US3449077A - Direct production of graphite fibers - Google Patents

Description

United States Patent 3,449,077 DIRECT PRODUCTION OF GRAPHITE FIBERS Dagobert E. Stuetz, Westfield, N.J., assignor to Celanese Corporation, New York, N .Y., a corporation of Delaware No Drawing. Filed Feb. 13, 1967, Ser. No. 615,374 Int. Cl. C01b 31/07 US. Cl. 23-2091 12 Claims ABSTRACT OF THE DISCLOSURE The continuous preparation of uniformly high modulus graphite fibers from preoxidized polybenzimidazole fibers by controlled passage of said-fibers through certain fuel/ oxidant flames.

In mans search for high performance materials, considerable interest has focused on graphite fibers. Graphite fibers can be defined as fibers which consist essentially of atomic carbon and which have an X-ray difl'raction pattern characteristic of graphite. Carbon fibers, on the other hand, can be defined as fibers in which the bulk of the fiber weight can be ascribed to atomic carbon and which show an amphorous X-ray diffraction pattern. Graphite yarns generally have a much higher modulus and higher tenacity than carbon fibers and in addition are electrically and thermally conductive.

Industrial high performance materials of the future will most likely involve reinforced composites, and graphite fibers theoretically have among the best properties of any fiber, including boron, for high strength reinforcement. Among these desirable properties are high corrosion and temperature resistance, low density, high tensile strength and most important, high modulus. Graphite is one of the very few known materials whose tensile strength increases with temperature. Uses for such graphite-reinforced composites include aerospace structural components, rocket motor casings, deep-submergence vessels and ablative materials for heat shields on re-entry vehicles.

One of the major factors retarding the large-scale use of graphite-reinforced composites is the extreme production costs of uniform high modulus graphite fibers. Although production of fibrous carbon by pyrolysis of hydrocarbon gases or by discharge between carbon electrodes has been reported, these methods are not suitable for industrial applications requiring good quality control. Graphitization of organic fiber precursors appears to be the only practical industrial route to graphite fibers.

Most of the prior art methods for producing graphite fibers involve long processing periods, high power requirements and/ or expensive and bulky heating apparatus such as closed furnaces. The processing and equipment costs often dwarf the fiber raw material costs. Often the fiber is of inferior quality due to damage in one or more steps of the treatment. For example, carbon yarn can be converted to graphite yarn by furnace graphitization in a high temperature vacuum oven. The hot zone material is generally a metal such as tungsten or tantalum. Besides being expensive and slow, this radiant heat method also may result in the deposition of foreign materials such as tungsten, tantalum and/ or carbides of these metals on the fiber during the high temperature treatment.

Another prior art approach to graphitizing carbon fiber, called conductive graphitization, involves passing the fiber over electrified rollers. For example, in one such method the carbonized fiber is advanced over a pair of spaced electrical rollers while supplying an electric current through the advancing fiber to raise it to graphitization temperature. A controlled atmosphere of hydrogen, carbon monoxide, ammonia or mixtures thereof generally must be provided around the fiber during passage from one roller to the other. Additionally, equipment must be ice is a high incidence of inhomogeneity along the finished yarn. Many of the fibers have low tensile strength and generally there is a wide distribution of individual break values. Apparently the combination of frictional contact of the yarn on the rolls during conductive graphitization and the arcing causes marked wear on the yarn such as to produce many flaws.

Another process for producing graphite fibers is disclosed in copending application Ser. No. 614,811 filed Feb. 9, 1967. According to said process suitable carbon fibers are passed through certain fuel/oxidant flames. Carbon fibers utilizable in such a process are characterized by (1) at least and preferably at least of its total weight attributable to atomic carbon, (2) a relatively amorphous X-ray diffraction pattern, and (3) structural integrity for at least two seconds at a fiber temperature of 1,900" C. Such suitable carbon yarns are in turn generally prepared by the controlled preoxidation of an organic fiber followed by a controlled pyrolysis. Such preoxidizing and carbonizing procedures are disclosed, for example, in U .5. Patents Nos. 3,053,775, 3,107,152, 3,116,975, and 3,235,323; French Patent NO. 1,430,803; Japanese Patent No. 12,615/63 and British Patent No. 1,034,542. A wide variety of organic fiber precursors of diverse chemical nature can be successfully employed for this purpose.

Graphite fibers produced in this manner have a relatively higher average tensile strength and a much narrower distribution of individual break values than is obtainable by conductive graphitization. This improvement is apparently due to two factors-the flame healing of flaws by ionized carbon fragments in a luminous flame and the reduced mechanical wear of the yarn in the flame is contrasted to the frictional contact of the rolls during conductive graphitization.

While the process of the aforementioned copending application itself is continuous, rapid, simple, and inexpensive and does not require bulky or expensive equipment, the preparation of the carbon fiber starting material itself is laborious and expensive. The pyrolysis procedures are generally slow and batch-wise. The necessary heating elements are bulky and expensive.

It is an object of this invention to continuously produce graphite fibers having uniformly good tensile properties without the need for a slow and expensive pyrolysis step.

I now have discovered that graphite fibers having the aforementioned desirable properties can be prepared directly from polybenzimidazole fibers preoxidized under controlled conditions. According to the method of this invention these preoxidized polybenzimidazole fibers are directly flame graphitized, i.e.. subjected to the flame graphitization step described in the aforesaid copending application. 'If preoxidized fibers other than polybenzimidazoles, such as cellulosics, are directly subjected to flame graphitization conditions they lost their structural integrity, i.e. they burn. Thus, by employing preoxidized polybenzimidazole fibers, graphite yarns can be directly, rapidly, easily and cheaply produced.

Polybenzimidazoles are a known class of heterocyclic polymers. They are prepared and described in Patent Nos. 2,895,948 and 3,174,947, for example. An espesially interesting subclass of polybenzimidazoles for fiber production consists of recurring units of the formula:

wherein R is an aromatic nucleus having each of the two depicted pairs of nitrogen atoms substituted upon adjacent carbon atoms of the said aromatic nucleus and R is a carbocyclic aromatic or alicyclic ring, an alkylene group, or a heterocyclic ring. Examples of such heterocyclic rings include pyridine, pyrazi'ne, furan, quinoline, thiophene and pyran. Preferred R groups are 3,3, 4,4-

bisphenylene and 1,2,4,5-phenyleneand wherein R" is wherein Z is an aromatic nucleus having the two depicted nitrogen atoms substituted on adjacent carbon atoms of said aromatic nucleus. Examplary of such polybenzimidazoles is poly-2,5 (6) -benzimidazole.

The most important polybenzimidazole commercially is poly 2,2 n1 phenylene 5,5 bibenzimidazole which consists of recurring units of the formula:

This species is commonly referred to as simply PB-I. A preparation of FBI is described in Example 11 of Patent No. 3,174,947. The fiber can be dry spun from dimethylacetamide, for example, in a manner known to the art.

The preoxidation step is conveniently carried out by heating the yarn in air at about 400500 C., preferably 430500 C., for about 2 to 15 minutes and preferably for 3 to 9 minutes. At the higher temperatures of this range a shorter exposure time can be employed. The preoxidation can also be effected chemically by the use of reagents such as nitric acid and potassium dichromate.

Undrawn, drawn and double drawn polybenzimidazole fibers can be preoxidized and flame graphitized according to the manner of this invention.

The nature of the reducing flame is not critical for operability but specific types of flame result in better tensile properties and/or greater ease of operation.

A highly preferred flame is that resulting from a acetylene and oxygen mixture. With this flame, the graphitizing step can be conducted in open atmosphere. A further advantage of the acetylene-oxygen flame is that it has a fairly constant high temperature which is independent, within limits, of the fuel/oxygen ratio. A carbon monoxide-oxygen flame also provides good results in an open atmosphere, although this of course requires safety provision for the operator. Hydrocarbon fuels such as propane and butane are operable but the process does proceed as smoothly as with carbon monoxide or acetylene. In the presence of an inert blanketing gas in the processing chamber, comparable stability is achieved with hydrocarbon fuels.

Molecular oxygen can be replaced in the combustion mixture by a gaseous oxidant such as nitrous oxide although generally it is not advantageous to do so since oxygen is so convenient. Fuel/oxidant combinations can be employed which do not contain a hydrocarbon such as a carbon monoxide-hydrogen mixture and a hydrogenchlorine mixture. Non-conventional flame sources such as augmented flames (cf. B. Karlovitz, International Science of Technology, June 1962, pp. 3641) and recombination flames such as the atomic hydrogen torch (cf. I. Langmuir, Industrial & Engineering Chemistry, June 1927, pp. 667-674), plasma torches and the like can also be employed to provide high temperatures. The temperature should not be so high, however, as to destroy the fiber.

In the context of this specification, temperatures in the flame zone refer to the temperature of the yarn as measured by an infra-red radiation thermometer and not to theoretical temperature under adiabatic conditions, i.e. without withdrawal of heat by immersing a body into the flame. The yarn temperature in the flame is generally significantly lower than the theoretical flame temperature. For example, the theoretical flame temperature of an oxyacetylene flame is about 3,100 C. An upper limit of about 2,500 C. for the yarn temperature is generally sufficient and safe.

The fuel to oxidant ratio generally is a significant parameter. The graphitizing tereatment is best carried out in a luminous flame obtained by keeping the amount of oxygen in the fuel mixture below the stoichiometric amount which is required to burn the fuel completely by oxidation if the oxidant-fuel ratio is too high. The luminosity of the flame is believed to be caused by ionized carbon fragments in the flame due to incomplete combustion. More pyrolytic carbon will be deposited at higher oxygen fuel ratios than at lower ratios. For certain applications a deposit of pyrolytic graphite is desirable since it increases the high temperature stability of the yarn; for others it is undesirable, e.g., where good adhesion to a matrix is desired. Hence, the flexibility of processing conditions allows the production of a variety of graphite yarn types. A surface protective layer of this kind can be formed in a separate step in which the yarn is heated to high temperatures in a controlled environment containing hydrocarbon vapors.

The volume flow of the fuel and oxidant through the burner should be as high as possible, consistant with good flame stability, in order to maximize the moduli of the fibers.

The carbon yarn must be passed through the flame at a fast enough rate to avoid breaking. As the flame temperature is increased, the minimum rate at which breakage is avoided also increases. This minimum speed can be determined for any given combination of yarn and flame. The longer the residence time, the greater the extent of graphitization. Optimum conditions are reached at the point where loss of fiber mass by burn-off is lowest and conversion of the remainder of graphite is highest. The two effects can be balanced favorably by adjusting resi deuce time and yarn temperature. Subject to the nature of the flame and other factors, residence times of 2 to 24 seconds, and preferably 6 to 17 seconds are generally suitable. An exemplary set of optimum conditions is a yarn temperature of 2,300 C. with a residence time of about 15 seconds.

Tension during flame treatment is important in achieving optimum yarn properties as it prevents the tendency of the yarn to shrink. Shrinkage usually leads to relaxation of ordered structures and, thereby, causes lowering of physical properties. Preservation of orientation and/or increase of orientation, depending on the magnitude of tension applied, increases both Youngs modulus and tensile strength. The tension applied should be at least sufiicient to avoid visible sagging. Beyond the optimum tension the fiber may be damaged by still higher tensions. The tension can be adjusted to a level where the denier size of the yarn is preserved or even slightly reduced by drawing. Tensions should be about 25 to 600 grams per 1,800 denier of precursor yarn and preferably between 150 and 300 grams. The amount of desirable tension depends on whether the precursor yarn is undrawn, single-drawn or double-drawn and increases generally in this direction.

Treatment of the polybenzimidazole yarn priorto flame passage with an aqueous boric acid solution (e.g. 20%) may result in a slight increase in the modulus of the graphite yarn as compared to non-treated yarns. Other flameproofing materials which can be optionally employed include silicone oil (DOW 700), antimony salts and the common bromine-and chlorine-containing flameproofiing compounds.

The necessary apparatus for flame graphitization is simple and should be so arranged that the yarn is exposed to a minimum of frictional contacts. A convenient setup is to feed the preoxidized polybenzimidazole yarn from a rotating-reciprocating bobbin through the flame to an identically functioning take-up mechanism. Starting at the correct reciprocating position on the bobbin, the yarn is unwound without traverse movement and analogously rewoun d at the take-up side. Hence, random yarn motions I are minor. Further positioning of the yarn in the flame can be accomplished with minimal action by two cylindrically shaped guides located before and after the bumers. Feed and take-up bobbins can be driven by, for example, solid-state controlled D.C. motors with r.p.m. generator/indicators. When an inert atmosphere is desired, a cruciform glass vessel fitted with cooling plates and passage opening for the yarn can be employed. Before entering the burner module, the yarn is put under constant tension as, for example, by passing it over a rubber-capped electro-magnetic clutch and a skewed roll. The latter separates individual yarn loops around the clutch and prevents abrasion by yarn to yarn contact.

The geometry of the burner is also a factor in maximizing the eflectiveness of the flame graphitization of this invention. Two impinging flames originating from two standard conical tips significantly raise the temperature of the yarn passing therethrough. A particularly preferred embodiment of the latter method is the impingement of the two flames on the tips of their inner cones at an angle of forty-fiive degrees. However the addition of more than two orifices does not have a beneficial eflect. A series of burners such as to form a continuous flame zone of increased lateral dimension may permit higher processing speeds. Since residence time in the flame is a major parameter, processing speed is significantly related to the length of the flame zone. Surrounding the conical tip with a cylindrical or globular reflector, constructed from" polished stainless steel sheets, for example, also significantly raises the yarn temperature.

The following tables illustrate the tensile properties of graphite fibers obtainable by the method of this invention as a function of (1) the type of PBI yarn employed, including type of preoxidizing and drawing, (2) the residence time in the flame, (3) the tension on the yarn during passage through the :flame, (4) the absolute and relative flow rates of the acetylene and oxygen and (5) the type of burner employed.

The yarn types designated in the tables are coded on the basis of the nature of their precursor PBI yarn and the preoxidization conditions as follows:

APrecursor 1,800 denier/450 filament, preoxidized at 485 C. for 5.0 minutes.

A Same as A, but impregnated with a 5% aqueous solution of boric acid and dried.

A Same as A, but coated with silicon oil (Dow Corning A Same as A, but preoxidized under gram ten- A -Same as A, but preoxidized under 200 gram ten- BPrecursor 1,800 denier/450 filament, preoxidized at 485 C. for 4.5 minutes.

B --Same as B, but preoxidized under 450 gram tension, at a profile of 435-485 C. (3-zone).

B Same as B but under 650 gram tension.

C-Precursor 1,800 denier/450 filament, preoxidized at 485 C. for 3.75 minutes.

BPrecursor 2,000 denier/600 filament, preoxidized at 485 C. for 3.75 minutes.

BPrecursor undrawn, 3,800 denier/450 filament, preoxidized at 485 C. for 5.0 minutes.

F-Precursor double drawn, 1,530 denier/450 filament,

preoxidized at 485 C. for 5.0 minutes.

HPrecursor double drawn, 1,530 denier/450 filament,

preoxidized at 485 C. for 6.0 minutes.

IPrecursor double drawn, 1,700 denier/500 filament,

preoxidized at 485 C. for 6.0 minutes.

K-Precursor double drawn, 1,700 denier/500 filament,

preoxidized at 485 C. for 5.0 minutes.

The FBI precursor itself has tensile properties as follows:

TABLE I Tensile strength Initial modulus Yarn type (10 p.s.i.) (10 p.s.i.)

Undrawn 33 0. 4-0. 8 Single Drawn- 87 1. 9 Double Drawn 2. 3

In the above code, the FBI precursor yarn is single drawn unless otherwise noted.

In the runs in Table II, the standard flame employed resulted from a combination of acetylene flowing at a rate of 1,150 mL/minute and oxygen flowing at the rate of 750 ml./minute. The burner had a standard conical tip, 0.050 inch in internal diameter.

Table III gives further data on the embodiment of this invention wherein two standard conical tips of the types used in Example I are employed in forming the flame zone. This listed flow rate represents the total flow rate from both burners.

TABLE III Time in Acetylene/ Tensile Initial Tension Flame 0; ratio strength modulus (grams) (sec) (ml/min.) (10 p.s.i.) (10 p.s.i.)

TABLE IV Tensile Initial Tension Time in strength modulus Yarn type (grams) flame (see) (10 psi.) (10 psi.)

B 200 6 155 25. 300 6 176 28. 0 400 6 208 28. 6 13m 200 6 176 29. 0 300 12 156 30. 0 E 200 3 142 17. 6 F 200 3 176 19. 0 300 3 120 18. 4 H 300 3 144 19. 0 450 3 110 21. 0 400 12 128 23. 2 600 12 146 24. 6 I 500 3 170 22. 0 K 400 3 150 22. 0

Tables II through IV illustrate the improved tensile properties achievable by the method of this invention.

Although the above discussion has focused on the use of continuous yarns, staple yarns may also be used. They will generally give correspondingly lower tensile properties than will continuous yarns.

Specific embodiments of the invention can be optimized for desired properties and/or processing parameters by simple experimentation and numerous relationships with other variables will become apparent. For example, other factors constant, the finer the denier the higher the modulus.

By the method of this invention one can form even substantial packages of graphite yarn, i.e. one containing more than 10 grams, in which all portions of the yarn have (1) an X-ray diffraction pattern characteristic of graphite, and (2) an average deviation from both the Youngs modulus value and the average tenacity value of less than -5%. Ten grams of the graphite yarn of this invention is of the order of magnitude of 100 feet in length. Such a package of uniform graphite yarn is particularlysuited in those applications where property uniformity and reliability are necessary.

Numerous other variants of the above process and product will be apparent to one skilled in the art within the spirit of this invention.

What is claimed is:

1. A rapid process for the production of uniform graphite fibers comprising the steps of preoxidizing a polybenzimidazole fiber and passing said p-reoxidized fiber through a reducing flame imparting to the yarn a minimum temperature of at least 1,900 C. 'at a speed sufiicient, to avoid breaking and under a tension at least sufiicient to avoid visible sagging.

2. A process according to claim 1 wherein said flame is generated by a fuel'oxidant mixture.

3. A process according to claim 2 wherein said oxidant is oxygen.

4. A process according to claim 3 wherein said fuel is acetylene.

5. A process according to claim 3 wherein said fuel is propane.

6. A process according to claim 1 wherein said polybenzimidazole is poly-2,2-m-phenylene-5,5'-bibenzimida zole.

7. A process according to claim 1 wherein said preoxidizing step is conducted by heating in air at about 400550 C. for about 2 to 15 minutes.

8. A process according to claim 1 wherein said preoxidizing step is conducted by heating in air at 430500 C. for 3 to 9 minutes.

9. A process according to claim 2 wherein the residence time of the fiber in the flame is from 2 to 24 seconds.

10. A process according to claim 3 wherein the ratio of oxygen and fuel is such that the amount of oxygen is below the stoichiometric amount required to completely oxidize the fuel.

11. A process according to claim 5 wherein an inert atmosphere is provided around said flame.

12. A process according to claim 8 wherein the fuel is acetylene and the oxidant is oxygen and the fiber is passed through the luminous portion of the flame for a residence time of 6 to 17 seconds.

References Cited UNITED STATES PATENTS 3,011,981 12/1961 SOltes 252-502 3,107,152 10/1963 Ford et a1. 23209.2 3,174,947 3/1965 Marvel et a1 26047 3,285,696 11/1966 Tsunoda 23209.1 3,304,148 2/1967 Gallagher 23209.2 X 3,313,597 4/1967 Cranch et al. 232093 EDWARD J. MEROS, Primary Examiner.

U.S. Cl. X.R.