Low temperature stabilization of thermoplastic fibers

Abstract

A low-temperature process of processing green fibers fabricated from thermoplastic precursors such as asphaltenes-based precursor material includes the step of exposing the green fibers to an NO2-containing gas mixture, at a temperature less than about 200°C to produce stabilized green fibers. The stabilized green fibers may be further stabilized at a temperature greater than 200°C in an oxygen-containing environment. Stabilized green fibers may then be carbonized to produce carbon fiber.

Description

LOW TEMPERATURE STABILIZATION OF THERMOPLASTIC FIBERSCross-Reference to Related

[0001] This application claims the priority benefit of U.S. Provisional Patent Application 63 / 688,692, filed on August 29, 2024, the entire contents of which are incorporated herein by reference, where permitted.Field of the Invention

[0002] This disclosure relates to methods of low temperature and / or short time stabilization of thermoplastic hydrocarbon fibers for the fabrication of oxidized fibers and / or carbon fibers.

[0003] The fabrication of carbon fibers starts from the production of precursors that can be spun into fibrous filaments, referred to as green fibers. The precursors are usually thermoplastic polymers that will melt to liquids upon heating. Because of their thermoplastic nature, the green fibers must be converted to non-fusible thermosets by a process called stabilization before further thermal processing into carbon fibers.

[0004] The process of stabilization involves oxidation and is conventionally performed at temperatures between 200°C and 350°C in an oxy gen-containing gas to cross-link the molecules to the point where the fibers do not melt or fuse together. The time required to achieve stabilization is sensitive to the nature of precursors. In general, the rate of reaction in stabilization becomes meaningful at temperatures above 200°C. At a stabilization temperature lower than 200°C, the reaction is very sluggish and is not economical as a result. However, a high oxidation temperature can lead to over-oxidation and poor mechanical properties of the resulting carbon fibers. Generally, a precursor with a higher softening point can be stabilized at higher temperatures with shorter stabilization times. It is preferred that the softening points of precursor material to be in the range of 230°C to 280°C.WSLEGAL\093616\00067\41644682v5

[0005] The green fibers after proper stabilization can be carbonized to produce carbon fibers, which usually take place at temperatures above 1000° C. Carbon fibers will also go through the process of etching and sizing before being delivered to the end users.

[0006] Various precursors such as polyacrylonitrile (PAN), pitch, rayon, or lignin can be used to produce carbon fibers. PAN is a widely used precursor to produce PAN-based carbon fibers, which takes up about 90% of market share for carbon fiber. The stabilization of PAN-derived green fibers is usually performed at temperatures at around 300°C in air for a period usually being controlled within 90 minutes.

[0007] Pitch-based carbon fibers are fabricated from precursors derived from the hydrocarbon feedstocks such as heavy oil, refinery residues, or coal-derived asphaltene. The components in crude oils are customarily categorized as saturate, aromatic, resin and asphaltenes (SARA) according to their polarizability and polarity. The saturate fraction consists of nonpolar material including linear, branched, and cyclic saturated hydrocarbons (paraffins). Aromatics, which contain one or more aromatic rings, are slightly more polarized. Both resins and asphaltenes have polar substituents but resins are miscible with heptane or pentane, whereas asphaltenes are insoluble in heptane or pentane.

[0008] Pitch precursors are comprised of aromatic components present in hydrocarbon feedstocks such as heavy oil, refinery residues, or coal-derived asphaltene. The aromatic components in hydrocarbon feedstocks can be processed to produce isotropic pitch and mesophase pitch. Asphaltenes in these pitch-based feedstocks are undesirable and result in lower quality of pitch-derived carbon fibers. The stabilization of green fibers derived from pitch precursors is typically performed at temperatures ranging from 230°C to 350°C in air.

[0009] Asphaltenes present in the hydrocarbon feedstocks have also been processed into precursors for carbon fibers. Asphaltenes are defined operationally as the n-heptane- insoluble, toluene-soluble component of a carbonaceous material, and therefore do not have a specific chemical formula. It has been shown that asphaltenes have a distribution of molecular masses in the range of 400 u to 1500 u, but the average and maximum values are2WSLEGAL\093616\00067\41644682v5difficult to determine due to aggregation of the molecules in solution (Podgorski, D.C. (2013), Energy & Fuels, 27(3): 1268-1276).

[0010] Asphaltenes can be formed into green fibers either by a melt-spinning or wet-spinning process. Melt-spinning produces precursor fibers by heating the precursors to temperatures about 30-40° C higher than their softening points and pressing them through nozzles of about 0.3-0.1 mm in diameter. Wet spinning process fabricates precursor fibers at room temperature using a solution of asphaltenes dissolved in a solvent. The precursor fibers are formed after the solvent is removed.

[0011] However, stabilization of asphaltenes-derived green fibers has been found to be challenging. Owing to their wide range of molecular sizes and weights, asphaltene green fibers can start to fuse at very low temperatures before they can be effectively stabilized by oxidation in air. To prevent the occurrence of fusion during heating for oxidative stabilization, a very slow heating rate must be employed, requiring a very long time of oxidative stabilization, for example, up to 20 hours.

[0012] The risk of fusion at relatively low temperatures may be mitigated by raising the softening point of asphaltenes to remove small molecular species, thereby allowing the green fibers to be heated to a higher temperature for oxidation at relatively higher heating rate. This would result in a reduction in overall time for oxidation and savings in production time and energy consumption.

[0013] Asphaltenes are a thermoplastic polymer, where single molecules are linked together by thermosensitive intermolecular attractive forces such as Van der Waals, dipole-dipole, and hydrogen bonding. The softening points and melting points of polymeric materials are generally related to their molecular mass, as well as the intra- and intermolecular forces within the polymer. Melting points of polymers with a molecular weight of 100 mol / g are well below -50° C, while those with a molecular weight larger than 400 mol / g are higher than 50° C. At very low temperatures, thermoplastic polymers are very brittle and exhibit high Young’s modulus, owing to the rigidity of crystalline and amorphous molecules. With temperature rise, there is usually a sudden drop of their rigidity and Young’s modulus,3WSLEGAL\093616\00067\41644682v5because of activation of molecular movement of the amorphous chain segments. The temperature point at which Young’s modulus drops quickly is termed as the glass point, Tg. This drop of Young’s modulus depends on the amorphous / crystalline ratio. Between Tg and Tm (melting temperature) is the rubber-fluid state, where the polymers are softened and can be pulled into filaments such as fibers.

[0014] The softening points of asphaltenes produced from solvent de-asphalting of Alberta oilsands bitumen is known to be in the range of 160° to 170° C, while those from metallurgic pitch, is well below 150° C. The melting points of these materials are usually about 30-40° C higher than their softening points, which is typically higher than their glass points.

[0015] Acid treatment of asphaltene-derived green fibers, particularly nitric acid treatments, can substantially shorten the time of stabilization in air. In a typical process of acid treatment, green fibers are soaked in acid solution, which forms a thin oxidized layer on the surface of green fibers. This preserves the fibrous shape of green fibers and minimizes the risk of fusion at fiber contacting points before the bulk thermoplastic precursor fibers are fully converted to thermosets during oxidative stabilization.Summary of the Invention

[0016] Disclosed herein are methods of stabilizing green fibers fabricated from thermoplastic precursors at a low temperature and / or with reduced times. The method is particularly suitable to stabilize green fibers fabricated from those thermoplastic precursors that comprise molecules with a wide range of molecular sizes and weights, such as asphaltene-based precursors.

[0017] In some embodiments, asphaltene-based green fibers can be stabilized at a temperature less than about 200° C, or preferably less than about 100° C, such as room temperature, for example, at 20°C, in a NCh-containing air. The stabilized green fibers can then be carbonized to produce carbon fibers.

[0018] Stabilization in an atmosphere free of NO2 is possible only when processing temperature is higher than 200°C, where the chemical reaction of green fiber with oxygen is4WSLEGAL\093616\00067\41644682v5kinetically meaningful. Therefore, in some embodiments, the stabilization of asphaltenes- based green fibers is carried out at temperatures below 200°C for a time less than two hours, preferably less than 90 minutes, in an atmosphere comprising NO2 and air.

[0019] Thus, described herein is a process of processing green fibers fabricated from thermoplastic precursors such as asphaltenes-based precursor material, comprising the steps of:(a) exposing the green fibers to an NCh-containing gas mixture, at a temperature less than about 200° C to produce stabilized green fibers.(b) optionally further stabilizing the stabilizing green fibers in the presence of oxygen, at a temperature greater than about 200° C to produce oxidized fibers; and / or(c) optionally carbonizing the stabilized green fibers from step (a) or step (b) in an inert environment, preferably in nitrogen, to produce carbon fibers.

[0020] In preferred embodiments, the stabilization of step (a) is carried out at a temperature less than about 100° C, such as at 20° C, in a closed system comprising an atmosphere comprising NO2 and air. Preferably the closed system is pressure controlled to reduce the time of stabilization, such as between about 0.05 MPa to about 0.3 MPa.

[0021] In preferred embodiments, the stabilization of step (a) is followed by further stabilization of step (b) to produce oxidized fibers with improved mechanical properties and higher limiting oxygen index of oxidized fibers.

[0022] In preferred embodiments, the stabilization of step (a) is carried out to achieve a threshold level of weight gain, which is dependent on the diameter of green fibers. Above the threshold level of weight gain, the resulting carbon fiber after carbonization will have a more uniform fiber diameter. In some embodiments, the threshold level of weight gain is at least 5% after the stabilization step (a), and preferably greater than about 8%.WSLEGAL\093616\00067\41644682v5

[0023] A process of producing stabilized, carbon or oxidized fiber described herein may comprise any new and inventive step, act, combination of steps and / or acts or subcombination of steps and / or acts as described herein.

[0024] Brief Description of The Drawings

[0025] Figure 1 is a schematic overview of the process of stabilization of green fibers to produce oxidized fibers and carbon fibers, respectively.

[0026] Figure 2 is a correlation of weight gain of green fiber after stabilization at 20°C in NCh-containing air with a combined factor that is defined as Soak time x NO2 concentration x soak pressure (minute •%• MPa).Detailed Description of Preferred Embodiments

[0027] The present invention relates to the methods to stabilize thermoplastic green fibers in NCh-containing air, instead of air alone as is conventional, which allows reduced stabilization temperatures and / or times.

[0028] As discussed above, conventional stabilization in air is performed at temperatures above 230°C, above which the reaction of green fibers with oxygen in air becomes kinetically meaningful for industrial production. Accordingly, two characteristic temperature regimes of stabilization described herein, one below 100°C, and the other between 100°C and 200°C, one or both of which is considered "low temperature stabilization".

[0029] Embodiments disclosed herein are not limited by the type of precursors used to produce the green fibers, provided the precursors are thermoplastic in nature and can be reacted to incorporate nitro-functional groups (-NO). A precursor can be reacted to incorporate nitro-functional groups if they are produced by nitration reactions involving nitric acid.

[0030] Examples of polymer precursors that could lead to nitration may include but not limited to 2-Nitroresorcinol, 2-Nitro-pphenylenediamin, 3-Nitro-o-phenylenediamine, 4- Nitro-o-phenylenediamine, m-Nitrophenylboric Acid, 6-Nitrobenzimidazole, Resorcinol, p- Phenylenediamine, Benzimidazole, Nadic Methyl Anhydride (NMA), N,N-6WSLEGAL\093616\00067\41644682v5Dimethylbenzylamhe, Epon 826, Epon 1031, Kopox 171, Kopox 171, Resorcinol Diglycidyl Ether, Diglycidyl Ether of l,b-Naphthalenediol, Diglycidyl Ether of 1,6-Naphthalenediol, Diglycidyl Ether of 2,7-Naphthalenediol, Diglycidyl Ether of 2-Nitroresorcinol, etc. These polymers were found to increase char yield due to the incorporation of nitro-group incorporation (Chao, J.; Wilhoit, R. C.; Zwolinski, B. J. Gas phase chemical equilibrium in dinitrogen trioxide and dinitrogen tetroxide. Thermochimica Acta, 1974, 10, 359-371).

[0031] It is known to stabilize thermoplastic green fibers including asphaltene-based fibers by soaking precursor fibers in an aqueous nitric acid solution to prevent fiber fusion during oxidative stabilization in air and to reduce the time of oxidative stabilization in air. Nitric acid is subject to thermal or light decomposition: 4HNOs —> 2H2O + 4NCh + O2. Therefore, without limitation to a theory, it is believed that soaking precursor fibers in nitric acid prior to heating the precursor fibers to the temperatures for stabilization in air, normally above 200°C, releases NO2, which is believed to play a role in stabilization achieved using nitric acid.

[0032] NO2 exists in temperature-dependent equilibrium with dinitrogen tetroxide (N2O4). At low temperatures, NO2 reversibly converts to the colourless N2O4. NO2 is brown and it may be observed that the intensity of the brown color decreases as the temperature decreases.Therefore, a decrease in temperature yields an increase in N2O4 and an increase in temperature shifts the equilibrium to NO2. The reaction will be spontaneous above 52.6°C. At 100° C and above, the NOx exists almost entirely as NO2 (Chao, J.; Wilhoit, R. C.; Zwolinski, B. J. Gas phase chemical equilibrium in dinitrogen trioxide and dinitrogen tetroxide.Thermochimica Acta, 1974, 10, 359-371). In a dry ice - acetone bath, N2O4 crystallizes as a white solid. At 150°C and above, NO2 tends to decompose with release of oxygen via an endothermic process: 2NCh^2NO + O2.

[0033] While this disclosure demonstrates processes with asphaltenes as a petroleum derived hydrocarbon material, embodiments described herein can be used for processing other types of hydrocarbons such as those derived from coal, or other processing activities due to their similar thermoplastic nature.7WSLEGAL\093616\00067\41644682v5

[0034] As used herein, "asphaltenes" are non-polar and non-volatile hydrocarbon compounds present in crude oil such as heavy oil and bitumen, which are insoluble in an n-alkane solvent such as pentane or hexane and are soluble in aromatic solvents such as benzene or toluene. The molecular structure of asphaltenes is difficult to determine because the molecules tend to stick together in solution. These materials are extremely complex mixtures containing a large number of individual chemical species. Asphaltenes do not have a specific chemical formula: individual molecules can vary in the number of atoms contained in the structure, and the average chemical formula can depend on the source - the exact molecular structures are difficult to determine. Given this limitation, asphaltenes are composed mainly of polyaromatic carbon ring units with oxygen, nitrogen, and sulfur heteroatoms, combined with trace amounts of heavy metals, particularly chelated vanadium and nickel, and aliphatic side chains of various lengths.

[0035] "Asphaltene-based precursor material" or "precursor material" is material produced from hydrocarbon feedstock, which is enriched in asphaltenes and which preferably have been treated to raise their softening point and be modified to be suitable for carbon fiber production. The hydrocarbon feedstock material can be any material with a thermoplastic nature that is liquid or becomes liquid upon heating and which may have varied contents of asphaltenes from 5 % up to 90 % (wt).

[0036] Suitable asphaltene based material with an elevated softening point may be produced with methods described in PCT International Application PCT / CA2021 / 000092, the entire contents of which are incorporated herein by reference. Thermoplastic precursor material may be processed by melt spinning into green fibers, at a temperature between about 250°C to about 350°C, and more preferably between 275°C to 325°C. Feedstocks with melt spinning temperature beyond 325°C may form carbonaceous coke particles during the melt spinning process, which is undesirable. Some suitable methods of melt-spinning asphaltene based materials are described in co-pending United States Provisional Patent Appl. 63 / 752,051, the entire contents of which are incorporated herein by reference.

[0037] The melt spinning apparatus includes heating chamber, a device to evenly distribute mass flow for all nozzles, filtering plates and finally, single- or multiple-hole nozzle with8WSLEGAL\093616\00067\41644682v5hole diameter which is preferably smaller than or equal to 0.3 mm. The heat-treated feedstock is extruded into fibrous shape at elevated temperatures and the extruded material is wound on a winding drum to be thinned to a desired diameter at different winding speeds.

[0038] The melt-spun fibers are then stabilized. Stabilization may comprise of exposing melt- spun fibers into an atmosphere comprising NO2 in a chamber, preferably air with NO2. Such a chamber can be sealed and vacuumed to a lower pressure or pressurized to a higher pressure.

[0039] The melt-spun fibers are loaded in the chamber when the chamber is filled up with air. The chamber is sealed and NO2 is injected into the chamber. This can be done either by injecting an NCh-air mixture with known NO2 concentration which then replaces all the air originally in the chamber. Alternatively, the chamber could be vacuumed to remove preexisting air before injecting an NCh-air mixture with a known NO2 concentration. In another alternative, pure NO2 can be injected into the chamber to a desired concentration.

[0040] The NCh-air mixture could be injected to reach a pressure below, equal to or above atmospheric pressure at the beginning of stabilization process. Combining the NO2 concentration in air and the pressure of NCh-air mixture, the partial pressure of NO2 can be calculated and is a variable that affects the stabilization time and temperature required to reach a given amount of incorporation of NCh-group into the green fibers.

[0041] During the process of stabilization, the chamber could be sealed without a further supply of NCh-air mixture, or alternatively additional NCh-air mixture may be added to the chamber continuously or periodically.

[0042] The temperature and the time required to achieve sufficient stabilization is sensitive to the partial pressure of NO2 in the air environment and the diameter of green fibers. Sufficient stabilization is judged based on several criteria. First, the stabilized fiber will yield fibers with a uniform diameter after stabilized fibers are carbonized to produce carbon fibers. Non- uniform diameter is an indication of non-uniform stabilization. Uniform fiber diameter refers to the diameter of a single fiber along its length, which does not vary substantially (ie. less than 10%, preferably less than 5% variance). Uniform diameters will not be achieved if a threshold level of weight gain is not reached after stabilization. Second, the stabilized fibers9WSLEGAL\093616\00067\41644682v5will yield suitably high tensile strength after carbonization if a threshold level of weight gain is reached after stabilization or when the fibers are over stabilized with very high weight gains. An optimized weight gain is sensitive to the diameter of green fibers and the temperature of stabilization.

[0043] The temperature of stabilization can be divided into two temperature regimes: a first temperature regime between 0° C and 100°C, and a second regime between 100°C and 200°C.

[0044] When stabilization in NCh-air mixture is performed at temperatures between room temperature and 100°C, the highest weight gain is achieved at lower temperatures, below about 40° C, and preferably about room temperature. This results from a balance of greater conversion of N2O4 to NO2, which is maximized when temperature reaches 100°C, and the degree of adsorption of NO2 to the surface of green fibers, which increases with decreasing temperature.

[0045] The weight gain decreases with an increase in the temperature of stabilization in NCh- air mixture, and the stabilized green fibers lose weight after stabilization in NCh-air mixture when placed in ambient environment, due to desorption of pre-adsorbed species.

[0046] We experimented by varying the partial pressure of NO2 during stabilization by using NCh-air mixture with different NO2 concentrations and stabilizing under different gas pressures. Weight gain during stabilization is correlated to a gas exposure factor defined as Soak time (min) x NO2 concentration (vol%) x soak pressure (MPa), as shown in Figure 2.

[0047] In one embodiment, the weight gains of green fibers per unit weight were observed to be related to fiber diameter. Green fibers with smaller average diameter achieved higher weight gains after stabilization under the same condition. Without restriction to a theory, because adsorption is the mechanism of stabilization which results in weight gain, the ratio of surface area of green fiber to weight is important. Green fibers with smaller diameter would have higher surface area provided that the weight of green fibers is kept the same. This further confirms the mechanism of adsorption that has played a role in stabilization.10WSLEGAL\093616\00067\41644682v5

[0048] When stabilization in NCh-air mixture is performed at temperatures between 100°C and 200°C, the resulting weight gains can be affected by the following factors:• the decomposition of NO2 to NO at 150° C that leads to a reduced NO2 partial pressure in the NO2-air mixture and, therefore, reduces the incorporation of NO2- groups based the mechanism of adsorption, and• the increased degree of conventional stabilization in air through the incorporation of O2, which increases kinetically with increasing temperature, although sluggish when temperature is below 200°C.

[0049] In preferred embodiments, substantial weight gain may result with conditions where the stabilization temperature was 140°C, 160°C and 180°C. The highest weight gains were observed when the temperature was 160° C.

[0050] The green fibers after at least one low temperature stabilization step can proceed directly to the stage of carbonization performed in an inert or oxygen-free environment, such as in nitrogen.

[0051] In some embodiments, the low temperature stabilization is followed by a further stabilization, when the first step is insufficient or sub-optimal such that the fiber diameter is seen to be non-uniform after carbonization, which causes inconsistent mechanical properties of carbon fibers. The green fibers after insufficient low temperature stabilization could be further processed to achieve additional stabilization at higher temperatures in air. In some embodiments, the additional stabilization in air was performed at higher temperatures, for example at around 260° to about 300°C. By varying the temperature and / or time of this additional stabilization in air, the mechanical properties and the limiting oxygen index of the resulting oxidized fibers can be improved. These stabilized fibers may then be carbonized as described herein and results in a more uniform fiber diameter.

[0052] "Limiting oxygen index" or LOI is a measure of flammability of a polymer material. LOI is the minimum concentration of oxygen, expressed as a percentage, that will support combustion of a polymer. It can be measured by standardized tests, such as the ISO 4589 and11WSLEGAL\093616\00067\41644682v5ASTM D2863. LOI values can be used to rank the relative flammability of different polymer materials.Examples

[0053] By way of further description of the process of the present disclosure, reference may be made to the following examples, which are intended to exemplify certain elements of the claimed invention, not limit them. Unless otherwise indicated, all parts and percentages are by weight.Example 1

[0054] About 10 grams of asphaltene-derived precursor were melt-spun at 310°C and wound at a winding speed of 981 m / min to produce green fibers (precursor fiber after melt-spinning before any thermal treatment). The diameter of the green fiber was measured with a high- resolution microscope. A total of 20 green fibers were measured and the average diameter of the green fibers was determined to be 7.7 pm.

[0055] 4 samples of green fiber, each with a weight of about 1 gram, as listed in Table 1, were placed in a tubular furnace. The furnace was sealed at both ends except for a gas inlet / outlet to inject a gas mixture containing 2% NO2 and air in molar concentration in balance at a flow rate of 80 ml / min through a tubular furnace with a volume of about 2.0 liters. After the flow of the gas mixture for 15 minutes, the furnace was heated to different temperatures ranging from 20°C up to 90°C, at a heating rate of 5°C / min, and held at the final temperature for 30 minutes. The green fiber samples were then weighed and percentage weight changes were recorded and tabulated in Table 1. The largest weight gain was recorded when the green fibers were treated at 20°C. Increasing soak temperatures reduced weight gain. These results suggest that gas adsorption decreases with increasing temperature.12WSLEGAL\093616\00067\41644682v5Table 1 Weight changes of asphaltenes-derived green fibers after low temperature stabilization in 2% NO2 plus air in balance.Example 2

[0056] The weight change of Sample 1 after exposure in 2% NO2 + air in balance as detailed in Example 1 was monitored after the sample was taken out of furnace. As shown in Table 2, Sample 1 experiences continuous weight loss with time but appeared stabilized 22 hours after testing. This indicates the process of NO2 desorption in air after exposure to NCh-air mixture.Table 2 Weight reduction with time after an asphaltenes-derived green fiber sample was stabilized at 20°C in 2% NO2 plus air in balance.Example 3

[0057] Green fibers were fabricated by melt-spinning following the same procedures as detailed in Example 1 except for a reduced winding speed to fabricate green fibers with an average green fiber diameter of 9.5 pm. About 1 gram of green fiber being fabricated was placed in a tubular furnace. The furnace was sealed at both ends except for a gas inlet / outlet to inject a gas mixture containing 2% NO2 and air in balance at a flow rate of 80 ml / min. After the flow of the gas mixture for 15 minutes, the furnace was heated to different temperatures ranging from 140°C up to 180°C at a heating rate of 5°C / min and held at the final temperature for different times ranging from 30 to 120 minutes. The green fiber samples were then taken out of the furnace to measure their weight. The percentage of weight changes13WSLEGAL\093616\00067\41644682v5was recorded and tabulated in Table 3. Weight gains were found to increase with exposure temperature in general. The highest weight gains were found when the test temperature was at 160°C.Table 3 Weight changes of asphaltenes -derived green fibers after low temperature stabilization in 2% NO2 plus air in balance.Example 4

[0058] Green fibers were fabricated by melt-spinning following the same procedures as detailed in Example 1. The fabricated green fibers were exposed to a gas mixture composed of NO2 and air in balance. The introduction of NCh-Air mixture was made after the sealed furnace was vacuumed to -0.09 MPa. As listed in Table 4, all the exposure tests were performed at 20° C but with varied NO2 concentration, exposure time and pressure.

[0059] The weight changes right after the gas exposure were measured. A combined factor is defined as the soak time (min.) x NO2 concentration (vol%) x Soaking pressure (MPa):Gas Exposure Factor = Minutes x % x MPaThe gas exposure factor can be correlated to the weight changes caused by the exposure to NO2-air mixture. As shown in Figure 2, the weight gain can be well correlated to the combined factor.14WSLEGAL\093616\00067\41644682v5Table 4 Weight changes of asphaltenes-derived green fibers after stabilization at 20°C in NCh-containing air under various stabilization conditions.Example 5

[0060] Green fibers were fabricated by melt-spinning following the same procedures as detailed in Example 1 except for a reduced winding speed to fabricate green fibers with an average diameter of 9.5 pm. About 1 gram of green fiber being fabricated was placed in a tubular furnace. The furnace was sealed at both ends except for a gas inlet / outlet to inject a gas mixture containing NO2 and air in balance at a varied flow rate. The introduction of NO2- air mixture was made after the sealed furnace was vacuumed to -0.09 MPa.

[0061] The furnace was heated to 160°C at a heating rate of 5°C / min and held at the temperature for 60 minutes. The green fiber samples were then taken out of the furnace to measure their weight. As tabulated in Table 5, a change of flow rate yields a minor impact on weight changes and lower flow rate appears to increase the weight gains slightly.

[0062] The green fibers after stabilization at 160°C under the conditions listed in Table 5 were carbonized at 1200°C for 2 hours in flowing nitrogen gas. The resulting carbon fibers were examined under high resolution optical microscope. Some carbon fibers were observed15WSLEGAL\093616\00067\41644682v5to have non-uniform diameter with much reduced fiber diameter along the fiber length. Fiber diameter uniformity was achieved when stabilized green fibers had reached higher weight gains, as seen in Table 5.Table 5 Effect of low temperature stabilization of the uniformity of fiber diameter after carbonization.Example 6

[0063] Green fibers were fabricated by melt-spinning following the same procedures as detailed in Example 1 except for a reduced winding speed to fabricate green fibers with an average diameter of 9.5 pm. About 1 gram of green fiber being fabricated was placed in a tubular furnace. The furnace was sealed at both ends except for a gas inlet / outlet to inject a gas mixture containing NO2 and air in balance at a varied flow rate. The introduction of NO2- Air mixture was made after the sealed furnace was vacuumed to -0.09 MPa. The furnace was heated to 160°C at a heating rate of 5°C / min and held at the temperature for 60 minutes. The green fiber samples were then taken out of the furnace to measure their weight.

[0064] The same green fiber samples after being stabilized at 160°C in NCh-Air gas mixtures were further stabilized at 260°C for 30 minutes in flowing air. This additional oxidative stabilization leads to relatively smaller weight changes, as seen in Table 6.16WSLEGAL\093616\00067\41644682v5

[0065] The green fibers after the above two-step stabilization were carbonized at 1200°C for 2 hours in flowing nitrogen gas. The resulting carbon fibers were examined under high resolution optical microscope. All the resulting fibers were observed to have uniform fiber diameter in their length direction.Table 6 Effect of additional stabilization in air on the uniformity of fiber diameter after carbonization.Example 7

[0066] Green fibers were fabricated by melt-spinning following the same procedures as detailed in Example 1 except for a reduced winding speed to fabricate green fibers with an average diameter of 8.4 pm. About 1 gram of green fiber being fabricated was placed in a tubular furnace. The furnace was sealed at both ends except for a gas inlet / outlet to inject a gas mixture containing NO2 and air in balance. The introduction of NCh-Air mixture was made after the sealed furnace was vacuumed to -0.09 MPa. The pressure of NCh-Air mixture was controlled to be either 0.1 MPa (1 atmospheric pressure) or 0.2 MPa (2 times of atmospheric pressure). Flow of NCh-air mixture was not applied after its introduction. Three different test schemes were carried out after the introduction of NCh-air mixture:17WSLEGAL\093616\00067\41644682v51) Hold the samples at 20°C for 75 minutes in NCh-air mixture before carbonizing at 1200°C for 2 hours in N2 gas.2) Heat the samples immediately to various temperatures ranging from 120°C to 160°C and hold at the final temperature for various lengths of time in NCh-air mixture before carbonizing at 1200°C for 2 hours in N2 gas.3) The combination of the stabilization steps in 1) and 2), followed by carbonization at 1200° C for 2 hours in N2 gas, except for a shorter hold time (15 min) at 20°C in 1) before stabilization in step 2).

[0067] The carbonized fibers were made into tensile specimens and tested to determine their tensile strength. About 20 individual fibers from each carbonized sample were tensile tested.The results of tensile strength were averaged and tabulated in Table 7.Table 7 Effect of low temperature stabilization in NCh-containing air on the yield of carbon fibers after carbonization and the tensile strength of the resulting carbon fibers.Example 8

[0068] Green fibers were fabricated by melt-spinning following the same procedures as detailed in Example 1 except for a reduced winding speed to fabricate green fibers with an18WSLEGAL\093616\00067\41644682v5average diameter of 10.3 gm. About 1 gram of green fiber being fabricated was placed in a tubular furnace. The furnace was sealed at both ends except for a gas inlet / outlet to inject a gas mixture containing NO2 and air in balance. The introduction of NCh-Air mixture was made after the sealed furnace was vacuumed to -0.09 MPa. The pressure of NCh-Air mixture was controlled to 0.1 MPa (1 atmospheric pressure). Flow ofNCh-air mixture was not applied after its introduction. Two-step of green fiber stabilization was carried out to produce oxidized fibers:• Step 1 All the green fiber samples were stabilized at 20°C for 120 minutes:• Step 2 Fiber samples after Step 1 were stabilized at temperatures ranging from 300°C to 330°C for 60 minutes in air.

[0069] The fibers after two-step stabilization were made into tensile specimens and tested to determine their tensile strength and tensile elongation. About 20 individual fibers from each stabilized sample were tensile tested. The samples were also tested to determine their limiting oxygen index. The results obtained were averaged and tabulated in Table 8.Table 8 Effect of additional stabilization in air on the mechanical properties and limiting oxygen index of resulting oxidized fibers.WSLEGAL\093616\00067\41644682v5Example 9

[0070] Green fibers were fabricated by melt-spinning following the same procedures as detailed in Example 1. The green fibers were wound at two different winding speeds to fabricate green fibers with an average diameter of 7.3 and 10.3 pm, respectively.

[0071] About 1 gram of green fiber being fabricated was placed in a tubular furnace. The furnace was sealed at both ends except for a gas inlet / outlet to inject a gas mixture containing NO2 and air in balance. The introduction of NCh-Air mixture was made after the sealed furnace was vacuumed to -0.09 MPa. The pressure of NCh-Air mixture was controlled to 0.1 MPa (1 atmospheric pressure). Flow of NCh-air mixture was not applied after its introduction. Two-step of green fiber stabilization was carried out to produce oxidized fibers:• Step 1 All the green fiber samples were stabilized at 20°C for 120 minutes:• Step 2 Fiber samples after Step 1 were stabilized at 320°C for 60 minutes in air.

[0072] The fibers after the two-step stabilization were made into tensile specimens and tested to determine their tensile strength and tensile elongation. About 20 individual fibers from each stabilized sample were tensile tested. The results of tensile strength and elongation were averaged and tabulated in Table 9.

[0073] As shown in Table 9, the weight change after each step of stabilization is very sensitive to green fiber diameter. Smaller green fiber diameter yielded higher weight gain after Step 1 but larger weight loss after Step 2. The final yield of oxidized fiber is seen to be less sensitive to initial green fiber diameter. The strength of oxidized fiber after Step 2 stabilization is dependent of the initial green fiber diameter.20WSLEGAL\093616\00067\41644682v5Table 9 Effect of green fiber diameter on the weight change of low temperature stabilization in NCh-containing air and the mechanical properties of oxidized fibers after additional stabilization in air.Interpretation

[0074] References, patent applications, literature or publications referred to herein are indicative of the level of skill of one skilled in the art, and are each incorporated herein by reference, where permitted, for all purposes.

[0075] The corresponding structures, materials, acts, and equivalents of all means or steps plus function elements in the claims appended to this specification are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed.

[0076] References in the specification to "one embodiment", "an embodiment", etc., indicate that the embodiment described may include a particular aspect, feature, structure, or characteristic, but not every embodiment necessarily includes that aspect, feature, structure, or characteristic. Moreover, such phrases may, but do not necessarily, refer to the same embodiment referred to in other portions of the specification. Further, when a particular aspect, feature, structure, or characteristic is described in connection with an embodiment, it is within the knowledge of one skilled in the art to affect or connect such module, aspect, feature, structure, or characteristic with other embodiments, whether or not explicitly described. In other words, any module, element or feature may be combined with any other21WSLEGAL\093616\00067\41644682v5element or feature in different embodiments, unless there is an obvious or inherent incompatibility, or it is specifically excluded.

[0077] It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for the use of exclusive terminology, such as "solely," "only," and the like, in connection with the recitation of claim elements or use of a "negative" limitation. The terms "preferably," "preferred," "prefer," "optionally," "may," and similar terms are used to indicate that an item, condition or step being referred to is an optional (not required) feature of the invention.

[0078] The singular forms "a," "an," and "the" include the plural reference unless the context clearly dictates otherwise. The term "and / or" means any one of the items, any combination of the items, or all of the items with which this term is associated. The phrase "one or more" is readily understood by one of skill in the art, particularly when read in context of its usage.

[0079] The term "about"can refer to a variation of ± 5%, ± 10%, ± 20%, or ± 25% of the value specified. For example, "about 50" percent can in some embodiments carry a variation from 45 to 55 percent. For integer ranges, the term "about" orcan include one or two integers greater than and / or less than a recited integer at each end of the range. Unless indicated otherwise herein, the term "about" oris intended to include values and ranges proximate to the recited range that are equivalent in terms of the functionality of the composition, or the embodiment.

[0080] As will be understood by one skilled in the art, for any and all purposes, particularly in terms of providing a written description, all ranges recited herein also encompass any and all possible sub-ranges and combinations of sub-ranges thereof, as well as the individual values making up the range, particularly integer values. A recited range includes each specific value, integer, decimal, or identity within the range. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, or tenths. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc.22WSLEGAL\093616\00067\41644682v5

[0081] As will also be understood by one skilled in the art, all language such as "up to", "at least", "greater than", "less than", "more than", "or more", and the like, include the number recited and such terms refer to ranges that can be subsequently broken down into sub-ranges as discussed above. In the same manner, all ratios recited herein also include all sub-ratios falling within the broader ratio.23WSLEGAL\093616\00067\41644682v5

Claims

CLAIMS1. A process of processing green fibers fabricated from thermoplastic precursors such as asphaltenes-based precursor material, comprising the steps of:(a) exposing the green fibers to an NCh-containing gas mixture, at a temperature less than about 200° C to produce stabilized green fibers;(b) optionally, further stabilizing the stabilizing green fibers in the presence of oxygen, at a temperature greater than about 200° C to produce oxidized fibers; and / or(c) optionally, carbonizing the stabilized green fibers from step (a) or step (b) in an inert environment, preferably in nitrogen, to produce carbon fibers.

2. The process of claim 1 wherein the NCh-containing gas comprises NO2 and air, with an NO2 concentration between about 0.05 to about 3 vol%.

3. The process of claims 1 or 2 wherein the green fibers are placed in a sealed chamber and vacuumed to a pressure of -0.08 MPa or lower before introducing the NCh-containing gas mixture.

4. The process of any one of claims 1-3, wherein step a) is performed at a temperature below 100° C, for example at about 20°C, for a time period in the range of 5 minutes to 150 minutes, for example about 30 minutes.

5. The process of any one of claims 1-3, wherein step a) is performed at a temperature between about 100° C to about 200° C, for between about 5 min to 120 min.

6. The process of any one of claims 1-5 wherein step (a) has a gas exposure factor of soak time (min.) x NO2 concentration (vol%) x soaking pressure (MPa) of greater than about 15, preferably greater than about 20, more preferably greater than about 25, and most preferably greater than about 30.

7. The process of claim 6 wherein the NO2 concentration is between about 1.0 to about 2.0 %.

8. The process of claim 6 or 7 wherein the soaking pressure is between about 0.05 MPa to 0.3 MPa.24WSLEGAL\093616\00067\41644682v59. The process of any one of claims 1-8, wherein step (b) is performed in air, at a temperature between about 230°C to about 350°C, for a period in the range of about 5 minutes to about 150 minutes, and preferably less than about 120 min.

10. The process of any one claims 1-9 wherein the stabilized green fibers from step (a) or step (b) are carbonized in step (c) to produce carbon fibers.

11. The process of any one of claims 1-10 wherein the precursor fibers are melt-spun to an average diameter of less than about 10 pm, preferably less than about 8 pm.

12. The process of any one of claims 1-11 wherein the thermoplastic materials comprises asphaltenes, polyacrylonitrile (PAN), pitch, rayon, lignin-based, or other thermoplastic polymers that can be nitrated with NO2.

13. The process of any one of claims 1-12 wherein the green fibers have a weight gain of at least 5% after the stabilization step (a), and preferably greater than about 8%.25WSLEGAL\093616\00067\41644682v5

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