WO2023023640A1

WO2023023640A1 - Carbon fibers having improved strength and modulus and an associated method and apparatus for preparing same

Info

Publication number: WO2023023640A1
Application number: PCT/US2022/075199
Authority: WO
Inventors: Anne RUMINSKI; Peter FERRIN
Original assignee: Hexcel Corporation
Priority date: 2021-08-20
Filing date: 2022-08-19
Publication date: 2023-02-23
Also published as: CN117795142A

Abstract

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

Description

CARBON FIBERS HAVING IMPROVED STRENGTH AND MODULUS AND AN

ASSOCIATED METHOD AND APPARATUS FOR PREPARING SAME

CROSS REFERENCE TO RELATED APPLICATIONS

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

FIELD OF THE INVENTION

[0002] The present invention relates generally to carbon fibers and more particularly to carbon fibers having improved strength and modulus, and a method and apparatus for making the carbon fibers.

BACKGROUND OF THE INVENTION

[0003] Carbon fibers have been used in a wide variety of structural applications and industries because of their desirable properties. For example, carbon fibers can be formed into a structural component that combines high strength and high stiffness, while having a weight that is significantly lighter than a metal component of equivalent properties. Carbon fibers can be manufactured by converting a precursor fiber, such as a spun polyacrylonitrile (PAN) fiber, in a multi-step process in which the precursor fiber is heated, oxidized, and carbonized to produce a fiber that is 90% or greater carbon. The resulting carbon fibers can be molded into high strength composite materials for structural applications, used in their pure form for electrical and friction applications, or can be further processed for use in adsorbent, filter, or other applications. In particular, composite materials have been developed in which carbon fibers serve as a reinforcing material in a resin, ceramic, or metal matrix.

[0004] Current trends in automotive, aerospace, structural, and other applications have continued a push for materials with ever-higher tensile strengths and modulus. Polyacrylonitrilebased carbon fiber has become a leading reinforcement in composites, both thermosets and thermoplastics, that satisfy this basic need. They have enabled current generations of ground and air transportation and structures that are more fuel -efficient than their predecessors due to weight reduction and other efficiencies.

[0005] However, there is still a need for even stronger and stiffer materials to enable further efficiency improvements. Higher strength and modulus allow for a resulting composite that can achieve the same strength at even lighter weights than current state-of-the-art technology, achieving a more fuel-efficient plane or car. Being able to make a stronger fiber, preferably without changing other properties of the material (e.g., cost, density) would allow for a pareto improvement in design of these structures.

[0006] Unfortunately, many methods of increasing strength and/or modulus come with significant tradeoffs to the fiber. For instance, it is well-known in the art that increasing final carbonization temperature during the conversion of polyacrylonitrile to carbon fiber will increase the modulus of the resulting fiber; however, after a certain temperature it also reduces the overall tensile strength of the fiber. It also increases the amount of fiber breakage and therefore builds up fuzz.

[0007] It is also well-known in the art that control of other important parameters, particularly tension and stretching of the fiber as it is converted from polyacrylonitrile (PAN) by first a slower oxidation process and then a multi-step carbonization process, will lead to significant improvements in the final tensile modulus and strength of the final fiber. Particularly important is the stretch in oxidation, which is imparted by a speed differential between the rolls that convey the fiber into the oxidation oven and those that convey the fiber outside the oven. It has previously been recognized in the prior art that the modulus of carbon fibers can be improved by stretching the fibers in a post-spinning step, oxidizing step, carbonizing step, or a combination thereof.

[0008] U.S. Patent No. 4,609,540 describes a method of determining the optimum stretch to be applied to a precursor fiber in an oxidizing atmosphere. According to the ’540 patent, the optimum amount of stretch corresponds to an inflection point that is determinable from a plot of % elongation versus tension, and that this optimum elongation also roughly corresponds to the maximum degree of crystalline orientation within the fibers. Beyond this inflection point, the ’540 patent teaches that any gains from further stretching are minimal and may result in the development of “fuzz” and possibly breakage.

[0009] U.S. Patent No. 8,591,859 describes a method of stretching a precursor fiber in an oxidizing atmosphere, where tension loads are distributed evenly across a plurality of passes through an oxidizing oven. In other words, instead of stretching a fiber only at the last pass through the oven, the fiber is stretched at each pass through the oven. This reference teaches that subjecting the fiber during conversion to different amounts of stretch with the aim of very high stretch in oxidation will allow for a further improvement in tensile strengths. [0010] There is room for further optimization of tension principles that may allow for even further improvement in tensile strength without damaging the fibers significantly. U.S. Patent No. 8,591,859 suggests that higher strength and modulus will be obtained if the fiber is allowed to stretch more in oxidation, but this will incur a significant penalty in throughput by reducing the overall mass per unit length of the carbon fiber. A method that increases the strength without affecting yields would be desired.

[0011] Thus, there exists a need for carbon fibers having both high tensile strength, and for a method and apparatus that can be used to prepare such carbon fibers with a high degree of repeatability.

BRIEF SUMMARY OF THE INVENTION

[0012] The present invention provides carbon fibers having improved strength and modulus and a method and apparatus that can be used to prepare the carbon fibers. In one embodiment, the method comprises advancing a precursor fiber through an oxidation oven wherein the fiber is subjected to controlled stretching in an oxidizing atmosphere in which tension loads are distributed across a plurality of passes through the oxidation oven. As a result, the overall cumulative stretch of the fiber can be increased by selecting stretch conditions that permit distribution of the tension loads over multiple passes. Distributing tension loads amongst a plurality of passes permits the fiber to be stretched to an extent greater than previously expected. This controlled stretching of the fibers during oxidation can help provide, for example, improvements in orientation, uniformity in oxidation, and reduction in the growth of flaw-inducing crystallites, which in turn can provide improvements in the modulus of elasticity and tensile strength of the resulting carbon fibers.

[0013] In another aspect, the invention is directed to an oxidation oven that is capable of subjecting a precursor fiber to a plurality of controlled stretching passes in an oxidizing atmosphere. In one embodiment, the oxidation oven includes a plurality of drive rolls and a plurality of idler rolls, wherein a drive roll and idler roll cooperate to define a fiber pass through the oxidation oven. In one embodiment, the drive rolls can be driven independently of each other so that the speed, and alternatively the tension, on at least two or more of the passes through the oxidation oven can be independently controlled. In some embodiments, the idler rolls include a tension measuring device, such as load cell, that permits continuous monitoring of fiber tension as the fiber is being advanced through the oxidation oven. [0014] After the oxidizing step, the remainder of the process for converting the fibers into carbon fibers can be carried out utilizing conventional methods. The fibers can be converted by advancing the oxidized fibers through a low temperature and a high temperature furnace. In one embodiment, controlled stretching of the fibers during oxidization permits further stretching of the fibers as they are advanced through the low temperature furnace by an amount, for example, between 5 and 40 percent.

[0015] Carbon fibers prepared in accordance with the invention can have a tensile strength that approaches and exceeds 1,000 ksi. In one embodiment, the invention provides a carbon fiber having a tensile strength of at least 950 ksi.

[0016] Thus, the invention provides carbon fibers having improved tensile strength, and a method and apparatus for making such carbon fibers.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

[0017] Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

[0018] FIG. l is a graphical illustration of a reaction process in which a PAN precursor fiber undergoes cyclization and oxidation to form a pyridone structure;

[0019] FIG. 2 is a graphical illustration of an exemplary oxidation oven that may be used in accordance with the invention;

[0020] FIG. 3 is a schematic illustration of a system that can be used to convert a precursor fiber into a carbon fiber;

[0021] FIGS. 4A and 4B are a side-by-side comparison of an oxidation oven using a single driven roll scheme (FIG. 4A) compared to an oxidation oven using the controlled pass-by- pass scheme of the present invention, including a plurality of maintenance rolls followed by a plurality of stretcher rolls (FIG. 4B); and

[0022] FIG. 5 is a graphical plot showing the stretch, by pass, generated by the ovens depicted in each of FIGS. 4 A and 4B.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.

[0024] Each embodiment disclosed herein is contemplated as being applicable to each of the other disclosed embodiments. All combinations and sub-combinations of the various elements described herein are within the scope of the embodiments.

[0025] In the following description, various components may be identified as having specific values or parameters, however, these items are provided as exemplary embodiments. Indeed, the exemplary embodiments do not limit the various aspects and concepts of the present invention as many comparable parameters, sizes, ranges, and/or values may be implemented. Further, 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.

[0026] It is understood that where a parameter range is provided, all integers and ranges within that range, and tenths and hundredths thereof, are also provided by the embodiments. For example, “5-10%” includes 5%, 6%, 7%, 8%, 9%, and 10%; 5.0%, 5.1%, 5.2%....9.8%, 9.9%, and 10.0%; and 5.00%, 5.01%, 5.02%....9.98%, 9.99%, and 10.00%, as well as, for example, 6-9%, 5. l%-9.9%, and 5.01%-9.99%. Similarly, where a list is presented, unless stated otherwise, it is to be understood that each individual element of that list, and every combination of components of that list, is a separate embodiment. For example, “1, 2, 3, 4, and 5” encompasses, among numerous embodiments, 1; 2; 3; 1 and 2; 3 and 5; 1, 3, and 5; and 1, 2, 4, and 5.

[0027] In one aspect, the present invention is directed to carbon fibers having improved tensile strength. In another aspect, the invention is directed to an apparatus and method of making the carbon fibers. Carbon fibers prepared in accordance with the method of the invention can have a tensile strength approaching and exceeding 6000 MPa.

[0028] As used herein, “about” in the context of a numerical value or range means ±10% of the numerical value or range recited or claimed.

[0029] An embodiment of the invention is a method of making a carbon fiber, the method comprising: [0030] advancing a carbon fiber precursor polymer through an oxidation oven to produce an oxidized fiber, the oven having an oxidizing atmosphere at a temperature between about 175 to 300° C;

[0031] wherein said advancing comprises

[0032] (i) subjecting the fiber to a first plurality of passes, wherein each of said first plurality of passes has a stretch % which is less than or equal to 0.5 %; and

[0033] (ii) subj ecting the fiber to a second plurality of passes, wherein each of said second plurality of passes has a stretch % which is greater than 0.5 %.

[0034] In an embodiment, the stretch in each of said first plurality of passes is between 0 and 0.1 %, inclusive. In an embodiment, the stretch in each of said first plurality of passes is 0 %. In an embodiment, the stretch in at least one of said first plurality of passes is negative. In an embodiment, each of said first plurality of passes has an identical stretch %.

[0035] In an embodiment, in said second plurality of passes, each successive pass has a

% stretch which is greater than that of the immediately preceding pass.

[0036] In an embodiment, the fiber exits the oxidation oven as part of a pass.

[0037] In an embodiment, the fiber comprises one or more comonomers selected from the group consisting of acrylonitrile, methyl acrylate, methacrylic acid, sodium methallyl sulfonate, and itaconic acid.

[0038] In an embodiment, the precursor fiber has a denier between about 0.6 to 1.53 dpf.

In an embodiment, the precursor fiber has a denier between about 0.6 to 0.8 dpf. In an embodiment, the precursor fiber has a denier between about 1.2 to 1.4 dpf.

[0039] In an embodiment, the the fiber is introduced into a plurality of oxidation ovens and wherein each successive oven includes an oxidizing atmosphere that is at a temperature that is as least as great as a preceding oxidation oven.

[0040] In an embodiment, said first plurality of passes comprises at least two passes. In an embodiment, said first plurality of passes comprises at least four passes. In an embodiment, said second plurality of passes comprises at least four passes. In an embodiment, said second plurality of passes comprises at least six passes.

[0041] In an embodiment, said method further comprises passing the oxidized carbon fiber through a low-temperature furnace at a temperature between about 350 and 800° C; and subsequently carbonizing the oxidized carbon fiber by passing the oxidized carbon fiber through a carbonizing furnace.

[0042] In an embodiment, the carbonization furnace is at a temperature between about

1150 and 2000° C. In an embodiment, the carbonization furnace is at a temperature between about 1200 and 2000° C. In an embodiment, the carbonization furnace is at a temperature between about 1300 and 1500° C.

[0043] In an embodiment, the low-temperature furnace is at a temperature between about 300 and 900° C. In an embodiment, the low-temperature furnace is at a temperature between about 400 and 800° C.

[0044] In an embodiment, the method further comprises the steps of surface treating and sizing the precursor fiber.

[0045] In an embodiment, the oxidation oven is at a temperature between about 150 to

600° C. In an embodiment, the oxidation oven is at a temperature between about 175 to 300° C. [0046] In an embodiment, upon exiting the oxidation oven, the fiber has an average diameter that is between 0 and 50% less than the fiber’s original diameter prior to entering the oxidation oven.

[0047] In an embodiment, the fiber comprises a fiber bundle having between about

1,000 and 50,000 individual filaments.

[0048] In an embodiment, the fiber comprises improved tensile strength and/or modulus of elasticity compared to a fiber prepared using an idled roll scheme with an identical total stretch. In an embodiment, the fiber comprises improved tensile strength and modulus of elasticity compared to a fiber prepared using an idled roll scheme with an identical total stretch. In an embodiment, the tensile strength of the fiber is measured in accordance with the procedures set forth in ASTM D-4018. In an embodiment, the modulus of elasticity of the fiber is measured in accordance with the procedures set forth in ASTM D-4018.

[0049] An embodiment is a carbon fiber prepared according to any of the above embodiments.

[0050] In an embodiment, the carbon fiber has a tensile strength greater than about

4500 MPa. In an embodiment, the carbon fiber has a tensile strength greater than about 5500 MPa. [0051] As discussed in greater detail below, carbon fibers in accordance with the invention can be prepared by subjecting a precursor fiber, such as a fiber comprising polyacrylonitrile (PAN), to a plurality of passes through an oxidizing atmosphere in which the fiber is controllably stretched in two or more of the passes through the oxidizing atmosphere. Upon completion of the oxidizing step, the fibers can be advanced through one or more additional furnaces, such as a low temperature furnace and a high temperature furnace, to complete conversion of the precursor fibers into carbon fibers. In the context of the invention the term “fiber” includes a single filament or a plurality of filaments that are bundled together, also referred to as a tow. A tow or bundle may include from about 1,000 to 100,000 individual filaments.

[0052] In the context of the invention, the term “precursor fiber” refers to a fiber comprising a polymeric material that can, upon the application of sufficient heat, be converted into a carbon fiber having a carbon content that is about 90% or greater, and in particular about 95% or greater, by weight. The precursor fiber can comprise both homopolymers and copolymers of acrylonitrile (AN), and may include copolymers such as 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.

[0053] In embodiments, the precursor fibers can be prepared by melt spinning, by solvating the precursor polymers in organic and/or inorganic solvents such as dimethylsulfoxide, dimethyl formamide, zinc chloride or sodium thiocyanate solutions to form a spinning solution. In a particular embodiment, the spinning solution is formed from water, acrylonitrile polymer and sodium thiocyanate at exemplary respective weight ratios of about 60: 10:30. This solution can then be concentrated through evaporation and filtered to provide the spinning solution. In one embodiment, the spinning solution comprises about 15% by weight of the acrylonitrile polymer. The spinning solution is passed through spinnerets using conventional spinning processes, such as dry, dry/wet or wet spinning, to form the polyacrylonitrile precursor. In a particular embodiment, PAN precursor fibers are made using a dry/wet spinning wherein a multitude of filaments are formed from the spinning solution and pass from the spinneret through an air gap or other gap between the spinneret and a coagulant, such as aqueous sodium thiocyanate. After exiting from the coagulant bath, the spun filaments are washed. In some embodiments, the spun filaments can be stretched up to several times their original length in hot water and steam. (See e.g., U.S. Pat. No. 4,452,860, which is incorporated herein by reference.) In addition, the polyacrylonitrile precursor fiber can be treated with sizing agents, such as silane compounds, to improve its handling during manufacture of the carbon fiber. Exemplary methods of preparing PAN precursor fibers are discussed in greater detail in U.S. Patent No. 5,066,433, the contents of which are incorporated herein by reference.

[0054] The precursor fibers can comprise polyacrylonitrile based fibers that are made from between about 85 and 99% by weight acrylonitrile and between about 15 and 1% of 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 that each comprise between about 3000 and 50,000 filaments per bundle, and in particular between about 3000 and 24,000 filaments per bundle. The filaments may have a mean average denier between about 0.50 and 1.50, and in particular between about 0.60 and 0.85, preferably with 95 % or more of the filaments in each bundle having differences in denier within ± 0.05 dpf. In one embodiment, the polyacrylonitrile starting material has a smooth surface, a round cross section, and an intrinsic viscosity of between about 1.5-2.5 deciliters per gram. Filament diameters prior to conversion may range from about 7.5 to 13.5 pm, and more generally from about 8.5 to 10.5 pm.

[0055] During oxidation, which is also referred to as oxidative stabilization, the PAN precursor fibers are heated in an oxidizing atmosphere at a temperature between about 150° to 600° C to cause the cyclization and oxidation of the PAN precursor molecules. In this regard, FIG. 1 illustrates in a step-wise manner the process of cyclizing and oxidizing the PAN precursor fibers. In step (A), the nitrile groups of the PAN become aligned. 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 in step (D) to form a polycyclic dihydropyridine. In step (E), the polycyclic dihydropyridine undergoes oxidation/dehydrogenation to form stabilized pyridone structures.

[0056] During oxidation, the extent to which the reactions illustrated in Fig. 1 occur for a given precursor are generally a function of temperature and filament diameter. This is believed to be due in part to the influence of oxygen diffusion into the filaments. At relatively low temperatures (e.g., about 240°C or less) and/or relatively small filament diameters (e.g., about 10 microns or less) the rate of oxygen diffusion into the filament cores is promoted relative to the rate of oxygen reaction with the filament surfaces. At higher temperatures and/or larger filament diameters, oxygen tends to react faster than it is able to diffuse, and a skin layer of oxidized fiber is formed around a core where only thermally induced reactions take place. The oxidized surface layer is thought to act as a diffusion barrier for oxygen transport into the filament cores. Its presence is undesirable because it leads to skin-core differences and both structural and chemical inhomogeneities in the resulting fibers. For example, skin-core differences and structural inhomogeneities in the oxidized fibers can result in the modulus of the outer layer being higher than that of the inner layer. This modulus distribution is caused by the difference in the progression of cyclization/oxidation between the inner and outer layers of the precursor fiber. The difference in the progression of cyclization/oxidation is considered to result from a reduction in oxygen permeation into the inner portions of the fiber due in part to the selective oxidation of the outer portions of the precursor fiber, which leads to the formation of a barrier to oxygen diffusion into the fiber.

[0057] With reference to FIG. 2, an exemplary oxidation oven that may be used to controllably stretch a precursor fiber is illustrated and broadly designated as reference number 20. The oxidation oven includes an interior 22 having an oxidizing atmosphere, such as air, that is maintained at an elevated temperature between that is generally between about 150 to 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 is advanced through the interior of the oven in a plurality of passes in which the tension applied to the carbon fiber in each of these passes can be independently controlled. In the context of the invention, the term “elevated temperature” means a temperature that is high enough to cause oxidation of the PAN precursor fibers, but is not so high as to cause undesirable effects in the fibers, such as the creation of structural defects, burning, melting, or breaking of the fibers, and the like.

[0058] The oxidation oven 20 includes a plurality of maintenance rolls (28a-28d), collectively referred to as reference number 28, and a plurality of stretcher rolls (30a-30h), collectively referred to as reference number 30. Each maintenance roll 28 and stretcher roll 30 is considered a drive roll, as it is driven at a set speed, as opposed to an idler roll, which rotates solely due to the force from fibers passing around it. The precursor fiber 24 is provided from a source, such as a creel (not shown), and is pulled forward by driven feed roll 26. Each idler roll 28 cooperates with one or more corresponding drive rolls 30 to define a fiber pass through the oxidation oven. For the purposes of this invention, a “pass” is defined as the path traveled by the fiber from an upstream drive roll to a downstream drive roll, with at least some portion of the fiber traveling through the oxidation oven. A pass thus 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 as it travels between a drive roll and a corresponding drive roll. For example, the fiber paths between maintenance roll 28b and maintenance roll 28c, or between maintenance roll 28d and stretcher roll 30a, each define a single fiber pass through the oxidation oven.

[0059] In some embodiments, the precursor fiber 24 may exit the oxidation oven between successive passes. In this regard, FIG. 2 illustrates an embodiment wherein the maintenance rolls 28 and the stretcher rolls 30 are disposed on the exterior of the oxidation oven. Permitting the fiber to exit the oxidation oven between successive passes can help to dissipate some of the exothermic heat released while stabilizing the PAN chains, and hence the fibers as they are being controllably stretched. Exterior rolls may also help reduce the tendency of the fibers to stick onto hot surfaces.

[0060] In other embodiments, the maintenance rolls, stretcher rolls, or both may be disposed in the interior of the oxidation oven. Also, it is not necessary that the passes be opposite one another. For example, assuming sufficient dwell time in the oven 20, the precursor fiber 24 may be driven in a straight line through the oven 20 by a succession of maintenance rolls 28 followed by a succession of stretcher rolls 30.

[0061] The stretcher rolls 30 are driven at successively higher speeds, so as to create stretch and tension between passes, on each stretcher roll 30. The amount of stretch or tension applied between passes can be independently controlled, so as to control the amount of stretch between passes. For example, stretcher roll 30b may be driven at a speed that does not differ much from the speed at which stretcher roll 30a is driven (producing a small % stretch), while, relatively speaking, stretcher roll 30h may be driven at a speed that differs more greatly from the speed at which stretcher roll 30g is driven (producing a larger % stretch). Independently controlling the speed on each stretcher roll 30 permits the % stretch on each pass to be independently controlled. As a result, the successive stretcher rolls 30 can be used to distribute tensions or strain rates across a plurality of fiber passes through the oxidation oven. In one embodiment, the fiber is exposed to a strain rate that is no greater than about 10% per minute per pass. [0062] In one embodiment, the maintenance rolls 28 and/or stretcher rolls 30 are each separately in mechanical communication with a motor for driving the rolls. Typically, the drive rolls are each gear-driven separately by an independent motor to provide improved control over the speed at which the rolls are driven, and as a result can provide improved control over the amount of tension that is applied to the fiber. Although chain drives can be used in some embodiments, this is generally less desirable because of variations in speed that may occur between the rolls.

[0063] The amount of maintenance rolls and stretcher rolls can be selected based on the desired properties of the resulting carbon fibers. In one embodiment, the oxidation oven may include from 2 to 20 maintenance rolls, and from 2 to 20 stretcher rolls. In other embodiments, the oxidation oven may include from 2 to 12 pairs of cooperating idler and drive rolls. In some embodiments, assemblies having more than one roll per inlet port, or configurations with rolls of different dimensions may be used to increase the contact angle between the fiber and the rolls and thus help reduce or eliminate slippage of the fiber during stretching. For example, a pair of rolls in close proximity to each other can define an S-shape in the fiber path, which can reduce slippage of the fiber.

[0064] In some embodiments, any of rolls 28 or 30 may include a tension measuring device, such as a load cell, which permits the tension on each pass to be continually monitored. The measured tension may then be used to separately control the tension applied to the precursor fiber in a given pass by adjusting the speed of the drive rolls with respect to each other.

[0065] Stretch for a given pass is calculated from the difference between outlet speed

(V2) and inlet speed (Vi) of successive drive rolls using Equation 1 :

% Stretch = 100 (V2/V1 - 1) (1)

For example, a 50% stretch can be attained if the ratio of relative speeds (outlet/inlet) = 1.50. Stretch can be adjusted by increasing V2 relative to Vi, decreasing Vi relative to V2, or varying both speeds simultaneously, until the ratio V2/V1 = 1.50. Note that a 50% stretch corresponds to a stretch ratio of 1.50. In the context of the present invention, a 50% stretch is referred to as “1.5X”, and “2X” stretch implies 100% stretch in comparison to the original length (IX) of the fiber. A “3X” stretch represents 200% stretch over the original length (i.e., three times as long as the original).

[0066] The “total” or “cumulative” stretch at a given pass may be calculated from the difference between outlet speed (V₂) at the driven roll in question and inlet speed (Vi) of the initial roll, prior to oxidation, using Equation 2:

% Stretch = 100 (V₂/Vi - 1) (2)

Thus, in determining the total stretch for a given scheme, V2 would be based on the outlet speed of the final roll (VF).

[0067] In embodiments, a “negative” stretch is created in one or more passes between maintenance rolls 28. In such cases, the outlet speed is less than the inlet speed; in other words, the the speed of a subsequent drive roll can be reduced with respect to a preceding drive roll, which results in a drop in tension in that pass. In some cases, a drop in tension can be used to permit shrinkage of the fiber during oxidation. As noted above, stretching in a reactive environment may help lock-in mechanical structural gains that are obtained as a result of the controlled stretching. A “negative stretch” may also be referred to as a “shrink” (i.e., a -3% stretch is a 3% shrink). As a result, in some embodiments, the properties of fibers may first be enhanced by controlled stretching and then the fibers are permitted to shrink without losing the gains provided by the stretching process. This may permit the recapture of filament denier or an increase in weight per unit length that was lost in the previous stretches.

[0068] For example, if a given pass has an outlet speed which is 99% of an inlet speed, the % stretch would equal 100 x (0.99-1), or -1%. It is to be understood that when a pass is described as having a stretch % which is “less than or equal to 0.5%,” for example, this includes negative stretch %s.

[0069] Upon exiting the oxidation oven, the fiber 24 can be advanced downstream to one or more additional oxidation ovens, an intermediate furnace, or the carbonization furnace. In this regard, FIG. 3 is a schematic illustration of a system and process that may be used in the conversion of the precursor fibers into carbon fibers. As shown, the precursor fiber 24 is provided via a supply roll 40. Alternatively, the precursor fiber can be provided from a plurality of precursor bundles that are combined into a single bundle with a creel. The precursor fiber is then passed through one or more oxidation ovens 20 where it is subjected to controlled stretching. [0070] In some embodiments, the system may include a plurality of oxidation ovens where successive ovens are maintained at a temperature that is generally at least as great as that of a preceding oxidation oven. In some embodiments, each successive oxidation oven may independently have a plurality of maintenance rolls followed by a plurality of stretcher rolls of increasing speed as in the first oxidation oven. When the system includes a plurality of oxidation ovens having an ascending temperature gradation, the temperature of successive ovens is typically between about 1° to 50° C higher than the temperature of a preceding oven, and more typically from 5° to 20° C higher. In some embodiments, a temperature gradient may be set up in a single oxidation oven by means of different heating zones within the oven. In other embodiments, the oxidation process can be carried out in environments wherein the oxygen concentration is richer or leaner than that of atmospheric air. In still other embodiments, oxidation processing steps may be preceded or interjected by non-oxidizing gas treatments, or may be enhanced by the addition of various stabilization promoters, flow pattern arrangements, and other methods known in the art. [0071] After passing through the oxidation oven or ovens, the stretched, stabilized, oxidized fiber is then passed through a low temperature furnace 42 or furnaces, also referred to as the tar removal furnace, followed by passage through a high temperature furnace 44 or furnaces, also referred to as a carbonization furnace. The low and high temperature furnaces contain an inert gas such as nitrogen. The temperature of the stabilized fiber in the low temperature furnace or furnaces ranges between about 300° C and 900° C, and more typically between 350° C and 800° C, or 350° C and 750° C. In an embodiment, the temperature varies throughout different parts of the low temperature furnace 42. In an embodiment, the temperature of the low temperature furnace 42 is lower at the fiber inlet (the bottom of the figure) than at the fiber outlet (the top of the figure). [0072] The low temperature furnace 42 is purged of volatile products issuing from the passing stabilized fiber undergoing carbonization. After leaving the low temperature furnace 42 or furnaces, the fiber is then exposed to still higher temperatures, e.g. between about 1150° C and 2000° C, and in particular between 1250° C and 1600° C or between 1250° C and 1500° C in the high temperature furnace 44 or furnaces. In a preferred embodiment, the high temperature furnace 44 is between about 1300 to 1500° C. In an embodiment, the temperature in the high temperature furnace 44 or furnace varies. In an embodiment, the fiber may be exposed to multiple temperatures in a single pass. [0073] During travel through the low and high temperature furnaces, the fiber can be subjected to further stretching so that its length is between about 0.1 and 40%, such as between 0.1 and 30%, and in particular between about 0.1 and 24%, longer upon its exit as compared to what it was upon entry into the low-temperature furnace. In some embodiments, during travel through the low and high temperature furnaces, the fiber can be subjected to shrinking so that its length is between about 0.1 and 8%, such as between 0.1 and 5%, and in particular between about 0.1 and 3%, shorter upon its exit as compared to what it was upon entry into the low-temperature furnace. After completion of carbonization, the carbonized fiber may then be subjected to one or more further treatments including graphitization, surface treatments, and/or sizing. Graphitization refers to heat treatments in one or more inert gas furnaces at temperatures exceeding 2000° C. Surface treatments include anodic oxidation in which the fiber is passed through one or more electrochemical baths. Surface treatments may aid in improving fiber adhesion to matrix resins and hence composite properties, as reflected by tests such as fiber-matrix interlaminar or short beam shear strength assessment. Sizing typically involves passing the fibers through a bath containing a water-dispersible material that forms a surface coating or film to protect the fiber from damage during its use. In composite applications, the water-dispersible material is generally compatible with matrix resin targeted for composite manufacture.

[0074] The desired amount of stretching in a given pass, the length of each pass, the number of passes in an oxidation oven, and the residence time of the fiber within the oxidation oven are dependent on the composition of the precursor fibers and the desired properties of the carbon fibers. In one embodiment, the precursor fibers may make between about 2 and 40 total passes through the oxidation oven, and in particular between about 2 and 15, such as between 4 and 12 passes through the oxidation oven. In some embodiments, the length of each pass may range between 4 and 40 feet. Generally, the residence time in the oxidation oven for each pass is between about 0.1 to 20 minutes, such as between about 1 to 12 minutes or 2 to 10 minutes.

[0075] In one embodiment, carbon fibers having improved strength can be prepared by advancing the precursor through the oxidation oven in multiple passes wherein the tension on the precursor fiber in at least two or more of the passes is between about 100 to 1,000 mg/den. In an embodiment, the maximum amount of % stretch to which the fiber is subjected to in a given pass is selected so that the strain rate is about 10%/minute or less, and in particular less than about 5%/minute per pass. Methods for determining the amount % stretch to apply in a given pass for a given fiber are discussed in greater detail below. The gains in mechanical properties attained through controlled stretching are not restricted by the initial diameter, denier, or chemical composition of the precursor fiber.

[0076] In one embodiment, carbon fibers having improved tensile strength can be prepared by subjecting a precursor fiber having a filament denier of about 1.5 dpf or less, and in particular less than 0.8 dpf to a cumulative % stretch that is between 0 and 100% and in particular between 5 and 60%. In yet another embodiment, the precursor fiber is subject to a cumulative % stretch that is between 0 and 70%, and more typically between 15 and 60%. In other embodiments, the precursor fiber is subjected to a plurality of controlled stretches that result in a 20 to 70% reduction in the fiber’s diameter in comparison to the original diameter of the fiber prior to the oxidation step. In still other embodiments, the precursor fiber has a reduction in diameter that is between 25 and 50%, and in particular, between 30 and 45%. In one embodiment, carbon fibers prepared in accordance with invention can have tensile strength in excess of 4500 MPa, and in particular in excess of 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.

[0077] All publications, patents and patent applications referred to herein 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 that the incorporated material is inconsistent with the express disclosure herein, in which case the language in this disclosure controls.

EXAMPLES AND DISCUSSION

[0078] Stretching the fiber to a certain extent in oxidation is understood to improve tensile strength of the resulting carbon fiber. However, not all stretching is equivalent. The initial heating of PAN fiber is accompanied by an entropic shrinking force on the fiber (Warner, S. B, Peebles, L. H., Uhlmann, D. R. Oxidative Stabilization of Acrylic Fibers. III. Stabilization Dyanamics. (Report #9 NR 356-534) Office of Naval Research). This force, if constrained, will strongly increase the tension on the fiber. The tension will relax upon further heating as the molecules rearrange. Upon application of further heating, if unconstrained, the fiber will begin to shrink because of the oxidation and cyclization reactions that begin to occur in the fiber itself; if constrained, the tension of the fiber will increase.

[0079] Because of this two-stage heating, the inventors have found that if allowed to stretch through an oven using a series of idled rolls followed by a single driven roll, the fiber will preferentially stretch in the first few passes through the oven, during the time when the fiber would preferentially be entropically shrinking.

[0080] FIGs. 4A and 4B show a side-by-side comparison of the two roll schemes used in the experiments discussed hereinbelow. If a stretch is created throughout the entire oven by having a driven roll at the last roller of the oven, following numerous idled rolls, as in FIG. 4A, the majority of the total stretch occurs in the first few rolls of the oven, when the fiber is undergoing the entropic shrinking force associated with initial heating of the PAN.

[0081] In FIG. 4A, the PAN 24 enters the first oxidation oven 22 at the bottom right of the figure. The PAN 24 passes over each of idled rolls 32a-32i while being pulled by the lone driven roll 34. This results in a significant amount of stretching occurring in the initial passes.

[0082] However, by using maintenance rolls instead of idled rolls, the amount of stretch in the initial passes can be changed. By manipulating the drive speeds of certain rollers, the stretch in a given pass can be minimized, reduced to 0% stretch in a given pass, or even made negative, so as to permit shrinking (by reducing the exit roll speed below that of the entrance roller speed), as demonstrated in FIG. 4B. Specifically, the PAN 24 enters the first oxidation oven 22 at the bottom right of the figure. The PAN 24 first passes over each of maintenance rolls 28a-28d, which run at a speed identical to each other in this embodiment, and to the speed of the incoming PAN 24. The PAN then passes over six successive stretcher rolls 30a-30f, at successively higher speeds, before exiting the first oxidation oven 22 on the way to further oxidation. [0083] As previously discussed, it has been commonly accepted that the amount of % stretch that can be applied to the fiber during oxidation is limited by the accumulation of tension loads in the fibers. However, the inventors have discovered that even when the overall cumulative % stretch of the fiber is the same, the strength of the stretched fiber can be increased by distributing the tension loads or stretches more evenly in a plurality of passes through the oxidation oven. As a result, the overall strength of the stretched fiber can be increased by selecting stretch conditions that permit a more even distribution of the tension loads over multiple passes. By minimizing or eliminating stretch in the early heating passes, or even employing a negative stretch in one or more early heating pass, the stretching may be distributed more advantageously across the full set of passes. This can further help to improve the tensile strength of the resulting carbon fibers. In the context of the invention, the term “cumulative stretch” refers to the overall % stretch of the fiber in comparison to the fiber prior to entering the oxidation oven. Cumulative stretch can be calculated from either the product of the stretches at each individual step or from the ratio of initial and final speeds within the section of interest.

[0084] In one embodiment, controlled stretching in the oxidation stage can be used in conjunction with further stretching in the low temperature furnace. It is believed that more uniformly oxidized fibers are less affected by differential shear strain accumulation and can therefore tolerate more tension and hence, can handle additional stretch during the low temperature stage, which can provide additional structural gains, e.g., in molecular orientation, to be achieved in the low temperature furnace. In one embodiment, the oxidized fiber can be subjected to a % stretch in the low temperature furnace that is between about 1 and 40%, e.g., between about 1 and 30%, and 1 and 24%.

[0085] The following Examples are provided for illustrating aspects of the invention and should not be construed as limiting the invention. Carbon fiber tensile strengths were measured according to the methods described in U.S. Patent No. 5,004,590, which is hereby incorporated by reference in its entirety.

[0086] By preventing stretch in the initial region, but then stretching in later passes when oxidative shrink is the driving force to tension increasing (using a pass-by-pass controlled stretch scheme as exemplified in FIG. 4B), we find a significant increase in tensile strength in the resulting fiber as compared to a fiber that is allowed to find its natural stretch in the first few passes of oxidation (as in an idled roll scheme exemplified by FIG. 4A). Importantly, the total stretch in both cases is identical, but by delaying the stretching, we see an increase in tensile strength. Tensile strengths of the carbon fiber samples were measured in accordance with the procedures set forth in ASTM D-4018.

[0087] In Examples 1 -7, PAN fibers were subj ected to an identical amount of total stretch in both a pass-by-pass controlled stretch roll scheme and an idled roll scheme. By passing through a series of 10 driven rolls (the first four of which were set to identical speeds), as in the scheme of FIG. 4B, the fiber stretch in each pass through the oven was controlled carefully to avoid stretch in the initial period of heating. Identical PAN fibers with the same stretch as the fiber in the corresponding fibers were also run over idled rolls, with the exception of the final roll, which was driven, as in the scheme of FIG. 4A, thus allowing the fiber to naturally stretch to the same total stretch as in the delayed-stretch case. Every other condition (including oxidation temperature, carbonization temperature, surface treatment, total stretch, and oven and furnace residence times) remains the same for idle-roll and driven roll case.

[0088] Each of Examples 1-7 is a fiber prepared using the same polymeric chemistry.

Examples 1-5 use smaller diameter PAN fiber, resulting in a smaller diameter carbon fiber, compared to Examples 6-7. Due to the difference in thickness, slightly more silicon oil is used to finish the fibers of Examples 6-7.

[0089] As shown in Table 1, in every case, the strength of the same fiber is greater in the case where the stretch is preferentially performed later in oxidation rather than if the fiber is allowed to stretch where it would naturally do so:

Table 1: Tensile strength of fiber prepared with idled rolls or pass-by-pass controlled stretch through the first oxidization zone.

[0090] Average fiber strength is an average of at least 120 assays of fiber strength in each

Example, for both the pass-by-pass controlled stretch and the idled roll scheme. As shown in Table 1, for each of Examples 1-5, the fiber had a greater strength when stretched using a pass-by-pass controlled stretch scheme, compared to an idled roll scheme, by an average of greater than 3%. Similarly, with respect to Examples 6 and 7, the pass-by-pass controlled stretch scheme produced fibers that were 2.4% and 2.9% stronger, respectively, compared to the idled roll scheme.

[0091] Examples 8-10 created different total amounts of stretch, but each pass-by-pass controlled stretch scheme included shrink in the first two passes (specifically, -1% or -1.2% stretch). These, too, produced fibers that were stronger than those produced by an idled roll scheme. The difference was particularly noticeable at the highest amount of stretching (Example 9, with a 22.0% stretch), which produced a 3.2% strength improvement.

[0092] Fig. 5 shows the relative stretch at each roller for the pass-by-pass controlled stretch scheme (diamonds) and the idled roll scheme (squares), for Example 6. These were calculated by measuring the difference in speeds between successive rolls. Speeds of each roll were measured by tracking the time to complete 10 revolutions and dividing by 10 times the circumference of the roll. Each scheme resulted in a 13.3% total stretch.

[0093] In the idled roll scheme, the most stretch (over 6%) occurred in the first pass, and more than half of the stretch occurred in the first two passes. In the pass-by-pass controlled scheme, there was no stretch over the first four passes, by design; more stretching was gradually introduced in each successive pass, until the final pass. As all other conditions were identical, including total stretch, the increased strength was due to the difference in where in the oven the stretching was performed.

[0094] Many modifications and other embodiments of the invention set forth 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. Therefore, it is to be understood that the inventions are 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

THAT WHICH IS CLAIMED:

1. A method of making a carbon fiber, the method comprising: advancing a carbon fiber precursor polymer through an oxidation oven to produce an oxidized fiber, the oven having an oxidizing atmosphere at a temperature between about 175 to 300° C; wherein said advancing comprises

(i) subjecting the fiber to a first plurality of passes, wherein each of said first plurality of passes has a stretch % which is less than or equal to 0.5 %; and

(ii) subjecting the fiber to a second plurality of passes, wherein each of said second plurality of passes has a stretch % which is greater than 0.5 %.

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

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

4. The method of any one of claims 1-3, wherein the stretch in at least one of said first plurality of passes is negative.

5. The method of any one of claims 1-4, wherein each of said first plurality of passes has an identical stretch %.

6. The method of any one of claims 1-5, wherein in said second plurality of passes, each successive pass has a % stretch which is greater than that of the immediately preceding pass.

7. The method of any one of claims 1-6, wherein the fiber exits the oxidation oven as part of a pass.

8. The method of any one of claims 1-7, 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.

-22-

9. The method of any one of claims 1-8, wherein the precursor fiber has a denier between about 0.6 to 1.53 dpf.

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

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

12. The method of any one of claims 1-11, wherein the fiber is introduced into a plurality of oxidation ovens and wherein each successive oven includes an oxidizing atmosphere that is at a temperature that is as least as great as a preceding oxidation oven.

13. The method according to any one of claims 1-12, wherein said first plurality of passes comprises at least two passes.

14. The method according to any one of claims 1-13, wherein said first plurality of passes comprises at least four passes.

15. The method according to any one of claims 1-14, wherein said second plurality of passes comprises at least four passes.

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

17. The method of any one of claims 1-16, further comprising passing the oxidized carbon fiber through a low-temperature furnace at a temperature between about 350 and 800° C; and subsequently carbonizing the oxidized carbon fiber by passing the oxidized carbon fiber through a carbonizing furnace.

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

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

20. The method according to any one of claims 17-19, wherein the low-temperature furnace is at a temperature between about 300 and 900° C.

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

22. The method of any one of claims 17-21, further comprising the steps of surface treating and sizing the precursor fiber.

23. The method according to any one of claims 1-22, wherein the oxidation oven is at a temperature between about 150 to 600° C.

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

25. The method according to any one of claims 1-24, wherein upon exiting the oxidation oven, the fiber has an average diameter that is between 0 and 50% less than the fiber’s original diameter prior to entering the oxidation oven.

26. The method according to any one of claims 1-25, wherein the fiber comprises a fiber bundle having between about 1,000 and 50,000 individual filaments.

27. The method according to any one of claims 1-26, wherein the fiber comprises improved tensile strength and/or modulus of elasticity compared to a fiber prepared using an idled roll scheme with an identical total stretch.

28. A carbon fiber prepared according to the method of any one of claims 1-27.

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

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