Note: Descriptions are shown in the official language in which they were submitted.
2~~~~~~~
-1_
RUBBER BLEND AND TIRE WITH TREAD THEREOF
Field
This invention relates to a pneumatic tire with a
tread composed of a blend of at least three rubbers
including 3,4-polyisoprene rubber, cis 1,4-polyisoprene
rubber and at least one additional diene-based rubber.
Background
Pneumatic rubber passenger and truck tires are
composed of elements which conventionally include a
tread of a rubber composition. The tread rubber is
sometimes desirably compounded to provide a tire with a
relatively low rolling resistance with reasonable wear
arid traction.
Although it may be desired to compound the tire's
tread composition to reduce the rolling resistance of
the tire without substantially reducing the tire's
traction features, tire traction might be expected to
be somewhat sacrificed as may be evidenced by its
decrease in wet and dry skid resistance.
Various rubber compositions have been prepared for
various purposes, some of which have included the tire
treads. Often tire treads are composed of synthetic
rubber or blends of synthetic rubber with natural
rubber for the purpose of achieving desirable tire
tread characteristics such as wear, traction and
reduction in rolling resistance. Various synthetic
rubbers have been used in the manufacture of tires with
such treads including styrene/butadiene copolymers
(prepared by emulsion or solution polymerization
methods) sometimes referred to as SBR, high cis 1,4
polybutadiene rubber as well as high and medium vinyl
(1,2-) polybutadiene rubbers. Sometimes a synthetic
CA 02047899 2002-08-O1
2
cis 1,4-polyisoprene may, at least in part., be
substituted for the natural rubber in tires tread
compositions.
Vinyl isoprene (3,4-polyisoprene> rubber has
S heretofore been taught to be useful for various purposes
such as, for example, as a blend with other rubber in
tire treads and use in industrial products. such as
vibration dampers, belts and shoe soles.
Viscoelastics properties of a rubber, or a rubber
blend, for a tire tread applications, are important. For
example, a tan. delta property is the ratio of the viscous
contribution to the elastic contribution for a
viscoelastic rubber subjected to a dynamic' deformation.
Such property is typically represented in the form of a
1S curve as a plot of tan. delta values versus temperature.
For a tire with low rolling resistance, a tread
rubber with tan.delta optimization for' a temperature in
the range of about 50°C to about 60°C is desired and a
tan. delta optimization for a temperature range of about
-20°C to about +10°C is desired for a tire with good wet
skid resistance. It is difficult to adjust a rubber blend
to achieve a tan. delta optimization substantially
simultaneously for both temperature ranges and, thus, for
both rolling resistance and wet skid xesis~tance at the
same time. Often, compromises have to be made.
By tan.delta optimization it is meant that the
tan.delta value for the rubber, or rubber blend, is
maximized in the region of approximately -20°C to about
+10°C for a tire tread to have high wet skid resistance
and tan. delta value is minimized in the region of about
60°C for a tire tread to have low calling resistance.
CA 02047899 2003-04-16
3
Although various rubber compositions are taught to
provide various benefits, some for tire treads, it
continues to be desirable to provide a pneumatic tire
having a rubber tread having an enhanced rolling
resistance and/or treadwear commensurate with reasonable
traction qualities.
Disclosure and Practice of Invention
In accordance with an aspect of the present
invention, there is provided a tire having a tread of a
rubber composition comprised of, based on parts by weight
per 100 parts by weight rubber (phr):
(A) a tri-blend of rubbers composed of:
(1) about 10 to about 25 phr 3,4-polyisoprene
rubber,
(2) about 40 to about 55 phr cis 1,4-
polyisoprene rubber, and
(3) about 20 to about 50 phr of cis 1,4-
polybutadiene rubber,
(B) a quatra-blend of rubbers composed of:
(1) about 10 to about 25 phr 3,4-polyisoprene
rubber,
(2) about 40 to about 55 phr cis 1,4-
polyisoprene rubber, and
(3) about 20 to about 50 phr of cis 1,4-
polybutadiene rubber and styrene/butadiene rubber, or
(C) a quatra-blend of rubbers composed of:
(1) about 10 to about 25 phr 3,4-polyisoprene
rubber,
(2) about 40 to about 55 phr cis 1,4-
polyisoprene rubber, and
(3) about 20 to about 50 phr of cis 1,4-
CA 02047899 2003-04-16
4
polybutadiene rubber and isoprene/butadiene copolymer
rubber;
wherein said 3,4 polyisoprene rubber is of a
structure containing about 50 to about 60 percent 3,4-
vinyl isoprene units, and in its uncured state, a glass
transition (Tg) temperature in a range of about -15°C to
about -20°C and a Mooney(ML1+4) value (100°C) in a range
of about 70 to about 90;
wherein said styrene/butadiene copolymer rubber has a
styrene/butadiene ratio of:
(A) about 5/95 to about 30/70, or
(B) about 10/90 to about 60/40;
wherein said isoprene/butadiene copolymer rubber an
isoprene/butadiene ratio of from about 30/70 to about
70/30.
In further accordance with this invention, the
rubber composition itself is contemplated.
The term "Tg" refers to the glass transition of the
identified rubber and is suitably determined by a
differential scanning calorimeter at a rate of 1°C per
minute.
Thus, the tread rubber is required to be a blend of
at least three rubbers.
Preferably, the cis 1,4-polyisoprene rubber (B) is
natural rubber.
Preferably, said other rubber (C) is selected from
at least one of the solution polymerization prepared
styrene/butadiene copolymer rubber and the
isoprene/butadiene copolymer rubber.
In contemplated embodiments, such tread may be
composed of, based on 100 parts by weight rubber, (i) a
tri-rubber blend comprised of (A) natural rubber, (B)
CA 02047899 2003-04-16
styrene/butadiene copolymer rubber (preferably solution
30 polymerization derived copolymer, sometimes referred
to herein as S-SBR) and (C) 3,4-polyisoprene rubber as
prescribed herein; (ii) a quatra-rubber blend comprised
5 of (A) natural rubber, (B) cis 1,4-polybutadiene rubber,
(C) isoprene/butadiene copolymer rubber and (D)
3,4-polyisoprene rubber as prescribed herein; or (iii)a
quatra-rubber blend comprised of (A) natural rubber, (B)
S-SBR, (C) cis 1,4-polybutadiene rubber and (D)
3,4-polyisoprene as prescribed herein.
In an aspect of the present invention, a specified
3,4-polyisoprene rubber is used with the prescribed
characteristics, particularly its Tg and Mooney (ML1+4)
viscosity limitations and, further, that the prescribed
3,4-polyisoprene is utilized as a minor component with
selected other rubbers in a tire tread composition and
that the 3,4-polyisoprene rubber is relatively
incompatible with the other rubbers in the tread
composition.
The Mooney (ML1+4) value in a range of about 70 to
90, preferably about 75 to about 85, for the 3,4-
polyisoprene in combination with the required Tg range is
considered to be important.
For processability of the 3,4-polyisoprene rubber it
would ordinarily be desirable for the rubber to have a
relatively low Mooney (ML1+4) value which is a measure of
its viscosity and, on a relative basis, of its molecular
weight.
However, for a purpose of achieving the desired low
tan.delta for the rubber blend in the region of 60°C,
indicating a low hysteresis of the rubber blend and
predicting a low rolling resistance for a tire with tread
CA 02047899 2003-04-16
6
of such rubber blend as well as good abrasion resistance
for the rubber blend in its compounded, sulfur cured
condition, a higher molecular weight 3,4-polyisoprene
polymer is required and, thus, one with the higher Mooney
(ML1+4) value prescribed for the 3,4-polyisoprene rubber
used in this invention.
Therefore, for the purpose of this invention, the
relatively narrow Tg and Mooney (ML1+4) ranges of values
are prescribed in combination with the specified
3,4-,1,2- and 1,4-contents, including the relatively
narrowly defined total of 3,4- and 1,2-units being from
56 to 63 percent.
The ML(1+4) is a measure or value well known to
those skilled in such art and typically determined by a
viscoelastic tester.
It is to be appreciated that the 3,4-polyisoprene
rubber for this invention is required to have the
aforesaid characteristics for preparing a tire tread to
enable a tire to have good treadwear and low rolling
resistance. Therefore, the rubber is required to have a
relatively high molecular weight, or Mooney (ML1+4) value
while still possessing a reasonably good processability.
The good processability of the rubber is a desirable
feature so long as the aforesaid good rolling resistance
and treadwear of the tire is not appreciably compromised.
It is preferred that the 3,4-polyisoprene, by having
the defined physical Tg characteristic, is relatively
incompatible in the rubber tread blend. By being
incompatible, it is meant that the 3,4-polyisoprene
rubber individually displays a second, or additional,
tan. delta hump, or upward bend of the curve, in addition
to the tan. delta peak for the dime rubbers (B) and (C),
CA 02047899 2003-04-16
6a
which appears when the 3,4-polyisoprene is blended with
rubbers (B) and (C), as evidenced by the viscoelastic
response of the cured blend to a dynamic deformation.
For further description and understanding of this
invention, reference is made to the accompanying
drawings.
FIG. 1 demonstrates a viscoelastic property of
3,4-polyisoprene rubbers of microstructures represented
by
~~~ ~'°,~
_7_
their Tg's of -11°C, -18°C and -25°C, respectively,
blended with cis 1,4-polyisoprene rubber (natural
rubber). It represents the relationships between
tan. delta versus temperature for the three sulfur cured
rubber blends for a temperature range of -80°C to +20°C
and compares them to a sulfur cured natural rubber/
styrene-butadiene control rubber composition.
FIG. 2 is a table demonstrating the determined
tan. delta for the respective sulfur cured rubber blends
of FIG. 1 at 60°C.
FIG. 3 demonstrates the tan.delta curve for
temperatures in the range of -60°C to +60°C for a
sulfur cured blend of 3,4-polyisoprene having a Tg of
-18°C,,natural rubber and S-SBR (identified as X(3,4
PI) and compares it to a control of a sulfur cured
blend of natural rubber and S-SBR (identified as
Y(Control).
Referring to FIG. 1, curves are shown for three
sulfur cured rubber blends of 3,4-polyisoprene rubber
and natural rubber, identified as experimental blend
(A) experimental blend (B) and experimental blend (C)
with the Tg's of the 3,4-polyisoprene being -18°C,
-11°C, and -25°C respectively, as referenced in the
following table where 3,4-PI refers to the 3,4-poly-
isoprene rubber containing sulfur cure blends. Each
blend was a sulfur cured compounded blend composed of
25 parts by weight 3,4 polyisoprene, and 75 parts by
weight natural rubber.
-8-
Rubber Blends
Blend Identification Tg of 3,4-polyisoprene
1. Experimental (A) -18°C
2. Experimental (B) -11°C
3. Experimental (C) -25°C
The curves of FIG. 1 taken with the data of FIG. 2,
demonstrate that a tan. delta curve with a maximization
in the range of -20°C to +10°C coupled with a
minimization in the 60°C region (shown in FIG. 2) for
the dual rubber blend is accomplished with a 3,4
polyisoprene having a Tg of -18°C, thus, indicating
that the 3,4-polyisoprene with a Tg of about -18°C is
the preferable rubber for the purposes of the tri-
rubber and quatra-rubber blends of this invention.
Thus, only compound (A) demonstrated optimized
tan.delta curves for both the 60°C and the -20°C to
+20°C ranges, thus, indicating the desirability of
utilizing the 3,4-polyisoprene with a Tg of about
-18°C.
Three pneumatic rubber tires having treads composed
of individual sulfur cured compounded blends of the
3,4-polyisoprene rubbers having Tg's of -11°C, -18°C
and -25°C plus cis 1,4-polyisoprene rubber (natural
rubber) in a 25/75 ratio were prepared and tested for
wet traction, or skid resistance, (20 mph) and for
rolling resistance. The results are shown in the
following table and compared with a control tire with a
tread composed of a sulfur cured compounded blend of
natural rubber and S-SBR in a 50/50 ratio:
~fl~v~~r~
_g_
TIRE PROPERTIES
Properties Control Tire Experimental Tires
Tg of 3,4-
Polyisoprene -11C -18C -25C
Rolling
Resistance) 100 I02 106 110
Wet Traction 100 108 l04 101
Treadwear? 100 80
1. An increase of the normalized value reflects a
reduction in rolling resistance which is considered
an improvement.
2. Data not taken for treadwear for -11°C and -25°C Tg
3,4-polyisoprene containing tire treads.
The values fox the tire properties for the control
tire were normalized to a value of 100 and the
properties of the experimental tire were compared to
the control tire's values.
While the dual rubber blend, utilizing the 3,4
polyisoprene with a Tg of -18°C, demonstrated enhanced
properties as shown in the preceding table, it also
demonstrated certain disadvantages for use as a tire
tread, namely, upon tire test with a tread composed of
such a dual rubber blend, an inadequate treadwear was
observed.
Consequently, a tri-blend of rubbers was prepared
and sulfur cured. The results are depicted in FIG. 3
which shows that a similar and desirable tan. delta
curve is obtained with suitable maximum and minimum
values if the 3,4 polyisoprene having a Tg of -18°C is
:3~
-10-
used. Subsequent tire tests with such a tri-rubber
blend for its tread yielded a tire with adequate
treadwear.
More specifically, referring to FIG. 3, tan. delta
versus temperature curves are shown for sulfur cured
rubber tri-blends of 3,4-polyisoprene rubber having the
Tg of -18°C, cis 1,4-polyisoprene natural rubber and
S-SBR rubber, identified as experimental blend X, and a
sulfur cured control rubber blend Y. Blend X was a
sulfur cured blend of cis 1,4-polyisoprene natural
rubber, solution polymerization prepared
styrene/butadiene rubber and 3,4 polyisoprene rubber
required by this invention and having a Tg of -18°C.
Control blend Y was a sulfur cured blend of cis 1,4
polyisoprene natural rubber and solution polymerization
prepared styrene/butadiene copolymer rubber,
The curves of FIG. 3 demonstrate that the tri-blend
sulfur cured rubber composition has a tan. delta curve
maximization in the range of -20°C to +10°C and a
minimization in the range of about 50°C to about 60°C.
The curve also exhibits a tan. delta peak in the region
of about -60°C to about -30°C and a second tan. delta
hump in the region of about -20°C to about +10°C
indicating that the 3,4-polyisoprene is substantially
incompatible with the remaining rubbers of the blend.
The departure of the curve in the -20°C to +10°C region
from a smooth curve to a curve containing an upward
bend demonstrates the relative incompatibility.
For some applications, it has further been found to
be practical to utilize a pneumatic rubber tire with a
quatra-rubber blend for its tread to more completely
optimize rolling resistance, traction and treadwear
including winter tire performance.
CA 02047899 2002-08-O1
11
While the contribution of various elements or
components of a composition are not always completely
understood, it is considered that an important and
significant component of the blend is the specified
3,4 polyisoprene with its Tg of about -18°C together with
its Mooney viscosity limitation properties which
apparently provides unique viscoelastic properties when
combined with the remainder of the rubbers (B) and (C),
particularly as compounded sulfur cured tri-rubber and
quatra-rubber blends for tire treads.
The samples of the sulfur cured rubber blends for
FIG. 1 were tested by a viscoelastic tester, obtained
from the Rheometrics Company, to determine the
relationship between tan.delta and temperature from -80°C
to +25°C. During the test, the samples are maintained
under tension (0.5% strain) and a cyclic da_formation is
applied to the sample at a frequency rate of one hertz.
The viscoelastic tester measures the response of the
sample to the applied deformation and calcvulates the
tan. delta values at the desired temperatures.
The samples of sulfur cured rubber blends were also
tested for FIG. 2 by a dynamic viscoelastic tester
provided by Imass, Inc., to determine their tan.deltas at
60°C.
Samples of cured rubber blends were tested for FIG.
3 by an automated dynamic viscoelastic tester Imass,
Inc., to determine the relationship of tan. delta versus
temperature from -60°C to +60°C for the indicated rubber
blend containing the 3,4-polyisoprene rubber with the Tg
of -18°C. A tension (strain) of 0,1% and frequency of 11
hertz was used.
~~~:~r~~J l
-12-
The objective for these tests is to measure the
viscoelastic response to an applied deformation of a
cured rubber sample under tension at a specified
strain, frequency and temperature, or temperature
range. The viscoelastic response is used by the
instrument to determine the storage modulus, E', which
is a measure of energy stored and recovered in cyclic
deformation, and the loss modulus, E", which is a
measure of energy dissipated as heat. The ratio of
E"/E' is the tan. delta fox a paxticul.ar temperature.
Thus, in effect, the tan.delta is a measure of a
compound's viscoelastic character and has been observed
to relate to tire tread performance. The tan. delta
versus, temperature characterization of rubbers is well
known to those having skill in such art.
As pointed out, in practice it has been observed
that, for pneumatic rubber tires, a high tan.delta in
the region of -20°C to +10°C is desirable for a tire
tread to provide a tire with good wet traction while a
low tan.delta in the region of 50°C to 60°C is
desirable for a tire tread to provide a tire with good
rolling resistance.
The curve for experimental tread rubber (A) of FIG.
1, taken with the data of FIG. 2, exhibits a high
tan. delta in the region of -20°C to +10°C, thus,
predictably suitable for providing a tire with good wet
traction and a low tan.delta in the region of 60°C,
thus, predictably suitable for a tire with good rolling
resistance for this rubber blend.
The importance of such phenomenon is that the
rubber blend of this invention enables a relative
optimization of the property of tan.delta for
prescribed temperatures while also maintaining or even
optimizing the property of a tire's rolling resistance
and wet skid resistance.
-13-
Conversely, the curve for rubber (B) of FIG. 1
taken with FIG. 2 for the 3,4-polyisoprene having a Tg
of -11°C shows a higher tan.delta in the -20°C to +10°C
range, thus, predicting a tread with better traction
than (A) and a tan. delta in the 60°C range being higher
than curve (A) predicting higher tire rolling
resistance, as compared to curve (A), for this rubber
blend.
Also conversely, the curve (C) of FIG. 1 taken with
FIG. 2 for a 3,4-polyisoprene with Tg of -25°C shows a
lower tan.delta in the 60°C temperature range but a
lower tan.delta at the -20°C to +10°C predicting a
lower tire rolling resistance with lower wet tread
traction for this rubber blend.
The advantages of such dual optimization are
several fold, particularly including rolling resistance
and skid resistance.
In the description of this invention, while the cis
1,4-polyisoprene rubber includes both natural and
synthetic rubber, as pointed out, the natural rubber is
preferred. The cis 1,4-polyisoprene rubber, natural or
synthetic, typically has a cis 1,4-content of about 96
to about 99 weight percent.
The polybutadiene rubber can be composed of about
95 percent or more of cis 1,4 structure when prepared
with Ziegler-type catalyst or can be composed at least
about 90 percent cis and trans 1,4 structure when
prepared with alkyl lithium catalyst. Such
polybutadiene rubbers are well known.
The terms butadiene and polybutadiene as used
herein refer to 1,3-butadiene and polymers derived from
1,3-butadiene, respectively.
The solution polymerization prepared styrene/
butadiene copolymer rubber (S-SBR) can be prepared by
~~ r~ ~ vj
-14-
copolymerizing styrene and butadiene in an organic
solvent in the presence of a suitable catalyst. It
typically has a substantially narrower average
molecular weight distribution than an emulsion
polymerization prepared styrene/butadiene copolymer
rubber (E-SBR) and, further, typically enhances or
improves a tire's treadwear and rolling resistance when
used as a rubber component of a tire tread.
The emulsion polymerization prepared styrene/
butadiene rubber is prepared as an emulsion
polymerization and sometimes referred to herein as
E-SBR.
Both the S-SBR and E-SBR are well known rubbers as
is their differences in molecular weight distributions.
As an embodiment of the invention, particularly for
tires to be used for somewhat conventional loads and
speeds such as passenger vehicle tires, although the
embodiment is not necessarily limited to such use, is a
pneumatic tire provided with such tread where said
tread is a sulfur cured rubber composition comprised
of, based on 100 parts by weight rubber, (A) about 10
to about 25 phr of the 3,4-polyisoprene; (B) about 40
to about 55 phr of said natural rubber; and (C) about
20 to about 50 phr of at least one of
isoprene/butadiene copolymer rubber and
styrene/butadiene rubber, preferably solution
polymerization prepared styrene/butadiene rubber, and
cis 1,4-polybutadiene rubber and styrene/isoprene/
butadiene terpolymer rubber.
Other exemplary embodiments can be such tire with
tread composed of rubber blend embodiments hereinbefore
exemplified.
Such pneumatic tires are conventionally comprised
of a generally toroidal shaped carcass with an outer
~~~~rU
-I5-
circumferential tread, adapted to be ground contacting,
spaced beads and sidewalls extending radially from and
connecting said tread to said beads.
The required 3,4-polyisoprene rubber for the rubber
components of this invention can suitably be prepared
by polymerizing isoprene, preferably on a continuous
reactor basis in the presence of an organo lithium
catalyst, such as butyl lithium, in an organic solvent, '
such as, for example, hexane, and a polar modifier,
such as tetramethylethylene diamine (TMEDA) and the
polymerization shortstopped with triisopropanol amine,
rosin acid, methanol or other suitable shortstop to
obtain the required Tg.
The amount of_ organo lithium catalyst is largely
dependent upon the molecular weight desired for the
resultant polymer.
As hereinbefore represented in the accompanying
drawing, several 3,4-polyisoprene rubber polymers were
prepared with various polymer configurations (Tg's,
etc.) and blended with various other rubbers in order
to achieve the present invention. Accordingly, it was
considered that the 3,4-polyisoprene rubber have a Tg
in the range of about -15°C to about -20°C in order to
provide a micro structure for the tire tread for wet
traction; a Mooney (MLl+4) value in the range of about
70 to about 90, preferably about 75 to about $5 in
order to contribute to lower heat generation for the
tread tri-rubber blend or quatra-rubber blend and,
thus, lower rolling resistance; a 3,4-content in the
range of about 50 to about 60 in order to provide
microstructure for polymer incompatibility with other
rubbers in the blend and, thus, improve wet traction
for the tire tread; a 1,2-content in the range of about
2 to about 10 in order to also assist in providing
._,,. ~~~ gyp'
-16-
microstructure for the tire tread's wet traction and a
sum of the 3,4 and 1,2 (vinyl) contents in the range of
about 56 to about 63 in order to aid in providing an
incompatibility factor for the (B) and (C) rubbers in
the tire tread to enhance its wet traction.
The prescribed 3,4-polyisoprene rubber is used in a
minor amount (less than about 35 phr) of the rubber
composition for the tread. Its primary contribution
relates to enhancing traction, particularly wet
traction for the tread. Larger amounts of the rubber
would be expected, or has been observed, to increase
rolling resistance and decrease tear resistance of the
tire tread.
The other rubbers are utilized as the major portion
of the tread rubber because the natural rubber
contributes to low rolling resistance and treadwear and
the third and fourth rubbers) of the tri-blend and
quatra-blend generally contributes) to wet traction
and treadwear.
While the rubbers used herein, particularly those
in the higher Mooney (MLl+4) viscosity range, can
optionally be individually oil extended before or
during mixing with various rubber compounding materials
for ease of processing; it is preferred in the practice
of this invention that oiI extension is not used. If
oil extension is used, usually about 10 to about 50 phr
of rubber processing oil is used, usually of the
aromatic or aromatic/napthenic oil type or paraffinic/
napthenic oil type.
It should readily be understood by one having skill
in the art that said tread portion of the pneumatic
tire as well as the rubber or other material in the
basic carcass, which normally contains reinforcing
elements in the tread region, can be compounded by
methods generally known in the rubber compounding art,
such as mixing the various sulfur-vulcanizable
constituent rubbers with various commonly used additive
materials such, as for example, curing aids, such as
sulfur, activators, retarders and accelerators,
processing additives, such as oils, resins including
tackifying resins, silicas, and plasticizers, fillers,
pigments, stearic acid, zinc oxide, waxes, antioxidants
and antiozonants, peptizing agents and reinforcing
materials such as, for example, carbon black. As known
to those skilled in the art, depending on the intended
use of the sulfur vulcanizable and sulfur vulcanized
material (rubbers), the certain additives mentioned
above are selected and commonly used in conventional
amounts.
Typical additions of carbon black comprise about 20
to 100 parts by weight of diene rubber (phr),
preferably 30 to 60 phr. Typical amounts of tackifier
resins, if used, comprise about 0.5 to IO phr. Typical
amownts of processing aids comprise 1 to 5 phr.
Typical amounts of silica, if used, comprise about 5 to
about 25 phr and amounts of silica coupler, if used,
comprise about 0.05 to about 0.25 parts per part of
silica, by weight. Representative silicas may be, for
example, hydrated amorphous silicas. A representative
coupling agent may be, for example, a bifunctional
sulfur containing organo silane such as, for example,
bis-(3-triethoxy-silylpropyl) tetrasulfide, bis-(3-
trimethoxy-silylpropyl)tetrasulfide and
bis(3-trimethoxy-silylpropyl)tetrasulfide grafted
silica from DeGussa, AG. Typical amounts of
antioxidants comprise 1 to about 5 phr. Representative
antioxidants may be, for example,
diphenyl-p-phenylenediamine and others, such as those
._'
~~~"lr~ ~;
-18-
disclosed in the Vanderbilt Rubber Handbook (1978),
pages 344-346. Suitable antiozanant(s) and waxes,
particular microcrystalline waxes, may be of the type
shown in the Vanderbilt Rubber Handbook (1978), pages
346-347, Typical amounts of antiozonants comprise 1 to
about 5 phr. Typical amounts of stearic acid comprise
1 to about 3 phr. Typical amounts of zinc oxide
comprise 2 to 5 phr. Typical amounts of waxes comprise
1 to 5 phr. Typical amounts of peptizers comprise 0.1
to 1 phr. The presence and relative amounts of the
above additives are not an aspect of the present
invention which is primarily directed to the
utilization of specified blends of rubbers in tire
treads as sulfur vulcanizable compositions.
The vulcanization is conducted in the presence of a
sulfur vulcanizing agent. Examples of suitable sulfur
vulcanizing agents include elemental sulfur (free
sulfur) or sulfur donating vulcanizing agents, for
example, an amine disulfide, polymeric polysulfide or
sulfur olefin adducts. Preferably, the sulfur
vulcanizing agent is elemental sulfur. As known to
those skilled in the art, sulfur vulcanizing agents are
used in an amount ranging from about 0.5 to 8 phr with
a range of from 1.5 to 2.25 being preferred.
Accelerators are used to control the time and/or
temperature required for vulcanization and to improve
the properties of the vulcanizate. In one embodiment,
a single accelerator system may be used, i.e., primary
accelerator. Conventionally, a primary accelerator is
used in amounts ranging from about .5 to 2.0 phr. In
another embodiment, combinations of two or more
accelerators which is generally used in the larger
amount (.5 to 1.0 phr), and a secondary accelerator
which is generally used in smaller amounts (.05-.50
~4r1~~
-19-
phr) in order to activate and to improve the properties
of the vulcanizate. Combinations of such accelerators
have historically been known to produce a synergistic
effect of the final properties of sulfur cured rubbers
and are often somewhat better than those produced by
use of either accelerator alone. In addition, delayed
action accelerators may be used which are less effected
by normal processing temperatures but produce
satisfactory cures at ordinary vulcanization
temperatures. Representative examples of accelerators
include amines, disulfides, guanidines, thioureas,
thiazoles, thiurams, sulfenamides, dithiocarbamates and
xanthates. Preferably, the primary accelerator is a
sulfenamide. If a second accelerator is used, the
secondary accelerator is preferably a guanidine,
dithiocarbamate or thiuram compound.
The tire can be built, shaped, molded and cured by
various methods which will be readily apparent to those
having skill in the art.
In the practice of this invention, the polymer
blend-tread can be integral with and adhered to various
tire carcass substrate rubber compositions. Typically,
such a rubber composition is at least one of a
butadiene/styrene copolymer rubber, cis 1,4
polyisoprene (natural or synthetic rubber) and 1,4
polybutadiene. Optionally, such a blend for a portion
of the tread, particularly where the tread is in the
region of the sidewall area of the tire may contain one
or more of butyl rubber, halobutyl rubber, such as
chlorobutyl or bromobutyl rubber, and
ethylene/propylene/conjugated diene terpolymer rubber,
polyisoprene and polybutadiene rubber.
In the further practice of this invention, the
tread can typically be applied in the building of the
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green tire in which the uncured, shaped tread is built
onto the carcass following which the green tire is
shaped and cured.
Alternately, the tread can be applied to a cured
tire carcass from which the previous tread has been
buffed or abraded away and the tread cured thereon as a
retread.
As previously discussed, an important contribution
of the prescribed 3,4-polyisoprene rubber for the tire
tread component is attributed to the increase of wet
skid resistance for the tread tri-blend or quatra-blend
with minimal increase in rolling resistance due to its
relatively high molecular weight as evidenced by its
relatively high Mooney (ML1+4) value. A contribution
of the natural rubber for the tire tread component is
attributed to a Lower rolling resistance and to
treadwear and to an improved tear resistance. A
contribution of the additional diene rubber is
attributed to some of the traction, treadwear and/or
rolling resistance for the tire.
The practice of this invention is further
illustrated by reference to the .following examples
which are intended to be representative rather than
restrictive of the scope of the invention. Unless
otherwise indicated, all parts and percentages are by
weight.
EXAMPLE I
Pneumatic tires of conventional construction
(grooved tread, sidewalls, spaced beads, and supporting
fabric-reinforced carcass) were built, shaped and cured
in a conventional tire mold. The tread was built onto
the uncured carcass as a pre-extruded element. The
~~'~
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tires were of the P195/75R14 type which indicates that
they were belted, radial ply passenger type tires.
One tire is identified herein as Control X and an
experimental tire identified as Experimental Y.
Control tire X had a tread composed of (A) 50 phr
butadiene/styrene rubber; and (B) 50 phr natural rubber
and is intended to represent a somewhat conventional
passenger tire tread.
Experimental tire Y had a tread composed of (A)
3,4-polyisoprene rubber prescribed herein and having a
Tg of about -18°C, and a vinyl 3,4-content of about 55
percent; (B) natural rubber; and (C) S-SBR.
Thus, the 3,4-polyisoprene rubber, basically,
replaced at least a part of the butadiene/styrene
rubber in the tread rubber blend.
The tires (X and Y) were mounted on rims, inflated
and submitted to testing. The test values for the
control were normalized to a value of 100 for
comparison purposes. The tire with the experimental
tread was tested and its test values compared to the
values of the control tire and reported relative to the
normalized values of 100.
The tire with the experimental tread rubber
composition Y exhibited a lower rolling resistance and
higher skid resistance while providing a similar
treadwear as compared to control tire X. These results
are considered to be an important departure from
results which might ordinarily be expected absent the
prior experimentation reported herein.
The tread compositions for tires X and Y were
comprised of materials shown in the following Table 1.
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Table 1
Partsl
Ex erimental
Rubber Compound Control (X)
Butadiene/styrene rubber 50 30
Natural rubber 50 55
3,4-polyisoprene rubber2 0 15
Oil, paraffinic/napthenic 7 4
Carbon black (GPT) 43 38
The rubber compound contained conventional amounts
of antioxidant, antiozonant, stearic acid, peptizer,
wax, silica and coupling agent, sulfur, accelerators)
and zinc oxide which are not considered as being the
aspect of this invention since the invention is
primarily directed to the rubber blend itself.
1. - Amounts rounded to nearest part.
2. - Polymer is composed of about 55 percent 3,4 units,
about 5 percent 1,2-units and about 40 percent
1,4-units and is the 3,4-polyisoprene described in
this specification, particularly of the type shown
in Experiment A of TABLE 2 herein. It had a Tg of
-18°C and a Mooney viscosity; ML(1~-4) at 100°C of
about 80.
Table 2 illustrates various characteristics of the
(Control X) and (Experimental Y) rubber compound.
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-23-
Table 2
Property Control X E_xp Y
300% Modulus (MN/m2) 10.5 10
Tensile (MN/m2) 19.2 18
Elongation (%) 510 500
Rebound (23C) 46 50
Rebound (100C) 64.3 67
Autovibron
Tan Delta (0C) 0.135 0.209
Tan Delta (60C) 0.093 0.083
Table 3 illustrates the rolling resistance, wet
skid resistance and treadwear values with the
Experimental Tire Y compared to values of Control Tire
X normalized to 100.
Table 3
Measured Values Control X Experimental Y1'-2
Rolling resistance 100 109 (Improved)
(67" wheel)
Wet skid resistance 100 108 (Improved)
(20 mph)
Treadwear 100 100
1. - a reduction in the Rolling Resistance is
represented by an increase in the relative
reported value and is considered an improvement.
2. - an increase in the Treadwear value is an
improvement.
The rolling resistance was measured by mounting and
inflating the tire on a metal rim and allowing it to be
turned by a 67 inch diameter dynamometer under about 80
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percent of its rated load at a rate equivalent to a
vehicular speed of 50 mph and the drag force measured.
The test is believed to be samewhat standard.
The skid resistance was a standard test in which
the tires are mounted on a weighted, drawn trailer at
various speeds and brakes of the trailer applied and
skid force (peak and slide) measured.
The treadwear was evaluated as a measure of
reduction in tread depth after about 20,000 kilometers
of test on an automobile.
The treadwear was compared by actually mounting
both control and experimental tires on a vehicle and
driving it under controlled conditions, (38 psig
inflation) with the position of the tires on the
vehicle being maintained while rotated through
vehicles.
In this Example, the 3,4-polyisoprene is prepared
as hereinbefore reported with hexane as a solvent.
While certain representative embodiments and
details have been shown for the purpose of illustrating
the invention, it will be apparent to those skilled in
this art that various changes and modifications may be
made therein without departing from the spirit or scope
of the invention.
