CN117795142A

CN117795142A - Carbon fibers having improved strength and modulus and related methods and apparatus for making same

Info

Publication number: CN117795142A
Application number: CN202280053903.9A
Authority: CN
Inventors: A·鲁明斯基; P·菲林
Original assignee: Hexcel Corp
Current assignee: Hexcel Corp
Priority date: 2021-08-20
Filing date: 2022-08-19
Publication date: 2024-03-29
Also published as: KR20240042676A; WO2023023640A1

Abstract

The present invention relates to carbon fibers having high tensile strength. The invention also provides a method and an apparatus for manufacturing the carbon fiber. The method includes advancing the precursor fiber through an oxidation oven via multiple passes, wherein stretching during an initial pass is minimized or completely eliminated or becomes negative, followed by controlled stretching in a series of passes using rollers of increasing speed.

Description

Carbon fibers having improved strength and modulus and related methods and apparatus for making same

Cross Reference to Related Applications

The present application claims the benefit of U.S. provisional patent application No. 63/235,529 filed 8/20 of 2021, the disclosure of which is hereby incorporated by reference in its entirety.

Technical Field

The present invention relates generally to carbon fibers and more particularly to carbon fibers having improved strength and modulus and methods and apparatus for making the same.

Background

Carbon fibers are used in a wide variety of structural applications and industries for their desirable properties. For example, carbon fibers may form structural components that combine high strength and high stiffness while having significantly lighter weight than equivalent-property metal components. Carbon fibers can be manufactured 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 of 90% or more carbon. The resulting carbon fibers may be molded into high strength composites for structural applications, in their pure form for electrical and friction applications, or may be further processed for adsorbents, filters, or other applications. In particular, composite materials have been developed in which carbon fibers are used as reinforcing materials in a resin, ceramic or metal matrix.

Current trends in automotive, aerospace, structural and other applications continue to drive materials with even higher tensile strengths and moduli. Carbon fibers based on polyacrylonitrile have become the primary reinforcement in composite materials (both thermoset and thermoplastic materials) that meet this basic need. Due to weight savings and other efficiencies, they enable contemporary ground and air transportation and construction to be more fuel efficient than their predecessor.

However, even stronger and stiffer materials are still needed to achieve further efficiency improvements. The higher strength and modulus allow the resulting composite to reach the same strength at even lighter weights than current state of the art, thereby achieving higher fuel efficiency for aircraft or automobiles. Being able to make stronger fibers, preferably without changing other properties of the material (e.g., cost, density), would allow for pareto improvements in the design of these structures.

Unfortunately, many methods of increasing strength and/or modulus have a significant compromise on the fiber. For example, it is well known in the art that increasing the final carbonization temperature during the conversion of polyacrylonitrile to carbon fibers will increase the modulus of the resulting fibers; however, after a certain temperature, it also reduces the overall tensile strength of the fiber. It also increases the amount of fiber breakage and thus gradually creates fuzz (fuzz).

It is also well known in the art that controlling other important parameters, particularly the tension and stretching of the fiber when it is converted from Polyacrylonitrile (PAN) first by a slower oxidation process and then by a multi-step carbonization process, will result in a significant improvement of the final tensile modulus and strength of the final fiber. Of particular importance is the stretching in oxidation, which is imparted by the difference in speed between the rollers that transport the fibers into the oxidation oven and the rollers that transport the fibers out of the oven. It has previously been recognized in the art that the modulus of carbon fibers can be improved by stretching the fibers in a post-spinning step, an oxidation step, a carbonization step, or a combination thereof.

Us patent No. 4,609,540 describes a method of determining the optimal 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 can be determined from a plot of% elongation versus tension, and the optimum elongation also corresponds approximately to the greatest degree of crystallographic orientation within the fiber. In addition to this inflection point, the' 540 patent teaches that any gain from further stretching is minimal and may lead to development of "fuzz" and possible breakage.

Us patent No. 8,591,859 describes a method of stretching precursor fibers in an oxidizing atmosphere, wherein the tension load is evenly distributed in multiple passes through the oxidizing oven. In other words, rather than stretching the fiber only at the last pass through the oven, the fiber is stretched at each pass through the oven. This reference teaches subjecting the fibers to different amounts of stretching during conversion for very high stretching in oxidation, which would allow further improvement of the tensile strength.

There is room for further optimization of the tension principle, which may allow for even further improvement of the tensile strength without significant damage to the fibers. Us patent No. 8,591,859 suggests that higher strength and modulus would be obtained if the fibers were allowed to stretch more in oxidation, but this would result in a significant loss of productivity by reducing the total mass per unit length of carbon fibers. A method to increase strength without affecting yield would be desirable.

Thus, there is a need for carbon fibers that have both high tensile strength and for methods and apparatus that can be used to produce such carbon fibers with high reproducibility.

Disclosure of Invention

The present invention provides carbon fibers having improved strength and modulus and methods and apparatus useful for making the carbon fibers. In one embodiment, the method includes advancing a precursor fiber through an oxidation oven, wherein the fiber is subjected to controlled stretching in an oxidizing atmosphere, wherein the tension load is evenly distributed in multiple passes through the oxidation oven. Thus, the total cumulative stretch of the fiber may be increased by selecting a stretching condition that allows the tension load to be distributed in multiple passes. Distributing the tension load in multiple passes allows the fiber to be drawn to a greater extent than previously expected. Such controlled stretching of the fibers during oxidation may help provide improvements in, for example, orientation, uniformity of oxidation, and reduction in defect-induced crystallite growth, which in turn may provide improvements in the modulus of elasticity and tensile strength of the resulting carbon fibers.

In another aspect, the present invention relates to an oxidation oven that is capable of subjecting a precursor fiber to multiple controlled stretching passes in an oxidizing atmosphere. In one embodiment, the oxidation oven includes a plurality of drive rollers and a plurality of idler rollers, wherein the drive rollers and idler rollers cooperate to define a pass of the fibers through the oxidation oven. In one embodiment, the drive rollers may be driven independently of each other such that the speed and, alternatively, the tension in at least two or more passes through the oxidation oven may be independently controlled. In some embodiments, the idler roller includes a tension measurement device, such as a load cell, that allows for continuous monitoring of fiber tension as the fiber is being advanced through the oxidation oven.

After the oxidation step, the remainder of the process of converting the fibers into carbon fibers may be carried out using conventional methods. The fibers may be converted by advancing the oxidized fibers through a low temperature and high temperature furnace. In one embodiment, controlled stretching of the fibers during oxidation allows the fibers to be further stretched, for example, by an amount between 5% and 40% as they are advanced through a low temperature oven.

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 a carbon fiber having a tensile strength of at least 950 ksi.

Accordingly, the present invention provides carbon fibers having improved tensile strength, and methods and apparatus for making such carbon fibers.

Drawings

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:

FIG. 1 is a graphical illustration of a reaction process in which PAN precursor fibers are subjected to cyclization and oxidation to form pyridone structures;

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

FIG. 3 is a schematic illustration of a system that may be used to convert precursor fibers to carbon fibers;

fig. 4A and 4B are side-by-side comparisons (fig. 4B) of an oxidation oven using a single drive roller scheme (fig. 4A) compared to an oxidation oven using a controlled pass-by-pass scheme of the present invention, the oxidation oven comprising a plurality of maintenance rollers followed by a plurality of stretching rollers; and

fig. 5 is a graphical diagram showing the stretching (in passes) produced by the oven depicted in each of fig. 4A and 4B.

Detailed Description

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.

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

In the following description, various components may be identified as having particular values or parameters, however, these items are provided as exemplary embodiments. Indeed, the exemplary embodiments are not limiting of the various aspects and concepts of the invention as many comparable parameters, sizes, 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.

It should be understood that where a range of parameters is provided, all integers and ranges within that range, as well as 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% … … 9.8.8%, 9.9% and 10.0%; and 5.00%, 5.01%, 5.02% … … 9.98.98%, 9.99% and 10.00%, and for example 6% -9%, 5.1% -9.9% and 5.01% -9.99%. Similarly, where a list is presented, it should be understood that each individual element of the list, and each combination of components of the list, is a separate embodiment unless otherwise specified. For example, in various embodiments, "1, 2, 3, 4, and 5" encompasses 1;2;3, a step of; 1 and 2;3 and 5; 1. 3 and 5; and 1, 2, 4 and 5.

In one aspect, the present invention relates to carbon fibers having improved tensile strength. In another aspect, the present invention relates to an apparatus and method for manufacturing carbon fibers. The carbon fibers prepared according to the method of the present invention may have tensile strengths approaching and exceeding 6000 MPa.

As used herein, "about" in the context of a numerical value or range means ± 10% of the numerical value or range recited or claimed.

An embodiment of the present invention is a method of manufacturing carbon fiber, the method comprising:

advancing a carbon fiber precursor polymer through an oxidation oven to produce oxidized fibers, the oven having an oxidizing atmosphere with a temperature between about 175 ℃ and 300 ℃;

wherein the propelling comprises

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

(ii) Subjecting the fiber to a second plurality of passes, wherein each of the second plurality of passes has a% elongation of greater than 0.5%.

In one embodiment, the stretching in each of the first plurality of passes is between 0 and 0.1%, inclusive. In one embodiment, the stretching in each of the first plurality of passes is 0%. In one embodiment, the stretching in at least one of the first plurality of passes is negative. In one embodiment, each of the first plurality of passes has the same% elongation.

In one embodiment, in the second plurality of passes, the% stretch of each successive pass is greater than the% stretch of the previous pass.

In one embodiment, the fibers leave the oxidation oven as part of a pass.

In one embodiment, the fiber comprises one or more comonomers selected from the group consisting of: acrylonitrile, methyl acrylate, methacrylic acid, sodium methallylsulfonate, and itaconic acid.

In one embodiment, the precursor fiber has a denier of between about 0.6 to 1.53 dpf. In one embodiment, the precursor fiber has a denier of between about 0.6 to 0.8 dpf. In one embodiment, the precursor fiber has a denier of between about 1.2 to 1.4 dpf.

In one embodiment, the fibers are introduced into a plurality of oxidizing ovens, and wherein each successive oven includes an oxidizing atmosphere having a temperature at least as high as the previous oxidizing oven.

In one embodiment, the first plurality of passes includes at least two passes. In one embodiment, the first plurality of passes includes at least four passes. In one embodiment, the second plurality of passes includes at least four passes. In one embodiment, the second plurality of passes includes at least six passes.

In one embodiment, the method further comprises passing the oxidized carbon fiber through a low temperature furnace having a temperature between about 350 ℃ and 800 ℃; and subsequently carbonizing the carbon oxide fibers by passing the carbon oxide fibers through a carbonization furnace.

In one embodiment, the temperature of the carbonization furnace is between about 1150 ℃ and 2000 ℃. In one embodiment, the temperature of the carbonization furnace is between about 1200 ℃ and 2000 ℃. In one embodiment, the temperature of the carbonization furnace is between about 1300 ℃ and 1500 ℃.

In one embodiment, the temperature of the cryogenic oven is between about 300 ℃ and 900 ℃. In one embodiment, the temperature of the cryogenic oven is between about 400 ℃ and 800 ℃.

In one embodiment, the method further comprises the step of surface treating and sizing the precursor fibers.

In one embodiment, the temperature of the oxidation oven is between about 150 ℃ and 600 ℃. In one embodiment, the temperature of the oxidation oven is between about 175 ℃ and 300 ℃.

In one embodiment, the average diameter of the fibers upon exiting the oxidation oven is between 0 and 50% smaller than the original diameter of the fibers prior to entering the oxidation oven.

In one embodiment, the fibers comprise fiber bundles having between about 1,000 and 50,000 individual filaments.

In one embodiment, the fibers have improved tensile strength and/or elastic modulus compared to fibers prepared using an idler roll protocol having the same total stretch. In one embodiment, the fibers have improved tensile strength and elastic modulus compared to fibers prepared using an idler roll protocol having the same total stretch. In one embodiment, the tensile strength of the fiber is measured according to the procedure set forth in ASTM D-4018. In one embodiment, the modulus of elasticity of the fiber is measured according to the procedure set forth in ASTM D-4018.

An embodiment is a carbon fiber prepared according to any of the embodiments described above.

In one embodiment, the carbon fibers have a tensile strength greater than about 4500MPa. In one embodiment, the carbon fiber has a tensile strength greater than about 5500MPa.

As discussed in more detail below, the carbon fiber according to the present invention may be prepared by: precursor fibers, such as fibers comprising Polyacrylonitrile (PAN), are subjected to multiple passes through an oxidizing atmosphere, wherein the fibers are controllably drawn in two or more passes through the oxidizing atmosphere. After the oxidation step is completed, the fibers may be advanced through one or more additional ovens, such as a low temperature oven and a high temperature oven, to complete the conversion of the precursor fibers to the carbon fibers. In the context of the present invention, the term "fiber" includes a single filament or multiple filaments bundled together, also known as a tow (tow). The tow or bundle may comprise about 1,000 to 100,000 individual filaments.

In the context of the present invention, the term "precursor fiber" refers to a fiber comprising a polymeric material that can be converted to carbon fibers having a carbon content of about 90% or more and in particular about 95% or more by weight upon application of sufficient heat. The precursor fibers can comprise both homopolymers and copolymers of Acrylonitrile (AN), and can comprise copolymers such as Methyl Acrylate (MA), methacrylic acid (MAA), sodium methallylsulfonate, itaconic Acid (IA), ethylene bromide (VB), isobutyl methacrylate (IBMA), and combinations thereof. In one embodiment, the precursor fiber comprises a Polyacrylonitrile (PAN) polymer formed primarily from acrylonitrile monomers.

In embodiments, precursor fibers can be prepared by melt spinning by solvating a precursor polymer in an organic and/or inorganic solvent such as dimethyl sulfoxide, dimethylformamide, zinc chloride, 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 respective weight ratio of about 60:10:30. The solution may then be concentrated by evaporation and filtered to provide a spinning solution. In one embodiment, the spinning solution comprises about 15% by weight of acrylonitrile polymer. The polyacrylonitrile precursor is formed by passing the spinning solution through a spinneret using conventional spinning methods, such as dry, dry/wet or wet spinning. In certain embodiments, PAN precursor fibers are prepared using dry/wet spinning, wherein a plurality of filaments are formed from a spinning solution and passed from a spinneret through an air gap or other gap between the spinneret and a coagulant (such as aqueous sodium thiocyanate). After leaving the coagulant bath, the spun filaments are washed. In some embodiments, the spun filaments may be drawn in hot water and steam to several times their original length. (see, e.g., U.S. patent No. 4,452,860, which is incorporated herein by reference.) additionally, the polyacrylonitrile precursor fiber can be treated with a sizing agent, such as a silane compound, to improve its handling during the carbon fiber manufacturing process. An exemplary method of preparing PAN precursor fibers is discussed in more detail in U.S. patent No. 5,066,433, the contents of which are incorporated herein by reference.

The precursor fibers may comprise polyacrylonitrile-based fibers made from between about 85% and 99% acrylonitrile and between about 15% and 1% other monomers such as methacrylic acid, acrylic acid, methyl acrylate, and methyl methacrylate, and combinations thereof, by weight. The polyacrylonitrile precursor fibers are in the form of bundles comprising between about 3000 and 50,000 filaments/bundles and in particular between about 3000 and 24,000 filaments/bundles. The filaments may have an average denier of between about 0.50 and 1.50 and particularly between about 0.60 and 0.85, preferably 95% or more of the filaments in each bundle have a denier differential 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 filament diameter prior to conversion may be in the range of about 7.5 to 13.5 μm and more typically about 8.5 to 10.5 μm.

During oxidation (also referred to as oxidative stabilization), the PAN precursor fibers are heated in an oxidizing atmosphere at a temperature between about 150 ℃ and 600 ℃ to cause cyclization and oxidation of the PAN precursor molecules. In this regard, fig. 1 illustrates the process of cyclizing and oxidizing PAN precursor fibers in a stepwise manner. In step (a), the nitrile groups of 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 polyethylenimine ring "ladder" structure that undergoes tautomerization in step (D) to form a polycyclic dihydropyridine. In step (E), the polycyclic dihydropyridines are subjected to oxidation/dehydrogenation to form stabilized pyridone structures.

During oxidation, the extent to which the reaction illustrated in FIG. 1 occurs is generally a function of temperature and filament diameter for a given precursor. This is believed to be due in part to the effect of oxygen diffusion into the filaments. At relatively low temperatures (e.g., about 240 ℃ or less) and/or relatively small filament diameters (e.g., about 10 microns or less), 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, the reaction of oxygen tends to be faster than it diffuses and forms a surface layer of oxidized fibers around the core where only the heat-induced reaction occurs. The oxidized surface layer is believed to act as a diffusion barrier for oxygen transport into the filament core. Its presence is undesirable because it leads to both skin-core differences and structural and chemical non-uniformities in the resulting fibers. For example, skin-core differences and structural non-uniformities in oxidized fibers may result in the modulus of the outer layer being higher than the modulus of the inner layer. This modulus distribution is caused by the difference in the cyclization/oxidation process between the inner and outer layers of the precursor fiber. The difference in the cyclization/oxidation process is believed to be due to the reduced penetration of oxygen into the inner portion of the fiber, in part due to the selective oxidation of the outer portion of the precursor fiber, which results in the formation of a barrier to oxygen diffusion into the fiber.

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

The oxidation oven 20 includes a plurality of maintenance rolls (28 a-28 d), collectively referenced 28, and a plurality of stretching rolls (30 a-30 h), collectively referenced 30. Each of the maintenance roller 28 and the stretching roller 30 is considered a drive roller because it is driven at a set speed, as opposed to an idler roller which rotates solely due to the force of the fiber passing therearound. The precursor fiber 24 is provided from a source such as a creel (not shown) and is pulled forward by a driven feed roll 26. Each idler roller 28 cooperates with one or more corresponding drive rollers 30 to define the passes of the fibers through the oxidation oven. For the purposes of the present invention, a "pass" is defined as the path that a fiber travels from an upstream drive roller to a downstream drive roller, wherein at least a portion of the fiber travels through an oxidation oven. The pass so defined may include an inflection point in the form of an idler roller, bar or other such device (not shown). In the illustrated embodiment, a fiber pass refers to the path of a fiber as it travels between a drive roller and a corresponding drive roller. For example, the fiber paths between the maintenance roll 28b and the maintenance roll 28c or between the maintenance roll 28d and the stretch roll 30a each define a single fiber pass through the oxidation oven.

In some embodiments, the precursor fibers 24 may exit the oxidation oven between successive passes. In this regard, fig. 2 illustrates an embodiment in which the maintenance roller 28 and the stretching roller 30 are disposed outside the oxidation oven. Allowing the fibers to exit the oxidation oven between successive passes may help dissipate some of the released heat while stabilizing the PAN chains and thus stabilizing the fibers as they are controllably drawn. The external rollers may also help reduce the tendency of the fibers to stick to the hot surface.

In other embodiments, the maintenance roll, the stretch roll, or both may be disposed inside the oxidation oven. In addition, the passes need not be opposite each other. For example, provided there is sufficient residence time in oven 20, precursor fibers 24 may be driven in a straight line through oven 20 by continuous maintenance roll 28, followed by continuous stretch roll 30.

The stretching rollers 30 are driven at successively higher speeds to create stretching and tension between passes on each stretching roller 30. The amount of stretching or tension applied between passes can be independently controlled to control the amount of stretching between passes. For example, the stretching roller 30b may be driven at a speed that is not much different from the speed at which the stretching roller 30a is driven (producing small% stretching), whereas the stretching roller 30h may be driven at a speed that is much different from the speed at which the stretching roller 30g is driven (producing large% stretching). Independently controlling the speed on each stretching roller 30 allows for independently controlling the% stretching in each pass. Thus, the continuous draw roll 30 may be used to distribute tension or strain rate among multiple passes of fibers through the oxidation oven. In one embodiment, the fiber is exposed to a strain rate of no greater than about 10%/min/pass.

In one embodiment, the maintenance roller 28 and/or the stretch roller 30 are each separately in mechanical communication with a motor for driving the rollers. Typically, the drive rollers are each individually driven by a separate motor through gears to provide improved control of the speed of the drive rollers, and thus, may provide improved control of the amount of tension applied to the fibers. Although chain drives may be used in some embodiments, this is generally less desirable because speed variations between rollers may occur.

The amount of maintenance and stretch rolls may be selected based on the desired characteristics of the resulting carbon fiber. In one embodiment, the oxidation oven may include 2 to 20 maintenance rolls and 2 to 20 stretching rolls. In other embodiments, the oxidation oven may include 2 to 12 pairs of cooperating idler and drive rollers. In some embodiments, an assembly of more than one roller per inlet or a configuration of rollers having different sizes may be used to increase the contact angle between the fiber and the rollers and thus help reduce or eliminate slippage of the fiber during stretching. For example, a pair of rollers in close proximity to each other may define an S-shape in the fiber path, which may reduce slippage of the fibers.

In some embodiments, either of the rollers 28 or 30 may include a tension measurement device, such as a load cell, that allows for continuous monitoring of the tension on each pass. The measured tension can then be used to individually control the tension applied to the precursor fiber in a given pass by adjusting the speed of the drive rollers relative to each other.

Stretching in a given pass is driven by the exit speed (V ₂ ) And inlet velocity (V) ₁ ) The difference between them is calculated using equation 1:

stretching% = 100 (V ₂ /V ₁ - 1) (1)

For example, if the relative speed ratio (outlet/inlet) =1.50, a stretch of 50% can be obtained. By relative to V ₁ Increase V ₂ With respect to V ₂ Reducing V ₁ Or both speeds are changed simultaneously up to the ratio V ₂ /V ₁ =1.50 to adjust the stretching. Note that 50% stretch corresponds to a stretch ratio of 1.50. In the context of the present invention, 50% stretch is referred to as "1.5X" and "2X" stretch means 100% stretch compared to the original length of the fiber (1X). "3X" stretch represents 200% stretch relative to the original length (i.e., up to three times the original length).

"Total" in a given pass "Or "cumulative" stretching may be determined by the exit velocity (V ₂ ) With the inlet velocity (V) on the initial roll before oxidation _i ) The difference between them is calculated using equation 2:

stretching% = 100 (V ₂ /V _i - 1) (2)

Thus, in determining the total stretch for a given regimen, V ₂ Will be based on the exit speed (V _F )。

In embodiments, a "negative" stretch is created in one or more passes between the maintenance rolls 28. In this case, the outlet velocity is less than the inlet velocity; in other words, the speed of the latter drive roller may be reduced relative to the former drive roller, which results in a reduction in tension in the pass. In some cases, the decrease in tension may be used to allow the fiber to shrink during oxidation. As noted above, stretching in a reactive environment can help lock out mechanical structure benefits obtained from controlled stretching. "negative stretch" may also be referred to as "shrinkage" (i.e., -3% stretch is 3% shrinkage). Thus, in some embodiments, the properties of the fibers may be first enhanced by controlled stretching, and then the fibers allowed to shrink without losing the benefits provided by the stretching process. This may allow for re-acquisition of filament denier or increase the weight per unit length lost in previous stretching.

For example, if the exit velocity for a given pass is 99% of the entrance velocity, the% stretch will be equal to 100x (0.99-1) or-1%. It should be understood that when a pass is described as having a% stretch of "less than or equal to 0.5%", for example, this includes% negative stretch.

Upon exiting the oxidation oven, the fibers 24 may be advanced downstream to one or more additional oxidation ovens, intermediate ovens, or carbonization ovens. In this regard, fig. 3 is a schematic illustration of a system and method that may be used to convert precursor fibers to carbon fibers. As shown, the precursor fibers 24 are provided via a supply roll 40. Alternatively, the precursor fibers may be provided from multiple precursor bundles combined with a creel into a single bundle. The precursor fiber is then passed through one or more oxidation ovens 20 where it is subjected to controlled stretching.

In some embodiments, the system may include a plurality of oxidation ovens, wherein successive oxidation ovens are maintained at a temperature that is generally at least as high as the temperature of a previous oxidation oven. In some embodiments, each successive oxidation oven may independently have multiple maintenance rolls followed by multiple stretching rolls that increase in speed as in the first oxidation oven. When the system includes a plurality of oxidation ovens with an ascending temperature gradient, the temperature of the successive ovens is typically between about 1 ℃ and 50 ℃ higher and more typically 5 ℃ to 20 ℃ higher than the temperature of the previous oven. In some embodiments, a temperature gradient may be established in a single oxidation oven by means of different heating zones within the oven. In other embodiments, the oxidation process may be performed in an environment where the oxygen concentration is more or less dense than the concentration of atmospheric air. In still other embodiments, the oxidation treatment step may be prior to or interposed by the non-oxidizing gas treatment or may be enhanced by the addition of various stabilization promoters, flow pattern arrangements, and other methods known in the art.

After passing through one or more oxidation ovens, the drawn, stabilized, oxidized fibers then pass through one or more low temperature ovens 42, also referred to as tar removal ovens, and then through one or more high temperature ovens 44, also referred to as carbonization ovens. The low temperature furnace and the high temperature furnace contain an inert gas such as nitrogen. In one or more low temperature ovens, the temperature of the stabilized fibers ranges between about 300 ℃ and 900 ℃ and more typically between 350 ℃ and 800 ℃ or between 350 ℃ and 750 ℃. In one embodiment, the temperature varies in different portions of low temperature furnace 42. In one embodiment, the temperature of the low temperature furnace 42 is lower at the fiber inlet (bottom of the figure) than at the fiber outlet (top of the figure).

Volatile products produced by the passing stabilized fibers subjected to carbonization are purged from the low temperature furnace 42. After exiting the one or more low temperature ovens 42, the fibers are then exposed to still higher temperatures, such as between about 1150 ℃ and 2000 ℃ and in particular between 1250 ℃ and 1600 ℃ or between 1250 ℃ and 1500 ℃, in the one or more high temperature ovens 44. In a preferred embodiment, the high temperature furnace 44 is between about 1300 ℃ and 1500 ℃. In one embodiment, the temperature in the one or more high temperature furnaces 44 varies. In one embodiment, the fibers may be exposed to multiple temperatures in a single pass.

During traveling through the low temperature furnace and the high temperature furnace, the fibers may be subjected to further stretching such that their length is between about 0.1% and 40%, such as between 0.1% and 30% and particularly between about 0.1% and 24% longer than they enter the low temperature furnace upon exiting. In some embodiments, the fibers may be subjected to shrinkage during travel through the low temperature furnace and the high temperature furnace such that their length is between about 0.1% and 8%, such as between 0.1% and 5% and particularly between about 0.1% and 3% shorter as they exit than as they enter the low temperature furnace. After carbonization is complete, the carbonized fibers may then be subjected to one or more further treatments, including graphitization, surface treatment, and/or sizing. Graphitization refers to heat treatment in one or more inert gas furnaces at temperatures exceeding 2000 ℃. The surface treatment includes anodic oxidation in which the fibers are passed through one or more electrochemical baths. The surface treatment may help improve the adhesion of the fibers to the matrix resin and thus improve the properties of the composite as reflected by tests such as fiber-matrix layer 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 fibers from damage during their use. In composite applications, the water-dispersible materials are generally compatible with the matrix resins oriented for composite fabrication.

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 composition of the precursor fiber and the desired properties of the carbon fiber. In one embodiment, the precursor fibers may pass through the oxidation oven for a total of between about 2 and 40 passes, and in particular between about 2 and 15 passes, 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. 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 2 to 10 minutes.

In one embodiment, carbon fibers having improved strength may be prepared by advancing a precursor through an oxidation oven in multiple passes, wherein the tension on the precursor fiber is between about 100 and 1,000mg/den in at least two or more passes. In one embodiment, the amount of maximum% elongation the fiber is subjected to in a given pass is selected such that the strain rate per pass is about 10%/minute or less and particularly less than about 5%/minute. Methods for determining the amount of% stretch applied by a given fiber in a given pass are discussed in more detail below. The mechanical property benefits obtained by controlled stretching are not limited by the initial diameter, denier or chemical composition of the precursor fiber.

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

All publications, patents, and patent applications mentioned 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 unless the incorporated material is inconsistent with the explicit disclosure herein, in which case the language in the disclosure controls.

Examples and discussion

Stretching the fiber to some extent in oxidation is understood to improve the tensile strength of the resulting carbon fiber. However, not all stretching is equivalent. Initial heating of the PAN fibers was accompanied by entropy shrinkage forces on the fibers (Warner, S.B, peables, l.h., uhlmann, d.r. oxidative Stabilization of Acrylic fibers.iii. stabilization dyanamics (Report #9NR 356-534) Office of Naval Research). If constrained, such forces will strongly increase the tension on the fibers. Upon further heating, the tension will relax as the molecules rearrange. Upon further heating, if unconstrained, the fibers will begin to shrink because the fibers themselves begin to undergo oxidation and cyclization reactions; if constrained, the tension of the fibers increases.

As a result of this two-stage heating, the inventors have found that if a series of idler rollers and a subsequent single drive roller were allowed to stretch through the oven, the fibers would preferentially stretch as they pass through the first few passes of the oven during the time that the fibers would preferentially entropy shrink.

Fig. 4A and 4B show a side-by-side comparison of two roller schemes used in the experiments discussed below. If the stretching is produced in the entire oven by having a drive roller (as shown in fig. 4A) at the last roller of the oven after a number of idler rollers, a large portion of the total stretching occurs in the first few rollers of the oven, where the fibers are subjected to the entropy shrinkage forces associated with the initial heating of the PAN.

In fig. 4A, PAN 24 enters first oxidation oven 22 at the bottom right of the figure. PAN 24 passes over each idler roller 32a-32i as it is pulled by a separate drive roller 34. This results in a significant amount of stretching occurring in the initial pass.

However, by using a maintenance roller instead of an idler roller, the amount of stretching in the initial pass can be varied. By manipulating the drive speed of certain rollers, the stretching in a given pass can be minimized, reduced to 0% stretching in a given pass, or even negative, in order to allow for shrinkage (by reducing the exit roller speed below the entry roller speed), as demonstrated in fig. 4B. Specifically, PAN 24 enters first oxidation oven 22 at the bottom right of the figure. The PAN 24 first passes over each of the maintenance rollers 28a-28d, which in this embodiment run at the same speed as each other and as the speed of the incoming tray 24. The PAN then passes through six successive stretching rolls 30a-30f at successively higher speeds and then exits the first oxidation oven 22 for further oxidation.

As discussed previously, it has been generally accepted that the amount of stretch% that can be applied to a fiber during oxidation is limited by the build up of tensile load in the fiber. However, the inventors have found that the strength of the drawn fiber can be increased by distributing the tension load or drawing more evenly in multiple passes through the oxidation oven, even when the total cumulative draw% of the fiber is the same. Thus, the overall strength of the drawn fiber can be increased by selecting a drawing condition that allows a much more uniform distribution of the tension load in multiple passes. By minimizing or eliminating stretching in early heating passes or even employing negative stretching in one or more early heating passes, stretching may be more advantageously distributed throughout the set of passes. This may further help to improve the tensile strength of the resulting carbon fibers. In the context of the present invention, the term "cumulative stretch" refers to the total percent stretch of the fiber compared to the fiber prior to entry into the oxidation oven. The cumulative stretch may be calculated from the product of the stretches in each individual step or from the ratio of the initial velocity to the final velocity within the portion of interest.

In one embodiment, controlled stretching in the oxidation stage may be used in combination with further stretching in a low temperature furnace. It is believed that the more uniformly oxidized fibers are less affected by the accumulation of different shear strains and thus can withstand greater tension and thus can handle additional stretching in the low temperature stage, which can provide additional structural benefits realized in low temperature ovens, such as in molecular orientation. In one embodiment, the oxidized fiber may be subjected to a draw% in a low temperature oven of between about 1% and 40%, such as between about 1% and 30% and between 1% and 24%.

The following examples are provided to illustrate various aspects of the invention and should not be construed as limiting the invention. The tensile strength of carbon fibers was measured according to the method described in U.S. patent No. 5,004,590, which is hereby incorporated by reference in its entirety.

By preventing stretching in the initial region, but then stretching in subsequent passes when oxidative shrinkage is the driving force for tension increase (using a pass-by-pass controlled stretching scheme as exemplified in fig. 4B), we have found that the tensile strength in the resulting fiber is significantly increased compared to a fiber that is allowed to naturally stretch in the first few oxidation passes (such as the idler roll scheme exemplified in fig. 4A). Importantly, the total stretch in both cases was the same, but by delaying the stretch we see an increase in tensile strength. The tensile strength of the carbon fiber samples was measured according to the procedure set forth in ASTM D-4018.

In examples 1-7, PAN fibers were subjected to the same total amount of stretch in both a pass-by-pass controlled stretch roll scheme and an idler roll scheme. The stretching of the fibers in each pass through the oven was carefully controlled via a series of 10 drive rollers (with the first four set at the same speed) as in the scheme of fig. 4B, to avoid stretching in the initial stages of heating. The same PAN fibers having the same stretch as the fibers in the corresponding fibers also run on idler rollers except for the last driven roller, as in the scheme of fig. 4A, thus allowing the fibers to naturally stretch to the same total stretch as in the case of delayed stretching. For the case of idler and drive rolls, each of the other conditions (including oxidation temperature, carbonization temperature, surface treatment, total stretching, and oven and furnace residence time) remained unchanged.

Each of examples 1-7 is a fiber prepared using the same polymer chemistry. Examples 1-5 used smaller diameter PAN fibers compared to examples 6-7, resulting in smaller diameter carbon fibers. Because of the difference in thickness, slightly more silicone oil was used to finish the fibers of examples 6-7.

As shown in table 1, in each case, in the case where stretching is preferentially performed in the oxidation process, the strength of the same fiber is higher than in the case where natural stretching of the fiber is allowed:

Table 1: tensile strength of fibers prepared by idler rollers or by controlled pass-by-pass stretching of the first oxidation zone.

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For both pass-by-pass controlled stretching and idler roll schemes, the average fiber strength is the average of at least 120 fiber strength determinations in each example. As shown in table 1, for each of examples 1-5, the fibers had an average greater strength change of greater than 3% when drawn using the pass-by-pass controlled drawing scheme as compared to the idler roller scheme. Similarly, with respect to examples 6 and 7, the pass-by-pass controlled stretching scheme resulted in fibers that were 2.4% and 2.9% stronger, respectively, as compared to the idler roller scheme.

Examples 8-10 produced different total amounts of stretch, but each pass-by-pass controlled stretching regimen included shrinkage in the first two passes (specifically, -1% or-1.2% stretch). These also produce fibers that are stronger than those produced by the idler roll approach. This difference is particularly noticeable at the highest amount of stretch (example 9, 22.0% stretch), which results in a 3.2% strength improvement.

Fig. 5 shows the relative stretching on each roll for the pass-by-pass controlled stretching scheme (diamond) and the idler roll scheme (square) for example 6. These are calculated by measuring the speed difference between successive rolls. The speed of each roller was measured by tracking the time to completion of 10 revolutions and dividing by 10 times the roller circumference. Each regimen resulted in a total stretch of 13.3%.

In the idler roller scheme, the maximum stretching (over 6%) occurs in the first pass and more than half of the stretching occurs in the first two passes. In a pass-by-pass controlled scheme, there is no stretching in the first four passes, depending on the design; more stretching is gradually introduced in each successive pass until the last pass. Since all other conditions are the same, including total stretching, the increase in strength is due to the difference in the locations in the oven where stretching is performed.

Many modifications and other embodiments of the inventions 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

1. A method of making carbon fibers, the method comprising:

Wherein the propelling comprises

2. The method of claim 1, wherein the stretching 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 stretching in each of the first plurality of passes is 0%.

4. A method according to any one of claims 1-3, wherein the stretching in at least one of the first plurality of passes is negative.

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

6. The method of any one of claims 1-5, wherein in the second plurality of passes, the% stretch of each successive pass is greater than the% stretch of the previous pass.

7. The method of any of claims 1-6, wherein the fibers leave 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 methallylsulfonate, and itaconic acid.

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

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

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

12. The method of any of claims 1-11, wherein the fibers are introduced into a plurality of oxidation ovens, and wherein each successive oven includes an oxidizing atmosphere having a temperature at least as high as the previous oxidation oven.

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

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

15. The method of any one of claims 1-14, 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. The method of any one of claims 1-16, further comprising passing the oxidized carbon fibers through a low temperature furnace having a temperature between about 350 ℃ and 800 ℃; and

the carbon oxide fibers are then carbonized by passing the carbon oxide fibers through a carbonization furnace.

18. The method of claim 17, wherein the temperature of the carbonization furnace is between about 1150 ℃ and 2000 ℃.

19. The method of claim 18, wherein the temperature of the carbonization furnace is between about 1300 ℃ and 1500 ℃.

20. The method of any one of claims 17-19, wherein the temperature of the cryogenic oven is between about 300 ℃ and 900 ℃.

21. The method of claim 20, wherein the temperature of the cryogenic oven is between about 400 ℃ and 800 ℃.

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

23. The method of any one of claims 1-22, wherein the temperature of the oxidation oven is between about 150 ℃ and 600 ℃.

24. The method of claim 23, wherein the temperature of the oxidation oven is between about 175 ℃ and 300 ℃.

25. The method of any one of claims 1-24, wherein upon exiting the oxidation oven, the average diameter of the fibers is between 0 and 50% smaller than the original diameter of the fibers prior to entering the oxidation oven.

26. The method of any one of claims 1-25, wherein the fibers comprise fiber bundles having between about 1,000 and 50,000 individual filaments.

27. The method of any one of claims 1-26, wherein the fiber has improved tensile strength and/or modulus of elasticity compared to a fiber prepared using an idler roll protocol having the same total stretch.

28. A carbon fiber produced 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 4500MPa.

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