Lightweight pneumatic tire

Abstract

The invention relates to a pneumatic tire (1) having a radial carcass reinforcement and two working crown plies (41, 42) which are the only plies for forming a crown reinforcement (4) over at least 40% of a width L5 of the tread (5), the absolute value of the difference between the absolute value of the angle [alpha] 2 and the absolute value of the angle [alpha] 1 being greater than 4 DEG, the absolute value of the alpha2 is greater than the absolute value of the alpha1, and the average angle alpha meets the relational expression 20 + 164 * exp (-L / 100) lt; [alpha] [lt]; and 23 + 164 * exp (-L / 100). According to the invention, the rubber mixture constituting said ply C comprises a composition comprising from 60 phr to 80 phr of a reinforcing filler, said reinforcing filler comprising at least 10 phr of pyrolytic carbon black.

Description

TECHNICAL FIELD

[0001] The present invention relates to a tire having a radial carcass reinforcement, and more particularly to a tire intended to be mounted on a vehicle (such as, for example, a truck, a tractor, a trailer or a bus) carrying heavy loads and traveling at a constant speed. BACKGROUND OF THE INVENTION

[0002] Generally, in tires for heavy-duty vehicles, the carcass reinforcement is anchored in the two bead areas on both sides and is covered radially by a crown reinforcement composed of at least two superimposed layers formed by filaments or cords, the filaments or cords being parallel within each layer and crossing from one layer to another, forming an angle between 10° and 45° with the circumferential direction. The working layer forming the working reinforcement may also be covered by at least one layer called a protection layer, the protection layer being formed by preferably metal and extensible reinforcing elements (called elastic reinforcing elements). It may also include a layer of metal filaments or cords having low extensibility, the metal filaments or cords forming an angle between 45° and 90° with the circumferential direction, and this ply is called a chafer ply and is located radially between the carcass reinforcement and the first crown ply (called the working ply), the carcass reinforcement and the first crown ply being formed by parallel filaments or cords with an absolute value of the angle of at most 45°. The chafer ply and at least the working ply together form a chafer reinforcement, the chafer reinforcement having little or no deformation under the various stresses to which it is subjected, and the chafer ply being substantially used to absorb the transverse compressive forces applied to all the reinforcing elements in the crown area of the tire.

[0003] A cord is said to be inextensible when it exhibits a relative elongation of at most 0.2% under a tensile force equal to 10% of the breaking force.

[0004] A cord is said to be elastic when it exhibits a relative elongation of at least 3% and a maximum tangent modulus of less than 150 GPa under a tensile force equal to the breaking load.

[0005] A circumferential reinforcing element is a reinforcing element that forms an angle within the range of +2.5° to -2.5° (relative to 0°) with the circumferential direction.

[0006] The circumferential direction or longitudinal direction of a tire is the direction tangent to the outer circumference of the tire and defined by the direction of travel of the tire.

[0007] The lateral direction or axial direction of a tire is parallel to the axis of rotation of the tire.

[0008] The radial direction is the direction that intersects and is perpendicular to the axis of rotation of the tire.

[0009] The axis of rotation of a tyre is the axis about which the tyre rotates during normal use.

[0010] The radial plane or the meridian plane is the plane containing the axis of rotation of the tyre.

[0011] The circumferential median plane or the equatorial plane is the plane perpendicular to the axis of rotation of the tyre and dividing the tyre into two halves.

[0012] The "modulus of elasticity" of a rubber compound shall be understood as the secant modulus of elongation at 10% elongation and ambient temperature.

[0013] For a rubber composition, the secant modulus of elasticity at 10% elongation is the modulus of elasticity measured on the compound at an elongation value of 0.1 (i.e. 10% elongation, expressed as a percentage) during a uniaxial tensile test. A uniaxial tension is applied to the specimen at a constant rate, and the elongation and the force are measured. The measurement is carried out using a tensile testing machine of the INSTRON type at a temperature of 23 °C and a relative humidity of 50% (standard ISO 23529). The conditions for measuring and using the results to determine the elongation and the stress are as described in NF ISO 37 standard: 2012 - 03. The stress at 0.1 elongation is determined, and the secant modulus of elasticity at 10% elongation is calculated by determining the ratio of this stress value to the elongation value. A person skilled in the art will know how to select and adjust the dimensions of the specimen according to the quantity of the compound available and usable, especially in the case where the specimen is taken from a finished product such as a tyre.

[0014] Due to the improvement of road networks and the expansion of the global highway network, some tyres currently called "road tyres" are designed to travel longer distances at higher speeds. As the wear on the tyres is reduced, the combined conditions of travel of such tyres will undoubtedly enable an increase in the number of kilometres travelled; on the other hand, the durability of such tyres, in particular the durability of the crown reinforcement, will be adversely affected.

[0015] This is because there are stresses in the crown reinforcement, more specifically shear stresses between the crown layers, and these stresses, together with a non - negligible increase in the operating temperature at the ends of the shortest axial crown layer, result in the appearance of cracks in the rubber and their propagation at said ends.

[0016] In order to improve the durability of the crown reinforcement of tyres of the type under study, solutions have been proposed relating to the structure and quality of rubber compound layers and / or shaping elements arranged between and / or around the ends of the ply layers (more specifically the ends of the shortest axial ply layers).

[0017] It is well known that a rubber compound layer is introduced between the ends of the working layers in order to produce a decoupling between the ends, thereby limiting the shear stress. However, such a decoupling layer needs to have very good cohesion. For example, such a rubber compound layer is described in patent application WO 2004 / 076204.

[0018] To improve the resistance of the rubber compound situated near the edges of the crown reinforcement to degradation, patent FR 1 389 428 proposes using, in combination with a low-hysteresis tread, rubber profiled elements covering at least the sides and edges of the crown reinforcement and consisting of a rubber compound having low hysteresis.

[0019] In order to avoid separation between the crown reinforcement plies, patent FR 2 222 232 teaches covering the ends of the reinforcements with rubber pads having a Shore A hardness different from that of the tread covering the reinforcements and greater than that of the profiled elements of rubber compound arranged between the carcass reinforcement and the edges of the crown reinforcement plies.

[0020] The tire produced thereby can effectively improve performance, particularly in terms of durability.

[0021] Furthermore, in order to produce tires with very wide treads or to confer greater load bearing capacity on a tire of a certain size, it is known to introduce a layer of circumferential reinforcing elements. For example, patent application WO 99 / 24269 describes the presence of such a layer of circumferential reinforcing elements.

[0022] The layer of circumferential reinforcing elements generally consists of at least one metal cord wound so as to form a coil having a lay angle of less than 2.5° with respect to the circumferential direction.

[0023] Furthermore, the use of the tires on heavy-duty vehicles of the "worksite supply" type means that the tires are subject to shock loads when driving over rocky terrain. These shock loads will of course have an adverse effect on the performance in terms of durability.

[0024] It is also known to those skilled in the art to increase the number of plies constituting the crown reinforcement in order to improve the durability of the tire with respect to these impact loads.

[0025] The presence of one or more additional layers of reinforcement elements results in greater tire mass and higher tire manufacturing costs.

[0026] A tire is also known from document WO 2017 / 149222, in which the crown reinforcement of the tire is lightened while also improving the durability properties of the tire with respect to such impact loads. However, the inventors have found that when driving on a ground that exerts great stress on the tire, the durability performance of such a tire may be reduced (especially at the shoulders of the tire), for example, it may be reduced under particularly severe driving conditions that combine vehicle speed, the load borne by the tire, and the nature of the ground. In fact, for example, when driving at a relatively high speed on a ground of the type supplied to construction sites, which is very demanding on the tire, a reduction in durability performance can be observed. Summary of the Invention

[0027] Therefore, the inventors set themselves the task of providing a tire for "heavy-duty" vehicles (such as vehicles of the "construction site supply" type), which has a limited total mass and whose durability performance with respect to the impact loads experienced by the tread is improved regardless of the nature of the ground and the driving conditions.

[0028] According to the present invention, this object is achieved by a tire comprising a radial carcass reinforcement intended to be mounted on a deep groove rim of the 15° deep groove type, the tire comprising a crown reinforcement, the crown reinforcement comprising two working crown layers, each working crown layer being formed by reinforcing elements, the reinforcing elements being interposed between two surface layers of rubber compound, crossing from one layer to the other, and forming an angle (α1, α2) greater than 8° with the circumferential direction, the angles α1 and α2 being oriented on either side of the circumferential direction respectively, a rubber compound layer C being arranged between at least the ends of the at least two working crown layers, the crown reinforcement being radially covered by a tread, the tread being connected to two beads via two sidewalls, the two working crown layers and the at least one circumferential reinforcement element layer being the only layers used to form the crown reinforcement over at least 40% of the axial width of the crown reinforcement, the absolute value of the angle α2 formed by the reinforcing elements of the radially outermost working layer with the circumferential direction being greater than the angle α1 formed by the reinforcing elements of the radially innermost working layer with the circumferential direction, the absolute value of the difference between the absolute values of the angles α2 and α1 being greater than 4°, and the average angle α satisfying the following relationship:

[0029] 12 + 131*exp(-L / 100) < α < 20 + 164*exp(-L / 100),

[0030] where α is defined by the relationship α = Arctan((tan(|α1|)*tan(|α2|)) 1 / 2 ) and L is the maximum width of the tire measured in the axial direction and expressed in mm, and the composition of the rubber compound forming the layer C contains 60 phr to 80 phr of reinforcing filler, the reinforcing filler comprising at least 10 phr of pyrolytic carbon black.

[0031] Within the meaning of the present invention, a deep groove rim of the 15° deep groove type or a deep groove rim with a safety bulge is a one-piece rim as defined in the ETRTO, wherein the base intended to receive the tire bead has a truncated conical shape, the angle formed with the axial direction being substantially equal to 15°. These bases are also extended by a rim flange with a reduced height compared to the flange of a flat base rim (whose rim base has a substantially cylindrical shape).

[0032] The expression "parts by weight / 100 parts by weight of elastomer" (or phr) should be understood to mean parts by mass / 100 parts by mass of elastomer or rubber (the two terms being synonymous).

[0033] In certain embodiments, the composition comprises 60 phr to 80 phr of a reinforcing filler, which is pyrolytic carbon black. Thus, it should be understood that the composition comprises pyrolytic carbon black as the sole reinforcing filler (thus, the composition does not contain any inorganic reinforcing fillers and other organic reinforcing fillers).

[0034] The reinforcing filler can be as described below.

[0035] Within the meaning of the present invention, the term "pyrolytic carbon black" is understood to mean carbon black produced by the pyrolysis process (for example, in the case of the recycling of such a material) of a material comprising at least one carbon-based polymer and carbon black (hereinafter referred to as the material to be pyrolyzed). Whatever its form, whether in powder, granular, strip form or any other form, and whether in a crosslinked state or an uncrosslinked state, the physical state of the material to be pyrolyzed provided is not important.

[0036] Preferably, the material to be pyrolyzed can be recovered from finished products or products produced during their manufacture / production (such as by-products or waste); these finished products can be selected from pneumatic tires, non-pneumatic tires, industrial conveyor belts, conveyor belts, rubber gaskets, rubber hoses, shoe soles and windshield wipers. Even more preferably, the pyrolytic carbon black usable within the scope of the present invention is carbon black obtained by the pyrolysis process of a material to be pyrolyzed derived from finished products selected from pneumatic tires and non-pneumatic tires.

[0037] Within the scope of the present invention, pyrolysis means any type of thermal decomposition carried out under anaerobic conditions, the raw material thereof being the material to be pyrolyzed as defined above. Thus, pyrolytic carbon black differs from so-called industrial carbon black and / or ASTM grade carbon black in that the carbon-based raw material used for pyrolysis is a material comprising at least a carbon-based polymer and carbon black, rather than a material derived from petroleum fractions or from oils of coal or natural origin.

[0038] The pyrolytic carbon black that can be used within the scope of the present invention differs from known carbon blacks (such as industrial carbon blacks, especially "furnace process" carbon blacks) particularly in that its ash content is higher than that of the "furnace process" carbon black. Relative to the total weight of the "furnace process" carbon black, the ash content of the "furnace process" carbon black is less than 1% by weight.

[0039] Preferably, relative to the total weight of the pyrolytic carbon black, the ash content of the pyrolytic carbon black that can be used within the scope of the present invention ranges from 5% by weight to 30% by weight, more preferably less than 25% by weight, and still more preferably less than 22% by weight.

[0040] Preferably, relative to the total weight of the pyrolytic carbon black, the sulfur content of the pyrolytic carbon black that can be used within the scope of the present invention is greater than 1.5% by weight, preferably greater than 2% by weight, and more preferably ranges from 2.5% by weight to 5% by weight.

[0041] Relative to the total weight of the "furnace process" carbon black, the sulfur content of the "furnace process" carbon black is less than 1.2% by weight.

[0042] Preferably, relative to the total weight of the pyrolytic carbon black, the zinc content of the pyrolytic carbon black that can be used within the scope of the present invention is greater than or equal to 2% by weight, and preferably ranges from 2.5% by weight to 8% by weight.

[0043] Relative to the total weight of the "furnace process" carbon black, the zinc content of the "furnace process" carbon black is substantially zero and particularly less than 0.5% by weight.

[0044] Preferably, the ST SA specific surface area of the pyrolytic carbon black that can be used within the scope of the present invention measured according to standard ASTM D6556 - 2021 is in the range of 20 m 2 / g to 200 m 2 / g, and more preferably in the range of 30 m 2 / g to 90 m 2 / g.

[0045] Preferably, the void volume of the pyrolytic carbon black that can be used within the scope of the present invention measured according to standard ASTM D7854 (2018) at a pressure of 50 MPa is in the range of 30 ml / 100 g to 60 ml / 100 g, and more preferably in the range of 35 ml / 100 g to 55 ml / 100 g.

[0046] The ash content was determined by calcination in a platinum dish in a muffle furnace at 825 °C according to the following protocol. Prior to each series of measurements, the platinum dish was pre-determined and the tare weight of the platinum dish was weighed to an accuracy of within 0.1 mg, and the mass was designated as P0. A 5 g sample of pyrolytic carbon black, accurately weighed to within 0.1 mg, was placed into the platinum dish; this mass was designated as P1. The platinum dish and its contents were pre-calcined using a Bunsen burner until smoke appeared and the product ignited. After the combustion of the product was complete, the platinum dish and its contents were placed into a muffle furnace heated to 825 °C for 1 h. After 1 h, the platinum dish was removed from the furnace and immediately placed into a desiccator at ambient temperature. When the platinum dish and the ash had returned to ambient temperature, the platinum dish was weighed again to obtain the mass P2. Finally, the ash content (ash %) could be obtained using the following formula:

[0047]

[0048] After the sample was calcined, the ash was absorbed in an acidic medium and the zinc content in the pyrolytic carbon black was determined by ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) analysis. The ash was obtained by performing the above protocol. Approximately 100 mg of ash (test sample) was taken and placed into a PFA (Perfluoroalkoxy) tube for a HotBlock hot plate. Then 8 ml of 37% concentrated hydrochloric acid, 3 ml of 65% concentrated nitric acid, and 0.5 ml of 40% hydrofluoric acid were added. The tube was sealed with a tube stopper and heated at 130 °C for 2 h. After cooling, the contents were then transferred to a 100 ml PTFE (Polytetrafluoroethylene) volumetric flask that already contained 2 g of boric acid (for neutralizing hydrofluoric acid) using ultrapure water. Ultrapure water was added up to the calibration mark. The solution obtained was diluted 100-fold by taking 1 ml of the solution and placing it into a 100 ml PFTE volumetric flask that already contained 8 ml of 37% concentrated hydrochloric acid, 3 ml of 65% concentrated nitric acid, 0.5 ml of 40% hydrofluoric acid, and 2 g of boric acid. Then, prior to analysis by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES), the diluted solution was filtered through a 0.45 μm GHP syringe filter. Prior to analyzing the diluted solution, at least 5 calibration samples with zinc concentrations of 0 mg / l, 0.5 mg / l, 1 mg / l, 2 mg / l, and 5 mg / l were analyzed by ICP-AES. These calibration samples were prepared by diluting a commercially available solution with a certified zinc concentration of 1 g / l in a 100 ml volumetric flask.

[0049] These volumetric flasks already contain 8 ml of 37% concentrated hydrochloric acid, 3 ml of 65% concentrated nitric acid, 0.5 ml of 40% hydrofluoric acid, and 2 g of boric acid. The standard solution is analyzed by ICP-AES at a wavelength of λZn = 202.613 nm. For each standard concentration (c), the intensity IZn of the zinc signal is plotted on a curve of IZn = f(c), which corresponds to a calibration line (of the y = ax + b type). Then, the sample solution of unknown concentration (diluted solution) is measured under the same conditions as the standard samples. The measured intensity is related to the concentration using the previously obtained calibration line. Since the sample and volume are pre-recorded, the concentration [c] is directly obtained by software. 灰分 (in mass %). The zinc concentration [c] in the pyrolytic carbon black is obtained by the following equation 炭黑 (in mass %):

[0050] [c] 炭黑 = [c] 灰分 * 100 * ash %.

[0051] The sulfur content in the pyrolytic carbon black is determined by a LECO furnace. The LECO sulfur analyzer is designed to measure the sulfur content in organic materials and / or inorganic materials, particularly by combustion and non-dispersive infrared detection. Before measuring the sulfur content in the sample, the boats are cleaned and the furnace is calibrated. The boats used for the LECO furnace are pre-cleaned: this involves analyzing an empty boat under the same conditions as the sample. A calibration curve is prepared based on a commercial standard sample called "BBOT" (whose purity is greater than 99.99%, and the contents of carbon (C), hydrogen (H), nitrogen (N), oxygen (O), and sulfur (S) are ensured). These contents are as follows: C% 72.52; H% 6.09; N% 6.51; O% 7.43, and S% 7.44. Weigh approximately 10 ± 3 mg, 20 ± 3 mg, and 40 ± 3 mg of BBOT on the boats. The standard sample / boat assembly is placed in the combustion furnace and adjusted to 1350 °C under pure oxygen. The combination of the furnace temperature and the analysis flow rate causes the combustion of the sample and releases sulfur and / or carbon in the form of SO2(g). After 20 s, oxygen begins to flow through the lance, thus accelerating the combustion of materials that are difficult to burn. Sulfur and / or carbon in the form of SO2(g) are entrained by the oxygen flow through the infrared detection unit. The software of the instrument plots a straight line that relates the mass of the standard sample introduced to the response (area) observed on the detector. Thus, the calibration line is obtained. After thoroughly cleaning the sampling equipment, weigh approximately 80 ± 5 mg of pyrolytic carbon black and place it in the boat used for the LECO furnace.

[0052] The area of the SO2 peak observed is related to the concentration through the calibration line. The software of the instrument then calculates the weight % of sulfur in the sample based on the weight of the sample placed in the boat.

[0053] Pyrolytic carbon black is sold, for example, by BlackBear under the reference number "BBCT30" or by Scandinavian EnviroSystems under the reference number "P550".

[0054] According to ETRTO, the maximum width L of the tyre is measured on a tyre mounted on its nominal rim and inflated to its nominal pressure and is expressed in millimetres.

[0055] The angles α1 and α2 (expressed in degrees) are measured in the cross-section of the tyre. According to the invention, the angles are measured in the circumferential median plane.

[0056] The rubber compound layer C can decouple the working crown layer, so that the shear stress is distributed over a greater thickness.

[0057] Within the meaning of the present invention, the working crown layer is said to be coupled if the distance by which the respective reinforcing elements of each layer are radially separated is less than the average diameter of the circle circumscribing the reinforcing elements, the rubber thickness being measured radially between the respective upper and lower generatrices of the reinforcing elements in the radial direction.

[0058] The average diameter of the circle circumscribing the reinforcing elements is defined as the average diameter of the circle circumscribing the reinforcing elements of each working crown layer.

[0059] According to a preferred embodiment of the present invention, the rubber compound layer C is an elastomeric compound based on natural rubber or on synthetic polyisoprene mainly having cis-1,4 bonds and optionally based on at least one other diene elastomer, natural rubber or synthetic polyisoprene being present in a major amount relative to the one or more other diene elastomers used in the blend.

[0060] Among the diene elastomers that can be used in blend with natural rubber or synthetic polyisoprene mainly having cis-1,4 bonds, mention may be made preferably of polybutadiene (BR) mainly having cis-1,4 bonds, styrene-butadiene copolymer (SBR) in solution or emulsion, butadiene-isoprene copolymer (BIR) and styrene-butadiene-isoprene terpolymer (SBIR). These elastomers can be elastomers modified during or after polymerization by a branching agent (such as divinylbenzene) or a star branching agent (such as carbonate, tin halide or silicon halide), or are elastomers modified by a functionalizing agent such that an oxycarbonyl or carboxyl functional group or an amine functional group is grafted to the chain or the chain ends, for example by the action of dimethylaminobenzophenone or diethylaminobenzophenone. In the case of a blend of natural rubber or synthetic polyisoprene mainly having cis-1,4 bonds with one or more of the above diene elastomers, natural rubber or synthetic polyisoprene is preferably used in a major amount, more preferably in an amount greater than 70 phr.

[0061] Also preferably, in addition to the pyrolytic carbon black, the rubber blend layer C further comprises a reinforcing filler consisting of:

[0062] a) carbon black used in an amount between 30 phr and 70 phr, preferably between 40 phr and 60 phr,

[0063] b) or a white filler of the silica and / or alumina type having SiOH and / or AlOH surface functional groups with a BET specific surface area between 30 m 2 / g and 260 m 2 / g, the white filler being selected from precipitated silica or pyrogenic silica, alumina or aluminum silicate, or carbon black modified during or after synthesis, and used in an amount between 30 phr and 70 phr, preferably between 40 phr and 60 phr,

[0064] c) or a blend of the carbon black described in (a) and the white filler described in (b), wherein the total filler content is between 30 phr and 70 phr, preferably between 40 phr and 60 phr.

[0065] The BET specific surface area is measured by the Brunauer, Emmet and Teller method described in “The Journal of the American Chemical Society” (Volume 60, page 309, February 1938), which corresponds to the NFT 45007 standard of November 1987.

[0066] If a transparent filler or a white filler is used, a coupling agent and / or a covering agent selected from reagents known to those skilled in the art must be used. As an example of a preferred coupling agent, alkoxysilane sulfides of the bis(3-trialkoxysilylpropyl) polysulfide type can be mentioned, in particular bis(3-triethoxysilylpropyl) tetrasulfide sold by Degussa under the names Si69 (pure liquid product) and X50S (solid product (blended with N330 carbon black at 50 / 50 by weight)). As an example of a covering agent, fatty alcohols, alkylalkoxysilanes (such as hexadecyltrimethoxysilane or hexadecyltriethoxysilane sold by Degussa under the names Si116 and Si216 respectively), diphenylguanidine, polyethylene glycol or silicone oil optionally modified by OH or alkoxy functional groups can be mentioned. The covering agent and / or the coupling agent are used in a weight ratio of ≥1 / 100 and ≤20 / 100 relative to the filler, preferably between 2 / 100 and 15 / 100 when the transparent filler accounts for all of the reinforcing filler, and between 1 / 100 and 20 / 100 when the reinforcing filler consists of a blend of carbon black and transparent filler.

[0067] As further examples of reinforcing fillers having the abovementioned morphology and SiOH and / or AlOH surface functional groups of silica and / or alumina type materials and which can be used as partial or total substitutes for these materials according to the invention, mention may be made of carbon blacks which are modified during the synthesis by adding a compound of silicon and / or aluminium to the feedstock oil of the furnace or after synthesis by adding an acid to an aqueous suspension of carbon black in a sodium silicate and / or sodium aluminate solution, so as to cover at least partially the surface of the carbon black with SiOH and / or AlOH functional groups. As non-limiting examples of carbon-based fillers of this type having SiOH and / or AlOH functional groups on the surface, mention may be made of the fillers of the CSDP type described in document No. 24 of the ACS Rubber Division Meeting (Anaheim, California, 6-9 May 1997), and those in patent application EP-A-0799 854.

[0068] When using a transparent filler as the only reinforcing filler, hysteresis properties and cohesion properties are obtained by using precipitated silica or pyrogenic silica or precipitated alumina or aluminium silicate having a BET specific surface area between 30 m 2 / g and 260 m 2 / g. As non-limiting examples of fillers of this type, mention may be made of silica KS404 from Akzo, Ultrasil VN2 or VN3 and BV3370GR from Degussa, Zeopol 8745 from Huber, Zeosil 175MP or Zeosil 1165MP from Rhodia, HI-SIL 2000 from PPG, etc.

[0069] The results obtained with the tyres according to the invention have effectively demonstrated that, regardless of the nature of the ground and the driving conditions, the performance in terms of durability can be improved in the case of a reduction in the weight of the tyre crown reinforcement. Moreover, the reduction in the weight of the tyre crown reinforcement is accompanied by a simplification of the manufacturing process and a reduction in the manufacturing costs.

[0070] Surprisingly, the results have effectively shown that the weight of the tyres according to the invention can be reduced by reducing the number of layers constituting the tyre crown reinforcement, while improving in particular the durability of the tyre crown with respect to the impact loads to which the tyre edges (especially at the shoulders of the tyre) are subjected, regardless of the nature of the ground and the driving conditions.

[0071] In particular, those skilled in the art are aware that, in order to improve the durability performance of a tyre crown reinforcement with respect to this type of impact load, the usual practice is to increase the number of layers of reinforcing elements and to increase the stiffness of the compound used to decouple the ends of the layers of reinforcing elements.

[0072] The inventors believe that these results can be explained by the fact that the angle formed by the reinforcing elements of the radially innermost working tread crown layer with the circumferential direction is an angle whose absolute value is smaller than the angle formed by the reinforcing elements of the radially outermost working tread crown layer, and that pyrolytic carbon black used as a filler is present within layer C. They found that the smaller angle of the reinforcing elements of the radially innermost working tread crown layer seems to cause a delay in the absorption of tension by the reinforcing elements when such impact loads occur. Also, the inventors have demonstrated that the presence of pyrolytic carbon black in the rubber compound constituting layer C gives the rubber compound a higher elongation at break value than more common compounds. Generally, if the impact load is the impact load observed when driving over a stony ground, more specifically impacting the shoulder of the tire, the breakage of the reinforcing elements (if such breakage occurs) is found in the radially innermost layer, more specifically at its ends. These observations seem to indicate that in the face of this type of attack, the difference in the angles of the reinforcing elements between the two working tread crown layers and the presence of pyrolytic carbon black in the rubber compound constituting layer C enable the improvement of the durability performance of the tire and at the same time reduce the number of layers in the tread reinforcement.

[0073] However, the use of pyrolytic carbon black in layer C results in a reduction in the stiffness of the rubber compound layer C. This lower stiffness is another factor that is disadvantageous to the durability of the radially innermost working tread crown layer when under high stress (such as when driving at a constant speed) compared to more common compounds.

[0074] More conventional tire designs effectively provide a rubber compound layer arranged between the ends of the working tread crown layer, whose secant modulus of elasticity at 10% elongation is greater than 8.5 MPa, especially in order to be able to limit the shear stress between the ends of the working tread crown layer, which has a very low circumferential stiffness at its ends. This modulus (which is usually even greater than 9 MPa) can prevent cracking from starting and spreading in the rubber compound at the ends of the working tread crown layer (more particularly at the ends of the narrowest working layer).

[0075] The compound of layer C with a lower stiffness based on pyrolytic carbon black helps to limit the temperature rise generated when applying shear stress.

[0076] Moreover, the weight reduction of the tread reinforcement results in a reduction in the total thickness of the tire tread. The inventors have also demonstrated that when driving at a constant speed, the temperature of the tire tread is lower compared to tires with a more conventional design.

[0077] The inventors have also demonstrated that the cohesion of layer C according to the present invention remains satisfactory.

[0078] Within the meaning of the present invention, a cohesive rubber blend is a rubber blend that is particularly resistant to cracking. Thus, the cohesion of the blend is evaluated by means of a fatigue cracking test carried out on "PS" (pure shear) specimens. The test consists in determining the variation of the crack propagation rate "Vp" (nm / cycle) as a function of the energy release rate "E" (J / m 2 ) over the experimental range covered by the measurement, which is at temperatures in the range from -20 °C to +150 °C, using an atmosphere of air or nitrogen. The stress on the specimen is a dynamic displacement with an amplitude between 0.1 mm and 10 mm applied in the form of a pulsed stress load ("half-sine" tangent signal), where the pause time is equal to the pulse duration; the frequency of the signal is on average about 10 Hz.

[0079] The measurement comprises three parts:

[0080] · Adaptation of the "PS" specimen to 1000 cycles at 27% deformation.

[0081] · Energy characterization to determine the "E" = f(deformation) law. The energy release rate "E" is equal to W0*h0, where W0 = energy supplied to the material per cycle and per unit volume, and h0 = initial height of the specimen. Thus, the acquisition of "force / displacement" data gives the relationship between "E" and the amplitude of the stress load.

[0082] · Measurement of the crack after notching the "PS" specimen. The data collected are used to determine the variation of the crack propagation rate "Vp" as a function of the applied stress load level "E".

[0083] The inventors have particularly demonstrated that the combination of weight reduction of the crown reinforcement and a blend of lower stiffness based on pyrolytic carbon black in layer C helps to reduce the variation in the cohesion of layer C. This is because more conventional tyre designs particularly include a layer of rubber blend arranged between the ends of the working crown layer with a secant modulus of elasticity greater than 8.5 MPa at 10% elongation, which causes a variation in the cohesion of the said layer of rubber blend arranged between the ends of the working crown layer, the cohesion tending to become weaker. The inventors have found that the combination of weight reduction of the crown reinforcement and a blend of lower stiffness based on pyrolytic carbon black in layer C limits the temperature rise and results in a slight variation in the cohesion of layer C. Thus, the inventors consider that the cohesion of layer C (which is less than that present in more conventional tyre designs) is satisfactory in the tyre design according to the present invention.

[0084] The combination according to the invention of a weight-reduced crown reinforcement and a lower-stiffness pyrolysis carbon black-based compound of layer C (a pyrolysis carbon black-based compound might seemingly impair the durability of the tyre under extreme service conditions) in fact enables the durability performance of the tyre to be maintained or even improved. The inventors believe that this result can be explained by the fact that on the one hand, the weight-reduced design of the crown reinforcement and on the other hand, the presence of the lower-stiffness pyrolysis carbon black-based compound in layer C, which have a cumulative effect on the tyre crown temperature, and the combination of these factors seemingly reverses the expected effect on durability.

[0085] Preferably, the thickness of the rubber compound layer C measured at the end of the narrowest working crown layer among the two working crown layers under consideration is preferably between 30% and 80% of the total thickness of the rubber compound between the cord generatrices of each of the two working crown layers: a thickness of less than 30% does not give conclusive results, and a thickness of more than 80% is useless for improving the resistance to separation between the layers and is also disadvantageous in terms of cost.

[0086] Also preferably, the axial width D of the rubber compound layer C between the axially innermost end of the rubber compound layer C and the end of the axially narrowest working crown layer satisfies:

[0087] 3.φ2 ≤ D ≤ 25.φ2

[0088] where φ2 is the diameter of the reinforcing element of the axially narrowest working crown layer. This relationship defines the joint area between the rubber compound layer C and the axially narrowest working crown layer. Such a joint below three times the diameter of the reinforcing element of the axially narrowest working layer may not be sufficient to obtain the decoupling of the working crown layers, especially in order to obtain a reduction in stress at the end of the axially narrowest working crown layer. A value of such a joint greater than twenty times the diameter of the reinforcing element of the axially narrowest working layer may lead to an excessive reduction in the cornering stiffness of the tyre crown reinforcement.

[0089] Preferably, the axial width D of the rubber compound layer C between the axially innermost end of the rubber compound layer C and the end of the axially narrowest working crown layer is greater than 5 mm.

[0090] The invention also preferably provides, at the axially outer end of the axially narrowest working crown layer, a rubber compound layer C, the thickness of which satisfies the following relationship for the radial distance d between the two working crown layers separated by the rubber compound layer C:

[0091] 3 / 5.φ2 < d < 5.φ2

[0092] where φ2 is the diameter of the reinforcing element of the axially narrowest working crown ply.

[0093] The distance d is measured from cord to cord (i.e., between the cords of the first working ply and the cords of the second working ply). In other words, the distance d includes the thickness of the rubber compound layer C and the respective thicknesses of the rubber surface compound located radially outside the cords of the radially inner working ply and radially inside the cords of the radially outer working ply.

[0094] Various thickness measurements are made on the meridian cross-section of the tire, so the tire is in an uninflated state.

[0095] Advantageously according to the invention, the fracture potential index F2 / FR2 of the radially outermost working ply is less than 1 / 6, where:

[0096] FR2 is the breaking force of each cord of the radially outermost working ply under uniaxial extension,

[0097] F2 = p2 * Tc * [(tan(|α1|) / (tan(|α1|)+tan(|α2|))) / cos 2 (|α2|)+C F , where

[0098] Tc = 0.078 * P * Rs * (1 - (Rs 2 -R L 2 ) / (2 * Rt * Rs)),

[0099] P is the nominal inflation pressure of the tire according to ETRTO,

[0100] C F = 0.00035 * (min((L - 80) / sin(|α1|),(L - 80) / sin(|α2|),480)-480),

[0101] p2 is the spacing of the laying of the reinforcing elements of the radially outermost working tread ply, the spacing being measured perpendicular to the reinforcing elements at the circumferential mid-plane,

[0102] Rs = Re - Es,

[0103] Re is the outer radius of the tire, measured at the radially outermost point on the tread surface of the tire, the surface being extrapolated to fill any voids that may be present,

[0104] Es is the radial distance between the radially outermost point of the tire and its orthogonal projection on the radially outer surface of the reinforcing elements of the radially innermost working tread ply,

[0105] R L is the average value of the radii of the axially outermost points of the main part of the carcass reinforcing ply on each side of the tire,

[0106] Rt is the radius of a circle passing through three points located on the outer surface of the tread outside the void and defined at axial distances from the shoulder end equal to 1 / 4, 1 / 2, and 3 / 4 of the axial width of the tread, respectively.

[0107] The thickness Es and the spacing p2 are measured in the cross-section of the tire and expressed in millimeters.

[0108] According to the invention, the meridian cross-section of the tire is defined such that the centroids of the bead wires form an axially oriented straight line, the distance between said centroids being equal to the width of the nominal rim J increased by 20 mm and minus twice the distance measured axially between the centroid of the bead wire and a point on the outer surface of the tire.

[0109] The inventors have also found that by choosing the absolute value of the difference between the absolute values of the above-mentioned angles α1 and α2 in combination with the average angle α and the fracture potential index F2 / FR2 (as defined for this advantageous embodiment of the invention), it is possible to dispense with a protective layer that is usually radially mounted on the outside of other layers of the crown reinforcement. Such a layer is usually present so that it can be sacrificed in the case of an attack of the cutting type to the tire, which can compromise the integrity of the metal reinforcement elements through corrosion phenomena related to the fatigue of said reinforcement elements. The inventors have actually found that when the tire is inflated or when it is used in normal running, the reinforcement elements of the radially outermost working crown layer of the tire according to the invention are subjected to less stress than the reinforcement elements of the radially outermost working crown layer of more conventional tires; said more conventional tires show a smaller angular difference in absolute value between the reinforcement elements of the different working layers, the absolute value of the angle formed by the reinforcement elements of the radially innermost working layer being greater than or equal to the angle formed by the reinforcement elements of the radially outermost working layer, and a higher fracture potential index F2 / FR2. Therefore, the reinforcement elements of the radially outermost working crown layer of the tire according to the invention have significantly better durability than more conventional tires; thus, the inventors have found that the protective layer can be omitted, thereby contributing to reducing the weight of the tire and limiting the temperature rise in the crown area of the tire.

[0110] According to a preferred embodiment of the invention, the absolute value of the difference between the absolute values of the angles α2 and α1 is greater than or equal to 10°, preferably greater than 14°. According to this embodiment and according to the explanations provided above, it is possible to further improve the durability performance of the reinforcement elements of the radially outermost working layer and / or to further improve the performance of the tire with respect to impact loads (such as those experienced when driving over stony ground).

[0111] Preferably, the absolute value of the difference between the absolute values of the angles α2 and α1 is less than 25°, more preferably less than 20°. Above these values, the tire is prone to uneven wear under certain operating conditions.

[0112] Advantageously according to the invention, the fracture potential index F2 / FR2 of the radially outermost working layer is less than 1 / 8. Such a fracture potential index F2 / FR2 further helps to improve the durability of the reinforcing elements of the radially outermost working layer during tire use.

[0113] Preferably according to the invention, the fracture potential index F1 / FR1 of the radially innermost working layer is less than 1 / 3, where:

[0114] FR1 is the breaking force of each cord of the radially innermost working layer under uniaxial extension,

[0115] F1 = p1*Tc*[(tan(|α2|) / (tan(|α1|) + tan(|α2|))) / cos 2 (|α1|) + C F , where

[0116] p1 is the pitch of the laying of the reinforcing elements of the radially innermost working crown layer, the pitch being measured perpendicular to the reinforcing elements at the circumferential mid-plane.

[0117] Also preferably, the fracture potential index F1 / FR1 of the radially innermost working layer is at least 30% higher than the fracture potential index F2 / FR2 of the radially outermost working layer.

[0118] According to an advantageous embodiment of the invention, the axially widest working crown layer is located radially inside another working crown layer.

[0119] According to a preferred embodiment for optimizing the weight reduction of the tire crown according to the invention, the two working crown layers are the only layers used to form the crown reinforcement over the entire axial width of the crown reinforcement.

[0120] According to a variant of the embodiment of the invention, at least one surface layer of at least one working crown layer is made of a rubber compound having a composition comprising 60 phr to 80 phr of a reinforcing filler, the reinforcing filler including at least 10 phr of pyrolytic carbon black.

[0121] According to a preferred embodiment of this variant of the invention, the rubber compounds of the surface layers of the two working crown layers have a composition comprising 60 phr to 80 phr of a reinforcing filler, the reinforcing filler including at least 10 phr of pyrolytic carbon black.

[0122] Typically, the elastic secant modulus of the surface layer of the working tread at 10% elongation is greater than 10 MPa. Such an elastic modulus is required to limit the compression of the reinforcing elements of the working tread, especially when the vehicle is moving along a winding route, in a parking lot, or when passing through a circuitous route. This is because the shear action in the axial direction acting on the tread in the area of contact with the ground causes the compression of the reinforcing elements of the working tread.

[0123] Therefore, the inventors have also found that the combination of the rubber compound based on pyrolytic carbon black of the calendered surface layer of the working tread, the compound of layer C according to the present invention, and the weight reduction design of the tread reinforcement in the variant of this embodiment of the present invention enables the durability performance of the tire to be maintained, or even improved, and even more significantly improved.

[0124] According to a preferred embodiment of this variant of the present invention, at least one surface layer of at least one working tread is an elastomer compound, the elastomer compound being based on natural rubber or on synthetic polyisoprene mainly having cis-1,4 bonds, and optionally based on at least one other diene elastomer. In the case of a blend, natural rubber or synthetic polyisoprene is present in a major amount relative to the content of one or more other diene elastomers used.

[0125] Also preferably, in addition to pyrolytic carbon black, at least one surface layer of at least one working tread further comprises a reinforcing filler consisting of:

[0126] a) carbon black used in an amount between 30 phr and 70 phr, preferably between 40 phr and 60 phr,

[0127] b) or a white filler of the silica and / or alumina type having SiOH and / or AlOH surface functional groups with a BET specific surface area between 30 m 2 / g and 260 m 2 / g, the white filler being selected from precipitated silica or pyrogenic silica, alumina or aluminum silicate, or carbon black modified during or after synthesis, and used in an amount between 30 phr and 70 phr, preferably between 40 phr and 60 phr,

[0128] c) or a blend of the carbon black described in (a) and the white filler described in (b), wherein the total filler content is between 30 phr and 70 phr, preferably between 40 phr and 60 phr.

[0129] According to one embodiment of the present invention, the elastomer compound constituting at least one surface layer of at least one working tread is the same as the elastomer compound of layer C arranged between at least the ends of the two working treads.

[0130] The metal element is preferably a steel cord.

[0131] According to a preferred embodiment of the present invention, the reinforcing element of the working crown layer is an inextensible metal cord. Description of the Drawings

[0132] Other details and advantageous features of the present invention will become apparent hereinafter from the description of the exemplary embodiments of the present invention given with reference to the accompanying drawings, which show a meridional view of a tire design according to an embodiment of the present invention.

[0133] For easier understanding, the drawings are not drawn to scale. The drawings show only a half view of the tire, which extends symmetrically about an axis XX', which represents the circumferential median plane or equatorial plane of the tire. Detailed Description

[0134] In the drawings, the tire 1 has a size of 275 / 80R 22.5. The tire 1 includes a radial carcass reinforcement 2 anchored in two beads (not shown in the figures). The carcass reinforcement is formed of a single layer of metal cord. This carcass reinforcement 2 is hoop-shaped by a crown reinforcement 4, which is formed from the inside to the outside in the radial direction by:

[0135] - A first working layer 41, which is formed of metal cords oriented at an angle equal to 18°,

[0136] - A second working layer 42 formed of metal cords, which are oriented at an angle equal to 30° and cross the metal cords of layer 41. The cords of each of the working layers 41, 42 are oriented on both sides of the circumferential direction, respectively.

[0137] The metal cords of the reinforcing elements constituting the two working layers are cords of formula 9.35. They are distributed within each working layer at a distance equal to 2 mm between the reinforcing elements, the distance being measured along a direction perpendicular to the bisector of the cords.

[0138] The crown reinforcement itself is covered by a tread 5.

[0139] The tire is inflated to a pressure of 8.5 bar.

[0140] The axial width L of the first working layer 41 41 is equal to 220 mm.

[0141] The axial width L of the second working layer 42 42 is equal to 200 mm.

[0142] The axial width L5 of the tread is equal to 215 mm.

[0143] The maximum axial width L is equal to 302 mm.

[0144] According to the invention, the first rubber compound layer C provides a decoupling of the ends of the working crown layers 41 and 42.

[0145] The layer C in the joining region between the two working crown layers 41 and 42 is defined by its thickness, or more precisely by the radial distance d between the ends of layer 42 and layer 41 and the axial width D of layer C between the axial inner end of said layer C and the end of the radially outer working crown layer 42. The radial distance d is equal to 2.8 mm, i.e. approximately 2.1 times the diameter φ2 of the reinforcing element of the working crown layer 42, and the diameter φ2 is equal to 1.35 mm. The radial distance D is equal to 19 mm, i.e. approximately 14 times the diameter φ2 of the reinforcing element of the working crown layer 42.

[0146] The combined mass of the two working layers 41, 42 and the circumferential reinforcing element layer 43 (including the mass of the metal cords and the surface compound) thus totals 10.1 kg.

[0147] According to the invention, layer C is made of an elastomeric compound containing pyrolytic carbon black.

[0148] The difference between the angle formed by the cords of the first working crown layer and the circumferential direction and the angle formed by the cords of the second working crown layer and the circumferential direction is equal to 12°.

[0149] The average angle is equal to 23.4° and is clearly between 20.4° and 28.0°.

[0150] The measured value of Re is equal to 541.7 mm.

[0151] The measured value of Es is equal to 22.3 mm.

[0152] The average value R of the measured radius L is equal to 410 mm.

[0153] The value Rt determined on the tyre is equal to 900 mm.

[0154] The calculated value of Tc is equal to 362 N / mm.

[0155] C F The calculated value is equal to -0.01.

[0156] The value of F1 is equal to 514.4 N.

[0157] The value of F2 is equal to 311.2 N.

[0158] The breaking forces FR1 and FR2 of the reinforcing elements of the working crown layer are equal to 2600 N.

[0159] The fracture potential index F2 / FR2 is equal to 12%.

[0160] The fracture potential index F1 / FR1 is equal to 19.8%.

[0161] The fracture potential index F1 / FR1 is 65% higher than the fracture potential index F2 / FR2.

[0162] Various tires according to the invention have been compared with various reference tires of the same size.

[0163] The first tire I1 according to the invention has a layer C made of compound 1 and a surface layer made of compound R1.

[0164] The second tire I2 according to the invention has a layer C made of compound 1 and a surface layer.

[0165] The first reference tire T1 differs from the tire I1 according to the invention in the characteristics of the compound for the layer C, which layers C are made of compound R1.

[0166] The various compounds used are listed below, each compound showing the secant modulus of elasticity and the elongation at break at 10% elongation.

[0167] Compound R1 Compound 1 NR 100 100 Carbon black N347 52 Carbon black RCB 60 Antioxidant (6PPD) 1.5 1.5 Stearic acid 0.65 0.9 Zinc oxide 9.3 7.5 Cobalt salt (AcacCo) 1.12 1.12 Sulfur 6.1 4.5 Accelerator DCBS 0.93 0.8 CTP blocker (PVI) 0.25 0.15 <![CDATA[MA 10 (MPa)]]> 10.4 6.1 Elongation at break (%) 280 444

[0168] The values of the components are expressed in phr (parts by weight per hundred parts of elastomer).

[0169] The pyrolytic carbon black (carbon black RCB) contains 20% ash, 1.8% sulfur and 4.5% zinc.

[0170] The carbon black N347 contains 0.5% ash, 1% sulfur and 0% zinc.

[0171] The content of the various components in compound I1 other than carbon black is adjusted according to the knowledge of those skilled in the art so as to obtain similar temperature and curing time conditions for the various tires and to be able to compare the properties of the tires.

[0172] The second reference tire T2 differs from the reference tire T1 in that the crown reinforcement they have is formed radially from the inside out by:

[0173] - a triangular layer consisting of two half-carcass plies and formed by non-wound 9.28 non-extendable metal cords, said metal cords being oriented at an angle equal to 65°,

[0174] - a first working layer formed by metal cords oriented at an angle equal to 26°,

[0175] - A second working layer formed of metal cords, the metal cords being oriented at an angle equal to 18° and intersecting the metal cords of the first working layer, and the cords of each layer in the working layer being oriented on both sides in the circumferential direction.

[0176] - A protective layer formed of elastic 6.35 metal cords.

[0177] The metal cords of the two working layers are cords of type 9.35. They are distributed within each working layer at a distance equal to 2 mm between the reinforcing elements, the distance being measured along a direction perpendicular to the bisector of the cords.

[0178] Inflate the reference tire T2 to a pressure of 8.5 bar.

[0179] The total axial width of the triangular layer is equal to 180 mm, and the width of each half ply is equal to 60 mm.

[0180] The axial width of the first working layer is equal to 220 mm.

[0181] The axial width of the second working layer is equal to 200 mm.

[0182] The axial width of the protective layer is equal to 136 mm.

[0183] The combined mass of the working layer, protective layer, and triangular layer of the reference tire T2 (including the mass of the metal cords and the surface compound) totals 10.0 kg.

[0184] The mass of the reference tire is 62.9 kg.

[0185] The absolute value of the difference between the absolute value of the angle formed by the cords of the first working crown layer and the circumferential direction and the absolute value of the angle formed by the cords of the second working crown layer and the circumferential direction is equal to 8°.

[0186] The average angle is equal to 21.7°.

[0187] The value of F1 is equal to 320 N.

[0188] The value of F2 is equal to 392 N.

[0189] The values of F1 and F2 are obtained by finite element simulation. The large number of reinforcing plies in the crown makes it impossible to use a simple analytical model.

[0190] The breaking forces FR1 and FR2 of the reinforcing elements of the working crown layer are equal to 2600 N.

[0191] The fracture potential index F2 / FR2 is equal to 15.1%.

[0192] The fracture potential index F1 / FR1 is equal to 12.3%.

[0193] The fracture potential index F1 / FR1 is 22.7% higher than the fracture potential index F2 / FR2.

[0194] A first (especially heat - required) durability test is carried out on a testing machine. Each tire rolls straight at a speed equal to the maximum speed rating (or speed index) specified for the tire under an initial load of 4000 kg. In order to shorten the duration of the test, the initial load is gradually increased.

[0195] Other (especially mechanically - required) durability tests are carried out on a testing machine, applying lateral forces and dynamic over - loads to the tire in a cyclic manner. The tires according to the invention are tested under the same conditions as those applied to the reference tires.

[0196] The tests thus carried out show that the distances covered by the tires according to the invention and the reference tires in each of these tests are substantially the same.

[0197] Tests were also carried out to characterize the breaking strength of the tire crown reinforcement subjected to impact loads. These tests include running a tire inflated to the recommended pressure and subjected to the recommended load over a cylindrical obstacle or denting tool, the diameter of which is equal to 1.5 inches (i.e., 38.1 mm), having a hemispherical head and a given height. The trajectory of the tire is adjusted so that the axis of the obstacle corresponds to the position of one of the axially outermost ribs of the tread. The breaking strength is characterized by the critical height of the denting tool, which is the maximum height at which the denting tool causes complete fracture of the crown reinforcement (i.e., all crown layers are fractured). These values represent the energy required to break the crown blocks. These values are expressed relative to a base number 100 (which corresponds to the value measured for the reference tire T2).

[0198] I1 125 I2 130 Reference T1 120 Reference T2 100

[0199] These results show that, despite the reduced weight of the tires (especially by reducing the mass of their crown reinforcements), the fracture energy of the tread surfaces of the tires I1 and I2 according to the invention under impact loads is higher than that of the tread surface of the reference tire T2 under impact loads and higher than that of the tread surface of the tire T1 under impact loads.

[0200] Final durability tests were carried out to reproduce driving conditions combining vehicle speed and particularly adverse ground. Thus, these tests reproduce the extreme conditions especially for "heavy - duty load" vehicles of the "site - supply" type.

[0201] This final test includes: a stage of driving on a test track at 100 km / h for 2 hours under the load and pressure conditions marked on the tire, and then a stage of driving on a stony road at 35 km / h for 12 minutes, repeated 25 times.

[0202] The purpose of the low-speed driving stage on a rocky road is to have an adverse effect on durability due to the repeated impact loads on the tread.

[0203] The purpose of the high-speed driving stage on a circular track is to raise the temperature of the tire. This makes the tire more sensitive to the effects of repeated impact loads on the tread and promotes crack propagation initiated during the stage of driving on a rocky road.

[0204] At the end of the driving, the tire is inspected using speckle shearing interferometry and peeled so that any damage can be analyzed. This is a visual analysis, allowing a comparison between any cracks and crack propagation. The tires are scored and compared with each other. A score greater than 100 corresponds to a tire with less damage. The most severely damaged tire receives a score of 100.

[0205] Fraction: I1 115 I2 135 Reference T1 100 Reference T2 100

[0206] At the end of the driving, tires I1 and I2 according to the invention exhibit a lower degree of damage than reference tires T1 and T2.

[0207] During these final durability tests, after driving for 2 hours on a circular track at 100 km / h in the first stage, the temperature of the tire at the end of the tread block is measured.

[0208] Temperature I1 93℃ I2 90℃ Reference T1 97℃ Reference T2 102℃

[0209] These results show that the temperature exhibited by the tires according to the invention is not actually as high as that of the reference tires.

Claims

1. A tire (1) including a radial carcass reinforcement (2) intended to be mounted on a deep - groove rim of the 15° deep - groove type, said tire including a crown reinforcement (4), said crown reinforcement (4) including two working crown plies (41, 42) having reinforcing elements, said reinforcing elements being inserted between two surface layers of rubber compound, crossing from one ply to the other, and forming an angle (α1, α2) greater than 8° with the circumferential direction, said angles α1 and α2 being oriented on either side of the circumferential direction respectively, a rubber compound layer C being arranged between at least the ends of said two working crown plies, said crown reinforcement (4) being radially covered by a tread (5), said tread being connected via two sidewalls to two beads (3), wherein: - said two working crown plies (41, 42) are the only plies forming the crown reinforcement (4) over at least 40% of the axial width of the crown reinforcement (4), - the absolute value of the angle (α2) formed by the reinforcing elements of the radially outermost working crown ply (42) with the circumferential direction is greater than the angle (α1) formed by the reinforcing elements of the radially innermost working crown ply (41) with the circumferential direction, - the absolute value of the difference in absolute values between the angle (α2) and the angle (α1) is greater than 4°, - the average angle α satisfies the following relationship: 12 + 131*exp(-L / 100) < α < 20 + 164*exp(-L / 100), α is defined by the relation α = Arctan((tan(|α1|) * tan(|α2|)) 1 / 2 ), where L is the maximum width of the tyre measured in the axial direction and expressed in mm, characterized in that the composition of the rubber compound forming said layer C contains 60 phr to 80 phr of reinforcing filler, said reinforcing filler including at least 10 phr of pyrolytic carbon black.

2. The tire according to claim 1, wherein, The ash content of the pyrolytic carbon black ranges from 5 wt% to 30 wt%, preferably less than 25 wt%, more preferably less than 22 wt% relative to the total weight of the pyrolytic carbon black, and the ash content is determined by calcination in a platinum dish in a muffle furnace at 825 °C according to the method described in the specification.

3. The tire according to claim 1 or 2, characterized in that, The sulfur content of the pyrolytic carbon black is greater than 1.5 wt%, preferably greater than 2 wt%, and preferably ranges from 2.5 wt% to 5 wt% relative to the total weight of the pyrolytic carbon black, and the sulfur content in the pyrolytic carbon black is determined by a LECO furnace according to the method described in the specification.

4. The tire (1) according to any one of claims 1 to 3, characterized in that, The zinc content of the pyrolytic carbon black is greater than or equal to 2 wt%, preferably ranges from 2.5 wt% to 8 wt%, and the zinc content is determined by analyzing the ash absorbed in an acidic medium after calcining the sample and by ICP - AES (Inductively Coupled Plasma Atomic Emission Spectrometry) according to the method described in the specification.

5. The tire according to any one of claims 1 to 4, characterized in that, Said layer C is an elastomeric compound based on natural rubber, or on synthetic polyisoprene mainly having cis - 1,4 bonds, and optionally based on at least one other diene elastomer, and in the case of a blend, natural rubber or synthetic polyisoprene is present in a major amount relative to the content of one or more other diene elastomers used.

6. The tire (1) according to any one of claims 1 to 5, characterized in that, In addition to pyrolytic carbon black, the rubber compound layer C further contains a reinforcing filler consisting of: a) carbon black used in an amount between 30 phr and 70 phr, preferably between 40 phr and 60 phr, b) or a white filler of the silica and / or alumina type having SiOH and / or AlOH surface functional groups with a BET specific surface area between 30 m 2 / g and 260 m 2 / g, selected from precipitated silica or pyrogenic silica, alumina or aluminosilicate, or carbon black modified during or after synthesis, and used in an amount between 30 phr and 70 phr, preferably between 40 phr and 60 phr, c) or a blend of the carbon black described in (a) and the white filler described in (b), wherein the total filler content is between 30 phr and 70 phr, preferably between 40 phr and 60 phr.

7. The tire (1) according to any one of claims 1 to 6, characterized in that, The fracture potential index F2 / FR2 of the radially outermost working tread crown layer (42) is less than 1 / 6, where: FR2 is the breaking force of each cord in the radially outermost working tread crown layer under uniaxial extension, F2 = p2 * Tc * [(tan(|α1|) / ((tan(|α1|) + tan(|α2|))) / cos 2 (|α2|) + C F , where Tc = 0.078 * P * Rs * (1 - (Rs 2 -R L 2 ) / (2 * Rt * Rs)), P is the nominal inflation pressure of the tire according to ETRTO, C F = 0.00035 * (min((L - 80) / sin(|α1|), (L - 80) / sin(|α2|), 480) - 480), p2 is the spacing of the reinforcement elements laid in the radially outermost working tread crown layer, measured perpendicular to the reinforcement elements at the circumferential mid-plane, Rs = Re - Es, Re is the outer radius of the tire, measured at the radially outermost point on the tread surface of the tire, and the surface is extrapolated to fill any voids that may exist, Es is the radial distance between the radially outermost point of the tire and its orthogonal projection on the radially outer surface of the reinforcement elements in the radially innermost working tread crown layer, R L is the average value of the radii of the axially outermost points on each side of the tire, Rt is the radius of the circle passing through the following three points, which are located on the outer surface of the tread outside the void and are defined by axial distances equal to 1 / 4, 1 / 2, and 3 / 4 of the axial width of the tread respectively from the shoulder end.

8. The tire (1) according to claim 7, characterized in that, The fracture potential index F2 / FR2 of the radially outermost working tread crown layer (42) is less than 1 / 8.

9. The tire (1) according to any one of claims 7 and 8, characterized in that, The fracture potential index F1 / FR1 of the radially innermost working tread crown layer (41) is less than 1 / 3, where: FR1 is the breaking force of each cord in the radially innermost working layer under uniaxial extension, F1 = p1 * Tc * [(tan(|α2|) / ((tan(|α1|) + tan(|α2|))) / cos 2 (|α1|) + C F , where p1 is the spacing of the reinforcement elements laid in the radially innermost working tread crown layer, measured perpendicular to the reinforcement elements at the circumferential mid-plane.

10. The tire (1) according to claim 9, characterized in that, The fracture potential index F1 / FR1 of the radially innermost working layer (41) is at least 30% higher than the fracture potential index F2 / FR2 of the radially outermost working layer (42).

11. The tyre (1) according to any one of the preceding claims, characterised in that The two working tread crown layers (41, 42) are the only layers used to form the tread reinforcement in the entire axial width of the tread reinforcement (4).

12. The tire (1) according to any one of the preceding claims, characterized in that, At least one surface layer of at least one working tread crown layer is made of a rubber compound, and the composition of the rubber compound contains 60 phr to 80 phr of reinforcing filler, and the reinforcing filler includes at least 10 phr of pyrolytic carbon black.

13. The tire (1) according to claim 12, characterized in that, At least one surface layer of at least one working tread crown layer (41, 42) is an elastomer compound, which is based on natural rubber or on synthetic polyisoprene mainly having cis-1,4 bonds, and optionally based on at least one other diene elastomer. In the case of a blend, natural rubber or synthetic polyisoprene is present in a major amount relative to the content of one or more other diene elastomers used.

14. The tire (1) according to any one of claims 12 and 13, characterized in that, In addition to pyrolytic carbon black, the at least one surface layer of at least one working tread crown layer further contains a reinforcing filler composed of: a) carbon black used in an amount between 30 phr and 70 phr, preferably between 40 phr and 60 phr, b) or a white filler of the silica and / or alumina type having SiOH and / or AlOH surface functional groups with a BET specific surface area between 30 m 2 / g and 260 m 2 / g, selected from precipitated silica or pyrogenic silica, alumina or aluminosilicate, or carbon black modified during or after synthesis, and used in an amount between 30 phr and 70 phr, preferably between 40 phr and 60 phr, c) a blend of the carbon black described in (a) and the white filler described in (b), wherein the total filler content is between 30 phr and 70 phr, preferably between 40 phr and 60 phr.

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