Claims:We claim:
1. A process (100) of producing an isotropic pitch precursor for carbon fiber, wherein process (100) comprises:
thermally oxidizing at least one of soft pitch, raw QI free coal tar pitch, raw coal tar pitch to obtain the isotropic pitch precursor, wherein the soft pitch, the raw QI free coal tar pitch, the raw coal tar pitch are prepared from coal tar, wherein the thermal oxidation is carried out at a temperature in the range of 250-325°C, for a time duration of 2-8 hours in an oxidizing atmosphere.
2. The process (100) of producing an isotropic pitch precursor for carbon fiber as claimed in the claim 1, wherein the oxidizing atmosphere is maintained by controlling the gas flow having oxygen content in the range of 20 to 30% by weight.
3. The process (100) of producing an isotropic pitch precursor for carbon fiber as claimed in the claim 2, wherein a residence time of 0.3 – 2 minutes is provided for reaction of the oxygen and pitch.
4. The process (100) of producing an isotropic pitch precursor for carbon fiber as claimed in the claim 3, wherein the residence time of 0.3 – 0.6 minutes is provided for reaction of the oxygen and pitch.
5. The process (100) of producing an isotropic pitch precursor for carbon fiber as claimed in the claim 1, wherein the thermal oxidation is carried out at a temperature in the range of 250-300°C, for a time duration of 4-7 hours.
6. The process (100) of producing an isotropic pitch precursor for carbon fiber as claimed in the claim 1, wherein the raw coal tar pitch has QI content of 1.9 to 2.4% by weight, and the raw QI free coal tar pitch has QI content of 0.03 to 0.06% by weight, wherein the soft pitch, the raw QI free coal tar pitch, the raw coal tar pitch pitches have a softening point in the range of 75-85 °C.
7. The process (100) of producing an isotropic pitch precursor for carbon fiber as claimed in the claim 1, wherein the resultant isotropic pitch precursors obtained have a softening point in the range of 150-240°C, a high carbon to hydrogen atomic ratio of 2.2 to 3.2, and fixed carbon content in the range of 50 to 75 percent by weight.
8. The process (100) of producing an isotropic pitch precursor for carbon fiber as claimed in the claim 1, wherein the resultant isotropic pitch precursors obtained have aromaticity value in the range of 0.50 to 0.58 measured from Fourier Transformed Infrared spectroscopy, degree of condensation in the range of 0.65 to 0.80, beta resin content in the range of 10-60 percent by weight and NMPI content in the range of 15-50 percent by weight.
9. A method (200) of preparing the soft pitch required for producing the isotropic pitch precursor as claimed in claim 1, wherein the method (200) comprises:
mixing coal tar pitch distilled from coal tar with a nitrogen containing heterocyclic solvent to obtain a pitch slurry;
heating the said pitch slurry under controlled temperature, wherein heating is done in closed environment under vigorous mixing;
filtering the said heated and stirred pitch slurry with a filter to obtain a filter cake and a filtrate; and
subjecting the filtrate to solvent removal operation for separating the soft pitch.
10. The method (200) of preparing the soft pitch as claimed in claim 9, wherein the solvent removal operation is performed at a temperature in the range of 130-160°C, under reduced pressure of 60 to 80 torr.
11. The method (200) of preparing the soft pitch as claimed in claim 10, wherein the solvent removal operation is performed at the temperature in the range of 140-150°C.
12. The method (200) of preparing the soft pitch as claimed in claim 9, wherein the coal tar pitch used has a quinoline insoluble content in the range of 0.2 to 2.5 % by weight, coking value of the coal tar pitch is in between 50 to 58 % by weight, a softening point in between 70 to 120 °C, and an atomic ratio of carbon to hydrogen less than two.
13. The method (200) of preparing the soft pitch as claimed in claim 12, wherein the coal tar pitch used has softening point in between 80 to 95 °C.
14. The method (200) of preparing the soft pitch as claimed in claim 12, wherein the coal tar pitch has atomic ratio of carbon to hydrogen in between 1.9 to 2.
15. The method (200) of preparing the soft pitch as claimed in claim 9, wherein the weight ratio of pitch to solvent during mixing is kept in between 1:2 to 1:10.
16. The method (200) of preparing the soft pitch as claimed in claim 15, wherein the weight ratio of pitch to solvent during mixing is kept in between 1:5 to 1:10.
17. The method (200) of preparing the soft pitch as claimed in claim 9, wherein the said pitch slurry is heated up to a temperature in the range of 120 to 200 °C.
18. The method (200) of preparing the soft pitch as claimed in claim 17, wherein the said pitch slurry is heated up to a temperature in the range of in 160-180 °C temperature.
19. The method (200) of preparing the soft pitch as claimed in claim 9, wherein the filter is a stainless-steel wire mesh of 75-micron nominal diameter of Tyler standard.
20. The method (200) of preparing the soft pitch as claimed in claim 9, wherein the nitrogen containing heterocyclic solvent is n-methyl 2- pyrrolidone.
, Description:FIELD OF INVENTION
[0001] The present invention relates to a process for producing pitch precursor suitable for preparation of carbon fiber.
BACKGROUND
[0002] Carbon fibers possesses a unique combination of properties like low bulk density and high tensile strength and tensile modulus. These properties made carbon fiber, a new age material for the application in many industries like automotive, aerospace, sporting goods, wind turbine. Carbon fiber production from coal tar pitch has been a very challenging research domain for the past few decades. As compared to polyacrylonitrile (PAN) based carbon fiber, pitch-based fiber involves more complicated steps for successful spinning of precursor. Depending on the precursor type, two different grades of fibers can be produced from coal tar pitch. Pitch based precursors can be divided into two types. One is isotropic pitch, and another is mesophase pitch.
[0003] Mesogens develop inside pitch matrix when it is subjected to thermal treatment. These mesogens grow to form coalesced mesophase. Mesophase can be separated from the isotropic pitch matrix to obtain pure mesophase and isotropic precursors. Isotropic pitch is used for preparation of general-purpose carbon fiber or GPCF. Several investigations are available on preparation of suitable isotropic pitch precursor preparation. Crucial thing to keep in mind while preparing isotropic precursor is that the precursor must have a softening point above 200 °C. This will ensure that the green fiber produced from the precursor will not melt in the subsequent heat treatment process. Another crucial criterion is the solubility parameters of the precursor. Solubility parameters describes the molecular weight distribution of the molecules present in the precursor. This also gives an estimate of the beta resin present in the precursor. Beta resins contents are very important for a precursor as it gives a fair estimation of how the green fiber will behave during stabilization.
[0004] From the early 1980, isotropic pitch precursor preparation method was limited to vacuum distillation or flash evaporation of the pitches. For example, in U.S pat no. 4,460,455, a process is disclosed, where heavy petroleum residue is subjected to vacuum distillation, followed by hydrogenation. The hydrogenated oil is then catalytically cracked to obtain a pitch precursor for carbon fiber manufacturing. Alternatively, in U.S pat no. 4,533,535, a process is disclosed, where the vacuum residue of petroleum oil is mixed with oil derived from catalytic cracking of residue oil. The mixture is subjected to thermal treatment at 370 to 460 °C under 2 to 50 bar pressure. Flash evaporation process is also disclosed in U.S pat no. 4,575,411. Petroleum pitch obtained by vacuum distillation of hydrogenated vacuum gas oil, is subjected to thin film evaporation at 2 mm Hg pressure for 8 minutes at 350 °C temperature. Product obtained had a softening point of 270 °C.
[0005] However, total yield of precursor, prepared by these processes were very low. Such low yield was not economic as it increases the precursor prices. So, to increase the yield of the precursor, further modified processes was investigated. For example, European Patent 0,358,048, disclosed a process where coal tar pitch and petroleum pitch are heated up to 400-600 °C at elevated pressure of 0.2 to 9.9 MPa. Resultant pitch is extracted with monocyclic aromatic compound Xylene. Xylene soluble pitch is distilled to remove the solvent and the lighter components and then heat treated to produce the isotropic pitch. Hence, solvent extraction and heat treatment process was introduced to obtain suitable precursors for carbon fiber. However, solubility of pitch in the solvent became a key characteristic as it determines the overall process yield. An alternative method is disclosed in U.S pat no. 4,927,620, where pitch is catalytically heated before the solvent extraction, so that the solubility of the pitch gets increased.
[0006] Alternate method for solvent extraction has also been investigated. air blowing has found to be one of the promising methods, but very high processing time is involved. For example, in European patent 0.508,318, air blowing method of pitch is described where first step of blowing is done for 12 hours and second step is done for further 5 hours. Though the resultant pitch had 250 °C softening point, but the processing time was very high as compared to solvent extraction and heat treatment method.
[0007] Isotropic pitch preparation method was limited to the above-described method up to late 90s. From then, various methods of isotropic pitch preparation have been explored. For example, U.S pat no. 7,318,890, a process is disclosed where a pitch with high softening point of 285 °C is prepared by direct heating of coal tar pitch in contact with molten metal. In another example, U.S pat no. 9,222,027, a process is explained where coal tar pitch is steam reformed in high-speed tubular reactor, followed by flash evaporation at 500 mm Hg pressure. Softening point of the resultant pitch can be controlled by the flash drum pressure. However, solvent extraction and heat treatment remained as a research topic for high quality precursor preparation. For example, in Korean patent 101543534B1, a process is disclosed where pitch is mixed with monocyclic component and heat treated to 450-900 °C to obtain isotropic pitch. In another Korean patent 20160136483A, coal tar pitch is mixed with polyvinyl molecules and subjected to heat treatment to obtain clear isotropic pitch.
[0008] Heat treatment under oxidizing atmosphere has been a recent development for isotropic precursor preparation process. But the process still required further investigations. Oxygen inhibits mesophase formation and isotropic pitch with high softening point can be obtained by heat treatment under oxidizing atmosphere. For example, in Korean patent 20180051078A, a method is disclosed where pyrolysis fuel oil, vacuum residue oil or coal tar is taken as the raw material and is subjected to air oxidation at 300-400 °C temperature under air flow of 6 mL/min.
[0009] Prior arts described discloses that there has been continuous change in the method of isotropic precursor formation. Starting from vacuum distillation to air oxidation, there has been significant modifications both in method and the product property. However, air oxidation has been the newest addition in the research field. This required further investigations. Product quality and also the yield must be considered as the key criteria for a method to succeed economically.

OBJECTIVE OF INVENTION
[0010] It is an object of the invention to solve the aforementioned problems of the prior art and to provide a process of producing an isotropic pitch precursor for the production of carbon fibers having improved yield of the precursor from the raw feedstock, heat stability, spinnability and infusibility.
[0011] Another object of the invention is to selectively extract a soft pitch, from the parent coal tar pitch, which has comparatively higher beta resin content as compared to the parent pitch.
[0012] Another object of this invention is to produce a macromolecular compound rich pitch precursor, essentially free from any mesophase, which have a higher bond strength compared to the raw coal tar pitch.
[0013] Another object of this invention is to produce the isotropic pitch precursor from coal tar pitch with a high softening point which is suitable for melt spinning process.
[0014] It is further object of this invention is to produce an isotropic pitch suitable for producing general purpose carbon fiber with yield in the range of 50% to 90%, through a process which involves thermal treatment of coal tar pitch in an oxidizing atmosphere.
SUMMARY OF INVENTION
[0015] This summary is provided to introduce concepts related to a process for producing an isotropic pitch precursor. The concepts are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.
[0016] In one aspect of the present invention, a process of producing an isotropic pitch precursor for carbon fiber is provided. The process comprises thermally oxidizing at least one of soft pitch, raw QI free coal tar pitch, raw coal tar pitch to obtain the isotropic pitch precursor. The soft pitch, the raw QI free coal tar pitch, the raw coal tar pitch are prepared from coal tar. The thermal oxidation is carried out at a temperature in the range of 250-325°C, for a time duration of 2-8 hours in an oxidizing atmosphere.
[0017] In an embodiment, the oxidizing atmosphere is maintained by controlling the gas flow having oxygen content in the range of 20 to 30% by weight.
[0018] In an embodiment, a residence time of 0.3 – 2 minutes is provided for reaction of the oxygen and pitch. In an embodiment, the residence time of 0.3 – 0.6 minutes is provided for reaction of the oxygen and pitch.
[0019] In an embodiment, the thermal oxidation is carried out at a temperature in the range of 250-300°C, for a time duration of 4-7 hours.
[0020] In an embodiment, the raw coal tar pitch has QI content of 1.9 to 2.4% by weight, and the raw QI free coal tar pitch has QI content of 0.03 to 0.06% by weight, wherein the soft pitch, the raw QI free coal tar pitch, the raw coal tar pitch pitches have a softening point in the range of 75-85 °C.
[0021] In an embodiment, the resultant isotropic pitch precursors obtained have a softening point in the range of 150-240°C, a high carbon to hydrogen atomic ratio of 2.2 to 3.2, and fixed carbon content in the range of 50 to 75 percent by weight.
[0022] In an embodiment, the resultant isotropic pitch precursors obtained have aromaticity value in the range of 0.50 to 0.58 measured from Fourier Transformed Infrared spectroscopy, degree of condensation in the range of 0.65 to 0.80, beta resin content in the range of 10-60 percent by weight and NMPI content in the range of 15-50 percent by weight.
[0023] In an embodiment, a method of preparing the soft pitch required for producing the isotropic pitch precursor is provided. The method comprises mixing coal tar pitch distilled from coal tar with a nitrogen containing heterocyclic solvent to obtain a pitch slurry. The method also comprises heating the said pitch slurry under controlled temperature. The heating is done in closed environment under vigorous mixing. The method further comprises filtering the said heated and stirred pitch slurry with a filter to obtain a filter cake and a filtrate. The method comprises subjecting the filtrate to solvent removal operation for separating the soft pitch.
[0024] In an embodiment, the solvent removal operation is performed at a temperature in the range of 130-160°C, under reduced pressure of 60 to 80 torr.
[0025] In an embodiment, the solvent removal operation is performed at the temperature in the range of 140-150°C.
[0026] In an embodiment, the coal tar pitch used has a quinoline insoluble content in the range of 0.2 to 2.5 % by weight, coking value of the coal tar pitch is in between 50 to 58 % by weight, a softening point in between 70 to 120 °C, and an atomic ratio of carbon to hydrogen less than two.
[0027] In an embodiment, the coal tar pitch used has softening point in between 80 to 95 °C.
[0028] In an embodiment, the coal tar pitch has atomic ratio of carbon to hydrogen in between 1.9 to 2.
[0029] In an embodiment, the weight ratio of pitch to solvent during mixing is kept in between 1:2 to 1:10. In an embodiment, the weight ratio of pitch to solvent during mixing is kept in between 1:5 to 1:10.
[0030] In an embodiment, the said pitch slurry is then heated up to a temperature in the range of 120 to 200 °C.
[0031] In an embodiment, the said pitch slurry is then heated up to a temperature in the range of in 160-180 °C temperature.
[0032] In an embodiment, the filter is a stainless-steel wire mesh of 75-micron nominal diameter of Tyler standard.
[0033] In an embodiment, the nitrogen containing heterocyclic solvent is n-methyl 2- pyrrolidone.
[0034] Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS
[0035] Figure 1 is a process flow diagram of thermal oxidation process, according to an embodiment of the present invention;
[0036] Figure 2 is a flowchart which illustrates a method of preparing the soft pitch required for producing isotropic pitch precursor, according to an embodiment of the present invention;
[0037] Figure 3 illustrates an optical image of raw pitch used in comparative example 4, according to an embodiment of the present invention;
[0038] Figure 4 illustrates an optical image of the obtained product of example 2, according to an embodiment of the present invention;
[0039] Figure 5 illustrates an optical image of the obtained product of comparative example 2, according to an embodiment of the present invention;
[0040] Figure 6 illustrates an optical image of the obtained product of example 4, according to an embodiment of the present invention; and
[0041] Figure 7 illustrates an optical image of the obtained product of example 5, according to an embodiment of the present invention.
[0042] The drawings referred to in this description are not to be understood as being drawn to scale except if specifically noted, and such drawings are only exemplary in nature.

DETAILED DESCRIPTION
[0043] The detailed description of various exemplary embodiments of the disclosure is described herein with reference to the accompanying drawings. It should be noted that the embodiments are described herein in such details as to clearly communicate the disclosure. However, the amount of details provided herein is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0044] It is also to be understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present disclosure. Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples, are intended to encompass equivalents thereof.
[0045] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof.
[0046] It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may, in fact, be executed concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved.
[0047] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
[0048] Figure 1 illustrates an exemplary process (100) for producing isotropic pitch precursor useful as a raw material for carbon fibers. The process (100) comprises thermally oxidizing at least one of soft pitch, raw QI free coal tar pitch, raw coal tar pitch to obtain the isotropic pitch precursor. The soft pitch, the raw QI free coal tar pitch, the raw coal tar pitch are prepared from coal tar. The soft pitch, the raw QI free coal tar pitch, the raw coal tar pitch pitches have a softening point in the range of 75-85 °C. The raw coal tar pitch has QI content of 1.9 to 2.4% by weight, and the raw QI free coal tar pitch has QI content of 0.03 to 0.06% by weight.
[0049] The thermal oxidation is carried out at a temperature in the range of 250-325°C, for a time duration of 2-8 hours in an oxidizing atmosphere. More preferably, the thermal oxidation is carried out at a temperature in the range of 250-300°C, for a time duration of 4-7 hours.
[0050] In the preferred embodiment, the oxidizing atmosphere is maintained by controlling the gas flow having oxygen content in the range of 20 to 30% by weight. In an embodiment, flow of the oxidizing media into the reactor is kept between 0.06 to 0.18 m3/h.
[0051] In an embodiment, a residence time of 0.3 – 2 minutes inside a heated reactor is provided for reaction of the oxygen and pitch. More preferably, the residence time of 0.3 – 0.6 minutes is provided for reaction of the oxygen and pitch.
[0052] Quality of the resultant products are checked through analysis of different insoluble contents. These insoluble contents help to determine the beta resin content of the product. Beta resin content signifies a certain cut of the pitch, which is very reactive and has a good spinnability. Softening point of the products are also determined to estimate the spinnability of the materials. Optical images of the samples (Figures 3 to 7) have been studied to confirm the isotropic morphology of the products.
[0053] The product obtained by the above-mentioned process is an isotropic pitch precursor suitable for melt spinning process. The resultant isotropic pitch precursors obtained using the process (100) have a softening point in the range of 150-240°C, a high carbon to hydrogen atomic ratio of 2.2 to 3.2, and fixed carbon content in the range of 50 to 75 percent by weight. The resultant isotropic pitch precursors obtained have aromaticity value in the range of 0.50 to 0.58 measured from Fourier Transformed Infrared spectroscopy, degree of condensation in the range of 0.65 to 0.80, beta resin content in the range of 10-60 percent by weight and NMPI content in the range of 15-50 percent by weight.
[0054] Figures 1 and 2 illustrate an exemplary method (200) for producing soft pitch required to produce isotropic pitch precursor useful as a raw material for carbon fibers.
[0055] At step (202), coal tar pitch distilled from coal tar is mixed with a nitrogen containing heterocyclic solvent to obtain a pitch slurry. Coal tar, free from quinoline insoluble content, is used as raw material. Coal tar pitch is obtained as the bottom product of coal tar distillation under vacuum. Distillation of the said coal tar is done at 300-380 °C, preferably at 350-375 °C. The coal tar pitch used has a quinoline insoluble content in the range of 0.2 to 2.5 % by weight, coking value of the coal tar pitch is in between 50 to 58 % by weight. Softening point of the coal tar pitch is in between 70 to 120 °C, preferably between 80 to 95 °C. Atomic ratio of carbon to hydrogen must be less than two, preferably in between 1.9 to 2.
[0056] In the preferred embodiment, the nitrogen containing heterocyclic solvent is n-methyl 2- pyrrolidone. The weight ratio of pitch to solvent during mixing is kept in between 1:2 to 1:10, preferably between 1:5 to 1:10. In another embodiment, the nitrogen containing heterocyclic solvent may be pyridine.
[0057] At step (204), the said pitch slurry is heated under controlled temperature. The heating is done in closed environment under vigorous mixing. The said pitch slurry is heated up to a temperature in the range of 120 to 200 °C, preferably in the range of in 160-180 °C temperature.
[0058] At step (206), the said heated and stirred pitch slurry is filtered with a filter to obtain a filter cake and a filtrate. The solvent insoluble part is obtained as filter cake on top of the filter. In the preferred embodiment, the filter is a stainless-steel wire mesh of 75-micron nominal diameter of Tyler standard.
[0059] At step (208), the filtrate obtained in step (106) is subjected to solvent removal operation for separating the soft pitch from the solvent. The said operation is done at 130 to 160 °C temperature range, preferably at 140 to 150 °C temperature range, under reduced pressure of 60 to 80 torr. Solvent is removed from the filtrate and soft pitch is obtained as the residue product in the reactor. Yield of the soft pitch is in the range of 63 to 90 % by weight and preferably between 65 to 75 % by weight.
[0060] The extraction of pitch through nitrogen containing heterocyclic solvent is done to selectively separate a part of the pitch which has a high beta resin content as compared to parent raw pitch. Beta resins are reactive towards thermal treatments. Therefore, enables to achieve high softening point by producing macromolecular compounds in short time and temperature as compared to raw pitch. Thus, high quality precursors are obtained.
[0061] The main advantage of the invention is (a) Air blowing of coal tar pitch, instead of coal tar is done to avoid the excessive material loss, in terms of volatile matters. (b) Yield of the process is in the range of 50% to 90% of the raw pitch. Apart from that, solvent extraction of the pitch is also done to extract a selected part of the pitch which is more reactive as compared to the raw pitch and raw QI free pitch. Hence, quality of the precursor produced from the said soft pitch is better than the other pitches.
[0062] Following examples further illustrates the details of the present invention. However, the present invention is comprised of but not limited to these illustrations.
EXAMPLE 1
[0063] A coal tar pitch with quinoline insoluble (QI) content of 2.3 percent by weight is taken for the said thermal oxidation process. The said coal tar pitch has a softening point of 83 °C, as per ASTM D-3104 standard. The said pitch has an N-methyl 2 pyrrolidone insoluble (NMPI) content of 12 % by weight and toluene insoluble content (TI) of 30 % by weight. The pitch has a volatile matter content of 57% by weight and has a C/H ratio of 2.65, calculated on dry mineral matter free basis. The said pitch is crushed to 75-micron particle size and subjected to thermal oxidation process at 300 °C. Oxidation was carried out for 2 hours. Oxidizing medium used for the said purpose is air. Air flow rate is maintained at 0.06 m3/h. After completion of the oxidation process, sample was cooled down under the same oxidizing atmosphere. Resultant product comprised of softening point of 222.4°C and yield from parent coal tar pitch is approximately 83% by weight. NMPI and beta resin content of the product are 48% and 19% by weight, respectively. Optical images exhibited a clear isotropic morphology with some patches.
COMPARATIVE EXAMPLE 1
[0064] Pitch mentioned in example 1 was treated in same manner as described in example 1. Air flow was increased from 0.06 m3/h to 0.18 m3/h. Resultant product comprised of 213 °C softening point and yield from coal tar pitch is approximately 82% by weight. NMPI and beta resin content of the products are 43% and 16% by weight, respectively. Comparison shows that, higher air flow rate decreases the residence time of oxygen in the pitch interface. Hence, it decreases the macromolecular formation kinetics and lowers the softening point and beta resin content.
EXAMPLE 2
[0065] Pitch mentioned in example 1 was treated in same manner as described example 1. Only difference was that the pitch was that the thermal oxidation was carried out at 250 °C for 2 hours. Resultant product comprised of 230 °C softening point and yield from coal tar pitch is approximately 87% by weight. NMPI and beta resin content of the products are 31% and 60% by weight, respectively. Higher beta resin content shows that the lower reaction temperature at 250 °C increases the oxygen insertion and hence the macromolecule formation. Optical image of the resultant product is shown in Figure 4.
COMPARATIVE EXAMPLE 2
[0066] Pitch mentioned in example 1 was treated in same manner as mentioned in example 2. To see the effect of presence of air, reaction was carried out in inert atmosphere, using nitrogen. Resultant product comprised of 189 °C softening point and yield from coal tar pitch is approximately 84% by weight. NMPI and beta resin content of the products are 21% and 40% by weight, respectively. This shows that, at the identical conditions, 3% by weight oxygen gets inserted in the pitch, when air is used as oxidizing media. Optical image of the resultant product is shown in Figure 5.
EXAMPLE 3
[0067] Pitch mentioned in example 1 was treated in same manner as mentioned in comparative example 1. Reaction time was decreased to 1 hour to understand the effect of time on reaction kinetics, other parameters were kept same as mentioned in comparative example 1. Resultant product comprised of 205 °C softening point and yield from coal tar pitch is approximately 83% by weight. NMPI and beta resin content of the products are 23% and 59% by weight, respectively. Thus, it is understood that the reaction time plays an important role in the kinetics of thermal oxidation process.
COMPARATIVE EXAMPLE 3
[0068] Pitch mentioned in example 1 was treated in same manner as mentioned in example 3. Reaction temperature is reduced to 250 °C to study the effect of oxidation temperature. Resultant product comprised of 218 °C softening point and yield from coal tar pitch is approximately 90% by weight. NMPI and beta resin content of the products are 18% and 50% by weight, respectively. Hence, product with reduced reaction temperature increases the softening point of the resultant product, but higher air flow rate inhibits further oxidation.
COMPARATIVE EXAMPLE 4
[0069] A coal tar pitch with quinoline insoluble (QI) content of 0.03 percent by weight is taken for the said thermal oxidation process. The said coal tar pitch has a softening point of 78 °C, as per ASTM D-3104 standard. The said pitch has an N-methyl 2 pyrrolidone insoluble (NMPI) content of 3 % by weight and toluene insoluble content (TI) of 30 % by weight. The pitch has a volatile matter content of 62% by weight and has a C/H ratio of 1.98, calculated on dry mineral matter free basis. The said pitch is treated in same manner as described in example 2. Resultant product comprised of 125 °C softening point and yield from coal tar pitch is approximately 92% by weight. NMPI and beta resin content of the products are 23% and 10% by weight, respectively. As the raw pitch taken was a low QI pitch, the product properties deviated significantly as compared to the pitch mentioned in example 1. This ensures that, further increasing the reaction conditions is required for pitch with lower QI content. As the NMPI and beta resin content in the low QI pitch is less, reactivity of the pitch is also less. Optical image of raw pitch is shown in Figure 3.
COMPARATIVE EXAMPLE 5
[0070] Coal tar pitch described in comparative example 4 was extracted with NMP to extract NMP soluble pitch fraction. The said pitch fraction is treated in same manner as described in example 2. Resultant product comprised of 133 °C softening point and yield from coal tar pitch is approximately 78% by weight. NMPI and beta resin content of the products are 24% and 67% by weight, respectively. As the raw pitch taken was a low QI pitch, the product properties deviated significantly as compared to the pitch mentioned in example 1. This ensures that, further increasing the reaction conditions is required for pitch with lower QI content. As the NMPI and beta resin content in the low QI pitch is less, reactivity of the pitch is also less.
EXAMPLE 4
[0071] Coal tar pitch described in comparative example 5 is thermally treated under oxidizing atmosphere at an elevated temperature of 270 °C for 6 hours under air flow of 0.06 m3/h. This elevated reaction condition is provided to ensure higher macromolecular formation for extracted soft pitch. Resultant product comprised of 211 °C softening point and yield from coal tar pitch is approximately 54 % by weight. NMPI and beta resin content of the products are 18% and 50% by weight, respectively. Optical image shows that the precursor obtained is a pitch free from any anisotropic content. Hence, clear isotropic pitch with high softening point for carbon fiber melt spinning. Optical image of the resultant product is shown in Figure 6.
EXAMPLE 5
[0072] Coal tar pitch described in comparative example 5 is treated in same manner as described in example 4. Thermal treatment temperature is increased to 290 °C to study the further effect of reaction temperature. Resultant product comprised of 248 °C softening point and yield from coal tar pitch is approximately 48 % by weight. NMPI and beta resin content of the products are 21% and 42 % by weight, respectively. Optical image as shown in Figure 7 shows that the precursor obtained is essentially an isotropic pitch with few patches of anisotropic domains. These patches are mostly due to over oxidation of few pitch regions at the elevated temperature.
Pitch Softening point
(°C) Volatile matter content
(% weight) C/H ratio
(dmmf basis) QI
(% weight) TI
(% weight) NMPI
(% weight)
Raw pitch in example 1 83 57 2.65 2.31 30 12
Raw pitch in comparative example 4 78 62 1.98 0.03 30 3
TABLE 1: Properties of raw pitch and raw QI free pitch

Pitch Softening point
(°C) Yield from raw coal tar pitch NMPI
(% by weight) Beta resin
(% by weight)
Product in example 1 224 83 48 19
Product in comparative example 1 213 82 43 16
Product in example 2 230 87 31 60
Product in comparative example 2 189 84 21 40
Product in example 3 205 83 23 59
Product in comparative example 3 218 90 18 50
Product in comparative example 4 125 92 23 10
Product in comparative example 5 133 69 24 22
Product in example 4 211 54 18 50
Product in example 5 248 48 21 42
TABLE 2: Precursors prepared under different experimental conditions
[0073] The pitch precursor produced using the above process is suitable for preparation of carbon fibers which are used for manufacturing any component relating to any industry, without departing from the scope of the present invention. The process improves yield of the precursor from the raw feedstock, and improves heat stability, spinnability and infusibility of the resultant product.
[0074] The present invention relates to a process (100) for producing isotropic pitch precursor for the production of carbon fibers having improved yield of the precursor from the raw feedstock, heat stability, spinnability and infusibility. The key invention for the said process is selection of suitable cut of the parent pitch that is highly reactive towards the thermal treatment under oxidizing atmosphere. The suitable cut is obtained by solvent extraction of the pitch with heterocyclic solvents under controlled kinetics. Another key invention of the process is determining and tuning the residence time of oxygen atoms on the pitch matrix. This helps in formation of macromolecular compounds, without mesophase formation or burning of the pitch due to rapid oxidation. Time and temperature of the thermal treatment is optimized for getting the suitable precursor with intended softening point and optical morphology.
[0075] Furthermore, the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be combined into other systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may subsequently be made by those skilled in the art without departing from the scope of the present disclosure as encompassed by the following claims.
[0076] The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.
[0077] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.