Tire with improved durability and rolling resistance performance

Abstract

The present invention relates to a tire comprising a crown reinforcement formed of at least one layer of circumferential reinforcing elements. According to the invention, the thermal conductivity of the layer of polymer mixture forming the tire tread is strictly less than 0.270 W / (m.K), said layer of polymer mixture forming the tire tread being formed from a rubber composition based on at least one elastomeric matrix and at least one reinforcing filler, said elastomeric matrix mainly comprising a styrene-butadiene-based copolymer having a glass transition temperature Tg strictly greater than -65°C and less than or equal to -30°C, and said reinforcing filler mainly consisting of an inorganic filler.

Description

TECHNICAL FIELD

[0001] The present application relates to tires having a radial carcass reinforcement, and more particularly to tires intended to equip vehicles carrying heavy loads and traveling at sustained speeds, such as trucks, tractors, trailers or buses. BACKGROUND

[0002] In tires of the heavy load type, the carcass reinforcement is generally anchored on both sides in the bead zone and is covered radially by a crown reinforcement consisting of at least two superimposed layers formed of filaments or cords that are parallel in each layer and cross from one layer to the next, forming an angle with the circumferential direction of between 10° and 45°. Said working layers forming the working reinforcement can also be covered with at least one layer called a protective layer formed of reinforcing elements that are advantageously metal and extensible, which are called elastic reinforcing elements. It can also include a layer of metal filaments or cords having low extensibility that form an angle with the circumferential direction of between 45° and 90°, this ply being called a triangular ply and being located radially between the carcass reinforcement and the first crown ply, called the working ply, formed of parallel filaments or cords having an angle of at most 45° in absolute value. The triangular ply forms a triangular reinforcement with at least said working ply, which has little deformation under the various stresses to which it is subjected, said triangular ply essentially serving to absorb the transverse compression forces exerted on all the reinforcing elements in the crown zone of the tire.

[0003] Radially external to the crown reinforcement is the tread, which is generally made of polymeric material and is intended to come into contact with the ground in the contact patch of the tire in contact with the ground.

[0004] A cord is said to be inextensible when it exhibits a relative elongation of at most 0.2% at a tensile force equal to 10% of the breaking force.

[0005] A cord is said to be elastic when it exhibits a relative elongation of at least 3% and a maximum tangent modulus of less than 150 GPa at a tensile force equal to the breaking load.

[0006] A circumferential reinforcing element is a reinforcing element forming an angle with the circumferential direction within the range of +2.5°, -2.5° relative to 0°.

[0007] The circumferential direction or longitudinal direction of the tire is the direction corresponding to the outer circumference of the tire and defined by the direction of travel of the tire.

[0008] The transverse direction or axial direction of the tire is parallel to the axis of rotation of the tire.

[0009] The radial direction is the direction that intersects the axis of rotation of the tyre and is perpendicular thereto.

[0010] The axis of rotation of the tyre is the axis about which the tyre turns in normal use.

[0011] The radial plane or meridian plane is the plane containing the axis of rotation of the tyre.

[0012] The circumferential median plane or equatorial plane is the plane perpendicular to the axis of rotation of the tyre and dividing it into two halves.

[0013] Thanks to the improvement of the road network and the expansion of the global highway network, some tyres today called "road" tyres are intended to travel at high speed for increasingly long distances. The combined conditions required for such tyres to travel undoubtedly increase the number of kilometres travelled as a result of the reduction in wear on the tyre; however, on the other hand, the durability of such tyres, and in particular of the crown reinforcement, is adversely affected.

[0014] This is because there are stresses in the crown reinforcement, more particularly shear stresses between the crown plies, coupled with a non-negligible increase in the working temperature at the ends of the axially shortest crown plies, which results in the appearance and propagation of cracks in the rubber at said ends.

[0015] In order to limit the excessive increase in temperature in the crown of the tyre, the material of which the tread is made is advantageously chosen to have a hysteresis loss suitable for the working conditions of the tyre.

[0016] Furthermore, in order to improve the durability of the crown reinforcement of tyres of the type under consideration, solutions have been provided relating to the structure and quality of the layers and / or shaped elements of rubber compound provided between and / or around the ends of the plies, more particularly the ends of the axially shortest plies.

[0017] In order to improve the resistance to deterioration of the rubber compound located near the edges of the crown reinforcement, patent FR 1 389 428 suggests using, in combination with a low-hysteresis tread, a shaped element of rubber that covers at least the sides and edges of the crown reinforcement and is composed of a low-hysteresis rubber compound.

[0018] In order to avoid separation between the plies of the crown reinforcement, patent FR 2 222 232 teaches covering the ends of the reinforcement in a rubber pad having a different Shore A hardness from that of the tread surrounding said reinforcement and higher than that of the shaped element of rubber compound arranged between the edges of the plies of the crown reinforcement and the carcass reinforcement.

[0019] French application FR 2 728 510 proposes, on the one hand, to provide between the carcass reinforcement and the working crown reinforcement closest to the axis of rotation, an axially continuous ply formed of inextensible metal cords forming an angle with the circumferential direction at least equal to 60° and having an axial width at least equal to that of the shortest working crown ply, and on the other hand, to provide between two working crown plies, an additional ply formed of metal elements oriented substantially parallel to the circumferential direction.

[0020] French application WO 99 / 24269 also proposes, on either side of the equatorial plane and in the immediate axial continuation of the additional ply having reinforcing elements substantially parallel to the circumferential direction, to couple, over a certain axial distance, two working crown plies formed of reinforcing elements intersecting from one ply to the next, then to decouple, at least over the remaining width common to said two working plies, by means of a shaped element of rubber compound.

[0021] The circumferential reinforcing element layer generally consists of at least one metal cord wound to form a set of turns, the angle of laying of said metal cord with respect to the circumferential direction being less than 2.5°.

[0022] This improvement in the endurance of the tyre makes it possible to envisage at least the possibility of retreading when the tread has been worn. In particular, when the tyre is intended to be retreaded after the tread has been worn away, in order to optimise the use of the new tread, the tyre to be retreaded must not be in an excessively advanced state of ageing.

[0023] In order to further increase the service life of the tyre, it is usual practice to choose a polymeric material having improved wear resistance properties to make the tread. Since such materials generally have a detrimental effect on the hysteresis properties, it is also known practice to make the tread of the tyre by radial superposition of two different materials, so as to obtain a satisfactory compromise between wear resistance - hysteresis properties for the envisaged application.

[0024] Such a tyre is described, for example, in document US 6,247,512. The document describes the superposition of two layers of material to form the tread, the outer material in contact with the ground having in particular better properties in terms of wear, while the inner material has hysteresis properties which make it possible to limit the increase in temperature of the tyre in the crown region.

[0025] The tyre prepared in this way is perfectly satisfactory for the road applications mentioned above. However, in order to ensure the possibility of carrying out a retreading operation, it is recommended to avoid excessive degradation of the inner material which wears relatively quickly compared to the outer material, so as not to risk damaging the reinforcements of the tyre.

[0026] Therefore, the use of the tread of such tyres cannot be optimised sufficiently, due to the risk of compromising the retreading options of the tyre. SUMMARY

[0027] It was therefore the object set to the inventors to be able to provide a tyre which allows a greater rolling distance before envisaging retreading, i.e. an improved wear performance, by improving the endurance performance of the tyre, regardless of the use, while still providing a satisfactory rolling resistance performance.

[0028] According to the application, this object is achieved by a tyre having a radial carcass reinforcement, said tyre comprising a crown reinforcement, said crown reinforcement itself being covered in the radial direction by a tread, said tread being joined to two beads via two sidewalls, said crown reinforcement having at least one layer of circumferential reinforcing elements, said tread having at least one layer of polymer compound, said layer of polymer compound forming a tread surface intended to come into contact with the road, the thermal conductivity of said layer of polymer compound forming the tread surface being strictly less than 0.270 W / (m.K), said layer of polymer compound forming the tread surface being formed from a rubber composition based on at least one elastomer matrix and at least one reinforcing filler, said elastomer matrix mainly comprising a copolymer based on styrene and butadiene and having a glass transition temperature Tg strictly greater than -65°C and less than or equal to -30°C, and at least one reinforcing filler mainly comprising an inorganic reinforcing filler.

[0029] The thermal conductivity of the polymer compound is measured according to the standard ISO 8302:1991, entitled Determination of thermal resistance - Steady-state heat flux, and related performance - Guarded hot plate method. The measurements are carried out on parallelepiped samples having a side length of 50 to 100 millimetres and a thickness of 2.5 millimetres at 25°C.

[0030] The term "elastomer matrix" or "matrix of elastomers" means all the elastomers present in the rubber composition.

[0031] A "diene" elastomer (or equivalently rubber), whether natural or synthetic, is understood to mean an elastomer consisting at least in part (i.e. homopolymer or copolymer) of diene monomers (i.e. monomers bearing two conjugated or non-conjugated carbon-carbon double bonds).

[0032] The term "copolymer based on styrene and butadiene" means in this case at least one copolymer of styrene monomers and at least one butadiene monomer having a glass transition temperature Tg strictly greater than -65°C and less than or equal to -30°C (and of course also any mixture of such copolymers); in other words, said copolymer based on styrene and butadiene comprises by definition at least styrene units (derived from styrene monomers) and butadiene units (derived from butadiene monomers) and has a glass transition temperature Tg strictly greater than -65°C and less than or equal to -30°C.

[0033] The value of the glass transition temperature "Tg" is measured in a known manner by DSC ("Differential Scanning Calorimetry") according to standard ASTM D3418 (1999).

[0034] The rubber composition used in the tyre of the application comprises at least one reinforcing filler, said reinforcing filler comprising mainly inorganic reinforcing filler, i.e. the mass of inorganic reinforcing filler represents at least 51 % of the total mass of the reinforcing filler component. Preferably, the mass of inorganic reinforcing filler represents more than 60 %, preferably more than 70 % of the total mass of the reinforcing filler.

[0035] The term "inorganic reinforcing filler" is understood here to mean any inorganic or mineral filler, whatever its colour and its origin (natural or synthetic), also called "white filler", "transparent filler" or "non-black filler" compared to carbon black, which is able to reinforce by itself alone the rubber composition intended for the manufacture of pneumatic tyres without means other than an intermediate coupling agent, in other words which is able to replace the conventional tyre-grade carbon black in terms of reinforcing action. Such fillers are generally characterized in a known manner by the presence of hydroxyl (-OH) groups on their surface, for which the use of a coupling agent or system intended to provide stable chemical bonds between the filler and the elastomer matrix is necessary in order to be used as reinforcing filler.

[0036] Mineral fillers of siliceous type, preferably silica (Si02), are particularly suitable for use as inorganic reinforcing fillers. The silica used can be any reinforcing silica known to the person skilled in the art, in particular having a BET specific surface area and a CTAB specific surface area both less than 450 m 2 / g, preferably between 30 and 400 m 2 / g, in particular between 60 and 300 m 2Any precipitated or fumed silica between 5 and 150 m2 / g. Highly dispersible precipitated silicas ("HDS") that can be mentioned are for example Ultrasil 7000 and Ultrasil 7005 silicas from Evonik, Zeosil 1165MP, 1135MP and 1115MP silicas and Zeosil Premium 200 silica from Solvay, Hi-Sil EZ150G silica from PPG, Zeopol 8715, 8745 and 8755 silicas from Huber, or silicas with a high specific surface area as described in application WO 03 / 016387.

[0037] Of course, the term "inorganic reinforcing filler" is also understood to mean a mixture of various inorganic reinforcing fillers, in particular a mixture of highly dispersible silicas as described above or a mixture of inorganic fillers of siliceous type with non-siliceous inorganic fillers. As non-siliceous inorganic fillers, mention can be made of mineral fillers of aluminous type, in particular alumina (AI2O3) or aluminum (oxide) hydroxide, or reinforcing titanium oxides, as described in US 6 610 261 and US 6 747 087. When present, the non-siliceous inorganic fillers represent a small amount in the reinforcing filler.

[0038] It is of no importance in which physical state the inorganic reinforcing filler is provided, whether it is in the form of a powder, microbeads, granules or beads.

[0039] According to one embodiment, the content of reinforcing filler in the rubber composition of the tire according to the application can be between 55 phr and 200 phr, preferably between 55 and 150 phr, more preferably between 55 and 80 phr. These preferred ranges apply to any one of the embodiments of the application.

[0040] The skilled person will understand that reinforcing fillers of another nature, in particular organic nature, such as carbon black, can be used as equivalent to the inorganic reinforcing fillers described in this section, provided that this reinforcing filler is covered with an inorganic layer such as silica or comprises functional sites, in particular hydroxyl sites, on its surface that require the use of a coupling agent to establish a bond between the filler and the elastomer. By way of example, mention can be made of carbon blacks such as those used for tires, as described in for example patent documents WO 96 / 37547 and WO 99 / 28380.

[0041] The tests carried out show that the tires according to the application have satisfactory wear and endurance properties, regardless of the conditions of use of the tires. In addition, they perform better in terms of rolling resistance than the more conventional tires described above.

[0042] This is because the inventors have been able to demonstrate that the wear rate of the tread layer according to the application is greatly improved compared with more conventional compounds used to obtain treads with low hysteresis losses.

[0043] This is because the rubber composition of the said polymeric compound layer forming the tread surface, together with the thermal conductivity, imparts good performance in terms of rolling resistance and wear, wherein the rubber composition is based on at least one elastomeric matrix and comprises at least one reinforcing filler, the elastomeric matrix mainly comprising a copolymer based on styrene and butadiene having a glass transition temperature Tg strictly greater than -65°C and less than or equal to -30°C, the reinforcing filler mainly being formed of inorganic reinforcing filler, the thermal conductivity being strictly less than 0.270 W / (m.K). However, the inventors have been able to demonstrate that the low thermal conductivity of the said polymeric compound layer forming the tread surface leads to an increase in the temperature of the tyre tread, since the heat generated during the running of the tyre is only very slowly evacuated. This temperature increase highlights the ageing of the polymeric compound which has been subjected to shear stresses between the working layers of the crown reinforcement, and therefore has an adverse effect on the performance in terms of endurance. However, the inventors have been able to show that the layer of circumferential reinforcing elements makes it possible to compensate for this temperature rise due to the low thermal conductivity of the compound, and makes it possible to maintain the performance of the crown reinforcement in terms of endurance as expected.

[0044] The inventors have been able to demonstrate, while remaining attentive to the risk of oxidation, in particular of the carcass reinforcement (which is linked to the presence of oxidizing agents within the pressurized tyre, which increase with the service life of the tyre), that the temperature increase of the compound forming the tread is transmitted by conduction to the compound of the wall formed between the carcass reinforcement and the tyre cavity (which is pressurized during inflation). This temperature increase, in particular of the polymeric compound in contact with the layer of carcass reinforcement, which is intended to fix the oxygen which migrates from the cavity, increases the reactivity of the said compound, which is thus able to fix more oxygen. The inventors thus believe that this explains the way in which the endurance performance of the carcass reinforcement is maintained even if the tyre is used more.

[0045] According to the application, it is also advantageous for the thermal conductivity of the said polymeric compound layer forming the tread surface to be strictly less than 0.265 W / (m.K).

[0046] According to one embodiment of the application, the rubber composition of the said at least one polymeric compound layer forming the tread surface of the tyre has a plasticizing system comprising from 0 to 15 phr, preferably from 0 to 9 phr, of at least one plasticizing resin having a glass transition temperature Tg greater than or equal to 20°C, and the total content of plasticizing system in the composition is from 0 to 17 phr, preferably from 0 to 11 phr.

[0047] The abbreviation "phr" (per hundred of rubber) means parts by weight of the elastomer(s) present in the rubber composition (if several elastomers are present, the sum of the elastomers) or of the rubber per hundred of parts.

[0048] According to the application, advantageously, the plasticizing system can comprise from 2 to 15 phr, preferably from 2 to 9 phr of plasticizing resin, the total content of plasticizing system in the composition being from 2 to 17 phr, preferably from 2 to 11 phr.

[0049] Preferably, the copolymer based on styrene and on butadiene can be obtained by solution polymerization.

[0050] Preferably, the copolymer based on styrene and on butadiene has a glass transition temperature Tg of between -60°C and -40°C.

[0051] Preferably, the copolymer based on styrene and based on butadiene can be a styrene-butadiene copolymer.

[0052] In one embodiment of the application, the elastomer matrix can further comprise at least one second diene elastomer, different from the copolymer based on styrene and on butadiene; in particular, the second diene elastomer can be chosen from polybutadiene, natural rubber, synthetic isoprene, butadiene copolymers different from butadiene-styrene copolymers, isoprene copolymers and mixtures of these polymers and copolymers; preferably, the second diene elastomer can be polybutadiene.

[0053] Preferably, the content of the second diene elastomer can be from 5 to 49 phr, preferably from 15 to 35 phr.

[0054] In one embodiment of the application, the elastomer matrix can further comprise at least one third diene elastomer, different from the copolymer based on styrene and on butadiene and from the second diene elastomer, in particular the third diene elastomer can be chosen from natural rubber and isoprene elastomers, preferably it can consist of natural rubber.

[0055] Preferably, the content of the second diene elastomer can be from 0.5 to 35 phr and the content of the third diene elastomer can be from 0.5 to 35 phr, preferably the content of the second diene elastomer can be from 9 to 31 phr and the content of the third diene elastomer can be from 4 to 24 phr.

[0056] Preferably, the composition can further comprise carbon black; in particular, the content of carbon black can be less than or equal to 10 phr, less than or equal to 5 phr, preferably the content of carbon black can be from 0.5 to 4 phr.

[0057] Preferably, the content of the reinforcing filler can be comprised between 55 and 200 phr, preferably between 55 and 150 phr, more preferably between 55 and 80 phr.

[0058] Preferably, the plasticizing resin can have a glass transition temperature Tg greater than or equal to 30°C, preferably between 30 and 100°C; in particular, the plasticizing resin can be selected from the group consisting of cyclopentadiene homopolymer or copolymer resins, dicyclopentadiene homopolymer or copolymer resins, terpene homopolymer or copolymer resins, C5 fraction homopolymer or copolymer resins, C9 fraction homopolymer or copolymer resins, homopolymer resins, in particular mixtures of C5 fraction homopolymer or copolymer resins and C9 fraction homopolymer or copolymer resins, alpha-methylstyrene copolymer resins, and mixtures of these resins.

[0059] Preferably, the plasticizing system can comprise between 0 and 2 phr of at least one plasticizer which is liquid at ambient temperature (23°C).

[0060] According to another embodiment, the rubber composition of the tire according to the application can be free of plasticizing system.

[0061] According to an advantageous alternative embodiment of the application, the layer of circumferential reinforcing elements has an axial width greater than 0.5 x W.

[0062] W is the maximum axial width of the tire when it is mounted on its service rim and inflated to its recommended pressure.

[0063] The axial width of the layer of reinforcing elements is measured in the cross-section of the tire, the tire thus being in un-inflated state.

[0064] According to a preferred embodiment of the application, since the crown reinforcement is composed of at least two layers of reinforcing elements, one circumferential layer of reinforcing elements is disposed radially between the two working crown layers.

[0065] According to this embodiment of the application, the circumferential layer of reinforcing elements makes it possible to limit the compression of the reinforcing elements of the carcass reinforcement to a greater extent than a similar layer located radially outside the working layers. It preferably separates the at least one working layer from the carcass reinforcement radially, thus limiting the stress on the reinforcing elements and avoiding the excessive fatigue of the reinforcing elements.

[0066] According to a preferred embodiment of the application, the at least two working crown layers have different axial widths, the difference between the axial width of the working crown layer which is axially widest and the axial width of the working crown layer which is axially narrowest being between 10 and 30 mm.

[0067] According to a preferred embodiment of the application, the working crown layer which is axially widest is located radially inside the other working crown layers.

[0068] According to the application, it is also advantageous for the axial width of the working crown layer adjacent to the layer of circumferential reinforcing elements to be greater than the axial width of said layer of circumferential reinforcing elements, and preferably for said working crown layer to be coupled in axial width adjacent to the layer of circumferential reinforcing elements on either side of the equatorial plane and in the immediate axial continuation of the layer of circumferential reinforcing elements, then to be decoupled over at least the remaining width common to the two working layers by a layer of rubber compound.

[0069] The presence of such a coupling between the working crown layers adjacent to the layer of circumferential reinforcing elements makes it possible to reduce the tensile stresses acting on the axially outermost circumferential elements at the location closest to this coupling.

[0070] According to an advantageous embodiment of the application, 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 120 GPa and a maximum tangent modulus of less than 150 GPa.

[0071] According to a preferred embodiment, the reinforcing elements have a secant modulus at 0.7% elongation of less than 100 GPa and greater than 20 GPa, preferably between 30 and 90 GPa, more preferably less than 80 GPa.

[0072] It is also preferable for the maximum tangent modulus of the reinforcing elements to be less than 130 GPa, more preferably less than 120 GPa.

[0073] The above-mentioned moduli are measured on a curve of tensile stress as a function of elongation (said curve being determined using a pre-load of 20 MPa), the tensile stress corresponding to the measured tension corrected for the cross-section of the metal of the reinforcing element. The cords are measured on a portion of the layer of circumferential reinforcing elements extending 50 mm in axial width from the axial end of said layer towards the inside of said layer.

[0074] The modulus of the same reinforcing elements can be measured on a curve of tensile stress as a function of elongation (said curve being determined using a pre-load of 10 MPa), the tensile stress corresponding to the measured tension corrected for the entire cross-section of the reinforcing element. The entire cross-section of the reinforcing element is the cross-section of the composite element composed of the metal and the rubber that has penetrated into the reinforcing element, in particular during the course of the curing of the tyre; this entire cross-section of the reinforcing element in question is approximately twice the cross-section of the metal of the reinforcing element.

[0075] According to this conception relating to the entire cross-section of the reinforcing element, the reinforcing elements of the axially outer portion and of the intermediate portion of at least one layer of circumferential reinforcing elements are metal reinforcing elements having a secant modulus at 0.7% elongation of between 5 and 60 GPa and a maximum tangent modulus of less than 75 GPa.

[0076] According to a preferred embodiment, the reinforcing elements have a secant modulus at 0.7% elongation lower than 50 GPa and greater than 10 GPa, preferably comprised between 15 and 45 GPa, more preferably lower than 40 GPa.

[0077] It is also preferred that the maximum tangent modulus of the reinforcing elements is lower than 65 GPa, more preferably lower than 60 GPa.

[0078] According to a preferred embodiment, the reinforcing elements of the at least one layer of circumferential reinforcing elements are metallic reinforcing elements having a curve of tensile stress as a function of relative elongation showing a flat gradient for small elongations and a substantially constant steep gradient for greater elongations.

[0079] The various characteristics of the reinforcing elements described above are measured on reinforcing elements taken from a tyre.

[0080] According to the present application, the reinforcing elements more particularly suitable for making the at least one layer of circumferential reinforcing elements are for example assemblies of gauge 21.23, the assembly being constructed 3 x (0.26 + 6 x 0.23) 4.4 / 6.6 SS; this stranded cord is composed of 21 elementary filaments and has gauge 3 x (1 + 6), in which 3 strands are twisted together, each strand being composed of 7 filaments, one filament forming the central core having a diameter equal to 26 / 100 mm and 6 wrapping filaments having a diameter equal to 23 / 100 mm. This cord has a secant modulus at 0.7% equal to 45 GPa and a maximum tangent modulus equal to 98 GPa, these moduli being measured on the curve of tensile stress as a function of elongation (determined using a pre-load of 20 MPa and corrected for the cross section of the metal of the reinforcing element), the tensile stress corresponding to the measured tension corrected for the cross section of the metal of the reinforcing element. On the curve of tensile stress as a function of elongation (determined using a pre-load of 10 MPa and corrected for the entire cross section of the reinforcing element), the tensile stress corresponding to the measured tension corrected for the entire cross section of the reinforcing element, this cord of gauge 21.23 has a secant modulus at 0.7% equal to 23 GPa and a maximum tangent modulus equal to 49 GPa.

[0081] In the same way, another example of reinforcing element is an assembly of gauge 21.28, the assembly being constructed 3 x (0.32 + 6 x 0.28) 6.2 / 9.3 SS. This cord has a modulus at 0.7% equal to 56 GPa and a maximum tangent modulus equal to 102 GPa, these moduli being measured on a curve of tensile stress as a function of elongation (the curve being determined using a pre-load of 20 MPa and corrected for the cross-section of the metal of the reinforcing element), the tensile stress corresponding to the measured tension corrected for the cross-section of the metal of the reinforcing element. On a curve of tensile stress as a function of elongation (the curve being determined using a pre-load of 10 MPa and corrected for the entire cross-section of the reinforcing element) (the tensile stress corresponding to the measured tension corrected for the entire cross-section of the reinforcing element), this cord of gauge 21.28 has a modulus at 0.7% equal to 27 GPa and a maximum tangent modulus equal to 49 GPa.

[0082] The use of such reinforcing elements in at least one layer of circumferential reinforcing elements makes it possible in particular to maintain a satisfactory stiffness of the layer even after the shaping and curing phases in the conventional manufacturing method.

[0083] According to a second embodiment of the application, the circumferential reinforcing elements can be formed from inextensible metal elements cut in such a way as to form portions of length much smaller than the circumference of the shortest layer but preferably greater than 0.1 times said circumference, the cut between the portions being axially offset from one another. Again preferably, the tensile modulus per unit of width of the additional layer is less than the tensile modulus per unit of width measured under the same conditions for the most extensible working crown layer. Such an embodiment makes it possible to impart to the layer of circumferential reinforcing elements in a simple way a modulus which can easily be adjusted (by choosing the spacing between the portions of the same row) but which in all cases is lower than the modulus of a layer composed of identical but continuous metal elements, the modulus of the additional layer being measured on the vulcanized layer of cut elements removed from the tyre.

[0084] According to a third embodiment of the application, the circumferential reinforcing elements are undulating metal elements, the ratio a / λ of the amplitude to the wavelength being at most equal to 0.09. Preferably, the tensile modulus per unit of width of the additional layer is less than the tensile modulus per unit of width measured under the same conditions for the most extensible working crown layer.

[0085] The metal elements are preferably steel cords.

[0086] According to a preferred embodiment of the application, the reinforcing elements of the working crown layers are inextensible metal cords.

[0087] According to the application, advantageously, the crown reinforcement is formed by at least two working crown layers having reinforcing elements described below, the reinforcing elements crossing from one layer to the other and forming an angle with the circumferential direction of between 10° and 45°.

[0088] In order to reduce the tensile stresses acting on the axially outermost circumferential elements, the application also advantageously provides that the reinforcing elements of the working crown layers form an angle with the circumferential direction of less than 30°, preferably less than 25°.

[0089] One preferred embodiment of the application also provides that the crown reinforcement is supplemented radially on the outside by at least one additional layer called a protective layer, said protective layer having reinforcing elements called elastic reinforcing elements, said elastic reinforcing elements being oriented at an angle of between 10° and 45° with respect to the circumferential direction and in the same angular direction as the non-stretchable elements of the radially adjacent working layer.

[0090] According to any one of the above embodiments of the application, the crown reinforcement can be further supplemented radially on the inside between the carcass reinforcement and the radially innermost working layer closest to said carcass reinforcement by a triangular layer made of non-stretchable metal reinforcing elements, said metal reinforcing elements being made of steel and forming an angle of more than 45° with the circumferential direction and in the same angular direction as the reinforcing elements of the radially closest layer of the carcass reinforcement. BRIEF DESCRIPTION OF DRAWINGS

[0091] Further details and advantageous features of the application will become apparent from the following description of exemplary embodiments of the application, given by way of example only, and with reference to the accompanying drawings in which, Figure 1 A meridian view of a tire according to the application is shown.

[0092] For ease of understanding, the drawings are not shown to scale. The drawings show only half-views of the tire, the tire extending symmetrically with respect to an axis XX’ representing the circumferential median plane or equatorial plane of the tire. DETAILED DESCRIPTION

[0093] Figure 1 A tire 1 of size 315 / 70R 22.5 is shown. Said tire 1 comprises a radial carcass reinforcement 2 anchored in two beads (not shown in the figures). The carcass reinforcement is covered by a crown reinforcement 3 formed radially from the inside to the outside by:

[0094] - a first working layer 31 formed of non-covered non-stretchable metal cords 9.30, said metal cords 9.30 being continuous over the entire width of the ply and oriented at an angle equal to 26°,

[0095] - a layer 32 of circumferential reinforcing elements formed from metal cords 21 x 23 of the "bi-modulus" type,

[0096] - a second working layer 33 formed from non-coated inextensible metal cords 9.30, continuous over the entire width of the ply, oriented at an angle equal to 18° and intersecting the metal cords of the layer 31,

[0097] - a protective layer 34 formed from elastic metal cords 6.35.

[0098] The crown reinforcement is itself covered by a tread 4 having a surface 5 intended to come into contact with the ground.

[0099] The maximum axial width W of the tyre is equal to 317 mm.

[0100] The axial width L of the first working layer 31 31 is equal to 252 mm.

[0101] The axial width L of the second working layer 33 33 is equal to 232 mm.

[0102] The axial width L of the layer 32 of circumferential reinforcing elements 32 is equal to 194 mm.

[0103] The final crown ply 34, called protective ply, has a width L 34 equal to 124 mm.

[0104] Tests were carried out on the tyre according to the application.

[0105] The same tests were carried out with the reference tyre.

[0106] The contents of the various components of the compositions presented in the table are expressed in phr. All the compositions (T1, T2 and C) comprise a crosslinking system, which is used conventionally in the manufacture of tyre treads; this crosslinking system comprises, inter alia, sulphur, ZnO, stearic acid and accelerators.

[0107]

[0108] (1) a non-functional and non-incremental solution SBR having 24% of 1,2-butadiene units, 26.5% of styrene units, and a Tg = -48°C, relative to the total butadiene units;

[0109] (2) a tin-functionalized and non-incremental solution SBR having 24% of 1,2-butadiene units, 15.5% of styrene units, and a Tg = -65°C, relative to the total butadiene units;

[0110] (3) natural rubber;

[0111] (4) Neodyne polybutadiene with 98% of cis-1,4-butadiene units and Tg = - 108°C;

[0112] (5) Zeosil 1165MP silica of HDS type from Solvay;

[0113] (6) Carbon black N234;

[0114] (7) Carbon black N134;

[0115] (8) Coupling agent: TESPT (Si69 from Evonik-Degussa);

[0116] (9) Diphenyl guanidine (Perkacit DPG from Flexsys);

[0117] (10) N-1,3-dimethylbutyl-N'-phenyl-p-phenylenediamine, sold under the name Santoflex 6-PPD by Flexsys;

[0118] (11) C5 / C9 fraction resin, sold under the name THER 8644 resin by Cray Valley (Tg = 44°C).

[0119] The thermal conductivity properties of the compositions are presented in the following table.

[0120]

[0121] According to the application, the tires I were prepared according to the following Figure 1 The tread of these tires was composed of the compound C.

[0122] The first reference tires P1 and P2 are similar to the tires of the application, their tread being composed of the compounds T1 and T2, respectively.

[0123] Other reference tires P3 were prepared. They are similar to the tires I of the application, the difference in their crown construction being that the crown reinforcement does not have any layer of circumferential reinforcing elements. They were made with a tread composed of the compound C.

[0124] A wear resistance test was carried out by fitting each of the tires I according to the application and the reference tires P1 and P2 on the same vehicle and comparing these tires.

[0125] The test was carried out under identical conditions in terms of load and pressure and on the same route representative of normal use. The result consists in measuring the weight loss.

[0126] The results of the test are presented in the following table.

[0127] The values are expressed in kg, the value 100 being assigned to the tyre P1. Values greater than 100 indicate better wear performance.

[0128] P1 P2 I Wear 100 105 115

[0129] These latter results confirm that the tyre I according to the application has better wear resistance.

[0130] The first endurance test was carried out on a test machine which forced each tyre to travel in a straight line at a speed equal to the maximum speed class (speed index) assigned to the tyre, under an initial load of 4000 kg which was gradually increased in order to reduce the duration of the test.

[0131] The second endurance test was carried out on a test machine which periodically applied lateral loads and dynamic overloads to the tyres. The tyre according to the application was tested under the same conditions as those applied to the reference tyres.

[0132] The tests thus carried out showed that the tyre I according to the application covered the same distance as the reference tyres P1 and P2 in each test. It is therefore evident that the tyre according to the application behaves as well as the reference tyres in terms of endurance.

[0133] The tyre P3 itself covered a much shorter distance, approximately equal to half the distance covered by the tyre I according to the application and by the reference tyres P1 and P2.

[0134] Finally, a rolling road endurance test was carried out on a test machine which applied a load of 4415 daN and a speed of 40 km / h to the tyres, in which the tyres were aerated with oxygen doping. The tyre I according to the application was tested under the same conditions as those applied to the reference tyres P1 and P2. The running operation was stopped as soon as the tyre showed carcass reinforcement deterioration.

[0135] The tests thus carried out showed that the tyre I according to the application covered a distance which was advantageous in each test, which amounted to at least 250,000 km (at which kilometre number the test was stopped), while the reference tyres P1 and P2 covered only 220,000 km.

[0136] Furthermore, rolling resistance measurements were carried out. These measurements involved the reference tyres P1 and P2 and the tyre I according to the application.

[0137] The results of the tests are presented in the table below.

[0138] The rolling resistance measurements are expressed in kg / t, the value 100 being assigned to the tyre P1. Values greater than 100 indicate better performance in terms of wear.

[0139] P1 P2 I Rolling resistance 100 96 103

[0140] From these tests it can be concluded that the tyre according to the present application makes it possible to improve the performance in terms of durability and wear and to exhibit satisfactory performance in terms of rolling resistance.

Claims

1. Tyre (1) having a radial carcass reinforcement (2), said tyre (1) comprising a crown reinforcement (3) itself covered in the radial direction by a tread (4) joined to two beads via two sidewalls, said crown reinforcement (3) having at least one layer of circumferential reinforcing elements (32), said tread having at least one layer of polymer compound forming a tread surface (5) intended to come into contact with the road, characterized in that, The thermal conductivity of the polymer compound layer forming the tread surface is strictly less than 0.270 W / (m.K), the polymer compound layer forming the tread surface is formed from a rubber composition based on at least one elastomer matrix and at least one reinforcing filler, the elastomer matrix mainly comprising a copolymer based on styrene and butadiene and having a glass transition temperature Tg strictly greater than -65°C and less than or equal to -30°C, the elastomer matrix also comprising 0.5 to 35 phr of at least one second diene elastomer and 0.5 to 35 phr of at least one third diene elastomer, the second diene elastomer being different from the copolymer based on styrene and butadiene and being chosen from polybutadiene, natural rubber, synthetic isoprene, butadiene copolymers different from butadiene-styrene copolymers, isoprene copolymers and mixtures of these polymers and copolymers, the third diene elastomer being different from the copolymer based on styrene and butadiene and from the second diene elastomer and being chosen from natural rubber and isoprene elastomers, and the reinforcing filler mainly comprising inorganic reinforcing fillers.

2. Tyre (1) according to Claim 1, characterized in that, The thermal conductivity of the polymer compound layer forming the tread surface is strictly less than 0.265 W / (m.K).

3. Tyre (1) according to claim 1 or 2, characterized in that, The inorganic reinforcing filler is silica.

4. Tyre (1) according to claim 1, characterized in that, The rubber composition of the at least one polymer compound layer forming the tread surface of the tyre has a plasticizing system comprising 0 to 15 phr of at least one plasticizing resin having a glass transition temperature Tg greater than or equal to 20°C and the total content of plasticizing system in the rubber composition is 0 to 17 phr.

5. Tyre (1) according to claim 1, characterized in that, The copolymer based on styrene and butadiene has a glass transition temperature Tg of -60°C to -40°C.

6. Tyre (1) according to claim 1, characterized in that, The copolymer based on styrene and butadiene is a styrene-butadiene copolymer.

7. A tyre (1) according to claim 1, said crown reinforcement (3) comprising at least two working crown reinforcement element layers (31, 33), characterised in that, The circumferential reinforcing element layer (32) is disposed radially between two working crown reinforcing element layers (31, 33).

8. Tyre (1) according to Claim 7, characterized in that, The axial width of the working crown reinforcing element layers (31, 33) adjacent to the circumferential reinforcing element layer (32) is greater than the axial width of said circumferential reinforcing element layer (32).

9. Tyre (1) according to Claim 8, characterized in that, On either side of the equatorial plane and in the immediate axial extension of the circumferential reinforcing element layer (32), the working crown reinforcing element layers (31, 33) adjacent to the circumferential reinforcing element layer (32) are coupled in axial width so as to be uncoupled at least over the common residual width of said working crown reinforcing element layers (31, 33) adjacent to the circumferential reinforcing element layer (32) by a shaped element of rubber composition.

10. Tyre (1) according to claim 1, characterized in that, The crown reinforcement (3) comprises at least two working crown reinforcing element layers (31, 33), wherein the reinforcing elements of the at least two working crown reinforcing element layers (31, 33) are inextensible, cross from one layer to the other and form an angle with the circumferential direction of between 10° and 45°.

11. Tyre (1) according to claim 1, characterized in that, The reinforcing elements of the at least one layer of circumferential reinforcing elements (32) are metal reinforcing elements having a secant modulus at 0.7% elongation of between 10 and 120 GPa and a maximum tangent modulus of less than 150 GPa.

12. A tyre (1) according to claim 1, said crown reinforcement (3) comprising at least two working crown reinforcement element layers (31, 33), characterised in that, The crown reinforcement (3) is supplemented radially on the outside by at least one additional layer called a protective ply (34) having "elastic" reinforcing elements oriented at an angle of between 10° and 45° with respect to the circumferential direction and having the same direction as the angle formed by the inextensible reinforcing elements of the working layer of crown reinforcing elements (33) radially adjacent to the protective ply.

13. Tyre (1) according to claim 1, characterized in that, The crown reinforcement (3) further has a triangular layer formed of metal reinforcing elements forming an angle of greater than 45° with the circumferential direction.

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