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Title:
GRAPHITIZATION PROCESS
United States Patent 3656904
Abstract:
An improved process is provided for the production of a continuous length of a graphitic fibrous material through the catalysis of the graphitization reaction. While a continuous length of fibrous material capable of undergoing graphitization is continuously passed through a heating zone provided with an inert gaseous atmosphere having a maximum temperature of at least 2,000° C., at least one gaseous stream containing a catalytic quantity of a volatile alkyl borate in vapor form which is capable of catalyzing the graphitization reaction is introduced into the heating zone.


Inventors:
RAM MICHAEL J
Application Number:
05/045160
Publication Date:
04/18/1972
Filing Date:
06/10/1970
Assignee:
Celanese Corporation (New York, NY)
Primary Class:
Other Classes:
252/502, 423/447.4
International Classes:
D01F9/14; D01F9/22; D01F9/32; D01F11/12; (IPC1-7): C01B31/07; C01B31/04
Field of Search:
23/209
View Patent Images:
US Patent References:
3552923N/A1971-01-05Carpenter et al.
3449007FOLDING GAFF1969-06-10Stuetz
3294489Process for preparing carbon fibers1966-12-27Millington et al.
2949430Process for the protection of carbon electrodes for electric furnaces1960-08-16Jorgensen
Foreign References:
GB1130304A1968-10-16
Other References:

Allen et al. "Nature" vol. 224 Nov. 15, 1969 pages 684-685 .
Klein "Chemistry and Physics of Carbon" vol. 2, 1966, pages 225-227.
Primary Examiner:
Meros, Edward J.
Claims:
I claim

1. In a process for the graphitization of a continuous length of a fibrous material capable of undergoing graphitization comprising continuously passing said continuous length of fibrous material through a heating zone containing an inert gaseous atmosphere having a maximum temperature of at least 2,000° C. until substantial graphitization occurs while preserving the original fibrous configuration essentially intact; the improvement of introducing into said heating zone at least one gaseous stream containing a catalytic quantity of a volatile alkyl borate in vapor form capable of catalyzing said graphitization of said fibrous material within said heating zone.

2. A process according to claim 1 wherein said continuous length of fibrous material capable of undergoing graphitization is a carbonaceous fibrous material containing at least about 90 per cent carbon by weight and having an essentially amorphous X-ray diffraction pattern.

3. A process according to claim 1 wherein said continuous length of fibrous material is a continuous multifilament yarn.

4. A process according to claim 1 wherein said inert gaseous atmosphere is selected from the group consisting of nitrogen, argon, and helium.

5. A process according to claim 1 wherein said heating zone contains an inert gaseous atmosphere having a maximum temperature of about 2,400° to 3,100° C.

6. A process according to claim 1 wherein said volatile alkyl borate is an alkyl borate of the formula B(OR)3 where R is an alkyl group having one to five carbon atoms.

7. A process according to claim 1 wherein said volatile alkyl borate is trimethyl borate.

8. A process according to claim 1 wherein said volatile alkyl borate in vapor form is introduced into said inert gaseous atmosphere surrounding said continuous length of fibrous material in a concentration of about 200 to 20,000 parts per million prior to heating said fibrous material above about 500° C. in said heating zone.

9. A process according to claim 7 wherein said trimethyl borate in vapor form is introduced into said inert gaseous atmosphere surrounding said continuous length of fibrous material in a concentration of about 200 to 2,000 parts per million.

10. In a process for converting a stabilized acrylic fibrous material which is non-burning when subjected to an ordinary match flame and derived from an acrylic fibrous material selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers which contain at least about 85 mol per cent of acrylonitrile units and up to about 15 mol per cent of one or more monovinyl units copolymerized therewith to a graphitic fibrous material while preserving the original fibrous configuration essentially intact comprising continuously passing a continuous length of said fibrous material through a heating zone containing an inert gaseous atmosphere and a temperature gradient in which said fibrous material is initially carbonized, and in which said carbonized fibrous material is heated to a maximum temperature of at least 2,000° C. until substantial graphitization occurs; the improvement of introducing into said heating zone at least one gaseous stream containing a catalytic quantity of volatile alkyl borate in vapor form capable of catalyzing said graphitization of said fibrous material within said heating zone.

11. A process according to claim 10 wherein said stabilized acrylic fibrous material exhibits a bound oxygen content of at least about 7 per cent by weight.

12. A process according to claim 10 wherein said stabilized acrylic fibrous material is derived from an acrylonitrile homopolymer.

13. A process according to claim 10 wherein said stabilized acrylic fibrous material is derived from an acrylonitrile copolymer which contains at least about 95 mol per cent of acrylonitrile units and up to about 5 mol per cent of one or more monovinyl units copolymerized therewith.

14. A process according to claim 10 wherein said continuous length of stabilized acrylic fibrous material is a continuous multifilament yarn.

15. A process according to claim 10 wherein said inert gaseous atmosphere is selected from the group consisting of nitrogen, argon, and helium.

16. A process according to claim 10 wherein said carbonized fibrous material is heated in said inert gaseous atmosphere to a maximum temperature of about 2,400° to 3,100° C. until substantial graphitization occurs.

17. A process according to claim 10 wherein said volatile alkyl borate is an alkyl borate of the formula B (OR)3 where R is an alkyl group having one to five carbon atoms.

18. A process according to claim 10 wherein said volatile alkyl borate is trimethyl borate.

19. A process according to claim 10 wherein said volatile alkyl borate in vapor form is introduced into said inert gaseous atmosphere surrounding said continuous length of fibrous material in a concentration of about 200 to 20,000 parts per million, and in a quantity of about 0.01 to 4 per cent by weight based upon the weight of the stabilized acrylic fibrous material introduced into said heating zone.

20. A process according to claim 19 wherein said volatile alkyl borate is trimethyl borate and said trimethyl borate is introduced into said inert gaseous atmosphere surrounding said continuous length of stabilized acrylic fibrous material in a concentration of about 200 to 2,000 parts per million prior to heating said fibrous material above about 500° C. in said heating zone.

21. In a process for converting a stabilized acrylic fibrous material which is non-burning when subjected to an ordinary match flame and derived from an acrylic fibrous material selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers which contain at least about 85 mol per cent of acrylonitrile units and up to about 15 mol per cent of one or more monovinyl units copolymerized therewith to a graphitic fibrous material while preserving the original fibrous configuration essentially intact comprising continuously passing a continuous length of said fibrous material through a heating zone containing an inert gaseous atmosphere and a temperature gradient in which said fibrous material is raised within a period of about 20 to about 300 seconds from about 800° C. to a temperature of about 1,600° C. to form a continuous length of carbonized fibrous material, and in which said carbonized fibrous material is subsequently raised from about 1,600° C. to a maximum temperature of at least about 2,400° C. within a period of about 3 to 300 seconds where it is maintained for about 10 seconds to about 200 seconds to form a continuous length of graphitic fibrous material; the improvement of introducing into said heating zone at least one gaseous stream containing a catalytic quantity of a volatile alkyl borate in vapor form capable of catalyzing said graphitization of said fibrous material within said heating zone.

22. A process according to claim 21 wherein said stabilized acrylic fibrous material exhibits a bound oxygen content of at least about 7 per cent by weight.

23. A process according to claim 21 wherein said stabilized acrylic fibrous material is derived from an acrylonitrile homopolymer.

24. A process according to claim 21 wherein said stabilized acrylic fibrous material is derived from an acrylonitrile copolymer which contains at least about 95 mol per cent of acrylonitrile units and up to about 5 mol per cent of one or more monovinyl units copolymerized therewith.

25. A process according to claim 21 wherein said continuous length of stabilized acrylic fibrous material is a continuous multifilament yarn.

26. A process according to claim 21 wherein said inert gaseous atmosphere is selected from the group consisting of nitrogen, argon, and helium.

27. A process according to claim 21 wherein said carbonized fibrous material is heated in said inert gaseous atmosphere to a maximum temperature of about 2,400 to 3,100° C. to form a continuous length of graphitic fibrous material.

28. A process according to claim 21 wherein said volatile alkyl borate is an alkyl borate of formula B(OR)3 where R is an alkyl group having one to five carbon atoms.

29. A process according to claim 21 wherein said volatile alkyl borate is trimethyl borate.

30. A process according to claim 21 wherein said volatile alkyl borate in vapor form is introduced into said inert gaseous atmosphere surrounding said continuous length of fibrous material in a concentration of about 200 to 20,000 parts per million, and in a quantity of about 0.04 to 4 per cent by weight based upon the weight of the stabilized acrylic fibrous material introduced into said heating zone.

31. A process according to claim 30 wherein said volatile alkyl borate is trimethyl borate and said trimethyl borate is introduced into said inert gaseous atmosphere surrounding said continuous length of stabilized acrylic fibrous material in a concentration of about 200 to 2,000 parts per million prior to heating said fibrous material above about 500° C. in said heating zone.

Description:
BACKGROUND OF THE INVENTION

Processes involving the boron catalyzed conversion of amorphous carbon to graphitic carbon have long been known. For instance, boron compounds such as boric acid have been incorporated in a mass of graphitizable carbon and the same baked to form massive graphite structures, such as graphite electrodes. Also, the pyrolytic codeposition of boron and carbon to form boron-containing pyrocarbons and boron pyrolitic graphite has been disclosed.

In the search for high performance materials, considerable interest has been focused upon graphitic fibrous materials. Graphite fibers are defined herein as fibers which consist essentially of carbon and have a predominant X-ray diffraction pattern characteristic of graphite. Amorphous carbon fibers or carbonized fibers, on the other hand, are defined as fibers capable of undergoing graphitization in which the bulk of the fiber weight can be attributed to carbon and which exhibit an essentially amorphous X-ray diffraction pattern. Graphite fibers generally have a much higher modulus and a higher tenacity than do amorphous carbon fibers and in addition are more highly electrically and thermally conductive.

Industrial high performance materials of the future are projected to make substantial utilization of fiber reinforced composites, and graphite fibers theoretically have among the best properties of any fiber for use as high strength reinforcement. Among these desirable properties are corrosion and high 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 fiber 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 fiber reinforced composites may be traced to the extreme costs commonly required for the production of high modulus graphite fibers suitable for use as reinforcement. Although the production of fibrous carbon by pyrolysis of hydrocarbon gases has been reported, this technique is generally not suitable for industrial applications requiring good quality control. Graphitization of amorphous carbon fibers derived from fibrous organic precursors appears to be the only practical industrial route available to form graphite fibers.

Many 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. For instance, when graphite tube furnaces are utilized the graphite tubes are of limited life and must be periodically replaced. The processing and equipment costs required to produce graphite fibers commonly dwarf the fiber raw material costs.

Amorphous carbon fibers have been graphitized in the past by heating for extended periods of time, e.g., several hours, while present in a boron doped crucible. In an effort to expedite the graphitization of a fibrous material, a technique has heretofore been proposed in which the fibrous material undergoing graphitization is first soaked in an aqueous solution of boric acid, washed with water, and dried prior to graphitization. While such a technique has proven to be effective in catalyzing the graphitization of the fibrous material, it has proven to be unduly time consuming. For instance, it has proven to be essential to wash the fiber following soaking in the solution of boric acid so that an excessive quantity of the boron compound is not deposited upon the surface of the fibrous precursor upon drying. It is essential also that the fibrous material be dried prior to heating at highly elevated temperatures in order to assure the attainment of adequate physical properties in the fibrous graphite product.

In commonly assigned U.S. Ser. No. 45,161, filed on June 10, 1970 in the name of Michael J. Ram, is disclosed a graphitization process which overcomes many of the disadvantages associated with prior art attempts at the utilization of boron catalysis in the formation of graphite fibers. In such a process a continuous length of fibrous material capable of undergoing graphitization is continuously passed through a heating zone containing an inert gaseous atmosphere having a maximum temperature of at least about 2,000° C. bounded by walls of graphitic carbon in intimate association with a boron compound capable of undergoing volatilization at a temperature below about 2,000° C. thereby enabling the volatilization of a catalytic quantity of boron capable of catalyzing graphitization within the heating zone.

It is an object of the invention to provide an improved graphitization process for the production of graphitic fibrous materials.

It is an object of the invention to provide an improved graphitization process which is capable of producing graphitic fibrous materials of superior tensile properties.

It is an object of the invention to provide an improved graphitization process which is capable of producing graphitic fibrous materials without sacrifice in tensile properties while reducing the maximum graphitization temperature as well as the accompanying power requirements.

It is an object of the invention to provide an improved graphitization process which is capable of producing graphitic fibrous materials while operating under conditions wherein the life of the apparatus utilized to carry out the process is substantially lengthened.

It is another object of the invention to provide a catalyzed graphitization process in which the quantity of catalytic compound introduced into the heating zone may be readily controlled and adjusted with precision.

It is a further object of the invention to provide an improved graphitization process for the production of graphitic fibrous materials which is expeditiously carried out on a continuous basis.

These and other objects as well as the scope, nature, and utilization of the invention will be apparent from the following description and appended claims.

SUMMARY OF THE INVENTION

It has been found that in a process for the graphitization of a continuous length of a fibrous material capable of undergoing graphitization comprising continuously passing the continuous length of fibrous material through a heating zone containing an inert gaseous atmosphere having a maximum temperature of at least 2,000° C. until substantial graphitization occurs while preserving the original fibrous configuration essentially intact, that improved results are achieved by introducing into the heating zone at least one gaseous stream containing a catalytic quantity of a volatile alkyl borate in vapor form capable of catalyzing the graphitization of the fibrous material within the heating zone.

In a preferred embodiment of the process the precursor is a stabilized acrylic fibrous material and the heating zone is provided with a temperature gradient in which both carbonization and graphitization are carried out.

DESCRIPTION OF THE DRAWING

The drawing is a schematic illustration of a representative apparatus arrangement capable of carrying out the process of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The continuous length of fibrous material utilized in the process of the present invention is capable of undergoing graphitization while retaining or preserving its original fibrous configuration essentially intact. The fibrous material undergoing graphitization in the process may be formed by conventional techniques and may be provided in a variety of physical configurations. For instance, the fibrous material may assume the configuration of a continuous length of a multifilament yarn, tape, tow, strand, cable, or similar fibrous assemblage. In a preferred embodiment of the invention the fibrous material is a continuous multifilament yarn.

The fibrous material which is treated in the present process optionally may be provided with a twist which tends to improve the handling characteristics. For instance, a twist of about 0.1 to 5 tpi, and preferably about 0.3 to 1.0 tpi may be imparted to a multifilament yarn. Also, a false twist may be used instead of or in addition to a real twist. Alternatively, one may select continuous bundles of fibrous material which possess essentially no twist.

The fibrous material which is graphitized in accordance with the present process may be carbonaceous, contain at least about 90 per cent carbon by weight, and exhibit an essentially amorphous X-ray diffraction pattern. As is known in the art, amorphous carbon fibrous materials suitable for graphitization may be formed by a variety of techniques. For instance, organic polymeric fibrous materials which are capable of undergoing thermal stabilization may be initially stabilized by treatment in an appropriate atmosphere at a moderate temperature (e.g., 200° to 400° C.), and subsequently heated in an inert atmosphere at a more highly elevated temperature, e.g., 900° to 1,000° C., or more, until a carbonized fibrous material is formed which exhibits an essentially amorphous X-ray diffraction pattern. The exact temperature and atmosphere utilized during the initial stabilization of an organic polymeric fibrous material commonly vary with the composition of the precursor as will be apparent to those skilled in the art. During the carbonization reaction elements present in the fibrous material other than carbon (e.g., oxygen and hydrogen) are substantially expelled. Suitable organic polymeric fibrous materials from which the fibrous material capable of undergoing graphitization may be derived include an acrylic polymer, a cellulosic polymer, a polyamide, a polybenzimidazole, polyvinyl alcohol, etc. As discussed hereafter, acrylic polymeric materials are particularly suited for use in the formation of the fibrous material capable of undergoing graphitization which is employed in the present process. Illustrative examples of suitable cellulosic materials include the natural and regenerated forms of cellulose, e.g., rayon. Illustrative examples of suitable polyamide materials include the aromatic polyamides, such as nylon 6T, which is formed by the condensation of hexamethylenediamine and terephthalic acid. An illustrative example of a suitable polybenzimidazole is poly-2,2'-m-phenylene-5,5'-bibenzimidazole.

A fibrous acrylic polymeric material prior to stabilization may be formed primarily of recurring acrylonitrile units. For instance, the acrylic polymer should contain not less than about 85 mol per cent of recurring acrylonitrile units with not more than about 15 mol per cent of a monovinyl compound which is copolymerizable with acrylonitrile such as styrene, methyl acrylate, methyl methacrylate, vinyl acetate, vinyl chloride, vinylidene chloride, vinyl pyridine, and the like, or a plurality of such monovinyl compounds.

During the formation of a preferred carbonized starting material for use in the present process multifilament bundles of an acrylic fibrous material may be initially stabilized in an oxygen-containing atmosphere (i.e., preoxidized) on a continuous basis in accordance with the teachings of U.S. Ser. No. 749,957, filed Aug. 5, 1968, of Dagobert E. Stuetz, which is assigned to the same assignee as the instant invention and is herein incorporated by reference. More specifically, the acrylic fibrous material should be either an acrylonitrile homopolymer or an acrylonitrile copolymer which contains no more than about 5 mol per cent of one or more monovinyl comonomers copolymerized with acrylonitrile. In a particularly preferred embodiment of the invention the fibrous material is derived from an acrylonitrile homopolymer. The stabilized acrylic fibrous material which is preoxidized in an oxygen-containing atmosphere is black in appearance, contains a bound oxygen content of at least 7 per cent by weight as determined by the Unterzaucher analysis, retains its original fibrous configuration essentially intact, and is non-burning when subjected to an ordinary match flame.

In the present process a continuous length of the fibrous material capable of undergoing graphitization is continuously passed through a heating zone having a maximum temperature of at least 2,000°, e.g., a maximum temperature of 2,000° to 3,100° C. (preferably 2,400° to 3,100° C.), containing an inert gaseous atmosphere for a residence time sufficient to substantially convert the fibrous material to graphitic carbon while retaining its original fibrous configuration essentially intact. Suitable inert gaseous atmospheres for the heating zone include nitrogen, argon, helium, etc. For instance, a continuous length of an amorphous carbon fibrous material, e.g., a multifilament yarn may be passed through the heating zone while at a graphitization temperature of at least 2,000° C. for a residence time of about 5 seconds to 4 minutes to produce graphitization. Longer graphitization heating times may be selected but generally yield no commensurate advantage. Preferred residence times while within about 50° C. of the maximum graphitization temperature commonly range from about 10 seconds to 200 seconds.

In a preferred embodiment of the process a continuous length of a stabilized acrylic fibrous material which is non-burning when subjected to an ordinary match flame and derived from an acrylic fibrous material selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers which contain at least about 85 mol per cent of acrylonitrile units and up to about 15 mol per cent of one or more monovinyl units copolymerized therewith is continuously passed through a heating zone containing an inert gaseous atmosphere and a temperature gradient in which said fibrous material is initially carbonized, and in which said carbonized fibrous material is heated to a maximum temperature of at least 2,000° C. until substantial graphitization occurs. Representative inert gaseous atmospheres for the heating zone in which both carbonization and graphitization are accomplished include nitrogen, argon, helium, etc.

When the fibrous material supplied to the heating zone is a stabilized acrylic fibrous material it may be carbonized and graphitized while passing through a temperature gradient in accordance with the procedures described in commonly assigned U.S. Ser. Nos. 777,275, filed Nov. 20, 1968 of Charles M. Clarke entitled "Process for the Continuous Carbonization of a Stabilized Acrylic Fibrous Material;" 17,780 filed Mar. 9, 1970 of Charles M. Clarke, Michael J. Ram, and John P. Riggs entitled "Improved Process for the Carbonization of a Stabilized Acrylic Fibrous Material;" and 17,832 filed Mar. 9, 1970 of Charles M. Clarke, Michael J. Ram, and Arnold J. Rosenthal entitled "Production of High Tenacity Graphitic Fibrous Materials." Each of these disclosures is herein incorporated by reference.

In accordance with a particularly preferred embodiment of the process a continuous length of stabilized acrylic fibrous material which is non-burning when subjected to an ordinary match flame and derived from an acrylic fibrous material selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers which contain at least about 85 mol per cent of acrylonitrile units and up to about 15 mol per cent of one or more monovinyl units copolymerized therewith is converted to a graphitic fibrous material while preserving the original fibrous configuration essentially intact while passing through a carbonization/graphitization heating zone containing an inert gaseous atmosphere and a temperature gradient in which said fibrous material is raised within a period of about 20 to about 300 seconds from about 800° C. to a temperature of about 1,600° C. to form a continuous length of carbonized fibrous material, and in which said carbonized fibrous material is subsequently raised from about 1,600° C. to a maximum temperature of at least about 2,400° C. within a period of about 3 to 300 seconds where it is maintained for about 10 seconds to about 200 seconds to form a continuous length of graphitic fibrous material.

The equipment utilized to produce the heating zone used to produce graphitization or carbonization and graphitization in the process of the present invention may be varied as will be apparent to those skilled in the art. It is essential that the apparatus selected be capable of producing the required temperature while excluding the presence of an oxidizing atmosphere.

In a preferred embodiment of the invention, the continuous length of fibrous material undergoing graphitization or carbonization and graphitization is heated by use of an induction furnace. In such a procedure the fibrous material may be passed in the direction of its length through a hollow graphite tube or other susceptor which is situated within the windings of the induction coil. By varying the length of the graphite tube, the length of the induction coil, and the rate at which the fibrous material is passed through the graphite tube, many apparatus arrangements capable of carrying out the graphitization or carbonization and graphitization may be selected. For large-scale production, it is of course preferred that relatively long tubes or susceptors be used so that the fibrous material may be passed through the same at a more rapid rate while being graphitized or carbonized and graphitized. The temperature gradient of a given apparatus may be determined by conventional optical pyrometer measurements as will be apparent to those skilled in the art. The fibrous material because of its small mass and relatively large surface area instantaneously assumes essentially the same temperature as the inert gaseous atmosphere of the heating zone through which it is continuously passed.

During the formation of graphitic carbon within the continuous length of fibrous material a tensional force may be optionally applied to the bundles undergoing graphitization in order to provide efficient handling of the fibrous material and/or to modify the physical properties of the same.

During the graphitization reaction the inert gaseous atmosphere is commonly caused to flow through the heating zone while preserving the requisite heating. For instance, when an induction furnace is employed, the inert gaseous atmosphere may be continuously introduced through one or more small apertures provided in the walls of a hollow graphite tube which is surrounded by an induction coil. The inert gaseous atmosphere accordingly exits from the graphite tube through the ends thereof. The flow of the inert gaseous atmosphere out of each end of the graphite tube accordingly substantially excludes the introduction of air or an oxidizing atmosphere within the heating zone.

At least one gaseous stream containing a catalytic quantity of a volatile alkyl borate in vapor form capable of catalyzing the graphitization of the fibrous material is preferably introduced into the heating zone so that the vapor stream directly impinges upon the continuous length of fibrous material, or at least is provided in the inert gaseous atmosphere of the heating zone immediately adjacent the fibrous material. Preferably the point of introduction of the stream containing the volatile alkyl borate in gaseous form is identical to that location where the fibrous material enters the heating zone or is in relatively close proximity thereto. Preferably the gas stream is introduced into the heating zone in close proximity to the fibrous material prior to raising the temperature of the fibrous material above about 500° C. For this reason the heating zone is preferably provided with a temperature gradient in which the fibrous material is progressively elevated to a maximum temperature of at least 2,000° C. where substantial graphitization is accomplished. If desired, an auxiliary graphite tube or susceptor may be provided in series with the main graphite tube of the induction furnace to extend the length of the entrance portion of the heating zone, and the stream of boron compound as well as the starting fibrous material introduced into such extended portion. It has been found that if the stream of boron compound in vapor form is introduced solely into that portion of the heating zone which is at a highly elevated temperature (i.e., at least 2,000° C.), then there is a tendency for the boron compound to undergo a prompt reaction with the walls of the heating zone and thereby to become at least partially unavailable in the catalysis of the graphitization of the fibrous material.

In a preferred embodiment of the process the continuous length of fibrous material is supplied to the heating zone in an essentially anhydrous form which is absorbent with respect to the volatile alkyl borate which contacts the same within the heating zone. If desired the stream of volatile alkyl borate in vapor form may be at least partially diluted with an inert gas, such as nitrogen, argon, or helium, which is preferably identical to the inert gaseous atmosphere otherwise supplied to the heating zone.

The volatile alkyl borate capable of catalyzing the graphitization reaction is preferably introduced into the inert gaseous atmosphere surrounding the continuous length of fibrous material in at least a portion of the heating zone in a concentration of about 200 to 20,000 parts per million by volume. In a particularly preferred embodiment of the process the volatile alkyl borate is introduced into the gaseous atmosphere surrounding the continuous length of fibrous material in at least a portion of the heating zone in a concentration of about 200 to 2,000 parts per million by volume. The quantity of the volatile alkyl borate introduced into the heating zone is preferably about 0.01 to 4 per cent by weight based upon the weight of fibrous material introduced into the heating zone. In a particularly preferred embodiment of the invention the quantity of volatile alkyl borate introduced into the heating zone is about 0.04 to 0.4 per cent by weight based upon the weight of the fibrous material introduced into the heating zone.

It is preferred that the alkyl borate possess substantial volatility at 500° C. or below and is conveyed to the heating zone while at a temperature not in excess of 500° C. In a particularly preferred embodiment of the process the alkyl borate possesses substantial volatility at about room temperature (i.e., about 25° C.) thereby facilitating convenient handling and introduction of the same without resorting to elevated temperatures.

Any alkyl borate which is capable of being introduced into the heating zone in vapor form may be selected for use in the process. The preferred volatile alkyl borates for use in the process are the alkyl borates of the formula B(OR)3 where R is an alkyl group having 1 to 5 carbon atoms. Such alkyl borates include: trimethyl borate, B(OCH3)3 sometimes identified as methyl borate, or trimethoxyborine; triethyl borate, B(OC2 H5)3 ; tripropyl borate, B(OC3 H7)3 ; triisopropyl borate, B [O(CH3)2 CH ]3 ; tributyl borate, B(OC4 H9)3 ; and triamyl borate, B(OC5 H11)3. The particularly preferred alkyl borate for use in the process is trimethyl borate.

Other representative alkyl borates possessing sufficient volatility for use in the present process include higher molecular weight boric acid esters such as tricyclohexyl borate, B(OC6 H11)3 ; tridodecyl borate, B(OC12 H25)3 ; and trihexylene glycol biborate, B2 (O2 C6 H12)3.

The introduction into the heating zone of a gaseous stream containing a volatile alkyl borate as defined herein enables the graphitization of the fibrous material to proceed in a more efficient manner. Graphitic fibrous materials exhibiting improved tensile properties, Young's modulus as well as tensile strength, may be formed on a continuous basis in accordance with the present process without modification of the graphitization heating profile. Alternatively, through the use of the present process it is possible to decrease the maximum temperature experienced by the fibrous material within the heating zone while still achieving highly acceptable tensile properties within the resulting product.

The ability for one to operate at a lower maximum graphitization temperature offers a substantial cost reduction since less power is required and the usable life of the apparatus utilized in the process is extended. For instance, the graphite tube or susceptor of an induction furnace may have its life extended many times (e.g., five to 10 times) by simply lowering the maximum graphitization temperature from about 2,900° C. to 2,700° C. Not only is one spared the cost of a replacement graphite tube, but down time is eliminated which would otherwise be consumed while replacing the graphite tube, starting up the furnace, and allowing it to again come to equilibrium conditions.

No immersing, rinsing, and drying of the starting fibrous material is required as has been practiced in a prior art boron catalysis technique in which the starting material is initially soaked in a solution of boric acid. Through the use of the present process the catalytic quantity of the boron compound introduced into the heating zone is readily controlled and subject to accurate adjustment throughout the duration of the graphitization process.

The following examples are given as specific illustrations of the invention. It should be understood, however, that the invention is not limited to the specific details set forth in the examples. Reference is made in the examples to the drawing.

EXAMPLE I

A continuous length of 1,600 fil unwashed dry spun acrylonitrile homopolymer continuous filament yarn having a total denier of 2,000 was selected as the starting material. The yarn was oriented and drawn to a single filament tenacity of about 3.2 grams per denier. The yarn was subjected to a healing treatment in which residual N,N-dimethyl formamide spinning solvent was evolved by passage for 6 minutes through a muffle furnace provided with air at 185° C. during which time the yarn shrank 10 per cent in length. The yarn was continuously stabilized in accordance with the teachings of U.S. Ser. No. 749,957, filed Aug. 5, 1968, of Dagobert E. Stuetz, which is assigned to the same assignee as the present invention and is herein incorporated by reference. During the stabilization reaction (i.e., preoxidation) the yarn was continuously passed in the direction of its length in the absence of shrinkage through a multi-wrap skewed roll oven provided with an air atmosphere at 270° C. for a residence time of 147 minutes. The resulting stabilized yarn was black in appearance, contained a bound oxygen content of 10.85 per cent by weight as determined by the Unterzaucher analysis, and was non-burning when subjected to an ordinary match flame.

The preoxidized yarn was stored in a forced air oven at 110° C. while wound upon a bobbin following stabilization. The yarn was next unwound from the bobbin and passed through a drying zone (not shown) wherein volatiles were substantially removed prior to introduction into the heating zone wherein carbonization and graphitization were accomplished. The drying zone consisted of one 12 inch muffle furnace provided with circulating air at 200° C.

The dried stabilized yarn 1 was next continuously passed at a rate of about 1.5 inches/minute in the direction of its length through a Lepel 450 KC induction furnace 2 utilizing a 20 KW power source wherein both carbonization and graphitization were accomplished. The induction furnace comprised a 10 turn water cooled copper coil 4 having an inner diameter of three-fourths and a length of 2 inches and a hollow graphite tube or susceptor 6 suspended within the coil having a length of 81/2 inches, an outer diameter of one-half inch and an inner diameter of one-eighth inch through which the yarn was continuously passed. Four 1/8 inch holes 5 were provided in the wall of graphite tube 6. The hollow graphite tube 6 was held in position by supports 7. The copper coil 4 which encompassed a portion of the hollow graphite tube 6 was positioned at a location essentially equidistant from the respective ends of the graphite tube. The copper coil 4 was connected to 20 KW power source 8. An auxiliary graphite tube 10 through which the fibrous material passed was placed at the entrance end of graphite tube 6. The auxiliary tube 10 had an overall length of 5 3/4 inches, one-half inch of which encompassed the end of graphite tube 6. The outer diameter of the auxiliary tube 10 was 1 inch and the inner diameter of the auxiliary tube was one-fourth inch. A stainless steel enclosure or housing 12 having a wall thickness of about one-fourth inch and an overall length of about 11 inches, a height of about 6 inches, and a width of about 6 inches surrounded the induction furnace 2. The resulting graphite yarn 14 as it left graphite tube 6 passed through cylindrical orifice 16 formed of graphite having an outer diameter of one-half inch and an inner diameter of one-eighth inch.

Nitrogen was passed through line 18 to rotometer 20 which delivered the nitrogen at a rate of 25 standard cubic feet per hour through line 22. Line 22 branched into line 24 which communicated with a centrally located orifice in the wall of housing 12, and line 26 which communicated with rotometer 28. A stream of nitrogen from line 24 entered the interior of the housing 12 at a rate of 24.976 standard cubic feet per hour. Nitrogen was delivered from rotometer 28 through line 30 at a flow rate of 0.024 standard cubic feet per hour. The stream of nitrogen from line 30 was bubbled through liquid trimethyl borate 32 present in vessel 34. Nitrogen gas as well as trimethyl borate in vapor form was withdrawn from the space 36 above the liquid trimethyl borate 32 through line 38. An ice bath 40 surrounded vessel 34 to insure a constant vapor pressure for the trimethyl borate. Approximately 10 per cent by volume of the gaseous stream in line 38 was trimethyl borate, and approximately 90 per cent by volume of the gaseous stream in line 38 was nitrogen. The gaseous stream containing the trimethyl borate in vapor form is introduced into the interior of auxiliary graphite tube 10 through an orifice in the wall thereof located 41/4 inches from the entrance end thereof.

The nitrogen gas which entered housing 12 through line 24 entered the graphite tube 6 through holes 5 as well as the open end 42 thereof. Approximately one-half of the gaseous atmosphere present within housing 12 exits through orifice 16 and approximately one-half of the gaseous atmosphere exits through the exposed end 44 of auxiliary graphite tube 10.

3.9 × 10-6 grams of trimethyl borate in vapor form entered graphite tube 10 via line 38 per minute and directly impinged upon the yarn. The yarn 1 was at about 100° C. when it entered the exposed end of graphite tube 10. The temperature of the yarn within auxiliary tube 10 at the point where the vapor stream of trimethyl borate impinged upon the same was about 350° C. 0.0085 grams of yarn passed the point where the vapor stream of trimethyl borate impinged upon the same per minute. Accordingly a stream of trimethyl borate in vapor form was supplied to the inert gaseous atmosphere of the heating zone in a quantity of about 0.045 per cent by weight based upon the weight of the yarn. The quantity of trimethyl borate in the inert atmosphere surrounding the yarn in the immediate area of the heating zone wherein the boron compound was introduced was about 200 parts per million.

While passing through the heating zone defined by the auxiliary graphite tube 10 and graphite tube 6, the yarn was raised from about 100° C. to a temperature of 800° C. in approximately 300 seconds, from 800° C. to 1,600° C. in approximately 60 seconds to produce a carbonized yarn, and from 1,600° C. to a maximum temperature of approximately 2,700° C. in approximately 40 seconds where it was maintained ±50° C. for approximately 40 seconds. While passing through the heating zone constant longitudinal tensions of 300, 400, and 500 grams were maintained upon portions of the yarn at various points in time. The resulting yarn 14 exhibited a graphitic carbon X-ray diffraction pattern and a specific gravity of about 2.0. The following single filament tensile properties were obtained for the various graphite yarn samples.

Tension in Grams Tensile Strength Young's Modulus 300 323×103 psi .6 psi 400 305×103 psi .6 psi 500 400×103 psi .6 psi

EXAMPLE II

Example I was repeated with the exceptions indicated.

A stream of nitrogen gas from line 24 entered the interior of housing 12 at a rate of 24.76 standard cubic feet per hour. Nitrogen was delivered from rotometer 28 through line 30 at a rate of 0.24 standard cubic feet per hour. 3.9 × 10- 5 grams of trimethyl borate in vapor form entered graphite tube 10 via line 38 per minute and directly impinged upon the yarn. Trimethyl borate in vapor form was supplied to the inert gaseous atmosphere of the heating zone in a quantity of about 0.45 per cent by weight based upon the weight of the yarn. The quantity of trimethyl borate in the inert atmosphere surrounding the yarn in the immediate area of the heating zone wherein the boron compound was introduced was about 2,000 parts per million.

The following single filament tensile properties were obtained for the various graphite yarn samples.

Tension in Grams Tensile Strength Young's Modulus 300 330×103 psi .6 psi 400 380×103 psi p.6 psi 500 430×103 psi p.6 psi

In a comparative graphitization procedure Examples I and II were repeated in an identical apparatus with the exception that no boron compound was introduced into the heating zone. A stream of nitrogen gas from line 24 entered the interior of housing 12 at a rate of 25 standard cubic feet per hour. The overall tensile properties of the resulting graphite yarn were generally lower than those obtained in Examples I and II. The following single filament tensile properties were determined for the various graphite yarn samples.

Tension in Grams Tensile Strength Young's Modulus 300 328×103 psi .6 psi 400 373×103 psi .6 psi 500 455×103 psi .6 psi

EXAMPLE III

Example I was repeated with the exceptions indicated.

While passing through the heating zone defined by the auxiliary graphite tube 10 and graphite tube 6, the yarn was raised from about 100° C. to a temperature of 800° C. in approximately 300 seconds, from 800° C. to 1,600° C. in approximately 60 seconds to produce a carbonized yarn, and from 1,600° C. to a maximum temperature of approximately 2,900° C. in approximately 40 seconds where it was maintained ±50° C. for approximately 40 seconds. The following single filament tensile properties were determined for the various graphite yarn samples.

Tension in Grams Tensile Strength Young's Modulus 300 430×103 psi p.6 psi 400 480×103 psi p.6 psi 500 330×103 psi p.6 psi

It will be noted that the tensile properties were generally improved when employing a higher maximum graphitization temperature.

In Examples I, II, and III no boron carbide was detected in the resulting graphite yarn by conventional X-ray diffraction studies.

In a comparative graphitization procedure Example III was repeated in an identical apparatus with the exception that no boron compound was introduced into the heating zone. A stream of nitrogen gas from line 24 entered the interior of housing 12 at a rate of 25 standard cubic feet per hour. The overall tensile properties of the resulting graphite yarn were generally lower than those obtained in Example III. The following single filament tensile properties were determined for the various graphite yarn samples.

Tension in Grams Tensile Strength Young's Modulus 300 323×103 psi .6 psi 400 303×103 psi .6 psi 500 465×103 psi p.6 psi

It will be noted that the tensile properties achieved in the above comparative graphitization run are generally comparable to those achieved in Examples I and II in accordance with the present invention when operating at a lower maximum graphitization temperature. By operating at a lower maximum graphitization temperature such as that employed in Examples I and II the useful life of graphite tube 6 is substantially increased.

EXAMPLE IV

Example I is repeated with the following exceptions.

The continuous length of continuous filament yarn which is introduced into the heating zone is a carbonaceous yarn derived from an acrylonitrile homopolymer containing about 99 per cent carbon by weight and exhibits an essentially amorphous X-ray diffraction pattern. Substantially similar results are achieved.

Although the invention has been described with preferred embodiments, it is to be understood that variations and modifications may be resorted to as will be apparent to those skilled in the art. Such variations are to be considered within the purview and scope of the claims appended hereto.