GB2150924A

GB2150924A - Carbon fibres

Info

Publication number: GB2150924A
Application number: GB08422594A
Authority: GB
Inventors: Yasuyuki Takai; Minoru Takabatake; Hideyuki Nakajma
Original assignee: Kashima Oil Co Ltd
Current assignee: Kashima Oil Co Ltd
Priority date: 1983-11-10
Filing date: 1984-09-07
Publication date: 1985-07-10
Also published as: IT8448824A0; GB8422594D0; JPS60104528A; JPH0144805B2; IT1208695B; GB2150924B; CA1227005A

Abstract

A process for producing filament yarns of high strength, high modulus carbon fibers having superior properties can be obtained from a pitch containing mesophase in an amount of 70% to 100% by using, in melt-spinning of the pitch, spinning nozzles having a cross-sectional area at the nozzle outlet part greater than the cross-sectional area of the narrowest part of the passage for spinning dope inside the nozzles, and subsequently thermosetting and then carbonizing the pitch.

Description

SPECIFICATION Process for producing carbon fibers Background of the invention 1. Field of the invention This invention relates to a process for producing carbon fibers. More particularly, it relates to a process for producing filament yarns of high strength, high modules carbon fibers having flowless, superior properties from a pitch containing a specified amount of mesophase, as a raw material, by melt-spinning using spinning nozzles (spinnerettes) having a specified structure in extrusion holes for spinning dope.

Aterm "mesophase" herein referred to is one of the components constituting the pitch and it means an optically anisotropic part of the pitch which shines brilliantly when the section of a lump of pitch solidified at a temperature close to room temperature is polished and observed through the crossed nicols of reflection type polarizing microscopy. A pitch mostly composed of mesophase is called mesophase pitch. The content of mesophase in a mesophase pitch is calculated from the percentage of the area of optically anisotropic part obtained by observation under a reflection type polarizing micron scope.

2. Description of the prior art Recently, there has been a demand for high strength and high modulus light-weight materials in various fields, e.g. in aircraft, motorvehicle and other industries, and in this connection, a demand for carbon fibers provided with the above mentioned properties is rapidly increasing. It is well known that the starting material for high strength, high modulus carbon fibers available now in the market are mostly polyacrylonitrile fibers. However these polyacrylonitrile fibers are not only expensive but also give only a low yield of carbon fibers, e.g.

about 45%. This fact also increases the production cost of the ultimate products of carbon fibers.

As one method for producing high strength, high modulus carbon fibers at a low cost, there are descriptions in the official gazette of Japanese Patent Publication No.1810(1979) issued to Union Carbide Corporation and it is a well known fact that mesophase-containing pitches are excellent raw materials for filament yarn's of high strength, high modulus carbon fibers. In the raw materials of high strength, high modulus carbon fibers, the content and the physical properties of mesophase give a great influence upon the physical properties of carbon fibers. The higher the mesophase content, the better the quality of mesophase, and the greater the improvement of the physical properties of carbon fibers.Further, pitch of low mesophase content is not adequate as a raw material for high strength, high modulus carbon fibers because both the strength and modulus of the carbon fibers obtained therefrom are low.

As for the structure of the cross-section of pitchderived carbon fibers, it has been known that roughly random shape (orderless), radial shape (radiated), onion shape (concentric circle shape), etc.

of carbon arrangement exist (Examples of literature: The 12th biennial conference on carbon, July 329 (1975); Pittburg and Ceramics 11 (1976) No. 7, Nos 612-621). These structures depend greatly upon the physical properties of raw material pitch. When melt-spinning is carried out by using a spinning nozzle in which a narrow channel, as a passage for molten pitch, is a straight tube having a circular cross-section as commonly used case, filaments of carbon fibers thus obtained show a structure in which carbonaceous material is radially oriented because the higher the mesophase content of a raw material pitch, the higher the orientation degree of carbonaceous material of the filament produced by melt-spinning, and afterthermosetting and carbonization, obtained carbon fibers have noticeable radial structure.Filaments of carbon fibers having radial structure form very often big cracks extending from the circumference of cross-section toward the center of a filament and resultant carbon fibers utterly lose theirvalue as articles of commerce.

It is an object of the present invention to provide a process for producing carbon fibers in which the above-mentioned problems of prior art processes for producing pitch-derived carbon fibers have been overcome and products having excellent properties can constantly be made without forming a crack or cracks.

It is another object of the present invention to provide spinning nozzles capable of providing high strength, high modulus carbon fibers having nearly circular cross-section and containing no crackes at all.

The above mentioned objects can be attained by the process and the nozzles of the present invention.

Summary of the invention The present invention resides in a process for producing high strength, high modulus filament yarns of carbon fibers which comprises subjecting a pitch having a mesophase content of 70% or higher to melt-spinning using spinning nozzles having a cross-sectional area at their nozzle outlet part greater than the cross-sectional area of the narrowest part inside the nozzles and subjecting the resultant filament yarns to thermosetting and carbonization to obtain high strength high modulus filament yarns without cracks in the cross-section of the carbon fibers.

In the process of the present invention, melt spinning is carried out at a temperature which is higher than the softening temperature of mesophase pitch by from 40"C to 140"C preferably from 55'Cto 120"C using spinning nozzles having a crosssectional area at their nozzle outlet greater than the cross-sectional area of the narrowest part inside the nozzles, preferably by two times or more.

Detailed description of the invention The inventor of the present application has discovered after comprehenisve studies that the properties of carbon fibers can be notably improved by eliminating the formation of cracks in the crosssection of carbon fibers made from raw material mesophase pitch containing mesophase in an amount of 70% or higher.

As a process for eliminating cracks in the crosssection of carbon fibers, it has been found that the use of spinning nozzles having a cross-sectional area of the outlet of the nozzles greater than the crosssectional area of the narrowest part of the thin passage for spinning dope inside the nozzles, preferably having a ratio of the cross-sectional area at the outlet of the nozzle to that of the narrowest part of thin passage for spinning dope inside the nozzles of two or greater as shown hereinafter in the drawing of Figures 1,2 and 3 and also the use of a spinning temperature higher than the softening temperature (as measured by Koka type flow tester) by 40 to 140"C, preferably by 55" - 120"C in the melt-spinning, followed by usual treatments of thermosetting and carbonization provides filament yarns of carbon fiber which are excellent in quality and have no crack at all, and thus the process of the present invention has been completed.

Detailed description will be given as to the above mentioned spinning temperature. Although optimum spinning temperature somewhat varies depending on the mesophase content in the mesophase pitch and physical properties of mesophase, the results of an experiment show that, if spinning is carried out at a temperature which is not higher than the softening point of mesophase pitch by 40"C, the viscosity of the mesophase pitch is too high for spinning resulting in poor spinnability.On the other and if spinning is carried out at a temperature higher than the softening temperature of the mesophase pitch by 1 400C or more, the reduction of viscosity of mesophase patch, increase of contamination of spinning nozzles, and change of properties of mesophase pitch occur, resulting in increase of breakage of spun filaments and stable spinning becomes difficult. Accordingly, it is proper to select a spinning temperature in the range of 40 to 140"C, preferably 55" to 1 200C higher than the softening temperature of the mesophase. The softening temperature of mesophase is in the range of 1 90 C to 240"C.

As for a raw material of mesophase pitch used in the process of the present invention, petroleumorigin heavy oil, such as topped crude (reduced C. or long residue), vacuum residue (short residue), residue of thermal catalytic cracking of vacuum gas oil, tar or pitch produced, as a by-product of heat treatment of these residues and a coal-origin heavy oil such as coal tar, coal tar pitch and a coal liquified product can be mentioned. Mesophase pitch can be produced by subjecting one or more of these raw materials to heat treatment under non-oxidative atmosphere to form mesophase, causing the resulting mesophase to grow by aging, and separating the part mostly consisting of mesophase.

The inventors of the present application have found that filaments of carbon fibers having superior qualities can be produced at an inexpensive price according to the process of the present invention if the content of mesophase in mesophase pitch is 70% or greater, preferably 90% or higher. A mesophase pitch containing lower than 70% mesophase, when subjected to spinning according to an usual manner and then to thermosetting and carbonization, provides carbon fiber filaments which do not form radial structure in cross-section due to its low degree of carbon orientation. Therefore, although no crack is formed, both tensile strength and modulus of resulting filaments are low, and the carbon fibers have little value as articles of commerce.

When mesophase pitch is used, as a raw material of filament yarn of carbon fibers, the higher the mesophase content, the better the quality of the carbon fibers.

When mesophase pitch containing 70% or more, preferably 90% or more mesophase is melt-spun by causing velocity change to the flow of mesophase pitch inside the nozzles by using spinning nozzles having a cross-sectional area at their nozzle outlet part greater than that of the narrowest part of the passage way for spinning dope inside the nozzles, preferably in a ratio of the areas of 2 or grater, filament yarns of carbon fibers free of crackes in the cross-section can be obtained.

Brief description of the drawings Figure 1 is a view of vertical cross-section through the center of one type of a nozzle of this invention.

Figure 2 is also a vertical cross-section through the center of another type of nozzle.

Figure 3 is also a vertical cross-section through the center of a further type of nozzle of this invention.

Figure 4 7is a photograph of a cross-section of the filament yarns of carbon fibers made by using the nozzle of the present invention and observed under a SEM.

Figure 8 is a photograph of the cross-section of the filaments of carbon fibers made by using the nozzle of the referential example.

Examples of nozzle shapes of the spinnerettes used in the process of the present invention are illustrated in the drawings. However, it is to be noted that the shape of the spinning nozzles used in the present invention should not be limited to those shown in these drawings. Further, the cross-section of the nozzle should not be limited only to circular shape. It is only limited to the condition defined in the scope of claim.

In each drawings, 1 is an inlet part of spinning dope. 2 is narrowest tube part. 3 is an outlet part of extruding hole.

Following examples are offered by way of illustration and not by way of limitation.

Example 1 A distillate fraction higher than 404"C, as initial distilling point, of residue of thermal calalytic cracking of vacuum gas oil was subjected to heat treatment at 420"C for 2 hours while sending there methane gas and further to heating at 320"C for 16 hours to cause mesophase to grow by aging and a part consisting mostly of mesophase was separated.

The mesophase content of this mesophase pitch was 91% according to the measurement under a reflection type polarizing microscope and the softening point (as measured by a Koka type flow tester) was 21 so.

Using this mesophase pitch as a raw material, and using spinning nozzles shown in Figure 1 (having 100 extrusion holes i.e. passage for spinningdope, a diameter at the inlet part of spinning dope of 2.5 mm, a diameter at the narrowest thin tube part of 0.15 mm, the length of the narrowest thin tube part of 0.3 mm, an angle of cone expanding toward the outlet part of 90"C, a diameter at the outlet part of 0.3 mm), spinning was carried out at a spinning temperature of 300 C, and a spinning velocity of 210 m/min.

Resultant filaments yarns of pitch fibers were subjected to thermosetting at 300"C and then to carbonization at 2500"C to produce products. When the cross-section of these filaments of carbon fibers was observed under a scanning type electron microscope (SEM), it was found that the structure of the cross-section thereof was of radial shape and there was no crack formed. Further, resultant filaments of carbon fibers had a tensile strength of 278 kg/mm2, a modulus of elasticity of 49 S/mm2 and an elongation of 0.57%.

Example2 Using the mesophase pitch as used in Example 1 as a raw material and using spinning nozzles of Figure 1, (100 extrusion holes), in which the diameter of spinning dope introducing part is 2.5 mmí, the diameter of the thinnest tube part is 0.1 mmri, the length of the thinnest tube part is 0.1 mm, the cone angle of a frustum expanding toward outlet is 45 and the diameter of outlet part is 0.2 mm, filament yarns of carbon fibers were produced by spinning at a spinning temperature of 307"C and at a spinning velocity of 500 m/min. followed by other processings in same manner as in Example 1.When the crosssection of the resultant carbon fiber was observed with a scanning type electron microscope, it has an onion like structure in the cross-ection as shown in Figure 5, and no crack was found.

Example 3 Using the mesophase pitch used as in Example 1 as a raw material, and using spinning nozzles of Figure 2 (100 extrusion holes) in which the diameter of spinning dope introducing part is 2.5 mm, the diameter of the thinnest tube part is 0.1 mm6, the length of the thinnest tube part is 0.1 mm4, and the diameter at the outlet part is 0.25 mm4. (Expanding by forming a half cube), filament yarns of carbon fibers were produced by spinning at a spinning temperature of 280"C and a spinning velocity of 180 m/min. followed by other processings in the same manner as in Example 1. The representative crossectional structure of resultant carbon fibers was a mixture of radial, onion, and random patterns as shown in Figure 6. There was found no crack at all.

Example 4 100% Mesophase pitch having a softening point of 235"C was obtained after the same processing as in Example 1 except that longer time was necessary for separating the mesophase pitch. Using this pitch and nozzles as used in Example 2, filament yarns of carbon fibers were produced by spinning at a spinning temperature of 304"C and a spinning velocity of 150 m/min. followed by other processings in the same manner as in Example 1. Representative cross-sectional structure of resultant fibers was a mixture of radial and random patterns as shown in Figure 7. There was found no crack at all.

Comparative Example 1 Using a mesophase pitch as used in Example 1 as a raw material and using spinning nozzles having extrusion holes, in which thin tube parts of the extrusion holes are of a straight tube having a diameter of 0.3 mm in cross-section and 0.3 mm in length and also having a diameter of 0.3 mm at the outlet part, filament yarns of carbon fibers were produced under the same conditions for spinning, thermosetting and carbonization as in Example 1.

When the cross-section of the resultant filaments of carbon fibers was observed under a scanning type electron microscope, the structure of the crosssection of the filaments yarn of carbon fibers was of radial shape as shown in Figure 8 but there were formed cracks at an angle of about 90 . Resultant filaments of carbon fibers had a tensile strength of 157 kg/mm2 a modulus of elasticity of 38 $/mm2 and an elongation of 0.41%.

Comparative Example 2 Using the mesophase pitch as used in Example 4 as a raw material, and using spinning nozzles having extrusion holes in which thin tube parts of the extrusion hole are of a straight tube having a cross-sectional diameter of 0.1 mm, a length of 0.1 mm and a diameter at the outlet being also 0.1 mm, filament yarns of carbon fibers were produced under the same condition for spinning, thermosetting and carbonization as in Example 4. Resultant filaments of carbon fibers had a structure of radial pattern in cross-section as in Comparative Example 1, and cracks were formed.

Claims

1. A process for producing high strength, high modulus filament yarns of carbon fibers which comprises subjecting a pitch having a mesophase content of 70% or higherto melt-spinning using a spinning nozzle having a cross-sectional area at the nozzle outlet greater than across-sectional area inside the nozzle and subjecting the resultantfila- ment yarns to thermosetting and carbonization.

2. A process for producing filament yarns of carbon fibers according to claim 1; in which a pitch is subjected to melt spinning at a temperature higher than the softening point of the mesophase pitch by 40"C to 140"C.

3. A process for producing filament yarns of carbon fibers according to claim 1; in which a pitch is subjected to melt spinning at a temperature higher than the softening point of the mesophase pitch by 55"C to 120 C.

4. A process for producing filament yarns of carbon fibers according to claim 1, in which the raw material pitch is 100% mesophase.

5. A process for producing filament yarns of carbon fibers according to claim 1, in which the raw material pitch has a mesophase content lower than 100%.

6. A process according to claim 5, in which the raw material pitch has a mesophase content over 90%.

7. A process for producing filament yarns of carbon fibers according to claim 1 in which the resultant filament yarns has a structure of radial pattern in the cross-section.

8. A process for producing filament yarns of carbon fibers according to claim 1 in which the resultant filament yarns has a structure of random pattern in the cross-section.

9. A process for producing filament yarns of carbon fibers according to claim 1 in which the resultant filament yarns have a structure of onionlike pattern in the cross-section.

10. A process for producing filament yarns of carbon fibers according to claim 1 in which the resultant filament yarns have a structure of partly radial, partly random, or partly onion pattern or a mixture of the above-mentioned patterns in the cross-section.

11. A process according to any of claims 1 to 10, in which nozzles are used having a ratio of crosssectional area of outletto cross-sectional area of internal nozzle of 7.5:1 to 8.5:1.

12. A spinneret nozzle for melt spinning of carbon fibres having an outlet cross sectional area greater than an internal cross-sectional area.

Amendments to the claims have been filed, and have the following effect: *(a) claims 7 and 8 above have been textually amended, as follows: Claim 7 line 3 delete "has" insert "have" delete "the" Claim 8 line 3 delete "has" insert "have" delete "the"