absorbent resin having a crosslinked structure. After preparing a water-soluble unsaturated monomer containing 50 to 100 mol% of an acrylate having a neutralization ratio of 30 to 100 mol%, water or an aqueous liquid is polymerized as a polymerization solvent, and dried if necessary, and further essential. Acrylate-based polymer obtained by cross-linking the vicinity of the surface of the obtained acrylate-based polymer to form a crosslinked surface, wherein the polymer is a water-absorbing resin having a cross-linked structure, β-hydroxypropionic acid and A water-soluble unsaturated monomer containing 1 to 1,000 ppm (based on monomer) of the total of the salts is used for the polymerization of the polymer. Acrylate-based polymers. Crosslinking near the surface can be achieved by polyhydric alcohols, polyhydric epoxy compounds, polyhydric amine compounds, polyhydric aziridine compounds, alkylene carbonate compounds, polyhydric aldehydes, polyhydric isocyanate compounds, polyhydric oxazoline compounds, haloepoxy compounds, polyvalent metals The acrylate-based polymer according to any one of claims 4 to 6, wherein at least one surface cross-linking agent selected from the group consisting of salts is used. After preparing a water-soluble unsaturated monomer containing 50 to 100 mol% of an acrylate having a neutralization ratio of 30 to 100 mol%, water or an aqueous liquid is polymerized as a polymerization solvent, and dried if necessary, and further essential. Is a acrylate polymer having an average particle diameter of 100 to 1000 μm in which the vicinity of the surface is crosslinked by crosslinking the vicinity of the surface of the obtained acrylate polymer, wherein the obtained polymer has a crosslinked structure A water-soluble unsaturated monomer which is a resin and has a total content of β-hydroxypropionic acid and a salt thereof of 1 to 1000 ppm (based on monomer) is used for the polymerization of the polymer, and dried after polymerization to obtain an average particle size. A water-absorbent resin powder having a cross-linked structure having a diameter of 100 to 1000 μm is obtained, and a polyhydric alcohol as a surface cross-linking agent is added to the vicinity of the surface of the obtained water-absorbent resin powder to obtain In the heat treatment, characterized in, salt polymers of acrylic acid. The acrylate-based polymer according to any one of claims 4 to 8, wherein the water-soluble unsaturated monomer is polymerized at a concentration of 25 to 50% by weight. The polymerization is carried out by polymerization using a radical polymerization initiator, and the radical polymerization is started. The acrylate polymer according to any one of claims 4 to 9, wherein an initiator is added in an amount of 0.001 to 2 mol% based on the water-soluble unsaturated monomer. A sanitary material using the acrylate polymer according to any one of claims 1 to 10 .