GB2118922A - Process for producing carbon fibers - Google Patents

Abstract

A process for producing carbon fibers, comprising melt spinning a hydrocarbon-derived precursor pitch having a specific content of mesophase and a specific softening point, treating the resulting pitch fibers at not higher than 400 DEG C under a tension of 0.005-1.0 g/d applied thereto in an oxidizing atmosphere and then carbonizing or, if necessary, graphitizing the thus treated pitch fibers in an inert gas atmosphere to obtain the carbon fibers.

Description

SPECIFICATION Process for producing carbon fibers This invention relates to a process for the production of carbon fibers.

At present, carbon fibers are produced mainly from polyacrylonitrile as the starting material.

However, polyacrylonitrile is disadvantageous in that it is expensive as such a starting material and exhibits an inferior carbonization yield when carbonized.

In view of this, there have recently been reported many processes for producing carbon fibers from pitch as the starting material. Since pitch is inexpensive as such a starting material and exhibits a high carbonization yield when carbonized, it is expected that pitch may be used in producing carbon fibers at a low cost. However, as compared with carbon fibers produced from polyacrylonitrile, those produced from pitch raise a problem as to their low tensile strength although they have high tensile modulus.

Thus, if said problem is solved or overcome and a technique for further enhancing said tensile modulus is found, then it will be possible to produce carbon fibers having high tensile strength and high tensile modulus from pitch at a low cost.

The primary object of this invention is to provide a process for producing carbon fibers having excellent performances from pitch.

The object may be achieved by carrying out a process comprising melt spinning a hydrocarbonderived precursor pitch containing 5100% of mesophase (a pitch containing mesophase portions being calied "precursor pitch" in this application) and having a softening point of 150--4000C to obtain pitch fibers, treating the thus obtained pitch fibers at not higher than 4000C under a tension of 0.005-1.0 g/d in an oxidizing atmosphere and then carbonizing or, if necessary, graphitizing the thus treated pitch fibers to obtain carbon fibers.

It has heretofore been known to apply some tension to spun polyacrylonitrile type fibers when they are cyclized (or thermally stabilized) at temperatures in the range of about 250-about 3000C; for example, Japanese Pat. Appln. Laid-Open Gazette 51-35727 discloses a tension of 0.05-1.0 g/d and Japanese Pat. Appln. Laid-Open Gazette 49-47622 a tension of 0.01--0.2 g/d. It has also been known that the thermal stabilization is effected in the presence of oxygen.On the other hand, as for pitch as the starting material, it has been known that pitch fibers obtained by spinning are treated at said temperatures in an oxidizing atmosphere to enhance their softening point thereby making them infusible; however, the pitch fibers so treated have low tensile strength and, therefore, they have been considered impossible to stand a heavy load without any further treatments (Refer to, for example, "CARBON FIBERS" written by Ohtani et al, published by Kindai Henshu Co. on June 1 5, 1972).

Japanese Pat. Appln. Laid-Open Gazette 47-6466 discloses that in a method comprising incorporating pitch or tar with a polymer and spinning the resulting mixture to obtain a spun product, the thus obtained spun product is oxidized under a slight tension applying thereto to make it infusible.

More particularly, the Gazette discloses that a mixture of coal extracts with polypropylene is spun and then oxidized under a tension of about 1 mg/fiber applied (Refer to Example 1 in the Gazette). Further, Japanese Pat. Appln. Publication Gazette 48-11286 discloses that in a method comprising spinning or extruding coal elutes or extracts to obtain fibers or the like and oxidizing the thus obtained fibers or the like to stabilize them, the oxidation is effected under a tension only enough to keep the fibers straightened. More particularly, the Gazette 48-11286 discloses that staple fibers of 10-20 cm in length are suspended in a furnace and then oxidized under a tension of about 1 mg per fiber to keep the fibers straightened.

The present inventors made intensive studies in an attempt to find a process for producing carbon fibers having excellent tensile modulus and strength from pitch as the starting material and, as a result of their studies, found that hydrocarbon-derived precursor pitch having specified properties are melt spun to obtain pitch fibers and the thus obtained pitch fibers are then treated in an oxidizing atmosphere while applying a specified tension to the fibers thereby to obtain carbon fibers having excellent tensile modulus and strength. This invention is based on this finding or discovery.

As previously mentioned, the process of this invention comprises melt spinning a hydrocarbonderived containing 5100% of mesophase and having a softening point of 150--4000C to obtain pitch fibers, treating the thus obtained pitch fibers at a temperature of not higher than 4000C in an oxidizing atmosphere while applying a tension of 0.005-1.0 g/d to the fibers and then carbonizing or, if necessary, graphitizing the thus treated pitch fibers to obtain carbon fibers.

The process for the production of carbon fibers according to this invention will be explained below in more detail.

The hydrocarbon-derived precursor pitch used in this invention is one which contains 5-1 00% of mesophase and has a softening point of 150--4000C, preferably 1 80-3000C. It is preferable that this pitch be a petroleum-derived one.

The hydrocarbon-derived precursor pitch used herein may be produced by a known process. More particularly, said precursor pitch is prepared by heat treating a starting pitch to effect mesophase formation therein. The reaction for producing the precursor pitch may be carried out at a temperature of usually 340450 C, preferably 370--4200C at atmospheric or reduced pressure. In this case, an inert gas, such as nitrogen may be used if possible. The time for effecting this reaction may vary depending on the temperature, amount of the inert gas passed and the like, and it may be in the range of usually 1-50 hours, preferably 3-20 hours. The amount of inert gas passed may preferably in the range of 0.7-5.0 scfh-lb pitch.

The preferable hydrocarbon-derived precursor pitches and processes for producing the same by subjecting a starting pitch to mesophase formation under known conditions, are illustrated in Japanese Pat. Appln. Laid-Open Gazettes 57-168987, 57-168981, 57-168990, 57-168989, 57-170990, 57-179285, 57-179286, 57-179287, 57-179288, 58-18419 and 58-18420.

The pitch fibers used in this invention may be obtained by melt spinning the hydrocarbon-derived precursor pitch. The melt spinning may be effected by heretofore known methods; for example, the hydrocarbon-derived precursor pitch is melted at a temperature higher than its softening point by 3O-8O0C and then extruding the resulting melted pitch through nozzles of 0.1-0.5 mm in diameter while winding the resulting pitch fibers at a speed of 300-2000 m/min. on a take-up bobbin.

The pitch fibers produced by the above methods are made substantially of hydrocarbons and will substantially neither deform nor shrink even under no tension applied thereto when treated under the following treating conditions in an oxidizing atmosphere. The above phrase "substantially neither deform nor shrink" is intended to mean that when the pitch fibers are treated under no tension applied thereto in the oxidizing atmosphere, their axial shrinkage will be 10% or less, preferably 7% or less.

The pitch fibers prepared by melt spinning are treated at not higher than 400 C under a tension of 0.005-1.0 g/d applied thereto in an oxidizing atmosphere. The treating temperature is preferably 1 5O-38O0C, more preferably 200350 C. The use of lower treating temperatures will result in lengthening the treating time, while the use of higher treating temperatures will result in causing undesirable fusion of the pitch fibers and loss thereof.

In the case of treatment of the pitch fibers in an oxidizing atmosphere, the reaction temperature is raised slowly to the predetermined temperature and then maintained at this temperature. Any temperature-raising rate will do until the treating temperature is raised to 1 500 C, while a temperatureraising rate after the treating temperature has been raised beyond 1 5O0C is in the range of 1 0C/min.-1 0O00C/min., preferably 50C/min.-50O0C/min. More particularly, the pitch fibers will increase in softening point by heating them in an oxidizing atmosphere. In this case, the temperatureraising rate is at least 5 C./min., preferably at least 1 00C/min. and more preferably at least 3O0C/min.

until the softening point of the pitch fibers reaches 4000 C.

The tension applied to the pitch fibers in the oxidizing atmosphere is in the range of 0.005-1.0 g/d, preferably 0.005-0.5 g/d. In a case where the tension applied is lower than said ranges and more specifically, for example, the tension applied is as low as about 1 mg/fiber or is one which is low just to keep the fibers straightened in a reaction furnace, carbon fibers to be obtained by carbonizing treatment will have insufficient tensile modulus and tensile strength. On the other hand, the application of a tension exceeding said ranges will tend to cause undesirable tearing or breakage of the fibers frequently during the treatment.The pitch fibers used in this invention will substantially not shrink during the oxidizing treatment and will be stretched in the axial direction to an extent of 0-100% of their original strength. It is preferable to select the temperature-raising rate or tension so that the fibers are stretched to an extent of 0-50%, preferably 0-30%, of their original length.

The oxidizing gases used herein usually include oxidizing gases such as oxygen, ozone, air, nitrogen, oxides, halogens and sulfur oxides such as SO2, SO3. They are used alone or in combination.

Prior to the treatment in the oxidizing atmosphere, the pitch fibers may be treated with, or may carry thereon, an organic acid, an inorganic acid, a metal or ammonium salt of each of these acids, a metal oxide or the like in order to shorten the treating time.

The pitch fibers so oxidized are then carbonized or graphitized in an inert gas atmosphere to obtain carbon fibers. The carbonization is effected at usually 800--25000C. The time needed for the carbonization is generally in the range of 0.5 minutes to 10 hours. Further, the graphitization is carried out at 2500--35000C for usually one second to one hour. In addition, if necessary, some load or tension may be applied to the fibers to be treated, at the time of carbonization or graphitization in order to prevent the shrinkage, deformation and the like of the fibers.

This invention will be better understood by the following non-limitative examples and comparative examples.

EXAMPLE 1 AND COMPARATIVE EXAMPLE 1 A petroleum hydrocarbon-derived precursor pitch containing 80% of mesophase and having a softening point of 291 OC was melt spun to obtain pitch fibers of 17 ,u in average diameter. The thus obtained pitch fibers were heated to 3000C at a temperature-raising rate of 5 C/min. in air under a tension of 8 mg/d applied to the fibers, maintained at this temperature for 30 minutes and then carbonized in an inert gas atmosphere at 10000C to obtain carbon fibers. The carbon fibers so obtained had a tensile strength of 98 Kg/mm2 and a tensile modulus of 10 ton/mm2 (Example 1). In addition, the same pitch fibers as used in Example 1 were heat treated under a tension of 0.4 mg/d (1 mg/fiber) applied thereto thereby to obtain carbon fibers (Comparative Example 1).

The thus obtained carbon fibers were graphitized in an inert gas atmosphere to obtain graphite fibers having the properties shown in Table 1.

TABLE 1

Comp.

Example 1 Example 1 Tension 8 mg/d 0.4 mg/d Tensile modulus (ton/mm2) 55 50 247 204 Tensile strength (Kg/mm2) 247 204 Elongation at failure (%) 0.5 0.4 1 Diameter of graphite fiber (,u) 10 11 EXAMPLE 2 AND COMPARATIVE EXAMPLE 2 A petroleum hydrocarbon-derived precursor pitch having a 25% content of mesophase and a softening point of 2520C was melt spun to obtain pitch fibers having an average diameter of 11 ,u. The thus obtained pitch fibers were heated at a temperature-raising rate of lO0C/min. to 300 C in oxygen while applying a tension of 0.2 g/d to the fibers, maintained at 3000C for 3 minutes and then subjected to carbonization treatment at 10000C thereby to obtain carbon fibers. The carbon fibers so obtained had a tensile strength of 102 Kg/mm2 and a tensile modulus of 11 ton/mm2 (Example 2). The same pitch fibers as used in Example 2 were heated treated under the same conditions as above under no tension applied thereto to obtain carbon fibers (Comparative Example 2).

The carbon fibers so obtained were graphitized at 2500 C in an inert gas atmosphere to obtain graphite fibers having the properties shown in Table 2.

TABLE 2

Comp.

Comp.

Example2 Example 2 Tension 0.2 g/d None Tensile modulus (ton/mm2) 70 56 Tensile strength (Kg/mm2) 31 9 277 l Elongation at failure (%) 0.5 0.5 l Diameter of graphite fiber () 9 10 EXAMPLE 3 AND COMPARATIVE EXAMPLE 3 A petroleum hydrocarbon-derived precursor pitch having a 20% content of mesophase and a softening point of 260 C was melt spun to obtain pitch fibers having an average diameter of 12,u. The thus obtained pitch fibers were heated at a rate of 1 OoC/min. to 3400C in oxygen under a tension of 20 mg/d applied thereto and then subjected to carbonizing treatment thereby to obtain carbon fibers having a tensile strength of 96 Kg/mm2 and a tensile modulus of 10 ton/mm2 (Example 3). In addition, the same pitch fibers as above were heat treated without a tension applied thereto to obtain carbon fibers (Comparative Example 3).

The carbon fibers so obtained were graphitized at 25000C in an inert gas atmosphere to obtain graphite fibers having the properties indicated in Table 3.

TABLE 3

Comp.

Example 3 Example 3 Tension 20 mg/d . None l Tensile modulus (ton/mm2) 60 50 Tensile strength (Kg/mm2) 280 198 Elongation at failure (%) 0.5 0.4 Diameter of graphite fiber (y) 9 10 As is seen from the foregoing Examples and Comparative Examples, the process for producing carbon fibers according to this invention makes it possible to provide carbon and graphite fibers having excellent properties.

Claims (6)

CLAIMS:

1. A process for producing carbon fibers, comprising melt spinning a hydrocarbon-derived precursor pitch having a 5100% content of mesophase and a softening point of 150--4000C to obtain pitch fibers, treating the thus obtained pitch fibers at not higher than 4000C under a tension of 0.005-1.0 g/d applied to the fibers in an oxidizing atmosphere and then carbonizing the thus treated pitch fibers in an inert gas atmosphere to obtain carbon fibers.

2. A process according to claim 1, further comprising grapnitizing the carbon fibers in an inert gas atmosphere.

3. A process according to claim 1, wherein the carbonization is effected at a temperature of 800-250O0C.

4. A process according to claim 2, wherein the graphitization is effected at 2500--35000C.

5. A process for producing carbon fibers, substantially as described in any one of the foregoing Examples 1 to 3.

6. Carbon fibers which have been produced by the process claimed in any one of the preceding claims.