WO2019197764A1

WO2019197764A1 - Tyre having improved rolling resistance and impact resistance properties

Info

Publication number: WO2019197764A1
Application number: PCT/FR2019/050816
Authority: WO
Inventors: Nathalie Salgues; Katia BARAN; Arnaud Gommez; Christophe Gayton
Original assignee: Compagnie Generale Des Etablissements Michelin
Priority date: 2018-04-09
Filing date: 2019-04-08
Publication date: 2019-10-17

Abstract

The invention relates to a tyre comprising a crown reinforcement formed of at least two working crown layers of reinforcing elements, a layer C of rubber compound being arranged between at least the ends of said at least two working crown layers. According to the invention, the maximum value of tan(δ), denoted tan(δ)_max, of the layer C is less than 0.055 and the elongation at break of the layer C is greater than 300%.

Description

PNEUMATIC HAVING IMPROVED ROLLING RESISTANCE AND SHOCK RESISTANCE PROPERTIES

The present invention relates to a tire with a radial carcass reinforcement and more particularly a tire intended to equip vehicles carrying heavy loads and traveling at a high speed, such as, for example, trucks, tractors, trailers or road buses.

In general, in heavy-vehicle type tires, the carcass reinforcement is anchored on both sides in the region of the bead and is radially surmounted by a crown reinforcement consisting of at least two layers. , superimposed and formed of son or parallel cables in each layer and crossed from one layer to the next in making with the circumferential direction angles between 10 ° and 45 °. Said working layers, forming the working armature, can still be covered with at least one so-called protective layer and formed of advantageously metallic and extensible reinforcing elements, called elastic elements. It may also comprise a layer of low extensibility wires or metal cables forming with the circumferential direction an angle of between 45 ° and 90 °, this so-called triangulation ply being radially located between the carcass reinforcement and the first ply of plywood. so-called working top, formed of parallel wires or cables having angles at most equal to 45 ° in absolute value. The triangulation ply forms with at least said working ply a triangulated reinforcement, which presents, under the different stresses it undergoes, few deformations, the triangulation ply having the essential role of taking up the transverse compression forces of which the object all the reinforcing elements in the area of the crown of the tire.

Cables are said to be inextensible when said cables have under a tensile force equal to 10% of the breaking force a relative elongation at most equal to 0.2%.

Cables are said elastic when said cables have under tensile force equal to the breaking load a relative elongation of at least 3% with a maximum tangent modulus of less than 150 GPa. [00051] Circumferential reinforcing elements are reinforcing elements which make angles with the circumferential direction in the range + 2.5 °, -2.5 ° around 0 °.

The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the rolling direction of the tire.

[00071] The transverse or axial direction of the tire is parallel to the rotational axis of the tire.

[00081] The radial direction is a direction intersecting the rotational axis of the tire and perpendicular to it.

The axis of rotation of the tire is the axis around which it rotates in normal use.

A radial or meridian plane is a plane which contains the axis of rotation of the tire. The circumferential mid-plane, or equatorial plane, is a plane perpendicular to the axis of rotation of the tire and which divides the tire into two halves.

The term "modulus of elasticity" of a rubber mix, a secant modulus of extension at 10% deformation and at room temperature.

As regards the rubber compositions, the modulus measurements are made in tension according to the AFNOR-NFT-46002 standard of September 1988: the secant modulus is measured in second elongation (ie, after an accommodation cycle). nominal (or apparent stress, in MPa) at 10% elongation (normal conditions of temperature and hygrometry according to AFNOR-NFT-40101 of December 1979).

Some current tires, called "road", are intended to run at high speed and on longer and longer journeys, due to the improvement of the road network and the growth of the motorway network in the world. The set of conditions, under which such a tire is called to roll, undoubtedly makes it possible to increase the number of kilometers traveled, with less tire wear; against the endurance of the latter and in particular of the crown reinforcement is penalized.

Indeed, there are constraints at the level of the crown reinforcement and more particularly shear stresses between the crown layers, allied to a non-negligible increase in the operating temperature at the ends of the crown layer axially. the shortest, which result in the appearance and propagation of cracks of the rubber at said ends.

In order to improve the endurance of the crown reinforcement of the tire type studied, solutions relating to the structure and quality of the layers and / or profiles of rubber mixtures which are arranged between and / or around the ends of the webs and more particularly the ends of the axially shortest ply have already been made.

It is in particular known to introduce a layer of rubber mixture between the ends of the working layers to create a decoupling between said ends to limit the shear stresses. However, such decoupling layers must have a very good cohesion. Such layers of rubber compounds are described, for example, in the patent application WO 2004/076204.

Patent FR 1 389 428, to improve the resistance to degradation of rubber compounds located in the vicinity of the crown reinforcement edges, recommends the use, in combination with a low hysteresis tread, of a rubber profile covering at least the sides and the marginal edges of the crown reinforcement and consisting of a rubber mixture with low hysteresis.

FR 2 222 232, to avoid separations between crown reinforcement plies, teaches to coat the ends of the frame in a rubber mat, whose Shore A hardness is different from that of the strip. rolling overlying said armature, and greater than the Shore A hardness of the rubber mix profile disposed between the edges of crown reinforcement plies and carcass reinforcement.

The tires thus produced can actually improve performance especially in terms of endurance. Furthermore, it is known to make tires with a very wide tread or to give tires of a given dimension greater load capacities to introduce a layer of circumferential reinforcing elements. The patent application WO 99/24269 describes for example the presence of such a layer of circumferential reinforcing elements.

The layer of circumferential reinforcing elements is usually constituted by at least one wire rope wound to form a turn whose laying angle relative to the circumferential direction is less than 2.5 °.

The inventors have demonstrated that such tires could be at risk of degradation after repeated shocks for example on sidewalks.

An object of the invention is to provide tires whose performance in terms of rolling resistance are improved to contribute to lower fuel consumption by vehicles equipped with such tires while imparting improved impact resistance properties .

This object is achieved according to the invention by a radial carcass reinforcement tire comprising a crown reinforcement formed of at least two working crown layers of reinforcing elements, crossed from one layer to the other by making with the circumferential direction angles between 10 ° and 45 °, a layer C of rubber mixture being disposed between at least the ends of said at least two working crown layers, the crown reinforcement being capped radially with a tread, said tread being joined to two beads via two sidewalls, the maximum value of tan (δ), denoted tan (δ) _max , of the layer C being less than 0.055 and the elongation at rupture of the layer C being greater than 300%. The loss factor tan (δ) is a dynamic property of the layer of rubber mix. It is measured on a viscoanalyzer (Metravib VA4000), according to ASTM D 5992-96. The response of a sample of vulcanized composition (cylindrical specimen 2 mm thick and 78 mm ² in section), subjected to sinusoidal stress in alternating simple shear, at the frequency of 10 Hz, was recorded at temperature of 100 ° C. A strain amplitude sweep of 0.1 to 50% (forward cycle) and then 50% to 1% (return cycle) are performed. For the return cycle, the maximum value of tan (δ) observed, denoted tan (δ) _{max, is indicated} .

The rolling resistance is the resistance that appears when the tire rolls. It is represented by the hysteretic losses related to the deformation of the tire during a revolution. The frequency values related to the revolution of the tire correspond to values of tan (δ) measured between 30 and 100 ° C. The value of tan (δ) at 100 ° C thus corresponds to an indicator of the rolling resistance of the rolling tire. [00281 The elongation at break (in%) is measured according to the standard

AFNOR-NF-T-46-002 of September 1988. The tensile measurements for determining the breaking properties are carried out at a temperature of 100 ° C. ± 2 ° C., and under normal hygrometry conditions (50 ± 5%). relative humidity), according to the French standard NF T 40-101 (December 1979). A measurement is made on samples taken directly from a new tire and another measurement is carried out on samples taken from a new tire and which have undergone an aging of 10 days at 110 ° C. under nitrogen. This aging simulates an extreme use of the tire throughout its lifetime.

The layer C rubbery mixture provides a decoupling of said working crown layers to distribute the shear stresses on a greater thickness.

Within the meaning of the invention, the coupled layers are layers whose respective reinforcing elements are radially separated by at most 1.5 mm, said rubber thickness being measured radially between the respectively upper and lower generatrices of said elements of enhancement.

The use of such mixtures whose tan (δ) _{max value} is less than 0.055 and the elongation value at rupture is greater than 300% to achieve the layer C will improve the properties of the tire in terms of endurance especially against repeated shocks on sidewalks while maintaining satisfactory rolling resistance properties.

According to a preferred embodiment of the invention, the layer C of rubber compound is an elastomeric mixture based on natural rubber or synthetic polyisoprene with a majority of cis-1,4 bonds and possibly at least one other diene elastomer, the natural rubber or the synthetic polyisoprene in case of cutting being present at a majority rate with respect to the rate of the other or other diene elastomers used and a reinforcing filler mainly comprising at least one carbon black, exhibiting a BET specific surface area at most equal to 30 m ² / g and a compressed samples oil absorption index (COAN) of at least 60 ml / 100 g, said elastomeric mixture not comprising, or comprising at most 20 phr, and preferably at most 10 phr of carbon black, the BET specific surface area is greater than 30 m ² / g and the COAN index is greater than 40 ml / 100 g, said mixture elastomeric composition not comprising, or comprising at most 20 phr, and preferably at most 10 phr of white filler, and the crosslinking density measured according to the equilibrium swelling method being between 13 × 10 ⁵ mol / cm ³ and 21 × 10 ⁵ mol / cm ³ in said elastomeric mixture constituting the calendering layers of said at least two working crown layers.

The measurement of BET specific surface area is carried out according to the method of BRUNAUER, EMMET and TELLER described in "The Journal of the American Chemical Society", vol. 60, page 309, February 1938, corresponding to standard NFT 45007 of November 1987.

The structure index of black COAN (Compressed Oil Absorption Number) is measured according to the ASTM D3493 standard. Within the meaning of the invention, a white filler is a filler of silica and / or alumina type comprising SiOH and / or AlOH surface functional groups chosen from the group formed by precipitated or pyrogenic silicas, aluminas or aluminosilicates. .

As other examples of reinforcing fillers having the morphology and the SiOH and / or AlOH surface functions of the silica and / or alumina type materials and capable of to be used according to the invention as a partial or total replacement thereof, there may be mentioned the modified carbon blacks either during the synthesis by addition to the furnace feed oil of a silicon compound and / or of aluminum after synthesis by adding, to an aqueous suspension of carbon black in a solution of silicate and / or sodium aluminate, an acid so as to at least partially cover the surface of the carbon black of SiOH functions and / or AlOH. As non-limiting examples of this type of carbonaceous feedstock with SiOH and / or AlOH functions at the surface, mention may be made of the CSDP type feeds described in Conference No. 24 of the ACS Meeting, Rubber Division, Anaheim, California, May 6-9. 1997 and those of the patent application EP-A-0 799 854. [00371 According to the invention, the crosslinking density measurements are made from the equilibrium swelling method. In order to measure the crosslinking density, the mixtures, prepared in the form of samples, are swollen in cyclohexane for 72 hours. The weight of the samples is measured immediately after removing the excess solvent with blotting paper. The swelling of the samples and the absorption of solvent is inversely proportional to the presence and therefore to the density of crosslinking bridges.

The samples are then dried under vacuum until a constant weight is reached. From the difference between the two measured weight values, a swelling rate is deduced. Blackness swelling restriction is dispensed with by applying Flory Rhener's theory described in "Journal of Applied Polymer Science, 2016, 133, 43932". It is thus possible to determine the pontic density in 10 ⁵ mol / cm ³ .

According to a preferred embodiment of the invention, the carbon black content whose BET specific surface area is at most equal to 30 m ² / g is between 20 and 80 phr, preferably between 40 and 65 phr. and more preferably between 45 and 60 phr. Advantageously according to the invention, said carbon black, whose BET surface area is at most equal to 30 m ² / g, has a CO AN index of at least 65 μm / 100 g, and preferably at least equal to 70 ml / 100 g. Advantageously also according to the invention, said carbon black whose BET surface area is at most equal to 30 m ² / g has a CO AN index of at most 90 ml / 100 g.

And preferably according to the invention, the BET specific surface area of said carbon black at most equal to 30 m ² / g is at most equal to 25 m ² / g, and preferably greater than 15 m ² / g.

In the case of using clear charge or white charge, it is necessary to use a coupling agent and / or covering selected from agents known to those skilled in the art. Examples of preferential coupling agents that may be mentioned are sulphurised alkoxysilanes of the bis (3-trialkoxysilylpropyl) polysulfide type, and of these, in particular, bis (3-triethoxysilylpropyl) tetrasulfide marketed by DEGUSSA under the Si69 denominations for pure liquid product and X50S for solid product (50/50 by weight blend with N330 black). Examples of coating agents that may be mentioned include a fatty alcohol, an alkylalkoxysilane such as hexadecyltrimethoxy or triethoxysilane respectively marketed by DEGUSSA under the names Sil 16 and Si 16, diphenylguanidine, a polyethylene glycol, a silicone oil which may be modified with OH or alkoxy functions. The covering agent and / or coupling agent is used in a weight ratio relative to the filler> at 1/100 and <at 20/100, and preferably between 2/100 and 15/100 when the clear filler represents the all of the reinforcing filler and between 1/100 and 20/100 when the reinforcing filler is constituted by a carbon black and clear charge cutting.

Among the diene elastomers which can be used in a blend with natural rubber or a synthetic polyisoprene with a majority of cis-1,4 linkages, a polybutadiene (BR) may be mentioned, preferably with a majority of cis-1 linkages, 4, a styrene-butadiene copolymer (SBR) solution or emulsion, a butadiene-isoprene copolymer (BIR) or even a styrene-butadiene-isoprene terpolymer (SBIR). These elastomers may be modified elastomers during polymerization or after polymerization by means of branching agents such as divinylbenzene or starch agents such as carbonates, halogenotins, halosilicons or else by means of functionalization leading to grafting on the chain or at the end oxygenated carbonyl, carboxyl or an amine function chain such as for example by the action of dimethyl or diethylamino benzophenone. In the case of blends of natural rubber or synthetic polyisoprene with a majority of cis-1,4 with one or more of the diene elastomers mentioned above, the natural rubber or the synthetic polyisoprene is preferably used at a majority rate. and more preferably at a rate greater than 70 phr.

Preferably, the thickness of the first layer C of rubber mixture, measured at the end of the least wide working crown layer of the two working crown layers considered, will preferably be between 30% and 80%. the overall thickness of the rubber mixture between the cable generators respectively of the two working crown layers: a thickness of less than 30% does not make it possible to obtain convincing results, and a thickness greater than 80% is useless with respect to improvement in the resistance to separation between layers and disadvantageous from the point of view cost.

According to an advantageous embodiment of the invention, the axially widest working crown layer is radially inside the other working crown layers.

Preferably according to the invention, the axial width D of the layer of rubber mix C between the axially innermost end of said layer of rubber mix C and the end of the top layer of axially the least wide work is such that:

3.f ₂ £ D <25. f ₂ with f2, diameter of the reinforcement elements of the axially least wide working crown layer. Such a relationship defines a zone of engagement between the layer of rubbery mixture C and the axially-smaller working crown layer. Such an engagement below a value equal to three times the diameter of the reinforcing elements of the axially smaller working layer may not be sufficient to obtain a decoupling of the working crown layer, in particular to obtain attenuation of the stresses. at the end of the axially lower working crown layer. A value of this commitment greater than twenty times the diameter of the elements Reinforcement of the axially less wide working layer can lead to an excessive reduction in the stiffness of the G crown reinforcement of the tire.

Preferably, the axial width D of the layer of rubber mix C between the axially innermost end of said layer of rubber mix C and the end of the axially narrower working crown layer. is greater than 5 mm.

[00491] The invention further preferably provides that the thickness of the layer of rubber mix C at the axially outer end of the axially least wide working crown layer has a thickness such that the radial distance d between the two Working crown layers, separated by the layer of rubber mix C, verify the relation:

3/5. f ₂ <d <5.f ₂ with f ₂ , diameter of the reinforcement elements of the axially least wide working crown ply.

The distance d is measured from cable to cable, that is to say between the cable of a first working layer and the cable of a second working layer. In other words, this distance d includes the thickness of the layer of rubber mix C and the respective thicknesses of the calendering rubber mixes radially external to the cables of the radially inner and radially inner working layer of the layer cables. radially outer working.

The various thickness measurements are made on a cross section of a tire, the tire is therefore in a non-inflated state.

According to an advantageous embodiment of the invention the crown reinforcement comprises at least one layer of circumferential metal reinforcing elements.

Advantageously again according to this variant of the invention, the elastic modulus under tension at 10% elongation of said mixing layer C rubbery material disposed between at least the ends of said at least two working crown layers is less than 6 MPa.

The most common tire designs provide for layers of rubber mixture disposed between the ends of the working crown layers with tensile elastic moduli at 10% elongation greater than 8.5 MPa, in particular to make it possible to limit the stresses. shearing between the ends of the working crown layers, the circumferential stiffnesses of said working crown layers being zero at their end. Such modules, which are often even greater than 9 MPa, make it possible to avoid the primers and crack propagation in the rubber mixes at the ends of said working crown layers and more particularly at the end of the most effective working layer. narrow.

The inventors have been able to demonstrate that the presence of at least one layer of circumferential reinforcing elements makes it possible to maintain performance, particularly in terms of endurance but also in terms of satisfactory wear with a modulus of elasticity. under tension at 10% elongation of layer C less than 6 MPa.

The inventors have further demonstrated that the cohesion of the layer C, when it has a tensile modulus of elasticity at 10% elongation of less than 6 MPa, remains satisfactory.

For the purposes of the invention, a cohesive rubbery mixture is a rubbery mixture particularly resistant to cracking. The cohesion of a mixture is thus evaluated by a fatigue cracking test performed on a specimen "PS" (pure shear). It consists in determining, after notching the specimen, the crack propagation speed "Vp" (nm / cycle) as a function of the energy release rate "E" (J / m ² ). The experimental area covered by the measurement is in the range -20 ° C and + 150 ° C in temperature, with an atmosphere of air or nitrogen. The stress on the test specimen is a dynamically imposed amplitude displacement of between 0.1 mm and 10 mm in the form of a pulsed type stress (tangential "haversine" signal) with a rest time equal to the duration of the pulse; the frequency of the signal is of the order of 10 Hz on average.

The measurement comprises 3 parts: • Line accommodation of the "PS" test tube, from 1000 cycles to 27% deformation.

• an energetic characterization to determine the law "E" = f (deformation). The energy release rate "E" is equal to W0 * h0, with WO = energy supplied to the material per cycle and per unit volume and hO = initial height of the test piece. Exploitation of "force / displacement" acquisitions thus gives the relationship between

"E" and the amplitude of the solicitation.

The measurement of cracking, after notching of the test piece "PS". The information collected leads to determining the propagation velocity of the crack "Vp" as a function of the imposed stress level "E". The inventors have in particular demonstrated that the presence of at least one layer of circumferential reinforcing elements contributes to a lesser evolution of the cohesion of the layer C. In fact, the more usual tire designs comprising in particular layers of rubber mixture disposed between the ends of the working crown layers with tensile modulus of elasticity at 10% elongation greater than 8.5 MPa, lead to a change in the cohesion of said layers of rubber mixture disposed between the ends of the layers of working top, the latter tending to weaken. The inventors find that the presence of at least one layer of circumferential reinforcing elements which limits the shear stresses between the ends of the working crown layers and furthermore limits the temperature increases leads to a slight change in the cohesion The inventors thus consider that the cohesion of the layer C, which is smaller than that which exists in the more usual tire designs, is satisfactory in the design of the tire according to the invention.

According to an advantageous embodiment of the invention, the layer of circumferential reinforcing elements has an axial width greater than 0.5xS.

S is the maximum axial width of the tire, when the latter is mounted on its service rim and inflated to its recommended pressure. The axial widths of the layers of reinforcing elements are measured on a cross section of a tire, the tire therefore being in a non-inflated state.

According to a preferred embodiment of the invention, at least two working crown layers have different axial widths, the difference between the axial width of the axially widest working crown layer and the axial width of the working layer. axially the axially the least wide working vertex being between 10 and 30 mm.

According to a preferred embodiment of the invention, the layer of circumferential reinforcing elements is radially arranged between two working crown layers.

According to this embodiment of the invention, the layer of circumferential reinforcing elements makes it possible to limit more significantly the compression of the reinforcement elements of the carcass reinforcement than a similar layer set up. radially outside the working layers. It is preferably radially separated from the carcass reinforcement by at least one working layer so as to limit the stresses of said reinforcing elements and do not strain them too much.

Advantageously again according to the invention, the axial widths of the working crown layers radially adjacent to the layer of circumferential reinforcement elements are greater than the axial width of said layer of circumferential reinforcement elements and preferably, said working crown layers adjacent to the layer of circumferential reinforcing elements are on either side of the equatorial plane and in the immediate axial extension of the layer of circumferential reinforcing elements coupled over an axial width, to be subsequently decoupled by said first layer of rubber mixture C at least over the remainder of the width common to said two working layers.

The presence of such couplings between the working crown layers adjacent to the layer of circumferential reinforcing elements makes it possible to reduce the tension stresses acting on the circumferential elements axially the outermost and located closest to the coupling.

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

According to a preferred embodiment, the secant modulus of the reinforcing elements at 0.7% elongation is less than 100 GPa and greater than 20 GPa, preferably between 30 and 90 GPa and more preferably less than 80 GPa. Also preferably, the maximum tangent modulus of the reinforcing elements is less than 130 GPa and more preferably less than 120 GPa.

The modules expressed above are measured on a tensile stress curve as a function of the elongation determined with a preload of 20 MPa reduced to the metal section of the reinforcing element, the tensile stress corresponding to a measured voltage brought back to the metal section of the reinforcing element.

The modules of the same reinforcing elements can be measured on a tensile stress curve as a function of the elongation determined with a preload of 10 MPa reduced to the overall section of the reinforcing element, the tensile stress corresponding to a measured voltage brought back to the overall section of the reinforcing element. The overall section of the reinforcing element is the section of a composite element made of metal and rubber, the latter having in particular penetrated the reinforcing element during the baking phase of the tire.

According to this formulation relating to the overall section of the reinforcing element, the reinforcing elements of the axially outer portions and the central portion of at least one layer of circumferential reinforcing elements are metal reinforcing elements having a secant modulus at 0.7% elongation between 5 and 60 GPa and a maximum tangent modulus of less than 75 GPa. According to a preferred embodiment, the secant modulus of the reinforcing elements at 0.7% elongation is less than 50 Gpa and greater than 10 GPa, preferably between 15 and 45 GPa and more preferably less than 40 GPa.

Also preferably, the maximum tangent modulus of the reinforcing elements is less than 65 GPa and more preferably less than 60 GPa.

According to a preferred embodiment, the reinforcing elements of at least one layer of circumferential reinforcing elements are metal reinforcing elements having a tensile stress curve as a function of the relative elongation having slight slopes for the elements. low elongations and a substantially constant and strong slope for the higher elongations. Such reinforcing elements of the additional ply are usually referred to as "bi-module" elements.

According to a preferred embodiment of the invention, the substantially constant and strong slope appears from a relative elongation of between 0.1% and 0.5%.

The various characteristics of the reinforcing elements stated above are measured on reinforcing elements taken from tires.

Reinforcement elements more particularly adapted to the production of at least one layer of circumferential reinforcing elements according to the invention are for example assemblies of formula 21.23, the construction of which is 3x (0.26 + 6x0.23). 4.4 / 6.6 SS; this strand cable consists of 21 elementary wires of formula 3 x (1 + 6), with 3 twisted strands each consisting of 7 wires, a wire forming a central core of diameter equal to 26/100 mm and 6 coiled wires of diameter equal to 23/100 mm. Such a cable has a secant module at 0.7% equal to 45 GPa and a maximum tangent modulus equal to 98 GPa, measured on a tensile stress curve as a function of the elongation determined with a preload of 20 MPa brought back to the section. of the reinforcing element, the tensile stress corresponding to a measured voltage brought back to the metal section of the reinforcing element. On a tensile stress curve as a function of the elongation determined with a preload of 10 MPa brought back to the overall section of the reinforcing element, the tensile stress corresponding to a measured tension brought back to the overall section of the element of reinforcement, this cable Formula 21.23 has a secant modulus at 0.7% equal to 23 GPa and a maximum tangent modulus equal to 49 GPa.

In the same way, another example of reinforcement elements is an assembly of formula 21.28, whose construction is 3x (0.32 + 6x0.28) 6.2 / 9.3 SS. This cable has a secant module at 0.7% equal to 56 GPa and a maximum tangent modulus equal to 102 GPa, measured on a tensile stress curve as a function of the elongation determined with a preload of 20 MPa brought to the cross section. metal of the reinforcing element, the tensile stress corresponding to a measured voltage brought back to the metal section of the reinforcing element. On a tensile stress curve as a function of the elongation determined with a preload of 10 MPa brought back to the overall section of the reinforcing element, the tensile stress corresponding to a measured tension brought back to the overall section of the element of reinforcement, this cable of formula 21.28 has a secant module at 0.7% equal to 27 GPa and a maximum tangent modulus equal to 49 GPa.

The use of such reinforcing elements in at least one layer of circumferential reinforcing elements makes it possible in particular to maintain rigidity of the satisfactory layer including after the shaping and baking steps in conventional manufacturing processes.

According to a second embodiment of the invention, the circumferential reinforcing elements may be formed of inextensible metal elements and cut so as to form sections of length much shorter than the circumference of the least long layer, but preferably greater than 0, 1 times said circumference, the cuts between sections being axially offset with respect to each other. More preferably, the tensile modulus of elasticity per unit width of the additional layer is less than the tensile modulus of elasticity, measured under the same conditions, of the most extensible working crown layer. Such an embodiment makes it possible to confer, in a simple manner, on the layer of circumferential reinforcement elements a module which can easily be adjusted (by the choice of intervals between sections of the same row), but in all cases weaker. that the modulus of the layer consisting of the same metallic elements but continuous, the modulus of the layer additional being measured on a vulcanized layer of cut elements, taken from the tire.

According to a third embodiment of the invention, the circumferential reinforcing elements are corrugated metal elements, the ratio a / l of the amplitude of waviness over the wavelength being at most equal to 0.09. . Preferably, the tensile modulus of elasticity per unit width of the additional layer is smaller than the tensile modulus of elasticity, measured under the same conditions, of the most extensible working crown layer.

[00841 The metal elements are preferably steel cables. According to a preferred embodiment of the invention, the reinforcing elements of the working crown layers are inextensible metal cables.

The invention also advantageously provides for reducing the tension stresses acting on the axially outermost circumferential elements that the angle formed with the circumferential direction by the reinforcement elements of the working crown layers is less than 30 ° and preferably less than 25 °.

[0087] A preferred embodiment of the invention further provides that the crown reinforcement is completed radially on the outside by at least one additional layer, called protective layer, of so-called elastic reinforcing elements, oriented with respect to the direction circumferential with an angle between 10 ° and 45 ° and in the same direction as the angle formed by the inextensible elements of the working layer which is radially adjacent thereto.

The protective layer may have an axial width smaller than the axial width of the less wide working layer. Said protective layer may also have an axial width greater than the axial width of the narrower working layer, such that it covers the edges of the narrower working layer and, in the case of the radially upper layer, being the smallest, as coupled, in the axial extension of the additional reinforcement, with the widest working crown layer over an axial width, to be subsequently, axially outside, decoupled from said widest working layer with profiles at least 2 mm thick. The protective layer formed of elastic reinforcing elements may, in the case mentioned above, be on the one hand possibly decoupled from the edges of said least-wide working layer by profiles of thickness substantially less than the thickness of the profiles separating the edges of the two working layers, and have on the other hand an axial width smaller than or greater than the axial width of the widest vertex layer.

According to any one of the embodiments of the invention mentioned above, the crown reinforcement may be further completed radially inwards between the carcass reinforcement and the radially inner working layer closest to the reinforcement. said carcass reinforcement, by a triangulation layer of steel non-extensible reinforcing elements making, with the circumferential direction, an angle greater than 60 ° and in the same direction as that of the angle formed by the reinforcing elements of the layer radially closest to the carcass reinforcement.

The tire according to the invention as it has just been described therefore has performance in terms of endurance, including resistance to shocks type sidewalks, improved while maintaining a satisfactory rolling resistance.

Other details and advantageous features of the invention will emerge below from the description of an exemplary embodiment of the invention with reference to the figure which shows a meridian view of a diagram of a tire according to the invention.

The figure is not shown in scale to simplify understanding. The figure represents only a half-view of a tire which is extended symmetrically with respect to the axis XX 'which represents the circumferential median plane, or equatorial plane, of a tire.

In the figure, the tire 1, of dimension 315/70 R 22.5, has a H / S form ratio equal to 0.70, H being the height of the tire 1 on its mounting rim and S its axial width. Max. Said tire 1 comprises a radial carcass reinforcement 2 anchored in two beads, not shown in the figure. The carcass reinforcement is formed of a single layer of metal cables. This carcass reinforcement 2 is fretted by a crown reinforcement 4, formed radially from the inside to the outside: a first working layer 41 formed of unstretchable, in-line metal cables 9.26, continuous over the entire width of the web, oriented at an angle equal to 18 °, of a layer of circumferential reinforcing elements 42 formed of cables metal 21x23 steel, of the "bi-module" type, - a second working layer 43 formed of unstretchable, inextensible metal cables 9.26, continuous over the entire width of the web, oriented at an angle equal to 22 ° and crossed with the metal cables of the layer 41, a protective layer 44 formed of elastic metal cables 6.35.

[00941 The crown reinforcement is itself capped with a tread 5.

The maximum axial width S of the tire is equal to 317 mm.

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

The axial width L ₄₃ of the second working layer 43 is equal to 232 mm.

The difference between the widths L and L ₄₃ is equal to 15 mm.

As for the axial width L ₄₂ of the layer of circumferential reinforcing elements 42, it is equal to 194 mm.

The last crown ply 44, called the protection ply, has a width L ₄₄ equal to 124 mm.

According to the invention, a layer of rubber mix C decouples the ends of the working crown layers 41 and 43. The engagement zone of the layer C between the two working crown layers 41 and 43 is defined by its thickness or more precisely the radial distance d between the end of the layer 43 and the layer 41 and the axial width D of the layer C between the axially inner end of said layer C and the end of the radially outer working crown layer 43. The radial distance d is equal to 3.5 mm or about 2.1 times the diameter f ₂ of the reinforcement elements of the working crown layer 43, the diameter f ₂ being equal to 1.65 mm. The axial distance D is equal to 20 mm, or about 12 times the diameter f ₂ of the reinforcement elements of the working crown layer 43.

The layer C is made from the following mixture I:

(1) Natural rubber

(2) Carbon black "S204" from Orion Engineered Carbon

(3) N-1,3-dimethylbutyl-N-phenylparaphenylenediamine "Santoflex 6-PPD" from the company Flexsys

(4) Stearin ("Pristerene 4931" from Uniqema)

(5) Zinc oxide, industrial grade, Umicore company

(6) Cobalt Naphthalate - Product No 60830 from the company Fluka

(7) N, N-dicyclohexylbenzothiazole-2-sulfenamide from Flexsys

(8) N-cyclohexylthiophthalimide sold under the name "Vulkalent G" by the company Lanxess

The values of the constituents are expressed in phr (parts by weight per hundred parts of elastomers).

According to the invention, the crosslinking density measured according to the equilibrium swelling method on the layer C is equal to ¹⁹ × 10 ⁵ mol / cm ³ and therefore comprised between 13 × 10 ⁵ mol / cm ³ and 21 × 10 ⁵ mol. / cm ³ .

The tan (δ) _{max value} of the layer C is equal to 0.050 and therefore less than 0.055. The modulus of elasticity under tension at 10% elongation of the layer C is equal to 5.7 MPa and therefore less than 6 MPa.

Tests have been carried out with a tire made according to the invention in accordance with the representation of the figure and compared with a reference tire T1 also according to the representation of the figure.

The tire T1 differs from the tire according to the invention by the nature of the mixture constituting the layer C.

The layer C of the reference tire T1 is made from the following mixture R1:

(1) Natural rubber

(2) Carbon black "N347" from Cabot.

(4) Stearin ("Pristerene 4931" from Uniqema)

(5) Zinc oxide, industrial grade, Umicore company

(6) Cobalt Naphthalate - Product No 60830 from the company Fluka

(7) N, N-dicyclohexylbenzothiazole-2-sulfenamide from Flexsys

The values of the constituents are expressed in phr (parts by weight per hundred parts of elastomers). The crosslinking density according to the equilibrium swelling method on the layer C of the tire T1 is equal to ²² × 10 ⁵ mol / cm ³ .

The value of tan (δ) _max of the layer C of the tire T1 is equal to 0.13.

The modulus of elasticity under tension at 10% elongation of the layer C of the tire T1 is equal to 10.4 MPa.

The elongation at break of the mixtures I and Rl is measured on samples made on a new tire.

A first measurement is performed on the sample taken. Another measurement is performed on a sample taken and then aged for 10 days at 110 ° C. under nitrogen. The measurements are presented in the following table:

An attempt was made to highlight the contribution that can have the invention on the protection of the shoulders of the tire. This test consists of a sidewalk shock, the tire passing on a border of a height of 12 cm quite violently. The tire undergoes a pre-rolling of 2 hours at 100 km / h then a 20% overload is applied to the tire inflated with nitrogen before attacking the sidewalk at the slowed speed of 3 Km / h with an angle of 10 ° . The number of passage on the sidewalk in these conditions is five.

The tires are then peeled to allow analysis of the calendering layers of the working layers 41 and 43 and the layer C. While the reference tires R1 have breaks in the layer C between the two With working layers 41 and 43 possibly leading to tire failure, the tires I according to the invention only have crack initiators at the ends of the layers 41 and 43.

In addition, rolling resistance measurements have been made. These measurements relate to a tire according to the invention as described above and to the reference tire T1 as described above.

[001201 The results of the measurements are presented in the following table; they are expressed in Kg / t, a value of 100 being attributed to the tire T1.

Claims

RE VENDIC ATION S

1 - Radial carcass reinforcement tire comprising a crown reinforcement formed of at least two reinforcing element working crown layers, crossed from one layer to the other by making with the circumferential direction angles between 10 ° and 45 °, a layer C of rubber mixture being disposed between at least the ends of said at least two working crown layers, G crown reinforcement being radially capped with a tread, said tread being joined to two with two flanks, characterized in that the maximum value of tan (δ), denoted tan (δ) _max , of the layer C is less than 0.055 and that the elongation at break of the layer C is greater than 300%.

2 - A tire according to claim 1, characterized in that the layer C of rubber compound is an elastomeric mixture based on natural rubber or synthetic polyisoprene predominantly with chains cis-1,4 and possibly at least one other elastomer diene, the natural rubber or the synthetic polyisoprene in case of cutting being present at a majority rate with respect to the rate of the other or other diene elastomers used and a reinforcing filler comprising mainly at least one carbon black, having a BET specific surface at most equal to 30 m ² / g and a compressed oil absorption index (COAN) at least equal to 60 ml / 100 g, in that said elastomeric mixture does not comprise, or comprises at most 20 phr of carbon black, the BET specific surface area is greater than 30 m ² / g and the COAN index is greater than 40 ml / 100 g, in that said elastomeric mixture does not comprise or comprises at most 20 phr of white filler and in that the crosslinking density measured according to the equilibrium swelling method is between 13 × 10 ⁵ mol / cm ³ and 21 × 10 ⁵ mol / cm ³ in said mixture elastomer constituting the calendering layers of said at least two working crown layers.

3 - A tire according to claim 2, characterized in that the carbon black content whose BET specific surface area is at most equal to 30 m ² / g is between 20 and 80 phr, and preferably between 40 and 65 phr. . 4 - A tire according to claim 2 or 3, characterized in that said carbon black whose BET surface area is at most equal to 30 m ² / g has a CO AN index of at least 65 rnl / 100 g, and preferably at least 70 ml / 100 g.

5 - A tire according to one of claims 2 to 4, characterized in that said carbon black whose BET surface area is at most equal to 30 m ² / g has a COAN index at most equal to 90 ml / 100 g .

6 - A tire according to one of claims 2 to 5, characterized in that the BET specific surface area of said carbon black at most equal to 30 m ² / g is at most equal to 25 m ² / g, and preferably greater than 25 m ² / g. 15 m ² / g. 7 - A tire according to one of the preceding claims, characterized in that the crown reinforcement comprises at least one layer of circumferential metal reinforcing elements.

8 - A tire according to claim 7, characterized in that the modulus of elasticity under tension at 10% elongation of said layer C is less than 6 MPa. 9 - A tire according to one of claims 7 or 8, characterized in that the layer of circumferential reinforcing elements is radially disposed between two working crown layers.

10 - A tire according to one of claims 7 to 9, characterized in that the reinforcing elements of at least one layer of circumferential reinforcing elements are metal reinforcing elements having a secant modulus at 0.7% of elongation of between 10 and 120 GPa and a maximum tangent modulus of less than 150 GPa.