Carbon fibers having improved strength and modulus, and methods and apparatus relating to their preparation - Patents.com

Abstract

The present invention is directed to carbon fibers having high tensile strength. The present invention also provides a method and apparatus for making carbon fibers, which includes passing a precursor fiber through multiple passes in an oxidation oven, where stretching during the initial passes is minimized or entirely eliminated or made negative using rollers of increasing speed, followed by controlled stretching throughout the series of passes.

Description

[Technical field]

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 235,529, filed August 20, 2021, the disclosure of which is incorporated by reference in its entirety herein.

[0002] FIELD OF THEINVENTION This invention relates generally to carbon fibers, and more specifically to carbon fibers having improved strength and modulus, as well as methods and apparatus for making carbon fibers. [Background technology]

[0003] Carbon fibers have been used in a wide variety of structural applications and industries due to their desirable properties. For example, carbon fibers can be formed into structural parts that combine high strength and hardness while weighing significantly less than metal parts of comparable physical properties. Carbon fibers can be produced by converting precursor fibers, such as spun polyacrylonitrile (PAN) fibers, in a multi-step process in which the precursor fibers are heated, oxidized, and carbonized to produce fibers with a carbon content of 90% or more. The resulting carbon fibers can be formed into high-strength composites for structural applications that are used solely in electrical and friction applications, or can be further processed for use in adsorbents, filters, or other applications. In particular, composites have been developed in which carbon fibers act as reinforcing materials in resin, ceramic, or metal matrices.

[0004] Current trends in automotive, aerospace, structural, and other applications have led to a continuing demand for materials with ever higher tensile strength and modulus. Polyacrylonitrile-based carbon fibers have become the primary reinforcement in both thermoset and thermoplastic composites to meet this fundamental demand. Their weight reduction and other properties have enabled modern ground and air transportation and structures to be more fuel efficient than previous generations.

[0005] However, there remains a demand for stronger and stiffer materials to enable further performance improvements. High strength and high modulus can result in composites that can achieve comparable strength at lighter weight than the current state of the art, resulting in more fuel efficient airframes or vehicle bodies. The ability to create stronger fibers, preferably without changing other material properties (e.g., cost, density), can enable Pareto improvements in the design of these structures.

[0006] However, many methods of increasing strength and / or modulus come with significant trade-offs for the fiber. For example, it is known in the art that increasing the final carbonization temperature during the conversion of polyacrylonitrile to carbon fiber can increase the modulus of the resulting fiber. However, after a certain temperature, it also decreases the overall tensile strength of the fiber. It also increases the amount of fiber breakage, which increases fuzziness.

[0007] It is also known in the art that control of other important parameters of the fiber as it is converted from polyacrylonitrile (PAN) through a multi-step carbonization process, following an initial slow oxidation process, particularly the amount of tension and stretch, can lead to significant improvements in the final tensile modulus and strength of the final fiber. In particular, stretching during oxidation, imparted by the speed difference between the rolls that transport the fiber into the oxidation oven and the rolls that transport the fiber out of the oven, is important. It has long been recognized in the prior art that the modulus of carbon fibers can be improved by stretching the fibers in post-spinning, oxidation, or carbonization steps, or a combination thereof.

[0008] U.S. Patent No. 4,609,540 describes a method for determining the optimal amount of stretch to apply to precursor fibers in an oxidizing atmosphere. According to the '540 patent, the optimal amount of stretch corresponds to an inflection point that can be determined from a plot of % elongation versus tension, which also corresponds approximately to the maximum degree of crystalline orientation within the fiber. Beyond this inflection point, the '540 patent teaches that any benefit from further stretching is minimized, which may lead to the development of "fuzz" and, in some cases, breakage.

[0009] U.S. Patent No. 8,591,859 describes a method of stretching precursor fibers in an oxidizing atmosphere, where the tensile load is evenly distributed over multiple passes through an oxidation oven. In other words, instead of stretching the fiber only on the final pass through the oven, the fiber is stretched on each pass through the oven. This reference teaches that subjecting the converting fiber to various amounts of stretch, with the goal of very high stretching during oxidation, may allow for further improvement in tensile strength.

[0010] There is room for further optimization of tension principles that may allow the tensile strength to be improved even further without significant damage to the fiber. US Patent No. 8,591,859 suggests that higher strength and modulus may be obtained if the fiber is stretched further during oxidation, but this may result in a significant penalty in throughput by reducing the overall mass per unit length of carbon fiber. A method to increase strength without affecting yield would be desirable. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] U.S. Patent No. 4,609,540 [Patent Document 2] U.S. Patent No. 8,591,859 [Non-patent literature]

[0012] [Non-Patent Document 1] Warner,SB,Peebles,LH,Uhlmann,DROxidative Stabilization of Acrylic Fibers.III.Stabilization Dynamics.(Report #9 NR356-534)Office of Naval Research Summary of the Invention [Problem to be solved by the invention]

[0013] Thus, a need exists for carbon fibers that have both high and low tensile strength, as well as methods and apparatus that can be used to reproducibly prepare such carbon fibers. [Means for solving the problem]

[0014] The present invention provides carbon fibers with improved strength and modulus, as well as methods and apparatus that can be used to prepare the carbon fibers. In one embodiment, the method includes passing a precursor fiber through an oxidation oven, where the fiber is subjected to a controlled stretch in an oxidizing atmosphere in which the tensile load is distributed over multiple passes through the oxidation oven. As a result, the overall cumulative stretch of the fiber can be increased by selecting stretch conditions that allow for the distribution of the tensile load over multiple passes. Distributing the tensile load among multiple passes allows the fiber to be stretched to a greater extent than would be expected. This controlled stretching of the fiber during oxidation can help, for example, improve orientation, uniformity of oxidation, and reduce the growth of defect-inducing crystallites, which in turn can improve the modulus and tensile strength of the resulting carbon fiber.

[0015] In another aspect, the invention is directed to an oxidation oven that allows a precursor fiber to be subjected to multiple controlled stretching passes in an oxidizing atmosphere. In one embodiment, the oxidation oven includes multiple drive rolls and multiple idler rolls that cooperate to define a fiber path through the oxidation oven. In one embodiment, the drive rolls may be driven independently of one another to allow independent control of the speed and / or tension of at least two or more passes through the oxidation oven. In some embodiments, the idler rolls include a tension measuring device, such as a load cell, that allows continuous monitoring of the tension of the fiber as it advances through the oxidation oven.

[0016] After the oxidation step, the remaining process of converting the fibers to carbon fibers can be carried out using conventional methods. The fibers can be converted by passing the oxidized fibers through a low temperature furnace and a high temperature furnace. In one embodiment, controlled stretching of the fibers during oxidation allows for further stretching of the fibers as they pass through the low temperature furnace, for example, by an amount between 5 and 40 percent.

[0017] Carbon fibers prepared according to the present invention may have tensile strengths approaching and exceeding 1,000 ksi, In one embodiment, the present invention provides carbon fibers having a tensile strength of at least 950 ksi.

[0018] Thus, the present invention provides a carbon fiber having improved tensile strength, as well as a method and apparatus for producing said carbon fiber.

[0019] Having thus described the invention in general terms, reference is now made to the accompanying drawings, which are not necessarily to scale. [Brief description of the drawings]

[0020] [Figure 1] 1 is a diagram of the reaction process in which PAN precursor fibers undergo cyclization and oxidation to form pyridone structures. [Diagram 2]1 is an illustration of an exemplary oxidation oven that may be used in accordance with the present invention. [Diagram 3] FIG. 1 is a schematic diagram of a system that may be used to convert precursor fibers to carbon fibers. [Figure 4A] An oxidation oven using a single driven roll scheme. [Figure 4B] An oxidation oven using the pass-by-pass controlled scheme of the present invention, including multiple maintenance rolls followed by multiple stretcher rolls, is shown for comparison in Figures 4A and 4B. [Diagram 5] 4C is a graphical plot showing the pass-by stretch produced by the ovens depicted in FIGS. 4A and 4B, respectively. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The present invention will now be described in more detail hereinafter, some of which are illustrated in the accompanying drawings, in which not all embodiments of the invention are shown. Indeed, these inventions may be embodied in many alternative forms and should not be construed as limited to the embodiments set forth herein, but rather, these embodiments should be presented so that this disclosure can satisfy applicable legal requirements. Like numbering refers to like elements throughout.

[0022] Each embodiment disclosed herein is understood to be applicable to each of the other embodiments disclosed. All combinations and subcombinations of the various elements described herein are within the scope of the embodiments.

[0023] In the following description, various components may be specified as having particular values ​​or parameters. However, these items are presented as exemplary examples. Indeed, the exemplary examples do not limit the various aspects and concepts of the present invention, since many comparable parameters, magnitudes, ranges, and / or values ​​may be implemented. Furthermore, the terms "a," "an," and "the" do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.

[0024] When a range of a parameter is given, it is understood that all integers and ranges within that range, as well as tenths and hundredths thereof, are also provided by way of example. For example, "5 to 10%" includes 5%, 6%, 7%, 8%, 9%, and 10%; 5.0%, 5.1%, 5.2%....9.8%, 9.9%, and 10.0%; 5.00%, 5.01%, 5.02%....9.98%, 9.99%, and 10.00%, as well as, for example, 6 to 9%, 5.1% to 9.9%, and 5.01% to 9.99%. Similarly, when a list is given, each individual element of that list, and each combination of the members of that list, is understood to be a separate embodiment, unless otherwise stated. For example, "1, 2, 3, 4, and 5" includes, among other embodiments, 1; 2; 3; 1 and 2; 3 and 5; 1, 3, and 5; and 1, 2, 4, and 5.

[0025] In one aspect, the present invention is directed to carbon fibers having improved tensile strength. In another aspect, the present invention is directed to an apparatus and method for making carbon fibers. The tensile strength of carbon fibers prepared according to the methods of the present invention can approach and exceed 6000 MPa.

[0026] As used herein, "about" in the context of a numerical value or range means plus or minus 10% of the cited or claimed numerical value or range.

[0027] One embodiment of the invention is a method of making carbon fibers, comprising the steps of: passing the carbon fiber precursor polymer through an oxidation oven to produce an oxidized fiber, said oven having an oxidizing atmosphere at a temperature between about 175 and 300° C.; (i) subjecting the fiber to a first plurality of passes, each of the first plurality of passes having a % draw that is 0.5% or less; (ii) subjecting the fiber to a second plurality of passes, each of the second plurality of passes having a % draw ratio greater than 0.5%; A process comprising: The method includes:

[0028] In some embodiments, the stretch percentage in each of the first plurality of passes is between 0 and 0.1%, inclusive. In some embodiments, the stretch percentage in each of the first plurality of passes is 0%. In some embodiments, the stretch percentage in at least one of the first plurality of passes is negative. In some embodiments, each of the first plurality of passes has the same stretch percentage.

[0029] In one embodiment, in the second plurality of passes, each subsequent pass has a % stretch greater than the % stretch of the immediately preceding pass.

[0030] In one embodiment, the fibers exit an oxidation oven as part of a pass.

[0031] In one embodiment, the fibers include one or more comonomers selected from the group consisting of acrylonitrile, methyl acrylate, methacrylic acid, sodium methallyl sulfonate, and itaconic acid.

[0032] In some embodiments, the precursor fibers have a denier between about 0.6 and 1.53 dpf. In some embodiments, the precursor fibers have a denier between about 0.6 and 0.8 dpf. In some embodiments, the precursor fibers have a denier between about 1.2 and 1.4 dpf.

[0033] In one embodiment, the fibers are introduced into a plurality of oxidation ovens, each successive oven containing an oxidizing atmosphere that is at least as hot as the preceding oxidation oven.

[0034] In some embodiments, the first plurality of paths includes at least two paths. In some embodiments, the first plurality of paths includes at least four paths. In some embodiments, the second plurality of paths includes at least four paths. In some embodiments, the second plurality of paths includes at least six paths.

[0035] In one embodiment, the method further includes passing the oxidized carbon fibers through a low temperature furnace at a temperature between about 350 and 800° C., and then carbonizing the oxidized carbon fibers by passing the oxidized carbon fibers through a carbonization furnace.

[0036] In some embodiments, the carbonization furnace is at a temperature between about 1150 and 2000° C. In some embodiments, the carbonization furnace is at a temperature between about 1200 and 2000° C. In some embodiments, the carbonization furnace is at a temperature between about 1300 and 1500° C.

[0037] In one embodiment, the low temperature furnace is at a temperature between about 300 and 900° C. In one embodiment, the low temperature furnace is at a temperature between about 400 and 800° C.

[0038] In an embodiment, the method further comprises a surface treatment step and a sizing step of the precursor fiber.

[0039] In some embodiments, the oxidation oven is at a temperature between about 150 and 600° C. In some embodiments, the oxidation oven is at a temperature between about 175 and 300° C.

[0040] In one embodiment, upon exiting the oxidation oven, the fibers have an average diameter that is between 0 and 50% lower than the original diameter of the fibers before entering the oxidation oven.

[0041] In one embodiment, the fibers include fiber bundles having between about 1,000 and 50,000 individual filaments.

[0042] In some embodiments, the fibers have improved tensile strength and / or modulus compared to fibers prepared using an idler roll scheme at the same total draw ratio. In some embodiments, the fibers have improved tensile strength and modulus compared to fibers prepared using an idler roll scheme at the same total draw ratio. In some embodiments, the tensile strength of the fibers is measured according to the procedures set forth in ASTM D-4018. In some embodiments, the modulus of the fibers is measured according to the procedures set forth in ASTM D-4018.

[0043] An embodiment is carbon fiber prepared according to any of the above embodiments.

[0044] In some embodiments, the carbon fibers have a tensile strength greater than about 4500 MPa. In some embodiments, the carbon fibers have a tensile strength greater than about 5500 MPa.

[0045] As discussed in greater detail below, carbon fibers according to the invention can be prepared by subjecting precursor fibers, such as fibers comprising polyacrylonitrile (PAN), to multiple passes through an oxidizing atmosphere where the fibers are controllably stretched in two or more passes through the oxidizing atmosphere. Upon completing the oxidation step, the fibers may be passed through one or more additional furnaces, such as low and high temperature furnaces, to complete the conversion of the precursor fibers to carbon fibers. In the context of the present invention, the term "fiber" includes a single filament or multiple filaments bundled together, sometimes referred to as a tow. A tow or bundle may include about 1,000 to 100,000 individual filaments.

[0046] In the context of the present invention, the term "precursor fiber" refers to a fiber comprising a polymeric material that can be converted into a carbon fiber having a carbon content of about 90% or more by mass, particularly about 95% or more, when sufficiently heated. The precursor fiber can include both homopolymers and copolymers of acrylonitrile (AN), and may include copolymers of methyl acrylate (MA), methacrylic acid (MAA), sodium methallyl sulfonate, itaconic acid (IA), vinyl bromide (VB), isobutyl methacrylate (IBMA), and combinations thereof. In one embodiment, the precursor fiber comprises a polyacrylonitrile (PAN) polymer formed primarily from acrylonitrile monomers.

[0047] In an embodiment, the precursor fiber can be prepared by melt spinning by solvating a precursor polymer in an organic and / or inorganic solvent, such as dimethylsulfoxide, dimethylformamide, zinc chloride solution, or sodium thiocyanate solution, to form a spinning solution. In a particular embodiment, the spinning solution is formed from water, acrylonitrile polymer, and sodium thiocyanate in an exemplary weight ratio of about 60:10:30. The solution can then be concentrated by evaporation and filtered to produce the spinning solution. In one embodiment, the spinning solution includes about 15% by weight of acrylonitrile polymer. The spinning solution is passed through a spinneret using a conventional spinning process, such as dry spinning, dry / wet spinning, or wet spinning, to form a polyacrylonitrile precursor. In certain embodiments, PAN precursor fibers are made using a dry / wet spinning process in which bulk filaments are formed from a spinning solution and pass through an air gap from the spinneret or other gap between the spinneret and a coagulant such as an aqueous solution of sodium thiocyanate. After exiting the coagulation bath, the spun filaments are washed. In some embodiments, the spun filaments can be stretched in hot water and steam to several times their original length. (See, for example, U.S. Pat. No. 4,452,860, which is incorporated herein by reference.) Additionally, the polyacrylonitrile precursor fibers can be treated with a sizing agent such as a silane compound to improve their handling during the manufacture of carbon fibers. Examples of methods for preparing PAN precursor fibers are discussed in great detail in U.S. Pat. No. 5,066,433, the contents of which are incorporated herein by reference.

[0048] The precursor fibers may include polyacrylonitrile-based fibers made from between about 85 and 99% by weight acrylonitrile and between about 15 and 1% other monomers such as methacrylic acid, acrylic acid, methyl acrylate, and methyl methacrylate, and combinations thereof. The polyacrylonitrile precursor fibers are in the form of bundles, each containing between about 3000 and 50,000 filaments per bundle, particularly between about 3000 and 24,000 filaments per bundle. The filaments may have a mean average denier of between about 0.50 and 1.50, particularly between about 0.60 and 0.85, of the filaments, preferably with denier differences of 95% or more of the filaments in each bundle within ±0.05 dpf. In one embodiment, the polyacrylonitrile starting material has a smooth surface, a circular cross-section, and an intrinsic viscosity of between about 1.5 and 2.5 deciliters per gram. The diameter of the filaments before conversion may range from about 7.5 to 13.5 μm, more usually from about 8.5 to 10.5 μm.

[0049] During oxidation, also referred to as oxidative stabilization, the PAN precursor fiber is heated at temperatures between about 150° C. and 600° C. in an oxidizing atmosphere to cause cyclization and oxidation of the PAN precursor molecules. In this regard, FIG. 1 illustrates the process of cyclization and oxidation of PAN precursor fiber in a step-by-step manner. In step (A), the nitrile groups of PAN begin to align. X may be any suitable polymerization initiator known in the art. In steps (B) and (C), the nitrile groups polymerize to form a polynaphthyridine ring "ladder" structure, which undergoes tautomerization to form a polycyclic dihydropyridine in step (D). In step (E), the polycyclic dihydropyridine undergoes oxidation / dehydrogenation to form a stabilized pyridone structure.

[0050] During oxidation, the degree to which a given precursor induces the reactions depicted in FIG. 1 is typically a function of temperature and filament diameter. This is believed to be due in part to the effect of oxygen diffusion into the filament. At relatively low temperatures (e.g., below about 240° C.) and / or relatively small filament diameters (e.g., below about 10 microns), the rate of oxygen diffusion into the filament core is promoted relative to the rate of oxygen reaction with the filament surface. At higher temperatures and / or larger filament diameters, oxygen tends to react faster than diffusion is possible, and a skin layer of the oxidized fiber forms around the core where only thermally induced reactions occur. The oxidized surface layer is believed to act as a diffusion barrier for oxygen migration into the filament core. Its presence is undesirable because it leads to both skin-core differences and structural and chemical inhomogeneities in the resulting fiber. For example, skin-core differences and structural inhomogeneities in the oxidized fiber can lead to the modulus of the outer layer being higher than that of the inner layer. This modulus distribution is caused by the difference in the progress of cyclization / oxidation between the inner and outer layers of the precursor fiber, which is believed to be due in part to the selective oxidation of the outer portion of the precursor fiber, resulting in reduced oxygen penetration to the inner portion of the fiber, which results in the formation of a barrier to oxygen diffusion into the fiber.

[0051] Referring to FIG. 2, an exemplary oxidation oven that can be used to controllably stretch precursor fibers is illustrated and generally designated by reference numeral 20. The oxidation oven includes an interior 22 having an oxidizing atmosphere, such as air, maintained at an elevated temperature, typically between about 150 and 600° C., particularly between about 175 and 400° C., and more particularly between about 175 and 300° C. In one embodiment, the precursor fiber 24 passes through the interior of the oven in multiple passes, where the tension that can be applied to the carbon fiber in each of these passes can be independently controlled. In the context of the present invention, the term "high temperature" refers to a temperature high enough to cause oxidation of the PAN precursor fiber, but not so high as to cause undesirable effects in the fiber, such as the occurrence of fiber structural failure, burning, melting, or breaking.

[0052] The oxidation oven 20 includes a number of maintenance rolls (28a through 28d), generally referred to as 28, and a number of stretcher rolls (30a through 30h), generally referred to as 30. Each of the maintenance rolls 28 and stretcher rolls 30 is considered a driven roll that drives at a set speed, as opposed to an idler roll, which rotates only due to forces from the fibers passing therearound. The precursor fiber 24 is fed from a source such as a creel (not shown) and is pulled forward by a driven feed roll 26. Each of the idler rolls 28 cooperates with a corresponding one or more drive rolls 30 to define a fiber path through the oxidation oven. For purposes of the present invention, a "pass" is defined as the path that the fiber, including at least some of the fibers moving through the oxidation oven, travels from an upstream drive roll to a downstream drive roll. A pass so defined may include deflection points in the form of idler rolls, bars, or other such devices (not shown). In the illustrated embodiment, a fiber pass refers to the path of the fiber traveling between a drive roll and a corresponding drive roll. For example, the fiber path between maintenance roll 28b and maintenance roll 28c, or the fiber path between maintenance roll 28d and stretcher roll 30a, each define a single fiber pass through the oxidation oven.

[0053] In some embodiments, the precursor fiber 24 may exit the oxidation oven between successive passes. In this regard, FIG. 2 illustrates an embodiment in which a maintenance roll 28 and a stretcher roll 30 are located external to the oxidation oven. Allowing the fiber to exit the oxidation oven between successive passes may help dissipate some of the heat released during stabilization of the PAN chains so that the fiber is controllably stretched. The external rolls may also help reduce the tendency of the fiber to stick to hot surfaces.

[0054] In other embodiments, the maintenance rolls, stretcher rolls, or both can be positioned within the oxidation oven, and the passes need not be in opposite directions. For example, assuming sufficient residence time within the oven 20, the precursor fiber 24 can be driven in a straight line through the oven 20 by a series of maintenance rolls 28 followed by a series of stretcher rolls 30.

[0055] The stretcher rolls 30 are driven at successively higher speeds to provide stretch and tension during each pass of the stretcher rolls 30. The amount of stretch or tension applied between passes can be independently controlled to control the amount of stretch between passes. For example, stretcher roll 30b can be driven at a speed not significantly different than the speed at which stretcher roll 30a is driven (producing a small % stretch), while comparatively speaking, stretcher roll 30h can be driven at a speed that is more different than the speed at which stretcher roll 30g is driven (producing a larger % stretch). Independently controlling the speed of each of the stretcher rolls 30 allows for independent control of the % stretch of each pass. As a result, successive stretcher rolls 30 can be used to distribute the tension or strain rate over multiple fiber passes through the oxidation oven. In one embodiment, the fiber is exposed to a strain rate of about 10% or less per pass per minute.

[0056] In one embodiment, the maintenance roll 28 and / or stretcher roll 30 are in mechanical communication with a motor for driving each roll separately. Typically, the drive rolls are gear driven separately by independent motors to provide improved control over the speed at which the rolls are driven, and thus the amount of tension applied to the fiber. In some embodiments, a chain drive can also be used, but this is usually less desirable due to speed variations that can occur between the rolls.

[0057] The amount of maintenance and stretcher rolls may be selected based on the desired properties of the resulting carbon fiber. In one embodiment, the oxidation oven may include 2 to 20 maintenance rolls and 2 to 20 stretcher rolls. In another embodiment, the oxidation oven may include 2 to 12 sets of cooperating idler and drive rolls. In some embodiments, assemblies having more than one roll per inlet or configurations having rolls of different dimensions may be used to increase the contact angle between the fiber and the rolls, helping to reduce or eliminate slippage during drawing of the fiber. For example, a set of rolls in close proximity to each other may define an S-shape in the fiber path that can reduce fiber slippage.

[0058] In some embodiments, roll 28 or any of rolls 30 may include a tension measuring device, such as a load cell, that allows for continuous monitoring of the tension of each pass. The measured tension may then be used to independently control the tension applied to the precursor fiber in a given pass by adjusting the speed of the drive rolls relative to one another.

[0059] The stretch rate for a given path is given by Equation 1: % stretching rate=100(V2 / V1-1) (1) is calculated from the difference between the exit speed (V2) and the entry speed (V1) of successive drive rolls using: ##EQU1## For example, a 50% draw ratio can be achieved when the relative speed ratio (exit / entry) is 1.50. The draw ratio can be adjusted by increasing V2 relative to V1, decreasing V1 relative to V2, or varying both speeds simultaneously until the V2 / V1 ratio is 1.50. It should be noted that a 50% draw ratio corresponds to a draw ratio of 1.50. In the context of the present invention, a 50% draw ratio is referred to as "1.5X" and a "2X" draw ratio means a 100% draw ratio compared to the original length (1X) of the fiber. A "3X" draw ratio represents a 200% draw ratio of the original length (i.e., 3 times the original length).

[0060] The "total" or "cumulative" stretch rate for a given pass is calculated using Equation 2: % stretching rate=100(V2 / V i -1) (2) Using this, the exit speed of the driven roll in question (V2) and the initial entry speed of the roll before oxidation (V i ) can be calculated from the difference between the final roll (V F ) may be based on the discharge speed.

[0061] In an embodiment, a "negative" stretch ratio is applied in one or more passes between the maintenance rolls 28. In such a case, the exit speed is less than the entry speed, i.e. the speed of the trailing drive roll may drop relative to the leading drive roll, which results in a drop in tension in that pass. In some cases, the drop in tension can be used to allow the fiber to shrink during oxidation. As mentioned above, stretching in a reactive environment can help lock-in mechanical structural advantages resulting from controlled stretching. A "negative stretch" can also be referred to as "shrinkage" (i.e., -3% stretch is 3% shrinkage). As a result, in some embodiments, the fiber properties can first be enhanced by controlled stretching, and then the fiber is allowed to shrink without losing the advantages provided by the stretching process. This can allow recovery of filament denier or weight per unit length lost in the previous stretch.

[0062] For example, if the exit speed of a given pass is 99% of the entry speed, then the % stretch can be equal to 100x(0.99-1), or -1%. When a pass is described as having a % stretch, such as "0.5% or less," this is understood to include negative % stretches.

[0063] Upon exiting the oxidation oven, the fibers 24 may proceed downstream to one or more additional oxidation ovens, intermediate furnaces, or carbonization ovens. In this regard, FIG. 3 is a schematic diagram of a system and process that may be used to convert precursor fibers to carbon fibers. As shown, the precursor fibers 24 are fed via a feed roll 40. Alternatively, the precursor fibers may be fed from multiple precursor bundles that are assembled into a single bundle using a creel. The precursor fibers then pass through one or more oxidation ovens 20 where they are subjected to controlled stretching.

[0064] In some embodiments, the system may include multiple oxidation ovens, with each successive oven typically maintained at a temperature at least as high as that of the preceding oxidation oven. In some embodiments, each successive oxidation oven may independently have multiple stretcher rolls of increasing speed followed by multiple maintenance rolls, as in the first oxidation oven. When the system includes multiple oxidation ovens with increasing temperature gradients, the temperature of each successive oven is typically between about 1° C. and 50° C. higher than that of the preceding oven, more typically between 5° C. and 20° C. higher. In some embodiments, the temperature gradient can be set in a single oxidation oven using different heating zones within the oven. In other embodiments, the oxidation process can be performed in a lean environment or with oxygen concentrations higher than atmospheric oxygen concentrations. In still other embodiments, the oxidation processing step can begin or be interjected with a non-oxidizing gas treatment, or can be enhanced by the addition of various stabilizing promoters, flow channel pattern arrangements, and other methods known in the art.

[0065] After passing through the oxidation oven or ovens, the drawn, stabilized, and oxidized fiber then passes through a low temperature furnace 42, also called a tar removal furnace, and then through a high temperature furnace 44, also called a carbonization furnace. The low temperature and high temperature furnaces contain an inert gas, such as nitrogen. The temperature of the stabilized fiber in the low temperature furnace ranges between about 300° C. and 900° C., and more usually between 350° C. and 800° C., or between 350° C. and 750° C. In some embodiments, the temperature is different at different parts of the low temperature furnace 42. In some embodiments, the temperature of the low temperature furnace 42 is lower at the fiber entry (lower part of the figure) than at the fiber exit (upper part of the figure).

[0066] The low temperature furnace 42 is purged of volatile products from the passing stabilized fiber undergoing carbonization. After exiting the low temperature furnace 42, the fiber is then exposed to a higher temperature in the high temperature furnace 44, for example between about 1150° C. and 2000° C., particularly between 1250° C. and 1600° C. or between 1250° C. and 1500° C. In a preferred embodiment, the high temperature furnace 44 is between about 1300° C. and 1500° C. In some embodiments, the temperature in the high temperature furnace 44 varies. In some embodiments, the fiber may be exposed to multiple temperatures in a single pass.

[0067] During the travel through the low and high temperature furnaces, the fibers may be subjected to further stretching such that the length of the fibers when they exit is between about 0.1 and 40%, e.g., between 0.1 and 30%, particularly between about 0.1 and 24%, longer than the length when they entered the low temperature furnace. In some embodiments, during the travel through the low and high temperature furnaces, the fibers may be subjected to shrinkage such that the length of the fibers when they exit is between about 0.1 and 8%, e.g., between 0.1 and 5%, particularly between about 0.1 and 3%, shorter than the length when they entered the low temperature furnace. After carbonization is completed, the carbonized fibers may then be subjected to one or more additional treatments, including graphitization, surface treatment, and / or sizing. Graphitization refers to a heat treatment in one or more inert gas furnaces at temperatures above 2000° C. Surface treatment includes anodization, in which the fibers pass through one or more electrochemical baths. Surface treatments can aid in improving adhesion of the fibers to the matrix resin, and thus composite properties, as reflected by tests such as fiber-matrix interlaminar shear strength or short beam shear strength evaluation. Sizing typically involves passing the fibers through a bath containing a water-dispersible material that forms a surface coating or film to protect the fibers from damage during use. In composite applications, the water-dispersible material is typically compatible with the matrix resin targeted for composite production.

[0068] The amount of stretch desired in a given pass, the length of each pass, the number of passes in the oxidation oven, and the residence time of the fiber in the oxidation oven depend on the precursor fiber composition and the desired properties of the carbon fiber. In one embodiment, the precursor fiber can have a total of between about 2 and 40 passes through the oxidation oven, particularly between about 2 and 15 passes through the oxidation oven, such as between 4 and 12 passes. In some embodiments, the length of each pass can range between 4 and 40 feet. Typically, the residence time in the oxidation oven for each pass is between about 0.1 and 20 minutes, such as between about 1 and 12 minutes or between 2 and 10 minutes.

[0069] In one embodiment, enhanced strength carbon fibers can be prepared by passing the precursor through an oxidation oven in multiple passes, with the precursor fiber having a tension between about 100 and 1,000 mg / den in at least two or more passes. In one embodiment, the maximum amount of % stretch imparted to the fiber in a given pass is selected such that the strain rate is less than about 10% / min, particularly less than about 5% / min per pass. Methods for determining the amount of % stretch imparted to a given fiber in a given pass are discussed in greater detail below. The mechanical property benefits achieved through controlled stretching are not limited by the initial diameter, denier, or chemical composition of the precursor fiber.

[0070] In one embodiment, carbon fibers with improved tensile strength can be prepared by subjecting precursor fibers having a filament denier of about 1.5 dpf or less, particularly less than 0.8 dpf, to a cumulative % stretch of between 0 and 100%, particularly between 5 and 60%. In yet another embodiment, the precursor fibers are subjected to a cumulative % stretch of between 0 and 70%, more usually between 15 and 60%. In another embodiment, the precursor fibers are subjected to multiple controlled stretches that result in a reduction in fiber diameter of 20 to 70% compared to the original diameter of the fiber before the oxidation step. In yet another embodiment, the precursor fibers are subjected to a reduction in diameter of between 25 and 50%, particularly between 30 and 45%. In one embodiment, carbon fibers prepared according to the present invention may have a tensile strength greater than 4500 MPa, in particular greater than 5000 MPa, 5100 MPa, 5200 MPa, 5300 MPa, 5400 MPa, 5500 MPa, 5600 MPa, 5700 MPa, 5800 MPa, 5900 MPa, 6000 MPa, 6100 MPa, 6200 MPa, 6300 MPa, 6400 MPa, or 6500 MPa.

[0071] All publications, patents, and patent applications mentioned in this specification are incorporated by reference in their entirety to the same extent as if each individual publication, patent, or patent application was specifically and individually indicated to be incorporated by reference in its entirety, except to the extent the incorporated material contradicts an explicit disclosure herein, in which case the language of the present disclosure will control. EXAMPLES

[0072] It is understood that stretching the fiber to a certain extent during oxidation will improve the tensile strength of the resulting carbon fiber. However, not all stretches are equal. Initial heating of PAN fiber is accompanied by an entropic contraction force acting on the fiber (Warner, SB, Peebles, LH, Uhlmann, DROxidative Stabilization of Acrylic Fibers. III. Stabilization Dyanamics. (Report #9 NR356-534) Office of Naval Research). This force, if restrained, can strongly increase the tension in the fiber. The tension can relax with further heating as the molecules rearrange. With further heating, if unrestrained, the fiber can begin to shrink due to oxidation and cyclization reactions that begin to occur within the fiber itself. If restrained, the tension in the fiber can increase.

[0073] Because of this two-stage heating, the inventors have discovered that when drawn through an oven using a series of idler rolls followed by a single driven roll, the fiber may preferentially draw in the first few passes through the oven, during which time the fiber may preferentially shrink entropically.

[0074] Figures 4A and 4B show a side-by-side comparison of the two roll schemes used in the experiments discussed below. When stretching is provided throughout the oven by having a driven roll at the last roller of the oven, after a number of idler rolls, as in Figure 4A, most of the total stretching occurs in the first few rolls of the oven, where the fiber experiences entropic shrinkage forces associated with the initial heating of the PAN.

[0075] In Figure 4A, PAN 24 enters the first oxidation oven 22 at the bottom right of the figure. The PAN 24 passes through each of the idler rolls 32a through 32i while being pulled solely by driven roll 34. This results in a significant amount of stretch occurring in the initial passes.

[0076] However, by using maintenance rolls instead of idler rolls, the amount of stretch in the initial passes can be varied. By manipulating the drive speed of certain rollers, the amount of stretch in a given pass can be minimized (by reducing the exit roll speed below that of the entrance roll speed) to allow for shrinkage, reduced to 0% stretch in a given pass, or even made negative, as demonstrated in FIG. 4B. Specifically, PAN 24 enters the first oxidation oven 22 at the bottom right of the figure. PAN 24 first passes through each of the maintenance rolls 28a-d, which in this example are identical to each other and run at the same speed as the speed of the entering PAN 24. The PAN then passes through six successive stretcher rolls 30a-f at successively faster speeds before exiting the first oxidation oven 22 for further oxidation.

[0077] As previously discussed, it has typically been acceptable that the amount of % stretch that can be applied to a fiber during oxidation is limited by the accumulation of tensile loads in the fiber. However, the inventors have discovered that even when the overall cumulative % stretch of the fiber is the same, the strength of the stretched fiber may be increased by distributing the tensile load or amount of stretch more evenly over multiple passes through the oxidation oven. As a result, the overall strength of the stretched fiber may be increased by selecting drawing conditions that allow for a more even distribution of the tensile load over multiple passes. By minimizing or eliminating stretch in early heating passes, or even employing a negative stretch in one or more early heating passes, said stretch may be more advantageously distributed throughout all passes. This may further aid in improving the tensile strength of the resulting carbon fiber. In the context of the present invention, the term "cumulative stretch" refers to the overall % stretch of the fiber relative to the fiber before it enters the oxidation oven. The cumulative stretch rate can be calculated either from the product of the stretch rates in each of the individual steps, or from the ratio of the initial and final speeds in the zone of interest.

[0078] In one embodiment, controlled stretching during the oxidation stage can be used in combination with further stretching in a low temperature furnace. It is believed that the more uniform oxidized fibers are less affected by accumulated differential shear strains and can tolerate higher tensions, thus allowing additional stretching to be engineered during the low temperature stage, which may provide additional structural benefits, such as molecular orientation, achieved in the low temperature furnace. In one embodiment, the oxidized fibers can be subjected to a % stretch ratio of between about 1 and 40%, e.g., between about 1 and 30%, and 1 and 24%, in the low temperature furnace.

[0079] The following examples are provided to illustrate aspects of the invention and should not be construed as limiting the invention: Carbon fiber tensile strength was measured according to the method described in U.S. Patent No. 5,004,590, which is incorporated herein by reference in its entirety.

[0080] By not stretching in the early regions and then stretching in the back passes where oxidative shrinkage is the driving force for the tension increase (using a pass-by-pass controlled stretching scheme as illustrated in FIG. 4B), we find a significant increase in tensile strength in the resulting fiber compared to a fiber that allows its natural stretch to be found in the first few oxidative passes (similar to the idler roll scheme illustrated in FIG. 4A). Importantly, the total stretch ratio in both cases is the same, but by delaying the stretching, we observe an increase in tensile strength. The tensile strength of the carbon fiber samples was measured according to the procedure described in ASTM D-4018.

[0081] In Examples 1 to 7, the PAN fiber was subjected to the same amount of total stretch ratio in both the pass-by-pass controlled stretch roll scheme and the idler roll scheme. The fiber stretch was carefully controlled in each pass through the oven by passing through a series of 10 driven rolls (the first 4 were set at the same speed), as in the scheme of FIG. 4B, to avoid stretching at the initial stage of heating. The same PAN fiber, which stretched similarly to the fiber in the corresponding fiber, was also naturally stretched to a total stretch ratio similar to the delayed stretch case by running it over idler rolls other than the last roll that was driven, as in the scheme of FIG. 4A. Each of the other conditions (including oxidation temperature, carbonization temperature, surface treatment, total stretch ratio, and residence time in the oven and furnace) remained similar in the idler roll and driven roll cases.

[0082] Each of Examples 1 through 7 are fibers prepared using similar polymer chemistry. Examples 1 through 5 use smaller diameter PAN fibers, resulting in smaller diameter carbon fibers than Examples 6 through 7. Due to the difference in thickness, some silicon oil is used to finish the fibers of Examples 6 through 7.

[0083] As shown in Table 1, in each case the strength of the same fiber is greater if stretching is performed preferentially later during oxidation, rather than if the fiber is stretched to the extent that it would naturally stretch. [Table 1]

[0084] The average fiber strength is the average of fiber strengths for at least 120 tests in each example for both pass-by controlled drawing and the idler roll scheme. As shown in Table 1, for each of Examples 1 through 5, the strength of the fibers was, on average, more than 3% higher when drawn using the pass-by controlled drawing scheme compared to the idler roll scheme. Similarly, for Examples 6 and 7, the pass-by controlled drawing scheme produced fibers with strengths that were 2.4% and 2.9%, respectively, higher than the idler roll scheme.

[0085] Examples 8 through 10 provided different total stretch amounts, but the controlled stretching scheme for each pass included shrinkage in the first two passes (specifically, −1% or −1.2% stretch). These also produced stronger fibers than those produced by the idler roll scheme. The difference was especially noticeable at the highest stretch amount (Example 9, where the stretch was 22.0%), which resulted in a 3.2% strength improvement.

[0086] Figure 5 shows the relative stretch at each roller for the pass-by-pass controlled stretching scheme (diamonds) and the idler roll scheme (squares) for Example 6. These were calculated by measuring the difference in speed between successive rolls. The speed of each roll was determined by tracking the time to complete 10 revolutions and dividing by 10 times the circumference of the roll. Each scheme resulted in a total stretch of 13.3%.

[0087] In the idler roll scheme, the most stretch (>6%) occurred in the first pass, and more than half the stretch occurred in the first two passes. In the pass-by-pass controlled scheme, no stretch was intentionally allowed over the first four passes, and then larger stretch ratios were gradually introduced in each subsequent pass until the final pass. Since all other conditions, including total stretch ratio, were the same, the increased strength was due to differences in the oven in which the stretching was performed.

[0088] Many modifications and other embodiments of the inventions described herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. It is to be understood, therefore, that the invention is not to be limited to the specific embodiments disclosed, and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

1. A method for producing carbon fibers, comprising: passing the carbon fiber precursor polymer through an oxidation oven to produce oxidized fibers, said oven having an oxidizing atmosphere at a temperature between about 175 and 300°C; (i) subjecting the fiber to a first plurality of passes, each of the first plurality of passes having a % draw ratio that is 0.5% or less; (ii) subjecting the fiber to a second plurality of passes, each of the second plurality of passes having a % draw ratio greater than 0.5%; a process comprising: A method comprising:

2. 10. The method of claim 1, wherein the stretch percentage in each of the first plurality of passes is between 0 and 0.1%, inclusive.

3. The method of claim 1 or 2, wherein the stretch ratio in each of the first plurality of passes is 0%.

4. The method of claim 1 , wherein the draw ratio in at least one of the first plurality of passes is negative.

5. The method of claim 1 , wherein each of the first plurality of passes has the same % stretch.

6. The method of claim 1 , wherein in the second plurality of passes, each subsequent pass has a % stretch greater than the % stretch of the immediately preceding pass.

7. The method of claim 1 , wherein the fibers exit the oxidation oven as part of a pass.

8. The method of claim 1 , wherein the fiber comprises one or more comonomers selected from the group consisting of acrylonitrile, methyl acrylate, methacrylic acid, sodium methallyl sulfonate, and itaconic acid.

9. The method of claim 1 , wherein the precursor fiber has a denier of between about 0.6 and 1.53 dpf.

10. 10. The method of claim 9, wherein the precursor fiber has a denier of between about 0.6 and 0.8 dpf.

11. 10. The method of claim 9, wherein the precursor fiber has a denier of between about 1.2 and 1.4 dpf.

12. 10. The method of claim 1, wherein the fibers are introduced into a plurality of oxidation ovens, each successive oven containing an oxidizing atmosphere that is at least as hot as the preceding oxidation oven.

13. The method of claim 1 , wherein the first plurality of paths includes at least two paths.

14. The method of claim 1 , wherein the first plurality of passes comprises at least four passes.

15. The method of claim 1 , wherein the second plurality of passes comprises at least four passes.

16. The method of claim 15 , wherein the second plurality of passes comprises at least six passes.

17. passing the oxidized carbon fibers through a low temperature furnace at a temperature between about 350 and 800°C; and then carbonizing the oxidized carbon fibers by passing the oxidized carbon fibers through a carbonization furnace. The method of claim 1 further comprising:

18. 18. The method of claim 17, wherein the carbonization furnace is at a temperature between about 1150 and 2000°C.

19. 20. The method of claim 18, wherein the carbonization furnace is at a temperature between about 1300 and 1500°C.

20. 18. The method of claim 17, wherein the low temperature furnace is at a temperature between about 300 and 900°C.

21. 21. The method of claim 20, wherein the low temperature furnace is at a temperature between about 400 and 800°C.

22. 18. The method of claim 17, further comprising a surface treatment step and a sizing step of the precursor fiber.

23. The method of claim 1 wherein the oxidation oven is at a temperature between about 150 and 600°C.

24. 24. The method of claim 23, wherein the oxidation oven is at a temperature between about 175 and 300°C.

25. 10. The method of claim 1, wherein upon exiting the oxidation oven, the fibers have an average diameter that is between 0 and 50% lower than the original diameter of the fibers before entering the oxidation oven.

26. The method of claim 1 , wherein the fiber comprises a fiber bundle having between about 1,000 and 50,000 individual filaments.

27. 10. The method of claim 1, wherein the fiber has improved tensile strength and / or modulus compared to fibers prepared at the same total draw ratio using an idler roll scheme.

28. Carbon fibers prepared according to the method of claim 1.

29. 30. The carbon fiber of claim 28, wherein the carbon fiber has a tensile strength greater than about 4500 MPa.

30. 30. The carbon fiber of claim 28, wherein the carbon fiber has a tensile strength greater than about 5500 MPa.

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