GB2170491A - Method of producing graphite fiber and product thereof - Google Patents

Abstract

In a method of manufacturing graphite fibers from precursor polyacrylonitrile (PAN)-based carbonized fibers, the steps of cyclically heating and coo(ing said fibers in a non-oxidizing atmosphere between graphitization temperatures and relatively cooler temperatures, through at least two cycles.

Description

SPECIFICATION Method of producing graphite fiber and product thereof The present invention relates to graphitized polyacrylonitrile (PAN) fibers and particularly PAN fibers having a substantially increased modulus of elasticity.

Graphitized PAN fibers are used to form composites with other materials, particularly where high strength-to-density and high modulus-to-density ratios are desired. However, such applications are limited by the ultimate strength, modulus of elasticity and diameter of graphitized PAN fibers. Thus, many attempts have been made to increase strength and particularly the elastic modulus of PAN fibers.

Present methods for the production of PAN-based graphite fibers call for the spinning of the PAN, followed by oxidation, carbonization and subsequent graphitization of the resulting PAN fibers. The acrylonitrile monomer can be made by several known methods including direct catalytic addition of hydrogen cyanide to acetylene or the addition of HCN to ethylene oxide to give ethylene cyanohydrin, followed by dehydration. Polymerization is usually carried out in an aqueous solution with the polymer precipitating from the system as a fine white powder.

Pure polyacrylonitrile is difficult to spin because it is not sufficiently soluble in many organic solvents, and its fibers are not readily dyed. Consequently, polymers other than a pure PAN homopolymer are usually produced commercially. Thus, a "PAN" fiber may actually be an acrylic polymer formed primarily of recurring acrylonitrile units copolymerized with a minor proportion of methyl methacrylate, vinyl pyridine, vinyl chloride and the like. These copolymers exhibit many properties substantially similar to an acrylonitrile homopolymer. By convention, if the fiber does not contain more than about 15 percent foreign material, it is referred to as polyacrylonitrile, and if more than 15 percent then as modified acrylonitrile. Examples of such copolymers include PAN fibers produced under trade names such as Orlon RTM (E.l.DuPont de Nemours), Courtelle RTM SAF (Courtaulds Ltd.) and Acrilan RTM (Chemstrand).

To effect conversion of the PAN fibers to carbon fibers, the PAN fibers are first oxidized by heating to about 220"C while exposed to oxygen or oxygen-containing gases such as air, nitrous oxide and sulphur dioxide. The heating encourages the formation of a ladder structure through a cyclization reaction, some of the CH2 groups being oxidized and HCN being evolved.

The PAN fibers may be further oxidized at higher temperatures up to about 300"C. Thereafter, to effect carbonization, the oxidized PAN fibers are heated to temperatures of 300"C to 800"C in a nonoxidizing atmosphere, such as nitrogen, argon, helium or hydrogen. During this stage, HCN and other products from the decomposition reaction of PAN and some oxygen adsorbed during oxidation are also released as gases. This release is accompanied by the build-up in the fiber of ribbons consisting largely of carbon atoms arranged in aromatic ring structures.

The strength and modulus of these carbonized fibers increases rapidly up to about 1400"C. However, while further heating beyond about 1400"C continues to increase the elastic modulus, tensile strength is reduced, apparently because the structure of the carbonized fibers becomes more representative of true graphite. Consequently, commercial fibers are usually offered in a carbonized form with low modulus and high strength, or in graphitized form with high modulus and low strength.

The presence in the oxidized fiber of oxygen adsorbed during oxidation promotes the formation of the graphite crystalline structure in carbonizing and graphitization processes. In the carbonization and graphitization stages of processing, the oxygen as well as carbon containing compounds are released during the formation of the micro-fibrillar structure associated with high modulus graphite fibers. A large portion of the oxygen is released above the 1400"C carbonization temperature. Direct high temperature graphitization of a 1400"C carbon fiber results in oxygen elimination during the formation of the high modulus fiber structure. The overall effect is degradation of the fiber as well as furnace hardware.In view of the relatively high temperatures involved, high inputs of energy have been reqired to obtain a graphitized fiber of a given modulus of elasticity.

Accordingly, primary objects of the present invention are to provide an improved graphite fiber prepared from a PAN precursor, and a process for producing the improved graphite fiber.

Yet another object of the present invention is to provide graphitized PAN fiber with very high modulus of elasticity.

A more specific object of the present invention is to provide an improved process for making high modulus graphite fibers derived from acrylic polymers consisting primarily of recurring acrylonitrile units.

Yet another object of the present invention is to provide such an improved process wherein such high modulus is achieved at process temperatures lower than those currently used to produce a graphitized fiber of substantially equal modulus from the same precursor.

To these ends, the present invention cycles the fiber temperature through multiple graphitization steps to remove the oxygen and purify the fiber resulting in a higher modulus fiber product, and a furnace with an extended operational life. The multiple step graphitization technique results in a purer atmosphere during final high temperature graphitization. Less erosion of the surface layers of the fibers occurs in this purer atmosphere, as compared to conventional one-step graphitization, resulting in graphite fibers of higher strength and modulus.

Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the processes involving the several steps and the relation and order of one or more of such steps possessing the features, properties and relation of elements which arse exemplified in the following detailed disclosure and the scope of the application all of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the present invention, reference should be had to the following detailed description taken in connection with the accompanying drawing wherein there is shown a schematic of the apparatus used to carry out one embodiment of the present invention.

Referring now to Figure 1, PAN filaments or fibers in the form of a multifilament sheet, web or tow 20, are pulled from fiber supply spool 22, by any known type of constant speed device 24. Tensioning device 28 of known type, typically comprising rollers 29, 30 and 31, in conjunction with take-up device 32, is intended to place the multifilament sheet, tow or web in sufficient uniform tension to draw the PAN fibers, particularly to the extent desired during. oxidation. Fiber tow 20 is transferred under tension through an oxidation chamber, such as gradient furnace 34 maintained at between about 240"C to 275 C.

The tow is moved through furnace 34 in an oxidizing atmosphere at a speed sufficient to provide a proper residence time in the furnance to insure adequate oxidation; i.e. about at 10 to 11 weight percent oxygen content in the fibers when the tow leaves the furnace.

Furnace 34 typically may comprise a number of heating zones, preferably ranging in temperature from a low of about 200"C to a high of about 260"C, but varying from as much as 180"C at the entrance to 300"C at the exit of the furnace. Of course, a series of separate furnaces with one or more heating zones may be employed to establish a series of temperature stages. Likewise, a single heating zone furnace held at a particular temperature may also be appropriate, depending upon the ultimate properties desired in the fiber product An oxygenation medium comprising oxygen and oxygen-containing gases such as air, nitrous oxide and sulphur dioxide is supplied to furnace 34 by line 36.Although only shown as supplied at the inlet of furnace 34, the oxygenation medium may be injected into the furnace at various points along the path of the fibers as they are oxidized.

Pressure relief and recirculation of the oxidation reaction and thermal decomposition products of PAN, as well as any unreacted gases, can be achieved by venting furnace 34 through line 38.

Upon leaving furnace 34, oxidized tow 20 is passed through carbonizing furnace 40. During such carbonization, the oxidized PAN filaments or fibers are heated in a nonoxidizing atmosphere, such as nitrogen, argon, helium or hydrogen, to temperatures of about 300"C up to about 900"C in carbonization furnace 40. The strength and modulus of the oxidized fibers increases rapidly during this stage as carbon dioxide, water, carbon monoxide, HCN, NH3 and other products are released and aromatic ring structures of carbon are formed. The fiber is then introduced into second carbonization furnace 41 to complete carbonization at a temperature of about 1400"C.

The carbonized fibers are cooled and may be stored for subsequent processing or directly graphitized under an inert gas at temperatures beyond 1400cm. In the present invention, graphitization is carried out cyclically through not less than two or more than four heating cycles in which the fiber is preferably rapidly (e.g. from about two to four minutes) raised from a comparatively low temperatures (e.g. room temperature or some other conveniently maintained temperature, typically below carbonization temperatures) to a graphitizing temperature (e.g. about 1800"C to about 2800"C), maintained very briefly-atthat graphitizing temperature (e.g. less than one-half minute and preferably about 15 to 20 seconds), and then rapidly cooled (e.g. in a few seconds) to around the original low temperature. Conveniently, this process can be carried out by moving the room temperature fibers at about one or two feet/minute through a small (e.g. 3") hot zone and then back into a-room temperature environment. Thus, the average residence time in the hot zone is in the neighborhood of 10 seconds, and the temperature gradients from cool to hot and hot to cool are extremely steep. Superior results are obtained when the maximum graphitization temperatures reached in each successive cycle is greater than the maximum temperature of the previous cycle, so that the final cycle is carried out at the highest temperature.

The graphitization cycles are each carried out in graphitizing furnace 42 in a controlled, nonoxidizing atmosphere (e.g. argon, nitrogen, etc.) that may optionally include a graphitization catalyst such as a mixture of triethylborane an hydrogen, or a mixture of diborane and methane, or the like. The catalyst enhances modulus improvement, apparently by healing flaws or voids left in the original fiber structure during processing. Importantly, during graphitization, the fiber should be prevented from shrinking by tensioning the fiber as it unwinds from the feed spool. Permitting the fiber to shrink would lead to a nonuniform graphite structure and failure to attain the highest possible modulus for a given graphitization temperature.

Furnace 42 typically includes a plurality of spaced-apart hot zones 44, 46 and 48 separated by room temperature zones, each hot zone having respective control means (shown generally at 50, 52 and 54) for controlling the temperature. Alternatively, furnace 42 can be formed of a series of separated individual furnaces, each with respective heat controls. Means in the form of take-up device 62 apply a predetermined tension to the fibers as they pass through furnace 42.

Use of cyclic graphitizing allows for the removal of oxygen as well as other volatile compounds prior to the final graphitization cycle. This is essentially a purification process with the goal of increasing the carbon content of the fiber from 95 to 100 percent. The major benefits are achievement of a pure carbon fiber prior to formation of the high modulus graphite structure, a high strength and modulus fiber, and improved furnace life which are results of decreased erosion from oxygen contamination.

The following examples further illustrate the invention and the advantages resulting therefrom. These examples are presented solely for illustration, such that the invention should not be construed as being limited to the particular conditions set forth in the examples. To establish base measurements, an experiment was conducted in which PAN-based fibers, carbonized originally at 1400"C, were graphitized directly at 2800"C in a nitrogen, hydrogen, and triethylborane atmosphere. The resulting properties were measured as follows: 56.8 x 106 psi for the modulus and 245 x 103 psi for the tensile strength.

Example I PAN-based precursor carbon fibers (previously having been fired to 1400 C) identical to those used to obtain the basic measurements, were graphitized to 2300"C for about 15 to 20 seconds and immediately subjected to room temperature cooling, followed by graphitization to 2800do and then cooling quickly to room temperature. At both elevated temperatures the atmosphere of the furnace contained nitrogen, hydrogen, and triethylborane.Measured properties for the fibers processed at each temperature were: 1400"C Modulus 40 x 106 psi Tensile Strength 500 x 103 psi 2300"C Modulus 50.6 x 106 psi Tensile Strength 390 x 103 psi 2800"C Modulus 69.5 x 106 psi Tensile Strength 368 x 103 psi Clearly, the modulus obtained by processing at the final temperature was considerably improved over the basic measurements.

Example II PAN-based precursor carbon fibers, identical to those used in Example I, were graphitized in three cycles in which the fibers were cycled from room temperature to process temperatures and back to room temperature quickly, through three cycles respectively at 1800"C, 2300 C and 2800 C, of about 15 to 20 seconds each. The furnace atmosphere at all three temperatures contained nitrogen, hydrogen and triethylborane.Properties were not measured at 1800"C; those at 2300"C and 2800"C were: 2300"C Modulus 59.1 x 106 psi Tensile Strength 405 x 103 psi 2800"C Modulus 72.0 x 106 psi Tensile Strength 369 x 103 psi It is interesting to note that the addition of the first heating cycle to 18000C resulted in a marked improvement in the modulus over the corresponding products of Example I.

Example III The same type of PAN-based precursor carbon fibers were graphitized cyclically according to the invention at respectively 1800"C, 2300"C and 2800"C for 15 to 20 seconds each. The furnace atmosphere at 1800"C was nitrogen only and at 2300"C and 2800"C was nitrogen, hydrogen and triethylborane. The measured results for all three process temperatures were: 1800"C Modulus 43.6 x 105 psi Tensile Strength 330 x 103 psi 2300"C Modulus 55.4 x 106 psi Tensile Strength 298 x 103 psi 2800"C Modulus 77.0 x 106 psi Tensile Strength 304 x 103 psi Since certain changes may be made in the above process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description of shown in the accompanying drawings shall be interpreted as illustrative and notion a limiting sense.

Claims (13)

1. The method of manufacturing graphite fibers from precursor polyacrylonitrile (PAN)-based carbonized fibers, said method comprising the steps of cyclically heating and cooling said fibers in a non-oxidizing atmosphere between graphitization temperatures and relatively cooler temperatures, through at least two cycles.

2. The method as set forth in claim 1 wherein cyclic heating and cooling is carried out through not more than four cycles.

3. The method as set forth in claim 1 wherein said relatively cooler temperature is below the carbonization temperature at which said carbonized fibers were formed.

4. The method as set forth in claim 3 wherein said carbonization temperature is about 1400"C.

5. The method as set forth in claim 1 wherein said fibers are maintained at said graphitization temperatures for less than about one-half minute during each cycle.

6. The method as set forth in claim 5 wherein said fibers are maintained at said graphitization tem peratures for about 15 to 20 seconds during each cycle.

7. The method as set forth in claim 5 wherein the temperatures of said fibers is altered between said relatively cooler temperatures and said graphitization temperatures in a few seconds.

8. The method as set forth in claim 1 wherein the graphitization temperature in each successive cycle is greater than the graphitization temperature of the next earlier cycle.

9. The method as set forth in claim 8 wherein the graphitization temperature of the last cycle is about 2800"C.

10. The method as set forth in claim 1 including the step of tensioning said fiber during graphitization sufficiently to prevent longitudinal shrinkage thereof.

11. Apparatus for manufacturing graphite fiber from precursor polyacylonitrile-based fibers, said apparatus comprising, in combination: means for heating said fiber under tension at temperatures up to about 1400"C to effect carbonization of said fiber; and means for cyclically rapidly heating and rapidly cooling said carbonized fibers between graphitization temperatures and a temperature below about 1400"C, through at ieast two cycles under tension.

12. The method of manufacturing graphite fibers from precursor polyacrylonitrile (PAN)-based carbonized fibers as claimed in any one of claims 1 to 10, substantially as hereinbefore described.

13. Apparatus for manufacturing graphite fiber from precursor polyacylonitrile-based fibers, substantially as hereinbefore described with reference to the accompanying drawing.