JP2004347091A - Heat insulating material and heat insulating body using the same - Google Patents

Abstract

A heat insulating material having excellent heat insulating properties is provided.
SOLUTION: While using silica,
(A) making the pore volume of the silica 1.6 ml / g or more;
(B) containing a specific element selected from the group consisting of elements of the fifth and sixth periods of the periodic table at a concentration of 1 wt ppm or more and 30 wt% or less;
(C) The bulk density of the heat insulating material is 0.5 g / ml or less.
[Selection diagram] None

Description

【００３３】
本発明のシリカは、固体Ｓｉ−ＮＭＲ測定によるＱ^４／Ｑ^３の値が、通常１．２以上、中でも１．４以上であることが好ましい。ここで、Ｑ^４／Ｑ^３の値とは、上述したシリカの繰り返し単位の中で、−ＯＳｉが３個結合したＳｉ（Ｑ^３）に対する−ＯＳｉが４個結合したＳｉ（Ｑ^４）のモル比を意味する。なお、上限値は特に制限されないが、通常は１０以下である。一般にこの値が高い程、シリカの熱安定性が高いことが知られており、ここから本発明のシリカが熱安定性に極めて優れていることが判る。
【００３４】
なお、Ｑ^４／Ｑ^３の値は、固体Ｓｉ−ＮＭＲ測定を行ない、その結果に基づいて算出することができる。また、測定データの解析（ピーク位置の決定）は、例えば、ガウス関数を使用した波形分離解析等により、各ピークを分割して抽出する方法で行なう。
【００３５】
本発明の断熱材は、上述のシリカの特徴に加えて、周期表の第５周期及び第６周期の元素からなる群より選ばれる特定の元素（以下適宜、単に「特定元素」という）を含有することを、最大の特徴としている。
【００３６】
ここで、本発明の原理について説明する。
一般に、２つの原子が結合している部分では、それらの原子間の原子量の差が大きいほど、熱の伝導が妨げられ、それらの原子間を熱が伝わる際の熱伝導度が小さくなる傾向がある。したがって、シリカの基本的な骨格を形成する酸素及びケイ素の原子量よりも大幅に大きな原子量を有する元素、即ち、周期表の第５周期及び第６周期に属する特定元素を、上述のシリカの内部又は周囲に配置することによって、この特定元素が熱伝導を阻止する機能を発揮することとなる。つまり、ケイ素の原子量と特定元素の原子量との差、又は、酸素の原子量と特定元素の原子量との差が大きいことにより、ケイ素と特定元素との間、又は、酸素と特定元素との間で熱伝導が妨げられるので、本発明の断熱材は、純粋にシリカのみで構成された断熱材と比較して、より優れた断熱性を奏することができるのである。
【００３７】
特定元素の具体例としては、Ｒｂ，Ｓｒ，Ｙ，Ｚｒ，Ｎｂ，Ｍｏ，Ｔｃ，Ｒｕ，Ｒｈ，Ｐｄ，Ａｇ，Ｃｄ，Ｉｎ，Ｓｎ，Ｓｂ，Ｔｅ，Ｉ，Ｃｓ，Ｂａ，ランタノイド類（Ｌａ，Ｃｅ，Ｐｒ，Ｎｄ，Ｐｍ，Ｓｍ，Ｅｕ，Ｇｄ，Ｔｂ，Ｄｙ，Ｈｏ，Ｅｒ，Ｔｍ，Ｙｂ，Ｌｕ），Ｈｆ，Ｔａ，Ｗ，Ｒｅ，Ｏｓ，Ｉｒ，Ｐｔ，Ａｕ，Ｈｇ，Ｔｌ，Ｐｄ，Ｂｉ，Ｐｏが挙げられ、好ましくはＰｂ，Ｒｂ，Ｂａ，Ｚｒ，Ｓｒ，Ｗ，Ｃｅ，Ｙ，Ｎｄ，Ｎｂ，Ｌａが挙げられ、より好ましくはＰｂ，Ｒｂ，Ｂａ，Ｚｒ，Ｓｒ，Ｗが挙げられる。また、これら特定元素は、１種を単独で用いてもよく、２種以上を任意の割合及び任意の組み合わせで併用してもよい。
【００３８】
本発明の断熱材は、上述の特定元素を、通常１重量ｐｐｍ以上、好ましくは２重量ｐｐｍ以上、更に好ましくは３重量ｐｐｍ以上、また、通常３０重量％以下、好ましくは２５重量％以下、更に好ましくは２０重量％以下の比率で含有している。特定元素の比率がこれよりも小さいと断熱材が充分な断熱性を発揮することができない虞があり、また、特定元素の比率がこれよりも大きいと、断熱材が有する空隙の体積が小さくなって断熱性が低下するためである。
【００３９】
より詳細には、特定元素の含有率の範囲は、使用される特定元素の種類により異なる。即ち、通常は特定元素の原子量が大きいほどシリカが含有する特定元素の物質量の割合は小さいことが好ましく、特定元素の原子量が小さいほどシリカが含有する特定元素の物質量は大きいことが好ましい。ただし、特定元素の含有率として重量比率に注目すると、同じ物質量で比較した場合には原子量が大きい特定元素ほどその重量も大きくなるのであるから、本発明の断熱材に用いる特定元素の含有率を重量比率で表わす場合には、特定元素の原子量と好ましい物質量とを考慮して決定すべきである。具体例としては、特定元素として原子量が大きいＰｂを用いる場合には、含有率は通常１重量ｐｐｍ以上、通常１０重量％以下である。また、特定元素として原子量が小さいＺｒを用いる場合には、含有率は通常１０重量％以上、通常３０重量％以下である。なお、特定元素として複数種のものを用いる場合は、用いる特定元素全体の平均原子量に応じて含有率を決定する。また、特定元素の含有率を測定する方法は任意であるが、例えば、金属分析に利用される公知の方法のほか、蛍光Ｘ線分析などにより測定することができる。
【００４０】
また、特定元素の含有率の範囲は、本発明の断熱材中における特定元素の存在状態によっても異なる。以下、本発明の断熱材中における特定元素の存在状態を説明し、特定元素の含有率についても説明する。
【００４１】
本発明の断熱材中における特定元素の存在状態は特に制限されず、任意の状態で存在していて構わない。特定元素の存在状態の類型としては、上述のシリカの骨格内に取り込まれた状態（図１（ａ））、シリカの粒子間に挟まれた状態（図１（ｂ））、シリカの細孔内に担持された状態（図２（ａ））、シリカの粒子の周囲に付着した状態（図２（ｂ））などが挙げられるが、これらの何れであっても良い。勿論、これらの特定元素の存在状態はあくまでも類型であり、その他の状態で特定元素が存在していても良い。また、二以上の存在状態が混在していても構わない。
【００４２】
但し、本発明では、少なくとも特定元素の一部が、断熱材の熱伝導路中に存在することが望まれる。ここで、断熱材の熱伝導路とは、断熱材の空隙以外の部分、即ち、主にシリカの骨格によって形成される断熱材の固体部分を指す。一般に、断熱材の空隙は減圧されて少量の気体が存在する状態又は真空状態とされているため、断熱材を伝わる熱は、主にシリカの骨格によって形成される断熱材の固体部分、即ち断熱材の熱伝導路を伝わることになる。このとき、断熱材の熱伝導路中に特定元素が存在すれば、特定元素による熱伝導阻止機能が有効に発揮されることとなり、効果的に断熱性を高めることができる。
【００４３】
上述の類型で言えば、特定元素がシリカの骨格内に取り込まれた状態（図１（ａ））や、シリカの粒子間に挟まれた状態（図１（ｂ））で存在すれば、特定元素が断熱材の熱伝導路中に位置しているので、特定元素による熱伝導阻止機能が有効に発揮されると考えられる。一方、シリカの細孔内に担持された状態（図２（ａ））や、シリカの粒子の周囲に付着した状態（図２（ｂ））で存在する場合には、特定元素が断熱材の熱伝導路中に位置しておらず、特定元素による熱伝導阻止機能は十分に発揮されないと考えられる。よって、特定元素の少なくとも一部、好ましくはその大部分が、図１（ａ）（ｂ）の類型に代表されるように、断熱材の熱伝導路中に位置する状態で存在することが望まれる。
なお、特定元素が断熱材の熱伝導路中に存在することは、電子プローブ微小分析法により確認することができる。
【００４４】
また、特定元素が断熱材の熱伝導路中に存在する場合でも、特に、特定元素がシリカの骨格中、即ち、シリカのＳｉ−Ｏ骨格中に存在することが好ましい。断熱材を熱が伝わる場合には、熱は主にＳｉ−Ｏ骨格を伝わることになるため、特定元素がシリカの骨格中に存在すれば、より確実に特定元素が熱の伝わる経路上に位置することができるようになり、したがって、より効果的に断熱性を高めることができる。
なお、特定元素がシリカの骨格中に存在することは、広域Ｘ線吸収微細構造（ＥＸＡＦＳ）により確認することができる。
【００４５】
なお、特定元素がシリカの骨格中に存在する場合、Ｓｉ−Ｏ−Ｍ又はＳｉ−Ｏ−Ｏ−Ｍ（Ｍは特定元素を表わす）などで表わされるように、特定元素がシリカの骨格中の酸素原子と結合していることが好ましい。これは、ＳｉとＯとの原子量の差よりも、特定元素とＯとの原子量の差の方が大きいため、その結果、特定元素による熱伝導阻止機能がより効果的に発揮されるからである。
【００４６】
ここで、特定元素を断熱材の熱伝導路中に存在させる方法は、特に制限されないが、具体例としては、（ｉ）シリカを製造する際に、特定元素の原子単体又は化合物を共存させる方法、（ｉｉ）シリカを製造した後、特定元素の原子単体又は化合物と混合して組成物とする方法などが挙げられる（これらの手法の詳細については後述する）。（ｉ）又は（ｉｉ）の何れの手法を用いた場合でも、特定元素は様々な位置で存在することになるが、（ｉ）の手法を用いた場合には、図１（ａ）の類型に示されるように、シリカの骨格中に取り込まれる特定元素の割合が高くなり、（ｉｉ）の手法を用いた場合には、図１（ｂ）の類型に示されるように、シリカの粒子間に挟まれて存在する特定元素の割合が高くなる。従って、（ｉ）の手法を用いる方が、より効率的に断熱性能を高めることができるが、（ｉｉ）の手法を用いる場合でも、特定元素の原子単体又は化合物の存在量を高くすることにより、同様の断熱性能を得ることができる。
【００４７】
具体的に、（ｉ）の手法を用いる場合、シリカが含む特定元素の原子単体又は化合物の含有率は比較的低くてもよく、具体的には通常１重量ｐｐｍ〜５重量％である。また、（ｉｉ）の手法を用いる場合、特定元素の原子単体又は化合物の含有率は比較的高くする必要があり、具体的には通常１重量％〜３０重量％である。
【００４８】
特定元素の化合物を用いる場合、その化合物の種類は特に制限されないが、例としては、特定元素のハロゲン化物、酸化物、水酸化物、硫化物、錯体など、或いは、特定元素の硫酸塩、炭酸塩等の各種の塩などが挙げられる。これらの化合物の中でも、特定元素−酸素結合を有している化合物が好ましく、特に酸化物が好ましい。
【００４９】
なお、特定元素の単体又はその化合物は、室温における電気伝導度が、通常２．５×１０^−３Ｓ／ｍよりも小さく、好ましくは２．０×１０^−３Ｓ／ｍ以下、より好ましくは１．５×１０^−３Ｓ／ｍ以下であることが望まれる。したがって、上述した特定元素の化合物の中でもＳｎＯ，ＨｇＴｅ，ＳｎＴｅ，ＣｄＴｅ等を主成分とする化合物は、２．５×１０^−３Ｓ／ｍ以上の電気伝導度を有することがあるため、通常はその使用を避けるべきである。一般に、電気伝導度が大きい化合物は価電子帯の最高エネルギーと導電帯の最低エネルギーとの差、即ち、いわゆるバンドギャップが小さいため、室温において電子もしくはホールによる電気伝導が起こる。電気伝導が起こると、電子により伝熱が起こるため、電気伝導度が大きい化合物を含有するシリカでは断熱性が低下する。したがって、本発明では特定元素又はその化合物として電気伝導度の小さい物質を用いることにより、本発明の断熱材の断熱性をより高くすることができる。なお、特定元素の電気伝導度は公知の手法により測定してもよいが、文献等に記載されているものはその文献の値を参照してもよい。
【００５０】
また、本発明の断熱材は、そのかさ密度が通常０．５ｇ／ｍｌ以上、好ましくは０．４５ｇ／ｍｌ以下、更に好ましくは０．４ｇ／ｍｌ以下であることを、もう一つの特徴としている。かさ密度が小さな値を取るほど、シリカ中における空隙の割合が高まり、シリカの断熱性が向上する。但し、かさ密度があまりに大きいと、シリカの強度を保つことが困難となり、断熱材としての実用性が却って低下してしまうので、その下限は通常０．０１ｇ／ｍｌ以上、好ましくは０．０２ｇ／ｍｌ以上、更に好ましくは０．０３ｇ／ｍｌ以上である。
【００５１】
より詳細には、かさ密度の範囲は、本発明のシリカの細孔径、及び、特定元素の種類により異なる。即ち、本発明のシリカの細孔径が大きいほどかさ密度は小さくなり、また、特定元素の分子量が大きいほどかさ密度は小さくなる。
【００５２】
具体例としては、原子量が比較的大きいＰｂ（原子量２０７．２ｇ／ｍｏｌ）を特定元素として用いる場合には、かさ密度は、通常０．５ｇ／ｍｌ以下、好ましくは０．４５ｇ／ｍｌ以下、より好ましくは０．４ｇ／ｍｌ以下である。また、原子量が比較的小さいＺｒ（原子量９１．２２ｇ／ｍｏｌ）を特定元素として用いる場合には、かさ密度は、通常０．３５ｇ／ｍｌ以下、好ましくは０．３ｇ／ｍｌ以下、より好ましくは０．２ｇ／ｍｌ以下である。なお、特定元素として複数種のものを用いる場合は、用いる特定元素全体の平均原子量に応じてかさ密度を決定する。また、かさ密度は、例えばＪＩＳＫ６７２１など、公知の任意の方法により測定することができる。
【００５３】
［２．断熱材の製造方法］
次に、本発明の断熱材の製造方法を説明する。
本発明の断熱材は、上述した物性を有する断熱材を製造することができる方法であれば特に制限はないが、主な方法としては、（ｉ）本発明のシリカを製造する際に、特定元素の原子単体又は化合物を導入する方法、（ｉｉ）本発明のシリカを製造した後、特定元素の原子単体又は化合物と混合して組成物とする方法などが挙げられる。
【００５４】
以下の記載では、まず、本発明の断熱材の製造方法の基礎となる、本発明のシリカの製造方法について説明した上で、これに特定元素を含有させる手法として、上記（ｉ）（ｉｉ）のそれぞれの場合について説明する。
【００５５】
［２−１．本発明のシリカの製造方法］
本発明のシリカは、通常、シリコンアルコキシドを加水分解する工程（加水分解・縮合工程）と、加水分解・縮合工程で得られたシリカヒドロゲルを水熱処理する工程（物性調節工程）と、物性調節工程で得られたシリカスラリーの液体成分中の水分含有量を５重量％以下とする工程（水分除去工程）と、水分除去工程で得られたシリカスラリーを乾燥する工程（乾燥工程）とを有する方法により製造される。この製造方法のいずれかの工程中、又は工程の前後において、特定元素をシリカに導入することによって、本発明の断熱材は製造される。
【００５６】
上記の本発明のシリカの製造方法は、具体的には、シリコンアルコキシドを加水分解し、得られたシリカヒドロゲルを、好ましくは実質的に熟成することなしに水熱処理し、次いで水分除去工程として親水性有機溶媒と接触させた後に、乾燥させることで、シリカ中の水分を除去することを特徴とする。
【００５７】
［２−１−１．加水分解・縮合工程］
加水分解・縮合工程においては、シリコンアルコキシドを加水分解してシリカヒドロゲルを得る。得られるシリカヒドロゲルは、その原料であるシリコンアルコキシドの高純度化が可能であり、シリカヒドロゲルへの不純物の混入を容易に防止できるので好ましい。
【００５８】
本発明のシリカの原料として使用されるシリコンアルコキシドとしては、トリメトキシシラン、テトラメトキシシラン、トリエトキシシラン、テトラエトキシシラン、テトラプロポキシシラン、テトラブトキシシラン等の炭素数１〜４の低級アルキル基を有するトリ又はテトラアルコキシシラン或いはそれらのオリゴマーが挙げられる。中でも、テトラメトキシシラン、テトラエトキシシラン及びそれらのオリゴマー、特にテトラメトキシシランやそのオリゴマーを用いると、良好な細孔特性を有するシリカが得られるので好ましい。その主な理由としては、シリコンアルコキシドは蒸留により容易に精製し、高純度品が得られるので、高純度のシリカの原料として好適であることが挙げられる。シリコンアルコキシド中の金属不純物の総含有量は、通常３０００重量ｐｐｍ以下、中でも２０００重量ｐｐｍ以下、更には１０００重量ｐｐｍ以下、特に１００重量ｐｐｍ以下が好ましい。これらの金属不純物の含有率は、一般的なシリカ中の不純物含有率の測定法と同じ方法で測定できる。このように金属不純物の含有率が少ないシリコンアルコキシドを用いれば、製造されるシリカ中の金属不純物の含有率を小さくすることができる。
【００５９】
本発明では、先ず、加水分解・縮合工程において、触媒の不存在下にシリコンアルコキシドを加水分解すると共に得られたシリカヒドロゾルを縮合してシリカヒドロゲルを形成する。
【００６０】
シリコンアルコキシドの加水分解は、シリコンアルコキシド１モルに対して、通常２モル倍以上、好ましくは３モル倍以上、特に好ましくは４モル倍以上、また、通常２０モル倍以下、好ましくは１０モル倍以下、特に好ましくは８モル倍以下の水を用いて行なう。シリコンアルコキシドの加水分解により、シリカのヒドロゲルとアルコールとが生成し、生成したシリカヒドロゾルは逐次縮合してシリカヒドロゲルとなる。
【００６１】
加水分解時の温度は、通常室温以上、１００℃以下であるが、加圧下で液相を維持することで、より高い温度で行なうことも可能である。加水分解に要する反応時間は反応液組成（シリコンアルコキシドの種類や、水とのモル比）並びに反応温度に依存し、ゲル化するまでの時間が異なるので、一概には規定されない。この反応時間は、本発明のシリカのように細孔特性に優れたシリカを得る為には、ヒドロゲルの破壊応力が６ＭＰａを越えない時間であることが好ましい。
【００６２】
なお、この加水分解反応系に、触媒として、酸、アルカリ、塩類などを添加することで加水分解を促進させることができる。しかしながら、かかる添加物の使用は、後述のように、生成したヒドロゲルの熟成を引き起こし、また、酸やアルカリ、塩類などには金属不純物が混合している虞があるため、本発明のシリカゲルの製造においてはあまり好ましいことではない。
【００６３】
上述したシリコンアルコキシドの加水分解に際しては、攪拌を充分に行なうことが重要となる。例えば、回転軸に攪拌翼を備えた攪拌装置を用いた場合、その攪拌速度（回転軸の回転数）としては、攪拌翼の形状・枚数・液との接触面積等にもよるが、通常は３０ｒｐｍ以上、好ましくは５０ｒｐｍ以上である。
【００６４】
また、この攪拌速度は、一般的に速過ぎると、槽内で生じた飛沫が各種のガスラインを閉塞させたり、また反応器内壁に付着して熱伝導を悪化させ、物性制御に重要な温度管理に影響を及ぼしたりする場合がある。更に、この内壁の付着物が剥離し、製品に混入して品質を悪化させる場合もある。この様な理由から、攪拌速度は２０００ｒｐｍ以下、中でも１０００ｒｐｍ以下であることが好ましい。
【００６５】
本発明に於いて、分液している二液相（水相、及びシリコンアルコキシド相）の攪拌方法は、反応を促進させる方法であれば任意の攪拌方法を用いることが出来る。中でも、この二液相をより混合させるような装置としては、例えば以下の▲１▼、▲２▼が挙げられる。
【００６６】
▲１▼：回転軸が液面に対し垂直又は僅かに角度を持って挿入され、上下に液の流動が生じる攪拌翼を有する装置。
▲２▼：回転軸方向を二液相の界面と略平行に設け、二液相間に攪拌を生じさせる攪拌翼を有する攪拌装置。
【００６７】
上述した▲１▼、▲２▼の様な装置を用いた際の攪拌翼の回転速度は、攪拌翼の周速度（攪拌翼先端速度）で、０．０５〜１０ｍ／ｓ、中でも０．１〜５ｍ／ｓ、さらには０．１〜３ｍ／ｓであることが好ましい。
【００６８】
攪拌翼の形状や長さ等は任意であり、攪拌翼としては例えばプロペラ型、平羽根型、角度付平羽根型、ピッチ付平羽根型、平羽根ディスクタービン型、湾曲羽根型、ファウドラー型、ブルマージン型等が挙げられる。
【００６９】
翼の幅、枚数、傾斜角等は反応器の形状、大きさ、目的とする攪拌動力に応じて適宜選定すればよい。たとえば反応器の槽内径（回転軸方向に対して垂直面を形成する液相面の最長径）に対する翼幅（回転軸方向の翼の長さ）の比率（ｂ／Ｄ）は０．０５〜０．２、傾斜角（θ）９０゜±１０゜、翼枚数３〜１０枚の攪拌装置が好適な例として挙げられる。
【００７０】
中でも、上述の回転軸を反応容器内の液面よりも上に設け、この回転軸から伸ばした軸の先端部分に攪拌翼を設ける構造が、攪拌効率及び設備メンテナンスの観点から好適に使用される。
【００７１】
上記のシリコンアルコキシドの加水分解反応では、シリコンアルコキシドが加水分解してシリカヒドロゾルが生成するが、引き続いて該シリカヒドロゾルの縮合反応が起こり、反応液の粘度が上昇し、最終的にゲル化してシリカヒドロゲルとなる。
【００７２】
［２−１−２．物性調節工程］
次いで、本発明では、物性調節工程として、実質的に熟成することなく、シリカヒドロゲルの水熱処理を行なう。シリコンアルコキシドを加水分解すると、軟弱なシリカヒドロゲルが生成する。なお、従来行なわれているような、このヒドロゲルの物性を安定させるべく、熟成、あるいは乾燥させ、次いで水熱処理を施すという方法では、本発明のシリカを製造することは困難である。
【００７３】
上記のように、加水分解により生成したシリカヒドロゲルを、実質的に熟成することなく、直ちに水熱処理を行なうということは、シリカヒドロゲルが生成した直後の軟弱な状態が維持されたままで、次の水熱処理に供するようにするということを意味する。
【００７４】
具体的には、シリカヒドロゲルが生成した時点から、一般的には１０時間以内に水熱処理することが好ましく、中でも８時間以内、更には６時間以内、特に４時間以内にシリカヒドロゲルを水熱処理することが好ましい。
【００７５】
また、工業用プラント等に於いては、大量に生成したシリカヒドロゲルを一旦サイロ等に貯蔵し、その後水熱処理を行なう場合が考えられる。この様な場合、シリカヒドロゲルは、シリカヒドロゲルが生成してから水熱処理に供されるまでの時間、いわゆる放置時間が、上述の範囲を超える場合が考えられる。この様な場合には、熟成が実質的に生じないように、サイロ内での静置中に、例えばシリカヒドロゲル中の液体成分が乾燥しないようにすればよい。
【００７６】
具体的には、例えば、サイロ内を密閉したり、湿度を調節したりすればよい。また、水やその他の溶媒にシリカヒドロゲルを浸した状態で、シリカヒドロゲルを静置してもよい。
【００７７】
静置の際の温度は、できるだけ低くすることが好ましく、例えば５０℃以下、中でも３５℃以下、特に３０℃以下で静置することが好ましい。また、熟成が実質的に生じないようにする別の方法としては、シリカヒドロゲル中のシリカ濃度が低くなるように、予め原料組成を制御してシリカヒドロゲルを調製する方法が挙げられる。
【００７８】
シリカヒドロゲルを実質的に熟成せずに水熱処理することにより奏する効果と、この効果が得られる理由を考察すると、以下のことが考えられる。
【００７９】
まず、シリカヒドロゲルを熟成させると、−Ｓｉ−Ｏ−Ｓｉ−結合によるマクロ的網目構造が、シリカヒドロゲル全体に形成されると考えられる。この網目構造がシリカヒドロゲル全体に有ることで、水熱処理の際、この網目構造が障害となり、メソポーラスの形成が困難となることが考えられる。またシリカヒドロゲル中のシリカ濃度が低くなるように、予め原料組成を制御して得られたシリカヒドロゲルは、静置中に生ずるシリカヒドロゲルにおける架橋の進行を抑制できる。その為、シリカヒドロゲルが熟成しないと考える。
【００８０】
よって、この製造方法では、シリカヒドロゲルを熟成することなく、水熱処理を行なうことが重要である。
【００８１】
更に、シリコンアルコキシドの加水分解で得られたシリカヒドロゲルは、水熱処理を行なうまえに、これを平均粒径１０ｍｍ以下、中でも５ｍｍ以下、更には１ｍｍ以下、特に０．５ｍｍ以下となるよう、粉砕処理等を施すことが好ましい。
【００８２】
上述の通り、本発明のシリカの製造方法としては、シリカヒドロゲルの生成の直後に、直ちにこれを水熱処理する方法が重要である。但し、この製造方法に於いては、水熱処理するシリカヒドロゲルが熟成していなければよいので、例えば暫時低温下で静置した後に水熱処理するなど、必ずしもシリカヒドロゲルの生成直後、直ちにこれを水熱処理することを必要としない。
【００８３】
このように、シリカヒドロゲルの生成の直後、直ちにこれを水熱処理しない場合には、例えばシリカヒドロゲルの熟成状態を具体的に確認してから水熱処理を行なえばよい。ヒドロゲルの熟成状態を具体的に確認する手段は任意であるが、例えば、後述の実施例に示すような方法で測定したヒドロゲルの硬度を参考にする方法が挙げられる。即ち、先述したとおり、この破壊応力が通常６ＭＰａ以下の柔らかい状態のヒドロゲルを水熱処理することで、本発明で規定する物性範囲のシリカを得ることができる。この破壊応力は、中でも３ＭＰａ以下であることが好ましく、特に２ＭＰａ以下であることが好ましい。
【００８４】
この水熱処理の条件としては、水の状態が液体、気体のいずれでもよいが、中でも、液体の水を使い、シリカヒドロゲルに加えてスラリー状として、水熱処理を行なうことが好ましい。水熱処理においては、まず、処理するシリカヒドロゲルに、シリカヒドロゲルの重量に対して通常０．１重量倍以上、好ましくは０．５重量倍以上、特に好ましくは１重量倍以上、また、通常１０重量倍以下、好ましくは５重量倍以下、特に好ましくは３重量倍以下の水を加えてスラリー状とする。そしてこのスラリーを、通常４０℃以上、好ましくは１００℃以上、中でも好ましくは１５０℃以上、特に好ましくは１７０℃以上、また、通常２５０℃以下、好ましくは２００℃以下の温度で、通常０．１時間以上、好ましくは１時間以上、また、通常１００時間以下、好ましくは１０時間以下にわたって、水熱処理を行なう。水熱処理の温度が低すぎると、細孔分布がシャープになり難く、また、細孔容積を大きくすることも困難となる場合がある。
【００８５】
なお、水熱処理に使用される水には、溶媒が含まれていてもよい。溶媒として、具体的には、例えば、低級アルコール類であるメタノール、エタノール、プロパノールなどが挙げられる。この溶媒は、例えばアルコキシシランを加水分解して得られたシリカヒドロゲルを水熱処理する際には、その原料であるアルコキシシランに由来するアルコール類であってもよい。もちろん、この溶媒としては金属不純物を含まない、若しくはほとんど含まないものを選択すべきであることは言うまでもない。
【００８６】
熱処理に用いる水における、この様な溶媒の含有量は任意だが、少ない方が好ましい。例えば、上述した様な、アルコキシシランを加水分解して得られたシリカヒドロゲルを水熱処理する際には、このシリカヒドロゲルを水洗し、水洗されたものを水熱反応に供することにより、１５０℃程度まで温度を下げて水熱処理を行なった場合でも、細孔特性に優れ且つ細孔容積の大きいシリカを製造することが出来る。また、溶媒を含んでいる水で水熱処理を行なっても、２００℃程度の温度での水熱処理を行なうことで、本発明のシリカを容易に得ることが出来る。
【００８７】
なお、加水分解反応の反応器を用い、続けて温度条件変更により水熱処理を行なうことも可能であるが、加水分解反応とその後の水熱処理では最適条件が通常は異なっているため、この様に水を新たに加えないで行なう方法では、本発明のシリカを得ることは一般的には難しい。
【００８８】
以上の水熱処理の条件において、温度を高くすると、得られるシリカの径、細孔容積が大きくなる傾向がある。水熱処理温度としては、１００〜２００℃の範囲であることが好ましい。また、処理時間とともに、得られるシリカの比表面積は、一度極大に達した後、緩やかに減少する傾向がある。以上の傾向を踏まえて、所望の物性値に応じて条件を適宜選択する必要があるが、水熱処理は、シリカの物性を変化させる目的なので、通常、前記の加水分解の反応条件より高温条件とすることが好ましい。
【００８９】
なお、ミクロ構造的な均質性に優れる本発明のシリカを製造するためには、水熱処理の際に、反応系内の温度が５時間以内に目的温度に達する様に、速い昇温速度条件とすることが好ましい。具体的には、槽に充填して処理される場合、昇温開始から目標温度到達までの平均昇温速度として、通常０．１℃／ｍｉｎ以上、中でも０．２℃／ｍｉｎ以上、また、通常１００℃／ｍｉｎ以下、中でも３０℃／ｍｉｎ以下、特に１０℃／ｍｉｎ以下の範囲の値を採用するのが好ましい。
【００９０】
熱交換器などを利用した昇温方法や、あらかじめ作っておいた熱水を仕込む昇温方法なども、昇温速度を短縮することができて好ましい。また、昇温速度が上記範囲であれば、段階的に昇温を行なってもよい。反応系内の温度が目的温度に達するまでに長時間を要した場合には、昇温中にシリカヒドロゲルの熟成が進み、ミクロ構造的な均質性が低下する恐れがある。
【００９１】
上記の目的温度に達するまでの昇温時間は、好ましくは４時間以内、更に好ましくは３時間以内である。昇温時間の短縮化のため、水熱処理に使用する水を予熱することもできる。
【００９２】
水熱処理の温度、時間を上記範囲外に設定すると、本発明のシリカを得ることが困難となる。例えば、水熱処理の温度が高すぎると、シリカの細孔径、細孔容積が大きくなりすぎ、また、細孔分布も広がる。逆に、水熱処理の温度が低過ぎると、生成するシリカは、架橋度が低く、熱安定性に乏しくなり、細孔分布にピークが発現しなくなったり、前述した固体Ｓｉ−ＮＭＲにおけるＱ^４／Ｑ^３値が極端に小さくなったりする。
【００９３】
なお、水熱処理をアンモニア水中で行なうと、純水中で行なう場合よりも低温で同様の効果が得られる。また、アンモニア水中で水熱処理すると、アンモニアを含まない水を用いた水熱処理と比較して、最終的に得られるシリカは一般に疎水性となる。この際、水熱処理の温度を３０℃以上、好ましくは４０℃以上、また、２５０℃以下、好ましくは２００℃以下という比較的高温とすると、特に疎水性が高くなる。ここでのアンモニア水のアンモニア濃度としては、好ましくは０．００１％以上、特に好ましくは０．００５％以上、また、好ましくは１０％以下、特に好ましくは５％である。
【００９４】
［２−１−３．水分除去工程］
上述した水熱処理を経て得られたシリカには、多量の水が含まれている。例えば、水熱処理後のシリカは、多量の水を含むシリカ（例えばシリカスラリー）として得られる。このシリカの製造方法においては、この水を除去することが重要である。具体的には、このシリカスラリーを親水性有機溶媒と接触させ、水を親水性溶媒で置換し、次いでシリカを乾燥するという操作が、最も重要である。
【００９５】
ここに示す製造方法においては、シリカに含まれる水を親水性有機溶媒と置換し、乾燥することによって、乾燥工程に於けるシリカの収縮を抑制し、シリカの細孔容積を大きく維持でき、細孔特性に優れ、且つ細孔容積の大きいシリカを得ることが出来るのである。この理由は定かではないが、以下のような現象によるものと考えられる。
【００９６】
水熱処理後のシリカスラリー中の液体成分の多くは水である。この水は、シリカと互いに強く相互作用しあっている為に、シリカから完全に水を除去するには大きなエネルギーが必要と考える。
【００９７】
多量の水分が存在する条件下で乾燥過程（例えば加熱乾燥）を行なうと、熱エネルギーを受けた水が未反応のシラノール基と反応し、シリカの構造が変化する。この構造変化のうち最も顕著な変化はシリカ骨格の縮合であり、縮合によってシリカが局所的に高密度化することが考えられる。シリカ骨格は３次元的構造を有するので、骨格の局所的な縮合（シリカ骨格の高密度化）はシリカ骨格により構成されているシリカ粒子全体の細孔特性に影響を及ぼし、結果的に粒子が収縮して、細孔容積や細孔径が収縮すると考えられる。
【００９８】
そこで、例えばシリカスラリー中の（水を多量に含む）液体成分を親水性有機溶媒で置換することで、このシリカスラリー中の水を除去し、上述したようなシリカの収縮を抑えることが可能となる。
【００９９】
本発明で用いる親水性有機溶媒とは、上述した考えに基づき、水を多く溶かすものであればよい。中でも、分子内分極の大きいものが好ましい。さらに好ましくは、比誘電率が１５以上のものがよい。
【０１００】
本発明のシリカの製造方法に於いては、純度の高いシリカを得るために、親水性有機溶媒にて水を除去した後の乾燥工程で、この親水性有機溶媒を除去する必要がある。よって、親水性有機溶媒としては、乾燥（例えば加熱乾燥や真空・減圧乾燥等）により容易に除去可能な低沸点のものが好ましい。親水性有機溶媒の沸点としては、１５０℃以下、中でも１２０℃以下、特に１００℃以下のものが好ましい。
【０１０１】
具体的な親水性有機溶媒としては、メタノール、エタノール、プロパノール、ブタノール等のアルコール類、アセトン、メチルエチルケトン、ジエチルケトン等のケトン類、アセトニトリルなどのニトリル類、ホルムアミド、ジメチルホルムアミド等のアミド類、アルデヒド類、エーテル類等が挙げられる。中でも、アルコール類やケトン類が好ましく、特に、メタノール、エタノール、プロパノール等の低級アルコール類やアセトンが好ましい。本発明では、これら例示の親水性有機溶媒のうち、一種を単独で使用しても良く、二種以上を任意の組み合わせ及び任意の割合で混合して使用してもよい。なお、親水性有機溶媒としては金属不純物を含まない、若しくはほとんど含まないものを選択すべきであることは言うまでもない。
【０１０２】
なお、水の除去が可能であれば、使用する親水性有機溶媒中に水が含まれていてもよい。もっとも、親水性有機溶媒における水分含有量は当然少ない方が好ましく、通常２０％以下、中でも１５％以下、更には１０％以下、特に５％以下であることが好ましい。
【０１０３】
本発明に於いて、上述の親水性有機溶媒による置換処理時の温度及び圧力は任意である。処理温度は、通常０℃以上、中でも１０℃以上、通常１００℃以下、中でも６０℃以下とすることが好ましい。処理時の圧力は常圧、加圧、減圧のいずれでもよい。
【０１０４】
シリカスラリーと接触させる親水性有機溶媒の量は任意である。但し、用いる親水性有機溶媒の量が少な過ぎると水との置換進行速度が充分でなく、逆に多過ぎると水との置換効率は高まるが、親水性有機溶媒の使用量増加に見合う効果が頭打ちとなり、経済的に好ましくない。よって、用いる親水性有機溶媒の量は、シリカの嵩体積に対して通常０．５〜１０容量倍である。この親水性有機溶媒による置換操作は、複数回繰り返して行なうと、水の置換がより確実となるので好ましい。
【０１０５】
親水性有機溶媒とシリカスラリーとの接触方法は任意であり、例えば攪拌槽でシリカスラリーを攪拌しながら親水性有機溶媒を添加する方法や、シリカスラリーから濾別したシリカを充填塔に詰めて、この充填塔に親水性有機溶媒を通液する方法、また、親水性有機溶媒中にシリカを入れて浸漬し、静置する方法などが挙げられる。
【０１０６】
親水性有機溶媒による置換操作の終了は、シリカスラリーの液体成分中の水分測定を行なって決定すればよい。例えば、定期的にシリカスラリーをサンプリングして水分測定を行ない、水分含有量が通常５％以下、好ましくは４％以下、さらに好ましくは３％以下となった点を終点とすればよい。水分の測定方法は任意であり、例えばカールフィッシャー法が挙げられる。
【０１０７】
［２−１−４．乾燥工程］
親水性有機溶媒による置換操作の後、シリカと親水性有機溶媒とを分離し、乾燥することで、本発明のシリカを製造することが出来る。この際の分離法としては、従来公知の任意の固液分離方法を用いればよい。即ち、シリカ粒子のサイズに応じて、例えばデカンテーション、遠心分離、濾過等の方法を選択して固液分離すれば良い。これらの分離方法は、一種を単独で用いても良く、また二種以上を任意の組み合わせで用いてもよい。
【０１０８】
得られたシリカは、通常４０℃以上、好ましくは６０℃以上、また、通常２００℃以下、好ましくは１２０℃以下で乾燥する。乾燥方法は特に限定されるものではなく、バッチ式でも連続式でもよく、且つ、常圧でも減圧下でも乾燥することができる。中でも、真空乾燥を行なうことで、乾燥が迅速に行なえるのみならず、得られるシリカの細孔容積、比表面積が大きくなるので好ましい。
【０１０９】
必要に応じ、原料のシリコンアルコキシドに由来する炭素分が含まれている場合には、通常４００〜６００℃で焼成除去することができる。また、表面状態をコントロールするため最高９００℃の温度で焼成することもある。更に、必要に応じて粉砕、分級することで、最終的に本発明のシリカを得る。
【０１１０】
［２−２．特定元素を含有させる方法］
本発明の断熱材に特定元素を含有させる主な方法としては、（ｉ）上述の製造方法により本発明のシリカを製造する際に、特定元素の原子単体又は化合物を導入する方法、（ｉｉ）上述の製造方法により製造した本発明のシリカを、特定元素の単体又は化合物と混合して組成物とする方法が挙げられる。
【０１１１】
まず、（ｉ）の方法について説明する。
特定元素は、単体又は化合物のいずれの状態で使用してもよいが、通常は、特定元素の酸化物、硫化物、又は窒化物を用いることが好ましい。なお、以下の（ｉ）の方法における特定元素の導入方法の説明においては、特に説明しない限り、導入する特定元素の状態が単体であるか化合物であるかを区別せず述べる場合には特定元素の単体及び化合物をまとめて特定物質という。
【０１１２】
上述したシリカの製造工程のうち、どの工程において特定物質を導入するかは任意であるが、通常は、▲１▼加水分解・縮合工程中に、シリコンアルコキシドに本発明の特定物質を導入する、▲２▼物性調節工程中に、シリカスラリーに本発明の特定物質を導入する、▲３▼水分除去工程、乾燥工程、又は乾燥工程の後に、本発明の特定物質を導入する、という方法が挙げられる。なお、▲１▼、▲２▼、及び▲３▼の方法は、それぞれ単独で行なってもよく、２種以上を組み合わせて行なってもよい。
【０１１３】
▲１▼の方法では、シリコンアルコキシド及び特定物質の共存下にシリカを加水分解・縮合させる。これにより、シリカが縮合してシリカ骨格が形成される際（つまりシリカ骨格の縮合が生じる際）、この縮合が生ずる周辺に本発明の特定元素が存在するので、特定元素が断熱材の骨格中に存在しやすくなる。
【０１１４】
つまり、▲１▼の方法では、特定元素の多くが断熱経路中に存在できるようになるため、使用する特定元素が比較的少量であっても確実に断熱性を発揮することができる。したがって、▲１▼の方法では、Ｓｉと使用する特定元素との重量比率が、特定元素／（Ｓｉ＋特定元素）の重量比率で、通常１重量ｐｐｍ以上、好ましくは３重量ｐｐｍ以上、より好ましくは５重量ｐｐｍ以上、また、通常１５％以下、好ましくは１２％以下、より好ましくは１０％以下である。
【０１１５】
▲２▼の方法では、加水分解・縮合工程で製造されたシリカスラリー及び特定物質の共存下で、吸い熱処理を行なう。これによっても、▲１▼と同様に、シリカが縮合してシリカ骨格が形成される際（つまりシリカ骨格の縮合が生じる際）、この縮合が生ずる周辺に本発明の特定元素が存在するので、特定元素が断熱材の骨格中に存在しやすくなる。
【０１１６】
よって、▲２▼の方法によっても、特定元素の多くが断熱経路中に存在できるようになるため、使用する特定元素が比較的少量であっても確実に断熱性を発揮することができる。したがって、▲２▼の方法では、シリカスラリー中のシリカ固形分と使用する特定元素との重量比が、特定元素の重量／シリカ固形分の重量で、通常１％以上、好ましくは３％以上、より好ましくは５％以上、また、通常３０％以下、好ましくは２９％以下、より好ましくは２８％以下である。
【０１１７】
▲３▼の方法では、物性調節工程によってシリカ骨格が出来上がった後に、シリカ及び特定物質とを混合する。これにより、シリカの表面に特定物質が付着するが、特定物質の一部は、シリカ骨格内部に取り込まれる。
【０１１８】
▲３▼の方法では、特定元素の多くは断熱経路外に存在すると考えられるため、通常は、比較的多量の特定元素を用いるべきである。したがって、▲３▼の方法では、本発明のシリカと使用する特定元素との比が、特定元素の重量／シリカの重量で、通常１％以上、好ましくは３％以上、より好ましくは５％以上、また、通常３０％以下、好ましくは２９％以下、より好ましくは２８％以下である。
【０１１９】
なお、▲３▼の方法の具体的手法としては、含浸法＜吸着（ａｂｓｏｒｐｔｉｏｎ）法，ポアフィリング（ｐｏｒｅ−ｆｉｌｌｉｎｇ）法，”ｉｎｃｉｐｉｅｎｔ ｗｅｔｎｅｓｓ”法，蒸発乾固（ｅｖａｐｏｒａｔｉｏｎ ｔｏ ｄｒｙｎｅｓｓ）法，スプレー（ｓｐｒａｙ）法等＞、沈殿法＜共沈（ｃｏｐｒｅｃｉｐｉｔａｔｉｏｎ）法、沈着（ｐｒｅｃｉｐｉｔａｔｉｏｎ）法，混練（ｋｎｅａｄｉｎｇ）法等＞、イオン交換（ｉｏｎ ｅｘｃｈａｎｇｅ）法、機械的混合（メカノケミカル）法や、蒸着法（化学蒸着法、スパッタリング法）等が挙げられる。
【０１２０】
以上の操作により特定元素を導入した後、必要に応じて２００〜４００℃で焼成することにより、特定元素とシリカとの間の架橋反応を促進させることができる。また、必要であれば、特定元素を導入した後に酸化や還元等の化学処理を行ない、特定元素の化学状態を変化させても良い。
【０１２１】
なお、特定元素は、シリコンアルコキシドを加水分解し、熟成すること無しに水熱処理して得られたシリカに、上記の方法で担持させることができるが、その形態、化合物の種類、化合物の量は、目的異元素が機能性を発現する範囲であれば特に限定されず、これらの特定元素の単体、水溶性の塩、溶媒可溶性の塩、錯体化合物、加水分解性の化合物等の何れであってもよい。また、高純度な断熱材、即ち、金属不純物の含有率が小さい断熱材を得る観点から、特定元素も高純度なものを用いることが好ましい。
【０１２２】
続いて、（ｉｉ）の方法、即ち、上述の製造方法により製造した本発明のシリカを、特定元素の単体又は化合物と混合して組成物とする方法について説明する。特定元素は、単体又は化合物のいずれの状態で使用してもよいが、通常は、特定元素の酸化物、硫化物、又は窒化物を用いることが好ましい。なお、以下の（ｉｉ）の方法における特定元素の導入方法の説明においても、導入する特定元素が、単体であるか化合物であるかを区別せずにいう場合、単に特定物質というものとする。
【０１２３】
本発明のシリカと特定物質とを混合する際、特定物質の粒径は、通常、本発明のシリカの最頻細孔径よりも大きい。特定物質の粒径がこれ以下であると、特定物質が細孔に詰まり充分な断熱性を発揮することが難しくなる虞があるためである。ただし、粒径が本発明のシリカの最頻細孔径以下である特定物質であっても、その量が少なければ断熱性を大きく悪化させることはない。
【０１２４】
また、（ｉｉ）の方法において、本発明のシリカと使用する特定元素との重量比は、特定元素の重量／（シリカ＋特定元素）の重量で、通常１％以上、好ましくは３％以上、より好ましくは５％以上、また、通常３０％以下、好ましくは２９％以下、より好ましくは２８％以下である。この比が、これよりも小さいと充分な断熱性を発揮することができず、また、これよりも大きいと断熱材の空隙が小さくなり断熱性が低下するためである。
【０１２５】
（ｉｉ）の方法において、混合の手法は任意であるが、通常は、本発明のシリカ中に特定物質を均一に分散させることができる手法を用いる。具体的には、振倒混合などが挙げられる。
【０１２６】
なお、シリカの原料となりうる天然の鉱石のうち、特定元素を多く含むものがあれば、その鉱石から上述したシリカの製造方法などによってシリカを製造して、特定元素をシリカに含有させる工程を経ることなく、本発明の断熱材も製造を行なっても、本発明の断熱材を製造することは可能である。
【０１２７】
［１−３．その他］
さて、ここで、特許文献２にあるエアロゲルの製造方法と本願にかかるシリカの製造方法とを比較して、本願の断熱材について考察する。
【０１２８】
特許文献２では、シリカに導電性の金属酸化物を混合している。しかしながら、上述したように電気伝導度が大きい物質は一般に高い熱伝導性を有するため、そのまま断熱材として使用することは困難である。そこで特許文献２では、導電性の金属酸化物がシリカを破砕することに注目し、その破砕現象によって断熱材の断熱性を高めることを提案している。よって、この技術では、シリカの微細構造が破砕現象によって変動してしまうため、断熱材の強度や耐久性、成形性が損なわれてしまうという虞がある。
【０１２９】
一方、本発明は、原理の説明において述べたように、２種の原子が結合している場合にはその原子の原子量の差が大きいほど、２種の原子間を熱が伝わる際の熱伝導度が小さくなることに着目し、断熱材の断熱性を向上させている。つまり、本発明は、破砕という断熱材の構造制御を困難にする現象を生じることなく、特定元素を含有させるという比較的簡単な操作を加えることによって、断熱材の強度や耐久性、成形性を損なうことなく、優れた断熱性を発揮させることができる。また、その製造過程においてｐＨ調整等の煩雑な操作を用いていないことから、製造効率に優れており、製造コストの面でも有利である。
【０１３０】
本発明の断熱材は、以上の利点を備えていることから、保冷や保温など断熱性が必要な各種の用途に用いることができる。例えば、冷蔵庫、冷凍庫、保冷車、車の天井部、バッテリー、冷凍または冷蔵船、保温コンテナ、冷凍または保冷用ショーケース、携帯用クーラー、料理用保温ケース、自動販売機、太陽熱温水器、床暖房、床下、壁または壁内、天井部、屋根裏部屋等の建材、熱水または冷却水の配管、低温流体を移送する導管その他プラント機器類、衣料、寝具等の断熱材として好適に用いることができる。また、特にシリカは透明度が高いため、例えば建材であればガラスで囲って断熱用窓ガラスとするなど、透明性を生かした用途に用いることもできる。
【０１３１】
［３．断熱体］
本発明の断熱材の使用形態は特に制限は無く、上述の用途にそのまま用いても良いが、その用途によっては、本発明の断熱材を外装材で密閉した断熱体（本発明の断熱体）として用いるのも好ましい。
【０１３２】
外装材は、本発明の断熱材を密閉できるものであれば、特に限定無く様々なものを用いることができるが、特に、外装材で密閉された内部を外気から遮断することが可能なものが好ましい。
【０１３３】
通常、外装材により断熱材を密閉する場合、断熱材の存在する外装材内部を、通常は減圧状態、好ましくは真空状態として、断熱材であるシリカの空隙の熱伝導度を低下させることが望まれる。
【０１３４】
ここで、外装材を通して内部にガスが進入してくると、シリカの空隙の熱伝導度が上昇し、断熱材の断熱性が低下してしまうので、好ましくない。よって、外装材としては、ガスバリア性を有するガスバリア層を備えたものが好ましい。
【０１３５】
また、外装材を通して内部に水が浸入してくると、断熱材であるシリカが水を吸収して断熱性が低下してしまう虞があるので、やはり好ましくない。よって、外装材は非透湿性を有する非透湿層を備えたものが好ましい。
【０１３６】
更に、外装材自体も優れた断熱性を有する素材で形成されていることが好ましい。さらに、外装材は、筐体のように定形の形状を有するものでもよく、フィルムなど非定型のものであってもよい。
【０１３７】
外装材の具体例としては、ステンレススチール、アルミニウム、鉄などの金属の薄板や、アルミニウム箔などの金属箔などのほか、ポリエチレン、ポリプロピレン、アクリロニトリル−ブタジエン−スチレン共重合体（ＡＢＳ）、スチレン、ポリ塩化ビニル等の樹脂に、ポリ塩化ビニリデン、エチレンビニルアルコールフィルムや、アルミ等の金属やシリコンをラミネートしたラミネート材等が挙げられる。また、これらに表面保護層として、外装材の外表面にポリエチレンテレフタレートフィルム、ポリプロピレンフィルムの延伸加工品などの層を設けたり、ナイロンフィルムなどを設けると可撓性が向上し、耐折り曲げ性などが向上する。
【０１３８】
外装材により本発明の断熱材を密閉する際には、外装材の種類に応じて各種の手法を用いることができるが、上述の様に、断熱材の内部を減圧状態、好ましくは真空状態としながら密閉することが好ましい。
【０１３９】
なお、本発明の断熱材に加えて、その他の断熱材料等の各種の物質を、一緒に外装材内に密閉しても良い。これらの物質の種類は、断熱材の使用形態及び用途に応じて適宜選択すればよい。
【０１４０】
【実施例】
以下、本発明を実施例により更に詳細に説明するが、本発明はその要旨を超えない限り、以下の実施例に制約されるものではない。
【０１４１】
（１）実施例１：
（１−１）断熱材の製造：
＜加水分解・縮合工程＞
ガラス製で、上部に大気開放の水冷コンデンサが取り付けてある５Ｌセパラブルフラスコ（ジャケット付き）に、テトラエトキシシラン１８７５ｇを仕込んだ。攪拌翼先端速度２．５ｍ／ｓで撹拌しながら、これに０．００２ｍｏｌ／Ｌの塩酸を１６２ｇ仕込んだ。１時間攪拌を継続しセパラブルフラスコのジャケットには５０℃の温水を通水した。引き続き撹拌を継続しながら、ジルコニウムテトラブトキシド７３ｇを３分間かけて仕込んだ。３０分間攪拌した後、水８３８ｇを１０分かけて添加した。セパラブルフラスコのジャケットには５０℃の温水を通水した。引き続き攪拌を継続し、内容物が沸点に到達した時点で、撹拌を停止した。引き続き３０分間、ジャケットに５０℃の温水を通水して、生成したゾルをゲル化させた。
【０１４２】
その後、速やかにゲルを取り出し、目開き６００μｍのナイロン製網を通してゲルを粉砕し、粉体状のウェットゲル（シリカジルコニアヒドロゲル）を得た。
【０１４３】
＜物性調節工程＞
このシリカヒドロゲル３００ｇと水４５０ｇとを、１Ｌのガラス製オートクレーブに仕込み、２００℃で、３時間の密閉系水熱処理を実施した。
【０１４４】
＜水分除去工程＞
水熱処理して得られたシリカを濾紙（Ｎｏ．５Ａ）で濾過し、濾滓を別のセパラブルフラスコに無水メタノール６００ｇと共に加え、攪拌翼を用いて室温で１時間ゆっくり攪拌した。このスラリーをデカンテーションによって固液分離し、得られた固体について、無水メタノール６００ｇを用いて再度、上述と同様の液置換操作を行なった。
初回を合わせ、この操作を合計３回行なったところ、カールフィッシャー法により求められるシリカ中の水分含有量は２重量％以下であった。
【０１４５】
＜乾燥工程＞
この様にして得られたシリカを、１００℃で恒量となるまで減圧乾燥し、実施例１の断熱材とした。
【０１４６】
（１−２）断熱材（シリカ）の分析：
実施例１の断熱材について、その諸物性を以下の手法で測定した。
（１−２−１）細孔容積：
カンタクローム社製ＡＳ−１を用いて、断熱材であるシリカのＢＥＴ窒素吸着等温線を測定し、その細孔容積を求めた。具体的には、相対圧Ｐ／Ｐ_０＝０．９８のときの値を採用した。
【０１４７】
（１−２−２）最頻細孔径：
レーザー式粒度分布測定装置（セイシン企業社製 ＬＭＳ−３０）を用いて、断熱材であるシリカの最頻細孔径を測定した。
【０１４８】
（１−２−３）特定元素の含有量：
断熱材であるシリカ２．５ｇにフッ酸を加えて加熱し、乾涸させたのち、水を加えて５０ｍｌとした。この水溶液を用いてＩＣＰ発光分析を行ない、金属不純物の含有率を求めた。なお、ナトリウム及びカリウムの含有率については、フレーム炎光法で分析した。
【０１４９】
（１−２−４）断熱体の熱伝導度の評価：
断熱材を、金属薄膜層と熱可塑性ポリマー層とを有する容器に入れ、内部を１００Ｐａに減圧した後、開口部にヒートシールを行ない、断熱体とした。この断熱体の熱伝導度を、熱伝導率測定装置（英弘精機株式会社製 ＡＵＴＯ−∧）を用いて平均温度２５℃にて測定した。
（１−２−５）断熱材のかさ密度：
ＪＩＳＫ６７２１にて、断熱材のかさ密度を測定した。
【０１５０】
（２）比較例１：
＜加水分解・縮合工程＞の代わりに以下の操作を行なった他は実施例１と同様にして、比較例としての断熱材及び断熱体を製造し、諸物性を測定した。
【０１５１】
ガラス製で、上部に大気開放の水冷コンデンサが取り付けてある５Ｌセパラブルフラスコ（ジャケット付き）に、濃度２０％の硫酸５５０ｇを仕込んだ。これに、２．５倍に希釈したＪＩＳ３号ケイ酸ナトリウム１８５０ｇを徐々に添加してｐＨ４．５のシリカゾルを生成させた。このヒドロゾルは約１０分後にゲル化したが、更に一夜放置後、粗粉砕して１Ｎの硫酸中で８０℃２時間の酸処理を行ない、酸処理後のヒドロゲルをｐＨ５になるまで水洗いした。
【０１５２】
（３）結果：
得られた実施例及び比較例の諸物性を、表−１に示す。
【表１】

【０１５３】
表−１から明らかなように、細孔容積が大きいシリカと、周期表第５周期及び第６周期の元素から選ばれる特定元素とを含み、かさ密度が小さく、且つ、シリカ及び特定元素の合計に対する特定元素の含有率が所定範囲にある断熱材が、優れた断熱性を発揮することができることがわかる。
【０１５４】
【発明の効果】
本発明によれば、細孔容積が大きいシリカを用いるとともに、周期表第５周期及び第６周期の元素から選ばれる特定元素を所定の濃度で含有させ、且つ、断熱材のかさ密度を所定の値以下に抑えることにより、強度や耐久性、成形性に優れ、高い断熱性を発揮し、且つ、比較的簡単な方法で製造可能な、優れた断熱材を提供することができる。また、この断熱材を外装材で密閉して断熱体とすることにより、各種用途に好適に用いることが可能となる。
【図面の簡単な説明】
【図１】（ａ）（ｂ）はともに、本発明の断熱材における特定元素の存在状態の類型を、特にシリカとの関係に着目して模式的に表わした図である。
【図２】（ａ）（ｂ）はともに、本発明の断熱材における特定元素の存在状態の類型を、特にシリカとの関係に着目して模式的に表わした図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a heat insulating material suitable for use in electrical appliances and building materials, and a heat insulating body using the same, and more particularly, to a heat insulating material containing silica and a heat insulating material obtained by sealing the same with an exterior material.
[0002]
[Prior art]
BACKGROUND ART Conventionally, heat insulating materials have been developed for efficiently using heat in equipment for keeping and cooling a living environment, and for cooling and heating. In recent years, a large number of electric appliances equipped with an information processing device inside such as a notebook computer and a so-called information home appliance have been provided, but these devices have been designed to prevent internal heat from going outside. Also, it is necessary to prevent external heat from adversely affecting the internal information processing device, and it is necessary to use an excellent heat insulating material. As described above, there are more and more opportunities to use the heat insulating material. For this reason, research for developing an excellent heat insulating material has been conducted, and various heat insulating materials have been proposed.
[0003]
As a heat insulating material, for example, Patent Document 1 discloses that a silica hydrogel is produced by a sol-gel method using water glass as a raw material, and an aerogel of silica is produced by drying the silica hydrogel, and the aerogel is used as a heat insulating material. Has been proposed. According to this technique, a heat insulating material having excellent transparency and heat insulating properties can be obtained.
[0004]
Patent Literature 2 discloses a technology in which a conductive powder is added to a heat insulating material using silica to crush silica fine particles. According to this technique, the aggregated particles of silica are crushed by adding a conductive powder, the diameter of the void formed between the aggregated particles is reduced, the gas thermal conductivity can be reduced, and the vacuum heat insulation can be achieved. The heat insulation of the material over time can be improved.
[0005]
[Patent Document 1]
JP 2001-174140 A
[Patent Document 2]
JP 2003-074786 A
[0006]
[Problems to be solved by the invention]
However, the prior arts, including the techniques described in Patent Documents 1 and 2 described above, have not yet provided a heat insulating material having a satisfactory level of heat insulating property. In particular, in the material of Patent Document 2, since it is difficult to uniformly disperse the conductive powder, the conductive powder is unevenly distributed, and the unevenness of the strength due to the uneven distribution occurs. There is a possibility that the moldability is impaired. Further, both the materials disclosed in Patent Document 1 and Patent Document 2 require complicated operations such as pH adjustment during the manufacturing process, and therefore the manufacturing efficiency is low and the manufacturing cost is high. Therefore, development of an excellent heat insulating material which is excellent in strength, durability and moldability, has high heat insulating properties, and can be manufactured by a relatively simple method is desired.
The present invention has been made in view of the above problems, and provides an excellent heat insulating material that has excellent strength, durability, moldability, high heat insulation, and can be manufactured by a relatively simple method. The purpose is to do.
[0007]
[Means for Solving the Problems]
The inventors of the present invention have conducted intensive studies and, as a result, used silica having a large pore volume as a heat insulating material, and contained a relatively large molecular weight element belonging to the fifth or sixth period of the periodic table at a predetermined concentration. Further, they have found that excellent heat insulating properties can be obtained by suppressing the bulk density of the heat insulating material to a predetermined value or less, and completed the present invention.
[0008]
That is, the gist of the present invention is a heat insulating material containing at least silica, wherein (a) the pore volume of the silica is 1.6 ml / g or more, and (b) the fifth period and the fifth period of the periodic table. A specific element selected from the group consisting of elements having six periods is contained at a concentration of 1 wt ppm or more and 30 wt% or less, and (c) the heat insulating material has a bulk density of 0.5 g / ml or less. It exists in the heat insulating material (claim 1).
[0009]
Preferably, the specific element is present in the heat conduction path of the heat insulating material (claim 2).
It is preferable that at least a part of the specific element is bonded to oxygen (claim 3).
[0010]
Further, it is preferable that at least a part of the specific element is incorporated into the skeleton of the silica (claim 4).
[0011]
Further, it is preferable that the heat insulating material is obtained by introducing a simple substance or a compound of the specific element into the skeleton of the silica when producing the silica (claim 5).
[0012]
The silica is produced through a hydrolysis / condensation step of hydrolyzing a silicon alkoxide to form a silica hydrogel, and in the hydrolysis / condensation step, a simple substance or a compound of the specific element is allowed to coexist. Preferably, the specific element is introduced into the skeleton of the silica (claim 6).
[0013]
In addition, the silica is produced through a physical property adjusting step of performing a hydrothermal treatment on a silica hydrogel to form a silica slurry, and in the physical property adjusting step, coexisting a simple substance or a compound of the specific element to form the silica. Preferably, the specific element is introduced into the skeleton (claim 7).
[0014]
Further, the heat insulating material of the present invention is preferably a composition of a simple substance or a compound of the specific element and the silica (claim 8).
[0015]
The specific element or the compound has an electric conductivity at room temperature of 2.5 × 10 ^-3 It is preferably smaller than S / m (claim 9).
[0016]
In the heat insulating material of the present invention, the content of metal impurities in the second to fourth periods of the periodic table is preferably 1% by weight or less (claim 10).
[0017]
Further, the silica is preferably amorphous (claim 11).
Also, the mode pore diameter D of the silica _max Is preferably 5 nm or more (claim 12).
[0018]
The Q of the silica measured by solid-state Si-NMR ⁴ / Q ³ The value is preferably 1.2 or more (claim 13).
[0019]
Another gist of the present invention resides in a heat insulator characterized in that the heat insulator is hermetically sealed with an exterior material (claim 14).
[0020]
At this time, the exterior material preferably includes a gas barrier layer having gas barrier properties (Claim 15), and preferably includes a non-moisture permeable layer having non-moisture permeability (Claim 16).
[0021]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described. However, the present invention is not limited to the following description, and can be modified and implemented within the gist of the present invention.
[0022]
[1. Insulation material of the present invention]
The heat insulating material of the present invention is a heat insulating material made of at least silica,
(A) the pore volume of the silica is 1.6 ml / g or more;
(B) a specific element selected from the group consisting of elements of the fifth and sixth periods of the periodic table at a concentration of 1 wt ppm or more and 30 wt% or less based on the weight of the heat insulating material;
(C) the bulk density of the heat insulating material is 0.5 g / ml or less.
It is characterized by the following.
[0023]
Hereinafter, first, features of silica used for the heat insulating material of the present invention (hereinafter, appropriately referred to as “silica of the present invention”) will be described, and then, other features of the heat insulating material of the present invention will be described.
[0024]
[1-1. Characteristics of the silica of the present invention]
In the present invention, "silica" refers to hydrous silicic acid. Hydrous silicic acid is SiO ₂ ・ NH ₂ O is represented by the following formula. In the present invention, the effect is remarkable in so-called "silica gel" among silica, micelle template type silica, and the like.
[0025]
The silica of the present invention is characterized in that the pore volume exists in a range larger than a normal value. Specifically, the value of the pore volume measured by the nitrogen gas absorption / desorption method is 1.6 ml / g or more. Above all, it is preferably at least 1.8 ml / g, particularly preferably at least 1.85 ml / g. In the porous structure of silica, the voids formed by the pores have a lower heat insulating property than the silica matrix (the difference in the heat insulating property is further increased under reduced pressure or vacuum, as well as under normal pressure and air). growing). Therefore, as the pore volume increases, the ratio of voids in the silica increases, and the heat insulating property of the silica improves. However, if the pore volume is too large, it becomes difficult to maintain the strength of the silica, and the practicality as a heat insulating material is rather deteriorated. Therefore, the upper limit of the pore volume is usually 3.4 ml / g or less, particularly preferably. Is 3.2 ml / g or less, particularly preferably 3 ml / g or less. The pore volume can be determined from the adsorption amount of nitrogen gas at a relative pressure of 0.98 of the adsorption isotherm.
[0026]
In view of the three-dimensional structure of the silica of the present invention, in addition to the above-mentioned features of the pore structure, it is preferable that the silica is amorphous, that is, a crystalline structure is not recognized. This means that when the silica of the present invention was analyzed by X-ray diffraction, substantially no crystalline peak was observed. In this specification, the term “non-amorphous silica” refers to silica that shows at least one crystal structure peak at a position exceeding 6 Å (Å Units d-spacing) in an X-ray diffraction pattern. Amorphous silica is superior to crystalline silica in heat insulation and extremely excellent in productivity.
[0027]
In the silica of the present invention, the content of metals in the second to fourth periods of the periodic table (hereinafter, appropriately referred to as “metal impurities”) is usually 1% by weight or less, especially 0.7% by weight or less. Is preferably 0.5% by weight or less. When the metal impurities are composed of a plurality of metal species, the total content of the metal impurities is preferably within the above range. The fact that the influence of the metal impurities is small is one of the major factors in which the heat insulating material of the present invention can exhibit high heat insulating properties.
[0028]
Further, the silica of the present invention has an E.C. P. Barrett, L .; G. FIG. Joyner, P .; H. Haklenda, J .; Amer. Chem. Soc. , Vol. 73, 373 (1951), the pore distribution curve calculated by the BJH method, that is, the differential nitrogen gas adsorption amount (ΔV / Δ (logd) with respect to the pore diameter d (nm); ) Is plotted in the mode pore size (D _max ) Is usually at least 5 nm, especially at least 7 nm, further preferably at least 10 nm, and usually at most 50 nm, especially at most 30 nm. Most frequent pore size (D _max ) Is smaller than this range, the proportion of voids in the silica decreases, and the heat insulating property of the silica decreases. On the other hand, the most frequent pore diameter (D _max Is larger than this range, it becomes difficult to maintain the strength of silica, and the practicality as a heat insulating material is rather reduced.
[0029]
In connection with such characteristics, the silica of the present invention has a mode pore diameter (D) calculated by the BJH method described above. _max )), The differential pore volume ΔV / Δ (logd) is usually at least 2 ml / g, especially at least 3 ml / g, particularly preferably at least 5 ml / g, usually at most 40 ml / g, especially at most 30 ml / g, especially It is preferably at most 25 ml / g (where d is the pore diameter (nm) and V is the nitrogen gas adsorption volume). Those in which the differential pore volume ΔV / Δ (logd) is included in the above range are the most frequent pore diameters (D _max It can be said that the absolute amount of the pores arranged near ()) is extremely large.
[0030]
In addition, the silica of the present invention preferably has a small distortion in the structure of the silica constituting the silica. Here, the structural strain of the silica is Q in solid-state Si-NMR measurement. ⁴ It can be represented by the value of the chemical shift of the peak. Hereinafter, the structural distortion of silica and the aforementioned Q ⁴ The relationship between the peak and the value of the chemical shift will be described in detail.
[0031]
The silica of the present invention is a hydrate of silicic acid, SiO 2 ₂ ・ NH ₂ Although represented by the descriptive formula of O, structurally, it has a structure in which O is bonded to each vertex of the tetrahedron of Si, and Si is further bonded to these Os to spread in a net shape. Then, in the repeating unit of Si—O—Si—O—, part of O is replaced by another member (eg, —H, —CH ₃ And the like, and when attention is paid to one Si, as shown in the following formula (A), Si (Q ⁴ ), As shown in the following formula (B), Si (Q ³ (In the following formulas (A) and (B), the above tetrahedral structure is ignored, and the net structure of Si—O is represented in a plane.) In the solid-state Si-NMR measurement, the peaks based on each of the above Sis are sequentially Q ⁴ Peak, Q ³ It is called a peak.
[0032]
Embedded image

[0033]
The silica of the present invention has a Q ⁴ / Q ³ Is usually 1.2 or more, especially 1.4 or more. Where Q ⁴ / Q ³ Is the value of Si (Q in which three -OSi are bonded among the repeating units of silica described above. ³ ) Is bonded to Si (Q ⁴ ) Means the molar ratio. The upper limit is not particularly limited, but is usually 10 or less. In general, it is known that the higher the value, the higher the thermal stability of silica. From this, it can be seen that the silica of the present invention is more excellent in thermal stability.
[0034]
Note that Q ⁴ / Q ³ Can be calculated based on the result of performing solid-state Si-NMR measurement. The analysis of the measurement data (determination of the peak position) is performed, for example, by a method of dividing and extracting each peak by waveform separation analysis using a Gaussian function.
[0035]
The heat insulating material of the present invention contains a specific element selected from the group consisting of elements of the fifth and sixth periods of the periodic table (hereinafter, simply referred to as “specific element” as appropriate) in addition to the characteristics of silica described above. Is the biggest feature.
[0036]
Here, the principle of the present invention will be described.
Generally, in a portion where two atoms are bonded, the larger the difference in atomic weight between the atoms, the more the heat conduction is hindered, and the lower the thermal conductivity when heat is transmitted between those atoms tends to be. is there. Therefore, an element having an atomic weight significantly larger than the atomic weights of oxygen and silicon forming the basic skeleton of silica, that is, a specific element belonging to the fifth and sixth periods of the periodic table, is added to the above-mentioned silica or By arranging it around, this specific element exerts the function of blocking heat conduction. In other words, the difference between the atomic weight of silicon and the atomic weight of the specific element, or the large difference between the atomic weight of oxygen and the atomic weight of the specific element, between silicon and the specific element, or between oxygen and the specific element Since heat conduction is hindered, the heat insulating material of the present invention can exhibit better heat insulating properties as compared to a heat insulating material composed of purely silica.
[0037]
Specific examples of the specific element include Rb, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, I, Cs, Ba, lanthanoids (La , Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pd, Bi, Po, Pb, Rb, Ba, Zr, Sr, W, Ce, Y, Nd, Nb, La, and more preferably Pb, Rb, Ba, Zr, Sr, W Is mentioned. In addition, one of these specific elements may be used alone, or two or more thereof may be used in any combination at any ratio and in any combination.
[0038]
The heat insulating material of the present invention contains the above-mentioned specific element in an amount of usually 1 ppm by weight or more, preferably 2% by weight or more, more preferably 3% by weight or more, and usually 30% by weight or less, preferably 25% by weight or less. Preferably, the content is 20% by weight or less. If the ratio of the specific element is smaller than this, the heat insulating material may not be able to exhibit sufficient heat insulating properties, and if the ratio of the specific element is larger than this, the volume of the voids of the heat insulating material decreases. This is because the heat insulation is reduced.
[0039]
More specifically, the range of the content of the specific element differs depending on the type of the specific element used. That is, it is usually preferable that the larger the atomic weight of the specific element is, the smaller the ratio of the substance amount of the specific element contained in the silica is, and the smaller the atomic weight of the specific element is, the larger the substance amount of the specific element contained in the silica is. However, when attention is paid to the weight ratio as the content of the specific element, the weight of the specific element having a larger atomic weight is larger when compared with the same substance amount, so that the content of the specific element used in the heat insulating material of the present invention is Should be determined in consideration of the atomic weight of the specific element and the preferred amount of the substance. As a specific example, when Pb having a large atomic weight is used as the specific element, the content is usually 1 wt ppm or more and usually 10 wt% or less. When Zr having a small atomic weight is used as the specific element, the content is usually 10% by weight or more and usually 30% by weight or less. When a plurality of types of specific elements are used, the content is determined according to the average atomic weight of the whole specific element to be used. The method of measuring the content of the specific element is arbitrary, and for example, it can be measured by a known method used for metal analysis or by X-ray fluorescence analysis.
[0040]
Further, the range of the content of the specific element differs depending on the state of the specific element in the heat insulating material of the present invention. Hereinafter, the state of the specific element in the heat insulating material of the present invention will be described, and the content of the specific element will also be described.
[0041]
The state of the specific element in the heat insulating material of the present invention is not particularly limited, and may be in an arbitrary state. Examples of the type of the state of existence of the specific element include a state in which the specific element is incorporated in the skeleton of silica (FIG. 1A), a state in which the specific element is sandwiched between silica particles (FIG. 1B), and pores of silica. There is a state of being carried inside (FIG. 2 (a)) and a state of being adhered around silica particles (FIG. 2 (b)), and any of these may be used. Needless to say, the existence states of these specific elements are merely types, and the specific elements may be present in other states. Further, two or more existing states may be mixed.
[0042]
However, in the present invention, it is desired that at least a part of the specific element exists in the heat conduction path of the heat insulating material. Here, the heat conduction path of the heat insulating material refers to a portion other than the void of the heat insulating material, that is, a solid portion of the heat insulating material mainly formed by a skeleton of silica. In general, since the space in the heat insulating material is decompressed to a state in which a small amount of gas is present or a vacuum state, heat transmitted through the heat insulating material is mainly a solid portion of the heat insulating material formed by the silica skeleton, that is, a heat insulating material. It will travel along the heat conduction path of the material. At this time, if the specific element is present in the heat conduction path of the heat insulating material, the heat conduction preventing function by the specific element is effectively exerted, and the heat insulating property can be effectively improved.
[0043]
Speaking of the above-mentioned type, if the specific element is present in a state of being incorporated into the silica skeleton (FIG. 1 (a)) or being sandwiched between silica particles (FIG. 1 (b)), the specific element Since the element is located in the heat conduction path of the heat insulating material, it is considered that the heat conduction preventing function by the specific element is effectively exhibited. On the other hand, when the specific element exists in a state of being supported in the pores of silica (FIG. 2A) or attached to the periphery of the silica particles (FIG. 2B), the specific element of the heat insulating material It is considered that it is not located in the heat conduction path and the heat conduction preventing function by the specific element is not sufficiently exhibited. Therefore, it is desirable that at least a part, preferably most of the specific element exists in a state of being located in the heat conduction path of the heat insulating material as represented by the types of FIGS. 1 (a) and 1 (b). It is.
The presence of the specific element in the heat conduction path of the heat insulating material can be confirmed by electron probe microanalysis.
[0044]
Even when the specific element is present in the heat conduction path of the heat insulating material, it is particularly preferable that the specific element be present in the skeleton of silica, that is, in the Si—O skeleton of silica. When heat is transmitted through the heat insulating material, the heat is mainly transmitted through the Si—O skeleton. Therefore, if the specific element is present in the silica skeleton, the specific element can be more reliably positioned on the heat transmission path. Therefore, the heat insulating property can be more effectively improved.
The presence of the specific element in the skeleton of silica can be confirmed by a wide-area X-ray absorption fine structure (EXAFS).
[0045]
When the specific element is present in the skeleton of silica, the specific element is present in the skeleton of silica as represented by Si-OM or Si-O-OM (M represents a specific element). It is preferably bonded to an oxygen atom. This is because the difference in atomic weight between the specific element and O is larger than the difference in atomic weight between Si and O, and as a result, the heat conduction blocking function by the specific element is more effectively exhibited. .
[0046]
Here, the method for causing the specific element to be present in the heat conduction path of the heat insulating material is not particularly limited, but specific examples include (i) a method of coexisting a single element atom or a compound of the specific element when producing silica. , (Ii) a method of producing silica and then mixing it with a single element of a specific element or a compound to form a composition (the details of these methods will be described later). In either case of using the method (i) or (ii), the specific element exists at various positions. However, when the method (i) is used, the type shown in FIG. As shown in FIG. 1, the ratio of the specific element taken into the skeleton of the silica increases, and when the method (ii) is used, as shown in the type of FIG. The ratio of the specific element interposed between them increases. Therefore, the heat insulation performance can be more efficiently improved by using the method (i). However, even when the method (ii) is used, the abundance of the elemental element or the compound of the specific element is increased. , Similar heat insulation performance can be obtained.
[0047]
Specifically, when the method (i) is used, the content of the elemental element or the compound of the specific element contained in the silica may be relatively low, and is specifically 1 to 5% by weight. When the method (ii) is used, the content of the elemental element or the compound of the specific element needs to be relatively high, and specifically, it is usually 1% by weight to 30% by weight.
[0048]
When a compound of a specific element is used, the type of the compound is not particularly limited. Examples thereof include a halide, an oxide, a hydroxide, a sulfide, and a complex of the specific element, or a sulfate or a carbonate of the specific element. Various salts such as salts are exemplified. Among these compounds, compounds having a specific element-oxygen bond are preferable, and oxides are particularly preferable.
[0049]
In addition, the electric conductivity at room temperature of a simple substance of the specific element or a compound thereof is usually 2.5 × 10 ^-3 S / m, preferably 2.0 × 10 ^-3 S / m or less, more preferably 1.5 × 10 ^-3 S / m or less is desired. Therefore, among the compounds of the above-mentioned specific elements, a compound mainly composed of SnO, HgTe, SnTe, CdTe, etc. is 2.5 × 10 ^-3 Since it may have an electrical conductivity of S / m or more, its use should usually be avoided. In general, a compound having high electric conductivity has a small difference between the highest energy of the valence band and the lowest energy of the conduction band, that is, a so-called band gap, so that electric conduction by electrons or holes occurs at room temperature. When electric conduction occurs, heat transfer occurs due to electrons, so that the silica containing a compound having high electric conductivity has reduced heat insulation. Therefore, in the present invention, by using a substance having low electric conductivity as the specific element or the compound thereof, the heat insulating property of the heat insulating material of the present invention can be further enhanced. Note that the electric conductivity of a specific element may be measured by a known method, but the values described in a literature or the like may be referred to the values in the literature.
[0050]
Another feature of the heat insulating material of the present invention is that the bulk density is usually 0.5 g / ml or more, preferably 0.45 g / ml or less, more preferably 0.4 g / ml or less. . As the bulk density takes a smaller value, the ratio of voids in the silica increases, and the heat insulating property of the silica improves. However, if the bulk density is too large, it becomes difficult to maintain the strength of the silica, and the practicality as a heat insulating material is rather deteriorated. Therefore, the lower limit is usually 0.01 g / ml or more, preferably 0.02 g / ml. ml or more, more preferably 0.03 g / ml or more.
[0051]
More specifically, the range of the bulk density differs depending on the pore diameter of the silica of the present invention and the type of the specific element. That is, the larger the pore diameter of the silica of the present invention, the lower the bulk density, and the higher the molecular weight of the specific element, the lower the bulk density.
[0052]
As a specific example, when Pb (atomic weight 207.2 g / mol) having a relatively large atomic weight is used as the specific element, the bulk density is usually 0.5 g / ml or less, preferably 0.45 g / ml or less. Preferably it is 0.4 g / ml or less. Further, when Zr having a relatively small atomic weight (atomic weight 91.22 g / mol) is used as the specific element, the bulk density is usually 0.35 g / ml or less, preferably 0.3 g / ml or less, more preferably 0 g / ml or less. 0.2 g / ml or less. When a plurality of specific elements are used, the bulk density is determined according to the average atomic weight of the entire specific element to be used. The bulk density can be measured by any known method such as JIS K6721.
[0053]
[2. Method of manufacturing heat insulating material]
Next, a method for manufacturing the heat insulating material of the present invention will be described.
The heat insulating material of the present invention is not particularly limited as long as it is a method capable of manufacturing a heat insulating material having the above-mentioned physical properties, but the main method is as follows: (i) when producing the silica of the present invention, Examples include a method of introducing an elemental atom or a compound, and (ii) a method of producing the silica of the present invention, and then mixing with a specific elemental atom or a compound to form a composition.
[0054]
In the following description, first, the method for producing the silica of the present invention, which is the basis of the method for producing the heat insulating material of the present invention, will be described, and then the above-mentioned (i) and (ii) Each case will be described.
[0055]
[2-1. Method for producing silica of the present invention]
The silica of the present invention generally comprises a step of hydrolyzing a silicon alkoxide (hydrolysis / condensation step), a step of hydrothermally treating the silica hydrogel obtained in the hydrolysis / condensation step (physical property control step), and a property control step Having a water content of 5% by weight or less in the liquid component of the silica slurry obtained in step (water removal step), and drying the silica slurry obtained in the water removal step (drying step) It is manufactured by The heat insulating material of the present invention is manufactured by introducing a specific element into silica during, before, or after any of the steps of this manufacturing method.
[0056]
The method for producing silica of the present invention described above specifically comprises hydrolyzing silicon alkoxide, subjecting the obtained silica hydrogel to hydrothermal treatment, preferably without substantial aging, and then performing hydrophilic treatment as a water removing step. After contacting with an organic solvent, it is dried to remove water in the silica.
[0057]
[2-1-1. Hydrolysis / condensation step]
In the hydrolysis / condensation step, the silicon alkoxide is hydrolyzed to obtain a silica hydrogel. The obtained silica hydrogel is preferable because the raw material silicon alkoxide can be highly purified and impurities can be easily mixed into the silica hydrogel.
[0058]
As the silicon alkoxide used as a raw material of the silica of the present invention, a lower alkyl group having 1 to 4 carbon atoms such as trimethoxysilane, tetramethoxysilane, triethoxysilane, tetraethoxysilane, tetrapropoxysilane, and tetrabutoxysilane. Tri- or tetraalkoxysilanes or oligomers thereof. Among them, tetramethoxysilane, tetraethoxysilane and oligomers thereof, particularly tetramethoxysilane and oligomers thereof, are preferable since silica having good pore characteristics can be obtained. The main reason is that silicon alkoxide is easily purified by distillation to obtain a high-purity product, and thus is suitable as a raw material for high-purity silica. The total content of metal impurities in the silicon alkoxide is usually 3000 ppm by weight or less, preferably 2000 ppm by weight or less, more preferably 1000 ppm by weight or less, and particularly preferably 100 ppm by weight or less. The content of these metal impurities can be measured by the same method as that for measuring the content of impurities in general silica. By using a silicon alkoxide having a low content of metal impurities as described above, the content of metal impurities in the manufactured silica can be reduced.
[0059]
In the present invention, first, in a hydrolysis / condensation step, silicon alkoxide is hydrolyzed in the absence of a catalyst, and the obtained silica hydrosol is condensed to form a silica hydrogel.
[0060]
The hydrolysis of silicon alkoxide is usually at least 2 mole times, preferably at least 3 mole times, particularly preferably at least 4 mole times, and usually at most 20 mole times, preferably at most 10 mole times, per mole of silicon alkoxide. Particularly preferably, the reaction is carried out using 8 mol times or less of water. Hydrolysis of the silicon alkoxide generates a silica hydrogel and an alcohol, and the generated silica hydrosol is sequentially condensed into a silica hydrogel.
[0061]
The temperature at the time of hydrolysis is usually from room temperature to 100 ° C., but it is possible to carry out the hydrolysis at a higher temperature by maintaining the liquid phase under pressure. The reaction time required for the hydrolysis depends on the composition of the reaction solution (the type of silicon alkoxide and the molar ratio with water) and the reaction temperature, and the time until gelation is different, and thus is not specified unconditionally. In order to obtain silica having excellent pore characteristics like the silica of the present invention, the reaction time is preferably such that the fracture stress of the hydrogel does not exceed 6 MPa.
[0062]
The hydrolysis can be promoted by adding an acid, alkali, salt or the like as a catalyst to the hydrolysis reaction system. However, the use of such additives causes the aging of the produced hydrogel, as described later, and there is a possibility that the impurities such as acids, alkalis and salts may be mixed with metal impurities. Is not very desirable.
[0063]
In the hydrolysis of the silicon alkoxide described above, it is important to perform sufficient stirring. For example, when a stirrer provided with a stirring blade on the rotating shaft is used, the stirring speed (the number of rotations of the rotating shaft) depends on the shape, the number of the stirring blades, the contact area with the liquid, and the like. It is 30 rpm or more, preferably 50 rpm or more.
[0064]
In general, if the stirring speed is too high, the droplets generated in the tank may block various gas lines or may adhere to the inner wall of the reactor to deteriorate the heat conduction, and the temperature which is important for controlling the physical properties may be reduced. Or affect management. Further, the deposits on the inner wall may be peeled off and mixed with the product to deteriorate the quality. For such a reason, the stirring speed is preferably 2000 rpm or less, particularly preferably 1000 rpm or less.
[0065]
In the present invention, as a method of stirring the two liquid phases (aqueous phase and silicon alkoxide phase) that are separated, any stirring method can be used as long as the method promotes the reaction. Above all, examples of an apparatus for further mixing the two liquid phases include the following (1) and (2).
[0066]
{Circle around (1)} A device having a stirring blade in which the rotating shaft is inserted perpendicularly or slightly at an angle to the liquid surface and the liquid flows vertically.
{Circle around (2)} A stirrer provided with a rotating shaft in a direction substantially parallel to the interface between the two liquid phases and having a stirring blade for causing stirring between the two liquid phases.
[0067]
The rotation speed of the stirring blade when using the above-mentioned devices (1) and (2) is the peripheral speed of the stirring blade (tip speed of the stirring blade). ５5 m / s, and more preferably 0.1 to 3 m / s.
[0068]
The shape and length of the stirring blade are arbitrary, and examples of the stirring blade include a propeller type, a flat blade type, an angled flat blade type, a pitched flat blade type, a flat blade disk turbine type, a curved blade type, a faudler type, and the like. Bull margin type and the like.
[0069]
The blade width, the number of blades, the inclination angle, and the like may be appropriately selected according to the shape and size of the reactor and the desired stirring power. For example, the ratio (b / D) of the blade width (length of the blade in the rotation axis direction) to the inner diameter of the tank of the reactor (the longest diameter of the liquid phase surface forming a plane perpendicular to the rotation axis direction) is 0.05 to A stirrer having 0.2, a tilt angle (θ) of 90 ° ± 10 °, and 3 to 10 blades is a preferred example.
[0070]
Above all, a structure in which the above-described rotating shaft is provided above the liquid surface in the reaction vessel and a stirring blade is provided at the tip of the shaft extending from the rotating shaft is preferably used from the viewpoint of stirring efficiency and equipment maintenance. .
[0071]
In the above-mentioned hydrolysis reaction of silicon alkoxide, silicon alkoxide is hydrolyzed to produce silica hydrosol, but subsequently, a condensation reaction of the silica hydrosol occurs, the viscosity of the reaction solution increases, and gelation occurs finally. Into a silica hydrogel.
[0072]
[2-1-2. Physical property adjustment step]
Next, in the present invention, as a physical property adjusting step, hydrothermal treatment of the silica hydrogel is performed without substantial aging. Hydrolysis of the silicon alkoxide produces a soft silica hydrogel. Incidentally, it is difficult to produce the silica of the present invention by a conventional method of aging or drying and then performing a hydrothermal treatment to stabilize the physical properties of the hydrogel.
[0073]
As described above, performing the hydrothermal treatment immediately without substantially aging the silica hydrogel generated by the hydrolysis means that the next water is maintained while the soft state immediately after the silica hydrogel is generated is maintained. This means that it is subjected to heat treatment.
[0074]
Specifically, it is preferable that the hydrothermal treatment is generally performed within 10 hours from the time when the silica hydrogel is generated, and particularly, within 8 hours, further within 6 hours, particularly within 4 hours. Is preferred.
[0075]
Further, in an industrial plant or the like, a case where a large amount of silica hydrogel is temporarily stored in a silo or the like and then subjected to hydrothermal treatment may be considered. In such a case, the time from when the silica hydrogel is formed to when it is subjected to the hydrothermal treatment, that is, the so-called leaving time, may exceed the above range. In such a case, the liquid component in, for example, the silica hydrogel may be prevented from drying during standing in the silo so that ripening does not substantially occur.
[0076]
Specifically, for example, the inside of the silo may be sealed or the humidity may be adjusted. Alternatively, the silica hydrogel may be allowed to stand in a state where the silica hydrogel is immersed in water or another solvent.
[0077]
The temperature at the time of standing is preferably as low as possible. Further, as another method for substantially preventing aging, there is a method of preparing a silica hydrogel by controlling a raw material composition in advance so that the silica concentration in the silica hydrogel is reduced.
[0078]
Considering the effects achieved by hydrothermal treatment of the silica hydrogel without substantially aging the silica hydrogel and the reason why this effect can be obtained, the following can be considered.
[0079]
First, when the silica hydrogel is aged, it is considered that a macro network structure due to -Si-O-Si- bonds is formed in the entire silica hydrogel. When this network structure is present in the entire silica hydrogel, it is considered that this network structure becomes an obstacle during hydrothermal treatment, and it becomes difficult to form a mesoporous material. Further, the silica hydrogel obtained by controlling the raw material composition in advance so that the silica concentration in the silica hydrogel becomes low can suppress the progress of cross-linking in the silica hydrogel generated during standing. Therefore, it is considered that the silica hydrogel does not age.
[0080]
Therefore, in this production method, it is important to perform hydrothermal treatment without aging the silica hydrogel.
[0081]
Further, the silica hydrogel obtained by hydrolyzing the silicon alkoxide is subjected to a pulverizing treatment before hydrothermal treatment so that the silica hydrogel has an average particle size of 10 mm or less, particularly 5 mm or less, further 1 mm or less, particularly 0.5 mm or less. It is preferable to perform the above.
[0082]
As described above, as a method for producing the silica of the present invention, it is important to immediately carry out hydrothermal treatment of the silica hydrogel immediately after its production. However, in this production method, it is sufficient that the silica hydrogel to be hydrothermally treated is not aged. Therefore, for example, the hydrothermal treatment is performed immediately after the silica hydrogel is generated, for example, the hydrothermal treatment is performed after standing at a low temperature for a while. You don't need to.
[0083]
As described above, when the hydrothermal treatment is not performed immediately after the silica hydrogel is generated, for example, the hydrothermal treatment may be performed after specifically confirming the aging state of the silica hydrogel. Means for specifically confirming the aging state of the hydrogel is arbitrary, and examples thereof include a method in which the hardness of the hydrogel measured by a method described in Examples described later is referred to. That is, as described above, the hydrogel in a soft state having a breaking stress of usually 6 MPa or less is subjected to hydrothermal treatment, whereby silica having the physical property range defined in the present invention can be obtained. The breaking stress is preferably 3 MPa or less, and particularly preferably 2 MPa or less.
[0084]
As the condition of the hydrothermal treatment, the state of water may be either liquid or gas. Among them, it is preferable to perform hydrothermal treatment using liquid water in a slurry state in addition to silica hydrogel. In the hydrothermal treatment, first, the silica hydrogel to be treated is usually at least 0.1 times by weight, preferably at least 0.5 times by weight, particularly preferably at least 1 time by weight, and usually at least 10 times by weight based on the weight of the silica hydrogel. Water is added up to twice, preferably up to 5 times by weight, particularly preferably up to 3 times by weight to form a slurry. Then, the slurry is usually used at a temperature of 40 ° C. or higher, preferably 100 ° C. or higher, more preferably 150 ° C. or higher, particularly preferably 170 ° C. or higher, and usually 250 ° C. or lower, preferably 200 ° C. or lower. The hydrothermal treatment is performed for at least one hour, preferably at least one hour, and usually at most 100 hours, preferably at most 10 hours. If the temperature of the hydrothermal treatment is too low, it may be difficult to sharpen the pore distribution, and it may be difficult to increase the pore volume.
[0085]
Note that the water used for the hydrothermal treatment may contain a solvent. Specific examples of the solvent include lower alcohols such as methanol, ethanol, and propanol. When the silica hydrogel obtained by hydrolyzing the alkoxysilane is subjected to hydrothermal treatment, the solvent may be an alcohol derived from the alkoxysilane as a raw material. Of course, it goes without saying that a solvent containing no or almost no metal impurities should be selected.
[0086]
The content of such a solvent in the water used for the heat treatment is arbitrary, but is preferably as small as possible. For example, as described above, when hydrothermally treating a silica hydrogel obtained by hydrolyzing an alkoxysilane, the silica hydrogel is washed with water, and the washed product is subjected to a hydrothermal reaction, so that about 150 ° C. Even when the hydrothermal treatment is performed at a lower temperature, silica having excellent pore characteristics and a large pore volume can be produced. Further, even if the hydrothermal treatment is performed with water containing a solvent, the silica of the present invention can be easily obtained by performing the hydrothermal treatment at a temperature of about 200 ° C.
[0087]
It is also possible to perform hydrothermal treatment by continuously changing the temperature conditions using a reactor for the hydrolysis reaction, but the optimal conditions are usually different between the hydrolysis reaction and the subsequent hydrothermal treatment. It is generally difficult to obtain the silica of the present invention by a method in which water is not newly added.
[0088]
Under the above conditions of hydrothermal treatment, increasing the temperature tends to increase the diameter and pore volume of the obtained silica. The hydrothermal treatment temperature is preferably in the range of 100 to 200C. Further, with the treatment time, the specific surface area of the obtained silica tends to gradually decrease after reaching the maximum once. Based on the above tendency, it is necessary to appropriately select the conditions according to the desired physical property values, but the hydrothermal treatment is for the purpose of changing the physical properties of the silica, and therefore, usually, a higher temperature condition than the hydrolysis reaction conditions described above. Is preferred.
[0089]
In addition, in order to produce the silica of the present invention having excellent microstructural homogeneity, it is necessary to use a rapid heating rate condition so that the temperature in the reaction system reaches the target temperature within 5 hours during hydrothermal treatment. Is preferred. Specifically, in the case of processing by filling in a tank, the average heating rate from the start of heating to the target temperature is usually 0.1 ° C./min or more, especially 0.2 ° C./min or more, Usually, a value in the range of 100 ° C./min or less, particularly 30 ° C./min or less, particularly 10 ° C./min or less is preferably used.
[0090]
A heating method using a heat exchanger or the like or a heating method in which hot water prepared in advance is charged is also preferable because the heating rate can be reduced. If the rate of temperature rise is within the above range, the temperature may be raised stepwise. If it takes a long time for the temperature in the reaction system to reach the target temperature, the aging of the silica hydrogel proceeds during the temperature increase, and the microstructural homogeneity may decrease.
[0091]
The heating time until reaching the target temperature is preferably within 4 hours, more preferably within 3 hours. In order to shorten the heating time, water used for hydrothermal treatment can be preheated.
[0092]
If the temperature and time of the hydrothermal treatment are set outside the above ranges, it becomes difficult to obtain the silica of the present invention. For example, if the temperature of the hydrothermal treatment is too high, the pore diameter and pore volume of the silica become too large, and the pore distribution is widened. Conversely, if the temperature of the hydrothermal treatment is too low, the resulting silica will have a low degree of crosslinking and poor thermal stability, no peak will appear in the pore distribution, or the Q in the solid-state Si-NMR described above. ⁴ / Q ³ The value becomes extremely small.
[0093]
When the hydrothermal treatment is performed in ammonia water, the same effect can be obtained at a lower temperature than when the hydrothermal treatment is performed in pure water. In addition, when hydrothermal treatment is performed in ammonia water, silica finally obtained is generally hydrophobic compared to hydrothermal treatment using water containing no ammonia. At this time, if the temperature of the hydrothermal treatment is set to a relatively high temperature of 30 ° C. or higher, preferably 40 ° C. or higher, and 250 ° C. or lower, preferably 200 ° C. or lower, the hydrophobicity becomes particularly high. Here, the ammonia concentration of the aqueous ammonia is preferably 0.001% or more, particularly preferably 0.005% or more, and preferably 10% or less, particularly preferably 5%.
[0094]
[2-1-3. Water removal process]
The silica obtained through the above-described hydrothermal treatment contains a large amount of water. For example, silica after hydrothermal treatment is obtained as silica containing a large amount of water (for example, silica slurry). In the method for producing silica, it is important to remove the water. Specifically, the operation of bringing the silica slurry into contact with a hydrophilic organic solvent, replacing water with the hydrophilic solvent, and then drying the silica is the most important.
[0095]
In the production method shown here, the water contained in the silica is replaced with a hydrophilic organic solvent, and drying is performed, whereby the shrinkage of the silica in the drying step is suppressed, and the pore volume of the silica can be largely maintained. It is possible to obtain silica having excellent pore characteristics and a large pore volume. The reason for this is not clear, but is considered to be due to the following phenomena.
[0096]
Most of the liquid components in the silica slurry after the hydrothermal treatment are water. Since this water interacts strongly with the silica, it is considered that large energy is required to completely remove the water from the silica.
[0097]
When a drying process (for example, heating and drying) is performed under a condition in which a large amount of water is present, water that has received thermal energy reacts with unreacted silanol groups, and the structure of silica changes. The most remarkable change in the structural change is condensation of the silica skeleton, and it is considered that the condensation locally increases the density of silica. Since the silica skeleton has a three-dimensional structure, local condensation of the skeleton (densification of the silica skeleton) affects the pore characteristics of the entire silica particles constituted by the silica skeleton, and as a result, the particles are It is thought that it shrinks and the pore volume and the pore diameter shrink.
[0098]
Therefore, for example, by replacing the liquid component (containing a large amount of water) in the silica slurry with a hydrophilic organic solvent, it is possible to remove water in the silica slurry and suppress the above-described silica shrinkage. Become.
[0099]
The hydrophilic organic solvent used in the present invention may be any solvent that can dissolve a large amount of water based on the above-mentioned concept. Among them, those having large intramolecular polarization are preferable. More preferably, the material has a relative dielectric constant of 15 or more.
[0100]
In the method for producing silica of the present invention, in order to obtain high-purity silica, it is necessary to remove the hydrophilic organic solvent in a drying step after removing water with a hydrophilic organic solvent. Therefore, as the hydrophilic organic solvent, those having a low boiling point that can be easily removed by drying (for example, drying by heating or drying under vacuum or reduced pressure) are preferable. The boiling point of the hydrophilic organic solvent is preferably 150 ° C. or lower, more preferably 120 ° C. or lower, and particularly preferably 100 ° C. or lower.
[0101]
Specific hydrophilic organic solvents include alcohols such as methanol, ethanol, propanol and butanol, ketones such as acetone, methyl ethyl ketone and diethyl ketone, nitriles such as acetonitrile, amides such as formamide and dimethylformamide, and aldehydes. And ethers. Among them, alcohols and ketones are preferable, and lower alcohols such as methanol, ethanol, and propanol and acetone are particularly preferable. In the present invention, one of these exemplified hydrophilic organic solvents may be used alone, or two or more of them may be used in any combination and in any ratio. Needless to say, the hydrophilic organic solvent should be selected from those containing no or almost no metal impurities.
[0102]
Note that as long as water can be removed, water may be contained in the hydrophilic organic solvent used. However, the water content of the hydrophilic organic solvent is naturally preferably as small as possible, usually 20% or less, especially 15% or less, further preferably 10% or less, particularly preferably 5% or less.
[0103]
In the present invention, the temperature and the pressure at the time of the substitution treatment with the above-mentioned hydrophilic organic solvent are arbitrary. The treatment temperature is usually 0 ° C. or higher, preferably 10 ° C. or higher, usually 100 ° C. or lower, particularly preferably 60 ° C. or lower. The pressure during the treatment may be any of normal pressure, pressurization and decompression.
[0104]
The amount of the hydrophilic organic solvent to be brought into contact with the silica slurry is arbitrary. However, if the amount of the hydrophilic organic solvent used is too small, the rate of progress of substitution with water is not sufficient, and if the amount is too large, the efficiency of substitution with water increases, but the effect corresponding to the increase in the amount of the hydrophilic organic solvent used is It has reached a plateau and is not economically favorable. Therefore, the amount of the hydrophilic organic solvent used is usually 0.5 to 10 times the volume of the bulk volume of silica. It is preferable to perform the replacement operation with the hydrophilic organic solvent a plurality of times, since the replacement of water becomes more reliable.
[0105]
The method of contacting the hydrophilic organic solvent with the silica slurry is optional, for example, a method of adding the hydrophilic organic solvent while stirring the silica slurry in a stirring tank, or filling the silica filtered from the silica slurry into a packed tower, A method in which a hydrophilic organic solvent is passed through the packed tower, a method in which silica is immersed in a hydrophilic organic solvent, immersed, and then left standing are exemplified.
[0106]
The completion of the replacement operation with the hydrophilic organic solvent may be determined by measuring the water content in the liquid component of the silica slurry. For example, a silica slurry is periodically sampled to measure the water content, and the point at which the water content is usually 5% or less, preferably 4% or less, more preferably 3% or less may be set as the end point. The method for measuring moisture is arbitrary, and includes, for example, the Karl Fischer method.
[0107]
[2-1-4. Drying process]
After the replacement operation with the hydrophilic organic solvent, the silica and the hydrophilic organic solvent are separated and dried, whereby the silica of the present invention can be produced. As a separation method at this time, any conventionally known solid-liquid separation method may be used. That is, solid-liquid separation may be performed by selecting a method such as decantation, centrifugal separation, or filtration according to the size of the silica particles. One of these separation methods may be used alone, or two or more of them may be used in any combination.
[0108]
The obtained silica is dried at usually 40 ° C. or higher, preferably 60 ° C. or higher, and usually 200 ° C. or lower, preferably 120 ° C. or lower. The drying method is not particularly limited, and may be a batch type or a continuous type, and can be dried under normal pressure or under reduced pressure. Above all, vacuum drying is preferred because not only can drying be performed quickly, but the pore volume and specific surface area of the obtained silica also increase.
[0109]
If necessary, when a carbon content derived from silicon alkoxide as a raw material is contained, the carbon content can be removed by baking at 400 to 600 ° C. In addition, firing may be performed at a maximum temperature of 900 ° C. to control the surface state. Further, the silica of the present invention is finally obtained by pulverizing and classifying as required.
[0110]
[2-2. Method for containing specific element]
The main methods for causing the heat insulating material of the present invention to contain a specific element include (i) a method of introducing a single element atom or a compound of the specific element when producing the silica of the present invention by the above-mentioned production method, and (ii) There is a method in which the silica of the present invention produced by the above-mentioned production method is mixed with a simple substance or a compound of a specific element to form a composition.
[0111]
First, the method (i) will be described.
The specific element may be used in any state of a simple substance or a compound, but usually, it is preferable to use an oxide, sulfide or nitride of the specific element. In the following description of the method of introducing a specific element in the method (i), unless the state of the specific element to be introduced is a single element or a compound, the specific element is referred to unless otherwise specified. Are referred to as specific substances.
[0112]
In the silica production process described above, it is optional to introduce the specific substance in any of the steps. Usually, (1) the specific substance of the present invention is introduced into silicon alkoxide during the hydrolysis / condensation step. (2) a method of introducing the specific substance of the present invention into the silica slurry during the physical property adjusting step; and (3) a method of introducing the specific substance of the present invention after the water removal step, the drying step, or the drying step. Can be The methods (1), (2), and (3) may be performed alone or in combination of two or more.
[0113]
In the method (1), silica is hydrolyzed and condensed in the presence of a silicon alkoxide and a specific substance. Accordingly, when the silica is condensed to form a silica skeleton (that is, when the silica skeleton is condensed), the specific element of the present invention exists around the occurrence of the condensation. It is easy to exist.
[0114]
That is, in the method (1), since most of the specific elements can be present in the heat insulating path, the heat insulating property can be surely exerted even if the specific elements used are relatively small. Therefore, in the method (1), the weight ratio of Si to the specific element to be used is usually at least 1 ppm by weight, preferably at least 3 ppm by weight, more preferably at least 3 ppm by weight in the weight ratio of the specific element / (Si + specific element). It is at least 5 ppm by weight, usually at most 15%, preferably at most 12%, more preferably at most 10%.
[0115]
In the method (2), the absorption heat treatment is performed in the presence of the silica slurry and the specific substance produced in the hydrolysis / condensation step. Also in this case, similarly to (1), when the silica is condensed to form a silica skeleton (that is, when the silica skeleton is condensed), the specific element of the present invention exists around the occurrence of the condensation. The specific element is likely to be present in the skeleton of the heat insulating material.
[0116]
Therefore, according to the method (2), since most of the specific elements can be present in the heat insulating path, the heat insulating property can be surely exerted even when the specific elements to be used are relatively small. Therefore, in the method (2), the weight ratio of the solid content of silica to the specific element used in the silica slurry is usually 1% or more, preferably 3% or more in terms of the weight of the specific element / the weight of the silica solid content. It is more preferably at least 5%, and usually at most 30%, preferably at most 29%, more preferably at most 28%.
[0117]
In the method (3), after the silica skeleton is completed in the physical property adjusting step, the silica and the specific substance are mixed. Thereby, the specific substance adheres to the surface of the silica, but a part of the specific substance is taken into the silica skeleton.
[0118]
In the method (3), since most of the specific elements are considered to exist outside the adiabatic path, a relatively large amount of the specific elements should usually be used. Therefore, in the method (3), the ratio of the silica of the present invention to the specific element to be used is usually at least 1%, preferably at least 3%, more preferably at least 5% by weight of the specific element / weight of the silica. Also, it is usually at most 30%, preferably at most 29%, more preferably at most 28%.
[0119]
In addition, specific methods of the method (3) include an impregnation method <absorption method, pore-filling method, “incipient wetness” method, evaporation to dryness method, and spray (evaporation to dryness) method. spray method), precipitation method <coprecipitation method, deposition method, kneading method, etc.>, ion exchange method, mechanical mixing (mechanochemical) method, and vapor deposition method (Chemical vapor deposition method, sputtering method) and the like.
[0120]
After the specific element is introduced by the above-described operation, by baking at 200 to 400 ° C. as necessary, a crosslinking reaction between the specific element and silica can be promoted. If necessary, a chemical treatment such as oxidation or reduction may be performed after the introduction of the specific element to change the chemical state of the specific element.
[0121]
The specific element can be supported on the silica obtained by hydrolyzing the silicon alkoxide and hydrothermally treating it without aging by the above-described method, but its form, type of compound, and amount of the compound are as follows. There is no particular limitation as long as the target foreign element expresses functionality, and any of these specific elements alone, a water-soluble salt, a solvent-soluble salt, a complex compound, a hydrolyzable compound, etc. Is also good. From the viewpoint of obtaining a high-purity heat insulating material, that is, a heat insulating material having a small content of metal impurities, it is preferable to use a high-purity specific element.
[0122]
Subsequently, the method (ii), that is, a method of mixing the silica of the present invention produced by the above-described production method with a simple substance or a compound of a specific element to form a composition will be described. The specific element may be used in any state of a simple substance or a compound, but usually, it is preferable to use an oxide, sulfide or nitride of the specific element. In the following description of the method of introducing a specific element in the method (ii), when the specific element to be introduced is not distinguished whether it is a simple substance or a compound, it is simply referred to as a specific substance.
[0123]
When mixing the silica of the present invention and a specific substance, the particle diameter of the specific substance is usually larger than the mode pore diameter of the silica of the present invention. If the particle diameter of the specific substance is less than this, the specific substance may be clogged in the pores and it may be difficult to exhibit sufficient heat insulating properties. However, even for a specific substance having a particle size equal to or less than the most frequent pore diameter of the silica of the present invention, if the amount is small, the heat insulating property is not significantly deteriorated.
[0124]
In the method (ii), the weight ratio of the silica of the present invention to the specific element to be used is usually 1% or more, preferably 3% or more, by weight of the specific element / (silica + specific element). It is more preferably at least 5%, and usually at most 30%, preferably at most 29%, more preferably at most 28%. If the ratio is smaller than this, sufficient heat insulating properties cannot be exhibited, and if the ratio is larger than this, the voids of the heat insulating material become small and the heat insulating properties deteriorate.
[0125]
In the method (ii), the method of mixing is arbitrary, but usually, a method capable of uniformly dispersing the specific substance in the silica of the present invention is used. Specifically, shaking mixing and the like are mentioned.
[0126]
In addition, among natural ores that can be a raw material of silica, if there is one containing a large amount of a specific element, silica is produced from the ore by the above-described method for producing silica and the like, and a step of including the specific element in the silica is performed. Without producing the heat insulating material of the present invention, the heat insulating material of the present invention can be manufactured.
[0127]
[1-3. Others]
Now, here, the heat insulating material of the present application will be considered by comparing the method for manufacturing an airgel disclosed in Patent Document 2 with the method for manufacturing silica according to the present application.
[0128]
In Patent Document 2, a conductive metal oxide is mixed with silica. However, as described above, a substance having high electric conductivity generally has high thermal conductivity, and thus it is difficult to use the substance as it is as a heat insulator. Therefore, Patent Document 2 focuses on the fact that conductive metal oxides crush silica, and proposes to enhance the heat insulating properties of the heat insulating material by the crushing phenomenon. Therefore, in this technique, since the fine structure of silica fluctuates due to the crushing phenomenon, the strength, durability, and moldability of the heat insulating material may be impaired.
[0129]
On the other hand, according to the present invention, as described in the explanation of the principle, when two types of atoms are bonded, the larger the difference in the atomic weight of the atoms, the greater the heat conduction when heat is transmitted between the two types of atoms. Focusing on the fact that the degree becomes smaller, the heat insulation of the heat insulating material is improved. In other words, the present invention improves the strength, durability, and formability of a heat insulating material by adding a relatively simple operation of containing a specific element without causing the phenomenon of crushing that makes it difficult to control the structure of the heat insulating material. Excellent heat insulation can be exhibited without any loss. In addition, since complicated operations such as pH adjustment are not used in the production process, the production efficiency is excellent and the production cost is also advantageous.
[0130]
Since the heat insulating material of the present invention has the above-mentioned advantages, it can be used for various applications requiring heat insulating properties such as cooling and heat keeping. For example, refrigerators, freezers, refrigerated vehicles, car ceilings, batteries, refrigerated or refrigerated boats, insulated containers, refrigerated or refrigerated showcases, portable coolers, cooking insulated cases, vending machines, solar water heaters, floor heating It can be suitably used as a building material such as under the floor, inside a wall or inside a wall, a ceiling portion, an attic, hot water or cooling water piping, a conduit for transferring a low-temperature fluid, and other heat insulating materials for plant equipment, clothing, bedding and the like. . Since silica is particularly high in transparency, it can be used for applications utilizing transparency, for example, if it is a building material, it is surrounded by glass to form a window glass for heat insulation.
[0131]
[3. Insulation)
The use form of the heat insulating material of the present invention is not particularly limited and may be used as it is for the above-mentioned applications. However, depending on the use, a heat insulator in which the heat insulating material of the present invention is sealed with an exterior material (heat insulating material of the present invention) It is also preferred to use
[0132]
As the exterior material, various materials can be used without particular limitation as long as they can seal the heat insulating material of the present invention. In particular, those capable of shielding the interior sealed by the exterior material from the outside air can be used. preferable.
[0133]
Usually, when the heat insulating material is hermetically closed by the outer material, it is desirable to reduce the thermal conductivity of the voids of the silica as the heat insulating material by setting the inside of the outer material where the heat insulating material exists, usually in a reduced pressure state, preferably in a vacuum state. It is.
[0134]
Here, if gas enters inside through the exterior material, the thermal conductivity of the voids in the silica increases, and the heat insulating property of the heat insulating material decreases, which is not preferable. Therefore, it is preferable that the exterior material includes a gas barrier layer having gas barrier properties.
[0135]
In addition, if water enters the interior through the exterior material, silica as a heat insulating material may absorb water and the heat insulating property may be deteriorated. Therefore, it is preferable that the exterior material includes a moisture-impermeable layer having moisture-impermeable properties.
[0136]
Further, the exterior material itself is preferably formed of a material having excellent heat insulating properties. Further, the exterior material may have a fixed shape such as a housing, or may be an irregular shape such as a film.
[0137]
Specific examples of the exterior material include metal sheets such as stainless steel, aluminum, and iron, and metal foils such as aluminum foil, as well as polyethylene, polypropylene, acrylonitrile-butadiene-styrene copolymer (ABS), styrene, and polystyrene. Examples thereof include polyvinylidene chloride, ethylene vinyl alcohol film, and a laminated material obtained by laminating a metal such as aluminum or silicon on a resin such as vinyl chloride. In addition, when a layer such as a stretched product of a polyethylene terephthalate film or a polypropylene film is provided on the outer surface of the exterior material as a surface protective layer, or a nylon film or the like is provided, flexibility is improved, and bending resistance and the like are improved. improves.
[0138]
When sealing the heat insulating material of the present invention with the exterior material, various methods can be used depending on the type of the exterior material, but as described above, the inside of the heat insulating material is in a reduced pressure state, preferably in a vacuum state. It is preferable to seal while holding.
[0139]
Note that, in addition to the heat insulating material of the present invention, various substances such as other heat insulating materials may be sealed together in the exterior material. The types of these substances may be appropriately selected according to the usage form and application of the heat insulating material.
[0140]
【Example】
EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples unless it exceeds the gist.
[0141]
(1) Example 1
(1-1) Production of heat insulating material:
<Hydrolysis / condensation step>
1875 g of tetraethoxysilane was charged into a 5 L separable flask (with a jacket) made of glass and having a water-cooled condenser open to the atmosphere at the top. While stirring at a stirring blade tip speed of 2.5 m / s, 162 g of 0.002 mol / L hydrochloric acid was charged into the stirring blade. Stirring was continued for 1 hour, and warm water at 50 ° C. was passed through the jacket of the separable flask. While continuing stirring, 73 g of zirconium tetrabutoxide was charged over 3 minutes. After stirring for 30 minutes, 838 g of water was added over 10 minutes. Hot water at 50 ° C. was passed through the jacket of the separable flask. Subsequently, the stirring was continued, and when the contents reached the boiling point, the stirring was stopped. Subsequently, warm water at 50 ° C. was passed through the jacket for 30 minutes to gel the generated sol.
[0142]
Thereafter, the gel was immediately taken out, and the gel was pulverized through a nylon mesh having an opening of 600 μm to obtain a powdery wet gel (silica zirconia hydrogel).
[0143]
<Physical property adjustment step>
300 g of this silica hydrogel and 450 g of water were charged into a 1 L glass autoclave, and subjected to a closed hydrothermal treatment at 200 ° C. for 3 hours.
[0144]
<Moisture removal process>
The silica obtained by the hydrothermal treatment was filtered through filter paper (No. 5A), the filter cake was added to another separable flask together with 600 g of anhydrous methanol, and the mixture was slowly stirred for 1 hour at room temperature using a stirring blade. This slurry was subjected to solid-liquid separation by decantation, and the obtained solid was subjected to the same liquid replacement operation as above using 600 g of anhydrous methanol.
When this operation was performed three times in total, including the first time, the water content in silica determined by the Karl Fischer method was 2% by weight or less.
[0145]
<Drying process>
The silica thus obtained was dried under reduced pressure at 100 ° C. until a constant weight was obtained, to obtain a heat insulating material of Example 1.
[0146]
(1-2) Analysis of thermal insulation (silica):
The physical properties of the heat insulating material of Example 1 were measured by the following methods.
(1-2-1) Pore volume:
The BET nitrogen adsorption isotherm of silica as a heat insulating material was measured using AS-1 manufactured by Cantachrome, and the pore volume was determined. Specifically, the relative pressure P / P ₀ The value at the time of 0.98 was adopted.
[0147]
(1-2-2) Mode pore diameter:
Using a laser type particle size distribution measuring device (LMS-30 manufactured by Seishin Enterprise Co., Ltd.), the most frequent pore diameter of silica as a heat insulating material was measured.
[0148]
(1-2-3) Content of specific element:
Hydrofluoric acid was added to 2.5 g of silica as a heat insulating material and heated to dryness, and then water was added to 50 ml. ICP emission analysis was performed using this aqueous solution to determine the content of metal impurities. The contents of sodium and potassium were analyzed by a flame flame method.
[0149]
(1-2-4) Evaluation of thermal conductivity of heat insulator:
The heat insulating material was placed in a container having a metal thin film layer and a thermoplastic polymer layer. The thermal conductivity of this heat insulator was measured at an average temperature of 25 ° C. using a thermal conductivity measuring device (AUTO-∧ manufactured by Eiko Seiki Co., Ltd.).
(1-2-5) Bulk density of heat insulating material:
According to JIS K6721, the bulk density of the heat insulating material was measured.
[0150]
(2) Comparative Example 1:
In the same manner as in Example 1 except that the following operation was performed instead of the <hydrolysis / condensation step>, a heat insulating material and a heat insulating body as comparative examples were manufactured, and various physical properties were measured.
[0151]
550 g of sulfuric acid having a concentration of 20% was charged into a 5 L separable flask (with a jacket) made of glass and having a water-cooled condenser open to the atmosphere at the top. To this, 1850 g of JIS No. 3 sodium silicate diluted 2.5 times was gradually added to generate a silica sol having a pH of 4.5. This hydrosol gelled after about 10 minutes, but was further allowed to stand overnight, coarsely ground, and subjected to acid treatment in 1N sulfuric acid at 80 ° C. for 2 hours, and the acid-treated hydrogel was washed with water until the pH reached 5.
[0152]
(3) Result:
Table 1 shows the physical properties of the obtained examples and comparative examples.
[Table 1]

[0153]
As is evident from Table 1, it contains silica having a large pore volume and a specific element selected from the elements of the fifth and sixth periods of the periodic table, has a low bulk density, and is a total of silica and the specific element. It can be seen that a heat insulating material having a specific element content with respect to a predetermined range can exhibit excellent heat insulating properties.
[0154]
【The invention's effect】
According to the present invention, silica having a large pore volume is used, a specific element selected from the elements of the fifth and sixth periods of the periodic table is contained at a predetermined concentration, and the bulk density of the heat insulating material is set to a predetermined value. By suppressing the value to the value or less, it is possible to provide an excellent heat insulating material which is excellent in strength, durability and moldability, exhibits high heat insulating properties, and can be manufactured by a relatively simple method. In addition, by sealing this heat insulating material with an exterior material to form a heat insulator, the heat insulating material can be suitably used for various applications.
[Brief description of the drawings]
FIGS. 1A and 1B are diagrams schematically showing types of specific elements in a heat insulating material of the present invention, particularly focusing on the relationship with silica.
FIGS. 2 (a) and 2 (b) are diagrams schematically showing types of specific elements in the heat insulating material of the present invention, particularly focusing on the relationship with silica.

Claims (16)

At least a heat insulating material made of silica,
(A) the pore volume of the silica is 1.6 ml / g or more;
(B) containing a specific element selected from the group consisting of elements of the fifth and sixth periods of the periodic table at a concentration of 1 wt ppm or more and 30 wt% or less;
(C) A heat insulating material, wherein the heat insulating material has a bulk density of 0.5 g / ml or less. The heat insulating material according to claim 1, wherein the specific element is present in a heat conduction path of the heat insulating material. The heat insulating material according to claim 1, wherein at least a part of the specific element is bonded to oxygen. The heat insulating material according to any one of claims 1 to 3, wherein at least a part of the specific element is incorporated in the skeleton of the silica. The heat insulating material according to any one of claims 1 to 4, wherein the heat insulating material is obtained by introducing a simple substance or a compound of the specific element into the skeleton of the silica when producing the silica. . The silica is produced through a hydrolysis / condensation step of hydrolyzing a silicon alkoxide to form a silica hydrogel, and in the hydrolysis / condensation step, a simple substance or a compound of the specific element is allowed to coexist, whereby the silica is produced. The heat insulating material according to claim 5, wherein the specific element is introduced into a skeleton of silica. The silica is produced through a physical property adjusting step in which a silica hydrogel is subjected to a hydrothermal treatment to form a silica slurry, and in the physical property adjusting step, a simple substance or a compound of the specific element is allowed to coexist to form a skeleton of the silica. The heat insulating material according to claim 5, wherein said specific element is introduced into said heat insulating material. The heat insulating material according to any one of claims 1 to 3, wherein the heat insulating material is a composition of a simple substance or a compound of the specific element and the silica. The heat insulating material according to any one of claims 5 to 8, wherein the electric conductivity of the simple substance or the compound of the specific element at room temperature is smaller than 2.5 × 10 ⁻³ S / m. The heat insulating material according to any one of claims 1 to 9, wherein the content of metal impurities in the second to fourth periods of the periodic table is 1% by weight or less. The heat insulating material according to any one of claims 1 to 10, wherein the silica is amorphous. The heat insulating material according to any one of claims 1 to 11, wherein the mode pore diameter _Dmax of the silica is 5 nm or more. The heat insulating material according to any one of claims 1 to 12, wherein the silica has a Q ⁴ / Q ³ value measured by solid-state Si-NMR of 1.2 or more. A heat insulator, wherein the heat insulator according to any one of claims 1 to 13 is hermetically sealed with an exterior material. The heat insulator according to claim 14, wherein the exterior material includes a gas barrier layer having a gas barrier property. The heat insulator according to claim 14 or 15, wherein the exterior material includes a moisture-impermeable layer having moisture-impermeable properties.

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