Functional diene elastomer, and rubber composition containing same

Abstract

The invention relates to a diene elastomer, which diene elastomer is a copolymer of 1,3-butadiene and styrene and is functionalized by pendant imidazole groups, the diene elastomer containing 1,4-trans chain butadiene units and 1,2-chain butadiene units representing respectively more than 70 mol % of the butadiene units of the diene elastomer and less than 10 mol % of the butadiene units of the diene elastomer. The diene elastomer provides a rubber composition, in particular for a tyre tread, with excellent crack propagation resistance properties.

Description

[0001] Functional diene elastomer and rubber composition containing same

[0002] The present invention relates to a functional diene elastomer for use in a rubber composition, in particular a tire rubber composition.

[0003] The Applicant has described copolymers of 1,3-butadiene and styrene having a very low content of 1,2-chain, typically less than 8 mol% of the butadiene units, and a high content of 1,4-trans chain, typically greater than 70 mol% of the butadiene units, while retaining an elastomeric character. The synthesis of such elastomers, for example described in documents FR 2273822 A1 and

[0004] WO 2004099277 Al, generally requires the use of a catalytic composition containing three organometallic compounds, namely a barium alcoholate, an organoaluminum and an organolithium. These elastomers are distinguished from emulsion elastomers, diene elastomers synthesized by radical emulsion, in particular by their low 1,2-chain content and their high 1,4-trans chain content. They are also known to give a rubber composition mechanical properties at break that are much superior to rubber compositions containing emulsion diene elastomers, as demonstrated in document FR 227822 Al.

[0005] There is still a need to improve the mechanical properties of rubber compositions containing diene elastomers having a very low content of 1,2-chain and a high content of 1,4-trans chain.

[0006] The inventors discovered that the crack propagation resistance of rubber compositions containing a copolymer of 1,3-butadiene and styrene having a very low 1,2-chain content and a high 1,4-trans chain content was improved by the introduction of pendant imidazole functions into the copolymer.

[0007] Thus, a first subject of the invention is a diene elastomer which is a copolymer of 1,3-butadiene and styrene and which is functionalized by pendant imidazole groups of formula (I) in which:

[0008] Yi represents a hydrogen atom or a carbon chain which may contain at least one heteroatom,

[0009] Y2 denotes an attachment to a butadiene unit of the diene elastomer, which diene elastomer contains 1,4-trans butadiene units and 1,2-trans butadiene units, the 1,4-trans butadiene units representing more than 70 mol% of the butadiene units of the diene elastomer, the 1,2-trans butadiene units representing less than 10 mol% of the butadiene units of the diene elastomer.

[0010] A second subject of the invention is a rubber composition which comprises a diene elastomer in accordance with the invention, a reinforcing filler and a crosslinking system.

[0011] A third subject of the invention is a process for preparing a rubber composition in accordance with the invention, which process comprises the following steps: - during a so-called non-productive step, kneading a starting diene elastomer and a 1,3-dipolar compound by thermomechanical kneading, the starting diene elastomer being a copolymer of 1,3-butadiene and styrene which contains more than 70 mol% of 1,4-trans chained butadiene units and less than 10 mol% of 1,2-chained butadiene units, the molar percentages of the 1,4-trans chained butadiene units and the 1,2-chained butadiene units being calculated relative to the total number of butadiene units in the starting diene elastomer, the 1,3-dipolar compound being a compound containing a dipole chosen from a nitrile oxide,a nitrile imine and a nitrone and containing a group of formula (II) in which Zi represents a hydrogen atom or a carbon chain which may contain at least one heteroatom and Z2 denotes an attachment to the dipole,

[0012] - then add the reinforcing filler, if applicable the other ingredients of the rubber composition with the exception of the crosslinking system by thermomechanically mixing until reaching a maximum temperature of between 130 and 200°C to obtain a mixture,

[0013] - cool the mixture to a temperature below 100°C,

[0014] - then incorporate the crosslinking system,

[0015] -kneading the mixture and the crosslinking system up to a maximum temperature below 120°C. A fourth subject of the invention is a tire comprising a rubber composition preferably in its tread, which rubber composition is a rubber composition in accordance with the invention or obtained according to a process in accordance with the invention for preparing said composition.

[0016] Detailed description

[0017] The abbreviation "pce" means parts by weight per hundred parts of elastomer (of the total elastomers if more than one elastomer is present).

[0018] On the other hand, 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., excluding the limits a and b), 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 limits a and b).

[0019] The compounds mentioned in the description may be of fossil or bio-sourced origin. 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 in use, 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.

[0020] In the present invention, the term "tyre" means a pneumatic or non-pneumatic tire. A pneumatic tire usually comprises two beads intended to come into contact with a rim, a crown composed of at least one crown reinforcement and a tread, two sidewalls, the tire being reinforced by a carcass reinforcement anchored in the two beads. A non-pneumatic tire, for its part, usually comprises a base, designed for example for mounting on a rigid rim, a crown reinforcement, ensuring the connection with a tread and a deformable structure, such as spokes, ribs or cells, this structure being arranged between the base and the crown. Such non-pneumatic tires do not necessarily comprise a sidewall. 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 bandage.

[0021] The diene elastomer according to the invention has the essential characteristic of being a functionalized copolymer of 1,3-butadiene and styrene which contains 1,4-trans chained butadiene units and 1,2-chained butadiene units. In the diene elastomer according to the invention, the 1,4-trans chained butadiene units represent more than 70 mol% of the butadiene units of the diene elastomer according to the invention, the 1,2-chained butadiene units representing less than 10 mol% of the butadiene units of the diene elastomer according to the invention. Preferably, the 1,4-trans chained butadiene units represent more than 75 mol% of the butadiene units of the diene elastomer according to the invention. Also preferably, the 1,2-chain butadiene units represent less than 8 mol% of the butadiene units of the diene elastomer in accordance with the invention.The diene elastomer in accordance with the invention preferably comprises between 5% and 45% by mass of styrene, more preferably between 15% and 40% by mass of styrene, even more preferably between 25% and 35% by mass of styrene. Conventionally and well known to those skilled in the art, the level of styrene in an elastomer is given per 100 g of the elastomer. For example, an elastomer containing 10% styrene is an elastomer in which the proportion of styrene in the form of a monomer unit is 10 g per 100 g of this elastomer.

[0022] The diene elastomer according to the invention also has the other essential characteristic of carrying functional groups. The functional groups are pendant imidazole groups of formula (I) in which Yi represents a hydrogen atom or a carbon chain which may contain at least one heteroatom, Y2 denotes an attachment to a butadiene unit of the diene elastomer in accordance with the invention. A person skilled in the art understands that the functional groups are attached to the diene elastomer in accordance with the invention by covalent bonding. The attachment to a butadiene unit may be direct or indirect, i.e. via a group of atoms. It is preferably indirect. In the present application, the term “group of atoms” means a chain of atoms covalently bonded to form a chain.

[0023] Advantageously, Yi is a hydrogen atom or an alkyl group. When the symbol Yi represents an alkyl group, the alkyl group is preferably a C 1 -C 6 alkyl, more preferably a methyl. By C 1 -C 6 alkyl is meant an alkyl which contains from 1 to 6 carbon atoms. Very advantageously, Yi is a methyl group.

[0024] According to any embodiment of the invention, the pendant imidazole groups of formula (I) are preferably randomly distributed along the diene elastomer chain.

[0025] The molar content of pendant imidazole groups of formula (I) in the diene elastomer in accordance with the invention is preferably at most 3 mol% of the repeating units constituting the diene elastomer in accordance with the invention in order to obtain the best compromise between the properties of the rubber composition containing the diene elastomer in accordance with the invention and its additional cost compared to a rubber composition containing a diene elastomer devoid of pendant imidazole groups. It preferably varies in a range which goes from more than 0% to 3% by mole of the repeating units constituting the diene elastomer in accordance with the invention, for example from 0.02% to 3% by mole of the repeating units constituting the diene elastomer in accordance with the invention, even more preferably from 0.1% to 3% by mole of the repeating units constituting the diene elastomer in accordance with the invention.These preferred ranges may apply to any of the embodiments of the invention.

[0026] According to a particular embodiment of the invention, the diene elastomer in accordance with the invention may be a mixture of several elastomers which may differ from one another by their microstructure and their macrostructure. Each of the elastomers making up the mixture meets the definition of the diene elastomer in accordance with the invention both from the point of view of functionalization and microstructure.

[0027] According to a particularly preferred embodiment of the invention, the diene elastomer in accordance with the invention is obtained by modification of a starting diene elastomer by a grafting reaction of a 1,3-dipolar compound. The 1,3-dipolar compound contains a dipole chosen from a nitrile oxide, a nitrile imine and a nitrone and contains a group of formula (II) in which Zi is identical to Yi of formula (I) and Z2 denotes an attachment to the dipole. The dipole is preferably a nitrile oxide.

[0028] Preferably, the symbol Zi represents a hydrogen atom or a carbon chain which may contain at least one heteroatom, in particular an alkyl, more particularly a C 1 -C 6 alkyl. Advantageously, the alkyl group represented by Zi is a methyl.

[0029] The starting diene elastomer is a copolymer of 1,3-butadiene and styrene which contains more than 70 mol% of 1,4-trans chained butadiene units and less than 10 mol% of 1,2-chained butadiene units, the molar percentages of 1,4-trans chained butadiene units and 1,2-chained butadiene units being calculated relative to the total number of butadiene units in the starting diene elastomer. Preferably, the starting diene elastomer is a copolymer of 1,3-butadiene and styrene which contains more than 75 mol% of 1,4-trans chained butadiene units and less than 8 mol% of 1,2-chained butadiene units, the molar percentages of the 1,4-trans chained butadiene units and the 1,2-chained butadiene units being calculated relative to the total number of butadiene units in the starting diene elastomer.The starting diene elastomer preferably comprises between 5% and 45% by mass of styrene, more preferably between 15% and 40% by mass of styrene, even more preferably between 25% and 35% by mass of styrene. The starting diene elastomer can be synthesized according to the method described in patent applications EP 0037617 A1, WO 03037946 A2, WO 2004099277 A1.

[0030] In the synthesis of the diene elastomer according to the invention, the dipole of the 1,3-dipolar compound typically reacts with the carbon-carbon double bonds of the butadiene units of the starting diene elastomer. The 1,3-dipolar compound useful for the purposes of the invention is preferably an aromatic nitrile monooxide, i.e. an aromatic compound which contains a single nitrile oxide dipole, in particular a compound which contains a benzene ring and a single nitrile oxide dipole and in which the benzene ring is substituted by the nitrile oxide dipole. Advantageously, the benzene ring is substituted ortho to the dipole.

[0031] Advantageously, the 1,3-dipolar compound contains a unit of formula (III) in which four of the six symbols Ri to Re, identical or different, are each an atom or a group of atoms, knowing that the fifth symbol represents an attachment to the group of formula (II) and the sixth symbol a direct attachment to the dipole.

[0032] According to a particularly preferred embodiment, in formula (III) Re represents a direct attachment to the dipole, the symbols Ri and Rs are both different from a hydrogen atom, which makes it possible to confer greater stability to the 1,3-dipolar compound and thus easier use of the 1,3-dipolar compound. According to this particularly preferred embodiment, the symbols Ri, R3 and R5 each preferably represent a hydrocarbon group, more preferably an alkyl, even more preferably a methyl or an ethyl and one of the symbols R2 and R4 is preferably a hydrogen atom. With a benzene ring thus substituted, the synthesis of the 1,3-dipolar compound can then be carried out from a relatively easy synthesis route from a commercially available precursor, for example mesitylene, as described in particular in document WO 2015059269.

[0033] In formula (III), the fifth symbol is attached to the group of formula (II) preferably via a group of atoms called a spacer. The spacer is preferably a carbon chain which may contain at least one heteroatom. The spacer preferably contains 1 to 6 carbon atoms, in particular 1 to 3 carbon atoms. The spacer is more preferably an alkanediyl group, better still methanediyl.

[0034] According to any one of the embodiments of the invention, the 1,3-dipolar compound is advantageously the compound 2,4,6-trimethyl-3-((2-methyl-1 / 7-imidazol-1-yl)methyl)benzo-nitrile oxide of formula (III-a) or the compound 2,4,6-triethyl-3-((2-methyl-1 / 7-imidazol-1-yl)methyl)benzo-nitrile oxide of formula (III-b), more advantageously the compound of formula (III-a).

[0035] The diene elastomer in accordance with the invention is intended to be used in a rubber composition, in particular a rubber composition for a tire, another subject of the invention.

[0036] The rubber composition also has the essential characteristic of including a reinforcing filler.

[0037] The total content of reinforcing filler in the rubber composition can vary widely. It is adjusted according to the intended use of the rubber composition, particularly in a tire. The total content of reinforcing filler is preferably between 40 and 80 phr. A content less than or equal to 40 phr may lead to a reinforcement of the rubber composition after crosslinking which may be considered insufficient for use in a tire tread. A content greater than or equal to 80 phr may lead to a level of hysteresis which may be considered too high for use in a tread, particularly for a tire intended to equip a heavy goods vehicle or a civil engineering vehicle.

[0038] The reinforcing filler may be any type of so-called reinforcing filler, known for its ability to reinforce a rubber composition usable for the manufacture of tires, for example carbon black, a reinforcing inorganic filler such as silica with which a coupling agent is associated in a known manner, or a mixture of these two types of filler. Such a reinforcing filler typically consists of nanoparticles whose average size (by mass) is less than one micrometer, generally less than 500 nm, most often between 20 and 200 nm, in particular and more preferably between 20 and 150 nm.

[0039] The reinforcing filler preferably contains a carbon black. Suitable carbon blacks are all carbon blacks, in particular the blacks conventionally used in tires or their treads (so-called tire-grade blacks). Among the latter, mention may be made more particularly of the reinforcing carbon blacks of the 100, 200, 300 series, or the blacks of the 500, 600 or 700 series (ASTM grades, also known respectively by the acronyms SAF, ISAF, HAF, FF, GPF and SRF), such as for example blacks N115, N134, N234, N326, N330, N339, N347, N375, N550, N683, N772). These carbon blacks can be used in the isolated state, as commercially available, or in any other form, for example as a carrier for certain of the rubber additives used.

[0040] The carbon black preferably represents more than 50% by mass of the reinforcing filler, more preferably more than 90% by mass of the reinforcing filler. According to any one of the embodiments of the invention, the carbon black advantageously represents 100% by mass of the reinforcing filler.

[0041] The rubber composition may comprise an elastomer other than the diene elastomer in accordance with the invention. In particular, this other elastomer may be an unmodified diene elastomer, for example the starting diene elastomer used in the preparation of the diene elastomer in accordance with the invention.

[0042] The level of the diene elastomer in accordance with the invention in the rubber composition is preferably at least 50 phr, more preferably at least 75 phr, even more preferably at least 90 phr. The level of diene elastomer in accordance with the invention in the rubber composition therefore preferably varies from 50 to 100 phr, more preferably from 75 to 100 phr, even more preferably from 90 to 100 phr. The higher the level of the diene elastomer in accordance with the invention in the rubber composition, the more the crack propagation resistance of the rubber composition is improved. These preferred levels of diene elastomer in accordance with the invention in the rubber composition can be applied to any of the embodiments of the invention.

[0043] The rubber composition contains a crosslinking system for the diene elastomer according to the invention and, where appropriate, a diene elastomer other than the diene elastomer according to the invention. The crosslinking system may be a vulcanization system or may be based on one or more peroxide compounds, for example conventionally used in rubber compositions suitable for the manufacture of tires. The crosslinking system is preferably a vulcanization system, i.e. a system based on sulfur (or a sulfur-donating agent) and a primary vulcanization accelerator.To this basic vulcanization system are added, incorporated during the first non-productive phase and / or during the productive phase as described later, various known secondary accelerators or vulcanization activators such as zinc oxide, stearic acid or an equivalent compound, guanidine derivatives (in particular diphenylguanidine), or known vulcanization retarders. Sulphur is used at a preferential rate of between 0.5 and 5 pce, in particular between 1 and 3 pce. The primary vulcanization accelerator is used at a preferential rate of between 0.5 and 5 pce, in particular between 0.5 and 3 pce.

[0044] The rubber composition may further contain other additives known to be used in rubber compositions for tires, such as pigments, processing agents, antiozonants, antioxidants. Those skilled in the art will be able to adjust the formulation of the rubber composition according to their particular needs.

[0045] The rubber composition in accordance with the invention is typically manufactured in suitable mixers, using two successive preparation phases well known to those skilled in the art: a first working phase, a thermomechanical mixing step (so-called "non-productive" phase) at high temperature, up to a maximum temperature of between 130°C and 200°C, followed by a second mechanical working phase (so-called "productive" phase) down to a lower temperature, typically below 110°C, for example between 40°C and 100°C, a finishing phase during which the crosslinking system is incorporated.

[0046] The rubber composition according to a particular embodiment of the invention can be manufactured according to a method which comprises the following steps:

[0047] - during a so-called non-productive step, kneading a starting diene elastomer and a 1,3-dipolar compound by thermomechanical kneading, the starting diene elastomer being a copolymer of 1,3-butadiene and styrene which contains more than 70 mol% of 1,4-trans chained butadiene units and less than 10 mol% of 1,2-chained butadiene units, the molar percentages of 1,4-trans chained butadiene units and 1,2-chained butadiene units being calculated relative to the total number of butadiene units in the starting diene elastomer, the 1,3-dipolar compound being as defined above according to any one of the embodiments,

[0048] - then add the reinforcing filler, if applicable the other ingredients of the rubber composition with the exception of the crosslinking system by thermomechanically mixing until reaching a maximum temperature of between 130 and 200°C to obtain a mixture,

[0049] - cool the mixture to a temperature below 100°C,

[0050] - then incorporate the crosslinking system,

[0051] -mix the mixture and the crosslinking system up to a maximum temperature below 120°C.

[0052] The contact time between the starting diene elastomer and the 1,3-dipolar compound that are thermomechanically mixed is adjusted according to the thermomechanical mixing conditions, particularly according to the temperature. The higher the mixing temperature, the shorter this contact time. Typically, it is 1 to 5 minutes for a temperature of 100 to 130°C.

[0053] According to this particular embodiment of the invention, an antioxidant is preferably added to the starting diene elastomer before thermomechanically kneading it, in particular at the end of the synthesis of the starting diene elastomer as is done conventionally.

[0054] The final composition thus obtained can then be calendered, for example in the form of a sheet, a plate or even extruded, for example to form a rubber profile used for the manufacture of a semi-finished product, such as for example a tire tread.

[0055] The rubber composition according to the invention, which may be either in the raw state (before crosslinking or vulcanization) or in the cured state (after crosslinking or vulcanization), may be used in a semi-finished article for a tire. Preferably, the tire contains the rubber composition in its tread.

[0056] In summary, the invention is advantageously implemented according to any one of the following embodiments 1 to 28:

[0057] Mode 1: Diene elastomer which is a copolymer of 1,3-butadiene and styrene and which is functionalized by pendant imidazole groups of formula (I) in which:

[0058] Yi represents a hydrogen atom or a carbon chain which may contain at least one heteroatom,

[0059] Y2 denotes an attachment to a butadiene unit of the diene elastomer, which diene elastomer contains 1,4-trans butadiene units and 1,2-trans butadiene units, the 1,4-trans butadiene units representing more than 70 mol% of the butadiene units of the diene elastomer, the 1,2-trans butadiene units representing less than 10 mol% of the butadiene units of the diene elastomer.

[0060] Mode 2: Diene elastomer according to mode 1 in which the 1,4-trans chain butadiene units represent more than 75 mol% of the butadiene units of the diene elastomer. Mode 3: Diene elastomer according to mode 1 or 2 in which the 1,2-chain butadiene units represent less than 8 mol% of the butadiene units of the diene elastomer.

[0061] Mode 4: Diene elastomer according to any one of modes 1 to 3 which contains between 5% and 45% by mass of styrene.

[0062] Mode 5: Diene elastomer according to any one of modes 1 to 4 which contains between 15% and 40% by mass of styrene.

[0063] Mode 6: Diene elastomer according to any one of modes 1 to 5 which contains between 25% and 35% by mass of styrene.

[0064] Mode 7: Diene elastomer according to any one of modes 1 to 6 in which the molar rate of pendant groups of formula (I) is at most 3% by mole of the constituent repeat units of the diene elastomer.

[0065] Mode 8: Diene elastomer according to any one of modes 1 to 7 in which Yi is a hydrogen atom or an alkyl group.

[0066] Mode 9: Diene elastomer according to mode 8 in which the alkyl group represented by Yi contains from 1 to 6 carbon atoms.

[0067] Mode 10: Diene elastomer according to mode 8 or 9 in which the alkyl group represented by Yi is methyl.

[0068] Mode 11: A diene elastomer according to any one of modes 1 to 10 in which the pendant imidazole groups are randomly distributed along the diene elastomer chain.

[0069] Method 12: Diene elastomer according to any one of methods 1 to 11, which diene elastomer is obtained by modification of a starting diene elastomer by a grafting reaction of a 1,3-dipolar compound, the starting diene elastomer being a copolymer of 1,3-butadiene and styrene which contains more than 70 mol% of 1,4-trans chained butadiene units and less than 10 mol% of 1,2-chained butadiene units, the molar percentages of 1,4-trans chained butadiene units and 1,2-chained butadiene units being calculated relative to the total number of butadiene units in the starting diene elastomer, the 1,3-dipolar compound being a compound containing a dipole chosen from a nitrile oxide, a nitrile imine and a nitrone and containing a group of formula (II) in which Zi represents a hydrogen atom or a carbon chain which may contain at least one heteroatom and Z2 denotes an attachment to the dipole,

[0070] Mode 13: Diene elastomer according to mode 12 in which the 1,3-dipolar compound is an aromatic compound which contains a single nitrile oxide dipole.

[0071] Mode 14: Diene elastomer according to mode 12 or 13 in which the 1,3-dipolar compound is a compound which contains a benzene ring and which contains a single nitrile oxide dipole and in which the benzene ring is substituted by the nitrile oxide dipole, preferably ortho.

[0072] Mode 15: Diene elastomer according to any one of modes 12 to 14 in which the 1,3-dipolar compound contains a unit of formula (III) in which four of the six symbols Ri to Re, identical or different, are each an atom or a group of atoms, knowing that the fifth symbol represents an attachment to the group of formula (II) and the sixth symbol a direct attachment to the dipole.

[0073] Mode 16: Diene elastomer according to mode 15 in which Re represents a direct attachment to the dipole, the symbols Ri and R5 are both different from a hydrogen atom.

[0074] Mode 17: Diene elastomer according to mode 15 or 16 in which the symbols Ri, R3 and R5 each represent an alkyl.

[0075] Mode 18: Diene elastomer according to any one of modes 15 to 17 in which the symbols R1, R3 and R5 each represent a methyl or ethyl, and one of the symbols R2 and R4 is a hydrogen atom.

[0076] Mode 19: Diene elastomer according to any one of modes 15 to 18 in which the fifth symbol is attached to the group of formula (II) via an alkanediyl group, preferably methanediyl. Mode 20: Diene elastomer according to any one of modes 15 to 18 in which the 1,3-dipolar compound is of formula (III-a) or (III-b), preferably (III-a).

[0077] Mode 21: A rubber composition which comprises a diene elastomer which is a copolymer of 1,3-butadiene and styrene and which is functionalized with pendant imidazole groups, a reinforcing filler and a crosslinking system, which diene elastomer is defined in any one of modes 1 to 20.

[0078] Mode 22: Rubber composition according to mode 21 in which the reinforcing filler contains a carbon black.

[0079] Mode 23: Rubber composition according to mode 22 in which the carbon black represents more than 50% by mass of the reinforcing filler.

[0080] Mode 24: Rubber composition according to mode 22 or 23 in which the carbon black represents more than 90% by mass of the reinforcing filler.

[0081] Mode 25: Rubber composition according to any one of modes 21 to 24 in which the total level of reinforcing filler is between 40 and 80 pce.

[0082] Mode 26: Rubber composition according to any one of modes 21 to 25 in which the crosslinking system is a vulcanization system.

[0083] Mode 27: A process for preparing a rubber composition defined in any one of modes 21 to 26, which process comprises the following steps:

[0084] - during a so-called non-productive step, kneading a starting diene elastomer and a 1,3-dipolar compound by thermomechanical kneading, the starting diene elastomer being a copolymer of 1,3-butadiene and styrene which contains more than 70 mol% of 1,4-trans chained butadiene units and less than 10 mol% of 1,2-chained butadiene units, the molar percentages of the 1,4-trans chained butadiene units and the 1,2-chained butadiene units being calculated relative to the total number of butadiene units in the starting diene elastomer, the 1,3-dipolar compound being a compound containing a dipole chosen from nitrile oxide, nitrile imine and nitrone and containing a group of formula (II) in which Zi represents a hydrogen atom or a carbon chain which may contain at least one heteroatom and Z2 denotes an attachment to the dipole,

[0085] - then add the reinforcing filler, if applicable the other ingredients of the rubber composition with the exception of the crosslinking system by thermomechanically mixing until reaching a maximum temperature of between 130 and 200°C to obtain a mixture,

[0086] - cool the mixture to a temperature below 100°C,

[0087] - then incorporate the crosslinking system,

[0088] -mix the mixture and the crosslinking system up to a maximum temperature below 120°C.

[0089] Mode 28: Tire comprising a rubber composition preferentially in its tread, which rubber composition is defined in any one of modes 21 to 26 or obtained according to a process defined in mode 27.

[0090] The above-mentioned characteristics of the present invention, as well as others, will be better understood upon reading the following description of several exemplary embodiments of the invention, given for illustrative purposes.

[0091] Examples ll.l-Measurements and tests

[0092] Determination of the microstructure and function rate of elastomers by nuclear magnetic resonance (NMR):

[0093] The microstructure is determined by NMR 1 H. The molar ratio of grafted nitrile oxide compound is also determined by NMR analysis. The samples are solubilized in a mixture of carbon disulfide and deuterated benzene (CS2 / C6D6 (50 / 50)) in order to obtain a lock signal. The spectra are acquired on a 500 MHz BRUKER spectrometer equipped with a “CryoSonde BBFO-zgrad-5 mm”. The NMR experiment 1Quantitative H uses a single 30° pulse sequence and a 5-second repetition delay between each acquisition. 2D NMR experiments allow us to verify the nature of the grafted pattern using the chemical shifts of the carbon and proton atoms.

[0094] Glass transition temperature of elastomers:

[0095] The glass transition temperature (Tg) is measured using a differential scanning calorimeter according to ASTM D3418 (1999).

[0096] Mooney Viscosity:

[0097] Mooney viscosity is measured using an oscillating consistometer as described in ASTM D1646 (1999). The measurement is carried out according to the following principle: the sample analyzed in the raw state (i.e., before curing) is molded (shaped) in a cylindrical enclosure heated to a given temperature (100°C). After 1 minute of preheating, the rotor rotates within the specimen at 2 revolutions / minute and the torque needed 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).

[0098] Resistance to crack propagation:

[0099] The cracking rate was measured on rubber composition specimens using a cyclic fatigue machine (“Elastomer Test System”) type 381 from MTS, as explained below.

[0100] Cracking resistance is measured using repeated tractions on a specimen initially accommodated (after a first traction cycle), then notched. The tensile specimen consists of a parallelepipedal rubber plate, for example with a thickness between 1 and 2 mm, a length between 130 and 170 mm and a width between 10 and 15 mm, the two lateral edges each being covered lengthwise with a cylindrical rubber bead (diameter 5 mm) allowing anchoring in the jaws of the tensile machine. The specimens thus prepared are tested in the new state. The test was carried out in air, at a temperature of 80°C. After accommodation, 3 very fine notches between 15 and 20 mm long are made using a razor blade, halfway across and aligned along the length of the test piece, one at each end and one in the centre of the latter, before starting the test.At each tensile cycle, the deformation rate of the specimen is automatically adjusted so as to maintain the energy release rate (quantity of energy released during crack progression) constant, at a value less than or equal to approximately 300 J / m. 2 . The crack propagation rate is measured in nanometers per cycle. The crack propagation resistance will be expressed in relative units (ur) by dividing the propagation rate of the control by that of the mixture, the rates being measured at the same energy release rate. A value higher than that of the control, arbitrarily set at 100, indicates an improved result, i.e. a higher crack propagation resistance.

[0101] II.2-Preparation of rubber compositions:

[0102] The formulation of rubber compositions (in pce) is shown in Table 1.

[0103] For the manufacture of compositions T1 to T4, the procedure is as follows: the elastomer, then the reinforcing filler, as well as the various other ingredients except for the vulcanization system, are introduced into an internal mixer (final filling rate: approximately 70% by volume), whose initial tank temperature is approximately 80°C. Thermomechanical work (non-productive phase) is then carried out in one step, lasting approximately 5 min to 6 minutes, until a maximum "fall" temperature of 160°C is reached. The mixture thus obtained is recovered, cooled and then sulfur and a sulfenamide-type accelerator are incorporated on a mixer (homo-finisher) at 23°C, mixing everything (productive phase) for an appropriate time (for example between 5 and 12 min).

[0104] In the case of rubber compositions C1 to C4, the procedure is as mentioned for compositions T1 to T4 except that before introducing the reinforcing filler, the 1,3-dipolar compound is introduced which is kneaded alone with the elastomer for 1 minute at 120°C, which makes it possible to functionalize the elastomer with pendant groups of formula (I) before introducing the other ingredients of the rubber composition.

[0105] The rate of 1,3-dipolar compound introduced into the internal mixer to functionalize the elastomer before the introduction of the other ingredients of the rubber composition is shown in Table 1 and is expressed in phr. The rate introduced corresponds to a molar rate of modification of 0.5 moles per 100 moles of the monomer units of the functional elastomer.

[0106] 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 150°C (cooked state), or in the form of profiles which can be used directly, after cutting and / or assembling to the desired dimensions, for example as semi-finished products for tires, in particular as tire treads.

[0107] Rubber compositions C1 to C4 all contain an elastomer functionalized by imidazole functions, since the elastomer is modified by the 1,3-dipolar compound of formula (III-a).

[0108] Rubber compositions T1 to T4 do not contain any functional elastomer and are the control compositions of compositions C1 to C4 respectively.

[0109] Only composition C1 contains a diene elastomer in accordance with the invention which is a copolymer of styrene and 1,3-butadiene functionalized by pendant imidazole functions of formula (I) and in which the 1,4-trans chained butadiene units represent more than 70 mol% of the butadiene units of the diene elastomer and the 1,2-chained butadiene units represent less than 10 mol% of the butadiene units of the diene elastomer. Composition C1 is a composition in accordance with the invention. Compositions C2 to C4 are not in accordance with the invention.

[0110] II.3-Results:

[0111] The results of the cured properties of the rubber compositions are shown in Table 2.

[0112] The greatest improvement in crack propagation resistance resulting from the functionalization of the diene elastomer is observed for composition C1. Indeed, compared to its control composition, the crack propagation resistance is multiplied by a factor of 1.7 whereas the compositions not in accordance with the invention C2 and C3 which also contain an SBR have their performance well below that of their respective control. It is recalled that composition C1 differs from compositions C2 and C3 in that the SBR in C1 is an SBR in which the 1,4-trans chain and 1,2-chain butadiene units represent respectively more than 70 mol% and less than 10 mol% of the butadiene units of the elastomer.

[0113] Composition C4, which also contains an elastomer with high stereospecificity, in this case a BR containing more than 98% 1,4-cis bond, has a performance identical to that of its control T4.

[0114] In summary, only the performance improvement is observed in the case of an SBR elastomer functionalized with pendant imidazole groups of formula (I) in which the 1,4-trans and 1,2-trans butadiene units represent respectively more than 70 mol% and less than 10 mol% of the butadiene units of the elastomer and which carries pendant imidazole functions.

[0115] Table 1

[0116] (1) Copolymer of styrene and 1,3-butadiene of Tg -52°C and ML 54, containing 29.5% by mass of styrene; 79% of the butadiene units being 1,4-trans chained; 5% of the butadiene units being 1,2 chained,

[0117] (2) Copolymer of styrene and 1,3-butadiene (SBR 1500) with a Tg of -52°C and ML 51, containing 23.5% by mass of styrene; 70% of the butadiene units being 1,4-trans chained; 16% of the butadiene units being 1,2 chained

[0118] (3) Copolymer of styrene and 1,3-butadiene with a Tg of -48°C and ML 54, containing 26.5% by mass of styrene; 50% of the butadiene units being 1,4-trans chained; 24% of the butadiene units being 1,2 chained

[0119] (4) Homopolymer of 1,3-butadiene with Tg -108°C and ML 44; 98% of the butadiene units being 1,4-cis chained,

[0120] (5) HAF carbon black, (6) Alkylphenol tackifying thermoplastic resin (“SP1068” from SI Group-

[0121] Bethune)

[0122] (7) N-(l,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine from Flexsys / 2,2,4-trimethyl-l,2-dihydroquinoline « Pilnox TMQ w from Nocil 60 / 40 (by mass)

[0123] (8) Stearin “Pristerene 4931” from the company Uniquema (9): Industrial grade Zinc Oxide - Umicore company

[0124] (10) “Santocure” from Flexsys (N-cyclohexyl-2-benzothiazyl sulfenamide)

[0125] (11) 2,4,6-trimethyl-3-((2-methyl-1 / 7-imidazol-1-yl)methyl)benzo-nitrile oxide synthesized according to the procedure described in patent application WO 2015059269.

[0126] Table 2

Claims

Claims 1. A diene elastomer which is a copolymer of 1,3-butadiene and styrene and which is functionalized with pendant imidazole groups of formula (I) in which: Yi represents a hydrogen atom or a carbon chain which may contain at least one heteroatom, Y2 denotes an attachment to a butadiene unit of the diene elastomer, which diene elastomer contains 1,4-trans butadiene units and 1,2-trans butadiene units, the 1,4-trans butadiene units representing more than 70 mol% of the butadiene units of the diene elastomer, the 1,2-trans butadiene units representing less than 10 mol% of the butadiene units of the diene elastomer.

2. Diene elastomer according to claim 1 in which the 1,4-trans chain butadiene units represent more than 75 mol% of the butadiene units of the diene elastomer.

3. Diene elastomer according to any one of claims 1 to 2 in which the 1,2-linked butadiene units represent less than 8 mol% of the butadiene units of the diene elastomer.

4. Diene elastomer according to any one of claims 1 to 3 which contains between 5% and 45% by mass of styrene, preferably between 15% and 40% by mass of styrene, more preferably between 25% and 35% by mass of styrene.

5. Diene elastomer according to any one of claims 1 to 4 in which the molar content of pendant groups of formula (I) is at most 3 mol% of the repeating units constituting the diene elastomer.

6. Diene elastomer according to any one of claims 1 to 5 in which Yi is a hydrogen atom or an alkyl group.

7. Diene elastomer according to claim 6 in which the alkyl group represented by Yi contains from 1 to 6 carbon atoms, is preferably methyl.

8. A diene elastomer according to any one of claims 1 to 7 wherein the pendant imidazole groups are randomly distributed along the chain of the diene elastomer.

9. A rubber composition which comprises a diene elastomer which is a copolymer of 1,3-butadiene and styrene and which is functionalized with groups pendant imidazole, a reinforcing filler and a crosslinking system, which diene elastomer is defined in any one of claims 1 to 8.

10. A rubber composition according to claim 9 wherein the reinforcing filler contains a carbon black.

11. Rubber composition according to claim 10 in which the carbon black represents more than 50%, preferably more than 90% by mass of the reinforcing filler.

12. Rubber composition according to any one of claims 9 to 11 in which the total level of the reinforcing filler is between 40 and 80 pce.

13. Rubber composition according to any one of claims 1 to 12 in which the crosslinking system is a vulcanization system.

14. A process for preparing a rubber composition defined in any one of claims 9 to 13, which process comprises the following steps: - during a so-called non-productive step, kneading a starting diene elastomer and a 1,3-dipolar compound by thermomechanical kneading, the starting diene elastomer being a copolymer of 1,3-butadiene and styrene which contains more than 70 mol% of 1,4-trans chained butadiene units and less than 10 mol% of 1,2-chained butadiene units, the molar percentages of 1,4-trans chained butadiene units and 1,2-chained butadiene units being calculated relative to the total number of butadiene units in the starting diene elastomer, the 1,3-dipolar compound being a compound containing a dipole chosen from nitrile oxide, nitrile imine and nitrone and containing a group of formula (II) in which Zi represents a hydrogen atom or a carbon chain which may contain at least one heteroatom and Z2 denotes an attachment to the dipole, - then add the reinforcing filler, if applicable the other ingredients of the rubber composition with the exception of the crosslinking system by thermo-mechanically mixing until reaching a maximum temperature between 130 and 200°C to obtain a mixture, - cool the mixture to a temperature below 100°C, - then incorporate the crosslinking system, -mix the mixture and the crosslinking system up to a maximum temperature below 120°C.

15. Tire comprising a rubber composition preferably in its tread, which rubber composition is defined in any one of claims 9 to 13 or obtained according to a process defined in claim 14.

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