JPH0250211B2 - - Google Patents

Description

[Detailed description of the invention]

FIELD OF THE INVENTION The present invention relates to a method of manufacturing carbon fibers that avoids critical steps previously considered essential manufacturing steps for the manufacture of carbon fibers. In particular, the present invention relates to a method in which a separate step of infusible mesophase pitch fibers prior to the carbonization step for producing carbon fibers is eliminated. The invention also relates to novel carbon fibers and composite materials containing such fibers. BACKGROUND OF THE INVENTION Mesophase pitch-based carbon fibers are well known in the art since the publication of US Pat. No. 4,005,183. A large number of patent documents have been published relating to the production of mesophase pitches suitable for the production of carbon fibers. Such patents include U.S. Pat. No. 4,026,788 and U.S. Pat.
Examples include No. 3976729 and No. 4303631. Mesophase pitches suitable for spinning pitch fibers contain at least 40% by weight mesophase such that the mesophase is the continuous phase, and the mesophase pits have domains (domains) of at least 200 microns in size upon static heating. ) is known in the art. Spinning mesophase pitch into continuous pitch fibers for the production of carbon fibers is generally carried out using spinning equipment that simultaneously spins several hundred fibers, typically 1500 to 2000 pitch fibers. The average diameter of pitch fibers is approximately 13 microns. For example, 2000 pitch fibers are processed together in a subsequent step. A bundle of continuous fibers is usually referred to in the industry as a “yarn”.
It is called. Usually carbon fiber is manufactured by
It is packaged for transport and used in composites as yarn. Such yarns are often referred to as "carbon yarns." As used herein, the term "yarn" refers to a plurality of continuous fibers spun and processed together, and includes the terms "pitch yarn,""infusibleyarn,""carbonyarn," and "graphite yarn."
are used to represent yarns at various stages of the manufacturing process. In general, the method for producing carbon fiber from meso pitch is to spin meso phase pitch into a plurality of pitch fibers (pitch yarn), to infusible the pitch fibers (pitch yarn), and then to the infusible pitch. It includes the steps of producing carbon fibers (carbon yarn) by subjecting the fibers to a carbonization step in a substantially non-reactive atmosphere. It is known from the prior art that the process of infusibleizing pitch fibers is essential for the production of carbon fibers. This is because it allows the carbonization step to be carried out relatively quickly. The carbonization process typically heats the yarn to at least about 1000°C.
must be heated to a temperature of It would be desirable to be able to increase the temperature of the yarn from about room temperature to a final temperature of, for example, 1000° C. in a short period of time without causing fiber deformation, interfiber fusion, or deterioration of the mechanical properties of the carbon yarn. In the prior art, the infusibility step is particularly important in the production of carbon fibers based on mesophase pitch. Carbon fibers derived from mesophase pitch are characterized by excellent mechanical properties such as tensile strength and yarn modulus. This is because the aromatic molecules of the mesophase pitch tend to be oriented parallel to the pitch fibers during spinning of the mesophase pitch fibers. Increasing the temperature of non-infusible mesophase pitch fibers to the softening point of pitch fibers leads to the deorientation of aromatic molecules, thereby offering a substantial possibility of obtaining carbon fibers with excellent mechanical properties. The top is gone. In the prior art, in order to avoid the unusually long time it takes to raise the temperature of the yarn from room temperature to the carbonization temperature without degrading the quality of the carbon yarn being produced, it is necessary to prepare the mesophase pitch yarn prior to the carbonization step. It has been emphasized that it is necessary to make it infusible. Also, according to the prior art, it is essential to infusible the non-mesophase pitch fibers in order to avoid softening of the fibers resulting in fusion between the fibers of the yarn. The process of making pitch yarn infusible is also referred to as a "thermal curing process" in the art. The infusibility process is an exothermic reaction, and the heat generated by this reaction can soften or deform the fibers. This heat can cause the fibers in the yarn to stick or stick together, which reduces the tensile strength of the resulting carbon yarn as well as the properties of composites made with the carbon fibers. This problem,
It has been considered in US Pat. No. 4,275,051 and US Pat. No. 4,276,278. The production of carbon fiber as reflected in the patent literature is
“Carbon and Graphite Fibers,” published (1980) by Noyce Data Corporation, Burke Ridge, New Jersey, USA.
Manufacture and Applications'' (edited by Marshall Schitteg), which describes the historical development of carbon fibers as derived from various precursor materials and the patented techniques for their manufacture. In addition, this book describes various fiber processing processes, matrices used with carbon yarns to make composites, and other materials that can be included in combination with carbon fibers to make effective composites. The use of carbon fibers as reinforcing materials and in the manufacture of textile structures is briefly described.
Committe for Characterization and
"First Publication, of 30 Definitions" in Carbon, Vol. 20, pp. 445-449 (1982) to clarify the definitions of many terms used in this field.
announced. This international committee has established that “carbon fiber”
“Carbonization of either organic synthetic or natural fibers (PAN or others) or fibers drawn from organic precursors such as resins or pitches and subsequent heat treatment of the carbonized fibers (up to temperatures of approximately 3000 K)” filament made of non-graphitic carbon obtained by The International Committee also defines “non-graphitic carbon” as “a six-membered ring network in which carbon atoms are arranged in a quadratic regularity, but apart from more or less parallel stacking in a third direction (C ``all variants of substances consisting primarily of the element carbon that have no measurable crystalline regularity in the -direction''. In the art, the term "graphite fiber" is used to describe heat-treated carbon fibers of 2500-3000 ml.
The International Commission pointed out that such fibers remain non-graphite carbon in most cases and that the common name "graphite fibers" is inaccurate. However, the International Commission has stated that ``the term ``graphitic carbon'' refers to the use of three-dimensional crystalline regularity in a material by diffraction techniques, regardless of the volume fraction and distribution uniformity of such crystalline domains. is justified if it can be detected. According to the prior art, the infusibility step is carried out in an oxidizing atmosphere, preferably at elevated temperatures to increase the rate at which the fibers become infusible.
US Pat. No. 4,389,387 discloses the problem of rapidly and efficiently infusible pitch fibers. This patent discloses that 10,000 pitch fibers are preferably assembled into a tow with a diameter of 10 to 30 mm prior to infusibility treatment. The pitch fibers are placed on a net belt conveyor and passed through a mixture of air and a gaseous oxidizing agent such as oxygen, ozone, sulfur dioxide, nitrogen dioxide, etc. This gaseous oxidant is 0.1-10% by weight of the gas mixture. The temperature of the infusibility step in this patent is at least 5 to 50°C lower than the softening point of pitch fibers. This patent also states that the time for infusibility is 1 to 4.
time and are disclosed. The patent also states that the problem of making the pitch infusible is overcome by passing a gaseous mixture through the filled pitch. Nevertheless, this patent avoids making the filling height of pitch fibers too large in order to avoid insufficient removal of the heat generated. South African patent application 71/7853 entitled “Improvements in the manufacture of carbon fibres” filed on 4 November 1971
No. 1, No. 1, No. 1, No. 1, No. 1, No. 1, No. 1, No. 1, 2003, 1, 2, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, and 4, disclose a method for infusibleizing fibers after they have been spun and prior to the carbonization step. The infusibility step in this patent is referred to as "stabilization." That is, "stabilization" and infusibility are the same and are used interchangeably in the patent.
Precursors disclosed in this patent include solutions or extracts of coal as well as pitch, pitch-like materials and tars (particularly if they are derived from coal). The South African patent application states that “a spun or extruded fiber, filament or film of organic material is prepared by treating it with either an aqueous solution of bromine or an aqueous solution of nitric acid containing at least 25% by weight of HNO 3 and preferably at least 40% by weight of HNO <sub>3</sub> . The spun or extruded fibers, filaments or films can be stabilized by heat treatment by contacting for at least a sufficient time to stabilize the spun or extruded fibers, filaments or films against heat treatment. Furthermore, this patent application discloses that the stabilized fibers can be further stabilized against heat treatment by oxidation using an oxidizing gas, preferably one containing molecular oxygen, at elevated temperatures. The above South African patent application states that nitric acid reacts with coal and similar substances to decompose the coal, and that the reaction between nitric acid and coal is a surface action and that in some cases nitric acid interacts with coal violently or even explosively. Discloses that it will react. According to the above South African patent application, “If nitric acid is reacted with spun or extruded fibers, filaments or films of organic material for an excessive period of time, nitric acid will react with spun or extruded fibers, filaments or films of organic material and decompose it. If the organic material is a coal solution or extract as described above, the nitric acid may react with the coal solution or extract to form a coal solution or extract. The coal solution or extract has smaller molecules.This is the process by which the coal solution or extract is spun or extruded into or made from fibers, filaments or films. Therefore, spun or extruded fibers, filaments or films of solutions or extracts of coal or other organic materials are stable in aqueous solutions of either bromine or nitric acid. The reaction should not be carried out for such a long time as to have a significant adverse effect on the properties of the composite fibers, filaments or films, or of the carbon fibers, filaments or films produced therefrom." The South African patent application provides one example of the use of aqueous nitric acid. Example 1 is 30
It is disclosed that a single filament with a micron diameter was cut to length and immersed in a solution containing 50% by weight nitric acid at ambient temperature (approximately 20°C). The number of lengths cut is not stated in the patent. The fiber lengths are then washed with water to remove nitric acid and suspended in a vertical furnace and heated in nitrogen at a heating rate of 300°C/hr to a temperature of approximately 260°C, after which the nitrogen atmosphere is replaced with oxygen. Then, it was replaced for 5 minutes. Subsequently, the fibers were heated in nitrogen at a rate of 80°C/hr to a temperature of 1000°C and held at this temperature for 1 hour. The remaining two examples of the South African patent application disclose the use of bromine in water instead of aqueous nitric acid. In each of these examples, the temperature increase rate of the carbonization step was 50°C/hr to a final temperature of 1000°C. The South African patent application also discloses that in order to avoid deterioration of the fibers due to nitric acid, it is essential to wash the nitric acid from the fibers. Industrial application of the disclosure of the South African patent application requires a nitric acid treatment followed by a washing step and, after the nitric acid treatment, a heat treatment in oxygen similar to Example 1 above. Significantly, each of the embodiments in the South African patent application discloses that the temperature is 50°C or 1000°C at a rate of 80°C/hr for separate cut lengths of fiber suspended in a furnace.
This indicates that the temperature has been raised to ℃. In contrast, a typical industrial carbonization process for producing carbon fiber typically involves heating a yarn with at least 1000 filaments to a temperature of about 1000°C in a furnace through which it passes. It means heating. The yarn undergoes a change from room temperature to carbonization temperature and back to room temperature. The time the yarn is subjected to the carbonization temperature is on the order of about 1 second or less. Patent No. 564,648, based on Japanese Patent Publication No. 45-2510, dated February 3, 1969, discloses a method for producing carbon fiber from carbonized petroleum sludge having a sulfuric acid content of less than 30%. The spun fibers are subjected to surface treatment by exposure to a stream of chlorine gas or by immersion in hydrogen peroxide, hydrochloric acid or nitric acid solutions at temperatures between room temperature and 60°C. Subsequently, the fibers are heated to above 200°C in an oxidizing atmosphere to complete the infusibility process. The final step is a heat treatment in which the treated fibers are carbonized to produce carbon fibers. The Japanese patent discloses that surface treatment is necessary because direct heating of spun petroleum sludge fibers in an oxidizing atmosphere results in conversion and deformation of the fibers. U.S. Pat. No. 3,595,946 discloses performing an oxidation treatment on filaments in pitches, either continuously as the filaments exit the spinning machine or in batches on the filaments wound into packages. The hot filament from the spinning machine is
An oxidizing atmosphere such as air, ozone, nitrogen oxide, etc. is passed through. This patent discloses that the filament from the spinning machine can be cooled to a temperature below its thermal melting point and then passed through a liquid oxidizing bath such as nitric acid, sulfuric acid, chromic acid, permanganate solutions and the like. There is. This patent also discloses that an oxidation treatment can be applied to a batch of filaments wound into a package. Also,
The patent states that "the support of the filament package must be of such a nature and/or structure that it yields or caves in as the wound filament shrinks during the oxidation process." Additionally, this patent states: “Oxidation of filament wound into a package must be subject to a rather critical heating regime if superimposed adjacent loops of filament cannot be fused together. will vary depending on the pitch, its prior oxidation history, and the type and amount of additives, if any, present.The best heating rate and soaking temperature for a given material will, of course, be This is difficult to determine, since the melting temperature of the pitch changes as oxidation progresses.Nevertheless, the preferred type of heat-treated pitch, as described above, has a temperature of 15 to 100°C within minutes (non-critical process), hold the filament at 100°C for about 20 hours, increase the temperature from 100°C to 195°C at a preferred rate of about 5°C/hr, and bring the filament to the latter temperature. Approximately 60 to 120 in
It has been found that holding for a period within a range of hours (with the upper part of this range being preferred) produces successfully oxidized filaments. It should be noted that for certain materials, temperature increase rates of up to 10° C./hr can be tolerated. In any event, the temperature at any time during the oxidation treatment should preferably be no more than 10°C below the softening point of the pitch at any given time. This batch oxidation is best carried out in a circulation furnace through which a constant flow of both fresh and recycled air-oxygen gas, preheated to the desired temperature, is passed. Such heating schedules are extremely time consuming even after testing has been carried out to optimize the process and avoid fusion between filaments. In view of the prior art, it has been found that it is essential to carry out a separate infusibility step prior to the carbonization step and that considerable care must be taken in infusibility of pitch yarns in order to avoid sticking or fusing of the fibers. It seems that you have to pay. Many attempts have been made in the art to simplify and speed up the infusibility process. However, the prior art does not disclose any method of infusibility other than infusibility of the yarn as a separate step. Moreover, the prior art requires an oxidizing atmosphere to infusible the pitch fibers even after treating the pitch fibers with an oxidizing liquid such as nitric acid. SUMMARY OF THE INVENTION The present invention encompasses a method of making mesophase pitch-based carbon yarn. The method involves spinning mesophase pitch into a plurality of continuous fibers, combining the plurality of fibers to form pitch yarn, contacting the pitch yarn with an oxidizing liquid composition, gathering the pitch yarn into a bulky form, and The process involves subjecting the pitch yarn in a bulky form to a substantially heat treatment (in an inert, non-reactive atmosphere) to produce a carbon yarn in a bulky form. The oxidizing liquid composition enables the pitch yarn to be infusible in the method according to the invention, and the "size" or " sizing”
It also acts as. The terms "size" and "sizing" are used interchangeably in the art. In this regard, "sizing" the pitch yarn is intended to keep the pitch fibers in the pitch yarn together, thereby minimizing separation of the pitch fibers from the body of the pitch yarn. It is desirable to keep the pitch fibers in the pitch yarn closely together for handling of the pitch yarn in manufacturing operations. DETAILED DESCRIPTION OF THE INVENTION The present invention substantially simplifies the production of mesophase pitch-based carbon yarns and greatly reduces production costs. This can be better appreciated by comparing the present invention with conventional carbon yarn manufacturing methods. Traditional methods of producing carbon yarns based on mesophase pitches use multiple operations and expensive equipment. The following is a general description of conventional manufacturing operations. The spinning apparatus produces 2000 continuous mesophase pitch fibers that are individually drawn with a draw ratio of about 50:1 to have an average fiber diameter of about 12 microns. Drawdown is necessary to obtain a small diameter. This is because spinnerets approximately 12 microns in diameter are expensive to manufacture and easily clog. It is well known in the art that carbon fibers with small diameters generally have better mechanical properties than carbon fibers with relatively large diameters. Small diameter pitch fibers are used to obtain small diameter carbon fibers. 2000 pitch fibers are sized and gathered together to form pitch yarn. The infusibility process is carried out by placing the pitch yarn in a uniform pattern on a conveyor belt, which conveys the pitch yarn to a furnace. Pitch fibers are mechanically weak and must therefore be handled with considerable care. Thus, the scheme for loading pitch yarn onto a conveyor belt is complex and subject to speed limitations. The spinning device is located above the conveyor belt. The pitch yarn enters a moving device that moves across a conveyor belt to evenly deposit the pitch yarn. This mobile equipment is referred to in the industry as a "travalling godet" and, even with careful design, is subject to speed limitations and can damage pitch yarn due to its tendency to stick to rolls within the equipment. there's a possibility that. Such adhesion is due to surface tension resulting from the sizing used to hold the pitch fibers together to form the pitch yarn. Following the traveling godet, a device called a "transvector" is placed. The transvector picks up the pitch yarn from the last roll in the traveling godet under suction and directs the pitch yarn downward toward the conveyor belt. Transvectors are not subject to speed limitations, but the pitch fibers can be damaged by air pressure moving in the vicinity of the pitch yarn. Following the transvector is a "laydown tube" that places the pitch yarn on the conveyor belt in a predetermined pattern.
A poor pattern distribution or too high stacking of pitch yarns can lead to extremely high localized heating due to exothermic reactions during the infusibility step. This laydown tube presents another potential problem. This is because pitch yarn wet from sizing sometimes sticks to the sides of the tube for a short time and this interferes with the laydown pattern on the conveyor belt. A conveyor belt conveys the pitch yarn to a large furnace with an oxidizing atmosphere and a predetermined thermal gradient to infusible it with little damage to the pitch yarn to accommodate industrial operations. This heat treatment can be performed for several hours. Furnace costs as well as energy costs are extremely high. Subsequently, the infusible yarn is pulled from the belt and deposited on a bobbin for ease of handling and storage. This operation uses what is referred to as a "downstream drive" and can be troublesome because the infusible pitch yarn is less strong than the pitch yarn. The infusible pitch yarn must be collected at a speed that corresponds to the spinning speed. The present invention eliminates the need for traveling godets, transvectors, conveyor belts, large furnaces and downstream drives. In a preferred embodiment of the invention, the spinning apparatus produces a plurality of pitch fibers, for example 2000 pitch fibers, and these pitch fibers are drawn together while being wound onto a bobbin after being sized with an oxidizing liquid composition. . The combination of drawing the pitch yarn while collecting it on the bobbin greatly simplifies operation and eliminates many expensive parts of the equipment. The bobbin wound with the sized pitch yarn is then heat treated in a substantially non-reactive atmosphere to produce carbon yarn.
In contrast to the prior art, which required heat treatment in oxygen or air or the like before heat treatment in a substantially non-reactive atmosphere, pitch yarn according to the present invention requires heat treatment in an oxidizing atmosphere. Not needed at all. Carbon yarns made in accordance with the present invention provide more efficient precursor utilization than carbon yarns made in accordance with the prior art. Prior art infusibility processes introduce significant amounts of oxygen into the pitch yarn, on the order of 18% by weight or more. During carbonization heat treatment,
It is expected that some of the oxygen that is driven off will escape with the carbon atoms. As a result, carbon yarn produced according to prior art methods is less than 18% by weight of pitch yarn. In contrast, carbon yarn made in accordance with the present invention is approximately 90% by weight of pitch yarn. Thus, the present invention provides higher product yields than the prior art in addition to simplifying the operations required to produce carbon yarn. The oxidizing liquid composition can serve many functions in addition to its use in thermal processing. The composition can provide pitch yarn lubrication to minimize friction between the pitch yarn and the parts of the equipment that contact the pitch yarn during manufacturing operations.
The composition can also provide adhesion between the fibers so that the fibers stay together as yarns. In a preferred embodiment, the oxidizing liquid composition comprises aqueous nitric acid. Concentrations of aqueous nitric acid of 10-50% by volume are preferred, but concentrations of 15-35% by volume are more preferred. Preferably, deionized water is used in the aqueous nitric acid to avoid the introduction of undesired ions into the pitch fibers. Aqueous nitric acid is relatively cheap;
It was found to be excellent in obtaining carbon yarn. The concentration of nitric acid depends on how long the nitric acid is in contact with the pitch yarn before carrying out the heat treatment. A concentration of about 25% by volume is suitable for industrial operations where the time between application of nitric acid to pitch yarn and heat treatment is 1 to 5 days. For oxidizing liquid compositions, see U.S. Pat. The former patent states that the present invention "in processing a multifilament bundle of pitch fibers, such as yarn or tow, to prepare such multifilament bundle for further processing, By applying a dispersion obtained by dissolving a first compound consisting of a water-soluble oxidizing agent and another second compound consisting of a water-soluble surfactant in a dispersion in which graphite or carbon black is dispersed in water. ``Provides a method for processing multifilament bundles of pitch fibers''. Other patents feature water-soluble surfactants that can also function as oxidizing agents. Both of these patents use the term "oxidizing agent" as a source of oxygen to the fibers to infusible them. As used herein, the term "oxidizing liquid composition" includes an oxygen source to infusible the fibers. If necessary, reference is made to the disclosures of these patents. The oxidizing liquid composition can include a water-soluble oxidizing agent such as an aqueous acid or a peroxygenated compound. Some examples of water-soluble oxidizing compounds include sodium peroxide, potassium peroxide, ammonium peroxide,
Mention may be made of sodium persulfate, potassium persulfate, ammonium persulfate, sodium pyrosulfate and sodium nitrate. Preferably, aqueous nitric acid is used in the composition. The oxidizing liquid composition of the present invention may also include carbon black particles or colloidal graphite and a surfactant. The carbon black particles or colloidal graphite tend to separate pitch fibers from one another, thereby minimizing the occurrence of "sticking" or fusing between fibers during heat treatment in accordance with the present invention. be. One of the functions of the surfactant is to maintain a dispersion of carbon black particles. Mechanical means according to conventional techniques can be used to maintain a homogeneous dispersion of carbon black particles dispersed in the composition. In addition, surfactants are
Improves the flow of the composition onto the fibers. Surfactants may be water soluble and may be anionic or nonionic. Such surfactants are well known and typically include sodium tetramethyl oleate, tetramethyl ammonium oleate, sodium tetramethyl laurate, tetramethyl ammonium laurate, sodium laurate, and lauric acid. Examples include ammonium acid. The oxidizing liquid composition can be applied to pitch fibers using conventional methods for applying sizing. Preferably, the composition is applied by contacting the pitch yarn with a rotating wheel that passes through the solution and conveys a portion of the solution on its surface to the pitch yarn. Such a wheel is known as a “kiss wheel” in this field.
wheel) and rotates to carry fresh solution to the yarn and to minimize friction with the yarn. After the kissing wheel, the yarn can be deposited for subsequent processing. Composition The composition can be applied to the yarn by passing the pitch yarn through a bath of the composition. This has drawbacks as the high velocity may result in damage to the fibers by the chemicals in the bath. Other ways to apply things to pitch yarn are:
To improve the distribution of the composition to the fibers, a spray of the composition is applied to the pitch fibers before the fibers are brought together to form a yarn. The yarn can be wound onto the bobbin using a water size to the pitch fibers and spraying the layer of yarn to be produced on the bobbin with a spray of the composition. Alternatively, the composition can be applied to the pitch yarn after it has been deposited. For example, pitch fibers can be wound onto a bobbin and the composition applied to the yarn together with the bobbin. This can be done simply by placing the bobbin in a bath of composition. In this case, by winding the yarn relatively loosely, the composition can flow easily through the winding, thereby improving processing. A bobbin with multiple holes may be used to force the composition under pressure from the core to the outer layer using a suitable arrangement such that fibers near the bobbin are not deliberately omitted from processing. It has the benefit of being able to.
For this process, it is desirable to use some sizing on the pitch yarn before winding it onto the bobbin. It is convenient to use water for this sizing. A suitable bobbin is described in US Pat. No. 4,351,816. The bobbin can be shaped like a cylinder or a cylinder with end faces. The bobbin is subjected to elevated temperatures to carbonize the yarn on the bobbin. Therefore, the bobbin should be made of a material suitable for practicing the invention. Preferably, the bobbin consists of a cylinder made of stainless steel, refractory alloy, ceramic, boron nitride or preferably graphite material. That is, the bobbin is made of a material that is thermally and mechanically stable at the temperatures used to carbonize the pitch yarn on the bobbin. U.S. Pat. No. 4,351,816, cited above, describes a method for making mesophase pitch-based carbon yarns in which thermoset yarns are wound onto bobbins as described above and heat treated in a substantially non-reactive atmosphere to produce carbon yarns. ing. The pit yarn contacted with the oxidizing liquid composition reacts and combines with oxygen. Tests were conducted to investigate the range of oxygen binding in pitchyarn over a period of 0.1 to 70 hours. The yarn had 2000 pitch fibers with an average diameter of 13.5 microns. Nitric acid with a concentration of 25% by volume was used. After contacting the yarn with nitric acid, the scheduled time at room temperature was allowed to elapse, after which the yarn was washed with water for this test and dried for 16 hours at 125° C. before testing for oxygen content. Surprisingly, the range of oxygen bonds is from 1.5 to 0.1 to 7.0 hr
4.8% by weight, and most of the oxygen binding took place during the first 24 hours. The test points are essentially
The following relationship is defined: Oxygen bond (wt%) = 1.2355 log (hr) + 2.5278. Thus, fluctuations in pitch yarn after contact with nitric acid at this concentration are not expected to have a significant impact on industrial operations.
That is, the treated pitch yarn can be stored prior to carbonization. This is beneficial in industrial manufacturing. The heat treatment of the treated pitch yarn can be carried out in batches in closed furnaces or as a continuous process, for example using conveyor belt furnaces or so-called "walking beam furnaces" in which the bobbins can be continuously moved in and out of the furnace. Can be done. The furnace should be able to provide sufficient heat to pyrolyze the yarn and maintain a substantially non-reactive atmosphere so that the yarn is not consumed. The non-reactive atmosphere inside the furnace is nitrogen,
It may be argon, helium or the like. about
For temperatures above 2500°C, argon and helium are preferred. Preferably, the heat treatment is carried out in a completely non-reactive atmosphere, which is achieved by thoroughly purging the furnace. Small amounts of oxygen do not appear to be harmful, especially if the temperature is not raised too quickly. It will be appreciated that the wet yarn from treatment with the oxidizing liquid composition creates an atmosphere of steam that must be purged before reaching a temperature at which the steam is no longer substantially reactive. Boron or similar graphitizing materials can be used in the furnace atmosphere and are therefore considered non-reactive when used in the present invention. In practicing the invention, air was purged from the furnace before raising the temperature of the yarn. The purging step can be carried out by applying reduced pressure inside the furnace and then filling the inside with nitrogen. The heat treatment according to the invention has three broad ranges that are important in determining the heating schedule for the rate of temperature increase. The rate of temperature increase to about 400°C should take into account that the pitch fibers do not reach a fully infusible state until about 400°C. 400
If the temperature rises too quickly to ℃, softening,
Deformation of the fibers may occur due to fusion between fibers and/or deorientation of mesophase molecules. Temperature increases above 400°C may be at high rates, but taking into account that most of the gas loss in the pyrolysis or carbonization process occurs when heating the fibers between about 400 and about 800°C. I have to put it in. Rising too quickly can cause damage due to the gases generated. The temperature increase above 800°C may be as large as desired. Typically, the final temperature is between 1300 and 2700°C depending on the intended use of the carbon yarn. Generally, the heat treatment according to the present invention is carried out in a substantially non-reactive atmosphere and the temperature is about 100°C.
It can be raised from room temperature to 800°C at a rate of °C/hr. Thereafter, the temperature can be increased at the desired rate to the intended final temperature. The rate of increase in temperature to 400° C. depends in part on the sizing used, the contact time between the pitch yarn and the oxidizing liquid composition, the softening point of the pitch, the diameter of the fibers, and the composition of the pitch. Preferably, the furnace heating schedule is approximately 400 m
25°C/hr to about 800°C, then 50°C/hr to about 800°C. Thereafter, the temperature can be increased at the desired rate to the intended final temperature. Preferably, the pitch yarn is deposited on a refractory bobbin. This has the advantage that after heat treatment the yarn has become a dispersion suitable for storage and local transportation for industrial use. The yarn can be wound onto the bobbin with orthogonal or essentially parallel windings. The tension is
It can be monitored and controlled according to conventional techniques. This is particularly important for cases where stretching is carried out at the same time as winding onto the bobbin. Example 1 A mesophase pitch having a mesophase content of about 78% by weight and a Mettler softening point of about 324.6°C was spun into 2000 filaments according to conventional spinning techniques. The average diameter of stretched Pituchi fibers is approximately 13
They made it micron and then pulled all the fibers together to make pitch yarn. Nitric acid with a concentration of 20% by volume was applied to the rapidly moving pitch yarn using two rotating kiss wheels. The acid-treated yarn was wound onto a motor-driven rotating graphite bobbin. This bobbin has
It provided the force necessary to pull the fibers and support the movement of the pitch yarn. The graphite bobbin also had a layer of graphite felt on its surface for receiving yarn. The bobbin also had a length of 28 cm and an outer diameter of about 9 cm. The yarn was wrapped in an orthogonal pattern with a tension of approximately 85 g. The bobbin after each winding is approx.
It had a diameter of 14 cm and had approximately 2.5 Kg (dry weight) of yarn. The composition of the yarn, based on the dried sample, is as follows. Carbon 81.5% Hydrogen 3.3% Sulfur 0.8% Nitrogen 2.8% Oxygen 11.6% The yarn on the bobbin was heat treated in a nitrogen atmosphere in a furnace. The furnace was purged with nitrogen for 4 hours before increasing the temperature. The temperature of the furnace was increased at a rate of 100°C/hr from room temperature until reaching a temperature of 1300°C and this temperature was maintained for 2 hours. Thereafter, the furnace was cooled down to room temperature. Analysis of the carbon yarn produced by heat treatment showed that the carbon fibers were approximately 90.5% by weight of the original dry pitch compared to the original dry pitch fiber. In addition, the composition of this carbon yarn was determined to be 98.0% carbon, 0.1% hydrogen, 0.8% sulfur, 0.5% nitrogen, and 0.5% oxygen. Mechanical properties of the yarn were measured and the average carbon fiber had a tensile strength of 1.54 GPa and a Young's modulus of about 152 GPa. The carbon fibers obtained have a very high carbon content and such carbon fibers are found to be useful in the production of composites made of carbon yarns. Example 2 The carbon yarn produced in Example 1 had traces of a winding pattern as a result of shrinkage during heat treatment. To remove these traces and improve the carbon yarn, the carbon yarn from Example 1 was placed in a threadline carbonization furnace with a temperature of 2400°C. The carbon yarn was unwound and stretched between rolls along a straight path through a nitrogen purge seal to the carbonization furnace and out through a nitrogen purge seal. The furnace was heated by electrical resistance. Threadline carbonization is well known in the art. For example, U.S. Pat.
Please refer to No. 4301136. The carbon yarn was then sized, dried and wound onto a second spool for storage. The mechanical properties of the obtained carbon fibers include an average fiber tensile strength of 2.03GPa and
The average fiber Young's modulus was 434 GPa. The fiber is
It had an excellent appearance similar to carbon fiber produced by expensive conventional methods. Example 3 In a conventional spinning apparatus as used in Example 1, modified to produce 1000 pitch fibers with different spinnerets, a mesophase content of 80% by weight and a Mettler softening point of about 325°C. 1000 pitch fibers were produced using the mesophase pitch having the following properties. The pitch fibers were coated with the oxidizing liquid solution using a rotating graphite kissing wheel. This solution is
25% by volume nitric acid, 75% by volume deionized water and 20% by volume
g/g of carbon black.
The rotational speed of the kissing wheel was adjusted so that the pitch fiber adsorbed an amount of solution equal to approximately half its weight. The pitch fibers were drawn from the spinning apparatus by rotating a rotating mandrel (winder) to direct the pitch fibers onto the mandrel such that the pitch fibers were drawn to an average diameter of about 13 microns. This mandrel is 28cm long, 9cm in diameter and
It was a graphite bobbin with a wall thickness of 0.6 cm.
A 0.6 cm layer of carbon felt was wrapped around the bobbin core. The pitch yarn was wound in a cross-wound pattern. The yarn is approx.
It had a linear velocity of 250 m/min. Each bobbin is
It had pitch yarn of 0.5-1.0Kg. The bobbin with the damp pitch yarn was placed in a closed plastic container at a temperature of about 30° C. for 24 hours, after which time it was removed from the plastic bag and placed on a graphite track. The rack along with the bobbin was then placed in an electric induction furnace. The furnace was purged with nitrogen for 4 hours to ensure that a substantially non-reactive atmosphere was present for carbonization. Additionally, nitrogen was continuously supplied to the furnace during the heat treatment to create a non-reactive atmosphere. Temperature 400℃ at 25℃/hr
The temperature was then increased to 800°C at 50°C/hr, and finally the temperature was increased to 1300°C in about 1 hour. The final temperature was maintained for 2 hours and the furnace was allowed to cool back to room temperature. The carbon yarn produced according to this example was extremely soft and easily removed from the bobbin. This indicates that no fusion occurred between the yarn sections. Analysis of the carbon yarn using scanning micrographs showed that less than about 5% of the carbon fibers had some attachment to adjacent fibers. Carbon yarns produced by prior art methods typically include:
Indicates a level of 10% or higher. The carbon yarn in this example exhibited cross-wound impressions. As in Example 2, using a conventional thread line furnace with a temperature of 2400°C, the line speed was approximately
12 m/min and the line tension was about 300 g.
When the fiber is straightened, its mechanical properties are
It exhibited an average fiber tensile strength of 2.34 GPa and an average fiber Young's modulus of 407 GPa. Example 4 The mesophase pitch and process steps used in Example 3 were repeated, except that the oxidizing liquid solution contained 0.001% of a surfactant sold by DuPont as "FC-170C". The carbon yarn obtained from the furnace heat treatment was excellent and had almost no interfiber sticking. Subsequent threadline heat treatment at 2400°C resulted in carbon yarns with an average fiber tensile strength of 2.55 GPa and an average Young's modulus of 345 GPa. Comparative Examples A to C: Approximately 13 microns in diameter according to conventional methods.
A mesophase pitch yarn with 2000 pitch fibers was made and heat cured. The thermoset yarn was collected on a bobbin made from grain graphite and having an inside diameter of about 3 inches, a length of about 11 inches, and a layer of carbon felt about 1/4 inch thick. This bobbin had no collar and the winding tension was about 150 g.
A winding angle of approximately 20° was used. For each example sample, the yarn collected was approximately 6000 ft long, and the pyrolysis and carbonization process involved increasing the temperature in a nitrogen atmosphere at a rate of approximately 50°C/hr until it reached 1300°C. This was carried out by keeping the temperature for about 2 hours. The temperature was returned to room temperature and the pyrolyzed yarn was passed through a thread line carbonizer having a nitrogen atmosphere and a furnace temperature of approximately 2400° C. to further carbonize the yarn. The average line tension in the carbonizer was about 800 g. Tensile strength and Young's modulus (modulus) for each example
are shown in Table 1. Table 1 below also shows the tensile strength and modulus properties of Examples 3 and 4 of the invention as well as the properties of Comparative Examples A to C.

Table 1 It is clear that the tensile strength properties of carbon fibers produced according to the method of the present invention are generally greater than those produced by methods taught in the prior art. As mentioned above, the prior art teaches that the fibers can be further heat treated to complete the oxidation by heating the fibers. A portion of the heating cycle takes place in oxygen.
The heating step in this prior art is carried out only to temperatures of 1000° C. and by heating individual fibers suspended in a gas stream. No attempt has been made to heat the acid-treated fiber after it has been wound onto a spool. The difficulty and danger of heat treating carbon fibers is illustrated by the following Examples D-F. Comparative Example D A pitch fiber yarn with 2000 filaments was spun from mesophase pitch. Fiber is 2lb
extrusion speed of 12lb/hr to obtain a total weight of
The yarn was spun for 10 minutes at a take-up speed of 852 ft/min. The fibers were wound at a small orthogonal angle onto a graphite bobbin covered with a 1/4 inch thick carbon felt pad to obtain a 3.5 inch diameter. The final spool or package of fiber had an outside diameter of 5 inches. The spools were stored in plastic bags to maintain their moisture content during storage. The spool was placed in an air circulation oven partially surrounded by a metal shield to avoid direct air flow and then heated at an average rate of 2.4°C/min to a temperature of 400°C and held for 2 hours. After cooling, when the furnace was opened, most of the yarn on the spool melted, flowed to the bottom of the furnace and carbonized to form a porous body, while the remaining fibers on the spool were melted. And a carbon form was formed. As illustrated by Example D, the tendency of pitch fibers to melt easily makes it difficult to heat the fibers in bulky form on the bobbin without first performing a heat curing operation on the fibers. Make it. Comparative Example E A mesophase pitch having 2000 filaments was spun from a mesophase pitch. The fibers were extruded at an extrusion rate of 12lb/hr to obtain a total fiber weight of 3.7lb.
It was spun for 18.5 minutes at a winding speed of 852 ft/min. Aqueous nitric acid (25
% by weight) and 30 grams/carbon black was applied to the fibers, thus adding 2.4 lbs to the final weight of the pitch fibers. The fiber is
It was wound at a small orthogonal angle onto a graphite bobbin covered with a 1/4 inch thick carbon felt pad to obtain a 3.5 inch diameter. The final spool or package of fibers tapered to 10 inches at the bottom and 4 inches at the top, and had an outside diameter of 6.5 inches. Spin dry the spool to reduce the liquid content to approximately 9% by weight.
then placed in an air circulation oven partially enclosed with a metal shield to avoid direct air flow, then heated to a temperature of 400 °C at an average rate of 3.7 °C/min and kept for 2 hours. . After cooling, the furnace was opened and the yarn on the spool was found to have disintegrated into a black fuzzy dust material. Additional tests were carried out by winding heat-cured fibers on a bobbin in the conventional manner and heating them in an oxidizing atmosphere as in Comparative Example E, which showed that fiber combustion and complete loss of the fiber package were detected. Brought to you. Thus, infusible yarns in bulky form, whether obtained by conventional methods or by treatment with nitric acid, can be heated without baking in an oxidizing atmosphere. It has been shown that this is not possible. As-spun or “untreated” pitch yarn wound onto a bobbin is
When subjected to these same oxidative heat treatments, they underwent global melting. As mentioned above, the prior art teaches that pitch fibers can be thermally cured in an oxidation process carried out by heating in air. This method involves spreading the fibers, for example on a wire mesh or net belt conveyor, to allow air to flow between the fibers to control the fiber temperature by removing the heat of reaction generated during oxidation of the pitch fibers. Therefore, it is preferable to carry out this method. In the following comparative example, such precautions were taken in an attempt to oxidize pitch fibers in a bulky form. Comparative Example E A mat of pitch fibers was made by blow-spinning from mesophase pitch having a softening temperature of 365°C. This is approximately
It had a thickness of 1.5 inches and an areal density of 0.1 lb/ft ² pine. Matsuto's 9in×
The 12-inch section was placed in a stainless steel wire mesh basket, placed in a circulating hot air oven, and the heated air
It was confined to be forced through the pine at an average speed of 3.6 ft/min. The initial temperature of the furnace is 165
The temperature was increased at a rate of 250°C/hr. The furnace and fiber mat temperatures were monitored.
After about 1 hour at a furnace temperature of approximately 425°C, the temperature of the fiber suddenly rose to a peak temperature of about 675°C,
This indicates that the oxidation reaction of the pitch fibers generated heat beyond control. Most of the fiber was consumed in uncontrolled oxidation. It is thus clear that heating pitch fibers must be carried out with considerable care in order to prevent the fibers from melting or oxidizing and being destroyed. As shown by Comparative Example F, even when heating is carried out under conditions that promote the thermosetting reaction by providing adequate air flow to remove excess heat, the exotherm Oxidation can become uncontrolled, probably due to the presence of hot spots that initiate rapid oxidation. In the prior art, such as in the South African patent mentioned above, this problem is avoided by heating only the single fibers or yarns by suspending them in a gas stream or by thread line processes. Heating of individual fibers in air can be done with good temperature control and without oxidative losses, and if the heating rate is controlled the fibers can easily be thermoset without being melted. It will become clear. However, as shown by the comparative examples, heating the pitutsi fibers to bulky form is difficult due to the complete difficulty of temperature control and also due to fiber melt fracture and uncontrolled exothermic oxidation reaction. It has long been taught that this should be avoided due to the potential for destruction. In carrying out the method of the invention, the fibers are first treated with aqueous nitric acid and then heated after winding onto a suitable bobbin to carbonize the pitch fibers (in the substantial absence of an oxidizing atmosphere). According to the knowledge in the art for the production of pitch-based carbon fibers at the time of the present invention (as illustrated by Comparative Examples D-F), such methods are quite dangerous and result in destruction of pitch fibers. It is expected that
The success of the method of the present invention in producing pitch-based carbon fibers is therefore unexpected and surprising. As previously mentioned, prior art methods of oxidizing pitch fibers in air introduce very high levels of oxygen into the fibers, on the order of 18% by weight or more. When carbonizing this oxygen-containing fiber, the fiber loses more than 20% of its weight. Therefore, the yield of fibers decreases. As previously mentioned, in the process of the present invention the amount of oxygen in the fiber is typically less than 5% by weight and the fiber retains more than 90% of its weight upon carbonization. Thus, the yield of fibers produced by the method of the invention is considerably improved over the prior art. It will also be appreciated that large weight losses during the carbonization process result in severe shrinkage, thus producing thinner yarns.

Claims (1)

[Claims] 1. Mesophase pitch is spun to produce a plurality of continuous pitch fibers, the plurality of continuous pitch fibers are combined to produce pitch fiber yarn, and the pitch fiber yarn is prepared from 15 to 25% by weight aqueous nitric acid. the treated yarn is treated with an aqueous oxidizing liquid composition, the treated yarn is wound onto a bobbin, and the pitch yarn wound on the bobbin is treated with a substantially non-containing gas formed from a gas selected from nitrogen, argon, helium, and mixtures thereof. A process for producing carbon yarn consisting of steps in which carbon yarn is produced by heat treatment to a final temperature above 1300°C in a reactive atmosphere. 2. The method according to claim 1, wherein the oxidizing liquid composition further contains graphite or carbon black. 3. The method according to claim 1, wherein the oxidizing liquid composition further comprises an aqueous surfactant.