Pitch for carbon fibers, process for production of said pitch, and process for production of carbon fibers using said pitch

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a process for produc­ing pitch for use in production of carbon fibers, and more particularly to a process for efficiently producing suitable pitch for production of carbon fibers having a high strength.

[0002] The present invention further relates to novel pitch for use in production of carbon fibers as obtained by the above process and a process for producing carbon fibers using said pitch. More particularly, it is concerned with pitch capable of providing carbon fibers having a high tensile strength and a high knot strength, and a process for producing the above high performance carbon fibers using said pitch.

[0003] Carbon fibers known as a composite reinforcing material for plastics, ceramics and metal have heretofore been produced by calcining polyacrylonitrile fibers. This process, however, has disadvantages in that the starting material is expensive and the carbonization yield at the time of calcination is low. For this reason, a number of processes for production of carbon fibers using pitch, for example, as the starting material have been proposed in recent years (Japanese Patent Application Laid-Open Nos. 141488/1982, 196293/1983, 142976/1983, 164687/1983, 196721/­1988, etc.).

[0004] In the methods disclosed in Japanese Patent Application Laid-Open Nos. 141488/1982 and 196293/1983, a relatively low boiling fraction having a boiling point of 150 to 500°C is used as the starting material. This starting material, however, requires a long time in the polycondensation reaction to precursor pitch for carbon fibers. Thus the distribution of molecular weight of pitch formed is broadened, no pitch for good spinnability and no high strength carbon fiber can be obtained.

[0005] The method disclosed in Japanese Patent Application Laid-Open No. 142976/1983 uses a relatively high boiling fraction having a boiling point of 540°C or more. However, since a vacuum distillation residue is used, there is a pos­sibility that coke and inorganic impurities remain. Moreover, the distribution of molecular weight is shifted to a higher molecular weight side, which may be accompanied by difficulty in production of high strength carbon fibers and a reduction in spinnability.

[0006] The methods disclosed in Japanese Patent Application Laid-Open Nos. 164687/1983 and 196721/1988 are such that a mesophase is once formed, is separated from a non-meso phase portion utilizing a difference in gravity and so on therebetween, and is used as a pitch for carbon fibers. These methods, however, have disadvantages in that the separation of the non-mesophase portion is complicated and is low in effciency, and they are not desirable from an economic standpoint.

[0007] The present invention is intended to overcome the above prior art problems, and an object of the present invention is to provide a process for producing pitch which is suitable for production of carbon fibers having a high tensile strength and a high degree of elongation, and which has also excellent spinnability.

[0008] In recent years, a demand for carbon fibers as a raw material or a resin reinforcing material, for example, in various fields of air craft parts, car parts, sports goods, and so forth has been markedly increasing because the carbon fibers have excellent characteristics such as high tensile strength, high modulus of elasticity and light weight.

[0009] Carbon fibers can be divided into PAN-based carbon fibers and pitch-based carbon fibers. The former PAN-based carbon fibers are produced using polyacrylonitrile as the starting material and usually have a high tensile strength and an intermediate modulus. However, some of carbon fibers calcined at a temperature of more than 2,000°C exhibit a modulus of about 400 GPa at most. These PAN-based carbon fibers, however, are limited in an increase of degree of graphiti­zation because of their turbostratic graphite arrangement. Thus it is essentially difficult to produce carbon fibers with a super high modulus from the PAN-based carbon fibers. Moreover, the PAN-based carbon fibers have a disadvantage in that the production cost is inevitably high.

[0010] On the other hand, pitch-based carbon fibers are low in production cost and thus are advantageous from an economic standpoint, because they are produced using carbonaceous pitch as the starting material. In particular, graphite fibers obtained from a liquid crystal mesophase pitch and calcined at a temperature of about 3,000°C have features such as a super high modulus of about 700 GPa or more. However, such optically anisotropic pitch-based carbon fibers are brittle as compared with their strength, because their modulus is great, and fiber itself and their yarns cannot be bent to a small curvature. That is, the pitch-­based carbon fibers have a serious disadvantage that knot strength and elongation are small and it is difficult to produce a molding in a complicated shape.

[0011] Heretofore, high modulus fibers have been developed from mesophase pitch-based carbon fibers utilizing their high orientation properties. However, since they have the aforementined diadvantages, they are limited in their ap­plication fields and thus inevitably limited in use. In order to find a wide variety of applications, it has been strongly desired to develop inexpensive pitch-based carbon fibers having a high tensile strength, a high degree of elongation and a high knot strength.

[0012] Pitch-based carbon fibers are usually produced by melt spinning carbonaceous pitch, subjecting the resulting pitch fibers to stabilization treatment, and then carbonizing the stabilized pitch fibers. Various improvements in producing such pitch-based carbon fibers have been made. For example, with regard to carbonaceous pitch to be used as the starting material, a method in which a solubility in a solvent is specified as an index is proposed (Japanese Patent Publica­tion No. 1810/1979). In production of carbon fibers, the pitch is melt spun at a high temperature and undergoes various heat hysteresis in stabilization treatment and calcination treatment. Thus the behavior of the pitch at a high temperature becomes a greatly important factor. However, a study on the behavior of such pitch at a high temperature is not almost reported except for in limited literatures. In fact, a study on the behavior of pitch in relation with carbon fibers is not reported at all.

[0013] In connection with the stabilization treatment, for example, (1) a method in which air containing nitrogen dioxide is used (Japanese Patent Publication No. 42696/1973 and Japanese Patent Application Laid-Open No. 259629/1985), (2) a method in which the stabilization treatment is applied under specified conditions (Japanese Patent Application Laid-­Open Nos. 120112/1988, 145419/1988 and 264917/1988), etc. are proposed. The object of the method (1) is to increase the speed of stabilization and productivity, and conditions for increasing physical properties of carbon fiber are not specified. In accordance with the method (2), carbon fibers of high modulus and high tensile strength can be obtained. However, there cannot be found any disclosure about a rela­tion between properties of pitch and strength, and knot strength and elongation.

SUMMARY OF THE INVENTION

[0014] An object of the present invention is to provide pitch for carbon fibers, which is capable of providing carbon fibers having a high tensile strength and a knot strength.

[0015] Another object of the present invention is to provide high performance carbon fibers to be produced from the above pitch.

[0016] It has been found according to the present invention that the above objects can be attained by using carbonaceous pitch having specified properties that the optically an­isotropic liquid crystal (mesophase) content is at least 90% by weight.

[0017] The present invention relates to pitch for carbon fibers, having a quinoline insoluble content of not more than 50% by weight, a number average molecular weight of at least 1,000, a ratio of weight average molecular weight to number average molecular weight of 1.3:1 to 1.8:1, a mesophase content of at least 90% by weight, a softening point of 250 to 380°C, a 5% weight reduction temperature as determined by a thermo­gravimetric analysis (TGA) of at least 470°C, and a weight reduction at 800°C of not more than 25%.

[0018] The present invention further relates to a process for producing carbon fibers which comprises melt spinning the above pitch at a temperature of 280 to 400°C, subjecting the pitch fibers to oxidative stabilization, and then carbonizing the pitch fibers.

[0019] The present invention further relates to a process for producing pitch to be used in production of carbon fibers which comprises subjecting a pitch feed to a first heat treatment at a temperature of 380 to 500°C under a pressure of 1 to 1,500 mmHg and a second heat treatment at a tempera­ture of 450 to 550°C under a pressure of 0.1 to 20 mmHg wherein the pitch feed has a number average molecular weight of 300 to 500, a weight average molecular weight/number average molecular weight ratio of 1.5:1 to 2.2:1, a boiling point of at least 450°C, a residue when raised in temperature up to 800°C at a temperature-raising speed of 10°C/min in nitrogen gas of not more than 5% by weight, and a peak area at the starting point where it is developed on a silica gel thin layer with a mixed solvent of 95% by volume of dichloro­methane and 5% by volume of methyl alcohol of not more than 1%.

[0020] This process provides pitch for carbon fibers, having the specified characteristics as described above.

DETAILED DESCRIPTION OF THE INVENTION

[0021] The process for production of pitch for carbon fibers is first explained below.

[0022] The starting material to be used in production of pitch for carbon fibers is not critical, and those commonly used in production of pitch can be used. As described hereinafter, petroleum heavy oil is preferred, and in particular, residual oils from the catalytic cracking of petroleum distillate is suitable.

[0023] For this pitch is required that the mesophase content is at least 90% by weight. In order to produce pitch having a high mesophase content, that is, mesophase pitch, the aforementioned pitch starting material is subjected to treat­ment to increase the mesophase content. The term "mesophase" as used herein means as optically anisotropic phase, and the mesophase content can be determined by a proportion of optically anisotropic phase or optically isotropic phase as determined by observing under crossed nicols of a polariza­tion microscope and taking a photograph.

[0024] A method of preparing mesophase pitch is not critical, and it can be prepared by the known methods, such as a method in which the starting material oil for pitch is once polycondensed to a mesophase precursor, and then the precursor is treated at a temperature as high as about 450 to 550°C in a vacuum as high as about 0.1 to 20 mmHg until the mesophase content is more than 90% by weight and preferably 100%. If the mesophase content is less than 90% by weight, an isotropic phase is introduced in production of fibers and, therefore, no fiber having sufficiently high tensile strength and knot strength can be obtained and the objects of the present invention cannot be attained.

[0025] The process for production of pitch for carbon fibers of the present invention is characterized in that a specified pitch starting material is used and it is subjected to two stage heat treatment.

[0026] As the pitch starting material, a pitch having a number average molecular weight (measured by VPO) of 300 to 500, preferably 350 to 480, and a weight average molecular weighr/­number average molecular weight, Mw/Mn, (as measured by GPC) of 1.5:1 to 2.2:1, preferably 1.5:1 to 2.0:1 is used. The maximum molecular weight is desirably not more than 1,500. If the maximum molecular weight is more than 1,500, homogeneity is lost.

[0027] The boiling point as determined according to ASTM D1160 of the pitch starting material is at least 450°C. If the boiling point is less than 450°C, the yield undesirably drops.

[0028] The residue of the pitch starting material when it is heated to 800°C at a temperature raising speed of 10°C per minute (10°C/min) in a nitrogen gas by the use of a thermo­gravimetric analysis (TGA) apparatus is not more than 5% by weight and preferably 4.5 to 0.5% by weight. A high residue content means that the pitch starting material contains a large amount of high molecular weight material or component readily forming a high molecular weight material in the heat treatment. If the residue is more than 5% by weight, homogeneity, of prepared mesophase pitch are lost, undesirably causing a reduction in tensile strength of carbon fibers.

[0029] The peak area at the starting point as developed on a silica gel thin layer with a mixed solvent of 95% by volume of dichloromethane and 5% by volume of methyl alcohol by the use of a flame ionization detection-thin layer chromato­graphy (FID-TLC) of the pitch starting material is not more than 1%. If the peak area at the starting material is more than 1%, the content of impurities such as high molecular weight material is high, an incidence of defect in carbon fiber increases, and a reduction in strength is produced.

[0030] The outline of the FID-TLC method will hereinafter be explained. This testing method is to quantitatively measure the amounts of saturated hydrocarbons, aromatic hydrocarbons, resin and asphaltene among hydrocarbons of heavy oil having an initial boiling point of more than 200°C. A sample diluted with a suitable solvent is spotted to a base point of an activated silica gel thin layer bar. This thin layer bareis developed successively in three developing vessels containing different solvents, i.e., in the first vessel containing n-hexane, the second vessel containing 20 vol% of n-hexane and 80 vol% of toluene, and the third vessel containing 95 vol% of dichloromethane and 5 vol% of methyl alcohol to separate the saturated fraction, the aromatic fraction and the resin in this order, and the spot residue, i.e., a peak appearing at the starting point is referred to as "asphaltene". Then, the solvent is removed by drying, and the sample is placed on a spot detecting apparatus and is subjected to spot detection under specified conditions. The percent of area of each component is calculated from a height of integrated curve drawn simultaneously with the chromatograph.

[0031] A pitch starting material satisfying the aforementioned requirements can be used in production of carbon fibers ac­cording to the process of the present invention. It is preferred that the softening point is 50 to 150°C, the contents of Si and Al are each not more than 3 ppm, the aromatic index (fa) is at least 0.75, the n-heptane insoluble content is 25 to 80% by weight, and the toluene insoluble content is approximately o%by weight.

[0032] The pitch starting material can be produced from a petroleum-based residual oil having a high aromatic hydro­carbon content, such as the residual oils from the cata­lytic cracking of petroleum fractions and residual oils from thermal cracking of naphtha, for example, by (1) a method in which the residul oil is vacuum distilled to remove fractions having a boiling point of not less than about 400°C, and the pitch thus obtained is extracted with a mixed solvent of toluene and hexane or is subjected to super critical extraction to remove a heavy fraction, or (2) a method in which the residual oil is reacted in a batch type reactor at 380 to 500°C under 1 to 50 mmHg for 0.05 to 30 hours, and a relatively heavy cracked oil formed at a later stage of the reaction is recovered, or (3) a method in which a continuous two stage reactor is used, and the reaction is carried out in the first reactor at 380 to 500°C under not more than atmospheric pressure for 0.5 to 10 hours and then in the second reactor at 420 to 550°C under not more than 20 mmHg for 0.05 to 6 hours.

[0033] In accordance with the process for production of pitch for carbon fibers of the present invention, the aforementioned pitch starting material is subjected to two stage heat treat­ment.

[0034] The first stage heat treatment is carried out at a temperature of 380 to 500°C under a pressure of 1 to 1,500 mmHg for a time of 0.1 to 10 hours and preferably at a tempe­rature of 390 to 480°C under a pressure of 5 to 1,500 mmHg for a time of 0.2 to 8 hours. It is desirable that the heat treatment is carried out so that an optically anisotropic phase (mesophase) is not substantially formed and the material is made heavier until the toluene insoluble content reaches 5 to 35% by weight. If the temperature of the first stage heat treatment is less than 380°C, the rate of reaction is slow, requiring a long time in the heat treatment. On the other hand, if the temperature is in excess of 500°C, the amount of volatile portion removed is increased, leading to a reduction in yield and formation of coke. In addition, high molecular weight material is undesirably formed. If the pressure of the first stage heat treatment is less than 1 mmHg, the amount of volatile portion removed is increased, leading to a reduction in yield. On the other hand, if the pressure is in excess of 1,500 mmHg, a light portion is removed only insufficiently, thereby undesirably broadening a distribution of molecular weight.

[0035] Isotropic pitch obtained by the first stage heat treat­ment is subjected to a second stage heat treatment to convert it into mesophase pitch.

[0036] In the second stage heat treatment, the isotropic pitch (which may contain a small amount of mesophase pitch) is treated under severer conditions than in the first stage heat treatment. The second stage heat treatment is carried out at a temperature of 450 to 550°C under a pressure of 0.1 to 20 mmHg for a time of 0.2 to 30 minutes and preferably at a temperature of 460 to 500°C under a pressure of 0.1 to 10 mmHg for a time of 1 to 20 minutes to obtain pitch with a mesophase content of at least 90%, preferably 95 to 100%. In this second stage heat treatment, if the temperature is less than 450°C, a light fraction becomes difficult to remove, increasing the treating time. On the other hand, if it is in excess of 550°C, the yield is decreased, coke is formed and the rate of reaction becomes difficult to control. If the pressure in the second stage heat treatment is less than 0.1 mmHg, the yield is decreased and a vacuum apparatus becomes large-sized. On ther other hand, if the pressure is in excess of 20 mmHg, a light fraction is removed only insufficiently and thus the distribution of molecular weight is undesirably broadened.

[0037] In this manner, the pitch for carbon fibers of the present invention can be produced.

[0038] For the mesophase pitch thus prepared is required to have a quinoline insoluble content (QI) of not more than 50% by weight, preferably 5 to 40% by weight, a softening point of 250 to 380°C, preferably 260 to 350°C, a number average molecular weight (Mn) of at least 1,000, preferably 1,000 to 1,400, a ratio of weight average molecular weight (Mw) to number average molecular weight (Mn), Mw/Mn, of 1.3:1 to 1.8:1, preferably 1.35:1 to 1.75:1, a 5% weight reduction (T_5%) temperature as determined a thermogravimetric analysis (TGA) of at least 470°C, and a weight reduction at 800°C of not more than 25%. The value determined by the thermogravimetric analysis is a value measured by raising in temperature to 800°C in nitrogen atmosphere at a temperature-raising speed of 10°C/min.

[0039] Moreover, the toluene insoluble content of the mesophase pitch is 60 to 95% by weight.

[0040] This mesophase pitch is stable at an elevated tempera­ture because the volatile content at an elevated temperature is small. Although the light fraction is small, since the quinoline insoluble content is also small, fluidity is good and spinnability are good. Moreover, since the mesophase content is high, homogeneity and orientation characteristics are good, and the carbon fibers produced therefrom when calcinated at a high temperature exhibits high tensile modulus and thus has a high tensile strength and a high knot strength.

[0041] In the present invention, pitch for carbon fibers, comprising the aforementioned mesophase pitch is melt spun at a temperature of 280 to 400°C by the use of a spinneret preferably having a capillary diameter of 0.1 to 0.5 mm, thereby producing pitch fibers having a diameter of about 5 to 20 µm, and then subjecting the pitch fibers to the stabilization treatment. This stabilization treatment may be carried out by the use of air or oxidizing gas containing 0.1 to 30 vol%, preferably 0.5 to 15 vol% of nitrogen dioxide. The latter oxidizing gas is preferably used because it provides carbon fibers having a high tensile strength and a high knot strength.

[0042] In the case of the oxidative stabilization treatment using air, the treatment is usually carried out at a temper­ature of 200 to 400°C for 5 to 300 minutes. In the case of the stabilization treatment using the aforementioned nitrogen dioxide-containing oxidizing gas, the treatment is usually carried out at a temperature of 150 to 350°C, preferably 180 to 320°C for about 10 to 600 minutes.

[0043] To the fibers subjected to the stabilization treatment is then applied carbonization treatment. If necessary, preliminary carbonization treatment may be applied in an inert gas (e.g., nitrogen or argon) atmosphere at a tempera­ture of 350 to 800°C.

[0044] In the carbonization treatment, the fibers subjected to the stabilization treatment or further to the preliminary treatment, if necessary, is calcined in an inert gas (e.g., nitrogen or argon) atmosphere at a temperature of 1,000 to 3,000°C to thereby obtain carbon fibers.

[0045] The present invention enables to produce carbon fibers having a tensile strength of at least 350 Kg/mm² and a knot strength of at least 1,500 gf/3K-strand, while on the other hand in pitch-based carbon fibers produced by the conven­tional methods, the tensile strength is about 200 to 350 Kg/mm² and the knot strength is about 1,200 gf/3K-strand at most.

[0046] The process of the present invention can provide pitch for carbon fibers, from which high strength carbon fibers having a tensile strength after calcination of at least 400 kg/mm² can be produced.

[0047] The carbon fibers have a high degree of elongation and can overcome the defect of carbon fibers produced from pitch obtained by the conventional methods, i.e., brittleness.

[0048] In the process of the present invention, it is not necessary to separate mesophase pitch and isotropic pitch from each other and, moreover, the two stage heat treatment is carried out under severe conditions. The softening point of the pitch obtained is relatively high, and production efficiency is high; for example, the time for the stabilization treatment can be shortened.

[0049] Furthermore, the pitch obtained by the process of the present invention is narrow in distribution of molecular weight, is uniform in properties, and has good fluidity. Accordingly, high strength fibers can be produced with high spinning efficiency.

[0050] In the present invention, use of pitch for carbon fibers having a mesophase content of at least 90% by weight permits production of carbon fibers which are of high stability at a high temperature, are good in fluidity and are easy in melt spinning, are free from defects because of homogeneity and high orientation, have good modulus, and have a high tensile strength and a high knot strength. In particular, the stabilization treatment in NO₂-containing oxidizing gas provides carbon fibers excellent both in tensile strength and knot strength. These carbon fibers can be used in production of fabrics and so on, and also suitably used as material for composite materials.

[0051] The present invention is described in greater detail with reference to the following examples.

[0052] The knot strength of carbon fiber was measured as follows:

[0053] A strand comprising 3,000 of carbon fibers having a diameter of 5 to 15 µm is produced. A knot is provided on the strand in the same manner as in measurement of knot strength of single fiber (JIS L-1013). The strand is placed on a tensile tester with a chuck distance of 25 mm and while keeping so that the knot is neary in the center of the strand. A strength at break (gf) is measured at a tensile speed of 50 mm/min and is defined as a knot strength (gf/­3K-strand).

[0054] In measurement of molecular weight, VPO and GPC of the pitch starting material was measured without hydrogenation of the pitch. On the other hand, in measurement of VPO and GPC of measophase pitch, the pitch was measured after hydro­genation thereof.

EXAMPLE 1

[0055] Catalytic cracking residual oil of petroleum (Decant oil) was vacuum distilled to remove a fraction having a boiling point of 466°C or more, thereby obtaining pitch. This pitch was extracted with a mixed solvent of toluene: n-hexane = 7:3 to remove a heavy fraction, and further the solvent was dried to obtain a pitch starting material. This pitch starting material had the following properties.
(1) Number average molecular weight: 440(VPO measurment)
(2) Weight average molecular weight/Number average molecular weight = 1.77 (GPC measurement)
(3) Maximum molecular weight: 1,420
(4) Distillation properties (according to ASTM D1160)

	5%	10%	20%	30%	40%	50%
Initial Boiling point	480°C	491°C	504°C	520°C	537°C	559°C
466°C

(5) Residue when heated up to 800°C in nitrogen gas at a temperature-raising speed of 10°C/min by the use of TGA: 4.3% by weight
(6) Peak area at the starting point in FID-TLC (developed on a silica gel thin layer with a mixed solvent of 95 vol% of dichloromethane and 5 vol% of methyl alcohol) analysis: 0.8%
(7) Softening point: 105°C
(8) Aromatic Index (fa): 0.789
(9) n-Heptane insoluble content: 65 wt%
(10) Toluene insoluble content: 0 wt%
(11) Si: not more than 1 ppm
(12) Al: not more than 1 ppm

[0056] The pitch starting material having the above properties was polycondensed by subjecting to the first stage heat treatment in which it was maintained at a temperature of 420°C in a vacuum of 10 mmHg for 1 hour, and a mesophase precursor pitch having a toluene insoluble content of 17.0% by weight and a softening point of 183°C and substantially not containing a mesophase.

[0057] Subsequently, the above precursor pitch was subjected to the second stage heat treatment at a temperature of 460°C in a vacuum of 1 mmHg for 15 minutes, and a 100% mesophase pitch with a quinoline insoluble content of 34.5% by weight and a softening point of 330°C was obtained. In connection with the properties of the mesophase pitch, the toluene insoluble content was 89% by weight, the number average molecular weight was 1,130, and the weight average molecular weight/number average molecular weight ratio was 1.57:1.

[0058] A thermogravimetric analysis (TGA) in which the pitch was heated to 800°C at a rate of 10°C/min in nitrogen gas showed that the 5% weight reduction temperature T_5% was 485°C and the weight reduction when heated to 800°C was 22.3%.

[0059] The 100% mesophase pitch as obtained above was spun at a take-up speed of 400 m/min by the use of a spinneret with an capillary diameter of 0.2 mm to obtain pitch fiber with a diameter of 11 µm.

[0060] These pitch fibers were subjected to stabilization treatment by heating from 200°C to 360°C for 10 minutes in air and then maintaining at 360°C for 10 minutes. Then they were carbonized by maintaining at 1,500°C for 5 minutes. They were further graphitized by maintaining at 2,400°C for 5 minutes. A tensile strength and a modulus in tension of the carbon fiber thus obtained are shown below.

	Tensile Strength	Modulus in Tension	Fiber Diameter	Elongation
	(kg/mm²)	(ton/mm²)	(µm)	(%)
1,500°C Carbonized Product	407	20.5	9.2	2.0
2,400°C Graphitized Product	435	58.5	8.9	0.75

EXAMPLE 2

[0061] Catalytic cracking residual oil of petroleum was raised in temperature at a rate of 2°C/min in a batch reactor and finaly reacted at 420°C in 2 mmHg for 1 hour to obtain meso­phase pitch.

[0062] A relatively heavy fraction formed at a later stage of the reaction was recovered and used as a pitch starting material. This pitch starting material had the following properties.
(1) Number average molecular weight: 380
(2) Weight average molecular weight/number average molecular weight = 1.92/1
(3) Maximum molecular weight: 1,030
(4) Distillation characteristics

Initial Boiling Point	10%	20%	30%	40%	50%	60%	70%
453°C	487°C	495°C	505°C	517°C	532°C	548°C	568°C

(5) Residue when heated at a rate of 10°C/min up to 800°C in nitrogen gas by the use of TGA: 3.2% by weight
(6) Peak area at the starting point in TLC-FID analysis: 0.3%
(7) Softening point: 73°C
(8) Aromatic Index (fa): 0.75
(9) n-Heptane insoluble content: 32% by weight
(10) Toluene insoluble content: 0% by weight
(11) Si: 2 ppm
(12) Al: not more than 1 ppm

[0063] The pitch starting material having the above properties was subjected to the first stage heat treatment by maintain­ing at 400°C in a vacuum of 40 mmHg for 4 hours. At this point, the toluene insoluble content was 20.3% by weight and the softening point was 152°C.

[0064] Subsequently, the second stage heat treatment was ap­plied at a temperature of 480°C in a vacuum of 3 mmHg for 4.5 minutes to obtain a 100% mesophase pitch having a quinoline insoluble content of 25.5% by weight and a soften­ing point of 322°C. In connection with the properties of the mesophase pitch, the toluene insoluble content was 78% by weight, the number average molecular weight was 1,070, and the weight average molecular weight/number average molecular weight ratio of 1.67/1.

[0065] TGA in which the pitch was heated to 800°C at a rate of 10°C/min in nitrogen gas showed that the 5% weight reduction temperature T_5% was 486°C and the weight reduc­tion when the pitch was heated up to 800°C was 22.0% by weight.

[0066] The 100% mesophase pitch thus obtained was spun, sub­jected to stabilization treatment, and calcined in the same manner as in Example 1 to obtain carbon fibers. A tensile strength and a modulus in tension of the carbon fiber ob­tained are shown below.

	Tensile Strength	Modulus in Tension	Fiber Diameter	Elongation
	(kg/mm²)	(ton/mm²)	(µm)	(%)
1,500°C Carbonized Product	405	18.9	8.8	2.2
2,400°C Graphitized Product	416	62.5	8.5	0.67

[0067] Both in Examples 1 and 2, the 1,500°C carbonized product had a high tensile strength in comparison with modulus in tension, and the elongation was as great as before and after 2%. The 2,400°C graphitized product had a high modulus in tension. The results show that the present invention can provide carbon fibers of high tensile strength, high modulus and high elongation which could not be obtained by the conventional methods.

EXAMPLE 3

[0068] The same pitch starting material as used in Example 1 was made heavier by subjecting to the first stage heat treat­ment in which it was maintained at a temperature of 480°C in a vacuum of 100 mmHg for 10 minutes, and a mesophase precursor pitch having a toluene insoluble content of 15.8% by weight and a softening point of 169°C, and substantially not containing a mesophase was obtained.

[0069] Subsequently, the mesophase precursor pitch was sub­jected to the second stage heat treatment at a temperature of 520°C in a vacuum of 10 mmHg for 30 seconds to obtain a 100% mesophase pitch. In connection with the properties of the mesophase pitch, the quinoline insoluble content was 8.5% by weight, the softening point was 333°C, the number average molecular weight was 1,080 (VPO measurement), and the weight average molecular weight/number average molecular weight ratio was 1.48/1 (measured by GPC).

[0070] TGA in which the pitch was heated up to 800°C at a rate of 10°C/min in nitrogen gas showed that the 5% weight reduc­tion temperature T_5% was 476°C and the weight reduction when the pitch was heated up to 800°C was 20.6% by weight.

[0071] The pitch thus obtained was spun at 358°C at a take-up speed of 800 m/min by the use of a spinneret with an capil­lary diameter of 0.15 mm to obtain pitch fibers with a diameter of 11.8 µm.

[0072] This pitch fiber was subjected to stabilization treat­ment and then carbonized in a nitrogen atmosphere under the conditions shown below to obtain carbon fiber. This carbon fiber was measured for tensile strength, modulus in tension and knot strength.

(1) Stabilization treatment in air, and carbonization treatment

[0073] 

[0074] The above pitch fiber was heated in air from 200°C to 280°C at a rate of 10°C/min, and then maintained at 280°C for 1 hour to subject it to stabilization treatment.

[0075] Then the stabilized pitch fiber was carbonized by maintaining at 1,500°C for 10 minutes in a nitrogen gas and then graphitized by maintaining at 2,500°C for 2 minutes in an argon gas to obtain carbon fiber. The physical properties of the carbon fiber thus obtained are shown below.

	Tensile Strength	Modulus in Tension	Knot Strength
	(kg/mm²)	(ton/mm²)	(gf/3k·(9.1 µm))
1,500°C Carbonized Product	363	23.5	285
2,500°C Graphitized Product	389	58.5	68

(2) Stabilization treatment in NO₂-containing air

[0076] The above pitch fiber was subjected to stabilization treatemtn by maintaining at 220°C for 180 minutes in air containing 1 vol% of NO₂. Then it was carbonized by main­taining at 1,500°C for 10 minutes in nitrogen gas, and then graphitized by maintaining at 2,500°C for 2 minutes in argon gas to obtain carbon fiber. The physical properties of the carbon fiber are shown below.

	Tensile Strength	Modulus in Tension	Knot Strength
	(kg/mm²)	(ton/mm²)	(gf/3k·(9.7 µm))
1,500°C Carbonized Product	453	26.0	2,600
2,500°C Graphitized Product	455	61.2	86

EXAMPLE 4

[0077] Pitch having the properties shown below as obtained by heat treatment of catalytic residual oil of petroleum was used as a pitch starting material.
(1) Number average molecular weight: 465 (VPO measurement)
(2) Weight average molecular weight/number average molecular weight = 1.53/1 (GPC measurement)
(3) Maximum molecular weight: 940
(4) Distillation characteristics (according to ASTM D1160)

Initial Boiling Point	5%	10%	20%	30% or more
468°C	511°C	528°C	546°C	impossible to measure

(5) Residue content when the pitch was heated up to 800°C at a rate of 10°C/min by the use of TGA: 2.3% by weight
(6) Peak area at the starting point in TLC-FID (developed with a mixed solvent of 95 vol% of dichloromethane and 5 vol% of methyl alcohol on a silica gel thin layer) analysis: 0.3%
(7) Softening point: 133°C
(8) Aromatic Index (fa): 0.831
(9) n-Heptane insoluble content: 73 wt%
(10) Toluene insoluble content: 0 wt%
(11) Si: less than 1 ppm
(12) Al: less than 1 ppm

[0078] The pitch starting material with the above properties was made heavier by subjecting to the first stage heat treat­ ment in which it was maintained at 460°C in 1,200 mmHg for 25 minutes, and a mesophase precursor pitch having a toluene insoluble content of 22% by weight and a softening point of 173°C and substantially not containing a mesophase was obtained.

[0079] Subsequently, the mesophase precursor pitch was sub­jected to the second stage heat treatment at 500°C in 0.5 mmHg for 2 minutes to obtain 100% mesophase pitch. In con­nection with the properties of the mesophase pitch, the quinoline insoluble content was 39% by weight, the softening point was 348°C, the number average molecular weight (VPO measurement) was 1,190, and the weight average molecular weight/number average molecular weight (measured by GPC) was 1.41.

[0080] TGA in which the mesophase pitch was heated up to 800°C at a rate of 10°C/min in nitrogen gas showed that the 5% weight reduction temperature T_5% was 494°C, and the weight reduction when the pitch was heated to 800°C was 18.1%.

[0081] The pitch was spun at 372°C at a take-up speed of 600 m/min by the use of a spinneret with an capillary diameter of 0.20 mm to obtain pitch fiber with a diameter of 10 µm.

[0082] This pitch fiber was subjected to stabilization treat­ment and then carbonized in nitrogen atmosphere under the conditions shown below to obtain carbon fiber. This carbon fiber was measured for tensile strength, modulus in tension and knot strength.

(1) Stabilization treatment in air, and carbonization treatment

[0083] The above pitch fiber was subjected to stabilization treatment by heating from 200°C to 280°C at a rate of 10°C/­min in air, and then maintaining at 280°C for 1 hour.

[0084] The above pitch fiber was carbonized by maintaining at 1,500°C for 10 minutes in nitrogen gas, and then graphitized by maintaining at 2,500°C for 2 minutes in argon gas to obtain carbon fiber. The physical properties of the carbon fiber are shown below.

	Tensile Strength	Modulus in Tension	Knot Strength
	(kg/mm²)	(ton/mm²)	(gf/3k·(9.1 µm))
1,500°C Carbonized Product	413	24.3	265
2,500°C Graphitized Product	425	55.7	53

(2) Stabilization treatment in NO₂-containing air

[0085] The above pitch fiber was subjected to stabilization treatment by maintaining at 180°C for 540 minutes in air containing 0.5% by volume of NO₂. Subsequently, the pitch fiber was carbonized by maintaining at 1,500°C for 10 minutes in nitrogen gas, and then graphitized by maintaining at 2,500°C for 2 minutes in argon gas to obtain carbon fiber. The physical properties of the carbon fiber are shown below.

	Tensile Strength	Modulus in Tension	Knot Strength
	(kg/mm²)	(ton/mm²)	(gf/3k·(9.1 µm))
1,500°C Carbonized Product	458	26.5	3,200
2,500°C Graphitized Product	465	60.5	75

COMPARATIVE EXAMPLE 1

[0086] Catalytic cracking residual oil of petroleum was distilled under reduced pressure in the same manner as in Example 1 to remove a fraction having a boiling point of 455°C or more, and a residue pitch was obtained. The properties of the pitch are as follows.
(1) Number average molecular weight: 420
(2) Weight average molecular weight/number average molecular weight = 2.36/1
(3) Maximum molecular weight: 2,500
(4) Distillation characteristics

Initial Boiling Point	5%	10%	20%	30%	40%	50%	60%
455°C	503°C	508°C	517°C	527°C	540°C	553°C	573°C

(5) Residue content when the pitch was heated up to 800°C at a rate of 10°C/min in nitrogen gas by the use of TGA: 11 wt%
(6) Peak area at the starting point in TLC-FID analysis: 2.3%
(7) Softening point: 104°C
(8) Aromatic Index (fa): 0.779
(9) n-Heptane insoluble content: 63 wt%
(10) Toluene insoluble content: 1.3 wt%
(11) Si: 2 ppm
(12) Al: less than 1 ppm

[0087] The pitch starting material with the above properties was made heavier by subjecting to the first stage heat treat­ment in which it was maintained at 420°C in 10 mmHg for 4.5 hours, and a mesophase precursor pitch having a toluene in­soluble content of 16.3% by weight and a softening point of 181°C, and substantially not containing a mesophase was obtained.

[0088] Subsequently, the precursor pitch was subjected to the second stage heat treatment under the same conditions as in Example 1 to obtain a 100% mesophase pitch having a quinoline insoluble content of 36.3% by weight and a softening point of 335°C. In connection with the properties of the mesophase pitch, the toluene insoluble content was 68% by weight, the number average molecular weight was 1,150, and the weight average molecular weight/number average molecular weight was 1.83/1.

[0089] The 100% mesophase pitch thus obtained was spun, sub­jected to stabilization treatment and calcined in the same manner as in Example 1 to obtain carbon fiber. The tensile strength and modulus in tension of the carbon fiber thus obtained were as follows.

	Tensile Strength	Modulus in Tension	Fiber Diameter	Elongation
	(kg/mm²)	(ton/mm²)	(µm)	(%)
1,500°C Carbonized Product	274	22.3	9.1	1.2
2,400°C Graphitized Product	295	55.5	8.8	0.53

EXAMPLE 5

[0090] Fluid catalytic cracking residual oil was made heavier by heat treatment to obtain isotropic pitch with a softening point of 160°C and a toluene insoluble content of 5.5% by weight. This isotropic pitch was subjected to heat treatment at 460°C and 1 mmHg for 12 minutes to obtain a 100% mesophase pitch with a quinoline insoluble content (QI) of 28.5% by weight, a number average molecular weight of 1,140, Mw/Mn = 1.45/1, and a softening point of 310°C. In TGA (thermogravi­metric analysis) of the pitch, when the pitch was heated up to 800°C at a rate of 10°C/min in nitrogen, T_5% was 483°C and the weight reduction when the temperature was raised to 800°C was 22.0% by weight.

[0091] The pitch thus obtained was melt spun at 343°C by the use of a spinneret with a capillary diameter of 0.3 mm to obtain pitch fiber with a diameter of 9 µm. This pitch fiber was subjected to stabilization treatment and carbonized in nitrogen under the conditions shown below to obtain carbon fiber. This carbon fiber was measured for tensile strength, modulus and knot strength.

(1) Stabilization treatment in air

[0092] The above pitch fiber was subjected to stabilization treatment by heating from 200 to 300°C at a rate of 20°C/min in air and then maintained at this temperature for 30 minutes and, thereafter, carbonized at 1,500°C or 2,500°C to obtain carbon fiber. The physical properties of the carbon fiber are shown below.

	Tensile Strength	Modulus in Tension	Knot Strength
	(kg/mm²)	(ton/mm²)	(gf/3k (7.4 µm))
1,500°C Carbonized product	365	25.5	900
2,500°C Graphitized product	409	63	85

(2) Stabilization treatment in NO₂-containing air

[0093] The above pitch fiber was subjected to stabilization treatment by maintaining at 200°C for 180 minutes in air containing 2.5% by volume of NO₂ and then carbonized at 1,500°C or 2,500°C to obtain carbon fiber. The physical properties of the carbon fiber are shown below.

	Tensile Strength	Modulus in Tension	Knot Strength
	(kg/mm²)	(ton/mm²)	(gf/3k (7.5 µm))
1,500°C Carbonized product	383	21.5	3,100
2,500°C Graphitized product	415	71.0	130

COMPARATIVE EXAMPLE 2

[0094] Fluid catalytic cracking residual oil was heated at 420°C and 50 mmHg for 1 hour to prepare pitch having a meso­phase content of about 15% by weight. The mesophase was removed from the pitch utilizing a difference in gravity. The resulting isotropic pitch was subjected to heat treat­ment of 420°C and 10 mmHg for 2.5 hours to obtain pitch having a softening point of 305°C, QI of 31.5% by wegiht, a number average molecular weight of 960 and Mw/Mn of 1.90/1. TGA of the pitch showed that T_5% was 421°C and the weight reduction at 800°C was 27.0%.

[0095] This pitch was melt spun in the same manner as in Example 5 to obtain pitch fiber with a diameter of 10 µm. The pitch was then subjected to stabilization treatment under the conditions shown below and carbonized in a nitrogen atmosphere to produce carbon fiber. This carbon fiber was measured for physical properties.

(1) Stabilization treatment in air

[0096] The above pitch fiber was subjected to stabilization treatment by heating from 200°C to 300°C at a rate of 20°C/­min in air and then maintaining at this temperature for 30 minutes, and thereafter carbonized at 1,500°C or 2,500°C to obtain carbon fiber. The physical properties of the carbon fiber are shown below.

	Tensile Strength	Modulus in Tension	Knot Strength
	(kg/mm²)	(ton/mm²)	(gf/3k (8.2 µm))
1,500°C Carbonized product	285	23.2	220
2,500°C Graphitized product	310	55.0	54

(2) Stabilization treatment in NO₂-containing air

[0097] The above pitch fiber was subjected to stabilization treatment by maintaining at 280°C for 20 minutes in air containing 10% by volume of NO₂, and then carbonized at 1,500°C to obtain carbon fiber. The physical properties of the carbon fiber are shown below.

	Tensile Strength	Modulus in Tension	Knot Strength
	(kg/mm²)	(ton/mm²)	(gf/3k (8.2 µm))
1,500°C Carbonized product	295	22.5	900

EXAMPLE 6

[0098] The mesophase pitch separated by difference in gravity in Comparative Example 2 was subjected to devolatization treatment of 380°C and 0.1 mmHg for 20 minutes. In this mesophase pitch, QI was 34.5% by weight, the softening point was 315°C, the number average molecular weight was 1,090, and Mw/Mn was 1.67/1. TGA measurement showed that T_5% was 477°C and the weight reduction at 800°C was 23.8%.

[0099] This pitch was spun in the same manner as in Comparative Example 2 to obtain pitch fiber with a diameter of 10.3 µm.

[0100] This pitch fiber was subjected to stabilization treat­ment under the conditions shown below, and carbonized in a nitrogen atmosphere to produce carbon fiber. This carbon fiber was measured for physical properties.

(1) Stabilization treatment in air

[0101] The above pitch fiber was subjected to stabilization treatment by heating from 200 to 300°C at a rate of 20°C/min in air and then maintaining at this temperature for 30 minutes and, thereafter, carbonized at 1,500°C or 2,500°C. The physical properties of the carbon fiber are shown below.

	Tensile Strength	Modulus in Tension	Knot Strength
	(kg/mm²)	(ton/mm²)	(gf/3k (8.1 µm))
1,500°C Carbonized product	329	24.5	280
2,500°C Graphitized product	348	57.5	120

(2) Stabilization treatment in NO₂-containing air

[0102] The above pitch fiber was subjected to stabilization treatment by maintaining at 280°C for 20 minutes in air containing 10% by volume of NO₂ and then carbonized at 1,500°C to obtain carbon fiber. The physical properties of the carbon fiber are shown below.

	Tensile Strength	Modulus in Tension	Knot Strength
	(kg/mm²)	(ton/mm²)	(gf/3k (8.1 µm))
1,500°C Carbonized product	360	22.3	1,600

Claims

1. Pitch for use in production of carbon fiber, having a quinoline insoluble content of not more than 50% by weight, a number average molecular weight of at least 1,000, a ratio of weight average molecular weight to number average molecular weight of 1.3/1 to 1.8/1, a mesophase content of at least 90% by weight, a softening point of 250 to 380°C, a 5% weight reduction temperature as determined by a thermogravimetric analysis of at least 470°C, and a weight reduction at a temperature of 800°C of not more than 25%.

2. The pitch as claimed in Claim 1, having a quinoline insoluble content of 5 to 40% by weight, a number average molecular weight of 1,000 to 1,400, and a ratio of weight average molecular weight to number average molecular weight of 1.35/1 to 1.75/1.

3. A process for producing pitch for use in production of carbon fiber which comprises subjecting a pitch starting material to a first stage heat treatment at a temperature of 380 to 500°C under a pressure of 1 to 1,500 mmHg and then to a second stage heat treatment at a temperature of 450 to 550°C under a pressure of 0.1 to 20 mmHg, wherein the pitch starting material has a number average molecular weight of 300 to 500, weight average molecular weight number average molecular weight of 1.5/1 to 2.2/1, a boiling point of at least 450°C, a residue content as determined by heating up to 800°C in nitrogen gas at a rate of 10°C per minute of not more than 5% by weight, and a peak area at the starting point as determined by developing on a silica gel thin layer with a mixed solvent of 95% by volume of dichloro­methane and 5% by volume of methyl alcohol of not more than 1%.

4. The process as claimed in Claim 3 wherein the pitch starting material has a number average molecular weight of 350 to 480, weight average molecular weight/number average molecular weight of 1.5/1 to 2.0/1, and a residue content as determined by heating up to 800°C in nitrogen gas at a rate of 10°C per minute of 4.5 to 0.5% by weight.

5. The process as claimed in Claim 3 wherein the first stage heat treatment is carried out at a temperature of 390 to 480°C and under a pressure of 5 to 1,500 mmHg.

6. The process as claimed in Claim 3 wherein the second stage heat treatment is carried out at a temperature of 460 to 500°C and under a pressure of 0.1 to 10 mmHg.

7. A process for producing carbon fiber which comprises melt spinning the pitch of Claim 1 at a temperature of 280 to 400°C to form pitch fiber, subjecting the pitch fiber to stabilization treatment, and then carbonizing the pitch fiber.

8. The process as claimed in Claim 7 wherein the pitch fiber is subjected to stabilization treatment by heating at a temperature of 200 to 400°C for 5 to 300 minutes.

9. The process as claimed in Claim 7 wherein the pitch fiber is subjected to stabilization treatment by heating at a temperature of 150 to 350°C for 10 to 600 minutes in an oxidizing gas atmosphere containing 0.1 to 30% by volume of nitrogen dioxide.