Carbon fiber cord for rubber reinforcement

A carbon fiber cord for rubber reinforcement comprising a carbon fiber bundle having coated thereon from 5 to 50% by weight, based on the total weight of the carbon fiber bundle thus treated, of a resin composition comprising a mixture of 100 parts by weight of a butadiene-acrylonitrile copolymer having a carboxyl group on at least both terminals of the molecule of the copolymer and from 5 to 40 parts by weight of an epoxy resin, or a reaction mixture obtained therefrom, and a curing agent for the epoxy resin, and further coated thereon from 0.5 to 5% by weight, based on the total weight of the carbon fiber bundle and the resin composition coated thereon, of a mixture of a phenolformalin-rubber latex type adhesive for rubber. The carbon fiber cord exhibits excellent adhesion to rubber and fatigue resistance.

This invention relates to a carbon fiber cord for reinforcing rubber, which 
is excellent in adhesion to rubber. 
BACKGROUND OF THE INVENTION 
Rubber reinforcing cords have been made of organic fibers, such as rayon, 
polyamide, polyester, and, as the lastest addition, aramide, or inorganic 
fibers such as glass fiber, steel fiber, etc. For particular use in tires, 
cords for rubber reinforcement are desirably made of fibrous materials 
having high strength, high modulus, and lightweight, from the standpoint 
of controllability and running stability of tires when driving, 
comfortability to ride on, durability of tires, and fuel cost. 
Carbon fibers have a lower density, higher modulus of elasticity, and 
higher strength as compared with the above-recited glass fiber and steel 
fiber and are, therefore, highly promising as an excellent reinforcing 
material for rubber. 
However, carbon fibers have a disadvantage of poor adhesion to rubber. In 
order to overcome this disadvantage, various improved processes for 
producing reinforcing cords from carbon fibers have hitherto been 
proposed, such as a process comprising twisting elastomer-impregnated 
carbon fibers as disclosed in U.S. Pat. No. 3,648,452; a process 
comprising treating carbon fibers with an epoxy compound and then with an 
adhesive, such as a mixture of a resorcinol-formaldehyde condensate and a 
rubber latex (hereinafter referred to as RFL) as disclosed in Japanese 
Patent Application (OPI) No. 102678/75 (the term "OPI" as used herein 
means "unexamined published application"); a process comprising treating 
carbon fibers with a first treating bath containing a polyisocyanate and 
then with a second treating bath containing RFL as described in Japanese 
Patent Application (OPI) No. 102679/75; and the like. Nevertheless, none 
of these attempts have completely succeeded in attaining fully 
satisfactory adhesion to rubber. In particular, carbon fiber cords are 
inferior in resistance to repeated fatigue, such as flexual, compression, 
and the like, due to the high modulus of elasticity of carbon fibers. 
According to the inventors' study, it was found that the insufficient 
performances of the above-described conventional carbon fiber cords arise 
from insufficient adhesion or bonding between carbon fibers and rubber, 
the elastomer, or the polyisocyanate. Moreover, although bond of adhesion 
between epoxy resins and carbon fibers is relatively strong, the epoxy 
resin-treated carbon fibers do not have softness any more, and, when 
applied to rubber reinforcement, exhibit rather deteriorated resistance to 
flexing fatigue. 
Since the carbon fibers as treated with the epoxy resin have poor adhesion 
to rubber, an additional treatment with RFL is required. According to this 
technique, the uncured epoxy resin is reacted with RFL to increase 
adhesion strength to rubber, but there is noted a tendency that the 
resulting cord has reduced softness and reduced fatigue performance. 
SUMMARY OF THE INVENTION 
One object of this invention is to provide a carbon fiber cord for rubber 
reinforcement which exhibits excellent adhesion to rubber. 
Another object to this invention is to provide a carbon fiber cord for 
rubber reinforcement which exhibits softness and excellent resistance to 
flexing fatigue. 
It has now been found that the above objects can be accomplished by a 
carbon fiber cord for rubber reinforcement comprising a carbon fiber 
bundle having coated thereon from 5 to 50% by weight, based on the total 
weight of the thus treated carbon fiber bundle, of a resin composition 
comprising (a) a mixture of 100 parts by weight of a 
butadiene-acrylonitrile copolymer having a carboxyl group on at least both 
terminals of the molecule of the copolymer and from 5 to 40 parts by 
weight of an epoxy resin, or a reaction mixture, obtained therefrom, and 
(b) a curing agent for the epoxy resin, and further coated thereon from 
0.5 to 5% by weight (dry weight), based on the total weight of the carbon 
fiber bundle and the resin composition coated thereon, of a mixture of a 
phenolformalin-rubber latex type adhesive for rubber.

DETAILED DESCRIPTION OF THE INVENTION 
The carbon fibers which can be used in the present invention include both 
carbon fibers having a high carbon content and carbonaceous fibers having 
a relatively low carbon content. Usually, such carbon fibers have a carbon 
content of at least 70% by weight. 
The carbon fiber bundle which can be used in the present invention can be 
prepared by known processes, such as the process disclosed in U.S. Pat. 
No. 4,069,297. For example, the carbon fiber bundle can be prepared by 
oxidizing known polymer fibers comprising mainly acrylonitrile (acrylic 
fiber) in an oxidizing atmosphere, e.g., air, at a temperature of from 
200.degree. to 300.degree. C. for a period of from 0.1 to 100 minutes, and 
then carbonizing the oxidized fibers in an inert gas atmosphere, e.g., 
N.sub.2, argon or helium at a temperature of from 600.degree. to 
3,000.degree. C. 
The carbon fiber bundle may also be prepared by forming fibers from a pitch 
of petroleum or coal, rendering the fibers infusible, and carbonizing the 
infusible fibers in an inert gas atmosphere, e.g., nitrogen, argon, 
helium, etc., at a temperature of from 600.degree. to 3,000.degree. C. 
The bundle to be used usually comprises from 100 to 100,000 filaments each 
having a cross section area of from 2.times.10.sup.-4 to 5.times.10.sup.-6 
mm.sup.2. The carbon fiber bundle preferably has a specific resistivity of 
from 10.sup.3 to 10.sup.-4 .OMEGA.cm, a tensile strength of not less than 
100 kgf/mm.sup.2, a modulus of elasticity of not less than 
10.times.10.sup.3 kgf/mm.sup.2, and tensile ductility of at least 1.7. 
Examples of the epoxy resin which may be used in the present invention are 
listed below: 
1. Glycidylamine type epoxy resins 
Those having an average epoxy equivalent (molecular weight of resin/number 
of epoxy group in a molecule; hereunder simply referred to as an epoxy 
equivalent) of 110 to 150, preferably from 120 to 135, are used. Such 
epoxy resins include, for example, 
N,N,N',N'-tetraglycidyldiaminodiphenyl-methane (shown by formula (I)), 
N,N-diglycidylmethaaminophenol glycidyl ether, and a mixture with 
oligomers (degree of polymerization is 2-4) thereof, which are 
commercially available under the trade names Araldite MY 720 (manufactured 
by Ciba-Geigy Corporation) or Epototo YH 434 (Toto Kasei Co.) and YDM 120 
(Toto Kasei Co.), respectively. It is preferred to use an epoxy resin 
mixture containing the oligomers in an amount of 10 to 40 wt % based on 
the resin. 
##STR1## 
2. Novolak type epoxy resins 
(1) Phenolic novolak type epoxy resins 
Those having an epoxy equivalent preferably of 160 to 200, more preferably 
from 170 to 190, are used, and they include, for example, Epikote 152 and 
154 (Shell Chemicals Corp.), Araldite EPN 1138 and 1139 (Ciba-Geigy 
Corporation), Dow Epoxy DEN 431, 438, 439 and 485 (Dow Chemical Company), 
EPPN 201 (Nippon Kayaku Co., Ltd.) and Epicron N 740 (Dainippon Inki 
Kagaku Kogyo Co.). 
(2) Cresol Novolak type epoxy resins 
Those having an epoxy equivalent preferably of from 180 to 260, more 
preferably, from 200 to 250 are used. Examples of such resins include, 
Ciba-Geigy ECN 1235, ECN 1273, ECN 1280 and ECN 1299 (manufactured by 
Ciba-Geigy Corporation), EOCN 102, 103 and 104 (manufactured by Nippon 
Kayaku Co.). 
3. Bisphenol A.F and S types epoxy resins 
Those having an epoxy equivalent preferably of from 150 to 1,000, more 
preferably from 300 to 600, are used, and the heat resistance of bisphenol 
A type epoxy resins having an epoxy equivalent more than about 1,000 is 
somewhat low. Illustrative bisphenol A type epoxy resins include Epikote 
828, 834, 827, 1001, 1002, 1004, 1007 and 1009 (Shell Chemicals Corp.), 
Araldite CY 205, 230, 232 and 221, GY 257, 252, 255, 250, 260 and 280, 
Araldite 6071, 7071 and 7072 (Ciba-Geigy Corporation), Dow Epoxy DER 331, 
332, 662, 663U and 662U (Dow Chemical Company), Epicron 840, 850, 855, 
860, 1050, 3050, 4050 and 7050 (Dainippon Inki Kagaku Kogyo Co.), and 
Epototo YD115, 115-CA, 117, 121, 127, 128, 128 CA, 128 S, 134, 0012, 011, 
012, 014, 014 ES, 017, 019, 020 and 002 (Toto Kasei Co.). 
4. Brominated bisphenol A type epoxy resins 
Those having an epoxy equivalent preferably of from 200 to 600, more 
preferably from 220 to 500, are used. Examples of such epoxy resin include 
Araldite 8011 (Ciba-Geigy Corporation) and Dow Epoxy DER 511 (Dow Chemical 
Co.). 
5. Urethane-modified bisphenol A type epoxy resins 
Those having an epoxy equivalent preferably of from 200 to 1,000, more 
preferably from 250 to 400, are used. Examples include Adeka Resin EPU-6, 
10 and 15 (Asahi Denka Co., Ltd.). 
6. Alicyclic epoxy resins 
Those having an epoxy equivalent preferably of from 110 to 300, more 
preferably from 130 to 280, are used. Examples are Araldite CY-179, 178, 
182 and 183 (Ciba-Geigy Corporation). 
In the present invention epoxy resins may be used either alone or in 
combination. A resin composition containing at least 50 wt %, preferably 
at least 70 wt %, based on the total epoxy resin, of at least one of 
N,N,N',N'-tetraglycidyl diaminodiphenylmethane and N,N-diglycidyl 
meta-aminophenyl glycidyl ether provides particularly high heat 
resistance. These epoxy resins are preferably combined with a novolak type 
epoxy resin, bisphenol A type epoxy resin, brominated bisphenol A type 
epoxy resin or urethane-modified epoxy resin. 
Examples of preferable epoxy resin to be used in the present invention 
includes glycidyl ether or glycidylamine of bisphenol A, bisphenol F, or 
bisphenol S, etc. Specific examples of preferred epoxy resins are MY720 
(produced by Chiba Geigy A.G.) and Epototo YH-434 (produced by Toto Kasei 
K.K.). 
The curing agents for these epoxy resins advantageously include imidazole 
type and polyamide type curing agents in view of their performance to 
complete a curing reaction in a short period of time. Specific examples of 
these curing agents are 2-ethyl-4methylimidazole as an imidazole type, and 
Tomide (produced by Fuji Kasei Co., Ltd.) and dicyandiamide as a polyamide 
type. Of these, 2-ethyl-4-methyl-imidazole is particularly preferred. 
The butadiene-acrylonitrile copolymer having carboxyl groups on at least 
both terminals is preferably liquid to facilitate the reaction with the 
epoxy resin and to provide a coating of good quality. More specifically, 
the copolymer preferably has a viscosity of about 500 to 8,000 poise, more 
preferably from about 1,000 to 7,000 poise, at 27.degree. C. The 
acrylonitrile monomer content of the copolymer is generally from 10 to 35 
wt %, preferably from 15 to 30 wt %. The copolymer may contain up to 3 
carboxyl groups including those at the two terminals, and such copolymer 
can be prepared by using at least one of acrylic and methacrylic acid as a 
comonomer. 
The above-described copolymer can be obtained by radical copolymerization 
using a catalyst having carboxyl groups. When a compound represented by 
the following formula (II) is used as a catalyst for the production of a 
butadiene-acrylonitrile copolymer having terminal moieties including 
carboxyl groups as shown in the following formula (III) can be obtained. 
##STR2## 
Examples for the above-described copolymerization catalysts include 
4,4'-azobis-(4cyanopentanoic acid) and 
2,2'-azobis-(4-carboxy-2methylbutyronitrile). 
The preparation of the copolymer can also be conducted by using an anion 
copolymerization catalyst, for example, organic dilithium compound such as 
dilithium tetraphenylethane, dilithium trans-stilbene, dilithium 
polyisoprene, 1,4-dilithium butene or 1,5-dilithium pentane. After a 
butadiene-acrylonitrile copolymer is produced, the copolymer is subjected 
to a reaction with CO.sub.2 gas, and then to a reaction with an acid such 
as HCl to produce the copolymer having carboxylic acid groups on at least 
both terminals of the molecular of the copolymer. The reactions proceed as 
shown below: 
##STR3## 
Examples of the copolymer include Hycar CTBN (manufactured of B.F. Goodrich 
Chemical Co.). 
When it is desired to increase the viscosity of the coating compositions in 
order to improve coating performances, the butadiene-acrylonitrile 
copolymer is reacted with an epoxy resin so that at least terminal 
carboxyl groups react with epoxy rings. The reaction is carried out by 
using a reaction mixture containing at least 1 equivalents of the epoxy 
group per equivalent of the carboxyl group (i.e., the ratio of the total 
number of epoxy groups in the epoxy resin to the total number of 
carboxylic acid groups in the copolymer is at least 1). The conditions for 
the reaction between the copolymer and epoxy resin vary with the type of 
the epoxy resin. Usually, the reaction is effected at a temperature 
between 50 and 170.degree. C. for 1 to 2 hours in the absence or presence 
of a catalyst such as triphenylphosphine. Curing agents for epoxy resins 
as recited above may also be added to the reaction system. 
The above-described reaction mixture containing the reaction product of the 
copolymer and the epoxy resin and a mixture of the copolymer and the epoxy 
resin should comprise from 5 to 40 parts by weight, preferably from 7 to 
20 parts by weight of the epoxy resin per 100 parts by weight of the 
copolymer. If the amount of the epoxy resin is less than 5 parts by 
weight, adhesion between a carbon fiber bundle and the mixture or reaction 
mixture is reduced, resulting in inferior flexing fatigue performance for 
use as rubber reinforcing material. 
On the other hand, if it exceeds 40 parts by weight, the resulting carbon 
fiber cord has reduced softness and deteriorated flexing fatigue 
performance. Such a carbon fiber cord, when used for rubber reinforcement, 
is liable to cracking, buckling or breaking. 
The epoxy resin curing agent is preferably used in an amount of from 1 to 
5% by weight based on the weight of the epoxy resin in the coating 
composition. When the proportion of the curing agent is less than 1% by 
weight, the carbon fibers are not sufficiently bundled up so that they are 
readily unbound and cut during use for rubber reinforcement, thus 
exhibiting deteriorated flexing fatigue performance. When it exceeds 5% by 
weight, curing of the copolymer and the epoxy resin tends to excessively 
proceed to reduce adhesion to adhesive that is coated subsequently, and 
the resulting carbon fiber cord may have reduced flexing fatigue 
performance. 
The resin composition comprising the reaction mixture or mixture of the 
copolymer and the epoxy resin and the curing agent for the epoxy resin is 
coated on a carbon fiber bundle to a coverage of from 5 to 50% by weight, 
preferably from 10 to 30% by weight based on the thus treated carbon fiber 
bundle. A coverage of less than 5% by weight results in poor flexing 
fatigue performance. A coverage exceeding 50% by weight tends to harden 
the cord, resulting in deterioration of flexing fatigue performance. 
Application of the resin composition to the carbon fiber bundle can be 
carried out by dissolving the above-described components in an appropriate 
solvent, e.g., acetone, methyl ethyl ketone, methyl cellosolve, etc., 
either separately or in any combination to prepare a uniform solution, and 
coating the solution by a known coating technique, such as dip coating, 
spray coating, and the like. Dip coating is particularly preferred in 
order to let the coating solution penetrate deep into the inside of the 
fiber bundle in such a manner that the individual single fibers 
constituting the bundle may be coated with the solution. 
The coating solution usually has a solid content of from about 3 to 70% by 
weight, though somewhat varying depending on the conditions of dip 
coating. The temperature of the solution is preferably low from 
considerations of solution stability and concentration stability, usually 
ranging from 10.degree. to 30.degree. C. After coating, the carbon fiber 
bundle is dried to remove the solvent. The drying temperature preferably 
ranges from 80.degree. to 150.degree. C. It is desirable that drying 
starts at a lower temperature and the drying temperature is gradually 
elevated because sudden removal of the solvent at a high temperature is 
apt to cause formation of voids in the inside of the fiber bundle. After 
drying, the cord is then subjected to heat treatment usually at a 
temperature of from 150.degree. to 230.degree. C. for a period of from 1 
to 30 minutes. The fiber cord may be heat-treated in either non-contact or 
contact with a heating means. It should be noted that the resulting cord 
has a round form in the former case or a rather flat form in the latter 
case. Accordingly, conditions of heat treatment should be selected taking 
this fact into consideration and depending on the final intended use of 
the product. It is preferable to conduct the heat treatment after complete 
removal of the solvent in order to prevent void formation inside of the 
cord or blister formation on the surface of the cord. By virtue of this 
heat treatment, the epoxy resin is cured and, at the same time, the 
copolymer and the epoxy resin are completely reacted so that the coating 
becomes insoluble in the solvent remaining in the cord, e.g., methyl ethyl 
ketone. 
The carbon fiber cord having coated thereon a resin composition of the 
copolymer, the epoxy resin, and the curing agent is then coated with a 
phenol-formalin-rubber latex type adhesive for rubber. 
The phenol-formalin-rubber latex type adhesive used in the present 
invention is a conventionally used adhesive for adhering fibers with a 
rubber. 
The phenol compound which is preferably used in the adhesive is a compound 
represented by the following formula (IV) 
##STR4## 
wherein a represents 1 or 2, R represents H or an alkyl group preferably 
having from 1 to 4 carbon atoms, and b represents 1 or 2. 
Examples for the phenol include phenol, o-cresol, m-cresol, p-cresol, 
3,5-xylenol, isothimol, thimol, catechol, and resorcinol. 
The adhesive is prepared by mixing a phenol compound with formalin and 
allowing the mixture to react at room temperature (from about 20.degree. 
to 30.degree. C.) for from about 6 to 30 hours in the presence of a 
catalyst for a condensation reaction, such as alkaline catalyst or an acid 
catalyst to form a so-called primary condensation product which can be 
shown by formula (V) 
##STR5## 
wherein a, b, and R each represents as defined above and n represents 0 or 
an integer of from 1 to 3, and then a rubber latex is added to the 
reaction mixture. 
The molar ratio of the phenol compound and the formaldehyde is preferably 
from 1/0.1 to 1/8, and more preferably from 1/0.5 to 1/5. As the catalyst 
it is preferable to use an alkali such as sodium hydroxide or potassium 
hydroxide. 
In the adhesive, a condensate represented by formula (VI) shown below may 
be used in place of a part of the amount of a phenol in order to improve 
adhesion to the rubber 
##STR6## 
wherein X represents a methylene group, --S.sub.m -- (wherein m is an 
integer of from 1 to 8), or an oxygen atom; Y represents 1 or 2; Z 
represents a hydrogen atom, a halogen atom, an alkyl group preferably 
having from 1 to 4 carbon atoms, an allyl group, an allyloxy group, or an 
alkoxy group preferably having from 1 to 4 carbon atoms; and n represents 
0 or an integer of from 1 to 15. 
The preferable amount of the condensate represented by formula (VI) is up 
to 70 weight% based on the total weight of the condensate and a phenol. In 
order to obtain the effect of improvement of adhesion, it is preferable 
that the amount is not less than 30 weight %. 
A method for producing such a condensate is disclosed in Japanese Patent 
Application (OPI) No. 109684/83. 
The rubber latex to be used in the adhesive includes a natural rubber 
latex, a styrene-butadiene copolymer latex, a 
vinylpyridine-styrene-butadiene terpolymer latex, a nitrile rubber latex, 
a chloroprene rubber latex etc., and a mixtures thereof. Of these, a 
vinylpyridine-styrene-butadiene ter-polymer latex (preferable molar ratio: 
10-15/15-20/60-70) is particularly preferred. 
In the preparation of the adhesive, the reaction mixture and the rubber 
latex are mixed at a weight ratio of from 1/1 to 1/15, and preferably from 
1/3 to 1/12, on a solid basis, and the resulting mixture is dispersed in 
water so as to have solids content of from 10 to 35% by weight. 
The thus prepared adhesive is preferably used within 100 hours, preferably 
48 hours after mixing a phenol and formalin because the condensation 
reaction proceeds also after mixing the latex with the reaction mixture, 
and thereby the viscosity of the adhesive is gradually increased to make 
processability worse. 
In carrying out the impregnation of the adhesive to the carbon fiber bundle 
having coated thereon the epoxy resin composition, the aqueous dispersion 
of the adhesive having the above-recited solid content, is applied thereto 
by, for example, dipping at room temperature (usually from 10 to 
25.degree. C.). If necessary, the amount to be applied can be adjusted by 
means of squeeze rollers. 
If the adhesive coverage is less than 0.5% by weight based on the total 
weight of the carbon fiber bundle and the resin composition coated thereon 
the resulting cord shows poor adhesion to rubber and reduced flexing 
fatigue performance. In cases where the adhesive coverage exceeds 5% by 
weight, the reaction of the adhesive with epoxy groups is accelerated, 
resulting in excessive hardness, which leads to deterioration of flexing 
fatigue performance. 
If desired, the resin composition comprising a mixture of the copolymer and 
an epoxy resin or a reaction mixture thereof containing a reaction product 
and a curing agent for the epoxy resin may further contain various 
additives, such as viscosity modifiers, conductivity modifier, colorants, 
and the like, preferably in an amount of from 10 to 30 % by weight based 
on the weight of the composition. 
Examples of such additives include a polyethylene glycol, polypropylene 
glycol and a diglycidyl ether of an ethylene-propylene block copolymer. 
As described above, the carbon fiber cords in accordance with the present 
invention are composed of firmly bound fibers so that cracking hardly 
occurs. Further, they are excellent in adhesion to rubber and especially 
in resistance to repeated fatigue. Therefore, the carbon fiber cords of 
the present invention are useful as reinforcement for rubber while taking 
advantage of high strength and high modulus of elasticity possessed by 
carbon fibers. In particular, they are capable of greatly improving 
running stability and fuel efficiency of automobiles when used for tires. 
The carbon fiber cord according to the present invention is useful as a 
reinforcing material for commonly employed rubbers, such as natural rubber 
and synthetic rubbers, e.g., styrene-butadiene rubber, isoprene rubber, 
isobutylene isoprene rubber, nitrile-butadiene rubber, etc. 
The rubber latex to be used in the adhesive is appropriately selected 
according to the kind of rubber to which the cord is applied. For example, 
a nitrile-butadiene rubber latex is used for nitrile-butadiene rubber; and 
for other rubbers a natural rubber latex, a styrene-butadiene rubber 
latex, a vinylpyridine-styrene-butadiene rubber latex, etc., can be used 
suitably. 
The above-described rubber to which the cord is applied may contain various 
additives, such as carbon black, sulfur, a vulcanization accelerator, an 
antioxidant, zinc oxide, stearic acid, a process oil, and the like. 
The cord according to the present invention can be used for reinforcement 
of rubber in a conventional manner. For example, the cord or the cord in 
the form of a woven fabric may be sandwiched between two sheets composed 
of a rubber composition containing the above enumerated additives, 
followed by heating under pressure to effect vulcanization simultaneously 
with molding. 
Use of the cord according to the present invention makes it possible to 
produce rubber products having high durability because of the excellent 
adhesion of the cord to rubber. 
This invention is now illustrated in greater detail with reference to the 
following examples, but it should be understood that these examples are 
not intended to limit the present invention. 
In these examples, all the percents and parts are given by weight unless 
otherwise indicated. In the following examples, the adhesive strength 
between carbon fiber cords and rubber was evaluated by a drawing test and 
a two-ply peel test according to the following method. Further, carbon 
fiber cords were evaluated for flexing fatigue performance according to 
the following test method. 
Drawing Test 
A carbon fiber cord was embedded in a length of 8 mm in an unvulcanized 
rubber compound having the following composition, and the rubber was 
vulcanized at 150.degree. C. under a pressure of 30 kg/cm2 for 30 minutes. 
The force required for drawing the cord from the vulcanized rubber was 
measured. 
______________________________________ 
Rubber Compounding: 
______________________________________ 
Natural rubber RSS #3 100 parts 
Zinc oxide 5 parts 
Stearic acid 2 parts 
Carbon black (GPF) 50 parts 
Antioxidant (Santoflex 13 produced 
1 part 
by Mitsubishi Monsanto Chemical Co., 
Ltd.; N--(1,3-dimethylbutyl)-N'-- 
phenyl-paraphenylenediamine) 
Aromatic oil 
Sulfur 2.25 parts 
Vulcanization accelerator DM 
1 part 
(dibenzothiazolyl disulfide) 
______________________________________ 
Two-Ply Peel Test 
Twenty cords were placed on an unvulcanized rubber sheet having the same 
composition as used above (width: 25 mm; length: 200 mm; thickness: 1.0 
mm) in parallel to the lengthwise direction of the sheet. Another 
unvulcanized rubber sheet of the same composition was piled thereon, and 
20 cords were aligned on this sheet in the same manner as above. Finally, 
a rubber sheet of the same composition was placed thereon to build up a 
so-called two-ply structure of rubber/cord/rubber/cord/rubber. After the 
structure was heated at 150.degree. C. under a pressure of 30 kg/cm2 for 
30 minutes to effect vulcanization, two cord layers were peeled apart in 
the lengthwise direction to evaluate adhesion of the cord to rubber. FIG. 
1 illustrates the two-ply structure used in this test. In FIG. 1, symbols 
a and b indicate a rubber layer and a cord layer, respectively. 
Flexing Fatigue Test 
Three cords were inserted between two unvulcanized rubber sheet (width: 
25.4 mm; length: 76.2 mm; thickness: 3.2 mm) having the same composition 
as described above in the lengthwise direction f the sheet at 6.35 mm 
intervals. The rubber sheets having the cords embedded therein were 
vulcanized at a temperature of 150.degree. C. under a pressure of 30 
kg/cm2 for 30 minutes to prepare a rubber block. The rubber block was 
fitted to a de Mattia type flex fatigue test machine, and the rubber block 
was subjected to 100,000 flexes with a stroke of 30 mm (30 mm=l.sub.1 
-l.sub.2 in FIG. 2). The rubber block was cut into three equal parts, in 
the direction parallel to the cords as shown by doted lines in FIG. 1 and 
the rubber block having the cord was pulled at a rate of pulling of 300 
mm/min at a distance of 30 mm between chucks to measure the tensile 
strength after the flexing fatigue. A percentage of the tensile strength 
after the flexing fatigue to that before the test was obtained to evaluate 
flexing resistance of the cord (distance between chucks: gauge length). 
EXAMPLE 1 
A hundred parts of a carboxyl-terminated butadiene-acrylonitrile copolymer 
(Hycar CTBN 1300.times.13, produced by Goodrich Co.) and 18 parts of a 
glycidylamine type epoxy resin (MY720, Ciba Geigy A.G.) were mixed and 
allowed to pre-react at 110.degree. C. for 2 hours. Subsequently, the 
resulting reaction mixture was dissolved in methyl ethyl ketone so as to 
result in a solid content of 20%. To the resin solution was added 
2-ethyl-4-methylimidazole as a curing agent in an amount of 2.5% based on 
the weight of the resin, followed by stirring. 
The resulting resin solution was continuously impregnated into a carbon 
fiber bundle composed of 3,000 filaments each having a diameter of 7 .mu.m 
(tensile strength: 410 kgf/mm.sup.2 ; tensile modulus of elasticity: 24 x 
103 kgf/mm.sup.2), followed by drying at 120.degree. C. for 3 minutes. The 
dried fiber bundle was then subjected to curing at 200.degree. C. for 2 
minutes. The resulting carbon fiber bundle was found to have a resin 
composition coverage of 19.5%. 
The carbon fiber bundle was then continuously dipped in a 25% (content of 
materials in water) RFL bath having the following formulation at 
25.degree. C. 
______________________________________ 
RFL Bath Formulation: 
______________________________________ 
i Soft water 387.6 parts 
ii Sodium hydroxide (10% aqueous solution) 
6.3 parts 
iii Resorcinol 23.1 parts 
iv Formalin (37% formaldehyde aqueous 
25.6 parts 
solution) 
Nipol 2518FS (solids content: 40%) 
543.5 parts 
(vinylpyridine-styrene-butadiene 
copolymer rubber latex produced by 
Nippon Geon Co., Ltd.) 
v Aqueous ammonia (28%) 13.9 parts 
Total: 1000.0 parts 
______________________________________ 
The reaction product was obtained by mixing components (i), (ii), (iii), 
and (iv) and the mixture was stirred at 25.degree. C. for 6 hours. 
Components (v) and (vi) were added to the reaction mixture, and the thus 
obtained mixture was allowed to stand at the room temperature for 20 
hours. 
After drying at 85.degree. C. for 2 minutes, the carbon fiber bundle was 
subjected to heat treatment at 210.degree. C. for 2 minutes. The resulting 
fiber cord was found to have an RFL coverage of 3% based on the weight of 
the carbon fiber bundle having coated with the resin composition thereon. 
As a result of evaluation of performances, the carbon fiber cord had a 
drawing strength of 19.5 kg/8 mm, a two-ply peel strength of 25.9 kg/25 
mm, and a flexing fatigue strength retention of 85%. 
EXAMPLE 2 and COMATIVE EXAMPLES 1 to 6 
Carbon fiber cords were obtained in the same manner as described in Example 
1 except that the amounts of MY-720 and 2-ethyl-4-methylimidazole and the 
coverage of the resin composition were varied as indicated in Table 1. 
Each of the resulting cords was evaluated for performance properties, and 
the results obtained are shown in Table 1. It can be seen from Table 1 
that the cords according to the present invention are excellent in 
adhesion to rubber and fatigue resistance. 
TABLE 1 
__________________________________________________________________________ 
Amount 
Amount of 
Coverage Two-Ply 
Flexing Fatigue 
of 2-Ethyl-4- 
of Resin 
Drawing 
Peel Strength 
Example 
MY720 
Methylimidazole 
Composition 
Strength 
Strength 
Retention 
No. (part) 
(%) (%) (kg/8 mm) 
(kg/25 mm) 
(%) 
__________________________________________________________________________ 
Comparative 
2* 25 19.5 17.0 23.3 65 
Example 1 
Comparative 
20 25 3.2* 8.3 10.5 42 
Example 2 
Example 2 
20 25 19.3 19.2 26.1 84 
Comparative 
20 25 63.1* 19.0 25.8 49 
Example 3 
Comparative 
20 0.25* 19.1 19.1 25.6 72 
Example 4 
Comparative 
20 17.5* 19.3 19.3 25.8 68 
Example 5 
Comparative 
43* 1.2 19.4 19.0 26.1 41 
Example 6 
__________________________________________________________________________ 
Numbers with an asterisk (*) in Table 1 and hereafter indicates that the 
number is outside of the scope of the present invention. 
EXAMPLE 3 and COMATIVE EXAMPLES 7 and 8 
Carbon fiber cords were obtained in the same manner as in Example 1 except 
for varying the RFL coverage as indicated in Table 2. 
Each of the resulting cords was evaluated for performance properties, and 
the results obtained are also shown in Table 2. As can be seen from Table 
2, the carbon fiber cord according to the present invention exhibits 
excellent adhesion to rubber and fatigue resistance. 
TABLE 2 
______________________________________ 
Two-Ply Flexing 
Drawing Peel Fatigue 
Example RFL Strength Strength 
Strength 
No. Coverage (kg/8 mm) (kg/25 mm) 
Retention 
______________________________________ 
Comparative 
0.1* 13.0 13.5 48 
Example 7 
Example 3 
3.3 19.5 26.8 88 
Comparative 
7.5* 19.5 26.5 49 
Example 8 
______________________________________ 
EXAMPLE 4 
A carbon fiber cord was obtained in the same manner as in Example 1, except 
that the mixture of the copolymer and the epoxy resin was not pre-reacted. 
As a result of evaluation of performance properties, the resulting cord had 
a drawing strength of 19.4 kg/8 mm, a two-ply peel strength of 25.9 kg/25 
mm, and a flexing fatigue strength retention of 85%. These results are 
substantially equal to those obtained in Example 1, indicating that the 
resulting cord is also excellent in adhesion to rubber and fatigue 
resistance. 
EXAMPLE 5 
Acrylic fibers comprising 98% acrylonitrile, 1% methyl acrylate, and 1% 
itaconic acid (3,000 filaments each having a diameter of 10 .mu.m; tensile 
strength: 6.5 g/d; elongation: 15%) were air-oxidized at 250.degree. C. 
for 25 minutes under a load of 180 mg/d, followed by carbonizing in a 
nitrogen atmosphere at 850.degree. C. for 3 minutes under a load of 200 
mg/d to obtain a carbonaceous fiber bundle. The resulting carbonaceous 
fiber bundle was designated as Bundle (A). 
Bundle (B) was prepared in the same manner as for Bundle (A), except that 
the load in the air-oxidation was changed to 100 mg/d. Properties of the 
carbonaceous fibers of Bundles(A) and (B) are shown in Table 5. 
TABLE 5 
______________________________________ 
Tensile 
Bonded Oxygen 
Modulus Tensile Carbon 
Content of Elasticity 
Elongation 
Content 
Bundle 
(%) (kgf/mm.sup.2) 
(%) (%) 
______________________________________ 
(A) 6.3 16,000 1.9 78 
(B) 6.4 14,000 1.7 79 
______________________________________ 
The thus obtained carbonaceous fiber bundles were treated with a resin 
solution, dried, and cured in the same manner as Example 1 to obtain 
carbonaceous fiber bundles having a resin composition coverage of 19.5%. 
The thus obtained carbonaceous fiber bundles were treated with RFL, dried 
and heat treated in the same manner as in Example 1 to obtain bundles 
having RFL coverages as shown in Table 6. 
TABLE 6 
______________________________________ 
Bundle (A) 
Bundle (B) 
______________________________________ 
RFL Coverage (%) 20.3 19.8 
Two-Ply Peel Strength 
26.3 26.4 
(kg/25 mm) 
Drawing Strength (kg/8 mm) 
18.3 16.5 
Flexing Fatigue Strength 
88 86 
Retention (%) 
______________________________________ 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.