optimized civil engineering pneumatic architecture

Abstract

A civil engineering tire (1) comprises a tread (2) consisting of at least two rubber compounds (23, 24), a first rubber compound (23) intended to contact the ground and a second rubber compound (24) radially inside it, and a crown reinforcement (3). On either side of the equator plane (OXZ), a crown packing compound (6) is located between the end of the widest working layer (321) and the carcass reinforcement (4), and crown decoupling compounds (5) are located between the axial ends of the crown layers (311, 312, 321, 322, 331, 332) and the nearest crown layer. The elastomeric materials of the decoupling compounds (5) or crown packing compounds include silica as a reinforcing filler. The thermal conductivity of the at least second rubbery compound (24) is at least 0.35 W / mK. (Shorthand 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 more particularly concerns the top of such a tire.

[0002] Radial tires intended to equip a heavy vehicle of the civil engineering type are designated as such in the sense of the standard of the European Tyre and Rim Technical Organisation (European Tyre and Rim Technical Organisation) or ETRTO.

[0003] For example, a radial tire for heavy-duty construction equipment, as defined in ETRTO 2020, is intended to be mounted on a rim with a diameter of at least 25 inches. The invention is more particularly intended for tires for large construction dump trucks and therefore for tires with a rim diameter of at least 57 inches.

[0004] Since a tire has a geometry of revolution about 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 denote 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 what follows, the expressions "radially inside" and "radially outside" respectively mean "closer" and "further" from the axis of rotation of the tire. By "axially inside" and "axially outside," respectively, we mean "closer" and "further" 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, a tire includes a tread, intended to come into contact with a ground via a tread surface, the two axial ends of which are connected via two sidewalls to 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 heavy-duty construction vehicles typically comprises at least one carcass layer including reinforcements, generally metallic, coated with a polymeric material of the elastomeric or elastomeric type, also known as a rubber compound, obtained by mixing and called calendering compound or calendering rubber. A carcass layer includes a main part, connecting the two beads and generally wrapping, within each bead, from the inside to the outside of the tire around a circumferential reinforcement element, most often metallic, called a bead, to form a inversion. The metallic reinforcements of a carcass layer are substantially parallel to each other and form an angle of between 80° and 90° with the circumferential direction.

[0009] The crown reinforcement of a radial tire for construction vehicles comprises a superposition of crown layers extending circumferentially, radially outside the carcass reinforcement. Each crown layer consists of reinforcements, generally metallic, parallel to each other and coated with a polymeric material of the elastomer type or a coating compound (or rubber).

[0010] Among the top layers, we usually distinguish the protective layers, which are part of the protective reinforcement and radially the outermost, and the working layers, which are part of the working reinforcement and radially located 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 aggressions, which may propagate through the tread radially towards the inside of the tire.

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

[0013] The working structure, often comprising at least two working layers, serves to encircle the tire and provide it with rigidity and road holding. It absorbs both mechanical inflation stresses, generated by the tire's inflation pressure and transmitted by the carcass reinforcement, and mechanical rolling stresses, generated by the tire rolling on a surface and transmitted by the tread. A recurring problem in tire crowns is cracking due to shear stresses, related to rolling, in the rubber compounds, crown coating compounds, or other compounds, at the crown edges. These cracks impact the tire's durability and reduce its lifespan. A conventional solution to prevent this cracking The goal is to decouple the crown layers, particularly the working layers at their axial ends, as shown in Figures 1 to 3 of document EP3297851. The crown reinforcement must also resist impacts and punctures, thanks to its inherent design, including its flexibility and, in particular, that of the protective reinforcement. Furthermore, the tire must have sidewall flex, or drift stiffness under transverse load, to ensure proper vehicle handling on curves.

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

[0015] To reduce the mechanical inflation stresses transmitted to the working reinforcement, it is known to place a shrink-fit reinforcement radially outside the carcass reinforcement. The shrink-fit reinforcement, whose function is to absorb at least some of the mechanical inflation stresses, improves the durability of the top reinforcement by stiffening it. The shrink-fit 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 confinement reinforcement may comprise two radially superimposed confinement layers formed of metallic reinforcements, parallel to each other in each layer and crossed from one layer to the next, forming angles of no more than 10° with the circumferential direction. Another embodiment of the confinement reinforcement consists of a circumferential winding of a confinement wire or a continuous confinement strip forming angles of no more than 5° with the circumferential direction.

[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 %), known as the force-elongation curve. From this force-elongation curve, tensile mechanical characteristics of the metal reinforcement are deduced, such as the structural elongation As (in %), the total elongation at break At (in %), the breaking strength Fm (maximum load in N), and the tensile strength Rm (in MPa), these characteristics being measured according to ASTM D 2969-04:2014.

[0018] The total elongation At of the metallic reinforcement is, by definition, the sum of its a Structural, elastic, and plastic elongations (At = As + Ae + Ap), particularly at break, where each elongation is non-zero. The structural elongation As results from the relative positioning of the metal wires constituting the reinforcement under a small tensile force. The elastic elongation Ae results from the elasticity of the metal wires constituting the reinforcement, considered individually, the metal's behavior following Hooke's law. The plastic elongation Ap results from the plasticity, that is, the irreversible deformation, beyond the elastic limit, of the metal wires considered 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] A tensile modulus, expressed in GPa, is also defined at every point on the force-stretch curve of a metallic reinforcement. This modulus represents the slope of the line tangent to the force-stretch curve at that point. In particular, the tensile modulus of the linear elastic portion of the force-stretch curve is called the elastic tensile modulus or Young's modulus.

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

[0021] An elastic metal reinforcement, in its unglued state, is characterized by a structural elongation As of at least 0.5% and a total elongation at break At of at least 3%. Furthermore, an elastic metal reinforcement has a tensile elastic modulus of at most 180 GPa, and is usually between 40 GPa and 150 GPa.

[0022] A non-stretchable 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-stretchable metal reinforcement has an elastic modulus in tension usually between 150 GPa and 200 GPa.

[0023] One method for significantly improving endurance is to increase not the stiffness of the apex rubbers but their elongation at break, even though this apex area operates under imposed deformation. For apex endurance, it is important to have certain minimum values ​​for the elongation at break of the apex elastomeric materials; these minimum values ​​vary depending on the position of said rubbers, specifically whether these elongations concern the apex packing rubber or the apex decoupling rubbers.

[0024] The tread block, referred to as the apex tread block, is located on either side of the equatorial plane (OXZ), or median circumferential plane OXZ, perpendicular to the axis of rotation of the tire and passing through the center of the tread, which is between The axial end of the working layer with the greatest axial width and the carcass reinforcement. Composed of at least one elastomeric material, it links the curved part of the carcass reinforcement under the shoulder and the apex reinforcement whose optimal shape is roughly parallel to the axis of rotation, notably to optimize shoulder wear or rolling resistance.

[0025] Decoupling rubbers are positioned between the ends of the top layers and the nearest top layer, or possibly the carcass layer for the innermost radially positioned top layer if this is not the widest working layer. They reduce shear stress at the ends of the top layers, particularly working layers with cross-bracing, which generate significant shear stress. They can extend from the ends of the top layers and aggregate with each other to form a substantial volume in the shoulder area.

[0026] As shown in document WO2023 / 110499 Al, rubbery compounds or Elastomeric materials exhibiting these desirable deformation-at-break properties contain silica. Applying different elongation-at-break requirements to either of these rubbers allows for improved peak endurance performance. In the cases relevant to this invention, peak failure often arises from crack junctions extending from the tips of the peak layers through the peak packing rubber. Preventing, delaying, or blocking crack propagation within the peak packing rubber results in a significant endurance gain. It is also important, when increasing the elongation-at-break values, not to increase the temperature at the peak; therefore, it is also necessary to limit the dynamic loss of either of these rubbers.

[0027] However, the use of such elastomeric materials containing silica, a highly thermally insulating material, requires an increased 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 especially 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 packing 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 construction vehicles defining an internal cavity intended to receive an inflation gas when the tire is mounted on a rim, comprising: - a tread designed to come into contact with a ground by via a tread surface having an axial end El, - the tread having a recommended wear limit at shrinkage and comprising at least 2 elastomeric materials, a first material, called the 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 the diffusion material, - a crown reinforcement, radially inside the tread and radially outside a carcass reinforcement comprising crown layers including metallic reinforcing elements coated in elastomeric materials, called calendering compounds, - the distance between the axial end El of the rolling surface and the internal cavity in a meridian plane (OYZ) being at least equal to 130 mm, - the top reinforcement comprising at least one working layer whose reinforcing elements form an angle with the median circumferential plane or equatorial plane (OXZ) of at least 15°, - on either side of the equatorial plane (OXZ), a packing rubber called the apex packing rubber, located between the axial end of the working layer with the greatest axial width and the carcass reinforcement, composed of at least one elastomeric material, - on either side of the equatorial plane (OXZ), so-called vertex decoupling gums located between the axial ends of the vertex layers and the vertex layer closest to said end, the decoupling gums being of a radial thickness at least equal to 0.5mm, - elastomeric materials of the apex decoupling gums comprising silica as a reinforcing filler at a rate of at least 40 parts per cubic meter and / or elastomeric materials of the apex packing gum comprising silica as a reinforcing filler at a rate of at least 30 parts per cubic meter, - 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 of overcoming the problem of overheating of off-road tires made of elastomeric materials containing silica by using a portion of the tread to better dissipate heat 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 that characterizes a material's ability to allow heat transfer by conduction. It represents the amount of heat transferred per unit area and time, under a Thermal conductivity is a temperature gradient of 1 Kelvin per meter. It is expressed in Wm⁻²·K⁻¹. Thus, a thermal conductivity of 1 Wm⁻²·K⁻¹ represents the amount of heat that propagates through a material by thermal conduction, across a surface area of ​​1 m², over a distance of 1 m. Thermal conductivity is measured at ambient temperature (23°C) on a Hotdisk TPS 2500 thermal analyzer with a 5501 type probe, according to ISO 22007-2:2015. The dimensions of the measuring specimen will be adapted by a person skilled in the art to the amount of rubber taken from the cured tire.

[0031] Such materials do not have the appropriate properties to constitute the part of the tread intended to come into contact with the ground, whether in terms of wear, rolling resistance, or resistance to damage. They also lack the crack resistance properties of silica compounds, and it is therefore not suitable for their use as crown filler or crown decoupling compound. However, they can be used between the contact material of the tread and the crown reinforcement.

[0032] This is effective insofar as the tire is intended for civil engineering use 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 barely perceptible. On the other hand, in civil engineering applications, the inventors were surprised to find that not only was it possible to maintain the same curing time as for a tire with silica-free crown sealant or crown decoupling sealant, but also with an improvement in the tire's thermal performance. A civil engineering tire can also be described as having a radial tread depth, measured from the tread surface to the bottom of the tread grooves, of at least 65 mm.The recommended removal limit refers, for example, to the limit indicated by wear indicators present in the tread pattern or by tire manufacturers' data providing specific recommendations for tire removal based on expert opinions; these recommendations are not necessarily indicated on the tire.

[0033] An advantageous solution is that the axial distance from the outermost axial 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, particularly due to the thickness of The crown layer is minimal at the center of the tread. Indeed, given two characteristics—namely, the greater local thickness of the tire and the presence of elastomeric materials containing silica—the diffusing elastomeric material should be positioned at the ends of the crown layers, whose dimensions are proportional to the axial width of the tread. This material allows for better local diffusion of the curing heat. This diffusion helps maintain the curing time, ensuring optimal curing of the elastomeric materials despite the presence of silica-filled elastomeric materials.

[0034] Advantageously, on either side of the equatorial plane, the axial width of the diffusion material is at least equal to 80% of the axial width of the decoupling and / or packing rubbers containing silica, this axial width being the axial width between the outermost axial point of the decoupling and / or packing rubbers containing silica and the innermost axial point of the decoupling and / or packing rubbers containing silica. This axial width of the diffusion material makes it possible to reduce the thermal insulation 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 equatorial plane. The radial thickness of the diffusion material, measured in a meridian plane OYZ at the center of the tread, is at least 10 mm. This configuration of the diffusion material appears to allow for better relative heating of the center and shoulders. With such a good conductor for an elastomeric material, the heat reaching the shoulder comes from the hot elements of the mold, but also more from the central part of the top, which heats up more quickly due to its approximately 30% thinner thickness. This minimal thickness allows for a minimal heat flux to have a noticeable effect.

[0036] Advantageously, the maximum radial thickness of the diffusion material measured in a meridian plane (OYZ) is at least 15 mm, preferably at least 40 mm. Starting from a maximum diffusion material thickness of 10 mm at the shoulder, it is possible to reduce the curing time by approximately ten minutes, and this reduction can be sustained up to a maximum diffusion material thickness of approximately 40 mm. Beyond this thickness, the gains in curing time and rolling resistance become more significant, exceeding 20 minutes for the curing time, even with higher hysteresis properties of the diffusion material than the reference tire material. For a diffusion material thickness of less than 10 mm at the shoulder, the inventors have not observed any significant measurable impact.

[0037] Advantageously, when the tread comprises branches including walls and a base, the diffusion material must be radially inside all the bases of the tread grooves 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 pc (percent parts of elastomer) of a diene elastomer selected from the group of isoprene elastomers, butadien elastomers and mixtures of these diene elastomers, the reinforcing filler comprising predominantly a carbon black, the graphite having a crystallite size Le in the range of 80 to 500 nm, more preferably 90 to 400 nm, more preferably 100 to 300 nm, the total filler content being less than or equal to 60 pc.

[0039] The term "diene" elastomer (or, indiscriminately, "rubber"), whether natural or synthetic, is to be understood, in a known manner, as 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 diene elastomers are copolymers, they are statistical polymers. Diene elastomers can be classified into two categories: "essentially unsaturated" or "essentially saturated".The term "essentially unsaturated" generally refers to a diene elastomer derived at least in part from conjugated diene monomers, having a proportion of diene motifs or units (conjugated dienes) greater than 15% (mol%). Thus, diene elastomers such as butyl rubbers or EPDM-type diene-alpha-olefin copolymers do not fall under the preceding definition and can be described, in particular, as "essentially saturated" diene elastomers (low or very low proportion of diene motifs, always less than 15% mol%). The diene elastomers usable 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 parts per cent of a diene elastomer selected from the group of isoprene elastomers, butadien elastomers, and mixtures of these diene elastomers. For example, the elastomeric matrix may comprise an isoprene elastomer or a mixture of isoprene elastomers, a buta- diene elastomers, a mixture of butadien elastomers, or a mixture of isoprene and butadien elastomers. These diene elastomers must have a diene motif or unit content (conjugated dienes) exceeding 15 mol%. Thus, the elastomeric matrix of the diffusion material composition must comprise at least 50 parts per cent of a diene elastomer with a diene motif or unit content (conjugated dienes) exceeding 15 mol%; this elastomer must be selected from the group consisting of isoprene elastomers, butadien elastomers, and mixtures of these diene elastomers. In addition to the aforementioned diene elastomers, the elastomeric matrix may also include thermoplastic elastomers.

[0041] The elastomeric composition of the diffusion material comprises fillers, including a reinforcing filler and graphite. The term "filler" here refers to any type of filler, whether reinforcing, non-reinforcing, or inert. Reinforcing fillers include carbon black and inorganic reinforcing fillers such as silica or alumina. Non-reinforcing or inert fillers include 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 reinforcing and non-reinforcing filler content) in the elastomeric composition of the diffusion material is less than or equal to 60 parts per million (ppm), preferably less than or equal to 55 ppm, preferably less than or equal to 50 ppm, preferably within a range of 20 to 55 ppm, and more preferably within a range of 25 to 50 ppm. At this total filler content, a good compromise of resistance to damage, stiffness, deformation at break, and thermal conductivity is obtained, making it suitable for use in the tread.

[0042] The elastomeric composition of the diffusion material includes a reinforcing filler, this reinforcing filler being predominantly carbon black. In addition to carbon black, which is the predominant reinforcing filler, the elastomeric composition of the diffusion material may optionally also include a second reinforcing filler such as silica, for example. The total reinforcing filler content, that is, the sum of the carbon black content and the silica content when present, is less than or equal to 55%, more preferably less than or equal to 50%, more preferably less than or equal to 45%, more preferably less than or equal to 44%. This reinforcing filler content is advantageously greater than or equal to 20%, more preferably greater than or equal to 25%, more preferably greater than or equal to 30%.Preferably, the rate of reinforcing fillers in the elastomeric composition is within a range. ranging from 20 pc to 55 pc, more preferably from 25 pc to 50 pc, more preferably from 30 pc to 45 pc.

[0043] Preferably, for the diffusion material, carbon black represents more than 55% by weight of the total weight of the reinforcing filler, more preferably more than 60% by weight, more preferably 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 ratio is less than or equal to 55 pc, more preferably less than or equal to 50 pc, more preferably less than or equal to 45 pc 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] More preferably, the rate of reinforcing fillers in the elastomeric composition of the diffusion material is in a range of 20 pc to 55 pc, more preferably from 25 pc to 50 pc, more preferably from 30 pc to 45 pc and carbon black represents more than 55% by weight of the total weight of the reinforcing filler, more preferably more than 60% by weight, more preferably more than 80% by weight, more preferably still represents 100% by weight of the total weight of the reinforcing filler.

[0046] The diffusion material composition includes 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 stack of graphene sheets, graphene being a sheet of atom-thickness in which the carbon atoms are arranged in a predominantly 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 notably be obtained by a complex process of heating 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, denoted Le, in the range of 80 to 500 nm, more preferably from 90 to 400 nm, and even more preferably from 100 to 300 nm. The crystallite size is determined by X-ray diffraction according to the ASTM D5187-10 method adapted by those skilled in the art for samples from tires.

[0049] Graphite suitable for the diffusion material may have a BET specific surface area in the range of 10 to 50 m² / g; preferably from 15 to 40 m² / g, and even more preferably from 20 to 30 m² / 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 / pO₂ = 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 capacity can be determined. Based on the cross-sectional area of ​​the nitrogen molecule, the monolayer capacity, and the sample weight, the specific surface area can then be calculated.

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

[0051] The graphite usable for the diffusion material may have an apparent density (Scott density) greater than or equal to 0.10 g / cilft, preferably greater than 0.12 g / cm3. The apparent density, or Scott density, is determined by passing the dry graphite powder through the Scott volumemeter according to ASTM B329-98 (2003).

[0052] Preferably the graphite usable within the framework of the present invention is expanded graphite.

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

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

[0055] Surprisingly, the inventors identified that the use of the elastomeric material in the cited ranges, with a staggered trade-off of stiffness, strain at break and thermal conductivity properties, as a diffusion material, leads to a performance trade-off 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 parts per cent of silica composing the top packing 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 top sealant containing a percentage of silica making it insulating, but, for optimal performance, this same top sealant 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 top decoupling gums comprising at least 40 pce of silica is at least equal to 500%, and the maximum dynamic loss tanô of said top decoupling gums, 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 top decoupling gums are optimized from the point of view of top endurance in particular of the cracking of the mixtures (or gum) at the ends of the top layers.

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

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

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

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

[0062] It is also advantageous that the decoupling gums have a radial thickness of at least 1.5mm.

[0063] The endurance of the apex can be further improved if the apex reinforcement includes at least one layer of reinforcement comprising metallic reinforcements forming, with the circumferential direction tangent to the circumference of the tire, an angle whose absolute value is at most 10°, and whose axial width is at most 0.7 times the axial width of the working layer with the smallest axial width. The presence of such a compression layer limits the rise of the tire's crown during inflation and increases the effectiveness of the other features of the invention.

[0064] The dynamic mechanical properties of rubber compositions (rubbers, compounds) are measured on bonded specimens extracted from the tire. Specimens such as those described in ASTM D 5992-96 (version published in September 2006, originally approved in 1996) in Figure X2.1 (circular embodiment) are used. The diameter "d" of the specimen is 10 mm [0 to +0.04 mm], and the thickness "L" of each portion of the rubber compound is 2 mm [1.85-2.20] if possible. Those skilled in the art will be able to select and adapt the dimensions of the specimen according to the amount of accessible and available compound, particularly in the case of specimens taken from a finished product such as the tire. These properties are measured on a Metravib VA4000 viscoanalyzer. The terms complex, elastic and viscous moduli refer to dynamic properties well known to those skilled in the art.The "complex modulus" G* is defined by the following relation: G* = ^G'2 4- G”2 ) where G' represents the elastic modulus and G” represents the viscous modulus. The phase angle θ between the force and the displacement, translated into dynamic loss tanθ, is equal to the ratio G” / G'. The response of a vulcanized rubber composition sample subjected to sinusoidal alternating simple shear loading at a frequency of 10 Hz with imposed stress, symmetrically around its equilibrium position, is recorded. The specimen is accommodated prior to the temperature sweep measurement. For this purpose, the specimen is subjected to sinusoidal shear loading at 10 Hz, at 100% peak-to-peak strain, at 100°C.

[0065] The features of the invention are illustrated by Figures 1 and 2, which are schematic and not shown to scale, with reference to a tire of size 40.00R57.

[0066] Figures 1 and 2 show meridional cross-sections of the crown of heavy-duty construction vehicle tires (1) differing in the geometries of the diffusion material (24). This material is present only around the ends of the crown layers (311, 312, 321, 322, 331, 332) of the crown reinforcement (3) for [Fig. 1], whereas it is continuous across the entire axial width of the tread (2) for [Fig. 2]. Of course, the invention is not limited to the two geometric configurations shown.

[0067] Figures 1 and 2 therefore represent a tire apex comprising a A crown reinforcement (3) is radially internal to a tread (2) and radially external to a carcass reinforcement (4). The crown reinforcement (3) comprises, radially from the outside in, a protective reinforcement (31), a working reinforcement (32), and a reinforcing reinforcement (33). The protective reinforcement has two protective layers (311, 312) comprising elastic metal reinforcements embedded 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, embedded in an elastomeric material, are parallel to each other and form, with the circumferential direction XX', equal angles between 15° and 50°, and are intersected from one working layer to the next.The protective layer (312) is axially overhanging with respect to the working layer of greatest axial width, here the innermost radially working layer (321). The confinement reinforcement (33) comprises two confinement layers (331, 332) whose respective metallic reinforcements, embedded 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 confinement layer to the next.

[0068] The tread comprises a contact material (23) intended to come into contact with the ground via a tread surface (21) having an axial end El and a tread height H. The contact material is radially external to a diffusion material (24) radially internal to a recommended wear limit (221) to shrinkage here represented by a wear indicator in the bottom (222) of a groove or channel (22) on the [Fig. 1].

[0069] A top-filling gum (6) is disposed between the wider working layer and the frame reinforcement. Decoupling gums (5) are disposed between the ends of the top layers and the top layer nearest to the end in question. They comprise either or both silica.

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

[0071] Fig. 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 outermost 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). - the axial width (563) of the decoupling rubbers (5) and / or packing rubber (6) comprising silica between the outermost axial point (561) here of the decoupling gums (5) and the innermost axial point (562) here of the decoupling gums (5) comprising silica. - the axial width (233) (in the axial direction YY') of the diffusion material (24) between its two axial ends (241, 242). - top decoupling rubbers (5) between the top layers of the shrink frame (33) which extend from the end of the top layers and aggregate with each other to form a significant volume in the shoulder area. The invention includes such an arrangement of the vertex decoupling gums also for the other ends of the other vertex layers.

[0072] Fig. 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 widest top layer among other possible architectural variants 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 40.00R57 commercial tires of the "Michelin XDR3" type. The distance (11) between the axial end E1 of the tread surface (21) and the internal cavity (7) in a meridian plane (OYZ) is 217 mm, and the tread height (H) is 116 mm. The radial thicknesses of the crown decoupling rubbers are approximately 8 mm. The crown decoupling rubbers extend to form a thickness of 35 mm above the crown packing rubber. The crown packing 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 level of 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 tread patterns, the metallic reinforcement elements, and the elastomeric materials are the same except for the diffusion material for the different tires and the materials of the crown sealant and crown decoupling sealants.

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

[0077] Reference tires R2 and R3 are respectively identical and have compound top-filling rubber and top-release decoupling rubber including silica. The elastomeric materials of the crown sealant of these tires have an elongation at break of 680% and a maximum dynamic loss tanô of said sealant at a temperature of 100°C at 10 Hz of 0.06. The elastomeric materials of the decoupling sealant have an elongation at break of 600% and a maximum dynamic loss tanô of said sealant at a temperature of 100°C at 10 Hz of 0.055. These properties were obtained by using silica as a reinforcing filler for each of the aforementioned sealant at a ratio of at least 30 parts per 100 parts of elastomer for the crown decoupling sealant and at least 40 parts per 100 parts of elastomer for the crown sealant.The R2 tire is cured by extending the curing time by approximately 10 minutes compared to the RL tire. The R3 tire is cured by increasing the curing temperature by 5°C compared to the RI tire in order to compensate for the insulating properties of silica in the tires, while maintaining the same curing time as the RL tire. As expected, given the addition of silica to the crown filler and crown decoupling rubbers, the R2 and R3 tires offer superior crown durability, particularly in terms of compound (or rubber) cracking at the crown layer edges. The R2 tire offers better thermal performance and rolling resistance than the RI and R3 tires, but at the cost of increased production equipment usage time.

[0078] The tire according to the invention is identical to the RI or R2 tire except that the reference tires have a diffusion material that is graphite-free, while the tires according to the invention have a diffusion material comprising 7.5 parts per cubic inch of expanded natural graphite with a thermal conductivity of at least 0.38 W / mK and on average 0.46 W / mK. The tires according to the invention are cured at the temperature of the reference RI tire in order to reduce the energy required for curing and / or with a curing time 20 minutes shorter than that of the reference RI tire, with the aim of reducing the required curing time.

[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 3 other tires; which demonstrates the interest of the invention.

Claims

1. Demands Tire (1) for construction vehicles 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 running surface (21) having an axial end El, - the tread (2) having a recommended wear limit (221) at the point of withdrawal and comprising at least 2 elastomeric materials (23, 24), a first material (23), called the 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 the diffusion material, - a crown reinforcement (3), radially internal to the tread (2) and radially external to a carcass reinforcement (4) comprising crown layers (311, 312, 321, 322, 331, 332) comprising metallic reinforcing elements embedded in elastomeric materials, known as calendering compounds, - the distance (11) between the axial end El 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) of at least 15°, - on either side of the equatorial plane (OXZ), a packing rubber (6) called the top packing rubber (6) included between the axial end of the working layer of greatest axial width (321) and the carcass reinforcement (4), composed of at least one elastomeric material, - on either side of the equatorial plane (OXZ), so-called vertex decoupling gums (5) located between the axial ends of the vertex layers (311, 312, 321, 322, 331, 332) and the vertex layer closest to said end, the decoupling gums (5) being of a radial thickness at less than or equal to 0.5mm, - the elastomeric materials of the top decoupling gums (5) comprising silica as a reinforcing filler at a rate of at least 40 pce and / or the elastomeric materials of the top packing gum (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 outermost axial 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. Pneumatic (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 gums (5) and / or packing gum (6) comprising silica, this axial width being the axial width between the outermost axial point (561) of the decoupling gums (5) and / or packing gum (6) comprising silica and the innermost axial point (562) of the decoupling gums (5) and / or packing gum (6) comprising silica.

4. Tire (1) according to any one of the preceding claims, wherein the diffusion material (24) is continuous between its outermost axial ends (241) located on either side of the equatorial 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. Pneumatic (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, of preferably at least equal to 40 mm.

6. Pneumatic (1) according to any one of the preceding claims, wherein 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 parts per anth of a diene elastomer selected from the group of isoprene elastomers, butadien elastomers and mixtures of these diene elastomers, the reinforcing filler comprising predominantly carbon black, the graphite having a crystallite size Le in the range of 80 to 500 nm, more preferably in the range of 90 to 400 nm, even more preferably in the range of 100 to 300 nm, the total filler content being less than or equal to 60 parts per anth.

7. Pneumatic (1) according to claim 6, wherein the graphite included in the diffusion material (24) has a specific surface area BET in the range of 10 to 50 m2 / g, preferably in the range of 15 to 40 m2 / g, more preferably in the range of 20 to 30 m2 / g.

8. Pneumatic (1) according to any one of the preceding claims, the top packing rubber (6) comprising at least 30 parts per cent of silica, wherein the elongation at break at 100°C according to standard NF T 46-002, of at least one elastomeric material composing the top packing 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. Pneumatic (1) according to any one of the preceding claims, the apex decoupling gums (5) comprising at least 40 parts per cent of silica in which the elongation at break at 100°C according to standard NF T 46-002 of the apex decoupling gums (5) is at least equal to 500%, and the maximum dynamic loss tanô of said apex decoupling gums, 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. Pneumatic (1) according to any one of the preceding claims the top packing rubber (6) comprising at least 30 pc of silica, wherein the elastic modulus G' at 35% strain at 100°C and 10 Hz of the top packing rubber (6), measured according to ASTM D 5992-96, is at most equal to 1.2 MPa.

11. Pneumatic (1) according to any one of the preceding claims, decoupling rubbers (5) comprising at least 40 parts per cent of silica, wherein the elastic modulus G' at 35% strain at 100°C and 10 Hz of the decoupling rubbers (5), measured according to ASTM D 5992-96, is at most equal to 2.2 MPa.

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