Note: Descriptions are shown in the official language in which they were submitted.
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ViJLCANIZABLE ELASTOMERIC
COMPOSITIONS FOR USE AS TIRE TREADS
S Technical Field
This invention relates generally to vulcanizable elastomeric compounds having
enhanced viscoelastic properties, and, more particularly, to improved
elastomeric
compositions having 100 parts by weight of at least one dime-based elastomer
and from
30 to 160 phr (parts per hundred parts of rubber by weight) of fillers, which
include
compounds of zinc, barium and/or titanium.
Background Art
Elastomeric compounds are so-called viscoelastic materials. This means that
the
properties which they exhibit depend on the duration (time or frequency) and
on the
temperature at which external stresses or deformations (strains) are applied
to them.
The balance and level of such viscoelastic properties determine the
processibility
and the range of end-use characteristics of these elastomeric compounds, and,
therefore,
their practical applications. With present technologies, a wide range of
applications is
possible due to the fact that the basic constituents of these elastomeric
compounds,
namely, naturally-occurring or synthetically-produced rubbers, can be mixed or
com-
pounded with numerous chemicals and other additives, so as to tailor and
customize their
viscoelastic properties. The technology of developing vulcanizable elastomeric
com-
pounds for specific applications has achieved a high degree of sophistication
over the
past years.
As a result of developments in electronics and dynamic test equipment, great
advances have also been made in the precise measurement of viscoelastic
properties of
elastomeric compounds, and also in the correlation of such measurements with
the
performance of these compounds in engineered products, such as tires.
Pertinent viscoelastic parameters, which are measured in laboratory tests, are
the
elastic moduli, E' (in tension or compression) and G' (in shear), the viscous
moduli E"
(in tension or compression) and G" (in shear), and the ratio of the viscous
and elastic
moduli, otherwise known as the loss tangent or tan 8. These parameters are
determined
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under dynamic conditions at specific temperatures, frequencies, strain rates,
and stress-
or strain- amplitudes. Another important parameter is the glass transition
temperature,
Tg, which is the temperature below which the elastomeric composition becomes
"glass-
like" or brittle.
With present testing technology, these viscoelastic parameters can be
determined
with great precision, and these parameters can be confidently correlated to
practical
performance characteristics in, say, tires. For instance, with tire tread
compounds, the
tan aof a compound, measured within a temperature range of about 50-70
°C, correlates
directly with the rolling resistance of a tire. That is, the lower the tan ~
the lower the
rolling resistance of a tire tread. Similarly, the magnitude of the tan ~ or
E", measured
at about 0 °C, or at the respective Tg of a tire tread compound, relate
to certain traction
characteristics of a tire, whereas the magnitude of the tan 8 at very low
temperatures of
about -65 °C is indicative of the abrasion or wear characteristics of a
tire tread. Regard-
ing traction and wear, the greater the values of the respective viscoelastic
parameters, the
better the performance of the compounds in tire treads.
To be specific, with respect to predicting the potential rolling resistance of
tire
tread compounds, differences in tan 8 of 0.005 are significant and beyond
experimental
error, while changes of 0.015 and greater are significant with respect to
certain traction
characteristics and can be observed in actual tire performance tests.
In practice, there are opposing performance trends of elastomeric compounds,
which usually requires compromises when optimizing their viscoelastic
properties. For
instance, improvements in the tan 8 leading to a lower rolling resistance of
tire tread
compounds are generally also accompanied by a reduction of tan 8 at other
relevant
temperatures, thus resulting in a potentially lower wet traction performance
of the tire
tread. Similarly, there are generally also opposing trends with respect to
certain proper-
ties, such as traction capabilities of tire tread compounds and their abrasion
or wear
resistance characteristics.
Much effort has been spent on developing compounding technologies and new
compound additives to ameliorate this problem of opposing property trends
while raising
the overall performance levels. Great progress has been made through what is
now
commonly referred to as "silica compounding" technology, but there is still
need for
further technical improvements. The present invention demonstrates that this
is now
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possible.
Disclosure of the Invention
The present invention relates generally to the field of vulcanizable
elastomeric
compositions.
In one aspect, an improved elastomeric composition includes 100 parts by
weight
of at least one dime-based elastomer, and from 30 to 160 phr of filler, the
filler compris-
ing at least about 7 phr of zinc sulfate.
In another aspect, an improved elastomeric composition includes 100 parts by
weight of at least one dime-based elastomer, and from 30 to 160 phr of filler,
the filler
comprising at least 7 phr barium sulfate.
In yet another aspect, the improved elastomeric composition includes 100 parts
by weight of at least one dime-based elastomer, and from about 30 to 160 parts
of filler,
the filler comprising at least 8 phr titanium dioxide, and also containing at
least one
compound selected from the group consisting of silica, carbon black, clay,
calcium
carbonate, and talc.
The mean particle size of zinc sulfate, barium sulfate and titanium dioxide
particles is between about 0.2 and 1.6 microns and accounts for between 10 and
30
weight percent of the filler.
Accordingly, the general obj ect of the invention is to provide improved
vulcaniz-
able elastomeric .compositions.
Another object is to provide improved elastomeric compositions employing zinc
sulfate, barium sulfate or titanium dioxide as filler material.
These and other obj ects and advantages will become apparent from the
foregoing
and ongoing written specification, and the appended claims.
Description of the Preferred Embodiments
The present invention deals with a novel compounding technology through the
use of a new class of additives. As a result, the opposing property trends of
conventional
elastomeric compounds are minimized, while the overall performance level of
com-
pounds is greatly improved.
In general, the nature of the elastomer determines the basic properties of
vulcanizable elastomeric compounds. With current technologies, these
properties are
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modified by the kind and amount of compounding ingredients that are used.
These
ingredients include processing aids, fillers, softeners, vulcanizing
chemicals, chemicals
protecting against aging, blowing agents, etc. All these conventionally-used
materials
are compatible with compounds of the present invention. However, the present
invention
uses an new group of compounding aids to achieve the desired novel balance and
level
of viscoelastic properties.
Commonly-used elastomers, which are compatible with the novel compounding
technology of this invention, include natural rubber, or synthetic elastomers,
based on
mono, copolymers or terpolymers from butadiene, isoprene, isobutylene,
styrene, acrylo-
nitrile, chlorobutadiene, ethylene, propylene, dicyclopentadiene, ethylene
norbornene,
hexadiene, vinyl acetate, chlorosulfonyl ethylene, epichlorohydrin, ethylene
oxide, or
propylene oxide. Blends from these elastomers can also be employed within
useful
blend ratios. In addition fluoroelastomers, silicone rubbers, polysulfide
rubbers, and
polyurethane rubbers are also compatible with the compounding technology of
this
invention.
Fillers, which are compatible with the novel compounding technology of this
invention can be generally classified as carbon blacks or light-colored
fillers. The
carbon blacks comprise a wide range of grades, and there is no restriction on
surface
area, surface activity, particle size, or aggregate structure. Light fillers
include colloidal
silica, calcium silicate, aluminum silicate, alumina gel, clay, talcum, or
calcium carbon-
ate (i. e., chalk). Again, there is no restriction on the particle size,
aggregate size, or the
surface activity of these light fillers. The surface activity of carbon blacks
and light
fillers can also be modified with appropriate chemicals according to current
technolo-
gies. It is also possible to employ blends of carbon black grades, light
fillers, or carbon
blacks and light fillers in compounds of the present invention.
It is also possible to use plasticizers, process aids, factices, mineral oils,
bonding
resins, reinforcing resins, tackifiers, blowing agents and various aging-,
fatigue-, and
ozone- protective agents.
The elastomeric compositions of this invention can be vulcanized using current
technologies. These include accelerated sulfur systems, sulfur donors,
peroxides, curing
resins and high energy radiation. Combinations of any of these systems can
also be
employed.
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The following examples will illustrate the nature of the novel compounding
technology. In these examples, all compounds contained 100 parts by weight of
at least
one dime-based elastomer.
Example 1:
Six compounds were prepared under identical conditions to demonstrate the
properties of rolling resistance, traction, and wear of tire tread compounds
using modern
compounding technology based on carbon black and silica fillers, and the
significant and
unexpected improvements that can be obtained with the compounding technology
of the
present invention.
All compounds had the same type and concentration of elastomers, namely a
blend of two solution SBR's (synthetic styrene-butadiene rubber), and the same
sulfur/
accelerator cure system, protective agents, oils, and process aids. The total
filler concen-
tration was held constant at 35% by weight throughout, but different filler
types were
I S used.
Compound A (Control 1 ) is a modern low-rolling-resistance, high traction
passenger tire tread, based on the latest silica compounding technology. Here,
the filler
was a blend of 45 phr silica with a surface area of 180 m2 per gram of silica.
The surface
of the silica was modified with a silane coupling agent and of 25 phr carbon
black
(ASTM grade N134), which has a high surface area and high structure (i.e.,
degree of
aggregation of primary particles).
Compound B (Control 2) is a modern long-wearing, high-traction passenger tread
compound using carbon black filler only (70 phr of ASTM Grade N134).
Compound C (Control 3) is a modern high-traction, low-rolling-resistance
passenger tread using carbon black filler with lower surface area than that of
Control 1
(a blend of 35phr of ASTM GradeN343 carbon black, and 35 phr of ASTM Grade
N351
carbon black).
Compounds D, E and F were compounded using the novel technology of this
invention. The fillers in Compound D were 61.5 phr carbon black (i. e., blend
of N343
and N351) plus 8.5 phr zinc sulfate with mean particle size of 0.7 microns. In
Com-
pound E, the fillers were 61.7 phr carbon black (blend of N343 and N351) plus
8.3 phr
barium sulfate with mean particle size of 1.6 microns. In Compound F, the
fillers were
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60.8 phr carbon black (blend of N343 and N351) plus 9.2 phr titanium dioxide
of mean
particle size of 0.2 microns.
The viscoelastic properties of these six compounds were determined using a
Rheometrics RSA dynamic tester in uniaxial extension over a temperature range
from -
70 °C to +60 °C, and a dynamic strain amplitude of 0.5% with a
10% prestrain at a
frequency of 10 Hertz. From these measurements, the glass transition
temperature Tg
and the tan ~ values at -25 °C, 0 °C and 50 °C were
determined. These data are summa-
rized in Table 1:
Table 1:
Compound CompoundCompoundCompound CompoundCompound
A B C D E F
Controll Control2Control3
Tg C -28.4 -28.1 -30.1 -28.3 -28.5 -28.2
tan 8 0.20 0.28 0.23 0.19 0.19 0.19
@50 C
tan b 0.34 0.44 0.37 0.32 0.34 0.33
@0C
tan 8 0.86 0.92 0.96 0.90 0.90 0.87
@ -25C
When comparing the results for the Compound A (silica filler) and Compound B
(high surface area carbon black), the predicted advantage of Compound A for
lower
rolling resistance (lower tan ~ @ 50 °C ) is obvious, albeit at the
expense of traction
(lower tan 8 @ 0 °C) and treadwear.
If we next compare Compound B (high surface area carbon black) and Com-
pound C (carbon black with lower surface area), we observe the expected
improvement
in rolling resistance, although not to the level of the silica compound
(Compound A), but
a reduced level of traction.
Therefore, these data demonstrate the limitations of current compounding
technology due to the opposing property trends as mentioned above.
Now consider the significant results for Compounds D, E and F, which incorpo-
rate the technology of the present invention. Here, the consistently lowest
tan d @ 50
°C values (lowest rolling resistance) is apparent even when compared to
the silica
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compound (Control 1 ), without sacrificing traction or wear. It should also be
noted that
the compounds based on the technology of this invention offer significant cost
savings
over silica-based compounds.
Example 2:
This example was included to demonstrate that the technology of the present
invention can also beneficially be applied to compounds formulated with a
different
elastomer system, and that compounds with a high-surface-area carbon black can
also be
significantly improved with respect to rolling resistance at largely
equivalent traction
capabilities.
To this end, five compounds were prepared under identical conditions. Common
to all compounds was the elastomer system, namely a blend of 70 parts by
weight of two
solution SBR's (synthetic styrene-butadiene rubber) and 30 parts by weight of
natural
rubber. The same protective agents, oils, and process aids and
sulfur/accelerator cure
system were also used throughout, but for the silica-based Compound A, the
sulfur/accelerator ratio had to be adjusted to maintain equivalent cure rates
of the
compounds in this series. The total filler concentration was held constant at
33% by
weight throughout, but different filler types were used.
Compound A (Control 1 ) is based on the latest silica compounding technology
to give a very low-rolling-resistance, high traction passenger tire tread,.
Here, the filler
was a blend of 45 phr silica with a surface area of 180 m2 per gram of silica.
The surface
of the silica was modified with a silane coupling agent and of 25 phr carbon
black
(ASTM grade N134), which has a high surface area and high structure.
Compound B (Control 2) represents a long-wearing, high-traction passenger
tread compound using carbon black filler only (70 phr N 134).
Compounds C, D and E were compounded using the novel technology of this
invention. The fillers in Compound C were 61.5 phr carbon black (N134) plus
8.5 phr
zinc sulfate with mean particle size of 0.7 microns. In Compound D, the
fillers were
61.7 phr carbon black (N134) plus 8.3 phr barium sulfate with mean particle
size of 1.6
microns. In Compound E, the fillers were 60.8 phr carbon black (N134) plus 9.2
phr
titanium dioxide of mean particle size of 0.2 microns.
The viscoelastic properties of these five compounds were determined using the
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same test conditions as listed in Example 1. From these measurements, the
glass transi-
tion temperature Tg and the tan 8 values at 0 °C and 50 °C were
determined. These data
are summarized in Table 2:
Table 2:
Compound Compound Compound Compound Compound
A B C D E
Controll Control2
T C -28.4 -28.1 -28.3 -28.5 -28.2
tan 8 0.19 0.29 0.27 0.26 0.25
@ 50 C
tan 8 0.32 0.44 0.43 0.43 0.41
@ 0 C
The results demonstrate again the opposing property trends, which cannot be
eliminated with current compounding technology. Compared with the high-surface-
area
carbon black compound (Control 2), the silica compound (Control 1 ) has a
significantly
lower rolling resistance in passenger tire treads (i. e., lower tan 8 @ 50
°C) but at consid-
erable expense of tire tread traction (i. e., lower tan 8 @ 0 °C).
On the other hand, compared with Control 2, the compounds based on the
technology of the present invention (Compounds C, D and E) have a
significantly lower
rolling resistance (i. e., lower tan a @ 50 °C) without the precipitous
loss in traction (i. e.,
tan 8 @ 0 °C.
Therefore, while certain specific compositions have been specifically
described,
and certain changes and modifications discussed, persons skilled in this art
will readily
appreciate that various additional changes and modifications thereof may be
made
without departing from the spirit of the invention, as defined and
differentiated by the
following claims.
