optimized civil engineering pneumatic architecture

Abstract

A tire (1) for a civil engineering vehicle comprising a tread (2) intended to come into contact with the ground via a tread surface (21) having an axial end E1, the distance (11) between the axial end E1 of the tread surface and the internal cavity (5) in a meridian plane being at least equal to 120 mm. The tread (2) comprises 2 rubber compounds (23, 24), a first material, intended to come into contact with the ground and a second (24), called diffusion material, radially inside the contact material (23). The thermal conductivity of the second rubber compound (24) is at least equal to 0.35 W / mK. Abstract figure: figure 1.

Description

Title of the invention: 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 vehicle of the civil engineering type 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 relative 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", respectively "radially outward" mean "closer", respectively "further from the axis of rotation of the tire". By "axially inward", respectively "axially outward", is meant "closer", respectively "further from the equatorial plane of the tire", the equatorial plane of the tire being the plane passing through the middle of the rolling 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 mixture 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 reinforcing 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 type 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 reinforcement, often comprising at least two working layers, has the function of surrounding the tire and giving it rigidity and road holding. It absorbs both the mechanical stresses of inflation, generated by the inflation pressure of the tire and transmitted by the carcass reinforcement, and the mechanical stresses of rolling, generated by the rolling of the tire on the ground and transmitted by the tread. A recurring problem in tire crowns is rolling resistance and thermal resistance given the thicknesses of these tires due to very large tread thicknesses linked to the size of the tires and the users' need for durability.

[0014] The working reinforcement usually comprises two working layers, radially su perposed, 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 flexion on edge for acceptable vehicle behavior.

[0015] To reduce shear and reduce thermal stress with 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 take up 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 hoop layers, radially superimposed, 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 hoop strip 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 standard ASTM D 2969-04 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 al lengths 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] We also define, at any point on the force-elongation curve of a metal reinforcement, an extension modulus, expressed in GPa, which represents the slope of the line tangent to the force-elongation curve at this point. In particular, the elastic extension modulus or Young's modulus is the extension modulus of the linear elastic part of the force-elongation curve.

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

[0021] An elastic metal reinforcement, in its unrubberized 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] Another well-known method for improving rolling resistance and crown thermal is to lower the hysteresis of the crown materials, filler rubber, edge rubber placed at the end of the crown layers, tread, calendering mixtures, without being exhaustive. Using more hysteretic materials at the crown to gain rolling resistance is, however, totally counter-intuitive.

[0024] The inventors aimed to improve the rolling resistance performance of civil engineering tires by using more thermally conductive elastomeric materials in the tread even if their hysteresis is degraded.

[0025] 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 the ground via a rolling surface having an axial end El, - the tread having a recommended wear limit upon removal and comprising at least 2 rubber compounds, a first rubber compound, called contact material, intended to come into contact with the ground and at least a second compound rubbery, called diffusion material, radially inside the recommended wear limit and the contact material, - a crown reinforcement, radially inside the tread and radially outside a carcass reinforcement, - the distance between the axial end El of the rolling surface and the internal cavity in a meridian plane, being at least equal to 120 mm, - the thermal conductivity of the diffusion material being at least equal to 0.35 W / mK, - the maximum radial thickness of the diffusion material measured in a meridian plane (OYZ) being at least equal to 10 mm.

[0026] The invention consists of using a portion of the tread to better diffuse calories thanks to a particularly high conductivity, at least equal to 0.35 W / mK. This would tend to improve the curing of the tire and the general performance of the rubber compounds of the tire. Indeed, the gain in rolling resistance obtained was counter-intuitive and the inventors assume that the phenomena of curing state gradients of the rubber compounds in the tire are significantly lowered.Usually, with civil engineering tires, given the different thicknesses from the crown to the shoulder (area near the axial end of the tread) as in the center, for the rubber compounds at the heart of the tire to be cured (sufficiently crosslinked), it is inevitable that the rubber compounds in contact with the heating elements of the mold are in a higher and therefore less than optimal state of curing than those in the center, except perhaps by curing at a low temperature but for such a long time that this solution is unusable industrially. By placing a highly conductive rubber compound near the heating elements, in particular the parts of the mold creating the grooves, the curing gradient must be sufficiently reduced so that the effect is measurable on the tire.

[0027] On the other hand, such highly conductive materials do not have the appropriate properties to constitute the part of the tread intended to come into contact with the ground, either in terms of wear, rolling resistance, or resistance to aggression. They also do not have the crack resistance properties of the mixtures used near the end of the crown layers. 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.

[0028] This is effective to the extent that the tire is intended for civil engineering and the distance between the axial end El of the rolling surface and the internal cavity in a meridian plane (OYZ) is at least equal to 130 mm. For larger tires small as heavy goods vehicle tires, whose curing time is sometimes 10 times shorter, the effect obtained would be imperceptible. On the other hand, in civil engineering, inventors were surprised to note an improvement in rolling resistance despite an increase in the hysteresis of the rubber compound used. A civil engineering tire can also be described as having a radial tread height, measured from the rolling surface to the bottom of the groove, at least equal to 65 mm. By recommended shrinkage limit we mean, for example, the limit given by wear indicators present in the tread or data from tire manufacturers giving specific recommendations for shrinkage of the tire based on expert opinions, these recommendations not necessarily being indicated on the tire.

[0029] An advantageous solution is that the axial distance from the axially outermost 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 E1 of the contact surface to the median circumferential plane (OXZ). Indeed, taking into account two characteristics, namely the greater local thickness of the tire and sometimes the presence of elastomeric materials comprising silica, as in document WO2023 / 110499 A1, it is at the end of the crown layers whose dimension is proportional to the axial width of the tread, that the elastomeric diffusion material should be positioned which will locally allow better diffusion of the baking heat.This diffusion makes it possible to maintain the curing time or even reduce it to achieve optimal curing of the elastomeric materials despite the local thickness of the tire. The invention works even if the diffusion material does not constitute a continuous layer from one shoulder to the other on either side of the meridian plane, because the optimum curing time of a tire is determined by that of its thickest zones and therefore for the crown, that of the shoulder.

[0030] Interestingly, the diffusion material is continuous between its axial ends located on either side of the equator plane (at the median circumferential plane (OXZ) passing through the center of the tread), the radial thickness of the diffusion material measured in a meridian plane (OXY) at the center of the tread is at least equal to 10 mm, preferably at least equal to 15 mm. 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. This configuration of the diffusion material seems to allow better relative curing of the center and the shoulders. With such a good conductor material 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 top which rises in temperature more quickly due to its lower thickness of around 30%. A minimal thickness allows a minimal thermal flow to have a visible effect.

[0031] 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.

[0032] Advantageously, when the tread comprises grooves comprising walls and a bottom, the diffusion material is 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.

[0033] Advantageously, the contact material comprises at least 30 pce of silica. Given its insulating properties, contact materials comprising at least 30 pce of silica are complicated to use for civil engineering. They allow if they have lower hysteretic properties than carbon black-based contact materials, which is often the case if they are formulated for this purpose. They therefore allow the rolling resistance to be reduced, but their insulating properties prevent the calories produced at the shoulder block from being evacuated. Tests have shown that the reduction in rolling resistance brought about by these contact materials was accompanied by an increase in the shoulder temperature of 10°C, reducing the endurance of the tire below what is reasonable. Furthermore, at a constant curing temperature, silica-based contact materials require a longer curing time of almost 30 minutes.The use of a diffusion material as defined in the invention based on graphite makes it possible not to be penalized in terms of cooking time or shoulder temperature and therefore to benefit from the interest in rolling resistance of silica-based materials as contact materials.

[0034] The conductivity level 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 (percent part 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 in a range from 80 to 500 nm, more preferably from 90 to 400 nm, more preferably from 100 to 300 nm, the total filler content being less than or equal to 60 pce.

[0035] By “diene” elastomer (or indistinctly rubber), whether natural or synthetic, must be understood, in a known manner, an elastomer consisting at least in part (i.e., a homopolymer or a copolymer) of diene monomer units (monomers carrying two carbon-carbon double bonds, conjugated or not). 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 that 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. .

[0036] The elastomeric matrix of the elastomeric composition of the diffusion material comprises at least 50 pce of a diene elastomer chosen 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 (conjugated dienes) 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 patterns or units 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.

[0037] 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 of the silica or alumina type. 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.

[0038] The elastomeric composition of the diffusion material comprises a reinforcing filler, this reinforcing filler comprising predominantly a carbon black. In addition to carbon black which is predominantly as 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 it is 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 within a range from 20 pce to 55 pce, more preferably from 25 pce to 50 pce, more preferably from 30 pce to 45 pce.

[0039] 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.

[0040] Thus, preferably, for the diffusion material, the rate of reinforcing filler is less than or equal to 55 pce, more preferably less than or equal to 50 pce, more preferably less than or equal to 45 pce 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.

[0041] 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.

[0042] 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.

[0043] Graphite is generally understood to mean a set of stacked graphene planes, graphene being a sheet of atomic thickness in which the carbon atoms are organized in an essentially hexagonal network. 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 temperature. 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 consideration in the calculation of the total filler content.

[0044] The graphite usable for the diffusion material has a crystallite size noted Le within a range from 80 to 500 nm, more preferably from 90 to 400 nm, more preferably from 100 to 300 nm.

[0045] The graphite that can be used for the diffusion material can have a BET specific surface area ranging from 10 to 50 m2 / g; preferably ranging from 15 to 40 m2 / g, more preferably still ranging from 20 to 30 m2 / g.

[0046] The graphite usable in this context may have a particle size distribution D90 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.

[0047] The graphite usable for the diffusion material may have a density apparent (Scott density) greater than or equal to 0.10 g / cm3, preferably greater than 0.12 g / cm3.

[0048] Preferably, the graphite that can be used in the context of the present invention is an expanded graphite.

[0049] 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.

[0050] Preferably, the mass ratio of graphite relative 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.

[0051] Surprisingly, the inventors identified that the use of the elastomeric material in the cited ranges, having a shifted compromise of stiffness, strain at break and thermal conductivity properties, as diffusion material, brings about a performance compromise greater than expected.

[0052] 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 Multhnolecular Layers, J. Am. Chem. Soc, 1938, 60, 309-319), the monolayer capacity can be determined. Based on the cross-section of the nitrogen molecule, the monolayer capacity and the weight of the sample, the specific surface area can then be calculated.

[0053] Apparent density, or Scott density, is determined by passing dry graphite powder through the Scott volumeter according to ASTM B329-98 (2003).

[0054] 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.

[0055] The thermal conductivity of a material is a physical quantity that characterizes the ability of a material to allow heat transfer by conduction. It represents the quantity of heat transferred per unit area and time, under a temperature gradient of 1 degree Kelvin and per meter. It is expressed in Wm *.K *. Thus a thermal conductivity of 1 Wm '.K ' represents the quantity of heat that propagates through a material by thermal conduction, through a surface area of ​​1 m2, over a distance of 1 m. The thermal conductivity is measured at room temperature (23°C) on a Hotdisk TPS 2500 thermal analyzer with a type 5501 probe, according to the ISO 22007-2: 2015 standard. The dimensions of the measuring specimen will be adapted by a person skilled in the art to the quantity of rubber taken from the cured tire.

[0056] To protect all the top layers (working layers, top layer, hooping, triangulation layer) of the hammering phenomenon caused by rolling 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 for this use 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.

[0057] The endurance of the crown can be 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 ATE 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.

[0058] 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 know how to choose and adapt the dimensions of the test piece according to the quantity of accessible and available mixture, in particular in the case of taking samples from a finished product such as the tire. These properties are measured on a viscoanalyzer of the Metravib VA4000 type. 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 relationship: G* — ^C2 + G"2 ) in which G' represents the elastic modulus and G" represents the viscous modulus. The phase angle ô 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 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 specimen is carried out prior to the temperature scanning measurement. The specimen is therefore subjected to sinusoidal shear at 10 Hz, at 100% peak-peak strain, at 100°C.

[0059] The characteristics of the invention are illustrated by [Fig.l] schematically and not shown to scale, with reference to a tire of size 40.00R57.

[0060] [Fig.l] represents the meridian section of a tire crown (1) for a heavy vehicle of the civil engineering type comprising a diffusion material (24) between the contact material (23) and the crown reinforcement (3). Of course, the invention is not limited to this single geometric configuration represented, one can imagine for example and in a non-exhaustive manner several contact materials, a radial thickness of diffusion material varying axially several diffusion materials...

[0061] [Fig.l] therefore represents the crown of a tire 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 comprising elastic metal reinforcements coated in an elastomeric material or coating mixture, parallel to each other. The working reinforcement (32) comprises two working layers 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.A protective layer is axially projecting relative to the working layer of greater axial width, here the radially innermost working layer. The hoop reinforcement (33) comprises two hoop layers 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.

[0062] 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) on withdrawal here represented by a wear indicator in the bottom (222) of a groove or furrow (22).

[0063] The tire delimits an internal cavity (5) intended to receive an inflation gas when the tire is mounted on a rim.

[0064] [Fig.l] shows: - the distance (11) between the axial end El of the rolling surface (21) and the internal cavity (5) - the axial distance (112) from the axial 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).

[0065] The invention also works with only 5 top layers and a protection layer which would not be the top layer of greatest width among other possible architectural variants included by the invention.

[0066] [Fig.l] represents 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 El of the tread surface (21) and the internal cavity (7) in a meridian plane (OYZ) is equal to 217 mm, the tread height (H) is equal to 97 mm. The radial thickness of the diffusion material is equal, near the axial end El of the tread surface, to 48 mm and at the median plane, to 23 mm.

[0067] 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.

[0068] The reference tire RI comprises a standard mixture for diffusion material, without graphite having a thermal conductivity equal to 0.28 W / mK and a maximum dynamic loss tanô, at a temperature of 100°C at 10 Hz, is equal to 0.052, as a usual measure of its hysteretic performance. The tire RI has as contact material for a mine for hard soils and high thermal, namely that the contact material has a maximum dynamic loss tanô measured according to the same standard ASTM D 5992 - 96, at a temperature of 100°C and at 10 Hz, equal to 0.063 and an elastic modulus G' at 35% deformation at 100°C and at 10 Hz, measured according to the standard ASTM D 5992 - 96 equal to 1.141 MPa.

[0069] The tire according to the invention is identical to the RI tire except that the diffusion material is a material comprising 7 pce of graphite for an anisotropic thermal conductivity at least equal to 0.38 W / mK and on average equal to 0.46 W / mK and has a maximum dynamic loss tanô, at a temperature of 100°C at 10 Hz, equal to 0.055. The curing time was adapted according to the thermal conductivity characteristics of the diffusion material and was reduced by approximately 40 minutes.

[0070] The rolling resistance measurements surprisingly give an improvement in the performance of the tire according to the invention at 10% of the RI tire and at least 10° in thermal crown 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 RL tire. This demonstrates the interest of the invention.

Claims

Claims

1. A tire (1) for a civil engineering vehicle delimiting an internal cavity (5) intended to receive an inflation gas when the tire is mounted on a rim, comprising: - a tread (2) intended to come into contact with the 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 rubber compounds (23, 24), a first rubber compound (23), called contact material, intended to come into contact with the ground and at least one second rubber compound (24), called diffusion material, radially inside the recommended wear limit (221) and the contact material (23), - a crown reinforcement (3), radially inside the tread (2) and radially outside a carcass reinforcement (4),- the distance (11) between the axial end El of the rolling surface (21) and the internal cavity (5) in a meridian plane, being at least equal to 120 mm, - 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. A tire according to claim 1 in which the radial height (H) of the tread pattern is at least equal to 65 mm.

3. A tire according to claim 1 or 2, the tread comprising grooves (22) comprising walls and a bottom (222) in which the diffusion material (24) is radially inside all the bottoms (222) of the grooves (22) of the tread (2).

4. Tire (1) according to any one of the preceding claims, wherein the radial thickness (e) of the diffusion material (24) measured in a meridian plane (OYZ) at the center of the tread is at least equal to 10 mm, preferably at least equal to 15 mm.

5. A 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. A tire (1) according to any one of the preceding claims wherein the axial distance (112) from the axially outermost end 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).

7. A tire (1) according to any preceding claim, wherein the contact material comprises at least 30 phr of silica.

8. Tire (1) according to any one of the preceding claims, in which the diffusion material (24) is based at least on an 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, more preferably from 100 to 300 nm, the total rate of fillers being less than or equal to 60 pce.

9. Tire (1) according to claim 8, wherein the total charge rate of the diffusion material (24) is less than or equal to 55 pce, preferably less than or equal to 50 pce, preferably within a range from 20 to 55 pce, more preferably within a range from 25 to 50 pce.

10. Tire (1) according to claims 8 or 9, 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 from 15 to 40 m2 / g, more preferably still from 20 to 30 m2 / g.

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