Electrically conductive crown architecture for a tire of a heavy duty civil engineering vehicle

Abstract

A radial tire (10), with the sidewalls thereof (20), and the tread thereof (30) arranged for minimizing the temperature of the tire while guaranteeing its electrical conductivity. The tread (30) comprises two wings (311, 312) and a central portion (32). These components rest on a base layer (33) radially on the inside of the tread (30). The base layer (33) contains a lateral portion (331, 332) partly in contact with a tread wing (311, 312). This structure of the crown of the tire, in contact with the carcass reinforcement makes it possible to constitute a preferential conductive pathway of the electric charges between the rim and the ground when the tire is mounted on its rim and flattened on the ground.

Claims

1. A tire for heavy vehicle of construction plant type comprising: a tread comprising two axial end portions or tread wings axially separated by a central portion; a base layer, radially on the inside of the tread, comprising at least one lateral portion at least partly in contact with the tread wing; a crown reinforcement, radially on the inside of the base layer, comprising at least one crown layer, having metallic reinforcers that are coated in an electrically-conductive elastomeric compound; two sidewalls in contact at least partly with the tread wings connecting the tread wings to two beads, adapted to come into contact with a mounting rim by means of a bead layer made of electrically-conductive elastomeric compound; each said sidewall being axially on the outside of a carcass reinforcement comprising at least one carcass layer having metallic reinforcers that are coated in an electrically-conductive elastomeric coating compound; at least one tread wing having a first elastomeric compound M.sub.1 having a thermal conductivity λ.sub.1 and an electrical resistivity ρ.sub.1; the central tread portion having a second elastomeric compound M.sub.2 having a viscoelastic loss tgδ.sub.2; the base layer having a third elastomeric compound M.sub.3 having a thermal conductivity λ.sub.3 and an electrical resistivity ρ.sub.3; each said sidewall having a fourth elastomeric compound M.sub.4 having a viscous dynamic shear modulus G″4, wherein the first elastomeric compound M.sub.1 of at least one said tread wing has a thermal conductivity λ.sub.1 at least equal to 0.190 W/m.Math.K, wherein the second elastomeric compound M.sub.2 of the central tread portion has a viscoelastic loss tgδ.sub.2 at most equal to 0.06, wherein the third elastomeric compound M.sub.3 of the base layer has a thermal conductivity λ.sub.3 less than the thermal conductivity λ.sub.1 of the first elastomeric compound M.sub.1 of the at least one tread wing, wherein the electrical resistivities ρ.sub.1 and ρ.sub.3 respectively of the first elastomeric compound M.sub.1 and of the third elastomeric compound M.sub.3 are at most equal to 106 ′Ω.Math.cm, so that the bead layer, the elastomeric coating compound of the carcass layer, the coating compound of the at least one crown layer, the base layer, and the tread wing constitute a preferential conductive pathway of the electric charges between the rim and the ground when the tire is mounted on its rim and flattened on the ground, and wherein the fourth elastomeric compound M.sub.4 of each said sidewall has a viscous dynamic shear modulus G″.sub.4 at most equal to 0.125 MPa.

2. The tire according to claim 1, at least one said lateral portion of the base layer having an axial width, wherein the axial width of the lateral portion of the base layer is at least equal to 200 mm.

3. The tire according to either of claim 1, wherein the base layer is formed by two separate said lateral portions each having an axial width L.sub.331 and L.sub.332.

4. The tire according to either of claim 3, wherein the base layer is formed by two separate said lateral portions, respectively formed by the same third elastomeric compound M.sub.3.

5. The tire according to one of claim 1, wherein the base layer is formed by two separate said lateral portions, the respective axial widths of which L.sub.331 and L.sub.332 are equal.

6. The tire according to either of claim 1, wherein the base layer is formed by a single portion, in continuous contact with the entire central tread portion and in contact at least partly with at least one said tread wing.

7. The tire according to any one of claim 1, wherein at least one said lateral portion of the base layer is in contact at least partly with a said tread wing over a length at least equal to 10 mm.

8. The tire according to any one of claim 1, wherein the first elastomeric compound M.sub.1 of at least one said tread wing is an electrically-conductive rubber composition based at least on polyisoprene, on a crosslinking system and on at least one reinforcing filler comprising carbon black, having a BET surface area at least equal to 110 m.sup.2/g, and a content at least equal to 30 phr and at most equal to 80 phr.

9. The tire according to any one of claim 1, wherein the second elastomeric compound of the central tread portion is a rubber composition based on at least one diene elastomer, on a crosslinking system, and on a reinforcing filler comprising carbon black, having a BET surface area at most equal to 115 m.sup.2/g, and a content at most equal to 40 phr, and silica, at a content at most equal to 15 phr.

10. The tire according to any one of claim 1, wherein the second elastomeric compound of the central tread portion is a rubber composition based on at least one diene elastomer, on a crosslinking system, and on a reinforcing filler, at an overall content at most equal to 40 phr, and comprising carbon black, and silica.

11. The tire according to any one of claim 1, wherein the second elastomeric compound M.sub.2 of the central tread portion is an electrically-conductive rubber composition based on at least one diene elastomer, on a crosslinking system, and on a reinforcing filler comprising carbon black, having a BET surface area at least equal to 120 m.sup.2/g, and a content at least equal to 35 phr and at most equal to 80 phr, and silica, at a content at most equal to 15 phr.

12. The tire according to any one of claim 1, wherein the third elastomeric compound M.sub.3 of the base layer of the tread is an electrically-conductive rubber composition based at least on polystirene, on a crosslinking system, and on at least one reinforcing filler comprising carbon black, having a BET surface area at least equal to 110 m.sup.2/g, and a content at least equal to 30 phr and at most equal to 80 phr.

13. The tire according to any one of claim 1, wherein the elastomeric compound of each sidewall has a rubber composition based on at least one blend of polyisoprene, natural rubber or synthetic polyisoprene, and polybutadiene, on a crosslinking system, and on a reinforcing filler, at an overall content at most equal to 45 phr, and comprising carbon black, at a content at most equal to 5 phr, and, predominantly, silica, at a content at least equal to 20 phr and at most equal to 40 phr.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The architecture of the tire according to the invention will be better understood with reference to FIG. 1, not to scale, which represents a meridian half section of a tire.

(2) FIG. 1 schematically represents a tire 10 intended to be used on Dumper type vehicles.

(3) FIGS. 2, 3, and 4 represent the various possible configurations of the tread wings, of the base layer relative to the central portion.

DETAILED DESCRIPTION OF THE DRAWINGS

(4) In FIG. 1, the tire 10 comprises a radial carcass reinforcement 50, anchored in two beads 70 and turned up, in each bead, around a bead wire 60. Each bead 70 comprises a bead layer 71 intended to come into contact with a rim flange. The carcass reinforcement 50 is generally formed of a single carcass layer, consisting of metal cords coated in an elastomeric coating compound. Positioned radially on the outside of the carcass reinforcement 50 is a crown reinforcement (not referenced), itself radially on the inside of a tread 30. The tread 30 comprises, at each axial end, an axial end portion or tread wing 311 and 312, axially on the outside of a central tread portion 32. The tread 32, and the tread wings 311, 312 rest on a radially inner base layer 311, 312. Each tread axial end portion 311, 312 is connected to a bead 70 via a sidewall 20.

(5) The radially outer top end of the tread wing 31 is in contact with the central tread portion 32 over its entire thickness. Its radially inner bottom end is in contact with the radially inner base layer of the tread over a length (L.sub.c1, L.sub.c2) at least equal to 10 mm.

(6) The base layer (33) is formed by two separate lateral portions (331, 332) each having an axial width L.sub.331 and L.sub.332. In the example used this width is equal to 200 mm.

(7) The objective is to ensure a permanent contact between the electrically-conductive elastomeric compounds, in twos, in order to guarantee the continuity of the pathway for discharging the electrostatic charges, taking into account the manufacturing tolerances.

(8) FIG. 2 represents a tread that is symmetrical relative to the equatorial plane comprising two axial end portions or tread wings that are actually separated by a central portion. The inner end of the tread wing, in the axial direction, is located at a given distance L.sub.1 relative to the equatorial plane. The other outer end of the tread wing, still in the axial direction, is positioned at a distance of L.sub.2 of the same equatorial plane. For the base layer, the axially inner (respectively axially outer) end is located at a distance L.sub.3 (respectively L.sub.4) from the equatorial plane.

(9) FIG. 3 represents a tread laid in a continuous base that spreads over the entire width of the crown of the tire.

(10) FIGS. 4 and 5 represent a tread that is not symmetrical relative to the equatorial plane. In FIG. 4, the tread wing is positioned only on the vehicle exterior side (reference 100), and in FIG. 5, it is positioned only on the vehicle interior side (reference 110).

(11) The invention has more particularly been studied on a tire for a Dumper type vehicle, of dimensions 59/80 R63, in accordance with the invention, and as represented in FIG. 1.

(12) The results calculated on the tire produced according to embodiments of the invention are compared to those obtained for a reference tire of the same dimensions, comprising a sidewall having a single layer, and a tread made of a single portion, in accordance with the prior art.

(13) The inventors have established the connection between the chemical composition of the elastomeric compounds and the physical parameters such as the electrical resistivity, the thermal conductivity, and the viscoelastic loss. By way of example, represented on the graph from the appended FIG. 6, for two elastomeric compounds, are the curves of thermal conductivities as a function of the amount of reinforcing fillers in phr. These curves show that the elastomeric compound filled with silica is optimized for the hysteresis, but with a thermal conductivity that is relatively lower than the elastomeric compound filled with carbon black, for which the electrical conductivity property is favoured.

(14) According to the curve from FIG. 6, for a given content of filler, for example carbon black, it is possible to predict the value of the thermal conductivity of the elastomeric compound. The thermal conductivities are measured at an ambient temperature of from 23° C. to 25° C. The dependency of the thermal conductivity relative to the temperature is not taken into account here.

(15) The inventors determined the composition of the elastomeric compounds, constituting the sidewall layer, the tread wings, the central portion of the tread and the base layer by finding a compromise between the following physical parameters: the dynamic viscoelastic loss or the viscous shear modulus which are directly connected to the viscoelastic heat sources; the thermal conductivity which controls the thermal conduction of the heat in the compounds; the electrical conductivity which must be at a level sufficient for discharging electrostatic charges.

(16) In the example studied, the compositions of the elastomeric compounds, resulting from this compromise, are summarized in Table 1 below:

(17) TABLE-US-00001 TABLE 1 Elastomeric Elastomeric Elastomeric compound M.sub.1 compound M.sub.2 Elastomeric compound M.sub.4 of the tread of the central compound M.sub.3 of the sidewall Composition wing tread portion of the base layer layer Elastomer NR 100 100 *  100 50 (Natural Rubber) Elastomer BR NC NC NC 50 (Butadiene Rubber) Carbon black N330 NC NC NC NC Carbon black N234 35 35 *  35 3 Silica (2) 10 10   10 29 Plasticizer (3) NC NC NC 10 Wax NC NC NC 1 Antioxidant 3 3   3 3 ZnO 2.7 2.7 2.7 2.5 Stearic acid 2.5 2.5 2.5 1 Sulfur 1.25  1.25 1.25 1 Accelerator 1.4 1.4 1.4 0.8 * elastomeric compound M.sub.2 obtained via a liquid route (2) “Zeosil 1165MP” silica sold by Rhodia (3) “Vivatec 500” TDAE oil from Klaus Dahleke

(18) Table 2 brings together the physical parameters of the elastomeric compounds, measured on test specimens and resulting from choices of chemical composition:

(19) TABLE-US-00002 TABLE 2 Elastomeric Elastomeric Elastomeric compound M.sub.1 compound M.sub.2 Elastomeric compound M.sub.4 of the tread of the central compound M.sub.3 of the sidewall Composition wing tread portion of the base layer layer Thermal  0.240  0.240  0.240 0.208 conductivity at 25° C. (W/m .Math. K) Electrical 5.7  10.4  5.7  11.6   resistivity in Log (′Ω .Math. cm) Viscous shear NC NC NC 0.125 modulus G″ max at 60° C. and 10 Hz (in MPa) Elastic shear 1.33 1.16 1.33 NC modulus G*max (50%, 100° C. and 10 Hz) Dynamic loss 0.10 0.06 0.10 NC tgδ.sub.max (50%, 100° C. and 10 Hz)

(20) In a construction plant tire, the amount of elastomeric compound of the tread represents around 35% to 40% of the total mass of elastomeric compounds of the tire. The tread is thus one of the main sources of hysteresis, and it therefore contributes greatly to the increase in temperature of the tire. Consequently, the elastomeric compound M.sub.2 of the central tread portion is designed to have a low hysteresis with a dynamic viscoelastic loss of the order of 0.06, measured at a temperature of 100° C., and at a frequency of 10 Hz.

(21) In one preferred embodiment of the invention, the elastomeric compound M.sub.2 of the central tread portion has a composition which comprises at least one diene elastomer and a reinforcing filler that may be a blend of carbon black and silica, so that the carbon black has a content at most equal to 40 phr and a BET surface area at most equal to 115 m.sup.2/g and the silica has a content at most equal to 15 phr. The elastomer—carbon black mixture is obtained beforehand preferentially via a liquid route.

(22) In this embodiment, the central tread portion is electrically insulating. The discharging of the electrostatic charges is then carried out along the conduction pathway defined by the invention which passes through the tread wings in contact with the ground and which are always electrically conductive.

(23) For the elastomeric compound M.sub.1 of the running tread wings, the overall filler content being 45 phr, with 35 phr of carbon black and 10 phr of silica, guarantees an electrical resistivity of less than or equal to 10.sup.6 ′Ω.Math.cm, and a suitable thermal conductivity. In the example dealt with here, the thermal conductivity of the tread wing is equal to 0.240 W/m.Math.K. The same elastomeric compound M.sub.1 is used for the two tread wings positioned at the two ends of the tread, but the invention still remains valid if different materials are used. The required condition is to have at least, at one of the two axial ends of the tread, an elastomeric compound with an electrical resistivity of less than or equal to 10.sup.6 ′Ω.Math.cm.

(24) In a tire for a construction plant vehicle, the mass of the elastomeric compounds of the sidewall is of the order of 15% of the total mass of compounds of the tire. The option selected by the inventors is to have an elastomeric compound for the sidewall of low hysteresis with a viscous shear modulus fixed at 0.125 MPa. Since the sidewall is not involved in the pathway for conducting electrostatic charges, there is therefore no need to fill the compound with for example carbon black. A filler mainly with silica in a proportion of 29 phr, vs 3 phr carbon black is used to achieve the target of low hysteresis.

(25) The results on tires were obtained by finite element calculations is in order to determine the viscoelastic heat sources, the temperature and the electrical resistivity.

(26) Finite element calculations were carried out on the tires of the invention and reference tires respectively. The reference tire comprises a standard sidewall layer and a standard tread. For example, the standard sidewall layer is an elastomeric compound formulation according to the following proportions:

(27) TABLE-US-00003 TABLE 3 Standard compound of the sidewall layer Composition of the reference tire NR (Natural Rubber) 50 BR (Butadiene Rubber) 50 Carbon black N330 55 Carbon black N234 NC Silica (2) NC Plasticizer (3) 18 Wax 1 Antioxidant 3 ZnO 2.5 Stearic acid 1 Sulfur 0.9 Accelerator 0.6

(28) The standard tread of the reference tire comprises neither tread wings nor base layer. It is made of a single portion.

(29) The results of calculations for the reference tire are represented below in Table 4:

(30) TABLE-US-00004 TABLE 4 Tread made of a Results Single sidewall layer single portion Electrical 4.4 5.7 resistivity Log (′Ω .Math. cm) Viscoelastic 4520 5100 sources (W) Maximum 99.8 90 temperature° C.

(31) The reference tire is electrically conductive with an average operating temperature of the order of 90.4° C.

(32) For the tire of the invention, the results of the finite element calculations are summarized in Table 5:

(33) TABLE-US-00005 TABLE 5 Single sidewall Base Tread Central tread Results layer layer wing portion Electrical 11.6 5.7 5.7 10.4 resistivity Log (′Ω .Math. cm) Viscoelastic 2270 514 751 4540 sources (W) Maximum 88.9 78.5 65.5 85.9 temperature° C.

(34) The finite element calculations confirm the electrically insulating nature of the sidewall layer and of the central tread portion. The tread wing in contact with the ground and the underlayer are, on the other hand, electrically conductive. The evaluation of the electric potential confirms the conduction pathway with levels of electrical resistivity ranging from 10.sup.4 ′Ω.Math.cm to 10.sup.6 ′Ω.Math.cm for the elastomeric compounds constituting the pathway for discharging the electrostatic charges.

(35) For the tire of the invention, relative to the reference tire, the viscoelastic loss sources were halved in the sidewall of the tire, and in the tread the reduction is also significant.

(36) As a consequence of the drop in the viscoelastic loss sources, the calculation of the temperature field of the tire of the invention shows a maximum level of 88.9° C., which corresponds to a difference of 10% relative to the reference tire. This difference is sufficient for a significant improvement in the endurance of the tire of the invention by prolonging its service life by around 30%.

(37) The invention has been presented for a tire for a construction plant vehicle, but it can in fact be extrapolated to other types of tire.

(38) The scope of protection of the invention is not limited to the examples given hereinabove. The invention is embodied in each novel characteristic and each combination of characteristics, which includes every combination of any features which are stated in the claims, even if this feature or combination of features is not explicitly stated in the examples.