BACKGROUND OF THE INVENTION

[0001] This invention relates generally to a tire. More specifically, this invention relates to tires for heavy trucks meant to often go off-road, especially tires for military vehicles.

[0002] One problem with tires being used off-road is the occurrence of punctures and crown cable ruptures especially those caused when rolling over sharp rocks.

SUMMARY OF THE INVENTION

[0003] The invention provides for an off-road heavy truck tire comprising a sub- casing, a belt package and a rubber tread, the belt package comprising at least two working plies, the at least two working plies comprising reinforcement cables and defining a belt package width, the rubber tread comprising tread blocks and tread grooves, the rubber tread comprising a radially lower layer and a radially upper layer, the radially upper layer being intended to come into contact with the ground and the radially lower layer being interposed between the belt package and the radially upper layer, the radially upper layer consisting in an upper layer rubber compound, the radially lower layer consisting in a lower layer rubber compound which is different from the upper layer rubber compound, the lower layer rubber compound having a thickness of at least 2.0mm over the belt package width wherein the lower layer rubber compound has a complex shear modulus for 50% strain (G*50) at 60°C of at least 1.45 MPa.

[0004] In another embodiment, the lower layer rubber compound has a max tan(5) not greater than 0.095.

[0005] In another embodiment, the lower layer rubber compound has a complex shear modulus by 50% strain (G*50) at 60°C of at least 1.55 MPa. [0006] In another embodiment, the thickness of the lower layer is greater in tread blocks than under the tread grooves.

[0007] In another embodiment, the thickness of the lower layer in the tread blocks is greater than 50% of a tire tread depth, the tread depth being defined by the tread grooves.

[0008] The foregoing and other objects, features and advantages of the invention will be apparent from the following more detailed descriptions of a particular embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which:

[0010] FIG. 1 is a section view of an embodiment of a heavy truck tire according to the invention.

[0011] FIG. 2 is a partial section view of a heavy truck tire according to another embodiment.

[0012] FIG. 3 is a partial section view of a heavy truck tire according to another embodiment.

[0013] FIG. 4 is a partial section view of a heavy truck tire according to another embodiment.

[0014] FIG. 5 is a partial section view of a heavy truck tire according to another embodiment.

[0015] The use of the same or similar reference numerals in the figures denotes the same or similar features. DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0016] Reference will now be made in detail to embodiments of the invention, examples of which are illustrated in the drawings. These examples are provided by way of explanation of the invention.

[0017] As shown in section by FIG. 1, a heavy truck tire 1 generally comprises a crown portion 11 connected by respective sidewalls 12, 12' to beads portions 13, 13'. More specifically, one or more body plies 15 run from a bead core 14 in a bead area 13 to an opposite bead core 14' in an opposite bead portion 13'. Tire 1 is substantially symmetrical about the equatorial plane EP. Therefore, equatorial plane EP bisects tire 1 into opposing halves of substantially the same construction.

[0018] Beads, body ply(s) and sidewalls are together generally referred to as the tire sub-casing. In the crown portion of the tire, belts 16, 17 and 18, are wound circumferentially around the body ply or plies 15. Belts 16 and 17 are often called "working plies" because they provide reinforcements at angle to each other and to the body ply(ies) 15. Belt 18 is often referred to as a "protector ply". As used herein, the term "ply" or "plies" refers to a reinforcement layer of the tire and is not limited to a particular method of manufacturing a tire or the ply itself. The assembly 19 of those circumferential belts is referred as a "belt package". A belt width BW is defined as the greatest axial distance between remote belt edges of any belt in the belt package 19. This reinforced structure may comprise more plies or a different arrangement of them depending upon the exact tire type and upon its manufacturing process but this principle is widely known and used in most radial tires. The assembly of the sub-casing and the belt package is generally referred as the casing.

[0019] The tread 1 is attached around the above described reinforced structure. The tread is the part of the tire that is coming into contact with the ground when the tire is rolling. The tread may comprise any number of grooves 23 in any shape or form to provide grip over different kind of surfaces and to drain water from the contact patch. In a new tread, the depth of those grooves is often referred to as the tread depth.

[0020] According to this embodiment, the tread 2 is composed of at least two different rubber compounds generally disposed one radially outside the other. Because treads and tires are often represented in a section view as in FIG 1, those two layers are referred as being a lower layer 22 and an upper layer 21. The lower layer 22 can also be referred to as an "under-tread" and the upper layer 21 can also be referred to as an "upper- tread". Such treads composed of two layers may also be described as having a "base/cap" arrangement.

[0021] The lower layer 22 covers at least the width BW of the belt package 19 and is at least 2.0 mm thick over the same width.

[0022] As far as the invention is concerned, the tires and their treads are substantially symmetrical about equatorial plane EP. Therefore, FIGS 2 to 5 showing other embodiments are partial views limited to the top left part of the view of FIG 1.

[0023] In the embodiment of FIG 2, the lower layer 22 is going up to the bottoms of the grooves 23, thus having a thickness in the order of at least 5mm over the full width of the belt package.

[0024] In the embodiment of FIG 3, the lower layer 22 is going up even higher than the bottoms of the grooves 23, having a thickness in the order of 10mm outside the grooves.

[0025] In the embodiment of FIG 4, the lower layer 22 is going up to the bottom of the grooves 23 as in FIG 3 and is going higher than the bottoms of the grooves in the parts of the tread forming the tread blocks.

[0026] In the embodiment of Figure 5, the lower layer 22 is going higher than the bottom of the grooves in the parts of the tread forming the blocks or the tread as in FIG 4 but it is limited to a similar thickness to the one in FIG 1 in the regions facing the bottoms of grooves 23. The lower layer compound is not exposed in the grooves just as in FIG 1 despite its much greater thickness in the blocks.

[0027] In particular embodiments disclosed herein, the lower layer may be characterized as being of a material that is generally more rigid and having a higher hysteresis than those tires of the prior art that were also provided with a tread lower layer. The inventors have discovered improved tire performance may be achieved by replacing the traditional soft, low hysteresis tread lower layer with a more rigid, higher hysteresis material.

[0028] The loss factor "tan(5)" is measured in accordance with ASTM D5992-96 and the complex shear modulus is captured on the go or outward cycle of the sweep of the shear, is a dynamic property of the rubber compound. It is measured on a viscosity analyzer (Metravib VA4000) according to Standard ASTM D5992-96. The response of a test specimen consisting of two cylindrical pellets each 2 mm thick and one centimeter in diameter is recorded (the test specimen is made from samples taken from a tire mid-way up the height of the layer concerned as close as possible to the region of the equatorial plane in a region that is thick enough to be able to form the test specimen), the specimen being subjected to simple alternating sinusoidal shear loadings at a frequency of 10 Hz, at a temperature of 60° C. The sweep covers amplitude of deformation from 0.1 to 50% peak to peak (on the outbound cycle) then from 50% to 1% peak to peak (on the return cycle). During the outbound cycle, the maximum value of tan(5) that is observed is denoted "max tan(5)".The complex dynamic shear modulus is denoted "G*50" in reference to the 50% strain applied during the test.

[0029] More particularly the tread lower layer may be described as having a complex shear modulus at 50% strain G*50 at 60° C that is at least 1.45 MPa or alternatively at least 1.5 MPa or at least 1.55 MPa. The complex shear modulus is also measured in accordance with ASTM D5992-96 and the complex shear modulus is captured on the go or outward cycle of the sweep of the shear.

[0030] It should be noted that the complex shear modulus of the tread lower layer at the higher rigidity may be close to or even greater than the complex shear modulus of the tread upper layer. The tread upper layer, being designed for contact with the road, is typically made of a higher rigidity material than that of the tread lower layer of the prior art tires but surprisingly the treads disclosed herein provide the improved performance with a much more rigid tread lower layer. As such, an upper limit for the complex shear modulus of the tread lower layer may be, in particular embodiments for example, less than 1.95 MPa or alternatively less than 1.90 MPa or 1.85 MPa. It may be noted that these upper limits of the complex shear modulus defined herein may be combined with any of the lower limits to define the acceptable complex shear modulus range for the lower tread layer material for particular embodiments.

[0031] While the tread lower layer may not be characterized as having as low a hysteresis as those skilled in art may have thought was required, the lower tread layer still has a fairly low hysteresis. Particular embodiments of the rubber compositions disclosed herein may be characterized, in addition to the requirement of the complex shear modulus discussed above, as having a max tan delta of less than 0.095 or alternatively less than 0.09 or less than 0.08.

[0032] The tread lower layer and the tread upper layer may be made of typical materials used in the tire industry. Such materials are based upon a rubber composition reinforced with a filler such as carbon black, an inorganic filler or combinations thereof. The term "based upon" as used herein recognizes that the treads or other rubber articles are made of vulcanized or cured rubber materials that were, at the time of their assembly, uncured. The cured rubber material is therefore "based upon" the uncured rubber composition. In other words, the cross-linked rubber composition is based upon the cross -linkable rubber composition. The term rubber and elastomer are used interchangeable.

[0033] Useful elastomers include diene elastomers. Diene elastomers or rubber is understood to mean those elastomers resulting at least in part (i.e., a homopolymer or a copolymer) from diene monomers (monomers bearing two double carbon-carbon bonds, whether conjugated or not). Essentially unsaturated diene elastomers are understood to mean those diene elastomers that result at least in part from conjugated diene monomers, having a content of members or units of diene origin (conjugated dienes) that are greater than 15 mol.%.

[0034] Thus, for example, diene elastomers such as butyl rubbers, nitrile rubbers or copolymers of dienes and of alpha-olefins of the ethylene-propylene diene terpolymer (EPDM) type or the ethylene-vinyl acetate copolymer type do not fall within the preceding definition, and may in particular be described as "essentially saturated" diene elastomers (low or very low content of units of diene origin, i.e., less than 15 mol. %). Particular embodiments of the present invention include no essentially saturated diene elastomers.

[0035] Within the category of essentially unsaturated diene elastomers are the highly unsaturated diene elastomers, which are understood to mean in particular diene elastomers having a content of units of diene origin (conjugated dienes) that is greater than 50 mol.%. Particular embodiments of this disclosure may include not only no essentially saturated diene elastomers but also no essentially unsaturated diene elastomers that are not highly unsaturated.

[0036] The rubber elastomers suitable for use with particular embodiments of the treads disclosed herein include highly unsaturated diene elastomers, for example, polybutadienes (BR), polyisoprenes (IR), natural rubber (NR), butadiene copolymers, isoprene copolymers and mixtures of these elastomers. The polyisoprenes include synthetic cis-1,4 polyisoprene, which may be characterized as possessing cis-1,4 bonds at more than 90 mol.% or alternatively, at more than 98 mol.%.

[0037] Also suitable for use in particular embodiments are rubber elastomers that are copolymers and include, for example, butadiene-styrene copolymers (SBR), butadiene- isoprene copolymers (BIR), isoprene- styrene copolymers (SIR) and isoprene-butadiene- styrene copolymers (SBIR) and mixtures thereof. [0038] Suitable highly unsaturated elastomers for the rubber compositions may include polybutadienes (BR), polyisoprenes (IR), natural rubber (NR), butadiene copolymers, isoprene copolymers, butadiene- styrene copolymers (SBR), butadiene-isoprene copolymers (BIR), isoprene- styrene copolymers (SIR) and isoprene-butadiene-styrene copolymers (SBIR) and mixtures thereof.

[0039] The treads disclosed herein may be constructed of a single diene elastomer or a mixture of several diene elastomers, the diene elastomer(s) possibly being used in association with any type of synthetic elastomer other than a diene one, or even with polymers other than elastomers, for example thermoplastic polymers.

[0040] In addition to the elastomer the rubber compositions further include a reinforcing filler, such filler being inorganic, organic or combinations thereof. The inorganic reinforcing filler is to be understood here to mean any inorganic or mineral filler, whatever its color and its origin (natural or synthetic), also referred to as "white" filler or sometimes "clear" filler in contrast to carbon black. Such inorganic filler is capable, on its own, without any other means than an intermediate coupling agent, of reinforcing a rubber composition intended for the manufacturing of a rubber article in its reinforcement function. Such fillers may include, for example, a filler of the siliceous or aluminous type, or a mixture of these two types of fillers.

[0041] The silica (S1O2) used may be any reinforcing silica known to the person skilled in the art. Particular embodiments include any precipitated or pyrogenic silica having a BET surface area and a specific CTAB surface area both of which are less than 450 m /g, or from 30 to 400 m /g. Highly dispersible precipitated silicas (referred to as "HD") are included in particular embodiments, in particular for those embodiments used for the manufacturing of tires having a low rolling resistance. "Highly dispersible silica" is understood in known manner to mean any silica having a substantial ability to disagglomerate and to disperse in an elastomeric matrix, which can be observed in known manner by electron or optical microscopy on thin sections. Non-limitative examples of such highly dispersible silicas include the silicas BV3380 and Ultrasil 7000 from Degussa, the silicas Zeosil 1165 MP and 1115 MP from Rhodia, the silica Hi-Sil 2000 from PPG, the silicas Zeopol 8715 or 8745 from Huber, and treated precipitated silicas such as, for example, the aluminum- "doped" silicas.

[0042] The reinforcing alumina (AI2O3) used in particular embodiments is a highly dispersible alumina having a BET surface area from 30 to 400 m /g, or between 60 and 250 m /g, an average particle size at most equal to 500 nm, or at most equal to 200 nm. Non- limitative examples of such reinforcing aluminas are in particular the aluminas A 125 or CR125 (from Baikowski), APA-100RDX (from Condea), Aluminoxid C (from Degussa) or AKP-G015 (Sumitomo Chemicals).

[0043] The physical state in which the reinforcing inorganic filler is present is immaterial, whether it is in the form of a powder, micro-beads, granules, balls or any other densified form.

[0044] Of course "reinforcing inorganic filler" is also understood to mean mixtures of different reinforcing inorganic fillers, in particular of highly dispersible siliceous and/or aluminous fillers such as described above.

[0045] The amount of reinforcing inorganic filler may be between 10 and 120 phr, or between 20 and 100 phr approximately, in particular when the tread is intended for a passenger-car tire. The person skilled in the art will readily understand that the optimum will be different according to the nature of the reinforcing inorganic filler used and according to the type of tire in question, for example a tire for a motorcycle, passenger vehicle or alternatively for a utility vehicle such as a van or a heavy vehicle. The amount of reinforcing inorganic filler is not meant to be limited and may be at any quantity suitable for a particular purpose.

[0046] Carbon black, which is an organic filler, may be used as a sole filler or in combination with one or more inorganic fillers. The compounding amount of the carbon black in the elastomer composition is not limited. In particular embodiments of the present invention, the compounding amount of the carbon black may be up to about 200 phr or between about 10 and about 180 phr. Other useful ranges of carbon black loading may include between 30 and 120 phr in some embodiments or alternatively between 10 and 50 phr.

[0047] Suitable carbon blacks are any carbon blacks, in particular the blacks of the type HAF, ISAF and SAF, which are conventionally used in tires, and particularly in treads. Non-limitative examples of carbon blacks include, for example, the N115, N134, N234, N330, N339, N343, N347, N375, N550, N650, N665 and N787 carbon blacks.

[0048] In addition to the elastomer, reinforcement filler, particular embodiments of the rubber composition may further include all or part of the additives usually used in sulfur- cross-linkable diene rubber compositions intended for the manufacturing of treads or other rubber articles, such as, for example, plasticizers, including plasticizer oils and/or resins, pigments, protective agents of the type antioxidants, antiozonants, a cross-linking system based either on sulfur or on sulfur and/or peroxide and/or bismaleimide donors, vulcanisation accelerators, vulcanisation activators, extender oils, and so forth. There may also be associated with the reinforcing inorganic filler, if necessary, a conventional non-reinforcing white filler, such as for example particles of clay, bentonite, talc, chalk, kaolin or titanium oxides.

[0049] The rubber compositions that are embodiments of the present invention may be produced in suitable mixers, in a manner known to those having ordinary skill in the art, typically using two successive preparation phases, a first phase of thermo-mechanical working at high temperature, followed by a second phase of mechanical working at lower temperature.

[0050] The first phase of thermo-mechanical working (sometimes referred to as "non-productive" phase) is intended to mix thoroughly, by kneading, the various ingredients of the composition, with the exception of the vulcanization system. It is carried out in a suitable kneading device, such as an internal mixer or an extruder, until, under the action of the mechanical working and the high shearing imposed on the mixture, a maximum temperature generally between 110° C and 190° C, more narrowly between 130° C and 170° C, is reached.

[0051] After cooling of the mixture, a second phase of mechanical working is implemented at a lower temperature. Sometimes referred to as "productive" phase, this finishing phase consists of incorporating by mixing the vulcanization (or cross-linking) system (sulfur or other vulcanizing agent and accelerator(s)), in a suitable device, for example an open mill. It is performed for an appropriate time (typically between 1 and 30 minutes, for example between 2 and 15 minutes) and at a sufficiently low temperature lower than the vulcanization temperature of the mixture, so as to protect against premature vulcanization.

[0052] It is well within the skill set of those skilled in the art to compound rubber compositions that are suitable for use as tread lower layer materials. As one example, rubber compositions were prepared using the components shown in the following materials table in accordance with standard procedures known in the art.

Materials Table

[0053] All the components except for the sulfur were mixed in a Banbury mixer until a temperature of between 130° C and 170° C was reached. The mixture was then dropped and cooled on a mill where the sulfur was added. The rubber composition was cured, cut into testing plaques and then tested in accordance with the procedures provided above. The test results are provided in materials table.

[0054] The inventive rubber composition Fl is significantly more rigid with a G*50 of 1.6 MPa compared to the witness tread lower layer material of Wl that had a G*50 of 1.1 MPa. As is noted below, the inventive rubber composition Fl was used in a tire test.

[0055] The typical cable break caused by sharp rocks happens when a stone is pressed and becomes wedged into a tire groove. The stone slowly develops a crack in the groove bottom rubber material and creeps radially inward towards the carcass while straining the belt cables. When they become strained above their mechanical limits, cables eventually break, being ruptured in tension. Tensile ruptures are easily recognized because they typically show signs of necking due to a reduction of the cross-sectional area at the break surface as opposed to fatigue type ruptures that do not show a reduction of cross-sectional area.

[0056] To compare cable break resistance between different tires, a specific test consists in rolling those tires at their maximum rated load and pressure on a rocky track and comparing the number of broken cables in the crown portion of each tire. The rolling must cover a sufficient distance for a significant number of damages to appear. The rolling may also be stopped in the event of a non-repairable perforation of one of the test tires.

[0057] After the test is complete the tires are cut open for inspection. Each crown belt is peeled away and divided into sectors by marking. Each sector is visually inspected for cable breaks. The number of broken cables is recorded for each sector and the results are averaged over all sectors of a given tire. This average can then be prorated according to the test distance for comparing to other test sessions.

[0058] Using this test, a tire according to the invention was compared to a commercially available reference tire. The reference tire used was a MICHELIN XZL in size 395/85R20. The test tire is identical to the reference tire except for the tread using a test lower layer rubber compound Fl as described above and in the configuration shown on FIG 5. The test was stopped after 1185 km when the reference tire suffered a non-repairable perforation.

[0059] The test results are summarized in the table below:

[0060] Compared to the reference tire, the test tire shows a significant improvement at resisting cable breaks in all three plies of the belt package. Globally, the test tire suffered less than half the number of cable breaks found in the reference tire.

[0061] An additional benefit was found with configurations like the one of FIG 5 where the thickness of the lower layer is greater in the blocks than under the grooves. This additional benefit resides in the possibility to increase the speed rating of the tire. This is believed to be caused by significantly lower energy dissipation in the base of the tread blocks.

[0062] It should be understood from the foregoing description that various modifications and changes may be made to the embodiments of the present invention without departing from its true spirit. The foregoing description is provided for the purpose of illustration only and should not be construed in a limiting sense. Only the language of the following claims should limit the scope of this invention. [0063] As used herein, "radially-outward' refers to a radial direction away from the axis of rotation while "radially-inward" refers to a radial direction towards the axis of rotation.

[0064] The present invention may be used with treads of other constructions having a different arrangement of ribs, grooves, tread blocks, and combinations thereof.