Method for preparing a diene rubber composition

Abstract

The invention relates to a method for preparing a rubber composition which comprises an elastomer matrix that is made up of natural rubber and a synthetic diene elastomer, a modification agent comprising a nitrile oxide dipole and an N-substituted imidazole function, a reinforcing filler comprising more than 50% by weight of a silica, a silane coupling agent and a vulcanisation system, the method comprising a step of pre-mixing the elastomer matrix alone before proceeding to the steps of non-productive and productive mixing. The method offers a good trade-off between the hysteresis and rigidity of the composition and the amount of modifying agent used.

Description

[0001] Process for preparing a diene rubber composition.

[0002] The field of the present invention is that of processes for preparing silica-reinforced diene rubber compositions for use in a tire tread.

[0003] Ideally, a tread should provide the tire with a very good level of road behavior on a motor vehicle. This level of road behavior can be provided by the use in the tread of a rubber composition judiciously chosen because of its rather high cured stiffness. To increase the cured stiffness of a rubber composition, it is known for example to increase the filler content or to reduce the plasticizer content in the rubber composition or to introduce styrene and butadiene copolymers with a high styrene content in the rubber composition. But some of these solutions generally have the disadvantage of increasing the hysteresis of the rubber composition.

[0004] The Applicant has described in document WO2015059274 that the introduction of a 1,3-dipolar compound which contains a nitrile oxide dipole and an N-substituted imidazole function into silica-reinforced diene rubber compositions makes it possible to significantly reduce their hysteresis without reducing their cured rigidity.

[0005] Seeking to further increase the rigidity of these rubber compositions, the Applicant has discovered a new process for preparing a rubber composition which achieves this goal without degrading their hysteresis property.

[0006] Thus, an object of the invention is a process for preparing a rubber composition which comprises an elastomer matrix, a modifying agent which comprises a nitrile oxide dipole and an N-substituted imidazole function, a reinforcing filler which comprises more than 50% by mass of a silica, a silane coupling agent and a vulcanization system, which process comprises the following successive steps a, b, c and d:

[0007] - a) incorporating into the elastomer matrix during a so-called non-productive step the modifying agent, the reinforcing filler and the silane coupling agent by kneading until a maximum temperature of between 110°C and 190°C is reached,

[0008] - b) cool the assembly to a temperature below 100°C,

[0009] - c) then incorporate the vulcanization system,

[0010] - d) kneading the whole up to a maximum temperature below 120°C, characterized in that the elastomer matrix comprises natural rubber and a synthetic diene elastomer and in that the process comprises a step of thermomechanical kneading of the elastomer matrix alone prior to step a). Description

[0011] 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).

[0012] 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.

[0013] 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.

[0014] In the present invention, the term “elastomer matrix” means all of the elastomers in the rubber composition.

[0015] The abbreviation "pce" means parts by weight per hundred parts of the elastomer matrix.

[0016] By synthetic "diene" elastomer (or indistinctly rubber) is meant, in a known manner, an elastomer which is other than natural rubber and which is made up at least in part (i.e. a homopolymer or a copolymer) of diene monomer units (monomers carrying two carbon-carbon double bonds, conjugated or not).

[0017] The term synthetic diene elastomer capable of being used in the compositions in accordance with the invention is particularly understood to mean:

[0018] (a) - any homopolymer of a diene monomer, conjugated or not, other than natural rubber;

[0019] (b) - any copolymer of a diene, conjugated or not, and at least one other monomer.

[0020] By copolymer of a diene, conjugated or not, and at least one other monomer, we mean a copolymer of a diene and one or more other monomer(s). Other monomers include ethylene, an olefin and a diene, conjugated or not, different from the first diene.

[0021] Suitable conjugated dienes are conjugated dienes having 4 to 24 carbon atoms, in particular 1,3-dienes having 4 to 12 carbon atoms, more particularly 1,3-butadiene and isoprene.

[0022] Suitable olefins are vinylaromatic compounds having 8 to 20 carbon atoms and aliphatic α-monoolefins having 3 to 12 carbon atoms. An aliphatic α-monoolefin is an aliphatic α-olefin containing a single double bond. Suitable vinylaromatic compounds include, for example, styrene, ortho-, meta-, para-methylstyrene, the commercial mixture "vinyl-toluene", para-tert-butylstyrene. Suitable aliphatic α-monoolefins include, in particular, acyclic aliphatic α-monoolefins having 3 to 18 carbon atoms.

[0023] More particularly, the synthetic diene elastomer useful for the purposes of the invention is:

[0024] (a') - any homopolymer of a conjugated diene monomer other than natural rubber, in particular any homopolymer obtained by polymerization of a conjugated diene monomer having from 4 to 12 carbon atoms;

[0025] (b') - any copolymer obtained by copolymerization of one or more conjugated dienes with each other or with one or more vinylaromatic compounds having from 8 to 20 carbon atoms.

[0026] The synthetic diene elastomers useful for the purposes of the invention are preferably chosen from the group consisting of polybutadienes, synthetic polyisoprenes, butadiene copolymers and isoprene copolymers.

[0027] The synthetic diene elastomers useful for the purposes of the invention may contain an oil called an extension oil which is generally introduced at the end of the elastomer synthesis process and are then referred to as oil-extended elastomers. As is known, elastomers generally contain antioxidants, usually introduced at the end of their synthesis process.

[0028] Preferably, the level of natural rubber in the rubber composition is greater than 50 parts by weight per hundred parts of the elastomer matrix, phr, and less than 90 phr and the level of the synthetic diene elastomer in the rubber composition is greater than 10 phr and less than 50 phr. According to any one of the embodiments of the invention, the synthetic diene elastomer is preferably an SBR. Any SBR containing from 1 to 40% by weight of styrene, in particular from 15% to 35% by weight of styrene, may be suitable as SBR. The synthetic diene elastomer is more preferably a solution SBR (SSBR). Preferably, the total level of natural rubber and SBR is equal to 100 phr; in other words, the elastomers of the elastomer matrix are preferably natural rubber and an SBR.

[0029] The modifying agent is typically a 1,3-dipolar compound that contains a nitrile oxide dipole and an N-substituted imidazole function.

[0030] The N-substituted imidazole function is preferably of formula (I) in which the symbol Zi represents a hydrogen atom or an alkyl having 1 to 6 carbon atoms, the symbol Z2 denotes an attachment to the nitrile oxide dipole. By attachment to the nitrile oxide dipole is meant a bond or a group which makes it possible to covalently connect the 5-membered ring of the imidazole function to the dipole.

[0031] The alkyl represented by Zi preferably contains 1 to 3 carbon atoms, more preferably is methyl.

[0032] Preferably, the modifying agent is an aromatic nitrile oxide, a compound comprising an aromatic group substituted by a nitrile oxide dipole and by a group containing the N-substituted imidazole function.

[0033] Advantageously, the modifying agent is a compound which contains a unit of formula (II) in which R'i represents the nitrile oxide dipole, one of the symbols R'2 to R' 6 represents a saturated group having 1 to 6 carbon atoms and covalently bonded to one of the nitrogen atoms of the 5-membered ring of the N-substituted imidazole function, the other symbols, identical or different, representing a hydrogen atom or a substituent. The substituent in formula (II) can be any group as long as it does not react with the dipole. The substituent in formula (II) can form a ring with the substituent of the neighboring carbon. Preferably, the substituent in formula (II) is an alkyl having 1 to 3 carbon atoms, preferably methyl or ethyl, more preferably methyl.

[0034] The saturated group in formula (II) allows the imidazole function to be covalently linked to the benzene nucleus substituted in particular by the nitrile oxide dipole. It may contain one or more heteroatoms. It preferably contains 1 to 3 carbon atoms. The saturated group in formula (II) is preferably an alkanediyl, more preferably an alkanediyl having 1 to 3 carbon atoms, even more preferably a methanediyl.

[0035] In formula (II), R'2 and R'e are preferably different from a hydrogen atom. Preferably, R'2, R'4 and R'e are all different from a hydrogen atom and are identical. R'2, R'4 and R'e are more preferably alkyls having 1 to 3 carbon atoms, even more preferably methyls.

[0036] In formula (II), R' 3 and R' 5 are each preferably a hydrogen atom.

[0037] The synthesis of the 1,3-dipolar compound can be carried out using a relatively easy synthesis route from a commercially available precursor, for example mesitylene, as described in particular in document WO 2015059269.

[0038] Advantageously, the 1,3-dipolar compound is the compound 2,4,6-trimethyl-3-((2-methyl-1 / 7-imidazol-1-yl)methyl)benzo-nitrile oxide of formula (III) or the compound 2,4,6-triethyl-3-((2-methyl-1 / 7-imidazol-1-yl)methyl)benzo-nitrile oxide of formula (IV), more advantageously the compound of formula (III).

[0039] (III) (IV)

[0040] The silica used may be any reinforcing silica known to those skilled in the art, in particular any precipitated or pyrogenic silica having a BET specific surface area and a CTAB specific surface area both less than 450 m 2 / g, preferably within a range of 30 to 400 m 2 / g, especially from 60 to 300 m 2 / g. In this paper, the BET specific surface area is determined by gas adsorption using the Brunauer-Emmett-Teller method described in "The Journal of the American Chemical Society" (Vol. 60, page 309, February 1938), and more precisely according to a method adapted from the standard NF ISO 5794-1, annex E of June 2010 [multipoint volumetric method (5 points) - gas: nitrogen - vacuum degassing: one hour at 160°C - relative pressure range p / in: 0.05 to 0.17], The CTAB specific surface area values ​​were determined according to the standard NF ISO 5794-1, annex G of June 2010. The method is based on the adsorption of CTAB (N-hexadecyl-N,N,N-trimethylammonium bromide) on the "external" surface of the reinforcing filler.

[0041] Any type of precipitated silica may be used, in particular highly dispersible precipitated silicas (called "HDS" for "highly dispersible" or "highly dispersible silica"). These precipitated silicas, whether highly dispersible or not, are well known to those skilled in the art. Examples include the silicas described in applications WO03 / 016215-A1 and WO03 / 016387-A1. Among the commercial HDS silicas, it is possible to use in particular the silicas "Ultrasil ® 5000GR", "Ultrasil ® 7000GR" from the company Evonik, the silicas "Zeosil ® 1085GR", "Zeosil® 1115 MP", "Zeosil® 1165MP", "Zeosil® Premium 200MP", "Zeosil® HRS 1200 MP" from the company Solvay.As non-HDS silica, the following commercial silicas can be used: silicas “Ultrasil ® VN2GR”, “Ultrasil ® VN3GR” from Evonik, silica “Zeosil® 175GR” from Solvay, silicas “Hi-Sil EZ120G(-D)”, “Hi-Sil EZ160G(-D)”, “Hi-Sil EZ200G(-D)”, “Hi-Sil 243LD”, “Hi-Sil 210”, “Hi-Sil HDP 320G” from PPG.

[0042] The reinforcing filler may include any type of so-called reinforcing filler other than silica, known for its ability to reinforce a rubber composition usable in particular for the manufacture of tires, for example a carbon black. Suitable carbon blacks include all carbon blacks, in particular the blacks conventionally used in tires or their treads. Among the latter, we will mention more particularly the reinforcing carbon blacks of the 100, 200, 300 series, or the blacks of the 500, 600 or 700 series (ASTM D-1765-2017 grades), such as for example blacks N115, N134, N234, N326, N330, N339, N347, N375, N550, N683, N772. These carbon blacks can be used in their isolated state, as commercially available, or in any other form, for example as a carrier for some of the rubber additives used.When carbon black is used in the rubber composition, it is preferably used at a rate less than or equal to 10 phr (for example, the carbon black rate may be in a range from 1 to 10 phr).

[0043] Advantageously, the carbon black content in the rubber composition is less than or equal to 5 pce. Within the indicated ranges, the coloring (black pigmenting agent) and anti-UV properties of carbon blacks are benefited from, without otherwise penalizing the typical performance provided by silica.

[0044] The silica represents more than 50% by mass of the reinforcing filler. In other words, the proportion of silica in the reinforcing filler is greater than 50% by weight of the total weight of the reinforcing filler. Preferably, the silica represents more than 85% by mass of the reinforcing filler. The total content of reinforcing filler can vary over a wide range, for example from 30 phr to 150 phr. According to a first embodiment, the total content of reinforcing filler varies over a range from 30 phr to 60 phr. According to a second embodiment, the total content of reinforcing filler varies over a range from more than 60 phr to 150 phr. The first embodiment is preferred to the second embodiment for use of the rubber composition in a tread having very low rolling resistance.Any of these total reinforcing filler content ranges may be applicable to any embodiment of the invention.

[0045] To couple the silica to the functional highly saturated diene elastomer, a coupling agent (or bonding agent), a silane, is used in a well-known manner, at least bifunctional, intended to ensure a sufficient connection, of a chemical and / or physical nature, between the silica and the diene elastomer. In particular, at least bifunctional organosilanes or polyorganosiloxanes are used. By "bifunctional", we mean a compound having a first functional group capable of interacting with the inorganic filler and a second functional group capable of interacting with the diene elastomer. For example, such a bifunctional compound may comprise a first functional group comprising a silicon atom, said first functional group being capable of interacting with the hydroxyl groups of an inorganic filler and a second functional group comprising a sulfur atom, said second functional group being capable of interacting with the diene elastomer.

[0046] Preferably, the organosilanes are chosen from the group consisting of polysulfurized organosilanes (symmetrical or asymmetrical) such as bis(3-triethoxysilylpropyl) tetrasulfide, abbreviated to TESPT, marketed under the name “Si69” by the company Evonik, or bis-(triethoxysilylpropyl) disulfide, abbreviated to TESPD, marketed under the name “Si75” by the company Evonik, polyorganosiloxanes, mercaptosilanes, blocked mercaptosilanes, such as S-(3-(triethoxysilyl)propyl) octanethioate marketed by the company Momentive under the name “NXT Silane”, oligomers having at least one blocked mercaptosilane unit, at least one mercaptosilane unit and at least one hydroxyalkoxysilyl or dia group Cyclic icoxysilyl marketed by the company Momentive under the name “NXT-Z”. Of course, mixtures of the coupling agents previously described could also be used.

[0047] Polysulfurized silanes, called "symmetrical" or "asymmetrical" depending on their particular structure, are for example described in applications W003 / 002648 (or

[0048] US 2005 / 016651) and W003 / 002649 (or US 2005 / 016650). In particular, without the following definition being limiting, polysulfurized silanes corresponding to the general formula (V) Z - A 1 - S x - HAS 1 - Z (V) in which: - x is an integer from 2 to 8, preferably from 2 to 5;

[0049] - the A symbols 1 , identical or different, represent a divalent hydrocarbon radical (preferably a C1-C16 alkylene group or a C6-C12 arylene group, more particularly a C1-C10 alkylene, in particular

[0050] C1-C4, especially propylene);

[0051] - the symbols Z, identical or different, correspond to one of the three formulas below:

[0052] R1 R1 R2

[0053] — If— R1 ; — If— 2 ; —If— R 2

[0054] R 2 R 2 R2 in which

[0055] - the R radicals 1 , substituted or unsubstituted, identical or different from each other, represent a C1-C16 alkyl, C8-C18 cycloalkyl or C1-C18 aryl group (preferably C1-C16 alkyl, cyclohexyl or phenyl groups, in particular C1-C4 alkyl groups, more particularly methyl and / or ethyl),

[0056] - the R radicals 2 , substituted or unsubstituted, identical or different from each other, represent a C1-C18 alkoxyl or C5-C18 cycloalkoxyl group (preferably a group chosen from C1-C8 alkoxyls and Cs-Cs cycloalkoxyls, more preferably still a group chosen from C1-C4 alkoxyls, in particular methoxyl and ethoxyl).

[0057] The denomination in C n -C mqualifying a group refers to the number of carbon atoms constituting the group which contains n to m carbon atoms, n and m being integers with m greater than n.

[0058] In the case of a mixture of polysulfurized alkoxysilanes corresponding to formula (6) above, in particular usual commercially available mixtures, the average value of "x" is a fractional number preferably between 2 and 5, more preferably close to 4. But the invention can also be advantageously implemented for example with disulfurized alkoxysilanes (x = 2).

[0059] Examples of polysulfurized silanes include bis-(alkoxyl(Ci-C4)-alkyl(Ci-C4)silyl-alkyl(Ci-C4)) polysulfides (especially disulfides, trisulfides or tetrasulfides), such as bis(3-trimethoxysilylpropyl) or bis(3-triethoxysilylpropyl) polysulfides. Among these compounds, bis(3-triethoxysilylpropyl) tetrasulfide, abbreviated to TESPT, of formula [(CzHsO SifCHzhSzh or bis-(triethoxysilylpropyl) disulfide, abbreviated to TESPD, of formula [(CzHsO SifCHzhS .

[0060] Bifunctional POSS (polyorganosiloxanes) or hydroxysilane polysulfides are, for example, described in patent applications WO 02 / 30939 (or US 6,774,255), WO 02 / 31041 (or US 2004 / 051210). Mercaptosilanes, blocked or not, are, for example, described in patents or patent applications US 6,849,754, WO 99 / 09036, WO 2006 / 023815, WO 2007 / 098080, WO 2010 / 072685 and WO 2008 / 055986. In the rubber composition according to the invention, the level of the silane coupling agent is adjusted by a person skilled in the art according to the chemical structure of the coupling agent, according to the specific surface area of ​​the silica used in the rubber composition and according to the level of silica in the rubber composition. It is preferably within a range from 1 to 15 phr, more preferably from 1.5 to 10 phr, even more preferably from 2 to 8 phr.

[0061] The rubber composition according to the invention has the other essential characteristic of containing a vulcanization system, that is to say a sulfur-based crosslinking system. The sulfur is typically provided in the form of molecular sulfur or a sulfur donor agent, preferably in molecular form. Sulfur in molecular form is also referred to as molecular sulfur. A sulfur donor is understood to mean any compound which releases sulfur atoms, combined or not in the form of a polysulfide chain, capable of being inserted into the polysulfide chains formed during vulcanization and bridging the elastomer chains. In addition to the vulcanization system, various known secondary accelerators or vulcanization activators such as zinc oxide, stearic acid, guanidine derivatives (in particular diphenylguanidine), etc. are incorporated during the first non-productive phase and / or during the productive phase.The sulfur content is preferably between 0.5 and 4 pce, that of the primary accelerator is preferably between 0.5 and 5 pce. These preferential rates can apply to any of the embodiments of the invention.

[0062] Any compound capable of acting as a vulcanization accelerator (primary or secondary) that can act as a vulcanization accelerator for diene elastomers in the presence of sulfur may be used, in particular accelerators of the thiazole type and their derivatives, accelerators of the sulfenamide type for primary accelerators, and accelerators of the thiuram, dithiocarbamate, dithiophosphate, thiourea and xanthate type for secondary accelerators. Examples of primary accelerators include, in particular, sulfenamide compounds such as N-cyclohexyl-2-benzothiazyl sulfenamide ("CBS"), N,N-dicyclohexyl-2-benzothiazyl sulfenamide ("DCBS"), N-tert-butyl-2-benzothiazyl sulfenamide ("TBBS"), and mixtures of these compounds.

[0063] The primary accelerator is preferably a sulfenamide, more preferably N-cyclohexyl-2-benzothiazyl sulfenamide. Examples of secondary accelerators include 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.

[0064] The vulcanization is carried out in a known manner at a temperature generally between 130°C and 200°C, for a sufficient time which may vary, for example, between 5 and 90 min depending in particular on the curing temperature, the vulcanization system adopted and the vulcanization kinetics of the composition in question. The rubber composition in accordance with the invention may also comprise all or part of the usual additives usually used in elastomer compositions intended for the manufacture of tires, in particular pigments, protective agents such as anti-ozone waxes, chemical anti-ozonants, antioxidants, plasticizers such as plasticizing oils or resins. These additives are generally incorporated into the rubber composition before step b) of the process in accordance with the invention, for example during step a).

[0065] The rubber composition is typically manufactured in suitable mixers, using two successive preparation phases according to a procedure well known to those skilled in the art: a thermo-mechanical working or kneading phase (sometimes referred to as a "non-productive" phase) at high temperature, up to a maximum temperature of between 110°C and 190°C, preferably between 130°C and 180°C, followed by a mechanical working phase (sometimes referred to as a "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.

[0066] For example, the non-productive phase is carried out in a single thermomechanical step during which all the necessary ingredients, any additional processing agents and other various additives, with the exception of the vulcanization system, are introduced into a suitable mixer such as a conventional internal mixer. The total mixing time in this non-productive phase is preferably between 1 and 15 min. After cooling the mixture thus obtained during the non-productive phase, the vulcanization system is then incorporated at low temperature, generally in an external mixer such as a roller mixer, and the whole is then mixed (productive phase) for a few minutes, for example between 2 and 15 min.

[0067] According to the invention, a step of thermomechanical kneading of the elastomer matrix alone is carried out before proceeding to step a). Thermomechanical kneading of the elastomer matrix alone prior to step a) means that the elastomers of the elastomer matrix are kneaded together before step a). Preferably, the thermomechanical kneading of the elastomer matrix alone is carried out until a maximum temperature of between 80°C and 160°C is reached, preferably ranging from 110°C to 130°C. The thermomechanical kneading of the elastomer matrix alone typically takes place in a conventional internal mixer which can also be used to proceed to step a).

[0068] The modifying agent is added to the elastomer matrix to be incorporated therein and to react with the elastomer matrix by a grafting reaction according to a [3+2] cycloaddition of the dipole on the diene units of the elastomer matrix. Preferably, in step a) the modifying agent is incorporated into the elastomer matrix before the other ingredients of the rubber composition. According to a first variant, the incorporation of the modifying agent is carried out by thermomechanical mixing in a conventional internal mixer which may be the one used for the preliminary mixing of the elastomer matrix.Thermomechanical mixing is carried out until a preferred maximum temperature of between 80°C and 160°C is reached, preferably ranging from 110°C to 130°C to allow both the incorporation of the modifying agent into the elastomer matrix and the grafting of all or part of the modifying agent onto the elastomer matrix, for example 60 to 80% of the added quantity of the modifying agent. The contact time between the elastomer matrix and the modifying agent during thermomechanical mixing is adjusted according to the thermomechanical mixing conditions, in particular 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 110°C to 130°C.Thermomechanical mixing in the internal mixer can be followed by mixing on an external mixer, for example, to carry out sampling intended to characterize the product resulting from the thermomechanical mixing of the elastomer matrix and the modifying agent. To carry out mixing on an external mixer safely, a cooling step is generally carried out on the product extracted from the internal mixer to avoid burns during handling.

[0069] According to a second variant, the incorporation of the modifying agent is carried out on an external mixer, such as a roller mixer, at a temperature below 60°C and is followed by a heat treatment, for example in a press or in an oven, at temperatures ranging from 80°C to 200°C to graft the modifying agent onto the elastomer matrix.

[0070] According to a third variant, the incorporation of the modifying agent is carried out on an external mixer such as a cylinder mixer at a temperature above 60°C and does not require additional heat treatment to graft the modifying agent onto the elastomer matrix.

[0071] The amount of the modifying agent added in step a) may vary within a range of between 0 and 3 mol% of the monomer units of the elastomer matrix. It is adjusted by a person skilled in the art depending on the amount of silica in the rubber composition and depending on the use of the rubber composition. The lower the hysteresis of the rubber composition is sought, the higher the amount of modifying agent. The amount of modifying agent added in step a) is preferably less than 2%, more preferably than 1%, even more preferably than 0.5 mol% of the monomer units of the elastomer matrix. It is preferably greater than 0.02%, more preferably than 0.05 mol% of the monomer units of the elastomer matrix. Low levels of silica are generally associated with the lowest levels of modifying agent.The rubber composition may be calendered or extruded in the form of a sheet or plate, in particular for laboratory characterization, or in the form of a semi-finished (or profiled) rubber usable in a tire. The composition may be either in the raw state (before crosslinking or vulcanization), or in the cured state (before crosslinking or after vulcanization). It may constitute all or part of a semi-finished article, in particular intended to be used in a pneumatic or non-pneumatic tire which comprises a tread, in particular in the tread of the tire.

[0072] The rubber composition manufactured according to the process according to the invention is advantageously vulcanized, preferably at a temperature above 110°C, in particular after having been extruded or calendered in the form of a semi-finished article such as a tire tread.

[0073] In summary, the invention is advantageously implemented according to any one of the following embodiments 1 to 25:

[0074] Method 1: Process for preparing a rubber composition which comprises an elastomer matrix, a modifying agent which comprises a nitrile oxide dipole and an N-substituted imidazole function, a reinforcing filler which comprises more than 50% by mass of a silica, a silane coupling agent and a vulcanization system, which process comprises the following successive steps a, b, c and d:

[0075] - a) incorporating into the elastomer matrix during a so-called non-productive step with the modifying agent, the reinforcing filler and the silane coupling agent by kneading until a maximum temperature of between 110 and 190°C is reached,

[0076] - b) cool the assembly to a temperature below 100°C,

[0077] - c) then incorporate the vulcanization system,

[0078] - d) kneading the whole to a maximum temperature below 120°C, characterized in that the elastomer matrix comprises natural rubber and a synthetic diene elastomer and in that the process comprises a step of thermomechanical kneading of the elastomer matrix alone prior to step a).

[0079] Mode 2: Process according to mode 1 in which the thermomechanical mixing of the elastomer matrix alone is carried out until a maximum temperature of between 80°C and 160°C is reached.

[0080] Mode 3: Process according to mode 1 or 2 in which the thermomechanical mixing of the elastomer matrix alone is carried out until a maximum temperature ranging from 110°C to 130°C is reached.

[0081] Method 4: Process according to any one of methods 1 to 3 in which the modifying agent is incorporated into the elastomer matrix before the other ingredients of the rubber composition. Method 5: Process according to method 4 in which the incorporation of the modifying agent into the elastomer matrix alone is carried out by thermomechanical mixing in an internal mixer until a maximum temperature of between 80°C and 160°C is reached, preferably ranging from 110°C to 130°C.

[0082] Mode 6: Process according to mode 4 in which the incorporation of the modifying agent is carried out in an external mixer at a temperature below 60°C and is followed by a heat treatment at temperatures ranging from 80°C to 200°C.

[0083] Mode 7: Process according to mode 4 in which the incorporation of the modifying agent is carried out in an external mixer at a temperature above 60°C.

[0084] Mode 8: Process according to any one of modes 1 to 7 in which the level of natural rubber in the rubber composition is greater than 50 parts by weight per hundred parts of the elastomer matrix, pce, and less than 90 pce and the level of the synthetic diene elastomer in the rubber composition is greater than 10 pce and less than 50 pce.

[0085] Mode 9: Process according to any one of modes 1 to 8 in which the synthetic diene elastomer is an SBR.

[0086] Mode 10: Process according to any one of modes 1 to 9 in which the synthetic diene elastomer is a solution SBR.

[0087] Mode 11: Process according to any one of modes 1 to 10 in which the synthetic diene elastomer is an SBR containing 1 to 40% by weight of styrene.

[0088] Mode 12: Process according to any one of modes 1 to 11 in which the synthetic diene elastomer is an SBR containing 15 to 35% by weight of styrene.

[0089] Method 13: Process according to any one of methods 1 to 12 in which the added quantity of the modifying agent in step a) varies within a range of between 0 and 3% by mole of the monomer units of the elastomer matrix.

[0090] Mode 14: Process according to any one of modes 1 to 13 in which the silica represents more than 85% by mass of the reinforcing filler.

[0091] Method 15: Process according to any one of methods 1 to 14 in which the N-substituted imidazole function is of formula (I) in which the symbol Zi represents a hydrogen atom or an alkyl having 1 to 6 carbon atoms, the symbol Z2 denotes an attachment to the nitrile oxide dipole.

[0092] Method 16: Process according to method 15 in which the alkyl represented by Zi contains 1 to 3 carbon atoms.

[0093] Method 17: Process according to method 15 or 16 in which the alkyl represented by Zi is methyl.

[0094] Mode 18: Process according to any one of modes 1 to 17 in which the modifying agent is an aromatic nitrile oxide, compound comprising an aromatic group substituted by a nitrile oxide dipole and by a group containing the N-substituted imidazole function.

[0095] Method 19: A method according to any one of methods 1 to 18 wherein the modifying agent is a compound which contains a unit of formula (II) wherein R'1 represents the nitrile oxide dipole, one of the symbols R'2 to R' 6 represents a saturated group having 1 to 6 carbon atoms and covalently bonded to one of the nitrogen atoms of the 5-membered ring of the N-substituted imidazole function, the other symbols, identical or different, representing a hydrogen atom or a substituent.

[0096] Method 20: Process according to method 19 in which the saturated group is an alkanediyl.

[0097] Method 21: Process according to method 19 or 20 in which the saturated group is an alkanediyl having 1 to 3 carbon atoms.

[0098] Mode 22: A process according to any one of modes 19 to 21 wherein the saturated group is methanediyl. Mode 23: A process according to any one of modes 19 to 22 wherein R'2, R'4 and R'e are each alkyl.

[0099] Mode 24: A process according to any one of modes 19 to 23 wherein R'2, R'4 and R'e are each methyl or ethyl.

[0100] Mode 25: Method according to any one of modes 19 to 24 in which R' 3 and R' 5 are each a hydrogen atom.

[0101] 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 and non-limiting purposes.

[0102] Examples

[0103] The dynamic properties tanô(max) are measured on a viscoanalyzer (Metravib VA4000), according to the ASTM D 5992-96 standard. The response of a sample of vulcanized composition (cylindrical specimen 4 mm thick and 400 mm 2 section), subjected to sinusoidal stress in alternating simple shear, at a frequency of 10 Hz, at 60°C. A strain amplitude sweep is carried out from 0.1% to 100% (forward cycle), then from 100% to 0.1% (return cycle). The results used are the tanô loss factor and the modulus difference (AG*) between the values ​​at 0.1 and 100% strain (Payne effect). For the return cycle, the maximum tanô value observed, noted tanô(max), is indicated. The results are expressed on a base of 100 relative to a control. A value greater than that of the control, arbitrarily set at 100, indicates a measured quantity greater than that of the control.

[0104] Tensile tests:

[0105] Tensile tests are used to determine yield stresses and properties at break. Unless otherwise stated, they are carried out in accordance with French standard NF T 46-002 of September 1988. Processing of the tensile records also allows the modulus curve to be plotted as a function of elongation. The modulus used here is the nominal (or apparent) secant modulus measured at first elongation, calculated by referring to the initial section of the specimen. The nominal secant moduli (or apparent stresses, in MPa) are measured at first elongation at 100% elongation, noted MSA100. The stresses at break (in MPa) and elongations at break (in %) are measured at 23°C ± 2°C according to standard NF T 46-002. The results are expressed on a base of 100 relative to a control. A value higher than that of the control, arbitrarily set at 100, indicates an improved result, that is to say a measured quantity higher than that of the control.rubber coms:.

[0106] The formulations (in pce) of the prepared rubber compositions are described in Table 1.

[0107] The elastomer matrix is ​​a mixture of 75 pce natural rubber and 25 pce SBR. The SBR is a solution SBR containing 26% styrene unit and 56% 1,2-butadiene unit.

[0108] A 1,3-dipolar compound, 2,4,6-trimethyl-3-((2-methyl-1H-imidazol-1-yl)methyl)benzonitrile oxide, the synthesis of which is described in patent application WO 2015059274, is used as the modifying agent.

[0109] Preparation of an Al rubber composition according to a process not in accordance with the invention:

[0110] The natural rubber, the SBR, then the silica, then the silane coupling agent are introduced into an internal mixer (final filling rate: approximately 70% by volume), whose initial tank temperature is approximately 110°C, and the mixture is mixed for one to two minutes, then the various other ingredients are added, with the exception of the vulcanization system. A thermomechanical process is then carried out (non-productive phase) 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, then sulfur and a sulfenamide-type accelerator are incorporated into a mixer (homo-finisher) at 23°C, mixing everything (productive phase) for an appropriate time (for example, between 5 and 12 min).

[0111] Preparation of a rubber composition A2 according to a process not in accordance with the invention:

[0112] The process for preparing rubber composition A2 is identical to that of Al except that thermomechanical mixing of the two elastomers is carried out for 30 seconds before adding the silica.

[0113] Preparation of a rubber composition A3 according to a process not in accordance with the invention:

[0114] The process for preparing rubber composition A3 is identical to that of Al except that the natural rubber and the SBR are each previously modified by a modifying agent according to the following procedure:

[0115] The elastomer to be modified, in this case natural rubber or SBR, is introduced into an internal mixer (final filling rate: approximately 70% by volume), whose initial tank temperature is approximately 110°C, followed by the modifying agent, and then mixed for two minutes at 120°C. The resulting product is recovered, cooled and homogenized on an external mixer (roller tool) at 30°C (15 portfolio passes).

[0116] The amount of modifying agent is 1.13 pce in the case of natural rubber (i.e. 0.3 mol% of the monomer units of natural rubber); it is 1.24 pce in the case of SBR (i.e. 0.3 mol% of the monomer units of SBR). It follows that in the A3 process, the silica and the other ingredients of the rubber composition are incorporated into the elastomer matrix which is made up of the modified natural rubber and the modified SBR.

[0117] Preparation of an A4 rubber composition according to a process not in accordance with the invention:

[0118] The process for preparing rubber composition A4 is identical to that of A3 except that only the natural rubber is previously modified by the modifying agent (1.13 pce).

[0119] Preparation of a rubber composition A5 according to a process not in accordance with the invention:

[0120] The process for preparing rubber composition A5 is identical to that of A3 except that only the SBR is previously modified by the modifying agent (1.24 pce instead of 1.13 pce, or 0.3 mol%).

[0121] Preparation of an A6 rubber composition according to a process in accordance with the invention: The natural rubber and the SBR are introduced into an internal mixer (final filling rate: approximately 70% by volume), the initial tank temperature of which is approximately 110°C, kneaded for 30 seconds, then the modifying agent (1.13 pce) is added, kneaded for two minutes at 120°C, then the silica is added, then the silane coupling agent is added, kneaded for one to two minutes, then the various other ingredients are added with the exception of the vulcanization system. Thermomechanical work (non-productive phase) is then carried out in one step, which lasts approximately 5 min to 6 minutes, until a maximum "drop" temperature of 160°C is reached.The mixture thus obtained is recovered, cooled and then sulfur and a sulfenamide type accelerator are incorporated on an external mixer (homo-finisher) at 23°C, mixing everything (productive phase) for an appropriate time of 5 to 6 minutes.

[0122] Preparation of a rubber composition A7 according to a process in accordance with the invention: The process for preparing the rubber composition A7 is identical to that of A6 except that 0.75 pce of the modifying agent is added instead of 1.13 pce.

[0123] The process for preparing rubber compositions A6 and A7 differs from that of A2 in that a modifying agent is added to the elastomer matrix to be incorporated into the elastomer matrix and to be grafted onto the elastomer matrix.

[0124] 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, 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, in particular for treads. Crosslinking is carried out at 150°C. The results are shown in Table 2.

[0125] Table 1

[0126] (I) Natural rubber (2) SBR containing 26% by weight of styrene and 56% of 1,2-butadiene units

[0127] (3) NR modified according to the procedure described in the preparation process of A3

[0128] (4) SBR modified according to the procedure described in the preparation process of A3

[0129] (5) 2,4,6-trimethyl-3-((2-methyl-1H-imidazol-1-yl)methyl)benzonitrile oxide

[0130] 6) "Zeosil 1165 MP" from Solvay-Rhodia in the form of microbeads (7) Carbon black N234

[0131] (8) Liquid silane triethoxysilylpropyltetrasulfide (TESPT) “Si69” from Evonik

[0132] (9) Diphenylguanidine “Perkacit DPG” from Flexsys

[0133] (10) Anti-ozone wax “VARAZON 4959” from the company Sasol Wax

[0134] (II) Tetramethylquinone (12) N-(l,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine "Santoflex 6PPD" from the company

[0135] FLEXSYS

[0136] (13) Stearic acid “Pristerene 4931” from Uniqema (14) Industrial grade Zinc Oxide from Umicore

[0137] (15) N-cyclohexyl-2-benzothiazol-sulfenamide “Santocure CBS” from Flexsys

[0138] Table 2:

[0139] Composition A6 in which the elastomers are kneaded together before the introduction of 1.13 pce of modifying agent and the other ingredients has a medium deformation stiffness (MSA100 at 60°C) which is higher than composition A3 without increasing the hysteresis seen through tanô(max) at 60°C. From the comparison of rubber compositions A3 and A6, it is found that the process according to the invention which leads to the production of composition A6 succeeds in increasing the cured stiffness of the rubber compositions without penalizing the hysteresis. This result is obtained even though the quantity of modifying agent used is only 1.13 pce in A6 instead of 2.37 pce in A3.

[0140] Even compositions A4 and A5, which were prepared with lower levels of modifying agent than A3, 1.13 pce and 1.24 pce respectively, do not exhibit such low hysteresis and such high stiffness as those of A6.

[0141] The comparison between compositions A1 and A2 which are not in accordance with the invention and which differ from each other by a step of thermomechanical mixing of the two elastomers together in the process of A2 shows that only the step of thermomechanical mixing of the two elastomers together is not at the origin of the compromise of properties between hysteresis and rigidity which is obtained by the implementation of the process in accordance with the invention.

[0142] It is also observed that composition A7 prepared according to the process in accordance with the invention and with a modifying agent content of only 0.75 pce also presents an interesting property compromise between rigidity and hysteresis compared to compositions A4 and A5, however, prepared with a higher modifying agent content, respectively 1.13 pce and 1.24 pce.

[0143] The method according to the invention proves to be the most effective method for obtaining the best compromise between the properties of hysteresis, rigidity and the quantity of modifying agent.

Claims

Demands 1. A process for preparing a rubber composition comprising an elastomeric matrix, a modifying agent comprising a nitrile oxide dipole and an N-substituted imidazole function, a reinforcing filler comprising more than 50% by mass of a silica, a silane coupling agent and a vulcanizing system, which process comprises the following successive steps a, b, c and d: - a) incorporate into the elastomer matrix, during a so-called non-productive step, the modifying agent, the reinforcing filler, the silane coupling agent by mixing until a maximum temperature of between 110 and 190°C is reached, - b) cool the assembly to a temperature below 100°C, - c) then incorporate the vulcanization system, - d) kneading the whole up to a maximum temperature below 120°C, characterized in that the elastomer matrix comprises natural rubber and a synthetic diene elastomer and in that the process includes a thermomechanical kneading step of the elastomer matrix alone prior to step a).

2. A method according to claim 1 wherein the thermomechanical mixing of the elastomer matrix alone is carried out until a maximum temperature of between 80°C and 160°C is reached, preferably from 110°C to 130°C.

3. A method according to claim 1 or 2 wherein in step a) the modifying agent is incorporated into the elastomer matrix before the other ingredients of the rubber composition.

4. A method according to any one of claims 1 to 3 wherein the proportion of natural rubber in the rubber composition is greater than 50 parts by weight percent parts of the elastomer matrix, pc, and less than 90 pc and the proportion of synthetic diene elastomer in the rubber composition is greater than 10 pc and less than 50 pc.

5. A method according to any one of claims 1 to 4 wherein the synthetic diene elastomer is an SBR, preferably a solution SBR.

6. A method according to any one of claims 1 to 5 wherein the amount added of the modifying agent in step a) varies in a range between 0 and 3% by mole of the monomer units of the elastomer matrix.

7. A method according to any one of claims 1 to 6 in which silica represents more than 85% by mass of the reinforcing filler.

8. A method according to any one of claims 1 to 7, wherein the N-substituted imidazole function has formula (I), in which the symbol Zi represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, and the symbol Z2 denotes an attachment to the nitrile oxide dipole. A process according to any one of claims 1 to 8, wherein the modifying agent is an aromatic nitrile oxide compound comprising an aromatic group substituted by a nitrile oxide dipole and by a group containing the N-substituted imidazole function. A process according to any one of claims 1 to 9, wherein the modifying agent is a compound containing a motif of formula (II) in which R'i represents the nitrile oxide dipole, one of the symbols R'2 to R'e represents a saturated group having 1 to 6 carbon atoms and covalently bonded to one of the nitrogen atoms of the 5-membered ring of the N-substituted imidazole function, the other symbols, identical or different, representing a hydrogen atom or a substituent. A method according to claim 10, wherein the saturated group is an alkanediyl, preferably an alkanediyl having 1 to 3 carbon atoms. A method according to any one of claims 10 to 11, wherein R'2, R'4, and R'e are each an alkyl, preferably methyl or ethyl. A method according to any one of claims 10 to 12, wherein R' 3 and R' 5 are each a hydrogen atom.

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