JPH0233755B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing mesophase pitches. In particular, the present invention relates to a method for producing a mesophase pitch which is useful in the production of high modulus and high strength carbon fibers, which is characterized by using reduced pressure. Thus, according to the present invention, it is possible to develop the mesophase content at a given temperature in a substantially shorter period of time than heretofore possible. As a result of the rapidly expanding growth of the aircraft, spacecraft, and missile industries in recent years, a need has arisen for materials that exhibit unique and special combinations of physical properties. Thus, for use in applications such as the construction of aircraft, atmospheric reentry vehicles and spacecraft, as well as in the construction of submarine submerged pressure vessels and the like, materials characterized by high strength and stiffness and at the same time are lightweight. was needed. Current technology cannot supply such materials;
Research to satisfy this need has focused on the fabrication of composite articles. One of the most promising materials proposed for use in composite form is high-strength, high-modulus carbon-spun fibers, and this carbon fiber is rapidly gaining traction in the aircraft, spacecraft, and missile industries. It was brought to market just as growth was occurring. Such spun fibers have been incorporated into both plastic and metal matrices to produce composites with unusually high strength--and high elasticity-to-weight ratios, as well as other exceptional properties. however,
The high cost of producing high-strength, high-modulus carbon-spun fibers for use in such composites is a major impediment to their widespread use, even though such composites exhibit remarkable properties. It was hot. Producing high-modulus, high-strength carbon fiber at low cost 1
Two recently proposed methods are described in US Pat. No. 4,005,183 entitled "High Modulus High Strength Carbon Fibers Made from Mesophase Pitch." Such a method involves first spinning carbonaceous fibers from a carbonaceous pitch which has previously been partially transformed into a liquid crystalline or so-called "mesophase" state, and then incubating the fibers so produced in an oxygen-containing gas. The thermoset fibers are heat cured by heating for a sufficient time to render them fusible, and finally the thermoset fibers are removed from hydrogen and other volatiles in an inert atmosphere and the substantially all carbon fibers are cured. carbonization by heating to a temperature elevated enough to produce carbon. The carbon fibers produced in this manner have a highly oriented structure characterized by the presence of carbon crystallites aligned predominantly parallel to the fiber axis and It is a graphitizable material that develops graphite-like properties such as the three-dimensional alignment properties of polycrystalline graphite and its associated high density and low resistivity upon heating. At all stages of development from the as-drawn state to the graphitized state, the fiber is composed of large oriented elongated graphitizable domains (domains or small area). When a natural or synthetic pitch with an aromatic base is heated at a temperature of about 350-500°C, either at a constant temperature under static conditions or at a gradually increasing temperature, small insoluble liquid spheres appear in the pitch. initially, and its dimensions gradually increase as heating continues. Examination by electron diffraction and polarization techniques shows that these spheres consist of layers of oriented molecules aligned in the same way. As these spheres continue to grow in size as they continue to heat, they come into contact with each other and gradually coalesce into each other to create a larger mass of aligned layers. As coalescence continues, domains of aligned molecules are formed that are each larger than the original sphere. These domains (domains or subregions) together form a cohesive mesophase, where 1
The transition from one orientational domain to another often occurs smoothly and continuously through gradually curved lamellae and often through more sharply curved lamellae. Orientation differences between each domain result in a complex alignment of polarized absorption profiles in the aggregated mesophase that correspond to various types of linear discontinuities in the molecular alignment. The final dimensions of the resulting orientation domains depend on the viscosity of the mesophase from which they were formed and the rate of increase in that viscosity, which depends on the particular pitch and heating rate. In some pitches, domains are formed with dimensions from over 200 microns to over 1000 microns. At other pitches, the viscosity of the mesophase is such that only limited coalescence and structural transformation of the layers occurs such that the final domain size does not exceed 100 microns. The highly oriented, optically anisotropic, insoluble materials produced by treating pitches in this manner are given the term "mesophase" and pitches containing such materials are referred to as "mesophase". It is known as ``mesophase pitch''. When such pitches are heated above their softening point, they form two essentially immiscible liquids: an optically anisotropic oriented mesophase portion and an isotropic non-mesophase portion. It becomes a mixture with. The term “mesophase” comes from the Greek “meso” or “intermediate”
and exhibits the pseudocrystalline nature of this highly oriented optically anisotropic material. Carbonaceous pits having a mesophase content of about 40% to about 90% by weight are suitable for spinning into fibers that can be converted by heat treatment into carbon fibers having high elastic Young's modulus and high tensile strength. However, in order to obtain the desired fibers from such pitches, it is not only necessary that such an amount of mesophase be present, but also that it has a large coalesced domain or dimension under static conditions. It is also necessary to form a homogeneous aggregated mesophase with domains (domains or subregions) of aligned molecules ranging from 200 microns or more to 1000 microns or more. Static conditions mean settling without stirring. Pitches that have small oriented regions rather than large coalescing domains and form fibrous, aggregated mesophases under static conditions are unsuitable. Such pitches form mesophases with high viscosity that undergo only limited coalescence, insufficient to produce large coalesced domains with dimensions of 200 microns or more. Instead, the small oriented domains of the mesophase agglomerate to form clumps or filaments where the final domain size does not exceed 100 microns. There are certain pitches of this type that polymerize very rapidly. Similarly, pitches that do not form a homogeneous aggregated mesophase are unsuitable. The latter phenomenon is caused by an infusible solid that is surrounded by the coalescing mesophase and serves to disrupt the homogeneity and heterogeneity of the coalescing domains as well as the boundaries between them (which present or developed upon heating). Another requirement is that the pitch is non-thixotropic under the conditions used during spinning into fibers, i.e. it must exhibit Newtonian or plastic flow behavior such that the flow is uniform and well behaved. It is. When such pitches are heated to a temperature at which they exhibit a viscosity of about 10 to about 200 poise, uniform fibers can be readily spun therefrom. On the other hand, pitches that do not exhibit Newtonian or plastic flow behavior at the spinning temperature cannot be spun therefrom into uniform fibers that can be converted by heat treatment into carbon fibers with high elastic Young's modulus and high tensile strength. Carbonaceous pitches having a mesophase content of about 40% to about 90% by weight are prepared by drying carbonaceous pitches in an inert atmosphere with a mesophase content of about 350% by weight in accordance with known techniques such as those disclosed in the above-mentioned U.S. Patent Application No. 338,147. It can be prepared by heating at a temperature above 0.degree. C. for a time sufficient to produce the desired amount of mesophase. The term "inert atmosphere" means an atmosphere such as nitrogen, argon, xenon, helium, etc. that does not react with the pitch under the heating conditions used. The heating period required to produce the desired mesophase content varies depending on the pitch type and temperature used, with lower temperatures requiring longer heating periods than higher temperatures. At 350° C., which is the minimum temperature typically required to produce mesophase, at least one week of heating is typically required to produce a mesophase content of about 40%. At temperatures of about 400 DEG to 450 DEG C., the conversion to meso phase proceeds rapidly, and at such temperatures a meso phase content of 50% can usually be produced within about 1 to 40 hours. For this reason, such temperatures are commonly used. Temperatures higher than about 500°C are undesirable, and heating at this temperature should not be used for more than about 5 minutes to avoid pitch conversion to coke. Although the time required to produce a mesophase pitch with a given mesophase content decreases as the preparation temperature increases, heating at elevated temperatures reduces the mesophase and non-mesophase portions of the pitch. It was found that altering the molecular weight distribution of both had an adverse effect on the rheological properties of the pitch. Thus, heating at elevated temperatures tends to increase the amount of high molecular weight molecules in the mesophase portion of the pitch. At the same time, heating at such temperatures also results in an increase in the amount of low molecular weight molecules in the non-mesophase portion of the pitch. As a result, a mesophase pitch of a given mesophase content prepared at an elevated temperature in a relatively short period of time will be less effective than a mesophase pitch of a similar mesophase content prepared over a longer period of time at a moderate temperature. It was found that the mesophase portion of pitch had a higher average molecular weight and the non-mesophase portion had a lower average molecular weight. This wide molecular weight distribution was found to have an adverse effect on the flowability and spinnability of the pitch. This is apparently due to the low degree of compatibility between the very high molecular weight portions of the mesophase portion of the pitch and the very low molecular weight portions of the non-mesophase portion of the pitch. Very high molecular weight substances in the mesophase portion of the pitch can only be adequately plasticized at very high temperatures where the tendency of very low molecular weight molecules in the non-mesophase portion of the pitch to volatilize is greatly increased. As a result, when attempting to heat such pitches to a temperature at which they have a suitable viscosity for spinning and produce fibers therefrom, excessive removal of volatiles occurs, which is Severely interferes with the processing of fibers into small and uniform diameter fibers. For this reason, means have been sought to reduce the time required to produce mesophase pitches at relatively moderate production temperatures that impart favorable rheological properties to the pitches. Thus, in accordance with the present invention, a mesophase pitch having a given mesophase content can be produced with a mesophase content that is substantially lower than previously possible at a given temperature if the pitch is subjected to a vacuum during the formation of the mesophase. It was found that it can be prepared in a short time. Vacuum treatment of the pitch in this manner helps remove volatile low molecular weight components initially present along with low molecular weight polymerization by-products of the pitch, and more efficiently converts the precursor pitch into a mesophase pit. bring about a transformation. In this manner, approximately 40
A mesophase pitch having a mesophase content of from % to about 90% by weight will, at a given temperature, absorb mesophase at a rate up to twice or more than that normally required in the absence of such treatment, i.e. It can be prepared in less than half the time normally required when produced in the absence of reduced pressure. Furthermore, according to the invention, a mesophase pitch having a given mesophase content can be produced in a substantially shorter time period at a given temperature than heretofore possible if an inert gas is passed through the pitch during the formation of the mesophase. I found out that it can be prepared in a very short amount of time. Inert gas treatment of the pitch in this manner assists in removing volatile low molecular weight components initially present along with low molecular weight polymerization by-products of the pitch and infusing the precursor pitch into the mesophase pitch. Bring about efficient conversion. In this embodiment, a mesophase pitch having a mesophase content of about 40% to about 90% by weight can be used at a given temperature at a rate of up to twice or more than that normally required in the absence of such treatment. That is, the mesophase can be prepared in less than half the time normally required when producing the mesophase without passing an inert gas through the pitch. As the carbonaceous pit is heated to a temperature high enough to form a mesophase, the volatile low molecular weight molecules present therein are gradually volatilized from the pitch. If heating continues above the temperature at which the mesophase is formed, reactive, high molecular weight molecules polymerize to form even higher molecular weight molecules.
These then orient themselves to form a mesophase.
Although less reactive low molecular weight molecules that are not volatilized can also be polymerized, they often produce hydrogenated and/or displacement polymerization byproducts with molecular weights below about 600 that do not align to form mesophases. do. Although these low molecular weight polymerization by-products gradually volatilize as the pitch continues to heat, the presence of large amounts of these by-products during much of the time that the pitch is being converted to the mesophase
It has been found that the formation of mesophase by the reactive molecules is hindered, thereby considerably lengthening the time required to produce a pitch of a given mesophase content. Furthermore, these polymerization by-products, because of their small size and aromatic nature, are not readily compatible with the large, high molecular weight, aromatic molecules present in the mesophase portion of the pitch, and these high molecular weight The lack of compatibility between the molecules and low molecular weight molecules adversely affects pitch flow and spinnability. As pointed out earlier,
The extremely high molecular weight part of the mesophase part of Pituchi is
Very low molecular weight molecules of the non-mesophase portion of pitches can only be properly plasticized at very high temperatures where the tendency to volatilize is greatly increased, and pitches with large amounts of such materials can only be properly plasticized when they are spun. When heating to a temperature at which it has a suitable viscosity and attempting to make fibers from it, excessive removal of volatiles occurs, and this greatly interferes with the processing of the pitch into fibers of small and uniform diameter. . The present invention combines the mesophase-forming molecules present in the pitch with low molecular weight components that do not form a mesophase in order to cause the removal of undesirable volatile low molecular weight substances and to rapidly convert the pitch to the mesophase. It takes advantage of the difference in molecular weight and volatile content between the polymer and polymerization by-products. Molecules that do not convert to the mesophase have lower molecular weights than high molecular weight mesophase-forming molecules, and these may be affected by the vacuum that exists during the conversion of the pitch to the mesophase or by the conversion of the pitch to the mesophase. During the formation of the mesophase, the mesophase is preferentially volatilized from the pitch, so that the pitch can obtain a predetermined mesophase content in a substantially shortened period of time. Thus, in addition to reducing the time required to produce a pitch of a given mesophase content, this operation reduces the amount of low molecular weight molecules in the non-mesophase portion of the pitch and increases its average molecular weight. It has the effect of increasing Therefore, such pitches can be easily spun into small and uniform diameter fibers with little or no evolution of volatiles. Removal of volatile components of the pitch that do not convert to the mesophase is accomplished by subjecting the pitch to a pressure of about 100 mm Hg or less, preferably 30 mm Hg or less, during the formation of the mesophase. As previously described, the removal of undesirable volatile low molecular weight materials promotes the conversion of the pitch to the mesophase, and in this manner, producing the mesophase under vacuum allows the pitch of a given mesophase content to be heated to a given temperature. The time required to produce 1 is reduced by more than half of the time normally required in the absence of reduced pressure. In general, the time required to produce a pitch of a given mesophase content is the same as when the mesophase is prepared under vacuum as described above, as opposed to when prepared under the same conditions but in the absence of reduced pressure. When you do, it will be shortened by at least 25%, usually about 40-70%. In addition, to remove volatile components of the pitch that do not convert to the mesophase, inert gas is supplied to the pitch at least 0.5 ft ³ /hr, preferably from 0.7 to
This is done by passing at a rate of 5.0 scfh/lb pitch. "sch" is the standard unit of ft ³ /hr.
To facilitate removal of these components, any inert gas that does not react with the pitch under the heating conditions used can be used. Examples of such gases are nitrogen, argon, xenon, helium, steam, etc. As mentioned above, the removal of undesirable volatile low molecular weight substances promotes the conversion of the pitch to the mesophase, but generating the mesophase in this manner while passing an inert gas through the pitch results in the formation of a predetermined mesophase. The time required to produce a dense pitch at a given temperature is reduced by more than half the time normally required in the absence of such treatment. In general, the time required to produce a pitch of a given mesophase content will be longer than when the pitch is inert as described above, as opposed to when prepared under the same conditions but in the absence of such treatment. When preparing the mesophase while passing gas, it is shortened by at least 25%, usually by as much as 40-70%. 350 to convert the precursor pitch into the mesophase.
Although any temperature higher than 500°C up to about 500°C can be used, the mesophase pitch is between 50 and 65°C.
380-440 to produce a mesophase content of wt%
It has been found that it has improved rheological and spinning properties when prepared at a temperature of 380-410°C, most preferably 380-410°C. Typically, 2 to 60 hours of heating is required to produce the desired amount of mesophase at such temperatures. The mesophase pitch prepared under these conditions was
It was found that the mesophase pitch had a smaller difference between the number average molecular weights of the mesophase and non-mesophase portions of the pitch than a mesophase pitch with the same mesophase content prepared in a short time at an elevated temperature. The rheological and spinning properties associated with this narrow molecular weight distribution have been found to substantially facilitate processing of the pitch into small and uniform diameter fibers. Mesophase pitches prepared under favorable conditions, i.e. by heating at temperatures of 380-440°C to yield a mesophase content of 50-65% by weight, can be prepared in a short time at higher temperatures. has fewer high molecular weight molecules in the mesophase portion of the pitch and fewer low molecular weight molecules in the non-mesophase portion of the pitch than a mesophase pitch with the same mesophase content, and a lower number of molecules in the mesophase portion of the pitch. It has a high average molecular weight and a high number average molecular weight in the non-mesophase part of pitch. When a mesophase pitch is prepared under such conditions, less than 50% of the molecules in the mesophase portion of the pitch have a molecular weight greater than 4000, whereas the remaining molecules have a number average molecular weight between 1400 and 2800. have The molecules in the non-mesophase portion of such pitches have a number average molecular weight of 800 to 1200, and less than 20% of such molecules have a molecular weight of less than 600. The ringing pitch
When prepared by heating in the most preferred temperature range of 380-410°C, 20-40% of the molecules in the mesophase portion of pitch have a molecular weight greater than 4000, whereas the remaining molecules It has a number average molecular weight of 1400-2600. The molecules in the non-mesophase portion of pitches prepared by heating in the most preferred temperature range have a number average molecular weight between 900 and 1200, and 10-16% of such molecules have a molecular weight below 600. have On the other hand, the mesophase pitch was heated to 440℃.
When prepared at temperatures exceeding .
Molecules in the non-mesophase portion of the pitch have a number average molecular weight of less than 800, whereas molecules in the mesophase portion of the pitch that do not have a molecular weight exceeding 4000 have a number average molecular weight of 1400 to 2800. 380 to yield a mesophase content of 50-65% by weight
Mesophase pitches prepared by heating at temperatures of ~440°C are
It typically exhibits a viscosity of 200 poise and can be easily spun into small, uniform diameter fibers at such temperatures with little volatile evolution. Owing to their excellent rheological properties, such pitches are highly suitable for spinning carbonaceous fibers which can be converted by heat treatment into fibers with high elastic Young's modulus and high tensile strength. To produce pitches with favorable mesophase content and molecular weight characteristics, carbonaceous pitches must be
at a temperature of 440°C for at least 2 hours, preferably from 2 to
Heating for 60 hours is usually required. 65% by weight
Excessive heating should be avoided so as not to produce a mesophase content exceeding 100% or to adversely affect the desired molecular weight distribution. To obtain the desired molecular weight properties, it is also necessary to stir the pitch during mesophase formation to produce a homogeneous emulsion of immiscible mesophase and non-mesophase portions of the pitch. Such agitation may be effected by any means sufficient to efficiently mix the mesophase and non-mesophase portions of the pitch, such as by stirring or rotating the pitch. The degree of conversion of pitch to the mesophase can be easily determined by polarized light microscopy and solubility testing. Except for certain non-mesophase insoluble components present in the original pitch or developed upon heating in some cases, the non-mesophase portion of pitch is readily soluble in organic solvents such as quinoline and pyridine; The mesophase fraction, on the other hand, is essentially insoluble (% quinoline insoluble (QI) of a given pitch is determined by quinoline extraction at 75°C; % pyridine insoluble (PI) is determined by boiling pyridine (QI) (measured by Soxhlet extraction at 115°C). In the case of pitches that do not develop non-mesophase insoluble components during heating,
The insoluble content of the heat-treated pitcher, which exceeds the insoluble content of the pitcher before heat treatment, essentially corresponds to the mesophase content (the insoluble content of the untreated pitcher is generally 1
% (with the exception of certain coal tar pitches) and consists primarily of coke and carbon black found in the original pitch. ). In the case of pitches that develop non-mesophase insolubles upon heating, the insoluble content of heat-treated pitches that exceeds the insolubles content of pitches before heat treatment is due not only to the conversion of pitches to mesophase but also due to the increase in the insoluble content during heat treatment. It represents the non-meso phase insoluble matter that is generated together with the meso phase. As previously described, infusible non-mesophase insolubles (either present in the original pitch or generated by heating) in an amount sufficient to prevent the formation of a homogeneous aggregated mesophase. ) are unsuitable for use in the present invention. Generally, such infusible material is
Pitch containing more than % by weight is inappropriate. The presence or absence of such homogeneously aggregated mesophase domains as well as the presence or absence of infusible non-mesophase insolubles can be visually observed by polarized light microscopy examination of pitches (e.g., G.D. “The Formation of
Some Graphitizing Carbons”, Chemistry and
Physics of Carbon, Volume 4, Marcel Tetzker Incorporated, New York.
1968, pp. 243-268, and “The Carbonaceous Mesophase Formed” by G. Dubois, C. Agachie and G. L. White.
in the Pyrolysis of Graphitizable Organie
Materials”, Metallography 3 , 1970, No. 337~
See page 369. ). In this manner, each of these substances can be most visually calculated. Conventional molecular weight analysis techniques can be used to determine the molecular weight properties of mesophase pitches prepared in accordance with the present invention. In order to be able to carry out molecular weight measurements independently of both the mesophase and non-mesophase portions of the pitch, the two phases can be conveniently separated by the use of suitable organic solvents. As mentioned above, except for certain non-meso phase insolubles present in the original pitch or, in some cases, generated upon heating, the non-meso phase portion of the pitch is dissolved in organic solvents such as quinoline and pyridine. The mesophase portion is readily soluble, whereas the mesophase portion is essentially insoluble (the non-mesophase portion of the pitch is extracted by extraction with quinoline at 75°C or by boiling pyridine (115°C).
(can be easily separated from the mesophase part by Soxhlet extraction). After separation of the two phases by the solvent in this manner, the non-mesophase portion of the pitch can be recovered from the solvent by vacuum distillation of the solvent. One means used to determine the number average molecular weight of mesophase pitches prepared according to the present invention was:
Includes the use of a gas phase osmometer. The use of this type of instrument for molecular weight measurements has been described by A.P. Bratzday et al.
Brazday, Huff and J.D. Maccugain, J.Phys. & Coll.Chem. 55 ,
304 (1951)). An osmometer measures the difference in electrical resistance between a sensitive standard thermistor in contact with a pure solvent and a second thermistor in contact with a solution of said solvent in which a known concentration of the substance whose molecular weight is to be measured is dissolved. do. The difference in electrical resistance between the two thermistors is caused by the temperature difference between the thermistors caused by the different vapor pressures of the solvent and solution. By comparing this value with the resistance difference obtained with the solvent and a standard solution of the solvent containing a compound of known molecular weight at a known concentration,
It is possible to calculate the molecular weight of a solute substance.
One drop of a pure solvent and a drop of a solution of the solvent in which the substance whose molecular weight is to be measured is dissolved at a known concentration are placed next to a standard thermistor and a sample thermistor, respectively housed in a sealed thermostatic chamber saturated with solvent vapor. and measure the resistance of the two thermistors and record the difference between them. Since a solution of a given solvent always has a lower vapor pressure than the pure solvent, there will be differential mass transfer between the two droplets and the solvent vapor phase, such that the solution droplet (coming from evaporation is low) resulting in a large total condensation. This difference in mass transfer causes a temporary temperature difference between the two thermistors (due to the difference in heat of vaporization loss between the two droplets), which is due to the difference in vapor pressure between the two droplets. proportional to the difference. The difference in vapor pressure between the two droplets, hence 2
Difference in temperature and resistance (ΔR) between two thermistors
Since N depends only on the number of molecules of the solute substance dissolved in the solvent and is independent of the chemical composition of the molecules, the mole fraction of solute in solution (N) It can be determined from a plot of ΔR versus N for a solution of such a solvent containing a known concentration (the mole fraction of a given substance in a solution is the number of moles of such substance in the solution + the number of moles of the solvent). (means the number of moles of such substance in solution divided by ). ΔR and N have a direct proportional relationship to each other, and from the determination of N it is possible to calculate the calibration constant (K) for the solvent used from the following equation: K=ΔR/NK Once the value of K is determined, the molecular weight of the substance can be calculated using the following formula, M _x = (K-ΔR)・M _y・W _x /ΔR・W _y [Here, M _x is the value of the measurement is the molecular weight of the substance to be tested, K is the calibration constant of the solvent used, ΔR is the resistance difference between the two thermistors, M _y is the molecular weight of the solvent, and W _y is the weight of the solvent. , and W _x is the weight of the substance whose molecular weight is to be measured]. Although the molecular weight of the soluble portion of pitch can be determined directly in its solution in order to determine the molecular weight of the insoluble portion, it may be necessary to first solubilize it by chemical reduction of the aromatic bonds of such material, e.g. with hydrogen. is necessary. Suitable means for solubilizing coal and carbon by reduction of the aromatic bonds of these materials have been described by G.D. Brutz et al. (G.D. Brooks and H. Silver). Mr. Mann’s “The
Chemical Reduction of Some Cokes and
"Chars", Fuel 41 , 1962, pp. 67-69). This method involves the use of hydrogen generated by the reaction of lithium with ethylenediamine and
It has been found that aromatic bonds in carbonaceous materials can be efficiently reduced without breaking carbon bonds. Also,
Such a method was preferably used to solubilize the insoluble portion of pitches prepared according to the present invention. Another means used to determine the molecular weight characteristics of mesophase pitches prepared according to the present invention is gel permeation chromatography (GPC). This technique is described by L.R. Snyder (L.R. Snyder's “Determination of Asphalt Molecular Weight”).
Distributions by Gel Permeation
Chromatography”, Anal.Chem. 41 , 1969, No.
pp. 1223-1227). Gel permeation chromatography is used to separate solutions of polymers or polymer-related molecules of various sizes, and the molecular weight distribution of the sample is measured linearly with respect to solute concentration using a differential refractometer or a differential ultraviolet absorption spectrometer. It is measured with the help of responsive detection system. In order to be able to carry out molecular weight measurements independently on both the mesophase and non-mesophase parts of the pitch, as in the case of gas phase osmotic techniques, the two phases are treated in a suitable organic solvent. They must first be separated by use. Again, while the molecular weight of the soluble portion of pitch can be measured directly in its solution, in order to measure the molecular weight of the insoluble portion it is first necessary to solubilize it. Fractionation of a sample whose molecular weight distribution is to be determined involves dissolving the sample in a suitable solvent, passing the solution through a chromatograph, and collecting the measurement fraction of the solution eluted through the separation column of the chromatograph. It is carried out by. A given volume of solvent is required to pass molecules of a given molecular size through the chromatograph, such that each fraction of the solution eluted from the chromatograph contains molecules of a given molecular size. The fractions that flow through the column contain molecules of high molecular weight first, whereas the fractions that take the longest to elute through the column contain molecules of low molecular weight. After fractionating the sample, the concentration of solute in each fraction is
It is measured by a suitable detection system such as a differential refractometer or a differential ultraviolet absorption spectrometer. When using a differential refractometer, the refractive index of each fraction is automatically compared to that of the pure solvent by two photocells sensitive to the intensity of light passing through such fraction and the solvent;
The difference in signal intensity between the two cells is then automatically plotted against the cumulative elution volume of the solution. Since the magnitude of these signal intensity differences is proportional to the weight concentration of solute molecules present, the relative weight concentration of the molecule in each fraction is calculated by dividing the signal intensity difference of that fraction into the total integrated signal intensity difference of all fractions. It can be determined by dividing by . This relative concentration can be graphed by plotting the signal intensity difference of each fraction against the cumulative elution volume of the sample. The molecular weight of each fraction's molecules can then be determined by standard techniques, such as by the osmotic pressure technique described above. Most common pitches consist of the same molecular species, so once the molecular weight of each fraction of a particular sample has been determined, that sample can be used as a standard, and the molecular weight of the next sample fraction can be compared to the same standard. It can be determined from the known molecular weight of the fraction. Thus, it is not necessary to repeat the molecular weight determination for each fraction of each sample, which can be obtained from the molecular weight determined for the same fraction of a standard. For convenience, a molecular weight distribution curve showing the relationship between the molecular weight of the standard and the elution volume can be constructed by plotting the molecular weight measured for the standard fraction against the cumulative elution volume of the standard. Then,
From this curve, the molecular weights of the molecules of the various chromatographic fractions of a given sample can be read directly.
As previously described, the relative weight concentration of solute molecules in each fraction can be determined by differential reflectance measurements. To facilitate molecular weight determination, the signal intensity difference and elution volume values obtained for a given sample are processed by a computer together with the previously determined molecular weight data for each chromatographic fraction of the standard pitch, and It can be rewritten as a complete molecular weight distribution analysis value. This procedure provides a complete printout of the number average molecular weight (Mn), weight average molecular weight (Mw), molecular weight distribution parameters (Mw/Mn), as well as the molecular weight and weight of the solutes present in each chromatographic fraction of the sample. % compiler provided mechanically. For the preparation of mesophase pits that can be used to produce fibers that can be heat treated to produce fibers with high elastic Young's modulus and high tensile strength, a carbon content of about 92 to about 96% by weight. and aromatic-based carbonaceous pits having a hydrogen content of from about 4 to about 8 weight percent are generally preferred. Elements other than carbon and hydrogen, such as oxygen, sulfur and nitrogen, are undesirable and should not be present in excess of about 4% by weight. The presence of extraneous elements in greater than such amounts disrupts the formation of carbon crystallites during subsequent heat treatment and prevents the development of graphite-like structures within fibers made from these materials. In addition, the presence of extraneous elements reduces the carbon content of the pitch and therefore the final yield of carbon fibers. When such extraneous elements are present in amounts of about 0.5 to about 4% by weight, the pitch generally has a carbon content of about 92 to 95% by weight, with the balance being hydrogen. Preferred starting materials for preparing the mesophase pitch used to make the fibers of the invention are petroleum pitch, coal tar pitch and acenaphthylene pitch, which are pitches that graphitize well. be. Of course, oil pits are
It is a residual carbonaceous material obtained from distillation of crude oil or catalytic cracking of petroleum distillate. Similarly, coal tar pitch is obtained by distilling coal. Both of these materials are commercially available natural pitches from which the mesophase can be easily produced, and for this reason they are preferred. Acenaphthylene pitch, on the other hand, is a synthetic pitch, which is preferred because of its ability to produce superior fibers. Acenaphthylene pitch was patented by Mr. Edstrom with US Patent No.
It can be produced by thermal decomposition of an acenaphthylene polymer as described in No. 3,574,653. Some pitches, such as fluoroanthracene pitch, polymerize very easily on heating and do not find large, coalescing domains of mesophase, so they are not suitable precursor materials. Similarly, pitches that are highly infusible, non-mesophase and have a content that is insoluble in organic solvents such as quinoline or pyridine, or that develop highly infusible, non-mesophase insoluble content upon heating, are described above. It should not be used as a starting material. This is because these pitches produce the homogeneous agglomerated mesophase necessary to produce highly oriented carbonaceous fibers that can be converted by heat treatment into carbon fibers with high elastic Young's modulus and high tensile strength. Because you can't. For this reason, approximately 2
Pitch having an infusible, quinoline-insoluble or pyridine-insoluble content (determined as described above) of greater than or equal to % by weight should not be used or the material must be heated before heating to form the mesophase. Should be taken to remove. Preferably, such pitches are used when they contain about 1 weight or more of such infusible material. Most petroleum pitches and synthetic pitches have a low infusible content and can be used directly without further modification. On the other hand, most coulter pitches have a high infusible content and require filtration before their use. When pitchi is heated to a temperature of 350-500°C to produce a mesophase, the pitchi is of course heat-treated to some extent, and the composition of the pitchi is varied depending on the temperature, heating time, and composition and structure of the starting materials. . Generally, however, after heating the carbonaceous pitch for a period sufficient to produce a mesophase content of about 40 to about 90% by weight, the resulting pitch is about
It has a carbon content of 94-96% by weight and a hydrogen content of about 4-6% by weight. When such pitches contain elements other than carbon and hydrogen in amounts of about 0.5 to about 4% by weight, mesophase pitches generally contain about 92 to 95% by weight.
It has a carbon content of , with the balance being hydrogen. After preparing the desired mesophase pitch, it is spun into fibers by conventional techniques, such as by melt spinning, centrifugal spinning, blow spinning, or in other well known manners. As previously described, in order to obtain highly oriented carbonaceous fibers that can be heat treated to produce carbon fibers with high elastic Young's modulus and high tensile strength, Pitzi requires large coalescing domains under static conditions. It must form a homogeneous, cohesive mesophase with a 100% homogeneous structure and must be non-thixotropic under the conditions used during spinning. Furthermore, in order to obtain uniform fibers from such pitches, the pitches should be agitated immediately prior to spinning to effectively mix the immiscible mesophase and non-mesophase portions of the pitches. The temperature at which the pitch is spun will, of course, depend on the temperature at which the pitch exhibits a suitable viscosity. Since the softening temperature of the pitch and its viscosity at a given temperature increases as the mesophase content of the pitch increases, the mesophase content should not be increased to the point of raising the pitch's softening point to an excessive level. For this reason, pitches with a mesophase content of more than about 90% are generally not used.
Pits with a mesophase content of about 40% by weight usually have a mesophase content of about 40% by weight.
It has a viscosity of about 200 poise at 300°C and about 10 poise at about 375°C, whereas pitch with a mesophase content of about 90% by weight exhibits a similar viscosity at temperatures above 430°C. Within this viscosity range, fibers can be conveniently spun from such pitches at speeds of about 50 ft/min to about 1000 ft/min and even up to about 3000 ft/min. Preferably, the pitch used has a mesophase content of about 50 to about 65% by weight and a temperature of about 340 to about 380°C.
It exhibits a viscosity of about 30 to about 150 poise at a temperature of .
At such viscosities and temperatures, uniform fibers having diameters from about 5 to about 25 microns can be readily spun. However, as mentioned earlier,
In order to obtain the desired fibers, it is important that the pits be non-thixotropic and exhibit Newtonian or plastic flow behavior during spinning of the fibers. The carbonaceous fibers produced in this manner are highly oriented graphitizable materials with a high degree of preferred orientation of their molecules parallel to the fiber axis. The term "graphitizable" means that these fibers are thermally (usually at temperatures above about 2500°C, e.g. about 2500 to about 3000°C) means that it can be converted (by heating). The fibers produced in this manner, of course, have the same chemical composition as the pitch from which they are drawn, and similar pitches contain from about 40 to about 90 percent by weight mesophase. When examined under magnification by polarized light microscopy techniques, the fibers exhibit tissue variations that give them a "minicomposite" appearance. Large elongated anisotropic domains with a fibrillar appearance distributed throughout the fiber are seen. These anisotropic domains are highly oriented and aligned preferentially parallel to the fiber axis. We believe that these anisotropic domains (which are elongated by the shear forces exerted on the pitch during fiber spinning) are not composed entirely of mesophase, but also of non-mesophase. It will be done. Apparently, the non-mesophase is oriented during spinning and drawn into elongated domains by these shear forces and the orientation effect exerted by it as the mesophase domains are elongated.
Also, although anisotropic domains may exist, they may not be visible and are therefore difficult to distinguish from anisotropic domains that coincidentally exhibit extinction. Characteristically, the oriented elongated domains have diameters greater than 5000 Å, typically from about 10000 to about 40000 Å, and because of their large dimensions, when examined at 1000x magnification by conventional polarized light microscopy techniques. easily observed. The maximum resolution of a standard polarizing microscope with a magnification of 1000 times is only a few tenths of a micron (1 micron = 10,000 Å).
Therefore, anisotropic domains with dimensions below 1000 Å cannot be detected by this technique. Fibers spun from pitches containing more than about 85% by weight mesophase often retain their shape when carbonized without preliminary heat curing; Fibers spun from pitch require some heat curing before carbonizing them. Thermal curing of fibers is
This is readily accomplished by heating the fibers in an oxygen-containing atmosphere for a sufficient period of time to render them infusible. The oxygen-containing atmosphere used may be pure oxygen or an oxygen-enriched atmosphere. Most conveniently, air is used as the oxidizing atmosphere. The time required to cause thermosetting of the fibers will, of course, depend on such factors as the type of oxidizing atmosphere used, the temperature, the diameter of the fibers, the particular pitch in which the fibers were made and the mesophase content of such pitches. Varies depending on factors. However, in general, heat curing of the fibers takes place in a relatively short period of time, usually from about 5 to about 60
It can be done in minutes. The temperature used to cause thermosetting of the fibers is
Of course, the temperature at which the fiber softens or deforms cannot be exceeded. Thus, the maximum temperature that can be used depends on the particular pitch into which the fiber was spun and the mesophase content of such pitch. The higher the mesophase content of the pitch, the higher its softening temperature and the higher the temperature that can be used to cause thermosetting of the fiber. Of course, at higher temperatures, fibers of a given diameter can be heat-set in a shorter time than is possible at lower temperatures. On the other hand, fibers made from pitch with low mesophase content require relatively long heat treatments at somewhat lower temperatures to render them infusible. Maximum temperatures of at least 250°C are generally required to efficiently heat cure carbonaceous fibers made in accordance with the present invention. Temperatures above 400°C can cause melting and/or excessive burning of the fibers and should be avoided. Preferably, about
Temperatures from 275 to about 350°C are used. At such temperatures, heat curing can generally be accomplished within about 5 to about 60 minutes. Since it is undesirable to oxidize the fibers beyond that required to render them completely infusible, the fibers are generally oxidized for longer than about 60 minutes or
It will not heat up at temperatures above 400℃. After thermosetting the fibers, the infusible fibers are enriched in an inert atmosphere, such as those described above, sufficiently to remove hydrogen and other volatiles and produce substantially all carbon fibers. It is carbonized by heating to a certain temperature. Fibers having a carbon content of about 98% by weight or higher can generally be produced by heating to temperatures of about 1000°C or higher, and at temperatures of about 1500°C or higher, the fibers are fully carbonized. Typically, carbonization is carried out at a temperature of about 1000 to about 2000<0>C, preferably about 1500 to about 1900<0>C. Generally, residence times of about 0.5 to about 25 minutes, preferably about 1 to about 5 minutes are used. Although longer heating times can be used with better results, such residence times are uneconomical and, as a practical matter, there is no benefit to using such long periods. To ensure that the weight loss rate of the fibers is not so excessive as to destroy the fiber structure, the fibers are heated to approximately 700 to
Preferably, heating is carried out for a short period of time at a temperature of about 900°C. Residence times of about 30 seconds to about 5 minutes at these temperatures are usually sufficient. Preferably, the fibers are heated at a temperature of about 700°C for about 0.5 minutes and then at a temperature of about 900°C for the same period of time. In any case, the heating rate must be controlled so that volatilization does not proceed at an excessive rate. In a preferred method of heat treatment, a continuous filament of fiber is passed through a series of heating zones held at successively higher temperatures. If desired, the first of these zones can contain an oxidizing atmosphere and heat curing of the fibers takes place therein. Several arrangements of devices can be used to form a series of heating zones. Thus, one furnace can be used, passing the fiber several times and increasing the temperature each time. Alternatively, subsequent furnaces may be maintained at a higher temperature than the preceding furnace, and the fibers may be passed through some furnaces only once. It is also possible to use a single furnace with several heating zones maintained at continuous high temperatures in the direction of fiber travel. The carbon fibers produced in this manner have a highly oriented structure characterized by the presence of carbon crystallites preferentially aligned parallel to the fiber axis, and when heated to the graphitization temperature It is a graphitizable material that sometimes exhibits graphite-like properties such as the three-dimensional alignment properties of polycrystalline graphite and its associated high density and low resistivity. If desired, the carbonized fibers are further heated in an inert atmosphere as described above to an even higher temperature within the range of about 2500 to about 3300°C, preferably about 2800 to about 3000°C to Fibers can be produced that have the structural characteristics of polycrystalline graphite as well as a high degree of preferred orientation of parallel carbon crystallites. Although residence times of about 1 minute are satisfactory, both shorter and longer times can be used, such as from about 10 seconds to about 5 minutes or more. Although residence times longer than 5 minutes are sterile and unnecessary, they can be used if desired. Fibers produced by heating at temperatures above about 2500°C, preferably above about 2800°C, are characterized as having a three-dimensional array of polycrystalline graphite. This three-dimensional arrangement is determined by the X-ray diffraction diagram of the fiber, specifically the presence of (112) crossed lattice lines and the presence of two distinct lines (100) in the (10) band.
and (101). The short circular arcs that make up the (00l) band of the diffractogram indicate that the carbon crystallites of the fibers are preferentially aligned parallel to the fiber axis. Microdensitometric scanning of the (002) band of exposed X-ray film reveals that this preferred orientation is at best approximately
10° is typically about 5° to about 10° (expressed as the full width at half the maximum of the azimuthal intensity distribution). The apparent layer size (La) and apparent deposition height (Lc) of the crystallites exceed 1000 Å, thus it is too large to be measured by X-ray techniques. The layer distance (d) of the microcrystal is equivalent to (00
) Calculated from the diffraction arc, it is at most 3.37 Å, generally 3.36-3.37 Å. The following examples are provided for illustrative purposes so that those skilled in the art may better understand the invention. However, it is merely an example;
It should not be construed as limiting the invention in any way. Example 1 A pitchch having a mesophase content of approximately 57% by weight was prepared using commercially available petroleum pitchch. The precursor pitch has a number average molecular weight of 400, a density of 1.23 g/cc, and a density of 120
It had a softening temperature of °C and contained 0.83% by weight of quinoline insolubles (QI determined by quinoline extraction at 75 °C). According to chemical analysis, carbon content of 93.0%, hydrogen content of 5.6%, 1.1%
It showed a sulfur content of 0.044% and an ash content of 0.044%. The mesophase pitch is prepared by heating the precursor petroleum pitch at approximately 5°C/
The pitch was prepared by heating at a rate of Hr to a temperature of 415°C and maintaining the pitch at this temperature under a nitrogen atmosphere for a further 5 hours. To illustrate that the conversion to mesophase proceeds rapidly under vacuum, mesophase pitches were prepared from the same precursor pitches and in the same manner except that the pitches were heated to a pressure of less than 1 mm Hg. The resulting pitch had a pyridine insolubles content of 71% compared to only 57% for the pitch prepared in the absence of vacuum. Commercially available petroleum pitch was used to prepare pitch with a mesophase content of approximately 53% by weight. The precursor pitch has a number average molecular weight of 400, a density of 1.23 g/cc, 120
It had a softening temperature of 0.degree. C. and contained 0.83% by weight of quinoline insolubles (QI determined by quinoline extraction at 75.degree. C.). According to chemical analysis, carbon content of 93.0%, hydrogen content of 5.6%, 1.1%
It showed a sulfur content of 0.044% and an ash content of 0.044%. The mesophase pitch is prepared by heating 60 g of the precursor pitch in an 86 c.c.
The pitch was prepared by increasing the temperature from about 200°C to about 400°C at a rate of 100 hr and maintaining the pitch at about 400°C for an additional 12 hours. The pitch was continuously stirred during this time and the pitch was continuously bubbled with nitrogen at a rate of 0.2 scfh. The pitch prepared in this manner had a pyridine insoluble content of 53%, indicating a mesophase content close to 53%. Pitch can be easily spun into fiber, and significant amounts of fiber have been produced by spinning pitch through a spinneret (0.015 inch diameter hole) at a temperature of 368°C. The filament passed through a nitrogen atmosphere as it exited the spinneret and before it was wound by the reel. A portion of the fiber produced in this manner was heated in oxygen at 390°C for 6 minutes. The resulting oxidized fibers were completely infusible and could be heated at elevated temperatures without sagging. After heating the infusible fibers in an air atmosphere for about 10 minutes, the fibers were found to have a tensile strength of 171 x ¹⁰³ psi and an elastic Young's modulus of 46 x ¹⁰⁶ psi. Note that the tensile strength and Young's modulus are the average values of 10 samples. For comparison purposes, mesophase pitches were prepared from the same precursor pitches and in the same manner as described above, except that no nitrogen was bubbled into the pitches while preparing the pitches under a nitrogen atmosphere. Heating at 400° C. for 32 hours was required to prepare a mesophase pitch with 50% pyridine insoluble content.

Claims (1)

[Claims] 1 A carbonaceous pitch was heated at a temperature of 350 to 450°C to reduce the mesophase content while applying reduced pressure during the formation of the mesophase.
A method for producing a mesophase pitch consisting of heating to a range of 40 to 90% by weight.