WO2019197763A1

WO2019197763A1 - Light-weight tyre comprising a layer of circumferential reinforcing elements

Info

Publication number: WO2019197763A1
Application number: PCT/FR2019/050815
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 made up of at least two working crown layers, each made of reinforcing elements inserted between two rubber-mixture skim coats, the crown reinforcement further comprising at least one layer of circumferential reinforcing elements. According to the invention, the working layer reinforcing elements are in the form of cords which have a diameter of less than 1.1 mm and which are disposed at intervals of less than 2.1 mm, the stiffness of the cords being greater than 190 GPa, and the skim coats being composed of an elastomer blend that contains, as a reinforcing filler, predominantly at least one carbon black having a maximum specific surface area BET no greater than 30 m²/g and a COAN of at least 60 ml/100 g, the crosslinking density, measured according to the equilibrium swelling method, ranging from 13.10^-5 mol/cm³ to 21.10^-5 mol/cm³ in the skim coats.

Description

ALLEGE PNEUMATIC COMPRISING ONE (01 (111 OF ELEMENTS OF

CIRCUMFERENTIAL REINFORCEMENT

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 of at most equal to [00041] Cables are said elastic when said cables have under a 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.

The radial direction is a direction intersecting the rotational axis of the tire and perpendicular thereto.

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). [00141 Some current tires, called "road", are intended to run at high speed and on longer and longer journeys, because of 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 allows an increase in the number of kilometers traveled, the wear of the tire being less; 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 compounds which are arranged between and / or around the ends of the tire. plies 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 over said reinforcement, and greater than the Shore A hardness of the rubber mix profile disposed between the edges of crown reinforcement plies and carcass reinforcement.

[00201 The tires thus made effectively improve performance, particularly 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 °.

An object of the invention is to provide lightened tires whose properties including endurance are preserved regardless of the use. This object is achieved according to the invention by a radial carcass reinforcement tire comprising a crown reinforcement formed of at least two layers of working crown each formed of reinforcing elements inserted between two layers of calendering rubber mix crossed from one layer to another by making with the circumferential direction angles between 10 ° and 45 °, the crown reinforcement being capped radially with a tread, said tread being joined to two beads by means of two sidewalls, the crown reinforcement comprising at least one layer of circumferential reinforcement elements, the reinforcement elements of said at least two working layers being cables whose diameter is less than 1.1 mm, the pitch of distributing said cables in said at least two working layers being less than 2.1 mm, the rigidity of said cables of said at least two layers of the working layer being greater than 190 GPa and the calendering layers of the at least two working crown layers consisting of an elastomeric mixture based on natural rubber or synthetic polyisoprene predominantly with cis-1,4 and optionally 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 diene elastomer or elastomers used and a reinforcing filler comprising predominantly at least one carbon black, having a BET specific surface at most equal to 30 m ² / g and a compressed sample 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 plus 10 phr of carbon black whose BET surface area is greater than 30 m ² / g and the COAN index is greater than 40 ml / 100 g, said elastomeric blend not comprising, or comprising at most 20 phr, and preferably at most 10 phr of white charge 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 layers calendering said at least two working crown layers.

The BET surface area measurement 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.

For the purposes 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 SiOH and / or AlOH surface functions of the silica and / or alumina materials and can be used according to the invention as a partial or total replacement thereof, it is mention may be made of the modified carbon blacks either during the synthesis by addition to the feed oil of the oven of a silicon and / or aluminum compound or after the synthesis by adding to an aqueous suspension of carbon black. carbon 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 and / or AlOH functions. As non-limiting examples of this type of carbonaceous feedstock with SiOH and / or AlOH functions at the surface, there may be mentioned typical feeds. CSDP described in Conference No. 24 of Meeting ACS, Rubber Division, Anaheim, California, May 6-9, 1997 as well as those of patent application EP-A-0 799 854.

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, the BET specific surface area of which is at most equal to 30 m ² / g, has a CO AN index at least equal to 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 at most equal to 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 recovery selected from the known agents of the skilled person. 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 of the chain of oxygen functions carbonyl, carboxyl or an amine function 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.

The cables of the working layers of the tire according to the invention, having smaller diameters than in the most usual designs, help to lighten the pneumatic, although the distribution pitch of said cables are smaller than those of said more usual designs. Furthermore, these smaller cables associated with smaller steps lead to a lightening of the rubbery mass due to a reduced mass of calender layers, the size of the intervals between cables being reduced in two directions.

The inventors have yet been able to demonstrate that the choice of elastomeric mixtures comprising a black carbon type filler having a BET specific surface area at most equal to 30 m ² / g and a sample oil absorption index. compressed (COAN) at least equal to 60 ml / 100 g makes it possible to give the layers of calenders properties including elongation breaking to maintain endurance properties of the tire despite the use of small diameter cables. It is indeed known to those skilled in the art that such cables associated with pitch between cables smaller than the usual designs lead to performance in terms of reduced endurance during heavy demands. Conventional mixtures which have a lower break elongation do not effectively reduce the distance between the cables at the risk of promoting the propagation of cracks that initiate cable ends.

According to an advantageous embodiment of the invention, the elongation at break of the calendering layers of said at least two working crown layers is greater than 300%.

The elongation at break (in%) is measured in accordance with the AFNOR-NF-T-46-002 standard of September 1988. The tensile measurements for determining the breaking properties are carried out at a temperature of 100 ° C. C ± 2 ° C, and under normal humidity 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 inventors have yet been able to demonstrate that this choice of mixture having elongation values at break greater than those of the more usual mixtures, leading to inferior stiffnesses to those of said more usual mixtures, is allowed because of the presence the layer of circumferential reinforcing elements. According to a preferred embodiment of the invention, the tensile modulus of elasticity at 10% elongation of the calendering layers of said at least two working crown layers is less than 8.5 MPa.

The use of such mixtures whose elastic modulus are less than 8.5 MPa will further improve the properties of the tire in terms of satisfactory endurance.

[0044] Usually, the tensile modulus of elasticity at 10% elongation of the crown layer calendering is greater than 8.5 MPa and most often greater than 10 MPa. Such elastic moduli are in particular required to make it possible to limit the compressing of the reinforcing elements of the working crown layers, especially when the vehicle follows a winding path, when maneuvering in the car parks or during the passage of circles. points. Indeed, the shears in the axial direction that occur on the tread in the area of the ground contact surface lead to the compression of reinforcing elements of a working crown layer. The inventors have been able to demonstrate that the circumferential reinforcement element layer allows the choice of modulus of elasticity of the rubber mixes of the calender layers of the lower work crown layers without adversely affecting the endurance properties of the pneumatic due to the compression of the reinforcing elements of said working crown layers as described above.

The inventors have also been able to demonstrate that the cohesion of the layers of calendering of the working crown layers, when it has a tensile modulus of elasticity at 10% elongation less than 8.5 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.

[00481 The measurement comprises 3 parts:

• An 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 W0 = energy supplied to the material per cycle and per unit volume and hO = initial height of the test piece. The exploitation of acquisitions "force / displacement" thus gives the relation between "E" and the amplitude of the solicitation.

• The measurement of cracking, after notching of the "PS" test piece. 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 layers of calendering of the working crown layers. Indeed, the more usual tire designs comprising, in particular, layers of calendering of the working crown layers with tensile modulus of elasticity at 10% elongation greater than 8.5 MPa, lead to an evolution of the cohesion of said layers of the worktop layers, which tend to weaken. The inventors find that the presence of at least one layer of circumferential reinforcing elements which contributes to limiting the compression of the elements it - reinforcing the working crown layers especially when the vehicle follows a winding course and furthermore limits the temperature increases leads to a small change in the cohesion of the layers of calendering. The inventors thus consider that the cohesion of the caliper layers of the working crown layers, which is lower than that which exists in the more usual tire designs, is satisfactory in the design of the tire according to the invention.

More preferably according to the invention, the maximum value of tan (δ), denoted tan (δ) _max , of said calendering is less than 0.055.

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, at a temperature of 100 is recorded. ° 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.

The use of such mixtures whose tan (δ) _max is less than 0.055 will improve the properties of the tire in terms of rolling resistance. According to an advantageous embodiment of the invention, said reinforcing elements of at least one working crown layer are saturated layer cables, at least one inner layer being sheathed with a layer consisting of polymeric composition such as a non-crosslinkable, crosslinkable or crosslinked rubber composition, preferably based on at least one diene elastomer. So-called "layered cords" or "multilayer" cables are cables consisting of a central core and one or more layers of strands or substantially concentric son arranged around this central core.

For the purposes of the invention, a saturated layer of a layered cable is a layer consisting of wires in which there is not enough room to add at least one additional wire.

The inventors have demonstrated that the presence of the cables as just described as strengthening elements of the working crown layers can contribute to better performance in terms of endurance. Indeed, it appears as explained above that the rubber mixtures of the calenders of the working layers can reduce the rolling resistance of the tire. This results in lower temperatures of these rubber mixes, when using the tire, which can cause less protection of the reinforcing elements vis-à-vis the oxidation phenomena in some cases of use of the tire. In fact, the properties of the rubber compounds relating to the oxygen blocking decrease with temperature and the presence of oxygen can lead to a gradual degeneration of the mechanical properties of the cables, for the most severe driving conditions, and can alter the lifetime of these cables. The presence of the rubber sheath within the cables described above compensates for this potential risk of oxidation of the reinforcing elements, the sheath contributing to the blockage of oxygen.

By the expression "composition based on at least one diene elastomer" is meant in known manner that the composition comprises in majority (i.e. in a mass fraction greater than 50%) this or these diene elastomers.

Note that the sheath according to the invention extends continuously around the layer it covers (that is to say that this sheath is continuous in the "orthoradial" direction of the cable which is perpendicular to its radius), so as to form a continuous sleeve of cross section which is preferably substantially circular. It will also be noted that the rubber composition of this sheath may be crosslinkable or crosslinked, that is to say that it comprises by definition a crosslinking system adapted to allow the crosslinking of the composition during its cooking (ie its hardening and not its fusion); thus, this rubber composition can be described as infusible, since it can not be melted by heating at any temperature.

By elastomer or "diene" rubber is meant in known manner an elastomer derived at least in part (i.e. a homopolymer or a copolymer) of monomers dienes (monomers carrying two carbon-carbon double bonds, conjugated or not).

Preferably, the crosslinking system of the rubber sheath is a so-called vulcanization system, that is to say based on sulfur (or a sulfur donor) and a primary vulcanization accelerator . To this basic vulcanization system may be added various known secondary accelerators or vulcanization activators. The rubber composition of the sheath according to the invention may comprise, in addition to said crosslinking system, all the usual ingredients that can be used in tire rubber compositions, such as reinforcing fillers based on carbon black and / or a reinforcing inorganic filler such as silica, anti-aging agents, for example antioxidants, extension oils, plasticizers or agents facilitating the use of the compositions in the raw state, acceptors and donors of methylene, resins, bismaleimides, known adhesion promoter systems of the "RFS" type (resorcinol-formaldehyde-silica) or metal salts, especially cobalt salts.

Preferably, the composition of this sheath is chosen identical to the composition used for the calender layer of the working crown layer that the cables are intended to reinforce. Thus, there is no problem of possible incompatibility between the respective materials of the sheath and the rubber matrix.

According to a variant of the invention, said cables of at least one working crown layer are cables with building layers [L + M], comprising a first layer C1 to L son of diameter di wound together helically in a pitch pi with L ranging from 1 to 4, surrounded by at least one intermediate layer C2 to M son of diameter d ₂ wound together in a helix in a step p ₂ with M ranging from 3 to 12, a sheath consisting of a non-crosslinkable, crosslinkable or crosslinked rubber composition based on at least one diene elastomer, covering, in the construction, said first layer Cl.

Preferably, the diameter of the wires of the first layer of the inner layer (Cl) is between 0.10 and 0.5 mm and the diameter of the wires of the outer layer (C2) is between 0.10 and 0.5 mm and more preferably the diameter of the wires of the layers (C1) and (C2) is greater than 0.2 mm. More preferably, the pitch of the winding helix of said son of the outer layer (C2) is between 8 and 25 mm.

For the purposes of the invention, the pitch of the helix represents the length, measured parallel to the axis of the cable, at the end of which a wire having this pitch performs a complete revolution around the axis of the cable; thus, if the axis is divided by two planes perpendicular to said axis and separated by a length equal to the pitch of a wire of a constituent layer of the cable, the axis of this wire has in these two planes the same position on the two circles corresponding to the layer of the wire considered.

Said cables according to the invention may be obtained according to various techniques known to those skilled in the art, for example in two steps, firstly by sheathing via an extrusion head of the core or layers Cl, step followed in a second step of a final operation of wiring or twisting the remaining M son (layer C2) around the layer Cl and sheathed. The problem of stickiness in the green state posed by the rubber sheath, during any intermediate operations of winding and uncoiling can be solved in a manner known to those skilled in the art, for example by the use of a spacer film. plastic material.

Such cables of at least one working crown layer are for example chosen from the cables described in patent applications WO 2006/013077 and WO 2009/083212. According to an advantageous embodiment of the invention, the axially widest working crown layer is radially inside the other working crown layers.

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

[00751 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 layer. axially the least wide axially working vertex being between 10 and 30 mm. According to a preferred embodiment of the invention, the layer of circumferential reinforcing elements is radially disposed 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 set 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 reinforcing elements are greater than the axial width of said layer of circumferential reinforcing elements and preferably, said top layers of work 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 then decoupled at least on the remainder. of the width common to said two layers of work. The presence of such couplings between the working crown layers adjacent to the layer of circumferential reinforcing elements makes it possible to reduce 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 brought back 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 of the central portion of at least one layer of circumferential reinforcement elements are metal reinforcing elements having a Secant modulus at 0.7% elongation between 5 and 60 GPa and a maximum tangent modulus 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 to the 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 mentioned above are measured on reinforcing elements taken from tires.

Reinforcing 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 metal of the element of reinforcement, 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 21.23 has a secant module at 0.7% equal to 23 GPa and a maximum tangent modulus equal to 49 GPa.

In the same way, another example of reinforcing elements is an assembly of formula 21.28, the construction of which 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 the 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 that can easily be adjusted (by the choice of intervals between sections of the same row), but in all cases weaker than the module of the layer consisting of the same metallic elements but continuous, the module of the additional layer 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 / 1 of the amplitude of waviness on 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.

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 reinforcing elements of the working crown layers is less than 30 ° and preferably less than 25 °.

[0099] A preferred embodiment of the invention further provides that the crown reinforcement is completed radially outwardly by at least one additional layer, so-called protective, of so-called elastic reinforcing elements, oriented relative 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 layer of work. According to any one of the embodiments of the invention mentioned above, the crown reinforcement can be further completed radially inwards between the carcass reinforcement and the radially inner working layer closest to 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 thus has a reduced mass in comparison with more usual tires while retaining comparable performance in terms of endurance.

In addition, the lower elasticity moduli of the rubber mixes of the working crown layer calenders make it possible to soften the crown of the tire and thus limit the risks of attacking the top and corrosion of the reinforcing elements of the layers. of crown reinforcement when for example pebbles are retained in the sculpture grounds.

Other details and advantageous features of the invention will emerge below from the description of the exemplary embodiments of the invention with reference to the figure which shows a meridian view of a diagram of a tire according to a mode. embodiment of the invention. [00105] 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, 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, of a second working layer 43 formed of non-shrinking, inextensible metal cables 9.26, continuous over the entire width of the ply, 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.

[001071 The crown reinforcement is itself capped with a tread 6. [001081 The maximum axial width S of the tire is equal to 318 mm.

[00109] 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.

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, referred to as protection ply, has a width L ₄₄ equal to

124 mm.

According to the invention, the cables of the working layers 41 and 43 are two-layer assemblies consisting of wires of 0.26 mm. The cables thus formed have a diameter d of 1 mm and therefore less than 1.1 mm. The steel wires forming the cables have a SHT grade.

The distribution pitch of said cables in the two working layers 41 and 43 is equal to 2 mm and therefore less than 2.1 mm.

The rigidity of the cables of the two working layers 41 and 43 is equal to 196 GPa and is greater than 190 GPa. The calendering layers of the two working layers 41 and 43 are 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 Society

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). The cumulative mass of the working layers 41 and 43 comprising the mass of the metal cables and calendering mixtures is equal to 5.4 Kg.

The mass of the tire I is equal to 58.1 Kg.

According to the invention, the crosslinking density measured according to the equilibrium swelling method on the calendering layers of the two top layers of Work 41 and 43 is equal to 19.10 ⁵ mol / cm ³ and therefore between 13.10 ⁵ mol / cm ³ and 21.10 ⁵ mol / cm ³ .

The tensile modulus of elasticity at 10% elongation of the calendering layers of each of the working layers 41 and 43 is equal to 5.7 MPa and therefore less than 8.5 MPa.

The value of tan (δ) _max of the calender layers of each of the working crown layers 41 and 43 is equal to 0.050 and therefore less than 0.055.

Tests have been made with different tires 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 on the one hand, by the nature of the constituents constituting the calendering layers of the working layers 41 and 43 and on the other hand, by the cables of the working layers 41 and 43 which are 9.35 cables whose diameter is equal to 1.35 mm, the steel wires forming said cables having a grade SHT.

The distribution pitch of said cables in the two working layers 41 and 43 of the tire T1 is equal to 2.5 mm.

The rigidity of the cables of the two working layers 41 and 43 of the tire T1 is equal to 185 GPa. The calendering layers of the two working layers 41 and 43 of the reference tire T1 are made from the following mixture R1:

(1) Natural rubber

(2) "N683" carbon black from Cabot.

(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

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

The cumulative mass of the working layers 41 and 43 of the tire R1 comprising the mass of the metal cables and calendering mixtures, is equal to 7 Kg.

The mass of the tire T1 is equal to 59.7 Kg.

The crosslinking density measured according to the equilibrium swelling method on the calendering layers of the two working crown layers 41 and 43 of the tire T1 is equal to 25 × 10 ⁵ mol / cm ³ .

The modulus of elasticity under tension at 10% elongation of the calendering layers of each of the working layers 41 and 43 of the tire T1 is equal to 6.12 MPa. The value of tan (δ) _max of the calendering layers of each of the working crown layers 41 and 43 of the tire T1 is equal to 0.056.

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 elongation at break (in%) is measured in accordance with the AFNOR-NF-T-46-002 standard 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 humidity conditions (50 ± 5% relative humidity), according to the French standard NF T 40-101 (December 1979).

[001371 The measurements are presented in the following table:

[00138] First endurance tests were performed on a test machine requiring each tire to run a straight line at a speed equal to the maximum speed index prescribed for said tire (speed index) under an initial load of 4000 Kg gradually increased to reduce the duration of the test.

These first tests showed that the distances traveled during the tests are substantially identical for the tires I according to the invention and the tires Rl of reference.

[00140] Other endurance tests were performed on a test machine imposing cyclically a transverse force and a dynamic overload to the tires. The tests are carried out over a distance of 5000 Km.

The tires are then peeled to allow the analysis of the calendering layers of the working layers 41 and 43. While the reference tires R1 have highly propagated cracks that can potentially lead to tire failure, the tires according to invention only present crack initiators at the ends of the layers 41 and 43, these being only very slightly propagated.

Claims

RE VENDIC ATION S

1 - Radial carcass reinforcement tire comprising a crown reinforcement formed of at least two working crown layers each formed of reinforcing elements inserted between two layers of calendering rubber mix, crossed from one layer to another by making with the circumferential direction angles between 10 ° and 45 °, G crown reinforcement being capped radially tread, said tread being joined to two beads by means of two flanks, the crown reinforcement comprising at least one layer of circumferential reinforcing elements, characterized in that the reinforcement elements of said at least two working layers are cords whose diameter is less than 1.1 mm, in that the pitch of said cables in said at least two working layers is less than 2.1 mm, in that the rigidity of said cables of said at least two layers of work garlic is greater than 190 GPa and in that the calendering layers of the at least two working crown layers consist of an elastomeric blend based on natural rubber or synthetic polyisoprene predominantly with cis-1,4-chains and optionally 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 diene elastomer or diene elastomers used and a reinforcing filler comprising mainly at least one diene elastomer; at least one carbon black, having a BET specific surface area at most equal to 30 m ² / g and a compressed oil absorption index (COAN) of at least 60 ml / 100 g, in that said The elastomeric blend does not comprise, or comprises, at most 20 phr of carbon black having a BET specific surface area greater than 30 m ² / g and a COAN number 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 elastomeric mixture constituting the calendering layers of said at least two working crown layers.

2 - A tire according to claim 1, characterized in that the carbon black content whose BET surface area is at most equal to 30 m ² / g is between 20 and 80 phr, and preferably between 40 and 65 phr. . 3 - A tire according to claim 1 or 2, 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.

4 - tire according to one of the preceding claims, characterized in that said carbon black whose BET surface area is at most equal to 30 m ² / g has a subscript

COAN at most equal to 90 ml / l 00 g.

5 - A tire according to one of the preceding claims, 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 15 m ² / g. 6 - tire according to one of the preceding claims, characterized in that said reinforcing elements of at least one working crown layer are saturated layer cables, at least one inner layer being sheathed with a layer consisting of a polymeric composition such as a non-crosslinkable, crosslinkable or crosslinked rubber composition, preferably based on at least one diene elastomer. 7 - A tire according to one of the preceding claims, characterized in that the layer of circumferential reinforcing elements is radially disposed between two working crown layers.

8 - tire according to one of the preceding claims, 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% elongation included between 10 and 120 GPa and a maximum tangent modulus less than 150 GPa.

9 - A tire according to one of the preceding claims, characterized in that the reinforcing elements of the working crown layers are inextensible.

10 - tire according to one of the preceding claims, characterized in that the angle formed with the circumferential direction by the reinforcing elements of the working crown layers is less than 30 ° and preferably less than 25 °.

11 - tire according to one of the preceding claims, characterized in that the crown reinforcement is completed radially outside by at least one additional ply, called for protection, so-called elastic reinforcing elements oriented with respect to the circumferential direction with an angle of between 10 ° and 45 ° and in the same direction as the angle formed by the inextensible elements of the radial working ply adjacent.

12 - A tire according to one of the preceding claims, characterized in that the crown reinforcement further comprises a triangulation layer formed of metal reinforcing elements forming with the circumferential direction angles greater than 60 °.