CA1284261C

CA1284261C - Process for producing carbon fibers

Info

Publication number: CA1284261C
Application number: CA000499690A
Authority: CA
Inventors: Takeshi Imamura; Yasuhiro Yamada; Hidemasa Honda; Ryoichi Nakajima; Masatoshi Tsuchitani
Original assignee: Agency of Industrial Science and Technology; Maruzen Petrochemical Co Ltd
Current assignee: Maruzen Petrochemical Co Ltd; National Institute of Advanced Industrial Science and Technology AIST
Priority date: 1985-01-19
Filing date: 1986-01-16
Publication date: 1991-05-21
Anticipated expiration: 2008-05-21
Also published as: JPS61167022A; EP0189150B1; US4818449A; AU576654B2; DE3670515D1; EP0189150A3; EP0189150A2; JPH0637725B2; AU5222886A

Abstract

ABSTRACT OF THE DISCLOSURE
When preparing carbon fibers from mesophase pitches, we usually encounter the formation of cracks along the fiber axis. The crack formation is the most serious and troublesome problem. A simple process and apparatus which can effectively prevent the crack formation are disclosed.
In the process for preparing the carbon fibers from mesophase pitches by melt spinning, it is sufficient only to give a rotatory motion to the molten mesophase pitches just before extrusion substantially around the axis of a spinning nozzle hole. The apparatus is essentially the same as a usual nozzle plate having a pitch introducing tube and a spinning nozzle hole provided that the apparatus further contains a plug member having an outer spiral groove such as a drill point or a worm gear and the plug member is inserted within the pitch introducing tube.

Description

~6i 1 The present invention relates to a process for producing carbon fibers with high strength and high modulus of elasticity such as Young's modulus from mesophase pi~ches. More specifically, the present invention relates to an excellent economical process for producing high quality carbon fibers from mesophase pitches by melt spinning, wherein the pitch is spun while being subjected to rotatory motion. The characteristic feature of the apparatus suitable for carrying out the process of the present invention is that the apparatus contains a plug member having a spiral groove thereon, such as a drill point or a worm gear-like structure and the plug member is positioned within a path of pitch flow near a spinning nozzle hole so aq to give a rotatory motion to the pitch.
In the specification, the words "pitch-based carbon fibers" mean carbon fibers made from pitches.
Carbon fibers are useful materials, and they are recently gathering interest as an important material of the next generation. ~he carbon fibers may be classified into two groups: a high performance grade carbon fiber with high strength and high modulus of elasticity which is used as composite material in the fabrication of aircraft structures, sports goods, and the like, and a general purpose grade carbon fiber which is mainly used as heat ;

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,, 1 insulator because of its low strength and modulus of elasticity.
High performance grade carbon fibers have been produced mainly by spinning a polyacrylonitrile (PAN) fiber, converting the PAN fiber to infusible state under oxidizing conditions, and subsequently carbonizing or graphitizing it under an inert atmosphere. In contrast to these PAN-based carbon fibers, pitch-based carbon fibers which are produced from pitches, have been regarded as unsuitable for use as structure materials because of their lower strength and modulus of elasticity than PAN-based carbon fibers.
Recently however, pitch-based carbon fibers received attention because of the low cost of the starting material and because high yields are attainable when they are rendered infusible or are carbonized. Vigorous studies ~urrently being made concerning the process for producing of high performance carbon fibers from pitches as the starting material. Several processes have been proposed which permit production of pitch-based carbon fibers showing equal properties to those of PAN-based carbon fibers or even showing a far superior modulus of elasticity.
Various processes for the production of pitch-based high performance carbon fibers have been .

- 3 ~84~6~

1 elaborated: in one process (Japanese Patent Laid-Open No.
Sho 58 (1983)-196292 to Yamada et al published November 15, 1983), pitches for spinning are produced by hydrogenation and subsequent thermal treatment; in another process (Japanese Patent Laid-Open No. Sho 5B (1983)- -113292) to Miyazaki et al published July 6, 1983, pitches for spinning are produced by fractional solvent extraction of pitches and subsequent thermal treatment of specific fractions thus fractionated. In still another process (Japanese Patent Laid-Open No. Sho 53(1978)-86717 to McHenry et al published July 31, 1978), pitches for spinning are produced by submitting pitches to a thermal treatment for a prolonged period of time at a relatively low temperature. A characteristic feature common to these prOCeSge3 i9 that all of the pitches for spinning are so-called me~ophase pitch, which contain the mesophase showing an optical anisotropy when examined on a polarized light microscope as the main component.
The mesophases described above are liquid crystals and are formed on heating heavy oils, tars or pitches. in the specification, the words "heavy oil" mean an oil having high boiling point and high specific gravity. It i8 considered that these mesophases show an optical anisotropy because planar aromatic molecules, developed by thermal polymerization, are aligned in a li - ' . .

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1 layered structure. When fibers are produced from such mesophase pitches by melt spinning, the planar aromatic molecules are aligned parallel to the fiber axis by the stress exerted on passing through a spinning nozzle hole. This oriented structure is not disturbed and is maintained throughout the states of rendering the fibers infusible and their carbonization. Therefore, the carbon layers in the carbon fibers thus produced are also oriented along the fiber axis. Such highly oriented carbon fibers show high tensile strength, and when they are graphitized, they show high modulus of elasticity which is not attainable with PAN-based carbon fibers.
In order to improve the performance of pitch-based carbon fibers, it is essential to produce and to use mesophase pitches which will permit formation of well-oriented planar aromatic molecules during the spinning stage, and most of the prior proposals on the processes for the production of high performance carbon fibers relate to processes for the production of high quality mesophase pitches.
When fibers are spun with high quality mesophase pitches which induoo well-oriented planar aromatic molecules, the molecules are oriented not only along the fiber axis, but also specifically on the cross section of the fiber. When a fiber is spun through an ordinary ~, i .

1 spinning nozzle hole with a circular cross section, it willhave a circular cross section. When this cross section is examined, the planar aromatic molecules take the so-called radial orientation, which means that they are oriented radially from the center of the circle. On carbonization, the planar aromatic molecules shrink to form carbon layers, while evaporating off volatile components. The degree of this shrinkage is markedly greater than in the direction which is perpendicular to the plane of the planar aromatic molecules. Therefore, in the case of a fiber with a radial orientation, there is a large difference in the degree of shrinkage between that near the surface and that near the center of the fiber.
This causes large cracks to take place along the fiber axi~ on carbonization and drastically lowers the commercial value.
The fibers also suffered from the crack formation along the fiber axis on carbonization, but after intensive studies aimed at solving these problems, we successfully came to the present invention.
Thus, we found that the cracks can be completely prevented by a simple process and the characteristic feature of the process comprises giving to the molten pitch just before the extrusion, a rotary motion substantially around the axis of the spinning nozzle ~, 1 hole. We also found a spinning apparatus with a simple construction which is suitable to the practice of the process described above.
Therefore, the fist object of the present invention is to provide a process for producing high performance pitch-based carbon fibers which can effectively prevent the crack formation.
Thus, the gist of the present invention resides in a process for producing carbon fibers from a ~esophase pitch by melt spinning which comprises extruding a molten mesophase pitch through a spinning nozzle hole, bringing the extruded pitch fibers thus obtained into an infusible state by heating under an oxidizing atmosphere and then carbonizing or graphitizing the pitch fibers by heating under an inert atmosphere, wherein the spinning is accomplished by constraining the pitch to flow past a plug means forming a spiral passage substantially around the axis of said spinning nozzle hole to give a rotary motion to the molten mesophase pitch just before extruding the pitch through the nozzle hole.
Figure 1 shows the orientation and cracks on a cross section of a pitch-based carbon fiber produced by a conventional method, wherein broken lines show the orientation of aligned carbon layers; and the right hand side of Figure 2 is a side view of the fiber, and the left - 128~:1 1 hand side thereof is a cross sectional view of the fiber showing schematically the alignment of carbon layers;
Figure 3 shows the orientation on a cross section of a pitch-based carbon fiber produced by the process of the present invention, wherein broken lines show the orientation of aligned carbon layers; and the right hand side of Figure 4 is a side view of the fiber, and the left hand side thereof is a cross sectional view of the fiber showing schematically the alignment of carbon layers; and Figure 5 shows a side view of an essential part of an example of the spinning apparatus suitable for carrying out th2 process of the present invention, which is a partially cross sectional view for the sake of a ready understanding of the structure.
Any mesophase pitch may be used which contains mesophase as the main component. The mesophase shows an optical anisotropy when examined on a polarized light microscope. The production of the mesophase pitch is not restricted to any specific process. Therefore, coal tar, naphtha tar, pyrolysis tar, decant oil, or other pitch-like substances produced by distillation or thermal treatment of these heavy oils, or the like may be used as the starting material for the production of mesophase pitches.

l As described before, several processes have been known to the art concerning the process of production of mesophase pitches. For example, a mesophase pitch with a low softening temperature and with good spinning properties may readily be produced by 1) hydrogenating a pitch by mixing l weight part of the pitch with 2 - 3 weight parts of tetrahydroquinoline and heating the mixture of at 400 - 450C under an autogeneous pressure, and then 2) subjecting the hydrogenated pitch to a brief thermal treatment at a high temperature with bubbling of an inert gas.
The spinning properties of mesophase pitches are greatly dependent upon the softening temperature and the ratio of constituents. Mesophase pitches with a very high softening temperature are not preferable because they require a high spinning temperature which causes degradation and decomposition of pitches. Even if pitches with a low softening temperature are used, if they are such that their main components are isotropic materials and if a ~mall amount of mesophase materials.~e present and dispersed as spheres, they show poor spinning properties because the pitches become heterogeneous due to the large difference in the viscosities of the isotropic and anisotropic materials in the spinning temperature range. When isotropic pitches which do not contain . ,- , ..

$~8~
~o ~h,~s 1 mesophase ~ t least one-half are spun, ~ will not meet the object of the present invention, because the aromatic molecules in this case are not ~arge enough to orient themselves distinctly by the stress on passing through the spinning nozzle hole. Preferred mesophase pitches are those which contain more than 60%, and more preferably more than 80% of components showing an optical anisotropy when observed on a polarized light microscope. Mesophase pitches with softening temperature of 250 - 320C are preferred.
It has been known to the art ~hat the crack formation during conversion to infusible state, carbonization, or graphitization step of a spun pitch may be prevented by raising the spinning temperature, which induces the molecules within the fiber to take an onion-like orientation instead of a radial orientation.
However, when mesophase pitches which themselves show high softening temperature of 250 - 300 C are spun by the process just described above, the spinning temperature should be maintained, at least, above 350C to prevent the crack formation. At this temperature, many organic compounds will decompose, and when degradation and decomposition of pitches are taken into account, it is not necessarily a preferred method to use a higher temperature in order to establish an onion-like orientation, since - 128~

1 degradation and decomposition of the pitches produce tremendous adverse effects to the properties of carbon fiber products. The use of a spinning nozzle hole with a complex cross section may also reduce the radial orientation of molecules and prevent the crack formation along the fiber axis at the carbonization stage. In this case, however, forming the spinning nozzle hole requires a special technique, and problems may arise concerning the precision of the spinning nozzle hole or cleaning up of the spinning nozzle after the use.
On the other hand, the process of the present invention can use a conventional spinning nozzle with a circular spinning nozzle hole, requires no further processing to the spinning nozzle hole itself, and yet it can shift the orientation of molecules within the fibers only by giving a rotatory motion to the pitch just before the extrusion, and in this way, can completely prevent the crack formation along the fiber axis at the carbonization or graphitization stage.
The means to g$ve a rotatory motion to the pitch just before the extrusion are not restricted, but it is advantageous to use the apparatus described below.
Thus, the simplest means to give a rotatory motion to a pitch just before the extrusion is to use a ~5 spinning apparatus in which a plug member with an outer 1 spiral groove is inserted into the pitch introducing tube which is connected substantially coaxial with a spinning nozzle hole, the plug member substantially fitting into the pitch introducing tube.
The most preferred structure of the above apparatus is a pitch introducing tube having a circular cross section and a plug member having a circular cross section with the same or slightly smaller diameter as that of the pitch introducing tube. In this case, the shape of the plug member looks like a drill point or a worm gear.
Thus, when a simple construction is desired, it will be achieved by connecting above the spinning nozzle hole, a pitch introducing tube with a diameter of a few mm and having a length of greater than ten mm, into which a conventional drill point commonly used as a tool or worm gear is in~erted.
Generally, the diameter of a pitch introducing tube of a nozzle plate decreases as it nears the spinning nozzle hole and the top of the pitch introducing tube is conically shaped. The top of a drill point i8 also conically shaped but generally with an obtuse angle.
Therefore, a small space can remain near the top of the pitch introducing tube, even after the insertion of a drill point into the pitch introducing tube. In general, drill groove is a very loose spiral with a pitch of a few 8426~L

1 mm per rotation. It was hardly predictable that such a slow rotation of the flow at a rate of a few mm per rotation in the pitch introducing tube would influence the orientation of the produced fibers, because it is believed that the molecules in the spun fibers are oriented by the stress at the spinning nozzle hole with a diameter as small as 0-1 - 0.5 mm. However, when fiber spun from the apparatus described above was carbonized or graphitized and its appearance was examined on a scanning electron microscope, it was found, as shown in Figs. 3 and 4, that no crack formation occurred along the fiber axis.
Further, as shown in Fig. 3, examination of the orientation of carbon layers on the cross section showed that even though the orientation looked radial, it neverthelesY was influenced by the effect of spiral rotation effect within the pitch introducing tube, and it was found to have the feature that the carbon layers were curved like an impeller blade of a centrifugal pump.
It has not been elucidated fully why such a slow rotation, caused by a drill point inserted into a pitch introducing tube should result in such a remarkable effect. At present, we consider that as the cross sectional area is reduced to 1/10 - 1/1000, usually 1/100 - 1/1000 in the conical portion of the pitch introducing tube, the flow wou~d rotate very rapidly at the spinning 1284;~61 1 nozzle hole in just the same manner as a flow rotates quite rapidly at the conical part of a cyclone separator.
It is as yet impossible to offer a precise reason why a shift of orientation from a raaial to impeller-type can completely prevent crack formation at the carbonization stage, it may be assumed that the latter orientation has an effect similar to that of the onion-like orientation.
The present invention has a high commercial value because, by the use of the process of the present invention, the crack formation at the carbonization stage, which waR the most troublesome problem in the spinning of mesophase pitch, can easily be prevented. Moreover, the purpose of the present invention can be achieved by use of a conventional nozzle plate without any special processing thereof when it is equipped with a pitch introducing tube, by simply inserting into the pitch introducing tube, a plug member with a shape similar to a drill point. Also, the spinning nozzle hole can be cleaned up readily by the conventional method without any modification.
By the use of the process of the present invention, carbon fibers without crack formation can be constantly produced independent of the characteristics of 14 lZ8~

1 the mesophase pitch employed, spinning conditions, and the conditions of conversion to infusible state and carbonization.
A preferred embodiment of the apparatus suitable ror carrying out the process of the present invention is exemplified by Fig. 5. A side view of the structure of the essential part of an example of the spinning apparatus is shown in Fig. 5, which partly shows a cross section for the sake of an easy understanding of the structure.
The spinning apparatus consists of a nozzle plate 1 and a plug member 2. In the Figure, for the convenience of explanation, the nozzle plate 1 is shown in a cross sectional view. A spinning nozzle hole 3 is provided at the top of a pitch introducing tube 4. The pitch introducing tube 4 forms a conically shaped part 5 near the spinning nozzle hole 3, and the other side of the pitch introducing tube 4 is expanded to form a funnel-shaped part 6. The outer diameter of the plug member 2 is made substantially equal to the inner diameter of the pitch introducing tube 4. A spiral groove 7 is provided along the outer surface of the plug member 2, and a molten pitch, pumped downward, is given a rotatory motion as it flows down along this spiral groove. In general, the apparatus has a spinning nozzle hole of 0.1 to 1 mm diameter and O.S to 1.5 mm length, above which is equipped ~4~

1 with a pitch introducing tube with an inner diameter of 2 to 10 mm. More specifically, the,apparatus shown in the Figure has a spinning nozzle hole of 0.25 mm diameter and 0.75 mm length, above which is provided a .

16 ~;2 ~ iL

pitch introdu¢ing tube ~ith an inner diameter Or 2.5 ~m. ~he cro~e 4ection~1 ar~a Or the pitch $ntroducin~ t~b~ i~ 10 to 1000 timea Or the croas sectional area o~ the 4p$nning nozzle hole.
The plug ember 2 i4 a co~mercial drill point (Japanese Indu4t-rial Standard (JIS); straight ahan~ drill) ~ith an outer dia~oter Or 2.5 nm. Tho pitch Or the spiral groo~e Or tho plug member i4 5 ~ to 30 ~. 2he nozzle plate 1 ~a4 not proceased apeciricall~
~or the purpose Or the present in ention, but had been used rormerl~ ror spinnin~ ~ithout in4ertion Or a plug mem~er 2 until the preaent in-e~tlo~.
The proceJe Or the pro~ont in ontion will be explained ln more deta~`l b~ the rollo~ln~ Example~.
Examplo 1 coal tar pitch (200 g) and totrah~droquinoline (400 g) ~a8 ¢har~od into a 1 liter autocla e and the mixture, ~rtor pur~in~ ~ith nitro~e~, ~as h~drogenated b~ heating ror ~0 ~in.
at 4aoc under an autogenoous pre~sure. ~ hydrogonated pitch ~aa obtainea b~ riltration Or the trested li~uid to eliminate ~naolublo ateriala, ~ollo~od b~ romo-al of the 801 ent u~dor reduced pro4sure. Thi~ h~drogenated pitch (100 g) ~a4 charged to a ~00 ~1 pol~erization rlaa~, a~d ~as heated ror 10 min. i~
a rolten aalt bath at 510C, then it ~a~ heated rurther ror 105 ; in. i~ a ~olten aalt bath at 440C, during theae heat treat-enta, a stre~ Or nitrogen ~a~ bubbled through at a rate Or 5 liter/min.
Thia proccaa arrordcd a pitch ror ~pinning ~ith a sortening temperature Or a6sc. On e~amination Or thi~ pitch on a pol-rl~ed 11 =t 1cro-cope, lt ~ round that th- -in co-ponent "
' ;, ` ~ , . .

: .

17 ~284261 o~ the pitch i~ a mesophase ~ho~ing ~n optical anisotrop~, and a ~all a~ount of isotropic ~aterial ~a8 disper~ed as apher~s in the pitch.
The spinning apparatus co~pri~ing (a) a nozzle plate ha~ing a pitch introducing tubo (internal ai~meter o~ 2.5 ~) ~hich haa a ~pinning nozzle hole (dia~eter Or 0.25 ~ and a hole length Or 0.75 ~m) at the top and (b) a drill point ~JIS
atraight shan~ drill) ha~ing outer diameter Or 2.5 ~c ~hich ~a~
poaitioned ~ithin the pitch introducing tube by insertion, sho~n in Fig. 5 ~a~ used. The ~e60phaae pitch prepsred abo~e wa~
chargod into the spinning apparatuJ and pitch riber~ ~ere produced b~ ~pinning at a terperat~re Or 340C at a apin rato Or 400 ~/~in. ~heJ ~ore ~ade inruaiblo by heating up to ~20C
in tho air, ~nd then heated up to 1000C under an at~o~pherc Or nitrogen to gi~e carbon riber~. Firty ~onofilamenta ~ere rando-l~ ta~en out rrO~ the abo~e carbon riber aa~ple and they ~. .
~ere e~a ined by ~¢anning olectron icroscope at a ragniricatio~
Or ~000. It ~aa ro~nd that tho~ had an a~erage di-~eter o~ 7.8 ~, and nono Or %he abo~e rirt~ aarplec Jho~od the pre~e~ce Or crac~-.
Tho ~tructu~p Or tho cro~ ~ection Or thi~ ple had a i~peller-t~pe orientation aa ~ho~n in Fig. ~.
Couparatl e Example 1 Pitch ~iber~ ~ere produced b~ Jpinniu6 a ~e~opha~e pitch ~ith a Jortening teuperaturc Or 268C, produced b~ the came ~ethod a~ Exa ple 1, b~ the ca e nozzle plate a~ Esueple 1 but ~ithout inJertion Or a drill point, at a te~per-ture Or ~40C ~ith a ~pin~ing rate Or 400 hin. The~ ~ere uade inru~ible and 18 ~ ~ ~L

carbonized under the ~a~e condition~ ao tho~e o~ Esa-ple ~, and then rirt~ ~ono~ila~ent~ were r~ndonl~ ta~en out ana their appearanc~ ~ere es~ined at a uagnification of 3000. ~he~ had un a~erago diumotor Or 7.9 ~, and 2~ ~onorila~ent~ out Or rirt~
showed the pre~ence o~ crac~ to the direction parallel to the riber axi~. Becau~e onl~ ono sido of the ~a~ple could be esaminod b~ a ~canning olectron microsc,opo, the ract that 23 out o~ rifty sa~ples sho~od crac~ ~ay be con~idered aa suggest-ing that al~ost all samples had crac~ ho structuro Or the cros~ eoction Or thoso sampleJ had a radial orientation a8 sho~n 1~ Pig. 1.
Esa~ple 2.
Two ~ind~ Or pitch ribor~ woro produced b~ ~pinning the ~a~o ~esophaso pit¢h a~ Examplo 1 ~ith a softening tenperaturo Or a6sc, by an apparatu~ ~ith tho ~aae nozzle plate as Esu~plo 1 ~lth in~rtion o~ tho drlll point, at a te~porsture Or ~40C
with a spinnin~ rato Or 200 n ~nd 100 /~in., ro~pecti~el~.
Who~ rondorod inruaiblo a~d carbonized under the ~ane condition~
a- tho~o o~ Exuupl- 1, tho carbon riber~ thus produced had ao a~er-go dia~otera Or 9.9 ~ and 12.4 ~, respecti~cl~. E-ch rirt~
~ ronorilaaent~ ~oro ra~do~lr ta~on out rro~ each ~u~ple, ~d ; thoir ppoar~nco~ wero exs~inod at a ra~nirication Or ~000. In ^ oach C8~0, nono ~howod crac~ along tho ribor a~i8.
Exa~plo ~
Pitch ribors were produced rron the ~auo resophase pitch a~ Esanplo 1 with a ~ortening terporature Or 268C, by an apparatu~ with tho a o ~ozzlo plate UJ Exuaplo 1 with in~ertion , ", ,, . ~, ' ., -`- ~8421~1 1 of the drill point, at a temperature of 370C with a spinning rate of 500 m/min. After being rendered infusible and carbonized under the same conditions as those of Example 1, fifty monofilaments were randomly taken out, and their appearances were examined at a magnification of 3000. The carbon fibers thus produced had an average diameters of 10.1 u. None showed cracks along the fiber axis.

Example 4 A spinning pitch with a softening temperature of 285C was produced by charging a hydrogenated pitch (200 g) which was hydrogenated by the same method as Example 1, into a 500 ml polymerization flask, heated for 10 min. in a molten salt bath kept at 510C and then heated for 1 hr.
in a molten salt bath kept at 460C. Pitch fibers were produced from this pitch by an apparatus with the same nozzle plate as Example 1 with insertion of the drill point, at a temperature of 350C with a spinning rate of 300 m/min. They were rendered infusible by heating up to 340C in the air, and carbonized at 1000C under the same conditions as those of Example 1. The carbon fibers thus produced had an average diameter of 11.6 u. When the appearance3 of fifty monofilaments were examined in the same way as Example 1, none of the fifty monofilaments showed the presence of cracks.

-~ 20 ~8~

1 Example 5 Pitch fibers were produced from the same mesophase pitch as Example 1 with a softening temperature of 268C, and by using a spinning apparatus with a spinning nozzle hole of 0.5 mm diameter and 1.0 mm length and a pitch introducing tube with an inner diameter of 2.5 mm to which was inserted the same drill point as Example 1, at a temperature of 340C with a spinning rate of 300 m/min. They were rendered infusible and carbonized under the same conditions as those of Example 4. The carbon fibers thus produced had an average diameter of 13.4 u.
When examined as described in Example 1, none of the fifty monofilaments showed the pre~ence of cracks.
The left hand side of Fig. 2 shows schematically the alignment of carbon layers in the carbon fiber of the Comparative Example I and the right hand side thereof shows the appearance of the fiber, and the left hand side of Fig. 4 shows schematically the alignment of carbon layer~ in the carbon fiber produced by Example 1 and the right hand side thereof shows the appearance of the fiber. Comparison of the two Figures shows that the general pattern of alignment of carbon layers is similar _ in that the carbon layers are aligned parallel to the fiber axis. However, as seen in Figs. l ,~ ~ ,;

. . .
~;

21 ~ ~ ~

l and 3, they differ from each other in that while the aligned layers are oriented radially in Fig. 1 (Comparative Example l), they have a curved orientation like an impeller blade in Fig. 3 (Example 1).

: i . , :

Claims

1. A process for producing carbon fibers from a mesophase pitch by melt spinning which comprises extruding a molten mesophase pitch through a spinning nozzle hole, rendering the extruded pitch fibers thus obtained into an infusible state by heating under an oxidizing atmosphere and then carbonizing or graphitizing said pitch fibers by heating under an inert atmosphere, wherein the spinning is accomplished by constraining said pitch to flow through a plug means forming a spiral passage substantially around the axis of said spinning nozzle hole to give a rotary motion to said molten mesophase pitch just before extruding said pitch through said nozzle hole, to thereby effectively prevent crack formation in said fibers.

2. The process as claimed in Claim 1, wherein said mesophase pitch has a mesophase content of 60 to 100%.

3. The process as claimed in Claim 1, wherein said mesophase pitch has a mesophase content of 80 to 100%.

4. The process as claimed in Claim 1, wherein said mesophase pitch has a softening temperature of 250 - 320°C.

5. The process as claimed in Claim 1, wherein said pitch is constrained to flow through a spiral passage formed by an inner side of a pitch introducing tube and a plug member having an outer spiral groove.

6. The process as claimed in Claim 5 wherein said pitch introducing tube is provided with a cross-sectional area 10 to 1000 times the cross-sectional area of said spinning nozzle hole.

7. The process as claimed in Claim 6, wherein the pitch of said spiral groove of said plug member is 5 mm to 30 mm and the outer diameter of said plug member is 2 mm to 10 mm.

8. The process as claimed in Claim 5, wherein the cross sections of said pitch introducing tube and said spinning nozzle hole are circular and said plug member is a drill point or a worm gear.

9. The process as claimed in Claim 8, wherein said plug member is a drill point.

10. The process as claimed in Claim 5, wherein the pitch of said spiral groove of said plug member is 5 mm to 30 mm and the outer diameter of said plug member is 2 mm to 10 mm.