Carbon Fiber Manufacturing

Abstract

A method for identifying stabilization conditions for producing thermally stabilized polyacrylonitrile (PAN)-based carbon fiber precursors from any batch of precursors used in a carbon fiber manufacturing process is useful for reducing the cost of carbon fiber production. The method for producing carbon fibers includes the steps of providing a batch of isothermally stabilized polyacrylonitrile (PAN)-based precursor fibers, the batch exhibiting no evidence of tow burning or tow breakage and having a structural transformation index selected from the range of 0.5 to 0.7, and subjecting the provided batch of isothermally treated precursor fibers to a carbonization process to produce carbon fibers.

Description

[Technical field]

[0001] The present invention relates to an improvement in the process for thermo-oxidative stabilization of PAN-based carbon fiber precursors in air, which improvement reduces the time required for stabilization of PAN-based carbon fiber precursors. [Background technology]

[0002] Carbon fibers have desirable mechanical properties and can be broadly classified into ultra-high modulus (>500 GPa), high modulus (>300 GPa), intermediate modulus (>200 GPa), low modulus (100 GPa), and high strength (>4 GPa) carbon fibers.

[0003] Carbon fibers can also be classified based on the final heat treatment temperature as Type I (heat treated at 2,000°C), Type II (heat treated at 1,500°C) and Type III (heat treated at 1,000°C).

[0004] Type II PAN carbon fibers are typically high strength carbon fibers, whereas most high modulus carbon fibers belong to type I. The structure and properties of carbon fibers strongly depend on the development of fiber structure during the heat stabilization process applied to PAN-based precursors. Moreover, the structural changes of the fibers during this process make them flame-retardant and able to withstand high temperatures during carbonization. Therefore, heat stabilization is considered to be a critical step in the production of carbon fibers.

[0005] The process of producing carbon fibers from PAN-based precursor fibers involves subjecting the precursor fibers to a number of processing steps, including a carbonization step that involves thermal stabilization / oxidation in an air atmosphere followed by an initial low-temperature carbonization that proceeds to high-temperature carbonization.

[0006] Among these stages, the initial heat stabilization of the PAN-based precursor fibers is a critical step in the overall carbon fiber manufacturing process involving PAN-based precursor fibers. Thermal stabilization of the PAN precursor involves various exothermic reactions and the formation of polymer ladder structures (see Figure 1), which ultimately transform into turbostratic graphite structures during the carbonization stage of the carbon fiber manufacturing process.

[0007] During thermal stabilization, the PAN-based precursor fibers undergo various structural changes due to the progression of reactions such as cyclization, dehydrogenation, and oxidation (see Figure 1). The cyclization reaction leads to the formation of closed ring structures, while the dehydrogenation reaction leads to the release of hydrogen in the form of H2O (i.e., water) and H2 (i.e., hydrogen gas), which aids in the formation of double bonds between carbon atoms. Finally, the oxidation reaction leads to the formation of C=O (carbonyl) functional groups on the polymer ladder structure. Overall, these structural changes increase the fiber density and make the PAN-based precursor fibers thermally stable to the high temperatures applied in the subsequent carbonization step.

[0008] The cyclization and oxidation reactions that occur in the stabilization process are exothermic in nature, and therefore, unless process parameters such as temperature, time and tension are balanced, excessive heating can cause processing failures (e.g., burning or breaking of fiber tows), resulting in downtime during the manufacturing process that creates additional costs.

[0009] In a typical oxidation step in commercial carbon fiber manufacturing processes, stabilization of the PAN-based precursor fiber is performed gradually in a continuous process by exposing the PAN-based precursor fiber to an increasing temperature gradient in multiple successive stabilization ovens (typically 4-8 or more ovens) (see Figure 2). The stabilization ovens are maintained at increasing temperature steps and utilize zone-specific processing parameters such as residence time and tow tension to ensure a smooth and controlled structural transformation of the PAN-based precursor fiber into a thermally stable fiber ready for carbonization.

[0010] However, considering the time typically required to stabilize the fibers (i.e., about 50 min) and the associated energy consumption, the traditional multi-step PAN-based precursor fiber stabilization process is one of the reasons for the high costs associated with the production of carbon fibers. Therefore, a deeper understanding of the stabilization processing steps and identifying opportunities for reducing the cost of carbon fibers is key to improving the widespread use of carbon fibers in mass market (both automotive and aerospace), renewable energy, and structural applications.

[0011] Considering that the current stabilization processes in the production of PAN-based carbon fibers are expensive and energy consuming, providing a rapid and more cost-effective method for stabilizing PAN-based precursor fibers is an important improvement and a key step towards producing low-cost carbon fibers. Summary of the Invention

[0012] In a first aspect, the present invention provides a method for identifying stabilization conditions for producing a thermally stabilized polyacrylonitrile (PAN) based carbon fiber precursor from any batch of precursor for use in a carbon fiber manufacturing process, comprising: (A) heat stabilizing a sample tow from a particular batch of polyacrylonitrile (PAN)-based precursor fiber under study, (i) a first temperature (T x 1 ) and processing time within 30 minutes (P z 1 ), subjecting the sample tow to a first set of stabilization conditions by applying a single isothermal heat treatment to the sample tow to produce an isothermally treated precursor sample tow; (ii) The T x 1 and P z 1 is the optimal stabilization condition for the sample tow, If such evidence is found, x 1 and P z1 is not optimal for the treated precursor sample tows and that the resulting isothermally treated precursor sample tows are not suitable to withstand the subsequent carbonization step required to produce carbon fibers; In the absence of such evidence, further methods to identify stabilizing conditions are required. (B) determining a structural transformation index (SCI) associated with the isothermally treated precursor sample tow, Obtaining Fourier transform infrared (FTIR) spectra of the isothermally treated precursor sample tows; The following equation:

[0013]

number

[0014] (In the formula, Abs(1595) corresponds to the C=N functional group at 1595 cm -1 is the absorbance peak intensity at wavenumber Abs(2243) is the 2243 cm -1 (where is the absorbance peak intensity at wavenumber determining the SCI by calculating the SCI using If the SCI is determined to be less than 0.5, this indicates that the isothermally treated precursor sample tow is not sufficiently heat stabilized to withstand the subsequent carbonization step to produce carbon fiber; or Determining the SCI to be between 0.5 and 0.7 provides a way to indicate that the isothermally treated precursor sample tows are sufficiently stabilized to withstand the subsequent carbonization process to produce carbon fibers.

[0015] It should be understood that the phrase "producing carbon fibers" also encompasses "producing carbon fibers" that are more than one carbon fiber, e.g., multiple tows are subjected to the methods described herein.

[0016] In a second aspect, the present invention provides a method for producing carbon fibres comprising precursor stabilisation conditions identified by the method of the first aspect. In a third aspect, the present invention provides a method for producing carbon fibers, comprising the steps of: subjecting a batch of polyacrylonitrile (PAN) based precursor fibers to the stabilization conditions for that batch of precursor fibers identified by the method of the first aspect to produce an isothermally treated batch of precursor fibers; and subjecting the batch of isothermally treated precursor fibers to a carbonization process to produce carbon fibers.

[0017] In a fourth aspect, the present invention provides a carbon fibre obtainable or obtained by the method of the second or third aspect of the invention. In a fifth aspect, the present invention provides the use of the heat-stabilised polyacrylonitrile (PAN) based carbon fibre precursor of the fourth aspect in the manufacture of carbon fibres.

[0018] In a sixth aspect, the present invention provides the use of carbon fibres according to the fifth aspect in applications in the automotive, aerospace, sports, nuclear technology, renewable energy sectors and / or in the chemical engineering sector. [Brief description of the drawings]

[0019] [Figure 1] Figure 1 shows the widely accepted mechanism for the development of ladder polymer structures in heat-stabilized PAN fibers. (Source: Rahaman, MSA, AFIsmail, and A. Mustafa, A review of heat treatment on polyacrylonitrile fiber. Polymer Degradation and Stability, 2007. 92(8):1421-1432.) [Diagram 2]This diagram shows an example of a research-scale continuous line for carbon fiber (Source: Nunna, S. et al., A Pathway to Reduce Energy Consumption in the Thermal Stabilization Process of Carbon Fiber Production. Energies, 2018.11:1145 (1-10)). [Diagram 3] FIG. 1 shows the relationship between stabilization temperature and conversion index for a) in-house precursor fiber 1, b) in-house precursor fiber 2 (Note: 24 min residence time was used at each temperature step), and c) commercial precursor fiber 2 (Note: 12 min residence time was used in this study). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: The present invention relates to an improved method for producing PAN-based carbon fibers that is faster and more cost-effective than current / conventional methods for producing PAN-based carbon fibers that rely on processing steps including thermal stabilization / oxidation of carbon fiber precursors in an air atmosphere, followed by carbonization with an initial low temperature carbonization proceeding to high temperature carbonization in an inert atmosphere, typically N2.

[0021] In particular, the present invention provides an improved method for the thermal stabilization (e.g., by oxidation) of suitable carbon fiber precursors that is much faster and more cost-effective than current methods for thermal stabilization of carbon fiber precursors, which heretofore involve slow stabilization times.

[0022] In a related aspect, the present invention provides a means for identifying optimal heat stabilization treatment conditions for any particular batch of available polyacrylonitrile (PAN)-based precursor fibers.

[0023] Thus, the present invention further provides a method for providing a heat-stabilized polyacrylonitrile (PAN)-based carbon fiber precursor capable of withstanding the temperatures applied in the subsequent carbonization step in the carbon fiber manufacturing process to produce carbon fibers.

[0024] The present invention extends to a heat-stabilized polyacrylonitrile (PAN)-based carbon fiber precursor obtainable by the method of the first aspect, which is free of evidence of tow burning or tow breakage and has a structure transformation index selected from the range of 0.5 to 0.7, more preferably 0.60 to 0.65.

[0025] The present invention further relates to a method of producing carbon fibers, the method comprising the steps of: providing a batch of isothermally stabilized polyacrylonitrile (PAN) based precursor fibers that exhibit no evidence of tow burning or tow breakage and have a structural transformation index selected from the range of 0.5 to 0.7, more preferably 0.60 to 0.65; and subjecting the provided batch of isothermally treated precursor fibers to a carbonization process to produce carbon fibers.

[0026] The present invention extends to a method of producing carbon fibers, the method comprising the steps of: subjecting a batch of polyacrylonitrile (PAN) based precursor fibers to precursor stabilization conditions for the batch of precursor fibers identified as optimal for the batch by applying the method of the first aspect to sample tows from the batch of precursor fibers, producing a batch of isothermally treated precursor fibers that exhibits no evidence of tow burning or tow breakage and has a structural transformation index selected from the range of 0.60 to 0.65; and subjecting the batch of isothermally treated precursor fibers to a carbonization process to produce carbon fibers.

[0027] Heat stabilization of any given batch of polyacrylonitrile (PAN)-based precursor fibers. Polyacrylonitrile (PAN)-based carbon fiber precursors are subjected to a specially designed or "personalized" heat stabilization process that can be applied to any particular PAN precursor starting material. The optimized heat stabilization step involves the application of a single isothermal heat treatment step to the precursor, which treatment is specifically devised or designed for the particular batch of polyacrylonitrile (PAN)-based precursor fibers under consideration. In some cases, the heat stabilization step may further include controlling the specific time for which the isothermal heating step is applied. The optimal heat stabilization step may further include applying a specific tension to the PAN precursor fibers during the treatment. When properly heat stabilized, the resulting isothermally treated precursor fibers are optimized for the subsequent carbonization step to produce carbon fibers.

[0028] Desirably, the polyacrylonitrile (PAN)-based precursor fiber is a virgin polyacrylonitrile (PAN)-based precursor fiber. Polyacrylonitrile (PAN)-based precursor fibers include, but are not limited to, precursor fibers including homopolymers of acrylonitrile monomers (regardless of stereoregularity or molecular weight), copolymers containing acrylonitrile monomers, or compositions containing at least one of these materials. For example, a precursor containing acrylonitrile, methyl acrylate, and itaconic acid is included in the class of polyacrylonitrile (PAN)-based precursor fibers referred to herein.

[0029] An important part of the present invention is to identify the optimal parameters of the isothermal heat treatment step performed on any given batch of polyacrylonitrile (PAN)-based precursor fibers, which makes the precursor fibers suitable for the subsequent carbonization step, i.e. the parameters that make the specific polyacrylonitrile (PAN)-based precursor fibers more ideally conditioned to withstand the subsequent carbonization step. The specifically optimal isothermal heat treatment parameters identified are specific to a given batch of polyacrylonitrile (PAN)-based precursor fibers. In some embodiments, the temperature applied as well as the residence time at that temperature and / or the fiber tension may also be important. The present invention provides a simple test method that can be quickly and conveniently applied to samples (single tow or several tows) from the specific batch of precursor fibers under consideration. If a sample from a batch meets the criteria defined herein in terms of achieving optimal heat stabilization, it indicates that the sample is optimized for the subsequent carbonization step to produce carbon fibers.

[0030] The present invention provides a simple test method that can be quickly and conveniently applied to samples (one tow or several tows) from a particular batch of precursor fibers under consideration. If a sample from a batch meets the criteria defined herein in terms of achieving optimal heat stabilization, it indicates that the sample is optimally stabilized and suitable for the subsequent carbonization step to produce carbon fibers. Initial tests can be conveniently performed in a laboratory oven or the like. The isothermal heat treatment parameters so identified can then be applied on a larger scale (e.g., pilot line and / or commercial production line) to the particular precursor batch from which the test sample originates.

[0031] Accordingly, the present invention relates to a method for identifying stabilization conditions for producing a thermally stabilized polyacrylonitrile (PAN)-based carbon fiber precursor from any batch of precursor for use in a carbon fiber manufacturing process, the method comprising: (A) heat stabilizing a sample tow from a particular batch of polyacrylonitrile (PAN)-based precursor fiber under study, (i) a first temperature (T x 1 ) and processing time within 30 minutes (P z 1 ), subjecting the sample tow to a first set of stabilization conditions by applying a single isothermal heat treatment to the sample tow to produce an isothermally treated precursor sample tow; (ii) The T x 1 and P z 1 is the optimal stabilization condition for the sample tow, If such evidence is found, x 1 and P z 1 is not optimal for the treated precursor sample tows and that the isothermally treated precursor sample tows produced are not suitable to withstand the subsequent carbonization step required to produce carbon fibers; If no such signature is present, the method further (B) determining a structural transformation index (SCI) associated with the isothermally treated precursor sample tow, Fourier transform infrared spectroscopy (FTIR) spectra of the isothermally treated precursor sample tows were obtained and calculated according to the following equation:

[0032]

number

[0033] (In the formula, Abs(1595) corresponds to the C=N functional group at 1595 cm -1 is the absorbance peak intensity at wavenumber Abs(2243) is the 2243 cm -1 (where is the absorbance peak intensity at wavenumber Calculate the SCI using If the SCI is determined to be less than 0.5, this indicates that the isothermally treated precursor sample tow is not sufficiently heat stabilized to withstand the subsequent carbonization step to produce carbon fiber; or If the SCI is determined to be 0.5-0.7, it indicates that the isothermally treated precursor sample tow is sufficiently stabilized to withstand the subsequent carbonization process to produce carbon fibers. Steps and The present invention relates to a method comprising the steps of:

[0034] It will be appreciated that the above method may be applied to the particular batch of polyacrylonitrile (PAN) based precursor fiber under consideration. Typically, batch-to-batch variations in polyacrylonitrile (PAN) based precursor fibers mean that there is no ideal set of parameters applicable to the heat stabilization process in the production of carbon fibers.

[0035] It will be understood by one of ordinary skill in the art that evidence of tow burn refers to the visual (at least to the naked eye) observation of one or more of burning, charring, blistering, expansion, and / or other forms of degradation of the precursor material compared to its new, initial condition.

[0036] It will be understood by one of ordinary skill in the art that evidence of tow failure means visual (at least to the naked eye) observation of one or more of the following conditions: breaking, shattering, splitting, fraying, caving in, curling, wrinkling, and / or other forms of mechanical degradation compared to the initial, new condition of the precursor material.

[0037] If the isothermally treated precursor sample shows evidence of tow burning and / or tow breakage after being subjected to the specific stabilization conditions (i) and (ii) above, it indicates that the specific stabilization conditions applied are not suitable (i.e., not optimal) to ensure that the resulting stabilized precursor fiber can be safely subjected to a subsequent carbonization step to produce carbon fiber. If the resulting sample tow is not good enough for the subsequent carbonization step, it is clear that different stabilization conditions need to be considered and applied to another sample taken from the particular batch under consideration.

[0038] Preferably, if the SCI is determined to be less than 0.5, the method further comprises: The method further includes repeating steps (A)(i) and (A)(ii) on a new sample tow from the same batch of polyacrylonitrile (PAN)-based precursor fiber, the repeated step (A)(i) further comprising: x 2 >T x 1 At a temperature of, optionally, P z 2 >P z 1 A treatment time of isothermal heating is included, In the repeated step (A)(ii), identification of evidence of tow combustion and / or tow breakage in the isothermally treated precursor sample tow is performed by T x 2 and / or P z 2 is not optimal for the freshly treated sample tows with respect to subsequent carbonization, therefore, Step (A)(i) is repeated until no evidence of tow burning and / or tow destruction is found; and The step (A)(ii) is repeated until a structural transformation index selected from the range of 0.5 to 0.7 is identified. T x n+1 >T x n At successively higher temperatures, optionally T z n+1 >T z n+1 Repeating steps (A)(i) and (A)(ii) stepwise on additional sample tows from the batch for successively longer treatment times (n is 2, 3, 4, 5, 6, etc.) may be required as necessary.

[0039] It will be appreciated that a structural transformation index in the range of 0.5-0.7 indicates that the treated precursor sample is sufficiently stabilized to withstand the subsequent carbonization step to produce carbon fibers. Stabilization conditions that do not result in tow breakage or burning and that result in a structural transformation index in the range of 0.5-0.7 can then be utilized in the batch process for producing carbon fibers as defined herein.

[0040] Similarly, if the SCI is determined to be greater than 0.7, the method further comprises: Repeating steps (A)(i) and (A)(ii) using additional sample tows from the batch, the repeated step (A)(i) further comprising: x 2 <T x 1 At a temperature of, optionally, P z 2 <P z 1 A treatment time of isothermal heating is included, If, in the repeated step (A)(ii), evidence of tow combustion and / or tow destruction in the isothermally treated precursor sample tow is confirmed, T x 2 and / or P z 2 indicates that the further treated sample tow is not optimal for subsequent carbonization; therefore, Step (A)(i) is repeated until no evidence of tow burning and / or tow destruction is found; and The step (A)(ii) is repeated until a structural transformation index selected from the range of 0.5 to 0.7 is identified. T x n+1 <T x n At successively lower temperatures, optionally T z n+1 <T z n+1Repeating step (A)(i) and (A)(ii) on additional sample tows for successively shorter treatment times (n is 2, 3, 4, 5, 6, etc.) may be required as necessary.

[0041] It will be appreciated that a structural transformation index in the range of 0.5-0.7 indicates that the treated precursor sample is sufficiently stabilized to withstand a subsequent carbonization step to produce carbon fibers. Stabilization conditions that do not result in tow breakage or burning and that result in a structural transformation index in the range of 0.5-0.7 can then be utilized in a batch process to produce carbon fibers as defined herein.

[0042] In some embodiments, the upper temperature limit in the stabilization step is 300° C., 299° C., 298° C., 297° C., 296° C., 295° C., 294° C., 293° C., 292° C., 291° C., 290° C., 289° C., 288° C., 287° C., 286° C., 285° C., 284° C., 283° C., 282° C., 281° C., 280° C., 279° C., 278° C., 279 ... The heating temperature may be selected from among 270°C, 276°C, 275°C, 274°C, 273°C, 272°C, 271°C, 270°C, 269°C, 268°C, 267°C, 266°C, 265°C, 264°C, 263°C, 262°C, 261°C, 260°C, 259°C, 258°C, 257°C, 256°C, 255°C, 254°C, 253°C, 252°C, 251°C, or 250°C.

[0043] Suitably, in step (A)(i), the processing time (P z 1 ) is about 30 minutes or less, preferably less than about 15 minutes. Desirably, in step (A)(i), the treatment time (P z 1 ) is about 24 minutes or about 12 minutes. "About" means ±0.25 minutes.

[0044] In some embodiments, the upper limit of processing or residence time in the stabilization step may be selected from 29 minutes, 28 minutes, 27 minutes, 26 minutes, 25 minutes, 24 minutes, 23 minutes, 23 minutes, 21 minutes, 20 minutes, 19 minutes, 18 minutes, 17 minutes, 16 minutes, 15 minutes, 14 minutes, 13 minutes, 12 minutes, 11 minutes, 10 minutes, 9 minutes, 8 minutes, 7 minutes, 6 minutes, 5 minutes, 4 minutes, 3 minutes, 2 minutes, or 1 minute.

[0045] Preferably, the structural transformation index is selected from the range of about 0.60 to about 0.65, where "about" means ±0.05 minutes. In some embodiments, in step (A)(i), the sample of precursor fiber tow is under tension.

[0046] In some embodiments, the fibers are tensioned at one or more stages of the process to a value of up to 3500 cN or up to 3000. In some embodiments, the tension may be selected from 25 to 3000 cN, preferably 50 to 2700 cN. Examples of suitable tensions for the isothermal stabilization step include those in the range of up to 3000 or up to 2000 cN, preferably 200 to 1750 cN. In some embodiments, tow tensions of 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500 or 1600 are particularly preferred. Particularly suitable tensions for the isothermal stabilization step are 200, 1100 and 1600 cN. However, in some embodiments, the isothermally treated precursor fibers are tensioned to a value of at least 3000 cN.

[0047] Preferably, step (A)(i) is carried out in a single oven. Suitably, step (A)(i) is carried out in the presence of air, more preferably oxygen. Preferably, step (A)(ii) is carried out visually and / or with the aid of a microscope.

[0048] The present invention further extends to a heat-stabilised polyacrylonitrile (PAN) based carbon fibre precursor obtainable or obtained by the process of the invention using the identified optimised parameters for heat stabilisation.

[0049] The present invention also relates to the use of the heat-stabilized polyacrylonitrile (PAN)-based carbon fiber precursor of the present invention (obtained from heat stabilization using the parameters specified in the method described herein) in the manufacture of carbon fibers.

[0050] The present invention further relates to a method of producing carbon fibers, the method comprising the steps of: subjecting a batch of polyacrylonitrile (PAN) based precursor fibers to the stabilization conditions for that batch of precursor fibers identified in the method according to the first aspect to produce an isothermally treated batch of precursor fibers; and subjecting the batch of isothermally treated precursor fibers to a carbonization process to produce carbon fibers.

[0051] The initial process may be conveniently carried out in a laboratory oven or the like, so that the steps can then be applied on a larger scale (e.g., pilot lines and / or commercial manufacturing lines).

[0052] Suitably the carbonisation process comprises a two-stage process comprising an initial low temperature carbonisation step and a subsequent high temperature carbonisation step. Suitably, the initial low temperature carbonization step involves heating the treated precursor fibre to one or more low carbonization temperatures between 400°C and 1000°C, preferably between 425°C and 900°C, more preferably between 450°C and 850°C. Suitably, two or more, preferably at least three temperatures are selected and the stabilized fibre is treated for a total residence time encompassing all selected temperatures. For example, the initial low temperature carbonization step may involve applying three zones of temperatures from these ranges, e.g. 450°C-650°C-850°C, for a total residence time ranging, for example, from about 3 minutes to about 8 minutes. "About" means ±0.05 minutes, i.e. ±3 seconds.

[0053] Desirably, the initial low temperature carbonization step involves heating in a suitable furnace at one or more of the required low carbonization temperatures for a total residence time ranging from 3 minutes to about 8 minutes. Particularly preferred residence times are in the range of about 3.6 to about 7.2 minutes. In some embodiments, residence times in the low temperature furnace of about 3.6, about 5.4, or about 7.2 minutes are particularly preferred. "About" means ±0.05 minutes, i.e., ±3 seconds.

[0054] Desirably, the subsequent high temperature carbonization step involves heating the isothermally treated precursor fiber to a temperature of at least 1000° C. based on the final carbon fiber property requirements. Desirably, the subsequent high temperature carbonization step involves heating the isothermally treated precursor fiber to a temperature of from 1000° C. to 1600° C., preferably from 1100° C. to 1500° C., more preferably from 1250° C. to 1450° C. Suitably, at least two temperatures are selected and the stabilized fiber is treated for a residence time encompassing all the selected temperatures.

[0055] Desirably, the high temperature carbonization step involves heating in a suitable furnace at one or more of the required high carbonization temperatures for a total residence time ranging, for example, from 1 minute to about 6 minutes. Particularly preferred residence times are in the range of 2 to 5 minutes. In some embodiments, residence times of about 2.4, about 3.6, and about 4.8 minutes in the high temperature furnace are particularly preferred. "About" means ±0.05 minutes, i.e., ±3 seconds.

[0056] In some preferred embodiments, the high temperature carbonization step may include applying two temperature zones, for example from the range of 1200° C. to 1400° C. or 1200° C. to 1450° C., for a total residence time ranging from about 1 minute to about 5 minutes, where "about" means ±0.05 minutes, i.e., ±3 seconds.

[0057] In some embodiments, the fibers are subjected to tension in one or more steps of the above process up to a value of 3500 cN, in some embodiments the tension may be selected from 25 to 3000 cN, preferably 50 to 2700 cN.

[0058] Examples of suitable tensions for the first low-temperature carbonization step include tensions preferably up to 1500 cN, preferably in the range of 50 to 1300 cN. Particularly suitable tensions for the isothermal stabilization step are 55, 60, 350, 550 and 1200 cN.

[0059] Examples of suitable tensions for the second high temperature carbonization step include tensions preferably up to 3000 cN, preferably in the range of 75 to 2750 cN. Particularly suitable tensions for the isothermal stabilization step are 280, 2300 and 2700 cN.

[0060] Preferably, the isothermally processed precursor fibers are under tension, especially in processes carried out on a larger scale (eg, pilot and / or commercial manufacturing lines). One aspect of the present invention focuses on the development of a method for the rapid production of carbon fibers from PAN precursors, and as a result of the inventive process, the inventive manufacturing process involves a reduction in the number of ovens and / or a shorter residence time in the stabilization ovens, resulting in significant cost and / or footprint savings.

[0061] The present invention was developed as a result of investigating the thermal resistance of various PAN-based precursor fibers as well as the structural transformation response of the PAN-based precursor with respect to the process parameters, particularly temperature, and especially the combination of temperature and exposure time, applied in the stabilization / oxidation step of a typical carbon fiber manufacturing process.

[0062] By applying the methods of the invention described herein, it has been discovered that significant efficiencies and cost savings can be achieved using the improved carbon production methods described herein compared to typical carbon fiber manufacturing processes. For example, the investment cost of ovens in a newly designed carbon fiber manufacturing plant can be reduced by up to a factor of X, where X is the number of ovens used in a conventional process (typically 4-8). Furthermore, because the methods described herein can be applied to existing equipment, production rates can be increased very significantly by utilizing the methods, thus significantly increasing overall plant productivity.

[0063] Unlike conventional methods of carbon fiber production, the improved method of the present invention based on the improved thermal stabilization step described herein requires only one oven to complete the stabilization process in some cases as little as 12 minutes. The efficiency is due to the use of balanced process parameters in the stabilization stage to rapidly achieve the required structural transformation of PAN to stabilized (oxidized) PAN prior to the carbonization process step.

[0064] Depending on how the process is implemented, either capital investment can be significantly reduced or production capacity can be increased, or both, either of which reduces the overall production costs of carbon fiber.

[0065] Description of the Preferred Embodiments Previously, due to the exothermic reactions that occur during the stabilization of PAN-based precursor fibers, it was necessary to subject the precursor fibers to a gentle and controlled thermal stabilization in the form of a temperature ramp applied by exposing the precursor fibers to multiple temperature steps at temperatures between 200°C and 300°C in order to prevent fiber tow breakage or burning during stabilization.

[0066] Surprisingly, the inventors have found that rapid heat stabilization can be achieved by exposing PAN-based precursor fibers to an isothermal thermal process that can be conveniently achieved in a single stabilization zone, resulting in sufficient structural transformation of the PAN-based precursor fibers into the desired polymeric ladder structure. Unexpectedly, the rapid structural transformation of the PAN-based precursor fibers is achieved without burning or damaging the fiber tows. Rapid heat stabilization is achieved by applying a specially designed heating regime to a particular batch or form of PAN-based precursor fibers. The specially designed heat stabilization regime to be adopted is unique to any particular precursor used and is determined prior to heat treatment by developing an understanding of the thermal resistance of the particular precursor of interest, along with the inherent ability of the fiber to produce the necessary structural transformation prior to carbonization.

[0067] Advantageously, the rapid stabilization strategy involves application of maximum tolerable heat for any given fiber precursor at a given residence time of less than about 30 minutes, where "about" means ±0.05 minutes. Unexpectedly, this strategy was found to rapidly promote thermal stabilization (oxidation) of the precursor fibers to an optimal extent, such that the stabilized (oxidized) fibers could withstand the subsequent extremely high temperatures required for the conversion of the stabilized precursor fibers to carbon fibers via a carbonization process.

[0068] The discovery that stabilization can be affected by the rapid application of heat as nearly as possible without precursor degradation runs counter to all previous methods and understanding of PAN-based precursor fiber stabilization methods. EXAMPLES

[0069] [Experiments and Results] The heat resistance of any batch or type of PAN precursor fiber can be established by exposing individual sample tows of the precursor fiber, not under tension, to various isothermal temperatures, e.g., in a laboratory oven, for residence times corresponding to the residence times that the fiber would typically experience in one stabilization oven at a constant speed (e.g., about 24 minutes of oven residence time in a continuous line). The precursor is then visually inspected to determine whether tow breakage or tow burning occurred due to exposure to the specific test conditions. If the untensioned fiber precursor passes the applicable tests, it has been found that the application of tension to the fiber does not adversely affect the results after application of the same parameters.

[0070] [Example 1] SUMMARY STEPS Samples of virgin precursor fiber tows were subjected to a first set of stabilization conditions, which involved isothermal heating at about 200° C. for 24 minutes, and then visually inspected for fiber burn or tow breakage.

[0071] If no signs of fiber burning or tow breakage are observed, in the next step, another sample of fresh precursor tow from the same precursor batch is isothermally treated for 24 minutes at a higher temperature than that applied in the first step, and further observations are made as to whether fiber burning or tow breakage occurs in the fiber under the second set of stabilization conditions.

[0072] This procedure is continued for subsequent samples of tows from the same precursor batch under consideration until it is determined that the fiber tow will burn at a specific temperature, which is defined as the material's inherent upper heat resistance limit.

[0073] After obtaining the maximum temperature that a new sample precursor fiber can be exposed to without burning, the temperature of the first zone in the continuous production line can be set. Based on the assessment made on the structural transformation of the fiber by FTIR, the temperature requirements for the next carbonization step can be determined. Once the fiber is sufficiently / optimally heat stabilized, it can be carbonized first in a low temperature furnace in various temperature steps ranging from 350-1000°C, and then in a high temperature furnace in various temperature steps ranging from 900-2500°C or even above 2500°C if carbon fiber is to be produced.

[0074] The inventors have found that in order to rapidly promote thermal stabilization of the fibers to a degree that the stabilized precursor can withstand the temperatures applied in the subsequent carbonization process, a rapid stabilization strategy should apply maximum heat at the thermal upper limit of the precursor for a shorter time than current methods.

[0075] [Example 2] Type I precursors As an example, in this study, the method was evaluated using type I precursor from company A. Interestingly, it was found that the heat resistance of the sample tow from the precursor was very high, and no traces of burning and tow breakage were observed, even at temperatures of the order of about 280°C in the laboratory oven. Due to this observation, as an initial test, only two ovens maintained at about 270°C and about 295°C were used for the heat stabilization process with a total residence time of about 24 minutes at an average tension of about 1950cN. In this case, the residence time in each oven is 12 minutes at isothermal temperature. It will be understood that any desired tension can be applied by varying the oven inlet and outlet roller speeds in the process line.

[0076] However, after carbonization of the fibers stabilized in this way (see conditions in Table 1), the carbon fibers produced were brittle and had poor tensile properties. This is believed to be because the Type I precursor fibers were over-stabilized or under-stabilized, preventing proper and / or complete conversion to carbon fibers. Overall, the inventors believe that the extremely high temperatures used in the first oven meant that there was a rapid structural transformation in the fiber precursor, and the fibers were believed to be over-stabilized.

[0077] Therefore, the first oven temperature was reduced to about 265° C. and the process was repeated for additional sample tows, but using only one oven instead of two. The precursor fiber sample tows were subjected to heat stabilization conditions for various times ranging from about 12 to 24 minutes.

[0078] Interestingly, even with a very short treatment time of 12 min (compared to the conventional stabilization process of 50–90 min depending on the line speed), the properties of the final carbonized carbon fibers were significantly improved (see adopted conditions and results in Table 1).

[0079] [Example 3] General Illustrative Steps for Establishing a Rapid Stabilization Method A sample of virgin precursor fiber tow was isothermally treated at a temperature selected within the range of about 200° C. to about 300° C. (e.g., about 235° C. ±1%) for a time period of about 30 minutes or less (e.g., (i) about 24 minutes or less, or (ii) about 12 minutes or less if period (i) is not optimal) (depending on the precursor fiber under consideration). "About" means ±0.05 minutes.

[0080] After the heat stabilization step, the tows of treated precursor fibers were then inspected for evidence of fiber burning or tow breakage, and the extent of structural transformation of the fibers in each sample tow after heat treatment.

[0081] If the inspection shows no signs of fiber burning, it indicates that the precursor is thermally stabilized. The sample is then subjected to further analysis to examine the extent of structural transformation to the desired polymeric ladder structure for the heat-treated fiber. The extent of structural transformation is evaluated by calculating the structural transformation index (Equation 1) using at least one sample spectrum obtained from a Fourier Transform Infrared Spectroscopy (FTIR) test performed on the isothermally treated fiber. For example, the FTIR test is performed herein in ATR mode using a Bruker Lumos FTIR equipped with a germanium crystal. The FTIR spectrum of the sample is measured using a 4 cm -1 Resolution of 600-4000 cm -1 Each spectral data set is the average of 64 co-added scans, and the structure transformation data for each sample is the average of data obtained from three different locations on the fiber tow sample.

[0082] In general, for PAN-based precursor fibers thermally stabilized in air, a structural transformation index between 0.5 and 0.7, more preferably 0.60 to 0.65, has been found to be associated with stabilized precursor fibers suitable for subsequent carbonization processes required to convert the stabilized precursor to carbon fibers. The inventors have found that if the structural transformation index examined falls within the proposed range, the treated precursor fibers can be safely carbonized in the next step to produce carbon fibers.

[0083] The structural transformation index (also known as reaction progress) is calculated according to the following formula:

[0084]

number

[0085] (wherein “Abs(1595)” refers to the 1595 cm -1 and "Abs(2243)" is the FTIR absorbance peak intensity at the wavenumber of 2243 cm associated with the C≡N functional group. -1 (where is the FTIR absorbance peak intensity at wavenumber It is calculated according to:

[0086] If the structural transformation index falls outside the proposed range, it indicates that the treated precursor fiber is not suitable for carbonization and that the applied isothermal processing conditions are not optimal for the particular precursor in question, in other words, the isothermal processing conditions were detrimental to the PAN-based precursor fiber.

[0087] If an incompatible structural transformation index is observed, the isothermal treatment step is repeated using a new sample of fresh precursor tow from the batch under consideration. For example, the new sample is isothermally treated at a temperature (e.g., about 240°C) slightly higher than that applied in the first isothermal treatment step for no more than (i) 24 minutes or (ii) 12 minutes. After this, the structural and physical state of the new PAN-based precursor fiber and the extent of structural change are examined as described above. In some embodiments, the slightly higher temperature may be 5°C, 4°C, 3°C, 2°C, 1°C, or 0.5°C higher than the previous temperature tested. The magnitude of the difference in the higher temperature compared to the previously applied temperature depends on how close the result is to the desired SCI. For SCI results within 20% of the upper or lower limit of the SCI range, a larger change to the higher temperature may be applied compared to the magnitude required when the desired SCI is within 1% of the lower upper or lower limit of the SCI range and a smaller increase in temperature is applied.

[0088] This procedure is continued until conditions are identified that result in isothermally processed PAN-based precursor fibers that achieve the required structural transformation without burning or destruction in the stabilization oven. If sufficient structural transformation does not occur (as indicated by a mismatched structural transformation index) and the fiber burns at a given temperature, the above procedure can be applied again at the same temperature, but with a shorter residence time than previously applied.

[0089] At the end of this work, it is possible to identify the optimal temperature and optional residence time that any given PAN-based precursor fiber can be exposed to in the first zone of the stabilization stage to achieve the necessary degree of structural transformation that allows the treated fiber to proceed to the subsequent carbonization step. In some embodiments, the preferred initial residence time is 24 minutes. Shorter residence times are applied only if there is no evidence of burning or destruction or SCI is met using the 24 minute residence time. In some cases, the residence time can be as short as 12 minutes.

[0090] Commercially available PAN-based precursor fibers are available on the market. Other proprietary PAN-based precursor fibers have been developed for in-house use only. However, each precursor has a different chemical composition and properties, and each exhibits different thermal behavior under the same conditions. Therefore, the maximum temperature and / or residence time that any given PAN-based precursor fiber can be exposed to in the isothermal stabilization step can vary widely between precursors and should be defined by developing an understanding of the thermal tolerance of the selected PAN precursor using the methods described herein. The exact composition of the precursor used is not a factor in determining the optimal stabilization conditions for a particular precursor, since thermal tolerance is explored based on the application of the screening method of the present invention to screen any particular fiber precursor for temperature and residence time tolerance.

[0091] The improved PAN-based precursor fiber isothermal stabilization method described herein has been validated using four different PAN-based precursor fibers, two of which are commercially available and two of which are in-house developed PAN-based precursor fibers. Details of the two in-house precursor fiber compositions are provided in the last two rows of the table.

[0092] As an example, the correlation between the stabilization temperature and the structural transformation index is shown in FIG. [result] As shown in Table 1, all PAN-based fiber precursors tested were found to achieve the required structural transformation index when isothermally processed under the optimal processing conditions identified using the methods described herein.

[0093] In general, the optimal conditions for the tested PAN-based fiber precursors shown in Table 1 included an isothermal stabilization temperature (depending on precursor type) of between about 260°C and about 270°C ("about" means ±1%) with a residence time of either about 12 minutes or about 24 minutes ("about" means ±0.05 minutes) applied in a single stabilization step.

[0094] After employing the proposed rapid heat stabilization protocol described herein to obtain optimal stabilization conditions for the fibers of the sample tows of each of the four precursors, the heat stabilized fibers of each precursor type were subsequently carbonized to successfully produce carbon fibers under the conditions disclosed in Table 1. The properties of the resulting carbon fibers are shown in Table 1.

[0095] Although carbon fibers were successfully produced in all cases of the precursors subjected to the method of the present invention, the mechanical properties of the carbon fibers produced from the commercial precursor 1 were significantly higher than those of the other carbon fibers.

[0096] It should be noted that the properties of many of the carbon fibers produced from this study are close to those of the T300 commercial fiber. The properties of the preferred carbon fibers exceed those required for automotive use.

[0097] The commercial precursor 1 was first stabilized at 265 °C for 12 min in an air atmosphere under a tension of about 1600 cN to ensure that the fiber was thermally stable enough to withstand the high temperatures during the subsequent carbonization step. The structural transformation index of the treated precursor fiber was evaluated using FTIR technique and found to be 0.55.

[0098] The stabilized fiber is then subjected to a first carbonization stage in a low temperature furnace having three temperature zones maintained at 450, 650 and 850° C. at a tension of about 550 cN for a total residence time of about 3.6 minutes.

[0099] These fibers are subsequently further carbonized in a high temperature furnace having two temperature zones maintained at 1200 and 1400° C. The total residence time and tension applied in this further carbonization step are 2.4 minutes and approximately 2300° C., respectively.

[0100] The above work starting from the commercially available precursor 1 resulted in desirable carbon fibers with a tensile strength of 3.68 GPa, a tensile modulus of 223.7 GPa and a % elongation of 1.77.

[0101] [Table 1]

Claims

**Claim 1** A method for specifying stabilization conditions for producing a heat-stabilized polyacrylonitrile (PAN)-based carbon fiber precursor from any batch of precursors used in a carbon fiber manufacturing process, comprising: (A) heat stabilizing a sample tow from a particular batch of polyacrylonitrile (PAN)-based precursor fibers under consideration, (i) A first temperature (T selected from 200 °C to 300 °C ｘ １ ) and a processing time (P within 30 minutes ｚ １ ), by applying a single isothermal heat treatment to the sample ear during this period, subjecting the sample ear to a first set of stabilization conditions to produce an isothermally treated precursor sample ear, (ii) By inspecting the isothermally treated precursor sample dough for the presence or absence of signs of dough combustion or dough destruction, T ｘ １ and P ｚ １ evaluate whether are the optimal stabilization conditions for the sample dough, and if such signs are confirmed, T ｘ １ and P ｚ １ are not optimal for the treated precursor sample dough and that the resulting isothermally treated precursor sample dough is not suitable to withstand subsequent carbonization steps necessary for manufacturing carbon fibers, including steps indicating If there is no such trace, the method further comprises (B) determining a structure conversion index (SCI) associated with the isothermally treated precursor sample tow, obtaining a Fourier transform infrared spectroscopy (FTIR) spectrum of the isothermally treated precursor sample tow, The following equation: 【Number 1】 (wherein Abs(1595) is the absorbance peak intensity at a wavenumber of 1595 cm corresponding to the C=N functional group. －１ and Abs(2243) is the absorbance peak intensity at the wavenumber of 2243 cm related to the C≡N functional group) －１ .) is used to calculate the SCI, If it is determined that the SCI is less than 0.5, it indicates that the isothermally treated precursor sample tow is not sufficiently heat-stabilized to withstand subsequent carbonization steps for producing carbon fibers, or If it is determined that the SCI is between 0.5 and 0.7, it indicates that the isothermally treated precursor sample tow is sufficiently stabilized to withstand subsequent carbonization for producing carbon fibers), including the steps of the method. **Claim 2** The method according to claim 1, wherein if it is determined that the SCI is less than 0.5, the method further comprises Including a step of repeating steps (A)(i) and (A)(ii) for a new sample tow from the same batch of polyacrylonitrile (PAN)-based precursor fibers, wherein the repeated step (A)(i) involves subjecting the new sample tow to isothermal heating at a temperature of T ｘ ２ > T ｘ １ and, optionally, at a pressure of P ｚ ２ > P ｚ １ for a treatment time, In the repeated step (A)(ii), when traces of ear combustion and / or ear destruction in the isothermally treated precursor ear sample are confirmed, T ｘ ２ and / or P ｚ ２ indicate that they are not optimal for the new treated ear sample with respect to subsequent carbonization, Therefore, until no signs of tow combustion and / or tow breakage are found in the repeated step (A)(i), and until a structure conversion index selected from the range of 0.5 to 0.7 is confirmed in the repeated step (A)(ii). T ｘ ｎ＋１ > T ｘ ｎ at successively higher temperatures, optionally T ｚ ｎ＋１ > T ｚ ｎ＋１ at successively longer processing times (n being 2, 3, 4, 5, 6, etc.), the method being required to repeat the stepwise steps (A)(i) and (A)(ii) to further sample ears from said batch as necessary. **Claim 3** The method according to claim 1, wherein if it is determined that the SCI is greater than 0.7, the method further comprises Repeat steps (A)(i) and (A)(ii) using further sample ears from said batch, with repeated step (A)(i) comprising isothermally heating said further sample ears at a temperature of T ｘ ２ < T ｘ １ and, optionally, for a processing time of P ｚ ２ < P ｚ １ ; In the repeated step (A)(ii), when traces of ear combustion and / or ear destruction in the isothermally treated precursor sample ear are confirmed, the T ｘ ２ and / or P ｚ ２ indicate that it is not optimal for the further processed sample ear with respect to subsequent carbonization, Therefore, until no signs of tow combustion and / or tow breakage are found in the repeated step (A)(i), and until a structure conversion index selected from the range of 0.5 to 0.7 is confirmed in the repeated step (A)(ii). T ｘ ｎ＋１ <T ｘ ｎ at successively lower temperatures, optionally T ｚ ｎ＋１ <T ｚ ｎ＋１ at successively shorter processing times (n being 2, 3, 4, 5, 6, etc.), and repeating step (A)(i) and (A)(ii) in a stepwise manner to an additional sample pod as required, said method. **Claim 4** In step (i), the processing time (P ｚ １ ) is less than 25 minutes, preferably less than 15 minutes, according to the method of claim 1 or 2. **Claim 5** In step (i), the processing time (P ｚ １ ) is 24 minutes or 12 minutes, the method according to claim 1 or 2. **Claim 6** The method according to any one of claims 1 to 3, wherein the structure conversion index is selected from the range of 0.60 to 0.

65. **Claim 7** The method according to any one of claims 1 to 3, wherein in step (i), the sample of the precursor fiber tow is under tension. **Claim 8** The method according to any one of claims 1 to 3, wherein step (i) is performed in a single oven. **Claim 9** The method according to any one of claims 1 to 3, wherein step (i) is carried out in the presence of air.

10. The method according to any one of claims 1 to 3, wherein step (ii) is carried out visually and / or using a microscope.

11. A method for producing carbon fibers, comprising: providing a batch of isothermally stabilized polyacrylonitrile (PAN)-based precursor fibers, which shows no signs of tow combustion or tow breakage and has a structure conversion index selected from the range of 0.5 to 0.7, more preferably 0.60 to 0.65; subjecting the provided batch of isothermally treated precursor fibers to a carbonization process to produce carbon fibers.

12. The method according to claim 11, wherein the carbonization process comprises a two-step process including an initial low-temperature carbonization step and a subsequent high-temperature carbonization step.

13. The method according to claim 12, wherein the initial low-temperature carbonization step comprises heating the isothermally treated precursor fibers to a temperature in the range of 350 °C to 1000 °C.

14. The method according to claim 12, wherein the subsequent high-temperature carbonization step comprises heating the isothermally treated precursor fibers to a temperature in the range of 900 °C to 2500 °C.

15. A method for producing carbon fibers, comprising: subjecting a batch of polyacrylonitrile (PAN)-based precursor fibers to precursor stabilization conditions that are determined to be optimal for the batch of precursor fibers by applying the method according to claim 1 to a sample tow from the batch of precursor fibers, to produce a batch of isothermally treated precursor fibers that shows no signs of tow combustion or tow breakage and has a structure conversion index selected from the range of 0.60 to 0.65; subjecting the batch of isothermally treated precursor fibers to a carbonization process to produce carbon fibers.

16. The method according to claim 15, wherein the carbonization process comprises a two-stage process including an initial low-temperature carbonization step and a subsequent high-temperature carbonization step.

17. The method according to claim 16, wherein the initial low-temperature carbonization step comprises heating the isothermally treated precursor fibers to a temperature between 350 and 1000 °C.

18. The method according to claim 16, wherein the subsequent high-temperature carbonization step comprises heating the isothermally treated precursor fibers to a temperature between 900 and 2500 °C.

19. The method according to claim 11 or 15, wherein the isothermally treated precursor fiber is under tension. **Claim 20** The method according to claim 11 or 15, wherein the isothermally treated precursor fiber is under a tension of up to 3000 cN or a value exceeding 3000 cN.

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