JP3579998B2 - Manufacturing method of density gradient airgel - Google Patents

Description

【００１４】
【化２】

【００１５】
【化３】

【００１６】
【化４】

【００１７】
【化５】

【００１８】
本発明に係るゲル状化合物（湿潤アルコゲル）は、前記の一般式▲３▼〜一般式▲５▼で表されるアルコキシシランからなる群から選択される少なくとも１種、又は前記の一般式▲３▼〜一般式▲５▼で表されるアルコキシシランからなる群から選択される少なくとも１種と前記の一般式▲２▼で表されるアルコキシシランとを含有する混合物を加水分解し、縮重合することによって得られる。
【００１９】
本発明で用いられる前記の一般式▲２▼〜一般式▲４▼で表されるアルコキシシランの具体例を挙げると、２官能アルコキシシランとしては、例えば、ジメチルジメトキシシラン、ジメチルジエトキシシラン、ジフェニルジエトキシシラン、ジフェニルジメトキシシラン、メチルフェニルジエトキシシラン、メチルフェニルジメトキシシラン、ジエチルジエトキシシラン、ジエチルジメトキシシラン等があり、３官能アルコキシシランとしては、例えば、メチルトリメトキシシラン、メチルトリエトキシシラン、エチルトリメトキシシラン、エチルトリエトキシシラン、フェニルトリメトキシシラン、フェニルトリエトキシシラン等があり、４官能アルコキシシランとしては、例えば、テトラメトキシシラン、テトラエトキシシラン等があるが特に、限定されない。
【００２０】
前記の一般式▲５▼で表されるアルコキシシランのオリゴマーとしては、重合度が１０（以下重合度がｎのものはｎ量体と記す。）以下であることが好ましいが、無色透明な液状であれば、これに限定されない。前記アルコキシシランのオリゴマーは、この重合度が均一な化合物である必要はなく、重合度の分布が存在したり分子構造が鎖状、分岐状及び環状で混在していても構わない。物質としての安定性や、ゲル状化合物を作製するための反応時間を考慮すれば、２〜６量体のものが好ましい。前記アルコキシシランのオリゴマー内のＲはアルキル基、フェニル基を表し、中でも、メチル基（−ＣＨ_３ ）、エチル基（−Ｃ_２ Ｈ_５ ）が好ましい。具体的にはメトキシシランのオリゴマーの場合には平均分子量が２５０〜７００、エトキシシランのオリゴマーの場合には平均分子量が３００〜９００のオリゴマーが好ましい。
【００２１】
本発明で前記アルコキシシランを効率よく加水分解し、縮重合を行うためには、このアルコキシシランを含む反応溶液に、予め触媒を添加しておくことが好ましい。このような触媒としては、酸性触媒、塩基性触媒等が挙げられる。具体的に述べると、酸性触媒としては、塩酸、クエン酸、硝酸、硫酸、フッ化アンモニウム等が用いられ、塩基性触媒としては、アンモニア、ピペリジン等が用いられるが、これらに限定されるものではない。
【００２２】
アルコキシシランの加水分解、縮重合に用いられる溶媒としては、通常、原料となるアルコキシシランと水とを均一に溶解混合するために、アルコ−ル、アセトン等が用いられるが、これらに限定されるわけではなく、アルコキシシランと水との両方が溶解しやすい溶媒であればよい。しかし、ゲル状化合物の生成過程の加水分解反応でアルコ−ルが生成すること、また、超臨界乾燥のことを考慮すると、溶媒としては、例えば、エタノール等のアルコールが好ましい。
【００２３】
本発明に係る密度傾斜性エアロゲルは、後述する方法で得たゲル状化合物を疎水化剤との反応により疎水化処理した後に、超臨界乾燥を施して作製される。密度傾斜性エアロゲル作製のためのゲル状化合物の作製方法としては、例えば、次のような方法があるが、これらに限定されるものではない。
【００２４】
図１は第１の実施の形態の概略説明図である。図１（ａ）に示すように、アルコキシシラン若しくはアルコキシシランのみをエタノール等の溶媒に溶解させたアルコキシシラン溶液、又は、水とアンモニア等の触媒若しくは水と触媒とを溶媒に溶解させた溶液を、例えば、フェルト状の多孔質材等のフィルターに含浸させておき、この溶液含浸フィルター１を容器２内で、アルコキシシラン、水及び触媒を溶媒に溶解させた反応物であるゾル３と接触させた状態で、ゾル３をゲル化させることにより、図１（ｂ）に示すように、ゲル状化合物４を得る。すなわち、溶液含浸フィルター１の溶液からゾル３に、シリカの原料となるアルコキシシランが供給されるか、又は、アルコキシシランのシリカへの化学変化を促進させる水及び触媒が供給され、結果的に、溶液含浸フィルター１に接している面に近い程、シリカ濃度が高く、溶液含浸フィルター１に接している面から遠ざかる程、シリカ濃度が低くなるというシリカ濃度分布を有するゲル状化合物４が得られる。従って、このゲル状化合物４を疎水化処理後、超臨界乾燥すると、図３に示すように、溶液含浸フィルター１に接していた当接面に近い程、密度が高く、溶液含浸フィルター１に接している面から遠ざかる反対面に近い程、密度が低くなるという、密度に傾斜性を有するシリカエアロゲルが得られる。
【００２５】
図２は第２の実施の形態の概略説明図である。図２（ａ）に示すように、アルコキシシラン、水及び触媒を溶媒に溶解させた反応物であるゾル３を容器２内に流し込み、図２（ｂ）、図２（ｃ）に示すように、ゾル３をゲル化した直後のゲル５の片面又は両面に前記溶液含浸フィルター１を当接して、さらに所定時間放置して、縮重合反応をさせることにより密度傾斜性のゲル状化合物４を得る。すなわち、溶液含浸フィルター１の溶液からゾル３に、アルコキシシランのシリカへの化学変化を促進させる水及び触媒が供給され、結果的に、溶液含浸フィルター１に接している面である当接面に近い程、シリカ濃度が高く、溶液含浸フィルター１に接している面から遠ざかる反対面に近い程、シリカ濃度が低くなるという、シリカ濃度分布を有するゲル状化合物４が得られる。従って、ゲル５の両面に前記溶液含浸フィルター１を当接した場合には、前記ゲル状化合物４を疎水化処理後、超臨界乾燥すると、図４に示すように、溶液含浸フィルター１に接していた上面及び下面に近い程、密度が高く、溶液含浸フィルター１に接していた面から遠ざかる程、すなわち、中心に近い程、密度が低くなるという、密度に傾斜性を有するシリカエアロゲルが得られる。
【００２６】
また、ゾル調製の際の配合比は限定されず、透明性、断熱性、比表面積、密度、光屈折率など要求される性能によって様々に変化させることが可能である。本発明で作製される密度傾斜性エアロゲルの密度は特に限定されないが、０．０１〜０．５ｇ／ｃｍ^３ であることが好ましく、さらには、０．０４〜０．２５ｇ／ｃｍ^３ であることがより好ましい。すなわち、このエアロゲルの密度が０．０１ｇ／ｃｍ^３ 未満の場合には、ゲル化させるのに多大の時間を要し、あるいは、超臨界乾燥時の収縮が避けられず、あまりに軽量な為、作業者の手や容器、部材など接触するものに付着して割れる等、現実的には取扱いが困難なものとなるといった問題が生じ、０．５ｇ／ｃｍ^３ を越える場合には、エアロゲルの熱伝導率もやや大きくなり、断熱性の点でさほど優れた素材ではなくなり、透光性等の性能も低下し、さらにはエアロゲルの調製が困難となる。したがって、超臨界乾燥後の密度が前記範囲であるようにゾルの配合比が決定される。
【００２７】
得られたゲル状化合物に対しては、超臨界乾燥前に疎水化処理を行う。疎水化剤は、ゲル状化合物が有するシラノール基に対して反応する官能基と疎水基とを有しているものを用いる。シラノール基に対して反応する官能基としては、例えば、ハロゲン、アミノ基、イミノ基、カルボキシル基、アルコキシル基及び水酸基が挙げられる。疎水基としては、例えば、アルキル基、フェニル基及びそれらのフッ化物等が挙げられる。疎水化剤は前記官能基及び疎水基を、それぞれ１種のみを有してもよいし、２種以上を有してもよい。具体的には、ヘキサメチルジシラザン、ヘキサメチルジシロキサン、トリメチルクロロシラン、トリメチルメトキシシラン、トリメチルエトキシシラン、トリエチルエトキシシラン、トリエチルメトキシシラン、ジメチルジメトキシシラン、ジメチルジクロロシラン、ジメチルジエトキシシラン、メチルトリメトキシシラン、メチルトリクロロシラン、エチルトリクロロシラン等の有機シラン化合物が挙げらる。
【００２８】
ここで、疎水化処理は、超臨界乾燥を行う前に予め、液体を媒体として行うか、後述する超臨界乾燥中に超臨界流体を媒体として行う。これらの媒体としては、疎水化剤との反応性が低く、かつ、疎水化剤を溶解するものであればよく、特に限定されない。液体で疎水化処理を行う場合はアルコールなどの溶媒に前記疎水化剤を溶解させたものを疎水化溶液として調製し、この疎水化溶液中にゲル状化合物を浸漬することで行う。また、反応については必要に応じて加熱を行う。
【００２９】
超臨界乾燥を行う際に用いられる溶媒としては、特に限定されないが、例えば、エタノ−ル、メタノ−ル、イソプロパノール、ジクロロジフルオロメタン、二酸化炭素、水等の単独系または２種以上の混合系を挙げることができる。混合系ではなく単一の溶媒で超臨界乾燥を行う場合は、一般的にはオ−トクレ−ブ中に溶媒と、同一の溶媒に溶媒置換を行ったゲル状化合物を一緒に入れ、その溶媒の臨界点以上の温度、圧力まで上昇させた後に溶媒を徐々に除き、最終的に常温常圧の状態に戻すことによって乾燥を終了する。また、２種以上の混合系で超臨界乾燥を行う場合は、乾燥容器内でその混合系での超臨界状態になるよう設定した温度、圧力まで上昇させる方法、乾燥容器内でゲル状化合物の第１の溶媒から超臨界状態にしたい第２の溶媒に置換し、ほぼ溶媒置換を完結させてから、第２の溶媒の超臨界状態で溶媒を除去する方法等がなされている。
【００３０】
この発明のエアロゲルは、非常に微細なシリカ粒子からなる構造体で、その粒子径は光の波長よりもはるかに小さく空隙構造も非常に均質であることから、多孔体であるにもかかわらず透明性を有する。ここで光透過性とは、例えば、可視光波長領域等に対する視覚的な透明性や、赤外領域に対する透過性であるが、これに限定されない。しかも疎水性を有するため耐湿性に優れ、性能や寸法が経時的に安定な材料である。
【００３１】
本発明に係る密度傾斜性エアロゲルの製法は、縮重合性を有するアルコキシシランを加水分解したゾルを、縮重合反応によりゲル化させることによってゲル状化合物とし、このゲル状化合物を疎水化剤との反応により疎水化処理した後に、超臨界乾燥を施してエアロゲルにするエアロゲルの製法において、アルコキシシラン又は加水分解重合を促進させる触媒のいずれか一方のみを含む溶液を含浸させた溶液含浸フィルターと、アルコキシシラン、触媒、水及び溶媒を含む反応物とを当接した状態で、反応物を縮重合反応させることによりゲル状化合物にするので、高密度部分から低密度部分へと密度が徐々に変化する密度傾斜性エアロゲルが得られる。すなわち、少なくとも一方の外殻部の密度が大きいために取り扱い性が良く、しかも低密度部分で断熱性を発現するので、良好な透明性断熱材料となる。また、本発明による密度傾斜性エアロゲルでは、低密度部分と高密度部分の明確な界面が存在しないため、取り扱いによる界面での剥離等による破損もなくなる。
【００３２】
【実施例】
以下、本発明を実施例により具体的に説明する。
【００３３】
以下に、この発明の具体的な実施例及び比較例を示すが、この発明は、下記実施例に限定されるものではない。
【００３４】
（実施例１）
図１に示すように、溶液として、平均分子量４７０のテトラメトキシシランのオリゴマー〔コルコート株式会社製；商品名メチルシリケート５１〕を用い、この溶液をフェルトに含浸させた溶液含浸フィルター１を容器２内に敷いた。
【００３５】
前記テトラメトキシシランのオリゴマーと水と２８重量％のアンモニア水溶液とをエタノールに溶解させたものを、モル比がテトラメトキシシランのオリゴマー：水：アンモニア：エタノール＝１：２０：０．１４：３３の混合比になるように調製し、反応物であるゾル３を得た。このゾル３を前記溶液含浸フィルター１を敷いた容器２内に流し込んだ後、室温下で放置することでゲル状化合物４を得た。次に、このゲル状化合物４を、容器３内から取り出し、さらに溶液含浸フィルター１も取り外した後に、ゲル状化合物４をエタノール洗浄し、その後、ヘキサメチルジシラザン〔トーレダウコーニングシリコーン株式会社製；品番ＳＺ６０７９〕の１．２モル／リットルのエタノール溶液中に浸漬し、疎水化処理を施した。
【００３６】
次に、高圧容器内にこのゲル状化合物を入れ、さらにエタノールを満たした後、この容器内に１８℃、５５ｋｇ／ｃｍ^２ の二酸化炭素を添加し、ゲル内及び容器内のエタノ−ルを二酸化炭素に置換する操作を２〜３時間行った。その後、容器内を二酸化炭素の超臨界条件である、８０℃、１６０ｋｇ／ｃｍ^２ にして、超臨界乾燥（溶媒除去）を２時間行った後に減圧し、ゲル状化合物に含まれるエタノールを除去して、密度傾斜性シリカエアロゲル試料を作製した。
【００３７】
（実施例２）
実施例１において、溶液含浸フィルター１に含浸させる溶液として、テトラメトキシシランのオリゴマーに代えて、１Ｎのアンモニア水を用いて、溶液含浸フィルター１を作製したこと以外は、実施例１と同様にして密度傾斜性シリカエアロゲル試料を作製した。
【００３８】
（実施例３）
図２（ａ）に示すように、テトラメトキシシランのオリゴマーと水と２８重量％のアンモニア水溶液とをエタノールに溶解させたものを、モル比がテトラメトキシシランのオリゴマー：水：アンモニア：エタノール＝１：２０：２．２：１２０の混合比になるように調製し、ゾル３を得た。このゾル３を前記容器２内に流し込んだ後、室温下で放置することで反応物であるゲル５を得た。溶液としてテトラメトキシシランのオリゴマーを用い、この溶液をフェルトに含浸させた溶液含浸フィルター１を図２（ｂ）に示すように、別の容器２内に敷いた。この溶液含浸フィルター１の上に前記ゲル５を当接し、さらに、このゲル５の上にも、前記溶液含浸フィルター１を当接し、さらに室温で放置して、図２（ｃ）に示すように、ゲル状化合物４を得た。
【００３９】
次にこのゲル状化合物４を、ジメチルジメトキシシラン〔トーレダウコーニングシリコーン株式会社製；品番ＡＹ４３−００４〕の２．４モル／リットルエタノール溶液に浸漬し、疎水化処理を施した。さらに、このゲル状化合物を高圧容器内に入れ、エタノールを満たした後、この容器内に二酸化炭素を添加しながら加熱し、８０℃、１６０ｋｇ／ｃｍ^２ にした。この条件で容器内のエタノールを二酸化炭素に置換した後に減圧し、ゲル状化合物４に含まれるエタノールを除去して、密度傾斜性シリカエアロゲル試料を作製した。
【００４０】
（実施例４）
実施例３において、溶液含浸フィルター１に含浸させる溶液として、テトラメトキシシランのオリゴマーに代えて、５Ｎのアンモニア水を用いて、溶液含浸フィルター１を作製したこと以外は、実施例３と同様にして、密度傾斜性シリカエアロゲル試料を作製した。
【００４１】
（比較例１）
テトラメトキシシランのオリゴマーと水と２８重量％のアンモニア水溶液とをエタノールに溶解させたものを、モル比がテトラメトキシシランのオリゴマー：水：アンモニア：エタノール＝１：２０：２．２：１２０の混合比になるように調製し、ゾル３を得て、このゾル３を前記容器２内に流し込んだ後、室温下で放置することでゲル状化合物４を得た以外は、実施例１と同様にして、疎水化処理及び超臨界乾燥を施し、シリカエアロゲル試料を作製した。
【００４２】
（比較例２）
テトラメトキシシランのオリゴマーと水と０．０７５Ｎのアンモニア水溶液とをエタノールに溶解させたものを、モル比がテトラメトキシシランのオリゴマー：水：アンモニア：エタノール＝１：７：０．０２７：３．８の混合比になるように調製し、ゾル３を得て、このゾル３を前記容器２内に流し込んだ後、室温下で放置することでゲル状化合物４を得た以外は、実施例３と同様にして、疎水化処理及び超臨界乾燥を施し、シリカエアロゲル試料を作製した。
【００４３】
実施例１〜実施例４並びに比較例１及び比較例２で得たシリカエアロゲルの密度、可視光透過率、熱伝導率及び曲げ強度を測定した。
【００４４】
ここで、熱伝導率は、英弘精機（株） 製の定常法による熱伝導率測定装置を使用して、ＡＳＴＭ−Ｃ５１８に準拠した方法で、設定温度２０℃と４０℃の条件で測定した。可視光透過率は、１ｃｍ厚み試料について、可視光域の光透過率分布を測定し、可視光透過率をＪＩＳ−Ｒ３１０６に基づいて求めた。
【００４５】
試料の主な内容及び測定結果を表１に示した。
【００４６】
【表１】

【００４７】
表１の結果、実施例の密度傾斜性シリカエアロゲルは、比較例のシリカエアロゲルに比べて、断熱性、光透過性及び機械的強度のバランスに優れており、かつ、取り扱い性も良好なものであった。
【００４８】
本発明によって得られる密度傾斜性シリカエアロゲルは、断熱性等、多孔質材料に特有の機能に優れ、かつ光透過性に優れている。しかも本発明による製法により得られる密度傾斜性シリカエアロゲルは、特性を最大限に生かしながら、取り扱い性等の強度も十分に有するものが得られる。
【００４９】
本発明によって得られるエアロゲルは、例えば、太陽光集熱に有用な光透過性断熱材、または、音響材料、触媒担体、チェレンコフカウンター媒体等の様々な用途に用いることができる。
【００５０】
【発明の効果】
本発明の請求項１乃至請求項５に係る密度傾斜性エアロゲルの製法は、アルコキシシラン又は加水分解重合を促進させる触媒のいずれか一方のみを含む溶液を含浸させた溶液含浸フィルターと、アルコキシシラン、触媒、水及び溶媒を含む反応物とを当接した状態で、反応物を縮重合反応させることによりゲル状化合物にするので、本発明の請求項１乃至請求項５に係るエアロゲルの製法によると、高密度部分から低密度部分へと密度が徐々に変化する密度傾斜性エアロゲルが得られ、少なくとも一方の外殻部の密度が大きいために取り扱い性が良く、しかも低密度部分で断熱性を発現するので、良好な透明性断熱材料となるとともに、低密度部分と高密度部分の明確な界面が存在しないため、取り扱いによる界面での剥離等による破損が低減される。すなわち、透明性、断熱性に優れ、かつ、取り扱い性良好な強度を有する密度傾斜性エアロゲルが得られる。
【図面の簡単な説明】
【図１】本発明の第１の実施の形態に係る密度傾斜性ゲル状化合物の作製方法の概略説明図である。
【図２】本発明の第２の実施の形態に係る密度傾斜性ゲル状化合物の作製方法の概略説明図である。
【図３】本発明の第１の実施の形態に係る密度傾斜性シリカエアロゲルの密度分布図である。
【図４】本発明の第２の実施の形態に係る密度傾斜性シリカエアロゲルの密度分布図である。
【符号の説明】
１ 溶液含浸フィルター
２ 溶液
３ ゾル
４ ゲル状化合物
５ ゲル[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a density gradient aerogel in which a density distribution composed of a porous skeleton of silica is present, and in particular, for example, a lighting property to be filled between a transparent glass plate or a hollow container, The present invention relates to a method for producing a density gradient airgel that can be used for various uses such as a filling material constituting a high-strength transparent heat-insulating layer having excellent light transmittance, transparency and heat insulating properties.
[0002]
[Prior art]
Conventionally, an airgel made of silica has been known as a material having a low thermal conductivity and light transmissivity. This aerogel is obtained by hydrolyzing and polymerizing alkoxysilane (also referred to as silicon alkoxide, alkyl silicate, etc.) as disclosed in US Pat. Nos. 4,402,927, 4,432,956 and 4,610,863. The wet gel compound comprising the skeleton is dried in the supercritical state above the critical point of this solvent in the presence of a solvent (dispersion medium) such as alcohol or liquefied carbon dioxide, and if necessary, Thereafter, it can be manufactured by further performing a heat treatment at about 500 to 750 ° C. or higher. Further, as in US Pat. Nos. 5,137,927 and 5,124,364, a gel-like compound can be similarly obtained using sodium silicate as a raw material, and the gel-like compound can be produced by supercritical drying. The airgel obtained by such a manufacturing method is a material useful as, for example, a heat-insulating material having light transmittance.
[0003]
However, such aerogels are very lightweight, have low strength and are brittle, and thus have the drawbacks that they are liable to crack or break and are very difficult to handle.
[0004]
[Problems to be solved by the invention]
Therefore, the present inventors have focused on the fact that aerogels having higher densities generally have higher strength, and have succeeded in producing high-density aerogels as disclosed in Japanese Patent Application No. 7-105299. However, when the density of the airgel is too high, there is a problem that the essential heat insulating properties and transparency tend to be impaired. In addition, the inventors have invented an airgel having a sandwich structure of a high-density portion and a low-density portion as disclosed in Japanese Patent Application No. 6-216870. There is a problem that peeling is likely to occur at the interface of.
[0005]
The present invention has been made in view of the above-mentioned facts, and it is an object of the present invention to provide a method for producing a density gradient airgel having excellent transparency, excellent heat insulation, and good strength in handling. .
[0006]
[Means for Solving the Problems]
In the method for producing a density gradient airgel according to claim 1 of the present invention, a sol obtained by hydrolyzing an alkoxysilane having polycondensability is gelled by a polycondensation reaction to form a gel-like compound. In the method for producing an airgel, which is subjected to supercritical drying after being subjected to a hydrophobizing treatment by reaction with an agent, a solution impregnation in which a solution containing only one of an alkoxysilane and a catalyst for promoting hydrolysis polymerization is impregnated. A gel-like compound is obtained by subjecting the reaction product to a condensation polymerization reaction in a state where the filter is in contact with a reaction product containing an alkoxysilane, a catalyst, water and a solvent.
[0007]
The method for producing a gradient airgel according to claim 2 of the present invention is characterized in that the reactant is a sol.
[0008]
The method for producing a gradient airgel according to claim 3 of the present invention is characterized in that the reactant is a gel.
[0009]
The method for producing a gradient airgel according to claim 4 of the present invention is characterized in that the solution-impregnated filter is brought into contact with both surfaces of the gel.
[0010]
The method for producing a density gradient airgel according to claim 5 of the present invention is characterized in that the catalyst is ammonia.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail.
[0012]
The alkoxysilane used in the present invention is an alkoxysilane represented by the following general formula (1), and more specifically, a bifunctional alkoxysilane represented by the following general formula (2), Examples include trifunctional alkoxysilanes represented by the general formula (3), tetrafunctional alkoxysilanes represented by the following general formula (4), and oligomers of alkoxysilanes represented by the following general formula (5). .
[0013]
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[0014]
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[0015]
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[0016]
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[0017]
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[0018]
The gel compound (wet alcogel) according to the present invention is at least one selected from the group consisting of alkoxysilanes represented by the general formulas (3) to (5) or the general formula (3). A mixture containing at least one selected from the group consisting of alkoxysilanes represented by formulas (1) to (5) and the alkoxysilanes represented by formula (2) is hydrolyzed and polycondensed. Obtained by:
[0019]
Specific examples of the alkoxysilanes represented by the general formulas (2) to (4) used in the present invention include, for example, dimethyldimethoxysilane, dimethyldiethoxysilane, and diphenyl. Diethoxysilane, diphenyldimethoxysilane, methylphenyldiethoxysilane, methylphenyldimethoxysilane, diethyldiethoxysilane, diethyldimethoxysilane, and the like.Examples of trifunctional alkoxysilanes include, for example, methyltrimethoxysilane, methyltriethoxysilane, Ethyltrimethoxysilane, ethyltriethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, etc., and as the tetrafunctional alkoxysilane, for example, tetramethoxysilane, tetraethoxysilane, etc. In particular, but are not limited.
[0020]
The oligomer of the alkoxysilane represented by the above general formula (5) preferably has a degree of polymerization of 10 or less (hereinafter, those having a degree of polymerization of n are referred to as n-mers). However, it is not limited to this. The oligomer of the alkoxysilane need not be a compound having a uniform degree of polymerization, and may have a distribution of the degree of polymerization or may have a mixed molecular structure of a chain, a branch, and a ring. In consideration of the stability as a substance and the reaction time for producing a gel compound, a dimer to hexamer is preferable. R in the oligomer of the alkoxysilane represents an alkyl group or a phenyl group, and among them, a methyl group (—CH ₃ ) and an ethyl group (—C ₂ H ₅ ) are preferable. Specifically, in the case of an oligomer of methoxysilane, an oligomer having an average molecular weight of 250 to 700 is preferable, and in the case of an oligomer of ethoxysilane, an oligomer having an average molecular weight of 300 to 900 is preferable.
[0021]
In order to efficiently hydrolyze the alkoxysilane and carry out polycondensation in the present invention, it is preferable to add a catalyst in advance to the reaction solution containing the alkoxysilane. Examples of such a catalyst include an acidic catalyst and a basic catalyst. Specifically, as the acidic catalyst, hydrochloric acid, citric acid, nitric acid, sulfuric acid, ammonium fluoride and the like are used, and as the basic catalyst, ammonia, piperidine and the like are used, but not limited thereto. Absent.
[0022]
As a solvent used for hydrolysis and condensation polymerization of alkoxysilane, alcohol, acetone and the like are usually used in order to uniformly dissolve and mix alkoxysilane as a raw material and water, but are not limited thereto. However, the solvent may be any solvent in which both the alkoxysilane and water are easily dissolved. However, considering that alcohol is produced by a hydrolysis reaction in the process of producing a gel-like compound and that supercritical drying is performed, an alcohol such as ethanol is preferable as the solvent.
[0023]
The density gradient aerogel according to the present invention is produced by subjecting a gel compound obtained by a method described below to a hydrophobizing treatment by a reaction with a hydrophobizing agent and then performing supercritical drying. Examples of a method for producing a gel compound for producing a density gradient airgel include the following methods, but are not limited thereto.
[0024]
FIG. 1 is a schematic explanatory diagram of the first embodiment. As shown in FIG. 1A, an alkoxysilane solution in which alkoxysilane or only alkoxysilane is dissolved in a solvent such as ethanol, or a solution in which water and a catalyst such as ammonia or a solution in which water and a catalyst are dissolved in a solvent is used. For example, a filter such as a felt-like porous material is impregnated, and the solution-impregnated filter 1 is brought into contact with a sol 3 which is a reactant obtained by dissolving alkoxysilane, water and a catalyst in a solvent in a container 2. In this state, the sol 3 is gelled to obtain a gel compound 4 as shown in FIG. 1 (b). That is, from the solution of the solution impregnated filter 1, to the sol 3, the alkoxysilane as a raw material of silica is supplied, or water and a catalyst for promoting a chemical change of the alkoxysilane to silica are supplied. As a result, A gel compound 4 having a silica concentration distribution is obtained in which the silica concentration is higher as the surface is in contact with the solution-impregnated filter 1 and the silica concentration is lower as the surface is in contact with the solution-impregnated filter 1. Therefore, when the gelled compound 4 is subjected to a hydrophobizing treatment and then subjected to supercritical drying, as shown in FIG. 3, the closer to the contact surface that was in contact with the solution-impregnated filter 1, the higher the density, and the more contact with the solution-impregnated filter 1. Silica airgel having a gradient in density is obtained, in which the density decreases as the distance from the opposite surface increases.
[0025]
FIG. 2 is a schematic explanatory diagram of the second embodiment. As shown in FIG. 2 (a), a sol 3 which is a reactant obtained by dissolving alkoxysilane, water and a catalyst in a solvent is poured into a container 2, and as shown in FIGS. 2 (b) and 2 (c). The solution impregnated filter 1 is brought into contact with one side or both sides of the gel 5 immediately after the sol 3 is gelled, and further left for a predetermined time to cause a condensation polymerization reaction to obtain a gel compound 4 having a density gradient. . That is, water and a catalyst that promote the chemical change of alkoxysilane to silica are supplied from the solution of the solution impregnated filter 1 to the sol 3, and as a result, the contact surface that is in contact with the solution impregnated filter 1 is A gel-like compound 4 having a silica concentration distribution is obtained, in which the silica concentration is higher as the surface is closer and the silica concentration is lower as the surface is further away from the surface in contact with the solution-impregnated filter 1. Therefore, when the solution-impregnated filter 1 is brought into contact with both surfaces of the gel 5, the gel-like compound 4 is subjected to a hydrophobizing treatment and then subjected to supercritical drying, as shown in FIG. The higher the density is, the closer the surface is to the upper surface and the lower surface, the higher the density is.
[0026]
Further, the compounding ratio at the time of preparing the sol is not limited, and can be variously changed depending on required properties such as transparency, heat insulation, specific surface area, density, and light refractive index. The density of the gradient airgel produced in the present invention is not particularly limited, but is preferably 0.01 to 0.5 g / cm ³ , and more preferably 0.04 to 0.25 g / cm ^3. Is more preferred. That is, when the density of this aerogel is less than 0.01 g / cm ³ , it takes a lot of time to gel, or shrinkage during supercritical drying is inevitable, and the airgel is too light. In actuality, it becomes difficult to handle, for example, by sticking to the hands, containers, members, etc. that come into contact with the objects, such as cracks, etc. If it exceeds 0.5 g / cm ³ , the heat conduction of the aerogel The ratio also becomes somewhat large, the material is not so excellent in terms of heat insulation, the performance such as translucency is reduced, and the preparation of aerogel becomes difficult. Therefore, the mixing ratio of the sol is determined so that the density after supercritical drying is within the above range.
[0027]
The obtained gel compound is subjected to a hydrophobic treatment before supercritical drying. As the hydrophobizing agent, one having a functional group that reacts with the silanol group of the gel compound and a hydrophobic group is used. Examples of the functional group that reacts with the silanol group include a halogen, an amino group, an imino group, a carboxyl group, an alkoxyl group, and a hydroxyl group. Examples of the hydrophobic group include an alkyl group, a phenyl group, and a fluoride thereof. The hydrophobizing agent may have only one type of the functional group and the hydrophobic group, or may have two or more types. Specifically, hexamethyldisilazane, hexamethyldisiloxane, trimethylchlorosilane, trimethylmethoxysilane, trimethylethoxysilane, triethylethoxysilane, triethylmethoxysilane, dimethyldimethoxysilane, dimethyldichlorosilane, dimethyldiethoxysilane, methyltrimethoxysilane Examples include organic silane compounds such as silane, methyltrichlorosilane, and ethyltrichlorosilane.
[0028]
Here, the hydrophobization treatment is performed in advance using a liquid as a medium before performing supercritical drying, or using a supercritical fluid as a medium during supercritical drying described later. These media are not particularly limited as long as they have low reactivity with the hydrophobizing agent and dissolve the hydrophobizing agent. When performing the hydrophobic treatment with a liquid, a solution obtained by dissolving the hydrophobic agent in a solvent such as alcohol is prepared as a hydrophobic solution, and the gel-like compound is immersed in the hydrophobic solution. In addition, heating is performed as necessary for the reaction.
[0029]
The solvent used when performing supercritical drying is not particularly limited. For example, a single system or a mixture of two or more of ethanol, methanol, isopropanol, dichlorodifluoromethane, carbon dioxide, and water is used. Can be mentioned. When supercritical drying is performed using a single solvent instead of a mixed system, generally, a solvent and a gel-like compound that has been subjected to solvent replacement in the same solvent are put together in an autoclave, and the solvent is added. After the temperature and the pressure are raised to the critical point or higher, the solvent is gradually removed, and finally the state is returned to the normal temperature and normal pressure state, thereby completing the drying. When supercritical drying is performed in a mixed system of two or more types, a method in which the temperature and pressure are set to a supercritical state in the mixed system in a drying container, and a method in which the gel-like compound is There is a method in which the first solvent is replaced with a second solvent to be brought into a supercritical state, the solvent is almost completely replaced, and then the solvent is removed in a supercritical state of the second solvent.
[0030]
The aerogel of the present invention is a structure composed of very fine silica particles, the particle size of which is much smaller than the wavelength of light, and the void structure is also very homogeneous, so that the airgel is transparent despite being porous. Has the property. Here, the light transmittance is, for example, visual transparency in a visible light wavelength region or the like, or transmittance in an infrared region, but is not limited thereto. Moreover, since it has hydrophobicity, it is excellent in moisture resistance, and is a material whose performance and dimensions are stable over time.
[0031]
The method for producing a density gradient airgel according to the present invention is a method in which a sol obtained by hydrolyzing an alkoxysilane having polycondensability is gelled by a polycondensation reaction to form a gel-like compound. After a hydrophobizing treatment by a reaction, in a method of producing an aerogel by performing supercritical drying and aerogel, a solution impregnated filter impregnated with a solution containing only one of alkoxysilane or a catalyst for promoting hydrolysis polymerization, In the state where the reactant containing the silane, the catalyst, water and the solvent is in contact with the reactant, the reactant is converted into a gel-like compound by a condensation polymerization reaction, so that the density gradually changes from a high-density portion to a low-density portion. A density gradient airgel is obtained. That is, since the density of at least one of the outer shell portions is large, the handleability is good, and the heat insulating property is exhibited in the low density portion, so that a good transparent heat insulating material is obtained. Further, in the density gradient airgel according to the present invention, since there is no clear interface between the low-density portion and the high-density portion, damage due to separation at the interface due to handling is also eliminated.
[0032]
【Example】
Hereinafter, the present invention will be described specifically with reference to examples.
[0033]
Hereinafter, specific examples and comparative examples of the present invention will be described, but the present invention is not limited to the following examples.
[0034]
(Example 1)
As shown in FIG. 1, an oligomer of tetramethoxysilane having an average molecular weight of 470 [manufactured by Colcoat Co., Ltd .; trade name: methylsilicate 51] was used as a solution, and a solution-impregnated filter 1 in which this solution was impregnated with felt was placed in a container 2. Laid.
[0035]
A solution obtained by dissolving the oligomer of tetramethoxysilane, water, and an aqueous ammonia solution of 28% by weight in ethanol has a molar ratio of oligomer of tetramethoxysilane: water: ammonia: ethanol = 1: 20: 0.14: 33. The mixture was adjusted to have a mixing ratio to obtain a sol 3 as a reaction product. The sol 3 was poured into the vessel 2 on which the solution-impregnated filter 1 was spread, and then left at room temperature to obtain a gel compound 4. Next, the gel compound 4 is taken out of the container 3, and after removing the solution impregnated filter 1, the gel compound 4 is washed with ethanol, and then hexamethyldisilazane [manufactured by Toray Dow Corning Silicone Co., Ltd .; [Part number SZ6079] was immersed in a 1.2 mol / liter ethanol solution, and subjected to a hydrophobic treatment.
[0036]
Next, this gel compound is put in a high-pressure vessel, and after filling with ethanol, carbon dioxide at 18 ° C. and 55 kg / cm ² is added to the vessel, and the ethanol in the gel and in the vessel is removed. The operation of replacing with carbon was performed for 2 to 3 hours. After that, the inside of the container was set to 80 ° C. and 160 kg / cm ² , which is the supercritical condition of carbon dioxide, and subjected to supercritical drying (removal of solvent) for 2 hours, followed by pressure reduction to remove ethanol contained in the gel compound. Thus, a density gradient silica airgel sample was prepared.
[0037]
(Example 2)
In Example 1, in the same manner as in Example 1 except that the solution impregnated filter 1 was prepared using 1N ammonia water instead of the tetramethoxysilane oligomer as the solution to be impregnated in the solution impregnated filter 1. A density gradient silica airgel sample was prepared.
[0038]
(Example 3)
As shown in FIG. 2 (a), a tetramethoxysilane oligomer, water and a 28% by weight aqueous ammonia solution dissolved in ethanol, and the molar ratio of tetramethoxysilane oligomer: water: ammonia: ethanol = 1 : 20: 2.2: 120 to obtain a sol 3. The sol 3 was poured into the container 2 and then left at room temperature to obtain a gel 5 as a reaction product. As a solution, an oligomer of tetramethoxysilane was used, and a solution-impregnated filter 1 in which this solution was impregnated with felt was placed in another container 2 as shown in FIG. The gel 5 is brought into contact with the solution-impregnated filter 1, and the solution-impregnated filter 1 is also brought into contact with the gel 5 and left at room temperature, as shown in FIG. Thus, a gel compound 4 was obtained.
[0039]
Next, this gel compound 4 was immersed in a 2.4 mol / liter ethanol solution of dimethyldimethoxysilane [manufactured by Toray Dow Corning Silicone Co., Ltd .; product number AY43-004] to perform a hydrophobic treatment. The gelled compound was placed in a high-pressure container, filled with ethanol, and then heated while adding carbon dioxide to the container to reach 80 ° C. and 160 kg / cm ² . After replacing the ethanol in the container with carbon dioxide under these conditions, the pressure was reduced and the ethanol contained in the gel compound 4 was removed to prepare a density gradient silica airgel sample.
[0040]
(Example 4)
In Example 3, in the same manner as in Example 3 except that the solution impregnated filter 1 was prepared by using 5 N ammonia water instead of the tetramethoxysilane oligomer as the solution to be impregnated in the solution impregnated filter 1. , A gradient silica airgel sample was prepared.
[0041]
(Comparative Example 1)
A mixture of an oligomer of tetramethoxysilane, water and a 28% by weight aqueous ammonia solution dissolved in ethanol, and a molar ratio of oligomer of tetramethoxysilane: water: ammonia: ethanol = 1: 20: 2.2: 120. In the same manner as in Example 1 except that the sol 3 was prepared so as to have a ratio, and the sol 3 was poured into the container 2 and then left at room temperature to obtain a gel compound 4. Then, a hydrophobic treatment and supercritical drying were performed to prepare a silica airgel sample.
[0042]
(Comparative Example 2)
A solution obtained by dissolving an oligomer of tetramethoxysilane, water and an aqueous 0.075N ammonia solution in ethanol, and having a molar ratio of oligomer of tetramethoxysilane: water: ammonia: ethanol = 1: 7: 0.027: 3.8. The sol 3 was prepared in such a manner that the mixing ratio of the sol 3 was obtained. The sol 3 was poured into the container 2 and then left at room temperature to obtain a gel-like compound 4. Similarly, a hydrophobic treatment and supercritical drying were performed to prepare a silica airgel sample.
[0043]
The density, visible light transmittance, thermal conductivity, and bending strength of the silica airgel obtained in Examples 1 to 4 and Comparative Examples 1 and 2 were measured.
[0044]
Here, the thermal conductivity was measured at a set temperature of 20 ° C. and 40 ° C. by a method based on ASTM-C518, using a thermal conductivity measuring device by a steady method manufactured by Eiko Seiki Co., Ltd. The visible light transmittance was obtained by measuring the light transmittance distribution in the visible light range of a 1 cm thick sample, and determining the visible light transmittance based on JIS-R3106.
[0045]
Table 1 shows the main contents of the samples and the measurement results.
[0046]
[Table 1]

[0047]
As a result of Table 1, the density gradient silica aerogel of the example is excellent in the balance of heat insulation, light transmission and mechanical strength, and has good handleability as compared with the silica aerogel of the comparative example. there were.
[0048]
The density gradient silica airgel obtained by the present invention has excellent functions such as heat insulation and the like, which are unique to porous materials, and also has excellent light transmittance. In addition, the gradient silica aerogel obtained by the production method according to the present invention can be one having sufficient strength such as handleability while maximizing the properties.
[0049]
The airgel obtained by the present invention can be used for various applications such as a light-transmitting heat insulating material useful for solar heat collection, an acoustic material, a catalyst carrier, and a Cherenkov counter medium.
[0050]
【The invention's effect】
The method for producing a gradient airgel according to any one of claims 1 to 5 of the present invention includes a solution impregnated filter impregnated with a solution containing only one of an alkoxysilane and a catalyst for promoting hydrolysis polymerization, In a state where the reactant containing the catalyst, water and the solvent is in contact with the reactant, the reactant is subjected to a polycondensation reaction to form a gel-like compound. Therefore, according to the method for producing an airgel according to claims 1 to 5 of the present invention, , A density gradient aerogel whose density gradually changes from a high-density part to a low-density part is obtained, and because at least one of the outer shell parts has a large density, it is easy to handle and expresses heat insulation in the low-density part As a result, the material becomes a good transparent heat-insulating material, and there is no clear interface between the low-density part and the high-density part. It is. That is, it is possible to obtain a density gradient airgel having excellent transparency and heat insulating properties and having strength with good handleability.
[Brief description of the drawings]
FIG. 1 is a schematic explanatory view of a method for producing a density gradient gel-like compound according to a first embodiment of the present invention.
FIG. 2 is a schematic explanatory view of a method for producing a density gradient gel-like compound according to a second embodiment of the present invention.
FIG. 3 is a density distribution diagram of a density gradient silica airgel according to the first embodiment of the present invention.
FIG. 4 is a density distribution diagram of a density gradient silica airgel according to a second embodiment of the present invention.
[Explanation of symbols]
1 solution impregnated filter 2 solution 3 sol 4 gel-like compound 5 gel

Claims (5)

The sol obtained by hydrolyzing the alkoxysilane having polycondensability is gelled by a polycondensation reaction to form a gel-like compound, and the gel-like compound is hydrophobized by a reaction with a hydrophobizing agent. In the method for producing an airgel to be subjected to aerogel, a solution impregnation filter impregnated with a solution containing only one of alkoxysilane or a catalyst for promoting hydrolysis polymerization, and a reaction product containing an alkoxysilane, a catalyst, water and a solvent. A method for producing a density gradient aerogel, wherein a gelled compound is obtained by subjecting a reaction product to a polycondensation reaction in a state where the aerogel is in contact with the airgel. The method for producing a gradient airgel according to claim 1, wherein the reactant is a sol. The method for producing a gradient airgel according to claim 1, wherein the reactant is a gel. The method for producing a density gradient aerogel according to claim 3, wherein the solution-impregnated filter is brought into contact with both surfaces of the gel. The method for producing a density gradient airgel according to any one of claims 1 to 4, wherein the catalyst is ammonia.

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