JP2004217485A - Carbon fiber for carbon-fiber reinforced carbon composite material, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a carbon-fiber, having an excellent interlayer shearing strength without reducing tensile strength, for a carbon-fiber reinforced carbon composite material, and to provide a manufacturing method for the carbon-fiber. <P>SOLUTION: The carbon-fiber has ≥ 1 nm and ≤ 20 nm arithmetic mean roughness (Ra) of the surface, ≥ 0.5 m<SP>2</SP>/g and ≤ 1.2 m<SP>2</SP>/g specific surface area measured by the BET method by the adsorption of krypton gas, and 0.13 ≥ and ≤ 0.27 oxygen concentration ratio O/C, measured by an X-ray photoelectron spectroscopy, on the surface of the carbon-fiber. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

また、かかる束状の炭素繊維は、そのストランド弾性率が２００ＧＰａ以上４００ＧＰａ以下であることが、高強度、高接着なＣ／Ｃ複合材料を得るという点で好適である。ここでいうストランド弾性率は、上記ストランド強度測定方法と同様の方法で引張試験を行い、荷重−伸び曲線の傾きから求めることができる。
【００２８】
次に、本発明のＣ／Ｃ複合材料用炭素繊維の製造方法について説明する。
【００２９】
本発明のＣ／Ｃ複合材料用炭素繊維は、レーヨン、ポリアクリロニトリル、ピッチなどの繊維を炭素化した繊維、或いはそれらをさらに高温で熱処理した黒鉛化繊維が主として用いられる。高強度なＣ／Ｃ複合材料を得るには、高強度な炭素繊維が得られやすいアクリロニトリル繊維を用いるのが好ましい。
【００３０】
前述したクリプトンガス吸着から求めたＢＥＴ法比表面積、Ｘ線光電子分光法によって求められる表面酸素濃度比Ｏ／Ｃを有するＣ／Ｃ複合材料用炭素繊維は、電解酸化処理によって得ることができる。電解酸化処理は、処理ムラを制御し、かつ短時間で炭素繊維表層深く酸化処理ができるため、比表面積および表面酸素濃度を制御し易い。かかる電解酸化処理に使用される電解液は、酸性、アルカリ性のいずれでもよい。
【００３１】
具体的には、電解液の電気伝導度が１５ｍＳ／ｃｍ以上１００ｍＳ／ｃｍ以下に調整された電解液槽中で、炭素繊維の単位重量あたりの電流値が０．５Ａ以上１００Ａ以下、好ましくは２Ａ以上８０Ａ以下、かつ処理時間が０．５秒以上２０秒以下、好ましくは１秒以上１０秒以下に制御することにより、前述したクリプトンガス吸着から求めたＢＥＴ法比表面積、Ｘ線光電子分光法によって求められる表面酸素濃度比Ｏ／Ｃを制御できる。電解液の電気伝導度が１５ｍＳ／ｃｍ未満であると、電流が流れにくく電流値の制御が困難であり、１００ｍＳ／ｃｍを超えると、高濃度であるため、後の水洗工程での電解質の除去が不十分になる場合がある。また、炭素繊維の単位重量あたりの電流値が０．５Ａ未満であると、電流値が小さいため処理斑が生じ、１００Ａを超えると、炭素繊維表面の過剰な酸化処理により繊維自体の強度が低下する場合がある。さらに処理時間が２０秒より長いと、炭素繊維表面の酸化処理がゆっくり進行するため、表面凹凸ができにくく、０．５秒より短いと処理斑が生じ、Ｃ／Ｃコンポジットの剪断強度が低くなる場合がある。上記、電解酸化処理の際には、炭素繊維を陽極、白金板を陰極とし、かかる処理を行うことができる。
【００３２】
電解酸化処理の後、水洗及び乾燥を行った後、炭素繊維表面の酸素濃度比Ｏ／Ｃが前述した値を越える場合は、さらに、不活性雰囲気中において温度を２５０℃以上１０００℃以下、好ましくは２５０℃以上７５０℃以下、処理時間は３０分以上５時間以下の範囲で加熱処理をおこなうことで酸素濃度比Ｏ／Ｃを既定の範囲内に制御することもできる。
【００３３】
さらに必要に応じて、炭素繊維にサイジング剤を付与することもできる。サイジング剤としては、例えばエポキシ樹脂、フェノール樹脂、アルキド樹脂、ウレタン樹脂等の熱硬化性樹脂や、ポリエチレン、ポリ塩化ビニル、ポリアミド等の熱可塑性樹脂や、コールタールピッチ、石油ピッチ等のピッチを用いることができる。
【００３４】
上記、本発明のＣ／Ｃ複合材料用炭素繊維を適用することにより、力学特性とりわけ引張強度と層間せん断強度に優れたＣ／Ｃ複合材料を作製することができる。かかるＣ／Ｃ複合材料は強化繊維方向が実質的に一方向であるＣ／Ｃ複合材料とすることもできるし、強化繊維方向が９０°、０°方向に交差したような二方向Ｃ／Ｃ複合材料、その他目的に応じて繊維方向を任意に設定することができる。特に前記Ｃ／Ｃ複合材料用炭素繊維を用いることにより、引張強度５００〜７００ＭＰａかつ層間剪断強度１０〜２０ＭＰａである一方向Ｃ／Ｃ複合材料を得ることができる。
【００３５】
ここでかかる一方向Ｃ／Ｃ複合材料の引張強度は繊維軸方向に長さ５０±１ｍｍ、繊維軸方向に幅１０±１ｍｍ、厚さ２．０±０．２ｍｍに平板を切断して得られた試験片を、ゲージ部１０ｍｍ、試験速度０．５ｍｍ／分として測定し、繊維体積含有率（Ｖｆ）４０％に換算した場合の強度である。また、一方向Ｃ／Ｃ複合材料の層間剪断強度は上記と同様の試験片を、ＪＩＳ Ｋ７０７８に規定する試験方法に従って測定するものである。
【００３６】
かかる本発明のＣ／Ｃ複合材料は前記炭素繊維に各種バインダーを含浸せしめ、加熱硬化することで得ることができる。Ｃ／Ｃ複合材料の製造に用いるバインダーの種類については特に制限はなく、フェノール樹脂、エポキシ樹脂、アルキド樹脂、ウレタン樹脂、フラン樹脂等の熱硬化性樹脂、ポリエチレン、ポリ塩化ビニル等の熱可塑性樹脂、あるいはコールタールピッチ、石油ピッチ等のピッチなどの任意のものを使用することができ、これらバインダーを炭素繊維に混合あるいは含浸させた後乾燥して炭素繊維とバインダーからなる組成物を得る。その際、バインダーはアルコール、アセトン、アントラセン油等の溶媒に溶解して適当な粘度に調整したものを使用することができる。また、加熱硬化する温度や時間はバインダーの種類によって適宜設定することができるが、例えば、フェノール樹脂を用いた場合、１２０〜１５０℃、５〜１０ＭＰａにて加圧加熱成形した後、空気中で１７０〜２５０℃まで加熱処理して後硬化を行った後、さらに窒素中５００〜２０００℃にて炭素化処理、２０００〜３０００℃にて黒鉛化処理を行う。また、Ｃ／Ｃ複合材料中の繊維体積含有率としては３０〜７０％が好ましく、４０〜６０％が好ましい。かかる範囲の炭素繊維繊維含有量とすることにより、引張強度に優れかつ、剪断強度に優れたＣ／Ｃ複合材料とすることができる。
【００３７】
【実施例】
以下、実施例により本発明を具体的に説明するが制限されるものではない。
【００３８】
尚、本実施例で用いた特性の測定方法は以下の通りである。
＜算術平均粗さ（Ｒａ）の測定＞
表面の算術平均粗さ（Ｒａ）は次のようにして測定した。測定試料としては、炭素繊維を長さ数ｍｍ程度にカットしたものを使用した。銀ペーストを用いて基板（シリコンウエハ）上に固定し、原子間力顕微鏡（ＡＦＭ）によって各単繊維の中央部において、３次元表面形状の像を得た。原子間力顕微鏡としてはＤｉｇｉｔａｌ Ｉｎｓｔｕｍｅｎｔｓ社製 ＮａｎｏＳｃｏｐｅ ＩＩＩａにおいてＤｉｍｅｎｓｉｏｎ ３０００ステージシステムを使用した。観測条件は下記条件とした。
【００３９】
・走査モード：タッピングモード
・探針：シリコンカンチレバー
・走査範囲：０．６μｍ×０．６μｍ
・走査速度：０．３Ｈｚ
・ピクセル数：５１２×５１２
・測定環境：室温、大気中
各試料について、単繊維１本から１箇所ずつ観察して得られた像について、繊維断面の丸みを３次曲面で近似し、得られた像全体を対象として、算術平均粗さ（Ｒａ）を算出した。単繊維５本について、算術平均粗さ（Ｒａ）を求め平均した。
＜炭素繊維のＢＥＴ法比表面積の測定＞
炭素繊維のＢＥＴ法比表面積は次のようにして測定した。試料としては炭素繊維を長さ数十ｃｍ程度にカットしたものを使用した。試料を精秤後、試験管に封入し、クリプトンガスの吸着によりＢＥＴ法比表面積を測定した。ガス吸着に際しては、日本ベル（株）製 高精度全自動ガス吸着装置「ＢＥＬＳＯＲＰ ３６」を使用し、測定条件は下記の通りとした。
【００４０】
・吸着ガス：Ｋｒ
・死容積：Ｈｅ
・吸着温度：液体窒素温度（７７Ｋ）
・測定前処理：２００℃
・測定モード：等温での吸着
・測定範囲：相対圧（Ｐ／Ｐ０）＝０．０１〜０．４
Ｐ：測定圧
Ｐ０：吸着ガスの飽和蒸気圧
・平衡時間：各平衡相対圧につき１８０ｓｅｃ
比表面積の計算法はＢＥＴ理論を適用した。同理論式に従ってＢＥＴプロットの約０．０５〜０．３の相対圧域を解析して比表面積を算出した。
＜ストランド引張強度および弾性率の測定＞
束状の炭素繊維に下記組成の樹脂を含浸させ、１３０℃で３５分間硬化させた後、ＪＩＳ Ｒ７６０１に基づいて引張試験を行った。
【００４１】

＜炭素繊維の表面酸素濃度比（Ｏ／Ｃ）＞
表面酸素濃度比Ｏ／Ｃは、次の手順に従ってＸ線光電子分光法により求めた。試料となる炭素繊維は、適当な長さにカットしてステンレス製の試料支持台上に拡げて並べた。光電子脱出角度を９０゜とし、Ｘ線源としてＭｇＫα１，２を用い、試料チャンバー内を１×１０^−８Ｔｏｒｒの真空度に保った。測定時の帯電に伴うピークの補正として、Ｃ１Ｓの主ピークの結合エネルギー値を２８４．６ｅＶに合わせた。Ｃ１Ｓピーク面積は、２８２〜２９６ｅＶの範囲で直線のベースラインを引くことにより求め、Ｏ１Ｓピーク面積は、５２８〜５４０ｅＶの範囲で直線のベースラインを引くことにより求めた。表面酸素濃度Ｏ／Ｃは、上記Ｃ１ｓピーク面積に対するＯ１Ｓピーク面積の比を、装置固有の感度補正値で割ることにより算出した原子数比で表した。なお、本実施例ではＸ線光電子分光測定装置として島津製作所（株）製ＥＳＣＡ−７５０を用い、かかる装置固有の感度補正値は２．８５であった。
＜Ｃ／Ｃ複合材料の引張強度＞
一方向Ｃ／Ｃ複合材料の平板を切断し、繊維軸方向に長さ５０±１ｍｍ、繊維軸方向に幅１０±１ｍｍ、厚さ２．０±０．２ｍｍからなる試験片を作製した。試験片の中央部１０ｍｍを残して両端の両側に厚さ１ｍｍのアルミ製タブを接着して、引張強度用の試験片とした。試験速度は０．５ｍｍ／分とした。試験数は５つとし、その平均を引張強度とした。尚、試験機には、荷重測定誤差が±１％を超えない、クロスヘッド移動速度を一定に保てる形式の適当な材料試験機を用いるが、本実施例においては、試験機としてインストロン（登録商標）試験機４２０８を用いた。Ｃ／Ｃ複合材料の切断にはダイヤモンドカッターを用いた。
＜Ｃ／Ｃ複合材料の層間剪断強度＞
一方向Ｃ／Ｃ複合材料の平板を切断し、繊維軸方向に長さ１４±１ｍｍ、繊維軸方向に幅１０±０．２ｍｍ、厚さ２±０．２ｍｍの試験片を作製した。支点間距離１０ｍｍ、試験速度１ｍｍ／分として、ＪＩＳ Ｋ７０７８に規定する試験方法に従って測定した。本実施例においては、試験機としてインストロン（登録商標）試験機４２０８を用いた。
【００４２】
（実施例１）
ストランド強度が４．９ＧＰａ、ストランド弾性率が２４０ＧＰａの束状のポリアクリロニトリル系炭素繊維（単繊維直径６．９μｍ、単繊維数１２０００本／束）を陽極とし、白金を陰極として、電気伝導度が１５ｍＳ／ｃｍになるように調整された硫酸水溶液中で、炭素繊維１ｇ当たりの電流１０Ａで２秒間電解処理した後、水洗し、２６０℃の加熱空気中で乾燥した。得られた炭素繊維の表面の算術平均粗さＲａ５．０ｎｍ、比表面積０．６２ｍ^２／ｇ、表面酸素濃度比０．２２であった。
【００４３】
この束状の炭素繊維を金型中に置いた後、レゾール系フェノール樹脂（住友ベークライト社製スミライトレジン（登録商標ＰＲ−５００８７））を含浸して、１５０℃で９．８ＭＰａ（１００ｋｇｆ／ｃｍ^２）にて加圧加熱成形して、厚さ２．０ｍｍの成形板を得た。さらにこの成形板を空気中で昇温速度３℃／ｈｒにて２５０℃まで加熱処理して後硬化を行った後、窒素中にて１５℃／ｈｒで１０００℃まで加熱して炭素化処理を行った。さらに１５℃／ｈｒで２０００℃まで加熱して２４時間保持し、黒鉛化処理を行った。このＣ／Ｃ複合材料の繊維体積含有率は４０％、引張強度は５７０ＭＰａ、層間剪断強度は１４．６ＭＰａで、優れた引張強度、層間剪断強度が得られた。
【００４４】
（比較例１）
前記実施例１において、０．２Ａ、１００秒で電解処理した以外は実施例１と同じ製造方法により束状の炭素繊維および一方向Ｃ／Ｃ複合材料を製造した。得られた炭素繊維の表面の算術平均粗さＲａ３．１ｎｍ、比表面積０．４５ｍ^２／ｇ、表面酸素濃度比０．１６であった。また、Ｃ／Ｃ複合材料の繊維体積含有率は４０％、引張強度は５４０ＭＰａ、層間剪断強度は９．１ＭＰａであった。
【００４５】
（実施例２）
前記実施例１において、１５Ａ、２秒で電解処理した以外は実施例１と同じ製造方法により束状の炭素繊維および一方向Ｃ／Ｃ複合材料を製造した。得られた炭素繊維の表面の算術平均粗さＲａ５．５ｎｍ、比表面積０．７４ｍ^２／ｇ、表面酸素濃度比０．２５であった。また、Ｃ／Ｃ複合材料の繊維体積含有率は４０％、引張強度は５８８ＭＰａ、層間剪断強度は１４．１ＭＰａであり、良好な引張強度、層間剪断強度が得られた。
【００４６】
（比較例２）
前記実施例１において、１２０Ａ、１秒で電解処理した以外は実施例１と同じ製造方法により一方向Ｃ／Ｃ複合材料を製造した。得られた炭素繊維の表面の算術平均粗さＲａ６．３ｎｍ、比表面積０．９４ｍ^２／ｇ、表面酸素濃度比０．２９であった。また、Ｃ／Ｃ複合材料の繊維体積含有率は４０％、引張強度は４９３ＭＰａ、層間剪断強度は１２．５ＭＰａであった。
【００４７】
実施例１〜２、比較例１〜２の結果を表１にまとめて示す。
【００４８】
【表１】

【００４９】
表１から明らかなように、実施例１〜２は、比較例１〜２に比して、Ｃ／Ｃ複合材料の層間剪断強度および引張強度の両方が著しく優れていることがわかる。
【００５０】
【発明の効果】
本発明によれば、引張強度を低下させることなく層間剪断強度を向上させたＣ／Ｃ複合材料およびその製造方法、更にはかかるＣ／Ｃ複合材料用の炭素繊維、およびその製造方法を提供することができる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a carbon fiber for a carbon fiber reinforced carbon composite material and a method for producing the same. More specifically, the present invention relates to a carbon fiber for a carbon fiber reinforced carbon composite material capable of obtaining a high interlaminar shear strength without lowering the tensile strength of the carbon fiber reinforced carbon composite material, and a method for producing the same.
[0002]
[Prior art]
Conventionally, carbon fiber reinforced carbon composite materials (hereinafter referred to as C / C composite materials) are generally composed of carbon fibers of long fibers or short fibers made of polyacrylonitrile, pitch, rayon, or the like as phenol resin or furan. A method of impregnating or mixing a binder such as a thermosetting resin such as a resin, or a thermoplastic resin such as a pitch, and carbonizing or graphitizing the molded product in a non-oxidizing atmosphere such as an inert gas, or a method using a chemical gas. It is manufactured by the method of filling pyrolytic carbon between carbon fibers by phase vapor deposition method, and has attracted attention as a structural material, friction material, and conductive material because it has heat resistance, specific strength and light weight that can not be reached with metals etc. I have.
[0003]
In the manufacturing process of the C / C composite material, the process that affects the mechanical properties of the molded product is a carbonization process. Usually, this carbonization step is performed in the range of 500 to 2000 ° C. In this step, the binder shrinks in volume to a greater extent than the carbon fibers due to the carbonization thereof, so that thermal stress is generated between the matrix carbon and the carbon fibers. This thermal stress increases as the bonding force between the matrix carbon and the carbon fiber increases, and this large thermal stress causes a crack at the interface between the matrix carbon and the carbon fiber, and the crack grows rapidly and the interface peels off. It is easy for this phenomenon to occur. As a result, when the interlaminar shear strength of the C / C composite material is improved, a decrease in tensile strength occurs.
[0004]
As a method for solving these problems, a method of not performing a surface treatment on carbon fibers and a method of performing a heat treatment at 1500 ° C. or more in an inert gas after performing a surface treatment have been disclosed (for example, Patent Document 1). , 2). These reduce the bonding force at the interface between the matrix carbon and the carbon fiber by eliminating functional groups effective for bonding to the matrix carbon existing on the carbon fiber surface, and reduce the thermal stress in the carbonization process at the interface. By mitigating by severe peeling, a fatal large crack at the interface is prevented, and as a result, the tensile strength is improved. However, in these methods, while the tensile strength was improved, the decrease in the interlaminar shear strength was remarkable, and this method could not be applied to a structural material requiring shear characteristics.
[0005]
Further, a method of irradiating ultraviolet rays in an ozone atmosphere to modify the carbon fiber surface to be hydrophilic (for example, see Patent Document 3), a method of fluorinating the carbon fiber surface (for example, see Patent Document 4), Various surface treatment methods for treating with hydrogen peroxide (for example, see Patent Document 5) are disclosed. These are intended to improve the adhesion between the carbon matrix and the carbon matrix of the C / C composite material which is a molded product by introducing a functional group into the surface of the carbon fiber to improve the adhesion between the carbon matrix and the carbon fiber. I have. However, although the shear strength was improved by these surface treatment methods, the tensile strength as a structural material was not sufficient.
[0006]
That is, there is a trade-off relationship between the tensile strength and the interlaminar shear strength of the C / C composite material. In the above-mentioned conventional method, carbon fibers that simultaneously improve both the tensile strength and the interlaminar shear strength have not been obtained. Was the current situation.
[0007]
[Patent Document 1] JP-A-6-157139 (page 3)
[0008]
[Patent Document 2] JP-A-59-107913 (page 2)
[0009]
[Patent Document 3] JP-A-3-8866 (page 3)
[0010]
[Patent Document 4] Japanese Patent Application Laid-Open No. 4-175266 (page 2)
[0011]
[Patent Document 5] Japanese Patent Application Laid-Open No. 1-145375 (page 2)
[0012]
[Problems to be solved by the invention]
In view of the background of the related art, the present invention provides a carbon fiber for a C / C composite material that can obtain a high interlaminar shear strength without lowering the tensile strength of the C / C composite material, and thus has excellent fatigue resistance and the like. It provides a C / C structure material, a friction material, a conductive material, and the like.
[0013]
[Means for Solving the Problems]
The present invention employs the following means in order to solve such a problem. That is, the arithmetic average roughness (Ra) of the surface is 1 nm or more and 20 nm or less, the BET specific surface area obtained from krypton gas adsorption is 0.5 m ² / g or more and 1.2 m ² / g or less, and X-ray photoelectron spectroscopy. The carbon fiber for carbon fiber reinforced carbon composite material, wherein the surface oxygen concentration ratio O / C of the carbon fiber obtained by the method is 0.13 or more and 0.27 or less.
[0014]
Further, in order to solve such a problem, the following means is adopted. That is, in an electrolytic solution having an electric conductivity of 15 mS / cm or more and 100 mS / cm or less, a current value per unit weight of carbon fiber is 0.5 A or more and 100 A or less, and a processing time is 0.5 This is a method for producing a carbon fiber for a carbon fiber reinforced carbon composite material, which is subjected to an electrolytic oxidation treatment by applying a current for at least 20 seconds and at most 20 seconds.
[0015]
Furthermore, in order to solve the above-mentioned problems, the following means are adopted. That is, the arithmetic average roughness (Ra) of the surface is 1 nm or more and 20 nm or less, the BET specific surface area obtained from krypton gas adsorption is 0.5 m2 / g or more and 1.2 m2 / g or less, and X-ray photoelectron spectroscopy is used. A carbon fiber reinforced carbon composite material containing carbon fibers having a required surface oxygen concentration ratio O / C of 0.13 or more and 0.27 or less.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
The present inventors have conducted intensive studies and found that the arithmetic average roughness (Ra) of the surface is 1 nm or more and 20 nm or less, and the BET specific surface area obtained from krypton gas adsorption is 0.5 m ² / g or more and 1.2 m or more. ^{When a} carbon fiber having a surface oxygen concentration ratio O / C of not more than 0.13 and not more than 0.27, which is not more than ² / g and determined by X-ray photoelectron spectroscopy, was used for the C / C composite material, It is the result of solving the problem at once.
[0017]
The carbon fiber for a C / C composite material of the present invention needs to have an arithmetic average roughness (Ra) of 1 nm or more and 20 nm or less on the surface. In this range, the C / C structural material easily exhibits high tensile strength, and is suitable for a C / C composite material. Carbon fibers having a surface with an arithmetic average roughness of more than 20 nm have remarkable surface irregularities. Therefore, when used in a C / C composite material, physical adhesion called an anchor effect is increased, and tensile strength may be reduced in some cases. In addition, a carbon fiber having an arithmetic average roughness surface smaller than 1 nm may not be able to obtain a C / C composite material having a high interlayer shear strength because the surface is smooth. The thickness is more preferably 1 nm or more and 15 nm or less, and further preferably 1 nm or more and 10 nm or less.
[0018]
Here, the arithmetic mean roughness (Ra) of the surface is an index of irregularities on the surface of the carbon fiber, and the image of the three-dimensional surface shape of 600 nm × 600 nm measured by an atomic force microscope (AFM) is used to determine the roundness of the fiber. Is an arithmetic average roughness (Ra) calculated on the obtained three-dimensional surface shape image by using a value obtained by approximating the three-dimensional surface as an average line.
[0019]
C / C composite material for carbon fiber of the present invention, the BET specific surface area determined from the krypton gas adsorption of carbon fibers 0.5 m ² / g or more 1.2 m ² / g or less, more preferably 0.5 m ² / g and 0.8 m ² / g or less, and the surface oxygen concentration ratio O / C of the carbon fibers determined by X-ray photoelectron spectroscopy is 0.13 or more and 0.27 or less, more preferably 0.17 or more and 0.1 or less. Control is performed so as to fall within the range of 25 or less. When controlled in this manner, high tensile strength and high interlaminar shear strength can be obtained simultaneously. The reason for this is not clear, but cracks at the interface due to thermal stress generated between the matrix carbon and the carbon fibers in the carbonization process are prevented by balancing the fine voids in the carbon fiber surface layer and the amount of functional groups. It is estimated that it is. When the specific surface area is larger than 1.2 m ² / g, the carbon fiber surface has many defects and the strength of the carbon fiber itself decreases, and when the specific surface area is less than 0.5 m ² / g, the contact between the carbon matrix and the carbon fiber is sufficient. However, a C / C composite material having good adhesiveness cannot be obtained. On the other hand, if the surface oxygen concentration ratio O / C of the carbon fiber is less than 0.13, the affinity between the carbon fiber and the carbon matrix is insufficient, and a C / C composite material having good adhesion cannot be obtained. If the ratio O / C is larger than 0.27, the bonding strength between the carbon fiber and the carbon matrix becomes too large, the carbon fiber and the carbon matrix are integrated, the C / C composite material becomes brittle, and the tensile strength, etc. The mechanical properties of are significantly reduced.
[0020]
Here, the surface oxygen concentration ratio O / C of the carbon fiber referred to in the present invention can be obtained by X-ray photoelectron spectroscopy by the following method.
[0021]
If a post-treatment agent such as a sizing agent has adhered to the carbon fiber to be measured, wash it with a solvent such as methylene chloride, methyl ethyl ketone, acetone, ethanol, etc., rinse with distilled water, and perform ultrasonic cleaning as necessary. After removing the sizing agent and the like, the sample is cut into a suitable length, spread on a stainless steel sample support, and arranged, and then measured under the following conditions.
[0022]
In the case of measuring carbon fibers mixed with a binder or the like, the resin can be removed with a solvent such as methylene chloride, methyl ethyl ketone, acetone, or ethanol, and the carbon fibers can be taken out and measured in the same manner.
[0023]
X-ray source: AlKα1,2 or MgKα1,2
The correction of the peak due to the charging at the time of measurement is performed based on the binding energy value of the main peak of C1S. E. FIG. To 284.6 eV.
[0024]
Next, the C1s peak area [C1s] is obtained by drawing a linear base line in the range of 282 to 296 eV, and the O1s peak area [O1s] is obtained by drawing a linear base line in the range of 528 to 540 eV.
[0025]
The surface oxygen / carbon ratio (O / C) can be determined by the following equation from the ratio of the O1s peak area [O1s], the C1s peak area [C1s], and the sensitivity correction value unique to the apparatus.
[0026]
O / C = ([O1s] / [C1s]) / (sensitivity correction value)
Further, the carbon fiber for C / C composite material of the present invention may be a bundle-like carbon fiber in which the carbon fiber is bundled, preferably 1,000 to 100,000, more preferably 3,000 to 70,000, still more preferably It is preferable from the viewpoint of handleability and the like that the carbon fiber is a bundle-like carbon fiber in which 10,000 to 50,000, particularly preferably 12,000 to 24,000, single fibers are bundled. Such bundled carbon fibers have a strand strength of 4 GPa or more and 7 GPa or less, preferably 4.5 GPa or more and 6.5 GPa, so that the strength of the C / C composite itself can be increased, and it is particularly suitable for structural materials. It is. The strand strength can be obtained by impregnating a bundle of carbon fibers with a resin having the following composition, curing the resin at 130 ° C. for 35 minutes, and then performing a tensile test based on JIS R7601.
[0027]

Further, it is preferable that the bundle-like carbon fibers have a strand elastic modulus of 200 GPa or more and 400 GPa or less in that a C / C composite material having high strength and high adhesion is obtained. The strand elastic modulus referred to here can be determined from a slope of a load-elongation curve by performing a tensile test in the same manner as the above-described strand strength measuring method.
[0028]
Next, a method for producing the carbon fiber for C / C composite material of the present invention will be described.
[0029]
As the carbon fiber for the C / C composite material of the present invention, a fiber obtained by carbonizing fibers such as rayon, polyacrylonitrile, and pitch, or a graphitized fiber obtained by heat-treating the fiber at a higher temperature is mainly used. In order to obtain a high-strength C / C composite material, it is preferable to use acrylonitrile fibers from which high-strength carbon fibers are easily obtained.
[0030]
The carbon fiber for a C / C composite material having the BET specific surface area determined from krypton gas adsorption and the surface oxygen concentration ratio O / C determined by X-ray photoelectron spectroscopy can be obtained by electrolytic oxidation treatment. In the electrolytic oxidation treatment, since the treatment unevenness is controlled and the oxidation treatment can be performed deeply on the carbon fiber surface layer in a short time, the specific surface area and the surface oxygen concentration are easily controlled. The electrolytic solution used for such electrolytic oxidation treatment may be either acidic or alkaline.
[0031]
Specifically, a current value per unit weight of carbon fiber is 0.5A or more and 100A or less, preferably 2A or more in an electrolytic solution tank in which the electric conductivity of the electrolytic solution is adjusted to 15 mS / cm or more and 100 mS / cm or less. The BET specific surface area determined from the above-mentioned krypton gas adsorption by controlling the processing time to at least 80 A and the processing time to 0.5 to 20 seconds, preferably 1 to 10 seconds, and X-ray photoelectron spectroscopy The required surface oxygen concentration ratio O / C can be controlled. If the electric conductivity of the electrolytic solution is less than 15 mS / cm, it is difficult for the current to flow, and it is difficult to control the current value. If the electric conductivity exceeds 100 mS / cm, the concentration is high, so that the electrolyte is removed in the subsequent washing step. May be insufficient. When the current value per unit weight of the carbon fiber is less than 0.5 A, the current value is small, and the unevenness occurs due to the small current value. When the current value exceeds 100 A, the strength of the fiber itself decreases due to excessive oxidation treatment of the carbon fiber surface. May be. Furthermore, if the treatment time is longer than 20 seconds, the oxidation treatment of the carbon fiber surface proceeds slowly, so that it is difficult to form surface irregularities. If the treatment time is shorter than 0.5 seconds, unevenness in treatment occurs, and the shear strength of the C / C composite decreases. There are cases. In the above-described electrolytic oxidation treatment, such treatment can be performed using carbon fiber as an anode and a platinum plate as a cathode.
[0032]
When the oxygen concentration ratio O / C on the carbon fiber surface exceeds the above-mentioned value after performing the water washing and drying after the electrolytic oxidation treatment, the temperature is further increased to 250 ° C or more and 1000 ° C or less in an inert atmosphere. The oxygen concentration ratio O / C can be controlled within a predetermined range by performing a heat treatment at a temperature of 250 ° C. or more and 750 ° C. or less and a treatment time of 30 minutes or more and 5 hours or less.
[0033]
Further, if necessary, a sizing agent can be added to the carbon fibers. As the sizing agent, for example, a thermosetting resin such as an epoxy resin, a phenol resin, an alkyd resin, a urethane resin, a thermoplastic resin such as polyethylene, polyvinyl chloride, or polyamide, or a coal tar pitch or a pitch such as a petroleum pitch is used. be able to.
[0034]
By applying the carbon fiber for a C / C composite material of the present invention, a C / C composite material having excellent mechanical properties, particularly excellent tensile strength and interlaminar shear strength, can be produced. Such a C / C composite material can be a C / C composite material in which the direction of the reinforcing fiber is substantially one direction, or a bidirectional C / C in which the direction of the reinforcing fiber intersects the 90 ° and 0 ° directions. The fiber direction can be arbitrarily set according to the composite material and other purposes. In particular, by using the carbon fiber for C / C composite material, a unidirectional C / C composite material having a tensile strength of 500 to 700 MPa and an interlayer shear strength of 10 to 20 MPa can be obtained.
[0035]
Here, the tensile strength of the unidirectional C / C composite material is obtained by cutting a flat plate into a length of 50 ± 1 mm in a fiber axis direction, a width of 10 ± 1 mm and a thickness of 2.0 ± 0.2 mm in a fiber axis direction. The test piece was measured at a gauge portion of 10 mm and a test speed of 0.5 mm / min, and the strength was converted to a fiber volume content (Vf) of 40%. In addition, the interlaminar shear strength of the unidirectional C / C composite material is obtained by measuring the same test piece as described above in accordance with a test method specified in JIS K7078.
[0036]
Such a C / C composite material of the present invention can be obtained by impregnating the carbon fibers with various binders and curing by heating. There are no particular restrictions on the type of binder used in the production of the C / C composite material, and thermosetting resins such as phenolic resins, epoxy resins, alkyd resins, urethane resins, and furan resins, and thermoplastic resins such as polyethylene and polyvinyl chloride. Alternatively, a pitch such as coal tar pitch or petroleum pitch can be used. These binders are mixed or impregnated with carbon fibers and then dried to obtain a composition comprising carbon fibers and a binder. At that time, a binder dissolved in a solvent such as alcohol, acetone or anthracene oil and adjusted to an appropriate viscosity can be used. In addition, the temperature and time for heat curing can be appropriately set depending on the type of the binder. For example, when a phenol resin is used, after pressure and heat molding at 120 to 150 ° C. and 5 to 10 MPa, in air, After post-curing by heat treatment to 170 to 250 ° C., further carbonization treatment at 500 to 2000 ° C. in nitrogen and graphitization treatment at 2000 to 3000 ° C. are performed. Further, the fiber volume content in the C / C composite material is preferably 30 to 70%, and more preferably 40 to 60%. By setting the carbon fiber content in this range, a C / C composite material having excellent tensile strength and excellent shear strength can be obtained.
[0037]
【Example】
Hereinafter, the present invention will be described specifically with reference to Examples, but the present invention is not limited thereto.
[0038]
The method for measuring the characteristics used in this example is as follows.
<Measurement of arithmetic average roughness (Ra)>
The arithmetic average roughness (Ra) of the surface was measured as follows. As a measurement sample, a carbon fiber cut to a length of several mm was used. The substrate was fixed on a substrate (silicon wafer) using silver paste, and an image of a three-dimensional surface shape was obtained at the center of each single fiber by an atomic force microscope (AFM). As an atomic force microscope, a Dimension 3000 stage system was used in NanoScope IIIa manufactured by Digital Instruments. The observation conditions were as follows.
[0039]
・ Scanning mode: Tapping mode ・ Tip: Silicon cantilever ・ Scanning range: 0.6 μm × 0.6 μm
・ Scanning speed: 0.3Hz
-Number of pixels: 512 x 512
Measurement environment: At room temperature and in the air, for each sample in a single fiber, one image was obtained by observing one point at a time, and the roundness of the fiber cross section was approximated by a cubic curved surface. The arithmetic average roughness (Ra) was calculated. The arithmetic average roughness (Ra) was determined and averaged for five single fibers.
<Measurement of BET specific surface area of carbon fiber>
The BET specific surface area of the carbon fiber was measured as follows. As a sample, a carbon fiber cut to a length of about several tens cm was used. After precisely weighing the sample, it was sealed in a test tube, and the specific surface area by the BET method was measured by adsorption of krypton gas. At the time of gas adsorption, a high-precision fully automatic gas adsorption device “BELSORP 36” manufactured by Nippon Bell Co., Ltd. was used, and the measurement conditions were as follows.
[0040]
・ Adsorption gas: Kr
・ Dead volume: He
・ Adsorption temperature: liquid nitrogen temperature (77K)
・ Measurement pretreatment: 200 ° C
-Measurement mode: adsorption at isothermal temperature-Measurement range: relative pressure (P / P0) = 0.01 to 0.4
P: Measured pressure P0: Saturated vapor pressure of adsorption gas, equilibrium time: 180 sec for each equilibrium relative pressure
The calculation method of the specific surface area applied the BET theory. The specific surface area was calculated by analyzing the relative pressure range of about 0.05 to 0.3 on the BET plot according to the same theoretical formula.
<Measurement of strand tensile strength and elastic modulus>
A bundle of carbon fibers was impregnated with a resin having the following composition, cured at 130 ° C. for 35 minutes, and then subjected to a tensile test based on JIS R7601.
[0041]

<Carbon fiber surface oxygen concentration ratio (O / C)>
The surface oxygen concentration ratio O / C was determined by X-ray photoelectron spectroscopy according to the following procedure. The carbon fibers used as the samples were cut to an appropriate length, spread on a stainless steel sample support, and arranged. The photoelectron escape angle was 90 °, MgKα1,2 was used as the X-ray source, and the inside of the sample chamber was kept at a vacuum of 1 × 10 ⁻⁸ Torr. As a correction of a peak due to charging at the time of measurement, the binding energy value of the main peak of C1S was adjusted to 284.6 eV. The C1S peak area was determined by drawing a linear baseline in the range of 282 to 296 eV, and the O1S peak area was determined by drawing a linear baseline in the range of 528 to 540 eV. The surface oxygen concentration O / C was represented by an atomic ratio calculated by dividing the ratio of the O1S peak area to the C1s peak area by a sensitivity correction value unique to the apparatus. In this embodiment, an ESCA-750 manufactured by Shimadzu Corporation was used as an X-ray photoelectron spectrometer, and the sensitivity correction value unique to the device was 2.85.
<Tensile strength of C / C composite material>
A flat plate of the unidirectional C / C composite material was cut to prepare a test piece having a length of 50 ± 1 mm in the fiber axis direction, a width of 10 ± 1 mm in the fiber axis direction, and a thickness of 2.0 ± 0.2 mm. An aluminum tab having a thickness of 1 mm was adhered to both sides of both ends except for a central portion of 10 mm of the test piece to prepare a test piece for tensile strength. The test speed was 0.5 mm / min. The number of tests was 5, and the average was taken as the tensile strength. As the testing machine, a suitable material testing machine of a type capable of keeping the crosshead moving speed constant, with a load measurement error not exceeding ± 1%, is used. In this embodiment, Instron (registered) is used as the testing machine. (Trademark) testing machine 4208 was used. A diamond cutter was used for cutting the C / C composite material.
<Interlaminar shear strength of C / C composite material>
A flat plate of the unidirectional C / C composite material was cut to prepare a test piece having a length of 14 ± 1 mm in the fiber axis direction, a width of 10 ± 0.2 mm in the fiber axis direction, and a thickness of 2 ± 0.2 mm. The measurement was performed according to a test method specified in JIS K7078, with a distance between supports of 10 mm and a test speed of 1 mm / min. In this example, an Instron (registered trademark) test machine 4208 was used as a test machine.
[0042]
(Example 1)
A bundle of polyacrylonitrile-based carbon fibers having a strand strength of 4.9 GPa and a strand elastic modulus of 240 GPa (single fiber diameter: 6.9 μm, number of single fibers: 12,000 / bundle) is used as an anode, platinum is used as a cathode, and electric conductivity is made. In a sulfuric acid aqueous solution adjusted to 15 mS / cm, an electrolytic treatment was performed at a current of 10 A per 1 g of carbon fiber for 2 seconds, followed by washing with water and drying in heated air at 260 ° C. The arithmetic average roughness Ra of the surface of the obtained carbon fiber was 5.0 nm, the specific surface area was 0.62 m ² / g, and the surface oxygen concentration ratio was 0.22.
[0043]
After the bundled carbon fibers are placed in a mold, they are impregnated with a resole-based phenol resin (Sumilite Resin (registered trademark PR-50087) manufactured by Sumitomo Bakelite Co., Ltd.), and 9.8 MPa (100 kgf / cm) at 150 ° C. Pressing and heat molding was performed in ² ) to obtain a molded plate having a thickness of 2.0 mm. Further, after heat-treating this molded plate in air at a heating rate of 3 ° C./hr to 250 ° C. to perform post-curing, it is heated in nitrogen at 15 ° C./hr to 1000 ° C. to carry out carbonization treatment. went. Further, it was heated to 2000 ° C. at a rate of 15 ° C./hr and held for 24 hours to perform a graphitization treatment. The C / C composite material had a fiber volume content of 40%, a tensile strength of 570 MPa, and an interlayer shear strength of 14.6 MPa, and excellent tensile strength and interlayer shear strength were obtained.
[0044]
(Comparative Example 1)
In Example 1, a bundle of carbon fibers and a unidirectional C / C composite material were manufactured by the same manufacturing method as in Example 1 except that the electrolytic treatment was performed at 0.2 A for 100 seconds. The arithmetic average roughness Ra of the surface of the obtained carbon fiber was 3.1 nm, the specific surface area was 0.45 m ² / g, and the surface oxygen concentration ratio was 0.16. The fiber volume content of the C / C composite material was 40%, the tensile strength was 540 MPa, and the interlaminar shear strength was 9.1 MPa.
[0045]
(Example 2)
In Example 1, a bundle of carbon fibers and a unidirectional C / C composite material were produced by the same production method as in Example 1 except that the electrolytic treatment was performed at 15 A for 2 seconds. The arithmetic mean roughness Ra of the surface of the obtained carbon fiber was 5.5 nm, the specific surface area was 0.74 m ² / g, and the surface oxygen concentration ratio was 0.25. The fiber volume content of the C / C composite material was 40%, the tensile strength was 588 MPa, and the interlaminar shear strength was 14.1 MPa, and good tensile strength and interlaminar shear strength were obtained.
[0046]
(Comparative Example 2)
In Example 1, a one-way C / C composite material was manufactured by the same manufacturing method as in Example 1 except that the electrolytic treatment was performed at 120 A for 1 second. The arithmetic average roughness Ra of the surface of the obtained carbon fiber was 6.3 nm, the specific surface area was 0.94 m ² / g, and the surface oxygen concentration ratio was 0.29. The fiber volume content of the C / C composite material was 40%, the tensile strength was 493 MPa, and the interlaminar shear strength was 12.5 MPa.
[0047]
Table 1 shows the results of Examples 1 and 2 and Comparative Examples 1 and 2.
[0048]
[Table 1]

[0049]
As is clear from Table 1, Examples 1 and 2 show that both the interlayer shear strength and the tensile strength of the C / C composite material are remarkably superior to Comparative Examples 1 and 2.
[0050]
【The invention's effect】
According to the present invention, there is provided a C / C composite material having improved interlaminar shear strength without lowering the tensile strength, a method for producing the same, a carbon fiber for the C / C composite material, and a method for producing the same. be able to.

Claims (6)

The arithmetic average roughness (Ra) of the surface is 1 nm or more and 20 nm or less, the BET specific surface area obtained from krypton gas adsorption is 0.5 m ² / g or more and 1.2 m ² / g or less, and X-ray photoelectron spectroscopy is used. A carbon fiber for a carbon fiber reinforced carbon composite material, wherein a required surface oxygen concentration ratio O / C of the carbon fiber is 0.13 or more and 0.27 or less. The carbon fiber for a carbon fiber reinforced carbon composite material according to claim 1, wherein the carbon fiber is a bundle having a strand strength of 4 GPa or more and 7 GPa or less. The carbon fiber for a carbon fiber-reinforced carbon composite material according to claim 1 or 2, wherein the carbon fiber is a bundle having a strand strength of 200 GPa or more and 400 GPa or less. In an electrolyte having an electric conductivity of 15 mS / cm or more and 100 mS / cm or less, a current value per unit weight of the carbon fiber is 0.5 A or more and 100 A or less, and a processing time is 0.5 second or more, using carbon fiber as an anode. The method for producing a carbon fiber for a carbon fiber reinforced carbon composite material according to claim 1, wherein the electrolytic oxidation treatment is performed by applying a current for 20 seconds or less. A carbon fiber reinforced carbon composite material containing the carbon fiber according to claim 1. A unidirectional carbon fiber reinforced carbon composite material having a tensile strength of 500 to 700 MPa and an interlayer shear strength of 10 to 20 MPa.