JP2004067880A - Fluorocarbon type ion exchange membrane - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fluorocarbon type ion exchange membrane, which is drawn and oriented and thereby gives high mechanical strengths, high conductivity, and low heat shrinkage. <P>SOLUTION: The fluorocarbon type ion exchange membrane is a copolymer substantially constituted by (A) and (B) of the recurring units, with 1-500 μm in film thickness and 0.0005 or more in degree of orientation (ΔP) of its film face. In the (A) and (B) of the recurring units, Z is H, Cl, F, or a 1-3C perfluoroalkyl group; m is an integer of 1-3; and W=CO<SB>2</SB>R<SP>1</SP>, COR<SP>2</SP>, or SO<SB>2</SB>R<SP>2</SP>(wherein R<SP>1</SP>is a 1-3C perfluoroalkyl group; R<SP>2</SP>is a halogen or OM; and M is H, an alkali metal atom, an alkaline earth metal atom, or an ammonium ion). <P>COPYRIGHT: (C)2004,JPO

Description

（ＺはＨ、Ｃｌ、Ｆまたは炭素数１〜３のパーフルオロアルキル基、ｍは１〜３の整数、Ｗ＝ＣＯ_２Ｒ^１、ＣＯＲ^２、ＳＯ_２Ｒ^２、Ｒ^１は炭素数１〜３のパーフルオロアルキル基、Ｒ^２はハロゲンまたはＯＭ、ＭはＨ、アルカリ金属原子、アルカリ土類金属原子またはアンモニウムイオン）
共重合体を用いて面配向度（ΔＰ）０．０００５以上に延伸配向を行うことで、機械強度、イオン伝導性、かつ耐熱寸法安定性に優れることを見出し本発明のフッ素系イオン交換膜を完成させるに至った。
【００１０】
すなわち本発明は、
（１）下記の繰り返し単位（Ａ）及び（Ｂ）により実質的に構成される
【化３】

（ＺはＨ、Ｃｌ、Ｆまたは炭素数１〜３のパーフルオロアルキル基、ｍは１〜３の整数、Ｗ＝ＣＯ_２Ｒ^１、ＣＯＲ^２、ＳＯ_２Ｒ^２、Ｒ^１は炭素数１〜３のパーフルオロアルキル基、Ｒ^２はハロゲンまたはＯＭ、ＭはＨ、アルカリ金属原子、アルカリ土類金属原子またはアンモニウムイオン）
共重合体であり、かつ膜厚１〜５００μｍ、フィルム面配向度（ΔＰ）が０．０００５以上であることを特徴とするフッ素系イオン交換膜、
（２）１６０℃における熱収縮率５０％以下であることを特徴とする（１）に記載のフッ素系イオン交換膜、
（３）（１）又は（２）に記載のフッ素系イオン交換膜を備えることを特徴とする膜電極接合体、
４．（１）又は（２）に記載のフッ素系イオン交換膜を備えることを特徴とする固体高分子電解質型燃料電池に関する。
【００１１】
以下に本発明を詳細に説明する。
まず、本発明のフッ素系イオン交換膜について説明する。
延伸配向したフィルムは高い機械強度を発現するが、多くの場合、熱収縮が大きいために高温加工を伴うような用途、特に燃料電池用途への適用に対して制限があった。これに対して本発明のフッ素系イオン交換膜は、通常のフッ素系イオン交換膜の優れた特性を失うことなく、高い機械強度、イオン伝導性および良好な耐熱寸法安定性を有することから、例えば燃料電池用イオン交換膜として特に好適に使用することが可能である。
【００１２】
（膜厚）
本発明のフッ素系イオン交換膜の膜厚は、１μｍ以上が好ましく、より好ましくは５μｍ以上、さらに好ましくは１０μｍ以上である。膜厚が１μｍより小さい場合は水素や酸素の拡散により前記したような直接反応の不都合が発生しやすいとともに、燃料電池製造時の取り扱いや燃料電池運転中の差圧・歪み等によって膜の損傷等の不都合が発生しやすい。また、５００μｍより大きい膜厚を有する膜は一般にイオン透過性が低いため、イオン交換膜として十分な性能を持たない可能性がある。膜厚の好ましい上限は５００μｍ以下が好ましく、より好ましくは１００μｍ以下、さらに好ましくは５０μｍ以下である。具体的に好ましい範囲としては、１〜５００μｍ、より好ましくは５〜１００μｍ、さらに好ましくは１０〜５０μｍである。
【００１３】
（面配向度）
本発明のフッ素系イオン交換膜の特徴として挙げられるのが面配向度であり、いくら配向を行ってもこの値が一定値以上を示さなければ、単なる薄膜化に過ぎず、機械強度、イオン伝導性の向上は望めない。一方で、一定値以上に延伸しても、面配向度は上昇せず、熱収縮率が上昇するのみで、実用性に欠ける。
また、イオン交換膜は燃料電池内で生成される水、あるいは供給される水による加湿の程度によって膨潤または収縮する。この膨潤収縮の寸法変化により機械的なダメージを受け強度が低下、その結果、燃料電池運転が不安定になる。面配向度０．０００５以上を示すイオン交換膜を用いた場合、寸法安定性に優れることから上記のような問題が解消され、良好な燃料電池運転が達成できるのではないかと考えられる。
【００１４】
フッ素系イオン交換膜の面配向度は機械強度および耐熱寸法安定性と密接に関係している。本発明のフッ素系イオン交換膜の面配向度は、０．０００５以上が好ましく、より好ましくは０．００１０以上、さらに好ましくは０．００１５以上、さらにより好ましくは０．００２０以上である。面配向度が０．０００５未満では機械強度が不十分であり、イオン交換膜として十分な性能を持たない可能性があるため好ましくない。本発明において面配向度の上限は特に設けないが、０．００５より大きい場合は、熱収縮が起こりやすくなり、例えばＭＥＡ作成時などに不都合が発生しやすい。具体的に面配向度の好ましい範囲は０．０００５〜０．００５、より好ましくは０．００１〜０．００５、よりさらに好ましくは０．００１５〜０．００５である。
【００１５】
（１６０℃における熱収縮率）
本発明のフッ素系イオン交換膜の１６０℃における熱収縮率は、５０％以下、好ましくは４５％以下、より好ましくは４０％以下である。熱収縮率が５０％より大きい場合、高温加工を伴うような用途において熱収縮が起こりやすく、例えばＭＥＡの製造時などに大きな支障をきたす場合がある。
（当量重量）
本発明のフッ素系イオン交換膜の当量重量（ＥＷ）は特に限定されないが、当量重量が低すぎると強度の低下が起きるため好ましくない。４００以上が好ましく、より好ましくは６００以上、さらに好ましくは７００以上である。一方、当量重量が大きくなると未配向膜でも機械強度が向上するが、同時にイオン交換基の密度が低くなるためにイオン伝導性が低下する。ＥＷの好ましい上限は、１４００以下であり、より好ましくは１２００以下、さらに好ましくは１０００以下である。具体的にＥＷの好ましい範囲は、４００〜１４００が好ましく、より好ましくは６００〜１２００であり、更に好ましくは７００〜１０００である。
【００１６】
（換算突刺強度）
本発明のフッ素系イオン交換膜の換算突刺強度（乾燥状態での突刺強度を２５μｍあたりに換算）は特に限定されないが、３００ｇ以上が好ましく、より好ましくは３５０ｇ以上、さらに好ましくは４００ｇ以上である。換算突刺強度が３００ｇより小さい場合は薄膜化のために必要な機械強度が不十分であり、膜を厚くする必要があるため好ましくない。本発明においては換算突刺強度の上限は特に設けないが、３０００ｇ以上の強度を有する膜は一般的に含水率が低いことが予想されるため、イオン交換膜として十分な性能を持たない可能性がある。
【００１７】
（２５℃水中における水平イオン伝導度）
本発明のフッ素系イオン交換膜の２５℃水中における水平イオン伝導度は、０．０５Ｓ／ｃｍ以上が好ましく、０．０７Ｓ／ｃｍ以上がより好ましく、０．１０Ｓ／ｃｍ以上がさらに好ましい。水平イオン伝導度が０．０５Ｓ／ｃｍより小さい場合は、燃料電池用イオン交換膜として使用する場合に内部抵抗が上昇するため好ましくない。
【００１８】
次に、本発明のフッ素系イオン交換膜の製造方法について説明する。
イオン交換膜は、一般的にイオン交換樹脂前駆体を膜状に成形した後、高温で加水分解を行うことによって作成される。従って、延伸を行う対象としては加水分解前のフッ素系イオン交換樹脂前駆体と加水分解後のフッ素系イオン交換樹脂とに大別できるが、本発明においては目的に応じていずれの膜に対しても延伸を行うことができる。両者は次のように選択することができる。
【００１９】
（フッ素系イオン交換樹脂前駆体の延伸）
本発明における好ましい延伸の形態の第一は、フッ素系イオン交換樹脂前駆体に対して為されるものである。フッ素系イオン交換樹脂前駆体の延伸において特に重視されるべき点は、延伸終了に伴なう配向緩和の防止である。これは次のような理由による。一般的に、フィルムの延伸温度は粘弾性測定におけるα分散温度を参考にして設定されることが多い。ここでいうα分散温度とはポリマー主鎖が熱運動を開始すると考えられる温度であり、延伸のようにポリマーに対して大きな歪みを与えながら加工する際の指標として、広く用いられている。例えば、ポリエステルやナイロンに代表されるようなポリマーのα分散温度は一般的に室温よりもはるかに高いため、延伸終了後にα分散温度以下に冷却することによって主鎖の熱運動を大きく減少させることが可能であり、これによって延伸配向を効果的に安定化することができる。
【００２０】
これに対して、フッ素系イオン交換樹脂前駆体のα分散温度は室温近辺に存在するため、こうした「延伸固定」が困難であり、延伸状態から拘束を外すと急速に収縮して延伸配向を失うことが多かった。本発明者らは、フッ素系イオン交換樹脂前駆体の配向緩和に関して鋭意検討を重ねた結果、当該前駆体に特有な製造工程である加水分解に着目することによって、α分散温度に依らない新規な延伸固定方法を見出した。すなわち、本発明においてはその好ましい延伸の形態の第一として、フッ素系イオン交換樹脂前駆体を延伸した後延伸配向を拘束した状態で加水分解することを特徴とする。
このような方法によって延伸固定が達成できる理由は明らかではないが、加水分解によって生成するフッ素系イオン交換樹脂のα分散温度は当該前駆体よりもはるかに高く、１５０℃近辺に存在すると考えられているので、延伸配向を維持しながら加水分解を行うことによってその進行と共に配向膜のα分散温度が上昇する過程で主鎖の熱運動が減少し、延伸固定を達成できたのではないかと考えられる。こうした延伸固定の方法を本発明においては「ケン化固定」と呼称する。
【００２１】
ケン化固定が達成できる理由としては、さらに次のように考えることもできる。フッ素系イオン交換樹脂前駆体を加水分解すると多量の水を吸水するようになるが、こうした水は樹脂内部に均一に存在するのではなく、微視的な水滴を形成しつつ局所的に存在すると考えられている。このような水滴はクラスターと呼ばれ、小角Ｘ線回折や透過型電子顕微鏡によって具体的に観察することができる。
１つのクラスターには複数の側鎖末端が含まれると予想されるが、フッ素系イオン交換樹脂前駆体を延伸した後、拘束を維持した状態でクラスターを形成させると、これらの側鎖末端同士が互いに水を介して結びつく一種の架橋点として機能することが期待できる。すなわち、α分散温度の上昇に加えて、延伸配向後に形成されるクラスターが疑似架橋点として機能することにより、ケン化固定がより良好に機能するものと考えられる。
【００２２】
（フッ素系イオン交換樹脂の延伸）
本発明における好ましい延伸の形態の第二は、フッ素系イオン交換樹脂に対して為されるものである。前述したように、フッ素系イオン交換樹脂のα分散温度は１５０℃近辺に存在すると考えられるために、冷却による延伸固定が容易であり、拘束を解いたあとも高い機械強度を維持することができる。特に、このような配向膜はケン化固定のような特殊な処理を必要としないために一般的な延伸技術を適用することができ、燃料電池用イオン交換膜の生産性向上の観点から好ましい。すなわち、本発明においてはその好ましい延伸の形態の第二として、フッ素系イオン交換樹脂前駆体を加水分解した後に延伸することを特徴とする。
【００２３】
（原料ポリマー）
本発明で使用されるフッ素系イオン交換樹脂前駆体は、一般式ＣＦ_２＝ＣＦ−Ｏ−（ＣＦ_２）_ｍ−Ｗで表されるフッ化ビニル化合物と、一般式ＣＦ_２＝ＣＦＺで表されるフッ化オレフィンとの、少なくとも二元共重合体からなる。ここでｍは１〜３の整数、ＺはＨ、Ｃｌ、Ｆ又は炭素数１〜３のパーフルオロアルキル基である。また、Ｗは加水分解によりＣＯ_２Ｈ又はＳＯ_３Ｈに転換し得る官能基であり、このような官能基としてはＳＯ_２Ｆ、ＳＯ_２Ｃｌ、ＳＯ_２Ｂｒ、ＣＯＦ、ＣＯＣｌ、ＣＯＢｒ、ＣＯ_２ＣＨ_３、ＣＯ_２Ｃ_２Ｈ_５が通常好ましく使用される。このようなフッ素系イオン交換樹脂前駆体は従来公知の手段により合成可能なものである。
【００２４】
上記フッ素系イオン交換膜樹脂前駆体は、従来から用いられているフッ素系イオン交換膜樹脂前駆体一般式ＣＦ_２＝ＣＦ−Ｏ（ＣＦ_２ＣＦＸＯ）_ｎ−（ＣＦ_２）_ｍ−Ｗで表されるフッ化ビニル化合物と、一般式ＣＦ_２＝ＣＦＺで表されるフッ化オレフィンとの、少なくとも二元共重合体からなるもの（ここでＸはＦ原子又は炭素数１〜３のパーフルオロアルキル基、ｎは０〜３の整数、ｍは１〜３の整数、ＺはＨ、Ｃｌ、Ｆ又は炭素数１〜３のパーフルオロアルキル基である。また、Ｗは加水分解によりＣＯ_２Ｈ又はＳＯ_３Ｈに転換し得る官能基である。）に比し、エーテル結合で繋がっている側鎖の長さが短く、これにより結晶性が増し、耐熱性も上昇している。この特性を利用することで、本発明では延伸配向を効果的に安定化し、高い機械強度と耐熱性に優れたフッ素系イオン交換膜を可能にした。
【００２５】
（成膜工程）
フッ素系イオン交換樹脂前駆体を膜状に成形する方法としては、溶融成形法（Ｔダイ法、インフレーション法、カレンダー法など）やキャスト法など、成形法として一般的に知られている方法であればいずれも好適に用いることができる。
キャスト法としては、フッ素系イオン交換樹脂を適当な溶媒に分散させたもの、又は重合反応液そのものをシート状に成形した後、分散媒を除去する方法を挙げることができる。Ｔダイ法による溶融成形を行う際の樹脂温度の下限は、１００℃以上が好ましく、さらに好ましくは２００℃以上である。上限温度については、樹脂の耐熱性を考慮して、３００℃以下が好ましく、さらに好ましくは２８０℃以下である。具体的には、１００〜３００℃が好ましく、さらに好ましくは２００〜２８０℃である。その下限は、１００℃以上が好ましく、さらに好ましくは１６０℃以上である。上限温度については、樹脂の耐熱性を考慮して、３００℃以下が好ましく、さらに好ましくは２４０℃以下である。具体的には、１００〜３００℃が好ましく、さらに好ましくは１６０〜２４０℃である。これらの方法で溶融成形されたシートは、冷却ロール等を用いることによって溶融温度以下の温度にまで冷却される。前駆体膜の膜厚は、配向工程における膜厚減少を見越した上で最適の膜厚に調整することが好ましい。たとえば配向工程で４×４倍延伸を行うとき、配向膜の膜厚を２５μｍとするためには前駆体膜の膜厚を４００μｍ付近で調整する必要がある。
【００２６】
（加水分解工程）
加水分解の方法としては、例えば日本特許第２７５３７３１号公報に記載のように水酸化アルカリ溶液を用いて配向膜のイオン交換基前駆体を金属塩型のイオン交換基に変換し、次にスルホン酸又は塩酸のような酸を用いて酸型（ＳＯ_３Ｈ又はＣＯＯＨ）のイオン交換基に変換する従来公知の方法を使用することができる。このような変換は当業者には周知であり、本発明の実施例に記載している。
加水分解工程の前に配向工程を実施する場合は加水分解工程を通してフッ素系イオン交換樹脂前駆体を拘束する必要がある。本発明における拘束とは、膜の熱収縮等による延伸配向の自発的な緩和を防ぐための拘束を意味しており、一定寸法での拘束だけではなく延伸を伴う拘束も含むと考えるべきである。加水分解工程の前に配向工程を実施しない場合は、加水分解に伴う吸水によって膜が膨張するため、特にロール、ベルト等を用いての連続処理を行う場合は皺の発生防止に努める必要がある。本発明においては、加水分解工程の途中に延伸や熱処理を実施してもかまわない。
【００２７】
（配向工程）
延伸の方法としては、フィルムの延伸方法として一般的に知られている方法であればいずれも好適に用いることができるが、このうちテンターによる横１軸延伸、テンター及び縦延伸ロールによる逐次２軸延伸、同時２軸テンターによる同時２軸延伸、インフレーション製膜装置によるブロー延伸がより好ましく、同時２軸延伸又はブロー延伸が更に好ましい。好適な延伸倍率は面積倍率で、下限については１．１倍以上、好ましくは２倍以上、さらに好ましくは４倍以上、上限については１００倍以下、好ましくは２０倍以下、さらに好ましくは１６倍以下である。このうち、横方向（機械方向に対して直角な方向）の延伸倍率が下限については１．１倍以上、好ましくは１．５倍以上、さらに好ましくは２倍以上、上限については１００倍以下、好ましくは１０倍以下、さらに好ましくは４倍以下である。具体的には１．１〜１００倍、好ましくは２〜２０倍、更に好ましくは４〜１６倍、横方向の延伸倍率については１．１〜１００倍、好ましくは１．５〜１０倍、更に好ましくは２〜４倍である。好適な延伸温度は前駆体膜の溶融温度以下であり、下限については（α分散温度−１００℃）以上、好ましくは（α分散温度−８０℃）以上、さらに好ましくは（α分散温度−５０℃）以上、上限については（α分散温度＋１００℃）以下、好ましくは（α分散温度＋８０℃）以下、さらに好ましくは（α分散温度＋５０℃）以下である。具体的には（α分散温度−１００℃）〜（α分散温度＋１００℃）、好ましくは（α分散温度−８０℃）〜（α分散温度＋８０℃）、さらに好ましくは（α分散温度−５０℃）〜（α分散温度＋５０℃）である。
なお、本発明における延伸とは延伸応力の発生を伴う伸長を意味しており、延伸応力の発生を伴わない伸長は拡幅と呼称される。たとえば加水分解工程の前に配向工程を実施しない場合は加水分解に伴う吸水によって膜が水平方向に大きく膨潤するが、この変化に追従して膜を伸長する場合は拡幅と考えることができる。
【００２８】
（熱処理工程）
より熱収縮を低減させたい場合は、必要に応じて熱処理を行うことによってフッ素系イオン交換膜の熱収縮を低減させることができる。熱処理の方法は、フィルムの熱処理方法として一般的に知られている方法であればいずれも好適に用いることができるが、フッ素系イオン交換膜を拘束した状態で熱処理することが好ましい。熱処理温度としてはα分散温度以上であることが好ましく、ＭＥＡ製造時のプレス温度のように高温加工を伴うような用途において暴露される最高温度が明確である場合は、それよりも高温であることがより好ましい。フッ素系イオン交換樹脂を３００℃以上に加熱すると変質することがあるため、熱処理温度としては３００℃以下にすることが好ましい。より具体的には、熱処理温度の上限は、プレス温度等の膜使用温度を基準として、その温度よりも５０℃高い温度以下の温度が好ましく、より好ましくは３０℃高い温度以下の温度、更に好ましくは２０℃高い温度以下の温度、更により好ましくは１０℃高い温度以下の温度である。また、熱処理温度の下限は、プレス温度等の膜使用温度を基準として、その温度よりも５０℃低い温度以上の温度が好ましく、より好ましくは３０℃低い温度以上の温度、更に好ましくは２０℃低い温度以上の温度、更により好ましくは１０℃低い温度以上の温度である。熱処理時間は熱処理温度に依存するが、概ね１秒〜１時間の範囲で好適に熱処理を実施することができる。熱処理時間が長く、熱処理温度が高いほど熱収縮率を低減させることが可能であるが、機械強度の低下やイオン伝導度の低下といった不都合が生じやすい。
【００２９】
（洗浄工程）
熱処理工程によってイオン伝導性が大きく低下する場合は、必要に応じてフッ素系イオン交換膜を洗浄することによってこれを回復させることができる。洗浄は、例えばフッ素系イオン交換膜を拘束下又は非拘束下で酸性水溶液に浸漬又は噴霧することによって行うことができる。使用する酸性水溶液の濃度はイオン伝導性の低下状況や洗浄温度、洗浄時間にも依存するが、例えば０．００１〜５規定の酸性水溶液が好適に用いることができる。洗浄温度は多くの場合は室温であれば、十分な洗浄効果を得ることができ、洗浄時間を短縮したい場合は酸性水溶液を加熱してもかまわない。洗浄処理が終了したら余分の酸性水溶液を除くためによく水洗した後、乾燥する。洗浄の効果は、例えば交換容量やイオン伝導度の回復として数値的に確認することが可能である。
【００３０】
（膨潤工程）
より高いイオン伝導性を発現させたい場合は、必要に応じて加水分解工程の後に膨潤処理を行うことによってフッ素系イオン交換膜の含水率を向上させることができる。例えば特開平６−３４２６６５号公報のようにフッ素系イオン交換膜を水又は水と水に可溶な有機溶剤の混合物中で加温することによって膨潤処理を行い、その後、酸型に戻すことによって高含水率のフッ素系イオン交換膜とすることができる。
【００３１】
（膜電極接合体の製造方法）
次に、膜電極接合体（ＭＥＡ）の製造方法について説明する。ＭＥＡはフッ素系イオン交換膜に電極を接合することにより作成される。電極は触媒金属の微粒子とこれを担持した導電剤から構成され、必要に応じて撥水剤が含まれる。電極に使用される触媒としては、水素の酸化反応及び酸素による還元反応を促進する金属であれば特に限定されず、白金、金、銀、パラジウム、イリジウム、ロジウム、ルテニウム、鉄、コバルト、ニッケル、クロム、タングステン、マンガン、バナジウム又はそれらの合金が挙げられる。この中では主として白金が用いられる。導電剤としては電子電導性物質であればいずれでもよく、例えば各種金属や炭素材料を挙げることができる。炭素材料としては、例えばファーネスブラック、チャンネルブラック、アセチレンブラック等のカーボンブラック、活性炭、黒鉛等が挙げられ、これらを単独又は混合して使用される。撥水剤としては撥水性を有するような含フッ素樹脂が好ましく、耐熱性、耐酸化性に優れたものがより好ましい。例えばポリテトラフルオロエチレン、テトラフルオロエチレン−パーフルオロアルキルビニルエーテル共重合体、テトラフルオロエチレン−ヘキサフルオロプロピレン共重合体を挙げることができる。このような電極としては、例えばＥ−ＴＥＫ社製の電極が広く用いられている。
【００３２】
前記電極とイオン交換膜からＭＥＡを作成するには、例えば次のような方法が行われる。フッ素系イオン交換樹脂をアルコールと水の混合溶液に溶解したものに電極物質となる白金担持カーボンを分散させてペースト状にする。これをＰＴＦＥシートに一定量塗布して乾燥させる。次に当該ＰＴＦＥシートの塗布面を向かい合わせにしてその間にイオン交換膜を挟み込み、熱プレスにより接合する。
熱プレス温度はイオン交換膜の種類によるが、通常は１００℃以上であり、好ましくは１３０℃以上、より好ましくは１５０℃以上である。
【００３３】
前記以外のＭＥＡの製作方法としては、「Ｊ．Ｅｌｅｃｔｒｏｃｈｅｍ．Ｓｏｃ．Ｖｏ．ｌ１３９，Ｎｏ２，Ｌ２８−Ｌ３０（１９９２）」に記載の方法がある。これによれば、フッ素系イオン交換樹脂をアルコールと水の混合溶液に溶解した後、ＳＯ_３Ｎａに変換した溶液を作成する。次にこの溶液に一定量の白金担持カーボンを添加してインク状の溶液とする。別途ＳＯ_３Ｎａ型に変換しておいたイオン交換膜の表面に前記インク状の溶液を塗布し、溶媒を除去する。最後に全てのイオン交換基をＳＯ_３Ｈ型に戻す事によりＭＥＡを作成する。本発明はこのようなＭＥＡにも適用することができる。
【００３４】
（燃料電池の製造方法）
次に、固体高分子電解質型燃料電池の製造方法について説明する。固体高分子電解質型燃料電池は、ＭＥＡ、集電体、燃料電池フレーム、ガス供給装置等から構成される。このうち集電体（バイポーラプレート）は、表面などにガス流路を有するグラファイト製又は金属製のフランジのことを言い、電子を外部負荷回路に伝達する他に、水素や酸素をＭＥＡ表面に供給する流路としての機能を持っている。こうした集電体の間にＭＥＡを挿入して複数積み重ねることにより、燃料電池を作成することができる。燃料電池の作動は、一方の電極に水素を、他方の電極に酸素又は空気を供給することによって行われる。燃料電池の作動温度は高温であるほど触媒活性が上がるために好ましいが、通常は水分管理が容易な５０℃〜１００℃で作動させることが多い。一方、本発明のような補強されたイオン交換膜については高温高湿強度の改善によって１００℃〜１５０℃で作動できる場合がある。酸素や水素の供給圧力については高いほど燃料電池出力が高まるため好ましいが、膜の破損等によって両者が接触する確率も増加するため適当な圧力範囲に調整することが好ましい。
【００３５】
【発明の実施の形態】
以下、本発明を実施例に基づいて更に詳細に説明するが、本発明は実施例に制限されるものではない。実施例において行われる評価方法は次の通りである。
（１）膜厚
酸型にしたイオン交換膜を２３℃・６５％の恒温室で１時間以上放置した後、膜厚計（東洋精機製作所：Ｂ−１）を用いて測定する。
（２）面配向度
酸型にしたイオン交換膜を２３℃・６５％の恒温室で１２時間以上放置した後、王子計測機器社製の自動複屈折計ＫＯＢＲＡ−２１ＡＤＨを用い、それぞれ３方向の屈折率を測定する。
【化４】

【００３６】
（３）１６０℃における熱収縮率
酸型にしたイオン交換膜を２３℃・６５％の恒温室で１２時間以上放置したあと、加熱前の膜面積を測定する。その後、１６０℃に加熱したシリコンオイルバスの中で１０秒間浸漬したあと取り出し、吸湿しないように注意しながら加熱後の膜面積を測定する。これらより、下記式を用いて１６０℃における熱収縮率Ｈ_ａ（％）を求める。
Ｈ_ａ＝（（Ａ_１−Ａ_２）／Ａ_１）×１００
Ａ_１：加熱前の膜面積（ｃｍ^２）、Ａ_２：加熱後の膜面積（ｃｍ^２）
【００３７】
（４）当量重量
酸型のイオン交換膜およそ２〜１０ｃｍ^２を３０ｍｌの２５℃飽和ＮａＣｌ水溶液に浸漬し、攪拌しながら３０分間放置した後、フェノールフタレインを指示薬として０．０１Ｎ水酸化ナトリウム水溶液を用いて中和滴定する。中和後得られたＮａ型イオン交換膜を純水ですすいだ後、真空乾燥して秤量する。中和に要した水酸化ナトリウムの当量をＭ（ｍｍｏｌ）、Ｎａ型イオン交換膜の重量をＷ（ｍｇ）とし、下記式より当量重量ＥＷ（ｇ／ｅｑ）を求める。ＥＷ＝（Ｗ／Ｍ）−２２
（５）メルトインデックス
ＪＩＳ　Ｋ−７２１０に基づき、温度２７０℃、荷重２．１６ｋｇで測定したフッ素系イオン交換樹脂前駆体のメルトインデックスをＭＩ（ｇ／１０分）とした。
【００３８】
（６）実延伸倍率
延伸前前駆体膜の膜厚Ｔ_ｂと換算突刺強度測定時の膜厚Ｔ_ａから、下記式を用いて実延伸倍率を求める。
実延伸倍率＝（Ｔ_ｂ／Ｔ_ａ）^０．５
（７）換算突刺強度
酸型にしたイオン交換膜を２３℃・６５％の恒温室で１２時間以上放置した後、ハンディー圧縮試験器（カトーテック社製：ＫＥＳ−Ｇ５）を用いて針先端の曲率半径０．５ｍｍ、突き刺し速度２ｍｍ／ｓｅｃの条件で突き刺し試験を行い、最大突き刺し荷重を突き刺し強度（ｇ）とした。また、突き刺し強度に２５（μｍ）／膜厚（μｍ）を乗じることによって換算突き刺し強度（ｇ／２５μｍ）とした。
【００３９】
（８）２５℃水中におけるイオン伝導度
酸型にしたイオン交換膜を２５℃の水中に３０分間浸漬した後、幅１ｃｍの短冊状に切り出し、その表面に直径０．５ｍｍの電極線を１ｃｍ間隔で平行に６本接触させる。２５℃９８％に調節した恒温恒湿槽に２時間以上保持したあと、交流インピーダンス法（１０ｋＨｚ）による抵抗測定を行い、電極間距離と抵抗から単位長さ当たりの抵抗値を測定する。これから、下記式を用いて２５℃における水平イオン伝導度Ｚ（Ｓ／ｃｍ）を求める。
Ｚ＝１／膜厚（ｃｍ）／膜幅（ｃｍ）／単位長さ当たりの抵抗値（Ω／ｃｍ）
【００４０】
【実施例１】（イオン交換樹脂前駆体の延伸−低倍率）
上記（原料ポリマー）で述べた一般式のフッ化ビニル化合物とフッ化オレフィンとの共重合体（但し、ｍは２であり、ＺはＦであり、ＷはＳＯ_２Ｆである。）からなるフッ素系イオン交換樹脂前駆体（ＥＷ：７２０、ＭＩ：３．６）をＴダイ法を用いて成膜し、厚さ１３０μｍの前駆体膜とした。当該前駆体膜を簡易式小型延伸機を用いて延伸温度５０℃で２×２倍に同時二軸延伸し配向膜とした。延伸後、簡易式小型延伸機に拘束したままの状態で当該配向膜を９５℃に加温した加水分解浴（ＤＭＳＯ：ＫＯＨ：水＝５：３０：６５）に１時間浸漬し、金属塩型のイオン交換基を有するフッ素系イオン交換膜を得た。これをよく水洗した後、６５℃に加温した２Ｎの塩酸浴に１５分間浸漬し、酸型のイオン交換基を有するフッ素系イオン交換膜を得た。これをよく水洗した後、膜を乾燥した。当該乾燥膜を拘束から外して、厚さ３６．４μｍの乾燥膜を得た。得られたフッ素系イオン交換膜について上記（１）〜（８）の特性試験を行った。その測定結果を表１に示す。
【００４１】
【実施例２】（イオン交換樹脂前駆体の延伸−高倍率）
延伸温度を９０℃、延伸倍率を４×４にした以外は実施例１と同様の方法を用いて厚さ１３．８μｍのフッ素系イオン交換膜を得た。得られた膜の上記測定結果を表１に示す。
【実施例３】（イオン交換樹脂の延伸−低倍率）
実施例１と同様のフッ素系イオン交換樹脂前駆体（ＥＷ：７２０、ＭＩ：３．６）をＴダイ法を用いて成膜し、厚さ１４０μｍの前駆体膜とした。当該前駆体膜を９５℃に加温した加水分解浴（ＤＭＳＯ：ＫＯＨ：水＝５：３０：６５）に１時間浸漬し、金属塩型のイオン交換基を有するフッ素系イオン交換膜を得た。
これをよく水洗した後、６５℃に加温した２Ｎの塩酸浴に１６時間以上浸漬し、酸型のイオン交換基を有するフッ素系イオン交換膜を得た。これをよく水洗した後、膜を乾燥した。当該乾燥膜を簡易式小型延伸機を用いて延伸温度１５０℃で２×２倍に同時二軸延伸し、厚さ４１．６μｍの乾燥膜を得た。得られたフッ素系イオン交換膜の上記測定結果を表１に示す。
【００４２】
【比較例１】（未延伸）
実施例１と同様のフッ素系イオン交換樹脂前駆体（ＥＷ：７２０、ＭＩ：３．６）をＴダイ法を用いて成膜し、未配向の状態で加水分解を行うことによって厚さ１４０μｍのフッ素系イオン交換膜を得た。当該フッ素系イオン交換膜の上記測定結果を表１に示す。
【００４３】
【表１】

【００４４】
【発明の効果】
本発明では、従来のフッ素系イオン交換膜と比べ、ポリマー主鎖が熱運動を開始するα分散温度が１５０℃付近と耐熱性に優れたフッ素系イオン交換樹脂前駆体及びフッ素系イオン交換樹脂を用いて、分子鎖を引き延ばしフィルム面配向度（ΔＰ）を０．０００５以上に配向させることで、高強度、低膜抵抗、低熱収縮を可能にすることができる。これにより、高耐熱性かつ高強度のフッ素系イオン交換膜を実現することができ、高耐久性、高性能の固体高分子電解質型燃料電池運転を実現することができる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a fluorine-based ion exchange membrane used as an electrolyte and a membrane of a polymer electrolyte fuel cell, and more particularly to a fluorine-based ion exchange membrane having excellent performance as an electrolyte and a membrane.
[0002]
[Prior art]
2. Description of the Related Art A fuel cell is a kind of power generation device that extracts electric energy by electrochemically oxidizing a fuel such as hydrogen or methanol, and has recently attracted attention as a clean energy supply source. Fuel cells are classified into phosphoric acid type, molten carbonate type, solid oxide type, solid polymer electrolyte type, etc., depending on the type of electrolyte used. Among them, solid polymer electrolyte fuel cells have standard operating temperatures. Is low as 100 ° C. or lower and has a high energy density, so that it is expected to be widely applied as a power source for electric vehicles and the like.
[0003]
The basic structure of a solid polymer electrolyte fuel cell consists of an ion-exchange membrane and a pair of gas diffusion electrodes joined to both sides of the membrane, supplying hydrogen to one electrode and oxygen to the other, and applying an external load between the two electrodes. The power is generated by connecting to a circuit. More specifically, protons and electrons are generated at the hydrogen side electrode, and the protons move inside the ion exchange membrane and reach the oxygen side electrode, and then react with oxygen to generate water. On the other hand, the electrons flowing out of the hydrogen-side electrode through the lead wire, after electric energy is extracted in the external load circuit, further travels through the lead wire to reach the oxygen-side electrode, and contributes to the progress of the water generation reaction. One of the required characteristics of ion exchange membranes is high ion conductivity, but protons move inside the ion exchange membrane and are thought to be stabilized by hydration of water molecules. High water content and water dispersibility as well as properties are important required characteristics. In addition, since the ion exchange membrane functions as a barrier for preventing a direct reaction between hydrogen and oxygen, low permeability to gas is required. Other required characteristics include chemical stability for withstanding a strong oxidizing atmosphere during operation of the fuel cell and mechanical strength for withstanding further thinning.
[0004]
As a material of the ion exchange membrane used in the polymer electrolyte fuel cell, a fluorine-based ion exchange resin is widely used because of its high chemical stability. Among them, a main chain is perfluorocarbon and a side chain terminal is used. "Nafion (registered trademark)" manufactured by DuPont having a sulfonic acid group is widely used. Such a fluorine-based ion exchange resin has generally well-balanced properties as a solid polymer electrolyte material, but further improvement in physical properties has been required as the battery has been put to practical use.
[0005]
For example, in order to achieve high current density and uniformity of moisture in the membrane at a high level, it is considered that thinning of the ion exchange membrane will become even more important in the future, but for this purpose, the mechanical strength of the ion exchange membrane will be improved. There is a need. Similarly, demands for higher strength are increasing from the viewpoint of improving long-term durability. The stretching technique is one of the effective means for improving the mechanical strength of a membrane or a film, and a method for obtaining a high-strength ion exchange membrane by stretching is already known. Japanese Patent Application Laid-Open No. 60-149631 discloses an ion exchange resin swelled with a liquid organic compound, or a melt processable precursor of an ion exchange resin swelled with a fluorine-containing liquid organic compound. A manufacturing method for stretching in at least one planar direction is disclosed.
[0006]
In Example 1 of the above publication, the mechanical strength was 2.8 × 10 by stretching the fluorine-based ion exchange resin 2 × 2 times in the vertical and horizontal directions at 125 ° C.⁷6.3 × 10 from Pa⁷It is disclosed that it rises to Pa. However, it has been revealed that the stretched film according to this example has a large thermal shrinkage in high temperature processing when the α-dispersion temperature is around 120 ° C. Then, problems such as the occurrence of large thermal shrinkage and relaxation of the orientation and the shrinkage of the film in hot water have been found. In Example 13 of the above publication, the mechanical strength was 3.3 × 10 by stretching the fluorine-based ion exchange resin precursor at 70 ° C. 2 × 2 times in the vertical and horizontal directions.⁷3.5 × 10 from Pa⁷It is disclosed that it rises to Pa. This is because the stretching temperature is high, and it is considered that the film is stretched without generation of stretching stress, that is, the degree of plane orientation is small. The increase in mechanical strength is significantly smaller than that of the stretched film of Example 1, Has not been achieved.
[0007]
Japanese Patent Publication No. 63-61337 discloses "an ion characterized by drawing a film made of a fluorine-containing ion exchange resin <precursor> containing uniformly dispersed fluororesin fibrillated fibers at a specific temperature to form a thin film. Method for producing exchange membrane ". However, since the above publication mainly aims at reducing the thickness of the ion exchange membrane, the mechanical strength is lower than that of the unstretched membrane as shown in Table 3 of the publication.
As described above, the prior art relating to the enhancement of strength is merely an attempt to elongate, and cannot be disclosed as an industrially useful technique as an ion exchange membrane for a fuel cell because of particularly large heat shrinkage. Was.
[0008]
[Problems to be solved by the invention]
An object of the present invention is to provide a fluorine-based ion exchange membrane having high mechanical strength and conductivity and low heat shrinkage by stretching and orientation.
[0009]
[Means for Solving the Problems]
Stretching orientation increases mechanical strength, but also increases heat shrinkage, which is a problem. The present inventors have conducted intensive studies in order to solve the above-mentioned problems, and as a result, are substantially constituted by the following repeating units (A) and (B).
Embedded image

(Z is H, Cl, F or a perfluoroalkyl group having 1 to 3 carbon atoms, m is an integer of 1 to 3, W = CO₂R¹, COR², SO₂R², R¹Is a perfluoroalkyl group having 1 to 3 carbon atoms, R²Is halogen or OM, M is H, alkali metal atom, alkaline earth metal atom or ammonium ion)
By performing stretching orientation to a degree of plane orientation (ΔP) of 0.0005 or more using a copolymer, it has been found that mechanical strength, ion conductivity, and heat resistance dimensional stability are excellent. It was completed.
[0010]
That is, the present invention
(1) Substantially constituted by the following repeating units (A) and (B)
Embedded image

(Z is H, Cl, F or a perfluoroalkyl group having 1 to 3 carbon atoms, m is an integer of 1 to 3, W = CO₂R¹, COR², SO₂R², R¹Is a perfluoroalkyl group having 1 to 3 carbon atoms, R²Is halogen or OM, M is H, alkali metal atom, alkaline earth metal atom or ammonium ion)
A fluorine-based ion exchange membrane, which is a copolymer, and has a film thickness of 1 to 500 μm and a degree of film plane orientation (ΔP) of 0.0005 or more;
(2) The fluorine-based ion exchange membrane according to (1), wherein the heat shrinkage at 160 ° C. is 50% or less.
(3) a membrane / electrode assembly comprising the fluorine-based ion exchange membrane according to (1) or (2);
4. A solid polymer electrolyte fuel cell comprising the fluorine-based ion exchange membrane according to (1) or (2).
[0011]
Hereinafter, the present invention will be described in detail.
First, the fluorine-based ion exchange membrane of the present invention will be described.
Stretch-oriented films exhibit high mechanical strength, but are often limited in applications involving high-temperature processing due to their large heat shrinkage, especially in fuel cell applications. On the other hand, the fluorine-based ion exchange membrane of the present invention has high mechanical strength, ion conductivity, and good heat-resistant dimensional stability without losing the excellent characteristics of a normal fluorine-based ion exchange membrane. It can be particularly suitably used as an ion exchange membrane for a fuel cell.
[0012]
(Thickness)
The thickness of the fluorine-based ion exchange membrane of the present invention is preferably 1 μm or more, more preferably 5 μm or more, and still more preferably 10 μm or more. If the film thickness is smaller than 1 μm, the above-described disadvantages of the direct reaction are likely to occur due to diffusion of hydrogen and oxygen, and the film may be damaged due to a difference in pressure and strain during handling during fuel cell operation and fuel cell operation. Disadvantages are likely to occur. Further, a membrane having a thickness greater than 500 μm generally has low ion permeability, and thus may not have sufficient performance as an ion exchange membrane. The preferable upper limit of the film thickness is preferably 500 μm or less, more preferably 100 μm or less, and still more preferably 50 μm or less. Specifically, a preferable range is 1 to 500 μm, more preferably 5 to 100 μm, and further preferably 10 to 50 μm.
[0013]
(Plane orientation degree)
The feature of the fluorine-based ion exchange membrane of the present invention is the degree of plane orientation. If this value does not exceed a certain value, no matter how the orientation is performed, it is merely a thin film, and the mechanical strength and ion conductivity The improvement of the nature cannot be expected. On the other hand, even if the film is stretched beyond a certain value, the degree of plane orientation does not increase, and only the heat shrinkage increases, which is not practical.
Further, the ion exchange membrane swells or contracts depending on the degree of humidification by water generated in the fuel cell or supplied water. The dimensional change of the swelling and shrinkage causes mechanical damage and lowers the strength, resulting in unstable operation of the fuel cell. It is considered that when an ion exchange membrane having a degree of plane orientation of 0.0005 or more is used, the above-mentioned problem is solved because of excellent dimensional stability, and good fuel cell operation can be achieved.
[0014]
The degree of plane orientation of the fluorine-based ion exchange membrane is closely related to mechanical strength and dimensional stability under heat. The plane orientation degree of the fluorine-based ion exchange membrane of the present invention is preferably 0.0005 or more, more preferably 0.0010 or more, further preferably 0.0015 or more, and still more preferably 0.0020 or more. When the degree of plane orientation is less than 0.0005, the mechanical strength is insufficient and the ion exchange membrane may not have sufficient performance, which is not preferable. Although the upper limit of the degree of plane orientation is not particularly set in the present invention, if it is larger than 0.005, heat shrinkage is likely to occur, and inconvenience is likely to occur, for example, at the time of MEA production. Specifically, the preferred range of the degree of plane orientation is 0.0005 to 0.005, more preferably 0.001 to 0.005, and even more preferably 0.0015 to 0.005.
[0015]
(Thermal shrinkage at 160 ° C)
The heat shrinkage at 160 ° C. of the fluorine-based ion exchange membrane of the present invention is 50% or less, preferably 45% or less, more preferably 40% or less. When the heat shrinkage ratio is larger than 50%, heat shrinkage is likely to occur in applications involving high-temperature processing, which may cause a great hindrance in, for example, the production of MEA.
(Equivalent weight)
The equivalent weight (EW) of the fluorine-based ion exchange membrane of the present invention is not particularly limited. However, if the equivalent weight is too low, the strength is reduced, which is not preferable. It is preferably 400 or more, more preferably 600 or more, and still more preferably 700 or more. On the other hand, when the equivalent weight is increased, the mechanical strength is improved even in the non-oriented film, but at the same time, the ion conductivity is reduced due to the reduced density of the ion exchange groups. The preferred upper limit of the EW is 1400 or less, more preferably 1200 or less, and even more preferably 1000 or less. Specifically, the preferable range of EW is preferably 400 to 1400, more preferably 600 to 1200, and still more preferably 700 to 1000.
[0016]
(Converted puncture strength)
The converted puncture strength of the fluorine-based ion exchange membrane of the present invention (the puncture strength in a dry state is calculated per 25 μm) is not particularly limited, but is preferably 300 g or more, more preferably 350 g or more, and further preferably 400 g or more. When the reduced puncture strength is less than 300 g, the mechanical strength required for thinning is insufficient, and the film needs to be thick, which is not preferable. In the present invention, the upper limit of the converted piercing strength is not particularly set, but a membrane having a strength of 3000 g or more is generally expected to have a low moisture content, and thus may not have sufficient performance as an ion exchange membrane. is there.
[0017]
(Horizontal ionic conductivity in 25 ° C water)
The horizontal ion conductivity of the fluorine-based ion exchange membrane of the present invention in water at 25 ° C. is preferably 0.05 S / cm or more, more preferably 0.07 S / cm or more, and further preferably 0.10 S / cm or more. When the horizontal ionic conductivity is smaller than 0.05 S / cm, the internal resistance increases when used as an ion exchange membrane for a fuel cell, which is not preferable.
[0018]
Next, a method for producing a fluorine-based ion exchange membrane of the present invention will be described.
Generally, an ion exchange membrane is formed by forming an ion exchange resin precursor into a membrane and performing hydrolysis at a high temperature. Therefore, the object to be stretched can be roughly classified into a fluorine-based ion exchange resin precursor before hydrolysis and a fluorine-based ion exchange resin after hydrolysis, but in the present invention, for any membrane according to the purpose. Can also be stretched. Both can be selected as follows.
[0019]
(Stretching of fluorinated ion exchange resin precursor)
The first preferred form of stretching in the present invention is performed on a fluorine-based ion exchange resin precursor. A point that should be particularly emphasized in the stretching of the fluorine-based ion-exchange resin precursor is the prevention of the relaxation of the orientation accompanying the completion of the stretching. This is for the following reasons. Generally, the stretching temperature of a film is often set with reference to the α dispersion temperature in viscoelasticity measurement. The α-dispersion temperature mentioned here is a temperature at which the polymer main chain is considered to start thermal motion, and is widely used as an index when processing while giving a large strain to the polymer such as stretching. For example, since the α-dispersion temperature of polymers such as polyester and nylon is generally much higher than room temperature, it is necessary to significantly reduce the thermal motion of the main chain by cooling below the α-dispersion temperature after drawing is completed. Is possible, whereby the stretching orientation can be effectively stabilized.
[0020]
On the other hand, since the α-dispersion temperature of the fluorine-based ion-exchange resin precursor is around room temperature, such “stretch-fixing” is difficult. When the constraint is removed from the stretched state, the α-dispersion rapidly shrinks and loses the stretch orientation. There were many things. The present inventors have conducted intensive studies on the relaxation of the orientation of the fluorine-based ion exchange resin precursor, and as a result, by focusing on hydrolysis, which is a production process unique to the precursor, a novel method independent of the α dispersion temperature. A stretching fixation method was found. That is, in the present invention, as a first preferred form of stretching, the fluorine-based ion exchange resin precursor is stretched and then hydrolyzed in a state where the stretch orientation is restricted.
Although it is not clear why stretching and fixing can be achieved by such a method, the α-dispersion temperature of the fluorinated ion exchange resin produced by hydrolysis is much higher than that of the precursor, and it is thought that the α-dispersion temperature is around 150 ° C. Therefore, it is thought that by performing the hydrolysis while maintaining the stretch orientation, the thermal motion of the main chain was reduced in the process of increasing the α dispersion temperature of the orientation film as the progress proceeded, and it was thought that the stretch fixation could be achieved. . Such a method of stretching and fixing is referred to as "saponification fixing" in the present invention.
[0021]
The reason why the saponification fixation can be achieved can be further considered as follows. Hydrolysis of the fluorinated ion-exchange resin precursor causes a large amount of water to be absorbed.However, such water does not exist uniformly inside the resin, but rather exists locally while forming microscopic water droplets. It is considered. Such water droplets are called clusters and can be specifically observed by small-angle X-ray diffraction or a transmission electron microscope.
It is expected that one cluster contains a plurality of side chain terminals. However, when a fluorine-based ion exchange resin precursor is stretched and then a cluster is formed in a state where constraints are maintained, these side chain terminals are mutually connected. It can be expected to function as a kind of cross-linking point linked to each other via water. That is, in addition to the increase in the α dispersion temperature, the cluster formed after the stretching orientation functions as a pseudo-crosslinking point, so that the saponification fixation functions more favorably.
[0022]
(Stretching of fluorine ion exchange resin)
The second of the preferred modes of stretching in the present invention is for a fluorine-based ion exchange resin. As described above, since the α-dispersion temperature of the fluorine-based ion exchange resin is considered to be around 150 ° C., stretching and fixing by cooling is easy, and high mechanical strength can be maintained even after the constraint is released. . In particular, since such an alignment film does not require a special treatment such as saponification fixing, a general stretching technique can be applied, which is preferable from the viewpoint of improving the productivity of an ion exchange membrane for a fuel cell. That is, in the present invention, as a second preferred mode of stretching, the stretching is performed after hydrolyzing the fluorine-based ion exchange resin precursor.
[0023]
(Raw polymer)
The fluorinated ion exchange resin precursor used in the present invention has a general formula CF₂= CF-O- (CF₂)_mA vinyl fluoride compound represented by the general formula CF₂= At least a binary copolymer with a fluorinated olefin represented by CFZ. Here, m is an integer of 1 to 3, and Z is H, Cl, F or a perfluoroalkyl group having 1 to 3 carbon atoms. Also, W becomes CO by hydrolysis.₂H or SO₃H is a functional group that can be converted to H. Such a functional group includes SO₂F, SO₂Cl, SO₂Br, COF, COCl, COBr, CO₂CH₃, CO₂C₂H₅Is usually preferably used. Such a fluorine-based ion exchange resin precursor can be synthesized by a conventionally known means.
[0024]
The fluorine-based ion-exchange membrane resin precursor is a conventionally used fluorine-based ion-exchange membrane resin precursor of the general formula CF₂= CF-O (CF₂CFXO)_n− (CF₂)_mA vinyl fluoride compound represented by the general formula CF₂= At least a binary copolymer with a fluorinated olefin represented by CFZ (where X is an F atom or a perfluoroalkyl group having 1 to 3 carbon atoms, n is an integer of 0 to 3, and m is Z is H, Cl, F or a perfluoroalkyl group having 1 to 3 carbon atoms, and W is CO by hydrolysis.₂H or SO₃It is a functional group that can be converted to H. ), The length of the side chain connected by the ether bond is shorter, whereby the crystallinity is increased and the heat resistance is also increased. By utilizing this property, in the present invention, the stretching orientation is effectively stabilized, and a fluorine-based ion exchange membrane having high mechanical strength and excellent heat resistance has been made possible.
[0025]
(Deposition process)
As a method of forming the fluorine-based ion exchange resin precursor into a film, any method generally known as a forming method such as a melt forming method (T-die method, inflation method, calendering method, etc.) and a casting method can be used. Any of them can be suitably used.
Examples of the casting method include a method in which a fluorinated ion exchange resin is dispersed in an appropriate solvent, and a method in which a polymerization reaction solution itself is formed into a sheet and the dispersion medium is removed. The lower limit of the resin temperature at the time of performing the melt molding by the T-die method is preferably 100 ° C. or higher, more preferably 200 ° C. or higher. The upper limit temperature is preferably 300 ° C. or lower, more preferably 280 ° C. or lower, in consideration of the heat resistance of the resin. Specifically, the temperature is preferably from 100 to 300C, more preferably from 200 to 280C. The lower limit is preferably 100 ° C. or higher, more preferably 160 ° C. or higher. The upper limit temperature is preferably 300 ° C. or lower, more preferably 240 ° C. or lower, in consideration of the heat resistance of the resin. Specifically, the temperature is preferably from 100 to 300C, more preferably from 160 to 240C. The sheet melt-formed by these methods is cooled to a temperature lower than the melting temperature by using a cooling roll or the like. It is preferable to adjust the thickness of the precursor film to an optimum thickness in anticipation of a decrease in the thickness in the alignment step. For example, when performing 4 × 4 stretching in the alignment step, it is necessary to adjust the thickness of the precursor film to around 400 μm in order to make the thickness of the alignment film 25 μm.
[0026]
(Hydrolysis step)
As a method of hydrolysis, for example, as described in Japanese Patent No. 2737531, an ion-exchange group precursor of an alignment film is converted into a metal salt-type ion-exchange group using an alkali hydroxide solution, and then sulfonic acid is used. Alternatively, using an acid such as hydrochloric acid, the acid form (SO₃H or COOH) can be used by a conventionally known method for converting to an ion exchange group. Such transformations are well known to those skilled in the art and are described in embodiments of the present invention.
When the orientation step is performed before the hydrolysis step, it is necessary to restrain the fluorine-based ion exchange resin precursor through the hydrolysis step. The constraint in the present invention means a constraint for preventing spontaneous relaxation of the stretching orientation due to thermal shrinkage of the film and should be considered to include not only a constraint in a certain dimension but also a constraint accompanied by stretching. . If the orientation step is not performed before the hydrolysis step, the film expands due to water absorption accompanying the hydrolysis, so it is necessary to strive to prevent the generation of wrinkles especially when performing continuous processing using a roll, a belt, or the like. . In the present invention, stretching or heat treatment may be performed during the hydrolysis step.
[0027]
(Orientation step)
As the stretching method, any method generally known as a method for stretching a film can be suitably used, and among these, transverse uniaxial stretching with a tenter and sequential biaxial stretching with a tenter and a longitudinal stretching roll are preferred. Stretching, simultaneous biaxial stretching with a simultaneous biaxial tenter, and blow stretching with an inflation film forming apparatus are more preferred, and simultaneous biaxial stretching or blow stretching is more preferred. A suitable stretching magnification is an area magnification. The lower limit is 1.1 or more, preferably 2 or more, more preferably 4 or more, and the upper limit is 100 or less, preferably 20 or less, more preferably 16 or less. It is. Among them, the stretching ratio in the transverse direction (direction perpendicular to the machine direction) is 1.1 times or more, preferably 1.5 times or more, more preferably 2 times or more, and the upper limit is 100 times or less. Preferably it is 10 times or less, more preferably 4 times or less. Specifically, it is 1.1 to 100 times, preferably 2 to 20 times, more preferably 4 to 16 times, and the transverse stretching ratio is 1.1 to 100 times, preferably 1.5 to 10 times, and furthermore Preferably it is 2 to 4 times. A suitable stretching temperature is equal to or lower than the melting temperature of the precursor film, and the lower limit is equal to or higher than (α dispersion temperature −100 ° C.), preferably equal to or higher than (α dispersion temperature −80 ° C.), and more preferably equal to (α dispersion temperature −50 ° C.). The upper limit is (α dispersion temperature + 100 ° C.) or less, preferably (α dispersion temperature + 80 ° C.) or less, more preferably (α dispersion temperature + 50 ° C.) or less. Specifically, (α dispersion temperature −100 ° C.) to (α dispersion temperature + 100 ° C.), preferably (α dispersion temperature −80 ° C.) to (α dispersion temperature + 80 ° C.), more preferably (α dispersion temperature −50 ° C.) ) To (α dispersion temperature + 50 ° C.).
In the present invention, the term “stretching” means elongation accompanied by generation of stretching stress, and elongation without occurrence of stretching stress is referred to as “widening”. For example, when the orientation step is not performed before the hydrolysis step, the film swells largely in the horizontal direction due to water absorption accompanying the hydrolysis. However, when the film is stretched following this change, it can be considered that the film is widened.
[0028]
(Heat treatment process)
When it is desired to further reduce the heat shrinkage, the heat shrinkage of the fluorine-based ion exchange membrane can be reduced by performing a heat treatment as needed. As the heat treatment method, any method generally known as a heat treatment method for a film can be suitably used, but it is preferable that the heat treatment be performed with the fluorine-based ion exchange membrane restrained. The heat treatment temperature is preferably equal to or higher than the α-dispersion temperature. If the maximum temperature to be exposed in applications involving high-temperature processing such as the press temperature during MEA production is clear, it should be higher than that. Is more preferred. If the fluorinated ion exchange resin is heated to 300 ° C. or more, it may be degraded. Therefore, the heat treatment temperature is preferably 300 ° C. or less. More specifically, the upper limit of the heat treatment temperature is preferably 50 ° C. or higher, more preferably 30 ° C. or lower, more preferably 30 ° C. higher, based on the film operating temperature such as the pressing temperature. Is a temperature not higher than 20 ° C., still more preferably a temperature not higher than 10 ° C. Further, the lower limit of the heat treatment temperature is preferably a temperature of 50 ° C. or lower, more preferably a temperature of 30 ° C. lower, more preferably 20 ° C. lower, based on the film operating temperature such as a pressing temperature. The temperature is not lower than the temperature, still more preferably not lower than 10 ° C. Although the heat treatment time depends on the heat treatment temperature, the heat treatment can be suitably performed in a range of about 1 second to 1 hour. Although the heat treatment time is longer and the heat treatment temperature is higher, the heat shrinkage can be reduced, but disadvantages such as a decrease in mechanical strength and a decrease in ionic conductivity are likely to occur.
[0029]
(Washing process)
When the ion conductivity is greatly reduced by the heat treatment step, it can be recovered by washing the fluorine-based ion exchange membrane as needed. The washing can be performed, for example, by immersing or spraying the fluorine-based ion exchange membrane in an acidic aqueous solution with or without constraint. The concentration of the acidic aqueous solution to be used depends on the state of decrease in ion conductivity, the washing temperature, and the washing time. For example, a 0.001 to 5N acidic aqueous solution can be suitably used. In many cases, if the washing temperature is room temperature, a sufficient washing effect can be obtained. If the washing time is to be shortened, the acidic aqueous solution may be heated. After the completion of the washing treatment, the substrate is thoroughly washed with water in order to remove excess acidic aqueous solution, and then dried. The effect of the cleaning can be numerically confirmed, for example, as the recovery of the exchange capacity and the ionic conductivity.
[0030]
(Swelling process)
When a higher ion conductivity is desired to be exhibited, the water content of the fluorine-based ion exchange membrane can be improved by performing a swelling treatment after the hydrolysis step, if necessary. For example, a swelling treatment is performed by heating a fluorine-based ion-exchange membrane in water or a mixture of water and an organic solvent soluble in water, as in JP-A-6-342665, and then returning to an acid form. A fluorine-based ion exchange membrane having a high water content can be obtained.
[0031]
(Method for producing membrane electrode assembly)
Next, a method for manufacturing a membrane electrode assembly (MEA) will be described. The MEA is made by bonding an electrode to a fluorine-based ion exchange membrane. The electrode is composed of catalytic metal fine particles and a conductive agent supporting the fine particles, and contains a water repellent as needed. The catalyst used for the electrode is not particularly limited as long as it promotes the oxidation reaction of hydrogen and the reduction reaction by oxygen, and platinum, gold, silver, palladium, iridium, rhodium, ruthenium, iron, cobalt, nickel, Chromium, tungsten, manganese, vanadium or alloys thereof are mentioned. Among them, platinum is mainly used. Any conductive agent may be used as long as it is an electron conductive material, and examples thereof include various metals and carbon materials. Examples of the carbon material include carbon black such as furnace black, channel black, and acetylene black, activated carbon, and graphite. These may be used alone or in combination. As the water repellent, a fluorine-containing resin having water repellency is preferable, and one having excellent heat resistance and oxidation resistance is more preferable. For example, polytetrafluoroethylene, a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, and a tetrafluoroethylene-hexafluoropropylene copolymer can be mentioned. As such an electrode, for example, an electrode manufactured by E-TEK is widely used.
[0032]
In order to form an MEA from the electrode and the ion exchange membrane, for example, the following method is performed. Platinum-supporting carbon serving as an electrode material is dispersed in a solution in which a fluorine-based ion exchange resin is dissolved in a mixed solution of alcohol and water to form a paste. A predetermined amount of this is applied to a PTFE sheet and dried. Next, the application surfaces of the PTFE sheet are faced to each other, and an ion exchange membrane is sandwiched therebetween, and the sheets are joined by hot pressing.
The hot pressing temperature depends on the type of the ion exchange membrane, but is usually 100 ° C. or higher, preferably 130 ° C. or higher, more preferably 150 ° C. or higher.
[0033]
As a method of manufacturing an MEA other than the above, there is a method described in "J. Electrochem. Soc. Vo. L139, No. 2, L28-L30 (1992)". According to this, after dissolving a fluorine-based ion exchange resin in a mixed solution of alcohol and water,₃Make a solution converted to Na. Next, a certain amount of platinum-supported carbon is added to this solution to form an ink-like solution. Separate SO₃The ink-like solution is applied to the surface of the ion exchange membrane that has been converted to the Na type, and the solvent is removed. Finally, replace all ion exchange groups with SO₃The MEA is created by returning to the H type. The present invention can be applied to such an MEA.
[0034]
(Fuel cell manufacturing method)
Next, a method for manufacturing a solid polymer electrolyte fuel cell will be described. A solid polymer electrolyte fuel cell includes an MEA, a current collector, a fuel cell frame, a gas supply device, and the like. The current collector (bipolar plate) is a graphite or metal flange having a gas flow path on the surface, etc., and supplies hydrogen and oxygen to the MEA surface in addition to transmitting electrons to an external load circuit. It has a function as a flow path. By inserting MEAs between such current collectors and stacking a plurality of them, a fuel cell can be produced. The operation of the fuel cell is performed by supplying hydrogen to one electrode and oxygen or air to the other electrode. The higher the operating temperature of the fuel cell is, the higher the catalyst activity is, which is preferable. However, the fuel cell is usually operated at 50 ° C. to 100 ° C. where the water management is easy. On the other hand, a reinforced ion exchange membrane as in the present invention may be able to operate at 100 ° C. to 150 ° C. by improving the high temperature and high humidity strength. The supply pressure of oxygen or hydrogen is preferably higher as the output of the fuel cell is higher, but it is preferable to adjust the supply pressure to an appropriate pressure range since the probability of contact between the two due to breakage of the membrane increases.
[0035]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited to the examples. The evaluation method performed in the example is as follows.
(1) Film thickness
After leaving the ion-exchange membrane in the acid form in a constant temperature chamber at 23 ° C. and 65% for 1 hour or more, the measurement is performed using a film thickness meter (Toyo Seiki Seisakusho: B-1).
(2) Plane orientation degree
After the acid-exchanged ion exchange membrane is allowed to stand in a thermostat at 23 ° C. and 65% for 12 hours or more, the refractive index in each of three directions is measured using an automatic birefringence meter KOBRA-21ADH manufactured by Oji Scientific Instruments.
Embedded image

[0036]
(3) Heat shrinkage at 160 ° C
After the acid-exchanged ion-exchange membrane is left in a thermostat at 23 ° C. and 65% for 12 hours or more, the membrane area before heating is measured. Thereafter, the film is immersed in a silicone oil bath heated to 160 ° C. for 10 seconds and then taken out, and the film area after heating is measured while taking care not to absorb moisture. From these, the thermal shrinkage H at 160 ° C. is calculated using the following equation._a(%).
H_a= ((A₁-A₂) / A₁) × 100
A₁: Film area before heating (cm²), A₂: Film area after heating (cm²)
[0037]
(4) Equivalent weight
Acid type ion exchange membrane about 2-10cm²Is immersed in 30 ml of a 25 ° C. saturated NaCl aqueous solution and left for 30 minutes with stirring, and then subjected to neutralization titration using a 0.01 N aqueous sodium hydroxide solution using phenolphthalein as an indicator. After the neutralization, the obtained Na-type ion exchange membrane is rinsed with pure water, dried in vacuum, and weighed. The equivalent weight of sodium hydroxide required for neutralization is defined as M (mmol), and the weight of the Na-type ion exchange membrane is defined as W (mg), and the equivalent weight EW (g / eq) is determined from the following equation. EW = (W / M) −22
(5) Melt index
The melt index of the fluorine-based ion exchange resin precursor measured at a temperature of 270 ° C. and a load of 2.16 kg was defined as MI (g / 10 minutes) based on JIS @ K-7210.
[0038]
(6) Actual stretch ratio
Thickness T of precursor film before stretching_bAnd the film thickness T when measuring the converted piercing strength_aThen, the actual stretching ratio is determined using the following equation.
Actual stretch ratio = (T_b/ T_a)^0.5
(7) Converted puncture strength
After leaving the acid-exchanged ion-exchange membrane in a constant temperature chamber at 23 ° C. and 65% for 12 hours or more, the radius of curvature of the tip of the needle was 0.5 mm using a handy compression tester (KES-G5, manufactured by Kato Tech). A piercing test was performed at a piercing speed of 2 mm / sec, and the maximum piercing load was defined as the piercing strength (g). Further, the piercing strength was multiplied by 25 (μm) / film thickness (μm) to obtain a converted piercing strength (g / 25 μm).
[0039]
(8) Ionic conductivity in 25 ° C water
After immersing the acid-exchanged ion-exchange membrane in water at 25 ° C. for 30 minutes, it is cut into strips having a width of 1 cm, and six electrode wires having a diameter of 0.5 mm are brought into parallel contact with the surface at 1 cm intervals. After holding in a thermo-hygrostat adjusted to 25 ° C. and 98% for 2 hours or more, the resistance is measured by the AC impedance method (10 kHz), and the resistance value per unit length is measured from the distance between the electrodes and the resistance. From this, the horizontal ionic conductivity Z (S / cm) at 25 ° C. is determined using the following equation.
Z = 1 / film thickness (cm) / film width (cm) / resistance value per unit length (Ω / cm)
[0040]
Example 1 (Stretching of ion exchange resin precursor-low magnification)
Copolymer of vinyl fluoride compound of general formula and fluorinated olefin described in the above (raw polymer) (where m is 2, Z is F, W is SO₂F. ) Was formed using a T-die method to form a precursor film having a thickness of 130 μm. The precursor film was simultaneously biaxially stretched 2 × 2 times at a stretching temperature of 50 ° C. using a simple small stretching machine to obtain an oriented film. After stretching, the alignment film is immersed in a hydrolysis bath (DMSO: KOH: water = 5: 30: 65) heated at 95 ° C. for 1 hour while being restrained by a simple small-sized stretching machine, and then subjected to a metal salt type. A fluorinated ion exchange membrane having an ion exchange group was obtained. After thoroughly washing this with water, it was immersed in a 2N hydrochloric acid bath heated to 65 ° C. for 15 minutes to obtain a fluorinated ion exchange membrane having an acid type ion exchange group. After thoroughly washing this with water, the membrane was dried. The dried film was removed from the restraint to obtain a dried film having a thickness of 36.4 μm. The characteristic tests (1) to (8) above were performed on the obtained fluorine-based ion exchange membrane. Table 1 shows the measurement results.
[0041]
Example 2 (Stretching of ion exchange resin precursor-high magnification)
A fluorine-based ion exchange membrane having a thickness of 13.8 μm was obtained in the same manner as in Example 1 except that the stretching temperature was 90 ° C. and the stretching ratio was 4 × 4. Table 1 shows the measurement results of the obtained film.
Example 3 (Stretching of ion exchange resin-low magnification)
The same fluorine-based ion exchange resin precursor (EW: 720, MI: 3.6) as in Example 1 was formed by a T-die method to form a precursor film having a thickness of 140 μm. The precursor membrane was immersed in a hydrolysis bath (DMSO: KOH: water = 5: 30: 65) heated to 95 ° C. for 1 hour to obtain a fluorinated ion exchange membrane having a metal salt type ion exchange group. .
After thoroughly washing this with water, it was immersed in a 2N hydrochloric acid bath heated to 65 ° C. for 16 hours or more to obtain a fluorinated ion exchange membrane having an acid type ion exchange group. After thoroughly washing this with water, the membrane was dried. The dried film was simultaneously biaxially stretched 2 × 2 times at a stretching temperature of 150 ° C. using a simple type small stretching machine to obtain a dried film having a thickness of 41.6 μm. Table 1 shows the measurement results of the obtained fluorine-based ion exchange membrane.
[0042]
Comparative Example 1 (Unstretched)
The same fluorine-based ion-exchange resin precursor (EW: 720, MI: 3.6) as in Example 1 was formed by a T-die method, and was hydrolyzed in an unoriented state to form a film having a thickness of 140 μm. A fluorinated ion exchange membrane was obtained. Table 1 shows the measurement results of the fluorine-based ion exchange membrane.
[0043]
[Table 1]

[0044]
【The invention's effect】
In the present invention, compared with the conventional fluorine-based ion exchange membrane, the α-dispersion temperature at which the polymer main chain starts thermal motion is around 150 ° C., and the heat-resistant fluorine-based ion-exchange resin precursor and fluorine-based ion-exchange resin are used. By using them to stretch the molecular chains and orient the film plane orientation (ΔP) to 0.0005 or more, high strength, low film resistance, and low heat shrinkage can be achieved. As a result, a fluorine-based ion exchange membrane having high heat resistance and high strength can be realized, and a high durability and high performance solid polymer electrolyte fuel cell operation can be realized.

Claims (4)

Substantially constituted by the following repeating units (A) and (B)

(Z is H, Cl, F or a perfluoroalkyl group having 1 to 3 carbon atoms, m is an integer of 1 to _{^{3, W = CO 2 R 1,}} COR 2, SO 2 R 2, R 1 is 1 to the number of carbon atoms 3 is a perfluoroalkyl group, R ² is halogen or OM, M is H, an alkali metal atom, an alkaline earth metal atom or an ammonium ion)
A fluorine-based ion exchange membrane, which is a copolymer, has a film thickness of 1 to 500 μm, and has a film plane orientation (ΔP) of 0.0005 or more. The fluorinated ion exchange membrane according to claim 1, wherein the heat shrinkage at 160 ° C is 50% or less. A membrane electrode assembly comprising the fluorine-based ion exchange membrane according to claim 1. A solid polymer electrolyte fuel cell comprising the fluorine-based ion exchange membrane according to claim 1.