Optimized structure of civil engineering tire

Abstract

The invention relates to a radial tyre for a mining vehicle having a rim diameter of at least 24 inches and a nominal load of at least 8000 kg, said tyre comprising a tread (22) having a radial thickness E of at least 0.04 times the outer radius of the tyre. The tread comprises two rubber compositions constituting at least 90% of the thickness E. The first composition (221) radially inside the second composition (222) constitutes at least 40% and at most 66% of the thickness E; the secant modulus at 23 DEG C under 10% deformation is greater than 5.2 MPa, the elongation at break at 100 DEG C is greater than 500%, and the maximum dynamic loss at 100 DEG C and 10 Hz is greater than 0.13. The weight content, expressed as phr, of the filler in the second composition is greater than the weight content, expressed as phr, of the filler in the first composition.

Description

Technical Field

[0001] The subject of the invention is a radial tyre intended to be fitted to heavy vehicles of the civil engineering type, more particularly heavy vehicles for underground mines, and more particularly to the tread of such a tyre. Background Art

[0002] Within the meaning of the European Tyre and Rim Technical Organisation or ETRTO standards, radial tyres intended to be fitted to heavy vehicles of the civil engineering type used in underground mines are designated as such tyres.

[0003] For example, within the meaning of the European Tyre and Rim Technical Organization or ETRTO standards, radial tires for heavy vehicles of the civil engineering type are intended to be mounted on rims with a diameter at least equal to 25 inches and with a nominal load at least equal to 8000 kg.

[0004] Since the geometry of the tire exhibits rotational symmetry around the axis of rotation, the geometry of the tire is usually described in a meridian plane containing the axis of rotation of the tire. For a given meridian plane, the radial direction, the axial direction and the circumferential direction respectively denote the direction perpendicular to the axis of rotation of the tire, the direction parallel to the axis of rotation of the tire and the direction perpendicular to the meridian plane. The circumferential direction is tangential to the circumference.

[0005] In the following, the expressions "radially inner" or "radially inside" and "radially outer" or "radially outside" mean "closer to the tire's axis of rotation" and "further away from the tire's axis of rotation", respectively. "Axially inner" or "axially inside" and "axially outer" or "axially outside" mean "closer to the tire's equatorial plane" and "further away from the tire's equatorial plane", respectively, the tire's equatorial plane being a plane passing through the middle of the tread surface and perpendicular to the axis of rotation.

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

[0007] The radial tire additionally comprises a reinforcement composed of a crown reinforcement radially on the inside of the tread and a carcass reinforcement radially on the inside of the crown reinforcement.

[0008] The carcass reinforcement of a radial tire for heavy vehicles of civil engineering type generally comprises at least one carcass layer, which generally comprises a metal reinforcement coated with a polymer material of an elastomeric type or an elastic type obtained by blending and referred to as a coating compound. The carcass layer comprises a main portion, which connects two beads together and is generally wound around a metal circumferential reinforcement element generally referred to as a bead wire in each bead from the inside to the outside of the tire, thereby forming a bead. The metal reinforcements of the carcass layer are substantially parallel to each other and form an angle between 80 ° and 90 ° with the circumferential direction.

[0009] The crown reinforcement of a radial tire for civil engineering type vehicles comprises a superposition of circumferentially extending crown layers radially outside the carcass reinforcement. Each crown layer is generally constituted by metal reinforcements parallel to one another and coated with a polymer material of the elastomeric type or of the coating compound type.

[0010] The treads of tires for underground mines have very specific conditions of use. Driving the tire down to the bottom of the mine and changing it are expensive and complicated operations that must be limited. Therefore, for the tread, it is necessary to offer the user a maximum volume of rubber constructed to come into contact with the ground or a maximum volume of wearable rubber in order to limit these operations. This is particularly the case when the tires used in underground mines are subjected to severe and repeated attacks. In addition, for safety reasons, speeds in underground mines are limited and the presence of tread patterns for drainage or improving grip is disadvantageous. Therefore, for this extreme use, tire manufacturers use treads that are smooth and very thick relative to the diameter of the tire casing.

[0011] From the wear point of view, having the maximum volume of wearable rubber is not enough to optimize the service life of the tire. Some rubber compounds have demonstrated good attack resistance properties in this use. Therefore, the use of synthetic compounds in the tread can significantly improve the attack resistance. All these technical solutions lead to an increase in the temperature of the tread and promote the formation of depressions inside the tread. Depending on their size, these depressions may lead to the loss of part of the tread and thus to tire failure. The expression "rubber compound" or "rubber composition" means a composition of rubber comprising at least an elastomer and a filler.

[0012] One solution is to reduce the operating speed of the vehicle, but this has a negative impact on mining capacity. To increase this productivity, manufacturers have proposed combining two rubber compounds, one with low hysteresis, radially on the inside of the abradable rubber compound, and a sipe tread for cooling this thick tread (WO2019 / 058084). This solution is effective in improving performance, but it is not enough, so these tires have problems with partial tread chipping at speeds allowed by mine safety. Summary of the invention

[0013] The inventors have set themselves the object of improving the chipping resistance of parts of the tread of radial tires for vehicles of the mining and civil engineering type.

[0014] This aim has been achieved by a radial tyre for mining vehicles, the radial tyre being intended to be mounted on a rim having a rim diameter at least equal to 24 inches and being intended to carry a nominal load at least equal to 8000 kg, the radial tyre comprising:

[0015] a tread intended to come into contact with the ground through the tread surface and connected by two sidewalls to two beads intended to come into contact with the rim,

[0016] a crown reinforcement, which is radially located on the inside of the tread and comprises a crown layer, which comprises a metal reinforcement,

[0017] a median plane, perpendicular to the axis of rotation of the tire, passing through the middle of the tread and having an intersection with the tread surface, the average distance of which from the axis of rotation of the tire is equal to Rm,

[0018] the tread has an axial width and a radial thickness in the median plane, the radial thickness being defined as the radial distance from the radially outermost point of the radially outermost crown layer to the tread surface, the radial thickness E of the tread being at least equal to 0.04*Rm,

[0019] the tread comprises at least a first rubber composition and a second rubber composition, the first rubber composition and the second rubber composition constituting at least 90% of the thickness of the tread,

[0020] The first rubber composition is located radially inward of the second rubber composition,

[0021] The second rubber composition has a secant tensile modulus E10_2 measured at 10% strain and 23° C. of at least 5.2 MPa,

[0022] the first rubber composition constitutes at least 40% and at most 66% of the tread thickness, preferably at least 45% and at most 60% of the tread thickness,

[0023] the first rubber composition has an elongation at break at 100° C. of at least 500% and a maximum dynamic loss tan δ of said first rubber composition measured at a temperature of 100° C. and a frequency of 10 Hz of at most 0.13,

[0024] the first rubber composition comprises a reinforcing filler in a proportion by weight at least equal to 30 phr (parts of filler per 100 parts of elastomer) and at most equal to 80 phr,

[0025] the second rubber composition comprises carbon black in a proportion by weight of at least 30 phr and at most 75 phr,

[0026] The total weight proportion of the reinforcing filler of the second rubber composition is greater than the total weight proportion of the reinforcing filler of the first rubber composition.

[0027] The particular characteristics of the mining tire in question are a relatively thick tread when the tire is new and a very low void fraction when the tire is new or when the tire is worn. Since the tread has an axial width W and a radial thickness E in the median plane, said radial thickness E being equal to the radial distance from the radially outermost point of the radially outermost crown layer to the tread surface, said radial thickness E of the tread is at least equal to 0.04*Rm, which for the target size corresponds to a radial thickness of the tread greater than 55 mm when the tire is new, and up to 120 mm for some sizes. The area void fraction expressed relative to the total surface area of ​​the tread (which corresponds to the axial width of the tread multiplied by the circumference of the tire) is calculated by evaluating the surface area of ​​the voids, grooves, sipes present in the tread surface of the tire when new. For tires with a circumferentially variable axial width of the tread, the average axial width can be used to calculate the tread surface area, and for tires with an axially variable circumference, the average circumference will be used to calculate the tread surface area. The void fraction of such tires is usually zero. This area void fraction reduces the temperature within the tread without adversely affecting the volume of wearable rubber.

[0028] The tread comprises essentially two rubber compounds or rubber compositions which constitute at least 90% of the thickness of the tread. "Constitute at least 90% of the thickness of the tread" is understood to mean that in a meridian section, the average radial thickness of the second composition in the axial direction plus the average radial thickness of the first rubber composition in the axial direction is greater than 90% of the average radial thickness of the tread in the axial direction. In a meridian section, the radial thickness of the tread is measured from the radially outermost point of the radially outermost reinforcing layer of the crown reinforcement to the point on the tread surface at a given axial distance from the median plane between the axial ends of the radially outermost layer of the reinforcement of the crown reinforcement. The thickness of the coating rubber of the radially outermost reinforcement is negligible relative to the thickness of the tread. For a point whose axial distance from the median point is greater than the axial half-width of the radially outermost layer of the reinforcement, the thickness of the tread is calculated as the radius of the tread surface at this axial distance from the median plane minus the radius of the axially outermost point of the radially outermost reinforcing layer of the crown reinforcement.

[0029] Other rubber compositions may be present in the tread, for example a fine rubber composition enabling the first rubber composition to adhere to the radially outermost coating compound of the crown reinforcement, or a layer of rubber composition enabling better bonding between the two main rubber compositions of the tread which together account for at least 90%, preferably 99%, of the tread thickness.

[0030] For good wear resistance, the second rubber composition radially located outside the first rubber composition needs to be at least equal to 5.2MPa in the secant tensile modulus E10-2 at 10% strain measured at 23°C on a sample taken from a cured tire. The tensile modulus of the rubber composition is usually measured on a sample manufactured for testing according to ASTM D 412 standards. Those skilled in the art will know how to select and adjust the size of the sample according to the amount that the compound can use and obtain when taking out the sample from the tire for all measurements of this measurement and material.

[0031] In order to improve the handling of the tire, it is also necessary to maintain a specific balance between the second rubber composition and the first rubber composition, since experience has shown that outside these ranges, the solutions are not optimal in terms of performance in terms of cracking or in terms of wear of the rubber composition of the tread. In order to optimally balance the performance properties, the first rubber composition constitutes at least 40% and at most 66% of the tread thickness, preferably at least 45% and at most 60% of the tread thickness.

[0032] In order to achieve the best balance, it is also necessary that the first rubber composition produces a small temperature rise when the tire is running, so the maximum dynamic loss tan δ at a temperature of 100°C and a frequency of 10Hz is at most equal to 0.13. The dynamic loss of the rubber composition is usually measured on a specimen made for the test according to the ASTM D5992-96 standard. Those skilled in the art will know how to select and adjust the size of the specimen according to the amount that the compound can be used and obtained when the specimen is taken out from the tire for all measurements of the measurement and material. It is also required that its crack resistance is high, more specifically, the elongation at break of the first rubber composition of the tread at 100°C is at least equal to 500%, preferably at least equal to 575%. The elongation at break of the rubber composition is usually measured on a specimen made for the test according to the NF T 460002 standard. Those skilled in the art will know how to select and adjust the size of the specimen according to the amount that the compound can be used and obtained when the specimen is taken out from the tire for all measurements of the measurement and material.

[0033] In order to achieve the objectives of the present invention, a balance must be found between the ability of the second rubber composition to exchange heat energy further with the outside, in particular, because it is in contact with the outside air through the tread surface, and having better performance in terms of wear (being radially located on the outside of the first rubber composition). This is because the second rubber composition is more likely to exchange heat energy with the outside, because it is in contact with the outside through the tread surface, rather than being in contact with the outside only through the axial edge of the tread, as the first rubber composition is. In addition, since it is radially located on the outside of the first rubber composition, for the same force applied to the tire through the tread surface, the second rubber composition will be further displaced, so at the end of the contact surface, it will slide for a longer length and therefore tend to wear to a greater extent, even if its composition is intended to avoid this problem. For this reason, the weight proportion of the total reinforcing filler expressed in phr of the second composition is greater than the weight proportion of the filler expressed in phr (relative to 100 parts of elastomer) of the first composition. Reinforcing filler is understood to mean carbon black or silica.

[0034] Taking into account the percentage of the radial thickness of the tread of the first composition, it is obvious that after the second rubber composition is worn, the rubber composition must be configured to have good performance in terms of driving. For this reason, it is important that its filler ratio is consistent with the filler ratio of the second rubber composition. Therefore, it is necessary that the first composition contains a reinforcing filler in a ratio of at least 30phr (relative to 100 parts of the filler portion of the elastomer) and at most 80phr. In addition, in order to make the filler ratios of the two rubber compositions consistent with each other, it is necessary that the second composition contains at least carbon black as a reinforcing filler, the carbon black in a ratio of at least 30phr (relative to 100 parts of the filler portion of the elastomer) and at most 75phr, preferably at least 40phr. Therefore, it is advantageous that the second rubber composition (222) contains a reinforcing filler in a total weight ratio of at least 40phr.

[0035] In order to avoid heat generation, it is not only necessary that the first rubber composition has low dynamic losses, but it is also preferred that it contains silica and carbon black as reinforcing fillers. This is because, for the same proportion of fillers, silica generates less heat. This feature makes it more heat-insulating, but considering the geometric position of the first rubber composition, the second rubber composition is already insulated from the external maximum exchange area (especially the tread surface), so this has little effect.

[0036] In order to obtain the target balance, a preferred solution is that the first rubber composition is a rubber composition based on a diene elastomer, which comprises at least one reinforcing filler, said reinforcing filler mainly comprising carbon black, said carbon black having a STSA specific surface area (STSA means statistical thickness surface area, or surface area determined by the statistical thickness method) greater than 100 m 2 / g. STSA specific surface area is greater than 100m 2 / g of carbon black is "fine" carbon black, which provides better crack growth resistance.

[0037] In order to obtain the target balance, a preferred solution is that the second rubber composition is a rubber composition based on a diene elastomer, comprising at least one reinforcing filler, said reinforcing filler mainly comprising carbon black, said carbon black having a relative humidity of 70 m / s. 2 / g to 140m 2 / g and a COAN (Compression Oil Absorption Number) value between 85 ml / 100 g and 105 ml / 100 g, the composition comprising a proportion of oil up to 20 phr and a proportion of tackifying resin (or specific hydrocarbon resin) up to 10 phr. This compound has better resistance to attack, and the more uniformly dispersed relatively fine carbon black is able to improve thermal conduction, thereby improving the release of thermal energy and thus reducing the temperature of the tread when the tire is in use.

[0038] The measurement of the STSA specific surface area or fineness of carbon black is well known to those skilled in the art of tires. It is a compound taken from a tire as a sample, the sample being suitable for the measurements proposed in the ASTM D-6556 standard. The Compression Oil Absorption Number (COAN) of carbon black is a measure of the ability of carbon black to absorb liquids. This property itself varies with the structure of the carbon black. The COAN value is determined using an absorptometer according to the ISO 4656 / 2012 standard using a compression sample of carbon black taken from a sample taken from a tire.

[0039] The tackifying resin for the purpose of the present invention can be selected from natural resins or synthetic resins. In synthetic resins, it can be preferably selected from aliphatic or aromatic or aliphatic / aromatic thermoplastic hydrocarbon resins of type, i.e. hydrocarbon resin according to the present invention comprises aliphatic unit or aromatic unit or aliphatic unit and aromatic unit. Suitable aromatic monomers are, for example, vinylbenzene, alpha-methylstyrene, adjacent to, between or to methylstyrene, vinyltoluene, to (tert-butyl) styrene, methoxystyrene, chlorostyrene, vinyl mesitylene, divinylbenzene, vinylnaphthalene or derive from C9 fraction (or more generally derive from C8 to C9 fraction). 10 Preferably, the vinyl aromatic monomer is styrene or any vinyl aromatic monomer obtained from the C9 fraction (or more generally from C8 to C 10 Preferably, the vinyl aromatic monomer is the minor monomer (expressed in mole fraction) in the copolymer under consideration.

[0040] The second rubber composition with improved electrical conductivity will be more effective if the porosity of the tread is not zero. It is important that it is kept low, specifically that the area porosity of the tread surface is less than 1%, so as not to significantly reduce the volume of wearable rubber. The optimal void distribution is to have two narrow grooves around the tire on both sides of the median plane so as not to reduce the volume of wearable rubber, while zigzagging from the axially outer side of the tread towards the median plane, so as to release thermal energy from all areas of the tread, especially the center and axial ends. More specifically, the preferred solution is that the tread includes at least two narrow grooves around the tire, each of which has a zigzag overall shape of wavelength L and amplitude A, the wavelength L ranges between 10% and 120% of the axial width W of the tread, the amplitude A ranges between 10% and 40% of the same axial width W, and the area porosity of the tread surface is less than 1%. Narrow grooves are understood to mean grooves with an average width of less than 10 mm.

[0041] Depending on the desired performance and the geometry chosen for the radial thickness of the second rubber composition and of the first rubber composition, a preferred solution is that the maximum depth of each zigzag narrow groove is at least equal to 20% and at most equal to 90% of the thickness E of the tread, preferably ranging between 40% and 70% of the thickness E.

[0042] The disadvantage of the grooves is that they increase the flexibility of certain parts of the tread, since this can lead to irregular wear of the tread. One solution to limit this phenomenon is to place bridges in these grooves, in particular in the longitudinal or transverse parts. Therefore, a preferred solution is to form a plurality of bridges in the zigzag narrow grooves, each bridge being formed from the bottom of the narrow groove and locally reducing the depth of these narrow grooves by at least 20% of the maximum depth of these narrow grooves. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] exist Figure 1 Features of the invention are illustrated in the Figures, which are schematic and not drawn to scale, with reference to a tire of size 29.5R29 used in underground mines.

[0044] Figure 1 A volumetric view of an inventive variant of a tire according to the invention is shown. In this variant, the tire 1 comprises a crown portion 2 extended on each side by sidewalls 3 terminating at the Figure 1 The tire has a bead not shown in the figure, and the carcass reinforcement 31 extends in the crown portion 2, the sidewalls 3 and the bead. The median plane M is perpendicular to the axis of rotation YY' of the tire and passes through Figure 1 The middle of the middle tread 22. DETAILED DESCRIPTION

[0045] The crown portion 2 is covered radially on the outside by a tread 22 having a tread surface 10, a width W equal to 680 mm and a thickness E equal to the radial distance from the radially outermost point of the radially outermost crown layer (211) to the tread surface (10) in the median plane M. The crown portion 2 comprises a crown reinforcement 21 consisting of a plurality of working layers 210 and protective reinforcements 211 radially on the outside of the working layers 210.

[0046] The tread 22 is formed by the superposition of two materials, a first rubber composition 221 and a second rubber composition 222, which is radially located on the outside of the first rubber composition 221 to come into contact with the ground when the tire is new, and has an average radial thickness equal to 62.5 mm. The first rubber composition 221 has an average thickness equal to 62.5 mm and is intended to be worn during driving after the second layer 222 has been completely worn.

[0047] The first rubber composition 221 radially inside the second rubber composition 222 is chosen to have low hysteresis values ​​(characterized by tan δ values) obtained under the conditions specified in the present application. In the present invention, this low hysteresis should be associated with higher values ​​of elongation at break.

[0048] An intermediate layer 20 may be interposed between the crown reinforcement 21 and the tread 22 ; this intermediate layer has in particular the function of connecting the tread to the rest of the tire. The radial thickness of this intermediate layer is less than 10% of the thickness E of the tread 22 .

[0049] Two zigzag narrow grooves 4, 5 having the same geometric shape are formed by molding in the tread 22. These zigzag narrow grooves are continuous and extend around the tire rotation axis (in the tread 22). Figure 1 The zigzag narrow grooves 4, 5 surround the tire (indicated by the direction YY' in the figure). These zigzag narrow grooves 4, 5 have the same maximum depth P equal to 70 mm and an average width equal to 6 mm, which is suitable for allowing the walls delimiting these zigzag narrow grooves 4, 5 to at least partially touch each other when the tire is running. When the tire is new, the zigzag narrow grooves are present, thus providing effective ventilation, and after the tread thickness is reduced, the zigzag narrow grooves disappear, so that they are less likely to heat up during running.

[0050] Each zigzag narrow groove 4, 5 has a sawtooth shape, for which a wavelength L (here equal to 240 mm) and an amplitude A (equal to 300 mm) are defined.

[0051] Due to the presence of these two narrow zigzag grooves 4, 5, a surface for heat exchange with the surrounding air is formed in the material, which heat exchange surface corresponds to the surface area of ​​each wall delimiting the narrow groove. When new, the surface area of ​​each narrow groove wall is approximately equal to five times the equivalent transverse surface area of ​​the tread, as evaluated by multiplying the width W of the tread by the thickness E of the wearable material, seen in section.

[0052] In the first part of the wear of the tire according to this variant, the radially outermost part of the tread is ventilated by the presence of these two narrow zigzag grooves 4, 5, which can close when they enter the contact patch, thus maintaining an appropriate level of stiffness when new. After sufficient partial wear has occurred to make the narrow zigzag grooves disappear, the good inherent qualities of the first rubber composition 221 provide the tire with good integrity.

[0053] The invention was tested or evaluated on tires of size 29.5R29. For each numerical evaluation, a tire according to the invention was compared with a reference tire of the same size. The tire was modeled by the finite element method and the calculations were carried out at a load of 14000 kg and an inflation pressure of 4.5 bar. The calculation results below illustrate the invention. These results are expressed as the distance in the time required to reach a maximum temperature of 120° C., which is the temperature at which the risk of cracks at any position of the tread increases significantly. Only the tread varies between the different tires; the crown reinforcement, the carcass reinforcement, the sidewalls and the beads are strictly identical between the reference tire and the different variants of the tire according to the invention.

[0054] The reference tire contained:

[0055] A first rubber composition comprising 70 phr of natural rubber and 30 phr of styrene / butadiene (SBR), and comprising 43 phr of STSA having a specific surface area of ​​85 m 2 / g of carbon black, the elongation at break at 100°C is 460%, the hysteresis at 100°C is equal to 0.08, and the secant tensile modulus E10_1 at 10% strain measured at 23°C is 5.1 MPa.

[0056] A second rubber composition comprising 20 phr of natural rubber and 80 phr of styrene / butadiene (SBR), and comprising 65 phr of STSA having a specific surface area of ​​125 m 2 / g and a COAN value equal to 95 ml / 100 g of carbon black and 16 phr of tackifying oil, the elongation at break at 100°C is 551%, the hysteresis at 100°C is equal to 0.25, and the secant tensile modulus E10_2 at 10% strain measured at 23°C is 5.0 MPa.

[0057] The tire according to the present invention comprises:

[0058] A first rubber composition comprising 100 phr of natural rubber and 40 phr of STSA having a specific surface area of ​​125 m 2 / g of carbon black and 15phr of silica, the elongation at break at 100°C is 625%, the hysteresis at 100°C is equal to 0.11, and the secant tensile modulus E10_1 at 10% strain measured at 23°C is 5.2MPa.

[0059] A second rubber composition comprising 100 phr of styrene / butadiene (SBR) and 60 phr of STSA having a specific surface area equal to 125 m 2 / g and a COAN value equal to 95ml / 100g of carbon black and 11phr of tackifying oil, the elongation at break at 100°C is 362%, the hysteresis at 100°C is equal to 0.22, and the secant tensile modulus E10_2 at 10% strain measured at 23°C is 5.7MPa.

[0060] The reference tire uses a technology with two rubber compositions, a first rubber composition with very low losses (0.08) and balanced mechanical properties between elongation at break and secant tensile modulus for this hysteresis and a second rubber composition with high elongation at break and average secant tensile modulus for resistance to attack. The optimal distribution of this type of stack is about 20% of the first rubber composition in the tread thickness.

[0061] For tires that are very specific in terms of wear and tread thickness, the idea is to find a different balance (reducing hysteresis but increasing elongation at break) by introducing silica in the filler of the first rubber composition and increasing the secant modulus of the second rubber composition (reducing its elongation at break). Surprisingly, by changing the thickness of the first rubber composition in the tread, the design optimum value of the crown temperature has changed significantly, and the synergy between the materials and the balance between the thickness of the rubber composition can improve performance. For smooth tires without zigzag narrow grooves, when the model only contains the rubber composition with the lowest hysteresis, the optimum value of the temperature closely related to crack propagation should be logical. In fact, surprisingly, the optimum value of this configuration is when 66% of the tread is the first rubber composition. Above this optimum value, the thermal parameters are slightly reduced. However, above this optimum value, the tire's resistance to attack is significantly reduced, especially due to the modulus of the first rubber composition, which reduces its resistance to attack. When more than 40% of the tread is the first rubber composition, the gain in thermal performance is about 40% compared to a tire in which 100% of the tread is the second rubber composition. Between these two limits, varying as the desired balance of properties is, the tire according to the invention remains superior to tires of the prior art in terms of resistance to attack and in terms of crack propagation.

[0062] For the tires arranged with said narrow grooves, the optimum in terms of temperature is no longer when 66% of the tread is the first rubber composition, but when 50% of the tread is the first rubber composition, and for the tires comprising only the first rubber composition, the performance is reduced by more than 10% relative to this optimum. Surprisingly, the interaction between the tread pattern and the tread composition enables to find an optimum that varies with the proportion of the first rubber composition, compared to tires without narrow grooves. The heat exchange effect is optimized due to the narrow grooves of the second rubber composition comprising carbon black as filler, which replaces the generation of thermal energy, and due to the reduction of the heat exchange of the material with low hysteresis comprising silica, which also has the effect of reducing the heat exchange. The specific characteristics of the tread, which has a very high thickness and a very low void fraction, make this optimum fully materialized. The optimum of a tire having a tread pattern according to the invention with narrow grooves and with 50% of the first rubber composition enables to convert the thermal performance:

[0063] Almost 80% improvement compared to a tire with 100% of the tread consisting of the second rubber composition and no narrow grooves

[0064] Almost 20% improvement compared to a tire in which 100% of the tread is composed of the first rubber composition and there are no narrow grooves

[0065] An improvement of almost 22% compared to a tire having 66% of the tread composed of the first rubber composition (which provides the best performance for tires without narrow grooves) and having no narrow grooves.

[0066] Around this optimum value of 50%, for a proportion of the first rubber composition between 40% and 66% of the tread thickness, the gain in thermal performance is greater than 70% compared to a tire with 100% of the tread consisting of the second rubber composition and without narrow grooves. Preferably, for a proportion of the first rubber composition between 45% and 60% of the tread thickness, the gain in thermal performance is greater than 75% compared to a tire with 100% of the tread consisting of the second rubber composition and without narrow grooves.

[0067] Expert knowledge and tests carried out on mining sites have shown that the tires according to the invention improve the resistance to attack by 10% to 20% compared to the reference tires, thanks to the high tensile modulus of the second rubber composition. Similarly, the tires according to the invention improve the performance in terms of cracking resistance by about 20% compared to the composition of the reference tire, thanks to the improvement in the elongation at break of the first rubber composition, although the hysteresis is adversely affected.

[0068] The invention thus makes it possible to find a significantly improved balance between the performance in terms of thermal performance, cracking performance and tread portion chipping compared to prior art tires.

Claims

1. A radial tyre (1) for a mining vehicle, the radial tyre (1) being intended to be mounted on a rim having a rim diameter at least equal to 24 inches and being intended to carry a nominal load at least equal to 8000 kg, the radial tyre (1) comprising: a tread (22) intended to come into contact with the ground via the tread surface (10) and connected via two sidewalls to two beads intended to come into contact with the rim, a crown reinforcement (21), the crown reinforcement (21) being located radially inward of the tread (22) and comprising a crown layer (210, 211), the crown layer (210, 211) comprising a metal reinforcement, a median plane (M), perpendicular to the axis of rotation of the tyre, passing through the middle of the tread and having an intersection with the tread surface (10) at an average distance from the axis of rotation of the tyre equal to Rm, The tread (22) has an axial width (W) and a radial thickness (E) in the median plane (M), wherein the radial thickness (E) is defined as the radial distance from the radially outermost point of the radially outermost crown layer (211) to the tread surface (10), the radial thickness E of the tread being at least equal to 0.04*Rm, The tread (22) comprises at least a first rubber composition (221) and a second rubber composition (222), wherein the first rubber composition (221) and the second rubber composition (222) constitute at least 90% of the thickness (E) of the tread (22), The first rubber composition (221) is located radially inside the second rubber composition (222), The second rubber composition (222) has a secant tensile modulus E10_2 measured at 10% strain and 23° C. of at least 5.2 MPa, · characterized in that the first rubber composition (221) constitutes at least 40% and at most 66% of the thickness (E) of the tread (22), the elongation at break of the first rubber composition (221) at 100°C is at least equal to 500%, and the maximum dynamic loss tan δ of the first rubber composition (221) measured at a temperature of 100°C and a frequency of 10 Hz is at most equal to 0.13, the first rubber composition (221) comprises a reinforcing filler in a proportion by weight of at least 30 phr (to 100 parts of elastomer) and at most 80 phr, The second rubber composition (222) contains carbon black in a weight proportion of at least 30 phr and at most 75 phr, The total weight proportion of the reinforcing filler in the second rubber composition (222) is greater than the total weight proportion of the reinforcing filler in the first rubber composition (221).

2. The tire according to claim 1, wherein: The first rubber composition (221) constitutes at least 45% and at most 60% of the thickness (E) of the tread (22).

3. The tire according to any one of claims 1 and 2, wherein: The first rubber composition (221) contains silica and carbon black as reinforcing fillers.

4. A tyre according to any one of the preceding claims, wherein: The first rubber composition (221) has an elongation at break at 100°C of at least 575%.

5. A tyre according to any one of the preceding claims, wherein: The first rubber composition (221) is a rubber composition based on a diene elastomer, comprising at least one reinforcing filler, wherein the reinforcing filler mainly comprises carbon black, and the STSA specific surface area of ​​the carbon black is greater than 100 m 2 / g.

6. A tyre according to any one of the preceding claims, wherein: The second rubber composition (222) is a diene elastomer-based rubber composition comprising at least one reinforcing filler, the reinforcing filler mainly comprising carbon black, the carbon black having a relative humidity of 70 m / s. 2 / g to 140m 2 / g and a COAN value between 85 ml / 100 g and 105 ml / 100 g, said second rubber composition comprising an oil in a proportion equal to at most 20 phr and a tackifying resin in a proportion equal to at most 10 phr.

7. A tyre according to any one of the preceding claims, wherein: The second rubber composition (222) comprises reinforcing fillers in a total weight proportion of at least 40 phr.

8. A tyre according to any one of the preceding claims, wherein: The tread (22) comprises at least two narrow grooves (4, 5) surrounding the tire, each narrow groove (4, 5) having a zigzag overall shape with a wavelength L and an amplitude A, the wavelength L ranging from 10% to 120% of the axial width (W) of the tread (22), the amplitude A ranging from 10% to 40% of the same axial width (W), and the area void ratio of the tread surface (10) is less than 1%.

9. The tire according to claim 8, wherein: The maximum depth (P) of each zigzag narrow groove (4, 5) is at least equal to 20% and at most equal to 90% of the thickness (E) of the tread (22), preferably ranging between 40% and 70% of the thickness (E).

10. The tire according to claim 8 or 9, wherein: A plurality of bridges are formed in the zigzag narrow grooves (4, 5), each bridge being formed from the bottom of the narrow groove and locally reducing the depth of the narrow grooves (4, 5) by at least 20% of the maximum depth of the narrow grooves (4, 5).

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