Selective control of oxidizing atmospheres in carbon fiber production.

Abstract

A method for producing carbon fiber that increases the tensile strength of the carbon fiber without dehumidifying the ambient air entering all of the oxidation ovens in a multiple oxidation oven system. If only the ambient air entering the first oxidation oven in the series is dehumidified, a positive effect on tensile strength is achieved. Furthermore, if one or more of the preceding oxidation ovens are operated with dehumidified air, the ambient air entering the last oven is not dehumidified.

Description

[Technical field]

[0001] The present invention relates generally to methods for producing carbon fibers. More particularly, the present invention involves selective control of humidity within an oxidation oven used in such methods to minimize production costs without adversely affecting the tensile strength of the carbon fibers. [Background technology]

[0002] Composite materials contain two main components: a fiber-reinforced structure and a resin matrix. Composite materials typically have a fairly high strength-to-weight ratio. As a result, composite materials are used in the aerospace industry, where the high strength and relatively light weight of composite structures are particularly important.

[0003] Carbon fiber is a common fiber reinforcement for composite materials. Carbon fiber is usually provided as a multifilament yarn, commonly called a "tow." Carbon fiber tows typically contain 1,000 to 50,000 individual filaments. Commercially available carbon fiber tows, for example, contain approximately 3000 filaments (3K), 6000 filaments (6K), 12000 (12K) filaments, or 24000 (24K) filaments. The linear weight of a single carbon filament typically ranges from 0.02 to 0.5 milligrams per meter. Carbon filaments are also commonly referred to as carbon fibers.

[0004] Carbon fibers are produced by converting precursor fibers, such as polyacrylonitrile (PAN) fibers, into fibers that are at least 90% carbon. The conversion process involves heating the precursor fibers in an oxidizing atmosphere, such as air, to form oxidized fibers, also called stabilized fibers. The oxidized fibers are further heated and carbonized in an inert atmosphere, such as nitrogen, to form the final carbon fibers.

[0005] In practice, the oxidation step is accomplished by passing the precursor fiber through a series of oxidation ovens maintained at temperatures between 150°C and 300°C. Ambient air surrounding the oxidation ovens is typically used as the oxidizing gas. Ambient air enters the oxidation ovens at several locations, including air that enters a heater box before entering the oxidation oven and air that enters through a gap where the precursor fiber enters the oven. Depending on the oven design and operation, there may be additional sources of ambient air that enter each oxidation oven. Carbon fiber production lines generally include between two and six oxidation ovens. A typical production line utilizes four oxidation ovens.

[0006] It is known in the art that oxidation conditions such as oven temperature and fiber residence time both in and between oxidation ovens affect the tensile strength of the resulting carbon fiber. Tensile strength is also affected by the atmosphere in the oxidation or stabilization oven. For example, stabilization in a low oxygen atmosphere tends to reduce the tensile strength of carbon fiber. The humidity of the ambient air introduced into the oxidation oven also affects tensile strength. For example, relatively humid ambient air having an absolute humidity of 12 gm water / Kg air or more has been shown to adversely affect tensile strength. See JP 6-264311, published Sep. 20, 1994.

[0007] An important goal in any carbon fiber production process is to provide carbon fiber with uniformly high tensile strength. This goal can be difficult to achieve when ambient air is used as the oxidizing gas due to seasonal and / or diurnal variations in humidity. To obtain uniformly optimal tensile strength in relatively high humidity environments (absolute humidity of 12 gm water / Kg air or greater), one may have no choice but to dehumidify all ambient air before it enters the oxidation oven. However, the cost, complexity, and labor associated with such dehumidification systems are significant. Thus, one is faced with a dilemma of either relocating production facilities to drier climates or investing in expensive dehumidification systems to ensure uniformly high tensile strength. Even in drier climates, challenges remain with regard to possible tensile strength variations that may occur due to diurnal and / or seasonal variations in absolute humidity.

[0008] In drier climates, absolute humidity typically varies from 2 to 10 gm water / Kg air over the course of a season and / or day. Little, if any, is known about the relationship between the tensile strength of carbon fiber and the absolute humidity of the ambient air used in oxidation ovens when the absolute humidity varies at these lower humidity levels.

[0009] It would be desirable to provide a method of manufacturing carbon fiber in drier climates that reduces the humidity of the ambient air entering the oxidation ovens, if necessary, to produce carbon fiber having uniformly high tensile strength. It would further be desirable for such a method to limit the amount of ambient air that is dehumidified as much as possible, to avoid the cost, complexity, and effort associated with dehumidifying the ambient air entering multiple oxidation ovens. Summary of the Invention

[0010] In accordance with the present invention, it has been discovered that the tensile strength of carbon fiber can be improved without dehumidifying the ambient air entering each oxidation oven in a multiple oven system. Instead, it has been discovered that dehumidifying only the ambient air entering the first oven in a series of oxidation ovens has a positive effect on tensile strength. Furthermore, it has been discovered that dehumidifying the ambient air entering the last oven in a series of oxidation ovens provides little improvement in tensile strength when each preceding oven uses dehumidified air. These discoveries make it possible to produce carbon fiber having uniformly high tensile strength by dehumidifying the atmosphere of at least the first oxidation oven while avoiding the cost, complexity, and effort associated with dehumidifying all of the oxidation ovens.

[0011] The invention is based on a method of producing carbon fibers from precursor fibers, in which the precursor fibers are subjected to an oxidation treatment in a plurality of oxidation ovens to form oxidized fibers. The oxidized fibers are then subjected to a carbonization treatment to form carbon fibers. The oxidation ovens are surrounded by ambient air. At least a first oxidation oven is provided, which defines a first oxidation zone having a first oxidation zone atmosphere. The first oxidation zone atmosphere is comprised of first oxidizing air entering the first oxidation zone via one or more first oxidation air inlets. The first oxidation zone atmosphere has a first oxidizing atmosphere humidity, and one or more temperatures within the first oxidation zone are within the temperature range of the first oxidation zone.

[0012] A final oxidation oven is provided that defines a final oxidation zone having a final oxidation zone atmosphere comprised of ambient air entering the final oxidation zone via one or more ambient air inlets, the ambient air having an ambient air humidity and one or more temperatures within the final oxidation zone being within a final oxidation zone temperature range that is higher than the temperature range of the first oxidation zone.

[0013] As a feature of the invention, the first oxidation air is comprised of dehumidified ambient air such that the humidity of the first oxidation atmosphere is lower than the humidity of the ambient air. It has been discovered that dehumidifying only the atmosphere of the first oxidation zone provides improvements in the tensile strength of carbon fibers produced using multiple oxidation ovens.

[0014] As a further feature of the present invention, when three or more oxidation ovens are used to carry out the oxidation of the precursor fiber, the oxidation air supplied to each oxidation oven except the final oxidation oven is comprised of dehumidified ambient air. It has been discovered that a relatively small improvement in tensile strength can be obtained by dehumidifying the ambient air entering the final oxidation zone in a series of three or more oxidation ovens. In many circumstances, such a small increase in tensile strength may not require the additional cost, complexity, and effort associated with dehumidifying the ambient air entering the final oxidation oven.

[0015] The present invention relates to a method for producing carbon fiber, operating multiple oxidation ovens using ambient air as the oxidizing gas. The present invention provides an effective and efficient way to ensure that the carbon fiber has the highest possible and uniform tensile strength, even when the absolute humidity of the ambient air fluctuates upwards. The present invention is particularly well suited for use in climates where the absolute humidity of the air surrounding the carbon fiber production facility varies from 2 to 10 gm water / Kg air.

[0016] The above and many other features and attendant advantages of the present invention will become better understood by reference to the following detailed description when taken in conjunction with the accompanying drawings. [Brief description of the drawings]

[0017] [Figure 1] FIG. 2 is a schematic diagram of an exemplary oxidation process according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The present invention is applicable to any process or method for producing carbon fibers from precursor fibers, such as polyacrylonitrile fibers. In general, such methods may include various pretreatments, which may include drawing the precursor fibers and / or applying a finishing oil. The treated precursor fibers are then passed through a number of oxidation ovens where they are oxidized in an oxygen-containing atmosphere to form oxidized precursor fibers. The oxidation step is also referred to as a stabilization step, which forms stabilized precursor fibers. The stabilized precursor fibers are then fed into one or more carbonization furnaces where they are carbonized in an inert atmosphere at high temperature to form carbon fibers. Carbon fibers are typically composed of at least 90 percent carbon by weight, preferably at least 92 percent carbon by weight.

[0019] The present invention is directed to controlling the humidity in the oxidizing atmosphere in a carbon fiber production process that uses multiple oxidation ovens to oxidize or stabilize precursor fibers. The method can be used in combination with any typical precursor fiber, precursor pretreatment step, and any typical carbonization method.

[0020] An exemplary oxidation system is shown diagrammatically at 10 in FIG. 1. Oxidation system 10 includes a first oxidation oven 11, a second oxidation oven 12, a third oxidation oven 13, and a fourth (final) oxidation oven 14. The following detailed description of the invention is limited to a preferred oxidation system and method that uses four oxidation ovens. Oxidation systems and methods that use as few as two oxidation ovens and as many as six oxidation ovens are also suitable. The oxidation ovens used in a particular oxidation system and method are typically all of the same or similar type and operated in the same manner, except that the temperature within the oxidation ovens typically increases from the first oven to the last oven.

[0021] The first oxidation oven 11 defines a first oxidation zone 16 having a first oxidation zone atmosphere formed by first oxidation air entering the first oxidation zone 16 through one or more first oxidation air inlets, as represented by arrows 18. First oxidation zone exhaust exits the first oxidation oven 11 through suitable exhaust ports, as represented by arrows 20.

[0022] The primary inlet for the first oxidizing air is through a first heater box (not shown) where the oxidizing air is heated before entering the first oxidation zone 16. This type of oxidizing air is referred to as "make-up" oxidizing air. Other possible inlets for the first oxidizing air include the gap where the precursor fiber enters and exits the oven, and the edge or other locations of the oven that are not sealed. This type of oxidizing air is referred to as "alternate" oxidizing air.

[0023] The amount of alternative oxidation air entering the first oxidation zone 16 can be minimized by controlling the pressure at the alternative air inlet. This can be done using fans and other devices known in the art. The amount of alternative oxidation air entering the first oxidation zone 16 is controlled such that the amount of first oxidation air (first make-up air) entering the first oxidation zone 16 through the first heater box is at least 60 volume percent of the total volume of first oxidation air entering the first oxidation zone 16. Preferably, at least 85 volume percent of the total volume of first oxidation air enters the first oxidation zone 16 through the heater box.

[0024] According to the present invention, the first oxidation air is primarily comprised of dehumidified ambient air. For purposes herein, ambient air is the atmosphere present in the carbon fiber production facility and surrounding the oxidation oven. The ambient air may or may not be conditioned, and the production facility may be open to the outside atmosphere to various degrees. Ambient air typically includes air recycled from the oxidation oven. Such recycled air may constitute a significant proportion of the ambient air. Ambient air has an ambient humidity that depends on several factors, including the climate at the production facility, the proportion of recycled air surrounding the oxidation oven, and the humidity of the recycled air. As used herein, "humidity" means absolute humidity unless otherwise specified. Absolute humidity is expressed as grams of water per kilogram of air (gm water / Kg air) and is the water vapor content of the atmosphere, regardless of temperature.

[0025] The present invention can be used in production facilities where the humidity of the ambient air is relatively high (12 gm water / Kg air or more). However, to ensure that the carbon fibers have a uniformly high tensile strength, it is preferred that the humidity of the ambient air be in the range of 2-10 gm water / Kg air. Ambient air with a humidity of 2-6 gm water / Kg air is more preferred, and ambient air with a humidity of 2-4 gm water / Kg air is most preferred.

[0026] The first oxidation zone 16 has a first oxidation zone atmosphere having a first oxidation atmosphere humidity. The humidity of the first oxidation atmosphere is determined primarily by the moisture in the make-up air and the replacement air entering the first oxidation zone 16. The humidity of the first oxidation atmosphere is also determined to a lesser extent by the inherent moisture released from the precursor fiber and the moisture generated by the oxidation reaction.

[0027] It is preferred to dehumidify only the ambient air (make-up air) entering through the first heater box. This simplifies the dehumidification process while still dehumidifying at least 60 volume percent (preferably at least 85 volume percent) of the total amount of first oxidation air entering first oxidation zone 16. The ambient air is preferably dehumidified before entering the first heater box.

[0028] Dehumidification of the ambient air entering the heater box can be accomplished using any type of dehumidification system capable of removing moisture from large volumes of air. Suitable dehumidification systems include those that condense and / or absorb moisture from the ambient air.

[0029] According to the present invention, the ambient air (first make-up air) passing through the first heater box into the first oxidation zone is sufficiently dehumidified to provide a first oxidation zone atmosphere having an absolute humidity of 1-3 gm water / Kg air, preferably 1-2 gm water / Kg air. The amount of moisture that must be removed from the ambient air (first make-up air) passing through the first heater box to provide the desired first oxidation atmosphere humidity depends on several factors including the relative amounts of make-up air and replacement air, the amount of moisture introduced into the oxidation oven by the precursor fiber, and the humidity of the ambient air. The first make-up air is sufficiently dehumidified such that the humidity of the first oxidation atmosphere is at least 1 gm water / Kg air lower than the ambient air. For example, if the humidity of the ambient air is 2, the humidity of the first oxidation atmosphere will be 1 gm water / Kg air or less.

[0030] The higher the humidity of the ambient air, the higher the level of dehumidification required. For example, ambient air with an absolute humidity of 8 gm water / Kg air needs to be dehumidified to remove at least 5-7 gm water / Kg air to provide a first oxidizing atmosphere humidity of 1-3 gm water / Kg air. As another example, ambient air with an absolute humidity of 4 gm water / Kg air needs to be dehumidified to remove at least 2-3 gm water / Kg air to provide a first oxidizing atmosphere humidity in the preferred range of 1-2 gm water / Kg air.

[0031] The humidity of the atmosphere in the first oxidation zone is preferably substantially the same as or slightly higher than the humidity of the dehumidified first oxidation air (first make-up air) entering through the first heater box. This is achieved by limiting as much as possible the amount of ambient air entering the first oxidation zone 16 as replacement air. If 60-85 volume percent of the total amount of first oxidation air enters the first oxidation zone 16 through the first heater box as first make-up air, the humidity of the atmosphere in the first oxidation zone will be slightly higher than the humidity of the dehumidified first oxidation air (first make-up air) entering through the first heater box. When 85-100 volume percent of the total amount of primary oxidation air (primary makeup air) enters the primary oxidation zone 16 through the primary heater box, the humidity of the atmosphere in the primary oxidation zone will be substantially the same as, or at most 0.9 gm water / Kg air higher than, the humidity of the dehumidified primary oxidation air (primary makeup air) entering through the primary heater box.

[0032] The second oxidation oven 12 defines a second oxidation zone 22 having a second oxidation zone atmosphere formed by second oxidizing air entering the second oxidation zone 22 through one or more second oxidizing air inlets, as represented by arrows 24. Second oxidation zone exhaust exits the second oxidation oven 12 through suitable exhaust ports, as represented by arrows 26.

[0033] The primary inlet for the second oxidizing air is through a second heater box (not shown) where the oxidizing air is heated before entering the second oxidation zone 22. As previously mentioned, this type of oxidizing air is referred to as "make-up" oxidizing air. Other possible inlets for the second oxidizing air include the gap where the precursor fibers enter and exit the oven, and the edge or other location of the oven that is not sealed. As previously mentioned, this type of oxidizing air is referred to as "alternate" oxidizing air.

[0034] In accordance with the present invention, the second oxidation air is preferably composed primarily of dehumidified ambient air to provide carbon fibers having as uniform and high tensile strength as possible. However, in situations where a slight reduction in tensile strength (less than 3%) can be tolerated to eliminate the cost, complexity, and labor associated with dehumidifying the ambient air entering the second oxidation oven, the second oxidation air may be composed primarily of ambient air.

[0035] If the ambient air is dehumidified, the second oxidation oven 12 is operated in the same manner as the first oxidation oven 11, so that the amount of alternative oxidation air entering the second oxidation zone 22 is minimized by controlling the pressure at the alternative air inlet. This can be done using fans and other devices known in the art. The amount of alternative oxidation air entering the second oxidation zone 22 is controlled such that the amount of second oxidation air (second make-up air) entering the second oxidation zone 22 through the second heater box is at least 60 volume percent of the total volume of second oxidation air entering the second oxidation zone 22. Preferably, at least 85 volume percent of the total volume of second oxidation air (second make-up air) enters the second oxidation zone 22 through the second heater box.

[0036] Dehumidification of the ambient air (second make-up air) entering the second oxidation zone 22 is accomplished in the same manner as described above for the first oxidation oven. Thus, the ambient air entering the second oxidation zone 22 through the second heater box is sufficiently dehumidified to provide a second oxidation atmosphere having an absolute humidity of 1-3 gm water / Kg air, preferably 1-2 gm water / Kg air. The amount of moisture that must be removed from the ambient air to form the second make-up air also depends on the relative amounts of make-up air and replacement air, the amount of moisture introduced into the oxidation oven by the precursor fiber, and the humidity of the ambient air. The humidity of the second oxidation atmosphere should be at least 1 gm water / Kg air lower than the ambient air, and may be 9 gm water / Kg air lower than the humidity of the ambient air.

[0037] If the ambient air entering the second oxidation oven is dehumidified, the humidity of the atmosphere in the second oxidation zone is preferably substantially the same as or slightly higher than the humidity of the dehumidified second oxidation air (second make-up air) entering through the second heater box. This is accomplished in the same manner as the first oxidation oven by limiting as much as possible the amount of ambient air entering the second oxidation zone 22 as replacement air. If 60-85 volume percent of the total amount of second oxidation air enters the second oxidation zone 22 through the second heater box as second make-up air, the humidity of the atmosphere in the second oxidation zone will be slightly higher than the humidity of the dehumidified second oxidation air (second make-up air) entering through the second heater box. When 85-100 volume percent of the total amount of second oxidation air (second make-up air) enters the second oxidation zone 22 through the second heater box, the humidity of the atmosphere in the second oxidation zone will be substantially the same as or at most 0.3 gm water / Kg air higher than the humidity of the dehumidified second oxidation air (second make-up air) entering through the second heater box.

[0038] The third oxidation oven 13 defines a third oxidation zone 28 having a third oxidation zone atmosphere formed by third oxidation air entering the third oxidation zone 28 through one or more third oxidation air inlets, as represented by arrows 30. The third oxidation zone exhaust exits the third oxidation oven 13 through suitable exhaust ports, as represented by arrows 32.

[0039] The primary inlet for the third oxidizing air is through a third heater box (not shown) where the oxidizing air is heated before entering the third oxidation zone 28. As previously mentioned, this type of oxidizing air is referred to as "make-up" oxidizing air. Other possible inlets for the third oxidizing air include the gap where the precursor fibers enter and exit the oven, and the edge or other location of the oven that is not sealed. As previously mentioned, this type of oxidizing air is referred to as "alternate" oxidizing air.

[0040] In accordance with the present invention, the third oxidation air is preferably composed primarily of dehumidified ambient air to provide carbon fibers having as uniform and high tensile strength as possible. However, in situations where a slight reduction in tensile strength (less than 3%) can be tolerated to eliminate the cost, complexity, and labor associated with dehumidifying the ambient air entering the third oxidation oven, the third oxidation air may be composed primarily of ambient air.

[0041] If the ambient air is dehumidified, the third oxidation oven 13 is operated in the same manner as the first oxidation oven 11, so that the amount of alternative oxidation air (third make-up air) entering the third oxidation zone 28 is minimized by controlling the pressure at the alternative air inlet. This can be done using fans and other devices known in the art. The amount of alternative oxidation air entering the third oxidation zone 28 is controlled such that the amount of third oxidation air (third make-up air) entering the third oxidation zone 28 through the third heater box is at least 60 volume percent of the total volume of third oxidation air entering the third oxidation zone 28. Preferably, at least 85 volume percent of the total volume of third oxidation air enters the third oxidation zone 16 through the third heater box.

[0042] Dehumidification of the ambient air entering the third oxidation zone 28 is accomplished in the same manner as described above for the first oxidation oven. Thus, the ambient air entering the third oxidation zone 28 through the third heater box is sufficiently dehumidified to provide a third oxidation atmosphere having an absolute humidity of 1-3 gm water / Kg air, preferably 1-2 gm water / Kg air. The amount of moisture that must be removed from the ambient air to form the third oxidation atmosphere depends on the relative amounts of make-up and replacement air, the amount of moisture introduced into the oxidation oven by the precursor fiber, and the humidity of the ambient air. The humidity of the third oxidation atmosphere should be at least 1 gm water / Kg air lower than the humidity of the ambient air, and may be as low as 9 gm water / Kg air lower than the humidity of the ambient air.

[0043] If the ambient air entering the third oxidation oven is dehumidified, the humidity of the atmosphere in the third oxidation zone is preferably substantially the same as or slightly higher than the humidity of the dehumidified third oxidation air (third make-up air) entering through the third heater box. This is accomplished in the same manner as the first oxidation oven by limiting as much as possible the amount of ambient air entering the third oxidation zone 28 as replacement air. If 60-85 volume percent of the total amount of second oxidation air enters the third oxidation zone 28 through the third heater box as third make-up air, the humidity of the atmosphere in the third oxidation zone will be slightly higher than the humidity of the dehumidified third oxidation air (third make-up air) entering through the third heater box. When 85-100 volume percent of the total amount of third oxidizing air (third make-up air) enters the third oxidation zone 28 through the third heater box, the humidity of the atmosphere in the third oxidation zone will be substantially the same as the humidity of the dehumidified third oxidizing air entering through the third heater box, or at most 0.3 gm water / Kg air higher.

[0044] The fourth and final oxidation oven 14 defines a fourth oxidation zone 34 having a fourth oxidation zone atmosphere formed by ambient air entering the fourth oxidation zone 34 through one or more ambient air inlets, as represented by arrow 36. The fourth and final oxidation zone exhaust exits the fourth oxidation oven 14 through suitable exhaust ports, as represented by arrow 38.

[0045] In accordance with the present invention, the ambient air entering the fourth oxidation zone 34 is not dehumidified. It has been discovered that dehumidifying the ambient air entering the fourth oxidation zone 34 as the fourth make-up air provides little, if any, improvement in the production of carbon fibers having uniformly high tensile strength. This is particularly true when the humidity of the ambient air is less than 10 gm water / Kg air, preferably less than 6 gm water / Kg air, and most preferably less than 4 gm water / Kg air. Furthermore, at least the first oxidation air must be dehumidified as described above. Preferably, the second and / or third oxidation airs are also dehumidified.

[0046] The continuous precursor fiber, after any pretreatment, is fed into the first oxidation zone 16, as represented by arrow 40. The precursor fiber can be any of the known fiber types suitable for the production of carbon fibers. Polyacrylonitrile is the preferred precursor fiber. Exemplary precursor fibers are described in U.S. Pat. Nos. 4,001,382, 4,009,248, 4,397,831, and 4,452,860, the contents of which are incorporated herein by reference.

[0047] The precursor fiber is preferably passed through the first oxidation zone at a rate that results in a residence time of the precursor fiber in first oxidation zone 16 (first oxidation time) of 5 minutes to 1 hour. Preferably, the first oxidation time is 10 to 40 minutes. The precursor fiber exits first oxidation zone 16 as a partially oxidized fiber, as shown at 42.

[0048] The partially oxidized fibers are exposed to ambient air as they travel from the outlet 42 of the first oxidation zone to the inlet 44 of the second oxidation zone 22. The residence time of the partially oxidized precursor in ambient air between the first and second oxidation ovens (first ambient air time) should be kept as short as possible for the particular type and design of oxidation oven used. A first ambient air time of less than 8 minutes is appropriate, with an ambient air time of 5 minutes or less being preferred.

[0049] The velocity at which the partially oxidized fiber passes through the second oxidation zone 22 is typically, but not necessarily, the same as the fiber velocity through the first oxidation zone 16. The residence time of the partially oxidized fiber in the second oxidation zone 22 (second oxidation time) is also between 5 minutes and 1 hour, preferably between 10 and 40 minutes.

[0050] The partially oxidized fibers exit the second oxidation zone 22 as first further oxidized fibers, as shown at 46. As the first further oxidized fibers travel from the exit 46 of the second oxidation zone to the entrance 48 of the third oxidation zone 28, they are again exposed to ambient air (second ambient air time). The second ambient air time should also be kept as short as possible for the particular type and design of oxidation oven used. A second ambient air time of less than 8 minutes is suitable, with a second ambient air time of 5 minutes or less being preferred.

[0051] The rate at which the first further oxidized fiber passes through the third oxidation zone 28 is typically, but not necessarily, the same as the fiber rate through the first and second oxidation zones 16 and 22. The residence time of the first further oxidized fiber in the third oxidation zone 28 (third oxidation time) is also preferably between 5 minutes and 1 hour, and preferably between 10 and 40 minutes.

[0052] The first further oxidized fibers exit the third oxidation zone 28 as second further oxidized fibers, as indicated at 50. The second further oxidized fibers are again exposed to ambient air as they travel from the exit 50 of the third oxidation zone to the entrance 52 of the fourth and final oxidation zone 34 (third ambient air time). The third ambient air time should also be kept as short as possible for the particular type and design of oxidation oven being used. A third ambient air time of less than 8 minutes is suitable, with a third ambient air time of 5 minutes or less being preferred.

[0053] The velocity at which the second further oxidized fiber passes through the fourth oxidation zone 34 is also typically, but not necessarily, the same as the fiber velocity through the first, second, and third oxidation zones 16, 22, and 28. The residence time of the second further oxidized fiber in the fourth oxidation zone 28 (fourth oxidation time) is also preferably between 5 minutes and 1 hour, and preferably between 10 and 40 minutes.

[0054] The second further oxidized fibers exit the fourth oxidation zone 34 as oxidized fibers, as shown at 54. The oxidized fibers are then passed through one or more carbonization furnaces to convert the oxidized or stabilized fibers into final carbon fibers, as is known in the art. Any of the typical carbonization furnace systems and processes can be used to convert the oxidized fibers into carbon fibers. Such furnace systems typically include multiple carbonization furnaces through which the oxidized fibers pass in sequence. The carbonization ovens have an inert atmosphere at a temperature high enough to convert the oxidized fibers into carbon fibers.

[0055] The four oxidation ovens can be any type of oxidation oven typically used in producing carbon fiber from precursor fibers. All four oxidation ovens are operated according to a typical carbon fiber production operating procedure in which multiple oxidation ovens are utilized. The operating parameters of the four oxidation ovens follow a typical carbon fiber production line, except that the first make-up air entering the first oxidation oven is dehumidified ambient air as described above. Preferably, the second and / or third make-up air entering the second and third oxidation ovens, respectively, are also dehumidified ambient air as described above. The fourth oxidation oven is operated in the same manner as the first three oxidation ovens, except that the fourth make-up air is non-dehumidified ambient air.

[0056] The four oxidation ovens are operated at conventional oxidation / stabilization temperatures of 150° C. to 300° C. (preferably 200° C. to 300° C.). As is well known, the temperature or temperatures within the ovens are gradually increased from the first oxidation oven to the fourth oxidation oven. Exemplary oxidation / stabilization processes are described in U.S. Pat. Nos. 5,256,344 and 9,121,112, the contents of which are incorporated herein by reference.

[0057] The temperature range of the first oxidation zone 16 is preferably 200°C to 250°C. The first oxidation zone 16 includes a first temperature subzone and a second temperature subzone, and the precursor fiber preferably passes through the first temperature subzone before passing through the second temperature subzone. The temperature of the second temperature subzone is 1 to 20°C higher than the temperature of the first temperature subzone. Preferably, the temperature of the second temperature subzone is 1 to 10°C higher than the temperature of the first temperature subzone.

[0058] The temperature range of the second oxidation zone 22 is preferably 210°C to 260°C. The second oxidation zone also preferably comprises a first temperature subzone and a second temperature subzone, the precursor fibre passing through the first temperature subzone before passing through the second temperature subzone. The temperature of the second temperature subzone is 1 to 20°C higher than the temperature of the first temperature subzone. Preferably, the temperature of the second temperature subzone is 1 to 10°C higher than the temperature of the first temperature subzone.

[0059] The temperature range of the third oxidation zone 28 is preferably 220°C to 265°C. The third oxidation zone also preferably includes a first temperature subzone and a second temperature subzone, the precursor fiber passing through the first temperature subzone before passing through the second temperature subzone. The temperature of the second temperature subzone is 1 to 20°C higher than the temperature of the first temperature subzone. Preferably, the temperature of the second temperature subzone is 1 to 10°C higher than the temperature of the first temperature subzone.

[0060] The temperature range of the fourth oxidation zone 34 is preferably 225°C to 275°C. The fourth oxidation zone also preferably includes a first temperature subzone and a second temperature subzone, the precursor fiber passing through the first temperature subzone before passing through the second temperature subzone. The temperature of the second temperature subzone is 1 to 20°C higher than the temperature of the first temperature subzone. Preferably, the temperature of the second temperature subzone is 1 to 10°C higher than the temperature of the first temperature subzone.

[0061] In accordance with the present invention, an exemplary oxidation method requires dehumidifying the make-up air entering the first of four oxidation ovens in order to provide the advantage of carbon fiber having relatively high uniform tensile strength as compared to carbon fiber produced using the same four ovens without any dehumidification of the make-up air. This advantage is provided without the cost, complexity, and effort required to dehumidify the make-up air entering all four ovens. The present invention provides a method for dehumidifying the make-up air entering the first of four oxidation ovens in order to provide the advantage of carbon fiber having relatively high uniform tensile strength as compared to carbon fiber produced using the same four ovens without any dehumidification of the make-up air. dehumidification This provides the additional advantage of eliminating the cost, complexity, and effort associated with dehumidifying the make-up air entering the final oxidation oven, based on the discovery that dehumidifying the make-up air entering the final oxidation oven results in little, if any, increase in the tensile strength of the carbon fiber when the make-up air entering the three preceding oxidation ovens is dehumidified.

[0062] The above features of the present invention provide the option of dehumidifying the make-up air entering the first three ovens to increase uniform tensile strength as much as possible while saving the cost and labor associated with unnecessarily dehumidifying the make-up air entering the final oxidation oven. Another option provided by the present invention is that only the make-up air entering the first oxidation oven can be dehumidified. This option incurs only the cost and labor associated with dehumidifying the first make-up air while substantially increasing the tensile strength of the carbon fiber. This option requires that a relatively small (5% or less) decrease in tensile strength of the carbon fiber can be accepted when compared to carbon fiber produced in which the make-up air entering three or more oxidation ovens is dehumidified. EXAMPLES

[0063] An example implementation is as follows:

[0064] Tests were conducted to convert polyacrylonitrile precursor fibers to carbon fibers using the same carbon fiber production line, operated under similar conditions for all tests. The carbon fiber production line included four oxidation ovens, each defining an oxidation zone containing two temperature subzones. The second subzone was 1-10°C hotter than the first subzone. For all tests, the temperature in the first oxidation zone was between 230°C and 250°C. The temperature in the second oxidation zone was between 235°C and 255°C. The temperature in the third oxidation zone was between 245°C and 260°C. The temperature in the final oxidation zone was between 250°C and 265°C.

[0065] The rate at which the fiber was passed through the oxidation ovens resulted in an oxidation residence time of approximately 30 minutes for the precursor fiber in each oxidation zone. The precursor fiber was passed through ambient air between the first and second oxidation ovens, between the second and third oxidation ovens, and between the third and fourth oxidation ovens. The rate at which the precursor fiber was passed through ambient air between the ovens resulted in an ambient air residence time of approximately 2 minutes between each oxidation oven pair.

[0066] The pre-oxidation treatment for all precursor fibers was the same, including the application of a finishing oil. Carbonization of all test fibers was carried out in a series of furnaces under nitrogen at temperatures increasing from 350 to 1450° C. The polyacrylonitrile precursor fibers used in all tests were 5 microns in diameter and had a denier of 0.8.

[0067] Various tests were run on the production line by selectively varying the humidity of the oxidation atmosphere in one or more of the four oxidation oven zones. Variations in the humidity of the oxidation atmosphere were obtained by either dehumidifying the ambient air entering the oxidation oven as make-up air, adding water to the ambient air entering the oxidation oven as make-up air, or operating the oxidation oven using only ambient air. For all tests, the humidity of the ambient air was approximately 3 grams of water per kilogram of air (gm water / Kg air). When the ambient air was dehumidified before entering the oxidation oven(s) as make-up air, the amount of water removed from the ambient air was controlled such that the oxidation atmosphere in the dehumidified oxidation zone(s) was approximately 1.4 gm water / Kg air. When water was added to the ambient air before entering the oxidation oven as make-up air (Test No. 6), the amount of water added to the ambient air to form the humidified air was approximately 21 gm water / Kg air. The make-up air entering each of the four oxidation ovens was 60-85 volume percent of the total amount of oxidation air (make-up air + replacement air) entering the oxidation ovens.

[0068] The tensile strength of the carbon fibers was measured according to the current version of ASTM D4018, as described in U.S. Patent No. 5,004,590, the contents of which are incorporated herein by reference. The results of the testing are shown in Tables 1 and 2. The type of air used as make-up air in each oven is identified in the table as either "ambient air," "dehumidified air," or "humidified air," which correspond respectively to the three air types as described above. [Table 1]

[0069] In Test No. 1, all four oxidation ovens were operated using ambient air. In Test No. 2, the first oxidation oven was operated with make-up air that was dehumidified air. The second, third, and fourth oxidation ovens were operated with ambient air. The tensile strength of the carbon fiber produced in Test No. 2 was 215 MPa higher than the tensile strength of the carbon fiber produced in Test No. 1. These tests indicate that it is only necessary to dehumidify the make-up air going into the first of the four oxidation ovens to provide an improvement in the tensile strength of the carbon fiber. [Table 2]

[0070] In Test No. 6, the second, third, and fourth oxidation ovens were operated using ambient air. Water was added to the make-up air for the first oven so that the absolute humidity of the atmosphere in the first oxidation oven was sufficiently higher than the humidity of the atmosphere in the second, third, and fourth oxidation ovens (humidified air). Increasing the humidity in the first oxidation oven alone resulted in a tensile strength of the carbon fibers that was reduced to 81% of the tensile strength of the fibers produced according to Test No. 5. In Test No. 5, the ambient air entering all oxidation ovens as make-up air was dehumidified. These tests show that high humidity in only the first oxidation oven of the four oxidation ovens has a substantial adverse effect on the tensile strength of the resulting carbon fibers.

[0071] In Test No. 3, the first and second oxidation ovens were operated with dehumidified air as make-up air, and the third and fourth oxidation ovens were operated with ambient air. The tensile strength of the carbon fiber produced in Test No. 3 was approximately 97% of that of the carbon fiber produced in Test No. 5. These tests show that it was only necessary to dehumidify the ambient air entering the first two of the four oxidation ovens to provide carbon fiber having a tensile strength that was only 3% lower than the tensile strength of the carbon fiber produced if all four oxidation ovens were dehumidified.

[0072] In Test No. 4, the first, second, and third oxidation ovens were operated with dehumidified air as make-up air, and the fourth oxidation oven was operated with ambient air. The tensile strength of the carbon fiber produced in Test No. 4 was approximately 99% of that of the carbon fiber produced in Test No. 5. These tests show that it was only necessary to dehumidify the first three of the four oxidation ovens to provide carbon fiber having a tensile strength only 1% lower than the tensile strength of the carbon fiber produced when the ambient air entering all four oxidation ovens was dehumidified.

[0073] Previous testing has demonstrated that using dehumidified air in the first oxidation oven alone results in increased tensile strength of the resulting carbon fiber. Furthermore, testing has demonstrated that dehumidifying the ambient air entering all four of the four oxidation ovens results in little, if any, improvement in tensile strength over that obtained when the ambient air entering only the first three of the four oxidation ovens is dehumidified. The present invention recognizes these findings and provides a method of dehumidifying the ambient air entering at least the first oxidation oven to provide enhanced tensile strength, but does not dehumidify the ambient air entering the final oxidation oven in order to reduce the cost, complexity, and labor associated with dehumidifying the final oxidation oven. This selective control of the atmosphere of the oxidation oven provides the advantage of increasing tensile strength while simultaneously reducing the cost, complexity, and labor associated with dehumidifying the ambient air before it enters all of the oxidation ovens.

[0074] Having thus described exemplary embodiments of the present invention, it should be noted that the disclosure is merely exemplary and that various other alternatives, adaptations, and modifications may be made by those skilled in the art within the scope of the present invention. Accordingly, the present invention is not limited to the above-described embodiments, but only by the appended claims. Various aspects or embodiments that can be included in the present invention are summarized as follows. [1]. 1. A method for producing carbon fibers from precursor fibers, comprising: the precursor fibers are subjected to an oxidation treatment in a plurality of oxidation ovens to form oxidized fibers, and then the oxidized fibers are subjected to a carbonization treatment to form the carbon fibers, the plurality of oxidation ovens being surrounded by ambient air; The oxidation treatment includes the following steps: providing at least a first oxidation oven defining a first oxidation zone having a first oxidation zone atmosphere including first oxidizing air entering the first oxidation zone via one or more first oxidation air inlets, the first oxidation zone atmosphere having a first oxidizing atmosphere humidity and one or more temperatures within the first oxidation zone being within a temperature range of the first oxidation zone; passing the precursor fiber through the first oxidation zone to form a partially oxidized fiber; providing a final oxidation oven defining a final oxidation zone having a final oxidation zone atmosphere comprising ambient air entering the final oxidation zone via one or more ambient air inlets, the ambient air having a humidity of ambient air, the first oxidation air comprising ambient air that has been dehumidified such that the humidity of the first oxidation atmosphere is less than the humidity of the ambient air, and one or more temperatures within the final oxidation zone being within a final oxidation zone temperature range that is greater than a temperature range of the first oxidation zone; passing the partially oxidized fibers through the final oxidation zone to form the oxidized fibers. A method comprising: [2]. 2. The method for producing carbon fibers according to claim 1, wherein at least a second oxidation oven is provided defining a second oxidation zone, wherein one or more temperatures within the second oxidation zone are within a second oxidation zone temperature range that is higher than the temperature range of the first oxidation zone and lower than the temperature range of the final oxidation zone, wherein the second oxidation zone has a second oxidation zone atmosphere comprising second oxidizing air, wherein the second oxidizing air enters the second oxidation zone via one or more second oxidizing air inlets, wherein the second oxidation zone atmosphere has a second oxidizing atmosphere humidity, wherein the second oxidizing air comprises ambient air that has been dehumidified such that the humidity of the second oxidizing atmosphere is lower than the humidity of the ambient air, wherein the partially oxidized fibers are passed through the second oxidation zone to form first further oxidized fibers, and wherein the first further oxidized fibers are passed through the final oxidation zone to form the oxidized fibers. [3]. 3. The method for producing carbon fibers according to claim 2, wherein at least a third oxidation oven is provided defining a third oxidation zone, wherein one or more temperatures within the third oxidation zone are within a third oxidation zone temperature range that is higher than a temperature range of the second oxidation zone and lower than a temperature range of the final oxidation zone, wherein the third oxidation zone has a third oxidation zone atmosphere comprising third oxidation air, wherein the third oxidation air enters the third oxidation zone via one or more third oxidation air inlets, wherein the third oxidation zone atmosphere has a third oxidation atmosphere humidity, and wherein the third oxidation air comprises ambient air that has been dehumidified such that the humidity of the third oxidation atmosphere is lower than the humidity of the ambient air, wherein the first further oxidized fibers pass through the third oxidation zone to form second further oxidized fibers, and wherein the second further oxidized fibers pass through the final oxidation zone to form the oxidized fibers. [4]. 2. The method for producing carbon fibers according to item 1, wherein the precursor fibers are polyacrylonitrile fibers. [5]. 2. The method for producing carbon fibers according to item 1, wherein the temperature(s) of the first oxidation zone is in the range of 200°C to 300°C, and the temperature(s) of the final oxidation zone is in the range of 200°C to 300°C. [6]. The method for producing carbon fiber according to item 2 above, wherein the temperature(s) of the first oxidation zone, the temperature(s) of the second oxidation zone, and the temperature(s) of the final oxidation zone are all within the range of 200°C to 300°C. [7]. 4. The method for producing carbon fiber according to item 3 above, wherein the temperature(s) of the first oxidation zone, the temperature(s) of the second oxidation zone, the temperature(s) of the third oxidation zone, and the temperature(s) of the final oxidation zone are all within the range of 200°C to 300°C. [8]. 2. The method for producing carbon fibers according to item 1, wherein the rate at which the precursor fiber passes through the first oxidation zone is such that the residence time of the precursor fiber in the first oxidation zone is between 10 minutes and 40 minutes, and the rate at which the partially oxidized fiber passes through the final oxidation zone is such that the residence time of the partially oxidized fiber in the final oxidation zone is between 10 minutes and 40 minutes. [9]. 3. The method for producing carbon fibers according to item 2 above, wherein the rate at which the precursor fiber passes through the first oxidation zone is such that the residence time of the precursor fiber in the first oxidation zone is 10 minutes to 40 minutes, the rate at which the partially oxidized fiber passes through the second oxidation zone is such that the residence time of the partially oxidized fiber in the second oxidation zone is 10 minutes to 40 minutes, and the rate at which the first further oxidized fiber passes through the final oxidation zone is such that the residence time of the first further oxidized fiber in the final oxidation zone is 10 minutes to 40 minutes.

[10] . 4. The method for producing carbon fibers according to item 3 above, wherein the rate at which the precursor fiber passes through the first oxidation zone is such that the residence time of the precursor fiber in the first oxidation zone is 10 minutes to 40 minutes, the rate at which the partially oxidized fiber passes through the second oxidation zone is such that the residence time of the partially oxidized fiber in the second oxidation zone is 10 minutes to 40 minutes, the rate at which the first further oxidized fiber passes through the third oxidation zone is such that the residence time of the first further oxidized fiber in the third oxidation zone is 10 minutes to 40 minutes, and the rate at which the second further oxidized fiber passes through the final oxidation zone is such that the residence time of the second further oxidized fiber in the final oxidation zone is 10 minutes to 40 minutes.

[11] . 2. The method for producing carbon fiber according to item 1, wherein the humidity of the ambient air is 2 to 10 grams of water per kilogram of air, and the humidity of the first oxidizing atmosphere is 1 to 3 grams of water per kilogram of air, and the humidity of the first oxidizing atmosphere is at least 1 gram of water per kilogram of air lower than the humidity of the ambient air.

[12] . 3. The method for producing carbon fiber according to item 2 above, wherein the humidity of the ambient air is 2 to 8 grams of water per kilogram of air, the humidity of the first oxidizing atmosphere and the humidity of the second oxidizing atmosphere are each 1 to 3 grams of water per kilogram of air, and the humidity of the first oxidizing atmosphere and the humidity of the second oxidizing atmosphere are each at least 1 gram of water per kilogram of air lower than the humidity of the ambient air.

[13] . 4. The method for producing carbon fibers according to item 3 above, wherein the humidity of the ambient air is 2 to 8 grams of water per kilogram of air, the humidity of the first oxidizing atmosphere, the humidity of the second oxidizing atmosphere, and the humidity of the third oxidizing atmosphere are each 1 to 3 grams of water per kilogram of air, and the humidity of the first oxidizing atmosphere, the humidity of the second oxidizing atmosphere, and the humidity of the third oxidizing atmosphere are each at least 1 gram of water per kilogram of air lower than the humidity of the ambient air.

[14] . 12. The method for producing carbon fibers according to item 11 above, wherein the ambient air has an ambient air humidity of 2 to 4 grams of water per kilogram of air.

[15] . 13. The method for producing carbon fibers according to item 12 above, wherein the ambient air has an ambient air humidity of 2 to 4 grams of water per kilogram of air.

[16] . 14. The method for producing carbon fibers according to item 13 above, wherein the ambient air has an ambient air humidity of 2 to 4 grams of water per kilogram of air.

[17] . 2. The method for producing carbon fibers according to claim 1, wherein the first oxidation zone comprises a first temperature subzone and a second temperature subzone, the precursor fiber passes through the first temperature subzone of the first oxidation zone before passing through the second temperature subzone of the first oxidation zone, and the temperature of the second temperature subzone is higher than the temperature of the first temperature subzone.

[18] . 2. The method for producing carbon fibers according to claim 1, wherein the final oxidation zone comprises a first temperature subzone and a second temperature subzone, the precursor fiber passes through the first temperature subzone of the final oxidation zone before passing through the second temperature subzone of the final oxidation zone, and the temperature of the second temperature subzone is higher than the temperature of the first temperature subzone.

[19] . 18. The method for producing carbon fibers according to claim 17, wherein the final oxidation zone comprises a first temperature subzone and a second temperature subzone, the precursor fiber passes through the first temperature subzone of the final oxidation zone before passing through the second temperature subzone of the final oxidation zone, and the temperature of the second temperature subzone of the final oxidation zone is higher than the temperature of the first temperature subzone of the final oxidation zone.

[20] . 18. The method for producing carbon fibers according to item 17, wherein the temperature of the second temperature subzone is 1° C. to 10° C. higher than the temperature of the first temperature subzone.

Claims

1. 1. A method for producing carbon fibers from precursor fibers, comprising: the precursor fibers are subjected to an oxidation treatment in a plurality of oxidation ovens to form oxidized fibers, and then the oxidized fibers are subjected to a carbonization treatment to form the carbon fibers, the plurality of oxidation ovens being surrounded by ambient air, the ambient air having an ambient air humidity of 2-10 gm water / Kg air; The oxidation treatment includes the following steps: providing at least a first oxidation oven defining a first oxidation zone having a first oxidation zone atmosphere comprising first oxidizing air entering the first oxidation zone via one or more first oxidizing air inlets, the first oxidation zone atmosphere having a first oxidizing atmosphere humidity, the first oxidizing air comprising ambient air dehumidified such that the humidity of the first oxidizing atmosphere is 1-3 gm water / Kg air, the humidity of the first oxidizing atmosphere being at least 1 gm water / Kg air less than the humidity of the ambient air, and one or more temperatures within the first oxidation zone being within a temperature range of the first oxidation zone; passing the precursor fiber through the first oxidation zone to form a partially oxidized fiber; providing a final oxidation oven defining a final oxidation zone having a final oxidation zone atmosphere including ambient air entering the final oxidation zone via one or more ambient air inlets, wherein the ambient air entering the final oxidation zone is not dehumidified and wherein one or more temperatures within the final oxidation zone are within a final oxidation zone temperature range that is higher than a temperature range of the first oxidation zone; passing the partially oxidized fibers through the final oxidation zone to form the oxidized fibers. A method comprising:

2. 2. The method for producing carbon fibers of claim 1, wherein at least a second oxidation oven is provided defining a second oxidation zone, one or more temperatures within the second oxidation zone are within a second oxidation zone temperature range that is higher than the temperature range of the first oxidation zone and lower than the temperature range of the final oxidation zone, the second oxidation zone has a second oxidation zone atmosphere comprising second oxidizing air, the second oxidizing air enters the second oxidation zone via one or more second oxidizing air inlets, the second oxidation zone atmosphere has a second oxidizing atmosphere humidity, the second oxidizing air comprises dehumidified ambient air such that the humidity of the second oxidizing atmosphere is 1-3 gm water / Kg air, the humidity of the second oxidizing atmosphere being at least 1 gm water / Kg air lower than the humidity of the ambient air, the partially oxidized fibers pass through the second oxidation zone to form first further oxidized fibers, and the first further oxidized fibers pass through the final oxidation zone to form the oxidized fibers.

3. 3. The method for producing carbon fibers of claim 2, wherein at least a third oxidation oven is provided defining a third oxidation zone, one or more temperatures within the third oxidation zone are within a third oxidation zone temperature range that is higher than a temperature range of the second oxidation zone and lower than a temperature range of the final oxidation zone, the third oxidation zone has a third oxidation zone atmosphere comprising third oxidizing air, the third oxidizing air enters the third oxidation zone via one or more third oxidizing air inlets, the third oxidation zone atmosphere has a third oxidizing atmosphere humidity, the third oxidizing air comprises dehumidified ambient air such that the humidity of the third oxidizing atmosphere is 1-3 gm water / Kg air, the humidity of the third oxidizing atmosphere being at least 1 gm water / Kg air lower than the humidity of the ambient air, the first further oxidized fibers are passed through the third oxidation zone to form second further oxidized fibers, and the second further oxidized fibers are passed through the final oxidation zone to form the oxidized fibers.

4. 10. The method for producing carbon fibers of claim 1 , wherein the precursor fiber is a polyacrylonitrile fiber.

5. 2. The method for producing carbon fibers according to claim 1, wherein the temperature(s) of the first oxidation zone are in the range of 200°C to 300°C and the temperature(s) of the final oxidation zone are in the range of 200°C to 300°C.

6. 3. The method for producing carbon fibers according to claim 2, wherein the temperature(s) of the first oxidation zone, the temperature(s) of the second oxidation zone, and the temperature(s) of the final oxidation zone are all within the range of 200°C to 300°C.

7. 4. The method for producing carbon fibers according to claim 3, wherein the temperature(s) of the first oxidation zone, the temperature(s) of the second oxidation zone, the temperature(s) of the third oxidation zone, and the temperature(s) of the final oxidation zone are all within the range of 200°C to 300°C.

8. 2. The method for producing carbon fibers of claim 1, wherein the rate at which the precursor fiber is passed through the first oxidation zone is such that the residence time of the precursor fiber in the first oxidation zone is between 10 minutes and 40 minutes, and the rate at which the partially oxidized fiber is passed through the final oxidation zone is such that the residence time of the partially oxidized fiber in the final oxidation zone is between 10 minutes and 40 minutes.

9. 3. The method for producing carbon fibers of claim 2, wherein the precursor fiber is passed through the first oxidation zone at a rate such that the residence time of the precursor fiber in the first oxidation zone is between 10 minutes and 40 minutes, the partially oxidized fiber is passed through the second oxidation zone at a rate such that the residence time of the partially oxidized fiber in the second oxidation zone is between 10 minutes and 40 minutes, and the first further oxidized fiber is passed through the final oxidation zone at a rate such that the residence time of the first further oxidized fiber in the final oxidation zone is between 10 minutes and 40 minutes.

10. 4. The method for producing carbon fibers of claim 3, wherein the precursor fiber is passed through the first oxidation zone at a rate such that the residence time of the precursor fiber in the first oxidation zone is between 10 minutes and 40 minutes, the partially oxidized fiber is passed through the second oxidation zone at a rate such that the residence time of the partially oxidized fiber in the second oxidation zone is between 10 minutes and 40 minutes, the first further oxidized fiber is passed through the third oxidation zone at a rate such that the residence time of the first further oxidized fiber in the third oxidation zone is between 10 minutes and 40 minutes, and the second further oxidized fiber is passed through the final oxidation zone at a rate such that the residence time of the second further oxidized fiber in the final oxidation zone is between 10 minutes and 40 minutes.

11. 3. The method for producing carbon fibers of claim 2, wherein the humidity of the ambient air is 2 to 8 grams of water per kilogram of air, the humidity of the first oxidizing atmosphere and the humidity of the second oxidizing atmosphere are each 1 to 3 grams of water per kilogram of air, and the humidity of the first oxidizing atmosphere and the humidity of the second oxidizing atmosphere are each at least 1 gram of water per kilogram of air lower than the humidity of the ambient air.

12. 4. The method for producing carbon fibers of claim 3, wherein the humidity of the ambient air is 2 to 8 grams of water per kilogram of air, the humidity of the first oxidizing atmosphere, the humidity of the second oxidizing atmosphere, and the humidity of the third oxidizing atmosphere are each 1 to 3 grams of water per kilogram of air, and the humidity of the first oxidizing atmosphere, the humidity of the second oxidizing atmosphere, and the humidity of the third oxidizing atmosphere are each at least 1 gram of water per kilogram of air lower than the humidity of the ambient air.

13. 2. The method for producing carbon fibers according to claim 1, wherein the ambient air has an ambient air humidity of 2 to 4 grams of water per kilogram of air.

14. 12. The method for producing carbon fibers according to claim 11, wherein the ambient air has an ambient air humidity of 2 to 4 grams of water per kilogram of air.

15. 13. The method for producing carbon fibers according to claim 12, wherein the ambient air has an ambient air humidity of 2 to 4 grams of water per kilogram of air.

16. 2. The method for producing carbon fibers of claim 1, wherein the first oxidation zone includes a first temperature subzone and a second temperature subzone, the precursor fiber passes through the first temperature subzone of the first oxidation zone before passing through the second temperature subzone of the first oxidation zone, and the temperature of the second temperature subzone is greater than the temperature of the first temperature subzone.

17. 2. The method for producing carbon fibers of claim 1, wherein the final oxidation zone includes a first temperature subzone and a second temperature subzone, the precursor fiber passes through the first temperature subzone of the final oxidation zone before passing through the second temperature subzone of the final oxidation zone, and the temperature of the second temperature subzone is greater than the temperature of the first temperature subzone.

18. 17. The method for producing carbon fibers of claim 16, wherein the final oxidation zone includes a first temperature subzone and a second temperature subzone, the precursor fiber passes through the first temperature subzone of the final oxidation zone before passing through the second temperature subzone of the final oxidation zone, and the temperature of the second temperature subzone of the final oxidation zone is greater than the temperature of the first temperature subzone of the final oxidation zone.

19. 17. The method for producing carbon fibers according to claim 16, wherein the temperature of the second temperature sub-zone is between 1°C and 10°C higher than the temperature of the first temperature sub-zone.

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