Carbon black for tire tread

The present invention discloses a carbon black having an N.sub.2 SA of 75 to 105 m.sup.2 /g and a compressive DBP absorption of at least 110 ml/100 g and, at the same time, having the following selective characteristic values: PA0 true specific gravity.ltoreq.1.9080-0.0016.times.N.sub.2 SA; PA0 void diameter of particle aggregates (nm).gtoreq.62.2-0.236.times.N.sub.2 SA; and PA0 range of aggregate size distribution [.DELTA.Dst (nm)].gtoreq.30.6+0.6118.times.Dst.

BACKGROUND OF THE INVENTION 
The present invention relates to carbon black for tire tread and more 
particularly to novel carbon black suitable for use as a reinforcement of 
rubber, such as tire tread of a passenger car, where high abrasion 
resistance and resilience are required. There are many kinds of carbon 
black for reinforcement of rubber having different characteristics. The 
characteristics of carbon black are a ma]or factor determining various 
properties of a rubber composition with which the carbon black is 
compounded. 
For this reason, in compounding carbon black with a rubber composition, 
carbon black having characteristics suitable for applications of the 
rubber composition has been selected and used. 
In recent years, in order to improve the fuel saving of automobiles, the 
development of a rubber composition which exhibits high abrasion 
resistance and excellent resilience when used as a tire tread has eagerly 
been desired. 
In general, hard-grade carbon black having a small particle diameter and a 
large specific surface area has been used for rubber, such as tire tread, 
where it should exhibit high abrasion resistance under severe conditions. 
On the other hand, in order to attain fuel saving through a decrease in the 
rolling resistance of the tire tread portion, it is necessary to increase 
the resilience of rubber by making use of carbon black having a large 
particle diameter and a small specific surface area or carbon black having 
a wide range of aggregate size distribution per given particle diameter. 
Thus it was difficult to impart both of the high abrasion resistance and 
the high resilience to rubber at the same time, because these requirements 
are contradictory with each other from the viewpoint of the capability of 
carbon black in reinforcing rubber. 
For this reason, various proposals have been made to impart both of the 
abrasion resistance and resilience to rubber through the compounding of 
the rubber with carbon black having particular properties, such as a 
particular specific surface area measured by the nitrogen adsorption 
method (hereinafter abbreviated to "N.sub.2 SA") and dibutyl phthalate 
(hereinafter abbreviated to "DBP") absorption, selected by evaluating 
various characteristics of carbon black in more detail on a microstructure 
level besides fundamental characteristics such as the particle diameter, 
specific surface area and structure of carbon black. 
For example, a proposal has been made on carbon black belonging to hard 
carbon black having an N.sub.2 SA of at least 60 m.sup.2 /g and a DBP of 
at least 108 ml/100 g and satisfying the following characteristics 
requirements: 
true specific gravity.ltoreq.1.8379-0.0006.times.N.sub.2 SA; 
tinting strength (%).gtoreq.0.6979.times.N.sub.2 SA-0.4278.times.DBP+203.3; 
and 
range of aggregate size distribution (hereinafter abbreviated to 
".DELTA.Dst").gtoreq.30.6+0.6118.times.Dst (see U.S. Pat. No. 4,478,973 
issued Oct. 23, 1984. 
According to this proposal, for example, the true specific gravity of a 
conventional carbon black is 1.7854 when the N.sub.2 SA is 93 m.sup.2 /g, 
while the true specific gravity of the proposed carbon black is as low as 
1.7541 when the N.sub.2 SA is substantially the same as that of the 
conventional carbon black, i.e., 92 m.sup.2 /g. 
Further, as is apparent from the above-described formulae, the proposed 
carbon black has the tinting strength and the range of aggregate size 
distribution each maintained above a given value. 
On the other hand, Japanese Patent Application No. 62-154474 proposes a 
rubber composition having a high abrasion resistance imparted thereto by 
making use of carbon black having an N.sub.2 SA of at least 140 m.sup.2 /g 
and a relatively enlarged void size per given specific surface area of 
particle aggregates to improve the dispersion of the carbon black in the 
rubber component. 
Despite the above-described proposals, the development of a rubber 
composition which has higher resilience while maintaining high abrasion 
resistance is still eagerly desired in the art. 
Under the above-described circumstances, the present inventors have found 
that it is possible to effectively impart both of the above-described 
contradictory characteristics to rubber without the necessity of 
maintaining the tinting strength above a given value through setting of 
the true specific gravity per given specific surface area at a smaller 
value and the intra-aggregate pore diameter at a larger value with an 
N.sub.2 SA of 75 to 105 m.sup.2 /g, which has led to the completion of the 
present invention. 
When the N.sub.2 SA is 75 to 105 m.sup.2 /g according to the present 
invention, the dispersion is better than that of the carbon black having 
an N.sub.2 SA of at least 140 mg/g described in the above-described 
Japanese Patent Application No. 62-154474. Accordingly, in the present 
invention, a definite preponderance of the abrasion resistance has been 
established by further increasing the void diameter of the particle 
aggregates per given N.sub.2 SA. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide carbon black which can 
impart not only high abrasion resistance but also high resilience to a 
rubber composition. 
A second object of the present invention is to provide carbon black for 
tire tread. 
The above-described objects of the present invention can be attained by 
carbon black having an N.sub.2 SA of 75 to 105 m.sup.2 /g and a 
compressive DBP absorption of at least 110 ml/100 g and, at the same time, 
having the following characteristic values: 
true specific gravity.ltoreq.1.9080-0.0016 [N.sub.2 SA]; 
void diameter of particle aggregates (nm).gtoreq.62.2-0.236 [N.sub.2 SA]; 
and 
range of aggregate size distribution .DELTA.Dst.gtoreq.30.6+0.6118 
[.DELTA.st].

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The carbon black for tire tread of the present invention has an N.sub.2 SA 
of 75 to 105 m.sup.2 /g and a compressive DBP absorption of at least 110 
ml/100 g. 
The above-described carbon black belongs to hard carbon black usually 
employed for tire tread. 
When the N.sub.2 SA is less than 75 m.sup.2 /g, no high abrasion resistance 
can be maintained, while when the N.sub.2 SA exceeds 105 m.sup.2 /g, the 
resilience is remarkably lowered. 
When the compressive DBP absorption is less than 110 ml/100 g, as with the 
above-described case, no high abrasion resistance can be maintained and 
the resilience is lowered. 
The carbon black of the present invention has the following characteristics 
besides the above-described ones: 
true specific gravity.ltoreq.1.9080-0.0016.times.N.sub.2 SA; 
void diameter of particle aggregates (nm).gtoreq.62.2-0.236.times.N.sub.2 
SA; and 
range of aggregate size distribution .DELTA.Dst 
(nm).gtoreq.30.6+0.6118.times..DELTA.st. 
When the true specific gravity exceeds 1.9080-0.0016 .times.N.sub.2 SA, not 
only high abrasion resistance cannot be attained but also the resilience 
is lowered. When the void diameter of particle aggregates is less than 
62.2-0.236 .times.N.sub.2 SA, the dispersion of the carbon black in the 
rubber component is lowered, which makes it impossible to attain high 
abrasion resistance. 
Further, when .DELTA.Dst is less than 30.6+0.6118.times..DELTA.st, it 
becomes difficult to impart high resilience. 
As described above, a decrease in the true specific gravity and an increase 
in the .DELTA.Dst are factors which contribute to an improvement in the 
resilience while maintaining the abrasion resistance. 
Further, the void diameter of the particle aggregates is a measure of the 
anisotropy of the aggregate structure of the carbon black. When the void 
diameter of the particle aggregates per given range of specific surface 
area is large, there occurs a strong interaction between the carbon black 
and the rubber component during the step of compounding with the rubber 
component. 
Thus, in the present invention, it became possible to impart high 
resilience while maintaining the abrasion resistance through a decrease in 
the true specific gravity and an increase in the .DELTA.Dst and, at the 
same time, an increase in the void diameter of the particle aggregates. 
The carbon black having the above-mentioned characteristic properties is 
produced by a Y-shaped oil-furnace process using a reactor as shown in the 
accompanying figure, in which reference numerals 1 and 1' each denote a 
generator comprising a wind box 4, a burner 5, a feedstock oil spray 
nozzle 6, a combustion chamber 7, and a pyrolysis duct 8. The generators 
are arranged in such a manner that the pyrolysis ducts converge at point P 
in the cylindrical main reaction zone 2. The hydrocarbon feedstock oil is 
sprayed into the combustion gas of fuel oil so that the oil spray is 
pyrolyzed. The burned gas streams from the generators enter the zone 2 at 
a high speed and impinge against each other. 
In operation, adequate adjustments are made for the supply of feedstock oil 
and other conditions for pyrolysis in the generators and for the position 
of water spray 3 in the main reaction zone 2 which determines the 
residence time for the burned gas to reach the water spray, whereby it is 
possible to produce the carbon black that has all the characteristic 
properties specified in this invention. 
The carbon black of the present invention is compounded with natural rubber 
or various synthetic rubbers such as styrene-butadiene rubber, 
polybutadiene rubber, isoprene rubber, and butyl rubber. The carbon black 
is compounded in an amount of 25 to 250 parts by weight, preferably 40 to 
80 parts by weight based on 100 parts by weight of the rubber component 
and kneaded together with necessary components such as commonly employed 
vulcanizer, vulcanization accelerator, antioxidant, vulcanization 
assistant, softening agent, and plasticizer. The resultant compound is 
vulcanized and then formed into a tire tread. As described above, the 
carbon black of the present invention is remarkably low in the true 
specific gravity value per N.sub.2 SA and remarkably high in the void 
diameter of the particle aggregates. Further, the carbon black of the 
present invention has a unique physico-chemical property in that the 
.DELTA.Dst is broad. 
Since the carbon black of the present invention has all of these 
characteristics, the compounding thereof with rubber enables a remarkable 
improvement in the resilience of the rubber while maintaining the abrasion 
resistance. 
The method of determining the above-described various characteristics of 
the carbon black according to the present invention will now be described. 
(1) Nitrogen adsorption specific surface area (N.sub.2 SA) was determined 
by ASTM D3037-78 "Standard Methods of Testing Carbon black--Surface Area 
by Nitrogen Adsorption", Method B. The nitrogen adsorption specific 
surface area (N.sub.2 SA) of Industry Reference Blacks (IRB)#5 found by 
this method is 80.3 m.sup.2 /g 
(2) DBP absorption number of compressed sample was determined by ASTM 
D3439-79 "Carbon Black-Dibutyl Phthalate Absorption Number of Compressed 
Sample". 
(3) True specific gravity: 
A carbon black sample is placed in a crucible with a drop lid and 
devolatilized at 650.+-.25.degree. C. for 5 min. 
A proper amount of carbon black is weighed into a pycnometer. After being 
wetted with a small quantity of benzene, the carbon black is deaerated 
under a vacuum of 2 to 5 mmHg until bubbling does not occur any longer. 
The pycnometer is filled with benzene and held in a high temperature water 
bath at 25.+-.0.1.degree. C. for 30 min. The mass of the pycnometer 
containing benzene and carbon black is measured. 
The true specific gravity of carbon black is calculated from the following 
equation: 
##EQU1## 
where A is the mass of the pycnometer, 
C is the mass of the pycnometer plus benzene, 
D is the mass of the pycnometer plus carbon black sample, 
E is the mass of the pycnometer plus carbon black sample plus benzene, and 
d.sub.4.sup.25 is the specific gravity of benzene. 
The true specific gravity of IRB No. 5 measured according to this method 
was 1.7781. 
(4) Void diameter of particle aggregates: 
A mercury porosimeter (Pore Sizer 9300) manufactured by Micromeritics is 
used for this purpose, and a cell for exclusive use (3 ml) is charged with 
0.2 g of a carbon black pellet of which the particle size has been 
adjusted to 250 to 500 .mu.m. Mercury is introduced into the cell under a 
pressure of 25 to 2,000 lb/in.sup.2 to determine a pressure at which the 
amount of mercury introduced under pressure is rapidly increased. The pore 
diameter is calculated from the pressure and regarded as the void diameter 
of the particle aggregates. 
The void diameter of the particle aggregates of IRB No. 5 (N.sub.2 SA: 80.3 
m.sup.2 /g) was measured by the above-described method and found to be 40 
nm. 
(5) Range of aggregates size distribution (Dst, .DELTA.Dst) 
A carbon black sample is dried according to JIS K622l (1975), Section 
6.2.1, Method A. The dried carbon black sample is accurately weighed out 
and dispersed into a 20% aqueous solution of ethanol containing a small 
quantity of surface active agent (dispersing agent) to prepare a 
dispersion of carbon black in a concentration of 50 mg/l. Complete 
dispersion is accomplished ultrasonically. 
Then the dispersion is subjected to centrifugal classification by a disk 
centrifuge (made by Joyes Loebl Co., England) set to 8000 rpm. 10 to 20 ml 
of spin liquid (2% aqueous solution of glycerin) is added and then 1 ml of 
buffer (aqueous solution of ethanol) is added. Finally, 0.5 to 1.0 ml of 
the dispersion of carbon black is added by means of a syringe. 
Centrifugation is started. Simultaneously, the recorder is also started to 
draw a distribution curve of aggregate Stokes diameter. The Stokes 
diameter of the maximum frequency in the distribution curve thus prepared 
is defined as Dst (m.mu.), and the difference between two Stokes diameters 
each obtained in a frequency of 50% of the maximum frequency is defined as 
.DELTA.Dst (m.mu.). The Dst and .DELTA.Dst of ASTM D-24 Standard Reference 
Black C-3 (N234) measured by the above-described method are 80 nm and 60 
nm, respectively. 
The present invention will now be described in more detail with reference 
to the following Examples. 
EXAMPLE 1 
As shown in the drawing, there was provided an oil furnace having a 
Y-shaped structure comprising a main reaction zone 2 composed of two 
connectively provided portions, i.e., a large-diameter rear portion 10 
having an inner diameter of 300 mm and a length of 4000 nm and a 
small-diameter front portion 9 having an inner diameter of 135 mm and a 
length of 850 mm; and, convergently linked to the front thereof at a 
crossing angle (.alpha.) of 60.degree., two generators, i.e., a first 
generator 1 equipped with a combustion chamber 7 having an inner diameter 
of 400 mm and a length of 800 mm (including a conical portion having a 
length of 200 mm) provided with a burner 5 and a feedstock oil spray 
nozzle 6 through a wind box 4 and a pyrolysis duct 8 having an inner 
diameter of 80 mm and a length of 700 mm and a second generator 1' 
equipped with a combustion chamber 7 having an inner diameter of 650 mm 
and a length of 800 mm (including a conical portion having a length of 200 
mm) similarly provided a stockoil spray nozzle 6 and a pyrolysis duct 8 
having an inner diameter of 110 mm and a length of 1000 mm. An aromatic 
hydrocarbon oil having a specific gravity of 1.0703 (15/4.degree. C.), a 
viscosity of 2.10 (Engler; 40/20.degree. C.), a benzene insoluble of 
0.03%, a coefficient of correlation (BMCI) of 140, and an initial boiling 
point of 103.degree. C. was used as a stockoil, and a hydrocarbon oil 
having a specific gravity of 0.903 (15.4.degree. C.), a viscosity (CST; 
50.degree. C.) of 16.1, a residual carbon content of 5.4%, a sulfur 
content of 1.8%, and a flash point of 96.degree. C. was used as a fuel 
oil. 
The carbon black of the present invention was prepared by making use of the 
above-described reaction furnace, stockoil, and fuel oil under generation 
conditions shown in Table 1. 
Various characteristics and calculated values of the carbon black thus 
prepared are shown in Table 2. 
In Table 1, Run Nos. 1 to 4 (examples of the present invention) are each 
carbon black prepared by properly controlling the conditions of each 
generator and, at the same time, cooling a formed carbon black gas steam 
at two stages, while Comparative Example No. 5 is carbon black prepared by 
conducting one-stage cooling. In Table 2, Run Nos. 6 to 8 shown as 
comparative examples are each carbon black having a specific surface area 
measured by the nitrogen adsorption method (N.sub.2 SA) and a compressive 
DBP absorption equal to those of the carbon black prepared by the 
conventional process. 
TABLE 1 
__________________________________________________________________________ 
fuel feed rate 1st cooling water 
total air 
fuel oil 
combustion 
of position of 
position of 
feed rate/2nd 
Run feed rate 
feed rate 
rate stockoil 
1st cooling 
2nd cooling 
cooling water 
No. 
generator 
(Nm.sup.3 /hr) 
(kg/hr) 
(%) (kg/hr) 
(mm) (mm) feed rate ratio 
__________________________________________________________________________ 
1 1 2,200 
72 300 687 850 1,500 1/7 
2 1,300 
51 250 488 
2 1 2,400 
84 280 625 850 1,500 2/9 
2 1,500 
59 250 516 
3 1 2,100 
82 250 474 1,250 2,000 1/4 
2 1,500 
49 300 549 
4 1 1,800 
77 230 354 1,250 2,000 1/3 
2 1,700 
67 250 539 
5 1 1,500 
82 180 471 -- 1,500 -- 
2 1,500 
82 180 623 
__________________________________________________________________________ 
Note: 
Position of cooling is expressed in terms of a distance from the inlet of 
the smalldiameter front portion (135 .phi. .times. 850 l). 
TABLE 2 
__________________________________________________________________________ 
Ex. of the present invention 
Comp. Ex. 
Run No. 1 2 3 4 5 6 7 8 
__________________________________________________________________________ 
N.sub.2 SA (m.sup.2 /g) 
77 88 95 103 80 87 98 101 
compressive DBP absorption (ml/100 g) 
116 114 112 111 115 109 113 110 
true specific gravity (measured) 
1.7814 
1.7541 
1.7460 
1.7392 
1.7842 
1.7723 
1.7621 
1.7643 
*1 true specific gravity (calculated) 
1.7848 
1.7672 
1.7560 
1.7432 
1.7800 
1.7688 
1.7512 
1.7464 
void diameter of particle 
48 46 43 42 41 38 37 37 
aggregate (nm) (measured) 
*2 void diameter of particle 
44.0 41.4 39.8 37.9 43.3 41.7 39.1 38.4 
aggregate (nm) (calculated) 
-- Dst (nm) 123 148 112 134 144 137 110 101 
.DELTA.Dst (nm) (measured) 
115 128 125 132 110 98 108 78 
*3 .DELTA.Dst (nm) (calculated) 
106 121 99 113 19 114 98 92 
__________________________________________________________________________ 
Note: 
*1: values obtained through calculation by making use of the equation 
1.9080 - 0.0016 [N.sub.2 SA 
*2: values obtained through calculation by making use of the equation 62. 
- 0.263 [N.sub.2 SA 
*3: values obtained through calculation by making use of the equation 30. 
+ 0.6118 [--Dst 
EXAMPLE 2 
Each carbon black sample shown in Table 2 was compounded with a synthetic 
rubber component comprising polybutadiene rubber (BR)/oil extended 
styrene-butadiene rubber (SBR) in a compounding ratio shown in Table 3. 
Rubber compositions prepared by vulcanization treatment (vulcanization 
time: 40 min) of the compounds shown in Table 3 at a temperature of 
145.degree. C. were subjected to measurement of various rubber 
characteristics. The results are shown in Table 4 together with Run Nos. 
of the compounded carbon black samples shown in Table 2. 
TABLE 3 
______________________________________ 
compounding components pts. wt. 
______________________________________ 
styrene-butadiene rubber (SBR) *1 
96.25 
polybutadiene (BR) *2 30 
carbon black 70 
aromatic oil (softening agent) 
10 
stearic acid 2 
(dispersion vulcanization assistant) 
zinc oxide (vulcanization assistant) 
3 
phenyl-.beta.-naphthylamine (antioxidant) 
0.9 
diphenyl-guanidine 0.5 
(vulcanization accelerator) 
dibenzothiazyl disulfide 
1.2 
(vulcanization accelerator) 
sulfur (vulcanizing agent) 
1.65 
______________________________________ 
Note: 
*1: JSR 1712 [a product of Japan Synthetic Rubber Co., 
*2: JSR BR01 [a product of Japan Synthetic Rubber Co., Ltd. 
TABLE 4 
__________________________________________________________________________ 
Rubber characteristics of BR/oil extended SBR compound 
__________________________________________________________________________ 
Ex. of the 
present invention 
Comp. Ex. 
Run No. 1 2 3 4 5 6 7 8 
__________________________________________________________________________ 
resilience (%) 46.2 
45.9 
44.3 
43.8 
44.1 
42.0 
40.8 
39.1 
abrasion 
Lambourn (24% slip) 
102 
105 
106 
108 
101 
102 
103 
110 
resistance 
Lambourn (60% slip) 
111 
114 
113 
116 
110 
112 
113 
120 
Pico abrasion resistance 
120 
127 
125 
130 
115 
119 
118 
127 
hardness (JIS Hs) 
60 
62 
62 
63 
61 
61 
62 
64 
tensile stress at 300% (kg/cm.sup.2) 
124 
128 
115 
120 
119 
113 
121 
125 
tensile strength (kg/cm.sup.2) 
223 
229 
218 
220 
215 
210 
217 
228 
elongation (%) 540 
530 
545 
535 
550 
530 
555 
520 
__________________________________________________________________________ 
Note to Table 4: 
Methods of abrasion test: 
1) Lambourn abrasion resistance was measured with a Lambourn abrasion 
tester (of 
mechanical slip type) under the following conditions. 
Test piece: 10 mm in thickness, 44 mm in outside diameter 
Emery wheel: GC type; grain size: 80; hardness: H 
Carborundum added: grain size: 80; rate of addition: approximately 9 
g/min 
Speed of revolution of test piece: 660 rpm 
Load of test piece: 4 kg 
Relative slipping: 24%, 60% 
2) Pico abrasion resistance was measured according to ASTM D-2228-76 
"Standard Test 
Method for Rubber Property-Abrasion Resistance (Pico Abrader)" under the 
following conditions. 
Load: 44 N 
Frequency of rotation: 1 Hz 
Speed of revolution: 160 rpm 
The values of the abrasion tests are expressed in terms of index, with 
the reference 
being 100 which is the abrasion of a rubber composition prepared by 
compounding IRB 
No. 5 (standard carbon black) with the basic rubber under the same 
conditions. 
Other characteristic properties than above were measured according to JIS 
K6301 
"General Test Methods for Rubbers". 
It is apparent from the results shown in Table 4 that when the carbon black 
used is Run Nos. 1 to 4 having all the requirements for the present 
invention, the rubber compositions exhibit not only abrasion resistance 
superior to that of the comparative rubber composition wherein the 
corresponding carbon black is Run Nos. 5 to 8 but also a significant 
improvement in the resilience.