TIRES FOR AGRICULTURAL VEHICLES

Description

[0001] The present invention relates to a tire for a vehicle for agricultural use, such as an agricultural tractor or an agro-industrial vehicle.

[0002] The present invention relates more particularly to the tread of such a tire, intended to come into contact with a ground via a rolling surface.

[0003] In the following, the circumferential, axial and radial directions respectively denote a direction tangent to the tread surface of the tyre and oriented in the direction of rotation of the tyre, a direction parallel to the axis of rotation of the tyre and a direction perpendicular to the axis of rotation of the tyre. By "radially inward, respectively radially outward" is meant "closer to, respectively further from the axis of rotation of the tyre". By "axially inward, respectively axially outward" is meant "closer to, respectively further from the equatorial plane of the tyre", the equatorial plane of the tyre being the plane passing through the middle of the tread surface of the tyre and perpendicular to the axis of rotation of the tyre.

[0004] The tread of an agricultural tractor tire generally comprises a plurality of lugs. The lugs are elements raised relative to a base surface which is a surface of revolution around the tire's axis of rotation.

[0005] A bar generally has a generally elongated parallelepiped shape, consisting of at least one rectilinear or curvilinear portion, and is separated from adjacent bars by grooves. A bar may consist of a succession of rectilinear portions, as described in documents US3603370, US4383567, EP795427 or have a curvilinear shape, as presented in documents US4446902, EP903249, EP1831034.

[0006] In the radial direction, a lug extends from the bottom surface to the tread surface, the radial distance between the bottom surface and the tread surface defining the lug height. The radially outer face of the lug, belonging to the tread surface, which comes into contact with the ground, when the lug passes through the contact patch of the tire, is called the lug contact face.

[0007] In the axial direction, a bar extends inwardly, toward the equatorial plane of the tire, from an axially outer end face to an axially inner end face.

[0008] In the circumferential direction, a lug extends, in a preferred direction of rotation of the tire, from a leading face to a trailing face. By preferred direction of rotation, we mean the direction of rotation recommended by the tire manufacturer for optimal use of the tire. For example, in the case of a tread comprising two rows of V-shaped or chevron-shaped lugs, the tire has a preferred direction of rotation according to the tip of the chevrons. The leading face is, by definition, the face whose radially outer edge or leading edge first comes into contact with the ground, when the lug passes through the contact surface of the tire with the ground, during the rotation of the tire.The trailing face is, by definition, the face whose radially outer edge or trailing edge last comes into contact with the ground, when the lug passes through the contact surface of the tire with the ground, during the rotation of the tire. Depending on the direction of rotation, the leading face is said to be in front of the trailing face.

[0009] A bar usually, but not necessarily, has a mean inclination angle, relative to the circumferential direction, close to 45°. Indeed, this mean inclination angle allows in particular a good compromise between field traction and vibration comfort. Field traction is all the better when the bar is axial, that is to say that its mean inclination angle, relative to the circumferential direction, is close to 90°, while vibration comfort is all the better when the bar is circumferential, that is to say that its mean inclination angle, relative to the circumferential direction, is close to 0°.It is well known that field traction is more strongly determined by the lug angle at the shoulder, which has led some tire designers to propose a very curved lug shape, leading to a lug that is substantially axial at the shoulder and substantially circumferential in the middle of the tread.

[0010] The tread of an agricultural tractor tire generally comprises two rows of lugs as previously described. This distribution of lugs inclined with respect to the circumferential direction gives the tread a V-shape commonly referred to as a herringbone pattern. The two rows of lugs have symmetry with respect to the equatorial plane of the tire, most often with a circumferential offset between the two rows of lugs, resulting from a rotation around the tire axis of one half of the tread relative to the other half of the tread. In addition, the lugs may be continuous or discontinuous, and distributed circumferentially with a constant or variable pitch.

[0011] The tread of an agricultural tractor tire thus comprises two types of elements: the lugs, which are the raised elements, and the grooves, which are the portions of the bottom surface separating the lugs. These two types of elements are stressed in very different ways. The lugs are particularly sensitive to wear in road use and to attack by stones in off-road or field use. The furrows, between the lugs, are mainly attacked by residual stubble after harvest, in field use, and are also sensitive to chemical attack by ozone to the extent that these furrows are not subject to wear.

[0012] Although not limited to this application, the invention will be more particularly described with reference to a multi-purpose agricultural machine, capable of driving both in fields and on roads, such as an agricultural tractor.

[0013] A tire for an agricultural tractor is intended to run on various types of soil such as more or less compacted earth in fields, unpaved access roads to fields and tarred road surfaces. Given the diversity of use, in fields and on roads, a tire for an agricultural tractor, and in particular its tread, must offer a compromise of performance, in particular between traction in the field, resistance to tearing, resistance to wear on the road, rolling resistance, vibration comfort on the road.

[0014] Furthermore, the demand for tires for agricultural vehicles that can travel at high speeds (up to around 65 km / h) on paved roads is constantly increasing, in order to reduce travel times between work areas (fields, forest areas, etc.) and storage locations (for the vehicle, agricultural equipment, harvested produce, etc.).

[0015] However, increasing speed is accompanied by an increase in fuel consumption. Thus, the impact of rolling resistance is becoming an increasingly considered component for agricultural vehicle tires. Solutions have been proposed to reduce rolling resistance when driving on asphalt without impacting the other properties of agricultural vehicle tires, by adjusting the inflation pressure directly depending on the nature of the soil on which the agricultural vehicle is traveling (WO2016 / 071158).

[0016] The increase in driving speed on asphalt also creates the need to take into consideration new performance characteristics that must be met by tires for agricultural vehicles intended to travel at high speeds on asphalt, in particular wet grip.

[0017] There is therefore a real need for agricultural vehicle tires that allow the vehicle to travel at high speeds on asphalt surfaces, which have both low rolling resistance, in order to reduce fuel consumption, and good grip, particularly on wet surfaces, without compromising the properties specifically linked to agricultural vehicle tires.

[0018] Continuing its research, the Applicant discovered that the use of a specific elastomeric matrix comprising a particular reinforcing filler mixture makes it possible to solve the aforementioned technical problem.

[0019] Thus, the subject of the present invention is a tire for an agricultural vehicle comprising a tread intended to come into contact with a ground which comprises a plurality of bars separated from each other by grooves, each bar extending radially outwards, over a radial height H, from a bottom surface to a contact face, the grooves being constituted by the portions of the bottom surface separating the bars, in which the tread comprises a rubber composition based on: of an elastomeric matrix comprising at least one butadiene-styrene copolymer having a glass transition temperature in a range from -60°C to -10°C, from 30 to less than 120 parts by weight per hundred parts by weight of elastomer, pce, of reinforcing filler, the reinforcing filler comprising a mixture of silica and carbon black, the reinforcing filler comprises between 33% and 100% by weight of silica relative to the total weight of the reinforcing filler, the silica content in the composition being between 10 and 80 pce, and a crosslinking system, the composition comprising from 35 to 65 pce of butadiene-styrene copolymer having a glass transition temperature in a range from -60°C to -10°C, from 20 to 40 pce of polybutadiene and from 20 to 40 pce of polyisoprene, the total content of butadiene-styrene copolymer having a glass transition temperature in a range from -60°C to -10°C, polybutadiene and polyisoprene, in the composition, being in a range from 80 to 100 pce. I- DEFINITIONS

[0020] The expression "composition based on" means a composition comprising the mixture and / or the reaction product in situof the different constituents used, some of these constituents being able to react and / or being intended to react with each other, at least partially, during the different phases of manufacturing the composition; the composition thus being able to be in a totally or partially crosslinked state or in a non-crosslinked state.

[0021] By the expression "part by weight per hundred parts by weight of elastomer" (or pce), it is meant, within the meaning of the present invention, the part, by mass per hundred parts by mass of elastomer.

[0022] In this document, unless expressly stated otherwise, all percentages (%) indicated are percentages (%) by mass.

[0023] On the other hand, any interval of values designated by the expression "between a and b" represents the range of values from more than a to 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 range of values from a to b (i.e., including the strict limits a and b). In this document, when describing an interval of values by the expression "from a to b", the interval represented by the expression "between a and b" is also and preferably described.

[0024] When a "majority" compound is referred to, within the meaning of the present invention, this compound is the majority among the compounds of the same type in the composition, that is to say that it is the one which represents the greatest quantity by mass among the compounds of the same type. Thus, for example, a majority elastomer is the elastomer representing the greatest mass relative to the total mass of the elastomers in the composition. In the same way, a so-called majority filler is the one representing the greatest mass among the fillers in the composition. For example, in a system comprising a single elastomer, this is the majority within the meaning of the present invention; and in a system comprising two elastomers, the majority elastomer represents more than half of the mass of the elastomers.Preferably by majority, we mean present at more than 50%, preferably more than 60%, 70%, 80%, 90%, and more preferably the “majority” compound represents 100%.

[0025] The carbon-containing 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. This includes, in particular, polymers, plasticizers, fillers, etc.

[0026] All glass transition temperature “Tg” values described herein are measured in a known manner by DSC (Differential Scanning Calorimetry) according to ASTM D3418 (1999). II- DESCRIPTION OF THE INVENTION II-1 Elastomeric matrix

[0027] According to the invention, the elastomeric matrix of the composition of the tire tread comprises at least one, i.e. one or more, butadiene-styrene copolymers (SBR) having a glass transition temperature (Tg) in a range from -60°C to -10°C.

[0028] Furthermore, the composition of the tread of the tire according to the invention comprises : from 35 to 65 pce of butadiene-styrene copolymer having a glass transition temperature in a range from -60°C to -10°C, from 20 to 40 pce of polybutadiene, and 20 to 40 pce of polyisoprene (preferably natural rubber), the total content of butadiene-styrene copolymer having a glass transition temperature in a range from -60°C to -10°C, polybutadiene and polyisoprene, in the composition, being in a range from 80 to 100 pce, preferably from 90 to 100 pce, more preferably being 100 pce.

[0029] Advantageously, the glass transition temperature of the butadiene-styrene copolymer having a glass transition temperature in a range from -60°C to -10°C is between -50°C and -15°C.

[0030] The butadiene-styrene copolymer (SBR) having a glass transition temperature (Tg) in a range from -60°C to -10°C may consist exclusively of styrene monomers and butadiene monomers, i.e. the sum of the molar percentages of styrene monomers and butadiene monomers is equal to 100%. It may also comprise, in a minor amount, monomers other than styrene and butadiene, preferably less than 20%, preferably less than 10%, more preferably less than 5%, by weight relative to the total weight of the copolymer.

[0031] It will be noted that the SBR can be prepared in emulsion (ESBR) or in solution (SSBR). Whether it is ESBR or SSBR, the SBR can be of any microstructure provided that it generates a Tg in a range from -60°C to -10°C, preferably from -50°C to -15°C. It is possible in particular to use an SBR having a low styrene content, for example from 35 to 55% by weight of the copolymer, and a vinyl bond content of the butadiene part of between 4% and 70%, preferably between 10 and 40% by weight of the butadiene part of the copolymer. Advantageously, the butadiene-styrene copolymer having a glass transition temperature in a range from -60°C to -10°C is a butadiene-styrene copolymer prepared in solution.

[0032] The butadiene-styrene copolymer (SBR) having a glass transition temperature (Tg) in a range from -60°C to -10°C, usable according to the invention, can for example be coupled and / or star-shaped or even functionalized with a coupling and / or star-shaped or functionalizing agent.For coupling with carbon black, examples that may be mentioned are functional groups comprising a C-Sn bond or amine functional groups such as benzophenone for example; for coupling to a reinforcing inorganic filler such as silica, examples that may be mentioned are silanol or polysiloxane functional groups having a silanol end (as described for example in FR 2 740 778 or US 6,013,718), alkoxysilane groups (as described for example in FR 2 765 882 or US 5,977,238), carboxylic groups (as described for example in WO 01 / 92402 or US 6,815,473, WO 2004 / 096865 or US 2006 / 0089445) or polyether groups (as described for example in EP 1 127 909 or US 6,503,973). Advantageously, the butadiene-styrene copolymer having a glass transition temperature in a range from -60°C to -10°C is functionalized with a coupling agent.Preferably, the coupling agent is 3-Mercaptopropyltriethoxysilane.

[0033] THE butadiene-styrene copolymer rate having a glass transition temperature in a range from -60°C to -10°C can also be in a range from 40 to 60 pce. II-2 Reinforcing charge

[0034] According to the invention, the composition of the tread of the tire comprises from 30 to less than 120 pce of reinforcing filler, the reinforcing filler comprising a mixture of silica and carbon black, the reinforcing filler comprises between 33% and 100% by weight of silica relative to the total weight of the reinforcing filler, the level of silica in the composition being between 10 and 80 pce.

[0035] Suitable carbon blacks are all carbon blacks, including those conventionally used in tires or their treads. Among the latter, we will particularly mention 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 the isolated state, as commercially available, or in any other form, for example as a carrier for some of the rubber additives used. Carbon blacks could, for example, already be incorporated into the diene elastomer, in particular isoprene, in the form of a masterbatch (see, for example, applications WO97 / 36724-A2 or WO99 / 16600-A1).The 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 / P0: 0.1 to 0.3]. Advantageously, the carbon black has a BET specific surface area of between 70 and 150 m 2 < / g, preferably between 100 and 120 m 2 < / g.

[0036] 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 from 30 to 400 m 2 < / g, in particular from 60 to 300 m 2 < / g.

[0037] In this presentation, 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 NF ISO 5794-1 standard, annex E of June 2010 [multipoint volumetric method (5 points) - gas: nitrogen - vacuum degassing: one hour at 160°C - relative pressure range p / po: 0.05 to 0.17].

[0038] For inorganic fillers such as silica, for example, the CTAB specific surface area values were determined according to standard NF ISO 5794-1, annex G of June 2010. The process is based on the adsorption of CTAB (N-hexadecyl-N,N,N-trimethylammonium bromide) on the "external" surface of the reinforcing filler.

[0039] Any type of precipitated silica may be used, in particular highly dispersible precipitated silicas (known as "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, we can notably use 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.

[0040] Those skilled in the art will understand that, as a replacement for the reinforcing inorganic filler described above, a reinforcing filler of another nature could be used, provided that this reinforcing filler of another nature is covered with an inorganic layer such as silica, or else has functional sites on its surface, in particular hydroxyl sites, requiring the use of a coupling agent to establish the bond between this reinforcing filler and the diene elastomer. By way of example, mention may be made of carbon blacks partially or completely covered with silica, or carbon blacks modified with silica, such as, without limitation, the “Ecoblack®” type fillers of the CRX2000 series or the “CRX4000” series from Cabot Corporation.

[0041] To couple the reinforcing inorganic filler to the diene elastomer, it is possible to use, in a well-known manner, an at least bifunctional coupling agent (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, 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.

[0042] 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”. More preferably, the organosilane is a polysulfurized organosilane.

[0043] Of course, mixtures of the coupling agents described above could also be used.

[0044] Typically the coupling agent content represents from 0.5% to 15% by weight relative to the amount of reinforcing inorganic filler. Its content is preferably within a range of 0.5 to 20 phr, more preferably within a range of 3 to 3 phr. This content is easily adjusted by a person skilled in the art according to the content of reinforcing inorganic filler used in the composition of the invention.

[0045] As indicated above, the composition of the tread of the tire according to the invention comprises from 30 to less than 120 phr of reinforcing filler, the reinforcing filler comprising a mixture of silica and carbon black, the reinforcing filler comprises between 33% and 100% by weight of silica relative to the total weight of the reinforcing filler, the silica content in the composition being between 10 and 80 phr. Advantageously, the reinforcing filler comprises between 33% and 70% by weight, preferably between 35% and 67% by weight, of silica relative to the total weight of the reinforcing filler. More particularly, the reinforcing filler preferably comprises more than 50% by weight, preferably between 50% and 67% by weight, of silica relative to the total weight of the reinforcing filler.

[0046] Advantageously, the silica content in the composition is between 15 and 60 phr, and / or the carbon black content in the composition is between 5 and 20 phr. Alternatively, advantageously, the silica content in the composition is between 15 and 60 phr, and / or the carbon black content in the composition is between 5 and 35 phr, preferably between 10 and 35 phr, more preferably between 11 and 20 phr.

[0047] Whatever the respective levels of carbon black and silica, the total level of reinforcing filler in the composition is advantageously between 35 and 90 pce.

[0048] Furthermore, the composition of the tread of the tire according to the invention may comprise a non-reinforcing filler but this is not preferable.

[0049] Thus, the composition of the tread of the tire according to the invention advantageously does not comprise any non-reinforcing filler or comprises less than 25 phr, preferably less than 20 phr, more preferably less than 10 phr, and more preferably less than 5 phr. In a particularly preferred manner, the composition of the tread of the tire according to the invention does not comprise any non-reinforcing filler.

[0050] Examples of non-reinforcing fillers (or inert fillers) known to those skilled in the art include those chosen from the group consisting of ash (i.e., combustion residues), microparticles of natural (chalk) or synthetic calcium carbonates, synthetic or natural silicates (such as kaolin, talc, mica, cloisite), titanium oxides, aluminas, aluminosilicates (clay, bentonite), and mixtures thereof. II-3 Crosslinking system

[0051] The crosslinking system may be any type of system known to those skilled in the art in the field of tire rubber compositions. It may in particular be based on sulfur, and / or peroxide and / or bismaleimides.

[0052] Preferably, the crosslinking system is sulfur-based, in which case it is referred to as a vulcanization system. The crosslinking system thus preferably comprises molecular sulfur and / or at least one sulfur-donating agent. At least one vulcanization accelerator is also preferably present, and, optionally, also preferably, various known vulcanization activators may be used, such as zinc oxide, stearic acid or equivalent compounds such as stearic acid salts and transition metal salts, guanidine derivatives (in particular diphenylguanidine), or even known vulcanization retarders.

[0053] Sulphur is used at a preferential rate of between 0.5 and 12 pce, in particular between 1 and 10 pce. The vulcanisation accelerator is used at a preferential rate of between 0.5 and 10 pce, more preferably between 0.5 and 5.0 pce.

[0054] Any compound capable of acting as an accelerator for the vulcanization of diene elastomers in the presence of sulfur may be used as an accelerator, in particular accelerators of the thiazole type and their derivatives, accelerators of the sulfenamide, thiuram, dithiocarbamate, dithiophosphate, thiourea and xanthate types. Examples of such accelerators include, but are not limited to, the following compounds: 2-mercaptobenzothiazyl disulfide (abbreviated as "MBTS"), N-cyclohexyl-2-benzothiazyl sulfenamide ("CBS"), N,N-dicyclohexyl-2-benzothiazyl sulfenamide ("DCBS"), N-tert-butyl-2-benzothiazyl sulfenamide ("TBBS"), N-tert-butyl-2-benzothiazyl sulfenimide ("TBSI"), tetrabenzylthiuram disulfide ("TBZTD"), zinc dibenzyldithiocarbamate ("ZBEC") and mixtures of these compounds. II-4 Other possible additives

[0055] The rubber compositions of the tire tread according to the invention may optionally also comprise all or part of the usual additives usually used in elastomer compositions for tires, such as for example plasticizers (such as plasticizing oils and / or plasticizing resins), pigments, protective agents such as anti-ozone waxes, chemical anti-ozonants, antioxidants, anti-fatigue agents, reinforcing resins (as described for example in application WO 02 / 10269).

[0056] The composition may in particular comprise a hydrocarbon plasticizing resin. The hydrocarbon plasticizing resin is chosen from the group consisting of cyclopentadiene or dicyclopentadiene homopolymer or copolymer resins, terpene homopolymer or copolymer resins, terpene phenol homopolymer or copolymer resins, C5 cut homopolymer or copolymer resins, C9 cut homopolymer or copolymer resins, alpha-methyl-styrene homopolymer and copolymer resins and mixtures thereof. Advantageously, the content of hydrocarbon plasticizing resin in the composition of the tread of the tire according to the invention is within a range from 2 to 20 phr, preferably from 2 to 10 phr. Such resins are described for example in paragraph I-4-1 of application WO 2016 / 202968.

[0057] Furthermore, the composition of the tread of the tire according to the invention advantageously does not comprise plasticizing oil or comprises less than 5 pce. As examples of plasticizing oils, mention may be made of the liquid plasticizers mentioned in paragraph I-4-2 of application WO 2016 / 202968. II-5 Preparation of rubber compositions

[0058] The rubber composition in accordance with the invention can be manufactured in suitable mixers, using two successive preparation phases well known to those skilled in the art: a first working phase or thermomechanical mixing (so-called "non-productive" phase), which can be carried out in a single thermomechanical step during which all the necessary constituents, in particular the elastomeric matrix, any fillers, any other various additives, with the exception of the crosslinking system, are introduced into a suitable mixer such as a conventional internal mixer (for example of the "Banbury" type). The incorporation of the possible filler into the elastomer can be carried out in one or more stages by thermomechanical mixing. The non-productive phase can be carried out at high temperature, up to a maximum temperature of between 110°C and 200°C, preferably between 130°C and 185°C, for a duration generally of between 2 and 10 minutes.a second phase of mechanical work (so-called "productive" phase), which is carried out in an external mixer such as a roller mixer, after cooling the mixture obtained during the first non-productive phase to a lower temperature, typically below 120°C, for example between 40°C and 100°C. The crosslinking system is then incorporated, and everything is then mixed for a few minutes, for example between 5 and 15 min.

[0059] Such phases have been described for example in applications EP-A-0501227, EP-A-0735088, EP-A-0810258, WO00 / 05300 or WO00 / 05301.

[0060] The final composition thus obtained is then calendered, for example, in the form of a sheet or plate, in particular for characterization in the laboratory, or extruded in the form of a semi-finished (or profiled) rubber usable, for example, as a tread for an agricultural vehicle tire. These products can then be used for the manufacture of tires, according to techniques known to those skilled in the art.

[0061] The composition can be either in the raw state (before crosslinking or vulcanization), or in the cooked state (after crosslinking or vulcanization), can be a semi-finished product which can be used in a tire.

[0062] The crosslinking of the composition can be carried out in a manner known to those skilled in the art, for example at a temperature between 130°C and 200°C, under pressure. II-6 Tires

[0063] The present invention relates to a tire for agricultural vehicles.

[0064] Agricultural vehicle tires are characterized in particular by their large diameter, generally ranging from 20 to 63 inches, preferably from 28 to 54 inches, and even more preferably from 36 to 42 inches.

[0065] Furthermore, their tread may have one or more grooves with an average depth ranging from 15 to 120 mm, preferably 65 to 120 mm. Furthermore, the average volumetric hollow rate over the entire tread may be in a range ranging from 5 to 40%, preferably from 5 to 25%. III- BRIEF DESCRIPTIONS OF THE FIGURES

[0066] The present invention will be better understood with the aid of the figure 1 , schematic and not shown to scale, attached in the appendix, representing a perspective view of a tire 1 for a vehicle for agricultural use, such as a tractor.

[0067] In this figure, the tire 1 has a tread 2, intended to come into contact with a ground via a rolling surface, and which comprises bars 3 separated from each other by grooves 4. Each bar 3 extends radially outwards, from a bottom surface 5 to a contact face 6, positioned in the rolling surface. The grooves 4 are constituted by the portions of the bottom surface 5 separating the bars 3.

[0068] In particular, in the case of a tire for an agricultural tractor as shown in the figure, the plurality of bars 3 of the tread is distributed in a first row and a second row of bars generally symmetrical with respect to the equatorial plane of the tire, passing through the middle of the tread 2 and perpendicular to the axis of rotation of the tire. IV- EXAMPLES IV-1 Measures and tests used Determination of the glass transition temperature

[0069] The glass transition temperature, Tg, is measured in the present application by the DSC (Differential Scanning Calorimetry) technique on a device called “Setaram DSC 131”. The temperature program used corresponds to a temperature rise from -120°C to 150°C at a speed of 10°C / min. Reference may be made to the method described in application WO 2007 / 054224 (page 11). Dynamic properties

[0070] The dynamic properties tan(δ)max at -20°C, 0°C and 60°C are measured on a viscoanalyzer (Metravib VA4000), according to the ASTM D 5992-96 standard. The response of a sample of crosslinked composition (cylindrical specimen 4 mm thick and 400 mm 2< in cross-section) is recorded, subjected to a sinusoidal stress in alternating simple shear, at a frequency of 10 Hz, under the defined temperature conditions, for example at -20°C, 0°C or 60°C according to the ASTM D 1349-99 standard, or depending on the case at a different temperature. A strain amplitude sweep is carried out from 0.1 to 50% (forward cycle), then from 50% to 0.1% (return cycle). The results used are the loss factor tan(δ). For the return cycle, we indicate the maximum value of tan(δ) observed, noted tan(δ)max. IV-3 Preparation of the compositions

[0071] The following tests are carried out as follows: the elastomer, the reinforcing fillers, and the various other ingredients, with the exception of the crosslinking system, are successively introduced into a paddle mixer (final filling rate: approximately 70% by volume), whose initial tank temperature is approximately 50°C. Thermomechanical work (non-productive phase) is then carried out in one step, which lasts a total of approximately 4 to 5 minutes, until a maximum "drop" temperature of 165°C is reached.

[0072] The mixture thus obtained is recovered, cooled and then the crosslinking system (peroxide or sulfur depending on the case) is incorporated, on a mixer (homo-finisher) between 23°C and 50°C respectively, mixing everything (productive phase) in a cylinder tool for an appropriate time (for example between 5 and 12 min).

[0073] The compositions thus obtained are then calendered either in the form of plates (thickness of 2 to 3 mm) or thin sheets of rubber for the measurement of their physical or mechanical properties. IV-3 Rubber composition testing

[0074] Six rubber compositions were prepared as indicated in point IV-2 above, three in accordance with the invention (hereinafter noted C1 to C3) and two not in accordance (control composition hereinafter noted T1 and T2).These six formulations all contain, in addition to the elastomeric matrix and the reinforcing fillers presented in Table 1 below: 4 pce of plasticizing resin (C5 / C9 resin, “ECR-373 Resin” from ExxonMobil), 1.5 pce of tackifying resin (“Escorez 1102” from EXXON (Mn 1370 g / mol; Ip= 2.3)), 1.6 pce of sulfur, 1.6 pce of vulcanization accelerator (N-ter-butyl-2-benzothiazyl sulfenamide, “Santocure TBBS” from Flexsys), 1.5 pce of industrial grade zinc oxide (Umicore), 1 pce of stearic acid (“Pristerene 4931” from Uniqema), 3 pce of antioxidant (N-1,3-dimethylbutyl-N-phenylparaphenylenediamine, “Santoflex 6-PPD” from Flexsys), 2 pce of anti-ozone wax (“VARAZON 4959” from Sasol Wax) and 0.2 pce of “CTP” (N-cyclohexylthiophthalimide marketed under the name, “Vulkalent G” by Lanxess).The nature and content (in pce) of the elastomeric matrix and the reinforcing filler system, as well as the properties of these formulations are presented in Table 1 below.

[0075] Control composition T1 differs from the compositions in accordance with the invention in that the reinforcing filler system is not in accordance with the invention. Control composition T2 differs from the compositions in accordance with the invention, furthermore, in that the glass transition temperature of the butadiene-styrene copolymer used is not within a range from -60°C to -10°C.

[0076] The tan(δ)max results are presented on a “base 100” basis relative to the control compound T1. For tan(δ)max values at -20°C and 0°C, the higher the value, the more the compound will have improved wet grip. Furthermore, the lower the tan(δ)max value at 60°C, the lower the hysteresis and therefore the improved rolling resistance of the compound. Table 1 T1 T2 C1 C2 C3 SBR1 (1) 50 - 50 50 50 SBR2 (2) - 50 - - - BR (3) 30 30 30 30 30 NR (4) 20 20 20 20 20 N234 (5) 47 47 30 24 17 Silica (6) - - 17 23 30 Coupling agent (7) - - 1.7 2.3 3.0 Tan(δ) max (60°C) 100 71 86 81 76 Tan(δ) max (0°C) 100 54 103 106 106 Tan(δ) max (-20°C) 100 50 115 119 110 (1) SBR (star-coated 3-Mercaptopropyltriethoxysilane) with 41% styrene unit and 24% 1,2-butadiene unit (Tg -25°C) (2) SBR (star-coated Sn) with 15.5% styrene unit and 35% 1,2-butadiene unit (Tg -65°C) (3) Polybutadiene with 0.5% 1,2-unit and 97% 1,4-cis (Tg = -108°C) (4) Natural rubber (5) Carbon black N234 (denomination according to ASTM D-1765 standard) (6) Silica “Zeosil 1165 MP” from Rhodia type “HDS” (7) TESPT (“Si69” from Degussa)

[0077] These results show that the substitution of a butadiene-styrene copolymer having a glass transition temperature of -25°C by a butadiene-styrene copolymer having a glass transition temperature of -65°C makes it possible to improve the hysteresis, but greatly penalizes the grip on wet ground. Thus, only the compositions in accordance with the invention, having at least one butadiene-styrene copolymer having a glass transition temperature in a range from -60°C to -10°C and the specific reinforcing filler system in accordance with the invention, make it possible to improve both the hysteresis and the grip on wet ground, without penalizing the expected properties of a tire for agricultural vehicles.

Claims

1. Tire (1) for agricultural vehicle comprising a tread (2) intended to come into contact with the ground, which comprises a plurality of bars (3) separated from each other by grooves (4), each bar (3) extending radially outward, over a radial height H, from a base surface (5) to a contact face (6), the grooves (4) being formed by the portions of the base surface (5) separating the bars (3), characterized in that the tread comprises a rubber composition based on: - an elastomeric matrix comprising at least one styrene-butadiene copolymer having a glass transition temperature in the range of -60°C to -10°C, the glass transition temperature being measured by DSC (Differential Scanning Calorimetry) according to ASTM D3418 (1999), - from 30 to less than 120 parts by weight per hundred parts by weight of elastomer, phr, of reinforcing filler, the reinforcing filler comprising a mixture of silica and carbon black, the reinforcing filler comprising between 33% and 100% by weight of silica relative to the total weight of the reinforcing filler, the silica content in the composition being between 10 and 80 phr, and - a crosslinking system, the composition comprising from 35 to 65 phr of styrene-butadiene copolymer having a glass transition temperature in the range of -60°C to -10°C, from 20 to 40 phr of polybutadiene, and from 20 to 40 phr of polyisoprene, the total content of styrene-butadiene copolymer having a glass transition temperature in the range of -60°C to -10°C, polybutadiene, and polyisoprene in the composition being in the range of 80 to 100 phr.

2. Tire according to any one of the preceding claims, wherein the styrene-butadiene copolymer having a glass transition temperature in the range of -60°C to -10°C is a solution-prepared styrene-butadiene copolymer.

3. Tire according to any one of the preceding claims, wherein the glass transition temperature of the styrene-butadiene copolymer having a glass transition temperature in the range of -60°C to -10°C is between -50°C and -15°C.

4. Tire according to any one of the preceding claims, wherein the total content of styrene-butadiene copolymer having a glass transition temperature in the range of -60°C to -10°C, polybutadiene, and polyisoprene in the composition is in the range of 90 to 100 phr, preferably 100 phr.

5. Tire according to any one of the preceding claims, wherein the reinforcing filler comprises between 33% and 70% by weight, preferably between 35% and 67% by weight, of silica relative to the total weight of the reinforcing filler.

6. Tire according to any one of the preceding claims, wherein the silica content in the composition is between 15 and 60 phr, and the carbon black content in the composition is between 5 and 35 phr, preferably between 10 and 35 phr.

7. Tire according to any one of the preceding claims, wherein the carbon black has a specific BET surface area in the range of 70 to 150 m2 / g, preferably between 100 and 120 m2 / g.

8. Tire according to any one of the preceding claims, wherein the total content of reinforcing filler in the composition is in the range of 35 to 90 phr.

9. Tire according to any one of the preceding claims, wherein the composition does not contain plasticizing oil or contains less than 5 phr.