Tyre comprising layer of circumferential reinforcing elements

Abstract

The invention relates to a tyre (1) with a radial carcass reinforcement, said tyre (1) comprising two working crown plies (41, 42) and a layer (43) of circumferential reinforcing elements. According to the invention, the rubber mixture of the at least one inner liner layer constituting the at least one working crown ply comprises a composition comprising 40 phr to 70 phr of a reinforcing filler having at least 20 phr of pyrolytic carbon black.

Description

TECHNICAL FIELD

[0001] The present invention relates to a tyre having a radial carcass reinforcement and more particularly to a tyre intended to be mounted on a vehicle (such as, for example, a truck, a tractor, a trailer or a bus) which supports a heavy load and travels at a constant speed. BACKGROUND ART

[0002] Generally, in tyres for heavy-duty vehicles, the carcass reinforcement is anchored in the two bead areas on either side and is covered radially by a crown reinforcement consisting of at least two superimposed layers formed by threads or cords which are parallel within each layer and cross 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, which is formed by reinforcement elements (called elastic reinforcement elements) which are advantageously metallic and extensible. It may also include a layer of metallic threads or cords having low extensibility which form an angle between 45° and 90° with the circumferential direction, and this ply is called a channelling 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 threads or cords with an absolute value of the angle of at most 45°. The channelling ply and at least the working ply together form a channelling reinforcement which hardly undergoes any deformation under the various stresses to which it is subjected, and the channelling ply is essentially used to absorb the transverse compressive forces applied to all the reinforcement elements in the tyre crown area.

[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] Circumferential reinforcement elements are reinforcement elements which form an angle in the range from +2.5° to -2.5° (relative to 0°) with the circumferential direction.

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

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

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

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

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

[0011] The circumferential median plane or the equatorial plane is the plane perpendicular to the axis of rotation of the tire and dividing the tire 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% (ISO 23529 standard). 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 available and usable quantity of the compound, especially in the case where the specimen is taken from a finished product such as a tire.

[0014] Due to the improvement of the road network and the expansion of the global highway network, some tires currently known as "road tires" are designed to travel longer distances at higher speeds. Since the wear on the tires is reduced, the combined conditions of travel of such tires can undoubtedly result in an increase in the number of kilometers traveled; on the other hand, the durability of such tires, especially the durability of the crown reinforcement, is 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 axially shortest 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 the type of tire under study, solutions have been proposed related to the structure and quality of rubber compound layers and / or shaping elements arranged between and / or around the ends of the ply (more specifically, the ends of the axially shortest ply).

[0017] It is a well-known practice to introduce a layer of rubber compound between the ends of the working ply in order to create a decoupling between said ends and thus limit the shear stress. However, this decoupling layer needs to have very good cohesion. For example, such a layer of rubber compound is described in patent application WO 2004 / 076204.

[0018] In order to improve the resistance to degradation of the rubber compound located near the edge of the crown reinforcement, patent FR 1 389 428 proposes using a rubber forming element in combination with a low hysteresis tread, said rubber forming element covering at least the sides and the edge of the crown reinforcement and being made of a rubber compound having low hysteresis.

[0019] In order to avoid separation between the plies of the crown reinforcement, patent FR 2 222 232 teaches covering the ends of the reinforcement with a rubber pad, the Shore A hardness of said rubber pad being different from the Shore A hardness of the tread covering said reinforcement and being greater than the Shore A hardness of the forming element of the rubber compound arranged between the edge of the carcass reinforcement and the plies of the crown reinforcement.

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

[0021] Moreover, in order to produce a tire with a very wide tread or in order to give a greater load-bearing capacity to a tire of a given size, it is a known practice 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, said metal cord being wound so as to form a coil with a laying angle relative to the circumferential direction of less than 2.5°.

[0023] The use of the tire on heavy-duty vehicles of the "job-site supply" type means that the tire is subjected to impact loads when driving over stony ground. These impact loads will of course have an adverse effect on the performance in terms of durability.

[0024] The inventors have particularly found that the durability performance of such a tire may be reduced (especially at the shoulders of the tire) when driving on a ground that exerts great stress on the tire, for example under particularly severe driving conditions combining vehicle speed, the load borne by the tire and the nature of the ground. In fact, for example, a reduction in durability performance can be observed when driving at a relatively high speed on a ground of the job-site supply type that is very demanding on the tire. Summary of the Invention

[0025] Accordingly, the inventors set themselves the task of providing tires for "heavy-duty" vehicles (such as vehicles of the "site supply" type), which tires have improved durability with respect to the impact loads to which the tread is subjected, regardless of the nature of the ground and the driving conditions, and which exhibit improved performance in terms of rolling resistance.

[0026] According to the invention, this object is achieved by a tire comprising a radial carcass reinforcement, said tire comprising a crown reinforcement which comprises two working crown layers and at least one layer of circumferential reinforcing elements, each working crown layer being formed by reinforcing elements which are interposed between two calendered surface layers of rubber compound, cross from one layer to the other, and form an angle between 10° and 45° 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 said at least two working crown layers, the crown reinforcement being radially covered by a tread which is connected to two beads via two sidewalls, the rubber compound of at least one calendered surface layer constituting at least one of the working crown layers comprising a composition comprising 40 phr to 70 phr of reinforcing filler, said reinforcing filler having at least 20 phr of pyrolytic carbon black.

[0027] The expression "parts by weight per hundred parts by weight of elastomer" (or phr) is to be understood as meaning parts by mass per hundred parts by mass of elastomer or rubber (the two terms being synonymous).

[0028] In certain embodiments, the composition comprises 40 phr to 70 phr of reinforcing filler, said reinforcing filler being 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 filler and other organic reinforcing fillers).

[0029] The reinforcing filler can be as described below.

[0030] 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 called the material to be pyrolyzed). The physical state of the material to be pyrolyzed provided, whether in the form of powder, granules, strips or any other form, and whether in a crosslinked state or not, is not important.

[0031] Preferably, the material to be pyrolyzed can be recovered from manufactured products or products generated during their manufacturing / production (such as by-products or waste); these manufactured 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 the carbon black obtained from the pyrolysis process of the material to be pyrolyzed derived from manufactured products selected from pneumatic tires and non-pneumatic tires.

[0032] Within the scope of the present invention, pyrolysis means any type of thermal decomposition carried out under anaerobic conditions, and its raw material is the material to be pyrolyzed as defined above. Therefore, pyrolytic carbon black is different 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 containing at least a carbon-based polymer and carbon black, rather than a material derived from petroleum fractions or oils from coal or natural sources.

[0033] The pyrolytic carbon black usable within the scope of the present invention is particularly different from known carbon blacks (such as industrial carbon black, especially "furnace black") in that its ash content is higher than that of the said "furnace black". Relative to the total weight of the "furnace black", the ash content of the "furnace black" is less than 1% by weight.

[0034] Preferably, relative to the total weight of the pyrolytic carbon black, the ash content of the pyrolytic carbon black usable within the scope of the present invention ranges from 5% to 30% by weight, more preferably less than 25% by weight, and even more preferably less than 22% by weight.

[0035] Preferably, relative to the total weight of the pyrolytic carbon black, the sulfur content of the pyrolytic carbon black usable 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% to 5% by weight.

[0036] Relative to the total weight of the "furnace black", the sulfur content of the "furnace black" is less than 1.2% by weight.

[0037] Preferably, relative to the total weight of the pyrolytic carbon black, the zinc content of the pyrolytic carbon black usable within the scope of the present invention is greater than or equal to 2% by weight, preferably ranges from 2.5% to 8% by weight.

[0038] Relative to the total weight of the "furnace black", the zinc content of the "furnace black" is substantially zero and particularly less than 0.5% by weight.

[0039] Preferably, the STSA specific surface area of the pyrolytic carbon black usable within the scope of the present invention measured according to standard ASTM D6556-2021 is between 20 m 2 / g and 200 m 2 / g, more preferably between 30 m 2 / g and 90 m 2within the range of / g.

[0040] Preferably, the pyrolytic carbon black that can be used within the scope of the present invention has a void volume measured at a pressure of 50 MPa according to standard ASTM D7854 (2018) in the range of 30 ml / 100 g to 60 ml / 100 g, more preferably in the range of 35 ml / 100 g to 55 ml / 100 g.

[0041] The ash content is determined by calcining in a platinum dish in a muffle furnace at 825 °C according to the following procedure. Before each series of measurements, the platinum dish is pre-determined and the tare weight of the platinum dish is weighed to an accuracy of within 0.1 mg, and the mass is denoted as P0. A 5 g sample of pyrolytic carbon black accurately weighed to within 0.1 mg is placed into the platinum dish; this mass is denoted as P1. The platinum dish and its contents are pre-calcined using a Bunsen burner until smoke appears and the product ignites. After the combustion of the product is complete, the platinum dish and its contents are placed into a muffle furnace heated to 825 °C for 1 h. After 1 h, the platinum dish is removed from the furnace and immediately placed into a desiccator at ambient temperature. When the platinum dish and the ash have returned to ambient temperature, the platinum dish is weighed again to obtain the mass P2. Finally, the ash content (ash %) can be obtained using the following formula:

[0042]

[0043] After the sample is calcined, the ash is absorbed in an acidic medium, and the zinc content in the pyrolytic carbon black is determined by ICP-AES (inductively coupled plasma atomic emission spectrometry) analysis. The ash is obtained by performing the above procedure. Take approximately 100 mg of the ash (test sample) and place it into a PFA (perfluoroalkoxy) tube for a HotBlock hot plate. Then add 8 ml of 37% concentrated hydrochloric acid, 3 ml of 65% concentrated nitric acid, and 0.5 ml of 40% hydrofluoric acid. Seal the tube with a tube stopper and heat it at 130 °C for 2 h. After cooling, then transfer the contents to a 100 ml PTFE (polytetrafluoroethylene) volumetric flask that already contains 2 g of boric acid (for neutralizing hydrofluoric acid) using ultrapure water. Add ultrapure water up to the calibration line. Dilute the obtained solution 100 times by taking 1 ml of the solution and placing it into a 100 ml PFTE volumetric flask that already contains 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, before analyzing the diluted solution by inductively coupled plasma atomic emission spectrometry (ICP-AES), filter the diluted solution through a 0.45 μm GHP syringe filter. Before analyzing the diluted solution, analyze 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 by ICP-AES. These calibration samples are prepared by diluting a commercial solution with a certified zinc concentration of 1 g / l in a 100 ml volumetric flask.

[0044] 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 the IZn = f(c) curve, 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 sample. The measured intensity is related to the concentration using the previously obtained calibration line. Since the sample and volume have been pre - recorded, the concentration [c] is directly obtained by the software 灰分 (in mass %). The zinc concentration [c] in the pyrolytic carbon black is obtained by the following equation 炭黑 (in mass %):

[0045] [c] 炭黑 = [c] 灰分 *100* ash content %

[0046] 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, especially 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 ensures the contents of carbon (C), hydrogen (H), nitrogen (N), oxygen (O), and sulfur (S)). 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 boat. 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 starts to flow through the lance, thus accelerating the combustion of difficult - to - burn materials. 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 introduced standard sample 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

[0047] The area of the SO2 peak observed is related to the concentration by 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

[0048] Pyrolytic carbon black is sold, for example, by BlackBear under the reference number "BBCT30" or by Scandinavian EnviroSystems under the reference number "P550".

[0049] The angle (expressed in degrees) of the working crown ply is measured in the cross-section of the tire. According to the present invention, the angle is measured at the circumferential mid-plane. These measurements can also be carried out by radiography.

[0050] According to a preferred embodiment of the present invention, the rubber compound of the calendered surface layer of the two working crown plies has a composition comprising 40 phr to 70 phr of reinforcing filler, and the reinforcing filler has at least 20 phr of pyrolytic carbon black.

[0051] Advantageously, according to the present invention, the maximum value of tan(δ) (denoted as tan(δ) max ) of at least one calendered surface layer of at least one working crown ply is less than 0.130, preferably less than 0.100.

[0052] The loss factor tan(δ) is a kinetic property of the rubber compound layer. It is measured on a viscosity analyzer (Metravib VA4000) according to the ASTM D5992-96 standard. Record the response of a sample of the vulcanized composition (with a thickness of 2 mm and a cross-section of 78 mm 2 cylindrical specimen) subjected to a simple alternating sinusoidal shear stress at a frequency of 10 Hz at a temperature of 100 °C. Perform a strain amplitude sweep from 0.1% to 50% (outward cycle) and then from 50% to 1% (return cycle). For the return cycle, the maximum observed tan(δ) value is shown, denoted as tan(δ) max .

[0053] In the case where the thickness of the material is between 1 mm and 2 mm, the loss factor tan(δ) is measured on a sample of the vulcanized composition in the form of a cylindrical specimen (with a thickness of 1 mm and a cross-section of 78 mm 2 ) according to the same method as described above and under the same conditions.

[0054] Rolling resistance is the resistance that occurs when the tire rolls. Rolling resistance is represented by the hysteresis loss associated with the deformation of the tire during rotation. The frequency value associated with the rotation of the tire corresponds to the tan(δ) value measured between 30 °C and 100 °C. Therefore, the tan(δ) value at 100 °C corresponds to the rolling resistance index when the tire rolls.

[0055] According to a preferred embodiment of the invention, the at least one calendered skin of the at least one working tread ply is an elastomeric compound based on natural rubber or on synthetic polyisoprene having mainly cis-1,4 bonds, and optionally on at least one other diene elastomer, in the case of a blend, the natural rubber or synthetic polyisoprene being present in a major amount relative to the content of one or more other diene elastomers used.

[0056] Among the diene elastomers that can be used in blend with natural rubber or synthetic polyisoprene having mainly cis-1,4 bonds, mention may be made preferably of polybutadiene (BR) having mainly cis-1,4 bonds, styrene-butadiene copolymer (SBR) in solution or emulsion, butadiene-isoprene copolymer (BIR) and styrene-butadiene-isoprene terpolymer (SBIR). These elastomers may be elastomers modified during or after polymerization by a branching agent (such as divinylbenzene) or a star branching agent (such as a carbonate, a tin halide or a silicon halide), or elastomers modified by a functionalizing agent so as to graft, for example by the action of dimethylaminobenzophenone or diethylaminobenzophenone, an oxycarbonyl or carboxyl functional group or an amine functional group to the chain or the chain ends. In the case of a blend of natural rubber or synthetic polyisoprene having mainly cis-1,4 bonds with one or more of the abovementioned diene elastomers, the natural rubber or synthetic polyisoprene is preferably used in a major amount, more preferably in an amount greater than 70 phr.

[0057] Also preferably, in addition to pyrolytic carbon black, the at least one calendered skin of the at least one working tread ply of the rubber compound further comprises a reinforcing filler consisting of:

[0058] a) carbon black used in an amount between 20 phr and 50 phr, preferably between 30 phr and 40 phr,

[0059] 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 aluminosilicate, or carbon black modified during or after synthesis, used in an amount between 20 phr and 50 phr, preferably between 30 phr and 40 phr,

[0060] c) or a blend of the carbon black described in (a) and the white filler described in (b), the total filler content being between 20 phr and 50 phr, preferably between 30 phr and 40 phr.

[0061] The BET specific surface area is measured by the Brunauer, Emmet and Teller method described in "The Journal of the American Chemical Society" (Vol. 60, page 309, February 1938), which corresponds to the NFT 45007 standard of November 1987.

[0062] If a transparent filler or a white filler is used, a coupling agent and / or a covering agent selected from the reagents known to those skilled in the art must be used. As examples of preferred coupling agents, mention may be made of alkoxysilane sulfides of the bis(3-trialkoxysilylpropyl) polysulfide type, in particular bis(3-triethoxysilylpropyl) tetrasulfide sold by Degussa under the name Si69 (pure liquid product) and the name X50S (solid product (blended with N330 carbon black in a 50 / 50 ratio by weight)). As examples of covering agents, mention may be made of fatty alcohols, alkylalkoxysilanes (such as hexadecyltrimethoxysilane or hexadecyltriethoxysilane sold by Degussa under the names Si116 and Si216 respectively), diphenylguanidine, polyethylene glycol or silicone oils optionally modified by OH or alkoxy functional groups. 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 constitutes 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.

[0063] As other examples of reinforcing fillers having the above morphology and SiOH and / or AlOH surface functional groups of materials of the silica and / or alumina type and which can be used as partial or total substitutes for these materials according to the invention, mention may be made of such carbon blacks which are modified during the synthesis process by adding compounds of silicon and / or aluminum 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 at least partially cover 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, May 6-9, 1997), and those in patent application EP-A-0799 854.

[0064] When using a transparent filler as the sole reinforcing filler, by using a BET specific surface area of 30 m 2 / g to 260 m 2 / g to obtain hysteresis properties and cohesive properties. 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.

[0065] The results obtained with the tire according to the invention have effectively demonstrated that the performance in terms of durability can be improved, regardless of the nature of the ground and the running conditions, and the performance in terms of rolling resistance is improved. In particular, the endurance performance of the crown of the tire is improved with respect to the impact loads at the tread edges, especially at the shoulders of the tire, regardless of the nature of the ground and the running conditions.

[0066] The inventors believe that these results can be explained by the presence of pyrolytic carbon black used as filler in said at least one calendered skin layer of at least one working crown layer. The inventors have been able to demonstrate that the presence of pyrolytic carbon black in the rubber compound constituting said at least one calendered skin layer of at least one working crown layer gives said rubber compound elongation at break values higher than those of more conventional compounds.

[0067] The use of pyrolytic carbon black in the at least one calendered skin layer of at least one working crown layer also results in a reduction in the stiffness of the at least one calendered skin layer of at least one working crown layer. This lower stiffness compared to more common compounds is a factor that is detrimental to the durability of the tire when subjected to high stresses, such as when running at sustained speed.

[0068] The more common tire designs effectively provide that the calendered surface layer of the working crown layer has a secant modulus of elasticity at 10% elongation greater than 10 MPa. Such a modulus of elasticity is necessary in order to be able to limit the compression of the reinforcing elements of the working crown layer, in particular when the vehicle moves along a winding route, in a parking lot or when passing through a circuitous route. This is because the shearing action in the axial direction acting on the tread in the area of contact with the ground leads to compression of the reinforcing elements of the working crown layer.

[0069] The inventors have also demonstrated that the layer of circumferential reinforcing elements makes it possible to use a rubber compound based on pyrolytic carbon black having a lower modulus of elasticity without compromising the durability of the tire due to compression of the reinforcing elements of the working crown layer as described above.

[0070] The inventors have also been able to demonstrate that the cohesion of the at least one calendered skin of at least one working crown layer according to the invention remains satisfactory.

[0071] Within the meaning of the present invention, a cohesive rubber compound is a rubber compound that is particularly resistant to cracking. Thus, the cohesion of the compound 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 ) after making a notch in the specimen. The experimental range covered by the measurements is the temperature 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.

[0072] The measurements comprise three parts:

[0073] · Adaptation of the "PS" specimen to 1000 cycles at 27% deformation.

[0074] · Energy characterization to determine the "E" = f(deformation) law. The energy release rate "E" is equal to W0*h0, where W0 = the energy supplied to the material per cycle and per unit volume, and h0 = the initial height of the specimen. Thus, the acquisition of "force / displacement" data gives the relationship between "E" and the amplitude of the stress load.

[0075] · Measuring the crack after making a notch in 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".

[0076] The inventors have particularly demonstrated that the presence of at least one circumferential reinforcing element layer contributes to reducing the variation in the cohesion of at least one calendered surface layer of at least one working crown layer. This is because the surface layer of the working crown layer, which is particularly included in more conventional tire designs, has a secant modulus of elasticity greater than 10 MPa at 10% elongation, which causes a variation in the cohesion of the rubber compound layer forming the calendered surface layer of the working crown layer, and the cohesion tends to become weaker. The inventors have observed that the presence of at least one circumferential reinforcing element layer (which limits the shear stress between the ends of the working crown layer and furthermore limits the temperature rise) results in a slight variation in the cohesion of the rubber compound layer forming the calendered surface layer of the working crown layer. Thus, the inventors consider that the cohesion of the rubber compound layer forming the calendered surface layer of the working crown layer (which is less than that of the layers present in more conventional tire designs) is satisfactory in the tire design according to the present invention.

[0077] Furthermore, the compound with a lower stiffness based on pyrolytic carbon black of the rubber compound layer forming the calendered surface layer of the working crown layer (which has a circumferential reinforcing element layer) contributes to limiting the temperature rise generated when subjected to stress.

[0078] The compound of the layer of the rubber compound of lower stiffness based on pyrolytic carbon black forming the working tread cap ply (which might seemingly impair the durability of the tire under extreme service conditions) actually enables the durability performance of the tire to be maintained or even improved. The inventors believe that this result can be explained by the fact that in the presence of the circumferential reinforcing element layer, the layer of the rubber compound of lower stiffness based on pyrolytic carbon black forming the working tread cap ply has an impact on the tire tread temperature, and the combination of these factors seems to reverse the expected effect on durability.

[0079] Compared with tires having a more conventional design, the performance in terms of rolling resistance is also improved with respect to these not-too-high working temperatures.

[0080] According to an advantageous embodiment of the invention, the axially widest working tread cap ply is located radially inside another working tread cap ply.

[0081] According to an advantageous embodiment variant of the invention, the axial width of the circumferential reinforcing element layer is greater than 0.5×S.

[0082] S is the maximum axial width of the tire when the tire is mounted on its service rim and inflated to its recommended pressure.

[0083] The axial width of the reinforcing element layer is measured in the cross-section of the tire, so the tire is in the non-inflated state.

[0084] According to a preferred embodiment of the invention, the circumferential reinforcing element layer is arranged radially between two working tread cap plies.

[0085] According to this embodiment of the invention, compared with a similar layer located radially outside the working layer, the circumferential reinforcing element layer can more greatly limit the compressive action on the reinforcing elements of the carcass reinforcement. It is preferably radially separated from the carcass reinforcement by at least one working layer, thereby limiting the stress on the reinforcing elements and avoiding excessive fatigue of the reinforcing elements.

[0086] Also advantageously according to the invention, the axial width of the working tread cap ply adjacent to the at least one circumferential reinforcing element layer in the radial direction is greater than the axial width of the at least one circumferential reinforcing element layer, and preferably, on each side of the equatorial plane and in the immediate axial extension of the at least one circumferential reinforcing element layer, the working tread cap ply adjacent to the at least one circumferential reinforcing element layer is axially connected, and then is disconnected at least in the remaining width common to the two working layers by the rubber compound layer C.

[0087] According to an advantageous embodiment of the invention, the reinforcing elements of at least one circumferential reinforcing element layer are metallic reinforcing elements having a secant modulus at 0.7% elongation between 10 GPa and 120 GPa and a maximum tangent modulus of less than 150 GPa.

[0088] According to a preferred embodiment, the secant modulus of the reinforcing element at 0.7% elongation is less than 100 GPa and greater than 20 GPa, preferably between 30 GPa and 90 GPa, and even more preferably less than 80 GPa.

[0089] Also preferably, the maximum tangent modulus of the reinforcing element is less than 130 GPa, more preferably less than 120 GPa.

[0090] The above moduli are measured on a curve of tensile stress as a function of elongation (the curve being determined using a preload of 20 MPa), the tensile stress corresponding to the measured tension corrected for the metallic cross-section of the reinforcing element. The measurement is carried out on cords taken from the tire on a portion of the circumferential reinforcing element layer extending axially inwards from the axial end of the layer with an axial width of 50 mm.

[0091] The moduli of the same reinforcing element can be measured on a curve of tensile stress as a function of elongation (the curve being determined using a preload of 10 MPa), the tensile stress corresponding to the measured tension corrected for the entire cross-section of the reinforcing element. The entire cross-section of the reinforcing element is the cross-section of a composite element composed of metal and rubber, the rubber having penetrated into the reinforcing element in particular during the curing of the tire.

[0092] According to this constitution related to the entire cross-section of the reinforcing element, the reinforcing elements of the axially outer portion and the intermediate portion of at least one circumferential reinforcing element layer are metallic reinforcing elements having a secant modulus at 0.7% elongation between 5 GPa and 60 GPa and a maximum tangent modulus of less than 75 GPa.

[0093] According to a preferred embodiment, the secant modulus of the reinforcing element at 0.7% elongation is less than 50 GPa and greater than 10 GPa, preferably between 15 GPa and 45 GPa, and even more preferably less than 40 GPa.

[0094] Also preferably, the maximum tangent modulus of the reinforcing element is less than 65 GPa, more preferably less than 60 GPa.

[0095] According to a preferred embodiment, the reinforcing elements of at least one circumferential reinforcing element layer are metallic reinforcing elements having a curve of tensile stress as a function of relative elongation that shows a gentle gradient for smaller elongations and a steeper, substantially constant gradient for larger elongations.

[0096] Each of the above characteristics of the reinforcing element was measured on a reinforcing element taken from a tire.

[0097] According to the invention, a reinforcing element which is more particularly suitable for preparing at least one circumferential reinforcing element layer is, for example, a component of formula 21.23, the structure of formula 21.23 being 3×(0.26 + 6×0.23) 4.8 / 7.5 SS; such a cord is composed of 21 elementary filaments of formula 3×(1 + 6), where three strands are twisted together, each strand being composed of 7 filaments, the diameter of one filament forming the central core being equal to 26 / 100 mm and the diameter of the six winding filaments being equal to 23 / 100 mm. Such a cord has a secant modulus at 0.7% equal to 45 GPa and a maximum tangent modulus equal to 98 GPa, these moduli being measured on a curve of tensile stress as a function of elongation determined with a 20 MPa preload, the tensile stress corresponding to the measured tension corrected for the metallic cross-section of the reinforcing element. On a curve of tensile stress as a function of elongation determined with a 10 MPa preload (the tensile stress corresponding to the measured tension corrected for the overall cross-section of the reinforcing element), the cord of formula 21.23 has a secant modulus at 0.7% equal to 23 GPa and a maximum tangent modulus equal to 49 GPa.

[0098] Similarly, another example of a reinforcing element is a component of formula 21.28, the structure of formula 21.28 being 3×(0.32 + 6×0.28) 5.6 / 9.3 SS. Such a cord has a secant modulus at 0.7% equal to 56 GPa and a maximum tangent modulus equal to 102 GPa, these moduli being measured on a curve of tensile stress as a function of elongation determined with a 20 MPa preload, the tensile stress corresponding to the measured tension corrected for the metallic cross-section of the reinforcing element. On a curve of tensile stress as a function of elongation determined with a 10 MPa preload (the tensile stress corresponding to the measured tension corrected for the overall cross-section of the reinforcing element), the cord of formula 21×28 has a secant modulus at 0.7% equal to 27 GPa and a maximum tangent modulus equal to 49 GPa.

[0099] Using such a reinforcing element in at least one circumferential reinforcing element layer makes it possible in particular to maintain a satisfactory stiffness of the layer (even after the shaping and curing steps of a conventional production method).

[0100] According to a second embodiment of the present invention, the circumferential reinforcing element may be formed from inextensible metal elements which are cut so as to form portions having a length much less than the shortest layer circumference but preferably greater than 0.1 times said circumference, the incisions between said portions being axially offset relative to one another. Also preferably, the tensile elastic modulus per unit width of the additional layer is less than the tensile elastic modulus measured for the most extensible working crown ply under the same conditions. Such an embodiment makes it possible to impart a certain modulus to the circumferential reinforcing element layer in a simple manner, said modulus being easily adjustable (by choosing the spacing between the portions of the same row), but always less than the modulus of a layer composed of the same but continuous metal elements, the modulus of the additional layer being measured on a vulcanized layer of cut elements taken from the tyre.

[0101] According to a third embodiment of the present invention, the circumferential reinforcing element is a corrugated metal element, the ratio a / λ of the amplitude to the wavelength being at most equal to 0.09. Preferably, the tensile elastic modulus per unit width of the additional layer is less than the tensile elastic modulus measured for the most extensible working crown ply under the same conditions.

[0102] The metal element is preferably a steel cord.

[0103] According to a preferred embodiment of the present invention, the reinforcing element of the working crown ply is an inextensible metal cord.

[0104] A preferred embodiment of the present invention also supplements the radially outer side of the crown reinforcement by means of at least one additional layer (referred to as a protection layer) having so-called elastic reinforcing elements which are oriented at an angle between 10° and 45° relative to the circumferential direction, and the direction of said angle is the same as the angle formed by the inextensible elements of the working layer radially adjacent thereto.

[0105] According to any of the above embodiments of the present invention, the radially inner side of the crown reinforcement between the carcass reinforcement and the radially inner working layer closest to said carcass reinforcement may also be supplemented by a triangular layer having inextensible metal reinforcing elements made of steel, forming an angle greater than 60° with the circumferential direction, and the direction of said angle is the same as the angle formed by the reinforcing elements of the layer radially closest to the carcass reinforcement. BRIEF DESCRIPTION OF THE DRAWINGS

[0106] In the following, other details and advantageous features of the present invention will become apparent from the description of embodiments of the invention given with reference to the accompanying drawings, which show a meridional view of the design of a tyre according to an embodiment of the present invention.

[0107] For the sake of easier understanding, the accompanying drawings are not drawn to scale. The drawings show only a half view of the tyre, which extends symmetrically about an axis XX', which represents the circumferential median plane or equatorial plane of the tyre. DETAILED DESCRIPTION

[0108] In the drawings, the tyre 1 has a size of 315 / 70R 22.5. The tyre 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 cords. This carcass reinforcement 2 is hoop-reinforced by a crown reinforcement 4 which is formed, from the inside to the outside in the radial direction, by:

[0109] - a first working layer 41 formed of metal cords oriented at an angle equal to 22°,

[0110] - a circumferential reinforcement element layer 43 formed of steel metal cords of type 21.23,

[0111] - a second working layer 42 formed of metal cords which are oriented at an angle equal to 18° and cross the metal cords of layer 41, the cords of each of the working layers 41, 42 being oriented on either side of the circumferential direction,

[0112] - a protective layer 44 formed of elastic 6.35 metal cords, the distance between the reinforcement elements measured along a direction perpendicular to the bisector of the cords being equal to 2.5 mm, the elastic 6.35 metal cords being oriented at an angle equal to 18° on the same side as the cords of the second working layer.

[0113] The metal cords of the reinforcement elements constituting the two working layers are cords of type 9.35. They are distributed within each working layer at a distance equal to 2.2 mm between the reinforcement elements, the distance being measured along a direction perpendicular to the bisector of the cords.

[0114] The crown reinforcement is itself covered by a tread 5.

[0115] The tyre is inflated to a pressure of 9 bar.

[0116] The axial width L of the first working layer 41 41 is equal to 252 mm.

[0117] The axial width L of the second working layer 42 42 is equal to 232 mm.

[0118] The axial width L of the circumferential reinforcement element layer 43 43 is equal to 194 mm.

[0119] The axial width L5 of the tread is equal to 266 mm.

[0120] The maximum axial width L is equal to 315.9 mm.

[0121] According to the invention, the calendered surface layers of the working crown plies 41, 42 are made of an elastomeric compound containing pyrolytic carbon black.

[0122] Various tires according to the invention were compared with various reference tires of the same size.

[0123] The tire I according to the invention has calendered surface layers of the working crown plies 41, 42 made of compound 1.

[0124] The reference tire T1 differs from the tire I according to the invention in the properties of the compounds for the calendered surface layers of the working crown plies, which calendered surface layers are made of compound R.

[0125] The various compounds used are listed below, each compound showing the secant modulus of elasticity, elongation at break and tan(δ) max values

[0126]

[0127]

[0128] The values of the components are expressed in phr (parts by weight per hundred parts of elastomer).

[0129] The pyrolytic carbon black (carbon black RCB) contains 20% ash, 1.8% sulfur and 4.5% zinc.

[0130] The carbon black N347 contains 0.5% ash, 1% sulfur and 0% zinc.

[0131] The contents of the various components other than carbon black in compound I1 are adjusted according to the knowledge of a person skilled in the art so as to obtain similar temperature and curing time conditions for the various tires and so that the properties of the tires can be compared.

[0132] A first (especially heat - required) durability test is carried out on a testing machine, each tire rolling in a straight line at a speed equal to the maximum speed rating (or speed index) specified for the tire under an initial load of 4000 kg, and in order to shorten the duration of the test, the initial load is gradually increased.

[0133] 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.

[0134] The tests thus carried out show that the distance covered by the tyre according to the invention and the reference tyre is substantially the same in each of these tests.

[0135] Tests were also carried out to characterise the breaking strength of the tyre crown reinforcement subjected to impact loads. These tests consisted of running a tyre inflated to the recommended pressure and subjected to the recommended load over a cylindrical obstacle or indentor tool having a diameter equal to 1.5 inches (i.e. 38.1 mm), a hemispherical head and a given height. The trajectory of the tyre was adjusted so that the axis of the obstacle corresponded to the position of one of the axially outermost ribs of the tread. The breaking strength is characterised by the critical height of the indentor tool, i.e. the maximum height at which the indentor tool causes complete breakage of the crown reinforcement (i.e. breakage of all the crown plies). These values represent the energy required to break the crown blocks. These values are expressed relative to a base of 100 (which corresponds to the value measured for the reference tyre T1).

[0136] I 105 Reference T1 100

[0137] These results show that the breaking energy of the tyre I according to the invention is higher than that of the tyre T1 during the impact of the tread surface.

[0138] Final durability tests were carried out to reproduce driving conditions combining vehicle speed and particularly adverse ground. These tests thus reproduce the extreme conditions especially for "heavy-duty load" vehicles of the "site supply" type.

[0139] This final test consists of a phase of driving on a circuit at 100 km / h for 2 hours under the load and pressure conditions indicated on the tyre, followed by a phase of driving on a stony road at 35 km / h for 12 minutes, repeated 25 times.

[0140] The purpose of the phase of driving at low speed on a stony road is to have an adverse effect on durability due to the repeated impact loads on the tread.

[0141] The purpose of the phase of driving at high speed on a circuit is to raise the temperature of the tyre. This makes the tyre more sensitive to the effects of repeated impact loads on the tread and promotes the propagation of cracks initiated during the phase of driving on a stony road.

[0142] At the end of the run, the tyres were inspected and dissected using shearography so that any damage could be analysed. This is a visual analysis, allowing a comparison to be made between any cracks and crack propagation. The tyres were scored and compared with each other. A score greater than 100 corresponds to a tyre with less damage. The most severely damaged tyre received a score of 100.

[0143] Score: I 120 Reference T1 100

[0144] At the end of the run, the tyre I according to the invention exhibits a lower degree of damage than the reference tyre T1.

[0145] During these final endurance tests, after driving for 2 hours on a circuit at 100 km / h in the first stage, the temperature of the tyre is measured at the ends of the tread blocks.

[0146] Temperature I 91℃ Reference T1 94℃

[0147] These results show that the temperature exhibited by the tyre according to the invention is not actually as high as that of the reference tyre.

[0148] Furthermore, the rolling resistance is measured.

[0149] The rolling resistance of each tyre is measured under the same driving conditions in accordance with Regulation No. 117 of the United Nations Economic Commission for Europe (UNECE). The measurement results are expressed in kg / t, with a value of 100 assigned to tyre T1. A value greater than 100 indicates better performance in terms of rolling resistance.

[0150] T1 I Rolling resistance 100 103

[0151] It is clear from these tests that the tyre according to the invention can improve the performance in terms of rolling resistance and shock load resistance in a satisfactory manner while exhibiting satisfactory endurance performance.

Claims

1. A tire (1) including a radial carcass reinforcement (2), said tire including a crown reinforcement (4), said crown reinforcement (4) including two working crown layers (41, 42) having reinforcing elements and at least one circumferential reinforcing element layer (43), the reinforcing elements being inserted between two calendered surface layers of rubber compound, crossing from one layer to the other, and forming an angle between 10° and 45° with the circumferential direction, the angle being oriented on both sides of the circumferential direction, a rubber compound layer C being arranged between at least the ends of the two working crown layers, the crown reinforcement (4) being radially covered by a tread (5), the tread being connected via two sidewalls to two beads (3), characterized in that, The rubber compound of at least one calendered surface layer constituting at least one working tread ply comprises a composition containing 40 phr to 70 phr of reinforcing filler, with the reinforcing filler having at least 20 phr of pyrolytic carbon black.

2. The tire according to claim 1, wherein, Relative to the total weight of the pyrolytic carbon black, 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%, 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, Relative to the total weight of the pyrolytic carbon black, 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%. 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, Relative to the total weight of the pyrolytic carbon black, 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%. 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 the preceding claims, characterized in that, The at least one surface layer of at least one working tread ply is an elastomeric 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, the content of natural rubber or synthetic polyisoprene is present in a major amount relative to the one or more other diene elastomers used.

6. The tire (1) according to any one of the preceding claims, characterized in that, In addition to the pyrolytic carbon black, the at least one calendered surface layer of at least one working tread ply of the rubber compound further comprises a reinforcing filler consisting of: a) carbon black used in an amount between 20 phr and 50 phr, preferably between 30 phr and 40 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 20 phr and 50 phr, preferably between 30 phr and 40 phr, c) or a blend of the carbon black described in (a) and the white filler described in (b), where the total filler content is between 20 phr and 50 phr, preferably between 30 phr and 40 phr.

7. The tire (1) according to any one of the preceding claims, characterized in that, The axial width of the two working tread plies (41, 42) is greater than the axial width of the at least one circumferential reinforcing element layer (43).

8. The tire (1) according to any one of the preceding claims, characterized in that, The at least one circumferential reinforcing element layer (43) is located radially between the two working tread plies (41, 42).

9. The tire (1) according to any one of the preceding claims, characterized in that, The reinforcing elements of the at least one circumferential reinforcing element layer (43) are metal reinforcing elements having a secant modulus at 0.7% elongation between 10 GPa and 120 GPa and a maximum tangent modulus of less than 150 GPa.

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