WO2015091929A1

WO2015091929A1 - Tire for vehicle bearing heavy loads

Info

Publication number: WO2015091929A1
Application number: PCT/EP2014/078696
Authority: WO
Inventors: Christophe Chouvel; José-Carlos ARAUJO DA SILVA
Original assignee: Compagnie Generale Des Etablissements Michelin; Michelin Recherche Et Technique S.A.
Priority date: 2013-12-20
Filing date: 2014-12-19
Publication date: 2015-06-25
Also published as: AU2014368468A1; FR3015499A1; AU2014368468B2; FR3015499B1; CA2933303A1

Abstract

The present invention relates to a tire for vehicles bearing heavy loads. The tread of said tire contains a composition made of at least one elastomer matrix containing a first diene elastomer and a thermoplastic styrene elastomer. Said thermoplastic styrene elastomer is at most 50 wt% of the elastomer matrix and contains at least one rigid styrene segment and at least one flexible diene segment. The at least one flexible diene segment contains at least 20 wt% combined diene units. The combined diene units can be totally or partially hydrogenated. Said first diene elastomer is at least 50 wt% of the elastomer matrix and is selected from among the group made up of polybutadienes, butadiene copolymers, and the mixtures thereof. Said composition is also made of: at least one reinforcing filler that contains a carbon black, which is more than 50 wt% of the reinforcing filler; and a cross-linking system. Such a tire has enhanced resistance to crack propagation.

Description

Pneumatic tire for vehicles intended to carry heavy loads

The field of the present invention is that of tires for vehicles intended to carry heavy loads, in particular buses, trucks, agricultural vehicles, civil engineering vehicles.

These tires are provided with treads which have, in relation to the tread thicknesses of the tires for light vehicles, in particular for passenger cars or vans, large thicknesses of rubber material. Typically the wearing part of the tread of a truck has a thickness of at least 15 mm, that of a civil engineering vehicle at least 30 mm, or up to 120 mm. During the running, a tread undergoes mechanical stresses and aggression resulting from direct contact with the ground. In the case of a tire mounted on a vehicle carrying heavy loads, the mechanical stresses and the aggressions suffered by the tire are amplified under the effect of the weight carried by the tire. This has the consequence that the crack initiators which are created in the tread under the effect of these stresses and these attacks, tend to spread more on the surface or inside the tread. The propagation of cracks in the tread can cause damage to the tread and thus reduce the service life of the tread or the tire. A tire rolling on stony ground is very exposed to crack initiation. The very aggressive nature of the stony ground exacerbates not only this type of attack on the tread, but also its consequences on the tread. This is particularly true for tires fitted to civil engineering vehicles that typically operate in mines. This is also true for tires that are mounted on agricultural vehicles because of stony soil on arable land. The tires that equip heavy-duty vehicles of construction sites that circulate as much on stony soils as on bituminous soils, also know these same aggressions. Due to the two aggravating factors that are the weight carried by the tire and the aggressive nature of the taxiway, the crack propagation resistance of a tread of a tire for a civil engineering vehicle, an agricultural vehicle or a heavy-duty construction vehicle is crucial to minimizing the impact of tread damage.

It is therefore important to have tires for heavy load vehicles, whose tread has a crack propagation resistance sufficiently high to minimize the effect of crack initiation on the life of the tread. rolling. For To solve this problem, tire manufacturers, for example, use natural rubber in the treads because of the crack-growth resistance properties of natural rubber as mentioned in "Table 3.7 Comparison of elastomers properties" p. 162-163, Hofmann Rubber Technology Handbook, Hanser Publishers (1989).

The Applicants have discovered that the combined use of a majority of carbon black as a reinforcing filler, a polybutadiene or a butadiene copolymer and a certain level of a specific thermoplastic elastomer in a strip of rolling makes it possible to improve the crack propagation resistance of the tread of a vehicle tire intended to carry heavy loads without substantially deteriorating other tread performance, such as wear and resistance to the tread. rolling.

Thus, a first object of the invention is a tire for vehicles intended to carry heavy loads whose tread comprises a composition based on at least:

an elastomer matrix comprising a first diene elastomer and a styrenic thermoplastic elastomer,

wherein the thermoplastic styrene elastomer represents at most 50% by weight of the elastomer matrix, and comprises at least one styrenic rigid segment and at least one diene flexible segment, the at least one diene flexible segment comprises at least 20% by weight of conjugated diene units, the conjugated diene units being wholly or partly hydrogenated,

wherein the first diene elastomer represents at least 50% by weight of the elastomer matrix and is selected from the group consisting of polybutadienes, butadiene copolymers and their mixtures,

a reinforcing filler which comprises a carbon black which represents more than 50% by weight of the reinforcing filler,

a crosslinking system.

The invention also relates to a method for preparing the tire according to the invention.

I. MEASUREMENTS AND TESTS USED

Resistance to crack propagation:

The cracking rate was measured on specimens of rubber compositions, using a 381 type "Elastomer Test System" of the MTS company, as explained hereinafter.

Resistance to cracking is measured by repeated tractions on a specimen initially accommodated (after a first traction cycle) and then scored. The tensile test piece consists of a parallelepiped shaped rubber plate, for example having a thickness of between 1 and 2 mm, a length of between 130 and 170 mm and a width of between 10 and 15 mm, the two lateral edges being each covered lengthwise by a cylindrical rubber bead (diameter 5 mm) ) allowing anchoring in the jaws of the traction machine. The test pieces thus prepared are tested in the new state. The test was conducted in air at a temperature of 20 ° C. After accommodation, 3 very fine cuts of between 15 and 20 mm in length are made using a razor blade, at mid-width and aligned along the length of the test piece, one at each end and one in the center of the latter, before starting the test. At each tensile cycle, the deformation rate of the specimen is adjusted automatically so as to keep the rate of energy restitution (amount of energy released during the progression of the crack) constant, at a value less than or equal to at about 500 J / m ² . The crack propagation rate is measured in nanometers per cycle. The resistance to crack propagation will be expressed in relative units (ur) by dividing the speed of propagation of the control by that of the mixture, the speeds being measured at the same rate of energy release. A value greater than that of the control, arbitrarily set at 100, indicates an improved result, that is to say a higher resistance to the propagation of cracks.

DETAILED DESCRIPTION OF THE INVENTION In the present description, unless expressly indicated otherwise, all the percentages (%) indicated are% by weight. The abbreviation "pce" means parts by weight per hundred parts of elastomers present in the elastomeric matrix, the elastomeric matrix designating all of the elastomers present in the rubber composition. On the other hand, any range of values designated by the expression "between a and b" represents the range of values greater than "a" and less than "b" (i.e., terminals a and b excluded). while any range of values designated by the term "from a to b" means the range of values from "a" to "b" (i.e. including the strict limits a and b). By the term "composition-based" is meant in the present description a composition comprising the mixture and / or the reaction product in situ of the various constituents used, some of these basic constituents (for example the elastomer, the filler or other additive conventionally used in a rubber composition intended for the manufacture of tire) being capable of, or intended to react with one another, at least in part, during the different phases of manufacture of the composition intended for the manufacture of a tire .

The elastomer matrix of the rubber composition has the essential feature of comprising a first diene elastomer selected from the group consisting of polybutadienes (B), butadiene copolymers and mixtures thereof. Suitable polybutadienes are in particular those having a content of 1,2 units of between 4% and 80% by weight of the mass of polybutadiene or those having a 1,4-cis bond content of at least 90% by mass of the mass. polybutadiene. Particularly suitable butadiene copolymers are copolymers of butadiene and styrene (SB). The copolymers can be prepared in emulsion (ESBR) or in solution (SSBR). Mention may be made of butadiene-styrene copolymers and in particular those having a glass transition temperature Tg, measured according to ASTM D3418, between 0 ° C. and -90 ° C. and more particularly between -10 ° C. and -80 ° C., a styrene content of between 5% and 60% by weight and more particularly between 5% and 40%, a content (mol%) in -1,2 bonds of the butadiene part of between 4% and 75% of the butadiene part; a content (mol%) of trans-1,4 bonds of between 10% and 80% of the butadiene part.

The first diene elastomer, whether it be a polybutadiene or a butadiene copolymer, may be modified with a modifying agent such as, for example, a coupling agent, starring agent or functionalization agent. As modifying agent, mention may be made of compounds comprising a C-Sn bond, or those comprising an amine, silanol or alkoxysilane function. Such elastomers are for example described in patents EP 0 778 311 B1, EP 0 890 607 B1 and EP 0 692 492 B1, EP 1 000 970 B1, EP 1 457 501 B1 or patent applications WO 2009/000750, WO 2009/133068.

According to a preferred embodiment of the invention, the first diene elastomer is a polybutadiene, preferably having a 1,4-cis bond ratio greater than or equal to 90% by weight of the polybutadiene mass. This preferred embodiment of the invention may be combined with any one of the embodiments of the invention.

The first diene elastomer represents at least 50% by weight of the elastomer matrix. According to this embodiment, for example, as an elastomer matrix, a mixture consisting of 40% by weight of the thermoplastic styrene elastomer, 55% by weight of the first diene elastomer and 5% by weight of a second diene elastomer, the percentages being calculated on the basis of the total mass of the elastomeric matrix.

By a second diene elastomer (or indistinctly rubber), one or more elastomers consisting of at least a part (Le., A homopolymer or a copolymer) of monomeric diene units (monomers carrying two double carbon bonds) must be understood in known manner. carbon, conjugated or not), the second diene elastomer being different from the first diene elastomer and not being a thermoplastic styrene elastomer.

According to a preferred embodiment of the invention, only the first diene elastomer and the styrenic thermoplastic elastomer constitute the elastomer matrix, which means that the The elastomeric matrix contains no other elastomers than the first diene elastomer and the thermoplastic styrene elastomer.

The styrenic thermoplastic elastomer comprises at least one styrenic rigid segment and at least one diene flexible segment comprising at least 20% by weight of conjugated diene units, the conjugated diene units being wholly or partly hydrogenated. The rigid and flexible segments can be arranged linearly, star or connected.

A flexible segment refers to an elastomeric type polymer block, a rigid segment refers to a thermoplastic type polymer block.

According to one embodiment of the invention, the styrenic thermoplastic elastomer is a diblock. The diblock comprises a single rigid styrenic segment connected to a single diene flexible segment. According to a preferred embodiment of the invention, the styrenic thermoplastic elastomer comprises at least two rigid styrenic segments. According to this preferred embodiment of the invention, generally at least two chain ends of the styrenic thermoplastic elastomer are each provided with a styrenic rigid segment and the styrenic rigid segments are connected by the one or more flexible diene segments. According to this preferred embodiment of the invention, the styrenic thermoplastic elastomer is preferably a triblock. The triblock then consists of two rigid styrenic segments and a flexible diene segment.

In the case where the styrenic thermoplastic elastomer is a diblock, the denomination of "the at least one rigid segment" designates the rigid segment present in the styrenic thermoplastic elastomer. In the different cases of a diblock, for example in the case of a triblock, the name "the at least one rigid segment" designates the rigid segments present in the thermoplastic styrene elastomer. In the case where the thermoplastic styrene elastomer is a diblock or a triblock, the name of "the at least one flexible segment" designates the flexible segment present in the thermoplastic styrene elastomer. In cases where the styrenic thermoplastic elastomer is neither a diblock nor a triblock, the denomination of "the at least one flexible segment" designates the flexible segments present in the thermoplastic styrene elastomer.

The at least one styrenic rigid segment is the homopolymer of a styrenic monomer or the block or random copolymer of several styrenic monomers or else the copolymer of one or more styrenic monomers and of another non-styrenic monomer such as 1,3-diene. By styrene monomer is to be understood in the present description styrene or substituted styrene. Examples of substituted styrenes which may be mentioned are methylstyrenes (for example Γ-methylstyrene, m-methylstyrene or p-methylstyrene, alpha-methylstyrene, alpha-2-dimethylstyrene, alpha-4-dimethylstyrene or diphenylethylene), para-tert-butylstyrene, chlorostyrenes (e.g., Γο-chlorostyrene, m-chlorostyrene, p-chlorostyrene, 2,4-dichlorostyrene, 2,6-dichlorostyrene or 2,4,6-dichlorostyrene). -trichlorostyrene), bromostyrenes (e.g., bromostyrene, m-bromostyrene, p-bromostyrene, 2,4-dibromostyrene, 2,6-dibromostyrene or 2,4,6-tribromostyrene), fluorostyrenes ( for example Γο-fluorostyrene, m-fluorostyrene, p-fluorostyrene, 2,4-difluorostyrene, 2,6-difluorostyrene or 2,4,6-trifluorostyrene) or para-hydroxy-styrene.

According to a preferred embodiment of the invention, the at least one styrenic rigid segment has a glass transition temperature greater than 80 ° C. Preferably, the at least one styrenic rigid segment is polystyrene.

The at least one diene flexible segment comprises at least 20% by weight of conjugated diene monomer units (also referred to as conjugated diene units). The at least one diene flexible segment can be the homopolymer of a conjugated diene or the random or block copolymer of several conjugated dienes or the copolymer of one or more conjugated dienes and at least one other non-diene monomer such as than a styrenic monomer.

The proportion of conjugated diene units which form the diene flexible segment is preferably at least 50%, more preferably at least 60%, even more preferably at least 70% by weight of the weight of the diene flexible segment. Advantageously, it is at least 80% by weight of the mass of the diene flexible segment. These rates, whether preferential or not, apply to any of the embodiments of the invention.

As conjugated diene units, 1,3-butadiene units and isoprene units are particularly suitable. The at least one diene flexible segment may be a polybutadiene, a polyisoprene or a copolymer of 1,3-butadiene and isoprene. The copolymer of 1,3-butadiene and isoprene may be of a block or random nature.

Suitable thermoplastic styrene elastomers are diblock copolymers such as styrene / butadiene (SB), styrene / isoprene (SI), styrene / butadiene / isoprene (SBI) block copolymers or the mixture of these copolymers. In this designation the diene soft block is a random or block copolymer.

Styrene / butadiene / styrene (SBS), styrene / isoprene / styrene (SIS) block copolymers are particularly suitable as styrenic thermoplastic elastomers. styrene / butadiene / isoprene / styrene (SBIS) or the mixture of these copolymers. In this designation the diene soft block is a random or block copolymer. Particularly suitable is a styrene / butadiene / isoprene / styrene block copolymer (SBIS). According to a first variant of the invention, a fraction of the conjugated diene units of the at least one diene flexible segment is hydrogenated. It will be appreciated by those skilled in the art that it may equivalently use a styrenic thermoplastic elastomer whose double bonds of a fraction of the conjugated diene units of the diene soft segment will have been reduced to a single bond by a process other than hydrogenation. Among the methods which make it possible to reduce the double bonds of the diene units in single bond, mention may be made of reductions with aluminum hydride or with diimine, for example.

According to a second variant of the invention, all of the conjugated diene units of the at least one diene flexible segment is hydrogenated. It will be understood by those skilled in the art that it may equivalently use a styrenic thermoplastic elastomer whose double bonds of all of the conjugated diene units of the diene flexible segment will have been reduced in a single bond by a process other than hydrogenation.

According to this second variant of the invention, the thermoplastic elastomer copolymers styrene / ethylene / butylene (SEB), styrene / ethylene / propylene (SEP), styrene / ethylene / ethylene / propylene (SEEP) or mixtures of these copolymers are suitable as thermoplastic elastomer . In this designation, the hydrogenated flexible diene block is a random or block copolymer.

According to this second variant of the invention, the styrene / ethylene / butylene / styrene (SEBS), styrene / ethylene / propylene / styrene (SEPS), styrene / ethylene / ethylene / propylene / styrene block copolymers (SEEPS) are also suitable as thermoplastic elastomer ) or mixtures of these copolymers. In this designation, the hydrogenated flexible diene block is a random or block copolymer. Any of the embodiments of the invention applies to the first variant of the invention or to the second variant of the invention.

Styrene thermoplastic elastomers are also suitable mixtures of a triblock copolymer mentioned above and a diblock copolymer mentioned above. Indeed, the triblock copolymer may contain a minority weight fraction of diblock copolymer consisting of a rigid styrenic segment and a diene flexible segment, the rigid block and the flexible block being respectively of the same chemical nature, in particular of the same microstructure, as the rigid and flexible blocks of the triblock. The presence of diblock copolymer in the triblock copolymer generally results from the synthesis process of the triblock copolymer which can lead to the formation of a secondary product. as the diblock copolymer. Most often the percentage of diblock copolymer in the triblock copolymer does not exceed 40% by weight of triblock copolymer.

According to a preferred embodiment of the invention, the mass ratio of the at least one styrenic rigid segment is between 5 and 40% of the mass of the thermoplastic styrene elastomer. Below the minimum indicated, the thermoplastic nature of the styrenic thermoplastic elastomer is likely to decrease significantly while above the maximum recommended, the elasticity of the composition can be affected. For these reasons, the mass ratio of the at least one styrenic rigid segment in the styrenic thermoplastic elastomer is preferably in a range from 10 to 35%, more preferably from 10 to 20% of the mass of the thermoplastic elastomer. styrene. These levels, whether preferential or not, apply to any of the embodiments of the invention, especially when the polystyrene forms the at least one styrenic rigid segment of the thermoplastic styrene elastomer.

The number-average molar mass (denoted Mn) of the styrenic thermoplastic elastomer is preferably between 50,000 and 500,000 g / mol, more preferably between 60,000 and 450,000 g / mol, more preferably between 80,000 and 300,000 g / mol. Advantageously it is between 100,000 and 200,000 g / mol. These preferred ranges of average molar mass values apply regardless of the embodiment of the invention.

The molar mass is determined in a known manner by steric exclusion chromatography (SEC). The sample is first solubilized in tetrahydrofuran at a concentration of about 1 g / l; then the solution is filtered on 0.45 μιτι porosity filter before injection. The equipment used is a chromatographic chain "WATERS alliance". The elution solvent is tetrahydrofuran, the flow rate 0.7 ml / min, the system temperature 35 ° C and the analysis time 90 min. A set of four WATERS columns in series, of trade names "STYRAGEL"("HMW7","HMW6E" and two "HT6E") is used. The injected volume of the solution of the polymer sample is 100 μΙ. The detector is a differential refractometer "WATERS 2410" and its associated software for the exploitation of chromatographic data is the "WATERS MILLENIUM" system. The calculated average molecular weights relate to a calibration curve made with polystyrene standards. The styrenic thermoplastic elastomer is present in a mass proportion of at most 50% of the mass of the elastomer matrix of the rubber composition of the tread. Above the indicated maximum value, there is no more benefit on the crack propagation resistance of the rubber composition forming the tread of a tire intended to carry heavy loads. The content of the thermoplastic styrene elastomer varies in a range from 5 to 50%, more preferably from 10 to 45%, more preferably 20 to 45% by weight of the mass of the elastomer matrix. Advantageously, it varies from 25 to 45% by weight of the mass of the elastomer matrix. When the styrenic thermoplastic elastomer is a mixture of unsaturated thermoplastic styrene elastomers according to the invention, the rates indicated apply to the mixture and not to each of the styrenic thermoplastic elastomers. These rates, whether preferential or not, apply to any of the embodiments of the invention.

According to a particular embodiment of the invention, the styrenic thermoplastic elastomer has a glass transition temperature below -20 ° C. This glass transition temperature is generally attributed to the glass temperature of the diene flexible segment of the styrenic thermoplastic elastomer. The glass transition temperature is measured by means of a Differential Scanning Calorimeter according to ASTM D3418 (1999). According to this particular embodiment of the invention, the styrenic thermoplastic elastomer has a Tg preferably less than -30 ° C, more preferably less than -40 ° C, even more preferably less than -50 ° C.

The reinforcing filler may be any type of so-called reinforcing filler, known for its ability to reinforce a rubber composition that can be used for the manufacture of tires, for example an organic filler such as carbon black, a reinforcing inorganic filler such as silica which is associated in a known manner a coupling agent, or a mixture of these two types of charges.

A reinforcing filler typically consists of nanoparticles whose average size (in 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.

According to the present invention, the reinforcing filler comprises a carbon black which represents more than 50% by weight of the reinforcing filler. Carbon black means one or more carbon blacks. Carbon black is then considered as the majority reinforcing filler.

The carbon black has a BET surface area preferably of at least 90 m ² / g, more preferably at least 100 m ² / g. As such, the blacks conventionally used in tires or their belts are suitable. bearing (so-called pneumatic grade blacks). Among these, more particularly include reinforcing carbon blacks of the series 100, 200, 300 (ASTM grade), such as blacks N115, N134, N234, N375. The carbon blacks can be used in the isolated state, as commercially available, or in any other form, for example as a carrier for some of the rubber additives used. The carbon blacks could for example already be incorporated into an isoprene elastomer in the form of a masterbatch (see for example WO 97/36724 or WO 99/16600). BET specific surface area of carbon blacks is measured according to standard D6556-10 [multipoint method (at least 5 points) - gas: nitrogen - relative pressure range P / PO: 0.1 to 0.3].

According to one embodiment of the invention, the reinforcing filler also comprises a reinforcing inorganic filler. "Reinforcing inorganic filler" means any inorganic or mineral filler, irrespective of its color and origin (natural or synthetic), also called "white" filler, "clear" filler or even "non-black" filler. as opposed to carbon black, capable of reinforcing on its own, without any other means than an intermediate coupling agent, a rubber composition intended for the manufacture of pneumatic tires, in other words able to replace, in its function reinforcement, a conventional carbon black of pneumatic grade; such a filler is generally characterized, in known manner, by the presence of hydroxyl groups (-OH) on its surface.

Suitable reinforcing inorganic fillers are in particular mineral fillers of the siliceous type, preferentially silica (SiO ₂ ). The silica used may be any reinforcing silica known to those skilled in the art, in particular any precipitated or fumed silica having a BET surface and a CTAB specific surface both less than 450 m ² / g, preferably from 30 to 400 m ² / g, especially between 60 and 300 m ² / g. Silica "Ultrasil VN3" sold by the company Evonik can be cited as an example of silica that is useful for the purposes of the invention. As highly dispersible precipitated silicas (called "HDS"), mention may be made, for example, of the "Ultrasil" 7000 and "Ultrasil" 7005 silicas of Degussa, the "Zeosil" 1165MP, 1135MP and 1115MP silicas of the company hodia, the "Hi-Sil" silica EZ150G from the company PPG, the "Zeopol" silicas 8715, 8745 and 8755 from the Huber Company, the high surface area silicas as described in the application WO 03/016387.

The physical state under which the reinforcing inorganic filler is present is indifferent, whether in the form of powder, microbeads, granules or beads. Of course, the term "reinforcing inorganic filler" also refers to mixtures of different reinforcing inorganic fillers, in particular of highly dispersible silicas as described above.

Those skilled in the art will understand that as an equivalent load of the reinforcing inorganic filler described in this paragraph, it would be possible to use a reinforcing filler of another nature, in particular an organic filler such as carbon black, since this filler reinforcing would be covered with an inorganic layer such as silica, or would comprise on its surface functional sites, including hydroxyl, requiring the use of a coupling agent to establish the connection between the filler and the elastomer. By way of example, mention may be made, for example, of carbon blacks for tires as described for example in documents WO 96/37547 and WO 99/28380. In the present description, with regard to silica, the BET surface area is determined in a known manner by gas adsorption using the method of Brunauer-Emmett-Teller described in "The Journal of the American Chemical Society" Flight . 60, page 309, February 1938, specifically according to the French standard NF ISO 9277 of December 1996 (multipoint volumetric method (5 points) - gas: nitrogen - degassing: time at 160 ° C - relative pressure range p / po: 0.05 at 0.17). The CTAB specific surface is the external surface determined according to the French standard NF T 45-007 of November 1987 (method B).

In order to couple the reinforcing inorganic filler to the diene elastomer, a coupling agent is used in a well-known manner, in particular an at least bifunctional silane (or bonding agent) intended to ensure a sufficient connection, of a chemical and / or physical nature. , between the inorganic filler (surface of its particles) and the diene elastomer. In particular, organosilanes or at least bifunctional polyorganosiloxanes are used. In particular, polysulfide silanes, called "symmetrical" or "asymmetrical" silanes according to their particular structure, are used, as described for example in the applications WO03 / 002648 (or US 2005/016651) and WO03 / 002649 (or US 2005/016650).

In particular, polysulphide silanes having the general formula (V) are not suitable for limiting the definition below.

Z - A - S _x - A - Z (V)

in which :

x is an integer of 2 to 8 (preferably 2 to 5);

- the symbols A, which are identical or different, represent a divalent hydrocarbon group (preferably an alkylene Ci-Ci ₈ or an arylene group C ₆ -C _2, more particularly alkylene Ci-Ci _0, in particular -C C ₄ , especially propylene);

the symbols Z, identical or different, correspond to one of the three formulas below:

in which:

- the radicals ^one, substituted or unsubstituted, identical or different, represent an alkyl group _Ci-8 cycloalkyl, C ₅ -C ₈ aryl or C ₆ -C ₈ (preferably alkyl, -C C ₆ , cyclohexyl or phenyl, especially C ₁ -C ₄ alkyl groups, more particularly methyl and / or ethyl).

- the radicals R ^2, substituted or unsubstituted, identical or different, represent an alkoxy group or Ci-Ci ₈ cycloalkoxy, C ₅ -C ₈ (preferably a group selected from alkoxyls -C ₈ cycloalkoxyl and C ₅ -C _8, even more preferably a group selected from alkoxyls -C _4, especially methoxyl and ethoxyl).

In the case of a mixture of polysulfurized alkoxysilanes corresponding to formula (I) above, in particular common commercially available mixtures, the average value of "x" is a fractional number preferably of between 2 and 5, more preferably close to 4. But the invention can also be advantageously implemented for example with disulfide alkoxysilanes (x = 2). As examples of silane polysulfides, are more particularly the bis (mono, trisulfide or tetrasulfide) of bis (alkoxyl (Ci-C ₄₎ alkyl (Ci-C ₄₎ silyl-alkyl (C ₄ -C )), such as polysulfides of bis (3-trimethoxysilylpropyl) or bis (3-triethoxysilylpropyl). Among these compounds, bis (3-triethoxysilylpropyl) tetrasulfide, abbreviated to TESPT, of formula

or bis (triethoxysilylpropyl) disulfide, abbreviated to TESPD, of formula [(C ₂ H ₅ O) 3 Si (CH ₂ ) 3 S] ₂ .

As coupling agent other than polysulfurized alkoxysilane, mention will in particular be made of bifunctional POSS (polyorganosiloxanes) or polysulfides of hydroxysilane as described in patent applications WO 02/30939 (or US Pat. No. 6,774,255), WO 02 / 31041 (or US 2004/051210) or silanes or POSS carrying azodicarbonyl functional groups, as described for example in patent applications WO 2006/125532, WO 2006/125533, WO 2006/125534.

The content of coupling agent is advantageously less than 20 phr, it being understood that it is generally desirable to use as little as possible. Typically the level of coupling agent is from 0.5% to 15% by weight relative to the amount of inorganic filler. Its level is preferably between 0.5 and 12 phr, more preferably in a range from 3 to 10 phr. This level is easily adjusted by those skilled in the art according to the level of inorganic filler used in the composition.

The rubber composition in accordance with the invention may also contain, in addition to the coupling agents, coupling activators, inorganic charge-covering agents or, more generally, processing aid agents which can be used in a known manner, by improving the dispersion of the filler in the rubber matrix and lowering the viscosity of the compositions, to improve their ability to implement in the green state, these agents being for example hydrolysable silanes such as alkylalkoxysilanes (especially alkyltriethoxysilanes), polyols, polyethers (for example polyethylene glycols), primary, secondary or tertiary amines (for example trialkanolamines), hydroxylated or hydrolyzable POSs, for example α, ω -dihydroxy - polyorganosiloxanes (especially α, ω-dihydroxy-polydimethylsiloxanes), fatty acids such as stearic acid.

According to a particular embodiment of the invention, the silica may be used at levels ranging from 2 phr to 35 phr, preferably from 3 to 25 phr, and in particular from 5 to 20 phr. According to this embodiment, preferably the rubber composition contains from 0 to less than 2 phr of a coupling agent, more preferably from 0 to less than 1 phr of a coupling agent, even more preferably it does not contain a coupling agent. In the case where the rubber composition does not contain a coupling agent, the silica is not considered as a reinforcing filler and the rubber composition preferably contains a coating agent which is preferably a polyethylene glycol. This particular embodiment of the invention, in its preferred or non-preferred forms, may be combined with any one of the embodiments of the invention. The level of reinforcing filler is preferably comprised in a range from 10 to 90 phr. Below 10 phr, the reinforcement of the rubber composition may be insufficient to provide an adequate level of cohesion or wear resistance of the rubber component of the tire comprising this composition. Beyond 90 phr, there is a risk of increasing the hysteresis of the rubber composition and therefore a risk of heating of the tread and the tire. The level of total reinforcing filler is more preferably 25 phr to 70 phr, more preferably 35 to 60 phr. These reinforcing filler levels, whether preferential or not, apply to any of the embodiments of the invention. The rubber composition may also comprise all or part of the usual additives usually used in elastomer compositions, for example plasticizers, pigments, protective agents such as anti-ozone waxes, chemical antiozonants, antioxidants, anti-fatigue, a crosslinking system, vulcanization accelerators or retarders, vulcanization activators. According to any embodiment of the invention, the crosslinking system is preferably based on sulfur, but it may also be based on sulfur donors, peroxide, bismaleimides or their mixtures.

The rubber composition can be manufactured in suitable mixers, using two successive preparation phases well known to those skilled in the art: a first phase of work or thermomechanical mixing (so-called "non-productive" phase) at high temperature, up to at a maximum temperature of between 130 ° C. and 200 ° C., followed by a second mechanical working phase (so-called "productive" phase) to a lower temperature, typically less than 110 ° C., for example between 40 ° C. ° C and 100 ° C, finishing phase during which is incorporated the crosslinking system. The method for preparing the tire according to the invention comprises for example the following steps:

adding, during a first so-called non-productive step to the first diene elastomer, the styrenic thermoplastic elastomer, the reinforcing filler by thermomechanically kneading until a maximum temperature of between 130 and 200 ° C. is reached,

- cool the assembly to a temperature below 70 ° C,

- then incorporate the crosslinking system,

- mix everything up to a maximum temperature below 90 ° C to obtain a mixture,

- Then calender or extrude the resulting mixture to form a tread.

Whatever the embodiment of the invention, the tire for vehicles intended to carry heavy loads according to the invention is preferably a tire off the road, pneumatic for vehicles rolling on non-bituminous soils such as vehicles civil engineering, construction heavy-duty vehicles, or agricultural vehicles. The tire is preferably a tire for a civil engineering vehicle regardless of the embodiment of the invention.

The invention relates to the tires previously described both in the green state (that is to say, before firing) and in the fired state (that is to say, after crosslinking or vulcanization).

The aforementioned features of the present invention, as well as others, will be better understood on reading the following description of several embodiments of the invention, given by way of illustration and not limitation.

III. EXAMPLES OF CARRYING OUT THE INVENTION

The formulation of the compositions T1, A and B is described in Table I, that of the compositions T2 and C in Table II.

The compositions A to C are in accordance with the invention in that the elastomer matrix comprises a polybutadiene or a butadiene copolymer and at most 50% by weight of a thermoplastic styrene elastomer according to the invention and that the reinforcing filler contains more than 50% by mass of a carbon black. A and B differ from each other in the nature of the thermoplastic styrene elastomer. C differs from A and B in that it contains an SB instead of a BR as a first diene elastomer.

The composition Tl devoid of styrenic thermoplastic elastomer is the control composition of compositions A and B; the composition T2 devoid of styrenic thermoplastic elastomer is the control composition of C.

The compositions T1, T2, A, B and C are prepared according to the method described above. The compositions thus obtained are then calendered, either in the form of plates (with a thickness ranging from 2 to 3 mm) or thin rubber sheets, for the measurement of their physical or mechanical properties, or in the form of directly usable profiles, after cutting and / or assembly to the desired dimensions, as a tire tread.

The results are shown in Table III for A and B and the control Tl in Table IV for C and the control T2.

The results shown in Table III and IV show a very strong improvement in crack propagation resistance for A, B and C compared to their respective control. The improvement is remarkable when the first diene elastomer is a polybutadiene, and all the more remarkable that the styrenic thermoplastic elastomer is an SBIS.

The invention makes it possible to significantly improve the service life of tires carrying heavy loads, especially those traveling off the road, such as tires fitted to heavy goods vehicles, in particular agricultural vehicles, civil engineering vehicles and vehicles. construction heavy trucks, since these tires become much less sensitive to crack propagation at their tread.

Table I

(1) BR having 4.3% of 1-2; 2.7% trans 1,4; 93% cis 1.4 (Tg -106 ° C)

(2) SBS "D1101" marketed by Kraton

(3) SBIS "D1170" marketed by Kraton

(4) "Ultrasil VN3" marketed by Evonik

(5) N115

(6) Mn 6000-20000 g / mol polyethylene glycol from Sasol Mari

(7) N-cyclohexyl-2-benzothiazol sulfenamide, "Santocure CBS", marketed by Flexsys

Table II

(1) SBR with 25% styrene (% by mass relative to the mass of SBR) and 40% 1,2-butadiene units (% by weight of the butadiene part)

(2) SBS "D1101" marketed by Kraton

(3) "Ultrasil VN3" marketed by Evonik

(4) Rhodia "Zeosil 1165 MP" (HDS type)

(5) TESPT "Si69" of Evonik's company

(6) diphenylguanidine "Perkacit" DPG from Flexsys

(7) N115

(8) Mn 6000-20000 g / mol polyethylene glycol from Sasol Mari

(9) N-cyclohexyl-2-benzothiazol sulfenamide, "Santocure CBS", marketed by Flexsys

Table III

Table IV

Claims

claims

Pneumatic tire for vehicles intended to carry heavy loads whose tread comprises a composition based on at least:

an elastomeric matrix comprising a first diene elastomer and a styrenic thermoplastic elastomer,

a crosslinking system.

A tire according to claim 1 wherein the elastomeric matrix consists of a blend of the first diene elastomer and the styrenic thermoplastic elastomer.

Tire according to any one of Claims 1 to 2, in which the content of thermoplastic styrene elastomer represents from 5 to 50% by weight, preferably from 10 to 45% by weight, more preferably from 20 to 45% by mass, and even more preferably 25 to 45% by weight of the elastomer matrix.

A tire according to any of claims 1 to 3 wherein the at least one rigid styrenic segment has a glass transition temperature of greater than 80 ° C.

A tire according to any one of claims 1 to 4 wherein the at least one styrenic rigid segment is polystyrene.

A tire according to any one of claims 1 to 5 wherein the conjugated diene units of the at least one diene flexible segment are 1,3-butadiene units or isoprene units.

Tire according to any one of claims 1 to 6 wherein the styrenic thermoplastic elastomer is a diblock comprising a single styrenic rigid segment connected to a single diene flexible segment.

8. A tire according to claim 7 wherein the styrenic thermoplastic elastomer is a block copolymer styrene / butadiene (SB), styrene / isoprene (SI), styrene / butadiene / isoprene (SBI) or the mixture of these copolymers. 9. A tire according to any one of claims 1 to 6 wherein the styrenic thermoplastic elastomer comprises at least two rigid styrenic segments.

10. A tire according to claim 9 wherein the styrenic thermoplastic elastomer is a triblock consisting of two rigid styrenic segments and a diene flexible segment.

11. A tire according to claim 10 wherein the styrenic thermoplastic elastomer is a styrene / butadiene / styrene block copolymer (SBS), styrene / isoprene / styrene (SIS), styrene / butadiene / isoprene / styrene (SBIS) or the mixture of these copolymers. The tire of claim 11 wherein the styrenic thermoplastic elastomer is a styrene / butadiene / isoprene / styrene block copolymer (SBIS).

13. A tire according to any one of claims 1 to 12 wherein a fraction of the conjugated diene units of the at least one diene flexible segment is hydrogenated.

14. A tire according to any one of claims 1 to 12 wherein all of the conjugated diene units of the at least one diene flexible segment is hydrogenated.

15. A tire according to claims 7 and 14 wherein the styrenic thermoplastic elastomer is a styrene / ethylene / butylene (SEB), styrene / ethylene / propylene (SEP), styrene / ethylene / ethylene / propylene (SEEP) block copolymer or mixed.

16. A tire according to claims 10 and 14 wherein the styrenic thermoplastic elastomer is a styrene / ethylene / butylene / styrene block copolymer (SEBS), styrene / ethylene / propylene / styrene (SEPS), styrene / ethylene / ethylene / propylene / styrene (SEEPS) or their mixture.

17. A tire according to any one of claims 1 to 16 wherein the styrenic thermoplastic elastomer has a glass transition temperature of less than -20 ° C, preferably less than -30 ° C.

18. A tire according to claim 17 wherein the styrenic thermoplastic elastomer has a glass transition temperature of less than -40 ° C, preferably less than -50 ° C.

19. A tire according to any one of claims 1 to 18 wherein the carbon black has a BET specific surface area of at least 90 m ² / g, preferably at least 100 m ² / g. 20. A tire according to any one of claims 1 to 19 wherein the composition comprises between 2 and 35 phr of a silica.

21. A tire according to claim 20 wherein the rubber composition contains from 0 to less than 2 phr of a coupling agent, preferably from 0 to less than 1 phr of a coupling agent.

22. The tire of claim 21 wherein the level of coupling agent is equal to 0 phr. The tire of claim 22 wherein the rubber composition contains a coating agent, preferably a polyethylene glycol.

24. A tire according to any one of claims 1 to 23 wherein the tire is a tire off the road.

25. The tire of claim 24 wherein the tire is a tire for a civil engineering vehicle.

The method for manufacturing the tire according to any one of claims 1 to 25 which comprises the following steps:

- cool the assembly to a temperature below 70 ° C,

- then incorporate the crosslinking system,

- mix everything up to a maximum temperature below 90 ° C to obtain a mixture,

- Then calender or extrude the resulting mixture to form a tread.