Pneumatic tire provided with electrically conductive coating

This invention relates to a pneumatic tire coated with a water-based electrically conductive coating, in which the coating contains a carbon black having a nitrogen adsorption specific surface area (N.sub.2 SA) of 70 m.sup.2 /g-180 m.sup.2 /g and a dibutyl phtalate (DBP) absorption of 70 ml/100 g-180 ml/100 g, a surface active agent and a rubber ingredient.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a pneumatic tire provided with a water-based 
electrically conductive coating. 
2. Description of Related Art 
Since a tread rubber containing silica as a filler is high in the specific 
resistance, for example, 10.sup.10 -10.sup.12 .OMEGA..multidot.m, a 
pneumatic tire provided with the tread is difficult to dissipate a static 
electricity generated in a vehicle body to a ground through the tread. 
In order to solve the above problem, there is proposed a tire provided with 
a tread coated with an organic solvent-based electrically conductive 
coating on a surface thereof. 
Such a tire is easily manufactured, but adversely influencing a working 
atmosphere owing to using the organic solvent. 
Further, EP A 0705722 discloses a tire having a tread coated with a 
water-based coating. A carbon black contained in the coating has a large 
surface area and a high electric conductivity, but high cost; and the 
coating containing the carbon black is low in the adhesion property to the 
surface of the tread. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the invention to provide a pneumatic tire 
provided with a water-based electrically conductive coating which has a 
high adhesion property to a tread surface as well as a good working 
atmosphere and a low cost. 
In order to attain the object mentioned above, a pneumatic tire according 
to the invention has the following constructions: 
The pneumatic tire according to the invention is characterized in that the 
pneumatic tire comprising a tread made of a rubber composition having a 
high electrical resistance, a member adjacent to the tread and made of a 
rubber composition having a low electrical resistance (for example, 
10.sup.5 -10.sup.6 .OMEGA.) and a water-based electrically conductive 
coating extended from an outer surface of the tread to an outer surface of 
the member, in which the water-based electrically conductive coating 
comprises a rubber ingredient, carbon black and a surface active agent, 
the carbon black having a nitrogen adsorption specific surface area 
(N.sub.2 SA) of 70 m.sup.2 /g-180 m.sup.2 /g and a dibutyl phtalate (DBP) 
absorption of 70 ml/100 g-180 ml/100 g. The N.sub.2 SA and DBP absorption 
of the carbon black are measured in accordance with ASTM D3037-93 and ASTM 
D2414-97, respectively, and the classification is shown in ASTM D1765-98. 
The rubber ingredient is preferably at least one rubber selected from the 
group consisting of styrene-butadiene copolymer rubber (SBR), natural 
rubber (NR), and butadiene rubber(BR); and preferably added as a rubber 
latex. The carbon black is preferably at least one selected from the group 
consisting of N100S, N200S, N300S, and N600S in accordance with the ASTM 
classification. Further, the carbon black is preferably an amount of 30-70 
parts by weight based on 100 parts by weight of the rubber ingredient. 
Further, the surface active agent is preferably nonionic. The surface 
active agent is preferably of an amount of 1-15 parts by weight based on 
100 parts by weight of the rubber ingredient. The rubber composition for 
the tread contains silica.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention will be explained in more detail below. Among colloidal 
properties of the carbon black to be used for the coating in accordance 
with the invention, the nitrogen adsorption specific surface area (N.sub.2 
SA) is defined to be 70 m.sup.2 /g-180 m.sup.2 /g. When it is less than 70 
m.sup.2 /g, a grain size of the carbon black is too large and an 
electrically conducting path is difficult to be formed. When it exceeds 
180 m.sup.2 /g, the electrical conduction property of the coating is high, 
but its cost is high, and the adhesion property of the coating to the 
tread outer surface and the like is degraded, resulting in a low 
durability of the coating. Further, among the other colloidal properties 
of the carbon black, the DBP absorption is defined to be 70 ml/100 g-180 
ml/100 g. When it is less than 70 ml/100 g, a structure of an aggregate of 
the carbon black grain dose not almost developed, therefore, not only the 
electrically conducting path is formed, but also the coating itself 
according to the invention lowers in the reinforcing property. When it 
exceeds 180 ml/100 g, it is outside of a range of the colloidal properties 
of the carbon black used for the tire and its cost is high. 
The carbon black to be preferably used in the invention is, concretely, 
N100S, N200S, N300S, and N600S in accordance with the ASTM classification. 
Further, a compounding ratio of the carbon black to the rubber ingredient 
is preferable to be 30-70 parts by weight, more preferably 40-60 parts by 
weight, and most preferably 45-55 parts by weight to 100 parts by weight. 
Because when it is too small, the electrical conduction property is not 
sufficient. When it is too large, not only the peeling of the coating from 
the tread rubber and the like readily generates, but also the electrical 
conduction property is not much improved. 
The water-based electrically conductive coating according to the invention 
contains a surface active agent in order to ensure a wettability between 
the electrically conductive coating and the surfaces of the tread or the 
members adjacent to the tread. That is, since the surfaces of the rubber 
compositions are hydrophobic, if a water-based coating without containing 
a surface active agent is applied thereonto, the coating forms spots on 
the surfaces of the rubbers, resulting in that it is almost impossible to 
form an uniform coating. Then, in order to form the uniform coating, an 
affinity to the surfaces of the rubbers is enhanced by adding a surface 
active agent to the water-based coating. A preferable amount of the 
surface active agent added is 1-15 parts by weight based on 100 parts by 
weight of the rubber ingredient. When it is less than 2 parts by weight, 
the coating may not be able to ensure the affinity (wettability) to the 
surface of the rubber members, while when it exceeds 30 parts by weight, 
the adhesion property after vulcanization tends to lower. 
The surface active agent according to the invention is preferable to be 
nonionic, and particularly its HLB (hydrophile-lipophile balance) is 
preferable to be 3-15. When an anionic or a cationic surface active agent 
is used, the aging of the coating tends to be accelerated. 
As such a surface active agent, mention may concretely be made of alkyl 
phenol types, sorbitan ester types, sorbitan esterether types, 
polycarbonate types, perfluoro alkyl types, acetylene diol types, 
polyoxyethylene alkyl ether types or the like. 
Further, the rubber ingredient of the water-based electrically conductive 
coating according to the invention is preferable to be at least one rubber 
selected from the group consisting of SBR, NR, and BR in view of 
co-vulcanization property of the coating to the rubber members such as the 
tread and the member adjacent to the tread. More preferably, the rubber 
ingredient is added as a rubber latex. 
Further, the pneumatic tire according to the invention is effective when 
the rubber composition constructing the tread contains silica. 
Irrespective of the construction of the tread such as the tread made of 
one kind of rubber composition, the tread having a cap-base structure or 
the like, the invention can be applied and the effect of the invention is 
obtained. 
In the invention, the amount of the carbon black contained in the coating 
and an amount of the coating applied are properly designed in accordance 
with a kind of the carbon black contained in the coating, a tire size, a 
specific resistance of the tread rubber itself, a method of coating or the 
like. In order to effectively dissipate the static electricity generated 
in the vehicle body, an electrical resistance of the tire as a product is 
preferable to finally be not more than 10.sup.8 .OMEGA.. 
The water-based electrically conductive coating according to the invention 
may have proper additives as conventionally used added to it. 
The coating according to the invention may be prepared by admixing a 
colloidal solution of the carbon black, the rubber latex, the surface 
active agent and the like according to a conventional method. 
The pneumatic tire according to the invention may be manufactured by 
coating the thus obtained coating over the surface of the tread and at 
least a part of the surface of the rubber member adjacent to the tread to 
ensure a discharging path, and vulcanizing it by a conventional method. 
Since the coating according to the invention remains both on a convex 
region and a concave region of the tread having a groove after 
vulcanization, the discharge path can be ensured not only in a new tire 
but also in a tire after the coating on the convex region is worn. This 
occurs by contact between the coating on the concave region and the 
ground. 
The invention will be explained on the basis of Examples and Comparative 
Examples below. 
By applying each of coatings having various recipes shown in Table 3 onto 
the surface of the tread rubber having a recipe shown in Table 1 and a 
side wall rubber having a recipe shown in Table 2, a pneumatic tire having 
a size of 196/65R15 is manufactured. The electrical resistance, a 
durability, and a peeling property between the side wall rubber and the 
tread rubber are examined in a new tire and in a tire after 20,000 km 
running. 
Rubber ingredients are used as rubber latices. The SBR latex has a rubber 
concentration of 40% by weight, the NR latex has a rubber concentration of 
60% by weight and the BR latex has a rubber concentration of 57% by 
weight. 
The tire is manufactured by applying the coating onto the outer surface of 
the tread and at least a part of the outer surface of the side wall rubber 
adjacent thereto of before vulcanization (a thickness of the coating is 
0.1-1.0 mm), and vulcanizing it according to the conventional process. 
Further, a wettability of the coating to the tread rubber, a specific 
resistance of the coating, and a peeling strength are measured by applying 
each of the coatings having the various recipes to rubber sheets having 
the above recipe for the tread and having the recipe for the side wall 
rubber, or the like. 
Wettability 
The rubber sheet having the recipe shown in Table 1 is made, and coated 
with each of the coatings having various recipes shown in Table 3, and 
then dried while the state of the coating is observed by a naked eye from 
the beginning of coating to drying of the coating. 
Evaluation standard 
.smallcircle.: This mark shows a good wettability from the beginning of 
coating to drying of the coating. 
.DELTA.: This mark shows that the coating wets the tread only at the 
beginning of coating but is repelled as drying. 
.times.: This mark shows that the coating is repelled from the beginning of 
coating. 
Specific resistance of coating 
A disc shaped sample having a radius (r) of 2.5 cm and a thickness (t) of 
0.2 cm for measuring the specific resistance of each of the coatings is 
formed by drying the coating and then being vulcanized through sulfur (in 
which sulfur is contained in two parts by weight based on 100 parts by 
weight of rubber ingredient). The sample is placed in an insulation 
resistance test box made by a Advantest Co., Ltd. as shown in FIG. 1 and 
then measured about its electrical resistance (R). Then, a specific 
resistance (.rho.) thereof is calculated according to the following 
equation. 
EQU .rho.=(.pi.r.sup.2 /t)R 
In FIG. 1, numeral 11 is the sample, numeral 12 a main electrode, numeral 
13 a guard electrode, and numeral 14 a counter electrode. 
Electrical resistance of tire 
An electrical resistance of each of a new tire and a tire after 20,000 km 
running is measured according to Wdk 110 Sheet 3 of German Association of 
Rubber Industry by placing an assembly 21 of a test tire and a rim on a 
steel plate 22 placed on an insulation plate 23. An electrical current of 
100 V between the rim and the steel plate to read as electrical resistance 
by means of a high resistance meter of Model HP4339A made by Hewlett 
Packard as shown in FIG. 2. 
Peeling strength 
The tread rubber sheet having the recipe shown in Table 1 and the side wall 
rubber sheet having the recipe shown in Table 2, respectively having a 
thickness of 2 mm before vulcanization are prepared. Then one surface of 
the tread rubber sheet is coated with an organic solvent-based cement (so 
called a under cement) to be used for bonding tire rubber members, on the 
other hand, one surface of the side wall-rubber sheet is coated with an 
organic solvent-based coating in Comparative Example 1 (see: *1), or with 
the water-based electrically conductive coating in each of Comparative 
Examples 2-4 and Examples 1-9. Then, two rubber sheets are united with the 
both coated surfaces opposite to each other, and vulcanized. The 
vulcanized and united sheets are cut out to a size of 10 mm.times.150 mm, 
the tread rubber sheet and the side wall rubber sheet are then 
respectively pulled in the opposite directions to measure a force 
necessary to peel at a speed of 100 mm per minute. The larger numerical 
value, the better adhesion property. 
Durability 
According to FMVSS (federal motor vehicle safety standard) -No. 109, a 
total running time before breakage of the tire is measured by repeating 
running on the drum while a load applied to the tire is changed stepwise. 
Durability is represented by an index value on the basis that Comparative 
Example 1 is 100. The larger the index value, the longer the running time 
and the better the durability. 
Peeling property between side rubber and tread rubber 
Each tire after the above drum test is observed at the joining portion 
between the side wall rubber and the tread rubber by a naked eye. 
Evaluation standard 
.times.: This mark shows that peeling is detected irrespective of a degree 
of the peeling. 
.smallcircle.: This mark shows that no peeling is detected, namely, the 
case other than the above. 
TABLE 1 
______________________________________ 
Tread rubber composition 
(unit: parts by weight) 
______________________________________ 
Styrene-butadiene rubber*.sup.1 
70 
Butadiene rubber*.sup.2 30 
ZnO 3 
Stearic acid 2 
Silica*.sup.3 85 
Aromatic oil 46 
Antioxidant*.sup.4 1.0 
Vulcanization accelerator*.sup.5 1.2 
Sulfur 1.5 
Silane coupling agent*.sup.6 8.5 
______________________________________ 
Note) 
*.sup.1 SBR #1500, trademark, manufactured by JSR Co. 
*.sup.2 BR01, trademark, manufactured by JSR Co. 
*.sup.3 Nipsil AQ, trademark, manufactured by NIPPON SILICA INDUSTRIAL Co 
Ltd. 
*.sup.4 N(1,3-dimethyl butyl)Nphenyl-p-phenylenediamine Santoflex 6PPD, 
trademark, manufactured by Flexsys Co., Ltd. 
*.sup.5 Nt-butyl-2-benzothiazolyl-sulfenamide SANTOCURE TBBS, trademark, 
manufactured by Flexsys Co., Ltd. 
*.sup.6 Si 69, trademark, manufactured by Degussa AG Co., Ltd. 
TABLE 2 
______________________________________ 
Side wall rubber composition 
(unit: parts by weight) 
______________________________________ 
Natural rubber 40 
Butadiene rubber*.sup.2 60 
ZnO 3 
Carbon black (N399) 45 
Aromatic oil 10 
Antioxidant*.sup.4 1.0 
Vulcanization accelerator*.sup.5 0.8 
Sulfur 1.0 
Stearic acid 2.0 
______________________________________ 
*.sup.2, *.sup.4 and *.sup.5 are the same as in Table 1. 
TABLE 3 
__________________________________________________________________________ 
Comparative Example 
Example 
1*.sup.13 
2 3 4 1 2 3 4 
__________________________________________________________________________ 
Coating Carbon Colloidal graphite*.sup.7 50 
(part by black Aquablack*.sup.8 50 
weight) N339*.sup.9 50 50 40 60 
N234*.sup.10 50 
Rubber SBR 100 100 100 100 100 100 100 
ingredient NR 
BR 
Surface active agent 
A.sup.*11 3 3 0 3 3 3 3 
B*.sup.12 
Solvent in Coating 
petrol.sup.#2 
water 
water 
water 
water 
water 
water 
water 
Wettability .largecircle. .DELTA. .DELTA. X .largecircle. .largecircle. 
.largecircle. .largecir 
cle. 
Specific resistance of coating (.OMEGA. .multidot. cm) 1 .times. 
10.sup.4 5 .times. 
10.sup.3 5 .times. 
10.sup.3 2 .times. 
10.sup.3 2 .times. 
10.sup.3 2 .times. 
10.sup.3 2 .times. 
10.sup.3 
Electrical 
new tire 1.0 .times. 10.sup.6 
7.0 .times. 10.sup.6 
5.0 .times. 10.sup.6 
1.0 .times. 10.sup.6 
1.0 .times. 10.sup.6 
1.0 .times. 10.sup.6 
1.0 .times. 10.sup.6 
resistance after 
20,000 km running 2.0 
.times. 10.sup.7 5.0 
.times. 10.sup.8 2.0 
.times. 10.sup.8 2.0 
.times. 10.sup.7 2.5 
.times. 10.sup.7 2.5 
.times. 10.sup.7 1.0 
.times. 10.sup.8 
of tire (.OMEGA.) 
Peeling strength (MPa) 
2.35 0.18 0.32 -- 2.30 2.01 2.08 1.76 
Durability (index) 100 68 64 -- 96 93 94 82 
Peeling property between side wall rubber .largecircle. X X -- .largecir 
cle. .largecircle. 
.largecircle. .largecir 
cle. 
and tread rubber 
__________________________________________________________________________ 
Example 
5 6 7 8 9 
__________________________________________________________________________ 
Coating Carbon Colloidal graphite*.sup.7 
(part by black Aquablack*.sup.8 
weight) N339*.sup.9 50 50 50 50 50 
N234*.sup.10 
Rubber SBR 70 100 100 
ingredient NR 100 30 
BR 100 
Surface active agent 
A.sup.*11 
3 3 3 
B*.sup.12 1 3 
Solvent in Coating 
water 
water 
water 
water 
water 
Wettability .largecircle. .largecircle. .largecircle. .largecircle. 
.largecircle. 
Specific resistance of coating (.OMEGA. .multidot. cm) 2 .times. 
10.sup.3 2 .times. 
10.sup.3 2 .times. 
10.sup.3 2 .times. 
10.sup.3 2 .times. 
10.sup.3 
Electrical 
new tire 1.0 .times. 10.sup.6 
1.0 .times. 10.sup.6 
1.0 .times. 10.sup.6 
1.0 .times. 10.sup.6 
1.0 .times. 10.sup.6 
resistance after 
20,000 km running 4.0 
.times. 10.sup.7 4.0 
.times. 10.sup.7 4.0 
.times. 10.sup.7 2.0 
.times. 10.sup.7 2.0 
.times. 10.sup.7 
of tire (.OMEGA.) 
Peeling strength (MPa) 
1.96 1.76 2.35 2.35 1.96 
Durability (index) 92 90 103 105 99 
Peeling property between side wall rubber .largecircle. .largecircle. 
.largecircle. .largecir 
cle. .largecircle. 
and tread rubber 
__________________________________________________________________________ 
*.sup.7 N.sub.2 SA 50 m.sup.2 /g, DBP absorption 80 ml/100 g 
*.sup.8 N.sub.2 SA 217 m.sup.2 /g, DBP absorption 200 ml/100 g Aquablack 
is a trademark, manufactured by Borden Inc. 
*.sup.9 N.sub.2 SA 96 m.sup.2 /g, DBP absorption 120 ml/100 g 
*.sup.10 N.sub.2 SA 126 m.sup.2 /g, DBP absorption 125 ml/100 g 
*.sup.11 nonionic surface active agent, HLB 12.9, NS210, trademark, 
manufactured by NOF CORPORATION 
*.sup.12 nonionic surface active agent, HLB 47, Surphinol SEF, trademark, 
manufactured by Air Products Co., Ltd. 
*.sup.13 Rubber composition which contains 65 parts by weight of carbon 
black (N134) instead of 85 parts by weight of silica in Table 1, is 
dissolved in a petrol #2(as classified in Japanese Industrial Standard 
K2201) at a rate of 13% by weight to prepare this coating. 
Besides, since the coating in Comparative Example 4 has a low wettability, 
a corresponding tire was not manufactured. 
As explained above, according to the invention, the pneumatic tire having 
the excellent dissipate effect and the high durability can be obtained 
without using a specific electrically conductive agent such as graphite, 
the electrically conductive carbon black or the like, and with no 
influence on the adhesion property between the tread rubber and the side 
wall rubber. 
Further, according to the invention, there are advantages such as a good 
working atmosphere and a low cost.