TIRES WITH IMPROVED ROLLING RESISTANCE

Description

[0001] The present invention relates to a tire, with a radial carcass reinforcement and more particularly a tire intended to equip vehicles carrying heavy loads and traveling at high speed, such as, for example, trucks, tractors, trailers or road buses.

[0002] Generally speaking, in heavy goods vehicle type tires, the carcass reinforcement is anchored on both sides in the bead area and is surmounted radially by a crown reinforcement consisting of at least two layers, superimposed and formed of parallel wires or cables in each layer and crossed from one layer to the next, making angles of between 10° and 45° with the circumferential direction. Said working layers, forming the working reinforcement, may also be covered with at least one so-called protective layer and formed of advantageously metallic and extensible reinforcement elements, called elastic.It may also comprise a layer of low-extensibility metal wires or cables forming an angle of between 45° and 90° with the circumferential direction, this ply, called the triangulation ply, being radially located between the carcass reinforcement and the first crown ply, called the working ply, formed of parallel wires or cables having angles at most equal to 45° in absolute value. The triangulation ply forms with at least said working ply a triangulated reinforcement, which, under the various stresses to which it is subjected, exhibits little deformation, the triangulation ply having the essential role of absorbing the transverse compression forces to which all the reinforcing elements are subjected in the area of ​​the crown of the tire.

[0003] Radially outside the crown reinforcement is the tread, usually made of polymeric materials intended to come into contact with the ground in the contact area between the ground and the tire.

[0004] Cables are said to be inextensible when, under a tensile force equal to 10% of the breaking force, the said cables exhibit a relative elongation of at most 0.2%.

[0005] Cables are said to be elastic when, under a tensile force equal to the breaking load, the said cables exhibit a relative elongation of at least 3% with a maximum tangent modulus of less than 150 GPa.

[0006] Circumferential reinforcing elements are reinforcing elements which make angles with the circumferential direction in the range + 2.5°, - 2.5° around 0°.

[0007] The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the rolling direction of the tire.

[0008] The transverse or axial direction of the tire is parallel to the tire's axis of rotation.

[0009] The radial direction is a direction intersecting the axis of rotation of the tire and perpendicular to it.

[0010] The tire's axis of rotation is the axis around which it rotates during normal use.

[0011] A radial or meridian plane is a plane that contains the tire's axis of rotation.

[0012] The circumferential median plane, or equatorial plane, is a plane perpendicular to the tire's axis of rotation and which divides the tire into two halves.

[0013] Some current tires, called "road tires", are designed to travel at high speeds and on increasingly long journeys, due to the improvement of the road network and the growth of the motorway network in the world. All the conditions under which such a tire is called upon to travel undoubtedly allow an increase in the number of kilometers traveled, the wear of the tire being less; on the other hand, the endurance of the latter and in particular of the crown reinforcement becomes penalizing.

[0014] There are indeed constraints at the level of the crown reinforcement and more particularly shear constraints between the crown layers, combined with a significant increase in the operating temperature at the ends of the axially shortest crown layer, which result in the appearance and propagation of cracks in the rubber at the said ends.

[0015] To limit excessive temperature increases at the top of the tire, the materials making up the tread are advantageously chosen with hysteretic losses adapted to the operating conditions of the tire.

[0016] Furthermore, in order to improve the endurance of the crown reinforcement of the type of tire studied, solutions relating to the structure and quality of the layers and / or profiles of rubber mixtures which are arranged between and / or around the ends of the plies and more particularly the ends of the axially shortest ply have already been provided.

[0017] Patent FR 1 389 428, to improve the resistance to degradation of rubber compounds located in the vicinity of the crown reinforcement edges, recommends the use, in combination with a low hysteresis tread, of a rubber profile covering at least the sides and marginal edges of the crown reinforcement and consisting of a low hysteresis rubber compound.

[0018] Patent FR 2 222 232, to avoid separations between crown reinforcement plies, teaches coating the ends of the reinforcement in a rubber mattress, the Shore A hardness of which is different from that of the tread surmounting said reinforcement, and greater than the Shore A hardness of the rubber mixture profile arranged between the edges of the crown reinforcement plies and the carcass reinforcement.

[0019] French application FR 2 728 510 proposes to have, on the one hand between the carcass reinforcement and the working crown reinforcement ply, radially closest to the axis of rotation, an axially continuous ply, formed of inextensible metal cables making an angle with the circumferential direction at least equal to 60°, and whose axial width is at least equal to the axial width of the shortest working crown ply, and on the other hand between the two working crown plies an additional ply formed of metal elements, oriented substantially parallel to the circumferential direction.

[0020] French application WO 99 / 24269 also proposes, on either side of the equatorial plane and in the immediate axial extension of the additional ply of reinforcing elements substantially parallel to the circumferential direction, to couple, over a certain axial distance, the two working crown plies formed of reinforcing elements crossed from one ply to the next and then to decouple them by rubber compound profiles at least over the remainder of the width common to said two working plies.

[0021] The layer of circumferential reinforcing elements is usually made up of at least one metal cable wound to form a set of turns whose laying angle relative to the circumferential direction is less than 2.5°.

[0022] In order to further increase the service life of tires, it is common to choose polymeric materials constituting the tread with improved wear resistance properties. Since such materials most often penalize hysteretic properties, it is still known to produce the tread of a tire by a radial superposition of two different materials to obtain a compromise of wear and hysteresis properties satisfactory for the intended applications.

[0023] Such tires are for example described in document US 6,247,512. This document describes the superposition of two layers of materials to form the tread, the external material coming into contact with the ground being in particular more efficient in terms of wear while the internal material has hysteretic properties making it possible to limit the temperature rises of the tire in the crown area.

[0024] Tires produced in this way are perfectly satisfactory for the above-mentioned road applications. However, such tire designs can be considered expensive.

[0025] Document US 4,398,582 A describes a tire comprising a tread allowing the compromise between rolling resistance, wet grip and wear to be modified.

[0026] Document FR 2 950 838 A1 describes a tire that improves the endurance performance of the carcass reinforcement with reduced production costs by reducing the thickness of the compounds forming the inner surface of the tire.

[0027] Document WO 2019 / 211565 A1 describes a tire comprising a rolling strip with low thermal conductivity making it possible to modify the rolling resistance / manufacturing cost compromise.

[0028] The inventors have thus set themselves the mission of being able to provide tires whose wear performance and endurance performance are maintained, or even improved for road use, and whose manufacturing cost is reduced while providing satisfactory rolling resistance properties.

[0029] This object is achieved according to the invention by a tire with a radial carcass reinforcement comprising a crown reinforcement, itself radially capped with a tread, said tread being joined to two beads by means of two sidewalls, the crown reinforcement comprising at least one layer of circumferential reinforcing elements, said tread comprising at least one layer of polymeric mixture forming a rolling surface intended to come into contact with a roadway, the thermal conductivity of said layer of polymeric mixture forming the rolling surface being strictly less than 0.270 W / (m.K) and said layer of polymeric mixture forming the rolling surface being formed from a rubber composition based on at least one elastomer matrix comprising predominantly a copolymer based on styrene and butadiene having a glass transition temperature Tg strictly greater than -65°C and less than or equal to -30°C and, in a radial plane, over at least 50% of the meridian profile of the tire, the thickness of rubber mixture between the inner surface of the cavity of the tire and the point of a metal reinforcing element of the carcass reinforcement closest to said inner surface of the cavity being less than 4.5 mm.

[0030] The thermal conductivity of polymer blends is measured in accordance with ISO 8302:1991, Thermal insulation - Determination of thermal resistance and related properties in steady state - Guarded hot plate method. The measurement is carried out at 25°C on parallelepiped samples of 50 to 100 millimeters on each side and with a thickness of 2.5 millimeters.

[0031] By "elastomer matrix" or "elastomeric matrix" is meant all of the elastomer(s) present in the rubber composition.

[0032] By "diene" elastomer (or indistinctly rubber), whether natural or synthetic, must be understood an elastomer consisting at least in part (i.e. a homopolymer or a copolymer) of diene monomer(s) (i.e., bearing(s) two carbon-carbon double bonds, conjugated or not).

[0033] By copolymer based on styrene and butadiene is meant here a copolymer of at least one styrene monomer and at least one butadiene monomer (and of course also any mixture of such copolymers) having a glass transition temperature Tg strictly greater than -65°C and less than or equal to -30°C; in other words, said copolymer based on styrene and butadiene comprises by definition at least styrene units (derived from the styrene monomer) and butadiene units (derived from the butadiene monomer) and has a glass transition temperature Tg strictly greater than -65°C and less than or equal to -30°C.

[0034] The glass transition temperature values ​​“Tg” are measured in a known manner by DSC (Differential Scanning Calorimetry) according to ASTM D3418 (1999).

[0035] For the purposes of the invention, the meridian profile of the tire is defined by the curvilinear abscissa of the inner surface of the tire, in contact with the cavity of the tire, measured between the two points of this surface forming an intersection with a straight line passing through the radially outermost points of each of the bead wires.

[0036] The measurement of the curvilinear abscissa of the inner surface of the tire is carried out on a section of a tire, the bead spacing of which is the same as when the tire is mounted on the mounting rim recommended by ETRTO, the latter therefore being neither mounted nor inflated.

[0037] The thickness of rubber mixture between the inner surface of the tire cavity and the point of a reinforcing element closest to said surface is equal to the length of the orthogonal projection of the end of the point of a reinforcing element closest to said surface on the inner surface of the tire cavity.

[0038] Rubber compound thickness measurements are made on a cross-section of a tire, with the tire in an uninflated state.

[0039] Advantageously according to the invention, in a radial plane, the thickness of rubber mixture between the inner surface of the cavity of the tire and the point of a metal reinforcing element of the carcass reinforcement closest to said inner surface of the cavity is less than 4.5 mm over at least 70% of the meridian profile of the tire, and more preferably over at least 80% of the meridian profile of the tire.

[0040] Advantageously according to the invention, the rubber composition forming the layer of polymeric mixture forming the rolling surface comprises at least one reinforcing filler comprising predominantly a reinforcing inorganic filler.

[0041] The rubber composition used in the tires of the invention comprises at least one reinforcing filler comprising predominantly a reinforcing inorganic filler, that is to say that the mass of the reinforcing inorganic filler represents at least 51% of the total mass of the constituents of the reinforcing filler. Preferably, the mass of the reinforcing inorganic filler represents more than 60%, preferably more than 70% of the total mass of the reinforcing filler.

[0042] By "reinforcing inorganic filler" is meant here any inorganic or mineral filler, whatever its color and origin (natural or synthetic), also called "white" filler, "clear" filler or even "non-black" filler ("non-black filler") as opposed to carbon black; this inorganic filler being 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 capable of replacing, in its reinforcing function, a conventional tire-grade carbon black. Such a filler is generally characterized, in a known manner, by the presence of hydroxyl groups (-OH) on its surface, requiring, in order to be used as a reinforcing filler, the use of a coupling agent or system intended to ensure a stable chemical bond between the filler and the elastomeric matrix.

[0043] Suitable inorganic reinforcing fillers are, in particular, siliceous mineral fillers, preferably silica (SiO 2 ). 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 surface area and a CTAB specific surface area both less than 450 m 2 < / g, preferably from 30 to 400 m 2 < / g, in particular between 60 and 300 m 2 < / g. Examples of highly dispersible precipitated silicas (known as "HDS") include "Ultrasil 7000" and "Ultrasil 7005" silicas from Evonik, "Zeosil 1165MP, 1135MP and 1115MP" silicas and "Zeosil Premium 200" silicas from Solvay, "Hi-Sil EZ150G" silicas from PPG, "Zeopol 8715, 8745 and 8755" silicas from Huber, and high specific surface silicas as described in application WO 03 / 016387.

[0044] Of course, the term inorganic reinforcing filler also means mixtures of different reinforcing inorganic fillers, in particular highly dispersible silicas as described above or a mixture of inorganic fillers of the siliceous type and non-siliceous inorganic fillers. As non-siliceous inorganic fillers, mention may be made of mineral fillers of the aluminous type, in particular alumina (Al 2 O 3 ) or aluminum (oxide)hydroxides, or even reinforcing titanium oxides, for example described in US 6,610,261 and US 6,747,087. The non-siliceous inorganic fillers, when present, are in the minority in the reinforcing filler.

[0045] The physical state in which the inorganic reinforcing filler is presented is indifferent, whether in the form of powder, microbeads, granules, or even balls.

[0046] According to one embodiment, the level of the reinforcing filler in the rubber composition of the tire according to the invention can range from 55 phr to 200 phr, preferably from 55 to 150 phr, more preferably from 55 to 80 phr. These preferred ranges apply to any of the embodiments of the invention.

[0047] A person skilled in the art will understand that, as a filler equivalent to the reinforcing inorganic filler described in this paragraph, a reinforcing filler of another nature, in particular organic such as carbon black, could be used, provided that this reinforcing filler is covered with an inorganic layer such as silica, or else comprises functional sites on its surface, in particular hydroxyl sites, requiring the use of a coupling agent to establish the bond 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 patent documents WO 96 / 37547, WO 99 / 28380.

[0048] The tests carried out have shown that the tires according to the invention have satisfactory wear and endurance performance regardless of the conditions of use of the tires. They also have rolling resistance performance superior to that of the more common tires described previously and could be produced at reduced costs.

[0049] The thickness of the rubber compound layer between the carcass reinforcement and the tire cavity being reduced compared to conventional tires and this constituting one of the most expensive components of the tire, the manufacturing cost of the tire is lower than that of a conventional tire.

[0050] The inventors have also been able to demonstrate that in comparison with more usual mixtures for obtaining a tread with low hysteretic losses, the wear rate of the tread layer according to the invention is considerably improved.

[0051] Indeed, the rubber composition based on at least one elastomer matrix comprising predominantly a copolymer based on styrene and butadiene having a glass transition temperature Tg strictly greater than -65°C and less than or equal to -30°C associated with a thermal conductivity of said layer of polymeric mixture forming the rolling surface strictly less than 0.270 W / (mK) and said layer of polymeric mixture forming the rolling surface being able to comprise at least one reinforcing filler formed predominantly from a reinforcing inorganic filler provides high rolling resistance performance and wear performance.The inventors were, however, able to demonstrate that the low thermal conductivity of said layer of polymeric mixture forming the rolling surface leads to an increase in the temperature of the tire tread, the heat generated during rolling being dissipated only very slowly. Such an increase in temperature accentuates the aging of the polymeric mixtures already subjected to shear phenomena between the working layers of the crown reinforcement and is therefore unfavorable to performance in terms of endurance. The inventors were, however, able to show that the layer of circumferential reinforcing elements makes it possible to compensate for this rise in temperature due to the low thermal conductivity of the mixture and makes it possible to maintain the endurance performance of the crown reinforcement at the expected level.

[0052] The inventors, still concerned about the risks of oxidation, particularly of the carcass reinforcement, linked to the presence of oxidizing agents within the pressurized tire which increase with the tire's lifespan, have also been able to demonstrate that the increase in temperature of the mixtures forming the tread is transmitted by conduction to the mixtures forming the wall between the carcass reinforcement and the tire cavity pressurized during inflation. This increase in temperature, particularly of the polymer mixture in contact with the carcass reinforcement layer which is intended to fix the oxygen which migrates from the cavity, reinforces the reactivity of said mixture which will be able to fix more oxygen.The inventors thus believe that the endurance properties of the carcass reinforcement are thus preserved while the thickness of the rubber mixture between the inner surface of the tire cavity and the point of a metal reinforcing element of the carcass reinforcement closest to said inner surface of the cavity is reduced in comparison with more usual tires.

[0053] According to a preferred embodiment of the invention, the rubber mixture between the tire cavity and the reinforcing elements of the radially innermost carcass reinforcement layer being made up of at least two layers of rubber mixture, in a radial plane, on at least the part of the meridian profile of the tire on which the thickness of rubber mixture between the inner surface of the tire cavity and the point of a metal reinforcing element of the carcass reinforcement closest to said inner surface of the cavity is less than 4.5 mm, the radially innermost layer of rubber mixture has a thickness of less than 2 mm and preferably less than 1.8 mm.As explained previously, this layer is usually partly composed of butyl in order to increase the airtightness of the tire and this type of material having a significant cost, the reduction of this layer is favorable.

[0054] More preferably according to the invention, in a radial plane, on at least the part of the meridian profile of the tire on which the thickness of rubber mixture between the inner surface of the cavity of the tire and the point of a metal reinforcing element of the carcass reinforcement closest to said inner surface of the cavity is less than 4.5 mm, the layer of rubber mixture radially adjacent to the radially innermost layer of rubber mixture has a thickness of less than 2.5 mm and preferably less than 2 mm. The thickness of this layer, the constituents of which in particular make it possible to fix the oxygen in the air, can also be reduced so as to further reduce the cost of the tire.

[0055] The thicknesses of each of these two layers are equal to the length of the orthogonal projection of a point of one surface onto the other surface of said layer.

[0056] Advantageously still according to the invention, the thermal conductivity of said layer of polymeric mixture forming the rolling surface is strictly less than 0.265 W / (mK).

[0057] According to one embodiment of the invention, the rubber composition of said at least one layer of polymeric mixture forming the rolling surface of the tire comprises a plasticizing system comprising from 0 to 15 phr, preferably from 0 to 9 phr, of at least one plasticizing resin having a glass transition temperature Tg greater than or equal to 20°C, and the total level of the plasticizing system in the composition of which ranges from 0 to 17 phr, preferably from 0 to 11 phr.

[0058] The abbreviation "pce" (usually "phr" in English for "per hundred part of rubber") means parts by weight per hundred parts of elastomers (of the total elastomers if more than one elastomer is present) or rubber present in the rubber composition.

[0059] Advantageously according to the invention, the plasticizing system may comprise from 2 to 15 pce, preferably from 2 to 9 pce of the plasticizing resin, the total level of the plasticizing system in the composition ranges from 2 to 17 pce, preferably from 2 to 11 pce.

[0060] Preferably, the styrene-butadiene based copolymer can be obtained by solution polymerization.

[0061] Preferably, the styrene-butadiene based copolymer has a glass transition temperature Tg ranging from -60°C to -40°C.

[0062] Preferably, the styrene-based and butadiene-based copolymer may be a styrene-butadiene copolymer.

[0063] In one embodiment of the invention, the elastomer matrix may further comprise at least one second diene elastomer different from the styrene and butadiene-based copolymer; in particular, said second diene elastomer being chosen from the group formed by polybutadienes, natural rubber, synthetic isoprenes, butadiene copolymers other than butadiene-styrene copolymers, isoprene copolymers and mixtures of these polymers and copolymers; preferably, said second diene elastomer may be a polybutadiene.

[0064] Preferably, the rate of the second diene elastomer can range from 5 to 49 pce, preferably from 15 to 35 pce.

[0065] In one embodiment of the invention, the elastomer matrix may further comprise at least one third diene elastomer different from the styrene and butadiene-based copolymer and the second diene elastomer, in particular the third diene elastomer may be chosen from natural rubber and an isoprene elastomer, preferably it may consist of natural rubber.

[0066] Preferably, the rate of the second diene elastomer can range from 0.5 to 35 pce and a rate of the third diene elastomer can range from 0.5 to 35 pce, preferably the rate of the second diene elastomer can range from 9 to 31 pce and a rate of the third diene elastomer can range from 4 to 24 pce.

[0067] Preferably, the composition may further comprise carbon black; in particular the carbon black content may be less than or equal to 10 pce, less than or equal to 5 pce, preferably the carbon black content may range from 0.5 to 4 pce.

[0068] Preferably, the level of the reinforcing filler can range from 55 to 200 pce, preferably from 55 to 150 pce, more preferably from 55 to 80 pce.

[0069] Preferably, the plasticizing resin may have a glass transition temperature Tg greater than or equal to 30°C, preferably ranging from 30 to 100°C; and in particular, the plasticizing resin may be chosen from the group consisting of cyclopentadiene homopolymer or copolymer resins, dicyclopentadiene homopolymer or copolymer resins, terpene homopolymer or copolymer resins, C5-cut homopolymer or copolymer resins, C9-cut homopolymer or copolymer resins, homopolymer resins, in particular mixtures of C5-cut homopolymer or copolymer resins and C9-cut homopolymer or copolymer resins and alpha-methyl-styrene copolymer resins and mixtures of these resins.

[0070] Preferably, the plasticizing system may comprise from 0 to 2 pce of at least one plasticizing agent which is liquid at room temperature (23°C).

[0071] According to another embodiment, the rubber composition of tires in accordance with the invention may be free of plasticizing system.

[0072] According to an advantageous embodiment of the invention, the layer of circumferential reinforcing elements has an axial width greater than 0.5xW.

[0073] W is the maximum axial width of the tire, when the latter is mounted on its service rim and inflated to its recommended pressure.

[0074] The axial widths of the reinforcing element layers are measured on a cross-section of a tire, the tire being in an uninflated state.

[0075] According to a preferred embodiment of the invention, the crown reinforcement being made up of at least two layers of reinforcing elements, a layer of circumferential reinforcing elements is radially arranged between two working crown layers.

[0076] According to this embodiment of the invention, the layer of circumferential reinforcing elements makes it possible to limit the compression of the reinforcing elements of the carcass reinforcement to a greater extent than a similar layer placed radially outside the working layers. It is preferably radially separated from the carcass reinforcement by at least one working layer so as to limit the stresses on said reinforcing elements and not to fatigue them excessively.

[0077] According to a preferred embodiment of the invention, at least two working crown layers have different axial widths, the difference between the axial width of the axially wider working crown layer and the axial width of the axially narrower working crown layer being between 10 and 30 mm.

[0078] According to a preferred embodiment of the invention, the axially widest working crown layer is radially inside the other working crown layers.

[0079] Advantageously still according to the invention, the axial widths of the working crown layers radially adjacent to the layer of circumferential reinforcing elements are greater than the axial width of said layer of circumferential reinforcing elements and preferably, said working crown layers adjacent to the layer of circumferential reinforcing elements are on either side of the equatorial plane and in the immediate axial extension of the layer of circumferential reinforcing elements coupled over an axial width, to then be decoupled by a layer of rubber mixture at least over the remainder of the width common to said two working layers.

[0080] The presence of such couplings between the working crown layers adjacent to the layer of circumferential reinforcing elements allows the reduction of tensile stresses acting on the axially outermost circumferential elements located closest to the coupling.

[0081] According to an advantageous embodiment of the invention, the reinforcing elements of at least one layer of circumferential reinforcing elements are metallic reinforcing elements having a secant modulus at 0.7% elongation of between 10 and 120 GPa and a maximum tangent modulus of less than 150 GPa.

[0082] According to a preferred embodiment, the secant modulus of the reinforcing elements at 0.7% elongation is less than 100 GPa and greater than 20 GPa, preferably between 30 and 90 GPa and more preferably less than 80 GPa.

[0083] Also preferably, the maximum tangent modulus of the reinforcing elements is less than 130 GPa and more preferably less than 120 GPa.

[0084] The moduli expressed above are measured on a tensile stress versus elongation curve determined with a prestress of 20 MPa, the tensile stress corresponding to a measured tension related to the metal section of the reinforcing element. The measurements are carried out on cables extracted from the tire on a part of the layer of circumferential reinforcing elements extending from an axial end of said layer over an axial width of 50 mm towards the inside of said layer.

[0085] The moduli of the same reinforcing elements can be measured on a tensile stress versus elongation curve determined with a prestress of 10 MPa, the tensile stress corresponding to a measured tension related to the overall section of the reinforcing element. The overall section of the reinforcing element is the section of a composite element made of metal and rubber, the latter having penetrated the reinforcing element during the curing phase of the tire; this overall section of the reinforcing element concerned is approximately twice the metal section of the reinforcing element.

[0086] According to this formulation relating to the overall section of the reinforcing element, the reinforcing elements of the axially outer parts and of the central part of at least one layer of circumferential reinforcing elements are metallic reinforcing elements having a secant modulus at 0.7% elongation of between 5 and 60 GPa and a maximum tangent modulus of less than 75 GPa.

[0087] According to a preferred embodiment, the secant modulus of the reinforcing elements at 0.7% elongation is less than 50 GPa and greater than 10 GPa, preferably between 15 and 45 GPa and more preferably less than 40 GPa.

[0088] Also preferably, the maximum tangent modulus of the reinforcing elements is less than 65 GPa and more preferably less than 60 GPa.

[0089] According to a preferred embodiment, the reinforcing elements of at least one layer of circumferential reinforcing elements are metallic reinforcing elements having a tensile stress curve as a function of relative elongation having low slopes for low elongations and a substantially constant and steep slope for higher elongations.

[0090] The various characteristics of the reinforcement elements stated above are measured on reinforcement elements taken from tires.

[0091] Reinforcing elements more particularly suitable for producing at least one layer of circumferential reinforcing elements according to the invention are, for example, assemblies of formula 21.23, the construction of which is 3x(0.26+6x0.23) 4.4 / 6.6 SS; this stranded cable is made up of 21 elementary wires of formula 3 x (1+6), with 3 strands twisted together each made up of 7 wires, one wire forming a central core with a diameter equal to 26 / 100 mm and 6 wound wires with a diameter equal to 23 / 100 mm. Such a cable has a secant modulus at 0.7% equal to 45 GPa and a maximum tangent modulus equal to 98 GPa, measured on a tensile stress versus elongation curve determined with a prestress of 20 MPa reduced to the metal section of the reinforcing element, the tensile stress corresponding to a measured tension reduced to the metal section of the reinforcing element.On a tensile stress versus elongation curve determined with a prestress of 10 MPa reduced to the overall section of the reinforcing element, the tensile stress corresponding to a measured tension reduced to the overall section of the reinforcing element, this cable of formula 21.23 has a secant modulus at 0.7% equal to 23 GPa and a maximum tangent modulus equal to 49 GPa.

[0092] Similarly, another example of reinforcing elements is an assembly of formula 21.28, whose construction is 3x(0.32+6x0.28) 6.2 / 9.3 SS. This cable has a secant modulus at 0.7% equal to 56 GPa and a maximum tangent modulus equal to 102 GPa, measured on a tensile stress versus elongation curve determined with a prestress of 20 MPa referred to the metal section of the reinforcing element, the tensile stress corresponding to a measured tension referred to the metal section of the reinforcing element. On a tensile stress versus elongation curve determined with a prestress of 10 MPa reduced to the overall section of the reinforcing element, the tensile stress corresponding to a measured tension reduced to the overall section of the reinforcing element, this cable of formula 21.28 has a secant modulus at 0.7% equal to 27 GPa and a maximum tangent modulus equal to 49 GPa.

[0093] The use of such reinforcing elements in at least one layer of circumferential reinforcing elements makes it possible in particular to maintain satisfactory layer rigidities even after the shaping and curing steps in standard manufacturing processes.

[0094] According to a second embodiment of the invention, the circumferential reinforcing elements may be formed from inextensible metal elements and cut so as to form sections of length much less than the circumference of the shortest layer, but preferably greater than 0.1 times said circumference, the cuts between sections being axially offset from each other. More preferably, the tensile modulus of elasticity per unit width of the additional layer is less than the tensile modulus of elasticity, measured under the same conditions, of the most extensible working crown layer.Such an embodiment makes it possible to confer, in a simple manner, on the layer of circumferential reinforcing elements a modulus which can easily be adjusted (by the choice of the intervals between sections of the same row), but in all cases, lower than the modulus of the layer made up of the same metallic elements but continuous, the modulus of the additional layer being measured on a vulcanized layer of cut elements, taken from the tire.

[0095] According to a third embodiment of the invention, the circumferential reinforcing elements are corrugated metal elements, the ratio a / λ of the corrugation amplitude over the wavelength being at most equal to 0.09. Preferably, the tensile modulus of elasticity per unit width of the additional layer is lower than the tensile modulus of elasticity, measured under the same conditions, of the most extensible working crown layer.

[0096] The metal elements are preferably steel cables.

[0097] According to a preferred embodiment of the invention, the reinforcing elements of the working crown layers of reinforcing elements are inextensible metal cables.

[0098] Advantageously according to the invention, the crown reinforcement is formed of at least two working crown layers of reinforcing elements, crossed from one layer to the other making angles of between 10° and 45° with the circumferential direction.

[0099] The invention further advantageously provides, in order to reduce the tensile stresses acting on the axially outermost circumferential elements, that the angle formed with the circumferential direction by the reinforcing elements of the working crown layers of reinforcing elements is less than 30° and preferably less than 25°.

[0100] A preferred embodiment of the invention also provides that the crown reinforcement is completed radially on the outside by at least one additional layer, called a protective layer, of so-called elastic reinforcing elements, oriented relative to the circumferential direction with an angle of between 10° and 45° and in the same direction as the angle formed by the reinforcing elements of the working crown layer of reinforcing elements which is radially adjacent to it.

[0101] According to any of the embodiments of the invention mentioned above, the crown reinforcement can also be completed, radially inside between the carcass reinforcement and the working crown layer of reinforcing elements radially inside closest to said carcass reinforcement, by a triangulation layer of inextensible steel metal reinforcing elements making, with the circumferential direction, an angle greater than 45° and in the same direction as that of the angle formed by the reinforcing elements of the working crown layer of reinforcing elements radially closest to the carcass reinforcement.

[0102] Other details and advantageous characteristics of the invention will emerge below from the description of the exemplary embodiments of the invention with reference to the figures which represent: figure 1 , a meridian view of a diagram of a tire according to the invention, figure 2, an enlarged partial view of part of the diagram of the figure 1 .

[0103] The figures are not drawn to scale to simplify understanding. figure 1 represents only a half-view of a tire which extends symmetrically with respect to the XX' axis which represents the circumferential median plane, or equatorial plane, of a tire.

[0104] On the figure 1 , the tire 1 of dimension 315 / 70 R 22.5 is shown. Said tire 1 comprises a radial carcass reinforcement 2 anchored in two beads, not shown in the figure. The carcass reinforcement is hooped by a crown reinforcement 3, formed radially from the inside to the outside: of a first working layer 31 formed of inextensible metal cables 9.30 not hooped, continuous over the entire width of the sheet, oriented at an angle equal to 26°, of a layer of circumferential reinforcing elements 32 formed of 21x23 steel metal cables, of the "bi-module" type, of a second working layer 33 formed of inextensible metal cables 9.30 not hooped, continuous over the entire width of the sheet, oriented at an angle equal to 18° and crossed with the metal cables of layer 31, of a protective layer 34 formed of elastic metal cables 6.35.

[0105] The crown frame is itself topped with a tread 4 having a surface 5 intended to come into contact with the ground.

[0106] The maximum axial width W of the tire is equal to 317 mm.

[0107] The axial width L 31 of the first working layer 31 is equal to 252 mm.

[0108] The axial width L 33 of the second working layer 33 is equal to 232 mm.

[0109] As for the axial width L 32 of the layer of circumferential reinforcing elements 32, it is equal to 194 mm.

[0110] The last top sheet 34, called protection, has a width L44 equal to 124 mm.

[0111] There figure 2 illustrates an enlargement of zone 6 of the figure 1and indicates in particular the thickness E of rubber mixture between the inner surface 10 of the cavity 7 of the tire and the point 12 of a reinforcing element 11 closest to said surface 10. This thickness E is equal to the length of the orthogonal projection of the point 12 of a reinforcing element 11 closest to said surface 10 on the surface 10. This thickness E is the sum of the thicknesses of the different rubber mixtures placed between said reinforcing element 11 of the carcass reinforcement 2; this is on the one hand the thickness of the radially inner calendering layer 13 of the carcass reinforcement and on the other hand, the thicknesses e 1 , e 2 of the different layers 14, 15 of rubber mixture forming the internal wall of the tire 1. These thicknesses e 1 , e 2 are moreover equal to the length of the orthogonal projection of a point of a surface on the other surface of the layer concerned respectively 14 or 15.

[0112] These thickness measurements are taken on a cross-section of the tire, which is therefore unmounted and uninflated.

[0113] The measured value of E is equal to 3.5 mm.

[0114] The values ​​of ei and e 2 are respectively equal to 1.5 mm and 1.5 mm.

[0115] These values ​​are measured over 100% of the meridian profile, as defined previously.

[0116] Tests were carried out with tires made according to the invention in accordance with the representation of the figures 1 And 2 , and others with so-called reference tires.

[0117] The rates of the different constituents of the compositions presented in the following table are expressed in pce. All compositions (T1, T2 and C) include a crosslinking system conventionally used in the manufacture of tire treads; this crosslinking system notably includes sulfur, ZnO, stearic acid and an accelerator. T1 T2 C1 SBR (1) - - 65.00 SBR (2) 40.00 40.00 NR (3) 40.00 40.00 15.00 BR (4) 20.00 20.00 20.00 Silica (5) - - 60.00 Carbon black (6) 58.00 - 4.00 Carbon black (7) - 58.00 - Coupling agent (8) - - 6.00 DPG (9) - - 0.90 Antioxidant (10) 2.00 2.00 2.00 Resin (11) 3.00 3.00 3.00 Paraffin 1.00 1.00 1.00 (1) Non-functional, non-extended SBR solution with 24% 1,2-butadiene units relative to total butadiene units; 26.5% styrene units and a Tg = -48°C; (2) SBR solution functionalized with tin, not extended, with 24% of 1,2-butadiene units compared to the total of butadiene units: 15.5% of styrene units, and a Tg = -65° C; (3) Natural rubber; (4) Polybutadiene Neodymium with 98% 1,4-cis butadiene units and a Tg = -108°C; (5) “Zeosil 1165 MP” type “HDS” silica from Solvay; (6) Carbon black N234; (7) Carbon black N134; (8) Coupling agent: TESPT (“Si69” from Evonik - Degussa); (9) Diphenylguanidine (“Perkacit” DPG from Flexsys company); (10) N-1,3-dimethylbutyl-N-phenyl-para-phenylenediamine marketed by the company Flexsys under the name "Santoflex 6-PPD"; (11) Resin cut C5 / C9 marketed by the company Cray Valley under the name “THER 8644 Resin” (Tg = 44°C).

[0118] The thermal conductivity properties of the compositions are presented in the table below. T1 T2 C Conductivity (W / (mK)) 0.294 0.294 0.263

[0119] The tires I according to the invention are produced in accordance with the figures 1 And 2 Their treads are made of compound C.

[0120] First reference tires P1 and P2 have treads made up of the mixtures T1 and T2 respectively. These reference tires P1 and P2 also differ from the tires according to the invention by a thickness E of rubber mixture between the inner surface of the cavity of the tire and the point of a reinforcing element closest to said surface being equal to 5 mm, each of the thicknesses ei and e 2 being respectively equal to 2.2 mm and 2.3 mm.

[0121] The manufacturing costs of these reference tires P1 and P2 are increased by approximately 7% compared to the tires according to the invention.

[0122] Furthermore, the tires according to the invention have the advantage of being lighter with a weight reduction of 6% compared to the reference tires P1 and P2.

[0123] Other P3 reference tires have been produced. They are similar to the tires I of the invention and their crown architecture differs in that the crown reinforcement does not include a layer of circumferential reinforcing elements. They are produced with a tread made of the C mixture.

[0124] Wear resistance tests were carried out by comparing the tires I according to the invention with the reference tires P1 and P2 by equipping identical vehicles with each of these tires.

[0125] The tests were carried out under identical conditions in terms of load and pressure and on identical routes, representative of normal use. The results consist of the measurement of weight loss.

[0126] The test results are presented in the following table.

[0127] The measurements are expressed in kg, with a value of 100 being assigned to the P1 tyre. Values ​​above 100 show better wear performance. P1 P2 I wear 100 105 115

[0128] These latest results highlight better wear resistance of the tires I according to the invention.

[0129] Initial endurance tests were carried out on a test machine requiring each of the tires to run in a straight line at a speed equal to the maximum speed index prescribed for the said tire (speed index) under an initial load of 4000 kg, gradually increased to reduce the duration of the test.

[0130] Second endurance tests were carried out on a test machine cyclically imposing a transverse force and a dynamic overload on the tires. The tests were carried out for the tires according to the invention under conditions identical to those applied to the reference tires.

[0131] The tests thus carried out showed that the distances covered during each of these tests are equivalent for the tires I according to the invention and the reference tires P1 and P2. It therefore appears that the tires according to the invention have performances in terms of endurance as good as those of the reference tires.

[0132] The P3 tires covered much shorter distances, approximately half those covered by the I tires according to the invention and the reference tires P1 and P2.

[0133] Final endurance tests on the steering wheel were carried out on a test machine imposing a load of 4415 daN on the tires and a speed of 40 km / h, with oxygen-doped tire inflation. The tests were carried out for tires I according to the invention with conditions identical to those applied to the reference tires P1 and P2. The running is stopped as soon as the tires show damage to the carcass reinforcement.

[0134] The tests thus carried out showed that the distances covered during each of these tests are equivalent for the tires I according to the invention and the reference tires P1 and P2 which have covered approximately 220,000 km.

[0135] Furthermore, rolling resistance measurements were carried out. These measurements concerned the reference tires P1 and P2 and the tires according to the invention I.

[0136] The test results are presented in the following table.

[0137] Rolling resistance measurements are expressed in kg / t, with a value of 100 being assigned to the P1 tyre. Values ​​above 100 show better rolling resistance performance. P1 P2 I Rolling resistance 100 96 104

[0138] These tests show that the tires according to the invention make it possible to improve performance in terms of endurance and wear and present satisfactory performance in terms of rolling resistance.

Claims

1. Tyre (1) with a radial carcass reinforcement (2), comprising a crown reinforcement (3), itself capped radially by a tread (4), the said tread being joined to two beads via two sidewalls, the crown reinforcement (3) having at least one layer of circumferential reinforcing elements (32), the said tread having at least one layer of polymeric compound forming a tread surface (5) intended to come into contact with a roadway, characterized in that the thermal conductivity of the said layer of polymeric compound forming the tread surface is strictly less than 0.270 W / (m.K), the thermal conductivity being measured in accordance with the standard ISO 8302:1991, in that the said layer of polymeric compound forming the tread surface is formed of a rubber composition based on at least one elastomer matrix predominantly comprising a copolymer that is based on styrene and butadiene and has a glass transition temperature Tg strictly greater than -65°C and less than or equal to -30°C, and in that, in a radial plane, over at least 50% of the meridian profile of the tyre, the thickness (E) of rubber compound between the inner surface (10) of the cavity (8) of the tyre and that point of a metal reinforcing element of the carcass reinforcement that is closest to the said inner surface of the cavity is less than 4.5 mm.

2. Tyre (1) according to Claim 1, characterized in that, in a radial plane, the thickness (E) of rubber compound between the inner surface of the cavity (7) of the tyre and the point of a metal reinforcing element of the carcass reinforcement that is closest to the said inner surface of the cavity is less than 4.5 mm over at least 70% of the meridian profile of the tyre, and preferably over at least 80% of the meridian profile of the tyre.

3. Tyre (1) according to Claim 1 or 2, characterized in that the rubber composition of the said at least one layer of polymeric compound forming the tread surface of the tyre includes at least one reinforcing filler predominantly comprising an inorganic reinforcing filler.

4. Tyre (1) according to Claim 3, characterized in that the inorganic reinforcing filler is silica.

5. Tyre (1) according to one of the preceding claims, the rubber compound between the cavity of the tyre and the reinforcing elements of the radially innermost carcass reinforcement layer being made up of at least two layers of rubber compound, characterized in that, in a radial plane, over at least that part of the meridian profile of the tyre in which the thickness of rubber compound between the inner surface of the cavity of the tyre and that point of a metal reinforcing element of the carcass reinforcement that is closest to the said inner surface of the cavity is less than 4.5 mm, the radially innermost layer of rubber compound (15) has a thickness (e2) less than 2 mm and preferably less than 1.8 mm.

6. Tyre (1) according to one of the preceding claims, the rubber compound between the cavity of the tyre and the reinforcing elements of the radially innermost carcass reinforcement layer being made up of at least two layers of rubber compound (14, 15), characterized in that, in a radial plane, over at least that part of the meridian profile of the tyre in which the thickness of rubber compound between the inner surface of the cavity of the tyre and that point of a metal reinforcing element of the carcass reinforcement that is closest to the said inner surface of the cavity is less than 4.5 mm, the layer of rubber compound (14) that is radially adjacent to the radially innermost layer of rubber compound has a thickness (e1) less than 2.5 mm and preferably less than 2 mm.

7. Tyre (1) according to one of the preceding claims, characterized in that the thermal conductivity of the said layer of polymeric compound forming the tread surface is strictly less than 0.265 W / (m.K).

8. Tyre (1) according to one of the preceding claims, characterized in that the rubber composition of the said at least one layer of polymeric compound forming the tread surface of the tyre has a plasticizing system comprising from 0 to 15 phr, preferably from 0 to 9 phr, of at least one plasticizing resin having a glass transition temperature Tg greater than or equal to 20°C, and for which the total content of the plasticizing system in the composition ranges from 0 to 17 phr, preferably from 0 to 11 phr.

9. Tyre (1) according to one of the preceding claims, characterized in that the copolymer based on styrene and butadiene has a glass transition temperature Tg ranging from -60°C to -40°C.

10. Tyre (1) according to one of the preceding claims, characterized in that the copolymer based on styrene and based on butadiene is a styrene-butadiene copolymer.

11. Tyre (1) according to Claim 10, characterized in that the elastomer matrix further comprises at least a second diene elastomer different from the copolymer based on styrene and butadiene.

12. Tyre (1) according to one of the preceding claims, the crown reinforcement (3) comprising at least two working crown layers of reinforcing elements (31, 33), characterized in that the layer of circumferential reinforcing elements (32) is positioned radially between two working crown layers of reinforcing elements (31, 33).

13. Tyre (1) according to one of the preceding claims, characterized in that the crown reinforcement (3) comprises at least two working crown layers of reinforcing elements (31, 33), preferentially inextensible, crossed from one layer to the other, making angles of between 10° and 45° with the circumferential direction.

14. Tyre (1) according to one of the preceding claims, characterized in that the reinforcing elements of at least one layer of circumferential reinforcing elements (32) are metal reinforcing elements having a secant modulus at 0.7% elongation of between 10 and 120 GPa and a maximum tangent modulus of less than 150 GPa.

15. Tyre (1) according to one of the preceding claims, characterized in that the crown reinforcement (3) is supplemented radially on the outside by at least one additional layer (34), referred to as a protective ply, of "elastic" reinforcing elements, oriented at an angle of between 10° and 45° relative to the circumferential direction and of the same direction as the angle formed by the inextensible elements of the working crown layer of reinforcing elements (33) which is radially adjacent thereto.