Tire tread rubber

An all season tire tread rubber blend comprises about 30 to 50 parts per hundred low vinyl polybutadiene; about 25 to 60 parts per hundred natural rubber and/or synthetic polyisoprene; and about 10 to 25 parts per hundred high styrene styrene/butadiene rubber and/or other diene polymer having a glass transition temperature of about -5.degree. to about -20.degree. C. A preferred embodiment comprises about 40 parts per hundred low vinyl solution polybutadiene, about 40 parts per hundred natural rubber, and about 20 parts per hundred high styrene solution styrene/butadiene. Tires employing the tire tread rubber of this invention exhibit improved wet and winter traction and superior abrasion resistance and treadwear without increased rolling resistance.

BACKGROUND OF THE INVENTION 
This invention relates to a new all season tire tread rubber comprising low 
vinyl polybutadiene, polyisoprene, and a diene polymer having a glass 
transition temperature of about -5.degree. to about -20.degree. C. 
Tire tread compositions are engineered to produce traction, speed, 
stability, and casing protection, simultaneously providing frictional 
contact for the transmission of driving, braking and cornering forces as 
well as wear resistance. Typical tread rubber compositions employ 
elastomers having a relatively high tensile strength and high abrasion 
resistance. However, compounding elastomers for improved traction may 
adversely affect abrasion resistance and tire rolling resistance; 
conversely, tire traction can suffer when trying to improve rolling 
resistance. To balance these counteracting considerations, blends of 
styrene/butadiene and butadiene rubber are commonly employed in passenger 
tires (Bhakuni, et al., Encyclopedia of Polymer Science and Engineering, 
2nd ed., volume 16, John Wiley, 1989, page 838); other elastomers such as 
natural rubber may be used to modulate cure, dynamic, or physical 
properties, or the flex fatigue life of the tire. Fillers, notably carbon 
black, are added to reinforce the elastomers and improve strength (Eirich, 
F., Science and Technology of Rubber, Academic Press, New York, 1978, 
pages 374 and 375 and Bhakuni, et al., cited above, page 842). 
For example, Studebaker and Beatty described three tread compositions: two 
had styrene/butadiene elastomers and one had natural rubber (Eirich, F.R., 
Science and Technology of Rubber, Academic Press, 1978, pages 374 to 375; 
compounds 188, 208 and 211 are treads). Bridgestone disclosed a tire 
rubber composition comprising 50 to 85 parts by weight of natural rubber 
or synthetic polyisoprene, 5 to 20 parts by weight of high styrene 
styrene/butadiene, and 5 to 45 parts by weight of low styrene 
styrene/butadiene (Jap. Pat. Ap. Pub. No. 2,129,240). Takino, et al., 
disclosed a tire tread rubber composition having a two-peak loss tangent 
curve and comprising isoprene containing more than 50% total 3,4- and 
1,2-vinyl bonds and styrene/butadiene in a ratio of 5/95 to 60/40, with 50 
to 200 parts by weight carbon black (U.S. Pat. No. 4,946,887). 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a new tire tread rubber. It is 
another object of this invention to provide tires having treads that 
exhibit good traction properties in all conditions (dry, wet, snow, and 
ice), good handling characteristics, and decreased rolling resistance 
without abnormal tire treadwear. 
These and other objects are accomplished by the present invention, which 
describes rubber blends comprising low vinyl polybutadiene, polyisoprene, 
and a diene polymer having a glass transition temperature of about 
-5.degree. to about -20.degree. C., in amounts effective to produce a 
blend suitable for use in automobile and light truck tire treads and tire 
retreads. Preferred rubber blends comprise about 30 to 50 parts per 
hundred low vinyl polybutadiene; about 25 to 60 parts per hundred natural 
rubber and/or synthetic polyisoprene; and about 10 to 25 parts per hundred 
high styrene styrene/butadiene rubber and/or other diene polymer having a 
glass transition temperature of about -5.degree. to about -20.degree. C. A 
preferred embodiment comprises about 40 parts per hundred low vinyl 
solution polybutadiene, about 40 parts per hundred natural rubber and 
about 20 parts per hundred high styrene solution styrene/butadiene. A 
method for producing tire tread rubber compositions according to this 
invention is also disclosed.

DETAILED DESCRIPTION OF THE INVENTION 
In the practice of this invention, rubber blends comprising low vinyl 
polybutadiene, polyisoprene, and a diene polymer having a glass transition 
temperature of about -5.degree. to about -20.degree. C. are employed in 
all season tire tread rubber compositions for passenger cars and light 
trucks, tire retreads and the like. Preferred rubber blends comprise about 
30 to 50 parts per hundred low vinyl polybutadiene; about 25 to 60 parts 
per hundred natural rubber and/or synthetic polyisoprene; and about 10 to 
25 parts per hundred high styrene styrene/butadiene and/or other diene 
polymer having a glass transition temperature of about -5.degree. to about 
-20.degree. C. Tires having the tread rubber blend of the invention 
exhibit a superior dynamic response in wet and winter driving conditions, 
good handling characteristics, and improved abrasion resistance without 
increased rolling resistance. 
Low vinyl polybutadiene is employed in the rubber blends of this invention. 
By the term "polybutadiene" is meant polymerized butadiene or butadiene 
rubber, herein abbreviated BR. The vinyl content of polybutadiene refers 
to the weight percent of monomer in the 1,2-configuration. By the term 
"low vinyl" is meant having a vinyl content of about 10 to about 30 weight 
percent. The vinyl groups, or 1,2-monomers, can occur anywhere in the 
butadiene polymers: in clusters or groups, or interspersed regularly or 
irregularly among 1,4-monomers. 
Polybutadiene prepared by any known means including free radical, emulsion, 
ionic, coordination, bulk, solution, or suspension polymerization of 
butadiene may be employed. However, low vinyl solution polybutadiene 
prepared in a solution polymerization process employing an organic solvent 
and using lithium or the like catalyst is preferred, or its chemical 
equivalent. 
Polyisoprene is also employed in the rubber blends of this invention. The 
polyisoprene may be natural or synthetic, or a mixture of the two. By the 
term "natural polyisoprene" is meant natural rubber obtained from natural 
sources, or its chemical equivalent, such as cis-1,4-polyisoprene. By the 
term "synthetic polyisoprene" is meant any polyisoprene produced 
synthetically, regardless of the isomeric configuration of the isoprene 
monomers. Blends of synthetic polyisoprenes with natural rubber can be 
used in the rubber blends of this invention. Natural rubber is employed in 
preferred embodiments. 
A diene polymer having a glass transition temperature of 
about -5.degree. to about -20.degree. C. is also employed in the rubber 
blends of this invention. Diene polymers having glass transition 
temperatures in this range include high styrene styrene/butadiene 
(hereinafter referred to as SBR) co-polymers, polymethylbutadiene polymers 
(methyl rubber), butadiene/acrylonitrile copolymers, 
styrene/isoprene/butadiene terpolymers, ethylene/ propylene/diene, high 
3,4-polyisoprene, and the like elastomers, including thermoplastic 
elastomers having a diene component. Mixtures of rubbers may also be 
employed. 
High styrene solution SBR is employed in preferred embodiments. By "high 
styrene" is meant about 30 to about 50 weight percent bound styrene. By 
"solution" is meant prepared by the polymerization of styrene and 
butadiene in a solution, or a process yielding a chemical equivalent 
thereof, rather than in a free radical, emulsion, ionic, coordination, 
bulk, or suspension polymerization process. 
In one embodiment of the invention, the tire tread rubber blend comprises 
about 30 to 50 parts per hundred low vinyl solution polybutadiene, about 
25 to 60 parts per hundred natural rubber, and about 10 to 25 parts per 
hundred high styrene solution styrene/butadiene. A preferred rubber blend 
comprises about 40 parts low vinyl solution polybutadiene, about 40 parts 
natural rubber and about 20 parts high styrene solution styrene/butadiene. 
Tire tread rubber blends of this invention are processed with effective 
amounts of processing aids, accelerators, cross-linking and curing 
materials, antidegradants, fillers and the like to make tire tread rubber 
compositions. Processing aids include, but are not limited to, processing 
oils, plasticizers, tackifiers, extenders, chemical conditioners, 
homogenizing agents and peptizers such as mercaptans, petroleum and 
vulcanized vegetable oils, resins, rosins, and the like. Accelerators 
include amines, guanidines, thioureas, thiazoles, thiurams, sulfenamides, 
thiocarbamates, xanthates, and the like. Cross-linking and curing agents 
include sulfur, zinc oxide, and fatty acids. Antidegradants include 
antioxidants and antiozonants. Fillers include carbon black and mineral 
fillers such as silica and clay. Example formulations are set forth 
hereinafter. 
The materials are mixed in a single step or in stages. Two or three-stage 
processes are preferred. For example, the rubber blend can be processed 
with filler, wax, some antidegradants, and effective amounts of processing 
oils in one step, with accelerators, curing and cross-linking agents, and 
the remaining antidegradants added in a second stage. Additional stages 
may involve incremental additions of filler and processing oils. 
The tire tread compositions of this invention are employed in tire treads. 
As is known to those skilled in the art, in the conventional manufacture 
of a radial ply tire, the extruded tread composition is wrapped over the 
belt area of a green tire in the final stages of tireassembly prior to 
curing. For further understanding of the position of these components, 
reference is made to accompanying FIG. 1, which depicts a cross-sectional 
view of tire 10 which employs the tire tread composition of this invention 
in tire tread component 20. 
The tire of FIG. 1 consists of a carcass structure comprising one or more 
carcass plies 50, provided with textile cords disposed according to a 
radial extension, folded back from the inside to the outside around a 
metal bead core 55 disposed in the area of the tire bead, over which, at a 
radially external position, a filling strip 60 of elastomeric material of 
substantially triangular cross-sectional shape is placed in a manner known 
by those skilled in the art. Disposed crown-wise to the tire carcass, in 
the usual manner, is a tread component or band 20, and inserted between 
the carcass and the tread band is a belt structure 30 consisting of two 
radially superposed layers 31 and 32 of cords disposed at angles with 
respect to the midcircumferential plane of the tire in opposite 
directions, preferably symmetrically. The belt layers are formed in any 
appropriate known manner and in particular can comprise textile and/or 
metal reinforcing cords. 
Tires so formed with the tire tread rubber blends of this invention have an 
array of desirable characteristics. In tire road tests comparing the 
performance of tires having the tread rubber of this invention with 
control tires, wet ice traction and dry ice traction was improved, and 
snow traction was significantly improved. The overall ride and handling 
was also improved. Abrasion resistance was good; preferred tire tread 
compositions exhibited superior treadwear and less abrasion when compared 
with controls. Goodyear-Healey rebound of preferred tread compositions 
embodiments show good resilience (e.g., .about.37% to .about.39% rebound 
at room temperature). The physical properties of example compositions are 
set out hereinafter, as are tire road test results. 
Measures of dynamic response of preferred compositions predict good 
traction without increased rolling resistance, especially in wet and 
winter conditions. An industry accepted measure of tire traction, for 
example, is hysteresis loss, denoted by loss factor tan .delta., as 
determined by any number of dynamic viscoelasticity tests. Traction 
improvement is obtained through an increase of the loss factor in the 
mid-range temperature region (e.g., between about, -20.degree. and 
30.degree. C.) without the rubber composition becoming too hard. On the 
other hand, increased hysteresis loss at higher, operating temperatures of 
the tire (e.g., about 30.degree. C. and above) results in higher rolling 
resistance and lower fuel economy. At very low temperatures (e.g., below 
about -20.degree. C.), flexible compositions are preferred over stiffer 
ones. To balance these counteracting considerations, the desirable tread 
compositions of this invention are flexible at very low temperatures and 
exhibit low hysteresis loss at higher temperatures, with hysteresis loss 
at intermediate temperatures (in the wet traction zone) superior to a 
control. Example hysteresis loss measurements, as well as dynamic modulus 
testing and loss modulus response, are plotted hereinafter. 
Thus, employment of low vinyl polybutadiene with polyisoprene and high 
styrene styrene/butadiene as the rubber in tire treads provides improved 
all season tires having superior wet and winter traction, abrasion 
resistance and treadwear. 
EXAMPLES 
The following examples are presented to further illustrate and explain the 
present invention and should not be taken as limiting in any regard. 
Unless otherwise indicated, all parts and percentages are by weight, and 
are based on the weight at the particular stage of the processing being 
described. 
EXAMPLE 1 
This example illustrates the preparation of a tire tread rubber composition 
comprising the rubber blend of this invention (C, below) and two control 
compositions (A and B below). 
TABLE I 
______________________________________ 
Composition, parts 
Material A B C 
______________________________________ 
Natural Rubber 25 40 40 
Low Vinyl Polybutadiene 
0 0 40 
(20% vinyl) 
High Styrene Styrene/Butadiene 
0 20 20 
(Tg, -10.degree.) 
Low Styrene Styrene/Butadiene.sup.1 
0 40 0 
Butadiene Rubber 17 0 0 
Styrene/Butadiene (1712 Type) 
48 0 0 
Polynorbornene 10 0 0 
Filler.sup.2 69 80 80 
Processing Oil 26 45 45 
Zinc Oxide 1.7 2.0 2.0 
Fatty Acid 1.7 2.0 2.0 
Sulfur 1.3 1.5 1.5 
Other.sup.3 6.43 7.4 7.4 
______________________________________ 
.sup.1 18% styrene 
.sup.2 Carbon black and/or silica 
.sup.3 Accelerators, antidegradants, tackifiers, peptizers, resins, waxes 
and the like 
EXAMPLE 2 
In this example, the physical properties of the rubber compositions of 
Example 1 are compared and contrasted. The values are presented in Table 
II below. 
TABLE II 
______________________________________ 
Composition 
Physical Property A B C 
______________________________________ 
Mooney Viscosity (ASTM D3646) 
ML 4' @ 100.degree. C. 
60.9 59.8 66.0 
Mooney Scorch @ 135.degree. C. (ASTM D1546) 
2 pt. rise 13.3 15.1 13.7 
5 pt. rise 14.4 16.3 15.5 
10 pt. rise 15.0 16.9 16.1 
Hardness IRHD (ASTM D1415) 
25' @ 148.degree. C. 66.9 69.2 68.4 
35' @ 148.degree. C. 67.7 68.9 67.6 
Specific gravity (ASTM D297) 
1.139 1.132 1.130 
Unaged Stress/Strain (ASTM D412) 
M 300% 25' @ 148.degree. C., MPa 
9.537 8.196 7.468 
35' @ 148.degree. C., Mpa 
9.531 8.876 8.199 
Tensile 25', MPa 17.584 17.306 16.731 
35', MPa 17.431 17.776 16.975 
Elong. @ Break, 25' 517 553 592 
35' 514 551 552 
Rheometer (1d .multidot. 100 cpm .multidot. 148.degree. C., ASTM D2084) 
M.sub.L 3.58 2.80 3.55 
M.sub.H.sbsb.2 18.42 16.48 19.10 
t.sub.R 6.69 7.77 7.38 
t.sub.50 8.73 9.42 9.07 
t.sub.90 14.81 14.57 14.06 
BFG Flexometer (35' @ 148.degree. C., ASTM D623) 
Final Temp, .degree.F. 
187 186 180 
% Set 11.65 13.22 11.94 
% Static Compression 38.05 37.28 38.91 
% Dynamic Compression 
37.05 35.29 33.43 
% Final Dynamic Comp. 
40.30 41.15 40.70 
Goodyear-Healey Rebound (35' @ 148.degree. C., ASTM D1054) 
% Rebound @ RT 40.30 38.30 40.30 
% Rebound @ 100.degree. C. 
59.00 56.40 58.40 
Penetration @ RT 0.192 0.191 0.195 
Penetration @ 100.degree. C. 
0.258 0.276 0.260 
______________________________________ 
EXAMPLE 3 
This example compares and contrasts tire tread compositions comprising a 
control rubber blend (composition D), rubber blends comprising low vinyl 
polybutadiene, polyisoprene and high styrene styrene/butadiene blended in 
particular proportions (compositions E, F, and G), and rubber blends 
comprising natural rubber and low vinyl polybutadiene (compositions H and 
I). Components of the compositions are set out in Table III and the 
physical properties of the compositions are given in Table IV. 
TABLE III 
______________________________________ 
Compositon, parts 
Material D E F G H I 
______________________________________ 
Low Styrene Styrene/ 
40.0 0 0 0 0 0 
Butadiene Rubber.sup.1 
Natural Rubber 40.0 40.0 25.0 60.0 25.0 75.0 
Low Vinyl Polybutadiene 
0 40.0 50.0 30.0 75.0 25.0 
(20% vinyl) 
High Styrene Solution 
20.0 20.0 25.0 10.0 0 0 
Styrene/Butadiene.sup.2 
Filler 80.0 80.0 80.0 80.0 80.0 80.0 
Processing Oil 45.0 45.0 45.0 45.0 45.0 45.0 
Antidegradants 2.75 2.75 2.75 2.75 2.75 2.75 
Curing and Crosslinking 
4.5 4.5 4.5 4.5 4.5 4.5 
Agents 
Other.sup.3 5.0 5.0 5.0 5.0 5.0 5.0 
______________________________________ 
.sup.1 18% styrene 
.sup.2 Glass transition temperature = -10.degree. C. 
.sup.3 Processing aids, fatty acids, waxes, and the like. 
TABLE IV 
______________________________________ 
Physical Composition 
Property D E F G H I 
______________________________________ 
Mooney Viscosity (ASTM D1646) 
ML 4' 67.3 69.3 69.1 66.7 76.3 67.4 
@ 100.degree. C. 
Specific 1.131 1.126 1.126 1.120 1.117 1.120 
Gravity 
(ASTM 
D297) 
IRHD Durometer (ASTM D1415) 
30' @ 151.degree. C. 
63.9 66.1 64.3 64.7 62.9 62.7 
Unaged Stress/Strain (ASTM D412) 
300% 5.991 6.354 5.738 6.666 5.478 6.257 
Modulus, 30' 
@ 151.degree. C., 
Mpa 
Tensile, 16.128 16.389 15.223 
16.555 
15.174 
16.193 
30', MPa 
Elong. @ 650 650 674 624 654 622 
Break, 30' 
Rheometer (1d .multidot. 100 cpm .multidot. 148.degree.C., ASTM D2084) 
M.sub.L 8.50 9.20 8.80 8.80 10.10 8.80 
M.sub.H.sbsb.2 
24.90 26.90 26.80 26.60 28.80 28.80 
t.sub.S 13.12 11.00 20.80 16.02 15.90 13.83 
t.sub.50 16.53 14.73 29.90 19.55 20.60 18.92 
t.sub.90 23.53 22.58 43.77 25.97 29.88 28.38 
BFG Flexometer (40' @ 151.degree. C., ASTM D623) 
.DELTA. T 
188 191 191 184 220 178 
% Set 11.36 12.21 12.69 1.09 8.03 9.57 
% Static 45.94 43.15 42.46 43.50 42.46 43.23 
Compression 
% Initial 
35.77 33.99 35.01 35.57 35.01 35.14 
Dynamic 
Compression 
% Final 39.22 40.68 41.86 41.42 42.95 40.76 
Dynamic 
Compression 
Goodyear-Healey Rebound (40' @ 151.degree. C., ASTM D1054) 
% Rebound 
37.0 37.0 34.9 40.3 44.2 43.7 
@ RT 
% Rebound 
54.4 52.0 53.5 56.4 54.9 57.9 
@ 100.degree. C. 
Penetration 
0.218 0.212 0.217 0.226 0.218 0.240 
@ RT 
Penetration 
0.285 0.274 0.281 0.291 0.267 0.296 
@ 100.degree. C. 
______________________________________ 
EXAMPLE 4 
This example illustrates the hysteresis response for compositions D and E 
of Example 3 compared by testing samples over a broad temperature spectrum 
ranging from -80.degree. C. to 80.degree. C. at 1 Hz and 0.1% strain using 
a Rheometric-Dynamic Analyzer RDAII. The viscoelastic curves are plotted 
in FIGS. 2 (composition E) and 3 (composition D). The tan .delta. peak at 
the low and intermediate temperature region for composition E indicates 
good traction, better than the control, while at the same time a lower 
than typical loss factor in the upper temperature range indicates good 
rolling resistance. At intermediate temperatures, the tan .delta. plot is 
higher than the control composition D, indicating superior wet traction. 
EXAMPLE 5 
In this example, professional drivers ranked performance of tires having 
treads comprising the control tread rubber composition D of Example 3 are 
compared with tires having treads comprising a tread rubber composition of 
this invention, composition E of Example 3 above. Cars equipped with four 
tires having the respective tread compositions were exhaustively road 
tested for handling and performance characteristics on various surfaces 
and seasonal conditions, including a high speed racetrack. 
In these tests, tires having the tread rubber of this invention performed 
better than control tires in the following categories: 
______________________________________ 
Superior by 
______________________________________ 
Snow Traction (Hill Climb) 
16% 
Wet Ice Traction 4% 
Dry Ice Traction 4% 
Treadwear, 32,000 miles 
5% 
Rolling Resistance 5% 
______________________________________ 
Moreover, on a scale of 10, a professional drive at a high speed racetrack 
gave tires having the tread rubber of this invention an overall ride and 
handling rating of 71/2, as compared to 7 for the control tires. 
Additional test results are summarized in Table V below. 
TABLE V 
______________________________________ 
Characteristics 
______________________________________ 
Composition 
D E 
Abrasion Resistace (DIN Abrader) 
Volume Loss, mm.sup.3 146 95 
Wet Traction (Skid Trailer) 
Superior by 
20 mph 6% 
40 mph equal 
Dry Traction (Skid Trailer) 
40 mph equal 
______________________________________ 
The above description is for the purpose of teaching the person of ordinary 
skill in the art how to practice the present invention, and it is not 
intended to detail all those obvious modifications and variations of it 
which will become apparent to the skilled worker upon reading the 
description. It is intended, however, that all such obvious modifications 
and variations be included within the scope of the present invention, 
which is defined by the following claims.