JPS628521B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for cleaning fibers derived from polyacrylonitrile (PAN) and pitch, and more particularly to cleaning the fibers using high frequency mechanical vibration.

Generally speaking, the usual method for producing carbon fibers derived from PAN or pitch is to spin fibers from PAN or mesophase pitch and infusible by heating the spun fibers in air. , and carbonizing the infusible fibers by heating in an inert gas environment. The infusible process is commonly referred to as a "thermosetting process" and the infusible fibers are referred to as "thermosetting fibers."

Generally, high quality technical carbon fibers are made from PAN and mesophase pitch. In industrial methods, thousands of fibers are spun into bundles called "yarns." Pitch yarn consists of multiple pitch fibers, whereas PAN yarn consists of multiple PAN fibers. Subsequent operations are performed on the yarn to produce a carbon yarn comprising a plurality of carbon fibers. Mesophase pitch yarns can typically have 2000 fibers. Each fiber typically has a diameter of about 14 microns or less.

Thus, in the present invention, in the industrial production of mesophase pitch-derived carbon yarns, hot spots can occur in the pitch yarn during the heat setting process, so that the individual fibers can melt or soften before they are heat set. Divided. These fibers tended to stick to other fibers and the yarn became stiff and brittle. The mechanical properties of carbon yarn produced under such conditions are relatively poor.

US Pat. No. 4,275,051 and US Pat. No. 4,276,278 relate to methods for overcoming these problems that can occur during heat curing operations. In general, these methods
The method is characterized in that graphite or carbon black particles are deposited on the surface of pitch fibers prior to the heat curing process. If necessary, reference is made to the disclosures of these patents.

The presence of carbon black particles during the heat curing process improved the quality of the resulting carbon yarn but posed new problems. Many industrial applications of carbon yarn take the form of composites. The presence of particles on the carbon fibers in the carbon yarn resulted in poor final products for certain composites.

Research into methods for removing particles from fibers began with a review of the prior art and consideration of conventional methods.

Prior art methods of cleaning objects by ultrasonic vibration have been found to be satisfactory for cleaning and removing particles from fibers, but have exhibited significant drawbacks. Conventional ultrasonic cleaning is carried out by placing an ultrasonic source of mechanical energy in a liquid medium containing the object to be cleaned. The liquid medium transmits ultrasonic vibrations from the ultrasonic source to the object. The prior art teaches that ultrasonic cleaning is accomplished by a phenomenon called "cavitation" and that a liquid medium is required to cause cavitation.

One problem with applying prior art ultrasonic cleaning to yarn cleaning is that the yarn must be dried after cleaning. It is undesirable to transport wet yarn, and it is especially important that the yarn be dry if the yarn is to be additionally carbonized or heat treated. The drying process requires a furnace, energy to operate the furnace, additional supervisory control and floor space in the manufacturing area. Another problem with applying prior art ultrasonic cleaning to yarn cleaning is that the production line speeds preferred for industrial operations can leave the fibers in the yarn in a damaged or broken state, resulting in poor quality. Carbon yarn is produced.

After researching this problem, it was found that fibers are uniquely amenable to simple solutions for cleaning yarns.

Instead of using a liquid medium, an ultrasound source was applied directly to the dry yarn. Surprisingly, the fibers were substantially cleaned and not appreciably damaged by the heat generated.

For mesophase pitch-derived fibers, the purification step can be carried out after heat curing the fibers. In industrial processes, thermoset fibers are commonly subjected to a coating step, but this coating or "finish" may interfere with the effectiveness of the cleaning step in accordance with the present invention.

The industrial process for carbonizing mesophase pitch-induced thermoset yarns is advantageously carried out in two stages. The thermoset yarn is subjected to a first heat treatment at a temperature of about 1300° C. in an inert atmosphere while drawing the yarn through a heating chamber. Since the first heat treatment substantially removes the finished surface, it is often referred to in the prior art as "pre-carbonization" even though it is a carbonization step. The yarn after the first heat treatment becomes gradually stronger and can be handled without much care. The yarn is then subjected to a second heat treatment at a temperature of about 1500 to about 3000<0>C in an inert atmosphere to produce the final product, carbon yarn.

None of the heat treatments remove the carbon black particles deposited on the fibers to improve the heat curing process.

The purification step of the present invention is preferably used between the first heat treatment and the second heat treatment. because,
Higher speeds and stronger cleaning can be carried out because the yarn has no surface finish and the yarn has good mechanical properties.

The excellent results obtained with mesophase-induced yarns suggested a broader scope of the invention. That is, the same purification operation can be used for PAN-induced yarns. This is because these yarns have performance comparable to mesophase pitch-induced yarns.

Although ultrasonic vibrations have been found to be very effective, the invention generally resides in the use of high frequency mechanical vibrations applied directly to substantially dry yarns or fibers.

Accordingly, in its broadest embodiment, the present invention relates to an improvement in the process of making carbon fibers, which comprises the steps of spinning a fiber, heat-setting the fiber, and carbonizing it, and after the heat-setting step. The dry fibers are purified by subjecting the dry fibers to high-frequency mechanical vibration.

The fibers may be derived from mesophase pitch or
The mechanical vibrations may be derived from PAN, and preferably the mechanical vibrations are in the ultrasonic frequency range.

Of course, instead of a single fiber, multiple fibers with a diameter of about 14 microns, typically about 2000
Yarns consisting of real fibers can be used.

Preferably, the carbonization is carried out in two stages, so that the purification step can be carried out after the pre-carbonization step.

Preferably, the yarn after the first heat treatment or pre-carbonized yarn is purified according to the invention immediately before entering the chamber for carrying out the final carbonization step. The yarn is stretched generally along the line path and pressed against the horn of the ultrasonic source with an effective force of about 10 to about 25 lbs. Preferably, the ultrasound source produces mechanical vibrations having an ultrasound frequency of about 20,000 Hertz.

Suitable amplitude and frequency ranges of mechanical vibrations for cleaning according to the present invention can be easily determined experimentally for the particular yarn used. Some of the factors to consider for the yarn are the degree of cleaning that is acceptable, the speed at which the yarn is to be moved, the degree of yarn damage that can be tolerated, and the economics of the cleaning operation.

Further advantages of the invention will become apparent from the description below.

The best mode for carrying out the invention is illustrated in the accompanying drawings. Referring now to the drawings, the thermoset mesophase pitch-guided yarn 1 exits the pre-carbonizer 2 operating at a maximum temperature of approximately 1300°C. Yarn 1 has 2000 fibers and each fiber has a diameter of approximately 10 microns.

Ultrasonic sources 3, 4 and 6 provide mechanical vibrations and clean the yarn 1 according to the invention. Yarn 1 moves at a speed of about 20 m/min and has a speed of about 550 m/min.
It has a tension of lb.

Each of the ultrasound sources 3, 4 and 6 has a frequency of approximately 20,000 Hertz and is of a commercially available type. Each of horns 7, 8 and 9 is an active output element of a respective ultrasonic source 3, 4 and 6, and each horn has a surface approximately 1.3 cm long in contact with the yarn, so that on each horn The residence time of yarn 1 at is approximately 0.04 seconds. Horns 7, 8 and 9
For each, yarn 1 is pressed with a force of about 10 to about 25 lbs.

There are two important aspects regarding the present invention. One is the degree of cleaning and the other is the degree of negative impact on mechanical properties.

Yarns purified according to the invention showed significant improvement in the absence of particles compared to yarns that were not purified.

In addition, carbon yarns purified according to the present invention had tensile strength and Young's modulus comparable to carbon yarns that were not purified at all. This is important. This is because this shows that the present invention does not degrade these two important mechanical properties.

Typically, carbon yarn made in accordance with the invention as described in the accompanying drawings has a pressure of 2.23 GPa.
The fibers had an average tensile strength of , and an average Young's modulus of 632 GPa, whereas the carbon yarns that were not purified at all had corresponding average values of 2.24 GPa and 600 GPa.

A higher degree of cleaning of the yarn, even at the intended power level of the ultrasound sources, can be obtained by increasing the number of ultrasound sources and/or by increasing the pressure of the yarn against the horn. Ultrasonic sources available on the market usually have a preset power level. In separate experiments, about 50 lb instead of about 10 to about 25 lb for each of the three horns
The cleaning process was carried out using a pressing force of . The carbon yarn fibers produced had a tensile strength of about 2.25 GPa and a Young's modulus of about 612 GPa. These results were also comparable to those obtained with unpurified yarns, indicating that there is considerable latitude in pressure selection.

It is interesting to note that the particles that are being mechanically removed from the yarn during the cleaning process according to the invention result in a visible haze in the area of each horn. Removal of particles from the purification area can be carried out with a suction device or similar device. The greatest haze was present in the first horn, horn 7.

When the ultrasonic sources 3, 4 and 6 were removed, haze no longer formed and the yarn was considerably degraded with fray damage. In practice, such fraying was expected, but it is surprising that this does not occur when the ultrasound sources 3, 4 and 6 are activated.

Although the present invention has been described in detail above, it is to be understood that the present invention is not intended to be limited to the configurations illustrated and described herein, as many modifications and changes will occur to those skilled in the art.

[Brief explanation of the drawing]

The accompanying drawings show a schematic side view of the apparatus implementing the invention, with 2 being the pre-carbonization zone, 3, 4 and 6 the ultrasound sources and 12 the final carbonization zone.

Claims (1)

[Claims] 1. A method for producing at least one carbon fiber, which comprises spinning at least one fiber, thermosetting the fiber, and carbonizing the fiber, wherein the fiber after the thermosetting step is treated as a dry fiber. A method for producing carbon fibers, characterized in that the carbon fibers are purified by subjecting them to high-frequency mechanical vibration to remove particles adhering to the surface of the fibers. 2. The method of claim 1, wherein the fibers are derived from polyacrylonitrile or pitch. 3. The method of claim 1, wherein the fibers are derived from mesophase pitch. 4. The method of claim 1, comprising spinning a plurality of fibers and forming them into yarn. 5. The method of claim 1, wherein at least one ultrasonic source is used to generate high frequency mechanical vibrations. 6. The method of claim 4, wherein the ultrasound source has a frequency of about 20,000 Hertz. 7 Spun multiple fibers into yarn,
2. The method of claim 1, wherein the high frequency mechanical vibrations are generated using at least one ultrasonic source. 8. The yarn is carbonized by a first heat treatment to about 1300°C followed by a second heat treatment to a temperature in the range of about 1500 to about 3000°C, and a cleaning step is carried out between said first heat treatment and said second heat treatment. 7. The method of claim 6 carried out in . 9 Spun multiple fibers into yarn,
The method of claim 1 further comprising the step of coating a portion of the surface of the fiber with graphite or carbon black prior to the heat curing step.