Tire tread containing 3,4-polyisoprene rubber

Abstract: This invention reveals a pneumatic tire having an outer circumferential tread wherein said tread is a sulfur-cured rubber composition comprised of, based on 100 parts by weight of rubber, (1) from about 5 to about 50 parts of 3,4-polyisoprene rubber, wherein said 3,4-polyisoprene rubber has (a) a 3,4-isomer content of from 75 percent to 95 percent, (b) a 1,2-isomer content of from 5 percent to 25 percent, (c) a glass transition temperature from 0° to 25°C and (d) a number average molecular weight which is within the range of 30,000 to 180,000; and (2) from about 50 to about 95 parts of a rubbery polymer which is co-curable with the 3,4-polyisoprene rubber. The subject invention further discloses a pneumatic tire having an outer circumferential tread wherein said tread is a sulfur-cured rubber composition comprised of, based on 100 parts by weight of rubber, (1) from about 20 to about 60 parts of natural rubber, (2) from about 5 to about 30 parts of high cis-1,4-polybutadiene rubber, (3) from about 10 to about 50 parts of styrene-butadiene rubber, and (4) from about 5 to about 30 parts of 3,4-polyisoprene rubber, wherein the 3,4-polyisoprene rubber has (a) a 3,4-isomer content of from 75 percent to 95 percent, (b) a 1,2-isomer content of from 5 percent to 25 percent, (c) a glass transition temperature from 0° to 25°C and (d) a number average molecular weight which is within the range of 30,000 to 180,000.

Description: Background of the Invention It is highly desirable for tires to exhibit good traction characteristics on both dry and wet surfaces. However, it has traditionally been very difficult to improve the traction characteristics of a tire without compromising its rolling resistance and tread wear. Low rolling resistance is important because good fuel economy is virtually always an important consideration. Good tread wear is also an important consideration because it is generally the most important factor which determines the life of the tire. The traction, tread wear and rolling resistance of a tire are dependent to a large extent on the dynamic viscoelastic properties of the elastomers utilized in making the tire tread. In order to reduce the rolling resistance of a tire, rubbers having a high rebound have traditionally been utilized in making the tire's tread. On the other hand, in order to increase the wet skid resistance of a tire, rubbers which undergo a large energy loss have generally been utilized in the tire's tread. In order to balance these two viscoelastically inconsistent properties, mixtures of various types of synthetic and natural rubber are normally utilized in tire treads. For instance, various mixtures of styrene-butadiene rubber and polybutadiene rubber are commonly used as a rubber material for automobile tire treads. However, such blends are not totally satisfactory for all purposes. In some cases, 3,4-polyisoprene can be used in tire tread compounds to improve tire performance characteristics, such as traction. For instance, US-A-5,104,941 discloses a method of improving the wet skid resistance of a rubber mixture for a tire tread, comprising adding from 5 to 35 parts by weight of a 3,4-polyisoprene to from 95 to 65 parts by weight of a sulfur-vulcanizable elastomer containing conventional fillers, oils, auxiliaries and vulcanizing agents, wherein said 3,4-polyisoprene is at least partially incompatible with said sulfur-vulcanizable elastomer and has (a) a 3,4-content from 55 to 75 percent, as determined by NMR spectroscopy, (b) a glass transition temperature from 0° to -25°C, determined by differential scanning calorimetry at a heating rate of 10°C/minute, (c) a number average molecular weight Mn, determined by gel permeation chromatography of 220,000 or higher, and (d) an inhomogeneity U of less than 1.8, the inhomogeneity being defined by the equation U=Mw/Mn-1, where Mw and Mn are determined by gel permeation chromatography. US-A-5,104,941 further discloses a method for improving the wet skid resistance of a rubber mixture, comprising adding from 5 to 35 parts by weight of a 3,4-polyisoprene to from 95 to 65 parts by weight of a sulfur-vulcanizable elastomer, further containing conventional fillers, oils, auxiliaries or vulcanizing agents, wherein said 3,4-polyisoprene is at least partially incompatible with said sulfur-vulcanizable elastomer and has (a) a 3,4-content of from 55 to 75 percent, determined by NMR spectroscopy, (b) a glass transition temperature from 0° to -25°C, determined by differential scanning colorimetry (calorimetry) at a heating rate of 10°C/minute, (c) a number average molecular weight, determined by gel permeation chromatography, of from 200,000 to 218,000, and (d) an inhomogeneity U of 1.4 or less. US-A-5,087,668 and US-A-5,300,577 disclose a pneumatic tire having an outer circumferential tread where said tread is a sulfur-cured rubber composition composed of, based on 100 parts by weight rubber, (a) 5 to 35, preferably 10 to 25 parts by weight 3,4-polyisoprene rubber, (b) 20 to 60, preferably 30 to 55 parts by weight cis 1,4-polyisoprene rubber, and (c) 10 to 50 parts by weight of at least one other rubber selected from at least one of solution polymerization formed styrene/butadiene copolymer rubber having a styrene/butadiene ratio in the range of 5/95 to 30/70, preferably 8/92 to 25/75, emulsion polymerization formed styrene/butadiene copolymer rubber having a styrene/butadiene ratio in the range of 10/90 to 60/40, preferably 15/85 to 35/65, cis 1,4-polybutadiene rubber, isoprene/butadiene copolymer rubber having an isoprene/butadiene ratio in a range of 30/70 to 70/30, styrene/isoprene rubber having a styrene/isoprene ratio in a range of 10/90 to 35/65 and styrene/isoprene/butadiene rubber; wherein said 3,4-polyisoprene rubber, in its uncured state, is characterized by having a glass transition temperature (Tg) in the range of -15°C to -20°C, a Mooney (ML1+4) value in the range of 70 to 90, preferably 75 to 85, and, further, a polymer structure containing 40 to 70 percent, preferably 50 to 60 percent, 3,4-polyisoprene units, 30 to 50 percent 1,4-cis and trans units and 2 to 10 percent 1,2-polyisoprene units with the total of its 3,4 and 1,2 units being in the range of 56 to 63 percent. US-A-5,239,023 and US-A-5,151,398 disclose a process for the synthesis of 3,4-polyisoprene which comprises polymerizing isoprene monomer in an organic solvent at a temperature which is within the range of -10°C to 100°C in the presence of a catalyst system which is comprised of (a) an organoiron compound which is soluble in the organic solvent, wherein the iron in the organoiron compound is in the +3 oxidation state, (b) a partially hydrolyzed organoaluminum compound which was prepared by adding a protonic compound selected from the group consisting of water, alcohols and carboxylic acids to the organoaluminum compound and (c) a chelating aromatic amine; wherein the molar ratio of the chelating amine to the organoiron compound is within the range of 0.1:1 to 1:1, wherein the molar ratio of the organoaluminum compound to the organoiron compound is within the range of 5:1 to 200:1, and wherein the molar ratio of the protonic compound to the organoaluminum compound is within the range of 0.001:1 to 0.2:1. US-A-5,231,153, US-A-5,336,739 and US-A-5,448,003 disclose a process for the synthesis of 3,4-polyisoprene which comprises polymerizing isoprene monomer in an organic solvent in the presence of a catalyst system which is comprised of (a) a lithium initiator and (b) an alkyltetrahydrofurfuryl ether modifier, wherein the alkyl group in the alkyltetrahydrofurfuryl ether modifier contains from 6 to 10 carbon atoms. US-A-5,534,592, discloses an initiator system which is comprised of (a) a lithium initiator, (b) a sodium alkoxide and (c) a polar modifier; wherein the molar ratio of the sodium alkoxide to the polar modifier is within the range of 0.1:1 to 10:1; and wherein the molar ratio of the sodium alkoxide to the lithium initiator is within the range of 0.01:1 to 20:1. Summary of the Invention It has been determined that certain 3,4-polyisoprene rubbers having a high glass transition temperature (Tg) can be used in tire tread rubber compositions to significantly improve traction characteristics without compromising tread wear or rolling resistance. These 3,4-polyisoprene rubbers have a Tg which is within the range of 0°C to 25°C and preferably have a Tg which is within the range of 5°C to 20°C. Particularly good traction characteristics can be attained by utilizing a combination of both such a high Tg 3,4-polyisoprene and a conventional low Tg 3,4-polyisoprene in tire tread compounds. The subject invention more specifically discloses a pneumatic tire having an outer circumferential tread wherein said tread is a sulfur-cured rubber composition comprised of, based on 100 parts by weight of rubber, (1) from 5 to 50 parts of 3,4-polyisoprene rubber, wherein said 3,4-polyisoprene rubber has (a) a 3,4-isomer content of from 75 percent to 95 percent, (b) a 1,2-isomer content of from 5 percent to 25 percent, (c) a glass transition temperature from 0° to 25°C and (d) a number average molecular weight which is within the range of 30,000 to 180,000; and (2) from 50 to 95 parts of a rubbery polymer which is co-curable with the 3,4-polyisoprene rubber. Detailed Description of the Invention The 3,4-polyisoprene rubbers which can be employed in the blends of this invention are typically synthesized by anionic polymerization in an organic medium. The polymerization is normally carried out in an inert organic medium utilizing a lithium catalyst. The 3,4-isomer content of the polyisoprene rubber made is controlled by the amount of modifier system present during the polymerization step. The inert organic medium which is utilized as the solvent will typically be a hydrocarbon which is liquid at ambient temperatures which can 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 liquids under the conditions of the polymerization. It is, of course, important for the solvent selected to be inert. The term "inert" as used herein means that the solvent does not interfere with the polymerization reaction or react with the polymers made thereby. Some representative examples of suitable organic solvents include pentane, isooctane, cyclohexane, normal hexane, benzene, toluene, xylene and ethylbenzene alone or in admixture. Saturated aliphatic solvents, such as cyclohexane and normal hexane, are most preferred. The lithium catalysts which can be used are typically organolithium compounds. The organolithium compounds which are preferred can be represented by the formula R-Li, wherein R represents a hydrocarbyl radical containing from 1 to 20 carbon atoms. Generally, such monofunctional organolithium compounds will contain from 1 to 10 carbon atoms. Some representative examples of organolithium compounds which can be employed include methyllithium, ethyllithium, isopropyllithium, n-butyllithium, sec-butyllithium, n-octyllithium, tert-octyllithium, n-decyllithium, phenyllithium, 1-napthyllithium, 4-butylphenyllithium, p-tolyllithium, 1-naphthyllithium, 4-butylphenyllithium, p-tolyllithium, 4-phenylbutyllithium, cyclohexyllithium, 4-butylcyclohexyllithium and 4-cyclohexylbutyllithium. Organo monolithium compounds, such as alkyllithium compounds and aryllithium compounds, are usually employed. Some representative examples of preferred organo monolithium compounds that can be utilized include ethyllithium, isopropyllithium, n-butyllithium, secondary-butyllithium, normal-hexyllithium, tertiary-octyllithium, phenyllithium, 2-napthyllithium, 4-butylphenyllithium, 4-phenylbutyllithium, cyclohexyllithium and the like. Normal-butyllithium and secondary-butyllithium are highly preferred lithium initiators. The amount of lithium catalyst utilized will vary from one organolithium compound to another and with the molecular weight that is desired for the 3,4-polyisoprene rubber being synthesized. 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 utilized. Since the 3,4-polyisoprene of this invention is of a relatively low molecular weight, the amount of lithium initiator employed will be about three times as much as is used in synthesizing conventional 3,4-polyisoprene rubber. As a general rule, from 0.02 phm (parts per hundred parts by weight of monomer) to 1 phm of the lithium catalyst will be employed. In most cases, from 0.03 phm to 0.3 phm of the lithium catalyst will be employed. It is typically most preferred to utilize from 0.06 phm to 0.2 phm of the lithium catalyst. Normally, from 5 weight percent to 35 weight percent of the isoprene monomer will be charged into the polymerization medium (based upon the total weight of the polymerization medium including the organic solvent and monomer). In most cases, it will be preferred for the polymerization medium to contain from 10 weight percent to 30 weight percent monomer. It is typically more preferred for the polymerization medium to contain from 20 weight percent to 25 weight percent monomer. The isoprene will be polymerized at a temperature which is within the range of 30°C to 100°C. The polymerization temperature will preferably be within the range of 40°C to 70°C for practical reasons and to attain the desired microstructure. Temperatures within the range of 50°C to 60°C are most preferred. The microstructure of the 3,4-polyisoprene rubber being prepared is somewhat dependent upon the polymerization temperature. The polymerization is allowed to continue until essentially all of the isoprene monomer has been exhausted. In other words, the polymerization is allowed to run to completion. Since a lithium catalyst is employed to polymerize the isoprene monomer, a living 3,4-polyisoprene rubber is produced. The living polyisoprene rubber synthesized will have a number average molecular weight which is within the range of 30,000 to 180,000 and a weight average molecular weight which is within the range of 40,000 to 300,000. The polyisoprene rubber synthesized will more typically have a number average molecular weight which is within the range of 50,000 to 150,000 and will preferably have a number average molecular weight which is within the range of 70,000 to 120,000. The 3,4-polyisoprene rubber will typically have a high glass transition temperature which is within the range of 0°C to 25°C. It will more typically have a glass transition temperature which is within the range of 5°C to 20°C. The 3,4-polyisoprene rubber will generally have a 3,4-isomer content which is within the range of 75 percent to 95 percent and a 1,2-isomer content which is within the range of 5 percent to 25 percent. A combination of a sodium alkoxide and polar modifier will normally be employed as the modifier system to attain the very high 3,4-isomer content and to greatly improve (increase) the rate of polymerization. It has been unexpectedly found that a combination of a sodium alkoxide and polar modifier acts synergistically to increase the 3,4-isomer content of polyisoprene rubber synthesized in its presence. The molar ratio of the sodium alkoxide to the lithium initiator will normally be within the range of 0.05:1 to 3:1 and the molar ratio of the polar modifier to the lithium initiator will normally be within the range of 0.25:1 to 5:1. It is generally preferred for the molar ratio of the sodium alkoxide to the lithium initiator to be within the range of 0.15:1 to 1:1 and for the molar ratio of the polar modifier to the lithium initiator to be within the range of 0.4:1 to 3:1. It is generally more preferred for the molar ratio of the sodium alkoxide to the lithium initiator to be within the range of 0.3:1 to 0.5:1 and for the molar ratio of the polar modifier to the lithium initiator to be within the range of 0.5:1 to 2:1. The sodium alkoxides which can be utilized in the modifier system will normally be of the formula NaOR, wherein R is an alkyl group containing from 2 to 12 carbon atoms. The sodium metal alkoxide will typically contain from 2 to 12 carbon atoms. It is generally preferred for the sodium alkoxide to contain from 3 to 8 carbon atoms. It is generally most preferred for the sodium alkoxide to contain from 4 to 6 carbon atoms. Sodium t-amyloxide (sodium t-pentoxide) is a representative example of a preferred sodium alkoxide which can be utilized in the modifier systems of this invention. Ethers and tertiary amines which act as Lewis bases are representative examples of polar modifiers that can be utilized. 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 ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, trimethylamine, triethylamine, N,N,N',N'-tetramethylethylenediamine, N-methyl morpholine, N-ethyl morpholine and N-phenyl morpholine. The modifier can also be a l,2,3-trialkoxybenzene or a 1,2,4-trialkoxybenzene. Some representative examples of 1,2,3-trialkoxybenzenes that can be used include 1,2,3-trimethoxybenzene, 1,2,3-triethoxybenzene, 1,2,3-tributoxybenzene, 1,2,3-trihexoxybenzene, 4,5,6-trimethyl-1,2,3-trimethoxybenzene, 4,5,6-tri-n-pentyl-1,2,3-triethoxybenzene, 5-methyl-1,2,3-trimethoxybenzene and 5-propyl-1,2,3-trimethoxybenzene. Some representative examples of 1,2,4-trialkoxybenzenes that can be used include 1,2,4-trimethoxybenzene, 1,2,4-triethoxybenzene, 1,2,4-tributoxybenzene, 1,2,4-tripentoxybenzene, 3,5,6-trimethyl-1,2,4-trimethoxybenzene, 5-propyl-1,2,4-trimethoxybenzene and 3,5-dimethyl-1,2,4-trimethoxybenzene. Dipiperidinoethane, dipyrrolidinoethane, tetramethylethylene diamine, diethylene glycol, dimethyl ether and tetrahydrofuran are representative of highly preferred modifiers. US-A-4,022,959 describes the use of ethers and tertiary amines as polar modifiers in greater detail. The utilization of 1,2,3-trialkoxybenzenes and 1,2,4-trialkoxybenzenes as modifiers is described in greater detail in US-A-4,696,986. The microstructure of the repeat units which are derived from isoprene monomer is a function of the polymerization temperature and the amount of modifier present. For example, it is known that higher temperatures result in lower 3,4-isomer contents (lower levels of 3,4-microstructure). Accordingly, the polymerization temperature, quantity of modifier and specific modifier selected will be determined with the ultimate desired microstructure of the polyisoprene rubber being synthesized being kept in mind. Coupling agents can optionally be used in order to improve the cold flow characteristics of the rubber and rolling resistance of tires made therefrom. Coupling can also lead to better processability and other beneficial properties. A wide variety of compounds suitable for utilization as coupling agents can be employed. Some representative examples of suitable coupling agents include multivinylaromatic compounds, multiepoxides, multiisocyanates, multiimines, multialdehydes, multiketones, multihalides, multianhydrides, multiesters which are the esters of polyalcohols with monocarboxylic acids, and the diesters which are esters of monohydric alcohols with dicarboxylic acids. Examples of suitable multivinylaromatic compounds include divinylbenzene, 1,2,4-trivinylbenzene, 1,3-divinylnaphthalene, 1,8-divinylnaphthalene, 1,3,5-trivinylnaphthalene and 2,4-divinylbiphenyl. The divinylaromatic hydrocarbons are preferred, particularly divinylbenzene in either its ortho, meta or para isomer. Commercial divinylbenzene which is a mixture of the three isomers and other compounds is quite satisfactory. While any multiepoxide can be used, it is preferred to use those which are liquid since they are more readily handled and form a relatively small nucleus for the radial polymer. Especially preferred among the multiepoxides are the epoxidized hydrocarbon polymers such as epoxidized liquid polybutadienes and the epoxidized vegetable oils such as epoxidized soybean oil and epoxidized linseed oil. Other epoxy compounds such as 1,2,5,6,9,10-triepoxydecane and the like, also can be used. Examples of suitable multiisocyanates include benzene-1,2,4-triisocyanate and naphthalene-1,2,5,7-tetraisocyanate. Especially suitable is a commercially available product known as PAPI-1, a polyarylpolyisocyanate having an average of three isocyanate groups per molecule and an average molecular weight of 380. Such a compound can be visualized as a series of isocyanate-substituted benzene rings joined through methylene linkages. The multiimines, which are also known as multiaziridinyl compounds, that can be employed are preferably those containing three or more aziridine rings per molecule. Examples of such compounds include the triaziridinyl phosphine oxides or sulfides such as tri(1-aziridinyl)phosphine oxide, tri(2-methyl-1-aziridinyl)phosphine oxide and tri(2-ethyl-3-decyl-1-aziridinyl)phosphine sulfide. The multialdehydes which can be utilized are represented by compounds such as 1,4,7-naphthalene tricarboxyaldehyde, 1,7,9-anthracene tricarboxyaldehyde, 1,1,5-pentane tricarboxyaldehyde and similar multialdehyde containing aliphatic and aromatic compounds. The multiketones are represented by compounds such as 1,4,9,10-anthracenetetrone and 2,3-diacetonylcyclohexanone. Examples of the multianhydrides include pyromellitic dianhydride and styrene-maleic anhydride copolymers. Examples of the multiesters include diethyladipate, triethylcitrate and 1,3,5-tricarbethoxybenzene. The preferred multihalides are silicon tetrahalides, such as silicon tetrachloride, silicon tetrabromide and silicon tetraiodide, and the trihalosilanes, such as trifluorosilane, trichlorosilane, trichloroethylsilane and tribromobenzylsilane. Also preferred are the multihalogen-substituted hydrocarbons, such as 1,3,5-tri(bromomethyl)benzene and 2,4,6,9-tetrachloro-3,7-decadiene, in which the halogen is attached to a carbon atom which is alpha to an activating group such as an ether linkage, a carbonyl group or a carbon-to-carbon double bond. Substituents inert with respect to lithium atoms in the terminally reactive polymer can also be present in the active halogen-containing compounds. Alternatively, other suitable reactive groups different from the halogen as described above can be present. Examples of compounds containing more than one type of functional group include 1,3-dichloro-2-propanone, 2,2-dibromo-3-decanone, 3,5,5-trifluoro-4-octanone, 2,4-dibromo-3-pentanone, 1,2,4,5-diepoxy-3-pentanone, 1,2,4,5-diepoxy-3-hexanone, 1,2,11,12-diepoxy-8-pentadecanone and 1,3,18,19-diepoxy-7,14-eicosanedione. In addition to the silicon multihalides as described hereinabove, other metal multihalides, particularly those of tin, lead or germanium, also can be readily employed as coupling and branching agents. Difunctional counterparts of these agents also can be employed, whereby a linear polymer rather than a branched polymer results. Broadly, and exemplarily, a range of 0.01 to 4.5 milliequivalents of coupling agent can be employed per 100 grams of polymer. It is normally preferred to use 0.01 to 1.5 milliequivalents of coupling agent per 100 grams of polymer. The larger quantities tend to result in production of polymers containing terminally reactive groups or insufficient coupling. One equivalent of treating agent per equivalent of lithium is considered an optimum amount for maximum branching, if this result is desired in the production line. The coupling agent can be added in hydrocarbon solution, e.g., in cyclohexane, to the polymerization admixture in the final reactor with suitable mixing for distribution and reaction. The polymerization can be terminated with a conventional noncoupling type of terminator, such as water, an acid, a lower alcohol or with a coupling agent. After the copolymerization has been completed, the 3,4-polyisoprene rubber can be . recovered from the organic solvent. The 3,4-polyisoprene rubber can be recovered from the organic solvent and residue by any means, such as decantation, filtration or centrification. It is often desirable to precipitate the 3,4-polyisoprene from the organic solvent by the addition of lower alcohols containing from 1 to 4 carbon atoms to the polymer solution. Suitable lower alcohols for precipitation of the 3,4-polyisoprene rubber from the polymer cement include methanol, ethanol, isopropyl alcohol, normal-propyl alcohol and t-butyl alcohol. The utilization of lower alcohols to precipitate the 3,4-polyisoprene rubber from the polymer cement also "kills" the living polymer by inactivating lithium end groups. After the 3,4-polyisoprene rubber is recovered from the solution, steam-stripping can be employed to reduce the level of volatile organic compounds in the polymer. In the alternative, it may be desirable to remove residual organic solvent from the polymer by evaporation which can be facilitated by the application of vacuum and elevated temperatures. There are valuable benefits associated with utilizing the high Tg 3,4-polyisoprene rubber of this invention in making tire tread compounds. For instance, traction characteristics can be significantly improved without compromising tread wear or rolling resistance. As a general rule, from 5 phr (parts per 100 parts of rubber) to 50 phr of the high Tg 3,4-polyisoprene will be included in tire tread compounds. Normally, such tire tread compounds will contain from 10 phr to 25 phr of the high Tg 3,4-polyisoprene. It is typically more preferred for tire tread compounds to contain from 12 phr to 20 phr of the high Tg 3,4-polyisoprene rubber. Such tire tread compounds will, of course, also contain at least one other rubber which is co-curable with the 3,4-polyisoprene. Some representative examples of other rubbers which are co-curable with the 3,4-polyisoprene rubber include natural rubber, thigh cis-1,4-polybutadiene rubber, high vinyl polybutadiene rubber, medium vinyl polybutadiene rubber, high trans-1,4-polybutadiene rubber, solution styrene-butadiene rubber, emulsion styrene-butadiene rubber, styrene-isoprene-butadiene rubber, styrene-isoprene rubber, isoprene-butadiene rubber and other types of 3,4-polyisoprene rubber. A preferred blend for high performance automobile tires is comprised of, based on 100 parts by weight of rubber, (1) from 20 to 60 parts of natural rubber, (2) from 5 to 30 parts of high cis-1,4-polybutadiene rubber, (3) from 10 to 50 parts of styrene-butadiene rubber and (4) from 5 to 30 parts of the high Tg 3,4-polyisoprene rubber of this invention. It is preferred for this blend to contain (1) from 30 to 50 parts of natural rubber, (2) from 10 to 20 parts of high cis-1,4-polybutadiene rubber, (3) from 20 to 40 parts of styrene-butadiene rubber and (4) from 10 to 20 parts of the high Tg 3,4-polyisoprene rubber. It is more preferred for this tire tread rubber formulation to contain (1) from 35 to 45 parts of natural rubber, (2) from 10 to 20 parts of high cis-1,4-polybutadiene rubber, (3) from 25 to 35 parts of styrene-butadiene rubber and (4) from 10 to 20 parts of the high Tg 3,4-polyisoprene rubber. In these compounds, it is normally preferred for the styrene-butadiene rubber to be a solution styrene-butadiene rubber (a styrene-butadiene rubber which was synthesized by solution polymerization). High cis-1,4-polybutadiene rubber which is suitable for use in such blends typically has a cis-isomer content of greater than 90 percent and can be made by the process described in CA-A-1,236,648. High cis-1,4-polybutadiene rubber which is suitable for employment in such blends is also sold by The Goodyear Tire & Rubber Company as Budene® 1207 polybutadiene rubber and Budene® 1208 polybutadiene rubber. In order to maximize tire performance characteristics, a combination of the high Tg 3,4-polyisoprene and a low Tg 3,4-polyisoprene can be employed in the tire tread compound. The low Tg 3,4-polyisoprene will have a Tg of less than -5°C. The low Tg 3,4-polyisoprene will typically have a Tg which is within the range of -55°C to -5°C. It will preferably have a Tg which is within the range of -30°C to -10°C and will most preferably have a Tg which is within the range of -20°C to -10°C. The low Tg 3,4-polyisoprene will also typically have a number average molecular weight of greater than 200,000. The low Tg 3,4-polyisoprene will generally have a number average molecular weight which is within the range of 200,000 to 500,000 and will preferably have a number average molecular weight which is within the range of 250,000 to 400,000. The weight ratio of high Tg 3,4-polyisoprene to low Tg 3,4-polyisoprene will typically be within the range of 0.1:1 to 10:1. It is normally preferred for the weight ratio of high Tg 3,4-polyisoprene to low Tg 3,4-polyisoprene to be within the range of 0.5:1 to 2:1. It is generally most preferred for the weight ratio of high Tg 3,4-polyisoprene to low Tg 3,4-polyisoprene to be within the range of 0.8:1 to 1.2:1. The high Tg 3,4-polyisoprene and low Tg 3,4-polyisoprene will normally be used in essentially equal amounts to attain optimal results. A highly preferred blend for high performance automobile tires is comprised of, based on 100 parts by weight of rubber, (1) from 20 to 60 parts of natural rubber, (2) from 5 to 30 parts of high cis-1,4-polybutadiene rubber, (3) from 10 to 50 parts of styrene-butadiene rubber, (4) from 5 to 15 parts of the high Tg 3,4-polyisoprene rubber and (5) from 5 to 15 parts of a low Tg 3,4-polyisoprene rubber. It is preferred for this blend to contain (1) from 30 to 50 parts of natural rubber, (2) from 10 to 20 parts of high cis-1,4-polybutadiene rubber, (3) from 20 to 40 parts of styrene-butadiene rubber, (4) from 5 to 10 parts of the high Tg 3,4-polyisoprene rubber and (5) from 5 to 10 parts of a low Tg 3,4-polyisoprene rubber. It is more preferred for this tire tread rubber formulation to contain (1) from 35 to 45 parts of natural rubber, (2) from 10 to 20 parts of high cis-1,4-polybutadiene rubber, (3) from 25 to 35 parts of styrene-butadiene rubber, (4) from 5 to 10 parts of the high Tg 3,4-polyisoprene rubber and (5) from 5 to 10 parts of a low Tg 3,4-polyisoprene rubber. In cases where it is desirable to maximize tire traction characteristics, the high cis-1,4-polybutadiene rubber can be eliminated from the blend. However, it should be appreciated that in such cases tread wear characteristics may be compromised to some degree. In any case, outstanding tire tread compounds for high performance tires can be made by blending, based on 100 parts by weight of rubber, (1) from 20 to 60 parts of natural rubber, (2) from 10 to 50 parts of styrene-butadiene rubber and (3) from 10 to 30 parts of the high Tg 3,4-polyisoprene rubber of this invention. In another scenario, the blend could be comprised of, based on 100 parts by weight of rubber, (1) from 20 to 60 parts of natural rubber, (2) from 10 to 50 parts of styrene-butadiene rubber, (3) from 5 to 15 parts of the high Tg 3,4-polyisoprene rubber and (4) from 5 to 15 parts of a low Tg 3,4-polyisoprene. In cases where tread wear and rolling resistance are of greater importance, the styrene-butadiene can be eliminated from the blend. However, it should be appreciated that in such cases traction characteristics may be compromised to some degree. Tire tread compounds of this type can be made by blending, based on 100 parts by weight of rubber, (1) from 20 to 60 parts of natural rubber, (2) from 10 to 30 parts of high cis-1,4-polybutadiene rubber and (3) from 10 to 30 parts of the high Tg 3,4-polyisoprene rubber of this invention. In another scenario, the blend could be comprised of, based on 100 parts by weight of rubber, (1) from 20 to 60 parts of natural rubber, (2) from 10 to 50 parts of high cis-1,4-polybutadiene rubber, (3) from 5 to 15 parts of the high Tg 3,4-polyisoprene rubber and (4) from 5 to 15 parts of a low Tg 3,4-polyisoprene. The 3,4-polyisoprene rubber containing blends of this invention can be compounded utilizing conventional ingredients and standard techniques. For instance, the polyisoprene rubber blends will typically be mixed with carbon black and/or silica, sulfur, fillers, accelerators, oils, waxes, scorch inhibiting agents and processing aids. In most cases, the 3,4-polyisoprene rubber blends will be compounded with 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 tackifier resin, optionally a reinforcing resin, optionally one or more fatty acids, optionally a peptizer and optionally one or more scorch inhibiting agents. Such blends will normally contain from 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 utilize insoluble sulfur in cases where bloom is a problem. Normally from 10 to 150 phr of at least one filler will be utilized in the blend with 30 to 80 phr being preferred. In most cases, at least some carbon black will be utilized in the filler. The filler can, of course, be comprised totally of carbon black. Silica can be included in the filler to improve tear resistance and heat build-up. Clays and/or talc can be included in the filler to reduce cost. The blend will also normally include from 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 generally be included in the tread compound blend in amounts ranging from 0.25 to 10 phr with amounts in the range of 1 to 5 phr being preferred. Processing oils will generally be included in the blend in amounts ranging from 2 to 100 phr with amounts ranging from 5 to 50 phr being preferred. The 3,4-polyisoprene blends of this invention will also normally contain from 0.5 to 10 phr of zinc oxide with 1 to 5 phr being preferred. These blends can optionally contain from 0 to 10 phr of tackifier resins, 0 to 10 phr of reinforcing resins, 1 to 10 phr of fatty acids, 0 to 2.5 phr of peptizers and 0 to 1 phr of scorch inhibiting agents. To fully realize the total advantages of the blends of this invention, silica will normally be included in the tread rubber formulation. The processing of the 3,4-polyisoprene rubber blend is normally conducted in the presence of a sulfur containing organosilicon compound to realize maximum benefits. Examples of suitable sulfur containing organosilicon compounds are of the formula: Z-Alk-S n -Alk-Z in which Z is selected from the group consisting of where R 1 is an alkyl group of 1 to 4 carbon atoms, cyclohexyl or phenyl; wherein R 2 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 which may be used in accordance with the present invention include: 3,3'-bis(trimethoxysilylpropyl) disulfide, 3,3'-bis(triethoxysilylpropyl) tetrasulfide, 3,3'-bis(triethoxysilylpropyl) octasulfide, 3,3'-bis(trimethoxysilylpropyl) tetrasulfide, 2,2'-bis(triethoxysilylethyl) tetrasulfide, 3,3'-bis(trimethoxysilylpropyl) trisulfide, 3,3'-bis(triethoxysilylpropyl) trisulfide, 3,3'-bis(tributoxysilylpropyl) disulfide, 3,3'-bis(trimethoxysilylpropyl) hexasulfide, 3,3'-bis(trimethoxysilylpropyl) octasulfide, 3,3'-bis(trioctoxysilylpropyl) tetrasulfide, 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(tripropoxysilylethyl) pentasulfide, 3,3'-bis (tricyclohexoxysilylpropyl) tetrasulfide, 3,3'-bis(tricyclopentoxysilylpropyl) trisulfide, 2,2'-bis(tri-2"-methylcyclohexoxysilylethyl) tetrasulfide, bis(trimethoxysilylmethyl) tetrasulfide, 3-methoxy ethoxy propoxysilyl 3'-diethoxybutoxysilylpropyltetrasulfide, 2,2'-bis(dimethyl methoxysilylethyl) disulfide, 2,2'-bis(dimethyl sec.butoxysilylethyl) trisulfide, 3,3'-bis(methyl butylethoxysilylpropyl) tetrasulfide, 3.3'-bis(di t-butylmethoxysilylpropyl) tetrasulfide, 2,2'-bis(phenyl methyl methoxysilylethyl) trisulfide, 3,3'-bis(diphenyl isopropoxysilylpropyl) tetrasulfide, 3,3'-bis(diphenyl cyclohexoxysilylpropyl) disulfide, 3,3'-bis(dimethyl ethylmercaptosilylpropyl) tetrasulfide, 2,2'-bis(methyl dimethoxysilylethyl) trisulfide, 2,2'-bis(methyl ethoxypropoxysilylethyl) tetrasulfide, 3,3'-bis(diethyl methoxysilylpropyl) tetrasulfide, 3,3'-bis(ethyl di-sec. butoxysilylpropyl) disulfide, 3,3'-bis(propyl diethoxysilylpropyl) disulfide, 3,3'-bis(butyl dimethoxysilylpropyl) trisulfide, 3,3'-bis(phenyl dimethoxysilylpropyl) tetrasulfide, 3-phenyl ethoxybutoxysilyl 3'-trimethoxysilylpropyl tetrasulfide, 4,4'-bis(trimethoxysilylbutyl) tetrasulfide, 6,6'-bis(triethoxysilylhexyl) tetrasulfide, 12,12'-bis(triisopropoxysilyl dodecyl) disulfide, 18,18'-bis(trimethoxysilyloctadecyl) tetrasulfide, 18,18'-bis(tripropoxysilyloctadecenyl) tetrasulfide, 4,4'-bis(trimethoxysilyl-buten-2-yl) tetrasulfide, 4,4'-bis(trimethoxysilylcyclohexylene) tetrasulfide, 5,5'-bis(dimethoxymethylsilylpentyl) trisulfide, 3,3'-bis(trimethoxysilyl-2-methylpropyl) tetrasulfide and 3,3'-bis(dimethoxyphenylsilyl-2-methylpropyl) disulfide. The preferred sulfur containing organosilicon compounds are the 3,3'-bis(trimethoxy or triethoxy silylpropyl) sulfides. The most preferred compound is 3,3'-bis(triethoxysilylpropyl) tetrasulfide. Therefore, as to formula I, preferably Z is where R 2 is an alkoxy of 2 to 4 carbon atoms, with 2 carbon atoms being particularly preferred; Alk is a divalent hydrocarbon of 2 to 4 carbon atoms with 3 carbon atoms being particularly preferred; and n is an integer of from 3 to 5 with 4 being particularly preferred. The amount of the sulfur containing organosilicon compound of formula I in a rubber composition will vary depending on the level of silica that is used. Generally speaking, the amount of the compound of formula I will range from 0.01 to 1.0 parts by weight per part by weight of the silica. Preferably, the amount will range from 0.02 to 0.4 parts by weight per part by weight of the silica. More preferably, the amount of the compound of formula I will range from 0.05 to 0.25 parts by weight per part by weight of the silica. In addition to the sulfur containing organosilicon, the rubber composition should contain a sufficient amount of silica and carbon black, if used, to contribute a reasonably high modulus and high resistance to tear. The silica filler may be added in amounts ranging from 10 phr to 250 phr. Preferably, the silica is present in an amount ranging from 15 phr to 80 phr. If carbon black is also present, the amount of carbon black, if used, may vary. Generally speaking, the amount of carbon black will vary from 5 phr to 80 phr. Preferably, the amount of carbon black will range from 10 phr to 40 phr. It is to be appreciated that the silica coupler may be used in conjunction with a carbon black, namely pre-mixed with a carbon black prior to addition to the rubber composition, and such carbon black is to be included in the aforesaid amount of carbon black for the rubber composition formulation. In any case, the total quantity of silica and carbon black will be at least 30 phr. The combined weight of the silica and carbon black, as hereinbefore referenced, may be as low as 30 phr, but is preferably from 45 to 130 phr. 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 precipitate 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. Such silicas might be characterized, for example, by having a BET surface area, as measured using nitrogen gas, preferably in the range of 40 to 600, and more usually in a range of 50 to 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 100 to 400, and more usually 150 to 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, the designation of Z1165MP; and silicas available from Degussa AG with, for example, designations VN2 and VN3. Tire tread formulations which include silica and an organosilicon compound will typically be mixed utilizing a thermomechanical mixing technique. The mixing of the tire tread rubber formulation 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 including sulfur vulcanizing agents 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) in the preceding non-productive mix stage(s). The rubber, silica and sulfur containing organosilicon 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 sulfur vulcanizable rubber composition containing the sulfur containing organosilicon compound, vulcanizable rubber and generally at least part of the silica should be subjected to a thermomechanical mixing step. The thermomechanical mixing step generally comprises a mechanical working in a mixer or extruder for a period of time suitable in order to produce a rubber temperature between 140°C and 190°C. The appropriate duration of the thermomechanical working varies as a function of the operating conditions and the volume and nature of the components. For example, the thermomechanical working may be for a duration of time which is within the range of 2 minutes to 20 minutes. It will normally be preferred for the rubber to reach a temperature which is within the range of 145°C to 180°C and to be maintained at said temperature for a period of time which is within the range of 4 minutes to 12 minutes. It will normally be more preferred for the rubber to reach a temperature which is within the range of 155°C to 170°C and to be maintained at said temperature for a period of time which is within the range of 5 minutes to 10 minutes. The 3,4-polyisoprene rubber blend containing tire tread compounds of this invention can be used in tire treads in conjunction with ordinary tire manufacturing techniques. After the tire has been built with the 3,4-polyisoprene rubber containing blend, it can be vulcanized using a normal tire cure cycle. Tires made in accordance with this invention can be cured over a wide temperature range. However, it is generally preferred for the tires of this invention to be cured at a temperature ranging from 132°C (270°F) to 166°C (330°F). It is more typical for the tires of this invention to be cured at a temperature ranging from 143°C (290°F) to 154°C (310°F). It is generally preferred for the cure cycle used to vulcanize the tires of this invention to have a duration of 10 to 20 minutes with a cure cycle of 12 to 18 minutes being most preferred. The 3,4-isomer content of the 3,4-polyisoprene referred to herein can be determined by nuclear magnetic resonance (NMR) spectroscopy. Glass transition temperatures can be determined by scanning calorimetry at a heating rate of 10°C/minute and number average molecular weight (Mn) can be determined by gel permeation chromatography. This invention is illustrated by the following examples. Unless specifically indicated otherwise, parts and percentages are given by weight. Example 1 In this example, an isoprene rubber having a high Tg (glass transition temperature) and a low molecular weight was synthesized by the technique of this invention using a mixed tertiary amine and sodium-t-amylate (STA) as the modifier system. In the procedure used, 2,000 grams of a silica/molecular sieve/aluminum dried premix containing 17.8 weight percent of isoprene in hexanes was charged into a one-gallon (3.8 liters) reactor. After the impurity of the premix was removed by an n-butyl lithium (n-BuLi) solution, 3.56 ml (milliliters) of a 2.0 M solution of TMEDA (N,N,N',N'-tetramethylethylenediamine in hexane), 3.56 ml of a 2.0 M solution of STA (in hexane) and 3.60 ml of a 0.96 M solution of n-BuLi (in hexane) were added to the reactor. The molar ratio of STA to TMEDA and to n-BuLi was 2:2:1. The target Mn (number averaged molecular weight) was 100,000. The polymerization was allowed to proceed at 40°C for an hour. An analysis of the residual monomer indicated that isoprene was all consumed. Then, 2.0 ml of a 2M solution of ethanol (in hexane) was added to the reactor to shortstop the polymerization and the polymer was removed from the reactor and stabilized with 1 phr (parts per 100 parts by weight of rubber) of antioxidant. After the hexane solvent was evaporated, the resulting isoprene rubber was dried in a vacuum oven at 50°C. The isoprene rubber was determined to have a Tg at +14°C. It was also determined to have a microstructure which contained 15 percent 1,2-polyisoprene units, 82 percent 3,4-polyisoprene units and 3 percent 1,4-polyisoprene units. The Mooney viscosity (ML 1+4) at 100 °C of the isoprene rubber made was determined to be 13. The molecular weight of this isoprene rubber was measured using GPC (gel permeation chromatography) to have an Mn (number averaged molecular weight) of 78,000 and an Mw (weight averaged molecular weight) of 99,000 and a molecular weight distribution (MWD) of 1.30. Example 2 The procedure described in Example 1 was utilized in this example except that the target number average molecular weight (M n ) of the isoprene rubber was increased from 100,000 to 200,000. To achieve this, the amount of the modifiers and n-BuLi was reduced to half; i.e., 1.78 ml of a 2 M solution of STA, 1.78 ml of a 2 M solution of TMEDA and 1.8 ml of a 0.96 M solution of n-BuLi were used in this experiment. The isoprene rubber formed was determined to have a Tg at +18°C. It was also determined to have a microstructure which contained 15 percent 1,2-polyisoprene units, 79 percent 3,4-polyisoprene units and 6 percent 1,4-polyisoprene units. The Mooney viscosity (ML 1+4) at 100°C of the isoprene rubber made was determined to be 37. The molecular weight of this isoprene rubber was measured using GPC to have a M n of 121,000 and a M w (weight averaged molecular weight) of 182,000 and a molecular weight distribution (MWD) of 1.50. Example 3 The procedure described in Example 1 was utilized in this example except that the STA to TMEDA to n-BuLi ratio was changed from 2:2:1 to 0.5:0.5:1. The isoprene rubber formed was determined to have a Tg at +10°C. It was also determined to have a microstructure which contained 12 percent 1,2-polyisoprene units, 73 percent 3,4-polyisoprene units and 12 percent 1,4-polyisoprene units. The Mooney viscosity (ML 1+4) at 100°C of the isoprene rubber made was determined to be 9. The molecular weight of this isoprene rubber was measured using GPC (gel permeation chromatography) to have a M n (number averaged molecular weight) of 72,000 and a M w (weight averaged molecular weight) of 80,000 and a molecular weight distribution (MWD) of 1.12. Example 4 The procedure described in Example 1 was utilized in this example except that the STA to TMEDA to n-BuLi ratio was changed from 2:2:1 to 0.25:3:1 and the polymerization temperature was increased from 40°C to 65°C. The GC (gas chromatography) analysis of the residual monomer indicated that 95 percent of isoprene monomer was consumed in 5 minutes. A complete monomer conversion was achieved after 20 minutes. The isoprene rubber produced was determined to have a Tg at +17°C. It was also determined to have a microstructure which contained 15 percent 1,2-polyisoprene units, 79 percent 3,4-polyisoprene units and 6 percent 1,4-polyisoprene units. The Mooney viscosity (ML 1+4) at 100°C of the polyisoprene rubber made was determined to be 19. This polyisoprene rubber was measured to have a Mn of 72,400 and a Mw of 86,000 and a molecular weight distribution (MWD) of 1.20. Example 5 The procedure described in Example 1 was utilized in this example except that the polymerization temperature was changed from 40°C to 30°C and the STA to TMEDA to n-BuLi ratio was changed from 2:2:1 to 0.25:3:1. It took 20 minutes to convert 95 percent of isoprene monomer to polyisoprene rubber. The Tg, microstructure and Mooney viscosity of the polyisoprene rubber made are shown in Table 1. Comparative Example 6 The procedure described in Example 1 was utilized in this example except that the polymerization temperature was changed from 40°C to 30°C and the modifier was changed from mixed TMEDA/STA to TMEDA alone. It took 280 minutes to convert 95 percent of isoprene monomer to polyisoprene rubber. The Tg, microstructure and Mooney viscosity of the polyisoprene rubber made are shown in Table 1. Comparative Example 7 The procedure described in Example 1 was utilized in this example except that the polymerization temperature was decreased from 40°C to 30°C and except that the modifier was changed from a mixed TMEDA/STA to diglyme (2-methoxyethyl ether). It took 180 minutes to convert 95 percent of the isoprene monomer to polyisoprene rubber. The Tg and microstructure of the polyisoprene rubber made are shown in Table 1. Comparative Example 8 The procedure described in Example 1 was utilized in this example except that the polymerization temperature employed was changed from 40°C to 30°C and the modifier was changed from mixed TMEDA/STA to ETE (ethyltetrahydrofurfuryl ether). It took 140 minutes to convert 95 percent of the isoprene monomer to polyisoprene rubber. The Tg, microstructure and Mooney ML-4 viscosity of the polyisoprene rubber made are shown in Table 1. Example 9 In this example, an isoprene rubber having an Mn of 300,000 was prepared by the technique of this invention using mixed tertiary amine and sodium-t-amylate (STA) as the modifier. In the procedure used, 2,250 grams of a silica/molecular sieve/aluminum dried premix containing 20 weight percent of isoprene in hexanes was charged into a one-gallon (3.8 liters) reactor. After the impurity of the premix was removed by a n-butyl lithium (n-BuLi) solution, 2.25 ml (milliliters) of a 2.0M solution of DPE (N,N'-1,2-dipiperidinoethan; in hexane), 1.5 ml of a 0.5M solution of STA (in hexane) and 1.56 ml of a 0.96M solution of n-BuLi (in hexane) were added to the reactor. The molar ratio of STA to DPE and to n-BuLi was 0.5:3:1. The polymerization was allowed to proceed at 50°C. An analysis of the residual monomer indicated that the polymerization was essentially complete (more than 95 percent of isoprene was consumed) after 30 minutes. The polymerization was continued for another 30 minutes for complete consumption of monomer. Then, 1.0 ml of a 2 M solution of ethanol (in hexane) was added to the reactor to shortstop the polymerization and the polymer was removed from the reactor and stabilized with 1 phr (parts per 100 parts by weight of rubber) of antioxidant. After the hexane solvent was evaporated, the resulting isoprene rubber was dried in a vacuum oven at 50°C. The isoprene rubber formed was determined to have a Tg at +30°C. It was also determined to have a microstructure which contained 89 percent 3,4-polyisoprene units, 9 percent 1,2-polyisoprene units and 2 percent 1,4-polyisoprene units. Example 10 The procedure described in Example 9 was utilized in this example except that 1,3-butadiene was used as the monomer in place of isoprene. It took 15 minutes to convert 95 percent of 1,3-butadiene monomer to butadiene rubber. The polybutadiene formed was determined to have a Tg at -9°C. It was also determined to have a microstructure which contained 93 percent 1,2-polybutadiene units and 7 percent 1,4-polybutadiene units. Comparative Example 11 The procedure described in Example 9 was utilized in this example except that the polymerization temperature was changed from 50°C to 70°C and the modifier was changed from STA/DPE to TMEDA alone. The Tg of the polyisoprene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 2. Comparative Example 12 The procedure described in Example 9 was utilized in this example except that the polymerization temperature was changed from 50°C to 70°C and the modifier was changed from STA/DPE to ETE alone. The Tg of the polyisoprene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 2. Example 13 The procedure described in Example 9 was utilized in this example except that the polymerization temperature was changed from 50°C to 70°C and the modifiers was changed from STA/DPE to STA/TMEDA. The Tg of the polyisoprene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 2. Example 14 The procedure described in Example 9 was utilized in this example except that the polymerization temperature was changed from 50°C to 70°C and the modifiers was changed from STA/DPE to STA/ETE. The Tg of the polyisoprene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 2. Ex. No. Modifier System PZN Temp Tg Time needed to reach 90+ % Conversion 11 TMEDA 70°C -16°C 240 minutes 12 ETE 70°C -15°C 30 minutes 13 STA/ETE 70°C -3°C 20 minutes 14 STA/TMEDA 70°C 0°C 15 minutes Comparative Example 15 The procedure described in Example 9 was utilized in this example except that 1,3-butadiene was used as monomer in place of isoprene and the modifier was changed from STA/DPE to TMEDA alone. The Tg of the polybutadiene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 3. Comparative Example 16 The procedure described in Example 9 was utilized in this example except that 1,3-butadiene was used as monomer in place of isoprene and the modifier was changed from STA/DPE to ETE alone. The Tg of the polybutadiene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 3. Example 17 The procedure described in Example 9 was utilized in this example except that 1,3-butadiene was used as the monomer in place of isoprene and the modifiers was changed from STA/DPE to STA/TMEDA. The Tg of the polybutadiene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 3. Ex. No. Modifier System PZN Temp Tg Time needed to reach 90+ % Conversion 15 TMEDA 50°C -40°C 60 minutes 16 ETE 50°C -36°C 45 minutes 17 STA/TMEDA 50°C -22°C 15 minutes Comparative Example 18 The procedure described in Example 9 was utilized in this example except that a premix containing 50:50 mixture of 1,3-butadiene and isoprene in hexanes was used in place of isoprene and the modifier was changed from STA/DPE to TMEDA alone. The Tg of the 50/50 IBR (isoprene-butadiene rubber) formed and the time needed to consume more than 90 percent of the monomers charged are listed in TABLE 4. Comparative Example 19 The procedure described in Example 9 was utilized in this example except that a premix containing 50:50 mixture of 1,3-butadiene and isoprene in hexanes was used in place of isoprene and the modifier was changed from STA/DPE to ETE alone. The Tg of the 50/50 IBR formed and the time needed to consume more than 90 percent of the monomers charged are listed in TABLE 4. Example 20 The procedure described in Example 9 was utilized in this example except that a premix containing 50:50 mixture of 1,3-butadiene and isoprene in hexanes was used in place of isoprene and the modifier was changed from STA/DPE to STA/ETE. The Tg of the 50/50 IBR formed and the time needed to consume more than 90 percent of the monomers charged are listed in TABLE 4. Example 21 The procedure described in Example 9 was utilized in this example except that a premix containing 50:50 mixture of 1,3-butadiene and isoprene in hexanes was used in place of isoprene and the modifier was changed from STA/DPE to STA/TMEDA. The Tg of the 50/50 IBR formed and the time needed to consume more than 90 percent of the monomers charged are listed in TABLE 4. Ex. No. Modifier System PZN Temp Tg Time needed to reach 90+ % Conversion 18 TMEDA 65°C -26°C 210 minutes 19 ETE 65°C -27°C 60 minutes 20 STA/ETE 65°C -25°C 20 minutes 21 STA/TMEDA 65°C -16°C 20 minutes Comparative Example 22 The procedure described in Example 9 was utilized in this example except that the polymerization temperature was changed from 50°C to 70°C and the modifier was changed from STA/DPE to DMAMP (2-dimethyl aminomethyl-1-methyl piperidine) alone. The Tg, microstructure, Mooney viscosity (ML 1+4) of the polyisoprene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 5. Comparative Example 23 The procedure described in Example 9 was utilized in this example except that the polymerization temperature was changed from 50°C to 70°C and the modifier was changed from STA/DPE to Sparteine alone. The Tg, microstructure, Mooney viscosity (ML 1+4) of the polyisoprene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 5. Example 24 The procedure described in Example 9 was utilized in this example except that the polymerization temperature was changed from 50°C to 70°C and the modifier was changed from STA/DPE to STA/DMAMP. The Tg microstructure, Mooney viscosity (ML 1+4) of the polyisoprene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 5. Example 25 The procedure described in Example 9 was utilized in this example except that the polymerization temperature was changed from 50°C to 70°C and the modifier was changed from STA/DPE to STA/Sparteine. The Tg, microstructure, Mooney viscosity (ML 1+4) of the polyisoprene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 5. Example 26 The procedure described in Example 9 was utilized in this example except that a premix containing 10:90 mixture of styrene and 1,3-butadiene in hexanes was used in place of isoprene and the modifier was changed from STA/DPE to STA/TMEDA. It took 10 minutes to consume more than 90 percent of the monomers charged. The 10/90 SBR (styrene-butadiene rubber) produced was determine to have a Tg at -20° C. It was also determined to have a Mooney viscosity (ML 1+4) at 100°C of 69. Example 27-31 The procedure described in Example 9 was utilized in this example except that isoprene monomer was replaced with 25:75, 30:70, 35:65, 40:60 and 45:55 mixtures of styrene and 1,3-butadiene in hexanes and the modifier was changed from STA/DPE to STA/TMEDA. The STA to TMEDA and to n-BuLi ratio was also changed to 0.5:0.3:1. It took about 15 minutes to consume more than 90 percent of the monomers charged at the polymerization temperature of 70°C for all these experiments. Based on proton NMR (nuclear magnetic resonance) measurements, the polystyrenes contained in these SBRs are randomly distributed in the polymer chains. The Tgs and microstructures of these SBRs made are listed in TABLE 6. Example 32-33 The procedure described in Example 9 was utilized in this example except that isoprene monomer was replaced with 40:60 and 45:55 mixtures of styrene and 1,3-butadiene in hexanes and the modifier was changed from STA/DPE to STA/TMEDA. The STA to TMEDA and to n-BuLi ratio was also changed to 0.5:0.5:1. It took about 15 minutes to consume more than 90 percent of the monomers charged at the polymerization temperature of 70°C for all these experiments. Based on proton NMR (nuclear magnetic resonance) measurements, the polystyrenes contained in these SBRs are randomly distributed in the polymer chains. The Tgs and microstructures of these SBRs made are listed in TABLE 7. Comparative Example 34 The procedure described in Example 9 was utilized in this example except that a premix containing 25:75 mixture of styrene and 1,3-isoprene in hexanes was used in place of isoprene and the modifier was changed from STA/DPE to ETE alone. The molar ratio of ETE to n-BuLi was 0.5:1. The Tg, microstructure of 25/75 SIR (styrene-isoprene rubber) formed and the time needed to consume more than 90 percent of the monomers charged are listed in TABLE 8. Example 35 The procedure described in Example 9 was utilized in this example except that a premix containing 25:75 mixture of styrene and 1,3-isoprene in hexanes was used in place of isoprene and the modifier was changed from STA/DPE to STA/ETE. The molar ratio of STA to ETE and to n-BuLi was 0.5:0.5:1. The Tg, microstructure of 25/75 SIR formed and the time needed to consume more than 90 percent of the monomers charged are listed in TABLE 8. Example 36-37 The procedure described in Example 9 was utilized in this example except that isoprene monomer was replaced with 10:90 mixture of styrene and isoprene in hexanes and the modifier was changed from STA/DPE to STA/TMEDA or STA/ETE. It took about 10 minutes to consume more than 90 percent of the monomers charged at the polymerization temperature of 70°C for all these experiments. Based on proton NMR measurements, the polystyrenes contained in these SIRs are randomly distributed in the polymer chains. The Tgs and microstructures of these SIRs made are listed in TABLE 9. Example 38 The procedure described in Example 9 was utilized in this example except that isoprene monomer was replaced with 25:50:25 mixture of styrene, isoprene and 1,3-butadiene in hexanes and the modifier was changed from STA/DPE to STA/ETE. It took about 12 minutes to consume more than 90 percent of the monomers charged at the polymerization temperature of 70°C. Based on proton NMR measurements, the polystyrenes contained in the SIBR (styrene-isoprene-butadiene rubber) are randomly distributed in the polymer chains. The SIBR made was determined to have a Tg at +1°C. It was also determined to have a microstructure which contained 24 percent polystyrene units, 13 percent 1,2-polybutadiene units, 9 percent 1,4-polybutadiene units, 7 percent 1,2-polyisoprene units, 11 percent 1,4-polyisoprene units and 36 percent 3,4-polyisoprene units. The SIBR was also measured to have a Mooney viscosity (ML 1+4) at 100°C of 75. Comparative Example 39 In this experiment, a blend was made by mixing 40 parts of natural rubber, 15 parts of high-cis-1,4-polybutadiene, 30 parts of solution styrene-butadiene rubber, 15 parts of a conventional low Tg 3,4-polyisoprene rubber, 38 parts of carbon black, 10 parts of Hi-Sil™ 210 silica, 2 parts of Degussa X50S coupling agent, 10 parts of a paraffinic process oil, 3.5 parts of antidegradants, 3.5 parts of zinc oxide, 2 parts of stearic acid, 1 part of N-cyclohexylbenzenethiazole-2-sulfenamide, 0.1 part of tetramethylthiuram disulfide, and 1.5 parts of sulfur. The low Tg 3,4-polyisoprene rubber utilized in this blend had a Tg of -16°C and a number average molecular weight of about 350,000. This rubber blend was subsequently cured and evaluated for physical properties which are reported in TABLE 10. Low tan delta values at 70°C are indicative of low hysteresis and consequently tire treads made with such rubbers exhibit lower rolling resistance than tires made with rubbers having higher tan delta values at 70°C. On the other hand, rubbers which have high tan delta values between -25°C and +25°C can be utilized in making tire treads which exhibit better traction characteristics than tires made with tread rubber compositions which have lower tan delta values over the -25°C to +25°C temperature range. It is preferred for a tread rubber composition to have the highest possible tan delta value at every temperature within the range of -25°C to +25°C to realize the best possible traction characteristics over all driving conditions and at all driving speeds. The rubber blend made in this experiment had a high tan delta value of greater than 0.25 at temperatures of less than -15°C. However, the tan delta values of this blend dropped rapidly with increasing temperatures above -10°C. At 0°C, this blend exhibited a tan delta value of about 0.18. This experiment shows that conventional low Tg 3,4-polyisoprene rubber can be used in making tire tread formulations which have high tan delta values at temperatures of less than 0°C. The low tan delta value at 70°C of less than 0.1 further shows that this rubber formulation can be used to make tire treads which exhibit good rolling resistance. However, it would be highly desirable for the tan delta value of this rubber blend to be higher at temperatures within the range of 0°C to 25°C to exhibit outstanding traction characteristics over all driving conditions and at all driving speeds. Example 40 In this experiment, a tire tread rubber formulation was made using the same procedure as was employed in Example 39 except for the fact that a high Tg 3,4-polyisoprene rubber was substituted for the low Tg 3,4-polyisoprene rubber utilized in Example 39. The high Tg 3,4-polyisoprene rubber employed in this experiment had a M n of 121,000, a M w of 182,000 and a M z of 245,000. It was synthesized by the procedure described in Example 2. The rubber formulation made in this experiment had the physical properties reported in TABLE 10. As can be seen, the tan delta values of the blend increased over the temperature range from -2°C to +23°C. This blend has a tan delta value at 70°C of 0.07. This low tan delta value at 70°C of less than 0.1 shows that this rubber formulation can be used to make tire treads which exhibit good rolling resistance. This unique combination of high tan delta values at low temperatures and low tan delta values at high temperatures is indicative of a tire tread formulation with both good traction characteristics and good rolling resistance. Generally as the glass transition temperature of a tire tread rubber formulation increases abrasion resistance is sacrificed. However, it has been unexpectedly found that this is not the case with the tire tread rubber formulations of this invention. In fact, it has been determined that the tire tread rubber formulations of this invention have DIN abrasion characteristics which are as good as or better than the lower Tg control compounds. This means that better tread life can be expected while also attaining improved traction characteristics. Example 41 In this experiment, a tire tread rubber formulation was made using the same procedure as was employed in Example 39 except for the fact that a high Tg 3,4-polyisoprene rubber was substituted for the low Tg 3,4-polyisoprene rubber utilized in Example 39. The high Tg 3,4-polyisoprene rubber employed in this experiment had a M n of 72,000 and a M w of 86,000. The rubber formulation made in this experiment had the physical properties reported in TABLE 10. As can be seen, the tan delta values of the blend increased over the temperature range from -5°C to +17°C. This blend had a tan delta value at 70°C of 0.08. This low tan delta value at 70°C of less than 0.1 shows that this rubber formulation can be used to make tire treads which exhibit good rolling resistance. This unique combination of high tan delta values at low temperatures and low tan delta values at high temperatures is indicative of a tire tread formulation with both good traction characteristics and good rolling resistance. Generally as the glass transition temperature of a tire tread rubber formulation increases abrasion resistance is sacrificed. However, it has been unexpectedly found that this is not the case with the tire tread rubber formulations of this invention. In fact, it has been determined that the tire tread rubber formulations of this invention have DIN abrasion characteristics which are as good as or better than the lower Tg control compounds. This means that better tread life can be expected while also attaining improved traction characteristics. Example 39 40 41 100% Modulus 1.9 MPa 2.0 MPa 2.0 MPa 300% Modulus 8.5 MPa 9.0 MPa 8.7 MPa Break Strength 16.2 MPa 14.6 MPa 14.3 MPa Elongation 515% 469% 469% Hardness (20°C) 58 60 60 Hardness (100°C) 52 52 51 Rebound (20°C) 49 40 39 Rebound (100°C) 67 66 65 DIN Abrasion 116 105 111 tan.delta @ -25°C 0.28 0.20 0.21 tan delta @ -20°C 0.26 0.17 0.18 tan delta @ -15°C 0.25 0.15 0.16 tan delta @ -10°C 0.24 0.14 0.15 tan delta @ -5°C 0.22 0.14 0.15 tan delta @ 0°C 0.18 0.14 0.15 tan delta @ 5°C 0.16 0.14 0.17 tan delta @ 10°C 0.13 0.16 0.21 tan delta @ 15°C 0.12 0.19 0.23 tan delta @ 20°C 0.11 0.23 0.23 tan delta @ 25°C 0.10 0.23 0.19 tan delta @ 70°C 0.07 0.07 0.08

Claims: A pneumatic tire having an outer circumferential tread wherein said tread is a sulfur-cured rubber composition which is characterized by being comprised of, based on 100 parts by weight of rubber, (1) from 5 to 50 parts of high Tg 3,4-polyisoprene rubber, wherein said high Tg 3,4-polyisoprene rubber has (a) a 3,4-isomer content of from 75 percent to 95 percent, (b) a 1,2-isomer content of from 5 percent to 25 percent, (c) a glass transition temperature from 0° to 25°C and (d) a number average molecular weight which is within the range of 30,000 to 180,000; and (2) from 50 to 95 parts of at least one rubbery polymer which is co-curable with the 3,4-polyisoprene rubber. A pneumatic tire as specified in claim 1 characterized in that said sulfur-cured rubber composition is comprised of, based on 100 parts by weight of rubber, (1) from 20 to 60 parts of natural rubber, (2) from 5 to 30 parts of high cis-1,4-polybutadiene rubber, (3) from 10 to 50 parts of styrene-butadiene rubber, (4) from 5 to 15 parts of said high Tg 3,4-polyisoprene rubber and (5) from 5 to 15 parts of a low Tg 3,4-polyisoprene rubber, wherein the low Tg 3,4-polyisoprene rubber has a glass transition temperature of less than -5°C. A pneumatic tire as specified in claim 1 characterized in that said sulfur-cured rubber composition is comprised of, based on 100 parts by weight of rubber, (1) from 20 to 60 parts of natural rubber, (2) from 5 to 30 parts of high cis-1,4-polybutadiene rubber, (3) from 10 to 50 parts of styrene-butadiene rubber and (4) from 5 to 30 parts of the high Tg 3,4-polyisoprene rubber. A pneumatic tire as specified in claim 1, 2 or 3 characterized in that the high Tg 3,4-polyisoprene has a weight average molecular weight which is within the range of 40,000 to 300,000 and a number average molecular weight which is within the range of 50,000 to 150,000. A pneumatic tire as specified in claim 3 characterized in that the styrene-butadiene rubber is solution styrene-butadiene rubber; wherein the rubber composition contains from 30 parts to 50 parts of natural rubber, from 10 parts to 20 parts of high cis-1,4-polybutadiene rubber, from 20 parts to 40 parts of styrene-butadiene rubber and from 10 parts to 20 parts of the high Tg 3,4-polyisoprene rubber; and wherein the high Tg 3,4-polyisoprene rubber has a glass transition temperature which is within the range of 5°C to 20°C. A pneumatic tire as specified in claim 5 characterized in that the rubber composition contains from 35 parts to 45 parts of natural rubber, from 10 parts to 20 parts of high cis-1,4-polybutadiene rubber, from 25 parts to 35 parts of styrene-butadiene rubber and from 10 parts to 20 parts of the high Tg 3,4-polyisoprene rubber; wherein the high Tg 3,4-polyisoprene has a number average molecular weight which is within the range of 70,000 to 120,000. A pneumatic tire as specified in claim 2 characterized in that the low Tg 3,4-polyisoprene has a number average molecular weight of greater than 200,000; wherein the low Tg 3,4-polyisoprene has a glass transition temperature which is within the range of -55°C to -5°C; wherein the blend contains (1) from 30 to 50 parts of the natural rubber, (2) from 10 to 20 parts of the high cis-1,4-polybutadiene rubber, (3) from 20 to 40 parts of the styrene-butadiene rubber, (4) from 5 to 10 parts of the high Tg 3,4-polyisoprene rubber and (5) from 5 to 10 parts of the low Tg 3,4-polyisoprene rubber; wherein the high Tg 3,4-polyisoprene has a weight average molecular weight which is within the range of 40,000 to 300,000; wherein the high Tg 3,4-polyisoprene has a number average molecular weight which is within the range of 50,000 to 150,000; and wherein the high Tg 3,4-polyisoprene rubber has a glass transition temperature which is within the range of 5°C to 20°C. A pneumatic tire as specified in claim 1 characterized in that the rubbery polymer which is co-curable with the high Tg 3,4-polyisoprene is a mixture of natural rubber and styrene-butadiene rubber; wherein the rubber composition is comprised of (1) from 20 parts to 60 parts of the natural rubber, (2) from 10 to 50 parts of the styrene-butadiene rubber and (3) from 10 to 30 parts of the high Tg 3,4-polyisoprene rubber. A pneumatic tire as specified in claim 1 characterized in that the rubbery polymer which is co-curable with the high Tg 3,4-polyisoprene is a mixture of natural rubber, low Tg 3,4-polyisoprene rubber and styrene-butadiene rubber; wherein the rubber composition is comprised of (1) from 20 parts to 60 parts of the natural rubber, (2) from 10 parts to 50 parts of the styrene-butadiene rubber, (3) from 5 parts to 15 parts of the high Tg 3,4-polyisoprene rubber and (4) from 5 parts to 15 parts of a low Tg 3,4-polyisoprene rubber; and wherein the low Tg 3,4-polyisoprene rubber has a glass transition temperature which is within the range of -55°C to -5°C. A pneumatic tire as specified in claim 1 characterized in that the rubbery polymer which is co-curable with the high Tg 3,4-polyisoprene is a mixture of natural rubber and high cis-1,4-polybutadiene rubber; and wherein the rubber composition is comprised of (1) from 20 parts to 60 parts of the natural rubber, (2) from 10 to 30 parts of the high cis-1,4-polybutadiene rubber and (3) from 10 to 30 parts of the high Tg 3,4-polyisoprene rubber. A pneumatic tire as specified in claim 1 characterized in that the rubbery polymer which is co-curable with the high Tg 3,4-polyisoprene is a mixture of natural rubber, low Tg 3,4-polyisoprene rubber and high cis-1,4-polybutadiene rubber; wherein the rubber composition is comprised of (1) from 20 parts to 60 parts of the natural rubber, (2) from 10 parts to 50 parts of the high cis-1,4-polybutadiene rubber, (3) from 5 parts to 15 parts of the high Tg 3,4-polyisoprene rubber and (4) from 5 parts to 15 parts of a low Tg 3,4-polyisoprene rubber; wherein the low Tg 3,4-polyisoprene rubber has a glass transition temperature which is within the range of -55°C to -5°C; wherein the high Tg 3,4-polyisoprene has a number average molecular weight which is within the range of 50,000 to 150,000; and wherein the high Tg 3,4-polyisoprene rubber has a glass transition temperature which is within the range of 5°C to 20°C.

Inventors: ALLEN GERALD LEROY, AUSTIN LAURIE ELIZABETH, HALASA ADEL FARHAN, HSU WEN-LIANG, ZANZIG DAVID JOHN, ALLEN, GERALD LEROY, AUSTIN, LAURIE ELIZABETH, HALASA, ADEL FARHAN, HSU, WEN-LIANG, ZANZIG, DAVID JOHN
Applicants: GOODYEAR TIRE & RUBBER, THE GOODYEAR TIRE & RUBBER COMPANY
Application year: 1997
Main classification: C08L21
IPC codes: B60C1, C08L9, C08L7, C08J3, C08L21