PROCESS FOR PRODUCING CARBON FIBERS

Abstract: There is disclosed a process for producing a carbon fiber, comprising the steps of spinning a fiber, thermosetting the fib­ er, and thereafter carbonizing the fiber, in which the fiber is cleaned subsequent to the thermosetting step by subjecting the dry fiber to high frequency mechanical vibrations.

Description: DESCRIPTION PROCESS FOR PRODUCING CARBON FIBERS The invention relates to a process for producing carbon fibers and in particular to a process for cleaning the fibers. Generally, the conventional process for producing a PAN or pitch derived carbon fiber comprises the steps of spinning a fiber from polyacrylonitrile (PAN) or mesophase pitch, infusibilizing the spun fiber by heating it in air, and carbonizing the infusible fiber by heating it in an inert gaseous environment. It is common practice to refer to the infusibilizing step as a ',thermosetting step" and the infusible fiber is referred to as a "thermoset fiber". Generally, high quality commercial carbon fibers are Made from PAN and mesophase pitch. In commercial practice, several thousand fibers are spun and combined into a bundle referred to as ',yarn11. The pitch yarn comprises a plurality of pitch fibers whereas PAN yarn comprises a plurality of PAN fibers. The subsequent operations are carried out on the yarn to produce a carbon yarn comprising a plurality of carbon fibers. A mesophase pitch yarn typically can have 2,C00 fibers. Batch fiber typically has a diameter of about 14 microns or less. It has been found that during the commercial production of mesophase pitch derived carbon yarn hot spots developed within a pitch yarn during the thermosetting step and as a result individual fibers melted or softened before they became thermoset. These fibers tended to adhere to other fibers and the yarn became stiff and brittle. The mechanical properties of the carbon yarn crcdced sunder sue circurstances are relatively poor. The U.S. Patents No. 4,275,51 and o. 4,276,278 relate to methods for overcoming these problems which can arise during the thermosetting operating. Generally, these methods feature depositing graphite or carbon black particles on the surfaces of the pitch fibers prior to the thermosetting step. The disclosures of these patents are incorporated herein by reference. The presence of carbon black particles during the thermosetting step improved the quality of the carbon yarn obtained but resulted in a new problem. Many commercial uses of carbon yarn take the form of composites. The presence of the particles on the carbon fibers of the carbon yarn resulted in poor end products for certain composites. The search for a process for removing the particles from the fibers started with a review of the prior art and an evaluation of conventional methods. The prior art method of cleaning objects with ultrasonic vibrations was found to be satisfactory for cleaning and removing particles from the fibers but presented serious drawbacks. Conventional ultrasonic cleaning is carried out by having an ultrasonic source of mechanical energy in a liquid medium containing the object to be cleaned The liquid medium communicates the ultrasonic vibrations from. the ultrasonic source to the object. The art teaches that the ultrasonic cleaning is accomplished by the phenomena called "cavitation" and a liquid medium is required to produce cavitation. One problem with the prior art ultrasonic cleaning as applied to cleaning yarn is that the yarn must be dried after being cleaned. It would be undesirable to ship wet yarn and it is particularlJ important to have dry yarn if the yarn is to be carbon end or heat treated additionally. The drying step requires an oven, energy to operate the oven, additional monitoring, and floor space in the manufacturing area. Another problem with the prior art ultrasonic cleaning as applied to cleaning yarn is that at production line speeds suitable for commercial operations, the fibers in the yarn become damaged or broken so a poor quality carbon yarn is produced. After studying this problem, it was realized that the fibers uniquely lend themselves to a simple solution for cleaning the yarn. According to the present invention therefore there is provided a process for producing a carbon fiber, comprising the steps of spinning a fiber, thermosetting the fiber, and carbonizing the fiber; and in which the fiber is cleaned subsequent to the thermosetting step by subjecting the dry fiber to high frequency mechanical vibrations. Instead of using a liquid medium, an ultrasonic source was applied directly to the dry yarn. Surprisingly, the fibers become substantially clean and were not appreciably damaged by the heat produced. For mesophase pitch derived fibers, the cleaning step can be carried out subsequent to the fibers being thermoset. In commercial practice, the thermoset fibers are usually subjected to a coating step and the coating or "finish" can interfere with the effectiveness of the cleaning step according to the invention. It happens that the commercial practice for carbonizing the mesophase pitch derived thermoset yarn is advantageously carried out in two stages. The thermoset yarn is subjected to a first heat treatment at a temperature of about iC0 0C in an inert atmosphere by the yarn being pulled through a heat chamber. The first heat treatnent substantially removes the finish and is sometimes called in the art t?precarbonization", even though it is a carbonization step. The yarn, after the first heat treatment, is much stronger and can be handled with considerably less care. Subsequently, the yarn is subjected to a second heat treatment at a temperature of from about 15000C to about 30000C in an inert atmosphere to produce the final product, carbon yarn. Neither of the heat treatments remove the carbon black particles which were deposited on the fibers to improve the thermosetting step. It is preferable to use the instant cleaning step between the first and second heat treatments because the yarn is free of the finish, and because the yarn has good mechanical properties so higher speeds and more intense cleaning can be carried out. The excellent results obtained for the mesophase derived yarn suggested the larger scope of the invention. The same cleaning operation can be used for PAN derived yarns because these yarns have performance comparable to the mesophase pitch derived yarn. Although ultrasonic vibrations have been found to be highly effective, generally the instant invention is the use of high frequency mechanical vibrations applied directly to a substantially dry yarn or fiber. The fiber can be meso?h2se pitch derived or PAN derived and preferably the mechanical vibrations are in the ultrasonic frequency r2nze. Of course, a yarn comprising a plurality cf fibers having dIameters of about 14 microns, typically about 2,000 fibers, can be used instead of a single fiber. Preferal, the carbonization is carried out in two stages so that the cleaning step can be carried out subsequent to the precarbonization step. Preferably, the yarn after a first heat treatment, precarbonized yarn, is cleaned according to the invention immediately prior to the yarn entering into a chamber for carrying out the final carbonization step. The yarn is extended generally along a linear path and presses against the horn of an ultrasonic source with an effective force of from about 10 to about 25 pond. Preferably, the ultrasonic source produces mechanical vibrations having an ultrasonic frequency of about 20,000 Hertz. The range of suitable amplitudes and frequencies of the mechanical vibrations for cleaning according to the invention can be determined easily experimentally for the particular yarn being used. Some of the factors to be considered for yarns are the degree of cleaning which is acceptable, the speed at which the yarn is to be moved, the amount of yarn damage which can be tolerated, and the economics of the cleaning operation. Further advantages of the invention will be set forth, in part, in the following specification and, in part, will be obvious therefrom without being specifically referred to, the same being realized and attained as pointed out in the claims hereof. For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description, taken in connection with the acompanying drawings, in which: The sole Figure shows a diagrammatic side view of components carrying out the instant invention. The bes mode for carrying out the instant invention is shown in the Figure. A mesophase pitch derived yarn 1 which has been thernoset exits a precarbonizing unit 2 which is 0 operating at a maximum temperature of about 1300 C. The yarn 1 has 2,000 fibers and each fiber has a diameter of about 10 microns. Ultrasonic sources 3, 4, and 6 produce mechanical vibrations and clean the yarn 1 in accordance with the invention. The yarn 1 moves at a rate of about 20 meters per minute and has a tension of about 55C pond. Each of the ultrasonic sources 3, 4, and 6 has a frequency of about 20,000 Hertz and is a commercially available model. Each of the horns 7, 8, and 9 is the active output element of the respective ultrasonic sources 3, 4 and 6 and each horn has a surface about 1.3 centimeters long in contact with the yarn so that the residence time for the yarn 1 on each horn is about 0.04 second. For each of the horns 7, 8, and 9, the yarn 1 is pressed with a force of from about 10 to about 25 pond. There are two important matters as to the instant invention. One is the degree of cleaning and the other is the degree of adverse influence on mechanical properties. The yarns cleaned by the invention showed significant improvements in the absence of particles as compared to yarns not cleaned. In addition, a carbon yarn which had been cleaned according to the invention possessed tensile strength and Young's modulus comparable to those of a carbon yarn which had not been cleaned at all. This is important because it shows that the invention does not degrade these two important mechanical properties. In a typical case, a carbon yarn made according to the Invention as described for the Figure, had fibers having an average tensile strength of 2.23 GPa and an average Young's modulus of 632 GPa whereas the carbon yarn which had not been cleaned at all had correspondnz average values of 2.24 GFa and 600 GPa. A higher degree of cleaning of the yarn can be obtained for predetermined output levels of the ultrasonic sources by increasing the number of ultrasonic sources and/or increasing the pressure of the yarn against the horn. Commercially available ultrasonic sources usually have preset output levels. The cleaning step was carried out in a separate experiment using a pressing force of about 50 pond for each of the three horns instead of from about 10 to about 25 pond. The fibers of the carbon yarn produced had a tensile strength of about 2.25 GPa and a Young's modulus of about 612 GPa. These results also compared favorably with those obtained for uncleaned yarn and show that considerable latitude exists for selecting the pressure. It is interesting that the particles being mechanically removed from the yarn during the cleaning step according to the invention appear in the form of visible clouds in the regions of the respective horns. Removal of the particles from the cleansing region can be accomplished with a suction device or the like. The largest cloud was present at the first horn, horn 7. When the ultrasonic sources 3, 4, and 6 were turned off, the clouds were no longer fored and the yarn developed fraying damage which seriously degraded the yarn. Actually, such fraying would be erected; however, it is surprising that when the ultrasonic sources 3, 4, and 6 are operating, the frayIng does not occur. We wish it to be understood that we do not desire to be elicited to the exact details of ccnstruction shown d described, for obvious odifications will occur to a person slllcd in the art. Havln" thus described the invention what we claim is new and desire to be secured by Letters Patent, is as follows:

Claims: CLAIMS 1. A process for producing at least one carbon fiber, comprising spinning at least one fiber, thermosetting the fiber, and carbonizing the fiber, characterised by cleaning the fiber subsequent to the thermosetting step, by subjecting the dry fiber to high frequency mechanical vibrations in order to remove particles to the surface of the fiber. 2. A process as claimed in claim 1, characterised in that said fiber is either polyacrylonitrile derived or pitch derived. 3. A process as claimed in claim 1, characterised in that said fiber is mesophase pitch derived. 4. A process as claimed in any one of the preceding claims, characterised in that a plurality of fibers are spun and combined to form a yarn. 5. A process as claimed in any one of the preceding claims, characterised in that at least one ultrasonic source is used to produce the high frequency mechanical vibrations. 6. A process as claimed in claim 4, characterised in that said ultrasonic source has a frequency of about 20,000 Hertz. 7. A process as claimed in any one of the preceding claims, characterised in that a plurality of fibers are spun and combined to form a yarn, and at least one ultrasonic source is used to produce the high frequency mechanical vibrations. 8. A process as claimed in claim 7, characterised in that said yarn is carbonized by a first heat treatment to about 13000C and subsequently by a second heat treatment to a temperature in the range of from about 15000C to about 30000C and said cleaning step is carried out between said first and second heat treatments. 9. A process as claimed in any one of claims 1 to 6, characterised in that a plurality of fibers are spun and combined to form a yarn and further comprising the step of coating a portion of the surfaces of said fibers with graphite or carbon black particles prior to said thermosetting step.

Inventors: BROOKSTEIN, DAVID STUART, SCHULZ, DAVID ARTHUR
Applicants: UNION CARBIDE CORPORATION
Application year: 1982
Main classification: D01F9
IPC codes: D01F9, D01D11