TIRE WITH A TREAD MADE OF SEVERAL ELASTOMER BLENDS

Abstract

The invention relates to a tire (1) whose tread (6) comprises at least two layers of elastomeric mixtures (61 62).According to the invention, the first layer (61) forming the radially outer part of the tread (6) extends radially inside the grooves (3) to a thickness (E) of at least 1 mm, and a second radially inner layer (62), in contact with the outermost radially reinforced top layer (53), comprises a central portion (621) and two axially outer portions (622), said central portion (621) being in contact with said outermost radially reinforced top layer (53) over an axial width less than the axial width of said outermost radially reinforced top layer, the thickness (H) measured along the radial direction of said central portion (621) being greater than or equal to 1 mm, and said central portion (621) being made of a second elastomeric compound satisfying the relation: 4.5*(MA100)0.95 + 16*(G'')0.55 + 6*(Dz)0.3 > 37.5. Figure for the abbreviation: Fig 1.

Description

Title of the invention: TIRE COMPRISING A TREAD MADE OF SEVERAL ELASTOMER BLENDS

[0001] The present invention relates to a tire, more particularly a tire intended to equip vehicles carrying heavy loads and traveling at sustained speed, such as, for example, trucks, tractors, trailers or road buses.

[0002] Generally, in heavy-duty tires, the carcass reinforcement is anchored on both sides in the bead area and is radially surmounted by a crown reinforcement consisting of at least two superimposed layers formed of parallel wires or cables in each layer and crossed from one layer to the next at angles between 10° and 45° with the circumferential direction. These working layers, forming the working reinforcement, may be further covered by at least one protective layer formed of advantageously metallic and extensible reinforcing elements, known as elastic elements.It may also include a layer of low-extensibility wires or cables forming an angle of between 45° and 90° with the circumferential direction. This layer, known as the triangulation layer, is radially located between the carcass reinforcement and the first crown layer, known as the working layer, which is formed of parallel wires or cables having angles of no more than 45° in absolute value. The triangulation layer, together with at least the aforementioned working layer, forms a triangulated reinforcement, which exhibits little deformation under the various stresses it is subjected to. The essential role of the triangulation layer is to resist the transverse compression forces exerted on all the reinforcing elements in the crown area of ​​the tire.

[0003] In the case of tires for "Heavy Goods Vehicles", a single protective layer is usually present, and its protective elements are, in most cases, oriented in the same direction and at the same absolute angle as those of the reinforcing elements of the outermost, and therefore radially adjacent, tread layer. In the case of off-road tires intended for use on more or less uneven terrain, the presence of two protective layers is advantageous, with the reinforcing elements being intersected from one layer to the next, and the reinforcing elements of the inner radial protective layer being intersected with the inextensible reinforcing elements of the radially outer working layer and adjacent to said radially inner protective layer.

[0004] Radially outside the top reinforcement, there is the tread usually made of elastomeric materials intended to come into contact with the ground in the contact area between the ground and the tire.

[0005] Cables are said to be inextensible when said cables exhibit, under a tensile force equal to 10% of the breaking force, a relative elongation of at most equal to 0.2%.

[0006] Cables are said to be elastic when said cables exhibit, 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.

[0007] 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.

[0008] The axis of rotation of the tire is the axis around which it rotates in normal use.

[0009] A radial or meridian plane is a plane which contains the axis of rotation of the tire.

[0010] The circumferential median plane, or equatorial plane, is a plane perpendicular to the axis of rotation of the tire and which divides the tire into two halves.

[0011] The transverse or axial direction of the tire is parallel to the axis of rotation of the tire. An axial distance is measured along the axial direction. The expression "axially inside, respectively axially outside" means "whose axial distance measured from the equatorial plane is less than, respectively greater than".

[0012] The radial direction is a direction intersecting the axis of rotation of the tire and perpendicular to it. A radial distance is measured along the radial direction. The expression "radially inside, respectively radially outside" means "whose radial distance measured from the axis of rotation of the tire is less than, respectively greater than".

[0013] With regard to rubber compositions, modulus measurements are carried out in tension according to the AFNOR-NFT-46002 standard of September 1988: the nominal secant modulus (or apparent stress, in MPa) is measured in the second elongation (i.e., after an accommodation cycle) at 10% or 100% elongation, at 60°C.

[0014] Some current tires, known as "road" tires, are designed for high-speed driving and increasingly long journeys, due to improvements in the road network and the growth of the motorway network worldwide. The overall conditions under which such a tire is intended to operate undoubtedly allow for an increase in the number of kilometers traveled, as tire wear is less; however, the endurance of the latter and in particular of the apex frame is penalized.

[0015] Indeed, there are stresses at the top reinforcement level, and more specifically shear stresses between the top layers, combined with a significant increase in operating temperature at the ends of the axially shortest top layer, which result in the appearance and propagation of cracks in the rubber at said ends. This problem exists in the case of the edges of two layers of reinforcing elements, said layers not necessarily being radially adjacent.

[0016] To limit excessive temperature rises at the top of the tire, the materials constituting the tread are advantageously chosen with hysteresis losses adapted to the operating conditions of the tire.

[0017] Furthermore, in order to improve the endurance of the crown reinforcement of the type of tire 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 layers and more particularly the ends of the axially shortest layer have already been provided.

[0018] French patent FR 1 389 428, to improve the resistance to degradation of rubber compounds located in the vicinity of the edges of the top reinforcement, recommends the use, in combination with a low hysteresis tread, of a rubber profile covering at least the sides and marginal edges of the top reinforcement and made of a low hysteresis rubber compound.

[0019] French patent FR 2 222 232, to avoid separations between layers of top reinforcement, teaches to encase the ends of the reinforcement in a rubber mat, the Shore A hardness of which is different from that of the tread surmounting said reinforcement, and greater than the Shore A hardness of the rubbery mixture profile disposed between the edges of the top reinforcement layers and carcass reinforcement.

[0020] French application FR 2 728 510 proposes to have, on the one hand, between the carcass reinforcement and the top reinforcement working layer, radially closest to the axis of rotation, an axially continuous layer, formed of inextensible metal cables making an angle of at least 60° with the circumferential direction, and whose axial width is at least equal to the axial width of the shortest top working layer, and on the other hand, between the two top working layers, an additional layer formed of metal elements, oriented substantially parallel to the circumferential direction.

[0021] French application WO 99 / 24269 further proposes, on either side of the equatorial plane and in the immediate axial extension of the additional layer of reinforcement elements substantially parallel to the circumferential direction, to couple, over a certain axial distance, the two top working layers formed of reinforcement elements crossed from one layer to the next and then to decouple them by rubber compound profiles at least over the remainder of the common width of said two working layers.

[0022] This improvement in tire durability makes it possible to consider, at the very least, the possibility of retreading when the tread is worn. Indeed, when it is desired to retread a tire after tread wear, it is necessary to be able to retread a tire whose aging is not too advanced in order to optimize the use of the new tread.

[0023] In order to further increase tire life, it is common practice to choose elastomeric materials for the tread that have improved wear resistance properties. Since such materials often negatively impact hysteresis properties, it is also known to produce the tread of a tire by radially layering two different materials to obtain a compromise between wear and hysteresis properties that is satisfactory for the intended applications.

[0024] Such tires are described for example in US document 6,247,512. This document describes the superposition of two layers of materials to form the tread, the outer material coming into contact with the ground being in particular more efficient in terms of wear while the inner material has hysteretic properties allowing to limit the temperature rise of the tire in the area of ​​the crown.

[0025] The durability properties of tires can be further impaired by tread perforations occurring during driving due to stones or other objects that may damage the tread. These perforations can lead to degradation of the crown reinforcement elements through oxidation, thus rendering the tire unretreadable. In cases of significant oxidation damage, the degradation of the reinforcement elements may necessitate changing the tire on the vehicle before the tread is completely worn.

[0026] It is particularly known to avoid the risks of damage to the working layers by providing a protective layer as described above, this playing a sacrificial role in the event of an attack such as perforation or damage to the tread and allowing for possible retreading, the crown reinforcement being preserved in addition.

[0027] These solutions do not prevent perforations and may not be sufficient, under certain particularly severe rolling conditions, to preserve the integrity of the entire top reinforcement.

[0028] The inventors have thus set themselves the task of proposing tires allowing an ever greater driving distance before considering retreading, and whose performance in terms of resistance to punctures and aggressions is improved compared to more common tires.

[0029] This objective has been achieved according to the invention by a tire, comprising a crown reinforcement having at least one layer of reinforcing elements, itself radially capped by a tread whose design has at least two circumferential grooves, joined to two beads by means of two sidewalls, said tread comprising at least two radially superimposed layers of elastomeric compounds, the first layer forming the radially outer part of the tread being made up of a first elastomeric compound and being present radially inwardly at the radially innermost points of said at least two grooves over a thickness measured along the radial direction greater than or equal to 1 mm, a second radially inner layer and, in contact with the radially outermost crown reinforcement layer, comprising a central part and two axially outer parts,said central part of the second layer being in contact with said radially outermost top reinforcement layer over an axial width less than the axial width of said radially outermost top reinforcement layer, the thickness measured along the radial direction of said central part being greater than or equal to 1 mm radially inward at the radially innermost points of said at least two grooves and said central part of the second layer being made of a second elastomeric mixture having a tensile modulus of elasticity at 100% elongation MA100, a viscous shear modulus G”, measured at 60°C, and a tear coefficient Dz (measured on a pre-cracked specimen and expressed in N / mm) satisfying the relation: 4.5*(MA100)0.95 + 16*(G”)0.55 + 6*(Dz)0.3 > 37.5,The tear resistance coefficient is the product of the breaking strength per unit thickness (expressed in N / mm of thickness) and the elongation at break (expressed in %), measured at 100 °C.

[0030] The breaking strength per unit thickness and the elongation at break in tearability are measured on a specimen stretched at 500 mm / min to cause the specimen to break on a tensile testing dynamometer, equipped with a system for measuring and acquiring the force and displacement of the cross member The tensile test specimen consists of a parallelepiped-shaped rubber plate, 2.5 mm thick, 145 mm long, and 10 mm wide. Before starting the test, three very fine cuts perpendicular to the length of the specimen are made with a razor blade, 3 mm deep, on one edge of the specimen: one in the middle and the other two on either side of the first, 6 mm apart. The force (expressed in N per mm of specimen thickness) required to break the specimen is determined, and the elongation at break (expressed as a percentage) is measured. The test was conducted in air at a temperature of 100°C. High values ​​indicate good cohesion of the rubber composition, even though crack initiation may be present.The dimensions of the test specimen and the notches can be multiplied by a single homothety factor with slightly less accuracy if this factor is less than 1.

[0031] The shear modulus G” is determined from the values ​​of the complex dynamic shear modulus (G*) and the maximum value of the loss factor tan(φ). The loss factor tan(φ) is a dynamic property of the rubber compound layer. It is measured on a viscoelastic analyzer (Metravib VA4000), according to ASTM D 5992-96. The response of a vulcanized composition sample (cylindrical specimen 2 mm thick and 78 mm² in cross-section) subjected to sinusoidal alternating simple shear loading at a frequency of 10 Hz and a temperature of 60°C is recorded. A strain amplitude sweep is performed from 0.1 to 50% (forward cycle), then from 50% to 1% (reverse cycle). The results used are the complex dynamic shear modulus (G*) and the loss factor tan(φ) measured on the return cycle. For the return cycle, we indicate the maximum value of tan(ô) observed, denoted tan(ô)max.

[0032] In the case where the thickness of the material is between 1 and 2 mm, the loss factor tan(ô) and the shear modulus G* are measured according to the same method and under the same conditions, as described above, on a sample of vulcanized composition which is in the form of a cylindrical specimen 1 mm thick and 78 mm2 in cross-section.

[0033] The invention relates more particularly to radial carcass tires.

[0034] Advantageously according to the invention, the second elastomeric mixture has a tensile modulus of elasticity at 100% elongation MA100, a viscous shear modulus G”, measured at 60°C, and a tear coefficient Dz (measured on a pre-cracked specimen and expressed in N / mm) satisfying the relation: 4.5*(MA100)0'95 + 16*(G”)055 + 6*(Dz)0'3 > 42.5, the tear coefficient being the product of the breaking force per unit thickness (expressed in N / mm of thickness) by the elongation at break (expressed in %), measured at 100 °C.

[0035] Advantageously, also according to the invention, the second elastomeric compound contains at least 50 parts per million (ppm) of natural rubber. Such a natural rubber content ensures the long-term preservation of the properties of the second elastomeric compound, particularly with regard to the thermo-oxidative aging undergone during tire use.

[0036] Tests carried out with tires conforming to the invention have shown that the tires have satisfactory performance in terms of endurance and in particular their performance in terms of resistance to punctures and aggressions is improved compared to more common tires.

[0037] The inventors have been able to demonstrate that the presence of the central part of the second layer which comes into contact with the top reinforcement, the thickness of which is at least 1 mm and which satisfies the relationship expressed above, makes it possible to contribute to the protection of the top reinforcement with regard to the risks of perforations and aggressions which are particularly important at the bottom of grooves, and makes it possible to limit the risks of tearing off part of the tread.

[0038] The inventors believe they understand that when driving on stony ground, tires are subjected to stresses that result in cuts, particularly on the surface of the tread and therefore in the first layer of polymer compound forming the contact patch with the ground. Driving on these stony surfaces, which subject the tread to high shear and compression stresses, promotes the propagation of these cuts in the form of cracks within the compound.

[0039] The propagation of these cracks can lead to the tearing of part of the tread pattern once they reach the tread base. The loss of tire integrity may then necessitate its replacement, although the tire can be retreaded.

[0040] When cracks reach the crown reinforcement, they can lead to oxidation of the reinforcing elements in the layers of reinforcing elements constituting the crown reinforcement of the tire. The tire's durability is then affected, and it may become necessary to replace the tire, which may not be retreadable due to the damage it has sustained.

[0041] The inventors believe they interpret the results obtained during their tests by the ability of the central part of the second layer to partially consume and Sufficiently absorbs the energy generated by rolling on stony ground due to the previously described shear and compression stresses on the tread, thus limiting the propagation of cracks initiated by cuts forming on the tread surface. On the one hand, the contact of the central part of the second elastomeric compound layer with the apex reinforcement, which immobilizes said central part of the second elastomeric compound layer when the tread is subjected to shear and compression stresses on stony ground, and on the other hand, the properties of the second compound, which satisfy the relation 4.5*(MAiOo)095 + 16*(G”)0’55 + 6*(Dz)0.3 > 37.5, allow for optimal absorption of the energy generated by the stresses borne by the tread, thereby limiting crack propagation or at least its propagation rate.

[0042] According to a preferred embodiment of the invention, the second elastomeric mixture constituting the central part of the second layer has a tensile modulus of elasticity at 10% elongation, measured at 60°C, less than or equal to 20 MPa.

[0043] Such an embodiment of the invention may in particular simplify the manufacture of the tire.

[0044] According to another embodiment of the invention, the second elastomeric mixture has a maximum value of tan(ô), denoted tan(ô)max, measured at 60°C, greater than or equal to 0.30.

[0045] According to this variant, the central portion leads to a performance compromise in which it is accepted that this part of the tread is not favorable to the overall rolling resistance of the tire. However, the inventors have shown that the central portion of the second layer, due to its location, is subject to little deformation and its effect on the overall rolling resistance of the tire remains small.

[0046] Rolling resistance is the resistance that appears when the tire is rolling and reveals the temperature rise of said tire. It is thus represented by the hysteretic losses related to the deformation of the tire during one revolution. The tan(φ) values ​​of the materials used are measured at 10 Hz between 30 and 100°C to incorporate the effect of the different deformation frequencies induced by the revolution of the tire. The tan(φ) value at 60°C thus corresponds to an indicator of the rolling resistance of the tire while rolling.

[0047] According to an advantageous embodiment of the invention, said two axially external parts of the second layer being made of a third elastomeric mixture having a maximum value of tan(ô), denoted tan(ô)max, measured at 60°C, less than 0.10.

[0048] This embodiment makes it possible to reduce the overall hysteresis of the tire, the elastomeric compounds of the axially external parts of the second layer possibly compensating for the hysteretic properties of the second compound constituting the central part of the second layer and especially the hysteretic properties of the first compound radially constituting the first layer of the tread usually chosen for its wear properties which are most often accompanied by hysteretic properties that are not very favorable for rolling resistance.

[0049] According to a preferred embodiment of the invention, the first layer forming the radially outer part of the tread has, radially inward at the radially innermost points of said at least two grooves, a thickness measured along the radial direction greater than or equal to 2 mm.

[0050] Advantageously, according to the invention, for even better performance in terms of resistance to punctures and attacks, the thickness measured along the radial direction of said central part of said second layer of the tread is greater than or equal to 2 mm radially internally at the radially innermost points of said at least two grooves.

[0051] According to an advantageous embodiment of the invention, the first elastomeric compound has a tensile modulus of elasticity at 10% elongation, measured at 60°C, of ​​less than or equal to 10 MPa. Advantageously, the tensile modulus of elasticity at 10% elongation, measured at 60°C, of ​​the first elastomeric compound is between 2.5 and 8 MPa, thus optimizing the wear properties of this first elastomeric compound.

[0052] According to an embodiment of the invention, in which the top reinforcement comprises at least two layers of reinforcing elements, coupled over at least part of their length along the axial direction, said central part being in contact with said radially outermost top reinforcement layer over an axial width between the ends of the coupling zone of said at least two layers of reinforcing elements and advantageously at most equal to the axial width of the coupling zone.

[0053] For the purposes of the invention, layers of top reinforcement elements are said to be coupled if the respective reinforcement elements of each of the layers are separated radially by no more than 1.5 mm, said rubber thickness being measured radially between the respective upper and lower generatrices of said reinforcement elements of each of the layers.

[0054] The inventors have further demonstrated that in the presence of two coupled working layers over a certain axial width and whose ends are decoupled, in particular to allow the absorption of shear stresses between the At the ends of the two working layers, it is preferable that the central part of the second layer, and more specifically the second elastomeric mixture, is not present in the decoupling zone to avoid excessive temperature rises.

[0055] According to one embodiment of the invention, the crown reinforcement of the tire is formed of at least two working crown layers of reinforcing elements, preferably inextensible, crossed from one layer to the other making angles with the circumferential direction between 10° and 45°.

[0056] According to other embodiments of the invention, the top reinforcement also comprises at least one layer of circumferential reinforcing elements.

[0057] Advantageously according to these other embodiments of the invention, the reinforcing elements of at least one layer of circumferential reinforcing elements are metallic reinforcing elements having a secant modulus at 0.7% elongation between 10 and 120 GPa and a maximum tangent modulus less than 150 GPa.

[0058] One embodiment of the invention further provides that the top reinforcement is completed radially on the outside by at least one additional layer, called a protective layer, of reinforcing elements called elastic, oriented with respect to the circumferential direction 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 to it.

[0059] According to any one of the embodiments of the invention mentioned above, the top reinforcement can be further completed, radially inside between the carcass reinforcement and the radially inner working layer closest to said carcass reinforcement, by a triangulation layer of metallic steel reinforcement elements, preferably inextensible, making, with the circumferential direction, an angle greater than 45° and advantageously in the same direction as that of the angle formed by the reinforcement elements of the layer radially closest to the carcass reinforcement.

[0060] Other advantageous details and features of the invention will become apparent from the description of an example of an embodiment of the invention with reference to [Fig.1], which represents a meridian view of a diagram of a tire according to an embodiment of the invention.

[0061] Fig. 1 is not shown to scale for ease of understanding. The figure represents only a half-view of a tire which extends symmetrically with respect to the axis XX' which represents the circumferential median plane, or equatorial plane, of a tire.

[0062] In [Fig. 1], the tire 1, with dimensions 12 R 22.5, comprises a radial carcass reinforcement 2 anchored in two flanges around beads, not shown in the figure. The carcass reinforcement 2 is formed of a single layer of metal cords. The carcass reinforcement 2 is held in place by a crown reinforcement 5, itself capped with a tread 6. The tread has four grooves 3 forming five ribs 4.

[0063] The lower areas and ridges of the tire 1 are in particular not shown in the figure.

[0064] In [Fig. 1], the vertex reinforcement 5 is formed radially from the inside out: - of a triangulation layer 51 formed of inextensible metal cables 9.28 (2+7x0.28) not crimped, continuous over the entire width of the layer, oriented at an angle of 65°, - of a first working layer 52 formed of inextensible 11.35 (3+8x0.35) unreinforced metal cables, continuous over the entire width of the layer, oriented at an angle of 26°, and - of a second working layer 53 formed of inextensible metal cables 11.35 (3+8x0.35) not crimped, continuous over the entire width of the layer, oriented at an angle equal to 18° and crossed with the metal cables of the first working layer.

[0065] The working layers 52 and 53, with respective widths of 210 mm and 190 mm, are coupled over an axial width D equal to 148 mm and are decoupled at their ends by a layer C allowing to limit the risks related to shear.

[0066] According to the invention, the tread 6 consists of a first layer 61, made up of a first elastomeric mixture, radially outer and which comes into contact with the ground during rolling.

[0067] The thickness H of said first layer 61 measured along the radial direction, radially inside a groove 3 is equal to 3.5 mm and therefore greater than 1 mm.

[0068] Radially inside the first layer 61, a second layer 62 is made up of a central part 621 and two axially outer parts 622. The central part 621 is made up of a second elastomeric mixture and the axially outer parts 622 are made up of a third mixture.

[0069] The axial width L of the contact with the layer 53 of the central part 621 of said second layer 62 is equal to 144 mm and therefore less than the axial width of the layer 53 and less than the axial width D.

[0070] The thickness E of the central part 621 of said second layer 62 measured along the radial direction, radially inside a groove 3, is equal to 2 mm and therefore greater than 1 mm.

[0071] Tires have been made based on the elastomeric mixtures described below with some of their properties. Mixture A Mixture B Mixture Ci Mixture C2 Mixture: C3 SR 70 100 ®î 100 100 SSR 30 20 B fi '20 S234 54 S326 52 N347 >2 3S ^'7 saies 10 7.9 T '7 AnS OïÿdaRt «5PD 1.8 2.9 2.0 1.5 1.5 SULFUR 6.0 15 1.1 S 6.1 AKêtérateisf 1.1 1.4 1.1 1 09 Other additives of «Atit, er sgerst de «vise en centre? 15 6 5 113 MAmm (MPa) 2.0 255 TA 6.1 S 5.75 4.85 5.65 20 10.8 G" ' 0.474 0.074 0.488 0.9 0.47 DziNlmm) 80 25.8 100 17.65 30 0.2Î 0.05 0.2 ü .2 0.25

[0072] The values ​​of the constituents are expressed in pce (parts by weight per hundred parts of elastomers).

[0073] The first tire is a reference tire R manufactured according to a configuration corresponding to common designs as described above, consisting of two radially superimposed layers, the inner radial layer being formed of a single elastomeric compound. It combines a compound A located radially on the outside of the tread and a compound B located radially on the inside. The volume of compound B is usually defined by those skilled in the art so that the operating temperature of the tire corresponds to the intended driving conditions with such a tire.

[0074] A tire Tl, according to the invention and as described in the figure, combines, to form the tread, the compound A which forms the radially outer part and corresponds to the first layer 61, the compound Cl, which corresponds to the central part of the second layer 62 and mixture B which corresponds to the axially external parts of the second layer 62.

[0075] A T2 tire, according to the invention and as described in the figure, combines, to form the tread, the compound A which forms the radially outer part and corresponds to the first layer 61, the compound C2, which corresponds to the central part of the second layer 62 and the compound B which corresponds to the axially outer parts of the second layer 62.

[0076] A T3 tire, according to the invention and as described in the figure, combines, to form the tread, the compound A which forms the radially outer part and corresponds to the first layer 61, the compound C3, which corresponds to the central part of the second layer 62 and the compound B which corresponds to the axially outer parts of the second layer 62.

[0077] The following table shows the values ​​obtained according to the formula 4.5*(MA100)095 + 16*(G”)055 + 6*(Dz)°'3 for mixtures B,Cl, C2 and C3. Mixture B Mixture Cl Mixture C2 Titmouse C3 31.1 44.2 54.4 48.0

[0078] To make a comparison, similar tests are carried out with the four tires.

[0079] The first tests consisted of evaluating the distance traveled by the tires before retreading.

[0080] The tests are carried out under defined load and speed conditions to induce tread wear on the reference tire R that allows retreading after a mileage, assigned a value of 100, covered under the said test conditions. Wear performance is evaluated on a heavy goods vehicle during driving on open roads over routes representative of typical heavy goods vehicle use. Values ​​below 100 indicate lower wear performance.

[0081] The results obtained are presented in the following table: Tire R Tire Tl Tire T2 Tire T3 Wear 100 102.95 102

[0082] These results show that the tires according to the invention allow a ride substantially equivalent to that of the reference tire before retreading.

[0083] Other tests consisted of driving 10,000 km on a track covered with sharp stones, then subjecting the tires to an oxidizing atmosphere that could lead to oxidation of the metallic reinforcing elements in the tread layers. The tires were then analyzed to count the number of tread perforations and the number of corroded areas in the tread layers. A base of 100 was set for the reference tire. Values ​​below 100 indicate better performance in terms of resistance to punctures and tread damage.

[0084] The results obtained are presented in the following table: Number of perforations; Number of corroded areas R tire 100 100 T1 tire 50 58 T2 tire 40 57 T3 tire 59 55

[0085] These results highlight that the performance in terms of resistance to punctures and to wear of the tread is significantly improved.

Claims

Demands

1. A tire (1) comprising a crown reinforcement (5) having at least one layer of reinforcing elements (51, 52, 53), itself radially capped by a tread (6) having a tread pattern comprising at least two circumferential grooves (3), joined to two beads by means of two sidewalls, said tread (6) comprising at least two radially superimposed layers of elastomeric compounds (61, 62), the first layer (61), forming the radially outer part of the tread (6), being made of a first elastomeric compound and being present radially inwardly at the radially innermost points of said at least two grooves (3) over a thickness (E), measured in the radial direction, greater than or equal to 1 mm, a second layer (62), radially inner and in contact with the radially outermost crown reinforcement layer (53),comprising a central portion (621) and two axially external portions (622), characterized in that said central portion (621) of the second layer (62) is in contact with said radially outermost top reinforcement layer (53) over an axial width less than the axial width of said radially outermost top reinforcement layer, in that the thickness (H) measured along the radial direction of said central portion (621) is greater than or equal to 1 mm radially internally at the radially innermost points of said at least two grooves (3), and in that said central portion (621) of the second layer (62) is made of a second elastomeric compound having a tensile modulus of elasticity at 100% elongation MAi00, a viscous shear modulus G”, measured at 60°C, and a tear coefficient Dz (measured on a pre-cracked specimen and expressed in N / mm) satisfying the relation: 4,5*(MA1oo)°'95 + 16*(G”)055 + 6*(Dz)03 > 37.5, the tear coefficient being the product of the breaking force per unit thickness (expressed in N / mm of thickness) by the elongation at break (expressed in %), measured at 100 °C.

2. Pneumatic (1) according to claim 1, characterized in that the second elastomeric compound has a tensile modulus of elasticity at 100% elongation MAioo, a modulus viscous shear strength G”, measured at 60°C, and a tear coefficient Dz (measured on a pre-cracked specimen and expressed in N / mm) satisfying the relation: 4.5*(MA1Oo)0'95 + 16*(G”)°'55 + 6*(Dz)03 > 42.5, the tear coefficient being the product of the breaking force per unit thickness (expressed in N / mm of thickness) by the elongation at break (expressed in %), measured at 100 °C.

3. Pneumatic (1) according to claim 1 or 2, characterized in that the second elastomeric mixture contains at least 50 parts natural rubber.

4. Pneumatic (1) according to any one of claims 1 to 3, characterized in that the second elastomeric mixture has a tensile modulus of elasticity at 10% elongation MAio, measured at 60°C, less than or equal to 20MPa.

5. Pneumatic (1) according to any one of claims 1 to 4, characterized in that the second elastomeric mixture has a maximum value of tan(ô), denoted tan(ô)max, measured at 60°C, greater than or equal to 0.

30.

6. Pneumatic (1) according to any one of the preceding claims, characterized in that said two axially outer parts (622) of the second layer (62) are made of a third elastomeric mixture having a maximum value of tan(ô), denoted tan(ô)max, measured at 60°C, less than 0.

10.

7. Tire (1) according to any one of the preceding claims, characterized in that the first layer (61) forming the radially outer part of the tread (6) has, radially inward at the radially innermost points of said at least two grooves (3), a thickness (E) measured along the radial direction greater than or equal to 2 mm.

8. Tire (1) according to any one of the preceding claims, characterized in that the thickness (H) measured along the radial direction of said central part (621) of said second layer (62) of the tread (6) is greater than or equal to 2 mm radially inward at the radially innermost points of said at least two grooves (3).

9. Pneumatic (1) according to any one of the preceding claims, the apex reinforcement (5) comprising at least two layers of reinforcing elements (52, 53), coupled over at least part of their length along the axial direction, characterized in that said central part (621) of the second layer (62) is in contact with said radially outermost top reinforcement layer (53) over an axial width between the ends of the coupling zone of said at least two layers of reinforcing elements (52, 53).

10. Pneumatic (1) according to any one of the preceding claims, characterized in that the top reinforcement (5) is formed of at least two top working layers of reinforcing elements (52, 53), preferably inextensible, crossed from one layer to the other making with the circumferential direction angles between 10° and 45°.

11. Pneumatic (1) according to any one of the preceding claims, characterized in that the top reinforcement (5) comprises at least one layer of circumferential reinforcing elements.

12. Pneumatic (1) according to one of the preceding claims, characterized in that the top reinforcement (5) is completed radially on the outside by at least one additional layer, called a protective layer, of reinforcing elements called elastic, oriented with respect to the circumferential direction 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 to it.

13. Pneumatic (1) according to any one of the preceding claims, characterized in that the top reinforcement (5) further comprises a triangulation layer (51) formed of metallic reinforcing elements making angles greater than 45° with the circumferential direction.

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