JPS5837410B2 - Carbon fiber manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing continuous carbon fibers (including graphite fibers) used in reinforced composite materials and the like.

Carbon fibers are made by preparing polyacrylonitrile or other organic precursor fibers in an oxygen-containing atmosphere, usually air, at 200 to 300%
℃ for 30 minutes to several hours, then heat treated in an inert atmosphere or non-oxidizing atmosphere for 100 minutes.
It is produced by heating to a carbonization temperature of 0 to 2000'C and, if necessary, to a graphitization temperature of 2000C or higher.

Carbon fibers produced in this manner are often used as a reinforced composite material in a composite with a thermosetting resin such as an epoxy resin, and therefore the adhesiveness with the resin is improved.

Usually, the fiber surface is oxidized by air oxidation, electrolytic oxidation, etc., or so-called surface treatment.

Furthermore, since multifilaments are usually used as organic precursor fibers, the multifilament carbon fibers obtained from these fibers are given a convergence illumination to improve their handling properties.
A so-called sizing process is performed.

Because carbon fiber has high specific strength and specific modulus, it is often used in composites with epoxy resins in fields that require high-performance materials, such as the aerospace industry and various sporting goods.

A common method for compounding carbon fibers with epoxy resin, etc. is to align carbon fibers in one direction, impregnate them with epoxy resin that serves as a matrix, and then semi-cure the resin to produce a so-called prepreg. There is a method in which this unidirectional prepreg is laminated or wound into a desired shape and molded, and the resin is heated and cured to form a carbon fiber composite material in a predetermined shape.

The problem with this method is how uniformly the resin is impregnated into the multifilament carbon fiber tow, and how evenly the tow can be arranged without gaps.

Unless both of these are achieved, a composite material that fully exhibits the performance of carbon fibers cannot be produced.

Furthermore, depending on the intended use, in addition to the above two points, it is also important to consider how thin a prepreg can be manufactured.

For example, when used in the aerospace field, the weight of carbon fiber that can be used is limited, and a composite material with as little anisotropy as possible must be produced using a limited amount.

Normally used prepreg is made by arranging carbon fibers in one direction, so in order to form a composite material with little anisotropy, it is necessary to stack thin prepregs in multiple directions.

In order to achieve this purpose, it is necessary to use fibers that have never been twisted before, or to use fibers that have a small number of filaments since twisted fibers have poor opening properties.

When manufacturing prepregs with the same thickness, for example, 0.
When manufacturing a prepreg with a thickness of 1 mm, a tow with a thickness of 10,000 filaments or more can be used by opening the untwisted yarn, but a tow with a thickness of 10,000 filaments or more can be used if the yarn is twisted 10 times/rn or more. must be used.

In general, organic precursor fibers are made up of extremely thin filaments of 8 to 16 microns, their strength decreases during the oxidation heat treatment process, and once they are turned into carbon fibers, they become brittle and easily break due to friction. , fluff is likely to occur during the manufacturing process.

In particular, when manufacturing with untwisted fibers, it is necessary to constantly monitor the running fibers to prevent the generated fuzz from getting wrapped around the guide rolls, or to remove the fuzz to ensure stability. Difficult to operate continuously.

For this reason, from the standpoint of manufacturing carbon fibers, it is desirable to apply as much twist as possible to reduce the generation of fuzz without deteriorating the performance of the fibers.

Further, from the viewpoint of productivity, it is desirable to use a tow with a large number of filaments because a large amount can be produced with the same number of filaments.

As described above, the requirements for twisting the fibers and the thickness of the tow are contradictory between those who manufacture carbon fibers and those who use them to manufacture prepregs.

We have arrived at the present invention as a result of intensive research into a method for producing carbon fibers that can be produced stably and with high productivity, and that also have an optimal shape when producing thin prepregs.

The present invention processes a plurality of multifilament tows of organic precursor fibers by twisting them together in the heat treatment process during the carbon fiber manufacturing process, where the generation of fuzz is a particular hindrance to continuous operation.
After the treatment or the subsequent heat treatment, this twisting is untwisted, that is, untwisted, to produce carbon fibers with excellent fiber opening properties and good resin impregnation properties during prepreg formation.

It is already known that organic precursor fibers consisting of multifilament tows are twisted and manufactured, and the resulting strand-like carbon fibers are untwisted and used before manufacturing prepreg (Japanese Patent Application Laid-Open No. 138197-1987). ) However, it is difficult to untwist the twisted carbon fibers produced by this method into completely untwisted yarns.

As described in JP-A-51-1.05419, carbon fibers produced (fired) while being twisted tend to retain their twist. This tendency is due to the number of twists (
This becomes more noticeable as the number of twists per unit length increases, so in order to create a completely untwisted yarn, the number of twists must be reduced.

On the other hand, from the perspective of the untwisting side, if the number of twists is constant in the length direction of the fiber, it is possible to obtain untwisted yarn by untwisting under certain conditions, but if the number of twists is small, for example 10 times/7 rL
If the number of twists in the longitudinal force direction of the yarn is compared, for example, when comparing the number of twists in the longitudinal force direction of the yarn, for example, Since it is no longer constant, it becomes difficult to untwist and create a non-twisted yarn.

If you untwist a carbon fiber tow with uneven twisting at the same rotational speed as when it was twisted, the twist will remain in the heavily twisted part, and the reverse twist will occur in the lightly twisted part. Because it will be.

In this case, it is conceivable to untwist while detecting the twist in each part of the tow made of multifilaments, but since the filaments of the tow are entangled with each other, it is practically impossible to untwist while detecting the twist during the connecting operation. It is possible.

To make strong, untwisted yarn, the twisted multifilament tow is divided. It is possible to perform untwisting while detecting the twist, but with this method, as mentioned above, the individual filaments are often intertwined with each other, so fuzz is likely to occur, and it is not stable at a practical speed. It is difficult to untwist it.

The present invention is a method of twisting a plurality of multifilament tows together and untwisting them after heat treatment.

Untwisting can be done by separating twisted tows, and this separation and untwisting is easy.

Since the number of multiple tows is not so large, unlike the case of untwisting multifilament described above, when separating, even if there is uneven twisting, as will be explained in detail later, the twisting unevenness can be detected while being separated. Easy to untwist continuously.

Next, embodiments of the present invention will be specifically described.

By twisting multiple multifilament tows together,
In this case, each tow may or may not be twisted, and furthermore, a combination of tows with and without twists is also possible.

Among these, the most desirable embodiment is that the number of twists and the direction of twisting of each tow are the same as the number of twists of each tow and the direction of twisting are the same because it is easy to operate and has a large fuzz prevention effect. This is the case.

First, a method of implementing this desirable embodiment will be explained with reference to the drawings.

Figure 1 is a schematic diagram showing an example of twisting and heat treating two tows, Figure 2 is a state diagram of the resulting fiber, and Figure 4 is the untwisting of the fibers in Figure 2. It is a schematic diagram showing an example of the case.

Two multifilament tows 6 and 7 of organic precursor fibers, which are raw materials for carbon fibers, are wound around bobbins 1 and 2, and they are fixed to an unwinding machine A having the same rotating shaft 8 and rotated. The shaft 8 is rotated L to pull out the tows, the two tows are twisted together and passed through the guide rolls 3, guided into the heat treatment furnace 4, subjected to flame-retardant treatment, etc., and then wound onto the bobbin 5.

At this time, adjust the traveling speed of the tow and the rotation speed of the rotating shaft,
Give a certain number of twists per unit length of tow.

Each tow 6, 7 leaving the unwinding machine A is given a fixed number of twists until it reaches the guide roll 3.

Since the tows intertwine with each other and make one rotation in the same direction while each tow rotates once, the number of twists and the direction of twisting of each tow are the same as the number of twists and the direction of twisting of the tows.

This state is shown in FIG. Untwisting these two tows is easy.

On the other hand, FIG. 3 shows the state of twisting when two tows are tied together using a known method and then twisted.

As mentioned above, it is difficult to completely untwist the twisted tow 9 shown in FIG. 3.

In the untwisting operation, as shown in FIG. 4, the tow wound in step 5 of FIG. 1 is applied to an unwinding machine B, and the tow is unwound without rotating the rotating shaft 11 in the direction of untwisting.

The unwound tow is separated into two original tows and each tow is separately wound onto two bobbins 12 and 13.

At this time, the speed at which the unwinding machine B is rotated should correspond to the number of twists applied to the two tows and the running speed of the tow if there is no uneven twisting, but if there is uneven twisting, The separation point 10 of the two tows may be detected to control the rotational speed of the unwinder, or if the rotational speed is constant, the running speed of the tows may be controlled.

The separation point 10 may be detected using a conventional detection device using a phototube or a limit switch.

According to the method shown in FIG. 1, since the twisting of each tow and the twisting of each tow are performed simultaneously, uneven twisting occurs in almost the same way between each tow and each tow.

Therefore, if untwisting is performed by detecting the twist between the tows, each tow will be untwisted in accordance with the twist irregularity, and the entire tow will be completely untwisted.

Although FIG. 1 shows the case of two tows, if there are three tows, it is sufficient to prepare three bobbins, and in the same manner, four or more tows can be used.

The present invention is based on twisting together a plurality of multifilament tows.

Various combinations are possible depending on whether each tow itself is twisted or not, but in order to increase the effect of preventing fuzz, it is preferable to use the present invention in which each tow itself is also twisted, as described above. This method is effective when the filament is thin.

In untwisting, the tows are untwisted, but depending on the method, each tow can be untwisted, or each tow can be untwisted.

It is generally preferable for carbon fiber to be untwisted, and as mentioned above, untwisted carbon fiber is a must in order to make thin Brypreg, but it can be used with twisting depending on the purpose. There are some things.

In such a case, the present invention is very convenient because it can separately produce twisted tow and untwisted tow.

Another advantage of the present invention is that completely untwisted yarn can be produced and at the same time productivity can be increased.

When continuously manufacturing carbon fiber without twisting, there is a certain distance between the tows that run in parallel to prevent problems such as spreading of the tows and wrapping of adjacent tows due to fuzz. A certain distance or more is required, and this interval must be larger than in the case where there is twist.

In the production of carbon fiber, which is mainly processed in a heating furnace with a certain effective width, increasing the distance between the tows will reduce the number of tows that can be run in parallel, that is, the number of tows to be processed. , productivity decreases.

On the other hand, in the method according to the present invention, the tows are twisted together and a plurality of tows are processed as one tow, so the actual number of tows to be processed is increased, and productivity is improved.

Furthermore, in the conventional method, when several tows are combined to make each tow thicker in order to increase productivity, reaction heat accumulates inside the tow during heat treatment, which can cause various troubles. S. In the present invention, the tows are twisted together, each maintains an independent state, and the surface area is correspondingly large. This will prevent problems like the ones above.

When polyacrylonitrile fiber, which is the most commonly used raw material for carbon fiber, is used as a precursor, heat treatment in an oxygen-containing atmosphere, the so-called flame-retardant process, is the rate-limiting process for productivity. In order to increase the temperature, it is important to quickly remove the heat generated by the reaction, otherwise the thread will burn out.

Although it is effective to blow wind onto the tow to prevent the accumulation of reaction heat, in the case of non-twisted yarns, it is not desirable to increase the wind speed above a certain value, for example 2 m/sec, from the viewpoint of generating fuzz.

On the other hand, if the material is twisted, there will be less fluff even at 5 m/'SeC.

Twisted yarns tend to accumulate reaction heat more easily than non-twisted yarns, but this can be prevented by increasing the wind speed. There is no decrease in productivity.

As a result of various studies regarding the number of filaments and the degree of twisting, we found that the specific implementation conditions of the present invention are that the total number of filaments after twisting is 50,000 filaments, preferably 30,000 filaments or less. , and each tow has no more than 15,000 filaments, preferably 10
000 filaments or less, and the number of twists is 15 twists/m or less, preferably 10 twists/m or less, from the viewpoint of improving productivity and not causing deterioration of the physical properties of the carbon fiber.

There is no particular restriction on the lower limit of the number of filaments, but the lower limit of the number of twists must be the number of times that the tow can be stably untwisted and separated, and may be at least 1 twist/m.

Regarding the number of tows to be twisted, should the number of filaments after twisting be 50,000 filaments, preferably 30,000 filaments or less? Although not particularly limited, it is desirable that the device used during heating and untwisting be simple, so the number of wires is about four, preferably two.

Heat treatment steps in the production of carbon fibers include flame resistance, carbonization, and, in some cases, graphitization. In the present invention, twisting is performed before any of these steps, and untwisting is performed after any of the heat treatments. Later.

Usually, flame-retardant treatment causes many problems such as the generation of fuzz, so it is better to twist the fabric before the flame-retardant treatment.

If the process from organic precursor fibers to graphitization is carried out in an integrated manner, untwisting may be performed after graphitization.

In this case, if carbonization or graphitization does not cause any problem without twisting, untwisting may be performed after flame resistance.

A special case is when the process from the production of organic precursor fibers to carbonized fibers is carried out continuously. Since twisting according to the present invention is practically impossible before flame resistance is achieved, twisting is carried out after flame resistance is achieved.

Actual Foil Example A polyacrylonitrile fiber tow consisting of 10,000 filaments (15,000 denier) was used as a precursor, and two of these tows were twisted together three times per meter using the unwinding machine shown in Figure 1. 4 in the direction perpendicular to the fiber axis.
In a flameproofing furnace that blows hot air at 7n/sec,
℃ for 60 minutes, followed by carbonization treatment at a maximum temperature of 1,300℃ using a conventional method to produce carbon fibers.

This carbon fiber was subjected to surface treatment and sizing according to conventional methods, and then wound onto a bobbin.

In this production, the fibers were passed over a large number of guide rolls and fixed guides, and there was almost no fluff, and stable continuous operation was possible.

When the wound twisted yarn was untwisted at a speed of lm/myn as shown in FIG. 4, it was possible to produce two completely untwisted yarns with little fuzzing of the fibers.

The untwisting device uses a limit switch to detect the separation point, and this switch controls the rotation axis of the unwinding L machine (1
1) A type that turns on and off the power of the motor connected to the motor was used.

The yarn take-up speed was controlled by a constant speed clover roll placed in front of the winder.

When the tensile properties of this yarn were measured using the strand method, the strength was 336 k9/mjt, and the elastic modulus was 23.2 tons/m.
In A, the characteristics were the same as those manufactured with twisting and those manufactured without twisting as shown in the comparative example.

When a prepreg with a thickness of 0.75 mm was produced using this yarn, it was possible to produce a prepreg with good fiber opening properties and no gaps between tows.

Comparative Example 1 Carbon fibers were manufactured under the same conditions as in Example except that one tow was used instead of two tows.

Since the tow is twisted 3 times per meter, continuous operation can be carried out stably, and the tensile properties are also strong 3 4 1.
kg/myi. , the elastic modulus was 2 3.3 t7/mjt, and the production amount of force S1 was 1/2 that of the example.

When we tried to manufacture a prepreg with a thickness of 1 by using 1 part of this yarn as it was and 1 part by untwisting it, we found that the fibers could not be opened sufficiently because the yarn used as is was twisted. However, it was not possible to obtain the desired thickness, and even after untwisting, some twists remained, and there were many gaps between the tows that were pulled together, and the thickness was uneven.

Comparative Example 2 Carbon fiber was manufactured under the same conditions as in the actual case, except that it was not twisted and two yarns were not doubled. However, the fluffing during the flame-retardation process was severe, and the fluffing was used as a guide. Stable continuous operation was not possible due to the winding around the rolls and adjacent tows.

The tensile properties of the obtained carbon fibers were as follows: strength 333 kg/1;
The elastic modulus was 23.3 tons/1, but when the number of twists was measured over a length of 60 m, there were places where there were up to 8 twists per 1 m, and the average was about ±2 twists. Ta.

When a prepreg with a thickness of 0.75 mm was manufactured using this yarn, gaps were generated between the tows due to the twisting, and the result was an unsatisfactory prepreg.

[BRIEF DESCRIPTION OF THE DRAWINGS] Fig. 1 is a schematic diagram showing an example of an apparatus for implementing the method of the present invention, Fig. 2 is a state diagram of carbon fiber obtained with the apparatus of Fig. 1, and Fig. 3 is a phase diagram of a known carbon fiber, and FIG. 4 is a phase diagram of a known carbon fiber.
The schematic diagram which shows one example of the apparatus which untwists the carbon fiber shown in a figure is shown. In the figure, 1, 2... bobbin, 4...
・Heat treatment furnace, 5...Bobbin, 6, 7.9...
... Fiber filament tow, 10 ... Separation point detection device, 12.13 ... Bobbin.

Claims (1)

[Claims] 1. In a method for producing carbon fiber by sequentially subjecting a plurality of multifilament tows of organic precursor fibers to flame-retardant and carbonization heat treatments, as well as necessarily graphitization using fireworks, before any of the heat treatments described above. Twist each tow into
The number of twists and the twist direction of the plurality of tows are the same as the number of twists and the twist direction of each tow, and the number of twists is 1 to 15 times/m.
A method characterized in that the strands are twisted together and the strands are untwisted after any subsequent heat treatment.