JP4498532B2 - Heterogeneous ion exchange membrane and method for producing the same - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、不均質イオン交換膜及びその製造方法に関する。
【０００２】
【従来の技術】
イオン交換膜は海水の濃縮、飲料水用の地下鹹水の脱塩や硝酸性窒素の除去、食品製造工程における塩分除去や医薬品の有効成分の濃縮など、現在、多種多様な用途に、イオンの分離膜として電気透析法、拡散透析法などで使用されている。これらに使用される有用なイオン交換膜は、主にスチレン−ジビニルベンゼン系の均質イオン交換膜であり、一価と二価のイオン選択、特定イオンの選択性アップ、低抵抗化など種々の技術が開発され、工業上有用な分離ができるまでに至っている。
【０００３】
しかしながら、この均質イオン交換膜は、イオンの輸率や膜抵抗、膜の強伸度など電気的・機械的特性は良好なものの、フィルム形成、交換基導入、補強材の導入など複雑でかつ煩雑な製造工程を必要とし、製造コストが比較的高くなる問題や後加工特性に劣る欠点があった。
一方、不均質イオン交換膜は、イオン交換膜の開発初期から研究されている。
例えば、米国特許２６８１３１９および２６８１３２０号公報には、イオン交換樹脂とバインダーポリマーからなる不均質なカチオン交換膜とアニオン交換膜が開示されている。特公昭４７−２４２６２号公報には、ポリプロピレンに低架橋イオン交換樹脂を混合分散し、酸・アルカリで処理したポリプロピレン樹脂複合体が、また、特公昭５１−１２３１３号公報、特公昭５２−３９１２号公報、特公昭５３−１８４７２号公報、米国特許４１６７５５１号公報には、ポリオレフィン樹脂に粉末状イオン交換性物質を混合して膜状成形した後に、熱水、アルカリ金属塩やアンモニウム塩の水溶液で熱水処理することで膜抵抗を低下したものが開示されているが、膜抵抗は十分低くない。
【０００４】
また、特開平９−０５９３９８号公報には、カチオン交換樹脂とバイン−ダポリマ−としてクロルスルホン化ポリエチレンを用いたもの、特開平１０−３６５３０号公報には、イオン交換樹脂の粒子とバインダー成分として低密度ポリエチレンとエチレン−プロピレンゴムからなるものが、特開平１０−３３０５１０号公報には、不均質イオン交換膜を複層化させたもの、特開平１１−１８８１１２０号公報には、不均質イオン交換膜の表面にイオン交換基を有するポリマー層を積層したもの、特開平１１−１８１１２１号公報には、不均質イオン交換膜の表面を機械的に粗面化したものが開示されているが、いずれも、膜抵抗は均質イオン交換膜の１０〜１００倍程度と高く問題がある。
【０００５】
さらには、中国特許１０４４４１１号公報には、バインダー成分として線状低密度ポリエチレンとエチレン酢酸ビニル共重合体やポリイソブチレンの混合物を使用したもの、特開平２−２６１８２８号公報には管状の不均質イオン交換体が、特表平８−５０４２２４号公報、米国特許５３４６９２４号公報には、バインダ成分として線状低密度ポリエチレンまたは高分子量高密度ポリエチレンを用いたものがあるがこれらは、比較的低い抵抗を示すものの、加工性に乏しい問題点があった。
【０００６】
線状低密度ポリエチレンまたは、高分子量高密度ポリエチレンのバインダーを用いた不均質イオン交換膜は、加熱成形工程において流動特性に劣り、加工上の問題があった。この問題を解決するために、エチレングリコールの如き第３成分を添加する試みもなされているが、加工性が改善されたものの、逆に膜抵抗が高くなるという性能上の悪化が見られ、好ましくない。
また、特開平１−２２９３２号公報、特開平６−３２９８１５号公報、特開平１１−３３５４７３号公報には、熱可塑性樹脂製多孔質膜の空隙部に、溶剤に溶解させたイオン交換樹脂を含浸させ溶剤を除去する方法や、単量体を含浸させた後に重合し交換基を導入してイオン交換樹脂を充填する方法などが開示されているが、熱可塑性樹脂製多孔質膜の内部にイオン交換樹脂を充填する際に、溶剤を除去したり、イオン交換樹脂を単量体から重合した後に反応させたりする必要があり、工程が複雑で高コストのイオン交換膜となってしまう欠点が有った。
【０００７】
【発明が解決しようとする課題】
本発明は、膜の電気抵抗を低く保ちながらも、イオン選択性に優れ、また、ファウリング耐性のある不均質イオン交換膜を提供することを目的とする。
【０００８】
【課題を解決するための手段】
本発明者は、鋭意努力の結果、前記課題を解決するため、イオン交換樹脂の微粒子およびバインダー樹脂からなる不均質イオン交換膜であって、該バインダー樹脂がエチレン−ビニルアルコール共重合体を含む不均質イオン交換膜が、良好なイオン交換膜特性を有しかつ製造が容易であることを見いだし、本発明をなすに至った。
【０００９】
すなわち、本発明は、下記の通りである。
（１）イオン交換樹脂の微粒子およびバインダー樹脂からなる不均質イオン交換膜であって、バインダー樹脂がエチレン−ビニルアルコール共重合体を含むことを特徴とする、不均質イオン交換膜。
（２）バインダー樹脂のメルトインデックスが１．０以下であることを特徴とする、（１）記載の不均質イオン交換膜。
（３）エチレン−ビニルアルコール共重合体を含むバインダー樹脂の粉末をバインダー樹脂成分の最も低い融点以下であらかじめ混合し、加熱混練成形すること特徴とする、不均質イオン交換膜の製造方法。
（４）加熱混練成形温度が、バインダー樹脂成分の最も高い融点より５〜４０℃高い温度であることを特徴とする、（３）記載の不均質イオン交換膜の製造方法。
【００１０】
公知方法のように、単にイオン交換樹脂の微粒子とバインダー成分の樹脂のペレットを混練り混合して、押し出し機などでフィルム状に成形するだけでは、極めて膜の電気抵抗の高い不均質イオン交換膜しか得られない。
これは、理由は明らかではないが、混練りし易いバインダー成分がイオン交換樹脂の周りを被覆し、フィルム中にイオン交換樹脂成分を含みながらも、イオンの移動するパスが少なくなり、絶縁性の高いものとなるからであると考えられる。
【００１１】
一方、流動性の低いバインダー成分を使用すると長い混練り時間を必要とし、良質な不均質イオン交換膜の押し出し成形が困難になる。
従って、公知の不均質イオン交換膜では、膜の抵抗を低く保ちながらフィルムの成形性を確保することは、達成できなかった。しかし、本発明者は、以下に述べるようなエチレン−ビニルアルコール共重合体を含むバインダー樹脂を用いることにより、目的の不均質イオン交換膜が得られるという知見を得た。この理由は、明らかではないが、親水性の素材であるエチレンービニルアルコール共重合体が、不均質イオン交換膜中のイオンの移動経路を確保したものではないかと考えられる。
【００１２】
エチレン−ビニルアルコール共重合体のバインダー樹脂に占める割合は、５％以上が好ましく、より好ましくは１０％以上である。エチレン−ビニルアルコール共重合体のバインダー樹脂に占める割合が５％に満たないと本発明の効果が小さくなり膜の抵抗が十分に小さくならない。
また、本発明のエチレン−ビニルアルコール共重合体を含むバインダー樹脂からなる不均質イオン交換膜は、膜表面でのファウリング耐性に優れ、有機イオン成分を含む水溶液の電気透析や拡散透析や電気式脱イオンの処理に適している。
【００１３】
本発明に用いるバインダー樹脂は、上記のエチレンービニルアルコール共重合体を含むものであり、バインダーの他の成分として、低密度ポリエチレン、線状低密度ポリエチレン、高密度ポリエチレン、超高分子量高密度ポリエチレン、ポリプロピレン、エチレン−酢酸ビニル共重合体などを含んでも良く、また、弾性体として各種ゴム成分であるエチレン−プロピレン系共重合体、生ゴム、スチレン−ブタジエン系ラテックス、クロロプレン、塩素化ポリエチレンなどを含んでも良い。エチレンービニルアルコール共重合体とこれらの混合物であるこのバインダー樹脂のメルトインデックス（試験方法：ＡＳＴＭ Ｄ１２３８）は、１．０ｇ／１０分以下であることが好ましく、特に好ましくは、０．６ｇ／１０分以下であると良い。バインダー樹脂の流動特性が低いと膜の抵抗が低くなる傾向があり好ましい。特にこのメルトインデックスの範囲が、低い電気抵抗の不均質イオン交換膜に適している。
【００１４】
逆にメルトインデックスが高いとバインダー樹脂の流動特性は良くなるものの膜の電気抵抗が高くなるので好ましくない。
また、このバインダー樹脂の形状は特に制約されないが、特に粉末状の樹脂が好んで用いられる。
バインダー樹脂の粉末の粒度は、１μｍ〜５００μｍが好ましく、特に好ましくは、１０〜２００μｍである。バインダー樹脂の粉末は、小さすぎるとハンドリング上難しくなる他に高コストになる。また、大きすぎると不均質イオン交換膜の膜抵抗が上昇するので好ましくない。本発明では、エチレンービニルアルコール共重合体の樹脂とその他のバインダー樹脂の成分などを上記の粒度範囲にして、これらの樹脂の融点以下の温度で事前に混合する事が特に好ましい。
【００１５】
本発明に用いるイオン交換樹脂の微粒子は、汎用のイオン交換樹脂を乾燥し、粉砕器で粉砕したものを用いても良いし、スチレン−ジビニルベンゼン系の懸濁重合物に交換基を導入したものを用いても良い。微粒子の大きさは、１００メッシュの篩をパスする１５０μｍ以下でかつ１μｍ以上の大きさのものが良い。イオン交換樹脂の微粒子の大きさが大きくなりすぎると膜の抵抗が高くなったり、強度が低くなるといった不具合を生じる。また、小さくなりすぎると微粒子が抜け出たりするので良くない。イオン交換容量は、乾燥樹脂１ｇあたり、１〜５ミリ当量のものが好ましく、ゲル型、ポーラス型であっても良い。イオン交換基の種類は、強酸型、弱酸型、強塩基型、弱塩基型、両性型、いずれのものでも良く、これらから２種以上任意に選択された混合物であっても良い。
【００１６】
本発明のイオン交換樹脂の微粒子の重量比率は乾燥不均質イオン交換膜重量あたり、４０〜８０％が良く、より好ましくは５０〜７０％である。イオン交換樹脂の微粒子の比率が小さすぎると膜抵抗が高くなり好ましくなく、高すぎると膜の機械的強度が弱くなったり、膜のイオン選択性が低下するので好ましくない。
本発明の不均質イオン交換膜は、上記のバインダー樹脂を用いて本発明の製造方法により、容易に製造できるものである。
【００１７】
本発明の製造方法を以下に説明する。まず、イオン交換樹脂の微粒子とエチレンービニルアルコール共重合体の微粒子と他のバインダー樹脂の微粒子を事前にバインダー樹脂成分の最も低い融点以下の温度で機械的に十分混合する。その後に混練りを所定の温度範囲内で行いシート状もしくは管状などに加工することが好ましい。ここでいう所定の温度範囲とは、バインダー樹脂成分の最も高い融点より５〜４０℃高い温度である。本発明の混練り温度は、バインダー樹脂成分の最も高い融点の５〜４０℃高い温度が好ましく、さらに好ましくは１０〜３０℃高い温度である。混練り温度が低すぎると十分に混練りできずに、イオン交換膜が脆くなる。また、バインダー樹脂の流動性を上げて加工性を良くするために、加工温度をバインダー樹脂の融点から４０℃以上高くすると膜の抵抗が高くなる。これは、バインダー樹脂粉末がイオン交換樹脂の微粒子の表面を覆ってしまったか、もしくは高温で加工した為にイオン交換樹脂の交換基が減少したり、エチレンービニルアルコール共重合体の親水性基が減少した為と考えられる。
【００１８】
このようにして得られた混練り物の流動性が低い場合、押し出し機でシート状のものを得ても良いが、混練りした混合樹脂をミキサーや凍結粉砕器で３００μ以下に粉砕した後、所定の厚みのシート状に厚み設定してプレス装置などでシート状や管状に成形しても良い。この時のプレス温度は、本発明の混練り温度と同じく、バインダー樹脂成分の最も高い融点の５〜４０℃高い温度が好ましく、さらに好ましくは１０〜３０℃高い温度である。プレス圧力は、０．２〜０．５ＭＰａ程度の圧力で良く、この圧力より高圧になっても良い。
【００１９】
本発明の不均質イオン交換膜の膜厚みは、１０μｍ〜３０００μｍの厚みが好ましく、特に好ましくは、１００〜１５００μｍである。
また、本発明の不均質イオン交換膜には、機械的強度保持の為に、不織布、織布などを埋め込んでもかまわない。
本発明の製造方法によって得られた不均質イオン交換膜は、使用にあたって水に平衡する。この時、６０℃〜８０℃の熱水中で１時間程度処理すると膜の電気抵抗をさらに下げることができる。
【００２０】
このようにして得られた不均質イオン交換膜は、通常の電気透析用の分離膜として使用できるし、電気式脱イオン装置のイオン交換膜としても使用できる。均質イオン交換膜に比較して特に加工性に富む為、任意の形状の電気透析スタックの製造にも有利である。
【００２１】
【発明の実施の形態】
本発明について、以下具体的に説明する。
膜の性能を以下のように評価した。即ち、熱プレスで得られた不均質イオン交換膜を４０℃の温水中に３時間平衡する。次いで室温の純水に戻す。膜抵抗は、２５℃の０．５Ｎの塩化ナトリウム水溶液中で周波数１０００Ｈｚの交流抵抗より求めた。また静的輸率は、２５℃の０．５Ｎの塩化ナトリウム水溶液と０．００５Ｎの塩化ナトリウム水溶液を用いてイオン交換膜で分離し、電位を銀−塩化銀電極で塩橋を用いて測定した。
また、本発明の実施例に用いるバインダー樹脂のメルトインデックス（ｇ／１０分）は、ＡＳＴＭ Ｄ１２３８の試験方法で求めた。
本発明を実施例に基づいて説明する。
【００２２】
【実施例１】
イオン交換容量が乾燥樹脂１ｇあたり４ミリ当量の市販の陽イオン交換樹脂を熱風乾燥機で６０℃で２４時間乾燥後、真空乾燥機で４０℃で１０時間乾燥した。この乾燥したイオン交換樹脂をサンプルミルで粉砕し、１００メッシュの篩をパスした微粒子を得た。このイオン交換樹脂の微粒子６ｇとエチレンービニルアルコール共重合体ポリマー（エチレン共重合比率３２％、メルトインデックス＝０．６、融点１８３℃：クラレ社製を粉砕し、平均粒子経１００μｍの粉末）４ｇを室温で１分間、手で混合した。さらに、この混合粉体をラボプラストミルで、１９５℃で１０分間混練りした。この混練りした樹脂を厚さ５００μｍのアルミの型枠内に入れ、１９５℃、プレス面圧０．５ＭＰａで１分プレス成形してシート状の不均質イオン交換膜を得た。この膜を室温で純水中に戻し、膜の抵抗を測定すると、１０Ω・ｃｍ2であった。静的輸率は９７％であった。
【００２３】
【実施例２】
実施例１と同じイオン交換容量が乾燥樹脂１ｇあたり４ミリ当量の市販の陽イオン交換樹脂を熱風乾燥機で６０℃で２４時間乾燥後、真空乾燥機で４０℃で１０時間乾燥した。この乾燥したイオン交換樹脂をサンプルミルで粉砕し、１００メッシュの篩をパスした微粒子を得た。このイオン交換樹脂の微粒子６ｇに、エチレンービニルアルコール共重合体ポリマー（エチレン共重合比率３２％、メルトインデックス＝０．６、融点１８３℃：クラレ社製を粉砕し、平均粒子経１００μｍの粉末）０．８ｇと高密度ポリエチレン粉末（ＳＨ８００：旭化成社製、平均分子量２５万、メルトインデックス＝０．０８、融点１３５℃、平均粒子経１００μｍ）３．２ｇを室温で１分間、手で混合した。さらに、この混合粉体をラボプラストミルで、１９５℃で１０分間混練りした。この混練りした樹脂を厚さ５００μｍのアルミの型枠内に入れ、１９５℃、プレス面圧０．５ＭＰａで１分プレス成形してシート状の不均質イオン交換膜を得た。この膜を室温で純水中に戻し、膜の抵抗を測定すると、２５Ω・ｃｍ2であった。静的輸率は９７％であった。
【００２４】
【比較例１】
実施例１と同じイオン交換容量が乾燥樹脂１ｇあたり４ミリ当量の市販の陽イオン交換樹脂を熱風乾燥機で６０℃で２４時間乾燥後、真空乾燥機で４０℃で１０時間乾燥した。この乾燥したイオン交換樹脂をサンプルミルで粉砕し、１００メッシュの篩をパスした微粒子を得た。このイオン交換樹脂の微粒子６ｇとエチレンービニルアルコール共重合体ポリマー（エチレン共重合比率３２％、メルトインデックス＝４．４、融点１８３℃：クラレ社製を粉砕し、平均粒子経１００μｍの粉末）４ｇを室温で１分間、手で混合した。さらに、この混合粉体をラボプラストミルで、１９５℃で１０分間混練りした。この混練りした樹脂を厚さ５００μｍのアルミの型枠内に入れ、１９５℃、プレス面圧０．５ＭＰａで１分プレス成形してシート状の不均質イオン交換膜を得た。この膜を室温で純水中に戻し、膜の抵抗を測定すると、８０Ω・ｃｍ2であった。静的輸率は９７％であった。
【００２５】
【比較例２】
実施例１と同じイオン交換容量が乾燥樹脂１ｇあたり４ミリ当量の市販の陽イオン交換樹脂を熱風乾燥機で６０℃で２４時間乾燥後、真空乾燥機で４０℃で１０時間乾燥した。この乾燥したイオン交換樹脂をサンプルミルで粉砕し、１００メッシュの篩をパスした微粒子を得た。このイオン交換樹脂の微粒子６ｇに、エチレンービニルアルコール共重合体ポリマー（エチレン共重合比率３２％、メルトインデックス＝０．６、融点１８３℃：クラレ社製を粉砕し、平均粒子経１００μｍの粉末）４ｇの混合粉体をラボプラストミルで、２３０℃で１０分間混練りした。この混練り樹脂を厚さ５００μｍのアルミの型枠内に入れ、２３５℃、プレス面圧０．５ＭＰａで１分プレス成形してシート状の不均質イオン交換膜を得た。この膜を室温で純水中に戻し、膜の抵抗を測定すると、９０Ω・ｃｍ2であった。静的輸率は９７％であった。
【００２６】
【発明の効果】
本発明の不均質イオン交換膜は、膜の電気抵抗を低く保ちながらも、イオン選択性に優れ、また、ファウリング耐性があり、かつ加工性の良好なものとなる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a heterogeneous ion exchange membrane and a method for producing the same.
[0002]
[Prior art]
Ion exchange membranes are currently used for a wide variety of applications such as seawater concentration, desalination of groundwater for drinking water and removal of nitrate nitrogen, salt removal in food manufacturing processes, and concentration of active pharmaceutical ingredients. It is used as a membrane in electrodialysis and diffusion dialysis. The useful ion exchange membranes used in these are mainly styrene-divinylbenzene-based homogeneous ion exchange membranes. Various technologies such as monovalent and divalent ion selection, increased specific ion selectivity, and low resistance. Has been developed to achieve industrially useful separation.
[0003]
However, although this homogeneous ion exchange membrane has good electrical and mechanical properties such as ion transport number, membrane resistance, and strong elongation of the membrane, it is complicated and cumbersome such as film formation, exchange group introduction, and reinforcement material introduction. A manufacturing process is required, and the manufacturing cost is relatively high and the post-processing characteristics are inferior.
On the other hand, heterogeneous ion exchange membranes have been studied from the early stages of ion exchange membrane development.
For example, U.S. Pat. Nos. 2,681,319 and 2,683,320 disclose heterogeneous cation exchange membranes and anion exchange membranes comprising an ion exchange resin and a binder polymer. Japanese Patent Publication No. 47-24262 discloses a polypropylene resin composite prepared by mixing and dispersing a low-crosslinking ion exchange resin in polypropylene and treating with an acid / alkali, and Japanese Patent Publication No. 51-12313 and Japanese Patent Publication No. 52-3912. In Japanese Patent Publication No. 53-18472 and US Pat. No. 4,167,551, a powdered ion exchange material is mixed with a polyolefin resin to form a film, and then heated with hot water, an aqueous solution of an alkali metal salt or an ammonium salt. Although the film resistance is reduced by water treatment, the film resistance is not sufficiently low.
[0004]
Japanese Patent Application Laid-Open No. 9-059398 discloses a cation exchange resin and a chlorosulfonated polyethylene as a binder polymer, and Japanese Patent Application Laid-Open No. 10-36530 discloses low ion exchange resin particles and a binder component. Japanese Patent Application Laid-Open No. 10-330510 discloses a multi-layered heterogeneous ion exchange membrane, and Japanese Patent Application Laid-Open No. 11-1881120 discloses a heterogeneous ion exchange membrane. No. 11-181121, in which a surface of a heterogeneous ion exchange membrane is mechanically roughened, is disclosed in which a polymer layer having ion exchange groups is laminated on the surface of The membrane resistance is as high as about 10 to 100 times that of a homogeneous ion exchange membrane.
[0005]
Further, Chinese Patent No. 1044411 uses a mixture of linear low density polyethylene and ethylene vinyl acetate copolymer or polyisobutylene as a binder component, and JP-A-2-261828 discloses a tubular heterogeneous ion. In Japanese Patent Publication No. 8-504224 and US Pat. No. 5,346,924, there are exchangers using linear low-density polyethylene or high-molecular-weight high-density polyethylene as a binder component, but these have a relatively low resistance. Although shown, there was a problem with poor workability.
[0006]
A heterogeneous ion exchange membrane using a linear low-density polyethylene or a high-molecular-weight high-density polyethylene binder is inferior in flow characteristics in the thermoforming process and has a problem in processing. In order to solve this problem, an attempt has been made to add a third component such as ethylene glycol. However, although the processability has been improved, on the contrary, there is a deterioration in performance that the film resistance is increased, which is preferable. Absent.
JP-A-1-22932, JP-A-6-329815, and JP-A-11-335473 are impregnated with an ion exchange resin dissolved in a solvent in a void portion of a porous film made of thermoplastic resin. A method of removing the solvent and impregnating the monomer and then polymerizing and introducing an exchange group to fill the ion exchange resin are disclosed. When filling the exchange resin, it is necessary to remove the solvent or to react after polymerizing the ion exchange resin from the monomer, which has the disadvantage that the process is complicated and the ion exchange membrane becomes expensive. It was.
[0007]
[Problems to be solved by the invention]
An object of the present invention is to provide a heterogeneous ion exchange membrane having excellent ion selectivity and having fouling resistance while keeping the electrical resistance of the membrane low.
[0008]
[Means for Solving the Problems]
As a result of diligent efforts, the present inventor has developed a heterogeneous ion exchange membrane comprising fine particles of an ion exchange resin and a binder resin, and the binder resin contains an ethylene-vinyl alcohol copolymer. It has been found that a homogeneous ion exchange membrane has good ion exchange membrane characteristics and is easy to produce, and has led to the present invention.
[0009]
That is, the present invention is as follows.
(1) A heterogeneous ion exchange membrane comprising ion exchange resin fine particles and a binder resin, wherein the binder resin contains an ethylene-vinyl alcohol copolymer.
(2) The heterogeneous ion exchange membrane according to (1), wherein the binder resin has a melt index of 1.0 or less.
(3) A method for producing a heterogeneous ion exchange membrane, characterized in that a binder resin powder containing an ethylene-vinyl alcohol copolymer is mixed in advance at a temperature not higher than the lowest melting point of the binder resin component, followed by heating and kneading.
(4) The method for producing a heterogeneous ion exchange membrane according to (3), wherein the temperature of the heat-kneading molding is 5 to 40 ° C. higher than the highest melting point of the binder resin component.
[0010]
Just by kneading and mixing the fine particles of the ion exchange resin and the resin pellet of the binder component as in a known method, and forming it into a film with an extruder or the like, a heterogeneous ion exchange membrane with extremely high electrical resistance of the membrane Can only be obtained.
The reason for this is not clear, but the binder component that is easy to knead coats around the ion exchange resin, and while the film contains the ion exchange resin component, there are fewer paths for ions to move, and the insulating property This is considered to be expensive.
[0011]
On the other hand, if a binder component having low fluidity is used, a long kneading time is required, and it becomes difficult to extrude a high-quality heterogeneous ion exchange membrane.
Therefore, with known heterogeneous ion exchange membranes, it has not been possible to ensure film formability while keeping the membrane resistance low. However, the present inventor has obtained the knowledge that a desired heterogeneous ion exchange membrane can be obtained by using a binder resin containing an ethylene-vinyl alcohol copolymer as described below. The reason for this is not clear, but it is considered that the ethylene-vinyl alcohol copolymer, which is a hydrophilic material, may ensure the ion migration path in the heterogeneous ion exchange membrane.
[0012]
The proportion of the ethylene-vinyl alcohol copolymer in the binder resin is preferably 5% or more, more preferably 10% or more. If the ratio of the ethylene-vinyl alcohol copolymer to the binder resin is less than 5%, the effect of the present invention is reduced and the resistance of the film is not sufficiently reduced.
In addition, the heterogeneous ion exchange membrane made of the binder resin containing the ethylene-vinyl alcohol copolymer of the present invention is excellent in fouling resistance on the membrane surface, and is electrodialysis, diffusion dialysis, or electric type of an aqueous solution containing an organic ion component. Suitable for deionization treatment.
[0013]
The binder resin used in the present invention contains the above-mentioned ethylene-vinyl alcohol copolymer, and as other components of the binder, low density polyethylene, linear low density polyethylene, high density polyethylene, ultrahigh molecular weight high density polyethylene. , Polypropylene, ethylene-vinyl acetate copolymer, etc., and as an elastic body, various rubber components such as ethylene-propylene copolymer, raw rubber, styrene-butadiene latex, chloroprene, chlorinated polyethylene, etc. But it ’s okay. The melt index (test method: ASTM D1238) of this binder resin which is an ethylene-vinyl alcohol copolymer and a mixture thereof is preferably 1.0 g / 10 min or less, particularly preferably 0.6 g / 10. It should be less than a minute. Low flow characteristics of the binder resin are preferred because the resistance of the film tends to be low. This melt index range is particularly suitable for heterogeneous ion exchange membranes with low electrical resistance.
[0014]
On the contrary, a high melt index is not preferable because the flow resistance of the binder resin is improved, but the electric resistance of the film is increased.
The shape of the binder resin is not particularly limited, but a powdery resin is particularly preferred.
The particle size of the binder resin powder is preferably 1 μm to 500 μm, particularly preferably 10 to 200 μm. If the binder resin powder is too small, it becomes difficult to handle and high cost. On the other hand, if it is too large, the membrane resistance of the heterogeneous ion exchange membrane increases, which is not preferable. In the present invention, it is particularly preferred that the ethylene-vinyl alcohol copolymer resin and the other binder resin components are in the above particle size range and mixed in advance at a temperature not higher than the melting point of these resins.
[0015]
The fine particles of the ion exchange resin used in the present invention may be those obtained by drying a general-purpose ion exchange resin and pulverizing with a pulverizer, or by introducing an exchange group into a styrene-divinylbenzene suspension polymer. May be used. The size of the fine particles is preferably 150 μm or less and 1 μm or more passing through a 100 mesh sieve. If the size of the ion exchange resin fine particles is too large, the membrane resistance increases and the strength decreases. On the other hand, if it becomes too small, the fine particles may come off, which is not good. The ion exchange capacity is preferably 1 to 5 milliequivalents per gram of dry resin, and may be a gel type or a porous type. The type of ion exchange group may be any of strong acid type, weak acid type, strong base type, weak base type, and amphoteric type, or a mixture arbitrarily selected from two or more of these.
[0016]
The weight ratio of the fine particles of the ion exchange resin of the present invention is preferably 40 to 80%, more preferably 50 to 70% per dry heterogeneous ion exchange membrane weight. When the ratio of the fine particles of the ion exchange resin is too small, the membrane resistance is undesirably high, and when it is too high, the mechanical strength of the membrane is weakened or the ion selectivity of the membrane is deteriorated.
The heterogeneous ion exchange membrane of the present invention can be easily produced by the production method of the present invention using the above binder resin.
[0017]
The production method of the present invention will be described below. First, ion exchange resin fine particles, ethylene-vinyl alcohol copolymer fine particles, and other binder resin fine particles are mechanically mixed in advance at a temperature below the lowest melting point of the binder resin component. Thereafter, kneading is preferably performed within a predetermined temperature range to form a sheet or a tube. The predetermined temperature range here is a temperature 5 to 40 ° C. higher than the highest melting point of the binder resin component. The kneading temperature of the present invention is preferably 5 to 40 ° C., more preferably 10 to 30 ° C. higher than the highest melting point of the binder resin component. If the kneading temperature is too low, the ion exchange membrane cannot be sufficiently kneaded and the ion exchange membrane becomes brittle. Further, in order to increase the fluidity of the binder resin and improve the processability, the film resistance increases when the processing temperature is increased by 40 ° C. or more from the melting point of the binder resin. This is because the binder resin powder has covered the surface of the fine particles of the ion exchange resin, or the exchange groups of the ion exchange resin are reduced due to processing at a high temperature, or the hydrophilic groups of the ethylene-vinyl alcohol copolymer are reduced. This is thought to be due to a decrease.
[0018]
When the kneaded material obtained in this way has low fluidity, it may be obtained in the form of a sheet with an extruder, but after kneading the mixed resin to 300 μm or less with a mixer or freeze pulverizer, The thickness may be set to a sheet shape with a thickness of 5 mm, and the sheet may be formed into a sheet shape or a tubular shape with a press device. The pressing temperature at this time is preferably 5 to 40 ° C., more preferably 10 to 30 ° C. higher than the highest melting point of the binder resin component, like the kneading temperature of the present invention. The press pressure may be a pressure of about 0.2 to 0.5 MPa, and may be higher than this pressure.
[0019]
The thickness of the heterogeneous ion exchange membrane of the present invention is preferably 10 μm to 3000 μm, and particularly preferably 100 to 1500 μm.
The heterogeneous ion exchange membrane of the present invention may be embedded with a nonwoven fabric, a woven fabric or the like in order to maintain mechanical strength.
The heterogeneous ion exchange membrane obtained by the production method of the present invention equilibrates with water in use. At this time, the electrical resistance of the film can be further reduced by treating in hot water at 60 ° C. to 80 ° C. for about 1 hour.
[0020]
The heterogeneous ion exchange membrane thus obtained can be used as a separation membrane for ordinary electrodialysis, and can also be used as an ion exchange membrane for an electric deionization apparatus. Compared to homogeneous ion exchange membranes, it is particularly easy to process, and is therefore advantageous for the production of electrodialysis stacks of any shape.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be specifically described below.
The performance of the membrane was evaluated as follows. That is, the heterogeneous ion exchange membrane obtained by hot pressing is equilibrated in warm water at 40 ° C. for 3 hours. Then return to room temperature pure water. Membrane resistance was determined from AC resistance at a frequency of 1000 Hz in a 0.5N sodium chloride aqueous solution at 25 ° C. In addition, the static transport number was separated by an ion exchange membrane using a 0.5N sodium chloride aqueous solution and a 0.005N sodium chloride aqueous solution at 25 ° C., and the potential was measured with a silver-silver chloride electrode using a salt bridge. .
The melt index (g / 10 min) of the binder resin used in the examples of the present invention was determined by the test method of ASTM D1238.
The present invention will be described based on examples.
[0022]
[Example 1]
A commercially available cation exchange resin having an ion exchange capacity of 4 milliequivalents per gram of the dried resin was dried at 60 ° C. for 24 hours with a hot air dryer and then dried at 40 ° C. for 10 hours with a vacuum dryer. The dried ion exchange resin was pulverized with a sample mill to obtain fine particles that passed through a 100-mesh sieve. 6 g of this ion exchange resin fine particle and 4 g of an ethylene-vinyl alcohol copolymer polymer (ethylene copolymer ratio 32%, melt index = 0.6, melting point 183 ° C., crushed by Kuraray Co., Ltd., powder having an average particle diameter of 100 μm) Were mixed by hand for 1 minute at room temperature. Furthermore, this mixed powder was kneaded at 195 ° C. for 10 minutes with a lab plast mill. This kneaded resin was placed in an aluminum mold having a thickness of 500 μm and press-molded at 195 ° C. and a press surface pressure of 0.5 MPa for 1 minute to obtain a sheet-like heterogeneous ion exchange membrane. The membrane was returned to pure water at room temperature, and the resistance of the membrane was measured to be 10 Ω · cm 2 . The static transport rate was 97%.
[0023]
[Example 2]
A commercially available cation exchange resin having the same ion exchange capacity as in Example 1 at 4 milliequivalents per gram of the dried resin was dried at 60 ° C. for 24 hours with a hot air dryer and then dried at 40 ° C. for 10 hours with a vacuum dryer. The dried ion exchange resin was pulverized with a sample mill to obtain fine particles that passed through a 100-mesh sieve. Ethylene-vinyl alcohol copolymer polymer (ethylene copolymer ratio: 32%, melt index = 0.6, melting point: 183 ° C .: powder made by Kuraray Co., Ltd., average particle diameter: 100 μm) 0.8 g and 3.2 g of high-density polyethylene powder (SH800: manufactured by Asahi Kasei Corporation, average molecular weight 250,000, melt index = 0.08, melting point 135 ° C., average particle size 100 μm) were mixed by hand at room temperature for 1 minute. Furthermore, this mixed powder was kneaded at 195 ° C. for 10 minutes with a lab plast mill. This kneaded resin was placed in an aluminum mold having a thickness of 500 μm and press-molded at 195 ° C. and a press surface pressure of 0.5 MPa for 1 minute to obtain a sheet-like heterogeneous ion exchange membrane. The membrane was returned to pure water at room temperature, and the resistance of the membrane was measured to be 25 Ω · cm 2 . The static transport rate was 97%.
[0024]
[Comparative Example 1]
A commercially available cation exchange resin having the same ion exchange capacity as in Example 1 at 4 milliequivalents per gram of the dried resin was dried at 60 ° C. for 24 hours with a hot air dryer and then dried at 40 ° C. for 10 hours with a vacuum dryer. The dried ion exchange resin was pulverized with a sample mill to obtain fine particles that passed through a 100-mesh sieve. 6 g of this ion exchange resin fine particle and 4 g of an ethylene-vinyl alcohol copolymer polymer (ethylene copolymer ratio 32%, melt index = 4.4, melting point 183 ° C., crushed by Kuraray Co., Ltd., powder having an average particle size of 100 μm) Were mixed by hand for 1 minute at room temperature. Furthermore, this mixed powder was kneaded at 195 ° C. for 10 minutes with a lab plast mill. This kneaded resin was placed in an aluminum mold having a thickness of 500 μm and press-molded at 195 ° C. and a press surface pressure of 0.5 MPa for 1 minute to obtain a sheet-like heterogeneous ion exchange membrane. The membrane was returned to pure water at room temperature, and the resistance of the membrane was measured to be 80 Ω · cm 2 . The static transport rate was 97%.
[0025]
[Comparative Example 2]
A commercially available cation exchange resin having the same ion exchange capacity as in Example 1 at 4 milliequivalents per gram of the dried resin was dried at 60 ° C. for 24 hours with a hot air dryer and then dried at 40 ° C. for 10 hours with a vacuum dryer. The dried ion exchange resin was pulverized with a sample mill to obtain fine particles that passed through a 100-mesh sieve. Ethylene-vinyl alcohol copolymer polymer (ethylene copolymer ratio: 32%, melt index = 0.6, melting point: 183 ° C .: powder made by Kuraray Co., Ltd., average particle diameter: 100 μm) 4 g of the mixed powder was kneaded at 230 ° C. for 10 minutes with a lab plast mill. This kneaded resin was placed in an aluminum mold having a thickness of 500 μm and press-molded at 235 ° C. and a press surface pressure of 0.5 MPa for 1 minute to obtain a sheet-like heterogeneous ion exchange membrane. When this membrane was returned to pure water at room temperature and the resistance of the membrane was measured, it was 90 Ω · cm 2 . The static transport rate was 97%.
[0026]
【The invention's effect】
The heterogeneous ion exchange membrane of the present invention has excellent ion selectivity, fouling resistance and good processability while keeping the electrical resistance of the membrane low.
Claims (4)
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Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS50131688A (en) * | 1974-04-06 | 1975-10-17 | ||
| JPS50145379A (en) * | 1974-05-15 | 1975-11-21 | ||
| US5868976A (en) * | 1997-03-14 | 1999-02-09 | Koch Membrane Systems, Inc. | Process of making a dialysis membrane |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS50131688A (en) * | 1974-04-06 | 1975-10-17 | ||
| JPS50145379A (en) * | 1974-05-15 | 1975-11-21 | ||
| US5868976A (en) * | 1997-03-14 | 1999-02-09 | Koch Membrane Systems, Inc. | Process of making a dialysis membrane |
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