JPH042688B2

JPH042688B2 -

Info

Publication number: JPH042688B2
Application number: JP61223789A
Authority: JP
Priority date: 1986-05-02
Filing date: 1986-09-24
Publication date: 1992-01-20
Also published as: KR870011290A; KR960012991B1; JPS63120112A

Description

【発明の詳細な説明】[Detailed description of the invention]

〔産業上の利用分野〕本発明はピツチ系炭素繊維およびその製法に係
り、より詳しく述べると、低い焼成温度で製造し
て高弾性率を達成したピツチ系炭素繊維に関す
る。高弾性率炭素繊維はプラスチツク、金属、炭
素、セラミツクス等との複合材料として軽量構造
材料（航空機、宇宙船、自動車、建築物等）、高
温材料（ブレーキデイスク、ロケツト等）に使用
されるほか、金属、セラミツクスの補強などに使
用される。〔従来の技術〕ポリアクリロニトリルを原料として高強度、中
弾性率のPAN系炭素繊維が製造されており、
2000℃以上で焼成された繊維は最大400GPa程度
の弾性率を示すものである。しかしながら、
PAN系炭素繊維は原料コストが高いという欠点
もさることながら、難黒鉛化性であるため結晶化
度（黒鉛化度）の向上には限界があり、本質的
に、超高弾性率を達成することは困難である。ピツチ系炭素繊維は原料が安価で経済性に優れ
ているのみならず、石油系液晶ピツチより製造
し、3000℃付近で焼成したものは黒鉛繊維と呼ば
れ、700GPa程度の超高弾性率を示す（特公昭59
−3567号公報）。また、ピツチ系炭素繊維において、強度、弾性
率等の特性を改良することを目的として、繊維の
横断面において結晶が繊維の表層部では円周方向
に配列し、中心部では放射状またはモザイク状に
配列している組織をもつ炭素繊維（特開昭59−
53717号公報）や、特に表面強度を高めるために
繊維の外周表層部がラジアル配向構造を無し、内
核部がオニオンライク配向構造を成している炭素
繊維（特開昭60−239520号公報）、などが提案さ
れている。〔発明が解決しようとする問題点〕上記の如く、液晶系ピツチを用いて超高弾性率
の炭素繊維の製造が可能であり、また繊維の特性
を改良するいくつかの手法が提案されているが、
いずれの方法においても超高弾性率を達成するた
めには3000℃付近の高温での焼成が必要である。
高温の焼成温度は設備費や製造コストの低減の障
害であると共に、焼成温度を高くすると、繊維の
引張強度が低下するという不都合がある。〔問題点を解決するための手段および作用〕本発明者らは、低温で焼成して超高弾性率を有
する炭素繊維を得るべく鋭意研究開発する過程
で、繊維の外表層部より内部で炭素繊維の結晶性
を高くすることによつてそれが可能であることを
見い出し、本発明を完成した。すなわち、本発明は、繊維の横断面内におい
て、繊維の外表層部より繊維の内部において結晶
子の大きさが10％以上大きいことを特徴とする高
弾性率ピツチ系炭素繊維にある。また、本発明
は、このようなピツチ系炭素繊維を製造する方法
として、光学的異方性部分が90％以上の炭素質ピ
ツチを紡糸し、得られる炭素質ピツチ繊維を繊維
の横断面内において外表層部だけを選択的に不融
化し、然る後その外表層部のみ不融化した繊維を
焼成することを特徴とする高弾性率ピツチ系炭素
繊維の製法にも係る。炭素繊維の結晶性が良くなると弾性率が向上す
ることは公知である。そして、700GPa程度の超
高弾性率を発現するまで結晶性を高めるために、
従来の炭素繊維では3000℃付近の高温での焼成が
必要とされていた。これに対して、本発明によれ
ば、従来よりも約500℃低い焼成温度で従来と実
質的に同等の弾性率を有する炭素繊維を得ること
が可能である。これは、従来の黒鉛化炭素繊維の製法では、紡
糸した液晶ピツチ繊維は不融化の際にその結晶性
が乱されて低下していたが、本発明では、この不
融化の際にピツチ繊維の横断面内外表層部のみを
選択的に不融化することによつて、焼成時の繊維
の融着を防ぐ最低限の不融化を達成しながらピツ
チ繊維の横断面内部の結晶性を実質的に乱さず高
いままに保持しておくことによつて、従来より低
い焼成温度でも従来と同等あるいはそれ以上の弾
性率の高い炭素繊維を製造することが可能にされ
たからである。液晶ピツチから製造したピツチ繊維の不融化反
応についての研究は非常に限られており、一応、
酸化による架橋反応により高分子化が進むことに
より不融化が達成されると考えられている。さら
に、不融化過程における結晶構造変化については
研究がほとんど行なわれていない。本発明者等
は、不融化過程における結晶構造変化をＸ線回折
で詳細に検討した結果、液晶ピツチから製造した
結晶性の良いピツチ繊維の場合には、不融化過程
で結晶性が乱され低下することを見い出した。こ
の不融化過程での結晶性の低下は炭化後の炭素繊
維の結晶構造も低下させるので、必要最小限に抑
えることがより良い性能の炭素繊維を得るために
重要である。本発明者らは、また、炭化過程での
融着を防止するための不融化の達成と結晶構造の
低下を最小限に抑えることとを両立させるために
は、不融化過程で繊維の外表層部を選択的に不融
化すればよいことを見い出した。このように不融
化した繊維は外表層部は不融化が達成されている
ので、続く炭化過程で表面が融けて繊維同士が融
着することがなく、内部は結晶構造の乱れが少な
いため全体として結晶構造の低下は最小限に抑え
られるのである。こうして、結晶性の高いピツチ繊維の横断面内
において外表層部のみを選択的に不融化した繊維
を炭化して得られる炭素繊維は、一般的に、繊維
の横断面内において、繊維の外表層部より繊維の
内部において結晶性が高い。炭素繊維の結晶性の
低い外表層部は炭化焼成時に繊維が融着すること
を防止するために不融化する部分であるから、最
低限の肉厚があればよいが、それより肉厚が大き
くても繊維の内部に結晶性の高い部分すなわち不
融化されない部分が残る限りにおいて本発明の効
果は達成される。また、繊維の外表層部と内部の
結晶性の変化は急峻である必要はなく、漸進的な
変化でもよいことは勿論である。なお、不融化す
べき外表層部の肉厚さは繊維径に依存して増加し
ないので、一般に繊維径を大きくするほど、繊維
内部の結晶性の高い部分の割合を増加させること
ができ、炭素繊維の弾性率も向上させることがで
きる。炭素繊維の外表層部と内部の結晶性の差は、紡
糸するピツチの種類と品質、不融化の条件と程
度、炭化の条件などに依存するが、本発明では内
部の方が外表層部より結晶子の大きさで10％以上
大きい。結晶子の大きさは、制限視野電子線回折
法で測定した回折パターンをマイクロデンシトメ
ーターで計測し、回折強度の半価幅の逆数で比較
する。この差が10％以下の場合は余り効果が期待
できない。次に、本発明による上記の如きピツチ系炭素繊
維の製法について説明すると、紡糸する炭素質ピ
ツチは光学的異方性部分が90％以上をなす結晶性
の高いものを使用し、好ましいものは、特開昭57
−88016号公報、同58−45277号公報、同58−
37084号公報等に記載されているような軟化点230
〜320℃、光学的異方性部分が90〜100％の炭素質
ピツチであるが、これに限定されない。紡糸は慣
用手法によることができるが、上記の好ましい炭
素質ピツチは280〜370℃の範囲内の一定温度で紡
糸することが好ましい。こうして紡糸されて結晶性を有するピツチ繊維
は、本発明に従つて、繊維の横断面内において外
表層部のみを選択的に不融化する。この目的のた
めには、ピツチ繊維に通常より短い特定の範囲内
の短時間の不融化を行なうと良い。例えば上記の
好ましい態様によつて得られた5μｍ〜20μｍ、好
ましくは、9μｍ〜14μｍの径のピツチ繊維を空気
中で不融化する場合、不融化開始温度を150℃〜
200℃とし、昇温速度１℃／分以上好ましくは、
１℃／分〜２℃／分で最終温度250℃〜350℃まで
昇温し、昇温後直ちに室温まで冷却する。昇温速
度が１℃／分より遅いと、最終温度に達するまで
に時間がかかり、繊維内部まで不融化が進行して
しまう。また、２℃／分より速いと、不融化の過
程で繊維が融着してしまう。昇温速度を上記の温
度の範囲内で１℃／分〜２℃／分にすると、融着
を起こさずに比較的短時間で最終温度まで到達す
ることができ、その結果不融化は外表層部のみで
行なわれ、繊維内部の結晶性は乱されずにすむ。不融化の雰囲気としては空気以外に酸素、オゾ
ン、二酸化窒素等でもよく、酸化性の強いガスを
使用する場合には、昇温速度もそれだけ速い範囲
内で行ない、また最終温度を下げることができ
る。ピツチ繊維を炭化焼成時に融着させないために
不融化すべき外表層部の最小限の厚さはピツチ繊
維の種類、不融化の程度などにも依存するが、例
えば、1μｍ〜3μｍの程度であると考えられ、ま
たこの厚さは繊維の径にはあまり依存しないこと
が見い出されている。こうして繊維の外表層部のみを選択的に不融化
したピツチ繊維は、常法に従い焼成して炭化する
ことができる。この炭化焼成において、不融化さ
れていなかつた繊維内部は結晶性が高いまま焼成
されるため、繊維の外表層部より結晶性が高くな
る。焼成条件は、例えば、昇温速度20℃／分〜
500℃／分、最終温度2000℃〜3000℃、焼成時間
４分〜150分である。本発明の方法によれば従来
700GPaの弾性率を達成するために必要とされて
いる3000℃より約500℃低い2500℃の焼成温度で
同じく700GPaの弾性率を有する超高弾性率の炭
素繊維が得られるが、本発明の焼成温度はこれに
限定されるわけではない。本発明による炭素繊維は、低温焼成で超高弾性
率を達成できるほか、引張強度も高い。さらに、
本発明による炭素繊維は繊維横断面内において外
表層部より内部が結晶性が良いという特異な構造
を有し、従来にない特性を奏し得るものである。
また、本発明による炭素繊維は出発ピツチ原料、
紡糸条件、炭化焼成条件などに加えて、特に繊維
径と選択的不融化の割合を選択することによつ
て、得られる炭素繊維の特性をある程度任意に変
更し得るという利点がある。〔実施例〕実施例において炭素繊維の特性は下記の如きパ
ラメータあるいは測定方法を採用した。Ｘ線構造パラメータ配向角（φ）、積層厚さ（L_C002）、層間隔
（d₀₀₂）は広角Ｘ線回折より求められる炭素繊維
の微細構造を表わすパラメータである。配向角
（φ）は結晶の繊維軸方向に対する選択的配向の
程度を示すもので、この角度が小さい程配向が良
いことを意味する。積層厚さ（L_C002は炭素微結
晶中の（002）面の見掛けの積層の厚さを表わし、
層間隔（d₀₀₂は微結晶の（002）面の層間隔を表
わす。一般に積層厚さ（L_C002）が大きい程、層
間隔（d₀₀₂）が小さい程結晶性が良いと見なされ
る。配向角（φ）の測定は繊維試料台を使用し、繊
維束が計数管の走査面に垂直になつている状態
で、計数管を走査して（002）回折帯の強度が最
大となる回折角2θ（約26°）を予め求める。次に計
数管をこの位置に保持した状態で、繊維試料台を
360°回転することにより（002）回折環の強度分
布を測定し、強度最大値の1/2の点における半価
幅を配向角（φ）とする。積層厚さ（L_C002）、層間隔（d₀₀₂）は繊維を乳
鉢で粉末状にし、学振法「人造黒鉛の格子定数お
よび結晶子の大きさ測定法」に準拠して測定・解
析を行ない、以下の式から求めた。 L_C002＝Kλ／β cosθ d₀₀₂＝λ／２ sinθ Ｋ＝1.0、λ＝1.5418Å θ：（002）回折角2θより求める β：補正により求めた（002）回折帯の半価幅透過型電子顕微鏡（TEM）観察法及び電子線回
折測定法炭素繊維試料をその繊維軸方向に引きそろえて
加熱硬化型エポキシ樹脂に包埋し、硬化する。硬
化した炭素繊維包埋ブロツクを包埋された繊維が
露出するようトリミングした後、ダイヤモンドナ
イフを装備したウルトラミクロトームを用いて、
厚さが1000オングストローム（Å）以下の超薄切
片を作製する。この超薄切片を粘着処理したグリ
ツド上に載置し、電子顕微鏡を用いて明視野像、
暗視野像の撮影を行なう。明視野像とは通常の透
過型電子顕微鏡（TEM）写真のことで、暗視野
像とは特定の結晶面により回折した電子線を結像
させることによつて、その結晶面の集合状態を観
察するものである。実施例の（002）暗視野像は、
明視野像と同一視野において、直径約10μｍの対
物絞りを用いて、（002）結晶面により回折した電
子線を結像させることによつて、（002）結晶面の
集合状態を観察したものである。写真で（002）
結晶面は白く光つて観察される。従つて白く光る
部分が太い所は（002）結晶面がよく発達してい
る所で結晶性が良い所と考えられる。繊維内の結晶性の内外差を調べるために、制限
視野電子線回折法を使用して特定部分からの電子
線回折像を測定する。測定条件は加速電圧
200KV、直径約1.7μｍの制限視野絞りで、上記超
薄切片の繊維軸に対して垂直な方向にエツジから
エツジまで連続的に電子線回折写真を撮影する。
得られた回折パターンをマイクロデンシトメータ
ーを使用して、電子線回折像の（002）、（004）、
（100）、（110）回折線について赤道並びに子午線
の２方向の回折強度の走査プロフアイルを測定す
る。このようにして得られた走査プロフアイルの
各々の回折強度の半価幅（△Ｓ）を計測する。結
晶子の大きさＬはScherrerの式Ｌ＝Ｋ／△Ｓから
求められる。Ｋは定数で各回折線により異なつた
値をとる。この式から明らかなように同一回折線
では結晶子の大きさは半価幅と反比例の関係にあ
るので、各測定点において計測した半価幅の逆数
を計算し結晶子の大きさの比較をすることができ
る。実施例１光学的異方性相（AP）を約50％含有する炭素
質ピツチを前駆体ピツチとして使用し、これをロ
ーター内有効容積200mlの円筒型連続遠心分離装
置で、ローター温度350℃に制御しつつ遠心力
10000GでAP排出口より光学的異方性相に富むピ
ツチを抜き出した。得られた光学的異方性ピツチ
は、光学的異方性相を99％以上含み、軟化点は
271℃であつた。次に得られた光学的異方性ピツチをノズル径
0.3mmの溶融紡糸機で315℃で紡糸した。得られたピツチ繊維を空気雰囲気で開始温度
180℃、最終温度290℃、昇温速度２℃／分で不融
化した。不融化処理の終了後、アルゴン雰囲気中で昇温
速度を100℃／分、最終温度2500℃で炭化を行な
い直径約13μｍの炭素繊維を得た。この炭素繊維は表１に示したように配向角
（φ）が6.8°、積層厚さ（L_C002）が210Å、層間隔
（d₀₀₂）が3.395Åであり、弾性率は736GPaであつ
た。また引張強度は2.77GPaであつた。第１図は得られた炭素繊維の横断面を示す走査
型電子顕微鏡写真であるが、断面配向構造に内部
と外表層部で差が認められる。第２図アは得られ
た炭素繊維の縦断面の透過型電子顕微鏡による
（002）暗視野像で、外表層部より内部の方が光つ
ている部分が太く見えるが、これは内部の方が
（002）積層厚さが大きく結晶性が良いためと考え
られる。第２図イは同じ縦断面の透過型電子顕微
鏡による明視野像（通常のTEM写真）で、これ
も繊維内部の方が外表層部より結晶性が良いこと
を示している。事実、電子線回折パターン中の
（002）回折線の半価幅を測定して前述の如く半価
幅の逆数から求めたところ、繊維内部の方が外表
層部より結晶子が大きい割合は21％であつた。比較例１実施例１で得られた光学的異方性ピツチを同じ
溶融紡糸機で、315℃、ピツチ吐出量を実施例１
の約1/2で紡糸した。得られたピツチ繊維を実施例１と同一条件で不
融化、炭化を行ない、直径約9μｍの炭素繊維を
得た。この炭素繊維は表１に示したように、配向角
（φ）が8.9°、積層厚さ（L_C002）が160Å、層間隔
（d₀₀₂）が3.401Åであり、弾性率は573GPa、引張
強度は2.74GPaであつた。繊維横断面の走査型電子顕微鏡写真（第３図）
では断面配向構造に内部と外表層部で差が認めら
れない。透過型電子顕微鏡による繊維の縦断面の
暗視野像（第４図ア）と明視野像（第４図イ）で
は、繊維内部と外表層部とで結晶性に差がないこ
とが認められる。事実、電子線回折パターン中の
（002）回折線の半価幅の測定より求めると、繊維
内部の方が外表層部より結晶子が大きい割合は
0.3％であり、内外差なしとみなされる。比較例２実施例１と同一のピツチ繊維を空気雰囲気で開
始温度180℃、最終温度290℃、昇温速度0.3℃／
分で不融化した。不融化処理の終了後、実施例１と同一条件で炭
化を行ない直径約13μｍの炭素繊維を得た。この炭素繊維は表１に示したように、配向角
（φ）が7.0°、積層厚さ（L_C002）が190Å、層間隔
（d₀₀₂）が3.399Åであり、弾性率は685GPa、引張
強度は2.37GPaであつた。繊維の横断面の走査型電子顕微鏡写真（第５
図）では断面配向構造に内部と外表層部で差が認
められない。また、透過型電子顕微鏡による繊維
の縦断面の暗視野像（第６図ア）と明視野像（第
６図イ）でも、繊維の内部と外表層部で結晶性に
差は認められない。事実、電子線回折パターン中
の（002）回折線の半価幅の測定より求めると、
繊維内部の方が外表層部より結晶子が大きい割合
は−0.2％であり内外差なしとみなされる。比較例３これは市販のピツチ系超高弾性炭素繊維（ユニ
オンカーバイド社の商品UCC−P100）である。この繊維の横断面の走査型電子顕微鏡写真（第
７図）は断面配向構造に内部と外表層部に明瞭な
差がないことを示す。また、透過型電子顕微鏡に
よる繊維の縦断面の暗視野像（第８図ア）と明視
野像（第８図イ）でも繊維の内部と外表層部で差
がないことが認められる。電子線回折パターン中
の（002）回折線半価幅の測定より求めると、繊
維内部の方が外表層部より結晶子が大きい割合は
−５％であり、むしろ内部の方が多少結晶子が小
さい傾向にある。 [Industrial Field of Application] The present invention relates to a pitch-based carbon fiber and a method for producing the same, and more specifically, to pitch-based carbon fiber that is produced at a low firing temperature and achieves a high modulus of elasticity. High modulus carbon fiber is used as a composite material with plastics, metals, carbon, ceramics, etc. for lightweight structural materials (aircraft, spacecraft, automobiles, buildings, etc.) and high-temperature materials (brake discs, rockets, etc.). Used for reinforcing metals and ceramics. [Prior art] PAN-based carbon fibers with high strength and medium modulus of elasticity are manufactured using polyacrylonitrile as a raw material.
Fibers fired at temperatures above 2000°C exhibit a maximum elastic modulus of about 400GPa. however,
PAN-based carbon fibers have the disadvantage of high raw material costs, and are difficult to graphitize, so there is a limit to how much crystallinity (graphitization) can be improved. That is difficult. Pitch-based carbon fibers are not only inexpensive raw materials and highly economical, but also produced from petroleum-based liquid crystal pitch and fired at around 3000℃, which is called graphite fiber and exhibits an ultra-high modulus of elasticity of about 700 GPa. (Tokuko Showa 59
-3567). In addition, for the purpose of improving properties such as strength and elastic modulus in pitch-based carbon fibers, in the cross section of the fiber, crystals are arranged in the circumferential direction on the surface layer of the fiber, and in a radial or mosaic pattern in the center. Carbon fiber with an arranged structure
53717), and carbon fibers in which the outer peripheral surface layer of the fiber has no radial orientation structure and the inner core has an onion-like orientation structure (Japanese Patent Application Laid-Open No. 60-239520), in order to increase the surface strength. etc. have been proposed. [Problems to be solved by the invention] As mentioned above, it is possible to produce carbon fibers with ultra-high modulus of elasticity using liquid crystal pitch, and several methods have been proposed to improve the properties of fibers. but,
In either method, sintering at a high temperature of around 3000°C is necessary to achieve ultra-high elastic modulus.
A high firing temperature is an obstacle to reducing equipment costs and manufacturing costs, and increasing the firing temperature also has the disadvantage that the tensile strength of the fibers decreases. [Means and effects for solving the problem] In the process of intensive research and development to obtain carbon fibers having an ultra-high modulus of elasticity by firing at low temperatures, the present inventors discovered that carbon fibers are removed from the outer surface layer of the fibers and inside the fibers. They discovered that this was possible by increasing the crystallinity of the fibers, and completed the present invention. That is, the present invention resides in a high modulus pitch-based carbon fiber characterized in that, in the cross section of the fiber, the size of crystallites inside the fiber is 10% or more larger than in the outer surface layer of the fiber. In addition, the present invention provides a method for producing such pitch-based carbon fibers by spinning carbonaceous pitches having an optically anisotropic portion of 90% or more, and spinning the resulting carbonaceous pitches in the cross section of the fiber. The present invention also relates to a method for producing a pitch-based carbon fiber with a high elastic modulus, which is characterized in that only the outer surface layer portion is selectively made infusible, and then the fiber with only the outer surface layer portion made infusible is fired. It is known that when the crystallinity of carbon fibers improves, the elastic modulus improves. In order to increase the crystallinity until it exhibits an ultra-high elastic modulus of about 700GPa,
Conventional carbon fibers require firing at high temperatures around 3000°C. In contrast, according to the present invention, it is possible to obtain carbon fibers having substantially the same elastic modulus as conventional carbon fibers at a firing temperature approximately 500° C. lower than conventional carbon fibers. This is because in the conventional method for producing graphitized carbon fiber, the crystallinity of the spun liquid crystal pitch fibers was disturbed and decreased during the infusibility process, but in the present invention, the pitch fibers were reduced during the infusibility process. By selectively infusible only the inner and outer surface layers of the cross section, the crystallinity inside the cross section of the pitch fiber is substantially disturbed while achieving the minimum amount of infusibility to prevent fibers from fusing during firing. This is because by keeping the temperature high, it is possible to produce carbon fibers with a high elastic modulus equal to or higher than conventional ones even at a lower firing temperature than conventional ones. There is very limited research on the infusibility reaction of pitch fibers made from liquid crystal pitch.
It is believed that infusibility is achieved by the progress of polymerization due to a crosslinking reaction caused by oxidation. Furthermore, little research has been conducted on changes in crystal structure during the infusibility process. As a result of a detailed study of crystal structure changes during the infusibility process using X-ray diffraction, the present inventors found that in the case of pitch fibers with good crystallinity produced from liquid crystal pitch, the crystallinity is disturbed and deteriorated during the infusibility process. I found something to do. This reduction in crystallinity during the infusibility process also reduces the crystal structure of the carbon fiber after carbonization, so it is important to minimize the crystallinity to obtain carbon fibers with better performance. The present inventors also discovered that in order to achieve both infusibility to prevent fusion during the carbonization process and to minimize deterioration of the crystal structure, it is necessary to We have discovered that it is sufficient to selectively infusible portions. Since the outer surface layer of the fibers made infusible in this way has been made infusible, the surface will not melt during the subsequent carbonization process and the fibers will not fuse together, and the internal crystal structure will not be disturbed as a whole. Deterioration of the crystal structure is minimized. In this way, carbon fibers obtained by carbonizing fibers in which only the outer surface layer in the cross section of highly crystalline pitch fibers is selectively infusible are generally Crystallinity is higher inside the fiber than in the other parts. The outer surface layer of carbon fiber with low crystallinity is the part that becomes infusible to prevent the fibers from fusing during carbonization firing, so it is sufficient to have a minimum thickness, but it is necessary to have a larger thickness than that. However, the effects of the present invention can be achieved as long as a highly crystalline portion, that is, a portion that is not infusible, remains inside the fiber. Further, the change in crystallinity between the outer surface layer and the inside of the fiber does not need to be steep, and it goes without saying that the change may be gradual. Furthermore, since the thickness of the outer surface layer to be infusible does not increase depending on the fiber diameter, generally speaking, the larger the fiber diameter, the more the proportion of the highly crystalline part inside the fiber can be increased, and the carbon The elastic modulus of the fibers can also be improved. The difference in crystallinity between the outer surface layer and the inside of carbon fiber depends on the type and quality of the spinning pitch, the conditions and degree of infusibility, the carbonization conditions, etc., but in the present invention, the crystallinity of the inner layer is higher than that of the outer surface layer. More than 10% larger in crystallite size. The size of the crystallites is determined by measuring the diffraction pattern using a selected area electron diffraction method using a microdensitometer, and comparing the results using the reciprocal of the half width of the diffraction intensity. If this difference is less than 10%, no significant effect can be expected. Next, to explain the method for producing the above-mentioned pitch-based carbon fiber according to the present invention, the carbon pitch to be spun is a highly crystalline pitch having an optically anisotropic portion of 90% or more, and the preferable one is: Unexamined Japanese Patent Publication 1987
-88016 publication, 58-45277 publication, 58-
Softening point 230 as described in Publication No. 37084 etc.
It is a carbonaceous pitch with a temperature of ~320°C and an optically anisotropic portion of 90 to 100%, but is not limited thereto. Although spinning can be carried out by conventional techniques, it is preferred that the preferred carbonaceous pitch described above be spun at a constant temperature within the range of 280 to 370°C. According to the present invention, the spun fibers having crystallinity are selectively rendered infusible only in the outer surface layer within the cross section of the fibers. For this purpose, the pitch fibers are preferably infusible for a short time within a specific range, which is shorter than usual. For example, when pitch fibers having a diameter of 5 μm to 20 μm, preferably 9 μm to 14 μm, obtained by the above-mentioned preferred embodiment are infusible in air, the infusibility starting temperature is 150° C.
200℃, preferably at a temperature increase rate of 1℃/min or more,
The temperature is increased at a rate of 1°C/min to 2°C/min to a final temperature of 250°C to 350°C, and immediately cooled to room temperature. If the heating rate is slower than 1° C./min, it will take time to reach the final temperature, and infusibility will progress to the inside of the fiber. Moreover, if the speed is faster than 2° C./min, the fibers will fuse together during the infusibility process. If the heating rate is set to 1°C/min to 2°C/min within the above temperature range, the final temperature can be reached in a relatively short time without fusion, and as a result, the infusibility is reduced to the outer surface layer. The crystallinity inside the fiber is not disturbed. In addition to air, the infusible atmosphere may be oxygen, ozone, nitrogen dioxide, etc. If a strongly oxidizing gas is used, the heating rate must be kept within a correspondingly faster range, and the final temperature can be lowered. . The minimum thickness of the outer surface layer that must be infusible to prevent pitch fibers from fusing during carbonization and firing depends on the type of pitch fiber, the degree of infusibility, etc., but is, for example, about 1 μm to 3 μm. It has been found that this thickness does not depend much on the fiber diameter. The pitch fibers in which only the outer surface layer of the fibers has been selectively infusible can be fired and carbonized by a conventional method. In this carbonization firing, the interior of the fiber that has not been made infusible is fired while remaining highly crystalline, so that the crystallinity is higher than that of the outer surface layer of the fiber. Firing conditions include, for example, a temperature increase rate of 20°C/min.
The firing time is 500°C/min, the final temperature is 2000°C to 3000°C, and the firing time is 4 minutes to 150 minutes. According to the method of the present invention, conventional
Although an ultra-high modulus carbon fiber having an elastic modulus of 700 GPa can be obtained at a firing temperature of 2500°C, which is about 500°C lower than the 3000°C required to achieve a modulus of 700 GPa, the firing of the present invention The temperature is not limited to this. The carbon fiber according to the present invention can achieve an ultra-high modulus of elasticity by low-temperature firing, and also has high tensile strength. moreover,
The carbon fiber according to the present invention has a unique structure in which the inner part has better crystallinity than the outer surface layer within the cross section of the fiber, and can exhibit unprecedented characteristics.
Further, the carbon fiber according to the present invention can be obtained from starting pitch raw materials,
In addition to the spinning conditions, carbonization firing conditions, etc., there is an advantage that the properties of the obtained carbon fibers can be arbitrarily changed to some extent by particularly selecting the fiber diameter and the rate of selective infusibility. [Example] In the examples, the following parameters or measurement methods were used to measure the characteristics of carbon fibers. X-ray structure parameters The orientation angle (φ), the lamination thickness (L _C002 ), and the interlayer spacing (d ₀₀₂ ) are parameters representing the fine structure of carbon fibers determined by wide-angle X-ray diffraction. The orientation angle (φ) indicates the degree of selective orientation of the crystal with respect to the fiber axis direction, and the smaller this angle, the better the orientation. Lamination thickness (L _C002 represents the apparent lamination thickness of the (002) plane in the carbon microcrystal,
The layer spacing (d ₀₀₂ ) represents the layer spacing between the (002) planes of microcrystals. Generally, the larger the lamination thickness (L _C002 ) and the smaller the layer spacing (d ₀₀₂ ), the better the crystallinity. Orientation angle To measure (φ), use a fiber sample stage, scan the counter with the fiber bundle perpendicular to the scanning plane of the counter, and scan the (002) diffraction angle 2θ at which the intensity of the diffraction band is maximum. (approximately 26°).Next, while holding the counter in this position, place the fiber sample stand.
By rotating 360°, the intensity distribution of the (002) diffraction ring is measured, and the half-width at half the maximum intensity value is taken as the orientation angle (φ). Lamination thickness (L _C002 ) and layer spacing (d ₀₀₂ ) were determined by grinding the fibers into powder in a mortar and measuring and analyzing them in accordance with the Gakushin method "Lattice constant and crystallite size measurement method of artificial graphite". , was calculated from the following formula. L _C002 = Kλ/β cosθ d ₀₀₂ = λ/2 sinθ K=1.0, λ=1.5418Å θ: (002) obtained from the diffraction angle 2θ: β: half-width transmission electron of the (002) diffraction band obtained by correction Microscope (TEM) observation method and electron diffraction measurement method Carbon fiber samples are aligned in the fiber axis direction, embedded in heat-curable epoxy resin, and cured. After trimming the cured carbon fiber embedded block to expose the embedded fibers, an ultramicrotome equipped with a diamond knife was used to
Ultrathin sections with a thickness of 1000 angstroms (Å) or less are prepared. This ultrathin section was placed on an adhesive-treated grid, and bright-field images were obtained using an electron microscope.
Take a dark field image. A bright-field image is a normal transmission electron microscope (TEM) photograph, whereas a dark-field image is a photograph taken by focusing an electron beam diffracted by a specific crystal plane to observe the aggregated state of that crystal plane. It is something to do. The (002) dark field image of the example is
The aggregated state of the (002) crystal plane was observed by imaging the electron beam diffracted by the (002) crystal plane using an objective aperture with a diameter of approximately 10 μm in the same field of view as the bright field image. be. In photos (002)
The crystal planes are observed to glow white. Therefore, the areas where the white glowing part is thick are considered to be areas where the (002) crystal plane is well developed and the crystallinity is good. In order to investigate differences in crystallinity within the fiber, electron diffraction images from specific portions are measured using selected area electron diffraction. Measurement condition is accelerating voltage
Electron diffraction photographs are taken continuously from edge to edge in the direction perpendicular to the fiber axis of the ultrathin section using a selected area aperture of approximately 1.7 μm in diameter at 200 KV.
The obtained diffraction pattern was analyzed using a microdensitometer to obtain electron beam diffraction images (002), (004),
For the (100) and (110) diffraction lines, scan profiles of diffraction intensities in two directions, ie, the equator and the meridian, are measured. The half width (ΔS) of the diffraction intensity of each of the scanning profiles thus obtained is measured. The crystallite size L is determined from Scherrer's formula L=K/ΔS. K is a constant and takes a different value depending on each diffraction line. As is clear from this equation, the crystallite size is inversely proportional to the half-width for the same diffraction line, so the reciprocal of the half-width measured at each measurement point is calculated and the crystallite sizes are compared. can do. Example 1 A carbonaceous pitch containing approximately 50% optically anisotropic phase (AP) was used as a precursor pitch, and was heated to a rotor temperature of 350°C in a cylindrical continuous centrifugal separator with an effective volume of 200 ml in the rotor. centrifugal force while controlling
At 10,000G, a pit rich in optically anisotropic phase was extracted from the AP outlet. The optically anisotropic pitch obtained contains more than 99% of the optically anisotropic phase and has a softening point of
It was 271℃. Next, the obtained optical anisotropy pitch is determined as the nozzle diameter.
Spun at 315°C on a 0.3mm melt spinning machine. The obtained pitch fiber is heated to the starting temperature in an air atmosphere.
It was made infusible at a temperature of 180°C, a final temperature of 290°C, and a heating rate of 2°C/min. After the infusibility treatment was completed, carbonization was carried out in an argon atmosphere at a heating rate of 100° C./min and a final temperature of 2500° C. to obtain carbon fibers having a diameter of about 13 μm. As shown in Table 1, this carbon fiber had an orientation angle (φ) of 6.8°, a laminated thickness (L _C002 ) of 210 Å, a layer spacing (d ₀₀₂ ) of 3.395 Å, and an elastic modulus of 736 GPa. Moreover, the tensile strength was 2.77 GPa. FIG. 1 is a scanning electron micrograph showing a cross section of the obtained carbon fiber, and a difference in the cross-sectional orientation structure is recognized between the inner and outer surface layers. Figure 2A is a (002) dark-field image of the longitudinal section of the obtained carbon fiber taken with a transmission electron microscope.The shiny part appears thicker in the inner part than in the outer surface layer, but this is because the inner part is thicker than the outer surface layer. (002) This is thought to be due to the large layer thickness and good crystallinity. Figure 2A is a bright-field image (ordinary TEM photograph) of the same longitudinal section taken with a transmission electron microscope, and this also shows that the inside of the fiber has better crystallinity than the outer surface layer. In fact, when we measured the half-width of the (002) diffraction line in the electron diffraction pattern and calculated it from the reciprocal of the half-width as described above, we found that the proportion of crystallites in the interior of the fiber is larger than that in the outer surface layer is 21. It was %. Comparative Example 1 The optically anisotropic pitch obtained in Example 1 was processed using the same melt spinning machine at 315°C and the pitch discharge amount was the same as that of Example 1.
The yarn was spun at approximately 1/2 of the original amount. The obtained pitch fibers were made infusible and carbonized under the same conditions as in Example 1 to obtain carbon fibers with a diameter of about 9 μm. As shown in Table 1, this carbon fiber has an orientation angle (φ) of 8.9°, a laminated thickness (L _C002 ) of 160 Å, a layer spacing (d ₀₀₂ ) of 3.401 Å, an elastic modulus of 573 GPa, and a tensile strength of was 2.74 GPa. Scanning electron micrograph of fiber cross section (Figure 3)
No difference was observed in the cross-sectional orientation structure between the inner and outer surface layers. Dark-field images (FIG. 4A) and bright-field images (FIG. 4B) of the longitudinal section of the fibers taken with a transmission electron microscope show that there is no difference in crystallinity between the inside of the fiber and the outer surface layer. In fact, as determined by measuring the half-width of the (002) diffraction line in the electron diffraction pattern, the proportion of crystallites in the interior of the fiber is larger than that in the outer surface layer.
It is 0.3%, and it is considered that there is no difference between Japan and Japan. Comparative Example 2 The same pitch fiber as in Example 1 was prepared in an air atmosphere at a starting temperature of 180°C, a final temperature of 290°C, and a heating rate of 0.3°C/
It became infusible in minutes. After the infusibility treatment was completed, carbonization was performed under the same conditions as in Example 1 to obtain carbon fibers with a diameter of about 13 μm. As shown in Table 1, this carbon fiber has an orientation angle (φ) of 7.0°, a laminated thickness (L _C002 ) of 190 Å, a layer spacing (d ₀₀₂ ) of 3.399 Å, an elastic modulus of 685 GPa, and a tensile strength of was 2.37 GPa. Scanning electron micrograph of the cross section of the fiber (No. 5
In Figure), there is no difference in the cross-sectional orientation structure between the inner and outer surface layers. Moreover, no difference in crystallinity is observed between the inside and outer surface layer of the fiber in the dark field image (FIG. 6A) and bright field image (FIG. 6B) of the longitudinal section of the fiber using a transmission electron microscope. In fact, when determined from the measurement of the half-width of the (002) diffraction line in the electron diffraction pattern,
The percentage of crystallites larger inside the fiber than in the outer surface layer is -0.2%, and it is considered that there is no difference between inside and outside. Comparative Example 3 This is a commercially available pitch-based ultra-high modulus carbon fiber (Union Carbide's product UCC-P100). A scanning electron micrograph (FIG. 7) of a cross section of this fiber shows that there is no clear difference in cross-sectional orientation structure between the inner and outer surface layers. Further, it is recognized that there is no difference between the inside and outer surface layer of the fiber in the dark field image (FIG. 8A) and the bright field image (FIG. 8B) of the longitudinal section of the fiber using a transmission electron microscope. Based on the measurement of the half width of the (002) diffraction line in the electron diffraction pattern, the proportion of crystallites in the interior of the fiber is larger than that in the outer surface layer is -5%. They tend to be small.

〔発明の効果〕〔Effect of the invention〕

本発明によれば、700GPa以上の超高弾性率繊
維を従来より低い焼成温度で製造できるので、製
造設備費、製造コストを大幅に低減し得る。ま
た、繊維の内部が外表層部より結晶性が高い特異
な構造および物性を有する超高弾性繊維が提供さ
れる。そのほか、引張強度が向上すること、従来
より太い繊維径の製品のため生産効率が良くなり
取扱いも楽になること、などの効果がある。 According to the present invention, ultra-high modulus fibers of 700 GPa or more can be produced at a lower firing temperature than conventional ones, so that production equipment costs and production costs can be significantly reduced. Further, an ultra-high elastic fiber having a unique structure and physical properties in which the inside of the fiber has higher crystallinity than the outer surface layer is provided. Other benefits include improved tensile strength, and because the product has a larger fiber diameter than conventional products, it improves production efficiency and is easier to handle.

【図面の簡単な説明】[Brief explanation of drawings]

第１図は実施例１で得られた炭素繊維の横断面
を含む走査型電子顕微鏡による斜視写真、第２図
は実施例１で得られた炭素繊維の縦断面の透過型
電子顕微鏡によるそれぞれ暗視野像および明視野
像（普通のTEM写真）、第３図は比較例１で得ら
れた炭素繊維の横断面を含む走査型電子顕微鏡に
よる斜視写真、第４図は比較例１で得られた炭素
繊維の縦断面の透過型電子顕微鏡によるそれぞれ
暗視野像および明視野像（普通のTEM写真）、第
５図は比較例２で得られた炭素繊維の横断面を含
む走査型電子顕微鏡による斜視写真、第６図は比
較例２で得られた炭素繊維の縦断面の透過型電子
顕微鏡によるそれぞれ暗視野像および明視野像
（普通のTEN写真）、第７図は比較例３の炭素繊
維の横断面を含む走査型電子顕微鏡による斜視写
真、第８図は比較例３の炭素繊維の縦断面の透過
型電子顕微鏡によるそれぞれ暗視野像および明視
野像（普通のTEM写真）である。第９図は実施
例２の炭素繊維の特性の繊維径依存性を示すグラ
フ図である。 FIG. 1 is a perspective photograph taken with a scanning electron microscope of a cross section of the carbon fiber obtained in Example 1, and FIG. 2 is a dark photograph taken with a transmission electron microscope of a longitudinal section of the carbon fiber obtained in Example 1. Field image and bright field image (ordinary TEM photograph); Figure 3 is a perspective photograph taken by a scanning electron microscope including a cross section of the carbon fiber obtained in Comparative Example 1; Figure 4 is a perspective photograph of the carbon fiber obtained in Comparative Example 1. A dark-field image and a bright-field image (ordinary TEM photograph) of a longitudinal cross-section of carbon fiber taken with a transmission electron microscope, respectively. Figure 5 is a perspective view taken with a scanning electron microscope of a cross-section of the carbon fiber obtained in Comparative Example 2. The photograph, Figure 6, is a dark-field image and bright-field image (ordinary TEN photograph) of the longitudinal cross-section of the carbon fiber obtained in Comparative Example 2 taken by a transmission electron microscope, respectively, and Figure 7 shows the carbon fiber of Comparative Example 3. A perspective photograph taken by a scanning electron microscope including a cross section, and FIG. 8 are a dark field image and a bright field image (ordinary TEM photograph) taken by a transmission electron microscope of a longitudinal section of the carbon fiber of Comparative Example 3, respectively. FIG. 9 is a graph showing the fiber diameter dependence of the characteristics of the carbon fiber of Example 2.

Claims

【特許請求の範囲】１繊維の断面内において、繊維の外表層部より
繊維の内部において結晶子の大きさが10％以上大
きいことを特徴とする高弾性率ピツチ系炭素繊
維。２光学的異方性部分を90％以上含む炭素質ピツ
チを紡糸し、得られる炭素質ピツチ繊維を繊維の
断面内の外表層部だけを選択的に不融化し、然る
後その外表層部のみを不融化した繊維を焼成する
ことを特徴とする高弾性率ピツチ系炭素繊維の製
法。[Scope of Claims] 1. A high modulus pitch-based carbon fiber characterized in that, in the cross section of the fiber, the size of crystallites inside the fiber is 10% or more larger than in the outer surface layer of the fiber. 2 Spinning carbonaceous pitch fibers containing 90% or more of optical anisotropy, selectively infusible only the outer surface layer within the cross section of the fiber, and then A method for producing high modulus pitch-based carbon fiber, which is characterized by firing fibers that have been rendered infusible.