Resin composite material containing graphite fiber

Provided herein is a resin composite material containing graphite fiber which has good electrical conductivity and changes only a little in electrical resistance after processing. These desired properties are achieved by incorporating a synthetic resin matrix with a specific type of graphite fiber. The graphite fiber has a structure characterized by that the hexagonal network planes of carbon atoms are oriented substantially parallel to the fiber axis and like the annual ring. It is produced by bringing a hydrocarbon compound into contact, at a high temperature, with a metallic catalyst in the form of ultrafine particles to yield carbon fiber grown in the gas phase, and subsequently graphitizing thus obtained carbon fiber.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a conductive resin composite material 
suitable for the production of conductive molded articles. 
2. Description of the Prior Art 
The recent development in electronics technology needs shielding for static 
charge and electromagnetic waves which is made from a conductive resin 
composite material composed of a synthetic resin or rubber and carbon 
particles or fibers, having light weight, high strength, high 
conductivity, and good moldability. Unfortunately, a conductive resin 
composite material incorporated with carbon particles such as carbon black 
has a disadvantage that it needs to contain a large amount of carbon 
particles if it is to have a low resistance. The carbon particles 
incorporated in large quantities greatly increase the viscosity of the 
resin composite material, which leads to poor moldability. An additional 
disadvantage of the carbon-containing resin composite material is that the 
structure of carbon black is broken by shear at the time of mixing or 
molding, which leads to the deviation of resistivity. On the other hand, a 
conductive resin composite material incorporated with carbon fibers formed 
from organic fibers such as polyacrylonitrile fibers by the carbonization 
and subsequent graphitization also has a disadvantage that it does not 
provide a desired conductivity because carbon fibers themselves are short 
of conductivity. 
In order to overcome these disadvantages, there was proposed in Japanese 
Patent Laid-open No. 218661/1986 a resin composite material having a low 
resistance and good moldability, which is formed by incorporating plastics 
or rubber with carbon fibers 0.05 to 4 .mu.m in diameter, with an aspect 
ratio (length-to-diameter ratio) of 20 to 1000, and having a uniform 
thickness with very few branches. Such carbon fibers are produced by 
pyrolyzing a hydrocarbon introduced into a reaction zone together with a 
specific organometallic compound (and, if necessary, a carrier gas). The 
carbon fibers undergo heat treatment according to need. Even this resin 
composite material needs a large amount of carbon fiber if it is to have a 
low resistance. The incorporation of carbon fiber in large quantities 
adversely affects the formability. In addition, however large the quantity 
of carbon fiber may be, the resistivity achieved by it is of the order of 
10.sup.-2 .OMEGA..cm at the lowest. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a resin composite 
material which has good electrical conductivity and changes only a little 
in electrical resistance after processing. These desired properties are 
achieved by incorporating a synthetic resin matrix with a specific type of 
graphite fiber. The gist of the present invention resides in a resin 
composite material containing graphite fiber which comprises a synthetic 
resin matrix and an intercalated compound of graphite fiber and bromine, 
said graphite fiber having a structure characterized in that the hexagonal 
network planes of carbon atoms are oriented substantially parallel to the 
fiber axis and like annual rings. 
DETAILED DESCRIPTION OF THE INVENTION 
The graphite used as a constituent of the composite material of the present 
invention is obtained in two steps. The first step involves the thermal 
decomposition of a hydrocarbon in gas phase which yields carbon fiber. The 
second step involves the heat treatment of the carbon fiber in an 
atmosphere of inert gas. The hydrocarbon for thermal decomposition 
includes aromatic hydrocarbons such as toluene, benzene, and naphthalene, 
and aliphatic hydrocarbons such as propane, ethane, and ethylene. 
Preferable among them are benzene and naphthalene. The thermal 
decomposition is performed by bringing a gasified feedstock, together with 
a carrier gas (such as hydrogen), into contact with a catalyst at 
900.degree. to 1500.degree. C. The catalyst is iron, nickel, or 
iron-nickel alloy etc. in the form of ultrafine particles 100 to 300 .ANG. 
in diameter which is supported on a ceramics or graphite substrate. 
Alternatively, the thermal decomposition is performed by brining a 
gasified feedstock, together with a carrier gas (such as hydrogen), into 
contact with a catalyst suspended in a reaction zone at 900.degree. to 
1500.degree. C. The catalyst is iron, nickel, or iron-nickel alloy etc. in 
the form of ultrafine particles 100 to 300 .ANG. in diameter. 
The carbon fiber obtained in this manner is ground using an adequate 
grinding machine or crusher such as ball mill, rotor speed mill, and 
cutting mill, if necessary. This crushing is not a must but desirable 
because the crushed carbon fiber forms an intercalated compound easily and 
disperses readily into a resin matrix. 
The ground carbon fiber is subsequently heated in an atmosphere of inert 
gas such as argon at 1500.degree. to 3500.degree. C., preferably 
2500.degree. to 3000.degree. C., for 10 to 120 minutes, preferably 30 to 
60 minutes. This heat treatment yields graphite fiber having a crystal 
structure characterized in that the hexagonal network planes of carbon 
atoms are oriented substantially parallel to the fiber axis and like 
annual rings. With a heating temperature lower than 1500.degree. C., the 
heat treatment does not fully develop the crystal structure of carbon 
atoms. Conversely, with a heating temperature higher than 3500.degree. C., 
the heat treatment does not produce any additional effect and hence it is 
uneconomical. With a heating time shorter than 10 minutes, the heat 
treatment does not uniformly develop the crystal structure of carbon 
atoms. Conversely, with a heating time longer than 120 minutes, the heat 
treatment does not produce any additional effect. 
In the subsequent step, the graphite fiber is treated with bromine at 
0.degree. to 50.degree. C. for 10 minutes or more. The bromine used for 
this purpose should have as high a concentration as possible, preferably 
99% or higher, free of water. The bromine may be in the form of liquid or 
vapor when brought into contact with graphite fiber. In the case of liquid 
bromine, the bromine treatment may be accomplished by dipping the graphite 
fiber into bromine. The liquid bromine used in this manner should be free 
of impurities, because impurities would prevent bromine from infiltrating 
and diffusing into the space between the lattice plane of graphite crystal 
or impurities themselves get into the space between the lattice plane of 
graphite crystal. This holds true also in the case where bromine is used 
in vapor form. An advantage of using bromine vapor is that nonvolatile 
impurities are removed by vaporization and this eases restrictions on the 
purity and form of the bromine vapor source. 
The graphite fiber should be brought into contact with bromine at 0.degree. 
to 50.degree. C., preferably 5.degree. to 30.degree. C. If the temperature 
is excessively low, it takes a long time for bromine to diffuse into the 
space between the lattice plane of graphite crystal and the temperature 
control is difficult. If the temperature is excessively high, the handling 
of bromine is difficult and the graphite fiber is liable to break or lose 
mechanical strength. 
The graphite fiber should be kept in contact with bromine for at least 10 
minutes, preferably 30 minutes to 72 hours. Contacting for less than 10 
minutes does not permit effective time control and hence results in 
quality variation. In addition, the reduction of contacting time does not 
produce any economical effect. 
The bromine-treated graphite fiber thus obtained is finally uniformly 
incorporated into a synthetic resin or rubber. Among the synthetic resins 
that can be used in the present invention are thermoplastic resins such as 
polyethylene, polypropylene, polyvinyl chloride, ethylenevinyl acetate 
copolymer, and ethylene-acrylic ester copolymer etc., and thermosetting 
resins such as silicone resin, phenolic resin, urea resin, and epoxy resin 
etc. Among the synthetic rubbers that can be used in the present invention 
are chloroprene, chlorosulfonated polyethylene, chlorinated polyethylene, 
ethylene-.alpha.-olefin rubber, ethylene-propylene rubber, silicone 
rubber, acrylic rubber, and fluororubber etc. 
The dispersion of bromine-treated graphite fiber into the synthetic resin 
or rubber may be accomplished by using an ordinary mixing machine such as 
two-roll mixer, kneader, internal mixer, and Banbury mixer. The amount of 
the bromine-treated graphite fiber is not specifically limited; but it 
should be in the range of 5 to 200 parts by weight, preferably 10 to 100 
parts by weight, for 100 parts by weight of the synthetic resin or rubber 
from the standpoint of electrical resistivity, processability, and 
moldability. 
The thus obtained composite material can be formed into a desired shape by 
extrusion, injection molding, transfer molding, or press molding etc., 
which are properly selected according to the base resin of the composite 
material and the shape of the molded article. The base resin of the 
composite material may be incorporated with additives such as filler, 
processing aid, antioxidant, and cross-linking agent.

EXAMPLES 
The invention will be described in more detail with reference to the 
following examples, which are intended to restrict the scope of the 
invention. 
REFERENTIAL EXAMPLE 1 
Carbon fiber, 2 to 10 mm long and 10 to 50 .mu.m in diameter, was prepared 
by the thermal decomposition of benzene (introduced together with 
hydrogen) at 1000.degree. to 1100.degree. C. in a horizontal tubular 
electric furnace containing a metallic iron catalyst 100 to 300 .ANG. in 
diameter supported on a mullite ceramics plate. This carbon fiber was 
crushed using a satellite ball mill (Model P-5 provided by Fritsch Japan 
Co., Ltd.) for 20 minutes at 500 rpm. 
The ground carbon fiber was graphitized by heating at 2960.degree. to 
3000.degree. C. for 30 minutes in an electric furnace with an argon 
atmosphere. The resulting fiber was examined by X-ray diffratometry and 
observed under an electron microscope. It was found to have a crystal 
structure characterized by that the hexagonal network planes of carbon 
atoms are oriented substantially parallel to the fiber axis and like the 
annual ring. It was also found to have a length of 70 to 100 .mu.m. 
The graphite fiber was placed in a glass container, into which was poured 
bromine. With the glass container tightly stoppered, the graphite fiber 
and bromine were allowed to stand at 23.degree. C. for 48 hours. Thus 
there was obtained bromine-treated graphite fiber (A). 
REFERENTIAL EXAMPLE 2 
Carbon fiber, 10 to 1000 .mu.m long and 0 1 to 0.5 .mu.m in diameter, was 
prepared by the thermal decomposition of benzene (introduced together with 
hydrogen) at 1000.degree. to 1100.degree. C. in a vertical tubular 
electric furnace containing a metallic iron catalyst powder 100 to 300 
.ANG. in diameter suspended by an upward hydrogen stream. This carbon 
fiber was ground and then graphitized in the same manner as in Referential 
Example 1. The thus obtained graphite fiber was examined by X-ray 
diffratometry and observed under an electron microscope. It was found to 
have a crystal structure characterized in that the carbon hexagonal 
network face is substantially parallel with the axis of the fibers and is 
oriented coaxially. It was also found to have a length of 3 to 5 .mu.m. 
The graphite fiber was treated with bromine in the same manner as in 
Referential Example 1. Thus there was obtained bromine-treated graphite 
fiber (B). 
EXAMPLE 1 
Each of bromine-treated graphite fiber (A) and (B) obtained in Referential 
Examples 1 and 2, respectively, was mixed with low-density polyethylene 
("Mirason 3530" provided by Mitsui Petrochemical Co., Ltd.) at a ratio of 
20 or 40 parts by weight to 100 parts by weight, at 140.degree. to 
150.degree. C. for 30 minutes, using a 6-inch roll. 
The resulting composition was press-formed into a sheet, 70 mm long, 10 mm 
wide, and 1 mm thick. The specimen, with both edges 10 mm wide coated with 
a conductive silver paint, was examined for electrical resistivity using a 
Wheatstone bridge method. The composition containing 40 parts by weight of 
bromine-treated graphite fiber for 100 parts by weight of low-density 
polyethylene was pelletized using a pelletizer, and the pellets were 
extruded to form a 0.5 mm thick coating on a radiation-crosslinked 
polyethylene-coated electric wire using a 20 mm type extruder at 
200.degree. to 250.degree. C., so as to evaluate the formability. 
For the purpose of comparison, three kinds of composite materials were 
prepared in the same manner as above except that the bromine-treated 
graphite fiber was replaced by conductive carbon black ("Ketjenblack EC" 
provided by Lion Akzo Co., Ltd.), PAN-based carbon fiber ("Milled Fiber 
MLD-30" provided by Toray Industries, Inc.), or bromine-free graphite 
fiber (C) prepared by Referential Example 1. The specimens were examined 
for resistivity. The results are shown in Table 1. 
The extrusion formability of the composite materials numbered 2, 4, 6, 8, 
and 10 in Table 1 was evaluated. The results are shown in Table 2. 
It is noted from Tables 1 and 2 that the composite materials pertaining to 
the present invention have a very low resistivity and outstanding 
formability. 
TABLE 1 
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(Formulation, parts by weight) 
Component 1 2 3 4 5* 6* 7* 8* 9* 10* 
__________________________________________________________________________ 
Synthetic resin 
100 
100 
100 
100 
100 
100 
100 
100 
100 
100 
Graphite fiber (A) 
20 
40 
-- -- -- -- -- -- -- -- 
Graphite fiber (B) 
-- -- 20 
40 
-- -- -- -- -- -- 
Graphite fiber (C) 
-- -- -- -- 20 
40 
-- -- -- -- 
Carbon black 
-- -- -- -- -- -- 20 
40 
-- -- 
Carbon fiber 
-- -- -- -- -- -- -- -- 20 
40 
Resistivity, .OMEGA. .multidot. cm 
1.4 
0.018 
1.8 
0.032 
6.8 
0.24 
34.5 
4.6 
49.8 
11.3 
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*Comparative Examples 
TABLE 2 
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No. Results 
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2 Permitted continuous operation for 4 hours without any 
trouble. Gave a coating with a good appearance. 
4 Permitted continuous operation for 4 hours without any 
trouble. Gave a coating with a good appearance. 
6* Permitted continuous operation for 4 hours without any 
trouble. Gave a coating with a good appearance. 
8* Gave a coating thicker than 0.5 mm owing to increase 
in melt viscosity. 
10* Gave a 0.5-mm thick coating, at the sacrifice of a smooth 
surface. 
______________________________________ 
*Comparative Examples 
EXAMPLE 2 
A resin composite material was prepared from 100 parts by weight of epoxy 
resin ("Epikote 828" provided by Yuka Shell Epoxy Co., Ltd.), 110 parts by 
weight of acid anhydride hardener ("Epicure YH-307" provided by Yuka Shell 
Epoxy Co., Ltd.), 1 part by weight of hardening accelerator ("Epicure 
EMI-24" provided by Yuka Shell Epoxy Co., Ltd.), 50 or 100 parts by weight 
of bromine-treated graphite fiber (A) or (B) obtained in Referential 
Example 1 or 2, respectively. The resin composite material was made into a 
dumbbell specimen (conforming to JIS K-6301, No. 4) by transfer molding, 
with curing at 80.degree. C. for 3 hours. 
The mixing of the components was performed as follows: At first, the epoxy 
resin was placed in a mixing pot and then the graphite fiber was added. 
They were mixed for 60 minutes, and the resulting mixture was passed 
through a three-roll type mill five times. To the milled mixture were 
added the hardener and hardening accelerator, and the resulting mixture 
was passed through a three-roll type mill five times. The thus obtained 
composite material was finally fed to a transfer molding machine. 
The specimens were examined for resistivity and the moldability of the 
composite material was evaluated. The results are shown in Table 3. 
For the purpose of comparison, four kinds of composite materials were 
prepared in the same manner as above except that the bromine-treated 
graphite fiber was replaced by bromine-free graphite fiber (C) or 
PAN-based carbon fiber. The composite materials were evaluated in the same 
manner as above. The results are shown in Table 3. 
It is noted from Table 3 that the composite material pertaining to the 
present invention has a very low resistivity and outstanding moldability. 
TABLE 3 
__________________________________________________________________________ 
(Formulation, parts by weight) 
No. of composite materials 
Component 11 12 13 14 15* 16* 17* 18* 
__________________________________________________________________________ 
Graphite fiber (A) 
50 100 -- -- -- -- -- -- 
Graphite fiber (B) 
-- -- 50 100 -- -- -- -- 
Graphite fiber (C) 
-- -- -- -- 50 100 -- -- 
Carbon fiber 
-- -- -- -- -- -- 50 100 
Resistivity, .OMEGA. .multidot. cm 
0.074 
0.0079 
0.093 
0.0092 
0.43 
0.098 
1.43 
0.83 
Moldability 
Good 
Good 
Good 
Good 
Good 
Good 
Fair** 
Fair** 
__________________________________________________________________________ 
*Comparative Examples 
**with a rough surface 
EXAMPLE 3 
A coating material containing 25% solids was prepared from 100 parts by 
weight of chlorosulfonated polyethylene ("Hypalon 45" provided by DuPont) 
and 50 parts by weight of bromine-treated graphite fiber (A) or (B) 
obtained in Referential Example 1 or 2, respectively. The components were 
kneaded by using a two-roll type mill and then a mixing machine. The 
mixture was further mixed in a mixing pot with toluene for 48 hours. 
During mixing, 2.5 parts by weight of antioxidant was added. 
For the purpose of comparison, two kinds of coating materials were prepared 
in the same manner as above except that the bromine-treated graphite fiber 
was replaced by bromine-free graphite fiber (C) or PAN-based carbon fiber. 
The coating material was applied to a polyester film to form a thin coating 
film. The resistivity of the thin coating film was measured by the aid of 
blade electrodes pressed against the thin coating film. The results are 
shown in Table 4. 
It is noted from Table 4 that the coating material pertaining to the 
present invention has a very low resistivity. 
TABLE 4 
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(Formulation, parts by weight) 
No. of composite materials 
Component 19 20 21* 22* 
______________________________________ 
Graphite fiber (A) 
50 -- -- -- 
Graphite fiber (B) 
-- 50 -- -- 
Graphite fiber (C) 
-- -- 50 -- 
Carbon fiber -- -- -- 50 
Resistivity, .OMEGA. .multidot. cm 
0.034 0.059 0.68 1.92 
______________________________________ 
*Comparative Examples 
As mentioned above, the resin composite material containing graphite fiber 
of the present invention has a low resistivity and outstanding 
processability and moldability owing to the bromine-treated graphite fiber 
in which is formed an intercalated compound. According to the present 
invention, it is possible to produce a composite material of stable, high 
quality.