WO1992003601A2

WO1992003601A2 - Carbon fiber and process for its production

Info

Publication number: WO1992003601A2
Application number: PCT/US1991/004853
Authority: WO
Inventors: James Jay Dunbar; Gene Clyde Weedon; Thomas Yiu-Tai Tam
Original assignee: Allied-Signal Inc.
Priority date: 1990-08-08
Filing date: 1991-07-10
Publication date: 1992-03-05
Also published as: WO1992003601A3

Abstract

The present invention provides a carbon fiber, made from a precursor fiber of polyethylene having an intrinsic viscosity of about 2 to 30 in decalin, and characterized by a tenacity of greater than about 30 g/d and by a tensile modulus of at least about 2000 g/d (ASTM D 4018). The process for producing this fiber features the steps of stabilizing the polyethylene precursor fiber under a tension sufficient to minimize shrinkage of the fiber without breaking the fiber followed by carbonizing the stabilized polyethylene fiber by heating the fiber in an atmosphere of an inert gas or in vacuum while under a tension sufficient to minimize shrinkage of the fiber and for a time period sufficient to achieve the carbon fiber characterized above. The present invention also provides a graphite fiber produced by heating the carbon fiber of the invention at a temperature of from about 2000° to 3000° in an atmosphere of an inert gas or in a vacuum.

Description

CARBON FIBER AND PROCESS FOR ITS PRODUCTION

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a process for the production of carbon and graphite fibers, as well as to the carbon and graphite fibers produced therefrom. More particularly, the present invention relates to a process for the production of carbon and graphite fibers from a precursor fiber of polyethylene having an intrinsic viscosity of about 2 to 30 in decalin. 2. Prior Art

The high strength and light weight of carbon and graphite fibers makes them highly desirable, especially in the composite field. Carbon fiber is a man-made fiber converted from another man-made, organic fiber. Precursor fibers that have been considered for producing carbon fibers include cotton, polyacrylonitrile (PAN) , rayon, polyvinylidene chloride (saran) , aromatic polyamide, polybenzamidazole, polyoxadiazole, polyphenylene, lignin, cross-linked polyethylene, and various pitches from PVC, petroleum, or coal tar. See Katz, H.S., Carbon-Graphite Filaments, Handbook Fillers Reinforced Plastics, Van Nostrand-Reinhold, 1978, 562-82, and Spencer, J.N., New kid on the fiber block, America's Textiles: Reporter/Bulletin Edition, 1981, 10-12, both of which are hereby incorporated by reference. The Katz article lists in Table 29-2 selected carbon-graphite filament patents.

Polyethylene, with its high carbon content and easy spinnability, has been used as a precursor fiber to carbon-graphite fiber in two processes.- In the process of Japanese Patent Publication No. 16681/1964, hereby incorporated by reference, polyethylene fiber is treated with radiation or a peroxide compound to thereby effect cross-linking between the molecules, followed by carbonizing the product. In the process of U.S. Patent 4 070 446, hereby incorporated by reference, an ordinary melt spun polyethylene fiber is stabilized through sulfonation, followed by carbonizing (with heat) the stabilized product. This patent claims a higher carbon fiber yield over the prior Japanese process. The highest strength and elastic modulus values for carbon fiber produced according to this latter patent are 25.8 t/cm²

(Example 2, about 30 g/d) and 1500 t/cm² (Example 4, about 1740 g/d) , respectively.

Although not directed to formation of carbon fibers, U.S. 4 778 633, hereby incorporated by reference, is also of interest. This patent teaches a method of making a high tensile strength (≥ 70,000 psi) , high creep recovery polyethylene fiber. The fiber is cross-linked and then heated to a temperature which is above the second order transition temperature and below the crystalline melting temperature of the polyethylene. The heated fiber is drawn to a draw ratio > 2 after which the fiber is cooled. The highest weight average molecular weight for the polyethylene taught is "at least about 100,000," and the fiber can be formed either by melt spinning or solution spinning. The preferred manner of cross-linking is irradiation of the undrawn fiber.

The present invention was developed in a search for carbon fibers having even better mechanical properties than those of the prior art. SUMMARY OF THE INVENTION

It has been discovered that carbon and graphite fibers of extraordinary mechanical properties can be prepared by using a precursor fiber of polyethylene having an intrinsic viscosity of about 2 to 30, more preferably about 14 "to 26, in decalin (ASTM D 4020).

Accordingly, the present invention provides a carbon fiber, made from a precursor fiber of polyethylene having an intrinsic viscosity of about 2 to 30 in decalin, and characterized by a tenacity of greater than about 30 g/__, preferably at least about 35 to 60 g/d, and by a tensile modulus of at least about 2000 g/d, preferably at least about 2500 to 4000 g/d (ASTM D 4018) . The weight average molecular weight of the polyethylene precursor fiber ranges from high to ultra high. It is at least about 300,000, preferably at least about a million to six million, and more preferably between about two million and about five million. The polyethylene precursor fiber is preferably further characterized by a tenacity of at least about 20 g/d, more preferably about 28 to 85 g/d, a tensile modulus of at least about 1000 g/d, more preferably about 1200 to 3000 g/d (ASTM D 2256) , and a C axis orientation function of at least about 0.90.

The present invention is also a process for producing carbon fiber, comprising the steps of: stabilizing a precursor fiber of polyethylene having an intrinsic viscosity of about 2 to 30 in decalin under a tension sufficient to minimize shrinkage of the fiber without breaking the fiber; and carbonizing the stabilized polyethylene fiber by heating the fiber in an atmosphere of an inert gas or in a vacuum while under a tension sufficient to minimize shrinkage of the fiber and for a time period sufficient to achieve a carbon fiber characterized by a tenacity of greater than about 30 g/d and by a tensile modulus of at least about 2000 g/d (ASTM D 4018) . The polyethylene precursor fiber is as previously described. During the stabilizing step, the polyethylene fiber preferably is initially under a tension of from about 3 to 4 g/d, this tension is subsequently reduced as stabilization takes place, to thereby substantially prevent shrinkage of the fiber. The carbonization step is preferably carried out at a temperature of from about 600° to 2000 ' C for a time period sufficient to substantially completely carbonize the fiber. The preferred stabilizing step is the sulfonation of the precursor fiber with a sulfonating agent selected from the group consisting of chlorosulfonic acid, sulfuric acid, fuming sulfuric acid and mixtures thereof.

In an alternate embodiment, the process for producing carbon fiber comprises the steps of: stabilizing a solution spun polyethylene precursor fiber under a tension sufficient to minimize shrinkage of the fiber without breaking the fiber; and carbonizing the stabilized polyethylene fiber by heating the fiber in an atmosphere of an inert gas or in a vacuum while under a tension sufficient to minimize shrinkage of the fiber and for a time period sufficient to achieve a fiber characterized by a tenacity of greater than about 30 g/d and by a tensile modulus of at least about 2000 g/d (ASTM D 4018). In all other respects, the process is as described above.

The present invention also provides graphite fibers produced by heating the carbon fibers made as above at a temperature of from about 2000" to 3000"C in an atmosphere of an inert gas or in a vacuum.

DETAILED DESCRIPTION OF THE INVENTION The polyethylene precursor fibers of the present invention may be prepared by polyethylene solution spinning processes described, for example, in U.S. Patents 4 137 394 or 4 356 138, or by spinning from a solution to form a gel structure as described in German Off. 3 004 699, GB 2 051 667 and especially as described in U.S. 4 413 110, all of which are hereby incorporated by reference. These fibers can be differentiated from ordinary polyethylene fibers in being made from polyethylene having an intrinsic viscosity of about 2 to 30, preferably about 14 to 26, in decalin (ASTM D 4020) . This corresponds to a weight average molecular weight of at least about 300,000, preferably at least about one million to six million, more preferably between about two million and about five million.

The term polyethylene shall mean a predominantly linear polyethylene material that may contain minor amounts of chain branching or co-monomers not exceeding 5 modifying units per 100 main chain carbon atoms and that may also contain admixed therewith up to about 25 weight percent of one or more polymeric additives, such as alkene-1-polymers, in particular low density polyethylene, polypropylene or polybutylene, copolymers containing mono-olefins as primary monomers, oxidized polyolefins, graft polyolefin copolymers and polyoxymethylenes, or low molecular weight additives, such as antioxidants, lubricants, ultraviolet screening agents, colorants and the like, which are commonly incorporated therewith. See U.S. 4 455 273, hereby incorporated by reference.

The precursor fiber comprises polyethylene having a density of at least about 0.97 g/cm³, measured according to ASTM D 1505-85, modified as follows: for density range of 0.94 to 1.01 g/cm³. Flask A contains 280 ml carbon tetrachloride and 470 ml heptane, and Flask B contains 190 ml carbon tetrachloride and 560 ml heptane; carbon tetrachloride concentration should be adjusted for heavier densities, as would be obvious to one of ordinary skill in the art.

Tensile properties for the polyethylene precursor fiber are measured according to ASTM D 2256, using an Instron Model TTC equipped with modified Model G-61-4D cord and yarn grips using 90 psig air pressure, crosshead speed of 10 + 1/4 in/min, 10 + 1/16 in nip-to-nip gage length, and using 10 breaks per sample. The precursor fiber is characterized by a tenacity of at least about 20 g/d, more preferably about 28 to 85 g/d; a tensile modulus of at least about 1000 g/d, more preferably about 1200 to 3000 g/d; and a C axis orientation function of at least about 0.90. For detail on C axis orientation function, see L.E. Alexander, X-Ray Diffraction Methods in Polymer Science, John Wiley,

241-252, 1969, hereby incorporated by reference. These properties, and especially the preferred and more preferred forms of these properties, are best achieved by the process of European Patent Application 64 167, the disclosure of which is incorporated herein by reference. These properties are generally much higher than for prior art polyethylene precursor fibers.

The fibers can be prestabilized (through controlled cross-linking of the polyethylene) by incorporating a peroxide compound with the polymer prior to and/or during fiber formation. The fibers can also be prestabilized by irradiation, acetylene permeation followed by irradiation or silane cross-linking. The prestabilization should not be so complete that fiber formation is made difficult. Prestabilization permits faster stabilization of the precursor fiber, i.e., higher heating rates. The preferred manner of stabilizing, sulfonation, is set forth in U.S. 4 070 446, the disclosure of which is hereby incorporated in toto. As set forth there, the starting fiber is sulfonated with chlorosulfonic acid, sulfuric acid, fuming sulfuric acid, or a mixture of two or more kinds thereof while giving a tension to the fiber. 98% sulfuric acid is the preferred sulfonating agent, and the reaction temperature may range from about 80"C up to a temperature which will not melt the fiber, preferably 100" to 250"C, more preferably 120" to 180"C (this temperature range is preferred with the sulfuric acid as sulfonating agent) . The sulfonation may be carried out at a temperature lower than 80"C, but it takes a long time and is therefore less economical. At temperatures higher than 250^βC, the sulfonation reaction is completed more quickly but also more violently and may result in fiber with inferior characteristics. When the polyethylene precursor fiber is sulfonated at a temperature higher than its melting point (about 130*C) , it should either be pretreated at a lower temperature followed by raising the reaction temperature or pretreated in some other fashion to prestabilize the precursor fiber. The reaction time varies according to the reaction temperature, sulfonating agent and diameter of the fiber, and preferably is such that a fully stabilized fiber results which neither melts nor burns when exposed to a match flame (match test) .

After sulfonation, the stabilized fiber is washed to remove the sulfonating (stabilizing) agent, followed by drying. The wash may be with water or any other suitable solution or solvent. See, e.g., the teachings of U.S. 4 070 446. An air dry is satisfactory, but any drying temperature which will not shrink the yarn is acceptable.

The stabilized fiber is then carbonized by heating to about 600^βC or higher, preferably to about 600 to 2000^βC, more preferably from about 600 to 1200"C, in an atmosphere of an inert gas, e.g., nitrogen, helium, argon, or in a vacuum until a carbon fiber characterized by a tenacity of greater than about 30 g/d and by a tensile modulus of at least about 2000 g/d is achieved.

Sufficient tension must be applied to the fiber during the sulfonation (stabilization) , washing, drying and carbonization steps to minimize, and preferably substantially prevent feeder yarn shrinkage. Preferably a sufficient amount of tension is applied to actually draw the yarn, especially during stabilization and carbonization. The appropriate tension will be a function of the particular precursor fiber chosen; generally, a fiber which is less oriented on a denier per filament basis will require less tension than a fiber which is more oriented.

Graphite fiber of the present invention can be achieved by heating the carbon fiber of the present invention at a temperature of at least about 2000"C, preferably about 2000 to 3000*C, in an atmosphere of an inert gas or in a vacuum for a time period of about 10 seconds to 5 minutes, preferably for about 1 to 2 minutes. The following examples are presented to provide a more complete understanding of the invention, the specific techniques, conditions, materials, proportions and reported data being set forth to illustrate the principles of the invention and not to limit the scope thereof. In the examples, fibers were tested for burning behavior under a match flame. Thermal analyses were done using the Perkin Elmer DSC4. TGA was done using TGS2 by Perkin Elmer. EXAMPLE 1 (FEASIBILITY STUDY)

Several trials were carried out to study the feasibility of producing carbon fiber from either SPECTRA^R900 or SPECTRA^R1000 high strength, extended chain (solution spun) polyethylene fibers (precursor fibers) . SPECTRA 900, produced by Allied-Signal Inc., has a reported yarn tenacity of approximately 30 g/d, a tensile modulus of approximately 1400 g/d, a yarn denier of approximately 1200 and an individual filament denier of about 10 (120 filaments, untwisted yarn) , and an elongation of about 3.5%. SPECTRA 1000, produced by Allied-Signal Inc. , has a reported yarn tenacity of approximately 35 g/d, a tensile modulus of approximately 2000 g/d, a yarn denier of approximately 650 and an individual filament of about 5.4 (120 filaments, untwisted yarn), and an elongation of about 2.7%. Both of these fibers are produced from polyethylene having an intrinsic viscosity of about 14 to 26 in decalin (weight average molecular weight of about two to five million) . The densities of the SPECTRA 900 and SPECTRA 1000 fibers were 0.97 g/cm³ and 0.98 g/cm³, respectively. Melt index was zero and C axis orientation was greater than about 0.90.

The trials were carried out using an electrically heated, stainless steel sulfonation bath, the temperature of which was controlled by varying the power output from a variac. The stabilizing agent used was 98% sulfuric acid. The fiber sample was formed into a loop, one end of which was hung on a glass rod submerged in the treating bath. The free end of the loop was passed under a second submerged glass rod, brought out of the bath and passed over a third glass rod. A weight (100 mg/d) was hung from the free end of the loop. As the loop was freely hanging over the third rod, the fiber sample inside the bath could shrink or stretch. In all of the trials, the samples were immersed into the bath when the bath temperature was 120^βC, and the temperature of the bath was raised at a rate of 30"C per hour. The sample treatment length (submerged portion) , was about one foot. In several of the trials, the fiber samples broke when the bath temperature reached about 155°C. Even when the tension was lowered (by changing the weight to 50% and 70% of the original) the fiber samples broke when the bath temperature reached about 160"C. It was noted that the samples broke at what appeared to be the air-liquid interface for the bath, and the broken tip of the sample was fused. The fiber samples apparently started to shrink at these temperatures (about 155^βC) , which resulted in the unsulfonated portion of the sample entering the bath. As the unsulfonated fiber melts at these temperatures, the fiber was breaking at the interface.

Subsequently, trials were conducted wherein the second glass rod was moved, in steps, toward the first glass rod once the bath temperature reached 150"C. At all times the black sample which was previously in the bath remained at the interface. Both a SPECTRA 900 and a SPECTRA 1000 fiber sample were treated in this manner, and the bath temperature elevated to 180*C without breakage.

The samples shrank by about 40%. Thus, we discovered that sufficient tension had not been provided to prevent shinkage. The samples were washed thoroughly in running water for about 20 minutes, and were then dried in air. Both samples passed the match test. The completely stabilized sample (SPECTRA 1000 fiber) was carbonized using a small tubular furnace with a quartz tube inside. The sample was tied to carbon fiber at both ends and was kept under a small tension of about 70 mg/d. The sample was heated in a nitrogen environment and the temperature was raised at a rate of about 600"C per hour. The sample was heated up to 900"C and then taken out (to be within the limits of the quartz tube's tolerance) . Although the sample was not completely carbonized, it was sufficiently carbonized to prove that the sample could be completely carbonized. The sample was looked at with a scanning electron microscope at 15 KV, and the carbonized portion appeared substantially defect-free, which would lead one to conclude that the carbonized portion would have excellent tensile properties.

EXAMPLE 2 SPECTRA 900 fiber as used in Example 1 is immersed in 98% sulfuric acid while raising the temperature from 120° to 180^βC at a rate of 30^βC per hour. An initial tension of about 3 to 4 g/d is put on the fiber. This tension is subsequently reduced as sulfonation takes place to substantially prevent shinkage of the fiber without breaking the fiber. The fiber thus obtained is washed with water and air dried. This fiber is carbonized by raising the temperature from room temperature to about 1500" to 1800^βC at a rate of 600"C per hour in a nitrogen environment while maintaining a tension on the fiber sufficient to substantially prevent shrinkage. It is expected that a carbon fiber characterized by a tenacity of greater than about 30 g/d and by a tensile modulus of at least about 2000 g/d (ASTM D 4018) will be produced. EXAMPLE 3

Example 2 is repeated with the temperature of carbonization being taken up to over 2000^βC. It is expected that a graphite fiber will be produced.

DISCUSSION The carbon molecule arrangement of the extended chain polyethylene precursor fiber is highly oriented and resembles the arrangement of carbon fibers and graphite fibers. This arrangement is substantially maintained throughout the process of the present invention to produce carbon and graphite fibers having substantially defect-free surfaces and thus, excellent tensile properties.

Claims

WE CLAIM:

1. A carbon fiber, made from a precursor fiber of polyethylene having an intrinsic viscosity of about 2 to 30 in decalin, and characterized by a tenacity of greater than about 30 g/d and by a tensile modulus of at least about 2000 g/d .

2. The carbon fiber of claim 1 characterized by a tenacity of at least about 35 g/d and a tensile modulus of at least about 2500 g/d.

3. The graphite fiber produced by heating the fiber of claim 2 at a temperature of from about 2000" to 3000"C in an atmosphere of an inert gas or in a vacuum.

4. The carbon fiber of claim 1 wherein the precursor fiber is of polyethylene having an intrinsic viscosity of about 14 to 26 in decalin.

5. A process for producing carbon fiber, comprising the steps of a. stabilizing a precursor fiber of polyethylene having an intrinsic viscosity of about 2 to 30 in decalin under a tension sufficient to minimize shrinkage of the fiber without breaking the fiber; and b. carbonizing the stabilized polyethylene fiber by heating the fiber in an atmosphere of an inert gas or in a vacuum while under a tension sufficient to minimize shrinkage of the fiber and for a time period sufficient to achieve a carbon fiber characterized by a tenacity of greater than about 30 g/d and by a tensile modulus of at least about 2000 g/d.

6. The process of claim 5 wherein the polyethylene precursor fiber is characterized by a tenacity of at least about 20 g/d, a tensile modulus of at least about 1000 g/d, and a C axis orientation function of at least about 0.90.

7. The process of claim 5 wherein said stabilizing step comprises sulfonating the polyethylene precursor fiber with a sulfonating agent selected from the group consisting of chlorosulfonic acid, sulfuric acid, fuming sulfuric acid and mixtures thereof.

8. The process of claim 7 wherein the sulfonation is carried out at a temperature of from about 100* to 250-C.

9. The process of claim 5 further comprising the step of heating the carbonized fiber at a temperature of about 2000* to 3000*C in an atmosphere of an inert gas or in a vacuum to produce a graphite fiber.