Optimised architecture of a civil engineering tyre

Abstract

The invention relates to a civil engineering tyre (1) comprising a tread (2) comprising at least two rubber compounds (23, 24), a first rubber compound (23) intended to come into contact with the ground and a second rubber compound (24) that is radially inward from same, and a crown reinforcement (3). On either side of the equatorial plane (OXZ), a crown filler rubber (6) is comprised between the end of the working layer (321) of largest width and the carcass reinforcement (4), and so-called crown decoupling rubbers (5) are located between the axial ends of the crown layers (311, 312, 321, 322, 331, 332) and the closest crown layer. The elastomeric materials of the decoupling rubber (5) or crown filler rubber comprise silica as reinforcing filler. The thermal conductivity of the at least second rubber compound (24) is at least equal to 0.35 W / mK.

Description

Description Title: Optimized architecture of civil engineering tires

[0001] The present invention relates to a radial tire, intended to equip a heavy vehicle of the civil engineering type, and relates more particularly to the crown of such a tire.

[0002] Radial tires intended to equip a heavy civil engineering vehicle are designated as such within the meaning of the European Tire and Rim Technical Organization (ETRTO) standard.

[0003] For example, a radial tire for a heavy vehicle of the civil engineering type, within the meaning of the ETRTO 2020 standard, is intended to be mounted on a rim whose diameter is at least equal to 25 inches. The invention is more particularly intended for tires for large civil engineering dump trucks and therefore for tires whose rim diameter is at least equal to 57 inches.

[0004] A tire having a geometry of revolution with respect to an axis of rotation, the geometry of the tire is generally described in a meridian plane containing the axis of rotation of the tire. For a given meridian plane, the radial, axial and circumferential directions respectively designate the directions perpendicular to the axis of rotation of the tire, parallel to the axis of rotation of the tire and perpendicular to the meridian plane. The circumferential direction is tangent to the circumference.

[0005] In the following, the expressions "radially inward" and "radially outward" respectively mean "closer" and "further" respectively from the axis of rotation of the tire. "Axially inward" and "axially outward" respectively mean "closer" and "further" respectively from the equatorial plane of the tire, the equatorial plane of the tire being the plane passing through the middle of the tread surface and perpendicular to the axis of rotation.

[0006] Generally speaking, a tire comprises a tread, intended to come into contact with the ground via a rolling surface, the two axial ends of which are connected via two sidewalls with two beads ensuring the mechanical connection between the tire and the rim on which it is intended to be mounted.

[0007] A radial tire further comprises a reinforcing reinforcement, consisting of a crown reinforcement, radially inside the tread, and a carcass reinforcement, radially inside the crown reinforcement.

[0008] The carcass reinforcement of a radial tire for a heavy vehicle of the civil engineering type usually comprises at least one carcass layer comprising generally metallic reinforcements, coated with a polymeric material of the elastomer or elastomeric type, otherwise called a rubber compound, obtained by mixing and called a calendering compound or calendering rubber. A carcass layer comprises a main part, connecting the two beads together and generally winding, in each bead, from the inside to the outside of the tire around a circumferential reinforcement element, most often metallic, called a bead wire, to form a turn-up. The metallic reinforcements of a carcass layer are substantially parallel to each other and form, with the circumferential direction, an angle of between 80° and 90°.

[0009] The crown reinforcement of a radial tire for a civil engineering vehicle comprises a superposition of crown layers extending circumferentially, radially outside the carcass reinforcement. Each crown layer is made up of generally metallic reinforcements, parallel to each other and coated with a polymeric material of the elastomer type or coating mixture (or rubber).

[0010] Among the crown layers, a distinction is usually made between the protective layers, constituting the protective reinforcement and radially the outermost, and the working layers, constituting the working reinforcement and radially between the protective reinforcement and the carcass reinforcement.

[0011] The protective reinforcement, comprising at least one protective layer, essentially protects the working layers from mechanical or physicochemical attacks, likely to propagate through the tread radially towards the inside of the tire.

[0012] The protective reinforcement often comprises two protective layers, radially superimposed, formed of elastic metal reinforcements, parallel to each other in each layer and crossed from one layer to the next, forming, with the circumferential direction, angles at least equal to 10°.

[0013] The working frame, often comprising at least two working layers, has the function of surrounding the tire and giving it rigidity and road holding. It takes up both mechanical inflation stresses, generated by the inflation pressure of the tire and transmitted by the carcass reinforcement, and mechanical rolling stresses, generated by the rolling of the tire on a ground and transmitted by the tread. A recurring problem in tire crowns is cracking due to shearing, linked to rolling, of rubber compounds, coating compounds for the crown layers or other compounds, at the end of the crown layers. These cracks impact the endurance of the tire and reduce its service life. A classic solution to avoid this cracking is to decouple the crown layers, in particular the working layers at their axial ends as shown in figures 1 to 3 of document EP3297851.The crown reinforcement must also be resistant to impacts and punctures, thanks to its intrinsic design, in particular its flexibility and, in particular, that of the protective reinforcement. Furthermore, the tire must have edge flexion, or drift rigidity under transverse force so as to ensure correct vehicle behavior on bends.

[0014] The working reinforcement usually comprises two working layers, radially superimposed, formed of non-extensible metal reinforcements, parallel to each other in each layer and crossed from one layer to the next, forming, with the circumferential direction, angles at most equal to 50°, and, preferably, at least equal to 15° and at most equal to 45°. The two-layer, consisting of these two working layers, generally ensures a sufficient level of bending on the edge for acceptable vehicle behavior.

[0015] To reduce the mechanical inflation stresses transmitted to the working reinforcement, it is known to arrange, radially outside the carcass reinforcement, a hoop reinforcement. The hoop reinforcement, the function of which is to absorb at least part of the mechanical inflation stresses, improves the endurance of the crown reinforcement by stiffening the crown reinforcement. The hoop reinforcement can be positioned radially inside the working reinforcement, between the two working layers of the working reinforcement, or radially outside the working reinforcement.

[0016] In Civil Engineering applications, the hoop reinforcement may comprise two radially superimposed hoop layers formed of metal reinforcements, parallel to each other in each layer and crossed from one layer to the next, forming, with the circumferential direction, angles at most equal to 10°. Another embodiment of the hoop reinforcement consists of a circumferential winding of a hoop wire or a continuous hooping band forming, with the circumferential direction, angles at most equal to 5°.

[0017] With regard to metal reinforcements, a metal reinforcement is mechanically characterized by a curve representing the tensile force (in N), applied to the metal reinforcement, as a function of its relative elongation (in %), called the force-elongation curve. From this force-elongation curve are deduced the mechanical tensile characteristics of the metal reinforcement, such as the structural elongation As (in %), the total elongation at break At (in %), the breaking force Fm (maximum load in N) and the breaking strength Rm (in MPa), these characteristics being measured according to the ASTM D 2969-04 standard of 2014.

[0018] The total elongation At of the metal reinforcement is, by definition, the sum of its structural, elastic and plastic elongations (At = As + Ae + Ap) and particularly at break where each of the elongations is non-zero. The structural elongation As results from the relative positioning of the metal wires constituting the metal reinforcement under a low tensile force. The elastic elongation Ae results from the very elasticity of the metal of the metal wires, constituting the metal reinforcement, taken individually, the behavior of the metal following a Hooke law. The plastic elongation Ap results from the plasticity, that is to say from the irreversible deformation, beyond the elastic limit, of the metal of these metal wires taken individually. These different elongations as well as their respective meanings, well known to those skilled in the art, are described, for example, in documents US5843583, WO2005 / 014925 and WO2007 / 090603.

[0019] At any point on the force-elongation curve of a metal reinforcement, a modulus in extension, expressed in GPa, is also defined, which represents the slope of the line tangent to the force-elongation curve at that point. In particular, the elastic modulus in extension or Young's modulus is called the modulus in extension of the linear elastic part of the force-elongation curve.

[0020] Among the metallic reinforcements, a distinction is usually made between elastic metallic reinforcements, such as those used in protective layers, and non-extensible or inextensible metallic reinforcements, such as those used in working layers.

[0021] An elastic metal reinforcement, in its unraveled state, is characterized by a structural elongation As at least equal to 0.5% and a total elongation at break At at least equal to 3%. In addition, an elastic metal reinforcement has an elastic modulus in extension at most equal to 180 GPa, and usually between 40 GPa and 150 GPa.

[0022] A non-extensible metal reinforcement is characterized by a total elongation At, under a tensile force equal to 10% of the breaking force Fm, at most equal to 0.2%. Furthermore, a non-extensible metal reinforcement has an elastic modulus in extension usually between 150 GPa and 200 GPa.

[0023] One method to significantly improve endurance is to increase not the stiffness of the crown rubbers but their elongation at break, although this crown area operates at imposed deformation. It is important for crown endurance to have certain minimum values ​​of the elongations at break of the elastomeric materials of the crown, these minimum values ​​being different depending on the position of said rubbers, depending on whether these elongations concern the crown filling rubber or the crown decoupling rubbers.

[0024] The filling rubber, called the crown filling rubber, is, on either side of the equator plane (OXZ), or median circumferential plane OXZ, perpendicular to the axis of rotation of the tire and passing through the center of the tread, between the axial end of the working layer of greatest axial width and the carcass reinforcement. Composed of at least one elastomeric material, it forms the link between the curved part of the carcass reinforcement under the shoulder and the crown reinforcement, the optimal shape of which is roughly parallel to the axis of rotation, in particular to optimize shoulder wear or rolling resistance.

[0025] The decoupling rubbers are positioned between the ends of the crown layers and the nearest crown layer, possibly the carcass layer for the radially innermost crown layer if this is not the widest working layer. They reduce shear at the ends of the crown layers and in particular working layers whose reinforcements are crossed, which generate significant shear. They can extend from the end of the crown layers and aggregate with each other to form a large volume in the shoulder area.

[0026] As shown in WO2023 / 110499 A1, rubber compounds or elastomeric materials suitable for having these interesting properties of deformation at break contain silica. Applying different requirements to elongations at break on one or other of these rubbers allows a gain in crown endurance performance, the failure of the crown often coming, in the cases which interest the invention, from the junction of cracks from the ends of the crown layers through the crown filling rubber. Avoiding, delaying, blocking their propagation in the crown filling rubber brings an interesting gain in endurance. It is also important, when increasing the values ​​of the elongation at break, not to increase the temperature at the crown; it is therefore also necessary that the dynamic loss of one or other of these rubbers is limited.

[0027] However, the use of such elastomeric materials including silica, a highly thermally insulating material, requires an increase in the curing time. This problem is very specific to civil engineering tires where curing times are particularly long due to the thickness of the crown across its entire width but particularly due to the thickness at the shoulder between the axial end of the tread surface and the internal cavity of the tire.

[0028] The inventors aimed to improve the endurance performance of civil engineering tires using elastomeric materials containing silica either in the crown filler rubber or in the various crown decoupling rubbers without degrading the curing time.

[0029] This objective has been achieved, according to the invention, by a tire for a civil engineering vehicle delimiting an internal cavity intended to receive an inflation gas when the tire is mounted on a rim, comprising: - a tread intended to come into contact with a ground via a rolling surface having an axial end El, - the tread having a recommended wear limit upon withdrawal and comprising at least 2 elastomeric materials, a first material, called contact material, intended to come into contact with the ground and at least a second elastomeric material radially inside the recommended wear limit and the contact material, called diffusion material, - a crown reinforcement, radially inside the tread and radially outside a carcass reinforcement comprising crown layers comprising metallic reinforcing elements coated in elastomeric materials, called calendering mixtures, the distance between the axial end El of the tread surface and the internal cavity in a meridian plane (OYZ) being at least equal to 130 mm, the crown reinforcement comprising at least one working layer whose reinforcing elements make an angle with the median circumferential plane or equator plane (OXZ) at least equal to 15°, on either side of the equator plane (OXZ), a filling rubber called crown filling rubber between the axial end of the working layer of greatest axial width and the carcass reinforcement, composed of at least one elastomeric material, on either side of the equator plane (OXZ), so-called crown decoupling rubbers located between the axial ends of the crown layers and the crown layer closest to said end, the decoupling rubbers having a radial thickness at least equal to 0.5mm, the elastomeric materials of the crown decoupling rubbers comprising silica as a reinforcing filler at a rate of at least 40 pce and / or the elastomeric materials of the crown packing rubber comprising silica as a reinforcing filler at a rate of at least 30 pce, the thermal conductivity of the diffusion material being at least equal to 0 35 W / mK, maximum radial of the diffusion material measured in a meridian plane (OYZ) being at least equal to 10 mm.

[0030] The principle of the invention consists in overcoming the problem of curing civil engineering tires comprising elastomeric materials comprising silica by using a part of the tread to better diffuse the calories thanks to a particularly high thermal conductivity, at least equal to 0.35 W / mK. The thermal conductivity of a material is a physical quantity which characterizes the ability of a material to allow heat transfer by conduction. It represents the quantity of heat transferred per unit of surface area and duration, under a temperature gradient of 1 degree Kelvin and per meter. It is expressed in W.m'hK' 1 . Thus a thermal conductivity of 1 W.m'hK' 1 represents the amount of heat that propagates through a material by thermal conduction, across a surface of 1 m 2, over a distance of 1 m. Thermal conductivity is measured at room temperature (23°C) on a Hotdisk TPS 2500 thermal analyzer with a type 5501 probe, according to ISO 22007-2: 2015 standard. The dimensions of the measuring specimen will be adapted by the person skilled in the art to the quantity of rubber taken from the cured tire.

[0031] Such materials do not have the adequate properties to constitute the part of the tread intended to come into contact with the ground, either in terms of wear, rolling resistance, resistance to aggression. They also do not have the crack resistance properties of silica compounds and it is therefore not relevant to use them as crown filler rubber or crown decoupling rubber. However, they can be used between the contact material of the tread and the crown reinforcement.

[0032] This is effective as long as the tire is intended for civil engineering and the distance between the axial end El of the tread surface and the internal cavity in a meridian plane OYZ is at least 130 mm. For smaller tires such as truck tires, whose curing time is sometimes 10 times shorter, the effect obtained would be very little perceptible. On the other hand, in civil engineering, the inventors were surprised to find that not only was it possible to maintain the same curing time as for a tire with crown filler rubbers or crown decoupling rubbers without silica, but with an improvement in the thermal performance of the tire. A Civil Engineering tire can also be described as having a radial tread height, measured from the tread surface to the bottom of the tread grooves, at least equal to 65 mm.Recommended withdrawal limit means, for example, the limit given by wear indicators present in the tread pattern or data from tire manufacturers giving specific recommendations for tire withdrawal based on expert opinions, these recommendations not necessarily being indicated on the tire.

[0033] An advantageous solution is that the axial distance from the most axially outer end of the diffusion material to the median circumferential plane OXZ passing through the center of the tread is at least equal to 90% of the axial distance from the axial end El of the contact surface to the median circumferential plane OXZ. Advantageously, the radial thickness of the diffusion material measured in a meridian plane OXY at the center of the tread is the minimum thickness of the diffusion material over its axial width, in particular due to the crown thickness which is minimal at the center of the tread.Indeed, taking into account two characteristics, namely the greater local thickness of the tire and the presence of elastomeric materials comprising silica, it is at the end of the crown layers whose dimension is proportional to the axial width of the tread, that the diffusion elastomeric material should be positioned which will allow better diffusion of the curing heat locally. This diffusion makes it possible to maintain the curing time to have optimal curing of the elastomeric materials despite the presence of the silica-filled elastomeric materials.

[0034] Advantageously, on either side of the equator plane, the axial width of the diffusion material is at least equal to 80% of the axial width of the decoupling rubbers and / or packing rubber comprising silica, this axial width being the axial width between the axially outermost point of the decoupling rubbers and / or packing rubber comprising silica and the axially innermost point of the decoupling rubbers and / or packing rubber comprising silica. This axial width of the diffusion material makes it possible to reduce the thermal insulating impact of the silica-filled elastomeric materials over their entire axial width.

[0035] Interestingly, the diffusion material is continuous between its axial ends located on either side of the equator plane, the radial thickness of the diffusion material measured in an OYZ meridian plane at the center of the tread is at least equal to 10 mm. This configuration of the diffusion material seems to allow better relative curing of the center and the shoulders. With such a good conductor for an elastomeric material, the calories arriving at the shoulder come from the hot elements of the mold but also more from the part of the center of the crown which rises more quickly in temperature due to its lower thickness of around 30%. A minimal thickness allows a minimal heat flow to have a visible effect.

[0036] Advantageously, the maximum radial thickness of the diffusion material measured in a meridian plane (OYZ) is at least equal to 15 mm, preferably at least equal to 40 mm. From 10 mm of maximum thickness of the diffusion material, located at the shoulder, it is possible to reduce the curing time around ten minutes and this on a level up to approximately a maximum thickness of the diffusion material of 40 mm, thickness from which the gain in curing time and rolling resistance becomes more significant beyond 20 minutes for the curing time, even with hysteretic properties of the diffusion material higher than for the reference tire materials. For a thickness of the diffusion material less than 10 mm at the shoulder, the inventors have not observed any significant measurable impact.

[0037] Advantageously, when the tread comprises grooves comprising walls and a bottom, the diffusion material must be radially inside all the bottoms of the grooves of the tread in order to avoid coming into contact with the ground, which would cause rapid wear.

[0038] This level of conductivity of the diffusion material was obtained by an elastomeric material based on at least one elastomeric matrix, fillers including graphite and a reinforcing filler, and a vulcanization system, the elastomeric matrix comprising at least 50 pce (per hundred parts of elastomer) of a diene elastomer chosen from the group of isoprene elastomers, butadiene elastomers and mixtures of these diene elastomers, the reinforcing filler mainly comprising a carbon black, the graphite having a crystallite size Le comprised in a range from 80 to 500 nm, more preferably from 90 to 400 nm, more preferably from 100 to 300 nm, the total rate of fillers being less than or equal to 60 pce.

[0039] By “diene” elastomer (or indistinctly rubber), whether natural or synthetic, is meant, in a known manner, an elastomer consisting at least in part (i.e., a homopolymer or a copolymer) of diene monomer units (monomers bearing two carbon-carbon double bonds, conjugated or not). In the present application, diene elastomers are by definition non-thermoplastic. Preferably, when the diene elastomers are copolymers, they are random polymers. Diene elastomers can be classified into two categories: “essentially unsaturated” or “essentially saturated”.The term “essentially unsaturated” generally means a diene elastomer derived at least in part from conjugated diene monomers, having a content of units or patterns of diene origin (conjugated dienes) which is greater than 15% (mol %); thus, diene elastomers such as butyl rubbers or copolymers of dienes and alpha-olefins such as EPDM do not fall within the preceding definition and may in particular be described as “essentially saturated” diene elastomers (low or very low content of patterns of diene origin, always less than 15 mol %). The diene elastomers which can be used in the diffusion material according to the invention are essentially unsaturated diene elastomers. Butyl elastomers are therefore not usable in this context since they are essentially saturated diene elastomers.

[0040] The elastomeric matrix of the elastomeric composition of the diffusion material comprises at least 50 pce of a diene elastomer selected from the group of isoprene elastomers, butadiene elastomers and mixtures of these diene elastomers. For example, the elastomeric matrix may comprise an isoprene elastomer or a mixture of isoprene elastomers, a butadiene elastomer or a mixture of butadiene elastomers or even a mixture of isoprene and butadiene elastomers. These diene elastomers have a content of units or patterns of diene origin (dienes conjugated) which is greater than 15 mol%. Thus the elastomeric matrix of the composition of the diffusion material comprises at least 50 pce of a diene elastomer having a rate of units or patterns of diene origin (conjugated dienes) which is greater than 15 mol%; this elastomer being chosen from the group consisting of isoprene elastomers, butadiene elastomers and mixtures of these diene elastomers. In addition to the aforementioned diene elastomers, the elastomeric matrix may also comprise thermoplastic elastomers.

[0041] The elastomeric composition of the diffusion material comprises fillers including a reinforcing filler and graphite. By filler is meant here any type of filler, whether reinforcing or non-reinforcing or inert. As a reinforcing filler, it is known to those skilled in the art that it is carbon black or inorganic reinforcing fillers such as silica or alumina. As a non-reinforcing or inert filler, it is known to those skilled in the art that it is graphite, clay, talc, mica, etc. In this context, zinc oxide is not considered a non-reinforcing filler. It is an activator of the vulcanization system.The total filler content (i.e. the sum of the content of reinforcing fillers and non-reinforcing fillers) in the elastomeric composition of the diffusion material is less than or equal to 60 phr, preferably less than or equal to 55 phr, preferably less than or equal to 50 phr, preferably within a range from 20 to 55 phr, more preferably within a range from 25 to 50 phr. At this total filler content, a good compromise of properties of resistance to aggression, rigidity, deformation at break and thermal conductivity is obtained, allowing it to be used in the tread.

[0042] The elastomeric composition of the diffusion material comprises a reinforcing filler, this reinforcing filler comprising mainly a carbon black. In addition to carbon black which is the predominant reinforcing filler, the elastomeric composition of the diffusion material may optionally also comprise a second reinforcing filler such as silica, for example. The total level of reinforcing filler, i.e. the sum of the level of carbon black and the level of silica when present, is less than or equal to 55 phr, more preferably less than or equal to 50 phr, more preferably less than or equal to 45 phr, more preferably less than or equal to 44 phr. This level of reinforcing filler is advantageously greater than or equal to 20 phr, more preferably greater than or equal to 25 phr, more preferably greater than or equal to 30 phr. Preferably, the level of reinforcing fillers in the elastomeric composition is included in a range from 20 pce to 55 pce, more preferably from 25 pce to 50 pce, more preferably from 30 pce to 45 pce.

[0043] Preferably, for the diffusion material, the carbon black represents more than 55% by weight of the total weight of the reinforcing filler, more preferably still more than 60% by weight, more preferably still more than 80% by weight, more preferably still represents 100% by weight of the total weight of the reinforcing filler.

[0044] Thus, preferably, for the diffusion material, the reinforcing filler content is less than or equal to 55 phr, more preferably less than or equal to 50 phr, more preferably less than or equal to 45 phr and the carbon black represents more than 55% by weight of the total weight of the reinforcing filler, more preferably still more than 60% by weight, more preferably still more than 80% by weight, more preferably still represents 100% by weight of the total weight of the reinforcing filler.

[0045] Even more preferably, the level of reinforcing fillers in the elastomeric composition of the diffusion material is within a range from 20 phr to 55 phr, more preferably from 25 phr to 50 phr, more preferably from 30 phr to 45 phr and the carbon black represents more than 55% by weight of the total weight of the reinforcing filler, more preferably still more than 60% by weight, more preferably still more than 80% by weight, more preferably still represents 100% by weight of the total weight of the reinforcing filler.

[0046] The composition of the diffusion material comprises at least one graphite. The elastomeric composition of the diffusion material may contain a single graphite as described below or a mixture of several graphites as described below.

[0047] Graphite is generally understood to mean a set of stacked graphene planes, graphene being an atomic-thick sheet in which the carbon atoms are organized in an essentially hexagonal lattice. Unlike the aforementioned carbon black, graphite therefore has a crystalline structure. Graphite can be natural or synthetic. When graphite is synthetic, it can be obtained, in particular, by a complex process of cooking petroleum coke at very high temperatures. Graphite is not considered a reinforcing filler and is therefore not taken into account in the calculation of reinforcing fillers. Graphite is a filler and is therefore taken into account in the calculation of the total filler content.

[0048] The graphite usable for the diffusion material has a crystallite size noted Le ranging from 80 to 500 nm, more preferably from 90 to 400 nm, even more preferably from 100 to 300 nm. The size of the crystallites is determined by X-ray diffraction according to the ASTM D5187-10 method adapted by those skilled in the art to samples from tires.

[0049] The graphite usable for the diffusion material can have a BET specific surface area ranging from 10 to 50 m 2 / g; preferably ranging from 15 to 40 m 2 / g, more preferably still ranging from 20 to 30 m 2 / g. The method for measuring the BET specific surface area of ​​graphite is based on recording the absorption isotherm of liquid nitrogen in the range p / pO = 0.04-0.26 at 77 K. Following the procedure proposed by Brunauer, Emmet and Teller (Adsorption of Gases in Multimolecular Layers, J. Am. Chem. Soc, 1938, 60, 309-319), the monolayer capacitance can be determined. Based on the cross-section of the nitrogen molecule, the monolayer capacitance and the weight of the sample, the specific surface area can then be calculated.

[0050] The graphite usable in this context may have a particle size distribution Dç>o comprised in a range from 40 to 110 nm, more preferably from 50 to 100 nm, even more preferably from 60 to 90 nm. D90 corresponds to the 90th percentile of the mass distribution of particle size, that is to say that 90% by mass of the particles have a size less than D90 and 10% by mass of the particles have a size greater than D90. It is expressed in nm.

[0051] Graphite suitable for diffusion material may have an apparent density (Scott density) greater than or equal to 0.10 g / cm 3 , preferably greater than 0.12 g / cm 3 Apparent density, or Scott density, is determined by passing dry graphite powder through the Scott volumeter according to ASTM B329-98 (2003).

[0052] Preferably, the graphite usable in the context of the present invention is an expanded graphite.

[0053] Preferably, the rate of graphite in the elastomeric composition for the diffusion material is within a range from 1 to 12 pce, more preferably from 1 to 11 pce.

[0054] Preferably, the mass ratio of graphite to carbon black in the elastomeric composition is within a range from 0.05 to 0.5 pce, preferably from 0.06 to 0.4 pce.

[0055] Surprisingly, the inventors identified that the use of the elastomeric material in the cited ranges, with a shifted compromise of stiffness, strain at break and thermal conductivity properties, as a diffusion material, brings a performance compromise higher than expected.

[0056] Advantageously, the elongation at break at 100°C according to standard NF T 46-002, of at least one elastomeric material comprising at least 30 pce of silica composing the crown filling rubber is at least equal to 650% and the maximum dynamic loss tanô, of said elastomeric material, measured according to standard ASTM D 5992 - 96, at a temperature of 100°C at 10 Hz, is at most equal to 0.07. The invention will make it possible to improve a tire with a crown filling rubber comprising a percentage of silica making it insulating, but, to have optimal behavior, this same crown filling rubber must have adequate elongation at break properties.

[0057] Similarly, the elongation at break at 100°C according to standard NF T 46-002 of the crown decoupling rubbers comprising at least 40 pce of silica is at least equal to 500%, and the maximum dynamic loss tanô of said crown decoupling rubbers, measured according to the same standard ASTM D 5992 - 96, at a temperature of 100°C and at 10 Hz, is at most equal to 0.06, so that the crown decoupling rubbers are optimized from the point of view of crown endurance, in particular cracking of the mixtures (or rubber) at the ends of the crown layers.

[0058] The elongation at break (or breaking) measurements are carried out at 100°C and in accordance with the French standard NF T 46-002 of September 1988. The breaking test specimens are of type H2 as described in the standard NF ISO 37 of March 1, 2012 with the exception of the size of the test specimen extracted from the tire which is a test specimen 40 mm long, 20 mm wide and 0.3 mm thick. The force to be exerted to obtain rupture is determined (breaking stress, in MPa (in N / mm)) and the elongation at break is measured (in %).

[0059] To protect all the crown layers (working layers, hooping layer, triangulation layer) from the hammering phenomenon caused by driving on stony ground, it is advantageous for a protective layer to have the greatest axial width of all the crown layers. Advantageously for civil engineering tires optimized against crown impacts, the reinforcing elements of at least one protective layer have a diameter at least equal to 2.5 mm, having an extension modulus at most equal to 100 GPa and the calendering mixture of said crown layer comprises natural rubber.

[0060] Likewise, for optimal operation, it is particularly interesting that the elastic modulus G' at 35% deformation, at 100°C and at 10 Hz, of the packing rubber top comprising at least 30 pce of silica, measured according to standard ASTM D 5992 - 96, or at most equal to 1.2 MPa.

[0061] For better optimization, it is also advantageous that the elastic modulus G' at 35% deformation, at 100°C and at 10 Hz, of the decoupling rubbers comprising at least 40 pce of silica, measured according to the ASTM D 5992 - 96 standard, is at most equal to 2.2 MPa

[0062] It is also advantageous for the decoupling rubbers to have a radial thickness of at least 1.5 mm.

[0063] The crown endurance can be further improved if the crown reinforcement comprises at least one hooping layer comprising metal reinforcements forming, with the circumferential direction, tangent to the circumference of the tire, an angle whose absolute value is at most equal to 10°, and whose axial width is at most equal to 0.7 times the axial width of the working layer of smaller axial width. The presence of such a hooping layer makes it possible to limit the rise during inflation of the crown of the tire and increases the effectiveness of the other characteristics of the invention.

[0064] The dynamic mechanical properties of the rubber compositions (rubbers, mixtures) are measured on glued test pieces extracted from the tire. Test pieces such as those described in the ASTM D 5992 - 96 standard (version published in September 2006, initially approved in 1996) in Figure X2.1 (circular embodiment) are used. The diameter "d" of the test piece is 10 mm [0 to + 0.04 mm], the thickness "L" of each of the portions of rubber composition is 2 mm [1.85- 2.20] if possible. A person skilled in the art will be able to choose and adapt the dimensions of the test piece according to the quantity of accessible and available mixture, particularly in the case of taking samples from a finished product such as the tire. These properties are measured on a Metravib VA4000 type viscoanalyzer. The terms complex, elastic and viscous moduli designate dynamic properties well known to those skilled in the art.The "complex modulus" G* is defined by the following relation: G*= (G'2 + G"2 ) in which G represents the elastic modulus and G" represents the viscous modulus. The phase angle 5 between the force and the displacement translated into dynamic loss tanô is equal to the ratio G” / G'. The response of a sample of vulcanized rubber composition subjected to a sinusoidal stress in alternating simple shear, at a frequency of 10 Hz with imposed stress, is recorded symmetrically around its equilibrium position. An accommodation of. the test piece is made prior to the temperature scanning measurement. The test piece is therefore subjected to sinusoidal shear at 10Hz, at 100% peak-peak strain, at 100°C.

[0065] The characteristics of the invention are illustrated by figures 1 and 2, schematic and not shown to scale, with reference to a tire of dimension 40.00R57.

[0066] Figures 1 and 2 represent meridian sections of the crown of tires (1) for heavy vehicles of the civil engineering type distinguished by particular geometries of the diffusion material (24). This is present only around the ends of the crown layers (311, 312, 321, 322, 331, 332) of the crown reinforcement (3) for Figure 1, whereas it is continuous over the entire axial width of the tread (2) for Figure 2. Of course, the invention is not limited to the only 2 geometric configurations represented.

[0067] Figures 1 and 2 therefore represent a tire crown comprising a crown reinforcement (3), radially inside a tread (2) and radially outside a carcass reinforcement (4). The crown reinforcement (3) comprises, radially from the outside to the inside, a protective reinforcement (31), a working reinforcement (32) and a hoop reinforcement (33). The protective reinforcement has two protective layers (311, 312) comprising elastic metal reinforcements coated in an elastomeric material or coating mixture, parallel to each other. The working reinforcement (32) comprises two working layers (321, 322) whose respective non-extensible metal reinforcements, coated in an elastomeric material, are parallel to each other and form, with the circumferential direction XX', equal angles of between 15° and 50°, and are crossed from one working layer to the next.The protective layer (312) is axially projecting relative to the working layer of greater axial width, here the radially innermost working layer (321). The hoop reinforcement (33) comprises two hoop layers (331, 332) whose respective metal reinforcements, coated in an elastomeric material, parallel to each other and forming, with the circumferential direction XX', an angle of between 5° and 10°, are crossed from one hoop layer to the next.

[0068] The tread comprises a contact material (23) intended to come into contact with the ground via a rolling surface (21) having an axial end E1 and a tread height H. The contact material is radially outside a diffusion material (24) radially inside a recommended wear limit (221) at the withdrawal here represented by a wear indicator in the bottom (222) of a groove or furrow (22) in figure 1.

[0069] A crown filler rubber (6) is arranged between the wider working layer and the carcass reinforcement. Decoupling rubbers (5) are arranged between the ends of the crown layers and the crown layer closest to the end in question. They comprise one or the other or both of the silica.

[0070] The tire delimits an internal cavity (7) intended to receive an inflation gas when the tire is mounted on a rim.

[0071] Figure 1 shows: - the distance (11) between the axial end El of the rolling surface (21) and the internal cavity (7) - the axial distance (112) from the axially outermost end (241) of the diffusion material (24) to the median circumferential plane (OXZ) passing through the center of the tread (2) - the axial distance (113) from the axial end El of the contact surface (21) to the median circumferential plane (OXZ). - the axial width (563) of the decoupling rubbers (5) and / or packing rubber (6) comprising silica between the axially outermost point (561) here of the decoupling rubbers (5) and the axially innermost point (562) here of the decoupling rubbers (5) comprising silica. - the axial width (233) (in the axial direction YY') of the diffusion material (24) between its two axial ends (241, 242). - crown decoupling rubbers (5) between the crown layers of the hoop reinforcement (33) which extend from the end of the crown layers and aggregate with each other to form a large volume in the shoulder area. The invention includes such an arrangement of the crown decoupling rubbers also for the other ends of the other crown layers.

[0072] Figure 2 shows the radial thickness (e) (in the axial direction ZZ') of the diffusion material (24) measured in a meridian plane (OXY) at the center of the tread.

[0073] The invention also works with only 5 top layers and a protective layer which would not be the top layer of greatest width among other possible architectural variations included by the invention.

[0074] Figures 1 and 2 represent only one example among others of the possible architectures of the civil engineering tire. The invention was tested or evaluated on tires of dimension 40.00R57 of commercial type "Michelin XDR3". The distance (11) between the axial end E1 of the rolling surface (21) and the internal cavity (7) in a meridian plane (OYZ) is equal to 217 mm, the tread height (H) is 116 mm. The radial thicknesses of the crown decoupling rubbers are of the order of 8 mm. The crown decoupling rubbers extend to form above the crown filling rubber a thickness of 35 mm. The crown filling rubber itself is approximately 35 mm thick. The radial thickness of the diffusion material is equal, near the axial end El of the rolling surface, to 48 mm and at the median plane, to 23 mm.

[0075] The tires according to the invention are compared to reference tires of the same size for each of the tests. The sculptures, the metallic reinforcement elements, the elastomeric materials are the same except for the diffusion material for the different tires and the materials of the crown filling rubber and the crown decoupling rubbers.

[0076] The RI reference tire includes standard compounds, without silica for the crown filler rubber and crown decoupling rubbers and without graphite for the diffusion material and having a thermal conductivity equal to 0.28 W / mK. Its curing time is the reference of the study and considered equal to 100.

[0077] The reference tires R2 and R3 are respectively identical and have for the crown filling rubber and crown decoupling rubbers mixtures comprising silica. The elastomeric materials of the crown filling rubbers of these tires have an elongation at break equal to 680% and a maximum dynamic loss tanS, of said rubber, at a temperature of 100°C at 10 Hz, is equal to 0.06. The elastomeric materials of the decoupling rubbers have an elongation at break equal to 600% and a maximum dynamic loss tanô, of said rubber, at a temperature of 100°C at 10 Hz, equal to 0.055. These properties were obtained by using silica as a reinforcing filler for each of the mentioned gums at rates of at least 30 pce (parts of filler per 100 parts of elastomer) for the crown decoupling gums and at least 40 pce for the crown filling gum.The R2 tire is baked by extending the baking time by approximately 10 minutes compared to the RL tire. The R3 tire is baked by increasing the baking temperature by 5° compared to the RI tire in order to compensate. the insulating properties of silica in tires, while maintaining the same curing time as for the RI tire. As expected, given the addition of silica for the crown filler rubbers and crown decoupling rubbers, the R2 and R3 tires are better in crown endurance, particularly in cracking of the compounds (or rubber) at the ends of the crown layers. The R2 tire is better in thermal and rolling resistance than the RI and R3 tires but at the cost of an increase in the use time of the industrial tool.

[0078] The tire according to the invention is identical to the RI or R2 tire except that the reference tires have as diffusion material a material without graphite, while the tires according to the invention have a diffusion material comprising 7.5 pce of expanded natural graphite for a thermal conductivity at least equal to 0.38 W / mK and on average equal to 0.46 W / mK. The tires according to the invention are cured at the temperature of the reference tire RI with the aim of reducing the energy required for curing and / or with a curing time 20 minutes shorter than the reference tire RI, with the aim of reducing the curing time required.

[0079] With these gains in cooking time and / or thermal energy gain, the tire according to the invention significantly improves its peak thermal performance although the hysteresis of the diffusion material of the tire according to the invention is significantly degraded by 6% compared to the diffusion material of the other 3 tires; which demonstrates the interest of the invention.

Claims

Claims 1. Tire (1) for a civil engineering vehicle delimiting an internal cavity (7) intended to receive an inflation gas when the tire is mounted on a rim, comprising: - a tread (2) intended to come into contact with a ground via a rolling surface (21) having an axial end E1, - the tread (2) having a recommended wear limit (221) upon withdrawal and comprising at least 2 elastomeric materials (23, 24), a first material (23), called contact material, intended to come into contact with the ground and at least a second elastomeric material (24) radially inside the recommended wear limit (221) and the contact material (23), called diffusion material, - a crown reinforcement (3), radially inside the tread (2) and radially outside a carcass reinforcement (4) comprising crown layers (311, 312, 321, 322, 331, 332) comprising metallic reinforcing elements coated in elastomeric materials, called calendering mixtures, the distance (11) between the axial end E1 of the tread surface (21) and the internal cavity (7) in a meridian plane (OYZ) being at least equal to 130 mm, the crown reinforcement (3) comprising at least one working layer (321, 322) whose reinforcing elements make an angle with the median circumferential plane or equator plane (OXZ) at least equal to 15°, on either side of the equator plane (OXZ), a filling rubber (6) called crown filling rubber (6) included between the axial end of the working layer of greater axial width (321) and the carcass reinforcement (4), composed of at least one elastomeric material,on either side of the equator plane (OXZ), so-called crown decoupling rubbers (5) located between the axial ends of the crown layers (311, 312, 321, 322, 331, 332) and the crown layer closest to said end, the decoupling rubbers (5) having a radial thickness at least equal to 0.5 mm, the elastomeric materials of the crown decoupling rubbers (5) comprising silica as a reinforcing filler at a rate of at least 40 pce and / or the elastomeric materials of the crown filling rubber (6) comprising silica as a reinforcing filler at a rate of at least 30 pce, characterized in that the thermal conductivity of the diffusion material (24) is at least equal to 0.35 W / mK, and in that the maximum radial thickness of the diffusion material (24) measured in a meridian plane (OYZ) is at least equal to 10 mm.

2. Tire (1) according to claim 1 wherein the axial distance (112) from the most axially outer end (241) of the diffusion material (24) to the median circumferential plane (OXZ) passing through the center of the tread (2), is at least equal to 90% of the axial distance (113) from the axial end El of the contact surface (21) to the median circumferential plane (OXZ).

3. Tire (1) according to claim 1 or 2, wherein on either side of the equator plane, the axial width (233) of the diffusion material (24) is at least equal to 80% of the axial width (563) of the decoupling rubbers (5) and / or packing rubber (6) comprising silica, this axial width being the axial width between the axially outermost point (561) of the decoupling rubbers (5) and / or packing rubber (6) comprising silica and the axially innermost point (562) of the decoupling rubbers (5) and / or packing rubber (6) comprising silica.

4. Tire (1) according to any one of the preceding claims, in which the diffusion material (24) is continuous between its most axially outer axial ends (241) located on either side of the equator plane, the radial thickness (e) of the diffusion material (24) measured in a meridian plane (OXY) at the center of the tread being at least equal to 10 mm.

5. Tire (1) according to any one of the preceding claims, wherein the maximum radial thickness of the diffusion material (24) measured in a meridian plane (OYZ) is at least equal to 15 mm, preferably at least equal to 40 mm.

6. Tire (1) according to any one of the preceding claims, in which the diffusion material (24) is based on at least one elastomeric matrix, fillers including graphite and a reinforcing filler and a vulcanization system, the elastomeric matrix comprising at least 50 pce of a diene elastomer chosen from the group of isoprene elastomers, butadiene elastomers and mixtures of these diene elastomers, the reinforcing filler mainly comprising a carbon black, the graphite having a crystallite size Le comprised in a range from 80 to 500 nm, more preferably from 90 to 400 nm, even more preferably from 100 to 300 nm, the total filler content being less than or equal to 60 pce.

7. Tire (1) according to claim 6, in which the graphite included in the diffusion material (24) has a BET specific surface area in a range from 10 to 50 m2 / g, preferably ranging from 15 to 40 m 2 / g, more preferably still ranging from 20 to 30 m 2 / g.

8. Tire (1) according to any one of the preceding claims, the crown filling rubber (6) comprising at least 30 pce of silica, in which the elongation at break at 100°C according to standard NF T 46-002, of the at least one elastomeric material composing the crown filling rubber (6) is at least equal to 650% and the maximum dynamic loss tanô, of said elastomeric material, measured according to standard ASTM D 5992 - 96, at a temperature of 100°C at 10 Hz, is at most equal to 0.

07.

9. Tire (1) according to any one of the preceding claims, the crown decoupling rubbers (5) comprising at least 40 pce of silica in which the elongation at break at 100°C according to standard NF T 46-002 of the crown decoupling rubbers (5) is at least equal to 500%, and the maximum dynamic loss tanô of said crown decoupling rubbers, measured according to the same standard ASTM D 5992 - 96, at a temperature of 100°C and at 10 Hz, is at most equal to 0.

06.

10. Tire (1) according to any one of the preceding claims, the crown filling rubber (6) comprising at least 30 pce of silica, in which the elastic modulus G' at 35% deformation at 100°C and at 10 Hz of the crown filling rubber (6), measured according to standard ASTM D 5992 - 96, is at most equal to 1.2 MPa.

11. Tire (1) according to any one of the preceding claims, decoupling rubbers (5) comprising at least 40 pce of silica, in which the elastic modulus G' at 35% deformation at 100°C and at 10 Hz of the decoupling rubbers (5), measured according to standard ASTM D 5992 - 96, is at most equal to 2.2 MPa.

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