Silica reinforced rubber composition and tire with tread

The present invention relates to a silica reinforced rubber composition and to pneumatic tires having treads comprised of the silica reinforced rubber compositions. The silica reinforced rubber composition comprises at least one elastomer, silica, a silica coupler, a silylating agent and, optionally, carbon black.

FIELD 
This invention relates to rubber compositions which are quantitatively 
reinforced with silica. In one aspect, the rubber composition is comprised 
of rubber, particularly sulfur curable, or cured, rubber, reinforced with 
a combination of silica, silica coupling agent, silylating agent and, 
optionally, carbon black. 
BACKGROUND 
For various applications utilizing rubber which require high strength and 
abrasion resistance, particularly applications such as tires and various 
industrial products, sulfur cured rubber is utilized which contains 
substantial amounts of reinforcing fillers. Carbon black is commonly used 
for such purpose and normally provides or enhances good physical 
properties for the sulfur cured rubber. Particulate silica is also often 
used for such purpose, particularly when the silica is used in conjunction 
with a coupling agent. In some cases, a combination of silica and carbon 
black is utilized for reinforcing fillers for various rubber products, 
including treads for tires. The use of such reinforcing fillers for 
elastomers, including sulfur curable elastomers, is well known to those 
having skill in such art. 
It is important to appreciate that, conventionally, carbon black is a 
considerably more effective reinforcing filler for rubber products, and 
particularly for rubber tire treads than silica if the silica is used 
without a coupling agent, or silica coupler as it may be sometimes 
referred to herein. 
Often coupling agents are composed of a silane which has at least one first 
constituent component, or moiety, (such as a silane portion) capable of 
reacting with the silica surface and, also, an additional moiety capable 
of interacting with the rubber, particularly a sulfur vulcanizable rubber 
which contains carbon-to-carbon double bonds, or unsaturation. Usually the 
additional moiety is sulfur in a form of a polysulfide and particularly a 
polysulfide bridge between said first moieties. In this manner, then the 
coupler acts as a connecting bridge between the silica and the rubber and 
thereby enhances the rubber reinforcement aspect of the silica. 
The rubber-reactive group component, namely the said additional moiety, of 
such coupler may be, for example, one or more of groups such as mercapto, 
amino, vinyl, epoxy, and sulfur groups, preferably a sulfur or mercapto 
moiety and more preferably sulfur in a form of a polysulfide as a 
polysulfide bridge between at least two of said first moieties. 
Numerous of such coupling agents are taught for use in combining silica and 
rubber, such as, for example, silane coupling agents containing a 
polysulfide component, or structure such as, for example, 
trialkoxyorganosilane polysulfides, such as for example 
bis-(3-trialkoxysilylorgano) polysulfides, containing from about 2 to 
about 8 sulfur atoms in a polysulfide bridge such as, for example, 
bis-(3-triethoxysilylpropyl)tetrasulfide and/or trisulfide. 
Various U.S. patents relating to silicas and silica reinforced tire treads 
include, for example, U.S. Pat. Nos. 3,451,458; 3,664,403; 3,768,537; 
3,884,285; 3,938,574; 4,482,663; 4,590,052 and 5,089,554. 
In one aspect, and in contrast to the aforesaid rubber reactive silica 
coupling agents, other silica-reactive materials might be utilized to 
interact with the silica which have only one constituent component, or 
moiety, and which is capable of reacting with the silica surface. Such 
materials are not readily reactive with the rubber so that they do not 
readily promote interaction between the silica and the rubber. Therefore, 
by itself, such material is not considered herein to enable the silica to 
satisfactorily reinforce the rubber. 
The term "phr" if used herein, and according to conventional practice, 
refers to "parts of a respective material per 100 parts by weight of 
rubber, or elastomer". 
In the description of this invention, the terms "rubber" and "elastomer" if 
used herein, may be used interchangeably, unless otherwise prescribed. The 
terms "rubber composition", "compounded rubber" and "rubber compound", if 
used herein, are used interchangeably to refer to "rubber which has been 
blended or mixed with various ingredients and materials" and such terms 
are well known to those having skill in the rubber mixing or rubber 
compounding art. 
SUMMARY AND PRACTICE OF THE INVENTION 
In accordance with this invention, a rubber composition is provided which 
comprises (A) 100 parts by weight of at least one diene-based elastomer, 
(B) about 50 to about 100, optionally about 60 to about 90, phr 
particulate reinforcing filler composed of precipitated silica and carbon 
black, comprised of about 60 to about 100, alternatively about 60 to about 
90, phr of precipitated silica which contains silanol groups thereon and 
correspondingly zero to about 10, alternatively zero to about 7 or about 3 
to about 7, phr of carbon black, wherein the weight ratio of silica to 
carbon black is at least about 6/1 and alternatively, at least about 10/1, 
(C) at least one silica coupler having a moiety reactive with said silica 
and another moiety interactive with said elastomer(s), (D) about 0.5 to 
about 10, alternatively about 2 to about 6 phr of a silanol-reactive, 
trialkylsilyl functional group-containing silylating agent having the 
formula: 
EQU R.sub.1 R.sub.2 R.sub.3 Si--X 
where X is a radical selected from at least one of the group consisting of: 
(1) halogen radical selected from one of chlorine, bromine and iodine 
(2) --NH--C.dbd.OR.sub.1, 
(3) --NR.sub.4 --A, 
(4) --NH--C.dbd.ONH--A, and 
(5) --N(R.sub.4).sub.2 
wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are individually selected 
from the group consisting of primary, secondary and tertiary alkyl 
radicals and alkaryl radicals having from 1 to 30, alternatively about 1 
to about 3, carbon atoms, wherein R.sub.4 is alternatively hydrogen, A is 
--SiR.sub.1 R.sub.2 R.sub.3. 
It is an important feature of this invention that a relatively highly 
silica loaded elastomer composition is provided which contains the 
silica-reactive, silylating agent together with the silica coupler rather 
than the silylating agent or silica coupler individually. 
The term "silylating agent" conventionally relates to materials which are 
capable of reacting with an active hydrogen-containing substrate, for 
example, a silanol group (SiOH) on a silica surface for this invention, to 
introduce a silyl functional group to the substrate material, or silica 
surface for this invention. 
The term "silylating agent" is used herein to mean the said trialkylsilyl 
functional group-containing material which can react, for example, with a 
silanol group (SiOH) on a silica surface to introduce the trialkylsilyl 
group onto the silica surface. 
By the term "silica-reactive silylating agent", it is meant herein that the 
trialkylsilyl groups of the agent react with silanol groups at the silica 
surface and, thus, chemically combine with the silica. 
A trialkylsilyl group from the silylating agent is, thus, placed on the 
silica surface in place of the hydrogen atom of the silanol group. 
It is important that the alkoxysilane component of a silica coupler which 
contains alkoxysilane units also reacts with the silanol groups at the 
surface of the silica and thereby also chemically combines with the 
silica. 
While it is recognized that both the silica coupler (the silane moiety of 
the coupler) and the silylating agent compete for the silanol groups on 
the silica surface, only the additional moiety of the silica coupler such 
as, for example, a polysulfide bridge, has reactivity with the diene-based 
elastomer. 
Because both the alkoxysilane moiety of a silane-containing silica coupler 
and the trialkylsilyl moiety of the silylating agent are reactive with 
and, thus, compete for the silanol groups on the surface of the silica, it 
is important that the silylating agent be added to the silica concurrent 
with or subsequent to the silica coupler. As a result, while the precise 
mechanism, or result, may not be completely known, it is considered herein 
that the silylating agent exerts a synergistic benefit to the silica 
coupler, but does not replace its use. Such a synergistic benefit might be 
observed, for example, as improved performance as might be exemplified by 
higher 300 percent modulus values, reduced hysteresis as evidenced by hot 
rebound values and/or improved abrasion resistance as may be evidenced by 
DIN abrasion test, when using a combination of both of the silylating 
agent and the silica coupler in the silica reinforced elastomer 
composition, relative to the performance obtainable by using either the 
silylating agent or the silica coupler individually. 
For the said silanol-reactive silylating agent, and for said R.sub.1, 
R.sub.2, R.sub.3 and R.sub.4, representative examples of primary alkyl 
radicals are those selected from at least one of methyl, ethyl, propyl, 
octyl, n-octadecyl, and n-hexadecyl radicals; representative examples of 
secondary alkyl radicals are those selected from at least one of isopropyl 
and sec-butyl radicals and representative examples of tertiary alkyl 
radicals are those selected from at least one of tert-butyl, and 
dimethylbutyl radicals. 
It is the trialkylsilyl moiety of the silylating agent that is reactive 
with the surface of the precipitated silica which is usually the silanol 
groups on the surface of the silica. 
Representative, although not intended to be limitative, of various 
silylating agents are, for example, trimethylsilyl diethyl amine, 
N,N'-bis(trimethylsilyl) urea, trimethyl chlorosilane, hexamethyl 
disilazane and monotrimethylsilyl acetamide. 
In the practice of this invention, it has been observed that the addition 
of the aforesaid silica silylating agent to the rubber silica composition, 
which contains the silica coupler, results in not only improved processing 
characteristics for the silica reinforced elastomer composition, such as 
reduced viscosity, but also in improved cured elastomer composition 
performance properties such as, for example, abrasion resistance, tear 
strength and rebound. 
Cured physical properties such as abrasion resistance, tear strength and 
rebound values may be simultaneously improved. This is considered herein 
to be beneficial because tire treads having such properties are 
anticipated herein to provide one or more of improved treadwear, rolling 
resistance and durability. 
It is recognized that various silylating agents have hereinbefore been used 
for various purposes in various elastomeric compositions such as, for 
example, silica and carbon black reinforced rubber compositions with 5-50 
phr of silica and 10-60 phr of carbon black. For example, see (Japanese 
patent publication abstract No. 5051484). 
It is considered herein that this invention is a departure from such prior 
practice because the silica silylating agent containing a trialkylsilyl 
moiety is used in combination with a silica coupler, which contains a 
silane moiety, in a substantially silica reinforced (eg. at least 60 phr 
of silica and less than 10 phr of carbon black) rubber composition. 
In one aspect, a synergistic performance has been observed when using both 
the said silylating agent and silica coupler which was not observed when 
using either of the silylating agent or silica coupler alone for a silica 
reinforced elastomer composition. Moreover, the synergistic effect seemed 
to be dependant upon using only a limited amount, or level, of the 
silylating agent. 
While, in the practice of this invention various silica couplers can be 
used, one preferred coupler is a bis-(trialkoxysilylalkyl) polysulfide 
containing from about 2 to about 8 sulfur atoms in the polysulfide bridge 
an hereinbefore described. For example, the silica coupler can be 
bis-(3-triethoxysilylpropyl) tetrasulfide having an average of about 4 
sulfur atoms in its polysulfide bridge or, in an alternative, a 
polysulfide having about 2 sulfur atoms in its polysulfide bridge. 
Conventionally a weight ratio of silica coupler to silica is in a range of 
about 0.01/1 to about 0.25/1. 
In practice, a molar ratio based on trialkylsilyl functionality of the 
silica silylating agent to silane functionality of the silica coupler in a 
range of about 0.1/1 to about 4/1 alternatively, a molar ratio of about 
0.5/1 to about 3/1, is preferred. 
In one aspect of the invention, the rubber composition contains a high 
level, or amount, of silica, namely at least about 50 phr as a dominant 
elastomer reinforcing pigment with less than 10 phr of carbon black as a 
secondary elastomer reinforcing material. 
The rubber composition itself can also be provided as being a sulfur cured 
composition through vulcanization of the uncured elastomer composition. 
The sulfur curing is accomplished in a conventional manner, namely by 
curing under conditions of elevated temperature and pressure for a 
suitable period of time. 
Further, and in accordance with this invention, a tire is provided having a 
tread of the rubber composition of this invention. 
The said curatives for the process are curatives conventionally used for 
sulfur curable elastomers which typically include sulfur and one or more 
appropriate cure accelerators and sometimes also a retarder. Such 
curatives and use thereof for sulfur curable elastomer compositions are 
well known to those skilled in the art. 
Further, sequential mixing processes for preparing sulfur curable rubber 
compositions in which elastomers and associated ingredients exclusive of 
curatives are first mixed in one or more sequential steps followed by a 
final mixing step for adding curatives are also well known to those 
skilled in the art. 
In the practice of this invention, as hereinbefore pointed out, the rubber 
composition is comprised of at least one diene-based elastomer, or rubber. 
Such elastomers are typically selected from homopolymers and copolymers of 
conjugated dienes and copolymers of conjugated diene(s) and vinyl aromatic 
compound. Such dienes may, for example, be selected from isoprene and 
1,3-butadiene and such vinyl aromatic compounds may be selected from 
styrene and alphamethylstyrene. Such elastomer, or rubber, may be 
selected, for example, from at least one of cis 1,4-polyisoprene rubber 
(natural and/or synthetic, and preferably natural rubber), 
3,4-polyisoprene rubber, styrene/butadiene copolymer rubbers, 
isoprene/butadiene copolymer rubbers, styrene/isoprene copolymer rubbers, 
styrene/isoprene/butadiene terpolymer rubbers, cis 1,4-polybutadiene 
rubber, trans 1,4-polybutadiene rubber (70-95 percent trans), low vinyl 
polybutadiene rubber (10-30 percent vinyl), medium vinyl polybutadiene 
rubber (30-50 percent vinyl) and high vinyl polybutadiene rubber (50-90 
percent vinyl). 
In one aspect, particularly for a tire tread, the rubber is preferably of 
at least two of diene based rubbers. For example, a combination of two or 
more rubbers is preferred such as cis 1,4-polyisoprene rubber (natural or 
synthetic, although natural is preferred), 3,4-polyisoprene rubber, 
styrene/isoprene/butadiene rubber, emulsion and/or solution polymerization 
derived styrene/butadiene rubbers, cis 1,4-polybutadiene rubbers and 
emulsion polymerization prepared butadiene/acrylonitrile copolymers. 
In one aspect of this invention, an emulsion polymerization derived 
styrene/butadiene (E-SBR) might be used having a relatively conventional 
styrene content of about 20 to about 28 percent bound styrene or, for some 
applications, an E-SBR having a medium to relatively high bound styrene 
content, namely a bound styrene content of about 30 to about 45 percent. 
The relatively high styrene content of about 30 to about 45 for the E-SBR 
can be considered beneficial for a purpose of enhancing traction, or skid 
resistance, of the tire tread. The presence of the E-SBR itself is 
considered beneficial for a purpose of enhancing processability of the 
uncured elastomer composition mixture, especially in comparison to a 
utilization of a solution polymerization prepared SBR (S-SBR). 
By emulsion polymerization prepared E-SBR, it is meant that styrene and 
1,3-butadiene are copolymerized as an aqueous emulsion. Such are well 
known to those skilled in such art. The bound styrene content can vary, 
for example, from about 5 to 50%. In one aspect, the E-SBR may also 
contain acrylonitrile to form a terpolymer rubber, as E-SBAR, in amounts, 
for example, of about 2 to about 30 weight percent bound acrylonitrile in 
the terpolymer. 
Emulsion polymerization prepared styrene/butadiene/acrylonitrile terpolymer 
rubbers containing about 2 to about 40 weight percent bound acrylonitrile 
in the copolymer are also contemplated as diene based rubbers for use in 
this invention, particularly if used in combination with other diene-based 
elastomers. 
The solution polymerization prepared SBR (S-SBR) typically has a bound 
styrene content in a range of about 5 to about 50, preferably about 9 to 
about 36, percent. The S-SBR can be conveniently prepared, for example, by 
organo lithium catalyzation in the presence of an organic hydrocarbon 
solvent. 
A purpose of using S-SBR may be to promote a reduction in tire rolling 
resistance as a result of lower hysteresis when it is used in a tire tread 
composition. 
The 3,4-polyisoprene rubber (3,4-PI) is considered beneficial for a purpose 
of promoting the tire's traction when it is used in a tire tread 
composition. 
The 3,4-PI and use thereof is more fully described in U.S. Pat. No. 
5,087,668 which is incorporated herein by reference. The Tg refers to the 
glass transition temperature which can conveniently be determined by a 
differential scanning calorimeter at a heating rate of 10.degree. C. per 
minute. 
The cis 1,4-polybutadiene rubber (BR) is considered to be beneficial for a 
purpose of promoting the tire tread's wear, or treadwear. 
Such BR can be prepared, for example, by organic solution polymerization of 
1,3-butadiene. 
The BR may be conveniently characterized, for example, by having at least a 
90% cis 1,4-content. 
The cis 1,4-polyisoprene and cis 1,4-polyisoprene natural rubber are well 
known to those having skill in the rubber art. 
The vulcanized rubber composition should contain a sufficient amount of 
precipitated silica reinforcing filler namely, greater than about 50 phr, 
to contribute a reasonably high modulus, high abrasion resistance and 
resistance to tear for the cured rubber composition. The amount of the 
silica, as hereinbefore referenced, may be as low as about 50 parts per 
100 parts rubber, but is preferably from about 60 to about 90 or even up 
to about 100 parts by weight. 
Carbon black may be present in levels below about 10 phr or not at all. 
Carbon black is not intended to serve as a principal reinforcing filler in 
the elastomer compositions of this invention. Herein, the carbon black if 
used, is used primarily as a colorant where a black colored elastomer 
composition is desired and/or as a carrier for liquid additives for 
elastomer composition such as, for example, the silica coupler if it would 
otherwise be added to the elastomer composition in a liquid form. 
The commonly employed siliceous pigments used in rubber compounding 
applications can be used as the silica in this invention, including 
pyrogenic and precipitated siliceous pigments (silica), although 
precipitated silicas are preferred. 
The siliceous pigments preferably employed in this invention are 
precipitated silicas such as, for example, those obtained by the 
acidification of a soluble silicate, e.g., sodium silicate, generally 
exclusive of silica gels. 
Such silicas might be characterized, for example, by having a BET surface 
area, as measured using nitrogen gas, preferably in the range of about 40 
to about 600, and more usually in a range of about 50 to about 300 square 
meters per gram. The BET method of measuring surface area is described in 
the Journal of the American Chemical Society, Volume 60, page 304 (1930). 
The silica may also be typically characterized by having a dibutylphthalate 
(DBP) absorption value in a range of about 100 to about 400, and more 
usually about 150 to about 300. 
The silica might be expected to have an average ultimate particle size, for 
example, in the range of 0.01 to 0.05 micron as determined by the electron 
microscope, although the silica particles may be even smaller, or possibly 
larger, in size. 
Various commercially available silicas may be considered for use in this 
invention such as, only for example herein, and without limitation, 
silicas commercially available from PPG Industries under the Hi-Sil 
trademark with designations 210, 243, etc; silicas available from 
Rhone-Poulenc, with, for example, designations of Zeosil 1165MP and 
silicas available from Degussa AG with, for example, designations VN2 and 
VN3, etc. 
It is readily understood by those having skill in the art that the rubber 
composition would 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, fatty 
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 additives 
mentioned above are selected and commonly used in conventional amounts. 
Low amounts of reinforcing type carbon blacks(s), for this invention, if 
used, are hereinbefore set forth. 
It is to be appreciated that the silica coupler and/or the silica 
silylating agent, if in a liquid form, may be used in conjunction with a 
carbon black carrier, namely, pre-mixed with a carbon black prior to the 
addition to the rubber composition, and such carbon black is to be 
included in the aforesaid amount of carbon black accounted for in the 
rubber composition formulation. 
Typical amounts of tackifier resins, if used, comprise about 0.5 to about 
10 phr, usually about 1 to about 5 phr. Typical amounts of processing aids 
comprise about 1 to about 50 phr. Such processing aids can include, for 
example, aromatic, napthenic, and/or paraffinic processing oils. Typical 
amounts of antioxidants comprise about 1 to about 5 phr. Representative 
antioxidants may be, for example, diphenyl-p-phenylenediamine and others, 
such as, for example, those disclosed in the Vanderbilt Rubber Handbook 
(1978), pages 344-346. Typical amounts of antiozonants comprise about 1 to 
5 phr. 
Typical amounts of fatty acids, if used, which can include stearic acid, 
palmitic acid, linoleic acid or mixtures of one or more fatty acids, can 
comprise about 0.5 to about 3 phr. 
Often stearic acid is used in a relatively impure state and is commonly 
referred to in the rubber compounding practice as "stearic acid" and is so 
referred to in the description and practice of this invention. 
Typical amounts of zinc oxide comprise about 2 to about 5 phr. Typical 
amounts of waxes comprise about 1 to about 5 phr. Often microcrystalline 
waxes are used. Typical amounts of peptizers, if used, comprise about 0.1 
to about 1 phr. Typical peptizers may be, for example, 
pentachlorothiophenol and dibenzamidodiphenyl disulfide. 
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 about 4 phr, or even, in some circumstances, up 
to about 8 phr, with a range of from about 1 to about 2.5, sometimes from 
about 1 to about 2, 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 and preferably, a primary accelerator(s) is 
used in total amounts ranging from about 0.5 to about 4, preferably about 
0.8 to about 2, phr. In another embodiment, combinations of a primary and 
a secondary accelerator might be used with the secondary accelerator being 
used in amounts of about 0.05 to about 3 phr in order to activate and to 
improve the properties of the vulcanizate. Combinations of these 
accelerators might be expected to produce a synergistic effect on the 
final properties and are somewhat better than those produced by use of 
either accelerator alone. In addition, delayed action accelerators may be 
used which are not affected by normal processing temperatures but produce 
a satisfactory cure at ordinary vulcanization temperatures. Vulcanization 
retarders might also be used. Suitable types of accelerators that may be 
used in the present invention are 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 presence and relative 
amounts of sulfur vulcanizing agent and accelerator(s) are not considered 
to be an aspect of this invention which is more primarily directed to the 
use of the prescribed silylating agent in combination with a silica 
coupler in a silica reinforced rubber composition. 
The presence and relative amounts of the other additives, as hereinbefore 
described, are not considered to be an aspect of the present invention 
which is more primarily directed to the utilization of the prescribed 
silylating agent(s), required to be used in combination with a silica 
coupler, in a silica reinforced rubber composition. 
The mixing of the rubber composition can be accomplished by methods known 
to those having skill in the rubber mixing art. For example, the 
ingredients are typically mixed in at least two stages, namely, at least 
one non-productive stage followed by a productive mix stage. The final 
curatives are typically mixed in the final stage which is conventionally 
called the "productive" mix stage in which the mixing typically occurs at 
a temperature, or ultimate temperature, lower than the mix temperature(s) 
than the preceding non-productive mix stage(s). The rubber, silica, silica 
coupler, silica silylating agent, and carbon black if used, are mixed in 
one or more non-productive mix stages. The terms "non-productive" and 
"productive" mix stages are well known to those having skill in the rubber 
mixing art. 
The rubber composition of this invention can be used for various purposes. 
For example, it can be used for various tire compounds. Such tires can be 
built, shaped, molded and cured by various methods which are known and 
will be readily apparent to those having skill in such art. 
The invention may be better understood by reference to the following 
examples in which the parts and percentages are by weight unless otherwise 
indicated.

EXAMPLE I 
In this Example, the trialkylsilyl-containing silylating agent was 
evaluated as a component of compounding ingredients for a quantitatively 
silica reinforced elastomer composition. 
Rubber compositions containing the materials set out in Table 1 were 
prepared in a Kobe.TM. internal mixer using two separate stages of 
addition (mixing), one non-productive mix stage and a productive mix stage 
to temperatures of 160.degree. C. and 100.degree. C. and times of 6 
minutes and 2 minutes, respectively. The amount of silylating agent is 
listed as being "variable" in Table 1 and is more specifically set forth 
in Table 2. 
In comparison with Sample 1, Samples 2 and 3, which were prepared with 
addition of 3 and 6 phr silylating agent, respectively, clearly show the 
processing advantages of lower compound viscosity, plus the cured property 
advantages in modulus, rebound and abrasion resistance. In particular, the 
tire tread performance indicator properties of Samples 2 and 3, which 
contain 3 and 6 phr of the silylating agent, are better than those of 
Sample 1, namely, the 300 percent modulus is higher and the abrasion 
weight loss is less, indicative of better treadwear potential, and the 
rebound is higher indicative of better (reduced) rolling resistance. 
TABLE 1 
______________________________________ 
Non-Productive Mix Stage 
______________________________________ 
E-SBR.sup.1 25 
IBR.sup.2 45 
Cis 1,4-Polybutadiene.sup.3 
20 
NR.sup.4 10 
Processing Oils, Waxes 24.9 
Zinc Oxide 2.5 
Fatty Acid 3 
Antioxidants.sup.5 3 
Silica.sup.6 80 
Bis-(3-triethoxylsilylpropyl) 
12.8 
tetrasulfide.sup.7 
N.N-bis (trimethylsilyl) urea.sup.8 
variable 
Productive Mix Stage 
Sulfur 1.4 
Accelerators, sulfenamide and 
3.7 
guanidine types 
______________________________________ 
1) Emulsion polymerization prepared styrene/butadiene copolymer rubber 
having a styrene content of about 40 percent and obtained from The 
Goodyear Tire & Rubber Company; 
2) isoprene/butadiene copolymer rubber containing about 50 percent isoprene 
and having a Tg of about -43.degree. C. obtained from The Goodyear Tire & 
Rubber Company; 
3) cis 1,4-polybutadiene rubber obtained as Budene 1254 from The Goodyear 
Tire & Rubber Company; 
4) natural cis 1,4-polyisoprene rubber; 
5) of the phenylene diamine types; 
6) Z1165MP from Rhone Poulenc; 
7) a 50/50 blend or composite of bis-(3-triethoxysilylpropyl) tetrasulfide, 
said composite commercially available from Degussa GmbH as X50S. 
Technically, the tetrasulfide is understood to be an organosilane 
polysulfide as a composite, or mixture, in which the average polysulfide 
bridge contains about 3.5 to about 4 connecting sulfur atoms, although the 
mixture may contain such polysulfides with about 2 to about 8 connecting 
sulfur atoms; 
8) silylating agent obtainable from Huls America, Inc. 
TABLE 2 
______________________________________ 
Sample # 1 2 3 
______________________________________ 
N,N'-bis (trimethylsilyl) 
0 3 6 
urea, phr 
Cured Physical Properties 
300% Modulus, MPa 
10.1 11.7 11.8 
Rebound (100.degree. C.) 
60 62 62 
DIN Abrasion Resistance, 
108 104 102 
rel. wt. loss 
Viscosity - Uncured (Mooney MS 1 + 1.5, 100.degree. C.) 
Non-productive mixed 
98 78 72 
elastomer composition 
Productive mixed 
55 52 50 
elastomer composition 
______________________________________ 
In particular, this Example shows that the silylating agent, 
N,N'-bis(trimethylsilyl) urea, as utilized in Samples 2 and 3, in 
conjunction with the silica coupler, can provide significant improvements. 
For example, the processing of the elastomer composition of Samples 2 and 
3, as compared to Control Sample 1, was improved as evidenced by the 
reduced uncured viscosities for both the non-productive mixed elastomer 
compositions and the productive mixed elastomer compositions. 
Further, the 300 percent modulus, hot rebound and DIN abrasion resistance 
values were improved for Samples 2 and 3 as compared to Control Sample No. 
1 which contained only the silica coupler without the silylating agent. 
EXAMPLE II 
Rubber compositions containing the materials set out in Table 3 and as 
described in Example I were prepared in a Kobe.TM. internal mixer using 
one non-productive mix stage and a productive mix stage, to temperatures 
of 160.degree. C. and 100.degree. C. and times of 8 minutes and 2 minutes, 
respectively. The amounts of both silica coupler and silylating agent are 
listed as variable in Table 3 and are more specifically set forth in Table 
4. 
Sample No. 7 which contains only the silylating agent at 6 phr and no 
silica coupler, when compared to Sample No. 4 which contains only silica 
coupler at 6.4 phr and no silylating agent, clearly shows much inferior 
cured physical properties, including 300 percent modulus, DIN abrasion 
resistance and room temperature rebound. 
Sample No. 5, which contains the silylating agent at 6 phr in addition to 
silica coupler, exhibits superior cured physical properties such as DIN 
abrasion resistance, tear strength and room temperature rebound which are 
indicative of improved tire performance such as durability, treadwear and 
rolling resistance. 
Sample No. 6, in contrast, and which contains the silylating agent at 12 
phr in addition to the silica coupler, has cured physical properties 
similar to those of Sample No. 4, and inferior to those of Sample No. 5. 
In particular, the abrasion resistance, room temperature rebound and tear 
strength of Sample No. 6 are inferior to those properties of Sample No. 5. 
This indicates that an upper limit exists for the beneficial synergistic 
activity of the silylating agent in conjunction with the silica coupler in 
a highly silica reinforced rubber composition. 
Thus, the use of silylating agent without silica coupler is observed to be 
inferior in the physical property reinforcing characteristics which 
corresponds to the tire tread treadwear and rolling resistance performance 
indicators. The use of the silylating agent at 6 phr in combination with 
silica coupler provided what is considered herein to be synergistic 
improvements in the cured physical properties such as abrasion resistance, 
modulus ratio, rebound and tear resistance. The use of the silylating 
agent at 12 phr in combination with the silica coupler was observed to be 
no longer optimum and did not show the synergistic improvements seen for 
the lower level of silylating agent. 
TABLE 3 
______________________________________ 
Non-Productive Mix Stage 
______________________________________ 
E-SBR 25 
IBR 45 
Cis 1,4-Polybutadiene 20 
NR 10 
Processing Oils, Waxes 24.9 
Zinc Oxide 2.5 
Fatty Acid 3 
Antioxidants 3 
Silica 80 
Bis-(3-triethoxylsilylpropyl) 
variable 
tetrasulfide 
NN-bis (trimethylsilyl) urea 
variable 
Productive Mix Stage 
Sulfur 1.4 
Accelerators, sulfenamide and 
3.7 
guanidine types 
______________________________________ 
TABLE 4 
______________________________________ 
Sample # 4 5 6 7 
______________________________________ 
N,N'-bis 0 6 12 6 
(trimethylsilyl) 
urea, phr 
Bis-(3- 6.4 6.4 6.4 0 
triethoxypropyl) 
tetrasulfide, phr 
Cured Physical Properties 
300% Modulus, MPa 
13.4 11.7 13.6 2.9 
Rebound (23.degree. C.) 
40 42 40 34 
DIN Abrasion 103 92 97 125 
Resistance. rel. 
wt. loss 
Strebler Tear 54 79 52 141 
Resistance, N 
______________________________________ 
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.