Metal-loaded carbon fibres

Description

(d) Background of Invention

1. Field of Invention

[0001] This invention relates to carbon fibers and a process for their production. More particularly, this invention relates to metal-loaded carbon fibers and a process for their production

2. Prior Art

[0002] Carbon fibers have attracted attention as a reinforcing material for various composite materials due to their high strength, chemical and heat resistance and low weight. Due to these favorable characteristics, they have successfully been employed as materials for aircraft, spacecraft, sports equipment and for many industrial uses. Although these fibers generally possess outstanding physical characteristics, they frequently suffer from poor oxidation resistance at higher temperatures in that they are completely ashed, for example, by contact with air at 500°C after about three hours.

[0003] To improve these oxidation characteristics, protective systems for the fibers have been prepared by using a combination of three general approaches: surface coating, oxygen diffusion barrier layers, and matrix modification.

[0004] Surface coating with ceramics is widely used, but the anisotropic thermal expansion of high strength/high modulus carbon fiber due to thermal cycling makes it difficult to maintain the integrity of protective barriers (with differing thermal expansions) around the reinforcing carbon fibers.

[0005] Barrier layers and matrix additives are also only effective until cracking occurs. In particular, in the event of a matrix breech, the exposed carbon fiber will possess the same deficiencies as non-modified carbon fiber, unless oxidation resistant carbon fibers are prepared which maintain their structural integrity at higher temperatures. Such fibers can be produced with an oxidation resistant additive and/or surface coating. For example, U.S. Patent No. 4,162,301 discloses a process for the production of metal loaded, carbon fibers produced by impregnating a preformed organic polymeric fiber with a solution of a specific metal compound, stabilizing that fiber by a system of heating and stretching, and carbonizing the fiber by heating it to a high temperature in a non-oxidizing atmosphere. The metal is introduced into the preformed organic polymeric fiber by dissolving the metal salt in a bath and immersing the fiber in that bath. Among the metals disclosed are metals from group IVB, group VB and group VIB of the periodic table, as well as boron, aluminum, silicon, thorium, uranium and plutonium. Although the '301 patent discloses the impregnation of a precursor fiber with certain metals, it fails to disclose the loading of carbon fibers with a combination of boron and certain heavy metals to produce greatly improved, metal loaded carbon fibers, the impregnating of the metal into a polyacrylonitrile precursor fiber or the specific process of this invention.

[0006] U.S. Patent No. 4,197,279 discloses acrylic carbon fibers containing a phosphorus component and/or a boron component and, in addition, containing a zinc component and/or a calcium component. Although this patent discloses the impregnation of precursor acrylic fibers, including polyacrylonitrile fibers, it fails to disclose the impregnation of a precursor fiber with a combination of boron and certain heavy metals, as is disclosed in this invention.

[0007] U.S. Patent No. 3,803,056 discloses a process for the production of carbon fibers containing titanium, tantalum, niobium, zirconium or other such heavy metals. However, the patent fails to disclose the polyacrylonitrile precursor fiber used in the present invention and requires the introduction of from about 0.05 to about 6 percent nitrogen to the precursor fiber. In addition, the patent process requires mixing the fibers in a hot metal salt solution and fails to suggest the combination impregnation of the fibers with heavy metals and boron. Thus, there are significant differences between this patent and that of the instant invention.

[0008] Accordingly, it is object of the present invention to prepare improved composite metal loaded carbon fibers.

[0009] It is a further object of this invention to prepare metal loaded carbon fibers containing boron and a heavy metal.

[0010] It is a still further object of this invention to disclose high thermal oxidation resistant and low degradation metal loaded carbon fibers wherein the fibers are impregnated with a combination of boron and certain heavy metals.

[0011] These and other objects, as well as the scope, nature and utilization of the product and the process to produce that product will be apparent to those skilled in the art from a review of the following detailed description and appended claims.

(e) Summary of Invention

[0012] In accordance with the present invention there is provided an improved composite metal loaded carbon fiber comprising a fiberous material containing from about 98.9 to about 70 percent carbon by weight, about 0.1 to about 15 percent boron and about 1 to about 15 percent a heavy metal selected from the group consisting of elements of Group IVB, Group VB and Group VIB of the periodic table.

[0013] There is also provided a process for the production of these improved composite metal loaded carbon fibers as follows:

a. preparing a polymeric filament spinning solution;

b. spinning said polymeric filament solution into filaments;

c. introducing into the filaments a combination of a boron compound and at least one heavy metal salt, wherein the heavy metal is selected from the group consisting of elements contained in Groups IVB, VB and VIB of the periodic table;

d. insolubilizing the combination of the boron compound and the heavy metal salt into the filaments to form a precursor fiber;

f. stabilizing the precursor fiber; and

g. carbonizing the precursor fiber to form a combination loaded boron and heavy metal carbon fibers.

[0014] These composite metal loaded carbon fibers have great utility because of their high strength and oxidation resistance at high temperatures for use as materials for aircraft and spacecraft and for other industrial uses where high oxidation resistance is important.

(f) Detailed Description of Invention

[0015] The precursor carbon filament of the present invention is derived from a polymeric fibrous material. While the polymeric fibrous material may be produced from a regenerated cellulosic material, such as rayon, in a preferred embodiment, the fiber-forming polymer is either an acrylonitrile homopolymer or an acrylonitrile copolymer. An acrylonitrile homopolymer is particularly preferred. Suitable copolymer commonly contain at least about 95 mol percent of recurring acrylonitrile units and up to about 5 mol percent of one or more monovinyl units copolymerized therewith. Representative monovinyl units which may be incorporated in the acrylonitrile copolymers include styrene, methyl acrylate, methyl methacrylate, vinyl acetate, vinyl chloride, vinylidene chloride, vinyl pyridine, and the like. The acrylic polymers may be formed by standard polymerization processes which are well known in the art. Minor quantities of preoxidation of graphitization catalysts may optionally be incorporated in the bulk acrylic polymer prior to spinning.

[0016] The solvent utilized to form the polyacrylonitrile spinning solution may be any conventional spinning solvent and, in a preferred embodiment, the solvent is dimethylacetamide. The standard technical or commercial grade of dimethylacetamide may be employed as the solvent in the formation of the spinning solution.

[0017] The spinning solution may be prepared by dissolving sufficient acrylic polymer in the dimethylacetamide solvent to yield a solution suitable for extrusion containing from about 10 to 30 percent acrylic polymer by weight based upon the total weight of the solution, and preferably from about 15 to 25 percent by weight. In a particularly preferred embodiment of the invention, the spinning solution contains the acrylic polymer in a concentration of about 16 to 20 percent by weight based upon the total weight of the solution. The low shear viscosity of the spinning solution should be within the range of about 90 to 3,000 poise measured at 25°C. If the spinning solution low shear viscosity is much below about 80 poise measured at 25°C, spinning breakdowns commonly occur. If the spinning solution low shear viscosity is much above about 3,000 poise measured at 25°C, extremely high spinning pressures are required and plugging of the extrusion orifice may occur.

[0018] The spinning solution additionally may use lithium chloride as a dope additive. The incorporation of lithium chloride serves the function of lowering and preserving upon standing the viscosity of the spinning solution. The desired solution fluidity and mobility are accordingly efficiently maintained even upon the passage of time. Inclusion of lithium chloride at a level of about 0.5 to about 2 percent by weight based on the total weight of the solution reduced the viscosity of the dope by about 50 percent and allows the preparation of an improved stability dope. However, this improvement of viscosity may be short lived requiring higher percentages of lithium chloride up to about 12 percent by weight when dopes of about 18 to 24 percent are used. However, these high percentage dopes may require excessive pressure at the spinneret and reduce the speed of spinning below those normally achieved by lower percentage dopes. Dopes containing acrylic polymer with a concentration of about 16 to 18 percent produce satisfactory spinning solutions without the addition of a lithium chloride additive. Thus, in a preferred embodiment acrylic polymer concentrations of about 16 to about 18 percent will contain no lithium chloride as an additive.

[0019] If lithium chloride is added to the solution, it may be dissolved in the dimethylacetamide solvent either simultaneously with the acrylic polymer or before or after the acrylic polymer is dissolved therein. Minor quantities of preoxidation or graphitization catalysts may optionally be incorporated in the spinning solution.

[0020] The spinning solution is preferably filtered, such as by passage through a plate and frame press provided with an appropriate filtration medium, prior to wet spinning in order to assure the removal of any extraneous solid matter which could possibly obstruct the extrusion orifice during the spinning operation.

[0021] The spinning solution, containing the fiber forming acrylic polymer dissolved therein, is extruded into a coagulation bath under conditions capable of forming an acrylic filament having an internal structure which is capable upon subsequent thermal treatment of yielding the improved carbon filaments of the present invention.

[0022] It has been found that acrylic filaments are produced when a coagulation bath is utilized having a temperature of about 0° to 45°C (preferably about 10° to 35°C) which consists essentially of about 45 to 85 percent by weight of a non-solvent for the acrylic filament and about 15 to 55 percent by weight of a solvent for the acrylic filament. The non-solvent useful for the coagulation bath include those conventionally used in a coagulation bath for acrylic filaments such as ethylene glycol, methanol, isopropanol, water etc. with methanol and ethylene glycol the preferred non-solvents.

[0023] The preferred solvent utilized in the coagulation bath is dimethylacetamide. When dimethylacetamide is used in concentrations greater than about 55 percent by weight, filament breakage tends to occur at the spinneret. When employing dimethylacetamide in the coagulation bath in concentrations less than about 15 percent by weight, the resulting filaments tend to lose their substantially round cross section and have a tendency to exhibit a more pronounced bean-shaped configuration. Thus, in a preferred embodiment the coagulation bath consists essentially of about 60 to 75 percent by weight of ethylene glycol or methanol and about 25 to about 40 percent by weight of dimethylacetamide.

[0024] The temperature of the spinning solution at the time of its extrusion should be within the range of about 10°C to about 90°C and preferably at about 20°C to 30°C. In a particularly preferred embodiment of the invention, the spinning solution is provided at room temperature, e.g. about 25°C, which thereby facilitates expeditious handling and storage of the same.

[0025] The spinneret utilized during the extrusion may contain a single hole through which a single filament is extruded, or preferably it contains a plurality of holes whereby a plurality of filaments may be simultaneously extruded in yarn or tow form. For instance, tows of up to 20,000 or more, continuous filaments may be formed. The spinneret preferably contains holes having a diameter between about 50 to 150 microns when producing relatively low denier filaments having as-spun denier of about 8 to 24 denier per filament, and holes of about 300 to 500 microns when producing relatively high denier filaments having an as-spun denier of about 100 to 1,500 denier per filament. Extrusion pressures between about 100 and 700 psi may be conveniently selected, and preferably between about 100 and 400 psi. Spinning or extrusion speeds of about 0.5 to about 25 meters per minute and preferably from about 7.5 to about 15 meters per minute may be used.

[0026] Throughout the extrusion process, the coagulation bath is preferably circulated. A relatively constant composition within the coagulation bath may be maintained through the continuous withdrawal and purification of the same. Alternatively, additional non-solvent may be continuously added to the coagulation bath to preserve the desired proportion of solvent to non-solvent within the same. The length of time for the fibers to be held in the coagulation bath is at least about 6 seconds. For instance, residence times of about 6 to 300 seconds may be conveniently selected. Residence times less than about 6 seconds tend to result in an insufficiently developed fiber structure within the as-spun filaments.

[0027] The resulting as-spun filament is immediately passed through an imbibition bath after emerging from the spin bath. For greater absorption of the boron and metal salt into the precursor fiber, the fiber should be preswelled to the greatest extent possible. For example, the precursor fiber could be preswelled in water or other such material prior to immersion in the concentrated imbibing solution. The preferred preswelling agent is an aromatic alcohol.

[0028] Imbibition or impregnation of the precursor fiber can be carried out by several methods. For example, when the metal salt is highly soluble in water, the imbibation step can be carried out merely by immersing the swollen organic fiber in a concentrated aqueous solution of such metal salt. In another example, to impregnate the blend with a boron compound, the preswollen precursor fibers may be sprayed with a weak boric acid solution with a concentration of about 15 percent or less.

[0029] A major advantage of the impregnation technique is the uniform dispersion of very fine particles within the resulting fiber. To increase the extent of the impregnation the temperature of the bath can be elevated as long as such elevation does not interfere with the non-solvent used in the bath. For example, when ethylene glycol is used as the non-solvent the temperature of the bath can be elevated within the range of about 50°C to about 150°C and preferably from about 80° to about 100°C. However, when methanol is used as a non-solvent such increases in temperature would not be proper. Some elevation in temperature may be useful to increase the dispersion of the particles but excess heating would not prove useful. To increase the extent of the impregnation, the temperature of the bath can be elevated, usually in the range of about 50°C to about 150°C and preferably from about 80°C to about 100°C depending on the nature of the non-solvent.

[0030] Immersion time in the imbibation bath will depend on several factors including the desired percentage of metal salt within the fiber, the temperature of the bath and the speed at which the particular metal salt will be absorbed into the precursor fiber. In preferred embodiment the immersion time at preferred temperatures is from about 0.5 to about 10 seconds. If the preferred metal salt is not readily dissolvable in water, other suitable solvents can be used, such as bromoform, carbon tetrachloride, diethyl ether, nitrobenzene, methanol and other alcohols and other organic materials that do not interferingly react with the precursor fiber.

[0031] The metal salts employed for impregnation are salts of carbide-forming heavy metals which have carbides stable at high temperatures. Included within the heavy metals are the group consisting of elements of groups IVB, group VB and group VIB of the periodic table and are preferably selected from the group consisting of titanium, tantalum, niobium, chromium, molybdenum, vanadium, hafnium, zirconium and thorium and, most preferably, tantalum, titanium, niobium, zirconium and hafnium.

[0032] The soluble salts of these metals useful for the purpose of this invention include salts with strong and weak acids, preferably those which have high solubility. Any salt of the heavy metals which is soluble in the imbibing fluid can be used. Typical metal salts employed include chlorides, fluorides, acetates, oxalates and any other metal salt readily soluble in the chosen imbibation bath. For example, when niobium is the chosen heavy metal, niobium pentachloride is a preferred niobium salt.

[0033] The amount of the metal salt to be added to the fiber is, of course, dependant upon the desired concentration of metal in the final product. The concentration of the metal in the fiber increases as the carbonization process is conducted. Thus, to produce carbon fibers with concentration of metal from about 0.5 to about 20 percent the concentration of the metal in the fiber prior to carbonation should range from about 0.5 percent to about 10 percent.

[0034] Although the concentration of the metal salt within the fiber following imbibation may vary, in a preferred embodiment the percentage of metal salt incorporated in the precursor fiber should be from about 0.5 to about 15 percent based on the total weight of the precursor fiber.

[0035] Loading boron into the precursor fiber is somewhat more complicated and usually requires exposure of the precursor fiber to more than one bath containing a boron compound. In a preferred process for adding boron to the precursor fiber, a solution of boric acid with a concentration from about 1 to about 15 percent may be added to the coagulation bath or incorporated into the metal salt imbibation bath. Further, boric acid can be combined with other liquids during subsequent drawing or drying operations to increase the concentration of the boron in and on the precursor fiber. After each treatment with the boric acid, the fibers are preferably lightly rinsed with a water wash to remove any excess boric acid from the fiber surface. By these additional treatments with boric acid, an increase in the percentage of boron in the precursor fiber is accomplished such that the precursor fiber prior to carbonization has a loading of from about 0.1 percent to about 5 percent boron based on the total weight of the precursor fiber.

[0036] Following the imbibition step, the imbibed metal must be insolubilized so it does not wash out of the fiber in subsequent treatments. Various techniques can be used to insolubilize the fibers containing the metal. For example, the fibers may be insolubilized by treating the fiber with a chemical to secure the metal within the fiber. For example when tantalum pentachloride is used as a salt, it is insolubled by converting it to tantalum oxide by treating the the fiber containing the tantalum pentachloride with aqueous ammonia. The aqueous ammonia is collected in a tray through which the fibers are passed around a skewed roll, effectively giving the yarn an alkaline dip treatment. This dip tray is constantly withdrawn and recirculated to keep the ammonia fresh. Many other procedures can be utilized for the insolublization of the fibers as long as the metal is sufficiently insolubilized that it does not wash out of the fibers on subsequent treatment. The concentration of the ammonia in the insolublization bath may range from about 10 percent to about 40 percent.

[0037] After insolubilization the fibers can be washed in a water bath to remove any excess ammonia. Various techniques may be used for the removal of the excess solution include blotting thoroughly with cloths, vacuum filtration and wash baths. In a preferred embodiment, the filament is next washed with water to remove not only excess solution but also all residual amounts of solvent, coagulation bath, and inorganic compound (e.g., lithium chloride).

[0038] The water wash treatment is conveniently conducted in an in-line operation after the filament leaves the imbibation and insolubilization bath. Conventional filament wash rolls may be utilized. The filament alternatively may be washed with water while wound upon a perforated bobbin, or by the use of other washing means, as will be apparent to those skilled in the art.

[0039] The as-spun and washed acrylic filament is drawn or stretched from about 1.5 times its original length up to the point at which the filament breaks, to orient the same and to thereby enhance its tensile properties. Total draw ratios from about 1.5:1 to 15:1 may commonly be selected. The drawing is commonly conducted at an elevated temperature and preferably at a total draw ratio of between about 2:1 and 12:1. The dense internal filament structure makes possible the use of the relatively high total draw ratios indicated. As will be apparent to those skilled in the art, the drawing of the as-spun and washed acrylic filament may be conducted by a variety of techniques. For instance, it is possible for the drawing to be conducted while the filament is (a) immersed in a heated liquid draw medium, (b) suspended in a heated gaseous atmosphere, (e.g., at a temperature of about 120° to about 200°C) or (c) in contact with a heated solid surface (e.g., at a temperature of about 130° to 170°C). If desired, the total draw imparted to the filament may be conducted by a combination of the foregoing techniques. When draw techniques (b) and (c) are utilized, it is essential that the acrylic filament be provided to the draw zone in an essentially dry form. When draw technique (a) is employed, the acrylic filament is subsequently washed to remove the draw medium and is dried. Additionally, the liquid draw medium may also serve a washing and/or coagulating function wherein residual quantities of dimethylacetamide are removed from the water washed fiber.

[0040] In a preferred embodiment of the invention, the washed acrylic filament is at least partially drawn while immersed in a hot liquid bath. In a preferred embodiment the filament is drawn while immersed in a hot water bath maintained at a temperature from about 80°C to about 100°C and at a draw ratio of about 1:1 to 3:1. When additionally boron is to be added to the fiber, the bath water may contain boric acid in the range from about 5 to about 20 percent. After the hot water/boric acid bath the filaments may be washed in a water wash at relatively cool temperatures from about 10 to about 50°C.

[0041] In another preferred embodiment of the invention, the filament may be drawn while immersed in a hot glycerin bath at a temperature of about 80° to about 110°C and at a draw ratio of about 1.5:1 to about 3:1, washed in cool water (e.g., at a temperature of about 10° to about 50°C), and subsequently drawn at a draw ratio of about 3:1 to about 6:1 while in contact with a hot shoe at a temperature of about 100° to about 220°C and preferably at a temperature of about 150° to about 160°C.

[0042] The drawn acrylic filaments optionally may be plied to form yarns or tows of increased total denier as will be apparent to those skilled in the art, prior to thermal conversion into the improved carbon filaments of the present invention, as described hereafter.

[0043] The metal/boron loaded precursor fiber is stabilized in preparation for the carbonization step. To stabilize the fibers the metal compound imbibed organic fibers are heated under controlled conditions, to a temperature of about 200°C to about 280°C, to render the fiber inflammable. It is necessary to heat the metal compound imbibed fiber at a rate sufficiently low to avoid ignition of the fiber.

[0044] The first heating step may be preformed in a nonoxiding inert atmosphere, as for example, that provided by nitrogen, helium, argon, neon and the like or a vacuum. However, if it is desirable to reduce the quantity of carbon remaining from the polymer pyrolysis step, a portion or all of the first heating step may be preformed in an oxygen containing atmosphere. In the stabilization or preoxidation process, the fiber undergoes a series of reactions which oxidize and cross link the molecules. The net reaction is exothermic and the oxidation reaction tends to form a skin core structure in the fiber cross section. To control the exotherm and the skin core formation, stabilization is done in a multi-stage process with incremental increases in temperature as the fiber slowly passes through the various stages. Controlled tension is maintained to prevent misorientation of the molecular chains and a corresponding loss in tensile property.

[0045] The precursory fiber is fed through a tensioning device and feed role stand to a dryer. The material is heated to a temperature from about 180°C to about 210°C to remove any residual solvent, moisture, and lubricant. During this drying the fibers usually undergoes a shrinkage of about 1 to approximately 10 percent. The yarn then passes through a series of ovens with incremental increases in temperature from about 250°C to about 280°C. An additional 10 to about 15 percent shrinkage is experienced as the yarn undergoes the stabilization. As indicated, roll speed are set to accommodate the shrinkage and to maintain an acceptable tension level. As the oxidation and the cross linking reactions progress, the yarn may change colors until it finally appears black. After heating for a period from about 1 to about 5 hours at the elevated temperature, the yarn is sufficiently oxidized and cross linked to proceed with the carbonization step.

[0046] The stabilized yarn is processed through an induction furnace purged with nitrogen. Carbonization temperatures range from about 1250°C to about 2500°C and are dependant upon the specific metal additive and the desired final species. During carbonization, elements such as hydrogen, oxygen and nitrogen are gradually evolved from the fibers structure. The result is a mass loss from the precursor fiber of varying percentages while the loaded metal is generally fully retained. The weight percent of the metal increases as the mass loss of the fiber increases. However, under certain circumstances the percentage concentration of the boron will not increase, because of its loss during the stabilization or carbonization processes. Periods from about 1 to about 4 hours may be employed, while much shorter times may also be employed.

[0047] The fibers produced by this process are comprised of fiberous materials containing by weight from about 99 percent to about 70 percent carbon and about 1 percent to about 30 percent a heavy metal selected from the elements contained in group IVB, group VB and group VIB of the periodic table and boron. When combination loading using both a heavy metal selected from the elements of groups IVB, group VB and group VIB of the periodic table and boron are used, the composite metal loaded carbon fiber is comprised of a fiberous materials containing by weight of about 98.5 to about 70 percent carbon, about 0.5 to about 15 percent boron and about 1 to about 15 percent the heavy metal.

[0048] The composite metal loaded fibers have shown surprisingly significant improvements over unloaded fibers or fibers loaded with a single metal. In particular, these fibers have significantly lower rates of oxidation than unloaded or single metal loaded carbon fibers and exhibit a higher retention of tensile strength after being exposed to an oxidizing environment.

[0049] These high strength, high modulus carbon fibers with boron and heavy metals impregnated therein can be of great use in those situations where high temperature stability is important such as in aircraft or space equipment. In particular, the ability to resist degradation at high temperature along with the high tensile strength are valuable qualities which are quite useful in materials for military equipment which is subjected to high temperature and stress. In addition, the fibers can be used to produce products which maintain their structural integrity without the use of protective coatings.

[0050] The following examples are given as specific illustrations of the invention. All parts and percentages are by weight unless otherwise stated. It should be understood however that the invention is not limited to the specific details set forth in the examples.

Example 1

[0051] A 16 weight percent polyacrylonitrile homopolymer dissolved in dimethylacetamide solution was extruded at a rate of 7.9 grams per minute through 2 - 300 hole, 100 micron diameter spinnerets into a spin bath containing 30 percent dimethylacetamide and 70 percent methanol. The swollen precursor fibers were then passed through a dip solution containing 2 weight percent TaCl₅, 7 percent H₃BO₃, 18 percent dimethylacetamide and 73 percent methanol to incorporate the tantalum and boron into the precursor fiber. The dip bath was maintained at a temperature of about 25°C. After passing through the dip bath the filaments were washed with a 28 percent aqueous ammonium solution and an additional water rinse at a rate of 10 meters per minute. The fibers were then subjected to a hot water draw containing a 10 percent H₃BO₃ solution at 87°C at a draw ratio of 2.8:1. The fibers were then washed again in a water wash bath at a line speed of 28 meters per minute and then dried at a 108°C on a heated roll. The fiber was then drawn at a draw ratio of 2.5:1 in a steam tube maintained at a temperature of 124°C . The fibers were then heated again at a temperature of 108°C, on a heated roll at a roll speed of 70 meters per minute. Analysis of the fibers showed 1 percent by weight of tantalum and 2.6 percent boron loading.

[0052] The fiber were then stabilized by feeding them through a drying stage. The feed rate was 10 inches per minute and the fibers were dried at 195°C for 9.8 minutes, 220°C for 22.1 minutes, 250°C for 86 minutes, 260°C for 91.2 minutes and 275°C for 96.1 minutes. The overall shrinkage of the fiber after this drying process was 16.5 percent.

[0053] The fibers were then carbonized at a temperature of about 2,050°C in an induction furnace for about 2 minutes. The feed rate of the fibers into the oven was 21.25 inches per minute. The atmosphere of the oven was flushed with nitrogen during the carbonization process.

[0054] The carbon fibers showing 1.5 percent tantalum and 3.9 percent boron exhibited a tensile strength of 270 Ksi and a modulus of 38.2 Msi. (ASTM D3379 and D1577). After oxidation at a temperature of 500°C for 24 hours (ASTM D4102), tensile strength was 236 Ksi with an initial modulus of 35.9 Msi. Thermogravometric analysis of the carbon fiber at a heating rate of 20° per minute in air showed a 5 percent weight loss at 850°C and a 70 percent weight retention at 1000°C.

Example 2

[0055] As a comparison, an unloaded polyacrylonitrile precursor fiber was prepared and processed into carbon fiber. A 16 weight percent polyacrylonitrile homopolymer solution in dimethylacetamide was extruded at a rate of 7.9 grams of polymer per minute through two 300 hole, 100 micron diameter spinnerets into a spin bath containing 30% dimethylacetamide, about 70 percent methanol. The swollen fibers were then washed in a water bath on two washrolls at a rate of 10 meters per minute. The fiber was then drawn in a hot water bath maintained at the temperature of 92°C at a draw ratio of 2.8 to 1. The fibers were then twice washed again in a water wash and then dried on a heated roll at a roll rate of 28 meters per minute at 108°C. The fibers were then drawn in a steam tube heated to a temperature 124°C at a draw ratio of 2.5 to 1 and rolled on heated rolls maintaining at a temperature of 108°C at a rate of 70.2 meters per minute.

[0056] These fibers were then stabilized by heating in an oven at temperatures of 195°C for 9.8 minutes, 235°C for 22.1 minutes, 250°C for 86.2 minutes, 260°C for 93.2 minutes and 275°C for 95.3 minutes. The fibers were then carbonized at a feed rate of 21.25 inches per minute at a temperature of about 2,050° in nitrogen atmosphere. The unloaded carbon fibers showed a 5 percent weight loss at 790°C and a residue of only 1.6 percent at 900°C.

Example 3

[0057] An 16 weight percent polyacrylonitrile homopolymer dissolved in dimethylacetamide solution was extruded at a rate 7.9 grams per minute through 2 - 300 hole, 100 micron diameter spinnerets into a spin bath containing 30 percent dimethylacetamide and 70 percent methanol. The swollen precursor fibers were then passed through a dip solution containing 14 percent ZrO[Ac]₂, 1.0 percent H₃BO₃, 18 percent dimethylacetamide and 67 percent water to incorporate the zirconium and boron into the precursor fiber. The dip bath was maintained at a temperature of about 25°C. After passing through the dip bath the filaments were washed with a 28 percent, aqueous ammonium solution and an additional water rinse at a rate of 10.5 meters per minute. The fibers were then subjected to a hot water draw solution at 92°C. The fibers were then washed again in a water wash bath at a line speed of 29.6 meters per minute and then dried at a 108°C on a heated roll. The fiber was then drawn at a draw ratio of 2.0:1 in a steam tube maintained at a temperature of 124°C. The fibers were then heated again at a temperature of 108°C on a heated roll at a roll speed of 60 meters per minute. Analysis of the fiber showed 2.12 weight percent zirconium and 0.2 percent boron.

[0058] The fiber were then stabilized by feeding them through a drying stage. The feed rate was 8.5 inches per minute and the fibers were dried at 210°C for about 10 minutes, 230°C for about 20 minutes, 255°C for about 70 minutes, 265°C for about 70 minutes and 275°C for about 70 minutes. The overall shrinkage of the fiber after this drying process was 16 percent.

[0059] The fibers were then carbonized at a temperature of about 2,050°C in an induction furnace for about 2 minutes. The feed rate of the fibers into the oven was 16 inches per minute. The atmosphere of the oven was flushed with nitrogen during the carbonization process.

[0060] The carbon fibers exhibited a tensile strength of 232.1 Ksi, a modulus of 32.1 Msi (ASTM D3379 and D1577) and elongation of 0.72 percent. After oxidation at a temperature of 520°C for 24 hours (ASTM D4102), tensile strength was 232.1 Ksi, initial modulus was 31.8 Msi and elongation was 0.76 percent. Thermogravometric analysis in air showed a 56 percent weight loss after 300 minutes of 700°C.

Example 4

[0061] A 16 weight percent polyacrylonitrile homopolymer dissolved in dimethylacetamide solution was extruded at a rate of 7.9 grams per minute through 2 - 300 hole, 100 micron diameter spinnerets into a spin bath containing 30 percent dimethylacetamide and 70 percent methanol. The swollen precursor fibers were then passed through a dip solution containing 18 percent ZrO[Ac]₂, 2.0 percent H₃BO₃, 18 percent dimethylacetamide and 62 percent water to incorporate the zirconium and boron into he precursor fiber. The dip bath was maintained at a temperature of about 25°C. After passing through the dip bath the filaments were washed with a 28 percent aqueous ammonium solution and an additional water rinse at a rate of 10.5 meters per minute. The fibers were then subjected to a hot water draw at 90°C. The fibers were then washed again in a water wash bath at a line speed of 29.6 meters per minute and then dried at a 108°C on a heated roll at a roll speed of 60 meters per minute. Analysis of the fiber showed 3.9 weight percent zirconium and 0.30 percent boron.

[0062] The fiber were then stabilized by feeding them through a drying stage. The feed rate was 8.5 inches per minute and the fibers were dried at 210°C for about 10 minutes, 230°C for about 20 minutes, 255°C for about 70 minutes, 265°C for about 70 minutes and 275°C for about 70 minutes. The overall shrinkage of the fiber after this drying process was 16 percent.

[0063] The fibers were then carbonized at at temperature of about 2,050°C in an induction furnace for about 2 minutes. The feed rate of the fibers into the oven was 16 inches per minute. The atmosphere of the oven was flushed with nitrogen during the carbonization process.

[0064] The carbon fibers exhibited a tensile strength of 224.7 Ksi, a modulus of 29.4 Msi (ASTM D3379) and D1577) and elongation of 0.76 percent. After oxidation at a temperature of 520°C for 24 hours (ASTM D4102), tensile strength was 188.6 Ksi, initial modulus was 29.0 Msi and elongation was 0.64 percent. Thermogravometric analysis showed a 36 percent weight loss after 300 minutes at 700°C in air.

Example 5

[0065] A 16 weight percent polyacrylonitrile homopolymer dissolved in dimethylacetamide solution was extruded at a rate of 7.9 grams per minute through 2 - 300 hole, 100 micron diameter spinnerets into a spin bath containing 30 percent dimethylacetamide and 70 percent methanol. The swollen percursor fibers were then passed through a dip solution containing 7 percent TaCl₅, 6 percent H₃BO₃, 18 percent dimethylacetamide and 69 percent methanol to incorporate the tantalum and boron into the precursor fiber. The dip bath was maintained at a temperature of about 25°C. After passing through the dip bath the filaments were washed with a 28 percent aqueous ammonium solution and an additional water rinse at a rate of 10 meters per minute. The fibers were then subjected to a hot water draw containing a 5 percent H₃BO₃ solution at 87°C. The fibers were then washed again in a water wash bath at a line speed of 28 meters per minute and then dried at a 108°C on a heated roll. The fiber was then drawn a second time at a draw ratio of 2.5 to 1 in a steam tube maintained at a temperature of 124°C. The fibers were then heated again at a temperature of 108°C on a heated roll at a roll speed of 70 meters per minute. Analysis of the fiber showed a 4.4 weight percent zirconium and 2.3 percent boron.

[0066] The fiber were then stabilized by feeding them through a drying stage. The feed rate was 8.5 inches per minute and the fibers were dried at 195°C for about 10 minutes, 220°C for about 20 minutes, 250°C for about 70 minutes, 260°C for about 70 minutes and 275°C for an overall shrinkage of the fiber after this drying process was 16 percent.

[0067] The fibers were then carbonized at a temperature of about 2,050°C in an induction furnace for about 2 minutes. The feed rate of the fibers into the oven was 10 inches per minute. The atmosphere of the oven was flushed with nitrogen during the carbonization process.

[0068] The carbon fibers exhibited a tensile strength of 273 Ksi, a modulus of 33.8 Msi (ASTM D3379 and D1577) and elongation of 0.81 percent. After oxidation at a temperature of 520°C for 24 hours (ASTM D4102), tensile strength was 259.8 Ksi, initial modulus was 32.4 Msi and elongation was 0.80 percent. Thermogravometric analysis in air showed a 40 percent weight loss after 300 minutes at 700°C.

Example 6

[0069] As a comparison, an unloaded polyacrylonitrile precursor fiber was prepared and processed into carbon fiber. A 16 weight percent polyacrylonitrile homopolymer solution in dimethylacetamide was extruded at a a rate of 18.1 grams of polymer per minute through four 300 hole, 100 micron diameter spinnerets into a spin bath containing 30% dimethylacetamide and about 70 percent methanol. The fibers were then washed in a water bath on two washrolls at a rate of 10.7 meters per minute. The fibers were then drawn in a hot glycerine bath maintained at the temperature of 95°C at a draw ration of 3.3 to 1. The fibers were then twice washed again in a water wash and then dried on a heated roll at a roll rate of 35 meters per minute at 108°C. The fibers were then drawn over a hot shoe heated to a temperature of 180°C at a draw ratio of 2.3 to 1 to a final roll speed of 80 meters per minute.

[0070] These fibers were then stabilized by heating in an oven at temperatures of 195°C for 9.8 minutes, 235° for 22.1 minutes, 250°C for 86.2 minutes, 260°C for 93.2 minutes and 275°C for 95.3 minutes. The fibers were then carbonized at a feed rate of 20 inches per minute at a termperature of about 2,050° in a nitrogen atmosphere. The unloaded carbon fibers exhibited a tensile strength of 322.4 Ksi, a modulus of 39.3 Msi, and an elongation of 0.82 percent. After oxidation at a temperature of 520°C for 24 hours, tensile strength was 149.6 Ksi, modulus was 36.4 Msi, and elongation was 0.41 percent. Thermogravimetric analysis in air showed a 98 percent weight loss after 300 minutes at 700°C.

[0071] As is apparent, the filaments produced from this process with a combination of boron and heavy metals exhibited enhanced properties, specifically enhanced oxidation characteristics, particularly after heating, and thermal stability. Further, the weight/strength retention was a significant improvement over prior art unloaded or single metal loaded fibers.

Claims

1. A composite metal loaded carbon fiber comprising a fibrous material containing, by weight, from 98.9 to 70 percent carbon, from 0.1 to 15 percent boron and from 0.5 to 15 percent of a heavy metal selected from elements of group IVB, group VB and group VIB of the Periodic Table.

2. The metal loaded carbon fiber of Claim 1 wherein the heavy metal is tantalum, titanium, niobium, hafnium or zirconium.

3. A process for the production of a composite metal loaded carbon fiber comprising:

a. preparing a polymeric filament spinning solution;

b. spinning said polymeric filament solution into a polymeric filament;

c. introducing into the polymeric filament a combination of a boron compound and at least one heavy metal salt selected from salts of elements in group IVB, group VB and group VIB of the Periodic Table;

d. insolubilizing the combination of the boron compound and the heavy metal salt within the polymeric filament to form a precursor fiber;

e. stabilizing the precursor fiber; and

f. carbonizing the precursor fiber to form a composite boron and heavy metal loaded carbon fiber.

4. The process of Claim 3 wherein the heavy metal is tantalum, titanium, niobium, hafnium or zirconium.

5. The process of Claim 3 or 4 wherein the polymeric filament is either an acrylonitrile homopolymer or an acrylonitrile copolymer.

6. The process of any of Claims 3-5 wherein the polymeric filament spinning solution contains from 10 to 30 percent polymeric material by weight based on the total weight of the solution.

7. The process of any of Claims 3-6 wherein the precursor fiber contains from 0.1 to 5% by weight of boron compound and from 0.5 to 15% by weight of heavy metal salt.