Pneumatic tire with improved snow and ice traction

A composite tread structure (22) comprises a low modulus at -20.degree. C. rubber outer layer (23) and a high modulus at -20.degree. C. rubber inner layer (24), thicker than the outer layer. The outer layer is preferably made from a rubber having a glass transition temperature lower than the glass transition temperature of the rubber comprising the inner layer. Such a tire generally improves snow and ice handling and maintains good wet and dry traction. A pneumatic tire (10) having improved snow and ice traction employs a composite tread (22) comprising a low modulus at -20.degree. C. rubber outer layer component (23) and a high modulus at -20.degree. C. rubber inner layer rubber component (24), thicker than the outer layer component. Finally, a method for improving the snow and ice traction of pneumatic tires comprises the step of applying to the tire carcass prior to curing, a composite tread comprising a low modulus at -20.degree. C. rubber outer layer component and a high modulus at -20.degree. C. rubber inner layer component, thicker than the outer layer component.

TECHNICAL FIELD 
The present invention is related to the field of pneumatic tires. More 
particularly, the invention relates to high performance pneumatic tires 
having good wet and dry handling characteristics and improved snow and ice 
traction. Specifically, this invention relates to pneumatic tires having a 
composite tread structure which comprises a thin, low modulus outer layer 
that improves snow and ice traction and a core, high modulus inner layer 
for maintaining wet and dry handling. 
BACKGROUND OF THE INVENTION 
Heretofore, pneumatic tires with good snow and ice traction have been well 
known in the marketplace. For example, U.S. Pat. No. 3,475,205 sought to 
reduce the slippage and skidding of rubber surfaces on ice, thereby 
increasing the traction of the vehicular tire, by incorporating anti-skid 
agents such as sulfur, selenium, and manganese dioxide into the tread 
composition. 
While such anti-skid agents may have been effective, the more practical 
manner in which to improve snow and ice traction has been through the 
proper selection of tread material. Tread materials such as natural rubber 
or other rubber having a low glass transition temperature (Tg) have been 
known to be particularly effective in providing traction in snow and ice. 
However, while these tread materials may provide good handling 
characteristics in snow and ice, it has long been known that they do not 
provide very good wet and dry traction. Thus, tires having tread materials 
of low Tg rubber often sacrifice good wet and dry traction for improved 
snow and ice traction. 
For good wet and dry traction and handling, a tread material of high 
modulus and high glass transition temperature (Tg) rubber, such as 
styrene-butadiene rubber (SBR), is commonly used. However, this kind of 
tread material usually exhibits poorer snow and ice handling 
characteristics, and therefore, is not effective in all weather 
conditions. 
It is also well known to make tires which have a composite tread structure. 
Basically, a composite tread uses two different tread materials 
geometrically arranged within the tire to provide a tread with better 
characteristics than would normally be achieved with one tread material in 
the tire. For example, U.S. Pat. No. 4,478,266 shows a composite tread for 
a vehicle tire which exhibits good traction and reduced rolling resistance 
upon wear. The composite tread has a low hysteresis component and a high 
hysteresis component arranged therein such that, upon wear, generally more 
of the high hysteresis component is exposed to impart better traction to 
the life of the tread. This patent does not suggest how to improve snow 
and ice traction, while maintaining significant wet and dry traction, 
however. 
Therefore, a need exists for an effective composite tread structure which 
will improve the snow and ice traction of a pneumatic tire and still 
maintain consistent traction in wet and dry conditions. 
SUMMARY OF INVENTION 
It is therefore, an object of the present invention to provide a tire 
having a composite tread structure which improves snow and ice handling 
performance of the tire. 
It is another object of the present invention to provide a tire having a 
composite tread structure which improves snow and ice traction, while 
maintaining excellent wet and dry handling characteristics for the tire. 
It is yet another object of the present invention to provide a tire having 
a composite tread structure showing excellent wet and dry traction as well 
as excellent snow and ice traction even after the tire has been worn. 
It is still another object to provide a method for improving the snow and 
ice traction of pneumatic tires, while maintaining excellent wet and dry 
handling characteristics. 
At least one or more of the foregoing objects, together with the advantages 
thereof over the existing art, which shall become apparent from the 
specification which follows, are accomplished by the invention as 
hereinafter described and claimed. 
In general, the present invention provides a composite tread structure 
comprising a low modulus at -20.degree. C. rubber outer layer component 
and a high modulus at -20.degree. C. rubber inner layer component which is 
thicker than the outer layer component. 
The present invention also provides a pneumatic tire having improved ice 
and snow traction which employs a composite tread comprising a low modulus 
at -20.degree. C. rubber outer layer component and a high modulus at 
-20.degree. C. rubber inner layer component that is thicker than the outer 
layer component. 
The present invention also includes a method for improving the snow and ice 
traction of pneumatic tires comprising the step of applying to the tire 
carcass prior to curing, a composite tread comprising a low modulus at 
-20.degree. C. rubber outer layer component and a high modulus at 
-20.degree. C. rubber inner layer component, thicker than the outer layer 
component.

PREFERRED EMBODIMENT FOR CARRYING OUT THE INVENTION 
As noted hereinabove, tire manufacturers of previous pneumatic tires have 
attempted to improve traction in snow and ice in a variety of ways. It has 
been known that snow and ice traction may be improved by the proper 
selection of tread material, but until now, this was done at the sacrifice 
of dry and wet traction. Conversely, a high performance tire with superior 
handling and excellent wet and dry traction has usually lacked good snow 
and ice traction. The present invention provides for improved snow and ice 
traction while maintaining excellent wet and dry traction and handling. 
The phrase, snow and ice traction is used loosely herein to describe the 
traction in the mixture of snow and ice that is likely to be present on 
the road. 
The snow and ice handling characteristics of the tire of the present 
invention is significantly enhanced by the use of a composite tread 
structure. More specifically, improved snow and ice traction of pneumatic 
tires is accomplished by introducing a layered structure onto the tread 
section of the tires. Importantly, this improved handling of the tire in 
snow and ice is achieved with very little reduction in the effectiveness 
of the tire's wet and dry traction, even after the tread is worn. Hence, 
the tire of the present invention achieves a better balance between snow 
and ice traction and wet and dry skid resistance than other conventional 
tires. 
As depicted in FIG. I of the attached drawings, a tire of the present 
invention, as generally indicated by the numeral 10, is shown in section. 
Tire 10 is typically pneumatic and comprises tread reinforcing belts 11, 
12, body ply 13, inner liner 14, bead 15, first bead apex 16, bead toe 18, 
second bead apex 19, sidewall 20 and shoulder 21. Because the tire design 
can vary, it is to be understood that the present invention is not to be 
limited by the construction of any particular tire, or to pneumatic tires 
as opposed to solid tires. 
In the manufacture of a conventional tire, a tread is customarily provided. 
In the present invention, tire 10 is provided with a composite tread 22 
which is made of at least two distinguishable materials. More 
particularly, the composite tread 22 includes a thin outer layer 23 and a 
core inner layer 24. Notably, the composition of the thin outer layer 23 
has physical properties different from those of the inner layer 24. 
The outer layer 23, according to the present invention, preferably 
comprises a rubber composition which is effective in enhancing snow and 
ice traction. Such a composition generally has a low modulus at 
-20.degree. C. and includes a low glass transition temperature (Tg) 
rubber. Suitable rubber compositions include natural rubber or high 
cis-polybutadiene, as well as styrene butadiene rubber, and mixtures 
thereof. 
In contrast, the core inner layer 24 of the present invention preferably 
has a high modulus at -20.degree. C. and includes a high glass transition 
temperature (Tg) rubber. Such rubber compositions have been shown to have 
excellent wet skid resistance. An example of such a composition is high 
styrene SBR (styrene butadiene rubber) which is the preferred rubber 
composition for the inner layer of the subject invention. Other high Tg 
rubber compositions useful in the present invention include high vinyl 
polybutadiene rubber, high vinyl SBR, butyl rubber and blends thereof as 
well as with other rubbers including natural rubber. 
With respect to the modulus of the tread layers, it has been found for 
practice of the present invention that a thin outer layer 23 whose modulus 
at -20.degree. C. is at least 30 percent, and preferably 50 percent, lower 
than the modulus of inner layer 24 at the same temperature is useful in 
the present invention. The lower modulus outer layer 23 is believed to be 
particularly effective in improving snow and ice traction, while the high 
modulus inner layer 24 appears to affect the wet and dry handling and 
traction of tire 10. 
In addition, the average Tg of the rubber in the tread compound of outer 
layer 14 is preferably lower than the average Tg of the rubber in the 
inner layer compound. Specifically, the average Tg of the rubber of the 
outer layer 23 is desirably at least 10.degree. C. lower than the average 
Tg of the rubber in the inner layer 24 and can be up to about 30.degree. 
C. lower. The average Tg of the high modulus rubber of the inner layer 24 
is preferably higher than -60.degree. C., desirably higher than 
-50.degree. C. and most desirably higher than -40.degree. C. 
Generally, the outer layer 23 of the composite tread has a thickness from 
about 0.01 inches to about 0.25 inches. However, it is preferred that the 
thickness of the outer layer range from about 0.02 inches to about 0.20 
inches, and even more desirably, from about 0.03 inches to about 0.15 
inches. 
As shown in FIG. 3, even upon wear, tire 10 of the present invention will 
still have improved snow and ice traction because, although the thin outer 
layer 23 may be worn away at the outermost portion of the treads as at 25, 
there is still sufficient outer layer tread material along the sides 26 of 
the tread structure 22, and within the grooves 28 of tire 10, to achieve 
relatively good snow and ice traction. As for wet and dry traction, it 
will be appreciated that as more of the high modulus, high Tg rubber is 
exposed, the traction will tend to improve with the life of the tire, 
somewhat compensating for tread wear. Therefore, both snow and ice 
traction and wet and dry traction are maintained throughout the life of 
the tire 10. 
Tire 10 is of substantially conventional design and is essentially produced 
as is known in the art. The composite tread 22 of a tire 10 is made by 
placing the two layers 23 and 24 onto the body plies 29 and molding the 
tire in the conventional manner. 
In order to exemplify practice of the invention, tires having composite 
tread structures made according to the concepts of the present invention 
were molded and tested for snow traction and handling. Specifically, a low 
modulus natural rubber/SBR blend, presented as Compound No. 1 in Table I, 
was employed as the outer layer component 23 for the composite tread of 
the tires. A high performance tread was selected as the high modulus inner 
layer component. The inner layer component 24, included a high Tg rubber, 
and was considered to provide excellent wet and dry traction, but had 
rather poor snow and ice traction. A high performance tread composition 
similar to the inner layer component actually employed in the tests is 
presented as Compound 2 in Table I. 
TABLE I 
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TREAD COMPOUND COMPOSITIONS 
Ingredients Compound 1 
Compound 2 
______________________________________ 
SBR 1 75 -- 
SBR 2 -- 100 
NR 25 -- 
ZnO 3.0 3.0 
Stearic Acid 2.0 2.0 
Antioxidant 1.0 1.0 
Aromatic Oil 20 10.5 
Wax 0.75 1.0 
Carbon black 40 48.5 
Sulfur 1.8 1.5 
Sulfenamide 0.5 0.75 
Morpholine Disulfide 
0.5 -- 
Benzothiazyl Disulfide 
-- 0.25 
______________________________________ 
The SBR 1 component comprised 35% styrene and 65% butadiene (8% 
1,2-structure and 57% 1,4-structure) and had a Tg of -50.degree. C. The 
SBR 2 component comprised 35% styrene and 65% butadiene (23% 1,2-structure 
and 42% 1,4-structure) and had a Tg of -35.degree. C. 
Data on the pertinent physical properties of the inner layer component and 
the outer layer component of the composite tread are provided in Table II 
hereinbelow. The modulus was measured with static compressional stress of 
0.45 MPa and dynamic sinusoidal stress of .+-.0.3 MPa imposed on the 
static stress. 
TABLE II 
______________________________________ 
PHYSICAL PROPERTIES FOR THE COMPONENTS 
OF THE COMPOSITE TREAD 
Modulus at -20.degree. C. 
Avg. Tg of rubber 
______________________________________ 
Inner layer component 
61 MPa -35.degree. C. 
(Compound 2) 
Outer layer component 
13 MPa -55.degree. C. 
(Compound 1) 
______________________________________ 
The modulus at -20.degree. C. of the core inner layer component was 61 MPa 
while the modulus at the same temperature for the outer layer component 
was 13 MPa. Furthermore, the average Tg of the inner layer component was 
-35.degree. C. as compared to -55.degree. C. for the outer layer 
component, a difference of 20.degree. C. Thus, the physical properties of 
the subject compounds fall within the parameters set forth hereinabove for 
the present invention. 
For testing purposes, three different tires were molded and tested for 
braking ability, acceleration ability, lateral maximum acceleration (acc), 
and handling in snowy conditions. The test tires have been characterized 
in Table III hereinbelow. 
TABLE III 
______________________________________ 
TIRE TREAD CHARACTERISTICS 
Test Tire Tire A Tire B Tire C 
______________________________________ 
Outer layer employed (inches) 
0.0 0.05 0.10 
______________________________________ 
Each of the three tires, designated A-C, employed a tread core, or inner 
layer, of high performance tread compound. For testing purposes, the tire 
treads were built up an additional 0.100 inches as follows: Tire A 
received two 0.050 inch layers of the high performance tread composition 
similar to Compound 2 and utilized in the core to provide a Control; Tire 
B received a tread composite comprising a 0.050 inch high performance 
inner layer and a 0.050 inch outer layer (Compound 1) and, Tire C received 
a tread composite comprising two 0.050 inch layers of outer layer material 
(Compound 1). 
The traction and handling data are provided in Table IV hereinbelow. 
TABLE IV 
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SNOW TRACTION AND SNOW HANDLING TESTS 
Tire A Tire B Tire C 
______________________________________ 
Braking (30-0 mph) 
Distance (feet) 
112 95 92 
Time (sec.) 5.20 4.38 4.38 
Acceleration (0-30 mph) 
Distance (feet) 
268 194 198 
Time (sec.) 10.47 7.96 7.88 
Lateral Max. Acc (g) 
0.245 0.35 0.345 
Handling 
linearity 6.5 7.5 7.5 
over steer 6.5 7.5 7.5 
recovery 6.5 7.5 7.5 
steering response 
6.0 7.5 7.5 
overall rating 
6.5 7.5 7.5 
Comments: Slow response 
Excellent Excellent 
rear good good balance 
good balance 
good response 
good 
response 
______________________________________ 
Braking ability was tested by driving a car equipped with the specific 
tires on a medium packed snow surface and measuring the distance and time 
it took to come to a complete stop from 30 miles per hour. Similarly, 
acceleration ability was tested by measuring the distance and time 
required to accelerate from 0 to 30 miles per hour on snow. Lateral 
maximum acceleration tests were performed by turning the car sharply and 
measuring the maximum centrifugal force, expressed in units of g 
(gravitational acceleration, 9.8 m.sup.2). 
Tests for handling covered linearity, oversteer, recovery and steering 
response, and included an overall rating. The tires were rated on a scale 
of 1 (poor) to 10 (superior) for the handling tests described. 
As can be determined from the test data presented in Table IV, snow 
traction and handling were significantly better for the tires which 
included the low modulus outer layer component (Tires B and C) as compared 
to the control tire (Tire A) which had only a high modulus layer of tread. 
Both Tires B and C exhibited excellent response and balance. Tire A, 
however, showed poorer response. 
Next, the tires tested in Table IV hereinabove were buffed. Notably, 0.115 
inches of tread material were buffed from each tire to make the tire act 
as if it had been partially worn. It should be appreciated that by buffing 
the tires to that extent, the outer layer of the tread had disappeared 
from the outermost portion of the tread. However, it should be noted that 
the outer layer had not disappeared completely inasmuch as there was at 
least some portion of that tread material remaining between the grooves. 
Snow traction and handling tests for the Tires A-C were conducted in the 
same manner as above described and the data reported as follows 
hereinbelow. 
TABLE V 
______________________________________ 
SNOW TRACTION AND SNOW HANDLING TESTS 
buffed buffed buffed 
Tire A Tire B Tire C 
______________________________________ 
Braking (30-0 mph) 
Distance (feet) 
140 133 129 
Time (sec.) 6.43 6.08 5.75 
Acceleration (0-30 mph) 
Distance (feet) 
400 318 306 
Time (sec.) 15.63 12.58 11.96 
Lateral Max. Acc (g) 
0.215 0.270 0.290 
Handling 
linearity 4.5 5.5 6.0 
over steer -- 5.0 5.5 
recovery 4.0 5.0 6.0 
steering response 
4.0 5.5 6.0 
overall rating 
4.5 5.5 6.0 
Comments: Very poor Slightly better 
Slow but 
response steering sure 
would not turn 
response response 
in turning 
______________________________________ 
As can be seen from the data presented in Table V, the tires having the low 
modulus outer layer of Compound 1 (Tires B and C) generally exhibited 
better snow traction and handling than the control tire (Tire A). The 
tires having the low modulus outer layer were able to stop more quickly 
and in a shorter distance than the tires without the particular outer 
layer. They also exhibited a greater lateral maximum acceleration and 
better overall handling. 
In addition, the buffed Tire A in Table V handled very poorly. In fact, the 
driver noted that the tires would not turn on the snow. While the buffed 
tires B and C did not show the same superior handling and traction as when 
they were new tires, the tires did respond significantly better during the 
tests. 
In conclusion, it should be clear for the foregoing tests and specification 
that a tire which has a composite tread structure of two different 
materials enhances both wet and dry handling performance and snow and ice 
traction. 
Thus it should be evident that the composite tread of the present invention 
is highly effective in both enhancing the snow and ice traction of the 
subject tire and maintaining the wet and dry handling performance of the 
same. The invention is particularly suited for use on automotive vehicles 
which are driven in all kinds of weather, but is not necessarily limited 
thereto. The composite tread of the present invention, as well as the 
pneumatic tires of the present invention, can be used separately on other 
vehicles and wagons which may required all weather traction. 
Based upon the foregoing disclosure, it should now be apparent that the use 
of the composite tread structure described herein will carry out the 
objects set forth hereinabove. It is, therefore, to be understood that any 
variations evident fall within the scope of the claimed invention and 
thus, the selection of specific component elements can be determined 
without departing from the spirit of the invention herein disclosed and 
described. In particular, rubber compositions of the present invention are 
not necessarily limited to those noted hereinabove. Other compositions of 
rubber having similar physical properties to those described above may be 
substituted therefor. Thus, the scope of the invention shall include all 
modifications and variations that may fall within the scope of the 
attached claims.