GB2084975A - Carbon fibres - Google Patents

Abstract

A process is provided for the formation of carbon-fibre precursor materials, primarily of acrylonitrile units (with increased modulus of elasticity and strength as well as decreased diameter. Preferably, the material is acrylonitrile, typically copolymerized with a minor amount of an acrylic monomer such as methyl acrylate) by oxidation of the fiber at an elevated temperature in the presence of a carboxylic acid, or its anhydride within the fiber. The acid and/or its anhydride which is formed at the oxidizing temperature, serves as a plasticizer and reduces the fiber yield stress and increases fiber plasticity so that the fibers may be drawn by as much as 300% or more in the presence of the acid and/or its anhydride during oxidation, thereby providing improvement in increased elasticity and strength.

Description

SPECIFICATION Method of producing carbon fiber and product thereof The present invention relates to polyacrylonitrile (PAN) fibers and particularly improved oxidized PAN fibers and the carbonized and graphitized forms thereof.

Polyacrylonitrile (-CH2CH(CN)-) constitutes a major component of many industrial textile fibers.

Oxidized PAN fiber is potentially useful to form heat protective fabrics as a substitute for asbestos.

Carbonized and graphitized polyacrylonitrile (PAN) fibers 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, elasticity and diameter of the carbonized and graphitized PAN fibers. Thus, it is not surprising that many attempts have been made to increase strength and elasticity and reduce the diameter of PAN fibers.

Particularly, the smaller the diameter of the fiber, the greater is the ratio of surface area of the fiber to either weight or volume. The greater ratio thus provides increased fiber-to-matrix interface area, distributing the loading on the composite over a greater area so as to improve interlaminar shear strength markedly for composite materials utilizing such smaller diameter fibers. Additionally, smaller diameter fibers of improved strength and elasticity are considerably more flexible than larger diameter fibers of similar strength and elasticity, permitting formation of desirably thin woven fabrics or even braided and knitted fabrics, as composite precursors.

Present methods for the production of PAN-based carbon fibers call for the spinning of the PAN, followed by oxidation and carbonization 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 is not readily dyed. Consequently, polymers other than a pure PAN homopolymer are often produced. 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 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 1 5% then as modified acrylonitrile. Examples of such copolymers include PAN fibers produced under trade names such as Orlon (E. I. DuPont de Nemours), Courtelle SAF (Courtaulds Ltd.) and Acrilan (Chemstrand).

The conversion of the PAN to fibers may be accomplished by either dry or wet spinning. In the latter a salt solution of the polymer is extruded through a spinneret into a liquid which can coagulate the PAN. In the dry spinning process a filament is formed by the evaporation of a volatile solvent from a PAN solution. In either case the filaments are subsequently stretched to several times their original length at a slightly elevated temperature, for example 1 000C, so as to draw out and align the main polymer chains and increase interchain adhesion. Thus, Patent No. 3,729,549 indicates that the fibers are sometimes oriented by hot drawing over a heated shoe at a draw ratio of about 3:1 to about 7:1.In one instance stretching PAN fibers some fourteen times reportedly produced a fiber with a Young's modulus and strength of 2.7 x 106 psi and 130 x 103 pus/, respectively.

The denier of the resulting PAN fibers, such as those used as precursors for oxidation and carbonization, generally measures from 1.3 to 3. The spinning and production of PAN fibers with a denier of less than 1.3 as such precursors has proven impractical, since such fibers have been heretofore too fragile for processing.

To effect conversion of the PAN fibers, the latter are heated to about 2200C 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, while some of the CH2 groups are oxidized and HCN is evolved. This may be ideally summarized as:

Some references in the prior art indicate that PAN fibers should be prevented from shrinking or drawn slightly during oxidation in order to restrain the PAN fibers from reverting to their weak unstretched state. However, U.S. Patent No. 4,100,004, issued to Moss et al., indicates that some fibers (not identified as to source) may generally be stretched by at least 50%, though no experimental values greater than 50% are revealed and stretching of the fibers is generally limited to resulting carbonized fiber diameters of 6 microns or more.The maximum amount of stretching indicated by the prior art is approximately 95%. However, even a slight amount of stretching during oxidation is generally avoided.

In fact, general industries practice is to allow the fibers to shrink slightly during oxidation in order to avoid any damage to the fibers at this stage of the process.

The PAN fibers may be further oxidized at higher temperatures up to about 3000 C. Thereafter, to effect carbonization, the oxidized PAN fibers are heated to temperatures of 300 to 1 4000C in a nonoxidizing atmosphere, such as nitrogen, argon, helium or hydrogen. During this stage HCN and other products from the decomposition reaction of PAN 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 14000 C.

However, while further heating beyond about 1 4000C continues to increase the elastic modulus, it reduces tensile strength, 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. For example, in one case, heating PAN fibers from about 1 4000C to about 24000C reportedly resulted in a decrease in strength from approximately 3.1 GN/m2 (4.48 x 105 psi) to 2.2 GN/m2 (3.2 x 105 psi), but an increase in the modulus from approximately 230 GN/m2 (33.4 x 106 pSi) to 500 GN/m2 (72.5 x 106 psi).

From the foregoing it can be seen that restrictions on the size of precursor fiber employed and the amount to which the fibers can be stretched during oxidation, place limits on the strength and elasticity as well as the diameter of the resulting oxidized, carbonized or graphitized fibers produced. Additionally, in view of the relatively high temperatures involved, high inputs of energy are required to obtain an oxidized PAN, carbon or graphitized fiber of a given modulus and elasticity.

Accordingly, a primary object of the present invention is to provide an improved fiber and method of making the same.

Another object of the present invention is to provide an improved oxidized PAN, carbon or graphite fiber as well as a process for producing the same.

Yet another object of the present invention is to provide an oxidized PAN, carbon or graphite fiber with increased tensile strength and modulus of elasticity.

Still another object of the instant invention is to provide an oxidized PAN, carbon or graphite fiber of reduced cross sectional area.

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

Yet another object of the present invention is to provide an improved process whereby smaller denier precursor fibers can be employed in the production of carbon and graphite fibers.

Other objects of the present invention are to provide an improved process for making carbon or graphite fibers wherein a precursor fiber of an acrylic polymer consisting primarily of recurring acrylonitrile units, is oxidized and then is stretched during oxidation: to provide carbon fibers of increased thermal stability which exhibit enhanced molecular structure; to provide an improved process for the formation of stabilized fibrous materials derived from acrylic polymers which results in a product which is suitable for carbonization, or carbonization and graphitization; and to provide a carbon or graphite fiber derived from an acrylic polymer, which carbon fiber retains desirable textile properties when used at elevated temperature, e.g. strength, ductiiity, stiffness, and abrasion resistance at temperatures as high as 5000 C.

These and other objects are basically accomplished by mixing the acrylic polymer with a carboxylic acid so that the polymer fiber is permeated therewith, the acid being one which, upon heating the fiber to an oxidizing temperature, will form the anhydride corresponding to that acid (and it is postulated that the anhydride will equilibrate within the fiber with the acid); oxidizing the treated fibers in an oxidizing atmosphere; and stretching the fibers during the oxidation process. The acid and/or its anhydride appears to act as a plasticizer and reduces the fiber yield stress and increases fiber plasticity such that during oxidation the treated fibers may be drawn by at least 40% and as much as 300% or more than similar but untreated fibers. The acid should be present in the fiber during oxidation, in a quantity sufficient to improve the stretchability the noted amount.The relative amount of acid used depends then upon such factors as the nature of the acid, the choice of the particular fibers as to constituents and diameter, the length of time allowed to permit the acid to permeate the fiber to some desired extent, etc, and can easily be determined empirically for each set of parameters.

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 with respect to each of the others, and the products possessing the features, properties and relation of elements which are 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: Fig. 1 is a schematic of the apparatus used to carry out one embodiment of the present invention; and Fig. 2 is a representation of a typical temperature gradient of an oxidation furnace employed in one embodiment of the instant invention.

Referring now to Fig. 1, PAN fibers in the form of a multifilament sheet, tow or web, 20, are pulled from fiber supply spool, 22, by constant speed device, 24, which comprises a pair of electric drive rollers, 25 and 26. Tensioning device 28 of known type, typically comprising three 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 to the extent desired during oxidation.In this regard it is preferable to stretch the PAN fibers during the oxidation process, by at least 40% and as much as 300% or more over the stretchability of the untreated fibers, since the greater the stretching accomplished, the more one will achieve the purposes of the invention to produce higher strength and modulus fibers.

The fiber tow 20, is then transferred under tension through an oxidation chamber such as multizone gradient furnace 34, so as to provide a proper residence time, as discussed below. Upon leaving furnace 34 the oxidized and stretched PAN fiber of tow 20 is taken up on known constant speed take-up device 32, before being passed to a carbonizing zone for further treatment.

During subsequent carbonization by well-known techniques, the oxidized PAN fibers are heated in a nonoxidizing atmosphere, such as nitrogen, argon, helium or hydrogen to temperatures of about 300 to 1 4000C. 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 carbonized fibers may then be further heated and graphitized under an inert gas at temperatures up to 30000 C. Both carbonization and graphitization may be carried out in one or more stages, during which the fibers are generally placed under some tension. Further details of such treatment are not believed to be required herein, since such are well known in the prior art as illustrated by Moss et al. in U.S.Patent No. 4,100,004 and Gump et al in U.S. Patent No. 3,729,549.

Multizone gradient furnace 34 comprises a number of heating zones preferably ranging in temperature from a low of about 2000C to a high of about 2600 C, but varying from as much as 180"C at the entrance to 3000C at the exit of the furnace. A typical temperature gradient is depicted in Figure 2 in terms of temperature of the furnace atmosphere at a given distance from the furnace entrance. 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, although it may be desirable to permit the gases in the furnace to remain relatively stagnant to encourage the postulated equilibrium between the vaporized acid and its anhydride in the fiber. A main component of these decomposition products is HCN, particularly during oxidation of the fibers. However, other components include CO, CO2, H20, NH3, as well as a number of intermediate hydrocarbons and nitriles including acetonitrile and acrylonitrile.

In accordance with the present invention, a carboxylic acid which, at the temperatures used to oxidize the fiber, will substantially vaporize and convert to the corresponding anhydride with which the acid can be in material equilibrium at least in the fiber in part, is mixed with the PAN fiber. Inasmuch as such temperatures are in the range of about 1 80 to 3000 C, it is apparent that formic acid is excluded inasmuch as it decomposes at such temperatures and does not form an anhydride. Other carboxylic acids may not vaporize at such temperatures or may not thermally form their anhydride in sufficient amounts to maintain a substantially balanced equilibrium i.e. the reaction goes virtually to completion in one direction or the other. Thus, both mono and polycarboxylic acids are useful.Typically, such diverse carboxylic acids as acetic acid and itaconic acid are acceptable for purposes of the invention. Mixing of the fiber and acid can occur in the original manufacturing process for the fiber, or the fiber can be permeated or impregnated with the carboxylic acid by imbibition in an appropriate solution of the acid.

The imbibition time to impregnate the fiber depends upon the composition of the fiber, particularly the nature of the interstitial voids provided by the introduction of copolymers and other materials into the original fiber. Typically, imbibition times of from one minute to several hours can be used, but the longer imbibition times seem to provide the better results.

The plasticizing action of the carboxylic acid and/or its an hydride is believed to facilitate molecular motion in the PAN fibers. In any event, some acids may be less readily absorbed in the PAN fiber than others, depending upon the steric aspects of the acid and the molecular structure of the particular fiber.

Thus, care must be taken to insure proper mixture of fiber polymer and concentration of the acid, both in amount and in time as the case may be, to allow absorption of the latter into the acrylic fibers.

Homopolymer acrylonitrile exhibits little if any permeation by carboxylic acids from a soaking bath, even over extended periods of time, so the desired acid should be incorporated into the fiber preferably at the time of spinning.

In accordance with the present invention, fiber residence time in the furnace should generally not be less than 2 minutes, and is preferably in the range from 2 minutes to 120 minutes, since the plasticizing effect is not immediate. Consequently, oxidizing the fibers slowly is favored.

In this regard the acrylic polymer which is utilized in the present process is formed either entirely of recurring acrylonitrile units, of or recurring acrylonitrile units copolymerized with a minor proportion of one or more vinyl units to produce a copolymer exhibiting properties substantially similar to an acrylonitrile homopolymer, particularly with regard to the time needed to undergo oxidation. As to the temperature used in the oxidation process, while acrylonitrile homopolymers can be used in the present process, other PAN copolymer fibers which oxidize over a wide temperature range are preferred.

In the prior art, the acrylic fibers were stretched, if at all, during oxidation by approximately not more than 95% their original length. In contrast the treatment of the fibers with carboxylic acid in accordance with the present invention, allows the treated acrylic fibers to be stretched during oxidation, as much as 300% or more compared to the untreated fiber, thus increasing the ultimate strength and modulus of the resulting carbon fibers by as much as 40% and 50% or more, respectively. The increase in the strength and elasticity of the oxidized acrylic fibers is believed to occur because improved extension of the fibers causes greater chain orientation than has been heretofore possible.

As readily appreciated by those skilled in the art, these improved properties are obtained at little or no increase in cost, since the plasticizing effect may be obtained by simple addition of inexpensive carboxylic acids to the fiber. Additionally, overall energy requirements to produce a given strength or modulus of oxidized or carbonized PAN fiber are reduced, since the fibers obtain greater strength and elasticity at an earlier stage of the process, thus reducing the number of stages and the extent of heating required. Concomitant equipment savings are likewise obvious to those skilled in the art.

Use of the carboxylic acid and/or its anhydride at the oxidizing temperatures as a plasiticizing medium also allows smaller size fiber precursors to be processed and smaller diameter oxidized PAN and carbon fibers to be produced than previously possible. Prior art processes required precursor fibers of at least approximately 1.3 denier, or greater, and produced oxidized PAN fibers of 12 microns or larger in diameter and carbon fibers of 6 microns or more in diameter. In contrast, the present invention allows the processing of fiber precursors of 1.2 denier or less and the production of oxidized PAN fibers as small as 3 microns in diameter as well as carbon fibers of as small as 2 microns in diameter with little or no increase in process requirements.

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.

EXAMPLE I E. I. DuPont Orlon brand fiber is a commercially available PAN fiber with copolymer units interspersed throughout the fiber structure. On information and belief, the comDosition of the fiber is 94% polyacrylonitrile and 6% methyl acrylate. A thermal gradient from 220 to 2400C was established in furnace 34 in several steps. Pure oxygen served as the oxygenation medium supplied through line 36.

The drawn and oxidized PAN fiber, untreated with any acid, exhibited a maximum draw ratio of 1.27, corresponding to a 2796 elongation.

EXAMPLE II Example I was repeated, but prior to oxidizing the PAN fiber, the latter was soaked for one minute in a 8.3% itaconic acid solution in a dip tank, then put thrnucjh squeeze rolls to express excess fluid, and placed into the oxidizing chamber. On drawing the fiber during oxidation to a maximum temperature of 255 C, a maximum draw ratio of 1.44 was obtained, an improvement of .20 in the draw ratio.

EXAMPLE Ill Example II was repeated but the fiber was after soaking, additionally pretreated in air saturated with 8.5 , itaconic acid solution at 1 900C for four hours before oxidation. In this instance, a maximum draw ratio of 1.56 was obtained.

EXAMPLE IV Courtauld's SAF Courtelle fiber (hereinafter referred to as SAF) is a commercially available PAN fiber with copolymer units interspersed throughout the fiber structure. This fiber is believed to differ from the DuPont Orlon brand of acrylic fiber used in Example I in that, on information and belief, the composition of the Courtelle fiber is 93% polyacryionitrile, 6% methyl acrylate and 1% itaconic acid. An SAF 3000 fiber tow of 1.2 d'tex of this Courtelle fiber was drawn in three stages to 300% during oxidation with a residence time of about 1 hour. A thermal gradient from 2200C to 2600C was established in furnace 34 in three steps of 10 to 20 degrees each (i.e. 220-230, 230-240, 240-260). Pure oxygen served as the oxygenation medium supplied through line 36.The maximum draw ratio was 3.08 resulting in 3 208% elongation at the end of the first oxidation stage (max. temp.

2430C), a considerable improvement over the similar DuPont fiber in which no carboxylic acid was present prior to treatment.

Four tows of the stretched fiber were collimated to make one 12,000 fiber two during the last step of the oxidation process. The drawn and oxidized PAN fiber was-then carbonized continuously between 300 and 9000C under nitrogen and thereafter graphitized under nitrogen at 1 4000C and at 23000C in two steps. The graphite fiber diameter was found to be approximately 3 microns and a tensile strength of 46 x 104 psi and tensile modulus of 64 x 106 psi were observed.This represents a 31% increase in tensile strength and a 30% increase in tne modulus, since a grnphltized tiber prepared under identical conditions, but without drawing and use of a plasticizing medium during oxidation, had a diameter of approximately 7 microns with a tensile strength of 35 x 104 psi and a tensile modulus of 50 x 106 psi.

EXAMPLE V Example IV was repeated, but prior to oxidizing the PAN fiber, the latter was soaked for one minute in a 6% itaconic acid solution in a dip tank, then put through squeeze rolls to express excess fluid, and placed into the oxidizing chamber. On drawing the fiber during oxidation, a draw ratio of 3.61 was obtained.

EXAMPLE VI Example V was repeated three times but in each case the PAN fiber was soaked in a only water.

Upon drawing the fiber during oxidation, respective draw ratios of 3.03, 3.03 and 3.17 were observed, in excellent agreement with the results of Example IV.

EXAMPLE VII Example V was repeated using 10% acetic acid solution as the dip. On fiber drawing the observed maximum draw ratio was 3.60.

EXAMPLE VIII Example V was repeated using a 1% acetic acid solution as the dip. A draw ratio of 3.37 was obtained upon drawing the fiber.

Claims (14)

1. A method of oxidizing fibres of an acrylic polymer comprising recurring acrylonitrile units, wherein the fibres are oxidized in an oxidizing atmosphere heated to a temperature range between about 2000C to 3000C preparatory to carbonization of the fibres and, during oxidation the fibres are drawn to substantially their normal limit of elongation, the polymer being permeated with a carboxylic acid capable of forming its anhydride when heated to within the temperature range, the permeated fibres being drawn under tension during oxidation of the fibres in the atmosphere within the temperature range, the carboxylic acid being present initially in the polymer to permit improved drawing of the fibre to at least 40% beyond its normal limit.

2. A process as claimed in claim 1, in which the polymer is acrylonitrile homopolymer.

3. A process as claimed in claim 3, in which the polymer is an acrylonitrile polymer containing at least about 85 mole percent of acrylonitrile units copolymerized with at least another material.

4. A process as claimed in claim 1, in which the acid is a monocarboxylic acid.

5. A process as claimed in claim 1, in which the acid is a discarboxylic acid.

6. A process as claimed in claim 1, in which the acid is acetic or itaconic acid.

7. A process as claimed in claim 1, in which the fibre is maintained in the atmosphere for a period of between 2 to 120 minutes.

8. A process as claimed in claim 1, in which a temperature gradient is established along the fibres from about 2200C to about 2600C.

9. A process as claimed in claim 1, in which the fibre is treated with the acid in a soaking bath before disposing the fibres with the oxygen-containing atmosphere.

10. A process as claimed in claim 1, in which the acid is incorporated in the fibres in the during or prior to the original formation of the latter.

11. A process as claimed in claim 1, in which initially the fibres are substantially less than 1.35 denier.

1 2. An oxidized fibre produced from a polyacrylonitrile precursor permeated with a carboxylic acid prior to oxidation, and being drawn by under tension during oxidation in an oxidizing atmosphere to more than 40% greater than the maximum draw of the untreated fibre.

1 3. A graphitized fibre produced from a polyacrylonitrile precursor fibre and having a tensile strength of approximately 49 x 104 psi, a modulus of approximately 42 x 106 psi and a diameter of about 5 microns or less.

14. A graphitized fibre produced from polyacrylonitrile precursor fibre and having a tensile strength of approximately 46 x 105 psi, a modulus of approximately 64 x 106 psi and a diameter of about 5 microns or less.

1 5. A process for producing a carbon fibre comprising steps of supplying oxygen to a multizone gradient furnace, maintaining a temperature gradient in the multizone gradient furnace by heating the latter from about 2200C at the inlet thereof to about 2600C at the outlet thereof, passing a multifilament tow of fibres formed of an acrylonitrile copolymer which contains about 85 mole percent acrylonitrile units and about 1 5 mole percent methyl acrylate, permeated with a carboxylic acid, through the multizone gradient furnace, placing the multifilament tow of fibres under uniform tension so as to draw the fibres while passing the multifilament tow through the oxygen in the heated furnace atmosphere and thereafter passing the multifilament tow through a second multizone gradient furnace with a temperature of about 4000C at the inlet to about 9000C at the outlet.

1 6. A method of oxidizing fibres of an acrylic polymer comprising recurring acrylonitrile units substantially as herein described.