EP0099425B1

EP0099425B1 - Method for producing a mesophase pitch derived carbon yarn and fiber

Info

Publication number: EP0099425B1
Application number: EP82200943A
Authority: EP
Inventors: David Arthur Schulz
Original assignee: BP Corp North America Inc; Union Carbide Corp
Current assignee: BP Corp North America Inc
Priority date: 1982-07-22
Filing date: 1982-07-22
Publication date: 1986-09-10
Also published as: EP0099425A1; DE3273187D1

Description

The invention relates to a method for producing a mesophase pitch derived carbon yarn, wherein the term "mesophase pitch" means a pitch containing at least about 40% by weight mesophase as the minimum level for which the pitch is capable of forming a continuous anisotropic phase when dispersed by agitation or the like, comprising the steps of:

forming a plurality of mesophase pitch fibers to define a mesophase pitch yarn,
thermosetting said mesophase pitch yarn to produce a thermoset yarn,
winding said thermoset yarn onto a bobbin and
subjecting said thermoset yarn to a predetermined heat treatment in an inert atmosphere to pyrolize and carbonize said thermoset yarn.

A method of this type is known from EP-A2-0 014 161. According to said prior art method the carbonizing of the thermoset fibers took place by winding said fibers on a bobbin or roller, transporting the roller to a carbonizing furnace and then unwinding the fibers from the roller and move them through the carbonizing furnace along a linear path, at a substantially constant speed and under a certain tension. Carbonizing accordingly took place with the thermoset yarn extending linearly and subjected to tension. The tension has been found in the prior art to be necessary to obtain good mechanical properties such as tensile strength and Young's modulus in the yarn and to avoid kinks and other surface defects of the fibers in the yarn.
The presence of surface defects in the carbon fibers of the carbon yarn adversely affect subsequent steps leading to the commercial use of the carbon yarn. For example, a typical manufacturing operation carried out on carbon yarn is the use of a finishing material on the carbon yarn. Surface defects and broken fibers in the carbon yarn tend to result in the carbon yarn retaining relatively large amounts of the finishing material. This makes it difficult to dry and wind the yarn onto a bobbin.
In addition, major commercial uses require the carbon yarn to be incorporated into a composite structure with a resin and the surface defects and broken fibers tend to cause the carbon yarn to retain excessive amounts of the resin used. This is undesirable.
Commercial economics have motivated efforts to increase the rate of production of carbon yarn. These efforts have resulted in attempts to reduce the time needed for pyrolysis by increasing the rate of movement of the thermoset yarn during the pyrolysis. It has now been found that when a certain speed is exceeded the occurrence of breaks in the fibers in the thermoset yarn is drastically increased. Thus, this represents a serious obstacle to high production rates according to the prior art methods.
A careful and extensive analysis of the possible causes of the breakage of thermoset yarns during pyrolysis has resulted in the discovery that the load-bearing capacity of thermoset mesophase pitch yarn reduces considerably during pyrolysis. In particular, it has been found that the threadline breaking strength for a thermoset mesophase pitch yarn declines from its value at room temperature to about one-fourth of this value as the temperature of the yarn in an inert atmosphere is raised until a temperature of from about 700°C to about 800°C is reached. At higher temperatures, the breaking strength of the yarn increases.
The discovery of this phenomena manifests a serious limitation on production rates according to prior art practices.
Non-prepublished experiments were carried out in an endeavor to avoid the aforementioned problem of raduced thread-line breaking strength at the elevated temperatures by collecting the thermoset mesophase pitch yarn onto a bobbin and subsequently pyrolyzing and carbonizing the pitch yarn on the bobbin. The carbon yarns obtained had broken fibers and surface defects and were not deemed satisfactory. Moreover, the bobbins used usually became damaged as a result of this heat treatment because of the force arising from the contraction of the thermoset yarn. Among the bobbins tested was a bobbin made of fine grain graphite. This was used in order to have the bobbin compatible with the yarn at the elevated temperatures.
Even a coreless package of the thermoset mesophase pitch yarn was used in the experiments in order to attempt to avoid problems which arose because of the use of a bobbin. Nevertheless, the coreless package produced unsatisfactory carbon yarn having surface defects.
Purpose of the invention is to provide a method avoiding the above-mentioned drawbacks.
According to the invention this purpose is achieved in that the winding of the thermoset yarn takes place onto a bobbin which is thermally and mechanically stable at the temperatures used to pyrolize and carbonize said yarn and which is chemically compatible with said thermoset yarn at stages of transition of said thermoset yarn and subjecting said thermoset yarn, while remaining on said bobbin to the said predetermined heat treatment to pyrolize and carbonize. Accordingly and contrary to the prior art method the yarn now is subjected to the pyrolyzing and carbonizing treatment while still present on the bobbin which bobbin, however, has to fulfill a number of requirements which effect is surprising taken into account prior unsuccessful experiments. With the invention not only the problems of the prior art are overcome but numerous surprising advantages are obtained as well.
The invention allows higher manufacturing rates than are possible according to prior art methods at lower costs and with fewer problems. Less effort by production workers is needed for the instant heat treatment and there is a more efficient use of energy for the invention as compared to the prior art methods.
Even more surprising is that the invention permits the production of exceptionally good carbon yarn as compared to the prior art methods, and the invention enables a good control on the mechanical properties of the carbon yarn.
Among the numerous advantages is the longer operating life for the thread-line furnace used for one embodiment of the instant invention and a more stable furnace operation.
As used herein, the therm "pitch" is a carbonaceous residue consisting of a complex mixture of primarily aromatic organic compounds derived from the thermal treatment of organic materials. Pitch is solid at room temperature and exhibits a broad melting or softening temperature and exhibits a broad melting or softening temperature range. When cooled from the melt, pitch becomes solidified without crystallization.
As used herein, the term "mesophase" is synonomous with liquid crystal; i.e. a state of matter which is intermediate between a crystal and an isotropic liquid. Ordinarily, material in this state exhibits both anisotropic and the liquid properties.
Pitches can contain varying amounts of mesophase. The mesophase regions in the pitch are recognized by the optical anisotropy in the liquid state and the anisotropy is maintained in the solid state.
Preferably, the invention is carried out using mesophase pitch having at least about 70% by weight mesophase.
The term "yarn" as used in the art describes a plurality of fibers. Generally, the number of fibers is at least about 1000 and usually about 2000. The number of fibers can be 5000 or more.
The terms "mesophase pitch yarn" and "pitch yarn" are used herein to identify the plurality of mesophase pitch fibers or "as-spun" fibers which define a yarn.
The term "thermoset yarn" is used herein to identify the pitch yarn which has been subjected to a thermosetting treatment.
The terms "pyrolyzed yarn" and "carbon yarn" are used herein respectively to identify the thermoset yarn which has been pyrolyzed and carbonized.
The term "winding angle" is used herein in connection with the operation of winding thermoset yarn onto a bobbin. In accordance with prior art usage this term refers to the angle defined by the portion of the yarn being wound onto the bobbin and a plane perpendicular to the axis of the bobbin.
The instant invention allows the production of mesophase pitch derived carbon yarn having an average Young's modulus of at least about 69x10⁶ kPa for the individual carbon fibers in the carbon yarn.
Generally, the bobbin can take the form of a cylinder or a cylinder having end faces. The bobbin comprises a body made of stainless steel, or a refractory alloy, or ceramic, or boron nitride, or preferably a graphite material. In addition, the bobbin features a layer of compressible resilient carbon material such as carbon felt around the cylindrical portion in order to absorb stresses arising from the expansion of the bobbin during the heat treatment and the contraction of the thermoset yarn during the pyrolyzing and carbonizing treatment.
Typically, the cylindrical portion of the bobbin can have an inside diameter of about 7,6 cm and an outside diameter of 8,9 cm with a length of about 27,9 cm.
The use of carbon felt for various commercial applications is well known and U.S. Patent No. 3,107,142 is a reference for such materials.
Preferably, the carbon felt has a thickness of from about 6,4 mm to about 12,7 mm thick.
The instant invention is particularly significant in connection with commercial production of mesophase pitch derived carbon yarn having about 2000 fibers. The handling of the yarn and the difficulties in maintaining acceptable qualities is demanding.
Generally, the tension on the thermoset yarn during the winding step is from about 7,35 Newtons to about 29,4 Newtons (75-300 g) and preferably from about 14,7 Newtons to about 19,6 Newtons (150-200 g).
The control over the amount of tension in the thermoset yarn as it is wound on the bobbin is not very important. If the tension is too low, the resulting loosely wound bobbin is difficult to handle in the manufacturing operations, and the fibers in the yarn do not develop the straightness required for good mechanical properties. If the tension is too high, the fibers near the core of the bobbin become distorted as well as other problems develop.
One of the embodiments of the invention teaches a heat treatment carried out by increasing the temperature by from about 50°C to about 100°C per hour until the temperature is about 1300°C for from about one to two hours. A similar thread-line heat treatment is known. This heat treatment will be referred to as in the art, as a "precarb" or "precarbonizing" although carbonizing actually takes place. The carbon yarn obtained from this treatment possesses many characteristics which make it suitable for a range of commercial uses. If, however, improved mechanical properties such as tensile strength and Young's modulus are desired, then a thread- line treatment as used in the prior art is carried out by unwinding the yarn and sending it through the thread-line treatment at a temperature of about 2500°C. Surprisingly, this thread-line treatment can be carried out using relatively high tension with very few of the fibers breaking.
The precarbonizing treatment may be followed by a thread-line treatment using generally higher tensions than the tension used according to the prior art methods. The use of higher tensions during the thread-line treatment generally results in carbon yarn having Young's modulus from 10% to 40% higher than the carbon yarn subjected to the same temperature treatment according to the prior art.
The use of a precarbonizing treatment has several other advantages. During this treatment, the thermoset yarn goes through well known stages of transition as gasses are driven out of the fibers. These gasses can be corrosive and highly reactive so that the heating units must be designed to resist these gasses. To some extent, the tendency for the gasses to be reactive at 1300°C is less than typically higher pyrolyzing and carbonizing temperatures such as 2500°C.
Thus, the two step pyrolysis and carbonizing operation allows the second heating unit to be less resistant to attack so that the second heating unit can be a less expensive unit and the unit used generally has a longer useful operating life.
Typically, mesophase pitch yarn is thermoset by subjecting it to a temperature of from about 200°C to about 400°C in air or some other oxidizing atmosphere. The winding of the thermoset yarn onto a bobbin can be carried out using a range of winding angles. It has been found that a relatively wide range of angles of from about 15° to about 30° can be used in connection with a bobbin having no end faces for the aforementioned precarb treatment which uses a maximum temperature of about 1300°C. A zero degree or parallel winding should be used with a bobbin having end faces in order to avoid having the yarn fall off the bobbin at the ends. The bobbin having parallel windings can be heat treated to about 3000°C to produce good quality yarn having a strength of more than about 2760x10³ kPa and a Young's modulus of greater than about 690x10⁶ kPa.
For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description, taken into connection with the accompanying drawings, in which:

Fig. 1 is a simplified block diagram of the commercial operations for producing carbon yarn;
Fig. 2 shows a block diagram of some of the steps of the instant invention;
Figs. 3A and 3B show two embodiments of bobbins used in the instant invention; and
Fig. 4 shows a simplified block diagram of the instant invention subsequent to the steps shown in Fig. 2.

In carrying the invention into effect, certain embodiments have been selected for illustration in the accompanying drawings and for description in the specification. Reference is had to Figs. 1 to 4.
Fig. 1 shows the principal steps in the commercial production of carbon yarn from mesophase pitch. Spinning apparatus 5 is used to spin 2000 mesophase pitch fibers with each fiber having a diameter of about 0,325 mm. The mesophase pitch fibers form a mesophase pitch yarn 6 which enters thermosetting unit 7. Thermoset yarn 8 is produced by the thermosetting unit 7 and is moved to pyrolyzing and carbonizing unit 9 for a heat treatment to produce carbon yarn 11 which is wound up on rolls in collection unit 12. Typically, winding and unwinding operations onto and off of a cardboard bobbin are carried out for the thermoset yarn 8 between the units 7 and 9.
Attempts to move the thermoset yarn 8 through the unit 9 at relatively high rates have resulted in load fluctuations on the thermoset yarn 8 and this in turn has produced poor quality carbon yarn 11.
Fig. 2 shows a spinning apparatus 13 which produces 2000 mesophase pitch fibers to define a mesophase pitch yarn 14. The mesophase pitch yarn 14 enters thermosetting unit 16 which produces thermoset yarn 17. Collecting unit 18 collects the thermoset yarn 17 onto a bobbin.
Figs. 3A and 3B show two embodiments of bobbins suitable for carrying out the instant invention. Bobbin 19 includes a body 21 and a carbon felt material 22 having a bias cut 23 wrapped around the body 21 and to provide a smooth and continuous joint. The carbon felt material 22 can be attached to the body 21 with an adhesive or even "masking" tape 24. The tape 24 at the high temperatures carbonizes and is only used to temporarily hold the carbon felt material 22 in place until the thermoset yarn is wrapped onto the bobbin 19.
Typically, the inside diameter of the body 21 is about 7,2 cm and the length of the body 21 is about 27,9 cm. The carbon felt material 22 has a thickness of about 6,35 mm.
Fig. 3B shows a bobbin 25 which can also be used in connection with the instant invention. Bobbin 25 differs from bobbin 19 in that it has end plates 26. The bobbin 25 allows a zero angle or parallel winding of thermoset yarn without encountering the problem of the yarn falling off at the ends of the bobbin 24.
In Fig. 4, a bobbin containing thermoset yarn is subjected to a heat treatment in pyrolyzing and carbonizing unit 27. For one embodiment of the invention no further heat treatment is carried out. Another embodiment has pyrolyzed yarn 28 subjected to a thread-line treatment at about 2400°C in carbonizing unit 29. This produces carbon yarn 30 which is moved to collecting unit 31 which winds the carbon yarn 30 onto another bobbin for storage and handling.
In the examples herein, the bobbins used were made from commercially available fine grain graphite.

Examples 1 to 8

A mesophase pitch yarn having 2000 pitch fibers each with a diameter of about 0,325 mm was produced and thermoset in accordance with conventional practices. The thermoset yarn was collected onto a bobbin made from fine grain graphite and having an inside diameter of about 7,6 cm, a length of about 27,9 cm, and a carbon felt layer about 6,35 mm thick. The bobbin had no end faces and the winding tension was about 14,7 Newtons (150 g).
A winding angle of about 20° was used. For each of the samples of the examples, the yarn collected was about 1828 cm in length and the pyrolyzing and carbonizing treatment was carried' out in a nitrogen atmosphere with the temperature being raised at the rate of about 50°C per hour until a temperature of 1300°C was reached and this temperature was held for about two hours.
The temperature was returned to room temperature and the pyrolyzed yarn was then moved through a threadline carbonizing unit which had an atmosphere of nitrogen and had a furnace temperature of about 2400°C in order to further carbonize the yarn. The average line tension in the carbonizing unit was about 78,4 Newtons (800 g).
The tensile strength and Young's modulus for each of the examples is given in Table 1.
The examples 1 to 8 resulted in carbon yarns which exhibited excellent mechanical properties and were visibly well collimated and substantially free of frays.

Example 9

A mesophase pitch yarn such as in Example 1 was made and wound onto a bobbin having end faces but otherwise similar to the bobbin used in Example 1. Parallel winding with a tension of about 19,6 Newtons (200 g) was used. The heat treatment rate was the same as in Example 1 except that the final temperature was about 3000°C. No thread-line treatment was used. The carbon yarn obtained had a tensile strength of about 2760x10³ kPa and a Young's modulus of greater than 690X106 kPa.

Claims

1. A method for producing a mesophase pitch derived carbon yarn, wherein the term "mesophase pitch" means a pitch containing at least about 40% by weight mesophase as the minimum level for which the pitch is capable of forming a continuous anisotropic phase when dispersed by agitation or the like, comprising the steps of:

forming a plurality of mesophase pitch fibers (6, 14) to define a mesophase pitch yarn,

thermosetting said mesophase pitch yarn to produce a thermoset yarn (8, 17).

winding said thermoset yarn onto a bobbin (21, 25) and

subjecting said thermoset yarn (8) to a predetermined heat treatment in an inert atmosphere to pyrolize and carbonize said thermoset yarn,

characterized in that the winding of the thermoset yarn (8, 17) takes place onto a bobbin (19, 25) which is thermally and mechanically stable at the temperatures used to pyrolize and carbonize said yarn (8, 17) and which is chemically compatible with said thermoset yarn (8, 17) at stages of transition of said thermoset yarn and subjecting said thermoset yarn (8, 17), while remaining on said bobbin (19, 25) to the said predetermined heat treatment to pyrolize and carbonize.

2. The method of claim 1, wherein the tension on said thermoset yarn (8, 17) during said winding is from about 7,35 Newtons to about 29,4 Newtons (75-300 g).

3. The method of claim 2, wherein the tension on said thermoset yarn (8,17) during said winding is from about 14,7 Newtons to about 19,6 Newtons (150-200 g).

4. The method of claim 2, wherein said heat treatment comprises raising the temperatures of said thermoset yarn (8,17) to about 1300°C.

5. The method of claim 2, wherein said heat treatment is carried out by increasing the temperatures by from about 50°C to about 100°C per hour until the temperature is about 1300°C and thereafter maintaining the temperature of about 1300°C for a predetermined period of time.

6. The method of claim 5, wherein the temperature of about 1300°C is maintained for from about one to about two hours.

7. The method of claim 1, further comprising unwinding the pyrolized yarn (28) and subjecting said pyrolized yarn (28) to a thread-line heat treatment in an inert atmosphere.

8. The method of claim 1, wherein said winding step is carried out with a winding angle of from about 15° to about 30°.

9. The method of claim 1, wherein said winding step is carried out with approximately parallel winding.

10. The method of claim 1, wherein said mesophase pitch yarn (6,14) comprises at least about 1000 mesophase pitch fibers.

11. The method of claim 1, wherein said mesophase pitch yarn (6,14) comprises at least about 2000 mesophase pitch fibers.

12. The method of claim 1, wherein said mesophase pitch yarn (6,14) comprises about 2000 mesophase pitch fibers; the tension on said thermoset yarn during said winding is from about 7,35 Newtons to about 29,4 Newtons (75-300 g); said winding step is carried out with a winding angle of from about 15° to about 30°; and said heat treatment is carried out by increasing the temperature by from about 50°C to about 100°C per hour until the temperature is about 1300°C and thereafter maintaining the temperature of about 1300°C for from about one to about two hours.