Crown architecture of an electrically conductive tire for a civil engineering vehicle

Abstract

The compromise between the performance aspects of endurance and wear of a tyre for construction plant vehicles is improved, while limiting the mean operating temperature thereof to an appropriate level of around 100° C., and while ensuring its capacity of being electrically conductive, that is to say of discharging electrostatic charges that have built up during running. For this purpose, the tread comprises three parts: two electrically conductive tread wings and a central portion that is optimized in terms of hysteresis and is therefore not electrically conductive. The pathway for discharging electrostatic charges connects the tread wings to the rim, passing via electrically conductive edging rubbers positioned at the axial ends of the layers of the crown.

Claims

1. A tire for a heavy vehicle of construction plant type, the tire comprising: a tread comprising two tread wings, the two tread wings being axial end portions that are axially separated by a tread central portion, and at least one tread wing being made of an electrically conductive elastomeric compound; a protective reinforcement, radially on an inside of the tread, comprising at least one protective layer made of metal reinforcers coated in an electrically conductive elastomeric coating compound, the protective layer being bordered at each of its axial ends by an edging rubber having a curved width L.sub.GBS1 and being made up of an electrically conductive elastomeric compound; a working reinforcement, radially on an inside of the protective reinforcement, comprising at least two working layers made of metal reinforcers coated in an elastomeric coating compound; two sidewalls connecting the two tread wings to two beads that are intended to come into contact with a mounting rim via a bead layer made up of an elastomeric compound; and a carcass reinforcement connecting the two beads together, passing via the two sidewalls, and comprising at least one carcass layer made of metal reinforcers coated in an electrically conductive elastomeric coating compound, wherein each compound has a viscoelastic loss tgδ defined as being a ratio of a viscous shear modulus to an elastic shear modulus, the moduli being measured at a frequency of 10 Hz at a temperature of 60° C. and each compound is characterized by static stiffness measurements at 10% deformation and at 100% deformation according to a standard NFT 46-002 of September 1988, wherein an elastomeric compound of the tread central portion has a composition based on an elastomeric matrix comprising at least one diene elastomer and on a reinforcing filler predominantly comprising a filler covered at least partially with silica, with a dispersion of the reinforcing filler in the elastomeric matrix having a Z score greater than or equal to 70, wherein the edging rubber of the at least one protective layer is at least partially in contact with the electrically conductive elastomeric coating compound of the at least one protective layer and with the axially closest tread wing, wherein a Shore A hardness of the edging rubber is greater than or equal to 60, wherein the static stiffness modulus of the edging rubber at 10% deformation is greater than or equal to 4.5 MPa, wherein the static stiffness modulus of the edging rubber at 100% deformation is greater than or equal to 2.0 MPa, and wherein electrical resistivities of the elastomeric compound of the at least one tread wing, of the elastomeric coating compound of the at least one protective layer, of the elastomeric coating compound of the at least one carcass layer, of the elastomeric compound of the bead layer, of the elastomeric coating compound of the working layers, and of the elastomeric compound of the edging rubber of the at least one protective layer, respectively, are at most equal to 10.sup.6 Ω.Math.cm, such that the at least one tread wing, the edging rubber of the protective layer, the elastomeric coating compounds of the at least one protective layer, the working layers, and the at least one carcass layer, and the elastomeric compound of the bead layer, respectively, constitute a preferential conductive pathway for the electric charges between ground and a rim when the tire is mounted on the rim and in contact with the ground.

2. The tire according to claim 1, wherein the edging rubber of the at least one protective layer has a curved width L.sub.GBS1 at least equal to 100 mm and at most equal to 350 mm.

3. The tire according to claim 1, wherein the edging rubber of the at least one protective layer has a thickness at least equal to 3 mm and at most equal to 20 mm, measured at its middle along a direction normal to a curve of a carcass.

4. The tire according to claim 1, wherein the elastomeric compound of the tread central portion has a viscoelastic loss with a value at most equal to 0.065.

5. The tire according to claim 1, wherein the elastomeric compound of at least one tread wing is an electrically conductive rubber composition based on at least 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.

6. The tire according to 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

(1) FIG. 1 schematically shows a tyre 10 intended to be used on dumper type vehicles.

(2) FIG. 2 shows a configuration of the invention in which the wings and the central portion of the tread have been laid on a base layer 37, radially on the inside of the tread.

(3) In FIG. 1, the tyre 10 comprises a radial carcass reinforcement 50 that is 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 made up of metal cords coated in an electrically conductive elastomeric coating compound. Positioned radially on the outside of the carcass reinforcement 50 is a crown reinforcement 40, itself radially on the inside of a tread 30. The tread 30 comprises, at each axial end, an axial end portion or tread wings 311 and 312. Each axial end portion 311, 312 of the tread is connected to a bead 70 by a sidewall 20. The crown reinforcement 40 is made up of a protective reinforcement that is made up of two protective layers 41 and is radially on the inside of the tread 30. The crown reinforcement also comprises a working reinforcement radially on the inside of the protective reinforcement and comprising working layers 42.

(4) The protective layers 41 are bordered at their axial ends by an edging rubber 36 on either side of the axis of symmetry (OZ). This edging rubber 36 is in contact with the tread wing 311, 312 along its entire axially outer edge.

(5) The working layers 42 are bordered at their axial ends by an edging rubber 35 on either side of the axis of symmetry (OZ).

(6) The top, radially outer, end of the tread wing 31 is in contact with the tread central portion 32 over its entire thickness. Its bottom, radially inner, end is in contact with the edging rubber 36.

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

(8) FIG. 2 shows a tread that is symmetric about the equatorial plane and comprises two axial end portions or tread wings that are axially separated by a central portion. The entire tread, namely the wings plus the central portion, is laid on a base layer 37.

(9) The edging rubbers are not necessarily made of the same elastomeric material. The curved length of the edging rubber of the protective layers is denoted L.sub.GBS1.

(10) The invention was studied more particularly in the case of a tyre for a dumper type vehicle, of size 59/80 R63, according to the invention, and as shown in FIG. 1.

(11) The results of the invention were found on a tyre produced according to the invention and compared with the simulation results obtained on a reference tyre of the same size, comprising a one-piece tread, according to the prior art. In the case of the reference tyre, the electrostatic charges are discharged by the tread, which is electrically conductive.

(12) Table 1 below shows an example of the composition of the elastomeric compound of such a standard electrically conductive tread:

(13) TABLE-US-00001 TABLE 1 Elastomer Carbon NR (Natural black Stearic Compositions Rubber) N234 Silica (1) Antioxidant Paraffin acid ZnO Accelerator Sulfur Tread of the 100 35 10 3 1 2.5 2.7 1.4 1.25 reference tyre (1) Zeosil 1165MP silica, sold by Rhodia

(14) Still on the reference tyre, the composition of the elastomeric compound of the sidewall is standard, as indicated below in Table 2:

(15) TABLE-US-00002 TABLE 2 Elastomer Elastomer Carbon NR (Natural BR (Butadiene black Plasticizer Stearic Composition Rubber) Rubber) N33O (1) Wax Antioxidant ZnO acid Sulfur Accelerator Elastomeric compound of 50 50 55 18 1 3 2.5 1 0.9 0.6 the sidewall of the reference tyre (1) TDAE oil, Vivatec 500 from Klaus Dahleke

(16) For the tyre according to the invention, the inventors proposed the following compositions of the elastomeric compounds of the edgings of the protective layers and working layers, compiled in Table 3:

(17) TABLE-US-00003 TABLE 3 Elastomer Carbon OPF NR (Natural black Stearic Epoxy Cobalt Composition Rubber) N326 Antioxidant ZnO acid Sulfur DCBS resin salt CTP Elastomeric compound of 100 47 1.5 7.5 0.9 5.63 0.8 0.5 1.5 0.15 the edging of the protective and working layers

(18) The same elastomeric compound is therefore used for the edgings of the protective layers and the working layers. This compound needs to have mechanical properties suitable for the function of blocking shear stresses at the axial ends of the protective layers and working layers, as set out in Table 4 below:

(19) TABLE-US-00004 TABLE 4 Elastomeric compound of the edging of the protective Results and working layers Electrical resistivity Log (Ω .Math. cm) 5.2 Shore A 67 MA10 5.2 MPa MA100 2.6 MPa

(20) The mechanical properties are consistent with those expected, and the electrical resistivity of less than 10.sup.6 ohm.Math.cm ensures that this compound is electrically conductive.

(21) The wings of the tread need to be an electrically conductive portion, and the central portion needs to be optimized in terms of hysteresis. Table 5 gives examples of compositions:

(22) TABLE-US-00005 TABLE 5 Elastomer Carbon Black covered NR (Natural black with silica Antioxidant Stearic ZnO Accelerator Compositions Rubber) (1) (2) (3) Paraffin acid (4) (5) Sulfur Wings of the tread 100 50 NA 1.5 1 1 2.7 1.7 1.2 (ML20874) Central portion of 100 NA 50 1.5 1 1 2.7 1.7 1.2 the tread (1) N134, sold by Cabot Corporation (2) CRX2125, sold by Cabot Corporation (3) N-(1,3-Dimethylbutyl)-N-phenyl-para-phenylenediamine, Santoflex 6-PPD, sold by Flexsys (4) Industrial grade zinc oxide, sold by Umicore (5) N-Cyclohexyl-2-benzothiazolesulfenamide, Santocure CBS, sold by Flexsys

(23) The properties of these elastomeric compounds, which are measured on test specimens and result from the choices of chemical composition, are compiled in Table 6:

(24) TABLE-US-00006 TABLE 6 Elastomeric Elastomeric compound of compound of the tread Properties the tread wing.sup.(1) central portion Electrical resistivity in Log (Ω .Math. cm) 5.7 >11.6 Z score 52 88 Tanδ.sub.max return 0.126 0.062 Wear performance 100 117 .sup.(1)On the reference tyre, the tread is made of this same compound

(25) In a construction plant tyre, the quantity of elastomeric compound of the tread represents around 35% to 40% of the total mass of elastomeric compounds of the tyre. The tread is thus one of the main sources of hysteresis, and therefore it contributes greatly to the increase in temperature of the tyre. Consequently, the elastomeric compound of the central portion of the tread is designed to have low hysteresis with a dynamic viscoelastic dissipation of around 0.062 measured in tan δ.sub.max return, for a temperature of 100° C. and for a frequency of 10 Hz.

(26) The elastomeric compound of the tread central portion has a composition based on at least one diene elastomer, a reinforcing filler predominantly comprising a filler covered at least partially with silica, an agent for coupling the filler to the elastomer and a crosslinking system, characterized in that the dispersion of the filler in the elastomeric matrix has a Z score greater than or equal to 70.

(27) The inventors have demonstrated clear synergy of the combination of a filler covered with silica with its very good dispersion of the composition, for obtaining a tyre for a heavy vehicle of construction plant type that exhibits both improved rolling resistance and improved wear resistance properties.

(28) In this embodiment, the tread central portion is electrically insulating. Electrostatic charges are thus discharged along the conduction pathway defined by the invention, which passes via the tread wings that are in contact with the ground and are always electrically conductive.

(29) For the elastomeric compound of the tread wings, with the overall filler content being 45 phr, with 35 phr of carbon black and 10 phr of silica, this ensures an electrical resistivity less than or equal to 10.sup.6 ′Ω.Math.cm. The same elastomeric compound 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 condition imposed is that of having at least, at one of the two axial ends of the tread, an elastomeric compound having an electrical resistivity less than or equal to 10.sup.6 ′Ω.Math.cm.

(30) The results on tyres were obtained by finite-element calculations in order to determine the viscoelastic heat sources, the temperature and the electrical resistance.

(31) Finite-element calculations were carried out on the tyre of the invention and the reference tyre.

(32) The standard tread of the reference tyre does not comprise tread wings or a base layer. It is in one piece.

(33) The results of calculations for the reference tyre are shown below in Table 7:

(34) TABLE-US-00007 TABLE 7 Results Reference tyre Electrical resistance of the tyre <1 Megaohm Viscoelastic sources (total of the tyre) 21 kW Maximum temperature ° C. 109° C.

(35) The reference tyre is electrically conductive with a maximum operating temperature of around 109° C.

(36) For the tyre of the invention, the results of finite-element calculations are summarized in Table 8:

(37) TABLE-US-00008 TABLE 8 Results Tyre of the invention Electrical resistance <10 Megaohm Viscoelastic sources (total of the tyre) 180 kW Maximum temperature ° C. 100° C.

(38) The tread wings in contact with the ground and the sublayer are electrically conductive. The evaluation of the electrical 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 making up the pathway for discharging electrostatic charges.

(39) For the tyre of the invention, compared with the reference tyre, the viscoelastic sources have been reduced by 14%.

(40) As a consequence of the reduction in viscoelastic loss sources, the calculation of the temperature field of the tyre of the invention shows a maximum level of 100° C., this corresponding to a difference of 10% compared with the reference tyre. This difference is sufficient for a significant improvement in the endurance of the tyre of the invention by extending its lifetime by around 30%.

(41) The invention has been presented for a tyre for a construction plant vehicle, but it can actually be extrapolated to other types of tyre.