JPH0434019A - Carbon short fiber and preparation thereof - Google Patents

Abstract

PURPOSE:To prepare the subject cotton-like fiber having a high strength and useful for composite materials in electronic industry, automobile industry, etc., without requiring a cutting process, by feeding a pitch into a spinning device, extruding the melted pitch while blowing out the melted pitch from the periphery of an extrusion hole and subsequently subjecting the spun fiber to a non- melting treatment and subsequently to a calcination. CONSTITUTION:An optically isotropic pitch and/or an optically anisotropic pitch is fed into a spinning device and the melted pitch is spun while being blown out from the periphery of an extrusion hole. The fiber spun thus is subjected to a non-melting treatment and to a calcination treatment to prepare the objective short fibers.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to cotton-like pitch-based short carbon fibers and a method for producing the same.

(Conventional technology) Carbon fibers are broadly classified into PAN type and pitch type.

Currently, industrially, PAN-based carbon fibers, which are produced by firing acrylonitrile under specific conditions, are mainly used as high-strength materials (
HP type). However, since PAN-based fibers have a low carbon content, decomposition gas is generated during the firing process, and the yield is as low as 50-60%. Furthermore, the graphite structure does not develop at high temperatures (due to the high strength It is easy to make products, but it is difficult to make products with a large elastic modulus.

On the other hand, pitch-based carbon fibers are made from coal, petroleum, etc. pitch, so the carbon content in the spun fibers is 9.
It has a high yield of about 5%, a high yield of 80 to 90%, and has an excellent property of exhibiting a large elastic modulus in terms of physical properties, so its development has progressed rapidly.

In addition, even with pitch-based carbon fibers, if the pitch is melt-spun and fired, optically isotropic carbon fibers can be produced, which can be used as a general-purpose type (GP product) carbon fiber at low cost and with a certain level of strength for reinforcement of structures. It is used for materials, etc. Carbon fibers with optical anisotropy (mesophase) are produced by spinning entirely crystalline pitch, so that the liquid crystal alignment becomes aligned in the fiber axis direction under the shear stress field during spinning, and by carbonizing this, giant graphite is formed. Crystals are generated, resulting in a carbon fiber having a high modulus (HM type).

Therefore, the application of products that suit each of these characteristics is progressing, and carbon fiber alone is used in filters, catalysts, electromagnetic shielding materials, etc., and as a composite, it is used in matrices of resins, metals, carbon, ceramics, etc. It is used as a reinforcing material and is widely used in space, aviation, leisure, sports, industrial, etc.

Recently, research has been underway 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, an inexpensive and high-strength material is required, especially when used in large quantities as a reinforcing material for a structure.

As mentioned above, among pitch-based carbon fibers, optically isotropic carbon fibers (GP products) are inexpensive, so they are being used in place of asbestos as wall reinforcement materials for high-rise buildings by combining them with cement. is being advanced.

In addition, a wide range of applications as structural reinforcing materials are being considered and implemented. However, due to its low strength, it has not always been possible to obtain satisfactory results when applied to composite materials with plastics, metals, ceramics, etc. that require high strength as structural members.

On the other hand, optically anisotropic carbon fiber has higher tensile strength and higher elastic modulus than optically isotropic carbon fiber, so it can somewhat compensate for the above-mentioned defects in terms of strength. However, the current situation is that it cannot satisfy the demands of the electronics industry, automobile industry, space industry, etc., which require materials with even higher fracture toughness. It has not yet reached a state where it can be widely used. Now, regarding the spinning method of optically isotropic carbon fiber, isotropic pitch is produced by applying molten polymer and glass fiber technology, such as a rotary spinner, an air sucker drawing method, and a vortex method. The fibers obtained by this method have a diameter of several lθ to 20 μm and a length of several tens to hundreds of mm. In order to use it as a reinforcing material, it must be further cut. On the other hand, anisotropic carbon fibers are sometimes produced using the method described above, but most of them are made into long fibers by winding and drawing to create roving fibers, which are then made into chopped fibers. The cut fibers are mixed with a matrix material such as thermoplastic plastic and used as a reinforcing material.First, the long fibers undergo a cutting process and are cut into a certain length.

When these carbon fibers are used as a composite material, the thermoplastic plastic is a ductile material, and the reinforcing fiber carbon fiber has high tensile strength and elastic modulus, but has low elongation, so it behaves as a brittle material. Therefore, once a crack occurs, it can easily progress to final failure, leading to a major accident, which is dangerous, so how to improve fracture toughness has become a major problem. Factors that cause the destruction of these carbon fiber reinforced plastics include destruction of the matrix, separation of the matrix and fibers, rupture of fibers, pulling out of fibers, etc., and actual destruction is thought to be due to a combination of these. . However, the major factors are delamination between carbon fiber and plastic and fiber pull-out.

The reason is that carbon fiber is a highly linear material,
It is possible that the surface is smooth and there is a problem with bonding 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 of filters, catalysts, etc. It was molded using a binder to make the parts. Even when nets are woven two-dimensionally and overlapped, it is difficult to maintain a constant space, and it is extremely difficult to create a three-dimensional structure with constant gaps.

Moreover, it could not be used at all in applications requiring a resilient structure.

Therefore, the present invention dramatically improves the strength of optically isotropic carbon fibers, optically anisotropic carbon fibers, or composite fibers thereof, which could not be solved with conventional techniques, and at the same time, To provide pitch-based short carbon fibers that are compact, cotton-like, can be directly spun to chopped fiber dimensions, and have excellent compatibility with matrix materials without the need for a cutting step, and a method for producing the same.

(Means for Solving the Problems) The short carbon fibers of the present invention for solving the above problems are made from pitch-based optically isotropic carbon fibers, pitch-based optically anisotropic carbon fibers, or composite fibers thereof. It is characterized by being a cotton-like short carbon fiber.

Further, the production method of the present invention for producing the cotton-like short carbon fibers includes:
Optically isotropic pitch, optically anisotropic pitch, or both pitches are supplied to a spinning device and spun from around the pitch discharge hole by gas pressure to obtain flocculent short fibers. It is characterized by being subjected to infusibility treatment and then firing.

(Function) As mentioned above (because the carbon short fibers of the present invention are flocculent, they have good compatibility with the matrix material (improved breaking strength due to peeling). According to the method of the present invention, the fibers that have been spun as long fibers are first cut into chopped strands, which are then cut into these dimensions. By being able to do this, it has become possible to significantly improve productivity.

Furthermore, the fiber strength obtained by the method of the present invention is higher than that obtained by conventional spinning methods for optically isotropic carbon fibers and optically anisotropic carbon fibers, respectively. Since the strength is approximately 1-14 to 2.5 times higher than that of the composite material, it is possible to improve the fracture toughness when made into a composite material.

Furthermore, in composite fibers of optically isotropic carbon fibers and optically anisotropic carbon fibers, due to the difference in their expansion coefficients, more twisting and curling occurs, which leads to poor bondability with the matrix material. will be further improved.

The fibers have an apparent bulk and elasticity due to twisting and curling, and the fibers alone can also be used as cotton-like fibers with a certain amount of fluff.

For the pitch-based short carbon fibers according to the present invention, heavy oils having many aromatic six-membered ring structures in their molecules, generally coal tar, petroleum cracking tar, steam cracker tar, and the like are used as starting materials. From these raw materials, select the one with the best purity and softening point, or if it does not meet the requirements, perform pretreatment such as solvent extraction or thermal modification. Generally, raw material heavy oil contains fine solids such as free carbon, which must be removed. One method is to dissolve it in an aromatic oil such as anthracene oil or an organic solvent such as quinoline and filter it. Another method is to generate mesocarbon microspheres that can sufficiently adsorb free carbon, mineral particles, and microsolids contained in the pitch by carbon thermal treatment, and then remove them by extraction filtration. . The pitch obtained by concentrating this filtrate is further subjected to a secondary heat treatment for polycondensation, and at the same time light components are removed to adjust the softening point and obtain optically isotropic pitch.

On the other hand, optically anisotropic pitch is obtained by diluting the pitch with tetrahydroquinoline to 2 to 3 times the amount, and subjecting the pitch to solvent hydrogenation at a temperature of 400 to 450° C. under an autogenous pressure of 10 to 30 kgf/cnf. 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. Another method is to heat-treat the heavy tar (FFC decant oil) created when producing gasoline using the fluidized contact fractionation method of soft petroleum oil to form mesofaces and remove light components to soften it. Obtain the mesoface pitch by adjusting the points.

The optically isotropic pitch thus obtained and the optically anisotropic pitch have different properties when made into carbon fibers. Generally, when optically isotropic pitch is spun and made into carbon fibers, the graphite crystals within the fibers after carbonization become fine and the alignment in the fiber axis direction becomes poor, so it is called a general-purpose type (GP product).
Tensile strength is generally around 100 kg/-1 and elastic modulus of 5 ton/-.

In the case of optical anisotropy, it is important not only to prepare the raw material pitch, but also to control the orientation of molecules, especially in order to obtain high-strength, high-elastic carbon fibers.
This is affected by molecular orientation control unique to liquid crystal pitch. Therefore, there is a wide range of mechanical properties depending on the conditions, and the tensile strength of currently available carbon fibers is 300 to 500 kg/-1, and the modulus of elasticity is 30 to 500 kg/-1.
70 tons/-.

Also, optically isotropic carbon fiber has a thermal expansion coefficient of 4 x 10'/K' at 1000°C, whereas optically anisotropic carbon fiber has a thermal expansion coefficient of 2 x 10-6/', which is half that. It is.

As mentioned above, the present invention dramatically improves the properties of optically isotropic carbon fibers and optically anisotropic carbon fibers, and further improves the final use shape by infusibility and sintering after spinning. This is a cotton-like pitch-based short carbon fiber that can be molded into.

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

Two types of pitch were used for spinning: an optically isotropic pitch with a softening point of 230°C and a mesoface pitch containing 98% of an optically anisotropic phase with a softening point of 267°C. The spinning device used had the structure shown in FIG. Here, the inner diameter of the pitch discharge hole 1 was 0.2 mm, and the hole diameter of the gas flow path nozzle 2 was 0.5 mm.

Optically isotropic pitch 3 or optically anisotropic pitch 4 as described above
Either of these was placed in the pitch reservoir 5, the inside of which was replaced with nitrogen gas, and then heated and melted with the heater 6. After the temperature of the pitch reached a predetermined temperature, nitrogen gas at the same pressure was introduced from the upper part of the pitch reservoir 5 and the gas introduction pipe 7. Then, fibrous pitch discharged from the lower part of the spinning device was collected. The collected fibrous pitch was heated to 320°C in air at a rate of 2°C per minute and held for 30 minutes to be infusible. Then, it was carbonized at 1000°C in a nitrogen gas stream,
Furthermore, graphitization treatment was performed at 2600° C. in an argon gas stream.

One carbon fiber obtained in this way has a fiber length of 5
It was attached to a mount so that the thickness of the sample was 3 mm, and the tensile strength was measured according to the single fiber method specified in JISR7601. In addition, the outer diameter of the fiber was measured by the laser diffraction method specified in the same regulations.

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

(The following is a blank space) As seen in the table above, as the pitch temperature and gas pressure increase, the fiber diameter becomes thinner and its length also becomes shorter. The length of fibers varies even within the same lot.
- Although it cannot be expressed generally, the longest length is 50 mm in implementation number 1-1.

FIG. 3 is a photograph (500x magnification) of the shape of the short carbon fiber of Example No. 2-2 in the above table, observed with a scanning electron microscope. The fiber diameter varies in size and is not constant.

Moreover, most of it is angled and not straight. The fact that the fibers are bent is the same for other implementation numbers. Furthermore, in order to examine the shape of the fiber of this execution number, the magnification was 400.
Figure 4 is a photograph observed at 0x magnification. As seen in this figure, the cross-sectional shape of the fiber is not circular (it is a flat ellipsoid).It is thought that the ellipsoid or flat ellipsoid is the reason why the fiber is curved. It will be done.

Fibers with a diameter of 10 μm or less are short and curved, so they tend to form lumps, but their feel is soft, elastic, and cotton-like. However, it has good defibration properties and is easily dispersed as single fibers in water.

In addition, the shape of this fiber is flat, twisted, curled, and random, and it is fluffy and bulky, so when placed in a certain container, it is fixed by the elasticity of the fiber even if it is not made into a mat. It was confirmed that it could be used as it is in filtration filters, etc. Furthermore, by creating a composite with an elastic plastic material, a rubber-like carbon fiber composite, which was previously impossible, was created, and a highly abrasion-resistant and elastic packing material was made possible.

In addition, in the method for producing carbon short fibers of the present invention, pitch-based short carbon fibers are produced using various cross-sectional shapes of the tip of the spinning hole and the discharge hole of the spinning device. A cross-sectional shape can be obtained.

Note that composite short carbon fibers made by combining optically isotropic carbon fibers and optically anisotropic carbon fibers can also be produced using a spinning apparatus provided with a partition plate 8 as shown in FIG.

(Effects of the Invention) As can be seen from the above explanation, the method for producing short carbon fibers of the present invention completely eliminates the need for the cutting process that was conventionally required to make chopped strands.
Improving productivity can bring significant economic benefits.

Conventionally, it was thought that optically isotropic carbon fibers could only be produced as general-purpose products (GP), but according to the method for producing short carbon fibers of the present invention, pitch-based optically isotropic carbon fibers can be produced. Since the strength is equivalent to or even higher than that of pitch-based optically anisotropic carbon fiber, manufacturing costs such as adjustment of pitch raw materials can be significantly reduced. In addition, optically anisotropic carbon fibers have strength comparable to or higher than fibers obtained by conventional spinning methods, so they are used as high-strength composite materials in the electronics industry, automobile industry, and space industry. It can be used as an extremely useful material for high-strength, high-precision structural parts such as metals, carbon, and ceramics for industrial applications.

In addition, because the cross-sectional shape of the fibers is flat, twisted, and curled, it has a cotton-like bulk as it is, and has elasticity and stretchability, so it is used in the production of composite materials. In this case, since it has good compatibility with the flexible matrix material, peeling between the pitch-based carbon fiber and the matrix material is less likely to occur. Therefore, by applying it as a composite material such as a stretchable conductive material, elastic packing, and engineering plastic, it is possible to create a composite material with unprecedented high strength.

Furthermore, according to the method for producing carbon fibers of the present invention, pitch-based short carbon fibers having the above-mentioned excellent effects can be easily produced in large quantities at one time, and the fibers can be spun at the tip of the spinning hole of the spinning device. By manufacturing using caps with various cross-sectional shapes, it is possible to appropriately manufacture fibers with various cross-sectional shapes.

[Brief explanation of drawings]

Fig. 1 is a diagram showing one embodiment of the manufacturing method of the present invention, Fig. 2 is a diagram showing another embodiment of the manufacturing method of the present invention, and Fig. 3 is a diagram showing the shape of the short carbon fiber of the present invention by scanning type. A 500x photograph taken with an electron microscope, and FIG. 4 is a 4000x photograph of the fiber in FIG. 3, which is further enlarged. 1... Pitch discharge hole, 2... Gas flow path nozzle, 3
...Optical isotropic pitch, 4... Optical anisotropic pitch,
Sochi, 5...Pitch storage section, 6...Converter 7...
Gas introduction pipe, 8...partition plate. Diagram

Claims (1)

[Scope of Claims] 1) It is characterized by being a cotton-like short carbon fiber made of pitch-based optically isotropic carbon fiber, pitch-based optically anisotropic carbon fiber, or a composite fiber thereof. Short carbon fiber. 2) Either optically isotropic pitch, optically anisotropic pitch, or both pitches are supplied to a spinning device, and the molten pitch is spun while being blown out with gas pressure from around the discharge hole, and is infusible and fired. A method for producing short carbon fibers characterized by: