Asphaltene-based melt-blown nanofiber mats

Abstract

Provided herein are methods of producing carbon fibers using an asphaltene-containing feedstock. This methodology allows for lower processing temperatures and solvent-and chemical-free processing, unlike the production of carbon fibers from pitch, which has a higher processing temperature. Carbon fibers and carbon fiber mats prepared using these methods are also provided.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 754,188, filed on Feb. 5, 2025, the disclosure of which is hereby incorporated by reference in its entirety.BACKGROUND OF THE INVENTION

[0002] Carbon fibers are widely used as the reinforcing material for high-performance composite materials for both structural and functional applications, and renowned for their outstanding strength-to-weight ratio, making them significantly stronger and lighter than many conventional building materials like steel and aluminum. With comprising over 92 wt. % carbon atoms by definition, and having a unique arrangement of graphitic layers oriented along the fiber axis, carbon fibers possess exceptional mechanical, electrical and thermal properties such as high tensile strength, high stiffness, high fatigue resistance, high chemical resistance, high electrical conductivity, and low thermal expansion. This unique combination of properties makes carbon fibers essential for various industries where high strength and lightweight are required, such as aerospace, automotive, sporting goods, defense, wind turbines, marine, infrastructure, energy storage, and more.

[0003] Currently, polyacrylonitrile (PAN) is the predominant precursor for the commercial-grade carbon fiber production, accounting for over 90% of market volume, while alternative sources such as petroleum pitch and rayon fulfill the remaining demand. However, the high costs associated with the raw materials and energy-intensive production of carbon fibers, especially for the PAN-based precursor fiber, is a major challenge that limit the use of carbon fibers mainly for niche applications. Furthermore, greenhouse gas (GHG) emissions and environmental footprint related to the manufacturing of the raw materials (e.g. acrylonitrile) and PAN-based precursor fibers (e.g. wet spinning using toxic solvents) are problematic in terms of sustainability concerns. A key strategy that has been widely proposed is the adoption of cost-effective precursor materials for carbon fiber production since the precursor cost constitutes more than 50% of the overall cost for the PAN-based carbon fiber. Lowering the cost of carbon fibers holds the potential to significantly broaden their use on a commercial scale, particularly in high-volume applications such as the automotive sector, where there is a pressing need for lightweighting to reduce fuel consumption, enhance electric vehicle performance, and reduce greenhouse gas (GHG) emissions.

[0004] Asphaltenes are defined as the heaviest fraction of crude oil or the fraction of bitumen that is soluble in toluene and insoluble in n-alkane solvents. Asphaltenes are generated at considerably large quantities as a by-product of the refining process that transforms bitumen or crude oil into usable fuels. Because of their complex structure, molecular weight, and tendency for aggregation and precipitation, asphaltenes cause issues during both production and transportation of crude oil. Thus, they are considered as a low-value waste, which are either burned for its heat value or disposed of in tailing ponds and landfills. However, these practices are not sustainable, causing environmental problems and increased GHG emissions. There is thus a pressing need to discover innovative value-added applications for asphaltenes, not only to address its economic and environmental consequences but also to mitigate its adverse effects on the oil industry.

[0005] Further, numerous methods exist for synthesizing carbon nanofibers. The most common, electrospinning, makes precursor fiber mats and then converts them into carbon through subsequent stabilization and carbonization treatments, like the novel method described hereinbelow. However, the manufacturing of the precursor via electrospinning requires extreme control over ambient humidity and air temperature conditions to produce consistent results.

[0006] Also, electrospinning uses solvents which disrupt the scalability of the technology.

[0007] Alternatively, carbon nanofibers can be grown, without the need for precursor fibers and thermal treatments. This process is called chemical vapor deposition (CVD). This process requires extreme precision in the manufacturing process and expensive equipment, making it extremely difficult to scale. Furthermore, fibers can only be produced in bundles with lengths in the micrometer to millimeter scale.

[0008] It would be desirable to have a method of synthesizing carbon nanofibers that is solvent-and chemical-free, and is scalable. It is with these and other deficiencies in mind that the current invention was developed.BRIEF SUMMARY OF THE INVENTION

[0009] An embodiment of the invention is a non-woven carbon fiber mat comprising carbon fibers which have a diameter of less than 1 um, wherein the carbon fibers are derived from green fibers produced by melt-blowing an asphaltene feedstock.

[0010] An embodiment of the invention is a non-woven carbon fiber mat comprising carbon fibers which have an average diameter of less than 1 um, wherein the carbon fibers are derived from green fibers produced by melt-blowing an asphaltene feedstock.

[0011] An embodiment of the invention is a carbon fiber having a diameter of less than 1 um, wherein the carbon fiber is derived from a green fiber produced by melt-blowing an asphaltene feedstock.

[0012] An embodiment of the invention is a carbon fiber having an average diameter of less than 1 um, wherein the carbon fiber is derived from a green fiber produced by melt-blowing an asphaltene feedstock.

[0013] An embodiment is a method of producing carbon fibers, comprising the steps of:

[0014] i) blowing an asphaltene feedstock at one or more melt processing temperatures below 350° C. to produce melt-blown fibers, and collecting the melt-blown fibers;

[0015] ii) oxidizing the melt-blown fibers to prevent fusion during subsequent heating;

[0016] iii) stabilizing the melt-blown fibers by subjecting the fibers to an air atmosphere to form stabilized melt-blown fibers; and

[0017] iv) carbonizing the stabilized melt-blown fibers by subjecting the fibers to an inert atmosphere at a carbonization temperature to produce the carbon fibers.

[0018] An embodiment is carbon fibers prepared by the above method.

[0019] An embodiment is a carbon fiber mat prepared by the above method.BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0020] Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

[0021] FIG. 1 depicts an SEM image of shots within a fiber mat (scale bar=300 um).

[0022] FIG. 2 depicts an SEM image of carbonized nanofibers (scale bar=15 um).

[0023] FIG. 3 depicts an SEM image of fused carbon fiber mats (scale bar=80 um).

[0024] FIG. 4 depicts precursor melt blown nanofiber mats, including green fibers collected on a drum.

[0025] FIG. 5 depicts SEM images of green fiber mats.

[0026] FIG. 6 depicts carbonized melt-blown nanofibers. The post-carbonization morphology shows fiber uniformity and surface porosity.

[0027] FIG. 7 depicts carbonized melt-blown nanofibers, and shows the physical resilience of fiber mats in both transverse and longitudinal directions.

[0028] FIG. 8 depicts carbonized asphaltene-based meltblown fibers spun with micron-die (0.430 mm) diameter spinnerets.

[0029] FIG. 9 depicts carbonized asphaltene-based meltblown fiber spun with submicron-die (0.120 mm diameter) spinnerets, having an L: D ratio less than 50:1.

[0030] FIG. 10 depicts carbonized asphaltene-based meltblown fiber spun with nano-die (0.120 mm diameter) spinnerets, having an L: D ratio greater than 50:1.DETAILED DESCRIPTION OF THE INVENTION

[0031] The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown.

[0032] Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.

[0033] Each embodiment disclosed herein is contemplated as being applicable to each of the other disclosed embodiments. All combinations and sub-combinations of the various elements described herein are within the scope of the embodiments.

[0034] In the following description, various components may be identified as having specific values or parameters, however, these items are provided as exemplary embodiments. Indeed, the exemplary embodiments do not limit the various aspects and concepts of the present invention as many comparable parameters, sizes, ranges, and / or values may be implemented. Further, the terms “a,”“an,” and “the” do not denote a limitation of quantity, but rather denote the presence of “at least one” of the referenced item.

[0035] It is understood that where a parameter range is provided, all integers and ranges within that range, and tenths and hundredths thereof, are also provided by the embodiments. For example, “5-10%” includes 5%, 6%, 7%, 8%, 9%, and 10%; 5.0%, 5.1%, 5.2% . . . 9.8%, 9.9%, and 10.0%; and 5.00%, 5.01%, 5.02% . . . 9.98%, 9.99%, and 10.00%, as well as, for example, 6-9%, 5.1%-9.9%, and 5.01%-9.99%. Similarly, where a list is presented, unless stated otherwise, it is to be understood that each individual element of that list, and every combination of components of that list, is a separate embodiment. For example, “1, 2, 3, 4, and 5” encompasses, among numerous embodiments, 1; 2; 3; 1 and 2; 3 and 5; 1, 3, and 5; and 1, 2, 4, and 5.

[0036] As used herein, “about” in the context of a numerical value or range means±10% of the numerical value or range recited or claimed.

[0037] As used herein, a composition which is “substantially free” of a component comprises at most 10% of that component, by weight.

[0038] An embodiment of the invention is a non-woven carbon fiber mat comprising carbon fibers which have a diameter of less than 1 um, wherein the carbon fibers are derived from green fibers produced by melt-blowing an asphaltene feedstock.

[0039] An embodiment of the invention is a non-woven carbon fiber mat comprising carbon fibers which have an average diameter of less than 1 um, wherein the carbon fibers are derived from green fibers produced by melt-blowing an asphaltene feedstock.

[0040] An embodiment of the invention is a carbon fiber having a diameter of less than 1 um, wherein the carbon fiber is derived from a green fiber produced by melt-blowing an asphaltene feedstock.

[0041] An embodiment of the invention is a carbon fiber having an average diameter of less than 1 um, wherein the carbon fiber is derived from a green fiber produced by melt-blowing an asphaltene feedstock.

[0042] In an embodiment, the asphaltene feedstock is substantially free of solvent. In embodiments, the asphaltene feedstock comprises no more than 10, 5, 2, 1, 0.1, or 0.01% by weight of solvent. In an embodiment, the asphaltene feedstock is free of solvent.

[0043] In an embodiment, the asphaltene feedstock is derived from bitumen. In an embodiment, the bitumen contains various ratios of asphaltenes to maltenes. In an embodiment, the asphaltene feedstock has an asphaltene: maltene ratio of 99:1 to 1:99 by weight. In an embodiment, the asphaltene feedstock has an asphaltene maltene ratio of 99:1, 95:5, 90:10, 80:20, 70:30, 60:40, 50:50, 40:60, 30:70, 20:80, 10:90, 5:95, or 1:99 by weight, or within a range defined by any two of these values.

[0044] In an embodiment, the asphaltene feedstock is derived from vacuum residue.

[0045] An embodiment is a method of producing carbon fibers, comprising the steps of:

[0046] i) blowing an asphaltene feedstock at one or more melt processing temperatures below 350° C. to produce melt-blown fibers, and collecting the melt-blown fibers;

[0047] ii) oxidizing the melt-blown fibers to prevent fusion during subsequent heating;

[0048] iii) stabilizing the melt-blown fibers by subjecting the fibers to an air atmosphere to form stabilized melt-blown fibers; and

[0049] iv) carbonizing the stabilized melt-blown fibers by subjecting the fibers to an inert atmosphere at a carbonization temperature to produce the carbon fibers.

[0050] In an embodiment, the carbon fibers are in the form of a nonwoven carbon fiber mat or a carbon fiber bundle.

[0051] In an embodiment, stabilizing the melt-blown fiber comprises thermal and chemical treatment. In an embodiment, the stabilizing the melt-blown fiber comprises thermal and chemical treatment, and the thermal and chemical treatment for stabilizing the melt-blown fiber occurs simultaneously or sequentially. In an embodiment, the thermal and chemical treatment for stabilizing the melt-blown fiber occurs simultaneously. In an embodiment, the thermal and chemical treatment for stabilizing the melt-blown fiber occurs sequentially. In an embodiment, the thermal treatment occurs before the chemical treatment. In an embodiment, the thermal treatment occurs after the chemical treatment.

[0052] In an embodiment, the stabilizing of the melt-blown fiber is performed at a stabilization temperature between about 200° C. and about 300° C. In an embodiment, the the stabilizing of the melt-blown fiber is performed at a stabilization temperature of about 200, about 210, about 220° C., about 230° C., about 240° C., about 250° C., about 260° C., about 270° C., about 280° C., about 290° C., or about 300° C., or within a range defined by any two of these values. In an embodiment, the stabilizing of the melt-blown fiber is performed at a stabilization temperature greater than about 250° C. In an embodiment, the stabilizing of the melt-blown fiber is performed at a stabilization temperature between about 250° C. and about 300° C.

[0053] In an embodiment, the stabilizing of the melt-blown fibers comprises:

[0054] i) heating the fibers to a stabilization temperature at a heating rate of about 0.1° C. / min to about 5° C. / min; and

[0055] ii) maintaining the fibers at the stabilization temperature for a time interval.

[0056] In an embodiment, the heating rate is about 0.1° C. / min, 0.5° C. / min, 1° C. / min, 1.5° C. / min, 2° C. / min, 2.5° C. / min, 3° C. / min, 4° C. / min, 4.5° C. / min, or 5° C. / min, or within a range defined by any two of these values.

[0057] In an embodiment, the time interval is about 10 minutes to about 24 hours. In an embodiment, the time interval is about 10, 20, 30, or 45 minutes, or 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, or 24 hours, or within a range defined by any two of these values. In an embodiment, the time interval is about 1-16, 4-12, or 6-10 hours. In an embodiment, the time interval is about 8 hours.

[0058] In an embodiment, the stabilizing of the melt-blown fibers comprises contacting the fibers with a stabilizing compound. In an embodiment, the stabilizing compound comprises vaporized iodine, ClO2, Cl2, ozone, a nitrogen oxide (NOx), HOBr, or Br2. In an embodiment, the fibers are contacted with the vaporized iodine in a sealed chamber.

[0059] In an embodiment, the stabilizing compound is an acid. In an embodiment, the acid comprises nitric acid or sulfuric acid. In an embodiment, the acid strength ranges from about 10% to about 50% (by weight percent in a solution, including but not limited to an aqueous solution).

[0060] In an embodiment, during at least a portion of the time period of the stabilization step, the fibers are placed under tension. In an embodiment, during at least a portion of the time period of the stabilization step, the edges of the carbon fiber mat are fixed, such that an internal tension force is created during stabilization. In embodiments in which the fibers are on a roll, or a plurality of rolls, the roll speeds may be used to apply tension to the fibers.

[0061] In an embodiment, the method further comprises pre-treating the sample before or after the selecting step. In an embodiment, the pre-treating step is performed after the selecting step and before the blowing step.

[0062] In an embodiment, the pre-treating step comprises treating the sample by one or more thermal treatment processes. In an embodiment, the one or more thermal treatment processes comprise heating the sample to a temperature of greater than about 100° C. to form a purified sample. In an embodiment, the temperature is between 100 and 280° C. In an embodiment, the temperature is about 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, or 280° C., or within a range defined by any two of these values. In an embodiment, the temperature is about 100-280, 120-260, 130-240, 140-200, or 150-170° C. In an embodiment, the temperature is about 160° C. In an embodiment, the thermal treatment process is performed under an inert gas flow.

[0063] In an embodiment, the carbonizing step comprises:

[0064] i) heating the fibers to a first carbonization temperature at a first heating rate; and

[0065] ii) optionally, heating the fibers to a second carbonization temperature at a second heating rate.

[0066] In an embodiment, the first carbonization temperature is lower than the second carbonization temperature. The second step, at the second carbonization temperature, is sometimes referred to as a graphitization step.

[0067] In an embodiment, the first heating rate is lower than the second heating rate. In an alternative embodiment, the first heating rate is equal to the second heating rate.

[0068] In an embodiment, the fibers are maintained at the first carbonization temperature for a time interval before heating the fibers to the second carbonization temperature. In an embodiment, this time interval is about 10-120 minutes. In an embodiment, the time interval is about 10, 20, 30, 30, 40, 50, 60, 70, 80, 90, 100, 110, or 120 minutes, or within a range defined by any two of these values. In an embodiment, the time interval is about 10-120, 20-90, or 30-60 minutes.

[0069] In an embodiment, the fibers are maintained at the second carbonization temperature for a time interval. In an embodiment, this time interval is about 10-120 minutes. In an embodiment, the time interval is about 10, 20, 30, 30, 40, 50, 60, 70, 80, 90, 100, 110, or 120 minutes, or within a range defined by any two of these values. In an embodiment, the time interval is about 10-120, 20-90, or 30-60 minutes.

[0070] In an embodiment, the first carbonization temperature is about 400 to about 600. In an embodiment, the first carbonization temperature is about 400 to about 500. In an embodiment, the first carbonization temperature is about 400, 410, 420, 430, 440, 450, 460, 470, 480, 490, 500, 510, 520, 530, 540, 550, 560, 570, 580, 590, or 600, or within a range defined by any two of these values.

[0071] In an embodiment, the second carbonization temperature is about 800 to about 2800. In an embodiment, the second carbonization temperature is about 1000 to about 2800, about 1000 to about 1800, or about 1800 to about 2800. In an embodiment, the second carbonization temperature is about 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2000, 2100, 2200, 2300, 2400, 2500, 2600, 2700, or 2800, or within a range defined by any two of these values.

[0072] In an embodiment, during at least a portion of the carbonizing step, the carbon fiber mat is placed under tension. In an embodiment, during at least a portion of the carbonizing step, an internal tension force is created during carbonization. This can be obtained by fixing the edges of a mat, or, if the fiber mat is brought into a carbonization oven on rolls, by changing relative speeds of the rolls to create tension.

[0073] In an embodiment, the feedstock is substantially free of solvent. In an embodiment, the feedstock is free of solvent.

[0074] In an embodiment, blowing the feedstock comprises supplying the feedstock into a die wherein pressurized hot air draws fibers onto a collection device. The collection device may be any such device known in the art for this purpose, including but not limited to a vacuum-assisted rotating collection drum or a conveyor belt.

[0075] In an embodiment, the asphaltene feedstock is obtained from bitumen.

[0076] In an embodiment, the asphaltene feedstock is in a form comprising a solid, a powder, a gel and / or a liquid.

[0077] In an embodiment, the asphaltene feedstock is in a form comprising solid masses with diameter of <5 mm.

[0078] In an embodiment, the zero-shear viscosity of the feedstock is between about 50 Pa·s and about 200 Pa·s. as measured at a temperature of 200-210° C., and fit with the Carreau equation from the complex viscosity data across different frequencies. In an embodiment, the zero-shear viscosity is measured at a temperature of 200, 205, or 210° C. In an embodiment, the zero-shear viscosity of the feedstock is about 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, or 200 Pa·s as measured at the temperature In an embodiment, the melt processing temperature in the blowing step is between about 110° C. and about 250° C. In a preferred embodiment, the melt processing temperature in the blowing step is between about 170° C. and about 250° C. In an embodiment, the melt processing temperature in the blowing step is about 110° C., 120° C., 130° C., 140° C., 150° C., 160° C., 170° C., 180° C., 190° C., 200° C., 210° C., 220° C., 230° C., 240° C., or ° C. 250, or within a range defined by any two of these values.

[0079] In an embodiment, the melt processing temperature is increased stepwise during the blowing step within a range between about 110° C. and about 250° C. In a preferred embodiment, the melt processing temperature is increased stepwise during the blowing step within a range between about 170° C. and about 250° C.

[0080] In an embodiment, blowing the feedstock at one or more melt processing temperatures to produce melt-blown fibers, comprises extruding fibers through a multi-hole die, wherein pressurized hot air draws the fibers onto a vacuum-assisted rotating collection drum. In an embodiment, collecting the fibers on a vacuum-assisted rotating collection drum comprises forming a nonwoven sheet.

[0081] In an embodiment, the method further comprises, prior to the blowing step, selecting at least one sample comprising asphaltene for supplying as a feedstock, wherein the selecting of the at least one sample comprises determining a melt processing condition of the at least one sample.

[0082] In an embodiment, the determining of the melt processing condition of the at least one sample comprises identifying one or more thermal and rheological properties of the sample.

[0083] In an embodiment, the determining of the melt processing condition of the at least one sample comprises performing at least one of dynamic rheology tests, differential scanning calorimetry (DSC) analysis, Thermogravimetric Analysis (TGA), or dilatometry and thermomechanical analysis (TMA).

[0084] In an embodiment, the determining of the melt processing condition of the sample comprises identifying one or more of a thermal softening temperature (Ts), thermal transitioning temperature, melt viscosity, zero-shear viscosity, complex viscosity, Thermogravimetric Analysis (TGA) decomposition temperatures, elemental compositions, and level of aromaticity of the sample.

[0085] In an embodiment, the method further comprises, following carbonization, activating the carbonized fibers by treatment with KOH, CO2, or air. This can be performed on the carbon fibers or on mats comprising the carbon fibers.

[0086] In an embodiment, the activation uses a KOH solution having a KOH concentration of 0.01-10 M, preferably 6 M. The fibers (or mats) are soaked in the KOH solution and then treated in a furnace. In an embodiment, the KOH-treated fibers are treated in an inert atmosphere. In an embodiment, the treatment lasts for 20-40 minutes. In an embodiment, the treatment is at a temperature of 800-1000° C.

[0087] In an embodiment, the activation uses CO2. In an embodiment, the CO2 treatment is at a temperature of 800-1000° C. In an embodiment, the CO2 treatment is for 15-60 minutes.

[0088] In an embodiment, the activation uses air. In an embodiment, the activation is at a target temperature from 300-450 C. In an embodiment, the activation is at 350 C. In an embodiment, the air is heated at a rate of about 5 C / min to reach the target temperature. In an embodiment, the air treatment lasts about 90-180 minutes at the target temperature.

[0089] An embodiment is carbon fibers prepared by any of the above methods.

[0090] An embodiment is carbon fibers prepared by any of the above methods, wherein the carbon fibers comprise an average diameter of less than about 1 um. In an embodiment, the carbon fibers have an average diameter of from about 100 nm to about 1 um. In an embodiment, the carbon fibers have an average diameter of about 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, or 1 um, or within a range defined by any two of these values.

[0091] An embodiment is a carbon fiber mat prepared by any of the above methods. In an embodiment, the carbon fiber mat is non-woven.

[0092] An embodiment is a carbon fiber mat prepared by any of the above methods, wherein the carbon fiber mat comprises an average surface area of about 1 to about 1500 m2 / g. In an embodiment, the carbon fiber mat comprises an average surface area of 400 to about 1500 m2 / g. In an embodiment, the carbon fiber mat comprises an average surface area of about 1, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, 350, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, or 1500m2 / g, or within a range defined by any two of these values. In an embodiment, the surface area is greater than about 400 m2 / g.

[0093] An embodiment is a carbon fiber mat prepared by any of the above methods, wherein the carbon fiber mat comprises an average electrical conductivity of from about 1 to about 10 S / cm. In an embodiment, the carbon fiber mat comprises an average electrical conductivity of about 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 S / cm, or within a range defined by any two of these values.

[0094] An embodiment is a carbon fiber mat prepared by any of the above methods, wherein the carbon fiber mat comprises a bulk density ranging from about 0.09 to about 2g / cm3. In an embodiment, the carbon fiber mat comprises a bulk density of about 0.09, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, or 2 g / cm3, or within a range defined by any two of these values.

[0095] An embodiment is a carbon fiber mat prepared by any of the above methods, wherein the carbon fiber mat is highly flexible with the ability to fold in half and spring-back to original shape.

[0096] An embodiment is a carbon fiber mat prepared by any of the above methods, wherein the carbon fiber mat exhibits shape memory.

[0097] All publications, patents and patent applications referred to herein are incorporated by reference in their entirety to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety, except to the extent that the incorporated material is inconsistent with the express disclosure herein, in which case the language in this disclosure controls.DISCUSSION AND EXPERIMENTAL

[0098] Herein is described a novel, scalable, and environmentally friendly method for producing nonwoven carbon nanofiber mats from asphaltene or other petroleum-derived aromatic feedstocks. These feedstocks comprise a mixture of asphaltenes and maltenes directly recovered from the heavy products from bitumen, or asphaltenes recovered from vacuum residue following the boiling off of lighter compounds (such as distillable hydrocarbons, like naphtha.). Heavier material, referred to as vacuum residue, is left afterwards, and contains a mixture of maltenes and asphaltenes. This is called vacuum residues. Asphaltenes can be directly isolated from bitumen or the vacuum residues. This is typically done on large scale using solvent deasphalting. The solids are separated and this is used.

[0099] The feedstock is unique from highly processed mesophase-containing pitch derived from processed petroleum that requires melt processing above 350° C. The process eliminates the need for chemical solvents or polymer additives, offering a sustainable pathway to fabricate high-performance nanofiber sheets. The precursor, referred to as “green” fiber sheets, is created using an innovative melt-blowing process, then turned into carbon fibers with thermal stabilization and carbonization. The resulting carbon nanofiber mats exhibit exceptional properties, including high surface area, electrical conductivity, flexibility, and lightweight structure, making them ideal for applications such as energy storage systems, advanced filtration, lightweight composites, thermal management, and electromagnetic interference (EMI) shielding.

[0100] This is the first instance of producing sub-micron carbon nanofiber mats from asphaltene or petroleum-derived aromatic feedstocks via a melt-blown process and without the use of solvents or chemical treatments.

[0101] Existing melt-blown processes focus on polymeric feedstocks, while carbon nanofiber production traditionally relies on solvent-based electrospinning, which is limited in scalability and environmental sustainability. This invention uniquely combines the melt-blown method with solvent-free processing of asphaltene, an underutilized petroleum by-product, to produce sub-micron carbon nanofiber mats. This approach offers significant environmental and cost advantages by eliminating solvents, reducing hazardous waste, and achieving a scalable production rate of ~30 g / hour.

[0102] In summary, below is described a novel solvent-free melt-blowing process at temperatures below 350° C. to produce nonwoven nanofiber precursor sheets solely from petroleum-derived feedstocks, such as asphaltene. The process eliminates the need for chemical additives, solvents, or polymer blending, ensuring a simple, cost-effective, and environmentally friendly approach to make sub-micron scale carbon fiber precursors. The precursor sheets are converted into nonwoven, carbon nanofiber mats. More details on the process steps are as follows, in non-limiting exemplary embodiments:Forming the Precursor Mats (Melt Blowing)

[0103] The melt blowing process creates precursor fiber mats (“green” mats, precursor mats) which are then processed into carbon nanofibers. The asphaltene may come from any source, including bitumen or from vacuum residue. In some embodiments, melt blowing begins with crushing asphaltene derived from solvent deasphalted bitumen into small chunks and placing them into a single-screw extruder. The material is then extruded through 3 progressively higher heating stages set at temperatures in the range of of 110-250° C., and preferably 150-210°C. These temperatures were determined to be the optimal temperatures for melt processing, based on detailed rheological characterization of the asphaltene sample which critical for control of the viscosity. In the work described hereinbelow, the three stages were 150, 190, and 200° C. The zero-shear rate viscosity of the feedstock material, determined at a temperature of 200-210° C. and calculated by fitting a Carreau model, at the point of injection into the die is between about 50 Pa·s and about 200 Pa·s. (or preferred units). Controlling the viscosity and melt rheology is crucial to producing fibers on the nano scale. For these experiments, the rheology was studied using an AR 2000 rheometer (TA instruments) with a 25 mm a plate-plate geometry (~1 mm measuring gap), Measurements were conducted at temperatures ranging from 180 to 230° C. A frequency sweep test was then conducted, ranging from 0.1 to 100 rad / s. Suboptimal rheological properties can introduce large, coagulated pieces called shots (>10 um), depicted in FIG. 1, can block the spinning channels, or can cause the asphaltene to drip like a Newtonian liquid. Nonfibrous structures in the mat cause defects during thermal processing.

[0104] When enough material has been added into the extruder, the melt reaches the spin head (die tip) which has holes that are 0.12 mm in diameter that limit the flow of the polymer, in the embodiment used in the work described hereinbelow. In a preferred embodiment, the melt travels through a channel with a length to diameter (L:D) ratio greater than 50:1, and exits the spinneret hole which has a 0.12 mm diameter. The chamber of the holes extends 18 mm into the capillaries of the die in this embodiment. The back pressure generated by this high length / diameter ratio as an important factor in producing consistent fibers on the nano scale. The die tip used in the current work has 100 holes per inch and is 4 inches in length. The length of the die tip can be scaled up to make wider mats. Other variations include a “submicron” die (loose tip plate), which has 70 holes per inch, and a “micron” die, which has 23 holes per inch.

[0105] Upon reaching the holes at the tip of the die, the melt encounters strong currents of hot air, which are blasted from either side of the die. This hot air is typically provided at a pressure of 1-5 PSI, though other conditions are possible (such as 1-10 or 5-10 PSI). The air travels through channels of a given width. This width is customizable, but 0.03 inches provides sufficient air pressure to make nano-sized fibers from the asphaltene precursor. These channels are angled to converge at the tip of the die.

[0106] The force from the jets of hot air flowing with the direction of the melt propels the melt out of the holes on the die tip. The melt's viscosity allows it to be drawn into a very fine fiber. In some cases these fibers are less than 1 um, for example ~0.1-0.9 um in diameter, and in some cases the average diameter of the fibers is less than 1 um, for example having an average diameter of 0.5 um. The fibers are then collected onto a porous leader sheet (such as a glass or nylon sheet) on top of a winding drum in a nonwoven mat. A suction force at the drum's center pulls air in through the collector's pores, which assists in forming the mats into a uniform shape. The drum is rotated at a rate of 10 rpm but is capable at spinning at rates up to 50 meters / minute (110 rpm). As the tip of the die contains about 100 holes per inch, this process of extrusion and blowing happens in hundreds of holes, side by side, which allows for quick creation of green fiber mats. The process allows us to produce carbon nanofiber precursor mats at an unprecedented rate. Other collector systems such as flat surfaces or conveyor belts can be used.

[0107] Note: Each fiber within the mat is blown continuously, which is distinct from existing carbon nanofibers which have only been synthesized in lengths under 1 meter (most commercial-grade nanofibers are millimeters in length). FIG. 4 depicts precursor melt blown nanofiber mats. FIG. 5 depicts SEM images of green fiber mats.Converting the Green Precursor Mats into Carbon Nanofibers

[0108] The green nonwoven mats are chemically treated with exposure to an oxidant to help control the thermal stabilization. Nano-scale fiber mats were successfully produced with nitric acid treatment or iodine gas treatment. Exposure to nitric acid vapor in a sealed desiccator at room temperature produces highly flexible mats with very little fiber fusion evident in SEM images. Exposure to iodine gas in a sealed container at 90 degrees also retains the mats' fibrous structure, but with noticeable fusion at fiber intersections. Without proper oxidative treatment, the fibers will fuse together into larger fibers during thermal treatment, bringing the diameters above the nano scale. Fused fibers are also incredibly brittle and fragile while the oxidized non-fused fibers remain highly flexible.

[0109] Instead of nitric acid or iodine gas, ozone pretreatment can also be applied to oxidize melt-blown (MB) mats before the stabilization process. As a non-limiting example, the process involves exposing the fiber mats to a gas stream of ozone-enriched oxygen inside a sealed container at a slightly elevated temperature (40-80° C.). The ozonation process allowed for a much shorter pre-oxidation time (4 hours vs 16 hours) and a less harmful environment than the other pretreatment processes with nitric acid vapor or iodine gas. The process can be further sped-up at higher temperatures of 80 to 120° C.

[0110] After this treatment, the fibers undergo thermal stabilization. The stabilization involves a controlled ramp up of heating of the fibers to a target temperature for a period of time, for example the thermal ramp up could be over 6 hours to 265° C., and then a 2-hour dwell at that temperature. The stabilization treatment occurs under an air atmosphere. This treatment allows the fibers to withstand the subsequent carbonization treatment, by allowing the aromatic rings in the fibers to cross-link with each other. The controlled ramp up and extended dwell is important to making quality fibers, ensuring that the internal and external sections of the fibers are thermally treated equally, but not over-oxidized or degraded. Inadequate thermal stabilization can cause fiber fusion and degradation in the high temperature carbonization step. Proper stabilization is necessary to keeping the fibers nano sized. The time and temperature of stabilization can be adjusted dependent upon the oxidation of the green fiber mat. During stabilization, the edges of the mats are fixed which induces internal tension on the mats. During this step, the internal tension prevents excessive shrinkage of the mats, retains the fiber morphology and uniform shape of the mats as well as prevents wrinkling. Alternatively, the mat can be sandwiched between mesh or graphite foil, to prevent defects.

[0111] The stabilized mats are converted into carbon fibers through a carbonization treatment. In the carbonization treatment, the fibers are heated to an elevated temperature, typically under an inert temperature. For example, the fibers are placed in a high temperature furnace and heated to 800-1000° C. and above at a controlled heating rate, ramping up over 8 hours. This step is followed by a 30-minute dwell, and then a controlled cooling rate down to 100° C. This treatment occurs under an inert nitrogen atmosphere, preventing the fibers from disintegrating and allowing the carbon atoms inside to form graphitic structures. Fixing the edges of the mats to apply internal force during this step was observed to help the mats keep a more uniform shape, and prevents wrinkling. Optionally, other carbonization methods can be conducted up to 1500° C. or higher, such as 1500-2800° C., which has been observed to improve electrical conductivity of the fiber mats. FIG. 2 shows an SEM image of the carbonized fibers using the correct thermal treatment protocol. For comparison, FIG. 3 shows an SEM image of fibers that are not correctly thermally treated, for example without use of oxidant, which leads to excessive fusion. FIG. 6 depicts carbonized melt-blown nanofibers. The post-carbonization morphology shows fiber uniformity and surface porosity. FIG. 7 depicts carbonized melt-blown nanofibers, and shows the physical resilience of fiber mats in both transverse and longitudinal directions.

[0112] FIGS. 8-10 depict carbonized asphaltene-based meltblown fibers spun with micron-die (0.430 mm) diameter spinnerets, submicron-die (0.120 mm diameter) spinnerets with channels having an L: D ratio of less than 50:1, and nano-die (0.120 mm diameter) spinnerets with channels having an L:D ratio of greater than 50:1, respectively. As can be seen, based on the 80 micron scale bar in each of FIGS. 8-10, these fibers exhibit low, medium, and high length to diameter (L:D) ratios, respectively.Conclusion

[0113] This invention represents a significant advancement in sustainable carbon nanofiber manufacturing, offering a solvent-and chemical-free, scalable process with superior material properties. The resulting carbon nanofiber mats provide properties that are desirable to multiple industries, including energy storage, lightweight composites, and advanced filtration.EXAMPLE 1Feedstock Preparation:

[0114] Solid petroleum-derived feedstock (5-10 cm chunks) was crushed into smaller pieces (<5 mm) suitable for melt processing. The feedstock was a mixture of asphaltenes and other products such as maltenes.Melt-Blowing Process:

[0115] The feedstock was melt-blown across four heating zones ranging between 150° C.-325° C.

[0116] Fibers were extruded through a die having a 0.120 mm spinneret and a channel with a large L:D ratio. Pressurized hot air drew the fibers onto a vacuum-assisted rotating collection drum, forming a nonwoven sheet.

[0117] Production rate: ~30 g / hour with current set-up. The productivity can be higher based on die type and size.Oxidation:

[0118] Oxidative treatments to enhance stability include a variety of known oxidants such as peroxides, halogens, ozone, oxygen, nitrogen oxides and chlorine dioxide. The specific oxidant and reaction conditions can be optimized to tailor structural and electrical properties.

[0119] The tested oxidative treatments were:

[0120] Iodine Gas Treatment: Solid iodine vaporizes at 90° C. in a sealed chamber, leading to an 87-131% mass increase due to adsorption.

[0121] Ozone Gas Treatment: MB mats were placed in the vacuum glass desiccator (13 L), and oxygen was passed through the ozone generator (Oxidation Technologies, Inwood, IA) into the reactor. The ozone concentration was 1-1.5 g / hr, and the flow rate was 2.5 L / min, with the gas at 1.5 bar. The desiccator was placed in the oven at 40-80° C., and the reaction was processed for 30 min-5 hours depends on the mat thickness (0.1-1 mm) and ozone concentration.Thermal Stabilization:

[0122] Nitric Acid Treatment: Mats were doused in 20-67% nitric acid, dried, and stabilized at heating rates between 0.1-1° C. / min.

[0123] The precursor mats were then thermally stabilized at temperatures between 200 to 300° C. for 1-2 hours, with heating rates optimized between 0.1° C. / min and 2° C. / min.Carbonization:

[0124] Stabilized mats were carbonized in a tube furnace under inert gas such as nitrogen or argon flow at 800-2500° C. A non-limiting example of the heating protocol is:

[0125] 1. Ramp-up at 2° C. / min to 500° C., dwell for 30 minutes;

[0126] 2. Further ramp-up at 5° C. / min to 800-1000° C., dwell for another 30 minutes; and

[0127] 3. Furnace is then cooled to room temperature over several hours.Activation:

[0128] Carbonized mats can be further activated with treatment of KOH, CO2, or air to enhance the surface area, mesoporosity and microporosity of the mats. Exemplary conditions for these activation methods include:

[0129] KOH: soak carbonized mats in KOH solution (KOH concentration: 6M), then treat in furnace under an inert atmosphere for 20-40 mins at 800-1000° C.

[0130] CO2: Carbonized mats are heated to temperatures ranging between 800-1000° C. and CO2 gas is fed into the oven at 100-200 ml / min for 15 to 60 mins.

[0131] Air: carbonized mats were heated to 300-450° C., preferably 350° C., from room temperature under air flow (200 ml / min) with 5° C. / min heating rate and hold 90-180 mins in the tube furnace. A typical muffle furnace may be used for air activation method, using a heating rate of 5° C. / min heating rate and a hold time 90-180 minutes at the preferred temperature.

[0132] Resulting surface area of mats are increased to 350 to 1500 m2 / g of material dependent upon activation conditions.Key Properties and ResultsFiber Morphology:

[0133] Precursor fiber diameters ranged from 0.1 um to 10 um, generally 169 nm to >1 um (average: 462 nm, σ=233 nm).

[0134] Carbonized fiber diameters ranged from 0.1 to 10 um, generally 191 nm to >1 um (average: 541 nm, σ=345 nm).Surface Area and Porosity:

[0135] High surface area: 260-1423 m2 / g (with KOH treatment), 600-1123 m2 / g (with CO2 treatment) and 687 m2 / g (with air treatment).

[0136] Porosity: Total pore volume=0.15-0.714 cm3 / g (with KOH treatment), 0.334-0.559 cm3 / g (with CO2 treatment) and 0.317 cm3 / g (with air treatment), including micropores (<2 nm) with 0.11-0.68 cm3 / g (KOH), 0.185-0.486 cm3 / g (CO2) and 0.274 cm3 / g (Air).Electrical Conductivity:

[0137] Conductivity of fibrous mats ranges from 0.79 to 8 S / cm.Mechanical Flexibility:

[0138] Nanofiber mats are lightweight and flexible, capable of being rolled around a 12 mm diameter cylinder, folded upon themselves, and handled without defect formation.Thermal Insulation and Thermal Management:

[0139] Through thickness effective thermal conductivity of 0.053 W / m·K. In plane thermal conductivity of 1.680 W / m·K. Anisotropic thermal conductivity.

[0140] Unique Advantages of the process described herein include:

[0141] Simplicity: Allows for direct fiber formation without prior feedstock chemical or heat treatments.

[0142] Solvent-Free Process: no solvents or chemical additives are used during the melt-blowing stage, reducing cost, complexity, and environmental impact.

[0143] Lower temperature Processing: Samples are processed into fibers at temperatures lower than 350° C.

[0144] Scalable and High-Yield: Current production of nonwoven carbon nanofiber sheets at ~30 g / hour with adjustable heating and extrusion parameters. This number is scalable with increased die size.Material Performance:

[0145] Combination of high surface area, electrical conductivity, and flexibility offers multifunctional applications in:

[0146] Energy Storage: Lithium-ion batteries, sodium-ion batteries, redox flow batteries, and supercapacitors.

[0147] Filtration: High-surface-area porous mats for advanced filtration systems.

[0148] EMI Shielding: Lightweight conductive materials for shielding applications.

[0149] Lightweight Composites: Flexible reinforcement layers or chopped fiber in hybrid materials.

[0150] Resistive Heating: Continuous carbon sheets can be used in resistive heating applications for localized heat production.

[0151] Catalyst Support: High surface area carbon materials are ideal for deposition of metals to be used as catalyst supports.

[0152] Thermal Insulation and Thermal Management: Insulation in inert high temperature environments, and general insulation uses, especially directional thermal management (in-plane and out-of-plane).Optimization Capability:

[0153] Post-processing conditions (stabilization, carbonization) can be tailored based on feedstock properties to achieve specific performance metrics. For example, mats carbonized at 800° C. exhibited higher flexibility, where mats carbonized at and above 1000° C. exhibited higher conductivity. Samples can be carbonized at higher temperatures, including 1500-2600° C., and still exhibit flexibility.

Claims

1. A carbon fiber having a diameter or an average diameter of less than 1 um, wherein the carbon fiber is derived from a green fiber produced by melt-blowing an asphaltene feedstock.

2. A non-woven carbon fiber mat comprising the carbon fiber of claim 1.

3. The carbon fiber of claim 1, wherein the asphaltene feedstock is substantially free of solvent.

4. The carbon fiber according to claim 1, wherein the asphaltene feedstock is derived from bitumen or vacuum residue.

5. A method of producing carbon fibers, comprising the steps of:i) blowing an asphaltene feedstock at one or more melt processing temperatures below 350° C. to produce melt-blown fibers, and collecting the melt-blown fibers;ii) oxidizing the melt-blown fibers to prevent fusion during subsequent heating;iii) stabilizing the melt-blown fibers by subjecting the fibers to an air atmosphere to form stabilized melt-blown fibers; andiv) carbonizing the stabilized melt-blown fibers by subjecting the fibers to an inert atmosphere at a carbonization temperature to produce the carbon fibers.

6. The method according to claim 5, wherein the carbon fibers are in the form of a nonwoven carbon fiber mat or a carbon fiber bundle.

7. The method according to claim 5, wherein stabilizing the melt-blown fiber comprises thermal treatment, chemical treatment, or thermal treatment and chemical treatment;and wherein if stabilizing the melt-blown fiber comprises thermal treatment and chemical treatment, the thermal and chemical treatment for stabilizing the melt-blown fiber occurs simultaneously or sequentially.

8. (canceled)9. The method according to claim 5, wherein the stabilizing of the melt-blown fibers comprises:i) heating the fibers to a stabilization temperature at a heating rate of about 0.1° C. / min to about 5° C. / min; andii) maintaining the fibers at the stabilization temperature for a time interval.

10. The method according to claim 5, wherein the stabilizing of the melt-blown fibers comprises contacting the fibers with a stabilizing compound.

11. (canceled)12. The method according to claim 5, wherein during at least a portion of the stabilization step, the fibers are placed under tension; or wherein during at least a portion of the stabilization step, the edges of the carbon fiber mat are fixed, such that an internal tension force is created during stabilization; or wherein during at least a portion of the carbonizing step, the carbon fiber mat is placed under tension.

13. The method according to claim 5, further comprising pre-treating the sample before the blowing step by one or more thermal treatment processes.

14. (canceled)15. The method according to claim 5, wherein the carbonizing step comprises:i) heating the fibers to a first carbonization temperature at a first heating rate; andii) optionally, heating the fibers to a second carbonization temperature at a second heating rate.

16. (canceled)17. The method according to claim 5, wherein the feedstock is substantially free of solvent.

18. The method according to claim 5, wherein blowing the feedstock comprises supplying the feedstock into a die wherein pressurized hot air draws fibers onto a collection device.

19. The method according to claim 5, wherein the asphaltene feedstock is obtained from bitumen; wherein the asphaltene feedstock is in a form comprising a solid, a powder, a gel and / or a liquid; or wherein the asphaltene feedstock is in a form comprising solid masses with diameter of <5 mm.

20. The method according to claim 5, wherein the zero-shear viscosity of the feedstock is between about 50 Pa·s and about 200 Pa·s as measured at a temperature of 200-210° C.

21. (canceled)22. The method according to claim 5, wherein blowing the feedstock at one or more melt processing temperatures to produce melt-blown fibers, comprises extruding fibers through a multi-hole die, wherein pressurized hot air draws the fibers onto a vacuum-assisted rotating collection drum.

23. (canceled)24. The method according to claim 5, further comprising, prior to the blowing step, selecting at least one sample comprising asphaltene for supplying as a feedstock, wherein the selecting of the at least one sample comprises determining a melt processing condition of the at least one sample.

25. (canceled)26. The method according to claim 5, further comprising, following carbonization, activating the carbonized fibers by treatment with KOH, CO2, or air.

27. Carbon fibers or a carbon fiber mat prepared by the method according to claim 5.28.-30. (canceled)

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