Ethylene-rich diene elastomers and rubber compositions containing them

Abstract

The invention relates to a star-shaped elastomer that is a random copolymer of 1,3-butadiene and ethylene containing more than 50% to less than 80% by mole of ethylene units and consisting of copolymer chains linked together by a tin atom. The invention also relates to a rubber composition containing it.

Description

Title of the invention: Ethylene-rich diene elastomers and rubber compositions containing them

[0001] The field of the present invention is that of diene elastomers rich in ethylene units and that of rubber compositions reinforced with carbon black and containing such elastomers and which are particularly intended for use in a tire.

[0002] It is known to use copolymers with lower oxidation sensitivity in tire rubber compositions, such as highly saturated diene elastomers, namely ethylene-1,3-butadiene copolymers containing more than 50 mole percent ethylene units. The use of these ethylene-1,3-butadiene copolymers in a tire tread is described, for example, in document WO 2014114607 A1 and results in an improved performance compromise between rolling resistance and wear resistance.

[0003] There is always a need to improve the wear resistance performance of tires made from such elastomers. To improve wear resistance, it is known that a certain stiffness of the tread is desirable; this stiffening of the tread can be achieved, for example, by increasing the reinforcing charge ratio in the rubber compounds constituting these treads. Unfortunately, experience shows that such stiffening of the tread negatively impacts rolling resistance properties in a known way, most often prohibitively so, due to a significant increase in the hysteresis losses of the rubber compound. Indeed, it is well known that low hysteresis of the rubber compound is required to minimize rolling resistance.

[0004] The inventors have discovered, surprisingly, that the use of a new copolymer of ethylene and 1,3-butadiene in a rubber composition reinforced mainly with carbon black makes it possible to further increase the rigidity of the rubber composition in order to improve the wear resistance performance of a tire without being at the expense of rolling resistance performance.

[0005] Thus, a first object of the invention is a star elastomer which is a statistical copolymer of 1,3-butadiene and ethylene, which copolymer contains more than 50% to less than 80% by mole of ethylene unit and is made up of copolymer chains linked together by a tin atom.

[0006] A second object of the invention is a rubber composition comprising a star elastomer according to the invention, a reinforcing filler containing more than 50% to 100% by mass of carbon black and a crosslinking system.

[0007] A third object of the invention is a tire which includes a tread, which tire includes a rubber composition according to the invention. Detailed description

[0008] Any interval of values ​​designated by the expression "between a and b" represents the domain of values ​​greater than "a" and less than "b" (i.e., bounds a and b excluded) while any interval of values ​​designated by the expression "from a to b" means the domain of values ​​from "a" to "b" (i.e., including the strict bounds a and b).

[0009] The compounds mentioned in the description may be of fossil origin or bio-based. In the latter case, they may be partially or totally derived from biomass or obtained from renewable raw materials derived from biomass. Similarly, the compounds mentioned may also come from the recycling of materials already used, that is to say, they may be partially or totally derived from a recycling process, or obtained from raw materials themselves derived from a recycling process.

[0010] The expression "based on" used to define the constituents of the catalytic system means the mixture of these constituents, or the product of the reaction of some or all of these constituents with each other.

[0011] The star-shaped elastomer according to the invention has as its essential characteristic that it is a statistical copolymer of 1,3-butadiene and ethylene. The constituent units of the copolymer are those resulting from the statistical polymerization of 1,3-butadiene and ethylene.

[0012] The star-shaped elastomer according to the invention also has the essential characteristic of containing more than 50 mole percent of ethylene units. The star-shaped elastomer also contains less than 80 mole percent of ethylene units, preferably less than 75 mole percent of ethylene units. The percentages of ethylene units in the copolymer are expressed relative to the total number of units resulting from the polymerization of 1,3-butadiene and ethylene.

[0013] Preferably, the star-shaped elastomer according to the invention contains 1,2-cyclohexane cyclic units. In other words, the star-shaped elastomer contains, in addition to ethylene units and 1,3-butadiene units, also preferably 1,2-cyclohexane cyclic units. The 1,2-cyclohexane cyclic units have formula (I). The cyclic units result from a particular insertion of the ethylene and 1,3-butadiene monomers. 1,3-Butadiene in the polymer chain, in addition to the conventional ethylene and 1,3-butadiene units, respectively -(CH2-CH2)-, (CH2-CH=CH-CH2)- and (CH2-CH(C=CH2))-. The mechanism for obtaining such a microstructure is described, for example, in Macromolecules 2009, 42, 3774-3779. CH?—CH? s CH2

[0014] When the star elastomer contains 1,2-cyclohexane units, it preferably contains at most 15% by mole, more preferably at most 10% by mole, the percentage being expressed in relation to the total number of units resulting from the polymerization of 1,3-butadiene and ethylene.

[0015] The star-shaped elastomer according to the invention also has the additional characteristic of being tin-linked. In other words, the copolymer chains constituting the elastomer according to the invention are linked together by a tin atom. The star-shaped elastomer preferably consists of 3-branch star chains linked together by a tin atom, 4-branch star chains linked together by a tin atom, or a mixture thereof. Preferably, the star-shaped elastomer consists of 4-branch star chains linked together by a tin atom.

[0016] Preferably, the star-shaped elastomer according to the invention has a Mooney viscosity ML(l+4) at 100°C greater than 30. A Mooney viscosity ML(l+4) at 100°C greater than 30 facilitates finishing operations such as spinning and drying the star-shaped elastomer before baling it in an elastomer production line. According to any one of the embodiments of the invention, the star-shaped elastomer preferably has a Mooney viscosity ML(l+4) at 100°C greater than 30 and less than 200.

[0017] The star-shaped elastomer according to the invention can be prepared by a process comprising successive steps a), b) and c), - step a) being the polymerization of a monomer mixture of a 1,3-diene and ethylene in the presence of a catalytic system based at least on a metallocene of formula (la) and an organomagnesium co-catalyst, {P(Cp1)(Cp2)Nd(BH4)(1+y>Ly-Nx} (la) Cp1 and Cp2, identical or different, being chosen from the group consisting of fluorenyl groups, cyclopentadienyl groups and indenyl groups, the groups being substituted or unsubstituted, P being a group bridging the two groups Cp1 and Cp2, and comprising a silicon or carbon atom, Nd designating the neodymium atom, L represents an alkali metal chosen from the group consisting of lithium, sodium, and potassium, N representing a molecule of an ether, x, an integer or not, being equal to or greater than 0, y, an integer, being equal to or greater than 0, - step b) being the reaction of tin tetrachloride with the reaction product of the polymerization of step a), - step c) being a chain termination reaction.

[0018] Step a) of the process is a polymerization reaction of a monomer mixture of 1,3-butadiene and ethylene which allows the preparation of statistical copolymer chains of 1,3-butadiene and ethylene, growing chains intended to react in the next step, step b), with a star-forming agent, tin tetrachloride.

[0019] Preferably, the monomer mixture of step a) contains more than 50% by mole of ethylene, the percentage being expressed in relation to the total number of moles of monomers in the monomer mixture of step a).

[0020] The copolymerization of the monomer mixture can be carried out in accordance with patent applications WO 2007054223 A2 and WO 2007054224 A2 using a catalytic system composed of a metallocene and an organomagnesium compound.

[0021] In the present application, the term metallocene means an organometallic complex in which the metal, in this case the neodymium atom, is linked to a molecule called a ligand and consisting of two groups Cp1 and Cp2 linked together by a P-bridge. These groups Cp1 and Cp2, identical or different, are chosen from the group consisting of fluorenyl groups, cyclopentadienyl groups and indenyl groups, these groups being able to be substituted or unsubstituted.

[0022] The metallocene used as a basic constituent in the catalytic system corresponds to the formula (la) {P(Cp1)(Cp2)Nd(BH4)(1+y)_Ly-Nx} (la) P being a group bridging the two groups Cp1 and Cp2, and comprising a silicon or carbon atom, Cp1 and Cp2, identical or different, being chosen from the group consisting of fluorenyl groups, cyclopentadienyl groups and indenyl groups, the groups being substituted or unsubstituted, Nd denoting the neodymium atom, L represents an alkali metal chosen from the group consisting of lithium, sodium, and potassium, N representing a molecule of an ether, x, whether an integer or not, being equal to or greater than 0, y, an integer, being equal to or greater than 0.

[0023] As an ether, any ether which has the power to complex the alkali metal is suitable, in particular diethyl ether, methyltetrahydrofuran and tetrahydrofuran.

[0024] Examples of substituted cyclopentadienyl, fluorenyl, and indenyl groups include those substituted by alkyl radicals having 1 to 6 carbon atoms, or by aryl radicals having 6 to 12 carbon atoms, or by trialkylsilyl radicals such as SiMe3. The choice of radicals is also guided by the accessibility of the corresponding molecules, namely the substituted cyclopentadienes, fluorenes, and indenes, because these are commercially available or easily synthesized.

[0025] Examples of substituted fluorenyl groups include those substituted at positions 2, 7, 3, or 6, particularly 2,7-ditertiobutyl-fluorenyl and 3,6-ditertiobutyl-fluorenyl. Positions 2, 3, 6, and 7 respectively designate the positions of the carbon atoms in the rings, as shown in the diagram below, with position 9 corresponding to the carbon atom to which the P-bridge is attached.

[0026] Examples of substituted cyclopentadienyl groups include those substituted at position 2 (or 5) as well as at position 3 (or 4), particularly those substituted at position 2, more specifically the tetramethylcyclopentadienyl group. Position 2 (or 5) refers to the position of the carbon atom adjacent to the carbon atom to which the P-bridge is attached, as shown in the diagram below. It should be noted that a substitution at position 2 or 5 is also referred to as an alpha-bridge substitution.

[0027] As examples of substituted indenyl groups, particular examples include those substituted at position 2, more specifically 2-methylindenyl and 2-phenylindenyl. Position 2 refers to the position of the carbon atom adjacent to the carbon atom to which the P-bridge is attached, as shown in the diagram below.

[0028] Preferably, Cp1 and Cp2, whether identical or different, are alpha-substituted cyclopentadienyls, substituted fluorenyls, substituted indenyls, or fluorenyls of formula C[3H8], or indenyls of formula C9H7. More preferably, Cp1 and Cp2, whether identical or different, are chosen from the group consisting of substituted fluorenyl groups and the unsubstituted fluorenyl group of formula CnH8. Advantageously, Cp1 and Cp2 are identical and each represent an unsubstituted fluorenyl group of formula Ci3H8, represented by the symbol Flu.The process in which Cp1 and Cp2, identical or different, are chosen from the group consisting of substituted fluorenyl groups and the unsubstituted fluorenyl group of formula CnH8, and advantageously, each represent an unsubstituted fluorenyl group of formula Ci3H8, gives access to the synthesis of ethylene and 1,3-butadiene copolymers which contain, in addition to ethylene units and 1,3-butadiene units, 1,2-cyclohexane cyclic units.

[0029] Preferably, the bridge P connecting the Cp1 and Cp2 groups has the formula ZR'R2, in which Z represents a silicon or carbon atom, and R1 and R2, identical or different, each represent an alkyl group comprising from 1 to 20 carbon atoms, preferably a methyl group. In the formula ZR'R2, Z advantageously represents a silicon atom, Si.

[0030] Most advantageously, R1 and R2 each represent a methyl group and Z represents a silicon atom.

[0031] Better, the metallocene has the formula (1-1), (1-2), (1-3), (1-4) or (1-5): [Me2Si(Flu)2Nd(p-BH4)2Li(THF)] (1-1) [{Me2SiFlu2Nd(p-BH4)2Li(THF)}2] (1-2) [Me2SiFlu2Nd(p-BH4)(THF)] (1-3) [{Me2SiFlu2Nd(p-BH4)(THF)}2] (1-4) [Me2SiFlu2Nd(p-BH4)] (1-5) in which Flu represents the Ci3H8 group.

[0032] The metallocene useful for the synthesis of the catalytic system may be in the form of a crystalline or non-crystalline powder, or in the form of single crystals. The metallocene may be in monomeric or dimeric form, these forms depending on the method of preparation of the metallocene, as described, for example, in patent application WO 2007054224 A2 or WO 2007054223 A2. The metallocene may be prepared in a traditional manner by a process analogous to that described in patent application WO 2007054224 A2 or WO 2007054223 A2, in particular by reacting, under inert and anhydrous conditions, the salt of an alkali metal of the ligand with a rare-earth borohydride, neodymium, in a suitable solvent, such as an ether, like diethyl ether or tetrahydrofuran, or any other solvent known to those skilled in the art.After the reaction, the metallocene is separated from the reaction by-products using techniques known to those skilled in the art, such as filtration or precipitation in a second solvent. The metallocene is then dried and isolated in solid form.

[0033] The organomagnesium compound, another basic constituent of the catalytic system, is the co-catalyst of the catalytic system. Typically, the organomagnesium compound can be a diorganomagnesium compound or a halide of an organomagnesium compound. Preferably, the organomagnesium compound has the formula (lia) in which R3 and R4, which may be identical or different, represent a carbon group. MgR3R4 (lia) A carbon group is defined as a group containing one or more carbon atoms. The carbon group can be a hydrocarbon group (hydrocarbyl group) or a heterohydrocarbon group, that is, a group containing one or more heteroatoms in addition to carbon and hydrogen atoms. The compounds described as transfer agents in patent application WO2016092227 AL are suitable examples of organomagnesium compounds with a heterohydrocarbon group. The carbon groups represented by the symbols R3 and R4 are preferentially hydrocarbon groups.

[0034] The carbon groups represented by R3 and R4 can be aliphatic or aromatic. They can contain one or more heteroatoms such as an oxygen, nitrogen, silicon, or sulfur atom. Preferably, they are alkyl, phenyl, or aryl. They can contain from 1 to 20 carbon atoms.

[0035] The alkyls represented R3 and R4 can contain 2 to 10 carbon atoms and include ethyl, butyl, octyl.

[0036] The aryls represented R3 and R4 can contain 7 to 20 carbon atoms and are in particular a phenyl substituted by one or more alkyls such as methyl, ethyl, isopropyl.

[0037] R3 and R4 are preferably alkyls containing 2 to 10 carbon atoms, phenyls or aryls containing 7 to 20 carbon atoms.

[0038] R3 may comprise a benzene ring substituted by the magnesium atom, one of the carbon atoms of the benzene ring in ortho to the carbon substituted by the magnesium atom being substituted by a methyl, an ethyl, an isopropyl or forming a ring with the carbon atom which is its nearest neighbor and which is in meta, the other carbon atom of the benzene ring in ortho being substituted by a methyl, an ethyl or an isopropyl and R4 is an alkyl, such as an ethyl, a butyl, an octyl.

[0039] R3 and R4 can be ethyl, butyl, octyl.

[0040] For example, suitable organomagnesium compounds include butylethylmagnesium, butylloctylmagnesium, ethylmagnesium chloride, butylmagnesium chloride, ethylmagnesium bromide, butylmagnesium bromide, octylmagnesium chloride, octylmagnesium bromide, 1,3-dimethylphenylbutylmagnesium, 1,3-diethylphenylethylmagnesium, butylmesitylmagnesium, ethylmesitylmagnesium, 1,3-diethylphenylbutylmagnesium, 1,3-diethylphenylethylmagnesium, 1,3-diisopropylphenylbutylmagnesium, 1,3-disopropylphenylethylmagnesium, 1,3,5-triethylphenylbutylmagnesium, 1,3,5-triethylphenylethylmagnesium, 1,3,5-triisopropylphenylbutylmagnesium, 1,3,5-triisopropylphenylethylmagnesium.

[0041] Grignard reagents, compounds of formula (Ha), are well known; some of them are even commercial products. For their synthesis, one can, for example, also refer to the collection of volumes of "Organic Synthesis".

[0042] Like any organomagnesium compound, the organomagnesium compound constituting the catalytic system, in particular of formula (lia), can be in the form of a monomeric entity or in the form of a polymer entity. By way of illustration, the organomagnesium compound (lia) can be in the form of a monomeric entity (R3-Mg-R4)i or in the form of a polymer entity (R3-Mg-R4)p, p being an integer greater than 1, in particular a dimer (R3-Mg-R4)2.

[0043] Furthermore, whether in the form of a monomeric or polymeric entity, the organomagnesium can also be in the form of an entity coordinated to one or more molecules of a solvent, preferably an ether such as diethyl ether, tetrahydrofuran or methyltetrahydrofuran.

[0044] Preferably, the organomagnesium has the formula (lia).

[0045] The quantities of co-catalyst and metallocene put into reaction are such that the ratio between the number of moles of Mg of the co-catalyst and the number of moles of the rare earth of the metallocene, neodymium, preferably goes from 0.5 to 200, more preferably from 1 to less than 20. The range of values ​​from 1 to less than 20 is in particular more favourable for obtaining copolymers of high molar masses.

[0046] The catalytic system can be prepared conventionally by a process analogous to that described in patent application WO 2007054224 A2 or WO 2007054223 A2. For example, the cocatalyst, in this case the organomagnesium compound, and the metallocene are typically reacted in a hydrocarbon solvent at a temperature ranging from 20 to 80°C for a duration of between 5 and 60 minutes. The catalytic system is generally prepared in a hydrocarbon solvent, aliphatic such as methylcyclohexane or aromatic such as toluene, preferably in an aliphatic hydrocarbon solvent such as methylcyclohexane. Generally, after its synthesis, the catalytic system is used as is for step a).

[0047] The catalytic system can also be prepared by a process analogous to that described in patent application WO 2017093654 A1 or in patent application WO 2018020122 A1: it then contains a preforming monomer selected from 1,3-dienes, ethylene, and mixtures thereof, and is said to be of the preformed type. For example, the organomagnesium compound and the metallocene are typically reacted in a hydrocarbon solvent at a temperature of 20 to 80°C for 10 to 20 minutes to obtain a first reaction product. This first reaction product is then reacted with a preforming monomer at a temperature of 40 to 90°C for 1 to 12 hours. The preforming monomer is preferably used in a molar ratio (preforming monomer / metal of the metallocene) of 5 to 1000, preferably 10 to 500.Before its use in polymerization, the preformed catalytic system can be stored under an inert atmosphere, specifically at a temperature ranging from -20°C to room temperature (23°C). The basic constituent of the preformed catalytic system is a preforming monomer chosen from among 1,3-dienes, ethylene, and mixtures thereof. In other words, the so-called preformed catalytic system contains, in addition to the metallocene and the co-catalyst, a preforming monomer. The 1,3-diene used as the preforming monomer can be 1,3-butadiene, isoprene, or a 1,3-diene with the formula CH2=CR5-CH=CH2, the symbol R5 representing a hydrocarbon group having 3 to 20 carbon atoms, in particular myrcene or 3-famesene. The preforming monomer is preferentially a 1,3-diene, more preferably 1,3-butadiene.

[0048] The catalytic system is typically present in a solvent which is preferably the solvent in which it was prepared, and the concentration of rare earth metal, i.e. neodymium, of metallocene is then in a range preferably from 0.0001 to 0.2 mol / L more preferably from 0.001 to 0.03 mol / L.

[0049] As with any synthesis carried out in the presence of an organometallic compound, the synthesis of the metallocene, the synthesis of the organomagnesium compound, and the synthesis of the catalytic system take place under anhydrous conditions in an inert atmosphere. Typically, the reactions are conducted using solvents and anhydrous compounds in anhydrous nitrogen or argon.

[0050] The polymerization of the monomer mixture is carried out in a reactor, preferably in solution, either continuously or batchwise. The polymerization solvent is typically a hydrocarbon solvent, preferably aliphatic. Methylcyclohexane is a particularly suitable example of an aliphatic hydrocarbon solvent. The monomer mixture can be introduced into the reactor containing the polymerization solvent and the catalytic system, or conversely, the catalytic system can be introduced into the reactor containing the polymerization solvent and the monomer mixture. The monomer mixture and the catalytic system can be introduced simultaneously into the reactor containing the polymerization solvent, particularly in the case of continuous polymerization. The polymerization is typically carried out under anhydrous conditions and in the absence of oxygen, possibly in the presence of an inert gas.The polymerization temperature generally varies in a range of 40 to 150°C, preferably 40 to 120°C. Those skilled in the art adapt the polymerization conditions, such as the polymerization temperature, the concentration of each of the reactants, and the pressure in the reactor, according to the composition of the monomer mixture, the polymerization reactor, and the desired microstructure and macrostructure of the copolymer chain.

[0051] Polymerization is preferably carried out at constant pressure in monomers. A continuous addition of each or one of the monomers can be made to the polymerization reactor, in which case the polymerization reactor is a fed reactor. This embodiment is particularly suitable for the statistical incorporation of monomers.

[0052] Once the desired monomer conversion rate is reached in the polymerization reaction of step a), step b is carried out.

[0053] Step b) of the process involves reacting the reaction product of step a) with tin tetrachloride. Step b) is a star-linking reaction of the copolymer chains, one end of which reacts with tin tetrachloride. After deactivation of the reactive sites by a polymer chain termination reaction (step c), it is obtained a copolymer of a 1,3-diene and ethylene which contain copolymer chains linked together by a tin atom, in particular 3-branch star chains linked together by a tin atom, 4-branch star chains linked together by a tin atom or a mixture thereof.

[0054] Preferably, step b) is carried out in an aliphatic hydrocarbon solvent, such as methylcyclohexane. Advantageously, it is carried out in the reaction medium from step a). It is generally performed by adding tin tetrachloride to the reaction product of step a) in its reaction medium under stirring. Steps a) and b) are advantageously carried out in an aliphatic hydrocarbon solvent.

[0055] Before adding the tin tetrachloride, the reactor is preferably degassed and inert. Degassing the reactor removes residual gaseous monomers and also facilitates the addition of the tin tetrachloride. Alternatively, the tin tetrachloride can be injected into the reactor under pressure. Inertizing the reactor, for example with nitrogen, prevents the carbon-metal bonds present in the reaction medium, which are necessary for the star-forming reaction of the copolymer chains, from being deactivated. The tin tetrachloride can be added pure or diluted in a hydrocarbon solvent, preferably aliphatic such as methylcyclohexane or aromatic such as toluene. The tin tetrachloride is left in contact with the reaction product of step a) for the time required for the star-forming reaction.The star-forming reaction can typically be monitored by chromatographic analysis to track the consumption of the star-forming agent or by measuring the viscosity of the polymer solution. The star-forming reaction is preferably carried out at a temperature of 23 to 120 °C for 1 to 60 minutes with stirring. Step b) is preferably conducted with a molar ratio of tin tetrachloride to carbon-magnesium bonds per mole of cocatalyst in the catalytic system ranging from 0.01 to 1, preferably between 0.1 and 0.5.The ratio of the number of moles of tin tetrachloride to the number of carbon-magnesium bonds per mole of cocatalyst in the catalytic system can vary depending on the desired level of star polymer in the polymer obtained at the end of step c), the desired number of branches in the star copolymer, and, in the case of obtaining a mixture of star copolymers with different numbers of branches, their respective proportions. A ratio between 0.1 and 0.5 favors the highest levels of star formation. Advantageously, step b), the star formation reaction, is carried out with a ratio of the number of moles of tin tetrachloride to the number of carbon-magnesium bonds per mole of cocatalyst in the catalytic system between 0.1 and 0.5. Typically, in a diorganomagnesium compound such as butylloctylmagnesium (BOMAG), there are two carbon-magnesium bonds per mole. of the magnesium. The ratio between the number of moles of tin tetrachloride and the number of carbon-magnesium bonds per mole of co-catalyst of the catalytic system therefore corresponds to the ratio between the number of moles of tin tetrachloride introduced into the reaction medium and the number of moles of carbon-magnesium bonds provided by the catalytic system.

[0056] Once the end of the chain has been modified, step b) is followed by step c).

[0057] Step c), the chain-terminating reaction, is typically a reaction that deactivates the reactive sites still present in the reaction medium from step b). In step c), a chain-terminating agent is brought into contact with the reaction product of step b), generally in its reaction medium, for example by adding the terminating agent to the reaction medium after step b) or by pouring the reaction medium obtained after step b) onto a solution containing the terminating agent. The terminating agent is generally in stoichiometric excess. The terminating agent is typically a protic compound, a compound that contains a relatively acidic proton. Examples of terminating agents include water, carboxylic acids (particularly C2-Ci8 fatty acids such as acetic acid and stearic acid), aliphatic or aromatic alcohols such as methanol, ethanol, and isopropanol, and phenolic antioxidants.

[0058] After reaction with a protic compound, the process leads to the star copolymer useful for the purposes of the invention. The copolymer thus prepared can be separated from the reaction medium of step c) by methods well known to those skilled in the art, for example by evaporation of the solvent under reduced pressure or by steam stripping. The recovered elastomer contains the copolymer of a 1,3-diene and ethylene useful for the purposes of the invention and may also contain copolymer chains that are not star-shaped. Their presence may result from the deactivation of some of the growing copolymer chains obtained in step a).

[0059] In the case where the star copolymer is made up of a mixture of star copolymers which differ from each other by their microstructures or by their macrostructures, the rate of star copolymer in the rubber composition relates to the mixture of star copolymer.

[0060] The star elastomer according to the invention is typically used in a rubber composition, another object of the invention, reinforced mainly by mass with carbon black.

[0061] The rubber composition according to the invention may contain, in addition to the star copolymer, a second diene elastomer. A diene elastomer is understood to be an elastomer consisting at least in part (i.e., a homopolymer or a copolymer) of diene monomer units (monomers bearing two double bonds). carbon-carbon bonds, conjugated or not). The second elastomer may be a copolymer of ethylene and 1,3-butadiene that is not star-shaped or does not contain tin atoms in its copolymer chains. The second diene elastomer may also be chosen from the group of highly unsaturated diene elastomers, consisting of polymers containing 1,3-butadiene units or isoprene units, such as polybutadienes, 1,3-butadiene copolymers, and isoprene copolymers. A highly unsaturated diene elastomer is defined as one that contains more than 50 mole percent diene units.

[0062] According to any one of the embodiments of the invention, the percentage of star copolymer in the rubber composition is preferably greater than 30 pc.

[0063] The essential characteristic of the rubber composition is that it contains a reinforcing filler. The reinforcing filler used for the purposes of the invention contains more than 50% to 100% carbon black by mass. The mass percentage of carbon black in the reinforcing filler is expressed relative to the mass of the reinforcing filler in the rubber composition. When the mass percentage of carbon black in the rubber composition is less than 100% of the mass of the reinforcing filler, the reinforcing filler may therefore comprise any type of filler other than carbon black that is also known for its ability to reinforce a rubber composition usable for the manufacture of tires, for example, reinforcing silica. Preferably, the reinforcing filler contains more than 80% carbon black by mass. Even more preferably, the reinforcing filler contains more than 90% carbon black by mass.

[0064] All reinforcing carbon blacks are suitable, particularly those conventionally used in tires or their treads (so-called tire-grade blacks). Among these, reinforcing carbon blacks of the 100, 200, and 300 series (ASTM grades) are particularly suitable, such as NI 15, N134, N234, N326, N330, N339, N347, and N375. Carbon black can also be a mixture of carbon blacks, in which case the mass percentage of carbon black refers to the mixture of carbon blacks. When the rubber composition according to the invention is used in a tread, the carbon black is preferably a carbon black of the 100 or 200 series. Preferably, the carbon black has a specific surface area (BET) greater than 90 m² / g. More preferably, the specific surface area BET of carbon black is greater than 100 m2 / g.Preferably, carbon black has a specific surface area BET of less than 145 m² / g. More preferably, carbon black has a specific surface area BET of less than 130 m² / g. The specific surface area BET is measured. typically according to ASTM D6556-09 [multipoint method (5 points) - gas: nitrogen - relative pressure range P / PO: 0.05 to 0.30].

[0065] The rate of reinforcing filler in the rubber composition varies preferably from 30 pc to 60 pc, preferably from 30 pc to 50 pc, more preferably from 35 pc to 45 pc.

[0066] The rubber composition contains a vulcanization system. The vulcanization system typically comprises sulfur and a vulcanization accelerator.

[0067] Sulfur is typically supplied in the form of molecular sulfur or a sulfur-donating agent, preferably in molecular form. Molecular sulfur is also referred to as molecular sulfur. A sulfur donor is defined as any compound that releases sulfur atoms, whether or not combined in a polysulfide chain, capable of inserting themselves into the polysulfide chains formed during vulcanization and bridging the elastomeric chains. The sulfur content in the rubber composition is preferably less than 2 parts per million (ppm), preferably between 0.3 and 1.5 ppm.

[0068] Any compound capable of acting as a vulcanization accelerator for diene elastomers in the presence of sulfur can be used as a vulcanization accelerator (primary or secondary). Examples include thiazole-type accelerators and their derivatives, sulfenamide-type accelerators for primary accelerators, and guanidine, thiuram, dithiocarbamate, dithiophosphate, thiourea, and xanthate-type accelerators for secondary accelerators. The vulcanization accelerator is used at a preferential rate of between 0.3 and 5 parts per million (ppm), more preferably between 0.5 and 2.5 ppm. Examples of primary accelerators include sulfenamide compounds such as N-cyclohexyl-2-benzothiazyl sulfenamide ("CBS"), N,N-dicyclohexyl-2-benzothiazyl sulfenamide ("DCBS"), N-ter-butyl-2-benzothiazyl sulfenamide ("TBBS"), and mixtures of these compounds.The primary accelerator is preferably a sulfenamide, more preferably N-cyclohexyl-2-benzothiazyl sulfenamide. Examples of secondary accelerators include thiuram polysulfides, preferably thiuram disulfides such as tetraethylthiuram disulfide, tetrabutylthiuram disulfide ("TBTD"), tetrabenzylthiuram disulfide ("TBZTD"), and mixtures of these compounds. The secondary accelerator is preferably a thiuram disulfide, more preferably tetrabenzylthiuram disulfide.

[0069] The vulcanization accelerator is preferably a sulfenamide. When the vulcanization accelerator is a sulfenamide, it is preferably N-cy clohexy 1,2-benzothiazy sulfenamide.

[0070] As is well known, the vulcanization system may also include vulcanization activators such as metal oxides like zinc oxide or fatty acids such as stearic acid.

[0071] The rubber composition according to the invention may also include all or part of the usual additives commonly used in elastomer compositions intended to constitute treads, such as pigments, protective agents such as anti-ozone waxes, chemical anti-ozonants, antioxidants.

[0072] The rubber composition according to the invention can be manufactured in suitable mixers, using two successive preparation phases according to a general procedure well known to those skilled in the art: a first thermo-mechanical working or mixing phase (sometimes referred to as the "non-productive" phase) at high temperature, up to a maximum temperature between 110°C and 190°C, preferably between 130°C and 180°C, followed by a second mechanical working phase (sometimes referred to as the "productive" phase) at a lower temperature, typically below 110°C, for example between 40°C and 100°C, a finishing phase during which the sulfur or sulfur donor and the vulcanization accelerator are incorporated. As an example, the first (non-productive) phase is carried out in a single thermomechanical step during which all the necessary constituents, any additional processing agents, and other miscellaneous additives, with the exception of sulfur and the vulcanization accelerator, are introduced into a suitable mixer, such as a standard internal mixer. The total mixing time in this non-productive phase is preferably between 1 and 15 minutes. After the mixture obtained in the first non-productive phase has cooled, the sulfur and the vulcanization accelerator are then incorporated at low temperature, generally in an external mixer such as a roller mixer. The mixture is then blended (productive phase) for a few minutes, for example, between 2 and 15 minutes.

[0073] After the incorporation of all the ingredients of the rubber composition, the final composition thus obtained can be calendered, for example in the form of a sheet or plate, in particular for characterization in the laboratory, or extruded, for example to form a rubber profile used as a rubber component or semi-finished product, in particular for the manufacture of a tire.

[0074] Thus, according to a preferred embodiment of the invention, the rubber composition according to the invention, which can be either in the raw state (before vulcanization) or in the cured state (after vulcanization), is in a tire, for example in a tire tread.

[0075] Vulcanization is carried out in a known manner at a temperature generally between 130°C and 200°C, for a sufficient time which can vary for example between 5 and 120 min depending in particular on the cooking temperature, the crosslinking system adopted and the crosslinking kinetics of the composition considered.

[0076] Preferably, the rubber composition according to the invention is extruded to form all or part of a tread profile of a tire. Then, during the assembly of a tire, which typically comprises, radially from the outside in, a tread, a crown reinforcement, and a carcass reinforcement, the tread is laid radially on the outside of the crown reinforcement. Radially means in a known direction radial to the axis of rotation of the tire.

[0077] The tire may be in its raw state (i.e., before the tire curing stage) or in its cured state (i.e., after the tire curing stage). The tire is preferably a tire for a vehicle carrying heavy loads, in particular a tire for a heavy goods vehicle or a tire for construction equipment, preferably a tire for a heavy goods vehicle.

[0078] In the present invention, the term "tire" (in English, "tire") refers to a pneumatic or non-pneumatic tire. A pneumatic tire typically comprises two beads for contact with a rim, a crown consisting of at least one crown reinforcement and a tread, and two sidewalls. The tire is reinforced by a carcass reinforcement anchored in the two beads. A non-pneumatic tire, on the other hand, typically comprises a base, designed, for example, for mounting on a rigid rim, a crown reinforcement connecting to a tread, and a deformable structure, such as spokes, ribs, or dimples, this structure being arranged between the base and the crown. Such non-pneumatic tires do not necessarily include sidewalls. Non-pneumatic tires are described, for example, in documents WO 03 / 018332 and FR2898077.According to any one of the embodiments of the invention, the tire according to the invention is preferably a pneumatic tire.

[0079] Preferably, in the tire according to the invention, the rubber composition according to the invention constitutes all or part of the tread of the tire.

[0080] The aforementioned features of the present invention, as well as others, will be better understood upon reading the following description of the examples of embodiments of the invention, given by way of illustration. Examples Size exclusion chromatography (SEC):

[0081] a) Principle of measurement: Size exclusion chromatography (SEC) separates macromolecules in solution according to their size using columns filled with a porous gel. The macromolecules are separated according to their hydrodynamic volume, with the largest being eluted first. Combined with three detectors (3D), a refractometer, a viscometer, and a 90° light scattering detector, SEC allows for the determination of the absolute molar mass distribution of a polymer. The various absolute molar masses, number average (Mn), weight average (Mw), and dispersity (D = Mw / Mn) can also be calculated.

[0082] b) Polymer preparation: Each sample is solubilized in tetrahydrofuran at a concentration of approximately 1 g / L. The solution is then filtered through a 0.45 µm porosity filter before injection.

[0083] c) SEC 3D Analysis: To determine the number-average molar mass (Mn), and where applicable the weight-average molar mass (Mw) and the polydispersity index (Ip or also noted D = Mw / Mn) of the polymers, the method below is used. The number-average molar mass (Mn), weight-average molar mass (Mw), and polydispersity index of the polymer (hereafter referred to as the sample) are determined in absolute terms by triple-detection size exclusion chromatography (SEC). Triple-detection size exclusion chromatography has the advantage of directly measuring average molar masses without calibration. To determine the average molar masses, a previously prepared and filtered 1 g / L tetrahydrofuran solution is injected into the chromatography system. The equipment used is a Wyatt chromatography system. The elution solvent is tetrahydrofuran containing 250 ppm BHT (2,6-diter-butyl 4-hydroxytoluene), the flow rate is 1 mL / min*, the system temperature is 35°C, and the analysis time is 60 min. The columns used are a set of three AGILENT columns, commercially known as "PL GEL MIXED B LS". The injected volume of the sample solution is 100 pL. The detection system consists of a Wyatt differential viscometer (commercial name "VISCOSTAR II"), a Wyatt differential refractometer (commercial name "OPTILAB T-REX") with a wavelength of 658 nm, and a static light scattering detector. Wyatt multi-angle with a wavelength of 658 nm and the trade name "DA WN HELEOS 8+". The data is acquired and processed using ASTRA software and the dn / dc of ethylene-based copolymers is considered to be 0.1000 by default.

[0084] Nuclear magnetic resonance (NMR): Ethylene-1,3-butadiene copolymers are characterized by ¹H and ¹³C NMR spectroscopy. NMR spectra are recorded on a Brüker Avance III 500 MHz spectrometer equipped with a 5 mm BBIz-grad "broadband" cryo-probe. The quantitative ¹H NMR experiment uses a single 30° pulse sequence and a 5-second repetition delay between acquisitions. 64 to 256 accumulations are performed. The quantitative ¹³C NMR experiment uses a single 30° pulse sequence with proton decoupling and a 10-second repetition delay between acquisitions. 1024 to 10240 accumulations are performed. The two-dimensional ¹H / ¹³C experiments are used to determine the polymer structure. The determination of the microstructure of copolymers is defined in the literature, according to the article by Llauro et al., Macromolecules 2001, 34, 6304-6311.

[0085] Inherent viscosity: The inherent viscosity at 25 °C of a 0.1 g / dL polymer solution in toluene is measured from a dry polymer solution. The inherent viscosity is determined by measuring the flow time t of the polymer solution and the flow time to of the toluene in a capillary tube. In an Ubbelhode tube (capillary diameter 0.46 mm, capacity 18 to 22 mL), placed in a water bath thermostated at 25 ± 0.1 °C, the flow time of the toluene and that of the 0.1 g / dL polymer solution are measured. The inherent viscosity is obtained using the following relationship: Pinh = [ln (t / to)] / C with : C: concentration of the polymer toluene solution in g / dL; t: flow time of the toluene polymer solution in seconds; t0: flow time of the toluene in seconds; Pinh: inherent viscosity expressed in dl / g.

[0086] Determination of the glass transition temperature of copolymers: The glass transition temperature (Tg) and the glass transition width AT are measured using a Differential Scanning Calorimeter (DSC) according to ASTM D3418 (1999).

[0087] Mooney Viscosity: Mooney viscosity is measured using an oscillating consistometer as described in ASTM D1646 (1999). The measurement is performed according to the following principle: the sample being analyzed in its raw state (i.e., before cooking) is molded (shaped) in a cylindrical chamber heated to a given temperature (100°C). After 1 minute of preheating, the rotor rotates inside the specimen at 2 revolutions per minute, and the torque required to maintain this movement is measured after 4 minutes of rotation. Mooney viscosity (ML) is expressed in "Mooney units" (MU, with 1 MU = 0.83 Newton-meters).

[0088] Dynamic properties: The dynamic properties, namely the dynamic loss, tanô (max) at 60°C, and the complex shear modulus, G* at 60°C, are measured on a Metravib VA4000 viscoanalyzer. The response of a vulcanized composition sample, in the form of a test specimen, is recorded under a sinusoidal alternating simple shear loading at a frequency of 10 Hz under specified temperature conditions (60°C) according to ASTM DI349-99. A strain amplitude sweep is performed from 0.1% cc to 100% cc (forward cycle), then from 100% cc to 0.1% cc (reverse cycle), cc meaning peak-to-peak. The tangent tanô of the phase angle ô between the force exerted on the sample and its displacement represents a dynamic loss and is equal to the ratio G” / G’. The maximum value tanô (max) of the tangent tanô of the phase angle ô observed over the forward deformation cycle is recorded. The complex dynamic shear modulus G* is defined as the square root of the sum of the squares of G' and G” where G' represents the elastic modulus and G” represents the viscous modulus. The complex shear modulus G* is measured at 50% cc of strain on the forward cycle. The specimen is of cylindrical cross-section as described in ASTM D 5992-96 (version reapproved in 2011, originally approved in 1996) in Figure X2.1 (circular embodiment) and has a diameter of 10 mm [0 to +0.04 mm] and a thickness of 2 mm [1.83-2.33]. The lower the value of G* at 50%, the lower the stiffness of the rubber composition and the lower the wear resistance performance; the higher the value of tanô(max), the higher the hysteresis of the rubber composition, and the lower the rolling resistance performance. Preparation of copolymers:

[0089] Metallocene [{Me2SiFlu2Nd(p-BH4)2Li(THF)}]2 is prepared according to the procedure described in patent application WO 2007054224. BOMAG butyl methylmagnesium (20% in heptane, at 0.88 mol L1) comes from Chemtura and is stored in a Schlenk tube under an inert atmosphere. The ethylene, of N35 grade, comes from the company Air Liquide and is used without prior purification. 1,3-Butadiene is purified over alumina guards. Tin tetrachloride is marketed by Fox Chemicals. The methylcyclohexane (MCH) solvent from BioSolve is dried and purified on an alumina column in a solvent fountain from mBraun and used under an inert atmosphere.

[0090] All reactions are carried out under an inert atmosphere.

[0091] Example 1: Preparation of an ethylene-1,3-butadiene copolymer EBR1 not in accordance with the invention: In a 70 L reactor containing methylcyclohexane (64 L), ethylene, and 1,3-butadiene at a butadiene / ethylene (btd / eth) mass ratio of 0.76, BOMAG butylclotylmagnesium is added in the amounts indicated in Table 1, followed by the catalytic system (0.006 mol). At this point, the reaction temperature is regulated to 80°C, and the polymerization reaction begins. The polymerization reaction proceeds at a constant pressure of 8 bar. The reactor is fed with ethylene and 1,3-butadiene at a butadiene / ethylene mass ratio of 0.76 throughout the polymerization process. The polymerization reaction is stopped by cooling, degassing the reactor, and adding ethanol. An antioxidant is added to the polymer solution. The copolymer is recovered after steam stripping and drying to a constant mass. The weighed mass allows the average catalytic activity of the catalytic system to be determined, expressed in kilograms of polymer synthesized per mole of neodymium metal per hour (kg / mol.h).

[0092] Example 2: Preparation of an ethylene and 1,3-butadiene copolymer EBR2 according to the invention: In a 70 L reactor containing methylcyclohexane (64 L), ethylene, and 1,3-butadiene at a butadiene / ethylene mass ratio of 0.76, BOMAG butylmagnesium is added in the proportions indicated in Table 1, followed by the catalytic system (0.006 mol). At this point, the reaction temperature is regulated to 80°C, and the polymerization reaction begins. The polymerization reaction proceeds at a constant pressure of 8 bar. The reactor is fed with ethylene and 1,3-butadiene at a butadiene / ethylene mass ratio of 0.76 throughout the polymerization process. After polymerization, the monomer injection is stopped, and a 71 g / L SnCl4 solution in methylcyclohexane is introduced into the polymerization medium, which is then stirred for 15 minutes. After the reaction, the reaction mixture is allowed to cool and ethanol is injected followed by an antioxidant. The copolymer is recovered after steam stripping and drying to a constant mass. The weighed mass allows the average catalytic activity of the catalytic system to be determined, expressed in kilograms of polymer synthesized per mole of neodymium metal per hour (kg / mol.h).

[0093] The catalytic system used in the preparation of EBR1 and EBR2 is prepared according to the following procedure: In a reactor containing 100 mL of methyl γ-clohexane (MCH) hydrocarbon solvent, the co-catalyst butyl methylmagnesium (BOMAG) is added with a molar ratio Mg / Nd = 2.2, then butadiene with a molar ratio butadiene / Nd = 90. Metallocene [Me2Si(Flu)2Nd(q-BH4)2Li(THF)] is then added to the reaction medium (0.71 mmol). The pre-formation takes place at a temperature of 80°C for 5 hours.

[0094] The copolymers are analyzed by SEC 3D, NMR, DSC.

[0095] The copolymerization and star-forming reaction conditions specific to each example, in particular the ratio between the number of moles of tin tetrachloride introduced into the reaction medium and the number of carbon-magnesium bonds provided by the catalytic system, are shown in Table 1, the characteristics of the synthesized copolymers are shown in Table 2. The SEC 3D method was used to determine the number average molar masses and the ethylene content determined by NMR and expressed as a molar percentage relative to all the monomer units of the copolymer.

[0096] The viscosity values ​​measured using an Ostwald viscometer and reported in Table 1 show that the EBR2 copolymer synthesized according to the process according to the invention (Example 2) has a higher viscosity than the EBR1 copolymer of Example 1 not according to the invention (Example 1) and indicate that the reaction with tin tetrachloride leads to the formation of star copolymers. The increase in average molar masses in number and reported in Table 2 observed between Example 2 and Example 1 confirms the formation of star chains, particularly 4-branch chains, since the average molar masses increased by a factor of approximately 4. The observed increases in viscosity reflect a change in the rheological properties of the copolymer of Example 2 compared to the non-star copolymer of Example 1, which confirms the star formation reaction and the formation of a star elastomer.The proportion of starred chains is quantified at approximately 30% according to the method described below.

[0097] SEC 3D analysis makes it possible to determine the intrinsic viscosity values ​​for each number-average molar mass over the entire polymer distribution. From the Mark-Houvink-Sakurada relationship linking intrinsic viscosity to molar mass number average according to the equation In(visco) = ln(K) + aln(Mn), the coefficient a, called the architecture coefficient, can be calculated for a population of polymer with average molar mass. It is known to those skilled in the art that the architecture of a polymer is related to the coefficient a. If this coefficient decreases, then it indicates that the viscosity of the polymer changes little as a function of its molar mass and that the polymer is architectural, that is to say, star-shaped. Exploitation of the Mark-Houvink-Sakurada relationship shows that the reaction with tin tetrachloride leads to a mixture containing approximately 30% by mass of 4-branched star polymers.

[0098] Finally, as reported in Table 2, the results of the DSC analysis indicate that each of the copolymers exhibits a single Tg value, as well as a relatively low AT value (less than 6°C). All of these data demonstrate the production of statistical copolymers.

[0099] Table 1: Example mass ratio (BOMAG) (BOMAG active if) (mmol) Molar ratio S n / C-Mg bond Mother copolymer mass (kg) Copolymerization time (min) 1 0.76 30.5 (27) 0 8.37 204 2 0.76 32.4 (29) 0.3 7.18 193

[0100] Table 2: Example Inherent viscosity (dL / g) Mn SEC 3 D (g / mol) ML(l+4 ) 100°C Cold flow (g) Tg (AT) (°C) Ethylene unit (% mol) Cyclic unit (% mol) 1 1.38 119100 66 4.2808 -42.4°C (5.7) 69 8 2 1.83 114800 445100 121 0 -43°C (5.8) 69 8

[0101] Preparation of Cl and C2 rubber compositions:

[0102] In an internal mixer with a volume of 3300 cm³ (final filling rate: approximately 70% by volume), whose initial tank temperature is approximately 40°C, the following ingredients are successively introduced: Telatomer, the reinforcing charge, and the various other ingredients, with the exception of sulfur and the primary accelerator. a thermomechanical process (non-productive phase) in one step, which lasts in total about 3 to 4 min, until reaching a maximum "drop" temperature of 165°C. The mixture thus obtained is recovered, cooled and then the sulfur and the primary accelerator are incorporated on an external mixer (roller mixer) at 30°C, mixing everything (productive phase) for 10 minutes.

[0103] The compositions thus obtained are then calendered, either in the form of plates (with a thickness ranging from 2 to 3 mm) or thin sheets of rubber, for the measurement of their physical or mechanical properties after vulcanization at 140°C (cured state), or in the form of profiles directly usable, after cutting and / or assembly to the desired dimensions, for example as semi-finished products for tires.

[0104] The details of the formulations of the rubber compositions Cl and C2 are shown in Table 3. Compositions Cl and C2 differ from each other in the nature of the elastomer. Composition C2, which contains the elastomer EBR2, conforms to the invention; composition Cl, which contains the elastomer EBR1, does not conform to the invention. Composition Cl is considered a control composition.

[0105] The results are shown in Table 3. The results are expressed as a base of 100 relative to the control composition ([value of the composition considered / value of the control composition]x100). A value greater than 100 indicates a value higher than that of the control.

[0106] Table 3: Composition Cl C2 EBR1 100 EBR2 100 Carbon black (1) 40 40 Paraffin 1 1 Antioxidant (2) 2 2 Stearic acid (3) 1.5 1.5 ZnO (4) 2.5 2.5 Accelerator (5) 0.9 0.9 Sulfur 0.9 0.9 Properties at tanô (max) 60°C 100 77 G*50%, 60°C 100 119

[0107] (1) Carbon black N234 (BET of 120 m2 / g) (2) N-(l,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine “Santaflex 6PPD” from the company Flexys (3) Stearine “Pristerene 4931” from the company Uniqema (4) Industrial grade zinc oxide from Umicore (5) N-cyclohexyl-2-benzothiazyl sulfenamide “Santocure CBS” from Flexys

[0108] It is observed that composition C2 exhibits both lower hysteresis than that of the Cl composition and a higher rigidity than that of the Cl composition. Surprisingly, substituting the tin-coated EBR2 star elastomer for the non-star elastomer in a rubber composition reinforced primarily with carbon black results in an increase in rigidity without an increase in hysteresis. On the contrary, it decreases. Using the C2 composition in a tire tread instead of the Cl composition improves its wear resistance performance while also improving its rolling resistance.

Claims

Demands

1. Star elastomer which is a statistical copolymer of 1,3-butadiene and ethylene, which copolymer contains more than 50% to less than 80% by mole of ethylene unit and is made up of copolymer chains linked together by a tin atom.

2. Star elastomer according to claim 1 which consists of 3-branch star chains linked together by a tin atom, 4-branch star chains linked together by a tin atom or a mixture thereof.

3. Star elastomer according to claim 1 or 2 which consists of 4-branch star chains linked together by a tin atom.

4. Star elastomer according to any one of claims 1 to 3 which contains less than 75% by moles of unit ethylene.

5. Star elastomer according to any one of claims 1 to 4 which contains 1,2-cyclohexane cyclic units.

6. Elastomer according to claim 5 containing at most 15% by mole of 1,2-cyclohexane cyclic units.

7. Elastomer according to claim 5 or 6 which contains at most 10% by mole of 1,2-cyclohexane cyclic units.

8. Star elastomer according to any one of claims 1 to 7 which has a Mooney viscosity ML(l+4) at 100°C greater than 30.

9. Rubber composition comprising a star elastomer according to any one of claims 1 to 8, a reinforcing filler containing more than 50% to 100% by mass of carbon black and a crosslinking system.

10. Rubber composition according to claim 9 in which the reinforcing filler contains more than 80% by mass of carbon black.

11. Rubber composition according to claim 9 or 10 wherein the reinforcing filler contains more than 90% by mass of carbon black.

12. Rubber composition according to any one of claims 9 to 11 wherein the reinforcing filler ratio varies from 30 pc to 60 pc, preferably from 30 pc to 50 pc, more preferably from 35 pc to 45 pc.

13. Tire comprising a tread, which tire comprises a rubber composition according to any one of claims 9 to 12.

14. Tire according to claim 13 wherein the rubber composition constitutes all or part of the tread of the tire.