Pneumatic tire tread of stable dynamic performance

Improvement of stability of dynamic performance of pneumatic tire tread composition, by adding at least one of some esters such as phosphoric acid ester, fatty acid ester in a definite ratio to the known composition comprising styrene-rich SBR added with a processing oil and carbon black respectively in higher ratio for improving dynamic performance.

BACKGROUND OF THE INVENTION 
The present invention relates generally to pneumatic tire tread made of 
styrene-rich styrene-butadiene copolymer rubber which may be blended with 
butyl rubber, butyl halide rubber or butadiene-acrylonitrile copolymer 
rubber or mixture thereof and which is added with petroleum-based 
processing oil and carbon black in high ratio for improving road gripping 
performance, and more particularly to such tread rubber composition 
further added with an ester so as to stabilize grip property despite of 
temperature fluctuation with keeping satisfactory mechanical strength of 
tire tread. 
Hitherto various proposals have been made with respect to dynamic 
performances of the tire tread, above all grip property which is naturally 
related to accelerative and braking properties from the view points not 
only of tread patterns but also of tread rubber compositions. 
As is well known to those skilled in the art, one of the important 
proposals is using of such rubber as higher glass transition temperature 
such as styrene-rich styrene-butadiene copolymer, and the others is the 
addition of petroleum-based processing oil and carbon black in higher 
ratio. 
However, the former has been found unsatisfactory in that the grip property 
considerably declines as the temperature of the tread rubber is raised, or 
in other words the dynamic performance, such as grip property is not 
stable. The latter has an important disadvantage in that mechanical 
strength is considerably deteriorated when such fillers are added in such 
a higher ratio as being able to improve grip property. 
SUMMARY OF THE INVENTION 
It is, thus, an object of the invention to provide an improved pneumatic 
tire tread composition comprising styrene-rich styrene-butadiene copolymer 
rubber as a main component added with processing oil and carbon black in 
high ratio but without defects as referred to above. 
It is the other object to provide such tread rubber capable of displaying 
stably improved dynamic performance, above all grip property despite of 
temperature fluctuation with keeping desired mechanical properties. 
Said objects and various advantages to be appreaciated by those skilled in 
the art may be attained by adding one or more of some esters used as 
plasticizer in the polymer industrial field to said known tread rubber 
composition in the amount of a definite range relative to styrene content 
in the composition.

DETAILED EXPLANATION OF THE PREFERRED EMBODIMENT 
The tread rubber to which ester is added according to the invention is 
styrene-butadiene copolymer containing styrene more than 25% by weight, to 
which butyl rubber, butyl halide rubber or butadiene/acrylonitrile 
copolymer or mixture thereof may be blended. 
As the petroleum-based processing oil to be added to said rubber material, 
paraffinic, naphthenic or aromatic oil or mixture thereof is preferably 
used, but aromatic processing oil is particularly preferable. 
Ester to be added according to the invention may be phthalic acid ester, 
fatty acid ester, glyceric acid ester, maleic acid ester, fumaric acid 
ester, phosphonic acid ester or phosphoric acid ester. Above all 
phosphoric acid ester such as trioctyl phosphate, tributyl phosphate, 
octyldiphenyl phosphate, dibutyl phosphate, tributhoxyethyl phosphate, 
trioleyl phosphate is preferably used, among which trioctyl phosphate is 
particularly preferable. Fatty acid ester such as octyl oleate and octyl 
adipate is also preferable. 
In order to improve grip property stability, it is important how much of 
ester is added. Such ester or mixture thereof is preferably added in the 
amount of 5-60 parts by weight, and 5-300 parts by weight together with 
said processing oil in relation to 100 parts by weight of rubber component 
in the tread. As a result of a series of experiments, it has been found 
that said amount must be in the range defined by a trapezium A-B-C-D shown 
in the coordinates of the accompanying drawing, in which such amount in 
parts by weight is taken along the ordinate while styrene content in % by 
weight of the rubber component is taken along the abscissa. 
From the above, the lower limit is calculated as [0.137.times.(styrene 
content %/w)+1.78] parts by weight in relation to 100 parts by weight of 
rubber component in the tread, while the upper limit is to be 
[0.685.times.(styrene content %/w+13.904] parts by weight in relation to 
100 parts by weight of rubber component in the tread. Said limits are of 
course not critical, but when the amount of added ester is less than said 
lower limit the desired improvement of grip property stability can not 
essentially be attained, and even if said amount is more than said upper 
limit further improvement according to the invention can not be expected. 
In order to confirm how far satisfactory grip property can be stabilized 
according to the invention, the tread rubber was prepared from 100 parts 
of usual styrene-butadiene rubber (SBR 1500) containing 23.5%/w styrene 
content, 20 parts of carbon black (ISAF), 80 parts of carbon black (SAF), 
80 parts of usual aromatic processing oil, 1 part of stearic acid as 
dispersant, 1 part of N-phenyl-N'-isopropyl-p-phenylenediamine as 
antioxydant, 3 parts of zinc white as accelerator and/or antioxidant, 0.3 
parts of diphenyl-guanidine as accelerator, 0.7 parts of 
dibenzothiazyl-disulfide as accelerator and 1.5 parts of sulphur, 
according to the usual process, as Control 1. 
All "parts" are by weight in the above and hereafter. 
With using pneumatic tires each being mounted with said tread, automobile 
running test was actually made in the circuit of 4.359 km circumference. 
The time lag between the first round running and the tenth round running 
is set as an index of 100. 
The tread was similarly prepared as Control 2 but with using SBR 
containing 30% styrene instead of SBR poor in styrene of Control 1. When 
dividing said time lag of Control 1 with that of Control 2 and multiplying 
by 100, the index for grip property stability 104 was obtained as shown in 
the following Table 1. 
The same test was made in respect to the tread as Example 1 which was 
prepared similar to Control 2 but with the addition of 20 parts of 
trioctyl phosphate according to the invention instead of 20 parts of 
processing oil. The index of 108 given in Table 1 shows improvement by the 
invention in comparison with an index of 104 of the prior art. 
Example 2 shows tread rubber composition in which octyl oleate was used 
instead of trioctyl phosphate in Example 1. The index 106 is inferior to 
108 of Example 2, but still superior to 104 of the prior art. 
As Control 3, the tread was prepared similar to Control 2 but by using SBR 
containing 40% styrene insted of SBR of 35% styrene in Control 2. The grip 
property stability index was 115 which is superior to 108 in Example 1 and 
106 in Example 2. Examples 3-9 in which the respective tread was prepared 
by using the same styrene-rich SBR as in Control 3 and adding the 
respective ester as shown in the Table 1 resulted in indexes ranging from 
118 to 128 which are all superior to the index of 115 of Control 3, as 
seen in Table 1. 
Comparison of Control 4 with Example 10 also shows further improvement of 
grip property stability by addition of ester according to the invention. 
As shown in Table 2, Examples 11, 12 and 13 in which butyl rubber, butyl 
halide rubber or butadieneacrylonitrile copolymer rubber is blended with 
SBR containing 40% styrene, also show improvement of stability of grip 
property in contrast to Control 3. 
TABLE 1 
__________________________________________________________________________ 
Ex- 
Ex- Ex- 
Ex- 
Ex- 
Ex- 
Ex- 
amp. 
amp. amp. 
amp. 
amp. 
amp. 
amp. 
Cont. 1 
Cont. 2 
1 2 Cont. 3 
3 4 5 6 7 Examp. 8 
Examp. 
Cont. 
Examp. 
__________________________________________________________________________ 
10 
SBR 1500 100 
SBR (Styrene 35%) 
100 100 
100 
SBR (Styrene 40%) 100 100 
100 
100 
100 
100 
100 100 
SBR (Styrene 45%) 100 100 
Carbon Black (ISAF) 
20 " " " " " " " " " " " " " 
Carbon Black (SAF) 
80 " " " " " " " " " " " " " 
Processing Oil 
80 80 60 
60 
80 75 
50 
35 
50 
50 
50 50 80 40 
(Aromatic) 
Trioctyl Phosphate 20 5 30 
45 40 
Tributyl Phosphate 30 
Octyl Oreate 20 
Octyl Adipate 30 
Octyldiphenyl 30 
Phosphate 
Dibutyl 30 
Phosphate 
Stearic Acid 
1 
Antioxydant (IPPD)*1 
1 
Zinc White 3 " " " " " " " " " " " " " 
Accelerator (DGM)*2 
0.3 
(DM)*3 0.7 
Sulphur 1.5 
Grip Property 
100 104 108 
106 
115 118 
127 
128 
125 
123 
124 123 120 125 
Stability (Index) 
__________________________________________________________________________ 
*1 N--PhenylN'--isopropylp-phenylenediamine 
*2 Diphenylguanidine 
*3 Dibenzothiazyldisulfide 
TABLE 2 
______________________________________ 
Examp. Examp. Examp. 
Cont. 3 
11 12 13 
______________________________________ 
Butyl Rubber 20 
Butyl Halide Rubber 20 
SBR (Styrene 40%) 
100 80 80 80 
Butadiene-acrylonitrile 20 
Copolymer Rubber 
Carbon Black (ISAF) 
20 " " " 
Carbon Black (SAF) 
80 " " " 
Processing Oil 
80 50 50 50 
(Aromatic) 
Trioctyl Phosphate 30 30 30 
Tributyl Phosphate 
Octyl Oreate 
Octyl Adipate 
Octyldiphenyl 
Phosphate 
Dibutyl 
Phosphate 
Stearic Acid 1 
Antioxydant (IPPD)*1 
1 
Zinc White 3 " " " 
Accelerator (DGM)*2 
0.3 
(DM)*3 0.7 
Sulphur 1.5 
Grip Property 115 125 125 123 
Stability (Index) 
______________________________________ 
*1 N--PhenylN'--isopropylp-phenylenediamine 
*2 Diphenylguanidine 
*3 Dibenzothiazyldisulfide