Method for producing carbon fiber, carbon fiber, prepreg,...

Details Method for producing carbon fiber, carbon fiber, prepreg,... Method for producing carbon fiber, carbon fiber, prepreg,...

1999-05-21

2001-01-09

Weisberger, Richard (Department: 1774)

Stock material or miscellaneous articles

Coated or structually defined flake, particle, cell, strand,...

C428S428000, C428S392000, C428S401000, C442S417000, C442S060000, C423S447600, C423S447700, C423S447800, C264S029200, C264S029600, C264S029700

Reexamination Certificate

active

06171696

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method for producing a carbon fiber, the carbon, and a prepreg produced by impregnating the carbon fiber with an epoxy resin. The present invention also relates to a molded article produced from the fiber-reinforced composite material using the foregoing prepreg.
BACKGROUND OF THE INVENTION
Fiber-reinforced composite materials are used in many fields of sporting goods and equipment for leisure time amusement. One of the important characteristics of these sporting goods is lowness of elastic modulus, namely excellence in flexibility. For example, a flexible tennis racket prevents elbow pain, and a properly flexible fishing rod makes the hands smoothly feel a bite of fish and facilitates the taking-up of the fish. Also, in the case of a golf club, a club equipped with a shaft capable of holding flexibility is beneficial to amateurs and female golfers. Although their swing speed is slow, the use of such a club enables them to increase the head speed of the club so that a longer flying distance of ball is obtained because of the pliant suppleness of the shaft.
As stated above, in order to impart flexibility to a molded article produced from the fiber-reinforced composite material, a glass fiber or the like having a low tensile elastic modulus has been hitherto used. However, the use of a glass fiber is associated with disadvantages. For example, since the density of a glass fiber is larger than a carbon fiber or the like, the use of the glass fiber brings about increase in weight. Further, when a glass fiber and a carbon fiber are used in a combination, the difference in thermal expansivity between the two tends to cause defective products due to bending particularly in golf shafts, fishing rods, and the like.
OBJECTS OF THE INVENTION
The object of the present invention is to solve the problems of the prior art and to provide a method for producing a carbon fiber, which has excellent flexibility and does not cause molding defects such as bending and the like, the carbon fiber, a prepreg, and a molded article produced from the fiber-reinforced composite material.
SUMMARY OF THE INVENTION
First, the present invention relates to a method for producing a pitch-based carbon fiber, comprising carrying out a first-stage infusibilization of a pitch-based fiber, which is obtained from mesophase pitch having a softening point of 200 to 400° C. and a true density of 1.30 to 1.38 g/cm
3
, in a mixed gas atmosphere having a nitrogen dioxide concentration of 1 to 5% by volume and an oxygen concentration of 5 to 50% by volume, the balance being an inert gas such as nitrogen or steam, at a temperature between 100 and 200° C.; and then carrying out a second-stage infusibilization of the product of the first-stage infusibilization in a mixed gas atmosphere having a nitrogen dioxide concentration of 0.1 to 5% by volume and an oxygen concentration of 5 to 40% by volume, the balance being an inert gas such as nitrogen or a mixture of the inert gas and steam, at a temperature between 200 and 350° C.
Second, the present invention relates to a pitch-based carbon fiber, preferably a continuous pitch-based carbon fiber, having a tensile elastic modulus of 9 to 16 tonf/mm
2
, a density of 1.5 to 1.9 g/cm
3
, a thermal expansion coefficient of −0.8×10
−6
to 0.0/K, a diameter of 4 to 12 &mgr;m, a coefficient of water absorption of 0 to 4%, and a strain at compressive break of 1.7 to 5%.
Third, the present invention relates to a prepreg produced by impregnating the above-described carbon fiber with an epoxy resin.
Fourth, the present invention relates to a molded article produced from a fiber-reinforced composite material using at least as part thereof the above-described prepreg.
PREFERRED EMBODIMENTS OF THE INVENTION
The present invention will now be explained in more details.
Mesophase pitch, which is characterized by easy graphitization, can be used as a starting material of the carbon fiber of the present invention.
In the present invention, the mesophase pitch means a pitch which exhibits anisotropy under a polarizing microscope. Preferably, the mesophase pitch has an anisotropic phase content of 70 to 100%.
Examples of the mesophase pitch include coal-based pitch such as coal tar and coal tar pitch; liquefied coal pitch; ethylene tar pitch; petroleum-based pitch such as decanted oil pitch obtained from residual oil in fluidized catalytic cracking; and synthetic pitch produced from naphthalene or the like by using a catalyst or the like.
The softening point of the mesophase pitch for use in the present invention is preferably 200 to 400° C. and more preferably 250 to 350° C.
The true density of the mesophase pitch for use in the present invention is 1.30 to 1.38 g/cm
3
and preferably 1.31 to 1.36 g/cm
3
.
If the density of the mesophase pitch for use in the present invention is below 1.30, the coefficient of water absorption of the carbon fiber to be obtained is unsuitably too large. To the contrary, if the density of the mesophase pitch for use in the present invention is above 1.38, the spinnability is undesirably reduced.
The carbon fiber of the present invention can be obtained by a process comprising extruding the mesophase pitch at a temperature, at which the pitch has a viscosity of 200 to 900 poise, from a nozzle having 1000 or more holes each constituting a capillary having a diameter of 0.05 to 0.12 mm by applying a pressure of about 5 to 40 kg/cm
2
while stretching the extruded pitch at a winding velocity of 100 to 500 m/min to obtain a pitch fiber bundle having a diameter of 5 to 15 &mgr;m and composed of 1000 to 100,000 filaments, infusibilizing the pitch fiber bundle thus obtained, and then thermally processing the infusibilized pitch fiber bundle.
If the spinning viscosity is below 200 poise, the crystal structure of the carbon fiber to be obtained is so coarse that an excellent compressive strength cannot be obtained. To the contrary, if the spinning viscosity is above 900 poise, the crystal system of the carbon fiber to be obtained is liable to be defective and therefore is not desirable from the standpoint of the expression of strength.
According to the method for producing a carbon fiber of the present invention, the infusibilizing process needs to comprise two or more stages described below each having a different infusibilization condition.
That is, a first-stage infusibilization is carried out in a mixed gas atmosphere having a nitrogen dioxide concentration of 1 to 5% by volume, preferably 1.5 to 5% by volume, and an oxygen concentration of 5 to 50% by volume, preferably 20 to 50% by volume, the balance being an inert gas such as nitrogen or steam, at a temperature between 100 to 200° C.
Further, a second-stage infusibilization is carried out in a mixed gas atmosphere having a nitrogen dioxide concentration of 0.1 to 5% by volume, preferably 0.2 to 2% by volume, and more preferably 0.2 to 1% by volume, and an oxygen concentration of 5 to 40% by volume, preferably 10 to 30% by volume, the balance being an inert gas such as nitrogen or a mixture of the inert gas and steam, at a temperature between 200 and 350° C. and preferably at a temperature between 210 and 350° C.
Particularly, it is preferable to lower the concentration of nitrogen dioxide and raise the temperature in the second-stage infusibilization, relative to the first-stage infusibilization.
In the thermal treatments, it is preferable to obtain primarily carbonized fiber bundles by carbonizing the infusibilized fiber bundles at a temperature between 350 and 850° C. in an inert gas atmosphere without loading any tension on the fibers.
Moreover,it is also possible to further thermally process the primarily carbonized fiber bundles in an inert atmosphere at 850 to 1700° C., preferably at 900 to 1500° C. while loading a tension of 0.1 to 5 gf/tex on the fiber bundles.
As a result of the above-described treatments, it is possible to obtain a pitch-based carbon fiber, preferably a continuous pitch-based carbon fiber, having a tensile elastic modulus

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