Pneumatic Tire Having a Lightweight Crown Reinforcement

Abstract

The invention relates to a tire (1) comprising a crown reinforcement formed of two working crown layers (41, 43), the calendering layers of which comprise carbon black, and of a layer of circumferential reinforcing elements (42).

According to the invention, the reinforcing elements of the working crown layers (41, 43) are metal cords having a diameter of less than 1.3 mm, at least one thread of each metal cord of at least one working crown layer is of at least UHT grade, the tensile elastic modulus at 10% elongation of at least the radially outermost calendering layer of at least the radially outermost working crown layer (43) is less than 8.5 MPa and at least said radially outermost calendering layer of at least the radially outermost working crown layer (43) has a macrodispersion Z value of greater than 85.

Claims

1.-15. (canceled)

16. A tire for a vehicle of heavy duty type, having a radial carcass reinforcement comprising a crown reinforcement formed of at least two working crown layers each comprising metal reinforcing elements inserted between two calendering layers of elastomer compound comprising a reinforcing filler consisting of at least carbon black, the crown reinforcement being capped radially by a tread, said tread being connected to two beads via two sidewalls, the crown reinforcement comprising at least one layer of circumferential reinforcing elements, wherein the reinforcing elements of the working crown layers are metal cords having a diameter of less than 1.3 mm, wherein at least one thread of each metal cord of at least one working crown layer is of at least UHT grade, wherein the tensile elastic modulus at 10% elongation of at least the radially outermost calendering layer of at least the radially outermost working crown layer is less than 8.5 MPa and wherein at least said radially outermost calendering layer of at least the radially outermost working crown layer has a macrodispersion Z value of greater than 85.

17. The tire according to claim 16, wherein the maximum value of tan(), denoted tan()max, of at least said radially outermost calendering layer of at least the radially outermost working crown layer is less than 0.080.

18. The tire according to claim 16, wherein at least said radially outermost calendering layer of at least the radially outermost working crown layer is an elastomer compound based on natural rubber or on synthetic polyisoprene with a predominance of cis-1,4-linkages and optionally on at least one other diene elastomer, the natural rubber or synthetic polyisoprene in the case of a blend being present at a predominant content relative to the content of the other diene elastomer(s) used and on a reinforcing filler consisting: a) either of carbon black used at a content of between 20 and 80 phr, b) or of a blend of carbon black and a white filler, in which the overall filler content is between 20 and 80 phr, and preferably between 40 and 60 phr, said white filler being of silica and/or alumina type, comprising SiOH and/or AlOH surface functions selected from the group consisting of precipitated or fumed silicas, aluminas or aluminosilicates, or else carbon blacks modified during or after synthesis, with a BET specific surface area of between 30 and 260 m.sup.2/g.

19. The tire according to claim 16, wherein the reinforcing elements of at least one working crown layer are cords comprising an internal layer of M internal thread(s) and an external layer of N external threads, the external layer being wound around the internal layer.

20. The tire according to claim 19, wherein M=1 and N=5 or 6, or M=2 and N=7, 8 or 9.

21. The tire according to claim 19, wherein the reinforcing elements of the working crown layers are cords comprising an internal layer of M internal thread(s) and an external layer of N external threads, the external layer being wound around the internal layer, with M=1 or 2 and N=5, 6, 7 or 8, at least one of the internal or external threads of each cord, exhibiting a mechanical breaking strength R expressed in MPa such that R41802130D, D being the diameter of the thread expressed in mm.

22. The tire according to claim 19, wherein the reinforcing elements of said at least two working layers are cords comprising an internal layer of M internal thread(s) and an external layer of N external threads, the external layer being wound around the internal layer, with M=1 or 2 and N=5, 6, 7 or 8, at least one of the internal or external threads of each cord, exhibiting a mechanical breaking strength R expressed in MPa such that R44002000D, D being the diameter of the thread expressed in mm.

23. The tire according to claim 16, wherein at least said radially outermost calendering layer of at least one protective layer has an electrical resistivity per unit volume such that log() is greater than 8.

24. The tire according to claim 16, wherein the metal reinforcing elements of at least said protective layer are cords having a flow rate of less than 5 cm.sup.3/min in the permeability test.

25. The tire according to claim 16, wherein the layer of circumferential reinforcing elements is positioned radially between two working crown layers.

26. The tire according to claim 16, wherein 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.

27. The tire according to claim 16, wherein the reinforcing elements of said at least two working crown layers are crossed from one layer to the other, making angles of between 10 and 45 with the circumferential direction.

28. The tire according to claim 16, wherein the reinforcing elements of said at least two working crown layers are inextensible metal cords.

29. The tire according to claim 16, wherein the crown reinforcement is supplemented radially on the outside by at least one additional ply, referred to as a protective ply, of elastic reinforcing elements, oriented at an angle of between 10 and 45 relative to the circumferential direction and of the same direction as the angle formed by the inextensible elements of the working ply which is radially adjacent thereto.

30. The tire according to claim 16, wherein the crown reinforcement also comprises a triangulation layer formed of metal reinforcing elements forming angles of greater than 60 with the circumferential direction.

Description

[0119] Further details and advantageous features of the invention will become evident hereinafter from the description of an exemplary embodiment of the invention given with reference to the FIGURE, which depicts a meridian view of a diagram of a tire according to one embodiment of the invention.

[0120] For ease of understanding, the FIGURE is not drawn to scale.

[0121] The FIGURE shows only a half-view of a tire which extends symmetrically about the axis XX, which represents the circumferential median plane, or equatorial plane, of the tire.

[0122] In the FIGURE, the tire 1, of size 295/80 R 22.5, comprises a radial carcass reinforcement 2 anchored in two beads, not shown in the FIGURE. The carcass reinforcement 2 is formed of a single layer of metal cords. It further comprises a tread 5.

[0123] In the FIGURE, the carcass reinforcement 2 is hooped in accordance with the invention by a crown reinforcement 4 formed radially, from the inside to the outside: [0124] of a first working layer 41 formed of metal cords oriented at an angle equal to 26, [0125] of a layer of circumferential reinforcing elements 42, formed of 2123 steel metal cords, of the bimodulus type, [0126] of a second working layer 43 formed of metal cords oriented at an angle equal to 18 and crossed with the metal cords of the first working layer, the cords of each of the working layers being oriented on either side of the circumferential direction, [0127] of a protective layer 44 formed of elastic 18.23 metal cords, in which the spacing at which the cords are distributed is equal to 2 5 mm, which are oriented at an angle equal to 18 on the same side as the cords of the second working layer.

[0128] The axial width L.sub.41 of the first working layer 41 is equal to 214 mm.

[0129] The axial width L.sub.42 of the layer of circumferential reinforcing elements 42 is equal to 154 mm.

[0130] The axial width L.sub.43 of the second working layer 43 is equal to 194 mm.

[0131] The axial width L.sub.44 of the protective layer 44 is equal to 162 mm.

[0132] The reinforcing elements of the two working layers are metal cords of formula 9.30 of UHT type, having a diameter equal to 1.23 mm. They are distributed in each of the working layers with a spacing P equal to 2.25 mm.

[0133] The threads constituting the metal cords have a mechanical breaking strength R equal to 3556 MPa and therefore satisfy the relationship R41802130D.

[0134] In accordance with the invention, the tensile elastic modulus at 10% elongation of the calendering layers of the protective layer 43 is less than 8.5 MPa and the macrodispersion Z value is greater than 85.

[0135] The value of log(), which expresses the electrical resistivity of the calendering layers of the protective layer 43, is greater than 8 ohm.Math.cm.

[0136] The maximum value of tan(), denoted tan()max, of the calendering layers of the working crown layers 42 and 43 is less than 0.080.

[0137] The cumulative weight of the working layers, of the protective layer and of the layer of circumferential reinforcing elements of the reference tire, comprising the weight of the metal cords and of the calendering compounds, amounts to 9.8 kg.

[0138] The tire I according to the invention is compared to a reference tire T1 of the same dimension which differs from the tire according to the invention by metal cords of the two working layers which are cords of formula 9.35 of SHT type, having a diameter equal to 1.35 mm. They are distributed in each of the working layers with a spacing equal to 2.5 mm.

[0139] The cumulative weight of the working layers, of the protective layer and of the triangulation layer of the reference tire T1, comprising the weight of the metal cords and the calendering compounds, amounts to 10.4 kg.

[0140] The reference tires T further differ from the tires I according to the invention by the calendering compounds of the working crown layers 41 and 43, especially their tensile elastic modulus at 10% elongation and the Z value.

[0141] The tire I according to the invention is further compared to a second tire T2 which differs from the tire according to the invention solely by the nature of the calenderings of the working layers, identical to those of the tire T1.

[0142] The various compounds used are listed below.

TABLE-US-00001 Com- Com- Com- Com- pound R1 pound R2 pound 1 pound 2 NR 100 100 100 100 Black N347 52 50 Black N326 47 Black N234 40 Antioxidant 1 1.5 1 1 (6PPD) Stearic acid 0.65 0.9 0.65 0.65 Zinc oxide 9.3 7.5 9.3 9.3 Cobalt salt 1.12 1.12 1.12 1.12 (CoAcac) Sulfur 6.1 4.5 6.1 6.1 Accelerator 0.93 0.8 0.93 0.93 DCBS Retarder CTP 0.25 0.15 0.25 0.25 PVI MA.sub.10 (MPa) 10.4 5.99 6.4 5.3 tan().sub.max 0.130 0.099 0.069 0.060 Resistivity 4 6 9 >10 (logrho) Z value 77 80 92 89

[0143] The tires I according to the invention are produced with working crown layers, the calenderings of which consist of compounds chosen from the compounds 1 and 2.

[0144] Reference tires T1 and T2 are produced with working crown layers, the calenderings of which consist of the compound R1 or of the compound R2.

[0145] Tests were carried out with tires I according to the invention and with reference tires T1.

[0146] First endurance tests were carried out on a test machine that forced each of the tires to run in a straight line at a speed equal to the maximum speed rating prescribed for said tire (the speed index) under an initial load of 4000 kg gradually increased in order to reduce the duration of the test.

[0147] Other endurance tests were carried out on a test machine that cyclically imposed a transverse loading and a dynamic overload on the tires. The tests were carried out for the tires according to the invention under conditions identical to those applied to the reference tires T1.

[0148] The tests thus carried out showed that the distances covered during each of these tests are substantially identical for the tires according to the invention and the reference tires T1. It is thus apparent that the tires according to the invention exhibit performance which is substantially equivalent in terms of endurance to that of the reference tires T1 when running on bituminous surfaces.

[0149] Tests aimed at characterizing the breaking strength of a tire crown reinforcement subjected to shock loadings were also carried out. These tests consist in pressing cylindrical-shaped polars against the tread of the tire inflated to a recommended pressure. The values express the energy required to obtain breakage of the crown block. The values are expressed with reference to a base 100, corresponding to the value measured for the reference tire.

TABLE-US-00002 T1 100 Invention 99
These results show that, despite lightening the tire by decreasing the mass of its crown reinforcement, the breaking energy during a shock loading on the surface of the tread is substantially equivalent.

[0150] Other tests corresponding to endurance tests were carried out by running with vehicles travelling on a running surface consisting of damaging stones that become trapped in the void regions of the tread pattern of the tire tread. The vehicles then move into a tank of saline solution in order to allow the corrosive liquid to propagate within the tire via the cracks formed due to the damage caused by the stones.

[0151] After sufficient running, the reinforcing elements of the working crown layers are analyzed. The measurements carried out correspond to corroded lengths of reinforcing elements and numbers of breakages of said reinforcing elements.

[0152] Identical measurements are carried out on the tires I produced according to the invention, after covering an identical distance to that covered by the tires T2 under the same conditions.

[0153] The results are expressed in the following table with reference to a base 100 fixed for the reference tires T2. One base 100 is fixed for corroded lengths of reinforcing elements and another base 100 for the count of breakages of reinforcing elements.

TABLE-US-00003 Tire T2 Tire I Corroded length 100 80 Number of breakages 100 70

[0154] These tests show especially that the design of the tires according to the invention makes it possible to delay the corrosion of the elements of the working crown layers and is therefore favorable to performance in terms of the endurance of the tires, despite lightening the tire by decreasing the mass of its crown reinforcement.

[0155] These same tests were reproduced on tires in accordance with the invention in which the elastomer compounds constituting the calendering layers of the protective layer are identical to the compounds used for the calendering layers of the working crown layers.

[0156] As above, after sufficient running, the reinforcing elements of the working crown layers, but also the reinforcing elements of the protective layer, are analyzed. As above, the measurements carried out correspond to corroded lengths of reinforcing elements and numbers of breakages of said reinforcing elements.

TABLE-US-00004 Tire T2 Tire I Corroded length in the protective 100 80 layer Number of breakages in the 100 70 protective layer Corroded length in the working 100 70 layers Number of breakages in the 100 60 working layers

[0157] It emerges from these tests that the choice of using identical compounds for the protective layer and the working layers makes it possible to delay the corrosion of the reinforcing elements of the protective layer and further delay the corrosion of the reinforcing elements of the working crown layers.

[0158] Moreover, rolling resistance measurements were taken.

[0159] The results of the measurements are given in the following table; they are expressed in kg/t, a value of 100 being assigned to the reference tire.

TABLE-US-00005 Reference 100 Invention 98