JPH0781210B2 - Method for producing short carbon fibers - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a cotton-like pitch-based short carbon fiber and a method for producing the same.

(Prior Art) Carbon fibers are roughly classified into PAN type and pitch type. Currently, PAN-based carbon fibers that are industrially manufactured by firing acrylonitrile under specific conditions are mainly high-strength materials (HP type).
Is used as. However, since PAN fiber has a low carbon content, decomposition gas is generated in the firing process, the yield is low at 50-60%, and the graphite structure is hard to develop at high temperatures, so it is a high strength product. Is easy to make, but it is difficult to make one with a large elastic modulus.

On the other hand, pitch-based carbon fibers are made from pitch such as coal and petroleum, so the carbon content in the spun fiber is 95%.
%, And the yield is as high as 80 to 90%, and in terms of physical properties, it has the characteristics of exhibiting a large elastic modulus, and therefore has been rapidly developed.

In addition, pitch-based carbon fibers can also be used as a so-called general-purpose type (GP product) carbon fiber at a low cost and with a certain level of strength, if the pitch is melt-spun and fired as it is. It is used for materials.
Carbon fiber with optical anisotropy (mesoface) is made by spinning a pitch of crystalline surface, and the liquid crystal alignment becomes the fiber axis direction alignment in the shear stress field during spinning. Crystals are generated, resulting in a high elastic modulus (HM type) carbon fiber.

Therefore, application of products suitable for each of these characteristics is promoted, carbon fiber simple substance is used for filters, catalysts, electromagnetic shielding materials, etc.
It is used as a reinforcing material for a matrix of metal, carbon, ceramics, etc., and is widely used for space, aviation, leisure, sports, industrial, etc.

Recently, research is being conducted to combine it with engineering plastics to make electronic parts, automobile parts, and structural materials.

(Problems to be Solved by the Invention) When these carbon fibers are used as a composite material, particularly when a large amount is used as a reinforcing material for a structure, an inexpensive and high-strength material is required.

As mentioned above, the pitch isotropic carbon fiber is an optically isotropic carbon fiber (GP product) because it is cheap, and its use as a substitute for asbestos as a wall reinforcing material for high-rise buildings by combining it with cement. Is being promoted.

In addition, a wide range of applications have been studied and implemented as other structural reinforcing materials. However, because of its low strength, satisfactory results have not always been obtained when applied to composite materials such as plastics, metals and ceramics, which require high strength as structural members.

On the other hand, the optically anisotropic carbon fiber has a higher tensile strength and a higher elastic modulus than the optically isotropic carbon fiber, so that the above-mentioned defects can be somewhat compensated in terms of strength. Although it is possible, the demands of the electronic industry, automobile industry, space industry, etc. that require materials with higher fracture toughness are still unsatisfactory, and in terms of quality stability, mass productivity, and economic efficiency, Has not reached a state where it can be widely used. Now, regarding the spinning method of the optically isotropic carbon fiber, isotropic pitch is a rotary spinner, a method of stretching with an air sucker, a vortex method, etc., glass fiberization technology is applied to molten polymer, etc. The fibers obtained by these methods have tens to 20 μm and lengths of tens to 20 μm.
It is a fiber of several hundred mm. It must then be further cut in order to be used as a stiffener. On the other hand, the anisotropic carbon fiber may be manufactured by the above-mentioned method,
Most of them are made into long fibers by winding and stretching to make roping fibers, and to make them chopped fibers, mix them with a matrix material such as thermoplastics by cutting them about 10 to 15 mm or shorter.
It is used as a reinforcing material, and the work of once cutting the long fiber through a cutting process and cutting it into a certain length is performed.

When these carbon fibers are used as composite materials, thermoplastics are ductile materials, and carbon fibers that are reinforcing fibers have high tensile strength and elastic modulus, but have low stretchability, so they behave as brittle materials. Therefore, once a crack occurs, it is dangerous to proceed to final fracture as it is and lead to a serious accident, and it is dangerous. Therefore, how to enhance fracture toughness is a big problem. The factors that cause the destruction of these carbon fiber reinforced plastics include the destruction of the matrix, the separation of the matrix and the fibers, the fracture of the fibers, the withdrawal of the fibers, etc., and it is considered that the actual destruction is due to a combination of these. However, the peeling between the carbon fiber and the plastic and the pulling out of the fiber are major factors.

The reason is that carbon fiber is a highly linear material,
It is considered that the surface is smooth and there is a problem with the bondability at the interface.

In addition, when using carbon fiber alone, it is necessary to provide more surface area and more space within a certain volume such as a filter and catalyst. Weave it as a net or pick it up in a mat to create a space. It was molded using a binder for making parts. It was difficult to keep the space constant even when the nets were woven flatly, and it was very difficult to make a net structure with a constant gap. Furthermore, it could not be used at all in applications requiring an elastic structure.

Therefore, the present invention dramatically improves the strength of an optically isotropic carbon fiber, an optically anisotropic carbon fiber or a composite fiber thereof, which cannot be solved by conventional techniques, and at the same time, causes twisting and curling. (EN) Provided are pitch-based short carbon fibers having excellent compatibility with a matrix material that does not require a cutting step and can be directly spun into cotton, and can be directly spun into the dimensions of chopped fibers, and a method for producing the same.

(Means for Solving the Problems) The production method of the present invention for solving the above problems is an optically isotropic pitch, an optically anisotropic pitch, or both pitches supplied to a spinning device to obtain a pitch. A cotton-like short fiber is obtained by spinning at a high gas pressure of 6 kg / cm ² or more and 20 kg / cm ² or less that is blown out in parallel to the pitch discharge direction from around the discharge hole, infusibilized, and then fired. It is characterized by doing.

(Operation) As described above, the short carbon fibers produced by the production method of the present invention are cotton-like, and thus have good compatibility with the matrix material and improved fracture strength due to peeling. Further, conventionally, in order to obtain such a cotton-like short carbon fiber, in order to make a fiber once spun into a chopped strand, a cutting step is added to finish the size to the present invention. According to the method of (1), the final shape can be directly obtained, which makes it possible to significantly improve the productivity.

Furthermore, the fiber strength obtained by the method of the present invention is the same as that obtained by the conventional spinning method in the case of the optically isotropic carbon fiber and the optically anisotropic carbon fiber, respectively. Since the strength is about 1.4 to about 2.5 times that of the composite material, the fracture toughness of the composite material can be increased.

Furthermore, in the composite fiber of the optically isotropic carbon fiber and the optically anisotropic carbon fiber, twisting or curling is further caused due to the difference in the expansion coefficient between them, and thereby the bondability with the matrix material is increased. Is further improved. The fibers have apparent bulkiness and elasticity due to twisting and curling, and the fibers can be used alone as cotton-like fibers having a certain bulge.

In the method for producing a pitch-based carbon fiber according to the present invention, heavy oil having a large amount of aromatic six-membered ring structure in its molecule, generally coal tar, petroleum cracking tar, steam cracker tar, etc. are used as the starting material. From these raw materials, the optimum one in terms of purity and softening point is selected, or if it does not meet the requirements, pretreatment such as solvent extraction or thermal reforming is performed.
In general, raw heavy oil contains fine solids such as free carbon, which needs to be removed. As one of them, it is dissolved in an aromatic oil such as anthracene oil or an organic solvent such as quinoline and filtered. On the other hand, the primary heat treatment produces mesocarbon microspheres sufficient to adsorb free carbon, fine particles of minerals, and fine solids contained in the pitch, and then removes them by extraction filtration. The pitch obtained by concentrating the filtrate is further subjected to a secondary heat treatment to be polycondensed, and at the same time, the light component is removed to adjust the softening point and obtain an optically isotropic pitch.

On the other hand, the optically anisotropic pitch is diluted with tetrahydroquinoline in an amount of 2 to 3 times, and the pitch is 10 to 10 at a temperature of 400 to 450 ° C.
Hydrogenate the solvent under autogenous pressure of ~ 30 kgf / cm ² . This is filtered to sufficiently remove free carbon and the like, and then the solvent is removed. Finally, heat treatment is performed at a temperature of 450 to 500 ° C. to obtain an optically anisotropic (mesophase) pitch. Alternatively,
When gasoline is produced by the fluid catalytic contacting method of soft oil of petroleum, heat treatment of heavy tar (FFC decant oil) produced as a by-product to form mesophase and at the same time, to remove soft matter and adjust the softening point To obtain the mesophase pitch.

The optically isotropic pitch thus obtained and the optically anisotropic pitch have different properties when made into carbon fiber. Generally, when an optically isotropic pitch is spun into carbon fiber, the graphite crystals in the fiber after carbonization become fine and the alignment in the fiber axis direction deteriorates, so it is called a general-purpose type (GP product) and has tensile strength. Generally, the elastic modulus is 100 kg / mm ² , and the elastic modulus is around 5 ton / mm ² . In the case of optical anisotropy, it is of course necessary to prepare the raw material pitch, but in order to obtain high strength and high elasticity carbon fiber,
The control of the molecular orientation is important, and the temperature during spinning, the nozzle shape, and the molecular orientation control peculiar to the liquid crystal pitch influence. Therefore, the mechanical properties have a wide range under that condition, and the tensile strength of the carbon fiber currently obtained is 300 to 500 kg / mm ² , and the elastic modulus is 30 to 70 ton /
mm ² .

Also, the optically isotropic carbon fiber has a coefficient of thermal expansion of 4 x at 1000 ° C.
Carbon fiber with optical anisotropy is 2 × 10 ^-6 for 10 ^-6 / K °
/ K ° and half the coefficient of thermal expansion.

INDUSTRIAL APPLICABILITY The present invention dramatically improves the properties of the optically isotropic carbon fiber and the optically anisotropic carbon fiber as described above. It makes it possible to produce cotton-like pitch-based short carbon fibers that can be molded into.

(Examples) Specific examples of the present invention will be described below.

Two types of spinning pitches were used: an optically isotropic pitch having a softening point of 230 ° C. and a mesophase pitch containing 98% of an optically anisotropic phase having a softening point of 267 ° C. The first spinning device
The structure shown in the figure was used. Here, the inner diameter of the pitch discharge hole 1 is 0.2 mm, and the hole diameter of the gas channel nozzle 2 is
It was 0.5 mm. Either the optically isotropic pitch 3 or the optically anisotropic pitch 4 was placed in the pitch reservoir 5, the inside was replaced with nitrogen gas, and then heated and melted by the heater 6. After the pitch temperature reaches a predetermined temperature, the pitch reservoir 5
Nitrogen gas having the same pressure was introduced through the upper part of the and the gas introduction pipe 7. Then, the fibrous pitch discharged from the lower part of the spinning device was collected. The collected fibrous pitch was heated to 320 ° C. in the air at a temperature rising rate of 2 ° C./min and held for 30 minutes for infusibilization. Then, carbonization was carried out at 1000 ° C. in a nitrogen gas stream, and further graphitization treatment was carried out at 2600 ° C. in an argon gas stream.

One carbon fiber thus obtained has a fiber length of 5 m.
It was attached to a mount so as to be m, and the tensile strength was measured according to the single fiber method defined in JIS R7601. The outer diameter of the fiber was measured by the laser diffraction method of the same standard.

The physical properties of the carbon fibers obtained by changing the pitch type and spinning conditions are summarized in the table below.

As can be seen from the above table, as the pitch temperature and the gas pressure increase, the fiber diameter becomes smaller and the length becomes shorter. The length of the fiber is short and long even within the same lot,
Although it can not be expressed unconditionally, it is about the longest with implementation number 1-1
It is 50 mm.

FIG. 3 is a photograph (magnification: 500 times) of the carbon fiber shown in Table No. 2-2 in the above, which was observed with a scanning electron microscope. The fiber diameter is large and small, and is not constant. Moreover, it is mostly curved, not straight. The fact that the fiber is bent is the same for other execution numbers. Furthermore, in order to examine the shape of the fiber of this execution number, the photograph observed at a magnification of 4000 is the fourth.
It is a figure. As seen in this figure, the cross-sectional shape of the fiber is not a circle but a flat ellipsoid. The reason why the ellipsoid or flat is an ellipsoid is considered to be the reason why the fiber is bent.

Fibers having a fiber diameter of 10 μm or less have a short length and tend to be clumped due to bending, but the feel is soft, elastic and cotton-like. However, it has good defibration property and easily disperses as single fibers in water.

Also, the shape of this fiber is flat, twisted, and randomly curled, and it is cotton-like and has a high bulk, and when placed in a certain container, it is fixed by the elasticity of the fiber without forming a mat-like shape. Therefore, it was confirmed that the filter can be used as it is for filtration. Further, by making a composite with an elastic plastic material, a rubber-like carbon fiber composite, which has been impossible in the past, can be produced, and a highly wear resistant and elastic packing or the like can be produced.

Incidentally, the composite short carbon fiber obtained by combining the optically isotropic carbon fiber and the optically anisotropic carbon fiber to form a composite can also be manufactured by a spinning device provided with a partition plate 8 as shown in FIG.

(Effect of the invention) As can be seen from the above description, the method for producing a short carbon fiber of the present invention is a chopped strand in the related art.
Greater economic effect can be achieved by improving productivity.

Conventionally, it was thought that only the general-purpose products (GP) can be used to produce optically isotropic carbon fibers, but according to the method for producing short carbon fibers of the present invention, pitch-based optically isotropic carbon fibers are used. Since the strength is equal to or higher than that of the pitch-based optically anisotropic carbon fiber, the manufacturing cost for adjusting the pitch raw material can be significantly reduced. In addition, since the carbon fiber having optical anisotropy has strength equal to or higher than that of the fiber obtained by the conventional spinning method, it can be used as a high-strength composite material in the electronics industry, the automobile industry, the space industry. It can be used as an extremely useful material as a structural component with high strength and high precision such as metal, carbon and ceramics for industrial applications.

In addition, since the fiber cross-sectional shape is flat and can be twisted or curled, it has a cotton-like bulkiness as it is, and since it has elasticity and elasticity, it was used in the production of composite materials. At this time, since the compatibility with the flexible matrix material is good, peeling between the pitch-based carbon fiber and the matrix material does not easily occur. Therefore, by applying it as a composite material such as a stretchable conductive material, an elastic packing and an engineering plastic, it is possible to produce a high strength composite material which has never been obtained.

Further, according to the method for producing carbon fibers of the present invention, pitch-based short carbon fibers having the above-described excellent effects can be easily produced in a large amount at one time.

[Brief description of drawings]

FIG. 1 is a diagram showing one embodiment of the production method of the present invention, FIG. 2 is a diagram showing another embodiment of the production method of the present invention, and FIG. 3 is a scanning type of the shape of the carbon short fiber of the present invention. Taken with an electron microscope
Photograph of 500 times, Fig. 4 is a further enlarged 400 of the fiber of Fig. 400.
It is a 0x photo. 1 ... Pitch discharge hole, 2 ... Gas flow path nozzle, 3 ...
Optical isotropic pitch, 4 ... Optical anisotropic pitch, 5 ...
... Pitch reservoir, 6 ... Heater, 7 ... Gas introduction pipe, 8 ... Partition plate.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Susumu Shimizu 2-6-6 Nihonbashi Kayabacho, Chuo-ku, Tokyo Tanaka Kikinzoku Kogyo Co., Ltd. Ruka Fuchino (56) Reference JP-A-2-169725 ( JP, A) JP 63-85116 (JP, A) JP 1-222156 (JP, A)

Claims (1)

[Claims] 1. An optically isotropic pitch, an optically anisotropic pitch, or both pitches are supplied to a yarn preventing device, and a melted pitch is blown out from the periphery of a discharge hole in parallel to the pitch discharge direction. A method for producing a short carbon fiber, which comprises spinning, infusibilizing and firing while blowing at a high gas pressure of not less than cm ^{2 and not} more than 20 kg / cm ² .