MXPA99008758A - Rolling band rubber for high tracc rims - Google Patents

Abstract

The present invention is based on the unexpected discovery that tires have excellent dry and wet traction characteristics, including wet skid resistance, can be made by incorporating certain mixtures of styrene-butadiene (SBR) and isoprene-butadiene rubber (IBR) to the treads of the same without sacrificing greatly the characteristics of rolling resistance and wear of tread. This invention more specifically describes a tire tread rubber composition which is comprised of (a) about 50 phr to about 50 phr of an SBR and (b) about 15 phr to about 50 phr of an IBR, wherein IBR is comprised of repeating units that are derived from about 20 weight percent to about 50 weight percent isoprene and from about 50 weight percent to about 80 weight percent 1,3-butadiene, where the repeating units derived from isoprene and 1,3-butadiene are essentially in random order, wherein from about 3 percent to about 10 percent of the repeating units in the IBR are 1,2-polybutadiene units in where from about 50 percent to about 70 percent of the repeating units in the IBR are units of 1,4-polybutadiene, where from about 1 percent to about 4 percent of the units Repetition dats in the IBR are units of 3,4-polyisoprene, where from about 25 percent to about 40 percent of the repeating units in the IBR are 1,4-polyisoprene units, where the IBR has a temperature of glass transition that are on the scale of about -90 ° C to about -75 ° C and where the IBR has a Mooney viscosity that is within the range of about 55 to about 1

Description

ROLLING BAND RUBBER FOR HIGH TRACTION RIMS BACKGROUND OF THE INVENTION The ability to maintain good traction in both wet and dry pavements is a highly desirable feature for a tire. It is also usually desirable that the rim provide good tread wear and low rolling resistance. In order to reduce the rolling resistance of a tire, rubbers can be used which has a high bounce when manufacturing the tire treads. The tires made with these rubbers undergo less loss of energy during the bearing. The traditional problem associated with this approach is that the characteristics of the wet traction rim and the wet skid resistance are compromised. This is due to the fact that the good rolling resistance that favors low characteristics of low energy loss and good traction that favor the loss of high energy are viscoelastically inconsistent properties. In order to balance these two viscoelastically inconsistent properties, mixtures of various types of synthetic and natural rubber are commonly used in tire rolling bands. For example, various mixtures of styrene-butadiene rubber and polybutadiene rubber are commonly used as a rubbery material for automotive tire treads. To further improve the tensile characteristics, silica is also commonly included in the tread rubber as a filler. However, these mixtures are not entirely satisfactory for all purposes, US Pat. No. 4,843,120 discloses that tires having improved performance characteristics can be prepared using rubbery polymers having multiple transition temperatures of glass such as tread rubber, these rubbery polymers having multiple glass transition temperatures exhibit a first temperature of glass transition which is within the range of about -110SC to -20BC and exhibits a second glass transition temperature which is within the range of about -50aC to 0SC. In accordance with the Patent of E.U.A. Number 4,843,120, these polymers are made by polishing at least one conjugated diolefin monomer in a first reaction zone at a temperature and under conditions sufficient to produce a first polymeric segment having a glass transition temperature that is between -110aC and - 20aC and subsequently continuing the polymerization in a second reaction zone at a temperature and under conditions sufficient to produce a second polymeric segment having a glass transition temperature that is between -20SC and 20SC. These polymerizations are usually catalyzed with an organolithium catalyst and are usually carried out in an inert organic solvent. U.S. Patent 5,137,998 describes a process for preparing a rubbery terpolymer of styrene, isoprene and butadiene having multiple glass transition temperatures and having an excellent combination of properties for use in the manufacture of rim treads comprising terpolymerize styrene, isoprene and 1,3-butadiene in an organic solvent at a temperature of not more than about 40SC in the presence of (a) at least one member selected from the group consisting of tripiperidinophosphine oxide and alkali metal alkoxides and (b) an organolithium compound, U.S. Patent 5,047,483 discloses an air tire having an outer circumferential tread, wherein the tread is a sulfur cured rubber composition comprised of, based on 100 parts by weight of rubber (phr), (A) about 10 to about 90 parts by weight of a terpene rubber styrene olimer, isoprene, butadiene (SIBR) and (B) about 70 to about 30 weight percent of at least one 1,4-polyisoprene rubber and 1,4-polybutadiene cis rubber wherein the SIBR rubber is comprised (1) about 10 to about 35 weight percent bound styrene, (2) about 30 to about 50 weight percent bound ioprene and (3) about 30 to about 40 weight percent butadiene bound and characterized because it has a single glass transition temperature (Tg) that is on the scale of about -10SC to about -40aC and, in addition, the bonded butadiene structure contains about 30 to about 40 percent units of 1,2-vinyl, the bound isoprene structure contains about 10 to about 30 percent of 3,4-units and the sum of the percentage of 1,2-vinyl units of the bound butadiene and the percentage of 3,4 -Isolated isoprene units are on the scale of around 40 to about 70 percent. U.S. Patent 5,272,220 discloses a styrene-isoprene-butadiene rubber that is particularly valuable for use in manufacturing tire rims for truck that exhibit improved rolling resistance and tread wear characteristics. the rubber being comprised of repeat units that are derived from about 5 weight percent to about 20 weight percent styrene, from about 7 weight percent to about 35 weight percent isoprene and about from 55 weight percent to about 88 weight percent 1,3-butadiene, wherein the repeat units derived from styrene, isoprene and 1,3-butadiene are essentially in random order, where of about 25 per cent. one hundred to about 40 percent of the repeating units derived from 1,3-butadiene are from the cis- microstructure, where from about 40 percent to about 60 percent of the repeating units derived from 1,3-butadiene are of the trans- microstructure, where from about 5 percent to about 25 percent d © the repeating units derived from 1,3-butadiene are from the vinyl microstructure, where around from 75 percent to approximately 90 percent of the repeating units derived from isoprene are from microstructure 1,4-, where from about 10 percent to about 25 percent of the repeating units derived from isoprene are from the microstructure 3,4-, wherein the rubber has a glass transition temperature that is within the range of about -90 ° C to about -70 ° C, where the rubber has a number-average molecular weight that is within the range from about 150,000 to about 400,000, wherein the rubber has a weight average molecular weight of about 300,000 to about 800,000, and wherein the rubber has a lack of homogeneity that is within the range of about 0.5 to about 1.5 . U.S. Patent 5,405,927 discloses an isoprene-butadiene rubber which is reported to be particularly valuable for use in manufacturing tire treads for trucks, the rubber being comprised of repeat units that are derived from about 20 percent by weight. weight to about 50 weight percent isoprene and from about 50 weight percent to about 80 weight percent 1,3-butadiene, wherein the repeat units derived from isoprene and 1,3-butadiene are in essentially random order, wherein from about 3 percent to about 10 percent of the repeating units in the rubber are 1, 2-polybutadiene units, where from about 50 percent to about 70 percent of the units of repetition in rubber are 1, 4-polybutadiene units, wherein from about 1 percent to about 4 percent of the repeating units in the rubber are 3-4-polyisoprene units, where from about 25 percent to about 40 percent of the repeat units in the polymer are 1,4-polyisoprene units, wherein the rubber has a glass transition temperature that is within the range of -about -90aC to about -75SC and where the rubber has a Mooney viscosity that is within of the scale from about 55 to about 140. U.S. Patent 5,405,927 specifically discloses mixtures of this isoprene-butadiene rubber with natural rubber or 3,4-polyisoprene.

SUMMARY OF THE INVENTION It has unexpectedly been found that tires having marked dry and wet traction characteristics can be made by incorporating certain mixtures of styrene-butadiene rubber (SBR) and isoprene-butadiene rubber (IBR) into the treads of the tires. same. More importantly, this improvement in traction characteristics, including wet skid resistance, can be achieved without greatly sacrificing rolling resistance and tread wear characteristics. These rubber blends will typically contain from about 50 phr to about 85 phr of the SBR and from about 15 phr to about 50 phr of the IBR. The IBR will normally be comprised of repeating units that are derived from about 20 weight percent to about 50 weight percent isoprene and from about 50 weight percent to about 80 weight percent of 1, 3- butadiene, wherein the repeating units derived from isoprene and 1,3-butadiene are essentially in random order, wherein from about 3 percent to about 10 percent of the repeating units in the isoprene-butadiene rubber are units of 1, polybutadiene, wherein from about 50 percent to about 70 percent of the repeating units in the isoprene-butadiene rubber are 1,4-polybutadiene units, wherein from about 1 percent to about 4 percent of the repeating units in the isoprene-butadiene rubber are 3,4-polyisoprene units, where from about 25 percent to about 40 percent of the repeating units The rubber in isoprene-butadiene are 1,4-polyisoprene units, wherein the isoprene-butadiene rubber has a glass transition temperature that is within the range of about -90 ° C to about -75 ° C and where the isoprene-butadiene rubber has a Mooney viscosity that is within the range of about 55 to about 140. The present invention more specifically describes a tire tread rubber composition which is comprised of (a) about 50 phr at about 85 phr of a styrene-butadiene rubber and (b) about 15 phr to about 50 phr of an isoprene-butadiene rubber, wherein the isoprene-butadiene rubber is comprised of repeat units which are derived from about 20 weight percent to about 50 weight percent isoprene and from about 50 weight percent to about 80 weight percent 1,3-butadiene, wherein the s repeat units derived from isoprene and 1 ', 3-butadiene is essentially in random order, wherein from about 3 percent to about 10 percent of the isoprene-butadiene rubber repeat units are units of 1, 2 Polybutadiene, wherein from about 50 percent to about 70 percent of the repeating units in the isoprene-butadiene rubber are 1,4-polybutadiene units, wherein from about 1 percent to about 4 percent of the repeating units in the isoprene-butadiene rubber are 3,4-polyisoprene units, wherein from about 25 percent to about 40 percent of the repeating units in the isoprene-butadiene rubber are units of 1. , 4-polyisoprene, wherein the isoprene-butadiene rubber has a glass transition temperature that is within the range of about -902C to about -759C and where isoprene-butadiene rubber or has a Mooney viscosity that is within the range of about 55 to about 140. The present invention further discloses an air tire having an outer circumferential tread where the tread is a sulfur-cured rubber composition comprised from (a) about 50 phr to about 85 phr of a styrene-butadiene rubber and (b) about 15 phr to about 50 phr of an isoprene-butadiene rubber, wherein the isoprene-butadiene rubber is comprised of repeating units which are derived from about 20 weight percent to about 50 weight percent isoprene and from about 50 weight percent to about 80 weight percent 1,3-butadiene weight, wherein the repeating units derived from isoprene and 1,3-butadiene are essentially in random order, wherein from about 3 percent to about 10 percent of the repeating units in the rubber of isoprene-butadiene are 1,2-polybutadiene units, wherein from about 50 percent to about 70 percent of the repeating units in the isoprene-butadiene rubber are 1,4-polybutadiene units, wherein about 1 percent to about 4 percent of the repeating units in the isoprene-butadiene rubber are units of 3-polyisoprene, where from about 25 percent to about 40 percent e the repeating units in the isoprene-butadiene rubber are 1,4-polyisoprene units, wherein the isoprene-butadiene rubber has a glass transition temperature that is within the range of about -90 to about C. 75SC and wherein the isoprene-butadiene rubber has a Mooney viscosity that is within the range of about 55 to about 140. The IBR used in the tire tread rubber blends of this invention is prepared by polymerization by solution using an organolithium initiator. The process used to synthesize this IBR is conducted as a continuous process that is carried out at a temperature that is within the range of about 70aC to about 140aC. The gel buildup can be inhibited by conducting such polymerizations in the presence of a vestigial amount of a polar modifier, such as N, N, N ', N'-tetramethylethylenediamine (TMEDA).

Detailed Description of the Invention The IBR used in the tire tread rubber blends of this invention can be synthesized by solution polymerization using the process described in U.S. Patent 5,405,927. The teachings of U.S. Patent 5,405,927 are hereby incorporated by reference in their entirety. These solution polymerizations will normally be carried out in a hydrocarbon solvent which may be one or more aromatic, paraffinic or cycloparaffinic compounds. These solvents will normally contain from 4 to 10 carbon atoms per molecule and will be liquid under the polymerization conditions. Some representative examples of suitable organic solvents include pentane, isooctane, cyclohexane, normal hexane. benzene, toluene, xylene, ethylbenzene and the like, alone or as a mixture. In the solution polymerizations used in the synthesis. the IBR will usually be from about 5 to about 35 weight percent of monomers in the polymerization medium. These polymerization media, of course, are comprised of the organic solvent, 1,3-butadiene monomer and isoprene monomer. In most cases, it will be preferred that the polymerization medium contain from 10 to 30 weight percent of monomers It is generally more preferred that the polymerization medium contain 20 to 25 weight percent monomer. The monomer loading compositions used in the synthesis of the IBR used in the tire tread rubber compounds of this invention will typically contain from about 20 weight percent to about 50 weight percent isoprene and about 20 weight percent. 50 weight percent to about 80 weight percent 1,3-butadiene monomer. It is typically preferred that the monomer charge composition contain from about 25 weight percent to about 35 weight percent isoprene and from about 65 weight percent to about 85 weight percent 1,3-butadiene .

The IBR is typically synthesized on a continuous basis. In said continuous process, the monomers and an organolithium initiator are fed continuously into a reaction vessel or series of reaction vessels. The pressure in the reaction vessel is typically sufficient to maintain a substantially liquid phase under the conditions of the polymerization reaction. The reaction medium will generally be maintained at a temperature that is within the range of about 70aC to about 140aC through the copolymerization. This is generally preferred so that the copolymerization is conducted in a series of reaction vessels and so that the reaction temperature is increased from the reaction vessel to the reaction vessel as the polymerization proceeds. For example, it is desirable to use a two-reactor system wherein the temperature in the first reactor is maintained within the range of about 70aC to 90aC and where the temperature in the second reactor is maintained within the range of about 90aC at approximately 100aC. Organolithium compounds that can be used as initiators include organomonolithium compounds and organomonofunctional lithium compounds. The multi-functional organ liquid compounds will typically be organodilithium compounds or organotrilithium compounds. Some representative examples of multifunctional organolithium compounds include 1,4-dilithiobutane, 1, 10-dilithiodecane, 1, 20-dilithioeicosane, 1-dithioxybenzene, 1,4-dilithioeicosane, 1,4-dilithiobenzene, 1,4-dilithylaphthalene, 8,1-dithioxythracene, 1,2-dilithio- l, 2-diphenylethane, 1,3,5-tritylthiopentane, 1,5, 15-tritylthioeicosane, 1,3,5-tritylthiocyclohexane, 1,3,5,8-tetraliiodecane, 1,5,10, 20-t-tralitioeicosane, 1, 2, 4, 6-tetralithiocyclohexane,, -dilithiobiphenyl and the like. The organolithium compounds that can be used are usually organomonolithium compounds. Preferred organolithium compounds can be represented by the formula R-Li, wherein R represents a hydrocarbyl radical containing from 1 to about 20 carbon atoms. Generally, these monofunctional organolithium compounds will contain from 1 to about 10 carbon atoms. Some representative examples of organolithium compounds that may be employed include methylthio, ethyllithium, isopropyllithium, n-butyllithium, sec-butyllithium. n-octyllithium, tert-octyllithium, n-decyllithium, phenyllithium, 1-naphthylthio, 4-butylphenyllithium, p-tolylthio, 1-naphthylthio, 4-butylphenyllithium, p-tolylthio, 4-phenylbutylthio, cyclohexylthio, 4-butylcyclohexillithium and 4-cyclohexylbutyllithium. The amount of organolithium initiator employed will depend on the molecular weight that is desired for the IBR being synthesized. An amount of organolithium initiator will be selected to result in the production of IBR having a Mooney viscosity ML1 + 4 which is within the range of 55 to 140. The amount of organolithium initiator will preferably be selected to result in the production of IBR having a Mooney viscosity ML1 + 4 which is within the range of 60 to 100. The amount of organolithium initiator most preferably will be selected to result in the production of an IBR having a Mooney viscosity ML1 + 4 which is IBR most preferably will have a Mooney viscosity ML1 + 4 which is within the range of about 80 to about 85. As a general rule in all anionic polymerizations, the molecular weight ( Mooney viscosity) of the polymer produced is inversely proportional to the amount of catalyst used. As a general rule, from about 0.01 to about 1 phm (parts per hundred parts of monomer by weight) of the organolithium compound will be employed. In most cases, it will be preferred to use from about 0.015 to about 0.1 phm of the organolithium compound, with it being more preferred to use from about 0.015 phr to 0.07 phm of the organolithium compound. To inhibit gelation, it is important to carry out these polymerizations in the presence of a vestigial amount of a polar modifier, such as N, N, N ', N'-tetramethylethylenediamine (TMEDA). Due to this reason, it is highly desirable to continuously feed a polar modifier to the reaction vessel used. The ethers and tertiary amines that act as Lewis bases are representative examples of polar modifiers that can be used. Some specific examples of typical polar modifiers include diethyl ether, di-n-propyl ether, diisopropyl ether, di-n-butyl ether, tetrahydrofuran, dioxane, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, tri ethylamine, triethylamine, N, N, N ", N '-tetramethylethylene diamine, N-methyl1 morpholine, N-ethyl morpholine, N-phenyl morpholine and the like, Dipiperidinoethane, dipyrrolidinoethane, tetraraethylene diamine, diethylene glycol, dimethyl ether, TMEDA and tetrahydrofuran are representative of highly preferred modifiers, US Patent 4,022,959 discloses the use of tertiary ethers and amines as polar modifiers in greater detail, optionally , 1,2-butadiene can also be continuously fed to the reaction zone, 1,2-butadiene will typically be present in the polymerization medium at a at a concentration that is within the range of 10 to about 500 ppm (parts per million parts). It is generally preferred that 1,2-butadiene be present at a level that is within the range of about 50 ppm to about 300 ppm. It is generally more preferred that 1,2-butadiene be present at a level that is within the range of about 100 ppm to about 200 ppm. The polar modifier will typically be present in a molar ratio of the molar modifier to the organolithium compound which is within the range of about 0.01: 1 to about 0.2: 1, a molar ratio of polar modifier to the organolithium initiator of more than approximately 0.2: 1 should not be exceeded because the polar modifier acts to increase the glass transition temperature of the IBR produced.

To maintain the glass transition temperature of the IBR within the desired range of about -90aC to about -75aC, the amount of polar modifier used must be the minimum amount required to inhibit gelation. A molar ratio of polar modifier to organolithium compound of more than about 0.2: 1 will typically not be exceeded because such high polar modifier ratios to the organolithium compound can result in the IBR produced having a higher glass transition temperature -70aC. As a general rule, a molar ratio of polar modifier to organolithium compound that is within the range of about 0.05: 1 to about 0.15: 1 will be employed. It is typically more preferred that the molar ratio of polar modifier to organolithium compound is within the range of about 0.08: 1 to about 0.12: 1. After a monomer conversion of about 70 percent to about 100 percent is achieved, the living intermediate polymer can optionally be partially copied with divinylbenzene, tin tetrachloride or silicon tetrachloride. This is typically done in a second reaction vessel. For example, the living intermediate polymer can be pumped from a first reaction vessel to a second reaction vessel where the coupling agent is added to the polymerization medium. The coupling agent is preferably added after a reaction agent has been reached. monomer conversion from 72 percent to 90 percent and more preferably added after a monomer conversion of 75 percent to 85 percent has been achieved. The coupling agent is added at a level which is sufficient to leap the molecular weight of the polymer to the desired degree without killing all the intermediate intermediate polymer chains. In the absence of coupling agents, all polymer chains can grow to completion (but molecular weight jump can not occur). At a molar ratio of organolithium initiator to coupling agent of 4 or greater, complete coupling is possible; but because the coupling is by termination, further polymerization and higher conversion levels can not be achieved. The optimum level, of course, is between these two extremes. As a general rule, the molar ratio of organolithium compound to the coupling agent will be within the range of about 6: 1 to about 20: 1. Molar ratios of the organolithium compound to the coupling agent which are within the range of about 8: 1 to about 12: 1 are preferred because they induce sufficient coupling to achieve the desired increase in molecular weight, while leaving a adequate number of live chains to reach acceptable conversion levels. Since there are fewer living chains after copulation, those that are still alive reach a higher molecular weight than they would have otherwise achieved had the coupling agent not been used, since the living intermediate polymer is only partially copied, Live polymer chains still exist after the copulation step. Consequently, in such a scenario, the copolymerization is allowed to continue with the polymer chains still alive increasing in molecular weight as the copolymerization continues. The copolymerization is then allowed to continue in this step until a conversion is reached in excess of about 90 percent. It is preferred that the conversion be in excess of about 95 per cent with essentially quantitative conversions of more than about 99 percent being preferably reached.The IBR produced is then recovered from the organic solvent.The IBR can be recovered from the organic solvent by techniques conventional, such, as decanting. filtration, centrifugation and the like. It is often desirable to precipitate the IBR from the organic solvent by the addition of lower alcohols containing from 1 to about 4 carbon atoms to the polymer solution. The lower alcohols suitable for IBR precipitation of the polymer cement include methanol, ethanol, isopropyl alcohol, n-propyl alcohol and t-butyl alcohol. The use of lower alcohols to precipitate the IBR from the polymer cement'- also "exterminates" the IBR chains alive by inactivating the lithium end groups. After the IBR is recovered from the organic solvent, steam scrubbing can be employed to reduce the level of volatile organic compounds in rubber. The IBR used in the tire tread rubber blends of this invention is characterized in that it is comprised of repeating units that are derived from about 20 weight percent to about 50 weight percent isoprene and from about 50 weight percent to about 80 weight percent 1,3-butadiene, wherein the repeating units derived from isoprene and 1,3-butadiene is in essentially random order, wherein from about 3 percent to about 10 percent of the repeating units in the rubber are 1,2-polybutadiene units, wherein from about 50 percent to about 70 percent of the repeating units in the rubber are 1, 4-polybutadiene units, wherein from about 1 percent to about 4 percent of the repeating units in the rubber are units of 3-polyisoprene, where from about 25 percent to about 40 percent The repeating units in the polymer are 1,4-polyisoprene units, wherein the rubber has a glass transition temperature which is within the range of about -90 aC to about -75 aC and where the rubber has a Mooney viscosity which is within the range of about 55 to about 140. The isoprene-butadiene rubber will preferably have a Mooney viscosity ML1 + 4 which is within the range of about 60 to about 100. The repeating units IBR will preferably be derived from about 30 weight percent to about 40 weight percent isoprene and from about 60 weight percent to about 70 weight percent 1,3-butadiene, wherein the repeating units derived from isoprene and 1,3-butadiene are essentially in random order, where from about 5 percent to about 8 percent of the repeating units in the rubber are joined. 1, 2-polybutadiene ades, wherein from about 55 percent to about 65 percent of the repeating units in the rubber are 1,4-polybutadiene units, wherein from about 1 percent to about 3 percent One hundred of the repeating units in the rubber are 3-polyisoprene units, wherein from about 28 percent to about 36 percent of the repeating units in the polymer are 1, 4-polyisoprene units, wherein the rubber has a glass transition temperature that is within the range of about -85SC to about -80aC and where the rubber has a Mooney viscosity that is within the range of about 70 to about 90, the repeating units which are derived from isoprene or 1,3-butadiene differ from the monomer from which they were derived in that a double bond was consumed by the polymerization reaction. The repeating units derived from isoprene and 1,3-butadiene are in the IBR in an essentially random order. The term "random" as used herein means that the repeating units that are derived from isoprene are well dispersed throughout the polymer and are mixed with the repeating units that are derived from it., 3-butadiene. for purposes of this "random" patent it means that more than 60 percent of the isoprene in the IBR is present in blocks of three or less repeating units. For the purposes of this patent application, the polymer microstructures are determined by nuclear magnetic resonance (NMR) spectrometry. The glass transition temperatures are determined by differential scanning calorimetry at a heating rate of 10aC per minute and molecular weights are determined by gel permeation chromatography (GPC) .The IBR of this invention is particularly valuable for use in making pneumatic tires that have better traction characteristics IBR is mixed with styrene-butadiene rubber (SBR) when making these tire tread compounds The SBR used is typically synthesized by an emulsion polymerization process. by emulsion polymerization it is frequently referred to as an "emulsion styrene-butadiene rubber" or "emulsion SBR." These mixtures will normally contain from about 50 phr (parts by weight per 100 parts by weight of rubber) to about 85 phr. from the SBR and from about 15 phr to about 50 phr from the IBR. Wheel bearings will typically contain more than about 55 phr to about 80 phr of the SBR and about 20 phr to about 45 phr of the IBR. It is preferred that the tread rubber compounds of this invention contain from about 60 phr to about 75 phr of the SBR and from about 25 phr to about 40 parts of the IBR. The most preferred tire tread rubber blends will contain from about 65 phr to about 70 phr of the SBR and from about 30 phr to about 35 phr of the IBR, these tread rubber mixtures containing IBR from this invention can be compounded using conventional ingredients and standard techniques. For example, mixtures containing IBR will typically be mixed with carbon black, sulfur, fillers, accelerators, oils, waxes, scorch inhibiting agents and processing aids. In most cases, rubber mixtures containing IBR will be composed of sulfur and / or a sulfur-containing compound, at least one filler, at least one accelerator, at least one antidegradant, at least one processing oil, zinc oxide. optionally a tackifying resin, optionally a reinforcing resin, optionally one or more fatty acids, optionally a peptizer and optionally one or more singe inhibiting agents. These mixtures will normally contain about 0.5 to 5 phr (parts per hundred parts of rubber by weight) of sulfur and / or a sulfur-containing compound with 1 phr to 2.5 phr being preferred. It may be desirable to use insoluble sulfur in cases where flowering is a problem, typically 30 to 150 phr of at least one filling will be used in the blend with 50 to 90 phr being preferred. Typically it is more preferred to include from 60 phr to about 80 phr of fillers in the tread rubber compound. In most cases, at least some of the carbon black will be used in the filling. The filling, of course, can be entirely carbon black. Silica can be included in the filler to improve wear resistance and heat buildup. The clays and / or talc can be included in the fill to reduce the cost. The mix will also normally include 0.1 to 2. 5 phr of at least one accelerator with 0.2 to 1.5 phr being preferred. Antidegradants, such as antioxidants and antiozonants, will usually be included in the mixture in amounts ranging from 0.25 to 10 phr with amounts on the scale of 1 to 5 phr being preferred! The processing oils will generally be included in the mixture in amounts ranging from 2 to 100 phr with amounts varying from 5 to 50 phr being preferred. The IBR-containing mixtures of this invention will also typically contain 0.5 to 10 phr of zinc oxide and 1 to 5 phr being preferred. These mixtures may optionally contain from 0 to 10 phr of tackifying resins, 0 to 10 phr of reinforcing resins, 1 to 10 phr of acid grades, 0 to 2.5 phr of peptizers and 0 to 1 phr of singe inhibiting agents. The silica that can be optionally used in the tread rubber compounds of this invention can be any of the common silicon pigments used in rubber composition applications, such as pyrogenic silicon pigments and precipitates (silica), even when they prefer precipitated silicas. Silicon pigments are preferred; for example, precipitated silicas such as those obtained by the acidification of a soluble silicate (sodium silicate). These silicas could 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 on a scale of about 50 to about 300 square meters per gram. The BET method for measuring surface area is described in the Journal of the American Chemical Society, Volume 60, page 304 (1930). The silica can also typically be characterized having a dibutyl phthalate (DBP) absorption value on a scale of from about 100 to about 400 and more usually about 150 to about 300. It could be expected that the silica would have an average final particle size, per example, on the 0.01 to 0.05 micron scale as determined by the electron microscope, even though 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, for example only herein, and without limitation, silicas commercially available from PPG Industries under the trademark Hi-Sil with designations 210, 243, etc; the silicas available from Rhone-Poulenc, for example, with designations of Z1165MP and Z165GR and silicas available from Degussa AG with, for example, designations VN2 and VN3.

In cases where the silica is included in the tread compound, the silica coupling agent will also normally be included in the mixture to fully realize the full benefit of including silica in the mixture. The silica coupling agent will typically be an organosilicon compound containing sulfur. Examples of suitable sulfur-containing organosilicon compounds are of the formula: Z-Alk-SB-Alk-Z (I) wherein Z is selected from the group consisting of: R1 R1 R2 Si- Si- -R2 If R2 R2 R2 R2 where R1 is an alkyl group of 1 to 4 carbon atoms, cyclohexyl or phenyl; wherein R2 is alkoxy of 1 to 8 carbon atoms or cycloalkoxy of 5 to 8 carbon atoms; and wherein Alk is a divalent hydrocarbon of 1 to 18 carbon atoms and n is an integer of 2 to 8. Specific examples of sulfur-containing organosilicon compounds that can be used in accordance with the present invention include: 3, 3 disulfide. '-bis (trimethoxysilylpropyl), 3, 3'-bis (triethoxysilylpropyl tetrasulfide. 3, 3'-bis (triethoxysilylpropyl) octasulfide, 3,3 '-bis (trimethoxysilylpropyl) tetrasulfide, 2'-bis (triethoxysilylethyl) tetrasulfide, 3,3'-bis (trimethoxysilylpropyl) trisulfide, trisulfide 3, 3'-bis (triethoxysilylpropyl), 3,3'-bis (tributoxysilylpropyl) disulfide, 3, 3'-bis (trimethoxysilylpropyl) hexasulfide, 3,3 '-bis (rimethoxysilylpropyl) octasulfide, tetrasulfide 3 , 3'-bis (trioctoxysilylpropyl), 3,3'-bis (trihexoxysilylpropyl) disulfide, 3,3'-bis (tri-2"-ethylhexoxysilylpropyl) trisulfide, 3, 3'-bis (triisooctoxysilylpropyl) tetrasulfide, 3, 3'-bis (tri-t-butoxysilylpropyl) disulfide, 2,2'-bis (methoxy diethoxy silyl ethyl) tetrasulfide, 2,2 '-bis pentasulfide (tripropoxysilylethyl), 3,3' -bis tetrasulfide riciclonexoxysilylpropyl), 3,3'-bis (tricyclopentoxysilylpropyl) trisulfide, 2,2'-bis (tri-2"-methylcyclohexoxysilylene ilo) tetrasulfide, bis (rimethoxysilylmethyl) tetrasulfide, 3-methoxy ethoxy propoxysilyl 3'-tetrasulfide dietoxybutoxysilylpropyl, 2,2'-bis (dimethylmethoxysilylethyl) disulfide, 2,2'-bis (di-ethyl sec. butoxysilylethyl) trisulfide, tetrasul 3, 3'-bis (methylbutylethoxysilylpropyl) furan, 3,3'-bis (di t -butylmethoxysilylpropyl) tetrasulfide, 2'-bis (phenylmethyl methylsilylethyl) trisulfide, 3,3'-bis (diphenyl) tetrasulfide isopropoxysilylpropyl), 3,3'-bis (diphenyl cyclohexoxysilylpropyl) disulfide, 3,3'-bis (dimethyl ethylmercaptosilylpropyl) tetrasulfide, 2,2'-bis (methyl dimethoxysilylethyl) trisulfide, 2,2'-bis tetrasulfide (methyl ethoxypropoxylysilyl), 3,3'-bis (di-methyl methoxysilylpropyl) tetrasulfide, 3, 30-bis (ethyl di-sec. butoxysilylpropyl, disulphide 3, 3'-bis (propyldiethoxysilylpropyl), 3,3'-bis (butyldimidoxysilylpropyl) trisulfide, 3,3'-bis (phenyl dimethoxysilylpropyl) tetrasulfide, 3-phenyl ethoxybutoxylysilyl 3'-trimethoxysilylpropyl tetrasulfide, tetrasulfide 4, '-bis (trimethoxysilylbutyl), 6,6'-bis (triethoxysilylhexyl) tetrasulfide, 12,12'-bis (triisopropoxysilyl dodecyl) disulfide, 18,18'-bis (trimethoxysilyloctadecyl) tetrasulfide, tetrasulfide 18, 18'-bis (ripropoxysilyloctadecenyl), 4,4'-bis (rimethoxysilyl-buten-2-yl) tetrasulfide, tetrasulfide, β-bis (trimethoxysilylcyclohexylene), 5, 5'-bis (dimethoimide) trisulfide ? tilsilylpentyl), 3,3 '-bis (trimethoxysilyl-2-methylpropyl) tetrasulfide and 3,3' - (dimethoxyphenylsilyl-2-methylpropyl) disulfide. Some of the preferred sulfur-containing organosilicon compounds include the 3, 3'-bis (trimethoxy or triethoxy silylpropyl) sulfides. A highly preferred compound is 3, 3'-bis (riethoxysilylpropyl) tetrasulfide, Therefore, as to Formula I, preferably Z is R2 Si-R2 wherein R2 is an alkoxy of 2 to 4 carbon atoms, with 2 carbon atoms being particularly preferred; Alk is that divalent hydrocarbon of 2 to 4 carbon atoms with 3 carbon atoms being particularly preferred; and n is an integer from 3 to 5 with 4 being particularly preferred. Another highly preferred class of sulfur-containing organosilicon compounds are the 3, 3'-bis (trimethoxy or triethoxy silyl propyl) polysulfides; such as 3, 3'-bis (riethoxysilylpropyl polysulphide.) The amount of sulfur-containing organosilicon-containing compound of Formula I in the rubber composition will vary, depending on the level of silica used.Speaking in general terms, the amount of the compound of Formula I will vary from about 0.01 to about 1.0 parts by weight per part by weight of the silica, preferably, the amount will vary from about 0.02 to about 0.4 parts by weight per part by weight of The silica more preferably, the amount of the compound of the Formula I will vary from about 0.05 to about 0.25 parts by weight per part by weight of the silica.It should be noted that the silica coupler can be used in conjunction with carbon black; to say, pre-mix with carbon black before the addition to the rubber composition, and said carbon black must be included in the aforementioned amount of carbon black for the rubber composition formulation. In cases where the tire tread rubber formulation includes silica and a silica coupling agent, it will typically be mixed using a thermomechanical mixing technique. The mixing of the tire tread rubber formulation can be achieved by methods known to those skilled in the rubber mixing art. For example, the ingredients are typically mixed in at least two stages; to say, at least a non-productive stage followed by a stage of productive mixing. Final curatives that include sulfur vulcanization agents are typically mixed in the final stage which is conventionally referred to as the "productive" mixing step in which the mixing typically occurs at a temperature, or final temperature, lower than the temperature (s). (s) of mixing than the previous non-productive mixing stage (s). The rubber, silica and organosilicon containing sulfur, and carbon black, if used, are mixed in one or more non-productive mixing stages. The terms "non-productive" and "productive" mixing stages are well known to those who have experience in the rubber mixing art. The vulcanizable sulfur rubber composition containing the organosilicon compound containing sulfur, vulcanizable rubber and generally at least part of the silica must be subjected to a thermomechanical mixing step. The thermomechanical mixing step generally comprises mechanical work in a mixer or extruder for an appropriate period of time in order to produce a rubber temperature between 140aC and 190aC. The appropriate duration of thermo-ecological work varies as a function of the operating conditions and the volume and nature of the components. For example, thermomechanical work can be for a duration of time that is within the range of about 2 minutes to about 20 minutes. Normally it will be preferred that the rubber reaches a temperature that is within the range of about 145 ° C to about 180 ° C and that it is maintained at that temperature for a period of time which is within the range of about 4 minutes to about 12 minutes. It will normally be more preferred that the rubber reaches a temperature that is within the range of about 155aC to about 170aC and is maintained at that temperature for a period of time that is within the range of about 5 minutes to about 10 minutes. . The IBR-containing rubber blends of this invention can be used in tire treads in conjunction with ordinary tire manufacturing techniques. The tires are built using conventional procedures with the IBR simply being replaced by the rubber compounds typically used as the tread rubber. After the tire has been built with the mixture containing IBR, it can be vulcanized using a normal tire curing cycle. Tires made in accordance with this invention can be cured through a wide range of temperatures. However, it is generally preferred that the tires of this invention are cured at a temperature ranging from about 132aC (270SF) to about 166aC (330aF). It is more typical that the tires of this invention are cured at a temperature ranging from about 143SC (290aF) to about 154SC (310aF). It is generally preferred that the curing cycle used to vulcanize the tires of this invention have a duration of about 10 to about 14 minutes with a curing cycle of about 12 minutes being more preferred. This invention is illustrated by the following examples which are merely for the purpose of illustration and should not be construed as limiting the scope of the invention or the manner in which it can be practiced. Unless specifically indicated otherwise, all parts and percentages are given by weight.

Example 1 In this experiment, the IBR was synthesized in a continuous system of two reactors (10 liters each) at 95aC. A premix containing isoprene and 1,3-butadiene in hexane was charged to the first polymerization reactor, continuously, at a rate of 100 grams / minute. The premix monomer solution containing a ratio of isoprene to 1,3-butadiene of 30:70 and had a total monomer concentration of d © 11 percent. Polymerization was initiated by adding a 0.107 M solution of N-beryl lithium to the first reactor at a rate of 0.32 grams / minute. The residence time for both reactors was adjusted to 1.16 hours. The average monomer conversions were determined as being 62 percent for the first reactor and 93 percent for the second reactor. The polymerization medium was continuously boosted from the second reactor to a holding tank containing methanol (such as a short cutter) and an antioxidant, The resulting polymer cement was then steam-stripped and recovered. vacuum until a temperature of 60aC. The distribution of isoprene in 1 IBR was randomized since the isoprene and butadiene monomers were pumped continuously to the reactors. It was determined that 1 polymer has a glass transition temperature at -84 ° C and that it has a Mooney viscosity ML-4 of 85. It was also determined to have a microstructure containing 6 percent 1,2-polybutadiene units, 60 percent of 1,4-polybutadiene units, 32 percent of 1,4-polyisoprene units and 2 percent of 3-polyisoprene units.

Example 2 In this experiment, isoprene-butadiene copolymer (IBR) having a low vinyl content was synthesized using an unmodified n-butyl-lithium catalyst. In the method used, 10,900 grams of a dry silica / molecular sieve / alumina premix containing isoprene and 1,3-butadiene in hexane was charged in a five gallon 819 liter reactor). The premix monomer solution contained a ratio of 1 to 3-butadiene to 1 -3-butadiene and the total monomer concentration was 19 percent. The solution of the mono-β-premix had previously been cleaned of impurities with a solution of n-butyl-lithium. Polymerization was initiated by the addition of 4.18 ml of a 1.6 M solution of n-butyllithium. The ST reactor maintained at a temperature of approximately 65aC until an essentially complete monomer conversion was achieved which took approximately three hours. The polymerization medium was then cut short with ethanol and the polymer was stabilized with 1 phr (parts per hundred parts of polymer) of an antioxidant.

After evaporation of 1 hexane, 1 recovered polymer was dried in a vacuum oven at 50 ° C. The isoprene-butadiene copolymer produced was determined that. has a glass transition temperature (Tg) at -88aC.

It was also determined that it has a mycostructure containing 7 percent of 1,2-polybutadiene units, 68 percent of 1,4-polybutadiene units, 1 percent of 3,4-polyisoprene units, and 24 percent of 1-polyisoprene units Example 3-7 The procedure described in Example 1 was used in these examples, except that the ratio of isoprene to butadiene was changed from 25:75 to 35:65, 40:60, 50:50, 60:40 and 75:25. The Tgs and microstructures of the resulting isoprene-butadiene copolymers are listed in the Table. í Table Copolymer d © Isopreno-Butadiene Low Tg Micro structure (%) Ex. Relations Tg 1,2- • PBd 1, 4-PBd 3,4-PI 1,4-PI No, Isop / Bd (fiC) - - 2 25:75 -88 7 68 1 24 3 36:65 -86 6 59 3 32 4 40:60 -84 6 54 3 37 50:50 -di 5 46 3 46 6 60: 40 -77 4 37 4 55 7 75:25 -72 4 24 4. 68 Example 8 In this experiment, a rim was constructed using the tread compound of this invention and compared to a rim that was constructed using a conventional tread compound. The conventional tread compound used in the control tire was a mixture of 70/30 d © SBR emulsion with BudeneíR) 1207 cis-1 rubber, 4-polybutadiene © levado. The emulsion SBR used contained 25 percent bound styrene and had a glass transition temperature of -52aC. The experimental rim was constructed using a tread compound that was a mixture of 70/30 dL SBR emulsion with an IBR. The IBR had a microstructure that included 9 percent repeat vinyl polybutadiene units, 29 percent repeat units of cis-polybutadiene, 3 percent repeat units of 3,4-polyisoprene, and 29 percent repeat units. repeat of cis-polyisoprene. The IBR had a glass transition temperature of -83aC, a Mooney viscosity ML1 + 4 of 82, a dilute solution viscosity of 3.08, an average molecular weight of 208,000 and a weight average molecular weight of d 484,000 . The experimental tire had a projected mileage of 46,356 miles (85,897.67 km) compared to the control tire that had a projected mileage of 51,475 miles (95,383.18 km). The experimental rim of this invention was also compared with the conventional rim to evaluate the traction characteristics. The results of this comparison are shown in Table II.

Frame II Traction Trailer Test Condition Rim © Pilot Control Rim 37.07 km / h Maximum, Wet 120+ 128+ 74.12 km / h Maximum, Wet 127+ 132+ 111.18 km / h Maximum, Wet 111+ 118+ 74.12 km / h Maximum, Dry 106+ 106+ 37.07 km / h Sliding, Wet 125+ 127+ 74.12 km / h Sliding, Wet 123+ 126+ 111.18 km / h Sliding, Wet 113+ 114+ 74.12 km / h Sliding, Dry 109+ 109+ As can be seen from Table II, the experimental rim constructed with the tread compound of this invention proved to have superior wet traction and dry traction equivalent to the control rim. The experimental tires were also compared to the control tires in a subjective wet driving test. In this test, the tires were mounted on a test car and subjectively evaluated for various characteristics. With the experimental tires, the test driver was able to achieve a better average lap time (57.12 seconds) than could be achieved with the control tires (57.69 seconds). The test driver reported that "the experimental tires had a better response to cornering, oversteer, oversteer without acceleration, oversteer with connected power, low turn without acceleration, traction transition, lateral grip and interruption action than the control tires. However, the test driver reported that the experimental tires exhibited equivalent acceleration traction and slightly lower resistance to hydroplaning of straight and lateral lines.In any case, the total characteristics of wet traction of the experimental tire made with compound 1 The tread rubber of this invention were superior to those of the control tire, although certain modalities and representative details have been shown in order to illustrate the present invention, it will be evident to those experienced in this field that they can be made various changes and modifications in it without abandoning r the scope of the present invention.

Claims (10)

1. - A rim tread rubber composition characterized in that it is comprised of (a) 50 phr at 85 phr of a styrene-butadiene rubber and (b) 15 phr at 50 phr d © a rubber isoprene- butadiene, wherein the isoprene-butadiene rubber is comprised of repeating units which are derived from 20 weight percent to 50 weight percent isoprene and 50 weight percent to 80 weight percent 1, 3-butadiene, wherein the repeating units derived from isoprene and 1,3-butadiene is in essentially random order, wherein from 3 percent to 10 percent of the repeating units in the isoprene-butadiene rubber are 1, 2-polybutadiene units, where from 50 percent to 70 percent of the repeating units in the isoprene-butadiene rubber are d © 1, 4-polybutanediun, where n © 1 through One hundred to four percent of the repeating units in isoprene-butadiene rubber are 3, 4-po units liisoprene, where from 25 percent to 40 percent of the repeating units in the isoprene-but'adiene rubber are 1,4,4-polyisoprene units, where isoprene-butadiene rubber has a temperature of Transition of glass that is within the range of -90aC to -? 5aC and where isoprene-butadiene rubber has a Mooney viscosity that is within the range of 55 to 140. 2.- A rubber composition of rim tread band as specified in claim 1, characterized in that the composition is further comprised of 30 phr to 150 phr of a filler. 3. A tire tread rubber composition as specified in claim 2, characterized in that the repeating units in the isoprene-butadiene rubber are derived from 30 weight percent to 40 weight percent of isoprene and 60 percent by weight and 70 percent by weight d © 1, -butadiene, where from 5 percent to 8 percent d © the repeating units in isoprene-butadiene rubber are units of 1, 2-polybutadiene, where from 55 percent to 65 percent of the repeat units in the isoprene-butadiene rubber are units of 1-polybutadiene, where from 1 percent to 3 percent of the units of Repetition in 1 isoprene-butadiene rubber are 3-4-polyisoprene units, and where from 28 percent to 36 percent of the repeating units in n-isoprene-butadiene rubber are units of 1, 4- polyisoprene. 4. A composition for tire tread rubber as specified in claim 3, characterized in that 1 isoprene-butadiene rubber has a Mooney viscosity ML 1 + 4 which is within the range of 60 to 100 and, where the rubber has a glass transition temperature that is within the range of -85SC to -80aC. 5. A rim tread rubber composition as specified in claim 3, characterized in that more than 60 percent of the isoprene in the isoprene-butadiene rubber is present in blocks of three or less repeating units. , where isoprene-butadiene rubber is present in an amount that is within the range of 20 phr to 45 phr and, where the. Stuffing is present at a level that is within the range of 50 phr to 90 phr. 6. A tire tread rubber composition as specified in claim 5, characterized in that 1 isoprene-butadiene rubber is present in an amount that is within the range of 25 phr to 40 phr, and where the filler is present at a level that is within the range of 60 phr to 80 phr. 7. A tire tread rubber composition as specified in claim 5, characterized in that the isoprene-butadiene rubber is present in an amount that is within the range of 30 phr to 35 phr and , where isoprene-butadiene rubber has a Mooney viscosity ML 1 + 4, which is within the range of 70 to 90. 8. A tire tread rubber composition as specified in claim 2, characterized in that the styrene-butadiene rubber TS a styrene-butadiene rubber in emulsion, wherein the filler includes silica and wherein the composition is comprised in addition to a silica coupling agent selected from the group consisting of 3, 3'-bis (trimethoxysilylpropyl) disulfide, tri-sulfide of 3, 3'-bis (riethoxysilylpropyl), 3, 3'-bis (triethoxysilylpropyl) octasulfide, 3, 3'-bis (trimethoxysilylpropyl) tetrasulfide, 2,2'-bis (triethoxysilylethyl) tetrasulfide, 3, 3 'trisulfide -bis (trimethoxysilylpropyl), 3,3'-bis (triethoxysilylpropyl) trisulfide, •. 3, 3'-bis (ributoxysilylpropyl) ddiissuullffuurroo, 3, 3'-bis (rim? toxysilylpropyl) hexasulfuro, 3,3'-bis (trim? toxysilylpropyl) octasulfide, 3,3 '-bis tetrasulfide trioctoxysilylpropyl), disulfide 3, 3'-bis (trih? Xoxysilylpropyl), trisulphide of, 3'-bis (tri-2"-ethylhexosilylpropyl), 3,3'-bis (triisooctoxysilylpropyl) tetrasulfide, 3,3'-bis (tri) disulfide -5-butoxysilylpropyl), 2,2'-bis (methoxy diethoxy silyl ethyl) tetrasulfide, 2,2'-bis (ripropoxysilylethyl) pentasulfide, 3,3'-bis (tricyclonexoxysilylpropyl) tetrasulfide, trisulfide of 3, 3 '-bis (tricyclopentoxysilylpropyl), 2,2'-bis (tri-2"-me ilcyclohexoxysilyl-ethyl) tetrasulfide, bis (rimethoxysilylmethyl) tetrasulfide, 3-methoxy-ethoxy-propoxysilyl 3" -dietoxybutoxy-silylpropyl tetrasulfide, 2-disulfide 2'-bis (dimethylmethoxysilylethyl), 2,2'-bis (dimethyl-sec.butoxysilylethyl) trisulfide, 3,3'-bis (methylbutylethoxysilylpropyl) tetrasulfide, 3,3'-bis (t) sulfide -butylmethoxysilylpropyl), 2,2'-bis (phenylmethylmethoxysilylethyl) trisulfide, 3'3'-bis (diphenyl isopropoxysilylpropyl) tetrasulfide, 3-disulfide, 3'-bis (diphenyl cyclohexoxysilylpropyl), 3,3'-bis (dimethyl ethylmercaptosilyl-propyl) tetrasulfide, 2,2'-bis (methyl dimethoxysilyl-ethyl) trisulfide, 2,2'-bis tetrasulfide (m © useful ethoxypropoxy silylethyl), 3,3'-bis (diethylmethoxysilylpropyl) tetrasulfide, 3,3'-bis (ethyl di-sec. butoxysilylpropyl), 3,3'-bis (propyldiethoxysilylpropyl) disulfide, 3,3'-bis (butyldimethoxysilylpropyl) trisulfide, 3,3'-bis (phenyldimethoxysilylpropyl) tetrasulfide, t? 3-phenyl ethoXibutoXysilyl 3'-trim? toxysilylpropyl transulfide, tetrasulfide of 4,4'-bis (trimethoxysilylbutyl), 6,6'-bis (ri? Toxysilylhyl) tetrasulfide, 12,12'-bis (triisopropoxysilyl dodecyl) disulfide, 18,18'-bis (trim? toysilyloctadryl), 18, 18'-bis (ripropoxysilyloctadecenyl) sulfide, 4,4'-bis (trimethoxysilyl-but-n-2-yl) tetrasulfide, 4,4'-bis (trimethoxysilylcyclohexylene) tetrasulfide ), 5, 5'-bis (dimethoxymethylsilylphenyl) trisulphide, 3,3'-bis (dimethoxysilyl-2-methylpropyl) tri-sulphide and 3,3'-bis (dime oxy-n-nyl-2-disulfide) -I ilpropyl). 9. A tire tread rubber composition as specified in claim 8, characterized in that the silica coupling agent is 3, 3-bis (triethoxysilylpropyl) tetrasulfide. 10.- A pneumatic tire for a truck that has an outer circumferential tread, where the tread is a rubber compound lasted with sulfur that is characterized because it is included d? , based on 100 parts by weight of rubber, (a) from 25 to 75 parts of an isoprene-butadiene rubber, the rubber being comprised of repeat units that are derived from 20 weight percent to 50 weight percent of isoprene and 50 weight percent to 80 weight percent 1,3-butadiene, wherein the repeat units derived from isoprene and 1,3-butadiene are essentially in random order, where 3 percent 10 percent of the repeating units in the rubber are 1,2-polybutadiene units, where 50 percent to 70 percent of the repeating units in the rubber are 1, 4-polybutadiene units, where from 1 percent to 4 percent of the repeating units in the rubber are 3,4-polyisoprene units, where from 25 percent to 40 percent of the repeating units in the polymer are units of 1, 4- polyisoprene, where the rubber has a glass transition temperature that is within the scale from -90aC to -75aC, and where? © 1 rubber has a Mooney viscosity that is within the range of 55 to 140; and (b) from 25 to 75 parts of natural rubber. SUMMARY OF THE INVENTION The present invention is based on the unexpected discovery that tires have excellent dry and wet traction characteristics, including wet skid resistance, can be made incorporating certain mixtures of styrene-butadiene (SBR) and rubber d? isoprene-butadiene (IBR) towards' treads of the same without sacrificing greatly the characteristics of rolling resistance and wear of tread. This invention more specifically describes a tire tread rubber composition that is comprised of (a) about 50 'phr to about 85 phr of an SBR and (b) about 15 phr to about 50 phr of an IBR, wherein the IBR is comprised of repeating units that are derived from about 20 weight percent to about 50 weight percent isoprene and from about 50 weight percent to about 80 weight percent of 1, 3-butadiene, wherein the repeating units derived from isoprene and 1,3-butadiene are essentially in random order, wherein from about 3 percent to about 10 percent of the repeating units in the IBR are units of 1, 2-polybutadiene, wherein from about 50 percent to about 70 percent of the repeating units in the IBR are 1, 4-polybutadiene units, where from about 1 percent to about 4 percent. One hundred of the repeating units in the IBR are 3, 4-polyisoprene units, where from about 25 percent to about 40 percent of the repeating units in the IBR are 1, 4-polyisoprene units, T ? where the IBR has a temperature of. glass transition that is on the scale of about -90aC to about -75aC and where the IBR has a Mooney viscosity that is within the range of about 55 to about 140.